Institute of Chemical Technology of Organic Materials

JOHANNES KEPLER

UNIVERSITY LINZ

Altenberger Str. 69

4040 Linz, Austria

www.jku.at

DVR 0093696

Submitted by

Annika Wagner

Submitted at

Institute of Chemical

Technology of Organic

Materials

Supervisor and

First Examiner

Univ. Prof. DI Dr. Christian

Paulik

Second Examiner

Univ. Prof.in Drin Sabine Hild

April 2019

Curing strategies for

polyimide-like and

foamable inks for 3D

printing

Doctoral Thesis

to obtain the academic degree of

Doktorin der technischen Wissenschaften

in the Doctoral Program

Technische Wissenschaften

April 1, 2019 Annika Wagner 2/118

STATUTORY DECLARATION

I hereby declare that the thesis submitted is my own unaided work, that I have not

used other than the sources indicated, and that all direct and indirect sources are

acknowledged as references.

This printed thesis is identical with the electronic version submitted.

Linz, 01.04.2019

Signature


PREFACE AND ACKNOWLEDGEMENTS

This Dissertation was acquired at Profactor GmbH and the Institute for Chemical Technology of Organic Materials at the Johannes Kepler University Linz. The work presented here was carried out as part of the EU funded project DIMAP (grant agreement number 685937) and the MultiLink project, funded by the Austrian Federal Ministry for Transport, Innovation and Technology (grant agreement number BMVIT-612.015). This work was partly funded within the Strategic Economic and Research Program “Innovative Upper Austria 2020” by the Government of Upper Austria.

I would like to thank my supervisor Univ. Prof. DI Dr. Christian Paulik for his help during

the last years and all the good discussions and ideas which resulted therefrom.

I am grateful to Anita Fuchsbauer, Leo Schranzhofer and Michael Mühlberger for their

project- and research related advice.

A big thanks to Helene Außerhuber, Sonja Kopp and Hannes Leichtfried, who helped

with measurements and were always ready to help when problems occurred. Of

course, I want to thank all my other colleagues from the FON group at Profactor for

their help and the exceptional working atmosphere.

Thanks to Fabian Haiböck, Matthias Kaltenbrunner, Marco Röcklinger, Victoria

Rudelstorfer and Christina Staudinger for helping with measurements.

I am also grateful to Univ. Profin. Drin. Sabine Hild for granting access to her Microtomy

and DMA equipment and to Prof. Dr. Milan Kracalik, who gave me a lot of theoretical

and practical advice about rheological measurements.

Furthermore, I would like to thank my family, especially my mother and grandparents

for their financial support and the motivation to continue this project.

Last but not least, thanks to my companion Michael for his love and support; he gave

me a lot of energy and reminded me of the important things besides career and

education.


ABSTRACT

PolyJetTM 3D printing is an inkjet-based process, which enables additive manufacturing of

complex multi-material polymer parts with high resolution. In this process, a 3D object is built

up by jetting layers of photo-curable liquid precursors from multiple printheads, each containing

a distinct material. Each layer is cured immediately after deposition by UV-irradiation from

mercury lamps which are mounted on the sides of the printhead-block. Characterization of the

UV-curing kinetics is an important part in the process development, as the properties of the

final printed object depend strongly on the extent of curing. The photo-reactivity of the ink can

be adjusted by changing the precursor types and the type and amount of photoinitiator (PI).

Up to now, the ink components for inkjet-based 3D printing processes are mostly based on

acrylates. Polyacrylates are limited in their thermal and chemical stability and their mechanical

properties. Thus, it is of great interest to develop new functional materials for the PolyJetTM

technology. The new inks also require a special curing strategy, as in contrast to the standard

UV-curable inks, photo-polymerization is not the only process leading to the final cured object.

Polyimide-like inks contain bismaleimide (BMI) oligomers, which are UV-curable to yield highly

crosslinked polyimides through polyaddition. The formulation requires solvent to decrease the

viscosity to allow ink-jettability. The solvent is evaporated before full curing of the layer to avoid

solvent trapping, achieved by a near-infrared (NIR) lamp in addition to the UV lamps. The

thermal- and UV curing kinetics of selected BMI oligomers were characterized by Fourier

Transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). The

polyimides had high thermal stability (>400°C) and chemical resistance, characterized by

thermogravimetry (TGA), Dynamic Mechanical Thermal Analysis (DMTA) and solvent

resistance tests.

Due to their molecular structure, BMIs can serve both as polymerizable monomers and as PIs.

For this reason, the applicability of BMIs for a PI-free ink containing acrylates was investigated.

A BMI-acrylate mixture with suitable viscosity for inkjet-printing was formulated and the

photopolymerization with mercury lamps and light emitting diodes (LEDs) was followed via

FTIR measurements. It was shown that BMI serves as a PI for the acrylate and significantly

reduces oxygen inhibition. The acrylate-BMI ink showed good printability and the resulting co-

polymer is highly thermally stable (380°C) due to their high crosslinking degree.

Foamable inks consist of an acrylate matrix with a blowing agent (BA) inside, which is

decomposed through thermal energy input, releasing a significant amount of gases to trigger

foaming of the ink. To stabilize the foam, the BA decomposition is followed immediately by a

UV curing step. A foamable ink with 2.5 wt% BA was formulated and a printing and curing

strategy was developed including low dose UV pinning to increase viscosity, NIR-induced

foaming and UV curing. The creation of stable foamed layers was possible and the resulting

foams were characterized by microscopy of cross-sections and with profilometry.


KURZFASSUNG

Der PolyJetTM 3D Druck basiert auf dem Inkjet-Verfahren und ermöglicht die additive Fertigung

von komplexen multimaterial-Bauteilen mit hoher Auflösung. Mittels dieses Prozesses wird ein

3D-Objekt Lage für Lage aufgebaut, indem UV-härtbare Polymervorstufen von mehreren

Druckköpfen gedruckt werden, wobei jeder Druckkopf ein anderes Material enthält. Jede Lage

wird sofort mittels an der Druckkopfeinheit montierten Lampen UV-gehärtet. Die

Charakterisierung der UV-Aushärtekinetik ist ein wichtiger Schritt in der PolyJetTM

Prozessentwicklung, da die Eigenschaften des gedruckten Objekts von dem Aushärtegrad

abhängen. Die Photoreaktivität der Tinte kann mittels Monomerauswahl und Anpassen des

Photoinitiators (PI) eingestellt werden. Die kommerziell erhältlichen Komponenten für den

inkjet-basierten 3D Druck sind meist Acrylate. Polyacrylate zeigen jedoch Grenzen in ihrer

thermischen und chemischen Stabilität, sowie den mechanischen Eigenschaften, weshalb es

von großem Interesse ist, neue funktionelle Materialien für den PolyJetTM Druck zu entwickeln.

Die neuen Tinten benötigen auch eine spezielle Aushärtestrategie, da im Gegensatz zu den

klassischen UV härtenden Tinten die Photopolymerisation nicht der einzige Prozess ist, der

zum finalen gehärteten Objekt führt. Polyimid-artige Tinten enthalten UV härtbare Bismaleimid

(BMI) Oligomere, die durch Polyaddition hochvernetzte Polyimide bilden. Die

Tintenformulierung enthält Lösungsmittel, um eine niedrige Viskosität für den Inkjet Druck zu

erreichen. Dieses muss vor der vollständigen Härtung entfernt werden, um

Lösemitteleinschlüsse zu vermeiden, was mittels einem Nahinfrarot (NIR) Emitter zusätzlich

zu den UV-Lampen erreicht wird. Die Aushärtekinetik von ausgewählten BMI Oligomeren

wurde mittels Fourier-Transform Infrarot (FTIR) Spektroskopie und Dynamischer

Differenzkalorimetrie (DSC) charakterisiert. Die Polyimide wiesen eine hohe thermische

Stabilität (>400 °C) und chemische Resistenz auf, was durch Thermogravimetrie (TGA),

Dynamisch mechanischer thermischer Analyse (DMTA) und Löslichkeitstests bestätigt wurde.

Da BMI Oligomere aufgrund ihrer molekularen Struktur sowohl als polymerisierbare Monomere

als auch als PI dienen können, wurde die Anwendbarkeit für eine PI-freie BMI-Acrylat Tinte

untersucht. Die UV-Härtung der BMI-Acrylattinte wurde mittels FTIR Messungen verfolgt. Es

bestätigte sich, dass das BMI als PI für das Acrylat diente und dabei die Sauerstoffinhibierung

signifikant erniedrigte. Die PI-freie Tinte zeigte eine gute Druckbarkeit und das resultierende

Copolymer war durch den hohen Vernetzungsgrad thermisch sehr stabil (>380 °C).

Schäumbare Tinten bestehen aus einer Acrylatmatrix mit Treibmittel, welches durch Hitze

zerfällt und dabei Gase freisetzt, die zum Schäumen der Tinte führen. Um den Schaum zu

stabilisieren, folgt sofort ein UV-Härtungsschritt. Für eine Tinte mit 2.5 w% Treibmittel wurde

eine Druck- und Aushärtestrategie entwickelt, die aus UV-Fixierung, NIR-induziertem

Schäumen und UV-Härten besteht. Es gelang die Herstellung von geschäumten Schichten

welche mittels Profilometrie und Querschnitt-Mikroskopie charakterisiert wurden.


TABLE OF CONTENTS

1. Introduction ........................................................................................................... 8

1.1. Objective of this thesis ................................................................................... 8

1.1.1. The DIMAP Project .............................................................................. 8

1.1.2. The MultiLink Project ............................................................................ 9

1.2. Inkjet printing and PolyJetTM-3D-printing ........................................................ 9

1.2.1. Theory of inkjet printing ........................................................................ 9

1.2.2. Inkjet ink formulation .......................................................................... 10

1.2.3. Inkjet printing of functional materials .................................................. 12

1.2.4. Inkjet-based 3D printing (PolyJetTM-3D-printing) ................................ 14

1.2.5. The process of photopolymerization .................................................. 15

1.2.6. Overview of UV sources for photopolymerization............................... 17

1.2.7. Curing in inkjet printing ....................................................................... 18

1.2.7.1. Cure monitoring .................................................................... 18

1.3. Polyimide-like inks for PolyJetTM-3D-printing ................................................ 26

1.3.1. Polyimides .......................................................................................... 26

1.3.2. Bismaleimides .................................................................................... 27

1.3.3. Polyimide-like ink formulation using Bismaleimides ........................... 31

1.4. Photoinitiator-free bismaleimide-acrylate based inks for Inkjet-printing ........ 33

1.4.1. Photoinitiators and Photosensitizers .................................................. 33

1.4.1.1. Type I Photoinitiators ............................................................ 33

1.4.1.2. Type II Photoinitiators ........................................................... 34

1.4.1.3. Exemplary Photointiators ...................................................... 34

1.4.2. Bismaleimides as photoinitiators ........................................................ 35

1.4.3. Electron donor-acceptor mechanism .................................................. 35

1.4.4. Oxygen inhibition ................................................................................ 36

1.4.5. Bismaleimides to reduce oxygen inhibition ........................................ 39

1.5. Foamable acrylate-based inks for PolyJetTM-3D-printing ............................. 41

1.5.1. Polymeric Foams ............................................................................... 41

1.5.1.1. Thermoplastic foams ............................................................ 41

1.5.1.2. Thermosetting foams ............................................................ 42

1.5.1.3. Blowing Agents ..................................................................... 42

1.5.1.4. Mechanism of foam formation .............................................. 43

1.5.1.5. Foam characterization .......................................................... 44

1.5.2. Strategies for 3D printing of foams ..................................................... 46


1.5.2.1. Closed cell foam using thermally expandable microspheres in

acrylate matrices .................................................................. 47

1.5.2.2. Open cell foam using chemical blowing agents in acrylate

matrices ................................................................................ 49

2. Results and Discussion ....................................................................................... 52

2.1. Polyimide-like inks for PolyJetTM-3D-printing ................................................ 52

2.1.1. Supporting Information for: Cure kinetics of bismaleimides as basis for

polyimide-like inks for PolyJet™-3D-printing ...................................... 69

2.2. Photoinitiator-free bismaleimide-acrylate based inks for Inkjet-printing ........ 73

2.2.1. Supporting Information for: Photoinitiator-free Photopolymerization of

acrylate-bismaleimide mixtures and their application for inkjet printing

........................................................................................................... 88

2.3. Foamable acrylate-based inks for PolyJetTM-3D-printing ............................. 90

2.3.1. Supporting Information for: Foamable acrylic based ink for the

production of light weight parts by inkjet based 3D printing ............. 110

3. Summary and Conclusions ............................................................................... 116

3.1. Polyimide-like inks for PolyJetTM-3D-printing .............................................. 116

3.2. Photoinitiator-free bismaleimide acrylate based inks for inkjet printing ...... 117

3.3. Foamable acrylate based inks for PolyJetTM-3D-printing ............................ 118

Introduction


1. Introduction

1.1. Objective of this thesis

The main goal of this thesis is the development of curing strategies for novel UV

curable polyimide-like inks and foamable acrylic based inks for inkjet printing and

inkjet-based 3D printing. The workflow divides into three parts:

- Polyimide-like inks for PolyJetTM-3D printing: Within this task, the selection

and adaptation of a suitable method for characterization of the thermal- and UV

curing behaviour of polyimide precursors is carried out. It includes

characterization of the influence of different parameters like type of precursor,

usage of photoinitiator in the case of UV curing and temperature in the case of

thermal curing. The goal is to understand the curing process of the precursors

and adapt it for use in a 3D printing application, using conventional curing

sources like mercury halide lamps and alternatives such as high-powered UV

LEDs with various wavelengths.1

- Photoinitiator-free bismaleimide-acrylate based inks for inkjet printing: In

this part, the usage of bismaleimides as photoinitiators and co-monomers for

acrylates is investigated. The central part focusses on the characterization of

the curing kinetics of bismaleimide-acrylate mixtures and the selection of proper

candidates for formulation of inkjet inks.2

- Foamable acrylate-based inks for PolyJetTM-3D-printing: This task includes

the characterization of the photopolymerization kinetics of selected ink

components and the final ink formulation including a blowing agent.

Furthermore, inkjet printing tests together with the development of a suitable

foaming and curing strategy shall be carried out. The resulting foams are

characterized with respect to density and pore morphology.3 The synthesis and

initial characterization of the blowing agents used for foaming of the ink was

carried out within the framework other theses (A. M. Kreuzer4, L. Göpperl5).

1.1.1. The DIMAP Project

The scope of the EU funded project DIMAP (Horizon 2020, “Novel nanoparticle

enhanced Digital Materials for 3D Printing and their application shown for the robotic

Introduction


and electronic industry”, grant agreement number 685937) was to enhance the

currently available possibilities for additive manufacturing via PolyJetTM-3D printing

with four types of functional ink systems: ceramic enhanced, electrically conductive,

high-strength polymeric, and light weight polymeric foams. In this thesis, the focus is

laid on the latter two ink systems. Within the DIMAP project, the developed ink systems

should be used in combination to fabricate two multi-material demonstrators, namely

a robotic arm and a customized luminaire. There, each ink fulfils a certain function, e.g.

the electrically conductive ink is used to produce the electrical connections for the robot

actuators or to power the LEDs of a luminaire. Other materials serve as a heat sink

(ceramic inks) or as structural components (high strength polymeric inks, polyimides

and light weight foamable inks).6

1.1.2. The MultiLink Project

The MultiLink project (“Multimaterial Multilayer Additive Manufacturing by Linking

Nanoimprinting and Inkjetprinting”), funded by the Austrian Federal Ministry for

Transport, Innovation and Technology (grant agreement number BMVIT-612.015),

aims to combine Inkjet-printing and Nanoimprinting to increase the resolution of inkjet

printing and to digitalize the nanoimprinting process. Among others, a big topic in this

project is to fabricate multimaterial-multilayer stacks, for example a combination of

electrically conductive ink in between insulating layers for electrical applications. Within

this project, a photoinitiator-free, UV curable ink, which is thermally very stable after

curing is developed and its application shown for a simple combination with electrically

conductive ink.

1.2. Inkjet printing and PolyJetTM-3D-printing

1.2.1. Theory of inkjet printing

Inkjet printing is commonly known for office and home applications or for graphical art

printing. In contrast to screen printing, where a mask is needed for printing of the

desired pattern, inkjet printing enables a locally defined, digital deposition of the ink,

leading to a high flexibility and freedom in design. Inkjet can be classified as continuous

(CIJ) or drop-on-demand (DOD). In CIJ, the drops are ejected continuously in a stream

by a piezo crystal vibrating with high frequency. An electrostatic field deflects unwanted

drops, which are subsequently collected and returned for reuse. In contrast, DOD inkjet

Introduction


uses a pulse to eject the droplets exactly where needed. This pulse is generated either

piezoelectrically, thermally or electrostatically (see Figure 1). In this work, only piezo

DOD printheads are used.

Figure 1 Principle of continuous inkjet (left): the drops are continuously ejected, while

unwanted drops are deflected and removed via an electrostatic field; and drop on

demand inkjet (right): the drops are generated exactly where needed via a piezoelectric

element, thermally or electrostatically. Reproduced with permission from the IMI

Europe homepage, copyright © 2019 IMI Europe Limited.7

1.2.2. Inkjet ink formulation

Formulating inks for inkjet printing is very demanding, because a number of

requirements have to be taken into account, as the inks need to be ejected through

very small nozzles. Most importantly, the ink should have a low viscosity (8-25 mPa s)

at jetting temperature and a suitable surface tension of about 28-32 N m-1. Classical

inkjet inks are subdivided into the following types:8

- Water-based: Many of the inkjet printers used for home and office applications

use water-based inks. These contain water as the primary solvent. Other

components include colorants, co-solvents, surfactants, polymeric binders and

additives like biocides or anti-corrosion agents. Water based inks are

environmentally- and operator-friendly as the main solvent used is water.

However, the storage stability and open time (the time the inkjet nozzles are still

free and not blocked when the ink is stored in the cartridge without printing) is

limited due to the evaporation of water.

- Solvent-based: Mostly, solvent-based inks are classified as inks, where the

organic solvent is evaporated by active or passive drying after the printing

process. Solvents commonly used in DOD piezo ink formulations include

ethylene- and propylene glycol ethers, or high boiling ketones, such as

Introduction


cyclohexanone. Very high boiling solvents lead to a stable jetting performance,

as solvent evaporation in the nozzles is minimized, but this in turn leads to

higher drying times after printing and more energy consumption. Thus, a

compromise has to be found between stable jetting and an appropriate drying

time. Besides the solvent, colorants, resins or binders and additives such as

surfactants to adapt the surface tension and light stabilizers to protect the

printed film from UV light are part of the formulation. When formulating or using

solvent based inks, several drawbacks have to be considered, such as solvent

smell, release of volatile organic compounds (VOCs) and the related health and

environmental issues.

- UV-curable: UV curable inks are mainly formulated for DOD print heads. These

inks are environmentally friendly, as little to no VOCs are released. Moreover,

the UV curing process is very fast and the curing lamps require less energy than

the conventional dryers and the printed layers are very robust, as the printed

polymers are crosslinked. UV curable ink formulations consist of monomers and

oligomers, colorants such as pigments or dyes, photoinitiators and additives.

The polymerization may occur either via the free radical or the cationic

mechanism. Most formulations used in industry polymerize via the free radical

mechanism using photoinitiators. The monomers mainly used for UV curable

ink formulations are acrylates with varying functionalities. There is a high variety

of acrylate components for ink formulations available on the market, including

mono-, di- and trifunctional acrylates, linear and cyclic, aromatic and aliphatic

monomers and oligomers with various molecular weights. The most commonly

used ones for inkjet applications are difunctional acrylate monomers, which

have usually suitable viscosities, high cure speeds and good film properties. As

an example, Tripropylene-glycol-diacrylate (TPGDA) is shown in Figure 2.8

Figure 2 TPGDA, a difunctional acrylate with low viscosity (15 mPa s at 25 °C)

commonly used in UV curable inkjet formulations.

Introduction


A higher number of functionalities in the precursor molecule increases the

crosslinking density of the resulting polymer. The increased crosslinking density

in turn increases the hardness, solvent resistivity and abrasion resistance, while

reducing the flexibility. Taking this into account, the formulation can be tuned to

the desired end-use of the ink.8 There is a huge diversity of photoinitiators

available for the use in radical photopolymerization. Depending on the desired

film thickness which should be cured, the chemistry of the monomers, and if

pigments are present in the formulation, various systems are available. The

photoinitiators are classified as Type I and Type II. As a result of UV irradiation,

type I photoinitiators undergo a unimolecular bond cleavage, building free

radicals. Type II photoiniators require a co-iniator, which reacts with the excited

state of the photoinitiator, generating free radicals. Type I photoinitiators are the

most used species in industry, and consist for example of aromatic ketones.8,9

The chemistry and working principle of photoiniators will be discussed in more

detail in section 1.4.

Moreover, over the last decades, a vast number of functional inkjet inks have been

developed, including ceramics, electrically conductive inks, or bio-inks containing

proteins and cells, which will be described more closely in the subsequent section.

1.2.3. Inkjet printing of functional materials

Printing functional inks via inkjet printing opens the way to produce components such

as electrical heating wires, batteries, light-weight and insulating components or

biomimetic structures digitally and without creating waste. This enables a very flexible

way of production and high freedom of design. Here, only two types of ink systems,

namely electrically conductive inks and ceramic inks will be discussed, as they were

also part of the DIMAP project and represent a major proportion of the functional inks

developed lately. Foamable inks to produce light weight structures and high-

performance polyimide-inks will be described in section 1.3. and 1.5. , respectively.

Electrically conductive inks used for printed electronics have become a major part of

the inkjet market in the last years. Both in academics and in industry, there has been

increasing interest in carrying out research on this topic, as inkjet printing of conductive

inks offers the possibility to fabricate electronic features down to sizes in the order of

microns and even submicron dimensions on a variety of substrates. In contrast to

currently available methods for metal deposition, such as photolithography or screen

Introduction


printing, significantly lower amounts of metal raw materials and other chemicals are

wasted and process time can be saved. Another advantage of using inkjet printing for

metal deposition is the possibility of digitalizing the process. The design can be

changed very fast and there is no need of fabricating masks for each process. Thus,

for small scale customized printing processes, inkjet printing clearly offers a flexibility

and cost advantage.

Electrically conductive inks can be water-based or organic solvent based. Instead of

pigments, like in the case for graphical printing, these inks contain metal nanoparticles,

for example silver, copper or gold, which provide the ink its desired functionality. In

addition, UV curable monomers or oligomers can be added to the formulation, if higher

layer thicknesses are desired, e.g. when the inks are formulated for 3D printing

applications. Alternatively, also conducive polymers or metal-complexes in solution

may be used as the conductive part of the ink. The most widely used metal

nanoparticles are silver and gold, as they are chemically stable and have low resistivity

after sintering. The particles need to be stabilized in the carrier fluid to yield a stable

dispersion, usually by adding suitable surfactants to the formulation. After the printing

process, conductive inks require a drying and a sintering step in order to remove the

solvent and make the printed film conductive. The sintering temperature of metal

nanoparticles is significantly lower than the melting point of the corresponding bulk

metals, which allows sintering temperatures of 100-300 °C. Thus, temperature

sensitive substrates, i.e. polymers can be used. The sintering may be performed via

exposure to heat, laser, UV or IR irradiation, microwave irradiation or plasma.10

Ceramic inks are used mainly for two applications, namely the decoration of ceramic

tiles and the 3D printing of ceramics.11 More recently, ceramic inks have been used

increasingly for biomedical applications, such as customized additive manufacturing of

bone implants.12,13 Analogously to electrically conductive inks, ceramic inks consist of

ceramic nanoparticles suspended or dispersed in a suitable solvent (water-glycol

mixtures or organic solvents) as liquid carrier. A high solid loading of the ceramic inks

is desired to avoid long drying times and to yield thick layers after printing and

drying/sintering.14 In order to have a stable dispersion without agglomeration and

sedimentation of the nanoparticles, there is a need to stabilize ceramic nanoparticles

in the carrier fluid. A successful stabilization approach for ceramic nanoparticles needs

to overcome the attractive intermolecular forces between the distinct particles.

Generally, there are two stabilizing mechanisms, the electrostatic and the steric

stabilization. In electrostatic stabilization, additional charges are brought to the

Introduction


nanoparticle surface by physical adsorption of charged species for example, leading

to an electric double layer due to counter ions present in the solution surrounding the

particles. This in turn results in the repulsion of particles and to the desired stability of

the dispersion. The approach of electrostatic stabilization is, however, only possible for

diluted, polar ink systems. In contrast, steric stabilization may be used also for inks

with high nanoparticle loading. It is based on attaching macromolecules to the

nanoparticle surface, which serve as a steric barrier between distinct particles.15

To exploit the properties of the specific ceramic materials, a sintering step is performed

after the deposition by inkjet printing. After sintering, the printed parts may be used as

a heat sink for example. Sintering is usually performed at high temperatures in an oven.

1.2.4. Inkjet-based 3D printing (PolyJetTM-3D-printing)

PolyJetTM-3D-printing is a method for additive manufacturing which uses the principle

of inkjet printing. This technique allows the fabrication of complex three-dimensional

multi-material objects from computer-aided Design (CAD) files. PolyJetTM uses multiple

inkjet-printheads, each filled with a distinct material. In the “slicing” process, the printer

software processes the CAD file of the 3D object in such a way, that it is transformed

into a set of files, each representing a single slice of the object. With this information,

the desired 3D object is printed layer by layer onto a build platform and each layer is

immediately solidified after deposition by UV-light, mostly by mercury halide lamps

mounted on the left and right sides of the printhead block (Figure 3).

Figure 3 Schematic depiction of the PolyJetTM printing process: printing tray, moving

downwards during the printing process (a); inkjet printheads moving in plane (b);

printed support material supporting overhanging structures (c); printed object out of the

desired material (d) and UV curing lamps (mercury halide lamps) on the left- and right-

hand sides of the printheads (e). Reproduced with permission from the American

Chemical Society, Chemical Reviews, Copyright © 2017 American Chemical Society.16

Introduction


In order to fabricate undercut structures, a support material is printed to stabilize the

overhangs. The support material is removable from the printed part via mechanical

force, a water jet or dissolving in water, depending on the type of material used.17

1.2.5. The process of photopolymerization

The process of photopolymerization plays a central part in the PolyJetTM-printing

process. It is important to understand the photo-curing kinetics in order to adjust the

printing parameters like printing speed and UV intensity.

As already mentioned, the dominating monomers and oligomers used for radical

photopolymerization in industry are acrylates. The vinyl bonds of acrylates react at high

rates with radicals present in the ink formulation, which makes them valuable for

photocurable formulations. The photopolymerization follows the classical radical

polymerization mechanism (Figure 4) and is usually started using a photoinitiator,

denoted as “In”. These molecules absorb the energy provided by UV irradiation and

decompose to build initiating radicals In•. The monomers M react with the initiating

radicals to build monomer radicals M1•. The radical formation is the rate determining

step for the initiation, as the rate for addition of primary radicals to the monomer

molecules is very high.18

Figure 4 Description of the photoinduced radical polymerization, adapted from

Andrzejewska (2016).18

Introduction


The Initiation is followed by the propagation step, where the polymer chain growth

occurs via the addition of monomer molecules to the radical centre. The propagation

rate depends on the monomer- and total reactive radical concentrations. As a side

reaction, chain transfer may occur, where a macroradical is protonated by a small

molecule such as an initiator or monomer or to a polymer and the radical functionality

is transferred to this molecule or polymer. This may lead to a decrease of molecular

weight or to branching of the resulting polymer. The chain growth is stopped by a

termination reaction, which may follow three different routes, as outlined in Figure 4.

The reaction can either be stopped by a reaction of a macroradical with an initiator

radical (primary termination). This applies when a large amount of photoinitiator (PI) is

present in the formulation or when high light powers are used. For this reason, it is

important to always choose a suitable amount of PI and UV dose for the specific

formulation. The dominant termination process is the bimolecular termination, where

two macroradicals are combining in a bimolecular reaction. Moreover, a

monomolecular chain termination can occur at high conversions or in the case of

network formation after reaching the glass transition temperature. In this case, the

polymer chain is trapped inside the network and is not available anymore for a

reaction.18

In this work, mainly multifunctional acrylates and bismaleimides are used to form highly

crosslinked thermoset polymer networks upon UV irradiation. The process of UV curing

and network formation is depicted in Figure 5.

Figure 5 Photopolymerization of low molecular monomers and oligomers present in

UV curable inkjet inks leading to a crosslinked polymer network.

Introduction


1.2.6. Overview of UV sources for photopolymerization

When the process of photopolymerization started to enter industry, the UV source of

choice was usually a mercury halide lamp (MHL). MHLs have broad emission spectra

over the whole UV range, being able to trigger a high number of photoinitiators (see

Figure 6). However, MHLs also have a number of disadvantages, which lead to a

necessity of developing alternatives. MHLs need a warm-up time of several minutes

until they have reached their full power, thus they are usually not switched on and off

during their application. Moreover, the use of mercury should gradually be decreased

due to environmental reasons, which is supported by governments. Within the last

decades, UV-LEDs as an alternative for MHLs started to become increasingly popular.

UV-LEDs are available with various peak wavelengths from 280 nm to 405 nm. The

most advanced LEDs are 395 nm and 365 nm. LEDs feature some obvious

advantages like better energy efficiency compared to MHLs, the possibility of switching

them on and off frequently and their environmental friendliness. Because of their

limited emission spectrum, however, LEDs often lead to longer curing times, which is

commonly compensated by very high powers.19-21

Figure 6 Overview of the emission spectra of mercury lamps and UV LEDs.

Reproduced with permission from the Phoseon Technology homepage, Copyright ©

2019, courtesy of Phoseon Technology.22

Introduction


1.2.7. Curing in inkjet printing

Curing is an important part in the process development in inkjet-printing. Curing is

defined as the solidification of a liquid system, which can be, depending on the type of

ink used, either through thermal energy input (evaporation of solvent, thermal

polymerization) or through UV-irradiation leading to photopolymerization, respectively

UV-curing. The extent of curing, that is to say the curing degree is important for the

final properties of the resulting polymer layer that is printed. If the layer is not cured to

a suitable degree, the layer is still sticky as there is still residual solvent or monomers

and oligomers, which can lead to migration problems. If residual solvent diffuses into

neighbouring layers, the layers might be damaged. Moreover, the mechanical

properties, such as tensile- and impact strength of not fully cured polymers can be

inferior to those of the reference polymer. For the reasons mentioned above, it is

important to characterize the extent of curing, whenever new inks are developed. For

monitoring the curing degree, several methods have been reported and extensively

used, which will be described more closely in the following section.

1.2.7.1. Cure monitoring

For monitoring the curing degree, several methods have been reported in literature,

including Fourier-Transform Infrared (FTIR) spectroscopy23-26, differential scanning

calorimetry (DSC)27-30, Photo-DSC, Rheology31, Raman spectroscopy32, UV-vis

spectroscopy33 and Nuclear Magnetic Resonance (NMR) spectroscopy34. Here, only

the most widely used technique, namely FTIR-spectroscopy will be described.

FTIR Spectroscopy is very popular as it is easy, quick and broadly applicable. Starting

from the 1990s, researchers have reported on the usage of FTIR for monitoring curing

processes, such as thermal and UV curing of thermosetting resins, like epoxides and

bismaleimides. For this purpose, the bands corresponding to double bonds of the

monomers and oligomers, which are polymerized during the thermal or UV curing

process, are observed. Most beneficial for an appropriate description of the curing is

to use an online-system (“Real-time FTIR”), which records a number of spectra while

the sample is cured. However, for an approximation of the curing kinetics also the

standard FTIR devices are a valuable tool. Figure 7 depicts the approach for

calculating the curing degree from FTIR spectra recorded after the sample has been

irradiated with increasing UV doses. To obtain reliable data, it is important to choose a

Introduction


reference band as an “internal standard”, which compensates phenomena like

shrinkage during curing.35 This band should correspond to bonds, which remain

unchanged during the polymerization process. In this case, the carbonyl bands were

chosen as the reference band.

Figure 7 Approach for evaluation of the curing behavior using FTIR spectroscopy

during thermal curing, shown for the bismaleimide oligomer BMI-689; details are

shown in section 2.1.

Using the approach mentioned above, several kinetic parameters can be extracted

from the experimental data obtained, including the curing degree α, the polymerization

rate Rp and the kinetic rate constant k. Together with the activation energy Ea, these

parameters are of central importance to describe a chemical reaction.

The curing degree α describes the extent of polymerization and is calculated using

decreasing band areas after various curing times according to:

Introduction


𝛼 = (1 −

Adb,t

Aref,t

Adb,0

Aref,0

) ∙ 100 (Equation 1)

𝛼 = (1 − Adb,t∙Aref,0

Adb,0∙Aref,t) ∙ 100, (Equation 2)

where Adb,0 and Adb,t denote the band areas before curing and after the cure time t,

respectively. Analogously, Aref,0 and Aref,t correspond to the band areas of the reference

bands before- and after the cure time t.23

Chemical kinetics describe the process of chemical reactions with respect to rates and

mechanisms. According to whether or how the reaction rate depends on the

concentration of one or more reactants, different reaction orders are defined.36

0 Order Reactions: Reactions belonging to 0. Order show a reaction rate independent

of the concentration of educt. Examples for such reactions include radioactive decay

or reactions involving a catalyst. In this case, the educt concentration decreases

linearly with reaction time. By plotting the concentration of educt [A]t versus the time t,

the kinetic rate constant k can be obtained from the slope of the linear fit.

v =d[A]

dt= - k (Equation 3)

d[A] = - k dt (Equation 4)

∫ d[A] = - k ∫ dtt

0

[A]t[A]0

(Equation 5)

[A]t = − k t + [A]

0 (Equation 6)

1st Order reactions: In reactions with 1st order kinetics, the reaction rate depends on

the concentration of one educt. The concentration decrease shows an exponential

behavior with proceeding time.

v =d[A]

dt= - k [A] (Equation 7)

Introduction


Rearrangement of same variables to the same side gives the following equation:

d[A]

[A]= - k dt (Equation 8)

Both sides are integrated and yield the integrated form of the time law for the reaction.

∫1

[A]∙ d[A] = - k ∫ dt

t

0

[A]t[A]0

(Equation 9)

[A]t = [A]

0∙e−kt (Equation 10)

Via plotting the logarithm of educt concentration [A]t at time t versus the time t, the

kinetic rate constant k is obtained, as shown in the following equation.

ln ([A]t

[A]0) = -k ∙ t (Equation 11)

2nd Order reactions: In reactions with second order kinetics, the reaction rate is

dependent on the concentration of two educts. In this case, two educts react to one or

more products.

v = -d[A]

dt= -

d[B]

dt= - k [A] [B] (Equation 12)

In the case of reactions involving one educt, the rate depends on the square of the

educt concentration.

v = -d[A]

dt= - k [A]

2 (Equation 13)

The time law for 2nd order reactions can again be used to determine the kinetic rate

constant by plotting the inverse concentration of educt 1

[A] against the time t.

1

[A] -

1

[A]0= k t (Equation 14)

As an approximation, 1st order kinetics can be used to describe most radical

photopolymerization reactions, where an exponential decrease of monomers is

observed, where in the beginning of the reaction, the rate is very high and with

increasing conversion, the rate decreases again.37

Introduction


The experimental determination of the progress of curing degree during the

polymerization reaction allows the first approximation of the chemical reaction

examined.

Based on these observations, the polymerization rate and kinetic rate constant can be

calculated using the 1st order kinetic rate equations.

As the reaction rate is defined as the decrease of reaction educts with time, the rate

Rp at a specific measuring time is calculated using the decrease in double bond area,

corresponding to the educt concentration, according to:

Rp=

Adb,t1

Aref,t1 -

Adb,t2

Aref,t2

t2- t1, (Equation 15)

where Rp is the rate of polymerization, Adb,t1 and Adb,t2 correspond to the double bond

band areas at cure times t1 and t2. Analogously, Aref,t1 and Aref,t2 denote the reference

band areas at cure times t1 and t2.1

The kinetic rate constant k for polymerization is determined graphically via plotting the

logarithm of the educt concentration [A]t at time t versus the curing time t (see

Equation 11). In case of a 1st order reaction, the resulting plot is linear for conversions

of up to 80 % and the slope of the function corresponds to the kinetic rate constant.

Figure 8 shows an exemplary plot of ln ([A]t

[A]0) versus t for thermal curing of a

bismaleimide oligomer (BMI-689) at 225 °C with a resulting rate constant of 0.068 ±

0.001 s-1. Analogously to the curing degree and the polymerization rate, the band area

ratios of double bond bands and reference bands are used for determination of the

rate constant k.1

Figure 8 Plot for determination of the kinetic rate constant for thermal polymerization

of an exemplary polyimide precursor (BMI-689).1

Introduction


References

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Materials, Linz, Austria.

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3D printer inks, Master thesis, Johannes Kepler University Linz, Institute of Chemical

Technology of Organic Materials, Linz, Austria.

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Introduction


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Introduction


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Molecular Structure to Chemical Reactivity; Elsevier: Amsterdam, 2006, Chapter 2, p.77.

37 G. Odian, Principles of Polymerization, 3rd edition, John Wiley & Sons, 1991, New York.

Introduction


1.3. Polyimide-like inks for PolyJetTM-3D-printing

1.3.1. Polyimides

Polyimides are a class of high-performance polymers which are known for their

excellent thermal stability (>500°C) and mechanical and dielectric properties. They are

widely used in the aerospace industry, for high performance fibres, or in the electronic

industry.

The classical polyimide synthesis consists of two steps: A tetracarboxylic acid

dianhydride is reacted with a diamine in a polar aprotic solvent, e.g.

N,N-dimethylformamide (DMF) or N-methylpyrrolidone (NMP) at temperatures of

15-75°C, resulting in a poly(amic acid). In the second step, the polyimide is formed by

cyclodehydration of the poly(amic acid) at elevated temperatures (see Figure 9).

Figure 9 Scheme of classical polyimide synthesis from an aromatic dianhydride and a

diamine.

The resulting polyimides are often insoluble in organic solvents, thus they are usually

processed in the form of the poly(amic acid).

Alternatively, polyimides which are soluble in organic solvents can be produced via a

one-step synthesis. The one-step synthesis is carried out by stirring dianhydride and

diamine in a high boiling organic solvent such as nitrobenzene or m-cresol at

temperatures of 180-220 °C. These conditions allow spontaneous chain growth and

imidisation. The one-step polyimide synthesis is useful for unreactive, sterically

hindered monomers like phenylated dianhydrides to produce high MW polyimides. In

contrast to the two-step synthesis, the one-step synthesis allows to produce polyimides

with higher crystallinity, as the high temperatures enable a good solvation and thus a

conformation favourable for packing.38

The classical polyimide synthesis route via a condensation mechanism is not

compatible with PolyJetTM-3D-printing, as it involves high temperatures, aggressive

solvents and slow reaction kinetics. In contrast, for a 3D printing process, moderate

curing temperatures, fast curing kinetics and the prevention or minimization of solvent

Introduction


amounts is desired. Because of the mentioned reasons, an alternative route using a

different chemistry is necessary for producing polyimides via PolyJetTM-3D printing.

Using bismaleimides (BMI) as the major ink component allows the production of

polyimides in a printing process without the mentioned disadvantages.

1.3.2. Bismaleimides

Bismaleimides (BMI) offer the opportunity to create polyimides in one step via an

addition mechanism without the elimination of low molecular side products. These

molecules feature two maleimide groups with reactive double bonds which can be

polymerized via thermal- or photopolymerization (see Figure 10).

Figure 10 Polymerization of BMI oligomers as a result of UV irradiation. R= aliphatic or

imide-extended.

The typical oligomers have molecular weights (MWs) in the range of

600 g mol-1-5000 g mol-1, where the lower MW oligomers are liquid and the higher MW

oligomers are powders. The properties of the resulting polyimides can be tuned by

chemical structure and length of the inner part of the BMI. Typical properties of cured

BMI resins include high glass transition temperatures (Tg=230-380 °C)39-41, excellent

dielectric properties, high thermal stability (>400 °C) and good hot-wet performance.

The BMIs can be either aliphatic- or imide-extended. Multiple aromatic moieties

enhance the properties of the cured polymers – they lead to a higher thermal stability

and increased impact strength.42 The novel BMI resins used in this study contain very

flexible backbones, leading to more favourable mechanical properties like increased

impact strength, while compromising with a lower glass transition temperature.

BMIs are classically produced by reacting a diamine with two equivalents of maleic

anhydride in DMF, followed by dehydration and cyclisation of the maleamic acid in

acetic anhydride and sodium acetate (see Figure 11).42

Introduction


Figure 11 Classical synthesis route for BMI resins.42

The maleimide double bonds can undergo a number of addition-type reactions, such

as ene-alder, Diels-Alder, Michael addition and free radical reactions. BMIs are also

capable of undergoing a homopolymerisation at elevated temperatures starting from

about 180 °C to form highly crosslinked thermoset networks, which are highly thermally

and chemically stable.42 Compared to thermoplastics, thermosets contain crosslinks,

which makes the polymer chains rigid and reduces the possibilities for energy

dissipation. In contrast, thermoplastics do not contain crosslinks, thus they are melt-

processable and have reduced brittleness (see Figure 12).43

Figure 12 Molecular structure of thermoplasts and thermosets. Reproduced with

permission from MDPI, Polymers, Copyright © 2015 MDPI.43

Due to their molecular structure, BMIs can also photopolymerize via a self-initiation

without additional photoinitiator. This makes them valuable for the photopolymerization

together with co-monomers like vinyl-ethers or acrylates, which will be described in

detail in the second part of this thesis (section 1.4. and 2.2. ).

Introduction


On the one hand, the highly crosslinked structure of the thermosetting polyimides leads

to thermal and chemical stability, on the other hand, the crosslinks result in a lack of

processability and cause brittleness. Thus, there has been a lot of research dealing

with improving the mechanical properties of cured BMI resins. The unmodified,

classical BMI resins, first reported in the 1980s, are referred to as “first generation

BMIs”, while the adapted BMIs are called “second generation BMIs”. First generation

BMIs have mostly been used as a matrix material for high performance composites

including glass-, carbon- or aramide fibers.44

The most common components of BMI systems of second generation are

4,4-bismaleimidodiphenyl methane (BDM) and diallyl-bisphenol A (DABA), depicted in

Figure 13.

Figure 13 4,4-bismaleimidodiphenyl methane (left) and diallyl-bisphenol A (right),

common components of BMI systems.

The combination of BDM and DABA yields polymers with superior thermal stability,

toughness and better processability of precursors. However, the mentioned

advantages are accompanied by a lower Tg of the polymer.

Other strategies for improving the toughness of BMI-resins include (I) the synthesis of

novel BMI building blocks, (II) the incorporation of thermoplastics, (III) blends with

thermosetting comonomers and (IV) the development of nanocomposites.

The approach of altering the backbone structure of BMIs has been widely used and

has led to a vast number of BMI variations. Increasing the length of the BMI backbone

results in higher MW and reduces the crosslink density of the final polymer. Thus, the

brittleness and shrinkage while curing can be reduced. The distance between two

maleimide functional groups can alternatively be extended by using chain extenders,

such as diamines. These modifications of BMIs result in a higher degree of molecular

freedom and thus increased energy absorbing ability, which lead to a higher impact

strength but at the same time, lower Tg. Other modifications were aimed at improved

processability, namely lower melting point and solubility in organic solvents, while

keeping the high thermal stability.

Introduction


Figure 14 shows various exemplary backbone structures for BMIs: Aromatic moieties,

such as naphthalene groups (a), are used to increase the thermal stability, ether

linkages (b) are used to introduce more flexible chains and bulky side groups (c)

increase the solubility in organic solvents and lower the melting point.

Figure 14 Exemplary backbone structures for BMIs: aromatic moieties (a), ether

linkages (b) and bulky sidegroups (c).42

The incorporation of thermoplastics into BMI resins leads to an increase in fracture

toughness, as the thermoplastic regions of the blend allow a way for energy dissipation.

When incorporating thermoplastics into BMI systems, it is important to choose

polymers with high thermal stability to avoid a lowering of the thermal stability of BMIs

and low MW to keep the viscosity of the uncured BMI resin as low as possible to ensure

processability. Promising candidates with good thermal stability for thermoplastic

modification of BMIs include polyetherimides45,46 and polyether-sulfones47. Further, it

is advantageous, when the thermoplastic modifiers include reactive endgroups

capable of reacting with the BMIs during the polymerization reaction to form a stable

network. Alternatively, reactive elastomers can be incorporated into the BMI system to

increase the flexibility of the cured resin. Carboxyl- or vinyl terminated acrylonitrile

butadiene (CTBN) was widely used for this purpose.48

Introduction


Blends of BMIs with other high-performance thermosetting resins enables the

exploitation of the high thermal stability of BMIs together with beneficial properties of

the secondary resin. As an example, epoxy resins have excellent processability, but

low glass transition temperatures, thus the combination of epoxies with BMIs leads to

resins with good processability and thermal stability. BMI / cyanate ester resins have

been widely used to produce substrates for printed circuit boards (PCBs) because of

the low dielectric constant and good moisture resistivity of cyanate esters and the

thermal stability of BMIs.42

Nanofillers, such as Carbon-Nanotubes (CNT), boron-nitride, nanoclay and graphene

have been used to improve the mechanical properties of BMI without losing the desired

high thermal stability.42

Despite the drawbacks of brittleness and reduced thermal stability compared to

classical polyimides, the thermosetting polyimides resulting from BMI polymerization

have thermal stabilities of >400 °C and excellent dielectric properties, which makes

them valuable for applications like dielectric layers in PCBs. Furthermore, the excellent

chemical- and radiation resistance enable the use in harsh environments and for

aerospace applications.

1.3.3. Polyimide-like ink formulation using Bismaleimides

In this work, BMI resins from Designer Molecules Inc. were used.49 These resins

contain very flexible backbones and are aliphatic- or imide-extended. Designer

Molecules Inc. offers a number of low MW BMI oligomers with relatively low viscosity,

which score with good processability. The resulting crosslinked polyimides show high

thermal stability (>400 °C) and excellent chemical resistivity. Chemical structures of

the BMI oligomers used in this study are summarized in section 2.1.

For formulating inkjet inks, the low molecular weight BMI-oligomers are good

candidates as they are liquid to be able to be ejected through the small inkjet nozzles

with the addition of a small amount of suitable solvent. After the ink is deposited, it is

irradiated with near-infrared (NIR) to remove the solvent and with UV (MHLs or LEDs)

to trigger the curing of the BMI. The drying- and curing lamps are mounted directly on

the printhead block, so that the ink is instantly dried and solidified as a result of

immediate exposure to NIR- and UV light after each printing pass.

Introduction


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47 Y. Liu, Y. Yu, M. Wang, L. Zhao, L. Li, S. Li, Study on the polyethersulfone/bismaleimide

blends: morphology and rheology during isothermal curing, J. Mater. Sci. 2007, 42, 2150-2156.

48 D. Wang, X. Wang, L. Liu, C. Qu, C. Liu, H. Yang, Vinyl-terminated butadiene acrylonitrile

improves the toughness, processing window and thermal stability of bismaleimide resin, High

Performance Polymers 2017, 29 (10), 1199-1208.

49 Designer Molecules Inc. Homepage,

http://www.designermoleculesinc.com/products.cfm, accessed 10.01.2019.

Introduction


1.4. Photoinitiator-free bismaleimide-acrylate based inks for Inkjet-

printing

Usually, in UV curable ink formulations based on acrylates, there is a need to use

photoinitiators (PIs) to start the photopolymerization reaction. However, the usage of

PIs can lead to a number of problems, including yellowing, odour, migration or

accelerated ageing of the resulting polymer. Thus, it is of great interest to enable a

PI-free photopolymerization of UV curable inks.

1.4.1. Photoinitiators and Photosensitizers

PIs for UV curabe coatings are designed to absorb light in the region of 200 – 400 nm

and as a consequence, the molecules transition into an excited state, followed by an

intersystem crossing and the formation of initiating species. PIs include a

chromophore, which leads to a high absorption in the UV region, and consist mainly of

aromatic ketone functionalities. PIs are available for radical-, cationic and anionic

polymerizations, but the dominating mechanism for industrial application is radical

polymerization.50 PIs for radical polymerization are classified into two types, based on

the initiating mechanisms: Type I, and Type II PIs.

1.4.1.1. Type I Photoinitiators

Type I PIs fragment upon UV irradiation to yield initiating species. The fragmentation

may occur via several different routes, depending on the functional group and its

location: α-cleavage occurs at the bond adjacent to the carbonyl group, β-cleavage

occurs at the bond in β position. Rarely, the cleavage occurs at a remote position. The

dominating fragmentation is α-cleavage, where the C-C bond between the carbonyl

group and the alkyl residue breaks, known as the Norrish Type I reaction.50,51

Figure 15 Cleavage at the α-position as a result of UV irradiation, known as the Norrish

Type I reaction.51

Introduction


1.4.1.2. Type II Photoinitiators

Type II PIs belong to the class of bimolecular initiating systems and consist of a

sensitizer and a coinitiator. The excited PI (sensitizer) can undergo two reactions:

abstraction of a hydrogen from hydrogen donors and photoinduced electron transfer,

which is followed by fragmentation. The sensitizer can act as the electron donor and

the coinitiator as electron acceptor or vice versa. Figure 16 depicts the process of

radical formation from type II PIs: The photosensitizer transitions into an excited state

as a result of photo-irradiation and abstracts a hydrogen from the hydrogen donor. This

leads to the formation of two radicals: the alkyl-radical and the ketyl radical. The latter

is not very reactive towards acryl or vinyl bonds and mostly undergoes recombination

reactions. However, the alkyl-radical is capable of initiating a radical polymerization

reaction.51

Figure 16 Formation of initiating radicals from type II PIs.51

The most important substance class used as type II PIs are organic carbonyl

compounds. Aromatic carbonyl compounds are most suitable as they have high

absorptions in the UV region in contrast to their aliphatic counterparts due to the

aromatic moieties, enabling the excitation with longer wavelength UV sources.51

1.4.1.3. Exemplary Photointiators

Exemplary structures of Type I (α-cleavage) and Type II (electron transfer hydrogen

abstraction type) PIs are depicted in Figure 17.52 Fouassier and Lalevée published a

comprehensive list of radical photoinitiating systems.53

Introduction


Figure 17 Examples of Type I (α-cleavage) and Type II (electron transfer hydrogen

abstraction type) PIs.52

1.4.2. Bismaleimides as photoinitiators

Due to their molecular structure, BMIs absorb radiation in the UV range and can serve

as a PI for homo- or co-polymerization with vinyl ethers or acrylates for example,

exploiting the electron- donor-acceptor mechanism. The usage of BMIs for PI-free

photopolymerizations started in the 1990s and has gained much interest in the

following years.54-57 Despite the moderate reaction rate compared to standard acrylate

formulations with PI, the PI-free photopolymerization may be beneficial for applications

where migration issues or biocompatibility are more important than process speed.

1.4.3. Electron donor-acceptor mechanism

As mentioned above, BMIs can act simultaneously as PIs and polymerizable

monomers. Upon UV irradiation, the BMI transitions into an excited state (T1), which is

able to abstract a hydrogen atom from either a ground state BMI or a comonomer. In

this process, the comonomer (acrylate, vinyl ether) serves as an electron donor, while

Introduction


the excited state BMI serves as the electron acceptor. Figure 18 depicts the formation

of initiating radicals via this route. As a result of H-abstraction, the BMI forms aminyl-

and succinimido radicals.55,58,59

Figure 18 formation of initiating radicals from the excited state maleimide and a

hydrogen donor, such as vinyl ether or acrylate.59

If a stoichiometric mixture of BMI and comonomer is subjected to UV irradiation, an

alternating copolymerization takes place resulting in highly crosslinked copolymer

networks. As the comonomer (acrylate, vinyl ether) does not absorb enough light in

the UV region, the initiation only results from the excited BMI and leads to co-

polymerization with the acrylate and homopolymerization of the BMI. This results in an

alternating copolymer with separate homopolymerized BMI parts. If an excess of the

acrylate or vinyl ether is used, a pure copolymer is achieved, as enough comonomers

are present to react with the excited state BMIs. The initiation rate depends on the

molecular structure of the BMI and on the ability of the donor to donate a hydrogen

atom. Aliphatic BMIs are generally more photoreactive compared to aromatic ones.59

The best hydrogen donors for BMIs are vinyl ethers, which have been reported to

enable photopolymerization speeds comparable with acrylate systems including PI.52

1.4.4. Oxygen inhibition

Oxygen inhibition is a well-known process, which slows down or inhibits the

photopolymerization of acrylate- containing ink formulations. The oxygen molecules

present in air can react with the species formed during radical photopolymerization,

leading to chain termination and shorter polymer chains and thus, incomplete curing.

This process happens on the surface of the film, which is exposed to open air, within

the upper 10-15 µm, where oxygen can diffuse into. Consequently, oxygen inhibition

becomes an increasingly important problem, when thin layers in the micrometre range

are deposited, as it is the case for inkjet printing.60

Introduction


Figure 19 shows how O2 molecules react with species formed during radical

photopolymerization and thus influence the polymerization speed. Upon UV irradiation,

the PI is creating initiating radicals, which can react with monomer molecules to form

again radicals and start the polymerization. However, oxygen reacts readily with the

formed radicals yielding peroxy-radicals ROO• which do not initiate the polymerization

reaction but abstract a hydrogen atom from other molecules present in the formulation.

The resulting radicals react with oxygen, again forming peroxy radicals. As this process

significantly slows down the polymerization speed, one always has to consider this

when designing a UV curing process.61

Figure 19 O2 molecules reacting with different species formed during radical

photopolymerization leading to chain termination – a process known as oxygen

inhibition.61

A number of strategies have been developed to reduce oxygen inhibition, which can

be either of physical or chemical nature.60-62 Physical methods do not require a change

in formulation, but rather a change in the process or hardware. Chemical methods

require a change in formulation or monomer structure, or the addition of compounds

such as oxygen scavenging molecules. Compared to physical methods, the chemical

methods lead to lower process or hardware costs, but also to higher costs for the

formulation itself. Which strategy one should chose for minimizing oxygen inhibition

depends on the process for which curing is optimized. As an example, for 3D-printing

applications it is much more complicated to use inert atmosphere, as the curing takes

place immediately after deposition of each layer and it would be necessary to adapt

Introduction


the whole printer setup. However, in a batch process, where a coating is applied and

can be afterwards cured in a separate chamber, it is much easier to realise such a

setup. Table 1 presents an overview of physical and chemical approaches to reduce

oxygen inhibition, including advantages and drawbacks.

Table 1 Methods for reducing oxygen inhibition in radical photopolymerization.

Method Physical

/Chemical Characteristics + -

High

UV irradiance

Physical produces a large

number of radicals

able to react with

O2 and increase

the curing speed

Easy; no

change in

formulation

necessary

Over-exposure

of film surface

compared to

bulk; maybe

change of lamp

system

Inert

atmosphere

Physical Keeps O2 away

from the uncured

coating

Allows

reduced PI

amount

Not always

applicable

Physical

barrier (floating

wax; foils)

Physical Keeps O2 away

from the uncured

coating

Easy, cheap Not widely

applicable

Amines Chemical Amines scavenge

O2 molecules

Very

effective

Yellowing, poor

weatherability

High PI

concentrations

Chemical Excess of PI

reacts with O2

easy Expensive;

migration of PI

Acrylate

monomer

structure

Chemical labile H atoms to

react with O2 or

increased viscosity

No additional

components

Change in

formulation

necessary

High reactivity Chemical Monomers react

faster than O2

diffuses

No additional

components

Does not help in

low viscous

formulations

Introduction


Method Physical

/Chemical Characteristics + -

Viscosity of

formulation

Chemical Slow diffusion of

O2 into viscous

resin

No additional

components

Higher viscosity

Oxygen

scavengers

Chemical Scavengers react

with singlet state

O2

Easy,

effective

Sometimes

colouring of film

1.4.5. Bismaleimides to reduce oxygen inhibition

Another advantage of using BMIs in acrylate-based formulations is that they are

capable of reducing the oxygen inhibition. The maleimide radicals formed through UV

irradiation and H-abstraction can react with oxygen, which leads to a regeneration of

the BMI monomer/oligomer and the formation of a peroxy-radical. The process of

H-abstraction does not lower the extent of alternating copolymerization.59

Figure 20 Reaction of O2 with a maleimide radical, leading to regeneration of the

maleimide molecule.59

Introduction


References

50 W. A. Green, Free Radical Chemistry, In: Industrial Photoinitiators- A Technical Guide, 2010,

CRC Press, Taylor & Francis Group, Boca Raton, London, New York, 17-31.

51 H. F. Gruber, Photoinitiators for Free Radical Polymerization, Prog. Polym. Sci. 1992, 17,

953-1044.

52 R. Schwalm, Raw Materials, In: UV Coatings, 2007, Elsevier, 114-125.

53 J. P. Fouassier, J. Lalevée, Part II: Radical Photoinitiating Systems, In: Photoinitiators for

Polymer Synthesis, Scope, Reactivity and Efficiency, 2012, Wiley VCH Verlag GmbH & Co.

KGaA, Weinheim, Germany, 123-268.

54 C. Decker, D. Decker, Photoinitiated radical polymerization of vinyl ether-maleate systems,

Polymer 1997, 38, 2229-2237.

55 F. Morel, C. Decker, S. Jönsson, S. C. Clark, C. E. Hoyle, Kinetic study of the photo-induced

copolymerization of N-substituted maleimides with electron donor monomers, Polymer 1999,

40, 2447-2454.

56 D. Burget, C. Mallein, J. P. Fouassier, Visible light induced polymerization of maleimide-vinyl

and maleimide-allyl ether based systems, Polymer 2003, 44, 7671-7678.

57 C. P. Vázquez, C. Joly-Duhamel, B. Boutevin, Photoinitiator-Free, Open-Air Acceptor/Donor

Copolymerization of Bismaleimides: Simple Polymerization Conditions for New Thermoplastic

Elastomer Production, Macromol. Chem. Phys. 2013, 214, 1621-1628.

58 S. Jönsson, K. Viswanathan, C. E. Hoyle, S. C. Clark, C. Miller, F. Morel, C. Decker, Recent

advances in photoinduced donor/acceptor copolymerization, Nucl. Instrum. Methods Phys.

Res. B 1999, 151, 268-278.

59 E. Andrzejewska, Photopolymerization kinetics of multifunctional monomers, Prog. Polym.

Sci. 2001, 26, 605-665.

60 R. Schwalm, Tackling the Drawbacks of UV Systems, In: UV Coatings, 2007, Elsevier, 179-

194.

61 K. Studer, C. Decker, E. Beck, R. Schwalm, Overcoming oxygen inhibition in UV curing of

acrylate coatings by carbon dioxide inerting, Part I, Prog. Org. Coat. 2003, 48, 92-100.

62 T. Y. Lee, C. A. Guymon, E. Sonny Jönsson, C. E. Hoyle, The effect of monomer structure

on oxygen inhibition of (meth)acrylates photopolymerization, Polymer 2004, 45, 6155-6162.

Introduction


1.5. Foamable acrylate-based inks for PolyJetTM-3D-printing

1.5.1. Polymeric Foams

The term „foam“ commonly refers to a dispersion of gaseous bubbles in a liquid in case

of a liquid foam or in a solid medium in the case of a solid foam.63,64 A great diversity

of foam types is known from nature, each with distinct porosities and physical

characteristics (e.g. mechanical stability). Examples include bones, sponges, sea

foams, foam nests, honeycombs or alveoli in the lungs.65,66

The usage of polymeric foams for industry started with the production of foamed

polystyrene in the 1930s. From there on, numerous types of foamed polymers for a

vast number of applications have evolved.

Polymeric foams are classified according to their densities, cell morphologies, rigidity

and processability. Based on their cell type, there are open cell, closed cell- or mixed

cell foams. In open cell foams, the pores are interconnected, while closed cell foams

have individually closed cells. The cell structure of open- and closed cell foams is

shown in Figure 21.67

Figure 21 Structures of open cell (left) and closed cell foams (right). Reproduced with

permission from Elsevier, European Polymer Journal, Copyright © 2015 Elsevier.67

Depending on the desired application, either open or closed foams are preferably used.

As an example, closed cells are wanted when gas tightness is important, e.g. for

thermal insulation or packaging purposes, while open cells with large specific area are

desired as filtration membranes.68

1.5.1.1. Thermoplastic foams

Thermoplastic polymeric foams are mostly produced by supersaturating polymer melts

with gases or blowing agents (BA). First, the thermoplastic is melted and a gas, e.g.

CO2, is injected into the melt under pressure, usually leading to a supersaturated one

Introduction


phase system. During the extrusion process, the pressure is released, which leads to

an initiation of the foaming process followed by cooling down after extrusion to yield a

solid foam. The expansion process stops as soon as the modulus of the cooling

polymer is high enough to stabilize the expansion pressure.69

1.5.1.2. Thermosetting foams

Thermosetting polymeric foams, such as polyurethanes are mostly produced via

chemical foaming, i.e. by generating gaseous bubbles while the polymerization and

crosslinking is taking place. Another possibility is to use reactive systems, such as

acrylates, which can be cured by UV-light or thermal treatment in combination with

thermally triggered BAs. For this purpose, typically low viscous monomers and

oligomers are used as starting materials, which makes the foam stabilization very

challenging. Through the low viscosity, a lot of bubbles can collapse or escape through

the surface before the polymerization increases the viscosity and stabilizes the gas

bubbles. To tackle this problem of instability, surfactants can be used to stabilize the

low viscous precursors and create a quasi-stable foam, as it is known from aqueous

surfactant-containing foams. This bridges the time until the polymeric foam is created

through crosslinking of the precursors.69

1.5.1.3. Blowing Agents

BAs can be either of physical- or chemical nature. Physical blowing agents (PBA) form

gaseous bubbles in the medium to be foamed through evaporation at elevated

temperature or reduced pressure. Examples for PBAs are volatile solvents like

isooctane, hexane or toluene.

Chemical blowing agents (CBA) are chemicals which produce gas as a result of a

chemical reaction, such as thermal decomposition. The decomposition temperature of

the CBA should be close to the hardening temperature of the polymer, the gas should

be liberated in a narrow temperature range, and the gas liberation should be triggered

by heat or pressure. There is a large number of inorganic and organic CBAs available

on the market. Widely used are ammonium carbonate, azo compounds and

sulfonylhydrazides, just to name a few examples.70 The chemical structure and the

decomposition reaction of 4,4-oxydibenzenesulfonylhydrazide (OBSH), a widely used

CBA, is depicted in Figure 22. The thermal degradation of OBSH is a two-step reaction

and leads to the formation of H2O, N2, and a sulphur-based oligomer.71

Introduction


Figure 22 Thermal decomposition reaction of OBSH, a widely used CBA.71

1.5.1.4. Mechanism of foam formation

The mechanism of foam formation usually involves several steps:

(i) dissolving of the BA

(ii) bubble nucleation

(iii) bubble growth

(iv) stabilization of the bubbles

The processes of bubble nucleation and growth are schematically depicted in Figure

23.

Depending on the system used for foaming, the bubble nucleation can be either

homogeneous or heterogeneous.72 When gases or PBAs/CBAs are dissolved without

any nucleating additives, the bubble nucleation is homogeneous and the critical step

is the nucleation of bubbles, as the energy barrier for forming a second phase needs

to be overcome. When nucleating agents are added to the polymer melt, the bubble

nucleation is heterogeneous and the interface of the nucleating particles is significantly

lowering the energy barrier for bubble formation (see Figure 23).67,73

Introduction


Figure 23 Free energy for homogeneous and heterogeneous bubble nucleation in

polymeric foam production. Reproduce with permission from Elsevier, European

Polymer Journal © Copyright 2015 Elsevier.67

1.5.1.5. Foam characterization

A number of physical characteristics influence the properties of foams and can be used

to evaluate foams for a specific application. Basic factors which are used to

characterize the properties of the foam are the density, porosity, expansion ratio, pore

size, pore morphology, and the specific surface area.74,75 A very important feature of a

foam is usually its density, which is especially critical if a foam should be used for light-

weight applications or thermal insulation.

The porosity Θ of a foam is the ratio of the volume of pores Vp to the whole volume of

the body Vt and is expressed as:

θ = Vp

Vt (Equation 16)

Porosity is the parameter which mostly influences the properties of a porous body, e.g.

the thermal conductivity, optical and acoustic properties, and mechanical properties

such as tensile strength.

Introduction


Analogously, the relative density ρr is defined as the ratio of the nominal density of the

body ρ* to the solid density ρs:

ρr=

ρ*

ρs

(Equation 17)

Easy experimental methods for characterization of the porosity include microscopic

analysis of the cross section of a foam or the mass-volume direct calculation.

When using microscopic methods for determining the porosity of a foam, a cross

section of the foamed sample is observed and by measuring the area of pores via an

imaging software in relation to the total sample area, the porosity can be calculated via

θ = Sp

St, (Equation 18)

where Sp corresponds to the pore surface area and St represents the total surface area

of the sample.

If a non-destructive method for analysing the porosity is desired, tomography is a

suitable alternative to microscopic techniques.

Another possibility, which does not need any expensive equipment, is to determine the

porosity via the mass-volume direct calculation. By measuring the mass m and volume

V of the sample, the porosity is obtained via

θ = 1 -m

V ρs

, (Equation 19)

where ρs corresponds to the solid density of the bulk material.

For the mass-volume direct calculation only a balance and a vernier caliper or

micrometer is required.

The cell morphology includes parameters like the type of cells (open- or closed cells),

the pore sizes and shapes and the cell wall thicknesses, all influencing the mechanical

properties of foams. The cell morphology can also be characterized by optical- or

electron- microscopy or tomographic methods.75

The volume expansion ratio Rv is defined as the ratio of the unexpanded polymer

density ρp to the density of the foamed polymer ρf:

Introduction


Rv= ρp

ρf

. (Equation 20)

The volume expansion ratio is an important measure for the efficiency of the foaming

process. For high-density polymer foams Rv is less than 1.5, middle density foams

have expansion ratios of 1.5-9.0, and low-density foams have expansion ratios higher

than 9.0.74,75

1.5.2. Strategies for 3D printing of foams

Within the last years, there has been extensive research on facilitating methods to 3D

print foams for obvious reasons to combine additive manufacturing with the benefits of

polymeric foams, such as their light-weight-, shock absorbing and thermal insulation

properties. Much research has been carried out on the 3D printing of foams using

Fused Filament Fabrication (FFF). FFF creates 3D structures by feeding polymer

filaments through a heated nozzle, moving across horizontal and vertical axes to

deposit molten polymer layer by layer. The dominating strategies for printing foams

with FFF include (i) direct printing of lattices76,77, (ii) supersaturating the filaments with

CO2, which is released upon filament deposition78 and (iii) the usage of two-component

filaments, where one component is removed after the printing procedure either by

leaching out79 or by thermal decomposition.80

Compared to FFF, PolyJetTM printing offers a much higher resolution in the order of

15-30 µm, as in an inkjet-based process the resolution is limited by the volume of a

single droplet.81 However, up to now, no method for producing foams in one printing

step via PolyJetTM has been reported. Within the framework of the DIMAP Project and

this thesis, two approaches for 3D printing of foams using the PolyJetTM- technology

have been considered:

(i) The use of thermally expandable microspheres embedded in a UV curable

acrylate ink matrix, which expand during NIR irradiation after printing and form

a stable closed cell foam together with the cured ink matrix;

(ii) the direct use of a chemical blowing agent inside the UV curable acrylate

matrix to form an open- or closed cell foam after thermal decomposition of the

CBA and UV curing of the ink matrix.

The two strategies will be described in the following sections.

Introduction


1.5.2.1. Closed cell foam using thermally expandable microspheres in acrylate

matrices

The usage of thermally expandable microspheres (TEMs) allows to produce a closed

cell foam from a TEM-containing ink through thermal energy input, followed by a UV

curing step. To produce a foamable ink suitable for PolyJetTM-3D-printing based on

TEMs, the TEMs are mixed with an acrylate ink matrix containing acrylate monomers

and oligomers, photoinitiators and other additives. It is important that the particles are

dispersed well in the ink matrix, so that no agglomerates can clog the inkjet printheads,

thus a dispersion step via a suitable dispersing agent is necessary. The TEMs need to

be small enough to fit through the inkjet nozzles. After printing of each layer, the

expansion of the particles is triggered through thermal energy input, which is achieved

through NIR-irradiation. The NIR emitter can be mounted directly on the sides of the

printhead block next to the UV curing lamps. After the particles have been expanded

through boiling or decomposition of the physical or chemical blowing agent,

respectively, a UV curing step has to follow to stabilize the expanded particles to form

a stable foam. (see Figure 24).

Figure 24 The closed cell foam strategy involves the incorporation of thermally

expandable microspheres (TEMs) into an acrylic ink matrix. The polymeric shell of the

TEMs softens during heat input, while the BA decomposes and leads to an expansion

of the cell. A UV curing step stabilizes the expanded particles and a polymeric foam is

formed.

Figure 25 shows an example of commercially available (a) and self-synthesized TEMs

(d). If the acrylate matrix with TEMs is only UV cured, the TEMs stay unexpanded

(Figure 25 b,e). If the TEM / acrylate mixture, is subjected to heat before the UV curing

step (Figure 25 c,f), the particles expand through boiling of the PBA and a stable foam

is generated. Compared to the commercially available TEMs, the self-synthesized

TEMs show a broader size distribution and inferior expansion efficiency.

Introduction


Figure 25 Commercially available TEMs (Expancel 031DU40, a-c) and self-

synthesized TEMs (d-f): pure TEMs (a,d) and cross sections of TEMs incorporated in

UV cured acrylate matrix (b,e) and cross sections of TEMs incorporated in UV cured

acrylate matrix after a thermal expansion step before UV curing (c,f). The scale bar

represents 50 µm.

Despite the working proof-of-concept it turned out that the approach of the closed cell

foam could not be realized within the scope of the DIMAP project. The commercially

available TEMs were too big and the self-synthesized TEMs also showed too large

particle sizes and size distributions for using them in inkjet ink formulations. The

creation of a stable dispersion of the particles in the ink would have led to a need in

more research efforts. Initial trials with a MicroJet-reactor setup for the creation of

emulsions with droplet sizes smaller than 1 µm led to a product containing some TEMs

with <1 µm diameter. However, an effective synthesis of TEMs with diameters smaller

than 1 µm would have required an upgrade of the MicroJet reactor setup with more

powerful pumps, which was not possible with the resources available. For the

mentioned reasons, it was decided to follow an alternative route which directly uses

the BA dissolved in an acrylate matrix to receive an ink without any particles.

Introduction


1.5.2.2. Open cell foam using chemical blowing agents in acrylate matrices

Figure 26 depicts a schematic description of the production of a foamable ink

containing a CBA and the foaming process triggered by thermal energy input. First, the

acrylate ink matrix consisting of acrylate monomers and oligomers, photoinitiators and

other additives is mixed with a CBA. The BA is dissolved in the ink matrix, yielding a

stable ink formulation without particles. Analogously to the closed cell foam approach,

after inkjet printing of each layer, the blowing agent decomposition via NIR-irradiation

leads to foaming of the layer and UV curing fixes the foam structure. A UV pinning step

before the foaming step helps to increase the viscosity of the layer due to initial

polymerization, which leads to a better stabilization of the generated gas bubbles.

Figure 26 Schematic description of the production of an open cell foam from a CBA

containing foamable acrylate-based ink.

With this approach, it is possible to generate a stable ink with suitable viscosity for

jetting in inkjet-printheads, while eliminating the danger of clogging the nozzles through

particle agglomeration. For the ink formulation, benzenesulfonyl-hydrazide and

4,4’-Oxydibenzenesulfonyl-hydrazide were modified to avoid a reaction of the

nucleophilic amine group with the acrylic double bonds in the ink during storage and

to ensure solubility of the BA in the ink matrix. The synthesis of the modified BAs was

carried out by L. Göpperl5 and A. M. Kreuzer4 as part of their theses. The ink formulated

with this strategy was inkjettable and foamable via NIR irradiation. The synthesis of

modified BAs, their characterization and the ink formulation as well as the production

and characterization of printed foams are summarized in section 2.3.

Introduction


References

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A Design Guide, 2017, Ed. William Andrew, Elsevier, Chapter 6, p. 131-188.

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Foams – Mechanisms and Materials, Eds. S. T. Lee, N. S. Ramesh, 2004, CRC Press, Boca

Raton, London, New York, Washington D.C., Chapter 3.

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2017, ChemTec Publishing Toronto, Chapter 7, p.79-82.

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9, p.411-492.

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77 S. R. G. Bates, I. R. Farrow, R. S. Trask, Compressive behaviour of 3D printed thermoplastic

polyurethane honeycombs with graded densities, Materials & Design 2019, 162, 130-142.

Introduction


78 M. G. M. Marascio, J. Antons, D. P. Pioletti, P. Bourban, 3D Printing of Polymers with

Hierarchical Continuous Porosity, Adv. Mater. Technol. 2017, 2, 1700145.

79 K. Parthy, patent application DE102013011243A1, 08.01.2015, Polymer mixture for the 3D

printing for creating objects with pore structures.

80 X. Song, Z. Zhang, Z. Chen, Y. Chen, Porous Structure Fabrication Using a

Stereolithography-Based Sugar Foaming Method, J. Manuf. Sci. Eng. 2017, 139, 031015.

81 J. Lee, J. An, C. Kai Chua, Fundamentals and applications of 3D printing for novel materials,

Applied Materials Today 2017, 7, 120-133.

Results & Discussion


2. Results and Discussion


The work of the author consists of studying the thermal- and UV curing kinetics of

selected BMI-oligomers which should be used for PolyJetTM ink formulation. This

includes the evaluation of activation energies via DSC and the development of a

method for characterizing the curing kinetics via Fourier Transform Infrared

Spectroscopy. Different UV sources with various powers and wavelengths were used

for this study. Further, the author characterized thermal- and thermomechanical

properties of the polyimides via thermogravimetry and dynamic mechanical thermal

analysis. The preparation of this manuscript, which was published in the Journal of

Applied Polymer Science, also was the task of the author.1

Cure kinetics of bismaleimides as basis for polyimide-like inks for PolyJet™-

3D-printing

Annika Wagner ,1 Irina Gouzman,2 Nurit Atar,2 Eitan Grossman,2 Mariana Pokrass,3 Anita

Fuchsbauer,1 Leo Schranzhofer,1 Christian Paulik4

1 Profactor GmbH, Im Stadtgut A2, 4407, Steyr-Gleink, Austria

2 Space Environment Department, Soreq NRC, Yavne, 81800, Israel

3 Stratasys Ltd., Haim Holtzman Street 1, Rehovot, 7670401, Israel

4 Institute of Chemical Technology of Organic Materials,

Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Linz, Austria

Correspondence to: A. Wagner (E-mail: [email protected])

ABSTRACT: Since polyimides are well known for their excellent chemical and thermal stability and outstanding mechanical

properties there is increasing interest in developing polyimide-based inks to produce additively manufactured parts with

properties superior to those of currently available materials. Usage of bismaleimides (BMI) as precursors allows polyimides

to be fabricated via PolyJet™ printing (Stratasys Ltd., Rehovot, Israel). Characterization of the curing kinetics is a central

part of process development, as fast curing initiated by UV light is desired. Here, a comprehensive study of thermal and UV

curing of BMI oligomers with various molecular weights and chemical structures is presented. Fourier transform infrared

spectroscopy serves as a tool for determining the curing degree. Furthermore, an estimation of the activation energy for

thermal curing is performed. UV curing of the selected BMIs leads to highly crosslinked, thermoset polymers with excellent

chemical resistance and thermal stability which are of great interest for PolyJet™ 3D printing. © 2018 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 47244.

KEYWORDS: differential scanning calorimetry; kinetics; photopolymerization; spectroscopy; thermal properties

Received 28 June 2018; accepted 18 September 2018 DOI: 10.1002/app.47244



INTRODUCTION

PolyJetTM 3D printing is a promising method

that enables multiple materials to be used

to create complex components which are

impossible to create in any other way. The

working principle of PolyJetTM 3D printing

includes (i) jetting layers of low viscosity

liquid photosensitive monomers and

oligomers onto a build tray and (ii)

subsequent curing by UV light.1

Polyimides (PIs) are desirable materials for

3D printing since they have a variety of

outstanding characteristics, including high

thermal stability, excellent mechanical

properties, wear resistance, vacuum

compatibility, radiation resistance,

inertness to solvents, a low dielectric

constant, and good adhesion strength.2,3 PI

synthesis routes include polycondensation

and polyaddition mechanisms. Aromatic

PIs are generally prepared from aromatic

diamines and aromatic tetracarboxylic

dianhydrides in a two-step procedure.

However, this polymerization by

condensation is less compatible with 3D

printing procedures than polyaddition,

mainly because of slow thermal curing.4

Bismaleimide (BMI) resins are a family of

high-performance thermosetting PIs that

have a range of attractive properties for

industrial applications, particularly in the

aerospace materials sector.5 Since BMIs

are polymerized by an addition mechanism,

they are promising candidates for PI ink

printing using the PolyJetTM technology.6

BMIs are low molecular weight oligomers

consisting of imide moieties with reactive

terminal or pendant groups that undergo

thermal or catalytic homo- and/or co-

polymerization.7,8 The rheological

properties of BMI-based materials can be

tuned by tailoring the molecular weights of

the oligomers.9 Typical properties

associated with cured BMI-based systems

include high glass-transition temperatures

(230-380°C), good hot-wet performance,

excellent electrical properties and low

flammability.10-15 The BMI-materials used in

this study have very flexible backbones,

resulting in low shrinkage during curing and

yielding polymers that show high thermal

stability (decomposition temperature

typically >400°C). At the same time, these

flexible BMI polymers have a low modulus

(<500 MPa) and lower glass-transition

temperatures than classical BMI resins.5

Due to the reactive maleimide groups, BMIs

can act both as polymerizable monomers

for thermal and photo-polymerization and

as photoinitiators. The maleimides form

initiating radicals upon UV exposure, which

makes them uniquely valuable for co-

polymerization with various co-monomers,

such as vinyl ethers and acrylates.16,17

Further, UV homo-polymerization of BMIs

is possible without an additional

photoinitiator. Upon UV exposure, the

maleimide double bonds of BMI undergo

cyclo-dimerization to form a cyclobutane

ring.18 BMI oligomers can be linked

together via this mechanism to yield

thermoset polyimides with excellent



thermal stability and chemical resistance.

Depending on whether a photoinitiator is

used for polymerization, the polymers are

formed either by photo-polymerization or by

photo-cyclodimerization (see FIGURE 1)

FIGURE 1 Cyclo-dimerization of N-functionalized maleimide (a, b) and photo-polymerization (c) of N-substituted maleimides.18

Customization of BMI-based materials for

inkjet printing allows additively

manufactured polyimide structures to be

produced by PolyJetTM 3D printing. High

photoreactivity and suitable viscosity for



jetting are the key parameters to be

considered in designing the ink formulation.

In this work, commercial BMI oligomers

with molecular weights (MWs) ranging from

689 g mol-1 to 5000 g mol-1 were

characterized in terms of their thermal

curing and UV curing behaviors. Oligomers

with MWs of up to 1700 g mol-1 are liquid,

while oligomers with MWs of 3000 g mol-1

and above are powders. The number in the

BMI name denotes the MW of the oligomer.

As an example, BMI-1500 has a MW of

1500 g mol-1. The types of oligomer used

were linear chain-extended aliphatic BMI

(FIGURE 2a) and imide-extended BMI

(FIGURE 2b, c and d). Using of BMI

precursors with higher MWs has the

advantage of increasing polymer chain

flexibility due to lower crosslinking density,

which results in an impact strength that is

higher than that of highly cross-linked BMI-

based polymers. The polyimides formed of

imide-extended BMIs have higher thermal

stability than those of aliphatic extended

BMIs.5 Due to their low viscosity,

low-molecular-weight products, such as

BMI-689, are promising PolyJetTM ink

candidates. Low-viscosity BMI oligomers

can be used in ink formulations with high

solid content, allowing jettability and

printability under typical PolyJetTM

conditions at printing temperatures of 50°C

to 70°C.

Designing new materials for PolyJetTM

printing involves characterizing the UV

curing kinetics, since fast polymerization of

oligomers upon UV irradiation is desired to

keep printing times short. At the same time,

optimal rheological properties of the ink are

to be maintained.

Various ways of studying curing reactions

have been reported in the literature: Fourier

Transform Infrared (FTIR) spectroscopy for

monitoring polymerization reactions has

become increasingly popular since the late

1990s.19-24 Other examples include Raman

spectroscopy25 and Differential Scanning

Calorimetry (DSC).26-31

FTIR spectroscopy is a very convenient

method for monitoring the curing reaction,

as it is fast, easy and broadly applicable.

The intensities of the bands corresponding

to the reactive double bonds are

considered in order to follow the curing

reaction, while the decrease in absorption

band area is used to assess the degree of

curing α according to

𝛼 = (1 - Adb,t∙Aref,0

Adb,0∙Aref,t) ∙ 100, (1)

where Adb,0 and Adb,t are, respectively, the

band areas of maleimide double bonds

before and after the curing time t.

Analogously, Aref,0 and Aref,t denote the

reference band areas before and after the

curing time t. Most reliable results are

obtained when internal reference bands

(e.g., carbonyl bands which remain

constant during polymerization) are used.32



FIGURE 2 Chemical structure of the liquid BMI oligomers BMI-689 (a), BMI-1500 (b), BMI-1700 (c) and the powder oligomers

BMI-3000 and BMI-5000 (d).

Here, we present a comprehensive study of

the curing kinetics of selected BMI

oligomers, including activation energy

measurements and the rate of thermal- and

photo-polymerization. Further, we describe

the influence on polymerization speed of

temperature in the case of thermal curing

and of wavelength of the UV source, UV



intensity and type of photoinitiator in the

case of photo-curing.

EXPERIMENTAL

Materials

BMI-oligomers with various molecular

weights (BMI-689, BMI-1500, BMI-1700,

BMI-3000 powder and BMI-5000 powder)

were purchased from Designer Molecules

Inc., San Diego, California, USA.

Dichloromethane, 1-,4-,6-trichlorobenzene

(TCB) and N-methylpyrrolidone (NMP)

were purchased from Sigma Aldrich,

Vienna, Austria. All solvents were in

analytical grade and used as received. The

photoinitiators, Omnirad 819 (O-819),

Omnirad 379 (O-379), Omnirad 907

(O-907), Omnirad TPO-L (O-TPO-L) and

Omnirad 2959 (O-2959) were provided by

IGM Resins.

Methods

FTIR spectra were recorded using a Bruker

Tensor 37 spectrometer with MIRacle ATR-

bridge. A Perkin Elmer DSC8000

differential scanning calorimeter was used

to obtain kinetic information about the BMI

curing reaction. A Pyris Series TGA4000

thermogravimetric analyzer (TGA) was

employed to determine the temperature

stability of cross-linked BMI-based

materials. UV-vis absorption was

measured by means of a Perkin Elmer

Lambda 35 spectrometer. Photo-DSC

measurements were carried out using the

OmniCure S2000 XL UV-Setup with a

200 W mercury arc lamp in connection with

a Netzsch DSC 204 calorimeter and a

Mettler AT200 balance.

For thermal curing of BMI resins, a

Nabertherm muffle furnace model

L3/11/P330 was employed. Thermal curing

of BMI resins was performed at 200°C,

225°C and 250°C under ambient

atmosphere.

UV sources used in this study include

UV-LED arrays (with various wavelengths

and powers) and mercury lamps (see

TABLE S1 in the supporting information).

The intensities of the UV sources were

measured by a UV-Micro Puck Multi

Integrator (UV-Technik Meyer, Ortenberg,

Germany).

A Sartorius CPA225D balance with

0.01 mg accuracy was employed in order to

measure the weight loss after solvent

resistance tests.

Dynamic Mechanical Analysis (DMA) was

performed by means of an Anton Paar

Physica MCR 501 at 0.1% deflection to

obtain information about the

thermomechanical properties of the cured

BMIs.

For thin-film preparation (1-2µm),

100 mg ml-1 solutions of BMI-oligomers in

dichloromethane were spin-coated on

glass and KBr substrates. Films of pure

liquid oligomers with 12 µm thickness were

produced by applying the doctor-blading

technique.

RESULTS AND DISCUSSION

Kinetic studies were carried out to gain

comprehensive information about the

curing reaction for both thermal- and UV-



induced BMI polymerization. In the

PolyJetTM printing process, the UV-curing

kinetics are of central importance and here,

we compared them to the kinetics of the

classical thermal curing process. In the

case of thermal curing, we carried out DSC

measurements at various scan rates to

obtain the apparent activation energy from

the peak shifts according to the Kissinger33

and Ozawa34 methods. The results were

compared with the activation energies

calculated from the classical Arrhenius

plots. FTIR spectra were used to obtain the

curing degree as a function of the curing

conditions.

Activation Energy of BMI

polymerization

Together with other kinetic parameters

(including the polymerization rate Rp and

the kinetic rate constant k), the activation

energy Ea for thermal curing of BMI resins

is essential to understanding the curing

reaction of this system. Methods for

determining the apparent activation energy

by DSC include the Kissinger33 and the

Ozawa34 models. Applying these methods,

we calculated the activation energy from

the peak shift of the exothermic

polymerization peak from DSC scans at

various heating rates [eq. (2) and (3)].

FIGURE 3 shows DSC scans of BMI-689,

where the shift of the polymerization peak

maximum depends on the heating rate β.

According to the Kissinger method, the

activation energy Ea can be determined by

plotting the logarithm of the heating rate β

over the squared peak temperature Tp

against the inverse peak temperature:

d[lnβ

Tp2]

d[1

Tp]

= -Ea

R , (2)

where R denotes the universal gas

constant.

The Ozawa method uses a plot of the

logarithm of the heating rate β versus the

inverse peak temperature Tp to calculate

the activation energy:

d[lnβ]

d[1

Tp]= -1.052

Ea

R . (3)

The Kissinger and Ozawa plots of BMI-689

are shown in the supporting information

(FIGURES S1 and S2).

FIGURE 3 DSC scans of BMI-689 showing a shift in the

polymerization peak maximum for various heating rates.

The values for the activation energy

determined by the Kissinger and Ozawa

methods were compared to those obtained

from the classical Arrhenius equation:

k = A e-EaRT , (4)

where A denotes the pre-exponential

factor.

The logarithm of the rate constants for

curing at three different temperatures



(200°C, 225°C and 250°C) were plotted

against the inverse curing temperatures

according to:

lnk = lnA - Ea

RT . (5)

The rate constants used for the activation

energy calculation are shown in

FIGURE 5c. We determined the activation

energies from the slopes of the Arrhenius

plots (see supporting information

FIGURE S3 for the BMI-689 and BMI-1500

plots). TABLE I lists the activation energies

of BMI-689 and BMI-1500 determined by

applying the Kissinger, Ozawa, and

Arrhenius methods. For thermal curing of

BMI-689, activation energies of 90 kJ mol-1

and 93 kJ mol-1 were calculated by means

of the Kissinger and the Ozawa methods,

respectively. The activation energy

determined by the Arrhenius equation from

the kinetic rate constants at three different

temperatures was 118 kJ mol-1. For

BMI-1500, the Kissinger, Ozawa, and

Arrhenius methods gave activation

energies of 112 kJ mol-1, 122 kJ mol-1 and

99 kJ mol-1, respectively. Taking

measurement accuracy into account, the

activation energies calculated by the

Kissinger and Ozawa models are in good

agreement with the experimental values

obtained from the Arrhenius plot.

Similar activation energies of 90-

160 kJ mol-1 have previously been reported

for thermal polymerization of BMI-based

materials in bulk.28,35-38 Compared to radical

polymerization of BMIs in solution, the

activation energies for bulk polymerization

are higher because viscosity is higher and

diffusion limitation plays a more significant

role. For polymerization of maleimides in

solution, the activation energy is around

20 kJ mol-1.3

TABLE I Activation energies of aliphatic BMI-689 and imide-extended BMI-1500 determined by the Kissinger, Ozawa and

Arrhenius methods.

Material Ea, Kissinger /

kJ mol-1

Ea, Ozawa /

kJ mol-1

Ea, Arrhenius /

kJ mol-1

BMI-689 90 93 118

BMI-1500 112 122 99

Evaluation of the BMI-curing kinetics

Based on experimental FTIR data, an

assessment of the curing kinetics of BMI

thermal- and photo-polymerization was

carried out. In order to determine the curing

degree α, we followed the polymerization

progress via the decrease in maleimide

double bond concentration [eq. (1)]. FTIR

spectra were recorded after various

exposures to thermal treatment or UV

doses. FIGURE S4 shows the FTIR

spectrum of BMI-689 as an example (see

supporting information).

During polymerization, the reactive

maleimide double-bond bands at 696 cm-1

and 825 cm-1 decreased in intensity, as

shown in FIGURE 4.



FIGURE 4 Decrease of bands at 696 cm-1 and 825 cm-1 of

neat BMI-689 at 225 °C as a function of curing time,

indicating a decrease in BMI double-bond concentration

during thermal polymerization.

FIGURE S5 in the supporting information

shows the decrease in maleimide double-

bond band area and the corresponding

curing degree during thermal

polymerization of BMI-689 at 225°C.

Assuming that, in systems consisting of

pure BMIs, the polymerization rate depends

only on the concentration of BMI

precursors, the kinetic rate constant is

obtained via the first-order kinetic

equation40 (see equations S1 – S3 and

FIGURE S6, supporting information).

Comparison of the experimental data with

the theoretical conversion curves

(calculated using kinetic rate constants)

shows that a first-order kinetic equation is a

good approximation of the BMI curing

reaction.

Thermal curing of BMIs

Thermal curing of BMIs was performed at

temperatures of 200°C, 225°C and 250°C

under ambient atmosphere. FIGURE 5

shows the corresponding curing degrees of

selected BMI-oligomers (a), the rates of

polymerization (b), and the kinetic rate

constants depending on temperature (c).

As an example, the curing degree and

polymerization rate obtained during

polymerization at 225 °C are shown. It can

be seen that the crosslinking speed is

influenced both by molecular weight and

chemical structure. For imide-extended

BMIs, smaller molecules with more reactive

double bonds relative to molecular weight

show lower crosslinking speeds, whereas

higher molecular-weight oligomers

crosslink faster. In contrast, the aliphatic

BMI-689 shows a different curing behavior

and a crosslinking speed between those of

BMI-1700 and BMI-3000 (FIGURE 5a, b).

Unlike other oligomers, BMI-1500 is not

fully cross-linked, but exhibits a plateau at

α=85 %, reflecting the rigidity of the

polymer chains. More thermal energy

would be needed to reach a higher

conversion of the double bonds in

BMI-1500. The polymerization rate is

highest for BMI-3000 and BMI-5000,

whereas for BMI-689, BMI-1500 and BMI-

1700, polymerization proceeds more

slowly. The kinetic rate constants for

thermal polymerization of BMI oligomers

show an exponential increase with

increasing temperature, as illustrated in

FIGURE 5c. The aliphatic oligomer

BMI-689 shows the highest increase in

kinetic rate constant with increasing

temperature.



FIGURE 5 Curing degree (a) and rate of polymerization (b) of various BMI oligomers during thermal curing at 225°C, and kinetic

rate constants (c) for thermal polymerization of BMI-oligomers at 200°C, 225°C and 250°C. The films were prepared by spin-

coating of 100 mg ml-1 solutions of BMI in dichloromethane. The symbols represent the experimental values, while the solid lines

show the corresponding calculated fits.

UV-curing of BMI

BMIs can be activated by UV light to form

initiating radicals for homo-polymerization

or co-polymerization with various

monomers. The UV absorbance of BMIs

studied in this work was highest at 230-

250 nm (see FIGURE S7 in the supporting

information). The parameters influencing

the photopolymerization rate of the BMI

resins include UV intensity and wavelength

and the addition of photoinitiators. Addition

of a photoinitiator increased the response

to UV irradiation and consequently the

curing speed.

Influence of UV intensity on BMI curing

In contrast to the standard precursors used

for PolyJetTM printing, which contain highly

reactive acrylic double bonds, BMI

oligomers have maleimide double bonds,

which -as is commonly known- require

more energy for polymerization to be

initiated.41 For this reason, high power is

necessary for, and beneficial to, fast curing



of BMI resins. FIGURE 6 shows the curing

degree of selected BMI oligomers after

exposure to various doses of UV light with

395 nm wavelength (FJ200, 11.4 W cm-2)

(a), the corresponding polymerization rates

(b), and the kinetic rate constants obtained

by using the corresponding first-order

kinetic equations, versus UV intensity (c).

Analogously to the case of thermal curing,

the polymerization rate increases with

increasing power for all BMI oligomers.

The photo-polymerization rate at 395 nm

wavelength is highest for BMI-689, BMI-

3000 and BMI-5000, while for BMI-1500

and BMI-1700, the polymerization speed is

slower, as illustrated in FIGURE 6.

Influence of UV wavelength on the

curing reaction

The wavelength of the UV source used for

crosslinking the BMI resin has a

considerable influence on the crosslinking

speed. FIGURE 7 shows the curing degree

of BMI-689 during exposure to LEDs with

340 nm, 365 nm and 395 nm peak

wavelength (a). At the same dose, using a

UV-LED with a wavelength of 365 nm

rather than 395 nm significantly increases

the reaction rate constant and exposure to

a UV-LED with a wavelength of 340 nm

results in a further boost. This increase in

polymerization speed is due to the UV

absorption maximum of BMI-689 being at

230 nm, as shown in the supporting

information (FIGURE S7). At lower

wavelengths, a significantly higher amount

of energy is taken up by the BMI-oligomer,

which leads to greater polymerization

speed. FIGURE 7b plots the curing degree

of BMI-689 during irradiation with a mercury

lamp with 0.45 W cm-2 intensity at an

irradiation distance of 15 mm. The broad

spectrum of the lamp allows very fast curing

of BMI, exploiting the highest reactivity of

the system, as BMI absorbs mostly in the

UV-C region of 230 to 250 nm.

As fast photopolymerization of BMI is

desired in PolyJetTM printing, the usage of a

mercury lamp is most effective.

Influence of Photoinitiator on BMI curing

The influence of type and amount of

photoinitiator (PI) on the curing speed of

BMI-689 was determined by Photo-DSC

measurements. FIGURE 8 plots the

exothermic polymerization peaks for

BMI-689 with and without addition of

various types and amounts of PI after 2 s

irradiation with a mercury halide lamp. Only

those curves are shown that exhibit an

increase in UV-response compared to that

of the pure BMI-689 oligomer. A specific

amount of PI is beneficial, while higher

concentrations can inhibit the curing

reaction. Pure BMI-689 shows a broad

exothermic peak after the UV pulse.

BMI-689 with 2 wt.% O-819 also exhibits a

broad peak after UV irradiation, but the

polymerization peak area is significantly

larger. In contrast, mixtures with other PIs

(i.e., O-TPO-L, O-369 and O-907) show a

very sharp response to the UV pulse.



FIGURE 6 Curing degree (a) and polymerization rate (b) of various BMI oligomers during UV polymerization at 395 nm and an

intensity of 11.4 W cm-2 and kinetic rate constants of polymerization versus UV power (c). The films were prepared by spin-coating

of 100 mg ml-1 solutions of BMI in dichloromethane. The symbols represent the experimental data, while the solid lines show the

corresponding calculated fits

FIGURE 7 Curing degree of 12 µm thick pure BMI-689 irradiated with UV light of various peak wavelengths (340 nm, 365 nm and

395 nm) with the fastest curing reaction at 340 nm (a); curing degree during irradiation with a mercury halide lamp with 0.45 W cm-2

at the substrate (b). The symbols represent the experimental values obtained for the curing degree, and the solid lines show the

corresponding calculated fits.



FIGURE 8 Photo-DSC measurements of BMI-689 with and

without addition of various photoinitiators. The photo-

response to a 2s pulse with a mercury lamp is shown.

FIGURE 9 compares the

photopolymerization peak areas of pure

BMI-689 with those of mixtures of BMI-689

with various types and amounts of PI.

FIGURE 9 Comparison of exothermic reactions of BMI-689

with and without addition of various types and amounts of

photoinitiator after a 2s UV pulse based on Photo-DSC

measurements.

Addition of 2 wt.% O-819 to BMI-689

results in the largest increase in the

polymerization peak, while higher amounts

of the PI are not beneficial. In the case of

O-TPO-L and O-369, the optimal amount is

5 wt.%. The photo-initiator O-907

decreases the reactivity of BMI-689 when

increasing amounts are added. In general,

O-819, and O-TPO-L show the best

reactivity in combination with BMI-689, as it

is illustrated by FIGURE 9. Amounts of

2 wt.% O-819 and 5 wt.% O-TPO-L are

most successful in creating a fast and high

UV curing response.

Properties of cured BMI-films

We observed that cross-linked BMI-based

films exhibit excellent chemical resistance

and have good thermomechanical

properties. Films of BMI-based materials

cured with the high-power UV-LED FJ200

(395 nm) without additional thermal post-

treatment were used for testing chemical

resistance. Fully cross-linked BMI-689 is

even resistant to harsh solvents at elevated

temperatures. The tests were carried out

with 2 wt.% polymerized BMI-689 in various

solvents at room temperature (RT) and

40°C and in an ultrasonic bath.42 The

solvent resistance test was followed by

solvent evaporation and determination of

weight loss. The solvents used were THF,

2,4,6-trichlorobenzene (TCB) and

N-methyl-pyrrolidone (NMP). In all tests, no

measurable loss or gain in weight was

observed. The parameters for the solvent

resistance tests are listed in TABLE II.



TABLE II Parameters of the solvent resistance tests of the fully cross-linked BMI-689 polymer.

Solvent Parameters

THF, TCB, NMP RT, 1.5 h

THF, TCB, NMP 40 °C, 1h

THF, TCB, NMP RT, ultrasonic bath, 30 min

THF, TCB, NMP 40°C, ultrasonic bath, 30 min

The thermal stability of the BMI-based

materials was assessed by TGA

measurements. UV-cured BMI-based films

exhibit very good thermal stability, as can

be seen in the TGA curves of the BMI

polymers shown in FIGURE 10. The film

prepared by UV curing of the aliphatic

extended BMI-689 shows a weight loss of

< 2 % at 271°C, while the film prepared

from the imide-extended BMI-3000 exhibits

the highest temperature stability with a

weight loss of < 2 % at 431°C; the other

BMI materials characterized in this study

have values between those of the former

two. According to the TGA measurements,

the BMI polymers have decomposition

temperatures between 422°C and 464°C.

FIGURE 10 TGA curves of BMI-polymers, cured by UV light

(365 nm).

TABLE III lists the temperatures for 2%

weight loss (T2wt%) and the decomposition

temperatures (Td) for the different

BMI-materials.

TABLE III Temperatures for 2 % weight loss and decomposition temperatures as determined by TGA for the BMI-polymers

analyzed.

Polymer T2wt% / °C Td / °C

BMI-689 271 422

BMI-1500 346 453

BMI-1700 358 450

BMI-3000 431 464

BMI-5000 411 450

FIGURE 11 shows the DMA curve of

UV-cured BMI-689 resin compared to the

curves of UV cured BMI resin with

additional thermal post-treatment and a

mixture of UV cured BMI-689 and

BMI-1500 resins with additional thermal



post-treatment. Films with dimensions of

40x10x1mm were cured in silicone molds

by 30 passes per side through a DYMAX

UVC conveyor. To ensure homogeneous

cross-linking of the thick film, thermal post-

curing was performed in an N2 atmosphere

at 200°C for 1 h. The storage modulus of

UV-cured BMI-689 of 340 MPa decreases

continuously until a temperature of 100°C,

when the rubbery plateau is reached and

the storage modulus stays at 60 MPa. Due

to decomposition, the storage modulus

starts to decrease again at 325°C. The UV

cured BMI-689 polymer, which was

subjected to an additional thermal

treatment, shows a slight increase in

storage modulus compared to the polymer

without additional treatment. This suggests

that thermal post-curing improves the

mechanical properties of the BMI material

due to a reorganization of the polymer

backbones above the glass-transition

temperature. Heating above the glass-

transition temperature can release stresses

that were built up in the fast UV-curing

process. The BMI-689/BMI-1500

co-polymer has a storage modulus of only

180 MPa at room temperature, which

decreases until 40 MPa at the rubbery

plateau. However, addition of the

imide-extended BMI-1500 leads to higher

thermal stability. The specimen containing

BMI-1500 begins to deteriorate

mechanically at higher temperature

(350°C) than the pure aliphatic BMI-689

polymer.

FIGURE 11 DMA curves of BMI polymers showing a

rubbery plateau at elevated temperatures until mechanical

deterioration begins at 325 °C (BMI-689 polymer) and

350 °C (BMI-689/BMI-1500 copolymer). The storage

modulus can be increased by additional thermal

post-treatment.

CONCLUSIONS

The curing kinetics of commercial aliphatic

(BMI-689) and imide-extended BMIs

(BMI-1500, BMI-1700, BMI-3000 and

BMI-5000) with various molecular weights

and structures were evaluated using the

FTIR, DSC, and Photo-DSC techniques.

The properties of the cured BMI-based

materials were assessed by TGA and DMA

measurements. We have shown that cured

BMI-based materials exhibit high

temperature stability, chemical resistance,

and good thermomechanical properties.

The BMI resins studied form highly cross-

linked thermoset polymers upon either

irradiation with UV or thermal curing at

temperatures of 200-250°C. The average

activation energies for thermal curing,

obtained by the Kissinger, Ozawa and

classical Arrhenius methods were

100 ± 15 kJ mol-1 for BMI-689 and

111 ± 12 kJ mol-1 for BMI-1500.



For thermal curing, kinetic rate constants in

the range of minutes were obtained, while

curing initiated by high-power mercury

halide lamps or UV-LEDs showed a faster

reaction with rate constants in the range of

seconds. The curing speed depends on

both molecular weight and chemical

structure of the BMI precursor. Imide-

extended BMIs with higher molecular

weight cure faster than lower molecular-

weight BMIs with a similar structure.

Aliphatic BMIs exhibit polymerization rates

between those of high molecular-weight

and low molecular-weight imide-extended

BMIs.

Due to its relatively low precursor viscosity

and suitable photoreactivity, the aliphatic

BMI-689 resin is the most promising

candidate for a PolyJetTM ink formulation.

ACKNOWLEDGEMENTS

This work was performed within the DIMAP

project, which received funding from the

European Union’s Horizon 2020 Program

for research and innovation under Grant

Agreement No. 685937. Part of this work

was funded within the Strategic Economic

and Research Program “Innovative Upper

Austria 2020” by the Government of Upper

Austria.

We would like to thank Sabine Hild and

Milan Kracalik for granting access to their

DMA equipment.

Furthermore, we thank Tiger Coatings

GmbH & Co KG for carrying out the Photo-

DSC measurements.

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2.1.1. Supporting Information for: Cure kinetics of bismaleimides as basis for polyimide-like inks for PolyJet™-3D-printing

Annika Wagner ,1 Irina Gouzman,2 Nurit Atar,2 Eitan Grossman,2 Mariana Pokrass,3 Anita

Fuchsbauer,1 Leo Schranzhofer,1 Christian Paulik4


2 Space Environment Department, Soreq NRC, Yavne, 81800, Israel

3 Stratasys Ltd., Haim Holtzman Street 1, Rehovot, 7670401, Israel




Summary of UV-sources used for

photo-polymerization of BMI-resins

The following table (TABLE S1) lists the UV

sources used to investigate the photocuring

process of BMI resins.

Activation energies of BMI

polymerization

The activation energies of aliphatic and

aromatic bismaleimides (BMI) were

determined from DSC scans at various

scan rates by means of the Kissinger and

Ozawa methods. FIGURE S1 and

FIGURE S2 show the corresponding plots.

From the slope of the Kissinger and Ozawa

plots, the activation energies can be

determined via eq. (2) and (3) in the main

paper. The values for the activation

energies as obtained by the Kissinger and

Ozawa plots were compared with the

activation energies obtained by the

Arrhenius equation [eq. (4) and (5) in the

main paper].

TABLE S1 UV sources used for curing of BMI resins.

Denotation Type Wavelength Power / W cm-2 Manufacturer

UV LED 365 nm 25 LED array 365 nm 0.08* assembled in house

UV LED 395 nm 25 LED array 395 nm 0.054* assembled in house

Firejet 200 High power UV LED 395 nm max. 16 Phoseon

Omnicure AC475 High power UV LED 365 nm max. 8 Excelitas

Omnicure AC450 High power UV LED 340 nm max. 0.6 Excelitas

MHL 250 Mercury lamp broad band 0.45* USHIO

DYMAX UVC

conveyor

Fe-doted mercury

lamp broad band DYMAX

* Intensity measured by means of a UV-Micro Puck Multi Integrator (UV-Technik Meyer)



FIGURE S1 Kissinger plot of BMI-689 for determining the

activation energy for thermal curing.

FIGURE S2 Ozawa plot of BMI-689 for determining the

activation energy for thermal curing.

The logarithm of the kinetic rate constant

ln(k) was plotted against the inverse

temperature T -1 (see FIGURE S3). From

the slope, the activation energy was

calculated using the Arrhenius equation.

Monitoring of the curing progress of BMI

Fourier Transform Infrared (FTIR) spectra

were used to obtain the curing degree

during thermal and photo-polymerization of

BMI. FIGURE S4 shows the FTIR

spectrum of BMI-689 as an example. The

bands at 2920 cm-1 and 2852 cm-1

represent C-H vibrations, the band at

1700 cm-1 corresponds to the carbonyl

groups of the terminal maleimide and – in

the case of imide-extended BMIs -

additionally to the imides in the inner part of

the oligomers.

FIGURE S3 Arrhenius plots of BMI-689 and BMI-1500 for

determining the activation energies based on the kinetic rate

constants for curing at 200°C, 225°C and 250°C.

The bands at 825 cm-1 and 696 cm-1

indicate C=C double bonds.

Low-molecular-weight BMI oligomers show

a higher number of maleimide double

bonds (696 cm-1) related to carbonyl bonds

than oligomers with higher molecular

weight. This is advantageous when

determining the curing degree, as a higher

signal-to-noise ratio – and thus greater

accuracy - is achieved.

The bands at 696 cm-1 and 825 cm-1

(maleimide double-bonds) decrease in

intensity with increasing polymerization as

a result of oligomer cross-linking (see

FIGURE S5). For determining the curing

degree, we chose the band at 696 cm-1 for

its high intensity.



FIGURE S4 FTIR spectrum of BMI-689, with characteristic

bands for maleimide double bonds at 696 cm-1 and

825 cm-1, a band corresponding to carbonyl bonds at

1700 cm-1, and bands for C-H bonds at 2920 cm-1 and

2852 cm-1.

Kinetic information was obtained by

calculating the double-bond conversion,

that is, the curing degree α, via eq. (1) (see

main paper). As an internal reference band,

the carbonyl bonds at 1700 cm-1 were used,

since they remain unchanged during

polymerization.

FIGURE S5 Curing degree of BMI-689 and decrease in

maleimide double bond band areas during thermal curing at

225°C.

The double-bond concentration decrease

as a function of time shows an exponential

behavior:

[A]t = [A]0∙e−kt. (EQ. S1)

The kinetic rate constant k was obtained by

plotting the logarithm of the educt

concentration at time t [A]t versus time t

according to:

ln ([A]t

[A]0) = -k ∙ t . (EQ. S2)

In the case of incomplete conversion of

double-bonds, we used an exponential

equation with a plateau to describe the

curing reaction:

[A]t=[A]

min+([A]0 − [A]

min)∙e−kt (EQ. S3)

where Amin is the minimum double bond

concentration reached after a certain curing

time t.

If the variance in experimental data is too

high for k to be determined by a linear plot

from eq. (5), the kinetic rate constant k* can

be obtained using a trial-and-error

approach by inserting values for k into the

corresponding rate equation to arrive at the

best approximation.

The kinetic rate constant for BMI

polymerization was determined from

equation S2 by plotting the logarithm of the

ratio of double-bond concentration at time t

to the initial double-bond concentration

versus time, as shown in FIGURE S6.

BMI-689 exhibits a kinetic rate constant of

0.068 ± 0.001 s-1 for curing at 225°C.



FIGURE S6 Plot for determining the polymerization rate

constant of BMI-689 cured at 225°C. The resulting kinetic

rate constant is 0.068 ± 0.001 s-1.

UV absorbance of BMI and

photoinitiators

FIGURE S7 shows absorption spectra of

various BMI-oligomers with absorption

maxima at 230 nm (BMI-689, BMI-1500

and BMI-1700) and 240 nm (BMI-3000 and

BMI-5000). FIGURE S8 presents UV

absorption spectra of various

photoinitiators, on the other hand.

FIGURE S7 UV absorbance of various BMI oligomers with

absorption maxima at 230 nm (BMI-689, BMI-1500 and

BMI-1700) and at 240 nm (BMI-3000 and BMI-5000).

FIGURE S8 UV absorbance of various photoinitiators used

for enhancing the photoreactivity of BMI oligomers.



2.2. Photoinitiator-free bismaleimide-acrylate based inks for Inkjet-

printing

The work of the author was the selection of ink components for a photoinitiator-free

bismaleimide-acrylate ink and the characterization of the formulations regarding

photoreactivity, viscosity and inkjet-printability. Printed and molded samples were

characterized regarding their thermal stability and thermomechanical properties. The

author prepared a manuscript summarizing the findings, which was published in the

Journal of Applied Polymer Science:2

Photoinitiator-free Photopolymerization of acrylate-bismaleimide mixtures

and their application for inkjet printing

Annika Wagner ,1 Michael Mühlberger,1 Christian Paulik2





ABSTRACT: There is growing interest in using photoinitiator-free systems in coating applications such as inkjet-printing, because

residual photoinitiator can alter the properties of the resulting polymer. Bismaleimides (BMI) offer the opportunity to

polymerize acrylates without addition of photoinitiators, as this class of molecules can serve both as polymerizable monomer

and as photoinitiator together with electron donor systems, like vinyl ether monomers or acrylates. The UV-induced

copolymerization of a low molecular weight BMI with various acrylate monomers and oligomers without any photoinitiator

was characterized. The BMI-acrylate systems show comparable polymerization speeds to widely used acrylic systems with

photoinitiator. Superior thermal stability as well as thermomechanical properties are achieved by enhancing acrylics with BMI.

Such photoinitiator-free systems lend themselves to be used for low-migration coatings as well as for high temperature

applications. Here, a characterization of selected BMI-acrylate mixtures regarding their photo-curing kinetics and their

application as inks for inkjet-printing is shown.

© 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47789.

KEYWORDS: kinetics; spectroscopy; thermal properties; thermogravimetric analysis (TGA); viscosity and viscoelasticity

Received 07 December 2018; accepted 16 March 2019 DOI: 10.1002/app.47789



Introduction

Inkjet printing enables the locally defined

and accurate digital deposition of materials.

Thus, it can be used not only for digital color

printing, but also as a digital manufacturing

tool, e.g. in printed organic electronics. In

inkjet printing, the material is ejected

through nozzles via a thermally or

piezoelectrically generated pulse (“Drop-

on-demand”).1

Photopolymerization, respectively

UV-curing is an important process used

widely for coating applications, adhesives

or printing processes like inkjet printing.

The photopolymerization process enables

very fast curing at room temperature, which

is beneficial for temperature sensitive

substrates and significantly reduces the

process time. Most of the UV-curable ink

formulations for inkjet printing or other

coating applications require a photoinitiator

(PI)-system to start the

photopolymerization reaction upon UV-

exposure. Commonly used systems are

based on phosphin-oxides, alkyl-phenones

or aromatic ketones.2

However, depending on the type and

amount of PI the usage of the latter

includes some known drawbacks, such as

yellowing, migration or altering the

properties of the resulting polymer.3

For this reason, there have been efforts in

realizing a PI-free photopolymerization in

order to avoid negative effects of the PIs on

the cured coatings.

There are various types of PI-free

photopolymerization reported in literature.

Bauer et al. reported on using a

monochromatic excimer lamp creating high

energy photons, which lead to a self-

initiation of acrylate polymerization.4

Halogenated acrylates or methacrylates

can be used as initiators for polymerization

by photochemically induced cleavage of

carbon-halogen bonds, forming halogen

radicals, which in turn start the

polymerization reaction.5 Wang et al.

reported about the production of UV

curable PI-free coatings via a Thiol-ene

photoclick reaction of a castor oil based

biothiol.6 Another promising approach for

PI-free photopolymerization, which does

not involve additional compounds or special

lamp systems like excimer lamps, has

already been reported in the 1990s. This

approach exploits the electron-donor-

acceptor mechanism by using

bismaleimides (BMI) as electron acceptors

together with suitable electron donors for

initiating the polymerization reaction.7-11

BMIs contain two reactive double bonds

and can serve both as polymerizable

monomer and as photoinitiator for other

monomers, such as acrylates or vinyl

ethers. As a result of UV-irradiation, BMIs

transition into an excited state (T1,

FIGURE 1), abstract a hydrogen atom from

the electron donor (e.g. acrylate monomer)

and an initiating radical is generated, which

starts the polymerization reaction (see

FIGURE 1).12



FIGURE 1 Generation of an initiating radical from BMI due

to UV absorption and subsequent hydrogen abstraction.12

Another major advantage of BMIs is that

they are capable of reducing the oxygen

inhibition during photo-polymerization,

which is a commonly known problem,

especially in the case of acrylate-based

systems. Oxygen molecules readily bind to

the free radicals which are formed as a

result of photolysis of the photoinitiator. The

peroxy-radicals formed in this process do

not react with the acrylic double bonds and

thus the polymerization reaction is

inhibited. There are several approaches on

how to overcome the oxygen inhibition13: (i)

Adding amines, that consume O2 via a

chain peroxidation reaction; (ii) using red

light irradiation in combination with a dye

sensitizer in order to convert the oxygen to

an excited singlet state; (iii) increasing the

formulation reactivity by adding higher

amounts of photoinitiator or using high

power UV light; (iv) using wax barrier coats

to exclude oxygen from the coating or (v)

carrying out the polymerization reaction

under inert conditions.

However, addition of components, for

example amines, often leads to additional

problems, such as the alteration of the

properties of the resulting polymer. These

include yellowing, creation of odors, or

decreasing the weathering resistance.13,14

Using inert conditions for the UV curing

process is not always easily applicable and

leads to additional process costs.

For the reasons mentioned above, the

usage of BMIs offers an elegant way to

produce polymeric coatings without the

addition of any PIs or additives to reduce

the oxygen inhibition, which might alter the

properties of the polymer.

Oxygen molecules lead to a regeneration of

the BMI, as shown in FIGURE 2. Thus, the

extent of copolymerization with acrylic

double bonds is not reduced.12

FIGURE 2 Reaction of oxygen molecules with BMI radicals,

which leads to a regeneration of the BMI.12

In this paper, we show the PI-free

photopolymerization of selected acrylate-

BMI mixtures and their application as inks

for inkjet printing. The photopolymerization

kinetics were characterized via

Fourier-Transform Infrared (FTIR)

spectroscopy. The thermal stability and the

thermomechanical properties of the cured

co-polymers were determined via

thermogravimetric analysis (TGA) and

dynamic mechanical thermal analysis

(DMTA), respectively. Furthermore, the

jetting performance of selected

BMI-acrylate ink formulations was

evaluated and test structures were printed.

Materials

BMI-689, average Mn=689 g mol-1, was

purchased from Designer Molecules Inc.,



San Diego, California, USA.

Tripropyleneglycol-diacrylate (TPGDA) and

Trimethylolpropane-ethoxylate triacrylate

(TMPETA), average Mn=692 g mol-1, were

purchased from Sigma Aldrich, Vienna,

Austria. Polyethyleneglycol-600-diacrylate

(PEG-600-DA; SR610), average

Mn=742 g mol-1, was acquired from

Sartomer Europe, Colombes Cedex,

France. Photoinitiators (Omnirad 819,

Omnirad TPO-L) were supplied by IGM

Resins B.V., RM Waalwijk, the

Netherlands. Sicrys I50TM-119 was

purchased from P.V. Nanocell Ltd, Migdal

Ha’Emek, Israel. FIGURE 4 summarizes

the chemical structures of the materials

used in this study.

Methods

FTIR-spectroscopy served as a tool to

determine the curing degree. A Bruker

Tensor 37 spectrometer with MIRacle

ATR-bridge was employed. Dynamic

mechanical thermal analysis (DMTA) was

used for characterization of

thermomechanical properties of the

polymer by means of an Anton Paar

Physica MCR 501 at 0.1 % deflection. A

Pyris Series TGA 4000 thermogravimetric

analyzer (TGA) was assessed for

determination of the thermal stability of the

crosslinked polymers.

Inkjet-printing tests were carried out using

a DIMATIX DMP2800 inkjet printer with

10 pL cartridges. The profiles of

inkjet-printed samples were measured with

a Dektak 150 profilometer (Veeco

Instruments Inc.). Films of non-printable

formulations were applied manually on

glass slides with the doctorblading

technique using a doctor blade with 12 µm

grooves.

A DYMAX UVC conveyor with an Fe-doped

mercury halide lamp was used for

photopolymerization of the ink mixtures.

Further, high power UV-LEDs with various

peak wavelengths were used: FireJet (FJ)

200 from Phoseon (395 nm) (i); Omnicure

AC475 from Excelitas (365 nm) (ii); and

Omnicure AC450 from Excelitas (340 nm)

(iii). The intensities of the UV sources used

were measured by a UV-Micro Puck Multi

Integrator (UV-Technik Meyer, Ortenberg,

Germany).

Viscosity measurements were performed

with the SV10 Vibro Viscosimeter (A&D

Company Ltd.).

A Keyence VHX 5000 microscope was

used for optical analysis of inkjet-printed

test structures. A Nabertherm muffle

furnace model L3/11/P330 was used for

thermal sintering of the silver ink on top of

the printed BMI-acrylate ink.

Results and Discussion

As aliphatic BMIs are capable of creating

initiating radicals as a consequence of UV-

irradiation, we used BMI together with

acrylate monomers to create co-polymers

by photopolymerization. FIGURE 3 shows

the UV absorption spectrum of BMI-689,

with an absorption maximum at 228 nm. At

this wavelength in the UVC region, the

highest energy uptake takes place. Thus,

the most effective photopolymerization can

be reached by irradiation with short



wavelength LEDs or broad band mercury

halide lamps.

FIGURE 3 UV absorption spectrum of BMI-689 with an

absorption maximum at 228 nm.

In the following, a characterization of the PI-

free photopolymerization kinetics using

BMI as PI and polymerizable monomer is

shown. Further, we characterized the

thermal and thermomechanical properties

of the polymers and evaluated the

application of selected liquid formulations

for inkjet-printing.

Selection of ink components

The acrylate monomers and oligomers for

the PI-free BMI-acrylate ink were selected

regarding ink properties, such as viscosity,

and properties of the cured copolymers,

including thermal stability and

thermomechanical properties. For inkjet

printing, the inks have to be low viscous

(8-25 mPa s) at the jetting temperature.

TPGDA is a difunctional acrylate monomer

widely used for UV-curable ink formulations

with very low viscosity (FIGURE 4b).

Further, we chose PEG-600-DA (FIGURE

4c), which has a similar structure, but with

a longer chain in between the two acrylate

groups, leading to increased distance in

between the crosslinking points in the

polymer. Using PEG-600-DA, in the

following referred to as “PEGDA”, enables

a higher flexibility of the BMI-acrylate

copolymer through the possibility of energy

dissipation. TMPETA (FIGURE 4d) was

chosen because it offers a compromise

between high crosslinking degree due to its

three functional groups and increased

distance between the crosslinks to enable

energy dissipation through the polymer

chains. Thus, a BMI-TMPETA copolymer

has high thermal stability because of the

cross-linked nature and beneficial

mechanical properties due to reduced

brittleness because of longer chains

compared to BMI-TPGDA.

FIGURE 4 Chemical structures of BMI-689 (a),

Tripropyleneglycol-diacrylate (b), Polyethylene-glycol-600-

diacrylate (c) Trimethylolpropane-ethoxylate triacrylate (d)

and the photoinitiators Omnirad 819 (e) and Omnirad TPO-L

(f).



Kinetics of the PI-free

photopolymerization

FTIR spectroscopy was used for

characterization of the kinetics of the

PI-free photopolymerization of

BMI-acrylate mixtures. The extent of

polymerization was calculated via the

decrease in band areas of the reactive

acrylic and maleimide double bonds.

FIGURE 5 shows the FTIR spectrum of a

stoichiometric mixture of BMI-689 and

TPGDA (a) and the decrease in reactive

double bond bands after exposure to

increasing UV doses using the DYMAX

UVC conveyor (b). The bands at 696 cm-1

and 827 cm-1 correspond to the maleimide

double bonds, the band at 810 cm-1

corresponds to the acrylic double bond of

TPGDA. The band at 1700 cm-1 represents

the carbonyl bonds, which remain constant

during polymerization and is therefore used

as internal reference band.

FIGURE 5 FTIR spectrum of an equimolar BMI-689/TPGDA mixture (a) and decrease of reactive double bond bands of acrylate

(810 cm-1) and BMI oligomers (827 cm-1, 696 cm-1) after exposure to increasing UV-doses (DYMAX UVC conveyor) (b).

As the bands of the maleimide- and

acrylate double bonds overlap, a

deconvolution of the bands was performed

using the Software Origin Pro (OriginLab

Corporation). FIGURE 6 shows the

deconvoluted bands of maleimide and

acrylate, decreasing after exposure to

increasing UV doses, indicating the

polymerization of the monomers and

oligomers.

The curing degree α is determined

according to


Adb,0∙Aref,t) ∙ 100, (1)

where Adb,0 and Adb,t are, respectively, the

band areas of maleimide double bonds

(696 cm-1, 827 cm-1) in the case of BMIs

and acrylate double bonds (810 cm-1) in the

case of acrylates, before and after the

curing time t.



FIGURE 6 Deconvolution of overlapping bands of acrylate

(810 cm-1) and BMI (827 cm-1) using the Origin Pro

Software.


reference band areas before and after the

curing time t.15

The device for photopolymerization of the

PI-free ink (DYMAX UVC conveyor) is

described in the supporting information.

The UV dose was controlled via the speed

of the conveyor and the exposure time was

calculated from the irradiation width of the

UV lamp and the conveyor speed (see

FIGURE S1 and FIGURE S2, supporting

information).

FIGURE 7 compares the conversion of

reactive double bonds of various

stoichiometric BMI/acrylate mixtures (a-c)

during UV-curing with the DYMAX UVC

conveyor to the conversion of pure BMI

without and pure acrylates with 1 wt.%

Omnirad 819 and 1 wt.% Omnirad TPO-L.

(g). The corresponding reaction rates Rp

were calculated by the decrease of reactive

double bond areas over time and are

shown in FIGURE 7d-f and h. We observed

that the BMI serves as a good PI for the

acrylates, especially for TPGDA and

TMPETA, where the BMI- and acrylate

double bonds are consumed at similar

rates. In the BMI/PEGDA formulation the

acrylate reaction rate and curing degree are

lower. In the BMI/TPGDA and

BMI/TMPETA mixtures, the acrylate

reaction rates are higher compared to the

pure acrylates. This can be attributed to the

fact that BMI significantly reduces the

oxygen inhibition by reacting with the

oxygen molecules present in air, as has

been presented in FIGURE 2. Pure TPGDA

with PI only reacts up to a conversion of

20 %, while in the BMI/TPGDA mixture, the

acrylate double bonds of TPGDA are

converted to 90%



FIGURE 7 Conversion of acrylate and BMI double bonds (DB) in equimolar BMI-689/acrylate mixtures with increasing UV dosage

through irradiation with a mercury halide lamp (DYMAX UVC conveyor) (a-c) and the corresponding reaction rates (d-f); conversion

of pure BMI and pure acrylates (g) and the corresponding reaction rates (h). The symbols represent experimental data obtained

by FTIR measurements, while the solid lines show the corresponding exponential fits. The dashed lines show the connection

between experimental points for better readability.

In FIGURE 8, the influence of varying the

wavelength of UV irradiation on the

reactivity of BMI-689/TPGDA 50/50 (w/w) is

shown. The films were irradiated with UV

LEDs with 340 nm, 365 nm and 395 nm

peak wavelengths. It was found that lower

wavelengths increase the reactivity of the

BMI-acrylate system. Irradiation with a

340 nm LED results in a polymerization

speed comparable to irradiation with the

broad band mercury halide lamp. However,

increasing the wavelength results in a

significant drop of polymerization speed.

It can be seen that the acrylate double

bonds are consumed slower compared to

the maleimide double bonds, as in the

50/50 (w/w) mixture there is an excess of

acrylate.



FIGURE 8 Conversion of acrylate and BMI double bonds of a BMI-689/TPGDA 50/50 (w/w) mixture during irradiation with UV

LEDs with 340 nm (curves 1. and 2.), 365 nm (curves 3. and 4.) and 395 nm (curves 5. and 6.) (a) and the corresponding reaction

rates (b).

Characterization of thermomechanical

properties

The thermal stability and the

thermomechanical properties of the PI-free

BMI-acrylate copolymers were

characterized using TGA and DMTA,

respectively. FIGURE 9 shows the TGA

curves of pure BMI- and pure acrylate

polymers, and of BMI/acrylate copolymers

photo-polymerized without PI. We

compared bulk samples with dimensions of

40x10x1 mm obtained by UV-curing in

molds to inkjet-printed samples. Films of

non-printable formulations were applied by

the doctorblading technique for

comparison. TABLE I summarizes the

temperatures of the maximum

decomposition rates Td,max of the

BMI/acrylate copolymers, obtained from

the first derivative of the TGA curves. It was

observed that in general, the bulk samples

have slightly higher thermal stability

compared to the thin films, and that the

BMI-homopolymer is more thermally stable

than the acrylate homopolymers. The

thermal stability of the PI-free BMI/acrylate

copolymers lies in between, where the

BMI/TMPETA copolymer shows the

highest thermal stability of the PI-free

copolymers with Td,max = 463 °C for the bulk

sample and Td,max = 454 °C for the

doctorbladed film sample. The thermal

decomposition of the BMI/TPGDA

copolymers proceeds in two steps,

indicating distinct acrylate- and BMI-

phases in the copolymer. This can be

explained by the low molecular weight

(MW) of TPGDA, enabling a polymerization

of the low MW monomers in between the

longer BMI-chains.



FIGURE 9 TGA measurement of BMI-689- and acrylate-homopolymers, and BMI-689/acrylate copolymers. Comparison of bulk

samples (a,b) to doctorbladed and inkjet-printed samples (c,d).

TABLE I Temperatures of maximum decomposition rate of printed/doctorbladed films and bulk samples of acrylate and BMI homo-

and copolymers obtained by the first derivative of the TGA curves.

Material Td, max of printed or doctorbladed film / °C Td, max of bulk sample 40x10x1 mm / °C

TPGDA polymer -a 417

PEGDA polymer 418b 424

TMPETA polymer 429b 437

BMI-689 polymer 474b 505

BMI-TPGDA copolymer

(50/50 w/w)

387, 488c 408, 481

BMI-TPGDA copolymer

equimolar

397, 485c 410, 505

BMI-PEGDA copolymer 454b 463

BMI-TMPETA copolymer 461b 473

a sample not cured due to oxygen inhibition of thin film

b 12 µm doctorbladed samples

c 5x5 mm printed samples, 5 layers, UV cured, two-step decomposition



The thermomechanical properties of the

BMI-acrylate copolymers were assessed

using DMTA. FIGURE 10 shows the

storage moduli E ’ of the pure BMI-polymer

and pure polyacrylates (a) and the BMI-

acrylate copolymers (b). All polymers have

a distinct rubbery plateau after the glass

transition, followed by a decrease in E ’ due

to mechanical deterioration and specimen

failure. Both the TMPETA and PEGDA are

polymers with low branching, having a low

modulus, which stays constant with

temperature increase. The BMI- and

TPGDA polymers are cross-linked and

have higher storage moduli at room

temperature, which decrease at elevated

temperatures until the rubbery plateau is

reached. The BMI-polymer features the

highest thermomechanical stability with a

decrease in modulus starting from 350 °C

until the specimen fails at 410 °C, while the

TPGDA polymer fails already at 310 °C.

The BMI-acrylate copolymer with the best

thermomechanical properties is

BMI/TMPETA, which is stable up to 350 °C,

followed by specimen failure at about

375 °C. The three acrylate groups in

TMPETA allow a high degree of

crosslinking, beneficial for the thermal

stability. At the same time the higher initial

MW enables enough distance between the

crosslinks to improve the mechanical

properties by offering a way for energy

dissipation through the polymer chains. In

contrast, lower MW monomers, such as

TPGDA lead to brittleness because of the

formation of a highly cross-linked polymer

network.

FIGURE 10 DMTA measurement of the pure polyacrylates and pure BMI-689 polymer (a) compared to the BMI/acrylate

copolymers (b). The copolymer with the best thermomechanical properties (BMI/TMPETA) fails at 375 °C. Specimens with

dimensions of 40x10x1 mm were produced by UV-curing the precursors in silicone molds in the DYMAX UVC conveyor.

Jetting characterization of the PI-free ink

In order to be printable via inkjet-printing,

an ink has to fulfill certain requirements.

Most importantly, the ink should have a

suitable surface tension and a viscosity in

the range of 8-25 mPa s.16 FIGURE 11

shows the viscosity of different

BMI/acrylate mixtures at a jetting



temperature of 60-70°C. Only mixtures of

the BMI-689 oligomer with the low viscous

TPGDA have suitable viscosities for jetting.

The most promising formulation is 50 w%

BMI-689 in TPGDA, but also the equimolar

BMI-689/TPGDA mixture is jettable at

66-70°C cartridge temperature.

FIGURE 11 Viscosities of BMI-acrylate mixtures at

temperatures of 60-70°C. Only mixtures with the low

viscous TPGDA are ink-jettable at elevated temperatures.

Jetting tests of BMI-TPGDA (50/50 w/w)

and equimolar BMI/TPGDA were

performed using the DIMATIX DMP2800

inkjet printer and are described in the

supporting information. The drop formation

as observed in the DIMATIX dropwatcher

feature is depicted in FIGURE S3

(supporting information). Further

characterization includes the measurement

of the drop mass and the drop velocity.

FIGURE S4 (supporting information)

depicts the drop mass and velocity of the

two jettable BMI/acrylate formulations

depending on the piezo voltage at 60 °C

and 70 °C cartridge temperature. The

printer’s manufacturer recommends setting

a drop velocity between 7 and 9 m s-1.17

Piezo voltages of 21-23 V (60 °C cartridge

temperature) in case of BMI/TPGDA

50/50 (w/w) and of 25-29 V (70°C cartridge

temperature) in case of equimolar

BMI/TPGDA allow reasonable drop

velocities in this given range. FIGURE S5

shows first simple test structures printed

with the DIMATIX DMP2800 inkjet printer.

Covering layers were printed using a

resolution of 2540 dpi (a) and distinct drops

were printed using a resolution of 127 dpi

(b). Printing of covering layers becomes

important, when the ink will be used as

insulating layer between electrically

conductive paths.

Inkjet-printed test samples

Printed samples of the two ink-jettable

formulations (BMI/TPGDA (50/50) (w/w)

and equimolar BMI/TPGDA) were

fabricated with the DIMATIX DMP 2800

inkjet printer. The test structures include

5x5 mm squares and 15x25 mm rectangles

with layer numbers of 1-5. After each layer

the samples were UV cured with 2 passes

at 1.5 m min-1 in the DYMAX UVC conveyor

and the final layer was cured with 5 passes

at 1.5 m min-1. FIGURE 12 shows the side

view of 5 layers of equimolar BMI/TPGDA

(a,b), the profiles of 1, 3 and 5 layers,

averaged from three measurement lines

across the samples (c) and the top view of

1, 3 and 5 layers of the printed samples (d).

The equimolar BMI/TPGDA formulation

features a better surface quality and better

dimensional stability of printed samples

compared to the formulation consisting of

BMI/TPGDA 50/50 (w/w), as it is depicted

in FIGURE 12e and f. In BMI/TPGDA 50/50

(w/w), there is an excess of TPGDA, which



partly evaporates due to the energy uptake

from UV irradiation during curing, leading to

bubbles on the surface of the cured film. In

the equimolar formulation, enough

BMI-molecules are present to serve as an

initiator for TPGDA, avoiding excessive

evaporation and enabling the production of

UV cured films with smooth surface.

Furthermore, due to the immediate curing

in the equimolar BMI/TPGDA ink,

spreading of the printed layer is reduced.

One possible use of this photo-initiator-free

ink is as dielectric layer in combination with

electrically conductive layers.

FIGURE 12 Inkjet-printed test structures of the PI-free ink: side view of 15x25 mm rectangle with 5 layers (a) and 5x5 mm squares

with 5 layers (b), profiles of samples with 1, 3, and 5 layers (c), top view of 15x25 mm rectangles and 5x5 mm squares with 1

(bottom), 3 (middle), and 5 (top) layers (d) and microscopic images of printed squares of BMI-TPGDA (50/50) (w/w) (e) and

equimolar BMI-TPGDA (f) showing the dimensional stability of 1, 3, and 5 layers. The scale bar represents 5 mm.

Its high thermal stability ensures that the

layer withstands the high sintering

temperatures commonly used for silver

sintering. Higher sintering temperatures

lead to faster accomplishment of the

desired conductivity by a faster sintering

process.18

FIGURE 13 shows an inkjet- printed test

structure consisting of a bottom layer of PI-

free ink and a top layer of silver ink (Sicrys

I50TM-119). The silver line was sintered at

200°C for 1 h in ambient atmosphere. The

resistance measured across the silver line

was 1.1 Ω, which lies within the expected

range. This example shows the applicability

of the PI-free ink in combination with

electrically conductive inkjet inks.

FIGURE 13 Layer of PI-free ink with inkjet-printed silver line

on top. The measured resistance after sintering at 200°C

was 1.1 Ω. The scale bar represents 5 mm.



Conclusions

Various PI-free BMI/acrylate mixtures were

characterized regarding their photo-

reactivity and their applicability as inks for

inkjet printing. We showed that the aliphatic

BMI-689 oligomer is capable of starting the

co-polymerization reaction upon UV

irradiation, while reducing oxygen inhibition

significantly compared to the pure acrylates

with PI. The reaction rates of the PI-free

mixtures were comparable to widely used

acrylate systems with PI.

The selection of acrylates was based on

parameters affecting the inkjet-printability,

such as low viscosity, and on properties of

the cured copolymers, including thermal

stability and thermomechanical properties.

Only the mixtures containing low MW

TPGDA were suitable for inkjet printing, as

they featured the desired low viscosity.

BMI-689/TPGDA 50/50 (w/w) and

equimolar BMI-689/TPGDA were

successfully used for fabricating inkjet-

printed test structures.

The best thermomechanical properties,

comparable to the pure BMI-polymer, were

obtained using a trifunctional acrylate

oligomer (TMPETA, MW=692 g mol-1),

which resulted in a copolymer with

mechanical stability up to 350 °C. The

BMI/TMPETA copolymer also showed the

highest thermal stability of the PI-free

copolymers with Td,max=454 °C for the

doctorbladed film sample. The equimolar

BMI/TPGDA copolymer, fabricated by

inkjet printing, showed a two-step

decomposition with Td,max,1=397 °C and

Td,max2=485 °C.

Acknowledgements

This work was performed within the

MultiLINK project, funded by the Austrian

Federal Ministry for Transport, Innovation

and Technology.

We are grateful for the financial support by

the Johannes Kepler Open Access

Publishing Fund.

We would like to thank Sabine Hild and

Milan Kracalik (Johannes Kepler University

of Linz, Institute for Polymer Science) for

granting access to their DMA equipment.

References

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2. Nowak, D., Ortyl, J., Kaminska-Borek, I., Kukula, K., Topa, M., Popielarz, R. Polym. Test. 2017, 64, 313-320.

3. Green, W. A.; In Industrial Photointiators – A Technical Guide; CRC Press, imprint of the Taylor & Francis Group: Boca Raton, London, New York, 2010; pp 75-109.

4. Bauer, F., Decker, U., Naumov, S., Riedel, C. Prog. Org. Coat. 2014, 77, 1085-1094.

5. Daikos, O., Naumov, S., Knolle, W., Heymann, K., Scherzer, T. Phys. Chem. Chem. Phys. 2016, 18, 32369-32377.

6. Wang, Q., Chen, G., Cui, Y., Tian, J., He, M., Yang, J. ACS Sustainable Chem. Eng. 2017, 5, 376-381.

7. Decker, C., Decker, D. Polymer 1997, 38, 2229-2237.

8. Morel, F., Decker, C., Jönsson, S., Clark, S. C., Hoyle, C.E. Polymer 1999, 40, 2447-2454.

9. Andersson, H., Hult, A. J. Coat. Technol. 1997, 69, 91-95.

10. Burget, D., Mallein, C., Fouassier, J. P. Polymer 2003, 44, 7671-7678.

11. Vázquez, C. P., Joly-Duhamel, C., Boutevin, B. Macromol. Chem. Phys. 2013, 214, 1621-1628.



12. Andrzejewska, E. Prog. Polym. Sci. 2001, 26, 605-665.

13. Studer, K., Decker, C., Beck, E., Schwalm, R. Prog. Org. Coat. 2003, 48, 92-100.

14 Khudyakov, I. V., Prog. Org. Coat. 2018, 121, 151-159.

15 Moraes, L., Rocha, R., Menegazzo, L., Araújo, E., Yukimitu, K., Moraes, J. J. Appl. Oral. Sci. 2008, 16, 145-149.

16 Magdassi, S., In The Chemistry of Inkjet Inks; Magdassi, S., Ed.; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2010; Chapter 2, pp 19-21.

17 Fujifilm DIMATIX, 2010; Dimatix Materials Printer DMP2800 Series User Manual, ed. U.S.A.: FUJIFILM Dimatix, Inc., p. 65.

18 Halonen, E., Viiru, T., Östman, K., Lopez Cabezas, A., Mäntysalo, M., IEEE Trans. Compon. Packag. Manuf. Technol. 2013, 3, 350-356.



2.2.1. Supporting Information for: Photoinitiator-free Photopolymerization of acrylate-bismaleimide mixtures and their application for inkjet printing

Annika Wagner ,1 Michael Mühlberger,1 Christian Paulik2





Setup for UV-curing of the PI-free ink

The characterization of the photo-reactivity

of the PI-free ink was carried out using a

DYMAX UVC conveyor with an Fe doped

mercury lamp.

FIGURE S1 Scheme of the DYMAX UVC conveyor showing

the irradiation width of 90 mm.

The UV intensity was measured by a UV

Micro Puck Multi Integrator. With this UV

conveyor system, the dosage can be tuned

by adjusting the conveyor speed – slow

conveyor speeds result in longer exposure

times and thus a high UV dose. A scheme

of the DYMAX UVC conveyor is shown in

FIGURE S1. The exposure time was

calculated from the exposure width of the

UV lamp (90 mm) and the speed of the

conveyor. As an example, one pass at

1.5 m min-1 corresponds to an exposure

time of 3.6 s. FIGURE S2 shows the

dependence of exposure time and UV dose

on the conveyor speed (a) and the UV dose

versus the exposure time (b).

FIGURE S2 Exposure time and UV dose dependent of the UV conveyor speed (a) and UV dose versus UV exposure time (b).

Jetting characterization of the PI-free ink

FIGURE S3 shows an exemplary picture of

drop formation of the BMI-acrylate ink (BMI-

TPGDA 50/50 (w/w) at 60°C cartridge

temperature and 16 V piezo voltage as

observed in the drop-watching feature. The



ejection of the drop after strobe delays from

0 µs to 90 µs is displayed.

FIGURE S3 Drop formation of the BMI-acrylate ink at 60°C

cartridge temperature and 16 V piezo voltage, as observed

in the Dimatix DMP2800 drop-watcher.

It can be seen that the tail combines with

the drop and no satellite or spray formation

occurs. Finally, one single drop is

generated. The drop masses and velocities

of the two inkjet-printable formulations were

determined using the DIMATIX DMP2800

inkjet printer and the corresponding graphs

are shown in FIGURE S4. The desired drop

velocities of 7-9 m s-1 are obtained at piezo

voltages of 21-23 V (60 °C) in case of

BMI-TPGDA 50/50 (w/w) and of 25-29 V

(70 °C) in case of equimolar BMI-TPGDA.

First printed test structures with dimensions

of 5x5 mm are shown in FIGURE S5.

FIGURE S4 Drop mass and drop velocity versus piezo voltage measured with the DIMATIX DMP2800 inkjet printer, shown for

BMI-TPGDA 50/50 (w/w) at 60 °C cartridge temperature and for equimolar BMI-TPGDA at 70 °C cartridge temperature.

To obtain covering layers, the printing

resolution has to be adjusted. A higher

resolution results in higher thickness of the

printed layers. FIGURE S5 shows two

examples with different resolutions: Using

2540 dpi leads to a fully covered layer (a),

while distinct drops are deposited at a low

resolution of 127 dpi (b). For further test

samples, a resolution of 2540 dpi was

chosen.

FIGURE S5 Test structures of the BMI-acrylate ink printed

with the Dimatix DMP2800 inkjet printer with dimensions of

5x5 mm on a microscope glass slide without pretreatment.

A resolution of 2540 dpi was used for printing covering

layers (a) and a resolution of 127 dpi was used for printing

distinct drops (b).



2.3. Foamable acrylate-based inks for PolyJetTM-3D-printing

The author’s tasks included the formulation and evaluation of acrylate based foamable

inks containing a modified BA. The ink characterization was carried out regarding

inkjet- printability, thermal stability and foaming performance. A strategy for printing

foamed layers, consisting of low dose UV-pinning, NIR-induced foaming and UV

curing, was developed. The resulting foams were characterized by profilometry and

microscopy of cross-sections. The author prepared the following manuscript, which

was published in the European Polymer Journal:3

Foamable acrylic based ink for the production of light weight parts by inkjet-based 3D printing

Annika Wagner ,1 Andreas M. Kreuzer,2 Lukas Göpperl2, Leo Schranzhofer,1 Christian

Paulik2





ABSTRACT: Foamable inks for inkjet-based 3D printing processes, like PolyJetTM printing, open the way to produce light-weight additive

manufactured parts in one process step. A new ink formulation, consisting of acrylic components and a modified blowing agent is presented.

The goal is to directly produce foam during the printing process in contrast to other methods for 3D printing of foams, which are currently

used. The presented characterization of the ink is focused on viscosity, decomposition temperatures of the blowing agents, and

UV-polymerization of the ink matrix. More advanced ink characterization includes jetting tests at different inkjet-printheads and foaming of

printed layers, as well as microscopic characterization of the foams. © 2019 The Authors. European Polymer Journal published by Elsevier Ltd. Eur. Polym. J. 2019, 115, 325-334.

KEYWORDS: photopolymerization, blowing agent, polymeric foam, inkjet-printing, additive manufacturing

Received 17 January 2019, Revised 5 March 2019, Accepted 14 March 2019, Available online 15 March 2019 DOI: 10.1016/j.eurpolymj.2019.03.031

mailto:[email protected]

https://doi.org/10.1016/j.eurpolymj.2019.03.031



INTRODUCTION

Inkjet printing is not only used for graphical

applications like art printing or product

coding, but has become largely popular as

a tool for digital fabrication of functional

structures and additive manufacturing. It

has been used to print water-based inks,

polymer precursors, particle dispersions

such as ceramics or electrically conductive

particles, and biomaterials, like proteins

and cells [1]. In inkjet printing, the drops are

generated either by a thermal or

piezoelectric pulse where they are needed;

this technique is called “drop on demand”

(DOD). It offers high resolution, high

flexibility in terms of design and lot numbers

and precise control of deposited ink

volumes, leading to less waste compared to

subtractive methods [2]. Despite the

mentioned advantages, there are also

some drawbacks like the very small

processing window of inkjet inks, as they

need to be ejected through very small

nozzles and the possibility of clogging the

latter. The inks should have viscosities of

about 8-25 mPa s at jetting temperature

and a suitable surface tension of

28-32 N m-1 [3].

PolyJetTM-3D-printing uses multiple

inkjet-printheads filled with photo-curable

inks to create complex-shaped

multi-material three dimensional parts. The

lamps for UV-curing of the deposited

material are mounted directly on the sides

of the printing block to enable an immediate

fixing of the droplets after deposition. The

3D-objects are printed on a build tray,

where each material is deposited digitally

only where needed and a gel-like support

material can be used to print overhanging

structures. The resolution is defined by the

layer thickness, which is in the order of

15-30 µm [4,5].

The reasons for enhancing 3D printing with

a technique to produce polymeric foams

are manifold and coincide partly with

conventional produced foamed products;

weight reduction, structural improvements,

shock absorbing properties, thermal

insulation or packaging to name a few [6].

Although there are several different

methods known to achieve foaming of

polymers, for this study the thermal foaming

with the use of a chemical blowing agent

(BA) was chosen, as it is the most

promising route to combine with PolyJetTM

technology [7]. Chemical blowing agents

are substances which decompose through

an energy input via a chemical reaction to

one or more gaseous products, thus, when

incorporated in a suitable polymeric

environment, expanding the latter while

producing foam. Several substance

classes are known to have excellent

foaming qualities, but for the specific task of

PolyJetTM printing, a sulfonylhydrazide

blowing agent was chosen. This choice was

made based on ink matrix compatibility and

technology compatibility, decomposition

temperatures, gaseous and residue

products and gas numbers [6-8]. The gas

number of a BA describes the volume of

gases which can be released per gram of

BA. As an example, 4,4'-

Oxydibenzenesulfonyl hydrazide (OBSH)

releases about 125 cm3 g-1 of mainly



nitrogen gas as a result of thermal

decomposition [9].

Polymeric foams can be classified

according to their cell structure (i), their

density (ii) and their mechanical properties

(iii) [10].

(i) Open-cell foams have

interconnected pores in the

foamed body and are often

permeable to gases. On the

other hand, closed-cell foams

consist of a polymeric material

featuring individual pores, which

are separated from each other.

Closed cell foams are gas-tight.

(ii) According to their density,

polymeric foams can be

classified as low

foaming/high-density foams (

> 0.4 g cm-3, gas-solid

expansion ratio < 1.5), as

moderate-foaming/ middle-

density foams ( =

0.1-0.4 g cm-3, gas-solid

expansion ratio=1.5-9.0), or as

high-foaming/low-density foams

( < 0.1 g cm-3, gas-solid

expansion ratio > 9.0)

(iii) Based on their rigidity,

polymeric foams can be divided

into rigid, semi-rigid and flexible

foams. The glass transition

temperature of rigid foams is

higher than room temperature

and the E-modulus is greater

than 700 MPa. Flexible foams

have a glass transition

temperature lower than room

temperature and an E-modulus

less than 70 MPa. Semi-rigid

foams fall in between rigid and

flexible foams.

Depending on the condition and the desired

application, there are several methods for

characterization of polymeric foams.

Important parameters, which influence the

physical and mechanical properties of

foams and thus, the application of the foam,

include the cell morphology, porosity,

density, and gas tightness.

The porosity of a foam is the ratio between

the pore volume and the whole volume of

the body. The porosity influences the

thermal conductivity, optical and acoustic

properties, tensile strength, and creep rate.

The porosity θ is defined via the following

equation, where Vp is the pore volume, Vt is

the total volume of the foam body and Vs is

the volume of the dense solid:

θ =Vp

Vt= (

Vp

Vs + Vp) (1)

The relative density r is the ratio of the

nominal density of the foam body * and the

solid density s:

𝜌𝑟 =𝜌∗

𝜌s (2)

There are several ways for characterizing

the porosity and density of a foam, for

example microscopic analysis, or the

mass-volume direct calculation. For

microscopic analysis, a cross section of the

foamed body is prepared and via the areas

of pores and solid material, the porosity can

be calculated by dividing the pore area Sp

by the total cross section area of the

sample St:



θ =Sp

St (3)

The mass-volume direct calculation uses

the known volume V and mass m of the

porous sample to calculate the porosity via

the following equation:

θ = 1 - m

V ρs

(4)

The mass of the sample is determined by

balance measurement, the sample volume

is measured by using a vernier caliper or

micrometer, or by microscopy. s is the

theoretical solid density. This method is

easy, fast and does not need any specific

equipment. Commonly used sample

shapes for volume calculation include

cubes, spheres or cylinders.

The pore sizes and morphologies are

commonly characterized using microscopy

of cross-sections. The prerequisites for an

adequate estimation of pore sizes and their

distribution are to analyze enough pores for

obtaining representative results within a

statistical analysis and to have a good

quality, smooth cross-cut allowing a good

image quality [11]. An alternative to

microscopy is to use tomographic methods,

which are non-destructive, do not require

specific sample preparation and have a

high spatial resolution [12].

In the last years, several approaches for

3D-printing of foams have been reported in

literature. Most of the methods use Fused

Filament Fabrication (FFF). Marascio et al.

used a filament saturated with supercritical

CO2, which foams during deposition of the

molten strands [13]. A similar approach was

used by Kakumanu and Sundarram, who

performed the CO2 saturation and foaming

of PLA after printing [14]. Singh et al.

reported on creating foams via using

hollow-particle filled HDPE filaments [15].

However, using FFF does not allow high

resolution. Another possibility is to directly

print lattice structures, for example by

selective laser sintering (SLS), as reported

by Abueidda et al. [16] and Maskery and

co-workers [17]. SLS offers greater

resolution as the laser can selectively melt

a defined area. However, by printing

lattices, it is difficult to obtain micropores.

Stereolithography (SLA) [18] or direct laser

writing (DLW) can be used for fabricating

lattices with smaller pores [19].

Another possibility is to use composites of

two materials, where one of the materials

can be removed after the printing process,

by either leaching out [20] or via thermal

decomposition, while the other material

stays and forms a porous network [21].

Up to now, there has not been any method

to print foams via inkjet-based 3D-printing,

or more specifically, PolyJetTM-3D-printing.

Using an acrylate based ink with a modified

BA inside opens the way to build

light-weight three dimensional complex

shaped structures in combination with other

materials by PolyJetTM-technology. In the

following, we report on the synthesis of

modified BAs and their incorporation into

acrylate based ink matrices. The BA

decomposition reaction was characterized

via Differential Scanning Calorimetry (DSC)

and Thermogravimetric Analysis (TGA),

while the ink was characterized by

viscometry and by Fourier Transform

Infrared (FTIR) spectroscopy to determine



the extent of photopolymerization.

Furthermore, we performed inkjet-printing

tests and NIR-induced foaming of printed

layers, as well as microscopic evaluation of

the printed foams.

EXPERIMENTAL

Materials

Photomer 3016 was purchased and the

photoinitiators Omnirad 819 and Omnirad

TPO-L were supplied by IGM resins B.V.,

RM Waalwijk, the Netherlands. Genorad 22

was supplied by Rahn AG, Zürich,

Switzerland. Trimethylolpropane ethoxylate

triacrylate and Tripropylene glycol

diacrylate (mixture of isomers) were

purchased from Sigma Aldrich, Vienna,

Austria. SR610 (Polyethylene-glycol-600-

diacrylate “PEG-600-DA”) was supplied by

Sartomer Europe, Colombes Cedex,

France. Benzenesulfonyl hydrazide 98%

(BSH), 4,4'-Oxydibenzenesulfonyl

hydrazide 95% (OBSH) and Methacryloyl

chloride 97% (MAC) were supplied by Alfa

Aesar and VWR. Triethylamine 99% purum

(TEA) was purchased from Riedel-de

Haën. Tetrahydrofuran ROTISOLV® HPLC

unstabilized was purchased from Carl Roth.

NaHCO3 p.a., MgSO4 99,5%, Acetyl

chloride p.a. (AC) and Lauroyl chloride p.a.

(LC) were purchased from Merck Millipore.

Benzoyl chloride 99% (BC) and

Cyclohexancarbonyl chloride 98% (CHC)

were obtained from Sigma Aldrich. All

chemicals were used without further

purification.

Methods and devices

Viscosity measurements were performed

with the SV10 Vibro Viscosimeter (A&D

Company Ltd.), using a vibration frequency

of 30 Hz.

Structure elucidation of the synthesized

BAs was conducted by 1H-NMR

spectroscopy (Bruker Avance III 300 MHz)

in deuterated DMSO.

The following UV-sources were used for UV

pinning and UV curing of the ink matrix: a

mercury halide lamp, MHL250 (USHIO)

with 250 W power output and a measured

power density of 0.45 W cm-2 at 15 mm

lamp-to-substrate distance; a UV-LED

assembled in house, consisting of 2 LEDs

with 395 nm peak wavelength and a power

density of 0.019 W cm-2 at 5 cm

lamp-to-substrate distance; and a Firejet

200 from Phoseon with 395 nm peak

wavelength and a maximum power density

of 16 W cm-2. The self-assembled UV-LED

has a housing and an inlet for purging with

inert gases. The intensity of the UV-sources

was measured by a UV-Micro Puck Multi

Integrator (UV Technik Meyer, Ortenberg,

Germany).

Fourier Transform Infrared (FTIR)

Spectroscopy (Bruker Tensor 37 with

MIRacle ATR bridge) was employed to

measure the photopolymerization extent

via the decrease in intensity of reactive

acrylic double bonds at 810 cm-1.

Thermogravimetric analysis (TGA) (Pyris

Series TGA4000) was used for determining

the thermal stability of the inks. DSC

measurements were carried out using a

DSC Q2000 (TA Instruments).



For foaming of the printed inks, an Adphos

NIR 50-50 lamp with 800 W power output

and a colour temperature of 3000 K was

assessed.

Jetting tests and drop mass and drop

velocity measurements were carried out

with the DIMATIX DMP2800 inkjet printer

using 10 pL cartridges. Further

inkjet-printing tests were carried out using a

Ricoh Gen4 MH2420 printhead, driven by

print driver boards from Ardeje, Valence,

France.

The Dektak 150 profilometer (Veeco

Instruments Inc.) was used for measuring

the thickness of printed layers. Cross

sections of printed layers were prepared by

embedding in epoxide (structural adhesive,

quick set epoxy from RS components) and

microtome cutting using a Leica Ultracut

UCT with glass knifes. The glass knifes

were prepared with Leica EM KMR2.

The surface and cross section of foams

were observed using a Keyence VHX5000

microscope.

RESULTS AND DISCUSSION

Modification of blowing agent

Sulfonylhydrazides are well established as

blowing agents in the plastics industry and

they are readily available from several

commercial producers. Both BSH and

OBSH have adequate gas numbers (BSH

115-130 cm³ g-1 and OBSH 125-

160 cm³ g-1) as well as a decomposition

temperature well below 200°C (BSH

between 130°C and 140°C and OBSH

between 155°C and 165°C) which is

desirable since the printed parts could be

damaged when exposed to excess heat

[22-24]. Unfortunately, when tested with the

developed ink formulation the mixtures start

to solidify within 48 hours. This can be

explained through an Aza-Michael

analogue reaction mechanism, where the

amine functionality attacks the acrylic

double bond of the inks monomers [25],

thus triggering the solidification of the ink.

FIGURE 1 shows the temperature

dependent viscosity of 5 wt.% BSH in

acrylate matrix. Starting at 75 °C, the

viscosity increases sharply, indicating a

reaction of the acrylic double bonds with the

amine groups of the BA.

FIGURE 1 Temperature-dependent viscosity of 5w% BSH

in acrylate matrix, showing a sharp increase of viscosity at

elevated temperatures, which indicates the addition of the

BA to the acrylic double bonds.

We successfully prevented this unwanted

reaction by blocking the amine groups of

OBSH through an amidation via a known

route which we altered accordingly

(experimental protocol available in the

supporting information) (FIGURE 2) [26,

27]. TABLE I lists the acid chlorides which

were used to synthesize the different

blowing agents, the yield of the reaction

and the solubility of the obtained product in



the ink matrix. The structures of the BAs

synthesized was confirmed by 1H-NMR

spectroscopy; the peak lists are available in

the supporting information. Although all of

the synthesized derivatives were stable in

the acrylate ink, some substituents resulted

in better solubility than others: long aliphatic

chains lead to a better solubility than short

chains, substituents with high electron

density (vinyl and aromatic) dissolve better

than saturated hydrocarbons. Analogously,

aromatic ring systems dissolve better than

cycloalkanes. According to literature MAC

may react via an Oxo-Alder reaction into a

dimeric MAC (DMAC) [28]. This explains

the different solubility when using different

batches from different suppliers, since the

first batch contained 15 % dimer. The best

result was achieved with benzoyl chloride,

which is completely soluble and has a

reaction yield of 96 %. FIGURE 3 shows

exemplary images of 2.5 wt.% of an

insoluble (DMAOBSH), a completely

soluble (DBOBSH) and a moderately

soluble (DCHOBSH) BA.

FIGURE 2 Reaction scheme for 1 OBSH. The products received are 2: DLOBSH - 4,4'-oxybis(N'-

dodecanoylbenzenesulfonohydrazide); 3: DMAOBSH - 4,4'-oxybis(N'-methacryloylbenzene-sulfonohydrazide); 4: CDMAOBSH -

N'-(6-chloro-2,5-dimethyl-3,4-dihydro-2H-pyran-2-carbonyl)-4-(4-((2-methacryloylhydrazineyl)sulfonyl)phenoxy)benzene-

sulfonohydrazide; 5: DAOBSH - 4,4'-oxybis(N'-acetylbenzenesulfonohydrazide), 6: DBOBSH - 4,4'-oxybis(N'-

benzoylbenzenesulfonohydrazide) and 7: DCHOBSH - 4,4'-oxybis(N'-(cyclohexanecarbonyl)benzenesulfonohydrazide).



TABLE I Different acid chlorides used in the synthesis of new blowing agents, the reaction yield and the solubility of these blowing

agents in the ink matrix: -- insoluble, - bad solubility, o moderate soluble, + completely soluble (Structures of the achieved BAs

are depicted in FIGURE 2; abbreviations for acyl chlorides are given in the experimental section).

Acyl chloride Product Number Product

Abbreviation

Reaction yield /

%

Solubility 2.5 wt.% in

matrix

MAC 3 DMAOBSH 92 - -

CDMAC/MAC 4 CDMAOBSH 92 +

BC 6 DBOBSH 96 +

CHC 7 DCHOBSH 93 o

LC 2 DLOBSH 86 -

AC 5 DAOBSH 80 - -

FIGURE 3 Examples of insoluble (a: DMAOBSH),

completely soluble (b: DBOBSH) and moderately soluble (c:

DCHOBSH) BAs. The mixtures of 2.5 wt.% BA in acrylate

matrix were stirred over night at room temperature. The

scale bar represents 5 mm.

Ink formulation

The foamable inks were produced by

dissolving the BA in the pre-mixed acrylate

matrix, which contains 1 wt.% of

Omnirad 819 (O-819) and 1 wt.% of

Omnirad TPO-L (O-TPO-L) as

photoinitiators and Genorad 23 as

stabilizer. The detailed composition of the

ink formulation is available in the supporting

information (TABLE S1). Together with

surface tension a suitable range of viscosity

of the ink is an important prerequisite for

jettability in inkjet-heads. FIGURE 4 shows

the influence of increasing amounts of

dissolved BA on the viscosity of the ink. The

viscosity is shown at 70 °C, which is the

typical jetting temperature of acrylic based

PolyJetTM inks by Stratasys Ltd.

FIGURE 4 Viscosity of acrylate matrix containing different

amounts of CDMAOBSH, DBOBSH and DCHOBSH at

70 °C.

Ink characterization

As in PolyJetTM-printing, each ink layer is

cured immediately after the deposition by

UV-light, the characterization of the

photoreactivity of the ink is an important

part during ink development. We used two

types of UV-sources in this study: Mercury

halide lamps (MHL) with a broad emission

spectrum, including all UV wavelengths and

UV-LEDs with a narrow emission spectrum.

Typically, in PolyJetTM-printing MHLs are



used as they can cure a wide range of

materials. However, LEDs have also

become popular lately for some obvious

advantages like the possibility of switching

them on and off frequently, environmental

friendliness and lifetime aspects, as well as

operation safety [29-31]. The

photoreactivity of the acrylate matrix was

evaluated via recording FTIR-spectra after

exposure to various UV doses. The curing

degree α was determined via the decrease

of reactive double bond areas according to

the following equation.


Adb,0∙Aref,t) ∙ 100 (5)

Adb,0 and Adb,t are, respectively, the band

areas of acrylate double bonds (810 cm-1)

before and after the curing time t.


reference band areas (carbonyl bands,

1725 cm-1) before and after the curing time

t [32]. In the supporting information

(FIGURE S1) exemplary FTIR spectra of

the acrylate matrix (a) and the decrease of

reactive double bond bands at 810 cm-1 (b)

and at 1410 cm-1 (c) after irradiation with

various UV doses are available.

FIGURE 5 shows the curing degree of the

acrylate matrix with and without addition of

2.5 wt% of BA with increasing dosage of

UV, the influence of doubling the amount of

photoinitiator (a) and the corresponding

reaction rates (b). It can be seen that the

addition of BA does not affect the

photoreactivity of the acrylate matrix.

Adding the doubled amount of photointiator

leads to an increased photoreactivity and a

higher polymerization speed resulting

therefrom. FIGURE 5 (c) and (d) depict the

curing degree and polymerization rates of

the acrylate matrix during irradiation with a

395 nm LED in ambient- and nitrogen

atmosphere. It is visible that in ambient

atmosphere, only a curing degree of

30-40 % is reached, while in nitrogen

atmosphere, the acrylate matrix is cured to

over 80 %, which is due to the reduced

oxygen inhibition. In ambient atmosphere,

the oxygen molecules readily bind to the

free radicals which have been formed by

the photolysis of the initiator, forming

peroxy radicals, which are not reacting with

the acrylate bonds but rather lead to a chain

termination by recombination with other

radicals [33]. The reactivity of the ink matrix

can be increased among others by

increasing the amount of photoinitiator or

through irradiation in an inert atmosphere to

avoid oxygen inhibition.

In contrast to standard acrylate-based

3D-printing inks, which are directly

solidified by UV light after deposition, the

foamable ink should be less reactive in

order to allow a foaming step when the ink

is not yet fully cured and still liquid. The final

curing step is performed via high power

UV-irradiation and thermal energy input via

NIR-irradiation.



FIGURE 5 Curing degree of the acrylate matrix (detailed composition available in the supporting information, TABLE S1) after

exposure to increasing amounts of UV doses through irradiation with a broad band mercury halide lamp (a) and the corresponding

reaction rates (b); curing degree of the acrylate matrix after various exposures to UV doses by a 395 nm LED, in ambient- and

nitrogen atmospheres (c) and the corresponding reaction rates (d). The symbols represent the values obtained from experimental

FTIR measurements, the solid lines show the corresponding exponential fits and the dashed lines show the connection between

experimental values for better readability.

Thermal stability of the foamable ink

As a thermal step is necessary to

decompose the blowing agent in the ink,

the thermal stability plays a significant role

in the foaming process. Adding increasing

amounts of BA into the acrylate matrix

leads to a stabilization of the ink

components, as can be seen in the TGA

and differential thermogravimetry (DTG)

curves depicted in FIGURE 6. Through

intermolecular forces, such as Van-der-

Waals interactions and hydrogen bridge

linkages between the BA-molecules and

acrylic ink components, the overall solution

is more stable, and thus more difficult to

evaporate [34]. It is desired to keep the

amount of monomer evaporation as low as

possible, which can be reached by a high

amount of BA and fast heating rates to

trigger the decomposition of BA before a lot

of low molecular monomers have

evaporated. The high volatility of the ink

matrix suggests that there might be a

foaming effect resulting from the

evaporation of monomers. However, it was

observed in initial bulk foaming



experiments with a larger amount of

material (FIGURE 7), that ink evaporation

takes place without the formation of

bubbles inside the material and only the

decomposition of BA at higher

temperatures leads to a vigorous foaming

reaction followed by curing of the ink matrix.

Due to the low amount of BA (0.5-5 wt.%)

compared to the acrylate matrix, the TGA

curves only show the evaporation of

acrylate components and not the thermal

decomposition of BA. The characterization

of the BA decomposition reaction inside the

ink matrix was carried out using DSC and

will be described in the following section.

FIGURE 6 TGA (a) and DTG (b) curve of the acrylate matrix (detailed composition available in the supporting information,

TABLE S1) with various amounts of BA, showing the evaporation of low molecular monomers starting from about 150 °C, followed

by decomposition of the polymer at 350 °C. Increasing amounts of BA stabilize the ink. The TGA curves were recorded with a

scan rate of 10 °C min-1.

Characterization of the foaming

reaction

The BA inside the ink decomposes as a

result of thermal energy input, creating a

large amount of gases. We use this

principle to create a stable acrylic based

foam. FIGURE 7 shows the same volumes

of foamable ink, containing 2.5 wt.% of BA,

after UV curing (a) and after thermal energy

input via heat gun, which lead to a

decomposition of the BA and thermal curing

of the acrylate matrix (b). Through the

thermal foaming step, a stable foam is

formed because the decomposition of BA

and curing of the matrix takes place at

similar temperatures. A significant height

expansion was observed after the foaming

step. The densities were estimated using

the mass-volume direct calculation. The

density decreased from 0.63 to 0.27, thus

the resulting polymer foam is defined as a

moderate-foam or middle-density foam

[10]. In order to design a printing and

foaming procedure, it is important to know

the decomposition temperature of the BA



and the onset for thermal curing of the

acrylate ink matrix.

FIGURE 7 150 µl of foamable ink containing 2.5 wt.%

CDMAOBSH, only UV-cured (a) and after thermal energy

input via heat gun, which lead to a decomposition of the BA

and thermal curing of the acrylate matrix, accompanied by

a significant height expansion. (b). The scale bar represents

5 mm.

As mentioned above, the ink should be

pinned by a low amount of UV light,

followed by rapid heating via a NIR source

and a final UV curing step to stabilize the

foam. The pinning step uses a low dose of

UV light to start the polymerization reaction

to increase the viscosity without fully curing

the ink. This allows increased dimensional

stability during the subsequent foaming

step and helps to keep the gas bubbles

inside the ink matrix, while the ink is still

liquid and not fully cured. The ink should be

stable at a printing temperature of 70 °C

and should not be cured before the BA

decomposes. FIGURE 8 depicts the DSC

scans of the pure BAs (a) and the acrylate

matrix without and with addition of 2.5 wt.%

of BA (b). DMAOBSH shows a two-step

elimination of gaseous compounds visible

through exothermic peaks with onsets at

100 °C and 200 °C. In contrast, the two BAs

with cyclic side groups (DBOBSH and

DCHOBSH) melt shortly before the

decomposition and elimination of gaseous

compounds. The onset for the thermal

decomposition of DBOBSH is at 210 °C

and of DCHOBSH at 250 °C, represented

by a sharp exothermic peak. These BAs are

promising candidates for inkjet-inks, as

they feature a very defined and fast

decomposition reaction, enabling a fast

foaming step during the printing process.

The DSC scan of the pure acrylate ink

matrix shows an endothermic signal

starting from about 150 °C, which

corresponds to the evaporation of low

molecular weight monomers in the ink

formulation. However, the incorporation of

BAs into the ink matrix leads to a

stabilization of the acrylate monomers and

thermal evaporation is reduced. This

observation is also supported by the TGA

measurements, which were presented

above (FIGURE 6). The pure ink matrix

shows an exothermic peak representing the

curing reaction of its monomers and

oligomers starting from 210 °C. The ink

containing DMAOBSH, which is insoluble in

the acrylic ink matrix, decomposes starting

from 100 °C, and also features a thermal

curing reaction of the matrix. However, this

curing reaction is not as pronounced as in

the pure acrylate, as the matrix has already

been cured to a significant extent in parallel

to the exothermic BA decomposition. The

other formulations, containing DBOBSH

and DCHOBSH, which are both soluble in

the acrylate matrix, have a higher foaming

onset at about 190 °C, respectively, which



is accompanied by and immediately

followed by thermal curing of the matrix.

Jetting tests

FIGURE 9 shows the drop formation of the

foamable ink containing 2.5 wt%

CDMAOBSH at 16 V piezo voltage and

70 °C cartridge temperature. The drop is

displayed 10 – 70 µs after being ejected

through the nozzle. The ink shows a good

drop formation: the ligament combines with

the drop without creating satellite drops and

finally, a single drop is formed.

FIGURE 8 DSC scan of the acrylate matrix without and with addition of exemplary BAs. DMAOBSH is insoluble in the acrylate

matrix, whereas DBOBSH and DCHOBSH are soluble. The thermal curing reaction of the ink matrix starts at 210 °C, while the

decomposition of the modified BAs DBOBSH and DCHOBSH starts at 190 °C. The insoluble BA DMAOBSH decomposes

accompanied by monomer evaporation starting at 100 °C. The DSC data were subjected to baseline correction and are normalized

to the sample weight.

FIGURE 9 Drop formation of foamable ink containing

2.5 wt% CDMAOBSH at 16 V piezo voltage and 70°C

cartridge temperature.

In FIGURE 10, the drop mass and

velocities of the foamable ink containting

2.5 wt% of CDMAOBSH (a) and DBOBSH

(b) is shown. The printer manufacturer

recommends to set a drop velocity of

7-9 m s-1 [35]. Drop velocities in this range

are obtained at piezo voltages of 17-19 V,

using the DIMATIX Model Fluid 1

Waveform. The drop masses range from

6.7 ng to 7.7 ng (CDMAOBSH) and from

6.1 ng to 7.1 ng (DBOBSH) at piezo

voltages of 17 – 19 V.

FIGURE 10 Drop mass and drop velocity vs. piezo voltage

at 70°C; 2.5 wt.% CDMAOBSH and 2.5 wt.% DBOBSH in

acrylate matrix. The drop mass was determined with the

DIMATIX DMP2800 printer by jetting a defined number of

drops (usually about 1.000.000) into a pan and dividing the

measured weight by the number of drops according to the

DIMATIX DMP2800 software [35].



Foaming of inkjet-printed layers

In order to produce foams directly in the

printing machine, the following approach

was used:

(i) inkjet- printing and low dose UV pinning

of the foamable ink

(ii) foaming via thermal decomposition of

the BA with high power NIR-irradiation

(iii) final high dose UV curing of the

foamable ink

The steps (ii) and (iii) need to be coupled

very closely in time to ensure a stabilization

of the bubbles inside the acrylate matrix

and thus produce a dimensionally stable

foam. These steps of foaming and curing

partly overlap, as through the high input of

thermal energy a thermal curing process is

triggered.

The process of printing foams via

PolyJetTM-printing was simulated using a

printing and curing stage, assembled in

house, consisting of a Ricoh Gen 4

MH2420 printhead, a Phoseon FireEdge

UV pinning unit (395 nm), an Adphos NIR

50-50 lamp, and a Phoseon FireJet 200

(395 nm). The substrate is transported to

the different stages of printing, foaming and

curing across a linear axis with defined

speed. Via the defined speed and the

jetting frequency, the printing resolution is

adapted. Details about the printing and

curing stage can be found in the supporting

information.

FIGURE 11 shows 5x5 mm test structures

printed and foamed inside the curing stage

using 2.5 wt.% DBOBSH in acrylate matrix

as foamable ink. As an unfoamed

reference, 10 layers of foamable ink were

printed with 1693 dpi and subsequently

UV-pinned and UV cured (a). For

production of foamed samples, 10 layers

were UV-pinned, NIR-foamed (20 s, 30 %

power) and UV-cured (b, c). FIGURE 11 d

shows 10 layers of foamable ink, printed

with 3386 dpi, which was only UV-pinned

and UV-cured. FIGURE 11 e-f shows the

analogous sample with a NIR-foaming step

of 5 s at 100 % power. It is visible that the

samples, which were only subjected to

UV-irradiation, have no visible foam

structure (a, d), whereas on samples which

were subjected to NIR-irradiation in

between pinning and curing, a foam

structure can be seen (b, e). A close up of

the foamed surface shows an open cell

foam surface (c, f).



FIGURE 11 5x5 mm squares of foamable ink (2.5 wt.% DBOBSH in acrylate matrix) printed with a Ricoh Gen4 MH2420 inside

the curing stage: 1693 dpi, 10 layers, only UV pinned and UV cured (a); 1693 dpi, 10 layers, UV pinned, foamed with NIR (20 s,

30 % power), and UV cured (b); close-up of foamed sample (c); 3386 dpi, 10 layers, only UV pinned and UV cured; (d) 3386 dpi,

10 layers, UV pinned, foamed with NIR (5 s, 100 % power), and UV cured (e) and close-up of foamed sample (f).

The height expansion of the printed

unfoamed and foamed films was

determined by measuring the profiles of the

films. FIGURE 12 shows the thicknesses of

selected unfoamed and foamed printed

films of foamable ink. It is evident, that only

thick layers can reach a height expansion

by NIR-induced foaming. In the case of thin

films, there is not enough bulk material to

keep the bubbles, which are formed by

gaseous decomposition products of the BA,

inside and thus the gases escape readily

through the surface. However, when the

printed films had thicknesses starting from

30-40 µm we could observe a height

difference between unfoamed and foamed

layers.

FIGURE 12 Thicknesses of unfoamed and foamed printed

samples of the foamable ink, measured by profilometry. The

average thicknesses were calculated out of 9

measurements across the layers. The symbols represent

measurement data, while the solid lines show the

corresponding linear fits.

The sample with the biggest height

expansion of 44 % was chosen for cross-

section analysis. FIGURE 13 shows the



cross section of the unfoamed (a) and

foamed (b) layer. The expanded sample

features a foam structure, while the

unfoamed sample does not have visible

pores. The height expansions and

estimated densities of the printed and

foamed films are summarized in TABLE II.

The height of the unfoamed layer hunfoamed

corresponds to the height of samples which

were only subjected to UV irradiation

without a foaming step. The height of the

foamed layer hfoamed shows the height after

expansion through NIR and curing through

UV irradiation.

FIGURE 13 Cross-section of the printed, foamed and cured foamable ink, produced by microtome cutting: a) 1693 dpi, 10 layers,

only UV pinned and UV cured; b) 1693 dpi, 10 layers, UV pinned, NIR foamed (20 s, 30 % power), and UV cured.

The percentage of expansion hexpansion

describes to which extent the layer

expanded through the foaming step with

reference to the unfoamed layer. The

density of unfoamed samples was

determined by the mass-volume direct

calculation of a bulk sample. The height of

the samples was measured by a caliper in

case of the bulk sample (FIGURE 7) and by

profilometry in case of the printed samples

(FIGURE 11).

TABLE II Height expansion and estimated densities of selected unfoamed and foamed samples. Pictures of the samples are

shown in FIGURE 7 (bulk sample) and FIGURE 11 (printed samples).

Sample

hunfoamed

/ µm

hfoamed

/ µm

hexpansion

/ %

est.,unfoamed

/ g cm-3

est.,foamed

/ g cm-3

Bulk 2000 ± 100 5000 ± 750 150 ± 50 0.629 ± 0.031 0.272 ± 0.041

10 layers, printed, 1693 dpi, 20s NIR,

30% power

45.5 ± 5.8 65.3 ± 2.8 44 ± 25 0.629 ± 0.031 0.438 ± 0.075

10 layers, printed, 3386 dpi, 5s NIR,

100% power

131.4 ± 10.2 154.1 ± 9.6 17 ± 16 0.629 ± 0.031 0.536 ± 0.075



CONCLUSIONS

We modified commercially available

blowing agents to make them compatible

with acrylic matrix materials for inkjet-based

3D printing. By substitution of the

nucleophilic amine groups, the modified BA

is stable in acrylates, as a reaction with the

acrylic double bonds is prevented. OBSH

substituted with benzoyl chloride

(DBOBSH) showed a good solubility and

foaming capability inside the acrylate

matrix. The ink, which was used for foaming

and printing tests consists of 2.5 wt% of BA

inside the acrylic matrix, including

photoinitiators and a stabilizer.

The thermal stability of the foamable ink

was determined by TGA measurements

and we found out, that increasing amounts

of BA dissolved in the ink matrix prevent the

evaporation of low molecular weight

monomers in the ink formulation.

The foam resulting from bulk foaming via

heat input had a density of about

0.27 g cm-3, as estimated by a

mass-volume direct calculation and is

characterized as moderate-foaming or

medium-density foam.

In order to produce foams via inkjet-based

3D printing, a more complex approach for

foaming is required, as the ink must be

dimensionally stable in the printing plane

and the resulting pores need to be

stabilized immediately after the foaming

process to create a stable foam. We used a

combination of low dose UV-pinning to fix

the ink, high power NIR-induced foaming

and subsequent full UV-curing via high

power UV-irradiation inside a

self-assembled printing and curing stage.

The principle of PolyJetTM-printing offers

another approach to stabilize the foamable

ink during the foaming process by the

design of a multi-material part, or the usage

of support material in a single material part.

The parts around the foamable ink, either

the support material, or the other UV

curable inks can act as a border to prevent

spreading during the foaming process. The

classical UV curable inks have higher

photoreactivity than the foamable ink and

thus are cured in the first printing pass,

while the foamable ink inside is still liquid

and can be foamed via one or more

additional passes with NIR irradiation

without printing. When using such an

approach, it has to be taken into account

that the parts surrounding the foamable ink

need greater layer heights to compensate

the height increase during foaming.

The printed foams have a densities of 0.438

± 0.075 g cm-3 and 0.536 ± 0.075 g cm-3,

depending on the foaming parameters.

The cross-section of a selected printed and

foamed sample with the highest density

reduction showed a porous structure in

contrast to the unfoamed sample, which

was not porous.

We showed a proof of concept for printing

of foams via inkjet-based 3D printing, which

is very promising for producing three-

dimensional light weight parts.

Nonetheless, a further modification of BAs

to allow even faster foaming and a further

process development regarding pinning,

foaming and curing parameters is still



necessary to push this technology further to

finally produce multi-material 3D parts

including foams inside a PolyJetTM-printer.

ACKNOWLEDGEMENTS

This work was performed within the DIMAP

project, which received funding from the

European Union’s Horizon 2020 Program

for research and innovation under Grant

Agreement No. 685937. Part of this work

was funded within the Strategic Economic

and Research Program “Innovative Upper

Austria 2020” by the Government of Upper

Austria.

We thank Sabine Hild, Head of the Institute

of Polymer Science (IPS) at Johannes

Kepler University Linz, for granting access

to her Leica Ultracut device. We are also

grateful to Oliver Brüggemann, Head of the

Institute of Polymer Chemistry (ICP) at

Johannes Kepler University Linz, who gave

us the possibility to use his DSC

equipment.

Last but not least, we thank Hans Martin

Leichtfried for carrying out the viscosity

measurements, Marco Röcklinger for

helping with jetting tests and FTIR

measurements, Christina Staudinger and

Victoria Rudelstorfer for helping with taking

microscope pictures.

DATA AVAILABILITY

The raw and processed data required to

reproduce these findings are available to

download from

http://dx.doi.org/10.17632/t35htyvf8z.1

(Wagner, Annika (2019), “Data for:

Foamable acrylic based ink for the

production of light weight parts by inkjet-

based 3D printing”, Mendeley Data, v1).

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2.3.1. Supporting Information for: Foamable acrylic based ink for the production of light weight parts by inkjet based 3D printing

Annika Wagner ,1 Andreas M. Kreuzer,2 Lukas Göpperl2, Leo Schranzhofer,1 Christian

Paulik2





FOAMABLE INK FORMULATION

The foamable ink consists of 2.5 wt.% of

BA in a mixture of acrylates with

photoinitiators and a stabilizer, described in

Table S1.

SYNTHESIS OF MODIFIED BLOWING

AGENTS

The acyl chlorides used for the

derivatization of OBSH and the reaction

schemes are summarized in the main

paper (TABLE I, FIGURE 2).

Synthesis of 4,4'-oxybis(N'-

dodecanoylbenzenesulfonohydrazide)

(DLOBSH, 2):

To a vigorously stirred solution of OBSH

(12.5 g, 34.9 mmol), TEA (0.5 ml,

3.5 mmol) in THF (70 ml), LC (17.5 ml,

74.0 mmol) was added dropwise. During

the addition, the temperature rose from

20°C to 30°C and the reaction solution

became milky white. After the addition was

complete, NaHCO3 (8.4 g, 100 mmol) in

30 ml of H2O was slowly added under the

release of CO2. During the addition, a

phase separation occurred without any

color change. The organic layer was

separated and dried over MgSO4.

Table S1 Components of the acrylate matrix for the foamable ink.

Material Fraction / wt.%

Photomer 3016, IGM Resins 15

SR610, Sartomer 20

Trimethylolpropane ethoxylate triacrylate, Sigma Aldrich 20

Tripropylene gycol diacrylate, mixture of isomers, Sigma

Aldrich

45

Omnirad TPO-L, IGM Resins 1 (added)

Omnirad 819, IGM Resins 1 (added)

Genorad 23, Rahn 0.5 (added)

mailto:[email protected]



After evaporation of the solvent, the product

solidified as white, crystalline powder with a

yield of 86 %.

1H-NMR (DMSO-d6, 300 MHz): δ 0.84 (t,

6H, J=6.31), 1.21 (m, 36H), 1.94 (t, 4H,

J=7.13 Hz), 7.16 (d, 4H, J=8.53 Hz), 7.82

(d, 4H, J=8.49 Hz), 9.77 (d, 2H, J=2.96Hz),

9.99 (d, 2H, 2.59Hz)

Synthesis of 4,4'-

oxybis(N'-methacryloylbenzene-

sulfonohydrazide) (DMAOBSH, 3):


(12.5 g, 34.9 mmol), TEA (0.5 ml,

3.5 mmol) in THF (70 ml), MAC (8 ml,




became transparent. After the addition was




phase separation occurred and the upper

organic layer turned light green, while the

water phase turned light pink. The organic

layer was separated and dried over MgSO4.



yield of 92 %.

1H-NMR (DMSO-d6, 300 MHz): δ 1.73 (s,

6H), 5.38 (s, 2H), 5.60 (s, 2H), 7.18 (d, 4H,

J= 8.55 Hz), 7.82 (d, 4H, J=8.52 Hz), 9.84

(m, 2H), 10.16 (m, 2H)

Synthesis of N'-(6-chloro-2,5-dimethyl-3,4-

dihydro-2H-pyran-2-carbonyl)-4-(4-((2-

metha -cryloyl-

hydrazineyl)sulfonyl)phenoxy)benzenesulf

onohydrazide (CDMAOBSH, 4):


(12.5 g, 35 mmol), TEA (0.25 ml,

1.75 mmol) in THF (70 ml), MAC (8 ml, 74,5

mmol) was added dropwise. During the

addition, the temperature rose from 20°C to

40°C and the reaction solution became

clear. After the addition was complete,

NaHCO3 (5.8 g, 70 mmol) in 20 ml of H2O

was slowly added under the release of CO2.

During the addition, a phase separation

occurred and the upper organic layer

turned light green, while the water phase

turned light pink. The organic layer was

separated and dried over MgSO4. After

evaporation of the solvent the product


yield of 90 %.

1H-NMR (DMSO-d6, 300 MHz): not

available (mixture of product 3 and 4 due to

up to 15 % dimer content in the MAC batch

used)


acetylbenzenesulfonohydrazide)

(DAOBSH, 5):


(12.5 g, 34.9 mmol), TEA (0.5 ml,

3.5 mmol) in THF (70 ml), AC (6.5 ml, 91.1

mmol) was added dropwise. During the

addition, the temperature rose from 20°C to

38°C and the reaction solution became



transparent. After the addition was




phase separation occurred without any

color change. The organic layer was

separated and dried over MgSO4. After

evaporation of the solvent, the product


yield of 80 %.

1H-NMR (DMSO-d6, 300 MHz): δ 1.17 (m,

6H), 7.21 (d, 4H, J= 8.50Hz), 7.84 (d, 4H,

J=8.56Hz), 9.77 (s, 2H), 10.0 (s, 2H)


benzoylbenzenesulfonohydrazide)

(DBOBSH, 6):


(12.5 g, 34.9 mmol), TEA (0.5 ml,

3.5 mmol) in THF (70 ml), BC (8.5 ml,









organic layer turned light yellow, while the





yield of 96 %.

1H-NMR (DMSO-d6, 300 MHz): δ 7.14 (d,

4H, J=8.52Hz), 7.56 (m, 10H), 7.85 (d, 4H,

J=8.50 Hz), 10.00 (d, 2H, J=3.04 Hz),

10.70 (d, 2H, J=2.68 Hz)


(cyclohexanecarbonyl)benzenesulfonohyd

razide) (DCHOBSH, 7):


(12.5 g, 34.9 mmol), TEA (0.5 ml,

3.5 mmol) in THF (70 ml), CHC (12 ml,









organic layer turned light green, while the





yield of 93 %.

1H-NMR (DMSO-d6, 300 MHz): δ 1.12 (m,

9H), 1.56 (m, 11H), 2.04 (m, 2H) 7.17 (d,

4H, J=8.60Hz), 7.80 (d, 4H, J=8.59Hz),

9.72 (s, 2H), 10.00 (s, 2H)



DETERMINATION OF THE CURING

DEGREE VIA FTIR

The curing degree of the acrylic ink matrix

was determined via the decrease in

reactive double bond band areas in the

FTIR spectra. FIGURE S1 shows a FTIR

spectrum of the uncured ink matrix (a) and

the acrylate double bond bands decreasing

in intensity because of exposure to

increasing UV doses (b, c).

PRINTING AND CURING STAGE

Printing and foaming of the foamable ink

was performed inside a printing and curing

stage, consisting of a Ricoh Gen 4

printhead and a collection of UV pinning,

NIR drying and UV curing and sintering

devices, as depicted in FIGURE S2. Each

module is adjustable in height and is

exchangeable through alternative devices

The printing process inside a PolyJetTM-3D

printer can be simulated by printing on a

substrate, which is moved by a conveyor at

a controlled speed, followed by immediate

UV-pinning, NIR-foaming and final high

power UV curing.

FIGURE S1 FTIR spectrum of the acrylate mixture used as matrix for the foamable ink with reactive double bond bands at 810 cm-1

and 1406 cm-1 (a) and decrease of the double bond bands at 810 cm-1 (b) and at 1406 cm-1 (c) during irradiation with a UV-LED

with 395 nm peak wavelength in N2 atmosphere.



The stage can be operated either in static

mode, where the substrate stops for a

defined time beneath the lamps, or in

dynamic mode, where the substrate passes

the lamps with a defined speed. The

printing resolution is tuned via the printing

speed and the jetting frequency of the

printhead.

Printing (Ricoh Gen4 MH2420)

Inkjet-printing of the foamable ink was

performed using a Ricoh Gen4 MH2420

printhead driven by print driver boards from

Ardeje. A printing speed of 100 mm s-1 was

set by moving the glass substrate beneath

the printhead.

UV pinning (Phoseon FireEdge FE400)

Directly after printing, the sample was

subjected to a low dose UV irradiation at

395 nm (100 ms, 10 % power,

lamp-to-substrate distance: 3 mm) to fix the

printed layer by increasing the viscosity.

Figure S2 Components of the printing and curing stage, assembled at Profactor GmbH.

NIR-induced foaming (Adphos NIR 50-

50)

The decomposition of the blowing agent is

induced via NIR-irradiation, leading to a

foaming of the ink matrix. The temperature

of the NIR 50-50 lamp was measured with

a type K thermocouple at various

lamp-to-substrate distances. FIGURE S3

shows the measured temperatures during

30 s irradiation with various lamp powers.

This device enables very fast heating up to

high temperatures, allowing a fast foaming

process.

UV Curing (Phoseon FireJet FJ200)

The final curing is achieved by a

combination of heat from the foaming step

and high-power UV-irradiation at 395 nm

(5 s, 50 % power, lamp-to-substrate

distance: 5 mm).



FIGURE S3 Temperatures of the Adphos NIR 50-50 lamp, measured with a type-K thermocouple at lamp-to-substrate distances

from 3-15 mm during 30 s irradiation at powers from 10 %, 50 % and 100 %.

Summary & Conclusions


3. Summary and Conclusions


In PolyJetTM-3D printing, a fast curing reaction upon UV light at moderate temperatures is

desired. Thus, for fabricating polyimides via this process, BMI-oligomers were chosen as

precursors. BMIs are UV curable without PI, but the curing speed can be increased by

adding PIs. The thermal- and photo-curing kinetics of selected BMI-oligomers was

characterized using FTIR spectroscopy. The oligomers tested varied in MW and chemical

structure. For aromatic BMIs, low MW oligomers showed slower curing reactions

compared to higher MW oligomers. The aliphatic BMI-689 showed a curing speed in

between the liquid and powder aromatic BMIs. For thermal curing, kinetic rate constants

in the range of minutes were obtained, while the UV curing process is much faster with

kinetic rate constants in the range of seconds. Different UV-sources including mercury

halide lamps with a broad spectrum and high-power UV-LEDs with various wavelengths

were tested for curing the BMI oligomers. The curing speed of the BMIs were strongly

affected by the wavelength of the UV source used. The UV-spectra of the BMIs showed

absorption maxima at 230-240 nm and the shorter the wavelength of the UV-irradiation,

the higher was the rate of photopolymerization.

The activation energy for thermal curing of BMIs was determined via the classical

Arrhenius method using the kinetic rate constants for thermal curing at three different

temperatures and via DSC measurements using the Kissinger and Ozawa plots.

The thermal properties of the cured polyimides were determined by TGA and DMTA. The

polymers showed excellent thermal stability, increasing with MW and content of aromatic

moieties in the BMI resins. The decomposition temperatures of the cured BMI resins were

between 422 °C (aliphatic extended BMI-689) and 450 °C (aromatic, imide extended

BMI-5000). Addition of aromatic BMIs to aliphatic ones improves the thermal stability and

thermomechanical properties. The DMTA curves of cured BMI-689 and a mixture of

BMI-689 with BMI-1500 showed a rubbery plateau of the storage modulus at elevated

temperatures until mechanical deterioration began at 325 °C (BMI-689 polymer) and

350 °C (BMI-689/BMI-1500 copolymer).

Due to the highly crosslinked nature of the thermosetting polyimides they have a very good

chemical resistance tested in THF, Trichlorobenzene, and NMP. Because of its low

viscosity and suitable UV-curing speed BMI-689 together with a low amount of eco-friendly

solvent was found to be a promising candidate for a PolyJetTM ink formulation.



3.2. Photoinitiator-free bismaleimide acrylate based inks for inkjet printing

As BMIs are known to serve both as polymerizable monomers and PIs for co-monomers

such as acrylates or vinyl ethers, the aliphatic, low viscous BMI-689 oligomer was used

with several acrylate monomers and oligomers to formulate PI-free inks for inkjet printing.

The BMI-acrylate mixtures were tested regarding viscosity to ensure printability.

The photocuring-kinetics of the BMI-acrylate mixtures were characterized using

FTIR-spectroscopy. The results were compared with the photo-reactivity of pure BMI-689

and pure acrylates with PI. It was shown that the BMI serves as a PI for the acrylate

monomers with moderate polymerization speeds. Furthermore, BMI decreased the oxygen

inhibition of the acrylate significantly, leading to a higher curing speed and conversion.

The resulting BMI-acrylate copolymers showed high thermal stability and good

thermomechanical properties. The thermal decomposition temperature of the copolymers

was up to 470 °C, which is in between the pure polyimides (500 °C) and the pure

polyacrylates (400 °C). The thermomechanical properties, as determined by dynamic

mechanical thermal analysis showed that the specimens deteriorate mechanically at lower

temperatures, compared to the decomposition temperature determined by TGA

measurements. All polymers showed a rubbery plateau starting from about 100 °C,

followed by mechanical deterioration at higher temperatures. The BMI-acrylate

copolymers showed thermomechanical stability up to a temperature of 350 °C.

The influence of structure and molecular weight of the acrylate monomers and oligomers

on the inkjet-printability and thermal properties of the resulting copolymers was

investigated. Due to their low viscosity, low MW monomers, such as TPGDA are beneficial

for the inkjet-printability, while higher MW precursors like PEG-600-DA and TMPETA do

not help to decrease the viscosity of the formulation. Compared to the difunctional

acrylates, TMPETA contains three acrylate groups, which lead to higher crosslinking

degree in the polymer. The higher degree of crosslinking, in turn results in higher thermal

stability. Long chains in between the acrylate groups of the precursors help to improve the

mechanical properties of the polymers through generating a way for energy dissipation.

The BMI-acrylate inks showed suitable viscosities of 12-25 mPas at 60-70 °C. The

applicability of the PI-free ink for inkjet printing was shown by printing test patterns and

combining electrically conductive ink lines with the PI-free ink as insulating layer. Because

of its high crosslinking degree, the BMI-acrylate copolymer withstands the high

temperatures necessary for sintering the conductive ink.



3.3. Foamable acrylate based inks for PolyJetTM-3D-printing

A foamable ink consisting of acrylate matrix with 2.5 wt.% of modified BA and other

additives was formulated. The amine functionalities of 4,4-oxybenzenesulfonylhydrazide

(OBSH) were blocked through the reaction with acrylic acid chlorides to prevent a reaction

with the acrylates during storage. The synthesis of various modified BAs was carried out

within other theses (A. M. Kreuzer4, L. Göpperl5).

The ink was characterized regarding its viscosity, thermal stability, photoreactivity, inkjet-

printability, decomposition of the BA, foamability and expansion efficiency.

A strategy for printing of foams was developed, consisting of low-dose UV pinning of the

ink, followed by NIR-irradiation to induce the foaming process and final high-dose UV

curing. The UV-pinning step increases the viscosity of the ink by initial polymerization,

which helps to stabilize the gaseous cells formed in the NIR-foaming step.

TGA measurements were carried out to determine the thermal stability of the foamable

ink. Evaporation of low MW acrylate monomers, especially TPGDA was observed during

the heating process, but decreased with increasing amount of incorporated BA through

the intermolecular forces which stabilized the ink. For a PolyJetTM printing process, it is

desired to have a fast foaming step to achieve reasonable printing speeds, which is also

beneficial for keeping the amount of monomer evaporation low.

DSC measurements showed the processes of monomer evaporation, BA decomposition

and thermal curing of the ink matrix. It is important that the BA decomposition happens

before thermal curing of the ink matrix to enable a foaming of the still liquid ink.

First bulk foaming experiments were performed in small cylindrical containers using a heat

gun for triggering the foaming process resulting in an expansion of 150 % and the density

decreased from 0.63 g cm-3 to 0.27 g cm-3, thus it is classified as a medium-density foam.

Printed and foamed samples were characterized using profilometry and microscopy of top-

and cross-sections views. Compared to layers, which were only UV cured, an expansion

of 44 % could be reached. In contrast to bulk foaming, the foaming of printed layers is

much more challenging, as the layers are thin and are not confined, so that the stabilization

of the foam is very difficult. The gaseous bubbles can easily escape through the surface

of the film and the viscosity decrease through the temperature rise during NIR-irradiation

can lead to spreading of the film. This problem can be tackled by adapting the design for

a PolyJetTM process: the printing of support material or other material close to the foamable

ink helps to confine the ink through borders and to facilitate the foam stabilization.

Documents

Institute of Chemical Technology of Organic Materials