Functionalizing stereolithography resins: effects of dispersed multi‐walled carbon nanotubes on physical properties

Functionalizing stereolithography resins:effects of dispersed multi-walled carbon

nanotubes on physical propertiesJ. Hector Sandoval and Ryan B. Wicker

W.M. Keck Border Biomedical Manufacturing and Engineering Laboratory, University of Texas at El Paso, El Paso, Texas, USA

AbstractPurpose – The present research investigates tailoring the physical properties of stereolithography (SL) epoxy-based resins by dispersing controlledsmall amounts of multi-walled carbon nanotubes (MWCNTs) directly in SL resins prior to layered manufacturing.Design/methodology/approach – A modified 3D Systems 250/50 SL multi-material machine was used where the machine was equipped with asolid-state (355 nm) laser, unique , 500 ml vat, overfill drain vat design that continuously flowed resin into the vat via a peristaltic pump, and 8.89 by8.89 cm2 platform. The vat did not include a recoating system. Pumping the composite resin assisted in maintaining the MWCNTs dispersed over longperiods of time (with MWCNT settling times on the order of one week). The research approach required developing a method for dispersing theMWCNTs in SL resin, determining new SL build parameters for the modified resin and SL machine, and building and testing tensile specimens.Findings – Mechanical mixing and ultrasonic dispersion provided simple means for dispersing MWCNTs in the SL resin. However, MWCNTagglomerates were observed in all the parts fabricated using the filled resins. Each concentration of MWCNTs resulted in a “new” resin requiringmodifications to the SL build parameters, EC and DP. Once characterized, the modified resins performed similar to traditional resins in the SL process.Small dispersions of MWCNTs resulted in improvements in the tensile strength (TS) (or ultimate tensile stress) and fracture stress (FS) of tensilespecimens as 0.025 percent (w/v) MWCNTs in DSM Somosw WaterShede 11120 resin resulted in increases in TS and FS of 5.7 percent and 26 percent,respectively, when compared to unfilled resin. Increasing the concentration of MWCNTs to 0.10 percent (w/v) resulted in increases in TS and FS of7.5 percent and 33 percent, respectively, over the unfilled resin. Transmission and scanning electron microscopy showed strong affinity between theepoxy resin and the MWCNTs.Research limitations/implications – Additional MWCNT type and concentrations in various SL resins should be investigated along with additionalmeans for dispersion to provide sufficient information on developing new SL resins for unique functional applications.Practical implications – It is anticipated that the methods described here will provide a basis for further development of advanced nanocomposite SLresins for end-use applications.Originality/value – This research successfully illustrated the dispersion and use of MWCNTs as a reinforcement material in a commercially availableSL resin.

Keywords Rapid prototypes, Composite materials, Resins

Paper type Research paper

1. Introduction

Recent advancements in the synthesis, processing and

understanding of nanostructured materials have generated

considerable interest in the materials research community

(Sandoval et al., 2005). Since, the discovery of carbon

nanotubes (CNTs), researchers have been investigating

methods for exploiting their extraordinary properties.

Previous studies have shown that a polymer matrix (such as

epoxy resins) seeded with CNTs could potentially exhibit

novel properties.A number of researchers have studied CNT-reinforced epoxy

systems and have shown increases in tensile strength (TS),

The current issue and full text archive of this journal is available at

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Rapid Prototyping Journal

12/5 (2006) 292–303

q Emerald Group Publishing Limited [ISSN 1355-2546]

[DOI 10.1108/13552540610707059]

The research presented here was performed at UTEP in the W.M. Keck BorderBiomedical Manufacturing and Engineering Laboratory (W.M. Keck BBMEL)using equipment purchased through Grant Number 11804 from the W.M. KeckFoundation. This material is based in part upon work supported by the TexasAdvanced Research (Advanced Technology/Technology Development andTransfer) Program under Grant Number 003661-0020-2003. The first author(JHS) was supported by the National Science Foundation, Louis Stokes Alliancefor Minority Participation (LSAMP) Bridge to Doctorate Fellowship. Supportfor UTEP was also provided through a research contract (Number 28643) fromSandia National Laboratories in the Laboratory Directed Research andDevelopment (LDRD) program under the technical direction of Dr JeremyPalmer. Sandia National Laboratories is a multi-program laboratory operated bySandia Corporation, a Lockheed Martin Company, for the United StatesDepartment of Energy’s National Nuclear Security Administration undercontract DE-AC04-94AL85000. Support was also provided through the UTEPendowed Mr and Mrs MacIntosh Murchison Chair I in Engineering. The use ofUTEP’s Metallurgical and Materials Engineering TEM and El Paso NaturalGas Corporation’s SEM is gratefully acknowledged. The researchers wish tothank Dr Lawrence Murr and Dr Stephen Stafford, UTEP Professors ofMetallurgical and Materials Engineering, for their assistance, expertise andinsight into the materials science and microscopy aspects associated with theresearch, and Dr Malcolm Cooke, UTEP Professor of Mechanical Engineering,for his assistance, expertise, and insight into mechanical testing and the resultspresented here.Finally, theauthors thankMsKarlaSoto forherassistanceduringthe use of the TEM, and Mr Francisco Medina and Mr Oswaldo Lozoya for theirassistance in developing the multiple material SL machine used in this study.

Received: 1 January 2006Revised: 1 March 2006Accepted: 29 June 2006

292

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electrical conductivity, and thermal conductivity of the

nanocomposite (Song and Youn, 2005; Sandler et al., 1999;

Biercuk et al., 2002; Gojny et al., 2003; Andrews andWeisenberger, 2004). These novel properties were measured

on test specimens produced by molding (injection or vacuum)

or were cut from sheets to achieve the final test specimen shape.

Furthermore, once the specimens were fabricated, they werepost-processed to completely cure the epoxy-based

nanocomposite (which often took up to 24 h).The next step in this evolution is the development of

methods for further exploiting the potential benefits from

CNTs in end-use applications. In the present research,ultraviolet (UV) photocurable stereolithography (SL) epoxy

resin was used as the nanocomposite matrix with multi-walled

carbon nanotubes (MWCNTs) as the reinforcing agents.Fabricating nanocomposites using SL takes advantage of the

accuracy and build speed of SL and requires minimal post-

processing.Furthermore, layered manufacturing allows for building

complex three-dimensional (3D) nanocomposite geometries

that cannot be fabricated by other means.Preliminary experiments conducted by Sandoval et al.

(2005, 2006) showed strong affinity between MWCNTs and

commercially available SL resins (DSM Somosw,WaterShede 11120 and WaterCleare 10110) using

primarily transmission and scanning electron microscopy.

This affinity resulted in an enhanced mechanical (uniaxialloading) performance at a MWCNT concentration of 0.05

percent (w/v) in WaterShede.At a concentration of 0.50 percent (w/v) in WaterCleare,

the characteristics of the filled resin changed considerably

when compared to the unfilled resin. Using a dynamic

mechanical analyzer and a ramp temperature test from ,25 to3008C, the filled resin exhibited considerably increased

mechanical strength at temperatures above the glass

transition temperature of the unfilled resin. These limitedpreliminary results motivated the current study where the

effects of MWCNT concentration on SL building and

mechanical properties (uniaxial loading) of layermanufactured parts were investigated under well-controlled

experimental conditions.The ultimate goal of this research was to develop novel

nanocomposite resins for high performance and functional SL

applications. The possibilities for tailoring the physical

properties of SL resins by varying the concentration ofMWCNTs in the resin provide enormous opportunities for

developing advanced SL materials.The following describes the methods used to disperse the

MWCNTs in SL resin, issues and solutions for fabricating

parts using the nanocomposite resin, and tensile test resultsillustrating the potential benefits derived from using

nanocomposite resins in SL.

2. Background

Rapid prototyping (RP) is a layer-based manufacturing

process whereby 3D shapes are sliced into “thin” layers,

and complex parts are fabricated by stacking these individuallayers together. As described in Wicker et al. (2004), the

ability of RP machines to replicate prescribed geometries is

dependent on the specific RP technology used to form thelayers, the thickness of the layers, and how well the part

features are represented by the files commanding the

movement of the RP machines. SL involves the curing or

solidification of individual layers of a photo-reactive resin

(contained in a vat) by means of a UV laser. Once the toplayer is photocured by passing the laser over the liquid

surface, the part is immersed deeper into the liquid polymervat and a new layer is built on top of the previous one. This

procedure continues until the complex 3D part is completed.CNTs were first observed and reported in the early 1990s

by Iijima (Harris, 1999). Since, then, great progress has beenmade toward the understanding of CNT nucleation,

processing and applications. There are essentially two types

of CNTs, including single-walled carbon nanotubes(SWCNTs) and MWCNTs. The structure of a SWCNT

can be visualized as a single sheet of graphite, which is rolledto form a cylindrical shape and capped at the ends by a half

Carbon fullerene molecule (icosahedral geometry).MWCNTs, rather, are basically a series of coaxial SWCNTs

with intertubular distances of ,0.34 nm, which is slightlylarger than the interplanar spacing in graphite.

Meyyappan (2005) and Harris (1999) summarized the

extraordinary properties that characterize MWCNTs, whereMWCNTs posses an exceptionally high aspect ratio

(,1,000:1), very low density (,2.6 g/cm3), high strength(TS , 150 GPa), high stiffness (above 1,000 GPa) and the

capability to conduct electricity (resistivity ,0.5mVm or lessdepending on tube diameter and chirality).

In general, CNTs can be produced or grown by non-catalytic and catalytic methods. After being produced, CNTs

contain impurities, mostly catalyst metal residues and

amorphous carbon nanoparticles. Therefore, CNTs mustundergo a purification process such as thermal oxidation,

hydrothermal treatment or acid etching. For furtherinformation on CNTs, the interested reader is encouraged

to review, for example, Meyyappan (2005) or Harris (1999).

3. Experimental setup and procedure

3.1 Materials and equipment

A commercially available epoxy-based SL resin, WaterShede

11120 (DSM Somosw, Elgin, IL) was used as the

nanocomposite’s matrix material. The nanocompositeMWCNT filler material used here was produced by a

chemical vapor deposition process using a ceramic oxidesupport. The MWCNTs were then purified by means of acid

etching to 95 percent by mass. According to the manufacturer(NanoLab Inc., Newton, MA), the MWCNTs were

characterized by mean outer diameter of 30 ^ 15 nm andlength of 5-20mm, which represents a surface area of

,220 m2 per gram of nanotubes. A sample of MWCNTs

was prepared on silicon monoxide/formvar-coated Cu meshgrids and examined under transmission electron microscopy

(TEM) using a Hitachi H-8000 analytical TEM.The TEM was fitted with a Noran energy-dispersive (X-

ray) spectrometer system with a goniometer-tilt stage, andoperated at 200 kV accelerating potential (Sandoval et al.,2005, 2006). TEM micrographs of the purchased rawmaterial and its diffraction pattern were obtained and

shown in Figure 1 to verify that the materials were indeed

MWCNTs.A modified 3D Systems 250/50 SL machine equipped with

a diode pumped solid-state laser (Xcytew, JDS Uniphase,Milpitas, CA) was used in this study (355 nm laser

wavelength). The modifications consisted of removing the

Functionalizing stereolithography resins

J. Hector Sandoval and Ryan B. Wicker


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sweeping mechanism and the original ,46 L vat of material

and retrofitting a rotating multi-vat carousel system. The

rotating vat carousel is composed of three vats (each

,500 ml) distributed circumferentially as shown in Figure 2

and attached to a manual rotary stage via a shaft. The original

25.4 by 25.4 cm2 platform was replaced with a smaller 8.89 by

8.89 cm2 platform, which was attached to the z-stage via an

extended assembly. The center of the smaller platform

remained in the center of the build envelope so that the

distribution of the laser beam’s focal point along the liquid

surface remained unchanged. The height of the z-stage

elevator sensor was adjusted and fixed (Min z ¼ 22.596 cm

and Max z ¼ 0.864 cm with respect to the default startingbuilding height) by changing the limit micro-switches locatedon the z-traverse mechanism. This multi-vat system is part ofthe proprietary multi-material SL machine developedpreviously (details of which can be seen in Wicker et al.,2004), and although only a single vat is required for thecurrent research, this multi-material system was selected fordevelopment of the nanocomposite because of its smaller vats(reducing total resin requirements) and unique vat fill systemused to maintain the dispersion, as is described in more detailbelow.

As shown in Figure 2, the vats contained two partitions.Partition A served to separate the main vat chamber from anoverfill vat chamber. This partition also served to maintain aconstant resin liquid level by continuously pumping resininto the main vat chamber using a peristaltic pump(Masterflex L/S Model 7550-30, Cole-Parmer, VernonHills, IL). Owing to the reduced vat size, the originalrecoating system was not used, and modifications to thenormal SL building procedures had to be made toaccommodate removal of the recoating system (details ofthese modifications are contained below). Partition B servedto isolate the entering resin flow from the main vat chamber,allowing air bubbles produced during the resin pumpingprocess to reside in the isolated chamber associated withPartition B. This resin recirculation system contributedsignificantly to successful fabrication because this system:. assisted in maintaining the MWCNTs well dispersed and

reducing MWCNT agglomerates due to the continuousmechanical mixing; and

. maintained a constant resin level (since the small vats didnot have a vat leveling system).

Additional details of this setup are contained in sections tofollow, and more complete details of this system can be foundin Wicker et al. (2004).

Figure 1 TEM micrograph of raw MWCNTs (diffraction pattern alsoshown as inset)

Figure 2 Photographs of multiple material SL setup




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The use of this modified SL system allowed for testing of

multiple MWCNT concentrations without the need forcontaminating and wasting large amounts of both SL resins

and MWCNTs. More importantly, since this system usesthe same controlling software, rim assembly (along with the

standard laser sensors) and SL building principles, themodified system used standard SL building procedures and

software and allowed for effective research and developmenton the optimal building parameters and concentrations of

MWCNTs using relatively small amounts of material.

3.2 Nanocomposite preparation

The MWCNTs were used as supplied and directly seededinto WaterShede 11120 SL resin by means of shear and

ultrasonic dispersion. The mixture was first stirredmechanically via a squirrel cage mixer (Squirrelw Mixer,

Site-b Company, Spokane, WA) until uniformity was visuallyachieved (Sandoval et al., 2005; Sandoval, 2006). To diminish

the formation of MWCNT agglomerates, the mixture waspoured into sealed glass containers and the sealed containers

were placed in an ultrasonic cleaning tank filled with water.The containers were ultrasonicated in three cycles of ,1 h. To

avoid overheating of the nanocomposite, care was takenduring this step by continuously monitoring the temperatureof the water in the ultrasonic tank. Once the water inside the

tank reached a temperature of ,308C, the water was replacedwith new fresh tap water with a temperature of ,208C.

Studies by Curran et al. (1998), Gojny et al. (2004), Andrewsand Weisenberger (2004), and others suggest negative effects

from localized ultrasonic dispersion on the integrity of CNTs.Among the reported negative effects are an effective length

reduction and physical damage to individual CNT structure.The shear and ultrasonic dispersion techniques used in this

research were non-localized, and thus, it is not expected thatthe CNTs experienced these negative effects. As a result, it isassumed here that the structure of the MWCNTs remained

unchanged in the process. Once dispersed, the solution held acolloidal state for a prolonged period (with settling times on

the order of one week) as no MWCNT sedimentation wasobserved in the unused solution until at least one week.

After preparation, the nanocomposite was poured into theSL vat for use. The peristaltic pump was set to run at 6 ml/

min to maintain a constant resin level and to provide meansfor constant mechanical mixing and steady recirculation of the

nanocomposite. This was one advantage of using the modifiedSL system as continuous mechanical mixing through

pumping helped ensure a well-mixed nanocomposite. Thepump flow rate was selected as the maximum flow rateallowed that did not wash layers away while building (that is,

higher flow rates affected part building as there was evidenceof shifted layers during fabrication). Plate 1 shows a

photograph illustrating the visible differences between pure(semi-transparent) WaterShede 11120 and the same resin

containing 0.10 percent (w/v) of MWCNT (black).

3.3 SL build procedures using the multi-material SL

machine

As described previously, a modified multi-material SL

machine was used in this study. As a result, somemodifications to the traditional SL build parameters for

WaterShede 11120 were required. For example, the z-waittime was increased from 10 to 210 s per layer, the pre-dip

delay was set to 10 s, and the elevator speed was set to

1.27 mm/sec. Also, the layer thicknesses used here were

0.203 mm in order to minimize build time since the operator

observed the entire build process. Finally, as described in

Wicker et al. (2005), parts were fabricated on top of Mylarw

sheets where an intermediate resin platform was first

fabricated. Wicker et al. (2005) found that building on

Mylarw sheets improved the surface finish and dimensional

integrity of the bottom surfaces of the fabricated parts. Once

the intermediate resin platform was built, the Mylarw sheet

was placed on top of it. To ensure proper attachment of the

Mylarw sheet to the intermediate resin platform, the laser was

scanned over the surface of the sheet twice to cure existing

resin under the sheet, serving to additionally bond the

intermediate platform and Mylarw sheet surfaces together.

After the Mylarw sheet was properly secured, the platform

was immersed deeper into the vat until a liquid thickness of

0.203 mm was measured using a wet film gage (Model 112,

Elcometerw, Manchester, UK). Once this procedure was

completed, SL manufacturing began.During SL manufacturing, a deep dip recoating process was

used where the platform was lowered 6.35 mm and raised

6.147 mm to provide the build layer thickness of 0.203 mm.

During this process, formation of air bubbles was a common

occurrence. As a result, the parts were manually swept with a

flexible fixture during the deep dip process to detach the air

bubbles from the part’s upper surface. This manual sweep

was a critical step in providing accurate parts without voids.

Once the part was completed, the Mylarw sheet was removed

from the intermediate resin platform, the part was removed

from the Mylarw sheet, and the part was subsequently cleaned

with isopropyl alcohol and post-cured for 30 minutes in a UV

oven.

3.4 Simple 3D part demonstration

As a simple demonstration of the procedures described in the

previous section, a common part manufactured by many SL

users, a chess rook, was selected for fabrication using the

nanocomposite (MWCNTs in WaterShede 11120) resin and

the multi-material SL setup. The resulting fabricated part

is shown in Plate 2. The chess rook is an excellent

demonstration for complex layered manufacturing as it is

characterized by many intricate and fine internal and external

feature details. Owing to decreased build envelope of the multi-

material SL machine, the original chess rook CAD file was

scaled to ,40 percent of its original size. The part was attached

to the intermediate platform via a Mylarw sheet and the

approximate build time for the 2.54 cm high rook was ,2.5 h.

Plate 1 Petri dishes containing pure DSM Somosw WaterShede 11120(left) and WaterShede 11120 with 0.10 percent (w/v) of MWCNTs




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As shown in the Plate, the procedures described above providean effective method for accurate layered manufacturing.

4. Results and discussion

4.1 Optimal resin parameters

As recommended by Jacobs (1992), the SL exposure should beadjusted to obtain equivalent depths of cure for each resin when

comparing two different resins. As a result, the Reverse

WINDOWPANEe protocol (part of 3D Systems AccuMaxetoolkit; 3D Systems, 1993) and its associated data analysis tool

were employed here to determine equivalent cure depths for

each of the resins. This procedure enabled the test samples usedin the mechanical testing (described in the next section) to be

manufactured similarly. The Reverse WINDOWPANEe

protocol manufactures a set of five single layer panes at thespecified critical exposure, EC, and penetration depth, DP,

values and the resulting part is measured for layer thicknessaccuracy (for details of the SL build parameters EC and DP, the

interested reader is encouraged to review Jacobs, 1992, 1996). If

the specified EC andDP values are correct, the panes will rendercure depths or pane thicknesses of 0.152, 0.191, 0.229, 0.267

and 0.305 mm with a tolerance of ^0.0254 mm.This procedure was followed using three resins: unfilled

WaterShede 11120 and filled resins with 0.025 percent (w/v)

MWCNT and 0.10 percent (w/v) MWCNT concentrations inWaterShede 11120. Once the Reverse WINDOWPANEe

parts were prepared, each panel was measured with a

micrometer (Model 342-711-30, Mitutoyo America Co.,City of Industry, CA) and four measurements were taken at

various locations of each pane. 3D Systems recommends

taking the measurements near the frame of the panes andmore specifically near its corners. AccuMaxe was designed as

a quality assurance tool to verify and optimize the

performance of SL systems. This technique was madeavailable to users for calibration and verification of build

parameters of commercially available resins to ensure the SLresin was continuing to perform as specified (3D Systems,

1993). Using this technique, the following procedure was

used to determine equivalent cure depths for each resinstudied in this work.

The commercial building parameters for pure WaterShede11120 were first confirmed and subsequently used as the

baseline for the filled resins. When developing the equivalent

cure depths for the filled resins, the DP value was heldconstant and EC was systematically increased (by

approximately 30 percent) until the required cure depths

within the specified tolerance were achieved. Using thisprocedure, it should be noted that the values determined were

not the optimum building parameters as determined using a

curve fit of the cure depth versus logarithm of the laser

exposure, also known as the working curve of the resin

(Jacobs, 1992). However, this method provided a simple

means for manufacturing samples with equivalent cure depths

for mechanical testing, and furthermore, the optimum

parameters using a curve fit procedure rendered nearly

equivalent building parameters (Sandoval, 2006). For

additional information on the use of AccuMaxe, the

interested reader is encouraged to refer to Jacobs (1992,

1996) and 3D Systems (1993).The default build parameters for WaterShede 11120

published by DSM Somosw are EC of 11.5 mJ/cm2 and DP of

0.16 mm. As shown in Table I, the measured EC values using

this simplified approach were 11.5, 15, and 19.5 for pure, 0.025

percent (w/v) MWCNT concentration and 0.10 percent (w/v)

MWCNT concentration resins, respectively. By examining

Table II, several important observations can be made. For

example, the increasing EC with MWCNT concentration

implies a “slower” resin (in terms of overall building time) as

more energy is required to photopolymerize a similar cure

depth. Even though small concentrations of MWCNTs were

used, arguably significant differences in resin build speeds

were observed. Thus, it is important to fully characterize

individual nanocomposite resins prior to use. This

characteristic is not unexpected as the CNTs clearly change

the optical characteristics of the resin (Plate 1). It is expected

that combined laser scattering and laser absorption by the

MWCNTs contribute to the resin optical (UV) characteristics.

To partly support these observations, Sun and Zhang (2002)

studied laser scattering effects due to nano to micron sized

particle dispersions in commercially available SL resins. It was

found that as the particles approached the wavelength of the

laser, laser scattering effects increased. The MWCNTs used

here have,30 nm diameter and length ranging from 1 to 5mm.

Clearly, individual particles and particle agglomerates are

contributing to laser scattering with the solid-state laser

wavelength of 355 nm.

4.2 Mechanical testing

To measure the effect of dispersing MWCNT in WaterShede

11120 SL resin on the mechanical properties of the resin,

nanocomposite test specimens for pure resin, 0.025 percent

(w/v) and 0.10 percent (w/v) MWCNT concentrations were

manufactured according to ASTM D-638 Type V tensile test

specimens (ASTM, 1994) with gage length of ,9.5 mm

in the multi-material setup described previously. The

nanocomposite specimens were mechanically tested for TS

or ultimate tensile stress, fracture stress (FS) (or breaking

strength), and fracture strain (or elongation at break). The

tensile specimens were fabricated according to the procedures

described previously using EC parameters that provided

equivalent cure depths for each resin. Four specimens were

Table I Measured critical exposure values for unfilled andnanocomposite resins

Ec(mJ/cm2)

Dp(mm)

WaterShede11120 11.5 0.16

WaterShede11120 (0.025 percent (w/v) MWCNTs) 15 0.16

WaterShede11120 (0.10 percent (w/v) MWCNTs) 19.5 0.16

Plate 2 Photographs of successfully fabricated chess rook withMWCNT-filled resin




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fabricated simultaneously, and once finished, the samples

were cleaned with isopropyl alcohol, post-cured in a UV oven

for approximately 1 h (,30 min on each side) and finally

tested. The testing was performed with a dual column Instron

5866 machine equipped with a calibrated 10 kN static load

cell with a digital readout with ^0.5 percent accuracy and 0.5

percent repeatability. The cross-head speed during testing was

set at 10 mm/min. Table III summarizes the mechanical

testing results for six samples each.Tensile test results for all samples are contained in

Figure 3. As can be seen in Table III and Figure 3, TS and

FS increased ,5.7 and ,26 percent, respectively, for the

0.025 percent (w/v) MWCNT concentration resin over the

unfilled resin. Similarly, by increasing the MWCNT

concentration to 0.10 percent (w/v), the increase was ,7.5

and ,33 percent for the TS and FS, respectively, over the

unfilled resin. The stress-strain curves in Figure 3 can be

examined to determine the variation of the experiments. As

can be seen, the common samples exhibited similar behavior.

Observing the differences between the nanocomposite resins

and the unfilled resin illustrate the quite remarkable

transformation of the nanocomposite test specimens to a

more brittle material (as a greatly reduced plastic deformation

region is seen for the nanocomposites). Additionally, it is

quite remarkable that the considerably more MWCNTs in the

0.10 percent (w/v) concentration resin did not perform

Table II Reverse WINDOWPANEe measurements for unfilled and filled resins

Pane 1

(mm)

Pane 2

(mm)

Pane 3

(mm)

Pane 4

(mm)

Pane 5

(mm)

Material Nominal thickness (^0.0254 mm) 0.152 0.191 0.229 0.267 0.305

WaterShede 11120 MWCNT (0.10 percent) Average measured thickness 0.172 0.198 0.23 0.258 0.283

WaterShede 11120 MWCNT (0.025 percent) Average measured thickness 0.172 0.209 0.235 0.266 0.295

WaterShede 11120 Average measured thickness 0.164 0.188 0.228 0.255 0.289

Table III Mechanical testing results for the unfilled and filled resins (numbers represent averages of six samples each ^ sample standard deviation)

Resin TS MPa FS MPa

Elongation at break

(percent)

Young’s Mod

(ksi)

WaterShede 11120 53 ^ 1 42 ^ 3.8 14 ^ 1.5 109

WaterShede 11120 (0.025 percent wt MWCNT) 56 ^ 1.60 53 ^ 3.5 10 ^ 1.6 98

WaterShede 11120 (0.10 percent wt MWCNT) 57 ^ 1.63 56 ^ 1.90 9.9 ^ 0.65 98

WaterShed 0.025 percent MWCNT 0.10 percent MWCNT

TS TS TS (Mpa)

7,205 8,226 8,036 0.006894757 52 1

7,581 8,027 8,316 56 2

7,290 8,398 8,299 57 2

7,550 7,918 8,379

7,584 7,968 8,614

7,750 8,469 7,964

Average 7,493 8,168 8,268

SD 205 232 237

FS FS FS

5,190 7,162 8,036 39 5

7,073 7,865 8,316 53 3

5,434 7,095 8,231 56 2

5,343 7,918 8,183

5,361 7,968 8,557

5,244 8,388 7,736

Average 5,608 7,733 8,177

SD 723 504 276

Percent Percent Percent

17.7 14.53 12.158

13.27 12.43 11.05

14.8 14.659 12.437

19.1 9.658 12.716

19.105 10.846 13.547

21.6 12.715 12.159

18 12 12

3 2 1




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Figure 3 Stress vs strain results




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appreciably different from the lower concentration, both in

mechanical properties and brittle behavior. Sandoval (2006)previously attributed the strengthening mechanisms in thenanocomposite to an assisted adhesion between layers due tothe random orientation and distribution of the nanotubes

(embedded within the polymer chains) and a reduction in themobility of the chains upon deformation. Theseimprovements suggest an effective load transfer via shearingmechanisms and strong interfacial bonding between thehighly cross-linked matrix to the randomly distributed filler

which delays fracture mechanisms. The increase in stiffnesswith the addition of MWCNTs might be explained by thehigh stiffness of the nanotubes (typically above 1,000 GPa)which improves the mechanical performance of thespecimens.

As mentioned above, the addition of MWCNTs in theepoxy matrix resulted in a more brittle material as can be seenin Figure 3. The nanocomposite specimens (for both

concentrations) experienced an elongation at break decreaseof ,28 percent when compared with unfilled resin. Thisreduction resulted in a brittle type fracture mode for thenanocomposite specimens. It is believed that the brittlefracture mode was a result of the constraints imposed by

the nanotubes in the polymer chains that did not allowmovement and molecular alignment upon mechanicaldeformation. This conclusion is supported by the ductilefracture mode observed in the unfilled resin control samples,

where signs of macroscopic plastic deformation were clearlyobserved on the test specimens.

4.3 Transmission electron microscopy

Sandoval et al. (2005, 2006) investigated the interface

between the MWCNTs and the epoxy matrix in thisnanocomposite resin and brief observations from this priorwork are contained here. Figure 4 shows TEM scans ofsamples from the fracture surfaces of the tensile testspecimens. The nanocomposite samples (both

concentrations) were sliced and sandwiched between two75 square mesh, copper grids and observed in the TEM.Figure 4 shows the strong affinity between the polymericmatrix and the filler as there is good wetting and a strong

interfacial bonding between the materials. Sandoval et al.(2006) observed and described a buckling of MWCNTsthroughout the characterized samples. The bucklingphenomenon also supports the idea that there is a stronginterface and affinity between the polymeric matrix and the

nanotubes as Lourie et al. (1998) estimated the stresses

required to collapse or buckle a nanotube to be in the range of

100-150 GPa. As a result, there appears to be an effective loadtransfer from the polymer matrix to the nanotubes, andtherefore, there has been an effective reinforcement of thepolymer by introduction of the MWCNTs. More importantly,

these observations are supported at the macroscopic level asan improvement in the mechanical properties when comparedto unfilled SL resin was observed in the present study.

4.4 Stereomicroscope observations

As mentioned previously, the MWCNT dispersions modifiedthe optical properties of the resin (from a clear appearance tomore opaque). As shown in Plate 3, this effect was more

obvious as the MWCNT concentration increased. Thesemodifications resulted in an increase of the energy exposurerequired to cure equivalent cure depths due to increased UVabsorption by the CNTs. To partly provide evidence of this,the MWCNT filled samples were analyzed under a

stereomicroscope (model MZ16, Leica Microsystems Inc.,Exton, PA).

Figure 5 shows the noticeable differences between the

resins containing 0.025 percent (w/v) and 0.10 percent (w/v)MWCNTs. These photographs also show the presence ofMWCNT agglomerates which can be seen even at a lowmagnifications (100x). MWCNT agglomerates were easily

observed in all the parts fabricated with the filled resins.Furthermore, the size and number of the agglomeratesincreased with MWCNT concentration.

It is well known that nanomaterials tend to aggregate in aneffort to reduce their total surface energy (Guz andRushchitskii, 2003; Cao, 2004). For instance, silvernanoparticles are normally coated or functionalized withcarbon to avoid particle agglomeration (Nanotechnologies,

Inc., 2006). When CNTs are produced, collected andpurified, they form tangled structures which are verydifficult to disrupt (Harris, 1999). Furthermore, CNTstypically do not go through a surface functionalization process

during production which could prevent further entanglement.Although the filled resins went through extended periods ofmechanical and ultrasonic dispersion, it is evident that theforces imposed by the viscosity of the resin (,260 cps at308C) were not sufficient to both separate pre-existing

agglomerates and deter the formation and growth of newMWCNT agglomerates. MWCNT agglomerates resulted instress concentration sites which most likely induced earlyfailure of the material upon deformation as shown in the

following section.

Figure 4 TEM micrographs taken from the fracture surface of the MWCNT nanocomposite SL resin test specimens




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4.5 Scanning electron microscopy

Once the samples were pulled to failure, their fracturesurfaces were studied under scanning electron microscopy(SEM). Samples were sputter coated with gold and thereafteranalyzed. Figure 6 shows an image of a microcrack formed atthe fracture surface of the sample (filled with a MWCNTconcentration of 0.025 percent (w/v)). The resultingmicrocrack was further analyzed at a higher magnification(5,000x) and revealed the presence of fibrous groups(presumably coated tubes) bridging the internal surface ofthe microcrack.

As mentioned previously, the size and density of theagglomerates increased with increasing MWCNTconcentration. Figure 7 shows evidence of a MWCNT-agglomerate induced fracture. This image was taken from thefracture surface of a sample containing a MWCNTconcentration of 0.10 percent (w/v). The images at thebottom of Figure 7 shows an agglomerate produced fracturedimple and its corresponding MWCNT agglomerate. Theapproximate size of the agglomerate is 50-60mm whichconcurs with the size of the agglomerates shown inFigure 5(b).

5. Concluding remarks

In an effort to develop a novel nanocomposite photocurableresin for functional SL applications, MWCNTs weredispersed using shear and ultrasonic dispersion in acommercially available SL resin, DSM Somosw

WaterShede 11120. A modified 3D Systems 250/50 SLmachine was used where the machine was equipped with asolid-state (355 nm) laser, unique ,500 ml vat without arecoater blade, overfill drain vat design that continuouslyflowed resin into the vat via a peristaltic pump, and 8.89 by8.89 cm2 square platform. Pumping the composite resinassisted in reducing the amount of MWCNT agglomerationand maintaining the MWCNTs well dispersed over longperiods of time as the MWCNTs experienced settling times

on the order of ,1 week. Each concentration of MWCNTs

required determination of new SL build parameters using

AccuMaxe techniques. As expected, increasing the

MWCNT concentration resulted in a “slower” resin as

critical exposure, EC, values were 11.5, 15, and 19.5 for

unfilled, 0.025 percent (w/v) MWCNT concentration, and

0.10 percent (w/v) MWCNT concentration resins,

respectively. Increasing MWCNT concentration most likely

resulted in a combination of increasing UV absorption due

to the CNTs and laser scattering due to the dispersed

phase. Once characterized, the modified resins performed

similarly to traditional SL resins and a sample chess rook

was manufactured to demonstrate successful layered

manufacturing using the nanocomposite resin. Tensile

testing was performed on specimens manufactured with

the unfilled and filled resins. Small dispersions of

MWCNTs resulted in improvements in the TS or ultimate

tensile stress and FS. For the 0.025 percent (w/v) MWCNT

concentration filled resin, TS increased from 53 to 56 MPa

(,5.7 percent) and FS from 42 to 53 MPa (,26 percent)

when compared to unfilled resin. Similarly, for the

Plate 3 Photographs of successfully fabricated tensile test specimenswith WaterShede 11120 (top); WaterShede with 0.025 percent (w/v)MWCNTs (middle), and WaterShede with 0.10 percent (w/v) MWCNTs(bottom)

Figure 5 Photographs of cured WaterShede 11120 filled with aMWCNT concentration of (a) 0.025 percent (w/v); and (b) cured 0.10percent (w/v)




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0.10 percent (w/v) MWCNT concentration filled resin, TS

and FS increased to 57 MPa (,7.5 percent) and 56 MPa

(,33 percent), respectively. It appears that even small

dispersions of MWCNTs provide improvement in

mechanical properties and that adding significantly more

MWCNTs may not appreciably improve the mechanical

properties over the lower concentrations. However, the use

of MWCNTs as a reinforcing agent also had a detrimental

effect on the elastic behavior (elongation at break reduction

of ,28 percent) of the nanocomposite samples when

compared to unfilled resin.The work presented here demonstrates the functionality

and capabilities of the multi-material SL system for the

development of nanocomposite SL resins. Although shear

and ultrasonic dispersion methods were successfully used to

disperse MWCNTs, the dispersion was not quantified.

Furthermore, stereomicroscope observations revealed visible

evidence of MWCNT agglomeration in the manufactured

parts (with increasing agglomeration as MWCNT

concentration in resin increased). SEM and TEM analyses

showed strong affinity and good wetting between the epoxy

resin and the MWCNTs. The TEM results suggested that

the MWCNT allowed for an effective load transfer from the

polymer matrix to the filler and SEM characterization

provided evidence that MWCNTs bridged microcracks.

However, further optical and SEM analyses provided

evidence of the detrimental effects of MWCNT

agglomerates on the mechanical performance of the

material. During this process, it was concluded that large

agglomerates (.,50mm) served as stress concentration or

crack nucleation sites which negatively impacted the

performance of the material upon deformation. Thus, it

was not possible to fully evaluate the reinforcing

capabilities of the MWCNT as the formation of

agglomerates precluded an optimal load transfer from the

matrix to the nanotubes.Additional means for dispersing the MWCNTs such as

chemical dispersion should be explored to better investigate

the potential improvements provided by more effectively

dispersing MWCNTs. These investigations should also

include a variety of SL resins to provide sufficient

information on developing new SL resins for unique

functional applications. In addition, the use of lower

purity MWCNTs should be explored to ultimately reduce

the practical cost of the resin. The possibilities for tailoring

the physical properties of SL resins by using dispersed

MWCNTs provide enormous opportunities for developing

functional SL resins. It is anticipated that the methods

described here will provide a basis for further development

of advanced nanocomposite SL resins for end-use

applications.

Figure 6 SEM micrographs taken from the fracture surface of a MWCNT filled specimen (0.025 percent (w/v)): (a) 350x; and (b) 5,000x, (note evidenceof MWCNT bridging the microcrack)




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References

3D Systems Educational Services (1993), AccuMaxe ToolkitUser Guide, 1st ed., 3D Systems Inc, Valencia, CA.

Andrews, R. and Weisenberger, M.C. (2004), “Carbonnanotube polymer composites”, Current Opinion in SolidState and Materials Science, Vol. 8, pp. 31-7.

ASTM (1994), “Standard 638 standard test method fortensile properties of plastics”, Annual Book of ASTMStandards, American Society for Testing and Materials,West Conshohocken, PA.

Biercuk, M.J., Llaguno, M.C., Radosvljevic, M., Hyun, J.K.and Johnson, A.T. (2002), “Carbon nanotube compositesfor thermal management”, Applied Physics Letters, Vol. 80,pp. 2267-9.

Cao, G. (2004), Nanostructures and Nanomaterials: Synthesis,Properties, and Applications, 1st ed., Imperial College Press,London.

Curran, S.A., Ajayan, P.M., Blau, W.J., Carrol, D.L.,Coleman, J.N., Dalton, A.B., Davey, A.P., Drury, A.,McCarthy, B., Maier, S. and Strevens, A. (1998),

“A composite from poly (m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) and carbon nanotubes:a novel material for molecular optoelectronics”, AdvancedMaterials, Vol. 10 No. 14, pp. 1091-3.

Gojny, F.H., Nastalczyk, J., Roslaniec, Z. and Schulte, K.(2003), “Surface modified multi-walled carbon nanotubes

in CNT/epoxy-composites”, Chemical Physics Letters,

Vol. 370, pp. 820-4.Gojny, F.H., Wichmann, M.H.G., Fiedler, B. and Schulte, K.

(2004), “Carbon nanotube-reinforced epoxy-composites:

enhanced stiffness and fracture toughness at low nanotube

content”, Composites Science and Technology, Vol. 64,

pp. 2363-71.Guz, A.N. and Rushchitskii, Y.Y. (2003), “Nanomaterials: On

mechanics of nanomaterials”, International Applied

Mechanics (translated from Prikladnaya Mekhanika), Vol. 39

No. 11, pp. 1271-93.Harris, P. (1999), Carbon Nanotubes and Related Structures,

Cambridge University Press, Cambridge.Jacobs, P.F. (1992), RP & M Technologies: Fundamentals of

Stereolithography, SME Press, Dearborn, MI.Jacobs, P.F. (1996), Stereolithography and other RP & M

Technologies: From Rapid Prototyping to Rapid Tooling, ASME

Press, New York, NY.Lourie, O., Cox, D.M. and Wagner, D.H. (1998), “Buckling

and collapse of embedded carbon nanotubes”, Physical

Review Letters, Vol. 81 No. 8, pp. 1638-41.Meyyappan, M. (2005), Carbon Nanotubes: Science and

Applications, CRC Press, Boca Raton, FL.Nanotechnologies, Inc. (2006), Products Overview [Online],

Nanotechnologies, Inc., Austin, TX, available at: www.

nanoscale.com/products.asp (accessed March 15, 2006)

Figure 7 SEM micrographs showing a MWCNT agglomerate as the fracture origin in WaterShede filled with a MWCNT concentration of0.10 percent (w/v)




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Sandler, J., Shaffer, M.S.P., Prasse, T., Bauhofer, W., Schulte,K. and Windle, A.H. (1999), “Development of a dispersionprocess for carbon nanotubes in an epoxy matrix and theresulting electrical properties”, Polymer, Vol. 40,pp. 5967-71.

Sandoval, J.H. (2006), Functionalizing Epoxy-BasedStereolithography Resins using Multi-walled CarbonNanotubes, University of Texas at El Paso, El Paso, TX,M.S. edition (Unpublished).

Sandoval, J.H., Soto, K.F., Murr, L.E. and Wicker, R.B.(2006), “Nanotailoring photocrosslinkable epoxy resinswith multi-walled carbon nanotubes for stereolithographylayered manufacturing”, Journal of Materials Science(in press).

Sandoval, J.H., Ochoa, L., Hernandez, A., Soto, K.F.,Murr, L.E. and Wicker, R.B. (2005), “Nanotailoringstereolithography resins for unique applications usingcarbon nanotubes”, Proceedings of the 16th Annual SolidFreeform Fabrication Symposium, University of Texas, Austin,TX, August 1-3, pp. 513-24.

Song, Y.S. and Youn, J.R. (2005), “Influence of dispersionstates of carbon nanotubes on physical properties of epoxynanocomposites”, Carbon, Vol. 43 No. 7, pp. 1378-85.

Sun, C. and Zhang, X. (2002), “The influence of the materialproperties on ceramic micro-stereolithography”, Sensors andActuators A, Vol. 101, pp. 364-70.

Wicker, R.B., Medina, F. and Elkins, C.J. (2004), “Multiplematerial micro-fabrication: extending stereolithography totissue engineering and other novel applications”, Proceedingsof the 15th Annual Solid Freeform Fabrication Symposium,University of Texas, Austin, TX, August 2-4, pp. 754-64.

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Further reading

Davis, J.R. (2004), Tensile Testing, ASM International,

Materials Park, OH.DSM Somos (2005), WaterShede 11120 – Product Data Sheet,

DSM Somos, New Castle, DE, available at: www.

dsmsomos.com (accessed October 27)

About the authors

J. Hector Sandoval is a Master of Science degree candidate

in Metallurgical and Materials Engineering, holds a Bachelor

of Science degree in Mechanical Engineering, and is a

Graduate Research Assistant in the W.M. Keck Border

Biomedical Manufacturing and Engineering Laboratory at

the University of Texas at El Paso, El Paso, TX 79968-0521;

Tel: þ01 915 747-7443, e-mail: [email protected]

Ryan B. Wicker is a Professor of Mechanical Engineering,

holds the endowed Mr and Mrs. MacIntosh Murchison Chair

I in Engineering, and is Director of the W.M. Keck Border

Biomedical Manufacturing and Engineering Laboratory at

the University of Texas at El Paso, El Paso, TX 79968-0521,

Tel: þ01 915 747-7099, Ryan B. Wicker is the corresponding

author and can be contacted at: [email protected]




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