Ink-Textile Interactions in Ink Jet Printing-The Role of Pretreatments

John Provost, Mike Freche, Ulrike Hees, Michael Kluge, Juergen Weiser BASF Aktiengesellschaft

Ludwigshafen,Germany

Abstract

Over the last few years we have seen major changes in the global textile printing market: more individual designs, shorter

run lengths and the movement of printing to Far East markets. The use of ink jet printing technology to reduce the overall

cost of sample and coupon printing costs has become well established in the textile printing industry in the developed

printing markets. Also the adoption of wide format ink jet printers for small scale textile print production such as the flag /

banner and sportswear industries is another area that ink jet printing is making “in-roads”. We are now seeing developments

in the use of industrial ink jet piezo “drop on demand” print heads capable of increasing production rates and the introduction

of more ink jet machines specifically developed for textile printing production. At the ITMA 2003 we saw over 15

machinery makers exhibiting textile ink jet printing machines, with the majority of these machines using piezo “drop on

demand” print head technologies of various types. With ink jet printing there are a wide range of print head technologies, each with specific ink physical and chemical

requirements that must be satisfied for the ink formulation to perform reliably in a specific printer platform. These physical

and chemical specifications are very precise and require the development of textile ink jet ink formulations, which differ

from normal printing pastes, in that they can not contain the majority of chemicals, required to achieve colour yield,

definition or colour fastness. In addition the textile ink jet ink formulations must be developed with the aim of achieving

excellent operability and firing performance, together with chemical compatibility with the materials used in the manufacture

of the print head.

This paper describes the ink-textile media interactions of a wide range of textile colourant systems and outlines the pre

and post treatments required to achieve the optimum print definition, colour fastness and colour yields.

Introduction

Ink jet printing has become the major printing technology in the desktop/network printing markets. The advent of digital

colour printing has opened up many new application areas for ink jet including wide-format graphic arts and increasingly

industrial applications such as textiles, which, until recently, were the exclusive domain of the traditional analogue printing

technologies.

As the printing industry moves towards these new industrial ink jet markets then the media, whether it be coated paper,

films or textiles, becomes an integral part of the technology and knowledge of the chemistry of the interaction of the ink,

colourants and the media becomes vitally important.

The ink jet formulation, the specific print head and the complex interactions between them have all to be considered when we

start to develop total ink jet solutions for industrial applications.

These interactions can be shown schematically in Figure 1.

IN K / T E X T IL E / P R IN T H E A D IN T E R A C T IO N S M A T R IX

im a g e q u a lity im a g e d u rab ility co lo u r ap p earan c e d ry tim e F astn ess T e x tile

P rin th e a d

op erab ility in k com p atib ility

C o lo u ra n t/In k

p h ys ica l an d ch em ic a l p rop erties p u rityin k s tab ility

Figure 1. Interaction of the key components involved in producing a textile ink jet printed image.

Overall, many of the factors are inter-related and the final ink jet ink selection must take account of the total system in

order to provide the final textile ink print performance required by the end user application. The most important areas are the

development of a stable ink jet ink formulation for a specific print head and the application of the necessary chemicals by

either a pre or post treatment. In textile ink jet printing, a fixation stage (heat or steam) is also required, together with

washing off the unfixed dye (and the removal of any excess pre treatment chemicals), with ink systems such as acid or

reactive ink jet inks.

Colourants in Textile Printing

The classification of colourants (see Figure 2) has been defined, according to their chemistry and application. The chemical

nature of the colourant largely determines the extent of the interaction with the textile substrate.

COLOURANT

Pigments Dyes

Acid Direct Reactive Disperse

Figure 2. Colourants used in ink jet printing

Acid Dyes These are low molecular weight anionic dyes that are relatively small in size. They have high aqueous

solubility and are used for the colouration of nylon and protein (wool and silk) fibres. Their poor image durability (wet-

fastness and light fastness) has led to their use in textile ink jet in specialized areas such as silk printing.

(1)

NN

N

HO

N

HO3S

CO2H

SO3H CI Acid Yellow 23

Direct Dyes These have higher molecular weights and are much larger in size than the acid dyes increasing their affinity

for cellulose media. Direct dyes are generally quite soluble in water. They tend to have large planar aromatic structures. They

are not as bright as acid dyes but their wet-fastness and light fastness is better. In textile ink jet printing, direct dyes have not

been used although are used in other areas of ink jet printing such as desktop and wide format ink jet printing.

(2)

NN

NSO3H

SO3H

N N

Me

NH

NHCH2CH2OH

NH

Me

N N

SO3H

SO3H

CI Direct Yellow 86

Reactive Dyes These are high chroma, water-soluble colourants, containing a reactive group, for printing onto cellulose

and protein substrates (such as wool and silk). The major advantage of using these colourants is that they form a covalent

link with the fibre (see Figure 3). This produces a bright durable color print with excellent wash fastness on cellulose fibres.

Due to the high temperature and pH required to fix the dye this interaction cannot be carried out on other media such as

paper or film.

NN

NO

NH

Dye

ReactiveDye

CottonCellulose

Figure 3 Reactive Mechanism (Nucleophilic Substitution – mono chloro-s- triazine types)

Disperse Dyes These are very low solubility in water but are solvent soluble. The dyes are applied as finely dispersed

aqueous ink jet inks to the media. They have excellent chroma, and good image durability. They are used for hydrophobic

substrates such as polyester, for both vapour phase transfer printing and direct printing (using high temperature steam or dry

heat fixation, followed by a wash-off process).

O NH2

O

(3) O OH

CI Disperse Red 60

Pigments These are insoluble colourants, which must be applied as fine particulate dispersions. They have in general

excellent light fastness and as they have no affinity for the substrates to which they are applied. The fastness to the textile

with conventional screen-printing depends on the binder and other print paste additives in the print paste. In ink jet printing

the application of the textile binder or the addition of a textile binder to an ink formulation is very dependent on the specific

print head technology being used (1).

In textile ink jet printing the first developments took place with water-soluble dyes such as reactive and acid dyes,

because the development was relatively easy and there was the availability of dye purification and filtration equipment

already available at the colourant manufacturers (2).

The development of textile ink jet inks based on dispersion technologies, for pigment and disperse dye inks are a recent

development because of the difficulties in developing stable dispersion based ink formulations at the sub micron level (3).

Colour-Textile Interactions

There are a number of factors to consider when a water-soluble colourant interacts with a substrate. The colourants consist of

hydrated anionic dyes and this hydration sphere must dissociate for the colourant to interact effectively with the substrate.

For the dye to be attracted to the textile the dye-textile combination must have a lower energy than the combination of the

hydrated dye and the hydrated substrate.

Hydrophobic substrates, such as polyester, do not interact with the water-soluble colorants and disperse type dyes are used,

with colouration through a solid solution mechanism.

The methods of interaction, between the colourant and the various textile substrates, are reviewed in order of decreasing

strength of interaction.

Covalent Bonding This is the strongest interaction that can occur and results from a chemical reaction between the colourant

and the substrate. The electrophilic reactive group on the dye reacts with a nucleophilic group (primary hydroxyl group on

the cotton) producing a stable chemical bond. This produces textile prints with excellent wash fastness properties. (See figure

3 for a schematic diagram of the mechanism).

Electrostatic or Ionic Interactions (coulombic attraction) Anionic dyes contain water solubilising groups such as, SO3-,

COO-, PO32- and these are attracted to amino groups in nylon and silk, under acid pH conditions.

Π... Π Interactions these are important in dye...dye interactions and can lead to dye aggregation or crystallization.

Chromophores such as the phthalocyanines aggregate using such interactions.

Hydrogen Bonding This is a fairly weak interaction however, for cellulose substrates this is often the most important

interaction between colourant and the textile. For large dye molecule there may be a large number of sites for these

interactions to occur increasing the strength of the binding.

Hydrophobic Interactions This attraction occurs for sparingly soluble (in water) dye systems such as disperse dyes that

contain hydrophobic groups such as alkyl chains. These interact with similar hydrophobic groups in the substrate (such as

polyester) when applied from the aqueous phase.

Dipole-Dipole Interactions These are relatively weak and result from the induced polarity in the interacting groups.

Van der Waals Forces These tend to be quite weak at long range but become stronger when the interacting groups are

brought close together. A weak repulsion occurs between cellulose substrates and anionic dyes when they are far apart,

however, as the water-soluble dyes are absorbed then the interaction between colourant and media becomes quite strong such

as in direct dyes.

Interaction of Colourants with Textile Substrates

Textile ink jet printing always requires an additional fixation step compared to other ink jet applications. This involves

fixation of the dye to the fibre at high temperature, either steam or heat fixation, followed by a washing off stage

The development of the ink formulation is critical as the co-solvents and ink additives must be compatible with both the

colourant type and the print head being used (4).

Table 2 summarizes the colourant requirements for the major fibre types.

A pigment/textile binder based ink formulation system would appear to offer a universal solution, however these

development of pigment ink jet inks depends on a number of factors, including the actual print head technology being used in

the textile ink jet printer.

Colourant Fibre Colour-Media Interaction

Reactive dye Cotton, silk and wool Covalent bonding

Acid dye Silk, wool and

polyamide (nylon)

Electrostatic, H bonding

Disperse dye Polyester Hydrophobic – Solid State

Mechanism

Pigment All fibers None –Complex Polymer

bonding Mechanism

Table 2 Colourant selections for textile substrates and the mode of interaction with the fiber.

The general process route with textile ink jet printing is to pad (or screen print) the necessary chemicals for fixation on the

fabric before the ink jet printing stage. Also, one of the other key requirements is the addition of thickening or other

chemicals to “hold” the ink jet in position before drying and fixation. With ink jet inks the viscosity is in the 3-15 mPas area

and an ink volume as small as 10-30 picolitres.

Figure 4 illustrates the general principle of pretreatment.

Figure 4 – Comparison between fabric pretreated (right) and not pre treated (left)

An ink jet print comparison on polyester is shown in Figure 5, where the polyester print on left has no pre treatment, and the

polyester print on the right has been given a specific pretreatment (as outlined later in Table 4)

Figure 5 Polyester Ink jet Printed with Disperse Dye Inks (Left Not Pretreated Right Pretreated (Table 5 Recipe)

Reactive Dye Ink Jet Printing The interaction of water-soluble based reactive dye ink to cellulose is by a covalent bonding mechanism. Due to the

requirement of ink stability, the mono chlorotriazine reactive type of reactive dyes is generally used in textile ink jet printing.

The reaction with the cellulose takes place with heat under alkaline conditions (Figure 3). It is important that the reaction

conditions are controlled so that the dye reacts with cellulose rather than being hydrolyzed by the alkali (Figure 6).

NN

N

Cl

NH

(1) OH /H 2O

(2) Cellulose O /H 2O

DyeN

NN

OH

NH

NN

N

O

NH

Cellulose

+ Cl -

+ Cl -Dye

Dye

Figure 6. (1) Hydrolysis of a reactive dye. (2) Fixation to cellulose.

As the ink jet ink formulation generally contains co-solvents/ humectants, which also contain hydroxyl groups, capable

of preferentially reacting with the dye, considerable development is required to produce a stable textile ink jet ink

formulation (5).

The pre-treatment also requires the application of other chemicals (i.e. alkali, urea, sodium alginate thickeners) necessary

to achieve the fixation of the reactive dye onto cotton. These cannot be included in the ink formulation as the physical

properties (such as viscosity for example) of such inks would make it unsuitable for jetting from the print head.

A typical starting recipe and process route for reactive dye inks is given in Table 3.

As with all textile-printing recommendations (conventional screen or textile ink jet) they are, at best, a starting point

and are included here as typical examples.

Table 3 Typical Pre Treatment Starting Recipes for Reactive dye Inks on Cotton

Pad (80-90% Expression) Sodium Alginate Solution 250 gms

Urea 100 grms

Ludigol (BASF AG) 25 grms

Sodium Bicarbonate 25 grms

Water to 1000grms

(For Viscose Rayon Fabrics increase the Urea to 200 grm/L)

Dry Controlled conditions-Temperature below 120 °C

Ink Jet Print Reactive Ink Jet ink Formulations

Fixation 1) Atmos. Steaming; 8 minutes at 102° C, or

2) Bake Fixation; 6-8 minutes at 130° C.

For heat fixation use 25gms/kilo of Sodium Carbonate. Heat

Fixation is not recommended for Viscose Rayon fabrics.

Wash off Cold Water Rinse (overflow)

Rinse in Boiling Water

Soap at the Boil, Cold Water Rinse

Dry

In the patent literature there are references to the addition of cationic agents to the pad liquors, which can increase the colour

yield of the subsequent reactive ink print. (6). However, the selection of the cationic agents has to be considered carefully to

avoid compatibility problems with the other anionic components of the pad recipe and secondly, the cationic agent can lead

to staining of the prints as the hydrolyzed reactive dye (which is anionic in nature) is attracted to the cationic pretreatment

during wash off.

Ink Jet Printing of Acid Dyes

The mechanism of the interaction of an acid dye textile ink jet ink and amino containing textile substrates such as nylon and

silk is by the electrostatic or ionic attraction mechanism previously discussed. Typical starting recipes and process routes

would include the use of an acid donor and a guar type-thickening agent in the pad recipe. The post treatment and washing

off process would be similar to conventional acid dye printing.

Table 4 Typical starting Recipe for Acid dye Ink Jet Inks

Pad (80-90% Expression) Guar Thickener Solution * 50 grms

Urea 100grms

Ammonium Tartrate (1:2 Soln) 50 grms

Water to 1000grms

(*silk fabrics use 250 grms/kilo Sod.Alginate Solution)

Dry Controlled conditions-Temperature below 100 °C

Ink Jet Print Acid Dye Ink Jet Ink Formulations

Fixation Atmospheric Steaming; 30-45 minutes at 102° C

Wash off The specific wash off will follow the same process route for

the particular acid dye on the nylon, silk or wool fabric.

Dry

Ink Jet Printing with Disperse Dyes

Polyester fibre will continue to grow significantly over the coming years and from our market surveys in the conventional

textile printing industry, the share of polyester will increase from 16% to 20%. This will also lead to increased demand for

disperse dye ink jet inks (see Figure 7).

05000

10000150002000025000300003500040000

1997

1999

2001

2003

2005

2007

2009

2011

CottonPolyesterPolyamidWool/Silk

Figure 7 Predicted World Fibre Growths to 2012 (ktonnes/annum)

Disperse dye ink jet inks are normally of two types, vapour phase transfer types, which have been available for a number of

years, and direct printing types for a wide range of market outlets, such apparel and the automotive industries. Vapour phase

transfer disperse inks have the simplest processing route: printing the disperse dye ink jet ink onto a specific paper, followed

by thermal transfer (at 210 °C for 30 seconds), because of transfer process efficiency, no wash off is required (7). For vapour

phase transfer printing with disperse dyes, there is no pre treatment of the polyester required.

Although, the vapour phase disperse ink jet transfer printing process has many advantages, particularly the no wash off stage,

there are requirements for specific direct disperse dye ink developments, such as for apparel with high fastness properties, or

automotive seat covers with high light fastness. In these cases specific disperse dye ink jet inks have been developed which

do require a pretreatment and a heat fixation (H.T. steam or Bake Fixation), followed by a wash off sequence.

A typical process route is given in Table 5.

Table 5 Typical Process Route for Direct Disperse Dye Ink Jet Printing

Pad Luprejet® HD(BASF AG) 440.5 grms

Defoamer DC(TensideChemie ) 0.2 grms

Water to 1000 grms

Drying not to exceed 100 °C

Ink Jet Print Ink Jet Print with Disperse Based Ink Jet Inks

Fixation (Specific Times/Temperatures depend

on the disperse dye types used in the textile ink

jet inks.)

High Temp. Steam (e.g. 170-180° C for 6-8 mins)

or, Thermosol (e.g. 190 °C for 60 seconds)

Wash Off Normal Polyester wash-off process, including a

reduction clear stage

Dry

Luprejet® HD (BASF AG) is a polyester pretreatment specially developed to retain print definition whilst at the same time

giving a textile print with good penetration (8). Other pretreatments for polyester include the padding of dilute solutions of

synthetic thickeners, followed by low temperature drying. The maximum storage time period before printing has to be

determined under the specific working conditions.

Textile Ink Jet printing with Pigments

Of the 24 billion square metres printed world wide per year (2002), 48 % is by pigment printing and in some markets such as

the USA the figure is well over 90 %. Textile printing is still predicted will grow over the next 10 years to 33 Billion square

metres, with pigment printing still being the dominant colouration printing method.

Figure 8 gives a breakdown of the colouration classes, together with predictions of growth for the next 10 years (BASF

Market study).

Figure 8 Conventional Textile Printing by Colourant Group (BASF AG Data)

As pigment printing is the major colouration technology in textile printing why has it been only recently that pigment ink

formulations have been introduced into the market?

The reason is that the development and the scale up of pigment formulations to the sub micron level for textile ink jet

printing with good operability, stable formulations and optimum firing performance is extremely difficult. Also the question

of whether a textile binder can be incorporated into the ink formulation is not straightforward as the many types of print

heads available have very different physical and chemical performance specifications, which can be influenced by the

addition of a textile binder to the ink jet ink formulation.

The ink jet print head technology “map” is very complex with many types of print head technologies available (9). The

textile ink jet printers in the market place have print head technologies from a wide range of technologies, including both the

“continuous “and “drop on demand” print head types. The specific print head technology being used will determine the

initial physical specification target (such as viscosity, surface tension, conductivity etc) that must be met to develop a

pigment ink jet ink formulation. Subsequent development must then take place with the actual print head system to develop

the final textile ink jet ink. Only by continuous operability testing with developed ink jet inks under “real time” print

conditions will the optimum pigment ink jet ink formulation be developed.

The following broad matrix (Table 6) indicates the possible approaches to pigment based ink jet formulations with the broad

Textile Printing 2002 by Colourant Class

48%

24%

2%

16%

4%

3%

3%

PigmentsReactiveDisperse-TransferDisperse-DirectVatAcidothers

ReactivesDisperse-TransferDisperse-DirectVatAcidOthers

Textile Printing 2012 by Colourant Class

48%

22%

2%

20%

3%

3%

2%

Pigments

categories of print heads currently in use, or in known development, for the textile ink jet market.

Continuous Ink Jet

Technology

Piezo “Drop on

Demand”

Technology-

Low Viscosity

Piezo “Drop on

Demand”

Technology-

Higher Viscosity

Thermal

“Drop on

Demand”

Technology

Small

Volume

Large

Volume

Pigment Ink Jet

Ink- No Textile

Binder

Yes Yes Yes No Yes

Pigment Ink Jet Ink

- With Textile

Binder

No Yes No No Yes

Table 6 Possible Pigment Ink Jet Formulation Approaches to Different Print Head Technologies

The definition of viscosity in Ink jet printing is in a different magnitude to conventional textile screen-printing.

Low viscosity in Table 5 generally refers to viscosity lower than 5 mPas, whilst the term higher viscosity refers to viscosities

in the order of 10 to 25 mPaS.

The same difference in magnitude also applies to continuous print head ink jet volumes, small volumes refer to volumes in

the order of 10 to 30 picolitres and higher volumes refer of 75 to 300 picolitres (a Picolitre is 10 ¯¹² of a litre).

The initial approach to the development textile pigment inks was to add already available textile binders to pigment ink jet

ink. However, these additions can influence the ink jet viscosity and at binder levels required to achieve acceptable

international colour fastness standards, the viscosity is outside the operability performance range of many of the piezo drop

on demand print heads. Therefore, a number of different approaches have been followed over the last few years.

1) A pre and post treatment to achieve the pigment fastness, the so called “Universal “ Pigment ink jet approach (1)

2) Develop Pigment ink jet inks with textile binder included for specific piezo print heads (10), i.e. a specific ” print head”

and pigment (with binder) ink jet ink approach

3) Develop a new chemical approach for pigment textile ink jet printing (11) which allows the low viscosity requirement to

be achieved without the use of textile binders. This approach has been recently introduced at the 2003 ITMA and allows

the pigment ink jet inks to be used on the low viscosity piezo “drop on demand” type print heads such as found in the

Mimaki TX series (Japan) of textile ink jet printers and printers that use the Epson Corporation piezo print head (12).

Conclusions

By understanding the chemical nature of the colourants, used in textile ink jet, together with their modes of interaction and

fixation mechanism to the textile, specific ink jet solutions (ink, pretreatment and post treatments) can be developed. By

following an integrated approach we can maximize the benefits of textile ink jet printing by improving the final printed

image quality (including print definition), colour yield, colour fastness and the “ handle” of the final textile ink jet print.

The development of water-soluble textile ink jet inks such, as reactive and acid dye inks was the initial starting point for

textile ink jet printing.

Dispersion based inks, particularly pigment textile ink jet inks offer new challenges, such as stability, operability and firing

performance ,and a number of new approaches are being introduced to the market (11).

Considerable developments are now taking place in both the colourant /ink chemistry, print head developments and printing

machines, which will see textile ink jet printing develop further over the next few years.

References (1) U.Hees, M.Freche, M.Kluge, J.Provost, J Weiser “Developments in Textile Ink Jet Printing with Pigment Inks”,

Image Science and Technology NIP 18 Digital Printing Conference, pages 242-245,San Diego, October 2002

(2) S Aston, H.Messelink, J.Provost “Jet Printing with Reactive Dyes”, JSDC, 109(1993) 147

(3) H.Hauser, N.Buehler, ”Fine Particle Pigment Concentrates in Ink Jet Printing Inks”, Image Science and Technology

NIP14 Digital Printing Conference p95 (1998).

(4) H.Kang, J Imaging Science & Technology, 35 (1991) 179

(5) B.Smith, E.Simonson,” Ink Jet Printing on Textiles” Textile Chemist &Colorist, 19(1987) No19, 23

(6) Zeneca PLC, EP 534660 (1992)

(7) K. Siemensmeyer, J.R.Provost, F-W Raulfs, J Weiser, “Thermodynamics of the Digital Transfer Printing Process “,

Image Science &Technology NIP 17 Digital Printing Conference, pages 426 –431, Ft Lauderdale, October 2001

(8) U.Hees, M.Freche, M.Kluge, J.Provost, J Weiser,” InkJet Druck:Neues Produkt zur Vorbereitung von Textilien,

International Textile Bulletin, No 2( May), 2003, p64

(9) P Hue,”Progress and Trends in Ink Jet printing Technology”, J. Imaging Science &Technology, 42(1998) 49

(10) A Grant, “Pigmented Jet-Inks for Textile Applications”, Image Science & Technology NIP 17 Digital Printing

Conference, Ft Lauderdale, October 2001 431

(11) U. Hees,M.Freche,M.Kluge,J.Provost,J.Weiser “Textile Ink Jet Printing with Low Viscosity Pigment Inks”, Image

Science & Technology NIP 19 Digital Printing Technologies ,Sept 2003 p626

(12) M.Usui, “Development of the new MACH (MACH with ML Chips)”, Image Science &Technology, NIP12 Digital

Printing Technologies, October 1996 p50