Digital textile printing on growth trajectory

Tag : Printing Corduroy

Digital technology is the fastest growing method of printingtextiles, according to a report in the latest issue of TextileOutlook International. Between 2000 and 2005 digitally printedtextile output rose by 300% to 70m square metres.

Although digital methods still account for less than 1% of theglobal market for printed textiles, their share is likely to expandto as much as 10% by the early part of the next decade.

Screen printing still dominates the textile printing market with80% of global output. However, this share is expected to fall asdigital printing gains more of a foothold in the market.

One reason for the expected growth in digital printing is that moretextile manufacturers are able to provide "prepared for print"(PFP) fabrics to digital printers. But much of the increase will bedue to improvements in digital printing equipment, such asprintheads and ink dispersion techniques. Growth will also stemfrom advances in pigment-based inks.

New inks for inkjet printing
Ink producers are offering broader ranges of inkjet inks.Furthermore, many of these inks have some of the favourableproperties enjoyed by inks used in conventional textile printing.

For example, DuPont offers the same colorants in its Artistri digital inkjet printeras those which are used in screen printing.

Huntsman has recently launched 11 new inkjet inks under its TerasilDI-HL brand. These have sufficiently high light-fastness to meetstandards set by automotive manufacturers for direct inkjetprinting on to polyester . The inks are also suitable for applications such as homefurnishings, outdoor fabrics and apparel.

Even silk can be printed using inkjet systems, and such techniquesare being utilised to print silk for the Italian and other Europeanhigh fashion sectors. DyStar is marketing inkjet inks for silk based on its Procion and Levafixreactive dyes.

Huntsman has extended its Novacron (formerly Cibacron) range ofdyes for inkjet inks. The company claims that these achieve muchgreater colour intensity than inks based on conventional dyes.

By employing dyes of higher colour strength, printers are able touse lower concentrations of dye to achieve the same depth ofcolour. This, in turn, results in technical advantages during theprinting operation.

Pigment-based inks
Technological improvements are enabling manufacturers to usepigments rather than dyes when making inks for printing textilesusing digital technology. Pigments are intrinsically morelight-fast and wash-fast than dyes, and are often less expensive.

Unfortunately, pigmented inks tend to flow less well than dye-basedinks. This is important when delivering ink through an inkjetprinthead. Fortunately, advances in pigment dispersion techniquesand printheads have mitigated this problem.

Furthermore, technological advances in digital printing have led toimprovements in the way that pigment-based inks adhere to thesurface of the fabric. Pigment particles are coated with advancedsurfactants in order to improve dispersion stability. And the sametechnology is used to help the inks adhere properly to the fabric.

Companies with long-established expertise in pigments - such asCiba, BASF, DuPont and Clariant - have become technological leaders in the development ofpigment-based inkjet inks for textiles.

Pigments have a number of advantages over dyes in inkjet printing,and these benefits have contributed to the growth in demand forpigment-based inks.

For example, pigment-based inks can be printed on to a broadvariety of fibres and fabrics, whereas dye-based inks arerestricted to specific types of fibres and fabrics.

Also, pigment-based inks are more robust than dye-based inks.

Furthermore, pigment-based inks do not need to go through afixation process using steam to ensure that the ink adheres to thefabric. This makes pigment-based printing more economical in termsof running costs.

Moreover, textiles which have been coloured with pigment-based inkshave higher levels of weather fastness and light fastness thanthose coloured with dye-based inks. Consequently, they areparticularly suitable for large fabrics which are printed foroutdoor use.

Dye-based inks suffer from a number of disadvantages compared withpigment-based inks. For example, they often contain impuritieswhich can disrupt the sensitive mechanisms of printheads. In thisregard, dye-based inks sometimes need to be purified, which canincrease operating costs.

Unlike dye-based inks, pigmented inks do not require a solvent todissolve the colorant. Such solvents are often based on volatileorganic compounds, which means that dye-based inks tend to be lessenvironmentally friendly than pigmented inks.
Pigment-based inks also offer textile printers the opportunity toemploy ultra-violet curing technology during the drying stage.Using this technology, the ink dries almost instantaneously and thefabric can be handled and further processed immediately.
Conventional printing processes use heat to dry the ink, which ismuch slower and less convenient. At present, only around 5% oftextile printing employs ultra-violet curing. However this share isexpected to exceed 10% within the next five years, according toTextiles Intelligence.

BASF is in the process of upgrading its range of pigment-basedinkjet inks. These products are sold under the company's Helizarinlabel, which is also used for pigment-based concentrates for screenprinting.

Helizarin inks contain multi-functional polymeric dispersingagents. These are said to produce prints which are comparable withresults obtained using screen printing.

Also, separate binders do not need to be used with Helizarin inksbecause the binders are already built in. The company claims thatthe elimination of a separate binder gives printed fabrics a softhandle, high levels of fastness, and good colour strength andbrilliance.

Helizarin inks contain pigments in the form of a low viscosityaqueous dispersion which is ideal for inkjet printing.

With the aid of a proprietary pre-treatment process, Helizarin inkscan be used on a wide variety of fabrics - including cotton,polyamide and polyester blends.

The valve-jet process
Another important development is the use of the valve-jet processto apply the ink to the fibres. This helps to prevent problemsencountered in inkjet printing when loose strands of fibresinterfere with the flow of ink through the nozzles.

Jets in the printhead are controlled by valves which open and shutindependently.
Streams of ink droplets are ejected at high velocity and thepressure from the valve is sufficient to ensure that inks penetratedeep into the surface of the fabric.

Austria-based printing machinery manufacturer J Zimmer Maschinenbauhas recently launched two new versions of its ChromoJet digitalvalve-jet textile printer, namely the ChromoJet 400 and theChromoJet 1200.

These machines are able to print spot colours, process colours andhalf-tones.
Process colour technology prints four colours - cyan, magenta,yellow and key (black) - in the form of small dots or half-tones.The colours may overlap, and are able to give a full range ofvisual effects. The technique is widely used to produce colouredillustrations in magazines and newspapers.

The ChromoJet 400 has valve-jets which can open and close up to 400times per second, and can print up to 48 colours with a resolutionof 1 million pixels per square metre. The machine can be used toprint on carpets, rugs, furs and blankets.

The ChromoJet 1200 has valve-jets which are capable of being openedup to 1,200 times per second. It prints to a resolution of 8mpixels per square metre and it can be used to print on towelling,upholstery, automotive textiles, corduroy and mats.

" Developments in Textile Colorants " was published in Issue No 127 of Textile Outlook International.
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