UV radiation curing has had little impact on the textile industry, despite the widespread use of polymer treatments on fabrics and garments. However the use of UV to modify the surface properties of fabrics, especially natural fibres, offers an alternative application with commercial potential.
Most of the difficulties in applying conventional UV polymer curing technology to textiles are associated with the polymer systems, their constituents and the means of application to the textile surface. One major problem is that most textiles are highly absorbent and capillary attraction of the fibres tends to draw the liquid polymer film away from the fabric surface and into the bulk textile. Since UV light cannot penetrate beyond the surface fibres, this results in uncured resin remaining inside the textile structure. This is unacceptable due to residual odour of acrylate polymers and also concern over the toxicity of certain chemical residues (such as photoinitiators and monomers) on fabrics that may be used for garments worn next to the skin. Another problem is the effect of the cured polymer on the handle and drape of the fabric. The high level of crosslinking common for UV-cured polymers tends to produce rigid inflexible fabrics, unsuitable for apparel use.
An alternative approach has recently been developed by CSIRO using UV radiation to modify the fabric surface whilst leaving the bulk textile unaffected. Surface fibres must either absorb UV radiation directly or a suitable photoinitiator must be applied to produce large numbers of highly reactive free radicals when the textile surface is exposed to UV. Surface modification is particularly useful on natural fibres such as wool and cotton, where the engineering of fibre and fabric properties, now commonplace for synthetics, is precluded.
For wool the UV-absorbing species are aromatic amino acid and cystine residues in the protein structure which absorb strongly below 350 nm. UVC radiation (200-280 nm) is the most effective range for modifying wool fibre surfaces. Cotton cellulose has no UVabsorbing chromophores present above 200 nm, as it has a simple linear carbohydrate structure of D-glucose units, so surface modification of cotton requires the presence of a suitable photoinitiator. For practical textile applications, the photoinitiator must be odourfree, non-toxic, inexpensive and easily removable by aqueous washing. This rules out most of the commercial aromatic photoinitiators used by the UV curing industry, but hydrogen peroxide, commonly used on textiles as a bleaching agent, absorbs in the UVC region below 300 nm to produce highly reactive hydroxyl radicals.
In the presence of air and free radicals, oxidation of surface fibres occurs which alters their physical, chemical and mechanical properties. Surface fibres form a sacrificial layer beyond which a brief exposure to high-intensity UV is unable to penetrate. This protects the bulk fibres responsible for fabric strength.
Applications for textiles
UV treatment can add value in colouration (dyeing and printing), since it is predominantly surface fibres in a fabric that absorb, reflect and scatter light. Photomodification of the surface fibres can allow:
Heavy black and navy shades on wool are important for formal business suits. Chrome dyes are preferred since they are inexpensive and have good evenness and fastness properties. However chrome dyeing requires long, energy intensive dyeing cycles which can cause significant fibre damage, colour matching is difficult and there are environmental concerns over the release of chromium residues in dyehouse effluent, particularly in the form of toxic Cr (VI). Recently deep black shades with excellent evenness have been achieved on UVtreated wool using dyes which require no Cr (VI) aftertreatment.
Preparation for printing on wool
Wool is a particularly difficult fabric to print, since each fibre is coated with a thin layer of hydrophobic lipid making the wettability of untreated wool similar to Teflon. To obtain good prints on wool it is essential to modify the fibre surface using a pretreatment, usually oxidative chlorination. Chlorination has limitations, both for the fabric (yellowing and poor evenness) and for the environment due to the release of adsorbable organohalogens (AOX) in effluent. The Siroflash process involves continuous UV exposure of wool, followed by a mild wet oxidative treatment with hydrogen peroxide.
Siroflash is a very even pretreatment, producing a bright, white base fabric for printing.
Prevention of pilling
The important factor in the mechanism of pilling is the existence of strong anchor fibres that bind individual pills to the fabric surface. By selectively weakening these surface fibres with UV, pilling of wool and cotton knitwear can be eliminated without affecting the strength of the garment.
Siroflash treatment can produce a dramatic improvement in pilling performance which has been confirmed in wear trials. For wool, the degree of improvement depends on the dose of UVC radiation applied to the knitted fabric surface. Applying dilute hydrogen peroxide solution onto knitted cotton and then exposing the wet fabric continuously to UV is a highly effective anti-pilling treatment.