John Mason, MD of Rutland International, looks at the current moves towards PVC free inks and provides expert advice on the available options
It doesn’t seem that long ago that specifiers started to express concern about the toxicity of phthalates and their use in textile screen inks and photostencil emulsions. Ink manufacturers went through a challenging time reformulating their products in order to provide equal performance without excessive price increases. With the increase in demand over time more raw material options have become available and at realistic prices, and now phthalate free compounds have become the norm.
Subsequently concerns over other chemical constituents have led to more extensive Restrictive Substance Lists (RSLs) being created with several specifiers including PVC amongst the list of chemicals they want excluded from the products they use or purchase. There have been many thousands of words written as to why or why not PVC should be a restricted substance and there are compelling arguments for both sides. However, the reality is that we are being asked to remove PVC from our inks by more and more specifiers.
Although the title of this article refers to PVC free inks, all of the product types discussed here comply with the RSLs from the major specifiers as they are typically free of heavy metals, formaldehyde, phthalates, alkyl phenyl ethoxylates, organotin compounds and the other substances of concern, including PVC.
The PVC free ink types that I outline below are, in some cases, already known to most printers, but recent changes in both the list of restricted substances and the physical performance characteristics demanded by the specifiers have resulted in constant upgrading of formulations by ink manufacturers in order to comply. A further issue is the increasing use of synthetic fabrics requiring ink systems that are resistant to dye migration and having lower cure temperatures to minimise fabric shrinkage. Add to this the complication of climate, lower labour costs and equipment set-up in Asia, which results in water-based systems being the ink of choice, while the USA and Western Europe still prefer plastisol, and you can begin to see the complex issues facing the ink maker.
Plastisols The ink type hit hardest by the exclusion of PVC is conventional plastisol. These inks are based on PVC resin and plasticisers without the use of any volatile components resulting in superb screen stability, wet-on-wet printability, high opacity and high build, which allows an extensive variety of special effects inks to be created. Much of the screen printing and curing equipment used in the Western world is set up to print conventional plastisol so the challenge was to formulate a similar product but without the use of PVC. Fortunately there are a number of acrylic resins that can be used as alternatives.
The first generation of these acrylic plastisols had quite a few limitations in terms of stability and performance, but now the latest formulations are very close in every aspect to conventional plastisol with only slight changes in application technique being necessary to get maximum performance. The increased cost of these inks is offset by the high productivity as a result of opacity and printability.
Water-based ink systems Conventional (low solids) water-based ink systems have been around for as long as textile inks have been printed. The latest formulations are PVC free and compliant with most RSLs. In general terms they are low in opacity but have a very soft hand making them especially suitable for white and light coloured fabrics. For dark fabrics there were two options – multiple layers and multiple flashing or, providing they have been suitably dyed, discharge inks: however, the conventional products result in formaldehyde being released which is listed as being undesirable in most RSLs. Nevertheless, formaldehyde free versions are now available from a limited number of suppliers.
When specifiers first stated that they wanted to eliminate PVC from their products the major manufacturers of water-based ink systems started to produce higher solids versions of their existing systems as an alternative to plastisol, misleadingly calling the products PVC free plastisol. Reducing the volatile component (usually water) will obviously increase the solids content and allow higher pigmentation and opacity, but it also exacerbates problems such as drying in the screen. This is the number one issue with this type of product and ink manufacturers continue to work on improving this aspect using glycol solvents, humectants and other additives, but it’s a delicate balance between the application techniques required and the performance parameters demanded.
High humidity in the print shop alleviates much of the drying in problem, and this type of product dominates the Asian market for the following reasons…
- It is eminently suitable for hand printing on long tables
- The lower productivity associated with some grades is countered by lower labour costs in the region
- Although much of the region has high humidity for a large part of the year Asian printers have also modified the work environment to provide a constant level of humidity, which is both conducive to printing and more comfortable for the work force
- It is an extremely cost effective option in terms of cost per kilo
These high solids water-based inks are based primarily on acrylic and urethane resin systems, which either cure just with heat – being self-crosslinking – or require the addition of a cross-linker to ensure full cure and maximum physical properties.
Work continues on these systems to improve open-time while maintaining maximum solids content, to make the ink more user friendly in low humidity conditions, as well as keeping cure temperatures as low as possible to minimise dye migration and shrinkage.
Silicone inks These are the latest PVC free option. Although they have been around for some while it is only recently, with the increased use of polyester and highly elastic fabrics, that they have become of interest. Silicone inks usually contain no volatile component, thus giving high build. They are typically two or three component systems requiring the use of a catalyst or cross-linker to cause the silicone resin to react, usually in the presence of heat, to give a highly elastic film with good bleed resistance.
Silicone inks are at the higher end of the pricing spectrum, but they do give remarkable performance on certain substrates.
Because of the limiting factors relating to the chemistries that can be used, and the increased physical performance characteristics now required, some aspects of PVC free ink can be compromised. Most typically this revolves around the printing technique.
With all three of these PVC free systems flash times and multicolour printing benefit from the use of hot pallets which have been preheated to anywhere between 50°C and 70°C prior to commencing the print run. Flash curing during the run then maintains the boards at these temperatures reducing the flash time of acrylic plastisols and silicone inks, while allowing several colours of high solids water-based ink to be printed before further flash curing is necessary.
PVC inks and dye migration
The increasing demand for casual clothing and sportswear, along with the limited supply of natural fibres (cotton), has resulted in a dramatic rise in the use of synthetic fibres such as polyester and elastane. The sublimation dyes used in conjunction with polyester give rise to dye migration issues and although ink systems such as silicone exhibit excellent resistance to dye migration, even they are not sufficient alone when printed over some fabrics printed by the dye sublimation transfer technique.
This swing to synthetic fabrics has coincided with the move away from PVC presenting the ink manufacturer with a further issue to combat and formulate around. The result has been the creation of ‘barrier’ coats in each ink system: typically black or grey and printed down first, they have the ability to absorb and hold sublimated dyestuff from the fabric and prevent it discolouring the final print.
John Mason is the managing director of Rutland International and international sales director of The Rutland Group.