UV-LED-curable flexible and stretchable coatings for technical textiles
Interview with Dr rer. nat. Ralf Lungwitz, STFI
1. Which challenges are involved in coating technical textiles and where did STFI direct its research on the topic? Drying and crosslinking of textile coatings takes place at higher temperatures ranging from 120-220 °C for several minutes. It serves to cure the coating as well as to provide later useability. The related energy costs are a significant financial factor in manufacturing coated textiles. The significance of this factor needs to be considered more critically in light of future environmental meas-ures such as CO2-taxes. Further drawbacks lie in the use of water as a valuable resource, or organic solutions as solvents.
In times of rising energy prices and growing ecological awareness, there is an impetus towards modern, ecological, energy and cost-efficient application and drying systems when converting textiles. One type of drying technology already widely in use in the lacquer, printing and paper industry is UV-curing. Application of UV-curable systems is a fast, environment-friendly and energy-efficient alternative to traditional thermal drying or curing processes.
The project aims at developing technological solutions for the application of UV-curable formulations to achieve flexible and stretchable coatings on technical textiles.
2. Which formulations were tested and why were they chosen? The tested coating formulations consist of urethane acrylates, crosslinkers and TPO-L as a photoinitiator. UV-curing was carried out using an energy-efficient and environment-friendly UV-LED- lamp (395 nm). To achieve special functionalities, such as abrasion resistance or an antimicrobial effect, different additives were incorporated. The formulation was applied to web substrates, knitted goods and nonwovens using spread coating. To improve curing, several different strategies for oxygen inhibition were examined.
3. Which strategies were examined? One problem with UV-curing technology (especially when using UV-LED-lamps in the UVA-range) which does arise when using thermal types of crosslinking, lies in partly incomplete curing of the surface. At the interface between the binder and air, a reaction occurs between the oxygen and the generated radicals, which leads to discontinuation of the polymerisation/crosslinking reaction on the surface. To avoid oxygen inhibition, we have tested two different approaches.
For one, we have added special additives functioning as oxygen scavengers to the formulations. Secondly, we carried out UV-curing in an inert gas atmosphere (nitrogen). We noted that inertisation is significantly more effective.
4. What are your results? Flexible and stretchable coatings based on UV-curable formulations were developed. Additives to achieve wear resistance, antimicrobial effects, thermal insulation and optical effects could be incorporated without compromising UV-curing. The formulations were successfully applied on polyester-backed, fibres, fabric, and spun-bonded polyester nonwovens using spread coating in direct and transfer processes. Coating application weights ranging from 46 g/m2 to 265 g/m2 could be realized.
Effective prevention of oxygen inhibition was achieved by UV-curing in inert gas (N2). In doing so, the amount of photoinitiator employed could be reduced from 5% up to 0.5%. At a residual oxygen concentration of < 50 ppm and 0,5%, TPO-L curing could still be achieved at web-speeds of up 10 m/min.
Coating was successfully up-scaled from the manual trials to roll-to-roll application on the lab coating line.
5. How was this research funded and which other parties were involved? We thank the German Federal Ministry for Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie) for its support of the research project UV-LED curable flexible and stretchable coatings for technical textiles UV-FlexTEX («UV-LED- vernetzbare flexible und dehnbare Beschichtungen für Technische Textilien – UV-FlexTEX») registered under VF160011 as part of the R&D support programme for non-profit-making external industrial research institutions in eastern Germany („FuE-Förderung gemeinnütziger externer Industrieforschungseinrichtungen in Ostdeutschland – Innovationskompetenz Ost (INNO-KOM-Ost) – Modul: Vorlaufforschung (VF)»). “
Dr Lungwitz, thank you for the interview.
Sächsisches Textilforschungsinstitut e.V. (STFI), An-Institut der Technischen Universität Chemnitz , D-09125 Chemnitz, www.stfi.de