UV-LED curing – eco-friendly and energy-saving technology

In times of increasing energy costs and a growing environmental awareness the textile industry tends to modern, ecological, energy and cost-efficient technologies. Currently the energy costs of textile finishing and coating companies are about 25% of the turnover. Accordingly, there is a strong need in novel energy saving methods. UV curing is a well-known and established technology in many industrial applications such as the graphics, wood, paper or varnish sectors. The application of UV curable coatings on various materials like textile, plastics, paper, metal or ceramics is an eco-friendly and energy efficient alternative to traditional thermal drying and curing technologies. The main advantage is the application of water and solvent-free 100% formulations as well as the low space and energy consumption. Up to 75% of energy can be saved in comparison to the classic thermal drying/curing UV curing is applied at room temperature so thermal sensitive materials can be used.

Currently, mercury medium pressure lamps are used as state-of-the-art equipment for UV curing. Next to many advantages of these UV source there are some disadvantages concerning the environment and occupational safety like the toxicity of mercury and ozone (formed during operation). More recent developments are towards the application of UV-LED emitters. This type of UV light source is mercury-free and emits a high intensity and narrow-band (about 30-40 nm) light in the wavelength range of UVA (315-400 nm). Hence, no IR (heat) and no UVC radiation are produced, making this UV light source very energy efficient, gentle to the material and safe. Further advantages are the long work life (> 50,000 hours), a stepless dimming from 100% to 0% and the UV-LED source need no run-up time.

UV curing

The active base ingredients of a UV curable formulation are a photo-initiator and a crosslinkable binder. According to the crosslinking mechanism or active species, the UV curing can be divided into radical und cationic curing. The photo-initiator generates either radicals or cations (super acids). Both are formed only during UV radiation, but the radicals are very reactive, and the crosslinking occurs almost during radiation. In contrast, the cations are more stable and the crosslinking continuous without further UV radiation (dark reaction). Radical curing is much faster than the cationic once. Accordingly, radical UV curing is mainly used in industry.

Principle of the radical UV curing (Source: STFI)

During UV radiation, the photo-initiator is split into 2 radicals, which reacts with the crosslinkable acrylate groups of the oligomers/prepolymers leading to a crosslinking/polymerization.

One of the most important point of UV curing is the selection of the photo-initiator. The absorption spectrum of the photo-initiator must fit to the emission wavelength of UV source, especially for the narrow band UV-LED lamps, in order to achieve a good surface curing and curing completeness.

The chemical and physical properties of the cured material are mainly determined by the binder formulation. The main component are oligomers/prepolymers, composed of crosslinkable acrylate/methacrylate groups and an epoxide, urethane, polyether, silicone, ester or acrylate backbone. Additionally, the formulations can obtain similar composed monomers as reactive diluents, multifunctional crosslinkers and additives.

UV curing in textile sector

At the Saxon Textile Research Institute (STFI), Chemnitz/Germany, technological solutions were developed for the use of UV curing in the field of textile coating, 3D printing on textiles and fiber-reinforced composites.

For these applications, a UV-LED lamp was used with an emission wavelength of 395 nm and 8 W/cm² power. Different photo-initiators were tested and TPO-L (Ethyl-(2,4,6-trimethylbenzoyl)phenylphosphinate) was evaluated as universally applicable in concentrations of 1-5%. This liquid photo-initiator can be incorporated very easily into a wide variety of formulations, has only a very slight yellow coloration, and leads to good surface and through curing.

UV-LED curable formulations for coating technical textiles

The aim of the research was to develop technological solutions for the application of UV curable formulations for the coating of technical textiles. The coatings should be soft, flexible and stretchable. Various formulations based on silicone and urethane acrylates have been developed. Special functionalities such as lightfastness, flame retardancy, abrasion resistance, thermal insulation, antimicrobial effect and color/optical effects were realized by the incorporation of specific additives. The formulations were applied by knife coating using the direct and transfer method. Woven fabrics, knitted fabrics, warp knitted fabrics and nonwovens made of polyester (PET), cotton and high-performance polyethylene were coated. By using different transfer papers, various surface optics, from high gloss to matt, could be realized.

Stretchable (elongation at break of up to 300%), abrasion-resistant (>50.000 abrasion cycles), flexible (> 200,000 bending cycles) and washable (5x60°C + tumble drying at 70 °C) coated technical textiles were produced. The application weights were between 40 and 300g/m². The industrial feasibility could be demonstrated on the laboratory coating plant (50 cm product width) in a roll-to-roll process. By using UV-LED curing in an inert atmosphere (nitrogen inertization chamber), the line-speed could be increased up to 5 m/min (maximum of the laboratory coating plant) while the photo-initiator concentration were decreased from 5 to 1%.

UV-LED curable formulations for 3D printing on textiles via dispenser

State-of-the-art in 3D printing on textiles, thermoplastic filaments (with poor thermal stability) are deposited by using the FDM (Fused Deposition Modeling) process or aqueous/solvent-based pastes, silicones or plastisols are applied with a dispenser. With the latter variants, time-consuming intermediate drying is necessary, which strongly reduces the process speeds. Based on the results of coating textiles with UV curable formulations, the advantages of UV curing were combined with the benefits of additive manufacturing (3D printing). The aim was to combine the high curing speed of the UV technology with the precise application using a dispenser to partially functionalize/coat textiles.

In principle, all developed UV curable formulations from the textile coating research can be used. The formulations were optimized with regard to application using a dispenser by adding reactive diluents or thickeners. Thereby, the most important point was the setting of the viscosity and rheological properties. The UV curing was realized simultaneous to the printing process by using 2 UV-LED spotlights with 395 nm emission wavelength.

3D printer at STFI

UV-LED-curable formulation for fiber reinforced composites

When using classic resins, curing is the most time-consuming step in composite production. This is associated with long process times and low production rates. The curing of the fiber composite materials is usually carried out at temperatures between 70 and 180°C. These 2 factors lead to high production costs and the manufacturers of composites are looking for energy-efficient alternatives that significantly shorten process times. Derived from this, the research goal was the development of a fast and energy-efficient curing technology by means of UV radiation.

In the research activities, various formulations were developed and tested as a matrix. In terms of mechanical properties, the best results were achieved with epoxy acrylate and polyester acrylate-based formulations. UV curing were performed by a UV-LED lamp with an emission wavelength of 395nm. The process parameters of the UV curing was determined and optimized. At this point, a good balance between the reactivity of the formulations and the UV-curing process (exposure time and UV dose) was important, because temperatures of >300°C can be easily reached within a second due to a strong exothermic crosslinking reaction of high reactive acrylates.

Glass fiber (GF) triaxial fabrics and nonwovens were initially used as fiber materials. Complete curing was possible with up to 10 layers (~8 mm sample thickness) in one UV curing step. The knowledge gained was applied to needle punched hybrid nonwovens made from glass and carbon fibers (CF). With optimized UV curing parameters, it was possible to cure composites, based on hybrid nonwovens with up to 40% carbon fiber content.

With the current state of research, the first sample components could be manufactured using the hand lamination process and vacuum bag technology. A 30 cm x 30 cm composite panel made of GF/CF-hybrid nonwoven (40% carbon fibers) was fully cured in less than 3 minutes. The production of a similar pure GF composites was even faster. In contrast, the complete crosslinking of the reference sample with a thermosetting resin took 32 hours. Pictures of the UV curing of a GF/CF-hybrid composite component and a UV cured GF panel are shown in the next figure.

UV curing of a GF/CF-hybrid composite component and a UV cured GF panel

Conclusion/Outlook

The UV-LED curing is an eco-friendly, energy-saving, fast and material gentle next-generation technology for textile industry. The potential of this technology could be shown for the fields of functional textile coating, 3D printing and fiber reinforced composites. Current research is focusing on the development of UV curable formulation for specific products for example artificial leather and bandages. In the field of fiber reinforced composites, work is being carried out on the UV curing of shaped glass and basalt fiber composites.  

 

Acknowledgements

The IGF projects 89 EBR and 193 EBR of the “Forschungskuratorium Textil e.V.“, Berlin/Germany, were funded by the AiF within the framework of a program supporting the “Industrielle Gemeinschaftsforschung und –entwicklung (IGF)“ by the Federal Ministry for Economic Affairs and Energy based on a resolution of the German Bundestag.

We would like to thank the Federal Ministry for Economic Affairs and Energy for funding the research projects with the reg. no. VF150006, VF160011, 49MF190112 and 49MF200068 within the funding „FuE-Förderung gemeinnütziger externer Industrieforschungseinrichtungen in Ostdeutschland – Innovationskompetenz Ost (INNO-KOM-Ost) - Modul: Vorlaufforschung (VF) and Marktvorbereitende Forschung (MF)“.