UV acrylic formulations cure to form thermoset resins and can provide a variety of properties from extremely flexible (>250% elongation) to very hard and rigid with a high modulus. The UV adhesives are offered in a selection of viscosities from thin liquids (less than 100 mPa-s) to highly thixotropic gel consistency with a viscosity greater than 50,000 mPa-s — making them suitable for a wide range of applications.
Some UV adhesives have a secondary cure feature so that if the adhesive is ‘shadowed’ from the UV light it will cure by another cure mechanism. The principal types of adhesives available with these characteristics are:
• UV-curing cyanoacrylates — these products cure by surface moisture.
• UV-curing anaerobics — although not widely used for plastics as anaerobic adhesives can stress crack many thermoplastics, these adhesives can be used on thermoset plastics and will cure due to absence of oxygen and metal part activity. The use of activators is sometimes required, especially if there are no metal parts present.
• UV-curing silicones — these room temperature vulcanising (RTV) products will release a by-product and cure due to the presence of atmospheric moisture.
• UV + heat cure — some UV adhesives can be heat cured — usually at temperatures >120 °C for around 15 minutes.