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Under the UV spotlight
12 December 2018
While UV light exposure can cause problems for polymers, it can also be used constructively to join plastics together or to protect them
Short-wavelength ultraviolet (UV-C) light which has higher energy and is the type most likely to affect plastics. However, unlike long wave UV, short wave UV is attenuated more quickly and cannot pass through ordinary glass or most plastics, so the effects that it causes is concentrated in the surface regions.
UV-C light is used for irradiation, disinfection and sterilisation. Light-cured materials are also used as adhesives (primarily for glass, plastic, metal, and ceramic), and as conformal coatings for electronics and thick decorative purposes.
Light-curable adhesives cure in seconds on exposure to longwave ultraviolet and/or visible light. They optimise assembly speed, increase product throughput and enable 100% in-line inspection. They are suitable for bonding plastics, metal, glass and other substrates. They find many applications in electronics, medical device assembly, aerospace, opto-electronics, optics, automotive and electronic displays.
Light-curing equipment includes spot lamps (for small areas), flood Lamps (for larger areas) and conveyors (for large-scale production). Radiometers are used for measuring light intensity.
Light-curable materials (LCMs) used as adhesives are usually one-component mixtures of oligomers, monomers, photoinitiators and modifiers (hardness modifiers, colourants, fluorescing agents, thickeners, wetting agents, etc.).
Since its inception, Dymax has developed over 3000 light-curable formulations, as well as a range of light curing equipment, including spot lamps (for small areas), flood Lamps (for larger areas), conveyors (for large-scale production) and radiometers (for measuring light intensity).
Over 95% of Dymax LCMs are acrylates (a urethane backbone with an acrylic functional group). The balances are cationic epoxies. Be aware that acrylates and cationic epoxies are, aside from being light curable, significantly different from traditional acrylics and epoxies. The two chemistries also vary in many ways from each other.
UV-curable acrylates generally offer:
- Faster and deeper cures
- Wider range of properties
- Adhesion to a wider range of substrates
- Complimentary cure mechanisms, including visible light and heat.
UV-curable cationic epoxies generally offer:
- Superior adhesion to certain substrates (ie PP, PE, silicone)
- Superior resistance to some solvents
- Moderate speed and depth of cure (UV-curable acrylates cure faster and deeper)
- Tack-free surface cures, even at very low intensity (some UV-curable acrylates exhibit a tacky surface due to oxygen inhibition).
Most recently, LEDs are being used to cure adhesives, coatings and temporary masking products. These are optimised for use with UV curing lamps manufactured with LEDs, which can offer significant advantages over conventional UV lamp curing systems. These include:
- cooler curing temperatures
- lower intensity degradation over time
- more consistent cure results
- lower energy consumption
- reduced costs.
However, LED-based UV curing lamps provide specific narrow bandwidth UV light, which is not always optimal for adhesives which have been designed to react to broad spectrum UV lamps.
Therefore, unique adhesives have been specially formulated or tested to cure with LED UV curing lamps. Cure times range from fast to ultra-fast, to accommodate specific medical device, electronic, and industrial assembly needs. Table 1 shows a range of LED-curable adhesives from the Dymax range, specifically targeting medical devices.
Testing UV resistance
It is a real challenge for the plastics industry to predict the durability of plastics and adhesively bonded joints when exposed to UV light. Climate chambers for components exposed to UV light are standardised using procedures detailing cool and heat cycles, damp heat cycles or temperature or humidity cycles. The existing standards often fall short, and there is still no adequate simulation of the effects of rain and wind on surface appearance. Therefore, current practice is to adapt the test duration and aggressiveness in extreme temperatures to the durability requirements and the final product demands. For instance, in the automotive sector, each car manufacturer has its own standards to simulate component behaviour that reflect the most extreme temperature and radiation exposure likely to be encountered. Even the location of a component inside the car cabin is taken into account.
A UV resistance test for plastics http://dryflex.it/wp-content/uploads/2015/04/7-UV-resistance-test.pdf
Comprehensive guide to Light curing technology https://intertronics-electricstudiolt.netdna-ssl.com/wp-content/uploads/2017/05/Lit008-Comprehensive-Guide-to-Light-Curing-Technology.pdf