The non-yellowing mechanism of optical clear adhesives
It is important that pressure sensitive adhesive (PSA) plays an essential material for adhesive in optoelectronic application. However, commercial demanding required UV cut and UV protection. It needs UV absorber to fulfill the demands, definitely.
Basically, PSA used several kinds of acrylic monomers for polymerization. Some monomers involve hydroxyl groups. Therefore, acrylic polymer can react with isocyanate to form PSA. This is the reason that polymerization of acrylic monomers is the key factor for PSA performance.
From this diagram, acrylic polymer does not have strong absorption after 250nm. However, adding Eversorb® benzotriazole UV absorbers into acrylic polymers can find dramatically different from different PSA suppliers. First, adding different UV absorbers into PSA, absorption spectrum of PSA around 350nm is similar to spectrum of UV absorbers. Detecting UV spectrum of 320nm to 250nm, the profiles showed very strong absorptions but UV absorbers did not have absorption in this range. It means that there are strong interactions between UV absorbers and PSA. Therefore, non-absorption area of PSA and UV absorbers can find strong absorptions. It did not mean all of PSA and UV absorbers would interact each other. Actually, we found some PSA products did not have this absorption spectrum when they added UV absorbers.
To make a brief conclusion, light absorption depends on chromophore. Chromophore structures came from unsaturated double bonds such as, C=C, C=O…etc. Therefore, absorption from 320nm to 250nm could come from unreactive monomers or C=O of PSA interacted with benzotriazole UV absorbers. Although we don’t know which one is key factors generated extra UV absorption, selecting monomer and polymerization of PSA are very important. Fortunately, extra UV absorption did not affect performance of PSA and UV absorbers. On the other side, the results can apply to test quality of PSA. After all, improvement of light resistance is depended on UV absorbers. However, key factor of light resistance is depended on light resistance of raw materials. If you could not find better light resistance materials, you could spend a lot of effort and might not get too much improvement. Everlight has a lot of experience for customer total solutions and provide tailor-made products. We hope we can help people to develop new products and generate brilliant solutions and save time and cost in the future.
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