Solventless UV crosslinkable acrylic pressure sensitive adhesives

The synthesis and characterization of solventless acrylic UV crosslinkable pressure sensitive adhesives are presented. Different prepolymers were synthesized using bulk polymerization procedure. The reaction mixture consisted of acrylic monomers (2-ethylhexyl acrylate, acrylic acid and t-butyl acrylate), azobisisobutyronitrile initiator, chain transfer agent n-dodecylmercaptan and unsaturated UV photoinitiator 4-acryloyloxybezophenone, which was copolymerized into polymer backbone. Different formulations were tested and the prepolymer was characterized by viscosity measurements and final monomer conversions. The prepolymers were coated onto PET foil and crosslinked by application of UV light source. Peel adhesion at 180° on glass plate was measured. Gel phase was determined using the Soxhlet extraction and copolymer glass transition temperatures (T_g) were analyzed by differential scanning calorimetry (DSC). Results showed that the final monomer conversions in highly exothermic bulk polymerization reached a level between 75% and 90%. Prepolymer viscosity was highly influenced by change in polymer molecular weight and by addition of acrylic acid as a comonomer. On the other hand, the viscosity remained at the low level when t-butyl acrylate was used. The amount of gel phase for all adhesives was above 60 wt.%. Peel strength measurements showed decrease in peel strength with decreasing polymer molecular weight and increase of peel, when acrylic acid was used as a comonomer. All adhesive coatings with t-butyl acrylate comonomer showed cohesive failure.