Synthesis of acrylic resins for high-solids coatings by solution and separation polymerization

Conventional solution polymerization under monomer-starved conditions was c ompared with separation polymerization, also known as monomer-starved, as a method for making acrylic resins with low polydispersity (D=M-w/M-n). Sepa ration polymerization employs aliphatic or cycloaliphatic solvents that are good solvents for the monomers but poor solvents for the resin; thus, the resin separates during polymerization. Various process conditions, initiato rs, chain-transfer agents, and solvents were studied, focusing mainly on a monomer line-up of methyl methacrylate, styrene, ethyl acrylate, and 2-hydr oxy ethyl methacrylate in a 15/15/40/30 weight ratio. Two initiators, t-amy l peroxy 2-ethyl hexanoate and t-butyl peroxy 2-ethyl hexanoate gave about equal, excellent results. 2-Mercapto ethanol was selected as a chain transf er agent. With these ingredients, the separation polymerization method is c apable of producing oligomeric acrylic polyol resins with polydispersities (D) of about 1.7 to 1.8 when M-n is in the range 1350 to 1600. These resins have substantially lower solution viscosities than a commercial benchmark resin, which has M-n = 1230 and D = 2.03. In preliminary tests of 2K polyur ethane coatings, the film properties obtained with acrylics made by separat ion polymerization were, on balance, superior to those obtained with a comm ercial benchmark resin.