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

Abstract

Conventional solution polymerization under monomer-starved conditions was compared with separation polymerization, also known as monomer-starved, as a method for making acrylic resins with low polydispersity (D=Mw/Mn). Separation 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, initiators, chaintransfer agents, and solvents were studied, focusing mainly on a monomer line-up of methyl methacrylate, styrene, ethyl acrylate, and 2-hydroxy ethyl methacrylate in a 15/15/40/30 weight ratio. Two initiators, t-amyl peroxy 2-ethyl hexanoate and t-butyl peroxy 2-ethyl hexanoate gave about equal, excellent results. 2-Mercapto ethanol was selected as a chain transfer agent. With these ingredients, the separation polymerization method is capable of producing oligomeric acrylic polyol resins with polydispersities (D) of about 1.7 to 1.8 when Mn is in the range 1350 to 1600. These resins have substantially lower solution viscosities than a commercial benchmark resin, which has Mn=1230 and D=2.03. In preliminary tests of 2K polyurethane coatings, the film properties obtained with acrylics made by separation polymerization were, on balance, superior to those obtained with a commercial benchmark resin.