Acrylic adhesives cure by free-radical polymerisation of unsaturated compounds, most often esters of methacrylic acid. Acrylic adhesives have the advantage of fast room temperature cure, fast strength build-up, good adhesion to a wide range of substrates. However, often the methacrylic acid ester monomers yield brittle polymers. In order to achieve structural adhesive bonds, the acrylic adhesive has to be toughened. This is usually achieved by addition of impact modifiers to the adhesive formulation. Examples are polychloroprene (U.S. Pat. No. 3,333,025), styrene-butadiene-styrene (U.S. Pat. No. 4,182,644), core-shell polymer particles (U.S. Pat. No. 4,942,201), chlorosulfonated polyethylene (U.S. Pat. No. 4,106,971), polyurethane oligomers with methacrylic functionality (U.S. Pat. No. 3,873,640), methacrylic functionalised butadiene rubbers (U.S. Pat. No. 4,769,419).
There exists the need for acrylic adhesives with enhanced toughness, particularly at sub-ambient temperature.