Effect of crosslinking agent on the two component acrylic structural adhesive based on methyl methacrylate

 Acrylic adhesives based on methyl methacrylate are thermoplastic polymers due to their linear polymerization. By adding two or more functional acrylate or methacrylate monomers it is expected that cross-linking may occur partially in leading to their chemical and thermal resistance improvements. It is also possible to increase their adhesion strength by adding acidic acrylate or acidic methacrylate additives.

Compounds containing different percentages of methyl methacrylate monomer and polymethyl methacrylate were prepared. By measuring viscosity and contact angle, a suitable formulation based on surface wetting property was selected. Gel time measurement was used to find optimum amount of the second component of these two-component adhesives. For improving the properties, some modifying monomers including acidic monomer for increasing the adhesion property and a two-functional monomer for crosslinking were added. Crosslink density was assessed by dynamic mechanical analysis. Thermal gravimetry analysis, chemical resistance and lap shear test were used to evaluate other properties.  

Results show that the lap shear strength of acrylic adhesive on aluminum substrate increased from 1.1 MPa to 2.7 MPa by adding acidic monomer as adhesion promoter. The strength increased to 3.5 MPa by using 4% of ethylene glycol dimethacrylate as a two-functional crosslinking agent. The decomposition temperature at 5% weight loss increased from 194 to 248 ᵒC. The chemical resistance of cured adhesives in water, ethanol, acetone and toluene was also studied. The results show that the specimen without crosslinking was dissolved in acetone while the crosslinked specimen showed 14.9% weight loss after 24 h immersion. Both the thermoplastic and thermoset adhesives showed good water resistance.