6.4 Structural Bonding
6.4.1 2C Methyl Methacrylate (MMA)
MMA structural adhesives are rubber toughened, 2C structural adhesives that are designed to tackle the challenges of bonding metals, plastics, composites and dissimilar substrates. MMA-based structural adhesives are designed to offer open time ranging from 3 min to 2 hours and may be used in larger gap applications up to 25.4 mm (1 in). Offering a combination of strength, toughness and ease of use, MMAs are proving their worth in numerous demanding applications in the marine, transportation and building and construction industries.
The key properties of a structural MMA adhesive are strength, toughness and flexibility. The latter is particularly important when bonding dissimilar materials with high differential flexibilities, such as rigid metal and a flexible plastic. The toughness and flexibility of an adhesive film depend on the rigidity and flexibility of the polymer and the crosslink density. Higher crosslinking leads to improved strength and chemical and heat resistance, but reduced flexibility (brittle bonds) and higher viscosity. Lower crosslinking leads to reduced chemical resistance and weaker bonds but improved flexibility.