Do reactive acrylics and anaerobics behave like epoxies in terms of curing?
Dave Dunn's March 2017 column.
Question: Do reactive acrylics and anaerobics behave like epoxies in terms of curing and properties like heat resistance?
Answer: There are some similarities, but many differences. Most epoxy adhesives are two-component highly crosslinked systems based on an epoxy resin component and a hardener component, which are mixed together before application. A range of hardeners is available, and the cured properties are strongly influenced by the nature of the hardener. They can be formulated to give adhesive bonds that fixture in a few minutes or in many hours. The adhesives cure from a liquid through a gel stage followed by eventual vitrification (to a glass-like solid). Systems that cure at room temperature suffer from incomplete conversion of resin to polymer as molecular mobility is severely restricted after vitrification and they have relatively low glass-transition temperatures (Tg) of 50-60°C. Heat curing can cause further reaction and give much higher bond strengths and very good heat resistance. Epoxies are particularly suitable for bonding metals and composites but are somewhat limited in the bonding of plastics.
Anaerobics are similar to epoxies in terms of being highly crosslinked systems, but are very different from a chemical point of view. They are single-component adhesives based on multifunctional methacrylate monomers, and cure is initiated by small amounts of catalysts, which interact with metal surfaces. Commonly used as threadlockers for fasteners, full cure typically takes 24 hours. They can be formulated to give varying final bond strengths and heat resistance up to about 250°C. Similar to epoxies, there is some evidence that incomplete curing takes place at ambient temperatures. Anaerobics are normally used on metals and will stress crack some thermoplastics. Both epoxies and anaerobics are characterized by forming somewhat brittle bonds with very high strength and excellent fluid and heat resistance.
Reactive acrylics are two-component adhesives originally formulated to address some of the weaknesses of epoxies. They cure very quickly, bond a wide range of substrates (including plastics) and are toughened with dissolved rubbers. However, particular monomers are chosen to give less crosslinking than in anaerobics, and this allows the formulation of tough, somewhat flexible adhesive bonds. There is, however, a trade-off in heat resistance, which is limited to around 120°C. Reactive acrylics are widely used for bonding unprepared metals, plastics and composites. ASI
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