Epoxy glue works by building one giant molecule that can never melt again
Squeeze out the two tubes of an epoxy kit and you are mixing a resin with a hardener. Once they meet, their molecules link up into a dense network, a thermoset that will not soften again when heated. That chemistry makes epoxy tough enough to insulate high-voltage lines and bind aircraft-grade composites.
Epoxy resins are built around the epoxide group, a tight three-membered ring of two carbons and an oxygen that chemists formally call oxirane. The strained ring is eager to open and bond. It can react with itself under a catalyst, or with a hardener such as an amine, acid, phenol, alcohol or thiol. That crosslinking reaction is known as curing, and it yields a material that is strong and resists heat and chemicals.
The resulting list of uses is long: protective coatings on metal, fibre-reinforced plastics, structural adhesives, electrical insulators, LEDs, the encapsulation of chips on circuit boards, even paintbrush manufacture. The flip side is that uncured compounds can irritate the skin, trigger allergies, and cause breathing problems from vapour or sanding dust.
Paul Schlack in Germany first reported and patented the reaction of epoxides with amines in 1934. Pierre Castan is among those credited with bisphenol A-based resins around 1943; his work was licensed to the Swiss firm Ciba, which grew into one of the world's three biggest producers. In 1946 Sylvan Greenlee of the Devoe and Raynolds Company patented a resin made from bisphenol A and epichlorohydrin.
That combination still dominates. Epichlorohydrin is attached to bisphenol A and then sodium hydroxide closes the rings, releasing salt and water. Chain length decides the product's form: resins with one or two repeating units are thick, clear liquids, while those with up to thirty are colourless solids at room temperature. Makers lengthen chains by heating liquid resin with extra bisphenol A and a catalyst to about 160 °C. Push further and the rings all but vanish, leaving phenoxy resins that behave more like ordinary plastics. Using bisphenol F instead gives runnier resins that, once cured, stand up better to chemicals.
Source: Epoxy
