The Chemistry behind Kevlar and Epoxy Resin

The chemistry behind epoxy resin

Epoxy resins are prepared by the reaction of active hydrogen-containing compounds with epichlorohydrin followed by dehydro-halogenation. Bisphenol A (BPA) (80-05-7), on reaction with epichlorohydrin (ECH) (106-89-8) in the presence of caustic soda, produces diglycidyl ether of bisphenol A (DGEBPA) (1675-54-3). Here n is nearly zero (0.2). The resin is liquid when n < 1 and solid when n > 2. (Tanner, 649-6)

Curing

The curing of the epoxy group takes place either between the epoxide molecules themselves or by the reaction between the epoxy group and other reactive molecules with or without the help of the catalyst. 20-24 The former is known as homopolymerization, or corrective curing; and the latter is an addition or catalytic curing reaction (Tanner, 649-6). Both reactions result in coupling as well as crosslinking (Scheme III).

Curing of DGEBPA with a diamine occurs in three stages: propagation of the linear chain, formation of a branched structure, and crosslinking (Magat, 463-472). Primary and secondary amines are widely used to cure epoxy resins. The reaction between the oxirane group of the epoxy resin with primary amines is shown in Scheme IV.

Tertiary amines also are used to bring about catalytic polymerization of epoxy resin and the mechanism given in Scheme V.

To suit the requirements of the end products, other nitrogen compounds used for curing are triamines (DETA, TETA), polyamides (two) obtained from vegetable oils, polyureas (two), polyisocyanates, dicyanamide, polyurethane, and imidazole. Polymercaptans, polyhydric alcohols, polyphenols, novalacs, and silanes also are used for epoxidations (Magat, 463-472). Magnetic fields and photoinitiation also are used for polymerization. Hydantoin-based epoxy resin (15336-81-9) is used to form DGEBPA. Glycidyl esters of dimerfatty acids can also be produced from vegetable oils. Curing agents such as cyclic acid anhydrides are used. The reaction is shown in Scheme VI.

DGEBPA also is produced from aliphatic diols such as butane-1,4-diol (2425-79-8), propylene glycol (16096-30-3), hydrogenated BPA (13410-58-7), triglycidyl adduct of p-aminophenol, hetrocyclic glycidyl amides and imides, and triglycidyl isocyanurate (2451-62-9). Lewis acids such as boron trifluoride complexes are also used as curing agents (Kadolph, 35-41). Cationic catalysts such as metal halides, coordination catalysts such as metal chelates and photoinitiation are used to bring about polymerization (Kadolph, 35-41).

As defined above the curing of an epoxy resin system normally takes place in stages. The first of these is the combining of the two reactive materials - the epoxy resin and the curing agent. Normally, these two components are kept apart until reaction is desired. The resin is normally a viscous liquid and the curing agent may be a liquid or a low-melting solid.

When combined and a catalyst and heat are added, the resin and curing agent react with a release of additional heat. Reactive epoxy systems always release heat when curing, thus, they are classified as exothermic reactions. The exothermic heat acts to speed reaction. This second stage of reaction results in formation of linear chains of combined epoxy resin and curing agent. As the linear chains form, the material is still in a ...