Fibreglass, also known as "glass-reinforced plastic" (GRP), consists of a plastic matrix reinforced by fine glass fibres woven into a flexible fabric. A tough and lightweight material, fibreglass is commonly produced by placing the fabric into a mould, followed by a resin---such as epoxy, polyester, and vinyl ester---and a suitable hardener. The hardener, also known as the curing agent or catalyst, speeds up the setting of the plastic matrix, and is added in a relatively small concentration compared to the resin. Hardeners are generally classified according to their main functional group.
Used for curing epoxy resins, polyamine hardeners constitute the biggest and most diverse group of fibreglass curing agents in the market. The most common polyamine hardeners are chains of tertiary amines. Epoxy consists of short-chain polymers with epoxide groups at either end, which form covalent bonds with the amine groups in the hardeners, producing a new and heavily crosslinked polymer. Polyamine hardeners, such as diethylenetriamine (DETA), triethylenetriamine (TETA), and tetraethylenepentamine (TEPA) can speedily cure epoxy even at room temperature.
Polyamide hardeners react with epoxy resins in as much the same way as polyamine hardeners, differing only in the main functional group involved. Often in the form of a clear, yellow to yellowish-brown liquid of medium to high viscosity, polyamide hardeners sometimes require the addition of heat to effectively cure epoxy. Compared to polyamine hardeners, they exhibit less durability and chemical resistance, but perform better in moist environments due to their water insolubility. Polyamine hardeners "blush" or form a waxy film when curing epoxy under moist conditions.
Anhydrides, another class of hardeners used in conjunction with epoxy resins, also require the addition of heat for curing, and often call for the use of accelerants. Commonly used for manufacturing fibreglass-reinforced epoxy pipes, poles, sporting goods, circuit boards and switch gears, these hardeners exhibit minimal moisture absorption after curing. Epoxy products cured using acid anhydrides such as phthalic anhydride, maleic anhydride, and methyl tetrahydrophthalic anhydride (MTHPA) show better heat, chemical and electrical resistance than those cured with polyamines and polyamides.
Organic Peroxide Catalysts
While epoxy resins have a choice from among several classes of hardeners, curing agents for polyester and vinyl ester resins are nearly always limited to organic peroxide catalysts. Organic peroxides form crosslinks at the carbon-carbon double bonds of the resins by producing free radicals. Methyl ethyl ketone peroxide (MEKP), the most common organic peroxide used for fibreglass, is a medium-activity hardener capable of curing polyester and vinyl ester at room temperature.