Anhydride curing

In many adhesives formulations, the resin portion is the same or similar and what determines its properties is the hardener or catalyst that is used to cure the resin. Thus, depending on the hardener, epoxy adhesives may be referred to as amine-cured, anhydride-cured, polyamide-cured, or novolac-cured. Polyurethanes may be polyol-cured or hydroxypolyester-cured. 

The resulting bisepoxy compounds are cross-linked cold with polyamines, if necessary with added accelerators. A hot cure can either be accomphshed with amines or anhydrides (eg, phthaUc acid anhydride). If suitable initiators are present, EP systems can also be cross-linked by radiation. 

Ch1orocarhony1trime11itic acid 1,2-anhydride [1204-28-0] (9), is used in the preparation of esters and amide—imide polymers. TriaHyl trimellitate [2694-54-4] (10) is used as a cross-linking or co-curing agent for ethylene-derived mbbers and plastics. 

Diallyl Isophthalate. DAIP polymerizes faster than DAP, undergoes less cyclization, and yields cured polymers of better heat resistance, eg, up to ca 200°C. Prepolymer molding materials such as Dapon M of EMC, are not sticky. Maleic anhydride accelerates polymerization, whereas vinyl isobutyl ether retards it and delays gelation in castings. Copolymers with maleic anhydride are exceptionally hard and tough and may scratch homopolymer surfaces. 

Retarders. The purpose of vulcanization retarders is to delay the initial onset of cure in order to guarantee sufficient time to process the unvulcanized mbber. Three main classes of materials are used commercially, including organic acids and anhydrides, cyclohexylthiophthalimide (Santogard PVI or CTP), and a sulfenamide material (Vulkalent E). 

Powder coatings are formulated from the reaction product of trimethylolpropane and IPDI, blocked with caprolactam, and polyester polyols. The saturated polyester polyols are based on aromatic acid diols, neopentyl glycol, and trimellitic anhydride for further branching. To avoid the release of caprolactam in the curing reaction, systems based on IPDI dimer diols are used. 

The polyester resins used in this technology are typically based on terephthaUc acid [100-21 -0] CgH O, or isophthaUc acid [121 -91 -5] CgH O, neopentyl glycol [126-30-7] branched using trimellitic anhydride (38). The most commonly used curing agents are adducts of isophorone... 

Additive Polyimides. Rhc ne-Poulenc s Kin el molding compound and Kerimid impregnating resin (115), Mitsubishi s BT Resins (116), and Toshiba s Imidaloy Resin (117) are based on bismaleimide (4) technology. Maleic anhydride reacts with a diamine to produce a diimide oligomer (7). Eurther reaction with additional diamine (Michael addition) yields polyaminohismaleimide prepolymer with terminal maleic anhydride double bonds. Cure is achieved by free-radical polymerization through the terminal double bonds. 

The bisphenol A-derived epoxy resins are most frequendy cured with anhydrides, aUphatic amines, or polyamides, depending on desired end properties. Some of the outstanding properties are superior electrical properties, chemical resistance, heat resistance, and adhesion. Conventional epoxy resins range from low viscosity Hquids to soHd resins. 

For solvent-based ambient cure systems polyamides are often the hardeners of choice. For heat-cured systems, anhydrides are used to provide higher heat-resistance systems but at the expense of flexibiHty. 

Anhydride name CAS Registry Number Melting point or viscosity at 25°C M ol wt Recommended concentration, phr" Curing cycles ASTM deflection temperature, °C... 

An important appHcation is for filament-wound glass-reinforced pipe used in oil fields, chemical plants, water distribution, and as electrical conduits. Low viscosity Hquid systems having good mechanical properties (elongation at break) when cured are preferred. These are usually cured with Hquid anhydride or aromatic-amine hardeners. Similar systems are used for filament-win ding pressure botdes and rocket motor casings. 

The prime function of the saturated acid is to space out the double bonds and thus reduce the density of cross-linking. Phthalic anhydride is most commonly used for this purpose because it provides an inflexible link and maintains the rigidity in the cured resin. It has been used in increasing proportions during the past decade since its low price enables cheaper resins to be made. The most detrimental effect of this is to reduce the heat resistance of the laminates but this is frequently unimportant. It is usually produced by catalytic oxidation of o-xylene but sometimes naphthalene and is a crystalline solid melting at 131°C. 

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