Phthalic anhydride, cross-linked with curing

The curing (cross-linking) of the prepolymer can be achieved by either reaction with primary amines (often aromatic) which attack the epoxy end groups, or carboxylic acid anhydrides (e.g. phthalic anhydride) which react with the pendant hydroxy-groups. 

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

There are two options for the other component of an epoxy resin system. Use of mono- or di-anhydrides as curing agents, usually catalyzed by a tertiary amine, causes reactions with the residual secondary hydroxyls in the repeating unit of the prepolymer forming esters and free carboxylic acids. The carboxylic acids formed also react with the epoxide end groups forming cross-links and further free secondary hydroxyl groups. Maleic anhydride, phthalic anhydride, or pyromellitic dianhydride are suitable for this process (Eq. 21.27). 

The saturated acid (phthalic anhydride) helps to reduce the cross-link density and, hence, the brittleness of the cured polyester resin. Resin composition can be varied so that product properties can be tailored to meet specific end-use requirements. For example, a resin with enhanced reactivity and improved stiffness at high temperatures is obtained by increasing the proportion of unsaturated acid. On the other hand, a less reactive resin with reduced stiffness is obtained with a higher proportion of the saturated acid. 

Furfuryl alcohol may also be converted into polymeric materials in the presence of acids and such products have found some commercial use. Typically, furfuryl alcohol is heated at about lOO C with an acid catalyst such as phosphoric acid. The reaction is extremely exothermic and efficient cooling is necessary to prevent premature cross-linking. When the required degree of reaction is reached the system is neutralized and dehydrated under reduced pressure. The product is a dark free-flowing liquid. Often urea and formaldehyde are also included in resin formulations. Final cure of the resin is effected in situ by addition of an acid just prior to application. Weak acids (e.g., phthalic anhydride and phosphoric acid) give mixtures with long pot life which cure at 100—200°C whilst strong acids (e.g., p-toluenesulphonic and sulphuric acids) are effective at room temperature. 

Self-curing acrylic rubbers have been announced which apparently function by the inclusion of acrylic cure sites. In one system two copolymers are blended together. One copolymer is made by copolymerizing ethyl acrylate, with possibly some alkoxyethyl acrylate to improve low temperature properties, with methylol acrylamide. The other copolymer is made from ethyl acrylate and acrylamide (or possibly an alkoxy methyl acrylamide). On heating cross-linking occurs between amine and methylol groups. The rate of cure may be accelerated by the use of an acidic material such as phthalic anhydride ... 

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