Crosslinking of Polyester

Saturated polyester may be converted to infusible and insoluble materials in a number of ways. Resin containing both free hydroxyl and free carboxyl group in substantially equal ratio, for example the glyptals, may be cured merely by the application of heat, the liquid polymer of low molecular weight, as produced by the manufacturer, being crosslinked to form a solid high molecular-weight polymer by continuation of the condensation reaction. If the resin contains a relatively large excess of free carboxyl or free hydroxyl groups, the addition of a crosslinking agent is necessary to cure it. Thus if a non-drying alkyd contain an excess of hydroxyl over carboxyl groups it may be reacted with an etherified amino resin at elevated temperatures, crosslinking taking place through a trans-etherification reaction, or with a polyisocyanate.

The saturated resins in which carboxyl groups are present in excess of hydroxyl groups do not form an important class. However, they are of interest in that they may be crosslinked with epoxide resins, normally at elevated temperature without splitting off any other compound.

A method of crosslinking saturated linear polyesters in which a peroxide catalyst is used to promote linking between methylene groups in adjacent polymer molecules, rather than between hydroxyl or carboxyl groups is described.

In the curing of the two main type of unsaturated resins, the “drying or semi-drying oil alkyds” and the “unsaturated polyesters,” different methods are used. The alkyds are compounded with metal soups, usually called “driers,” such as lead, manganese and cobalt naphthenates. If the resin is then exposed to air, especially in a thin film, oxygen is absorbed at the surface and curing may be affected at room temperature through an oxidative reaction (catalyzed by the metal soap), to form a solid, insoluble product. By contrast, however, the commercial unsaturated compounds, a polyester and an ethylenically unsaturated monomer such as styrene or an acrylate; these cure by copolymerization in the presence of an organic peroxide catalyst at either ambient or elevated temperatures. For example at ambient temperature, an accelerator (the “promoter”) such as cobalt naphtenate is required in addition to the peroxide catalyst (the “initiator”).

The unsaturated polyesters in which ethylene unsaturation is supplied by maleic or related acids cannot the readily homopolymerized. On the other hand, the unsaturated ester diallyl phthalate, having unsaturation in the alcoholic residue will homopolymerise.

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