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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Resin Properties and Applications

Phenolics

Properties:
Good strength, heat stability, and impact resistance, high resistance to chemical corrosion and moisture penetration, machinability.

Application:
Impregnating resin, brake lining, rubber resins, electrical components, structureal board, laminates, glue, adhesive binders, mold.

Aminos

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Properties:
Good heat resistance, solvent and chemical resistance, extreme surface hardness, resistance to discoloration.

Applications:
Molding componends, adhesives laminating resins, paper coating, textile treatments, plywood, dinnerware, decorating structures.

Polyesters

Properties:
Extreme versatility to processing: excellent heat, chemical, and flame resistance, low coat, excellent mechanical and electrical properties.

Applications:
Constructions auto repair puity, laminates, skis, fishing rods, boats and aircraft components, coatings, decorative fixtures, bottles.

Alkyds

Properties:
Excellent electrical and thermal properties, versatility, in flexibility or rigidity, good chemical resistance

Applications:
Electrical insulation, electronic compounents, putty, glass reinforced parts, paints.

Polycarbonates

Properties:
High refractive indexs, excellent chemical, electrical and thermal properties; dimensional stability, transparent, self extinguishing; resistant to staining; good creep resistance.

Applications:
Replacement for metals, safety helmets, lenses, electrical components, photographic film, die casting, insulators.

Polyester Reaction and Preparation of The Resin

Both the saturated and unsaturated types of polyester resin can be made by a number of different methods, but the one most commonly used is the direct reaction at high temperature between a polycarboxylic acid or anhydride and a polyhydric alcohol, preferably in an inert atmosphere. When making unsaturated polyesters, a substance which inhibits ethylenic polymerisation, for example hydroquinone, may be added to the reactants.

With both types the molecular ratio used is such that the numbers of carboxyl and hydroxyl groups are approximately equal. The esterification can be catalysed by addition of an acidic substance. If the acid and the alcohol are difunctional, the resulting polymer is a linier polymer and therefore thermoplastic, an example is polyethylene terephtalate. If, however, either the acid or the alcohol contains more than two functional groups the resulting polyester is capable of crosslinking and therefore potentially thermosetting; examples of this type are the polyesters known by the name of Glyptals (from glycerol and phthalic anhydride), and the unsaturated polyester.

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