Surface Coatings, Plasticisers and Rubbers

It has been estimated that in the United States somewhat over half of the surface coatings are of the polyester (alkyd) type. 

In modem manufacturing methods the oil is sometimes reacted directly with the glycerol to form a monoglyceride and this is then reacted with the acid to form the alkyd resin. When the resulting surface coating is applied to the substrate the molecules are substantially linear. However, in the presence of certain driers such as lead soaps there is oxidative cross-linking via the unsaturated group in the side chain and the resin hardens. 

The alkyd resins are of value because of their comparatively low cost, durability, flexibility, gloss retention and reasonable heat resistance. Alkyd resins modified with rosin, phenolic resin, epoxy resins and monomers such as styrene are of current commercial imporUince.  

The resins are commonly cured by the use of peroxide with or without cobalt accelerators, depending on whether the hardening is to be carried out at room temperature or at some elevated temperature. Electron irradiation curing, which can be completed within a few seconds, has, however, been introduced for coatings on large flat surfaces such as plywood, chipboard and metal panels. 

Low molecular weight liquid polyester resins are useful as plasticisers, particularly for PVC, where they are less volatile and have greater resistance to extraction by water than monomeric plasticisers. Examples of such plasticisers are poly(propylene adipate) and poly(propylene sebacate). In some cases monobasic acids such as lauric acid are used to control the molecular weight. 

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Reaction of polyhydroxy compounds with polybasic acids gives rise to condensation polymers containing ester (—COO—) groups. Because of the presence of these groups such polycondensates are known as polyesters and find use in such diverse applications as fibres, surface coatings, plasticisers, rubbers and laminating resins. These materials are discussed in detail in Chapter 25. 

Polyesters are eneountered in many forms. They are important as laminating resins, moulding compositions, fibres, films, surface coating resins, rubbers and plasticisers. The common factor in these widely different materials is that they all contain a number of ester linkages in the main chain. (There are also a number of polymers such as poly(vinyl acetate) which contain a number of ester groups in side chains but these are not generally considered within the term polyester resins.)... 

Rubber/cement (RC) failures indicate that the weakest point in the bonded part is at the interface between the rubber and adhesive. These failures are characterised by a relatively glossy and hard bonded surface with little or no rubber present. Common causes of RC failure are the incorrect choice of adhesive, insufficient dry film thickness of adhesive, failure to properly agitate the adhesive to achieve a uniform dispersion prior to application, pre-cure of adhesive caused by excessive dwell time in the mould cavity before introducing rubber, low mould pressure, undercure of the part, migration of plasticisers and other ingredients from the body of the rubber to the rubber/adhesive interface or contamination of the surface of the adhesive coated part. 

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