Coatings, properties

Plastic packagiag materials are thermoplastic, ie, reversibly fluid at high temperatures and soHd at ambient temperatures. These materials may be modified by copolymerization, additives ia the blead, aHoyiag, and surface treatment and coating. Properties of principal plastic packagiag materials are givea ia Table 1. 

Alkyd resins are produced by reaction of a polybasic acid, such as phthaUc or maleic anhydride, with a polyhydric alcohol, such as glycerol, pentaerythritol, or glycol, in the presence of an oil or fatty acid. The resulting polymeric material can be further modified with other polymers and chemicals such as acryhcs, siUcones, and natural oils. On account of the broad selection of various polybasic acids, polyhydric alcohols, oils and fatty acids, and other modifying ingredients, many different types of alkyd resins can be produced that have a wide range of coating properties (see Alkyd resins). 

Reaction spraying is also being widely explored. In this case new compositions or particle surface reactions providing enhanced coating properties, eg, nitrides for wear resistance, are achieved while the particle is in transit. 

Table 1. Physical and Coating Properties of Thermoplastic Powders ...

This brief summary illustrates the fact that a wide range of coating properties can be obtained by applying the same type of processing treatment to different materials. Table 12.4 shows the types of coating obtainable on ferrous materials. 

Owing to the thinness of the coatings, properties such as reflectivity and smoothness are chiefly governed by the substrate, although addition agents find useful application in improving these properties. 

Critical parts of the test programme are the preparation of test specimens, the selection of the exposure conditions (both in laboratory and field tests) and the selection of significant coating properties to be evaluated as a measure of deterioration with time. 

The above tests for characterising coating properties necessarily continue to involve a certain amount of empiricism. The intelligent use of these tests, however, has shown that wide variations of physical and electrochemical characteristics of coatings as a function of composition may be obtained, and further, that significant changes in these characteristics, that can be measured before the usual evidence of failure appears, occur upon natural and accelerated ageing. 

Table 5. Coating Properties of Pigmented Poly(HAMCL) Alkyd Paints...

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