Network polyesters for surface coatings

The original polyesters for coatings were prepared from phthalic anhydride and glycerol and were referred to as glyptals or glyptal resins  

The products from the above polyesterifications are brittle materials. They are therefore modified with oils, either drying or nondrying. Such oil-modified resins bear the names of alkyds. While glycerol is widely used, other polyhydroxy compounds (polyols) are also utilized. These may be trimethylolpropane, pentaerythritol, sorbitol, or others. Phthalic anhydride is usually used in alkyd preparations. Other dicarboxylic compounds, however, may also be included for modification of properties. Common modifiers might be isophthalic, adipic, or sebacic acids, or maleic anhydride. In addition, many other acid modifiers are described in the patent literature. 

The oils in alkyd resins are usually of vegetable origin. They are classified by the type and amount of residual unsaturation into drying, semidrying, and nondrying oils. The drying oils contain most residual unsaturation, while the nondrying ones contain mostly saturated fatty acids. 

Alkyds are also classified by the quantity of modifying oil that they contain into short, medium, or long oil alkyds. Short oil alkyds contain 30-50% oil and are usually baked to obtain a hard dry surface. Medium oil (50-65%) and long oil (65-75%) alkyds will air-dry upon addition of metal dryers. 

There are two main methods for preparation of alkyd resins. In the first one, called the fatty acid process, a free fatty acid is coesterified directly with the dibasic acid and the polyol at 200-240 °C. The reaction may be carried out without a solvent by first heating in an inert atmosphere. At the end, an inert gas may be blown into the resin from the bottom of the reaction kettle to remove water and unreacted materials. As a modification of this, a small quantity of a solvent may be used to remove water of esterification continuously by azeotropic distillation with the aid of moisture traps. 

7 Step-Growth Polymerization and Step-Growth Polymers 

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IPNs are found in many applications though this is not always recognised. For example conventional crosslinked polyester resins, where the polyester is unsaturated and crosslinks are formed by copolymerisation with styrene, is a material which falls within the definition of an interpenetrating polymer network. Experimental polymers for use as surface coatings have also been prepared from IPNs, such as epoxy-urethane-acrylic networks, and have been found to have promising properties. 

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