Hydroxylated polyesters durability

The hydroxylated polyesters react with special silicone intermediates to give silicone-polyester copolymers for baked finishes that provide high heat resistance and outstanding exterior durability (14). These resins are extensively used in coil coatings for steel and aluminum building panels where high durability is required (Figure 6). 

The increased durability obtained by siliconising a polyster resin comes from reacting a high hydroxyl value polyester with 20-30% of appropriate silicone resin. 

These requirements are usually met with two-pack paints based on hydroxyl-rich polyester or acrylic resins in the pigmented pack and aliphatic polyisocyanates in the activator pack. Cure with this type of finish is relatively fast and complete even at low ambient temperatures. An alternative finish is an acrylic lacquer, similar to the lacquer used for refinishing motor cars. These finishes are applied to the assembled aircraft by operators protected by air-fed hoods and using airless or conventional spray guns. High durability pigments are included. 

Chem. Descrip. Highly branched hydroxyl-terminated polyester polyol Uses Polyester for hard, durable exterior coatings and one-shot castables Features Lowm.w. for reaction with isocyanates, alkylated melamines, and other hydroxyl reactive curatives hydrolysis, heal, and chemical resistance, high hardness Rucoflex S-101 [Bayer]... 

Polyisocyanates are used with hydroxyl functional polyesters for ambient-cure two-component systems giving rise to films with an excellent combination of mechanical properties, solvent resistance, adhesion, durability and optical properties. More details can be found in Section 2.11. 

The polyhydroxy components of urethane sealants are mostly hydroxyl-terminated saturated polyesters or poly ethers. Most polyesters used in urethane sealants have been standard condensation products of dibasic acids (such as adipic acid or phthalic anhydride) with glycols and triols (such as propylene glycol, glycerin, or trimethylol propane). Polyester-based urethane sealants are hard and tough, have relatively good adhesion in joints, but are deficient in hydrolytic stability and exterior durability. 

Curing with phenolic resins has been shown to provide the excellent gloss retention and durability of alkyds together with the excellent adhesion and chemical- and heat-resistance properties of phenolics. Curing reactions mainly involve condensation reactions between the hydroxyl groups of the alkyd/polyester resins and the phenolic resins. The novolac type of phenolic resins are generally used for these purposes. The reaction mechanisms and their structural implications together with their influence on final paint properties will be treated in Section 6.3.3. 

---