Лако-красочные материалы — производство

The chemistry involved in synthesis of unsaturated polyester resins is very similar to that in synthesis of saturated polyester resins, except that part of the dicarboxylic acid is replaced by diacids con­taining unsaturation, such as maleic anhydride and fumaric acid. In addition to this unsaturated acid, the predominant saturated acids used are phthalic anhydride, terephthalic acid and isophthalic acid. Phthalic anhydride is more economical, but isophthalic acid is pre­ferred when better chemical resistance and mechanical properties are required. Aliphatic diacids such as adipic acid, azelaic acid and sebacic acid are also frequently used to modify resins requiring increased flexibility and toughness.

Low MW glycols such as ethylene glycol, propylene glycol, diethy­lene glycol and dipropylene glycol are more common polyols, but other diols such as neopentyl glycol may also be incorporated for superior properties. Monofunctional alcohols are also used as a chain stopper, towards the end of the reaction, to control acid or hydroxyl number.

Reactive diluent is a very important component of unsaturated polyester resins. It should act as a solvent for unsaturated poly­ester resins as well as copolymerize with unsaturated bonds in the polymer backbone. Styrene is the most commonly used diluent due to good solvency, low cost, low viscosity and good reactivity. Apart from styrene, other monomers such as vinyl toluene, methyl methacrylate and a-methyl styrene as well as some polyfunctional diluents such as 1,6-hexanediol diacrylate, diallyl phthalate, divinyl benzene, and trimethylolpropane triacrylate are used.

In unsaturated polyester resins, the amount of maleic anhydride in proportion to other diacids may range from 25 to 75 % on a molar basis, which influences the number of cross-linking sites and hence governs the properties of the final network. The unsaturated sites may be the maleate (c/s-conflguration) or fumarate (trans­configuration) groups in the backbone. During esterification, par­tial isomerization of maleic anhydride to fumaric acid takes place, and the extent of isomerization may reach 100 %, depending upon composition and reaction conditions. The influence of isomerization on final curing is vital, because copolymerization with styrene is favored with the trans-configured fumarate, while maleate gives more of the homopolymer of styrene.

Once unsaturated polyesters are synthesized, they are diluted in reactive diluents that are capable of polymerizing thermally. There­fore, care needs to be taken at this stage to keep the temperature as low as possible. As an additional precaution during thinning, as well as to have better stability on storage, a polymerization inhibitor such as p-fert-butylcatechol or hydroquinone are necessarily added to the monomers in the thinning tank before resin dissolution.