Chemistry of unsaturated polyester resins

The chemistry involved in synthesis of unsaturated polyester resins is very similar to that in synthesis of saturated polyester resins, 

Low MW glycols such as ethylene glycol, propylene glycol, diethylene glycol and dipropylene glycol are more common polyols, but other dlols 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. 

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-tert-butylcatechol or hydroquinone are necessarily added to the monomers in the thinning tank before resin dissolution. 

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