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Unsaturated polyester resins are generally linear low MW polyes- ters having unsaturation (reactive double bonds) within their back- bone. They are supplied as a solution of polymerizable monomers (reactive diluents), most commonly in styrene.

2.5.1 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,

except that part of the dicarboxylic acid is replaced by diacids con- taining 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 pre- ferred 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, diethy- lene 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 poly- ester 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 (cis-configuration) or fumarate (trans-

configuration) groups in the backbone. During esterification, par- tial 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

Once unsaturated polyesters are synthesized, they are diluted in reactive diluents that are capable of polymerizing thermally. There- fore, 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.

2.5.2 Curing aspects

Unsaturated polyester resins are low MW condensation polymers which are transformed to a cross-linked network via radical initia- ted polymerization. The double bonds in the backbone copolymerize with unsaturated monomer (reactive diluent) present in the system. During polymerization, relatively short low MW polyester chains are cross-linked by short bridges consisting of, on average, around two to three styrene units, forming a densely cross-linked polymer network (see Figure 2.17).

Figure 2.17: Curing reaction of unsaturated polyesters

Free-radical initiated copolymerization can be accomplished either by conventional initiation with organic peroxides or hydroperoxides or phytochemically by using a photoinitiator in UV light. A detailed discussion of UV cured systems is outside the scope of this section; therefore, only conventional initiation is discussed here. The conven- tional curing of unsaturated polyester resins can proceed either by

thermal curing or under ambient conditions using redox system acce- lerators and promoters, in addition to initiators. The latter approach is more common in the coating industry as two-pack systems where initiator solution or paste is supplied separately, while accelerators and promoters are normally mixed with the unsaturated polyester. Examples of some important initiators are ethyl methyl ketone peroxide, cyclohexanone peroxide, benzoyl peroxide and cumene peroxide. Accelerator is a reducing agent, such as cobalt octoate, which is added in very small quantity to catalyze decomposition of the initiator into free radicals. Aromatic amines such as dimethyl aniline or dimethyl p-toluidine are added to promote that reaction.

The two components are combined prior to application, ensuring even distribution of initiator in the system. The dosage of initiator, accelerator, promoter and inhibitor will determine the pot life, the

longest period of time during which mixture is still usable and can be applied.

Air inhibition of the curing reaction is one of the major limi-

tations of unsaturated polyester systems. Polymerization is consi- derably inhibited by oxygen, which is added to the terminal radical of growing polystyrene chains by forming a stable peroxide radical. In most coating applications, the top surface is exposed to the air and will remain tacky while the layers below are cured. This is nor- mally addressed by incorporating some insoluble semi-crystalline paraffin wax in the formulation to minimize the problem. Once the coating is applied, the low surface tension wax particles pre- ferentially migrate to the surface and provide a barrier between the oxygen and polymerizing coating, thereby minimizing the difficulty of surface cure. It also reduces the rate of styrene loss from the coating. However, it has tendency to reduce the gloss of the surface, and therefore, for high gloss finishes, the surface needs to be polished after application.

In another approach, to minimize air inhibition, reactive oxygen species are added to the system, which preferentially consume oxygen before it can interfere with the curing reaction. Allyl ethers such as trimethylolpropane diallyl ether, pentaerythritol monoallyl ether and allyl glycidyl ether are normally used for this purpose.

2.5.3 Applications of unsaturated

polyester resins

The more significant volume of unsaturated polyester resins are used in fiberglass reinforced plastics, but they also find some applications in coatings and lining materials. The important characteristics of unsaturated polyester systems in coatings are high hardness, high gloss, rapid setting, low volatile content (reactive diluent) and high build capacity. Their low flexibility and high volume shrinkage may result in poor adhesion in the absence of good surface preparation or suitable primers. They also have very good chemical resistance. In coatings, one of the main applications of unsaturated polyester resins is in furniture and wood finishes. They are also used in gel coats for boats and bathroom fixtures. Another important area of application is in automotive fillers and putties. Unsaturated polyes- ters are also widely used in chemical resistant coatings and linings for chemical and petrochemical storage tanks.