Linear Unsaturated Polyesters

The materials in this group are linear copolyesters. One of the dicarboxylic acids is an aliphatic unsaturated diacid. The unsaturation is introduced into the polymer backbone for the purpose of subsequent cross-linking. Unsaturated polyester technology was developed for use in glass fiber laminates, thermosetting molding compositions, casting resins, and solventless lacquers. Propylene glycol is often used as the diol. To a lesser extent are also used other glycols, like diethylene glycol, for greater flexibility, or neopentyl glycol for a somewhat better thermal resistance. Bisphenol A (2,2 bis(4-hydroxyphenyl) propane) is used when better chemical resistance is needed. Use of mixed diols is common. Many unsaturated dicarboxylic acids can be used, but maleic (as an anhydride) or fumaric acids are the most common. Chloro maleic or chloro fumaric acids are also employed. The saturated dicarboxylic acids act as modifiers. While aliphatic dicarboxylic acids can be used, the most common one is ortho phthalic acid (added to the reaction mixture as an anhydride). The acid improves compatibility with styrene that is polymerized in the presence of the polyester to form hard, rigid, cross-linked materials. Other modifiers are used to obtain special properties. When a flexible product is needed, adipic or sebacic acids may be used instead. For better heat resistance, endo- methylene tetrahydrophthalic anhydride (nadic anhydride) may be utilized. Flame retardency is achieved by using chlorinated dicarboxylic acids, like tetrachlorophthalic. Styrene is the most common monomer used in cross-linking unsaturated polyesters. When special properties are required, other monomers like methyl methacrylate may be employed. Sometimes this is done in combination with styrene. Diallyl phthalate and triallyl cyanurate form better heat-resistant products. An example of a typical batch preparation of a polyester is one where 1.2 moles of propylene glycol, 0.67 moles of maleic anhydride, and 0.33 moles of phthalic anhydride are combined. Propylene glycol is used in excess to compensate for loss during the reaction. The condensation at 150-200°C lasts for 6-16 h, with constant removal of water, the by-product. An aromatic solvent, like toluene or xylene, is often added to the reaction mixtures to facilitate water removal by azeotropic distillation. Esterification catalysts, like toluene sulfonic acid, reduce the reaction time. In addition, the reactions are blanketed by inert gases, like nitrogen or carbon dioxide, to prevent discoloration from oxygen at high temperatures. When molecular weights of 1,000-2,000 are reached, the products are cooled to 90°C and blended with vinyl monomers. Often the blends are mixtures of equal weights of the polyesters and the monomers. The structure of the above-described unsaturated polyester can be illustrated as follows:

It is interesting that unsaturated polyesters can actually be prepared by anionic alternating copolymerization of an epoxide and bicyclic bis(g-butyrolactone) bearing an isopropenyl group [34]. The reaction was illustrated as follows:

If the reaction is carried for a prolonged period of time, an insoluble, cross-linked product results.

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خبراء تغذية يكشفون "أفضل وقت لتناول الإفطار"

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خبير يكشف عن اقتراب العلماء من فك شفرات "لغات الحيوانات" !

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مواضيع ذات صلة

Copolyesters

Synthetic Methods

Linear Saturated Polyesters

Techniques of Polymer Preparation

Ring Formation in Step-Growth Polymerizations

Kinetic Considerations

Reactions of Functional Groups

Poly (vinyl alcohol) and Poly (vinyl acetal) s

Poly (vinyl acetate)

Poly (vinylidine chloride)

Copolymers of Vinyl Chloride

Poly (vinyl chloride)