Polyesters, linear unsaturated

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. 

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

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Glycols and epoxides react with maleic anhydride to give linear unsaturated polyesters (61,62). Ethylene glycol and maleic anhydride combine to form the following repeating unit. This reaction is the first step in industrially important polyester resin production (see Polyesters, unsaturated). 

Table I. Composition and Properties of Linear Unsaturated Polyesters with Different Molecular Weights...

Linear unsaturated polyesters contain aliphatic unsaturation that can result in subsequent cross-linking. The first such product was prepared in 1946 by the condensation of maleic anhydride... 

Ranby and Shi also studied hyperbranched methacrylated polyesters and their use in photopolymerizations of films and fiber-reinforced polymer composites. The resins were found to have low viscosities and higher curing rates than those of corresponding linear unsaturated polyesters [131-133]. 

On the other heind, the linear unsaturated polyester resins find use in memy commercial applications, such as in producing solventless lacquers, and thermosetting molding compounds. The resin is normally prepcured by the reaction of a saturated diol with a mixture of an unsaturated dibasic acid and a modifying dibasic acid or its corresponding anhydride. It is commonly referred to as... 

Polyester-based resins are also used in bulk plastics in fiberglass reinforced fabrications such as boats, auto body repairs, and for very large structures such as industrial vessels and pipes, and ships for mine sweeping [10]. This technology uses a combination of polycondensation and vinyl-type (chain reaction) technologies to obtain the final composite product. The viscous, still fluid resin used for layup is a linear unsaturated polyester of relatively low-molecular weight. Unsaturation is introduced into the backbone of the polymer by using maleic anhydride to replace a part of the phthalic anhydride (e.g., Eq. 21.4). 

During the last few years we have developed methods for rapid cross-linking of polyethylenes, ethylene-propylene-diene elastomers (EPDM)2 and linear unsaturated polyesters using UV light for initiation. Efficient crosslinking to high gel content has been obtained for these polymer systems, which contain small amounts of UV-absorbing initiator and a multifimctional monomer (a crosslinker),... 

Three photopolymerizable primer monomers, namely the linear hexanediol diacrylate (HDDA), the branched trimethylol propane triacrylate (TMPTA), and the linear tripropyleneglycol diacrylate (TPGDA), and a model of a linear unsaturated polyester/ alkyd varnish repeating unit were used for the study. A model of the two top chains of an elementary cellulose I crystallite was used as a substrate, the refined conformation of which had already been reported [12]. 

What are the linear unsaturated polyesters Explain. Show by chemical reactions how they are prepared. How are they crosslinked Explain with the help of chemical reactions. 

A large number of polyesters is commercially available. These polymers are conveniently classified into the following types - linear unsaturated polyesters linear saturated polyesters of low molecular weight linear saturated polyesters of high molecular weight network polyesters poly(allyl ester)s and polycarbonates. Each of these groups is considered separately in this chapter. 

Linear unsaturated polyesters are prepared commercially by the reaction of a saturated diol with a mixture of an unsaturated dibasic acid and a modifying dibasic acid (or corresponding anhydrides). In principle, unsaturation desired in a polyester can be derived from either an unsaturated diol or an unsaturated acid for economic reasons the latter is invariably preferred. As mentioned previously, the unsaturated acid provides sites for subsequent cross-linking the function of the modifying acid is to reduce the number of reactive unsaturated sites along the polymer and hence to reduce the cross-link intensity and brittleness of the final product. Some acids and anhydrides which are used to modify polyesters are, in fact, unsaturated but the double bonds are not sufficiently reactive to represent sites for subsequent cross-linking. 

Propylene glycol is the diol most widely used for the manufacture of linear unsaturated polyesters it is prepared by the hydration of propylene oxide (Section 8.4.1.2) ... 

Maleic anhydride is the most important unsaturated component used in the manufacture of linear unsaturated polyesters. It is generally obtained by the oxidation of benzene ... 

Styrene is the most widely used cross-linking monomer, being preferred because of its compatibility, low viscosity, ease of use and low price. Other materials are sometimes employed when special properties are required. For example, methyl methacrylate is used, often in conjunction with styrene, for the preparation of translucent sheeting. Diallyl phthalate (X) and triallyl cyanurate (XI) are used for heat resistant products. Partially polymerized diallyl phthalate (solid) is used as the cross-linking agent in moulding powders (the so-called alkyd polyester moulding powders) based on linear unsaturated polyesters. 

Linear unsaturated polyesters are prepared batch-wise by heating a mixture of the appropriate acidic and hydroxy components in a reactor jacketed for heating and cooling and fitted for distillation. A typical formulation for a general-purpose material might be as follows ... 

It will be noted that the cross-linking of linear unsaturated polyesters by vinyl monomers does not involve the elimination of any volatile by-products. Hence it is possible to cure the resins without the application of pressure. Since it is also possible to cure the resins without the application of heat, they are very useful in the manufacture of large structures such as boat hulls. 

Example 2.3 Show (a) how a linear, unsaturated polyester is produced from ethylene glycol (I) and maleic anhydride (11), and (b) how the linear, unsaturated polyester is cross-linked with a vinyl monomer such as styrene. 

Commercially, the degree of polymerization x is maintained low (say 8-10 repeat units) so that the product is a viscous liquid. The linear, unsaturated polyester is then diluted with a liquid vinyl monomer, most often styrene. Before use, an initiator chemical that promotes addition polymerization (as discussed in Chapter 10) is added, causing the vinyl monomer to undergo addition copolymerization with the double bonds in the polyester (which has a fimctionahty of 16-20, twice the number of double bonds). This forms a highly cross-linked rigid network. 

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