Polyesters, unsaturated cross-linking

Alkyds are formulated from polyester resins, cross-linking monomers, and fillers of mineral or glass. The unsaturated polyester resins used for thermosetting alkyds are the reaction products of polyfunctional organic alcohols (glycols) and dibasic organic acids. 

One major use of polyesters, the alkyd coating resins, is based on random prepolymers reacted with unsaturated fatty acids. Gelation phenomena have been studied extensively and the gelation theories of Carothers, Flory, and Stock-mayer were developed with polyester random prepolymers. Only recently, Gordon (iJ, 12) has extended the gelation theory to the structopendant polyester system cross-linked by vinyl monomers. 

Photooxidation and Photostabilization of Unsaturated Cross-linked Polyesters... 

Sloane, Boerio and Koenig, and McGraw have described the sampling and other instrumental considerations for Raman spectra of polymers (144). Other reports on Raman investigations of polymers include molecular orientation in bulk polyethylene terephthalate (145). crystallinity of ethylene-propylene rubber (146). and the structure of unsaturated polyester resins cross-linked with styrene (147). 

FIGURE 4.12 Curing of unsaturated polyesters, (a) Species in polyester resin ready for laminating, (b) Structures present in cured polyester resin. Cross-linking takes place via an addition copolymerization reaction. The value of n 2-3 on average in general-purpose resins. (After Brydson, J. A. 1982. Plastics Materials. Butterworth Scientific, London, UK.)... 

Cross-linking by addition polymerization is also used to a considerable extent. Unsaturated polyesters are cross-linked by copolymerization with styrene or methyl methacrylate. Cross-linking soft, natural rubber with sulfur gives the normally used hard, vulcanized rubber. Ethylene-propylene rubbers can be cross-linked with peroxides. The cross-linking of elastomers is also called vulcanization, since the classic cross-linking of natural rubber, cis-l,4-poly(isoprene), uses heat and sulfur, which were the elements assigned to the god Vulcan (see also Chapter 37). 

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. 

The bis(dienes) [19] were also useful as cross-linking agents for linear poly(dienes) and unsaturated polyesters. The cross-linking reaction, presumably a Diels-Alder type, was claimed to yield products that possessed an attractive combination of mechanical, electrical, and adhesive properties (22). 

Organic peroxides are used extensively for the curing of unsaturated polyester resins and the polymerization of monomers having vinyl unsaturation. The —O—O— bond is split into free radicals which can initiate polymerization or cross-linking of various monomers or polymers. 

The presence of the unsaturated substituent along this polyester backbone gives this polymer crosslinking possibilities through a secondary reaction of the double bond. These polymers are used in paints, varnishes, and lacquers, where the ultimate cross-linked product results from the oxidation of the double bond as the coating cures. A cross-linked polyester could also result from reaction (5.J) without the unsaturated carboxylic acid, but the latter would produce a gel in which the entire reaction mass solidified and is not as well suited to coatings applications as the polymer that crosslinks upon drying. ... 

Diacyl peroxides are used in a broad spectmm of apphcations, including curing of unsaturated polyester resin compositions, cross-linking of elastomers, production of poly(vinyl chloride), polystyrene, and polyacrjlates, and in many nonpolymeric addition reactions. 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

The second largest use at 21% is for unsaturated polyester resins, which are the products of polycondensation reactions between molar equivalents of certain dicarboxyhc acids or thek anhydrides and glycols. One component, usually the diacid or anhydride, must be unsaturated. A vinyl monomer, usually styrene, is a diluent which later serves to fully cross-link the unsaturated portion of the polycondensate when a catalyst, usually a peroxide, is added. The diacids or anhydrides are usually phthahc anhydride, isophthahc acid, and maleic anhydride. Maleic anhydride provides the unsaturated bonds. The exact composition is adjusted to obtain the requked performance. Resins based on phthahc anhydride are used in boat hulls, tubs and spas, constmction, and synthetic marble surfaces. In most cases, the resins contain mineral or glass fibers that provide the requked stmctural strength. The market for the resins tends to be cychcal because products made from them sell far better in good economic times (see Polyesters,unsaturated). 

Uses. About 35% of the isophthahc acid is used to prepare unsaturated polyester resins. These are condensation products of isophthahc acid, an unsaturated dibasic acid, most likely maleic anhydride, and a glycol such as propylene glycol. The polymer is dissolved in an inhibited vinyl monomer, usually styrene with a quinone inhibitor. When this viscous hquid is treated with a catalyst, heat or free-radical initiation causes cross-linking and sohdification. A range of properties is possible depending on the reactants used and their ratios (97). 

Performance Characteristics Polyester resins undergo a rapid transformation from a viscous Hquid to a soHd plastic state that comprises a three-dimensional cross-linked polymer stmcture. The level of polyester polymer unsaturation determines essential performance characteristics (Table 7), although polymer components can influence subtle features that affect thermal, electrical, and mechanical performance as defined by ASTM procedures. 

Polyester Resins. Reinforced polyester resins are thermosets based on unsaturated polyesters from glycols and dibasic acids, either or both of which contain reactive double bonds. The ratio of saturated to unsaturated components controls the degree of cross-linking and thus the rigidity of the product (see Polyesters, unsaturated). Typically, the glycols and acids are esterified until a viscous Hquid results, to which an inhibitor is added to prevent premature gelation. Addition of the monomer, usually styrene, reduces the viscosity to an easily workable level. 

Alkyds. Alkyd resins (qv) are polyesters formed by the reaction of polybasic acids, unsaturated fatty acids, and polyhydric alcohols (see Alcohols, POLYHYDRic). Modified alkyds are made when epoxy, sUicone, urethane, or vinyl resins take part in this reaction. The resins cross-link by reaction with oxygen in the air, and carboxylate salts of cobalt, chromium, manganese, zinc, or zirconium are included in the formulation to catalyze drying. 

Polyesters. Unsaturated polyester resins based on DCPD, maleic anhydride, and glycols have been manufactured for many years. At least four ways of incorporating DCPD into these resins have been described (45). The resins are mixed with a cross-linking compound, usually styrene, and final polymerization is accompHshed via a free-radical initiator such as methyl ethyl ketone peroxide. 

An alternative route to cross-linking is to start with a linear polymer and then cross-link the molecules by tying the molecule through some reactive group. For example, it is possible to cross-link unsaturated polyesters by an addition polymerisation across the double bond as shown schematically in Figure 2.12. 

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