Unsaturated Polyester Resins and Composites

The advantages of powder coating compared to the more classic type of coatings in solution are clear they are 100% solvent-free and thick layers up to 500 pm can be applied in one step. Moreover, they are very cost-eflRcient owing to a very high application efficiency, close to 100%, as over-sprayed powder can be easily recycled. 

Last but not least, their mechanical properties are usually equal or superior to those of their solvent-borne counterparts. 

Inhibitors hydroquinone / berrzoquinone tert butyl catechol 

Methyl ethyl ketone peroxide Benzoyl peroxide Cumene hydroperoxide tert Butyl perbenzoate 

Chopped rovings Chopped strand mat Woven rovings 

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Preparation of a Composite Material from an Unsaturated Polyester Resin and Glass Fibers... 

For commodity applications, there are four major classes of resins that are used in FRP applications. They are phenolic resin, epoxy resin, unsaturated polyester resin, and epoxy vinyl ester resins. A more complete description of these types of resins and their many variations can be found in Handbook of Thermoset Plastics. This is not a comprehensive list of resins used in composite manufacture, as commodity materials like polyurethanes and isocyanurate resins are sometimes used as well to make FRP parts. However, these materials are not covered in this chapter owing to their limited use, but, the principals of fire safety that apply for the resins described subsequently apply to these materials as well. 

Propylene oxide, made by reacting propylene with chlorine to form propylene chloro-hydrin which is then dehydrochlorinated with caustic soda or lime (Eqs. 3-5), is used in the production of polyether polyols used for producing urethane foam. It also finds use in propylene glycol for making unsaturated polyester resins and in the pharmaceutical and food industries. Epichlorohydrin (EPI), formed by chlorination of propylene to allyl chloride and then dehydrochlorination (Eqs. 6 and 7), is used to make epoxy resins for producing laminates, fiber-reinforced composites, protective coatings, and adhesives. 

Wood-plastic composites can be further divided into wood-thermosetting composites and wood-thermoplastic composites. Thermosets or thermoset polymers have the property of becoming pennanentiy hard and rigid once been heated or cured. Such polymers contain phenol resins, epoxy resins, unsaturated polyester resins, and so on. Among them, phenol formaldehyde (PF) and urea formaldehyde (UP) are the most widely used thermosets for wood composites. The birth of wood-thermosetting composites can date back to the early 1900s. In 1907, Dr. Leoh... 

The use of accelerated ageing tests and spectrophotometric techniques in studying the photooxidative degradation of unsaturated polyester resins and glass fibre-reinforced composites is examined. Results of accelerated and natural ageing tests are compared, and criteria to be taken into account in achieving accelerated test results representative of reality are discussed. 5 refs. 

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. 

Aromatic diacyl peroxides such as dibenzoyl peroxide (BPO) [94-36-0] may be used with promoters to lower the usehil decomposition temperatures of the peroxides, although usually with some sacrifice to radical generation efficiency. The most widely used promoter is dimethylaniline (DMA). The BPO—DMA combination is used for hardening (curing) of unsaturated polyester resin compositions, eg, body putty in auto repair kits. Here, the aromatic amine promoter attacks the BPO to initially form W-benzoyloxydimethylanilinium benzoate (ion pair) which subsequentiy decomposes at room temperature to form a benzoate ion, a dimethylaniline radical cation, and a benzoyloxy radical that, in turn, initiates the curing reaction (33) ... 

Ketone Peroxides. These materials are mixtures of compounds with hydroperoxy groups and are composed primarily of the two stmctures shown in Table 2. Ketone peroxides are marketed as solutions in inert solvents such as dimethyl phthalate. They are primarily employed in room-temperature-initiated curing of unsaturated polyester resin compositions (usually containing styrene monomer) using transition-metal promoters such as cobalt naphthenate. Ketone peroxides contain the hydroperoxy (—OOH) group and thus are susceptible to the same ha2ards as hydroperoxides. 

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). 

Thermosetting unsaturated polyester resins constitute the most common fiber-reinforced composite matrix today. According to the Committee on Resin Statistics of the Society of Plastics Industry (SPl), 454,000 t of unsaturated polyester were used in fiber-reinforced plastics in 1990. These materials are popular because of thek low price, ease of use, and excellent mechanical and chemical resistance properties. Over 227 t of phenoHc resins were used in fiber-reinforced plastics in 1990 (1 3). PhenoHc resins (qv) are used when thek inherent flame retardance, high temperature resistance, or low cost overcome the problems of processing difficulties and lower mechanical properties. 

Unsaturated polyester resins predominate among fiber-reinforced composite matrices for several reasons. A wide variety of polyesters is available and the composites fabricator must choose the best for a particular appHcation. The choice involves evaluation of fabrication techniques, temperatures at which the resin is to be handled, cure time and temperature desked, and requked cured properties (see Polyesters, unsaturated). 

The surface energy of fibers is closely related to the hydrophilicity of the fiber [38]. Some investigations are concerned with methods to decrease hydrophilicity. The modification, of wood cellulose fibers with stearic acid [43] hydrophobizes those fibers and improves their dispersion in polypropylene. As can be observed in jute-reinforced unsaturated polyester resin composites, treatment with polyvinylacetate increases the mechanical properties [24] and moisture repellency. 

Finally, glass-reinforced unsaturated polyesters are well known as building materials for boats, yachts and cars and generally termed fibre-glass resin . The preparation, properties and applications of these unsaturated polyesters are summarized in Chapter 21, while the chemistry and properties of a new unsaturated polyester resin for fibre-reinforced composite materials are discussed in Chapter 22. 

By far the most common form of reinforcement is fiberglass. Products using unsaturated polyester resin as a matrix and fiberglass fiber reinforcements are commonly referred to as composites , laminates or FRPs (fiber-reinforced plastics). The latter reinforcements are sold as continuous roving, which is continuously chopped in place with a liquid resin stream, chopped roving mat, woven... 

Sheet molding compounds (SMCs) and bulk molding compounds (BMCs) are the dominant materials used in automotive applications. These composites of unsaturated polyester resin, fillers and fiberglass have advantages of high stiffness, heat resistance and low coefficient of expansion. Coupled with low creep resistance, which is a distinct advantage over thermoplastic competition, and low-profile additives, which can yield Class A surfaces, these materials are well suited for applications from exterior body panels to under the hood components. 

New and promising composite pre-form technology which can be in situ molded with unsaturated polyester resins to further improve structure and performance have been demonstrated. 

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