Prepolymer unsaturated polyester

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

Ultralow-monol polyols, 223 Ultrapek, 327, 328 Ultraviolet (UV) radiation, 209 Ultraviolet spectroscopy, 490 Unimolecular micelle, 58 United States, phenolic production in, 375 Unsaturated maleate/O-phthalate/ 1,2-propanediol polyester prepolymer, 101-102 Unsaturated polyester resins (UPRs), 19, 29-30, 58-59... 

The crosslinking of unsaturated polyesters (Sec. 2-12a) is carried out by copolymerization [Selley, 1988]. Low-molecular-weight unsaturated polyester (prepolymer) and radical initiator are dissolved in a monomer, the mixture poured, sprayed, or otherwise shaped into the form of the desired final product, and then transformed into a thermoset by heating. Styrene is the most commonly used monomer. Vinyltoluene, methyl methacrylate, diallyl phthalate, a-methylstyrene, and triallyl cyanurate are also used, often together with styrene. 

In the case of unsaturated polyesters, nondegraded samples made from a prepolymer of molar mass M and a styrene mass fraction s have a chain-ends concentration b = [2(1 — s)/M]p, where p is the density. If ve is the actual concentration of elastically active network chains, an ideal network would be obtained by welding each chain end to another one, leading to... 

This approach can be illustrated by unsaturated polyesters based on an almost equimolar combination of maleate and phthalate of propylene glycol, crosslinked by styrene (45 wt%) (Mortaigne el al., 1992). Six samples differing by the prepolymer molar mass were analyzed. The chain-ends concentration, b, was determined by volumetric analysis of alcohols and acids in the initial reactive mixture. Then, the system was cured, elastic measurements were made in the rubbery state at Tg + 30° C, and the shear modulus G was plotted against chain-ends concentration (Fig. 14.7). The following relationship was obtained ... 

Resin Certain liquid prepolymer products, such as epoxies and unsaturated polyesters, which are subsequently cross-linked to form hardened polymer. 

Structopendant Prepolymers. The reactive or functional groups are distributed along the prepolymer chain. Examples are the epoxies, unsaturated polyesters, and the thermoset acrylics. 

The fourth type of composition is exemplified by unsaturated polyester prepolymers combined with a photoinitiator 1. A mixture of a tolylene dUsocyanate-modified maleic anhydride-phthalic anhydride-propylene... 

The experimental unsaturated polyester resins used in this study were supplied by several chemical companies since the PET chemical conversion into unsaturated polyester could not be done at the University of Texas laboratories. These resins were prepolymers with high viscosity. They were therefore diluted with styrene to reduce their viscosity and allow their further cure to a solid (polymer) upon the addition of suitable free radical initiators and promoters. The typical styrene content varied between 30 to 40% of the total resin weight, and viscosities were in the range of 100 to 1000 cps. 

Because of low cost, the most widely used unsaturated polyester polymer is in the form of 60% to 80% solutions of the prepolymer in copolymerizable monomers such as styrene and a mix of styrene with methyl methacrylate. During hardening, the polyester prepolymer and the monomer react through their unsaturated groups (double bonds). 

One of the major uses of diallyl esters is in the preparation of reinforced plastics. In this application, a lyoven fabric or mat prepared from glass or other fibers is saturated with a liquid resin or monomer containing a peroxide catalyst. The liquid resin may be a diallyl ester, a mixture of an unsaturated polyester resin and a vinyl or allyl monomer, or a mixture of monomer and prepolymer. The resih-monomir-fiber combination is placed in a mold and under the influence of heat and pressure is converted to a thermoset reinforced material. The steps are outlined in Fig. 15-42. 

Diallyl phthalate is used as a monomer as well as in prepolymer form in the preparation of reinforced plastics. Diallyl itaconate is also used as a cross-linking agent for unsaturated polyester resins as well as in cross-linking of vinyl copolymers. 

Unsaturated resins are usually mixtures of vinyl monomers and prepolymers, such as unsaturated polyesters, polyurethane acrylates, and ac-rylated epoxides of the bisphenol A type. Polymerization of styrene-based resins involves the formation of a three-dimensional network via the cross-linking of prepolymer chains by styrene radicals. These standard thermoset resins are therefore characterized by great hardness (Shore D over 80, DIN 53505, arbitrary scale 0-100 based on the penetration of a needle point in the tested material), do not melt, and are not soluble in organic solvents. 

Unsaturated polyester resins are produced by a crosslinking reaction in solution of a prepolymer dissolved in styrene initiated with peroxides. This curing reaction renders an insoluble and infusible crosslinked matrix with high rigidity, but fragile. This matrix is usually reinforced with glass fibers or fillers of micrometric size. Since the mechanical properties of such materials depend strongly on... 

Example 2.2 Unsaturated polyester resins, which are used as the matrix component of glass-fiber composites, may be obtained by the copolymerization of maleic anhydride and diethylene glycol. The low-molecular-weight product is soluble in styrene. Describe, with the aid of equations, the possible structures of the prepolymer and that of the polymer resulting from benzoyl peroxide-initiated polymerization of a solution of the prepolymer in styrene. 

Unsatuiated polyesters represent an excellent illustration of a polymeric system with a latent abihty to cross-link. Unsaturated polyesters are, as the name implies, unsaturated polyester prepolymers mixed with... 

The unsaturated polyester prepolymers are obtained from the condensation of polyhydric alcohols and dibasic acids. The dibasic acid consists of one or more saturated acid and/or unsaturated acid. The saturated acid may be phthalic anhydride, adipic acid, or isophthalic acid, while the unsaturated acid is usually maleic anhydride or fumaric acid. The polyhydric alcohols in common use include glycol (such as ethylene glycol, propylene glycol, diethylene glycol), glycerol, sorbitol, and pentaeiythritol (Equation 5.1). 

The reactive monomer responsible for the cross-linking reaction is normally styrene, vinyl toluene, methyl methacrylate, or diallyl phthalate. The unsaturated polyester prepolymers are, by themselves, relatively stable at ambient temperatures. When they are added to the cross-linking monomers, however, the blend is extremely unstable. To prevent premature gelation and to control the cross-linking process, inhibitors like hydroquinone are added to the blend. The cross-linking process is usually initiated by an organic peroxide added by the end user. The peroxide may be activated by heat. For room-temperature cure, accelerators and activators such as cobalt salts are also added. The cross-linking reaction can be represented as follows (Equation 5.2) ... 

As discussed in previous chapters, copolymraization may generate cross-linked structures. For example, dissolution of low-molecular-weight unsaturated polyesters, such as prepolymers with a radical initiator, in monomers such as methyl methacrylate or (hallyl phthalate leads, upon heating, to a cross-linked structure. 

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