Polyester prepolymers, unsaturated

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). 

Unsaturated Maleate/O-Phthalate/1,2-Propanediol Polyester Prepolymer... 

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... 

There are numerous possible monomers that can be used in the backbone of the polyester prepolymer. However, typical monomers are 1,2-propanediol, as just mentioned, with maleic acid (usually added as the anhydride) to provide the sites of unsaturation, and phthalic acid (again usually added as the anhydride) to act as the second of the two diacid species. The structures of these latter two substances are shown in Figure 4.1. 

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. 

Crosslinks can be controlled by the number of unsaturated sites in the polyester prepolymer. Theoretically if each molecule has only two reaction sites, then infinite, almost linear, chains could be obtained. Hence, average functionability and molecular weight distribution in the prepolymer are extremely important. Plasticizers can be used to advantage in adjusting the average properties of the binder as obtained in the solid propellant formulation. 

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... 

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). 

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) ... 

The three polyester prepolymers prepared having varying viscosity (different di/tri acid ratio) with essentially the same hydroxyl content were used to define optimum reaction conditions for introduction of unsaturated groups via addition of glycidyl methacrylate to the liable hydroxyls. 

CLASS Thermoset polymers (mixtures of polyester prepolymers with aliphatic unsaturation and a vinyl monomer)... 

The resulting dispersion is blended with an appropriate amount of an unsaturated polyester prepolymer prepared from orthophthalic acid, maleic anhydride, and neopentyl glycol, with a 5% excess of the last component, P3. Molding polymerizes the styrene component, P4, to form C34. The overall composition is 20% rubber for a total of 40% by weight graded-rubber particles and 60% by weight of polyester-styrene present in weight ratio 65 35. 

Condensation of an alcohol and an organic acid, forming an ester — This is the unsaturated polyester prepolymer. It is dissolved in an unsaturated monomer (usually styrene or a similar vinyl-type monomer) to form a resin. 

Crosslinking of the polyester prepolymer using the unsaturated monomer — A peroxide catalyst is added to the resin so that a free radical addition reaction can occur, transforming the liquid resin into a solid film [17]. 

The extent to which a particular polyester is vulnerable to alkali attack depends on the combination of reactants used to form the polyester prepolymer and the unsaturated monomer with which it is crosslinked. 

The so-called conventional unsaturated polyester prepolymer is characterized by the presence, along the backbone chains, of randomly distributed ester groups and carbon-carbon double bonds. The molecular structure may be schematically represented as follows ... 

It was found that PBNCO and the unsaturated polyester prepolymer easily react through a condensation reaction that leads to the formation of urethane (—O—C=0—NH—) linkages. 

Maleopimaric acid, diethylene glycol and maleic anhydride in the molar proportions 1 4 3 were used in the synthesis of unsaturated polyester prepolymer [15]. Levopimaric acid was obtained by the acid isomerization of natural rosin or abietic acid and used for the synthesis of maleopimaric acid (Fig. 3). The reaction of maleopimaric acid with a suitable excess of diethylene glycol, carried out at a temperature reasonably lower (220 °C) than the... 

The use of phthalic anhydride as comonomer for the preparation of polyester prepolymer lowers its unsaturation content and reduces the cross-link density of the... 

The cheapest glass-reinforcement material is E-glass, often used as a roving, or a collection of parallel continuous filaments. Among the polymer matrices, polyester and epoxy resins, which we discuss shortly, are commonly employed. An unsaturated polyester prepolymer is first prepared by reacting maleic acid with diethylene glycol ... 

---