Polyesters, linear unsaturated cross-linking

Copolymerization is by no means restricted to two monomeric components. Copolymerization is also the term used to describe the uniting of linear polymers or polycondensates that still possess a reactive component (trifunctional monomers) with polymerizable (bifunctional) monomers. The result is a cross-linked plastic, for example unsaturated polyester -i- styrene cross-linked polyester resin. 

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. 

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... 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. The most common unsaturated polyester is made by step growth polymerization of propylene glycol with phthalic and maleic anhydrides. Subsequent treatment with styrene and a peroxide catalyst leads to a solid, infusible thermoset. 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. 

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),... 

Polymers are normally classified into four main architectural types linear (which includes rigid rod, flexible coil, cyclic, and polyrotaxane structures) branched (including random, regular comb-like, and star shaped) cross-linked (which includes the interpenetrating networks (IPNs)) and fairly recently the dendritic or hyperbranched polymers. I shall cover in some detail the first three types, but as we went to press very little DM work has been performed yet on the hyperbranched ones, which show some interesting properties. (Compared to linear polymers, solutions show a much lower viscosity and appear to be Newtonian rather than shear thinning [134].) Johansson [135] compares DM properties of some hyperbranched acrylates, alkyds. and unsaturated polyesters and notes that the properties of his cured resins so far are rather similar to conventional polyester systems. 

The free radicals first react with the chemical inhibitor which has previously been added to the resin, since the inhibitor material must be chemically dissipated before any reaction between free radicals and the C=C double bonds can proceed [5]. Apparently, the free radicals serve to open the double bonds in the polyester linear chain to set in motion that portion of the polymerization process designated as initiation. Either the opened double bonds react with the vinyl groups of the monomer, or the free radicals serve to also open (add to) these latter unsaturated C=C bonds, permitting them to perform their cross-linking function, uniting the polyester chains into a three-dimensional network. There is further evidence that free radicals may also, to some degree, react with the unsaturated monomer to form various products of decomposition [5]. 

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. 

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

The usefulness of a number of plastics, such as unsaturated polyester, epoxy and phenolic resins, is limited unless their linear polymer chains are cross-linked or cured. The various curing agents and compounds used as initiators (accelerators or catalysts) are discussed under the various plastics. 

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. 

It is possible to cross-link unsaturated linear polyester chains directly one to another however, reaction is slow and a low degree of cross-linking is achieved. These limitations are overcome by the introduction of a material which forms bridges between the chains. The materials most commonly used to cross-link unsaturated linear polyesters in this way are vinyl monomers. The addition of a liquid vinyl monomer to the polymer also leads to a reduction in viscosity and this facilitates the impregnation of glass-fibre in the preparation of laminates. 

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. 

As has been noted previously, the cross-linking of unsaturated linear polyesters involves the reaction of the unsaturated sites in the polymer chain with a vinyl-type monomer. This reaction is analogous to conventional vinyl copolymerization and proceeds by an essentially similar mechanism. As carried out in commercial practice, cross-linking of unsaturated polyesters is invariably a free radical reaction. Two types of initiating systems are commonly employed for this reaction, namely those effective at elevated temperatures and those effective at room temperature. 

The cross-linking of an unsaturated linear polyester by means of a vinyl monomer such as styrene may be represented 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. 

Plastic is a material that can be plasticized into certain shapes under certain conditions (temperature, pressure, etc.) and can keep its shape unchanged at room temperature and normal atmosphere pressure. According to their performance after heat treatment, plastics can be divided into thermoplastic and thermosetting plastics. A thermoplastic plastic is generally a linear or branched polymer. It melts when heated and solidifies when cooled, and this kind of behavior can be repeated, so the plastic can be used multiple times. The main varieties are polyethylene, polypropylene, polyvinyl chloride, polystyrene, and acrylonitrile-butadiene-styrene terpolymer. Thermosetting plastic is a space network polymer, which is formed by direct polymerization of monomers or by cross-linking of linear prepolymers. Once the solidification is finished, the polymer cannot be heated back to the plasticizing state. The main varieties are phenolic resin, epoxy resin, amino resin, and unsaturated polyester. 

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. 

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). 

The polymers making up this first group of polyesters are linear polyesters containing aliphatic unsaturation which provides sites for subsequent cross-linking. A polymer of this type first became available in the USA in 1946 the polymer was prepared from diethylene glycol and maleic anhydride and... 

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