Polyesters, linear unsaturated preparation

Esters. Neopentyl glycol diesters are usually Hquids or low melting soflds. Polyesters of neopentyl glycol, and in particular unsaturated polyesters, are prepared by reaction with polybasic acids at atmospheric pressure. High molecular weight linear polyesters (qv) are prepared by the reaction of neopentyl glycol and the ester (usually the methyl ester) of a dibasic acid through transesterification (37—38). The reaction is usually performed at elevated temperatures, in vacuo, in the presence of a metallic catalyst. 

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... 

In a similar report utilizing triethylene glycol dimethacrylate, the polymer was described as a linear polyester.330 Again, this description is probably incomplete, though it is possible at high catalyst loadings to prepare methacrylic dimers. Under ideal conditions, this would lead to the linear, unsaturated polymer 97. 

Poly(ethylene terephthalate) is a linear polyester obtained from the reaction of difunctional monomers. Branched or network polyesters are obtained if at least one of the reagents is tri- or multifunctional. This can be achieved either by the use of polyols such as glycerol in the case of saturated polyesters (glyptal) or by the use of unsaturated dicarboxyhc acids such as maleic anhydride in the ease of unsaturated polyester. In the preparation of glyptal, glycerol and phthalic anhydride reaet to form a viseous liquid initially, which on further reaction hardens as a result of network formation (Equation 2.31). 

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

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. 

Propylene glycol is the diol most widely used for the manufacture of linear unsaturated polyesters it is prepared by the hydration of propylene oxide (Section 8.4.1.2) ... 

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. 

Linear unsaturated polyesters are prepared batch-wise by heating a mixture of the appropriate acidic and hydroxy components in a reactor jacketed for heating and cooling and fitted for distillation. A typical formulation for a general-purpose material might be as follows ... 

The products to be considered in this section are commonly called polyester resins. However, they need better definition to distinguish them from several other commercial polyesters which are also called resins. Perhaps the shortest definition is that an unsaturated polyester resin is composed of two major components, one of which is a linear polyester prepared from a glycol and an unsaturated dibasic acid, and the second is a vinyl monomer in which the first is soluble and with which it will copolymerize. The sirup so formed is commonly but incorrectly called a polyester resin. No really effective definition has been devised. The above definition is satisfactory for the sirup. The common name is used for both the copoly-merizable solution and the pol merized final product. In the latter state, the unsaturation has ceased to exist. 

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. 

Unsaturated low molecular weight (M between 1000 and 10000), often branched, used as macromonomers for synthesis of thermosets (polyester resins), or thermosetting materials by themselves (alkyd resins). They are prepared from several monomers, namely phthalic and maleic anhydrides, adipic acid, iso-phthalic acid, natural fatty acids or triglycerides, and a great variety of multifunctional alcohols. In a few special cases, they may be saturated and/or linear for use as macromonomers in the production of polyurethanes or other polymers. 

Unsaturated polyesters are based on macromolecules with a polyester backbone in which an unsaturated acid or combination of a saturated with an unsaturated acid are condensed with a glycol. A three-dimensional structure is produced when the macromolecule is crosslinked through the unsaturation. Commercial unsaturated polyester resin formulations, neglecting consideration of additives, initiators, extenders, and fibrous reinforcing materials, consist essentially of a linear resin, a crosslinking (reactive diluent) monomer (ca. 18-40 wt. %), and inhibitors to retard crosslinking until the resin is used by the fabricator. The simplest member of the polyester series, ethylene maleate (or ethylene fumarate), is prepared as follows ... 

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... 

These unsaturated, non-linear resins are similar to epoxies and phenolics in their surface-preparation requirements, basically requiring only sanding for good results. Polyesters have good resistance to oils and solvents. The method of surface preparation given for epoxies in Section 7.3.2 is suggested. 

---