Preparation of Unsaturated Polyesters

Safety precautions Before this experiment is carried out. Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. 

Polycondensation of a diol with a dicarboxylic acid, either of which may contain a double bond, results in an unsaturated polyester. For this purpose suitable starting compounds are maleic anhydride and 2-butylene-1,4-diol.These can also be used mixed with saturated dicarboxylic acids or diols (copolycondensation) in order to vary the number of double bonds per macromolecule and thereby the properties of the polyester. Unsaturated polyesters are generally prepared by melt condensation.The resulting products are often viscous or waxy substances of relatively low molecular weight. 

The incorporation of double bonds into polyesters provides the possibility for subsequent reactions. Of particular interest in this context are reactions that lead to crosslinking. 

The crosslinking copolymerization of unsaturated polyesters with styrene is utilized industrially for the production of surface coatings. More extensive are applications in fiber-reinforced plastics where the unsaturated polyesters are used as a matrix for inorganic and organic fibers (see Example 5-20). Areas of application are (large) parts in boats, vehicles, and sport equipment construction. 

1-2 g of the polyester (without styrene) are dissolved by warming with 50 ml acetone and, after cooling, are titrated as quickly as possible with 0.1-N alcoholic potassium hydroxide using phenolphthalein as indicator, until the red color remains 

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The largest single use of maleic anhydride is in the preparation of unsaturated polyester resins. It is first esterihed with a polyalcohol (two or more hydroxyls) and then the double bond is copolymerized (crosslinked) with a vinyl monomer such as styrene to form a rigid structure. Such resins are usually reinforced with hberglass (FRP). Maleic anhydride is also used to make oil additives and agricultural chemicals. 

GENERAL PROCEDURE FOR THE PREPARATION OF UNSATURATED POLYESTER RESIN FROM A POLYETHER POLYOL... 

Table 1. Recipes (by molar ratios) for preparation of unsaturated polyester resin from recycled PET...

Maleic anhydride can also be made by the air oxidation of butenes. Maleic anhydride is used in the preparation of unsaturated polyester resins. It can also be reduced to buty-rolactone and to 1,4-butanediol. It also offers a possible synthesis of phthalic anhydride by a Diels-Alder reaction with 1,3-butadiene (12.9). 

Researchers also used silane as a compatibilizing agent in the preparation of unsaturated polyester-clay nanocomposites [23]. A lot of commercially modified clays like the Closite series and Nanomer series are readily available in the market from Southern Clay Products and Nanomer Inc., respectively. 

Modifying components other than phthalic anhydride are frequently used in the preparation of unsaturated polyesters in order to impart special properties to the final product. Adipic and sebacic acids are employed to give flexible materials and isophthalic acid (VI) is used for tough products with higher heat distortion temperatures. The use of e (iomethylenetetrahydrophthalic anhydride ( nadic anhydride ) (VII), the Diels-Alder reaction product of cyclopentadiene and maleic anhydride, leads to a substantial improvement in heat resistance. Flame resistant materials are obtained by the use of chlorinated acids and anhydrides, e.g., tetrachlorophthalic anhydride (VIII) and chlorendic acid (HET acid) (IX). The latter is prepared by the Diels-Alder reaction of hexachlorocyclopentadiene and maleic anhydride the initial product is chlorendic anhydride but this rapidly absorbs water from the air to give the acid. 

In addition to having the vapor pressure advantage of vinyl-toluene, PMS also provides an Improvement in thermal properties (probably associated with the high amount of para-isomer) when used in the preparation of unsaturated polyester moldings ... 

Regardless of the reasons, however, the theoretical calculation usually gives the lower limit, or most conservative value, for the gel-point conversion, giving a margin of safety in practice. An apparent exception can sometimes occur. In concentrated solutions, physical entanglements of chains can cause a polymer to behave as a crosslinked gel at conversions lower than that predicted by this theory. Also, in the preparation of unsaturated polyester... 

Other acids and anhydrides. Modifying components other than phthalic anhydride are frequently used in the preparation of unsaturated polyesters in order to impart special properties to the final product. Adipic and sebacic acids are employed to give flexible materials and isophthalic acid (VI) is used... 

Chlorendic Acid. Chlorendic acid [115-28-6] and its anhydride [115-27-5] are widely used flame retardants. Chlorendic acid is synthesized by a Diels-Alder reaction of maleic anhydride and hexachlorocyclopentadiene (see CyclopentadlENE and dicyclopentadiente) in toluene followed by hydrolysis of the anhydride using aqueous base (60). The anhydride can be isolated directly from the reaction mixture or can be prepared in a very pure form by dehydration of the acid. The principal use of chlorendic anhydride and chlorendic acid has been in the manufacture of unsaturated polyester resins. Because the esterification rate of chlorendic anhydride is similar to that of phthalic anhydride, it can be used in place of phthalic anhydride in commercial polyester... 

Because of analogy of radical formation by iron(II) ion from either peroxides or oxaziridines, the latter were proposed repeatedly as initiators of radical chains, e.g. in styrene polymerization and in treatment of unsaturated polyesters. Oxaziridines appear to be easier to prepare than peroxides and to be less hazardous in handling (76MI50801). 

As small molecule fragments resulting from the initiator may plasticize the polymer and lower performance, approaches have been developed to avoid this. A dihydroxyamine can be used to form a polyester [52]. This accelerator gave a modest increase in the strength of unsaturated polyester resins. A polymerizable tertiary amine has been prepared by the reaction of A-methylaniline with glycidyl methacrylate [53] (Scheme 8). 

Preparation and Cure of Unsaturated Polyester/Styrene Resin... 

The chemicals used in syntheses were used as received from the supplier. The unsaturated polyesters were prepared by standard methods from the diacids and diols using butyl stannoic acid (Fascat 4100) as the catalyst. The water of condensation removed using a xylene azeotrope and after reaching the desired acid value the xylene was removed by distillation. The synthesis of unsaturated polyester A is representative. 

Synthesis of Unsaturated Polyester A. The unsaturated polyester oligomer A was prepared in a 500 milliliter 4-neck flask fitted with a variable speed stirrer, a heating mantle, a gas inlet tube, a thermometer, a Dean-Starke trap and a condenser. The flask was charged with 147.0 g (1.5 mol) of maleic anhydride (Amoco), 195 g (1.88 mol) of 1,5-pentane diol (BASF), about 35 ml of xylene and 0.3 g of Fascat 4100. Then heated slowly under a nitrogen blanket to 175° C while the water of condensation was removed... 

Variation of Polyester Molecular Weight. A series of unsaturated polyester (UPE) oligomers were prepared by reacting maleic anhydride with 1,5-pentane diol. (Scheme... 

Unsaturated polyesters are low-molecular-weight fumarate esters containing various chemical structures designed for their specific cost and performance purposes. The two most important features of unsaturated polyesters are the fumarates, which provide the active sites for radical cross-linking with the diluent monomer and the random, low molecular weight, irregular nature of the rest of the molecule, which provide the necessary solubility in the diluent monomer. The preparation of the polyester thus requires the following considerations ... 

The most widely used graft copolymer is the styrene-unsaturated polyester copolymer (Equation 7.35). This copolymer, which is usually reinforced by fibrous glass, is prepared by the free radical chain polymerization of a styrene solution of unsaturated polyester. 

Lecithin has some catalytic or cocatalytic effects in multiphase systems because of its surface-active properties. Lecithin is reported to be useful as an emulsifier in the curing of aqueous dispersions of unsaturated polyesters (337). The products are more easily removed from their molds and have improved mechanical properties when lecithin is used. In a fermentation application, 1.5% soybean lecithin acts as an inducer in the preparation of cholesterol esterase using a strain of Pseudomonas bacteria (338). Aside from its role as a catalyst, initiator, or modifier, lecithin may have ancillary uses in catalyst systems as part of a protective coating (339). 

In addition to the possible reuse of BHET in the preparation of fresh PET polymer, two other processes have been proposed starting from the PET glycolysis products preparation of unsaturated polyesters20-27 and synthesis of polyester polyols.28-32 The latter can be used in the formulation of polyurethanes and polyisocyanurate foams26-30 (see Figure 2.1). 

A wide range of chemical agents, catalysts and conditions for the glycolysis of unsaturated polyester resins, used in the manufacture of buttons, have been described in a recent patent.34 In addition to different metal acetates, the following compounds have been proposed to be catalytically active in PET glycolysis sodium methylate, sodium ethylate, sodium hydroxide, methane-sulfonic acid, magnesium oxide, barium oxide and calcium oxide. Different applications of the depolymerization products were described, e.g., preparation of fresh unsaturated polyesters by reaction with maleic acid, maleic acid/ phthalic anhydride or maleic anhydride/terephthalic acid or the synthesis of polyurethane resins by reaction with a diisocyanate. 

PET glycolysis, mainly by reaction with ethylene glycol, leads to the formation of BHET, which can be used as a monomer for the preparation of fresh PET or, alternatively, it can be utilized in the formulation of unsaturated polyesters and polyurethane and polyisocyanurate foams through the corresponding polyesterification reactions. 

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