Dihydroxy alcohols

Polyesters. Polyester is used in embedding resins for electronic components because of its low cost compared to siUcones and epoxides. Polyesters (qv) are condensation products of dicarboxyhc acids and dihydroxy alcohols the reaction provides a wide range of viscosities for polyesters. 

Polyesters. The polycondensation prods of di-carboxylic acids with dihydroxy alcohols. 

There has in the past been some confusion in the use of the term alkyd, which is said to have been derived from alcohol plus acid. The definition offered by Kienle [1], discussed later, is broad enough to include all polyesters derived essentially from diols and dicarboxylic acids, and consequently linear polyesters were initially included in this class of polymer. On the other hand, Bjorksten et al. [2], in their 1956 compilation of published information about polyesters, restrict the term polyester to the polycondensation products of dicarboxylic acids with dihydroxy alcohols, and say that this definition does not include materials commonly known as alkyds . At the present time, there are still problems of nomenclature in the fibre field arising from the use of polyester as a generic term to cover fibres containing only a very restricted range of chemical groups. 

The pnndpal unsaturated acids used are maleic and fumaric. Saturated acids, usually phthahe and adipic, may also be included. The function of these acids is to reduce the amount of unsaturation in the final resin, making it tougher and more flexible. The acid anhydrides are often used if available and applicable. The dihydroxy alcohols most generally used are ethylene, propylene, diethylene, and dipropylene glycols. Styrene and diallyl philialate are the most common cross-linking agents, Polyesters are resistant to corrosion, chemicals, solvents, etc. 

Dihydroxy alcohols and amino alcohols in which the functional groups are not separated by more than four carbon atoms are effective acceptors. There is a general difference in the behavior of these compounds, however, in that hydroxyl alcohols decrease the rate of formation of p-nitro-phenol from p-nitrophenyl phosphate and amino alcohols increase the rate. Amino alcohols are relatively good nucleophiles in reaction with phosphate esters, perhaps because the amino group serves as a general base catalyst (143, 144)-... 

Polyester A resin formed by the reaction between a dibasic acid and a dihydroxy alcohol, both organic. Modification with multi-functional acids and/or bases and some unsaturated reactants permit crosslinking to thermosetting resins. Polyesters modified with fatty acids are called alkyds. 

Certain oxidizing agents convert alkenes into compounds known as glycols. Glycols-are simply dihydroxy alcohols their formation amounts to yitp[)f jlition of two hydroxyl groups to the double bond. 

Actually, of course, we have already encountered numerous heterocyclic compounds cyclic anhydrides (Sec. 20.9) and cyclic Mdes (Sec. 20.14), for example lactones (Sec. 20.15) and lactams (Problem 28.3, p. 891) cyclic acetals of dihydroxy alcohols (Problem 23, p. 651) the solvents dioxane and tetrahydrofuran (Sec. 17.9). In all these, the chemistry is essentially that of their open-chain analogs. 

The soft segments are composed of prepolymerized polyethers or polyesters with terminal hydroxyl groups, the molecular weights usually falling within the range of 1000 to 3000. Polyethers are obtained from propylene oxide and tetrahydrofuran. Dihydroxy alcohols and dicarboxylic acids give polyesters with hydroxyl groups at each end as follows ... 

It does not form, however, an addition-product with ammonia similar to that derived from aldehyde, but complex condensation products. When reduced by nascent hydrogen generated by the action of sodium amalgam on water, addition takes place as indicated above, and isopropyl alcohol is formed, but at the same time a dihydroxy alcohol, which contains six carbon atoms, is produced in appreciable quantities. This substance, called pinacol, is the first member of a series of such alcohols which are formed by the reduction of ketones. The equation for the reaction in the case of acetone is,... 

II. Dihydroxy Alcohols III. Trihydroxy Alcohols IV. Polyhydroxy Alcohols... 

Glycols Aliphatic alcohols with two hydroxy groups attached to a carbon chain. Can be produced by oxidation of alkenes followed by hydration. Also ealled Dihydrie Alcohols and Dihydroxy Alcohols. 

Carotenoid esters have been reported from a few plant sources (Hitchcock and Nichols, 1971). Flower pigments, for example, are known to contain saturated fatty acid esters of carotenoid alcohols. These compounds are also found in green algae. Where dihydroxy alcohols are involved then both substituents are usually esterified, for example ... 

In 1937 Carlton Ellis found that unsaturated polyesters, which are condensation products of unsaturated dicarboxylic acids and dihydroxy alcohols, would freely co-polymerize with monomers which contained double bond imsaturation, yielding rigid thermosetting resins. The allylic resins became commercial resins suitable for compounding into a broad range of products with exceptional electrical, mechanical, thermal, and chemical properties. 

A good example of difference in molecular requirements for linear and network polymers is the formation of polyesters from dihydroxy or trihydroxy alcohols. As was shown on page 554 for diamines, reaction of a dihydroxy alcohol with a diacid gives a linear copolymer. If a trihydroxy alcohol is used instead, a network copolymer is formed. An example is the class of ALKYD resins (Fig. 26.3), produced from glycerol and phthalic acid, and used as ingredients of paints and varnishes. 

Dihydroxy alcohols (diols) and trihydroxy alcohols (triols) are viscous, hygroscopic, high boiling point liquids with a sweetish taste. Some of them are useful as solvents for pharmaceutical use. In the case of diols the two hydroxyl groups are usually attached to different carbon atoms. If the hydroxyl groups are attached to the same carbon an atom of water is eliminated to give a carbonyl compound (Fig. 5.5). There are exceptions to this for example, in the case of chloral hydrate the diol is stabilised by the electron-withdrawing chlorine atoms. 

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