Polyols hydroxy-terminated

Hie most representative member of this class of polyesters is the low-molar-mass (M 1000-3000) hydroxy-terminated aliphatic poly(2,2/-oxydiethylene adipate) obtained by esterification between adipic acid and diethylene glycol. This oligomer is used as a macromonomer in the synthesis of polyurethane elastomers and flexible foams by reaction with diisocyanates (see Chapter 5). Hydroxy-terminated poly(f -caprolactonc) and copolyesters of various diols or polyols and diacids, such as o-phthalic acid or hydroxy acids, broaden the range of properties and applications of polyester polyols. 

Polyester polyols account for only ca. 10% of the total polyol market, which is dominated by polyether polyols such as hydroxy-terminated polyoxyethylene or polyoxypropylene. Polyester polyols are preferred for applications where better mechanical properties, wear resistance, and UV stability are required. The largest application of polyester polyols is flexible specialty polyurethane foam in the furniture, packaging, and automotive industries. Polyester polyols are also used for nonfoam applications such as coatings, paints, sealants, and adhesives 47... 

Many diols and polyols like 1, 4-butanediol and hydroxy-terminated polyesters or polyethers or polyesteramides are used for reaction with diisocyanates commercially. 

Hydroxy-terminated natural rubber (HTNR) It is a honey-colored viscous liquid and is prepared by the depolymerization of masticated natural rubber (NR) in the presence of H202 in toluene. Similar to other polyols, it is curable with TDI/MDI. Some important attributes of HTNR are ... 

HO—OH2—G l-2— - OG2F., ) ( —OOh2—CH2OH Structure (4.15) Hydroxy-terminated perfluoropolyether polyol... 

The hydroxyl terminated polybutadienes lead to PU with physico-mechanical properties significantly lower than those of PU based on polyether or polyester polyols. The nonpolar polymeric chain and the extraordinary hydrophobicity mean that hydroxy terminated polybutadienes are used for special applications, due to their excellent electrical insulation properties which are equal or superior to epoxies or silicone elastomer systems. The hydrolytic stability of PU elastomers derived from hydroxyl terminated polybutadiene is superior to the majority of other types of PU. Thus, some specific applications of... 

These are typically represented by the chemical structure shown in Fig. 12.1. This structure has superior water-resistant properties to that of the conventional polyols used in polyurethane synthesis and it successfully competes with both them and room temperature vulcanizing (RTV) silicones on this basis, especially as the principal application market is the encapsulation of electronic components. Epoxy resins, widely used for encapsulation, change their mechanical properties widely with temperature and hydroxy-terminated polybutadiene-based urethanes are superior in this respect. 

As already mentioned, the soft segment is typically a hydroxy-terminated polyester or polyether, although other hydroxy-terminated materials have also been used and studied, including polyols based on polybutadiene... 

Most hydroxy-terminated polyesters are made by coesterifying two polyols (a diol and a triol) and two diacids (an aliphatic dibasic acid and an aromatic dicarboxylic acid or its anhydride). The ratio of moles of dibasic acid to polyol must be less than 1 so as to give terminal hydroxyl groups and avoid gelation. Molecular weight is controlled by this ratio the smaller the ratio, the lower the molecular weight. The molecular weight distribution Mn, and /n are all controlled by the diol-to-triol ratio. The ratio of aromatic to aliphatic dibasic acids controls Tg of the resin. 

Polyether Polyols. The polyether polyols used in the manufacture of polyurethanes are hydroxy-terminated macromolecules, with molecular weights ranging from 250 to 8000. Lyondell/Bayer has provided pilot plant diols/triols having molecular weights of 10,000 to 15,000 for lubricant and surfactant applications. The hydroxy functionality can range from 2 to 8. The economically attractive polyether polyols based on alkylene oxides are listed in Table 4. 

Hydroxy-terminated polyethers have now assumed a dominant role in the commercial production of polyurethanes. The most widely used polyethers are derivatives of propylene oxide and these polymers are described in Section 8.4.3.1. Linear, glycol-initiated propylene oxide polymers and propylene oxide-ethylene oxide block copolymers find some use in the preparation of elastomers. Polyether triols of relatively high molecular weight (about 3000) are extensively used for the production of flexible foams whilst polyols of low molecular weight (about 500) are used for rigid foams and surface coatings. Poly(oxytetramethylene) glycols prepared from tetrahydrofuran (Section 8.4.6) are used for the preparation of elastomers and spandex fibres. 

Polyurethane foams typically have a segmented structure of hard and soft segments. The hard segments consist of diisocyanate and chain extender units. The soft segments are hydroxy-terminated polyether or polyester-based polyols. 

The final type of polyurethane available is known as the two-part polyol system. One component contains the isocyanate containing compound such as a prepolymer or adduct and the second component is a hydroxy group terminated resin which may or may not contain a catalyst. The most commonly used hydroxy-terminated components are polyols, castor oil, hydroxy-terminated polyesters and some epoxy resins. By varying the components of such a system a wide range of cured properties can be achieved ranging from high flexibility to very hard or brittle. The curing reactions and properties of the different polyurethanes are summarised in Table 10.9. 

Carboxylic functional polyols can be prepared by reacting hydroxyl terminated polyethers with unsaturated acids and a free radical initiator. Also carboxylic groups can be added to poly(ester/ethers) or other hydroxy terminated polymers by reacting with dianhydrides. 

This type of polyol is used to extend polyureflianes with percentages of mineral oil and to improve resistance to hydrolysis, acids, and bases. Olefinic polyols such as hydroxy-terminated polybutadiene (Poly BD) are commonly used. An example is shown in Fig. 10, where the value of n is 57-65 and flie functionality is 2.4 and equivalent weight is about 1260 [24, p. 27]. Another hydrophobic polyol is castor oil. It is a triglyceride which contains a non-saturated acid as the acid component, ricinoleic acid, which has a free hydroxyl group used as the alcohol component for polyureflianes. 

A very convenient method to obtain a macromer (structure 6.13) is by using an unsaturated isocyanate [50] or by using some accessible raw materials, by the reaction of a hydroxy alkyl acrylate or methacrylate with one -NCO group of a diisocyanate for example, 2,4 toluene diisocyanate (TDI) and to react the remaining -NCO group with the terminal hydroxyl group of a polyether polyol ... 

It is very clear that only the first group of monomers, which lead to termination by recombination, can be used for synthesis of hydroxy-telechelic polymers which are useful in polyurethane fabrication and from the second group of monomers it is impossible to obtain oligo-polyols useful in PU. 

The radical containing an hydroxyethyl group which is formed (9.5), initiates the formation of polymeric chains which, by recombination, give hydroxy-telechelic polymers (reactions 9.6 and 9.7). Based on the principles mentioned various hydroxy-telechelic polymers were obtained by radical polymerisation of styrene [9], acrylonitrile [10], butyl acrylate or butadiene [10-14]. Of course, the oligo-polyols derived from styrene and acrylonitrile are solid and difficult to use in PU, but butyl acrylate and butadiene lead to liquid polymers with terminal hydroxyl groups, which are useful in polyurethane manufacture. 

The end hydroxy groups of the polymer are secondary groups and are ordinarily rather unreactive in the urethane reaction. Initially, this limitation was overcome by the preparation of isocyanate-terminated prepolymer and by the use of block copolymers with ethylene oxide. The latter products are known as tipped polyols and are terminated with primary hydroxy groups of enhanced activity. 

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