ABSTRACT:Polyurethane elastomers were prepared from structurally distinct hydroxyterminated polybutadiene and polyether-ester based prepolymers with diisocyanates such as hexamethylenediisocyanate, toluenediisocyanate, and diphenylmethanediisocyanate by the one short polymerizatiQn process. The effect of crosslinking by polyfunctional alcohols such as castor oil, trimethylolpropane, and poly(vinyl alcohol) on mechanical properties is compared in case of hydroxyterminated polybutadiene based polyurethane systems. The effect of crosslinking by trimethylolpropane on the mechanical properties was quantitatively studied in all the prepolymers based polyurethane systems. Crosslinking was also induced in the polyurethanes by taking higher NCO/OH ratio (R value) and the observations on mechanical properties are compared with those prepared with trimethylolpropane crosslinking. The thermal degradation of the polymers was studied in an oxygen atmosphere and interpreted with respect to their structure.KEY WORDS Polyurethane Elastomers / Crosslinked Polyurethanes / NCO/OH Ratio Effect / HTPB Based Polyurethanes / Copolyetherester Polyols / Polyurethanes are prepared by reacting polyols with diisocyanates. The most commonly used polyols include low molecular weight hydroxyterminated polymers of saturated and unsaturated aliphatic hydrocarbon, polyethers and polyesters. The functionality of the hydroxyl containing compounds as well as isocyanates can be increased to three or more to form branched or crosslinked polymers. Other structural changes can also be made by changing the type and molecular weight of the intervening groups. Polyurethanes are unique in their crosslinking, chain flexibility and intermolecular forces which can be varied widely. The abundance of original substances used for preparing urethane elastomers in conjunction with the wide variety of reactions taking place during their synthetic cause the highly complicated chemical structure of the resulting polyurethanes. 1 -5 Crosslinking of polyurethanes is of primary importance in controlling many of their properties. 6 The degree of crosslinking was controlled by the use of polyfunctional ingredients or by taking excess diisocyanate than that of the stoichiometric requirement. Crosslinking at higher diisocyanate ratio is reported to proceed through the formation of allophanate and isocyanurate linkages. 7 A significant change in properties by crosslinking may also be expected in the case of elastomers like polyether based polyurethanes.
SynopsisEther-and ester-based prepolymeric diols suitable for polyurethane elastomer formation were prepared from polyoxytetramethyleneglycol and polyoxytetramethylene-oxypropyleneglycol with dicarboxylic acids like adipic acid, maleicacid, and phthalic acid. Crosslinked polyurethane elastomers were prepared from these prepolymers using various concentrations of trimethylolpropane as a crosslinker and toluenediisocyanate as the curing agent. Crosslinked polyurethanes were also prepared by a method taking excess toluenedisocyanate other than that of the stochiometric requirement and curing it at a comparatively higher temperature. The data on mechanical properties of these elastomers show that both methods of crosslinking in polyurethane imparts a similar effect on mechanical properties.
SynopsisA series of halogen-containing poly (esterurethane) s was synthesized from the prepolymers, i.e., hydroxy-terminated polyesters containing chlorine and fluorine and the diisocyanates such as toluene diisocyanate (TDI) and hexamethylene diisocyanate (HMDI) . Polyesterification of halogenated phthalic anhydride with an excess of diols such as ethylene glycol, 1,2-propanediol, and 1,4-butanediol in the presence of trifluoroacetic anhydride (TFAA) was carried out to prepare prepolymers. The prepolymers and the poly( ester urethane)^ were characterized by infrared (IR), 'H nuclear magnetic resonance (NMR) , viscosity, end-group analysis, solubility and thermal analysis namely thermogravimetric analysis (TGA) , and differential scanning calorimetry (DSC) . Kinetics of thermal degradation was also studied. Resistance of the polymers to alkaline agents and combustion was also tested. EXPERIMENTAL MaterialsHexamethylene diisocyanate (HMDI) (Fluka AG) and 2,4-toluene diisocyanate (TDI) (Fluka AG) were purified by vacuum distillation. Phthalic anhydride ( AR, Sarabhai, India), tetrachlorophthalic anhydride ( Aldrich Chem.Co., ) , trifluoroacetic anhydride (TFAA) (BDH, England), and dibutyltin di-* Present address and address for communication:
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