Preparation, Properties, and Crosslinking Studies on Polyurethane Elastomers

Abstract

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.

Effects of crosslinking density on structure and properties of interpenetrating polymer networks from polyurethane and nitroguar gum

“…In these nonstoichiometric samples, the excess isocyanate groups react with urethane groups to form allophanate linkages [2]. Many studies have examined the effects of chemical crosslink on the material properties of PU elastomers such as tensile properties [3][4][5][6][7][8][9][10][11][12], swelling [8][9][10][11][12], thermal degradation [6,11], thermal properties [9][10][11], and morphology [11,12]. The studies showed that an increase of crosslinking density has two opposing effects on structure and properties.…”

Characterization of the network structure of hydroxyl terminated poly(butadiene) elastomers prepared by different reactive systems

“…Therefore, a lot of work has been devoted to the characterization of HTPB. [1][2][3][4][5] The most commonly used reactants for end linking of HTPB prepolymer are di-or polyfunctional isocyanates, and the resultant network has the polyurethane structure. 4 -7 The HTPB prepolymer ideally contains two hydroxyl end groups.…”

“…In the end-linking reactions, chain extension and branching can be controlled by adjusting the weight ratio of the two functional reactant to polyfunctional reactants. 4,5,7 Therefore, the primary objective of this study is to determine the thermodynamic solution properties of HTPB in two different solvents; second, to compare the effect of the type of reactive system on the endlinking of HTPB chains; and last, to characterize the network structure of HTPB elastomers by following the swelling, solubility and mechanical measurements.…”

Polyurethanes

“…Kothandaraman [13] compared the PU properties obtained from HMDI, TDI, and MDI. The weakest properties were obtained in the case of HMDIbased PU.…”