Relation of structure to properties in polyurethanes

Abstract

The present work was undertaken to assess the effect of crosslink structure on the properties of an elastic network derived from a linear polyetherurethane elastomer. In order to study this effect, a polyetherurethane was synthesized which contained two types of reactive sites suitable for establishing crosslinks : pendant hydroxyl groups for reaction with diisocyanate curatives and pendant double bonds for vulcanization with sulfur. Incorporation of both types of curing sites within the same linear polymer served to hold constant variables which might otherwise influence vulcanizate properties. Thus, the elastic networks formed by both curing systems were produced from the same polymer, and such factors as polymer molecular weight and molecular weight distribution, interchain forces and cure site distribution remained constant. Furthermore, the curing sites have been placed on pendant groups well removed from the main polymer chain in order to avoid degradation of the polymer by side reactions which may accompany sulfur vulcanization.It is recognized that common elastomers with internal unsaturation, such as SBR and natural rubber, can be crosslinked by more than one method (e.g., with sulfur, peroxides, or high energy radiation), but the extent and nature of side reactions which may occur is not known with certainty. There are possibilities of polymer degradation reactions with all of these curing systems, and the occurence of such degradation reactions would cloud any conclusions concerning the relation between crosslink structure and vulcanizate properties.Polyetherurethane I was synthesized for the present study : n n

Preparation, Properties, and Crosslinking Studies on Polyurethane Elastomers

“…In all these cases, both hardness and tensile strength increased on crosslinking while elongation decreased as per the expectations. 22 Even though same equivalent (as per the hydroxyl content) of the crosslinking agents was used, the increase in tensile strength with castor oil is not high compared to TMP and PV A. This may be explained 23 as due to its low hydroxyl functionality of 2.…”

Network characterization of “viton” ahv fluoroelastomer

“…At the same C, but different an, eqs. (7) and (10) give (YO -?I/(?/ -2 ) = a n o / a n t (11) where the subscript t refers to time of aging; 1'0 and a,,, are the corresponding parameters without chain scission.…”

Thermoplastic Urethane Chemical Crosslinking Effects

“…Alternatively, the sulfur-accelerator type of free radical curing system employed to vulcanize natural rubber and conventional unsaturated synthetic rubbers can be used to crosslink special-structured polyurethane millable gums [5,. Such polymers are rendered responsive to sulfur-accelerator cure by building occasional, special cure sites into the polymer chains which are more reactive in the free radical cure mechanism.…”

“…Such structures are readily incorporated in the polyurethane chains via ethylenically unsaturated glycols, e.g., glyceryl-a-ally1 ether, trimethylolpropane monoallylether, as chain extender[ 5, or as macroglycol polymerization initiator [17, 181.The sulfur-accelerator cure of urethane millable gums can be rapid[ 191. It likely proceeds according toFigure 4if analogy to the same cure of conventional hydrocarbon rubbers holds.…”