Effect of molecular weight distribution of poly(oxytetramethylene)glycol on cut growth resistance of polyurethane

Abstract

SynopsisA new method is proposed to improve the cut growth resistance of polyether-based polyurethane elastomers, i.e., polyethers used in this method are characterized by a double-humped distribution of molecular weight (DHDM), prepared by blending low-molecular-weight components and highmolecular-weight ones. The measurements of stress-strain curves, viscoelastic properties, and DSC thermograms of polyurethane elastomers before and after fatigue tests indicate that the destruction of the super molecular structure because of fatigue is smaller in the elastomer by our method, while in the conventional elastomer it is greatly destructed to change into fibrous structure. Further, the broad line NMR measurements show that the molecular motion of DHDM-type elastomers is more active, which is considered to contribute to the improved cut growth resistance of the elastomers. INTRODUCTIONFor practical importance, few articles dealing with flex crack resistance of polyurethane elastomers have been published. Several ways have been known to improve the flex crack resistance of polyurethane elastomers:(1) decreasing modulus by reducing the content of hard segments or compounding plasticizers;(2) decreasing chemical crosslink density by adjusting the molar ratio of active hydrogens to isocyanate groups to a value larger than 1.0 (ref. 1); and (3) decreasing the molecular weight of the polyurethane elastomer.2 McGillvary combined (1) and (2) to achieve so excellent a flex crack resistance as to be durable for tire t e s t i n g~.~ However, for tire application, not only crack initiation but also cut growth resistance is important. In this case, tear resistance might be considered to be one of the most important factors as well. Several attempt^"^ have been made to increase the tear resistance of segmental block polyurethane elastomers:(4) using chain extenders and diisocyanate compounds of bulky and symmetrical molecular structure, as shown by Buist and Gudgeon4;(5) employing a polymer glycol that is polar and that easily crystallizes by elongation, for instance, poly(ethy1ene adipate)glycol5; (6) increasing the length of hard segments by keeping their fraction constant6; and (7) modifying the polyurethane by the interpenetrating polymer network (IPN) r n e t h~d .~ In this article we would like to disclose that segmental polyurethane elastomers prepared from specially designed polyethers with double-humped distribution of molecular weight (DHDM) should be improved in cut growth resistance without any significant disadvantage to other mechanical properties of conventional polyurethane elastomers. This conclusion is derived through discussion of cut growth resistance, tensile properties, viscoelastic properties, heat of fusion, and half-width of broad line NMR comparing DHDM types of polyurethanes with conventional ones. EXPERIMENTAL MaterialsAll the poly(oxytetramethy1ene)glycols (PTMGs) employed were from Dainihon Ink Co., Ltd.; their molecular weights are listed in Table 11. PTMG6000, PTMG10000, and PTMG20000 were prepared ...

Improved Elastomers through Control of Network Chain-Length Distributions

“…This is consistent in a general way with some early results for polyurethane elastomers. 101 Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. A schematic representation of this is shown in Figure 12.…”

Fatigue Cut Growth of Thermoplastic Polyurethanes

“…Further work is required to detail these predictions for TPUs. 6. Fracture surface morphology of the failed samples showed two distinct regions : a flat featureless region with an apparent fast cut growth and a rough undulate region with an apparent slow cut growth.…”

“…From Equation (2): then, or for an initial crack of c. length growing to a c length in a N number of cycles. Equation [6] predicts that the number of cycles to failure decreases as a function of the strain energy density W. Materials with high energy density values should show higher cut growth rates and lower cycles to failure. These predictions work for elastomers as has been shown by different authors [11][12][13][14][15].…”

“…In these tests the sample is submitted to a combination of bending and stretching and the cycles required to break or to grow the crack to specific dimensions reported. Thermosetting polyurethane elastomers have been evaluated using these techniques [6]. Limited studies are available dealing with the generation of S-N fatigue curves for TPUs using tensile specimens [3,[7][8].…”