One of the most important phenomenon in material science is the reinforcement of rubber by rigid entities, such as dispersed particulate filler or phase-separated organic domains. In order to impart significant reinforcement, the size of the hard phase must be small, much less than a micron. The basis of this requirement is a major focus of this short review. Furthermore, the roles of energy dissipation and crack deflection in rubber reinforcement are considered. The final part of the review deals with nano-composite rubbers, in which rigid domain size is in the range of 1–10 nm.
Tack and green strength are two important properties of many elastomer compounds. Tack is the ability of two materials to resist separation after bringing their surfaces into contact for a short time under a light pressure. Two types can be defined: autohesive tack (autohesion), where both materials have the same chemical composition and adhesive tack, in which the two materials have dissimilar compositions. The green strength of an elastomer is its resistance to deformation and fracture before vulcanization. Rubber stocks that are used in tire manufacture (or other plied-up operations) must have a certain minimum level of tack and green strength. Tack is necessary so that the many components of a green tire will hold together until molding. This requires not only that the components exhibit quick stick when building, but also that the tack bonds have long term creep resistance, since the green tire may be hung on a rack several days before molding and vulcanization. In addition, an uncured tire must have good green strength so that it will not creep and hence distort excessively before molding or tear during the expansion that occurs upon molding (or in the second stage for a radial tire). Adhesive tack is an important property of pressure sensitive tapes. Although some of the basic criterion necessary to obtain high autohesive tack or high adhesive tack are similar, this review will focus primarily on autohesion. Thus, unless otherwise staled, the term “tack” will refer to autohesive tack. However, ideas and results will be presented from the literature on pressure sensitive adhesives when such information leads to a further understanding of autohesion.
SBR compounds containing ground vulcanizates of known composition and cure state were prepared and the cure behavior studied. Decreases in scorch times and maximum rheometer torques were observed when ground vulcanizates were added to the SBR compounds. Two primary phenomena are proposed to explain these findings: (1) migration of sulfur from the matrix rubber to the ground vulcanizate (causing torque reduction) and (2) migration of accelerator fragments from the ground vulcanizate to the matrix (causing decreased scorch time). The first proposal is based on direct measurement of sulfur concentrations in both ground particles and the matrix. The second is based on the detection, by high performance liquid chromatography, of mercaptobenzothiazole in the extract from ground vulcanizates. The second also is inferred from the fact that ground vulcanizate particles cured with peroxide do not alter scorch time. Moreover, a compound containing sulfur and ground (accelerated-sulfur vulcanized) rubber, but no added accelerator, nonetheless exhibits acceleration of cure.
Typical sulfur-cured vulcanizates of styrene-butadiene rubber (SBR) and natural rubber (NR) were prepared, and subjected to air-oven aging at 100 °C. Gum specimens exhibited an initial aging period in which stiffness was unchanged, while tensile strength and strain-to-break were significantly reduced. In contrast, black-filled vulcanizates stiffened during early aging. After intermediate aging times, NR specimens softened, while SBR stiffened. With prolonged aging, all compositions became hard and inextensible.
A review is given of the mechanics of peeling rupture of an adhesive joint, consisting of a flexible adhering strip peled away from a layer of adhesive. Attention is drawn to a number of anomalous results that cannot be accounted for solely, in terms of the thermodynamic work of formation of two new surfaces. The work of detachment is found to be generally much larger than the theoretically‐predicted amount. Moreover, the value obtained is greater for thicker layers of adhesive, and for detachment at a peel angle of 180° rather than at 90°. Also, it is found to increase with increasing thickness of the adhering strip, passing through a maximum value in some cases and then decreasing as the strip thickness is increased still further. All of these effects are attributed to dissipative processes, for example, plastic yielding, in one or both of the adhering layers as they are peeled apart. Some quantitative relationships are given for the additional peel forces arising from plastic yielding of the adherend or the adhesive.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.