Electrical and mechanical behavior of filled rubber. III. Dynamic loading and the rate of recovery

Following the earlier articles in this series, the changes in the electrical resistivity and mechanical behavior as a result of static and dynamic deformation have been studied. Cyclic shear and tensile loading were used to follow the changes in stress and resistivity with strain, including the recovery with time from the effects of a large strain as monitored by the small‐strain behavior. The recovery of resistivity from a prestrain was not complete even after 7 days at room temperature or at 50 °C, but swelling with a solvent and subsequent drying produced rapid recovery. It appears from the detailed results that there are two strain regions. Below about 10% the resistance and the modulus are strongly dependent on the filler–filler structure, which can break down and reform fairly readily, but the changes at higher strains are probably influenced by changes in the elastomer matrix and also by slippage at the filler–rubber interface. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1649–1661, 2005     

Electrical and mechanical behavior of filled elastomers. I. The effect of strain

Electrical and mechanical property tests have been used to examine the changes in the carbon black network structure that occur in a filled elastomer at large strains in tension and compression. These changes have been examined both in materials that have no previous loading history and in test pieces that have been subjected to a specific known prestrain. When a previously unstrained, filled elastomer specimen is stretched to moderate extensions, the electrical resistivity increases. This is ascribed to the breakdown of the carbon black network structure. At higher tensile extensions, the resistivity decreases. This reduction in the electrical resistivity is attributed to the alignment of the shaped carbon black aggregates under strain. During unloading, the resistivity behavior is different from that during loading, with the final unloaded electrical resistivity being significantly higher than that measured in the unstrained elastomer. This dramatic change in the electrical properties after unloading is in marked contrast to the relatively modest changes observed in the mechanical behavior. After the first cycle, the electrical behavior becomes much more reversible, and this indicates that the bulk of the damage experienced by the carbon black network is developed during the first cycle. After unloading from a large strain, the electrical anisotropy is small, whereas the mechanical anisotropy is more marked. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2079–2089, 2003     

Volume changes under strain resulting from the incorporation of rubber granulates into a rubber matrix

The strength of an elastomer is in part determined by the size of the intrinsic flaws that are present. It has been observed that the incorporation of rubber granulates into a virgin matrix results in a reduction in strength and this has previously been attributed to an increase in the intrinsic flaw size. The precise nature of this intrinsic flaw is the subject of this investigation. Fundamental questions concerning the change in flaw size with strain and the reduction in strength resulting from a weaker interface have been investigated using volume change experiments. Initial experiments on carbon black filled rubber with no granulates incorporated have shown no significant volume change under strain. This contrasts with granulate filled materials whose experimentally measured volume changes with strain were seen to be substantially greater. Microstructural finite element analysis has revealed how this change in volume might result from a net increase in the flaw size with increasing strain. This work suggests that flaw size increases in a characteristic way with strain for materials where the matrix and granulates have a similar modulus, whereas a modulus mismatch between the matrix and the recycled granulate results in much larger volume changes and hence greater flaw size which also appears to increase with strain. This work emphasizes the importance in practical applications of matching the modulus of recycled granulate materials to that of the new virgin material in the matrix. This article introduces a novel technique for examining small changes in the interfacial bonding mechanisms under strain such as that caused by surface modification techniques. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3169–3180, 2007     

Migration and blooming of waxes to the surface of rubber vulcanizates

When certain substances, notably waxes, are incorporated into rubber during vulcanization, the surface of the vulcanized rubber may subsequently become covered by a film of the substance diffusing out. This phenomenon, known as blooming, depends on the substance being soluble at the vulcanization temperature but only partially soluble at the temperature of blooming. A study has been made using pure waxes in natural rubber vulcanizates with a range of crosslink densities. The mass of bloomed material has been determined as a function of time, and the expected dependence on the square root of the time has been found to hold over the anticipated range. It has been shown that the kinetics of the process cannot be explained simply in terms of the degree of supersaturation of the wax in the rubber, the observed rates being much too low. This appears to be related to the precipitation of the wax in the body of the material. A theory has been developed based on a calculation of the stresses set up around such a precipitated particle and the effect of the consequent free-energy gradient on the rate of diffusion. Comparison with experiment shows satisfactory agreement with the theory.     

Rupture of rubber

Summary The theory of the tearing of rubber has been further confirmed by experiments on three test pieces of widely different shapes, for each of which the tearing energy can be calculated. This rupture concept has been used to interpret measurements of out growth under conditions of both static and dynamic loading.

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Zusammenfassung Die Theorie des Reißens von Gummi wird durch Versuche an drei Proben von wesentlich verschiedener Gestalt, für die jeweils eine Reiß-Energie berechnet werden kann, weiter erhärtet. Diese Vorstellung über den Bruchvorgang wird dazu verwendet, Meßergebnisse bezüglich des Einschnittwachstums (Rißvergrößerung) sowohl bei statischer als auch bei dynamischer Versuchsführung zu interpretieren.

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Presented at a meeting on Flow, Fatigue, and Failure held in Leeds on January 8th and 9th, 1959, by the British Society of Rheology.     

Tearing of unvulcanized natural rubber

The tear behavior of unvulcanized natural rubber has been studied by using established techniques normally adopted for the study of vulcanized rubbers. Unvulcanized rubber has been found to tear in a relatively steady manner, in contrast to the stick-slip tear behavior of the vulcanized rubber, the tearing energy being dependent on the rate of tearing. Crystallization seems to be an important factor in determining the tear behavior since it has not been found possible to tear unvulcanized SBR under the same conditions. The effect of the pronounced imperfect elastic nature of the material was studied under conditions where the driving force for tearing was solely governed by the rate of release of elastic energy. Under such conditions, it has been found that the tearing energy is determined not by the strain energy required to stretch the material but by the energy which can be recovered on retraction. The set developed in the test piece, due to imperfect elasticity, has also to be taken into account.     

The incipient characteristic tearing energy for an elastomer crosslinked under strain

In a previous paper a two-network model for an elastomer in which crosslinks have been introduced in the strained state, similar to that proposed by Green and Tobolsky, was used to calculate the dependence of the incipient characteristic tearing energy on the number of chain segments in each of the two networks, the number of links in these chain segments, and the deformation at which the crosslinking takes place. The tearing energies were calculated for tearing on planes perpendicular to the principal directions of this deformation. Here the calculations are extended to cover tearing on a plane with arbitrary orientation.