Determination of Static and Dynamic Bond Strength between Vulcanizates

Methods employed for the determination of bond strength between vulcanizates may be classified as static or dynamic. Each of these groups has its own advantages and shortcomings. One undoubted advantage of the majority of static methods is that the tests are carried out under conditions where the stresses and deformations which determine ply separation may be measured directly. It is at the same time easy to determine the temperature since the specimens undergoing ply separation may be thermostatically conditioned. Nevertheless the practical value of the results obtained in static tests is very limited. In the first place, the character of the stresses governing ply separation in no way reproduces the true picture of the service life of multiply vulcanized rubber articles (in particular, tires), and, secondly, the resistance to ply separation for a single loading, for a number of reasons, may not indicate the endurance of the articles under repeated deformation. In dynamic tests we use the endurance under repeated deformations as a criterion of bond strength, presuming that the dynamic pattern of testing in some way reproduces the pattern of service of the material in the article. Unfortunately, the index of service life of any test specimens depends, in a complex manner which we do not in general understand, not only upon the bond strength between the vulcanizates, but also upon the stresses, deformations and also temperatures developed in the boundary layer. These parameters (stress, deformation and temperature) depend in their turn upon the elastic and relaxation properties of the vulcanizates from which the test specimen is made up. If the testing of the specimens is carried out in a dynamic pattern essentially different from that of service, then the results may be quite contrary to the behavior of the materials in an article. The selection of a rational method of dynamic testing applicable to tire compounds is particularly complicated on account of inadequate knowledge of the dynamic pattern of the operation of the component parts of the tire. Summarizing, we see that in investigations and tests of bond strength between vulcanizates we are obliged to use both static and dynamic methods. The static methods, allowing a quantitative interpretation of the results, must be considered more reliable for investigations in the fields of adhesion and covulcanization, particularly in cases where we are studying the effect of factors which essentially alter the properties (above all, the elastic and hysteresis properties) of pliedup vulcanizates.