Evaluation of Rubber to Metal Bonds
Authors: Buist J. M. , Meyrick T. J. , Stafford R. L. 
Published: February 2011

Abstract
During the last ten years improvements in bonding agents and their technique of application have enabled industry to manufacture with greater surety a wider range of rubber to metal components. This welcome advance continues but as the level of bond strength is raised certain problems associated with testing become more acute. It has always been a simple matter to differentiate between poor and good bonding. However, considerable thought and care in testing and interpretation are required to differentiate between good bonding agents and techniques when the bond strength is high. Industry is faced with two types of testing problem; control of factory production and evaluation of improved techniques (including different bonding agents). The question of control of factory production is not dealt with in this paper although some of our conclusions could usefully be applied in the field of control testing. Examining each of the test methods employed in the present paper it may be useful to summarize the salient features that appear to us to have some importance. It is axiomatic that the strength of a rubber to metal bond can not be assessed without breaking the adhesion between the rubber and the metal. However, only in the case of poor adhesion, where separation occurs entirely at or within the bonded interface, are breakdown tests easily and unequivocally related to the bond strength. When good bonds of high breaking strength are obtained it becomes a matter of great difficulty to discriminate between different bonding systems. The simple straight pull test has been employed for many years in spite of the obvious criticisms that can and have been levelled at it. There is no doubt that simple measurements of tensile breaking load give a clear indication of whether or not a good bond has initially been obtained in the molding process. From Table IV it is deduced that the variability of this test should not exceed that given by a coefficient of variation of about 8 to 12 per cent of the mean value ; values greater than this indicate that more careful control of the bonding process is required. Interesting results are reported with testpieces tested in shear; on a tensile break test the brass bond appeared superior to both CDR and PI bonding agents but this superiority was not retained in the dynamic test where the CDR was better than either the brass or PI bond. A simple mechanical switch has been fitted to the impact machine to ensure that the falling weight is arrested after the first rebound. With transfer molded testpieces the CDR and PI systems are more resistant to multiple impacts than the brass bond. At low energy inputs (50 ft. lb.) the rubber stock appears to fatigue before the bond but at inputs of 70 ft. lb. and above it is believed that fatigue of the bond occurs. This observation is important as the data obtained under a variety of dynamic conditions in this paper emphasize the difficulty of discriminating clearly between the fatigue of the rubber and fatigue of the bond. Since different dynamic tests, e.g., impact and shear flexing, discriminate differently between bonding agents, classified as equally good by simple tensile break tests, it is clear that in laboratory comparisons of bonding systems a variety of tests is essential. Using a 90° peeling test, the chemical bonding agents could not be distinguished since all-rubber breaks occurred. The brass-plated bond however, failed at a lower level of peeling load with evidence of bond rupture. There is no doubt that very good bonds can be consistently obtained to 70/30 brass. Brass plating, however, is a much more variable system and is quite unsuitable as a standard of comparison in laboratory work. The CDR and PI systems are capable of giving results at least equal to those obtainable with solid brass, and one would hesitate to claim any clear superiority of one over the others. The process of obtaining a good bond between rubber and metal is not one which is easy to reproduce without careful control of the rubber stock, the interply solution, the metal surfaces, the bonding agent and the molding technique. This applies to all bonding systems at present known to us. Except in cases of very poor adhesion the appearance of the broken unit is indifferently related to the bond strength and cannot be used as a criterion in assessing bonding systems or bond strength.