A New Pull-Out Test for Tire Cord Adhesion—Part I. Hot Bonding

Abstract

Abstract A new pull-out cord adhesion test has been investigated. It consists of a cord partially embedded axially in a long steel-enclosed rubber cylinder. A strain-energy fracture mechanics approach was used to analyze the test in order to derive the energy of detachment, a more fundamental measure of the strength of adhesion than pull-out force, since the modulus and geometry terms are factored out. Good agreement was observed between strain-energy fracture mechanics theory and experiment. In the model system in which steel rods of various diameters were substituted for the cord, rubber thermal shrinkage after cure caused partial internal debonding along the rod if the level of adhesion was not sufficiently high. While no debonding could be detected with truck tire cords, the shrinkage probably reduced any friction effects leading to a more accurate measure of the energy of detachment/fracture. The disadvantage of the Coaxial Shear Pull-out Test geometry (for samples prepared by the hot-bonding procedure as described here) is that scientific discretion based upon measurements of axial stiffness and shear modulus calculations together with subsequent comparisons to shear moduli from an independent measurement, (e.g., hardness) must first be used in order to determine whether debonding has occurred. If debonding is present, then it should be corrected for as shown in the previous discussion section. If no debonding has occurred (e.g., certain brass-plated steel tire cord adhesion systems) then the analysis is more straightforward. Thus, in principle, providing the right analysis is performed on the result, this test is capable of yielding reasonably accurate estimates of the work of adhesion when failure is interfacial or the work of cohesive fracture when failure is in the rubber. In practice however, this pull-out test with samples prepared by the hot-bonding procedure should be reserved for measurements of strong adhesive systems, such as original adhesion of truck tire cords in commercial compounds. A mechanical solution for debonding, involving a cold-bonding procedure is advanced in Part II of this study.