Adhesion of carbon black filled natural rubber to glass

The adhesion of carbon black (CB) filled natural rubber to a clean glass slide was measured during loading and unloading and the results analyzed according to the model proposed Johnson, Kendall and Roberts (JKR). The JKR model views loading as crack healing and unloading as crack growth and accounts for the contribution of surface energy to the relation between contact size and applied load. Analysis of the data yields the modulus and the surface energy during loading and unloading. The modulus during unloading was always greater than during unloading by a factor of 1.5. The modulus during loading and unloading increased approximately linearly with CB from 0 to 50 phr. The work of adhesion was also greater during unloading than loading, but by a factor of about 50. Thus there was a large hystersis in the loading and unloading process. In fact there was a period from when unloading started that the contact size remained constant and the load decreased. CB increased the work of adhesion from 0 to about 15phr, but then it decreased with additional CB for both loading and unloading. The origin of the large hystersis is not clear, but may be due to viscous contributions or strain induced crystallization.