Adhesion measurement of interfaces between gelatin and poly(ethylene terephthalate) using microscratch technique

A microscratch technique was used to evaluate the adhesion between interfaces of a gelatin coating and poly(ethylene terephthalate) (PET) film. The interface was reinforced by nitrogen plasma treatment on the PET surface and subsequently by heat treatment of each gelatin/PET sample to promote interactions at the interface. In the microscratch test, a normal load controlled conical stylus with 50‐μm radius tip was drawn over the gelatin coating surface under a continuously increasing normal load until failure occurred in the sample. Optical microscopy and depth profiling of the scratch track were used to detect failure and the failure mechanism. The critical normal load (Fc) was defined as when gelatin detached from the PET substrate or when a complete removal or plowing of the gelatin coating on the PET substrate occurred. With increasing plasma treatment time and heating treatment temperature, the Fc for both debonding and coating removal increased, which showed that both failure mechanisms are related to the adhesion. Different thicknesses of the gelatin coatings were also prepared under the same plasma and heat treatment conditions. It was found that the Fc increased with increasing coating thickness. The result demonstrated that both failure mechanisms depended on the plastic deformation of the coating and substrate. The Fc for coating detachment increased linearly with increasing coating thickness whereas the Fc for coating removal increased sharply with increasing thickness. Annealing temperatures ranging from 20 to 80°C exhibited a strong effect on the Fc, which increased with increasing annealing temperature. These results demonstrate that the microscratch technique can be used to access interfacial adhesion and that the Fc is a qualitative parameter for the evaluation of adhesion strengths. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1960–1974, 2006