Dynamic characterization of surface topography of plastic films

It is well known that surface roughness plays an important role on the hand ling and winding of flexible polymer films such as PET which is widely used in various applications. In order to characterize the surface topography o f such materials, an air layer is squeezed between a rigid smooth substrate and a film sample. For that purpose a novel experimental set-up has been b uilt. Using an interferometric method and image processing, we have observe d the evolution of the air layer thickness and measured its reduction for s everal configurations and squeezing pressures. It is found that the reducti on of the central air layer thickness follows a linear law versus time allo wing a parameter, called "dynamic roughness", to be defined. This parameter , which characterizes the kinetics of the air layer being squeezed, represe nts the dynamic manifestation of the influence on the flaw of more conventi onal "static" parameters representative of the film roughness. We have deve loped a theoretical model based on the hypothesis of perfectly flexible fil m samples and on the concept of equivalent smooth surfaces. The predictions are in good agreement with the experimental results and for each film test ed the value of the characteristic parameter associated to its "dynamic rou ghness" is determined.