Characterization of the interfacial bond in paper-polypropylene laminates and the effects of aging under service conditions

This study focused on the behavior of the paper-polypropylene-paper (PPP) laminate while aging in hot oil in the absence of voltage stress. The results provide an understanding of both the quality of the interfacial bond and the performance of this bond during service. X-ray photoelectron spectroscopy performed on two different peeled laminates suggest that the bond failed primarily adhesively. Weibull statistical analysis of the peel strength data obtained on unaged laminates and those aged in polybutene oil at 90 degrees C for 120 hours showed that the strength loss is consistent with one failure mechanism and the failure rate increases with applied stress. For the aged sample, Weibull analysis results are consistent with the prior loss of peel strength due to the aging. Experiments on the solubility of the oil show that lamination reduces the amount of absorption in comparison to the unlaminated composite. Swelling experiments on the individual components show differential swelling between the paper and polypropylene to be the source of the strength loss. The polypropylene swells, and the paper shrinks. Measurements on the laminated show that both paper and polypropylene shrink, indicating that the paper governs the laminate swelling process. During aging, the differential swelling generates internal stresses on the interface. In addition to yielding the magnitudes of these stresses, finite element analysis also predicts plastic deformation and creeping of the polypropylene as well as tensile stresses between the paper and polypropylene at a free edge. Very likely these processes damage the bond and contribute to the loss of bond strength.