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Abstract

The micro- and macrostructural changes occurring in multi-layered substrates during three-dimensional forming were studied by atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical analyses. Particular attention was paid to heat-induced deformations at the interface between of polymeric coating layer and the paperboard. With excessive heat transfer, occasional delamination of polyethylene terephthalate (PET) coating from the paperboard was observed. The mechanism behind delamination was studied in detail in-situ with an AFM at temperatures relevant to the converting process. Based on the analysis, the delamination could partially be linked to the widening of the initially-existing nano-scale cracks at the coating-paperboard interface due to the high temperature, rigid and less adhesive PET crystallites close to the paperboard layer, and the emergence of fissures and tensile stresses in the coating. SEM images also revealed severe macro-scale delamination in the paperboard matrix after forming. However, the results were somewhat conflicting, since optical and machine vision analyses showed indisputably that both the visual quality and the dimensional accuracy of formed trays were better at the higher forming temperature.

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