Prediction of loss of barrier properties in cracked thin coatings on polymer substrates subjected to tensile strain

Abstract

Thin brittle coatings on polymer films are a potentially useful material combination for food packaging applications. The brittle coatings inevitably risk cracking when the package is converted. This strain-induced cracking leads to a loss of the key barrier properties. In design of packaging materials, it would be useful to predict the loss of the oxygen transmission rate (OTR) as a function of the applied tensile strain, which are linked by the crack opening and crack spacing in the coating. Previous works have presented a model that predicts the effect of strain on the OTR in the presence of cracks in the coating. This work uses an improved numerical model based on finite element method (FEM) to predict the oxygen permeability more accurately, especially for thin coatings with high crack densities. The numerical predictions show reasonable correspondence with experimental results for SiOx coatings. These results as well as predictions for previously tested metal-oxide coated polymer films show a significant increase in OTR at crack onset, which suggests that efforts should be made to make the coatings more ductile with higher crack onset strains if the barrier performance should be maintained in converted packages. The quantitative link from deformation over the damage state to barrier properties indicate that mechanics could provide a tool to aid the design of improved food packages with retained barrier capacity.