The influence of pigment type and loading on water vapor barrier properties of paper coatings before and after folding

Water borne barrier coatings for paper have great potential to replace extruded polymer coatings because they can be recycled and are more likely to break down in the environment. Pigments are usually added to these coatings to reduce the costs and to improve the barrier properties for packaging applications. When these coated papers are formed into a package, often the paper needs to be folded, which usually leads to decreased barrier performance of the coating. While barrier properties of coatings have been studied extensively, the barrier properties of these systems after folding has not been well reported in the literature. In this study, the influence of pigment type, shape, and pigment volume concentration (PVC) on the water vapor transmission rate (WVTR) and fold resistance were characterized for barrier coating formulations. A method to fold the sample was developed to investigate barrier properties after folding. Coating crack structure was observed by scanning electron microscopy (SEM) and compared with the observed WVTR results. The deformation of a coating layer on paper, including the influence of pigments, was modeled using a finite element code. Plate shaped pigments gave the best WVTR results before folding but tended to generate large cracks and poor water vapor barrier properties after folding. Spherical pigments yielded small cracks and allowed some retention of barrier properties after folding. The change of the WVTR after folding was greater than what would be expected based on crack area, and internal damage or delamination may explain this result.