Abstract
Clay or other minerals are coated onto fibrous paper to improve print and optical properties. The minerals are fixed to the fibrous substrate with a binder, either latex or starch. The coated paper is dried during manufacture, and if the binder migrates during this drying, there can be a deterioration in properties. Not only are there manufacturing considerations as to the choice of binder, but also environmental ones with regard to factors such as the energy used in drying. This work reveals new perspectives on binder migration which have been gained by a variety of approaches. Firstly, a critique is presented of the experimental methods which have been used by other workers. Then a comprehensive series of experiments is described. For ease of study, and because any effects are likely to be exaggerated, most of the experiments were carried out on samples which were 1.4 mm thick, some two orders of magnitude thicker than commercial coatings. However, the results were also replicated in less extensive tests on samples of thickness 55 μm. The experiments show that under a very wide range of conditions, including different coating thicknesses and drying temperatures, no latex migration is observed. Migration of starch was observed, however, and caused an increase in starch concentration at the surface during drying. If the sample was covered during the experiments, the system relaxed back and the concentration enhancement was reduced. The relationship between the particle size of the latex and starch, and the extent to which these particles might percolate through the void structures, was investigated by the application of the “Pore-Cor” software to mercury intrusion experiments. Also presented are the results of a mathematical continuum model, which accurately describes the migration of starch binder in terms of only two parameters, namely the evaporation efficiency and the diffusion coefficient of starch.
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