Experimental Investigation of Industrial Coating-Drying Process Parameters

Abstract

Effects on air impingement convective drying of coated paper from variation in coating composition, coating-to-drying delay time, and drying intensity were investigated using six combinations of these three independently controlled process parameters for a fixed coat weight of 10.2 ± 0.5 g/m2 on commercial 37.7 g/m2 LWC base paper. The reliability of these experimental results was established through critical tests employing 8 or 10 replicate incomplete drying experiments for each of the drying times used for each of the 6 combinations of 3 important process parameters. Comparison with measurements obtained earlier by another experimenter establishing remarkably good reproducibility for these determinations over the first ones of drying. Two industrial binders and two industrially relevant coating-to-drying delay times were tested. Drying intensity was varied by using drying air temperatures over the range 120–200°C. Drying history curves were obtained using 6 to 12 drying times for each set of these process parameters. This three-parameter-based study shows that over the range of the experimental plan used, drying shows major effects from the variation in coating formulation, is significantly affected by drying intensity but not by coating-to-drying delay time. The behavior with these binders is sharply different between the early and late stages of drying. In the former, drying is much faster with SBR binder, while in the latter stage of drying there is no significant difference. Two mechanisms were indentified which explain this difference. For early stage drying, the solubility of starch reduces the thermodynamic activity of water, thereby reducing drying rate. In the latter stage of drying, the big difference in coating layer porosity between the two binders imposes two counter effects, with the use of SBR binder decreasing coating layer thickness but also decreasing coating layer permeability, resulting in the later stage drying history being similar for the two binders.