Modelling and simulation of the electric field and particle trajectories in simultaneous two-sided dry surface treatment of paper

In surface treatment of paper substrates the development is towards more compact and all-online surface treatment processes. The dry surface treatment (DST) process combines both the conventional coating and calendering processes. In this process, the wetting–drying cycles of conventional surface treatment processes are excluded. In DST the dry coating powder is electrostatically applied on the paper surface and after that thermomechanically fixed. DST provides possibilities to considerably reduce both investment and production costs for a similar paper quality as with conventional water-based coating methods. The reduced investment costs can be achieved through a more compact process with combined application and surface smoothening in C2S configuration. This study focuses on the simultaneous dry surface treatment of both paper sides (e.g. C2S) and the application process solutions. The electric fields in the application units and particle trajectories in those fields were modelled in this study. A laboratory-scale application unit was constructed to study the behaviour of coating powder in simultaneous electrostatic application on both sides of the paper. The charge of the particles in relation to their size and the coating powder composition as well as the charge decay properties of base papers and coating layers were determined. Rather homogeneous electric field strength distributions were achieved. The effect of the electric field on the particles and on their trajectories was strong, which allows a compact application process. Charge decay properties indicated sensitive electrostatic behaviour and depended mainly on the moisture content of the base paper and the composition of the coating powder. However, decay properties enable electrostatic deposition in alternating process environments in dry surface treatment of paper substrates.