The effect of basis weight and freeness on sheet permeability and critical impulse drying temperature

A low thermal mass ceramic coated pressing surface in conjunction with paper webs of low hydrodynamic surface reduce the likelihood of sheet delamination during impulse drying. Darcian permeability measurements of sheets at three basis weights and two freeness levels relates sheet specific surface to the critical impulse drying temperature above which delamination occurs. Impulse drying effectiveness is also determined by comparing outgoing sheet dryness and compression strength to double felted extended nip pressed sheets. Results indicate that Darcian permeability is affected by basis weight, due to formation of nonuniformities found in light weight sheets and the increasing presence of short fibers and fines in the heavier weight sheets. The nonuniformities, termed macropores, artificially increase the sheet permeability. The macropores allow for higher critical temperature and outgoing sheet dryness by transporting larger quantities of fluid. The higher weight sheets filtered and retained more short fiber than the light weight sheets, hence dryness was maximized at low basis weight and high freeness. All impulse dried sheets achieved higher sheet dryness than those pressed in a simulated double felted extended nip press. Also, it is more advantageous to impulse dry 740 CSF sheets rather than to refine the pulp to 600 CSF and press in a double felted extended nip press.