Pulp Extrusion at Ultra-High Consistencies: Selection of Water-Soluble Polymers for Process Optimization, 2002 Fall Technical Conference

Pulp extrusion at ultra-high consistencies (20% to 40% solids) is a new process developed at USDA Forest Service, Forest Products Laboratory (FPL) to convert recovered papers, wastepaper, and papermill residuals into solid sheets or profiles for compression molding. This process requires adding a water-soluble polymer (WSP) to alter the rheological properties of the pulp and generate a paste that can be extruded. The variety of fibrous raw materials can have a significant impact on the efficiency of a WSP to alter viscosity. Therefore, an appropriate WSP must be selected that will rapidly hydrate and adhere to fiber surfaces, allowing flocs to disperse in the shear-intensive environment of an extruder. Also, temperature-dependent viscosity changes are important to control elongational flow for fiber alignment in the extrudate and to enhance the consolidation properties. This study presents a methodology for determining the rheological properties of fiber pastes. Selected WSP (natural and modified gums, cellulose derivatives, and gelatin) were added to a clean newsprint pulp and processed in a modified bowlmixer rheometer. The temperature of the bowlmixer was controlled and ramped between 20^°C and 100^°C to observe temperature-dependent viscosity changes. Various cations were also added to induce thermo-reversible viscosity changes. A blend of sodium carboxy-methylcellulose and hydroxypropyl-methylcellulose resulted in the best overall rheological properties.