Pulp extrusion at ultra-high consistencies
A new process for producing continuous lignocellulosic fiber products is currently under development at the USDA Forest Service, Forest Products Laboratory. The goal of this exploratory research is to utilize conventional pulping techniques to process recovered papers with high filler contents, retain fiber integrity, and produce an engineered extruded product. This process involves the extrusion of wet crumb pulp (primarily derived from recovered paper) at consistencies of 20% to 40% with small amounts of water-soluble polymers added as hydromodifiers. The water-soluble polymers appear to have two important rheological functions in the extrusion process: (a) they bind water to the fiber and (b) they add lubricity to the pulp. A torque rheometer was used to measure the apparent viscosity of various pulp suspensions with water-soluble polymers. The added polymers reduce viscosity dramatically and enhance fiber dispersion. Polymer addition levels ranging from 1% to 3% by weight dry fiber were evaluated. Selected pulp/polymer compositions were then processed by a twin screw extruder. A slit die was designed to produce a thin sheet that could be press-dried and cut into coupons for measurement of tensile properties. Fiber orientation appeared to be random, and tensile properties were nearly equivalent in the flow and cross-flow directions. For most extruded compositions, fail stress and modulus of elasticity ranged from 10 to 20 MPa and 3 to 5 GPa, respectively. These results indicate that pulp extrusion at ultra-high consistencies is possible and poses a viable alternative to the disposal of highly filled recovered papers and papermill sludges.