Cellulose Fibers Impart Structural Performance To Extruded Thermoplastic Wood Products

Long used to fill thermoset resins, wood-derived cellulose fibers are now making inroads into thermoplastic parts. The rap against celluose has always been its inability to stand up to high processing temperatures or disperse well in resins. Creafill Fibers Corp. (Chestertown, Md.) has addressed these issues by producing densified cellulose fibers and powders from reclaimed paper waste — newsprint, magazines, etc. The papermaking process, which combines wood with chemicals and hot water, extracts tannins, lignins and volatile organic compounds, such as terpenols, that can burn at low temperatures. The company also makes higher-quality alpha-celluose fibers directly from pulp purchased from papermakers, which can withstand temperatures up to 260°C/500°F. Therefore, "pulped" cellulose fibers are a better additive for extruded thermoplastic composites than wood flour from sawdust, says Creafill's director of marketing Edward Schut.

Creafill's low-density, ribbon-like ~3-mm/0.12-inch fibers have 20:1 to 30:1 aspect ratios and are coated with coupling agents and dispersants as they are compressed into friable, easily-dosed pellets. Coatings have been customized for a variety of resin types, including nylon, PE, PP, PET (polyethylene terephthalate) and PVC (polyvinyl chloride). Pulped celluose provides better structural reinforcement than particulate fillers, says Schut, not only because of its high-aspect fibrous form but also because hydroxyl groups present on the fiber enhance bonding to polar resins like PVC. They are nonabrasive and reduce machine wear, and have zero coefficient of thermal expansion. High fiber loadings improve part impact strength. And, low density and flexibility mean that fibers don't break down in the extruder under shear, he notes.

Colorite Polymers (Ridgefield, N.J.) incorporates Creafill pellets and an array of additives into pelletized PVC compounds, says Colorite's Phil Cascais. Outdoor products molder Royal Crown Ltd. (Milford, Ind.) extrudes Colorite's 12 percent Creafill fiber PVC to make its Harmony Select line of outdoor deck, rail and fence products. In the proprietary process, a vinyl skin coating is coextruded over the PVC, says Royal's national marketing/sales manager Deron Manwaring. Harmony Select products were introduced this year and are available at home stores.