Specialty additives will help plastic lumber gain wider acceptance and capture new markets, according to several speakers at the Progress in Woodfibre-Plastic Composites Conference 2002 in Toronto.
Using additives to optimize plastic lumber production can be more profitable than buying machinery to increase capacity, said Richard Heath, senior technical manager for Honeywell Inc.'s specialty chemicals division. Heath said his Morristown, N.J., firm studied profitability when one of its lubricant additive packages was used for producing plastic lumber on a conical, twin-screw system.
The company compared its formulation with a standard ethylene bis stearamide/zinc stearate lubricant.
``For a four-extruder plant, it's like getting a fifth extruder free,'' Heath said. The claim is based on a study of fixed and operating costs at full production of wood/plastic composite boards.
Lubricants increase composite output partly because they reduce shear in extrusion, said Joe Williams, team leader for technical services at Lonza Inc. of Annandale, N.J. Less shear also means lower processing temperatures, which lead to better appearance for plastic lumber, he added.
Williams said some composites producers use little lubricant, partly because they might rely on recycled resin as part of their raw materials. Recycled resins contain low-molecular-weight residues that act like lubricants in a plastic/wood-fiber mixture, he said. Lonza may enter the wood composite market as a lubricant and fungicide supplier.
Plastic lumber appearance during aging can be improved with ultraviolet-light stabilizers and antioxidants, said Thomas Steele, technical service representative for the polymer additives division of Cytec Industries Inc. Cellulose and lignin discolor when exposed to outdoor conditions, and plastic resins also change color when exposed to sunlight and air.
Cytec, based in Stamford, Conn., just started researching additives for plastic lumber. Steele said the company's preliminary testing involved purchasing a standard grade of plastic lumber and exposing it to accelerated aging. The product's color faded badly with the equivalent of only six months of Miami weather.
Technical specialist James Rediske of Pittsburgh-based Bayer Corp. said inorganic pigments seem to outperform organic colorants for longer-lasting, better-looking wood/plastic composites. Virgin resins in the composite formulation provide a more predictable response to pigmentation, he added. He also said UV-absorbing additives may help lengthen composite life.
``Homeowners who want a natural-appearing deck on their homes are not always satisfied with a `driftwood gray,' unless of course they live at the beach,'' Rediske quipped.
Chemical foaming agents can be useful to plastic lumber development, according to Vahe Karayan, technical manager of Clariant Additive Masterbatches of Winchester, Va. In addition to weight and cost reduction, the additives allow faster production on a lineal-foot basis. Machinability and the ability to take nails and screws also are better than for unfoamed types, he said.
Density of wood-fiber/plastic composites typically is 1.2-1.3 grams per cubic centimeter. Foaming agents can bring density to below 1, although final density will depend on how much vacuum is applied to the extruder during production. High vacuum will draw out some foaming-agent gas and limit how low the board density can go. Low board density also is difficult to achieve with formulations containing high wood-fiber loadings, Clariant research showed.
James Morton, a principal with market research and consulting firm Principia Partners of York, Pa., said several other additives are useful, or could be, for wood composites, including antimicrobials, coupling agents, flame retardants, heat stabilizers and impact modifiers.
Lubricants and colorants account for more than two-thirds of the volume and value of additives used in wood/plastic composites, Morton told conference delegates. The total additives market in 2001 was 42.2 million pounds, worth $59.8 million, he calculated.
Volume will more than double by 2006 as composite markets grow and as producers exploit processing and performance gains offered by the materials. About 35 firms supply additives for wood/plastic composites production, including 20 that do development work specifically targeting such products.