Demand for compounds filled with wood flour or other cellulose materials is booming. In the U.S., there is continued growth, primarily in decking, but other applications are ramping up. Growth is even higher in markets where the compounds are only now emerging, such as Europe, South America, and Asia.

As applications beyond decking manifest, wood flour and other natural fillers are expected to find their way into materials other than post-consumer polyethylene recyclate, which is currently the most common matrix material.

Wood-polymer compounds have been around for decades, but only in the last decade has the market taken off, as machine, material, and processing developments enabled higher production speeds. Exton, pa-based consultant Principia Partners predicts demand in North America, the leading market for wood-plastics composites, will more than double between 2001 and 2006. Building products will remain the top end-uses, with demand surging from 500 million lb (227,000 tonnes) to about 1.1 billion lb (500,000 tonnes) in 2006, but consumption in transportation, industrial, and consumer applications will more than double.

“Quality wood seems to be getting harder and harder to come by,” notes Tom Hughes, vp. of compounder Hughes Processing, Costa Mesa, ca. “As much as people love wood — and I’m one of them — at some point, plastics will take over certain markets.”

Hughes points to the spa industry as an example, where wood-plastics compounds are making inroads into what was exclusively a wood end-market. “The majority of spa manufacturers offer composite siding for a portion of their spas, and in some cases, that’s all they offer,” he remarks.

At die and compounds developer Strandex Europe Ltd., managing director Barry J. Davis adds, “In terms of awareness, the market is growing at a phenomenal rate.” In the U.S., “it’s blown aside the use of pvc for decking,” he says. Strandex helps processors with machinery, process, and market consulting.

Decking is the top end-use in North America, where pe is used as the matrix material in about 70% of compounds, followed by pvc at 18%, polypropylene at about 11%, and then some other materials. While Strandex has developed compounds based on other materials, it recommends pe. “[pe] is very forgiving (in processing), and there are vast amounts of recyclate available,” Davis explains.

Greater awareness of wood-filled materials is the prescription for further growth, especially in non-building products, says Bill Crostic, president of Onaga Composites, Onaga, ks, which compounds wood flour with virgin pe or virgin pp. He points out that, unlike talc- or glass-filled polymers, there was no pre-existing demand for wood-filled compounds, so new products must be developed or wood composites must displace entrenched materials.

The overall market is still small enough to have rapid growth for years to come. Citing statistics from consultant Kline & Co., Little Falls, nj, Crostic says, of the 5.5 billion lb of fillers used in North America last year, only 400 million lb were natural fibers, most of which went into decking. “Every 1% conversion from talc, calcium carbonate, or glass to wood equates to 50 million lb of filler or 100 million lb of compounded product.”

Inline compounding is an option, but a challenging one

Large North American wood-plastics decking processors compound materials in-house, which makes sense at very high consumption levels, but as more processors have entered the market, there is a burgeoning need for independent compounders.

Peter Bins, product manager at compounder North Wood Plastics, Sheboygan, wi, says most extruders, indeed, do their own compounding. “The decision to make or buy compounds is an economic one that depends on volume,” he notes. “Buying pelletized compounds slashes capital investment two ways. It completely eliminates the need for compounding equipment. Second, it allows use of lower-cost, single-screw extruders.”

Thomas Reußmann, of the Thuring Institute for Textile and Plastics Research, in Thuring, Germany, notes that the majority of processors do not have experience with natural fibers, which he says are much more difficult to compound than glass fiber, talc, and other fillers. Natural fibers, he explains, do not feed as well into the extruder hopper, due to variability in fiber thickness, stiffness, length, and diameter, as well as both friction between fibers and between the fibers and contact metal.

In North America, the supply of wood fiber or flour suited for composites making is growing, and prices have remained stable over the last six years, says Hughes. In other regions, it’s a different scenario.

Simon Lewis, sales development executive at WTL International Ltd., Macclesfield, England, claims his firm is the only U.K. supplier of wood fiber. Until recently, it simply sold fiber to processors and compounders, but last year it started selling compounds after installing a Cincinnati Extrusion twin-screw machine. WTL uses virgin pp in its compounds, and late last year began compounding a grade based on pvc. Lewis says, “Because we make our own fiber, our compound prices will be under competitors’ prices.”

Rudolph Wessely, general manager at Technoplast Kunststofftechnik, Michel-dorf, Austria, which has developed downstream profile extrusion equipment and dies especially for the market, notes that processors in countries where wood fiber is expensive or hard to obtain — he cites China, Malaysia, and Brazil — may turn to other cellulose fillers. “Our trials show no problem when using rice husks, and we’ve two or three projects in the works,” he says. Two Malaysian companies and one Brazilian firm are working with Technoplast on production of rice-filled profiles suitable for home-construction applications.

Technoplast works with extruder maker Cincinnati Extrusion, Vienna, Austria. Like any hot market, the list of extruder suppliers is long, growing, and includes Davis-Standard, American Maplan, Entek, ICMA San Giorgio, DGP Windsor, Bausano, and ExtrusionTek Milacron. Berstorff, Coperion, Farrel, and Leistritz, plus a few of the aforementioned manufacturers, supply compounding extruders for the market. Brabender Technologie and Colortronic are among manufacturers of feeding and blending systems for wood composites.

Alliances have formed. For instance, Strandex has non-financial relationships with Conair for downstream equipment and Cincinnati Extrusion for extruders; the company refers their equipment to its licensees. Onaga Composites works with American Maplan, Kansas American Tooling for dies, and downstream equipment maker ACS.

There is potential beyond decking and extrusion

Bins of North Wood Plastics says the importance of natural-fiber-filled compounds is that they yield higher-stiffness materials with lower densities than are obtainable with mineral fillers. Higher stiffness translates into lighter, lower-cost parts.

As wood-composites applications expands beyond decking, the number of non-pe-based compounds is likely to increase. Hughes says his firm’s abs- and asa-based compounds, though more expensive, produce higher-quality end-products than with pe-, pvc-, and pp-based compounds. He says some customers are in higher-end applications, such as horizontal blinds, shutters, and portable-spa siding. Others are in commodities like fencing and railing, where they foam the profiles to lower per-length costs.

Because the European decking market is small, at $200 million/yr or less, Strandex’s Davis says building products are the firm’s target market, especially doors and windows. The firm is also in discussions with luxury automaker Rolls Royce for profiles for car interiors. Natural-fiber compounds already are specified for use in compression molded interior-door-panel components.

Materials development may accelerate as major polyolefin suppliers focus on the market. Though he declined to identify the company, Davis says this month, Strandex will likely announce an agreement with “one of the largest polymer suppliers” to develop polyolefins tailored for wood composites.

Strandex has also received confirmation from a large European recycling company (not directly in plastics) for a Strandex license. It has asked Strandex to help it establish a compounding facility that uses post-consumer plastics recyclate. “Establishment of a large compounding facility in Europe will remove the reason many processors have avoided this market,” says Davis, as such a plant counters their concerns about material supply and making compounds themselves.

A Strandex license involves a one-time payment of about $750,000, with extrusion dies extra. Processors also pay a fee based on their output. “It’s not cheap to enter, and it’s not cheap to produce,” Davis admits, but he says Strandex can get compounders and processors into the market fast. “We take the entry time down from 9 months to 3 months.”

Injection molding may become a major process for wood composites. Onaga Composites’ Crostic predicts that injection molding will ultimately be a larger market than extrusion. “Today, about 30% of our business is injection molding. Next year it will grow to 50%.” This month, the compounder is installing an injection machine for product testing.

Compared to wood composites designed for profile extrusion, injection moldable wood-filled compounds require higher flow, lower wood loadings, and smaller-sized wood fiber/flour, he says.

Moisture remains a technical issue

With wood-fiber loadings typically at or over 50%, many of the fiber ends are exposed on profiles. For decking, this is an asset, since profiles are stained or painted, but they will weather and eventually fade. This does not seem to dissuade consumers, who are accustomed to some color change in natural wood decks, but in other applications, such metamorphosis may not be allowed.

Most of Hughes Processing’s customers encapsulate the wood compound with translucent capstock, which Hughes says ensures the wood color doesn’t fade over time and prevents rotting or fungal growth. Other processors use streakers and embossing to realize a wood-like appearance. But many new applications, such as furniture backing, take advantage of the lower weight and better strength of the composites with little concern for appearance as required in decking.

Removing moisture from fibers and keeping it away from compounded pellets is a major issue, say all observers. Hughes Processing stores and ships its pellets in sealed containers. Besides pre-drying, Bins of North Wood Plastics notes the wood component of the formulation is not as thermally stable as the polymer, so pellets need to be run cooler (under 400°F) to prevent fiber decomposition.

Desiccant drying is required, for 3 to 4 h at 170 to 180°F. Onaga Composites pre-dries wood fiber before compounding, a step Crostic believes makes his firm unique among independent compounders. (Some captive compounders also pre-dry fiber.) Pre-drying the material is needed since venting on extruders is not sufficient in removing moisture, says Crostic, though he allows that some new extruders with improved venting may prove sufficient.

He reckons moisture levels of 0.5% are acceptable for pe-based compounds, whereas pp-based composites require levels below 0.25%. Higher levels can be tolerated when foaming the composites, as long they remain constant.

Notes Crostic: “Since most wood composites are olefin-based, drying is a difficult but manageable problem. Due to the moisture-barrier properties of olefins, the compounds must be taken above the boiling point of water. With hdpe, this becomes challenging [as] the melt temperature is much lower than pp. Nevertheless, 60% wood-filled pe can be dried at 220°F. If moisture levels are above 2%, then a hot-air dryer should be used before a desiccant dryer.” Hughes says moisture levels in hdpe-based composites tend to be as high as 4 to 6%.

At lower loadings, drying is hardly an issue, says Crostic. “Any product that we compound with 30% wood is bone-dry and stays that way for weeks.”

For foaming wood-filled abs, moisture levels need to be below 0.04%, as moisture and foaming agents “don’t get along very well,” says Hughes, though he adds the levels are not difficult to achieve with proper drying systems.