In North America, building and con­struction applications still consume most of the total WPC volume. De­spite the slow housing market, WPC use continues to grow, replacing timber and other materials in decking, railing and fencing. “Growth is driven by the ongoing and increasing consumer desire for customized outdoor living space,” explains Van Kouteren. This trend has been gaining momentum, even spawning a whole new industry segment of dealers who specialize in outdoor living spaces. “Everyone is looking for low-maintenance materials, and the popularity of WPC is driving demand for more products in differ­ent colours and styles,” he adds. The North American market is segmented into low-end products, which are af­fected by the cost-differential between WPC and treated lumber, and high-end, premium products, purchased by affluent consumers who are less concerned about cost. One high-end product is Azek Building Products’ Procell™ technology, which uses foamed PVC filled with flax fibres. PVC is inherently more scratch-re­sistant than olefins, and using flax fibres rather than wood is reported to eliminate microbial growth and reduce colour fade. Despite its expense, the product has enjoyed success in the premium decking market. Encour­aged by Procell’s success, several other decking manufacturers have recently introduced PVC-based products. Other technical innovations in WPC for construction – foamed products, more colours and textures, weatherable coex­truded capstocks – are focused mostly on improved appearance and physical properties.

 

Direct or captive profile extrusion continues to consume most WPC volume in North America, but WPC pre-compounded pellets still have a place. JER Envirotech, which supplies custom ‘biocomposite’ compounds, sees very high growth in injection mould­ing applications and is penetrating new markets with WPC. “Our compound­ing and formulation technology offers highly dispersed WPC with consistently less than 0.5% internal moisture. This is news for injection moulders; many were not aware that this low moisture quality was available,” says Edward Trueman, president of JER Envirotech.

 

The WPC market in Europe is begin­ning to take off in a variety of appli­cations, with growth rates averaging 23% per year from 2003 to 2007 and predicted to continue at 26% per year through 2012, estimates Ap­plied Market Information (AMI) in a recently published WPC market study. In the last few years, decking has emerged to the point where it is carrying the market, consuming about 75% of WPC volume in Europe, says Mathias Daniel, product manager at Cincinnati Extrusion. “The wide gap between installed capacity and produc­tion volume is beginning to decrease because of decking growth,” he adds. Other profile extrusion applications for building applications - door and window profiles, fencing, cladding or siding, roofing, and marine structures - are also growing. “Sufficiently long-term experiences with WPC in North America are starting to build confi­dence in Europe that this new material could be an acceptable replacement for timber,” says Markku Vilkki at Fin­land-based extruder designer Conenor, Ltd., who notes that a predicted timber shortage and subsequent price increases have contributed to WPC growth. Andrew Aitchison at UK-based Wood Treatment Ltd. (WTL), which supplies both wood fibres and pelletized WPC compounds, sees growing interest from injection moulders and product designers for innovative uses of WPC. “Moulders are very interested, espe­cially in WPC’s sustainable features,” he comments.

 

The WPC industry is catching the ‘green’ fever and most are heavily promoting their products as being more suitable for the environment. Proponents point out that wood and natural fibre composites have multiple benefits: they replace a portion of the petroleum-based plastic with biomass, in some cases use recycled plastic, and often use wood flour (processed from lumber production waste) or another waste product as biomass. Where WPC are replacing timber, they help in the growing, global drive to preserve forests. Some wonder if consumers, particularly in North America, actu­ally consider sustainability in their purchasing decisions, but others say it does make a difference. “The green movement in England is strong,” reports Aitchison. He notes that build­ing projects for the 2012 Olympics in London are getting started, and that planners want to use sustainable mate­rials. “We hope this may be an impetus for using WPC,” says Aitchison. In Beijing, construction for the 2008 Olympics is a potential outlet for significant volumes of WPC, reports Mr. Brown.

 

Echoing an early driver for WPC development - finding an outlet for post-consumer, recycled polyethylene - some development today is driven by the desire to reuse a waste product rather than resorting to disposal such as landfill or burning. Rice hulls are being considered for plastic compos­ites in the mid-East and South Ameri­ca as a way to reduce the air pollution created when the hulls are burned in open fields, reports Mr. Vilkki. In another example, Vilkki notes that Conenor’s extrusion technology is be­ing used to reprocess plastic laminated paper and cardboard packaging waste from industrial sources into compos­ites for applications such as roadside sound barriers. The waste material is shredded and densified into pellets that can be fed to the extruder. JER’s Trueman adds: “Part of JER Envi­rotech’s mission is the utilization of waste products. Our customers look to us because they share this concern for sustainability.”

 

Many types of natural fibres or biomass are being considered, largely depending on local availability and economics. Re­searchers have found success in process­ing fillers such as nut shells, MDF (fibreboard) fibres, and straw, to name a few. Commercially, however, low-cost wood flour remains the most widely used for WPC in North America and Europe. Wood flour is plentiful and is a waste product captured by an existing infrastructure; it comes down to cost, concludes Al England at Strandex. Rice hulls are also being used commercially and are a relatively low-cost alternative, although their abrasive nature could be a drawback.

Natural fibres with a high aspect ratio, such as flax, add strength to composites. They also add cost, and so are finding a niche in premium construction products, notes Mr. Van Kouteren. JER’s Trueman comments: “We are looking at higher-perform­ing agricultural fibres in aiming to compete with engineering resins for injection moulded applications in consumer goods, transportation, and construction.”

 

Cellulose fibres, processed from post-consumer recycled pulp from the paper industry, offer several advantages for WPC, notes Edward Schut, market­ing director for CreaFill Fibers, which mills cellulose fibres for a range of applications, including WPC. CreaFill fibres are long, high-aspect ratio fibres that improve physical properties. Schut says that CreaFill fibres have colour stability because tannin and lignin, plant byproducts that cause degrada­tion, have been removed from pulp cellulose during the paper-making process. CreaFill developed a pelletized, compatibilized fibre product for easier dosing.

 

Several extruder manufacturers see a move in North America and Europe towards direct or in-line extrusion, rather than using melt-compounded pellets, in order to save manufacturing costs. Some define direct WPC profile extrusion processes as feeding all com­ponents directly to the extruder, while others include pre-mixing or agglomer­ating some or all components as direct extrusion. Asian WPC processors, which are beginning to demand more Western extrusion technology, typically produce profiles from pelletized com­pounds, comments Tom Brown, sales and marketing manager at Cincinnati Milacron.

 

Various extruder types are used in the WPC industry: co-rotating twin-screw extruders (TSE) and planetary extrud­ers have advantages in compounding, while conical and parallel counter-rotating TSE are commonly used for direct extrusion. In general, co-rotating TSE are very good at mixing, devola­tilizing, and providing high output, while parallel and conical counter-ro­tating are typically better at generating pressure for pumping material through complex profiles, explains Mr. Brown from Cincinnati Milacron, which builds all three types of TSE for WPC extrusion. When using a co-rotating TSE or planetary extruder for direct extrusion, a melt pump or single screw extruder is typically used to generate higher pressure for pumping through a profile die. Recent developments in­clude new designs and sizes of various extruder types.

 

New screw element geometries and de­signs in Entek’s E-MAX^™ 103mm TSE have enabled the company’s high-volume WPC deck and railing customers to achieve rates of 1361-1588 kg/hr (3,000-3,500 lbs/hr) or better, reports the company, which specializes in co-rotat­ing TSE and turnkey systems for direct extrusion of WPC. “We’re constantly working with our customers, optimizing our machines to meet their production goals,” says John Effman, director of sales and marketing at Entek.

 

At K2007, Cincinnati Extrusion GmbH added a Fiberex parallel, counter-rotating TSE to its line of conical, counter-rotating WPC extruders for profile extrusion. The new machine, with outputs up to 1000 kg/hr, includes two active degassing units with vacuum pumps that help remove moisture. The extruder is designed for premixed material in the form of melt compound, agglomerate, or regrind.

 

Milacron’s relatively new TIMBEREX^® co-rotating TSE incorporates a pat­ented discharge segment that generates pressure for direct extrusion without a separate melt pump. In this segment, the twin bores reduce in diameter and volume, creating two separate single-screw chambers that become efficient pumps, says Mr. Brown. Milacron also introduced its ‘Plug & Play’ Extrusion System that includes upstream mate­rial handling and downstream cooling and cutting. It provides a cost-effec­tive way to pre-mix WPC ingredients into an agglomerated particle and dry the particle upstream of the extruder, says Mr. Brown. He explains that the agglomerate breaks apart easily in the extruder without the shear needed to melt a melt-compounded pellet, add­ing: “The agglomerate can be fed into either a counter- or co-rotating twin, but we have focused on counter-rotat­ing because we feel it meets the needs of a greater portion of the market for pumping through more complex dies.”

 

Conenor’s Conex conical-rotor ex­truder, built by Maillefer Extrusion, is designed for extruding high fibre contents and waste materials into WPC. The two, nested rotors provide multilayer extrusion without a com­plex coextrusion die or a side extruder. This allows extrusion of recycled material in the core, surrounded by higher quality and appearance mate­rial, for example. “Real growth of WPC in Europe requires a substan­tially lower cost structure, beginning with direct extrusion and incorporat­ing low-cost, recycled raw materials,” Mr. Vilkki proposes. Although most ­extruder manufacturers recommend drying fibres prior to extrusion, saying that a twin-screw extruder is not moisture limits throughput, Vilkki counters that the additional energy a disadvantage and involves a poten­an efficient dryer and that excess cost of fibre sizing and pre-drying is a potential fire hazard. He says the Conex extruder is able to cope with the 10% moisture typically found in pelletized wood waste, widely available in Eu­rope from the woodworking industry and sold as biofuel pellets. In some cases, pre-heating wood pellets in the integral Conex-extruder feeders to release moisture from the fibres can be an efficient solution to increase the extrusion throughput at high line speeds, agrees Mr. Vilkki.

Planetary extruders offer low shear and high dispersion, which is an advantage for compounding WPC pellets, says Paul Godwin, director of sales and service at American Maplan, which builds a planetary extruder as well as counter-rotating TSE and single screw extruders for WPC. The planetary roller extruder creates a different type of dispersion; material is gently rolled between the central spindle, surrounding planetary spin­dles, and the barrel, explains German extruder manufacturer Entex Rust & Mitschke (see Figure 8). Planetary extruders can be used to compound PVC as well as polyolefins. Compared to TSE, the planetary roller extruder offers much better dispersion and fibre encapsulation, which is needed for improved physical properties and weathering durability, says Ralf Quack, process engineer at Entex/Tri­ad. In the Entex extruder, temperature is controlled by circulating a heating/cooling medium in the centre of the central spindle as well as in channels in the barrel, close to the material. “This unique design results in high heat transfer. The polymer is melted primarily with thermal energy rather than shear,” Mr. Quack explains. Because of this high heat transfer and the high surface area of the planetary roller extruder, fibres or wood flour can be fed to the extruder without pre-drying, says Mr. Quack.

Gala recently introduced a solution that allows underwater pelletizing of compounded WPC. Underwater pelletizing can handle materials that are difficult to pelletize with other methods, says Wayne Martin, proc­ess engineering manager at Gala Industries. However, moisture absorp­tion during underwater pelletizing of WPC has been a concern. Gala’s patent-pending i-heat process accel­erates pellet velocity after cutting in order to remove them from the water as quickly as possible. Remaining moisture is evaporated by the still-hot pellets. “We are able to get very low moisture levels that were previously unheard of in underwater pelletizing,” says Mr. Martin.