Canadian Plastics

Special Report: Wood-Plastics Composites

By Cindy Macdonald, associate editor   



WOOD-PLASTIC COMPOSITES CROSS THE THRESHOLD FROM NOVELTY TO CELEBRITY...

WOOD-PLASTIC COMPOSITES CROSS THE THRESHOLD FROM NOVELTY TO CELEBRITY

Under scrutiny from researchers and suppliers, wood-fibre plastic composites are just beginning to show their strengths.

Although one well-known industry expert, Jim Morton, says the wood-plastic composite industry “is credible now,” the 2004 Progress in Woodfibre-Plastics Composites conference had a somewhat defensive and introspective tone, as this young industry searches for an identity that goes beyond deck boards. The technical conference and exhibition was held May 10-11 in Toronto, presented by Materials and Manufacturing Ontario and the University of Toronto.

As research continues into WPC formulations and processing, it is becoming clear that these composite profiles excel in certain types of applications, and that decking, popular as it may be, is not the only thing WPC can be known for.

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“Structural components that simultaneously resist moisture intrusion and high loads arising from lateral forces present particular opportunity,” writes Micheal P. Wolcott in his presentation, Opportunities and challenges for wood-plastic composites in structural applications.

Wolcott, an associate professor with Washington State University, states that although the static mechanical properties of current polyolefin-based formulations are adequate for current applications, improved long-term load properties and reduced product weight will be necessary to exploit new structural applications.

Presenters in the markets/applications sessions emphasized that perceived product failures and shortcomings of WPC may not be due to the properties of the wood-plastic composite. There were comments that mold/mildew on WPC decking is often due to airborne contaminants, not mildewing of the WPC structure. Similarly, another expert stated that physical deformation (lifting, twisting, etc.) can often be traced to the substructure of a deck, and is not attributable to the WPC boards.

WPC shines in shear, compression tests

Wolcott and Paul M. Smith, a professor at Pennsylvania State University, assessed potential design values for various WPC formulations and compared these with common structural lumber grades. In all cases, Wolcott notes, WPCs perform best in shear, compression and dowel bearing strength when compared to structural lumber.

To some, this suggests possibilities for WPC in door sill (or threshold) applications. In fact, door sills are the first application of profiles produced by PSA Inc. (Guelph, ON) using the company’s second-generation oriented polypropylene extrusion technology. Jim Morton, senior partner with market research firm Principia Partners (Exton, PA), says that door sill applications have been very prolific in the past year. “This is an area that will likely move away from wood,” he predicts.

Morton is also bullish on the future of WPC in roofing products, which is good news for an Ontario-based producer of natural fibre-plastic composite roofing shingles. Wellington Polymer Technology Inc. of Chatham, ON, produces a cedar-like shake roofing product called Enviroshake using extrusion and compression molding. At the 2004 conference, Wellington’s Alex Dragomirescu presented some data on weathering and degradation of Enviroshake-type products, comparing the results for wood-fibre plastic composites versus flax-fibre plastic composites.

Tests conducted by Dragomirescu and associates showed that the flax composites were more sensitive to chemical attack, and held twice the moisture of the wood fibre composite. All weathered specimens passed the impact resistance test, and lost some flexural strength, but remained above the threshold of 80% retained value. None of the samples showed any signs of cracking, spalling or other forms of deterioration.

Dragomirescu reported that there is a good market for Enviroshake on the eastern coast of the U.S., in Ontario, and in British Columbia. Wellington Polymer Technology offers a 50-year warranty on the product.

Market changing but promising

Wood-plastic composites are the fastest growing segment of both the building products and plastics industries, according to Principia Partners’ Jim Morton. He says the building products industry is using this segment to “think outside the box”.

“Most companies came into this marketplace thinking they need to be in the deck and railing category. Now, some companies are wondering if they need to be in both, and how much differentiation there is between products.”

The market for alternative decking has exploded in recent years. Morton estimates there are currently at least 50 manufacturers of plastic and WPC decking in North America.

The influence of big box stores is starting to be felt in the deck and railing segments. Morton states that lumber yards are by far the most important distribution channel for WPC deck and railing products, and the effect of stores such as Home Depot and Loews adding these products to their product line is not yet known.

“These stores have a major say in product awareness to the average consumer,” says Morton. Following that train of thought, some major decking producers feel they don’t need to be carried in the “big box” do-it-yourself home centres, such as Home Depot, because awareness of the product category is all that’s needed for marketing purposes, he explains. A premium product such as WPC decking is generally purchased and installed by a contractor — the consumer need only know enough to ask for a composite material.

Morton notes that WPC producers had seen price migration downward, but that pricing is recovering as there is a growing belief that WPC can be sold as a premium product.

Building products producers are also deciding now where they will fit into the WPC value chain: materials supplier, WPC producer, distributor, retailer or end-user. Morton notes that some building products producers are now evaluating whether their position in this chain should be that of WPC producer, or whether they should fit in at the distribution level.

Regarding future applications, Morton is optimistic about roofing, but has a wait-and-see approach to WPC siding. “The consumer will hold companies to a higher standard for these applications.”

Some producers are seriously considering WPC for the window market, he says, and questioning whether there are possibilities for cap stock. Mikron and Anderson were both cited as users of WPC lineals in their window products.

A report published in April by the Freedonia Group Inc. (Cleveland, OH), demonstrates the shift from traditional construction markets for WPC to more broad applications. The report, Composite & Plastic Lumber (US$3900), predicts gains for WPC in fencing, window and door components, and siding, as well as decking.

Applications for WPC in windows and doors grew from $17 million in 1998 to $75 million in 2003. By 2008, this market is expected to reach $160 million, for a growth rate of 16.4%.

Young Minds at Work

Research posters at the 2004 Progress in Woodfibre-Plastic Composites conference showed some common avenues of exploration: use of nano-sized particles in WPC, advances in foaming, experiments with biodegradable products by pairing WPC with degradable polylactic acid (PLA) polymers, and the use of alternative fillers, such as Portland cement.

The winning poster detailed a project by three Queen’s University students which examined the competitiveness of various composite fillers for use in a composite lattice product for fencing applications. The project included analysis of physical properties, comparison with similar wood products, and a cost analysis. It was a joint effort of Andrea Adams (chemical engineering), Dan Prentice (civil engineering) and Jeff Bai (applied economics), with Frank Maine of PSA Inc. as the client supervisor.

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ADDITIVES CRITICAL TO ECONOMICS AND IMAGE

As the number of entrants in the wood composite and plastic lumber field has grown, so to have the formulations that are used to distinguish one brand from another.

“We have seen customer requir
ements change as competing brands offer more extensive and longer warranties on their products,” says Dennis Haff, new business development manager, Clariant Masterbatches. “For plastic lumber or a composite to be chosen over wood, the material has to demonstrate that it is markedly better. What the industry has learned in the last few years is that they need technical help in the additives area to make a strong case to both the lumber suppliers and the end consumer.”

The relationship between lubricants and coupling agents has become a key issue for the WPC industry as producers seek to maximize throughput by using lubricants, and improve physical properties through additives such as coupling agents. It has been found that traditional metallic stearate lubricants can compromise the effectiveness of coupling agents with respect to improving physical properties. There has been considerable interest in finding lubricant formulations that can improve throughput without inhibiting the function of coupling agents.

The new SXT products from Ferro appear to meet that need. At some customer trials, Ferro’s SXT 2000 and SXT 3000 non-metallic and low-metallic lubricant systems have increased output up to 1.5 times traditional output rates. As well, these lubricants can widen the process window, reduce viscosity, improve color distribution, and reduce melt pressure and processing temperatures.

Lonza has developed an all organic lubricant for wood-fibre plastic composites the provides higher extrusion output rates compared with combinations of Lonza’s Acrawax C and zinc stearate. In commercial equipment trials, Glycolube WP-2200 delivered a 100% increase in throughput rate versus an EBS/ZnSt lubricant system. The increased output was obtained with up to 33% less total lubricant. Glycolube WP-2200 is not yet available in Canada, but regulatory approvals are expected shortly.

Ivan Garneau, sales marketing manager for Clariant Masterbatches, says the use of dustless dry blends to color WPC is more prominent in the U.S. than Canada right now, but he expects Canadian processors will adopt this technique since it lowers raw materials costs and provides better color distribution.

Garneau also noted that a new coloring effect may soon be available for wood composites. Clariant is working on a method to add streaks of an alternate color to the WPC extrusion. “We have had some success with this effect. We’re working on optimizing the polymer carriers,” says Garneau. The research work is being conducted at Clariant’s Centre of Excellence for Wood-Plastic Composites in McHerny, IL.

On the additives side, Clariant’s Hydrocerol PLC series is a range of tailor-made chemical foaming agents for plastic lumber and wood-plastic composite materials which can help a manufacturer to increase production output, reduce the weight of the final product, and improve the “workability” of the end product.

Three chemical foaming agents from Techmer PM are suitable for polyolefin-based wood-plastic composites. TRCEX40400ES is a 40% loaded exothermic CFA. It has a gas yield of about 90 cc/g and a 400F decomposition temperature. TRCEX20400ES is a similar exothermic blowing agent, with a lighter 20% loading. For a lower decomposition temperature, in the 320F to 340F range, Techmer offers TRCEX20340ES.

Crompton’s Polybond 3029 coupling agent was specially designed for use in wood- and natural-fibre polyethylene composites. Compared with general-purpose grades of coupling agents, it provides improved tensile, flexural and impact strength, as well as lower water absorption.

A wood stabilizer that reduces oxidation and charring during processing is available from Reedy International. Saftec WSD contains unique process aids, including dispersants, which tend to improve both physical properties and surface features of highly filled systems.

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WHICH TO CHOOSE?

Twin screw extruders are the most dominant platform for a WPC system, but there’s infinite variety of choices and add-ons.

The industry is still undecided about which method is best for producing wood-fibre plastic composites, according to Robert Butts, sales engineer with American Leistritz Extruder Corp.

Within twin-screw extrusion systems, he says the two most common process configurations can be described as “pre-dry, split feed” and “wood first, split feed”.

In a paper presented at a wood-plastic conference in December 2000, Augie Machado and Stuart Kapp of Leistritz explained that the pre-dry, split feed method involves feeding the polymer and additives at the main throat of a twin-screw compounding-style extruder by means of loss-in-weight feeders. Pre-dried wood fibre is forced into the melt stream by one or two twin-screw side stuffers. The compounded material can then be pelletized or fed to a discharge pump to extrude a finished product.

In the wood-first, split feed method, Machado and Kapp explain, the devolatilization capabilities of a twin-screw extruder are used to dry the wood fibre in the early stages of the extruder. A second extruder can be used to melt, mix and pump the polymer/additive feedstream into the main extruder, or the polymer/additive blend can be side-stuffed in its dry/solid state.

Davis-Standard’s purpose-designed Woodtruder, for example, uses a single-screw side injection extruder to melt the polymer, and a paralell twin-screw extruder for drying, mixing and pumping. There’s no pre-drying or wood blending required, and a gravimetric feeder ensures process consistency. According to Davis-Standard, typical processing rates are 500-600 lb./hr. from a 94 mm Woodtruder, and up to 1500 lb./hr. on 140 mm model.

Cincinnati Milacron suggests its twin conical screw extruders (TC model) for wood-fibre applications, because the design provides a large intake volume for “fluffy” feedstocks such as wood fibre. The large-diameter feed zone contributes to output rates up to 1700 lb./hr. for WPC.

Milacron also offers its V-MEDS system to overcome the low density and moisture content of wood fibre. V-MEDS is essentially a co-rotating twin-screw preheating feeder system.

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