Wood-plastic composites need protection, too.The primary advantage of wood-plastic composites (WPC) as a replacement for wood in outdoor applications like decking and railing is their low maintenance and relative weather-resistance. While in the early days of WPC decking, expectations were that the product would maintain a new appearance without any maintenance, this has not been entirely the case. Some WPC marketers have tried to present a more realistic "low maintenance" rather than "no maintenance" picture, promoting regular cleaning and noting that some color fade should be expected. At the same time, the industry is raising the bar on aesthetics, using various strategies to improve colorfastness and to protect against microbial growth, scratching, and food or grease stains.
Additives for Protecting WPC Appearance
One strategy for improving weathering and appearance is to use additives to protect the polymer and wood. Indeed, formulations with weatherable pigments and UV stabilizers are being used to minimize color fading. Weatherable, inorganic oxide pigments are typically used to color WPC. Polyolefin composite decking can use iron oxide-based pigments, while PVC composite decking uses the same complex inorganic colored pigments (CICPs) that are used in vinyl siding. "Color fade" in WPC does not typically come from degradation of the pigment. The primary cause of color fade is the natural lightening of any exposed wood in the composite. Since only the plastic portion--30% to 50%--of the composite is pigmented, whitening or graying of the wood portion exposed to sunlight causes the overall color to fade.
A secondary cause of color fade can be degradation of the polymer. Surface cracking in olefinic composites or yellowing or chalking in PVC composites can mute the original color richness. UV light stabilizers, including hindered amine light stabilizers (HALS) or UV absorbers, are typically used to help protect the polymer from UV degradation. Pigments can also absorb or reflect UV light to help protect the polymer from degradation. Newer infrared reflective pigments are being used to reduce heat buildup in the deck, which is useful for consumer comfort, helps prevent warping, and may also help slow degradation. While existing additive technology does help make a good WPC product, there is still plenty of room for improvement of color retention, agree industry experts.
Penn Color has been working with producers to obtain improved weathering performance without adding significant costs by adjusting the pigment type and percent loading. These new coloring packages have shown improvements in weathering by up to 30%, while adding only a few cents per linear foot of deck board, says Bob Kaminski, industry manager for thermoplastics at Penn Color. The company is also experimenting with stabilizing the wood portion to prevent it from lightening. Techniques are available for coloring the wood fibers separately, but at an added cost and process complexity, say others.
Americhem has spent two years developing improved colorant solutions for olefin-based WPC that are a combination of additive, pigment, and carrier-resin technologies. The target is composite color that weathers like vinyl siding, with no drastic color fade in the first year of exposure, notes Laura Prexta, technology manager for Building and Construction at Americhem. The improved master-batches are undergoing accelerated weather testing and outdoor exposure testing in commercially produced deck boards, with promising initial results, she reports. Accelerated testing in a weatherometer includes variables of moisture, temperature, and light, and is very useful for ranking samples exposed to the same conditions. Although it doesn't directly correlate to outdoor exposure, which has many more variables, accelerated testing can be used as an indicator.
Besides aiming for improved color retention in standard colors, WPC manufacturers have introduced more natural wood-grain finishes and darker colors that imitate tropical hardwoods. Darker colors seem to weather better than lighter colors--although lab tests show a similar shift in color measurement numbers, the visual appearance of darker samples is better, notes Penn Color's Kaminski.
Although manufacturers note that decking should be washed periodically to remove dirt, which helps prevent mold stains, biocidal additives to minimize mold growth are also increasing in use. Visible mold stains on decking are caused by surface mold fungi that grow if given the right conditions of temperature, moisture level, and food source, such as exposed wood or dirt on the deck surface. In addition, decay fungi can grow if moisture contents are above 25%. While lab test methods show low composite moisture contents around 4%, field tests have recently shown much higher moisture contents near the surface and ends of boards--in some cases above 25% moisture content, notes Mark Manning, manager of preservation technology at U.S. Borax (Rio Tinto Minerals). Under these conditions, there is potential for decay fungi to grow on unprotected wood, he warns. Broad-spectrum biocides such as Vinyzene DCOIT-based formulations from Rohm and Haas reduce growth of surface fungi on both wood and plastic. Microban[R], a triclosan-type antimicrobial, is also being used. Borogard[R] ZB zinc borates from Rio Tinto Minerals act to both inhibit surface mold growth and preserve exposed wood from decay fungi. Borogard ZB is also reported to help reduce levels of UV degradation, particularly when used with hindered amine light stabilizers (HALS).
The improved performance brought by additives comes with added formulation expense. Teknor Color recently introduced TEK Deck Pro[TM] masterbatches, which combine pigments with UV absorbers and biocides, saving processors money by reducing additive-handling costs.
Encapsulating Exposed Wood During Compounding
The compounding process--coating the wood fiber with plastic and dispersing the wood, pigment, and additives throughout the plastic--can be key in obtaining improved aesthetics by encapsulating the wood fibers. Exposed wood at the composite surface will absorb grease or liquids like wine, causing stains that are difficult to remove. Unprotected wood also absorbs moisture, which can lead to microbial growth. Proper screw configuration is important for optimal mixing, notes Paul Andersen, director of process technology at Coperion Corp., which offers co-rotating twin-screw extruders for WPC extrusion. Drying of the wood component is also important, because excessive moisture can inhibit mixing and form an interface between the wood and polymer that inhibits encapsulation. Some extrusion system configurations use predried wood fibers, while others dry the wood component in the extruder prior to adding the polymer. Coperion supplies both configurations, and recently introduced a new easy-to-install side vent designed to minimize particle entrainment.
Additives play a role in compounding. Both lubricants and coupling agents aid in dispersing pigments and wetting-out wood fiber, which promotes uniform color and weathering. Coupling agents also help bond the polar wood with the nonpolar polymer. Coupling agents improve strength properties, as well as improving encapsulation and reducing moisture absorption, which helps prevent mold growth. Maleated polyolefins are the most common coupling agent used in WPC. Chemtura introduced a nondusting powder form of its Polybond[R] 3029 maleic anhydride-grafted PE coupling agent for PE composites. Polybond[R] 3039 powder is useful for WPC manufacturers who blend powdered additives prior to feeding to a counter-rotating twin-screw extruder. DuPont[TM] Fusabond[R] W PC-576D incorporates anhydride functional groups as a copolymer, giving improved performance at low additive levels. Although traditional metallic stearate lubricants are known to interfere with maleic anhydride coupling agents, non-interfering lubricants are available for these formulations. Dyneon has tested a polymeric additive that provides both lubricant and coupling properties through functional groups tuned to go to the interface of the wood and polymer. In accelerated and outdoor weathering tests, these developmental controlled-architecture materials (CAM) reduce color fade and moisture absorption, and improve retention of physical properties after weathering.
Some WPC lumber manufacturers point to their compounding techniques for improved weathering. Advanced Environmental Recycling Technologies (AERT) says that the process for their MoistureShield[R] professional grade decking heats and pretreats the wood fiber prior to extrusion and provides "total encapsulation" of the wood fiber. This makes the boards "virtually moisture-proof," minimizing the need for preservatives. Composatron Composite Technology uses Strandex technology to encase strands of wood flour in plastic, which, the company says, results in significantly less color fade and reduces water absorption.
Protective Coextrusions and Coatings
Other manufacturers have introduced products with a coextruded polymer cap or coating on the wood-plastic composite. The capstock protects the wood from the environment, preventing it from absorbing moisture and stains. For example, Correct Building Products coextrudes a coating of PP containing UV, scratch, stain-resistant, and antimicrobial additives over the PP-wood fiber composite using a patent-pending process for their CorrectDeck CX product. While the core of the board does contain antioxidants and UV stabilizers, putting higher levels of additives only in the surface layer makes it a more affordable solution. Composatron uses an ASA (acrylonitrile-styrene-acrylate) coextruded capstock for its Premier[TM] railing products. ASA is inherently more weather-resistant than polyolefins, and can also hold darker pigments. Royal Crown Limited also puts a protective cap layer on its Harmony Select[TM] decking, introduced in 2005 and made with Celucor[TM], a composite of natural fibers encapsulated with cellular PVC. CertainTeed's Panorama PVC capped composite, introduced in 2005 for railings, replicates a painted wood look with good weather resistance and the handling and feel of wood, says product compliance engineer Neil Sexton.
Rohm and Haas is experimenting with acrylic-based polymers for thin coatings to be applied to deck boards after extrusion. The primary goal for the coating would be short-term scuff, stain, and bacterial resistance to protect boards during transport to the building site, although longer-term protection is a possibility. Rohm and Haas is also looking at acrylic-based polymers for solid color and semi-transparent stains for after-market use on WPC decks. This would allow consumers to restain their composite deck like a wood deck. One difficulty is in finding a coating that will adhere to all the different resins used in composite decks, notes Michelle Toth, technical group leader for WPC at Rohm and Haas.
Advanced Environmental Recycling
Technologies, Inc. (AERT)
+1 800-951-5117
www.aertinc.comAmerichem
+1 800-228-3476
www.americhem.comBorax (Rio Tinto Minerals)
+1 661-287-5400
www.borax.comCertainTeed
+1 517-780-3185
www.certainteed.comChemtura
+1 203-573-2000
www.chemtura.comComposatron
+1 416-335-6500
www.composatron.comComposite Lumber Manufacturers Association
+1 202-207-0906
www.compositelumber.orgCoperion Corp.
www.coperion.comCorrect Building Products
+1 877-332-5877
www.correctdeck.comDuPont Industrial Polymers
+1 302-774-1000
www.dupont.com/industrial-polymers/woodalternativesDyneon
+ 1 800-810-8499
www.dyneon.comPenn Color
+1 866-617-7366
www.penncolor.comPrincipia Partners
+1 800-378-8330
www.principiaconsulting.comRohm & Haas
+1 800-356-3402
www.rohmhaas.comRoyal Crown Limited
+1 800-488-5245
www.royalcrownltd.comTeknor Apex Co.
+1 401-725-8000
www.teknorapex.comTrex Co.
+1 800-289-8739
www.trex.com