July 28, 2015
Since the 1990s, many technologies emerged to use recycled plastics as a replacement for conventional wood lumbers. Recycled plastic lumber was developed in the production of exterior deck boards, moldings, doorjambs, window casings, playground equipment, railway ties, pilings, posts, fencing products, as well as outdoor furnishings.
Recycled plastic lumber products can work with conventional carpentry tools and have several advantages over wood products. They resist rot, mildew, and termites; they do not require regular painting or staining, and they are, otherwise, low maintenance materials. Many plastic lumber products are highly attractive as they can be manufactured to meet a wide variety of design and appearance specifications. When wood or some other natural fiber source is incorporated into the material, many plastic lumber products can be painted or stained.
This section briefly reviews the major production systems that are being used to make recycled plastic lumbers — including single polymer systems, extrusion flow molding systems, fiberglass-reinforced RPL, PVC extrusion profiles, wood fiber plastic composites, oriented wood fiber polymer composites, and polymer-polymer products.
Single Polymer Systems, use (primarily) continuously extruded, structurally-foamed high-density polyethylene, represent a significant part of the deck board market. The producers tend to use natural HDPE from milk jugs that can be pigmented to produce decks with attractive colors.
Extrusion Flow Molding is one of the first processes to be utilized to manufacture plastic lumbers with technology that developed in Europe. These systems utilize mixed polymers with lower raw material costs. However, the earlier versions of this system suffered from low productivity with poor product quality, which led to a low growth for this process.
Fiberglass-Reinforced RPL can be used to produce structural components and has a growing list of applications: deck joists, marine break walls, bulkheads and pilings, railway ties, and more demanding structural components
PVC Extrusion Profiles are used in railing and deck board markets.
Wood-fiber–Plastic Composites are the largest and fastest-growing segments of the recycled plastic lumber market. In the early 1990s, products were commercialized using mixtures of polyethylene and wood to manufacture deck boards and other wood substitute products. They were manufactured with mixtures of 50 – 70% wood fiber and 30 – 50% polyethylene, either high or low-density PE or mixtures of the two polymers. The extruded deck boards and profiles exhibit higher modulus than pure polymer products (such as those made from HDPE) and can be painted and stained like wood. They are offered in natural colors that age to a gray shade similar to aged cedar, but can also be manufactured with blue, gray, or red pigments that simulate the popular wood deck stains.
Dramatic improvements in flexural strength and flexural modulus have been demonstrated by cold drawing extruded polypropylene-wood composite. The flexural modulus of an oriented polypropylene composite with 30% wood fiber can achieve 82.5% of the flex modulus of dried pine. The same material had a flexural strength that was more than double that of pine.
Polymer/Polymer Systems is an interesting new technology developed by Rutgers University, which discovered that specific blends of polymers, normally thought to be incompatible (such as polyethylene and polystyrene), can form composites with properties that dramatically exceed the expected performance of the blend. Under the right conditions of mixing and component levels, an inter-penetrating network of the polymer can achieve a better balance of modulus and impact strength.
The pure polymer extruded profiles have performance characteristics that reflect the properties of the polymer used in its fabrication. The stiffness of recycled HDPE is lower than that of a polyethylene-wood fiber composite. The performance issues associated with pure polymer systems center around the color fastness and UV resistance of the surface of the board stock. Many studies have shown that UV stabilizers and antioxidants improve performance. It is important to select metal-based pigments for long-term color stability. The organic pigments appear to be less stable during long-term exposure to the sun. The cross-section of extruded profiles is so massive that any UV degradation affects the surface of the extrusion, but has little effect on the bulk properties of the board. With the wood-fiber polymer composites, some properties being studied to determine long-term serviceability. Surface degradation effects, selection of pigments, and use of UV stabilizers and antioxidants are similar in their effect on performance as they are when added to pure polymer extrusions. Moisture uptake in composite boards is being studied extensively. Absorption of moisture occurs slowly over time. Equilibrium moisture content may take years to achieve. In water immersion tests, the use of maleic anhydride bonding agents appears to reduce moisture uptake. Finally, the addition of up to 20 percent glass fiber reinforcement will increase modulus significantly. Fiberglass-reinforced polyolefins are currently offered as structural grade components for deck building.
This article is based on the research report of Environment & Plastics Industry Council (EPIC): Recycled Plastic Lumber: A Strategic Assessment of its Production Use and Future Prospects, by David Climenhage.