Composites as High Performance Building Solutions
What Is Green Construction?
In both new construction projects and renovation work, design professionals are continuing to discover the advantages of green building solutions: plastic composite building products, including durability, light weight, corrosion resistance, high strength, and low maintenance requirements. These plastic materials obtain much of their versatility because they can be engineered to provide specific performance characteristics. Technically known as fiber-reinforced plastics or fiber-reinforced polymers (FRP), plastic composites generally comprise two components: a reinforcement fiber and a polymer binder (often called a matrix).
The size, shape, proportional weight/volume, and material of the reinforcing fibers typically determine the plastic composite’s mechanical properties, such as stiffness and strength. The type and proportion of the plastic resin matrix, on the other hand, lends the finished plastic composite its physical characteristics, including resistance to impact.
In each case, the plastic composite is designed to provide a combination of properties that are intended to be superior to the individual ingredient. Fillers or additives such as epoxies and silicones can also be used to lend the final plastic composite attributes such as resistance to ultraviolet (UV) rays or fire resistance. Advances in plastic composite formulations and manufacturing technology offer exciting opportunities to custom-design highperformance properties into a wide variety of commercial and residential building applications, not only for new designs, but also renovation projects.
Designers of plastic composites can choose from a wide variety of plastic resin systems and fiber reinforcements when making a specific product. Resins, also known as binders, are typically thermoset plastics (e.g. polyester, vinyl ester, modified acrylic, epoxy, phenolic, urethane) that serve as the glue holding the reinforcing fibers together in an orderly fashion. These fibers, which are embedded in the plastic resin matrix, are structured to overlap and help transfer the load within the plastic composite structure. Usually manufactured using a plastic molding process, the combination of fibers and resin matrix cures into a solid laminate.
The plastic composite’s structural properties depend primarily on the type of fibers used. While glass fibers are the main ingredients for many plastic composites, certain physical characteristics can be harnessed through the use of carbon, aramid, or boron fibers. These materials impart stiffness and strength to the finished plastic composite and can control, to varying degrees, the end product’s weight. A wide range of properties and performance levels can be achieved to match the requirements for a specific application.
Agricultural and wood fibers have also generated interest in the last decade, especially in the housing sector. Although polyethylene, polyvinyl chloride (PVC), and polypropylene are the dominant polymers used with natural-fiber composites, natural fibers can also be used with phenolics, polyester, polystyrene, polyurethane, and other polymer matrixes.
Many plastic composite building products are now produced by embedding natural fibers derived from the bast (i.e. outer stem) of certain plants—wheat-straw fiber, flax, jute, kenaf, sisal, hemp, and coconut—in a polyester or polypropylene matrix. The moderate mechanical properties of natural fibers typically prevent them from being used in high-performance applications, but their low specific weight results in relatively high specific strength and stiffness, and is generally a benefit for parts designed for bending stiffness.
Another natural fiber, wood, is used primarily in composite versions of building products such as decking, window and door profiles, decorative trim, railings, and panel products. These plastic composite building products can contain anywhere from 30 to 70 percent wood, depending on the application, with about 50 percent being typical. Regardless of the proportion, the fibers used in these plastic composites most often take the form of a particulate (e.g. wood flour), rather than the longer individual fibers commonly used with inorganic-fiber composites. Commonly used species include pine, maple, and oak.
The polymer matrix or plastic resin used in wood-fiber composites can consist of:
- polyethylene (common in external building components and being investigated for building profiles);
- Polyvinyl chloride (PVC) (historically used in window manufacture and now also being used in decking);
- polypropylene (PP);
- polystyrene; and/or
- acrylonitrile-butadiene-styrene (ABS).
Some plastic products manufacturers use other thermoset plastic resins to produce specialty plastic composites with high wood content.
As with inorganic-fiber composites, small amounts of other materials can be added to wood-plastic composites (WPCs) to improve processing and performance. Additives may include coupling agents, light stabilizers, pigments, lubricants, fungicides, and foaming agents.
Plastic composites as building materials
For both new construction projects and renovation/replacement work, lightweight and corrosion-resistant plastic composite materials compete with traditional materials in residential, commercial, and industrial construction applications, including pipe, sheet piling for retaining walls, shingles, and concrete reinforcements (e.g. rebar). When a structural element, such as a wall or beam, can benefit from additional support, FRP composites could present a solution. Typical projects lending themselves to the advantages of plastic composites include decks, walls, and roofs.
Plastic composites are used in prefabricated, portable, and modular buildings, as well as for exterior cladding panels, which can simulate masonry or stone. In interior applications, plastic composites find application in shower enclosures and trays, baths, sinks, troughs, and spas. Vanity units, bench tops, and basins can be made from cast composite products. Each type of plastic composite brings its own performance characteristics, as illustrated by the following paragraphs.
Plastic composite panel and lumber products
Polyurethane-based binders are used to make plastic composite panels in various configurations, including oriented-strand board (OSB), hardboard (HB), medium-density fiberboard (MDF) and strawboard, particleboard (PB), and laminated veneer lumber.
Often used as structural sheathing, flooring, and roofing in residential construction, oriented-strand board (OSB) is made by coating strands of wood with plastic resin and then pressing them together at high temperature and pressure to form panels. By orienting the milled wood strands in layers (with each successive layer roughly perpendicular to the previous one), oriented-strand board (OSB) panels can exhibit substantial structural strength per unit thickness, allowing them to meet the requirements of the many building codes for numerous specific applications.
Milled strands and veneers can also be used to fabricate engineered lumber that can replace components made of traditional materials for many applications. If strands are milled at longer lengths than those used in oriented-strand board (OSB), and then aligned longitudinally across the entire cross section, they can be formed into long-strand lumber (LSL). Long, narrow veneer strips can also be layered along their length to form laminated-veneer lumber (LVL).
Another structural component is the I-joist, which comprises an oriented-strand board (OSB) web integrally bonded to LSL, LVL, or sawn 2×4 flanges. I-joists offer an alternative to traditionally constructed joists because they typically weigh less. Furthermore, since their length is limited only by the size of manufacturing equipment and shipping considerations (rather than the height of a tree), I-joists can be produced in longer lengths than those typically available with traditional materials. When a space is being redesigned, longer I-joists can help increase building spans—this not only allows architects more aesthetic freedom, but can also possibly help reduce the number of intervening support walls or columns.
Decking, fencing, and railings
Wood-plastic composites are commonly used when it comes time to replace exterior decking and moldings, doorjambs, fencing, and other applications where durability is an important performance attribute when properly manufactured and installed, wood plastic composite (WPC) lumber rarely rots, cracks, warps, or splinters in most normal U.S. climatic conditions. (As with all building products, the specifier should consult the manufacturer’s documentation.)
Typically stain-resistant, waterproof, ultraviolet (UV) light-resistant, and impervious to insects, wood plastic composites can be made strong enough for applications such as load-bearing deck boards. They also tend to have good dimensional stability and a lower coefficient of expansion than solid plastics. Wood fiber, wood flour, and rice hulls are common organic fillers used in these applications.
Co-extruded wood composite railings— typically comprising a core of polyvinyl chloride (PVC) or ABS resin and wood fiber capped by a weatherable polyvinyl chloride (PVC) or acrylonitrile-styrene-acrylate (ASA) protective layer—are increasingly being offered as an additional measure of customization for contractors and DIYrenovators. Along with a variety of color offerings to complement wood plastic composite decks, these capped composite railings offer water, stain, fade, and UV resistance.
Windows and doors
Polyvinyl chloride (PVC) is often used as the thermoplastic matrix in window applications (although other plastics are also employed). The fiber employed in windows and doors is usually 80- to 200-mesh wood fiber—this produces a wood-filled polyvinyl chloride (PVC) product, which offers thermal stability, moisture resistance, and stiffness.
In new exterior applications, both vinyl and polyolefin (e.g. polyethylene and polypropylene) plastic resins have been combined with wood flour to form exterior trim that resists rot and weathering. The resulting material can be molded into a wide range of designs that typically do not require painting or the use of special cleaning agents. At least one window manufacturer is creating such trim by combining the post-industrial waste from both its wood and vinyl window manufacturing operations.
There is also a growing trend toward using high-end plastic composites for doorjambs. By eliminating some of the performance concerns of commonly used materials, wood-plastic composite doorjambs can offer a complete, virtually maintenance-free system.
Sandwich construction—another common type of plastic composite structure—combines a lightweight core material with laminated composite skins. Doors made of fiberglass reinforced plastic (FRP) skins surrounding rigid polyurethane foam or expanded polystyrene (EPS) cores are currently available for both residential and commercial projects. These composite sandwich doors offer high specific strength and stiffness, low weight, impact resistance, and uniform smooth or textured surfaces. The core stabilizes the facings and carries most of the shear load. (A lowdensity core made of honeycomb or foam materials can provide structural performance with minimum weight.) Other considerations, such as sound insulation, heat resistance, and vibration-damping, dictate the choice of core material.
Another kind of sandwich is also finding its way into residential and commercial construction. Structural insulated panels (SIPs) feature a core of expanded polystyrene (or in some instances, extruded polystyrene [XPS] or polyisocyanurate [polyiso]) insulation sandwiched between two thin slices of OSB. The resulting floor/wall/roof panel is strong, lightweight, and can be designed to have exceptional insulation properties. Additionally, since they are manufactured components, SIPs can be delivered to the job site sized for a specific application, with wiring chases and provisions for plumbing rough-in machined or molded into the foam core and the OSB outer panel.
Composite materials are also being used in renovation applications to help strengthen beams, slabs, walls, columns, chimneys, and other structural elements subjected to deterioration, additional service loads, or excessive deflection caused by change in use, construction or design defects, code changes, and seismic retrofit.
Wrapping masonry in fiber composite materials can significantly increase its strength. This is particularly important in many communities where seismic codes have been tightened and masonry and concrete walls must be upgraded to meet them. The advantages of fiberglass reinforced plastic (FRP) materials lie in their high tensile strength, low weight, and their ability to conform to varying shapes. The growing use of fiberglass reinforced plastic (FRP) composites in repair and retrofit of concrete and masonry structures has opened the door for similar applications in strengthening wood beams.
Many manufacturers offer roofing shingles made of both post-industrial and postconsumer recycled rubber, recycled plastic, and cellulose fiber.1 Bound together with polymer adhesives, these shingles can be made to look like cedar shakes or slate tiles, as well as standard asphalt shingles. They may help to lower building energy consumption, while providing long, virtually maintenance-free service life.
Glass-reinforced plastic (GRP) pipe is used for the transport of water and wastewater in pressure and non-pressure systems. Glass-reinforced plastic (GRP) is a filament-wound, structural plastic composite made with glass fiber and polyester resins. The pipe is lightweight, corrosion-resistant, and designed for ease-of-installation. Its corrosion resistance can give products made from the material a long, effective service life with low maintenance costs while making it a strong candidate for piping applications in environments with acidic soil content.
Kitchens and baths
Cast plastic polymers, which encompass cultured marble, cultured granite, cultured onyx, and solid-surface products, are chemically bonded and mineral-filled materials used in a wide range of household and commercial applications. Some of these applications include countertops, vanities, shower receptors, bathtubs, enclosure sets, fireplace surrounds, windowsills, wall panels, floor tiles, whirlpool baths, and molding accents.
Cast plastic polymers are strong and can be less brittle than some traditional materials. The plastic products manufacturing process permits a design latitude and complexity of shape in the finished product that can be difficult to achieve with many alternative products. Moreover, cast plastic polymer products can resist mildew and stains, and they are easily cleaned with non-abrasive cleaning agents. The onepiece construction of cast plastic polymer is also easy to maintain and can be resistant to breakage.
Specially formulated non-reinforced plastic polyester resins, known as gel coats, can improve the impact and abrasion resistance, as well as the surface appearance, of the final product. These are applied to a mold surface and gelled before lay-up of the plastic composite. For both new projects and upgrades, glass/acrylic units made with polymethyl methacrylate compete with products made from traditional materials in the tub and shower market.
Thermoset-based solid surface materials used in kitchen countertops and bathroom products are produced with highperformance plastic resin systems, such as acrylic and unsaturated polyester, formulated with a very high content of fillers, pigments, and catalysts. This mixture is cast into molds, forming the matrix into either flat panels or customized shapes. The finished plastic composite product can be machined like wood, routed to make decorative edging, and precisely cut and bonded to fit nearly any surface shape.
Increasing interest in high-performance building solutions should continue to drive the growth of these plastic composite hybrid materials in new construction and renovation, especially residential housing applications. Plastic c omposites differ from traditional materials in that the combination of their distinctly different components can make new high-strength, lightweight materials with corrosion resistance, long-term durability, and low maintenance requirements. Plastic composites also can offer design flexibility, good vibrational damping, and resistance to both fatigue and temperature extremes. Skilled designers and fabricators understand plastic composites’ unique properties and performance capabilities and are able to develop custom-designed plastic products for optimal material performance. The unique blend of properties designed into the final product allows designers and manufacturers to substitute high-performance plastic composites for traditional materials.
1 Recycling is not available in all areas.