The Promise and Pitfalls of Plastics in Construction
The world is literally drowning in waste plastics. Can we redirect the stream into long-lasting building materials?
I won’t spend a lot of time elaborating on our global plastic addiction. Suffice it to say, the stuff is taking over. We’ve polluted our oceans and seafood with it, laced our drinking water with it, and are now using (and throwing “away”) more of it than ever. We need an intervention.
Perhaps that intervention could come in the form of an embrace. Given the season of storms and flooding we just experienced, plastic-based building materials are consistently looking better. In the least, we could keep some trash out of landfills and waterways. At best, we might create a whole new way of building, with durable, flood-resistant materials.
Although not without its drawbacks, the right time is now for lumber and other building products made with recycled plastic.
Plastic, especially when recycled, is actually much less energy intensive to produce than cement or steel. Biochemist Anthony L. Andrady argues that “the benefits provided by plastics justify the 4 percent fossil fuel raw materials and another 3 percent to 4 percent energy resources devoted to manufacturing it. In building applications, plastics save more energy than they use.”
But Andrady acknowledges that the plastics industry “has its share of environmental issues.” It is based on a linear flow of nonrenewable fossil fuel resources via useful consumer goods into the landfills. Lack of cradle-to-cradle corporate responsibility and design innovations to allow conservation of resources is responsible for this deficiency.
“For instance,” he adds, “there is not enough emphasis on design options for recovery of post-use waste. The move toward bio-based plastics, an essential component of sustainability, is too slow, with not enough incentive to fully implement even what little has been achieved.”
For plastic to remain a viable, useful material, full transparency about the products and processes is needed. We must acknowledge plastic’s down side—the perils of toxic emissions, ocean contamination and harmful byproducts—and address them directly.
Which leads back to the question posed by this article: Can the linear life cycle of plastics be interrupted on a large scale by the building industry, diverting post-consumer plastics to be used in construction? I believe it’s possible. But it’s a shift that will require new thinking from industry and consumers. The dire problems we now face from plastic pollution may hold within them solutions to other issues around housing and resilience. Imagine durable plastic walls and framing required by code in flood-prone areas. Just hose it down after a flood event, with no rebuilding or untold tons of landfill waste.
A couple companies sell this material. One is in California: American Plastic Lumber, Inc. Its 3/4-inch-thick recycled plastic “sheet goods” (imported from Asia) have the following characteristics:
Typical tensile strength for “sheathing grade” plywood, according to MatWeb, is 4,000 to 5,000 psi. Its flexural modulus is about 149,000 psi. In other words, plywood and plastic sheets have similar strength characteristics when temperatures are mild. The full specs can be found here .
Before we get there, however, we need more honest, third-party research and a commitment to closing the life-cycle loop—not just more use-it-and-toss-it mentality.
Digging for Data
What do we really know about plastic’s potential in the built environment? Research on the topic tends to focus myopically on one engineering challenge (such as UV resistance) or entirely ignore the big questions of durability and end-of-life prospects.
Information on plastics is complex and variable. One challenge is that no two types of plastics have the same physical properties. For example, vinyl siding, made with 80 percent polyvinyl chloride, is probably the plastic material most familiar to homebuilders.
The industry-funded Vinyl Siding Institute (VSI) is the best-known source for information on the performance and fate of PVC siding. VSI, however, often narrows its product R&D to “silos” of inquiry. For example, a 10-year study of vinyl siding that started in 1994 (VS4W) looked at the “color ellipsoid” of installed product, not whether it became brittle, warped or caused any moisture damage to sheathing. Too often, the really big questions are ignored, skipped or buried in dense academic research that’s indecipherable to anyone but a chemist and/or engineer.
Plastics behave differently as the temperature increases. Each has different tensile strength and load-bearing ability at specific temperatures. These are key factors in determining how and where plastic-based materials can be used in construction.
Fortunately, the ASTM (previously known as the American Society for Testing and Materials) has been proactive in developing standards for recycled plastic lumber (RPL), primarily for decking (D 6662). During this process, the organization identified and attempted to remedy missing info and specs. Here are some key takeaways:
Dimensional Tolerances: Tolerance limits were established that would meet industry requirements and performance considerations.
Creep: The viscoelastic nature of RPL makes it susceptible to creep at sustained loads at elevated temperatures. A methodology was developed to use creep data per ASTM D 6112 to define design limits to avoid excessive deflection and creep in the decking boards.
Flammability: ASTM’s fire test method uses a small ignition source, as might be expected on a deck, when hot charcoal briquettes from a tipped over barbecue grill make contact.
Allowable Material Properties for Structural Design: A complete methodology was presented in the standard to determine allowable maximum span lengths for decking boards based on the material properties determined from the test methods listed above.
Like any building material, plastics must be able to withstand the rigors of their intended use. Standards such as those devised by ASTM answer many of those questions. But here’s the scoop in layman’s terms:
Some plastics get soft at relatively low temperatures. For example: HDPE, the plastic used in most milk jugs, begins to soften at 172°F under low stress, but about 114.8°F under high stress.
A Denver-area deck builder tested surface temperatures of various deck materials at 87°F ambient temperature in full sun. He found that virtually all deck boards, including cedar and plastic composites, soared above 150°F, well past the heat deflection point of all common plastics.
Of course, building standards, such as reducing the joist distance, can compensate for some polymer weaknesses. But designers who use RPL also face a new, emerging variable—extreme heat. Progressive temperature rise and temperature spikes are expected to increase in coming years, due to climate change.
Most thermoplastics do not fully melt until they reach about 250°F (HDPE melts at between 248°F and 356°F), but they become soft and bendable long before that benchmark.
Deck tests in Boston found that decks typically get up to 76°F hotter than the ambient air (in full sun). Of course, outdoor temperatures would need to approach 174°F before the plastics would completely melt, but in the American West, ambient temperatures frequently exceed 100°F. Sun-exposed HDPE products without additives could easily exceed their design limit (100°F + 76°F = 176°F). American Plastics, for example, lists 170°F as the maximum temperature acceptable for its recycled HDPE sheet polymer.
Another concern with plastics, of course, is fire. Polymers are not especially ignition prone, but they burn hot and fast, igniting at about 540°F. As high-carbon materials, however, plastics tend to give off a very dense black smoke. The level of toxicity varies by material, but at the very least it’s disorienting and difficult for firefighters to navigate.
As engineer Geoffrey Pritchard notes in Reinforced Plastics Durability, adding fire retardants to plastics has pros and cons. They will work, but “there can be adverse effects on processing, mechanical properties or chemical resistance.” In other words, the material becomes less workable.
The Eco-Argument for Plastic Lumber
Overcoming Recycling Hurdles
Healthy Building News reports that with regard to polyethylene, “proportionally less ‘good material’ is coming out of the plastic waste recycling stream due to the rising use of municipal single-stream recycling over the past decade. Mixed- and low-quality scrap materials that come from single-stream recycling centers are more likely to be exported than sorted and screened for high-quality polyethylene scrap. As a result, more recovered plastic bags are exported than processed domestically.”
The Healthy Building Network’s efforts to optimize recycling track how the building industry is doing. They note that the plastic lumber sector, unlike the plumbing manufacturers, are stepping up their recycling protocols despite a lack on industry-wide standards for post-consumer product quality. “This year, the Association of Plastic Recyclers (APR) posted the industry’s first testing protocols, benchmark specifications and a grading system for bales of collected HDPE. It prohibits many contaminants and restricts others.”
Another growing problem is the use of pouches that combine aluminum and plastic layers. They’re notoriously tough to recycle. Dow Chemical is reportedly working on a recycling solution.
Fortunately, new methods of marking and recycling plastics are being tested, at least outside of the U.S. For example, Ioniqua Technologies, based in the Netherlands, has developed a way to separate plastics from the additives that give them color or other properties.
As reported in a white paper by Ethical Corporation, “When PET is added to the magnetic smart liquid that Ioniqa has developed, and then heated, the PET depolymerises. The colourants and other contaminants are removed in a magnetic field, to leave the original building blocks of the polymer.” Those building blocks can be used to create new PET, over and over again. That’s a big advantage, because at the moment PET can only be recycled up to six times.
The total recycling rate of HDPE, LDPE and PET in the U.S. was 8 percent in 1996, and has now increased to only about 10 percent for HDPE, 5.3 percent for LPDE and 19.5 percent for PET.
But as WorldWatch Institute points out, plastic manufacturing has grown at a vastly faster rate. Recycling can’t keep up with the avalanche of waste: “From 1950 to 2012, plastics growth averaged 8.7 percent per year, booming from 1.7 million tons to the nearly 300 million tons of today.”
This is just one of many reasons for introducing recycled plastic-based products into construction. Others include:
Less treated wood. Plastic lumber can replace treated wood, which is still considered a hazardous waste material by most landfills. Although modern treated wood has far less (if any) chromium or arsenic, it is still infused with copper, making it hard to reuse.
Alternative to tropical and redwoods. Plastic wood has many of the desirable durability qualities of pricey (and sometimes endangered) South American and Asian wood species such as teak and ipe. It can take some of the pressure off of endangered forests, although other threats such as palm oil production have replaced lumber sales as the biggest source of deforestation.
Flood resistance. With its natural resistance to rot and mold growth, some plastic lumber is ideally suited for use in wet or high flood risk locations.
Garden friendly. Although more research is needed, some early studies of plastic lumber in marine environments found very little leaching, compared with treated wood alternatives. This makes it a good candidate for garden hardscapes and beds.
Lower GHG than concrete or metals. On a pound-for-pound basis, plastic is less resource intensive than creating Portland cement or melting down metals. Recycled material has far more of an edge than virgin production. And, of course, plastic has durability on its side. It should last for decades, before being recycled again.
Industry Leaders and Laggers
A few building product manufacturers have recognized the ethical importance of recycled plastic content—and its potential to improve their bottom line. Interface, for example, has recycled about 309 million pounds of plastics over the past 20 years. The late CEO of Interface, Ray Anderson, set a goal of freeing the company completely from use of virgin materials, and the company seems to have remained committed to that idea. Its carpet backing material, Glasbac, contains about 98 percent recycled material. But in the long view, Interface and others have barely scratched the problem. The carpet industry still dumps about 4.5 billion pounds of end-of-life product into landfills every year.
I’d also be remiss not to give a caveat to Trex. Back in the 1980s, this company arguably launched a whole industry of composite decking where none existed. It has proven that profit and recycling can be close bedfellows. And it has shown how to be more flexible, not less, in terms of the types of plastics that are eligible. Trex currently processes grocery bags, bread bags, case overwrap, dry cleaning bags, newspaper sleeves, ice bags, wood pellet bags, Ziploc and other re-sealable bags, produce bags, bubble wrap, salt bags and cereal bags into their 95 percent recycled decking products.
Where the Plastic Hits the Road
Could 100 percent recycled plastic roads become reality?
According to motortrend.com, it’s already happening. A company in the United Kingdom, MacRebur, is already testing at least two roads made with recycled plastic replacing much of the asphalt bitumen. Typically, bitumen accounts for about 10 percent of the asphalt mix, but it comes at a high environmental and financial cost.
Motortrend.com notes that “The material, dubbed MR6, is made with 100 percent recycled materials and can reduce the amount of plastic waste that ends up in landfills. Not only is it considered a greener alternative, but it’s also 60 percent stronger and lasts 10 times longer than standard asphalt.”
But not every plastic-lumber company can boast the same eco-awareness. It’s important to read the fine print. Companies such as TimberTech for example, use only virgin PVC in their decking, resulting in a far less sustainable product than Trex.
The same is true of roofing manufacturers. To my knowledge, there is no U.S. firm selling roofing tiles with significant recycled plastic content. DaVinci Roofscapes does offer a composite roofing, but like TimberTech, they work only with virgin PVC and “processed polymers” from manufacturing (essentially PVC cuttings and scrap). TimberTech also describes its product with terminology that borders on greenwashing: “We use 100 percent pure virgin resins in our roof tiles to guarantee a sustainable product.”
Why do manufacturers deal only with “virgin” plastic? They say it’s easier to work with and more consistent. But the American Society for Testing and Materials (ASTM) has created performance-based industry standards for plastic lumber products that ensure that recycled content does not reduce the performance of end products.
RPL: The State of the Industry
In December 2001, there were about 30 manufacturers of recycled plastic lumber in North America, according to ASTM. That number appears to have declined, but getting an accurate head count is tricky.
The last major third-party report on the industry by the Healthy Building Network was published in 2005. And activity by the Plastic Lumber Trade Association seems to have frozen in time at about 2007, with few updates to their listings or publications. I tried to contact a couple of the resellers of plastic lumber, but at time of writing, neither had responded. The RPL business can be rough and tumble. Many innovative upstarts have failed.
For example, Correct Deck, an RPL company that made composite decking with HDPE had a factory in Maine, but ran into liability issues—specifically related to mixing organic materials in the right proportion with plastic and “capping” deck surfaces anti-microbial material. (I installed one of their decks 12 years ago and it still looks great—just a little spotting.) Consumers are extremely intolerant of any discoloration in their surfaces.
The market opportunities for RPL have never been better. Sure, there are challenges to using these materials. But when you factor in the fact that plastic trash is ubiquitous, free and in overwhelming need of a cleanup, the building industry could provide an ideal solution. Imagine basements that never leak, lightweight roofing with cradle-to-cradle credentials, impact-resistant siding and rot-proof joists and 2-by-4s. All of these innovations are not only possible, but available now. It would be great to see some more American companies join the growing field of plastic lumber manufacturing.
An overseas company called Eco Tiles makes attractive floor tiles from recycled plastics, but their method of collecting and separating the right polymers raises health and safety concerns.
In other parts of the world, reusing plastic in construction is an up-and-coming niche.
This small machine below, which combines recycled plastic and sand to create roof shingles, has attracted interested buyers from all over the world.
Setup costs €25,000 ($29,000) according to the seller, Andrey Kolev, who is based in Bulgaria.
The company (http://plasticabg.net), can make the molds from a drawing; total prep time is three months. The unit requires 20 kW to operate and can make 60 pieces per hour. The total factory area required is about 215 square feet, not counting storage.
Effects of Additives, Fillers and Reinforcements on Polymer Properties
Stabilizers and resins can improve the performance to some degree. For example, the bendability of plastic lumber can be decreased by adding fine mineral fillers, such as talc to the mix. Generally, the smaller the particles added, the greater the boost in stiffness. But the original plastic resin begins to lose impact strength as the level of fillers increases. Here’s a chart showing effects of other additives on polymers.
How Plastic Lumber Is Tested
The sophistication of plastic composite testing continues to increase from when engineers initially sought to test and classify recycled plastic lumber. The ASTM has developed seven important testing standards:
- D 6108 (www.astm.org/Standards/D6108.htm), Standard Test Method for Compressive Properties of Plastic Lumber and Shapes;
- D 6109 (www.astm.org/Standards/D6109.htm), Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastic Lumber;
- D 6111 (www.astm.org/Standards/D6111.htm), Standard Test Method for Bulk Density and Specific Gravity of Plastic Lumber and Shapes by Displacement;
- D 6112 (www.astm.org/Standards/D6112.htm), Standard Test Methods for Compressive and Flexural Creep and Creep-Rupture of Plastic Lumber and Shapes;
- D 6117 (www.astm.org/Standards/D6117.htm), Standard Test Methods for Mechanical Fasteners in Plastic Lumber and Shapes;
- D 6341 (www.astm.org/Standards/D6341.htm), Standard Test Method for Determination of the Linear Coefficient of Thermal Expansion of Plastic Lumber and Plastic Lumber Shapes Between -30 and 140 °F (-34.4 and 60 °C); and
- D 6435 (www.astm.org/Standards/D6435.htm), Standard Test Method for Shear Properties of Plastic Lumber and Plastic Lumber Shapes.
Active Plastic Lumber Manufacturers
There no longer seems to be an industry-managed list of current recycled plastic lumber/building product manufacturers. I’ve done my best to collect a list of every active company I could find. You’ll need to read the fine print to determine which ones use recycled material (and what percentage). The only other list of this type that I’ve seen is rather dated, from a 2005 report. Contact us if you know of others that should be added to the online edition of this article.
A.E.R.T., Inc. (ChoiceDek) www.choicedek.com
Aeolian Enterprises (BreezeWood) www.aeo1.com
American Plastic Lumber (Ameriwood) www.american-plasticlumber.com
Bedford Technology (Select) www.plasticboards.com
Cascades (Perma-Deck Advantage +) http://bit.ly/2hJxdzF
CertainTeed (Boardwalk) www.certainteed.com
Eco-Tech (Eco-Tech) EcTch@aol.com
Engineered Plastic Systems (Bear Board) www.epsplasticlumber.com
Enviro-Curb Manufacturing (Enviro-Curb) www.envirocurb.com
Epoch Composite Products (Evergrain) www.evergrain.com
Fiber Composites (fiberon) www.fiberondecking.com
Green Tree Composites (Monarch) www.biewerlumber.com/greentree.htm
Louisiana-Pacific (WeatherBest Select) www.lpcorp.com
PlasTEAK (PlasTEAK) www.plasteak.com
Polywood (Polywood nonstructural) www.polywood.com
Renew Plastics Division (Evolve, Perma-Poly) www.RENEWPlastics.com
Resco Plastics (MAXITUF) www.rescoplastics.com
Synboard America (Synboard) www.synboard.com
Trex (Trex Origins) www.trex.com
3D Printing: The Next Frontier for Recycled Plastics
Fabricating doorknobs, hinges and tiles from recycled plastics could soon be the new normal.
The reliability and speed of 3D printers is increasing. At the same time, the cost for a household-sized unit is dropping. It’s a classic “Moore’s law” technology story. What does this mean to building pros? Maybe more than you think. I’ll reiterate my prediction that we’re moving toward a time of localized manufacture. It’s not quite Star Trek yet, where a device recombines materials at the atomic level to form matter, but maybe an early iteration of that idea.
And plastics—especially recycled plastics—are a widely available, free material that’s optimal for 3D printing today. The products shown, from Thingiverse, are just a small sample of what’s to come for 3D printing.
Moldable vs Non-Moldable Plastics
From an engineering perspective, plastics fall into two broad categories: thermoplastics and the thermosets.
Thermoplastics refer to those plastics that soften and flow on heating, allowing them to be molded or formed into different shapes. Thermoplastics can therefore be recycled as they can be melted and reformed into different products (Subramanian, 2000).
Thermoset plastics, on the other hand, are cross-linked polymers such as vulcanized rubber, polyurethanes, glass-reinforced polyester or epoxy resins that do not melt or flow on heating and cannot therefore be remolded into a different shape. When heated to high temperatures, the material simply degrades chemically into small molecular products. Source: Andrady