Used Plastic+Hemp=Lumber

UNCC researchers create a formula for recycling old bottles into new building materials

By Amber Veverka
Special Correspondent

A UNC Charlotte researcher with a passion for sustainability is creating a new building material out of recycled plastic bottles and an ancient grass.
Dr. Na Lu, an assistant professor at UNCC’s Department of Engineering Technology, has created a material she believes may outperform composite lumber and wood lumber in many uses, and which has potential to be used in the residential and light commercial building industry.
In her lab at UNCC, Luna, as she prefers to be called, holds a dog bone-shaped sample of her creation: a beige plastic woven with threads of what looks like horsehair. “Hemp,” Luna says, and points to a fluffy pile of the fibers on the table.
Unlike much present-day composite lumber, Luna’s product substitutes hemp fibers for more typical chipped wood often mixed with virgin plastic. And unlike pressure-treated wood, the hemp material contains no toxic heavy metals.
Wood fiber is structured like a bundle of straws, she said, but hemp’s crystalline structure gives it greater mechanical strength. She demonstrates by holding out a handful of hemp fibers to pull.
“This (hemp composite) material performs up to 4,000 to 6,000 psi (pounds per square inch),” Luna said. “That’s as strong as medium-strength concrete.”
At the same time, the hemp-recycled plastic material is lighter than regular composite lumber, she said.
Hemp may be a promising building material, but the stuff Luna uses isn’t going to get anyone arrested. It’s industrial hemp, with an extremely low content of THC, the psychoactive substance for which marijuana is known.
Hemp is just one key to the new material; the other is recycled plastic bottles. In the United States, about 20 billion plastic bottles are used annually, and just 18 percent of those get recycled, Luna said. “The niche of what we do here is … we used HDPE recycled plastic, as opposed to resin epoxy,” she said.
Where things get wet
Unlike regular lumber, the experimental material is moisture- and insect-resistant, and hemp grows a lot faster than wood. Hemp fiber polymers are being used in the automotive industry in Europe for car interiors, Luna said, but she sees a future for the material in buildings, particularly in places where wood rot is a problem.
“The first application I really would like to see is any point where there is water contact in a civil application – a retaining wall, decking, bridges,” she said.
While it would cost more to produce the material today than it does to produce wood lumber, the life cycle cost would be cheaper and, over time, with a greater scale of production, she believes the cost to the consumer would fall.
For Luna, an interest in accomplishing conventional goals through unconventional means came early. Born in China, she said she saw firsthand the difficulty of a heavily populated nation struggling with high energy costs. After moving to the States, Luna earned her doctorate from Clemson University. In the process, she worked with a professor in Arizona in constructing a school from straw bales coated with cement.
Testing, testing
To prepare hemp composite samples for testing, Luna and her student assistant, John Larson, first extrude pellets of recycled plastic. Larson, a rising sophomore from Stanley majoring in construction management, treats the hemp fiber to remove its oil and odor. He points out a tensile testing machine used to pull the fibers and take pictures with a high-speed camera of how the material reacts and deforms in each moment.
Larson and Luna sandwich the strands between layers of plastic, and test the finished sample under a static load and a dynamic load (a moving load, such as that produced by wind or water) for changes in strength at various temperatures and humidity levels.
“We tried chopping them up,” Larson said of one of many experiments with the fibers. That didn’t prove strong enough, so now they’re turning out samples with longer hemp strands.
“It’s tedious,” Luna said of the yearlong process of trial and error. “But once you see the material improve … you love it.”
Listening to Mother Nature
In designing materials for building, it makes sense to take cues from nature, Luna said. “Mother Nature is much smarter than us,” she said. “I really respect nature and how things are designed.”
In the lab, Luna and Larson demonstrate the testing of a sample of the hemp composite. The “dog bone” slides into a vise-like apparatus on a strength-testing machine and, as Luna watches a glowing computer screen, the machine pulls the sample until at last it snaps, at 5,692 psi.
“Wow!” Luna says, surprised. Larson peers at the computer with her and they marvel at the test results, which were achieved at 70 degrees Fahrenheit and 35 percent humidity – variables, Luna says, which are important because a material’s performance changes with moisture and heat.
The next challenge will be making the material more fireproof. But already a lumber company and an architectural firm have expressed interest in it, Luna said.
In addition to exploring hemp and recycled plastic as a lumber substitute, Luna is looking at combining recycled plastic with bamboo fibers. She’s also working on a new class of thermoelectric materials to harvest waste heat energy and convert it into electrical energy without moving parts.