Researchers from the University of Nebraska-Lincoln and Jiangnan University have discovered a step to the production of plant-derived, biodegradable plastic that could improve the properties and overcome the obstacles in the way of manufacturing this plastic commercially.
Yiqi Yang, from the University of Nebraska-Lincoln, and colleagues found that raising the temperature of bio-plastic fibers by several hundred degrees Fahrenheit and slowly allowing them to cool improved bio-plastic's weak resistance to heat and moisture.
The thermal approach allowed the team to bypass solvents and other expensive, time-consuming techniques that are typically needed to manufacture a commercially viable bio-plastic.
Yang said the approach could allow manufacturers of corn-derived plastic to produce biodegradable material on a scale that at least approaches petroleum-based plastic, which is the current industry standard. Recent studies have shown that about 90 percent of the plastic that is used in the U.S. goes unrecycled.
This new approach uses polylactic acid, a component of biodegradable plastic that can be fermented from corn starch, sugarcane, and other plants. Most plastics are made from petroleum but polylactide has become a new, environmentally-friendlier alternative.
But polylactide is weak to heat and moisture, particularly during the manufacturing process, which limits the material’s use in textiles and other industries. While searching for ways to address the issue, researchers discovered that mixing mirror-image polylactide molecules, generally referred to as “L” and “D,” could have stronger molecular interactions and better performance than using L or D alone.
However, there is a catch to this. Convincing a reasonable proportion of the L and D molecules to pair up permanently is difficult. This forces researchers to come up with costly complicated matchmaking schemes. The most common schemes involve using solvents or other chemical agents that cause environmental issues of their own.
"The problem is that people couldn't find a way to make it work so that you could use it on large scales," said Yang, Charles Bessey Professor of biological systems engineering and of textiles, merchandising and fashion design. "People use nasty solvent or other additives. But those are not good for continuous production. We don't want to dissolve the polymers and then try to evaporate the solvents, and then have to consider re-using them. That's just too expensive (and) not realistic."
Yang and colleagues decided to pursue another approach. They mixed pellets of the L and D polylactide and spun them into fibers and then the team rapidly heated them to 400° F.
The resulting bio-plastic resisted melting at temperatures more than 100 degrees hotter than the previous plastics that only contained L or D molecules. It maintained structural integrity and tensile strength after being submersed in water at more than 250° F, approximating the conditions that bio-plastics have to endure when being incorporated into dyed textiles.
The textile industry produces around 100 million tons of fiber every year. This means that a feasible green alternative to petroleum-based manufacturing that could pay off environmentally and financially.
"So we just used a cheap way that can be applied continuously, which is a big part of the equation," Yang said. "You have to be able to do it continuously in order to have large-scale production. Those are important factors."
The team has demonstrated continuous production on a smaller scale in Yang’s lab and they are planning to ramp it up soon to further show how the approach might be integrated into existing industrial processes.
A paper on these findings will be published in the Chemical Engineering Journal in November.