ORNL’s tough new plastic is made with 50 percent renewable content from biomass. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; conceptual art by Mark Robbins (hi-res image)
Your car’s bumper is probably made of a moldable thermoplastic polymer called ABS, shorthand for its acrylonitrile, butadiene and styrene components. Light, strong and tough, it is also the stuff of ventilation pipes, protective headgear, kitchen appliances, Lego bricks and many other consumer products. Useful as it is, one of its drawbacks is that it is made using chemicals derived from petroleum.
Now, Dept of Energy’s Oak Ridge National Lab researchers have made a better thermoplastic by replacing styrene with lignin, a brittle, rigid polymer that, with cellulose, forms the woody cell walls of plants. In doing so, they have invented a solvent-free production process that interconnects equal parts of nanoscale lignin dispersed in a synthetic rubber matrix to produce a meltable, moldable, ductile material that’s at least 10X tougher than ABS. The resulting thermoplastic -called ABL for acrylonitrile, butadiene, lignin – is recyclable, as it can be melted 3X and still perform well. The results may bring cleaner, cheaper raw materials to diverse manufacturers.
“The new ORNL thermoplastic has better performance than commodity plastics like ABS,” said Amit Naskar, ORNL. “We can call it a green product because 50% of its content is renewable, and technology to enable its commercial exploitation would reduce the need for petrochemicals.”
The technology could make use of the lignin-rich biomass byproduct stream from biorefineries and pulp and paper mills. With the prices of natural gas and oil dropping, renewable fuels can’t compete with fossil fuels, so biorefineries are exploring options for developing other economically viable products. Among cellulose, hemicellulose and lignin, the major structural constituents of plants, lignin is the most commercially underutilized.
“Lignin is a very brittle natural polymer, so it needs to be toughened,” explained Naskar, leader of ORNL’s Carbon and Composites group. “We need to chemically combine soft matter with lignin. That soft matrix would be ductile so that it can be malleable or stretchable. Very rigid lignin segments would offer resistance to deformation and thus provide stiffness.”
All lignins are not equal in terms of heat stability. They found hardwood lignin is the most thermally stable, and some types of softwood lignins are also melt-stable. Next, the researchers needed to couple the lignin with soft matter. Chemists typically accomplish this by synthesizing polymers in the presence of solvents. Because lignin and a synthetic rubber containing acrylonitrile and butadiene, called nitrile rubber, both have chemical groups in which electrons are unequally distributed and therefore likely to interact, Naskar and Chau Tran instead tried to couple the two in a melted phase without solvents.
In a heated chamber with two rotors, the researchers “kneaded” a molten mix of equal parts powdered lignin and nitrile rubber. During mixing, lignin agglomerates broke into interpenetrating layers or sheets of 10 to 200 nanometers that dispersed well in and interacted with the rubber. Without the proper selection of a soft matrix and mixing conditions, lignin agglomerates are at least 10X larger than those obtained with the ORNL process. The product that formed had properties of neither lignin nor rubber, but something in between, with a combination of lignin’s stiffness and nitrile rubber’s elasticity.
By altering acrylonitrile amounts in the soft matrix, the researchers hoped to improve the material’s mechanical properties further. They found 41% gave an optimal balance between toughness and stiffness. They found heating components between 140 and 160Cformed desired hybrid phase.
Scanning electron microscopy explored the surfaces of the materials. Jihua Chen and Tran characterized soft matter phases using transmission electron microscopy, placing a thin slice of material in the path of an electron beam to reveal structure through contrast differences in the lignin and rubber phases. Small-angle x-ray scattering by Jong Keum revealed repeated clusters of certain domain or layer sizes. Fourier transform infrared spectroscopy identified chemical functional groups and their interactions.
Future studies will explore different feedstocks, particularly those from biorefineries, and correlations among processing conditions, material structure and performance. Investigations are also planned to study the performance of ORNL’s new thermoplastic in carbon-fiber-reinforced composites.
https://www.ornl.gov/news/ornl-researchers-invent-tougher-plastic-50-percent-renewable-content
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