New thermoplastic is made partly from wood

Your car's bumper is probably made of a moldable thermoplastic polymer called acrylonitrile butadiene styrene (ABS). Light, strong and tough, ABS is also used to produce 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, researchers at the US Department of Energy's Oak Ridge National Laboratory have made a better thermoplastic by replacing the styrene in ABS with lignin, a brittle, rigid polymer that is an important component of the woody cell walls of plants. To do this, they invented a solvent-free production process that disperses nanoscale lignin in a synthetic rubber matrix. The end result is a meltable, moldable, ductile material that's at least 10 times tougher than ABS.

This novel thermoplastic, called acrylonitrile butadiene lignin (ABL), is also recyclable, able to be melted three times and still perform well. This work, reported in Advanced Functional Materials, may bring cleaner, cheaper raw materials to diverse manufacturers.

"The new ORNL thermoplastic has better performance than commodity plastics like ABS," said senior author Amit Naskar from ORNL's Materials Science and Technology Division, who along with co-inventor Chau Tran has filed a patent application on the production process for the new material. "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 by-product streams 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, which are the major structural constituents of plants, lignin is the most commercially underutilized. The ORNL study aimed to use it as a feedstock for a renewable thermoplastic with properties rivaling those of current petroleum-derived alternatives.

"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. To determine what type would make the best thermoplastic feedstock, the scientists evaluated lignin derived from wheat straw, softwoods like pine and hardwoods like oak. They found that hardwood lignin is the most thermally stable, while 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. However, lignin and a synthetic rubber containing acrylonitrile and butadiene, called nitrile rubber, both possess chemical groups in which the electrons are unequally distributed and likely to interact. So Naskar and Chau Tran (who performed melt-mixing and characterization experiments) 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 10–200nm 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 10 times 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 the acrylonitrile amounts in the soft matrix, the researchers hoped to improve the material's mechanical properties further. They tried 33%, 41% and 51% acrylonitrile and found that 41% gave an optimal balance between toughness and stiffness. They also wanted to know at what temperature the components should be mixed to optimize the material properties. They found heating components between 140°C and 160°C produced the desired hybrid phase.

Using resources at ORNL, including the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility, the scientists analyzed the morphologies of these blends. They used scanning electron microscopy to explore the surfaces of the materials and transmission electron microscopy to explore the soft matter phases. They also used small-angle x-ray scattering to reveal repeated clusters of certain domain or layer sizes and Fourier transform infrared spectroscopy to identify 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.

"More renewable materials will probably be used in the future," Naskar said. "I'm glad that we could continue work in renewable materials, not only for automotive applications but even for commodity usage."

This story is adapted from material from ORNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.