Scientists from the Oak Ridge National Laboratory (ORNL) have discovered that a by-product of biorefinery processes called – lignin composites; could serve as renewable 3D printing feedstock.
The new discovery has been outlined in the journal Science Advances, which extends ORNL’s achievements in the reducing the cost of bio-products by creating profitable new uses of lignin. The lignin is a form of complex organic polymers that creates structural materials in the support of tissues of certain plants and provides plant rigidity, but prevent the biomass from breaking down into valuable products.
According to Amit Naskar, leader of ORNL project, discovering the novel use of lignin composites can help improve economics of the overall biorefining process.
To create lignin composites, the research team mixed a melt-stable material of lignin with a low-melting nylon (conventional plastic) and carbon fiber. The resulting material had excellent mechanical properties with right characteristics for weld strength and extrusion between layers during the printing process.
Lignin chars easily and unlike workhorse composites such as acrylonitrile-butadiene-styrene (ABS) made of fuel-based thermoplastic, it requires to be heated at a particular temperature for softening and extrusion from a 3D-printing nozzle. In addition, prolonged heat exposure can also drastically increase the viscosity of lignin, making it too thick to be extruded easily.
Interestingly, when the lignin was combined with nylon, the researchers discovered that the temperature stiffness of the composite increased, while its melt viscosity decreased. Moreover, the lignin-nylon composite had lower viscosity compared to high impact polystyrene or conventional ABS, and tensile strength equivalent to nylon alone.
To investigate molecular structure of the composite, the researchers carried neutron scattering and used advanced microscopy at the High Flux Isotope Reactor and at the Center for Nanophase Materials Science respectively. The team found that a mixture of lignin and nylon appeared to have plasticizing or lubrication effect on the composite.
Ngoc Nguyen who collaborated ORNL’s project said that structural properties of lignin are critical to better 3D printability of the materials.
When nylon was mixed in an increased percentage of lignin about 40-50% by weight, a new achievement in the quest for lignin-based 3D printing material. Further, the team added 4-16% carbon fiber in the mix, which allowed the new composite to heat up more easily, flow at quicker pace to accelerate printing, and result in high-strength product.
According to Moe Khaleel, associate lab director for Energy and Environmental Sciences, world-class capabilities of ORNL in characterization and synthesis of materials are important to the challenge of transforming byproducts such as lignin to useful co-products, developing novel renewable composites for advanced manufacturing.