Scientists at the Department of Energy’s Oak Ridge National Laboratory have created a recipe for a renewable 3D printing feedstock that could spur a profitable new use for an intractable bio-refinery by-product: Lignin.
Researchers combined a melt-stable hardwood lignin with conventional plastic, a low-melting nylon, and carbon fiber to create a composite with just the right characteristics for extrusion and weld strength between layers during the printing process, as well as excellent mechanical properties.
The work is tricky. Lignin chars easily; unlike workhorse composites like acrylonitrile-butadiene-styrene (ABS) that are made of petroleum-based thermoplastics, lignin can only be heated to a certain temperature for softening and extrusion from a 3D-printing nozzle. Prolonged exposure to heat dramatically increases its viscosity, it becomes too thick to be extruded easily.
But when researchers combined lignin with nylon, they found a surprising result: the composite’s room temperature stiffness increased while its melt viscosity decreased. The lignin-nylon material had tensile strength similar to nylon alone and lower viscosity, in fact, than conventional ABS or high impact polystyrene.
The scientists conducted neutron scattering at the High Flux Isotope Reactor and used advanced microscopy at the Center for Nanophase Materials Science, both DOE Office of Science User Facilities at ORNL, to explore the composite’s molecular structure. They found that the combination of lignin and nylon appeared to have almost a lubrication or plasticizing effect on the composite.
Scientists were also able to mix in a higher percentage of lignin, 40 to 50% by weight, a new achievement in the quest for a lignin-based printing material. ORNL scientists then added 4 to 16% carbon fiber into the mix. The new composite heats up more easily, flows faster for speedier printing, and results in a stronger product.