Lignin-polyamide composite shows promise as 3D printing material

Scientists at the Department of Energy’s Oak Ridge National Laboratory have created a recipe for a partially renewable-based 3D printing raw material that could spur a profitable new use for an intractable biorefinery by-product: lignin. The discovery expands ORNL’s achievements in lowering the cost of bioproducts by creating novel uses for lignin—the material left over from the processing of biomass. Lignin gives plants rigidity and also makes biomass resistant to being broken down into useful products.

Researchers combined a melt-stable hardwood lignin with a low-melting point polyamide (PA) thermoplastic 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 was challenging. Lignin chars easily; unlike workhorse materials like acrylonitrile-butadiene-styrene (ABS) 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 PA, they found a surprising result: the composite’s room temperature stiffness increased while its melt viscosity decreased. The lignin-PA material had tensile strength similar to PA alone and lower viscosity than conventional ABS or high impact polystyrene (HIPS).

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 PA “appeared to have almost a lubrication or plasticizing effect on the composite,” noted Naskar.

Scientists were also able to mix in a higher percentage of lignin—40–50 percent by weight—a new achievement in the quest for a lignin-based printing material. ORNL scientists then added 4–16 percent carbon fiber into the mix. The new composite heats up more easily, flows faster for speedier printing, and results in a stronger product.

The lignin-nylon composite is patent-pending and work is ongoing to refine the material and find other ways to process it. The ORNL research team also included Sietske Barnes, Christopher Bowland, Kelly Meek, Kenneth Littrell and Jong Keum. The research was funded by DOE’s Office of Energy Efficiency and Renewable Energy’s Bioenergy Technologies Office.