Lignin is the material left over from the processing of biomass. It gives plants rigidity and also makes biomass resistant to being broken down into useful products. 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.
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 lignin was combined with nylon, 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 than conventional ABS or high-impact polystyrene. 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 percent by weight — a new achievement in the quest for a lignin-based printing material. Four to 16 percent carbon fiber was added into the mix. The new composite heats up more easily, flows faster for speedier printing, and results in a stronger product.