Lignin, nylon and carbon fiber produce ideal composite for 3D printing

Scientists at the US Department of Energy (DOE)'s Oak Ridge National Laboratory (ORNL) have created a recipe for a renewable 3D printing material that could spur a profitable new use for an intractable biorefinery by-product: lignin.

The discovery, reported in a paper in Science Advances, 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.

"Finding new uses for lignin can improve the economics of the entire biorefining process," said ORNL project lead Amit Naskar.

The 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. The composite also possessed sufficient weld strength between layers during the printing process, as well as excellent mechanical properties.

The work was 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 the 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 a tensile strength similar to nylon alone, but a lower viscosity than conventional ABS or high impact polystyrene.

The researchers 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," noted Naskar.

"Structural characteristics of lignin are critical to enhance 3D printability of the materials," said ORNL's Ngoc Nguyen, who collaborated on the project.

The researchers 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. They then added 4–16% carbon fiber into the mix to produce an improved composite that heated up more easily, flowed faster for speedier printing and resulted in a stronger product.

"ORNL's world-class capabilities in materials characterization and synthesis are essential to the challenge of transforming by-products like lignin into co-products, generating potential new revenue streams for industry and creating novel renewable composites for advanced manufacturing," said Moe Khaleel, associate laboratory director for Energy and Environmental Sciences at ORNL.