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3rd January 2019, Oak Ridge, TN

Lignin composites promise more profitable biorefining

A recipe for producing a renewable feedstock for 3D printed composites from lignin – an intractable biorefinery by-product – has been developed at the US Department of Energy’s Oak Ridge National Laboratory (ORNL).

The discovery expands ORNL’s achievements in lowering the cost of bioproducts by creating novel uses for lignin as 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.

ORNL scientists have created a new composite material for additive manufacturing that makes use of lignin, a biofuels by-product.

The researchers combined a melt-stable hardwood lignin with conventional plastic, a low-melting nylon, and carbon fibre 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 because lignin chars easily. Unlike workhorse composites such as 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 and 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.

Using as much as 50% lignin by weight, a new composite material created at ORNL is well suited for use in 3D printing.

The scientists conducted neutron scattering at the High Flux Isotope Reactor and used advanced microscopy at the Centre for Nanophase Materials Science – both DOE Office of Science User Facilities at ORNL –to explore the composite’s molecular structure.

“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,” added ORNL’s Ngoc Nguyen who collaborated on the project. 

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

“ORNL’s world-class capabilities in materials characterisation and synthesis are essential to the challenge of transforming by-products like lignin into coproducts, 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.

The lignin-nylon composite is patent-pending and work is ongoing to refine the material and find other ways to process it.


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