KEY CONCEPTS
• Lignin, an attractive material to work with in commercial applications, cannot be 3D printed using extrusion because of poor thermal stability.
• Melt mixing hardwood lignin with the engineering thermoplastic, nylon 12, produced a material that can be 3D printed. 
• The lignin acted as a lubricant to mobilize the nylon matrix during the 3D extrusion process. 

Lignin (see Figure 3) is an extremely abundant renewable material that is derived from plants. The function of lignin is to provide rigidity to plants and also enable them to resist decomposition. 


Figure 3. A new alloy prepared from lignin and nylon 12 has now been 3D printed through the use of extrusion. (Figure courtesy of the Department of Energy’s Oak Ridge National Laboratory.)

The chemical structure of lignin has made it very difficult to utilize this material in commercial applications. In a previous TLT article, researchers were able to develop a new extreme-pressure (EP) additive based on lignin that showed promise in greases (1). Four-ball EP and wear testing showed that the lignin-based EP additive exhibited better performance in an aluminum complex grease than molybdenum disulfide.

Additive manufacturing or 3D printing is assuming a more prominent place as a means to manufacture specific components. This technique can use a wide variety of materials. For example, a previous TLT article discussed how researchers were able to successfully 3D print the widely used aluminum alloys 6061 and 7075 (2). The past difficulty in 3D printing these aluminum alloys was overcome through the addition of nanoparticle grain refiners. 

Polymers are under evaluation for use in 3D printing. Dr. Amit Naskar, group leader and senior R&D staff at the Department of Energy’s Oak Ridge National Laboratory in Oak Ridge, Tenn., says, “Ideally, in 3D printing using extrusion, the polymer should have as low a viscosity as possible so that it can be pushed through the nozzle without difficulty. To accomplish this, the polymer should exhibit a low viscosity at high shear rate but then must immediately not flow once it is out of the nozzle after 3D printing is completed. This means that at a low-shear condition, the polymer should have a very high viscosity. Temperature is a factor if the polymer can be heated to reduce its viscosity while remaining stable.”

Naskar and his colleagues have been evaluating the possibility of 3D printing lignin for the past two years. He says, “In general, lignin exhibits poor thermal stability at elevated temperatures as this material tends to char, which resists flow. This prevents lignin from being used by itself. But we found that two types of lignin derived from hardwood and softwood plants demonstrated relatively good melt stability and may be able to be 3D printed when combined with other polymers.”

In evaluating the structures (by ¹³C nuclear magnetic resonance spectroscopy) and physical characteristics (by rheology and thermal analysis) of hardwood and softwood lignin, the researchers determined that the hardwood lignin displays reduced melt viscosity and a lower glass transition temperature due to the increased mobility found in its structure. Naskar says, “Hardwood lignin exhibits a few orders of magnitude lower viscosity compared to softwood lignin and is a viable candidate to be used in 3D printing.”

Naskar and his colleagues prepared a renewable alternative to the well-known thermoplastic polymer, acrylonitrile butadiene styrene (ABS) by replacing styrene with lignin. The newly developed polymer is known as acrylonitrile butadiene lignin (ABL). 

While ABL is a good, tough polymer that displays a high viscosity, the researchers were not able to 3D print with it. Naskar says, “In contrast to ABS, which has a glassy plastic matrix, ABL has an elastomeric matrix. The lignin acts to stiffen the polymer and was used at a weight percentage of 60% in our testing. Unfortunately, ABL alone could not be 3D printed due to buckling of the polymer filament while feeding through the printer. The problem was that ABL is a soft polymer that does not show sufficient stiffness to handle 3D printing by fused deposition modeling. The team is exploring a different path for ABL 3D printing.”

Additionally, the researchers moved to a new polymer, which when mixed with hardwood lignin produced an alloy that can be 3D printed.

Nylon 12
Naskar and his colleagues decided to melt mix hardwood lignin with nylon 12 in an effort to find a workable material. He says, “Nylon 12 proved to be an attractive polymer to work with because it is an engineering thermoplastic. The viscosity profile of nylon 12 is very low. We also recognize that nylon 12 can be produced from renewable raw materials meaning that the hardwood lignin/nylon 12 alloy has the potential to be a 100% renewable system.”

The researchers mixed hardwood lignin at loadings between 40%-60% with nylon 12. In the alloy, lignin particles with sizes ranging from nanometers to several microns are present in the nylon 12 matrix. The flexible ether and aliphatic chains present in the lignin improved the flow characteristics of the alloy by reducing melt viscosity. Naskar says, “The hardwood lignin acts in a manner of a lubricant by mobilizing the nylon matrix.”

3D printing of the alloy was conducted and found to be successful. A second factor enabling the 3D printing to be achieved was the alloy’s good room temperature stiffness that eased feeding of the alloy to the printer. Naskar says, “We added carbon fiber to the alloy to further increase the stiffness of the alloy.”

The alloy exhibited a glassy and smooth surface upon completion of the 3D printing process. Evaluation of a tiny sample showed that the bonding between the printed layers was strong. 

Naskar says, “We have been working with many different types of lignin. Our success with hardwood lignin is leading us in the future to see if we can figure out a process for 3D printing other lignins.”

Additional information on this research can be found in a recent article (3) or by contacting Naskar at naskarak@ornl.gov. 

REFERENCES
1. Canter, N. (2011), “Environmentally friendly extreme pressure additive,” TLT, 67 (10), pp. 10-11.
2. Canter, N. (2018), “Manufacturing of aluminum alloys by 3D printing,” TLT, 74 (1), pp. 12-13.
3. Nguyen, N., Barnes, S., Bowland, C., Meek, K., Littrell, K., Keum, J. and Naskar, A. (2018), “A path for lignin valorization via additive manufacturing of high-performance sustainable composites with enhanced 3D printability,” Science Advances, 4 (12), eaat4967.