By Delilah F. Wood, Lennard F. Torres, Zach McCaffrey, Tina G. Williams, Bor-Sen Chiou, Colleen M. McMahan and William J. Orts
Agricultural Research Service, Western Regional Research Center, United States Department of Agriculture (USDA), Albany, California
In response to the demand for biobased materials[3], our USDAARS team developed the “Zero Waste” program to develop technologies enabling conversion of nut shells and hulls into consumer goods (Figure 1a).
Thermal conversion of biomass provides an opportunity for the processing of agricultural waste into high-value products. Torrefaction (Figure 1b) is a low-temperature conversion process that may be used as a treatment for fillers in composites. Torrefaction benefits material properties by increasing miscibility, reducing moisture, providing high bulk and energy densities, increasing heating value, and improving grindability, thereby saving energy.
Advanced activated carbon is currently derived from imported coconut or petroleum-based precursors. Nutshellderived carbon has been proposed as an alternative feedstock source for production of biobased activated carbon, using a two-step process: pyrolysis and activation. Similar to torrefaction, pyrolysis (Figure 1b) is a thermal process that heats a material above its decomposition temperature, typically above 300 °C, in a low oxygen environment. The pyrolysis step removes all volatile materials, leaving only a carbon structure and trace element impurities behind. Activation is a high-temperature process that uses a gas to etch the carbon structure by oxidation of the carbon surface and increase pore volume. The longer the material is exposed to thermal energy and the process gas, the more surface area and pore volume is developed.
Nutshell activated carbon are porous structures that have high surface areas (high surface area = high reactivity) that are advantageous for advanced carbon applications including lithium-ion batteries, ultracapacitors, and absorbed natural gas storage. USDA-ARS and partners are investigating the technical and economic feasibility of using nut shells as a feedstock for the manufacture of domestic sustainable activated carbon, eliminating agricultural residues from landfills, and generating new revenue for nut growers and processors. It can also result in exciting new markets for nut byproducts.
Nutshells serve as multi-functional fillers in plastic composites. Sawdust and minerals, such as talc and calcium carbonate, are traditionally used to produce filled plastic composites. However, nutshell biomass can serve the same function. Biomass can be used to displace expensive plastic resin (thereby reducing dependency on petroleum use) and to lend strength and structure to composites. Although raw nut shells may be successfully incorporated into plastics, torrefaction of biomass creates fillers that are less hydrophilic (compared to raw biomass) and reduces issues with blend compatibility, odors, off-gassing, and microbial degradation associated with raw biomass fillers. Torrefied nut shell-plastic composites have higher stiffness and better heat resistance than unfilled plastics. Torrefied shells, usually a dark brown or black color, also serve as a pigment for a uniform composite color, an advantage when blending with multi-colored recycled plastics. Another advantage of using torrefied shells over raw biomass is that they remain stable when blended with plastics during higher temperature processing (typically above 130 oC).
Natural rubber composites from torrefied nut shells may be used as a “green” filler for rubber. Non-renewable carbon black is a commonly used filler in rubber compounds[4]. The use of torrefied almond shells as fillers in natural rubber compounds was evaluated for partial and full replacement of carbon black. At all replacement levels, mixing and processing proceeded generally without issues. When torrefied almond shells were used solely as the filler (full replacement of carbon black), the physical properties of the composite material were not equivalent to carbon black filled controls, as expected, mostly due to particle size differences between carbon black and the biomass-derived filler. The highly porous structure from torrefaction may allow entrapment of rubber during mixing (i.e., the rubber can “grip” the particle surface). Nevertheless, torrefied almond shell-filled natural rubber compounds may be suitable for some industrial applications and offer an important step toward 100% biobased rubber compounds, tipping the scale of novel filler sourcing from nonrenewable to more sustainable resources.
Research continues to develop processes to tailor biomass properties for incorporation into commercial products. The use of biomass meets increasing market demand for biobased content in materials and reduces the environmental impact of petroleum-based precursors.
References
1. World Centric https://www.worldcentric.com/ (accessed 1/26/2021).
2. Ford Motor Co. Ford to turn McDonald's coffee waste into sustainable auto composites.https:// www.compositesworld.com/articles/ford-toturn-mcdonalds-coffee-waste-into-sustainableautocomposites (accessed 1/26/2021).
3. van der Hoeven, D. New biobased materials Biobased News [Online]. https://news.bio-based.eu/new-biobased-materials/ (accessed 1/26/2021).
4. McMahan, C.; Torres, L.; Wood, D.; Orts, W.; Wagner, C., Almond production residues as natural rubber fillers. Rubber World 2020, pp 28-31.