Biocarbon from peanut hulls and their green composites with biobased poly(trimethylene terephthalate) (PTT)

Abstract

There are millions of tons of post-food processing residues discarded annually. Currently, these waste materials are discarded to landfill, used as animal feed or incinerated. This suggests that there are potential uses for these materials in value-added applications. This work focuses on the characterization and valorization of peanut hulls through the generation of green composites. Peanut hulls were pyrolyzed at 500 °C and analyzed to discover their unique surface morphology and relatively low ash content. Raman spectral analysis determined I D /i G values of 0.74 for the samples, suggesting greater graphitic content than disordered carbon content. Such results were confirmed in X-ray diffraction analysis by the presence of (002) and (100) planes. Partially biobased engineering thermoplastic, poly(trimethylene terephthalate) (PTT), was combined with 20 wt.% biocarbon. The tensile and flexural moduli improved with the addition of biocarbon, and the bio-content increased from 35 to 48 wt.% as compared to neat PTT. The higher temperature biocarbon was found to have superior performance over the lower temperature sample. The enhanced sustainability of these materials suggested that peanut hulls can be valorized via thermochemical conversion to generate value-added products. Future works could focus on the optimization of these materials for non-structural automotive components or electrical housings. Biomass is generated from a number of biological sources, including waste from the agricultural industry, as well as beverage and food processing industries. There have been substantial amounts of work to valorize these materials 1 by extracting compounds 2 , repurposing for value-added products 3 or burning as energy sources 4. Peanut hulls are an abundantly available, low-cost and sustainable biomass. PHs are the outer shell which encompasses the nut (Fig. 1a). The largest global producers of peanuts are displayed in Fig. 1b, however, they are grown in many other areas around the world. It was estimated in 2017 that there were 45 million metric tons of peanuts produced worldwide 5. Since PHs comprise 21-29% of overall peanut weight 6 , at least 9.4 million metric tonnes were produced in 2017. This study takes a comprehensive look at biomass generated from PHs that were grown in southern Ontario (Canada). PHs consist of four major structural components. These four layers are the pericarp, exocarp, mesocarp and endocarp arranged in order of outermost to innermost layers (Fig. 1a) 7. Together, these layers contain different ratios of cellulosic materials such as lignin, cellulose and hemicellulose, as well as small amounts of protein and pectin 8. Peanut hull biomass is abundantly available and to date has had limited use in value-added products. In fact, the PHs generated in Ontario are spread back on the fields after completion of the harvest season. On the global scale, this biomass is readily available in large quantities, obtained at a relatively low cost and is renewed each year. Further examination of current us...

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