Petroleum-based plastics and composites have caused serious environmental problems leading to the onset of an imbalance on the equilibrium of our ecosystem. At the same time, agricultural residues are produced in huge volumes all over the world, corresponding to more than 200 billion tonnes per year. These residues also cause significant environmental problems, for example they are often disposed by open burning. A more beneficial route would be to upcycle these residues for production of materials that can replace petroleum-based plastics. This chapter reviews selected agricultural residues and their utilization for production of biocomposites and bioplastics with high biobased content and potential biodegradability. In the case of biocomposites, mainly materials consisting of natural fibers from agricultural residues and polysaccharide-based matrices are highlighted, while biocomposites with synthetic biobased matrices or fossil-based non-degradable matrices are not discussed in this review. Finally, a brief outlook on future potential is presented.

Rice husk (RH), an abundant agricultural residue, was successfully chemically modified and used as a component in reprocessable and biodegradable epoxy thermosets. First, RH was subjected to alkaline treatment to increase the cellulose content followed by succinylation and curing with trimethylolpropane triglycidyl ether to form the thermoset films. The chemical structure of the different intermediates and thermosets was confirmed by Fourier transform infrared spectroscopy. The developed thermoset films had good solvent resistance against common organic solvents and good thermal stability as measured by thermogravimetry with peak temperatures of 347–387 ℃, char residues of 16–20% and limiting oxygen index values of 24–26%, respectively. The films could be thermally reprocessed by hot-pressing with excellent recovery of the mechanical properties (92–96% recovery of tensile stress). Furthermore, 80–84% biodegradation during 150 days under mesophilic home composting conditions was demonstrated by cumulative CO<inf>2</inf> evolution. These results indicate promising potential for the developed RHs thermosets as replacements for petroleum-based plastics in e.g. packaging and agricultural applications.

Pyrolysis of rice husk (RH), alkali-treated cellulose-rich rice husk (RHC), chemically modified RHC (RHCM) and RH-biocomposites by thermogravimetric analysis was carried out to determine combustion and kinetic parameters at three different heating rates of 20, 40 and 50 degrees C min-1. Combustion performance was analyzed from results of ignition temperature, burnout temperature, combustion rates, flammability index and combustion characteristic index. Increase in heating rate from 20 to 40 and further to 50 degrees C min-1 increased the onset of degradation, burnout and peak temperatures as observed by curve shifts to the right. Maximum combustion rates were around 0.57-0.59% min-1, 1.03% min-1 and 0.63-0.69% min-1 for RH, RHC and RHCM, respectively. For the RH-biocomposites, the maximum combustion rates were in a 0.76-0.97% min-1 range. Their average pre-exponential factors using KAS method were in the 2.24E-03-8.07E-03 range, respectively, while those for OFW method were in the 7.75E + 04-4.55E + 06 range, respectively. Average activation energies of RH-biocomposites were in the 41.0-58.2 kJ mol-1 and 48.3-67.7 kJ mol-1 ranges for KAS and OFW methods, respectively. The data were well fitting with coefficient of determination (R2) values close to 1. Average Delta G value ranges for RH-biocomposites ranged between 148.2 and 161.7 kJ mol-1. The low-energy barrier (<= 5.4 kJ mol-1) between activation energy and enthalpy changes indicated that reaction initiation occurs easily.

In the context of processing, utilization and disposal of polylactic acid composites, pyrolysis is a promising technique that addresses this complex synergy. In this work, pyrolysis kinetics and thermodynamic parameters of rice husks/clay fiber-reinforced PLA composites were investigated using Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW) at multiple heating rates (16, 25 and 34 °C min−1). PLA composites’ pyrolysis followed a single-step degradation process. The flammability indices, combustion characteristic indices and mean reactivities obtained for the PLA composites are much lower than those for neat PLA (2.00 × 10−5–2.44 × 10−5% min−1 °C−2, 0.87 × 10−8–1.79 × 10−8% min−2 °C−3 and 6.97 × 10−3–8.04 × 10−3% min−1 °C−1, respectively) which signals that rice husks and clay improved flame retardancy of accruing PLA composites. The average activation energy values obtained from the KAS method were found to be in ranges 137.83–143.99 kJ mol−1 and 124.51–133.95 kJ mol−1 for raw and modified rice husks/clay fiber-reinforced PLA composites, respectively. Corresponding activation energies for raw and modified rice husks/clay fiber-reinforced PLA composites from the OFW method were 141.24–146.92 kJ mol−1 and 128.17–137.50 kJ mol−1, respectively. By comparing activation energy and enthalpy, it was found that the composites were favored to format activated complex due to the low energy barrier.