Direct Use of Natural Antioxidant-rich Agro-wastes as Thermal Stabilizer for Polymer: Processing and Recycling

Antioxidant-rich agro-wastes such as grape pomace waste (GW), turmeric shavings and waste, coffee grounds, and orange peel waste are used as-received for the first time as thermo-oxidative stabilizers for polymer. Relative to neat low density polyethylene (LDPE), a well-dispersed hybrid made by solid-state shear pulverization with 4 wt % GW results in 62 and 44 °C increases in temperatures corresponding to 10 and 20% mass loss in air (T_10% and T_20%), respectively. Such enhancements are superior to those obtained by adding 1 wt % synthetic antioxidant Irganox I1010 to LDPE by melt mixing. Relative to neat LDPE, hybrids with well-dispersed agro-waste exhibit enhanced Young's modulus, equal or enhanced tensile strength, and relatively small reduction in elongation at break. Reprocessing or recycling sometimes leads to enhanced antioxidant activity: relative to a hybrid before melt extrusion, 92/8 wt% LDPE/TW exhibits major increases in T_10% and T_20% after two and six melt extrusion passes, which is consistent with formation of transformation products with improved antioxidant activity during multiple high-temperature reprocessing cycles. Natural antioxidants are effective in suppressing LDPE chain scission and branching. After ten extrusion passes, neat LDPE exhibits a 16% increase in zero-shear viscosity and reduction in elongation at break from 500% to 280%, whereas hybrids with agro-waste have zero-shear viscosity and elongation at break values close to those of unprocessed hybrids. Isothermal shear flow measurements also show the effectiveness of natural antioxidant in stabilizing LDPE: hybrids exhibit no sign of chain branching during 3000 s of melt flow at 200 °C whereas neat LDPE branches after ∼500 s.