Poly(lactic acid)-based Composites Incorporated with Spent Coffee Ground and Tea Leave for Food Packaging Application: A Waste to Wealth

Polymer composites of polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT) incorporated with spent coffee grounds (SCG) and tea leave (TL) were prepared by two-roll mill mixer. 4,4-methylene diphenyl diisocyanate (MDI), toluene 2,4-diisocyanate (TDI), and bis[3-(triethoxysilyl) propyl] tetrasulfide (TESPT) were used as coupling agents. The influences of coupling agent types, coupling agent content, and weight ratios of polymer to filler on the mechanical properties, melt flow index, and overall migration (OM) of the composites were studied. The results showed that MDI and TDI had better performance compared to TESPT for both tensile and elongation at break. The tensile strength and elongation at break of PLA+PBAT/SCG composites with weight ratio of polymer to filler = 70/30 increased from 19.6 MPa to ∼23.0 to 25.0 MPa, and 6.6 to ∼10.0%, respectively when using these coupling agents (MDI and TDI) of 3 g/100 g polymers. Moreover, the addition of MDI and TDI greatly increased the viscosity of the melted composites (4-fold), while TESPT made the viscosity decrease. However, the mechanical properties of the composites decreased drastically with increasing SCG proportion. Compared to PLA+PBAT/SCG, interfacial adhesion of PLA+PBAT/TL was higher confirming by tensile strength and SEM images. However, there was no significant difference between PLA+PBAT/TL and PLA+PBAT/SCG composites in terms of elongation at break, impact strength and melt flow index. The OM of PLA+PBAT/SCG and PLA+PBAT/TL composites with coupling agents were in the range of ∼0.03–0.28 mg/dm2 when using 3% acetic acid and 10% ethanol as food simulants, which not excess the migration limit (10 mg/dm2) according to Food Contact Materials EU No. 10/2011 legislation. It means that they might be safe for use as food contact materials for packaging and containers.