DOI: 10.14457/tu.the.2022.808
title: Polylactide bio-composites for smart agricultural applications

description: Bio-composites of degradable polylactide (PLA), also known as polylactic acid, with functional properties, i.e., enhanced toughness, high-water vapor permeability (WVP), gas-selectivity, UV-protection, wavelength-selection, and competitive price, have been developed for use as pre-harvested bagging materials for producing high-quality mangoes “R2E2” and Nam Dok Mai-Srithong (Golden-skin mango). Low-cost silica precipitated (SI), and fumed silica (FS) particles were introduced as reinforcing fillers and processing aid. The surface of the particles was modified before blending with commercial PLA resin to promote compatibility between the two components. The surface modification of the inorganic silica particles was conducted by coating them with chitosan-grafted-lactide copolymer (CLC). Several modification processes including phases inversion emulsification (PIE), sonication, and dry state coating, were employed, and compared. Thermal, mechanical, light transmission properties, gas perm-selectivity, and hydrophilicity of the resulting PLA/silica bio-composite films were optimized for use as agricultural and smart packaging films. The treatments by CLC can promote compatibility and dispersibility of the silica particles in PLA/ poly(ethylene glycol) (PEG) matrix, reflected by a drastic increase in elongation, with a slight drop in tensile strength and modulus. The glass transition temperature (Tg) of the blends containing PEG is about 20 °C lower than neat PLA. A difference in nucleating activity is also observed due to the presence of a nucleating agent and plasticizer. The CO2/O2 selectivity of the bio-composite films can be adjusted by varying the particle compositions and adding PEG. At 5.0 wt.% of modified silica, the highest decrease in light transmission at 4, 8, and 16% for UV-A, UV-B, and visible regions is observed. The melt strength and viscosity of plasticized PLA can be enhanced by including FS via promoting particle-particle and particle-filler interactions. A reactive blending and formation of poly(lactide-block-ethylene glycol) (PLA-b-PEG) copolymers have been developed and optimized by employing a transesterification reaction in a melt-mixing process. The effects of processing parameters, including PEG weight composition, temperature, catalyst content, and rotor speed, on the reaction conversion, molecular chain structure, and specific mechanical energy (SME) were investigated. 1H-NMR spectroscopy was employed in quantitative analysis. The process parameters were optimized using a central composite rotatable design (CCRD) response surface methodology (RSM). A quadratic interaction is observed between the PEG weight content and temperature, indicating that high reaction temperature leads to lower PEG conversions due to undesirable competing thermal-oxidative degradations of PEG in the presence of the catalyst. A maximized desirability for PEG weight percentage, temperature, catalyst content, and screw speed was found at 39.76 wt.%, 150 ˚C, 0.50 wt.%, and 42 rpm, at 0.98 desirability value. A confirmation experiment revealed that all response values are 95% confidence, except %AB. The results agree with the ANOVA results of the quadratic model of %AB in which Pred.R2 of the %AB is 0.4205. These confirm a good agreement between the experimental and predicted values; hence the model is validated. The model is a platform for effectively preparing PLA-b-PEG copolymers with designed molecular weight and chemical structures for use as toughening agents for PLA resins.A field study to examine the performances of the fruit bag products produced from bio-composites was implemented at a local mango farm in Ratchaburi province, Thailand. UV protection with high WVP materials was adopted to formulate pre-harvest bagging material for “R2E2” mangoes. A low WVP of a commercial PE could cause fruit to rot and drop due to high relative humidity (RH) inside the bags. In contrast, PLA and PLA/silica bio-composite bags can improve fruit quality through a reduction in disease and minimize mechanical injuries. However, fruit mass, flesh color, TSS, and TA were not affected by the bagging materials used in this study. The effect of wavelength-selection bags on fruit quality was investigated with Nam Dok Mai-Srithong mango (Golden skin mango). Toughening agents and modified FS were used as fillers for PLA before fabricating. The mangoes bagged with all wavelength-selection bagging materials (T2-T4) show significantly (P ≤ 0.05) higher weight and size values over the control (T1) and those from the commercial paper bags (T5). High TSS/TA values were observed in the mangoes bagged with the wavelength-selection bags (T2-T4), while lower TSS/TA values were observed in the control (T1) and the paper bagged (T5) counterparts. Moreover, the mango products from the bio-composite bags showed more saturated yellow peel with higher glossiness. Given the improved properties and unique performances, the environmental-friendly bio-composites have a high potential for high-value smart agriculture applications.