Effect of Pre-Treatments on The Kinetics of Subsequent aerobic and Anaerobic Biodegradation of Polylactic Acid (PLA)

Abstract

Evalúa el efecto de pretratamientos como hidrólisis ácida, hidrólisis básica, irradiación gamma, irradiación por haz de electrónica y sometimiento a presión de vapor para reducir el tiempo de biodegradación de envases termoformados a base de ácido poliláctico. El propósito de este trabajo fue evaluar tratamientos que aumentan la velocidad de biodegradación del ácido poliláctico (PLA) durante el compostaje y la digestión anaeróbica. Los tratamientos fueron elegidos pensando en reducir el peso molecular y afectar la integridad estructural del polímero. Este trabajo está subdividido en cinco secciones: (a) encontrar el tratamiento más eficaz para reducir el peso molecular del PLA, (b) determinar la cinética involucrada en el tratamiento seleccionado, (c) desarrollar un método simple para evaluar la biodegradación aeróbica del PLA en el compost, (d) evaluar los efectos de los pretratamientos en la digestión anaeróbica posterior, y (e) evaluar los efectos de los pretratamientos en posteriores procesos de compostaje. Se investigaron cinco tratamientos potenciales: irradiación gamma, irradiación con haz de electrones, inmersión en medios alcalinos, inmersión en medios ácidos y exposición al vapor. Se encontró que la exposición al vapor fue el tratamiento más eficaz con hasta un 94 % de reducción del peso molecular, usando vapor a 120 ° C durante 4 horas. Para determinar la cinética de la reducción del peso molecular del PLA después de la exposición al vapor, se propuso un modelo de reacción de primer orden invertido.

The purpose of this work was to evaluate treatments that increase polylactic acid (PLA) biodegradation during composting and anaerobic digestion. Treatments were chosen on the potential to reduce molecular weight and affect structural integrity of the polymer. Five potential treatments were investigate, and it was found that steam exposure was the most effective treatment with up to 94% molecular weight reduction with 120°C steam for 4 hours. To determine kinetics of PLA molecular weight reduction after exposure to steam, a first-order reaction model was proposed. It was found that the model, which is time and temperature dependent, fits experimental data well, with activation energy, Ea, of 52.3 KJ/mol. In addition, it was demonstrated that steam treatments hydrolyze polymer molecules, resulting in de-polymerization to lactic acid. At 120°C, weight loss of PLA was 84.7% after 24 hours, indicating significant conversion to lactic acid. To develop a simple method to evaluate PLA aerobic biodegradation in compost, the fundamentals of conversion were examined, and three approaches were designed and assessed. These methods are referred to as methods of “flexible bags”, “rigid containers with plastic lids” and “perforated jars”. It was found that the method of perforated jars was most reliable, simple and consistent to apply. To evaluate effects of treatment in subsequent anaerobic digestion, weight loss and biochemical methane potential (BMP) of treated samples were investigated under mesophilic and thermophilic anaerobic conditions. Untreated PLA did not degrade under mesophilic conditions. However, degradation did occur under thermophilic conditions, producing 187 ccCH4/g at 58°C after 56 days. The best scenario evaluated, was steam-treated PLA at 120°C for 3 hours subjected to anaerobic thermophilic conditions, where the yield of methane was 225 ccCH4/g after 56 days. While untreated PLA biodegrades more slowly than common organic feedstock, steam-treated PLA biodegraded faster. Results of this investigation show that treatment of PLA with steam at 120°C for 3 hours reached 60% degradation in 14 days. Compost was not altered by PLA conversion.