Modulating poly(lactic acid) degradation rate for environmentally sustainable applications.

The huge amount of plastics generated by the massive use of packaging makes it difficult to manage waste safely. Introducing biodegradable polymers, such as poly(lactic acid) (PLA), can at least partially reduce the environmental pollution from plastic waste. Biodegradable polymers must have a degradation rate appropriate for the intended use to replace durable plastics. This work aims to introduce PLA fillers that can modulate the degradation rate during hydrolysis and composting. For this purpose, fumaric acid and magnesium hydroxide have been proposed. The experimental findings demonstrated that magnesium oxide makes hydrolysis faster than fumaric acid. A model describing the hydrolysis reaction, which also considers the effect of crystallinity, is proposed. The model can capture the filler effect on the kinetic constants related to the autocatalytic part of the hydrolysis reaction. Degradation of the PLA and compounds was also conducted in a composting medium. The compound with fumaric acid shows faster degradation than the compound with magnesium oxide; this behavior is opposite to what is observed during hydrolysis. Degradation in a composting medium is favored in a narrow pH window corresponding to the optimum environment for microorganism growth. Magnesium oxide leads to a pH increase above the optimum level, making the environment less favorable to microorganism growth. Vice-versa, fumaric acid maintains the pH level in the optimum range: it represents an additional carbon source for microorganism growth.

PLA Melt Stabilization by High-Surface-Area Graphite and Carbon Black.

Small amounts of carbon nanofillers, specifically high-surface-area graphite (HSAG) and more effectively carbon black (CB), are able to solve the well-known problem of degradation (molecular weight reduction) during melt processing, for the most relevant biodegradable polymer, namely poly(lactic acid), PLA. This behavior is shown by rheological measurements (melt viscosity during extrusion experiments and time sweep-complex viscosity) combined with gel permeation chromatography (GPC) experiments. PLA's molecular weight, which is heavily reduced during melt extrusion of the neat polymer, can remain essentially unaltered by simple compounding with only 0.1 wt % of CB. At temperatures close to polymer melting by compounding with graphitic fillers, the observed stabilization of PLA melt could be rationalized by scavenging traces of water, which reduces hydrolysis of polyester bonds. Thermogravimetric analyses (TGA) indicate that the same carbon fillers, on the contrary, slightly destabilize PLA toward decomposition reactions, leading to the loss of volatile byproducts, which occur at temperatures higher than 300 °C, i.e., far from melt processing conditions.