Multiscale analysis on multiextruded poly(lactic acid)/organoclay nanocomposites: Insights into the underlying mechanisms of thermomechanical degradation

Abstract

Due to the growing market and the relatively high price of poly(lactic acid) (PLA), the possibility to recycle PLA has attracted a tremendous attention, aiming to reduce the environmental impact related to the life cycle of PLA-based products. However, the comprehensive structural and properties changes of PLA and PLA/clay nanocomposites induced by reprocessing are still not clear. In this work, PLA and its organoclay nanocomposites (PCN5) were subjected to repeated extrusion for 1 and 4 cycles to reveal their recycling characteristics by the observation of the changes in the microstructure, thermal and mechanical properties. Gel permeation chromatography, rheology and morphology analyses were utilized to deeply explore effects of the cycle number on the evolution of chemical structures and clay dispersion. It is found that reprocessing leads to a significant reduction in the weight average molecular mass of PLA by 24.0% and of PCN5 by 37.3%, whereas a more uniform dispersion of clay and a stronger clay-polymer interaction are also found after reprocessing. A direct correlation between the molecular chain scission and deterioration of thermal/ mechanical properties is observed both for PLA and PCN5 subjected to reprocessing cycles. The crystallization rate of PLA is suppressed by the addition of clay, while is accelerated by clay after their processing. Additionally, the presence of clay is found to reduce the differences in the thermal stability between the samples reprocessed after 1 and 4 cycles, which is ascribed to the competition between intensive chain scission and better dispersion of clay in the PLA matrix.

Review on the Degradation of Poly(lactic acid) during Melt Processing

“…Additionally, Meng et al [38] reported a decrease in M n and M w , from 92.6 kg/mol and 112 kg/mol to 83.8 kg/mol and 104 kg/mol, respectively. Other researchers [5,12,17,20,[28][29][30]43,[49][50][51][52][53][54][55][56][57][58][59] reported the same decreasing trend in molecular weight, which makes M n and M w good indicators to monitor process-induced degradation. Whether the decrease is small or large depends on the selected processing technique and parameters, as will be further discussed in Section 4.…”

“…Benvenuta-Tapia and Vivaldo-Lima [17], Jain et al [54], and Tesfaye et al [28] also reported an increase in the Ð due to degradation, but no clear explanation was given. In the work of Lv et al [52], a Ð decrease was reported from 1.95 for unprocessed PLA to 1.83 after 4 reprocessing cycles, which is explained by the degradation occurring mainly in the chains with a high molecular weight. Amini Moghaddam et al [49] also reported a decrease in Ð, but no explanation was given.…”

“…In addition to a change in 𝑇 , two papers also reported a change in 𝑇 . Lv et al [52] reported a decrease in 𝑇 from 103.0 °C to 97.1 °C and an increase in 𝑇 from 96.8 °C to 100.2 °C after 4 reprocessing cycles. In the work of Pillin et al [53], a decrease in 𝑇 was reported from 131.1 °C to 89.8 °C throughout 7 reprocessing cycles, whereas a 𝑇 was only observed from the 2nd reprocessing cycle on.…”

Comparative performance of fused deposit modeling3D‐printedand injection molded polylactic acid/thermoplastic starch/nanoclay bio‐based nanocomposites

“…4 It is widely used in fabricating food garbage bags, medical parts, and disposable containers and exhibits considerable potential for green packaging applications. 5,6 Nevertheless, its rather high brittleness, 7 low-heat distortion temperature, 5 low-biodegradability rate, 8 and high cost could be listed as limitations of PLA in comparison to other biodegradable polymers. 9 One of the key strategies frequently reported in the literature is to blend PLA with soft biodegradable polymers using coupling agents and plasticizers as seen in the binary and trinary polymer blend systems to modify its flexibility, thermal stability, toughness, and degradation rate.…”

Influence of the hydrophilicity of montmorillonite on structure and properties of thermoplastic wheat starch/montmorillonite bionanocomposites

“…Due to their high surface area and cation exchange, as well as their excellent mechanical properties and thermal resistance, MMT have been an effective filler for differentpolymer matrices, from rubbers 12 to synthetic polymers, such as polystyrene, 13 biopolymers, as polylactide 14 and starch, 15,16 as well as polymer blends 17,18 . It has been reported that the incorporation of a low level of MMT into polymer matrices can significantly improve a large number of physical properties of the virgin polymer, including mechanical and barrier properties, thermal stability, flammability resistance and degradability of biodegradable polymers 19–21 .…”