Polylactic acid and its blends with petroleum‐based resins: Effects of reprocessing and recycling on properties

Abstract

Environmental and economic reasons make the use of bioplastics and biocomposites increasingly coveted in sectors other than packaging. Recycling of all wasted or rejected durable plastics is highly desired and biobased plastics are no exception. Therefore, the investigation of pre-and post-consumer recycling of products made from biobased plastics is of great interest. Polylactic acid (PLA) and its blends have been chosen for this study because it is an excellent representative of mass-produced bioplastics for industrial applications. As part of the "Sustainable Recycling of 'Green' Plastics" project, the current study addresses the durability issues related to the reprocessing and post-consumer recycling of a PLA virgin resin and two commercially available blends of PLA namely one with polycarbonate (PC) and one with polyethylene (PE). The materials were investigated using methods that simulate postprocessing and post-consumer recycling. Accelerated ageing was performed at elevated temperature and humidity to simulate the usage period of the materials. The materials were analyzed using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and their mechanical strength was evaluated by tensile and impact testing. The flow properties of the materials were characterized by the melt flow index (MFI). Multiple processing of pure PLA did not affect the impact strength or the glass transition temperature (T g ), but caused crystallization and increase in the MFI, indicating that degradation occurred during processing. DSC thermograms of the blends revealed that the components in the blends were not miscible. Multiple processing of the blends did not significantly affect the elastic modulus of the materials, but affected the elongation at break. The results indicated that multiple processing of the PLA/HDPE blend caused increased dispersion and thus increased elongation at break, while the dominating mechanism in the PLA/PC blend was degradation that caused a decrease in elongation at break. Post-consumer recycling of the PLA/ PC blend was simulated and the results clearly showed that ageing corresponding to one year of use caused a significant degradation of PLA. Pure PLA was severely degraded after only one ageing cycle. Although the PLA/PC blend showed some improved mechanical properties and resistance to degradation compared with pure PLA, one ageing cycle still caused a severe degradation of the PLA and even the PC was degraded as indicated by the formation of small amounts of bisphenol A. V C 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43916.

Study of the Influence of the Reprocessing Cycles on the Final Properties of Polylactide Pieces Obtained by Injection Molding

“…Since the value of elongation is in the denominator and this was reduced to a greater extent, the overall effect on this ratio on the modulus was higher than stress. Therefore, the values of E t increased with the number of reprocessing cycles, which indicates greater rigidity of the material associated with brittleness [68]. The E t value of the PLA0 piece was 3580 MPa and it increased up to 3900 MPa in the PLA pieces obtained after the fourth reprocessing cycle (PLA4, PLA5, and PLA6).…”

“…These shorter chains could, however, offer improved mobility and, subsequently, they are considered to be more readily to pack in an ordered structure and increase the degree of crystallinity. This can potentially yield an increase in the degree of crystallinity and, thus, it can result in a more brittle material with a parallel reduction in the deformation capacity by which its energy absorption capacity is reduced [68].…”

“…These results are consistent with those described above concerning mechanical properties and corroborate that the first reprocessing cycle was not critical in PLA degradation, whereas the second and third cycles induced a slight influence. Nevertheless, above the fourth reprocessing cycle, the effect of degradation was clearly detectable in both mechanical properties and melt fluidity [53,58,68]. This fragmentation results in shorter chains and, evidently, these short chains possess increased mobility, which gives increased flow with temperature.…”

Recycling of Mixed Poly(Ethylene-terephthalate) and Poly(Lactic Acid)

“…There are publications about blends of petroleum-based and biopolymers but some of these [6][7][8] aimed to improve the unfavourable properties of PLA (e.g. rigidity, low tolerance of heat) [9,10], usually with the addition of polyethylene.…”

Estudio de la viabilidad del reciclado mecánico del poli(ácido láctico) y sus nanocomposites

“…Yarahmadi et al [114] utilizaron un protocolo de envejecimiento acelerado de 7 semanas, a 50 ºC y con una humedad relativa del 90%, para simular la degradación durante la vida útil del PLA y sus mezclas con otras resinas como polietileno o policarbonato. El reprocesado múltiple del PLA, hasta 6 ciclos, causó un incremento del 40% en el índice de fluidez del PLA puro, indicando la degradación del PLA.…”