Impact of Melt Processing Conditions on the Degradation of Polylactic Acid

Abstract

To reduce the degradation of polylactic acid (PLA) during processing, which reduces the molecular weight of PLA and its properties, prior studies have recommended low processing temperatures. In contrast, this work investigated the impact of four factors affecting shear heating (extruder type, screw configuration, screw speed, and feed rate) on the degradation of PLA. The polylactic acid was processed using a quad screw extruder (QSE) and a comparable twin screw extruder (TSE), two screw configurations, higher screw speeds, and several feed rates. The processed PLA was characterized by its rheological, thermal, and material composition properties. In both screw configurations, the QSE (which has a greater free volume) produced 3–4 °C increases in melt temperature when the screw speed was increased from 400 rpm to 1000 rpm, whereas the temperature rise was 24–25 °C in the TSE. PLA processed at low screw speeds, however, exhibited greater reductions in molecular weight—i.e., 9% in the QSE and 7% in the TSE. Screw configurations with fewer kneading blocks, and higher feed rates in the QSE, reduced degradation of PLA. At lower processing temperatures, it was found that an increase in melt temperature and shear rate did not significantly contribute to the degradation of PLA. Reducing the residence time during processing minimized the degradation of PLA in a molten state.

Poly(butylene adipate-co-terephthalate)/Poly(lactic acid) Polymeric Blends Electrospun with TiO2-R/Fe3O4 for Pollutant Photodegradation

“…For calculations of the heat involved in the processes and the melting and crystallization temperatures that occurred during heating and cooling, TA60WS software (Kyoto, Japan) was used. The calculation of polymer crystallinity was also performed using Equation (1), conceived by Rossin and Aldhafeeri [ 25 , 26 ]. X c = (Δ H m − Δ H c /Δ H ° m ) × 100 where X c is the degree of crystallinity of the polymer, Δ H cc is cold crystallization enthalpy of the sample, Δ H m is fusion enthalpy of the sample, Δ H° m is theoretical fusion enthalpy of the 100% crystalline sample; in the case of PBAT, Δ H° m = 114 J·g −1 , and, for PLA, Δ H° m = 93.7 J·g −1 .…”

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

“…At a higher screw rotation speed, the residence time and thus the exposure of PLA to high temperature and shear inside the extruder is shorter, resulting in reduced degradation. In the work of Atalay et al [86], a lower complex viscosity was reported when amorphous PLA was processed at 50 rpm than at 100 rpm, both at a processing temperature of 150 • C. In addition, Aldhafeeri et al [90] reported η 0 -values of 4806 Pa•s at 400 rpm and 5332 Pa•s at 1000 rpm due to the longer residence time at lower screw rotation speeds. As indicated before in the works of Mysiukiewicz et al [79] and Taubner and Shishoo [89], it is important to take the processing temperature into account when evaluating the effect of screw rotation speed.…”

“…Contradictory results about the influence of screw rotation speed on the degradation are found in the literature. Most articles [79,86,89,90] concluded that an increase in screw rotation speed resulted in a smaller decrease in molecular weight (see Figures 16 and 17), which can be explained by a shorter residence time. At a higher screw rotation speed, the residence time and thus the exposure of PLA to high temperature and shear inside the extruder is shorter, resulting in reduced degradation.…”

Molecular Pathways for Polymer Degradation during Conventional Processing, Additive Manufacturing, and Mechanical Recycling

“…The influence of single extrusion, multiple extrusion, AM, and injection molding on polymeric degradation has been broadly studied in the past decades. For instance, degradation has been assessed for PLA and PLLA [167,[173][174][175][176], poly(ethylene terephthalate) (PET) [177,178], PS [42,[179][180][181], ABS [182][183][184], HIPS [185][186][187], PP [93,151,159,[188][189][190][191], HDPE [139,188,192], LDPE [140], and poly(methyl methacrylate) (PMMA) [42]. To highlight the advances made, an overview of the most important findings is given in this section.…”

“…Structural changes were monitored via rheological measurements, perceived by the significant decrease in the zero-shear viscosity under these conditions. Aldhafeeri et al [175] discussed that the processing parameters which increase the residence time, i.e., lower screw rate, lower feed rates, a quad screw extruder, and kneading blocks, result in a more pronounced degradation of PLA, confirmed by the reduction in the zero-shear viscosity. Additionally, the PLA grades with a low starting viscosity were less susceptible to degradation than the ones with a high starting viscosity at higher screw speeds [174].…”