Correlation between Processing Parameters and Degradation of Different Polylactide Grades during Twin-Screw Extrusion

Abstract

This article presents the effect of twin-screw extrusion processing parameters, including temperature and rotational speed of screws, on the structure and properties of four grades of polylactide (PLA). To evaluate the critical processing parameters for PLA and the possibilities for oxidative and thermomechanical degradation, Fourier-transform infrared spectroscopy (FT-IR), oscillatory rheological analysis, and differential scanning calorimetry (DSC) measurements were used. The influence of degradation induced by processing temperature and high shearing conditions on the quality of the biodegradable polyesters with different melt flow indexes (MFIs)was investigated by color analysis within the CIELab scale. The presented results indicate that considering the high-temperature processing of PLA, the high mass flow index and low viscosity of the polymer reduce its time of residence in the plastifying unit and therefore limit discoloration and reduction of molecular weight due to the degradation process during melt mixing, whereas the initial molecular weight of the polymer is not an essential factor.

Effect of Extrusion Screw Speed and Plasticizer Proportions on the Rheological, Thermal, Mechanical, Morphological and Superficial Properties of PLA

“…Here, significant molecular weight loss of up to 25% is related to PLA plasticized with 20% ATBC from 178,000 to 135,000 g/mol at 60 and 150 rpm, respectively ( Supplementary Materials Table S1 ). Studies in PLA without plasticizer showed no incidence of critical process variables, allowing processing in wide ranges of speed (50–250 rpm) and temperature (180–220 °C) without affecting the molecular weight and thermomechanical characteristics of PLA [ 7 ]. However, plasticization of PLA with ATBC under higher shear conditions shows appreciable changes in viscosity and consequently in molecular weight.…”

“…The behavior of the mixtures is largely related to the extrusion speed used in this study, so this knowledge could contribute to obtaining future materials in which different conditions will be used in the process, knowing how speed influences function of the proportion of plasticizers more convenient to mix the PLA. Some studies report the extruded behavior of PLA at different speeds and process temperatures; however, the addition of plasticizer has not been considered [ 7 ].…”

“…Zero shear viscosity ( ) was determined by the Carreau–Yasuda model fitted to the experimental data [ 35 ]. was possible due to the rheological measurements in oscillatory mode, and the calculations that were performed applying the Trios 5.0 software [ 7 ] describe pseudoplastic flow with asymptotic viscosities at zero and infinite shear rates, according to Equation (1) [ 35 ]: where n is a power-law coefficient, a is the adjustable exponent, γ is a shear rate, and λ is characteristic time. The theoretical value of the molecular weight (Mw) of the PLA plasticized at different speeds based on rheological data ( , Equation (1)) according Equation (2) was estimated: where C(T) = 14.83, as revealed from literature data [ 36 ].…”

“…Nowadays, there is a great need to resolve problems associated with the high consumption of non-renewable resources, to produce non-degradable polymers, which is generating a high amount of plastic waste [ 1 ]; biodegradable aliphatic polymers, such as poly-hydroxyalkanoates (PHAs), poly-butylene succinate (PBS), poly-butylene succinate-co-adipate (PBSA), poly(butylene adipate-co-terephthalate) (PBAT) or poly-lactic acid (PLA), are an important alternative [ 2 , 3 , 4 ]. Among these biopolymers, PLA is the key and promising biopolymer to develop new materials both in industry and in academy [ 5 ], commercially is the most traded and it is obtained from the fermentation of renewable sources such as whey, corn, potato, molasses, or sugar feed stocks to produce the monomer lactic acid [ 6 , 7 ], which is subsequently polymerized to a linear aliphatic thermoplastic polyester by a ring-opening synthesis procedure [ 8 ].…”

“…The melt process is carried out by heating the material above its melting temperature to get the desired product [ 30 ] and also is necessary take into account during the blending process, the shearing conditions [ 7 ]. At lower processing speeds between 20 and 80 rpm, various blending systems were obtained from mixers or torque rheometers [ 8 , 9 , 13 , 24 , 25 , 26 , 31 ], or from twin screw extruders [ 14 , 16 , 20 ], using plasticizers such as ATBC, PEG or TBC, and PLA or a PLA-blend, varying only the plasticizer content in the final PLA-based blend; generally, for processing speeds above 100 rpm, twin screw extruders [ 1 , 2 , 6 , 7 , 10 , 15 , 22 , 23 ] are used to obtain the plasticized PLA-based blend material from ATBC, OLA, oligoesteres or malonate oligomers to plasticize the PLA or a PLA-blend. In the most of cases, especially with ATBC, the plasticizer decreases the glass transition temperatures and improve the ductile properties but there is always a migration of the plasticizer from the PLA matrix.…”