Improving the resistance to hydrothermal ageing of flame-retarded PLA by incorporating miscible PMMA

The key role of unique crystalline property in the hydrolytic degradation process of microcrystalline cellulose-reinforced stereo-complexed poly(lactic acid) composites

Stereo-complexed poly(lactic acid) (SC-PLA) has unique stereo-complexed crystallites (SC) and homogeneous crystallites (HC), but the effect of this special crystalline property on the hydrolytic degradation of SC-PLA has not been researched. In this study, the hygrothermal aging behaviour of injection-molded SC-PLA and SC-PLA/microcrystalline cellulose (MCC) composites at different temperatures (25 °C and 60 °C) was investigated from micro- and macroscopic perspectives. The results demonstrated that the hydrolysis of SC-PLA was sequentially dominated by the amorphous region, the homogeneous crystalline region, the stereo-complexed crystalline region (three stages). The hydrolytic degradation of SC-PLA only completed the first stage after 4 weeks aging at 25 °C, while it was in the third stage after 4 weeks aging at 60 °C. On this basis, the accelerating effect of 10 wt% MCC on the hydrolysis process of SC-PLA at different stages was investigated. It was found that MCC shortened the hydrolysis time in the stereo-complexed crystalline region by reducing the rearrangement of amorphous structure to form SC and causing cracks and interfacial deterioration by water absorption-swelling-degradation. In addition, the thermal properties and impact strength of SC-PLA and SC-PLA/MCC composites decreased dramatically due to rapid hydrolytic degradation at 60 °C. Overall, the results of this study can provide theoretical basis for the application of SC-PLA and SC-PLA/MCC composites in hygrothermal environment.

Effect of Aging on Tensile and Chemical Properties of Polylactic Acid and Polylactic Acid-Like Polymer Materials for Additive Manufacturing

Additive manufacturing, with its fast development and application of polymeric materials, led to the wide utilization of polylactic acid (PLA) materials. As a biodegradable and biocompatible aliphatic polyester, produced from renewable sources, PLA is widely used in different sectors, from industry to medicine and science. The aim of this research is to determine the differences between two forms of the PLA material, i.e., fused deposition modeling (FDM) printed filament and digital light processing (DLP) printed resin, followed by aging due to environmental and hygiene maintenance conditions for a period of two months. Specimens underwent 3D scanning, tensile testing, and Fourier transform infrared (FTIR) spectrometry to obtain insights into the material changes that occurred. Two-way Analysis of Variance (ANOVA) statistical analysis was subsequently carried out to determine the statistical significance of the determined changes. Significant impairment can be observed in the dimensional accuracies between both materials, whether they are non-aged or aged. The mechanical properties fluctuated for aged FDM specimens: 15% for ultimate tensile stress, 15% for elongation at yield, and 12% for elastic modulus. Regarding the DLP aged specimens, the UTS decreased by 61%, elongation at yield by around 61%, and elastic modulus by 62%. According to the FTIR spectral analysis, the PLA materials degraded, especially in the case of resin specimens. Aging also showed a significant influence on the elastic modulus, ultimate tensile stress, elongation at yield, elongation at break, and toughness of both materials, which was statistically shown by means of a two-way ANOVA test. The data collected in this research give a better understanding of the underlying aging mechanism of PLA materials.

Effect of Thermal and Hydrothermal Accelerated Aging on 3D Printed Polylactic Acid

In the new transformation of 'Industry 4.0', additive manufacturing technologies have become one of the fastest developed industries, with polylactic acid (PLA) playing a significant role. However, there is an increasing amount of garbage generated during the printing process and after prototypes or end-of-life parts. Re-3D printing is one way to recycle PLA waste from fused filament fabrication. To do this process successfully, the properties of the waste mixture should be known. Previous studies have found that PLA degrades hydrolytically, but the time at which this process occurs for 3D printed products is not specified. This work aims to establish the baseline of the degradation kinetics of 3D printed PLA products to predict the service time until which these properties are retained. To achieve this, 3D printed specimens were thermally and hydrothermally aged during several time intervals. Thermal and mechanical properties were also determined. This study reveals that tensile strength decreases after 1344 h of hydrothermal ageing, simulating 1.5-2.5 years of real service time. PLA therefore has the same thermo-mechanical properties before reaching 1.5-years of age, so it could be recycled.

Effect of Hygrothermal Ageing on the Mechanical and Fire Properties of a Flame Retardant Flax Fiber/Epoxy Composite

In engineering applications, natural fiber composites must comply with fire requirements including the use of flame retardant. Furthermore, biocomposites are known to be water sensitive. Whether flame retardants affect the water sensitivity and whether water absorption affects the fire behavior and the mechanical performance of biocomposites are the two main topics addressed in this work. In this study, a flax fiber/epoxy composite flame retardant with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or aluminum diethyl phosphinate (AlPi) was aged in humid atmosphere or by immersion in water. Water absorption kinetics revealed that DOPO induces an increase in equilibrium water content by approximately a factor of 2 due to its intrinsic hygroscopicity and/or its plasticizing effect on the epoxy matrix. In contrast, AlPi does not significantly change the water sensitivity of the biocomposite. Mechanical testing highlighted that, whatever the FR, the evolution of mechanical properties with ageing is governed by the moisture content. The drop of elastic modulus was attributed to a decrease in fiber rigidity due to plasticization, while the increase in tensile strength was assigned to an increase in fiber/matrix friction due to fiber swelling. As regards flame retardancy, only the highest water contents modified the fire behavior. For the AlPi containing biocomposite, the water release resulted in an increase by 50% of the time to ignition, while for the DOPO flame retardant biocomposite the water release was mainly postponed after ignition.

Polylactide as a Substitute for Conventional Polymers—Biopolymer Processing under Varying Extrusion Conditions

The polymer processing industry is paying more attention to biodegradable materials synthesized from renewable sources. One of the most popular of them is polylactide (PLA). Except the material from which a given product is made, particularly important is the process of manufacturing a polymer material, processing, use by the consumer, and finally, recycling it. Neither of these steps is indifferent to the environment. The processing of polymers can often lead to material degradation, which affects the properties of the material and leads to the generation of substantial amounts of post-production waste that cannot be reused by processors. The aim of this work is to evaluate selected properties of PLA subjected to the extrusion process under variable extrusion conditions. This is important due to the large losses of material and energy resulting from the extrusion of biodegradable polymers under poorly selected processing conditions, which, apart from the economic effects, has a negative impact on the environment. The research proved that both the temperature and the structure of the plasticizing system as well as the rotational speed of the screws affect the mechanical properties of the final product. For PLA optimization, this process will directly contribute to the improvement of the PLA processing process, and indirectly help to act for the benefit of the environment by reducing the consumption of energy, raw materials, and the amount of post-production waste. The obtained results allowed for the selection of appropriate parameters depending on the expectations regarding the properties of the final product. The conducted research will help to optimize processing processes and reduce the consumption of raw materials, which in the future will also affect the environment.

Phase Structure and Properties of Ternary Polylactide/Poly(methyl methacrylate)/Polysilsesquioxane Blends

Ternary blends of polylactide (PLA, 90 wt.%) and poly(methyl methacrylate) (PMMA, 10 wt.%) with functionalized polysilsesquioxanes (LPSQ-R) were obtained by solution blending. R groups in LPSQ containing hydroxyethyl (LPSQ-OH), methylglycolic (LPSQ-COOMe) and pentafluorophenyl (LPSQ-F5) moieties of different chemical properties were designed to modify PLA blends with PMMA. The effect of the type of LPSQ-R and their content, 1-3 wt.%, on the structure of the blends was studied with scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (SEM-EDS), dynamic mechanical thermal analysis (DMTA) and Raman spectroscopy. Differential scanning calorimetry (DSC) and tensile tests also showed various effects of LPSQ-R on the thermal and mechanical properties of the blends. Depth-sensing indentation was used to resolve spatially the micro- and nano-scale mechanical properties (hardness and elastic behaviour) of the blends. The results showed clearly that LPSQ-R modulate the structure and properties of the blends.

Influence of amorphous cellulose on mechanical, thermal, and hydrolytic degradation of poly(lactic acid) biocomposites

Eco-friendly materials such as poly(lactic acid) (PLA) and cellulose are gaining considerable interest as suitable substitutes for petroleum-based plastics. Therefore, amorphous cellulose (AC) was fabricated as a new reinforcing material for PLA biocomposites by modifying a microcrystalline cellulose (MCC) structure via milling. In this study, the mechanical properties, thermal properties, and degradability of PLA were analysed to compare the effects of both MCC and AC on PLA. The tensile and impact properties improved at an optimum value with AC at 8 wt% and 4 wt% fibre loading, respectively. Notably, a scanning electron micrograph analysis revealed improved AC fibre-matrix adhesion, compared with MCC fibre-matrix adhesion, as well as excellent interaction between AC and PLA. Both MCC and AC improved the hydrolytic degradation of PLA. Moreover, the biocomposites with AC exhibited superior degradation when the incorporation of AC improved the water absorption efficiency of PLA. These findings can expand AC applications and improve sustainability.

Filament Extrusion and Its 3D Printing of Poly(Lactic Acid)/Poly(Styrene-co-Methyl Methacrylate) Blends

Herein, we report the melt blending of amorphous poly(lactide acid) (PLA) with poly(styrene-co-methyl methacrylate) (poly(S-co-MMA)). The PLAx/poly(S-co-MMA)y blends were made using amorphous PLA compositions from 50, 75, and 90wt.%, namely PLA50/poly(S-co-MMA)50, PLA75/poly(S-co-MMA)25, and PLA90/poly(S-co-MMA)10, respectively. The PLAx/poly(S-co-MMA)y blend pellets were extruded into filaments through a prototype extruder at 195 °C. The 3D printing was done via fused deposition modeling (FDM) at the same temperature and a 40 mm/s feed rate. Furthermore, thermogravimetric curves of the PLAx/poly(S-co-MMA)y blends showed slight thermal decomposition with less than 0.2% mass loss during filament extrusion and 3D printing. However, the thermal decomposition of the blends is lower when compared to amorphous PLA and poly(S-co-MMA). On the contrary, the PLAx/poly(S-co-MMA)y blend has a higher Young’s modulus (E) than amorphous PLA, and is closer to poly(S-co-MMA), in particular, PLA90/poly(S-co-MMA)10. The PLAx/poly(S-co-MMA)y blends proved improved properties concerning amorphous PLA through mechanical and rheological characterization.