Polylactic acid | EPFL Graph Search

Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula (C3H4O2)n or [–C(CH3)HC(=O)O–]n, formally obtained by condensation of lactic acid C(CH3)(OH)HCOOH with loss of water (hence its name). It can also be prepared by ring-opening polymerization of lactide [–C(CH3)HC(=O)O–]2, the cyclic dimer of the basic repeating unit. PLA has become a popular material due to it being economically produced from renewable resources. In 2021, PLA had the highest consumption volume of any bioplastic of the world, although it is still not a commodity polymer. Its widespread application has been hindered by numerous physical and processing shortcomings. PLA is the most widely used plastic filament material in FDM 3D printing. Its low melting point, high strength, low thermal expansion, and good layer adhesion, although it possesses poor heat resistance unless annealed. Although the name "polylactic acid" is widely used, it does not comp

Recycled Glass Fiber Composites from Wind Turbine Waste for 3D Printing Feedstock: Effects of Fiber Content and Interface on Mechanical Performance

Kazem Fayazbakhsh, Rodolphe Henri

This research validates the viability of a recycling and reusing process for end-of-life glass fiber reinforced wind turbine blades. Short glass fibers from scrap turbine blades are reclaimed and mixed with polylactic acid (PLA) through a double extrusion process to produce composite feedstock with recycled glass fibers for fused filament fabrication (FFF) 3D printing. Reinforced filaments with different fiber contents, as high as 25% by weight, are extruded and used to 3D print tensile specimens per ASTM D638-14. For 25 wt% reinforcement, the samples showed up to 74% increase in specific stiffness compared to pure PLA samples, while there was a reduction of 42% and 65% in specific tensile strength and failure strain, respectively. To capture the level of impregnation of the non-pyrolyzed recycled fibers and PLA, samples made from reinforced filaments with virgin and recycled fibers are fabricated and assessed in terms of mechanical properties and interface. For the composite specimens out of reinforced PLA with recycled glass fibers, it was found that the specific modulus and tensile strength are respectively 18% and 19% higher than those of samples reinforced with virgin glass fibers. The cause for this observation is mainly attributed to the fact that the surface of recycled fibers is partially covered with epoxy particles, a phenomenon that allows for favorable interactions between the molecules of PLA and epoxy, thus improving the interface bonding between the fibers and PLA.

2019

Life cycle assessment of bio-based synthetic fibers: the case of polyester substitutes

Tijana Ivanovic

Numerous studies have shown that the textile sector has a high fossil-fuel dependency, emits excessive amount of greenhouse gases and uses vast amount of water. Currently, polyester (fossil-based) makes up about a half of the industry's fiber production and the volumes are expected to increase. Polyester fiber is traditionally made out of polyethylene terephthalate (PET) polymer. In order to break from the fossil-fuel dependency, textile industry has the possibility to use alternative feedstock for the polyester by reverting to bio-sourcing – strategy where the product's carbon backbone comes partially or fully from the bio-based feedstock. The polyester fiber can be entirely substituted in its current uses by three such bio-based substitutes: bio-based PET fiber as a drop-in substitute (chemically identical to the fossil counterpart); polytrimethylene terephthalate (PTT) fiber as an intra-family substitute with properties comparable to PET fiber and nylon; and polylactic acid (PLA) fiber as a novel fiber without a fossil counterpart and with properties comparable to PET fiber. All of these fibers are produced from the homonymous polymers via melt spinning and their carbon backbone comes partially or entirely from bio-based feedstock. The environmental effects of this feedstock substitution have not been studied abundantly, in spite of the existing political will to prioritize bio-based products, e.g. as the European Green Deal does. This project therefore performs a comparative, cradle-to-gate life cycle assessment of the conventional polyester fiber and these substitutes taking into account state-of-the-art production process. In order to do so Life cycle inventories (LCI) were modelled for 6 different scenarios for bio-based PET, 2 for bio-based PTT and 1 for PLA and were based on public literature and ecoInvent data. Impact assessment was performed by applying two methods – Swiss Ecological Scarcity and European Environmental Footprint, representative of the Swiss environmental policy and of a more scientific classification of environmental and health impacts respectively. The study finds that all three bio-based fibers are produced from the first-generation feedstock (crops). Currently, only the partially bio-based PET and PTT polymers are commercialized, such that the prevailing monomer, purified terephthalic acid (PTA) remains fossil-based for both. Then, monoethylene glycol (MEG) is produced from sugarcane or corn and 1,3-propanediol (PDO) from corn respectively. PLA is a fully bio-based polymer, produced from corn. Bio-sourcing for polyester is found to offer a limited improvement in a small number of impacts (with negligible contribution to the overall score) while causing substantial additional environmental burdens elsewhere (per kg of fiber). Namely, none of the bio-based alternatives perform better than the fossil-based PET fiber on the overall score. However, the partially bio-based PTT fiber may be a viable substitute for nylon (based on lower overall score). In the absence of carbon crediting for bio-based feedstock, climate impacts are found to be worse for the alternatives than the conventional polyester. Eutrophication, acidification, water scarcity impacts and land use rise with fiber's bio-content and largely stem for the agricultural practices (up to 88%, 74%, 97% and 100% respectively). It was also seen that PTA necessitates more precise modeling to raise the certainty of conclusions for the respective fully bio-based PET and PTT scenarios. At this stage, bio-based alternatives have a worse environmental performance than fossil-based polyester. The environmental performance of the bio-based fibers may be increased with alternative feedstock options or with improved agricultural practices.

2020

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.

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