Synthesis of High Molecular Weight Poly(l-lactic acid) via Melt/Solid State Polycondensation. II. Effect of Precrystallization on Solid State Polycondensation

Poly(lactic Acid): A Versatile Biobased Polymer for the Future with Multifunctional Properties-From Monomer Synthesis, Polymerization Techniques and Molecular Weight Increase to PLA Applications

Environmental problems, such as global warming and plastic pollution have forced researchers to investigate alternatives for conventional plastics. Poly(lactic acid) (PLA), one of the well-known eco-friendly biodegradables and biobased polyesters, has been studied extensively and is considered to be a promising substitute to petroleum-based polymers. This review gives an inclusive overview of the current research of lactic acid and lactide dimer techniques along with the production of PLA from its monomers. Melt polycondensation as well as ring opening polymerization techniques are discussed, and the effect of various catalysts and polymerization conditions is thoroughly presented. Reaction mechanisms are also reviewed. However, due to the competitive decomposition reactions, in the most cases low or medium molecular weight (MW) of PLA, not exceeding 20,000-50,000 g/mol, are prepared. For this reason, additional procedures such as solid state polycondensation (SSP) and chain extension (CE) reaching MW ranging from 80,000 up to 250,000 g/mol are extensively investigated here. Lastly, numerous practical applications of PLA in various fields of industry, technical challenges and limitations of PLA use as well as its future perspectives are also reported in this review.

Evaluation of the Technical Viability of Distributed Mechanical Recycling of PLA 3D Printing Wastes

3D printing PLA wastes were recovered from a well-known reference grade and from different sources. The recovered wastes were subjected to an energic washing step and then reprocessed into films by melt-extrusion, followed by compression molding to simulate the industrial processing conditions. The obtained materials were characterized and the optical, structural, thermal and crystallization behavior are reported. The mechanical recycling process leads to an increase of the crystallinity and a decrease of the intrinsic viscosity of the formulations, particularly in the sample based on blends of different 3D-PLA wastes. Moreover, the obtained films were disintegrated under composting conditions in less than one month and it was observed that recycled materials degrade somewhat faster than the starting 3D-PLA filament, as a consequence of the presence of shorter polymer chains. Finally, to increase the molecular weight of the recycled materials, the 3D-PLA wastes were submitted to a solid-state polymerization process at 110, 120, and 130 °C, observing that the recycled 3D-wastes materials based on a well-known reference grade experiences an improvement of the intrinsic viscosity, while that coming from different sources showed no significant changes. Thus, the results show that 3D printing PLA products provides an ideal environment for the implementation of distributed recycling program, in which wastes coming from well-known PLA grades can successfully be processed in films with good overall performance.

Thermoplastic PLA-LCP Composites: A Route toward Sustainable, Reprocessable, and Recyclable Reinforced Materials

Reprocessing of reinforced composites is generally accompanied by loss of value and performance, as normally the reinforcing phase is damaged, or the matrix is lost in the process. In the search for more sustainable recyclable composite materials, we identify blends based on poly(l-lactide) (PLA) and thermotropic liquid crystalline polymers (LCP) as highly promising self-reinforced thermoplastic composites that can be recycled several times without loss in mechanical properties. For example, irrespective of the thermal history of the blend, injection molded bars of PLA containing 30 wt % LCP exhibit a tensile modulus of 6.4 GPa and tensile strength around 110 MPa, as long as the PLA matrix has a molecular weight of 170 kg mol^-1 or higher. However, after several mechanical reprocessing steps, with the gradual decrease in the molecular weight of the PLA matrix, deterioration of the mechanical performance is observed. The origin of this behavior is found in the increasing LCP to PLA viscosity ratio: at a viscosity ratio below unity, the dispersed LCP droplets are effectively deformed into the desired fibrillar morphology during injection molding. However, deformation of LCP droplets becomes increasingly challenging when the viscosity ratio exceeds unity (i.e., when the PLA matrix viscosity decreases during consecutive reprocessing), eventually resulting in a nodular morphology, a poor molecular orientation of the LCP phase, and deterioration of the mechanical performance. This molecular weight dependency effectively places a limit on the maximum number of mechanical reprocessing steps before chemical upgrading of the PLA phase is required. Therefore, a feasible route to maintain or enhance the mechanical properties of the blend, independent of the number of reprocessing cycles, is proposed.