Polylactic Acid Processing Methodologies
Poly(lactic acid) or polylactide (PLA) has garnered significant attention as a biopolymer, owing to its emergence as an environmentally sustainable alternative to polymers derived from fossil fuels. PLA, classified as an aliphatic polyester, can be obtained from either natural or synthetic sources, primarily through industrial polycondensation of lactic acid and/or ring-opening polymerization of lactide. Melt processing serves as the predominant technique for mass-producing PLA products across diverse industries, including medical, textile, plasticulture, transportation, electronics, and packaging. However, the processing of PLA poses various challenges that necessitate attention to optimize both its processing and component qualities.
Applications of PLA (Credits https://www.mdpi.com/2073-4360/13/11/1854)
One of the principal challenges in PLA processing is its susceptibility to degradation under high temperatures and shear forces. With a relatively low glass transition temperature (Tg) ranging from 60-65°C, PLA softens at comparatively low temperatures, leading to compromised dimensional stability and reduced mechanical properties. Moreover, PLA exhibits hygroscopic properties, making it highly sensitive to elevated relative humidity (RH) and temperature. Prior to processing, it is imperative to subject PLA to thorough drying, reducing its water content to less than 100 ppm to prevent hydrolysis and molecular weight (Mw) reduction.
Process window and processing instructions for Polylactic acid
(Credit: https://www.ifbb-hannover.de/files/IfBB/downloads/EV_Processing-of-Bioplastics-2016.pdf)
To overcome these challenges, various methods have been developed to optimize PLA processing. Notably, melt extrusion emerges as a crucial technique for the continuous processing of high Mw PLA consumer goods. Extrusion in a heated screw serves as the initial step before subsequent processes such as injection molding, thermoforming, or fiber spinning takes place. While commercial PLA resins can be processed using conventional screws equipped with a general-purpose metering section and compression section, optimal processing conditions require careful screw design to ensure efficient melting and mixing of the PLA resin.
Another approach to optimize PLA processing involves blending it with other biodegradable and non-biodegradable resins. This blending enhances mechanical properties, such as toughness and impact resistance, as well as processability. For instance, blending PLA with polybutylene succinate (PBS) can improve melt strength and elongation at break, while blending it with polycarbonate (PC) enhances impact resistance. However, this blending may also result in non-biodegradability and increased cost of the resulting polymer compound.
Compounding PLA with fillers, such as fibers, micro, and nanoparticles, represents another method to optimize processing and improve properties. The addition of fillers enhances mechanical properties like stiffness and strength, as well as thermal and barrier properties. For instance, adding cellulose nanocrystals (CNCs) to PLA improves tensile strength and modulus, while adding graphene oxide (GO) enhances thermal conductivity. Nevertheless, adding fillers to PLA may lead to reduced processability and increased cost. However, the biodegradability will be impacted with the virtue of filler biodegradation characteristics. 
Tuning of PLA properties (Credits: https://www.mdpi.com/1996-1944/15/9/2989)
In addition to blending and compounding, modifying PLA through crosslinking, chain extension, or grafting can also enhance its processing and properties. Various modifiers, including organic peroxide, multifunctional epoxide, styrene maleic anhydride, isocyanurate + diisocyanate, and bisoxazoline + diisocyanate, have demonstrated the ability to increase the Mw of commercially available PLA. This increase in Mw improves melt properties such as shear and elongational viscosity, contributing to enhanced processability and component qualities.
Other processes such as foaming and fiber production are employed to create PLA products. The production of PLA foam parts is driven by the desire for lightweight materials with improved cushioning, insulation, and structural performance. Commercial foaming of PLA is typically achieved through batch or continuous processes, introducing physical or chemical foaming agents (PFA or CFA) into the PLA matrix. PFAs undergo a physical change during foaming, while CFAs, stable at room temperature, undergo a chemical reaction under specific temperature and pressure conditions to generate gas for nucleating bubbles inside the PLA. PLA fibers, particularly for suture applications, are produced through spinning, driven by the need for materials with lower water barrier properties. Critical criteria for fiber production include controlling moisture content to less than 50 ppm to avoid hydrolytic degradation and achieving appropriate crystallinity and strength. During spinning, careful control of spinning temperature (190-230°C) and drawing speed (200-9000 m/min) is necessary to obtain the desired crystallinity and strength in PLA fibers.
Polylactic acid (PLA), a biodegradable thermoplastic derived from renewable sources, encounters specific challenges across various processing methods. During injection molding, PLA's sensitivity to processing temperatures and rapid cooling rates necessitates careful temperature control to prevent degradation and ensure uniform mold filling. Adhesion to molds can pose challenges during demolding, necessitating proper release agents. In extrusion, PLA's shear sensitivity and susceptibility to die swell and melt fracture require attention to prevent polymer chain degradation and surface irregularities. Rapid cooling rates during extrusion impact crystallinity and morphology, affecting mechanical and thermal properties. Crosslinking PLA poses challenges due to limited crosslinkability, requiring careful selection of agents without inducing excessive degradation. In fiber spinning, high melt viscosity and the need for controlled processing conditions affect fiber uniformity and diameter, while drawing processes must balance improving strength with the risk of molecular weight reduction.
The processing of PLA introduces several challenges that demand attention to optimize both processing and component qualities. However, numerous methods, including blending and compounding with other resins and fillers, modifying through crosslinking, chain extension, or grafting, and utilizing foaming and fiber processes, have been developed to overcome these challenges. These methods contribute to enhancing the mechanical, thermal, and barrier properties of PLA, as well as its processability. As the demand for sustainable materials continues to rise, optimizing PLA processing will increasingly become crucial to meet the diverse needs of various industries.
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Author
Dr. Prashant Gupta
Faculty, Polymerupdate Academy
