Polylactic Acid

Polylactic Acid or Polylactide (PLA) is a biodegradable and bioactive thermoplastic polymer derived from renewable resources, such as corn starch, cassava roots, chips or satrch, or sugarcane. Polylactic Acid is biodegradable and has characteristic similar to polypropylene (PP), polyethylene (PE), or polystyrene (PS). PLA are available in different grades depending on the process i.e. thermoforming, injection molding, blown film,  injection stretch blow molding, continuous yarn and fiber.

Properties of Polylactic Acid

• High mechanical strength
• Low toxic level
• Good barrier properties (moisture, heat, etc.)
• High mechanical resistance
• Good and smooth appearance
• Resistant to chemicals
• UV resistant
• Low flammability & smoke formation

Manufacturing Process

There are two main methods for manufacturing polylactic acid (PLA) from lactic acid: direct polycondensation of lactic acid and ring opening polymerization of lactic acid cyclic dimmer, known as lactide.

Method 1: Direct Polycondensation

• In direct polycondensation, solvents and/or catalysts are used under high vacuum and temperatures for the removal of water produced in the condensation. The resultant polymer is a low to intermediate molecular weight material, which can be used as it is or coupled with isocyanates, epoxides or peroxide to produce a range of molecular weights. 

• Polycondensation method produces oligomers with several tens of thousands average molecular weights. Other side reactions can also occur, such as transesterification, resulting in the formation of ring structure as lactide.
• These reactions have a negative influence on properties of the final polymer. That subproducts production cannot be excluded, but can be controlled by the use of different catalysts and functionalization agenst, as well by varying the polymerization condition.e
• Lactic Acid direct condensation is carry out in three stages: removal of the free water, oligomer polycondensation and melt condensation of high molecular weight PLA. In the first and third stage, the removal of water is the rate-determining step. For the second one, the rate-determining step is the chemical reaction, which depends on the catalyst used. 
• In the sequential melt/solid-state polycondensation, beside the three mentioned steps, there is an additional stage. In the fourth stage, the melt-polycondensated PLA is cooled below its melting temperature, followed by particle formation, which then subjected to a crystallization process. 
• The direct polycondensation of lactic acid in bulk is not applied on  a large scale because of the competitive reaction of lactide formation and the simultaneously occurring degradation process.
• Chain extension is an effective way to achieve high molecular weight lactic acid-based polymers by polycondensation. In this method, the intermediate low molecular weight is to treat polymers with chain extenders which link the low molecular is to treat polymers with chain extenders which link the low molecular weight prepolymer into a polymer of high molecular weight

Method 2: Ring-Opening Polymerization (ROP)

• Ring-opening polymerization (ROP) is the most commonly route to achieve high molecular weight. This process occurs by ring opening of the lactic acid cyclic dimmer (lactide) in the presence of catalyst.
• The process consists of three steps: polycondensation, depolymerization and ring opening polymerization. This process requires additional steps of purification which is relatively complicated and expensive. Catalytic ring-opening polymerization of the lactide result in PLA with controlled molecular weight as an intermediate product. By controlling residence time and temperatures in combination with catalyst type and concentration, it is possible to control the ratio and sequence of D-Lactic and L-Lactic acid units in the final polymer.

• Unlike direct polymerization, ring-opening polymerization of lactide can be carried out in melt, bulk, or in solution and by cationic, anionic, and coordination-insertion mechanism depending on the catalyst. Various types of initiators have been successfully tested, but among them, stannous octoate is usually preferred because it is provide high reaction rate, high conversion rate, and high molecular weights, even under rather mild polymerization conditions. 

• Food Packaging

PLA is a Food and Drug Administration (FDA) approved polymer for use a food contact material. It can be used as a food packaging polymer for short shelf life products such as fruit and vegetables. The common packaging application of polylactides include as containers, drinking cups, sundae and salad cups, overwrap and blister packages.

• Healthcare and Medical Industy

Due to its biocompatibility and biodegradability, the healthcare and medical industry is making good use of polylactide to manufacture tissue engineering scaffolds, delivery system materials, or covering membranes and different bioabsorbable medical implants. Due to its versatility, PLA has been investigated for membrane application (e.g. wound covers), implants and medical devices (fixation rods, plates, pins, screws, sutures), and dermatological treatments (e.g. facial lipoatrophy and scar rejuvenation)

• Structural Application (Composites)

Biobased PLA and PLA-based composites are comparatively new for high-end automotive as well as electrical and electronics applications. These composites show better tensile strength and impact characteristics and hence could be used for interior parts of automobile or safety helmets. Improved material properties enable PLA suitable for use in floor mats, pillar cover, door trim, front panel and ceiling material. PLA biocomposites are proposed for use in cover spare wheel or translucent roof in hybrid concept vehicles.

• Fibers and Fabrics

PLA is also suitable for textile fiber applications such as shirts, carpets, bedding, mattress and sportswear. Due to its low moisture absorption, low smoke generation capability as well as good resistance to UV light. The polymer also has potential in mulch films and compostable garden waste bags, structural protective foams, and insulation materials.