Polylactic Acid or Polylactide (PLA)

Properties and Applications

Polylactic acid (PLA) is one of the most important biodegradable and biobased thermoplastics.^1,2 The monomer is derived from renewable resources such as corn starch, tapioca or sugar cane via fermentation of starch (dextrose) which yields two optically active enantiomers, namely D (-) and L (+) lactic acid (2-hydroxy propionic acid). Polymerization is carried out by either direct condensation of the lactic acid monomers or by ring-opening polymerization of the cyclic diesters (lactides).

Unmodified PLA resins are rather brittle, have a relative low glass transition temperature of about 50 to 60 °C and thus, have a rather low upper service temperature. They also have a relative narrow processing window of about 185-190°C because of the high melting point (175°C) of PLA.¹ The processability of PLA can be greatly improved when small amounts of lactite enantiomers of opposite configuration are randomly copolymerized with the monomer. For example, the addition of small amounts of D (-) lactide to L (+) lactide yields PDLA which has a noticeably lower melting point. However, PDLA resins are rather stiff and brittle (low impact resistance). These limitations can be overcome by blending PLA with low molecular weight plasticizers such as sorbitol or glycerol. PLAs are also often blended with other resins to lower cost and to increase its biodegradability (starch) or to further enhance its performance and processing properties.

Most commercial high-purity grades are semicrystalline, have high transmittance (> 90 %), and high yield and tensile strength (about twice of HDPE). These resins can be easily converted into films and sheets via standard forming methods. In fact, many commercial grades are specifically designed for thermoforming or extrusion / injection molding.

Typical applications of PLA are disposable tableware articles like drinking cups, cutlery, trays, food plates and food containers. Some other (potential) applications include soil retention sheething and other agriculture films, waste and shopping bags, and the use as packaging material in general. PLA can also be spun into fibers which could be used for the manufacture of woven and non-woven biodegradable one-use fabric articles such as disposable garments, feminine hygiene products, and diapers.

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