Polylactic acid

  Polylactic acid or Polylactide (PLA) is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources. Corn starch (in the U.S.) or sugarcanes (rest of world) are the common feedstock. Bacterial fermentation is used to produce lactic acid, which is oligomerized and then catalytically dimerized to make the monomer for ring-opening polymerization. It can be easily produced in a high molecular weight form through ring-opening polymerization using most commonly a stannous octoate catalyst, but for laboratory demonstrations tin(II) chloride is often employed.

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Due to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known as L-lactide). PLLA has a crystallinity of around 37%, a glass transition temperature between 50-80° C and a melting temperature between 173-178° C. The polymerization of a racemic mixture L- and D-lactides leads to the synthesis of poly-DL-lactide (PDLLA) which is not crystalline but amorphous.

Polylactic acid can be processed like most thermoplastics into fiber (for example using conventional melt spinning processes) and film. The melting temperature can be increased 40-50° C and the Heat Deflection temperature of PLLA can be increased from approximately 60° C to up to 190 ° C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA are known to form a highly regular stereocomplex with increased crystallinity. The maximum effect in temperature stability is achieved when a 50-50 blend is used, but even at lower concentrations of 3-10% of PDLA a substantial effect is achieved. In the latter case PDLA is used as a nucleating agent, thereby increasing the crystallization rate. Due to the higher crystallinity of this stereo-complex, the biodegradability will become slower. The interesting feature is that the polymer blend remains transparent.

The physical blend of PDLA and PLLA can be used to widen the application window and include applications such as woven shirts (ironability), microwavable trays, hot-fill applications and even engineering plastics (in this case the stereocomplex is blended with rubber-like polymer such as ABS). Even for improving form-stability of low-end packaging application while maintaining transparency the stereo-complex is of interest. Progress in bio-technology has resulted in the development of commercial processes for D(-), something that was not possible until recently. As of September 2006, PURAC[1], a wholly owned subsidiary of CSM located in the Netherlands is the primary producer of L(+) and D(-) lactic acid from carbohydrates through a fermentation process.

PLA is currently used in a number of biomedical applications, such as sutures, stents, dialysis media and drug delivery devices, but it is also evaluated as a material for tissue engineering. Being biodegradable it can also be employed in the preparation of bioplastic, useful for producing loose-fill packaging, compost bags, food packaging and disposable tableware. In form of fibers and non-woven textiles PLA also has many potential uses, for example as upholstery, disposable garments, awnings, feminine hygiene products and nappies. As of September 2006, the primary company producing PLA for medical applications is PURAC [2] in the Netherlands.

PLA is particularly attractive as a sustainable alternative to petrochemical-derived products, since the lactate from which it is ultimately produced can be derived from the fermentation of agricultural by-products such as corn starch or other starch-rich substances like maize, sugar or wheat. PLA is more expensive than many petroleum-derived commodity plastics, but its price has been falling as more production comes online. The degree to which the price will fall, and the degree to which PLA will be able to compete with non-sustainable petroleum-derived polymers, is uncertain.

As of December 2005, NatureWorks LLC [3], a wholly owned subsidiary of Cargill Corporation, was the primary producer of PLA in the United States. Other companies involved in PLA manufacturing are Toyota (Japan), Hycail (The Netherlands), Galactic [4] (Belgium) and several Chinese manufacturers.

One of the little known facts about PLA is that it is quite an old material, discovered in the 1890s, but it has only now found a universal route to market in the form of bio-degradable packaging. The world however will have to wait a little longer before all plastics are replaced by PLA, as there is a significant lack in "cracking plants", with the next one not coming on stream until 2008.

Packaging made from PLA is bio-degradable and reverts in less than 60 days in ideal conditions, namely in commercial composting installations. Much work is being undertaken by Vinalith (a printing company in England) to develop techniques that allow PLA to be used as a printed primary packaging material.

See also

• RepRap Project

Other biodegradable polymers:

• plastarch material
• polycaprolactone
• Polyetheretherketone
• polyglycolide
• poly-3-hydroxybutyrate
• zein