PLA / polylactic acid, polylactide
Polylactic polymers were first synthesised almost 80 years ago (1932, Wallace Hume Carothers, DuPont). Since they were expensive to produce, use of PLA plastics was long restricted to the world of medicine. Commercial plants for the production of this thermoplast only started to appear in 1994.
It is produced by fermenting sugar and starch with the aid of lactic acid bacteria or certain moulds. The intermediate product, lactic acid, is polymerised at around 180 °C through the addition of a catalyst (stannic oxide). The fermentation bacteria used determine the subsequent molecular structure (L- or D-lactide) and therefore the durability and degradability of the plastic. The range of the PLA's properties depends on the molecular mass and degree of crystallinity and can also be extended through copolymerisation with other monomers. PLA can be mixed with other bioplastics and fibres added. (Lactic acid can also be manufactured in a petrochemical process from ethine (acetylene) and ethene (ethylene). PLA can be directly synthesised from organic solvents by polycondensation.) Ultimately, pure PLA is an organic raw material which is not usable in this state. The usable blend is only produced through the addition of compounds / additives and tailored to the relevant application.
Polylactic acid is an aliphatic (basic hydrocarbon chain) polyester with good mechanical and physical properties. PLA plastics are characterised by high strength, transparency, clarity and sheen. They have a density of 1.25 g/cm³. Oriented PLA film can be folded like paper, has no odour and is very resistant to grease and oil. PLA is suited to packaging for dry products with a short shelf life. But its poor permeability to oxygen, CO2 and water makes it unsuitable for carbonated drinks or other liquids. It does not steam up when used to package fresh products and can be printed on in the same way as PET.
PLA and its copolymers are biodegradable, although the conditions required for this are usually only found in industrial composting plants. They do not decompose or only decompose very slowly in the open or in domestic composts.
