Basics
feedstock (Fig. 2). Consequently, PLA producers can also reap financial benefits by trading CO2 emission certificates (Fig. 3).
Process Routes to PLA Several Process Routes have been developed or are practised on industrial scale: Ring Opening Polymerisation (ROP), Direct Polycondensation in high boiling solvents (DP S), and Direct Polymerisation in bulk followed by chain extension with reactive additives. ROP is the route which delivers by far the highest proportion of PLA chips available on the market. The other routes produce only minor amounts or did not get past the pilot scale. Figure 4 depicts the steps of a ROP process, starting from lactic acid. In the first part lactide is formed, which – after fine purification – is converted by ROP to PLA.
Processing of PLA A major advantage of PLA is the possibility to process the polymer on common process equipment. Especially the converters of polyolefins do not require a change to other process equipment. They only need to change the handling of granulate. It is very important to dry the polymer before processing otherwise it will degrade. Water and high temperatures (up to 240°C) facilitate fast degradation.
7 6
[kg CO2 eq/kg]
If process energy is supplied by biomass, e.g. biogas, the fossil energy required for 1 kg PLA can be cut by half, thus duplicating the benefits from trading CO2 emission certificates. Additionally, significant potential exists for saving process energy by improving lactic acid and polymerisation technologies.
8
5 4 3 2 1 0
PA 6
HDPE
PET
PLA
Source: M. Patel, R.N arayan, in Natural Fibers, Biopolymers and Biocomposites, A. Mohanty , M. Misra , L. Drzal, Taylor & Francis Group, 2005, Boca Raton.
Figure 3: CO2 Emissions by PLA vs. polymers from fossil feedstock - ‘cradle to gate’
Lactic Acid Water to Hydrolysis
Evaporation/Distillation Water, Lactic Acid
Concentrated Lactic Acid Pre-condensation Purge
Oligomers
Pre-polymer Formation of Cyclic Dimer Crude Lactide
see Fig. 5
Lactide Purification Highly Purified Lactide Ring Opening Polymerisation
Dilactide
Polylactide with Monomer Demonomerisation/Stabilisation
PLA is a polymer which can be processed by: injection moulding sheet extrusion extrusion blow moulding thermoforming
Polylactide
Figure 4: Steps of a PLA Process with Ring Opening Polymerisation
stretch blow moulding injection stretch blow moulding fibre spinning non woven spinning, spun bonding
Properties of PLA PLA is a crystal clear, transparent material when amorphous that becomes the hazier the higher the crystallinity. Crystallized material is opaque. When producing lactide, meso-lactide is formed as a by-product. It is difficult to separate the meso-lactide from the Llactide in the purification step. When polymerizing L-
Figure 5: Ring opening Polymerisation
bioplastics MAGAZINE [01/09] Vol. 4
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