Basics Basics of PLA
Basics Basics of PLA Figure 1: Methods of PLA Recycling Total Fossil Energy [GJ/ t plastic] Industrial composting • • Article contributed* by Dr. Rainer Hagen, Vice President and Product Manager, Uhde Inventa-Fischer GmbH, Berlin, Germany Most attractive method of disposal based on public acceptance No recovery of material and energy Mechanical recycling • • Loss of product properties cannot be recovered ‘Downcycling’ Burning (energy recycling) • Recovers ‘green energy’ Chemical recycling • • Back into polymerisation Collecting and sorting to be solved yet 140 120 100 80 60 40 20 0 fossil fuel PA 6 fossil raw material HDPE Source: M. Patel, R. Narayan, in Natural Fibers, Biopolymers and Biocomposites, A. Mohanty, M. Misra, L. Drzal, Taylor & Francis Group, 2005, Boca Raton. PET Figure 2: Consumption of Fossil Resources by PLA vs. Polymers from Fossil Feedstock - ‘cradle to gate’ PLA Introduction Polylactide or Polylactic Acid (PLA) is a synthetic, aliphatic polyester from lactic acid. For industrial applications, such as fibres, films and bottles, the chain length n should be between 700 and 1400. This is significantly higher than with partially aromatic polyesters like PET and PBT where n is between 100 and 200. Therefore, the requirements on both raw material purity and technical effort are much higher. At temperatures below its glass transition point (e.g. 55°C, depending on comonomer content) PLA is as stable as PET or PBT. Only in an industrial composting facility, the high temperature (60°C) and humidity required for the hydrolysis are achieved. After hydrolysis, PLA is biologically degradable by common micro-organisms. Lactic acid, the monomer building block of PLA can frequently be found in plants and animals as a by-product or intermediate product of metabolism. Lactic acid is non-toxic. Non-depleting properties of PLA Lactic acid can be industrially produced from a number of starch or sugar containing agricultural products. Competition between human food, industrial lactic acid and PLA production is not to be expected: For example, using PLA as substitute for 5% of the German packaging plastics consumption requires only 0.5% (sugar beet) to 1.25% (wheat) of the agricultural area available. At the same time, approximately 30% of the available area lies fallow mainly for economic reasons. Research is in progress on processes and micro-organisms that produce lactic acid from cellulose coming from agricultural residues such as maize stalks or straw. Several recycling methods can be applied to waste PLA (Fig. 1). Composting allows only moderate benefits. In future, sorting, purification of PLA waste and re-feeding into the polymerisation plant seems to be the most attractive way of recovery. PLA – like other biopolymers – is often criticised for the need of process energy from fossil resources. Even if this is the case at present, 1 kg of PLA represents less energy equivalents than 1 kg of polymers from petrochemical 38 bioplastics MAGAZINE [01/09] Vol. 4
Basics feedstock (Fig. 2). Consequently, PLA producers can also reap financial benefits by trading CO 2 emission certificates (Fig. 3). 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 CO 2 emission certificates. Additionally, significant potential exists for saving process energy by improving lactic acid and polymerisation technologies. 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. PLA is a polymer which can be processed by: • injection moulding • sheet extrusion • extrusion blow moulding • thermoforming • 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 L- lactide in the purification step. When polymerizing L- [kg CO 2 eq/kg] 8 7 6 5 4 3 2 1 0 PA 6 HDPE 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: CO 2 Emissions by PLA vs. polymers from fossil feedstock - ‘cradle to gate’ Water to Hydrolysis Purge Concentrated Lactic Acid Oligomers Pre-polymer Crude Lactide Highly Purified Lactide Polylactide with Monomer Lactic Acid Figure 5: Ring opening Polymerisation PET Evaporation/Distillation Pre-condensation Formation of Cyclic Dimer Lactide Purification Ring Opening Polymerisation Demonomerisation/Stabilisation Polylactide see Fig. 5 Figure 4: Steps of a PLA Process with Ring Opening Polymerisation PLA Water, Lactic Acid Dilactide bioplastics MAGAZINE [01/09] Vol. 4 39
