Basics PLA (polylactide or polylactic acid) belongs to the group of biopolymers chemically prepared from biobased, renewable raw materials. In this class of materials PLA is today’s most important thermoplastic biopolymer on the market. PLA is an aliphatic polyester based on lactic acid, a natural acid, that is mainly produced by fermentation of sugar or starch with the help of micro-organisms. Lactic acid exists in two optically active enantiomeric forms, i.e., as L-(+)- or (S) lactic acid and as D-(―)- or (R)-lactic acid. STARCH, SUGAR, BIO- GENIC WASTE MATERIALS CONDITIONING OF SUBSTRATES Fermentation IsolATION MiCroorganismS InoCulATION LACTIC ACID ProduCt PROCESSING PLA MATERIAL Blending/ AdditivES PLA PolymeriZation SynthesIS LactidE (Source: [1]) Basics of PLA O O O O (R,R)- lactide or D-lactide (Source: Purac) By Michael Thielen This article is based on a chapter in the new book “Engineering Biopolymers” [1] as well as personal information of Sicco de Vos (Purac) and Andreas Grundmann (Uhde-Inventa-Fischer) O O O O (S,S)- lactide or L-lactide O O O O (R,S)- lactide or meso-lactide Polymerisation Most of the lactic acid today is being produced by fermentation. Here biological material is being converted with the aid of bacteria, fungal or cell structures, or by adding enzymes. However, to manufacture lactic acid and — in the next step — polylactide a certain amount of process engineering is necessary (see graph). The biological feedstock, this engineering as well as the purity of the lactic acid play an important role on the quality, the properties and not least the cost of the final PLA. In the last 10-15 years, mainly by optimising the process technology and the ‘economy of scale’ with larger manufacturing capacities, the price of PLA could be reduced significantly. Further significant reductions in the manufacturing cost seem possible in the future, especially when raw material costs are reduced, i. e., by the use of biogenic residues or wastes, such as whey, molasses, or wastes containing lignocellulose. In order to convert lactic acid into PLA, the lactic acid is in a first step prepolymerised to form small prepolymers by socalled oligopolycondensation and then depolymerised into cyclic lactides. This means two lactic acid molecules form a cyclic dimer, lactide, which, depending on the constituting isomers, can be a D-D-lactide, an L-L-lactide or a mesolactide (having one D and one L isomer). These lactides are then connected in a ringopening polymerization process, producing long, linear macromolecules: the PLA resin. This process can be performed using stirred tank cascades or horizontal reactors as they are known from polyester chemistry. The majority of the industrially relevant production processes for PLA have 54 bioplastics MAGAZINE [01/12] Vol. 7
Basics in common that they are continuous melt processes, operated at high temperatures without the use of solvents. The capacity of such plants varies from 5,000 to 140,000 tonnes per annum. Apart from some exceptions, like clear film and fiber, virgin PLA resin as it exits the polymerization reactor, cannot be directly processed into final plastic products. Hence, as is usual with most plastics, virgin PLA resin is modified for specific applications by compounding with functional additives and/or by blending with other polymers (bioplastics or traditional, oil-based polymers). Such modifications have already resulted in PLA compounds with sufficient performance to replace PET, HIPS, PP and even ABS. In order to prevent the PLA pellets from sticking together during storage and transportation, virgin resin pellets are commonly crystallized. The resulting semi-crystalline, heat resistant granulate can be shipped around the globe without problems. In its crystalline state the chemical stability of PLA – and PLLA homopolymer in particular - is higher and its water absorption, swelling behavior, and rate of biological degradation are lower than those of amorphous PLA. PLA production For the production of PLA approximately 0.1 to 0.25 ha (in Europe rather 0.2 to 0.5 ha) of agricultural area is needed for 1 tonne. For comparison, cotton requires almost 3x more land for the production of the same quantity. Hence, PLA exhibits very high land use efficiency and other comparisons can be found in [1, 2]. The world’s first large PLA production unit with a capacity of 140,000 tonnes per annum began production in the USA in 2002. Industrial PLA production facilities can now also be found in the Netherlands, Japan and China. For example one Dutch company is going to expand their 5,000 t/a capacity to 35 – 70,000 t/a. A recent announcement from China was about an expansion of their PLA capacity to 50,000t/a in 2013 from 5,000 t/a currently. In Germany a 500 t/a industrial pilot plant started operation in 2011 and in Switzerland a 1000 t/a industrial pilot plant will become operational in the first quarter of 2012. Gattinoni Obama Dress 100% NatureWorks Ingeo PLA (Picture: Gattinoni) Properties Advantages of PLA are its high level of rigidity, transparency of the film, cups and pots, as well as its thermoplasticity and good processing performance on existing equipment in the plastics converting industry. Nevertheless PLA has some disadvantages at the moment: as its softening point is around 60°C, the unmodified material is not suitable for the manufacture of cups for hot drinks. Modified PLA types can be produced by the use of additives like nucleating agents or impact modifiers, or by a blending PLLA and PDLA, the homopolymers of of L- and D- lactides (stereocomplexing), which then have the required morphology for use at higher temperatures (see bM 02/2008). A second characteristic of PLA together with other bioplastics is its low water vapour barrier. Whilst this characteristic would make it unsuitable, for example, for the production of bottles, its ability to “breathe” is an advantage in the packaging of bread or vegetables. Applications Transparent PLA is very similar to conventional mass produced plastics, like PS, PP, PET and PMMA, not only in its properties but it can also be bioplastics MAGAZINE [01/12] Vol. 7 55
