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02 | 2008

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Materials polymer

Materials polymer technologies, such as co-polymerization, blending, modification with additives, and combining materials or films with different properties. Further recognition of PLA in specific high-end applications is currently limited by a number of material properties that need improvement to meet the material requirements in these markets: 1. weak structural integrity at elevated temperatures, expressed as the heat deflection temperature (low HDT), 2. brittleness, i.e., low impact strength, Figure 1: PLA cup collapsed with hot coffee 3. gas barrier performance, in particular for bottle applications. The biggest issue is the low heat resistance of PLA. The material becomes soft and weak upon heating beyond temperatures of 50-60°C, which causes practical problems during storage, transportation and use of pellets and finished articles. When hot coffee is poured into a PLA cup, if collapses (Fig. 1). Clearly, amorphous – glassy – PLA loses its structural integrity completely when subjected to temperatures above its glass transition temperature. Due to the chiral (see box) nature of lactic acid, several distinct forms of polylactide exist: poly(L-lactide) (PLLA) is the product based on L(+) lactic acid or L-lactide, the major product of Purac. Likewise, polymerization of D-lactide produces PDLA. Today commercially available PLA grades are random copolymers of D- and L-lactic acid isomers with relatively slow nucleation and crystallization rates. As a result, most PLA materials will be amorphous – i.e., glassy and not crystalline – after melt processing. These materials become sticky and soft at temperatures above 60°C. Purac allows polymer producers to add value in a new way by offering L-lactide and D-lactide as solid flakes, available in bulk quantities from 2009. By combining these lactides smartly, new PLA grades with tailored physical properties – like improved heat-stability – can be made by polymer industry. • If lactides are combined in the same polymer chain by a one-pot polymerization of L- and D-lactides, polylactides with melting temperatures ranging from about 130-180°C can be made. At very low D-isomer content, semi-crystalline PLLA is obtained, while amorphous, optically clear PLA is made with D-contents higher than 10-15%. • Polymerization of only the D-lactide monomer produces PDLA. This PLA type is the mirror reflection of PLLA and can be mixed with PLA (co)polymer to improve the material’s heat resistance according to the stereocomplexation concept. 22 bioplastics MAGAZINE [02/08] Vol. 3

Materials A solution for the low heat-resistance while maintaining transparency would accelerate the acceptance of PLA and widen the application window. The use of PDLA results in the formation of PLA stereocomplex crystallites (sc-PLA) that act as a so-called nucleating agent and crystallization enhancer for PLA. Six years of innovative research and development at Purac have resulted in the commercial availability of D(-)-lactic acid and D-lactide, the monomer that enables large-scale utilization of PDLA. Concept of Stereocomplexation PLA stereocomplex crystals (sc-PLA) are formed by mixing polymers resulting from separately polymerized L-lactide and D-lactide. Melt-blending PLLA and PDLA produces crystals, by association of the polymers in a 1:1 ratio, with a melting temperature of about 230°C, i.e., at least 50°C higher than common PLA. This semi-crystalline sc-PLA is a suitable polyester for melt-spun fibers and biaxially stretched film. Melt-blending of PLA (copolymer) with a few percent of PDLA as an additive, produces sc-PLA crystallites in the PLA melt by racemic crystallization of the PDLA with an equivalent amount of PLA. Upon cooling the melt, e.g. during injection molding, the presence of the sc-PLA crystals promotes crystallization. Thus, the sc-PLA crystals act as heterogeneous nucleation sites for PLA crystallization and are nucleating agents. The nucleation efficiency of PDLA is superior to that of talc, a common filler and nucleating agent. PLA can crystallize 20-30 times faster by blending with only 1-5% (w/w) of PDLA. The resulting material will exhibit a higher degree of crystallinity, which will translate macroscopically into structural integrity up to higher temperatures and will be opaque. Unique features of Stereocomplex PLA Apart from the mixing ratio, key parameters that control stereocomplexation in PLA/PDLA blends are the molecular weight and stereochemical purity (L/D ratio) of the constituents. Homopolymers of either L- or D-lactide are stereochemically pure, while all intermediate compositions have reduced stereochemical purity. PDLA of relatively low molecular weight (

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