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02 | 2008

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Materials nometers per

Materials nometers per second, while the maximum crystallization rate of PLLA is 600nm/s. Commercial PLAs of reduced stereochemical purity exhibit even slower crystallization. The ability to act as nucleating agent for PLA homocrystallization can be seen from fig 2 and 3. The pictures show few crystals (white) and a lot of molten PLA (black) in the absence of PDLA (fig 2). Under the same conditions, PLA blended with 5% PDLA, exhibits much higher nucleation density as illustrated by the abundantly present white spots (crystallites) (fig. 3). The end result is faster crystallization in the PLA/PDLA blend compared to homogenously nucleated PLA. Improving heat resistance of PLA using poly(D-lactide) The heat deflection temperature of PLA can be increased from approximately 60°C to up to HDT values beyond 130°C by incorporating PDLA. Bioplastics with even higher values are possible upon stretching the material into oriented film or fiber, or upon incorporation of mineral fillers or natural fibers. The maximum effect in temperature stability is found in compositions with a 1:1 ratio of PLA and PDLA, as shown by Dutch company Hycail (2005). They demonstrated that potatoes can be fried in boiling oil in a transparent, heat resistant tray prepared from a 50/50 Mirror, mirror on the wall … the stereochemistry of lactic acid and PLA The person in the mirror looks just like you – same height, same eyes, yet there is a crucial difference: the left and right sides are switched around. Your hands are another good example of a mirrored pair. Similarly, certain molecules also exist in mirrored pairs, lactic acid being a famous example. The molecules exhibit chirality and the pairs are called enantiomers. Chiral substances are able to rotate polarized light. This phenomenon is called optical rotation. In a chiral substance with one chiral center, like lactic acid. both enantiomers (the two molecules of which the structure is the mirror image of the other) will give opposite optical rotations. This means that one enantiomer will give a rotation clockwise (positive) and the other a rotation of the same magnitude, but counter clockwise. Thus, the two enantiomers of lactic acid (2 hydroxypropionicacid) are: H 3 C OH O OH H 3 C OH Lactic acid enantiomers: L(+)-lactic acid (or S-lactic acid), and D(-)-lactic acid (R-lactic acid) A 1:1 mixture of both enantiomers is called a racemic mixture. It does not rotate polarized light and is optically not active. Over the last few years, Purac has successfully developed a fermentation process for the so-called ‘mirror image’ of traditional L(+)-lactic acid: D(-)-lactic acid. A unique feature of the Purac fermentation processes for lactic acids O OH is that they yield the lactic acid enantiomers with high stereochemical purity. The concept of racemic crystallization, called stereocomplexation for polymers, is not unique for polymers or PLA. An equimolar mixture of D- and L-lactide, the cyclic monomers of PDLA and PLLA, is a physical blend with a melting point of 125°C called DL-lactide. The melting point of the pure lactides is only 98°C. When the interaction between enantiomeric polymers existing as D and L-configurations prevails over that between polymers of identical configuration, association between polymer chains of opposite configuration can take place. Such association is described as stereocomplexation. Other polymers, like polypeptides, specific types of PMMA, polyesters and polyethers, also have the ability to stereocomplexation. Polymerization of D-lactic acid produces PDLA, a completely bio-based polyester like PLA. PDLA can be used as an additive to improve one of PLA’s main weaknesses – the low heat resistance. PLA already deforms at temperatures below 50°C. This creates major problems for the storage, transportation and usage of both granular and finished articles. A solution for the low heat stability while maintaining transparency would drive further acceptance of PLA and widen its application window. PLA’s heat stability can be increased from less than 60 to more than 130°C by combining it with Purac PDLA. The mirror molecules fit to each other, almost like hands in a handshake, to increase crystallinity and temperature resistance. 50/50 mixtures of PLLA and PDLA have the highest potential for applications where high temperature stability is a must. At lower PDLA loadings, PDLA allows for modifications of the physical properties of PLA, resulting in bioplastics that can be used for apparel with improved ironing qualities, microwaveable trays, hot-fill bottles and even engineering plastics for automotive applications. 24 bioplastics MAGAZINE [02/08] Vol. 3

PLLA/PDLA composition. At lower loadings (

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