Poly(lactic acid): plasticization and properties of biodegradable multiphase systems
Introduction
Poly(lactic acid) (PLA) has received much attention in the research of alternative biodegradable polymers [1], [2], [3]. PLA is a linear aliphatic thermoplastic polyester, produced from renewable resources and readily biodegradable [4], [5]. PLAs are produced by ring-opening polymerization of lactides and the lactic acid monomers used are obtained from the fermentation of sugar feed stocks [6]. Generally, commercial PLA grades are copolymers of poly(l-lactic acid) (PLLA) and poly(d,l-lactic acid) (PDLLA), which are produced from l-lactides and d,l-lactides, respectively. The ratio of l- to d,l-enantiomers is known to affect the properties of PLA, such as the melting temperature and degree of crystallinity. To date, PLA resins have mostly been used for biomedical applications such as drug delivery systems [7]. Thanks to mechanical properties comparable to those of polystyrene, PLA could reasonably substitute conventional polymers in domains such as packaging. However, the low deformation at break and quite elevated price of PLA limit its applications.
Considerable efforts have been made to improve the properties of PLA so as to compete with low-cost and flexible commodity polymers. These attempts were carried out either by modifying PLA with biocompatible plasticizers, or by blending PLA with other polymers. Varying types of chemicals, such as citrate esters, have been tried to plasticize PLA [8]. Recently, plasticizers such as poly(ethylene glycol) (PEG), glucosemonoesters and partial fatty acid esters [9], [10] were used to improve the flexibility and impact resistance of PLA. The resulting plasticized PLA materials gained in deformation and resilience. None of these studies proposed oligomeric lactic acid (OLA) and glycerol as plasticizers for PLA.
Blends of PLA with various non-biodegradable polymers have been investigated [11], [12], [13]. Biodegradable blends of PLA with other aliphatic polyesters such as poly(ϵ-caprolactone) [14], [15], [16], poly(butylene succinate) [17], [18] and poly(hydroxy butyrate) [19], [20] were also reported in the literature. Some of these blends were found to be immiscible, resulting in fairly poor mechanical properties. Surprisingly, none of these studies have investigated the use of thermoplastic starch (TPS) as the biodegradable blend component for PLA, although it offers unquestionable advantages in terms of cost and sustainability. TPS has been widely used in association with other polyesters because of its low cost, satisfactory properties, renewability and biodegradability. Blends of TPS with PCL [21], polyesteramide [22] and PHBV [23] have been reported. Some starch-based blends have been commercialized like Mater-Bi [24] (Novamont — Italy) or Bioplast [25] (Biotec — Germany).
In this study, we have investigated the influence of plasticizers such as PEG of various kinds, citrate ester (CITRO), OLA and glycerol on the physical properties of PLA. Moreover, we have blended thermoplastic wheat starch together with PLA. The properties of the subsequent materials were characterized through tensile testing, thermal and thermo-mechanical analysis and microscopy. The efficiency of the plasticizers as well as the compatibility of the TPS/PLA blends are discussed.
Section snippets
Materials
The PLA was obtained from Cargill-Dow, and consists of 92% l-lactide and 8% meso-lactide contents. The average molecular weight of 49,000 was determined by intrinsic viscosity measurements in chloroform at 25°C. The PLA pellets were transparent and amorphous, with a glass transition temperature of 58°C. The plasticizers selected for the study were purchased from Sigma. The choice of these plasticizers was based on requisites such as non-toxicity and biocompatibility. Their characteristics are
Thermal properties
The results obtained from differential scanning calorimetry of PLA and plasticized PLA are shown in Table 3. The pure PLA shows a clear glass transition at 58°C, and a very small melting endotherm at 152°C, corresponding to residual crystallinity. The crystallization behavior of PLA was checked through isothermal crystallization from the melt at 110°C, for various time intervals. As shown in Fig. 1, the crystallinity increases with increasing isothermal time intervals, and the heat capacity (ΔCp
Conclusions
PLA is a promising biodegradable polymer. However, because of its inherent brittle behavior, we have blended it with various low-molecular weight plasticizers. The efficiency of respective plasticizers was investigated through DSC and mechanical determinations. Glycerol was found to be the least efficient plasticizer. Thermal analyses demonstrated that OLA and the lower molecular weight PEG (PEG400) gave the best results. The glass transition temperature decreased from 58 to 12°C and 18°C for 20
Acknowledgements
This work was funded by Europol'Agro through a research program devoted to the development of packaging materials based on agricultural resources. The authors want to thank Patrice Dole (INRA-Reims, France) for his help with the characterization experiments.
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