PLA: a Bio-based and Compostable Packaging Plastic
Introduction
With the growing interest in sustainability of packaging systems has come increased interest in packaging materials that come from renewable feedstocks (bio-based) as well as packaging materials that can be broken down and assimilated by living organisms (biodegradable). There is particular interest in materials that are compatible with large-scale (industrial) composting operations. There are only a few types of commercially available materials that meet this combination of requirements. When this is coupled with requirements for transparency or for reasonably effective barrier performance and limited to materials that are economically viable for their intended use, the list gets much shorter. One of the materials that has been successful is polylactide (PLA).
While PLA has a very long history, with a ring-opening polymerization method patented by DuPont in 1954 (Kharas, Sanchez-Rivera, & Severson, 1994), its high cost limited its applications primarily to medical uses until first Cargill and later a 1997 joint venture between Cargill and Dow Chemical Company began to further develop these materials and succeeded in greatly reducing their cost. In 2005, Dow exited from the NatureWorks joint venture, and subsequently Cargill partnered with PTT Global Chemical of Thailand to continue PLA development and production. Cargill’s patented polymerization process uses pre-polymerization of lactic acid (currently derived from corn) to a low molecular weight polymer followed by catalytic conversion to lactide monomers, and then ring-opening polymerization to high molecular weight PLA. The properties of the polymer are determined by both its molecular weight and, importantly, by the proportion of D- and L- lactic acid enantiomers in the polymer (determined by the proportion of L-lactide and meso-lactide in the ring-opening polymerization process), as well as by processing conditions (Auras, Harte, & Selke, 2004). The basic structure of PLA is shown in Figure 1. Biological processes tend to produce predominantly the L isomer of lactic acid.
At the elevated temperatures associated with commercial composting operations, PLA first hydrolyzes and then biodegrades.
Figure 1. Structure of PLA
(asterisk indicates chiral carbon)
It does not efficiently biodegrade in “backyard” composting operations, as these do not get hot enough for rapid hydrolysis. Lactic acid, the main breakdown product from hydrolysis of PLA, is non-toxic (and, in fact, is produced in the human body when muscles are working hard and oxygen is temporarily in limited supply). As mentioned above, PLA has long been used in implantable medical devices intended to be resorbed by the body over time.
Packaging applications of PLA are growing, but are limited by its performance characteristics. There is a great deal of interest in modifying PLA in various ways to improve its usefulness as a packaging material. The main issues are brittleness, stability at elevated temperature, and poor water vapor barrier.
Mechanical Properties of PLA
As already mentioned, the properties of PLA are affected both by its molecular weight and by the proportion of L and D isomers. Higher L-lactide content tends to result in higher crystallinity, with PLA with less than 93% L-lactide being totally amorphous (Auras, Harte, & Selke, 2004). Table 1 provides a summary of some PLA properties.
Table 1. Typical PLA Properties (Auras, Harte, & Selke, 2004)
* Source for Tg (Lim, Auras, & Rubino, 2008)
Barrier Properties of PLA
The barrier properties of PLA are affected by crystallinity (which in turn can be affected by L/D ratio) as well as by temperature and factors such as orientation and moisture content. Literature information on the mass transfer properties of PLA remains incomplete and sometimes is inconsistent. Some of this is likely due to the varying crystalline forms that can be produced in PLA depending on processing conditions. Additionally, moisture content of PLA can affect the permeability of the material to gases, including oxygen. Permeation of gases and vapors is due to the combined effects of the solubility of the permeant in the material and the ability of the permeant to move (diffuse) within the polymer. These can be affected in different ways by moisture content as well as by temperature. Typically, for gases, the diffusion coefficient increases with increasing temperature, but solubility decreases. The net effect is most often that permeability increases with increasing temperature, but there can be anomalies. When changes in moisture content are added to the mix, the presence of water can increase mobility of other permeants by swelling the matrix, or it can compete with other permeants in occupying free volume, decreasing solubility (Almenar & Auras, 2010).
For carbon dioxide, reported permeation values differ dramatically, with reported values at 30oC of 1.52 x 10-17 kg m/(m2 s Pa) in one study, and 1.52 x 10-13 kg m/(m2 s Pa) in another (Almenar & Auras, 2010). Almenar and Auras speculate that differences in crystallinity and processing conditions may account for these differing results. However, additional research on CO2 is needed.
Oxygen permeability coefficients reported by different investigators also differ, with values at 30oC reported as 4.90 x 10-14 kg m/(m2 s Pa) in one study and as 4.948 x 10-16 kg m/(m2 s Pa) in another. Oxygen permeability of PLA has been found to decrease linearly with increasing water activity, especially at higher temperatures, with the oxygen diffusion coefficient increasing exponentially with water activity. The diffusion coefficient was higher for PLA with lower L-lactide content (Almenar & Auras, 2010).
Water vapor permeability of PLA was found to be little affected by RH in the range of 40-90% or by L-lactide content (94% or 98%), with measured values of 1.48-2.20 x 10-14 kg m/(m2 s Pa) (Almenar & Auras, 2010).
Limited information is available on barrier of PLA to organics. Permeability coefficients at 23oC of 53.4 x 10-14 kg m/(m2 s Pa) for ethyl acetate, 29.0 x 10-14 kg m/(m2 s Pa) for acetaldehyde, and 30.1 x 10-14 kg m/(m2 s Pa) for 2E-hexenal have been reported (Almenar & Auras, 2010).
PLA Modification
Increasingly, interest in PLA has focused on development of the racemic mixture of PLA (i.e., racemic mixture (50:50) of l- and d-lactides) and modifications of the base material through use of blends and composites, with the goal of improving its barrier properties, reducing brittleness, and reducing distortion at elevated temperatures. Use of PLA nanocomposites has been of particular interest. These modifications show promise of permitting PLA use in applications where the unmodified material has not been successful. It seems likely that use of PLA in packaging applications will continue to grow.
References
Almenar, E., & Auras, R. (2010). Permeation, Sorption, and Dikffusion in Poly(lactic acid). In R. Auras, L. Lim, S. Selke, & H. Tsuji, Poly(Lactic Acid): Synthesis, Structures, Properties, Processing, and Applications (pp. 155-179). Hoboken, NJ: Wiley.
Auras, R., Harte, B., & Selke, S. (2004). An Overview of Polylactides as Packaging Materials. Macromolecular Bioscience, 835-864.
Kharas, G., Sanchez-Rivera, F., & Severson, D. (1994). Polymers of Lactic Acids. In e. D. Mobley, Plastics from Microbes: Microbial Synthesis of Polymers and Polymer Precursors (pp. 93-137). Munich: Hanser.
Lim, L.-T., Auras, R., & Rubino, M. (2008). Processing technologies for poly(lactic acid). Progress in Polymer Science, 820-852.
Rafael Auras is an Associate Professor with the School of Packaging at Michigan State University. You may contact him via his contributor page.
Susan E. Selke is a Professor at Michigan State University and Director of the School of Packaging. You may contact her at sselke@msu.edu or visit her contributor page.
