Plasticized Poly (lactic acid) Films - Preparation and Properties

Abstract: The development of biodegradable polymers from renewable resources is of importance in order to prevent the depletion of fossil fuels and the increasing emissions of carbon dioxide to the atmosphere caused by the commodity plastics of today. However, to be able to replace these non-environmentally friendly materials, production costs of biodegradable polymers have to be decreased and their physical properties need to be enhanced. This research has been devoted to poly (lactic acid), PLA; a biodegradable, semicrystalline thermoplastic that can be produced from renewable resources. The main focus was to overcome the inherent problem of brittleness in PLA in order to enable it for potential use in the packaging material industry. Blending PLA with suitable plasticizers in order to enhance its flexibility has been the explored route and obtaining sufficiently compatible blends in order to prevent phase separation and migration has been an important issue when modifying the physical properties of the polymer. It was found that the presence of plasticizers in the PLA matrix contributed to a more complete crystallization of the polymer because of the increased chain mobility as a consequence of the depressed glass transition temperature, Tg. The more efficient the plasticizer, the larger the decrement in Tg and, thus, the faster the aging process leading to phase separation and migration of the plasticizer. It is clear that there exists a competition between the efficiency of the plasticizer and the speed of the aging/cold crystallization in the material and it is imperative to find an optimum where the Tg is as low as possible without the plasticized material cold crystallizing too fast. Low molecular weight plasticizers, such as tributyl citrate, TbC, and diethyl bishydroxymethyl malonate, DBM, drastically decreased the Tg of PLA and the extent of the decrement was larger with an increasing amount of plasticizer. However, the blends were not stable over time since rapid cold crystallization caused a size reduction of the amorphous domains in PLA. Consequently, the ability of PLA to accommodate the plasticizer diminished with the increase in crystallinity and migration of the plasticizer occurred. Increasing the molecular weight of the plasticizers by synthesizing oligoesters and olidoesteramides resulted in blends that displayed Tg depressions slightly smaller than with the low molecular weight plasticizers. The compatibility with PLA was dependent on the molecular weight of the oligomer and on the presence or not of polar amide groups that were able to positively interact with the PLA chains. Simulating current storage conditions in the packaging industry by aging the materials at ambient temperature revealed that cold crystallization did not take place in the films since the temperature was kept below the Tg:s of the blends. Thereby the enhanced flexibility in the plasticized films could be maintained.