Green Matter: New technology for greener PLA

An intriguing European project aimed at developing a new polymerization route for PLA will kick off this month at the Fraunhofer Institute in Pfinztal (Germany).

Among the many bioplastics available today, one of the best known is polylactic acid, or PLA. And it's certainly one of the fastest growing: in August of last year, Germany-based nova-Institute published the preliminary results of a multi-client market survey of the international bioplastics market, which projected a production capacity of over 800,000 tons of PLA by the year 2020, up from around 180,000 tons in 2012. As Michael Carus, managing director of nova-Institute, noted: "PLA is definitely a polymer for the future."

Global PLA capacity

What is PLA, exactly?

It's actually a biodegradable, aliphatic polyester that is derived from lactic acid. Lactic acid is easily obtained from inexpensive, renewable agricultural feedstocks, such as beets or corn. The naturally occurring sugars in the feedstock, or those that are produced by breaking down carbohydrates in the feedstock are fermented to lactic acid, from which lactide and the PLA resin are produced.

PLA is a high-strength and high-modulus thermoplastic that is easily processed using conventional processing techniques. Moreover, it is safe for use with food. As a result, PLA has fast become widely used in numerous applications; common uses are as loose-fill packaging, compost bags, food packaging, disposable tableware, non-woven textiles and films, to name but a few.

So why the new project?

Currently, most PLA on the market today is produced using a continuous process applying ring-opening polymerization to convert lactide to PLA. The process involves the use of tin catalysts, thus "completely eliminating the use of costly and environmentally unfriendly solvents" as one of the main PLA producers active in the market today points out. However, because of health and environmental concerns related to the use of tin, the InnoREX project is aimed at developing an alternative polymerization route for PLA that is nonetheless able to yield the same level of performance. This will involve a novel reaction concept involving the continuous, highly precise, metal-free polymerization via reactive extrusion.

Organic catalysts will replace the metal-containing catalysts currently in use. While these have been shown to efficiently control the lactide polymerization process, their activity must still be improved to meet industrial standards. The activity of these catalysts can be increased through the application of low-intensity, meticulously targeted alternative energy (ultrasound, microwave or laser light energy), using commercial co-rotating twin screw extruders as reaction vessels. The rapid response time of microwaves, ultrasound and laser light enables a precisely controlled and efficient, continuous polymerization of high molecular weight PLA, as both the reaction and the resulting molecular structure of the polymer can be dynamically controlled. Product quality is assured due to an in-line cleaning device that will purify the polymer to remove traces of any remaining catalyst and monomer residues.

Moreover, combining polymerization, compounding and shaping in a single production step will achieve significant energy savings.

Eleven partners including small and medium enterprises, research centers and trade associations from around Europe are participating in the InnoREX project. The project will run for 42 months, ending on May 31, 2016.