At the latest since the discovery of the Great Atlantic Garbage Patch, the topic of plastic waste has been back in the public debate. Since then, it has become clear that it is not enough to replace petroleum with bio-based resources in the production of plastics: That may protect the climate, but not nature. For the latter, the bio-based plastics must also be biodegradable under environmental conditions. The only plastic produced on an industrial scale to date that is heading in this direction is polyactide (PLA). But even it only decomposes in industrial composting plants in a reasonable time.The search for new, more suitable plastics therefore continues, and the PHB2Market research project has tested and optimized a promising candidate for its marketability.
Two PHB composites developed
At first glance, polyhydroxybutyrate (PHB) is a potential candidate, but not a convincing one: the polymer is biobased and readily biodegradable. It also has very good heat resistance of more than 100 degrees Celsius, which is important for beverage cups, for example, or for surfaces that are exposed to the sun for long periods of time. However, PHB has low impact strength in its unmodified state - i.e., it is relatively brittle - and changes its mechanical properties over time. With around 360,000 euros in funding from the German Federal Ministry of Education and Research (BMBF), a team led by the Fraunhofer Institute for Chemical Technology (ICT) has therefore set out to develop a PHB composite that addresses PHB's weaknesses without losing its strengths. The project ran from January 2017 to April 2020 and resulted in two practical composite formulations.
"We focused on two main objectives in the project," explains project manager Kevin Moser from ICT: "The incorporation of in-house developed plasticizer systems to increase impact strength and the optimization of long-term stability." PHB already outperforms polypropylene (PP) in other industrially important properties such as strength and Young's modulus. So if the research team achieved its two goals, it would have a sustainable, generally usable plastic that can be further individually modified depending on the target application - so the vision.
3D printing filaments and a frisbee
The functionality of the PHB composite was to be tested on two practical applications: the production of a Frisbee disc and filaments for the 3D printing of cubes or gear wheels. "The Frisbee is an injection-molded product, must be sufficiently flexible, not too brittle and look visually appealing," says Moser, explaining the choice.
Of course, compounders know the different additives that can be used to influence the properties of a plastic. But this is still a long way from finding the right formulation: "If you add a plasticizer to increase impact strength, this also influences all the other properties. You then have to balance that out and add other additives if necessary." Such a development process involves many years of experience, but also a great deal of testing.