A tougher bioplastic for food packaging

ADDING rubber to bioplastic to make it less brittle has led to the development of a new type of bioplastic that may provide a suitable alternative to petroleum-based plastic for food packaging.

Producing an alternative to petroleum-based plastic for food packaging is challenging as it needs to meet the standards of the food industry. So far, products have had limited success due to processing and economic constraints, and some have been too brittle to be used for food packaging.

Polyhydroxyalkanoates (PHAs) are biodegradable polymers produced from renewable resources. Polyhydroxybutyrate (PHB) is one of the most studied PHAs as it has similar properties to the petroleum-based plastic polypropylene (PP), however it is stiffer and more brittle than PP. Incorporating poly(3-hydroxyvalerate) produces poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) which appears promising but still remains brittle and expensive to produce. Researchers at The Ohio State University have now developed a process to combine natural rubber with the PHBV to improve the flexibility and toughness. Natural rubber is renewable and biodegradable, and is an ideal toughening agent due to its ductility, elasticity, availability, and low cost.

"Previous attempts at this combination were unsuccessful because the softness of the rubber meant the product lost a lot of strength in the process," said lead author Xiaoying Zhao, a postdoctoral researcher in Ohio State's Department of Food Science and Technology. The previous combinations were too weak to withstand shipping and handling, and couldn’t withstand microwaving and freezing.

The team melted rubber into the PHBV, together with organic peroxide and trimethylolpropane triacrylate (TMPTA) as a coagent. The end product was 100% more flexible than PHBV alone, and was 75% tougher, which makes it more suitable to shape into food packaging. The addition of the rubber causes a loss of strength in the PHDV, but their product only lost 30% strength compared to 80% in previous studies.

"Imagine trying to pull a block of concrete apart with your hands. That's testing its strength. But karate chopping it with your hand or foot is testing its toughness - how easily it breaks," said study co-author Katrina Cornish. "You can never pull it apart, but if you're strong enough you can break it."

The team is currently looking at biodegradable materials, such as used coffee grounds, tomato skins, and eggshells, as potential fillers to strengthen the mix. They are also looking at invasive grasses that need to be removed from waterways. "We want something that would otherwise go to waste that is sustainable and also relatively cheap," said the study's senior author, Yael Vodovotz.

Future work will also include trying to make the product ready to be adopted by the food industry. "As we get closer and closer to working with food manufacturers, there are specific questions our potential partners are asking," Vodovotz said. "We have to be very careful about what we use in this process in order to meet their needs, and they have very specific parameters." They are also looking to collaborate with researchers in other fields to expand the range of applications to industries such as construction and automotive.

Polymers http://doi.org/c5tf