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November 16, 2020 09:30 AM

Polymer researcher develops tough, flexible PLA that withstands boiling water

Karen Laird
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    The prototype PLA cup developed by Kumho Polymer Science Professor Shi-Qing Wang, Ph.D., is transparent and super tough, and does not shrink when filled with boiling water.

    For the past several years, University of Akron (UA) professor Shi-Qing Wang, Ph.D. has worked on the development of a PLA with improved mechanical properties.

    In his lab in UA’s School of Polymer Science and Polymer Engineering, he and his team are focusing on research into effective strategies for turning brittle polymers, such as PLA, into tough and flexible materials. Currently, the range of applications for these polymers have remained somewhat limited, because they break relatively easily under moderate load.

    While petroleum-based polymer materials offer well-known advantages such as strength and light weight, these materials are increasingly causing problems at the end of life.

    “Most cannot be recycled and therefore accumulate in landfills,” according to Wang, who has taught at UA for 20 years and currently serves as the Kumho Professor of Polymer Science r.  “Some promising biodegradable/compostable alternatives, such as PLA, are typically not strong enough to replace traditional fossil-fuel based polymers like poly(ethylene terephthalate) (PET) because these sustainable materials are brittle.”

    The group has now reported its first success: it has produced a prototype poly(lactic acid) (PLA) cup that is transparent, extremely tough and which does not shrink when filled with boiling water.

    Ramani Narayan, Ph.D., distinguished professor in Michigan State University’s Department of Chemical Engineering and Materials Science, and renowned scientist in the bioplastics space, said Wang’s research has the potential to be a breakthrough in the PLA market. Narayan was not involved in the research, but learned about the work being done by Wang when one of Wang’s former students, who is currently on the faculty at MSU, invited Wang to present a seminar there.

    “PLA is the world’s foremost 100% biobased and fully compostable polymer,” said Narayan. “But it has low toughness and a low heat distortion temperature. It softens and structurally collapses around 140 degrees Fahrenheit, making it unusable in many hot food packing applications and disposable containers. Dr. Wang’s research could be disruptive technology because his prototype PLA cup is tough, transparent, and yet rigid enough to hold boiling water.”

    The challenge facing Wang and his team was to gain insight into why one polymer is tougher than the other, and how to make these tougher, more resistant to breaking. They first looked at the origin of ductility in semicrystalline polymers. As Wang explains it, molten PLA, strands magnified a million times, are similar to intertwined cooked spaghetti, many meters in length. Wang calls this interwoven structure the “chain network”, and it is what gives thermoplastics (including PLA) their toughness. However, crystallisation can remove or disrupt the intertwining of these chains.

    Properly manipulated, the chain network ensures that the PLA beverage cup is mechanically strong without crystallization. But such a commercial cup collapses when boiling water is poured into it.  "Cups made from normally crystallized PLA can hold boiling water but are terribly brittle and opaque," said Wang.

    What Wang and his research group have done is to develop a way to limit crystals to nanoscopic scales in PLA while preserving the network, resulting in the clear, tough and heat resistant cup. Its optical transparency is a key characteristic. The cup can hold hot tea and coffee and has the potential to replace most plastic beverage cups on the market.

    In addition, compared to conventional crystalline PLA, this new form of PLA should be easier to undergo hydrolysis, the first step in its biodegradation, he says.

    “The impact of our new understanding could finally stimulate the PLA market to grow exponentially,” said Wang. “And if PLA finds wider use, the price will come down more. It’s a matter of making PLA more competitive in terms of its mechanical and other characteristics.”

    A U.S. patent on how to modify PLA based materials has been filed through UA’s Office of Technology Transfer.

    However, the team has not yet explored the possibility of finding industrial partners to commercialise the development.

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