Understanding the structural properties of PLA processing
PLA is the world’s premier 100% bio-based and fully compostable polymer. But it has low toughness and a low heat deflection temperature. It softens and structurally collapses at about 140 degrees Fahrenheit, making it unusable in many hot food packaging applications and disposable containers. This research could be disruptive technology because the prototype PLA cups are strong and transparent, but hard and cannot hold boiling water.
UA Professor Shiqing Wang has been working to build a knowledge base to understand the processing-structure-property relationships of various plastics and to apply the latest knowledge to deal with the brittleness of PLA.
The Science Behind the Tough PLA Cups
To explain the science behind how a prototype PLA cup gains ductility and achieves heat resistance, Wang uses the analogy of cooked pasta. If molten PLA is magnified a million times, each chain-like molecule will look like a string of spaghetti a few meters long. It is important for thermoplastics, including PLA, to be tough, i.e., the crystallization process cannot remove or break the “spaghetti strands” of tangles.
This interwoven structure, called the “chain network”, allows anyone to remove almost all of the spaghetti from the bowl with a pair of chopsticks, and this chain network, when done correctly, ensures that PLA beverage cups are mechanically strong and crystallization-free. However, such commercial cups can collapse when boiling water is poured into them.” Cups made from normally crystalline PLA can hold boiling water, but are very brittle and opaque.” Mr. Wang
investigated the origin of the ductility of semi-crystalline polymers, which led to the discovery of a way to confine crystals to the nanoscale while preserving the PLA network, forming a transparent, tough, and heat-resistant cup. These clear cups can hold hot tea and coffee and can replace most plastic beverage cups on the market.
“The impact of our new understanding will ultimately spur exponential growth in the PLA market,” Wang said.
