Better plastics with shish-kebabs

 

New research points to a novel structure in plastics, the "shish-kebab".

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Shish-kebabs are a tiny pattern that can form when polymers crystallise during flow, and when magnified a million times they resemble a skewer running through a stack of bell peppers.

Inside plastics, they make car body panels stiff and carpet fibres strong.

Shish-kebabs are responsible for the product's nice glossy finish and hardness, but come with a few problems.

Despite being scratch-resistant, they might also cause a layer to peel off, which is why scientists want to control them.

Now, Caltech Institute of Technology chemical engineering professor Julia Kornfield and Yoshinobu Nozue at Sumitomo, in Tokyo, have led a team that has uncovered certain properties of shish-kebabs that should lead to improved materials in the most widely used plastics.

"Our discovery is pertinent to the relatively strong and stiff plastics," said Kornfield.

"For example, it will allow manufacturers to make polymers for complex and beautifully shaped body panels with equal or better quality than currently available-and cheaper and faster."

Commonly used plastics

Shish-kebabs are made of polymers known as polyolefins, which make up half of all plastics used - over 100 million tonnes per year.

In addition to being used for car parts, polyolefins are also used to make pipes, wire, cable, carpets, fabrics, disposable syringes, and many other things.

Polyolefins are useful because manufacturers can custom-design their properties, Kornfield explained.

By varying the degree of crystallinity and the way the crystals come together, polyolefins can be altered so that they are as hard as steel or as soft as a rubber band.

"The plastics industry can tailor-make molecular distributions, but we don't know how to manipulate them," Kornfield explained.

"This discovery opens up a whole new neck of the woods that people didn't know they could explore, and they'll be able to create combinations of properties you couldn't get before."

Much as an inspiring leader can influence the action of thousands, the researchers discovered, some molecules (especially long ones) can shepherd many others to create the shish-kebab, which then direct the formation of kebabs.

This knowledge will allow for greater control of the creation process itself.

"In other words, you could make things by injection moulding that you couldn't make before, and injection moulding is a very cheap, fast process-you can pop a plastic bumper for an automobile out of its mould in a couple of minutes," explained Kornfield.

"So you bring down the cost of manufacturing and at the same time increase the throughput."

The lead author of the paper is Shuichi Kimata, a former postdoctoral researcher in Kornfield's Caltech lab.

He played a central role linking Kornfield's group at Caltech with Yoshinobu Nozue's group at Sumitomo and collaborators at the University of Tokyo.

The researchers reported their results in the 18 May issue of the journal Science in an article called 'Molecular Basis of the Shish-Kebab Morphology in Polymer Crystallization'.

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