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Materials science – Better disposable coffee cups | Science & technology


SUGAR CANE contains around 10% sugar. But that means it contains around 90% non-sugar—the material known as bagasse (pictured) which remains once the cane has been pulverised and the sugar-bearing juice squeezed out of it. World production of cane sugar was 185m tonnes in 2017. That results in a lot of bagasse.

At the moment, most of this is burned. Often, it fuels local generators that power the mills, so it is not wasted. But Zhu Hongli, a mechanical engineer at Northeastern University in Boston, thinks it can be put to better use. As she and her colleagues describe in Matter this week, with a bit of tweaking bagasse makes an excellent—and biodegradable—replacement for the plastic used for disposable food containers such as coffee cups.

Dr Zhu is not the first person to have this idea. But previous attempts tended not to survive contact with liquids. She thought she could overcome that by spiking the sugar cane pulp with another biodegradable material. She knew from previous research that the main reason past efforts fell to pieces when wet is that bagasse is composed of short fibres which are unable to overlap sufficiently to confer resilience on the finished product. She therefore sought to insert a suitably long-fibred substance.

Bamboo seemed to fit the bill. It grows quickly, degrades readily and has appropriately long fibres. And it worked. When the researchers blended a small amount of bamboo pulp into bagasse, they found that the result had a strong interweaving of short and long fibres. As a bonus, they also discovered that the hot pressing used as part of the process had mobilised some of the lignin in the fibres, and that this stiff, water-repelling material was now acting as an adhesive that bound the fibres together.

To put their new material through its paces, Dr Zhu and her colleagues first poured hot oil onto it and found that, rather than penetrating the material, as it would have with previous bagasse products, the oil was repelled by their invention. They also found that when they made a cup out of the stuff and filled it with water heated almost to boiling point, the cup remained intact for more than two hours. Though this is not as long as a plastic cup would last (it would survive indefinitely) it is long enough for all practical purposes. Moreover, the new material is twice as strong as the plastic used to make cups, and is definitely biodegradable. When Dr Zhu buried a cup made out of it in the ground, half of it rotted away within two months, and she reckons six months would have seen it gone completely.

Last, but by no means least, she estimates that cups made from the new material would cost $2,333 a tonne. That is half the $4,750 a tonne cost of biodegradable cups made from polylactic acid (fermented plant starch), and only slightly more than the $2,177 a tonne that it takes to make plastic cups. Overall, then, Dr Zhu argues that bagasse is an obvious choice for making coffee cups, straws, disposable plates, lightweight cutlery and so on. Once used, these could be dumped in landfills with a clear conscience.

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This article appeared in the Science & technology section of the print edition under the headline “Would you like sugar cane in that?”

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