Recycling thermosetting plastics
Thermoset plastics that can be remoulded and recycled several times over when heated to about 100ºC are possible, claim researchers in The Netherlands.
The self-healing polymers make use of the Diels-Alder and Retro-Diels-Alder reactions between thermosetting polyketones and bis-maleimide, allowing the strong covalent bonds of the thermoset materials to be broken and reformed.
‘The Diels-Alder reaction has been known for many years as an example of a perfectly reversible (with high temperatures) reaction,’ explains Professor Francesco Picchioni of the University of Groningen. ‘On the other hand, polyketones can be easily modified to produce functional analogues that have, as side chains, a variety of chemical groups. When you combine these two concepts together you get a thermally reversible crosslinked polyketone.’
This process has been applied to other polymers, he adds, ‘However, a near 100% efficiency in the recovery of mechanical properties (like in ours) has never been achieved’.
Thermoset plastics are traditionally used in products requiring stiffer and longer-lasting materials, such as computers, refrigerators, and electrical insulation. The strong crosslinks in these polymers generally decompose when heated to high temperatures, making the material almost impossible to recycle. The majority of end-products go to landfill.
In the Groningen composition, the polyketone and bis-maleimide’s links break when heated in a mould and rejoin when cooled, allowing it to be reformed. ‘We use quite a low molecular weight (2,000g/mol to 5,500g/mol), and this keeps the voscosity in the melt quite low,’ says Picchioni. This makes it easy to remould, and its mechanical properties remain comparable to an epoxy resin.
In tests, the group has remelted its polymer seven times, but theorise that it could be recycled many more times over without decomposing. The broad range of polyketone precursors also makes
the system cost-effective to replicate, the researchers add.
Dr Ezat Khosravi, a Reader in the Department of Chemistry at Durham University, UK, says, ‘Self-healing has been used before. But what [the Groningen team] has done is twist the application, and say that as well as self-healing, [Diels-Alder] can be used to make a thermoset turn to thermoplastic, and then back to thermoset. So they’ve made the whole thing reversible.’
Khosravi’s group is working on similar polymers based on the Diels-Alder reaction, which, he says, have not been able to demonstrate complete solubility. He wonders about the full solubility of the Groningen material, and adds that oxidation could be a problem with the remelted thermoset over time, as it currently is with thermoplastics.
Picchioni notes, ‘You can never fully exclude [degradation] effects. However, during our thermal cycles, chemical analysis did not show any relevant sign of oxidation, and the almost 100% recovery of thermal behaviour seems to indicate a lack of degradation’.
The group is modifying the polymers to demonstrate reversibility at higher temperatures.
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