Guilin, China – Rigid PU foam panels are a mainstay of building insulation, but they are tricky to recycle, with mechanical recycling giving low recovery, incineration and energy capture being polluting, and chemical recycling requiring precise control. Now, scientists at the Guilin University of Aerospace Technology have devised a novel way to recycle and reuse them using whisker-toughened aerogels.
SiO2 aerogels have good thermal insulation properties, but the limitations in their mechanical properties that result from their high porosity limit their application in construction. The team had previously introduced mullite whiskers to improve the aerogels’ thermal and chemical stability, and wondered whether they could be incorporated into recycled rigid PU during the chemical recycling process.
To create the nanocomposite, waste PU sheet was cleaned, dried and crushed, before undergoing an alcoholysis reaction with diethylene glycol and ethylene glycol, plus sodium hydroxide as the catalyst. The whisker aerogel was added, and the mixture heated for a couple of hours. The regenerated polyols were then mixed with a commercial polyether polyol in a 1:2 ratio, and used to make new foam.
They thought the addition of the aerogel could improve the performance of the recycled PU, because of its low thermal conductivity, better thermal insulation and heat preservation performance. This proved to be the case. The thermal conductivity of each version containing the was lower, thus improving insulation performance. The optimal results were achieved with the addition of 0.9% by weight of whisker content, after which the thermal conductivity goes up and the strength starts to decline. This, they said, showed the aerogel gives an agglomeration phenomenon, which is responsible for the fall-off in insulating performance.
They attributed the improvement in properties to the whiskers that bridge and transfer stress in aerogel materials, improving the toughness of the resulting recycled PU composite. There is also a synergistic effect.
The team concluded that the recycled PU foam composites have potential in building insulation. They could also provide a novel way of efficiently recycling waste rigid PU insulation panels.
The work has been published in the journal Gels.