Cold sintering upcycles plastics and ceramics
A repeatable recycling process using cold sintering can strengthen materials, suggest scientists at Penn State College of Engineering, USA.
Professor Enrique Gomez at the University says cold sintering enables two or more waste materials to be combined into a composite that can be infinitely recycled.
They have combined polypropylene (PP) with ceramic calcium sulphate – the main component of gypsum drywall – to create a composite for structural building materials such as drywalls or outdoor decks.
This would overcome the issue where plastic bottles, for example, are recycled into a textile that eventually ends up in landfill when the textile is thrown away.
Cold sintering combines powder-based materials into dense forms at low temperatures through applied pressure using solvents.
It differs from traditional sintering by using a liquid phase for the dissolution and precipitation of an increasingly small fraction of ceramic to fuse powders at sintering temperatures below 200°C.
The process mixes ground PP particles of 200-500µm size with calcium sulphate powders and 10wt.% of deionised water (of total mass) into a stainless-steel mould. The mix is then heated to 160°C and 200MPa for one hour under ambient conditions to provide a sufficient flow of PP. The study says it enables densification to more than 99% under the correct conditions.
The paper on Upcycling plastic waste into fully recyclable composites through cold sintering in Materials Horizons outlines how the modest processing temperature precludes chain scission and polymer degradation after reprocessing, enabling resintering of ground composites multiple times without significant mechanical degradation.
The team has reported inorganic-matrix composites with significant enhancements, with a range of compositions of 14-68% PP.
The paper claims the Young’s modulus as 30% higher after cold sintering than after heat press processing, at 1,473MPa instead of 1,112MPa. The ultimate tensile stress (UTS) is eight times larger, 3.55MPa rather than 0.43MPa.
The level at which the cold-sintered material breaks under strain is said to be nearly three times larger than heat press-made samples. This is despite identical materials and processing conditions, except the inclusion of water.
The researchers conclude that the UTS appears to increase when the PP volume fraction is above 40vol.%. Doctoral student Po-Hao Lai says this produces strong and tough composites that are perfect for construction.
Traditional recycling often leads to downcycling, says the team, as the material quality decreases with each cycle. This method supposedly overcomes this and the drawbacks of using brittle ceramics, with the paper concluding that the recycled composites have 'significant enhancements in tensile strength and toughness over pure gypsum, commonly found in construction'.
At their end-of-life, the building materials can be ground down and cold sintered again for reuse, which the team says they have demonstrated up to 10 times. They can be recycled multiple times by only adding water, offering lower energy and water demands compared to conventional construction materials.
The team reports no significant degradation in mechanical properties after reprocessing, with the elastic modulii said to be invariant. In fact, it is suggested that the elongation at break of all compositions increases after reprocessing, alongside the UTS for some composites. The researchers believe this is due to the polymer flow filling the pore structure better around the ceramic particles.
The paper says, 'Even when using virgin feedstocks, cold sintering displays lower energy requirements and decreased global warming potential, underscoring its efficacy in processing recyclable composites.'
