A team of researchers has turned waste materials into a new rubber material that can stick itself back together when cut into pieces. They claim the approach is a new concept in repairing and recycling sustainable rubber.
Elemental sulfur is produced during petroleum refinement, generating millions of tons of waste. Inverse vulcanisation, which requires mostly sulfur and a small amount of another additive, uses this waste as feedstock to create new materials with high sulfur content. These polysulfide materials have found increasing use in energy storage applications, IR optics and environmental remediation. They have a unique property: sulfur-sulfur bonds in their backbone can be reversibly broken and reformed when heated – usually to temperatures around 100°C.
Now a team from Flinders University, the University of Western Australia and the University of Liverpool have shown that polymers made by inverse vulcanisation can be reformed at room temperature for the first time (Chemical Science, doi: 10.1039/D0SC00855A). They made a model polymer from a polymerisation of sulfur, canola cooking oil and dicyclopentadiene from petroleum refining. The result was a soft, flexible black rubber material. When cut into pieces, they report it can be completely repaired and returned to its original strength in minutes using an amine catalyst. It appears that nucleophiles, such as amines, can induce rearrangement of S–S bonds in these polymers at room temperature, allowing repair.
The new rubber can be also used as a ‘latent adhesive’; rubber bricks made from the polymer could be chemically joined together by applying pyridine as a ‘catalytic mortar’.
‘The rubber bonds to the surface of another piece of rubber when the amine catalyst is applied to the surface,’ says University of Liverpool researcher Tom Hasell. ‘The adhesion is stronger than many commercial glues.’ Such chemically induced bonding has potential in additive manufacturing and assembly. For instance, the researchers say that polymer components could be moulded separately and then assembled into chemically bonded objects.
The team also tested pyridine as a catalyst for polymer recycling. Polymer chunks were ground into a powder, coated with pyridine and compressed at room temperature for 30min, to make a rubber mat. This technique differs from most other recycling methods because the material is not heated and reshaped, they say. Instead, the polysulfide groups on the polymer surface rearrange so they join the polymer pieces into a new shape.
Rhett Smith of Clemson University, South Carolina, US, says this is the first time that these emerging materials have been processed so easily and at room temperature. ‘The work is ground-breaking, and stands to open the door for facile commercialisation of these sulfur-organic materials using well-established compression moulding and additive manufacturing techniques. The clever use of nucleophilic solvents is a pioneering approach.’.