How the Size of Silica Capsules Influences their Stability During Rubber Processing
Katerina Filzer1,2, Katarzyna Bandzierz2,3 Wilma Dierkes4, Anke Blume2, Frederik R. Wurm1
1Sustainable Polymer Chemistry,, Department of Molecules and Materials, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands – Email: k.filzer@utwente.nl
2Elastomer Technology and Engineering, Department of Mechanics of Solids, Surfaces and Systems, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands
3Apollo Tyres Global R&D B.V., Colosseum 2, 7521 PT Enschede, The Netherlands
4Department of Mechanics of Solids, Surfaces and Systems, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands
Micro- and submicron capsules have gained significant attention in various fields,1,2 but have not yet been investigated to improve the recycling/environmental degradation of rubber. A big challenge of rubber recycling is dispersing a devulcanization aid (DA), which breaks rubber crosslinks, due to the dense rubber network. When using the DA shielded by a capsule, the DA can be better dispersed before rubber curing. Challenging here is the high shear force encountered during processing that the capsules have to endure which can severely impact requirements in capsule stability.
This study investigates the stability of silica capsules depending on their size within a rubber matrix. Silica capsules were prepared, in micro (4-7 µm) and submicron (300-700 nm) sizes by inverse miniemulsion using sol-gel chemistry from silica precursors, tetraethyl orthosilicate and (3-Aminopropyl)trimethoxysilane. Characterization was done by using Scanning Electron Microscopy, where they show a difference in shell morphology.
Subsequently, capsules of both sizes were incorporated separately into a rubber matrix using an internal mixer. It was shown that submicron-sized capsules stayed intact, while microcapsules broke under the high shear forces encountered in rubber processing showing that the stability of the silica capsules is dependent on their size. This research provides valuable insights for the development of a capsule-enhanced rubber system in which the shielded DA is well dispersed within the rubber matrix. The capsules are designed to release the DA after the first lifecycle of a rubber product, which yields an active DA able to break down rubber crosslinks and facilitate rubber recycling.