Sub-micrometer TPVs based on crosslinking-induced phase separation of polycaprolactone/oligoetherdimethacrylate blends

Thermoplastic vulcanizates (TPVs) are produced via dynamic vulcanization of immiscible blends of a semi-crystalline thermoplastic and a rubber. Most commercial TPVs are based on blends of polypropylene (PP) and ethylene/propylene/diene (EPDM) rubber and have typical rubber domain sizes of 1 – 3 mm. It has been shown that decreasing the rubber particle size in PP/EPDM-based TPVs from 100 to 1 mm results in strongly improved tensile properties. A further reduction of the rubber particle size in order to possibly further improve the properties is not feasible in a practical way, because ~1 mm is the smallest rubber particle size that can be achieved via dynamic vulcanization (shear forces in extruder cannot overcome further increase in surface tension).
In this project we explore the new concept of producing TPVs with sub-mm rubber dispersions via crosslinking-induced phase separation. A semi-crystalline thermoplastic is dissolved in a rubber precursor, the rubber precursor is polymerized/crosslinked and, consequently, phase separation occurs with the final morphology being fixated somewhere between the Å and mm scale. In a first attempt, poly(e-caprolactone) (PCL) was combined with poly(propyleneoxide)bisepoxides and crosslinking was performed with a polyamine. TPVs with a rubber dispersion of 0.5 up to 3 mm were obtained over a broad composition range, but due to connectivity between the rubber particles, the tensile properties were not as good as expected. In a follow up, PCL was combined with oligoetherdimethacrylates and crosslinking was initiated with a peroxide. This approach resulted in the successful preparation of a series of sub-mm TPVs with rubber particle sizes in the range of 80 to 800 nm. The polymer characteristics, such as rubber gel content, PCL graft level and PCL crystallinity and melting temperature, of these TPVs were determined and correlated with the physical properties, such as tensile properties, compression set and melt processability. The deformation mechanism was studied by in-situ small-angle X-ray scattering.
This work is part of the research program of the Dutch Polymer Institute (DPI), project #537 ‘sub-mm TPVs’.