Influence of the processing conditions on a two‐phase reactive blend system: EVA/PP thermoplastic vulcanizate

Abstract

The elaboration of a TPV based on copolymer of ethylene and vinyl acetate (EVA) and polypropylene (PP) as thermoplastic phase was investigated in a batch mixer. The crosslinking reaction is carried out through a transesterification reaction between ester groups of EVA and alcoxysilane groups of the crosslinker agent tetrapropoxyorthosilicate (TPOS). The main advantage of this crosslinking reaction is that it can be well controlled and suitable for different processing conditions. The aim of the present study is to get a better understanding of the dispersion mechanism and of the phase inversion of the EVA major phase during its dynamic vulcanization into the PP minor phase. It was proved that the initial viscosity ratio, λ = ηpp/ηEVA, between EVA and PP plays an important part in the morphology development of the reactive blend. The viscosity ratio must be close to the critical ratio expressed by Utracki's model of phase inversion mechanism. Furthermore, the influence of different processing parameters on the variation of the morphology and on the mechanical properties of the ultimate TPV was investigated. The main conclusion of this study is that the characteristic time of crosslinking must be of the same order than the time of mixing. Indeed, better mechanical properties are obtained when a progressive phase inversion occurred and when it is controlled by rheological aspects and transient morphology equilibrium of the two phases and not by the mechanical fragmentation of the crosslinked EVA. For example, in our experimental conditions (concerning the amounts of catalyst and crosslinker reagents), high shear rates can be avoided (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \dot \gamma} $\end{document} < 80 s−1) as the self‐heating of the blend under shear considerably increases (ΔT ≈ 50°C for \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \dot \gamma} $\end{document} = 225 s−1), leading to faster kinetics and consequently to a phase inversion controlled by the fragmentation of the crosslinked EVA phase.

Novel 3D porous multi-phase composite scaffolds based on PCL, thermoplastic zein and ha prepared via supercritical CO2 foaming for bone regeneration

“…For both the systems we observed that torque attains a maximum in the early stages of mixing and subsequently decreases to a rather stationary value, when also the melt temperature becomes quite constant. Similar trends were reported for melt blending processes of others thermoplastic polymers and fillers, indicating the achievement of uniform dispersions between polymers and HA particles [20,22].…”

Plasticization and Morphology Development in Dynamically Vulcanized Thermoplastic Elastomers

“…As a result, the stress transferred by the low viscosity thermoplastic phase becomes less and less effective in deforming the elastomeric domains and coalescence of the elastomeric domains becomes more and more hindered due to increased viscosity and elasticity of this phase. Consequently, the cross-linking reaction merely stabilizes the already existing, rather coarse dispersed morphology of the elastomeric domains, without any further morphological refinement [60][61][62]. With this in mind, although the co-continuity is a crucial factor in this process, a stable and unchangeable co-continuous morphology in the intermediate stage of dynamic vulcanization is not a desirable situation.…”

“…The fact that high and low viscosity ratio systems represent completely different initial morphological states prior to dynamic vulcanization means that the torque requirement for mixing could be substantially different from one system to another. Indeed, blends with high initial viscosity ratio with a dispersed thermoplastic phase in an elastomeric matrix have demonstrated a shoulder in the mixing torque during dynamic vulcanization process [61]. This shoulder appears while the blend structure passes through a co-continuous morphology prior to complete vulcanization and dispersion of the elastomeric component in the thermoplastic phase.…”

“…This additional increase in temperature could readily affect the morphology development through its influence on the rate of the cross-linking reaction. In a situation like that, the morphology development is said to be mainly controlled by the fragmentation of the cross-linked elastomer rather than by the transient equilibrium between coalescence and breakup of particles [61]. It has been pointed out that the significant effect of mixing intensity on the final morphology is through its effect on the rate of the cross-linking reaction [60,63].…”

“…For instance, a blend with extremely low viscosity minor thermoplastic phase (low viscosity ratio, η thermoplastic /η elastomer ) generally forms a dispersed/matrix morphology, where the thermoplastic phase encapsulates coarse elastomeric domains [61]. This is a condition which has to be avoided.…”