J.G.P. Goossens

The rubber particle size to control the properties-processing balance of thermoplastic/cross-linked elastomer blends

The influence of the number-averaged rubber particle size (dn) on the mechanical and rheological properties of thermoplastic/cross-linked elastomer blends was studied, with the focus on thermoplastic vulcanizates (TPVs) based on poly(propylene) (PP) and ethylene-propylene-diene (EPDM) rubber. Time-resolved small-angle X-ray scattering measurements revealed that the criteria to obtain tough behavior are very different for TPVs than for traditional rubber-toughened thermoplastics, since the deformation mechanism of TPVs under tensile conditions is dominated by yielding of the semi-crystalline, thermoplastic matrix without the occurrence of matrix crazing and internal rubber cavitation. The formation of interlamellar voids, as occurs in the unfilled thermoplastic during deformation, is more effectively suppressed with decreasing dn, which leads to a significant enhancement of the ultimate tensile properties. Additionally, the decrease in dn leads to an enhanced elastic recovery, but also increases the melt viscosity. These results demonstrate that dn is an important parameter to control the balance between the mechanical properties and the melt processability of blends based on thermoplastics and cross-linked elastomers.

Thermoplastic vulcanizates by reaction-induced phase separation of a miscible poly(e-caprolactone)/epoxy system

A new method for the prepn. of thermoplastic vulcanizates (TPVs) was successfully applied. Starting with a miscible system of a semi-cryst. thermoplastic polymer and an elastomer precursor, phase sepn. was induced by the increase in mol. wt. during selective crosslinking of the elastomer precursor. As a model system, the semi-cryst. thermoplastic poly(.epsilon.-caprolactone) (PCL) was used in combination with a rubbery epoxy resin based on poly(propylene oxide) (PPOn-epoxy). This approach enables the dispersion of up to 80% of crosslinked rubber in a thermoplastic matrix, providing morphologies that are typical for TPVs. A range in rubber particle size of 0.5-3 .mu.m was obtained by this new method, without the need of blend compatibilization or dynamic processing. The materials exhibit mech. and rheol. properties typical for TPVs and the rubber particles feature a high crosslink d. At high curing temps. and high PPOn-epoxy contents, connectivity of the rubber particles is obsd., which is attributed to the phase sepn. mechanism. Connectivity of the rubber particles has a neg. influence on both the mech. properties and the rheol. behavior.

Reactive compatibilization of ethylene- co -vinyl acetate/starch blends

AbstractThe dispersion of starch as a filler in hydrophobic ethylene-co-vinyl acetate (EVA) rubber is an issue. To obtain a fine dispersion of starch in EVA rubber, EVA/starch blends were prepared by reactive extrusion in the presence of maleic anhydride (MA), benzoyl peroxide (BPO), and glycerol. MA, BPO, and glycerol play the role of coupling agent, free-radical initiator, and plasticizer, respectively. Molau experiment and Fourier transform infrared spectroscopy (FTIR) results showed that EVA chains were grafted onto the surface of starch particles during reactive extrusion via a free-radical grafting mechanism. As a result, EVA-g-starch copolymers acted as a compatibilizer, leading to fine dispersion of starch and strong interfacial adhesion between the starch and the EVA matrix. Scanning electron microscope (SEM) images showed that the starch particle size reduced from hundreds micrometers in the case of physical blending to approximately 1 micrometer in the case of reactive blending, and consequently, the EVA rubber was effectively reinforced by the incorporation of starch and the reactive compatibilization (e.g., the tensile strength of the EVA/starch (50/50, wt/wt) was increased by a factor of 6 after the addition of 0.9–1.8 wt% MA). The property stability of starch compounds is usually an issue, while the mechanical properties of the (compatibilized) EVA/starch blends reported in this article were stable during storage.