Nanocomposite foams were fabricated from 60/40 wt% ethylene vinyl acetate (EVA)/natural rubber (NR) blends by using azodicarbonamide as a blowing agent. Two different nanofillers (sodium montmorillonite and organoclay) were employed to study their effects on foam properties. The results were also compared with conventional (china clay)‐filled foams. Transmission electron microscopy, X‐ray diffraction, scanning electron microscopy, and three‐dimensional Microfocus X‐ray computed tomography scanning analysis were performed to characterize the EVA/NR blend morphology and foam structures. The results revealed that the nanofiller acted as a blend compatibilizer. Sodium montmorillonite was more effective in compatibilization, generating better phase‐separated EVA/NR blend morphology and improving foam structure. Higher filler loading increased the specific tensile strength of rubber foams. The rubber nanocomposite foam showed superior specific tensile strength to the conventional rubber composite foam. The elastic recovery and compressive strength of the nanocomposite foams decreased with increasing filler content, whereas the opposite trend was observed for the conventional composite foams with china clay. The thermal conductivity measurement indicated that the nanofiller had better beneficial effect on thermal insulation over china clay filler. From the present study, the nanofillers played an important role in obtaining better blend morphology as compatibilizer, rather than the nucleating agent and the nanofiller content of 5 phr (parts by weight per hundred parts of rubber) was recommended for the production of EVA/NR nanocomposite foams. J. VINYL ADDIT. TECHNOL., 21:134–146, 2015. © 2014 Society of Plastics Engineers
Nanocomposite foams made of ethylene-vinyl acetate (EVA), natural rubber (NR) and nanoclay were fabricated by mean of melt mixing in an internal mixer, and later foaming using azodicarbonamide through compression molding. Effect of NR addition (10-40 phr) into the EVA nanocomposite foam was studied. Characterizations by using Oscillating disk rheometer, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical tests were performed. The XRD analysis showed that the clay was mainly intercalated by the rubber. The SEM analysis revealed that the EVA/NR nanocomposite foams had closed-cell structures. Foaming EVA/NR/clay with composition of 100/0/5 (phr/phr/phr) led to low cell density and large cell size. The EVA nanocomposite foam also had low tensile strength, low compressive strength and low elastic recovery. However, the foam cell structure, i.e., greater cell density and smaller cell size, was obtained when combined with NR. Moreover, the strength and elastic recovery of the EVA nanocomposite foam were improved when the NR was added, and the improvement level was increased with increasing NR content.
Elastomeric foams based on 60/40 EVA/NR blend nanocomposite were prepared by melt mixing EVA, NR and clay (5 phr) in an internal mixer, and later foamed with azodicarbonamide through compression molding. Epoxidized natural rubber (ENR) with epoxide content of 25 mole% was used as a compatibilizer to improve interfacial adhesion between EVA and NR. The addition effect of ENR concentration on the blending and properties of EVA/NR blend nanocomposite foams was studied. XRD and TEM results showed that the clay in the EVA/NR blends was intercalated and locally existed in the NR phase as well as at the interface between EVA and NR. The EVA/NR blend nanocomposites showed dispersed NR domains in continuous EVA phase. The addition of ENR into EVA/NR blend nanocomposites reduced the size of the dispersed NR phase, indicating that the blend miscibility was improved. Mechanistic model was proposed for the miscibility improvement of a two-phase structure of EVA/NR blend nanocomposite in the presence of ENR and clay dispersion. However, the improved blend morphology did not significantly affect the cell structure of EVA/NR blend nanocomposites. The observation of cell structure of EVA/NR blend nanocomposites suggested that the increased addition of ENR caused a little increase in cell density, and had no influence on cell size. Furthermore, with increasing addition level of ENR in the blend nanocomposite foams, the foam density slowly decreased, whereas the specific tensile strength slightly increased. The elastomeric recovery of blend nanocomposite foams decreased with increasing ENR content. The thermal conductivity of EVA/NR blend nanocomposite foams also decreased with more addition of ENR, which had beneficial result in improving thermal insulation property.
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