| Summary: | Ethylene-vinyl-acetate (EVA) polymeric foams are one of the most widespread used foams for athletic high-performance footwears. Low density, high impact peak force, low energy absorption, low thermal conductivity, high hydrophobicity, and high thermal stability are several crucial factors of a midsole polymeric foam. This project investigated the influence of carbon-based additives after incorporation in EVA polymeric foam. Five carbon-based additives, namely, carbon black, graphene, graphene oxide, carbon nanotube (CNT), and multiwalled carbon nanotubes (MWCNT) were incorporated into EVA matrix with varying weight percentages (1-5 wt%) and synthesized with compression moulding. Varying the weight percentages of additives enabled the observation of the trends of the various property’s changes. Morphology, density, mechanical, thermal, and surface properties were analyzed via a series of characterization processes. Effects of each carbon-based additives on various properties were studied and discussed. Comparison of effects between different additives were made to understand the compatibility of each additives with EVA polymeric matrix foams.
Characterization results indicated that incorporation of carbon-based additives into EVA matrix led to a lower foam porosity and higher density. It was observed that various properties of foams were highly correlated to the morphology of the foams. Foams with lower porosity generally displayed higher density, higher thermal conductivity, higher compressive stress, and higher energy absorption. In contemplation of reducing density and increase entrapment of gas phase in foams to allow optimization of properties, addition set of foam samples with increased amount of crosslinking reagent was synthesized. With the increase in degree of crosslinking, the densities of foams were lowered and mechanical properties such as compressive stress and energy absorption were enhanced. This was accredited to the increase of polymer melt strength and hence, better formation of pores and entrapment of gas phase.
Carbon black, graphene, and CNT foam samples with higher degree of crosslinking exhibited promising results. Despite their lower density (~10%) relative to pure EVA foams, they displayed better mechanical and thermal properties. These samples had high compressive stress, 10 to 15% higher than that of the reference sample. Their low densities allowed them to have low energy absorption, slightly higher relative to the reference sample (0.3-0.7N.s). High volume fraction of gas phase within these foam samples also enabled them to have lower thermal conductivity (5 to 25%) than the reference sample. This proved that with a optimized degree of crosslinking, these additives are highly compatible with EVA matrix and displayed great potential as high-performance footwear foams. Future research could be geared towards a deeper analysis of the molecular structure of the polymer chains via spectroscopy techniques. Analysis from these spectroscopy results would enable the attainment of an optimal and most suitable foam composition for each carbon-based additive.
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