Investigating novel elastomer blend foams for shoe soles

1 August 2017
Zhen Xiu Zhang and Prakashan K
Ethylene-vinyl acetate/polyurethane foams containing calcium carbonate fillers are produced via a batch foaming process, with the use of supercritical nitrogen.

Foamed polymer materials (i.e., polymer materials containing many very small voids that are filled with air or other gases) are commonly used in a number of applications because of their favorable characteristics, e.g., good cushioning, sealing, and thermal insulation properties. For example, ethylene–vinyl acetate (EVA) is a widely used thermoplastic elastomer in the production of films, wires, cables, and toys.1, 2 Although EVA is most commonly used as the material for the soles of shoes, virgin EVA foams exhibit poor compression set, wear, and tear properties. EVA is therefore normally blended with natural rubber (NR) in shoe-sole applications, but these blends are still subject to some limitations (e.g., coloring problems).

The development of new EVA-based blends is thus an ongoing quest in the footwear industry. For instance, in previous work,3 butadiene rubber has been blended with EVA to produce microcellular shoe-sole foams. It was found that the overall performance of these foams was comparable to commercial EVA/NR foams, but compatibility between the EVA and rubber was problematic. Moreover, most elastomer (e.g., EVA) foams are currently produced with the use of chemical blowing agents,3–7 which can cause harmful environmental and health effects. In addition, batch foaming processes—involving the use of more environmentally friendly supercritical gases, such as carbon dioxide (CO2) and nitrogen (N2)—are relatively unexplored for elastomers compared with thermoplastic foams.

The main aim of our study8 was therefore to produce—in an efficient and environmentally friendly manner—improved EVA-based foams that can be used as soles in the footwear industry. We used a batch foaming process to produce blends of EVA and polyurethane (PU) rubber in a variety of ratios. We chose PU for this purpose because of the polar groups on its polymer chain that provide excellent compatibility with EVA. For our batch foaming process we used supercritical N2 as the blowing agent. We also included calcium carbonate (CaCO3) fillers in a range of concentrations.

We subjected our EVA/PU samples to a variety of tests so that we could investigate their physicomechanical, friction, and wear properties. For example, we obtained scanning electron microscope (SEM) images and found that all the compounds contained spherical cells with a closed-cell morphology. As an example, the SEM image for the EVA/PU blend (in an 80/20 weight ratio) foam containing 30 parts per hundred rubber (phr) of CaCO3 is shown in Figure 1. Overall, the SEM images illustrate that the blend foams had lower average cell sizes than the neat EVA foam and that the cell sizes of the foams decreased with increased PU content.


Scanning electron microscope image of the ethylene–vinyl acetate/polyurethane (EVA/PU) blend (in an 80/20 weight ratio) foam containing 30 parts per hundred rubber calcium carbonate fillers.

The measured physicomechanical characteristics of our EVA/PU foams are given in Table 1. Our results show that the density, hardness, and resilience of the blend foams lie between those of the neat EVA and neat PU foams. In addition, the values of these parameters increase with increasing PU content of the blends. However, despite the good compatability of the EVA and PU phases, we did not measure any improvement in the tensile strength and tear strength of the blend foams compared with the EVA foam. This is probably because of a decrease in the effectiveness of the bis(t-butyl peroxy isopropyl) benzene that we used as a crosslinking agent in the blends. Our tests also revealed that the friction coefficient and abrasion resistance of the EVA/PU blend foams were significantly better than for the neat EVA foam, which is an important result for shoe-sole applications.

Measured physicomechanical properties of the EVA/PU blend foams. The EVA/PU blend ratio is provided for each foam.

FoamDensityHardnessResilienceCompression setTensile strengthTear strength
(blend ratio)(g/cm3)(Shore C)(%)(%)(MPa)(kN/m)
EVA/PU (100/0)0.30624815.63.721.3
EVA/PU (90/10)0.36634917.32.810.0
EVA/PU (80/20)0.37645024.43.211.7
EVA/PU (70/30)0.41665124.63.512.6
EVA/PU (60/40)0.46685324.83.615.5
EVA/PU (0/100)0.56725825.05.018.9

We found that incorporating CaCO3 up to a concentration of 40phr in the blends had a number of effects. First, it caused a decrease in the cell sizes, as well as increase in the density and hardness, of the foams. This increase in the density and hardness did not appear to be significant, because the foam expansion was not adversely affected by the dispersion of CaCO3 in the matrix of the blends. In addition, we found that the tensile strength and tear strength of the foams improved with increased CaCO3 content (i.e., up to 40phr).

In summary, we have investigated the morphological, physicomechanical, friction, and abrasion properties of EVA/PU blend foams—produced via batch foaming with supercritical N2—that contain CaCO3 fillers. Overall, our aim was to determine a compound formulation that provides improved friction and wear characteristics compared with current EVA-based foams, i.e., for shoe-sole applications. Our tests indicate that the hardness, resilience, friction coefficients, and abrasion resistance of the EVA/PU foams were better than the neat EVA foam, whereas their compression set, tensile strength, and tear strength were inferior. Furthermore, the incorporation of the CaCO3 fillers causes an increase in the density, hardness, tensile strength, and tear strength of the blend foams, but decreased their resilience, compression set, friction coefficients, and abrasion resistance. In our future studies we plan to explore the role of crosslink density in elastomer vulcanizates. We will also examine how foaming of vulcanizates is affected by the incorporation of a variety of platy fillers.


Authors

Zhen Xiu Zhang
Qingdao University of Science and Technology

Zhen Xiu Zhang is an associate professor in the School of Polymer Science and Engineering. His research interests include foaming of elastomers and plastics, rubber technology, and superhydrophobic polymer films.

Prakashan K
Qingdao University of Science and Technology

Prakashan K is a postdoctoral researcher in the School of Polymer Science and Engineering. His current research focus is foaming of elastomers with the use of supercritical gases.


References

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DOI:  10.2417/spepro.006937