In search of new materials to develop comfortable shoe sole, nowadays the researchers have resorted to the thermoplastic elastomers (TPEs) usually used as cushioning materials for its easy processability and adorable physical properties that can meet both the comfort as well as other functional requirements (i.e. fashion, ultralight, aging, etc.). TPEs are a class of copolymers or physical mixes of polymers and have the morphology of non-miscible blends of elastomer and thermoplastic matrix simultaneously. Low or room temperature elastomeric behavior and high temperature processablitiy makes TPE as a suitable material for developing lightweight and recyclable microcellular foams [1]. Most of the commercial TPE foams for footwear application are developed either from thermoplastic polyurethane (TPU) or ethylenevinyl acetate (EVA) copolymer in which the soft matrix provides sufficient free space for gas absorption whereas the rigid matrix prevents the gas diffusion during foaming process [2][3][4][5][6]. Styrene-(ethylene-cobutylene)-styrene polymers (SEBS) derived from the hydrogenation of styrene-butadiene-styrene (SES) polymers is a typical TPE which shows a better weather resistance, lower compression set, lower density, etc., can be a potential alternative of TPU or EVA [7,8]. SEBS consists of a soft midblock of ethylene-butylene (E-co-B) and hard end-blocks of 948 Abstract. Developing eco-friendly, flexible thermoplastic elastomeric foams based on poly(styrene-(ethylene-co-butylene)styrene) (SEBS) is a challenging task because of its poor melt strength. A promising approach to overcome this challenge is the use of synergistic technologies, such as combination of irradiation, supercritical fluid foaming, and steam-chest molding technologies. Herein, foamed beads were produced from pre-crosslinked SEBS beads using supercritical nitrogen as blowing agent, followed by subsequently efficient steam-chest molding to obtain midsole part. The crosslinking was accomplished under the assistance of electron beam. The rheology properties and foaming behavior reveals that the viscosity and modulus of the matrix increase with the increase of crosslinking resulting from increasing the irradiation dose (ID). With increasing the ID, successful foaming with larger expansion and improved cell morphology was achieved. SEBS bead foams were successfully obtained from 65 kGy-derived pre-crosslinked beads through steam-chest molding which showed a specific gravity of 0.252 g·cm -3 and comparable/superior mechanical properties to/than that of commercial thermoplastic polyurethane (TPU) or ethylene-vinyl acetate copolymer (EVA) foams. Especially, the higher elasticity and resilience of SEBS foams meet well the desirable properties for footwear application which supports SEBS to be an alternative for TPU or EVA.
The lightweight development of thermoplastic polyether ester elastomer (TPEE) has been limited by its low‐melt strength. In this work, the epoxy chain extender ADR was employed to improve TPEE's melt strength in order to enhance its foamability by supercritical N2 (sc‐N2). The results showed that the melt strength of TPEE increased gradually with the increase of ADR content, which led to the uniformity of the foam cell structure and the improvement of its mechanical properties, that is, a high resilience of 82%. However, when ADR content exceeded 1.6 phr, the viscosity was too high to induce foaming. Remarkably, the maximum stress retention of TPEE foam with 1.6 phr ADR remained at 90% after 50 cycles of compression, while the strain recovery rate was higher than 85%. The prepared TPEE foam has excellent resilience and dynamic mechanical properties, which broadens its application in fields of midsole, sports equipment and transportation.
Foams were prepared from acrylonitrile-butadiene rubber (NBR)/chlorinated polyvinyl chloride (CPVC) blend compounds using a chemical blowing agent during compression molding. The effect of varying NBR/CPVC blend ratio and the time on the foams were investigated. The curing parameters showed that both the maximum torque and the difference between the maximum and minimum torque decreased with increase of CPVC content. The limiting oxygen index (LOI) of the blend vulcanizate increased with increase of CPVC content, reached up to 30% for the NBR/CPVC blend with blend ratio 50/50. The SEM micrographs of the NBR/CPVC blend indicate semi-compatibility between the blend phases. The NBR/CPVC foams prepared using a chemical blowing agent, showed closed cell structures, which were uniformly distributed across the blend phases. The average cell sizes increased and foam density decreased with increase of CPVC content. Tensile strength and tear strength of both vulcanizates and foams increased with increase CPVC content. Hardness of the foams increased but resilience of the foams decreased with increase of CPVC content. Density, tensile strength, tear strength, and hardness of the foams increased but resilience decreased when the compression molding time of the foam was increased. J. VINYL ADDIT. TECHNOL., 00:000-000, FIG. 8. Effect of blend ratio on the average cell size and cell density of the NBR/CPVC blend foams.
In this study, a lightweight microcellular carbon‐based filler/poly(vinylidene fluoride) (PVDF) composite foam is fabricated with a 3D conductive network that is thermally insulating, electrically conductive, and fabricated on a large scale. This composite can be used for high‐efficiency thermal insulation and electromagnetic interference (EMI) shielding applications. The prepared composite demonstrates low density, high electrical conductivity, and excellent thermal insulation properties. The structure and density of the conductive network and the carbon‐based filler content has a significant influence on the electrical conductivity of the prepared composite foam. Although the composite comprises microcellular PVDF beads of the same density, the conductivity of the composite‐comprising strip beads is greater than that comprising spherical beads. In the same conductive network structure, as the size of the microcellular PVDF beads decrease, the conductive network becomes denser, which results in a higher conductivity. Furthermore, with an increase in the conductive filler content, the conductivity improves significantly. Excellent EMI shielding materials with optimal filler content and particle shapes, exhibiting EMI shielding effectiveness of up to 40–50 dB, are developed. The prepared composite foam possesses excellent application potential in the fields of ultra‐light thermal insulation, conductivity, and EMI shielding.
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