EMAA-Zn Ionomer의 가교 및 발포 특성에 관한 연구

Abstract

Crosslinked foam is commonly used in major shoe parts, such as the shoe sole, to provide both lightness and elasticity. To ensure the performance and competitiveness of shoes, it is crucial to design various functional polymer foams and secure manufacturing technology. Recently, in order to compensate for the shortcomings of existing foam sole manufacturing technology, the MIF (Molded-In Foaming) method, which allows the final product to be manufactured by completing the crosslinking and foaming process within the mold cavity, has been attracting attention. Due to the nature of the MIF process, it is very important to control the rheological properties of the polymer substrate over time, and for this, basic research on a cross-linking system suitable for the MIF process and the resulting foaming characteristics is essential. Thus, in this study, ethylene-co-methacrylic acid copolymer (EMAA) was selected as the polymer material. Subsequently, organic peroxide (dicumyl peroxide, DCP) and metal oxide (zinc oxide, ZnO) were chosen as cross-linking agents to analyze the cross-linking behavior of EMAA and its foaming characteristics. The research aimed to investigate the suitability of the MIF process for EMAA-Zn ionomer, which exhibits thermal reversible cross-linking points based on the characteristics of ionomers. The following are the results. To analyze the crosslinking characteristics of EMAA-Zn ionomer, which possesses a physical crosslinking point in the form of an ionic bond based on the characteristics of the ionomer, dicumyl peroxide (DCP) and zinc oxide (ZnO) were selected as crosslinking agents. Ethylene-vinyl acetate copolymer (EVA) was chosen as a comparative group for the purpose of assessing and comparing the crosslinking characteristics. When zinc oxide was applied as a crosslinking agent, EVA did not exhibit significant torque changes over time in the ODR graph. However, in the case of EMAA, meaningful torque changes were observed with zinc oxide additions of 5 phr or more. When 10 phr of ZnO was added to EMAA, the maximum torque value was similar to that of adding 2 phr of DCP to EMAA, and when 15 phr of ZnO was added to EMAA, the maximum torque value was similar to that of adding 3 phr of DCP to EMAA. When ZnO was added 20phr to EMAA, the maximum torque value was the same as 1.0phr of DCP added to EVA, and when ZnO was added 30phr to EMAA, the maximum torque value was 88% of that obtained when 1.5 phr of DCP was added to EVA. In addition, when ZnO was applied as a cross-linking agent to EMAA, unlike when DCP was applied as a cross-linking agent, tC10 increased by about four times, even though it exhibited a similar maximum torque value. It was judged that this would relatively increase the fluidity of the polymer at the beginning of foaming, thereby increasing the mold fillability when applying the MIF process. When blending ZnO and DCP, the maximum torque value was 1.2 to 1.8 times higher than the sum of the maximum torque values formed when the two cross-linking agents were added alone to EMAA due to the synergistic effect of DCP and ZnO. Infrared spectroscopic analysis was performed to evaluate whether an ionic aggregate was formed in the EMAA-Znionomer. The infrared spectroscopic analysis confirmed the formation of an ionic aggregate in the EMAA-Znionomer when ZnO was added to EMAA, as evidenced by the presence of a characteristic peak at 1585 cm-1. To analyze the foaming characteristics of EMAA-Zn ionomer, the foam expansion index (FEI) was defined and the change in FEI value according to foaming conditions was compared and analyzed. In the case of a foaming compound in which ZnO was applied as a cross-linking agent to EMAA, the FEI and specific gravity change patterns as the ZnO content increased showed the same trend as when DCP was applied as a cross-linking agent. When the thickness of the specimen was increased from 2 mm to 5 mm, the FEI value increased up to 500% at a ZnO content of 30 phr, and the effect on foaming time was not significant. Upon comparing and evaluating the cross-sectional morphology at FEI(0), it was concluded that a ZnO content of 15 phr or more was necessary to ensure cell stability. When the MH-ML values were similar, the FEI value of EMAA-Zn ionomer increased by 250% compared to the formulation in which DCP was added to EMAA and by 240% compared to the formulation in which DCP was added to EVA. This indicates that using ZnO as a cross-linking agent for EMAA results in superior mold filling properties compared to using DCP at similar cross-linking densities. To analyze the in-mold foaming characteristics of the selected cross-linking system, a preform with a mold filling ratio of 30% was used to foam in the mold, and then the mold filling characteristics and cell morphology of EMAA-Zn ionomer were compared and analyzed. During the foaming process of a compound utilizing the ZnO cross-linking system within a mold, it was noted that complete mold filling was hindered when the ZnO content surpassed 15 phr. Additionally, applying ZnO at more than 10 phr resulted in a rapid deformation of cell morphology due to an increase in the density of ion aggregates. Considering the above results, when manufacturing a foam sole using the MIF method, It was deemed useful to utilize the physical cross-linking system of EMAA-Zn ionomer. In addition, the optimal content of ZnO to ensure cell stability and mold filling of EMAA-Zn ionomer foam was determined to be 5 to 10 phr.