Recently, interest in sports and leisure activities has increased due to an improvement in the income level and greater need for leisure life. As a result, there has been rapid popularization of related sports as well as growth in the marine sports apparel market. Marine sports garments require high functionality such as shock absorption performance to safeguard the human body from extreme environments. Among the many materials that are used in these garments, neoprene, a class of synthetic rubber produced from chloroprene that accounts for more than 40% of diving wetsuit materials, has become a popular choice owing to its mechanical strength, and aging and thermal resistance. However, there is a growing need, domestically and globally, for the development of eco-friendly materials since the disposal of such wastes causes environmental pollution with a large CO2 footprint (Patagonia 2016).
Therefore, studies on eco-friendly elastomers such as bio-based polyurethane and chlorine-free synthetic rubber are being actively conducted. Studies on polyurethane elastomers include studies on compatibility of bio-based and biodegradable polymer blends (Imer and Pukanszky 2013), improvement in the mechanical and thermal properties of bio-based polyurethane and its composites for expansion of various applications (Lee et al. 2018; Sebastian et al. 2018; Kuranska et al. 2016; Gama et al. 2015), and performance enhancement of bio-based polyurethane foam using nanoclay additives (Pauzi et al. 2014). In addition, there has been a wide range of studies on the synthesis and composites of bio-based polyurethane, such as the compatibility of bio-based polyol and conventional petroleum-based polyol for polyurethane production (Zhang and Kessler 2015; Park and Kim 2014) and the synthesis of bio-based polyurethane foam with organic materials (Li et al. 2018).
On the other hand, studies on chlorine-free synthetic rubber have focused on EPDM, an ethylene propylene rubber developed to address the low resistance to oxygen, ozone, heat, and gas of synthetic butadiene rubber, which is composed of a diene monomer (Allen 1983). EPDM is an ethylene-propylene copolymer that contains a double bond (diene), and is thus characterized with superb shock absorption property (Fig. 1). It not only has similar properties as that of neoprene, but is also an economical elastomer with ozone and thermal resistance as well as electrical insulation; because of these reasons, it has recently attracted attention as an industrial material in the fields of electricity, buildings, and automobiles (Lee and Bae 2018). Particularly, the development of bio-based EPDM from sugarcane-derived ethylene can reduce CO2 footprint and fossil resource dependency (Eco-friendly rubber seal 2015; Bio-based rubber 2015), making it an environment-friendly alternative to neoprene. Therefore, an intensive study on the blowing agent and foaming method is required to maximize the properties of foam made of bio-EPDM.
A blowing agent is a material that, when added into a polymer such as plastic or rubber, creates a foam through bubble generation. Among various blowing agents, chemical and encapsulated blowing agents are more commonly used. The former creates foam from CO2 produced by the reaction of an isocyanate with a liquid such as water, whereas the latter creates foam by vaporization of liquefied hydrogen gas located at the core of an acrylic copolymer at high temperatures, which expands the capsule (Ha et al. 2014). The foam is used in vehicle interior, life jacket, shock absorber, flooring, and shoe materials owing to its properties such as high elasticity, thermal insulation, lightweight, and shock absorption after the formation of a micro foam structure.
Depending on the type of blowing agent, the decomposition temperature, amount of generated gas, and the gas discharge rate as well as the resulting property changes vary. Therefore, uniform cell formation, excellent quality, and stable productivity are important factors in foam development (Peyda et al. 2016). Unlike chemical blowing agents, which produce harmful gases upon thermal decomposition, encapsulated blowing agents do not generate gases and the surface of the produced foam is much smoother. It also creates uniform, independent foam that is easy to handle, and stable (Jonsson et al. 2010). In addition, since encapsulated blowing agents are noted for their ease of thin-foam processing and advantages in elasticity, thermal insulation, soundproofing, specific gravity degradation, and shock absorption, many attempts have been made to increase the use of microcapsules in some industries (Ha et al. 2014; Jonsson et al. 2006).
Most of the studies on chemical blowing agents have focused on determining the effect of blowing agent content in the formulation on the foaming and molding characteristics of foam and the decomposition temperature of composites (Kim and Youn 2009), or properties such as shock absorption and compression set by adjusting the additives or the mixing ratio of polymer. Such attempts were focused on maximizing the productivity through an optimal expansion ratio while obtaining properties suitable for application. However, efforts to prevent harmful gas generation or improve foam uniformity have not really been made. Meanwhile, research on microcapsule blowing agents include a basic study on the development of a lightweight foam for automobile interior materials using an expandable microcapsule (Ha et al. 2014) and a study on the characteristics of PU/MWNT foam films for electrostatic shielding (Park et al. 2012). Both the studies used a chemical blowing agent and a microcapsule blowing agent to compare the foam properties and cell morphology. However, no research has been conducted on foaming technology for the development of highly-elastic foams with improved foam uniformity and shock absorption performance that can replace neoprene.
The chemical composition of an elastomer as well as its properties such as chemical, ozone, flame, and oil resistance vary depending on the type of elastomer. In addition, a systemic research on elastomers is required for the development of eco-friendly high-elasticity materials for diving wetsuits that can protect the human body from accidents because the required performance depends on usage and use condition. Further, it is necessary to steer away from the existing productivity-oriented research and study various foaming technologies that use expandable microcapsules to obtain relatively uniform foam cells without generating harmful gases. That is, since a foam exhibits different properties depending on the type of blowing agent used for gas generation, we believe that an optimal foaming technology can be developed by appropriately adjusting the cell foaming gas generation method by mixing different blowing agents. Thus, in this study, we investigated the changes in mechanical properties, thermal stability, and the salt-water resistance of a bio-EPDM foam depending on the mixing ratio of blowing agents.