Vulcanization is an important procedure for elastomers processing. The effects of vulcanization system on the interfacial interaction and mechanical properties of magneto-rheological elastomers (MREs) based on natural rubber (NR) have not been researched, even though, there were many researches focusing on the natural rubber based MREs. In this work, sulfur and dicumyl peroxide (DCP) acted as curing agents preparing MREs based on NR. Scanning electron microscopy was used to characterize the interfacial adhesion of carbonyl iron particles (CIPs) in the matrix. Crosslink density provided evidence that the MREs with DCP system had higher crosslink density than that of MREs with sulfur system. The tensile strength of MREs with DCP system increased, while that of MREs with sulfur system decreased, when 30 vol% of CIPs filled into NR. Finally, the magneto-induced dynamic mechanical properties were tested using a modified dynamic thermo-mechanical analysis under magnetic fields.

엘라스토머 가공에서 가황반응은 매우 중요한 공정이다. 천연고무(NR)를 사용한 자기유변탄성체(MRE)에 관한 연구들은 많이 수행되어 왔지만 엘라스토머 경화 시스템이 천연고무계 MRE의 계면접착 및 기계적 특성에 미치는 영향에 대해서는 연구가 거의 이루어지지 않았다. 본 연구에서는 경화제로 황과 디큐밀퍼옥사이드(DCP)를 사용하여 이들이 NR계 MRE의 특성에 미치는 영향을 조사하였다. MRE 매트릭스에 분산된 카보닐 철 입자(CIP)와 매트릭스의 계면 접착력은 주사전자현미경을 사용하여 분석하였다. 결과에 따르면 경화제로 황을 사용할 경우보다 DCP를 사용할 경우 MRE의 가교도가 높았다. 또한 CIP가 30 vol% 배합되었을 때 DCP 경화 시스템의 경우는 인장강도가 증가하였지만, 황 경화 시스템의 경우는 감소하였다. 개선된 DMA 장치를 사용하여 자기장이 인가된 상태에서 MRE의 자기유변 특성이 조사되었다.

Keywords: magneto-rheological elastomer, natural rubber, vulcanization system, mechanical properties, interfacial adhesion

Magneto-rheological elastomers (MREs) are functional materials due to the dynamic and rheological properties that can be stimulated by an external magnetic field.^1,2 Therefore, MREs have been applied in a wide range, including adaptively tuned vibration absorbers,^3,4 suspension bushing,⁵ seismic protection, ⁶ magneto-resistor sensors,^7,8 noise barrier systems,⁹ and electric current active element.¹⁰ MREs are compose of magnetic particles, elastomers, and additives. Many types of elastomers have been applied for MREs fabrication, such as natural rubber,^11-16 silicone rubber,¹⁷ polyurethane,¹⁸ polychloroprene rubber,¹⁹ EPDM,²⁰ and cis-polybutadiene rubber (BR).²¹
Vulcanization is an important procedure in rubber processing. The rubber changes into elastic vulcanizate by vulcanization via chemical reactions. The fundamental aim of vulcanization is cross-linking of macromolecules, which results in a threedimensional network.²² Sulfur and peroxide cure system are two main curing systems for rubber vulcanization technologies. Peroxide vulcanization is initiated by radical to form carbon-carbon crosslink structure, and homogeneous or uniform networks in the rubber matrix. Sulfur vulcanization is crosslinked by sulfur to form sulfur crosslink structure, including polysulfides, disulfides, mono-sulfides etc. and inhomogeneity of network structures existed in the rubber matrix.²³ Therefore, different cure system showed different tensile strength and strain-induced crystallization behavior.
Natural rubber (NR) is harvested from Hevea brasiliensis, and is widely used in various fields because of its superior elasticity, flexibility, and resilience.²⁴ Hence, NR could not be totally replaced by other types of rubber in industry due to the excellent comprehensive performance. The advantageous properties of NR and its suitability for preparing MREs have attracted great attention. Chen¹¹ discussed the effects of carbon black on mechanical performances in MREs based on NR. However, the maximum content of carbon black was 7%, and they did not mention about vulcanization system. Chung¹² found that peptizer decreased viscosity and molecular weight of NR matrix with sulfur vulcanization system, and resulted in efficient orientation of carbonyl iron particles (CIPs) and higher MR effect. Ge¹³ used rosin glycerin ester to enhance wettability and dispersibility of CIPs in the NR. It decreased zero-field modulus of MREs and increased magnetic-induced storage modulus and MR effect of MREs. As with Chen’s work, they also did not mention about the additive. Choi’s group^14,15 fabricated the MREs using pure CIPs and surface modified CIPs. The results showed that storage modulus, loss modulus, and MR effect of MREs increased when the surface modified CIPs filled in the NR matrix. However, a curing agent was not used in the MREs matrix. Pickering¹⁶ modified the iron sand using bis-(3-triethoxysillylpropyl) tetrasulphane (TESPT), and prepared the MREs using surface modified iron sand based on NR with sulfur curing system. Overall, the previous studies of MREs based on NR have focused on improving interfacial compatibility and enhancing mechanical properties via surface modification of magnetic particles and plasticizer filled elastomers. To our knowledge there has not been any research focusing on the impact on the interface between CIPs and rubber molecules and the properties of curing agent system in the MREs based on NR.
The interfacial adhesion between inorganic particles and elastomers molecules is a common problem in the composites of elastomers matrix. How to solve this question is the main research in this domain. As we introduced previously, the research of interfacial interaction of MREs has focused on the surface modification of CIPs. Hence, we would like to discuss the effect of vulcanization system on interfacial interaction of MREs based on NR in this work.
According to finite-element analysis, the best volume percent of CIPs for good MREs performance is about 30 vol%.²⁵ In this work, 30 vol% of CIPs was filled and sulfur and peroxide (dicumyl peroxide) were used as curing agents for preparing NR based MREs. Mechanical properties, crosslink density, and morphology were characterized to prove that the interfacial structure between CIPs and NR molecules were impacted by the vulcanization system. Finally, the modified dynamic thermomechanical analyzer (DMA) was used to examine the magneto-induced dynamic properties of MREs.