2.3.2. Natural Rubber

Natural rubber (NR) is a polymer of isoprene made by solidifying latex obtained from the Hevea brasiliensis tree, being generally composed of cis-constituted C5H8 units (isoprene) with one double bond in each repeat unit. NRs as unsaturated elastomers have superior characteristics such as high strength, better resistance, and elongation at break [72]. However, due to the presence of double bonds in the chain, they are very sensitive to heat and oxidation. The improvement in rubber elasticity and strength is usually obtained by a vulcanization process in the presence of sulfur, accelerators and other compounding components, creating a three-dimensional network. NR, which has undergone through this vulcanization process, has higher mechanical performance than other rubbers, making it more suitable for practical applications [30]. Chen et al. [73] used NR to study the MRE properties according to temperature, plasticizer, and iron particle content. They observed that NR had better mechanical properties than SR, and as CI contents increased, the shear modulus of MRE increased, resulting in an increase in MRE performance. However, as CI contents increased, tensile strength and angle tear strength of MRE decreased. Aziz et al. [74] studied the e ffects of dispersing agents such as naphthenic oil, light mineral oil, and epoxidized palm oil on MR rubber based on NR. Compared to conventional petroleum-based dispersing aids, these eco-friendly dispersing aids, also increased mechanical properties such as MRE's magnetic behaviors, tensile strength, and elongation at break. This is because EPO increased the crosslink density of the NR matrix as dispersing aids.
