Influence of Metal Ions on Coordination Linkages of Epoxidized Natural Rubber

Abstract

The crosslinking reaction of epoxidized natural rubber (ENR) with ferric ion (Fe 3+) from ferric chloride (FeCl3) was investigated using ENR with two different levels of epoxide groups: 25 and 50 mol% (ENR-25 and ENR-50) and compared with unmodified natural rubber, air dry sheet (ADS). Mixing was performed in an internal mixer at 60°C and a rotor speed of 60 rpm. The results showed that the ENR-FeCl3 compound exhibited an increased torque response with time (or marching cure curve), while unmodified natural rubber (ADS) did not have any torque response. The observed increased torque during the curing process can be attributed to a chemical reaction between the oxirane rings of ENR molecules and Fe3+ ions. This results in the formation of coordination -O-Fe-O- linkages, which is supported by the detection of the Fe-O absorption peak at the wavenumber 565 cm-1 in the infrared spectrum. Furthermore, the extent of coordination crosslinking reaction in ENR molecules was found to increase with increasing concentrations of FeCl3 from 1 to 10 mmol. This leads to a more pronounced internal polymerization of the epoxy groups in the ENR molecular chains and an increased content of crosslinking networks. The results also indicated that the ENR-50 compound with FeCl3 concentrations greater than 7 mmol exhibited the superior mechanical properties compared to those ENR cured with the conventional sulfur vulcanization. This is likely due to the higher crosslink density achieved through coordination crosslinking between epoxide groups and Fe3+ ions. Next, the ENR-FeCl3 compound with a 7 mmol FeCl3 concentration was filled with varying amounts of carbon nanotubes (CNTs) at 1, 3, 5, 7, and 10 phr. The addition of CNTs was found to accelerate the vulcanization reactions by reducing the scorch time and cure time. Furthermore, ENR-FeCl3-CNTs nanocomposites exhibited higher mechanical, and dynamic properties, thermal resistance, and electrical conductivity than the compound without CNTs. This is due to chemical interactions between the epoxy groups of ENR and polar functional groups on the CNTs' surface. Additionally, the ENR-FeCl3-CNTs nanocomposites showed a finer dispersion of CNTs in the ENR matrix, resulting in outstanding mechanical, dynamic mechanical properties, thermal stabilities, and electrical conductivity. The percolation threshold concentration of ENR-FeCl3-CNTs nanocomposites was determined to be about 7 phr of CNTs. Therefore, this content of CNTs (7 phr) was selected to prepare the ENR-FeCl3 filled hybrid filler between CNTs and conductive carbon black (CCB) by varying CCB loadings at 2.5, 5.0, 7.0, 10.0, and 15.0 phr. The results indicated that the mechanical, dynamic, and electrical properties of ENR-FeCl3/CNTs/CCB nanocomposites were further enhanced due to the chemical interactions between the polar functional groups in ENR molecules, CNT surfaces, CNT bundles, and CCB particles. Moreover, the ENR-FeCl3/CNTs/CCB nanocomposites showed higher electrical conductivities than ENR-FeCl3/CNTs without CCB, and a finer dispersion of CNTs in the ENR matrix was observed due to the CCB hindering the agglomeration of CNTs in the ENR matrix.