Modification of curing, morphological, mechanical and electrical properties of epoxidised natural rubber (ENR-25) through the addition of copper calcium titanium oxide (CCTO)

Abstract

The limited applicability of very high dielectric permittivity ceramic materials such as copper calcium titanium oxide, commonly known as CCTO (εr = 100,000 at room temperature and nearly independent of frequency from 1 Hz to 1 MHz), could be improved through the fabrication of polymer matrix composites. Ceramic is brittle while polymers are materials that are ductile and have excellent flexibility but low εr. Another contrast between the two materials is the fabrication process where ceramic, specifically, requires pressing and a high sintering temperature. Hence, the right combination of ceramic and polymeric materials should theoretically produce a composite with excellent mechanical and electrical properties. Therefore, a study on CCTO ceramic powder blended with 25 mol% of epoxidised natural rubber (ENR-25) was carried out. The CCTO powder was initially synthesised through solid-state reaction followed by compounding with ENR-25 formulations with different CCTO loadings (0, 20, 40, 60, 80, 100, and 120 phr) in an internal mixer. Small blocks of the composite were cast into ~ 2 mm of mould thickness and then hot compressed into square shapes. Samples were characterised by curing, mechanical, electrical, and microstructural properties. As a result, the addition of CCTO was found to have lowered the curing time, i.e. tc90 at 20 phr, compared to composites without CCTO loading. Then, the curing time gradually increased with filler loading from 2.05 to 2.48 at 20 to 120 phr loading, respectively. Mechanical testing of the composites showed an increase in tensile strength from 5.91 to 16.46 MPa. However, with content higher than 40 phr content, the tensile strength's magnitude gradually decreased with increasing filler loading from 13.63 to 6.49 MPa. In comparison, hardness properties increased with an increase of filler loading from 30.5 to 44.7 Shore A. Meanwhile, LCR meter showed that increased CCTO content could improve εr value from 6.134 to 12.114 at 75 kHz and decrease the dielectric loss (tan δ) from 0.179 to 0.150 at 2 MHz. The composite's microstructure also shows CCTO crystals embedded in the ENR-25 with excellent surface contact. The surface morphology showed that samples with filler content of 60 phr onwards had a lot of CCTO particle pore agglomeration, which reduced its mechanical strength from 16.46 to 6.49 MPa.