Spherical CaCO₃: Synthesis, Characterization, Surface Modification and Efficacy as a Reinforcing Filler in Natural Rubber Composites

Natural rubber (NR), an important natural polymer derived from the Hevea brasiliensis tree, has been widely used in the rubber industry owing to its excellent elastic properties. However, it requires reinforcing fillers to improve its mechanical properties for the manufacturing of rubber products. Generally, calcium carbonate (CaCO₃) is employed as a non-reinforcing filler. This work aimed to synthesize spherical-shaped CaCO₃ at a submicrometric scale without and with surface treatment and explore its utilization as a reinforcing filler in NR composites. The morphological shape and polymorphic phase of CaCO₃ were investigated using SEM, TEM, XRD, ATR-FTIR and Raman techniques. The mechanical properties of various amounts (0 to 60 phr) of CaCO₃-filled NR composites were explored. As a result, the NR/treated CaCO₃ composites provided higher tensile strength than the NR/untreated CaCO₃ composites and pure NR at all filler loadings. This may have been due to the improved interfacial interaction between NR and CaCO₃ with the improved hydrophobicity of CaCO₃ after treatment with olive soap. The optimal filler loading was 20 phr for the highest tensile strength of the rubber composites. In addition, the elongation at break of the NR/treated CaCO₃ was slightly decreased. Evidence from SEM and FTIR revealed the vaterite polymorph and shape stability of CaCO₃ particles in the NR matrix. The results demonstrate that the particle size and surface treatment of the filler have essential effects on the mechanical property enhancement of the rubber composites. Synthesized spherical CaCO₃ could be a potential reinforcing filler with broader application in polymer composites.