Influence of Clay Soil Mineralogy on the Microstructure and Physico-Mechanical Properties of Natural Rubber Composites for Inner Tube Applications

The limited utilization of raw clays in the rubber industry is primarily attributed to an inadequate understanding of the complex interactions between clay minerals, their accessory minerals, and the polymer matrix. This study compares two local raw clays (SIT and KLE) with a reference industrial clay (REF) to demonstrate how differences in mineralogical composition affect the mechanical properties of natural rubber (NR)-based composites. Mineralogical characterization techniques (XRD, TGA-DSC, and IR) reveal distinct profiles: REF exhibits well-crystallized kaolinite as its dominant phase, whereas SIT contains a high proportion of quartz with less crystalline kaolinite, and KLE shows a complex mineral assemblage including swelling minerals (montmorillonite and chlorite). Microstructural analysis of NR/clay composites reveals a physical dispersion of clay particles within the polymer matrix without evidence of intercalation, with greater homogeneity observed for REF and SIT. Rheological properties indicate that curing times increase with increasing filler content, a trend that is particularly pronounced for KLE due to its interactions with the curing system. NR/REF composites outperform NR/SIT and NR/KLE in all evaluated mechanical properties, including the modulus at 300% elongation (6.1 MPa vs. 3.3 and 2.7 MPa), tensile strength, hardness, and tear resistance. This study establishes that reinforcement efficiency is directly linked to high kaolinite crystallinity, the absence of swelling minerals, and low concentrations of accessory minerals. Based on these findings, untreated local clay soils are not suitable for the manufacture of NR inner tubes and require appropriate pre-treatment prior to use.