Comparative Study on the Influence of Inorganic Pigments on the Thermomechanical and Combustion Performance of Different Elastomer Composites

Inorganic pigments are widely used in elastomer technology primarily as coloring agents; however, the broader effects of these additives on the structure–property relationships of rubber composites remain insufficiently understood, particularly across different elastomer matrices. This study addresses this gap by systematically evaluating and comparing the effects of zirconium cerulean blue (Pigment Blue 71, PB71) and earth pigment iron ocher (IO) in natural rubber (NR), acrylonitrile–butadiene rubber (NBR), and styrene–butadiene rubber (SBR) composites. The influence of pigment loading (2–10 phr, parts per hundred parts of rubber) on rheometric behavior, crosslink density, mechanical performance, thermo-oxidative aging resistance, and combustion behavior was investigated to determine whether inorganic pigments provide benefits beyond coloration. The incorporation of pigments did not adversely affect vulcanization behavior, with the crosslink density of NR increasing from approximately 1.5 × 10−5 mol cm−3 for the unfilled vulcanizate to 1.5–2.3 × 10−5 mol cm−3 for pigment-containing composites, while SBR exhibited an increase from approximately 1.0 × 10−5 to 1.5–2.0 × 10−5 mol cm−3. Selected formulations also exhibited improved tensile strength and enhanced resistance to thermo-oxidative aging. Pyrolysis combustion flow calorimetry (PFCF) demonstrated that the incorporation of 10 phr IO reduced the peak heat release rate (pHRR) by approximately 11% (NR), 10% (NBR), and 3% (SBR), whereas 10 phr PB71 decreased this parameter by approximately 13%, 8%, and 38%, respectively. These findings demonstrate that inorganic pigments can serve as multifunctional colorants, simultaneously improving mechanical performance, thermo-oxidative stability, and fire resistance while maintaining effective coloration.