Replacing synthetic fillers, which are commonly used to reinforce rubber, with bio-fillers has potential to improve the sustainability of rubber products. Eggshell (ES) (a powder with a maximum particle diameter of 9.4 μm and a median of 1.1 μm) was added to guayule natural rubber (GNR) composites to partially or fully replace bifunctionally silanized, high surface area, precipitated silica (BSS). The mixing energy consumption, mechanical properties, cross-link density, filler dispersion and final particle size, fracture surface morphology, and dyeability of GNR composites were characterized. ES filler effectively reinforced vulcanized GNR compared with unfilled vulcanized GNR. Energy consumption, modulus at 300% strain (M300), and hardness generally decreased with increasing ES fraction (decreasing BSS), but tensile strength, gel fraction, and elongation at break increased even where cross-link density and M300 were similar. Thus, composite cross-link density was not solely influenced by silane content as the ratio and loading of ES and BSS changed. The production of the composites reduced particle size to submicron size. Even a small amount of ES improved the dispersion of BSS filler particles in the composites and hence the mechanical properties. The contributions of the two fillers to the composite properties are explained. Linear mixed models were built to predict the mechanical properties of a broader range of GNR–ES–BSS composites, and r2 (the quality of the model predictability) was above 0.9 for all models. ES filled GNR, with or without BSS, can be dyed different colors for specific applications. The lower-cost, renewability, dyeability, and excellent performance of ES–GNR composites addresses the need for sustainable rubber products with low carbon footprint.

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