Compound‐Modified Calcium Sulfate Whiskers as a Green Reinforcing Agent for Natural Rubber: A Study on the Replacement of Carbon Black

ABSTRACT Carbon black (CB) is the primary reinforcing agent for traditional natural rubber (NR). However, as its production depends on petroleum resources and significantly affects the environment, green alternative materials need to be developed. Herein, calcium sulfate whisker hemihydrate (HH‐CSW) was synthesized from rare‐earth metallurgical wastewater via a hydrothermal method and modified with a double layer of tetraethyl orthosilicate and a titanate coupling agent to obtain composite‐modified calcium sulfate whiskers (2‐CSWs). Notably, the 2‐CSWs exhibited remarkable hydrophobicity (contact angles reaching up to 100.7°) and interfacial compatibility with the NR matrix. In NR, the 2‐CSWs significantly enhanced the mechanical and vulcanization properties. Specifically, addition of 2‐CSWs at 10 parts per hundred rubber (phr) showed the most significant improvement, with a 37% increase in tensile strength, 321.97% increase in vulcanization rate index, and 54.2% reduction in vulcanization time. Further, the hardness and elongation at break improved to 48 HA and 490.81%, respectively, demonstrating excellent overall performance. Compared with CB, 2‐CSWs resulted in a better reinforcing effect and processing performance. 2‐CSWs added at 10 phr increased the maximum torque by 37.6%, tensile strength by 20.92%, and vulcanization rate index by ~209.7% as well as reducing the vulcanization time by more than half. As a green alternative to CB, 2‐CSWs significantly improve the performance of NR. Additionally, they demonstrate the potential to reduce oil dependence and environmental pollution, thus representing a new idea for the sustainable enhancement of NR.