Stearic Acid-Modified Calcium Sulfate Whiskers as a Functional Filler for Rubber Enhancement

Calcium sulfate whiskers (CSWs) are fibrous crystals with uniform cross-section, well-defined morphology, and dense structure. Due to their low toxicity and low cost, CSWs have wide applications as additives in composite materials. In this work, CSWs prepared from desulfurized gypsum were used as raw materials. The mechanism of stearic acid (SA) surface modification of CSWs was investigated, and the influence of SA-modified CSWs on the mechanical properties of rubber was evaluated. Results show that SA effectively modifies CSW surfaces through a synergistic mechanism involving chemical bonding and physical adsorption. At lower SA concentrations, surface modification is primarily governed by chemical bonding, whereas physical adsorption becomes increasingly dominant at higher SA concentrations. Consequently, both the activation index and contact angle of modified CSWs initially increase but then decrease with rising SA content, peaking at a 4 wt.% SA dosage. At this optimal concentration, maximum values of 0.636 (activation index) and 110° (contact angle) were achieved. Furthermore, both unmodified and modified CSWs could improve the hardness, tensile strength, and elongation at break of the rubber. The optimal performance was achieved with 4 wt.% SA-modified CSWs, resulting in a hardness of 67°, a tensile strength of 21.92 MPa, and an elongation at break of 619%.