Rare earth gypsum is an industrial by-product generated from the treatment of wastewater produced by the rare earth metallurgical process, and its recycling is a strategic measure for the current green metallurgy and cleaner production. In addition, the filler reinforcing agent for natural rubber in polymers is mainly carbon black or other materials such as calcium carbonate, which will cause a large amount of carbon emissions in the production process. Calcium sulfate whisker (CSW) is a new reinforcing agent in rubber composites, while its surface is hydrophilic and has poor compatibility with organic matrix. This work aims at the above problem by using inorganic trisodium phosphate-organic stearic acid composite modification, thus changing the surface polarity of CSW and improving the compatibility with organic matrix.

20 g/L CSW was dispersed in 100 mL of trisodium phosphate solution (0.005, 0.010, 0.015, 0.020, 0.025 mol/L), then ultrasonicated for 2 min, and stirred at room temperature to make the solution mixed homogeneously. The solution was filtered and washed to obtain inorganic modified CSW (1-CSW). Subsequently, 1-CSW was dispersed in 100 mL of ethanol, and stearic acid (0, 1%, 6%, 11%, 16%, 21% of CSW addition) was added to the solution, and the slurry was heated (60, 70, 80, 90 ℃, and 100 ℃) in an oil bath for a certain time (10, 15, 20, 25 min, and 30 min) under magnetic stirring. The final product was cooled to room temperature, then filtered and washed with anhydrous ethanol for five times, and dried at 100 ℃ for 1 h to obtain the composite modified CSW (2-CSW).

The optimal modification conditions for inorganic-organic surface modification of CSW were determined by one-way experiments: CSW addition as 20 g/L, optimal amount of stearic acid as 6% of CSW, modification time as 10-15 min, modification temperature as 80 ℃, and concentration of trisodium phosphate as 0.02 mol/L. XRD patterns indicated that the modified products were mainly monoclinic hemihydrate (HH) calcium sulfate with the presence of a small amount of monoclinic dihydrate (DH) calcium sulfate, It can be seen that the surface modifier contributed to the transformation of CSW from dihydrate to hemihydrate calcium sulfate. TEM analysis was performed that the surface modifier existed in amorphous form on the surface of the whisker. A transparent coating layer on the surface of 2-CSW can be clearly observed. The enlarged TEM image shows that the average thickness of the hydrophobic layer on the surface of 2-CSW is about 121.26 nm. Infrared spectroscopy analysis shows that inorganic trisodium phosphate and organic stearic acid were successfully modified on the surface of CSW. Thermal stability analysis shows that the first mass loss of CSW is perfectly agreement with the theoretical water content in CSW. The second weight loss is due to the decomposition of stearic acid. XPS analysis of the surface elements and contents proves that trisodium phosphate and stearic acid are successfully encapsulated on the surface of 2-CSW.

The surface modification of CSW was successfully realized through a series of one-factor experiments to change the surface property of this whisker from hydrophilic to hydrophobic. The optimal modification conditions were as follows: concentration of CSW: 20 g/L, stearic acid modifier: 6% of CSW, trisodium phosphate: 0.02 mol/L; modification time: 15 min at 80 ℃. The morphology of CSW remained the original form after the modification, but the crystal structure of the whisker was transformed from DH to HH, and the surface property was changed from hydrophilic to hydrophobic. The contact angle of 2-CSW reached a maximum of 104.23 °, and the thickness of its hydrophobic layer was 121.26 nm. The modification mechanism is that the surface of CSW firstly generates CaHPO₃ and Ca(OH)₂, then undergoes a bonding reaction with stearic acid, generating a nanoscale hydrophobic layer with a C₁₇H₃₅COO-Ca -structured nanoscale. The layer protects CSW from denaturation by the external environment, maintains the unique properties, and confers compatibility with organic substrates.