Preparation of anhydrite whiskers from red gypsum and application in polyvinyl chloride

Using red gypsum (RG) as raw material, two types of anhydrite whiskers with aspect ratios of 10 (10AW) and 25 (25AW) were prepared by the hydrothermal method at solid–liquid mass ratios of 1:4 and 1:10, respectively, which were used as fillers in polyvinyl chloride (PVC) after being modified by stearic acid. The effect of stearic acid amount on the oil absorption values of the modified whiskers was studied. Additionally, the effect of the aspect ratio and the amount of filler on the mechanical properties of PVC composites was also investigated. The results showed that the optimum amounts of stearic acid for 10AW and 25AW were 4 wt%, the oil absorption value of modified 10AW (M-10AW) was 53.9 g/100 g, and that of modified 25AW (M-25AW) was 55.6 g/100 g. The melt flow rate (MFR) and tensile strength of the two PVC composites decreased as the doping amount of anhydrite whiskers increased, while the elongation at break, flexural strength, and impact strength initially increased and then decreased. The optimal addition amount was 5 wt% for both composites, and the mechanical properties of PVC composites were better with the addition of M-25AW compared to M-10AW. The elongation at break, flexural strength, and impact strength of PVC composites increased by 51.1%, 18.2%, and 19.3% on adding 5 wt% M-25AW, respectively.