硫酸钙晶须表面无机磷酸三钠-有机硬脂酸复合改性

Inorganic Trisodium Phosphate-Organic Stearic Acid Composite Modification of Calcium Sulfate Whisker

聚合物中天然橡胶(NR)的填充补强剂主要是炭黑等材料,炭黑等会在生产过程中造成大量的碳排放,因此可将硫酸钙晶须(CSW)作为新型补强剂应用到橡胶复合材料中替代炭黑。硫酸钙晶须表面是亲水性的,与有机基体相容性较差,本工作首先使用稀土石膏作为原材料制备硫酸钙晶须,然后采用无机磷酸三钠-有机硬脂酸对晶须表面进行改性。重点优化改性温度、时间和改性剂用量等工艺条件对改性效果的影响,实现了CSW稳定性和疏水性。采用X射线衍射、扫描电子显微镜、Fourier红外光谱、热重-差示量热分析和X射线光电子能谱等检测手段进行表征分析。结果表明:CSW加入量为20.00 g/L,硬脂酸加入量为CSW的6%,改性时间为15 min,改性温度为80℃和0.02 mol/L磷酸三钠为最佳改性工艺条件。改性后最佳产物的接触角达到104.23°,修饰层实现纳米级,其厚度为121.26 nm。改性机理为磷酸三钠在晶须表面生成CaHPO_(3)和Ca(OH)_(2),然后其与硬脂酸发生键合反应,生成C_(17)H_(35)COO-Ca-表面层。该改性工艺实现了CSW的疏水性,为纤维型晶须在NR中的应用提供了实验和理论基础。

Introduction 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.Methods 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).Results and diScussion 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.Conclusions 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 CaHPO_(3)and Ca(OH)_(2),then undergoes a bonding reaction with stearic acid,generating a nanoScale hydrophobic layer with a C_(17)H_(35)COO-Ca-structured nanoScale.The layer protects CSW from denaturation by the external environment,maintains the unique properties,and confers compatibility with organic substrates.