纳米二氧化硅是一种广泛用作填料的无机纳米颗粒，如沉淀法白炭黑、气相法白炭黑和硅溶胶等。目前，在未经预修饰的填料颗粒直接合成复合材料的工艺中，工业上常用的直接混炼法，存在颗粒分散性较差且无法保证修饰剂接枝率等问题；研究较多的溶胶凝胶法原位制备复合材料，存在原料成本较高和有机溶剂环境污染等问题。因此，对颗粒进行预修饰是必要的。但大部分有机修饰剂不溶于水，其修饰过程通常在有机溶剂体系中进行，同样存在污染与成本问题。本文针对工业上用量最大的硅烷偶联剂双-(γ-(三乙氧基硅)丙基)四硫化物(Si69)，利用Si69水解产物可溶于水的特性，通过微量注射的方法加快其水解速率，结合水相分散—喷雾干燥—热处理三步法，实现亲油疏水型修饰剂Si69的高效水相修饰，制备出高接枝密度、高分散性的纳米二氧化硅。由于制备工艺的局限，沉淀法白炭黑在生成的同时存在一定程度的硬团聚；由硅卤化物高温水解得到的气相法白炭黑成本较高。本文采用成本与沉淀法白炭黑相近且原始分散性较高的小粒径硅溶胶，对其进行Si69的水相有机修饰，将修饰结果与同样条件下修饰的沉淀法和气相法白炭黑进行对比，从修饰操作条件、疏水性变化、化学接枝密度和颗粒分散性等方面，证明经本工艺修饰后的硅溶胶要明显优于同样工艺修饰后的沉淀法白炭黑，且接近同样工艺修饰后的气相法白炭黑。在Silica:Si69 = 1:1时，修饰后的硅溶胶接触角为95.5°，接枝密度为3.25 nm-2，达到了高密度水相有机修饰的目标，且高于沉淀法白炭黑的90.4°和2.96 nm-2、以及气相法白炭黑的83.4°和3.07 nm-2。改变颗粒与修饰剂的质量比，在Silica:Si69 = 1:0.2时，修饰后的硅溶胶在非极性溶剂中的分散性稳定性最高，且高于Silica:Si69 = 1:1时的分散稳定性；在Silica:Si69 = 1:0.1时，气相法白炭黑、硅溶胶的Si69化学接枝质量分数达到90wt%。本文从颗粒表面带电特性、表面羟基特性以及表面酸碱性等方面，分析了这些特性对颗粒修饰过程与结果的影响。颗粒表面羟基活性和颗粒分散性是决定热处理前后接枝密度提升程度和热处理后最终能达到的接枝密度的主要因素。硅溶胶表面游离硅羟基占比最高，水相反应活性最强。沉淀法白炭黑颗粒表面总酸量最高，硅溶胶的平均酸性最强。三种颗粒进行对比，硅溶胶的中强Brønsted酸含量最高，反应活性最强，因此在水相反应阶段能达到的接枝密度最高，修饰完成后最终达到的接枝密度最高。
Nanosilica particles, such as precipitated silica, fumed silica, and colloidal silica, have been widely used as composite fillers. In a practical production process of directly mixing, the dispersion of silica is poor and the grafting amount of agent is unguaranteed. The in-situ synthesis of nanosilica particles in composite by using sol-gel method results in high costs and causes pollution problems during industrialization. Therefore, a pre-modification of the nanosilica particles is necessary to guarantee dispersion and affinity in the composite. The pre-modification of nanosilica is usually carried out in organic solvents since modifiers are mostly hydrophobic, which also costs high and causes pollution problems.An effective modification of nanosilica particles in aqueous with hydrophobic modifier bis[3-(triethoxysilyl)propyl]-tetrasulfide (Si69) is realized with high grafting density and high dispersion. The hydrolysis of Si69 is enhanced by microinjection for producing water soluble hydrolysate of Si69 to react with the silanol on the nanosilica particle surface. The modification is conducted through the route of aqueous mixing, spray drying and thermal treatment. Precipitated silica particles seriously aggregate during the synthesis and subsequent drying process. Fumed silica from hydrolyzing of silicon halide is only preferably used for special materials owing to its high cost. Colloidal silica displays a narrow particle size distribution and high dispersion in aqueous solutions, and costs similar to precipitated silica with the same particle size, ie. 20nm. Three kinds of nanosilica particles are modified by Si69 through the same modification procedures and the modification results are compared. The modified colloidal silica shows a better performance than the modified precipitated silica in terms of operating condition of modification, contact angle, grafting density and dispersion, and close to those of the modified fumed silica. The modified colloidal silica particles realize a high grafting density. At the mass ratio of Silica:Si69 = 1:1, the contact angle and grafting density of Si69-modified colloidal silica are 95.5° and 3.25 nm-2, respectively, higher than 90.4° and 2.96 nm-2 for precipitated silica and 83.4° and 3.07 nm-2 for fumed silica. The Si69 modified colloidal silica shows the best dispersion in the non-polar solvent at the mass ratio of Silica:Si69 = 1:0.2, better than that at the mass ratio of Silica:Si69 = 1:1. The chemical grafting proportion of Si69 reaches up to 90wt% at the mass ratio of Silica:Si69 = 1:0.1 for colloidal silica. The Si69 modified colloidal silica by this process is significantly better than modified precipitated silica and modified fumed silica in terms of the modification results.The different modification performances of the three kinds of silica are confirmed to be resulted from Zeta potential, silanol characteristics and Brønsted/Lewis acid sites on particle surface. The high Zeta potential of colloidal silica surface prevents the grafting reaction with negatively charged modifiers such as the anionic surfactant SDS. The different activities of surface silanol and the dispersion of the three particles affect the surface modification resulets. Colloidal silica has the most isolated silanol on the particle surface and the highest average activity of silanol. Colloidal silica has the highest capacity and overall intensity of medium-strong Brønsted acid, resulting in the highest activity of silanol. The modification for colloidal silica achieves the highest grafting density in aqueous mixing stage and the highest final grafting density among the three kinds of silica.
