用激光法测定石墨微粉粒度分布特性时单宁酸所起的分散作用

DISPERSING ACTION OF TANNIC ACID FOR DETERMINING GRANULARITY DISTRIBUTION CHARACTERISTICS OF GRAPHITE POWDER WITH LASER METHOD

以水为分散介质,用激光法测定了球形天然石墨炭包覆(C-NG)试样和人造石墨(AG)试样的粒度分布特性。实验结果表明:由于石墨表面的疏水性,其颗粒在水中会聚集成团,因此不可能准确地测定试样的粒度分布特性。在水中添加具有复极性分子结构的单宁酸后,由于其非极性端定向吸附于石墨颗粒表面,极性端面向水,与水分子结合形成水分子层,从而阻挡了石墨颗粒的团聚,起到了分散(隔离)作用;单宁酸的添加量必须适当,过量时将使石墨颗粒表面的饱和单分子层叠加为双分子层,使石墨颗粒表面又逐渐转变为疏水性,从而使石墨颗粒之间再次发生团聚;测试时增加分散(搅拌)时间有利于石墨颗粒的分散,但影响甚微。

The granularity distribution characteristics of carbon sample coated natural graphite （C-NG） and artificial graphite （AG） sample were determined by laser method in the medium of water. The experimental results show that the graphite particles aggregate in water due to their hydrophobic surfaces. Therefore the granularity distributions characteristics cannot be determined correctly. Tannic acid possesses the molecular structure of complex polarity. After adding it into water, its molecules will adsorb on the surfaees of graphite particles with their nonpolar ends. The polar ends will face to the water and eombine with water molecules to form a layer of water, which will hinder the aggregation of graphite particles and produce the dispersing （separating） action. The addition of tannic acid must be appropriate. Excess of it will transform the surfaces of graphite particles from saturated monomolecular layers to bimolecular layers. The surfaces of graphite particles will become hydrophobie gradually and the graphite particles will aggregate again. Increasing the dispersing （stirring） time will be benefit to disper graphite partieles in water, but the influences is very little.