Application of rutile titanium dioxide in Waterborne Coatings

一、 钛白粉在水性涂料中分散

1. Dispersion of titanium dioxide in water-based coatings

在涂料生产的过程中，钛白粉分散的好坏直接影响着水性涂料的储存稳定性、涂膜外观和涂装作业难易等。众所周知，钛白粉的分散包括钛白粉的润湿、机械粉碎和分散稳定性三个过程，在水性涂料的生产过程之中，钛白粉颜料的润湿过程是一个钛白粉—空气、钛白粉—钛白粉粒子界面被钛白粉一树脂溶液界面替换的过程，决定润湿效果的因素主要有树脂溶液在钛白粉颜料粒子表面的吸附，树脂向钛白粉颜料凝聚体空隙问的渗透。粉碎过程是钛白粉颜料粒子的凝聚体或者附聚体通过剪切力或者冲击力破坏为细小粒子的过程，它不仅与设备有关系，还与钛白粉颜料的聚凝状态相关。分散稳定是粒子问产生斥力而阻止粒子间再聚集的过程。

In the process of coating production, the dispersion of titanium dioxide has a direct impact on the storage stability of waterborne coatings, the appearance of coating and the difficulty in coating operations. It is well known that the dispersing of titanium white powder includes three processes of wetting, mechanical crushing and dispersing stability of titanium white powder. In the production process of water based coatings, the wetting process of titanium white powder is a process of replacing the interface of titanium white powder, air, titanium white powder and titanium white powder, to the interface of a titanium white powder and a tree fat solution. The main factors of wet effect are the adsorption of resin solution on the surface of titanium dioxide pigment particles and the infiltration of resin toward titanium dioxide pigment aggregates. The process of pulverization is the process of breaking into fine particles by the shear force or the impact force of the coagulant or polymer of the titanium pigment particles. It is not only related to the equipment, but also is related to the coagulation state of the titanium pigment. Dispersion stability is the process of particle repulsion to prevent particles from reassembling.

1．1双电层理论

1.1 double layer theory

双电层理论就是静电稳定机制，通过调整pH值或者加入电解质，使颗粒表面产生一定量的表面电荷，以增大双电层厚度和颗粒表面的Zeta电位值，使颗粒间产生较大的排斥力，进而实现颗粒的稳定分散。尽管钛白粉的Zeta电位有助于分散稳定性，但是，与位阻的排斥力相比，其作用还是相当的小的。通常的工艺要求浆液具有较高的固含量、较低粘度同时具有较好的稳定性。悬浮液的稳定性是其基本性质：首先粒子不能太大，否则重力会导致快速地沉降；另一重要因素是粒了间的吸引力。不管是否存在其它力，范德华力都存在于粒子间。如果粒子引力足够大，彼此就会相互粘附，导致粒子束快速沉降(如：絮凝)。通常采用的防止絮凝的方法是在粒子表面引入相互排斥的力：电荷间的排斥力(电稳定)、吸附的高分子问的斥力(空间稳定)、或两者的结合(空间稳定)。双电层稳定机制，即通过调节pH值使颗粒表面带上一定的表面电荷，形成双电层；通过双电层之间的排斥力使粒子之间的引力大大降低，从而实现粉体的分散。钛白粉的表面电性是由粉体表面的荷电离子，如OH-、H+等决定的，当钛白粉分散到溶液中的电性还与溶液的pH之一即溶液的离子类型有关系。粉体表面的荷电性影响颗粒之间、颗粒与无机离子之间、表面活性剂离子以及其他化学。

The double layer theory is the electrostatic stabilization mechanism. By adjusting the pH value or adding the electrolyte, the surface charge on the surface of the particle is produced to increase the thickness of the double layer and the Zeta potential value on the surface of the particle, so that the particles can be repulsive and the particles can be dispersed steadily. Although the Zeta potential of titanium dioxide contributes to the dispersion stability, it still has a very small effect compared with steric hindrance. The usual process requires that the slurry has higher solid content, lower viscosity and better stability. The stability of the suspension is the basic property: first particles can not be too large, or gravity will cause rapid settlement; another important factor is the attraction between particles. No matter the existence of other forces, the van Edward force exists between particles. If the particle gravity is large enough, they will adhere to each other, resulting in the rapid settling of particle bundles (such as Xu Ning). The commonly used method of preventing flocculation is to introduce mutually exclusive forces on the surface of the particle: the repulsive force between the charge (electric stability), the repulsion of the adsorbed polymer (space stability), or the combination of the two (space stability). The stability mechanism of the double layer is to make the surface charge on the surface of the particle by adjusting the pH value to form a double layer of electricity, and the particle's gravity is greatly reduced by the repulsion force between the double layers, thus the dispersion of the powder is realized. The surface electricity of titanium dioxide is determined by the charge ions on the surface of the powder, such as OH-, H+ and so on. The electrical property of the titanium white powder in the solution is also related to one of the pH of the solution, that is, the ionic type of the solution. The charge on the surface of the powder affects particles, particles and inorganic ions, surfactant ions and other chemicals.

根据双电层理论我们可以比较清晰的看出所谓的Zeta电位并不是颗粒的界面电位只是吸附层外侧的电位，Zeta电位与颗粒紧密层的电位十分的接近，可以近似为相等。热力学电位总是大于Zeta电位，吸附层越厚毛电位越低。如果颗粒表面上的负电荷数和固定层吸附的正电荷数相等，Zeta电位就变成了零，这时对应的溶液的pH值称为等电点。当溶液的pH值大于等电点时，粉体表面荷负电，小于等电点时，荷正电。当然，影HI,Zeta电位的因素很多，如粉体的化学成分、pH值、表面缺陷、溶剂、粒度分布等等。部分无机粉体的等电点如表1所示。

According to the double layer theory, we can clearly see that the so-called Zeta potential is not the potential of the particle's interface potential only on the outside of the adsorption layer, and the potential of the Zeta potential is very close to the potential of the compact layer of the particles, and it can be approximately equal. The thermodynamic potential is always greater than the Zeta potential. The thicker the adsorption layer is, the lower the Mao potential is. If the number of negative charges on the surface of the particle is equal to the number of positive charges adsorbed on the fixed layer, the Zeta potential becomes zero, and the pH value of the corresponding solution is called the isoelectric point. When the pH value of the solution is greater than that of the isoelectric point, the charged power on the powder surface is positively charged when it is smaller than the isoelectric point. Of course, there are many factors affecting HI, Zeta potential, such as chemical composition, pH value, surface defect, solvent, particle size distribution and so on. The isoelectric point of a part of the inorganic powder is shown as shown in Table 1.

1．2表面化学特性

1.2 surface chemical properties

实际上在水性涂料体系里面，Zeta电位是随着钛白粉的单位吸附量的增加而迅速增大的，但是当Zeta电位升高到一定的程度后，其增大的幅度逐渐的变缓。从而表明钛白粉颗粒必须达到一定的吸附量，才能够具有一定的电荷稳定性，这一点对实际的水性涂料配方设计有着深远的意义。随着钛白粉粒子的细微化，表面原子所占比例增加。但表面原子受到的是不对称力，即与其邻近的内部原子的非对称价键力和其它原子的范德华力的作用，其价键是不饱和的。根据钛白粉在水溶液中的pH值不同，可带正电、负电和电中性。当pH比较小时，粒子表面形成M-OH2，导致粒子表面带正电；当pH高时粒子表面形成M-O键，使粒子表面带负电；如果pH值处于中间值，则表面形成M-OH，粒子呈电中性。在不同的pH下，分散在水中的粉体的表面化学特性就由吸附到颗粒表面的H+和OH-粒子所决定。

In fact, in the water coating system, the Zeta potential increases rapidly with the increase of the unit adsorption capacity of the titanium dioxide, but when the Zeta potential increases to a certain degree, the increase gradually slows down. It shows that the titanium dioxide particles must reach a certain amount of adsorption, and can have a certain charge stability. This point is of profound significance to the design of the actual water-based coating formulation. The proportion of surface atoms increases with the fineness of titanium dioxide particles. However, the surface atom is subjected to asymmetric force, that is, its valence bond is unsaturated with the asymmetric valence bond force of its adjacent internal atoms and the van Edward force of other atoms. According to the pH value of titanium dioxide in aqueous solution, it can be positively charged, negatively charged and electrically neutral. When pH is relatively small, the surface of the particle forms M-OH2, causing the surface of the particle to bring positive electricity. When the pH is high, the particle surface forms the M-O bond, which makes the particle surface negative. If the pH value is in the middle value, the surface forms M-OH and the particle is electrically neutral. Under different pH, the surface chemical properties of powders dispersed in water are determined by H+ and OH- particles adsorbed onto the surface of particles.

1．3水性涂料分散剂对钛白粉粉体特性的影响

1.3 the influence of dispersant on the characteristics of titanium dioxide powder

在钛白粉含量不变的情况下，单位的吸附量与分散剂的含量成正比，根据公式a=Ax／(1+Ay)=x／(A-1+Y)可以推导出单位钛白粉上的吸附量。根据胶体稳定性的DLVO理论，胶体质点之间存在范德华吸引作用，而质点在相互接近时又因双电层的重叠而产生排斥作用，胶体的稳定性取决于质点之间吸引与排斥作用的相对大小。悬浮体的分散性和流动性与其Zeta电位值有很大关系，一般地，Zeta电位绝对值越大，越有利于悬浮体分散。在浆料的制备过程中，为获得高分散性稳定性好的浆料，通常选用一些高分子电解质来改善其悬浮性能。引入

When the content of titanium dioxide is constant, the adsorption quantity of the unit is directly proportional to the content of the dispersant, and the amount of adsorption on the titanium white powder can be deduced according to the formula a=Ax / (1+Ay) =x / (A-1+Y). According to the DLVO theory of colloid stability, there is a Fan Dehua attraction between the colloid particles, and the particles are repulsive when the particles overlap each other, and the stability of the colloids depends on the relative size of the attraction and repellent between the particles. The dispersion and fluidity of suspension have a great relationship with the Zeta potential value. Generally, the larger the absolute value of Zeta potential is, the more conducive to the dispersion of suspension. In the process of slurry preparation, some polymer electrolytes are usually selected to improve the suspension performance in order to obtain highly dispersed and stable slurry. Introduce