introduction
Pigments are designed and produced as insoluble particles that are used to impart color to a variety of materials. They can be divided into organic, inorganic, effect and function. The wide variety of pigment chemistries, end-use requirements, and wide color range present challenges for chemists in selecting the best pigment or additive for a specific application. The importance of proper pigment and additive selection in determining the performance of the finished coating as well as production efficiency cannot be overemphasized. Understanding the various pigment types, properties and chemical groups will help formulators determine effective pigment chemistry and manufacturing processes to avoid potential problems in final coatings and applications. This article focuses on pigment families and their applications in waterborne, solventborne and general purpose dispersions. Theories on processing, selection of wetting and dispersing agents, and differences between co-milling and single pigment dispersion are discussed.
Pigments vs Dyes - Does Solubility Matter?
Pigments are inorganic or organic colorants which are practically insoluble in the application medium, whereas dyes are colorants which are soluble in the application medium. Understanding this difference is an important concept to help choose the correct colorant. Dyes are organic molecules with brighter undertones, higher transparency, and they generally exhibit lower fastness to UV exposure than pigments. Organic pigments vary in chemical structure and surface treatment; some may, like dyes, lose their crystalline structure when exposed to dissolution conditions. This structural change will have a negative impact on pigment fastness properties. A good example is Pigment Yellow 74, widely used in architectural paints. When used in the above applications, this pigment has better fastness properties than most industrial coating applications. The difference in performance is due to the solvents used in these systems. Solvent based architectural coatings are based on acrylic and/or medium/long oil alkyd resins and the preferred solvent used in this application is mineral spirits. In water-based applications, acrylic emulsions or water-soluble acrylic resins are used. Pigment Yellow 74 is slightly soluble in mineral spirits, but insoluble in water (Figure 1), so it will retain its crystalline structure. However, in industrial coating applications, commonly used solvents are aromatics, esters and ketones, and Yellow 74 has a high solubility, which will negatively affect its fastness, such as thermal stability, weathering, overspray migration and opacity. Selecting the correct pigment for the paint formulation process should be the first step in the paint formulation process in order to formulate a stable system. It is recommended that formulators consult their pigment suppliers for appropriate pigment solubility information.
In addition to solubility, resistance to acids and bases, thermal stability and weather/light resistance, considerations should be taken to formulate a stable system that meets end-use requirements.
Figure 2 shows some important issues to consider during the initial formulation phase. Formulators should always keep in mind that the same pigment chemistry may not be balanced in all paint systems.
Organic vs. Inorganic - Are They Different?
Organic pigments are synthetic materials based on carbon, usually derived from petrochemicals. They are generally unstable at elevated temperatures and partially soluble in strong solvents, but insoluble in water. Inorganic pigments are metal salts and oxides, some natural and some synthetic, generally stable at elevated temperatures and insoluble in solvents. Due to their stable chemical structure, most inorganic pigments have better weather resistance, dispersibility and opacity than organic pigments, but they usually have lower chroma and tinting strength.
Pigment impurities
Pigments with the same color index identification produced by different manufacturers or by different manufacturing processes may have different properties despite being similar in color. This is due to impurities which may be unreacted raw materials, by-products, inorganic salts and pollutants in the water. These impurities may adversely affect weatherability, dispersion stability and solvent resistance.
To ensure that pigments are free from excess impurities, pigment manufacturers should test pH, conductivity, oil absorption and viscosity in specific test systems. They may even consider additional viscosity stability and dispersibility testing for certain pigments as needed. In addition to approving pigments for color, paint formulators should always determine the stability of the final paint to determine the appropriate pigment or pigments for a given system or application.
Factors Affecting Color Development
Pigment color development depends on five variables: dispersibility, energy input, process residence time, system components and pigment interactions. Proper attention to these variables will provide the highest probability of developing a stable formulation.
dispersion
Pigments need to be properly wetted, deagglomerated/dispersed and evenly distributed and stabilized (Figure 3) to achieve great color strength, gloss and hiding power. Stabilization of pigment dispersions takes time and energy. Dispersed pigments have a strong tendency to return to their original aggregated state. Due to this strong trend, the importance of proper selection of wetting and dispersing agents to obtain stable formulations cannot be overstated. Surfactants or wetting additives are generally defined as amphiphilic chemicals with low molecular weight, while dispersing additives are oligomers that stabilize pigments and avoid reagglomeration. Different types of wetting and dispersing agents are described in Table 1.
moisten
For proper wetting of the pigment, the air/solid interface needs to be replaced with a liquid/solid interface. Therefore, the less air is trapped in the system, the better the humidification will be. It's all about surface tension! For a liquid to wet a solid, its surface tension needs to be lower than the free surface energy of the solid. Therefore, liquids with low surface tension are more effective at wetting, which is why wetting additives are so valuable to formulators. They will reduce surface tension and adhere to the surface and coat the pigment to form an additive/liquid interface. In general, solvent-based systems wet pigments more readily due to the lower surface tension of the solvent compared to the higher surface tension of water (Table 2).
Depolymerization
During this stage of the dispersion process, pigment agglomerates are separated into smaller aggregates and primary particles. The lower the surface tension of the carrier into which the pigment is incorporated, the lower the energy required to disperse the pigment. Deagglomeration is achieved by using mechanical energy developed through the use of high speed dispersers and various types of grinding equipment. Cowles blades mounted on the shaft of a high speed mixer can be an effective method of dispersing pigments. A high shear blade is a highly recommended Cowles blade (Figure 4), which is excellent for pigment dispersion.
In order to ensure good laminar flow and improve dispersion efficiency, it is recommended that the diameter of the blade is about 1/3 of the diameter of the tank, and the blade is about 0.5 to 1.0 times away from the bottom of the tank. Recommended tip speeds for systems with viscosities between 70-100 Krebs units are between 4,000 - 6,000 fpm. The following equation can be used to determine tip speed: shaft RPM x 0.262 x blade diameter in inches. Pigments with a harder texture can be dispersed more finely by using media mills that generate significantly greater shear, such as horizontal, vertical and basket mills. In order to disperse pigments down to the nanoscale, it is recommended to use 0.3-0.5nm grinding media.
Stablize
Due to the increased surface area of the solid particles during the deagglomeration/grinding stage, the deagglomerated pigments require stabilization to avoid problems such as flocculation, color shift, settling and loss of stability. The stabilization process is carried out by introducing dispersing additives that achieve stabilization through the following mechanism.
Static stabilization
用于水基体系并且主要使用无机颜料，添加剂分子通过离子键合，氢键和/或偶极相互作用粘附到颜料表面，并使粒子通过静电力相互排斥。具有高导电性的颜料可能无法通过静电稳定来稳定。Zeta电位（浸入导电液体例如水中的固体颗粒表面与液体本体之间存在的电位差）用作关于配方如何稳定的参考。具有+ 30mV和-30mV之间的电势的颜料分散体具有很高的不稳定性。一旦将颜料掺入配方中，pH值就可以成为稳定性的良好指标; pH值在4到7之间的分散体。图5中的Zeta电位值更容易具有分散性/稳定性问题，因为它最可能在+30和-30mV之间。在某些情况下，可能需要添加pH调节剂。
建议在最终应用中pH低于4时分散体需要是酸性的配方，对于碱性分散体，pH高于7.5的配方是好的。Zeta电位越接近零，WB分散体越容易发生再凝聚。通过具有阳离子或阴离子分子基团的分散剂，例如季铵盐和烷基多胺（阳离子）或多元羧酸和磺化有机物质（阴离子）来实现静电稳定化。
空间稳定
用于水基和溶剂基体系中，添加剂锚定基团将粘附在颜料表面上。该系统的相容性取决于构成疏水部分的聚合物的功能链段。关于空间稳定化，添加剂将物理地降低颜料颗粒的流动性，因此避免/最小化絮凝或再附聚。这种类型的稳定化主要通过非离子分散剂完成。
表面活性剂选择量
太多或太少的表面活性剂可能对颜料分散体的稳定性有害。很好的水平的确定基于每平方米颜料表面所需的2-2.5mg聚合物分散剂的规则（图5）。需要首先应用此规则，然后建议配方设计师进行梯形研究，以According to粘度变化，溢流和漂浮的发生，烤箱稳定性以及必要时的冻融稳定性评估确定很好的水平。
分散剂选择
建议的分散剂粘合剂类型取决于颜料表面。以下建议可帮助配方设计师为所评估的颜料选择有效的分散剂化学品。
•？有机颜料（芳香表面处理） - 建议含有苯基或萘基的分散剂。
•？无机颜料（氧化物，硫化物，硅酸盐等） - 建议使用含有酸性基团的分散剂，即磷酸盐，羧酸盐或硫酸盐。
•？炭黑（重氮表面处理） - 含氮分散剂。
颜料分散性 - 共研磨与单一颜料分散
颜料化学品的质地不同，实现全色显色所需的能量数量也各不相同。确定颜料很好的分散时间的一个好方法是进行分散性研究，其中配方设计师将通过评估其颜色特性来确定将颜料发展到其全部强度所需的时间和能量。图6显示了所选PB 15：2，PG 7，PY 74和PY 65颜料的分散性。以30分钟的间隔评价这些颜料的强度和颜色，其中以15分钟的研磨时间制备的分散体用作标准。将颜料研磨至超过其很好的水平，这可在观察到颜色强度损失时得到证实。这种类型的评估对于确定所选配方中颜料的很好的处理时间是重要的。从图6中可以看出，一些颜料比其他颜料更快地发挥其强度和颜色。当共研磨颜料时，这是考虑的关键因素，因为通过这种方式可能无法实现很好的的颜色发展和一致性。因此，建议单一颜料分散体作为获得给定颜料的全色显色值的很好的方法。
另外，并非所有颜料在特定涂料配方中具有相同的行为，因为它们的物理（即表面积，吸油，粒度）和化学性质（即化学结构，表面处理）的变化如表3所示。应该注意的是，各种颜色指数的物理性质之间没有直接的相关性，这提供了为什么不建议为什么共同分散颜料的额外推理。
结论
颜料是涂料配方中的关键组分，适当的选择和分散对于确定涂料的性能重要性无庸赘述。在最近一项关于过去两年收到的技术援助申请的研究中，注意到近80％的配方问题是通过改变或改变配方中使用的颜料来解决的。假设所有颜料从一种配方到另一种配方的性能相同是错误的。对于涂层开发和制造过程，这种假设也可能证明是昂贵的。实现具有很好的性能和价值的稳定涂层的关键是：适当的颜料选择，适当的添加剂选择以及实现完全分散的适当方法。
参考文献
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3.表面涂层。伦敦：查普曼和霍尔，1993年。打印。
4. "Surface Tension Values of Some Commonly Used Test Liquids for Surface Energy Analysis." Np, nd Web June 29, 2017
thank you
The authors would like to thank Frank Lavieri, Mark Freshwater, Ralph Svenningsen, and Belinda DeSousa of LANSCO COLORS for their contributions.
