An Effective Method to Disperse Pigments

Summary

As problems associated with pigment dispersions become more frequent and the requirements for color reproducibility become more stringent, new requirements are placed on pigment wetting and dispersing agents to address these pigment stability issues. Over the past 20 years, advances in polymeric dispersants have made remarkable strides in the process of wetting and stabilizing pigments. As a result, a number of dispersant technologies that better stabilize small pigment particles and better prevent flocculation have emerged on the market. This article describes the pigment and dispersant interface and how coordination and optimization of these interactions will lead to better pigment dispersion.

introduce

Pigments from natural sources - materials such as clay, earth tones and minerals - were used to color paints before the development of synthetic pigments. Vegetable oils with a high lecithin content are used to moisturize natural pigments. Since then, technology has evolved very rapidly.

Dispersing pigments is sometimes seen as an art rather than a technical process. This is mainly due to the multiple steps that occur during the dispersion process. These processes can seem confusing, but breaking the steps down into a structured way can make them easier to understand and help find solutions faster.

The following research explains the science behind dispersing pigments and fillers, with the hope of leading to a better understanding of how to stabilize them more effectively. This study discusses the different types of wetting and dispersing additives that can be used to stabilize two notoriously difficult pigments.

Pigment and Additive Interactions

Pigments are tiny solid particles that have the ability to refract light.1 Pigments have two primary functions: the optical function of providing color, opacity, and luster; and the protective function of the underlying coating.

Pigment structure

Pigments come in three forms: primary particles, aggregates and agglomerates. Primary particles are individual small particles formed during synthesis.2 These are the smallest components of pigments and fillers, and they consist primarily of cuboid, rod-shaped, and spherical particles (Figures 1-2). During calcination, primary particles can form together to produce larger particles through chemical bonding. These particles are called aggregates, which are regions of an ordered lattice connected face-to-face from primary particles. Alternatively, particles formed by physical bonds rather than chemical bonds are called agglomerates. These particles are connected by edges, resulting in less surface area and more difficulty wetting the pigment.

Organic and Inorganic Pigments

Pigments are broadly classified as organic or inorganic pigments. Inorganic pigments are used for color properties as well as other properties such as antistatic and corrosion resistance properties. Inorganic pigments generally have a high refractive index, which means they have a greater ability to scatter light. Therefore, these pigments are very good at "hiding" the surface under the coating. Oxides such as titanium dioxide and iron oxide are typical examples of inorganic pigments. Carbon blacks are technically classified as inorganic pigments, but they require different anchor groups to adsorb onto the pigment surface

Organic pigments are intensely colored, so they are incorporated only for their color properties. Organic pigments are classified into azopigments, polycyclic pigments, and anthraquinone pigments.4 Their particle size tends to be smaller than that of inorganic pigments, so they are more transparent. They have a tendency to dissolve when moisture is present, which causes them to migrate and chalk to surfaces. Due to their smaller particle size, organic pigments are generally more difficult to disperse than inorganic pigments. Because most inorganic pigments have polar surfaces, they wet more easily.

decentralized technology

In order to understand wetting and dispersing agents, a basic understanding of the dispersion process is required. Dispersants stabilize deflocculated pigment particles. To stabilize these particles, a dispersant needs to be able to overcome the van der Waals attractions that continually move the pigment particles back together.2 The pigment dispersion process can be broken down into three steps: wetting, deagglomeration, and stabilization.

• Step 1: Wetting

The first step in the dispersion process involves wetting the pigment with a liquid. The liquid spreads over the surface of the paint and fills the voids and pores of the paint, expulsing any remaining air pockets. 2 For a paint to be wetted by a liquid, the surface tension of the liquid needs to be lower than the surface energy of the paint. .1 This interaction between pigment and liquid is described below by Young's equation. Liquids with low surface tension generally wet out pigments better than liquids with higher surface tension.

• Second step: depolymerization

After the pigments are wetted, they are broken down to obtain small particle sizes with large surface areas. This produces higher color strength, which is more cost-effective for paint manufacturers.

To grind to a smaller particle size, more energy is required. To break up agglomerates and increase the surface area (ΔA), an increase in energy input (ΔW) is required (Equation 2).1 This energy is proportional to the surface tension (Y) of the dispersion. The smaller the surface tension, the larger the surface area for a given amount of energy.3 

When dispersed, agglomerates are broken down into primary particles and small aggregates. When breaking up an aggregate, only the physical bonds are broken. Figure 3 shows typical energy content ranges for 1 mole of different types of chemical and physical bonds.

These energies range from 40-50 kJ/mol, which means that 40,000 to 50,000 joules are required to break these physical bonds.4 If you grind aggregates, you need about 600 to 1000 kJ/mol to break these chemical bonds.4 This is about grinding Agglomerates require ten times as much energy.

• Step 3: Stabilization

Therefore, when large surface areas and small pigment particles are present, the energy is very high and thermodynamically unstable. Solid particles will always tilt toward each other in Brownian motion to minimize their surface area and return to a more stable lower energy state2 (Figure 4).

If the particles are not well stabilized, they will flocculate together. In order to achieve good pigment stability, the dispersant needs to be able to adsorb on the surface of the pigment. Therefore, additives need to have anchor groups with high affinity for the pigment surface.

Stabilization of pigments can be achieved by electrostatic, steric or electronic stabilization (Figure 5).

Electrostatically stabilized for use in aqueous formulations with high dielectric charge. Dispersing additives adsorb on the pigment surface and dissociate into anionic and cationic moieties. This creates an electric double layer that prevents the pigments from flocculating together through electrostatic repulsion of similar charges

In contrast to electrostatic stabilization, steric stabilization uses polymer side chains to keep pigment particles stable in dispersion. When the pigment particles come closer together, the polymer side chains restrict their motion and reduce the entropy. 4 The result is a repulsive force between the two particles. These interactions also restrict the movement of viscosity-generating particles.

In some cases, it is not enough to use only static or only spatial stabilization. Pigment dispersions have complex requirements, so it is sometimes necessary to combine two stabilizers to produce electronic stabilization.

Wetting and Dispersing Agents

The terms wetting agents, wetting additives, dispersants or wetting and dispersing additives are often used without fully understanding their exact definitions. Each has very important differences in chemical structure and function.

Wetting agents are low molecular weight amphiphilic molecules having hydrophilic and hydrophobic segments. They help reduce surface tension and wet the surface, but most of the time, pigment stabilization is not achieved. Dispersants are oligomers or polymers that help stabilize pigments and fillers. At the heart of every dispersant, there needs to be a wetting agent to facilitate the first step in the dispersion process - wetting the pigment. The main difference between the two technologies is that dispersants utilize anchor groups and polymer side chains to stabilize pigments.

polymer side chain

The polymer side chains help to adsorb the dispersant onto the pigment surface. The side chain needs to be soluble in the medium. If they are insoluble, they may collapse onto the pigment surface causing flocculation.

Medium molecular weight polymers are good. If the quality is too low, in the case of wetting agents, it may not be effective enough to stabilize the pigment. On the other hand, if the mass is too large, then it may be incompatible and cause the viscosity to rise.

anchor

Anchor groups Anchoring groups are located at the ends of the polymer chains to attach to the surface of the pigment. Without these anchor groups, the polymer side chains would be useless. Specific chemical groups serve as anchors for certain pigment types.

Dispersants with aromatic rings have an affinity for the surface of organic pigments. They are adsorbed on the surface by van der Waals forces. Dispersants with hydroxyl, carbonyl or carboxyl groups have high affinity for the surface of inorganic pigments. They adsorb on surfaces through hydrogen bonding or induced dipole interactions. The amine groups have a high affinity for the carbon black surface. Without nitrogen, the applicability of carbon black is not high.

experimental design

In this study, two of the most difficult pigments to process were selected for the preparation of pigment concentrates: yellow iron oxide and organic violet (Table 1-2). Four 100% active additives were selected for testing, with additive solids levels in the pigment ranging from 10% to 30%.

• Additive A: low molecular weight alkoxylate

• Additive B: Medium weight polyether with aromatic groups

• Additive C: Medium weight polyether phosphate with acid groups

• Additive D: High molecular weight polymer with hyperbranched polyester chains with aromatic and acid groups

All samples were packaged in 8 oz glass jars containing 100 grams of material. Add glass beads with a size of 2.4-2.9 mm as grinding media in a ratio of 1:1. Formulations were processed on a Skandex shaker for 1 hour. After dispersion was complete, the sample was cooled to room temperature and filtered through a mesh cone filter.

Yellow Iron Oxide Formula

The yellow iron oxide formula consists of 55% pigment loading and 10% additive solid pigment (ASOP). Due to increased environmental demands, an exempt solvent (Oxsol 100) was chosen for this study.

Pigment Violet Recipe

Price is an important driving factor for the pigment concentrates market. Because organic pigments are usually more expensive, titanium dioxide is added to this formulation to keep the price down. Violet has a pigment loading of 6%, titanium dioxide at 30%, and 30% solids added to the pigment.

Yellow Iron Oxide Results

在配备有PP35 Ti L03 089板的Haake Rheostress 1流变仪上测量粘度，并在室温下在0.20mm间隙下测试（图6）。初始粘度研究表明，坯料和样品A具有非常好的剪切增稠行为，这意味着它们的粘度随剪切速率而增加。这对于颜料浓缩物来说并不称心，因为它可能会在着色白色基础涂料时堵塞配料机。样品B具有剪切稀化粘度直至1000 / s，其中粘度升高。这表明没有足够的分散剂来润湿剩余的颜料。样品C和D具有剪切稀化曲线，粘度对于样品D总体上是最低的。这对颜料浓缩物供应商是有吸引力的，因为低粘度意味着更多的颜料负载是可能的。

在处理一小时后，坯料，样品A和样品B没有合适的研磨（图7）。对于大多数颜料浓缩物供应商来说，合适的研磨是6 Hegman或更高。样品C和D均以Hegman标度研磨超过7。样品C具有较高的粘度，但在1小时后具有良好的研磨性; 这表明分散剂负载没有得到优化。进一步的工作将继续使用这种分散剂创造出优化的配方。对于样品D，存在非常低的粘度和高研磨度，表明这是该配方的合适的分散剂负载量。还将进行颜色接受和颜色强度研究以确认添加剂的性能。

紫 - 二氧化钛结果

紫色样品的粘度参数与前面提到的黄色氧化铁制剂的粘度参数相同（图8）。对于坯料和样品D，初始粘度非常低。样品A在低剪切速率下具有非常高的粘度，但是随着剪切速率增加，粘度降低。样品B和C都具有剪切稀化曲线。

使用Beckman Coulter LS 13-320激光衍射粒度分析仪测试样品的粒度（图9）。在研磨1小时后，坯料，样品B和样品C显示出最低的中值粒径。然而，对于有机颜料而言，结果仍然太高。（注意：这种配方中也含有二氧化钛。这可能会影响粒度测量。）这些配方可能需要更多的能量来分解剩余的附聚物和聚集体。将在更长的研磨时间段内进行额外的测量，以确定这是否会降低粒度。

使用4体积％的商业级建筑涂料评价颜料分散体的颜色接受性和颜色强度（图10-11）。在Leilta 3B卡上以3密耳湿度进行刮涂并使其风干，然后使用X-Rite 962分光光度计进行颜色测量。

坯料具有高色强度，但也具有非常高的ΔE。这表明虽然坯料可能看起来稳定，但在颜料被破坏后，它会失去稳定性。样品A和样品D都具有较低的色强度和较高的ΔE值。这对于涂料制造商来说是不可接受的，因为不仅颜料不能用于着色强度，它还会在涂覆时改变颜色。样品B和样品C具有很高的色强度和小于1的ΔE。在这种情况下，这些将是最稳定和很有吸引力的配方。

对于该配方，总体样品B和样品C具有很好的性能。将进行更多研究以优化这些配方。另外，所有分散样品将在50℃下进行高温测试1周。然后将再次测试粘度，粒度，颜色接受度和颜色强度以确认每种添加剂的性能。

结论

黄色氧化铁的结果表明，样品D是该配方的很好的分散剂。发现与超支化聚合物链组合的较高分子结构对于配方的粘度特别有用。这允许在研磨时间的一小部分实现更高的颜料负载。通过使用较合适的添加剂，更少的机器时间，更少的能量消耗和更少的劳动力成本大大降低了加工的复杂性。这有助于以更具成本效益的方式配制颜料浓缩物和涂料。

另一方面，样品D在紫 - 二氧化钛配方中表现不佳。很有可能的是，分子量太高，聚合物侧链不溶，导致与该系统不相容。这里，样品B和样品C在色强度和颜色接受性方面表现良好，并且通过充分利用颜料产生了经济的研磨。这两种添加剂使用“受控絮凝”基质保持紫色和二氧化钛稳定（图12）。这允许两种颜料结合在一起以通过使用分散剂作为两者之间的桥来防止进一步的絮凝。

颜料和添加剂相互作用可能难以确定，因为有很多变量在起作用。然而，了解分散剂和颜料结构以及作用于它们的力的能力可以更好地理解在开发油漆和颜料浓缩物时如何协调两者。CW 

参考文献

1.赢创公司，TEGO Chemie Service GmbH。TEGO Journal，第4版; 2012.第79-89页。

Muller, Bodo. Learn about additives. European Coatings Literature, Vincentz Network GmbH & Co. : 2010 Hannover, Germany

3. Heilen, Wernfried. Waterborne paint additive. European Coatings Literature, Vincentz Network GmbH & Co. : Hannover, Germany, 2009.

Winkler, Jochen. Disperses pigments and fillers. European Coatings Literature, Vincentz Network GmbH & Co. : Hannover, Germany, 2012.