Selection principles, dosage and usage of powder coating
Aug 31, 2024
The surface modification of powders is largely achieved through the action of surface modifiers on the powder surface. Therefore, the formulation of surface modifiers (variety, dosage and usage) has an important influence on the modification effect of the powder surface and the application performance of the modified product.
The formulation of surface modifiers is highly targeted, that is, it has the characteristics of "one key opens one lock", which mainly includes the selection of varieties, determination of dosage and usage.
1. Screening of surface modifiers
The main considerations for selecting surface modifier varieties are the properties of powder raw materials, the purpose or application field of the product, and factors such as process, price and environmental protection.
(1) Properties of powder raw materials
The properties of powder raw materials are mainly acidity, alkalinity, surface structure and functional groups, adsorption and chemical reaction characteristics, etc. Surface modifiers that can react chemically or chemically adsorb with the surface of powder particles should be selected as much as possible, because physical adsorption is easy to desorb under strong stirring or extrusion during subsequent application. For example, the surfaces of acidic silicate minerals such as quartz, feldspar, mica, and kaolin can bond with silane coupling agents to form a relatively strong chemical adsorption; however, silane coupling agents generally cannot chemically react or chemically adsorb with carbonate alkaline minerals, while titanate and aluminate coupling agents can chemically adsorb with carbonate alkaline minerals under certain conditions and to a certain extent. Therefore, silane coupling agents are generally not suitable for use as surface modifiers for carbonate alkaline mineral powders, such as light calcium carbonate and heavy calcium carbonate.
(2) Product use
The use of the product is the most important consideration in selecting a surface modifier. Different application fields have different technical requirements for the application performance of powders, such as surface wettability, dispersibility, pH value, hiding power, weather resistance, gloss, antibacterial properties, UV protection, etc. This is one of the reasons why surface modifiers should be selected according to their use. For example, inorganic powders (fillers or pigments) used in various plastics, rubbers, adhesives, oily or solvent-based coatings require good surface lipophilicity, that is, good affinity or compatibility with organic polymer base materials, which requires the selection of surface modifiers that can make the inorganic powder surface hydrophobic and oleophilic; inorganic pigments used in ceramic blanks are not only required to have good dispersibility in the dry state, but also require good affinity with inorganic blanks and be able to be evenly dispersed in the blanks; surface modifiers for inorganic powders (fillers or pigments) used in water-based paints or coatings require good dispersibility, sedimentation stability and compatibility of the modified powders in the water phase. The selection of inorganic surface modifiers is mainly based on the functional requirements of the powder material in the application field. For example, to make titanium dioxide have good weather resistance and chemical stability, SiO2 and Al2O3 should be used for surface coating (film), and to make white mica pigments have good pearlescent effect, TiO2 should be used for surface coating (film). At the same time, different application systems have different components. When selecting a surface modifier, the compatibility and compatibility with the components of the application system must also be considered to avoid the failure of other components in the system due to the surface modifier.
(3) Modification process
The modification process is also one of the important considerations for selecting a surface modifier, such as temperature, pressure and environmental factors. All organic surface modifiers will decompose at a certain temperature. For example, the boiling point of silane coupling agent varies between 100 and 310 °C depending on the type. Therefore, the decomposition temperature or boiling point of the selected surface modifier is preferably higher than the processing temperature during application.
Currently, the surface modification process mainly adopts two methods: dry method and wet method. For the dry process, there is no need to consider its water solubility, but for the wet process, the water solubility of the surface modifier must be considered, because only when it is soluble in water can it fully contact and react with the powder particles in a wet environment. For example, stearic acid can be used for dry surface modification of calcium carbonate powder (either directly or after dissolving in an organic solvent). However, in wet surface modification, if stearic acid is added directly, it is not only difficult to achieve the expected surface modification effect (mainly physical adsorption), but also the utilization rate is low. The surface modifier is seriously lost after filtration, and the organic matter emission in the filtrate exceeds the standard. Other types of organic surface modifiers also have similar situations. Therefore, for surface modifiers that cannot be directly soluble in water but must be used in a wet environment, they must be saponified, ammonized or emulsified in advance so that they can be dissolved and dispersed in aqueous solution.
(4) Price and environmental factors
Finally, the selection of surface modifiers should also consider price and environmental factors. On the premise of meeting the application performance requirements or optimizing the application performance, try to use a cheaper surface modifier to reduce the cost of surface modification. At the same time, pay attention to choosing a surface modifier that does not pollute the environment.
2. Dosage of surface modifier
Theoretically, the dosage required to achieve monolayer adsorption on the particle surface is the optimal dosage, which is related to the specific surface area of the powder raw material and the cross-sectional area of the surface modifier molecule, but this dosage is not necessarily the dosage of the surface modifier when 100% coverage is achieved. For inorganic surface coating modification, different coating rates and coating layer thicknesses may show different characteristics, such as color, gloss, etc. Therefore, the actual optimal dosage should be determined through modification tests and application performance tests. This is because the dosage of the surface modifier is not only related to the dispersion of the surface modifier during surface modification and the uniformity of coating, but also to the specific requirements of the application system for the surface properties and technical indicators of the powder raw materials.
For wet modification, the actual coating amount of the surface modifier on the powder surface is not necessarily equal to the dosage of the surface modifier, because there is always a part of the surface modifier that fails to react with the powder particles and is lost during filtration. Therefore, the actual dosage should be greater than the dosage required to achieve monolayer adsorption.
3. Method of using surface modifiers
The method of using surface modifiers is one of the important components of the surface modifier formula and has an important impact on the surface modification effect of powders. A good method of use can improve the dispersion degree of surface modifiers and the surface modification effect of powders. On the contrary, improper method of use may increase the amount of surface modifiers used and the modification effect cannot achieve the expected purpose.
The method of using surface modifiers includes preparation, dispersion and addition methods as well as the order of adding when using more than two surface modifiers.
(1) Preparation
The preparation method of surface modifiers depends on the type of surface modifiers, modification process and modification equipment. Different surface modifiers require different preparation methods. For example, for silane coupling agents, silanols are bonded to the surface of powders. Therefore, to achieve a good modification effect (chemical adsorption), it is best to hydrolyze before adding. For other organic surface modifiers that need to be diluted and dissolved before use, such as titanate, aluminate, stearic acid, etc., corresponding organic solvents such as anhydrous ethanol, toluene, ether, acetone, etc. should be used for dilution and dissolution. For organic surface modifiers such as stearic acid, titanate, aluminate, etc. that are not directly soluble in water used in the wet modification process, they should be saponified, ammonized or emulsified in advance to become products that can be dissolved in water.
(2) Addition method
The best way to add surface modifiers is to make the surface modifiers and powders contact evenly and fully to achieve a high degree of dispersion of the surface modifiers and uniform coating of the surface modifiers on the particle surface. Therefore, it is best to use a continuous spraying or dripping (addition) method linked to the powder feeding speed. Of course, only a continuous powder surface modifier can achieve continuous addition of surface modifiers.
The preparation method of inorganic surface modifiers is relatively special, and it is necessary to consider multiple factors such as solution pH, concentration, temperature, and additives. For example, when titanium dioxide is coated on the surface of muscovite, titanyl sulfate or titanium tetrachloride must be hydrolyzed in advance.
(3) Order of adding drugs
When more than two surface modifiers are used to treat the powder, the order of adding drugs also has a certain influence on the final surface modification effect. When determining the order of adding surface modifiers, we must first analyze the role of each of the two surface modifiers and the mode of action on the powder surface (whether it is mainly physical adsorption or chemical adsorption). Generally speaking, the surface modifier that plays the main role and is mainly chemically adsorbed is added first, and the surface modifier that plays the secondary role and is mainly physical adsorption is added later.
For example, when a coupling agent and stearic acid are mixed, generally speaking, the coupling agent should be added first and the stearic acid should be added later, because the main purpose of adding stearic acid is to enhance the hydrophobicity and lipophilicity of the powder and reduce the amount of coupling agent and the cost of the modification operation.
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