Powder surface modification uses physical, chemical, mechanical and other methods to purposefully change the physicochemical properties of the surface of the powder material according to the application needs to meet the needs of the development of modern new materials, new processes and new technologies. Therefore, the surface modification technology of powder is increasingly valued by engineers and technicians engaged in powder processing and application, and is increasingly used in industry.
Powder surface modification is a new technology that integrates powder processing, material processing, material properties, chemical industry and machinery. It has the characteristics of specificity or purpose, and has many process methods and many influencing factors. Therefore, careful analysis of these influencing factors will help to select the correct surface modification methods, processes, formulations and equipment to achieve the intended purpose.
1.The nature of the powder raw materials
The specific surface area, particle size and particle size distribution, specific surface energy, surface physicochemical properties and agglomeration of the powder raw materials all have an effect on the modification effect, which is an important factor to select the surface modifier formulation, process and equipment. One.
In the case of ignoring the porosity of the powder, the specific surface area of the powder is inversely proportional to its particle size. That is, the finer the particle size, the larger the specific surface area of the powder. In the case where a certain monolayer coating ratio is required and the same surface modifier is used, the finer the particle size of the powder, the larger the specific surface area, and the larger the amount of the surface modifier.
The surface physicochemical properties of the powder, such as surface electrical properties, wettability, functional groups or groups, solubility or hydrolysis characteristics, directly affect its interaction with the surface modifier molecules, thereby affecting the surface modification effect. At the same time, surface physicochemical properties are also one of the important considerations for the selection of surface modification processes.
Powder materials with a larger surface energy generally tend to agglomerate. If the agglomerates cannot be depolymerized during the surface modification process, the application properties of the surface modified powder products will be affected. Therefore, the powder having a strong tendency to agglomerate is preferably subjected to deagglomeration before the action with the surface modifier.
2.Surface modifier formula
The surface modification of the powder is largely achieved by the action of the surface modifier on the surface of the powder. Therefore, the formulation (variety, amount and usage) of the surface modifier has an important influence on the modification effect of the powder surface and the application performance of the modified product.
The surface modifier formulation is highly targeted, with the feature of a key to open a lock. The formulation of the surface modifier includes the selection of the variety, the determination of the dosage and the usage.
(1) Surface modifiers
The main considerations for selecting a surface modifier are the nature of the powder material, the use or application of the product, and the factors such as process, price and environmental protection.
The properties of the powder raw materials are mainly acid, basic, surface structure and functional groups, adsorption and chemical reaction characteristics, etc. Surface modifiers capable of chemically reacting or chemically adsorbing the surface of the powder particles should be selected as much as possible because of physical adsorption. It is easily desorbed by strong agitation or extrusion during application.
For example, the surface of an acidic silicate mineral such as quartz, feldspar, mica or kaolin can be bonded to a silane coupling agent to form a stronger chemical adsorption; however, the silane coupling agent generally cannot be alkaline with carbonate. Minerals undergo chemical reactions or chemisorption, while titanate and aluminate coupling agents can be chemically adsorbed with carbonate-based alkaline minerals under certain conditions and to some extent. Therefore, silane coupling agents are generally not suitable for use as carbonate-based alkaline mineral powders, such as surface modifiers for light calcium carbonate and heavy calcium carbonate.
The use of the product is the most important consideration in selecting a surface modifier. Different application fields have different technical requirements for powder application performance, such as surface wettability, dispersibility, pH value, hiding power, weather resistance, gloss, antibacterial property, UV protection, etc., which is to select surface modification according to the application. One of the reasons for the variety of sex agents.
For example, inorganic powders (fillers or pigments) used in various plastic, rubber, adhesive, oily or solvent-based coatings require good surface lipophilicity, ie good affinity or compatibility with organic polymer binders. This requires the selection of a surface modifier which can make the surface of the inorganic powder hydrophobic and oleophilic; the inorganic pigment used in the ceramic blank not only requires good dispersibility in the dry state, but also requires its affinity with the inorganic billet. It has good compatibility and can be uniformly dispersed in the billet. For the surface modifier of inorganic powder (filler or pigment) used in water-based paint or coating, the dispersibility and sedimentation stability of the powder in the aqueous phase are required after modification. And compatibility is good.
For inorganic surface modifiers, the functional requirements of the powder materials are mainly selected according to the application field. For example, in order to make the weather resistance and chemical stability of titanium dioxide good, surface coating (film) is carried out using SiO2 and Al2O3. In order to make the muscovite pigment have a good pearlescent effect, it is necessary to use TiO2 for surface coating (film).
At the same time, the composition of different application systems is different. When selecting the surface modifier, the compatibility and compatibility with the components of the application system must be considered to avoid the failure of other components in the system due to the surface modifier.
In addition, when selecting a surface modifier, process factors such as temperature and pressure should be considered. All organic surface modifiers will decompose at a certain temperature. For example, the boiling point of the silane coupling agent varies between 100 and 310 °C depending on the species. Therefore, the decomposition temperature or boiling point of the selected surface modifier is preferably higher than the processing temperature at the time of application.
The modification process is also one of the important considerations for the selection of surface modifiers. The current surface modification process mainly uses dry and wet methods. For the dry process, the water solubility problem is not considered, but the water solubility of the surface modifier should be considered for the wet process because only the water can be dissolved in the wet environment to sufficiently contact and react with the powder particles.
For example, the surface modification of calcium carbonate powder can be carried out by stearic acid (either directly or after dissolution with an organic solvent), but in the case of wet surface modification, such as direct addition of stearic acid, it is not only difficult to achieve. The expected surface modification effect (mainly physical adsorption), and low utilization rate, the loss of surface modifier after filtration is serious, and the organic matter in the filtrate exceeds the standard. A similar situation exists for other types of organic surface modifiers. Therefore, for surface modifiers which are not directly soluble in water and must be used in a wet environment, they must be previously saponified, ammonium or emulsified to dissolve and disperse in an aqueous solution.
Finally, the choice of surface modifiers should also consider the price and environmental factors, in order to meet the application performance requirements or application performance optimization, try to use less expensive surface modifiers to reduce the cost of surface modification. At the same time, care should be taken to select surface modifiers that do not pollute the environment.
(2) The amount of surface modifier
Theoretically, the amount required to achieve monolayer adsorption on the surface of the particles is the optimum amount, 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 amount is not necessarily 100% coverage. The amount of surface modifier used for inorganic surface coating modification, different coating ratios and coating layer thickness may exhibit different characteristics, such as color, gloss and the like.
Therefore, the determination of the actual optimum amount is still determined by the modification test and the application performance test, because the amount of the surface modifier is not only related to the dispersion of the surface modifier and the uniformity of the coating during surface modification. It is also related to the specific requirements of the application system for the surface properties and technical specifications of the powder raw materials.
For wet modification, the actual amount of surface modifier on the surface of the powder is not necessarily equal to the amount of surface modifier, because there is always a part of the surface modifier that fails to interact with the powder particles and is filtered. Lost. Therefore, the actual amount is greater than the amount required to achieve monolayer adsorption.
(3) How to use
The use of the surface modifier is one of the important components of the surface modifier formulation, which has an important effect on the surface modification effect of the powder. The good use method can improve the dispersion degree of the surface modifier and the powder The surface modification effect, on the contrary, the improper use method may increase the amount of the surface modifier, and the modification effect may not achieve the intended purpose.
The use of surface modifiers includes methods of formulation, dispersion, and addition, and dosing sequences when two or more surface modifiers are used.
The preparation method of the surface modifier should be determined according to the variety of the surface modifier, the modification process and the modification equipment. Different surface modifiers require different preparation methods. For example, for silane coupling agents, silanol is bonded to the surface of the powder. Therefore, it is best to add a good modification effect (chemical adsorption). The hydrolysis is carried out before.
For other organic surface modifiers that need to be diluted and dissolved before use, such as titanate, aluminate, stearic acid, etc., use appropriate organic solvents such as absolute ethanol, toluene, ether, acetone, etc. for dilution and dissolution. .
For the surface modification agent which is not directly soluble in water, such as stearic acid, titanate or aluminate used in the wet modification process, it is previously saponified, ammonium or emulsified into water-soluble product.
The best way to add a surface modifier is to provide a uniform and sufficient contact of the surface modifier with the powder to achieve a high degree of surface modification and uniform coating of the surface modifier on the surface of the particles. Therefore, it is preferable to use a continuous spray or drop (add) addition method in conjunction with the powder feed rate. Of course, only a continuous powder surface modification machine can be used to continuously add a surface modifier.
The preparation method of the inorganic surface modifier is special, and various factors such as pH value, concentration, temperature, and auxiliary agent need to be considered. For example, when the surface of the muscovite is coated with titanium dioxide, the titanium oxysulfate or titanium tetrachloride is previously hydrolyzed.
When two or more surface modifiers are used to treat the powder, the dosing sequence also has a certain effect on the final surface modification effect. In determining the order of addition of the surface modifier, it is first necessary to analyze the effect of each of the two surface modifiers and the mode of action on the surface of the powder (either physical adsorption or chemical adsorption). In general, the main role and chemical adsorption-based surface modifiers are added first, followed by secondary effects and surface modification agents based on physical adsorption.
For example, when a coupling agent and stearic acid are used in combination, generally, a coupling agent should be added first, followed by stearic acid, because the main purpose of adding stearic acid is to strengthen the hydrophobic and lipophilic properties of the powder and reduce the coupling. The amount of agent used, reducing the cost of modification operations.
3. Surface modification process
After the surface modifier formulation is determined, the surface modification process is one of the most important factors in determining the surface modification effect. The surface modification process should meet the application requirements or application conditions of the surface modifier, and the dispersion of the surface modifier is good, and the surface modifier can be uniformly and firmly coated on the surface of the powder; at the same time, the process is simple and the parameters are required. Good controllability, stable product quality, low energy consumption and low pollution.
Therefore, at least the following factors should be considered when choosing a surface modification process:
1 characteristics of surface modifiers, such as water solubility, hydrolysis, boiling point or decomposition temperature;
2 Is the front section crushing or powder preparation work wet or dry? If it is a wet process, a wet modification process may be considered;
3 surface modification method. The method determines the process. For the surface chemical coating, either a dry process or a wet process can be used; however, for the precipitation coating of the inorganic surface modifier, only the wet process can be used.
The following is a brief review of the characteristics and applicability of various processes for surface chemical modification of inorganic powders (fillers and pigments) using various organic surface modifiers.
(1) Dry process
The dry surface modification process is simple and suitable for various organic surface modifiers, especially various surface modifiers which are not water-soluble. In the dry modification process, the main process parameters are the modification temperature, the action of the powder and the surface modifier or the residence time. The dispersion of the surface modifier and the uniformity of the surface coating in the dry process depend to a large extent on the surface modification equipment.
(2) Wet process
The wet process has the characteristics of well-dispersed surface modifier and uniform surface coating, but requires subsequent dewatering (filtration and drying) operations, and is applicable to various water-soluble or hydrolyzable organic surface modifiers, inorganic surface modifiers, and The front section is the wet milling process and the latter section needs to be dried.
In the wet modification process, the main process parameters are temperature, slurry concentration, reaction time, drying temperature and drying time.
(3) Combining pulverization and surface modification into one process
The feature is that the process can be simplified, and in addition, some surface modifiers have certain grinding properties, which can improve the pulverization efficiency to some extent.
The disadvantage is that the temperature is not well controlled, it is difficult to meet the requirements of the modified process technology, and, because the coated particles are continuously pulverized during the pulverization process, a new surface is produced, the particle coating is uneven, and the coating rate is not high; In addition, if the heat dissipation of the pulverizing equipment is not good, the local excessive temperature rise during the ultrafine pulverization may decompose the surface modifier or destroy the molecular structure to some extent.
(4) Drying and surface modification combined into one process
Its characteristics can also simplify the process, but the drying temperature is generally above 200 ° C. The low-boiling surface modifier added during the drying process may not be able to evaporate with the water molecules when it interacts with the surface of the powder. Although the addition of the surface modifier can avoid evaporation of the surface modifier, the residence time is short, and it is difficult to ensure uniform and firm coating.
Although the wet surface modification process is also dried, the surface modifier has been adsorbed on the surface of the particles before drying, and the hydrated film on the surface of the particles is displaced. Therefore, when drying, the moisture of the periphery of the particles is first evaporated.
4. Surface modification equipment
The surface modification or surface treatment technology of the powder mainly includes surface modification methods, processes, surface modifiers and their formulations, and surface modification equipment. Among them, in the case of the surface modification process and the modifier formulation, the surface modification equipment becomes a key factor affecting the surface modification or surface treatment effect of the powder.
▲CLG continuous powder surface modification machine
The performance of the surface modification equipment is not the high or low speed or the complexity of the structure. The key lies in the following basic process characteristics:
1 dispersion of powder and surface modifier;
2 the opportunity to contact or act on the powder with the surface modifier;
3 modification temperature and residence time;
4 unit product energy consumption and wear;
5 dust pollution;
6 The operating status of the equipment.
The high-performance surface modification machine should be able to make the powder and surface modifier have good dispersibility, and the contact and action opportunities of the powder and the surface modifier are equal, so as to achieve uniform single-layer adsorption and reduce the amount of modifier. At the same time, it is convenient to adjust the modification temperature and reaction or residence time to achieve a firm coating and complete evaporation of the solvent or diluent (if a solvent or diluent is used); in addition, the energy consumption and wear per unit product should be low. No dust pollution (the powder overflows not only pollutes the environment, deteriorates working conditions, but also loses materials and increases production costs). The equipment is easy to operate and runs smoothly.
5. Development prospects
The surface modification of inorganic powder is a new technology that should be developed in response to the development of modern high-tech and new materials industries, especially the functional materials industry, to meet the needs of modern society for environmental protection, energy conservation, safety and health.
Inorganic powder surface modification products are the most promising functional powder materials, and the market demand is expected to average 8 in the next 10 years.
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