Powder Surface Modification

2022-11-11

Summary:With the development of science and technology, users have put forward more and more requirements for the performance of ultra-fine powder, such as requiring ul

With the development of science and technology, users have put forward more and more requirements for the performance of ultra-fine powder, such as requiring ultra-fine powder to have good dispersibility, good compatibility when mixed with other materials, To this end, ultrafine powder workers have devoted a lot of energy to the research on surface modification of ultrafine powders, and gradually formed another branch of ultrafine powder technology - surface science and technology. The research of ultrafine powder surface technology includes the research of surface modifier and the research of surface modification technology and modification equipment.

The physical and chemical properties of the surface of the powder are different due to its own properties and processing methods. Surface treatment of powder by physical or chemical methods is called powder surface modification. Methods of surface modification Many classification methods vary according to the angle of analysis of the problem.

A Chinese expert proposed that the classification method based on the nature of the modification process has its own unique characteristics. He divided the powder surface modification methods into six categories: ① Surface coverage modification. Use surfactants to cover the surface of the powder with polymers, inorganic substances, organic substances, etc. to achieve surface modification; ② Surface chemical modification. The surface modifier is used for chemical reaction or chemical adsorption on the surface of the powder to generate new functions on the surface; ③ mechanochemical modification. The surface activity of the particles is enhanced by methods such as pulverization, grinding, friction, etc. The surface of the powder with strong activity reacts with other substances and attaches to the surface to achieve the purpose of surface modification; ④ Capsule modification. Coating one or more layers of uniform films of other substances on the surface of the powder changes the surface properties of the powder; ⑤ Modification by high-energy treatment. Use corona discharge, ultraviolet light, plasma beam, etc. to modify the surface of the powder; ⑥Precipitation reaction modification. A method of using chemical reaction and depositing the product on the surface of powder to form one or more modified layers.

Among them, mechanochemical modification refers to enhancing the surface activity of particles by means of pulverization, grinding, friction, etc., so that the surface of the powder with strong activity can react and adhere to other substances to achieve the purpose of surface modification. The action of high-energy mechanical force will not only change the physical structure and crystalline state of minerals, but also lead to the breaking of chemical bonds, the generation of unsaturated groups, free ions and electrons, resulting in new lattice defects, and finally induce chemical reactions. The application research results of mechanochemistry provide a new method for the surface modification of powder, which can achieve the purpose of surface modification while making the powder ultra-fine. It has the characteristics of simple process, good product modification effect and high production efficiency.

1. Wet Mechanochemical Surface Modification

Wet mechanochemical surface modification means that the powder to be modified and the modifier are subjected to high-speed impact, shearing, extrusion, etc. Activated, and then physically and chemically react with the modifier to achieve the purpose of surface modification. After the modification is completed, the modified powder is obtained through solid-liquid separation, drying and other processes. Wet mechanochemical surface modification is usually accomplished by ball mills, stirring mills, vibration mills, etc. Among them, stirring mills are more commonly used.

The wet process has the characteristics of good dispersion of the surface modifier and uniform surface coating. It is suitable for occasions where the final product is used in the form of slurry, or where the first stage is wet milling process and the latter stage needs to be dried. However, if the product needs to be made into dry powder, solid-liquid separation, drying and other treatments are required, and the production process is more complicated. In the wet process, the surface modifier not only imparts special functions to the powder, but also acts as a grinding aid to improve the fluidity of the powder, thereby improving the pulverization efficiency.

2. Dry Mechanochemical Surface Modification

Dry mechanochemical surface modification is carried out in a single solid phase environment. Compared with wet modification, dry modification has the characteristics of high efficiency, large output and no pollution.

The dispersion of the surface modifier and the uniformity of surface coating in the dry process depend to a large extent on the surface modification equipment, but currently there are few special modification equipment, which are usually transformed from mixing equipment or pulverizing equipment, such as high-energy Ball mill, vibration mill, etc. The dry surface modification process is simple and suitable for various organic surface modifiers, especially water-insoluble surface modifiers. In the dry modification process, the main process parameters are the modification temperature, the action time of the powder and the surface modifier, etc.

In recent years, people have begun to pay attention to the research on the mechanochemical surface modification of powders using jet mills. Compared with other crushing equipment, the jet mill has outstanding advantages such as high crushing efficiency, low energy consumption, low wear, and narrow particle size distribution of non-polluting products. It can be used to crush high hardness materials. Therefore, the jet mill is the main equipment for the airflow The integrated process of pulverization and surface modification has been developed rapidly.

3. Integrated Process Of Jet Pulverization And Surface Modification

The integrated process of jet pulverization and surface modification (referred to as integrated process) utilizes the high turbulence effect of jet mill to complete the surface activation modification of powder while superfine pulverization of powder, which can effectively overcome jet pulverization and surface modification. When the two modification processes are completed separately, the modification agent has the disadvantages of poor affinity with the powder surface, weak adhesion and non-uniformity. In the integrated process, on the one hand, the powder produces a fresh surface with high activity under the action of mechanical force, so that the modifier can more effectively cover the surface of the powder to achieve the purpose of surface modification; Under the action of the modifier, the surface free energy of the powder is greatly reduced, and as a result, the pulverization efficiency is improved. The integrated process combines ultra-fine grinding and surface modification, and the two promote each other, which can reduce energy consumption and improve production efficiency.

Mechanochemical Characterization

In order to know exactly what changes have taken place in the substances of a system under the action of mechanical force, it is necessary to have corresponding detection methods. There are many detection methods for mechanochemical effects, which are summarized in the table below.
 

Detection methods Detection content
Archimedes principle Determination of bulk density of powders
X-ray Diffraction (XRD) It is used to semi-quantitatively analyze the grain size and lattice distortion of powders, and can also analyze changes in crystallinity.
Electron Microscopy (SEM) Observe the particle size distribution, agglomeration and structural changes of the powder material before and after stress
laser particle size analysis Used to determine the particle size distribution, specific surface area, etc. of powders
Thermal Analysis Technology (TG-DTA) It is used to judge whether there is any change before and after the material is subjected to force, especially for the analysis of the dehydration process and degree of dehydration of water-containing substances
Infrared Spectroscopy (IR) Detect the bond energy and bond properties of substances before and after mechanochemical action to infer the effect that occurs
Photoelectron Spectroscopy (XPR) Determination of the binding energy of different elements and oxygen to determine the changes in the system, it has high measurement accuracy and high resolution, and can be longitudinally peeled from the surface of the object for layer-by-layer analysis to determine the depth and degree of change.
Solid State Nuclear Magnetic Resonance Spectroscopy (NMR) Determine the state of an element in a substance

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