Surface modification methods of kaolin

Many main uses have special requirements for the surface or page characteristics of kaolin, and in order to better meet the application regulations, it is necessary to carry out surface modification. Surface modification is one of the key production and processing modification methods for kaolin, which refers to the physical, organic chemical, or mechanical equipment methods used to solve the surface of kaolin according to the necessity of application, in order to improve the fineness, chromaticity, surface vitality, or improve its solubility with high polymers.

The surface functional groups of kaolin are isomerized into Si-O or Al-O. Therefore, any physical or organic chemical method that can modify the Si-O or Al-O bonding mode on the surface of kaolin can complete the surface modification of kaolin. The key methods for surface modification of kaolin include calcination, coupling agents, adsorption, and surface encapsulation.

1. Calcination modification

Calcination modification refers to calcining kaolin at high temperature, adsorbing part or all of the methyl resin on the surface of kaolin to obtain unique surface characteristics, which will change the molecular structure of kaolin from orderly molecular structure to chaotic kaolin. The purpose of calcining kaolin to remove water and volatile chemicals depends on improving its purity and fineness.

The calcined kaolin has characteristics such as high fineness, low relative density, expanded specific surface area, improved oil absorption and solubility, high heat resistance, and insulation performance.

2 Coupling agent modification

The coupling agent solution utilizes the interaction between the active functional groups on the surface of kaolin and the coupling agent to achieve the goal of changing the surface characteristics of kaolin. There are generally two solutions for coupling agent solutions: wet solution and dry solution. The mechanism of coupling agent modification is that the coupling agent produces a bipropellant chemical substance with hydrophilic group (usually Si-OH) and hydrophilic functional group at the same time after hydrolysis reaction. The surface functional group of kaolin particles can cause chemical changes with hydrophilic groups, producing Chemical bond, while the hydrophilic functional group can closely combine with the polymer, or the two reactions can be carried out at the same time to transform into stronger ionic bonds, And then achieve the goal of modification.

Common coupling agents include Chlorosilane coupling agent and titanate coupling agent, in addition, aluminate coupling agent, phosphate coupling agent, azide coupling agent, organic chemical chromium coupling agent, zirconium coupling agent, and advanced fatty acid, alcohol, ester, etc. At present, only the action mechanism of Chlorosilane coupling agent and titanate coupling agent is very clear.

2.1 Modification of Chlorosilane coupling agent

A portion of the organic chemical functional group isomers react with high polymers such as epoxy resin and vulcanized rubber to form ion bonds; Alkyl groups connect a portion of organic chemical functional groups with a portion of silicone ester groups; After hydrolysis of the silicone ester group, the converted silicone alcohol group merges with the surface chemical substances of the inorganic filler material to form ion bonds. The key reflections are as follows:

(1) Addition reaction with hydrocarbon groups on the molecular structure chain of vulcanized rubber;

(2) Reflect with the chain oxygen free radical of vulcanized rubber molecular structure transformed by the force effect of mechanical equipment;

(3) Hydroxide ion ion migration reaction;

(4) Condensation reaction with acrylate;

(5) Reflected with saturated polymers.

In general, Chlorosilane coupling agent acts as a "highway bridge" to promote the solid fusion of organic chemical pregnant mother and inorganic powder by means of ionic bonding.

2.2 Titanate coupling agent

The development trend of titanate coupling agents has reached over 60 varieties so far, which can be divided into four categories: monoalkoxy pyrophosphate ester type, monoalkoxy type, coordination type, and chelating type. Among them, the first three types are suitable for modifying kaolin. Among them, the monoalkoxy type titanate coupling agent has a wide range of applications and diverse functions, with the key being used to solve the problem of dry calcined kaolin; The surface of kaolin powder with more water content can be modified with monoalkoxy pyrophosphate based titanate coupling agent with alcohol chelating group; Most of the coordination titanate coupling agents do not dissolve strong electrolytes, have good waterproof property, and do not produce Transesterification, which can be used to modify various types of calcined kaolin.

Generally speaking, titanate coupling agent should not be used independently in the surface modification of calcined kaolin, and the key point is to cooperate with the application of Chlorosilane coupling agent in order to obtain good actual effect of modification. Therefore, titanate coupling agent can be used as an auxiliary coupling agent for kaolin.

3. Surface modification of bread

This method is an early and simple traditional modification method, suitable for kaolin with low application standards. The basic principle is to achieve surface modification by overlaying a layer of organic chemical or inorganic polymer on the surface of kaolin based on physical or organic chemical adsorption methods.

The advantage of this method is that the modification of kaolin can improve the reliability of raw material structure and the activity of metal catalysts, weaken the agglomeration level of powder, and improve dispersion and flowability.

4 Surface reflection method modification

This method is a chemical transformation method, which refers to the reaction between the modifier and the methyl functional group on the surface of kaolin, resulting in hydrophobicity or further formation of hydrophilic functional groups, resulting in changes in surface characteristics. This not only reduces surface energy but also improves the hydrophobicity and reactivity of kaolin.

The modifier can immediately modify kaolin, and can first reflect and convert it into positive ions on its surface. Then, according to the ion exchange method, the modification target can be achieved. The key methods of this method include esterification reaction, halogenation, amination, etc.

Modification by 5-layer intercalation method

The intercalation modification method is to use layered structure to modify the solid layer and page characteristics of powder particles due to poor adhesion or exchangeable positive ions in the solid layer. Chemical changes or ion exchange methods are used to modify the solid layer and page characteristics of the powder. The kaolin cannot carry out positive ion exchange, but the kaolin solid layer has - OH and Si-O bonds that are easy to produce Covalent bond. The layer spacing is small, only a part of the optically active small molecular water is allowed. This optically active small molecular water can be inserted into the kaolin solid layer and its Covalent bond can be destroyed, and the layer spacing is extended, making the water absorption of the solid layer become hydrophobic, which is beneficial to other organic compounds and biological macromolecules according to the whole process of replacement, Promote the dispersion of kaolin into various substrates at the limit of nanotechnology.

Modification of mechanical equipment by mechanochemical methods

The mechanical equipment force organic chemical modification method essentially relies on the mechanical kinetic energy to stimulate the particles and surface modifiers to produce effects, so as to achieve the goal of converting the mechanical kinetic energy into Chemical energy, which can be completed according to the mixing, impact, milling and other methods of strong mechanical equipment force.

This method also makes a key contribution at the composite level of the powder, relying on the external force of mechanical equipment to coat the surface of the powder particles with a finer or more functional layer of powder particles. The surface coating modification is chemical precipitation, and this method is the chemical effect of mechanical equipment. The use of different equipment and their modification processing techniques in the organic chemical modification method of mechanical equipment results in different actual effects of powder modification. In addition to the above common ways, coagulation co precipitation, particle surface positive ion exchange, Irradiance modification and other ways are also reasonable ways of kaolin surface modification.