One article on the methods, processes, and influencing factors of surface modification of quartz powder

Surface modification of quartz powder is one of the effective ways and important technologies to optimize its properties, explore new adaptive fields, and improve industrial and added value. Generally divided into coating modification and surface chemical modification.

1. Quartz powder coating modification

Coating modification is a method of surface modification of quartz powder by coating the surface with high polymers or resins. If phenolic resin or other materials are used to coat quartz sand to improve the bonding performance of fine casting sand. This coated casting sand can achieve high investment casting speed while maintaining high resistance to rolling and cracking in mold and core production; Quartz sand coated with furan resin for oil well drilling can increase oil well production.

Coating modification is a simple method of treating the surface of powder. Taking resin coated quartz sand as an example, surface coating modification methods can be divided into two types: cold method and hot method.

(1) Cold film coating

Rinse or scrub and dry the quartz sand before coating treatment. Cold coated sand is prepared at room temperature by mixing powdered resin with sand, and then adding solvents (industrial alcohol, acetone, or furfural). The amount of solvent added depends on whether the sand mixer can be sealed; For closed individuals, the alcohol consumption is 40% -50% of the resin usage; The proportion of products that cannot be sealed is 70% -80%. Then, continue to mix and grind until it evaporates completely, and after drying, crush and sieve to obtain the product.

But this method uses a large amount of organic solvents and is only used for small-scale production.

(2) Hot method film coating

Thermal film coating is the process of heating sand for film coating. First, the quartz sand is heated to 140-160 ℃, and then mixed with resin in a sand mixer (where the resin dosage is 2% -5% of the quartz sand dosage). At this point, the resin is softened by hot frying and coated on the surface of the sand particles. As the temperature decreases, it becomes sticky. At this point, Urotropin is added to distribute on the surface of the sand particles and the sand is quenched (Urotropin can be used as a catalyst to solidify the resin during the formation of the shell mold). Add calcium stearate (to prevent clumping) and mix for a few seconds before sanding. Then grind, sieve, and cool to obtain the product.

This method is effective and suitable for large-scale production, but the process control is complex and requires specialized sand mixing equipment.

The main factors affecting surface coating include particle shape, specific surface area, porosity, type and dosage of coating agent, coating treatment process, etc.

2. Chemical modification of quartz powder surface

Quartz powder, silica micro powder, and other forms of silica powder used for plastics, rubber, and other resins must be surface treated in order to improve their compatibility with polymer substrates and enhance the overall performance and processability of the filling material.

The crushed quartz powder or other silica powder may exhibit functional groups such as Si-OH (silanol group), Si-O-Si (silicon ether group), and Si-OH ······ O (surface adsorbed free water) under the action of water and air. Therefore, it is easy to accept external functional groups such as amino groups, epoxy groups, methacrylene, trimethyl, methyl, and vinyl groups of silane, which lays a certain foundation for surface modification of silica powder.

(1) Surface modifier

The surface chemical modification of quartz powder, silica micro powder, or other silica powders mainly uses silane coupling agents, including amino, epoxy, methacryl, trimethyl, methyl, and vinyl silane coupling agents.

The principle of modifying quartz powder with silane coupling agent:

The - RO functional group of silane coupling agent can hydrolyze in water (including free water adsorbed on the surface of the filler) to produce silanol groups, which can chemically bind with SiO2 or combine with the existing silanol groups on the surface to form a homogeneous system. In this way, not only is the moisture on the surface of SiO2 removed, but it also forms silicon ether bonds with the oxygen atoms inside, thereby firmly covering the surface of quartz or silica particles with the organic functional group - R 'carried by the other end of the silane coupling agent, which has good affinity with the polymer, forming a reactive coating film.

Organic functional group - R 'and polymer materials such as epoxy resin have good affinity, which can reduce the surface energy of quartz or silica powder, improve the wettability with polymer matrix, and enhance the compatibility between powder and polymer matrix. In addition, the formation of this new interface layer can improve the rheological properties of the filled composite system.

In addition to silane coupling agents, zirconium aluminate coupling agents and polymers can also be used for surface treatment of silica powders. High porosity ultrafine silica used as a matting agent in coatings, leather products, cosmetics, etc. is generally modified by surface coating with fatty acids (such as stearic acid) and polyethylene wax.

Cationic surfactants, such as hexadecyltrimethylammonium bromide, can also be used for surface modification of silica powders. Due to the low isoelectric point of silica, particles are usually negatively charged in aqueous media, and as the pH value increases, the point of charge increases. After surface treatment with cationic surface modifiers, the negative charge on the particle surface gradually decreases and can be transformed into positively charged particles.

(2) Factors affecting surface modification

The main factors affecting the surface treatment effect of quartz powder and other silica powders include the variety, dosage, usage method, treatment time, temperature, pH value, etc. of silane coupling agent.

Due to the selectivity of the organic functional group - R 'of silane towards materials such as polymers or resins, the type of resin to be filled with quartz powder should be considered when selecting silane coupling agents.

The dosage of silane coupling agent can be determined based on the specific surface area of quartz or silica powder, or based on experiments.

(3) Surface modification process

The wet process involves mixing appropriate diluents and additives with silane coupling agents to form a treatment solution, heating or soaking quartz powder or silica powder in a stirred reactor or tank, and then removing moisture.

When preparing amorphous silica powder by chemical precipitation method, wet surface treatment process is often used.

The dry process involves adding a small amount of diluent and silane to form a treatment agent. Under the conditions of high-speed stirring, dispersion, and heating of quartz powder, the prepared silane treatment agent is sprayed into the mist. After a certain reaction time, the material is discharged. This method does not require dehydration and drying, and the process flow is simple.

The dry wet combination process is to first mix the acidified aqueous solution (deionized water is selected for high electrical performance requirements) with an appropriate amount of coupling agent in a strong stirring device to prepare a certain concentration of treatment solution. 8% -10% of the quartz powder feeding amount is added to the treatment solution. Then, it is mixed and ground for 1 hour in a small ball mill with corundum or silica balls as the grinding body, and the discharged material is dried at 110 ℃ and sieved for classification.

During the processing, the amount of water added and the mixing and grinding time have a significant impact on the treatment effect. Excessive water addition can cause clumping of the powder, requiring a longer mixing and grinding time, which can increase the phenomenon of powder re crushing and introduce impurities; Insufficient water addition is not conducive to the sufficient dispersion of coupling agents and the uniform interaction with quartz powder.

Meanwhile, the acidity or alkalinity of the solution directly controls the degree of hydrolysis of the coupling agent. The modified quartz powder treated by this method has been tested in resin mastics, resin anti-corrosion coatings, resin powder coatings, and PVC synthetic leather with good results.

3. Surface modification effect of quartz powder

Silicon dioxide powder treated with silane is commonly used in epoxy resin based plastics. This plastic is used as a high-voltage insulation material for outdoor applications, as well as for manufacturing instrument panels, molded resin transformers for power output, and bushings and insulators for outdoor transformers (with a rated voltage of up to 150kV). Silica treated quartz powder is used as a filler for polymer based electrical equipment materials, which can improve bending strength, obtain a smoother surface, enhance the ability to withstand ultraviolet radiation, and because it does not have severe blackening after breakdown, it has good resistance to small electrical leakage.

In addition, modified quartz powder is used in anti-corrosion materials and can also be used as lining filler for chemical pumps due to its chemical inertness. Due to the same reasons and the ability to maintain surface density for a long time, surface coated quartz powder can also be used in floor coatings in nuclear technology laboratories.

Surface modification of precipitated silica (white carbon black) with silane can further improve the application performance of white carbon black in rubber. Some high-quality white carbon black products have undergone surface treatment. The white carbon black treated with silane coupling agent has a hydrophobic and oleophilic surface, greatly improving its compatibility with rubber.