Surface modification is one of the effective ways and important technologies to optimize the properties of quartz sand in order to open up new application fields and improve its industrial value and added value. Its purpose is:
Change the original properties of the surface of quartz sand powder, such as: lipophilicity, wettability, oil absorption rate, viscosity, mixture, etc;
Improving the compatibility, affinity, processing dispersion and fluidity of quartz sand as a filler with organic polymers;
Improve the binding force between the quartz sand filler and the polymer phase interface;
The performance of the composite material has been significantly improved.
1. Coating modification
Coating modification mainly refers to the use of resin or polymer on the surface of quartz sand powder "film" to achieve surface modification method, is a simple method of powder surface treatment.
For example, the use of furan resin to coat quartz sand, and then used in oil well drilling, can improve the production of oil wells; with phenolic resin coating quartz sand, to improve the bonding properties of fine casting sand, this coating modified casting sand not only to obtain a high investment casting speed, but also to maintain the mold and mold core production to obtain a higher anti-cracking and anti-roll shell performance.
The method of coating modification is generally divided into two kinds: cold method and thermal method.Taking quartz sand coated with resin as an example, the quartz sand should be washed or polished and dried as necessary before coating. The cold method is to coat at room temperature. First, the resin is mixed with quartz sand, and then solvent (industrial alcohol, acetone, furfural) is added. The amount of solvent is related to whether the sand mixer is closed. If it can be closed, the amount of alcohol is 40% ~ 50% of the amount of resin. On the contrary, it cannot be closed to 70% ~ 80%, continue to mix until the solvent volatilizes, dry and sift to obtain the product. Since the cold method uses a large amount of solvent, it is only used for a small amount of production.
thermal coatingIs the first quartz sand heated to a certain temperature (120~140 ℃), and then mixed with the resin, the amount of resin is generally 2% ~ 5% of the amount of quartz sand. The resin is softened by hot sand, coated on the surface of quartz sand, and becomes sticky with the decrease of temperature. A curing agent (Urotropine) should be added, calcium stearate should be added to mix (to prevent caking), and then the product should be obtained after cooling and sieving. The coating effect of this method is good, suitable for mass production, but its process is more complex and difficult to control, need special sand mixing equipment.
The main factors affecting the effect of quartz sand coating modification are the shape of quartz sand particles, specific surface area, porosity, the type and amount of coating polymer, coating process and so on.
2. Surface chemical modification
The chemical surface modification of quartz sand powder mainly uses silane coupling agent. The-RO functional group in silane coupling agent can be hydrolyzed to produce Si-OH (silanol group). Because there is free water adsorbed on the surface of quartz sand, such group can be combined with SiO in quartz sand powder.2Chemical combination into a homogeneous system, then the other end of the silane coupling agent with a good affinity with the polymer functional group-R' is covered on the surface of the quartz sand particles, forming a reactive coating film. The organic functional group-R' has a good affinity with epoxy resin and other polymers to reduce the surface energy of quartz sand powder and improve the wettability and compatibility with polymers. At the same time, the rheological properties of such composites can be enhanced.
(1) Surface chemical modification process of quartz sand
At present, the quartz sand surface chemical modification process mainly has: dry process, wet process, composite process three categories.
dry processIs one of the most widely used powder surface modification process, refers to the dry quartz sand powder in high-speed stirring dispersion, adding a good configuration of the modifier treatment liquid, while heating conditions for surface modification treatment process. The dry process has the characteristics of simple process, flexible operation, less investment and good applicability of modifier.
wet processIt is a process of surface modification of quartz sand powder in stirring dispersion and certain temperature conditions by using a well-configured surface modifier and additive mixture. Compared with the dry process, the wet process has the characteristics of good dispersion of modifiers, uniform coating on the surface of the powder, but the wet process needs to remove moisture (filtration and drying), so it is suitable for a variety of water-soluble or hydrolyzed modifiers and the front section for the wet powder process and the back section needs to be dried.
Mechanochemical and chemical coating composite modification processRefers to the mechanical force or fine grinding, ultra-fine grinding process to add modifiers, with the quartz sand powder particle size decreases at the same time, its particles surface modification process. This composite modification process is characterized by a relatively simple process, some modifiers have the effect of grinding at the same time, improve the grinding efficiency, but its temperature is difficult to control, while the particles are constantly crushed, resulting in a new surface, the modifier coating is difficult to uniform, in addition, the crushing equipment in the process of strong mechanical force to produce local high temperature, can make part of the modifier decomposition or molecular structure is destroyed.
Precipitation reaction and chemical coating composite modification processIn essence, it is an inorganic-organic composite modification process, which mainly obtains composite powder materials required for certain special purposes. This composite modification process is now mainly used for the surface modification of composite titanium dioxide, that is, the first precipitation coating SiO.2or Al2O3Film, and then use the modifier to TiO.2/SiO2or Al2O3The surface of the composite powder was modified.
Drying and chemical coating composite modification processRefers to the wet powder drying process to add modifiers, that is, dehydration and powder surface modification at the same time the composite process. The process drying temperature is higher (above 200 ℃).
(2) Influencing factors of surface modification
The main factors affecting the surface treatment effect of quartz powder and other silica powder are: the variety, dosage, use method and treatment time, temperature, pH value of silane coupling agent.
Since the organic functional group-R' of silane is selective to materials such as polymers or resins, the type of resin to be filled with quartz powder should be considered when selecting a silane coupling agent.
The amount of the silane coupling agent can be determined according to the specific surface area of the quartz or silica powder, and the optimum amount can also be determined according to experiments.