Surface treatment of titanium dioxide

Titanium dioxide surface-treated titanium white powder flow modern plant schematically shown in Fig.

The surface treatment of titanium dioxide is generally carried out by an inorganic surface treatment followed by an organic surface treatment. In the case of inorganic surface treatment of the coating, depending on the application, the thickness of the film varies (Fig. 2). Generally, when the main purpose is to improve weather resistance, the thickness of the film is closely skin-like, so that the titanium dioxide particles can be isolated from the vehicle. The photocatalytic effect of titanium dioxide can be suppressed to the utmost extent. Although the rutile titanium dioxide weatherability is superior to anatase titanium dioxide, but the photochemical activity of rutile titanium dioxide iron without surface treatment will still affect the life of the coating, especially in recent years newly developed acrylic paint industry Resins such as silicone polyester, which can pass through ultraviolet light, are stable to light in the absence of titanium dioxide, so it is particularly necessary to reduce the photochemical activity of titanium dioxide, while ordinary alkyd resin is in sunlight. Under irradiation, even if there is no photocatalytic effect of titanium dioxide, they will degrade. The thickness of such inorganic film is mainly adjusted by the thickness of SiO ₂ . If the maximum hiding power of titanium dioxide is desired (such as titanium dioxide for flat emulsion paint), the film layer should be thick and spongy, which can be effective. The distance between the pigment particles is opened, so that the difference in refractive index between the pigment and the air interface (2.76~1.06) is greater than the difference between the refractive index of the resin and the resin in the paint (2.76~1.50), thereby improving the light scattering ability. The opacity is strengthened.

Different surface treatment agents and treatments, and even different treatment methods, will produce the properties (dispersibility, weather resistance, oil absorption, gloss, hiding power, surface area, isoelectric point, rheology, storage stability, etc.) of titanium dioxide. Table 1 shows the surface treatment and performance of several types of titanium dioxide from Bayer, Germany.

Table 1 Chemical composition and physical properties of TiO ₂ pigment

Organic surface treatment (Fig. 3) is mainly to improve the compatibility and dispersibility of various titanium dioxide media, and to improve its gloss, rheology and wettability. The organic surfactant is adsorbed on the surface of titanium dioxide particles, relying on electric repulsion. And the steric hindrance effect keeps the titanium dioxide pigment particles in a dispersed state.

(1) Inorganic surface treatment

The most commonly used inorganic surface treatment agents are hydrated oxides of aluminum and silicon. Currently, almost every brand of commercially available Jinhong plant type titanium dioxide has aluminum or silicon, of which Al ₂ O ₃ /SiO ₂ series are It is considered to be the most basic standard composition. This coated product not only has excellent weather resistance, but also enables titanium dioxide to have good dispersibility in water and organic media. In addition to this, depending on the application, a hydrous oxide such as titanium, zirconium , zinc , antimony , tin , or manganese may be added to coat the film.

a. Aluminum (Al ₂ O ₃ ) coating, the role of aluminum coating, in addition to forming a protective layer on the surface of titanium dioxide to isolate the direct contact between titanium dioxide and organic media, Al ₂ O ₃ can also reflect part of the ultraviolet light, avoiding titanium dioxide A large amount of ultraviolet light absorbs photochemical reactions. It has been mentioned in the former titanium dioxide properties that TiO ₂ has a strong reflection effect in the visible light range, but slightly absorbs violet light near 405 nm. It can be seen from Fig. 4 that Al can be seen. Although the reflectance of ₂ O ₃ to light is not strong by TiO ₂ , its reflection range to the visible spectrum is wider than that of TiO ₂ , and the absorption tends to start at 300 nm, so Al ₂ O ₃ can reflect more ultraviolet light.

In addition, the Al ₂ O ₃ coated TiO ₂ reacts with acidic functional groups in the paint to form salts. In the alkyd resin, Al ^{3+ is} adsorbed on the surface of TiO ₂ , and in the presence of a small amount of water, the aluminum salt dissociates to form a double The electric layer causes the surface of the TiO ₂ particles to be positively charged, and a resin layer is adsorbed on the surface thereof. The titanium dioxide pigment particles act as a dispersion due to the adsorption protection of the resin layer (Fig. 5).

By measuring the oil absorption and water absorption of titanium dioxide coated with Al ₂ O ₃ /SiO ₂ , it is found that the higher the ratio of Al ₂ O ₃ / Al ₂ O ₃ + SiO ₂ in the coating agent, the lower the oil absorption. It can be seen that Al ₂ O ₃ is lipophilic, which contributes to the dispersion of titanium dioxide in an organic medium, so there is a saying of “aluminum oleophilic, silicon new water” in the titanium white industry. [next]

Table 2 Resin adsorption capacity of titanium dioxide

In the measurement of the amount of resin adsorption of titanium dioxide (Table 2), it was also found that the amount of resin adsorbed by TiO ₂ coated with Al ₂ O ₃ was the largest. The aluminum coating can be carried out separately (single aluminum coating) or in combination with SiO ₂ , ZrO ₂ , TiO ₂ or the like. The coating mode of the aluminum salt can be expressed by the following chemical reaction formula.

Al ₂ (SO ₄ ) ₃ +6NaOH + (x-3) H2O → Al ₂ O ₃ · xH ₂ O + 3Na ₂ SO ₄

2NaAl(OH) ₄ +H ₂ SO ₄ +(x-5)H ₂ O→Al ₂ O ₃ ·xH ₂ O+NaSO4

b. Silicon (SiO ₂ ) coating, the silicon coating is generally not carried out separately, usually simultaneously with the aluminum coating, its most important role is to form a film on the surface of titanium dioxide, so that the titanium dioxide particles are isolated from the organic medium, reducing The contact area of ​​TiO ₂ with the medium blocks the photoactivation point on TiO ₂ , effectively protecting the titanium dioxide from ultraviolet rays and reducing its photochemical activity. There are two kinds of SiO ₂ films, one is a dense skin-like or glass-like film. It was developed by DuPont in the late 1960s and is one of the most outstanding achievements in the production of modern titanium dioxide. Particularly excellent weather resistance, mainly used in coatings, plastics and other fields with high weather resistance and high durability. At the same time, this film can also protect TiO ₂ particles from chemical attack, for example, using conventional A1 ₂ O ₃ /SiO. ₂ coated titanium dioxide, heated in 96% concentrated sulfuric acid at 175 ° C for 5 h, 95% of TiO ₂ is dissolved, while the dense SiO ₂ coated titanium dioxide, under the same conditions, only 40% of TiO ₂ is The sulfuric acid is dissolved. The other is a porous, high-dose sponge-like film, which is mainly used in flat-light emulsion paints. After the paint film of the latex paint is dried, the micropores originally occupied by water enter a certain amount of air to produce dry hiding power. To improve the total hiding power of the latex paint.

Total opacity = TiO ₂ true hiding power (oil hiding power) + TiO ₂ dry hiding power (air hiding power) + packing opacity

It is this dry hiding power that allows the general latex paint to achieve a satisfactory covering effect at a 20% pigment volume concentration.

c. Coating process of A1 ₂ O ₃ and SiO ₂ Titanium dioxide should be firstly beaten and dispersed before coating. According to the quantity and nature of soluble cation and anion impurities in titanium dioxide, the variety and addition amount of dispersant are determined. adjust the pH of the slurry to obtain a satisfactory dispersion effect, commonly used dispersants are: sodium silicate, sodium hexametaphosphate, phosphorus, and sodium hydroxide.

In the coating of the A1 ₂ O ₃ -SiO ₂ type, the ratio of A1 ₂ O ₃ to SiO ₂ is 1:1 to 2:1, and the process of the coating is to use an aluminum sulfate solution at 60 to 80 ° C. Adding to the titanium dioxide dispersion containing sodium silicate, and adjusting the pH of the end point to about 7, the coating time is 30~60min; the sodium silicate solution can also be added to the titanium dioxide slurry containing the aluminum sulfate solution according to the above operating conditions. in. The first method is also called alkaline coating, because alkaline sodium silicate is a good dispersing agent for titanium dioxide, and the titanium dioxide particles can be coated in a state of being sufficiently dispersed, and the obtained product has good weather resistance and phase with the resin. Capacitance is good, but the filtration performance of the coated product is poor; the second method is also called acid coating, because the aluminum sulfate solution is acidic, the coated product is easy to be filtered and washed, but there is no first method for weather resistance. it is good.

Either way, it seems to be very simple, just by adjusting the pH by acid-base neutralization to deposit hydrated alumina or silica from the solution onto the surface of the titanium dioxide. However, the chemical nature and crystal structure of hydrated alumina are particularly complicated, and the form of formation is closely related to temperature, pH, concentration, etc., and is generally considered to be an amorphous structure of the following reaction:

The hydrated alumina formed when the aluminum salt is coated, the actual composition is boehmite or pseudo-boehmite type alumina (γ-AlOOH), diaspore (a-AlOOH), and bayerite [Y- A mixture of Al(OH) ₃ ], but the process requirements are preferably to form boehmite or pseudo-boehmite-type hydrated alumina, because they form a filamentous or Ribbon-like structure, which facilitates the dispersion of the pigment.

According to the study by Rochelle and M. Cornell et al., in the case of alumina coating, the precipitate gel after neutralization of aluminum sulfate at pH 8 has an orderly fine rod structure, and the X-ray diffraction pattern is a boehmite structure; The alumina gel precipitated from aluminum sulfate has a network structure at pH 4~8.5, and the specific surface area of ​​the precipitated product under alkaline conditions is significantly higher than that of the precipitated product under acidic conditions, but it is too alkaline. In the case of (pH 12~7) precipitation, the obtained alumina coating is neither dense nor uniform, and the mesh structure is stretched between the particles under an electron microscope. [next]

Due to the complexity of the above chemical reactions, good agitation ensures that the pH is not in the local area beyond the process specification. The purity of the aluminum salt of the coating agent is very high. It is generally considered that the use of sodium metaaluminate as a coating agent is better than the use of aluminum sulfate as a coating agent because metal ions such as iron are in a soluble state in an acidic aluminum sulfate solution. In the case of coating, hydrolysis occurs together to precipitate onto the surface of titanium dioxide, while sodium metaaluminate solution is alkaline, impurities such as insoluble iron can be removed by filtration before use, and titanium dioxide is well dispersed under alkaline conditions. The envelope is relatively uniform.

For a single aluminum-coated product, the A1 ₂ O ₃ coating amount is generally 1% to 6% by mass of TiO _{2 , and} an aluminum sulfate solution containing 40 to 100 g/L of Al ₂ O ₃ is usually added to the dispersion. In the titanium dioxide slurry, it is neutralized with dilute NaOH or neutralized by cocurrent, that is, aluminum sulfate solution and alkali solution are added at the same time, and the pH is maintained at 8.5 to 11 during the neutralization process, so that TiO _{2 is} in the entire coating process. Disperse the state, then adjust the pH to neutral with acid to completely hydrolyze the aluminum salt, or use sodium metasilicate solution, neutralize with dilute sulfuric acid, or add sodium metaaluminate to complete the number under alkaline conditions. After ten minutes, the precipitate was neutralized with dilute acid to increase the dispersibility.

U.S. Patent No. 4,416,699 (DuPont) describes a method for operating a single aluminum-coated titanium dioxide having good anti-powdering properties, fading resistance, high gloss, and good dispersibility. The method uses a composite coating of dense alumina and boehmite alumina which accounts for 2% to 6% by mass of titanium dioxide, wherein the dense alumina (containing sulfate) imparts anti-pulverization property and anti-fading property to the pigment. And boehmite type alumina (a form of hydrated alumina) gives the pigment a satisfactory gloss and dispersibility.

The dense alumina is alumina which strongly adsorbs sulfate ions, and the sulfate is derived from an acid which precipitates hydrated alumina or an aluminum sulfate solution. The densification of alumina is completed when the alumina with sulfate ion is matured at a temperature of 30-65 ° C and pH 5.2-6.5, and the temperature during the precipitation or maturation period is too high (>90 ° C). Will cause insufficient sulfate. Boehmite is a crystal form of alumina containing adsorbed water, and water is usually sandwiched in an octahedral layer of Al(OH) in a random or regular manner. The operation of the invention is as follows.

Add 15.5L of deionized water to 6200g rutile titanium dioxide, raise the temperature to 60 ° C, pH 3.85, adjust the pH of the slurry to 1.5 with 63mL concentrated sulfuric acid, use 34mL 50% sodium hydroxide or 196mL aluminum in 15min A sodium solution (315 g/L in terms of A1 ₂ O ₃ ) was adjusted to pH 8.5 to form a boehmite-type alumina equivalent to 1% by mass of TiO ₂ . Then, the pH was adjusted to 5.5 with 6 mL of concentrated sulfuric acid, and then 393 mL of the above sodium aluminate solution was added in 30 min while maintaining the pH of 5.5 to 6.0 with 125 mL of concentrated sulfuric acid, after which the slurry was aged at 60 ° C and pH 5.7. 15min. Then the slurry was added to 75 mL of tetrapotassium pyrophosphate solution (containing 12.4 g of tetrapotassium pyrophosphate) while adding 2 mL of concentrated sulfuric acid to maintain the pH value of 5.7, and then aging at 60 ° C for 0.5 h, then filtering and washing (making It does not contain sodium sulfate), is dried at 120 ° C, and is finished after being pulverized by steam flow.

The above method is a platinum-type alumina, which is formed into a dense aluminum oxide film, or a dense aluminum oxide film, and then a pH value is used to form a boehmite-type aluminum oxide film.

The latest advancement of silicon coatings is the formation of dense SiO ₂ films, which are characterized by a uniform thickness of the film and a continuous dense structure rather than a porous sponge, so some describe it as the skin of the skin, or glass. a shell-like film that does not increase oil absorption or reduce gloss. The amorphous hydrated silica in this film is firmly bonded to the surface of titanium dioxide in the form of hydroxyl groups, that is, it is not a simple physical precipitation. The way the envelope is, but a chemical bond. Water vapor can permeate through the coating layer, but nitrogen is not permeable. The apparent wetting heat of such coating layer in water is very high, reaching 750~1200mJ/m ² , which is much higher than that of porous SiO ₂ coating sample. The heat of wetting (about 400 mJ/m ² ) is because the heat of wetting is calculated based on the specific surface area of ​​the nitrogen adsorption method.

The silicon coating agent usually uses sodium silicate (water glass), and the amount generally accounts for 1% to 10% (mass) of titanium dioxide. Generally, the amount used in the preparation of the dense film is low, and the amount of the sponge film is high, but the total coating amount is high. When it exceeds 12%, the mechanical properties of the pigment are lowered, the optical properties are degraded, and even the film layer is separated from the surface of the pigment. The chemical reaction formula of the silicon envelope is as follows:

Na ₂ SiO ₃ +H ₂ SO ₄ +(x-1)H ₂ O→SiO ₂ ·xH ₂ O+Na ₂ SO ₄

There is no metasilicate ion (SiO ₃ ^2- ) in the sodium silicate solution, only H ₃ SiO ₄ ^- and H ₂ SiO ₄ ^2- . When the sodium silicate solution is acidified with an acid, the silicic acid in the form of Si(OH)4 is initially precipitated. This monomeric form of orthosilicate has a great activity, and it will soon undergo polycondensation reaction. Polymeric silicic acid of siloxane chain . Monomeric silicic acid and low degree of polymerization of hydrated silica are active, also known as activated silicon, while polymers of high molecular weight silica are inactive. [next]

In order to prevent the silicic acid from being too fast in the coating process to form a dense film, the neutralization should be carried out very slowly during the operation, so that SiO ₂ gradually precipitates on the surface of TiO _{2 to} form a nuclear point, and further sinks with SiO ₂ . They are attracted to these nuclear sites and continue to grow at these nucleation sites, forming a continuous dense membrane. The precipitation of silica is a rapid adsorption (bonding process), the pH range of this process is narrow, and the most important thing in operation is to control its pH. From the relationship between the logarithm of the polymerization (or gelation) time of the silicic acid solution and the pH in Fig. 6, it can be seen that the polymerization rate of silicic acid has a significant N-shaped curve with respect to pH. The fastest polymerization rate is at the lowest point of the N-shaped curve (pH 7~8), and the polymerization rate is the slowest at the two highest points of the N-type curve (pH 2~3 and pH>11), due to the acid used in actual production. The difference between the types of sodium silicate and the temperature and concentration during operation, the actual value is generally biased to the left of the theoretical curve, which theoretically provides a basis for the production of dense membranes and porous sponge membranes.

As for why the low-pH method is not used in industrial production to produce dense membranes, mainly because titanium dioxide is strongly aggregated at low pH, which causes uneven coating, and the pH is just at the bottom of the N-shaped curve when neutral. At this time, the polymerization rate of silicic acid is too fast, and only at a high pH (pH 9 to 11), not only the polymerization speed of silicic acid is slow, but also the titanium dioxide is in a state of high dispersion, and the coating film is relatively uniform at this time. According to the above theoretical analysis, whether or not a dense SiO ₂ film can be formed in industrial production, attention should be paid to the following aspects.

(a) the pH of the titanium dioxide slurry is preferably maintained at 9 to 11;

(b) The temperature of the dense membrane reaction should be controlled at 80-100 ° C, and the reaction temperature of the porous membrane should be controlled at about 60 ° C;

(c) The concentration of the sodium silicate solution and the sulfuric acid solution for the dense film is low (sodium silicate is less than 1 mol/L in terms of Na ₂ O), and if the concentration is too high, the activity is increased and the polymerization phenomenon is accelerated;

(d) The reaction time of the dense membrane is long, generally up to 5~10h. If the reaction rate is too fast, sand-like SiO ₂ particles will be formed, which cannot be deposited on the surface of TiO ₂ . When the amount of active silicon is further increased, these sand grains The active silicon is adsorbed in advance, and finally accumulated into a sponge film, which destroys the preparation process of the dense film;

(e) The dense film of the titanium dioxide slurry is in a good dispersion state, so the stirring should be uniform, and it is not good to avoid the local pH and the temperature being too high or too low.

Dense membranes are good, but the reaction time is too long. Recently, many patents have reported methods for shortening the operation time of dense membranes. For example, in USP 4,125,412, the dispersed titanium dioxide slurry is heated to 80-100 ° C, and sodium silicate solution is rapidly added. And adjust the pH to 9~10.5, keep the SiO ₂ coating mature for 15~60min, then adjust the pH to 8, quickly add sodium aluminate, add sodium aluminate while adding acid to maintain pH7~9, finally Filter, wash, dry, and pulverize.

In European Patent Application 106706.9, the pH of the bismuth dioxide slurry is first adjusted to 9.8~, 10.1, then heated to 85 °C, and the acid is used to neutralize the sodium silicate to pH 9.6~9.8; 9.2~9.4; 3~ 4. Add sodium silicate and acid at pH 5~6.5, and finally adjust the pH to about 7.5, then filter, wash, dry and pulverize. The essence of the patent is to gradually neutralize, precisely control the pH of each neutralization, and precisely control the sedimentation rate of SiO ₂ so that the titanium dioxide and the soluble silicate can be in intimate contact before the precipitation of SiO ₂ begins.

The method of USP 5,041,162 is to first adjust the pH of the titanium dioxide slurry to 9.8 to 10.2 with ammonia or sodium hydroxide, and heat to 75 to 90 ° C, after which the whole process is maintained at this temperature, and then sodium silicate solution is added. After aging for 15~20min, SiO _{2 is} matured and precipitated on the surface of TiO ₂ . When the sodium silicate is added quickly, sulfuric acid is quickly added to make the pH reach 9.2~9.4, so that the dense SiO _{2 is} further precipitated and kept at 15~. 20min, then the second time with sulfuric acid to quickly adjust the pH to 2.8 ~ 3.2, quickly add sodium aluminate, when the pH rises to 5.5 ~ 6.5, sodium aluminate and acid are added at the same time, maintaining pH ≤ 7, so that A1 ₂ O ₃ sink It is precipitated on dense SiO ₂ and finally adjusted to neutral with alkali, then filtered, washed with water, dried, and pulverized by steam.

A1 ₂ O ₃ -SiO ₂ is coated with aluminum (A1 ₂ O ₃ ) and then coated with silicon (SiO ₂ ) in the order of the coating; or aluminum is coated first, then aluminum is first wrapped, then all of the aluminum is added. The silicate is then added to the other half of the aluminum salt. However, in the production of high weatherability pigments, aluminum is generally coated with aluminum, and the varieties used for waterborne coatings are sometimes coated with aluminum and then coated with silicon. [next]

The A1 ₂ O ₃ and SiO ₂ coatings are currently the most effective methods for preventing the photochemical activity of titanium dioxide, and their main mechanism of action is to provide a physical barrier to TiO ₂ . Another advantage of using an aluminum salt and a silicon salt mixed coating is that the compatibility between the two is good. The acidic aluminum sulfate solution and the alkaline sodium silicate solution are easily coprecipitated onto the surface of the titanium dioxide, and in the silica. The hydroxylated aluminum ions adsorbed on the gel particles prevent the silica gel particles from polymerizing, thereby preventing further growth of the gel particles. Figure 7 is an electron micrograph of titanium dioxide treated at the coating and uncoated.

d. The coating of other oxides, in the Al ₂ O ₃ -SiO ₂ coating series, there is often a coating composition of TiO ₂ , and some manufacturers indicate TiO _{2 in the} composition of the coating agent, and some products Although there is a coating layer of TiO ₂ in the specification, it is generally not indicated. The role of the titanium dioxide film is mainly to improve the adhesion between the surface of the titanium dioxide and the coating layer, so the titanium dioxide film is referred to as an adhesion promoting layer. The main raw material of the titanium dioxide coating layer is titanium tetrachloride or titanium sulfate solution. In operation, TiO ₂ is generally coated, followed by Al ₂ O ₃ and SiO ₂ , and the typical ratio between the three is 3:4:2. However, in recent years, some experts believe that the coating layer of hydrated titanium dioxide has a disadvantage that it may be reduced to Ti ₂ O ₃ during the coating process to reduce the whiteness of titanium dioxide. Instead of the TiO ₂ coating layer, a ZrO ₂ coating layer is used in many cases.

The zirconium dioxide coating layer not only has the adhesion promoting effect of the titanium dioxide film, but also masks the photoactive group (TiOH) on the surface of the titanium dioxide lattice, which can improve the powdering resistance and glossiness of the titanium dioxide, such as British Tioxide. Group TR-92, R-KB-6 from Bayer, Germany, and CR-97 from Japan's Ishihara Corporation all contain ZrO2 coatings, which are included in the ultra-durable products.

The zirconium salt used for the titanium dioxide coating mainly includes Zr(SO ₄ ) ₂ , ZrCl ₄ , ZrOCl ₂ , Zr(NO ₃ ) _{4 ,} etc., but Zr(SO ₄ ) ₂ is used more. The coating amount of ZrO ₂ generally accounts for 0.1%~2.5% (mass) of coated titanium dioxide (calculated as ZrO ₂ ), and the usual amount is 0.5%~1%. Too little effect on the durability and anti-pulverization of titanium dioxide Too much (more than 2.5%) of such transition metal oxides can have an adverse effect on the optical properties of the pigment. Titanium dioxide containing a zirconium salt hardly affects flocculation performance in an alkyd/melamine system.

The method of zirconia coating is roughly: after the titanium dioxide slurry is ground and dispersed, the slurry is heated to about 50 ° C, the pH is adjusted to about 5 with sulfuric acid, and then the zirconium sulfate solution (about 200 g / L) and 20% NaOH, While stirring, the slurry was maintained at a pH of about 8, and then a sodium metaaluminate solution was added to carry out an aluminum (Al ₂ O ₃ ) coating or a silicon (SiO ₂ ) coating.

Other coating agents there manganese oxide, cerium oxide, molybdenum oxide, chromium oxide gas, but rarely used, mostly for use only in particular species specific envelope above oxides, such as manganese oxide of anatase type sometimes used in Titanium dioxide for chemical fiber is specially supplied for nylon-6 or nylon-66 matting. TA-500 for nylon matt in Japan Fuji Titanium belongs to the Al ₂ O ₃ -SiO ₂ -MnO coating type; tin oxide is mostly used for mica. Pearlescent titanium dioxide pigment, which simultaneously promotes the conversion of rutile; cerium oxide acts similarly to zirconia, and a cerium oxide coating is sometimes used in laminated paper.

(2) Organic surface treatment

The main effect of the organic surface treatment is to improve the wetting, dispersing and rheological properties of titanium dioxide in various dispersion media, and to reduce the absorption of moisture during storage and to reduce the agglomerates in the pigment. The organic surface treatment mainly acts outwardly with the organic dispersion medium through the hydrophilic polar group in the organic surfactant, which not only reduces the surface tension, enhances the wettability with the dispersion medium (resin), but also due to its directional buffering. The effect can help the titanium dioxide to improve its dispersion in the medium. Another type of surfactant such as trimethylchlorosilane mainly relies on chemisorption, and the (CH) ₃ -Si-O- group in its molecule is bonded to the surface of titanium dioxide through an oxygen bridge to make titanium dioxide Hydrophilicity changed to lipophilicity. There is also a class of organic surfactants such as sodium alkylbenzene sulfonate, sodium hypomethylene naphthalene sulfonate, sodium lignin sulfonate, etc., which can ionize a large number of negatively charged ions in aqueous solution, so that they are in aqueous solution. The titanium dioxide double layer becomes thicker, thereby improving the dispersion performance of titanium dioxide in an aqueous medium.

Organic surface treatment agents for rutile titanium dioxide for coatings mainly include pentaerythritol, trimethylolpropane, trimethylolethane, triethanolamine, diisopropanolamine, monoisopropanolamine, and octyl glycol. The above surface treatment agent has a better effect on triethanolamine, but it will prolong the drying time of the paint film, and the effect of trimethylolpropane is good, but it cannot be used for papermaking titanium dioxide because it will foam in the pulp, so that The paper produces small holes in the surface when it is dry.

The method of adding the organic surface treatment agent can be generally added after the inorganic surface treatment, the slurry before drying, or the final air flow pulverization, and the latter is added in the industrial production, and the addition amount generally accounts for 0.1% of the titanium dioxide. ~1% (mass), the usual amount is 0.2%~0.6%.

The effect of titanium dioxide surface treatment is as described above to improve the weather resistance and durability of titanium dioxide, to compensate for the defects of photochemical activity of titanium dioxide, and to improve its dispersion properties in various media, but it cannot improve the whiteness of titanium dioxide, nor can it improve the whiteness of titanium dioxide. Significantly improve its opacity, achromatic power and other indicators (except for increasing the hiding power of latex paint), these indicators depend on the quality of the base pigment (untreated titanium dioxide pigment) before the surface treatment.