Technical requirements and selection principles of titanium dioxide for PVC products

Titanium dioxide is used as brightener and sunscreen in the formulation of PVC products. Anatase titanium dioxide has poor heat resistance and light resistance, and is mostly used for indoor PVC products, while rutile titanium dioxide is mostly used for outdoor PVC products. In fact, once PVC products are circulated to the market, it is difficult to control their scope of use.

Although different PVC products have different requirements for the properties of titanium dioxide used, the main indicators that need to be considered are:

Titanium dioxide: content, whiteness and background color, light stability and weather resistance, crystal form, heavy metal content, etc.

Titanium dioxide content is the preferred method to identify the quality of titanium dioxide.

Titanium dioxide: the content is too low. If it is not marked as composite titanium dioxide, it should be considered as doped titanium dioxide.

Titanium dioxide is the best brightener for PVC products. Its whiteness and tone (ground color) directly affect the whiteness and tone of PVC products. PVC products with different whiteness and hue have different usage habits. For example, the market prefers blue background color for PVC profiles, while most PVC pipes are light yellow background color (natural color). This requires the whiteness and hue of raw materials such as titanium dioxide to be controlled in the production process of PVC products. Gb/t1706-2006 does not require the hue of titanium dioxide, and the hue of titanium dioxide from different manufacturers and models is different.

The particle size, shape and particle size distribution of titanium dioxide, as well as the type and content of impurities have an impact on the hiding power, gloss, whiteness and hue of titanium dioxide. The particle size distribution range of titanium dioxide with good pigment performance is small, and it is mainly distributed in 0.2-0.3 μ m。 At this time, the achromatic power and hiding power of titanium dioxide are the largest, and the whiteness and hue are the best. Titanium dioxide: the particle size is mainly 0.2 μ M, titanium dioxide tends to be blue; The particle size is mainly 0.4 μ M, the color of titanium dioxide is red.

In the process of PVC processing, when the mixing shear force reaches the maximum, the abrasion effect of titanium dioxide is the most serious on the contact surface between the mixer and the mixed material before being wrapped by the polymer. During the molding process, the abrasion of titanium dioxide pigment in plastic is insignificant compared with the allowable amount, because in essence, there is no more direct contact between titanium dioxide and the inner wall of the machine.

Titanium dioxide, like other powder materials, also absorbs moisture from the surrounding environment. The amount of moisture absorption depends on the relative humidity of the atmosphere around the pigment. Too high the moisture content of the pigment makes the pigment easy to agglomerate and reduce its wettability. These two factors usually lead to poor dispersion of the pigment.

When using titanium dioxide without effective surface treatment, under the action of sunlight and water, PVC profiles are prone to gray, which seriously affects the appearance and color of products. Therefore, under the condition of lead stabilizer, titanium dioxide with good weather resistance after good surface coating treatment must be selected. (the reaction of water and oxygen adsorbed on the surface of titanium dioxide will produce primary free radicals, such as hydroxyl radical oh ˙ And peroxide hydroxyl radical hoo ˙。 Free radicals transfer from the pigment surface to the matrix, hydroxyl radical oh ˙ It can react with organic molecules to produce intermediate degradation products, while the peroxide radical hoo ˙ Lead salt can be reduced to low valence.)

Therefore, the general titanium dioxide used in plastics needs surface treatment. Different varieties of inorganic oxides coated on the surface of titanium dioxide have different effects on the performance of titanium dioxide. Aluminum oxide mainly improves the chemical stability of titanium dioxide, while silicon dioxide and zinc oxide mainly improve the weather resistance of titanium dioxide. The higher the amount of silicon in the coating, the better the weather resistance, but it affects the hiding power of titanium dioxide.

After inorganic surface treatment, the pigment will contain chemically bound water, but it can be released only when it is higher than the boiling point. This water generally has no impact on PVC processing, because it has not reached its boiling point under the usual PVC processing temperature.

In addition to inorganic oxide coating, the surface of titanium dioxide particles is treated with organic matter, which can make titanium dioxide easy to disperse in PVC-U system, and reduce the hygroscopicity of titanium dioxide at the same time.

The denser the inorganic coating on the surface, the lower the solubility of titanium dioxide in concentrated sulfuric acid, and the better the weather resistance of titanium dioxide. Therefore, acid solubility can be used as the basis for characterizing the weather resistance of titanium dioxide.

Super durable titanium dioxide has been designed as a dense silicon coating or zirconia stabilizer to treat the surface of titanium dioxide. This surface treatment reduces the interaction between free radicals and resin system generated by the core of titanium dioxide particles under ultraviolet radiation, but there are also some defects. The surface treatment method with dense silicon coating has excellent chalking resistance, but usually at the cost of reducing the dispersion, gloss retention and anticoagulation in the solvent system. Lower anti coagulation performance and higher surface treatment degree (usually only 89% titanium dioxide content) may reduce the initial gloss and hiding power of many resin systems. In contrast, the use of zirconium compounds in surface treatment can achieve higher hiding power, gloss and gloss retention, but at the expense of chalking resistance.

The light stability and weather resistance of titanium dioxide are determined by the crystal form of titanium dioxide and the quality of inorganic coating. Titanium dioxide itself has photoactive points, which can participate in the degradation of PVC products in the form of photocatalysis. The photochemical activity of anatase titanium dioxide is 10 times higher than that of rutile titanium dioxide. When the surface of titanium dioxide particles is not coated, or the coating effect is poor, under the action of ultraviolet light, the exposed ti4+ is reduced to ti3+, which can precipitate the lead in the lead salt, resulting in reversible discoloration and blackening of PVC products.

Good weather resistance can make titanium dioxide play the following roles in PVC products: better absorb ultraviolet rays to reduce the aging and discoloration of the resin inside the products caused by light and oxygen; Through the good scattering and refraction of visible light, it can cover the discoloration of PVC and reduce the negative effects of photocatalysis of titanium dioxide, such as loss of light, chalking and lead ash

The addition of anatase titanium dioxide into rutile titanium dioxide will reduce the light stability and weather resistance of PVC products, and increase the probability of blackening and deterioration of weather resistance of PVC products. Rutile titanium dioxide with good coating effect shall be selected for outdoor PVC products.

The content of heavy metals is an important control index for the hygienic performance of water related PVC products. The heavy metals in untreated titanium dioxide mainly come from ilmenite, and the main heavy metal elements are Cr and Pb. In the prior art, heavy metal salts are added to titanium dioxide in order to adjust the whiteness of titanium dioxide. In addition, both composite titanium dioxide and doped titanium dioxide may be mixed with heavy metal pigments. At present, two technical problems, the detection method of heavy metals in titanium dioxide used for PVC products involved in water and how to control the content of heavy metals in titanium dioxide, have not been solved.