Mixing Applications

Fumed Silica Dispersion

Fumed Silica (also called colloidal silica) is a fluffy white powder with an extremely low density, marketed under trade names such as Aerosil®* and Cab-o-sil®*. With both hydrophobic and hydrophillic grades available, it is widely used as a rheology modifier, imparting highly thixotropic properties at relatively low percentages. It can also provide increased tack, better stability in suspensions and prevents &#;sagging&#; and settling of solids in a liquid system. For this reason it is particularly suitable for coatings, inks, adhesives, resins, sealants and greases.

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Fumed silica is also used in some pharmaceutical and cosmetic applications. 

*Registered trademarks of Evonik Industries AG and Cabot Corp. respectively

The Process

Despite the diversity of end uses and the various processing methods and equipment used, (ranging from simple agitators to milling equipment) there are a number of common processing requirements which must be satisfied in order to obtain maximized &#;functionality&#;:

• Mixing equipment must be capable of rapidly incorporating and dispersing the powder into the base liquid.
• Shear is required to obtain functionality. The dispersion equipment used has as much of an effect on the viscosity obtained as the concentration of fumed silica. This must be taken into account when formulating products or altering processing conditions.
• Vigorous in-tank movement must be maintained as the viscosity rises.

Material selection

Generally speaking, hydrophobic fumed silica can be divided into two categories: hydrophilic and hydrophobic. For general-purpose ortho- and meta-benzene unsaturated resins, hydrophilic products are generally used, while for polar resins, hydrophobic products are recommended. In addition, the specific surface area of hydrophobic fumed silica, resin styrene content, additives, cobalt salts and transparency requirements, and the order of addition also have a considerable impact on the use of hydrophobic fumed silica.

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Dispersion

The key to making hydrophobic fumed silica work effectively is ensuring that it is properly dispersed in the resin.The better the decentralized design, the better the effectiveness. For the resin industry, the dispersion of hydrophobic fumed silica can use a variety of methods, such as milling, high-speed shearing, ball milling, sand milling, ultrasonic dispersion, etc., of which the first two methods are used more.As far as the main dispersing steps are concerned, there are also two types, the so-called one-step method and the two-step method. The process is simple, and it is more suitable for mass production and single product production. The two-step method is to first disperse the hydrophobic fumed silica in a relatively high proportion of the resin to make a masterbatch, and then add the resin to dilute to a certain proportion. The characteristics of the two-step method are that due to the high viscosity of the system, the shear force during dispersion is large, and the dispersion of hydrophobic fumed silica is better, which is suitable for the production of various varieties.

Store

It is well known that the effect of moisture on resin is considerable. In the production of glass fiber reinforced plastic products, some enterprises have requirements on the moisture content of glass fiber. However, in the current market, due to improper transportation, the damage to materials is quite serious, and the manufacturers have not paid enough attention to this and used them correctly, thus affecting the interests of FRP manufacturers. In fact, it is easier to judge whether the product is wet. In addition to the standard weighing test, visual inspection can also make basic judgments. Generally, when the product is not damp, the packaging is neat and the elasticity is better. Conversely, packaging deformation has no elasticity at all. In this regard, users of hydrophobic fumed silica are expected to pay enough attention.

Test

General users check the thixotropic index and viscosity of the final product, and it is enough to meet the standard, so as to determine the formula and process. What needs to be discussed here is whether the process and formula are the best and most reasonable. From the above analysis, we know that reasonable dispersion effect is an important factor to exert the performance of hydrophobic fumed silica when the material factor is determined. so. The author suggests that users of hydrophobic fumed silica should determine the process and ratio from its dispersion effect, not from the final effect, because other factors such as the amount of addition will also affect the final effect. During the production process, The dispersion effect analysis should be enhanced, the viscosity and thixotropic index of the final product is only a result.

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