Fumed silica for thixotropic liquids

Pyrogenic Silica for Thixotropic Liquids

What have pyrogenic silicas (HDK^®) to do with silicones? Well, for one thing, HDK^® is used for precisely adjusting the desired properties of silicones (see below). For another, the production of these products is closely linked to the Manufacture of silicones at WACKER. When chlorosilanes (tetrachlorosilane and others), which are essentially unwanted volatile by-products of the silicones production process, are introduced into a hydrogen flame, they react with water formed in situ to produce pyrogenic silica (HDK^®), whose composition approximates to SiO₂:

SiCl₄ + 2H₂ + O₂ ® SiO₂ + 4HCl

A carefully controlled process and short contact time in the flame ensures that the silica is produced in the form of nano-particles. Since this synthetic silica is formed in a flame and is a white powder, it is also referred to as white carbon black. Its configuration and properties are discussed in detail below.
First, though, let us explain what a thixotropic material is:

Paints must be free-flowing when they are brushed over a surface, but they should then become viscous as soon as possible when at rest so that no sagging or drips occur. Such paints are said to be thixotropic. They are notable for the fact that, when stirred or shaken, their viscosity increases or decreases after a certain time delay. A well known example of a thixotropic liquid is tomato ketchup. Like many food and personal hygiene products, it contains thixotropic agents to thicken it.

Silicone rubber, which is used for insulating metal wires and glass fibers, may contain up to 40 % pyrogenic silica. Here, the HDK^® ensures that the vulcanized silicone rubber becomes an elastic material that has excellent mechanical properties.

The flow properties of paints and coatings can be controlled precisely with pyrogenic silica. HDK^® is accordingly used in modern paints and coatings as a rheological additive.

HDK^® is used as an active filler in joint sealants and as a free-flow agent in fire extinguisher powders. In toothpastes, its function is to impart the desired consistency.

In modern laser printers, a latent, invisible image is first written with light on an electrically charged photoconductor. The toner powder particles must “sense” the charge differences in the latent image, such that they are deposited in certain places but not in others. The result is the powder image, which, in a subsequent step, is transferred to the paper and then fixed by fusing. Strict demands are made on the toner in respect of its free-flow quality and its electrical and thermal properties. Small additions of HDK^® pyrogenic silica to the toner make it possible to regulate its free-flow property, so that HDK^® effectively “fine-tunes” the toner in laser printers.

Structure and Properties of Pyrogenic Silica

Pyrogenic silica is a very efficient thixotropic agent. This property is based on its structural characteristics. Although it has essentially the same composition as sand and quartz , structural differences exist. The following model of the atomic lattice of silica shows that every silicon atom is surrounded by four covalently bonded oxygen atoms:

In sand and quartz, these SiO₄-SiO₄ tetrahedra are regularly arranged in a three-dimensional crystal lattice, but in a primary particle of pyrogenic silica they form a disordered amorphous system. A primary particle contains about 10.000 SiO₂ units and is a so-called nano-particle with a diameter between 5 and 30 nm (1 nm = 10^-9 m). The outwardly projecting oxygen atoms are bonded to hydrogen atoms. So a primary particle is like a hedgehog whose spines consist of -OH groups These groups render the surface of a primary particle hydrophilic because they can form hydrogen bonds with water molecules. However, they also cause many stationary primary particles to form extensive three-dimensional networks through aggregation (aggregates of nano-particles).

Fluids or gases can be stored and largely immobilized in the interstitial space of these aggregates. If the liquid is water or an aqueous solution, hydrogen bonds are formed between the water molecules and the -OH groups on the surface of the silica particles. In this state, the system is viscous to solid on a macroscopic scale. Mechanical stress, such as shaking or stirring, temporarily disturbs the silica nano-particle aggregates. This ruptures some of the hydrogen bonds holding the particles together. Macroscopically, the system becomes free-flowing (thixotropic effect). The longer the external force acts, the more bonds are ruptured and the "more fluid" the material becomes. When the force stops, three-dimensional aggregates are re-formed and the material becomes viscous to solid again (see diagram above).