Celebrations for an All-Rounder - Wacker Chemie AG

Celebrations for an All-Rounder

Pyrogenic silica is an all-rounder. Sold by WACKER under the name HDK®, it has a myriad of applications: reinforcing filler in silicone elastomers, anti-caking agent in pharmaceutical powders, and flow modifier for paints, surface coatings and adhesives. WACKER first started developing HDK® 50 years ago.

Pyrogenic silica is an all-round product widely employed in the chemical industry. Yet, despite being in everyday contact with it, consumers are usually unaware of its presence. In principle, HDK® – WACKER’s trade name for pyrogenic silica – is nothing more than a hyperpure, amorphous form of silica that has a high surface area and low density. It is used by the pharmaceutical industry, for example, to bestow optimum free-flowing properties on the products which patients subsequently consume in the form of tablets and capsules. It serves as a flow improver in toners, enhancing print definition and resolution. It is responsible in cosmetics for ensuring that lipstick does not melt and run in the heat. And it makes automotive coatings much easier to process.

“Pyrogenic silica is essential for many products and industrial production processes and therefore is in great demand,” says Maria-Anna Biebl, head of WACKER’s HDK® business in EMEA/India.

Just how important those seemingly nondescript additives are is shown by the example of the fast-growing market for wind turbines. Wind power alone now accounts for some 10% – and rising – of Germany’s electricity needs. Modern wind turbines can produce peak outputs of up to 7.5 megawatts and the blades can be more than 50 meters long.

HDK® Controls Rheology

Pyrogenic silica is obtained by combusting chlorosilanes with hydrogen and oxygen in an oxyhydrogen flame at over 1,000 degrees Celsius. The reaction produces silica and hydrogen chloride. The latter is fed back into the production loop as an auxiliary. The particles formed in the flame are initially just a few nanometers in size. On cooling, they fuse to form larger, highly branched, flaky aggregates, which ultimately become several micrometers in size – resulting in the large surface of HDK®.

As the turbines have grown in performance, so too have the demands imposed on their constituent materials. The tips of the 50-meter rotor blades rotate at speeds of up to 340 kilometers per hour. This generates centrifugal forces equivalent to 18 times the acceleration due to gravity. Consequently, the rotor blade half-shells made of glass-fiber-reinforced plastic must be joined together so stably that they will never tear apart. The adhesive holding the half-shells together plays a key role here. Its flow properties are controlled by HDK®, which prevents the ingredients from separating and the fillers from settling.

“Pyrogenic silica helps to create the optimum processing conditions,” says Biebl, summarizing the various positive effects of the additive. Without pyrogenic silica, the adhesive would sag because it would be too thin.

HDK® is made by the reaction of chlorosilanes with hydrogen and oxygen in an oxyhydrogen flame at over 1,000 degrees Celsius. The spherical primary particles initially formed in the flame (see diagram above) fuse together permanently to form larger, highly branched aggregates with a diameter of 100 to 500 nanometers. When these cool, they form flaky agglomerates several micrometers in size.

Experimenting with Water

Inside the HDK® filling facility in Burghausen: because pyrogenic silica has such a low density, it is very easy to transport the bags.

This year, WACKER’s hyperpure amorphous silica celebrates its 50th anniversary. The initial work aimed at producing silica coincided with research into hyperpure silicon in Burghausen back in 1955. At the time, chemists were attempting to convert silicon tetrachloride byproduct into pure silica by treating it with water. However, this approach proved too problematic and was abandoned.

Success came from a different quarter in 1966 following research conducted at the ESK foundry in Kempten, southern Germany. There, scientists led by Dr. Günter Kratel were attempting to convert low-grade silicon carbide powder into a useful form. They chlorinated the powder to tetrachlorosilane, which they then cleaned up and pyrolyzed with hydrogen. The outcome was a useful white ash – ultrapure silica in particularly finely divided form.

Industrial production of HDK® commenced in Kempten in 1972. Thereafter, production volumes went from strength to strength. Further plants were added in Burghausen (1978), Nünchritz (2002) and Zhangjiagang (2008), enabling WACKER to emerge as one of the world’s leading silica producers. The Kempten site was shut down in 2011 and production was switched to Burghausen. WACKER now estimates that the market for silica is growing by 3% to 6% every year.

In addition to its many other qualities, HDK® makes an excellent thermal insulator. A 15-mm-thick HDK® vacuum insulation panel, for instance, has an insulation performance comparable to that of a 100-mm polystyrene panel. Accordingly, wherever extremely high or low temperatures need to be regulated and space is at a premium, HDK®-based high-performance insulating materials have become the products of choice. Hardly surprising, then, that HDK® is used in high-temperature applications in industry. It can be found in everyday life, too – in radiant heaters for ceramic hobs and in refrigerators.

Hydrophilic and Hydrophobic

WACKER offers hydrophilic and hydrophobic variants of pyrogenic silica to suit different applications. Hydrophilic HDK® is produced by hydrolyzing chlorosilanes in an oxyhydrogen flame, whereas hydrophobic HDK® is made by chemically modifying hydrophilic HDK® with reactive silanes, such as siloxanes. The resulting water-repellent properties mean that hydrophobic silica cannot be dispersed in water.

WACKER processes some of the pyrogenic silica produced in Burghausen, Nünchritz and Zhangiagang to make fillers for the Group’s silicone elastomers and joint sealants. HDK® significantly improves the tensile strength of silicone rubbers. It is also used to adjust the viscosity of silicone fluids.

“Pyrogenic silica is essential for many products and industrial production processes.”

Maria-Anna Biebl Maria-Anna Biebl, head of HDK® business in EMEA/India at WACKER
Maria-Anna Biebl inside Burghausen’s HDK® filling facility. The plant is an essential part of the site’s integrated production system, because it processes surplus silanes resulting from the manufacture of silicones and polysilicon.

For WACKER itself, HDK® has a totally different function: it plays a vital role in the company’s integrated production system, where it serves to optimize the raw materials yield. The production of hyperpure polycrystalline silicon, the starter material for solar cells and computer chips, generates large amounts of tetrachlorosilane as byproduct. It can either be returned straight to the production process or be converted into pyrogenic silica. The hydrogen chloride gas (HCl) liberated in the process can be returned to the integrated production loop and made to react again with raw silicon to yield chlorosilanes. These are subsequently purified by distillation and converted into hyperpure polycrystalline silicon.

As the HCl molecules more or less act as auxiliaries in silicone and polysilicon production, the material can, with some effort, be returned to the production process without detriment to quality. The HDK® plant thus kills three birds with one stone: it boosts the added value at the site, it reduces waste, and helps with the recycling of HCl.

At first glance, HDK® may seem to be an unspectacular product. But a closer look reveals pyrogenic silica as having all kinds of applications. That is why Maria-Anna Biebl and her team are constantly looking for new applications for this seemingly innocuous white powder.