Chemical Technology April 2015

DESIGN & MATERIALS OF CONSTRUCTION

The Wacker Chemie Nünchritz site outside Dresden

a large surface area. Their pronounced particle-particle interactions are the reason behind the superior rheology effect of HDK ® . Another characteristic property of pyrogenic silica is that its affinity to water and its reactivity can be tailored via chemi- cal surfacemodification. Wacker offers two different product classes: hydrophilic and hydrophobic HDK ® . Hydrophilic pyrogenic silica finds use as a thixotropic agent in applications such as coatings, paints, adhesives, sealants and composites. For its purity and freeflow properties, the pharmaceutical and food industries frequently alsomake use of hydrophilic grades. Hydrophobic HDK ® does not generally react with polar resins, thus finds use, for example, as rein- forcing fillers in silicone elastomers and as thixotropic agents in coatings and polar epoxy, vinyl-ester or polyurethane resins. Application example: wind turbine Adhesives for wind-turbine rotor blades are a striking ex- ample of a HDK ® application. Rotor blades generally consist

the addition of pyrogenic silica to obtain the necessary mechanical properties. This makes HDK ® an important for- mulation ingredient for the manufacture of such silicones. Since the company produces the pyrogenic silica itself, it has a reliable rawmaterial supply for this very important additive; the Nünchritz site, as a result, thus also profits as a centre of competence for silicone sealants. As well as water-compatible HDK ® grades, the chemical plant produces water-repellent ones, so-called hydrophobic HDK ® . Here, the pyrogenic silica’s surface is modified with the aid of silanes or siloxanes, so that it repels water and controls or prevents chemical reactions with reactive resins. Nünchritz is equipped with the necessary processes and

facilities to manufacture hydrophobic HDK ® . Pyrogenic silica: properties and applications

HDK ® aggregates have some special features. For example, they possess an open structure, low specific density and

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Chemical Technology • April 2015