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Chemical Technology • April 2015
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
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
DESIGN & MATERIALS
OF CONSTRUCTION
The Wacker Chemie Nünchritz site outside Dresden