Use of aerogels for deadening structure-borne and/or impact sounds

DETAILED DESCRIPTION OF THE INVENTION In general, the aerogels used are those based on metal oxides which are suitable for the sol-gel technique (C.
J.
Brinker and G.
W.
Scherer: Sol-Gel Science 1990, Chapters 2 and 3), such as e.
g.
Si or Al compounds or those on the basis of organic substances suitable for the sol-gel techniques, such as melamine-formaldehyde condensates (U.
S.
Pat.
No.
5,086,085) or resorcinol-formaldehyde condensates (U.
S.
Pat.
No.
4,873,218).
Mixtures of the above-mentioned materials may also be used.
Used with preference are aerogels containing Si compounds, and SiO.
sub.
2 aerogels in particular.
In a particularly preferred embodiment the aerogel particles have permanently hydrophobic surface groups.
Suitable groups for permanent hydrophobization are e.
g.
silyl groups of general formula --Si [R].
sub.
n, where n=1, 2 or 3, and preferably trisubstituted silyl groups where the R radicals, generally independently of one another, are the same or different and are a hydrogen atom or a nonreactive organic linear, branched, cyclic, aromatic or heteroaromatic radical, preferably C.
sub.
1 -C.
sub.
18 -alkyl or C.
sub.
6 -C.
sub.
14 -aryl, especially preferably C.
sub.
1 -C.
sub.
6 -alkyl, cyclohexyl or phenyl, and particularly methyl or ethyl.
Especially advantageous for permanent hydrophobization of the aerogel is the use of trimethylsilyl groups.
The incorporation of these groups can take place as described e.
g.
in WO 94/25149 or German patent application 196 48 798.
6, or be carried out by gas-phase reaction between the aerogel and e.
g.
an activated trialkylsilane derivative, such as a chlorotrialkylsilane or a hexaalkyldisilazane (cf.
R.
Iler, The Chemistry of Silica, Wiley & Sons, 1979).
Compared with OH groups the hydrophobic surface groups prepared in this manner extensively reduce the dielectric loss factor and dielectric constant.
Depending on the air humidity, aerogel particles having hydrophilic surface groups can adsorb water, as a result of which the dielectric constant and dielectric loss factor can vary with air humidity