| http://www.w3.org/ns/prov#value | - The process as claimed in claim 1, wherein, in step c), the solvent-containing gel is reacted with at least one chlorine-free silylation agent of the formula I and/or II, where R1 and R2 are each identically or differently a methyl or ethyl group and R3 is a hydrogen atom or a methyl group.
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