Process for quenching or suppressing the fluorescence of substrates treated with fluorescent whitening agents

A process for quenching or suppressing the fluorescence of natural or synthetic polyamide substrates treated, or to be treated, with fluorescent whitening agents, which process comprises applying to said substrates, before or after the treatment with the fluorescent whitening agent, the step of applying a liquor containing at least 1% of ##STR12## wherein R.sub.1 is hydrogen, halogen, C.sub.1 -C.sub.12 -alkyl, C.sub.5 -C.sub.6 -cycloalkyl, C.sub.7 -C.sub.9 -phenylalkyl or sulfo.

• R.sub.2 is hydrogen, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy, halogen, hydroxy or sulfo,
• R.sub.3 is hydrogen, C.sub.1 -C.sub.12 -alkyl, C.sub.1 -C.sub.4 -alkoxy, phenyl, C.sub.1 -C.sub.8 -alkylphenyl, C.sub.5 -C.sub.6 -cycloalkyl, C.sub.2 -C.sub.9 -alkoxycarbonyl, halogen, carboxy-C.sub.1 -C.sub.4 -alkyl, C.sub.2 -C.sub.9 -phenylalkyl or sulfo, and
• X is a radical of formula ##STR13## wherein R.sub.4 is hydrogen, halogen, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy, C.sub.2 -C.sub.9 -alkoxycarbonyl, carboxy or sulfo,
• R.sub.5 is hydrogen or halogen and
• R.sub.6 and R.sub.7 are each independently of the other C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy, C.sub.5 -C.sub.6 -cycloalkyl, phenyl or phenyl which is substituted by C.sub.1 -C.sub.4 -alkyl and hydroxy,
• and fixing said UV absorber thereon.