Preparation of stable solutions of azo dyes of m-phenylenediamine by reaction with formic acid

DETAILED DESCRIPTION I claim: 1.
A process for preparing a stable solution of an azo dye obtained by diazotizing and coupling unsubstituted or C.
sub.
1 -C.
sub.
4 -alkyl- or C.
sub.
1 -C.
sub.
4 -alkoxy-substituted m-phenylenediamine in acetic acid, which comprises, after the reaction has ended, treating the resulting reaction mixture with from 0.
1 to 1.
2 moles of formic acid, based on 1 mole of m-phenylenediamine, at from 60.
degree.
C.
to the boiling point of the reaction mixture.
2.
A process as claimed in claim 1, wherein the reaction mixture is treated with from 0.
3 to 0.
9 mole of formic acid, based on 1 mole of m-phenylenediamine.




Description:
The present invention relates to a novel process for preparing a stable solution of an azo dye obtained by diazotizing and coupling substituted or unsubstituted m-phenylenediamine by treating the reaction mixture present at the end of the reaction with formic acid at elevated temperature.
Dyes of the vesuvin type are used in large amounts in the paper industry for producing yellow and brown colorings.
Vesuvin dyes are azo dyes obtained by diazotizing and coupling m-phenylenediamine or alkyl or alkoxy derivatives thereof.
They are generally not uniform dyes but mixtures of monoazo, disazo and polyazo dyes, since the diamine starting material and the products derived therefrom are not just singly but multiply diazotized and coupleable.
Specific examples are dyes of Colour Index (C.
I.
) numbers 21,000 (C.
I.
Basic Brown 1), 21,010 (C.
I.
Basic Brown 4) or 21,020 (Basic Dye).
Unfortunately, vesuvin dyes tend to undergo undesirable reddening in the dyeing of paper in the acid range, which severely limits their possible uses.
It is an object of the present invention to provide a process whereby this defect can be eliminated.
We have found that this object is advantageously achieved with a process for preparing a stable solution of an azo dye obtained by diazotizing and coupling unsubstituted or C.
sub.
1 -C.
sub.
4 -alkyl- or C.
sub.
1 -C.
sub.
4 -alkoxy-substituted m-phenylenediamine where, after the reaction has ended, the resulting reaction mixture is treated with from 0