Stabilised form of azo dispersion dye - made by heating to give 1-ethyl:hexyl 3-cyano 4-methyl 5-nitrophenyl:azo 6-hydroxy pyridone crystal form with improved rub fastness

New crystal form of the azo dispersion dye 1-(2'-ethylhexyl) 3-cyano 4-methyl 5-(o-nitrophenylazo)6-hydroxy pyrid-2-one with structure characterised by an X-ray diffraction diagram with a strong line at 8.9 a, medium line at 20.3A and medium-to-weak lines at 19.0, 14.3, 10.2, 9.6, 6.4 and 3.76A. They are made b heating the coupling prod. in water and/or organic fluid. Dyeing of aromatic polyester fibres in greenish-yellow shades. Excellent fastness to light, wet treatments and dry heat. Stability of aq dispersions during the dyeing process is increased by the crystal modification, giving improved resistance to rubbing on the fibres.