It's 40 years since I studied it, but acid dyes attach to the amide groups in wool or nylon.
They tend to block of from nylon when all the amide sites are occupied. I think the dyes are anionic and the amide groups are cationic, so creating any cationic groups will attract the dyes. Sometimes it is a problem with straight nylon to get the required depth of shade, so alloys of polypropylene will be even more difficult to dye to dark shades.
Regards
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