Message: Hi, I have black shorts that are 65% polyester / 35% cotton.  The shorts are fading and I would like to dye them back to black or possibly even blacker than before.  Also are there any products that can prevent fading after the dye?

I'm afraid it would cost a lot less to just buy a new pair of shorts. (If you buy a pair of 100% cotton shorts, you will be easily able to redye them if you ever want to.)

The problem is that polyester is not easy to dye. It requires a special polyester dye, because all-purpose and other cotton dyes will wash out. (See "Dyeing Polyester with Disperse Dyes".) Moreover, it requires a lot of heat to get this special dye into the polyester fiber. You can dye cotton will cool-water dyes, but polyester must be boiled with the dye for a while. This means that you need to invest in a very large cooking pot, large enough for your garment to move in freely as it boils with the dye, but you must not plan to reuse the pot for food. Dyes for polyester or cotton are not food-safe, and thus should not be used in cooking pots that you intend to reuse for food preparation.

You will not be able to buy polyester dyes in any local shops. The dyes that are locally available work only on natural fibers. You can mail-order disperse dyes from PRO Chemical & Dye in Massachusetts or Aljo Mfg. in New York. You can also mail-order Jacquard Products' low-energy disperse dye, iDye Poly, from a number of retailers, such as Blick Art Materials or Dharma Trading Company. 

If you try to dye your shorts with an easy-to-use cotton dye, such as Procion fiber reactive dye, the 35% cotton in the shorts will pick up the dye nicely, but the dye will just wash out of the 65% polyester. That means that you can get a color that is only about 1/3 as dark as the maximum you could get with 100% cotton. This would be fine if you wanted a pale color such as pink, but it's no good at all when you want a deep dark black.

There are some effective commercial dye fixatives for use on negatively charged dyes, such as the direct dyes that are often used on cotton, or the acid dyes usually used on wool or nylon, but these fixatives do not work on uncharged dyes. Since the disperse dyes used to dye polyester do not carry a negative charge, there is no reason to expect these cationic dye fixatives to work on polyester dyes. They do work well on the dyes used in most commercial garments made of cotton, rayon, or nylon, however; see my page on "Commercial Dye Fixatives".

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