With the exceptions of nylon, which should be dyed with protein fibre dye; polypropylene, which cannot be dyed at home; and spandex, which requires an acid dye, synthetic materials may be dyed with dispersion dye. Dispersion dye gets its name from the fact that it is used with a dispersing chemical that allows the dye to get into polyester and other synthetic fibres. Without the dispersing, or carrying, agent, many synthetics require a very high temperature for absorbing dye---hotter than boiling water can provide. Dyeing polyester and other synthetic fabrics is somewhat dangerous because of the heat necessary and the toxicity of the carrying agent.
Find dispersion dye online. You cannot use natural dyes or the RIT dye commonly found in stores. Instead, order dye from Pro Chemical and Dye or Aljo Dyes.
Machine wash your synthetic fabric in hot water (60 degrees C or hotter), or wash it by hand with soda ash and Synthrapol or a homemade version of Synthrapol. Synthrapol is a fabric cleanser that can help prepare the fabric for dyeing.
Mix dispersion dye powder with 1 cup hot water in the measuring cup. If you want to dye your fabric to a deep shade of your chosen colour, use more powder; for a lighter shade of that colour, use less powder. Read the package from the exact amount of dispersion dye you'll need.
Strain the dye-water through two layers of an old nylon stocking to remove particles.
Mix the dye carrier in the amount specified on the dye package to 1 cup of hot water in another measuring cup. The dye carrier is the toxic substance that allows the dispersion dye to work on the synthetics at a lower heat.
Set a pot of 120 degree water on the stove in a stainless steel or enamel pot. You can use the thermometer to check the temperature on the water.
Add to this pot the dye carrier, the disperse dye, water softener such as Metaphos if you have hard water, and any other ingredients required by the particular brand of dye you have, in the order listed in its instructions.
Add the synthetic fabric to the pot.
Bring the pot of dye to a boil over high heat, stirring the entire time.
Allow the dye bath to simmer for as long as needed to dye the fabric to the right shade. Twenty minutes will produce a lighter dye job and 45 minutes a darker dye job. Stir the fabric every so often.
Meanwhile, bring another large pot of water to a boil, for a rinse.
When the fabric is dyed, pull it from the dye bath and put it in the boiling rinse.
Hand wash the rinsed fabric in a third pot of 160 degree water with Synthropol added, stirring it with a stick or long wooden spoon. Again, use a thermometer to check the water temperature.
Rinse the fabric once more in hot water.
If you can still smell the carrying chemical at the end of the process, repeat steps 13 through 15. Dispersion dye can also be added to one side of synthetic fabric with a hot iron. It is not toxic like the carrying chemical.
The carrying chemical should only be used in a well ventilated space and handled with gloves. Boiling water should be handled with care to avoid burns. You cannot dye clothes at home if they are dry-clean only, or you risk ruining them. Never use dispersion dye utensils or containers for food, even once they have been washed out.