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Synthrapol is a special detergent used in pre-scouring fibers before dyeing, and in washing out fiber reactive dyes after dyeing. It is also sometimes used as a surfactant or wetting agent to improve dye penetration.
A detergent contains long, thin molecules which each have one end that "likes" oily substances, and another end that "likes" water. Detergent molecules will completely surround a tiny particle of something too oily to be washed away by water alone, leaving just their water-loving 'tails' sticking out, to be easily swept away by the water. Water is the strongest of all solvents, but it requires detergents to wash away oily substances.
The actual contents of Synthrapol are water, isopropanol (which is ordinary rubbing alcohol, a chemical "known to the state of California to cause reproductive harm", thus the alarming warnings on the label), and the detergent itself, a combination of ethoxylated and sulfated aliphatic alcohols.
The special thing about Synthrapol is that it's supposed to be exceptionally good at removing Procion dyes from fabric. Many dyers believe that Synthrapol is better than other detergents at keeping dye from coming off a dark section of fabric and landing onto a light section of fabric - having a medium pH, unlike the high pH of the dye bath, keeps the dye from binding to the cellulose fiber in the wrong place. However, this claim has been disputed and appears to be more tradition than fact. A far more effective way to prevent back-staining of fabric with fiber reactive dye is to allow excess time for the dye to react, so that no unreacted dye remains when you start washing out.
Another special thing about Synthrapol is its neutral pH. The neutral pH does not encourage further reaction of active fiber reactive dyes, and it is much kinder than regular laundry soap to protein fibers such as silk or wool. Ordinary laundry detergents typically have a high pH, often from large amounts of soda ash in the formula, which aids in ordinary laundering of cellulose fibers such as cotton but which can damage silk or wool.
Ordinary laundry detergents contain optical brighteners, which are poorly washfast fluorescent dyes that convert invisible ultraviolet light to make your fabric appear brighter. Using brightener-free detergent, such as Synthrapol, can make your blacks and dark colors appear deeper in color. However, there's no point in worrying about this if you are going to be using ordinary detergents to wash your dyed items later, because the optical brighteners will be added then in any case.
Synthrapol is also commonly used as a surfactant to improve the penetration of water (and therefore dye) into the fiber, especially when dyeing wool. Other surfactants will also work. You can substitute one drop of liquid hand dishwashing detergent for this purpose.
Dharma Trading Company recommends the use of one-quarter cup of Synthrapol per top-loader washing machine load. A quarter of a cup is 60 milliliters, or 12 teaspoons.
PRO Chemical & Dye recommends the use of a much smaller amount, 1/2 teaspoon (2.5 ml) of Synthrapol per pound of fabric for washing out after dyeing. They recommend the same amount of Synthrapol, plus 1/2 teaspoon of extra soda ash per pound of fabric, for pre-scouring your fiber before dyeing. An eight-pound washing machine load would call for four teaspoons of Synthrapol.
Front loading washers require much smaller amounts of detergent than top-loading washers. Using too much detergent will cause serious foaming trouble. In addition to using a much smaller amount of Synthrapol in a front-loading washing machine, seek out Synthrapol LF instead of the usual Synthrapol SP.
Yes, ordinary laundry detergent will work fine, if it's all you have access to. It usually has a high pH, so do not add it until after you have rinsed out the majority of the unattached dye without using any detergent.
In washing out excess unattached fiber reactive dye after dyeing, it is more important to use very hot water than it is to use Synthrapol. Synthrapol probably does not remove any more dye than ordinary detergent, but warm or cool water is highly inefficient. The hotter your water, the more effective your washout of unattached excess dye will be. Try to use water that is a minimum of 140°F (60°C) for washing out; some dyers prefer to use boiling water, instead, for a huge improvement in efficiency. (Never use hot water on all purpose or direct dye, and avoid its use in washing fiber dyed with acid dyes or basic dyes, as well.)
For most purposes any laundry soap will do pretty well. If you can get Synthrapol, you should, but if you cannot, just use whatever detergent you have access to. Remember to wash or rinse first in cold water, and then in the hottest water that the fabric can tolerate. Hot water is more important than detergent, in removing dyes, but it is important to rinse out all salt and auxiliary chemicals before using hot water. Try not to use ordinary high-pH detergents until after you've washed away as much dye as you can without any detergent at all. Synthrapol can be used even in the first washing.
For hand-washing silks and wools, you could use any shampoo you would use for
your hair, as it has the same advantage of non-alkalinity. However,
shampoo tends to produce a lot of bubbles, which should be avoided in
automatic washing machines, and the hair conditioners often found in shampoos are not recommended at all. A superior alternative, for hand-washing only, is pure sodium lauryl sulfate, sold in feed stores as Orvus Paste
G & S Dye, in Toronto, sells a non-ionic detergent called TNA Soap, which they describe as generic Synthrapol.
Dharma Trading Company, in California, has recently (late 2007) introduced their own equivalent to Synthrapol, Dharma Professional Textile Detergent, which is similar in function but lacks the isopropanol and the resulting overly alarmist warning labels. Dharma Professional Textile Detergent costs a little less than Synthrapol.
I have never heard anything to indicate that either of these substitutes is in any way inferior to the Synthrapol brand.
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