Synthrapol is used to suspend & remove excess dye from fiber. It removes any unfixed or unreacted dye to keep it from redepositing dye onto areas of the fabric that you don't want to be stained.
Invisible lubricants, dirt, oil, & other impurities can interfere with the dye and cause uneven dyeing.
Prewashing in Synthrapol helps remove all these before dyeing your fabric. T
ie-dyers and printers: your white areas run the risk of being stained in the final wash by the run-off excess dye.
Synthrapol in the wash keeps loose dye particles in suspension so they don't stain other areas of the fabric.
It is highly recommended for use as a pre wash to remove excess dye (especially in reds and blues), sizing, dirt and oils.