Pre-Scour vs. Pre-Bleach: The Best Way to Pretreat Cotton
All dyers should know that it is crucial to pretreat cotton prior to dyeing. This process eliminates the impurities in the cotton, along with any knitting oils and waxes. It also preps the cotton for the dyeing process. All crops of cotton are different and by pretreating, you are attempting to control an otherwise uncontrollable situation, as far as substrate consistency goes.
Two options are available, pre-scouring or pre-bleaching. Pre-scouring or pre-bleaching help promote better penetration of the dyes and aid in color evenness and levelness. I’ll discuss these two options so you as a dyer can decide what best fits your needs based on the end color.
Pre-Scouring
FSW offers Sora Scour LF-MD for pre-scouring. This product is a low-foaming wetting agent, with excellent emulsifying and extractive actions. It works to remove the impurities in cotton. After pre-scouring, the substrate will become a slightly creamier color, yet still the same color as natural cotton. A basic procedure for this chemical is very simple:
1.0% (owg) Sora Scour LF-MD
0.50% – 1.0% (owg) Caustic Soda 50%
Heat to 205°F and hold for 30 minutes. After, a simple cold rinse should complete the scouring process. Your cotton is now ready for dye. A pre-scour like this is a better, more efficient process when the end color is a dark shade, such as navy or black.
Pre-Bleaching
Pre-bleaching is more common when the end colors are pastel, light, or medium shades. Think of pinks, yellows, and baby blues. Pre-bleaching will help enhance the brightness of bright colors. Pre-bleaching is also a good idea when the quality of cotton is not superior. This is the best way to remove all impurities in cotton. This creates a white substrate and, in most cases, promotes a more level dyeing and consistency from lot to lot. There are a few different ways to pre-bleach cotton, but this is my preference:
1.0% (owg) Sora Scour LF-MD
1.0% (owg) Sora Stabilizer CP
2.0% (owg) Caustic Soda 50%
5.0% (owg) Hydrogen Peroxide 50%
Heat to 205°F and hold for at least 30 minutes. Afterwards, rinse thoroughly for neutralization in preparation for dyeing. You can do this with several cold rinses or, depending on final pH, you may need to add a small amount of acetic acid to neutralize.
By pre-scouring or pre-bleaching, you will have better substrate consistency. This will allow for more repeatability from lot to lot, as your colors will be more consistent.
About the Author: Scott Wilson
