Boiling and Bleaching Stains
Common defects in scouring include various stains: raw stains, alkali stains, calcium stains, yellow stains, and water glass stains. So the question arises: how do these stains form, and how can they be resolved?
1. Raw Stains
Defect Appearance
Local areas of the fabric are dark yellow; cottonseed hull residues may sometimes be present, and the capillary effect value is very low.
Causes
Insufficient dosage of chemical auxiliaries during scouring.
Too low scouring temperature.
Inadequate scouring duration.
Rapid pressure rise leading to fast temperature increase.
Too low liquor ratio of the scouring bath, or liquid leakage mid-process, leaving part of the fabric not fully immersed in scouring liquor.
Over-compacted fabric loading in the kier, blocked iron tubes in the heater, or insufficient flow rate and velocity of the liquid pump, resulting in poor circulation of the scouring liquor.
All the above factors cause uneven scouring and lead to raw stains.
Solutions
Formulate and strictly implement the scouring process. Special attention shall be paid to controlling the rates of pressure and temperature rise, as well as the scouring temperature.
Maintain a proper liquor ratio and prevent liquid leakage during scouring to avoid partial fabric not being immersed in scouring liquor.
Load fabric into the kier with moderate compactness. Adjust the liquid pump to a proper flow rate and velocity for smooth and uniform circulation of scouring liquor.
Remedial Method
Raw stains can be repaired by re-scouring. For re-scouring, the dosage of caustic soda and surfactants may be slightly reduced, and the scouring time can be shortened appropriately.
2. Alkali Stains
Defect Appearance
Dark brown stains with residual liquor on partial fabric areas.
Causes
Alkali stains are mainly formed by the drying of alkali liquor on fabric.
When the scouring liquor ratio is too low or liquid leakage occurs mid-process, part of the fabric is not submerged in scouring liquor. The residual scouring liquor dries on the fabric surface, forming alkali stains and even local alkali shrinkage.
After scouring, excessive and rapid liquor drainage, slow/delayed water feeding for washing, insufficient water volume, or partial fabric not immersed in wash water will cause residual liquor to dry on fabric, easily resulting in alkali stains and local alkali shrinkage.
If fabric still carries residual liquor after unloading from the kier, and acid pickling is delayed or incomplete, residual liquor will adsorb onto the fabric and form alkali stains.
Solutions
Maintain a proper liquor ratio and prevent liquid leakage during scouring to ensure full immersion of fabric in scouring liquor.
Guarantee the effect of acid pickling and water washing after scouring to avoid uneven or incomplete washing and residual alkali in local areas.
Remedial Method
Alkali stains can be removed by repeated washing with hot dilute acid or hot water. Severe cases require re-scouring. Repair is impossible if local alkali shrinkage has already occurred.
3. Calcium Stains
Defect Appearance
The color of calcium stains is close to that of fabric after scouring, so they are generally hard to identify visually. Calcium stains will make the fabric feel stiff and may even render the fabric water-repellent.
Causes
Excessively hard water used for scouring or post-scouring washing leads to adsorption of calcium soaps, magnesium soaps or inorganic calcium salts on fabric, forming calcium stains.
Solutions
Soft water shall be adopted for both scouring and water washing.
Remedial Method
If calcium stains are detected, repeatedly wash with hot dilute acid to dissolve calcium salts.
4. Yellow Stains
Defect Appearance
Dark brown discoloration on partial fabric areas.
Causes
Yellow stains tend to form if water for scouring, post-scouring acid pickling and washing contains large amounts of sediment or iron substances (especially when acid pickling and washing are carried out by spraying). Yellow stains caused by iron are sometimes difficult to distinguish from alkali stains. Test method: drop one drop of dilute hydrochloric acid on the stain, then add one drop of potassium thiocyanate solution. A red color indicates the yellow stain originates from iron.
Solutions
Control water quality and the condition of water supply pipelines.
Remedial Method
Yellow stains caused by sediment can be eliminated by repeated washing. For iron-induced yellow stains, treat the fabric with 2–3 g/L oxalic acid solution at medium temperature. Iron ions will convert into soluble iron oxalate complexes and be removed.
5. Water Glass Stains
Defect Appearance
Fabric feels stiff and becomes water-repellent.
Causes
Excessive water glass dosage in scouring liquor, together with delayed or insufficient water washing and acid pickling, causes water glass, silicon dioxide, calcium silicate and magnesium silicate to adsorb onto fabric and form water glass stains.
Solutions
Control the dosage of water glass, raise washing temperature and strengthen water washing.
Remedial Method
Repeatedly wash with hot dilute acid or hot water. Severe cases require re-scouring.
Post time: Sep-14-2026