Post-finishing Processes For Fabrics Dyed With Sulfur Black Dyes
Dec 23, 2025
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The core objectives of post-finishing processes for fabrics dyed with sulfur black dyes are to enhance color fastness, eliminate dyeing defects, improve fabric hand feel and appearance, while addressing common issues like fabric embrittlement, reddening, and unstable shade associated with sulfur black dyeing. These processes are mainly divided into two categories: basic finishing and functional finishing, with specific procedures and operational key points as follows:
I
Basic Finishing (Essential Processes to Ensure Dyed Fabric Quality)
These procedures are indispensable for sulfur black-dyed fabrics, directly impacting the stability and service performance of the finished products.
Reduction Cleansing + Soaping (Core Process for Removing Surface Dye)
- Function: Eliminate unfixed dyes, sulfide impurities on the fabric surface, prevent fading and reddening during subsequent use, and reduce fabric embrittlement.
- Operational Key Points:
- Reduction Cleansing: Use a mixed solution of sodium hydrosulfite and soda ash, control the temperature at 60–70℃, treat for 15–20 minutes, and maintain pH at 10–11. This disrupts the structure of surface dye to facilitate its removal.
- Soaping: Adopt neutral or weakly alkaline detergents, process at 80–90℃ for 20 minutes to further remove residual surface dye and size, thereby improving washing fastness.
- Note: Rinse thoroughly with clean water until neutral after cleaning to avoid fiber damage from residual chemicals.
Anti-embrittlement Treatment (Key Process for Cotton Fabrics)
- Function: During sulfur black dyeing, free sulfur is generated, which oxidizes to sulfuric acid during long-term storage or use, corroding cotton fibers and causing fabric embrittlement and tearing. Anti-embrittlement treatment neutralizes free sulfur and forms a protective film on fibers.
- Operational Key Points:
- Method 1 (Commonly Used): Urea Anti-embrittlement Method – Immerse the fabric in a 2%–3% urea solution at room temperature for 10–15 minutes, then dehydrate and dry. Urea can bind with free sulfur to prevent sulfuric acid formation.
- Method 2: Use a mixed solution of sodium acetate and trisodium phosphate to neutralize acidic residues on the fabric and enhance fiber flexibility.
Color Fixation Treatment (Stabilize Shade and Prevent Reddening)
- Function: Sulfur black-dyed fabrics tend to redden under humid or sun-exposed conditions. Color fixation treatment seals the reactive groups of dye molecules to stabilize the black shade.
- Operational Key Points:
- Copper Salt Fixation Method – Use copper sulfate or copper acetate solution with a concentration of 0.5%–1%, adjust pH to 4–5, and treat at room temperature for 10 minutes. Copper ions form stable complexes with the dye to inhibit reddening.
- Note: Do not use excessive copper salt, otherwise the fabric shade may turn greenish.
Softening Finishing (Improve Fabric Hand Feel)
- Function: Cotton fabrics dyed with sulfur black dyes usually have a stiff hand feel; softening finishing enhances fabric smoothness and wearing comfort.
- Operational Key Points:
- Select non-ionic or cationic softeners (avoid anionic softeners that may react with the dye), use a concentration of 1%–2%, immerse the fabric at room temperature for 15 minutes, then dehydrate and dry at 80–100℃.
- For heavy fabrics such as denim, adopt the padding-drying process to ensure uniform adhesion of the softener.
Preshrinking Finishing (Control Fabric Dimensional Stability)
- Function: Reduce the shrinkage rate of finished fabrics during washing, especially suitable for sulfur black-dyed cotton fabrics and denim used in garment production.
- Operational Key Points:
- Use a mechanical preshrinking machine; through the processes of moistening, squeezing, and drying, control the warp shrinkage rate of the fabric within 3%, meeting garment processing standards.
II
Functional Finishing (Optional to Expand Fabric Applications)
Additional functional finishing procedures can be added according to the end use of the fabric to meet specific requirements:
Anti-ultraviolet Finishing
- Applicable Scenarios: Outdoor sulfur black-dyed fabrics (e.g., workwear, denim jackets).
- Operational Key Points: Adopt nano-zinc oxide or ultraviolet absorbers, attach them to the fabric surface via padding method, and endow the fabric with UPF 50+ anti-ultraviolet performance.
Abrasion and Wrinkle Resistance Finishing
- Applicable Scenarios: Durable fabrics such as labor protection clothing and denim.
- Operational Key Points: Use polyurethane (PU) resins or cross-linking wrinkle-resistant finishing agents; after padding, cure at 120–140℃ to improve fabric abrasion resistance and wrinkle resistance without affecting hand feel.
Flame Retardant Finishing
- Applicable Scenarios: Industrial protective sulfur black-dyed fabrics.
- Operational Key Points: Adopt phosphorus-based flame retardants (e.g., phosphate esters) through the padding-curing process to make the fabric meet the GB 8965 flame retardant standard. Ensure compatibility between the flame retardant and sulfur black dyes to avoid shade changes.
III
Precautions for Finishing Processes
- Ensure compatibility between chemicals used in each process and sulfur black dyes; avoid strong acidic or oxidizing agents to prevent dye reduction and discoloration.
- Do not set the drying temperature too high (not exceeding 120℃ for cotton fabrics), otherwise fiber embrittlement and shade yellowing will be aggravated.
- Conduct sampling tests on key indicators after finishing: washing fastness (≥ Grade 3), rubbing fastness (≥ Grade 3), shrinkage rate (≤ 3%), and fabric strength (no significant reduction).
