The dyeing process for cotton knit fabric through the exhaust dyeing method is a meticulous journey. In this comprehensive guide, we dive deep into the intricate steps involved in transforming raw cotton into vibrant, dyed knit fabric. Explore the nuances of the exhaust dyeing method, from pretreatments and color application to fixation and finishing. Discover how this technique enriches cotton knits with vivid, long-lasting hues, unraveling the science and artistry behind this dyeing process. Exhaust dyeing is a batch method, so the dye gradually moves from the bath into the fabric until the required shade is reached.
What Is Exhaust Dyeing?
Exhaust dyeing is widely used for cotton knits because it gives good control over shade depth, levelness, and repeatability when the bath chemistry is managed correctly. For cotton knit fabric, the bath should be loaded evenly and circulated well, because rope marks and patchy pickup are harder to correct after fixation.
What Is 100% Cotton Knit Fabric?
100% cotton knit fabric is a type of textile material made completely from cotton fibers and produced by using a knitting process. Knitting involves interlocking loops of yarn to create a stretchy and breathable fabric. This results in a soft, comfortable, and versatile material that is commonly used for making T-shirts, underwear, sweaters, and various other clothing items. 100% cotton knit fabric types are s/j, rib, terry, fleece, pique, lacoste etc. Single jersey is often written as S/J. Because the structure is looped, knit fabric stretches more than woven fabric and usually needs careful shade control during dyeing. Single jersey usually levels more easily, while rib, terry, fleece, pique, and lacoste need tighter control because their structures absorb dye a little differently.
Cotton Knit Fabric Dyeing Process
1. Demineralization
We know that cotton grows naturally and that cotton absorbs minerals from the soil. Demineralization in cotton dyeing refers to the removal of mineral ions from the cotton fabric before the dyeing process. It is an essential step in the textile industry, as the presence of minerals in the fabric can interfere with the dyeing process and affect the quality of the dyed product. In this process, wetting agents and sequestering agents are commonly used. According to the American Water Works Association, hard water is mainly caused by calcium and magnesium, so a chelating system is often used before dyeing to keep the bath stable.
Why Demineralization Is Necessary
- Mineral Interference: Minerals in water, such as calcium and magnesium ions, can form insoluble compounds with certain dyes, preventing them from effectively binding to the fabric.
- Color Consistency: Removing minerals ensures uniform and consistent color uptake during the dyeing process, resulting in better color quality. If the mill uses hard water, this is one of the first checks before shade matching starts.
2. Scouring and Bleaching
Scouring is the initial step in preparing cotton fabric, and it involves the removal of natural impurities, waxes, oils, and other substances present in the cotton fibers with the help of NaOH. This process ensures that the fabric is clean and ready to accept dyes or finishes evenly. Then bleaching is done with H2O2 to remove the natural color from the cotton greige fabric. The real goal here is full absorbency, because cotton that wets evenly will take dye more uniformly in the next bath.
This process is described briefly below.
In this textile processing procedure, the process begins by filling the machine with water at a temperature of 40⁰C. Detergent, an anticreasing agent, and a sequestering agent are injected into the water, followed by loading the fabric. The water temperature is then raised to 60⁰C, and caustic soda is dosed into the mixture for 7 minutes. Afterward, the temperature is increased to 70⁰C, and hydrogen peroxide is injected. The water temperature is further raised to 98⁰C, and the machine runs for 60 minutes. Subsequently, the water is cooled to 78⁰C, and a sample of the fabric is checked for absorbency and brightness. If the sample meets the criteria, the water is drained.
Next, the machine is refilled with hot water at 80⁰C, and a 10-minute cycle is run to wash the fabric clear. After washing, the water is drained and refilled again. Acetic acid is added to neutralize any remaining chemicals, followed by the injection of a peroxide killer, and a 5-minute run cycle. The effectiveness of the peroxide treatment is checked using peroxide paper. Finally, the water is drained once more, and a final wash cycle is performed before refilling the machine. Residual peroxide must be removed completely, because even a small carryover can dull the next shade or interfere with fixation. These timings are typical machine-cycle examples, so the exact program should still be matched to fabric GSM, machine load, liquor ratio, and shade depth.
3. Biopolishing
Biopolishing is a chemical process where the protruding fibers and slubs in the knitted cotton fabric are weakened with the help of cellulase enzyme and removed with the help of water pressure. This step is often underestimated in practice, but too much cellulase can reduce fabric weight and strength, so time and temperature should be watched closely.
This process is also known as enzymatic singeing. The reason for saying enzymatic singeing is that in singeing, the hairiness on the surface of the fabric are removed with the help of flame. On the other hand, in biopolishing, the hairiness of the knitted cotton fabric is removed with the help of enzymes.
The changes that occur in the fabric by biopolishing are:
- Reduces fabric pilling.
- Gives the fabric a smooth and soft hand feel.
For knitwear, the benefit is usually most visible on single jersey and compact structures where surface fuzz is easy to notice after washing. Biopolishing is usually done at 45° to 55°C and at acidic pH. A hot wash is then given at 80°C to deactivate the enzyme.
4. Leveling
“Leveling” in cotton dyeing refers to the process of ensuring that the dye is evenly distributed and absorbed by the cotton fabric, resulting in uniform and consistent coloration.
A leveling agent is used before the dyeing step begins in the fabric. The purpose of using the leveling agent is to spread the dye chemicals evenly on the fabric.
In this step before fabric dyeing, sequestering agent, anti-creasing agent, defoaming agent etc. are also used. Here the pH is controlled at 6.5. Temperature 40°C for 5 mins. A relaxed fabric rope and steady circulation help prevent crease marks, which can otherwise show up as shade streaks after fixation.
5. Dyeing
After completing the leveling step, the fabric is ready for dyeing. Generally reactive dyes are used for cotton dyeing. Direct dyes and vat dyes are also used. Reactive dyes are usually preferred for cotton knit because they give bright shades and good wash fastness when fixation is controlled properly.
This step is described briefly below.
The initial salt dosing for 10 minutes, followed by a 10-minute run, the process continues with color dosing for 30 minutes and another 20-minute run. A sample check is conducted to assess dye shade percentages and ensure levelness. This sample check is where levelness problems are usually caught before the bath is fully exhausted, which saves re-dyeing. If all is satisfactory, the temperature is raised to 60⁰C.
Next, a 15-minute soda dosing is carried out, followed by a 15-minute runtime. The remaining 90% of soda dosing takes 20 minutes, followed by a 20-minute runtime. Another sample check is performed, and if everything meets the standards, the bath is drained, preferably rinsed. Salt is typically added in stages to drive exhaustion gradually, while soda ash raises the pH and helps the reactive dye bond with cellulose. The exact dosing time changes with fabric GSM, machine load, and shade depth, so the lab recipe should be confirmed before bulk dyeing.
A 5-minute wash and drain fill is executed at normal hot 70⁰C, followed by a 10-minute run and a clear wash. Acetic acid is used for neutralization, and a final wash and drain complete the process, ensuring water clarity. In bulk production, the lab dip should be matched against the buyer shade under standard light before the machine is charged.
6. After-Treatment
The last process of dyeing is after-treatment. This process is done in 3 steps:
1) Soaping
In this step, the fabric is soaped with the help of detergent or soaping agent as a result of which the unfixed dyes are removed from the fabric surface.
In this step the soaping agent is injected and then the temperature is raised at 90°C for 10 mins and then drained and washed. Soaping removes hydrolyzed dye as well as loose surface color, which is why the shade becomes cleaner and wash fastness improves. ISO 105-C06:2010 wash testing is commonly used to confirm that this step worked.
2) Fixing
In this step fixing agents are used to improve color fastness of the dyed fabric. Fixing agents are also known as fixative. In reactive cotton dyeing, fixative is the more accurate term here.
Dosing the fixing agent for 5 to 15 mins and then drain and wash. For reactive-dyed cotton, fixing agents should be treated as a fastness aid, not a replacement for proper alkali fixation during dyeing.
3) Softening
After fixing, the fabric is made soft and smooth with the help of softener. Use the softener carefully on absorbent knits, because heavy dosage can reduce moisture pickup and change the hand feel more than expected.
Conclusion
100% cotton knit fabric is known for its natural feel, breathability, and ability to provide good moisture absorption, making it a popular choice for casual and activewear.
This fabric is an excellent choice for anyone looking to create high-quality textiles. Its durability, softness, smoothness, and absorbency make it ideal for a wide range of products.
Advantages of 100% cotton knit fabric include comfort, breathability, and its natural, hypoallergenic properties.
Disadvantages include susceptibility to wrinkles, shrinkage, and pilling over time with wear and washing. For production, the real target is balanced shade, low pilling, and clean wash fastness after the first home washes.
Frequently Asked Questions (FAQs)
Q1. What is exhaust dyeing in textile?
Ans: Exhaust dyeing is a batch dyeing method where the dye moves from the liquor into the fiber over time. It is widely used for cotton knit fabric because it gives good shade control and even color.
Q2. Which dye is best for 100% cotton knit fabric?
Ans: Reactive dyes are usually the best choice for 100% cotton knit fabric because they give bright shades and good wash fastness. For deeper shades or special performance needs, vat or direct dyes may also be used.
Q3. What temperature is used for cotton knit fabric dyeing?
Ans: Many cotton knit reactive dyeing recipes start around 40°C to 60°C, then move into fixation based on the specific dye and machine. The exact temperature depends on the dye class, fabric weight, machine load, and shade depth.
Q4. How do you prevent uneven shade in cotton knit fabric dyeing?
Ans: Good scouring, proper leveling, and gradual chemical dosing are the main controls. A sample check before full exhaustion helps catch creases, patchiness, or rapid uptake early.
Q5. Why does cotton knit fabric bleed after washing?
Ans: Cotton knit fabric usually bleeds when the dye is not fully fixed or when after-treatment is weak. Proper soaping, correct alkali fixation, and wash fastness testing reduce bleeding sharply.
