In textile processing, a series of cleaning processes are performed before dyeing or printing, called textile pretreatment. After textile pretreatment, a neat and clean textile material is ready for further dyeing or printing, providing perfect dyed or printed articles. So, pretreatment is crucial for the best quality dyed and printed textile material. This article will guide you through knowledge of textile pretreatment.
Materials Of Textile Pretreatment
There are two main areas of pretreatment. The first is greige manufacturing, which means treating yarn to make it suitable for weaving, and the second is wet processing, which makes greige fabric ready for dyeing and printing. In yarn preparation, sizing agents are applied to warp yarn. In wet preparation, a series of steps, such as singeing, de-sizing, scouring, and bleaching, are followed to prepare greige fabric for dyeing or printing.
Pretreatment Of Yarn
To prepare yarn for weaving, sizing agents like starch, modified starch, or polyvinyl alcohol ( PVA) are applied to the yarn, which should be used as warp yarn in the weaving process. These sizing agents increase the strength of the yarn, which resists breaking, improve the process, and increase production. You should select the sizing agent, which should be easily decomposed during the resizing process after the completion of weaving.
Pretreatment Of Greige Fabric
Weaved fabric is called greige fabric. During wet processing, a series of steps are applied to remove natural and added impurities, such as dust, seeds, leaves, starch, machine oil, etc., from the surface of the greige fabric to make it ready for dyeing or printing. The following are the steps that are applied during wet processing.
Singeing
When a fabric is weaved on looms, many protruding fibers are on the fabric’s surface, which should be removed to improve overall appearance and even dyeing. Singing is the process of removing protruding fiber from the fabric’s surface and improving its appearance. The main purpose of singeing is to improve hydrophilicity, enhance dyeing quality, reduce the chances of pilling after finishing, remove fluff or lint from the fabric’s surface, and make it ready for the next step.
Methods and Precautions in Singeing
Three methods are widely used in the textile industry globally.
Plate singeing, rotary cylinder of roller singeing, and gas singeing. The singeing of yarn is called gassing. You should take care during singeing, such as keeping the flame bluish because a bluish flame provides maximum temperature. Keep the flame length and contact as per the construction of the fabric. You must check the nozzles and clear them to provide consistent flame length. The exhaust blower should be fully functional during singeing. Proper cold water circulation should be maintained to eliminate surface burning issues.
Desizing
Before the weaving process, sizing agents are applied in the yarn preparation step to increase the strength of the warp yarn and make it suitable for the weaving process. To remove that sizing agent, a step of desizing is added in wet processing called desizing. Desizing is the removal of a sizing agent from the warp yarn. If the sizing agent is not removed completely, it will create disturbing during dyeing, resulting in uneven dyeing. There are different types of desizing methods, such as enzymatic desizing, oxidative desizing, acidic desizing, etc. You should select desizing method as per your yarn quality. The pH of the bath during desizing depends on the type of desizing agents. Enzymatic desizing is usually applied in textile industries because it has many advantages, such as the is less chance of fiber damage, no need for strong chemicals and auxiliaries that damage fibers, many varieties of process possibilities, and high biodegradability which decrease hazardous in the effluent.
Scouring
Different types of impurities are attached to the surface of both natural and synthetic fibers. These impurities are oils, fats, waxes, minerals, knitting oil, mill dirt, grease, etc., attached to the fabric’s surface. So, to remove these impurities, a step is performed called scouring.
Scouring enhances the fabric’s hydrophilicity, which allows it to absorb water. It also makes the fabric neat and clean and removes all impurities that can create disturbances during the dyeing or printing process.
Scouring involves different processes, such as saponifiable oil attached to the surface of fabric being converted into soaps, protein being hydrolyzed into soluble degradation products, amine being converted into ammonia, and mineral matter being dissolved in the scouring effluent.
Scouring has many advantages, such as reducing the surface tension, making the fabric hydrophilic, which can easily absorb dyes and auxiliaries, helping in even dyeing, and improving the overall appearance of the fabric. The drawback of scouring is that the organic solvents used during the scouring process are highly inflammable and carcinogenic. Scouring is an expensive process.
Bleaching
There are many natural colors as yellowness, found in fiber, especially in cotton fibers. To remove this natural color and vegetable contamination, a process called bleaching is performed during wet processing. The main objective of this process is to ensure fabric absorption for water, dyes, and other textile auxiliaries, which will be used in further processes such as dyeing or printing and finishing. Usually, the applied impurities during weaving are hydrophobic, which doesn’t let the water absorb, so bleaching converts hydrophobic fabric into hydrophilic fabric and provides a uniform degree of whiteness.
Two types of bleaching agents are used in textile industries: oxidative and reductive. Oxidative bleaching is usually done with hydrogen peroxide (H2O2) and caustic soda (NaoH).
The advantage of hydrogen peroxide over other oxidative bleaching agents, such as hypochlorite, is that peroxide is less likely to damage the fiber, is suitable for a wide range of fibers, and provides whiteness that doesn’t convert into yellowness with time.
The disadvantage of hydrogen peroxide on hypochlorite is that peroxide is an expensive product that requires stabilizers and is used in high concentrations, which can cause skin irritation.
Conclusion
In textile industries, a series of steps are applied to prepare fabric for dyeing and printing. During the singeing process, all hanged and protruding fibers are burned out from the fabric’s surface, decreasing the chances of pilling and hairiness and providing an ideal surface for dyeing and printing. In de-sizing, the added sizing agent’s code is removed from the fabric’s warp yarn, making the yarn more absorbent. In scouring, all added and natural impurities are removed from the fabric’s surface, which provides an ideal and absorbent fabric surface for dyeing and bleaching. In bleaching, the whiteness of the fabric increases and makes it super absorbent for further processes.
FAQs
What is the purpose of textile pretreatment?
The purpose of the pretreatment is to remove all types of impurities from the fabric surface.
Why is textile pretreatment considered the heart of dyeing?
Pretreatment enhances fabric wettability and absorbency, both of which are crucial for successful dyeing.
What is the characteristic of an ideal sizing agent?
The characteristics of an ideal sizing agent are a decrease in fraction, optimal coating, and polarization. It is also affordable, easily available, and easily resizable.
How to prepare TEGEWA solution?
To prepare the TEGEWA solution, follow the steps given below.
- Weigh 0.65g of iodine crystals and add to a volumetric flask.
- Add 200 ml of ethanol to the flask.
- Weigh 5g of potassium iodide and add to the flask.
- Fill the flask with water to 1000 ml and shake well.
How will you check the results of singeing process?
To check the effectiveness of the singeing process, take before and after singed fabric, evaluate its surface, and focus on the protruding or hanging fiber attached to the fabric’s surface. If there are still hanged fibers on the fabric’s surface, repeat the process and if the surface is free from hanged fiber, the process is done successfully, and go on further process.
How will you check the results of desizing process?
Perform the TEGEWA test and evaluate the results using the TEGEWA scale. If the results are satisfactory, continue the process; if they are unsatisfactory, repeat the process.
Before Desizing, After DesizingHow will you check the results of the scouring process?
Perform the absorbency test and check the overall appearance of the before-and-after scoured fabric. If its absorbency and overall appearance improve, continue the process; if not, repeat it. After scouring, the fabric should be free from impurities.
How will you check the results of the bleaching process?
Check the whiteness and capillary test. If the fabric is white enough as required and its capillary test is as per requirement, proceed with the dyeing or printing process; if not, repeat the process. Whiteness is usually checked with the help of a spectrophotometer in quality control labs.
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