Textile Dyeing Preparation Processes

The quality of dyeing of any textile material is directly dependent on the quality of the textile fibers used to manufacture the product. Moreover, to maximize dyeing process efficiency and color performance properties, the textile fibers involved must be as clean as possible.

The central goal of preparation is the removal of both naturally occurring impurities and those which are added during yarn or fabric manufacturing. The dyeing process can be performed on textile fibers, yarns, fabrics, or garments, depending on the properties or for cost control. Preparation can also be performed at any processing stage depending on the dyeing technique employed. However, fabric preparation is typically most often used. Additionally, some commonly used optional preparation processes such as singeing or mercerization can only be performed on yarns or fabrics.

Complete fiber cleanliness is not easily achieved. The degree of cleanliness of the textile material is only determined by employing standard accepted testing methods and properly interpreting the data obtained. However, not all shades require the same degree of cleanliness to achieve high-quality color performance. Deep, dark shades of green, brown, blue, and black do not require the ultimate fiber cleanliness needed by whites, light pastel, and bright medium colors. Regardless, the often-quoted cliché’ still applies: “Well prepared is half dyed.”

Importance of Water Quality in Dyeing in Process

The purity of the processing water has a direct effect on the performance of the preparation processes which clean the fibers, the efficiency of the dyeing process, and the end product color properties.

Typical preparation processes used for cotton and cotton blend yarns, and fabrics include singeing, desizing, scouring, bleaching, mercerizing (cotton only), heat setting (synthetic fibers only), and cellulose enzyme treatments.   Each process will be covered in this document. All of these except singeing and heat settings are processed using water baths.

Poor quality processing water creates all types of problems during the preparation and dyeing processes including color defects in the end product. Regardless of the source, the process water should be routinely analyzed. Key issues include water hardness, alkalinity or acidity (also known as pH), turbidity (suspended solids), sediment, dissolved organic matter, dissolved minerals such as iron, copper, and zinc, chlorine content, and color of the process water.

Water hardness is especially important for preparation cleaning processes. Hard water means that there is a large amount of calcium and magnesium dissolved in the water. The dissolved ions from the minerals can interfere with the performance of detergents, soaps, and wetting agents used in preparation. Hard water is typically responsible for the formation of soap scum or the ‘ring around the tub’ effect. This can sharply reduce detergent cleaning action. Additionally, iron or copper present in the water can have a very negative effect on bleaching. Specific effects of water-based impurities will be included in the discussion of the preparation processes.

As a general rule, it is important to keep in mind that just because the process water is clear, does not mean that this water is clean enough for preparation or dyeing. Also, if the process water is obtained directly from lakes, rivers, streams or wells, the quality of the water can vary from season to season throughout the year. Regionally specific water sources may require unique additional testing to ensure the acceptable quality of the process water.

As a general rule preparation processes, as discussed earlier, can be thought of as cleaning the textile substrate prior to dyeing, printing, and/or finishing. The essential objective of this process is to produce a substrate that has the following properties:

• even and rapid absorption of water,
• total removal of cotton seed husks, and
• the ability to absorb dyes and chemicals

Also, many end products are required to have high whiteness in order to be printed, dyed to a light or pastel shade, or to be sold as a market white. Preparation processes must be carefully controlled to minimize fiber damage which can lead to fabric yellowing or even strength loss.

Poor results from preparation processes can be due to poor process control and/or poor quality of the textile substrate. Often preparation problems are hidden or latent. That is, these defects do not appear in the substrate until after further processing. For instance, streaks or blotches seen in the fabric after dyeing may actually be the result of poor preparation. Also, fabric yellowing which is seen in storage may be due to fiber damage that occurred during the preparation process sequence. Preparation processes are key to good quality but are often overlooked.

Singeing or ‘Gassing’

Singeing is one of the few preparation processes which is not a wet process. In this process, a textile yarn or fabric is passed rapidly through a gas flame. This operation burns the fuzzy fibers from the surface of the yarn or fabric. The process is also known as ‘gassing’ the yarn or fabric.

The purpose of this process is generally to process a smooth yarn of fabric surface to increase the luster or the smooth feel of the final end product. With other products, the main purpose of this process is to reduce the pilling tendency of the end product. Pilling occurs when an end product forms small fiber balls on the fabric surface yielding a rough feel and an uneven appearance. Pilling is considered a quality defect. When surface fuzz is removed from the yarn or fabric, pilling tendency is typically minimized.

Figure 2 shows the results of the singeing process. Notice the results of the singeing process.    Notice also that the yarns shown are plied yarns but singles yarns can also be gassed. Yarns are usually singed individually on a single end singer, then repackaged onto cones followed by further processing. These yarns typically are used for high-quality products.

Fabric singeing is more traditional than yarn singeing.   In Figure 3, the fabric is dry, open width, and passed through the gas flames of the singer burners.   These burners are essentially horizontal pipes with holes across the width of the pipe. Natural or propane gas is passed through the burner and ignited.

 

 

Figure 3 shows the schematic of a standard six-burner fabric singer. This burner configuration has three burners on the fabric front or face and three on the back. However, this burner configuration is variable. Most machines are capable of various configurations such as five burners on the face, one on the back, four on the back, with two on the face, etc. Key process variables include the speed of the fabric and the intensity of the gas flames.

Typically a sheeting weight fabric might be processed at speeds of 300-350 yards per minute (ypm) while heavyweight denim might run at 60-70 ypm. When yarns are processed, speeds can reach 1,000 ypm or more. However, yarn singeing is not as productive as fabric singeing and is considered a more costly and specialized process.   The speed of the yarns or fabrics is a key processing control factor. If the fabric runs too fast insufficient singeing takes place. If the fabric runs too slow, it can be scorched or even burned. This is also true of singeing yarns.

Control of the flame intensity is an additional key control point. Also, when singeing blends of cotton and thermoplastic synthetic fibers melt balls can form on the tips of the synthetic fibers. These melt balls can cause the fabric to feel rough or raspy. They also dye deeper than the base fabric which can lead to a ‘specky’ dyeing appearance on light or pastel shades. So these fabrics are typically singed after dyeing. This is not an issue for 100% cotton fabric because cotton burns to fluffy light ash. When singeing after dyeing, care must be taken to select dyes that are not affected by the heat of singeing for these shades.

Once signed, there are usually glowing sparks or embers left on the yarns or fabrics. These must be cooled below ignition temperature to prevent further burning of the substrate. Usually, these sparks are quenched by exposing them to cold air, cold water-filled steel cylinders, or a water bath, such as a resizing or scouring bath which is used in a continuous fabric preparation range. Vacuum systems can be employed to remove burnt fiber ash from the singed fabric. However, washing the ash away during the subsequent preparation processes is the standard method to remove the ash residue.

Desizing 

Desizing is a preparation process that is used only for woven fabrics. It is the wet process that degrades and removes the sizing materials which are applied to the warp yarns of a woven fabric. These sizing materials must be removed to enable the fabric to uniformly absorb dyes or finishing chemicals. Additionally, these materials, if not removed, can also interfere with various mechanical finishing processes resulting in poor end-product quality and/or finishing defects. This process is never used on knitted fabrics because sizing materials are not applied to yarns used for knitting.

The content of the warp size formulation including the chemical nature of the sizing materials is key factor in determining the type of desizing process used to completely remove these materials from the fabric. A typical sizing formulation can include a film former, such as vegetable starch, lubricants, binders, anti-stats, and waxes.

The film-forming component is normally the most difficult to remove. Vegetable starches like cornstarch or rice starch are mixed with water and cooked. They form a strong, smooth water-insoluble film. In order to desize these materials, they must be degraded to water-soluble components by using alpha-amylase enzymes. This is followed by rinsing the fabric multiple times with water to remove residue.

Synthetic sizing materials such as PVA (polyvinyl alcohol), CMC (carboxymethyl cellulose), and PAA (polyacrylic acid) are normally water-soluble. These materials can usually be easily removed by using a warm or hot washing bath which includes detergent and alkali. The specifics of the time and temperature of processing depending on the exact nature of the synthetic material and the amount of it that is present. Specifically, PVA is often captured from the desizing process and recycled. The vegetable starches cannot be recycled while the other synthetic sizing materials are not cost-effective when recycled.

Desizing can be accomplished in a batch dyeing machine using water at a temperature range of 40-60 minutes. This process can also be part of an open-width continuous preparation range. It often follows singeing. The desizing chemical bath is used to quench the sparks and embers from singeing.

The saturated fabric then travels into a fabric steamer where it is allowed to dwell for 10-20 minutes to allow the degradation of the sizing materials to occur. These components subsequently are removed by rinsing the fabric in a series of fabric washers. An important quality control factor is to test for residual sizing materials. There are various specific tests published for this process depending on the chemical nature of the sizing materials.

Many companies place a drop of iodine on the surface of the desized fabric. If the iodine turns blue, this is an indication of the presence of starch.   Further testing or reprocessing of this fabric may be required. Other spot tests are used for the other sizing materials. The desized fabric may then be held in the wet or dry state waiting for further processing. However, the fabric may go immediately into the next preparation process which is scouring.

Scouring

Scouring is the process, used on both knits and wovens, where the impurities in the fabric are removed by washing. Usually, the only major impurity not removed by this process is the inherent color of the fiber.

Table 1 gives a list of the various impurities found in typical cotton substrates.   The exact type and amount of these impurities can vary due to factors such as fiber quality, fiber harvesting methods, textile substrate storage, and handling as well as many other factors.

Table 1 – Components of Raw Cotton Fiber by Percentage.

 

These specific types of impurities and the approximate amounts are found in bales of cotton fibers. These impurities will pass through the yarn and fabric manufacturing processes. In addition, the textile substrate will also include many of the mill impurities listed in   Table 2.    All of these impurities should be thoroughly removed prior to dyeing or their presence can lead to poor dyeing quality, poor finishing performance, and end product defects. Unlike desizing, scouring is absolutely necessary for all fabrics and all dyed shades.

Impurities During Manufacturing Process

• Mill Impurities
• Fiber Finishes
• Tints
• Sizes
• Chalk Markers
• Lubricants (fiber and yarn)
• Oils
• Greases
• Dirt
• Rust
• Metals in Lubricants

The basic process involves using detergents and alkali to remove these impurities by washing. Like desizing, this process can be accomplished in batch form using standard dyeing equipment or as a section of a continuous preparation range. Many of the impurities listed in Tables 1 and 2 are not water-soluble.

For cotton, this is especially true of the outer waxy layer which, if not removed, will not allow water to penetrate the fiber. This produces serious negative effects on subsequent water-based processing such as dyeing or chemical finishing. Detergents have the ability to allow oily, greasy liquids and solids to mix with water. This dirty water is then drained from the dyeing equipment or washer and sent to the wastewater treatment system for final disposal.

Typical washing temperatures range from room temperature (70oF) to boiling (212oF). However, most processing is done at 120o-180oF. The fabric must be mechanically agitated during the process. In the case of continuous processing, the fabric is saturated with the scouring bath. The fabric then goes into a steamer where it dwells for 20-40 minutes. In both cases, the scoured fabric must be thoroughly rinsed multiple times to ensure the maximum removal of residual impurities.

It should be noted, most companies mix their own scouring formulations from basic chemicals to match the nature of their specific substrates. This is done to maximize cleaning efficiency while minimizing costs. Greige, unscoured substrates should never be stored laying on the floor of the plant or handled in a manner so that they pick up impurities.   These unknown impurities add uncontrolled variables to the process and may not be removed during processing. This can lead to various product defects and overall poor end-product quality.

The results from a well-executed scouring process include:

• a uniformly clean base for dyeing yarns and fabrics in all shade ranges,
• removal of fats, waxes, oils, greases, and dirt,
• better textile substrate absorbency resulting in more uniform subsequent processing,
• softening and preconditioning cotton motes for removal in bleaching, and
• a clean, neutral base for finishing

Scouring is the single, most critical, a process in preparation. Insufficient scouring results in poor performance in many subsequent processes; especially dyeing and chemical finishing. Residual impurities left from the poor scouring result in dye blotches, streaks, and spots, as well as, insufficient finishing performance. Residual oils left on the fabric can degrade during storage resulting in fabric yellowing. Additionally, degrading impurities can also result in the formation of noxious fabric odors. This can lead to poor end-product performance and strained customer relations.

The effectiveness of a scouring process is normally evaluated by using a laboratory method that involves the extraction of residual oils, greases and dirt. The amount of residual materials extracted is correlated to the degree of cleanliness of the substrate. However, companies often use a quick method where a few drops of clean water are placed on the dry, scoured fabric surface. The time taken for the drops to be absorbed is recorded. Normally, clean scoured fabrics should absorb the water in 1-3 seconds.

Bleaching

For pastel or bright shades, including market whites, scouring alone does not yield a sufficiently clean textile substrate. These products require bleaching, which is the preparation process where the inherent color of the fabric is removed. It is a destructive process where the color-producing molecules are destroyed by oxidation. This process also destroys oil and dirt residues left from the scouring process.

After properly bleached, 100% cotton substrates are 99.9% plus pure cellulose and are the cleanest substrates produced by preparation. However, the textile fiber can also be attacked by the bleaching process resulting in fiber damage. This fiber damage can be severe if the process is not well controlled. Bleaching is a balancing act between fiber cleanliness and whiteness versus potential fiber damage.

The oxidizing agents used for this process are known as textile bleaching agents. In the modern textile industry, there are two main bleaching systems employed. The most widely used bleaching agent is hydrogen peroxide.   This agent uses a form of oxygen as the oxidizing agent in the process. The other popular bleach is sodium hypochlorite and uses chlorine as the bleaching agent. The main goals of the bleaching process are to obtain a white, ultimately cleaned substrate in the shortest time (highest productivity) possible with minimum fiber damage. The goal is the same regardless of the bleaching agent chosen.

There are distinct differences between hydrogen peroxide and sodium hypochlorite. Hydrogen peroxide is the more expensive chemical on a cost per pound basis. However, it is the most popular bleach due to its versatility. Hydrogen peroxide can be used with virtually any fiber as long as the process conditions are carefully controlled. It also exhibits wide process temperature versatility being effective at low temperatures such as 120oF and high temperatures such as 212oF (boiling water). The temperature actually used in the process will depend on the type of fiber blend, the concentration of the bleach in the formulation, and the degree of whiteness required by the end-use.

Hydrogen peroxide is also the most environmentally friendly bleach. It is a very strong oxidizing agent and must be chemically stabilized during the process in order to prevent poor bleaching results. However, after the process, hydrogen peroxide will chemically break down in the wastewater to form only oxygen and water. These components not only are not polluting, but they actually improve the performance of wastewater treatment systems. Under the correct bath conditions, hydrogen peroxide is also safely used on fiber and yarn-dyed products. It is widely known as the “color safe” bleach. However, this is not absolute and should be evaluated on a dye lot to dye lot basis.

Sodium hypochlorite is less expensive than hydrogen peroxide. It is an excellent bleaching agent for cotton and other cellulose-based fibers. However, it is not nearly as versatile as hydrogen peroxide. It is well known that this bleach causes fiber yellowing when used on nitrogen-containing fibers such as spandex and wool. It also causes fiber damage due to the retaining of chlorine by the nitrogen in the fibers. This bleach destroys or severely damages most dyes that are used on cotton or cotton blends. It is not a “color-safe” bleaching agent.

It also has much more temperature and pH sensitivity than does peroxide being most effective at temperatures between 90-140oF. After bleaching an additional agent known as an antichlor must be added to one of the rinsing baths to prevent unused bleach from having negative effects on subsequent processing. In bleaching wastewater, sodium hypochlorite is a pollutant that must be carefully neutralized. Chlorine in the wastewater treatment system can form organochloro compounds which may be toxic. Also, improper waste treatment can liberate chlorine gas which is toxic and dangerous.

Both peroxide and hypochlorite bleaches can cause severe fiber damage during the bleaching process if the textile substrate contains metal contamination such as iron rust or copper shavings. This fiber damage can be so severe that holes can occur in the substrate where the metal particles exist prior to bleaching. This problem is due to the fact that these metals act as a catalyst for the bleaching action, accelerating it to the point that it severely weakens or even destroys the fibers during the process.

The best way to prevent this problem is to handle the substrate carefully to avoid contamination from rust particles, metal railings, or rusty drip marks from plumbing pipes. Maintenance personnel should be trained to avoid metal shavings contamination on the substrates when processing equipment is serviced. Dissolved metal contamination in processing water is also a major problem. To counteract this problem, a sequestering agent, which keeps dissolved metal ions from interacting with the bleach, is added to the bleaching formulation. Other agents added to a typical bleaching formulation include penetrants, bleach stabilizers, alkali, and pH buffering chemicals. The buffering chemicals are used to keep the bath between pH 9-11 during bleaching which is optimum for most formulations.

As with scouring and desizing, bleaching can be accomplished with batch equipment or as a step in a continuous preparation range. Key control parameters for the bleaching process include time, temperature, and concentration of the bleaching chemicals. Generally, with batch bleaching, the higher the temperature the shorter the time or the higher the degree of bleaching. Also, the higher the bleach concentration the shorter the bleaching time and the potential for more severe bleaching action. Over-bleaching causes fiber damage which often results in fabric yellowing and loss of fabric strength.

With continuous bleaching, the fabric is saturated with the bleaching bath, followed by steaming. The length of time in the steamer will be determined by the fiber content, the concentration in the bleach bath, and the degree of whiteness required by the end-use. Typical steamer dwell times vary between 15-40 minutes for most cotton products. Once out of the steamer the fabric goes through multiple washing steps to remove residual bleach and impurities.

In the case of some pastel or bright shades or market whites, the white obtained from bleaching may not be sufficient. In those cases, a fluorescent whitening agent (FWA) or optical brightening (OBA) may be added to the bleaching bath or in a rinse bath after the bleaching process. The FWA’s or OBA’s are colorless organic compounds that absorb invisible ultraviolet radiation and emit this energy in the visible light region. This property is known as fluorescence.

Figure 4 illustrates the fluorescent nature of FWA’s or OBA’s. Thus, the amount of light energy reflected from a textile substrate can actually exceed 100% of the incoming light energy illuminating the material. These materials have the ability to make white textile materials appear “whiter than white.” In the same way, they can make dyed materials appear “brighter than bright.” However, OBA’s are quality improvement compounds used with bleaching but they are not a substitute for the bleaching process.

 

These FWA’s or OBA’s have varying chemical natures and are often fiber specific. Anionic OBA’s which have a negative electronic charge in water are used with cotton, wool, and nylon. Cationic OBA’s which have a positive electronic charge in water are used with acrylic and some polyester fibers. Nonionic which have no electronic charge can be used with most synthetic fibers. These compounds vary in their fastness to washing and daylight.   They usually have poor wash-fastness but can be reapplied. Most commercial detergent formulations include a mixture of optical brightening agents and these are applied in the standard home laundry.

Mercerization

Mercerization is an optional quality improvement preparation process. It is used almost exclusively for cotton. It is one of the oldest modern finishing processes and was developed for cotton substrates by an English chemist, John Mercer in 1857. Today it is mainly used for cotton fabrics but can also be used for cotton yarns. This process involves the treatment of cotton yarns or fabric held under tension while being treated with a strong sodium hydroxide solution.

Figure 5 shows a schematic of the mercerization process. In the initial section of the range, the fabric is impregnated with sodium hydroxide solution. In this example, a 22-30% solution of caustic soda (sodium hydroxide) is applied to the cotton fiber held under tension at room temperature. The caustic soda causes the fiber to swell and the internal cellulose molecules rearrange to form the ‘Cellulose II’ structure from the standard ‘Cellulose I’ configuration. The subsequent swelling of the fiber molecules collapses the lumen. Then the fabric is passed over timing rollers to allow the caustic enough dwell time at ambient temperature to accomplish the process. This step is also called ‘skying’. The total dwell time of the caustic soda in the fiber prior to washing is normally 30 seconds to one minute.

In the stabilize section, the fabric is held by the edges in order to exert the tension necessary for the full development of the final mercerized fabric properties. Cascade rinsing is a counter-flow washing process where the cleanest water is introduced through the last cascade unit. The water in this process is heated to 200oF to remove the caustic soda. The first cascade washer uses the water which is most saturated with caustic. This “dirty” water is captured after passing through the fabric.   The caustic soda is separated from this wastewater and recycled. The mercerized fabric is thoroughly washed and any residual alkali is neutralized. The fabric may be stored wet or dry prior to dyeing and further processing.

Once the caustic soda is removed by thorough washing, the cotton substrate is dried and returns to approximately its original size but in a different cross-sectional shape.

 

Figure 6 is an artist’s concept of the changes in the cross-sectional shape of cotton fiber during the mercerization process. Notice the non-uniform uneven appearance and various shapes of the unmercerized fibers. Then note the uniformity of the rounded shape of the mercerized fiber. Also, it can be seen that the lumen has collapsed to various degrees.

 

Figure 7 is an actual microscopic photo of cotton fibers before and after the mercerization process.

The positive results of the mercerization process include increased dyeability and uniformity. Dye yield by the mercerized fibers can be as much as a 20-25% increase in shade depth over unmercerized. This can be major cost savings because a given shade may be obtained using approximately 20% less dye.

 

Mercerized cotton substrates show up to a 20% increase in tensile strength. These products are normally dimensionally stable and absorb water and chemicals more rapidly than non-mercerized substrates. Mercerization significantly improves the luster of the product because the substrate is kept high tensions. The fiber surface becomes significantly smoother and reflects light in a mirror-like fashion.

Many bales of cotton have some immature fibers. These fibers have a primary wall but due to some growing conditions, they did not grow a significant amount of cellulose in the secondary wall.   Some immature fibers have no secondary wall cellulose and will not dye and appear as white specks in dyed fabric. A name often applied to these fibers is ‘dead cotton’. Figure 8 shows immature cotton before treatment. Mercerization is the only process commonly used to remedy problems resulting from immature fibers.

This process will swell any small amount of cellulose present in the immature fibers so that they will be dyeable as is shown in Figure 9. Any non-cellulosic material in the fabric will also be destroyed and washed away during processing.

 

 

Yarn mercerization uses essentially the same type of mercerizing bath and similar processing conditions. However, yarn mercerization is a more complex process. The yarn is taken off creels to form yarn ropes which go through the mercerizing process. These ropes must then be dried and the mercerized yarns must be individually rewound onto packages prior to fabric manufacturing, typically knitting.   Yarn mercerization is only possible with plied yarns due to the high tensions used to create luster.

Best results are obtained when the yarns are singed (gassed) prior to mercerization. These yarns can be dyed prior to knitting or dyed after knitting in fabric or garment form. These yarns are often mercerized in fabric form after knitting. This produces a cotton product with extremely high luster.   This product is known as ‘double mercerized’. Figure 10 shows a schematic of the yarn mercerizing process.

 

 

Heat Setting

Thermoplastic synthetic fibers such as polyester or nylon are greatly affected by heating them above their glass transition temperature (Tg). This temperature is unique to each fiber type and variant. When the fiber is heated above its Tg but below its melting or degradation point, the internal fiber molecules flow and rearrange along lines of tension while the outside of the fiber remains solid. This process allows the manufacturer to set the dimensions of the substrate minimizing shrinkage. Special effects can be set into fabrics such as permanent designs or crinkles. In garments, permanent pleats and creases or a specific desired shape can be heat set into the textile product.

However, cotton and other cellulose-based fibers are not thermoplastic and not affected by the typical heat setting process.   Exposure to high temperatures above 400oF for long periods of time may scorch cotton and result in product degradation and yellowing. In the special case of spandex blends, heating processes are used to stabilize the spandex fiber prior to wet processing. After wet processing spandex blends are processed through a final heat treatment to set the amount of stretch in the final product. Great care must be taken in processing spandex blends in order to produce uniform product properties with satisfactory performance.

The dyeing performance of polyester and nylon is directly affected by heat settings. Uneven heating across the width of the tenter frame oven leads to uneven heat setting across the width of the fabric. Therefore side-center-side dyeing (shading) defects are heat set into the fabric. The uneven moisture content of the fabric entering the oven also can lead to non-uniform processing. The typical Tg for polyester fibers is approximately 330-350oF. For nylon, Tg is around 390-410oF. The Tg needs only to be exceeded for a few seconds for a uniform heat setting to take place. Cotton/polyester and cotton/nylon blends are normally heat set to obtain maximum dyeing uniformity of the synthetic component and acceptable product shrinkage properties. Heat setting can also be performed as a mechanical finishing process after dyeing. Dyes must be selected and evaluated to withstand this high-temperature process if heat setting after dyeing is used.

Cellulase Enzyme Processing

Cellulase enzyme processing is an optional preparation and/or washing technique that removes surface fiber fuzz and softens the feel or hand of the fabric or garment. Enzymes are organic substances that quicken certain natural reactions. They are usually obtained from natural sources such as bi-products from a biological process like fermentation. These enzymes are usually easily biodegraded in wastewater and thus are considered environmentally friendly processing chemicals.

Cellulase enzymes have the ability to selectively hydrolyze or dissolve cellulose. They are used for surface fiber fuzz removal, color wash down of specific types of dyed fabric, and increased fabric softness. However, fabric weight reduction with subsequent fabric strength loss results when the enzyme action of the process is too severe or is allowed to continue for too much time.

Figure 11 illustrates the improved fabric appearance obtained from cellulose enzyme treatment. The enzyme process removes the surface fiber fuzz. The treated fabric appears smooth and clean.   It also appears deeper in color and brighter in shade. Notice also that the surface of the treated fibers appears much smoother in the microscopic photo.

 

 

The overall enzyme action of the process will depend on many factors including the type of enzyme chosen, the temperature of the bath, the liquor ratio of the bath, the concentration of the enzyme, the type and nature of auxiliary chemicals used, the pH of the bath and the overall processing time. Enzymes are very sensitive to bath pH, temperature, and processing.    They work within a specific temperature range. Outside of these upper and lower limits, very little enzyme action is produced.   Additionally, some enzyme types are more aggressive than others.

Note that the acid cellulose reached peak activity at pH 4.3 while the neutral cellulose reached peak activity at pH 6.0. Both enzyme types exhibit reduced enzyme activity outside of their optimum pH range.

One other major factor in cellulase enzyme processing is the mechanical agitation of the processing equipment. Typically cellulase enzyme processing is accomplished in batch dyeing machines or garment washing machines such as the rotary drum machine. The mechanical agitation improves the removal of hydrolyzed material.     There are limits but in general, the higher the mechanical agitation level, the better the removal of hydrolyzed residue. This results in reduced processing times and improved process performance.

There are a few key parameters for optimizing the overall preparation processing. The operation must be systematic and consistent. Manufacturers should thoroughly understand their fabric properties and the capabilities of their processing equipment. All equipment should be properly serviced and maintained. All processes should be routinely monitored and published procedures should be closely followed. Process operators should be thoroughly trained and properly supervised. Well-prepared textile substrates are absolutely necessary in order to produce high-quality end products.