Textile Dyeing

Dyes and Dyeing of Textile Substrates

Dyes are the chemicals that are absorbed into the molecular structure of textile fibers which produce the color of the molecular structure of textile fibers which produce the color of the textile product. Dyeing is the process that places the dyes inside the fibers. Currently, dyeing of textile substrates takes place using water baths. Dyeing equipment is used to contain the textile substrate and the dye bath during the dyeing process.    The dyeing equipment is also used to control the necessary parameters of the dyeing process in order to maximize dyeing quality and productivity.    Pigments are a special case of textile coloration. They require a binder or glue to adhere them to the surface of the textile fiber. Not all dyeing equipment can be used to successfully apply pigments to textile substrates.

General Dyeing Factors 

The primary objective of the dyeing process is to produce the shade on the textile substrate that matches the color standard.   If the substrate is off-shade, it is of quality by definition. Next in importance is to produce the shade with the colorfastness properties which meet the performance specifications required by the end-user. For example, most garments must be able to be washed without color bleeding or fading. Outdoor furniture upholstery fabric should be highly resistant to light fading as another example. Each dye formulation must be evaluated for its colorfastness properties. Finally, dyeing companies are for-profit businesses.

The cost of the dyeing process both in terms of dyes, equipment, and processing times, greatly impacts the profitability of a manufacturing company. Other key factors considered when selecting dyes include ease of mixing with other dyes and chemicals, uniformity or level dyeing properties, dusting issues when using powdered dyes, and overall environmental impact. Environmental concerns are of particular importance. In some cases, certain dye formulations produce large volumes of highly colored wastewater. In this case, the volume and intensity of the color are the issues. Other formulations produce non-biodegradable or semi-toxic waste products.     These must be properly treated and made harmless prior to returning the treated water to the environment. These concerns exist worldwide.

Fiber Forms Used For Dyeing

Dyes are specific to the fiber involved and this factor will be discussed in some detail later in this document.   However, they are not specific to the type of substrate the fiber is used. For example, the dyeing process can be accomplished using textile fibers, yarns, fabrics, or garments. All of these different textile substrates can use the same dyes as long as the fiber content is the same. However, there are reasons for dyeing different fiber forms.

• Fiber dyeing is used as a styling technique. Natural fibers or staple synthetics are dyed in bundles or baskets. This process is known as ‘stock dyeing’. These fibers are often blended for shade in the yarn manufacturing process.   They are used to produce heather shades which are popular in many different apparel products.
• ‘Dope or solution dyeing’ is the process where color is mixed into the polymer solution prior to fiber extrusion. This process yields colored fiber with very high fastness properties. However, it is difficult to alter the extruded color if off shade. Certain synthetic fibers such as polyethylene can only be colored using this technique. These fibers are used where high color fastness is a must, such as geotextiles. In general, fiber dyeing is inflexible with regard to color flexibility. It is also a more expensive technique because large amounts of colored fiber waste are generated in the manufacturing process.
• Yarn dyeing is also used as a styling technique. Yarn dyeing is used to produce stripes, plaids, and some complex designs with 100% fiber content products. In many cases, the same designs could be obtained by textile printing. However, yarn dyeing produces products with higher color quality and color durability.   ‘Skein dyeing’ is the process where a single yarn is wound into long yardage loops or hanks. The hanks or skeins are held loosely in the dyeing machine. The dyebath cascades like a waterfall over the loops of yarn. This allows the yarn to develop more volume or bulk during the dyeing process. This technique is important for products like rugs or sweaters.
• In ‘package dyeing’, a single yarn is wound uniformly around a perforated tube or spool. Multiple of these packages are loaded into the dyeing vessel by placing them onto perforated carrier arms. The dyebath is pumped through the package inside-out and outside-in until the dyeing process is completed. This is the most versatile and highest productivity method for dyeing yarns. Yarns dyed in this way are used in a wide variety of products.
• ‘Beam dyeing’ is a technique where multiple yarns are wound side by side onto a single perforated beam. This can be a few hundred or even a few thousand yarns wound onto a single beam. This is dependent on the end product and manufacturing requirements. ‘Beam dyed’ yarn is often used for striped or plaid woven fabrics. Typically a single beam is placed in the dyeing vessel and the dyebath is pumped through the yarns in the inside-out and outside-in flow technique.
• ‘Space dyeing’ is a yarn dyeing method for placing color sections or blocks along the length of the yarn. The most popular method is a modification of package dyeing and will be discussed in more detail later in this document. Space dyeing is a highly specialized coloration technique. These yarns are often used for highly styled sweaters. Synthetic fibers dyed this way are used in the carpet industry.   In general, yarn dyeing is more expensive than fabric dyeing due to lower productivity and colored yarn waste.
• Fabric or piece dyeing is the most cost-efficient and highest productivity technique. Fabric dyeing machines include jet machines, dye becks, fabric beams, and jig dyeing machines.    Each type of machine has its own unique advantages and limitations. These will be discussed later. All of these machines as well as most of the yarn dyeing machines are batch or exhaust machines. Fabrics can also be dyed on continuous pad dyeing ranges or using the semi-continuous pad batch method. Textile printing is a localized continuous dyeing technique. Fabric dyeing produces the largest amount of dyed textile materials. However, there is colored cut and trim waste produced when cutting and sewing dyed fabrics into garments.
• Garment dyeing is a batch dyeing process and is very popular with certain types of products such as sweaters or hosiery. This method uses commercial laundry machines such as a rotary drum or paddle machine to dye garments. It is a slower, lower, productive, less efficient method than fabric dyeing. However, garment dyeing produces no colored waste and is the most color-responsive technique to the demands of the marketplace.

Factors in Exhaust Dyeing

The vast majority of textile substrates are dyed using the exhaust or batch method. Figure 14 illustrates the general process of batch or exhaustion dyeing.

The dyebath consists of water, dyes, and any necessary auxiliary chemicals.   The textile substrate is immersed in the dyebath. The dyebath is heated to the dyeing temperature and held at that temperature for the required amount of time. Simultaneously, the fabric is agitated during the process. During the process, the dye migrates and ‘adsorbs’ to the surface of the fiber. Subsequently, the dye molecules ‘absorb’ and diffuse into the fiber structure.     This process is known as exhaustion. Once all possible dye diffuses into the fiber, residual dye is removed by after-washing.

 

The actual color of the textile material obtained from this process is determined by the amount or weight of dye in the dyebath formulation. This is usually expressed as a percentage of the weight of the textile material being dyed and is known as ‘depth of shade’. Also, for most commercial dyes, only a fraction of the total dye in the bath actually exhausts into the fiber. This is known as ‘fractional exhaustion’ and is a key parameter of each dye in the formulation. The higher the fractional exhaustion, the better the use of the dye and the less dye or color in the wastewater.

Other key factors affecting exhaustion dyeing include substrate preparation, machine agitation, flow characteristics, and ‘liquor-to-goods ratio’ (LR). LR is a machine-based parameter defined as the weight of the dyebath compared to the weight of the textile substrate. Of course, with any textile dyeing process temperature, pH, chemical auxiliaries, and time of dyeing are key control factors. Other factors that influence the dyeing performance are related to the specific dyes and/or specific fibers used in the process. It is essential to have properly cleaned machines to prevent dye spots or cross-staining contamination from dye lot to dye lot.

Dyes for Cotton (Cellulose) and Blend Fibers

As mentioned earlier, dyes are fiber specific based on the chemical nature of the dye and the molecular nature of the fiber. Table 3 is a chart that shows the most commonly used textile fibers and what classes of dyes work with each fiber.

Table 3 – Comparison of Fibers and Dyestuffs Used
Dye classes are actually groups of dyes that work on a specific textile fiber in a particular way. These dyes within a group will have similar chemical compositions but they will not be identical. Dye classes can be thought of as application groups of dyes.

From Table 3, the dyes that work for cotton are reactive, directs, vats, and sulfurs. Naphthol dyes are not used on cotton and rayon as much as in previous years. Pigment colorant properties will also be discussed.   However, pigments are not dyes and they are not fiber specific. All the dyes used for cotton will also work for other natural cellulose fibers such as linen or ramie. They will also work for the synthetic cellulose fibers such as rayon, lyocell, and bamboo.    As mentioned earlier, the final color of any specific dye formulation on cotton will be different than the color on rayon, even if the fibers are dyed together.

Currently, reactive dyes are the most used for cotton blend fabric. These dyes react directly with the chemical structure of the cellulose. They form a strong covalent bond between the dye and the fiber resulting in very good to excellent wash fastness. With proper after washing, these dyes also exhibit good crock fastness. Their resistance to sunlight is variable, but most reactive dyes have acceptable lightfastness.

Reactive dyes were invented in 1956. They have the widest shade range with the brightest colors that are currently available for cotton.   They are highly water-soluble and are useful for both exhaust and continuous dyeing. There are many different types of chemistries used for reactive. The cold dyeing type can dye cotton at low temperatures. These dyes are used in the cold pad-batch energy-saving dyeing technique.    The hot dyeing type is more difficult to exhaust into the fiber requiring higher dyebath temperatures for long periods of time.   Many new reactive dyes can react at two positions on the fiber. These ‘bi-reactive dyes’ exhibit superior fastness properties but are more difficult to process. They typically require higher dyeing temperatures and longer processing.

Unfortunately, some of the reactive dye in the bath can react with the water under dyeing conditions and become deactivated. The deactivated dye cannot be recovered and becomes a colored wastewater pollutant. Normally, reactive dyes typically require large amounts of salt, such as sodium sulfate as a dyeing assistant. This salt can be an environmental concern in wastewater. Reactive dyes are not generally chlorine resistant; however, it is dye dependent. These dyes require large volumes of water for the processing which can be a negative cost factor. They are moderately expensive dyes; however, specific dye costs per pound can vary greatly. In general, newer patented dyes are more expensive than older generic dyes. Overall, reactive dyes are the cotton dyestuff of choice for most end uses. Their combination of a wide range of bright colors and good fastness properties, especially wash fastness, make them the best dye choice for most cotton products.

Direct dyes are a grouping of dye structures that have been used for many years with cotton substrates. They are highly water-soluble and used mainly in exhaust dyeing. They are usually temperature-sensitive with most being dyed at temperatures between 160-180oF. Like reactive dyes, they use salt as a dyeing assistant but need very small amounts compared to reactive dyes. They do not react with the fiber structure but rely on dye-to-fiber associations such as hydrogen-bonding and dipole moments to help with color fastness after dyeing.

As a group, they have a wide shade with fairly bright colors, but not as bright as reactive dyes.   They are easily exhausted during dyeing.   In general, they exhibit good, light fastness. However, they have poor wash fastness and poor wet crock fastness. Direct dyed medium and dark shades tend to bleed and fade in standard home laundry washing. Chemical after-dyeing treatments known as dye fixative agents (dye fixes) have been developed for direct dyes. Some of these have greatly improved the wash fastness of directs but can change the shade of the dyeing. In general, direct dyes are not a good choice for items that will be washed frequently. However, they are a good choice for items such as drapes or curtains where lightfastness is the major requirement. Finally, they use much less water, chemistry, and time compared to reactive dyeing.

Vat dyes are a group of older dye structures. These dyes are not water-soluble in their normal form.    In order to dye cotton, these dyes are made water-soluble by the chemical process known as ‘reduction’. This requires a reducing agent such as sodium hydrosulfite and an alkali, typically sodium hydroxide. The reduced form of the dye is known as the ‘leuco form’. For most vat dyes the leuco form is highly water-soluble and nearly colorless or very lightly colored. In the leuco form, the dye is highly attracted to the cotton fiber. It rapidly absorbs into the fiber structure. Once inside the fiber, the leuco form of the dye is ‘oxidized’ back to the water-insoluble original colored dye.

This process traps a water-insoluble dye inside the cotton fiber. The dyes are high wash fastness and usually very lightfast. With proper after washing they exhibit good to excellent wet and dry crock fastness. Most of the vat dye structures are highly resistant to chlorine. As a group, these dyes exhibit the best color fastness properties on cotton fiber.

Unfortunately, vat dyes are limited in their shade selection. There are very few yellow, orange, or red dyes available and these tend to be very dull. There are relatively bright dyes available in deeper colors, particularly blues, greens, and blacks.    However, vat dyes produce overall duller shades than reactive or direct dyes. The dyeing procedure is cumbersome involving several complex chemical steps. Control of these conditions determines whether the dyeing process is successful or not.

Additionally, the vat dyes are by far the most expensive dyes used for cotton on a cost-per-pound basis. Due to the complex dyeing process, vat dyes are used more with continuous dyeing than in exhaust processing. Vat dyes are a good choice where color fastness especially wash fastness is a prime requirement, but where a wide shade range of colors may not be as important. Vat dyes are typically used for work wear, towels, and high-end garments which must survive multiple items of washing such as men’s dress shirts and dress pants.

Indigo is an ancient natural dye that happens to be a vat dye.    Today, synthetic indigo is the dye mostly used by the textile industry. When dyed onto the cotton fiber, indigo creates a bright reddish blue shade. Indigo is used for denim and is, by volume, the most used dye in the textile industry. Indigo forms a ring of dye around the outer edges of the fiber. Figure 15 shows the ring dyeing nature of indigo.

 

Since the color ring of indigo is around the outer fiber surface or the outer surface of the yarn bundle, indigo leaves a white inner core. This dyeing can be abraded in garment finishing (stone washing, hand sanding, laser, etc.) to produce special color effects. Additionally, unlike other vat dyes, indigo can be destroyed or discharged with sodium hypochlorite bleach or potassium permanganate treatments. These special processes are performed on denim garments and are possible due to the chemical sensitivity and ring dyeing nature of indigo. Figure 16 illustrates a schematic of an indigo yarn dye range.

 

Denim is a yarn-dyed fabric where only the warp yarns are dyed. A ball warp is a package of 400 yarns made into a rope. A typical dye range dyes 10-36 ropes of yarn simultaneously. The ropes are fed into the preparation section of the range where detergent and alkali help clean impurities from the yarns so that the indigo can penetrate and dye the yarns more uniformly. As an option sulfur black dye can be applied (sulfur bottom) in this section to make the indigo dyed yarn appear deeper in color.   Indigo does not exhaust in normal processing.

To obtain deep blue color, layers of indigo are built up in ring form.    This is done by dipping the yarn in the reduced (leuco) indigo dyebath and then oxidizing this layer of dye by exposing the leuco indigo to oxygen found in the air (skying).    In this way, layer upon layer of indigo is built on the yarn and fiber.

Typically, 6-8 dips and skyings are required to produce standard denim color depths. In the final section of the range, an optional sulfur top of sulfur black dye can be applied to make the shade appear to be a little blacker. However, in standard processing, the final section of the range is only used for washing to remove unfixed, surface indigo.

Any indigo removed can be captured and reused. The final washing of the dyed yarns helps minimize color transfer during weaving and crocking problems in the final fabric. The dyed and washed yarns are then dried and segregated into individual ropes. The yarns of each rope will be separated and wound onto section beams, ready for the weaving process. There are many optional processes in producing modern denim products but the nature of indigo is still the key factor. Unlike other vat dyes, indigo is an average-priced dye.

Sulfur dyes are the least costly dyes available for cotton. Like vat dyes, in their normal form, sulfur dyes are water-insoluble.    Also, like vats, they are reduced to a water-soluble form, applied to the cotton fiber, then oxidized back to their water-insoluble form once the dye has been absorbed into the fiber structure.    Unlike vats, the reduced water-soluble form of the sulfur dye does not change color.    Sulfur dyes have a low color power so a large amount of dye is necessary to produce deep shades.   Sulfur dyes can be applied in either exhaust or continuous dyeing techniques. When properly dyed and after washed, these dyes exhibit good wash fastness. However, their crock fastness is limited, especially for deep shades.

Sulfur dyes have a fairly wide shade range but they produce dull colors.    In light shades, these dyes tend to have poor lightfastness. However, in dark shades, these dyes exhibit good lightfastness so they are often used for navy, blacks, and greens. In storage, particularly hot, humid conditions, sulfur-dyed products can form sulfurous and sulfuric acids which attack and destroy cellulose. Sulfur-dyed products must be finished in a manner to neutralize this acid formation. Otherwise, severe fabric strength loss will result from storage. Sulfur dyes are a good, inexpensive choice, particularly in deep shades, where wash fastness is important but shade brightness is not a major requirement. In woven bottom weights, sulfur black is the standard due to its rich shade.

Naphthol dyes are an older technology dye system. With this dyestuff class, a water-insoluble dye is created inside the fiber during the dyeing process. Basically, a component known as a naphthol is applied to the cotton material. The naphthol component is not a dye and can be washed out of the substrate. Next, a second component known as a coupling salt is joined to the naphthol during a process similar to exhaust dyeing. The color salt reacts with the naphthol to form a water-insoluble dye inside the fiber. The technology was developed to produce bright yellow, orange and red shades with vat dye fastness properties. However, this process is very complex. It is also difficult to shade match. These dyes have lost nearly all of their market share to reactive dyes. However, some naphthol dyes may still be used for some specialty products and in textile printing.

Pigments are totally water-insoluble colorants.   They have no affinity for textile fibers and must be attached to the fiber or yarn bundle surface with a binder or glue. The binders are not fiber specific and can be used on different fiber types simultaneously. Pigments do not penetrate into the fiber structure. In general, pigment coloration exhibits good lightfastness. Their washfastness properties are directly dependent on the binder selection.

Typically, they produce intense colors with a wide shade range. They are simple to use. Pigments are useful in printing, continuous pad or spray coloration. Some specialty types, known as exhaustible pigments, can be used in garment dyeing. They usually produce colored garments with distressed or garment washed appearance without using an extra garment washing process. However, they have become more expensive and sometimes stain the processing equipment. Many pigments are fluorescent and are used to produce ‘neon’ colors. Pigments typically are lower cost colorants than dyes.

Since the colorants are glued to the outer surface of the yarn in the fabric, pigments have limited crockfastness, especially dry crockfastness. The binder systems tend to make the feel or hand of the textile feel stiff or harsh. In general, the better the pigment fastness properties, the stronger the binder and the stiffer the hand. When applied by padding or spraying, pigments can migrate during drying. This leads to uneven color or even blotches in the colored product.   During processing, the binder may cause build- up on machinery rolls or may cause sticking of the substrate.

Pigments are also economical because of the limited number of processing steps. Blends can be colored to a solid or union shade with one pigment formulation which is applicable to all fibers. Pigments offer the possibility of combining coloration and finishing. However, the specific pigments and finishes must be chosen so that their normal processing conditions match.

Disperse dyes are water insoluble dyes which are used for thermoplastic synthetic fibers, primarily polyester.    They are only medium quality dyes for acrylic fibers and they are generally poor dyes for nylon. Disperse dyes are never water soluble. They are dispersed in particulate form during the polyester dyeing process.

During processing, the polyester fiber is swollen by either a chemical swelling agent known as a carrier or by high temperature in the range of 265-275oF. Carrier dyeing is processed with water up to a boil (212oF). In order to get water to those temperatures a high temperatures pressurized dyeing vessel is required such as a jet machine for fabric dyeing or a package machine for yarn dyeing.     Once the proper conditions are reached, the disperse dye diffuses throughout the polyester structure. After cool down, the polyester substrate is afterwashed to remove unfixed dye. Disperse dyes exhibit good washfastness, light fastness and crockfastness. They are typically used for all cotton/polyester blends.

Disperse dyes are an average priced dyes. They do not dye cotton fibers but may stain them.   This staining is easily removed during processing, but may require an additional step. Some disperse dyes have the capability of sublimation. Sublimation is the property where upon severe heating, the solid dye particle changes directly into a gas, without going into the liquid or melted state. The dyes then immediately revert back to the solid state under proper temperature conditions. Disperse dyes that sublimate are used in the continuous dyeing process known as Thermosol dyeing and in sublimation printing. Disperse dyes are the only textile dye class that can sublime.

Acid dyes are used in the dyeing of nylon, wool and silk. They are highly water soluble. Under dyeing conditions, these fibers generate a positively charged dye site on the fiber structure. At the same time, the acid dye develops a negative charge. This negative- positive charge attraction is the initial driving force to move the dye from the dye bath into the fiber structure. Once inside the fibers, other dye-fiber interactions account for the final overall colorfastness of the dyed fiber.

There is a wide range of acid dyes with a wide color range and a wide variety of colorfastness properties. However, acid dyes are the dyestuff of choice for use with cotton-nylon blends.   In general acid dyes do not dye cotton; however, staining can be a problem. This must be evaluated on a dye formulation by dye formulation basis. Any staining should be removed during processing.

Basic dyes are water soluble dyes used for dyeing acrylic fibers. Under dyeing conditions, the acrylic fiber generates negatively charged dye sites while the dye generates a positive charge. The positive-negative electronic attraction is the initial driving force to move the dye from the dyebath into the fiber.   Once inside the fiber, other dye-fiber interactions influence the final colorfastness properties of the dyeing. Basic dyes have a wide color gamut and exhibit a wide range of colorfastness properties. Several of these dyes are fluorescent (neon dyes) or produce extremely bright colors. These ultimate brightness dyes tend to have poor lightfastness. Basic dyes are the dyestuff of choice for acrylic fibers and are used with cotton-acrylic blends. They can produce severe staining on cotton and this must be taken into account when dyeing these blends.

There is always a range of properties for specific dyes within any dye class. Also colorfastness properties, in particular, can vary depending on the specific fiber content of the dyed textile substrate. For example, reactive dyes have better washfastness properties on cotton substrates than they do on rayon substrates.    However, reactive dyes are a good dye choice for both types of fibers. There are many other examples of this situation.

Dyeing Blends

A large portion of the dyed textile substrates are actually fiber blends. Worldwide, the most popular blend is cotton and polyester.    Other fiber blends are also very popular. Of particular interest are the cotton and spandex blends.   In the dyeing process, all fibers in the blended product must be considered individually. Using a cotton/polyester blend as an example, the cotton would be dyed with reactive dyes while the polyester portion is dyed with disperse dyes. The dyeing procedure is also modified to accommodate each fiber in the blend. For a cotton/polyester substrate, the polyester portion is usually dyed first followed the dyeing of the cotton.

The reason for this sequence is that the conditions necessary for dyeing polyester could damage dyed cotton fibers. However, the conditions needed to dye cotton have little or no effect on the dyed polyester fibers. Also dyeing times required for blends are typically much longer than the times needed for 100% fiber content substrates.

In practice, spandex does not dye but it will stain with various dyes. In many applications of spandex, it is normally covered with another dyeable fiber such as in core spun and cover yarns. In other products, the individual spandex filament is hidden on the back of the fabric. Normally, the end product requires that the spandex be stained enough to not be noticed. However, spandex colorfastness, particularly washfastness, is not good and this can present some quality issues in end products.

When different fibers in the blend are dyed different colors, this process is known as ‘cross dyeing’. Because of the fiber specific nature of dyes, this is a relatively easy dyeing process. For instance, out of a single dyebath disperse dyes will only dye the polyester and direct dyes will only dye the cotton. If reactive dyes are used, separate dyebaths will be used for the polyester and the cotton.

‘Union dyeing’ is the term used when the blended substrate is dyed to a uniform, solid color.   A true union requires that each fiber in the blend be the same color.    This can be very difficult and requires a high degree of skill on the part of the dyer. Using our cotton/polyester example, the dye formulation of disperse dyes on polyester will have to produce the same shade as the reactive dye formulation produces on the cotton fibers. One of the biggest problems with this is that the two fibers have a different brightness or luster.   Regardless of the difficulty, high quality union dyeings are produced every day on blended substrates.

When at least one fiber in the blend is left undyed, the term ‘reserve dyeing’ is used. If the blended substrates are intimate blends, then reserve dyeing produces a heather appearance.    In our example, the cotton fibers could be dyed with reactive dyes and the polyester left undyed. Alternatively, the polyester fibers could be dyed with disperse dyes and the cotton left undyed. Depending on the blend ratio of cotton to polyester, these two reserve dyeings could look very different when compared to each other. Also, staining of the undyed fiber may or may not be removed depending on the end product requirements.

When different fibers which dye with the same dye classes are blended together, a tone-on-tone effect results from the dyeing process.    For example, cotton and rayon can be blended together. They both dye with reactive dyes.    After dyeing both fibers will be the same hue or color, however, the rayon fibers will be a deeper shade than the cotton fibers. This is due to the fiber structural differences between cotton and rayon. Normally dye penetrates rayon more easily then cotton resulting in the deeper shade.

When dyeing blends, the properties of the weakest fiber in the blend must be considered. The standard dyeing method for one fiber may severely damage another fiber. Therefore, in many cases, certain dye formulations, chemical auxiliaries and dye bath conditions which are considered routine will be modified or eliminated due to the sensitive nature of the blend fiber.

Dyeing Equipments 

The purpose of dyeing equipment is to contain the textile substrate and the dyebath and to generate an exchange between the substrate and the dye. It also must monitor and control of all necessary dyebath parameters including temperature levels and degree of agitation. Most dyeing equipment is made of stainless steel in order to resist chemical attack and corrosion from the dyebath chemicals at the elevated temperatures required for most dyeing processes. Most dyeing machinery control systems incorporate microprocessors or computers. This has greatly improved the accuracy, monitoring, flexibility, and reproducibility of the control systems.

Most dyeing equipment is designed to process specific textile substrates. That is, there is dyeing equipment designed to dye yarns. These are not used to dye fabrics. There are machines designed for dyeing fabrics.   These are not used to dye yarns.    There are also machines designed for dyeing garments. These machines are not used for dyeing yarns, but can do limited amounts of fabrics.

Most dyeing equipment is designed for exhaust or batch dyeing.   These machines are targeted to use with yarns or fabrics or garments. There are continuous dye ranges designed for dyeing fabrics. Pad-batch is a semi-continuous dyeing range designed for dyeing fabrics. The indigo dye range described earlier is a continuous yarn dyeing range.

Yarn Dyeing Machines

Figure 17 illustrates a package yarn dyeing machine.   A single yarn is wrapped around a perforated dye tube to form a package. The size of the package is based on the weight of the yarn wound onto the package. Typical package weights used in most package machines today are 3 to 5 lbs. The capacity of the dyeing machine is expressed as a total yarn weight that can be dyed in a single dyeing. For example, one machine may be rated at 400 lbs., while a larger machine is rated at 1,000 lbs.

Package machines are batch, exhaust dyeing machines. The correct number of yarn packages is loaded onto perforated carrier tubes. The carrier is placed in the machine. The vessel has a lid which is sealed. Most package machines can be pressurized in order to dye polyester. During the process, the dyebath is pumped through the package with inside-out flow, then the flow is reversed to outside-in. This is done to insure color uniformity throughout the package.

Once dyed, the packages are washed and removed from the machine. They are then dried. The quality of the dyeing is evaluated by back-winding yarn from the inside, middle and outside of the package. This back-wound yarn is then knitted into a sleeve, and color uniformity as well as shade are evaluated. Typically, 3-7 packages will be evaluated per dye lot. Package dyeing is the highest production, most widely used method for dyeing yarn.

 

Space dyeing is a specialty yarn dyeing method where variable segments of different colors are dyed along the length of the yarn. Space dyeing is a semi-continuous technique that is a modification of package dyeing. In this method, a package of yarn is wound onto a dye tube that is placed in the space dyeing machine.    Dye is then injected into specific quadrants of the package using needles which are inserted into the package. Each needle injects a different color and inserted to different depths. The typical number of needles used is 4 or 8, but more or less can be used.

Once the dye is injected, the needles are removed and the packages are steamed to fix the dye. The fiber type and dyes used must be those that can be fixed with steam. The dyed packages are then loaded into a package dyeing machine and afterwashed, and then unloaded and dried. The yarn is then back wound onto cones for further processing. Space dyeing produces unique coloring effects that cannot be obtained on other dyeing equipment. Figure 18 shows the needle assembly used and a package of space dyed yarn. Figure19 shows a sweater with space dyed yarns.

 

 

Fabric Dyeing Machines

Most textile products in the market today are dyed in fabric form.    The most popular type of dyeing machines for fabrics is the ‘jet dyeing machine’. Figure 20 shows a schematic of a typical jet machine.

 

The fabric is loaded into the machine in rope form. The strand of fabric travels around the lifter reel at the top of the machine, then through the jet nozzle tube (venture), and onto the back of the machine. The cloth, typically 300-400 yards, is sewn into a continuous loop, and travels around the machine continually during the process. The length and weight of the fabric making up a continuous loop will be determined by the machine capacity. Many machines can dye many roped strands totally several thousands of pounds of fabric in a single dyeing.

As the fabric circulates within the machine, the dyebath is constantly pumped from the bottom of the machine through a filter and then through the jet nozzle. This venture provides the force to move the rope of fabric. Therefore, during the dyeing process, the dyebath is circulated and contacts the fabric in the venture tube, resulting in excellent agitation and high efficiency dyeing.

After dyeing, the fabric is afterwashed, unloaded and dried. Jet machines are sealed, pressurized batch, exhaust machines. Normally, they are computer-controlled and considered the most flexible machinery design for dyeing all types of fabrics, knits and wovens. The jet machine was invented in the 1960’s but is the dominant fabric dyeing machine today.

Jet machines are manufactured by various companies throughout the world. In many cases, new machines maintain the basic jet machine operations, but their designs are modified to meet specific textile fabric requirements. Figure 21 illustrates a horizontal- flow jet that can be compared to the conventional jet configuration in Figure 20.

 

The “dye beck” is one of the oldest types of dyeing machines used for fabric dyeing. The basic design for the dye beck was developed in Europe in the 1500’s. Until the invention of the jet dyeing machine in the 1960’s, the dye beck was the most widely used exhaust fabric dyeing equipment. Figure 22 is a schematic of the design of a typical dye beck.

 

Like the jet, the fabric is loaded into the machine in rope form over an idler reel at the front and around an elliptical reel at the back of the machine.    The dyebath is held in the bottom of the machine. The typical cloth strand length is approximately 100 yards but this can vary with machine capacity.    Production machines typically are rated from a few hundred to many thousand pound capacity using numerous strands of fabric.

During the dyeing process the fabric is pulled around the machine by the rotation of the elliptical reel. The dyebath remains stationary. This agitation creates stress on the fabrics during the process and tends to stretch and distort knit fabric structures. After dyeing the fabric is afterwashed and the cloth unloaded.   Dye becks are still widely used. They are simple to operate. Some newer designs are usable with knits. In general, dye becks use much more water and energy and generate more wastewater than the jet machines.

The ‘fabric beam dyeing’ machine is a batch, exhaust machine designed to process surface sensitive or easily distortable fabric such as lightweight warp knits and power stretch fabrics of all types with cotton/spandex blends. Figure 23 is a photograph of a typical fabric beam machine.

 

Using this machine, fabric is wound open-width around a perforated beam.   The beam is loaded into the machine and the door is sealed. During dyeing, the dye is pumped through the cloth in an inside-out flow. In this process the dye bath is circulated while the fabric is held stationary. This minimizes stress and distortion of the fabric during dyeing. The fabric can be dyed essentially wrinkle-free. After dyeing, the fabric is afterwashed, then unloaded and dried. The machine shown in the figure is sealed and pressurized in order to dye the polyester in cotton-polyester blends at high temperatures.

Overall, fabric beam dyeing machines can handle up to 3,000 pounds. They are considered to some degree specialized dyeing machines. Figure 24 is a schematic of the flow characteristics of a beam dyeing machine.

 

‘Continuous dye ranges’ are high production primarily for woven fabrics but are highly expensive dyeing equipment. Figure 25 shows a schematic of a standard dye range. Continuous dye ranges are actually a series of different machines that control certain dyeing conditions as the fabric is processed. From Figure 25, the open-width fabric is saturated with concentrated dye liquor, and then squeezed through pad rolls to remove any excess dye. Very accurate dyeings can be obtained by the ability to control the concentration and the amount of dye applied during padding. The fabric is then passed through a pre-drier to minimize dye migration. The fabric is then dried to a specified moisture level, and then passed through a chemical pad to apply the correct amount of auxiliary chemicals necessary for the dyeing process. In the case of reactive dyes on cotton fabric, salt and alkali are applied in the chemical pad. The fabric immediately goes into the steamer and dwells in this steam atmosphere long enough to fix the dye. Steamer dwell times for most dyeings vary from 10-30 minutes. After fixation, the fabric is passed through a series of washers to remove unfixed dye. After the washing process, the fabric is usually dried before further processing.

 

Continuous dye ranges are complex operations. They require large amounts of floor space and are very expensive to purchase, install and maintain. They require more down time than other dyeing machines for cleaning or maintenance. However, this is offset due to the large amounts of fabric that can be dyed in a short amount of time. Normal production speeds are up to a slightly above 100 ypm.   Another big advantage is they process the cloth flat and wrinkle free. Due to the high level of linear stress on the fabric, they are targeted toward wovens. As a general rule, they are shade inflexible in that they are only profitable for long runs of the same shade. In recent years, they have lost some market share to exhaust dyeing.

Today, energy, in the form of electricity, steam, and natural or propane gas, is a major cost item in the dyeing of textile substrates. Because these energy concerns, a ‘pad batch’ dyeing of cotton fabrics has become more common place. This system uses the specialized properties of certain reactive dyes. This new semi-continuous method is known as pad-batch dyeing and is illustrated in Figure 26.

 

 

In pad-batch dyeing, flat open-width cotton fabric is saturated with selected concentrated reactive dyes, squeezed through a pad and rolled up. The dye saturated fabric is then loaded onto a motorized A-frame. The cloth roll is covered with plastic to prevent surface evaporation and the roll is slowly rotated or batched to prevent gravity from moving the dyestuff and insuring uniform dyeing. The batch time can be from 4 to 24 hours depending of the type of reactive dye used. This entire process is at room temperature requiring very little energy. During the batching process, these special cold reacting reactive dyes fix onto the cotton fabric. Most hot dyeing reactive dyes will not work well with this process. After fixation, the dyed fabric is washed to remove unfixed dye and the fabric is dried.

Pad-batch dyeing is a simple, low energy method for dyeing cotton fabrics.   It only works with certain reactive dyes so there may be limitations in shade availability and fastness properties of the dyed fabric. This is an extremely energy efficient method capable of high productivity requiring a small amount of plant floor space. It is currently used worldwide for cotton fabrics.

‘Garment dyeing’ has become popular for certain types of garments such as sweaters and socks.   In addition, knit products such as t-shirts and knitted pants are dyed with this method. Recently, denim garments which have been stonewashed or discharged are over-dyed in garment form. Figure 27 is a photograph of the most popular type of machine for garment dyeing known as a rotary drum machine.

The rotary drum machine used for garment dyeing had its roots in the commercial laundry industry. These are batch, exhaust dyeing machines. They have a rotating basket inside the machine into which garments are loaded. As the basket rotates, the garments tumble creating agitation with the dye bath which is necessary for dyeing. The tumbling action can be modified depending on the internal design of the basket. In general, dyeing procedures are the same as with other exhaust machines. After dyeing, the garments are washed. Some modern machines have garment extraction spin cycles. The garments are unloaded and transported to further processing.

 

The justification for garment dyeing has been discussed earlier.   However, this method is lower productivity than fabric dyeing but produces no colored waste fabric since only the fabric that is sewn into the garment is dyed. Also, since the dyeing is closer to the retail industry in time (after cut and sew) the dyeing of unpopular colors can be minimized. This method is most effective for cotton knit structures; however, woven garments like denim are over dyed. Typically, exhaustible pigments are processed using this type of machine. Some rotary drum machines can be pressurized and are used for dyeing cotton-polyester blends. A big advantage of garment dyeing is that process time is reduced due to much less water being used and faster after washing.

Summary of Dyeing

The dyeing process, regardless of the textile substrate, is complex and requires a high degree of knowledge and skill on the part of the manufacturers. Dyeing requires clean textile substrates in order to maximize product quality. In preparation, desizing and scouring are required processes for woven fabric structures. Other textile substrates only require scouring. Scouring and bleaching are necessary for whites, pastels and very bright shades. Most other preparation processes are optional.   Clean process water is critical for producing quality products.

Dyeing can be complex and even cumbersome. Dyeing quality and performance are directly controlled by time, temperature, pH, and chemical auxiliaries. Dyes are fiber specific and have their individual advantages and disadvantages. They must be processed under careful control. What may be thought of as an insignificant variable can have a major impact on dyeing performance and dyed product quality. Dyeing equipment is designed for specific types of textile substrates and, in certain cases, specific dyeing conditions.

The goal of dyeing is to produce the highest quality on-shade product which meets all of the customer’s specification for colorfastness and color durability. This dyed product should be produced in the shortest amount of time, using the least amount of energy, generating minimum water waste. This produces the most profit for the manufacturer and the highest quality product for the customer.

References

1. Hudson, P., Joseph’s Introductory Science. 6th Orlando, Fl.: Harcount, Brace, Jovanovich; 1993.
2. Berns, Bill Meyer and Saltzman’s, Principles of Color Technology. 3rd Edition, New York, NY; John Wiley & Sons, Inc.; 2000.
3. Dyeing Primer, Research Triangle Park, NC: American Association of Textile Chemists and Colorists; 1981.
4. Hatch, K, Textile Science, Los Angeles, CA: West Publishing Company,