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Controlling Shade In Continuous Finishing Plants

With the complex nature of the process, understanding the details is crucial.

By Buck Bright

shade40_964T here is a reason why we refer to finishing plants by a specific name such as indigo, print, batch dye, piece dye, continuous dye, yarn dye and jet dye. It is obvious that these operations focus on the addition of color to some type of substrate. The difficulty and complexity of doing this is the major thrust of finishing plants. Color drives “the business” most consistently on a long-term basis. However, this isn’t to say that color application is the only important and complex application made in a textile finishing plant, but it certainly is the most important and complex.

The identification, number and control of variables are at the heart of good color shade control. As we know, different types of dyeing and application methods are more or less complex depending on the chosen substrate. For instance, blends create greater difficulty than one-fiber fabrics and synthetics are more consistent than natural fibers.

Continuous dyeing of cotton and poly/cotton blends presents a huge number of variables, and the difficulty of producing good shade quality is exacerbated by the very nature that the process is continuous. Every minute that one is running perfect shade and quality on the dye range, it is joy; and every moment that one is running the converse is absolute terror mixed with a lion’s share of anxiety.

Okay, we have the stage. Now what do we do? We’ll start by defining the variables. It is only through making them constants that we will achieve the level of consistent quality that we desire.

The factors which affect shade control are substrate, fabric preparation, dye-mix preparation, dye-range application, finishing application and specialty finishing where applicable.

Understanding Substrates

Examination of the substrate points out that the following factors bear significance relative to subsequent shade control and shade variation: fiber content, fiber specifications, yarn and fabric formation, chemical auxiliaries and the age of the substrate.

The contents of fabrics for continuous processing run the gamut from 100-percent cotton to blends of polyester and cotton. There are any number of blending combinations used. The most popular are spun poly/cotton yarns in ratios of 65-percent poly/35-percent cotton, 50-percent poly/50-percent cotton and 40-percent poly/60-percent cotton. There are also blended fabrics which combine blended spun yarns with textured or spun filament polyester yarns. Each and every one of these different mixes create variations in dyeability as well as different physical properties.

In addition to the content or type of fiber, the basic nature of those fibers present are factors that have an effect on dyeing or on dyeability. The most fundamental of these specifications are maturity, base color, fineness, crystallinity, orientation, ionic character and micronaire. With these characteristics, the most important point to remember is that it is the mixing of variations of them that contribute to shade variability.

Variations are introduced through the degree and type of blending, two or three phase drawing, spinning type and tensions as well as loom tensions, reed spread and so on. Slashing is critical in a number of ways. The size formula is very critical, as are differences in size pick-up, speed differences, creep speed, duration of creep and temperature settings. These are all factors in how well the size is removed. The product texturing, core-spinning or other specialty processes also have a large bearing on subsequent shade control.

It is very important that all chemical auxiliaries are selected carefully and are controlled with precision. Lubricants, waxes, oils (natural or synthetic) and warp-sizing chemicals are the most obvious of these assistants. In a number of ways, these products have a large effect on dye distribution. Some of them promote dye uptake while others are dye inhibitors. Another difficulty is that they do not affect different classifications in the same manner. Complete removal of all auxiliaries used in fabric preparation is extremely critical because redeposition of these agents will cause huge quality and/or aesthetic problems. Warp size specifications are of extreme importance. PVA, CMC and starch may be used, but they must adhere to guidelines that promote excellent removal characteristics.

How much, what proportions, viscosity, hydrolysis levels, fluidity ratings and mix percentages are some of the important specifications. Any size mix and/or additive that include agents that are not water-soluble will result in a myriad of problems later on. The use of animal fats or tallow is a certain disaster.

Several of the natural and synthetic components in the fiber/fabric tend to migrate over time. Levelness and degree of dye uptake are affected by the distribution as well as by the type of component that is present. Long-term storage of yarn or greige fabric has an effect on shade. Fabric stored for long duration in a hot warehouse is quite different than that processed more quickly. All chemicals, even moisture, will act as a carrier system.

Fabric Preparation

Singeing is required for two reasons: elimination of fuzz that will cause uneven dyeing and lint build-up in processing, and minimization of pilling in the final product. The effect of singeing on shade varies with the flame intensity, heat and the effectiveness and control of the cooling roll mechanism.

Full elimination of the size materials is necessary to allow all of the subsequent treatments to proceed on an even basis. If the size removal is not complete, unlevel and non-thorough dyeings will occur. Sometimes this is manifested as shade, streak and/or spot problems.

Proper scouring is the single most important step in the fabric preparation. It is this process where the majority of the waxes, oils, cotton hulls, waste products and insoluble salts are removed from the substrate, and the fabric’s absorbency is determined. This can only be rendered effective if full desize has allowed the scouring to take place. Side-to-center shading is the result of a number of things, but the uneven redistribution of non or partially soaponifiable waxes is very often the culprit.

Bleaching gives the fabric its base color or whiteness. This is the background, or starting point, on which each shade is built. It is an attempt to overcome the innate differences that may be caused by yarn-on-yarn related variations and those that were introduced by previous processing.

Mercerization is the process of trying to impart dyeability to fibers of cotton that are not fully mature. The process swells the cotton (while burning polyester in a detrimental way) so that it simulates a mature cotton fiber and will thus accept dyes and chemicals to a greater degree. Mercerization obviously has an effect on both mature and immature cotton, so the result is that one still has an unlevelness to contend with in the dyeing process.

The heat setting of polyester/cotton blends does positive things for width stability and for crease prevention, but has a variable effect on shade. The amount of heat that a polyester fiber has seen is proportional to the shade depth that one will receive upon thermosol dyeing of that fiber.

The type and amounts of chemicals used during the mechanical steps of preparation will have a profound effect on the substrate as introduced to dyeing or finishing. What type of chemical will be removed — which type of size? If there are mixed sizes on the yarn, which is the dominant and what is the best way to remove both (or all) of them? Very often, the chemical that is correct for one type of removal process may be a problem in the removal of another type.

Organic stabilizers are great for insuring level degree of oxidation in bleaching as long as they don’t produce spots or streaks on a short-term basis. The quality of cotton dictates the formula. How much sugar content is there? How much natural wax and oil is present? These types of characteristics dictate what chemical interaction is needed.

In each of the processes, it is imperative that constant control be established. Consistency is the single most important factor if a reliable product is to be produced. Control of absorbency, pH, degree of mercerization, whiteness and degree of heat-set and tension (warp and fill) throughout the process all have a variable effect on the subsequent dyeing and finishing process. These factors are the result of precise control of chemical choice, chemical flows, temperature maintenance, timed reactions and so on. It becomes obvious why there must be a very consistent substrate coming from greige manufacturing. The very nature of substrate preparation is to render that substrate as clean and consistent as possible.

Dye Processing

dispense42_967We now have a textile substrate that is ready to be dyed. The factors that first must be determined are the proper kind of dyestuffs to do either or all of the following: produce a precise color; produce a color that will give certain physical or quality characteristics; and/or produce a color at the least expensive level. This task is initiated in the color lab.

Prior to beginning the shade-matching process, a number of issues must be addressed. What is the substrate? What is the end use? What lights will be used by the customer to view the shade? What finishes and auxiliaries will be necessary? Will there be subsequent processing, other than cut-and-sew, after the customer receives the product?

For the purposes of this discussion, 100-percent cotton and regular dyeable polyester and cotton blends will be considered.

In general, matching 100-percent cotton (or 100-percent anything) is much easier than matching a blend. The degree of blend throws additional variables into the equation. Every dyer knows that a formula for a 65/35 will not be appropriate for a 70/30, or for a 40/60, or for a 50/50, etc.

Not only will the shade be different, but the appearance will also show varying degrees of acceptability. Other factors which bring about differences are open-end versus ring-spun, spun versus filament, twill versus poplin (i.e., warp-faced fabric versus evenly distributed fabric), and so on.

The selection of the dye type will have a very real effect on how well the shade will run in the subsequent production situation. For instance, sulfur dyes do not repeat well in medium shades and will not reproduce at all in light shades. Monochlorotriazine and vinyl sulfone reactive dyes are very different and certainly should not be mixed together. Trying to achieve brightness in a blend when one is matching against a single fiber submit is very difficult or impossible. There are bifunctional dyes that are rendered so on a molecular level, but that is not the same as a mix of two complete parent compounds.

In matching blends, the resultant shade and ultimate control of the shade are dependent on whether the polyester is dull, semi-dull or bright.

One other important point is that many dyes are sensitive to the moisture in the fabric. If one looks at or reads shade when the fabric is completely dry, it will give a look or reading that will be different when the fabric conditions change. Sometimes this difference is subtle and sometimes it is quite large. At times, that “conditioning difference” is large enough to render a sample completely off-shade when initially it may have appeared to be perfectly okay.

The dye lab must produce a formulation that can be reproduced in the production environment time after time. The perceived color includes such vague qualities as brightness, clarity and warmth. Brightness, clarity and warmth are achieved differently in the laboratory than they are on the range.

The task of taking a new shade to the production dye range is attacked in a number of ways. Often, there is good use made of a “mini-range” which is made by components that most nearly represent a scale of the parent range. For instance, a 10-percent mock-up would be made to run at 10 yards per minute and then the pad, predryer, ovens, steamer and washers would be roughly 10 percent of the size of a regular range. Care must always be taken to simulate the same immersion and dwell times. As with all chemically related reactions, they are a function of time and temperature.

Having dyed a patch on this equipment will not be an exact replication of a production run because the abilities of the equipment to reach equilibrium are never going to be the same due to equipment size, bath size and speed achieved. Also these mini- ranges are processing fabric that is only 18 in. to 22 in. wide versus the 60- to 72-in. production fabrics.

On the production ranges, there are a large number of variables that have an effect on shade versus standard as well as on side-to-center shading. A dye range consists of:
• a frame let-off;
• entry scray;
• dye pad;
• predryers;
• predryer cans or hot flue;
• thermosol oven;
• cooling cans;
• chemical pad (in the case of sulfur dyes, this is a dye pad);
• steamer;
• lip;
• ceiling;
• main chamber;
• exit seal (water);
• exit steamer nip;
• wash boxes;
• oxidation boxes;
• soap boxes;
• dry cans;
• inspection scray; and
• take up.

In discussing a dye range, it is important to consider all of the components. The entry section leading to the dye pad must have an even amount of tension or it will have a detrimental effect on the openness of the fabric as it picks up dye and the dye chemicals.

In batch dyeing, we are putting X lb of dye on Y lb of fabric, and we know to expect Z percent of exhaustion. In continuous dyeing, we are working with a concentration gradient, such as pounds of dye per gallon of mix. This means that the pick-up percentage must be very consistent. This pick-up is a function of the tension in the pad, the absorbency of the fabric, and the hardness of the dye pad rolls. Also the pad volume, the number of immersion rolls and arrangement of the immersed rolls are significant factors. Unless these are exactly the same, a fabric and mix made to produce a shade on another range will in all probability be somewhat different than that produced on another dye range.

It is also a factor that the bath and the dye range must come to equilibrium. From an equipment standpoint, that equilibrium is primarily a function of temperatures, air content and airflow. But from a chemical view, it is much more complex. In the dye pad itself, we find that dye and water are being picked up by the fabric at different rates. In fact, different dyes and chemicals are deposited at different rates. As this pick-up happens, the dye bath itself is changing concentration. Eventually an equilibrium is reached and the resultant shade becomes level as long as everything else on the dye range becomes level at its particular equilibrium. It is a general rule that vats and dispersed dyes build up during the first part of a dye run. It is also a fact that the reverse happens in chem pad sulfur dyeing.

Once past the dye pad, the fabric begins to be dried. It is very critical that this drying is complete and that it be done with exceptional results along the width of the fabric. The results of poor drying are poor appearance, poor dye and chemical migration, very bad side-to-center shading and also shade variation.

The predryers are typically gas-fired ceramics or electrical tubes. Electric predryers are great because they are easier to control and they don’t use an open flame. The problems are maintenance and very high operating costs. Most dye ranges in the United States use gas-fired predryers with ceramic or other heat radiating panels. It is the job of these predryers to render the incoming 70-percent wet fabric down to at least 30-percent moisture content as it exits this area. Some fabrics get as low as 10 percent at this point, but care must be taken to get into these low ranges very carefully because of appearance and migration problems.

The predryer cans or hot flue are used to complete the drying process because the fabric should be completely dry as it enters the thermosol section of the range. As a note, most predryers are set at about 1,500°F and the cans are set at 260° to 280°F. The thermosol oven is set so that the fabric reaches 400° to 425°F. The difference in these two oven temperatures is that some dyestuffs are low- to medium-energy users and some, such as navy and red, are high-energy dyes. These temperatures vastly affect the degree and permanency of the strike of these dyes.

The fabric achieves the glass-transition point of polyester and this allows the small molecular-sized dispersed dyes to seep into the fiber. Part of the problem is that various types of cotton dyes also seep into the fiber at varying degrees. It is obvious that the amount of heat and the time at which the fabric is held at temperature are critical factors in the dyeing on continuous ranges. This harsh environment is rather severe on various types of cotton dyes that are “just along for the ride” at this particular time. Heat always promotes migration of dyes. It is always that dyes will migrate toward the heat, so it is imperative that the heat and airflow be very level.

A note about thermosol oven temperatures — if the proper temperature is not achieved, the dyes will not totally dye the fiber. This may or may not be obvious at the exit of the dye range and may be manifested only upon final finishing or subsequent washing. If the temperature is too high or held too long, the fabric may be scorched or tendered. In addition, dyestuffs all have different sublimation rates and sublimation is bound to occur.

At this juncture in the dye range, the cotton dyes (vats and reactives) have been padded-on and dried only. They have stained both polyester and cotton fibers, but there is not true chemical link at this point. We have merely dyed the polyester, which by the way is best dyed at acid pH levels. Naturally, cotton dyeing is not physical as is the dyeing of polyester, but rather it is chemical in nature and is primarily done in an alkaline environment.

As the fabric comes out of the thermosol oven, it is very important to cool it down as rapidly as possible because it is very detrimental to the chemical bath that will render the fabric and the vat dyestuffs reduced so that cotton dyeing can take place. If the fabric is not cool, and it transfers heat into the caustic and sodium hydrosulfite bath, that bath will dissipate very rapidly and “out of reduction” will occur.

If everything has gone according to plan, the dyestuff is reduced and the cellulose is reduced to the sodium leuco form and there develops an attraction between the dye and the fiber. They have not yet reacted, but are attracted by a bond such as Van der Waal forces or hydrogen bonds. The dye steamer catalyzes this reaction.

The conditions in the steamer are really quite critical when dyeing vats or sulfurs. The temperature and moisture content of that steam are absolutely critical if full and consistent shade is to be achieved. If the air content exceeds 400 parts per million by volume, then the dye reactions will not occur.

As with the thermosol oven, time and time at the proper temperature are factors in the steamer’s effect on dye shade. Also a factor is the flow control and temperature maintenance of the steamer’s water seal. If the seal flow is too fast, and if the seal temperature is too high, the dyestuff will be washed off to varying degrees. At best, you will have an unlevel dyeing. Both shade and side-to-center will be affected.

Upon exiting, the steamer oxidation begins to occur, but the first several washes are dedicated to washing off residual and/or unreduced dyestuffs.

After the first several washers, the next two boxes are used for peroxide (or other) oxidation. It is at this point that the true bonding takes place between the vat or sulfur dyes and the cotton fibers. The final chemical requirement on the dye range is to neutralize the fabric so that it is on the acid side going to the finishing department. In reactive dyeing, soap is applied that actually helps to develop the color in a visual sense, since unreacted or water-reacted dyes promote dullness at this point. The dyeing of cotton with reactives, naphthols, soluble vats or others has its own sets of variables.

Finishing Departments

range44_965In finishing, there are a variety of chemicals that are applied. Their effect on shade is varied. Resins have a great impact on shade change and the various resins have different effects that make each of them unique as to exactly how they do this effecting. Glyoxal resins produce varying shade changes depending on the amount of resin and on the catalyst system that is used. Heavy durable press requirements have dictated needs at the 22-percent level where precured, softer hand needs use resin levels in the 8-percent area.

Not only do these levels produce differences in final shade, but also the use of magnesium chloride versus zinc nitrate versus combinations or other types of “hot” catalysts, such as aluminum, lead to quite a difference in the final color. Different dyes in the same class of dye (much less different classes) react quite differently to these finish chemicals. Low formaldehyde resins don’t produce the same shade change, as do higher formaldehyde resins. Of course, they also produce vastly different finished results as far as functionality and physical properties.

The other types of finish resins and chemicals such as carboxylic acid (BTCA), fluorocarbon, melamine, reactive silicone and scavengers all react and, therefore, manifest differently than the DMDHEU types. They are not only different than glyoxal, but they have the same type of variation in their own right. The indiscriminate use of softeners is another issue that introduces a shade variable and a performance variable as well.

In the finishing department, the control of the chemicals and the set-up of the machine variables are as important to shade as is a dye range. The pH of the fabric on entering a finish pad is critical because the varying chemicals will not tolerate excessively wide pH ranges. The mix content is also a great factor. For one to simply say to add a little hand builder or add softener or sewing lubricant, one must know exactly the effect that that decision has on shade as it is seen and also shade as it is maintained in the final and functional product. To make it softer may sound good, but softeners of various types will liberate the dyestuff on heating, curing or washing.

If the fabric is precured one has a good idea (if not exact) of the final shade at that point. But if the goods are post-cured, tunnel-finished, stonewashed, enzyme washed, etc., the final shade is really an educated guessing game.

As with preparation and with dyeing, process control in finishing is a key to consistency. Both temperatures, vacuum levels, preheat and/or predry controls, oven temperatures and cooling rates all must be kept within certain tolerance levels or they will have varying effects on the final shade, not to mention on the final finishing characteristics.

Specialty Finishing

Specialty finishing may include sanding or sueding, napping, chemical coating or chemical foaming. These applications are used to impart some unique effect. Hopefully, these are the types of value-added applications that would give the products the differential advantage for which we all look.

However, these special effects also have a variable effect on the shade of the final product. These imparted qualities must be understood and valued, but for sure, controlled so as to minimize the variation in the final product.

Drug Room And Mixing Facilities

  condition46_966It is all wasted if we don’t have perfect mixing and weighing. The greatest effect and worst performance in controlling shade is in mixing facilities and procedures.

In a truly modern dyeing and finishing plant, up-to-date and mechanized weighing and dispensing systems are in place and have been in place. In today’s manufacturing environment, it is mandatory that we have a plan to get to these modern drug rooms automated to achieve consistent quality and efficiency. Antiquated mixing rooms contribute vastly to product variability. And where we walk on a borderline, it will certainly produce negative results some of the time. Properly designed and statistically proven experiments have been run to demonstrate where shade variation is introduced and to what degree. The purpose of these experiments was not to answer what causes off-quality for shade. Those two questions are close and they are certainly related, but they are not the same.

These trials were repeated several times and they were additionally repeated at different facilities and even at different companies. In each case, the product was a 7.5-oz, 65-percent poly/35-percent cotton twill that was dispersed/vat-dyed and finished in a post-cured resin finish.

The experiments demonstrated that shade variation contributions were allocated to the various disciplines: 23 percent greige and substrate; 6 percent preparation (fully modern); 36 percent dye department; and 35 percent finishing department.

The results demonstrate that we must address and tie down the variables that exist in our operations. No flexibility, and thus variability, without testing, trial and verification is acceptable. If our objective is a consistently performing final product, it is necessary that we develop our equipment and our people in such a way as to allow the realization of that objective.

Shade control in continuous finishing plants takes total commitment from the entire breadth of manufacturing. As with most objectives of achieving excellence, it requires dedicated team effort to attain these goals.

Editor’s Note: Buck Bright has more than 30 years of industry experience including technical, management and executive positions with Milliken & Co., Springs Industries and Avondale Mills. He has also consulted domestically as well as internationally in various disciplines of textile manufacturing and marketing. Bright has been a member of ATMI’s Dyeing and Finishing Committee and served a number of years as chairman of The Institute of Textile Technology Dyeing and Finishing Committee.
April 2000