Study On Continuous Dyeing Process For Cotton

 

Continuous Dyeing Process For Cotton

The first step in continuous dyeing involves padding the dye solution onto full-width woven fabric. This is a very low liquor ratio dyeing method that gives the expected improved exhaustion and fixation, under appropriate conditions. The very rapid fixation (<60) at high temperatures that are possible in fully continuous dyeing gives high productivity for long runs of a given color. On the other hand, the slow dye fixation achieved during storage of a roll of impregnated fabric at room temperature results in better dye diffusion and penetration into the fibers.

Several problems must be considered in continuous dyeing with reactive dyes:

1. Dyes of low substantivity are desirable to avoid preferential dye absorption during padding and the initial color tailing that it causes. This is less of a problem with reactive dyes than with direct or sulfur dyes, which have a greater molecular size and therefore higher substantivity. Tailing caused by dyes with different substantivities can result in an initial drift of the hue. These effects can be minimized by adjusting the initial concentration and composition of the feed solution of dyes.
2. When using a prepared alkaline solution of reactive dyes reasonable stability of the dye or alkaline mixture is essential. Any premature hydrolysis of the dye reactive group in the reservoir or pad bath will result in a loss of fixed color. This can be controlled using a dye solution that is not sufficiently stable. and it cannot be stored in a reservoir without an unacceptable degree of hydrolysis. metering pumps deliver known flow rates of separate neutral dye and alkali solution, which are mixed just before entering the pad bath. A constant padding temperature must be maintained for an invariable color yield. Having two separate padding stages solves the problem of dyes that are very susceptible to hydrolysis. In this case, a neutral solution of the reactive dye is first padded onto the goods and after drying, the alkali solution is padded on just before the fixation stage. In some cases, a wet-on-wet two-stage padding is possible.
3. When dyeing very deep shades concentrated dye solutions are required. Dye solutions containing more than 50 gm/l to 60 % solution pick up. The desired concentration may exceed the dye solubility. , particularly if an appreciable amount of electrolyte is present. Low dye solubility can be improved by the addition of a relatively large amount of urea 100 gm/l. Urea helps to break up dye aggregates by hydrogen bonding to the dye molecules. It is sprinkled onto the color dye solution before the salt and stirred to dissolve it.
4. After padding with a neutral solution of reactive dyes, the fabric will often be dried. This must be done under conditions that minimize the migration of unfixed dye to the fiber and yarn surface. Heating for the wet material must be as uniform as possible to avoid side-to-center and back-to-face color differences due to migration. The padding solution will usually contain salt as an antimigrant and also a thickening agent such as a sodium alginate
5. In full operation, the final rinsing and soaping of goods dyed with reactive dyes are also carried out continuously. These processes take place at high fabric speeds and the goods are only in contact with the washing solution for short periods. Therefore, washing off is usually less effective than washing in a dyeing machine where longer washing time and fresh baths are more easily arranged. The usual eight-box washing range allows three dilute raises 2-3 soapings and 2-3 final rinses. For initial rinsing, static baths, into which no water flows are more economical. The concentration of salts accumulating in the static bath must not become so high that soaping efficiency falls because the unfixed dye is too substantive. It is common to have a counter-common flow of water through each short series of boxes before the last bath overflows to the brain. The flow rate of water must provide the required degree of dilution and unfixed dye removal so that the final fabric has acceptable fastness properties.  

Pad-Batch Dyeing

Pad-batch dyeing involves padding the fabric with a pre-mixed alkaline solution of the reactive dyes and then winding up the impregnated fabric on a suitable roller. Fixation occurs during the storage of batched fabric at ambient temperature. The dye and alkali solutions are usually mixed just before padding using metering pumps to maintain the correct ratio. A 4:1 dye-to-alkali solution volume ratio is very common. During storage, the roller may be continuously rotated at a low speed to avoid drainage of the internal liquid within the batch.

Obviously, the dye must have adequate reactivity under the batching conditions to give good fixation within about 24 hr. The more reactive dyes give effective fixation within 2-6 hr. To avoid evaporation from the exposed surfaces and edges of the roll, the fabric is stitched to a somewhat wider end cloth that is padded and finally wrapped around the entire wet roll before it is covered with a plastic film. When padding on solutions with relatively high Ph values, which are necessary to promote fixation of less reactive dyes, there is always a risk of carbon dioxide absorption on the selvage of the material even when these have been accurately wound edge on edge. This decreases the solution Ph since the hydroxide ion is converted into a carbonate ion. The rate of fixation is then lowered. The effect of this only becomes apparent after the final washing when the selvages appear paler than the rest of the material. In pad-batch dyeing with remazol dyes (Dystat), the buffering action of sodium metasilicate added to the NaOH solution is claimed to virtually eliminate the problem of paler selvages from carbon dioxide absorption.

After storing, the batch of material must be thoroughly washed to remove unfixed dye and chemicals. If the fabric is wound on a perforated beam, this can be carried out using a beam dyeing machine. Alternatively, other types of batch dyeing machines or continuous washing can be used.

The semi-continuous pad-batch process offers many advantages and is now widely used in place of jig dyeing. Provided that space is available for storage of the fabric batches, the energy requirements for dyeing are minimal and the process is more economical than batch dyeing for lots in the 1000-10000 m range. The fabric also often has a better handle and surface appearance because it is not continuously circulating around in a winch or jet dyeing machine. The relatively slow fixation process results in good dye penetration into the fibers. 

  Pad-Heat Dyeing

The pad-dye process is only suitable for reactive dyes with fairly high reactivity. For dyes of lower reactivity, the dried fabric must be baked to promote further fixation. In pad-dry dyeing, the fabric is first padded with a dye solution containing sodium bicarbonate. During dyeing, bicarbonate is converted into carbonate, which gives a higher Ph and more effective fixation. The dye solution also contains a high concentration of urea. This helps the cotton fiber retain water during drying, possibly provides a fluid medium for dye diffusion in the fibers in low water contents, and increases dye solubility. Unfortunately, all the urea is removed on washing after fixation and poses a pollution problem. Urea is a source of nitrogen nutrients for algal growth.

Pad Heat Dyeing

                                          2 NaHCO3 = Na2CO3 + H2O + CO2

In the pad dry process, typically with the reactive DCT dyes, the drying rate is fairly low and the cotton retains at least 18% water. If the fabric is fully dried, the reaction increases so the drying time must be longer than that for effective reaction with the cotton. Typical conditions are 2-5 min at 105 C in a hot flue or on the heated cylinder at 105 C. The fabric temperature will not exceed 70 C. The initial drying cylinders may be much cooler to avoid excessively high initial evaporation rates and migration.

The pad-dry-bake process is suitable for low-reactive dyes such as MCT dyes. Today, this type of process is used mainly in combination with the application of dispersed dye to cotton /polyester materials using the thermosol process. Baking temperatures of 200-220 C for 1 min is typical but may be lower (150-170) C if only cotton is being dyed. When dyeing deep shades, the degree of fixation is usually lower. The fabric tends to be yellow at high baking temperatures and this can influence the hue of pale shades. The most popular of these variant processes for dyeing cotton or polyester fabrics is the neutral thermofix method. For this, a solution of reactive and dispersed dye containing sodium bicarbonate is padded onto the cotton/polyester using dicyanodiamide, in place of urea. Both fibers are dyed simultaneously by heating at around 200C for 1 min. As in all reactive dyeing processes, the final step is through washing to remove unfixed dyes.

Pad-Steam Dyeing

In this process, the goods are padded with a solution containing the reactive dyes, salts, and the appropriate alkali. Again only selected dyes are suitable and the manufacturer's recommendations should be followed. The hot humid conditions during streaming tend to excessive hydrolysis of the reactive group and thus lower the color yield. In dyeing terry toweling and other pile fabrics, a two-stage wet-on-wet padding version of this process is used to avoid an intermediate drying step. The toweling is first padded with a neutral solution of the reactive dyes, and then with a solution of the alkali before steaming. In wet-on-wet padding, the pick-up of the second solution should be sufficiently high to give good fixation, and color bleeding into the second pad bath must not be excessive.

Pad Steam Dyeing

Pad-dry-pad-steam dyeing

This is the predominant fully continuous reactive dyeing process. The fabric is padded with a neutral solution of the reactive dyes, dried, and then padded with the alkali solution containing salt before steaming. The solution is quite stable because there is no alkali present in the dye bath. Some salt and anti-migrant agent helps to minimize the migration of the dyes to the yarn surfaces during the initial stages of the drying. The intermediate drying ensures a uniform and high pick-up of the alkaline salt solution and minimizes the bleeding of dye into the chemical pad. After drying, the fabric must be cooled on cold cylinders to avoid heating the chemical pad solution. Since sodium carbonate is difficult to dissolve in salt solution, sodium hydroxide is the preferred alkali. There are several variations of this type of process and it is also incorporated into the continuous dyeing of cotton /polyester materials using dispersed and reactive dyes.