Dyeing methods for polyester and cellulose fibers

Polyester fiber and cellulose fiber blended fabrics are often dyed with reactive and dispersed dyes. The dyeing methods can generally be divided into two bath method, one bath two-step method, and one bath one-step method.

 

The two bath method includes dyeing with disperse dyes at high temperature and pressure or carrier dyeing followed by reactive dyes dyeing, and dyeing with reactive dyes followed by dyeing with disperse dyes at high temperature and pressure or carrier dyeing; The one bath two-step method also includes two methods: dispersion followed by activity, and activation followed by dispersion. When one dye is dyed or fixed, the dye solution is not discharged completely, and only the pH value and temperature are adjusted before adding the second dye for dyeing; One bath one step method, also known as one bath method, refers to the process of dyeing and fixing two types of fibers under acidic, neutral, alkaline, or sliding pH conditions when two dyes are mixed into the same dyeing bath, followed by washing and soaping to complete the entire dyeing process.

The main advantages of one bath dyeing are: shortening dyeing time; Save energy and water, reduce sewage; Simplify the operation of dyeing solution preparation and feeding, and reduce the related problems caused by this. This article mainly reviews the polyester/cellulose one bath dyeing process and the neutral fixing reactive dyes used.

Difficulties in One Bath Dyeing with Disperse/Reactive Dyes

  1. The diffusion rate of disperse dyes in polyester fibers is very low at dyeing temperature, so the dyeing temperature is generally 110-130 ℃, which is much higher than the dyeing temperature of commonly used reactive dyes. When using carrier dyeing, although it can be around 100 ℃, this is still relatively high for reactive dyes. Most of the dyeing temperature of reactive dyes is between 60-90 ℃, and high temperature not only reduces the fixation efficiency, but also makes it difficult to dye dark colors after the dye’s directness is reduced.
  2. Most reactive dyes are fixed under alkaline conditions, while high-temperature and high-pressure dyeing with dispersed dyes should be carried out in weakly acidic media, and carrier method dyeing is also carried out in weakly acidic media. Due to the fact that most disperse dyes are prone to hydrolysis or reduction damage under alkaline conditions, resulting in color change or fading of the dyes. However, reactive dyes, if fixed under weak acidic conditions, not only have low fixation efficiency, but also have a fast breaking rate of covalent bonds between the dyes and fibers.
  3. In order to improve the dyeing rate and percentage during immersion dyeing with electrolyte reactive dyes, a certain amount of neutral electrolyte needs to be added, especially when the dyeing temperature is high and the bath ratio is large. Disperse dyes are dispersed in a suspended form in the dye solution by dispersants. The dispersion stability decreases with the increase of electrolyte content in the dye solution. A large amount of electrolyte causes the dispersed dye particles to increase and even precipitate, thereby reducing its dyeing rate and leveling effect.
  4. The interaction between dyes or between reactive dyes and dispersants. Many dispersed dyes and dispersants react with reactive dyes at high temperatures, resulting in a decrease in the fixation rate of both dyes. They are particularly prone to occur at high dyeing or fixation temperatures, such as high-temperature and high-pressure dyeing. When using the carrier method for dyeing, the carrier will also react with reactive dyes.
  5. Dyeing fastness: Due to the fact that two types of dyes not only dye the corresponding fibers, but also severely stain the other fiber, especially the staining of dispersed dyes on cellulose fibers, such as cotton fibers, it greatly reduces their soaping and friction fastness. This is also an important reason for the poor color fastness of such blended fabrics at present.
  6. When dyeing a blend fabric of low polyester fibers and cellulose fibers with difficulty in dyeing and reproducibility in one bath, whether it is a disperse dye or a reactive dye, they can only dye one type of fiber. Therefore, the actual dyeing bath ratio is much larger than that of dyeing pure textiles, and the bath ratio varies with the blending ratio. For example, if the bath ratio of reactive dye dyeing pure cotton fabric is 20:1, and the same dyeing solution dyeing a blend fabric with a cotton blending ratio of 40%, the bath ratio increases to 50:1, Because only 40% of the original weight of cotton fibers are dyed in the same amount of dye solution. The increase in bath ratio reduces the dye uptake rate, which in turn reduces the dye uptake and fixation rate, thus increasing the difficulty of dyeing. In fact, not only does the bath ratio increase, but for blended fabrics, the fibers that blend together also have mutual mechanical resistance to dyeing. When reactive dyes are applied to cellulose fibers, the polyester fibers near the cellulose fibers will hinder the reactive dyes from approaching the surface of the cellulose fibers, and the dye solution in the capillaries between the fibers inside the yarn is difficult to flow. At this point, the dye actually diffuses in the diffusion boundary layer, and this mechanical hindrance is very obvious.

At present, the biggest difficulties in dyeing polyester/cellulose fiber blended fabrics with dispersed/reactive dyes in one bath are severe staining, poor wet treatment fastness, and low reproducibility. Figure 1 shows a comparison of the staining of dispersed dyes in polyester fiber cotton blended fabrics using two bath, one bath two-step, and one bath dyeing methods. Staining is the main reason for poor wet treatment fastness.

Disperse dyes not only dye polyester fibers, but also stain two types of fibers. If the two bath dyeing process is used to dye dispersed dyes first, reduction cleaning and soap washing can be used to remove the staining of dispersed dyes. Whether the dyed product is

The residual staining dyes on cotton or polyester fibers can be basically removed, so the product has good soaping and friction fastness. If it is a one bath or one bath two-step dyeing method for disperse dyes and reactive dyes, if there are reactive dyes present on the fabric, reduction cleaning cannot be used, and only soap washing can be used to remove the stained dyes. Due to poor washing effect, the stained disperse dyes on the two fibers are not easy to clean, resulting in poor color fastness.

Dye requirements

One bath dyeing requires the first selection of dispersed dyes with weak dyeing properties, and the use of reactive dyes with high acid resistance and high temperature dyeing rate, which can be used for low salt and low alkali fixation. At the same time, a reasonable dyeing process should be formulated, dyeing time should be shortened, appropriate dyeing auxiliaries should be selected to improve even dyeing, color fastness, and dyeing reproducibility.

  1. Reactive dyes are required to dissolve in neutral water at 80-90 ℃, and both acidic and alkaline water can cause dye decomposition. Dissolve separately with dispersed dyes and mix for use. According to the level of affinity, it is divided into three levels: high, medium, and low. High affinity is suitable for dyeing dark colors, but poor levelness; Low affinity is suitable for dyeing light colors, but good levelness.
  2. Disperse dyes require stability in a high-temperature neutral bath, minimal influence from sodium sulfate, good compatibility with reactive dyes, good dye absorption, no need for reduction cleaning, less decrease in color fastness after fixation and post-treatment, high color fastness, pH value that cannot fluctuate, and good color reproducibility.

Theoretical basis of one bath dyeing

The reaction rate of reactive dyes not only depends on the active group, but also varies with pH and temperature. Temperature and pH are two related factors. Under alkaline conditions, the fixation temperature is lower, while at higher temperatures, the fixation pH is lower. It can be fixed under weak alkaline or neutral conditions. According to the study of reaction kinetics, the effect of a temperature difference of 20 ℃ on the fixation rate is equivalent to changing one pH value unit. Therefore, under high temperature conditions of dispersed dyes, the fixation pH value of reactive dyes can be reduced.

Both difluoro chloropyrimidine and monochlorotriazine reactive dyes exhibit the same fixation rate in the range of 40 ℃, pH 10-11 to 100 ℃, and pH around 7; The fixation pH of vinyl sulfone reactive dyes should be 11-12 at 40 ℃, and 9-10 at 100 ℃ for the same fixation rate; Trichloropyrimidine reactive dyes have low reactivity. At a pH of 13, the fixation temperature is 60-70 ℃, and even at 100 ℃, the fixation pH is about 11-12.

The contradiction of fixation rate can be solved by selecting high direct dyes, controlling the heating rate appropriately, and adding more electrolytes to compensate, as the direct nature of dyes decreases with increasing temperature. Adding neutral electrolytes can improve the dye uptake and fixation rate of reactive dyes. The higher the dyeing temperature, the higher the amount of salt needed to be added. The one bath dyeing method with dispersed/reactive dyes has a high dyeing temperature, with polyester dyeing at 130 ℃ and PTT fibers also dyeing at 110 ℃. Therefore, it is often necessary to add more sodium sulfate to promote dyeing, in order to compensate for the direct decrease in salt (sodium sulfate) concentration required by the dye at higher temperatures. The higher the dye concentration, the higher the salt concentration required, and the higher the dyeing temperature, the higher the salt concentration required.

However, the addition of electrolytes is very detrimental to the dyeing of dispersed dyes, as it not only reduces their dispersion stability, but also lowers their fixation rate, migration and levelness, increases the staining of cellulose fibers, and reduces dyeing fastness.

One bath dyeing process with dispersed/reactive dyes

  1. Weakly acidic high-temperature high-pressure one bath process

When reactive dyes and disperse dyes undergo fixation and hydrolysis reactions in weak acidic baths with the presence of acid acceptors, their pH value does not decrease rapidly. They can be dyed in one bath with disperse dyes in the range of 100-130 ℃. To ensure a high fixation rate, more electrolytes need to be added to the dyeing bath, and it is required that the reactive dyes have sufficient stability and directness at high temperatures.

The one bath dyeing process with weak acidic high temperature and high pressure starts from around 70 ℃, and NaH2P04 and NaH2P04 pH buffer systems are added to the dyeing solution. The pH value of the dyeing should not be too low. Depending on different types of active dyes, those with strong reactivity and good hydrolysis stability can be slightly lower. For most dyeing materials, the pH value should not be lower than 5. The electrolyte required for fixing reactive dyes is always added to the dyeing bath from the beginning. Cycle the solution several times before adding reactive and dispersed dyes, and finally add a buffer and alkali releasing agent. Due to the low pH value, the salt dosage is relatively high compared to the standard process, reaching 60-150g/L. The salt dosage depends on the color depth, and this process is mainly suitable for dyeing light colored products.

  1. The dyeing process of the weak alkaline high temperature and high pressure method one bath process is the same as that of the weak acidic bath. When selecting dispersed dyes, dyes with good alkali resistance and stability should be selected. Due to its high pH value, reactive dyes have a high fixation rate and are not only suitable for dyeing light colored products, but also for dyeing medium and dark colored products. When dyeing in a weakly alkaline bath (pH=7-8), the color of phthalocyanine structured emerald blue and bright green dyes dyed at 130 ℃ is much darker than that of the usual method. This is because high temperatures are conducive to the diffusion of these larger structured dyes into the fibers.

Due to the poor alkali resistance of most dispersed dyes, careful selection of dispersed dyes is necessary, as many dispersed dyes are not suitable for this process. Another advantage of alkaline dyeing is its good dyeing fastness. This is because alkaline dyeing can not only achieve a certain degree of scouring effect, but also remove oligomers or residual hydrolysis products from PET fibers, and remove dispersed dyes from cellulose. Therefore, it can improve the levelness, reproducibility, washing and rubbing fastness of dyeing.

  1. The one bath dyeing method involves dyeing in a weak acidic bath first, followed by alkaline high-temperature and high-pressure dyeing. After dyeing at 130 ℃, the temperature is lowered to a suitable temperature for the fixation of reactive dyes, and then alkaline fixation is added. Unlike the one bath two-step method, both dispersed dyes and reactive dyes are added to the dyeing solution before the dyeing temperature rises, which greatly simplifies the operation and does not require the addition of reactive dyes midway, Just add an alkaline agent midway to adjust the pH value of the dye to the desired alkalinity. It is a process that falls between the two-step one bath method and the complete one bath method. This process not only avoids the disadvantage of alkaline hydrolysis damage to dispersed dyes, but also reduces the adverse effects of electrolytes on the dyeing of dispersed dyes, reduces the staining of dispersed dyes on cellulose fibers, and improves dyeing reproducibility and color fastness.

According to this dyeing process, in the early stage, including the dyeing period when the temperature is raised to 130 ℃, the pH value of the dyeing solution should be controlled between 5-6 (preferably 5.5), and the pH value should not exceed 6 or be lower than 5. This way, both dispersed and active dyes are relatively stable, and hydrolysis damage is less at 130 ℃. In addition to using NaH2P04, acetic acid and sodium acetate can also be used as pH buffering agents.

Dispersants are used to prevent the dyeing of cellulose fibers by dispersed dyes, and they need to be dispersion leveling agents that are resistant to sodium hydroxide. The application of these agents can improve the leveling effect and color fastness. To prevent the reduction damage of cellulose fibers and dispersants to disperse dyes, causing fading and discoloration, weak oxidants are added to prevent reduction damage. After dyeing, wash in cold water, neutralize, soap, and then wash in hot or cold water. Appropriate detergents should be used to thoroughly remove contaminated dispersed dyes.

This process has the advantages of one bath two-step dyeing and can simplify the operation. Therefore, it is currently widely used and is particularly suitable for low salt reactive dyes with high directness. Due to the fact that the dyeing process is carried out in an alkaline bath in the later stage and anti staining agents are added, all fastness after soaping are good.

  1. The one bath process of pH sliding high-temperature and high-pressure method The one bath process of pH sliding high-temperature and high-pressure method is developed from the one bath two-step method. The two types of dyes are dyed and fixed on the fiber successively, but it is not necessary to adjust the pH value in the dyeing process, just select the appropriate pH regulator, also known as pH sliding agent. It is added together with the dye at the beginning of dyeing, and then, like one bath dyeing, the two types of dyes can be sequentially dyed and fixed under the optimal dyeing conditions by controlling the heating process. Generally, the pH is slid from alkaline to acidic, that is, in the early stage of the heating process, the dyeing solution remains alkaline, mainly resulting in the uptake and fixation of cellulose fibers by reactive dyes. When the temperature rises to 90-100 ℃, the uptake and fixation of reactive dyes are basically completed, and the pH of the dyeing solution automatically slides to the acidic range, preventing dispersed dyes from being hydrolyzed and reduced, maintaining a weakly acidic state for dyeing, and maintaining the optimal conditions for the uptake of dispersed dyes, Until the dyeing is completed (the highest dyeing temperature is generally 130 ℃). This not only has the advantages of a one bath two-step process, but also eliminates the need for sudden cooling or heating during dyeing, intermediate feeding, and pH adjustment, making it easy to operate with a one bath method. The fixation rate of these two dyes is higher than that of the one bath method in acidic or alkaline baths. Compared with the two bath and one bath two-step methods, its dyeing time is significantly shortened. And since the dyeing and fixation of reactive dyes overlap in the heating stage of dispersed dye dyeing, no additional energy consumption is required. Although the heating stage takes a long time, it is beneficial for dispersed dyes to undergo interface migration (70-90 ℃) and improve the leveling effect.

The key to pH sliding one bath dyeing is the application of pH sliding auxiliaries, which are a type of auxiliaries that gradually release acid during the dyeing process. They are usually mixtures of ester compounds that undergo hydrolysis as the ester compounds gradually heat up in an alkaline bath. The released acid neutralizes the alkali in the dyeing bath and lowers the pH value. Finally, they can be buffered in neutral or weak acidity to maintain a suitable dyeing pH value for dispersed dyes.

Quick links:

Skycron® Disperse Dyes:https://www.tiankunchemical.com/Skycron-Disperse-Dyes-pl3625703.html
Reactive dyes:https://skygroupchem.com/product/skyzol-reactive-dyes/
Dyes Products:https://skygroupchem.com/products/dyes-products/
Agent Proudcts:https://skygroupchem.com/products/textile-auxiliaries-products/
Textile solutions:https://skygroupchem.com/solutions/
Service Case:https://skygroupchem.com/services/case/

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