What Should I Do If The Dyeing Depth of Polyester And Its Blends Cannot Reach The Required Level, And The Imitation Color Proofing Cannot Match The Customer’s Sample?

During the dyeing and finishing process of knitted fabrics, some unexpected quality defects often occur. If they cannot be dealt with in a timely and effective manner, they will affect the progress of the entire process and cause serious economic losses to production. Some young printing and dyeing workers have little production experience and do not have a thorough understanding of the properties and uses of dyeing materials, which will inevitably cause long-term confusion. This article lists some quality problems and solutions that occur in actual production, hoping to be helpful to printing and dyeing technicians.

01 Analysis and Discussion of Dyeing Defects

1. The problem of dye discoloration

In the dyeing of polyester and its blended knitted fabrics, printing and dyeing workers have raised issues with the use of dyes, mainly the discoloration problem of disperse dye polyester. For example, dispersed red FB changes color to purple, dispersed dark blue H-GL changes color to brown, etc. Disperse Red FB has bright hue and good dyeability. However, when there are metal ions such as copper and iron in the dye bath, they will combine with the dye to form a metal complex structure, making the dyed light blue and purple, and the dyeability is poor.

In addition, certain metal ions in the dye bath can trigger oxidation-reduction reactions in acidic and high-temperature environments, destroying the structure of the dye and directly leading to no color.

The solution is:

① Add chelating dispersant to the dye bath to complex metal ions in the water so that it does not affect the structure of the dye.

②Choose dispersed red ACE with similar hue instead. The latter is more stable than the former.

Dispersed Deep Blue H-GL (CI. Dispersed Blue 79) has been used for a long time due to its good dyeing depth and color fastness to sublimation, but occasionally the problem of large color deviation occurs. This is mainly due to the fact that dispersed deep blue H-GL is particularly sensitive to the pH value of the dye bath. Usually the PH value of the dye bath is controlled at 4.5 to 5. If the PH value is greater than 7, the color will change greatly, from reddish to brown, or even to earthy yellow, and the depth will drop greatly. Preventive measures: The amount of acetic acid used for dyeing must be accurate, and the dyeing bath ratio must be strictly controlled.

The color change problem of dyes is not only a problem of a single dye, but also has a certain relationship with the dye combination selected when color matching. Low-temperature dyes do not have high requirements for pH value and can even be dyed in alkaline dyes, while some high-temperature dyes are particularly sensitive to pH value. The other two types of dyes have very different exhaustion rates and dyeing temperature requirements, so they should not be used in color combinations.

Pay attention to the change of dyeing shade caused by auxiliaries. Leveling agents, anti-wrinkle agents, moisture wicking finishing agents, color fixing agents, silicone softeners and other additives added during the dyeing process of knitted fabrics must ensure that they have no impact on the shade and color depth of the fabric. Generally, pioneer experiments are done before being used in large-scale production. Properly adjusting the setting temperature can also reduce or avoid fabric discoloration.

2. Improper post-dyeing treatment, resulting in very different color depths

Sometimes, in order to pursue a certain style, polyester-cotton knitted fabrics only require one fiber to be dyed alone and the other fiber to be left blank. In production, we often encounter such problems. The color imitation of the sample was very successful, but the color of the large machine was different after dyeing, either darker or lighter. This is mainly manifested in the black and linen gray color of polyester and cotton, which is mostly light in color and difficult to deepen and correct. The above problems make it difficult to achieve one-time attachment of samples through color stripping or adding materials.

The reasons for the problem are as follows:

① When using disperse dyes to dye polyester, the post-processing operation of the small sample does not match the operation of the large machine. Sample reduction and cleaning often do not pay attention to the feeding temperature, and the temperature at which the insurance powder is added to the dye cup is arbitrary. The optimal temperature for dye stripping of insurance powder is between 70°C and 80°C. If the temperature is too low, it will decompose too early and become ineffective during the heating process, and the cleaning effect on floating colors is very low. Disperse dye stained on cotton fibers will make the entire fabric look darker, seeming to achieve standard style and depth. The large machine adds insurance powder at 70°C to 80°C, and the intensity of restoration cleaning is much greater than that of the sample. The floating color on the cotton fiber is cleaned relatively cleanly, making it look whiter, and the overall color of the cloth surface looks lighter. In this case, overdying polyester won't help. The only option is to overdye the cotton fiber with a little reactive dye to enhance the depth and achieve a sample style.

②The imitation color of the sample must be made from mass-produced fabrics. Some factories don't pay much attention to the use of fabrics when imitating samples, and just use a piece of fabric to do it. However, they don't know that the color depth and dyeing style of fabrics vary greatly due to different proportions of materials. Once the sample is confirmed, it will be difficult for large-scale production to attach the sample! For example, the styles of 65/35 polyester-cotton fabrics and 60/40 CVC polyester-cotton fabrics dyed with the same color are completely different. The latter has more cotton points and the color looks lighter. This must be paid attention to in practical work.

The selected disperse dyes have poor lifting power, washing fastness, thermal migration fastness, etc. For the above reasons, the methods to overcome the problem are:

1. Use large-scale machine-produced fabrics as much as possible to imitate colors

2. Sample reduction and cleaning should be carried out according to process requirements.

3. Single dyeing of polyester or single dyeing of cotton is attached, and a sample analysis is required before making a decision.

4. When using disperse dyes, choose high-quality products with high fastness and high lifting power.

3. Color difference problem of main and auxiliary materials

Taking polyester-cotton knitted fabric dyeing as an example, we often find this phenomenon during production:

For knitted fabrics in the same dye bath, the colors of the main materials and auxiliary materials are very different, and the color difference requirements cannot be met.

The main reasons are:

①The composition ratio of main materials and auxiliary materials is different. The auxiliary materials have high polyester content or high cotton content.

②The weight of the main and auxiliary materials is inconsistent, and the coloring rate varies greatly.

Prevention methods are:

① When the fabrics are put into storage, the composition and weaving ratio of the main and auxiliary materials should be recorded to avoid color differences caused by different fabric compositions.

②For fabrics with the same main and auxiliary material composition and weaving ratio, if the weight difference is too large, a sample dyeing test should be done in advance. The difference in color depth contrast is large, so separate processes need to be formulated and dyeing in separate pots (generally the color of the auxiliary materials depends on the main material).

③ Fabrics whose main and auxiliary materials have different compositions and weaving ratios should be dyed in separate pots in principle. If you want to dye in one pot, you must find out in advance how big the color difference will be when dyed in the same pot and whether the color difference requirements can be met. The method of inspection is to simulate the mass ratio of the main ingredients and auxiliary materials in the same pot according to the actual pot arrangement plan of large-scale production, and then observe the coloring situation. Only when the color difference can meet the quality requirements can the same pot be dyed.

4. The problem of holes in fabric

Knitted fabrics occasionally have holes in the fabric after dyeing, and the location and distribution of the holes are uneven. After eliminating the causes of fabric weaving and equipment, the main thing is to check the operation of the dyeing process. The holes that appear on the polyester-cotton knitted fabric are mainly caused by the caustic soda added in the pre-treatment, which is directly added to the dyeing machine without being diluted and dissolved in the chemical tank. Undissolved solid particles adhere to the surface of the fabric, causing local alkalinity to be too strong, damaging the fibers and forming holes (mostly due to the pre-treatment process).

The holes on the brocade cotton fabric are mainly caused by the formic acid or glacial acetic acid and other additives used for dyeing being added directly into the dyeing machine without dilution. Nylon is resistant to alkali but not acid. Strong acid will corrode the nylon locally and form holes. Ordinary viscose fiber has poorer alkali and acid resistance than cotton, and its strength in the wet state is lower. Strong alkali can cause fabric damage and strength reduction. The use of caustic soda should be strictly controlled in the scouring and oxygen bleaching process of dyeing production. To prevent the occurrence of the above problems, the main thing is to strengthen the operation management of workers, standardize the use of strong acids and strong alkali and other additives, and the process must use the reasonable dosage of acid and alkali and other additives.

5. The problem of insufficient dyeing depth

During the dyeing process, we often encounter the problem that the dyeing depth cannot meet the requirements. In addition to the error between small sample and large sample dyeing, this is also closely related to fabric pre-treatment, dye bath pH control, dyeing temperature, dye bath ratio, dye lifting power and stability, different fabric composition, etc.

Some disperse dyes with high fastness have strict dyeing pH requirements. Generally, the pH value is 4.5 to 5. If the pH value of the dye bath is slightly higher, the color yield will decrease. The dyeing properties of the same type of dyes vary greatly depending on where they are produced. For some fabrics containing spandex, the dye pick-up rate will decrease after pre-styling. If the dyeing temperature is too low, the high-temperature dyes in the color matching formula will not reach the optimal dyeing temperature, and the dyeing rate will decrease, affecting the overall depth of the color.

The dyeing liquor ratio not only affects reactive dyes, but also disperse dyes. If the liquor ratio of individual varieties of disperse dyes is too large, the dye uptake rate will also be reduced. Therefore, only by strengthening the accuracy of sample color imitation, strictly managing the operation of the machine, and scientifically using dyes can we achieve satisfactory results. Here we take the alkali reduction processing and dyeing of black silk suede as an example to illustrate.

Black silk suede (polyester/polyester microfiber double-sided fabric, the ratio of black silk to white silk is 85/15)

Dyeing process: Raising and pre-shaping → alkali reduction/dyeing in the same bath → pre-shaping → sanding → rolling of the finished product.

Dyeing formula: 1g/L refining degreaser, 1g/L alkaline leveling agent, 10g/L alkali, alkali-resistant disperse dye HA-6RS 0.8%, alkali-resistant disperse dye HA-2G ruby 0.5%, alkali-resistant disperse dye HA- 2B black 6.0%,

Dyeing conditions: Liquor ratio 1:15, 130℃×40min. The factory dyed according to this process. It was found that the color of the large product was quite different from the color of the small sample. It was not black, the depth was shallow, it was yellowish brown with less blue light, and was close to brown.

At first, the factory changed the process formula, extended the dyeing time, and increased the amount of black dye, but this problem could not be solved. Later, joint research with technicians from the dye production plant found the root cause of the problem. In the alkali reduction dyeing process of black silk suede in the same bath, most of the disperse dyes used in black silk are not resistant to alkali. Because it will gradually fade in a strong alkali environment, even if the blackness depth of the white silk is reached, the black silk will undergo major changes, which will affect the final depth and color of the cloth.

The reason why small samples can be attached is that the dyeing time is short and the alkali treatment time is short and the color fades lightly. However, large machine reduction dyeing takes a long time, the dye is highly destructive, the depth and hue changes are not fixed, and the dispersed black dye used for dyeing itself is also reddish. Since the lack of blue light cannot be supplemented, the problem of yellowish and reddish dyeing on the main machine has been caused. After experiments, the alkali-resistant disperse dye HA-2BR black was used instead of HA-2B black, and the large-scale dyeing was more stable. HA-2BR black has more blue light than HA-2B black. The lack of blue light is supplemented during the dyeing process, so the color of the cloth meets the requirements.

6. The dielectric proofing fails to match the standard sample.

The problem of dither proofing not matching the standard sample is mainly manifested in the fact that the color light is not attached to the light, the dyeing is not bright enough, and the hue deviates too much. The main reason is that the selection of dyes and the color matching formula are unreasonable. First of all, before dithering, the sampler must observe the incoming sample, especially bright colors, and check whether it contains fluorescence under a standard light source. If it does, add a whitening agent to adjust it. If the whitening agent still cannot achieve the brightness, you have to consider using fluorescent dyes to match the color.

Dyes with the same specifications and names have different dyeing properties due to different manufacturers. Some of them have big yellow light, some have big blue light, and some have big red light. There are also differences in dyeing speed and lifting power. Improper selection will affect the color matching effect. For a black standard with a heavy red light, it is best to use a black dye with a strong red light when copying, but it is unreasonable to use a black dye with a strong green light for color matching. The basic principle of color matching proofing is to minimize the number of dyes used in color matching. For example, if a bright red color can be created using a red dye with a higher yellow light intensity and a red dye with a higher light intensity of the blue light, there is no need to use dyes of the three primary colors to spell the color.

When using three dyes to dye one color, the dye type and dyeing performance must be basically the same. In order to avoid the phenomenon of light jumping, the metamerism of the color matching dyes used must be observed. Some dyes display different colors on different fabrics, which requires samplers to have rich dye application knowledge and color matching experience.

02Notes

In short, during the dyeing and finishing process, printing and dyeing technicians should follow up and observe more, test and analyze more problems that arise, and combine theoretical knowledge to solve some shortcomings in actual production. This is critical to improving factory production efficiency and product quality.

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