Causes And Solutions To Common Nylon Dyeing Problems

Nylon is a hydrophobic fiber that contains amino and carboxyl groups. It can be dyed with disperse dyes, acid dyes, especially weak acid dyes. Its dyeing has the characteristics of fast dyeing rate and high exhaustion rate, but due to the different spinning process specifications, the fiber structure is very different, resulting in large differences in dyeing performance and easy color difference.

Therefore, if there is a slight negligence in the preparation of the sample process, it may cause uneven dyeing, color difference, color stains, color spots, dark and light edges, stripes, color mismatch and poor dyeing fastness.

1. Color flowers and color differences caused by competitive dyeing

Cause analysis:

This is caused by improper selection of dyes and chemicals. The dyeing saturation value of nylon is very low, so when dyeing dark colors, competitive dyeing between different dyes is very prominent. If the selected dyes have large differences in dyeing rate and affinity, the color of the fiber dyed will be very different during different dyeing times, resulting in color difference and poor reproducibility between large and small samples.

Preventive and remedial measures:

Select dyeing materials with similar dyeing curves and affinity, good compatibility, and suitable for production machines. Samplers are required to fully understand the dyeing properties of various dyes. When selecting dyes, they should comprehensively consider factors such as dyeing rate, dyeing curve, leveling, color fastness, and sensitivity to temperature and leveling agents.

1. Fully consider the compatibility of dyes

When using several dyes for dyeing, it is necessary to select suitable dyes and control the amount of dyes. Generally, dyes from the same company should be selected as much as possible. Even if dyes from different companies have to be selected, dyes with similar dyeing curves, similar initial dyeing temperatures, and similar sensitivity to temperature and leveling agents should be selected as much as possible to avoid competitive dyeing.

2. Pay attention to the differences in competitive dyeing of large and small samples of dyes

Some dyes are not obviously competitive when dyeing small samples, but they are completely exposed in large production. For example, when producing lake green and peacock blue, if acid turquoise blue and acid yellow are selected for combination, similar problems will occur. This is because the molecular structure of acid turquoise blue is large, and the dyeing curve is very different from that of acid yellow, which causes competitive dyeing. If acid turquoise blue is used instead and acid green with yellow light is used, the competitive dyeing problem can be basically solved.

3. Pay attention to the adaptability of the machine to dyes

Dyeing machines include jet, warp and roller dyeing machines. In the jet dyeing machine, the dye liquid is in full contact with the fabric, the leveling property is good, the product feels full, and the reproducibility is good, the cylinder difference is small, but its wet treatment fastness is relatively poor. Weak acid dyes with good fastness but slightly poor leveling property or 1:2 metal complex acid dyes can be used for dyeing. The product width of the warp dyeing machine is simple to control and the color fixation is easy, but it is easy to have problems such as deep and shallow layers and head and tail color difference. Dyes with good leveling property but slightly poor fastness can be selected, and the amount of leveling agent can be slightly increased, and the color fixation can be strengthened after dyeing.

2. Defects caused by unreasonable process

Nylon dyeing has extremely high process requirements. Process conditions are important factors that affect the color and levelness of dyed products. For example, temperature, bath ratio, pH value, etc., will affect the quality of the product. Unreasonable processes are prone to produce defects such as poor levelness, color spots, color willows, color differences, and poor fastness.

1. Control the initial dyeing temperature and heating rate

Temperature is an important factor in controlling dyeing. The temperature will affect the expansion degree of the fiber, the performance of the dye (solubility, dispersibility, dyeing rate, color, etc.) and the performance of the auxiliaries. Nylon is a thermoplastic fiber. The dyeing rate is very slow at low temperatures. When the temperature exceeds 50°C, the fiber swelling increases with the increase of temperature.

The effect of temperature on the dye uptake rate also varies depending on the dye. The dyeing rate of level dyes gradually increases with the increase of temperature; the dyeing rate of shrinkage-resistant dyes will only begin to increase rapidly with the increase of temperature after the dye bath temperature is higher than 60°C. Especially in the temperature range of 65-85℃, controlling the heating rate is the key to the success of nylon dyeing. If it is not properly controlled, it will cause problems such as fast coloring, poor dyeing, easy to flower and difficult to repair. If nylon is dyed with shrinkage-resistant dyes, the initial dyeing temperature should be room temperature. In the temperature range of 65-85℃, the heating rate should be strictly controlled at about 1℃/min, and a leveling agent should be added to adopt a step-by-step heating method; then the temperature should be raised to 95-98℃ and kept warm for 45-60min. In addition, the dyeing properties of this fiber also vary with the heat treatment conditions before dyeing, and the fiber dyeing rate after dry heat setting is significantly reduced.

2. Determine the appropriate bath ratio

Due to equipment limitations, the bath ratio of small samples will be larger than that of large production, but too large a bath ratio will reduce the dyeing rate and cause color difference between large and small samples. The bath ratio of thin taffeta is generally 1:50, and the bath ratio of thicker fabrics is 1:20, based on the fabric being completely immersed in the dye solution.

3. Control the pH value

The pH value of the dye bath has a great influence on the dye uptake rate, and the dye uptake rate will increase rapidly as the pH value decreases. When dyeing nylon with weak acid dyes, the pH value of light-colored dyes is generally controlled at 6-7 (commonly adjusted with ammonium acetate), and the amount of leveling agent is increased to enhance leveling and avoid dyeing, but the pH value cannot be too high, otherwise the color will wither; the pH value of dark-colored dyes is 4-6 (commonly adjusted with acetic acid and ammonium acetate), and an appropriate amount of acetic acid is added during the heat preservation process to lower the pH value and promote dye uptake.

4. Pay attention to the selection and dosage of leveling agents

In view of the poor leveling and coverage of nylon dyeing, a small amount of anionic or non-ionic leveling agents should be added to the dye bath, among which anionic surfactants are the main ones. It can be used in the same bath with the dye during dyeing, or the leveling agent can be used to treat nylon before dyeing. Anionic leveling agents dissociate into negative ions in the dye bath and enter the fiber. They first occupy the limited dyeing sites on the nylon fiber, and then are gradually replaced by the dye as the temperature rises during the dyeing process, reducing the binding speed between the dye and the fiber and achieving the purpose of leveling; nonionic leveling agents hydrogen bond with the dye in the dye bath, and then gradually decompose and release the dye during the dyeing process and are adsorbed by the fiber.

Through experiments, it was found that the addition of leveling agents can significantly improve the leveling and covering ability, but with the increase of the concentration of the auxiliary agent, the dyeing rate decreases, resulting in a decrease in the exhaustion rate to varying degrees, so the amount of leveling agent should not be too much. Because the leveling agent has a dyeing effect in addition to the leveling effect during the dyeing process, it also has a dyeing blocking effect. Excessive use of leveling agents will reduce the dyeing rate of acid dyes, increase the concentration of the dyeing residual liquid, and cause color difference and poor reproducibility of large and small samples. Generally, the amount of leveling agent is larger when dyeing light colors; when dyeing dark colors, the amount of leveling agent is less.

For a long time, the industry has always believed that the control of pH value is the key to the success or failure of nylon dyeing. After years of production experience, we found that after the introduction of the buffer system, the selection and dosage of the leveling agent plays a decisive role in controlling the color difference between the large and small samples. The leveling agent should be used in conjunction with the corresponding dye category, but the dosage must be adjusted according to the actual situation. When producing small samples, the dosage of the leveling agent is controlled at 0.2-1.5 g/L, that is, under the premise of achieving a good leveling effect, if the residual liquid rate of light colors is 2%-3% and the medium and dark colors are 5%-15%, then the dosage of the leveling agent is the required amount. In large production, the dosage can be corrected according to the dosage of the small sample to achieve a good effect.

3. Color difference of large and small samples

When dyeing nylon, there are many reasons for the color difference of large and small samples, such as the different grey fabrics, dyes and chemicals used in the large and small samples, and the different process conditions of the large and small samples. The prevention and remedies that can be taken are: reduce the impact of the environment and light source, standardize the proofing and color matching operations; analyze the differences between large and small samples, and correct the data of the small sample.

1. Small sample color matching must be strict and accurate

(1) The color matching and color matching environment design of the laboratory should use a series of colors such as black, white and gray as much as possible, so as to prevent the "afterimage" caused by the environmental color on the eye physiology and affect the color matching. The lighting of the color matching environment must be sufficient to prevent the hue change caused by the light source during color matching. A "conditional" fixed light source should be used, or a light box that meets international standards should be equipped. If the probability of light source change is relatively high, such as the color matching environment of the laboratory is open, the light source outside the window will affect the color matching efficiency due to the change of different light sources at different times (such as different light sources in the morning and afternoon, and different light sources on cloudy days and sunny days).

(2) Before making a confirmation sample, you should first clarify the various requirements of the customer, such as the original sample color light bias, whether there is special finishing, whether the dye for fiber dyeing is specified, etc.

(3) The water used for small sample production should be consistent with that for large sample production, and the water quality and pH value should be tested daily and adjusted to the process requirements to avoid color light differences.

(4) The dyeing properties of nylon fiber also vary with its heat history before dyeing. Different heat setting conditions will cause different color absorption rates of the fabric, resulting in color differences between batches of fabric. The process control of nylon fabric pretreatment also has a great influence on the dyeing effect, so the organizational specifications of the large and small samples should be the same, and the process conditions of the semi-finished products before dyeing should be consistent. It is best to use the same batch of semi-finished products.

(5) The small sample should use dyes and chemicals from the same origin, the same factory, the same product name, and the same batch number as the large sample. The dye compatibility values selected during color matching should be basically consistent, so as to ensure the proportion of each dye in the dye solution during the dyeing process, which is conducive to the stability and reproducibility of the dyeing color. When matching colors, the main color dye should be fixed, and the dye used to adjust the color should be changed to ensure that the color of the large and small samples is consistent. Dyes that are easy to cause discoloration during the dyeing process should not be used.

2. Standardize the color matching operation in the laboratory

(1) Generally speaking, the closer the color is to the gray series, the more difficult it is to judge its gray saturation, because the absorption hue it contains is relatively complex and often requires three dyes to be mixed. Therefore, for hues close to the gray series, only the feeling of yellow, red, and blue can be used to make color choices when matching. The stronger the color feeling, the more important the color's vividness and purity are to the judgment of hue. Therefore, when matching colors, you must first make a correct judgment and choose the right dye.

(2) When matching colors, pay attention to observing the changes in the angle of the sample and the light to maintain consistency.

(3) Understand the degree of drying of the sample cloth after dyeing. Excessive drying will cause the color light to be irreversibly reddish; insufficient drying will affect the color light saturation of the color sample. Both situations will cause color light deviation.

(4) When imitating colors, you should pay attention to the classification of varieties and color systems, keep samples, accumulate data and establish a color sample library (it is better to have corresponding actual production samples).

3. Strictly control the consistency of the process of small and large samples

(1) The pH value and heating process of the sample dye bath should be as consistent as possible with that of the large production. Due to the cross-use of water quality and direct steam or indirect steam in large sample production, the boiler steam often brings in alkalinity, which makes the pH value of the dye bath higher. The use of buffers or equipping with a pH value online monitor can solve this problem.

(2) The insulation time of small sample dyeing should be consistent with that of the large sample to avoid color difference due to poor dye penetration.

(3) Since color fixation will also affect the color light, the color light must be adjusted after the small sample is fixed before entering the process formulation of large production.

Conclusion:

There are many factors that affect the quality of nylon dyeing products, mainly including equipment, process, dyes and chemicals, color difference between large and small samples, and operation. Production practice has proved that by grasping the above links, the accuracy and stability of nylon dyeing can be improved, and the one-time success rate of small sample placement can reach more than 90%.

If you want to know more about dyeing solutions, please contact: info@tiankunchemical.com