Causes and measures of common dyeing problems in nylon


Guide:Nylon is a hydrophobic fiber with amino and carboxyl groups on the fiber. It can be dyed with disperse dyes, acid dyes, especially weak acid dyes.


Its dyeing has the characteristics of fast dye uptake and high exhaustion rate. However, due to the different spinning process specifications, the fiber structure is very different, which leads to large differences in dyeing performance and easy to produce color differences.


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

Causes and measures of common dyeing problems in nylon
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Causes and measures of common dyeing problems in nylon

 Molecular formula of Nylon 6 and Nylon 66


(Picture: Molecular formula of Nylon 6 and Nylon 66)


1. Color flower and color difference caused by competitive dyeing


1.1 Cause analysis


This is caused by improper selection of dyestuffs. The dyeing saturation value of nylon is very low, so the competition between different dyes is very prominent when dyeing thick colors. If the selected dyes differ greatly in dye uptake and affinity, the color of the fiber will be very different in different dyeing time, resulting in color difference and poor reproducibility of the size sample.

 Color flower and color difference caused by competitive dyeing

1.2 Preventive and remedial measures


Choose dyeing and chemical series with similar dyeing curve and affinity, good compatibility, and suitable for production machine. Proofers are required to fully grasp the dyeing properties of various dyes. When choosing dyeing materials, they must comprehensively consider the dye uptake rate, dyeing curve, leveling property, color fastness performance, and sensitivity to temperature and leveling agents. Factors such as sex.


1.2.1 Fully consider the compatibility of dyes


When using several dyes for patch dyeing, the appropriate dyes should be selected and the amount of dyes should be controlled. Generally, you should try to choose the same series of dyes from the same company. Even if you have to use dyes from different companies, you should try to choose dyes with similar dyeing curves, similar initial dyeing temperatures, and similar sensitivity to temperature and leveling agents. Avoid competing infections.

Compatibility of dyes

1.2.2 Pay attention to the difference in the size of the dye in the competition


Some dyes are not obvious when dyeing small samples, but they are completely exposed in large-scale production. For example, in the production of lake green and peacock blue, if the acid turquoise blue and acid yellow are used together, similar problems will arise. This is due to the large molecular structure of acid turquoise blue, which is very different from the dyeing curve of acid yellow, which causes competition. If the acid green basket and the yellowish acid green are used together, the competition dyeing problem will be basically solved.


1.2.3 Pay attention to the adaptability of the machine to dyes


Dyeing machines include jet, warp beam and jigger etc. In the jet dyeing machine, the dye liquor is in full contact with the fabric, the levelling property is good, the product feels full, and the reproducibility is good, the tank difference is small, but its wet fastness is relatively poor. We can choose weak acid dyes or 1:2 metal complex acid dyes with good fastness but poor levelness for dyeing. The product width of the warp beam dyeing machine is easy to control and easy to fix, but it is prone to problems such as depth and color difference between the head and tail. Dyes with good levelling properties but poor fastness can be selected, and the amount of levelling agent should be increased slightly, and the fixation can be strengthened after dyeing.

Dyeing machine

2. Defects caused by unreasonable technology


Nylon dyeing requires extremely high craftsmanship. Process conditions are important factors that affect the color and level of dyeing products, such as temperature, bath ratio, PH value, etc., will affect the quality of the product. Unreasonable technology is easy to produce defects such as poor level dyeing, color stain, color willow, color difference, and poor fastness.


2.1 Control the initial dyeing temperature and heating rate


Temperature is an important factor in controlling dyeing. The temperature will affect the degree of fiber puffing, dye performance (solubility, dispersibility, dye uptake, shade, etc.) and the performance of additives. Nylon is a thermoplastic fiber. When the temperature is low, the dyeing rate is very slow. When the temperature exceeds 50℃, the swelling of the fiber increases continuously with the increase of temperature.


The effect of temperature on the dye uptake rate is also different for different dyes. The dye uptake rate of level dyeing dyes gradually increases with the increase of temperature; the dye uptake rate of fulling-resistant dyes should be higher than 60 It starts to increase rapidly with the increase of temperature after ℃. Especially in the temperature range of 65~85℃, controlling the heating rate is the key to the success or failure of nylon dyeing. If it is not controlled properly, it will cause the problems of fast coloring, poor dye migration, easy to repair and difficult to repair. If using fulling-resistant dye to dye nylon, the initial dyeing temperature should be room temperature, in the temperature range of 65~85℃, strictly control the heating rate of about 1℃/min, and add leveling agent, adopt a stepped heating method; then increase the temperature to 95 ~98℃, keep for 45~60min. In addition, the dyeing performance of this fiber also changes with the heat treatment conditions before dyeing, and the dyeing rate of the fiber after dry heat setting is significantly reduced.


2.2 Determine the appropriate liquor ratio


Due to the limitation of equipment, the liquor ratio of the small sample will be larger than that of the large-scale production, but if the liquor ratio is too large, the dye uptake rate will be reduced and the color difference of the sample size will be caused. The bath ratio of light and thin taffeta is generally 1:50, and the bath ratio of heavier fabrics is 1:20, subject to the fact that the fabric can be completely immersed in the dye solution.


2.3 Control PH value


The pH value of the dyeing 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 dyeing light colors is generally controlled at 6-7 (usually adjusted by ammonium acetate), and the amount of leveling agent is increased to strengthen leveling and avoid dyeing flowers, but the pH value cannot be too high. High, otherwise the shade will be dark; the pH value of dyeing dark colors is 4-6 (usually adjusted by acetic acid and ammonium acetate), and an appropriate amount of acetic acid is added during the heat preservation process to reduce the pH value and promote dye uptake.

Weak acid dye dyeing

2.4 Pay attention to the selection and dosage of levelling agent


In view of the poor leveling and coverage of nylon dyeing, a small amount of anionic or non-ionic leveling agent should be added to the dyeing bath, of which anionic surfactants are the main ones. It can be used in the same bath with dyes during dyeing, or pre-dyeing nylon with levelling agent. The anionic leveling agent dissociates into negative ions in the dye bath, enters the fiber, first occupies a limited dye seat on the nylon fiber, and then is gradually replaced by the dye as the temperature rises during the dyeing process, reducing the bond between the dye and the fiber Speed, to achieve the purpose of leveling; non-ionic leveling agent hydrogen bond with the dye in the dyeing bath, and then gradually decompose to release the dye during the dyeing process, and is absorbed by the fiber.


It is found through experiments that the addition of levelling agent can significantly improve leveling and covering dyeing ability, but with the increase of auxiliary agent concentration, the dye uptake rate decreases, resulting in a decrease in the exhaustion rate to varying degrees. Therefore, the amount of leveling agent should not be too much. Because the leveling agent not only has the leveling effect during the dyeing process, but also has the effect of blocking the dyeing. Excessive amount of leveling agent will reduce the dye uptake rate of acid dyes, increase the concentration of dyeing residue, 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 accumulation of production experience, we found that after the introduction of the buffer system, the selection and dosage of levelling agents play a decisive role in controlling the color difference of 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. In the production of small samples, the amount of leveling agent is controlled at 0.2~1.5g/L, that is, under the premise of achieving a good leveling effect, if the residual liquid rate of light colors is 2% to 3%, and the medium and dark colors are at 5% ~15%, the amount of the leveling agent is the required amount. Large-scale production can be modified according to the sample amount to achieve good results.


3. Color difference of size sample lofting

When nylon is dyed, there are many reasons for the color difference of the size sample, such as the gray fabric used in the size sample, the dyeing material, and the different process conditions of the size sample. The precautions 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 the large and small samples, and correct the data of the small samples.


3.1 The color matching of the sample must be strict and accurate


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

 The sample color

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


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


(4) The dyeing performance of nylon fiber also changes with its heat history before dyeing. Different heat setting conditions will result in different color absorption rates of fabrics, resulting in color difference between batches of fabric. The process control of the pretreatment of nylon fabrics also has a great influence on the dyeing effect, so the size and size of the organization should be the same, and the process conditions of the semi-products before dyeing should be consistent. It is best to use the same batch of semi-products.


(5) The small sample should be dyed with the same origin, the same factory, the same product name, and the same batch number as the large sample. The compatibility value of the dyes selected during the color matching should be basically the same, so as to ensure the proportional relationship of the dyes in the dyeing solution during the dyeing process, which is beneficial to the stability and reproducibility of the dyeing shade. When matching colors, the main color dyes should be fixed, and the dyes used for color shading should be adjusted in a variable manner so that the color shading of the size samples is consistent. Dyes that are likely to cause discoloration during the dyeing process are not used.


3.2 Standardize the color matching operation in the laboratory


(1) Generally speaking, the closer to the color of the gray series, the more difficult it is to judge its gray saturation, because the absorption hue it contains is more complicated, and three kinds of dyes are often required to mix, so it is close to the hue of the gray series. Make the choice of color bias with the feeling of yellow, red, and cyan. The stronger the color perception, the more important it is to judge the hue of the freshness and purity of the color. Therefore, when matching colors, you must first make a correct judgment and choose the correct dye.

(2) Pay attention to the change of the angle between the observation sample and the light to keep the color matching.

(3) Grasp the drying degree of the sample fabric after dyeing. Excessive drying will cause irreversible reddish tint; insufficient drying will affect the color saturation of the color sample. Both conditions will cause color light deviation.

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


3.3 Strictly control the consistency of the size and sample process


(1) The pH value and heating process of the proofing dye bath should be as consistent with mass production as possible. Due to the cross-use of water quality and direct steam or indirect steam in large-scale production, the boiler steam is often brought into alkalinity and the pH value of the dye bath is high. The use of buffers or an online pH monitor can solve this problem.


(2) The holding time of the small sample dyeing should be the same as that of the large sample, so as to avoid color difference due to poor dye penetration.


(3) Since the color fixation will also affect the color light, the color light must be adjusted after the sample is fixed in order to enter the process of mass production.


4. Conclusion


There are many factors that affect the quality of nylon dyeing products, including equipment, technology, dyeing materials, color difference between sizes and operations. Production practice has proved that the accuracy and stability of nylon dyeing can be improved by grasping the above-mentioned links, so that the one-time success rate of sample lofting can reach more than 90%.

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