Technical analysis of dyeing and finishing chromatic aberration

There are many and complex factors leading to the color difference of dyed finished fabric. To strictly control the color difference within the standard range and keep it stable for a long time, it involves all aspects of the production and processing process. It is necessary to make a comprehensive analysis of each factor and influencing factors, formulate corresponding preventive measures and strictly standardize the means of implementation. Control the chromatic aberration of dyed fabrics.
The color difference of dyed products is generally divided into four categories: original color difference, front and back color difference, left middle right color difference and positive and negative color difference (commonly known as Yin and yang). The original color difference refers to the difference in color phase, color light and color depth between dyed fabrics and customers'samples or standard color card samples; the former and latter color difference refers to the difference in color phase, color light and depth between the fabrics dyed by the same process; the left-middle-right color difference refers to the color phase, color light and color of the left-middle-right part of the weft-to-left part of the dyed fabrics. The difference of luster depth exists between the positive and negative sides (yin and Yang sides) refers to the difference between the two sides of plain weave and the depth of luster. Combining with production practice, the causes of these four types of color difference are briefly analyzed, and the preventive and control measures are put forward.
Main factors causing color difference
1, pre-processing
With the progress and development of printing and dyeing production and processing technology, the influence of pretreatment capacity, treatment quality and treatment effect on the quality of finished products is becoming more and more prominent. Industry experts believe that about 70% of the product quality should be controlled in the pre-processing process.
In the process of fabric printing and dyeing, the defects of semi-finished products after pretreatment are generally concealed to some extent, especially those affecting the intrinsic quality index of the fabric. It is difficult to judge and identify them. The main factors affecting the color difference of finished fabrics in pretreatment are the uneven and unclean desizing and impurity removal, the inconsistent gross effect before and after desizing, the large batch difference of whiteness, the uneven mercerization, the poor washing effect and the excessive alkalinity of semi-finished fabrics.
Semifinished products are unevenly distributed and unclean.
After scouring and bleaching, the pigments have been removed. It is difficult to find the quality problems of desizing, impurity removal and impurity removal only from the fabric surface. Bleaching and light-colored varieties produced by this semi-finished product are prone to yellowing, discoloration, color instability (fading, darkness), poor brightness and other problems. In addition, it will affect the mercerization and dyeing effect of the fabric, resulting in defects such as mercerization and white core dyeing on the surface of the fabric. When mercerizing, if the fabric surface appears patches before the first alkali concentration trough of mercerizing machine and the first rolling point, it shows that the alkali absorption of the fabric is not uniform during mercerizing, which is the performance of uneven and unclean desizing of semi-finished products. Such semi-finished products will also cause uneven color absorption when the bottling machine is rolling the dye solution.
Wool and whiteness of semi finished products
We usually emphasize that the pretreatment effect of semi-finished fabrics is good. It is not only about desizing and cleaning impurities, high gross efficiency and good whiteness, but also emphasizes the consistency and stability of semi-finished fabrics'quality. In the pretreatment process, the fluctuation of equipment status, process conditions and parameters will affect the quality consistency of semi-finished fabrics. For example, the large pressure difference between the left and right of the wash rolling car or the serious concave core of the roll will make the wash effect of the two sides and the middle of the fabric quite different, and the color difference between the left, middle and right of the fabric will be caused by the inconsistency of the capillary effect and whiteness of the fabric in dyeing. Similarly, the left and right pressure difference or concave center of the rolling mill will cause the discrepancy of the rolling allowance between the two sides and the middle of the fabric, resulting in the color difference of the fabric after dyeing. In addition, the instability of additives and additives, the fluctuation of washing temperature and speed will lead to the inconsistency of gross effect and whiteness of semi-finished products before and after the pretreatment, which will lead to the color difference between the front and back of the fabric. Especially for dyes with high directness, such as reactive dyes, Hindicosine dyes and Nafto dyes, it is easy to produce chromatic aberration, even for reductive suspensions and disperse dyes with low directness. This is mainly because the semi-finished products with high capillary effect have better moisture absorption, higher dyeing rate and darker color. On the contrary, the semi-finished products with low capillary effect have poor moisture absorption, low dyeing rate and light color, which cause uneven dyeing. It should be emphasized that the inconsistent whiteness of the semi-finished products to be dyed will also have a great impact on the color and light of the fabric, especially in the production of bleaching and light-colored dyeing varieties.
Semifinished mercerizing process
The mercerizing effect has a great influence on the dyeing rate of cotton fibers. The instability of mercerizing process conditions and parameters has a significant impact on fabric dyeing. For example, the large fluctuation of alkali concentration in concentrated alkali tank and the speed of production and processing will affect the mercerizing effect of cotton fibers, which will result in the color difference of fabric dyeing. In addition, the unstable pH value of mercerized fabric, i.e. the alkali removal, poor washing effect or instability of mercerized fabric, will also have a negative impact on fabric dyeing. Especially in the production of polyester-cotton dyed fabrics and dyeing with alkali-sensitive dyes, the high or unstable pH value will cause great changes in the color light of the fabric after dyeing, resulting in darkness, unstable color phase, poor brightness, resulting in the original color difference of the fabric.
2, dyed
Dyeing process is one of the key processes in fabric dyeing and finishing. It is also a process in which the color difference of fabrics is concentrated. There are many factors affecting the color difference of fabrics and it is also complex. According to production practice, it can be roughly divided into four main factors: dye performance, dye solution preparation, process conditions and equipment performance.
Dyestuff properties
There are two basic methods to select dyes before dyeing. One is to select a dye with similar dyeing luster.Then add a small amount of other dyes to adjust its color and lustre; another method is to use three primary colors, which can spell out most of the color and lustre. This method is one of the main methods of dyeing cloth selection, but the use of too many kinds of dyes will affect the color, depth and brightness of the fabric. If the color change of the fabric is large after dyeing, it will cause the original color difference of the fabric, that is, it does not match.
Some dyes have thermal denaturation, that is, high temperature will cause dyed or colored changes of dyed fabrics. Reactive dyes and Schilling dyes have obvious thermal denaturation. It takes a long time for them to return to normal color light. It even takes more than ten hours or more for individual dyes. For example, green dyes are blue when heated and yellow when cooled, while Shilinlian IR is red when heated and blue when cooled. This will result in the instability of the color and light after dyeing, or difficult to determine the color and the sample, affecting the pattern of the fabric, easy to produce the original color difference.
Dyeing liquid preparation
The preparation of dye solution consists of weighing, grinding and chemicals. The accuracy of weighing and chemicals in the preparation of dye solution and the standardization of abrasive and chemical operation have obvious effects on the color difference of the original sample and the color difference before and after the preparation. For example, equipment errors, the use errors of symmetrical instruments, operation errors in weighing, abrasive and chemical materials, errors caused by inconsistent moisture regain of dyes, inadequate dyestuff force distribution, and different batches of dyes used in the same bill of lading, all of these will result in color variation and color aberration of dyed fabrics.
The moisture absorption problem of dyes should be emphasized here. In the process of production management, this factor is often neglected. According to relevant information, the additives in powdery dyes have strong hygroscopicity. When the relative humidity of air is high, the hygroscopicity of dyes is very serious. In practical production, if such dyes are used for color matching, the color and luster depth of dyed fabrics will be greatly affected. At the same time, due to the relative reduction of absolute use of dyes, the phenomenon of color difference and discrepancy between before and after dyeing will easily occur.
Fluctuation of process parameters
In the process of pad dyeing, the dye concentration in the working fluid will be different from the initial concentration after reaching the dynamic equilibrium due to the great difference of the dye's directness to the fabric in the dyeing solution. In practical production, dye solution is often thickened (suspension dye solution is 10%) or diluted according to the different dilution coefficients of dyes (aqueous solution) before the fabric is dipped in pad dyes, that is, before the initial start-up. However, due to the unstable process parameters, the color difference of dyed fabrics will still be caused.
Padding and priming machine is the fluctuation of technological conditions and parameters of thermosol dyeing machine and color developing soaping machine, which directly affects the color difference of dyed fabrics. If the speed is not stable, it will disturb the dynamic balance of dye solution, resulting in the change of color and light after dyeing, resulting in color difference before and after dyeing. The change and fluctuation of air pressure and oil pressure of uniform rolling mill will result in the discrepancy of the left, middle and right rolling allowances of dyed fabrics, resulting in the left, middle and right color difference of dyed fabrics, and in serious cases, the obvious color difference between the front and back of dyed fabrics. In addition, the infrared and hot air drying chamber temperature, air volume is not uniform, such as the fabric after rolling, rapid drying will cause swimming, resulting in the left, middle and right color difference of the fabric or uneven fabric surface.
The temperature of reduction evaporator, soap evaporator and water washing tank, the instability of technological parameters such as reducing liquid, oxidizing liquid and fixing agent of reactive dyes during vapor-setting, or the large fluctuation beyond the scope, will have a serious impact on the colour light and colour fastness of dyed fabrics. Inadequate washing of vat dyes will affect the normal color of dyes. More than a dozen dyes, such as reduced yellow G, grey BG, grey M, brown BR, blue VB and violet 2R, will change their color when they are dyed in dark, while olive green B needs to strengthen its soaping to achieve normal color. In addition, the soaping time of reactive dyes will also affect the fixation rate.
3, finishing
The influence of finishing process on fabric color is also very obvious. For example, after calendering, the shade and shade of dyed fabrics will change greatly; after coating, the shade and brightness of dyed fabrics will change obviously. In addition, the effect of temperature on the thermal dyes is obvious during the process of finishing. Even with general soft, waterproof and stiff finishing, the color and light of dyed fabrics will be affected to varying degrees. In serious cases, the original color difference of dyed fabrics, even the left, middle and right color difference, will be caused.
Prevention and control of chromatic aberration of dyed fabric
In actual production, on the basis of strengthening management and operation, the following preventive and control measures are summarized through the analysis of various factors causing color difference in fabric dyeing.
1. Prevention and control of original color difference
The color difference of dyed fabrics is mainly caused by three factors: unreasonable selection of dyes for proofing and color matching, improper adjustment of dyeing prescription on production machine, and great fluctuation of process conditions and parameters in dyeing process.
Reasonable choice of dyestuff dyes
When making small samples, the dyes in the prescription should be as few as possible. Because different dyes have different dyeing properties, reducing the types of dyes can reduce the interference between dyes. The color phase of the dyes selected in the prescription should be as close as possible to the original, and the dyes with similar dyeing performance and consistent dyeing curve should be selected as far as possible.
In addition, it should be emphasized that the force and color of the imitated dyes should be as consistent as possible with those used in the workshop. Due to the difference in color and strength between products from different manufacturers and batches of the same dyestuff, attention should be paid to the uniformity of dyestuff usage, especially the dyestuff of combination dyestuff and toner light.Otherwise, the color of the fabric will be affected. Such as reduction ash BG, ash M and so on. Therefore, it is necessary to strengthen control in actual production so as to imitate, set out and dye normally. The dyes selected are the same batch number of the same manufacturer in order to avoid color difference caused by improper selection of dyes.
Reasonable amendment of prescription according to dye characteristics
Attention should be paid to the effect of dye uptake on color and light during dyeing process. If the affinity or dyeing rate of the dyestuffs are quite different, the color-light pattern of the product will appear at the beginning of dyeing, and the color-light will change after a period of time, that is, the so-called phenomenon of gradually weakening the color-light. In addition, attention should be paid to the interference of the Thermochromic Properties of dyes. As mentioned above, generally green dyes are blue when heated and yellow when cooled. Therefore, in the production of modified dyeing prescriptions for imitation, cart sample and normal dyeing, the dye solution should be added or diluted reasonably according to the properties of dye diffusion, affinity and dye uptake rate. At the same time, the thermal denatured dyes should be judged after the dyed fabrics are cooled, and then the working prescription is revised to avoid the influence and interference of the thermal denaturation of dyes.
In addition, the process conditions and parameters must be strictly controlled and standardized in order to effectively control the color difference.
Strengthening the imitation and lofting management of the machine
Because the dyeing conditions in the laboratory are quite different from the actual dyeing conditions in the production workshop, the normal dyeing prescriptions should be revised in the following order: sample prescription revision-rarr; cart sample prescription revision-rarr; normal dyeing prescription revision, and then normal dyeing production can be carried out.
In the process of modifying the dyeing prescription, we should strengthen management, strengthen technology tracking, and grasp the situation of carding lofting. Laboratory samples and normal dyeing production should be in accordance with the prescribed process conditions and parameters, can not change the process parameters to adjust the color; the process conditions and parameters of cart samples should be consistent with normal dyeing production.
2, prevention and control of chromatic aberration before and after
In dyeing and finishing process, the color difference before and after dyeing is mainly affected by ingredients, chemicals, semi-finished product quality, process conditions, fluctuation of parameters and mechanical properties of equipment. Therefore, to prevent and control the color difference before and after dyeing, management and implementation control should be strengthened from the following aspects:
Strictly control the quality of semi-finished products
Good pretreatment effect, even, clean and transparent desizing and bleaching, consistent capillary effect and whiteness, mercerized foot, and stable pH value of 6.5-7 are the fundamental factors for successful dyeing of fabrics. It must be emphasized that the same pretreatment process should be used for dyeing fabrics of the same colour and lustre, and the grey fabrics of the same manufacturer should be selected as far as possible. In addition, when dyeing light and bright colors, it is necessary to choose the same whiteness of the dyed blank, because the whiteness of the dyed blank determines the color of the dyed product.
Standardization of dyeing preparation
Before weighing, check whether weighing apparatus is accurate; weigh materials accurately according to the production process prescription; after weighing, cover and seal the remaining dye barrels to prevent the dyes from absorbing moisture in the air and affecting the net value of dyes. Especially for dyes with toner light, the dyes should be weighed and ground at one time according to the amount needed, and then used equally. In addition, the unification and standardization of chemical operation methods are effective means to prevent and control the color difference of dyed fabrics before and after dyeing.
Strengthening production site and operation management
The influence of moisture content of semi-finished products on fabric dyeing production is quite obvious, and the management of semi-finished products before dyeing must be strengthened. According to the bill of lading and production schedule, the workshop can control the stock of semi-finished products, implement sealing management for semi-finished products, and ensure that the humidity of semi-finished products is consistent or within a certain range, so as to avoid the color difference before and after dyeing. In addition, the fluctuation of the production process and parameters will also cause the color difference before and after dyeing. Therefore, strengthening the management of production site and operation is one of the effective means to solve and prevent the color difference of dyed fabrics before and after dyeing.
3. Prevention and control of chromatic aberration in the left and right sides of dyed cloth.
The main factors causing the left, middle and right color difference in the continuous pad dyeing process of dyed fabrics are: the left, middle and right color difference of semi-finished products, whiteness, size content of fabric surface, pH value of fabric surface, uniform rolling pressure, heat treatment of fabrics in pre-oven, improper baking of reactive dyes, thermal dyeing and supplementary dyeing solution, etc. Therefore, we should take precautions and control measures from the following aspects.
Quality control of semi-finished products
Strengthen the management of pretreatment semi-finished products, especially emphasize the consistency of the effect and quality of semi-finished products, that is, uniform cloth. Otherwise, it will cause the left, middle and right color difference of dyed fabrics, which is difficult to control and solve in dyeing. In addition, the quality of semi finished fabrics requiring special dyeing and finishing should be strictly controlled. For example, in the dyeing and finishing of abrasive products, if the abrasion of semi-finished products is uneven or the left-middle-right capillary effect is inconsistent, the left-middle-right color difference will occur after the fabric is dyed. Therefore, it is the key to provide qualified semi-finished products for dyeing and finishing.
Control of uniform rolling process and parameters
The bottoming machine is one of the key machines in the process of dyeing and finishing of fabrics.It is necessary to maintain the uniform left and right side pressure of the rolling mill. In production, the following tasks should be done well: periodically checking and testing whether the oil pressure and air pressure are normal; periodically checking the accuracy of the instrument; periodically checking the wear condition of the rollers; at the same time, it is required that the staff of the machine platform should not adjust the oil pressure and air pressure of the rolling mill at will or adjust the color of the fabric by adjusting the process parameters of the rolling mill pressure. The above measures should be strictly controlled in order to avoid the color difference between left, middle and right in the process of rolling.
Control of fabric heating
Fabric pre drying or baking and fixing after dyeing and dyeing should make the fabric get the same amount of hot air in the zonal direction. The dyes are evenly dipped and rolled on the fabric under the action of rolling pressure. If the fabric is heated inconsistently, the dyes will migrate during the drying process, resulting in the left, middle and right color difference of the fabric, especially the thin fabric or polyester thin fabric. Generally, the following measures are taken to solve this kind of color difference: periodically check the transmission system of the circulating fan to ensure that the left and right wind pressures of the circulating fan are the same; timely cleaning of the dust-proof mesh plates of the left and right heating plates when changing the type of car brushing to ensure that the dust-proof mesh plates are clean, so as not to affect the air volume of the left and right circulating fan.
In addition, in the process of high temperature thermosol dyeing of polyester-cotton blended fabrics, the difference of temperature in the inner edge of the hot melting box will also cause the difference of color in the middle edge of the fabrics. When the dyeing machine is empty, the hot air nozzle sprays uniformly inward. Whether it is a pre-oven or a hot-melt oven, the middle temperature is always higher than the temperature on both sides. The reason is that the two walls of the box and the two ends of the guide roller have heat dissipation phenomena in the air. When the dyeing machine is running normally, the temperature of the whole box will become low between the high and middle sides because of the wet cloth entering the pre-oven, high heat consumption, fast exchange of hot air on both sides and easy drying of the fabric. The temperature of the drying oven is the opposite, the drying cloth consumes less heat, so the temperature of the whole box is still higher than the temperature of both sides. Some equipments will have left, middle and right color difference due to lack of symmetry or large deviation of feeding.
The difference of temperature between left, middle and right in the thermolysis tank will lead to the discrepancy of fixing rate and sublimation degree of disperse dyes or reactive dyes on the cloth surface. If the fabric contains more impurities or alkali, the yellowing degree of left, middle and right will be different, resulting in color difference of monochrome varieties; if the thermal sensitivity of various dyes is different, the left, middle and right colors of the fabric will also be different. Therefore, only by thoroughly eliminating the temperature difference in the box can we solve the problem of chromatic aberration.
In the process, the dyes with flat thermal dissolution curve can be chosen, because under the same dyeing process conditions, the dyes have different properties, showing different forms of edge-to-center color difference or left-to-right color difference. For example, after disperse red 3B dyeing, light to medium edge color difference often occurs, while when stained with Fulong scarlet S-BWFL, light to medium to dark edge color difference often occurs. This is because disperse red 3B is a low temperature disperse dye and Fulong red S-BWFL series high temperature disperse dyes. Low temperature disperse dyes can fix at lower temperature, while high temperature will cause the sublimation of dyes, affect the fixing of dyes, resulting in color difference between edges and mid-edges. High temperature disperse dyes need a higher temperature to fix. If the temperature is not reached, the dye fixing will be affected. The color is lighter. Therefore, high temperature disperse dyes often have light to dark color difference.
The color difference caused by the inconsistency of baking temperature is obvious in high temperature disperse dyes and low temperature disperse dyes. This is mainly due to the fact that most of the heat-soluble solid-color curves of the former are not very flat and sensitive to temperature.
In production, the width of the product should be lower than the width of the air nozzle, so that there is sufficient space on both sides of the baking room, and there is more room for swing and mixing of heating air in the baking room. In addition, movable valves can be installed on the equipment to adjust the intermediate air intake, or to block the middle of the air nozzle about 60 cm to reduce the intermediate temperature. Through these measures, the left and right color differences of the fabric can be effectively solved.
4. Prevention and control of positive and negative chromatic aberration.
Positive and negative chromatic aberration is more obvious in plain fabric dyeing. The main reason for the positive and negative color difference is the inconsistent heating of the positive and negative sides of the fabric during dyeing and finishing, which is usually caused by the migration of dyes or fixing of dyes in the drying process. Therefore, in order to solve this kind of color difference, the infrared pre-drying device can be installed on the base machine, the distance between the air nozzle in the pre-drying room and the cloth surface can be checked, and the air volume of the fan in the pre-drying room can be controlled.
Chromatic aberration is a common quality problem in fabric dyeing and finishing. It is necessary to take corresponding measures according to different conditions over a long period of time. Because the color difference is produced in the process of production and processing, it is necessary to comprehensively analyze various factors and formulate effective prevention and control measures according to the situation of production site in order to achieve better results.
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