With today’s increasing requirements for energy conservation and emission reduction, and the gradual increase in water costs, cellulose fiber fabrics such as cotton are more likely to produce semi-finished gray fabrics before dyeing and finishing. The phenomenon of alkali contamination on the cloth surface even occurs from time to time in dyed products with a PH value above 8, and some even exceed 9. Among them, the continuous long car pad dyeing production line occurs more frequently than the intermittent dip dyeing method. .
The phenomenon of alkali in semi-finished gray fabrics after dyeing and before post-processing will cause greater defects in many types of additives in post-processing and post-finishing. The effect is sometimes far-reaching (for example, for some textile products dyed with reactive dyes that are not resistant to alkali hydrolysis, discoloration problems such as wind marks will still occur after a few months due to alkali on the cloth surface).
Because defects are often revealed during post-processing and post-cleaning, this makes it difficult to find the cause of the problem and can easily lead to misleading and misjudgment. , and focus blindly on post-processing, finishing aids, etc.
1. Impact on some post-processing additives
Post-processing generally refers to After the dyeing process is completed, washing, soaping, color fixing (or cross-linking agent treatment) and other processes are included. For dyeing with reactive dyes, only after the floating color has been fully removed by soaping can the true and stable dyeing color be revealed. Fixing is done to improve intrinsic quality indicators such as color fastness. Although some fixing agents will cause a certain shift in hue and light, this should be controlled in advance during lofting work. Increasing color fastness is The first purpose of color fixation.
However, most of the commonly used important color fixing agents and cross-linking agents are weakly acidic with a suitable working bath pH value of 5 to 6.5. In an alkaline medium, the higher the pH value, the worse the effect. After reaching a certain alkalinity, precipitates may even precipitate.
Poor working bath conditions will not achieve the desired effect, so that indicators such as color fastness cannot be satisfactorily improved. At this time, even if the dosage of color-fixing agent or cross-linking agent is increased, the improvement in fastness is not synchronized, so the significance of increasing the dosage of additives is lost.
2. The impact of some auxiliaries on finishing
Finishing is dyeing and finishing The last link among the three major links of the project is of great significance to the expansion of textile functions, extension of functions, and improvement of quality.
Some important additives in finishing, such as silicone softeners, non-iron resin finishing agents, etc., are also related to color fixing agents and cross-linking agents. In similar situations, it is more appropriate to process it in a weakly acidic working bath. The difference is that the taboo of silicone softeners is not that the alkali agent can directly destroy the silicone oil that makes the fiber soft, but that it can have a demulsifying effect on the emulsified softener. Pure silicone oil cannot be used directly on textiles, so it is necessary to use emulsification method to make an emulsion (or microemulsion) with a silicone oil content of generally 10 to 15%, in order to become a truly usable commercial softener.
Alkali agents can demulsify emulsified softeners, causing the emulsifier to fail and silicone oil to precipitate. As time goes by, the silicone oil separates and precipitates. Dots, after repeated re-aggregation, form oil droplets with larger diameters that are visible to the naked eye. In severe cases, oil drifting may even occur. When these separated silicone oils are stained on fabrics, they will form oil spots and blemishes of varying sizes. What’s more serious is that silicone oil is firmly bonded to fibers, making it very difficult to remove and repair such defects. Therefore, only prevention is better than cure, which has positive practical significance.
In addition, the non-iron finishing resin, which plays an important role in post-finishing, often requires acidic catalytic conditions when used, so the PH value of the rolling tank working bath must be ensured. Within the prescribed scope, it is quite critical.
Various post-finishing projects basically use the continuous padding method of padding. Its advantages are high efficiency, fast speed, and almost close to for 100% raw material utilization. If the semi-finished gray cloth contains alkali, it will have a cumulative impact on the working bath in the rolling tank, causing the pH value of the working fluid to gradually rise. Therefore, there are often no problems when driving at the beginning. After 2000 to 3000m, problems appear gradually and become more and more serious. For those who lack on-site experience, it is easy to create misunderstandings and misjudgments. Starting from the superficial phenomenon and looking for the cause solely from the additives will not only leave the problem unresolved, but also end up in confusion.
Most of the various finishing projects have excellent fastness and durability, but there are few supporting chemicals that can be effectively used for stripping. , which makes it difficult to effectively repair defects once they occur, so the consequences are more serious than post-processing problems.
3. Predisposing factors that cause alkali on the cloth surface
4.1 Cellulose fiber in During pre-treatment and dyeing, basicallyAll are carried out in a working bath with a higher pH value. Due to the action of alkali agents such as caustic soda and soda ash, they have great directness (affinity) to cellulose fibers. In addition, cellulose fibers have extremely excellent water absorption properties. Therefore, the alkali agents that have fully penetrated and adsorbed on the fibers must be used. When cleaning, a large amount of clean water is needed for washing. In addition to a large amount of cold water, hot water is also needed, and sometimes acid and alkali neutralization is required for coordination. Since the water consumption, energy consumption and removal efficiency of the alkali removal process are basically proportional, pre-treatment and dyeing are also the main links that consume water and energy. This is determined by the performance characteristics of cotton and other natural cellulose fibers and alkali agents, as well as their relationship with each other. So far, fabric dyeing and finishing processing is inseparable from alkali agents.
4.2 The working mode of mechanical equipment also directly determines the water washing efficiency. Different water washing methods, such as immersion type, spray type, backwash overflow type , oscillating (ultrasonic), etc., all have their own adaptability to different varieties, and the washing efficiency effects of different equipment types are different. In production, water washing equipment is required to save water and steam, and to have good cleaning effects. However, there is a certain contradiction between the two. For fiber cleaning and alkali removal, ensuring product quality is the basic principle. Otherwise, if repairs are made after defects occur, the gain will outweigh the losses and result in greater consumption and waste of water, energy, and other raw materials. To ensure first-time success is one of the fundamental measures to save water and energy.
4.3 Saving water and energy in dyeing and finishing is a goal with almost no end point. First of all, the design of mechanical equipment must be considered. The starting point, purpose and goal of its design, as well as the working principle of equipment operation, should meet the requirements of the dyeing and finishing process as much as possible. As for the cleaning efficiency and water-saving and energy-saving effect, the conclusions can only be reliable after large-scale production use. The process plan for dyeing and finishing applications must also develop appropriate targeted processes based on the performance characteristics of the equipment. Therefore, in addition to understanding the properties and performance of relevant chemical raw materials, dyeing and finishing personnel also need to understand the equipment. Reasonable process plans and processes can not only ensure washing efficiency, but also make it possible to reduce the number of washes and process time.
4.4 The pH value of the cloth surface of the dyed semi-finished products (including final dyed products) after dyeing and before post-processing can be found through testing, even in the same production line It also shows ups and downs. Generally speaking, defects in mechanical equipment and process design will occur in batches once problems occur, while problems in operation and on-site management will change dynamically. Therefore, this is also a concern that cannot be missed in management. After long-term observation, it can also be found that the problems in water washing are similar to those in the pre-treatment process. They have a certain seasonal periodicity. In winter when the weather temperature is low, fluctuations are particularly frequent.
The water temperature of water washing has a significant impact on the dissolution of chemical raw materials (including additives) such as alkali agents, so hot water washing is indispensable in the cleaning work. You can see some liquid additives. Many varieties are in the form of thin solutions that flow easily in summer and can be easily transformed into uniform working fluids. However, the same additives will become thick and highly viscous in winter. It becomes more difficult to mix materials. Correspondingly, when it comes to the cleaning process, the cleaning efficiency of different water temperatures will not be the same.
Even in summer, when soda ash is hydrated at room temperature, it is easy to absorb crystal water molecules, and form a solid old alkali that is insoluble in low-temperature water and precipitates out. , when the material is heated with hot water of 50 to 60°C or above, it will be fully dissolved and no old alkali will be generated. In winter, the temperature of cold water is very low. If the water temperature and frequency of washing cannot be distinguished, the cleaning efficiency will naturally decrease accordingly. The permeability of hot water to cellulose fibers is much better than that of cold water, so there is a saying that “the cheapest penetrant is hot water”. Since hot water can improve solubility and promote permeability, it further improves and strengthens the basis for cleaning with alkaline agents. Therefore, we can see that similar pre-treatment defects are often related to poor chemical materials and poor post-cleaning, and are often more likely to occur in winter.
Therefore, if the front-line vehicle operators do not have enough understanding of the water washing problem, they think that water washing has nothing to do with the overall situation and treat it casually, or even think that it has little impact on the color, in the If process parameters such as water temperature, water volume, number of washes, and wash time cannot be paid enough attention, the negative consequences will be difficult to avoid.
4.5 Due to the long-term treatment of fabrics in the working bath in the two major stages of pre-treatment and dyeing, the penetration and absorption of alkali agents on the fibers have become extremely difficult. It is clear that in the continuous pad-dyeing production line where the gray fabric is subject to great tension, there is still a problem that surface cleaning is easy to occur, especially for some high-density and heavy gray fabrics. In this regard, in addition to strengthening cleaning, neutralization treatment is also a useful supplement.
It should also be noted that in addition to the variability of gray cloth and fiber types, changes in water quality, failures in equipment and supporting facilities are often possible The objective phenomenon, coupled with the possibility of subjective interference such as process execution, causes the alkali of semi-finished gray fabrics to be dynamic, and may often vary in levels and irregularities. For this reason, the regular detection and correction of the pH value of the rolling tank working bath should be set as a process procedure in on-site management.
IV. Conclusion
pH value is one of the key conditions for chemical reactions, so it is also listed as one of the four major process conditions in dyeing and finishing production. The alkali of dyed semi-finished products is important for post-processing and finishing. Although the mechanisms of the negative effects of many additives are different, they all have the same point of causing sequelae.
On the surface, the cause of alkali on semi-finished gray fabrics is related to washing, but different washing machines, process measures and washing efficiency effects are inextricably linked to it. This problem It may seem like a small matter, but the negative impact is far-reaching. Many quality problems in dyed products are related to this, so it must be given full attention.
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