Dyeing Color Depth not Reaching Target: Add Additives, Dyes, or Extend Temperature Holding Time?

In actual production, we often encounter insufficient color depth. What should we do? First, we need to understand some basic principles and fundamental theories of deep color effects.

01 Dye Coloration Theory

This theory states that the selective absorption of different light by substances results in various colors. The color of a substance is the complement of the light waves it absorbs. The same applies to dyes; their color is the complement of the light waves they absorb, reflecting the absorption characteristics of light in human visual perception.

In the molecular structure of dyes, some chromophores absorb light in the 380-780 nm wavelength range. Additionally, some auxochromes enhance the effect of chromophores, such as -NH2, -NR2, etc.

The synergistic action of chromophores and auxochromes causes selective absorption of light waves by the dye. When certain groups in the structure favor dye coloration, the dye's light absorption shifts towards longer wavelengths. Coloration theory refers to this effect of increasing absorption wavelength as the bathochromic effect.

This theory is particularly helpful in researching and developing dark-colored dyes. For instance, achieving deep colors on nylon and acetate fabrics is challenging, typically limited to light and medium shades.

In traditional processes using acid and dispersed dyes, attempts to achieve deep, rich colors mainly focus on increasing dye dosage, with limited effectiveness and significant dye waste. Foreign reports indicate that triazole disperses dyes for nylon fiber dyeing significantly deepens the color and provides high wash and light fastness. The primary method involves substituting the hydrogen atom on the triazole ring with a trimethyl group (CH3S-), shifting the dye's light absorption towards longer wavelengths, thus producing a bathochromic effect and achieving deeper coloration without increasing dye usage.

1. Surface Coloration and Reflection Theory

This is a commonly used method for evaluating the color depth of dyed fabrics. Generally, the K/S value (Kubelka-Munk function value) is calculated at the maximum absorption wavelength (λmax) in the reflectance or transmission spectrum of the colored object, representing the relative depth of color. Guided by this theory, post-dyeing finishing techniques are used to alter the microscopic surface structure of the dyed fabric, inducing diffuse reflection of light waves to achieve a deep color effect.

2. Dye Uptake and Diffusion Theory

Dyeing theory suggests that the key to achieving deep color effects is improving the fabric's dye uptake rate. The uptake rate refers to the ratio of dye absorbed by the fabric to the amount of dye introduced into the dye bath. As this evaluation method for achieving deep color effects is clear and produces evident results, it is always one of the primary considerations in the dyeing process. Much of the research on deep colors and deep shade dyeing focuses on improving fabric dye uptake, providing practical guidance for production.

02 Conventional Dyeing Methods for Dark Fabrics

For easily dyeable fibers like cotton, with appropriate process conditions and correct dye selection, dyeing medium to dark colors generally poses no technical issues. However, for polyester and nylon fabrics with higher requirements, achieving deep colors is typically more challenging, such as with extra-black polyester, which requires special dyeing methods. The traditional approaches for dyeing dark fabrics are summarized as follows:

1. Increasing Dyeing Temperature

Raising the dyeing temperature can cause the fiber structure to swell and accelerate dye molecule movement, increasing the opportunity for dye diffusion into the fiber. Thus, when dyeing dark colors, we often try to increase dyeing temperature to improve dye uptake. However, unilaterally increasing temperature may affect fabric strength and potentially cause issues such as color changes or hydrolysis of certain dyes at high temperatures, color specks in synthetic fibers, and decreased dye uptake for some dyes as temperature rises (desorption phenomenon). Higher dyeing temperatures also place greater demands on dyeing equipment. Therefore, raising the temperature to improve dye uptake is not always scientifically sound.

2. Increasing Dye Dosage

Some factories achieve dark colors by increasing dye dosage. For instance, after sample checking, if the color is not deep enough, dye workers may add more dye. However, the final result is often unsatisfactory. Conversely, the large amount of dye used increases the difficulty of wastewater treatment, and even if a deep color is achieved, the dyed fabric's colorfastness may be poor due to increased surface dyeing. This is why some dark-colored garments in the market fade quickly when washed, especially common in dyeing factories with weak technical capabilities.

3. Adding Electrolytes as Dyeing Promoters

In reactive and direct dyeing processes, electrolytes like NaCl and Na2SO4 are often added to promote dyeing. In acid dyeing, glacial acetic acid (HAC) or H2SO4 is used. These methods can improve dye uptake rate and percentage to some extent.

However, in dark-color dyeing, due to the relatively large amount of dye used, the amount of dyeing promoters added is also typically high. Excessive addition of electrolytes not only reduces the brightness of the dyed fabric but may also cause dye aggregation and other quality issues. Therefore, even with large dye dosages, there is still a certain ratio for the use of dyeing promoters in actual dyeing processes.

03 Other Major Approaches for Achieving Deep Color Effects

1. Starting from Pre-treatment of Fabrics to be Dyed

Effectively improving dye uptake is one means of achieving deep color effects, and the improvement of dye uptake is closely related to the fiber's affinity for dyes, fabric structure, surface conditions, etc.

1.1 Enhancing impurity and contamination removal treatment of the material to be dyed

Thorough pre-treatment improves fabric wettability and capillary effect, potentially increasing the diffusion ability of dye molecules into the material, thereby improving dye uptake. Therefore, enhancing impurity and contamination removal can achieve this goal.

1.2 Mercerization of cotton fabrics

Mercerized cotton fabrics show significantly improved dye adsorption capacity, achieving deeper medium to dark colors compared to unmercerized cotton fabrics, along with improved surface luster.

1.3 Alkaline weight reduction treatment of polyester fabrics

Polyester macromolecules have high crystallinity, tight fiber structure, high refractive index, surface reflectance, and smooth fiber surface, mainly reflecting light specularly. When dyed black with dispersed dyes, a large amount of reflected light enters the human eye as white light from the fabric surface, making it difficult to achieve deep black. After alkaline weight reduction treatment, the originally smooth polyester surface becomes rough, forming an irregular reflection layer with peaks and valleys, known as the "etching" effect, producing a certain deep color effect. Additionally, the fabric's freedom of movement increases after weight reduction treatment and dye affinity improves.

1.4 Application of low-temperature plasma technology

Plasma technology is a modern dyeing technique involving dry physical processing of fabric surfaces. For example, polyester fibers are treated with plasma and then graft polymerized to form a low refractive index film on the fiber surface, significantly improving dye uptake and color depth. Similarly, for hard-to-dye linen fabrics, plasma treatment increases the capillary effect by 1-1.5 times compared to untreated fabrics, greatly increasing dye adsorption and facilitating deep, rich colors with noticeable deep color effects.

1.5 Ultrasonic technology treatment

Ultrasonic waves are vibration waves inaudible to human ears, with normal frequencies ranging from 2×104 to 2×109 Hz. Frequencies above 109 Hz are called hypersonic or microwave ultrasonic waves. After ultrasonic treatment before dyeing, fiber concentration loosens to some extent, increasing the specific surface area inside the fibers, thereby increasing fiber dye adsorption and improving fabric dye uptake, enabling medium to deep color effects. Research shows this technique is particularly effective for wool fibers with scale layers and tightly structured linen fabrics.

2. Starting from Fiber Modification

Fiber grafting and modification is one of the most researched topics among dyeing professionals. Some fibers with low dye uptake that can only be dyed in light colors show significantly improved dye uptake after grafting or modification treatments, enabling medium to deep color shades.

2.1 Modification of cellulose fibers

Cellulose fiber modification includes physical and chemical treatments, with chemical modification being the primary method. The modification process can be carried out before or after dyeing. Modified cellulose fibers can greatly increase their adsorption capacity for anionic dyes such as reactive and direct dyes. Modification is generally achieved through amination or quaternary ammoniation treatments, introducing -NH2 or quaternary ammonium groups into cellulose macromolecules that have an affinity for dyes with anionic groups. Additionally, the introduction of these active groups can improve the fixation rate of reactive dyes.

2.2 Cationic modification of ramie fabrics

Cationically modified ramie fabrics change the dyeing mechanism of dyes on ramie fibers, transitioning from reliance on van der Waals forces and hydrogen bonding to primarily electrostatic attraction, greatly improving dyeing affinity. Some reactive groups in reactive dye molecules can also form covalent bonds with hydroxyl groups on linen fibers. The emergence of multiple bonding forms increases dye uptake and fixation rates. Research shows that ramie fibers modified with quaternary ammonium groups, under the same dye dosage, increase surface color depth (K/S value) by 50% to 468%, generally 100% to 200%. Therefore, when dyeing to the same depth, cationically modified fibers can save 30% to 90% of dye usage, making this a promising modification dyeing technology.

3. Starting from Dyeing Auxiliaries

3.1 Using rare earth elements in dyeing to improve fabric dye uptake

Rare earth elements refer to 15 elements with atomic numbers 57-71, plus scandium (Sc) and yttrium (Y), totaling 17 elements. In dyeing, rare earth oxides or mixtures are generally used as dyeing auxiliaries, which have been widely studied by dyeing professionals in recent years with corresponding achievements.

Linen fabrics are difficult to dye, but after treatment with rare earth chlorides, the amorphous regions of the fibers increase, the fabric feels fluffy, and rare earth elements can interact with reactive dye molecules to form colored complexes, deepening the dye solution. Additionally, rare earth elements interact with linen fibers to increase active groups, acting as bridges in the dyeing process, improving the uptake of reactive dyes and achieving deep color effects.

Experiments have shown that with a disperse dye concentration of 3%, rare earth usage of 0.44, pH around 6, dyeing temperature of 130°C, and holding time of 30 minutes, some disperse dyes can increase the depth grade by 0.5 to 2 levels on polyester knits.

Furthermore, with the increasing use of plant dyes today, improving the colorfastness and dye uptake of plant dyes is also a consideration.

3.2 Application of specialized deepening agents

To enhance color-deepening effects, specialized deepening agents are currently available in the market. If these agents are used simultaneously with dyes, they may generally cause dyeing defects. They are more commonly used in post-treatment baking processes. Their deepening principle is mainly to change the surface reflection intensity of the treated fabric, and they are mostly applied to synthetic fibers. For example, after dyeing polyester extra-black, deepening agents (added resins) can be used for treatment. Due to their high adsorption on polyester fabrics, they can form deposits on the fiber surface, creating an uneven diffuse reflection effect after baking, improving blackness.

3.3 Low refractive index treatment of dyed fabrics

Organosilicon and polyurethane resins are commonly used. These compounds have a lower refractive index than polyester, greatly reducing the surface refractive index of polyester fabrics, and resulting in more saturated colors. The dyeing and finishing process involves padding the dyed fabric with resin compounds, and then baking to form a low refractive index reflection layer on the surface. Measurements show that the fabric depth index can increase by about 30% to 40%. The deepening principle of specialized color-deepening auxiliaries in the market is the same.

Additionally, considering the tight molecular structure of polyester fibers, breaking away from traditional high-temperature, high-pressure dyeing methods or toxic carrier dyeing methods, safe polyester dyeing auxiliaries similar to rare earth elements, such as high-efficiency dyeing auxiliary P, can be used. As the auxiliary molecules are smaller than dye molecules, they quickly penetrate polyester fibers in the dye bath, causing fiber swelling, which has a certain promoting effect on dyeing deep-colored fabrics.

04 Conclusion

In addition to the above methods for achieving deep color effects in fabrics, dye selection and improvement of dye properties are also very important approaches. Improving the reactivity and fixation rate of reactive dyes, developing medium to deep color dyes with excellent dye uptake properties, etc., are all issues we should research.

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