A Comprehensive Guide To The Causes And Prevention Of Color Differences In Fabrics

Introduction

Color difference refers to the difference between two colors, specifically the difference between three factors: hue, saturation, and brightness. Broadly speaking, any difference between two colors is called color difference. In textile processing, due to improper implementation and quality control of spinning, weaving, and dyeing processes, color differences can exist between different parts of the textile or between individual pieces.

Common Fabric Color Difference Classification

The resulting dyed products have varying shades and colors.

1. Color difference within the same batch

Within the same batch of products, there may be color differences between boxes, pieces, packages, rolls, and cylinders of the same color number.

2. Color difference within the same roll

The most common color difference is the color difference between the left, center, and right sides (including light and dark edges) of the same roll of fabric.

Arranging the fabric according to the color difference can slightly adjust the effect of the color difference, but it cannot completely eliminate the color difference; the color difference between the front and back of a roll of fabric can generally be adjusted within 2 meters in the warp direction when arranging the fabric; color difference between the front and back sides;

3. Inconsistent color

This includes color differences between the sample and the product, and color differences between the sold sample and the product.

Color difference is one of the most common and frequently occurring defects in dyeing and finishing plants, which seriously affects the quality of dyed finished products.

Causes of fabric color difference

1. The dye is initially unevenly distributed on the fabric

If the dye is unevenly distributed across the fabric before fixing, color differences will inevitably occur after fixing. The main reasons for this phenomenon are:

Fabric factors: Due to differences in fiber properties or insufficient uniformity in pretreatment processes such as annealing, scalding, bleaching, and spinning, the permeability of the pre-dyed semi-finished products is uneven, resulting in differences in the degree of dye absorption.

Liquid absorption factors: Due to mechanical structure issues or improper operation, the liquid absorption rate varies across different parts of the fabric, resulting in color differences. Uneven roller pressure and uneven addition of dyes and chemicals can also cause uneven dye absorption by the fabric.

Pre-drying factors: During pre-drying after padding with dye liquor, the inconsistent drying rate and degree cause the dye to migrate to different degrees, resulting in uneven distribution of the dye on the fabric.

2. The degree of dye fixation on the fabric varies

Although the dye is initially evenly distributed on the fabric, if the conditions are not properly controlled during the fixation process (such as temperature, time, dye concentration, etc.), the dye in some parts of the fabric may not be fully fixed and will be removed during the post-treatment soaping, resulting in color difference.

3. The color of the dye changes

This difference is not caused by uneven dye distribution on the fabric, but rather by changes in the color of some dyes on the fabric due to certain reasons. These reasons generally include the following:

Pre-dyeing factors: Uneven whiteness or significant differences in pH value of semi-finished products often result in color differences after dyeing.

Dyeing factors: For example, excessively high melting temperature of disperse dyes can cause the color of some dyes to become dull; excessive reduction of vat dyes can also cause differences in color.

Post-dyeing factors: During the finishing process, such as resin finishing, high-temperature stretching, and different pH values ​​on the fabric, the color of the dye will change to varying degrees.

Soap washing: If soap washing is not sufficient, the color will be insufficient and the color will be inaccurate.

Overcome methods

1. Improve the uniformity of the rolls

For dyeing using ordinary rollers, the roller surface should be ground into an olive-shaped roller with a slightly larger diameter in the middle and slightly smaller diameters at both ends; the hardness of the rubber roller should be selected appropriately.

2. Using uniform rolling mill

When producing a single variety and color product, using olive-shaped rollers is a feasible method to solve color differences between the edge and center or between the left and center. However, when producing small batches of multiple varieties and multiple colors, more factors need to be considered, often requiring frequent roller changes, which brings trouble to the process. Using a uniform rolling mill is a better way to solve color differences between the edge and center, and between the left, center, and right.

3. Equipment

Select appropriate equipment and improve its condition to ensure that the dyed material is evenly colored throughout the dyeing process.

4. Improve the uniformity of fabric drying

When fabric enters a hot air dryer, the speed of the airflow and the temperature at the nozzles have varying effects on the rate of moisture evaporation. Areas with higher airflow and temperatures experience faster moisture evaporation, causing the dye liquor to migrate to these areas and resulting in color differences.

When processing thick fabrics, a temperature difference of less than 10°C has little effect on color difference, while when processing thin fabrics, a temperature difference greater than 5°C will result in obvious color difference.

A wind speed difference of less than 4 m/s between the left, center, and right sides has little effect on thick fabrics. However, a wind speed difference of 2 m/s will result in noticeable color differences in thin fabrics. Therefore, it is essential to carefully manage the wind speed and ensure that the fabric surface temperature and heating rate are consistent to guarantee uniform drying and prevent color variations .

The hot air section uses a horizontal guide roller to feed the fabric, which is advantageous for preventing color differences between the front and back sides because the hot air blowing onto both sides of the fabric comes from the same air duct. If a hot air dryer with vertical fabric feeding is used, the equipment needs to be modified so that the air volume and speed at the upper and lower air inlets can be appropriately adjusted, which is beneficial for correcting color differences.

Drum dryers are contact dryers, meaning the parts in contact with the drying drum heat up and dry quickly, which can also cause migration problems. Therefore, it’s best to start with a slightly lower temperature in the feed drum and gradually introduce the fabric into the high-temperature drying drum. This helps reduce the possibility of color differences.

Using methods such as infrared pre-drying and microwave drying, dyed products can be dried uniformly inside and out. This helps to avoid color differences caused by migration. When drying fabrics with infrared rays, the infrared radiation on both sides must be as uniform as possible, and the distance between the infrared radiator and the fabric should not be too close to prevent excessive drying and the resulting color difference between the front and back sides.

In addition, the drying of semi-finished products before dyeing should be uniform so that the amount of liquid absorbed during dyeing is consistent. Uneven moisture content in the semi-finished products can lead to uneven absorption of dyes and chemicals, resulting in color differences.

5. Appropriate dyes should be selected during dyeing

Dyes with similar dyeing curves should be selected whenever possible to minimize color difference. Dyes used for dyeing should be stored separately according to their origin and batch number. Thorough testing and analysis should be conducted to ensure a clear understanding of the color of each batch of dyes, and they should be used in batches to prevent color variations.

6. Add leveling agent during dyeing

Adding a cotton leveling agent during the dyeing of cotton knitwear helps reduce color difference. When pad-dyeing polyester/cotton fabrics, adding an appropriate anti-migration agent to the dye bath (such as sodium alginate or polyacrylic acid derivatives) is effective in preventing color difference caused by migration.

7. Strengthen the management of raw materials to provide qualified semi-finished products for dyeing

① The type, quality, ratio, and sizing of the greige fabric have a significant impact on dyeing. Greige fabrics from different textile mills often produce inconsistent colors under the same dyeing process conditions. Therefore, the same greige fabric must be used for the same batch of dyed products.

② Different pretreatment equipment and inconsistent process conditions result in variations in the color of the resulting semi-finished products after dyeing. The process must be adjusted according to different varieties and equipment to ensure even and thorough desizing, scouring, bleaching, and mercerizing, guaranteeing that the rinsing and bleaching effects meet the dyeing requirements of the finished product.

③ Fabrics that require pre-dyeing setting should be set evenly and consistently to avoid color differences during dyeing due to uneven setting.

④ The pH value of the fabric after mercerizing is one of the most significant factors affecting color difference in pretreatment. A high pH value after mercerizing will not only cause the fabric to yellow during drying, but will also cause the dye to undergo color changes under high temperature and alkaline conditions, resulting in color difference.

8.Dyeing process

The dyeing process must be reasonable. The process should be correctly formulated according to the characteristics of different dyed products and the properties of dyes and chemicals, and the stability of the process should be ensured during processing to prevent color difference in dyeing.

9.Finishing aids

Finishing auxiliaries have different effects on color when finishing conditions change. Therefore, the selection of finishing auxiliaries should be made carefully, and the finishing process conditions must be strictly controlled and consistent in order to avoid color difference.