Complete knowledge of fabric color fastness

Color fastness refers to the degree to which a garment’s color fades and stains. The mandatory national standard GB18401-2003 “National Basic Safety Technical Specifications for Textile Products” stipulates that the various color fastness indicators of qualified products must be greater than or equal to level 3.

Color fastness ratings are: Level 1, Level 1-2, Level 2, Level 2-3, Level 3, Level 3-4, Level 4, Level 4-5, Level 5. Level 5 is grade 9, grade 1 is poor, grade 5 is good (no fading). Level 3 can only be a basic level, an intermediate level.

Textile color fastness usually includes:

Resistance to soaping, friction, light, bleaching or oxidants (reducing agents), ironing resistance, perspiration resistance and perspiration-to-light color fastness, etc.

Among them, soaping resistance, friction resistance, light resistance, water resistance and perspiration resistance are several color fastness indicators that most buyers pay high attention to in actual production and trade.

Color fastness to soaping

1. Comparison of test methods for color fastness to soaping

Color fastness to soaping is one of the common color fastness assessment items. Color fastness to soaping refers to the degree of fading of colored fabrics after soaping under specified conditions. It includes two evaluation contents: original fading and white cloth staining. Original fading refers to the fading of colored fabrics before and after soaping; white cloth staining refers to the situation where white cloth and colored fabrics are sewn together in a certain way, and after soaping, the white cloth is stained due to the fading of the colored fabrics. The degree of fading or staining should be tested under the specified light source and rated with a standard gray card. The results are divided into 5 levels, with level 5 being good and level 1 being poor.

2. The relationship between dye structure, dyeing and post-treatment technology and soaping color fastness

In daily tests, a considerable number of textiles, including cotton, wool, polyester, nylon and their blended fabrics, as well as elastic fabrics containing spandex, stained nylon and acetate fiber linings at level 3 or below. Although the dyes and printing and dyeing processes used in different fiber materials are different, the problems that arise are quite similar. This is mainly related to the floating color on the cloth surface and the transfer of some colored fiber particles, and is therefore related to the type of dye used, the dyeing process and the post-processing process.

3. The color fastness of reactive dye dyeing products to soaping depends on the unfixed dye

Taking reactive dyes as an example, theoretically speaking, due to the covalent bonding between dyes and fibers, washing with water cannot easily cause dye desorption, fading and bleeding. Therefore, the soaping color fastness of reactive dye dyeing products is determined by The amount of unfixed dye (hydrolyzed dye and a small amount of unreacted dye). If the hydrolyzed dye cannot be completely removed by soaping, the color will continue to fade after subsequent washing. Soaping fastness is also related to the bonding stability of bonded dyes, and bond-broken dyes will also fade when washed. Therefore, the most important factors that affect the color fastness to soaping are the dye structure and properties, followed by dyeing and post-dye treatment processes. Reactive dyes have a high fixation rate or a slow hydrolysis rate, resulting in a small amount of hydrolyzed dye and a small amount of dye that needs to be removed by washing. Unfixed dyes and hydrolyzed dyes have low directness, good water solubility, are not easy to stain, and are easy to wash off. However, if the dye concentration is high and the amount of residual dye is large, it is difficult to wash off.

4. Soaping fastness is also closely related to the dyeing process

In addition, soaping fastness is also closely related to the dyeing process. The dye adsorption and diffusion are sufficient, the color fixation rate is high, the residual dye and hydrolyzed dye are less, and it is easy to wash off. The dyeing process is reasonable, the covalent bond between the dye and the fiber is not easy to break during dyeing and post-processing, and the color fastness to soaping is good.

Color fastness to rubbing

1. Comparison of test methods for color fastness to rubbing

The rubbing color fastness test refers to a test in which a colored sample is rubbed with a dry rubbing cloth and a wet rubbing cloth respectively, and then the staining degree of the rubbing cloth is evaluated. The test results are divided into 5 levels, with 5 being good and 1 being poor. Although the test process is simple, it is the basic color fastness assessment index of textile products. It is almost one of the items that buyers from all over the world must assess when placing orders. The technical conditions of rubbing fastness testing standards in various countries are very similar, but there are also some differences.

2. Main factors affecting color fastness to rubbing and their control measures

During the friction process between textiles and other objects, the color shedding or the degree of staining of the rubbed objects is affected by many factors.

There are two ways for color to fall off and stain:

First, the dye on the textile falls off or fades, and is stained on the surface of the friction object;

Second, the dyed fibers fall off and adhere to the surface of the friction object.

3. In practice, dye shedding is the main cause of staining

Although there are certain differences in the covalent bond strength and adhesion between reactive dyes with different chemical structures and cellulose fibers, their effects on the wet rubbing fastness of dyed fabrics are basically the same. When dyed fabrics are wet rubbed, the covalent bonds formed between the dye and the fiber will not break.��Produces floating color. The transferred dye usually does not form a covalent bond with the fiber, but only relies on van der Waals forces to produce adsorption, that is, floating color.

Color fastness to light

1. Comparison of test methods for light fastness

The light fastness test refers to placing a textile sample and a set of blue wool standard samples under artificial light sources for exposure according to specified conditions, and then comparing the discoloration of the two to evaluate the color fastness.

2. Ways to improve light fastness

The photofading mechanism of dyes is very complex, but the main reason is that the dye is excited after absorbing photons, and a series of photochemical reactions occur to destroy the structure, resulting in discoloration and fading. The light fastness of textiles mainly depends on the chemical structure of the dye, as well as its aggregation state, combination state and mixed color matching. Therefore, the rational selection of dyes is very important.

3. Select dyes according to fiber properties and textile uses

For cellulose fiber textiles, dyes with better oxidation resistance should be selected; for protein fibers, dyes with better resistance to reduction or containing weak oxidizing additives should be used; for other fibers, dyes should be selected based on their impact on fading. In order to enhance the photooxidation stability of the azo group in the dye molecular structure, during the dye synthesis process, some strong electron-withdrawing groups are usually introduced at the ortho-position of the azo group, thereby reducing the electron cloud density of the azo gas atom. In addition, hydroxyl groups can also be introduced into the two ortho-positions of the azo group, and their coordination ability can be used to complex with heavy metals, thereby reducing the electron cloud density of the hydrogen atoms of the azo group and shielding the azo group, ultimately increasing the Color fastness of dye to light.

4. The dye should be selected according to the color depth

A large number of tests have proven that the light fastness of reactive dyes on cellulose fibers is directly proportional to the depth of the dye, that is, the darker the color, the better the light fastness. This is because the higher the concentration of the dye on the fiber, the greater the aggregation of the dye molecules, the smaller the surface area of the same amount of dye exposed to air, moisture and light, and the lower the chance of the dye being photo-oxidized. On the contrary, the lighter the color, the dye is mostly highly dispersed on the fiber, and the probability of being exposed to light is higher, resulting in a significant decrease in light fastness. Therefore, when dyeing light-colored varieties, dyes with higher light fastness should be used. In addition, many finishing agents such as softeners and anti-wrinkle finishing agents are added to the fabric, which will also reduce the light fastness of the product. Therefore, dyes that are not sensitive to these finishing agents should be selected.

5. Color matching dyes with good light stability and compatibility should be selected

Different dyes have different fading properties and even different photofading mechanisms. Sometimes the presence of one dye can sensitize the fading of another dye. When color matching, you should choose dyes that will not sensitize each other and can even improve light stability. This is especially important when dyeing dark colors such as black. If one of the three primary colors of dye fades too quickly, it will quickly cause the dyed fiber or fabric to discolor, and the faded dye residue will also affect the light stability of the other two dyes that have not faded. Reasonable control of the dyeing process to fully combine the dye with the fiber and try to avoid hydrolyzed dyes and unfixed dyes remaining on the fiber is an important way to obtain higher light fastness.

Water/sweat color fastness

1. Comparison of test methods for color fastness to water and perspiration

The color fastness to water reflects the discoloration of the textile itself and the staining of the adjacent fabric under the combined action of a certain amount of moisture, pressure and temperature. During the test, after the sample and the backing fabric are fully soaked, they are placed in an oven at a certain temperature for a certain period of time under a certain pressure, then taken out to dry, and the discoloration and staining levels are evaluated. The discoloration and staining grades are divided into 5 levels, with level 5 being good and level 1 being poor.

The color fastness to perspiration reflects the discoloration of the textile itself and the staining of the adjacent fabric under the combined action of pressure and temperature in different test solutions containing histidine. Use a gray card to evaluate the discoloration of the sample and the staining of the backing fabric. The results are divided into 5 levels, with level 5 being good and level 1 being poor.

2. Ways to improve color fastness to water and perspiration

The main way to improve the color fastness of fabrics to water and sweat resistance is to rationally select dyes, especially dyes with high color fixation rate and good stability. Properly formulate and control the dyeing process, strengthen the fixation conditions, and form highly stable covalent bonds, which can fully fix the dye. For example, when dyeing less reactive dyes, a catalyst can be used, an appropriate fixing agent can be used, or the color can be fixed at a higher temperature.

(Source: Fabric 114)