Textile fabric color fastness testing
The color fastness of fabrics (referred to as dye fastness) refers to the color fastness of colored or printed fabrics to external factors (squeezing, friction, washing, rain, etc.) during use or finishing. The degree of fading under the action of rain, exposure, exposure to sunlight, seawater immersion, saliva immersion, water stains, sweat stains, etc.) is an important indicator of fabrics. If the dyeing fastness is good, the fabric will not easily fade during finishing or use; if the dyeing fastness is poor, fading, slight discoloration, or staining will occur, causing a lot of trouble.
The dyeing fastness of fabrics usually includes:
Soaping resistance, friction resistance, light resistance, bleaching resistance or resistance to oxidants (reducing agents), ironing resistance, sweat resistance and sweat resistance, dyeing fastness, etc. Among them, soaping resistance, friction resistance, light resistance, water resistance and perspiration resistance are several dyeing fastness indicators that most buyers pay high attention to in actual production and trade.
1. Frequently asked questions about dyeing fastness of fabrics
In daily inspections and consumer complaints, common fabric dyeing fastness problems include the following aspects:
1) Sunlight fastness is unqualified. During the wearing process of the jacket and work clothes, the color of the parts that are exposed to more sunlight will become lighter or discolored (usually the back and shoulders), while the color of the parts that are not or less exposed to sunlight will remain the same or change slightly, resulting in the original color being the same. The product has different colors and can no longer be used.
2) The dye fastness to washing, soap washing and dry cleaning are unqualified. High-end silk work clothes, wool work clothes, and pure cotton work clothes are prone to this problem.
3) The friction and dyeing fastness is unqualified. During the use of fabrics, because different parts of the product receive different degrees of friction, the degree of discoloration will be different. For example, the elbows of tops, sleeves, collars and armpits are easy to fade. In addition, the buttocks and knees of pants are also prone to fading.
4) The dyeing fastness of sweat stains is unqualified. It is mainly caused by the discoloration of summer clothes or underwear when they are soaked in sweat.
2. Factors affecting dyeing fastness and improvement methods
Products with poor dyeing fastness will fade during wearing, which will affect other work clothes worn on the body, or contaminate other clothes when washing with other clothes, affecting the appearance and wearing performance; on the other hand, , whether the dyeing fastness is good or bad is directly related to the health and safety of the human body. The dye molecules and heavy metal ions on products with poor dyeing fastness may be absorbed by the body through the skin and harm the skin and even harm the health of the body.
The factors that affect the dyeing fastness of fabrics are divided into internal factors and external factors. Internal factors refer to the firmness of the combination of dye and fiber, and external factors refer to the external forces exerted on the product by external factors or the environmental conditions provided during the use of the product. External factors cannot be controlled, so producers must work hard to improve the dye fastness of the product itself. Here are some solutions based on personal practical experience for everyone to learn from.
2.1 Selection of dyes
The dyeing fastness of a product is particularly important in the selection of dyestuffs. If the dyeing materials are not properly selected, no matter how good the additives or the coloring process are, there is no way to achieve high-quality dye fastness. Only when you choose the right dye can you talk about the next step.
2.1.1 Select dyes based on fiber characteristics
Different types of dyes and fibers have different combination forms, and the strength of the bond is also different. After the dye type is determined, choose a dye with high coloring performance. For example, when dyeing wool fabrics, the same strong acid dyes are used, but the domestically produced strong acid dyes are not as good as the imported strong acid dyes. Not only is the color not good, but the bonding strength is not as good as the latter; or the domestic strong acid dyes are also used. , different dyes have different bonding fastness to wool and different color brightness. For example, the dyeing fastness of wool yarn dyed with weak acid dyes is higher than that of strong acid dyes; for pure cotton fabrics or regenerated cellulose fiber fabrics, either direct dyes or reactive dyes can be used; in addition to silk fabrics, weak acid dyes can be used. In addition to acid dyes and some reactive dyes, individual direct dyes can also be used.
2.1.2 Select dyes according to color depth
After determining the major categories of dyes, it is necessary to further determine which dye to use based on the color system and depth of the dyed color. , select the dye whose color finish is close to the desired color. If there is a deviation, use other dyes to adjust the color; secondly, look at the dye fastness index of the selected dye itself. The dye itself has poor dyeing fastness, and the dyeing fastness can be improved by half-level processing. Thirdly, it depends on whether the saturation of the dye can reach the required color depth. If you choose a dye with a very low dye pick-up rate, it can be temporarily achieved even after finishing.
Different dyes have different fading properties and even different fading 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 the durability and stability. This is especially important when dyeing dark colors such as black.
If one of the three primary colors fades too quickly, it will quickly cause the colored fiber or fabric to discolor, and the faded dye residue will also affect the integrity of the other two dyes that have not faded. . Reasonable control of the coloring process to fully combine the dye with the fiber and completely avoid hydrolyzed dyes and unfixed dyes remaining on the fiber is an important way to obtain higher dyeing fastness.
Water/sweat fastness
1. Comparison of testing methods for water- and perspiration-resistant dyeing fastness
Water-resistant dyeing fastness reflects the discoloration of the fabric itself and the staining of the adjacent fabric under the combined action of certain 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 progression are evaluated. The discoloration and staining grades are divided into 5 levels, with level 5 being good and level 1 being poor.
The dyeing fastness to perspiration reflects the discoloration of the fabric 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 water- and perspiration-resistant dyeing fastness
The main way to improve the water-resistant and sweat-resistant dyeing fastness of fabrics is to rationally select dyes, especially dyes with high fixation rate and good stability. Properly formulate and control the coloring process, strengthen color fixation conditions, and form highly stable covalent bonds, which can fully fix the dye. For example, when coloring 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.
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