Cotton Sateen Stretch Fabric gray CP4103

Warning: You’re Losing Money by Not Using Color Fastness The Ultimate Guide PART 2 (B)

5.2.5 How to improve the light fastness of textiles? Three reliable methods

The light-fading mechanism of dyes is very complicated. But simply put, it is due to the dyes being excited after absorbing photons, and the occurrence of a series of photochemical reactions to destroy the fundamental dye structure, which ultimately leads to discoloration and fading. The light fastness of textiles mainly depends on the chemical structure of the dye and its aggregation state, combination state and mixed color matching. Therefore, selecting dyes rationally is very important.

  • Choose dyes according to fiber properties and textile applications. For cellulose fiber textiles, it is advisable to select dyes with good oxidation resistance; for protein fibers, it’s best to select dyes with good reduction resistance or containing weak oxidizing additives. And for other fibers, dyes should be selected based on their effect on fading. Enhancing the azo group’s photooxidation resistance in the dye molecular structure can be achieved during the dye synthesis process. Some strong electron-withdrawing groups are usually introduced at the azo group’s ortho position, which would reduce the electron cloud density of the azo nitrogen atom. Also, it is possible to introduce hydroxyl groups at the two ortho positions of the azo group and use its coordination ability to form complex structures with heavy metals, thereby reducing the electron cloud density of the azo group hydrogen atoms, shielding the azo group, and ultimately improving the dye Fastness to light.
  • Dyes should be selected according to the color depth.Many experiments have proved that the light fastness of reactive dyes on cellulose fibers is directly proportional to the depth of the dyed luster, 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 degree of aggregation of dye molecules, the smaller the surface area of the same amount of dye in contact with air, moisture and light, and ultimately, the lower the probability of the dye being oxidized by light. On the contrary, when the colors are lighter, the dyes are mostly in a highly dispersed state on the fiber. The probability of exposure to light is higher, which will eventually reduce the Color Fastness to light. Therefore, for dyeing light-colored varieties, dyes with higher light fastness should be used. Besides, many finishing agents such as softeners and anti-wrinkle finishing agents are added to the fabric, which will also reduce the product’s light fastness. Therefore, dyes that are not sensitive to these finishing agents should be selected.
  • Dyes with good light resistance stability and compatibility should be used for color matching. The fading properties and light fading mechanisms of various dyes are different. Sometimes, the presence of one dye sensitizes the fading of another dye. When matching colors, dyes that do not sensitize each other and can even improve light resistance’s stability should be used. This is especially important when dyeing dark varieties such as black. One of the three primary colors fades too fast, and this will quickly cause the dyed fiber or fabric to change color. The faded dye residue will also affect the light stability of the other two dyes that are not faded. To obtain higher light fastness, it is expedient to apply reasonable control of the dyeing process, fully combining the dye and fiber, and avoid hydrolyzed dyes and unfixed dyes lingering on the fiber as much as possible.

5.2.6 FAQs On light fastness Test

The light fastness tester in textile testing equipment is relatively conventional, but it is also one of the most important Color Fastness test items.

The light fastness test project is not difficult to carry out, but various problems are often encountered in the actual operation process. Here, we analyze common problems in the national standard, ISO and AATCC light fastness test standards for your reference.

Q1. What is the difference between blue wool cloth 1-8 and L2-L9? Can they replace each other?

In GB/T 8427 and ISO 105 B02, blue wool standard samples, 1-8 and L2-L9, are described in detail. They are all wool and have eight levels of blue labels, and each higher numbered blue label target or reference has light fastness that is about one time higher than the previous number. But different dyes and manufacturing processes are used.

Blue labels 1-8 are dyed with eight dyes of different light fastnesses, and this is suitable for European exposure conditions specified in GB/T8427 and ISO105 B02. For L2-L9, the fibrils are dyed with two dyes, and then the two dyed fibers are made into blue label L2-L9 in different proportions. This fits the US exposure conditions specified in GB/T8427 and ISO 105 B02. And it is suitable for AATCC TM 16. Nonetheless, the blue labels 1-8 and L2-L9 cannot be mixed, and the test results cannot be interchanged.

Q2. The light fastness tester has relative humidity in the cabinet. Why do we need to calibrate it with humidity control standards?

At present, most light fastness testers can display the relative humidity in the cabinet. Still, GB/T 8427 and ISO 105 B02 stipulate that the cabinet’s humidity should be calibrated with humidity control standard samples every day. The reason is that the standard sample calibration of the humidity control cloth is not the “relative humidity” in the cabinet, but the “effective humidity.” Effective humidity is also called absolute humidity. It is defined by combining air temperature, sample surface temperature, and relative air humidity that determines the moisture content on the sample surface during exposure. “Effective humidity” directly affects the test results of the light fastness of humidity-sensitive samples. Therefore, the GB and ISO standards stipulate to check the humidity in the cabinet every day.

The humidity control standard is cotton fabric dyed with red azo dye. The method of use is as follows:

  • Put a piece of humidity control cloth (not less than 45×10mm) together with the blue wool standard sample on the hard cardboard and place it in the middle of the sample holder as much as possible.
  • Expose the partially covered humidity control standard sample and the blue wool standard sample at the same time until the color difference between the exposed and unexposed parts of the humidity control standard sample reaches level 4 of the color change sample card.
  • At this time, the blue wool standard sample is used to evaluate the color change of the humidity control cloth and which blue wool standard sample is consistent. The color difference should be the same as that of grade 5 blue wool cloth. If it is inconsistent, you need to re-adjust the controller to maintain the specified blackboard temperature and humidity.

Q3. What is the role of the xenon reference fabric in AATCC TM 16?

Xenon Reference Fabric is a purple polyester fabric. Its function is to determine whether the temperature in the box is correct. The used method is to place the xenon reference fabric on the sample holder for continuous exposure for 20±2h. If the xenon reference fabric’s color change is consistent with the xenon reference standard sample or the color difference is 20±1.7 CIELAB units by measuring the color with the instrument, the proof box body temperature is normal.

Q4. In some product standards, some light fastness requirements require method 3, and the quality requirements are intermediate levels, such as 3-4. How should we test it?

Certain product standards do have such a requirement, and some experts say that this formulation is completely wrong because experimenters cannot choose the blue wool standard. However, because some current product standards are stipulated in this way, it is recommended for operators to use Method 3 for experiments, and then refer to Method 1 for evaluation. For example, if the standard requirement is 3-4, we choose the 4th and 3rd blue wool standard to use the requirement experiment specified in Method 3. When grading, you can refer to the method one grading method because there are 4 and 3 blue wool Standard samples. Theoretically, we can judge whether the test result of the sample reaches 3-4 levels.

Q5. What is the unit of AFU in the American AATCC standard? What is the relationship with the number of hours?

AFU is an energy unit, and it is the acronym for the “AATCC Fading Unit.” It is defined as 1/20 of the exposure energy required to make the L4 blue wool standard fade to level 4 of the color-changing gray card. This means 20 AFU of energy is needed to make the L4 blue wool standard fade to level 4 color change. The AFU and radiant energy values that make L2-L9 blue wool reach grade 4 color change are shown in the table below.

The relationship between AFU and hours can be calculated by a formula, assuming that when the xenon arc lamp is operated under 1.10W/m2•nm conditions, the energy required to make L4 reach level 4 color change is 85 kJ/m2.

85 kJ/m2=1.10 W/m2 x 3.6 x (hours)

Hours = (85 kJ/m2) / (1.10W/m2x3.6) = 21.5

This shows that when the radiant energy of the xenon lamp changes, the number of hours from the day to the specified AFU will also change. Only when the lamp is operated at 1.10 W/m2•nm can the energy of 20 AFU be reached in 21.5 hours.

5.3 Color Fastness to washing

Washing is one of the most common cleaning and maintenance methods for clothes. The Color Fastness to washing determines the color firmness of textiles in different detergents and different washing environments. There are many ways to test Color Fastness to washing. The general principle is to imitate the state of household or commercial washing. Under the specified time and temperature conditions, after stirring, rinsing, and drying, use a gray sample card or instrument to compare the original sample to evaluate the color change of the sample and the lining fabric’s staining. Various methods may have certain differences in temperature, test solution, washing procedures, drying procedures and a decision to add steel balls or not.

5.3.1 Comparison of common textile Color Fastness standards:

Chinese standard: GB/T 3921-2008; GB/T12490-2007

International standards: ISO 105C10:2006; ISO105C06:2010

EU standards: EN ISO 105C10:2007; ENISO 105C06:2010

British Standard: BS EN ISO 105C10:2007; BSEN ISO 105C06:2010

American Standard: AATCC 61-2010

Australian Standard: AS 2001.4.15-2006

German standard: DIN EN ISO 105C10:2007; DINEN ISO 105C06:2010

Japanese Standard: JIS L 0844:2011

Take GB/T 3921-2008 “Textile Color Fastness test Color Fastness to soap” as an example to introduce you.

5.3.2 Test process of Color Fastness to soap and washing:

(1) Sample: Take a 100mm×40mm sample with the front side in contact with a 100mm×40mm multi-fiber lining fabric, stitched along a short side to form a combined sample. Or take a 100mm×40mm sample, sandwich it between two 100mm×40mm single fiber lining fabrics, and stitch along a short side to form a combined sample.

(2) Preparation of test solution: 5 grams of soap per liter of tertiary water is used for tests A and B, and 5 grams of soap and 2 grams of sodium carbonate per liter of tertiary water are used tests C, D and E, respectively.

(3) Test: Put the combined sample and the specified number of steel balls in the container, according to the standard test conditions. Then inject the required amount of soap solution preheated to the test temperature ±2℃, so that the bath ratio is 50:1. Close the container, adjust the temperature and time according to the standard and start the machine. Remember to start timing when the container is closed.

(4) Washing and drying: For all tests, take out the combined samples after washing, wash them twice in tertiary water, and then wash them in running water until they are clean. Squeeze the excess water from the combined sample by hand, flatten the sample between two unused filter papers to remove the excess water, and then hang it to dry in the air of temperature not exceeding 60℃. The sample is only connected at the position of the suture.

(5) Grading: Use the gray sample card or the instrument to compare the original sample to evaluate the sample’s discoloration and the staining of the lining fabric.

(6) Result report

5.3.3 Analysis of the reasons for the unqualified test of Color Fastness to the washing of textile fabrics

  • The fabric structure, fiber composition and corresponding dyeing process conditions are the primary factors that affect its Color Fastness to washing.Generally, the Color Fastness of single-component fabrics is better than that of blended fabrics. All polyester, cotton, nylon and all wool fabrics are less likely to have color problems. The dyes corresponding to the fabric components are selected. After processing by the dyeing factory, they can usually achieve 3-4 grades or more. Cleaning and curing the color and other processes are properly controlled, and level 4 can be easily achieved.

For blended fabrics, the Color Fastness to washing after dyeing is generally lower than that of the corresponding single-component fabrics, especially when the dyeing process is improperly controlled. For example, when a polyester-spandex blended fabric is dyed in dark colors such as black and red, the Color Fastness to washing often fails.

Using disperse dyes (A class of non-soluble, nonionic dyes that depend on dispersing agents to spread color in synthetic fabrics) to dye polyester fibers generally stains spandex seriously. When selecting disperse dyes, it is necessary to consider whether they have good dyeing properties, and at the same time, to ensure that the staining on spandex is relatively easy to remove.

Some dyeing factories in Xiaoshao, in a one-sided pursuit of output, often use high temperatures to quickly shape the finished product. But when the temperature is too high, the dye molecules will easily escape from the polyester molecular structure. Various surfactants on the fiber surface accelerate this process, resulting in the floating color’s appearance on the surface of the finished product. For polyester, nylon, acetate, and spandex dyeing with disperse dyes, the thermal migration phenomenon is an important reason for these fabrics’ unsatisfactory Color Fastness and the heavy staining of acetate lining and nylon lining in six-fiber staining. The thermal migration of disperse dyes makes the dyes that have penetrated into the fiber to migrate to the fiber’s surface and accumulate on the surface of the fiber. The deeper the dyeing depth and the higher the post-setting temperature, the more obvious the above phenomenon is. After dyeing, it is important to reduce cleaning treatment to remove the disperse dyes contaminated by the spandex component and the floating color on the polyester surface. At present, acid reduction cleaning with thiourea dioxide or alkaline reduction cleaning with sodium hydroxide and soda ash can improve the washing fastness of polyester-spandex blended fabrics to above level 4, which should meet the requirements of high-end brand clothing.

In the actual production and life process, nylon, silk and wool blended fabrics often encounter poor washing fastness. Nylon is similar to protein fibers such as silk and wool. It contains a certain amount of amino and carboxyl groups in its molecular structure, so acid dyes are generally used for dyeing. As a water-soluble dye, the acid dye has a small molecular volume, which makes the result of a wet treatment fastness test on fibers dyed with acid dye poor, especially its Color Fastness to washing. The dissolution of the dye is relative to the water solubility of the dye molecule. The sulfonic acid group in the dye binds to the fiber by hydrogen bonding. When the dyed fabric is immersed in water, the hydrogen bond is cut off, and part of the swollen dye will leave the fiber and dissolve in water. When the dyed material is stirred in the washing machine, the pigment of insoluble dye will fall off from the fiber due to the stirring. The higher the washing temperature, the larger the bath ratio, the longer the time, the more intense the stirring, and the more the dye falls off. In addition, the presence of surfactants in the washing process will greatly increase the dissolution of the dye from the fiber. The main reason for dye removal is that the pigment molecules have lipophilicity, which can co-dissolve with the surfactant’s hydrophobic base, thereby pulling the dye away from the fiber. To increase the washing fastness of acid dyes, we must first choose dyes with larger dye matrix and relatively few water-soluble groups to limit the dye’s mechanical movement inside the fiber and try to increase the bond between the dye matrix and the fiber strength. Furthermore, the tannin method and synthetic fixative method can be used, and after dyeing, the fabric is fixed.

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