Testing For Color Fastness Of Textile
5. Detailed explanation of various Color Fastness tests
5.1 Color Fastness to rubbing test
Color Fastness to rubbing is a type of textile Color Fastness inspection, and it is generally one of the most common inspection types in the textile trade. It refers to the ability of the color of textiles to resist friction, and that is both dry friction and wet friction.
5.1.1 Test template of Color Fastness to rubbing
The model generally followed to test for Color Fastness to rubbing textiles to fix the specified size textile sample on a friction tester platform with a clamping device. Then, rub it with a dry friction cloth and a wet friction cloth, respectively. In the end, the degree of staining of white cloth is used as the evaluation basis, and it is graded against a set of standard Color Fastness to staining gray scales.
The gray sample card used to determine the fastness rating is divided into five grades; the higher the grade, the better the rubbing fastness. A fabric with poor rubbing fastness could rub off dyes on basically anything, and that is undesirable for end-users.
5.1.2 Dry friction test
Put a piece of (50×50) mm rubbing cloth (standard white cotton cloth) on the rubbing head under standard atmosphere (temperature 20℃±2℃, humidity 65%±4%), humidity control for more than 4 hours. Make sure the direction of the friction cloth is consistent with the movement direction of the friction head. Adjust the running speed of the rubbing head to one reciprocating friction cycle per second, ten times, which amounts to a total of 10 cycles of friction. The friction stroke or reciprocating movement on the sample should be (104±3) mm, and the applied direction is vertically downward. This downward force should be (9±0.2) N. After the entire ten cycles are completed, remove the friction cloth, adjust the humidity (over 4 hours), and remove any excess fibers on the friction cloth that may affect the rating. As shown:
5.1.3 Wet friction test
Immerse the weighed piece of friction cloth completely in distilled water, take it out, and reweigh the friction cloth to ensure that the moisture content of the friction cloth reaches 95%-100%. Then apply the same operating method as in the dry friction test.
5.1.4 Textile rub resistance test rating
After performing the above test process, we need to take the moistened friction cloth to the grading room and place it in the standard light source box, then use the gray sample card to evaluate the staining grade of the friction cloth.
Place three layers of friction cloth on the back) as shown in the figure:
5.1.5 Comparison of common standards of textile Color Fastness to rubbing
The commonly used standards for Color Fastness to rubbing are GB/T 3920-2008, AATCC 8-2007, AATCC 116-2010 and JIS L 0849-2004. We can analyze the similarities and differences between these four standards through the following table.
It can be seen from this table that different testing standards have different requirements depending on the size of the sample cloth. When the customer prepares the sample cloth, if the tested sample is a fabric or carpet: prepare two sets of samples with a size not less than 50 mm × 140 mm and group two pieces of each set. (When sampling, one piece of fabric is parallel to the warp yarn, and the other piece of fabric is parallel to the weft yarn.) Another sampling method is to sample at a certain angle from the sample’s length to the warp and weft of the fabric.
If it is a pile fabric and the pile is easy to distinguish, the pile’s direction when cutting the sample should be consistent with the length of the fabric. Under normal circumstances, the national standard and the European standard adopt the method of taking one piece in each of the latitude and longitude directions. In contrast, the American standard adopts the method of sampling with an inclination to 45 degrees.
If the tested fabric is the yarn: it needs to be woven into a fabric with a sample that is not less than 50 mm × 140mm. Alternatively, the yarn can be wound parallel to a cardboard of the same size as the sample and along the length of the cardboard.
5.1.6 Analysis of factors affecting Color Fastness to rubbing
- The influence of fabric surface morphology
Under dry conditions, it is very easy to perform dry rubbing on a rough surface or sanded and raised fabrics, such as hemp fabrics, denim fabrics and pigment printing fabrics, because unfixed dyes are the main cause of poor Color Fastness to rubbing. This is because the dye, paint, or other colored substances accumulated on the fabric’s surface are ground down. Some colored fibers are even broken while colored particles are formed, which further reduces the Color Fastness to dry rubbing. For sanded or raised fabrics, the fluff on the surface of the fabric and the surface of the friction cloth are at a certain included angle, which is not parallel, so that the friction resistance of the friction head during reciprocating motion increases, making this kind of fabric resistant to drying. Hence, the Color Fastness of rubbing would decrease.
- The influence of fabric structure
The surface of a sample of light and thin fabric (usually synthetic fiber or silk fabric), due to the relatively loose fabric structure, during dry friction, the sample will slip with the movement of the friction head under the action of pressure and friction. The slippage partially increases the frictional resistance and improves friction efficiency as well. But in wet friction with these chemical fibers, the reaction is different from the reaction with cellulose fibers.
Due to the extremely low moisture absorption of the fiber or the insignificant water puffing effect, and the presence of water as a lubricant, the Color Fastness to wet rubbing of such light and thin fabrics is significantly better than their Color Fastness to dry rubbing.
Therefore, it is not uncommon for certain fabrics to have better Color Fastness to wet rubbing than dry rubbing. In these cases, the selected dye types, dye performance, dyeing, finishing process conditions etc. will also affect the Color Fastness to rubbing. But when their influence is compared with the influence of physical factors such as the texture and surface morphology of the fabric, they seem quite unimportant.
However, studies revealed that this effect usually pertains to products with dark colors, such as black, red and navy blue. Of course, due to the dyes, printing and dyeing processes of corduroy, twill and other pigment printing fabrics, under wet conditions, usually have a Color Fastness to wet rubbing grade of level 2, or lower and this is not superior to their fastness to dry rubbing.
- The influence of reactive dye chemical structure
When a cellulose fiber fabric dyed with reactive dyes is subjected to the wet rubbing fastness test, two main factors could cause color transfer: the water-soluble dye is transferred to the rubbing fabric during rubbing, causing the original color to fade, therefore staining the rubbing cloth. The second is that some of the dyed fibers break during rubbing, forming tiny colored fiber particles that are transferred to the rubbing fabric, which, of course, stains it.
Factors that may affect the Color Fastness of reactive dyes to wet rubbing include the structure and characteristics of the reactive dyes themselves, the fabric’s properties, the effect of pretreatment, cloth surface damage, surface finish, etc. Furthermore, the dyeing process and the effect of soaping after dyeing the fabric, the effect of fixing treatment, and the effect of dyeing fabric finishing can all contribute to influencing the Color Fastness of reactive dyes.
Studies show that the covalent bond strength, bond stability and adhesion formed by reactive dyes of varying chemical structure on cellulose fibers are different. On the other hand, there is no significant difference in the effect of the Color Fastness to wet rubbing of dyed fabrics. When the dyed fabric is wet rubbed, the covalent bond formed between the dye and the fiber will not break; hence, floating colors won’t be produced. The transferred dyes are usually supersaturated dyes that do not form a covalent bond with the fiber and only rely on van der Waals forces to produce adsorption, and these are the so-called floating color.
- Influence of reactive dyeing degree
The Color Fastness to wet rubbing of reactive dye-dyed fabrics is closely related to the depth of dyeing in the sense that, when wet rubbing, the amount of color transfer and the depth of dyeing are almost in a good linear relationship. Excessive dyes cannot be combined entirely with fibers. They will only accumulate on the fabric’s surface to form floating colors, which seriously affects the Color Fastness to wet rubbing of the fabric.
Cotton fibers without special treatment and under wet conditions, will swell, increase friction, and decrease fiber strength. These create favorable conditions for the breakage, shedding and transfer of color in colored fibers. Therefore, we can improve the fabric’s surface finish and hair effect through pretreatment of the cellulose fiber before dyeing. Some of these pretreatment processes include; mercerizing, singeing, cellulase finishing, scouring, bleaching, washing, and drying. This will reduce friction resistance and reduce floating color, thereby effectively improving the Color Fastness to wet rubbing of the fabric.
- The effect of softener
We can improve the Color Fastness of reactive dye printing through soft finishing. Softeners have a lubricating effect when applied to the fabric and can reduce the coefficient of friction to prevent the dye from falling off. Cationic softeners can also form lakes with anionic dyes, and the dyes won’t easily fall off. Simultaneously, the formation of the color lake eventually reduces the dye’s solubility and improves the wet rubbing fastness. However, softeners with hydrophilic groups will most likely deter the improvement of Color Fastness. In the production practice process, the dye’s water-soluble group can be blocked using a fixing agent. With this, the pH value of the finished colored cloth’s fabric surface can be controlled, floating color can be removed, smoothness of the fabric can be improved, and the wet rubbing fastness of the fabric can also be improved. Proper pre-bake at the front stage can avoid dye “migration.”
During pretreatment, the factors and properties that require the most attention are the amount of alkali, steaming time, washing method, sufficient soaping, etc. The first two are closely related to the degree of hydrolysis of the dye, and the latter two are directly related to the dye’s floating color.
The dyed fabric, especially the long-car pad dyeing, must undergo sufficient washing, soaping, and other processes to remove the floating color and the unreacted and hydrolyzed dyes on the fiber surface. This will avoid undesirable effects on the fabric’s Color Fastness, but it will result in poor Color Fastness, and the shade will be darker when not paid the necessary attention.
Among the factors mentioned above affecting the Color Fastness of fabrics to rubbing, their respective principle of action and the degree of influence is very different. The Color Fastness problem seems simple, but the factors involved are quite complicated. Over the years, whether in dye research and production or textile dyeing and finishing, people have invested a lot of manpower and material resources to solve the problem of Color Fastness in textile products. Thankfully great progress has been made. Although dyestuffs, new processes and new additives continue to emerge, there are still many problems that need resolving.
5.2 Textile light fastness test
The light fastness of textiles has been paid progressively more attention at home and abroad. Presently, China’s textile industry product standards (especially the new standards endorsed in recent years, excluding underwear standards) all use light fastness as one of the assessment standards. For example, the silk product standards publicized by China before did not stipulate the assessment of light fastness. Still, the promulgated standards have now taken the light fastness of elastic silk as the assessment index. With chemical fiber like silk fabric and cotton product standards, light fastness is also taken as an important evaluation index, and some product standards even take light fastness as an evaluation index.
5.2.1 Comparison of common test methods and standards for light fastness
There are many test methods for light fastness. The following table lists several commonly used standard methods.
5.2.2 China’s current effective light fastness test method standard
- -Color Fastness to light: This is done by exposure to sunlight under the specified conditions and not exposed to rain. The Color Fastness to light of the sample in question is then evaluated with reference to the blue wool standard.
- -Color Fastness to artificial light: Xenon arc testing refers to the exposure of a sample to an artificial light source equivalent to sunlight under specified conditions. The Color Fastness to light of the sample is then evaluated with reference to the blue wool standard.
- -Color Fastness to weather: This means outdoor exposure of a sample without any protection and under specified conditions. The sample is then compared with the blue wool standard to evaluate its Color Fastness.
- -Color Fastness to human-made weather: This refers to using xenon arc to spray exposure in a xenon lamp tester under specified conditions. The exposed sample is then compared with the blue wool standard to evaluate the Color Fastness.
- -Composite Color Fastness to light and perspiration: Expose the sweat-treated sample in the instrument, and then evaluate its Color Fastness to light and perspiration to determine its sensitivity. Among them, the xenon arc in the Color Fastness to artificial light is the most used standard. Most textiles in China are tested according to this standard method when assessing the Color Fastness to light.
5.2.3 The relevant standards of the American light fastness tester
Color Fastness to light is suitable for indoor textiles. Weather resistance of fabric (through xenon arc) is suitable for outdoor textiles. Among them, light fastness is a widely used textile light resistance and stability standard.
Its main measuring range:
- light fastness test of dyes: light fastness test of dye is measured under light (wavelength 380nm～750nm) and certain temperature and humidity conditions.
- Weather fastness test of dyes: under the combined influence of light (wavelength 380nm～750nm) and a certain temperature, humidity and rain, the weather-fastness of dyes is measured.
- Weather fastness test of textiles: Under the combined action of light (wavelength: visible light = 380nm～750nm; ultraviolet light = 300nm～380nm) and climatic conditions, the weather-fastness of textiles is measured.
5.2.4 The best way to test light fastness
The fabric’s reaction to light happens to be one of the most important test aspects in all Color Fastness tests. Of course, for manufacturers, the performance of fabrics in washing, dry cleaning, rubbing, sweat absorption, and exposure to different solutions is crucial information. However, because fabrics are sensitive to light and their reactions to light are difficult to predict, accurate and reliable testing is critical.
For many manufacturers, light fastness testing is an important part of R&D and quality control. However, it is also one of the most difficult tests to become proficient at. This is because most materials take months or even years to respond under sunlight. It is not feasible to test the fabric under natural light in an actual application environment. What’s more, we can’t say many R&D departments are willing to wait patiently.
Many manufacturers choose laboratories for testing. They simulate natural light to speed up the test process. The most successful technique used is the xenon long arc radiation method. This singular reliable method of simulating and reconstructing the natural light spectrum uses various filters to reproduce special optical conditions.
The light fastness test is not as simple as placing the fabric under a specific light source to observe its reaction. Temperature and humidity must also be considered because both temperature and humidity influence the fabric’s response to light more so. Therefore, the light fastness tester must control these environmental factors, that is, create corresponding environmental conditions for each test, and keep them constant throughout the process.
Also, if a certain fabric is used outdoors, the simulation of weather factors, especially rain, is another issue that must be considered. Therefore, the equipment must include a water sprinkler system that simulates rainy days and simulates climate conditions in different parts of the world.