Definition and basic concept of color fastness
Color fastness refers to the ability of the color of textiles to resist the action of various physical and chemical factors during processing and use to maintain the original color. Essentially, it measures a comprehensive measure of how firmly the dyed fabric bonds to the dye. In daily life, consumers often encounter the phenomenon of washing and fading, sun fading, sweat stains and other phenomena, which are specific manifestations of color fastness problems. In laboratory testing, color fastness is subdivided into several independent evaluation dimensions, including soap washing color fastness, friction color fastness, sweat color fastness, light fastness, water color fastness, saliva color fastness, etc. Each color fastness is evaluated using a five-grade nine-grade system (the lightfastness color fastness is eight grades and nine grades), of which level 1 is the worst, level 5 (or level 8) is the best, and grades 3 to 4 are usually regarded as the basic threshold for textile quality qualification.
Physicochemical principles of color fastness
The quality of color fastness depends on the nature of the interaction between the dye molecule and the fiber and the chemical stability of the dye molecule itself. Dyes are able to impart color to fibers because they contain specific chromophores in their molecular structure and can be attached to the inside or surface of fibers through physical adsorption, hydrogen bonding, ionic bonds, or covalent bonds. When textiles are subjected to external factors, these binding bonds may be broken, or the structure of the dye molecules themselves may be altered, leading to color changes.
From a thermodynamic point of view, the dyeing process is a process in which the dye is redistributed between the fibrous and liquid phases and achieves equilibrium. When external conditions such as temperature, pH or solvent environment change, the original balance is broken, and the dye will migrate from the fiber phase to the external medium, causing fading or staining. The mechanism involved in lightfastness is more complex, and the dye molecules absorb light energy and enter the excited state, which leads to photooxidation or photoreduction reactions, resulting in the destruction of the chromophore structure. The sensitivity of dyes with different chemical structures to light was significantly different, and azo dyes were more prone to photodegradation, while phthalocyanine and anthraquinone dyes usually had better photostability.
The principle of friction color fastness mainly involves the transfer of dye particles under mechanical action. When rubbing the white cloth to dye the surface of the fabric with a certain pressure and stroke, the dye particles or colored fiber fluff that are not fixed on the surface of the fabric will be mechanically peeled off and transferred to the white cloth, causing staining. During the wet friction process, the infiltration of water expands the fibers, increases the friction coefficient, and water as the medium promotes the dissolution and migration of soluble dyes, so the wet friction fastness is usually lower than that of dry friction.
The color fastness rating is based on a standardized visual comparison, and its core tool is the standard gray sample card. The color-changing gray card consists of five pairs of small dull gray cards, which are divided into five fastness levels according to the distinguishable color difference, one of which is medium gray, and the other becomes lighter according to the increasing color difference. The stained gray card consists of five pairs of white and gray cards, and is graded by evaluating the degree of comparison between the stained white cloth and the original white cloth. Under standard light source conditions, laboratory technicians can determine the grade closest to the gray card through visual comparison, and can evaluate the middle half level if necessary.
Measurement method of color fastness
The detection of color fastness is divided into a variety of standardized test procedures according to the different external effects of simulation. The following introduces several core colorfastness measurement methods.
Color fastness to frictionIt is the most basic color fastness test item. According to the GB/T 3920 standard, the specimen is fixed flat on the bottom plate of the friction fastness tester during the test, and the standard friction white cloth is fixed on the friction head, so that the friction head rubs the friction head on the surface of the specimen 10 times with a pressure of 9 Newtons along a straight trajectory, and the reciprocating distance is 100 mm. Dry friction and wet friction tests are carried out respectively, and the white cloth needs to be impregnated in distilled water to make the water content reach 95% to 105% during wet friction. After the test is completed, the white cloth is removed, and the degree of staining of the white cloth is evaluated with a color-stained gray card to obtain the corresponding grade.
Soap washing fastnessSimulate the color change of textiles during washing. The specimen is sewn into a combination specimen with standard lining fabric, put into a stainless steel container, and a soap solution containing standard soap powder and a certain number of stainless steel beads are added to simulate mechanical agitation. The container is placed in a washability fastness tester and rotated at the specified temperature (commonly 40°C, 50°C or 60°C) for 30 minutes. After washing, the combination specimen is taken out, and after cleaning and drying, the color change grade of the specimen is evaluated with the color-changing gray card, and the color-staining grade of the lining fabric is evaluated with the color-dipping gray card.
Sweat resistance and color fastnessSimulate the effect of human sweat on textile color. According to the GB/T 3922 standard, two types of artificial sweat - alkaline and acidic test solution need to be prepared to simulate the sweat composition of people with different physiques. The specimen is soaked in the test solution after being soaked with the lining fabric, and after it is completely soaked, it is taken out and placed between the specimen plates of the sweat resistance tester, and the specified pressure is applied and placed in a 37°C incubator for 4 hours. After taking it out, it is hung to dry, and finally the color discoloration of the specimen and the color staining grade of the lining are evaluated with gray cards.
Lightfastness and color fastnessXenon arc lamps are used to simulate sunlight irradiation. According to the GB/T 8427 standard, the specimen and a set of blue wool standards with known fastness levels are placed on the specimen rack of the lightfastness tester at the same time, and exposed to continuous exposure under the set radiation intensity and blackboard temperature conditions. After exposure to the specified stage, the degree of discoloration of the specimen is compared with that of the blue wool standard to evaluate the color fastness grade of the specimen. The color fastness of light fastness is rated from 1 to 8, with grade 8 being the best, indicating that the material has excellent light stability.
Resistant to saliva color fastnessIt is a mandatory testing item unique to infant textiles. According to the GB/T 18886 standard, the specimen is stitched with the lining fabric, put into artificial saliva for treatment, and then placed in a 37°C incubator for a certain period of time, and the discoloration and staining grade are evaluated after drying. China's mandatory standard GB 31701 clearly stipulates that the color fastness to saliva of textile products for infants and young children must reach level 4 or above to ensure the safety of infants and young children when biting clothes.
Key factors that influence color fastness measurements
The color fastness measurement results are affected by the characteristics of the material itself, the pretreatment process, the dyeing conditions and the post-treatment process. First, the chemical structure of the dye is an intrinsic factor that determines the color fastness. Dyes containing more water-soluble groups are more likely to dissolve and fall off during wet treatment, and their washing and sweat fastness are usually low. Dyes with photooxidizing groups in their molecular structures have poor light fastness. For example, azo dyes generally have lower light fastness than anthraquinone dyes, and direct dyes generally have lower wet fastness than reactive dyes.
Secondly, the dyeing process parameters directly affect the existence of dyes on the fibers. The higher the dyeing concentration, the more aggregates the dye forms inside the fiber, which helps to improve the lightfastness, but at the same time, excessive dye may lead to an increase in floating color and reduce friction fastness. The control of dyeing temperature and time determines the degree of penetration of the dye, and the dye with sufficient penetration is more firmly combined with the fiber and has better fastness. The soaping process after dyeing is very important, and the residual hydrolyzed dye and surface floating color when the soap is not washed thoroughly are the main reasons for the decrease in wet fastness.
Third, the physical structure and chemical properties of fibers have a significant impact on color fastness. Synthetic fibers such as polyester have a tight structure, and disperse dyes enter the fiber under high temperature conditions, making it difficult to migrate after cooling, so the washing fastness is better. Natural fibers such as cotton have a loose structure, and the dye is easy to enter but is also easy to desorption during wet treatment. The flatness of the surface of the fabric directly affects the friction fastness, and fabrics with more hairiness on the surface and loose tissue structure are more prone to staining when rubbed.
Finally, the finishing process may alter the chemical environment of the dye. Some color fixers can improve the fastness of wet treatment, but may lead to a decrease in light fastness. Softeners affect the friction characteristics of fabric surfaces at the same time as improving the feel. Environmental factors such as temperature, humidity, and pH of the test medium can also interfere with the results, so it is important to strictly control the conditions during the testing process to ensure the accuracy of the results.
Application of color fastness in the textile industry
Color fastness index runs through all links of the textile industry chain and is the core element of quality control. In the field of dye research and development, color fastness testing is used to screen and evaluate the comprehensive performance of new dyes, and color fastness data is one of the core parameters in the product technical data provided by dye suppliers. R&D personnel systematically test the fastness performance of dyes with different structures to provide direction for the optimization of dye molecular structure.
In the process of printing and dyeing production, color fastness testing is an important means of process control and product quality monitoring. The dyeing factory conducts various color fastness tests on semi-finished products and finished products to verify the compatibility of dyes, the rationality of the dyeing process and the post-treatment effect. When a certain fastness is found to be not up to standard, the process personnel need to analyze and adjust from the dye selection, dyeing conditions, soap washing process and color fixation treatment until the product meets the quality standard. For dark and dark products, wet friction fastness is often the focus of technical research.
In the procurement of clothing and home textile products, brands and buyers determine the color fastness items and grade requirements that need to be assessed according to the end use of the product. Underwear and summer clothing need to focus on sweat resistance and washing fastness; outdoor clothing and sunshade products have high requirements for lightfastness and color fastness; Infant clothing must meet the requirements of saliva color fastness in mandatory standards. The technical clauses in the procurement contract usually specify the minimum acceptance standards for each color fastness.
In international trade, the color fastness index is an important basis for conformity assessment. There are differences in the standard system of different countries and regions, the European Union generally adopts ISO standards, the United States follows AATCC standards, and Japan adopts JIS standards. Exporters need to understand the specific requirements of their target markets to ensure that their products meet the regulations of the importing country and customer requirements. The color fastness test report has also become an important document for quality proof in international trade.
In the field of consumer guidance, color fastness level information can help consumers make reasonable choices. Although high color fastness products may be expensive, they can maintain long-term aesthetics and performance, especially for children's clothing and underwear that require frequent washing, as well as curtains and outdoor products that require long-term exposure to sunlight.
Summary and outlook
As the core index to measure the color stability of textiles, the evaluation system covers dozens of test methods that simulate actual use conditions, from sun exposure, washing, friction to sweat stains and saliva. This system is not only a comprehensive reflection of dye properties, fiber characteristics, dyeing processes and finishing technology, but also directly related to consumer experience and health safety. Through standardized testing methods and a unified rating system, color fastness testing provides a comparable and traceable quality language for the textile industry chain.
Looking forward to the future, color fastness detection technology is developing in the direction of intelligence, speed and precision. The gradual popularization of instrument rating systems based on computer vision and spectrophotometry technology is expected to reduce the subjective differences of traditional visual ratings and improve the stability and reproducibility of detection results. At the same time, with the continuous improvement of consumers' requirements for textile functionality and ecological environmental protection, the demand for new functional dyes and high-fastness dyes continues to grow, and the attention to special fastness indicators such as light-resistant sweat composite color fastness, chlorine water color fastness, and seawater color fastness is also increasing.
Driven by the concept of sustainable development, color fastness testing is increasingly closely linked to green manufacturing. Improving dye on-dye and fixation rates by optimizing the dyeing process not only helps to improve color fastness, but also reduces dye usage and wastewater discharge. In the future, the color fastness prediction model based on big data analysis is expected to assist printing and dyeing enterprises to achieve precise control of process parameters, reduce resource consumption while ensuring product quality, and promote the development of the textile industry in a higher quality and more sustainable direction.