Main factors affecting the color fastness of fabrics to sun exposure and wet rubbing

How to improve fastness to dyeing has become a research topic for engineers and technicians in the printing and dyeing industry. In particular, the sun fastness of reactive dyestuff dyeing light-coloured fabrics, the wet friction fastness of deep and rich-coloured fabrics; the decrease in wet treatment fastness caused by heat migration after dyeing of disperse dyestuff; and high chlorine fastness, sweat light fastness, etc., and there are many factors affecting colour fastness, and there are many ways to improve colour fastness, how by choosing suitable dyeing and chemical additives, improving the dyeing and finishing process, strengthening process control, etc., we have explored Some methods and measures have been developed to improve the fastness to dyeing and basically meet the market demand.

Sun fastness of light coloured fabrics with reactive dyes

 It is well known that reactive dyestuffs dyed on cotton fibres are attacked by ultraviolet light under sunlight and the colour-emitting or colour-assisting groups in the structure of the dyestuff are damaged to varying degrees, resulting in colour changes or light colours, i.e. problems of sunlight fastness. For example, GB/T411-93 standard for cotton printing and dyeing fabrics stipulates that the light fastness of reactive dyestuff dyeing fabrics is 4-5, and that of printing fabrics is 4; GB/T5326 standard for combed polyester-cotton blended printing and dyeing fabrics and FZ/T14007-1998 standard for cotton-polyester blended printing and dyeing fabrics both stipulate that the light fastness of disperse/reactive dyeing fabrics is 4, and the printed fabric is also level 4. It is difficult to achieve this standard with reactive dyestuff dyed light coloured printed and dyed fabrics.

1、The relationship between dyeing parent structure and sun fastness The sun fastness of reactive dyestuffs is mainly related to the parent structure of the dyestuff. 70-75% of the parent structure of reactive dyestuffs is azo, the rest is insecure quinone, phthalocyanine and a. The light fastness of the azo type is poor, while the light fastness of the anthraquinone, phthalocyanine and a type is better. Yellow reactive dyes have an azo molecular structure, with the best solar fastness of pyrazolinone and naphthalene trisulphonic acid as the parent colouring body, blue spectrum reactive dyes have an anthraquinone, phthalocyanine and a parent structure with excellent solar fastness, and red spectrum reactive dyes have an azo molecular structure. Sun fastness is generally low, especially in light colours.

2, the relationship between dyeing concentration and sun fastness The sun fastness of the dyed specimen will vary with the change in dyeing concentration, the same dye in the same fiber dyed specimen, its sun fastness with the increase in dyeing concentration and improve, mainly due to changes in the distribution of dyestuff on the fiber's aggregate particle size caused by. The larger the aggregate particles, the smaller the area per unit weight of dye exposed to air-moisture, etc., and the higher the solar fastness. An increase in dyeing concentration increases the proportion of large aggregates on the fibre and the sun fastness increases accordingly. Light coloured fabrics with low dyeing concentration have a low proportion of dyestuff aggregates on the fibre and most of the dyestuff is in a single molecule state, which means that the decomposition of the dyestuff on the fibre is high and each molecule has the same chance of being affected by light and air and moisture, and the fastness to sunlight decreases accordingly. ISO/105B02-1994 standard sun fastness is divided into 1-8 grade standard assessment, our national standard is also divided into 1-8 grade standard assessment, AATCC16-1998 or AATCC20AFU standard sun fastness is divided into 1-5 grade standard assessment. The sun fastness of all dyestuffs shown on the colour cards provided by all dyestuff manufacturers or companies, including the Dyestuff Index, is the data measured at a dyeing depth of 1/1, i.e. on medium coloured fabrics with a dye concentration of about 20-30 g/L. Light coloured fabrics do not reach this level.

Measures to improve solar fastness

1、The most important factor affecting the sun fastness of light coloured fabrics is the dyestuff itself, so the choice of dyestuff is of paramount importance. When choosing dyestuffs for colour combinations, it is important to ensure that the light fastness of each component chosen is comparable, as long as the light fastness of any one of the components, especially the component with the lowest dosage, does not meet the requirements of the light coloured dye, the light fastness of the final dye will not be up to standard.

2、Other measures

❶ Influence of floating dyes. The light fastness of the dyed material is also affected by incomplete soaping, unsettled dyes and hydrolysed dyes remaining on the fabric surface, which have a significantly lower light fastness than set reactive dyes. The more thorough the soaping process, the better the light fastness.

❷ Influence of fixatives and softeners. Cationic, low-molecular or polyamine condensed resin-based fixatives and cationic softeners used in the finishing of fabrics will reduce the sun fastness of the dyed material. Therefore, when choosing fixing agents and softeners, attention must be paid to their effect on the sun fastness of the dyed material.

❸ Influence of UV absorbers. UV absorbers are often used recently to improve the sun fastness of light coloured dyes, but they must be used in large amounts to have some effect, which not only increases costs but also causes yellowing and strong damage to the fabric, so it is best not to use this method.

3 Wet rubbing fastness to wet treatment of reactive dyestuffs in deep rich colours

Reactive dyestuffs give excellent fastness to dyeing through covalent bonding, but fading and staining often occur in deep, rich coloured dyestuffs. These phenomena occur in addition to the decomposition of azo groups, the detachment of complexed metal ions and the oxidation of amino groups caused by light, heat, sweat, acid gases and oxidants in part of the dye's parent structure. Breakage of the dye-fibre bond can also cause discolouration and staining. And floating colours on the dyed material are responsible for the wash fastness of deep, rich coloured dyed material.

1、All dyestuffs leave floating dyestuffs on the fabric after dyeing. The floating dyestuffs of reactive dyestuffs contain partially hydrolysed dyestuffs, dyestuffs adsorbed on the fibres without participating in the covalent reaction. The greater the directness of the reactive dyes, the less likely they are to be removed from the fibres, and the more water-soluble they are, the more they will dissolve in water and stain other fabrics. They are the main reason for the failure of the wet fastness of reactive dyestuff dyed fabrics, including water washing, soaping and wet rubbing. To remove the floating dyestuff, both the choice of dyestuff and the rationalisation of the process need to be considered.

①Dye selection

Dyestuffs are selected by printers based on the fastness properties provided on the dye colour card and on small sample tests to determine the level of wet treatment fastness of the dye. As the fastnesses provided on the colour cards are for dyeing at a depth of 1/1 of the dye concentration, they can only be used as a reference and the latter is the main factor. Therefore, the dyestuff for deep and intense dyed matter must be preferentially selected, and imported reactive dyestuffs with better fastness in all respects, high fixation rates and good dye transmission must be chosen.

② process points of attention to strengthen the quality of pre-treatment control, pre-treatment is mainly desizing, burning, boiling and bleaching, mercerization and other processes, and the control of each process directly affects the quality of dyeing and colour fastness.

(1) The sizing must be fully receded from the internal fibres, failure to do so will affect the evenness and permeability of the dyeing, hinder the diffusion of the dye into the fibres and cause the dye to float on the surface of the fibres, affecting the colour fastness.

(2) Burning hair burning hair to burn clean, burning hair is not clean will directly affect the surface finish and smoothness of the fibres, increase the coefficient of friction, produce a certain resistance, reduce friction fastness.

(3) The bleaching and boiling process should be carried out thoroughly, and the required whiteness should be achieved, impurities and cotton seed shells should be removed, and the standard wool efficiency of 8-10cm/30min or more should be achieved. The better the wool effect, the better the dyeing permeability and evenness, the higher the dyeing rate and fixing rate, the deeper the colour, the less the floating colour floating on the surface, the better the rubbing fastness and washing fastness, which can save the cost of dyeing and chemical materials and improve the dyeing fastness.

(4) Mercerisation is very important for dyeing fabrics, in addition to making the fabric obtain a stable width size, it can also increase the dyeing depth, improve the surface gloss and smoothness of the fabric, improve the wet friction fastness and staining fastness, especially for dyeing dark fabrics, mercerisation is more important, therefore it is necessary to control the alkali concentration of mercerisation to achieve the full mercerisation effect.

2、Difference in dyeing methods

The different methods of dyeing, the different shades of colour, evenness, transmission and fixation effects, are analysed and discussed below.

3、Wash treatment

Only when these impurities are removed can the dyed material achieve optimum dye fastness and colour vibrancy. The aim is to remove the hydrophilic floating colours, to wash away electrolytes and alkalis and to reduce the concentration of electrolytes in order to increase the electrostatic repulsion between the floating dyes and the fibres, so that the floating dyes can be easily removed from the fibres; washing away the alkalis is to prevent the hydrolysis of the fixed dyes caused by the alkalis in the high temperature soaping. Washing after soaping is to remove the large amount of floating dyes that have been dispersed by the soaping agent. Water quality is very important in the post-treatment process, if the water hardness is high, there will be more Ca2+, Mg2+ and other heavy metal ions, they make the water-soluble sodium sulphonate salt into water-insoluble calcium (magnesium) sulphonate salt precipitate, thus weakening the hydrophilic nature of the floating dyes, so the water quality has a significant impact on the fastness of the dye post-treatment.

4. Finishing

Inherent deficiencies are made up for later, the colour fastness of the dyed fabric does not meet the requirements and can only be made up for and improved by the use of colour fixing agents and enhancers in the finishing. Special colour fixing agents can improve the wet rubbing fastness of reactive dyestuffs by 0.5-1.0 grade.

4 chlorine fastness and sweat - light fastness reactive dyes generally poor chlorine fastness, mainly depends on the molecular structure of the chromophore, the presence of sulfonic acid or carboxylic acid groups adjacent to the diazo group of the dye, or the presence of sulfonic acid or carboxylic acid groups adjacent to or opposite the hydroxyl group of the coupling component, because of its spatial site resistance effect, reducing the ability of CI-attack -NH- or -N-, therefore improving the chlorine fastness. According to the water fastness test method for chlorine-containing swimming pools of GB/T8433-1998, the effective chlorine concentration is divided into 20mg/L, 50mg/L and 100mg/L, with special requirements of 200mg/L. As the effective chlorine concentration increases, the chlorine fastness decreases. Therefore the choice of dyestuff is very important, but the fastness to chlorine can also be improved by using colour fixing agents in the finishing process, but only by 0.5-1.0 grade. The fastness of reactive dyes to sweat and light has received a lot of attention in recent years. Some reactive dyes have good fastness to sunlight, but poor fastness to sweat and light. This is because the fading mechanism is different under the dual action of sweat and sunlight, as the amino acids or related substances in the sweat chelate with the metal ions of the metal complex dyes, causing them to detach from the parent dye, and the sun fastness of the parent dye before complexation would have been poor, so they fade or change colour.

5 Reduced fastness due to heat migration after dispersion dyeing

The phenomenon of thermal migration is a redistribution of disperse dyes in a two-phase solvent, therefore all auxiliaries that can dissolve disperse dyes, can produce thermal migration effects.

The heat migration phenomenon is due to the solubilisation of the dyestuff by the auxiliaries in the outer layer of the fibre at high temperatures. The dyestuff migrates from the inside of the fibre through the fibre capillaries at high temperatures and broadens to the surface of the fibre, causing a series of effects such as colour change, staining of other fabrics during ironing, rub resistance, washing resistance, perspiration resistance, dry cleaning resistance and sunlight fastness reduction. Practice shows that the amount of heat migration of dyestuff from inside the fibre to outside the fibre is directly related to the dyeing depth, the deeper the dyeing depth of the fibre, the more the amount of dyestuff heat migrating from inside to outside, the greater the impact on the dyed material; high temperature treatment after dyeing has a greater impact on the heat migration of disperse dyestuff, the higher the temperature, the greater the impact.

For this reason, the finishing of dyed fabrics is as gentle a process as possible. The heat migration of disperse dyes is related to the molecular structure of the dyes themselves. In production practice it has been found that the widely used non-ionic surfactants are the main cause of the heat migration phenomenon of disperse dyes.

Amino silicone emulsions as softeners are by far the most used softeners because to make microemulsions, 40-50% of the total silicone needs to be applied to non-ionic surfactants such as fatty alcohol ethoxylates or alkylphenol ethoxylates as emulsifiers. With the widespread use of amino silicone emulsions, the post-dye heat migration of disperse dyes is more severe. The solution is to select disperse dyes with high lifting power and absorbency, especially with good wet fastness after heat fixation of the dyed material.

Dyes attached to the surface of the fibres (floating colours) can exacerbate the effects of dye heat migration on the colour fastness of the dyed fabric. Therefore, after dyeing (especially for dark colours) a reducing agent and alkali should be used for reducing cleaning to completely remove the floating colour.