Some key points affecting the color fastness to rubbing of reactive digital printing

The color fastness to rubbing is one of the tests of printing color fastness, and it is also one of the most common tests in the process of digital printing trade. it refers to the fading degree of printed fabrics after rubbing, and it is also used to evaluate the resistance of fabrics to mechanical rubbing.  The method for testing the rubbing fastness comprises the following steps of: fixing a textile sample with a specified size on a rubbing tester platform by a clamping device, rubbing the textile sample with a dry rubbing cloth and a wet rubbing cloth respectively, and finally taking the staining degree of white cloth as an evaluation basis, and comparing with a standard grey sample card, dividing the textile sample into five grades in total, wherein the larger the grade is, the better the rubbing fastness is indicated.  There are many factors affecting the color fastness to rubbing of fabrics, several of which are analyzed below.

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1, fabric surface roughness of dry grinding fastness will be poor

Because the unfixed dye is the main cause of poor color fastness to rubbing, in dry condition, for rough or ground wool fabric, wool fabric, hard as hemp fabric, denim fabric and paint printing fabric, if dry friction easily will be deposited on the surface of the fabric dye, paint or other colored material grinding down, even cause part of the colored fiber fracture and form colored particles, make the color fastness to dry friction further decline.  For wool or wool fabric, the wool on the surface of the fabric and the friction cloth surface is a certain angle, is not parallel, so that the friction resistance of friction head in reciprocating motion increases, make this kind of fabric dry friction fastness is reduced.

2, the influence of reactive dye chemical structure

There are two main factors that cause color transfer in the color fastness to wet rubbing of cellulose fiber fabrics dyed with reactive dyes:

First, the water-soluble dye is transferred to the friction fabric during friction, so as to fade the original color and make the friction fabric stained with color;

Second, the partially dyed fibers break during friction, forming tiny colored fiber particles which are transferred to the friction fabric to cause staining.

When the dyed fabric is wet rubbed, the covalent bond formed between the dye and the fiber does not break and float.  Whereas the dyes to be transferred are typically supersaturated dyes which do not form covalent bonds with the fibers and which produce adsorption by van der waals forces alone, i.e. so-called floats.

If the process specification, pretreatment and soaping is sufficient, the color fastness and hand feeling of digital printing products made of active digital printing ink are better, so most of the high-end products are more inclined to use active printing.

3, dyeing degree ( color depth )

The practical experience shows that the color fastness to wet rubbing of fabrics dyed with reactive dyes is closely related to the dyeing depth, that is, the transfer amount of color and the dyeing depth are almost in good linear relationship when wet rubbing is carried out, in which supersaturation of dyes is the most important factor during dyeing, and the concentration of dyes used in dyeing dark colors is higher, but the saturation value cannot be greatly exceeded, because excess dyes can not be combined with fibers, but can only be accumulated on the surface of fabrics to form floating color, which seriously affects the color fastness to wet rubbing of fabrics.

In addition, the cotton fiber without special treatment will swell under wet condition, the friction force will increase, the fiber strength will decrease, all of these create a good condition for the colored fiber breaking, falling off and color transfer.  Therefore, proper pretreatment of cellulose fiber before digital printing, such as mercerizing, singeing, cellulase finishing, scouring, bleaching, washing and drying, can improve the smoothness and wool effect of the surface of the fabric, reduce friction resistance and reduce floating color, thus effectively improving the color fastness to wet friction of the fabric.

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4, the influence of fabric structure

Little attention has been paid to the effect of fabric structure and surface morphology on the color fastness to rubbing.  Because the fabric structure is relatively loose on the sample surface of light and thin fabrics ( usually synthetic fibers or silk fabrics ), when dry friction is carried out, the sample will slide partially along with the movement of the friction head under the action of pressure and friction force, so that the friction resistance is increased and the friction efficiency is improved.

However, when wet friction is performed, the situation is completely different from that of cellulose fibers.  Because the moisture absorption of fibers is very low or the swelling effect of water is not obvious, and the presence of water acts as a lubricant, the color fastness to wet friction of such fabrics is obviously better than the color fastness to dry friction, which is in obvious contrast to the concept that the color fastness to dry friction of textile products should be better than the color fastness to wet friction, and often leads to people's doubts.

Therefore, it is not uncommon for certain fabrics to have a better color fastness to wet rubbing than to dry rubbing.  At this time, the selected dye variety, dye properties, dyeing and finishing process conditions, although will also have an impact on the color fastness to rubbing, but compared with the fabric structure and surface morphology and other physical factors, it is not very important.

Statistics show that the majority of such cases or dark products, such as black, red and navy blue.  Of course, for fabrics such as corduroy, twill and paint printing, the color fastness to wet rubbing is usually grade 2 or even lower under wet conditions due to the dyes and printing and dyeing processes used by the fabric itself, and is not better than the color fastness to dry rubbing.

5, the influence of softener

Improve that color fastness of reactive dye printing through soft finis.  Softening agent has lubricating effect, can reduce the coefficient of friction, so as to prevent the dye from falling off.  Cationic softeners and anionic dyes can also form lake, dye is not easy to fall off.  At that same time, the solubility of dye is reduce by the lake, and the wet rubbing fastness can be improve.

However, softening agents with hydrophilic groups are not conducive to the improvement of wet rubbing fastness.  In the process of production practice, the water-soluble group of the dye can be blocked by using the fixing agent, the ph value of the cloth cover of the finished color cloth can be controlled, the floating color can be removed, the smoothness of the fabric can be improved, and the wet friction fastness of the fabric can be improved.  Proper pre-drying of the front section can avoid dye " swimming".  The factors needing attention are alkali dosage, steaming time, water washing method, sufficient soaping, etc. the former two are closely related to the degree of hydrolysis of dye, and the latter two are directly related to the floating color of dye.

Summary: in order to optimize the color fastness of reactive digital printing, reasonable pretreatment process of fabric, high quality reactive digital printing ink, ink amount within the range of fabric can bear, reasonable steaming and sufficient washing and soaping are needed.  Softeners and fixatives can be added if desired.

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