Quality Requirements For Pretreatment Of Pure Cotton Medium And Heavy Fabric Before Printing And Dyeing

The basic requirements of pre-processing can be summarized as four words: "uniform, pparent, clean and white".

"Uniform" means uniform boiling, and the wool effect between fabric matches is basically the same. "Penetration" means full boiling and no "sandwich" phenomenon. "Net" means that the impurities such as pectin, cotton wax, ash and cottonseed shell on the cellulose fiber after being boiled are fully removed.

Theoretically, not all colors need to be dyed before bleaching, but the whiteness of the fabric will affect the reproducibility of the color. Therefore, "white" also requires a uniform whiteness of the fabric that has been boiled and bleached.

"Uniform, pparent, clean and white" is only a general requirement for the quality of semi-finished products. In actual production, it is necessary to control the quality of semi-finished products by means of specific detection.

Capillary effect

The semifinished product used for dyeing is not the higher the wool efficiency, the better the wool fabric is 10cm/30min.

Two points should be noted. One is to determine the instantaneous wool effect of half finished products.

Because in continuous pad dyeing, the impregnation of fabric impregnated instantaneous, the dyeing effect is easy to cause impermeable, forming surface color or color bar.

The time of fabric impregnation in the dye solution is only a few seconds. There is no specific index for measuring the instantaneous wool effect. Generally, the method of visual inspection is used. That is to say, dropping the water droplet from the fabric about 15cm at the same time by using the rubber head dropper, if the water droplet is impregnated instantly on the fabric surface and has a uniform circle, that is, the instantaneous wool effect is qualified; conversely, if the water droplet drops on the fabric surface for a short time, even if it is short for a short time, it also indicates that the instantaneous wool effect is not qualified.

The two is to test the whole wool effect of the fabric instead of taking 10cm on the left and right sides of the fabric.

The test of the whole wool effect requires that the 30min final test line is in a straight line or nearly straight.

The difference between the left and right sides should be controlled within 0.5cm, otherwise the right middle left chromatic aberration will be easily produced during dyeing.

Whiteness

It can be tested by ZBD whiteness tester.

For semifinished products used for dyeing, the whiteness is not as good as white. The whiteness should be determined according to the varieties and colors.

After normal boiling and bleaching, the whiteness of coarse and denser varieties is slightly worse, while the whiteness of finer and thinner fabrics is slightly better.

Using oxygen bleaching to increase whiteness is at the expense of strength.

Therefore, when formulating the actual production process, we should give full consideration to the strength reduction of the fabric.

The whiteness value should be above 75, and the dyeing activity should be light. The whiteness value should not be less than 80.

In addition, white consistency is also required. For medium light colored semi dyed products, the whiteness value of the same dyed semi finished product should be less than 3, and the difference between the left and right white values of the fabric is less than 1.5.

Strength

After boiling and bleaching, the cotton fiber is affected by various chemical additives, and the strength decreases. But if the technology is formulated reasonably, it can be controlled within the allowable range of strength drop.

Because of the poor quality of the cotton fabric, sometimes the strength of the fabric is not up to the national standard. Therefore, it is necessary to test the strength of the fabric before putting into production. After the boiling and bleaching, the strength drop can generally be controlled within 5%.

Weft deviation

In fabric

In processing, pretreatment is one of the processes that tend to cause weft deviation.

The main reason is that the process of desizing and bleaching is longer, and the tension of the fabric is relatively large. When the guide roller is not parallel, the weft is easily caused by uneven force.

In general, corduroy and medium thickness Khaki fabrics are skewed at 2%-4% after scouring and bleaching.

If the weft is too large, the machine must be inspected thoroughly.

Hydrogen peroxide residue

After oxygen bleaching, even if the water is fully washed, a certain amount of hydrogen peroxide will remain on the fabric, which will affect the dyeing of reactive dyes.

At present, there is no specific standard for the allowable value of residual hydrogen peroxide on fabrics.

According to practical experience, the amount of H2O2 residue on fabric is lower than 10mg/kg, and its distribution is uniform. It has no obvious effect on dyeing. When the residue is more than 35mg/kg, the reactive dyes will be dyed lighter and darker.

The residue can be tested with special test paper.

When the fabric is boiled and washed into the dryer, the test paper is pressed on the surface of the fabric to observe the change of the color of the test paper.