Pure cotton fabrics' four keys to the pretreatment

Cotton knitting off-white, whitening is a type of dyeing with more varieties processed in the dyeing factory, and it is also an important link in reactive dyeing. If the scouring and bleaching is not done well, the time will be wasted later, then, what are the requirements for scouring and bleaching?

Quality requirements for pre-treatment of cotton printing and dyeing

The basic requirements for the pretreatment of pure cotton fabrics can be summarized as "even, transparent, clean and white".

"Even" means that the scouring is even, and the wool effect between the fabric pieces is basically the same;

"Through" means sufficient cooking and no "pinning" phenomenon;

"Clean" means that impurities such as pectin, cotton wax, ash, and cotton seed hulls on the cellulose fiber are fully removed after scouring;

"White" means that the fabric has the whiteness required for dyeing after pretreatment.

"Even, transparent, clean, and white" is only a general requirement for the quality of semi-products. In actual production, it is necessary to control the quality of semi-products through specific testing methods.

Gross effect

For dyeing semi-finished products, the higher the wool effect, the better, the wool effect of pure cotton fabric is 10cm/30min. Two points should be paid attention to. One is to determine the instantaneous gross effect of the semi-finished product.

Because in continuous pad dyeing, the instantaneous wool effect of fabric impregnation is poor, and it is easy to cause impermeability during dyeing, resulting in surface color or color streaks.

The time for the fabric to be immersed in the dye solution is only a few seconds, and there is no specific index to test the instantaneous wool effect.

Visual method

The method of visual inspection is generally adopted:

That is, use a glue-tip dropper to drop the water droplets about 15cm away from the fabric. If the water droplets instantly infiltrate on the surface of the fabric and form a uniform circle, the instantaneous hair effect is qualified; on the contrary, if the water droplets on the surface of the fabric become water droplets for a short time, Even a short period of time indicates that the instantaneous gross effect is unqualified.

The second is to test the overall wool effect of the fabric, instead of taking 10cm from the left, middle and right sides of the fabric for testing.

The entire gross effect test requires 30 minutes, and the final test line is in a straight line, or basically close to a straight line.

The difference between left, middle and right hair effects should be controlled within 0.5cm, otherwise it is easy to produce left, middle and right color difference during dyeing.

White degree

For the semi-products used for dyeing, it is not that the whiter the whiter the better, the whiteness required should be determined according to the different varieties and the different dyed colors.

After the normal scouring and bleaching process, the whiteness of the coarser yarn count and denser varieties is slightly worse; while the finer yarn count and lighter density fabrics have better whiteness. The use of oxygen bleaching to improve whiteness basically comes at the expense of strength.

Therefore, when formulating the actual production process, the strength reduction of the fabric should be fully considered.

The whiteness value should generally be above 75, for dyeing reactive light colors, the whiteness value should not be lower than 80.

In addition, the whiteness is also required to be uniform. For medium and light dyed semi-products, the whiteness value difference of the same dyed semi-product should be less than 3, and the whiteness value difference of the left, middle and right fabrics should be less than 1.5.

Strong

After scouring and bleaching, the cotton fiber is affected by various chemical auxiliaries, and its strength is reduced. However, if the process is formulated reasonably, it can be controlled within the allowable range of strength reduction.

Due to the poor quality of cotton fabrics for medium and thick cotton fabrics, and sometimes the strength of the grey fabrics does not meet the national standards, the strength of the grey fabrics must be tested before being put into production, and the strength reduction after scouring and bleaching can generally be controlled within 5%.

Weft skew

In the fabric printing and dyeing process, pre-treatment is one of the processes that is easier to cause weft skew. The main reason is that the process of desiccation, scouring and bleaching is longer, and the tension on the fabric is relatively large. When the fabric guide rollers of the machine are not absolutely parallel, the fabric is prone to weft skew due to uneven force. Generally, corduroy and medium-thick khaki fabrics have a weft skew of 2%-4% after cooking and bleaching. If the weft skew is too large after cooking and bleaching, a comprehensive inspection of the machine must be carried out.

Residual hydrogen peroxide

After the fabric has been bleached by oxygen, even if it is fully washed, a certain amount of hydrogen peroxide will remain on the fabric, which has an impact on the dyeing of reactive dyes. In severe cases, it will cause partial rejection and form white spots.

According to practical experience, if the residual amount of hydrogen peroxide on the fabric is less than 10mg/kg, and the distribution is uniform, it has no obvious effect on the dyeing; when the residual amount is greater than 35mg/kg, the reactive dye will be dyed lightly and the shade will be dark. The residual amount can be tested with special test paper. After the fabric is boiled, rinsed and washed before entering the dryer, put the test paper close to the surface of the fabric and observe the color change of the test paper.