Polyester yarn

50/2 high tenacity polyester yarn for sewing

Quality requirements of polyester yarn dyeing pretreatment

The basic requirements of cotton fabric pretreatment can be summarized as "uniform, transparent, clean and white".

"Uniform" means that the scouring is uniform, and the wool effect of fabrics is basically the same;

"Thorough" means that the cooking is sufficient and there is no "sandwich" phenomenon;

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

"White" refers to the whiteness of the fabric after pretreatment.

"Uniform, transparent, 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 through specific detection methods.

1. Gross effect

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

Because in continuous pad dyeing, the wool effect is poor in the instant of impregnation, which is easy to cause penetration and form surface color or stripe during dyeing.

However, the time of fabric immersion in the dye solution is only a few seconds, and there is no specific index to test the instantaneous wool effect.

Visual inspection method

Generally, visual inspection is used

That is to say, drop the water drops about 15cm away from the fabric with a rubber tip dropper. If the water drops instantly infiltrate on the surface of the fabric and form a uniform circle, it means that the instantaneous wool effect is qualified; on the contrary, if the water drops form into water drops on the surface of the fabric for a short time, it indicates that the instantaneous wool effect is not qualified.

The other is to test the whole width wool effect of the fabric, instead of taking 10 cm from the left, middle and right of the fabric.

The final test line is a straight line or nearly straight line in 30 minutes.

The difference of hair effect between left, middle and right should be controlled within 0.5cm, otherwise, the color difference between left, middle and right is easy to occur.

2. Whiteness

For the semi-finished products used for dyeing, the whiteness is not the better. The whiteness should be determined according to different varieties and colors.

After the normal scouring and bleaching process, the whiteness of the fabric with thicker count and higher density is slightly worse, while that of the fabric with thinner count and thinner density is slightly better. Using oxygen bleaching to improve whiteness is basically at the expense of strength.

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

The whiteness value should be above 75, and the whiteness should not be lower than 80.

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

3. Strength

After scouring and bleaching, the strength of cotton fiber decreases due to the effect of various chemical additives. However, if the process is reasonable, it can be controlled within the allowable range of strength decline.

Due to the poor cotton blending quality of medium and heavy cotton fabrics at present, sometimes the strength of grey cloth can not reach the national standard. Therefore, the strength of grey fabric must be tested before putting into production. The strength drop after boiling and bleaching can be controlled within 5%.

4. Declination

In the dyeing and printing process of fabrics, the pretreatment is one of the processes which can easily cause weft skew. The main reason is that the desizing, scouring and bleaching process is long, and the tension of the fabric is relatively large. Under the condition that the guide rollers of the machine are not absolutely parallel, the fabric is easy to cause weft skew due to uneven force. After scouring and bleaching, the weft deviation of corduroy and medium thickness Khaki fabrics is 2% - 4%. If the weft deviation is too large after boiling and bleaching, the machine must be checked comprehensively.

5. Residual hydrogen peroxide

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

According to practical experience, if the residual hydrogen peroxide content is less than 10mg / kg and evenly distributed, it has no obvious effect on dyeing; when the residual hydrogen peroxide content is more than 35mg / kg, the reactive dyes will be light and dull. The residual amount can be tested with special test paper. After the fabric is washed by boiling and bleaching water and before entering the dryer, the test paper shall be closely attached to the surface of the fabric to observe the color change of the test paper.