100% cotton knitted off-white, whitening is a type of dyeing that has many varieties in dyeing factories. It is also an important link in reactive dyeing. If the boiling and bleaching is not done well, the subsequent time will be wasted. So, what are the requirements for boiling and bleaching?
Quality requirements for pre-treatment of printing and dyeing
The basic requirements for pre-treatment 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 pieces of fabric is basically the same;
“Transparent” means It means that the scouring is sufficient and there is no “cold” phenomenon;
“Clean” means that impurities such as pectin, cotton wax, ash, and cottonseed hulls on the cellulose fibers after scouring have been removed. Fully removed;
“White” means that after pre-treatment, the fabric has the whiteness required for dyeing.
“Even, transparent, clean and white” is only a general requirement for the quality of semi-finished products. In actual production, specific testing methods are needed to control the quality of semi-finished products.
1. Wool effect
Semi-finished products used for dyeing, It is not that the higher the wool efficiency, the better. The wool efficiency of pure cotton fabrics is 10cm/30min. Two points should be noted. One is to measure the instantaneous gross effect of the semi-finished product.
Because during continuous pad dyeing, the instantaneous wool effect of fabric impregnation is poor, which can easily cause impregnation during dyeing, resulting in surface discoloration or color stripes.
The fabric is immersed in the dye solution for only a few seconds, and there is no specific indicator for testing the instantaneous wool effect.
Visual inspection method
Generally, the visual inspection method is used:
Ready to use Use the dropper to drop the water droplets about 15cm away from the fabric. If the water droplets instantly penetrate the surface of the fabric and become uniformly round, the instantaneous wool effect is qualified; conversely, if the water droplets briefly form droplets on the surface of the fabric, even if they are very A short time also indicates that the instantaneous gross effect is unqualified.
The second is to test the wool effect of the entire fabric, instead of taking 10cm from the left, middle and right sides of the fabric for testing.
The test of the entire wool effect requires that the final test line for 30 minutes should be a straight line, or basically close to a straight line.
The difference between the left, middle and right hair effects should be controlled within 0.5cm, otherwise it is easy to produce color difference between the left, middle and right during dyeing.
2. Whiteness
For semi-finished products used for dyeing Generally speaking, the whiter the whiter, the better. The required whiteness should be determined according to different varieties and dyeing colors.
After normal boiling and bleaching process, the whiteness of fabrics with thicker yarn count and higher density will be slightly worse; while the whiteness of fabrics with finer yarn count and thinner density will be slightly better. . Using oxygen bleach to improve whiteness basically comes at the expense of strength.
So when formulating the actual production process, the strength reduction of the fabric must be fully considered.
The whiteness value should generally be above 75. For dyeing reactive light colors, the whiteness value should not be less than 80.
In addition, the whiteness is also required to be uniform. For medium and light dyed semi-finished products, the difference in whiteness value of the same dyed semi-finished product should be less than 3. The left, middle and right parts of the fabric are white. The degree difference is less than 1.5.
3. Strength
After boiling and bleaching, cotton fibers are subjected to various The effect of this kind of chemical additives has reduced the strength, but if the process is properly formulated, it can be controlled within the allowable range of strength reduction.
Due to the poor cotton quality of the current pure cotton medium-thick fabrics, sometimes the strength of the gray fabric does not meet the national standards. Therefore, the strength of the gray fabric must be tested before being put into production. The final strength drop can generally be controlled within 5%.
4. Weft bias
In fabric printing and dyeing processing, the front Processing is one of the processes that is more likely to cause weft skew. The main reason is that the deboiling and bleaching process is long and the tension on the fabric is relatively large. When the machine’s cloth guide rollers are not absolutely parallel, the fabric is prone to skew due to uneven stress. Generally, the weft bias of corduroy and medium-thick khaki fabrics after boiling and bleaching is 2%-4%. If the weft skew is too large after cooking and bleaching, the machine must be fully inspected.
5. Residual amount of hydrogen peroxide
After oxygen bleaching of the fabric, Even if the fabric is washed thoroughly, a certain amount of hydrogen peroxide will remain on the fabric, which will affect the dyeing of reactive dyes. In severe cases, it will cause local dye 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 is evenly distributed, it will have no obvious impact on dyeing; when the residual amount is greater than 35mg/kg, it will cause Reactive dyes produce light colors and faded colors. The residual amount can be tested with special test paper. After the fabric is boiled, rinsed and washed before entering the dryer, place the test paper close to the surface of the fabric and observe the color change of the test paper. </p
