1 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 that the scouring is sufficient and there is no “crush” phenomenon; “clean” refers to the pectin on the cellulose fibers after scouring , cotton wax, ash, cotton seed hulls and other impurities are 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.
2 Wool effect
Semi-finished products used for dyeing, and 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.
3 Whiteness
For semi-finished products used for dyeing It is said that 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. Therefore, 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 colored semi-finished products, the difference in whiteness value of the same dyed semi-finished product should be less than 3, and the difference in whiteness value between the left, middle and right sides of the fabric should be less than 1.5.
4 Strength
After boiling and bleaching, cotton fiber is subjected to various The effect of chemical additives has reduced 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%.
5 Weft bias
In fabric printing and dyeing processing, pre-treatment It 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.
6 Hydrogen peroxide residual amount
After oxygen bleaching, even if the fabric If the fabric is washed sufficiently, 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
