Denim has been popular from its birth till now, and the competition is getting more and more intense globally. How to apply new technology and new process to improve the quality of denim is a great concern for denim manufacturers. Due to the limitation of the characteristics of denim itself, it is necessary to adopt the process of large tension and strong weft beating in the weaving process, and the loss of strength in the process of dyeing the warp yarns should be compensated by sizing, therefore, sizing of the warp yarns is an indispensable part of the production process of denim fabrics. At the same time, sizing in compensation for the strength of the warp yarn at the same time, can also be covered by the yarn surface hair, is conducive to the smooth progress of the weaving process. The effect of sizing depends on the selection of sizing material and reasonable sizing process.
Table of Contents
The Significance Of Sizing Material Selection
Types of Sizing Materials
Traditional woven fabric warp sizing used in the paste mainly starch, vinyl (PV) and acrylic three major types of paste.
- Original starch pulp The original starch is mainly produced from wheat, corn and other crops, its pulp on cotton, hemp and other cellulose fibers have good adhesion properties, in the existing pulp application is also the most. However, the pulp film is brittle and hard, the elongation is small, the viscosity of the slurry is not stable enough, the slurry yarn has a rough feel, and the adhesion performance to synthetic fibers is poor, so the use is limited to a certain extent.
- Modified starch slurry Modified starch is a kind of slurry made by denaturing the original starch through physical, chemical or biological methods. After the original starch is acidified, oxidized, cross-linked, esterified and etherified, it improves the flow properties and the adhesion properties to hydrophobic fibers to meet the requirements of the development of textile products and sizing process. Deformed starch has a simple denaturation method, low cost, wide range of adaptability, in the country is also applied to pure cotton fabric warp sizing raw materials.
- Polyvinyl alcohol (PVA) Polyvinyl alcohol is a water-soluble synthetic adhesive, it has good adhesion to a variety of fibers, slurry viscosity stability, not easy to deteriorate, slurry film strength, abrasion resistance and flexural strength than other slurry, so it is widely used in polyester/cotton blended fabrics and cotton fine number of high-density fabrics warping sizing, is a widely adaptable general-purpose slurry.
- Acrylic paste Acrylic paste compared with other pastes, its biggest advantage is excellent adhesion to hydrophobic fibers, and good water solubility, easy to desizing, not easy to skin, small environmental pollution. However, its hygroscopicity and re-adhesion is strong, in the workshop when the humidity is high, the yarn is easy to bond with each other, resulting in unclear openings, affecting the weaving efficiency, so it can only be used as an auxiliary slurry.
- Combined slurry Combined slurry is also known as ready-to-use slurry or disposable slurry. Combination of slurry components are usually modified starch and PVA or modified PVA as the main body, supplemented by other slurries, adhesives, emulsifiers and so on. It can be used directly in the sizing process, with the advantages of stable quality, easy to use and accurate dosage, widely welcomed by denim manufacturers.
Sizing Solution Requirements
The slurry should have good permeability and coating properties. So that the color yarn can withstand high tension stretching and reed friction during the weaving process. The slurry film formed in the slurry has good elasticity to increase its elongation; the slurry film is soft and tough to reduce the breakage of color yarns; the slurry film has high transparency, so that the denim weaved is bright in color and good in appearance. The slurry should have high concentration and low viscosity, and a certain degree of stability, to ensure that the slurry enough penetration ability, so that the sizing is uniform, to reduce the uneven sizing caused by the color difference. Generally, the viscosity of high concentration and low viscosity modified starch is controlled at about 0.05Pa-s, the temperature is 95℃, and the solid content of the slurry is controlled at 9%~10% to maintain the elasticity and elongation of the yarn.
Slurry Formulation
- Main slurry material and proportioning Because in the process of denim sizing and dyeing, the slurry is required to have the characteristics of high concentration and low viscosity, modified starch is often used instead of common starch, and acrylic slurry is used instead of PVA slurry. After the color yarn is sized, the handfeel is soft and will not be brittle in the process of weaving, and the film is tough and elastic at the same time. The ratio of modified starch and acrylic paste is 7:3~9:1, and at the same time, according to the specific requirements, the appropriate amount of composite paste is added. Specific ratios depending on the production situation, such as: thicker yarn color yarn can be appropriate to increase the proportion of acrylic slurry in the main slurry.
- Auxiliaries Auxiliaries are gum powder, denaturant, catalyst, emulsifying oil, penetrant, wax flakes, caustic soda and so on.
Slurry Preparation
When making raw pulp, calculate the net weight, solids content and steam water index. Before cooking pulp should add enough water, so that the pulp is completely dissolved, and does not constitute a pulp spot. When mixing pulp, we should grasp the dry basis weight of each pulp, swelling degree and steam water index. Strictly in accordance with the pulp formula to calculate the amount of material and water. Pour in the material, stir evenly, and open the steam cooking pulp. Cooked slurry mixing time should not be too long, otherwise the slurry hydrolysis, so that the viscosity is reduced and lead to light slurry. The cooked slurry has been adjusted can not be added starch or gum powder to prevent the undissolved gum powder to stick to the warp yarn, affecting the smooth progress of the weaving process.
Optimizing the Warp Sizing Process
Roller Pressure Adjustments
Due to the poor sizing performance of color yarn, it is necessary to use higher pressure sizing process. Because the higher rolling pressure can not only save steam, to adapt to the needs of high-speed sizing, but also to ensure the integrity of the film, so that the hair can be appressed to the color yarn, but also can be appropriate to reduce the rate of sizing, to ensure that the weaving of the smooth progress. Pressure control in 25 ~ 30kN, slow rolling pressure for the normal rolling pressure of about 20% if the pressure roller pressure is too small, in the stop, the pulp tank surface cooling of the pulp will be left on the surface of the color yarn white marks and affect the quality of production.
Sizing Speed
Because denim sizing and dyeing process, in order to improve the yield, the speed will inevitably be increased, and denim is a heavy type of fabric, too high a speed so that the yarn in the pulp drying room stay time becomes shorter, the color yarn sizing is difficult to dry, to meet the requirements of the moisture rate, and more detrimental to the separation of stranded. At the same time, the high pressure of the sizing roller and high speed will increase the tension of the yarn, so the speed should not be too high. In addition, the same variety of yarn sizing and dyeing process, the stopping time should not be too long, which may make part of the cooled pulp solidification, resulting in poor circulation.
Application Rate
Referring to the standard of sizing rate of coarse special yarn card fabrics, considering the need of heavy denim fabrics with large tension weaving, the sizing rate of warp yarns is 10%~12% when adopting starch-based mixed sizing. On the basis of good sizing, appropriate reduction of sizing rate is conducive to the yarn’s elongation and flexibility.
Tension and Elongation Control
Control of sizing yarn elongation and sectional tension. Ball warp dyeing process of the warp color yarn after ball warp, dyeing, re- warping three processes, the elongation of the color yarn up to 1.8%-2%. In order to ensure that the color yarn sizing elasticity, sizing should maximize the control of elongation, generally in the actual production of strict control below 0.5%. Reduce sizing elongation is mainly to control the yarn wet state tension, control sizing machine on the segment tension. Production practice has proved that it is related to the strength of the yarn piece, the number of special warp yarns and the total number of warp roots.
Drying Temperature Settings
Lower drying temperature is conducive to the pulp film intact, therefore, the drying cylinder temperature should not exceed 130 ℃.
Others
When adjusting the slurry, you can add an appropriate amount of glue in the slurry to improve the fastness of the film and the hairiness of the coated properties, increase the friction resistance of the yarn, but also reduce pilling, so that the opening is clear and the fabric surface is smooth after weaving. Friction resistance of the yarn, but also to reduce pilling, so that the opening is clear and the fabric surface is smooth after weaving.
Addressing Common Sizing Challenges
Issues of Uneven Sizing
In the process of sizing, low viscosity of slurry, long storage time of slurry, insufficiently high temperature of the slurry tank, and slurry blistering, etc., will cause light slurry. Light sizing will affect the loom tension, thick sizing will affect the color of the cloth surface, and make the hand feel hard. Rolling pressure is too large, too fast in the sizing process, uneven pulp concentration or circulating obstruction, etc. will cause light slurry, causing certain difficulties to the weaving. Pulping roller pressure is too small, short yarn caused by stopping or slowing down the speed of the car, so that the pulp tank surface of the pulp cooling, in the surface of the piece of yarn will form a section of the obvious white marks. Parking time is too long, poor winding, pulp tank pipe clogging, poor circulation pump circulation, etc., will form thick pulp. It can be solved by adjusting the speed of the car, the pressure of the rollers, controlling the viscosity of the slurry, and stirring the slurry.
Sizing Streaks
The quality of sizing will affect the color and feel of the cloth. The first factor affecting the sizing quality is the viscosity and concentration of the slurry, followed by the sizing speed, pressure and drying temperature. If the sizing operation is not properly controlled, the solidified small pieces in the slurry or not completely dissolved slurry will cause the formation of slurry spots, which seriously affects the weaving quality of denim. If the temperature of the sizing tank is low, the pulp on the surface of the slurry will solidify and form lumps or crusts, and the pressure of the sizing roller is too low or the surface of the sizing roller is coiled back on the silk will also produce sizing spots. Sizing rate should be based on the actual production requirements of the process. The main thing that affects the stability of sizing rate is the driving speed. Before each batch of yarn starts sizing, the speed should be adjusted according to the total feeding speed and the amount of sizing material used. If the speed is reduced in the middle, it will definitely affect the sizing rate of color yarn. At the same time can also be adjusted by the method of rolling pressure to reduce the sizing machine high and low speed sizing rate gap, so that the sizing rate to maintain stability.
Color Consistency
Color yarn after sizing, in the yarn surface to form a film, the transparency of different sizing film is different, even if the use of high transparency of tapioca starch will have a certain effect on the contrast. With high sizing rate, the color yarn is glaucous gray; with low sizing rate, the color yarn is reddish blue. Using modified starch and keeping the stability of sizing formula and sizing process can stabilize the color after sizing. After sizing the color yarn in the pulp drying cylinder to stay too long will cause the dye on the dyeing of the permanent shift and the formation of color differences. Therefore, in the production process should minimize stopping and maintain a stable speed.
Others
Improper operation or improper sizing process and formula will make the color yarn in the sizing process produce sizing stripe flower, and head, twisted head, hair and other defects. Can be solved by selecting good quality and transparency of the pulp, reduce the viscosity of the slurry, adjust the slurry temperature method.
Disposal of residual slurry
If the residual pulp after sizing of each variety of color yarn is not handled well, it will cause waste and increase production costs. The residual pulp can be reused after a short period of time, but it must be used only after removing the appropriate amount of water in the residual night, and the residual liquid that has been reused for many times cannot be used again. Residual liquid reuse process is more troublesome, so each variety of color yarn production should be based on the process before the calculation of the amount of pulp, in order to reduce the waste of residual liquid.
Conclusion
As denim is color yarn sizing, different from the ordinary warp sizing process, has its own special characteristics, so it is necessary to develop a special process to improve product quality. In the actual production process, according to the use of machinery and equipment to develop a reasonable process, and to ensure a stable and orderly production environment. Improve the technical level of operators, regular maintenance of the machine can ensure the quality of production. At the same time should use environmentally friendly paste, to avoid pollution and waste, in order to adapt to the contemporary requirements of energy saving and environmental protection.
