Spandex, scientific name is polyurethane fiber, English name is spandex, abbreviation is PU. In 1937, the German Bayer Company successfully developed it; in 1959, the American DuPont Company began industrial production; in 1989, China’s first spandex production factory, the Yantai Spandex Factory, was put into operation. Now, China has become the world’s largest producer of spandex.
Molecular structure of spandex Δ
1. Overview of chemical properties of spandex
Spandex is resistant to reducing agents, hydrogen peroxide bleaching, low concentration acids and alkali, can be dry cleaned (such as tetrachlorethylene, carbon tetrachloride, etc.), not resistant to chlorine and high polarity Organic oil agents (such as dimethylformamide DMF, dimethylacetamide DMA, cyclohexanone, phenol, etc.). Under normal conditions, polyether spandex has good alkali resistance, but poor chlorine resistance and thermal oxidation resistance; polyester spandex has better chlorine resistance and thermal oxidation resistance, but poor alkali resistance.
Spandex is divided according to different molecular structures: a mosaic copolymer of aromatic diisocyanate and polyester segments containing hydroxyl groups (referred to as polyester spandex), which is composed of aromatic diisocyanate and Polyether segment embedded copolymer containing hydroxyl groups (referred to as polyether spandex).
The resistance of spandex to various common chemicals is shown in Δ
2. Chlorine resistance of spandex
As can be seen from the above chart, sodium hypochlorite will severely damage spandex. Therefore, spandex is not resistant to chlorine. That is to say, chlorine, sodium hypochlorite, sodium chlorite and other substances containing available chlorine will cause damage to the strength of spandex.
3. Alkali resistance of spandex
Different from the alkali resistance of cotton fiber, the strength of cotton fiber will increase after high-concentration alkali treatment or mercerization; high-concentration alkaline conditions will reduce the strength of spandex.
For polyether spandex, experiments have shown that it can withstand 40g/L NaOH padding and steaming, but the strength is slightly damaged. However, the damage caused by the strength of spandex treated before dip-dyeing with a large liquor ratio is greater than that of open-width long cars.
Polyether spandex can withstand 25% concentrated NaOH treatment for a short time at room temperature, which means that cotton-spandex fabrics can be mercerized.
4. Acid resistance of spandex
Under acidic conditions, the strength of spandex will also be slightly reduced, and yellowing will occur. But relatively speaking, spandex’s acid resistance is better than alkali resistance. Because the strength of cotton fibers will decrease under acidic conditions, try to use neutral polishing enzymes when polishing cotton/ammonia fabrics or yarns.
5. The expression of spandex in textiles
Spandex usually exists in four forms in textiles: bare yarn, core-spun yarn, covered yarn, and twisted yarn.
Spandex naked silk
The earliest The developed polyurethane elastic fiber variety has good stretch and recovery properties and can be produced without spinning processing, so it has the advantage of low cost. Due to the large friction coefficient and poor sliding properties of bare yarn, it is rarely used directly for weaving fabrics. It is generally suitable for interweaving with other chemical fiber filaments on knitting machines. The main textile products include tights, sportswear, leggings, surgical bandages, stocking cuffs, cuffs, etc.
Core-spun yarn
Bag The core yarn uses spandex yarn as the yarn core and is wrapped with one or several non-elastic short fibers. Yarns with natural fibers as the wrapped fibers have good hygroscopicity. And because of this coating, the core yarn is generally stretched when stretched. It is not exposed, so the comfort is good, and dark-colored products will not affect the same color due to the light spandex color. However, the strength of core-spun yarn is relatively low. Generally, the strength of a single yarn is only equivalent to 80% to 90% of the same specification of outer fiber yarn alone.
It has three significant characteristics: First, it has the same feel and appearance as the outer sheath (cotton, wool, polyester, cotton, etc.), and the outer sheath is composed of natural fibers. It has good hygroscopicity; secondly, it has good elasticity; thirdly, the elasticity percentage can be controlled below 20%.
Compared with other elastic yarns, spandex core-spun yarn has a significant feature that the core yarn is not exposed when the yarn is stretched tightly. Therefore, the dyeing effect is good and it is suitable for making products of various colors including dark colors.
Compared to other types of stretch yarns, the strength of core-spun yarns is relatively low. Under normal circumstances, the strength of a single yarn is only equivalent to 80% to 90% of that of the same specification of outer fiber when it is individually spun into yarn. The first thing to break in the core-spun yarn is the outer fiber, and the strength formed by the outer fiber is the strength of the core-spun yarn. Spandex yarn is a low-strength and high-elongation filament. It does not contribute to the strength of the yarn in the core-spun yarn, but instead destroys the strength between the fibers.Sometimes spandex-containing fabrics will form “cold setting” during long-term storage, causing the fabric to produce permanent wrinkles that cannot be eliminated during post-processing. Therefore, in addition to reducing the storage time of spandex-containing knitted fabrics, it is more important to open the width immediately after weaving and then roll it in an open width to avoid the chance of creasing.
For knitted fabrics containing spandex, the spandex inside the fiber will have a certain amount of internal stress formed during weaving after it is removed from the machine. Therefore, the purpose of preheating and setting is to eliminate stress through the relaxation process of the fabric, fully stabilize the size of the fabric, and eliminate permanent creases in the fabric during post-processing.
If the second loop is to the left of the first loop when forming a loop in cylinder weaving, then the fabric feed direction should also be 7% to 10% (relative width) to the left during preforming , so that the twist is consistent with the finished product.
Predetermined
Therefore, the best pre-shaping plan is: relaxation → steaming (rolling hot water 80℃ without steaming conditions) → Over-feeding and pre-shaping → (enter fabric pre-processing → shaping → printing).
Because most dyeing factories do not have steaming equipment, they directly squeeze water into the shape. When the temperature is low (winter), creases are easy to occur on the fabric. After the fabric is relaxed in the steaming box and then shaped, you will get satisfactory results. The heat setting temperature is preferably between 185 and 190°C, and the setting time is 45 to 60 seconds. If the setting temperature is too high, the fabric will turn yellow.
At the same time, if the impurities and additives on the fabric are partially degraded due to the influence of baking, once the degraded substances migrate to the inside of the fiber and cause stains, these stains will be difficult to remove in subsequent processing. During dyeing, the oil spots inside the fiber will first absorb the dye, but during the heat preservation and dyeing process, they cannot escape in time to form dark spots. Even if the color is stripped and re-dyed, it will still remain on the fabric. Therefore, avoid 195 Preforming at high temperatures above ℃ (note: the softening temperature of spandex is approximately above 200°) will also affect the fastness of spandex. The width during shaping is 10% to 15% wider than the blank, allowing it to fully recover during pre-processing in the dye vat and completely eliminating the internal stress of the fabric.
Pre-treatment
Knitted fabrics that need to be printed also need to go through Pre-treatment removes cotton stains, impurities, grease, wax, etc., increases the whiteness of the fabric and improves the wetting and water-absorbing properties of the fabric, thereby achieving good printing effects.
The pre-treatment process can be divided into two categories, namely traditional alkali treatment and modern biological enzyme treatment. Biological enzyme treatment meets the requirements of environmentally friendly processing, but currently there are still many requirements for fabrics with high whitening or bleaching requirements. It is difficult to meet the requirements, so the conventional alkali treatment process is still widely used. It is very necessary to gradually cool down the water after the pre-treatment of spandex-containing fabrics to prevent the sudden cooling from causing fiber shrinkage and creases.
Requirements for the shaping of semi-finished products
The shaping of semi-finished products must be It is carried out according to the original design process. Generally speaking, the shaping width is about 3% to 5% wider than the finished product width. The meridional shrinkage is controlled within the shrinkage required by the finished product. The elongation of the patch is equal to the enlargement ratio during tracing. . If it is a single-sided knitted fabric, then paddle trimming and optimal twist control are also required to reduce the printing weft skew caused by twist. The width of the pulp edge is about 1cm, and the breaking point is pulp. It should not be too thin or too thick. If it is too thin, it will not work as it should. If it is too thick, it will affect the printing effect near the edge of the cloth. In pigment printing, the solid slurry remaining on the cloth surface will relatively increase the square meter mass of the fabric, so under the conditions of consistent width and shrinkage, the gram mass can be lighter. </p
