Types and Identification Methods of High-Quality Denim Fabrics

　　1. Authentic denim uses indigo dye, with the fabric showing no discoloration or streaking. After stonewashing, it has a bright and vivid feel, with a slight reddish glow, and the pocket fabric shows no significant staining.

　　2. After stonewashing, the fabric feels soft and full, with a fuzzy texture. The stitching and edges are faded, while the flat areas show no abrasion marks, and the color is uniform and consistent.

　　3. Double stitching is used, with chain stitching at the waist, and 215/3 pure polyester thread is used, making the stitching strong and resistant to unraveling.

　　4. Metal hooks, buttons, and rivets are intact, with no wear on the coating. The back is reinforced with a backing fabric to prevent cracking or damage.

　　Ring-spun denim

　　With the development and application of new technologies such as high-speed ring spinning, large-package winding, fine-count yarn, and knotless yarn, the shortcomings of coarse-count yarn, such as short spinning length, low production efficiency, and many knots, have been solved. The situation where denim yarn is replaced by air-jet yarn is rapidly changing, and ring-spun yarn is making a strong comeback. Due to some superior properties of ring-spun denim, such as hand feel, drape, and tear strength, and also due to people's psychological return to nature and pursuit of the original denim style, and more importantly, because ring-spun denim garments, after washing and processing, will show a hazy bamboo-knot style, which meets the personalized needs of today's denim wear. In addition, bamboo-knot yarn denim is currently very popular in the market, and ring-spun bamboo-knot yarn can produce shorter and denser bamboo knots, which also promotes the development of ring-spun denim.

　　Bamboo-knot denim

　　When different yarn counts, different bamboo-knot thicknesses (compared to the base yarn), bamboo-knot lengths, and spacings are used in the design, using single warp or single weft, and both warp and weft are equipped with bamboo-knot yarn, with appropriate proportions and arrangements with the same or different counts of normal yarn, various bamboo-knot denim can be produced. After garment washing and processing, various hazy or clearer striped styles of denim can be formed, which are welcomed by consumers with personalized needs. Early bamboo-knot denim was almost all made with ring-spun bamboo-knot yarn, because it can produce yarn with shorter length, smaller spacing, and relatively high density of bamboo knots, which is easy to form a denser dotted effect on the fabric surface, and mainly uses warp bamboo knots. With the development of market consumer demand, warp and weft double-sided bamboo-knot denim is currently popular, especially double-sided bamboo-knot denim products with weft elasticity, which are very popular in both domestic and international markets. For some varieties, as long as the fabric structure is well designed, the warp can use a single variety of ring-spun yarn, and the weft can use an appropriate proportion of bamboo-knot yarn, which can also achieve the effect of warp and weft double-sided bamboo knots.

　　Weft-elastic denim

　　The use of spandex elastic yarn has brought denim to a new field, making denim clothing both close-fitting and comfortable. Combined with bamboo knots or different colors, denim products are more suitable for fashion and personalized consumer needs, thus having great development potential. Currently, most elastic denim is weft-elastic, with elasticity generally ranging from 20% to 40%. The magnitude of elasticity depends on the fabric structure design. The looser the warp and weft density on the loom, the greater the elasticity. Conversely, under the condition of fixed warp density, the tighter the weft elastic yarn, the smaller the elasticity. When the weft density reaches a certain level, it may even lose its elasticity. In addition, a prominent problem with current elastic denim finished fabrics is excessive weft shrinkage, generally more than 10%, and some even as high as more than 20%. The unstable fabric width brings great difficulties to garment production. One solution is not to make the elasticity too large in product design, generally taking 20% to 30%, that is, maintaining a certain warp and weft density, and using a method of increasing tension appropriately during preshrinking finishing to make the fabric width have a larger shrinkage, thus obtaining a lower residual shrinkage rate in the weft direction of the finished fabric; another solution is to perform heat setting treatment after the elastic denim is preshrunk, which can obtain a more uniform and consistent fabric width and a more stable and lower weft shrinkage rate, meeting the requirements of garment processing production.

　　Special color denim

　　Because garments made of super indigo dyed or extra dark indigo dyed denim can obtain a special effect of vivid and bright color after washing and processing, they are widely welcomed by consumers.

　　“Super indigo” dyed denim has two main characteristics: extremely deep dyeing and excellent wash fastness. The former refers to the amount of indigo dye dyed on the yarn per unit weight (generally expressed as the percentage of dye to the dry weight of the yarn, referred to as dyeing depth%) is particularly large. For example, the indigo dyeing depth of conventional denim warp yarns is between 1% and 3%, while the “super indigo” dyeing depth needs to reach more than 4% to be called super indigo or extra dark indigo. The latter refers to the fact that “super indigo” dyed denim needs to withstand repeated washing for more than 3 hours, and its color can still reach or exceed the color depth of conventional dyed denim before washing, and its color is much more vivid and bright than conventional dyed denim. For the wash fastness of indigo dyed denim, the essence is the degree to which the dye penetrates the yarn, not the wash fastness of the dye itself (the wet rubbing fastness of indigo is only grade 1), that is, the better the penetration, the better the wash fastness.

　　In the past, the so-called “indigo dyeing rapid washing process” actually intentionally made the penetration of indigo dye into the fiber very shallow during yarn dyeing. In this way, when the denim garment is washed and processed, the extremely thin layer of dye on the surface of the yarn is rubbed off, revealing more white yarn core, making the color fade quickly, thus achieving the effect of immediate fading after a short time of washing. However, the “super indigo” dyeing process is the opposite, requiring the dye penetration to be particularly good, so that the denim garment can obtain a deep, rich, and bright color after washing and processing.

　　Because the dyeing depth of "super indigo" dyed denim products is more than 60% higher than that of conventional traditional denim dyeing, the indigo concentration of the dye liquor will also increase exponentially, even reaching 3-4g/L, to obtain a deeper color. This increases the viscosity of the dye liquor and reduces its fluidity, affecting the penetration ability of the dye chromophores, reducing the wash fastness of the denim, and failing to meet the final depth requirements of garment production. Therefore, some enterprises adopt the design of increasing the dyeing depth again, which further increases the indigo concentration of the dye liquor, and the penetration performance becomes worse. This back-and-forth forms a vicious cycle, still failing to meet the requirements of "super indigo" color. The higher the indigo concentration in the dye liquor, the heavier the red light of the dyed material, and the darker the color, which does not have the effect of "super indigo". Therefore, many manufacturers are currently or are preparing to transform their dyeing and sizing equipment to solve this problem by increasing the number of dyeing passes. For example, increasing the number of dyeing passes to 8 or even 10 not only increases investment costs and the consumption of dyes and chemicals, but also increases the difficulty of operation and environmental pollution. A better way to solve this contradiction is to appropriately reduce the proportion of insurance powder or caustic soda, especially the control of caustic soda, so that the pH value of the dye liquor is stable between 11 and 12, the dyeing rate is the highest and the color is stable, and at the same time, the dyeing tension of the warp yarn is appropriately reduced, so as to obtain a better "super indigo" dyeing effect.

　　Indigo-based color-matched denim

　　In order to increase the color and gloss variations of indigo denim varieties, various color-matched denim varieties are currently very popular. For example, indigo overdyed sulfur black, indigo overdyed sulfur green, sulfur black green, sulfur blue, etc., meet the personalized needs of the market. It also allows denim factories to have their own patented and characteristic new denim varieties to enhance market competitiveness. In this aspect, it is necessary to control the concentration of the mother liquor as much as possible to prevent excessive overflow of the dye liquor, which will cause dye waste and increase environmental pollution.

　　Colorful (various colors) denim

　　Mainly includes bromoindigo (commonly known as turquoise blue in the market) denim and sulfur black denim, as well as coffee, turquoise green, gray, khaki, and sulfur blue denim using sulfur dyes for color matching, and a small amount of bright red, peach red, and concubine color denim dyed with naphthol dyes or reactive dyes, etc. Although the production batch is not large, the market demand is urgent, and often the needs cannot be met. The main problems are that the color and gloss are not stable enough, and the dyeing fastness is also poor, which is unsatisfactory to garment factories. This is certainly related to the small production batch and the large number of colors, but the difficulty of producing various colors by using a combined dyeing and sizing machine is relatively large, with high consumption, high cost, and difficult wastewater treatment, which also has a certain impact. The solution is to reduce the number of color matching as much as possible during design, using two-color matching as much as possible, and not exceeding three-color matching at most, or using other more stable dye varieties to replace them, so as to adapt to the production characteristics of the combined dyeing and sizing machine and obtain more stable dyeing effects; the more thorough solution is to use the production process of large-capacity yarn dyeing and separate warping in color-woven factories to produce various colors of denim, which is more ideal.