c c The finishing operations performed on fabrics include all the operations carried out to provide a fabric with those properties that the customer desires after the fabrics leave the textile factories. With the modern textile finishing operations that are applied today, it is possible to provide cotton fiber with a structure similar to the superior properties of synthetic fibers. One of the finishing operations that change the physical and characteristic properties of cotton fiber is mercerization. Mercerization is a process in which textiles (typically cotton) are treated with a caustic (NaOH) solution to improve properties such as fiber strength, shrinkage resistance, luster, and dye affinity. The caustic actually rearranges the cellulose molecules in the fiber to produce these changes. Higher-end fabrics may be double or triple mercerized for added benefits. Effective Mercerization requires attention to variables such as caustic strength, dwell time (feed rate), temperature, and neutralization. The feed rate of the fabric may also be limited by its strength and weight and is usually run at 80 to 120 yards (73 to 110 m) per minute.
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% D ) % * % + D ) p p ? There are THREE main methods. 1-Chain Mercerizing 2-Chain less Mercerizing 3-Slack Mercerizing 1-CHAIN MERCERIZATION: In order to make up for the shortenings of the roller mercerizing machine,a clip stenter is used for post-mercerization treatment, in which a widthwisetension is applied then most alkali is showered off the fabric kept by throughalkali removal and neutralization using an openwidth washing machine. 2-CHAINLESS MERCERIZATION: This method of mercerization running fabric through a number of rollerswithout the use of a clip stenter is also called roller mercerization. The machinehas a number of stainless rollers, or stainless and rubber rollers of a relativelylarge diameter tiered zigzag in close content to each other inside a long troughwith the lower tier designed to submerge in alkaline solution for mercerization.
3-SLACK MERCERIZATION: Slack mercerizing is done by exhaust. The fabric is slacked and treatedwith NaOH solution by this process of luster is obtained but abs orption isincreased.Slack mercerizing = Loose mercerizing or mercerizing without tension. c c c c c 1. Temperature: Temperature has a great effect on swelling. Optimum swelling takes place at around 10 - 15°C. However, the swelling speed of fiber at low temperatures falls due to the high viscosity of caustic soda. The heat that emerges during the reaction prevents swelling and therefore, solution is cooled in order to achieve the best mercerization effect. The temperature of the solution must be uniform. Fig 2:
2. Concentration: The gloss that is obtained as a result of the mercerization processes applied on the same type of fabric with caustic soda at various concentrations at 8 and 18°C is shown in Figure 2. As seen in the Figure 2, the best gloss is obtained with solution of 27 ʹ 30°B.
? ? ? ? ? ? ? ? ? ? 3. Time period: It has been determined that when a scoured fabric is dipped into the mercerization solution, the time required for complete swelling of fibers is 60 s. However this period can be reduced with the aid of mechanical effects, as is in the mercerization machines. In a good mercerization machine without chain, the time period required for the full impregnation of fibres with the caustic soda solution is 12 s for dry fabrics and 20 s for wet fabrics. 4. DRYING: In comparison to untreated fiber, absorption of dyestuff is twice as high after mercerization, and 2.4 times as high after tensionless mercerization. Furthermore, the absorption of dyestuff is
reduced by one third after natural drying in air, and by nearly one half after drying at 110°C in comparison with non-dried fiber, which suggests the contraction of the air gaps inside the fiber.
| ? ? ? ? ? ? ? ? ? 5. Tension: The luster of cotton fibers is decided by the ratio between the long and short axes of the cross section of the cellulose hair, and it improves as the cross section becomes more circular. On applying tension, the alignment of cellulose chainsis proper and uniform. Also, rounder fibers have less surface area. As the uniformity increases and surface area decreases, there is better reflection and less scattering of light and consequently, luster increases. Also, fiber shrinkage in width and length-wise direction is inversely proportional to the tension applied.
c c c 4 sections: D Mercerizing Section : Impregnation with Caustic D Intermediate squeezing aggregate: Divides mercerizing section and stabilizing section. Here itself the required time factor is taken care of to allow the Caustic to penetrate and mercerizing to take place. D Stabilizing Section: Water treatment and thinning down concentration of the lye. At this zone, structural modifications will take place.
D Intermediate squeezing aggregate: Divides stabilizing section and washing section. D Washing section: Washing off the alkali and neutralization. CHAINLESS MERCERIZING MACHINE: D Running fabric through a number of rollers without the use of a clip stenter. Also called roller mercerization. D The machine used has a number of stainless rollers, or stainless and rubber rollers, of a relatively-large diameter tiered zigzag in close contact to each other inside a long trough, with the lower tier designed to submerge in alkaline solution for mercerization. D The absorption of alkaline solution and fabric swelling take place as fabric sequentially glides through the surface of these rollers, and, although this movement from roller to roller in close contact with them reduces the widthwise contraction to a minimum, the resulting fabric expansion remains within a limited range, thus displaying the mechanism of mercerization at fixed length. D A similar device is used for the removal of most alkali following this initial stage of alkali penetration and fabric swelling, and an open-width soaping machine for further removal and neutralization. D Therefore, the machinery required is extremely concise and the cost is low, in comparison with the chain mercerization. However, this method is subject to a considerable number of constraints due to inflexible widthwise control over fabric depending on the kind and use. CHAIN MERCERIZING MACHINE:
D In order to make up for the shortcomings of the roller mercerizing machine, a clip stenter is used for post-mercerization treatment, in which a widthwise tension is applied. D The critical alkali content is removed from the fabric during the passage of the fabric through the stenter, followed by thorough alkali removal and neutralization using an open-width washing machine. D In practice, a heavy padding mangle is used for the application of alkaline solution using a 2 dip, 2 nip mangle, while allowing the fabric to have sufficient time for penetration and swelling of the cellulose in a timing cylinder, instead of undergoing an operation using so many rollers and so much solution as in roller/chainless mercerization, to ensure reduced use of the alkali. D Since the chain mercerizing machine operates at an extremely high speed of 120-200m/min, a clip stenter is commonly used after two consecutive treatments of alkali application/penetration. D It is a device of considerable size, capable of holding, while maintaining a widthwise tension, 70-90m fabric at a speed of 120m/min., or 117-150m at 200/m, so that sufficient time is allowed, approximately 35 seconds for polyester/cotton blends, and 45 seconds for 100% cotton, between the initial application of alkali solution and the subsequent start of showering the alkali of the fabric.
