The primary purpose of sizing is to produce warp yarns that will weave satisfactorily without suffering any consequential damage due to abrasion with the moving parts of the loom. The other objective, though not very common in modern practice, is to impart special properties to the fabric, such as weight, feel, softness, and handle. However, the aforementioned primary objective is of paramount technical significance and is discussed in detail herein. During the process of weaving, warp yarns are subjected to considerable
tension together with an abrasive action. A warp yarn, during its passage from the weaver’s beam to the fell of the cloth, is subjected to intensive abrasion against the whip roll, drop wires, heddle eyes, adjacent heddles, reed wires, and the picking element, as shown in Fig.1 . The intensity of the abrasive action is especially high for heavy sett fabrics. The warp yarns may break during the process of weaving due to the complex mechanical actions consisting of cyclic extension, abrasion, and bending. To prevent warp yarns from excessive breakage under such weaving conditions, the threads are sized to impart better abrasion resistance and to improve yarn strength. The purpose of sizing is to increase the strength and abrasion resistance of the yarn by encapsulating the yarn with a smooth but tough size film. The coating of the size film around the yarn improves the abrasion resistance and protects the weak places in the yarns from the rigorous actions of the moving loom parts.
Fig 1:- Parts of the loom and major abrasion points.
The functions of the sizing operation are
1. To lay in the protruding fibers in the body of the yarn and to cover weak places by encapsulating the yarn by a protective coating of the size film. The thickness of the size film coating should be optimized. Too thick a coating will be susceptible to easy size shed-off on the loom.
2. To increase the strength of the spun warp yarn without affecting its extensibility. This is achieved by allowing the penetration of the size into the yarn. The size in the yarn matrix will tend to bind all the fibers together, as shown in Fig. 4.18. The increase in strength due to sizing is normally expected to be about 10 to 15% with respect to the strength of the unsized yarn. Excessive penetration of the size liquid into the core of the yarn is not desirable because it affects the flexibility of the yarn.
Fig2 : -Fiber–size binding in a yarn (not to scale).
3. To make a weaver’s beam with the exact number of warp threads ready for weaving.
Fig.3 Schematics showing size distribution; (a) too much penetration, no surface coating; (b) too much penetration, more size added to provide surface coating; (c) too little penetration, no anchoring of yarn structure; (d) optimal distribution.
Figure 3 illustrates various possible conditions that may occur in practice depending upon the properties of the size employed. This emphasizes the importance of an optimal balance between the penetration of the size into the yarn and providing a protective coating around the yarn, as shown in Fig. 3d. The flow properties of the size liquid and the application temperature have important effects on the distribution of the size within the yarn structure. More size at the periphery of the yarn will tend to shed off on the loom under the applied forces because the size is not well anchored on the fibers. Too much penetration, as shown in Fig. 3a, may leave too little size around the yarn surface to protect it against the abrasive action. To rectify such a condition, a higher size add-on is required to provide the required protective surface coating.