PHYSICAL FINISHES AND FINISHING
Physical finishing methods for textiles include optical finishing, brushing and napping, softening, shearing, and compacting of the textile structure.
Optical Finishes
Luster may be imparted to a fabric by physical means. The techniques basically involve flattening or smoothing of the surface yarns using pressure.
Beating of the fabric surface or passing the fabric between hard calendering rolls under pressure and with some friction will tend to flatten out the yarns and lower light scattering by the fabric surface, thereby improving reflectance and luster. Luster may be improved further if the calendering rolls are scribed with closely spaced lines which will be imprinted on the fabric to reinforce light striking and reflecting from the fiber surface. Similar techniques can be used to impart optical light interference patterns to the fabric. Thermoplastic fibers which can deform under heat and press can most readily be modified to impart luster.
Brushing and Napping
Physical delustering of a fabric as well as bulking and lofting of the fabric can be achieved by treatments which roughen the fiber surface or raise fibers to the surface.
Fiber raising processes such as brushing and napping involve use of wires or brushes which catch yarns in the textile structure and pull individual fibers partly from the yarn structure. The resulting fabric is warmer, more comfortable, and softer.
Softening and Shearing
During calendaring or beating of a fabric interaction between individual fibers within yarns may be lessened and the textile structure softened.
Also, when a smooth textile structure free of raised surface fibers or hairiness is desired, the fabric may be sheared by passing the fabric over sharp moving cutting blades or by passing the fabric over a series of small gas jets which singe and burn away raised fibers.
Compacting
During fabric formation processes, stresses often are introduced into a textile. Such stresses can be controlled by drying the finished fabric on a stenter frame, which controls the width of the fabric and the tension on the fabric during the drying process. A second method involves compression of the fabric structure, as in the Sanforizing process. In this process, the fabric and backing blanket (rubber or wool) is fed between a feed roller and a curved braking shoe, with the blanket being under some tension.
The tension on the blanket is released after passing the fabric and blanket between the roller and braking shoe. The net result is compaction of the fabric being carried along in the system. Such a simple technique permits fabrication of the fabric of finished textile goods without fear of excessive shrinkage on laundering. Protein hair fibers such as wool, and thermoplastic fibers such as polyester, can be compacted by felting action. The scale structures on protein fibers entangle and stick on agi tation,particularly in the presence of moisture. The resulting "ratcheting" effect causes the fibers to compact and felt. Many processes for wool take advantage of this effect, and nonwoven felt structures are produced by this method. Compaction of thermoplastic structure occurs when the fibers are raised to near their softening point. At a sufficiently high temperature the fibers shrink and contract, causing compaction of the textile structure.
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