Warping absorption

With disk diameters above 5.25 in., all parameters, eg, water absorption and thermal expansion, become more critical which aggravates the expansion or warp of disks. If in the future disk rotation speeds have to be increased significantly to boost data transfer rates, higher demands will be placed on warp (tilt angle) and modulus to avoid creeping (ie, irreversible elongation in radial direction). A survey of the requirement profile for the substrate material of optical disks is given in Table 5 (182,186,187,189). 

Poly(methyl methacrylate). PMMA offers distinct advantages over BPA-PC with respect to significandy lower birefringence, higher modulus, and lower costs, but has not been successhil as a material for audio CDs and CD-ROM as well as a substrate material for WORM and EOD disks because of its high water absorption (which makes it prone to warp) and its unsuitabiUty for metallising, and less so because of its low resistance to... 

An extremely useful technique for measuring the amount of size applied is non-contact on-line determination of water absorption [ 194]. The moisture content of sized warps can be derived from microwave absorption by the water present. 

This leaves PMMA and PC as the contenders. The disc design is asymmetric, with an impermeable aluminium coating on one side, so dimensional changes caused by water diffusing into the polymer make the disc bow. The water absorption of PMMA at saturation relative humidity is 2.1wt.%, compared with 0.4% for PC. The diffusion constants at 23 °C for water are 0.5 x 10 and 4.8 x 10 mm s , respectively. The CD surface warping must be less than 0.6°, for the laser spot to focus properly. This rules out a PMMA disc with one side sealed, because it would expand over a period of tens of days as water diffuses through the 1mm thick... 

A wicking test is used to determine the absorptive velocity of water in fabrics. It is known as the capillary rise test (DIN EN 53924). With respect to time, the capillary rise of absorptive fabrics in the warp and weft directions is measured by dipping them in distilled water. The absorptive velocity depends on the capillary force, which determines the transport of liquid contrary to gravity. For fabric the wicking velocity, the path of liquid distribution and the final wet figure can be different in the warp and weft directions. 

Weft-knitted and warp-knitted spacer fabrics can be used as sound absorbers to reduce the noise level in building, automotive and other places (Liu and Hu, 2010). Figure 6.28 shows a weft-knitted spacer fabric (a) of multifilament spacer yams and a warp-knitted spacer fabric (b) of monofilament spacer yams for sound absorption. Whereas the weft-knitted spacer fabric exhibits the typical sound absorption behaviour of porous absorbers, the warp-knitted spacer fabric exhibits the typical sound absorption behaviour of microperforated panel (MPP) absorbers. Figure 6.29 shows the noise absorption coefficients of the two spacer fabrics without and with air-back cavity. The combinations of weft-knitted and warp-knitted spacer fabrics can significantly improve their sound absorbability, as shown in Figure 6.30. 

Figure 6.28 Weft-knitted spacer fabric (a) and warp-knitted spacer fabric (b) for sound absorption.

Figure 6.29 Noise absorption coefficients of spacer fabrics (a) without air-back cavity and (b) with air-back cavity (A, weft-knitted B, warp-knitted).

Figure 6.30 Noise absorption coefficients of the combinations of weft and warp spacer fabrics.

D knitted textiles have already been widely used as technical textiles in different fields. The future work should be on the development of new 3D knitted structures with more extra functions to meet the requirements of new application fields. For example, warp-knitted spacer fabrics have great structural variations. By using different structures and fibre materials, they have been developed to have various physical functions to be used in different fields, such as cushioning, sound absorption, smart textiles and thermal collection. New potential application fields should be identified first in the future, and then fabric stmctures can be designed to better match the specific applicatimis. 

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