Cooling melt spinning

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Fig. 9.11. Ribbons or wires of amorphous metal con be mode by melt spinning. There is on upper limit on the thickness of the ribbon if it is too thick it will not cool quickly enough and the liquid will crystallise.

Like nylon, when it is made PET can be extruded, cooled, cut into chips, and stored for later melt spinning. It also can go directly to the spinneret. The downstream operations of the spinneret are much the same as with nylon. 

Figure 1U9. Comparison between calculated ( ) glass-forming ranges at 10 K sec and observed ( ) glass-forming ranges at melt spin cooling rates for (a) Pd-Si, (b) Zr-Be, (c) Cu-Zr and (d) Pd-P (Saunders and Miodownik 1988).

The polymer is extruded from the bottom of the polymerizer through a slot or holes on to the surface of a water-cooled drum. The ribbon is cut to chips and dried before melt spinning in a manner similar to that described for nylon. The filaments are stretched, with the application of heat, to about 3 to 6 times their original length. 

Fibers emerging from the spinneret are cooled under controlled conditions, passing over guides and rollers to a take-up spool or bobbin. Often a finish is applied before windup to control static electricity and friction (Stevens 1993). Large-scale production machinery produces fiber at a rate of thousands of feet a minute. A schematic diagram of a melt-spinning apparatus is drawn in Figure 8-10. 

The polymerized product is an extremely insoluble material and must be melt-spun, as discussed later. Therefore, should a delustered or precolored fiber be desired, it is necessary to add the titanium dioxide or colored pigment to the polymerization batch prior to solidification. For ease of handling, the batch of nylon polymer may be extruded from the autoclave to form a thin ribbon, which is easily broken down into chips after rapid cooling. But, whenever possible, the liquid polymer is pumped directly to the fiber melt spinning operation (see Fig. 12.14). 

Rapid solidification processing (RSP), for example, by melt spinning onto a cooled copper drum, which is capable of retaining many alloys in a glassy (amorphous) state or in nanocrystalline form ... 

Figure 15 Two selected methods for metallic glass preparation (a) in the melt-spinning process the molten alloys are projected against a spinning wheel (b) in the twin-roUer quenching technique the melt is cooled between two rotating wheels. In both processes, the aim is to achieve ultrafast cooling...

Transmet Corporation manufactures flakes by a Rapid Solidification Technology. There are two variations of this method Melt spin and Spinning cup methods. In the Melt spin method, molten metal of any composition (pure metal or alloy) is driven through an orifice and the shape formed in the orifice (continuous sheet) is rapidly cooled on a chilling block. This metal sheet is cut into segments in the form of flakes (square and rectangular), flat fibers, and ribbons of desired... 

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