Filament solidification

The rapid extraction of the solvent prevents proper filament formation and makes the filament crumbly. That is why good results may be obtained by methylene chloride extraction after solidification with water. A wet spinning process with solvent extraction and stretching consists of [43] extrusion, filament solidification, first solvent extraction, first water stretching, second solvent extraction, second hot air stretching, takeup. 

A particularly important mechanism of solvent transfer in dry spinning, and one that has been shown to be the rate controlling factor in filament solidification, is the process of diffusion. The diffusion process for a binary polymer-solvent system is described by the Fick equation ... 

Voids often look similar to air bubbles. The appearance of voids in filaments or films, however, results for different reasons. Voids can be produced during stretching in the area of necking by a kind of folding mechanism. The formation of voids may also depend on the generation of a radial gradient structure during solidification of the threads. 

In dry spinning, the dope is extruded through spinnerets located at the top of a tower. As the uncoagulated filaments flow down the tower, they are brought into contact with an inert gas heated above the boiling point of the dope solvent. The solvent evaporates from the filaments as they pass down the column and solidify. Conceptually, dry spinning can be considered to be a special case of melt spinning, in which the polymer solidification or crystallization temperature has been depressed by the solvent. The solidification temperature of the filaments will continuously increase as the solvent evaporates until solid filaments are formed. The filaments are continuously removed from the bottom of the tower, washed free of solvent, and then for the most part processed like the wet-spun fibers. 

The original Cyanamid process used a single phase fusion melt of an acrylonitrile copolymer (molecular weight 30,000 60,000) and water and extruded, at about 50 bar, through a spinneret (hole size 60-160 pm) directly into a steam pressurized solidification zone, such that the subsequent release of water from the extrudate avoided any deformation of the extruded filaments and their condition remained such that they could be stretched up to some 25 times. The fiber was cooled with water and passed to the atmosphere by way of a pressure lock. 

Melt-spinning techniques that use a solid quenching medium can only be used to produce filaments of flat cross-sections. Various techniques that differ in small details (single roller, twin roller) are in use. They all have in common that the ribbon forms from a liquid puddle that remains in a fixed position on the moving quench medium which is usually a rotating Cu wheel. The width of the ribbon varies with the width of this puddle and the thickness with the penetration depth of the solidification front that moves into the puddle. Only the solidified metal moves with the wheel and this... 

The process mainly consists in the winding of continuous filaments coated with resin on a rotating body, called a mandrel, whose shape corresponds to the geometry of the part to be produced. Resin solidification is obtained by placing the component in an oven or autoclave. The fundamental factors underlying this production technology which strongly impact the properties conveyed to the final composite product are ... 

Wet-spun fibers are formed by extruding a highly viscous polymer solution through a spinneret into an appropriate liquid bath, where it is solidified. The solidification is brought about by a diffusional interchange between the extruded polymer filaments and the bath. In this process, called coagulation, one or more components from the bath diffuse into the fiber while, in turn, solvent leaves the forming filaments. The net result is a solid fiber. In some fiber systems, such as viscose rayon, there is also a chemical reaction superimposed on the diffusional process [60-62]. In others, the situation is strictly diffusional. Only the diffusional process will be considered in this treatment. 

Polymer melt spinning is one of the major processes for the manufacture of synthetic fibers. Typically, a polymer melt is extruded vertically downward through a spinneret or die, which may have hundreds of openings. The resulting filaments are simultaneously cooled by crossflow air and are stretched by the action of rollers. On solidification, the yam is wound onto a bobbin. Different aspects of this operation have been described in the literature.(, 59-63) When the fiber-spinning process is used as an extensional viscometer, the setup is simplified and only a single filament of circular cross section is employed. This is shown schematically in Fig 6. Also, the operation is run in an isothermal manner by spinning into an isothermal oven. 

The initial research focus was to study two possible approaches for creating the EGA. The first approach is to bond the filament fibres with a flexible polymeric resin to hermetically seal off the chip area. The second method is to use a side-by-side thermal-bonding process, which would interlock the filament fibres, thus sealing off the chip area. The behaviour of the EGA would depend on the individual properties of the components, their relative proportions, the degree of homogeneity, the properties of the interface between the components and the rate of solidification of each component. 

Commercial melt spinning processes involve substantial filament cooling, and control of the quench profile is one of the practical considerations for design and operation. Laboratory experiments are often designed to operate isothermally, however, typically by spinning in a temperature-controlled chamber with rapid solidification effected at a fixed position (by spinning into a water bath, for example) or by taking... 

The blown film process was briefly described in Section 1.2.6. The process is shown schematically in Figure 10.7. There are many similarities between the blown film and the fiber spinline because of the free surface and the very small transverse dimension relative to the distance between melt extrusion and solidification, and thin sheet equations analogous to the thin filament equations are typically used, although the hoop stress must now be taken into account. The equations for a Newtonian fluid were first published by Pearson and Petrie in 1970, and their approach has been used by nearly all investigators since. There are two steady-state momentum equations because variations in both thickness and width in the stretching direction are important. The mechanics of the solid region above the ill-defined freezeline are... 

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