Skin/core

To establish the degree of oxidation in an oxidized PAN fiber, it is advantageous to prepare a section for examination under the microscope. The following technique has been adapted from one used in hospitals to look at sections of bone. 

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An aqueous PVA solution containing a small amount of boric acid may be extmded into an aqueous alkaline salt solution to form a gel-like fiber (15,16). In this process, sodium hydroxide penetrates rapidly into the aqueous PVA solution extmded through orifices to make it alkaline, whereby boric acid cross-links PVA molecules with each other. The resulting fiber is provided with sufficient strength to withstand transportation to the next process step and its cross section does not show a distinct skin/core stmcture. 

Fig. 1. Microstereo view of the cross section of skin core filaments.

Highest thermal performance with PPS compounds requires that parts be molded under conditions leading to a high level of crystallinity. Glass-filled PPS compounds can be molded so that crystalline or amorphous parts are obtained. Mold temperature influences the crystallinity of PPS parts. Mold temperatures below approximately 93°C produce parts with low crystallinity and those above approximately 135°C produce highly crystalline parts. Mold temperatures between 93 and 135°C yield parts with an intermediate level of crystallinity. Part thickness may also influence the level of crystallinity. Thinner parts are more responsive to mold temperature. Thicker parts may have skin-core effects. When thick parts are molded in a cold mold the skin may not develop much crystallinity. The interior of the part, which remains hot for a longer period of time, may develop higher levels of crystallinity. 

The blends of thermotropic LCPs and thermoplastics are generally two-phase systems where the dispersed LCP phase exists as small spheres or fibers within the thermoplastic matrix. Often a skin/core morphology is created with well-fibrillated and oriented LCP phases in the skin region and less-oriented or spherical LCP domains in the core. 

The composites injection molded at the lower temperature (180°C) did not exhibit any skin/core effect, but rather contained fibers throughout. 

The basic steps of the IM process produce unique structures in all molded products, whether they are miniature (micro) electronic components, compact discs, or large automotive bumpers. These structures have frequently been compared to plywood with several distinct layers, each with a different set of properties. In all IM products, a macroscopic skin-core structure results from the flow of melt into an empty cavity. Identifiable zones or regions within the skin are directly... 

The basics observed in molded products are always the same only the extent of the features varies depending on the process variables, material properties, and cavity contour. That is the inherent hydrodynamic skin-core structure characteristic of all IM products. However, the ratio of skin thickness to core thickness will vary basically with process conditions and material characteristics, flow rate, and melt-mold temperature difference. These inherent features have given rise to an increase in novel commercial products and applications via coinjection, gas-assisted, low pressure, fusible-core, in-mold decorating, etc. 

Frische, S., Maunsbach, A. B., and Vollrath, F. (1998). Elongate cavities and skin-core structure in Nephila spider silk observed by electron microscopy. Journal of Microscopy 189, 64-70. 

Hobbs, S. Y. and Pratt, C. F., The effect of skin-core morphology on the impact fracture of polybutylene terephthalate, J. Appl. Polym. Sci., 19, 1701-1722 (1975). 

The fibers exhibit a fine structure consisting of large crystals separated by areas of poorly oriented, amorphous regions with many crystal nuclei at a pronounced skin/core character, which is a consequence of a complex structure formation mechanism. The nuclei grow into large crystals upon heating. 

Figure 13.1 Yarn break caused by skin-core differences (brittle fracture and crazes) [9]. Photograph provided by W. Goltner...

Figure 5.74 Skin-core morphology of an injection-molded polypropylene structural foam. Reprinted, by permission, from P. R. Hornsby, in Two-Phase Polymer Systems, L. A. Utracki, ed., p. 102. Copyright 1991 by Carl Hanser Verlag.

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

Composite fibers have been produced for a number of years to create structures with enhanced properties or combinations of properties. Different configurations of the materials selected for composite fibers are matrix-fibril, side-by-side, and skin-core. In the latter configuration, the core component can be made to dominate the mechanical properties of the fiber, while the skin controls surface properties. This permits decoupling of the two types of properties and closer control of the overall characteristics of the final product. 

In previous work, a model skin-core composite fiber with a nylon core and a rayon skin was produced by a coating process fl, 2], The composite fibers exhibited the mechanical properties of the nylon core, while the moisture regain was proportional to the thickness of the rayon skin. 

Figures 11 to 13 are dark field micrographs of 66 polyamide monofilaments. Figure 11 show an Ag-S stained filament. Silver sulfide precipitates, which appear as black areas (as they did in bright field images) as well as polyamide crystallites (bright spots) are visible. Figure 12 corresponds to a type 4 fiber (with skin-core morphology) where there is a lower density of crystallites in the skin region. Figure 13 corresponds to the case of type 5 fiber which has smaller crystallites.

Figure 13. Dark-field micrograph of me cross section of a Type 4 PA 66 fiber. No silver sulfide deposits notice the skin-core effect and the dimensions of the...

Whatever their origin, the bands reflect the susceptibility of the fibrils to transverse kinking or buckling. Development of a skin-core morphology in the coagulation process may explain the... 

The substrate also has an important influence on diffusion of the dye. The diffusion rate increases with caustic soda treatment or mercerization of cotton. However, with regenerated cellulose fibers, which have a marked skin-core structure, the outer parts of the fiber can act as a diffusion barrier. 

Two polymers can be used in sequence to fill the mold, forming skin-core sandwich molded articles. Air can also be introduced in partially melt-filled molds, and pressurized to form a polymer skin-air core sandwich structure, through the gas-assist injectionmolding process. In all injection molding processes, the polymer is melted, mixed, and injected from the injection unit of the machine. 

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