Fiber skin core, structure

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. 

Figure 2.12 Schematic of a skin core structure in a high speed spun fiber showing a greater degree of crystallization in the skin than in the core.

Kevlar 149 has a substantially higher degree of crystallinity and larger crystal size than for Kevlar 49. The unit cell dimensions for the two also differ significantly. In addition, while there are present skin-core structure in both Kevlar 29 and Kevlar 49 fibers, there is no definable difference between the skin and core for Kevlar 149. 

Morphological level (nm to um). This level describes the organization of the microfibrils and macrofibrils into layers and walls the existence of distinct cell wall layers in native cellulose libers or in skin core structures in man-made cel-lulosic fibers are discussed. 

It is seen that ultrasonic techniques have two major advantages high accuracy and the ability to determine the complete set of elastic constants for a sample of small size. As a result of these capabilities, all five independent stiffness constants have been obtained for extruded rods of PLC of high draw ratio (2 = 15) and small diameter (0.8 mm). Moreover, it has been possible to study the skin-core structure in injection molded PLC by monitoring the variation in stiffness with position. For blends of a PLC and a thermoplastic or glass fiber-reinforced PLC, successful correlation has been obtained between the modulus data and the orientation of the PLC fibrils or glass fibers. 

There is clear evidence of a skin-core structural differentiation in PpPTA fibers [119,125], which for a product of a wet-spinning process is not surprising. Fig. 13d shows, for example, the change of the pleat spacing along the radius of the fiber cross section. 

Si-C-N(O) fibers derived from HPZ precursor fibers are nanoporous and heterogeneous with a skin/core structure. The composition changes from SiOxCy in the external porous surface to SiNxC, in the core. The molecuiar formuia of this fiber is close to 4 mol. >4 SiOa, 81 mol.% SiNxCy (x = 1.02, y = 0.23) and 15 mol.% free C [22]. The presence of complex tetrahedral units is supported by the Si NMR spectrum which shows a broad signal covering the chemical shift region expected for silicon oxycarbide, siiicon oxynitride and silicon carbonitride units [21]. The occurrence of free carbon, expected from the nature of the precursor, is supported by the C Is XPS pattern [22]. 

Most success was obtained using a multi-stage oxidation process. To implement this work, microsections of the oxidized fiber were taken and examined for skin/core structure and the work revealed some interesting facts concerning the oxidation rate ... 

It is generally believed that the fine fiber structure comprises fringed micelles and fringed fibrils (Figure 4.18). The presence of zinc in the bath contributes to a skin-core structure (Figure 4.19) and proportions of skin and core can be varied. Skin contains small crystallites and is stronger than the core, which contains fewer, but larger crystallites [175]. 

Mochida I, Zeng SM, Korai Y, Toshima H, The introduction of a skin core structure in mesophase pitch fibers by oxidative stabilization. Carbon, 28, 193, 1990. 

Lu YG, Wu D, Zha QF, Liu L, Yang CL, Skin-core structure in mesophase pitch based carbon fibers Cause and prevention. Carbon, 36(12), 1719-1724, 1998. 

The higher-order structures (morphologies) of injection-molded polypropylene (PP), which should be considered, are crystalline form, lamellar thickness, spherulite size, crystallinity, molecular orientation, crystal orientation, dispersion state of a blended polymer, length and orientation of reinforcing fibers, crystalline texture (skin-core structure), etc. 

When a cross-section of an injection-molded PP is observed under a polarizing microscope, a skin-core or skin-shear-core structure is seen as shown in Figure 1. A cross-section cut perpendicular to the flow direction (MD) shows a skin-core structure, while a cross-section cut parallel to the MD shows a skin-shear-core structure. The skin layer is composed of a kind of fiber structure oriented in the MD, the shear layer is composed of a row structure oriented in the MD, and the core layer is composed of almost unoriented spherulites. There are also papers claiming that, in injection-molded PP, more than five different layers can be resolved through the thickness. 

Keywords weldline, injection molding, cold weldline, hot weldline, flow, skin-core structure, weldline strength, fiber orientation, fountain flow, gate. 

Fiber Structure. An extensive description of the structure of PPTA (Kevlar) fibers has been provided in a 1993 book (47), including a description of the crystal lattice (48), estimates of apparent crystallite size and percent ciys-tallinity, a description of fibrillar and pleat structure, and evidence of a skin-core structure. Crystal lattice structure and dimensions for PBA and MPDI (Nomex) fibers are also included, as shown in Table 3. The structure of Twaron is essentially the same (10), while the crystal structure ofTeijinconex is nearly identical to the MPDI fiber in Table 3. PPTA fibers are highly crystalline, ranging from 68 to 95% crystalline depending on the heat treatment of the fiber and the crystallinity measurement technique. MPDI fibers are also highly crystalline, although the crystal lattice is quite different from that of PPTA (30). 

The skin-core structure is a macroscopic analogue of the partitioned structure within the fiber. Since fiber stresses become concentrated in the oriented regions, there is a loss of participation of some of the interior molecules to resist subsequent strains. Under fiber extension, the taut molecules will break first, triggering rupture of the fiber before the unoriented molecules contribute much resistance. A loss of overall fiber strength and tenacity results. 

As mentioned above, the manufactming process leaves aramid fibers with a skin/core structure, reflected in the model of Morgan et al [32]. Apparently, the coagulation creates a differential in density, voidage and fibrillar orientation along the fiber cross section. The fiber surface cools more rapidly, and this, combined with the effects of solvent evaporation, leaves a skin layer with an average thickness between 0.1-0.6 pm, having low... 

Hydrophylicity of amide linkage leads to moisture absorption by all aramids. The skin-core structure of the p-aramid fibers plays an important role in the moisture absorption, which is critical for many structural applications of the FRP on their basis. Thus, Fhkuda and Kawai found that the ultrahigh modulus Kevlar 149 has a moisture uptake of 1% (20°C, 55% relative humidity), while in the regular brand Kevlar 29 it is -7% under the same conditions [34], Apparently, above a certain concentration, the water molecules could upset the intermolecular hydrogen bond formation (Figure 8.2) and affect the mechanical properties, as seen by the comparison of the mechanical properties of Kevlar 29 and Kevlar 149. The same authors found that the diffusion coefficient through the skincore structure of the Kevlar fiber is also of importance in the skin the trend is Kevlar 149 > Kevlar 29 > Kevlar 49 and in the core it is reversed Kevlar 29 > Kevlar 49 > Kevlar 149. 

Cross Section. The cross-section structure of mesophase-pitch carbon fibers is one of the four types shown in Fig. 8.12 and is determined by the spinning method, the temperature of stabilization, and the partial pressure of oxygen.The formation of a skin-core structure or skin effect is often observed. This structure is similar to that of the PAN-based carbon fiber shown In Fig. 8.8 above. 

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