Aramids fiber skin-core structure

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. 

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... 

Fi. 4. A two-dimensional representation of the lamellar structure (or lurbosiraiic structure) of a carbon fiber. The cross-section of carbon fiber has essentially parallel basal planes in the skin region, but extensive folding of layer planes can be seen in the core region. It is thought that this extensive interlinking of lattice planes in the longitudinal direction is responsible for better eompre.ssive properties of carbon fiber than aramid fibers. 

The model extends the structural hierarchy proposed by Dobb, Johnson and Saville [374] for the aramids. Three distinct fibrillar elements have been noted microfibrils, on the order of 50 nm in size fibrils, on the order of 500 nm in size and macrofibrils, about 5 pm (5000 nm) across. The importance of this structural model is that it not only describes the structure of uniaxially oriented fibrous materials, but it also shows the fine structure of the thicker LCP forms of moldings and extrudates. In these thicker materials, process history and temperature affects macrostructures, such as skin-core, bands and layering (Fig. 5.85). The fiber structural model shows the arrangement of the fine structure within those macro units. This structural model improves the understanding of relationships between processes, structure and properties in LCPs. 

Young et al performed a detailed study of PBO fibres using electron microscopy and Raman microscopy. Micrographs of longitudinal sections taken through as-spun and heat-treated PBO fibers show a fibrillar structure on a 100 nm to 200 nm scale. Unlike the aramids, no banding pattern has been observed. Skin-core differences similar to those formed in aramid fibers were also found in the PBO fibers. As-spun and heat-treated fibers showed a better orientation in the skin than in the core. 

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