Fiber elongation, polyamides

The incorporation of glass fiber info polyamide 6 increases the tensile strength from 40 to 145 GPa (Table 3.1), accompanied by an increase in flexural modulus (Table 3.2) from 1.0 to 1.6 MPa and hardly any change in elongation at break (Table 3.4). 

To accommodate the various uses in 100% form and in blends, the tenacities and elongations of the nylon staple offerings range from 0.3 to 0.6 N /tex (3—7 g/den) and from 50 to 100% elongation. Most other fiber properties of nylon staple differ tittle from those of the continuous filament property characteristics of nylon-6 and nylon-6,6 are similar (see Polyamides, general). 

Fibers are polymers with very high moduli and very high resistance to deformation therefore, they elongate very little. Some examples of polymers used as fibers are nylon (polyamide), polyesters, and polyacrylonitrile (acrylic fiber). 

Fibers spun from polyvinyl alcohol, polybenzimidazoles, polyamides, and aromatic polyamides have been used as carbon fiber precursors. However, at present, the most attractive precursors are made from acrylonitrile copolymers and pitch, and a small amount from rayon. Today more than 95% of the carbon fibers produced for advanced composite applications are based on acrylic precursors. Pitch-based precursors are generally the least expensive, but do not yield carbon fibers with an attractive combination of tenacity (breaking strength, modulus, and elongation as those made from a acrylic precursor fiber). The acrylic precursors provide a much higher carbon yield where compared to rayon, typically 55% versus 20% for rayon, and this translates directly into increased productivity. 

Aromatic polyamides are, for example, wet spun from hot 100% sulfuric acid into cold water. The lateral self-association of the macromolecules in the nematic mesophase is retained after precipitation of the filaments and leads to the very high moduli of elasticity and good tensile strengths of such fibers. In these cases, the properties essentiaUy depend on the draw ratio during spinning, that is, on the ratio of the filament diameter at the spinneret to that at the first winding roller. The modulus of elasticity increases and the elongation at break decreases with increase in this ratio. 

Table 3. Modulus (E), strength (O],), elongation at break (8],) of filaments and the density (q) of para-aromatic polyamide, PBO and PBT fibers...

Polyamide-imides enjoy, as the name suggests, a positive synergy of properties from both polyamides and poljdmides, such as flexibility, melt processability, elongation, dimensional stability, and toughness. Polyamide-imide polymers can be processed into a wide variety of forms, from injection- or compression-molded parts and ingots to coatings, films, fibers, and adhesives. Generally these articles reach their maximum properties with a subsequent thermal cure process. 

Well known examples of orientation induced crystallization are the stretching of rubber, fiber orientation in polyamides and polyesters, and the biaxial stretching of polycarbonate filuK. An example of thermal effects during induct crystallization in rubber is shown in Fig. 45, where the heat flow (Q) for successive steps of elongation and retraction is plotted as a function of time. 

The heating of polyamides in the presence of oxygen or air leads to considerable changes in their chemical composition and physicochemical properties. This is accompanied by a loss of valuable physico-mechanical properties in the polyamide materials decrease in the breaking strength and breaking elongation for fibers and films, intensification of brittleness. 

Fig. 131. Variation of the breaking elongation (a) and breaking strength (b) of polyamide fibers as a function of the time of irradiation in an atmosphere of nitrogen. 1) Polyhexa-methyleneadipamide 2) polycaproamide.

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