Drawn fibers melting point

The process competes with the traditional method of fiber production in which the precursor material is melted, usually in an arc furnace, then drawn through spinnerets and spun or impinged by high pressure air. The melt-spin process is not well suited to materials with high melting points such as zirconia, silicon carbide, or pure alumina. 

Marx, Smith, Worthington and Dole (1955) observed a sudden change of slope of the cv— T curve of drawn 6-Nylon at 115° C. Although this break in the value of dcjd T was reproducible (observed during three different experiments), it disappeared on annealing the drawn fibers just below the melting point. 

The formation of anisotropic thermorever-sible gels was observed with this polyester, for example, in 3-phenoxytoluene as the solvent. These mixtures also retained their anisotropy above the gel melting point at a concentration of 35% or more [29]. In highly aligned fibers spun from the nematic melt and post-drawn, a tensile modulus of 40-45 GPa and a tensile strength of up to 550-650 MPa was reached [28c]. 

Poly(vinyl alcohol) is completely water soluble and thus is used as a thickener in some suspensions and emulsions. Although usually amorphous, it can be drawn into a semicrystalline fiber. However, the melting point of the crystallites is above the thermal degradation temperature. Some typical properties are Glass Transition Temperature, Tg = 85° C Amorphous density at 25° C = 1.26 g/cc Crystalline density at 25° C = 1.35 g/cc Molecular weight of repeat unit = 44 g/mole. 

The polymers, which are of poor solubility, can only be spun from a methanolic solution of calcium thiocyanate. Spun fibers are highly crystalline even when not drawn. Since the polymers withstand oxidation and possess high melting points, they are used as sewing yam for industrial sewing machines. With polymers of lower melting points, the yam fiber would melt when in contact with the very hot sewing needles, especially... 

Obviously, the constrained-state and free-to-shrink measurements may give vastly different results depending on the orientation of the fiber. The melting point of drawn fibers measured in a constrained state is always higher than in the free-to-shrink measurements. It can be intuitively understood that in a constrained measurement, the entropy increase is smaller than in the free-to-shrink measurement, since in the constrained measurement the macromolecules are stretched, and they will remain partially stretched in the melt. They will lose their orientation in the melt, but that will require some time. Then, supposing that the heat of fusion is the same in both types of measurement (which is not always true, but can be accepted as a first approximation)... 

---