Fiber-induced crystallization

Huihui L, Liu J, Wang D, Yan S (2003) A comparison study on the homogeneity and heterogeneity fiber induced crystallization of isotactic polypropylene. Colloid Polym Sci... 

Both stress-induced crystallization and orientational crystallization can be used for the preparation of polymer materials with mechanical property values (e.g. tenacities and elastic moduli) much higher than those for polymer films and fibers obtained by conventional processing. We believe that the advantage of orientational crystallization over more complex methods consists in the possibility of obtaining samples of elastic moduli and tenacities in a one-step continuous process. 

Although PBT fiber also has a plateau region in the stress-strain curve [4], the crystalline chains do not respond to external strain in the first few percent of deformation. They increased in length only when the strain is above 4% (see Figure 11.13). Therefore, initial macroscopic deformation involved viscous flow of the amorphous phase. Furthermore, PBT undergoes strain-induced crystal transformation at moderately low strains of 15-20% [75], The differences in their microscopic crystalline chain deformation explained why PTT has a better elastic recovery than PBT even though both have contracted chains and knees in their stress-strain curves [4, 69],... 

The stress-induced crystallization influences the ability to sustain the stress of deformation occurring in the spin line. Optical micrographs of fluffs allow the assumption of different deformation or stress behavior of core and skin layers of spun fibers, as shown in Figure 13.1. The reason for this kind of yam break can... 

These solvents include tetrahydrofuran (THF), 1,4-dioxane, chloroform, dichloromethane, and chlorobenzene. The relatively broad solubility characteristics of PSF have been key in the development of solution-based hollow-fiber spinning processes in the manufacture of polysulfone asymmetric membranes (see Hollow-fibermembranes). The solvent list for PES and PPSF is short because of the propensity of these polymers to undergo solvent-induced crystallization in many solvents. When the PES structure contains a small proportion of a second bisphenol comonomer, as in the case of RADEL A (Amoco Corp.) polyethersulfone, solution stability is much improved over that of PES homopolymer. 

As long as thermally induced crystallization (see above) is avoided, PCTFE exhibits excellent mechanical properties. It also has an excellent resistance to creep.83 The addition of glass fibers (typically 15%) improves high-temperature properties and increases hardness, but also increases brittleness.86... 

Crystallinity indexes calculated according to the method described by Segal et al. (32) showed that the old cotton has a crystallinity of about 38 . Aqueous treatments increased the crystallinity of the historic cotton sample to about 45 . However, the crystallinity of contemporary cotton, which is about 70 , was not reached (30). This increase suggests that water acts as an internal plasticizer and allows a segmental reorientation which leads to an increase in crystallinity. Water-induced crystallization of amorphous cellulose fibers has been reported (17). Kalyanaraman (33) investigated orientation factors of cotton fibers from historic samples and found that the orientation values of the museums samples are smaller than the values of present-day cottons. He opined that cotton may have lost its orientation over time. In view of this... 

For this comparison, a melt-spinning process was chosen. Each special thermoplastic process influences the structure and thus the properties of the obtained polymer samples differently. This is particularly pronounced for fibers, since especially melt spinning is a process which makes extremely high demands on the deformation ability of the polymer melts at high deformation speeds. Particularly the tensile stress within the fiber formation zone is a very important factor to reach a high orientation of the macromolecules along the fiber axis and a stress-induced crystallization. This crystallization should be discussed in relation to PLA and PHB multifilaments, and at the same time the general property spectrum of these polymers should be represented. 

Tsebrenko et al. [1976] reported on fibrillation of POM in a copolyamide (CPA) matrix, as a result of flow through a capillary. Fine fibrils with diameters of about 20 j,m and length 3.2 mm were obtained during extrusion at T = T (POM) H- 6°C. The low extmsion temperature facilitated stress-induced crystallization of the POM fibers, preserving the morphology engendered at the entrance to the spinneret. 

Similarly, at T > coalescence of semicrystalline dispersed domains combined with stress-induced crystallization leads to formation of long fibers. This effect has been explored for the performance improvement of blends comprising liquid crystal polymers, LCP [La Mantia, 1993]. 

This report summarizes recent work on a process for the physical modification of polyester fiber surfaces via solvent Induced crystallization (SING). New experimental results show ... 

Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat the orientation-induced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion. 

In strain-induced crystallization the polyester material is stretched at a suitable rate and temperature to achieve crystallization within the polyester. With polyester compositions dealt with here, typical temperatures are about 80°C-140°C typical stretch rates are about 300 to about 1500%/s. The stretch ratio suitable is about 8 to about 24. For fiber stretching or orientation, the stretch ratio of about 2 to about 8 is suitable. 

Wide-angle X-ray scattering and DSC measurements of cold drawn PEN stretched from an amorphous fiber that is spun at low speeds indicate that the strain-induced crystallization can occur at a temperature be-... 

Fiber processing, spinning, raw nucleation The mechanism by which stress-induced crystallization is initiated usually during fiber spinning or hot drawing. 

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