Cold stretch

J.J. Kim, T.S. lang, Y.D. Kwon, U.Y. Kim and S.S. Kim, Structural Study of Micro-porous Polypropylene Hollow Fiber Membranes Made by the Melt-Spinning and Cold-Stretching Method, J. Membr. Sci. 93, 209 (1994). 

SC = Single crystals SF = Single fibrils (cold stretched) SM = Shearing region V = Voids... 

Cold stretched films were made by deforming compression molded 40/60 PET/POB films In a T. M. Long biaxial stretching device. Specimens were originally 60 mm x 60 mm. Following a five minute preheat, the films were unlaxlally stretched 2 x 1 at 750%/mln. with... 

Cold stretched films were difficult to produce and upon stretching exhibited little. If any, orientation. From Table III It Is obvious that of the ten samples stretched only specimens U2 and U8 did not fall upon being loaded. These samples were stretched at 90, and 110 c. SALS was performed on all the samples, but only samples U2 and U8 were used In the SAXS, UAXD, and SEM experiments. Figure 9 shows the resulting Hv SALS patterns for some of the cold stretched films. The patterns are circular Indicating no orientation. Confirming this lack of orientation are SAXS patterns that showed cylindrical symmetry and UAXD patterns of the films that yielded circular diffraction rings. 

Another more recent technology is the cold forming of a foil sandwiched between two plastic plies. The process involves the forming of a relatively shallow, well radiused draw by a mechanical (cold) stretching action between male/female dies, or air... 

Fig. 10. Sketch of the transformations of the oriented chains during deformation, in-between are additional amorphous chains a) lamellae with some tie-molecules, b) elastic shear-deformation of lamellae imder small load and reorientation with respect to the load, c) fracture of lamellae into smaller blocks due to local stress concentration caused by tie molecules, d) stretched aligned, but not re-crystallised chains between the blocks during stretching at higher temperatures, e) some of the fibrillar arranged molecules crystallise, final stage in the case of hot stretched iPP, f) further dissolution of the blocks creating more extended chains at room temperature, g) finally, there are several strands of extended chains, not crystallised, with some amorphous regions in between, final stage in the case of cold stretched rPP.

Fig. 16. iPP crystallised within a rheometer without shear and cold stretched sample with fibrillar strands (stretching direction diagonal, see arrow)... 

SC Single crystals SF single fibrils Cold stretched)... 

It is noted that the cold-stretching induced microstmcture change depends on the temperature and stretching rate due to the famous time-temperature equivalent principle. During the stretching process, the molecule chains experience two competing processes. One is the extension, that is, the... 

Figure 5.9 Inliared spectra of the cold-stretched SMPF with the polarizer parallel with and perpendicular to the fiber direction. Source [10] Reproduced with permission from the Royal Society...

Figure 5.23 RVE (a) a nonstretched SMPF and (b) a cold-stretched SMPF. Source [13] Reproduced with permission from ASME...

The collection of diffraction data of polymers in a solid mesophase for the structural analysis is generally performed on imiaxially oriented samples (fibers) [152-154]. Oriented fibers of high polymers in a mesomorphic form may be easily obtained by extrusion procedures from solution or melt and/or by cold stretching procedures. 

Cold drawing (cold stretching) n. A stretching process performed at a temperature below a thermoplastic s melting range to orient the material and improve the tensile modulus and strength. 

Cold stretch n. Pulling operation, usually on extruded filaments, to improve tensile properties. 

I cold stretching heattog-cooling-heating raet m = Jlijs... 

Figure 2.12 CP/MAS NMR spectrum of syn-polypropylene (a) Spectrum of a crystalline powder (b) a sample of cold-stretched sf-polypropylene, showing eiongation with necking (c) sample of cold-stretched sf-polypropylene, far from the neck region (positions 1 and 2), and (cf) spectrum of a cold-stretched sf-polypropylene, in the necking region (position 3). (Reproduced, courtesy of P. Sozzani).

The formation of the hollow fiber membrane via TIPS process has not been studied very much. Kim et al. [13] prepared poypropylene hollow fiber membrane from polypropylene/soybean oil mixture. The membrane was formed by the TIPS process and subsequent cold-stretching. The hollow fiber became more oriented by increasing the melt-draw ratio defined as the ratio of take-up speed to the extrusion of the polymer solution. The cold-stretching of the hollow fiber membranes remarkably increased the membrane porosity. 

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