Orientation, oriented stretched sheet

Similarly, oriented crystallisation can be induced by stretching sheets or films of polymers in two directions simultaneously. The resulting materials have biaxially oriented polymer crystals. Typical examples of such materials are biaxially stretched poly(ethylene terephthalate), poly(vinylidene chloride), and poly (propylene). Since the oriented crystals do not interfere with light waves, such films combine good strength with high clarity, which makes them attractive in a number of applications. 

Heat-shrinkable sheets (thickness 0.040 to 0.120 inch, or 1 to 3 mm) and films (thickness 0.001 to 0.020 inch or 0. 025 to 0.5 mm) are fabricated from many of the same materials as shrinkable tubing.91 They are produced by extrusion as a tube, sheet or blown film. Irradiation is done by the equipment shown in Figure 8.10. Orientation (stretching) after irradiation can be done by several methods, namely by differentially heated and driven rolls (in the machine direction) or by a tenter frame (see Figure 8.11) in the transverse direction. If desired, biaxial stretching can be done. 

Figure 11.1 A typical OPS (oriented polystyrene sheet) tenter frame process with stretching in the machine (MD) and transverse (TD) directions. Rolls 1-8 are heated and driven. Sheet is heated above and below in the ovens. Sheet geometry as a function of position in process ...

Hou, Z., Wang, K., Zhao, R, Zhang, Q., Yang, C., Chen, D. et al. 2008. Structural orientation and tensile behavior in the extrusion-stretched sheets of polypropylene/multi-walled carbon nanotubes composite. Polymer 49 3582-3589. 

Transverse direction n. (1) In extruding sheet or film, the direction of the width, crosswise to the direction of extrusion. (2) In a uniaxially oriented plastic, either direction perpendicular to the direction of orientation (stretching). In a biaxially oriented sheet, the direction perpendicular to both axes (the plane) of orientation. (3) In a fiber-reinforced laminate the thickness direction or, in a laminate with unidirectional reinforcement, either of the two directions perpendicular to the fiber lengths. 

The ease of sample handling makes Raman spectroscopy increasingly preferred. Like infrared spectroscopy, Raman scattering can be used to identify functional groups commonly found in polymers, including aromaticity, double bonds, and C bond H stretches. More commonly, the Raman spectmm is used to characterize the degree of crystallinity or the orientation of the polymer chains in such stmctures as tubes, fibers (qv), sheets, powders, and films... 

Biaxial orientation effects are of importance in the manufacture of films and sheet. Biaxially stretched poly(ethylene terephthalate) (e.g. Melinex),... 

In addition to orientation in one direction (mono-axial orientation), biaxial orientation is possible. This is achieved when sheet is stretched in two directions resulting in layering of the molecules. This can increase the impact strength, tensile strength and solvent cracking resistance of polymers and with crystalline plastics the polymer clarity may also be improved. 

Sheet forming processes, such as vacuum forming, do have effects on the product. The designer should be aware that these will affect the performance of one s product and one should learn how to modify the design to minimize any deleterious effects. Probably the most serious problem encountered in formed film or sheet products results from the fact that the materials are made from film or sheet at temperatures well below the melt softening point of the plastic, usually near the heat distortion temperature for the material. Forming under these condition when the draw down ratio is exceeded for a specific plastic can result in over stretched orientation of the material, the production of frozen-in stresses, poor product reproducibil-... 

Orientation In addition to where the product tends to revert to the original shape, the effects of the stretching of the sheet materials result in two other impairments of the product. Highly stretched sections are usually thin and highly oriented. It has a decided tendency to split in these areas in a direction parallel to the stretching that took place. The designer should consider this potential situation and consider thicker material so that the product will perform adequately in use. 

The most common examples of uniaxially oriented materials include fibers, films, and sheets hot-stretched in one direction and composites containing fibers all aligned in one direction. Some injection-molded objects are also primarily uniaxially oriented, but most injection-molded objects have a complex anisotropy that varies from point to point and is a combination of uniaxial and biaxial orientation. 

Fig. 18. Time-resolved small-angle X-ray scattering patterns from polypropylene sheet under quick stretch in the horizontal direction. A speed of stretch was 233 mm/min (367 % stietch/min). An exposure time for each pattern was 0.1 s. Intervals between exposures were 0.2 s. An X-ray wavelength was 0.155 nm. A slight deformation of the symmetric SAXS pattern was already observed in the second patterns, suggesting some degree of orientation was brought about in quite an early stage. The SAXS patterns changed abruptly and drastically in the sixth pattern just when the sample began to yield (when the tension began to decrease).

---