Draw ratio, natural

Table 2. Lamellar thickness 1, as function of annealing temperature at 5.3 Kbar for PE cold-drawn to a natural draw ratio X 8.

As mentioned before, the conventional drawing process leads to the well-known fibrillar structure, which still contains a majority of folded chains (Fig. 19.14). In drawing above the "natural" draw ratio further unfolding takes place. Such a process at very high draw ratios must be conducted with the utmost care since critical concentrations of stress on the folded chain surface of the crystal blocks must be avoided they lead to fracture. The super-drawing can be carried out in one and two stages. 

Spectacular results have been obtained with two-stage drawing of polyoxymethylene by Clark and Scott (1974). In the first stage and polymer is drawn to the natural draw ratio of about 7. The second stage takes place at a very low velocity (50% elongation per minute) up to an ultimate draw ratio of about 20. The optimum temperature for the second step is about 30 °C below the standard melting point. A modulus of 35 GPa, about 70% of the theoretical value has been obtained in this way. The drawn material did not show folded chain periodicity anymore. 

A quantity often used in the description of drawing phenomena is the natural draw ratio. As can be seen in Fig. 24.10, simple drawing of a material with the given stress-strain curve results in a fixed draw ratio corresponding to point B on the curve. This is called the natural draw ratio. 

The natural draw ratio is not constant, but dependent on experimental conditions temperature, rate of deformation, etc. Nevertheless, the order of magnitude of the natural draw ratio gives an indication of the stretchability of a given material. Very broadly speaking, the polymers can be divided into three categories ... 

Typically amorphous polymers, such as polystyrene and polysulphone they generally show a weak strain-hardening effect and their natural draw ratios are about 1.5-2.5... 

Polymers with a certain degree of crystallinity in the drawn state to this group belong the important synthetic fibres like polyesters and polyamides with natural draw ratios of about 4—5... 

The typically high-crystalline polymers, such as polyethylene and polypropylene here high natural draw ratios (5-10) are found in combination with gel-spinning they may be as high as 50... 

In principle, a simple bench-drawing test may be used to obtain an impression of the stretchability and of the natural draw ratio of a given polymer. However, as the rate of deformation in the bench test is appreciably lower than under technical drawing conditions, testing should be done below the technical drawing temperature. An impression of the order of magnitude of this temperature difference may be obtained by application of the Williams-Landel-Ferry equation (see Chap. 13). The temperature difference may be more than 20 °C. 

Natural draw ratio, 738 Nature of the glass transition, 151 Neck/Necking, 463, 728, 813 zones, 33... 

Thus the combination of pre-orientation to suppress crazing and to reduce the natural draw ratio and rubber block introduction to bias the deformation response toward yielding has converted a brittle polymer to a moderately ductile one. Although the BPFC domain Tg has been reduced by roughly 30 °C, the improved balance of mechanical properties makes this sacrifice worthwhile. 

Duckett, R. A. The Natural Draw Ratio, to appear in Proc. Internat. Spring School on Plastic Deformation of Amorphous and Semicrystalline Materials, Les Houches, France, April 19-29, 1982 (ed. B. Escaig, C. G Sell), (Les Editions de Physique, Les Ulis, 1983) p. 253 Gent, A. N., Thomas, A. G. J. Polymer Sci. A2 10, 571 (1972)... 

In the discussion of the shear yielding and crazing behavior of PC (in Sect. 3.2 and 4.1), the existence of characteristic extension ratios has become apparent (1) The extension ratio, 7. , after shear yielding referred to as natural draw ratio, (2) the extension ratio, V, of craze I fibrils and (3) the extension ratio, at craze II initiation. 

It was established that each polymer has a maximum draw ratio, X, also called natural draw ratio or maximum strain at break. Owing to the influences of one polymer on another, the relationship between in the blends and that of the components is not straightforward. values larger than that of either component have been reported in blends. 

In seetion 8.2.2 the phenomenon of cold drawing and the associated natural draw ratio are described. This process is referred to again in section 10.2.2 and discussed further in section 10.2.4. 

Nadella et al. [204] studied the drawing of samples H-0042 and 11-1200, which were produced earlier in their spinning studies [79]. The drawing was carried out on an Instron tensile testing machine. One-inch samples were stretched to three different lengths 1 in. beyond the natural draw ratio (disappearance of neck), approximately 1 in. prior to fracture, and between the above two. If necking was not observed, the first stage corresponded to a... 

MEASURED VALUES OF EXTENSIONAL MODULI OF POLYETHYLENE TEREPHTHALATE FILAMENTS (IN ON ra -) FOR A RANGE OF NATURAL DRAW RATIOS (NDR) AND INITIAL BIREFRINGENCE (A/i) COMPARED WITH REUSS AND VOIGT PREDICTIONS FOR AN AGGREGATE MODEL. 

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