Draw temperature

The mechanical properties can be studied by stretching a polymer specimen at constant rate and monitoring the stress produced. The Young (elastic) modulus is determined from the initial linear portion of the stress-strain curve, and other mechanical parameters of interest include the yield and break stresses and the corresponding strain (draw ratio) values. Some of these parameters will be reported in the following paragraphs, referred to as results on thermotropic polybibenzoates with different spacers. The stress-strain plots were obtained at various drawing temperatures and rates. 

These differences on the stress-strain behavior of P7MB and PDTMB show the marked influence of the mesomorphic state on the mechanical properties of a polymer. When increasing the drawing temperatures and simultaneously decreasing the strain rate, PDTMB exhibits a behavior nearly elastomeric with relatively low modulus and high draw ratios. On the contrary, P7MB displays the mechanical behavior typical of a semicrystalline polymer. 

Figure 14 Stress-strain plots of several PTEB specimens, stretched at I cm/min and two drawing temperatures. The inset shows the two CR specimens at low deformations.

A common feature of the three PTEB samples is that the yield stress decreases as the drawing temperature increases (Table 2), whereas it does not change significantly with the strain rate. The Young modulus does not change with the strain rate but it decreases and the break strain increases as the drawing temperature increases. The main conclusion is that the behavior of PTEB-RT is intermediate between the other two samples, with the advantage of a considerable increase in the modulus in relation to sample PTEB-Q and without much decrease in the break strain (Table 2). 

Fig. 7. Gauche/trans content versus

In order to ensure uniform drawing temperatures and avoid excessive wear on the dies and mandrels used, it is essential that a suitable lubricant be applied during drawing. A wide variety of lubricants are used for this purpose. Heavier draws may require oil-based lubricants, but oil-in-water emulsions are used for many applications. Soap solutions may also be used for some of the lighter draws. Drawing oils are usually recycled until their lubricating properties are exhausted. 

Figure 11.15 PTT stress-strain curves at draw temperatures below and above the glass transition temperature [77]. Reprinted in part with permission from Chuah, H. H., Macromolecules, 34,6985-6983 (2001). Copyright (2001) American Chemical Society...

This feature will be increasingly Important as battery manufacturers continue to increase the cell capacity with thinner separators. The pore structure is usually influenced by polymer composition, and stretching conditions, such as drawing temperature, drawing speed, and draw ratio. In the wet process, the separators produced by the process of drawing after extraction (as claimed by Asahi Chemical and Mitsui Chemical) are found to have much larger pore size (0.24—0.34 fixxi) and wider pore size distribution than those produced by the process of extraction (0.1—0.13 after drawing (as claimed by Tonen). ... 

More than 50% water is converted into ice crystals in ice cream at -5°C to -6°C which is the common drawing temperature for correctly operated continuous freezers. This portion of the water freezes very rapidly, often in less than one minute. Fast freezing induces the formation of small ice crystals, a critical prerequisite for smooth ice cream. At slightly higher temperatures (such as -4°C which is the common drawing temperature for batch freezers), less than 40% water is frozen and the freezing time will be longer. This is one of the reasons why ice cream frozen continuously is smoother in texture than batch-frozen products. 

The viscosity of ice cream mix is important for processing in the ice cream freezer and must be within certain limits. Factors which may increase viscosity are increased %solids content, particularly hydrocolloids and protein, and low drawing temperatures in the freezer. Viscosity is usually measured on a simple comparison basis using a 50 to 100 ml capacity pipette, marked at an arbitrary place below the bulb. The flow time required to discharge the sample to the lower mark may be determined for water and then for the sample being tested for comparative purposes, and recorded in seconds62. More sophisticated rotation viscometers may also be used. 

Fig. 5 Stress-strain curves for solution-cast UHMW-PE films compared to melt-crystallized films, k refers to the draw ratio. Films were drawn at a drawing temperature of Td = 115 °C...

An interesting example of the difference in drawing behaviour between amorphous and crystalline yam is the drawing of crystalline poly(ethylene terephthalate). It is often stated that crystalline PETP cannot be drawn. It is tme that the material breaks if drawn at a temperature of 80 °C, which is a drawing temperature normal for the amorphous polymer. Mitsuishi and Domae (1965), however, were able to draw crystalline PETP to a draw ratio of 5.5 at a temperature of 180 °C. 

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. 

The optimum draw ratio is close to the micropore saturation level (if any), since further drawing would lead to an undesirable embrittlement of the fiber without substantial gains in the amount of microvoids. By analogy with the results obtained with stretched films, a maximum of micropores is expected for drawing temperatures around 80-110 °C [128,129]. However, no incontestable experimental evidence has been provided up to now to support this assertion. 

Fig. 12. Effect of draw temperature on the maximum draw ratio attainable (solid line) for high molecular weight (M = 8 X lO ) LPE. Also shown (broken line) the room temperature modulus of samples drawn to the maximum draw ratio at each temperature...

It was also shown that the draw temperature affects G, and the best prospects for producing oriented PE sheets with adequate tearing resistance are obtained by draw-... 

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