Stretch rate factor

The factor having the strongest effect is the elongation imparted in the process of production stretching. Second, the overall orientation is affected by the stretching rate. For the same draw ratio, the overall orientation grows with an increase in the stretching rate. The effect of the draw ratio on the value of Hermans function of orientation is illustrated by the values of/o, established by the authors and depicted in Table 7. 

One major consequence of the Ml project was the development of a modified filament stretching instrument by Sridhar. In this device, the test sample is held horizontally between two Teflon discs and pulled equally at both ends at a programmable exponential rate such that a constant strain rate is achieved and the stress growth at a constant stretch rate is obtained (40). It appears though that the test sample has to adhere to the plates as the technique does not use aids to clamp samples. Consequently, it is not clear if the technique can be applied to products that are non-sticky or exhibit slip, which could be limiting factors for testing food products. 

Enough experimental data are now available that a clear picture of polymer behavior in extension is emerging. For Newtonian liquids, it is found, by both experiment and theory, that the extensional viscosity equals exactly three times the shear viscosity. For non-Newtonian polymer melts, though, the extensional viscosity can exceed three time the zero-shear viscosity by more than a factor of ten. The extensional viscosity, however, is a function of the stretch rate and is frequently a decreasing function of the stretch rate, especially at large values of the stretch rate (see Figure 5). 

Step-strain stress-relaxation measurements have been frequently used to determine Sr(X) for polymer melts > . Equation (6) shows that if separability of time and strain effects is possible for the melt under consideration, the stress after a step elongational strain can be factored into a time-dependent function, the linear shear relaxation modulus G(t), and a strain-dependent function, the nonlinear strain measure Sr(X). Also other types of experiment may be oerformed to obtain Sr(X), such as constant-strain-rate experiments "", creep under constant stress and constant-stretching-rate experiments but these methods require more involved analytical and/or numerical calculations. 

They studied solutions of polyethylene oxide (a flexible coil, water-soluble polymer similar physically to HPAM) flowing through porous beds of different grain sizes and reported the onset of elastic behaviour at maximum stretch rate to be of the order 100 s and shear rates of the order 1000 s" More recently, Durst and Haas have made an extensive study of the flow of viscoelastic polymers in porous media (Durst et al, 1981, 1982. Haas et al, 1981a, b). They characterised the onset of high resistances by using a product of a resistance factor / and the Reynolds number Re, defined as ... 

The effects of mechanical degradation for uniform materials can be correlated from the stretch rate of the fluid. Fig. 5.49 is the correlation between screen-factor loss and a group deter-... 

To estimate the maximum rate polymer can be injected, the maximum polymer stretch rate was calculated to occur in the reservoir matrix just outside the perforations. The difficulty in estimating the true velocity (and hence shear degradation) of the polymer bank in the reservoir is the uncertainty in diameter of the perforations and the extent they protrude into the sand matrix. Calculations based on Maerker s work indicate that the sandpack data of Fig. 5 should undergo a 78% reduction in screen factor at 352 ft/D (true velocity). 

A covalent bond (or particular nomial mode) in the van der Waals molecule (e.g. the I2 bond in l2-He) can be selectively excited, and what is usually observed experimentally is that the unimolecular dissociation rate constant is orders of magnitude smaller than the RRKM prediction. This is thought to result from weak coupling between the excited high-frequency intramolecular mode and the low-frequency van der Waals intemiolecular modes [83]. This coupling may be highly mode specific. Exciting the two different HE stretch modes in the (HF)2 dimer with one quantum results in lifetimes which differ by a factor of 24 [84]. Other van der Waals molecules studied include (NO)2 [85], NO-HF [ ], and (C2i J )2 [87]. 

Prices of spandex fibers are highly dependent on thread size selling price generally increases as fiber tex decreases. Factors that contribute to the relatively high cost of spandex fibers include (/) the relatively high cost of raw materials, (2) the small size of the spandex market compared to that of hard fibers which limits scale and thus efficiency of production units, and (J) the technical problems associated with stretch fibers that limit productivity rates and conversion efficiencies. 

In semi-cristalline polymers, rate-enhancement under stress has been frequently observed, e.g. in UV-photooxidation of Kapron, natural silk [80], polycaprolactam and polyethylene terephthalate [81]. Quantitative interpretation is, however, difficult in these systems although the overall rate is determined by the level of applied stress, other stress-dependent factors like the rate of oxygen diffusion or change in polymer morphology could occur concurrently and supersede the elementary molecular steps [82, 83], Similar experiments in the fluid state showed unequivocally that flow-induced stresses can accelerate several types of reactions, the best studied being the hydrolysis of DNA [84] and of polyacrylamide [85]. In these examples, hydrolysis involves breaking of the ester O —PO and the amide N —CO bonds. The tensile stress stretches the chain, and therefore, facilitates the... 

If V v = 0, e,s(l/2)m in two-dimensional (2D) flows and (2/3)1/2 in three-dimensional (3D) flows, where i = A,r/. The efficiency can be thought of as the specific rate of stretching of material elements normalized by a factor proportional to the square root of the energy dissipated locally. 

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