Stress-to-break

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 3001bf/in (2.1 MPa). 

For thermosets with molecular weight between crosslinks M, the crosslink density Px, is described by Px N /Mx- As Px increases, the nets become tighter and stiffer, and thus require more stress to break via... 

The large drop (% 45%) in the stress to break between the non-oxidized sample and the oxidized ones, and... 

From this figure, safe wall stresses at various temperatures and loading times can be determined by extrapolation. We extrapolate the curves found at higher T to lower Ts. Here a ten years life (87,660 hours) at 80 °C is required. Extrapolation of the curve for 80 °C results in an estimated stress to break of 28 MPa. To remain at the safe side, we stay somewhat below this value, so that 20 MPa looks a reasonably safe wall stress for this purpose. 

The stress to break the bonds between the floes may be calculated as the difference between the static, aos, and the dynamic, aod, yield stresses of the samples with undisrupted and disrupted structure, respectively. 

Lake and Thomas supposed that no primary bond in a crt -linked network can fracture unless all the bonds in that particular network chain (ie. between adjacent cross-links) are stressed to breaking point. Thus, if there are n inter-atomic bonds between cr< s-links (on average) the minimum energy to cause one bond to fracture is not the dissociation energy of one bond but n times that energy. Their prediction takes the form... 

The intermolecular forces of attraction in hexatriacontane cited above and in polyethylene are essentially van der Waals forces. However, in polymers of large molecular size, there are so many intermolecular contacts that the sum of van der Waals forces holding each molecule to its neighbors is appreciable. It thus requires a much- larger stress to break a polymeric material as this involves separating the constituent molecules. Deformation of a polymeric structure for the same reason requires more force as the macromolecules become larger. 

Other important parameters of load-elongation curves are the Hookean limit (Figure 8-2, Point A), the turnover point (Figure 8-2, Point B), the percentage extension to break, the stress to break (the tensile strength), the... 

The stress at break provides a lower value than the elastic modulus. The equation given for Es may be used to calculate both values, and by convention, the El term is in force/mm elongation for the elastic modulus, while it is in force units for the stress to break. 

Several duties involving the dispersion of fine solids and emulsification cannot be carried out in the conventional mechanically agitated vessel because it is not possible to generate sufficiently high stresses to break>down the aggregated particles to achieve the required dispersion quality or to create a stable emulsion. 

Our interest lies in the loci of failure points as a function of temperature. The family of curves of stress to break (t, at various temperatures (of which the dashed-line portion of Figure 1.22 is representative) is plotted schematically in Figure 1.23 (Scott, 1967). Use may now be made of the time-temperature superposition principle and the WLF equation (Section 1.5.7) to construct a master curve, as shown in Figure 1.24. 

The strain to break 6, may be plotted against the stress to break in the master curve to yield a failure envelope, shown schematically in Figure 1.25 (Scott, 1967). The failure envelope is independent of temperature, time to break, and strain rate. It is a universal curve independent (at least ideally) of the type of rupture test. If is further divided by the crosslink density, the resulting failure envelope is also approximately independent of both the degree of crosslinking and the chemical structure of the elastomer. The latter... 

The ultimate utility of any material lies in its performance. Let us examine now the types of reinforcement obtained with IPN s. Stress-strain curves for random (R) PB/PS IPN s are shown in Figure 8.22. As is well known, random cis-trans mixture) polybutadiene homopolymer is very weak, breaking at a rather low elongation. Both ultimate elongation and stress to break are increased by addition of the polystyrene network the work required to break, as measured by the area under the curves, is vastly increased. Further, the shapes of the curves are affected by the presence... 

Figure 8.22. Stress-strain behavior of random PB/PS IPN s. Both ultimate elongation and stress to break are significantly increased. (Curtius et a/., 1972.)...

Effect of Physical Properties on Bubble Breakup and Size High surface tension liquids tend to stabilize bubbles. This is evident from Equation 7A.3, which indicates that a higher surface tension of the liquid requires higher turbulent stress to break the bubble. 

The mixing section should have a high stress region, HSR, where the material is subjected to high, preferably elongational, stresses to break down agglomerates and droplets. 

There are several terms in use that describe the strength of a polymer. The tensile strength describes the stress to break the material, usually in elongation, for example. The tensile strength is certainly important, but in engineering practice a polymer is rarely stressed so greatly that it breaks immediately. The toughness of the polymer is frequently a more useful parameter. 

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