Polymer ductile

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Our principal concern is often the polymer s mechanical properties. For instance, the requirements of the handle of an electrician s screwdriver are very different from those of wire insulation. In the former application, we are free to choose stiff polymers of many types, including glassy amorphous polymers. In contrast, wire insulation must be flexible, which limits our choice to ductile polymers. 

IMPACT Resid Cracking Catalyst, 11 683 performance of, ll 684t Impact resistance, of ductile polymers, 20 355... 

Figure 5.58 The stress-strain behavior of brittle polymer (curve A), ductile polymer (curve B), and highly elastic polymer (curve C). Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 475. Copyright 2000 by John Wiley Sons, Inc.

The stress-strain curves for cortical bones at various strain rates are shown in Figure 5.130. The mechanical behavior is as expected from a composite of linear elastic ceramic reinforcement (HA) and a compliant, ductile polymer matrix (collagen). In fact, the tensile modulus values for bone can be modeled to within a factor of two by a rule-of-mixtures calculation on the basis of a 0.5 volume fraction HA-reinforced... 

In an initially ductile polymer, failure properties (ultimate elongation, fracture toughness, impact resistance) decrease rapidly during a chain-scission aging process, whereas elastic and yield properties are practically unaffected at the embrittlement point. 

For a given rate of chain scission, the failure properties of an initially brittle polymer decrease at a considerably slower rate than for an initially ductile polymer. 

For an initially ductile polymer, the rate of decay of failure properties decreases abruptly after the embrittlement point. 

It is clear that if a relative end-life criterion (e.g., pf = Po/2) is chosen, the ductile polymer will systematically appear less stable than the brittle polymer. However, from the practical point of view, this distinction has no sense. 

During the deformation of ductile polymers there is often an increase in stress with deformation this is known as work-hardening. If at some point the stress is removed, the material recovers along a path nearly parallel to the linear region the sample then shows a permanent plastic deformation. 

Fig. 18. Dependence of stress-strain relationships on relaxation times. The change in the deformational mechanism from brittle to ductile polymers depends on the time scales in which a glassy polymer is measured (t) and relaxed (t)...

Figure 14.2 Nominal stress, a , versus strain, for a ductile polymer and consequent change in the dimensions of the specimen. (X indicates final fracture.)...

Toughness assessment of ductile polymers is still a matter of debate. A sensitive way to characterise the mechanical performance of these materials, and to rank them, is to determine their ductile-brittle transitions. Test speed can thus be varied over several decades of test speed, while keeping the temperature constant, or a wide range of temperature can be scanned in controlled steps at given velocity. In the first case, the higher the speed at which the tough-to-brittle transition occurred, the better the grade in terms of fracture resistance. In the latter case, the lower the temperature at which the brittle-to-ductile transition occurred, the more suited the material for impact applications. 

The possibility to get geometry independent parameters constitutes a master trump to characterise properly a ductile polymer. However, whereas far from the ductile-brittle transition the evaluation of Kes is achieved easily, it is more challenging closer to it. The difficulties to determine reliable effective toughness values in this latter case and thus precise ductile-brittle transitions will be illustrated with grade iPP/EPR-1 tested at room temperature. 

A.S.Saleemi, J.A. Naim, The Plain-Strain Essential Work of Fracture as Measure of the Fracture Toughness of Ductile Polymers, Polym. Eng. Sci., 30 (1990), 211-218. 

Owing to the difficulty of moulding thick sheets, most fixture measurements on pdymers have been made on specimens less than 7 mm thick. This thickness is not sufficient to produce plane strain conditions in most tou or ductile polymers. The difference between plane stress and plane strain fracture has hitherto been widely regarded as unimportant in HIPS and other rubber-modified jdastics, since crazing is itself a plane-strain process, and little attempt has been made to work with thick specimens. However, recent work by Parvin and Williams has shown that there is a very marked transition between plane stress and plane strain fracture in and it is clear that this aspect of rubber-toughening will requite closer attention in the future. 

It is often observed in filled ductile polymers that if adhesion is poor and deformation is accompanied by particle debonding followed by void formation... 

Release of naltrexone. The release of naltrexone using poly (ortho ester) II has already been discussed in Sect. 4.2.5. Since it was not possible to form highly loaded devices that had useful mechanical properties due to an interaction between naltrexone and the polymer at the device fabrication temperatures, it was decided to use the ductile polymer since naltrexone could be mixed into the polymer at room temperature and with appropriate mechanical mixing, high drug loadings could be achieved. 

ZYLAR series of alloys of brittle polymer, ductile polymer and rubbery polymer are available from Novacor Chemicals, Inc. The components are brittle polymer, styrene (70 wt%) and methyl methacrylate (30 wt%) (Novacor s NAS30) ductile polymer, block copolymer derived from styrene (75 wt%) and butadiene (25 wt%) and rubbery polymer, a tapered polymer derived from styrene (43 wt%) and butadiene (57 wt%). No specific gamma stabilizers were added to these polymer alloys. 

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