Polycarbonate, crazing yield

LeGrand, D.G. Crazing, yielding, and fracture of polymers 1. Ductile-brittle transition in polycarbonate. J. Appl. Polym. Sci. 1969, 13, 2129-2147. 

The mechanisms of crack propagation in poly(methyl methacrylate) eure particularly amenable to analysis. The situation is not so simple for other polymers. For example, in polystyrene there is usually multiple crazing in the vicinity of the crack tip. In tougher polymers such as polycarbonate shear yielding as well as crazing often takes place at the crack tip. In these cases crack propagation does not occur in such a well-controlled manner as in poly (methyl methacrylate) and it is more difficult to analyse. 

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

Failure Morphologies. Ductile failure of notched polycarbonate specimens has long been recognized to occur with shear yielding from the notch tip (6). This occurs for the block polymers for all rates of test. Hull and Owen (5) recently reported from micrographic studies of impact fracture surfaces that the brittle failure of polycarbonate involves the formation and breakdown of a craze at the notch tip. The ductile-... 

Using the cathetometer the force at initiation was measured on notched bars of % inch thick extruded Lexan polycarbonate sheet over the widest temperature range possible at one rate of deformation. These initiation forces and the forces at failure are shown in Figure 7a. Brittle failure parallels craze initiation in its temperature dependence. Yield force has a greater temperature dependence—one that approximately parallels the measured values of yield stress in linear tensile tests (data... 

Failure Mechanisms. BPF polycarbonate develops crazes at ascending stresses and fractures in a pseudo-brittle manner similar to polystyrene or PMMA. At room temperature the block polymers develop few separate crazes. As the yield is approached, shear bands grow from the edges. Fracture initiates at an edge from a point where the two shear bands initiated. When a neck forms, the plastic strain in the neck is ca. 80% however fracture occurs shortly after the neck is formed so that the ultimate elongation of the specimen is only 10 or 12%. The shear bands and necks show some stress whitening (Figure 9). 

Glassy polymers with highercohesiveness, like polycarbonate and cross-linked epoxies, preferentially exhibit shear yielding [7], and some materials, such as rubber-modified polypropylene, can either craze or shear yield, depending on the deformation conditions [8]. Application of a stress imparts energy to a body which... 

Fig. 20a—d. Surface stress profiles for polytertbutylstyrene [PTBS], poly(styrene-26% acrylonitrile) [PSANl] and poly(styrene-65% methylmethacrylate) [PSMMAj. The stress at the craze tip S, is plotted vs. Ve in d. The value of the shear yield stress Y of polycarbonate is indicated... 

Tg—TJ and change with the size of the molecular segment involved in yield a single value corresponding to the Y of polycarbonate at 300 K is indicated on Fig. 20. For polymers with Ve above about 5 x 10 m" shear yield should oceur in preference to crazing. Because of individual variations in Y from polymer to polymer, in isolated instances crazing may be favored over shear even above this value this may be the reason for the anomalous behavior of PMMA. 

As could be expected, the mechanical properties of a crazed polymer differ from those of the bulk polymer. A craze containing even 50% microcavities can still withstand loads because fibrils, which are oriented in the direction of the load, can bear stress. Some experiments with crazed polymers such as polycarbonate were carried out to get the stress-strain curves of the craze matter. To achieve this aim, the polymer samples were previously exposed to ethanol. The results are shown in Figure 14.24 where the cyclic stress-strain behavior of bulk polycarbonate is also illustrated (32). It can be seen that the modulus of the crazed polymer is similar to that of the bulk polymer, but yielding of the craze occurs at a relatively low stress and is followed by strain hardening. From the loading and unloading curves, larger hysteresis loops are obtained for the crazed polymer than for the bulk polymer. 

Crazes do not form unless the tensile strain exceeds a critical value, approximately 2% for polycarbonate and 0.4% for polystyrene, in air. We will see in Chapter 10 that these values are reduced when the polymer is exposed to certain liquids. If the applied strain barely exceeds the critical value, the crazes are widely spaced. The craze spacing decreases as the applied strain increases. This is further evidence that crazing is a yield process. 

Lazzeri and Bucknall [131] have proposed that the pressure dependence of yield behaviour caused by the presence of microvoids can explain the observation of dilatation bands in rubber-toughened epoxy resins [132], rubber-toughened polycarbonate [133] and styrene butadiene diblock copolymers [134]. These dilatation bands combine in-plane shear with dilatation normal to the shear plane. Whereas true crazes contain interconnecting strands, as described in Section 12.5.1 above, dilatation bands contain discrete voids that, for rubber-toughened polymers, are confined to the rubber phase. 

Keywords crazing, engineering resins, impact modifiers, impact strength, nylon 6, nylon 6,6, polycarbonate, poly(butylene terephthalate), poly(ethylene terephthalate), polyester, shear yielding, toughening, Izod, Charpy, alloys. 

Fig. 11.6 Craze region at tip of plastic yield zone ahead of an edge crack in thin sheet of polycarbonate. (The white region along the crack and the long plastic zone is reflected light due to the oblique angle of exposure (out of the plane of the page).

Figure 12.21 The strain rate dependence of the octahedral shear stress r at atmospheric pressure using data from torsion (o), tension (A), and compression. (Reproduced with permission from Duckett, R.A., Coswami, B.C., Smith, LS.A. et al. (1978) Yielding and crazing behavior of polycarbonate In torsion under superposed hydrostatic-pressure. Brit. Polym. J., 10, 11. Copyright (1978) Society of Chemical Industry.)...

Duckett, R.A., Goswami, B.C., Smith, L.S.A. et al. (1978) Yielding and crazing behavior of polycarbonate in torsion under superposed hydrostatic-pressure. Brit. Polym. 10, 11. 

In several glassy polymers [22,23], such as the polycarbonate shown in Figure 13.9, a complication occurs in that a thin line of material called a shear lip forms on the fracture surface where the polymer has yielded. Analogous to the behaviour of metals, it has been proposed that the overall strain energy release rate G is the sum of the contribution from the craze and that from the shear lips. To a first approximation, we would expect the latter to be proportional to the volume of yielded material. If the total width of the shear lip on the fracture surface is w, 5 is the specimen thickness and the shear lip is triangular in cross section, then... 

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