Polycarbonate fracture

Finger-like advance of cracks lying in a plane (a) Surface of polycarbonate fractured in a carbon tetrachloride environment the liquid has advanced down parallel channels, (b) Sketch of a proposed method of craze advance. 

The latter equation contains constants with well-known values and can therefore be used to predict the fracture stress of most polymers. For example, the bond dissociation energy Do, is about 80 kcal/mol for a C-C bond. For polystyrene, the modulus E 2 GPa, A. 4, p = 1.2 g/cm, = 18,000, and we obtain the fracture stress, o A1 MPa, which compares well with reported values. Polycarbonate, with similar modulus but a lower M. = 2,400 is expected to have a fracture stress of about 100 MPa. In general, letting E 1 GPa, p = 1.0 g/cm, and Do — 335 kJ/mol, the tensile strength is well approximated by... 

Heterogeneous compatible blends of preformed elastomers and brittle plastics are also an important route for the development of blends of enhanced performance with respect to crack or impact resistance. Polycarbonate blends with preformed rubber particles of different sizes have been used to provide an insight into the impact properties and the fracture modes of these toughened materials. Izod impact strength of the blends having 5-7.5 wt% of rubber particles exhibits best overall product performance over a wide range temperature (RT to -40°C) [151-154]. 

Figure 1 Freeze-fracture electron micrographs of egg phosphatidylcholine large unilamellar vesicles prepared by extrusion through polycarbonate filters with pore sizes of (A) 400 nm, (B) 200 run, (Q 100 nm, (D) 50 nm, and (E) 30 nm. The bar in panel (A) represents 150nm. Source From Ref. 7.

Fig. 6.8. Fracture toughness, K, of short glass fiber-thermoplastics injection molded composites as a function of weight fraction of fiber, fVr. (O) and (A) polyethylene terephthalate (PET) matrix ( ) and (A) polycarbonate (PC) matrix. Notches made transverse (O, ) and parallel (A, A) to the mold fill direction,...

The purpose of this paper is to investigate the mechanical properties (plastic deformation, micromechanisms of deformation, fracture) of several amorphous polymers considered in [1], i.e. poly(methyl methacrylate) and its maleimide and glutarimide copolymers, bisphenol A polycarbonate, aryl-aliphatic copolyamides. Then to analyse, in each polymer series, the effect of chemical structure on mechanical properties and, finally, to relate the latter to the motions involved in the secondary transitions identified in [ 1] (in most cases, the p transition). 

Bisphenol A polycarbonate (BPA-PC), whose the chemical structure is shown in Fig. 66a, has very interesting fracture properties, exhibiting quite a high toughness for a pure amorphous polymer. At a very low temperature (- 100 °C at 1 Hz) it presents a secondary fi transition, shown in Fig. 67, which has been analysed in detail in [1] (Sect. 5). 

The fracture energy GIc of unsaturated polyesters (UP), vinyl esters (VE), and phenolic resins, is less than 200 Jm 2 at room temperature. Epoxy networks can exhibit higher values but always lower than those of thermoplastics of similar Tg, as polycarbonate, polyetherimide, or polyphenylene ether. 

Measurements. The morphology of the blends was studied by optical microscopy (Leitz Dialux Pol), transmission electron microscopy (Jeol 100 U), and scanning electron microscopy (Cambridge MK II). Ultramicrotome sections were made with an LKB Ultratome III. Samples for scanning electron microscopy were obtained by fracturing sheets at low temperature. The fracture surfaces were etched with a 30% potassium hydroxide solution to hydrolyse the polycarbonate phase. Stress-relaxation and tensile stress-strain experiments were performed with an Instron testing machine equipped with a thermostatic chamber. Relaxation measurements were carried out in flexion (E > 108 dyn/cm2) or in traction (E < 108 dyn/cm2). Prior to each experiment, the samples were annealed to obtain volumetric equilibrium. 

The first secondary transition below Tg, the so called fj-relaxation, is practically important. This became evident after Struik s (1978) finding that polymers are brittle below Tp and establish creep and ductile fracture between Tp and Tg. The p-relaxation is characteristic for each individual polymer, since it is connected with the start of free movements of special short sections of the polymer chain. In view of more recent data of Tp Boyer s relation, Eq. (6.29), is very approximate and fails completely for amorphous polymers with high Tg s (e.g. aromatic polycarbonates and polysulphones). Some rules of thumb may be given for a closer approximation. 

The nanotubes were first oxidized in nitric acid before dispersion as the acidic groups on the sidewalls of the nanotubes can interact with the carbonate groups in the polycarbonate chains. To achieve nanocomposites, the oxidized nanotubes were dispersed in THF and were added to a separate solution of polycarbonate in THF. The suspension was then precipitated in methanol and the precipitated nanocomposite material was recovered by filtration. From the scanning electron microscopy investigation of the fracture surface of nanotubes, the authors observed a uniform distribution of the nanotubes in the polycarbonate matrix as shown in Figure 2.3 (19). 

Figure 2.3. Scanning electron microscopy image of the fracture surface of the polycarbonate nanocomposite. Reproduced from reference 19 with permission from American Chemical Society.

Figure 2.6. SEM micrographs of fracture surfaces of polycarbonate nanotube composites containing (a) 20 wt% filler and (b) 15 wt% filler. Reproduced from reference 33 with permission from Elsevier.

A significant improvement in the administration of a lipid-containing emulsion was achieved with a special additive to PC. As shown in Fig. 22b, the improved behavior of the new polycarbonate PC2 proved in practice to be verifiable by fracture mechanical fatigue crack growth experiments. In the presence of the fat emulsion the more lipid-resistant PC2 shows a higher fatigue threshold value AK as well as an improvement by a factor of two in AKcf in comparison to PCI used so far. 

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-... 

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

Tensile Yield Stresses of Cast Films. At room temperature all of the BPFC-DMS polymers investigated (with one exception) reached their yield stresses before fracturing. BPF polycarbonate on the other hand is brittle, breaking at about 11,000 psi. Traces of residual chloroform make the homopolymer ductile however the yield stress decreased linearly with chloroform content. Extrapolation of these results to a dry polymer gives a yield stress of 14,000 psi. 

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). 

The parameters which apply to plane strain fracture are G c and Ki, where the subscript 1 indicates that the crack opening is due to tensile forces. K]c is measured by applying Eq. (11-52) to data obtained with thick specimens. To illustrate the differences between plane stress and plane strain fracture modes, thin polycarbonate specimens, with thicknesses <3 mm reported to have G values of 10 kJ/m, while the Gic of thick specimens is 1.5 kJ/m. ... 

Figure 14.8 Stress-strain curves for polycarbonate at T = 11 K determined under tension and uniaxial compression. The nominal stress curves, cs , correspond to the dashed lines, and those for the true stress, o- correspond to solid lines. The material tested under tension fractures immediately after reaching yield, unlike the situation that occurs under compression. (From Ref. 12.)...

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

A rectangular bar of polycarbonate of thickness B = 2Q mm and width W = 20 mm is loaded in a three-point bending test with an 160 mm span. Calculate (a) the force needed to fracture the bar if it has a notch of length a= 10 mm and (b) the minimum notch length, a, needed to initiate brittle fracture before yield occurs. 

---