Engineering plastics ductility

Copolymers are typically manufactured using weU-mixed continuous-stirred tank reactor (cstr) processes, where the lack of composition drift does not cause loss of transparency. SAN copolymers prepared in batch or continuous plug-flow processes, on the other hand, are typically hazy on account of composition drift. SAN copolymers with as Httle as 4% by wt difference in acrylonitrile composition are immiscible (44). SAN is extremely incompatible with PS as Httle as 50 ppm of PS contamination in SAN causes haze. Copolymers with over 30 wt % acrylonitrile are available and have good barrier properties. If the acrylonitrile content of the copolymer is increased to >40 wt %, the copolymer becomes ductile. These copolymers also constitute the rigid matrix phase of the ABS engineering plastics. 

Several flexible polymers, such as natural rubber (NR) synthetic rubber (SR) polyalkyl acrylates copolymers of acrylonitrile, butadiene, and styrene, (ABS) and polyvinyl alkyl ethers, have been used to improve the impact resistance of PS and PVC. PS and copolymers of ethylene and propylene have been used to increase the ductility of polyphenylene oxide (PPO) and nylon 66, respectively. The mechanical properties of several other engineering plastics have been improved by blending them with thermoplastics. 

PTFE is a tough, flexible material of moderate tensile strength (2500-3800 psi, i.e., 17-21 MPa) at 23°C. Temperature has a considerable effect on its properties. It remains ductile in compression at temperatures as low as 4 K (—269°C). The creep resistance is low in comparison to other engineering plastics. Thus, even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 300 psi (2.1 MPa). 

The term plastic also refers to a material that has the physical characteristics of plasticity and toughness. See ductility plasticity toughness, plastic, advanced A high-performance material. Also called advanced reinforced plastic or advanced plastic composite. See engineering plastic reinforced plastic, advanced. 

Finally, another type of transition is provided by changing particle size at a fixed volume fraction of modifier. Figure 6 [6] illustrates these issues. Only particles with diameters between approximately 0.2 pm and 0.6 pm enhance toughness sufficiently for the matrix to fracture in a ductile manner. Therefore, an optimum particle size range is required for effective toughening. This type of behavior is expected to occur in other engineering plastics, although it has not been as systematically studied in matrices other than nylon. 

Frequently, engineering plastics are reinforced with glass fibers to enhance their stiffness and creep resistance. The presence of glass fibers in the matrix significantly decreases notched Izod impact values. No glass reinforced sample behaves in a ductile fashion. The addition... 

The properties presented in the table show average impact resistance values for engineering plastics and alloys. Final fabrication conditions can significantly affect their impact resistance, particularly where crystallinity is present, as is the case in most engineering polymers. Also noteworthy in this series is the impact behavior of polycarbonate, which responds in a ductile fashion in thin samples, even under notched conditions. Thicker specimens are brittle. Tougheners have to be incorporated to re-establish the original ductility. A similar behavior is observed for the other systems, although not to the same extent as in polycarbonate. 

GE Plastics has introduced Lexan EXE, an extra tough polycarbonate with added impact resistance and low temperature ductility. These attributes, plus its light weight, make it a great material for a variety of applications including telecommunications, portable electronics and outdoor equipment. It can replace metal and other engineering plastics. Lexan EXE sheet is finding new opportunities in vacuum formed parts, sound barriers and architectural applications. 

Rubbers are exceptional in behaving reversibly, or almost reversibly, to high strains as we said, almost all materials, when strained by more than about 0.001 (0.1%), do something irreversible and most engineering materials deform plastically to change their shape permanently. If we load a piece of ductile metal (like copper), for example in tension, we get the following relationship between the load and the extension (Fig. 8.4). This can be... 

Thermal Gradients may be measured or calculated by means of heat flow formulas, etc. After they are established it is likely to be found from the formula that for most cyclic heating conditions the tolerable temperature gradient is exceeded. This means that some plastic flow will result (for a ductile alloy) or that fracture will occur. Fortunately, most engineering alloys have some ductility. However, if the cycles are repeated and flow occurs on each cycle, the ductility can become exhausted and cracking will then result. At this point it should be recognized that conventional room temperature tensile properties may have little or no relation to the properties that control behavior at the higher temperatures. 

Equations 8.24 and 8.25 only apply to elastically brittle solids such as glass. However, many engineering materials only break in a truly brittle manner at very low temperature and above these temperatures failures are pseudo-brittle. These have many of the features of brittle fracture but include limited ductility. This plastic work can be included in the above equations, i.e. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

Many engineering thermoplastics such as nylon, high-impact PS, polyesters, and toughened plastics exhibit responses similar to those shown in the next two curves in Figure 3.3, designated ductile. Here the stress achieves a maximum called a yield stress at a specific strain. As strain increases beyond... 

Figure 3.3 Stress-strain plots representative of brittle, ductile, and elastomeric polymeric materials. Failure is denoted by . (After J. Fried, Plastics Engineering, July 1982, with permission.)...

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

One of the most important features of engineering materials is ductility which depends mainly on the competition between two macroprocesses — plasticity and fracture. 

---