Mechanical properties of plastics

Plastics with stress-strain curves of the type in Fig. 20.1a are rigid and brittle. The former term refers to the high initial modulus. The latter refers to the area under the stress strain curve, which represents the energy per unit volume required to cause failure. These materials usually fail by catastrophic crack propagation at strains on the order of 2%. Since Wdness correlates well with tensile modulus, it is another valuable property of this type of plastic. Examples of this class are polystyrene, polymethyl methacrylate, and most thermosets. 

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Mechanical properties of plastics can be determined by short, single-point quaUty control tests and longer, generally multipoint or multiple condition procedures that relate to fundamental polymer properties. Single-point tests iaclude tensile, compressive, flexural, shear, and impact properties of plastics creep, heat aging, creep mpture, and environmental stress-crackiag tests usually result ia multipoint curves or tables for comparison of the original response to post-exposure response. 

Determining and Reporting Dynamic Mechanical Properties of Plastics... 

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

The mechanical properties of plastics materials may often be considerably enhanced by embedding fibrous materials in the polymer matrix. Whilst such techniques have been applied to thermoplastics the greatest developents have taken place with the thermosetting plastics. The most common reinforcing materials are glass and cotton fibres but many other materials ranging from paper to carbon fibre are used. The fibres normally have moduli of elasticity substantially greater than shown by the resin so that under tensile stress much of the load is borne by the fibre. The modulus of the composite is intermediate to that of the fibre and that of the resin. 

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

Mechanical properties of plastics are invariably time-dependent. Rheology of plastics involves plastics in all possible states from the molten state to the glassy or crystalline state (Chapter 6). The rheology of solid plastics within a range of small strains, within the range of linear viscoelasticity, has shown that mechanical behavior has often been successfully related to molecular structure. Studies in this area can have two objectives (1) mechanical characterization of... 

The mechanical properties of plastics enable them to perform in a wide variety of end uses and environments, often at lower cost than other design materials such as metal or wood. This section reviews the static property tests. Chapter 5 provides more information on the meaning of these type data. 

Torsion property As noted, the shear modulus is usually obtained by using pendulum and oscillatory rheometer techniques. The torsional pendulum (ASTM D 2236 Dynamic Mechanical Properties of Plastics by Means of a Torsional Pendulum Test Procedure) is a popular test, since it is applicable to virtually all plastics and uses a simple specimen readily fabricated by all commercial processes or easily cut from fabricated products. 

Time dependence Viscoelastic deformation is a transition type behavior that is characterized by the occurrence of both elastic strain and time-dependent flow. It is the time dependence of the mechanical properties of plastics that makes the behavior of these materials difficult to analyze by mathematical theory. 

Compounding to change and improve the physical and mechanical properties of plastics makes use of a wide variety of... 

Tuchmann, D. and Rosen, S.L., The mechanical properties of plastics containing cryogenicaby ground tire, J. Blast. Plast., 10, 115, 1978. 

The intrinsic mechanical properties of plastics and composites are different from those of conventional materials ... 

ASTM D 4065-95, Standard Practice for Determining and Reporting Dynamic Mechanical Properties of Plastics, ASTM, West Conshohocken, PA, 1997. 

Diels, K., and R. Jueckel (H. Adam and I. Edwards, trans.), 1966, Leybold Vacuum Handbook, Pergamon, New York. This is an English translation of the second German edition. It contains information on the gas desorption and mechanical properties of plastics and elastomers. 

Of course in general the mechanical properties of plastics film and sheet will be rather different from those of metal foil and strip, whether ferrous or non-ferrous, and equipment and conditions may have to be adapted to allow for this. Sometimes cold techniques are modified to take advantage of the thermal properties of plastics but in their simplest forms they are limited to the compounds with which cold working is possible—that is, those of suitable hardness, ductility, and finish—and that can be fed in the forms of sheet, roll, or strip. So as to avoid frequent interruptions of the cycle the stock must be uniform and any strain distributed in a regular manner. Fast processes requiring the application of pressure are not suitable for hard, brittle materials, nor for any produced in such a way as to give concentrations of strain, perhaps in areas that differ with circumstances. 

Van den Tempel, M. 1961. Mechanical properties of plastic disperse systems at very small deformations. J. Colloid Sci. 16, 284-296. 

Busweii, Solid Propellent Rheology The Effect of Confining Pressure on the Mechanical Properties of Plastic Propellant , Rept No RPE-TM-519, TRC BR-18655, Westcott (Engl) (1970) 12) C.J. Brett A.T. Roff-Jarrett,... 

Kambour, R. P., Robertson, R. E. The mechanical properties of plastics, in Polymer Science, (ed. Jenkins, A. D.), Amsterdam—London, North-Holland Publ. Co. 1972, p. 687... 

In conclusion, many different processes occur during fat crystallization, at widely varying rates, all depending on temperature (history) and on fat composition. This makes it very difficult to predict the mechanical properties of plastic fats. However, the various processes involved have been identified, and their dependencies on composition and several external variables have been established. This means that trends can often be predicted. 

Structure and mechanical properties of plastic fats are discussed by... 

D 4065 (1995) Determining and reporting dynamic mechanical properties of plastics... 

Figure 3.9 Dynamic mechanical properties of plastics vs. temperature...

Compounding to change and improve the physical and mechanical properties of plastics makes use of a wide variety of materials as reviewed throughout this book. The major and large market for these materials, such as additives, fillers, and reinforcements, continues to expand as the demand for reducing the cost of plastics, plastics to function in wider or extreme markets, and under stricter regulatory regimes continue to expand. 

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