Plasticity differences

The friction and wear of plastics are extremely complex subjects which depend markedly on the nature of the application and the properties of the material. The frictional properties of plastics differ considerably from those of metals. Even reinforced plastics have modulus values which are much lower than metals. Hence metal/thermoplastic friction is characterised by adhesion and deformation which results in frictional forces that are not proportional to load but rather to speed. Table 1.7 gives some typical coefficients of friction for plastics. 

The age-old problem of predicting what will happen to any material after it is subjected to service also exists with plastics. Different data on plastics are available, but typical of so-called progress, there is never sufficient or adequate useful information to predict the service life of products being designed. It is suggested that rather than assume that a lack of data exists, one should determine what is logically available and apply it most efficiently. A potential example of improper design with plastics concerns toys. 

Fiber-reinforced plastics differ from many other materials because they combine two essentially different materials of fibers and a plastic into a single composite. In this way they are somewhat analogous to reinforced concrete, that combines concrete and steel. However, in the RPs the fibers are generally much more evenly distributed throughout the... 

This subject effects designers since many products have the requirement by regulations or otherwise to use recycled plastics. Different methods are used to recycle materials to provide plastics with a continuing life. Method used is influenced by factors such as costs, quantity involved, weight involved, size and shape, complexity of mixed types of plastics, extended of contamination such as metallic particles, continued availability of material, etc. (Recognize that they can also be used as energy sources through incineration that can be combined with production of electricity and/or hot water for example). 

For plastics, heat capacity is usually reported during constant pressure heating. Plastics differ from traditional engineering materials because their specific heat is temperature sensitive. 

A compromise among properties, cost, and manufacturing process generally determines the material of construction. Selecting a plastic is very similar to selecting a metal. Even within one class, plastics differ because of varying formulations, just as steel compositions vary (tool steel, stainless steel, etc.). There are, of course, products for which no plastics is satisfactory, and the interests of the producer and consumer alike are best served by using some other material. 

Although they have an endless variety of properties, polymers can be divided into three general categories, based on their form and resistance to stretching. These are plastics, fibers, and elastomers. Plastics differ in form from fibers whereas plastics exist as blocks or sheets, fibers have been drawn into long threads. Unlike plastics or fibers, elastomers can be stretched without breaking. Polyethylene packaging films and polyvinylchloride (PVC) pipe are examples of plastics. Orion carpets are made from polymer fibers, and mbber bands are elastomers. Some polymers, such as Nylon, can be formed into both plastics and fibers. 

Plastics differ because of their morphology, structure, rheology, etc. The geometry of a part and the complexity of the corresponding mould vary. Consequently, the tolerances of a part depend on ... 

Another important parameter is the choice between thermoplastic and thermoset composites, which are competing for multiple applications. Apart from their characteristic properties, the two families of plastics differ by general features such as ... 

Reinforced plastics differ from high-pressure laminates in that little or no pressure is employed. For instance, in making formed shapes, impregnated reinforcing material is cut to a desired shape, the various layers are added to a mold, which is then heated. This process is favored over the high-pressure process because of the use of a simpler, lower cost mold and production of strain-free products. 

The equilibrium gas concentration, and thus die equilibrium sample deflection of plasticized polystyrene samples, should be inversely proportional to the diffusivity of the gas in a plasticized polymer. The measured diffusivities for hydrogen in unplasticized polystyrene and polystyrene containing 20% plasticizer differ by a factor of about 2 (Figure 16). From the 0 and 20% plasticizer points in Figure 13, curve C, we see that the equilibrium deflections also differ by a factor of about 2 to 3. 

Rigid masses have a remaining height of more than 25 mm and for soft ones it is less than 16 mm. In figure 9.11 you can read off the desired plasticity (remaining height) and the required water content. The desired plasticity differs for every product, table 9.3 supplies some examples. 

Many variables used and phenomena described by fracture mechanics concepts depend on the history of loading (its rate, form and/or duration) and on the (physical and chemical) environment. Especially time-sensitive are the level of stored and dissipated energy, also in the region away from the crack tip (far held), the stress distribution in a cracked visco-elastic body, the development of a sub-critical defect into a stress-concentrating crack and the assessment of the effective size of it, especially in the presence of microyield. The role of time in the execution and analysis of impact and fatigue experiments as well as in dynamic fracture is rather evident. To take care of the specihcities of time-dependent, non-linearly deforming materials and of the evident effects of sample plasticity different criteria for crack instability and/or toughness characterization have been developed and appropriate corrections introduced into Eq. 3, which will be discussed in most contributions of this special Double Volume (Vol. 187 and 188). 

The location of the glass transition temperature will depend on the nature of the polymer. Generally, a plastic differs from a rubbery material due to the location of its glass transition temperature. A plastic has a T above room temperature, while a rubber has a Tg below room temperature. As previously mentioned the flexibility of the chain will affect the value of T. Flexible groups will tend to lower the T, whife stiffening groups will act to increase it. Side groups can also affect the value. The... 

To meet different formed products with different processable plastics different variations of this basic process are used. An example is a heated sheet clamped over a billow box (enclosed) chamber. Air pressure in the chamber is used causing the sheet to billow. 

Coating compounds are used to cover the surfaces of many materials from plastic to paper to fabric to metal to concrete and so on. Many plastics produced are consumed as coating materials, including paints, primers, varnishes, and enamels. Metals may be surface coated to improve their workability in mechanical processing. Substrates protected from different environmental conditions basically include the metals (steel, zinc, aluminum, and copper), inorganic materials (plaster, concrete, and asbestos) and organic materials (wood, wallboard, wallpaper, and plastics). Different technical developments continue to occur in the... 

Table 17.5 Mass balance of the pyrolysis of mixed plastics (different fractions A and B see Table 17.4) in a fluidized bed with different feedstocks and plants using pyrolysis gas for fluidization...

Plasticizers differ significantly in their fusion rates and effects on physical properties. Figure 10 illustrates the differences in solvation between three phthalate plasticizers observed at various temperatures in contact with PVC on a hot-stage microscope. Elongation buildup at 150 °C as a function of fusion time, for three different plasticizers, is illustrated in Figure 11. 

Phenomenology of corrosion for metals and polymer materials is not the same. For example, the terms such as corrosion rate, pitting, end grain attack, and intergranular attackusQd to describe metals behaviors are not applicable to plastics. Not only are the terms for plastics different blistering, discoloration, cracking, etc.) but they are based on a naked-eye observation and not at a microscopic level as for metals. 

Common packaging plastics differ considerably in properties, and therefore are suitable for differing uses. A summary of properties for some of the major plastics... 

In many cases, natural resins such as copal resins, amber, shellac, and colo-phonium, which were most widespread in the early period of plastics, differ widely in terms of their properties and chemical composition depending on their origins and production methods. Consequently, it is extremely difficult or even impossible to identify these materials by simple techniques. Instead, it would be advisable to conduct several laboratory tests. In the following, a few hints are given for a brief introduction. More detailed investigations may be based on the works of Thinius and Hummel, which mostly involve a higher test complexity. 

---