Plasticization, mechanical property changes

The glass transition temperature, T, is one of the most important of all properties of polymers. It is the temperature region where not only the modulus but almost all the physical and mechanical properties change rapidly during heating or cooling. It is the temperature at which the plastics material changes to become almost rubber-like. 

To determine whether PPy content influenced the mechanical properties, mechanical analysis was performed (Fig. 8.14), the results show that after the PPy addition, the mechanical properties change. Increasing the PPy concentration changed the properties of the composite films from elastomeric to ductile plastic-like and finally to brittle. 

Plasticizers influence PVC resistance to ionizing radiation mechanical properties change more slowly, because the amount of cross-connections is smaller due to the plasticizer content. As Table 5.33 shows, both the amount and the chemical composition of the plasticizer plays a role. Many plasticizers — such as diallyl phthalate and triallyl cyanurate — accelerate crosslinking considerably, because they also act as crosslinking agents [32]. 

Ta bl e 5.3 3 Influence of plasticizer on mechanical property change in soft PVC under ionizing radiation [32]... 

A large number of plastics have been exposed to the Earth s lower orbit and subsequently characterized. The radiation dominant in this environment in combination with atomic oxygen leads to changes in physical appearance and a reduction in mechanical properties. Changes in molecular weight have also been detected [748]. 

For a part to exhibit stmctural stiffness, flexural moduH should be above 2000 N/mm (290,000 psi). Notched l2od impact values should be deterrnined at different thicknesses. Some plastics exhibit different notch sensitivities. For example, PC, 3.2 mm thick, has a notched l2od impact of 800 J/m (15 fdbf/in.) which drops to 100 J/m (1.9 fflbf/in.) at 6.4-mm thickness. On the other hand, one bisphenol A phthalate-based polyarylate resin maintains a 250-J /m (4.7-fdbf/in.) notched l2od impact at both thicknesses. Toughness depends on the stmcture of the part under consideration as well as the plastic employed to make the part. Mechanical properties, like electrical properties, ate also subject to thermal and water-content changes. 

The mechanical properties of ionomers can be appreciably altered by the manner in which the ionomer is prepared and treated prior to testing. Some of the factors that are influential are the degree of conversion (neutralization) from the acid form to the salt form, the nature of the thermal treatment or aging, the type of counterion that is introduced, the solvent that is used for preparation of thin films, and the presence and nature of any plasticizers or additives that may be present. In the scope of this chapter, it is not possible to provide a complete description of the influence of each of these variables on the wide variety of ionomers that are now commercially available or produced in the laboratory. Instead, one or more examples of the changes in properties that may be induced by each of the processing variables is presented and discussed. 

The mechanical properties of composites are mainly influenced by the adhesion between matrix and fibers of the composite. As it is known from glass fibers, the adhesion properties could be changed by pretreatments of fibers. So special process, chemical and physical modification methods were developed. Moisture repel-lency, resistance to environmental effects, and, not at least, the mechanical properties are improved by these treatments. Various applications for natural fibers as reinforcement in plastics are encouraged. 

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

The time/temperature-dependent change in mechanical properties results from stress relaxation and other viscoelastic phenomena that are typical of these plastics. When the change is an unwanted limitation it is called creep. When the change is skillfully adapted to use in the overall design, it is referred to as plastic memory. 

The main considerations of mechanical properties of metals and alloys at low temperatures taken into account for safety reasons are the transition from ductile-to-brittle behavior, certain unconventional modes of plastic deformation, and mechanical and elastic properties changes due to phase transformations in the crystalline structure. 

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