Toughness plastics mechanical behavior

Chemically both wax and polyethylene can be described as polymethylene -(-CH2-)n-, but while the former is a brittle solid the latter is a tough plastic. What are the reasons for this difference in mechanical behavior ... 

While IPNs can be and have been made extremely tough and impact resistant, many of the proposed applications involve such diverse fields and sound and vibration damping, biomedical materials, and non-linear optics. This is because the presence of crosslinks in both polymers reduces creep and flow, allowing relatively stable materials with a wide range of moduli to be prepared. Thus, those materials with leathery mechanical behavior, combinations of elastomers and plastics, are especially interesting to scientists, inventors, and engineers. 

The mechanical behavior of these Be-rich phases and its variation with temperature has been studied by means of hardness tests, bending stress-rupture tests, tension tests and compression tests (Ryba, 1967 Marder and Stonehouse, 1988 Fleischer and Zabala, 1990 c Nieh and Wadsworth, 1990 Bruemmer etal., 1993). The observed brittle-to-ductile transition temperatures are of the order of 1000°C. The low-temperature fracture toughness has been found to be between 2 and 4 MN/m with practically no macroscopic ductility (Bruemmer et al., 1993), though there are indications of local plasticity at... 

The chemical and physical characteristics of plastics are derived from the four factors of chemical structure, form, arrangement, and size of the polymer. As an example, the chemical structure influences density. Chemical structure refers to the types of atoms and the way they are joined to one another. The form of the molecules, their size and disposition within the material, influences mechanical behavior. It is possible to deliberately vary the crystal state in order to vary hardness or softness, toughness or brittleness, resistance to temperature, and so on. The chemical structure and nature of plastics have a significant relationship both to properties and the ways they can be processed, designed, or otherwise translated into a finished product (Figures 3.8 and 3.9). 

The greatest differences between polyethylene and wax lie in their mechanical behavior, however. While wax is a brittle solid, polyethylene is a tough plastic. Comparing resistance to break of a child s birthday candle with a wash bottle tip, both of about the same diameter, shows that the wash bottle tip can be repeatedly bent whereas the candle breaks on the first deformation. 

While existing products span the possible physical and mechanical behavior spectrum, it is interesting to speculate on which classes of material are most usefully commercialized in IPN form. Modulus will be taken as a basis. All elastomers must be crosslinked by some mechanism, lest they flow or creep excessively. Plastics are crosslinked into thermoset materials only to prevent flow on heating. It seems that leathery IPN compositions based on one elastomer and one plastic form materials which are tough, flexible and yet do not flow. The modulus is adjustable by control of the overall composition, and by phase continuity aspects. 

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