Processing Influencing Service Properties

During processing, only chemical, no physical aging processes occur. Processing, however, determines the physical structure of the finished part and thus influences physical aging and diffusion-controlled degradation during service. Fig. 4.1. 

There are two basic ways to influence harmful degradation reactions  

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Polystyrene degradation has considerable influence on its processability and service properties. The degradation process, which proceeds mainly via radical intermediate steps, can cause molecular weight loss, branching or crosslinking, and the emission of monomer units and other low-molecular substances. The presence of oxygen creates oxidation products that lead to discoloration and catalyze further degradation. 

The study of dislocations in semiconductors is strongly connected to the advent and development of the electronics industry. Indeed, it was very soon observed that the presence of crystalline defects, including dislocations, was detrimental to the processing and service life of devices. Thus, the very early research focused on the link between dislocation properties and their influence on the potential applications of semiconductors. 

Since TPEs typically consist of two incompatible phases at service temperature, the hnal properties of TPEs are influenced by the properties of the individual phases and the interface. It is therefore necessary to consider several factors, like the constituents nature, their molecular weight and relative amounts, processing history, etc., to understand the properties of TPE. The influences of various factors on TPEs properties are discussed below. 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

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