Types of degradation

Polymer degradation is a frequent problem in extrusion. Degradation usually manifests itself as discoloration, loss of volatile components (smoking), or loss of mechanical properties. According to the mode of initiation, the following types of degradation can be distinguished  

Degradation processes are generally quite complex often more than one type of degradation is operational, e.g., thermo-oxidative degradation, thermo-mechanical degradation, etc. This situation is quite similar to wear in extruders, where usually more than one wear mechanism is operational at any one time. 

In extrusion, the first three types of degradation are the most important thermal, mechanical, and chemical degradation. 

Thermal degradation occurs when a polymer is exposed to an elevated temperature in an inert atmosphere under exclusion of other compounds. The resistance against such degradation depends on the nature and the inherent thermal stability of the polymer backbone. There are three main types of thermal degradation depolymerization, random chain scission, and unzipping of substituent groups. 

Depolymerization or unzipping is a reduction in length of the main chain by sequential elimination of monomer units. Polymers that degrade by this mechanism are polymethylmethacrylate, polyformaldehyde, polystyrene, etc. Polystyrene unzips to 

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Impaired appearance of wood due to staining by contaminating substances can also be considered as a type of degradation, and its avoidance is important in furniture, panelling and high-class joinery. 

Oxidative degradation of polypropylene chemically incorporates oxygen to the polymeric chains. Why would an isotactic polypropylene be less susceptible to this type of degradation than one that is atactic ... 

This type of degradation can also be referred to as photodegradation or ultraviolet (UV) degradation. It includes photo-oxidation. It produces some of the more familiar signs of degradation of plastics embrittlement, discoloration and loss of transparency. 

As was thf case with dieldrin, photodecomposition of C-parathion and Cl-toxaphene was relatively extensive, and the addition of FMN has almost no effect on the enhancing or decreasing the rate of degradation to water-soluble products. Since the organic-soluble products were not analyzed in the case of parathion and toxaphene, it is difficult to speculate on the type of degradation products formed and the pathways involved on... 

As an additional example of high practical significance, we refer here to copper depKJsits when used in microelectronics, mirrors, and other optical applications. Those deposits have been observed to soften in time even when stored at room temperature for only 4 to 6 weeks. Also, mirrors and other precision objects made of copper will undergo surface deformation after a few months. This type of degradation can be counterbalanced by a suitable metal overcoating. Another, not always practical way is heat treatment to about 300°C. These phenomena are the direct results of microstructural instabilities, often referred to as recrystallization in the copper. It is worth stressing that recrystallization is not limited to copper (5). 

One important aspect of any cleanup technique, is the type of degradation products that are produced. These products must be known in order to assess their potential environmental impact and toxicological hazards. One of the major degradation products of diazinon, oxypyrimidine was measured in soil after treatment with parathion hydrolase. Figure 6 shows that oxypyrimidine increases in soil as the diazinon is degraded by the enzyme. 

As of this time, there is no real satisfactory test for degradation in any polymeric system. Complete fate analysis is needed. Definitions have to be established and degree of analysis accepted for many different polymers, something that will take time. Tests will be required for each type of degradation mentioned earlier. 

Many drugs are quite stable but functional groups such as esters and lactam rings which occur in some drugs are susceptible to hydrolysis and functional groups such as catechols and phenols are quite readily oxidised. The most common types of degradation which occur in pure and formulated drugs obey zero or first order kinetics. 

First order kinetics of drug degradation has been widely studied. This type of degradation would be typical of the hydrolysis of a drug in solution. Such reactions are pseudo first order since the concentration of water is usually in such large excess that it is regarded as constant even though it does participate in the reaction. In first... 

Sample Preparation. PBS-MP20, obtained in powder form from Mead Chemical Co., was the primary source of the samples used in this study. A portion of the MP20 powder was dissolved in 2-methoxyethyl acetate (Mead Chemical) and films were spin-coated on silicon wafers. Films were baked at 120 C for 1 hour prior to irradiation. MP20 samples, both in powder and film form, were irradiated under vacuum at 30 C in a Co source. Films were spun from methoxyethyl acetate solutions of the irradiated powders. These films were also baked at 120 C for 1 hour. The above procedure created two types of degraded MP20 samples (1) those irradiated in film form and (2) those irradiated in powder form. The first type of films were designated as IF-type films. Films spun from solutions of the irradiated powders were designated as IP-type films. 

Mechanochemical Degradation. Mechanochemical degradation occurs in polymers as the result of an applied mechanical force. This type of degradation is quite common in machining processes such as grinding, ball milling, and mastication. 

In addition to the free-radical chain degradation described for polyolefins, another type of degradation (dehydrohalogenation) also occurs with chlorine-containing polymers, such as PVC. As shown by the following equation,... 

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