Other Form of Degradation

The situation with some forms of biological deterioration is somewhat different. Where the agent is macrobiological, as in the case of rodents, insects, and marine borers, the attack is physical in nature, such as by gnawing or boring. The attack is not at the atomic or molecular level. Any breaking of molecular bonds such as in polymer chain shortening is thus accidental. The attack may be said to be at the material s structural level, not the polymer molecule level. 

An important item to note is that most commercially used plastics are not single component pure substances. Practically always, the basic polymer itself, rarely if ever a single molecular species, is compounded with other components such as plasticizers, pigments, antioxidants, and other additives. More often than not, then, biological susceptibility is due to the nonpolymer component. 

Fungal and bacterial deterioration are identified as microbiological and have always caused problems to materials. Fungal attack on plastics has received a great deal of attention beginning with the early days of World War II, when the tropical theaters served to focus attention on the overall problem of materials deterioration. 

Microbial deterioration of plastics is intimately involved with the moisture problem, especially with regard to plastics in electronic equipment. For this reason much of the literature treats the two problems together. Furthermore, there is often confusion between the deterioration of the electrical properties of plastics, more often than not a moisture phenomenon, and actual deterioration of the substance of the polymer. 

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As with the other forms of degradation, an excellent description is given in [1],... 

Hydrolysis and other forms of degradation. Degradation of some species creates electroactive products. Acceleration of these reactions to completion could be used to allow polarographic analysis. 

Internal Pressure This theory considers hydrogen embrittlement to be caused by the formation of molecular hydrogen and the resulting buildup of pressure at internal voids or other internal surfaces [142, 143]. It provides an explanation for blistering in low-strength steels but does not adequately account for other forms of degradation such as HSC. 

PTFE is close to 2.0 and the tan 5 is less than 0.001. This is a significant improvement over aU other materials. In addition to offering excellent electrical properties, PTFE has excellent thermal properties. It is a thermal plastic that can operate at temperatures above 300 C without softening, oxidation, or other forms of degradation. PTFE is naturally fire retardant, so it does not need bromine addition to achieve the UL rating of V-0. It has very low moisture absorption. 

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

Structural changes in the polymer, which will accompany the formation of small molecule products from the polymer, or may be produced by other reactions, can cause significant changes to the material properties. Development of colour, e.g. in polyacrylonitrile by ladder formation, and in poly(vinyl chloride) through conjugated unsaturation, is a common form of degradation. 

Another source of chlordecone release to water may result from the application of mirex containing chlordecone as a contaminant and by the degradation of mirex which was used extensively in several southern states. Carlson et al. (1976) reported that dechlorinated products including chlordecone were formed when mirex bait, or mirex deposited on soil after leaching from the bait, was exposed to sunlight, other forms of weathering, and microbial degradation over a period of 12 years. Chlordecone residues in the soil could find their way to surface waters via runoff. 

Other signs of degradation of the specific dosage forms must be observed and reported. 

Since chemiluminescence is a very sensitive method of studying oxidative degradation, it has been used to measure the effect of stress on oxidation of polymers, i.e. stress-induced chemiluminescence (SCL). SCL is by definition a type of triboluminescence, and it is likely that SCL and other forms of tri-boluminescence can occur at the same time. SCL is, however, the only type of tribo-induced luminescence that is oxygen dependent and can therefore be sorted out by measurements in inert and oxidative atmospheres. 

Predicted and calculated flux of fenitrothion from water were similar although values were arrived at independently. Both results suggest that volatilization from water is slow compared to other paths of degradation of the insecticide which confirms predictions of the two-film theory of volatilization (17)(18). Losses of fenitrothion from surface films have been shown to be very rapid (2 ) but a surface film was not formed in the present work because the insecticide was mixed into the upper 10 cm of the water column. 

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