Degradation: physical factors

With rubber base adhesives, it is necessary to prevent their properties from changing during service life. Oxidative changes induced by thermal, ozone exposure and UV light can dramatically affect service life of rubber base adhesives. More precisely, the rubber and the resin are quite susceptible to oxidative degradation. Environmental and physical factors exert detrimental effects on rubber base adhesive performance. These effects can be mitigated by the incorporation of low levels of stabilizers during the fabrication process of the adhesive. 

Considerable studies have been done on the effects of the most important chemical and physical factors involved in the degradation of anthocyanins (temperature, light, pH, SO2, metal, sugar, and oxygen) in model systems and food extracts. In addition, anthocyanin concentrations, its chemical structures, and media compositions are fundamental factors influencing stability. 

Data on exposure and environmental fate are needed, not to determine toxicity, but to provide information that may be useful in the prediction of possible exposure in the event that the chemical is toxic. These tests are primarily useful for chemicals released into the environment such as pesticides, and they include the rate of breakdown under aerobic and anaerobic conditions in soils of various types, the rates of leaching into surface water from soils of various types, and the rate of movement toward groundwater. The effects of physical factors on degradation through photolysis and hydrolysis studies and the identification of the product formed can indicate the rate of loss of the hazardous chemical or the possible formation of hazardous degradation products. Tests for accumulation in plants and animals and movement within the ecosystem are considered in Section 21.7. 

It must be therefore taken into consideration that the physical loss of stabilizers may influence the r ults of the accelerated ageing test This is true mainly in tests where the polymer degradation is accelerated by physical factors having very different intensities to those applied under practical conditions. 

The faster ductile failure in water/phosphoric acid solution is thought to be due to swelling, and thus making it a physical (ESC) process. The failure times in water and phosphoric acid solution are comparable. Apparently, the physical factor is the same in both water and phosphoric acid solution, as already indicated by the similar saturation times and degradation times (Fig. 1). 

Approaches used for achieving programmed or pulsatile release may be physical mechanisms such as swelling with bursting or chemical actions such as enzymatic degradation. Capsules have been designed that burst after a predetermined exposure to an aqueous environment. Physical factors that can be controlled are the radius of the sphere, osmotic pressure of the contents, and wall thickness as well as elasticity. 

Factors such as fiber diameter, compactness, and the difficult solubility of collagen would seem of importance in determining such properties as resistance to hydrolytic attack by enzymes. This inertness is probably not entirely derived from the chemistry of collagen but depends also on physical factors which prevent an enzyme from gaining access to its substrate. Pepsin, acting under acid conditions on swollen fibrils, is able to cause degradation (195), and increased temperature or physical maceration facilitates even the attack by trypsin in neutral or slightly alkaline environments (194, 200). 

Chemical and physical factors known to degrade other carotenoids, including exposure to light, oxygen, elevated temperature, extremes in pH, and active surfaces, apply to lycopene as well. In the presence of oxygen, lycopene, and carotenoids in general, tend to... 

Microbial contamination of polymer emulsions is discussed, and is shown to depend on a large number of chemical and physical factors. An integrated approach to prevention and cure is recommended, and attention to raw material and water quality, plant design and hygiene, and the use of broad spectrum biocides such as those based on isothiazolin-3, is suggested. Conditions of temperature, pH and redox should be considered, it is stated, when using such biocides to avoid degradation. 

Polymer degradation is highly dependent on chemical and physical factors. The chemical factors vill be examined in relation to thermal degradation (see Section 15.4.2). This section vill be focused exclusively on the influence of physical factors on polymer degradation. 

As Rogers et al. [21] quoted to The U.S. Nuclear Regulatory Commission, Service Life of Concrete , conplied in 1989, there are at least seven major chemi-cal/physical factors reported to be major causes of concrete degradation ... 

In conclusion, the literature shows that in most cases LA/GA polymers exhibited comparable in vitro and in vivo degradation behaviours. Examination of corresponding data suggests that whenever differences are observed, effects of physical factors (temperature, stirring, pH) or physiological ones related to implantation sites (subcutaneous, intramuscular or in bony tissues) should be considered prior to invoking enzymatic activities. 

Antioxidants and stabilizers were used on a trial-and-error basis more than 100 years ago. Real scientific and technological progress is relatively recent. It was ushered in by the development of a basic understanding of the underlying mechanisms of polymer degradation. This has made it possible to ascribe specific chemical and physical functions to antioxidants and stabilizers according to their mode of action in the oxidative process. An important aspect in the development of antioxidants is that their effectiveness does not only depend on the chemical inhibition process but also on physical factors such as solubility of antioxidants and their compatibility with the polymer, diffusion and migration phenomena within the polymer matrix, volatility and extractability of antioxidants. Some historical perspectives of this development has been reviewed recently. ... 

Another factor in oxidative degradation is ultraviolet radiation, of which sunlight is a rich source. The oxidation of parylene appears to be enhanced by ultraviolet radiation. 02one may play a mechanistic role in the ambient temperature exposure of parylenes to ultraviolet radiation in the presence of oxygen. For the best physical endurance, exposure of the parylenes to ultraviolet light must be minimised. 

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