Oxygen, exposure conditions

Activated carbons contain chemisorbed oxygen in varying amounts unless special cate is taken to eliminate it. Desired adsorption properties often depend upon the amount and type of chemisorbed oxygen species on the surface. Therefore, the adsorption properties of an activated carbon adsorbent depend on its prior temperature and oxygen-exposure history. In contrast, molecular sieve 2eohtes and other oxide adsorbents are not affected by oxidi2ing or reducing conditions. 

Films or membranes of silkworm silk have been produced by air-drying aqueous solutions prepared from the concentrated salts, followed by dialysis (11,28). The films, which are water soluble, generally contain silk in the silk I conformation with a significant content of random coil. Many different treatments have been used to modify these films to decrease their water solubiUty by converting silk I to silk II in a process found usehil for enzyme entrapment (28). Silk membranes have also been cast from fibroin solutions and characterized for permeation properties. Oxygen and water vapor transmission rates were dependent on the exposure conditions to methanol to faciUtate the conversion to silk II (29). Thin monolayer films have been formed from solubilized silkworm silk using Langmuir techniques to faciUtate stmctural characterization of the protein (30). ResolubiLized silkworm cocoon silk has been spun into fibers (31), as have recombinant silkworm silks (32). 

Despite all these safeguards to extend the service life of the antifreeze, fluid replacement is requited periodically. Typically, fluids are replaced because of irreversible damage caused by one of four conditions contamination, gel formation because of glycol/siUcate reaction, extensive glycol degradation caused by overheating or excessive oxygen exposure, or inhibitor depletion. 

Figure 4.44. Comparison of the oxygen TPD spectra from Ag deposited on YSZ obtained with the three different modes of adsorption, gaseous, electrochemical and mixed, under the same conditions. TadS=300°C, 5.4 kL of oxygen exposure (for gaseous and mixed adsorption), and 4.6x1 O 9 mol O of electrochemically supplied oxygen (for electrochemical and mixed adsorption), i.e. 5 pA for 180 s. (UWR=+0.7 V, A=+1.3 eV).31 Reprinted with permission from Academic Press.

MSDS data are likely to pertain only to ambient and fire-exposure conditions. If the material is handled under different or unusual conditions, such as at high pressure or in an oxygen-enriched atmosphere, the data may be useless or misleading. 

Due to the variability in redox conditions commonly found in nearshore systems, the term diagenetic oxygen exposure time was proposed as an indicator for organic matter preservation based on the amount of time sinking POM resides in the oxic porewaters of sediments. 

The main problem is related to exposure of these materials to high temperatures in the presence of oxygen. These conditions favor the oxidative degradation of the elastomer, which leads to opposing changes in its structure. On the one hand, chain scission can occur, and on the other hand, cross-links can be generated. If chain scission dominates, the hardness and modulus of the elastomer will decrease. If, however, cross-links are generated, both the hardness and the modulus increase and the strain at fracture will decrease. Other mechanical properties are altered by an increase in... 

---