Reversibility, useful life

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

Flavor Stability. The oil ingredient must have the identified degree of resistance to oxidative and lipolytic flavor degradation to maintain a bland flavor and odor throughout the shelf life and use life of the prepared food product. Cottonseed oil reverts to a nutty flavor that is not as objectionable as the reversion flavors of the other oils with high levels of linoleic and oleic fatty acids. 

The end of useful life for a thoriated tungsten tube occurs when most of the carbon has evaporated or has combined with residual gas, depleting the carbide surface layer. Theoretically, a 3% increase in filament voltage will result in a 20 K increase in cathode temperature, a 20% increase in peak emission, and a 50% decrease in tube life because of carbon loss. This cycle works in reverse, too. For a small decrease in temperature and peak emission, the fife of the carbide layer—and hence, the tube may be increased. 

The useful life of a polymer has to be defined for conservation. Feller developed a rule of thumb for classifying materials by their photochemical stability, a useful tool that should inform aU stages in the conservation process, from planning to use (Table 2.4). In most fields, it is expected that a conservation treatment will have to be reversed in the future. For instance, picture varnishes have traditionally been replaced every 80-120 years and stained glass window installations every 2-300 years. It is likely that a material used for temporary fixing may never be entirely removed (Section 1.3). It is therefore necessary that even these have long-term stabihty. 

Note that it has become standard practice in some plants to reverse the pinion or bullgear in an effort to extend the gear set s useful life. While this... 

Failure, permanent A non-reversible condition that makes the battery essentially useless. There is no recognized standard method of rating or defining exactly when a cell has reached the end of its life however, the end of useful life is defined as that condition when the cell cannot perform above 50% of its rated ampere hour capacity with all charging and discharging done at room temperature. 

Although these composite fibers were developed for reverse osmosis their acceptance in the desalination industry has been limited due to insufficient selectivity and oxidative stabiUty. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerisation (20) have gained wide industry approval. HoUow fibers using this technique to give equivalent properties and life, yet to be developed, should be market tested during the 1990s. 

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