Chemical changes, undesirable

Since it is important to control the thermally activated side reactions in addition to the temperature, the time of the PEB must also be controlled. It should be noted that excessive time at an elevated temperature can result in undesirable additional chemical changes in the polymer such as crosslinking or oxidation that result in solubility changes of both the exposed and unexposed regions. 

Chemical changes in a polymeric material that usually result in undesirable changes in the in-use properties of the material. 

Inhibitor a substance, the presence of which, in small amounts, in a petroleum product prevents or retards undesirable chemical changes from taking place in the product, or in the condition of the equipment in which the product is used. 

During processing and storage of foods, a number of chemical changes involving proteins may occur (Hurrell 1984). Some of these may be desirable, others undesirable. 

Components in the feed stream can be damaged or altered by heating. These alterations can include chemical degradation, increases in chemical reactivity and polymerization reactions. This chemical change can cause fouling in the equipment which reduces the effective lifetime as well as the production of undesirable components. 

A much more difficult type of application is one in which a foreign odor is to be removed from honey, maple syrup, or other materials that have a bouquet or natural flavor composed of constituents present in minute quantities. Such applications require what may be termed fractionating adsorption. Unfortunately an undesired fractionation often results, and the removal of desired components of the bouquet leaves the product with an unbalanced flat flavor however, success has at times been achieved. A method occasionally found helpful is to allow the liquid to remain in contact with minute quantities of carbon the time of contact may range from a day to a month or more. Off-flavored brandies, whiskies, and other alcoholic beverages have been so treated. One explanation of the improvement is that the desired type of fractionation takes place during the longer contact another interpretation is that activated carbon catalyzes beneficial chemical changes. 

A catalyst is a substance that changes the speed of a chemical reaction without undergoing a permanent chemical change itself. Most reactions in the body, the atmosphere, and the oceans occur with the help of catalysts. Much industrial chemical research is devoted to the search for more effective catalysts for reactions of commercial importance. Extensive research efforts also are devoted to finding means of inhibiting or removing certain catalysts that promote undesirable reactions, such as those that corrode metals, age our bodies, and cause tooth decay. 

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