Chemical prevention

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

Contacting Uie manufacturer would help clarify any uncertainUes concerning the chemicals being used mid could also, in Uie event Uiat it has been discovered Uiat Uic manufacturer lias made an error in the production or delivery of a certain chemical, prevent a potential catastrophe elsewhere caused by the use of the same chemical. 

Triethylenetetramine (redistilled, Eastman Organic Chemicals) prevents bumping and foaming and acts as a chaser for the distillation. 

Sullivan FM, Barlow SM. 1985. The relevance for man of animal data on reproductive toxicity of industrial chemicals. Prevention of Physical and Mental Congenital Defects, Part B Epidemiology, Early Detection and Therapy, and Environmental Factors. Alan R. Liss, Inc. 

Antioxidants, Protective Plant Chemicals, Preventing Diseases... 

An enzyme involved in cell cycle control has been identified as the target of a compound supplied by broccoli, cabbage, and Brussels sprouts. One of the glucosinolates of these foods is the source of a chemical called indole 3-carbinol. Indole-3-carbinol enters the cell and influences the activity of a spixiat protein kinase that is used to control the cell cycle. The chemical prevents the synthesis of the enzyme, and thus reduces its activity. The action of this protein kinase (cyclin-dependent protein kinase 6) normally functions to make the cell embark on the G phase of the cell cycle, and thus to stimulate progression through the cell cycle. The chronic decrease in activity of this protein kinase serves to prevent cell division, but also prevents spontaneous cancers. The effect of mdoJe-3-carbinol has been shown to occur in cultured cells, as well as in animals (Cover ef o/ 1998). 

Barrier theories. Fire-retardant chemicals prevent the escape of volatile products by forming a glassy barrier. This barrier also prevents oxygen from reaching the substrate and insulates the wood surface from high temperatures. 

Thermal theories. Fire-retardant chemicals may increase the thermal conductivity of the wood to dissipate the heat from the surface faster than it is supplied by the igniting source, or they may affect chemical and physical changes so that heat is absorbed by the chemical, preventing the wood surface from igniting. 

Metal ion deactivators. Certain chemicals prevent the initiation reactions catalysed by transition metal ions, e.g. Ca, Fe, Zri and Mri. Unless the activity of these ions is restricted they can provide a source of free radicals for the oxidation chain reactions described by Equations 11.4 to 11.6. The activity of these metal ions in generating free radicals may be generalised by the following equations [Crawford and Miller 1963]... 

Chemical asphyxiates are toxic agents which enter into reactions to cause histotoxic hypoxia. These chemicals prevent the red blood cells from carrying oxygen. Some more familiar chemical asphyxiates are carbon monoxide, nitrites, hydrogen sulhde, and aniline. 

Biocontrol combined with other physical and chemical preventive actions along the barley-to-beer chain could result in a successful strategy for controlling toxigenic fnngi. Several new potential techniques have been studied in the laboratory or pilot scale and transfer of technologies into the industrial scale requires further studies. 

Simple asphyxiants are usually relatively nontoxic, but present a special hazard if used in a confined space since they can displace oxygen and make the atmosphere unsuitable to sustain life. (Chemical asphyxiants, such as carbon monoxide, function by the chemical preventing oxygen uptake.)... 

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