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Selection of organic peroxides |
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Organic peroxides have various structures and decompose at different temperatures to generate free radicals.
Accordingly, to choose the most suitable organic peroxides for a purpose, it is necessary to know the properties of a peroxide.
Data of characteristic values such as active oxygen content, half-life, and activation energy of peroxides are important information.
Moreover, organic peroxides are supplied as both the pure form and as mixtures with a diluent to increase stability. In the latter case, the effect of the diluent on conditions of polymerization must be considered.
The diluent can be specified by the customer, unless it influences the stability of the organic peroxide. |
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Active oxygen content |
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Active oxygen content represents not only the amount of free radicals produced from the peroxide but also the concentration or the purity of the products, i.e., the theoretical active oxygen content of 100% pure peroxide is given (Table 1) by the percentage of atomic weight of active oxygen to the molecular weight of the peroxides. |
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Half-life and activation energy |
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Half-life is a convenient index which represents the decomposition rate of organic peroxides from the initial active oxygen content of the peroxide to half of that value by decomposition at a specific temperature.
Half-life is measured using a solution of 0.1mol/l (occasionally 0.05mol/l) of peroxide with a solvent relatively inert to radicals, e.g. benzene, under nitrogen sealed in a glass ampoule, and immersed in a constant temperature bath set to the temperature required.
Generally, decomposition of organic peroxide can be treated approximately as a first order reaction as follows: |
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Half-life (t1/2) is the time required to reduce (a) to (a/2)
by decomposition, so equation (3) can be obtained by substituting (a/2) for (x) in equation (2). |
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Therefore, organic peroxide decomposition at a specific temperature has a linear relationship between time (t) and [ln a/(a-x)], so (k) can be obtained from the slope of the straight line. Then, half-life (t1/2) can be obtained at the specific temperature from equation (3).
The rate constant (k) is given as follows: |
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Measurement of (k) at several temperatures provides the relationship between (lnk) and (1/T), so activation energy can be obtained from the slope of the straight line.
The straight line relationship of (lnt1/2) to (1/T) can be used to find the half-life of organic peroxide at any temperature, or the decomposition temperature required for a particular half-life. |
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Decomposition products |
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During the polymerization of monomer and crosslinking of polymers, decomposition products of organic peroxides sometimes cause contamination, and have a bad influence on the physical properties of the polymer. Therefore, when selecting the organic peroxide, it is necessary to consider what decomposition products are generated. |
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