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Organic peroxides & polymerization...

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Organic Peroxides....

Organic Peroxides are especially useful as free radical sources because of the wide range of temperatures under which they undergo cleavage to form radicals.

The reactivity of the peroxide is often expressed in terms of half life as a means of comparing the rate of decomposition relative to other peroxides. The rate of decomposition for most peroxides is temperature dependent and follows first order kinetics.

Half life data for virtually all of the commercial peroxides have been determined. Usually 1-hr and 10-hr half-lives are reported.

Promoters and accelerators are commonly used to alter the rate of decomposition of some initiators, thereby extending the temperature range under which they can be employed.

Peroxides are classified according to several general chemical types. Major classes include hydroperoxides, alkyl peroxides, acyl peroxides, ketone peroxides, peroxyesters, peroxydicarbonates, peroxyketals and peroxyacids. Some of these classes contain many peroxides having a wide range of reactivities and so may be found in a variety of applications.

In the ensuing discussion on the various applications of Organic Peroxides, those peroxides which are readily available in India by virtue of indigenous manufacture will be marked in bold print.

Applications in Polymerizations :

Polymerization applications involve the use of Organic Peroxide initiators in suspension, emulsion and bulk polymerization systems. Peroxides are often used alone as free radical sources, but metal redox catalysts may also be used. Polythene, the highest volume thermoplastic resin, is produced in several forms high, medium, low, linear low, high molecular high density. LDPE is made with peroxyesters such as butyl per benzoate. t-butyl peroctoate, t-butyl per pivalate, t-butyl peracetate as well as decanoyl peroxide and di-t butyl peroxide. Medium density PE is manufactured with peresters such as butyl perpivalate t-butyl perisobutyrate, tbutyl perneodecanoate and may also use peroxydicarbonates such as di (secbutyl) peroxydicarbonate and diisopropyl peroxydicarbonate. Combinations of these peroxides are also used.

The development of LLDPE has replaced some LDPE because of greater toughness and tensile strength resulting from the more oriented structure of linear PE. LLDPE is manufactured under several licenced processes using metal catalysts replacing organic peroxides. HDPE also involves the use of catalysts other than organic peroxides.




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