Free-radical formation during melt processing

1 Free-radical formation during melt processing 

This difference between the behaviour of the solid PE and the solution of PMMA was attributed to the fundamental mechanism of scission, such that in the solid state there was amplification of the effect of cmck growth from the initial site of chain scission whereas in PMMA it was chain entanglement that produced the mechano-scission. 

In order to rationalize this result for PE, a simple calculation of the energy required for scission of main-chain bonds (E c c) the frictional dissipation between chains due to mechanical action was performed. The latter was calculated from the activation energy for viscosity of the monomer repeat unit, E, determined from a low-molar-mass analogue of polyethylene. This was then used to determine the number of repeat units, n, necessary to exceed the C-C bond energy of 349 kJ/mol. From the value of 4.22 kJ/mol for E, an estimate for n of 83 was made, which was considered a good approximation to the experimental result of 100 (Sohma, 1989b). However, such a simple approach is not amenable to extension to systems of structural complexity such as chain scission during the mastication of rubber. 

Casale and Porter (1978) brought together much of the early work on the theory of mechanical generation of free radicals. Much of the theory for radical formation in the rubbery state bas developed from tbe ideas of Buecbe regarding tbe scission of entangled cbains. Tbe entangled chain does not permit tbe rotation of tbe entire chain to dissipate energy when shear is applied and, furthermore, tensions reach a maximum at the centre of the chain. The number of scissions will decline exponentially with distance from the centre of the chain such that at link q from the centre the ratio of the number of scissions, ng, to that at the centre, no, is 

The approach of Zhurkov and Bueche is an activation-volume argument in which the presence of the applied stress lowers the activation energy for chain scission, so increasing the value of the rate coefficient, k. The effect is to increase the prohahility, P, of scission of the central hond from Pq to (Casale and Porter, 1978) 

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The vector fluid concept was first suggested for a polyethylene (PE)/polyamide (PA) reactive blending system [12], as mentioned earlier in this chapter. This concept is interesting because it has the potential to provide a compatibilization method for polymers that have no chemical functionalities suitable for copolymer formation during melt blending (e.g. the case of polyolefin and polystyrene). It has been seen that the blends of polyolefin/polystyrene are difficult to compatibilize in situ by simply adding a free radical initiator into the blending process. Usually, flie pre-made block or reactive polymers or copolymers, which can be expensive, are needed for polyolefin/polystyrene compatibilization [15-17]. If a suitable vector fluid can be found for the polyolefin/ polystyrene/peroxide in situ compatibilization, the process could become more controllable and more cost efficient. 

One of the obvious features of the oxidation of polypropylene is the formation of hydroperoxides (reaction (3) in Scheme 1.55) as a product. The initiation of the oxidation sequence is usually considered to be thermolysis of hydroperoxides formed during synthesis and processing (shown as the bimolecular reaction (1 ) in Scheme 1.55). The kinetics of oxidation in the melt then become those of a branched chain reaction as the number of free radicals in the system continually increases with time (ie the product of the oxidation is also an initiator). Because of the different stabilities of the hydroperoxides (e.g. p-, s- and t- isolated or associated) under the conditions of the oxidation, only a fraction of those formed will be measured in any hydroperoxide analysis of the oxidizing polymer. The kinetic character of the oxidation will change from a linear chain reaction, in which the steady-state approximation applies, to a branched-chain reaction, for which the approximation might not be valid since the rate of formation of free radicals is not... 

Preventive and secondary antioxidants decompose hydroperoxides without intermediate formation of free radicals, preventing chain branching [20]. They are termed secondary because their best performance is achieved in the presence of primary antioxidants. They also contribute to melt flow and odor stabilization during processing. Aliphatic phos-ph(on)ites esters act only as secondary HD antioxidants while sterically hindered ortho-tcrt-alkylated aromatic compounds are capable of acting also as a primary radical chain breaking reaction. 

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