Melt flow index, changes during

The melt flow index of filled PP changes during reprocessing (Figure 9.32). The magnitude of the change depends on the amount of talc added. Any level of talc addition improved the polymefs resistance to reprocessing but the optimum conditions were at low concentrations of filler. 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Figure 9.4 Variation of weight-average molecular weight of branched polyethylene versus logarithm of melt flow index when changes in the autoclave pressure are made during the polymerization. (From Ref. 2.)...

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