The mechanisms of melt stabilisation of polyolefins
The effects of melt stabilisers on the oxidative degradation of polyolefins
(polypropylene, low density polyethylene) have been studied
tmeer a variety of processing conditions. The changes in the both
chemical and physical properties of unstabilised polymers occurring
during processing were found to be strongly dependent on the amount
of oxygen present in the mixer.
2 ,6 ,3' ,5' -tetra-tert-butyl-4'-phenoxy-4-methylene-2, 5-cyclohexadienel-
one (galvinoxyl), iodine, nitroxyl radicals and cupric stearate
were found to be very efficient melt stabilisers particularly when
processed in a restricted arrount of air. The mechanisms of their
melt stabilising action have been investigated and a mITITOn cyclical
regenerative mechanism involving both chain-breaking electron acceptor
(CB-A) and chain-breaking electrcn donor (CB-D) antioxidant activity
was found to be involved in each case.
2,6,3' ,5'-tetra-tert-butyl-4'-hydroxy phenyl-4-methylene-2,5-cyclohexadienel-1-one (hydrogalvinoxyl), 4-hydroxy, 2,2,6, 6-tetra methyl-
N-hydroxy piperidine and hydrogen iodide were formed together with
olefinic unsaturation in the substrates during the melt processing
of the polymers containing galvinoxyl, 4-hydroxy, 2,2,6, 6-tetra methyl piperidine oxyl and iodine respectively.
No bonding of the melt stabilisers to the polymers was found to occur.
Cupric stearate was found to undergo a similar redox reaction during
its action as a melt stabiliser with the formation of unsaturation in
the polymer. Evidence for the above processes is presented.
The behaviours of melt stabilisers in the subsequent thermal and photooxidation
of polyolefins have also been studied.
Galvinoxyl which is very effective under both mild and severe processing
conditions has been found to be an effective antioxidant during
thermal oxidation (oven ageing) and it is also moderately good as a
photo-stabiliser. Iodine and cupric stearate acted efficiently during melt stabilisation of polymers, however they were both ineffective as
thermo-oxidative antioxidants and UV stabilisers. Although the melt
stabilisaticn effectiveness of stable nitroxyl radicals (e.g. 4-hydroxy,
2,2,6,6-tetra methyl piperidineoxyl and Bis-(2,2,6 ,6-tetra methyl-4-
piperidinyl-N-oxyl) sebacate) is not as high as that of galvinoxyl
during processing particularly in excess of air, they have been found
to be much more efficient as UV stabilisers for polyolefins. The
reasons for this are discussed.