Melt stabilisation

Effect of Polyolefin Processing Temperatures on the Melt Stabilising Activity of Vitamin E... 

Due to their peroxidolytic activity, phosphites are normally used in combination with hindered phenols in order to deactivate hydroperoxides formed as a consequence of the antioxidant function of hindered phenols, see reaction 1. The enhanced melt stability of tocopherol-containing PP extruded in the presence of a phosphite is clearly illustrated in Fig. 5a with an optimum melt stabilising performance at a 1 2 w/w ratio in favour of the phosphite. 

In addition to their use in combination with other antioxidants as melt stabilisers, phosphites are also used to reduce discoloration of polymer melts [37, 39, 40, 41, 42]. Using the phosphite Ultranox U-626 with tocopherol at a 2 1 w/w ratio, respectively, has resulted in a considerable colour reduction in the polymer (Fig. 5a-inset) [33]. This has also led to a 30% reduction in the total oxidation products formed in the polymer with only three products formed trimers, tocoquinone and aldehydes. This significant reduction in product formation results in an excellent retention of tocopherol in the polymer of over 95%. The preservation of the initial tocopherol con-... 

Based on the evidence obtained from the amount and nature of transformation products formed, a mechanism of melt stabilising action of tocopherol in PP and PE has been proposed, see Scheme 6 [34]. It is well known that, like other hindered phenols, a-tocopherol is rapidly oxidised by alkylperoxyl radicals to the corresponding tocopheroxyl radical (a-Toe, Scheme 6a). Further oxidation of the tocopheroxyl radical in the polymers leads to the formation of coupled and quinonoid-type products, e.g. SPD, TRI, DHD (see Figs. 8 and 9). Dimerisation of the intermediate o-quinone methide (QM) leads to the formation of the quinonoid-type dimeric coupled product, SPD (Scheme 6 reaction d). 

Scheme 6 Mechanism of melt stabilising action of a-tocopherol in polyolefins [34], TRI trimer, SPD spirodimer, DHD dihydroxydimer, QM quinine methide, ALD aldehyde, TQ tocoquinone...

The outstanding melt stabilising efficiency of tocopherol in polyolefins, coupled with the safety aspect associated with the use of such a biological antioxidant, should improve its future position in the antioxidant market and encourage its wide-spread use in many applications, especially those related to medical and other human-contact applications. 

Melt stabilisers such as phosphoric acid and its salts and esters are added to the polymerisation process to reduce thermal degradation and colour formation (yellow). They also have the added effect of stabilising the polymer when it is being later processed into food packaging. Many of the phosphorus based stabilisers are listed in 2002/72/EC for use in food contact applications and are covered by the basic polymer doctrine of the US EDA. 

Henman, T. J. (1979) Melt stabilisation of polypropylene, in Scott, G. (Ed.) Developments in Polymer Stabilisation - 1, London Elsevier Applied Science. 

Thermal stabilisation of aromatic polyester melts is a goal which, on paper at least, appears to be very difficult to achieve. If, as proposed by many researchers, that the main route of breakdown of the polyester chains is via concerted reaction via a cyclic intermediate, little may be achieved by using standard melt-stabilising additives. The non-radical nature of the reaction, and its likely high rate in a molten polymer under high temperature and shear conditions, provides no point of attack for conventional stabilisers. However, the potential for metal salt impurities such as catalyst residues to accelerate the chain scissions has been alluded to, and could provide a target for stabilising additives. 

As with virtually all organic polymers, some oxidative damage may have occurred in the polyester before the melt process under consideration. There may be hydroperoxides present which would be extremely useful to target, and to decompose to inert products by use of a melt stabiliser. 

General Electric Corporation [86] found that the addition of small amounts of a polybutadiene, with vinyl content over 80% resulted in substantial melt stabilisation of linear aromatic polyesters such as PET. 

Formed during processing as a result of the Irgafos 168 performing its role as a melt stabiliser this is a benign material taking no further part in the stabilisation process. 

Kriston I, Orban-Mester A, Nagy G, Staniek P, Foeldes E, Pukanszky B. Melt stabilisation of Phillips type polyethylene. Part I the role of phenolic and phosphorous antioxidants. Polym Degrad Stab 2009 94(4) 719-29. 

Mariani P, Carianni G, Balducci D, Roccasal-vo S, La Mantia FP. Melt stabilisation of high-density film-grade polyethylene. Macromol Symp 2001 176 73-82. [1st International Conference on Polymer Modification, Degradation and Stabilisation, 2000]. 

Al-Malaika S, Goodwin C, Issenhuth S, Bmdick D. The antioxidant role of a-tocopherol in polymers 11. melt stabilising effect in polypropylene. Polym Degrad Stab 1999 64 145-56. 

---