Stabilisers hindered phenols

In addition to stabilisers, antioxidants and ultra-violent absorbers may also be added to PVC compounds. Amongst antioxidants, trisnonyl phenyl phosphite, mentioned previously, is interesting in that it appears to have additional functions such as a solubiliser or chelator for PVC insoluble metal chlorides formed by reaction of PVC degradation products with metal stabilisers. Since oxidation is both a degradation reaction in its own right and may also accelerate the rate of dehydrochlorination, the use of antioxidants can be beneficial. In addition to the phenyl phosphites, hindered phenols such as octadecyl 3-(3,5-di-tcrt-butyl-4-hydroxyphenyI)propionate and 2,4,6-tris (2,5-di-rcrt-butyl-4-hydroxybenzyl)-1,3,5-trimethylbenzene may be used. 

Standardisation of EPDM characterisation tests (molecular composition, stabiliser and oil content) for QC and specification purposes was reported [64,65]. Infrared spectroscopy (rather than HPLC or photometry) is recommended for the determination of the stabiliser content (hindered phenol type) of EP(D)M [65]. Determination of the oil content of oil-extended EPDM is best carried out by Soxhlet extraction using MEK as a solvent [66], A round robin test was reported that evaluated the various techniques currently used in the investigation of unknown rubber compounds (passenger tyre tread stock formulations) [67]. 

Kellum [115] has described a class-selective oxidation chemistry procedure for the quantitative determination of secondary antioxidants in extracts of PE and PP with great precision (better than 1 %). Diorgano sulfides and tertiary phosphites can be quantitatively oxidised with /-chloropcroxybenzoic acid to the corresponding sulfones and phosphates with no interference from other stabilisers or additives. Hindered phenols, benzophenones, triazoles, fatty acid amides, and stearate... 

Wieboldt et al. [560] have described SFE-SFC-FTIR analysis of hindered phenol primary antioxidants and phosphite secondary antioxidants in PE. SFE is more selective for the lower-range low-MW polymer than Soxhlet-type extraction. This yields a chromatogram with less interference from low-MW polymer peaks in the region where the additive components elute. As a result, SFE appears to be a better choice than Soxhlet-type extraction for the selective removal of additives from flaked polymer. SFE and dissolution/precipitation methods were compared for a PVC/stabiliser system [366]. 

New developments are hydroxylamines and lactones (for processing stability), which operate at an earlier stage during stabilisation. Lactone (benzofuranone) chemistry has been identified as commercially viable, and marks a revolutionary advance in comparison to hindered phenols and phosphites [18]. New lactone chemistry (Figure 10.1) provides enhanced additive compatibility, reduced taste and odour (organoleptics), resistance to irradiation-induced oxidation, and inhibition of gas fade discoloration. The commercial introduction of fundamentally new types of stabilisers for commodity and engineering polymers is not expected in the near future. 

Figure 6. Scanning electron micrograph of tensile fractured surface of 30% glass reinforced MA-modified PP (stabilised with hindered phenol, 1C1-HWM25).

Sidwell and Zondervan [10] used LC-MS with APCI detection for the identification and quantification of extractable antioxidants from food-contact plastic materials. Identification is based on the presence of the molecular ion (M + FI)+, (M—H) , other key ions or on further ion breakdown (MSn) transitions. The following antioxidant/stabiliser types were examined hindered phenols,... 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

Similarly, the antioxidant activity of vitamin E is centreed on its chainbreaking donor activity in-vitro rate studies on a-tocopherol have shown that it is one of the most efficient alkylperoxyl radical traps, far better than commercial hindered phenols such as BHT, 2,6-di- ferf.butyl-4-methylphe-nol. Its efficiency was attributed [30, 31] to the highly stabilised structure of tocopheroxyl radical (which is formed during the rate-limiting step, reaction 3) because of favourable overlap between the p-orbitals on the two oxygen atoms. 

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. 

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). 

The use and benefits of liquid, partially hindered phenols, such as Irganox 1141, as effective chain-stoppers and antioxidants in PVC polymerisation and as heat stabilisers for MBS (impact modifiers for PVC) and PVC plasticisers are demonstrated. 

Use of both primary and secondary antioxidants usually provides a synergistic effect, where the combined effect of two or more stabilisers is greater than the sum of the effects of the individual stabilisers. It is common practice to include both a phosphite, such as tris(t-butylphenyl)phosphite and a hindered phenol, such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyljpropionate to provide improved heat stabilisation in polyolefin formulations. 

Two groups of anti-oxidants are of practical importance hindered phenol light stabilisers (HPLS) and hindered amine light stabilisers (HALS). Examples are shown in Fig. 13.7 and the mechanism of the radical trapping reaction is shown in Fig. 13.8. They are often combined with UV absorbents (for partly synergistic effects) and mostly used for the fibre protection. 

An ionic 2-chloro-l,3,2-diazaphospholene (72) was prepared from a silicon precursor (73). Since the saturated analogue 74 was covalent, the phosphenium ion 72 is probably stabilised by having an aromatic 6n electron structure. The phosphadiazonium compound 75 with a sterically hindered phenol or aniline gave the phosphenium ions 76 this constitutes a new preparative route to phosphenium ions. A series of phosphenium ions (77), stabilised by two intramolecular dative P-N bonds, has been prepared, and the X-ray crystal structure of one (77, X = H, Y = PFg) determined. ... 

Removal of the latter in the cyclic regeneration mechanism of piperidine stabilisers by reaction with hindered phenol radical intermediates is also believed to be important in antagonism. During thermal oxidation the interactions were more favourable and synergism prevailed due here to the complementary behaviour of nitroxyl free radicals and antioxidant radicals in free radical scavenging processes. 

It has been demonstrated that the nitroxyl radical, X, reacts with a secondary alkyl radical to form XI which, under high-temperature conditions (>120°C), regenerates the original diphenylamine molecule, Reaction (4.36). In essence, this group of stabilisers acts catalytically by scavenging alternately peroxy (ROO-) and alkyl radicals (R-). As stated earlier, sterically hindered phenols deactivate only two peroxy radicals per phenol molecule. Hence, under high-temperature conditions, aromatic amines are far superior to their phenolic counterparts. As shown in Table 4.3, the stoichiometric factor of the diphenylamines depends on the substituents in the para position [33]. The efficacy of the diphenylamine antioxidant is improved by alkylating the para positions. The stabilisation mechanism for phenyl-a-naphthylamines. Reaction sequence (4.37) [34], is described as follows ... 

The very reactive radicals R are blocked by being transformed into a neutral molecule (RH) and a radical of very low reactivity stabilised by conjugation. In that way the chain reactions are interrupted. Both antioxidants (hindered phenols and diphenylamines) are consumed after hydrogen radical donation (sacrificial antioxidants) [140]. 

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