Peroxidolytic stabilizers

The antioxidant stage of peroxidolytic stabilizers is frequently preceeded by a prooxidant stage which varies in intensity (see Section 19.3.2.2). In the case of metal dithiolates, however, (during both thermal and photooxidation), the prooxidant stage is not observable at the low stabilizer concentrations used normally in polymers. The radical scavenging ability of metal dithiolates may be responsible for the absence of this stage. 

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. 

These polar transformation products and sulfur oxides (SO2, SO3) arising in the ultimate stages of the transformation process are formed in trace amounts in the aged polymer matrix. Volatile products may be sources of undesirable organoleptic problems. This limits the use of organic thiocompounds in odor-sensitive applications. Organic S-proto-nic acids 85, 86 deactivate basic stabilizers (HAS). The peroxidolytic effect of 85, 86 is reduced in the presence of some antiacids or fillers, e.g., calcium carbonate. 

Organotin stabilizers containing sulfur (e.g. 45) or their transformation products 96, 97 are considered as hydroperoxide-decomposing antioxidants (Al-Malaika, 1989). Within this stabilizing function, thiol 97 is oxidized by hydroperoxide in disulfide 99 and sulfenic acid 100, having peroxidolytic properties. 

The elimination of labile chlorine atoms from the polymer backbone is the most important stabilization mechanism for PVC. Dialkyl tin maleates (e.g., DBTM) and thioglycollates (e.g., DOTG, AO 36, Table 1) function by eliminating HCl, see Reaction 7. In addition, the maleates also act by removing the unsaturation and limiting color development, whereas the thioglycollates have an additional peroxidolytic function. " ... 

Scott, G. Peroxidolytic antioxidants sulphur antioxidants and autosynergistic stabilizers based on... 

Amongst the simplest peroxidolytic antioxidants are the alkyl and aryl phosphites which also act as good melt stabilizers in PP (Table 1). Phosphites reduce hydroperoxides to alcohols with a 1 1 stoichiometry and are therefore referred to as stoichiometric peroxide decomposers (PD-s), Figure 5(a). In addition to their stoichiometric peroxidolytic activity, some phosphite esters also behave as catalytic peroxide decomposers (PD-c) in addition to having some chain-breaking (CB) activity ... 

Peroxide Decomposers as Processing Stabiiizers. Alkyl and aryl phosphite esters (see Scheme 12) are effective melt stabilizers. They are often used in combination with hindered phenols (see Table 6). Phosphites generally function by a stoichiometric peroxidolytic mechanism (PD-S) Table 7 illustrates the benefits of using the commercial phosphites TNPP (AO 15) and Irgafos P-EPQ (AO 17, Table 3) for melt stabilization (95). The unique phosphite AO 37 and its phosphate transformation products AOs 38 and 39, (Table 7), which were shown (104,133-135) to operate by a catalytic mechanism (PD-C), are particularly effective at low concentration of the parent stabilizer molecule (see AO 37, Table 7) (95). Phosphites are, however, generally susceptible to hydrolysis. For example, hydrolysis of aryl phosphites leads to the formation of low molecular mass phenol and a... 

Table 7. Effect of Some Peroxidolytic Antioxidants as Melt Stabilizers for PP at 270°...

Another example of highly effective melt stabilizers are the thiolate metal complexes (e.g. MDRC, MDRP, MRX, M = Ni, Zn) which are also thermal and UV stabilizers for polyalkenes (see Section 19.3.3.1.V, 19.4.2.2.i and 19.4.2.2.ii) and no hydroperoxides can be detected in polymers containing them after processing. In addition to their peroxidolytic function dithiolates have the ability to trap alkyl peroxyl radicals The contribution of this trapping mechanism to... 

It has been known for a long time that, in the case of sulfur-containing antioxidants, the effective peroxidolytic catalysts are not the parent compounds but their oxidation products (see Section 19.4.2.2.i). The realization that melt-processing operations exert a profound effect on the light stability of polymer articles containing them is, however, relatively recent. The main reason for this is the fact that earlier mechanistic studies were mainly conducted in model compounds or hydrocarbon oils. 

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