Peroxide Decomposers PDs

PD antioxidants may destroy hydroperoxides either stoichiometrically or catalytically. The commonly used processing stabilisers, the alkyl or aryl phosphite esters are representative of the first class  

However, hydroperoxide decomposers may act by much more complicated mechanisms. Many sulfur compounds, like the thiodipropionate esters (DRTPs) or the metal dialkyldithiocarbamates (MRDCs) are oxidised to sulfur acids (sulfinic, sulfonic and SO3) which are ionic catalysts for the non-radical decomposition of hydroperoxides. The MRDCs are particularly important since, unlike the phosphites, they also contain complex transition metal ions and when M is a transition metal ion e.g. Ni) they are also UVAs. 

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Metal peroxide decomposer (PD-S) ions Catalytic peroxide decomposer (PD-C)... 

Preventive antioxidants (sometimes referred to as secondary antioxidants) act by interrupting the secondary oxidation cycle to prevent or inhibit the generation of free radicals. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulfur-containing compounds, e.g., AOs 15-24, Table 1, are the most important groups of peroxide decomposers. Phosphite esters are known as stoichiometric peroxide decomposers (PD-S) they reduce hydroperoxides to... 

Three different classes of compounds form the major and most important commercial categories of photostabilizers for PP. These are based on nickel complexes (those containing sulfur ligands function primarily as peroxide decomposers, PD), UV-absorbers (UVA), e.g. based on 2-hydroxybenzophenone and 2-hydroxyphenylbenztriazole, and sterically hindered amine light stabilizers (HALS). 

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 ... 

Metal complexes of dithioic acids, eg, dithiocarbamates, dithiophosphates, xanthates (MDRC, MDRP, MRX, respectively) (see AOs 21-24, Table 3), which are highly effective catalytic peroxide decomposers (PD-C) and excellent melt stabilizers, are generally effective photostabilizers (101,105,136,137,139,149). Their... 

A variety of transition metal ions accelerate the oxidative degradation of the carbon-chain polymers by catalysing both the formation and the decomposition of hydroperoxides. Typically, cobalt-catalysed oxidation of hydrocarbons is used in the manufacture of terephthalic acid from p-xylene. These prooxidant reactions also accelerate the breakdown of polymer molecules to smaller fragments (see Fig. 12.2) but are effectively inhibited by metal deactivators. All antioxidants have some retarding effect, but the most effective are the peroxide decomposers (PD) that remove hydroperoxides as they are formed by ionic (non-free radical) reactions.Deactivated transition metal ions (e.g. 

Synergism can also arise from cooperative effects between mechanistically different classes of antioxidants, e.g., the chain breaking antioxidants and peroxide decomposers (heterosynergism) [42]. For example, the synergism between hindered phenols (CB—D) and phosphites or sulphides (PD) is particularly important in thermal oxidation (Table 2). Similarly, effective synergism is achieved between metal dithiolates (PD) and UV-ab-sorbers (e.g., UV 531), as well as between HALS and UV-absorbers, (Table 3). 

Scheme 2 Schematic presentation of the cyclical oxidation process and some of the main reactions/products formed from the propagating radicals. The antioxidant mechanisms interrupting the oxidative cycles are also shown. AO antioxidant, CB-A chain breaking acceptor, CB-D chain breaking donor, PD peroxide decomposer, UVA UV-absorber, MD metal deactivator...

In order for a stabilizer to function well it must also be sufficiently stable to UV light to survive the irradiation process. Photolysis studies were performed with a number of representative antioxidants of the hydrogen donor and peroxide decomposer type (Table III). When the stabilizers are photolyzed alone in n-hexane, conversions range from 10-22% with the hindered phenol, HD-1, the least stable and the aryl phosphite, PD-4, the most stable. When solutions containing both the antioxidant and the photoinitiator are photolyzed, the photoinitiator accelerates the decomposition of the antioxidants by a factor of about five. This results in the total decomposition of the HD-1 which can no longer be detected. The other antioxidants are not completely decomposed. 

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... 

The second antioxidant mechanism, the peroxidolytic or peroxide decomposing process (PD) removes the hydroperoxides (POOH) that are the main source of initiating radicals by reactions that do not produce free radicals. 

CB-A Chain breaking (electron acceptor) CB-D Chain breaking (electron donor) PD Preventive (peroxide decomposing) UVA UV absorbers Q Quenchers MD Metal deactivators... 

A 10 g sample is roasted at 650°C and decomposed with hydrochloric acid/hydrogen peroxide. The Pt and Pd in the solution is pre-concentrated using adsorbent materials which are composed of active charcoal and anion resin. The adsorbent materials are washed sequentially with 2% ammonium bifluoride, 5% hydrochloric acid and distilled water, and subsequently ashed in a muffle furnace at 650°C. The total residue of ca. 0.25 mg is dissolved with 2 ml fresh aqua regia, then diluted to 5ml using 10% hydrochloric solution, and determined using ICP-MS, which has a detection limit of 0.2 ppb for Pt and Pd. The residue can also be mixed with a spectral buffer, and determined by DC-arc ES, which has detection limits of 0.3 ppb for Pt and 0.2 ppb for Pd. 

Ten percent additional product can often be obtained from the filtrate after decomposing the hydrogen peroxide (e.g., using Pd/C). 

Pd/C, 25 °C) to give methyl 4-deoxy-6-aldchydo-J3-D-glucoside. Water is the medium and for all reactions pH is 7. Furthermore the enzyme catalase is added to decompose the hydrogen peroxide formed in the first step. 

Powerful synergism is achieved, for example, in the melt and thermal stabilisation of PP by using combinations of hindered phenols (CB-D) and phosphites and/or phosphonites (PD) and sulfur compounds (PD) in fact phosph(on)ites are seldom used alone. The latter enhance the melt stabilizing effect of hindered phenols (Table 1) and reduce discoloration of the polymer caused by phenol transformation products. The cooperative effect of hindered phenols (PhOH) and phosphites (P) occurs through two steps, whereby phenols scavenge alkylperoxyl radicals and phosphites decompose peroxides in a nonradical reaction which leads to enhanced melt stability of the polymer (Figure 7). Further interaction between the colored transformation products of phenol and the phosphite antioxidant, or its product(s), results in noncolored products, hence... 

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