Antioxidants, secondary

A secondary antioxidant is used in conjxmction with a primary antioxidant to improve the thermal oxidative stability of polyethylene. The secondary antioxidant is a hydroperoxide decomposer that reduces the peroxy group to an alcohol or ether, consequently preventing the homolytic cleavage of the ROOH group to RO and OH. 

Sedlar and co-workers [27] has described a gas chromatographic procedure, outlined below, for the determination of dilauryl p, P thiodipropionate antioxidant and its primary oxidation products in polyolefins, ethylene-vinyl acetate co-polymers, acrylonitrile-butadiene-styrene resins and high impact polystyrene. These workers examined the conditions under which di-lauryl-thiodipropionate and its oxidation products are hydrolysed, by 5 N methanolic potassium hydroxide at 80 °C, quantitatively to lauryl alcohol, thus making gas chromatographic determination possible. The method described next has the advantages of being rapid, accurate and simple. It has been applied successfully to the 

Substances separated Stationary phase Column temperature, °C Other details Ref. 

6-Di-t-butyl-p-cresol, 2-(2- hydroxy-5-methyl-phenyl) benzo-triazole 25% LAC-2R/446 (adipate ester) + 2% H3PO4 on Chromosorb 135 Hj carrier gas, FID Error 1% [8] 

6-Di-f-butyl-p-cresol, 2,6-di-f-butyl-phenol 10% Apiezon N on Celite 545 164 H2 carrier gas, FID, 10- M in the presence of others can be detected m 

Low bp phenols Capillary column coated with 10% xylenol phosphate 125 FID [12] 

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The effects of release additives on bulk properties must also be carefully considered, particularly with integral additives to plastics. Eor example, partial solubiHty usually confers some plastici2ing effect. This may improve impact strength but could reduce the heat distortion temperature. Some release additives such as metallic soaps have secondary antioxidant and heat-stabiH2er benefits. Such effects are exploited in multipurpose formulations. 

Another method for slowing oxidation of rubber adhesives is to add a compound which destroys the hydroperoxides formed in step 3, before they can decompose into radicals and start the degradation of new polymer chains. These materials are called hydroperoxide decomposers, preventive antioxidants or secondary antioxidants. Phosphites (phosphite esters, organophosphite chelators, dibasic lead phosphite) and sulphides (i.e. thiopropionate esters, metal dithiolates) are typical secondary antioxidants. Phosphite esters decompose hydroperoxides to yield phosphates and alcohols. Sulphur compounds, however, decompose hydroperoxides catalytically. 

When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

Process 5, the conversion of hydroperoxides to alkoxy and hydroxyl radicals, can be interrupted by incorporation of a secondary antioxidant such as phosphites (e.g. Irgafos 168) or thioesters (e.g. Evanstab 12). These materials act as reducing agents, converting hydroperoxides to alcohols and themselves being converted to phosphates or sulfoxides, respectively (see Fig. 16). 

Meyer, M.,. Schreck, R., and Baenerle, P. A. (1 993). H2O2 and antioxidents have opposite effects on activation of NF-kappa B and AP-1 in intact cells . AP I as secondary antioxidant respon Sive factor. EMBO J. 12(5), 2005-2015. 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

The mechanism of secondary stabilization by antioxidants is demonstrated in Figure 15.5. TnT-nonylphenyl phosphites, derived from PCI3 and various alcohols, and thio-compounds are active as a secondary stabilizer [21], They are used to decompose peroxides into non-free-radical products, presumably by a polar mechanism. The secondary antioxidant is reacting with the hydroperoxide resulting in an oxidized antioxidant and an alcohol. The thio-compounds can react with two hydroperoxide molecules. 

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]. 

Figure 9.3 Chemical stmctures for the phosphorous-containing secondary antioxidants Naugard 524 (A) tris(nonylphenyl)phos-phite (B) and Irgafos P-EPQ (C). After Brandolini et al. [49]. Reprinted from Spectroscopy, 7(3), AJ. Brandolini et al., 34-39, Copyright (1992), with permission from IOS Press...

Anti-oxidants can be divided into two classes depending on which part of the radical chain they quench. Primary anti-oxidants are radical scavengers and will react with alkyl chain radicals (R ) or hydroperoxides (ROOH). Secondary antioxidants work in combination with primary anti-oxidants and principally act by converting peroxide radicals (ROO ) into non-radical stable products. Synergism often works when both classes are used together. 

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. 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Effects of Secondary Antioxidants on Stored Ground Beef... 

Secondary antioxidants, i.e., sequestrants or chelators, are important compounds in the prevention of lipid oxidation. The effect of chelators tested varied with the different compounds. Of those chelators tested, ethylenediaminetetraacetic acid (EDTA, tetrasodium salt) and sodium phytate were the most effective inhibitors of lipid oxidation (so indicated by low hexanal and TEARS values) and MFD (as seen by high CBB and low PIT and CBD intensity values), see Table 5. Sodium phytate was previously shown to chelate iron and thus, was proposed as a food antioxidant(7J). Sodium citrate at a concentration of 500... 

Table 5. Effect of secondary antioxidants on chemical and sensory properties of cooked beef patties stored 2 days at 4" C...

A dipeptide Met- , derived from sardine muscle (Matsufuji et ah, 1994), stimulates expression of the antioxidant defense protein HO-1 in a concentration-dependent manner. Previous findings revealed that HO-1 protein expression is accompanied by the induction of a secondary antioxidant protein, ferritin. In a present study, the effect of Met- on the expression of the antioxidant stress proteins, heme oxygenase-1 (HO-1), and ferritin in endothelial cells derived from the human umbilical vein and their contribution to the decrease in radical formation that occurs under the influence of this dipeptide were studied and reported potential activity (Erdmann et ah, 2006). 

Dioxathiolane. Y-oxidc, benzo-l,3,2-dioxathiolene. Y-oxidc, and other cyclic sulfites have been studied as secondary antioxidants <1997MI209>. They decompose hydroperoxides in a nonradical way at a faster rate than phosphites, and may be used for the protection of polymers against aging. 

Secondary antioxidants or hydroperoxide decomposers (see Scheme 2.1) are typified by organosulfur species having reducing properties such as sulfides and thioethers. Tertiary phosphites also fall into this category (see Scheme 2.9). 

Very often, antioxidants are used in combinations to ensure maximum activity and typically, a commercial additive system may comprise both a primary and secondary antioxidant species, although the total concentration remains <1 wt%. Scheme 2.10 schematically shows... 

R + R. OH - RH + R. o flow a combination of primary and secondary antioxidants functions in a SCHEME2.8 Stabilizing polyolefin matrix.82 Some metal-chelate scavengers may also be based on activity of chain-breaking, a tertiary phenolic structure, thereby introducing two antioxidant properprimary antioxidants. ties into the same molecule. 

SCHEME 2.9 Stabilizing activity of hydroperoxide-decomposing secondary antioxidants. 

SCHEME 2.10 Combined stabilizing activity of primary and secondary antioxidants. 

The hindered phenolics are primary antioxidants and are normally used in conjunction with the thiodipropionate esters. These combinations are synergistic. They are of only limited value as heat stabilizers when used alone the thiodipropionate esters are secondary antioxidants, also of only limited value when used alone. 

The aryl phosphites also are secondary antioxidants and protect against process shearing. These chemicals have little value against high... 

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