Phosphates secondary antioxidant

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. 

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). 

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... 

Examples of widely used secondary antioxidants are phosphites, phosphonites, and sultides (Fig. 11.7). Usually, secondary antioxidants are used in combination with primary antioxidants to benetit from a synergistic effect. The main action of phosphites and phosphonites is the oxidation to the corresponding phosphates by reacting with hydroperoxides. These P compounds are mainly used as melt stabilizers during processing. Sulfur compounds act as well as hydroperoxide decomposers via sulfur oxide and sulfenic acid formation. Sulfur compounds are preferably used in combination with phenolic antioxidants to improve the long-term thermal stability of polymers at temperature ranges between 100 and 150 °C. 

The secondary antioxidants are usually sulfur compounds (mostly thioethers and esters of thiodipropionic acid) or trimesters of phosphorous acid (phosphates). A remedy for many types of discoloration in plastics is often the use of a phosphite or a thioether. Both have the ability to react with hydroperoxides, as those formed in Reaction 1.72, to yield noruadical products, following heterolytic mechanisms. The reaction of phosphites to phosphates as an example ... 

Typical processing stabilizers for polypropylene and butylated hydroxy-toluene (BHT) as the primary antioxidant and phosphates and phosphonates as secondary antioxidants. Examples of the latter that are commonly used are tetrakis-(2,4-di-terr-butyl-phenyl)-4-4 -bisphenylylenediphosphonite, distearyl-pentaerythrityl-diphosphonite, tris-(nonylphenyl)-phosphite, tris-(2,4-di-teft-butyl-phenyl)-phosphite and bis(2,4-di-ferr-butyl-phenyl)- pentaerythrityl-diphosphite. In commercial polypropylenes, phosphorous compounds are always used together with a sterically hindered phenol. The compounds are commonly added in concentrations between 0.05 and 0.25%. 

Organophosphites are secondary antioxidants which reduce hydroperoxides to alcohols. They inhibit the discolouration reaction experienced by phenolics. Tris-nonyl phenyl phosphite (TNPP) is the most commonly used. The disadvantage of phosphates is their high hygroscopicity. Thioesters act as secondary antioxidants by destroying hydroperoxides to form stable sulphur derivatives. In addition, thioesters impart high heat stability to polyolefins, polystyrene and its copolymers. The m or disadvantage of thioesters is their unpleasant odour which is transferred to the host polymer. 

Among common antioxidants for plastics, there are phenolics and amines (primary antioxidants), and phosphates and thioesters (secondary antioxidants). 

U626 is a rather complex molecule. Very interestingly the phosphite moieties in U626 can have also an important role in the stabilization. Phosphites are well-known stabilizers and they are called secondary antioxidants, while hindered phenol-based stabilizers are known as primary antioxidants. Phosphites have the ability to react with hydroperoxides to yield phosphates according to scheme 5.[20,38] U626 combines primary and secondary stabilizers in the same molecule. 

An apparently anomalous peak at m/z = 662 is observed in the 266 nm photoionisation MS (see Figure 1.21a) which is due to a phosphate antioxidant which is generally due to an oxidation product of the Irgaflox 168 phosphite secondary antioxidant, i.e., it is possible to determine not only the active phosphate level in the polymer but also the... 

While phosphorus is not a common element in polymeric materials, other than in the polyphosphazenes, it does occur in a number of additives, such as secondary antioxidants and some plasticizers. The ehemieal-shift correlation chart of Fig. 26 illustrates that 8p depends largely on the oxidation state of the phosphorus, with P(III) species (such as phosphites) resonating between —450 and +250 and P(V) species (such as phosphates), between —50 and +100 ppm [17,18,60]. These shifts are relative to the reference... 

Preventive antioxidants (sometimes referred to as secondary antioxidants), on the other hand, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals. The most important preventive mechanism is the non-radical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g. AO 12-18 in Table 1, are the most important classes of peroxide decomposers. The simple trialkyl phosphites (e.g. Table 1, AO 12) decompose hydroperoxides stoichiometrically (PD-S) to yield phosphates and alcohols, see reaction 4. Sulphur compounds, e.g. thioethers and esters of thiodipropionic acid and metal dithiolates (Table 1, AO 15-18, 31, 32), decompose hydroperoxides catalytically (PD-C) whereby one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids, see reaction 5. References 1 and 2 give detailed discussion on antioxidant mechanisms. 

Protection of PET from thermal degradation is obtained by removal or deactivation of transesterification catalysts present in PET. Esters of phosphoric acid such as trialkyl or triaryl phosphates can be used for this purpose. Organic phosphites, which are well known to decompose hydroperoxides as secondary antioxidants, are also reported to deactivate metal ions (traces and/or catalyst residues) in polyolefins [4]. In PET, it is suspected that they also deactivate the metal species that catalyse the transesterification reaction. 

Another type of antioxidant sacrificially reacts with oxygen, probably in the form of a peroxide. This type of antioxidant is often called a secondary antioxidant. Commonly they are used in conjunction with a primary antioxidant such as a hindered phenol. They are typically phosphites, which upon reaction with oxygen become phosphates or sulfides that can react with oxygen to form sulfoxides or sulfones, depending on the degree of oxidation. The same concerns about polymer compatibility and volatility apply here also. 

The rate of Equation 6.2 is much greater than the rate of Equation 6.3 hence, deactivation of the R-O-O species constitutes a key step in oxidation inhibition, which is the role of the primary antioxidant. The homolytic cleavage of the hydroperoxide species, ROOH, Equation 6.4, leads to the autocatalytic nature of the oxidation. Thus, both the rates of Equations 6.3 and 6.4 account for the rate of oxidation. The role of a secondary antioxidant such as a phosphite, PCOR), is to act as a peroxide decomposer by oxidation of the phosphite to a phosphate (0R)jP=O. 

Although it would be interesting to study s-NMR for rubber vulcanisates, this nucleus has such low abundance and sensitivity that it is now not possible. On the other hand, P s-NMR is of more interest because of the sensitivity of the nucleus and lack of polymeric matrix interference the spectra can usually be acquired in a relatively short time. The main applications in polymer/additive deformulation are found in the analysis of phosphorous containing additives such as secondary antioxidants (e.g. Irgafos 168 and Sandostab P-EPQ), flame retardants and transesterification suppressants, as well as in quantitative determinations. P s-NMR is an efficient tool for the stmctural analysis of insoluble polyphosphates and melamine phosphates. 

Phosphite and phosphonite esters act as antioxidants by three basic mechanisms depending on their structure (1). Basically, phosphites and phosphonites are secondary antioxidants that decompose hydroperoxides. Their performance in hydroperoxide decomposition decreases from phosphonites, alkyl phosphites, aryl phosphites, down to hindered aryl phosphites. Five membered cyclic phosphites act catalytically by the formation of acidic hydrogen phosphates. In contrast, hindered aryl phosphites are interrupting the branched kinetic chain. 

Stabilizers With ionic antistats, it is necessary to avoid reactions with heavy metals that form insoluble salts, such as the use of sulfates with barium or of phosphates with lead. Oxidizing anions such as nitrates should not be used with organophosphitcs or divalent sulfur secondary antioxidants (e.g., DLTDP). As with choice of resin and plasticizer, product and process considerations determine stabilizer selection, and antistatic behavior is usually secondary. 

Citric acid is used in soft drinks, candies, wines, desserts, jellies, jams, as an antioxidant in frozen fruits and vegetables, and as an emulsifier in cheese. As the most versatile food acidulant, citric acid accounts for about 70 percent of the total food acidulant market. It provides effervescence by combining the citric acid with a biocarbonate/carbonate source to form carbon dioxide. Citric acid and its salts are also used in blood anticoagulants to chelate calcium, block blood clotting, and buffer the blood. Citric acid is contained in various cosmetic products such as hair shampoos, rinses, lotions, creams, and toothpastes. More recently, citric acid has been used for metal cleaning, substituted for phosphate in detergents, for secondary oil recovery, and as a buffer/absorber in stack gas desulfurization. The use of sodium citrate in heavy-duty liquid laundry detergent formulations has resulted in a rapid increase in the use of citric acid. 

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