Secondary Antioxidants Hydroperoxide Decomposers

Substances capable of reacting with hydroperoxides without creating radical decomposition products are called secondary antioxidants. These are in particular phosphites and phosphonites as well as sulfur costabilizers. Sulfur compounds are already very active at room temperature, while phosphites are active only at increased temperatures. The active application range of phosphites/phosphonites is therefore essentially limited to common processing temperatures. 

In combination with primary antioxidants, secondary antioxidants delay the consumption of the former by decomposing hydroperoxides to non-radical compounds and by preventing the formation of new radical decomposition products. Many stabilizer types supplement each other, thus protecting against too rapid consumption and increasing their effectiveness multiple times. That is why secondary antioxidants are very often called synergists or co-stabilizers . Detailed descriptions of the secondary antioxidants mechanisms can be found in [518], [519], and [523]. 

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If primary antioxidants compete with free radicals for hydrogen abstraction from the polymer and then scavenge another free radical (Eqs. 15.3 and 15.4), secondary antioxidants directly decompose unstable hydroperoxides (Eq. 15.6), thereby preventing with free radicals formation (Eq. 15.7) ... 

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. 

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. 

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. 

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. 

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). 

SCHEME 2.9 Stabilizing activity of hydroperoxide-decomposing secondary antioxidants. 

Two principal classes of antioxidant are effective in thermal oxidation. Chainbreaking or primary antioxidants limit the rate of the chain propagation steps (Eqs. 3-2 and 3-3) by trapping carbon- or oxygen-centered free radicals. Hydroperoxide decomposing or secondary antioxidants prevent chain initiation by interfering with ROOH. Photoantioxidants protect plastics exposed to photo-oxidation. 

Antioxidants are classified as primary or secondary, depending upon how they react. Hindered phenols are primary antioxidants and function by donating a hydrogen to convert a peroxy radical to a hydroperoxide. Phosphites are among what are called secondary antioxidants and function as hydroperoxide decomposers. The ultimate outcome of these reactions is to convert the polymer bound radical to derivatives that are less destructive to the polymer. 

Antioxidants interrupt the degradation process in two ways, depending on structure by chain-terminating primary antioxidants and by hydroperoxide decomposing secondary antioxidants. 

Secondary antioxidants react with hydroperoxides to produce non-radical products and are therefore often termed hydroperoxide decomposers . They differ from priniaiy phenols and amines in that they are decomposed by reaction with hydroperoxide, rather than containing it. They are particularly useful in synergistic combinati(His with primary antioxidants.Systems that do not contain a phenolic... 

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. 

Two large, basic groups of antioxidants are normally distinguished (1) chain terminating or primary antioxidants, and (2) hydroperoxide decomposers or secondary antioxidant, frequently called synergists. 

Sulfoxides themselves yield, on further oxidation, even more powerful hydroperoxide decomposers than the original sulfides, in that they are able to destroy several equivalents of hydroperoxides. This catalytic effect is explained by the intermediate occurrence of sulfenic acids and sulfur dioxide. The fact that the phenomenon of synergism, which is defined as a cooperative action such that the total effect is greater than the sum of two or more individual effects taken independently, is often observed when primary and secondary antioxidants are combined ahs been explained with the concept of the simultaneous occurrence of the radical reactions (e.g.. Equation 1.72) and the nonradical hydroperoxide decomposition (e.g.. Equation 1.74 and Equation 1.75). 

There are two major classes of antioxidants and they are differentiated based on their mechanism of inhibition of polymer oxidation chain-terminating or primary antioxidants and hydroperoxide-decomposing secondary antioxidants [5]. Primary or free-radical scavenging antioxidants inhibit oxidation via very rapid chain-terminating reactions. The majority of primary antioxidants are hindered phenols or secondary aryl amines. Generally, hindered phenols are nonstaining, nondiscoloring, and are available in a wide... 

The performance of a primary antioxidant can be improved by the use of a secondary antioxidant. Secondary antioxidants or peroxide decomposers do not act as radical scavengers but undergo redox reactions with hydroperoxides to form nonradical stable products (Fig. 2). This class of antioxidants (Table 3) includes phosphites such as tris(nonylphenyl)phosphite (PS-1) and thiosynergists or thioesters such as dilauryl... 

The probable key to the continued success of the phosphorus-based additives in aromatic polyesters is their ability to take part in various processes beneficial to the non-oxidative heat stability of their host polymers. They are known hydroperoxide decomposers, and thus could safely destroy such species present in the polyester. They are, for the same reason, excellent secondary antioxidants, especially if used in conjunction with primary antioxidants such as hindered phenols, in a wide variety of polymers. Their ability to react with catalyst residues and prevent these contributing to degradation reactions of the polymer is also important. They would also appear to be capable of reacting with the polyester chain ends, leading to end-capping-and consequent reduction of the amount of volatiles such as acetaldehyde and acrolien-or even providing chain extension. 

The antioxidant additives present in the formulation of a polymer resin may have the function of protecting it against degradation by deactivating the free radicals formed (primary antioxidants) or by decomposing hydroperoxides formed (secondary antioxidants). The former are useful during the service life of the material at ambient temperatures, whereas the latter are most useful when processing the material at elevated temperatures [75]. 

Primary antioxidants, usually sterically hindered phenols, function by donating their reactive hydrogen to the peroxy free radical so that the propagation of subsequent free radicals does not occur. The antioxidant free radical is rendered stable by electron delocalization. Secondary antioxidants retard oxidation by preventing the proliferation of alkoxy and hydroxy radicals by decomposing hydroperoxides to yield nonreactive products. These materials are typically used in synergistic combination with primary antioxidants. 

Outstanding properties thermally and hydrolytically stable secondary antioxidant, decomposes peroxy radicals and hydroperoxides formed during the oxidation process ... 

Secondary antioxidants work by preventing the formation of free radicals. Some of them will decompose hydroperoxides by a safe reaction before they get the chance to generate fiee radicals. Hydroperoxide decomposers fall into two categories some act by a catalytic mechanism. These include the sulfur-containing acids that are formed by the oxidation of thiodipropionate esters or metal dialkyldithiocarbamates. The last-mentioned, if they contain a transition metal, are also ultraviolet light absorbers. An alternative type of hydroperoxide decomposer acts by a stoichiometric mechanism, namely the phosphite esters. 

Preventative or secondary antioxidants act at the initiation stage of the radical chain mechanism to prevent the formation of radical products. Their mechanism involves the decomposition of hydroperoxides to form stable nonradical products. In the absence of peroxide scavengers, hydroperoxides thermally or photolyti-cally decompose to radical products and accelerate decomposition. The most common secondary antioxidants are sulfur-based thiosynergist or phosphorus-based phosphites. ... 

Stabilisers Primary antioxidants (sterically hindered phenols, secondary aryl amines) Hydroperoxide decomposers (organophosphites, thioesters) Acid absorbers (lead salts, Ca/Ba-Ba/Cd-Ba/Sn salts, organotins, epoxidised oils)... 

To inhibit the oxidation process, certain chemical compounds (antioxidants or stabilisers) are added to the polyethylene, see Sect. 5.2.2. The so-called chain-breaking-donors or primary antioxidants, also called inhibitors, react with the chemical radicals. They thus intermpt the reaction chain. The so-called hydroperoxide decomposer or secondary antioxidants react with the hydroperoxide before it can disintegrate into radicals. Thus they prevent the start of new reaction chains. 

The so-called secondary antioxidants X convert the hydroperoxides into innocuous non-radical compounds. Thus the chain branching by the start of new kinetic chain reactions is prevented. Therefore these substances are also called hydroperoxide decomposers. The general reaction pattern reads as ... 

Antioxidants fall into two classes, according to their mechanism in interrupting the degradation process (i) chain-terminating primary anti-oxidants and (ii) hydroperoxide-decomposing secondary anti-oxidants. 

Traditional antioxidants are classified as either primary or secondary types depending on their mode of action. Primary antioxidants act by trapping free radicals, usually hydroperoxy radicals, through donation of a labile hydrogen to the radical species. Secondary antioxidants interfere with the propagation steps of au-toxidation by decomposing hydroperoxides to form stable, nonradical species. It is quite common for a combination of primary and secondary antioxidants to be used to provide the maximum stabilization of a plastic. Use of antioxidants in plastics is ubiquitous, since nearly all resin types require some form of stabilization in order to provide useful and durable materials. 

Secondary antioxidants, also called peroxide decomposers, inhibit oxidation of PP by decomposing hydroperoxides. Phosphites and thioesters are commonly used as secondary antioxidants. Secondary antioxidants are usually combined with primary antioxidants to produce a synergistic effect on oxidation. With proper selection of two antioxidants, it is possible to achieve protection against oxidation which is greater than the sum of protection given by the two antioxidants when working separately. 

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