Stabilizers hydroperoxide decomposers

Preventive antioxidants3, comprising light stabilizers, hydroperoxide decomposers, and deactivators of metals, take part in the deactivation of further species shown in Scheme 1. 

An unsubstituted hydroxylamine is a powerful hydroperoxide decomposer and peroxyl radical scavenger, and could play an important role in photo-stabilization even if present at only a low concentration after dark intervals. 

In this connection it is interesting to note that the rapid oxidation of phenylhydrazones with peracids generally leads to cis azoxy compounds, while the slower air oxidation (particularly when exposed to light) evidently affords hydroperoxides of some thermal stability which decompose to azo compounds [30]. 

Processing Additive Stabilizers Primary antioxidants (sterically hindered phenols, seoaryl amines) hydroperoxide decomposers (organophosphites, thioesters), acid absorbers (lead salts, Ca/Ba-Ba/Cd-Ba/Sn salts, organotins, epoxidized oils)... 

The stabilizers chosen for evaluation include different types of heat and light stabilizers selected to represent different mechanisms of action as well as chemical compositions (ArJi). Types of stabilizers evaluated include benzotriazole and benzophenone light stabilizers [ultraviolet (UV) light absorbers], hindered amine light stabilizers (HALS, catalytic radical scavengers), hindered phenol heat stabilizers (antioxidant radical scavengers), and thioester heat stabilizers (antioxidant hydroperoxide decomposers). 

SCHEME 2.9 Stabilizing activity of hydroperoxide-decomposing secondary antioxidants. 

Table I. Percentage of Hydroperoxide Decomposed in Chlorobenzene Solutions Containing Tetralin Hydroperoxide (2.5 X 10-2 mole/liter) and Various Stabilizers (0.1 X 10 2 mole/liter) at 70°C.

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. 

Polymers are generally doped with low amounts of preservatives during fabrication, prcK sing or confectioning. The individual classes of stabilizers involve chainbreaking (CB-AO) and hydroperoxide decomposing antioxidants (HD-AO) [3-5],... 

The strucmre of the aryl groups is designed to increase the hydrolytic stability of the compound. The sulfur compounds which are the most efficient hydroperoxide decomposers... 

Different types of antioxidants are used for the stabilization of polymers H-donors, radical scavengers and hydroperoxide decomposers. They interfere in different ways during the auto-oxidation cycle of polymeric materials (Scheme 2). 

Antioxidants, hydroperoxide decomposers, and processing stabilizers should be added in quantity required by the process, and service time under given exposure. The level of residual stabilizers should be determined prior to addition of the new ones. No antagonistic effects are expected. 

Effects of the additives specifically applied to stabilize polymers e.g., antioxidants, hydroperoxide decomposers, light and heat stabilizers) have been already discussed. Fillers, reinforcements, pigments, plasticizers and lubricants may affect the processability, but usually do not influence the stabilization strategy. Owing to different chemical characters some flame retardants and dyes may... 

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. 

According to the four possibilities of UV protection mechanism described above, the light stabilizer classes can be designated as (1) UV absorbers, (2) quenchers of excited states, (3) hydroperoxide decomposers, and (4) free radical scavengers. It must be mentioned, however, that this classification is a simplification and that some compounds may be active in more than one way and often do so. 

Primary phenohcs (free-radical scavengers) or secondary phosphite (hydroperoxide decomposer) may be used as heat stabilizers for WPC, while light stabilizers commonly used in WPC include UV absorbers (e.g., benzotriazole or benzophenone), radical scavengers or hydroperoxide decomposers, and hindered amine light stabilizers (HALS). 

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. 

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