Radical Scavengers and Hydroperoxide Decomposers

Sterically hindered amine light stabilizers (HALS) constitute another class of light stabilizers that efficiently reduce UV-induced degradation. They are compounds that intercept chemically highly reactive radicals formed hy photochemical reactions and transform them into harmless reaction products. Fig. 3.8. Their effectiveness is not based on a competing physical absorption effect, therefore no minimum layer thickness is required for HALS effectiveness, unlike for the UV absorbers. That makes them suitable for applications such as films and fibers (527). In particular, they provide surface protection, which plays a special role in visible automotive parts. Another essential difference to UV absorbers is the fact that photooxidation is not prevented, but after initial damage its propagation is either reduced or suppressed [510], 

A key role in HALS stabilization is played by nitroxyl radicals that can be recreated [65]. In recent years, a number of possible mechanisms have been suggested to explain the working principle of HALS s UV stabilizing effect [38], [186], [540]  

Heat stabilization, process stability, discoloration, chemical resistance 

In reality, the circumstances are probably best explained by the interaction of several mechanisms it is generaiiy assumed that the interception of free radicals is the most important step [544]. Based on this mechanism, HALS are used successfully as thermal-oxidative stabiiizers as well [527]. 

HALS often exhibit synergetic effects in combination with UV absorbers, because more than one harmful degradation reaction path is blocked [544]. 

---

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). 

HALS hardly absorb UV light but act most likely as radical scavengers and hydroperoxide decomposers. Chemical structures are mainly based on piperidines. Sterically hindered piperazines are known as well (Fig. 11.15). Secondary amines are the most common structures, but alkyl-amines or, more recently, alkoxyamines are commercially... 

As PE films, like all synthetic polymers, suffer from decomposition by environmental influences such as light and atmospheric oxygen, special photoprotective systems are added to delay these effects. Systems that are commonly used are ultraviolet (UV) light absorbers, quenchers, radical scavengers, and hydroperoxide decomposing agents. 

A mixture of antioxidants that function by different mechanisms might be synergistic and provide a higher degree of protection than the sum of the stabilizing activities of each component. The most fi equently used S5uiergistic mixtures are combinations of radical scavengers and hydroperoxide decomposers. 

An overview of stabilizer mechanisms and t5q>es of chemicals used for each mechanism is provided in Table 3. Further descriptions of these mechanisms follow. The three major classes of UV absorbers are 2-hydroxybenzophenones, 2-hydroxyphenylbenzotriazoles, and the newer 2-hydroxyphenyl-s-triazines. The nickel phenolates not only have excited state quenching capability, but also provide radical scavenging and hydroperoxide decomposing activity. The radical scavengers include hindered benzoates and hindered amines. The hindered amines have the added capability to decompose hydroperoxides. Another class of... 

Cytec s family of light stabilizers includes Cyasorb UV absorbers, which prevent photo-degradation of polymers by competitive UV light absorption, energy quenchers, radical scavengers, and hydroperoxide decomposers. 

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). 

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. 

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). 

In addition to the above light stabilizers, improved light stability is obtained also with antioxidants, which are efficient free radical scavengers, and with phosphites which decompose hydroperoxides. 

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... 

In addition to UV absorbers and HALS, other types of UV stabilizers which inhibit degradation processes subsequent to photoinitiation are used. These include excited state quenchers, hydroperoxide decomposers, radical scavengers, and acid acceptors. 

Hydroxylamines are multifunctional. They can act both as C-radical scavengers and as hydroperoxide decomposers. They reach full effect even in low concentrations without negative effects on color. That is why hydroxylamines are mainly used as processing stabilizers for color-critical applications. Generally, hydroxylamines are combined with sterically hindered amines [538]. 

Interception of free radicals by radical scavengers and/or hydroperoxide decomposers, such as sterically hindered amines and hydroxylamines... 

Within the last decade, there have been new approaches to polymer stabilization. One new stabilizer chemistry is based on the hydroxylamine functionality that can serve as a very powerful hydrogen atom donor and free radical scavenger as shown in Figure 15(a) and hydroperoxide decomposer as shown in Figure 15(b). 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

The synergistic effect of a hydroperoxide decomposer, eg, dilauryl thiodipropionate [123-28-4] (34), and a radical scavenger, eg, tetrakis[methylene(3,5-di-/ f2 butyl-4-hydroxyhydrocinnamate)]methane (9), ia protecting polypropylene duting an oxygen-uptake test at 140°C is shown ia Table 3. 

An antioxidant ties up the peroxy radicals so that they are incapable of propagating the reaction chain or to decompose the hydroperoxides in such a manner that carbonyl groups and additional free radicals are not formed. The former, which are called chain-breaking antioxidants, free-radical scavengers, or inhibitors. are usually hindered phenols or amines. The latter, called peroxide decomposers, are generally sulfur compounds or... 

Molybdenum dialkyldithiocarbamates Molybdenum dialkyldithiocarbamates are multifunctional lubricant additives as anti-wear, anti-friction and antioxidants. Molybdenum dialkyldithiocarbamates are also multifunctional antioxidants due to the hydroperoxide decomposing ability of dialkyldithiocarbamates, see Section 4.4.2, and the radical scavenging capacity of molybdenum. The best established structure for molybdenum dialkyldithiocarbamates is a six-coordinate complex of a dinuclear molybdenum centre with each molybdenum bonded to terminal oxygen or sulphur atoms, two bridging oxygen or sulphur atoms and one dialkyldithiocar-bamate ligand. Fig. 4.6 [44] ... 

Together with dinuclear molybdenum dialkyldithiocarbamates, trinuclear dialkyldithiocarbamates have recently been introduced for lubricant applications, specifically for use in engine oils [48]. These molecules are based upon [Mo3S4]" and [MosSy] cores complexed by four dialkyldithiocarbamate ligands. It has been reported that these products decompose hydroperoxides more effectively than do dinuclear molybdenum dialkyldithiocarbamates. The patent literature discloses trinuclear molybdenum dialkyldithiocarbamates that are most effective as antioxidants when combined with radical scavengers such as organocopper compounds and diphenylamines [49]. 

---