Antioxidant hydroperoxide decomposer

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). 

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... 

Antioxidants, hydroperoxide decomposers, and processing stabilizers should be added in quantity required by the process and service time under given exposure. 

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. 

This hydroperoxide decomposes slowly, avoiding accumulation. However, if the conditions are ideal for peroxidation (heat, prolonged time exposure to air, solar light), the hydroperoxide converts into extremely dangerous peroxides. Phenolic antioxidants inhibit this peroxidation efficiently. If tetrahydrofuran is peroxidised, it is not possible to destroy peroxides with ferrous salts or sulphites since tetrahydrofuran dissolves in water. Alumina or active carbon (passing over an alumina column or activated carbon at 20-66 C with a contact period of two minutes) are used, or by stirring in the presence of cuprous chloride. 

Synergism based on the mixture of a chain-breaking antioxidant (sterically hindered phenol) and a hydroperoxide decomposer (organic sulfides or phosphites). 

Synergism of Chain Termination and Hydroperoxide Decomposing the Antioxidants... 

Hydroperoxide decomposing antioxidants. These are compounds that react with hydroperoxides without forming free radicals sulfides, phosphites, arsenites, thiophosphates, carbamates, and some metal complexes. Reactions with hydroperoxides can be either stoichiometric (typical of, for example, sulfides and phosphites) or catalytic (typical of chelate metal complexes). 

SYNERGISM OF CHAIN TERMINATION AND HYDROPEROXIDE DECOMPOSING THE ANTIOXIDANTS... 

In contrast to the DSC method, the colorimetric method determines the hydroperoxide decomposing capabilities of the antioxidant species in the oil sample (Enmanji, 1979). 

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)... 

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. 

Sn2 displacement on the peroxidic bond 0-0 accounts for the reduction of alkyl-hydroperoxides and oxidation of the hydroperoxide-decomposing antioxidant (HD, Eq. 3-17) in products, generally HD(O) or HD(0)2. 

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],... 

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