Stabilizers preventive antioxidants

Chemical and thermal stabilizers both inactivate the byproducts of degradation processes, preventing them from causing further damage to the polymer. Their chemical structure and mobility in the part define their effectiveness in any given polymeric system. The most common type of chemical stabilizers are antioxidants. 

Shelton J R (1981) Organic sulfur compounds as preventive antioxidants. In Developments in polymer stabilization-4, Scott G (Ed), Applied Science Publishers London, pp 23-70. 

Ideally, a solution for injection should be an isotonic solution with a neutral (physiological) pH. However, the pH of a radiopharmaceutical is very important for its stability, and for labelled compounds, the pH for optimal stability is not always equivalent to physiological pH. For iodide solutions, the pH should be alkaline to prevent loss of radioiodine. Reducing agents, such as thiosulfate, are often added to radioiodide solutions to help this situation. A preservative can act as a stabilizer, an antioxidant, or a bactericidal agent. 

Preventive antioxidants reduce the rate of the chain initiation. The most important initiation suppressors are metal deactivators that chelate metal ions. Metal deactivators used for stabilizing edible fat and lipid-containing foods include citric, phosphoric, tartaric acids, and phospholipids. Peroxide destroyers also are preventive... 

Plastics that are commonly processed by extrusion include acrylics (polymethacrylates, polyacrylates) and copolymers of acrylonitrile cellulosics (cellulose acetate, propionate, and acetate butyrate) polyethylene (low and high density) polypropylene polystyrene vinyl plastics polycarbonates and nylons. The material properties and extrusion properties have been reviewed by Whelan and Dunning.Additives that may be included to modify or enhance proper-ties include lubricants and antislip agents to assist processing during extrusion plasticizers to achieve softness and flexibility stabilizers and antioxidants to retard or prevent degradation and dyes and pigments. 

Linear-Low Density Polyethylene (LLDPE) and Ethylene -Propylene Diene (EPDM) Stabilization. Prevention of discoloration of LLDPE and EPDM are prime objectives in addition to thermal stabilization. While thermal stabilization is readily achieved by the use of phenolic antioxidants, prevention of discoloration requires a co-additive such as a phosphite. 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Since stabilization against photooxidation and metal-catalyzed oxidation are covered elsewhere in this symposium, this discussion is restricted to protection against thermal autoxidation. I will first review the mechanism by which typical chain-breaking antioxidants function and then describe some of our current studies on the way in which certain organic sulfur compounds act as preventive antioxidants. 

From the results obtained it can be concluded that the phenol (Vila) is regenerated by the H-donating action of the preventive antioxidant. A prerequisite for this regeneration seems to be the stability of the phenoxyl radical (XVa) or its homologs. 

Uses stabilizer and antioxidant in acrylic monomers to prevent polymerization A... 

UV stabilizers are used to prevent or terminate the oxidation of plastics by UV light. They therefore act to protect the moulded product during its life, and are particularly used for building products. They act by absorption of energy, deactivating the by-products of oxidation, and decomposition of by-products (or a combination of these). The action is similar to heat stabilizers and antioxidants, and some types also offer these functions. 

Antioxidants are most effective in stabilizing oxidation-prone drug formulations. They have the ability to inhibit or slow down chain reaction oxidative processes at relatively low concentrations. This property of the antioxidant substances is of considerable importance with respect to formulations because of the large number of chemically diverse medicinal agents known to undergo oxidative decomposition. Antioxidants are classified as preventive antioxidants or chain-breaking antioxidants based upon their mechanism. Preventive antioxidants include materials that act to... 

After several minutes, the unstable end-groups were removed and a stable polymer resulted. More recently (1981), this technique has been refined by researchers at Mitsubishi Gas Chemical Co. who performed this controlled thermal unzippering in a vented extruder[10]. By either approach, this must be carried out in the presence of thermal stabilizers, especially antioxidants, to prevent random oxidative attack of the polymer chain. 

Hindered phenols, in fact, act as chain autoxidation inhibitors thiodipropionic acid esters, on the other hand, act as preventive antioxidants. This difference in the mechanism of the stabilizers permits them to act independently and synergistically, providing... 

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