Polymer stabilization primary antioxidants

Antioxidants function by preferentially reacting with the radical intermediates, thereby protecting the polymer and extending its usage life. There are two types of antioxidants that are typically used in commercial polymer stabilization primary and secondary. The majority of primary antioxidants are either hindered phenolics or secondary aryl-amines. Both hindered phenolics and aryl-amines have one or more reactive OH and NH groups. The hydrogen atoms which are liberated from the alcohol or amine groups readily react with free radicals to form stable species. Secondary antioxidants are usually phosphites or thioesters. 

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. 

Heat stabilizers are used to prevent degradation of polymers when they are exposed to heat, especially during processing. For most polymers, the primary mode of reaction is oxidation, so antioxidants function effectively as heat stabilizers. For a few sensitive polymers, however, other forms of reaction are more important. In particular, a major disadvantage of PVC is its poor thermal stability. Degradation takes the form primarily of dehydrochlorination, yielding HCl and resulting in formation of a double bond in the main polymer chain. 

CAS 6683-19-8 EINECS/ELINCS 229-722-6 Uses Primary antioxidant, stabilizer for plastics, styrenics, polyolefins, PVC, urethanes, acrylics, adhesives, elastomers, coatings, food-grade polymers food pkg. adhesives... 

Uses Primary antioxidant for syn. rug-backing, latex paints, rosin, ester gums, in gasoline and aviation fuels, insulating oiis, paraffin wax, polymer stabilization Manuf/Dlstrib. Rhodia Trade Names Uvi-Nox 1494... 

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. 

Secondary antioxidants are used in conjunction with primary antioxidants to provide added stability to the polymer. Typical compounds contain sulfur or phosphorus. The more popular secondary antioxidants are thioesters (thiodipropionic acid derivatives and polythiodipropionates) and organophosphites. 

The base additive packages required for the stabilization of polyolefins usually comprise combinations of phenolic antioxidants (primary antioxidants, radical scavengers), phosphites or phosphonites (secondary antioxidants) and acid scavengers. The co-operative performance of such an additive combination is certainly influenced by the proper choice and concentration of all individual components. Even acid scavengers can play an important role in melt viscosity retention during processing as well as in the long term stability of the final polymer article. 

They tend to react preferentially, preventing the regeneration of new free radicals from the decomposition of the hydroperoxides. Primary and secondary antioxidants are often used together because they exhibit a synergism which provides an effective mechanism for polymer stabilization. 

Physical effects which influence the performance of the small molecular materials in polymers are not well known yet. Experimental results proved that the protection time of primary antioxidants, UV stabilizers, etc., is related to the ratio S /D where S denotes the solubility, and D the rate of diffusion. The additives with low solubility and high diffusion rate migrate rapidly towards the surface, so the polymer loses the additive due to volatilization and leaching of additive from the surface. Therefore some applications are... 

Antioxidants and stabilizers are added to most polymer systems to improve durability. Antioxidants fall into two major categories, primary and secondary, (Petrie 2004) and within primary antioxidants there are two main chemical types, hindered phenolics and aromatic amines. Primary antioxidants function by donating their reactive hydrogen to the peroxy free radicals so that the propagation of subsequent free radicals does not occur. 

The most widely used antioxidants for polymer protection are phenolics. These products generally resist staining or discoloration. However, they may form quinoid structures upon oxidation, which lead to yellowing. Phenolic antioxidants include simple phenolics, bisphenolics, polyphenolics, and thiobisphenolics. Hindered phenolics, such as butylated hydroxy toluene (BHT), high molecular weight phenolics, and thiobisphenolics, are the most popular of the primary antioxidants. They are suitable as long-term stabilizers in almost all cases. 

Formulation-designed hot melt adhesives use a number of materials as the base polymer for the formulation. The most widely used material for this type of adhesive is polyethylene-co-vinyl acetate (EVA). Other materials used are polyethylene (low density), block copolymers (styrene-iso-prene-styrene), phenoxy resins, polypropylene, and paraffin waxes. Paraffin, the oldest hot melt adhesive, was used as a sealing wax. As with PSAs, tackifiers are also added to hot melt adhesives to lower their melt viscosity as well as to provide tack during the resolidification of the adhesive. If one examines some hot melt adhesive formulations, one finds that the tackifier is a primary component of the adhesive sometimes used to the same level or possibly in excess of the hase polymer. Another component is the addition of a wax. The wax is added not only to decrease the melt viscosity of the adhesive but also to aid in the recrystallization process. As one might be able to recount from statements made above about heat stability, an antioxidant is an important additive to hot melt adhesive formulations. Final additives indude colorants and fillers. 

A large number of hindered phenoHc antioxidants are based on the Michael addition of 2,6-di-/ f2 -butylphenol and methyl acrylate under basic catalysis to yield the hydrocinnamate which is a basic building block used in the production of octadecyl 3-(3,5-di-/ f2 butyl-4-hydroxyphenyl)propionate, [2082-79-3], tetrakis(methylene-3(3,5-di-/ f2 butyl-4-hydroxylphenyl)propionate)methane [6683-19-8], and many others (63,64). These hindered phenolic antioxidants are the most widely used primary stabilizers in the world and are used in polyolefins, synthetic and natural mbber, styrenics, vinyl polymers, and engineering resins. 2,6-Di-/ f2 -butylphenol is converted to a methylene isocyanate which is trimerized to a triazine derivative... 

It is an important fact, that there is an explicit synergistic effect observed by combining primary and secondary antioxidants. A well known example of such synergistic mixtures is the use of thiodipropionates together with sterically hindered phenols for long-term stabilization of polyolefins. Of course, the different polymers and different applications require laborious optimization of each case. 

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