Polymer stabilization secondary antioxidants

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. 

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. 

Organophosphites are secondary antioxidants which reduce hydroperoxides to alcohols. They inhibit the discolouration reaction experienced by phenolics. Tris-nonyl phenyl phosphite (TNPP) is the most commonly used. The disadvantage of phosphates is their high hygroscopicity. Thioesters act as secondary antioxidants by destroying hydroperoxides to form stable sulphur derivatives. In addition, thioesters impart high heat stability to polyolefins, polystyrene and its copolymers. The m or disadvantage of thioesters is their unpleasant odour which is transferred to the host polymer. 

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. 

Thioesters. Derived from aliphatic esters of B-thio dipropionic acid, thioesters act as secondary antioxidants and also provide high heat stability to a variety of polymers. Thioesters fimction as secondary antioxidants by destroying hydroperoxides to form stable hexavalent sulfur derivatives. Thioesters act as synergists when combined with phenolic antioxidants in polyolefins. The major disadvantage of thioester antioxidants is their inherent odor which is transferred to the host pol5uner. 

A new phosphite secondary antioxidant, based on butyl ethyl propane diol, reputedly yields high activity, solubility, and hydrolytic stability in a range of polymers. This would allow the producer to use lower levels of additives to achieve similar results. 

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. 

Free-flowing solid phosphite secondary antioxidant used in adhesives. Excellent hydrolytic stability in addition to excellent in polymer melt flow and color stability. Because of its high melting point, it can be utilized in applications that require processing at elevated temperatures. 

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. 

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. 

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. 

The best-known representative of the class of segmented polyether esters is the combination of polybutylene terephthalate as the hard component with polyether glycol as the soft component. The presence of polyether components causes a sensitivity to thermal-oxidative degradation in this class of materials. In non-stabilized form, these polymers cannot be processed. Their main oxidation product is formic acid. Moreover, re-formed monomers (terephthalic acid) of the polymer can collect on the surface and form a white, hard-to-remove deposit that changes the gloss level and color [512], Oxidative degradation reactions can be inhibited by primary and secondary antioxidants. Acidolysis caused by the formic acid can be controlled by adding acid acceptors that will bind either the precursor of formic acid, formaldehyde, or the acid itself. Acid amides, urethane, or urea are utilized as acid acceptors [86]. 

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

---