Polymers will change over time when exposed to radiation, excessive heat and/or corrosive enviroments. These changes are the result of oxidative degradation caused by free radicals which form through hydrogen abstration or homolytic scission of carbon-carbon bonds when polymers are exposed to heat, oxygen, ozone, or light. These changes can have a dramatic effect on the service life and properties of the polymer.
To prevent or slow down degradation, antioxidants and UV stabilizers are often added. The two main classes of antioxidants are free-radical scavengers and peroxide scavengers. The free-radical scavengers are sometimes called primary antioxidants or radical chain terminators whereas peroxide scavengers are often called secondary antioxidants or hydroperoxide decomposers.
As the name suggests, free-radical scavengers react with chain-propagating radicals such as peroxy, alkoxy, and hydroxy radicals in a chain terminating reaction. To be more specific, these antioxidants donate hydrogen to the alkoxy and hydroxy radicals which converts them into inert alcohols and water respectively.
Typical commercial primary antioxidants are hindered phenols and secondary aromatic amines. These compounds come in a wide range of molecular weights, structures, and functionalities.
The most widely used primary antioxidants are sterically hindered phenols. They are very effective radical scanvengers during both processing and long-term thermal aging, and are generally non-discoloring. Many also have received FDA approval. The mechanism of scavenging oxy radicals is shown below:
The most effective primary antioxidants are secondary aromatic amines. However, they cause noticeable discoloation and can only be used if discolation is not a problem, like carbon filled rubber products. They also function as antiozonants and metal ion deactivators.
As the name suggests, peroxide scavengers (secondary antioxidants) decompose hydroperoxides (ROOH) into nonreactive products before they decompose into alkoxy and hydroxy radicals. They are often used in combination with free radical scavengers (primary antioxidants) to achieve a synergistic inhibition effect.
The most common secondary antioxidants are trivalent phosphorus compounds (phosphites). They reduce hydroperoxides to the corresponding alcohols and are themselves transformed into phospates. The general mechanism of peroxide decomposition is shown below.
Another class of secondary antioxidants are thioethers or organic sulfides. They decompose two molecules of hydroperoxide into the corresponding alcohols and are transformed to sulfoxides and sulfones:
Organic sulfides are very effective hydrogen peroxide decomposers
during long-term thermal aging and are often used in combination with other antioxidants that provide good protection during processing, like hindered phenols.
In order to choose the most effective stabilizer package, it is important to know what temperature range the polymer will be exposed to. A good stabilizer package should protect the plastic during both processing, where high temperatures are encountered to melt and form the resin, and during lifetime when exposed to its upper service temperature.