Why should antioxidants be used?
Virtually all polymeric materials undergo oxidation reactions. Oxidation can occur at every stage of the life cycle of a polymer, i.e. during manufacturing and storage of the material or during processing and end-use. Typical manifestations of oxidation of polymers can be change of viscosity during processing, appearance and loss of mechanical properties such as elongation, impact strength, tensile strength and flexibility. Antioxidants protect polymers against oxidation by controlling molecular weight changes that lead to a loss of physical, mechanical and optical properties.
2.1 Mechanisms
> Oxidation
Organic materials react with molecular oxygen in a process called "autoxidation". Autoxidation is initiated by heat, light (high energy radiation), mechanical stress, catalyst residues, or reaction with impurities to form alkyl radicals. The free radical can, in turn, react and result in the degradation of the polymer. This is depicted below:
> lnhibited Polyolefin Oxidation
Antioxidants interrupt the degradation processes in different ways, depending on their structure. The two major classifications are: - chain terminating primary antioxidants and - hydroperoxide decomposing secondary antioxidants.
> Primary Antioxidants
| Primary antioxidants react rapidly with peroxy and alkoxy radicals and are, therefore, called "Radical Scavengers". The majority of primary antioxidants for polymers are sterically hindered phenols. |
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| Secondary aromatic amines are primary antioxidants mainly used in carbon-black filled rubbers and some polyurethane applications. The high discoloration of the polymer and the lack of indirect food contact approvals prohibits the use in larger applications such packaging materials. |
> Secondary Antioxidants
| Secondary antioxidants react with hydroperoxides to yield non-radical products and are, therefore, frequently called hydroperoxide decomposers. Secondary antioxidants are particularly useful in synergistic combinations with primary antioxidants. Blends of stabilizers with different mechanisms are state-of the-art today. |
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Phosphites are most effective during processing and protect both the polymer and the primary antioxidant. Hydrolytically stable phosphites are the most frequently used processing stabilizers in high-performance additive systems.
Thioethers are useful only in increasing the long-term thermal stability in conjunction with phenolic antioxidants. The use of thioethers is limited to areas where their possible effect on odor or taste and their negative interaction with HALS is not important. |
> Alkyl radical scavengers
A third class of antioxidants are the alkyl radical scavengers.
Scavenging the alkyl radicals immediately stops the oxidation. However, the extremely high reaction rate of oxygen with alkyl radicals limits the use of alkyl radical scavenger to oxygen deficient conditions, such as extrusion.
> Multifunctional stabilizers
In multifunctional stabilizers,
several functions are combined in the same molecule: the most widely used combination are sterically hindered phenols with sulfur substituents or sterically hindered phenols with copper chelating function (metal deactivators).
Polymer molecules undergo different reactions during degradation, leading to structural changes. Bond scission in the polymer background and crosslinking reactions will result in changes in average molecular weight and molecular weight distribution of the polymer chain. The chemical pathway of degradation and the manifestation of the aging are unique for a specific polymer and defined conditions. Test methods and failure criteria are therefore varying for different polymers and applications. Typically, a polymer property related to the structural changes of the polymer molecules is monitored during a thermal or thermomechanical aging.
Selection guides
Find the suitable antioxidants for your polymer and application