In order to prevent the degradation of polymers during processing or use, polymer stabilizers are employed in accordance with the degradation mechanism of the relevant polymer. Due to the large differences in degradation mechanisms of polymers containing chlorine(stabilizers for polyvinyl chlorides), and those without chlorine, stabilizers that are suitable for each type of polymer should be chosen.
In general, the same polymer stabilizers are used in most plastics, such as polypropylene, polyethylene, acrylonitrile butadiene styrene (ABS), and engineering plastics. The key to preventing the oxidative degradation of plastics is to terminate the radical reaction in the early stage of degradation, thereby terminating autoxidation. To achieve this, at least one, and preferably several, of the factors associated with the degradation scheme need to be eliminated.
Many stabilizers prevent one of the elementary degradation reactions of plastics, thereby contributing to their stabilization. Stabilizers are categorized into a radical chain initiation inhibitor, a radical scavenger, and a peroxide decomposer, depending on the autoxidation stage at which the radical chain is inhibited.
On the other hand, for this reason, the PVC stabilizers are designed to scavenge the chlorine atoms and hydrogen chloride formed in the degradation process.
Designing Polymer Stabilizer Formulations
Since the degradation reaction is complex, eliminating only one of the factors responsible for degradation is not sufficient to stabilize the polymer. Stated alternatively, simultaneously eliminating multiple degradation factors produces a large stabilizing effect.
For example, in the case of polypropylene products for outdoor use, a phenolic antioxidant (radical scavenger) and phosphorus antioxidant (peroxide decomposer) are blended in to act as thermal stabilizers during processes such as molding, and an ultraviolet absorber and HALS are also blended in to provide improved weatherability during use.
As described above, for plastics, several types of stabilizers are often used in combination. The effects vary greatly depending on the combination of stabilizers. For this reason, the technique of blending appropriate stabilizers is crucial to practical plastics applications.