Meaning
High-temperature chemical cleavage of alkyl substituent groups represents a common degradation pathway for plastic additives. The process of thermal dealkylation occurs when hindered phenol antioxidants or phosphite stabilizers are subjected to extreme temperatures during melt processing. This reaction removes the protective alkyl groups, such as tert-butyl groups, that are responsible for the stabilizer’s solubility and performance in the polymer matrix.
It leads to a reduction in the additive’s efficiency and can cause discoloration.
Chemical Reaction
Thermally driven elimination reactions proceed at elevated melt temperatures, typically above two hundred and fifty degrees Celsius. During thermal dealkylation, the carbon-carbon bonds holding the bulky alkyl substituents to the aromatic ring of the antioxidant are broken. This cleavage releases volatile alkenes, such as isobutylene, which can generate gas bubbles and void defects in molded parts.
The loss of these protective groups leaves the aromatic core of the stabilizer more vulnerable to oxidation.
Stabilizer Degradation
Consuming the antioxidant package reduces the long-term thermal stability of the plastic. When thermal dealkylation occurs, the stabilizer loses its ability to scavenge free radicals effectively. This premature depletion of the stabilizer package accelerates the degradation of the polymer chains themselves, leading to a loss of mechanical properties.
Molders can detect this degradation by monitoring changes in the oxidation induction time of the processed resin.
Process Consequence
Processing parameters must be optimized to prevent this degradation mechanism from occurring. Molders can reduce the risk of thermal dealkylation by lowering the melt temperature and minimizing the residence time of the resin inside the extruder barrel. This action is particularly important when processing recycled materials or regrind, which have already undergone at least one heat cycle and may have depleted stabilizer levels.
Preserving the integrity of the additive package is essential for maintaining consistent part quality and preventing cosmetic defects across production runs. It helps ensure that the polymer can withstand subsequent manufacturing steps.
