Meaning
Antioxidant compounds used in thermoplastic compounding protect the polymer from thermal degradation during processing and subsequent service life. In this context, bht is a synthetic phenolic antioxidant that scavenges free radicals generated by shear and heat during injection moulding or extrusion. It acts by donating a hydrogen atom to alkyl-radical species, halting the chain reaction that leads to molecular weight reduction.
This protection is limited to temperatures below the sublimation point of the additive, as it tends to volatilise during high-temperature melt processing.
Additive Migration
Volatile loss and surface transfer define the movement of low molecular weight stabilizers in finished polymer products. Because of its relatively low molecular weight, bht frequently migrates to the surface of polyolefins, creating a potential risk of yellowing when it reacts with nitrogen oxides in the surrounding air. This phenomenon, known as gas fading, affects the appearance of light-coloured moulded parts stored in warehouse environments.
Packaging producers monitor this surface accumulation to prevent contamination of sensitive products, particularly in medical and food applications where strict purity limits apply.
Regulatory Limit
Food contact notifications establish the permissible quantities of compounding additives that may migrate from a container into a packaged substance. Standardized extraction tests verify that bht levels remain below the migration thresholds defined by global food safety agencies. These testing procedures employ chemical simulants to measure the additive concentration that transfers under conditions representing prolonged storage or elevated temperature.
High pressure liquid chromatography determines the extracted amount, ensuring that the compound remains within compliant boundaries for food packaging.
Compounding Practice
Stabilizer selection involves balancing cost with thermal stability during compounding. Processors combine bht with higher molecular weight stabilizers to provide synergistic protection. This approach reduces formulation costs.
However, if melt temperatures exceed the thermal limit, degradation occurs.
