Meaning
Condensation of an aldehyde or ketone with an alcohol under acid catalysis generates a diether known as an acetal. This organic process of acetal formation occurs during the thermal degradation of polyoxymethylene resins when excessive residence time in the injection moulding barrel releases formaldehyde which then reacts with hydroxyl groups on other polymer chains or additives. The secondary reaction modifies the molecular weight distribution of the melt.
Reaction Pathway
Acid-catalyzed addition of alcohol molecules to carbonyl groups proceeds through a hemiacetal intermediate before completing the conversion. During thermoplastic processing, acetal formation represents a degradation route where generated formaldehyde undergoes secondary reactions rather than escaping as a gas. The presence of trace moisture or acidic residues on the metal surfaces of the screw and barrel accelerates this chemical transformation.
Elevated temperatures during the compounding stage provide the necessary activation energy for these reactions.
Processing Outcome
Generation of low molecular weight volatile species during the melt phase alters the viscosity of the polymer. When acetal formation proceeds unchecked, it reduces the melt strength of the resin and leads to structural instability in the molded part. Moulded components suffer from reduced tensile strength and impact resistance.
The cost of regrind recovery rises because the degraded material cannot be blended with virgin pellets without compromising the mechanical performance of the final product. Moulders face increased rejection rates during quality control because the degraded polymer produces surface blemishes that fail visual inspections. This increases scrap rates by up to twelve percent, which directly impacts the raw material budget of the production run.
Moulding Prevention
Thermal stabilizers added to the resin hinder the initial depolymerization of the polymer chain. Regular purging of the moulding cylinder prevents the accumulation of degraded material. Properly dried feedstocks minimize the hydrolysis reactions that initiate the breakdown of the polymer matrix.
