Volatile Extraction and Titration Frameworks for Recycled Polyolefin Pellets
Coupling thermal desorption solvent extraction with non aqueous potentiometric titration quantifies volatile contaminants and acid values in recycled polyolefins.
Carboxylic Acid Groups
Meaning
Reactive chemical endgroups consisting of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group terminate the chains of linear polyesters and polyamides. The concentration of these carboxylic acid groups influences both the polymerization rate during manufacturing and the hydrolytic stability of the resin during melt processing.
Polymer Chemistry
Condensation polymerization relies on a delicate balance between amine or hydroxyl groups and carboxylic acid groups to achieve high molecular weight polymers. If the ratio of these groups drifts during synthesis, the resulting resin exhibits a lower viscosity and reduced mechanical properties. In polyamides, carboxylic acid groups act as reactive sites that can bind with moisture or colorants, altering the performance of the molded part.
Degradation Pathway
Elevated temperatures and residual moisture in the extruder barrel catalyze the reaction between ester bonds and carboxylic acid groups, leading to chain scission. This hydrolysis reaction increases the number of acid endgroups, which in turn accelerates further degradation in a self-catalyzing loop. As the molecular weight plummets, the melt flow rate rises uncontrollably, causing part defects such as silver streaks and structural weakness.
In injection molding, this degraded melt cannot support the high pressure required to pack the mold cavity, resulting in dimensional instability and sink marks.
Processing Impact
Minimizing the concentration of carboxylic acid groups in the melt is accomplished by thorough desiccant drying before the polymer enters the hopper. Dry resins preserve their mechanical properties, whereas wet resins degrade rapidly, generating excessive scrap during start-up.