Reactive extrusion of cast polyamide for thermoforming Free

Cast polyamides, which are synthesized through anionical polymerization in a forming mold, have a high molecular weight compared to polyamides synthesized through polycondensation. However, high amounts of wastes (e.g. unusable sprues, post-processing wastes) occur with this production method. Because of this and due to the high molecular weight of the cast polyamide, compounding of the wastes and a further processing via methods like thermoforming is a potential approach for the recycling of polyamide wastes. However, when the polyamide wastes are recycled in a twin-screw extruder, the remaining catalyst, which is embedded within the wastes, can lead to a degradation of the polyamide chains. This degradation results in molecular weights respectively melt viscosities, which are suitable for processing via injection molding but not suitable for methods like thermoforming, for which high viscosities are necessary. In this work, cast polyamide wastes were compounded in a twin-screw extruder with additives to obtain maximum viscosities. We considered (mono- and di-)carboxylic acids, which act as catalyst catchers, as well as different additives, which increase the molecular weight, (linear chain extenders and multifunctional polymeric backbones). After the compounding, rheological measurements were conducted to examine the degree of the viscosity increase and thermal measurements to estimate a processing window for thermoforming. The rheological measurements showed that the increase is strongly dependent on the waste batches, which were used for the respective test series. However, the thermal properties were not influenced through the polymer modification. Overall, viscosities could be reached, which were high enough to produce semi-finished parts out of the compounds. Additionally, thermoformed polyamide parts with exact forming shape as well as constant wall thicknesses were realized.