Investigating the Efficacy of Non-Ionic Surfactants as Compatibilizing Agents for Polyethylene-Polyamide Sintered Polymer Blends

Abstract

A novel strategy was considered in this work to improve the physical properties of rotationally molded parts formed using a thermodynamically immiscible physical blend of high-density polyethylene (HDPE) and polyamide 11 (PA11). Morphology of the melted system of the two polymers dry-mixed in the mold was dictated by the selected non-ionic surface-active agents. Through a preliminary evaluation of numerous non-ionic surfactants, Span 85 and Tween 20 showed the most promise at concentrations between 0.1 - 1.0 wt% for influencing how the two polymers spread over the mold surface in a uniaxial rotational molding unit; the two surfactants demonstrated favourable interparticle cohesion at 235 °C and were chosen on the basis of investigating the hydrophilic-lipophilic balance (HLB) as a predictive metric for polymer migration during rotomolding. For a comparison to a more traditional (and expensive) approach of preparing a molded part with blends of these two polymers, extrusion-mixed blends were first compounded using a twin-screw extruder, pelletized, ground, and finally rotomolded. The good compatibility of the two constituent polymers with Tween 20 was demonstrated by decreasing zero-shear viscosities with increasing surfactant concentration. In the molded samples, porosity for 50/50 dry-mixed blends increased at low concentrations of Tween 20, but ultimately the surfactant demonstrated its beneficial nature on sintering with a downward trend observed in porosity with increasing surfactant concentration; molded samples with 1.0 wt% Tween 20 showed a 1% improvement in porosity relative to uncoated blends, whereas 1.0 wt% Span 85 coated blends showed an undesirable increase in porosity instead. Similar trends were observed for 75/25 dry-mixed blends, except that the porosities for all Tween 20 concentrations were lower than those of uncoated blends. Low porosities were observed for all extrusion-mixed blends (with or without 1.0 wt% Tween 20) relative to the dry-mixed blends. As more favourable results for this new approach, Tween 20 coated dry-mixed blends showed an increase in impact strengths for both blend ratios, whereas extrusion-mixed blends showed a drastic decrease instead. Conversely, the inverse trend was found with flexural strengths. The results were reconciled through morphological analysis of the molded samples which demonstrated that a moderate degree of polymer migration occurred (i.e. aggregation of PA11 at the wall boundaries and HDPE near the centre) in the presence of the surfactant for 50/50 dry-mixed blends, whereas observations for the 75/25 blends were inconclusive. Therefore, based on the observations for density/porosity, impact strength, flexural strength, and morphology (i.e. polymer migration) modifying the blend-ratio, mixing strategy, and surfactant concentration for rotomolded HDPE-PA11 physical blends enables us to reliably predict and control the polyblends’ resultant properties and cater them to meet the requirements of a wide range of unique and specific use-case scenarios.