Characteristics of Exfoliated Graphite Nanoplatelets-Polyamide12 Nanocomposites Processed by Extrusion Injection Molding

Abstract

Melt-mixing processes such as extrusion-injection molding induce shear mechanical forces to enhance the melt blend of nanofiller-polymer and have been considered as time and cost efficient in commercial processing of polymer nanocomposites (PNCs). Extensive research has been conducted so far to investigate the overall performance of melt-processed PNCs. However there is lack of systematic studies on the nanomaterial induced phenomena that dominate the properties of the end-use melt mixing processed parts leading to engineered high quality PNCs. Furthermore, studies for exploration of the structure-property relationships in the melt processed PNCs are also limited. In this work, nanocomposites of Polyamide-12 (PA12) reinforced with exfoliated graphite nanoplatelets (xGnP™) up to 15wt% were fabricated using two different compounding techniques: coating followed by melt mixing and direct melt mixing. Effects of the compounding methods on the reinforcing efficiency of xGnP within the PA12 matrix were investigated. To understand the reinforcing mechanisms contributing to alteration of the mechanical properties, rheological behavior of the PNCs were investigated in the linear viscoelastic region and correlated with the state of nanofiller-matrix interfacial interaction and the nanofiller dispersion. The results suggest that xGnP coating of the PA12 powder resulted in improved flexural strength of the PNCs with respect to that of the pure PA12 and of PNCs made by direct melt mixing. The rheological low-frequency measurements demonstrate that addition of xGnP resulted in a larger increase in the melt dynamic viscosity of the coated-melt mixed PNCs than the direct melt-mixed ones. This suggest stronger xGnP-PA12 interfacial interaction and better dispersion state of the nanofiller in the coated-melt mixed PNCs than the latter PNCs. Addition of xGnP content resulted in improved storage modulus for PNCs made by either compounding method. However, the results support the evidence of post-processing induced re-agglomeration or suppressed dispersing efficiency of the coating process at extreme xGnP content as was elucidated through the shear thinning behavior of the melt and the flexural modulus of the highly reinforced parts.