The Effect of Graphite Nanoplatelets on the Barrier and Mechanical Properties of LLDPE Films

Polymer nanocomposites for packaging applications have gained significant research interest over the previous decade. More recently, graphite nanoplatelets (GNP) have attracted interest as potential multipurpose fillers in polymer matrices because of their superb mechanical, thermal, and gas barrier properties that could potentially be transferred to a polymer composite at relatively low loadings. The purpose of this research was to determine the effect that GNP had on the mechanical and barrier properties of 3.5 ± 0.4 mil Dow® Elite™ 5230G Enhanced Polyethylene (EPE) films that were produced by letting down a 20% GNP filled Dow® Elite™ 5230G masterbatch into a base of 5230G resin via a 24:1 single screw cast film extruder. Films were extruded into batches consisting of 0.5%, 1.0%, 2.0%, 3.0%, and 5.0 % by weight of graphite nanoplatelet content as well as a 0.0% control film batch. Optical microscopy revealed poor distribution as well as regions of agglomeration due to a lack of shear provided by single screw extrusion and poor distribution of GNP in the masterbatch. X-ray diffraction results revealed that the films displayed a phase separated morphology, without intercalation or exfoliation of GNP throughout the films. TEM imagery revealed that agglomerations existed in the film and masterbatch samples, but small isolated regions of exfoliated graphite were also present. DSC analysis revealed that the presence of GNP in the matrix acted as a nucleation aid for the LLDPE structure, where cold crystallization occurred 8.0°C higher in the 0.5% film sample and at a maximum of 11.0° C higher in the 5.0% sample. The LDPE crystallinity remained unchanged. OTR results indicated a reduction in oxygen transmission with increasing GNP content. The maximum reduction of OTR was found in the 5.0 % sample, where the OTR was reduced from 166.4019 ± 2.434 CC / [100in² - day] to 135.2405 ± 7.38 CC / [100in² - day]. WVTR results displayed no significant changes with added GNP content. Tensile analysis revealed a reduction in the plasticity of the films with added GNP content. Increases in 1.0% Secant moduli were observed in all samples containing GNP content, with a maximum increase of 87.5% in the 2.0% GNP sample in the MD and 94.3% in the TD. Insignificant changes in tensile strength were observed in both the MD and TD. Puncture resistance testing displayed similar reductions in compressive extension prior to puncture in samples containing GNP content. The reductions in elongations during puncture and tensile testing may be due to the GNP reducing polymer mobility, while the reduction in tensile strength and puncture resistance is likely due to an inhomogeneous distribution of GNP agglomerates within the matrix, leading to premature cracking and subsequent failure. These agglomerates are likely present due to inhomogeneous distribution of GNP in the masterbatch and lack of shear applied by single screw extrusion