Graphene–polyamide-6 composite for additive manufacture of electromagnetic interference shielding components
Creators: name: LEE, Kok Peng Marcian
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401499
subject: Manufacturing engineering not elsewhere classified
classificationCode: 401605
subject: Functional materials
classificationCode: 401609
subject: Polymers and plastics

description: There is tremendous research interest in graphene because it is the strongest material ever measured and it possesses useful properties such as excellent electrical and thermal conductivities. Graphene has been used to imbue various polymer matrices with enhancements in mechanical properties, thermal stability and electromagnetic shielding effectiveness (EMI SE). Advances in manufacturing processes are complementary areas of development towards meliorated materials. Additive manufacturing processes, including material extrusion (ME), can allow manufacturers to surpass the design limitations faced by traditional manufacturing methods. This research aims to develop and characterise a novel graphene-polyamide-6 composite filament for ME applications. The investigation is motivated by the exceptional properties of graphene and its potential application in additive manufacturing of multifunctional components. The research objectives represent progressive steps from material formulation to application.<br><br>

Low-defect graphene (LG), carboxyl-functionalised graphene (FG), and graphene nanoplatelets (GNP) were considered as candidate filler for polyamide-6 (PA6). Composites with graphene concentrations ranging from 0.1 to 15.0 %·w/w were prepared and characterised in terms of their mechanical properties and EMI shielding effectiveness (SE). At equivalent graphene concentrations, LG gives the greatest elastic modulus reinforcement while FG gives the highest tensile strength at yield enhancement. While improvements to elastic modulus were observed in GNP-PA6 composites, high GNP concentrations resulted in a tensile strength at yield that is lower than that of neat PA6. Analysis of the mechanical properties with the Halpin-Tsai micromechanical model indicates that synergistic reinforcement was achieved in all composites when graphene concentration was low. Analysis of the tensile stress-strain curve showed that FG composites were the toughest, followed by LG then GNP composites. However, the reverse was true for EMI SE, where GNP composites showed the highest EMI SE, followed by LG then FG. The observations were attributed to the difference in purity of the graphenes. GNP was selected as the most suitable filler for filament preparation.<br><br>

A modified rule-of-mixture approach was developed for assessing the quality of the filler-matrix interface. Synergistic reinforcement FG and GNP composites at 0.1 %·w/w filler concentration were confirmed, but the quality of the filler-matrix interface deteriorates with increasing graphene loadings. Observation of the tensile fracture surface through scanning electron microscopy supports the assessment of the synergistic reinforcement at low graphene concentrations. On the other hand, it was indicated that the quality of the filler-matrix interface in LG composites was not ideal and challenged the reliability of the Halpin-Tsai approach for justifying instances of synergistic reinforcement. It is likely that, as a result of its high purity, the inherently large aspect ratio of LG could not be maintained following composite preparation.<br><br>

The formation of hydrogen bonds between LG and PA6 was indicated by Fourier transform infrared spectroscopy. The role of graphene functionalisation on improving interfacial bonding was confirmed by scanning electron microscopy. Competing effects of LG addition on polymer crystallinity and thermal stability were observed. The change in reinforcement trend was found to be related to changes in the interfacial quality as indicated by the modified rule-of-mixtures. <br><br>

GNP-PA6 composite filaments with 17.0 %·w/w GNP concentration were prepared by melt-mixing and twin-screw extrusion. Both filaments could be successfully printed via ME. Based on superior mechanical properties, melt-mixing is a more effective method for GNP dispersion in PA6 and subsequent work was carried with this filament (referred to as "GC" now). When compared to compression moulded (CM) specimens, ME-printed specimens tend to have reduced mechanical properties and EMI SE. Alignment of GNP as a result of high shear during printing resulted in an improved tensile strength at yield in GC (ME) over neat PA6 (ME), when the opposite was observed in CM specimens. ME-printing was found to have a greater negative influence on the absorption mechanism of EMI SE, while EMI reflection was almost unaffected. <br><br>

Increasing specimen thickness of GC (ME) did not improve EMI SE due to the detrimental effect of impedance matching on EMI reflection. The impedance matching condition was avoided by the introduction of suitable internal structural features in the ME-printed specimens. The EMI SE of ME-printed specimens with internal structural features show a unique frequency-dependent response depending on the feature design. While simulations could successfully predict the EMI SE of ME-printed specimens of up to 5 mm thickness, there is poor agreement between experimental and simulation results in most specimens with internal structural features. This is attributed to difficulty in printing at height, as evidenced by micro-computer tomography images, without sufficient support from preceding layers.<br><br>

The rheological properties of PA6 was significantly altered by GNP addition. Oscillatory rheometry revealed that the addition of 0.1 %·w/w GNP concentration resulted in a decrease in the dynamic viscoelastic properties when compared with neat PA6. This indicated that the composite possessed a more liquid-like rheological behavior following the addition of GNP. However, increasing GNP concentration to 6.0 %·w/w and above resulted in a significant enhancement to said properties, signifying a transition towards more solid-like behaviour. Further analysis indicated that the rheological percolation threshold lies between 6.0 and 10.0 %·w/w GNP concentration. The Carreau model was used to describe the complex viscosity of the materials. <br><br>

Capillary rheometry was applied to assess the steady shear rheology of neat PA6 and GC at high shear rates characteristic of printing conditions. Differences between viscosity measurements by oscillatory and capillary rheometry highlight the importance of selecting the appropriate rheological characterisation methods and shear rates when applying melt rheology for assessing the printability of a ME feedstock. A method to predict the printability of a ME filament feedstock, based on fundamental equations describing the material flow through the printer nozzle, in the form of a printing envelope was developed and verified experimentally. It was found that designing filaments with steady shear viscosities of approximately 15.0 % of the maximum printable viscosity at the desired printing conditions may be advantageous for easy ME processing.