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CNT: Carbon Nanotube(s) - Cylindrical carbon molecules 13 nm in diameter and 10 μm in length
PLA: Polylactic Acid - A thermoplastic aliphatic polyester
FDM: Fusion Deposition Modeling - An additive manufacturing process using layer-by-layer deposition of a plastic filament material extruded through a nozzle
MFR: Melt Flow Rate - A measure of the ease of flow of melted plastic
Tg: Glass Transition Temperature - The temperature range where the polymer substrate changes from a rigid glassy material to a soft (not melted) material
Tcc: Cold Crystallization Temperature - The temperature at which there is a change in the physical structure which leads to an increase in the degree of crystallization
Tm: Melting Temperature - The temperature at which a substance changes from solid to liquid
DSC: Differential Scanning Calorimetry - A thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature
SEM: Scanning Electron Microscope - A type of microscope that produces an image by scanning a focused electron beam over the surface of a specimen
Owing to the microstructural anisotropy and the layer-by-layer effect of the forming process, the mechanical behaviors and the fabrication qualities of the finished part formed by the FDM process are poorer than those of the finished part formed by conventional manufacturing techniques.
One way to conquer this restriction is to exploit new materials with excellent properties.
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This article examines how the addition of CNT affects the thermal, mechanical, and electrical properties of PLA 3D printed parts.
The filament used to make the specimens was created by blending PLA pellets and different wt% CNT. The mixture was then dried for 8 hours at 80 °C before it was extruded using the double screw extruder with a screw length to diameter ratio of 40/1. The strands produced by the double screw extruder were granulated and extruded using the single screw extruder to produce the final filaments.
Using the 3D printer, three different types of test specimens were made: tensile, flexural, and resistivity. During the FDM process, the nozzle diameter, liquefier temperature, filling velocity, and layer thickness were set at 0.8 mm, 215 °C, 50 mm/s, and 0.2 mm, respectively. The tensile specimens were made to the ISO 527:2012 standard, and the flexural specimens were made to the ISO 178:2010 standard. The resistivity samples were 15 mm x 15 mm x 2 mm in size. Each type of specimen was made with 0, 2, 4, 6, & 8 wt% CNT, however only the resistivity specimens could be reliably printed with the 8 wt% CNT.
Five different types of experiments were conducted to examine the thermal, mechanical, and electrical properties of the PLA/CNT composites. The crystallization-melting behavior and MFR were tested to examine the printability of the composite, the tensile and flexural strengths were tested to determine the mechanical properties of the composite, and the electrical resistivity was tested to determine the electrical properties of the composite.
The crystallization-melting behavior was tested using the differential scanning calorimeter. Six milligram samples were first heated from 20 °C to 300 °C at a rate of 20 °C/min and kept at 300 °C for 1 min to reset the thermal history. They were then cooled at a rate of 10 °C/min to 20 °C and kept at 20 °C for 1 min, and finally they were reheated from 20 °C to 300 °C at a rate of 10 °C/min. The DSC curves were obtained, and from the curves of the second heating step, the glass transition temperature, melting temperature, and cold crystallization temperature were determined. This was done for each of the filaments, including the 8 wt% CNT.
| Tg/°C | Tcc/°C | Tm/°C |
|---|---|---|
| 0 wt% | N/A | 151.09 |
| 2 wt% | 117.55 | 151.92 |
| 4 wt% | 111.50 | 159.14 |
| 6 wt% | 108.68 | 160.38 |
| 8 wt% | 109.92 | 172.48 |
The melt flow rates of the PLA/CNT composites were determined using the melt flow rate tester following the ISO 1133:2011 standard. The MFR was determined for each of the filaments, including the 8 wt% CNT.
| CNT Content | MFR (g/10 min) |
|---|---|
| 0 wt% | 29.38 |
| 2 wt% | 20.17 |
| 4 wt% | 16.54 |
| 6 wt% | 11.85 |
| 8 wt% | 6.91 |
The results of the tensile testing show that the tensile strength and elastic modulus increased with increasing CNT content. The addition of 6 wt% CNT produced a 64.12% increase in tensile strength from that of the pure PLA. This is because the force applied upon the PLA/CNT may be effectively transferred to the CNT, resulting in a significant increase in the tensile strength and elastic modulus of the PLA/CNT printed parts.
The SEM of the fracture interface shows that the interfacial adhesion is better and that the layer-by-layer effect becomes weaker with the addition of CNT.
The results of flexural testing show that the flexural strength and flexural modulus increased with increasing CNT content. The addition of 6 wt% CNT produced a 29.29% increase in flexural strength and a 17.39% increase in flexural modulus from that of the pure PLA. The mixture of PLA and CNT increased the proportion of the continuous phase and thereby increased the flexural strength.
The results of the electrical resistivity testing show that the electrical resistivity decreases with increasing CNT content. When the CNT content is increased from 0 to 2 wt% CNT, the electrical resistivity reduces from 1x1012 to 1x106 Ω/m2. The increase of CNT content in the PLA/CNT composite can promote the formation of permeable conduction paths and distribution of charge on the surface of the composites because of the excellent electrical conductivity of the CNT.
The results demonstrate that the CNT content has a significant influence on the mechanical and conductive properties of the PLA/CNT composite. The addition of CNT increases the mechanical and conductive properties but is disadvantageous to the printability of the PLA/CNT composite.
While they did look at the effect of filling velocity, liquefier temperature, and layer thickness on the electrical resistivity, they should have looked at the effect of those on the mechanical properties as well. I think there could have been an effect of layer thickness on the mechanical properties. I would have liked to see pictures of their specimens. I would have liked more information on their 3D printer, did they use it exactly as it came or was it modified? I would have liked to know how they ensured an even distribution of CNT in the filament.
We have been looking into the mechanical properties of molded specimens at Mercer, and so we could look into the effect of adding CNT to these specimens. We could look into adding the CNT into a coating, such as XTC-3D, instead of the filament, which would allow for a higher wt% of CNT. We could look at the differences when 3.0 mm filament is used instead of 1.75 mm filament.
Enhancing PLA 3D Printed Parts with CNT Composites Study. (2024, Jan 12). Retrieved from https://studymoose.com/document/enhancing-pla-3d-printed-parts-with-cnt-composites-study
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