Graphite sheets that are rolled into seamless tubes are called carbon nanotubes. With diameters of just a few nanometers and lengths up to centimeters, carbon nanotubes have received a great deal of attention because of their unique thermal, mechanical, and electrical properties.
CNTs hold numerous potential applications, particularly in the field of polymer compounds, where they are used to enhance electrical and mechanical properties. Polymer nanocomposites are often used in the aviation and automotive industries, and also in construction materials for windmill blades.
Thorough dispersion of the CNTs in the polymer matrix is crucial for unleashing the unique properties of the polymer nanocomposites. The desired property refinements can be realized only when the CNT particles are dispersed homogeneously within the polymer and the formation of larger clusters is prevented. Dynamic mechanical thermal analysis (DMTA), which can be carried out, for instance, with a rotational rheometer, can be used to test the enhanced mechanical properties of the final compound1.
The use of CNT suspensions for the extrusion process is one approach that can lead to a homogeneous distribution of the CNT particles within the polymer matrix. This is achieved by functionalizing the CNTs (i.e. by amination) and subsequently dispersing it in a carrier liquid like ethanol through ultra sonic treatment or high shear mixing. The CNT suspension, which was obtained, is then fed into the extrusion process. Moreover, using CNT suspensions in the extrusion process prevents the formation of CNT dust in the laboratory environment.
This article intends to show that CNT suspensions can be used to produce polymer nanocomposites using twin screw extrusion. With the described process, a homogeneous distribution of the CNTs in the polymer matrix can be attained to acquire the required property improvements for the polymer nanocomposite.
Material and Methods
- Two CNT-Ethanol suspensions with different functionalization (Rescol/France)
- Base Polymer: Polypropylene Metocene HM562S (LyondellBasell)
- Co-rotating twin screw extruder Thermo Scientific™ HAAKE™ Rheomex PTW16 OS System (L/D = 40)
- Torque rheometer system Thermo Scientific™ HAAKE™ PolyLab OS System
- Liquid feeding pump for the suspensions
- Gravimetric RotoTube feeder for pellets
- Strand line with Varicut pelletizer
- Vacuum pump
- Temperature profile: 20°/230°/250°/250°/230°/220°/220°/200°/200°
- Screw speed: 250 rpm
- Feed rate CNT-suspension: 0.114 kg/h (equivalent to 0.5% CNT in PP)
- Feed rate PP: 0.919 kg/hour
Figure 1 depicts the whole extruder and screw configuration. In the first stage (zone 1), the polypropylene was added and molten in the first mixing section (zone 2). A liquid feeding pump was used to dose the CNT suspension into the second feeding port (zone 3) into the polypropylene melt. The ethanol of the suspension was removed from the extruder by means of an atmospheric venting port and a vacuum venting in zones 4 and 9, respectively.
Figure 1. Extruder and screw configuration.
Figure 2. Extrusion conditions.
The polypropylene and the CNTs were completely mixed and sheared in two mixing sections in zone 8 and zones 5/6.
The melt pressure at the die was measured during the test. Figure 3 depicts this melt pressure as an overlay of three different extrusion tests. One test was carried out with the pure polypropylene, one test with the addition of CNT suspension “1”, and one with CNT suspension “4”.
The pressure evidently increased when a CNT suspension was added. The pressure difference between the two different suspensions itself was insignificant. The extruded material was subsequently formed into a strand. The strand was cooled down in a water bath and cut into pellets with a pelletizer.
Using the Thermo Scientific™ HAAKE™ MiniJet System – a mini injection moulding machine – those pellets were injection moulded into specimens like DMTA bars and disks for further analysis.
Shown in Figure 4 is a microscopic picture taken from specimens made from the PP compound containing 0.5% CNT from suspension “1”. In this picture, no agglomeration can be observed and the CNTs appear to be uniformly distributed in the polymer matrix.
Figure 5 depicts a microscopic picture taken from the PP compound containing 0.5% CNT from suspension “4”. A large amount of agglomerates is seen in this picture. The dispersion appears to be much worse than the result obtained from suspension “1”.
Figure 3. Pressure at Die-Had.
Figure 4. PP with 0.5% of CNT “1”.
Figure 5. PP with 0.5% of CNT “2”.
The PolyLab System with the lab scale twin screw compounder, Rheomex PTW16, can be used to prepare compounds from CNTs and polymers using CNT suspensions.
These test results reveal considerable differences between the compounds made with differently functionalized CNTs.
Thermo Fisher Scientific thanks the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart, Germany for their input, and the Research Company RESCOLL in Pessac, France for their kind co-operation and supplying the CNT-suspensions. Thermo Fisher also thanks Volker Räntzsch from the Karlsruhe Institute of Technology in Germany for providing the microscopic images.
References and Further Reading
- Thermo Scientific Application note V241 “Dynamic mechanical thermal analysis (DMTA) on polymer nanocomposites“ Fabian Meyer, Klaus Oldörp and Frits de Jong
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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