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Electrical Contact Properties of Micro-Injection Molded Polypropylene/CNT/CB-Composites on Metallic Electrodes

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The investigations carried out under this work deal with a new field of application for large-scale production of electric contacting processes for micro-electro-mechanical systems (MEMS) using the micro-injection molding technology. The focus of this article is the analysis of process-related influential factors of micro-injection molding that determine both the electrical resistivity and the flowability of polymer nano composites filled with carbon nano tubes (CNT) and carbon black (CB). Therefore, the viscosity and the electrical conductivity as a function of different CNT-and CB-contents and their combination were investigated in a manufacturing study for Polypropylene. The results of the investigations answered questions regarding material science and technical processes. Thereby, optimal rheological properties for the formation of micro-injection molded conductive patterns with high aspect ratios on the one side and with the best possible conductivity of the nano composites on the other side can be set.
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Electrical contact properties of micro-injection molded
Polypropylene/CNT/CB-composites on metallic electrodes
Michael Heinrich
1,a*
, Ricardo Decker
1,b
, Joerg Schaufuss
2,c
,
Juergen Troeltzsch
1,d
, Jan Mehner
2,e
and Lothar Kroll
1,f
1
Technische Universität Chemnitz, Faculty of Mechanical Engineering, Reichenhainerstrasse 70,
09126 Chemnitz, Germany
2
Technische Universität Chemnitz, Faculty of Electrical Engineering and Information Technology,
Reichenhainerstrasse 70, 09126 Chemnitz, Germany
a
michael.heinrich@mb.tu-chemnitz.de,
b
ricardo.decker@mb.tu-chemnitz.de,
c
joerg.schaufuss@etit.tu-chemnitz.de,
d
juergen.troeltzsch@mb.tu-chemnitz.de,
e
jan.mehner@etit.tu-chemnitz.de,
f
lothar.kroll@mb.tu-chemnitz.de
Keywords: Nano composites, Polypropylene, micro-injection molding, MEMS, CNT, Carbon black
Abstract
The investigations carried out under this work deal with a new field of application for large-scale
production of electric contacting processes for micro-electro-mechanical systems (MEMS) using
the micro-injection molding technology.
The focus of this article is the analysis of process-related influential factors of micro-injection
molding that determine both the electrical resistivity and the flowability of polymer nano
composites filled with carbon nano tubes (CNT) and carbon black (CB). Therefore, the viscosity
and the electrical conductivity as a function of different CNT- and CB-contents and their
combination were investigated in a manufacturing study for Polypropylene. The results of the
investigations answered questions regarding material science and technical processes. Thereby,
optimal rheological properties for the formation of micro-injection molded conductive patterns with
high aspect ratios on the one side and with the best possible conductivity of the nano composites on
the other side can be set.
Introduction
The increasing miniaturization of products including electronic and mechatronic assemblies
leads to a maximal integration of functions and developments of manufacturing technologies for
very small multi-component devices. For this purpose, the micro-injection molding technology for
plastic-based micro parts is outstandingly appropriate. This technology is able to produce plastic
parts with a high degree of shape-freedom in high quantity without post-processing. The flexibility
of this process allows the combination of different materials and the integration of metallic and
ceramic inserts as well as functional textiles and electronic or magnetic parts [1, 2].
Therefore, functionalized polymers are becoming ever more prevalent in the field of electrical
engineering by the micro-injection molding technology, especially in polymer electronics for the
dissipation of electrostatic charges, as well as for current-carrying switching functions [3]. In
addition, further applications have opened up for these nano composites. These include applications
in the automotive industry for packaging processes to protect sensitive circuits with sensors or
actors like micro-electro-mechanical systems (MEMS) [4-7].
Compared to inherent conductive plastics, filled polymers gain their electrical conductivity
through additives, e.g. metals or carbon in different modifications [8]. Depending on the volume
fraction and filling material, the electrical resistivity can be influenced in a wide range. Especially
carbon nano tubes and carbon black are appropriate to build a conductive network with a low filling
material concentration, the so-called percolation threshold [9, 10]. Those nano composites have
outstanding features compared to polymers with usual fillers: low percolation threshold, excellent
Advanced Materials Research Vol. 1103 (2015) pp 77-83 Submitted: 09.10.2014
© (2015) Trans Tech Publications, Switzerland Revised: 05.02.2015
doi:10.4028/www.scientific.net/AMR.1103.77 Accepted: 09.02.2015
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 134.109.240.95-11/03/15,15:50:41)
particle-particle-interdependence, high deployment density within polymer matrix, very high
surface to volume ratio, small distance between particles and small influence on the polymer
structure resulting from being stored in the polymer-chains.
With regard to a shift of the percolation threshold to higher levels of nano filler material with an
increasing orientation of the additives in the polymer matrix, it is important to minimize this
negative influence by a process optimization [10-13]. The first detailed results for that purpose
provide Villmow et al. [11]. The focus of the investigation of the influence of injection speed, melt
temperature, cavity temperature and holding pressure was laid on the orientation of carbon nano
tubes within the polymer matrix. Hence, mainly the injection speed and the melt temperature have
decisive influence on the conductivity of the composites.
In this paper, an overview is given of how to prepare nano composites based on polypropylene
and how the electrical characteristics as well as the rheological properties will be influenced by
different filler contents of CNT and CB.
Experimental
Materials. The starting material for the investigations was a polypropylene homopolymer
Moplen HP 500V, supplied by LyondellBasell. The high-fluidity polymer was measured with a
melt flow rate (MFR) of 120 g/10 min (ISO 1133; 230 C/2.16 kg). It allows the production of
compounds with high amounts of CNT and CB and good processability. A PP-based CNT-
masterbatch with 20 wt% CNT (Plasticyl PP2001, Nanocyl S.A., Sambreville, Belgium) and CB-
pellets (Ketjenblack EC-600JD, Akzo Nobel GmbH, Düren, Germany) were used as conductive
fillers. According to the suppliers the CNTs have a diameter of 5 to 50 nm and a length of 1 to 10
µm and the CB has a pore volume of 480 to 510 ml/100g.
Compound preparation. The compounds were made by blending the PP with the CNT-
masterbatch and the CB-pellets in a HAAKE MiniLab II micro compounder (Thermo Fisher
Scientific Inc., Waltham, MA USA). The materials were compounded for 5 minutes at 230 °C with
co-rotating screws at 80 min-1 (Rotations per minute). 16 different compounds with various mass
fractions of CNT and CB were prepared and are summarized in table 1.
Table 1: CNT- and CB-loadings of the different compounds
Compound description CNT [wt%] CB [wt%]
Total filling level [wt%]
PP-0-0
0 0 0
PP-4-0
4 0 4
PP-6-0
6 0 6
PP-9-0
9 0 9
PP-12-0
12 0 12
PP-0-4
0 4 4
PP-0-6
0 6 6
PP-0-9
0 9 9
PP-0-12
0 12 12
PP-2-2
2 2 4
PP-2-4
2 4 6
PP-4-2
4 2 6
PP-4-4
4 4 8
PP-4-8
4 8 12
PP-8-4
8 4 12
PP-8-8
8 8
16
For composites with single filler, four compounds with 4 to 12 wt% of the additive were made.
In addition, different compounds with both fillers, CNT and CB, were prepared. The total
percentage of additives ranges from 4 to 16 wt%, with an individual content of the components
78 Micro- and Nanotechnologies for Sustainable Development
varying from 2 to 8 wt%. Compounds with more than 16 wt% of CNT or CB can’t be blended with
the used micro compounder due to the high viscosity.
Measurement of viscosity. Following the blending process, the viscosity of the compound was
measured in the compounder using the two integrated pressure sensors on the capillary section of
the compounding circuit. The screw rotation speed started at 0 min
-1
and increased in 15 linear steps
up to 360 min
-1
. The shear rate ranges from approximately 90 s
-1
to 1280 s
-1
. The melt temperature
was set to 230 °C. Then, the compounds were extruded and processed by micro-injection molding.
Sample preparation. The samples were prepared using a HAAKE MiniJet II micro-injection
molding machine (Thermo Fisher Scientific Inc., Waltham, MA USA). The injection pressure was
set to 600 bar for 5 seconds and the post pressure was set to 400 bar for 5 seconds. The melt
temperature was 230 °C and the tool temperature 50 °C.
Two different types of samples were made: a prismatic bar with four contact pins for the 4-wire-
sensing resistance measurements; and circuit boards with holes, where the electrical contact is made
by the injection molded compound, for the contact resistance measurements. The prismatic bar is
shown in figure 1. The dimension of the cross section is 24.6 mm² and the electrodes feature a
spacing of 10 mm. The contact pins are gold-plated to obtain the best electrical properties and
smallest disturbing influences.
Fig. 1: Sample for 4-wire-sensing
The circuit boards are shown in figure 2. In the experimental preparation different layouts with
one to four holes with a diameter of 0.3 to 0.9 mm and three different electrode materials (Cu, Sn,
Au) were tested.
Fig. 2: a) Drawing of circuit board with cross section of micro-injection molded nano composite
structure and b) circuit board sample with and without micro-injection molded nano
composite structure
The gold electrodes always showed the best mechanical and electrical properties, regardless of
the used compounds. With at least two holes, the mechanical coupling of the compound on the
circuit board is stable enough, which also improves the electrical contact. By the contact set up from
the top to the bottom of the circuit boards, a compressive force is given on the electrodes due to the
material shrinkage of the compound (cf. figure 2a), which enhances the contact resistance.
Advanced Materials Research Vol. 1103 79
Therefore, circuit boards with gold electrodes and 3 holes with 0.4 mm diameter were used for the
further experiments to study the specific and the contact resistance of the compounds.
Measurement of electrical resistance. The electrical resistance of the samples was measured
with an Agilent 34410A multimeter (Agilent Technologies, Santa Clara, CA USA). The 4-wire-
sensing method was used to determine the volume resistance of the prismatic bar without the
influence of contact resistance between the electrodes and the compound. The test current was 1
mA. The total resistance on the circuit boards was measured using the 2-wire-sensing method.
Results and Discussion
Electrical resistivity. The evaluation of the 4-wire sensing shows an exponential reduction of
the resistivity ρ of the CNT-PP- and CB-PP-compounds with an increasing loading of CNT or CB
(cf. figure 3). The CNT-loaded samples always show better conductivities than the samples with an
equal amount of CB. Especially at low filling levels of up to about 6 wt%, the resistivity of CNT-
PP-compounds (5.81 Ωcm) is much smaller than the resistivity of CB-PP-compounds (15.38 Ωcm).
The difference between CNT and CB decreases with the increase of the filling content. From about
9 wt% there are only small differences between the resistivity of CNT-loadings (2.40 Ωcm at
12 wt%) and CB-loadings (3.01 Ωcm at 12 wt%). In the range of low filler loadings, a small
increase of the filler content leads to a high reduction of the resistivity. With the raise from 4 wt%
to 6 wt%, the resistivity drops by 62.2% (15.38 to 5.86 Ωcm) for CNT-filling and by 55.6% (32.78
to 14.54 Ωcm) for CB-filling. At higher loadings the reduction slows down.
A combination of both fillers, CNT and CB, leads to a lower resistivity in comparison to
compounds with the same amount of CNT or CB alone (cf. figure 3). At lower loadings, this effect
is more evident than at higher ones. At a total loading of 4 wt% the resistivity decreases from
15.38 Ωcm (CNT) and 32.78 Ωcm (CB) to 12.44 Ωcm when using 2 wt% CNT and 2 wt% CB. At a
total loading of 12 wt%, the resistivity reaches 1.34 Ωcm at 8 wt% CNT and 4 wt% CB in
comparison to 2.40 Ωcm for the CNT-PP-compound and 3.01 Ωcm for the CB-PP-compound.
Therefore, the optimal mixture ratio CNT:CB shifts from 1:1 to 2:1.
Fig. 3: Specific resistance in response to CNT- and CB-loading; a) separate and b) combined
At 4 wt% of CB the variance of the measured values is much higher than the variance at higher
CB-loadings. It can be assumed that the percolation threshold for CB is about 4 wt%. The variance
of the measured values of the CNT-PP-samples is for all loadings approximately the same. So the
percolation threshold must be lower than 4 wt%.
Contact resistance. To determine the contact resistance between the Au-electrodes and the
conductive polymer the circuit boards were used to measure the overall resistance between the
electrodes after the filling process of the holes with the conductive polymer. By knowing the
geometrical parameters of the polymer and the resistivity ρ (determined in the previous chapter), the
resistance of the polymer itself can be calculated and subtracted from the averaged overall
80 Micro- and Nanotechnologies for Sustainable Development
resistance to receipt the contact resistance normed to the contact area between the polymer and the
electrodes.
In figure 4a the contact resistance of the polymer in dependency of the filling level with the
single materials CB and CNT can be seen. Independent of the filling material the resistance
decreases exponentially with the filling level. It is apparent that at a filling level of at least 6% of
CB it leads to a much higher reduction of the contact resistance in comparison with the same
amount of CNT.
Fig. 4: Contact resistance in response to CNT- and CB-loading; a) separate and b) combined
The development of the contact resistance in dependency of a combination of both filling
materials can be seen in figure 4b. For filling levels below 6% lower contact resistances can be
reached with a combination of the filling materials CNT and CB than with a single filling material.
The optimal mixture ratio CNT:CB shifts from 1:1 to 1:2 within the total filling level of 4% to 6%.
Beyond a filling level of 6% the contact resistance of the single material CB is lower than of the
mixture.
Viscosity measurement. In the measured shear rate range from 100 up to 1000 s
-1
the typical
shear thinning for thermoplastic melts can be observed. The pure PP and the compounds with CNT
and CB show the behavior of a power-law-fluid, the viscosity decrease appears in the double
logarithmic plot as a linear curve (cf. figure 5). In the region of the measured shear rates, which
typically occurs in injection molding processes, the adding of the CNT and CB only leads to a shift
of the viscosity curve but doesn’t changes the behavior of shear thinning. It corresponds to an
exponential increase of the viscosity due to the narrowing of the flow passage caused by the filler
that is transported by the fluid and the interaction between the filler particles.
The CNT and CB on its own lead to a distinctive increase of the viscosity, whereby the CNT
raises the viscosity at the same weight content compared to CB higher because of the huge aspect
ratio. Only for 12 wt% of filler is the viscosity nearly the same. In general a convergence for high
shear rates >>1000 s
-1
is indicated, which is typical for all suspensions with solid filler.
The compounds with combinations of CNT and CB exhibit remarkable differences in the
viscosity in comparison to the compound with one of the filler at same total weight content. Due to
the aspect ratio the viscosity behavior is dominated by the weight content of CNT. Compounds with
same weight content of CNT and CB show a higher viscosity than compounds with the same total
weight content consisting of only CNT (cf. figure 5). For 12 wt% filler even a lower content of
CNT leads to a higher viscosity in comparison to the compound with 12 wt% of only CNT (PP-4-8
in figure 5), this compound also shows some irregularities in the shear thinning behavior. The
viscosity increase for the combination of CNT and CB indicates some filler interaction with
influence on the viscosity.
Advanced Materials Research Vol. 1103 81
Fig. 5: Viscosity plots of the measured compounds with combinations of CNT and CB
Conclusion
It is stated that for high portions of CNT the contact resistance is the dominating part of the
overall resistance, while high portions of CB dominate the polymer resistance (cf. figure 6).
Through changing the electrode and polymer geometries the resistance ratio can be shifted into
other regions. For example larger electrode areas but the same polymer dimensions reduce the
resistance that can be assigned to the contact resistance. In consequence, the resistivity of the
polymer gets more important for the overall resistance. The actual values for certain assemblies can
be calculated by knowing the resistivity ρ and the contact resistance R
cont
normed to the contact area
given in this paper.
Fig. 6: Ratio of polymer resistance and the contact resistance relating to the experimental setup
described in this paper
The necessity of the combination of CNTs and CB in nano composites, relating to optimized
overall resistance, leads to an increase of viscosity compared to the compound with one of the filler
at same total weight content. This behavior indicates some filler interaction with influence on the
viscosity and will be more broadly analyzed in further investigations.
Acknowledgement
This work was performed within the Federal Cluster of Excellence EXC 1075 “MERGE
Technologies for Multifunctional Lightweight Structures” as well as the CRC/Transregio 39 “PT-
Piesa” and supported by the German Research Foundation (DFG). Financial support is gratefully
acknowledged.
10
100
1000
100 1000
PP-4-0 PP-0-4 PP-2-2
Viscosity in Pa·s
Shear rate in s
-1
Compound with 4%wt CNT/CB
10
100
1000
100 1000
PP-12-0 PP-0-12 PP-4-8 PP-8-4
Viscosity in Pa·s
Shear rate in s
-1
Compound with 12%wt CNT/CB
82 Micro- and Nanotechnologies for Sustainable Development
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Advanced Materials Research Vol. 1103 83
... Analogously, the CB-fraction dominates the contact resistance between the metallic electrodes and the polymer. However, higher amounts of fillers increase the viscosity of the compounds significantly [11]. Based on previous experiments [11] the combination of 8 wt% CNT and 8 wt% CB is a good compromise between low electrical resistivity, low contact resistance and good processability of the compounds by micro injection moulding. ...
... However, higher amounts of fillers increase the viscosity of the compounds significantly [11]. Based on previous experiments [11] the combination of 8 wt% CNT and 8 wt% CB is a good compromise between low electrical resistivity, low contact resistance and good processability of the compounds by micro injection moulding. Higher filling levels lead to strongly increasing viscosities and thus the compounds cannot be processed. ...
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In this work a MEMS based acoustic emission (AE) sensor is fabricated in a modified high aspect ratio micromachining (HARM) technology. The improved band-pass sensor consisting of four single resonators is developed to detect AE signals in the increased frequency range of 100 kHz-130 kHz. Additionally the sensor's sensitivity is reduced below the working frequencies to avoid overloads of the signal conditioning circuitry caused by disturbing low frequency vibrations. The sensor will be used for failure detection in hybrid components like wind turbines and to monitor conveyor belts in terms of wear and friction.
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Polypropylene composites with different contents of multiwalled carbon nanotubes (MWCNT) were microinjection molded to investigate the influence of melt temperature and injection velocity on the final mechanical properties. Therefore, samples of various MWCNT loadings from 1 wt% to 5 wt% were prepared by diluting commercial masterbatches. Microinjection molding was used to prepare micro tensile bars under different processing conditions. The nano composites expose mechanical properties significantly influenced by nanotube loading, injection velocity and melt temperature.
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A polymer-based sensor for low frequency acceleration detection is fabricated by using microinjection molding technologies. Finite Element simulations and characterization of the sensing functionality are done. Due to an out-of-plane acceleration a force is applied to a seismic mass (length and width each 3.2 mm, thickness 1 mm), which leads to a deformation of a connected plate with dimensions of 1 mm x 1 mm x 50 μm. Thus, charge separation at the electrodes of integrated piezoelectric polyvinylidene fluoride (PVDF) copolymer sheets occur and can be measured as sensor signal. A charge sensitivity of 0.57 pC/g is determined which is in good agreement with the simulation results. A resonance frequency of 660 Hz was measured. Furthermore, the sensor concept as well as preparation technologies to assemble a compound structure containing piezoelectric layers and the system integration by micro injection molding are discussed. In addition, different bonding techniques for the assembly of the functional components are investigated and described.
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Two polycarbonate (PC) composites with 2 and 5 wt% multi-walled carbon nanotube (MWNT) content were injection molded using a two-level, four-factor factorial design to evaluate the influences of holding pressure, injection velocity, mold temperature, and melt temperature on the electrical surface and volume resistivities. For both composites variations in resistivity of the injection-molded plates up to six orders of magnitude were found. The highest impact was determined for the injection velocity followed by the melt temperature and the interaction of both. The resistivity varied also locally within the plates showing differences up to five orders of magnitude for 2 wt% and up to two orders for 5 wt% MWNT. Thereby, areas of equal resistivity are formed in a semicircular shape with values increasing with the flow path. Transmission electron microscopy (TEM) investigations indicated a skin layer with highly oriented nanotubes in case of high injection velocity and low melt temperature, but a network-like structure even in the skin area at low injection velocity and high melt temperature.
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We investigated the effect of flow field and deformation rate on the nanotube alignment and on the properties of PC/multiwalled carbon nanotube nanocomposites. Samples of various MWCNT loadings were prepared by diluting a commercial masterbatch containing 15 wt% nanotubes using optimized melt mixing conditions. Different processing conditions were then used to systematically change the degree of nanotube alignment, from random orientation to highly aligned. Morphological studies and Raman spectroscopy analysis revealed that the nanotubes are preferentially aligned in the flow direction, particularly at large injection or compression rates. Rheological measurements corresponding to high shear rate conditions showed drastic changes in the viscoelastic behavior. The complex viscosity significantly decreased and percolation threshold notably rose. High degrees of nanotube alignment also resulted in a significant increase in the electrical percolation threshold. The mechanical properties of the nanocomposites for different nanotube loadings were also shown to depend on the processing conditions, and somehow improved when the material was processed at higher rates. Finally, we used a power-law type equation to correlate the percolation behavior and the nanotube alignment. The estimated percolation threshold and the power index, q, significantly increase with the degree of nanotube alignment as determined by Raman analysis.