All-Elasto-Plastic Polyisoprene/Nanostructured Carbon Pressure Sensing Element
Maris Knite, Juris Zavickis, Gatis Podins, Raimonds Orlovs and Kaspars Ozols
Institute of Technical Physics, Riga Technical University, Azenes str. 14/24, Riga, Latvia
Keywords: Completely Flexible Pressure Sensor, Polyisoprene, High-Structured Carbon Black, Hybrid Composite.
Abstract: Our further achievements in the design, processing and studies of physical properties of elastomer – nano-
structured carbon composites as prospective compressive strain sensor materials for robotic tactile elements
as well as for other automatic systems are presented. Composites made of polyisoprene matrix and high-
structure carbon nanoparticle filler have been designed and manufactured to develop polymer nano-
composites for flexible, entirely polymeric pressure sensing elements. Electrical resistance of the
composites as a function of mechanical strain and pressure is studied. SEM pictures of cross-section surface
of sensing elements are analyzed.
In our previous study we have already shown the
possibility to manufacture entirely flexible PNC
sensing element with glued conductive rubber
electrodes (Knite, 2008). Such elements show good
mechano-electrical properties but they have one
drawback – possible delamination of flexible
electrodes during operation. In this study we further
developed the technology of preparing all-elasto-
plastic (AEP) strain sensing element with vulcanized
flexible electrodes made of polyisoprene-
nanostructured carbon (PNC) composite. Recently,
some promising results have been presented
regarding the application of polymer/conductive
filler composites as strain and pressure sensors as
well as selective gas sensors (Knite, 2002; Knite,
2004; Qu, 2007; Li, 2008; Knite, 2007; Sakale,
2009). Interesting and excellent properties have been
obtained in case the composite contains dispersed
nano-size conducting particles. If the size of carbon
particle and specific surface area of carbon black are
between 60 to 200 nm and 16-24 m
/g, respectively
(low-structure carbon nano-particles (LSNP)), the
electrical resistance of natural rubber composites
slowly decreases with applied pressure (Job, 2003).
The effect is explained by the increasing number of
conductive channels due to the increase of external
pressure. Resistance of natural polyisoprene-carbon
nanocomposites grows very rapidly and reversibly
for both – tensile and compressive strain when high-
structure carbon nano-particles (HSNP) (specific
surface area 950 m
/g, mean diameter 25 nm) are
used as the filler (Knite, 2007). The sensing
elements described in all mentioned papers contain
metallic electrodes that reduce the flexibility of the
whole element as well as delamination of electrodes
can be possible due to bending. In this paper our
recent success in the design, processing and studies
of properties of vulcanized foliated composite sensor
element is reported.
The polyisoprene – nano-structured carbon black
composite was made (see Figure 2) by rolling high-
structure PRINTEX XE2 (DEGUSSA AG) nano-
size carbon black (CB) and necessary additional
ingredients (sulphur and zinc oxide) into a Thick
Pale Crepe No9 Extra polyisoprene (MARDEC,
Inc.) matrix and vulcanizing under 3 MPa pressure
at 155 ˚C for 20 min. The mean particle size of
PRINTEX XE2 is 30 nm, DBP absorption – 380
ml/100 g, and the BET surface area – 950 m
The sensor element was made as follows. Two
blends of polyisoprene accordingly with 30 and 10
phr (parts per hundred rubber) carbon black have
been mixed. Initially 30 phr of PRINTEX have been
Knite M., Zavickis J., Podins G., Orlovs R. and Ozols K. (2009).
PROSPECTIVE ELASTO-PLASTIC PRESSURE SENSORS - All-Elasto-Plastic Polyisoprene/Nanostructured Carbon Pressure Sensing Element.
In Proceedings of the 6th International Conference on Informatics in Control, Automation and Robotics - Robotics and Automation, pages 409-412
DOI: 10.5220/0002200604090412
used for obtaining PNC composite electrodes, but
the tests of mechanical and electrical properties
showed, that electrodes made from PNC composites
with 20 phr of PRITEX were as much conductive as
30 phr carbon black/polyisoprene electrodes but had
better elasticity as well as superior adhesion to
active element. Three semi-finished rounded sheets
made from mentioned above two PNC composite
blends have been formed and fitted onto special steel
die. Those are two sheets for conductive electrodes
(30 phr CB) and one sensitive sheet (10 phr CB) for
pressure-sensing part. Each of these three sheets
were separately pre-vulcanized under 3 MPa
pressure and 110°C temperature to obtain flat
surfaces. This operation lasts 10 minutes. After that
the components were cooled and cleaned with
ethanol. Further, all three parts were joined together
in one sensor element and were placed into the steel
die and vulcanized under pressure of 30 MPa and
155° C temperature for 20 minutes vulcanization
(previous attempts (Knite, 2008) to create sensor
element with conductive glue were shown to be
relatively ineffective). To study mechano-electrical
properties small brass foil electrodes were added
before vulcanization. Finally, disc shape sensor 50
mm in diameter and 3 mm thick was obtained. From
this preparation we cut out useful sensor elements
for testing (Figure 1).
Figure 1: The accomplished all-elasto-plastic sensor
element with brass foil extensions.
A modified Zwick/Roell Z2.5 universal testing
machine, HQ stabilized power supply and a
KEITHLEY Model 6487 Picoammeter/Voltage
Source was used for testing mechano-electrical
properties of sensor elements. All devices were
synchronized with the HBM Spider 8 data
acquisition logger. Resistance R versus compressive
force F was examined. Uniaxial pressure was
calculated respectively.
Before testing the accomplished sensor element, we
measured the electrical properties of separate
vulcanized electrode layers. We also separately
tested the mechano-electrical properties of
vulcanized active element layer to see whether it has
expected sensing capabilities. The active element of
the sensor (nano-structured carbon black composite
with 10 phr) belongs to the region of the percolation
threshold (specific electrical resistance ρ = 12 ·m).
The specific resistance for flexible electrodes is in
the order of 0.1 ·m, which is noticeably above the
percolation threshold.
Let’s look closer at the conductivity properties of
sensors. Measurement results for electrical
resistance versus pressure for small pressure range
are given in Figure 2.
Figure 2: Electrical resistance of the all-elasto-plastic
sensor element as function of pressure (lower pressure
range, T = 294
Measurement results for relatively large pressure
range are shown in Figure 3. The observed positive
piezoresistance effect can be explained by transverse
slip of nano-particles caused by external pressure
leading to disarrangement of the conductive
channels. The volume concentration of conductor
particles V
at which the transition proceeds is called
the percolation threshold or the critical point.
According to the statistical model, conductor
particles, in the vicinity of V
, assemble in clusters.
Upon approaching V
, the correlation radius
average distance between two opposite particles of a
cluster) diverges as
where ν is the critical index (Roldughin, 2000).
ICINCO 2009 - 6th International Conference on Informatics in Control, Automation and Robotics
In the vicinity of the percolation threshold,
electrical conductivity of the composite changes as:
where t is the critical index (Roldughin, 2000).
Under mechanical deformation of composites
change. This is the reason causing
the piezoresistive effect.
Because of higher mobility of HSNP compared
to LSNP the electro-conductive network in the
elastomer matrix is easily disarranged by very small
tensile, compressive or shear strain. We suppose this
feature makes the elastomer–HSNP composite an
option for flexible sensitive tactile elements for
robots and automatics.
Figure 3: Electrical resistance of the all-elasto-plastic
sensor element as function of pressure (higher pressure
range, T = 294
The scanning electron microscopy (SEM) was
used to check the quality of joined regions of three
PNC sheets of the AEP sensor element. SEM
micrographs of breaking surface of the sensor
element are shown in Figure 4. To prepare the
sample for SEM investigations the sensor element
was frozen in liquid nitrogen and then broken. Good
quality of joining of all three PNC sheets can be
clearly visible in SEM images with different scale
(Figure 4). Pale regions correspond to electrically
more conductive PNC composite with 30 phr CB
and dark regions cover the PNC composite with 10
phr CB.
The pale particles, which are visible in the
bottom picture (Figure 4), are carbon nano-particles.
A functioning model of low-pressure-sensitive
indicator was made. The block diagram of pressure
indication circuit is shown on Figure 5. The sensor is
connected to power supply (PS) via resistor (R) and
to the input of amplifier (Amp). Transistor-based
two-stage amplifier includes integrating elements.
Figure 4: SEM micrographs of sensor element. Sensor
element was frozen in liquid nitrogen and then broken in
two. One of the broken sides is shown in different scales:
20 μm, 5 μm and 2 μm. Boundary between two PNC
composite layers with 10 and 30 phr (parts per hundred
rubber) carbon black are shown.
PROSPECTIVE ELASTO-PLASTIC PRESSURE SENSORS - All-Elasto-Plastic Polyisoprene/Nanostructured Carbon
Pressure Sensing Element
These elements are necessary to avoid noise from
induced currents and to flatten the wavefronts. The
first stage amplifies the signal in linear mode. The
second stage works in saturation mode. The output
of the amplifier is connected to the comparator
(Comp), which forms sharp wavefronts.
The research has been supported by Latvian
National Research program in Materials Science.
The authors are thankful to Mr. Dmitrij Jakovlew
from the Institute of Biomaterials and Biomechanics
of the Riga Technical University for the TEM
These signals are passed to the differential circuit
and they form a sharp pulse, which is passed further
to the one-shot multivibrator (OSM).
The duration of the pulse of the OSM is
adjustable. The OSM is necessary to form the
determined length of pulse which is independent
from AEP sensor element deformation time. The
output of OSM is connected to performing device
PD (indicator/counter or actuator).
Knite, M., Podins, G., Zike, S., Zavickis, J., Tupureina, V.,
2008. Elastomer – carbon nanostructure composites as
prospective materials for flexible robotic tactile
sensors. In Proc. of 5
International Conference on
Informatics in Control, Automation and Robotic, 1:
Knite, M., Teteris, V., Polyakov, B., Erts, D., 2002.
Electric and elastic properties of conductive
polymeric nanocomposites on macro- and nanoscales.
Materials Science & Engineering C, 19: 5-19.
Knite, M., Teteris, V., Kiploka, A., Klemenoks, I., 2004.
Reversible tenso-resistance and piezo-rezistance
effects in conductive polymer-carbon nanocomposites.
Advanced Engineering Materials, 6: 742-746.
Qu, S., Wong, S., C., 2007. Piezoresistive behaviour of
polymer reinforced by expanded graphite. Composites
Science and Technology, 67, 231-237.
Li, X., Levy, C., Elaadil, L., 2008. Multiwalled carbon
nanotube film for strain sensing. Nanotechnology, 19:
045501 (7pp).
Figure 5: Block diagram of pressure-sensitive indication
circuit with completely elasto-plastic sensing element.
Sakale, G., Knite, M., Teteris, V., Tupureina, V., 2009.
Polyisoprene – nanostructured carbon composite
(PNCC) material for volatile organic compound
detection, Proc. of the International Scientific
Conference on Biomedical electronics and Devices
(BIODEVICES 2009), Porto, Portugal, 117.
Completely flexible polyisoprene – high-structured
carbon black all-elasto-plastic sensing element has
been designed, prepared and examined.
Job, A.E., Oliveira, F.A., Alves, N., Giacometti, J.A.,
Mattoso, L.H.C., 2003. Conductive composites of
natural ruber and carbon black for pressure sensors.
Syntetic metals, 135-136: 99-100.
The sensor element was composed of two
electrically conductive composite layers (electrodes)
and piezoresistive PNC layer (active element)
between them. A method for curing three-layer
hybrid composite for pressure sensing application
was developed. The joining in-between conductive
flexible electrodes and sensitive sensor material was
remarkably improved.
Knite, M., Klemenok, I., Shakale, G., Teteris, V., Zicans,
J., 2007. Polyisoprene-carbon nano-composites for
application in multifunctional sensors, Journal of
Alloys and Compounds, 434-435: 850-853.
Roldughin, V., I., Vysotskii, V., V., 2000. Percolation
properties of metal-filled films, structure and
mechanisms of conductivity, Progres in Organic
Coatings, 39: 81-100.
Hybrid three-layer polyisoprene/high-structure
carbon black composite has shown good pressure
sensing properties. Functioning model of low-
pressure-sensitive indication circuit which can turn
on suitable actuator has been made.
ICINCO 2009 - 6th International Conference on Informatics in Control, Automation and Robotics