PROSPECTIVE ELASTO-PLASTIC PRESSURE SENSORS

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.

1 INTRODUCTION

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.

2 PREPARATION OF SAMPLES

AND THE EXPERIMENT

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

/g.

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

409

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

 SciTePress

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.

3 RESULTS AND DISCUSSION

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 Ω·m).

The specific resistance for flexible electrodes is in

the order of 0.1 Ω·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

K).

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

(the

average distance between two opposite particles of a

cluster) diverges as

∼

V-V

(1)

where ν is the critical index (Roldughin, 2000).

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410

In the vicinity of the percolation threshold,

electrical conductivity of the composite changes as:

∼

V-V

(2)

where t is the critical index (Roldughin, 2000).

Under mechanical deformation of composites

and,

consequently,

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

K).

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

411

ACKNOWLEDGEMENTS

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

investigations.

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).

REFERENCES

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:

234-238.

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.

4 CONCLUSIONS

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.

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