PROSPECTIVE ROBOTIC TACTILE SENSORS

Elastomer-Carbon Nanostructure Composites as Prospective Materials for Flexible

Robotic Tactile Sensors

Maris Knite, Gatis Podins, Sanita Zike, Juris Zavickis

Institute of Technical Physics, Riga Technical University, Azenes str. 14/24, Riga, Latvia

Velta Tupureina

Institute of Polymer Materials, Riga Technical University, Azenes str. 14/24, Riga, Latvia

Keywords: Flexible, pressure sensor, polyisoprene, carbon black, CNT.

Abstract: Our recent 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

are presented. Composites made of polyisoprene matrix and high-structured carbon black or multi-wall

carbon nano-tube filler have been designed and manufactured to develop completely flexible conductive

polymer nano-composites for tactile sensing elements. Electrical resistance of the composites as a function

of mechanical strain and pressure is studied.

1 INTRODUCTION

Sensors of strain and pressure are important in many

fields of science and engineering. One of the main

confines of the existing conventional sensors is

being discrete-point, fixed-directional and inflexible.

For design of pressure sensors the most often used

material is piezoelectric ceramics made separated

from the material or structure being monitored.

There is a demand for new flexible large-area

sensors that can be embedded, for example, into the

flexible skin material of robotic fingers and used for

sensing multiple locations. Due to increasing

application in the machine-building, especially in

aerospace industry, polymer – carbon black

composites are most actively studied materials

(Manson, 1976; Sichel, 1982). Such composites are

mostly used as the so-called inactive materials in

electrical heating elements, resistors, and antistatic

charge materials or shields of electromagnetic

radiation (Wessling, 1986). Conductive rubber

composites produced using carbon black, are still

attracting attention as active materials due to the

effectiveness in applications such as strain and

pressure sensors, temperature sensors and selective

gas sensors (Aneli, 1999; Zhang, 2000; Das, 2002;

Job, 2003; Knite, 2002; Knite, 2004; Dohta, 2000;

Dharap, 2004). New interesting properties are

expected 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 polyisoprene – carbon nano-

composites grows very rapidly and reversibly for

both – tensile and compressive strain when high-

structured carbon nano-particles (HSNP) (specific

surface area 950 m

/g, mean diameter 25 nm) are

used as the filler (Knite, 2002; Knite, 2004).

In this paper our recent success in the design,

processing and studies of physical properties of

polymer – nano-structured carbon (PNC) composites

is presented with regard to prospective flexible

compressive strain and pressure-sensing materials

for soft robotic tactile elements.

Superior mechanical and electrical properties of

carbon nano-tubes offer attractive possibilities for

new sensors. So far most studies have been related

234

Knite M., Podins G., Zike S., Zavickis J. and Tupureina V. (2008).

PROSPECTIVE ROBOTIC TACTILE SENSORS - Elastomer-Carbon Nanostructure Composites as Prospective Materials for Flexible Robotic Tactile

Sensors.

In Proceedings of the Fifth International Conference on Informatics in Control, Automation and Robotics - RA, pages 234-238

DOI: 10.5220/0001497802340238

 SciTePress

to mechanical deformation and change of nano-scale

electrical properties. An attempt to use the strain

sensing capability of single-wall carbon nano-tubes

(SWCNT) on the nano-scale level in a macro-scale

strain sensor was made by Dharap et al.. We present

in this paper an attempt to use the multi-wall carbon

nano-tubes (MWCNT) to devise a flexible

composite for macro-scale pressure indicators

(relative pressure difference sensors) or robotic

tactile elements.

2 DESIGN PRINCIPLES OF THE

STRUCTURE OF MATERIALS

On the basis of the results of other authors (Aneli,

1999; Zhang, 2000; Das, 2002; Job, 2003) we have

developed the following principles designing the

structure of materials to obtain most sensitive

multifunctional elastomer–carbon nano-composites:

1) Polyisoprene (natural rubber) of the best

elastic properties has to be chosen as the matrix

material;

2) High-structured carbon nano-particles (HSNP)

providing a fine branching structure and a large

surface area (better adhesion to polymer chains

compared to LSNP) should be taken as the filler.

Because of a higher mobility of HSNP compared

with LSNP the electro-conductive network in the

elastomer matrix in this case is easily destroyed by

very small tensile or compressive strain. We suppose

this feature makes the elastomer–HSNP composite

an option for more sensitive tactile elements in

robots.

3) The highest sensitivity is expected in the

percolation region of a relaxed polyisoprene

composite. The smallest mechanical strain or

swelling of the composite matrix remarkably and

reversibly increases resistance of such a composite.

From the thermodynamic point of view the sensing

of strain is based on the shift of percolation

threshold, for example, under tensile strain as shown

in Figure 1 (Knite, 2002; Knite, 2004).

As seen from Fig. 1, the shift of the percolation

threshold at less than 30% strain changes electric

resistance of the composite with 10 mass parts of

HSNP filler more than 10

times. The highest strain

sensitivity is expected in the percolation region (9-

10 mass parts of the HSNP filler) of a relaxed PNC

composite. Thus, the maximum sensitivity of PNC

composite materials to thermodynamic forces is

supposed to occur near the percolation threshold of

electric conductivity.

To obtain completely flexible tactile sensing

elements of large area (relative to rigid piezoelectric

sensors) a layer of the active PNC composite is fixed

between two conductive rubber electrodes by means

of a special conductive rubber glue.

6 8 10 12 14

The maximum

sensitivity

region

lgR, Ω

Carbon black, mass parts

streched 30%

streched 40%

relaxed

Figure 1: Shift of the percolation threshold in polyisoprene

– high-structured carbon nano-particle composite under

tensile strain (Knite, 2002; Knite, 2004).

3 PREPARATION OF SAMPLES

AND ORGANISATION OF THE

EXPERIMENT

The polyisoprene – nano-structured carbon black

(PNCB) composite was made by rolling high-

structured PRINTEX XE2 (DEGUSSA AG) nano-

size carbon black and necessary additional

ingredients – sulphur and zinc oxide – into a Thick

Pale Crepe No9 Extra polyisoprene (MARDEC,

Inc.) matrix and vulcanizing under 30 atm pressure

at 150 ˚C for 15 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 polyisprene – carbon nanotube (PCNT)

composites containing dispersed multi-wall carbon

nanotubes (MWCNT) were prepared as follows. The

size of MWCNT: OD = 60-100 nm, ID = 5-10 nm,

length = 0.5-500 μm, BET surface area: 40-

300 m

/g. To increase the nano-particles mobility

and to obtain a better dispersion of the nano-

particles within the matrix the matrix was treated

with chloroform. The prepared matrix was allowed

to swell for ~ 24 h. The MWCNT granules were

carefully grinded with a small amount of solvent in a

china pestle before adding to the polyisoprene

matrix. Solution of the polyisoprene matrix and the

concentrated product of nano-size carbon black was

mixed with small glass beads in a blender at room

temperature for 15 min. The product was poured into

a little aluminum foil box and let to stand for ~ 24 h,

PROSPECTIVE ROBOTIC TACTILE SENSORS - Elastomer-Carbon Nanostructure Composites as Prospective Materials

for Flexible Robotic Tactile Sensors

235

dried at 40 ºC and vulcanized under high pressure at

160ºC for 20 min.

Discs of 16 mm in diameter and 6 mm thick

were cut from the vulcanized PNCB composite

sheet. Conductive polyisoprene – HSCB (30 mass

parts) composite electrodes were prepared and

fastened to the disc with special conductive adhesive

(BISON Kit + 10 mass parts of HSCB) as shown in

Figure 2.

Figure 2: Picture of completely flexible strain sensing

element made of PNCB composite with conductive rubber

electrodes.

Aluminum electrodes were sputtered on opposite

sides of the sensing element (20 × 11.5 × 2.4 mm)

made of the PCNT composite as shown in Figure 3.

Figure 3: picture of a strain sensing element made of

PCNT composite with sputtered Al electrodes.

Electrical resistance of samples was measured vs

mechanical compressive strain and pressure on a

modified Zwick/Roell Z2.5 universal testing

machine, HQ stabilized power supply, and a

KEITHLEY Model 6487 Picoammeter/Voltage

Source all synchronized with an HBM Spider 8 data

acquisition logger. Resistance R of the composites

was examined with regard to compressive force F

and the absolute mechanical deformation Δl in the

direction of the force. Uniaxial pressure and relative

strain were calculated respectively.

4 EXPERIMENTAL RESULTS

AND DISCUSSION

The percolation thresholds of PNCB and PCNT

composites were estimated in the first place. Of all

the composites examined, the best results were

obtained with samples containing 14.5 mass parts of

MWCNT and 10 mass parts HSCB, apparently

belonging to the region slightly above the

percolation threshold. Dependence of electrical

resistance on uniaxial pressure first was examined

on a PNCB composite disc without the flexible

electrodes. Two brass sheets 0.3 mm thick and 16

mm in diameter were inserted between the disc and

electrodes of the testing machine.

036912151821242730

200

400

600

800

1000

1200

ΔR/R

Pressure, MPa

First cycle

Figure 4: Electrical resistance (in relative units) of an

element (without flexible electrodes) of PNCB composite

containing 10 mass parts of HSCB as function of pressure.

T = 293 K.

0369121518212427

200

400

600

800

1000

1200

ΔR/R

ε, %

First cycle

Figure 5: Electrical resistance (in relative units) of an

element (without flexible electrodes) of PNCB composite

containing 10 mass parts of HSCB as function of

compressive strain ε. T = 293 K.

The piezoresistance effect in PNCB composite is

reversible and positive ((ΔR)/R

>0) (Figure 4 and

Figure 5).

As a next the measurements of the

piezoresistance effect observed in an element of

PNCB composite with flexible electrodes attached is

illustrated in Figure 6 and Figure 7 showing that the

piezoresistance effect decreases approximately 10

times but remains positive.

The positive effect can be explained by

transverse slippage of nano-particles caused by

ICINCO 2008 - International Conference on Informatics in Control, Automation and Robotics

236

external pressure leading to destruction of the

conductive channels.

0246810121416182022

ΔR/R

Pressure, MPa

First cycle

Figure 6: Electrical resistance (in relative units) of an

element (with flexible electrodes) of PNCB composite

containing 10 mass parts of HSCB as function of pressure.

T = 293 K.

0 3 6 9 1215182124

ΔR/R

ε, %

First cycle

Figure 7: Electrical resistance (in relative units) of an

element (with flexible electrodes) of PNCB composite

containing 10 mass parts of HSCB as function of

compressive strain ε. T = 293 K.

As seen from Figures 8, 9 and 10, the electrical

resistance of the sensing element of PCNT

composite decreases monotonously with uniaxial

pressure and compressive strain. In this case the

piezoresistance effect is considered as negative

((ΔR)/R

<0). Compared with a sensing element of

the PNCB composite with flexible electrodes the

piezoresistance effect – the absolute value of

(ΔR)/R

of a sensing element of the PCNT

composite (Figure 7 and Figure 9) is more than 10

times smaller. Thus, the PNCB composite is more

sensitive to mechanical action than the PCNT

composite. The latter exhibits a more monotonous

dependence of electrical resistance on compressive

strain.

Moreover, only insignificant changes of

disposition of the curve were observed during 20

cycles (Figure 10). We explain the negative

piezoresistance effect by formation of new

conductive channels of MWCNT under external

pressure.

0,00 0,03 0,06 0,09 0,12 0,15 0,18 0,21

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

ΔR/R

pressure, MPa

Max compressive strain 5 %

Figure 8: Electrical resistance (in relative units) of an

element (with Al electrodes) of PCNT composite

containing 14.5 mass parts of MWCNT as function of

pressure. T = 293 K.

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

ΔR/R

ε, %

First cycle

Figure 9: Electrical resistance (in relative units) of an

element (with Al electrodes) of PCNT composite

containing 14.5 mass parts of MWCNT as function of

compressive strain ε. T = 293 K.

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

ΔR/R

ε, %

1 cycle

1st cycle

20th cycle

Figure 10: Electrical resistance (in relative units) of an

element (with Al electrodes) of PCNT composite

containing 14.5 mass parts of MWCNT as function of

compressive strain ε. 20 loading cycles. T=293 K.

PROSPECTIVE ROBOTIC TACTILE SENSORS - Elastomer-Carbon Nanostructure Composites as Prospective Materials

for Flexible Robotic Tactile Sensors

237

Consequently, the PNCB composite could be a

prospective material for pressure-sensitive indication

while the PCNT composite can be considered as a

prospective material for pressure sensors.

5 CONCLUSIONS

Completely flexible sensing elements of

polyisoprene – high-structured carbon black and

polyisoprene – multi-wall carbon nanotube

composites have been designed, prepared and

examined. The first composite having a permanent

drift of its mean electrical parameters is found to be

a prospective material for indication of pressure

change. The other composite has shown good

pressure sensor properties being capable to

withstand many small but completely stable and

reversible piezoresistive cycles.

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