The Dependence of Piezoresistivity of Elastomer/Nanostructured

Carbon Composites on Dynamic Mechanical Load Frequency

Artis Linarts, Imants Barons and Maris Knite

Institute of Technical Physics, Rigas Technical University, Paula Valdena 3, Riga, Latvia

Keywords: Dynamic Mechanical Loading, Piezoresistivity, Nanostructured Carbon Composites, Sensor Materials,

Piezoresistive Sensor.

Abstract: The aim of this article is to determine piezoresistive sensitivity of elastomer nanostructured carbon

composites at dynamic loading tests and show the piezoresistive effect correlations to various frequencies of

applied mechanical force in a manner that could provide a parameter of the highest detectible dynamic load

frequency. This parameter is crucial when determining sensor’s usability in possible applications. There are

only few articles on conductive polymer composite sensitivity in dynamic mechanical loading tests.With

this article we are trying to estimate the values of dynamic loading frequencies in which sensor would be

functional.

1 INTRODUCTION

Rapid detection of mechanical forces in tactile

sensing devices is very important for precise and

cautious automation. Electrically conductive

polymer composites (CPCs), being stretchable,

bendable, light weight, low cost, and soft, offer

several key advantages soft compared to their solid

state alternatives. Applications of CPCs are now

widely used in different research fields such as

stress/strain sensors (Knite et al., 2004), mechanical

damage self-monitoring materials (Nanni et al.,

2011), gas sensors (Knite et al., 2007), health control

(Sebastian et al., 2014), and tactile sensing skin for

human robotic application (Canavese et al., 2014).

If insulating polymer matrix is filled with

electrically conductive fillers like carbon nanotubes

(Wang and Cheng, 2014), carbon blacks (Zhou et

al., 2008; Wang et al., 2011; Knite et al., 2004;

Nanni et al., 2011; Aldraihem et al., 2009), metallic

particles (Jung et al., 2013), or hybrid fillers

(Zavickis et al., 2011) at concentrations just above

percolation transition also known as critical volume

fraction, piezoresistive effect in CPCs can be

observed (Zhou et al., 2008). Piezoresistive effect in

general describes the electrical conductivity

variation upon an influence of external force.

Conductivity of the piezoresistive CPCs can be

easily tuneable by changing the content of the

nanoparticles. However it should be noted that the

critical volume fraction as well as piezoresistive

sensitivity critically depends on the efficiency of

filler dispersion method used to produce these

composites (Zha et al., 2014). Piezoresistive CPCs

are starting to become one of the most widely

researched materials for possible sensor manufacture

because of simple and low cost preparation, simple

electronics and low power consumption (Stassi et

al., 2014). For example, tactile sensing skin for

human robotic application was made from nickel

particles and poly-dimethylsiloxane rubber. Particle

size was from 3.5 micrometers to 7 micrometers in

diameter. On particle surfaces were sharp spikes

ranging under hundred nanometers in height. Results

of this study revealed that this metal–polymer

composite is capable of sensing pressure

deformations with speeds from 2.5 up to 250 mm/s

(Canavese et al., 2014).

Structural health monitoring for observations of

other structural materials in buildings is another field

where CPC`s would be very helpful. These systems

were made from glass fibers with conductive carbon

tubes as spikes on their surface in the insulating

matrix of epoxy. Strain deformation experiments

revealed that these systems could sense different

types of deformation including longitudinal,

transverse and off-axis orientated (Sebastian et al.,

2014).

416

Linarts A., Barons I. and Knite M..

The Dependence of Piezoresistivity of Elastomer/Nanostructured Carbon Composites on Dynamic Mechanical Load Frequency.

DOI: 10.5220/0005121704160420

In Proceedings of the 11th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2014), pages 416-420

ISBN: 978-989-758-039-0

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

Conductive polymer composite with 10 parts by

weight carbon black embedded in a polyurethane

matrix was tested to confirm the usage of these

sensor materials for paint sensors. These sensors

could be applied to structures as paint and observe

structural stresses as well as noise levels in towns.

The experiments were conducted using compression

deformation with oscillation frequency under 100

Hz (Aldraihem et al., 2009).

In our Institute conductive polymer composites

are made from natural rubber as matrix and carbon

black is mostly used as the conductive filler. The

piezoresistive effect for this sensor material is stable

in temperatures from 20 to70 degrees Celsius. Knite

et al. have investigated polyisoprene nanostructured

carbon composites as sensor materials in quasistatic

loading tests where the loading happens once and

after it the relaxation is observed. Relaxation and its

velocity are strongly connected to dynamic loading,

so these studies were the base from which this

research was developed (Knite et al., 2004).

2 PREPARATION OF SAMPLES

AND THE EXPERIMENT

In our case CPCs were made by roll mixing of extra-

conductive carbon black (CB) Printex XE2 (specific

surface area 950 m2/g, average primary particle

diameter 30 nm, DBP absorption 380 ml/100 g)

together with natural raw rubber – polyisoprene (Pi)

and necessary vulcanization ingredients. Raw rubber

chemical composition expressed in parts per

hundred rubbers (p.h.r.) is shown in Table 1.

Table 1: Chemical composition of raw rubber composite.

Component Content, p.h.r.

Natural polyisoprene 100

Sulfur 3,5

Cyclohexyl-

benzothiazolesulfonamide

0,8

Zinc oxide 5

Stearic acid 1

Carbon black 8

The electrical conductivity of these composites is

highly dependent on the conductive filler

concentration and external influences like pressure,

tension, temperature. Since this CPCs has got a

positive piezoresistive effect (the electrical

resistivity increases on external influence) then

concentration of conductive filler should be slightly

over percolation threshold so that it would be easier

to monitor resistivity during testing. In this study to

achieve relatively low electrical resistivity we used

concentrations of 8 p.h.r. CB (further in text refered

to as PiCB8). To get an excellent electrical contact

for resistivity monitoring brass electrodes with

thickness of 0.05 millimetres where chemically

bound to both sides of the sample during

vulcanization. After curing, the samples were shelf

aged at room temperature for at least 24 hours before

any measurements were made.

Equipment for creating high frequency dynamic

loading was specially made for this research to study

the piezoresistivity of our sensor materials. Device

produces oscillating movements with frequency

from 3 to 45 Hz and the amplitude of movement

differentiates from 3 to 40 mm giving deformation

from 4.3 to 57.1% of samples length.

Device is electrically powered from the 220 V

AC grid. Rotation frequency of an electric motor is

controlled manually with a transformer that changes

electric voltage. The rotation frequency is monitored

using stroboscope. The electric motor has a gear at

the end of the shaft which is connected to an

eccentric part by timing belt. The eccentric part is

round and has a T-shape channel on its surface.

Through it a T-shape figure is moved to differentiate

the amplitude of the clamp movement. A connecting

rod is bolted to the T-shape figure which can be

fixated in different positions. The other end of the

connection rod is cylindrical and is gliding through a

pipe-like control channel, which turns the eccentric

movement in to linear movement. A clamp for

holding a sample is permanently connected to this

end of the connecting rod and is moving during the

experiment. The other clamp for holding the sample

is electrically insulated from all the other

components and stays static during the experiment.

Electrical resistivity is calculated from voltage

measured by an‘’HBM Spider8’’ data acquisition

system capable of 4,8 kHz reading frequency on 4

parallel channels which is connected to the sample

and a personal computer where data is stored.

Resistivity measurements where conducted with 1.2

kHz frequency. Data acquisition system is connected

to the sample as is shown in figure 1.

Figure 1: Circuit diagram for sample monitoring.

TheDependenceofPiezoresistivityofElastomer/NanostructuredCarbonCompositesonDynamicMechanicalLoad

Frequency

417

Two resistances– our sample and an etalon resistor

are in series with a nine volt voltage source which

produces the current for the circuit. Data acquisition

system ‘’Spider 8’’ is connected in parallel to the

etalon resistance to measure the voltage drop when

our sample is deformed. For more precise

measurements, the etalon resistance is almost

matched to the sensor’s resistance. Electrical

resistivity of the sample is at least one order higher

than the wire and contact resistances combined

therefore they are not taken in to account. The same

is true for the measuring equipment resistances.

Resistivity of the sample is calculated from the

formula:

R 













 1

, (1)

where U

is the power source voltage and U

– the

voltage measured by Spider, R

is the resistance of

sensor sample and R

is the etalon resistance.

The piezoresistive effect at slow loading

frequency tests (0.005-0.1 Hz) was determined using

a Zwick/Roell Z2.5 universal material testing

machine coupled with an Agilent 34970A data

acquisition/switch unit. Images of the loading

equipment are shown in Figure 2.

Figure 2: Equipment used for dynamic mechanical loading

for low (top) and high (bottom) frequencies; where 1 –

electric motor; 2 – eccentric sheave; 3 – sample; 4 –

stroboscope; 5 – HBM spider 8.

Figure 2: Equipment used for dynamic mechanical loading

for low (top) and high (bottom) frequencies; where 1 –

electric motor; 2 – eccentric sheave; 3 – sample; 4 –

stroboscope; 5 – HBM spider 8. (cont.)

3 RESULTS AND DISCUSSION

Mechanical loading frequency influence on the CPC

piezoresistivity were conducted in room temperature

at 0 to 6.6%deformationfrom 0,005 to 45 Hz. For

deformation frequencies higher than 3 Hz the

desired loading frequency is manually set using the

transformer and the stroboscope. After at least 5

minutes of mechanical loading the change of sample

resistivity is measured in time.

The same was done for frequencies lower than 3

Hz, however in this case automated frequency

control was used. PiCB8 piezoresistive effect at

0.01, 3 and 40 Hz loading frequencies in 0 to 6.6%

deformation are shown in Figure 3.

The observed piezoresistive effect can be

explained by transverse slippage of nanoparticles in

the composite structure caused by external strain

leading to disarrangement of the conductive

channels. When the frequency is increased the

minimal and maximal resistivity values or resistance

at zero and 6.6 % deformation increases as well,

however the overall piezoresistivity tends to

decrease with increasing frequency as shown in

figure 4.

When the sample is deformed the average

distance between particles in the strain direction

increases leading to an increase in overall resistivity

of the sample, however at constant deformation in

time the polymer macromolecules reconfigure

(process known as strain relaxation) in strain

direction leading to a decrease of average layer

thickness between conductive particles and

subsequent increase of tunnelling current or decrease

of overall resistivity.

ICINCO2014-11thInternationalConferenceonInformaticsinControl,AutomationandRobotics

418

Figure 3: Change of resistivity in 0.01; 3 and 40 Hz

deformation frequencies.

To explain these results the relaxation of

sample’s resistivity at constant 6.6% strain

deformation was carried out in room temperature

(Figure 5). At constant deformation the decrease of

resistivity in time can be explained as fallows. The

3D conductive grid throughout the composite

structure is composed from carbon black particles

which mostly are separated by a thin layer of

polymer. The conductivity in this case is ensured by

tunnelling currents between particles.

Figure 4: The dependence of piezoresistive sensitivity

versus loading frequency.

Figure 5: Change of resistance in time at constant 6.6 %

strain deformation.

Luheng Wang et.al (14) described the change of

resistivity in time using a mathematical model

similar to stress relaxation in time:













∞





∑





∗exp













(2)

The exponents in equation (2) represent the

resistance relaxation times related to the movement

of the polymer composites constituent parts. We

found the best fitting of experimental curve using in

equation (2) three exponents i = 3 with three mean

relaxation times τ

= 1754, τ

= 197, and τ

= 22s

that could represent relaxation of carbon

nanoparticle aggregates, polymer chains, and chain

segments respectively.

Based on the additional investigation of

relaxation processes (Figures 5 and 6) and analysis

of the acquired values of relaxation time 

, 

, and



we explain the strain sensitivity ∆R/R

dependence on load frequency f as follows. At very

low tension load frequencies both the

macromolecular chain and the CB nanoparticle

TheDependenceofPiezoresistivityofElastomer/NanostructuredCarbonCompositesonDynamicMechanicalLoad

Frequency

419

Figure 6: Fitting of experimental curve of change of

resistance in time at constant 6.6 % strain deformation

with equation (2) taking into account three mean

relaxation times: 

= 1754, 

= 197, and 

= 22s.

aggregates have enough time to relax, therefore R

also relaxes with time when the mechanical loading

and the strain sensitivity is the highest. At higher

(>1Hz) frequencies CB aggregates have a difficulty

to follow the macromolecular chains during loading,

so the strain sensitivity decreases versus frequency.

4 CONCLUSIONS

It was shown that the polyisoprene high structure

carbon black composite samples can be used for

periodically changed mechanical load or mechanical

vibration testing. The maximal and minimal values

of resistivity increase with mechanical load

frequency while the piezoresistivity effect decreases.

These can be explained by analysing the

experimentally determined values of relaxation

times of carbon black aggregates and polymer

chains. The limiting factors for use of the developed

sensors at higher mechanical load frequencies are

the rise of maximal and minimal resistivity as well

as the decrease of piezoresistivity versus frequency.

ACKNOWLEDGEMENTS

This study was supported by ESF Grants Nr.

1DP/1.1.1.2.0/13/APIA/VIAA/021.

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