Journal Publication 20Jul Els

8/3/2019 Journal Publication 20Jul Els

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Modification of impact strength of polycarbonate composites with carbon nanotubes

Prashant Jindal1*, Rajesh Kumar1, Prince Sharma2, Pradeep Chandel2, Vikas Mangla2,Shailaja

Pande3, Anisha Chaudhary3, B P Singh3, R B Mathur3, Meenakshi Goyal4 and V K Rattan4

1 University Institute of Engineering & Technology, Panjab University, Chandigarh-160014,

INDIA

2 Gun Group, Terminal Ballistics Research Laboratory,

Sector-30, Chandigarh, INDIA

3 Carbon Technology Unit, Division of Engineering Materials, National Physical Laboratory,

Dr. K.S. Krishnan Marg, New Delhi, 110012, INDIA

4 University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh-

160014, INDIA

Abstract:-

A polymer based polycarbonate material has been used as base material to form composites

using varying concentrations of carbon nanotubes (CNT) and subjected to impact to determine

its dynamic strength. Split Hopkinson Pressure Bar was deployed as the instrument for impact

testing of all these samples. It has been found, that a CNT concentration of around 0.5% is

enough to enhance the impact strength of the polycarbonate by about 10% at a true strain of

25%. There are some other interesting features at lesser concentrations. We have also studied

these composites under varying strain rates to study changes in their true stress-strain curve.

The effect of concentration on impact strength has been analyzed by studying the SEM

images.

Keywords:-

A. Carbon Nanotubes; B. Impact behavior; B. stress/strain curves


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Introduction:-

Light weight and impact resistant composite materials are being extensively studied under

high and variable loading conditions so that they can be used on large scale to manufacture

shields, jackets, resistant surfaces, shock and impact absorbers etc. [1, 2] Composites like

graphite, PMMA and epoxy laminates have been tested over the past few years for their static

and dynamic strengths[3-7]. The applications of such materials can be wide, depending on

their load bearing capacity. Dynamic loading with large variation of strain rate applied to

different composite has been discussed and published widely over the past decade or so [5 -7].

Generally, dynamic strength of such materials increases with increase in strain rate. Although,

Hosur et al[5] report some deviations in this general behavior where the dynamic strength has

been observed to fall after certain strain rate in some materials. Additionally, effect of

direction of loading, geometry of specimen fibers, angular orientation of laminates and type of

fracture for carbon/epoxy laminate composites on the stress strain behavior has also been

studied. At smaller angles of orientation of laminates the impact strength is much higher at

strain rates of nearly 1000/s or even higher. Laminates loaded along 00 possess higher impact

strengths than the ones loaded along 900 under dynamic strain rates of nearly 800/s [5, 6].W.

Chen et al [7] has worked on Epon and PMMA to find true stress strain variation under tensile

and compressive loading and showed that dynamic strength for PMMA is nearly 110MPa for

strain rates varying in the region of 3300/s and for Epon its 175MPa at a strain rate of 2500/s.

However, it has been stressed that more varied database is needed to have a consensus on the

pattern of the results.


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Among the thermoplastic group of polymers, polycarbonates have attracted a great deal of

attention due to their ability to be easily worked upon and mould ability. Their capability to

resist temperature and impact makes them a common application material in house wares,

laboratories and industries. A modification in their properties to suit specific requirements is

an interesting proposition.

Ever since the synthesis of carbon nanotubes [8] and study that followed exploring

mechanical and structural properties of carbon nanotubes [9-11], there has been wide ranging

interests in scientific and engineering communities to exploit these for varying applications.

The unusual mechanical strength of the carbon nanotubes revealing them as about 100 times

stronger than steel motivates to fabricate and modify useful materials which are cheaply

available in bulk form by embedding in these carbon nanotubes in various forms to make

composites which have desired mechanical properties.

M. Kwiatowaska et al [12] have used thermal analysis techniques like dynamic mechanical

thermal analysis (DMTA) and Differential Scanning Calorimeter (DSC) to find changes in

mechanical properties of pure PBT and its composite with different concentrations of Carbon

Nanotubes. DMTA results show change in elastic modulus with rise in temperature. At

temperatures above 750C the modulus for pure PBT is about 30% lower than PBT with

0.2%CNT. The stress-strain curve also depicts higher stress for PBT-CNT composite than

pure PBT.


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We have chosen to exploit the useful properties of polycarbonates in combination with high

strength of Carbon Nanotubes by making their composites, and hence analyze their dynamic

properties. We fabricated polycarbonate composites with various concentrations of CNTs and

subjected them to high strain rate impact using Split Hopkinson Pressure Bar (SHPB).

Split Hopkinson Pressure Bar is a very useful equipment to study the behavior of materials

under impact loading in the lab [13]. One obtains stress- strain behaviour of the specimen

when subjected to impact or dynamic loading. Specimen undergoes a strain rate of 100 to

10,000/s by using this instrument and the specimen in the form of a disc has a diameter range

between 10 to 20mm and thickness range between 5 to 10mm.

Although the details of working of split Hopkinson bar set up are widely available in literature

[14], however for the sake of clarity and completeness, we reproduce the main features here.

The SHPB apparatus consists of two long slender bars as we call them input and output bars

that sandwich a short specimen between them. A block diagram of a typical SHPB is shown in

Fig. 1.

Fig.1 represents a schematic block diagram of Split Hopkinson Pressure Bar

Striker/Projectile Input Bar Output Bar

Strain measuring

Gauge A

Strain measuring

Gauge B

Specimen


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High gas pressure usually acts as a source of impact which propels a projectile or a striker

which is used to strike one end of the input bar. A compressive stress wave is generated that

immediately begins to traverse towards the specimen. When this wave hits the specimen, it

partially gets transmitted through it and reaches the output bar while some part is reflected

back in the input bar. Usually, an irreversible plastic deformation is caused in the specimen

due to this complete process which lasts less than a millisecond. The reflected pulse is

reflected as a wave in tension or compression, whereas the transmitted pulse remains opposite

to the reflected pulse which is based on the impedance of the sample. The wave signal

measurements are done with the help of strain gauges A (measuring incident and reflected

components) and B (measuring transmitted component) attached on the input and output bars

respectively. The waves are a measure of strains which are calibrated to find stress and strain

in the specimen.

The incident strain ( ) and reflected strain ( ) add algebraically to transmitted strain ( )

as:-

(1)

The force on specimen ( ) due the impact of striker on input bar is the mean of force on the

interface of specimen-input bar ( ) and force on the interface of specimen-output bar ( )

as


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(2)

Stress on the specimen ( ) is related to force on the specimen through the cross sectional

area of the specimen ( /4) facing the input bar as:-

(3)

Force expression on input and output bars can also be written in the form of elastic modulus

(E), strains and diameter ( ) of the bars.

(4)

(5)

These equations result in relationship of stress in the specimen to the transmitted strain as

(6)


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Similarly the strain rate in the specimen (ds/dt)is related to the wave velocity ( ) inside the

bar, transmitted strain and length ( ) of the bar [14] as

(7)

The strain gauges pick the transmitted and reflected strains and hence generate the stress

strain curves based on above equations. Strain can be calculated from equation (7) by

integrating over the time period of impact.

In the subsequent sections we focus on our experimental procedure, present results using

SPHB, and topography images of impacted and un impacted samples of varying

concentrations. Finally, discussion and conclusion has been presented.

2. Experimental

2.1 Synthesis of MWCNT

Multi walled carbon nanotubes(MWCNT) were synthesized by thermal decomposition of

toluene in presence of iron catalyst obtained from organometallic ferrocene. The details

of the experimental set up are given elsewhere [15]. The diameter of the tubes is in the

range of 1060 nm and their lengths ranging in several microns. The purity of these tubes

as determined from Thermogravimetric analysis (TGA) was ~90 %.

2.2 Preparation of MWCNT-Polycarbonate composite


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As-synthesized MWCNT were ultrasonically dispersed in Tetrahydrofuran (THF) for 2h to

obtain a stable suspension of CNTs in THF. The suspensions were then mixed with solutions

of polycarbonate (PC) in THF to obtain a series of mixtures of MWCNT /PC containing

different volume percent (vol. %) of MWCNT varying from 0.1 to 2 vol. %. The mixtures

were then stirred on a magnetic stirrer for 24h to obtain a uniform dispersion of MWCNT in

PC. Thin polymer films were casted on a petri dish (Diameter 4 ) and allowing the solvent to

evaporate over 24hrs followed by drying in oven. The resulting films had a thickness of about

0.25-0.3mm. Blank PC films were also cast by the same technique. MWCNT-PC bulk

composites were prepared by a two-step method of solvent casting followed by compression

molding using as-synthesized MWCNT. In this method solvent casted films were cut into

pieces and stacked in a mold of diameter around 10 mm with 5mm thickness. The final

samples were prepared by the compression moulding in Hydraulic press at temperature 1700C.

The polycarbonate composites were fabricated by polymerization process [3] at National

Physical Laboratory, New Delhi. We used these composites of polycarbonates in the shape of

a cylindrical disc with diameter around 10mm and thickness around 5mm with different

concentrations of CNTs and compared their dynamic strengths at high strain rates using

SHPB. The variation parameter here is only the concentration not the geometry or orientation

of the inner structure of specimen.

Our setup for SHPB at Terminal Ballistic Research Laboratory, Chandigarh comprised of two

high strength maraging steel with yield strength ~ 1750MPa, diameter 20mm and length

2000mm. The projectile diameter was 20mm and length was 300mm. Strain gauges of 120,


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900 tee rosette precision stain gauges designated as EA-06-125TM-120) wee used. For wave

shaping a 1.5mm OFHC Copper wave shaper was used.

This projectile of length 300mm was hit on samples of different compositions one by one

which were sandwiched between the two bars. The projectile was shot at different velocities

for various samples, producing stress-strain curves for different strain rate. Strain rates for our

experiment varied in the range 1576 to 4017/s. Some of the data so collected had to be

discarded due to non compatibility with dynamic equilibrium. Data in which force curves on

the two surfaces of the sample do not match is not in dynamic equilibrium. A sample which

was out of dynamic equilibrium has been shown in Fig.2 which shows large variations

between force on the surface of the sample (F1) facing input bar and force on surface of the

sample (F2) facing output bar.


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Fig.2 Dynamic force history for pure polycarbonate sample, where F1 is the force on surface

of the sample facing input bar and F2 is the force on surface of the sample facing output bar.

Then we took a longer projectile of length 600mm to increase the loading duration which

resulted in achieving dynamic equilibrium. We show in Fig.3 a typical measured data for a

sample where dynamic equilibrium was achieved for a short span of time, for which readings

were valid and considered for further analysis.


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Fig.3 Dynamic force history for pure polycarbonate sample, where F1 is the force on surface

of the sample facing input bar and F2 is the force on surface of the sample facing output bar.

3. Results

The experimental procedure explained above was performed on samples of different

concentrations for different strain rates. Table-1 shows the specifications of the samples and

results of the experiments performed.


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Table-1 Specifications of the samples and results of the experiments performed.

Parameters Measured

Value

Specimen

dimensions(mm)

Remarks

Diameter Thickness

Polycarbonate samples

Max Strain rate (s-1)

Max True Stress(MPa)

Max. True Strain (%)

1994

104

25

10 5 Sample not in

dynamic

equilibrium




3900

96

229

9.8 4.8




1576

99

45

10 4.92




3300

92

154

9.97 4.9 Sample broke into

pieces




2778

94

112

9.95 4.82 Sample was

totally crushed


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1350

94

39

9.90 4.80

Polycarbonate samples with 1% CNT




2018

93

27

10 5




2547

102

92

5.16 9.88




2926

102

116

5.15 9.87




3133

102

136

9.96 5.10




1643

104

53

10.04 5.09 Sample not in

dynamic

equilibrium

Polycarbonate samples with 0.1% CNT




2778

95

108

10 5




2609

92

99

10 4.9 Sample failed


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3200

92

144

10 5 Sample failed

Max Strain rate (s-1

)



4017

91

287

9.50 4.61

Polycarbonate samples with 0.5% CNT




2186

106

73

9.98 4.93

Max Strain rate (s-1

)



2768

105

108

10 5




3599

106

179

10 5




2032

110

66

9.97 5




2845

109

113

10 5

We pick the data for samples of all concentrations (0%, 0.1%, 0.5% and 1.0% of CNTs in

polycarbonates), which were under similar strain rates. This helps us to make a proper

comparison. We plot, in Fig.4 a comparative true stress-strain curve for all such samples

under a strain rate in the range of 2700 to 3000/s.


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0

20

40

60

80

100

120

140

160

0 10 20 30 40 50

True Strain (%)

TrueStress(MPa

Sample with 0%CNT at 2778/s

Sample with 1%CNT at 2926/s

Sample with 0.1%CNT at 2778/s

Sample with 0.5%CNT at 2768/s

Fig.4 representing comparison of true stress and strain for samples of different concentrations

of CNT in polycarbonate.

An analysis of the true stress as a function of concentration of CNTs in polycarbonates as

picked up from Fig.4, at 25% true strain has been shown separately in Fig.5

As can be observed from Fig.5, there is transition of decrease of flow stress by nearly 4%

from 0% to 0.1% and then an increase of flow stress from 0.1 to 0.5% by nearly 14%,


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Fig.5 Represents variation of flow stress vs concentration of CNT in polycarbonates at a true

strain of 25%.

It is also evident that flow stress which is a measure of dynamic strength increases up to

105MPa by making composites of polycarbonates with CNTs concentration of only 0.5%. We

notice a small dip in strength for concentrations up to 0.1% of CNTs.

Scanning Electron Microscope (SEM) images were also taken of the samples used in Fig. 4

and 5 to analyze any difference in topography under impact. The SEM images under nearly

similar impact conditions on all the samples have been given in Fig. 6. We also present pure

and 1.0% CNT composite in Fig. 7 for comparison.


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6(a) 6(b)

6(c) 6(d)

Fig-6. SEM Images of samples of varying concentration of CNT under impact. 6(a) has CNT

concentration of 0% under strain rate-2778/s, 6(b) of 0.1% under strain rate-2778/s, 6(c) of

0.5% under strain rate-2768/s and 6(d) of 1.0% under strain rate-2926/s


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7(a) 7(b)

Fig.7 SEM Images of samples of varying concentration of CNT under no impact. 7(a) has

CNT concentration of 0% and 7(b) of 1%.

Although, we do not have here all topographic images under un impacted conditions, but it

seems that low concentration of CNT (0.1%) as shown in Fig. 6(b) indicates highly

fragmented topology. At such low concentrations, the inter tube distances are significantly

large and bind to only local domains of polycarbonate. They do not seem to interact strongly

between inter-domains. That seems to be the reason for reduced impact strength of the

composite at concentration lower to around 0.1%. On the other hand, the samples of higher

concentration of CNT show SEM images with reasonable integrity and due to decrease in

inter-tube distance.

4. Discussion and Conclusion


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This paper, reports the results of change in dynamic strength of polycarbonates due to varying

concentration of carbon nanotubes in them. The measurements of impact strength have been

done by using Split Hopkinson Pressure Bar. The stress-strain curves for various compositions

have been presented. Further, impact strength of different compositions has been compared.

It has been observed, that concentrations above 0.1% of CNTs in polycarbonates tend to

increase the dynamic strength. Measurements have been performed only up to 1.0% of CNT

in polycarbonates. The SEM images indicate that above concentrations of 0.1% there is a

significant interlinking provided by CNTs with their base material. It would be interesting to

observe the effect by further increasing the CNT concentration before deciding the practical

usability of polycarbonates.

ACKNOWLEDGEMENTS

The authors wish to thank Director, NPL for his support and permission to publish the results.

We would also like to thank Dr. Rajeev Patnaik for his help in obtaining SEM images on their

SEM in Geology Department, Panjab University. We would like to express our gratitude to

Professor V.K. Jindal for guidance and suggestions at various levels.

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