Processing and Failure Behavior of Carbon-NanotubeComposites in Sheet Forms

Wen-Shyong Kuo,1 Hung-Chih Lai,2 Ting-Wei Chang,2 Chan-Chao Wen2

1Department of Aerospace and Systems Engineering, Feng-Chia University, Taichung 407, Taiwan, Republicof China

2Ph.D. Program of Mechanical and Aeronautical Engineering, Feng-Chia University, Taichung 407, Taiwan,Republic of China

In their original forms, carbon nanotubes (CNT) are spa-tially entangled. To make CNT papers (CNTPs), the CNTagglomerates must be dispersed and re-entangled intoa planar sheet. The processing characteristics are verydifferent from those of traditional buck-form nanocom-posites. This article examines the processing, micro-structures, and failure behavior of the CNTP compo-sites. The CNTPs were first made by a dispersion andfiltering process. Then, an epoxy resin was added intothe CNTP by using a vacuum bag method. DifferentCNT weights were employed to make the CNTPs withdifferent thicknesses and areal weights. The CNTPallows direct resin impregnation along the thicknessdirection and avoids the difficulty of dispersing CNTs inthe viscous resin. The CNT content can be much higherthan that attainable in traditional bulk CNT composites.Both tensile and tearing tests were conducted, and thefracture behaviors were examined. POLYM. COMPOS.,00:000–000, 2014. VC 2014 Society of Plastics Engineers

INTRODUCTION

Nanocomposites based upon carbon nanotubes (CNTs)

have been the focus of research in the past two decades.

Like many other fibers, CNTs can be employed to make

paper-like preforms, also known as buckypapers, CNT

papers (CNTPs), or CNT sheets [1–5]. CNTs are naturally

entangled in the form of agglomerates. Compared with

their original forms of irregular agglomerates, the CNTPs

are uniform in thickness and can be manipulated easily as

they are integrated. Because most CNTs are oriented in

the in-plane directions, their mechanical performance

[6–10], thermal and electrical conductive behaviors

[11–16] along the in-plane directions are inherently better

than bulk-type CNT composites. Because of the thin

thickness and the entangled CNT network, the CNTP

composites are highly flexible and can be sharply bent

without disintegration. By adding a suitable matrix mate-

rial, they become CNT composites in the sheet form. The

most notable feature of this nanocomposite is the much

higher CNT content achievable because of the different

processing methods.

In making CNT composites in bulk forms, the inher-

ently entangled CNTs must be properly dispersed and

embedded within a resin [17–19]. However, the viscosity

of the CNT/resin mixture increases drastically with the

CNT loading. Beyond a certain CNT fraction, the mixture

will become too sticky to be processed by typical disper-

sion facilities, such as high-speed mixers, sonication, and

three-roller mills [17, 20]. The previous works indicated

that when the CNT loading reached 5 wt%, the mixture

became a visco-elastic material, and dispersion of CNTs

becomes difficult for the three-roller mill [20, 21]. With-

out proper dispersion, many CNTs are unable to be wet-

ted and bonded by the resin, and the resulting mechanical

behavior can be deteriorated.

In comparison, the high-viscosity problem can be

avoided in CNTP composites because of different proc-

essing approaches. CNTPs are made before resin impreg-

nation. CNTs are dispersed within an ethanol solution

rather than within a polymer resin. At this stage, the pur-

pose of CNT dispersion is to break apart the CNT

agglomerates. After filtration, the individual CNTs can be

deposited and re-entangled to form a CNTP. In this work,

we adopted sonication, and the processing time is critical.

If the processing time is insufficient, the agglomerates

remain too large to form a CNTP. On the other hand, if

the CNTs are overly sonicated, the CNTs can be fractured

and shortened, and the resulting composite properties can

be lowered. Then the CNTP can be impregnated by a

resin and cured to make a CNTP composite. Typical resin

impregnation methods, such as resin transfer molding or

Correspondence to: Wen-Shyong Kuo; e-mail: wskuo@fcu.edu.tw

Contract grant sponsor: National Science Council of Taiwan, R.O.C;

contract grant number: 100-2221-E-035-034-MY3.

DOI 10.1002/pc.23327

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2014 Society of Plastics Engineers

POLYMER COMPOSITES—2014

resin infusion, are unsuitable for making CNT compo-

sites, because the entangled CNTs are too dense for the

resin to penetrate into. A vacuum-bag method is more

suitable for this, and the resin is permeable along the

thickness direction.

From the microstructure point of view, CNTs in a

CNTP composite are preferentially oriented in the in-

plane directions, in contrast to the 3D random orientation

in buck CNT composites. In theory, the in-plane stiffness,

strength, thermal, and electrical conductivities can be

improved. The impregnated CNTPs are prepreg-like and

can be embedded within carbon-fiber prepregs to make

hybrid composites [22]. The inter-laminar strength and

damping capability can be benefited. Because of the

excellent energy-storage properties of CNTs, potential

applications of the CNTP composites include super-

capacitors and the electrodes for lithium batteries [23,

24]. Because of the conductive, acid-resistant gas-permea-

ble characteristics, CNTPs have the potential to be used

in the gas diffusion layers for fuel cells [25, 26].

The CNTs in CNTPs are dense and integrated in a pla-

nar form, which can be manipulated like typical carbon-

fiber prepregs. These features combine to make this mate-

rial attractive. In this work, CNTPs with different thick-

nesses were made, aiming to study the effect of thickness

on the processing and properties. Since the materials are

thin, they are vulnerable to some forms of loads. The ten-

sile and tearing properties and associated fracture behav-

ior are examined.

MATERIAL PREPARATION

Preparation of CNTP

The preparation of the CNTP is briefly described.

Multi-walled CNTs were employed for making the

CNTP. The CNTs were mixed with ethanol solution for

dispersion. To treat 100 mg of CNTs, 300 ml of the solu-

tion was used. They were first mixed by a magnetic stir-

rer, and then a sonicator (1,500 W, 20 kHz) was

employed to disperse the CNTs. The processing lasted for

30 min. The CNT agglomerates were torn apart into

smaller ones or into individual CNT filaments. The CNT

solution was then filtered by using a porous film placed

within a buffer container. A negative pressure was applied

to the container to enhance CNTP deposition. The CNTP

with the filtering film was dried at 50�C for 24 h. Then

the CNTP was carefully peeled from the film. The CNTP

is circular and 9-cm in diameter. Samples with different

CNT weights were made. The nominal weights for the

specimens were 100, 150, 200, 250, and 300 mg. The

weight of each sample was measured and recorded. A

typical CNTP is shown in Fig. 1.

Resin Infiltration

The method of vacuum bag was adopted for resin

impregnation and curing. A controlled amount of the epoxy

resin was directly applied on one surface of the CNTP. The

amount was just sufficient for a saturated resin impregna-

tion. The assembly for resin infiltration consisted of, from

the top to the bottom, a vacuum bag, a non-porous releasing

film, the wetted CNTP, a porous releasing film, a bleed

cloth, a non-porous releasing film, and a flat steel plate.

Vacuum was applied and a roller was used to roll and

evenly press the entire assembly. The resin on the CNTP

was forced to penetrate into the CNTP along the thickness

direction. The excess resin can be absorbed by the bleed

cloth under the CNTP. The resin was cured under the vac-

uum condition. After curing, the CNTP composite was

removed from the assembly. The weight and the thickness

of the CNTP composite were measured. Table 1 lists the

measured data for some specimens. To ensure resin

impregnation, the sample was cut by a knife and the cross-

section was examined by a Scanning Electron Microscope

(SEM). A typical SEM is shown in Fig. 2. The CNTs were

uniformly distributed through the thickness, and the resin

impregnation was generally satisfactory.

Tensile Test

The CNTP composite is thin and can be readily cut by

a knife to make specimens. The tensile specimen was an

end-tab type, as shown in Fig. 3a. The specimens were

5-mm wide and 60-mm long. The test section in the mid-

dle was 20 mm. A thin aluminum plate was used as the

end-tab material. The end-tabs were tightly bonded on the

specimen. A testing machine with a 1-kN load cell was

used. The loading speed was set at 0.5 mm/min. The

FIG. 1. Typical SEM image showing the surface of a CNTP before

resin impregnation.

TABLE 1. Material samples made in this study.

Sample

CNTP

weight (g)

Composite

weight (g)

Thickness

(mm)

CNT

(wt%)

C100 0.102 0.960 0.195 10.62

C150 0.156 1.368 0.262 11.40

C200 0.204 1.787 0.338 11.41

C250 0.252 2.085 0.391 12.08

C300 0.308 2.651 0.452 11.61

2 POLYMER COMPOSITES—2014 DOI 10.1002/pc

displacement and the load were recorded at a rate of 10

data per second. At least five specimens were tested for

each type of the CNTP composite. After the test, the frac-

tured sections were examined by a SEM.

Tearing Test

The specimen for the tearing test was a double-cantilever

type, as shown in Fig. 3b. The specimen sizes were 60 mm

3 10 mm, and a 30-mm artificial precut was made to the

specimen by using a sharp knife. Figure 4b shows a speci-

men clamped by the grips. The clamped portion of each

cantilever beam was 10 mm, leaving a 20-mm portion

deflected by 90�. The composite was thin and flexible and

can be clamped without incurring noticeable damage. The

pulling load by the grips became the tearing force at the

crack tip. The loading speed was set at 3 mm/min. The dis-

placement and the load were recorded. After a distance of

grip moving, the crack started to propagate. The test was

terminated after the tearing crack extended 10 mm. After

the test, the fractured sections induced by the tearing were

observed.

RESULTS AND DISCUSSION

Geometry and Processing Characteristics

Figure 1 shows a CNTP surface and reveals the inter-

mingling of CNTs. The CNTPs are generally flat and the

thickness is uniform. Before dispersion, CNTs were mostly

in the form of ball or rope agglomerates. The microscopic

observations indicated that the agglomerates can no longer

exist in the CNTP. The agglomerates must be eliminated

or significantly reduced in size in order to make CNTPs

with a sufficient cohesive strength. The sonication power

and processing time are thus important for this purpose.

FIG. 2. A typical cross-section of the CNT composite after resin impreg-

nation and consolidation. CNT pullouts are popular on the section.

FIG. 3. Specimens specifications. (a) Tensile test. (b) Tearing test. (c)

Illustration of the tearing test.

FIG. 4. Testing specimens. (a) Tensile specimens after test. (b) The

tearing specimen clamped by the grips. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—2014 3

Once the dispersed CNTs are filtered, the CNTs can be

entangled again and form the paper. If the CNTs are not

well entangled or the amount of CNTs is insufficient, the

CNTP can be easily broken during the processing.

Figure 2 shows a typical through-thickness section of a

CNTP composite after resin infiltration and curing. This

SEM sample was prepared by cutting a CNTP composite

by using a sharp knife. The section shows dense CNT

packing, and many CNT pullouts can be observed. Holes

left by the CNT pullouts can be seen too. Under the

vacuum-assisted process, the resin can effectively infiltrate

into the CNTP. However, the resin is still unable to reach

some small voids entrapped among CNTs. When vacuum

was applied, the resin tended to infiltrate into the CNTs

through the larger flowing paths. For a dense CNT pack-

ing, resin becomes difficult to flow, and small voids in

nanoscales can be formed. Because of the direct resin infil-

tration, the CNT contents of the present materials can be

much higher, typically between 10 and 12 wt%, much

higher than the value in typical bulk CNT composites [20].

Unlike bulk CNT composites in which the CNT/resin

ratio can be varied from zero up to its maximum limit,

the thickness of the CNTP reaches a minimum after the

vacuum is applied. Thus, the space for resin is a constant.

If the resin supply is excess and more than the CNTP can

absorb, resin layers can be accumulated on the CNTP

surfaces. In the present work, the resin amount was con-

trolled to maximize the resin impregnation without form-

ing resin layers on the surfaces. This condition is termed

as saturated impregnation. On the other hand, if the resin

supply is insufficient, more voids can be formed. For the

CNTP composites, the amount of resin is a processing

parameter and can be changed for different purposes. The

saturated impregnation can result in the best mechanical

properties. Other potential applications such as the gas

diffusion layers for fuel cells require the presence of

voids [25, 26]. A controlled wetting below the saturated

impregnation is necessary.

Tensile Behavior

Figure 5 shows typical loading curves of the speci-

mens. The curves become nonlinear at high strains. Typi-

cal fractured specimens are shown in Fig. 4a. The crack

cuts the specimen along the width direction. No necking

was observed, despite the non-brittle behavior and high

failure strains. The failure strain increases with the CNT

areal weight. The nonlinearity is in part because of the

pullout of CNTs that hinder crack growth. In comparison,

the neat epoxy is more brittle. The thinnest sample, C100,

is slightly weaker than the pure epoxy. The results of the

composite strength are shown in Fig. 6. The strength is

increased with the CNT areal-weight. The C300 is about

67% stronger than the C100. The major reason is that the

CNT packing is denser for specimens with a higher CNT

weight. The second reason is that the presence of voids

affects more the thinner specimens than the thicker ones.

Figure 7 shows typical fractured surfaces of the tensile

specimens. Unlike Fig. 2, the fractured sections are uneven

and stepped. The stepped surface was possibly because of

cracks located in different positions. When the tensile strain

increases, each crack extends along the normal direction

(the solid arrows in Fig. 7b). When two cracks of different

altitudes approach each other, the induced transverse shear

stress at the crack front can deflect the crack into the axial

direction (the open arrow in Fig. 7b). These two cracks can

be linked and a step is formed. Similar to Fig. 2, CNT pull-

outs are popular on the fractured surface. Extensive pullouts

deter crack growth and extend the failure strains.

Tearing Behavior

The tearing test is stroke-controlled, and the crack grows

in a stable manner. The loading response is illustrated in

Fig. 8. The loading curve is composed of two parts. The

first part is a gradually increased curve due to the deflec-

tion and elastic deformation of the cantilever beams. In

theory, no damage is induced in this part. The last point is

FIG. 5. Typical loading curves of the tensile test. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.

com.]

FIG. 6. Tensile strengths for the specimens with different CNT areal-

weights.

4 POLYMER COMPOSITES—2014 DOI 10.1002/pc

the peak of the curve, at which the tearing crack starts to

grow from the tip of the precut. This point refers to the

crack triggering, and the corresponding load is termed trig-

gering load. The second part of the curve is due to crack

growth. Once the tearing is triggered, the crack will grow

with the displacement of the grip. While the grip moves at

a constant speed, the crack grows in a stop-and-go manner,

and the load changes up and down. This process continues

until the crack cuts the entire specimen.

Figure 9 shows some loading curves of the tests. The

slope and the peak increase with the composite thickness.

Nearly all specimens experienced minor drops in the load

before the peak point. The presence of defects and voids

in the material can trigger minor and local damage before

the crack starts to grow. As expected, the load in the sec-

ond part of the curve goes up-and-down within a range.

For most specimens, the crack grows straight along the

center-line of the specimen (the dotted line in Fig. 3c).

However, in some specimens, the crack grew away from

the center-line. The fractured section became inclined to

FIG. 7. Typical fractured sections of the tensile test. (a) The stepped sur-

face. (b) A step on the surface. Two horizontal cracks approach each other

(open arrows). One of the crack is deflected to the vertical direction due to

the transverse shear (open arrow), forming the step. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 8. Illustration of the loading response of the tearing test. The

curve comprises two parts separated by the dotted line. The first part

involves deflection and elastic deformation of the cantilever beams. The

second is the stage of crack growth, and the curve goes up-and-down

with the displacement.

FIG. 9. Typical loading curves of the tearing tests. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.

com.]

FIG. 10. Normalized results of the triggering load for the specimens.

DOI 10.1002/pc POLYMER COMPOSITES—2014 5

FIG. 11. Fractured section at the tip of the precut (C250). The dotted line indicates the tip. The crack

started from the dotted line and grew to the right.

FIG. 12. Typical fractured sections of the tearing specimens, showing a divergent pattern of streamlines.

(a) C200. (b) C300. (c) Schematic illustration of crack growth. The crack grows intermittently. The stream-

line is where the crack stops. The numbers indicate the sequence of the crack growth. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

the loading direction, and the crack was no longer a pure

Mode-III. The test was terminated if this happened.

Figure 10 lists the results of the normalized triggering

load, defined as the load divided by the specimen thickness.

The normalized load is increased with the composite thick-

ness. The reason is that the stress concentration at the tip is

less intense in thicker specimens. The growing of the crack is

sensitive to the stress at the crack tip, and the stress concentra-

tion is disproportionately higher in thinner specimens. On the

fractured section, the CNTs were pulled out, rather than cut-

to-break by the tearing stress. The CNT pullouts represent

energy consumption, which hinders the growth of the crack.

Figure 11 shows a fractured section around the precut tip,

revealing the morphological differences between the precut

and the tearing-induced fracture surfaces. The original loca-

tion of the precut tip is indicated. The left region is the precut

surface opened by the cutting knife. The crack started from

the precut tip and grew toward the right. The fractured sur-

face around the tip is generally irregular because of the

geometry of the precut tip. However, once the crack grows

stably in the tearing mode, the fractured surface becomes reg-

ular. Figure 12 shows two typical tearing-induced surfaces.

The most notable feature is the streamlines on the surface.

Figure 12c illustrates how the streamlines were formed. The

crack grows in a stop-and-go manner, depending on the stress

level and the strain energy. The streamline is where the crack

stops. Once the stored strain energy reaches a critical level,

the crack will quickly propagate to the next streamline and

stop. The numbers indicate the sequence of the crack move-

ment. The upper half and the lower half are symmetric in the

pattern. According to the SEM observations, most CNTs on

the fractured surface were pulled out, and very few were frac-

tured because of shear-cut or stretching. A typical SEM

micrograph is shown in Fig. 13. CNT pullouts and holes left

by the pullouts are abundant on the surface.

CONCLUSIONS

This work examines the processing and fracture behav-

ior of the CNT composite. The two-step approach allows

CNT dispersion and resin infiltration to be conducted sepa-

rately, and the high-viscosity problem in the CNT/resin

mixture can be avoided. The nominal CNT content can

reach 10 wt%, which is much higher than that of traditional

bulk-type CNT composites. The resulting nanocomposite is

paper-like and flexible. Both tensile and tearing properties

have been examined. The tensile strength increases with

the CNT weights, and the C300 specimens are about 38%

stronger than the neat resin. In comparison with the linear

response and brittle failure in the pure epoxy specimens,

the CNTP composites show nonlinear responses at higher

strains, and the fractured surfaces are unsmooth. Pullout of

CNTs is responsible for the nonlinear behavior. The tearing

response is composed of two parts. The first involves

deflection and elastic deformation of the double cantilever

beams, although minor damage could also appear. The sec-

ond part is due to the crack growth. The crack grows in a

stop-and-go manner, leaving symmetric streamlines on the

fractured surfaces. The normalized triggering load is

increased with the CNTP thickness. CNT pullout is the

dominant mode in the fracture section.

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