Cure Rate and Mechanical Properties of a DGEBF Epoxy Resin Modified with Carbon Nanotubes

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2009 28: 937 originally published online 16 May 2008Journal of Reinforced Plastics and CompositesAnnamaria Visco, Luigi Calabrese and Candida Milone

NanotubesCure Rate and Mechanical Properties of a DGEBF Epoxy Resin Modified with Carbon

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Cure Rate and Mechanical Properties of a DGEBFEpoxy Resin Modified with Carbon Nanotubes

ANNAMARIA VISCO,* LUIGI CALABRESE AND CANDIDA MILONE

Contrada Di Dio, Vill. S. Agata, Messina 98166, Italy

ABSTRACT: Nanocomposites, made by epoxy resin and multi wall carbon nanotubes (CNT)(0.5–1.5wt%), were studied in this work. Mechanical and physical analyses were performed in orderto investigate both mechanical performance and the cure kinetic. Experimental results suggested thatthe presence of carbon nanotubes modified the polymeric resin properties. The addition of a smallamount (0.5%) of carbon nanotubes increased the cure reaction kinetic of the epoxy resin butinduced a little embrittlement. Higher concentrations (1.0–1.5%) of CNTs reduced the molecularchains mobility, the cure reaction kinetic and the mechanical performance. The results presentedin this work evidenced the necessity to increase the resin/nanotube interaction with an appropriatefunctionalization.

KEY WORDS: carbon nanotube, thermoset resin, nanocomposite, cure kinetic, mechanicalcharacterization.

INTRODUCTION

CARBON NANOTUBES (CNT) are characterized by excellent mechanical, thermal,and electrical properties. The values of modulus and tensile strength are about

250–1200GPa and 10–200GPa, respectively [1]. Such values are remarkably higher thanthat of the traditional materials, as for example carbon fibers (whose modulus is about500GPa and the strength less than 1GPa).

However, as for carbon fibers, carbon nanotubes must be used as structural componentsonly through the support of a matrix. In fact carbon fibers are incorporated generallywithin a thermoplastic or thermosetting polymer matrix with the aim to obtain highperformances composite materials [2,3]. Due to their extensive application, epoxy resinshave been widely used as matrices for nanotubes based composites [3–5].

In order to optimize the nanocomposite performance, carbon nanotube dispersion mustbe maximum and homogenous into the polymeric matrix. Manufacturing of nanocompo-sites is, hence, a crucial factor. A common production technique of nanocompositesconsists of the nanotubes dispersion into a solvent/resin mixture in order to form a stablesolution. Subsequently the solvent is evaporated in order to obtain the solid material [6].The main problem of this procedure is that the nanotubes aggregate between each other.This hinders their dispersion and involves the production of a composite material withlower mechanical characteristics than those designed [7]. Due to the nanometer dimensions

*Author to whom correspondence should be addressed. E-mail: [email protected] 6 appears in color online: http://jrp.sagepub.com

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and high superficial area of the nanotubes, a small amount is enough to increase theviscosity of the nanotube/matrix blend. This provokes problems in the blend dispersion,workability, and, consequently, in composite manufacturing [7]. So, the incorporation ofCNT as reinforcement in the thermoset matrix will enhance the mechanical propertiesof the resin but it will also modify its processing behavior.

A calorimetric and rheological study of the CNTs modified resins is important in orderto optimize the process parameters and to elucidate the phase transitions occurring duringthe polymer processing.

Some authors have studied the cure reaction of nanocomposites containing a diglycidylether of bisphenol A (DGEBA). Puglia et al. [8] and Tao et al. [9] point out that theincorporation of single-wall carbon nanotubes (SWCNTs) have a relevant catalytic effectupon an epoxy resin cure reaction.

Other authors have studied the mechanical performance of the nanocompositematerials, DGEBA resin based [6,10,11].

In the present work the influence of the CNTs content on the cure rate and on themechanical properties of a nanocomposite containing a low viscosity epoxy resin(diglycidyl ether of bisphenol F, or DGEBF) has been investigated with the aim to finda relationship between workability and mechanical performance. The kinetics of the curereaction has also been evaluated by calorimetric and rheological analyses. The mechanicalperformance of the nanocomposites have been evaluated by flexural and impact tests.

MATERIALS AND METHODS

Nanocomposite Preparation

A low viscosity epoxy resin, based on diglycil ether of bisphenol F (DGEBF) (code DER354, supplied by DOW Chemicals), was used. A diethyl toluene diamine (DETDA) wasused as hardener. The epoxy resin was reinforced with different wt% of carbon nanotubes(from 0 to 1.5%) having a multi wall structure [12]. They were produced by the chemicalvapor deposition (CVD) technique. The catalyst was a 5% nickel on �-alumina withhigh superficial area and ethanol as reagent gas. The nanotubes were purified in orderto eliminate traces of catalyst inclusions by suspending the nanotube in a saturatedsolution of potassium hydroxide (KOH, Fluka) at the boiling temperature of 1058C for12 h. Acid etching in HNO3 favored the catalyst metal dissolution. The thermo gravimetricanalysis (TGA) of the purified CNT (by a TA Instruments TGA Q50), evidenced aresidual purity degree higher than 90% [12].

Multi-wall carbon nanotubes were suspended in 10mL of acetone. Then they wereadded with the DGEBF and sonicated in an ultrasonic bath for 2 h in order to obtain anhomogeneous distribution of CNT into the epoxy resin. The blend was mixed undermagnetic stirring for 15min at 508C in order to reduce its viscosity and to favor theremoval of gas bubbles. Finally the DETDA was added and kept in a magnetic stirrer for15min at room temperature.

Four nanocomposite samples were studied in this article. They were codified asfollows: 0.0CNT (pure resin) 0.5CNT (resin containing 0.5wt% of CNT), 1.0CNT (resincontaining 1.0wt% of CNT), and 1.5CNT (resin containing 1.5wt% of CNT).

The pristine resin and the nanocomposites were investigated by physical analyses(calorimetric, rheological, electron microscopy) and mechanical tests (flexural, impact),described in the following.

938 A. VISCO ET AL.



Calorimetric Analyses

A differential scanning calorimeter (DSC), TA Instruments, model Q100, was used tomonitor the cure kinetics. Experiments were performed in nitrogen flow, in the range oftemperature 25–2008C, with a heating rate of 58C/min. Isothermal curing tests wereperformed at different temperatures in the range of 130–1708C. The heat of reaction wasemployed to calculate the resin conversion during this period.

These data let us calculate the activation energy (E). In fact, the curves of conversionvs log (time) collected at different temperature can be overlapped simply translatingeach curve along the log (time) axis with respect to an arbitrary curve. The amount oftranslation is the shift factor A(t):

AðT Þ ¼ lnðtrefÞ � lnðtTÞ ð1Þ

where tref and tT are respectively the time of the reference temperature, Tref, and of thegeneric temperature, T. The shift factor can be used to calculate the cure activation energythrough the relationship:

AðT Þ ¼ lnðtrefÞ � lnðtTÞ ¼ ln kðT Þ � ln kðTrefÞ ¼ �E

RTþ

E

RTrefð2Þ

where E is the activation energy, R the gas constant, and T is the absolute temperature.The relationship between the shift factor and the inverse of the absolute temperature (K) islinear. The cure activation energy (E) can be calculated by the slope of this line.

Rheological Analyses

The rheological properties of the resin during the cure reaction were measured by aparallel plate rheometer SR5 (Rheometric Scientific). Rheological tests were performedin a dynamical way from 25 up to 2008C (heating rate 58C/min). Isothermal tests wereperformed at different temperatures in the range of 130–1708C. The gel time, tgel, wasidentified as the G0 with G00 crossover.

The activation energy was also calculated by the rheological analysis data and comparedwith the activation energy value obtained by the calorimetric data. The gel time can berelated with the activation energy by the equation [13]:

ln tg ¼ const:þE

R

1

T: ð3Þ

A linear relationship between ln tg and 1/T determine the activation energy by theslope calculation.

Scanning Electron Microscopy

A JEOL mod. JSM–5066 LV Scanning Electron Microscope (SEM) with anacceleration voltage of 20 kV was used to observe the surface fracture of the pure resinsand composites.

Properties of a DGEBF Epoxy Resin 939



The surface fracture was obtained by a three-point flexural test. It was cut and mountedon an aluminum stab with a conductive adhesive film. When necessary, samples werecoated with gold before the morphological analysis.

Mechanical Characterization Analyses

The three-point flexural test was performed on the nanocomposite samples withdimensions 4� 6� 40mm. It was used on a Lloyd Universal Testing Machine, modelLR10K, with a span length of 23mm and crosshead speed of 1mm/min. The impact tests,according to the ASTM D 256 standard, were carried out on the nanocomposite sampleswith dimensions 5� 12.7� 64mm. It was used on a CEAST Resil Impactor 50J. For eachanalytical condition adopted, five samples were tested and the average measurements werecompared.

RESULTS AND DISCUSSION

Figure 1 shows the conversion values (�) of the pure resin as a function of reactiontime. In particular, Figure 1(a) shows the isothermal curves, in the range of 130–1708C.The conversion degree increases with increase of the temperature. It follows the typicalsigmoidal trend due to the structural and molecular variations that occur during the cureprocess. Initially the gelation process involves a progressive free volume reduction.The viscosity of the resin and the conversion quickly increase [14]. Then, with theproceeding of the cure, vitrification takes place. The cure process completely ends and theconversion curve reaches a plateau. The curves of Figure 1(a) are progressively shifted tothe left, i.e. toward lower reaction times, when the temperature increases (see the arrowshown on the picture).

The highest conversion is obtained at 1708C, therefore this temperature value has beenchosen for further investigations. The maximum conversion values (�max) are summarizedin Table 1. The �max values at 1708C ranges between 0.94 and 0.98.

1

0.8

0.6

0.4

0.2

010 100

Time (s)

1000 10000 10 100

Time (s)

1000 10000

Con

vers

ion

α

1(a) (b)

0.8

0.6

0.4

0.2

0

Con

vers

ion

α

130°C

130°C

140°C150°C160°C170°C

170°C

T=170°C

0.0

0.51.0

1.5

Figure 1. Conversion vs time of pure resin at different temperatures (a) and of nanocomposites at thetemperature of 1708C (b).

940 A. VISCO ET AL.



The sigmoidal isothermal curves, as a function of the amount of CNTs, are plotted inFigure 1(b).

It is shown that the addition of 0.5% of nanotube causes a translation of the pristineresin curve (0.0 CNT) toward the left. This result suggests that the reaction rate increases.Changes in cure kinetics can be related to several factors, such as the thermal properties ofthe carbon nanotubes and/or the presence of chemical species on CNT surfaces. The highthermal conductivity of the carbon nanotube enhances the cure kinetic even at low CNTamount. In this case, the degree of dispersion of carbon nanotubes within the epoxy matrixmust be the highest [8]. Chemical species on the CNT surface, like the –OH groups, can bepresent as consequence of the nanocomposite preparation. The –OH groups are able toreact with the epoxy resin opening the epoxidic rings [15].

The addiction of higher CNT concentrations (1.0 and 1.5%) causes, vice versa, a curvetranslation toward the right that means a lowering of the reaction rate. This result could bedue to the high viscosity of these nanocomposites that reduces the molecular mobility.As a consequence, the resin has a great difficulty to cure. Different mechanisms occur innanocomposites containing high CNT amounts. In this case the nanotube preferentiallycoalesce in agglomerates rather than react with the resin [10]. The resulting materials havea low homogeneity. This inhibits the heat dispersion into the material and the reaction ratedecreases as a consequence.

Figure 2 shows two curves of conversion at 1708C versus the carbon nanotube amount.The conversion values were checked during the cure reaction (after 800 s) and at about theend of reaction (after 3000 s). The 0.5CNT sample obtained the highest conversion valueamong all the nanocomposites studied in this article. This result evidences again that a lowCNT amount of presence is useful in order to increase the kinetic of the cure reaction,regardless of the reaction time.

Table 1. Rheological and calorimetrical parameters of pureresin and of the nanocomposites.

T (8C) �max tgel (s) �gel

0.0 CNT 130 0.86 3946 0.67140 0.87 2893 0.67150 0.90 1759 0.64160 0.93 1275 0.63170 0.94 1035 0.67

0.5 CNT 130 0.87 3154 0.60140 0.90 2145 0.62150 0.92 1566 0.60160 0.97 1128 0.57170 0.99 784 0.53

1.0 CNT 130 0.84 3928 0.64140 0.89 2638 0.66150 0.94 1791 0.67160 0.97 1409 0.70170 0.97 911 0.63

1.5 CNT 130 0.81 4891 0.62140 0.88 3028 0.64150 0.93 1981 0.63160 0.95 1607 0.64170 0.96 1059 0.62




SEM micrographs of the pure resin and of the nanocomposite are shown in Figure 3.The white area is the polymeric matrix (Figure 3(a)) and the black one is the carbonnanotube (Figure 3(b)–(d)). The polymeric progressive white areas decreased with increaseof the CNT amount. The pictures evidenced a non-homogeneous CNT distribution withinthe matrix.

The shift factors of each resin versus the temperature were plotted in Figure 4. The slopeof the interpolating line relative to the 0.5CNT sample is the lowest among all the studiedsamples, while that relative to the 1.5CNT sample is the highest one. This result suggestedthat the 0.5CNT sample had the lowest activation energy and the 1.5CNT had the highestone (Table 2). The activation energy values numerically confirm the qualitative resultsdescribed in Figure 1(b). They are in good agreement with that reported in the literaturefor other similar nanocomposite materials [16].

Figure 5(a) shows the trend of the complex viscosity (�*) versus the reaction timerelative to the 0.0CNT sample in the temperature range of 130–1708C. Initially the �*value remains constant. Then, in correspondence with the gel time (tgel), it growsexponentially. Furthermore, the gel time decrease with increase of the cure temperature,as evidenced by the directions of the arrow.

Figure 5(b) compares the rheological features at 1708C of all the studied samples (pureresin and nanocomposites). The presence of CNT in the resin induces two effects.The viscosity of the thermoset resin initially increases proportionally to the CNT amount.It is maximum in the 1.5CNT nanocomposite where work ability becomes very difficult.At high reaction times (4500 s) the viscosity quickly increases corresponding to gel time.The gel time of the resin changes depending on the CNT amount present in thenanocomposite. It decreased when it was added to a 0.5% CNT and increased whenadded to higher CNT amounts (1.0 and 1.5%). The gel time values are listed in Table 1.The tgel values at 1708C are within the 784 s–1059 s range. The tgel of the 0.5CNT sampledecreased about 24% compared with that of the pure resin, while that of the 1.5CNTincreased by about 2%.

0.50 0.5

CNT (%)

1 1.5

0.55

0.6

0.65Con

vers

ion

0.7

0.9

0.95

13000 s

800 s

Figure 2. Conversion value, after 800 s and 3000 s, as a function of the CNT content.

942 A. VISCO ET AL.



The conversion value at the gel time (�gel) of the studied samples ranged from about0.5 to 0.7 (Table I). The �gel values were calculated by interpolating the conversion valuethat corresponds to the gel time value in the plot of Figure 1. They indicate the conversionvalues at which the resin gelation occurs.

The activation energy of all investigated resins were also calculated by rheologicalanalysis data (Table 2). The values follow the same trend as the activation energy valuescalculated by means of the calorimetric analysis, described above. In any case, those

(a)

(c)

0.0 CNT (b)0.5 CNT

1.0 CNT (d)1.5 CNT

Figure 3. SEM micrographs of the sample 0.0CNT (a), 0.5CNT (b), 1.0CNT (c), and 1.5CNT (d).

1.5 CNT

0.5 CNT

1.6

1.2

0.8

0.6

00.0022 0.0023

1/T (1/K)

0.0024 0.0025

Shi

ft fa

ctor

Figure 4. Shift factor vs temperature for the pure resin.




calculated by rheological analysis are much higher than those obtained by means of thecalorimeter ones. This discrepancy can be explained by considering that the DSC tests arevery sensible to small variations in the sample volume that induces great uncertainties inthe final values [12,16].

All these results suggested that a low CNT amount in the nanocomposite (0.5%)increases the cure reaction rate without too much increase in its initial viscosity and soaffecting the resin work ability.

In Figure 6, Young’s modulus, stress at failure, and failure strain values of the pristineresin and the nanocomposites, are compared.

The elastic modulus and the failure stress increased with increasing carbon nanotubeamount (Figure 6(a, b). The average modulus and the failure stress of the pristine resinwere about 2000MPa and about 93.5MPa, respectively. The addition of 1.5% of CNTincreased the modulus of about 35%, raising to 2750MPa, and the average stress of about50%. The failure strain decreased with increasing the CNT amount (Figure 6(c)). Theaddiction of 1.5% of CNT reduces the average strain of about 28%.

Figure 7(a) shows the impact energy of the pristine resin and the nanocomposites.The material resilience decreases with increasing the CNT content. In particular thepristine resin showed an average energy absorbed value equal to 0.0058 J/mm2 and that ofthe 1.5 CNT sample was of 0.0015 J/mm2. So the addition of 1.5% of CNT to the pristineresin reduced its resilience to about 74%.

1001 1 100 1000 10000

170°C 130°C

T=170°C

170°C160°C150°C140°C130°C

1000

Time (s) Time (s)

10000

1,E+07 1000(b)(a)0.0 CNT

0.5 CNT

1.0 CNT

1.5 CNT

100

10

1

0.1

1,E+04

1,E+01

1,E−02

Vis

cosi

ty (

Pa*

s)

Vis

cosi

ty (

Pa*

s)

Figure 5. Complex viscosity vs time at different temperature of the pristine resin (a) and of thenanocomposites at the temperature of 1708C (b).

Table 2. Activation energy obtained by calorimetric and rheologicalanalysis of pure resin and of the nanocomposites.

Activation energy (kJ/mol) Activation energy (kJ/mol)Shift factor method Gel time method

0.0% CNT 51.93 52.040.5% CNT 48.43 50.871.0% CNT 55.51 52.721.5% CNT 57.37 54.97

944 A. VISCO ET AL.



These mechanical results suggested that nanotubes act as inclusion rather thanreinforcement. They make the system locally anisotropic. Stresses are concentrated inproximity of the CNT reinforcement, inducing a premature sample fracture at lower loadsthan the unmodified one. In particular, the results of impact test suggested the presence ofvoids at the interface filler/matrix, as schematized in Figure 7(b). The voids enhance thecrack propagation inside the sample. This is confirmed by the lower impact energyrequired in the CNT reinforced nanocomposites compared with the pure resin.

3000(a)

(b)

(c)

2500

2000

1500

160

120

80

40

10

8

6

4

0.0 0.5

% CNT

1.0 1.5

0.0 0.5

% CNT

1.0 1.5

0.0 0.5

% CNT

1.0 1.5

Mod

ulus

(M

Pa)

Failu

re s

tres

s (M

Pa)

Failu

re s

trai

n (M

Pa)

Figure 6. Flexural modulus (a), failure strain (b), failure stress (c), vs the CNT amount of the pure resinand nanocomposites.




Nanotubes are positioned between the resin molecules. It creates an obstacle to thesliding of molecular motion and the increase in brittleness. The polymeric matrix changedits mechanical behavior from ductile to fragile increasing the nanotube amount.

This was clearly evidenced by SEM micrographs of Figure 8. The nanocompositesshowed a rough fracture surface characterized by the presence of thick river patterns inthe crack propagation direction (Figure 7(a)). Instead, the fracture surface of a thermosetresin appears typically cleavage-like [17].

The river patterns are formed when the cleavage fracture is forced to re-initiate,via a stepwise process, in a different direction. In this way a brittle fracture appearscharacterized by many micro flow lines (Figure 8(c)) [18].

An additional fracture region can be evidenced near the edges of the nanocomposite(Figure 8(b)). Here, ‘shear lips’ are formed, as a consequence of a change of stress state

0.008(a)

(b)

0.006

0.004

0.002

0.0CNT

0.5CNT

1.5CNT

00 0.5

% CNT1 1.5

Impa

ct e

nerg

y (J

/mm

2 )

Figure 7. Impact energy versus the CNT amount of the pure resin and nanocomposites.

946 A. VISCO ET AL.



from triaxial tension (plane-strain) in the sample central region to biaxial tension (plane-stress) in the edge regions [18–19]. A micrograph of the shear lip shows a rough surfacewith extensive shear deformation areas. So, the central region of the specimens fractures ina brittle mode and the edge regions in a ductile one.

CONCLUSIONS

In this work an epoxy resin, pure and reinforced with a multi wall carbon nanotube, wascharacterized by mechanical tests and physical analyses.

The experimental results evidenced that a low CNT amount present in the epoxyDGEBF matrix (0.5%) accelerated its cure reaction rate, increased the conversion degree,and maintained its low viscosity. On the other hand, the mechanical properties were lowercompared with those of the pristine resin.

Higher CNT amounts in the epoxy matrix (1.0 and 1.5%) instead reduced the cure rateand enhanced the initial viscosity. Furthermore, they increased the material stiffness andstrength but highly reduced its resilience and deformability.

These results clearly suggested that the design of the carbon nanotube based compositesmust consider two parameters simultaneously: its work ability and mechanicalperformance. With this aim, this study evidenced the importance of the simultaneousphysical and mechanical parameters such as the viscosity, cure reaction rate, toughness,tensile strength, deformability, and stiffness.

The experimental results performed on the DGEBF based nanocomposite evidencedthat work ability and mechanical performance are in competition with each other.For an application in which high stiffness and mechanical resistance are required, thebest percentage of carbon nanotube could be high (1.0 or 1.5%). However, in thiscase the nanocomposite becomes difficult to process due to its high viscosity.

(a)

(b)

(c)

Figure 8. SEM micrographs of fracture surface after three-point flexural tests of a nanocomposite (a) andmagnification of two areas of the picture (b, c).




Alternatively, an application in which lower mechanical properties are required employs alow reinforcement amount (0.5%); in this case the reaction rate will be faster comparedwith that of the pure resin.

A compromise between the two situations described above could be obtained by using alow amount of CNT but well sealed to the matrix. This should enhance the mechanicalproperties of the nanocomposite, as well as the kinetics ones.

An oxidation process, with a mixture of concentrated nitric and sulfuric acid at differentpercentages, can be performed in order to functionalize the CNT [20,21]. With thistreatment the chain ends and external shell of CNT are activated with various oxygencontaining groups (mainly carboxyl groups) that react with the epoxy resin.

Further studies are in progress in order to enhance the interaction between the carbonnanotube and the polymeric matrix. With this aim, the CNT will be functionalized beforethe nanocomposite preparation. These samples will be mechanically compared with thenon-functionalized ones in order to discriminate if the CNT functionalization is a keyfactor for the material performance.

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Documents

Cure Rate and Mechanical Properties of a DGEBF Epoxy Resin Modified with Carbon Nanotubes