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Polymer Degradation and Stability 94 (2009) 1972–1978

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Effects of modified multi-walled carbon nanotubes on the curingbehavior and thermal stability of boron phenolic resin

Lin Liu a,b,*, Ziping Ye a

a School of Materials Science and Engineering, Tongji University, Shanghai 200092, Chinab Key Laboratory of Advanced Civil Engineering Materials, Ministry of Education, Shanghai 200092, China

a r t i c l e i n f o

Article history:Received 23 June 2009Received in revised form23 July 2009Accepted 26 July 2009Available online 30 July 2009

Keywords:Boron phenolic resinCarbon nanotubesCuring reactionThermal stability

* Corresponding author at: School of Materials SciUniversity, Shanghai 200092, China. Tel./fax: þ86 21

E-mail addresses: llsp@tongji.edu.cn, 0720060019

0141-3910/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2009.07.022

a b s t r a c t

One kind of boron phenolic resin (BPR) was prepared from the solvent-less reaction of resoles with boricacid. X-ray photoelectron spectroscopy (XPS) showed that the reaction degree of boric acid was 83.8%.Multi-walled carbon nanotubes (MWCNTs) were modified by nitric acid, 4,40-Diaminodiphenyl methaneand boric acid. The effect of modification was determined by Fourier transform infrared (FT-IR), ther-mogravimetric analysis (TGA) techniques and XPS. The cure kinetics and thermal behavior of BPR andmodified multi-walled carbon nanotubes (m-MWCNTs)/BPR were studied. It was found that the curingapparent activation energy (Ea) decreased with the increasing amount of m-MWCNTs. But there was noobvious change in the orders of curing reactions. The results of TGA showed that 1.0 wt% of the m-MWCNTscould increase the thermal decomposition temperature (Td) and the char yield of m-MWCNTs/BPRnanocomposites by 36.7 �C and 6.2%. These critical enhancements will definitely help to attract moreresearches on this area.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Common phenolic resins (PR) formed from phenol and form-aldehyde in the presence of alkali are widely used as polymercomposites and coating materials. Generally, PR is used incombination with organic or inorganic fibers and fillers. Thesecompounds possess remarkable thermal stability, flame retardancyand heat resistance properties [1–4]. The rapid development of theapplications of PR has attracted the very attention of manyresearchers to improve its related properties. To improve the flameretardancy and thermo-oxidative resistance of PR, the addition ofboron, phosphorus, silicon or other compounds has been reportedby some researchers [5–7]. BPR has excellent performances, suchas thermal stability, mechanical strength and dielectric properties[8–10].

MWCNTs are molecular-scale tubes of graphitic carbon withoutstanding properties. In particular, excellent mechanical strength,thermal stability, and thermal conductivity of MWCNTs have led toa large amount of activity in materials research for potential appli-cations. The carbon nanotubes possess high thermal conductivity,

ence and Engineering, Tongji65980009.@tongji.edu.cn (L. Liu).

All rights reserved.

which is believed to be 3000 Wm�1 K�1in multi-wall systems [11].But MWCNTs have great potential to agglomerate, because of theirtiny size and high specific surface area. The agglomeration is foundto hinder the performance of MWCNTs [12], as a result, manyresearches are taken to improve the dispersion of MWCNTs in thepolymer matrix [13–16]. To optimize the potential applications ofMWCNTs, it is essential to modify MWCNTs with functional groupsin order to integrate the MWCNTs into desired matrix.

A series of polymer matrices have been studied and greatpromotion have been reported in varied fields with the additionof carbon nanotubes. The matrices include epoxy resin, naturalrubber, polybenzoxazine and so on [17–21], but rarely BPR.

In this work, MWCNTs were firstly dealt with nitric acid tointroduce carboxyl groups. Benzene rings were then introduced toimprove the compatibility between carbon nanotubes and BPR,since the main chains of BPR consist of benzene rings and methy-lene groups. Finally, boric acid was used to bring reactive groupsonto the sidewall of carbon nanotubes. The m-MWCNTs werecharacterized by means of FT-IR, TGA and XPS. In the second part,we incorporated m-MWCNTs into BPR via in-situ polymerization.The effects of m-MWCNTs on the curing kinetics and thermalstability of the BPR are reported and discussed in detail. Our studyprovides a simple and economical method which will facilitate theindustrial realization of the m-MWCNTs/BPR nanocomposites withenhanced thermal stability.

L. Liu, Z. Ye / Polymer Degradation and Stability 94 (2009) 1972–1978 1973

2. Experiments

2.1. Materials

Phenol, 37 wt% aqueous formalin, boric acid, NaOH, acetone,toluene, 68 wt% nitric acid, N,N-dicyclohexyl carbodiimide (DCC),4,40-Diaminodiphenyl methane (DDM), and N,N-dimethylaceta-mide (DMAC) were all of analytical grade, and were supplied bySinopharm Chemical Reagent Co., Ltd. MWCNTs, Outside Diameter8–15 nm, length<50 mm, Special Surface Area>233 m2/g, ChengduOrganic Chemicals Co. Ltd. PP microporous membrane, 0.45 mm,Xingya Purification Material Co., Ltd.

2.2. Synthesis of BPR

Gao [3] reported an aqueous preparation method for the BPRin a basic medium. In the mole ratio, phenol:formaldehyde:boricacid ¼ 3:3.6:0.8. The product was solved in the solvent of buta-none and precipitates were removed. After the butanone volatil-ized completely, the powder was analyzed by FT-IR and XPSspectroscopy.

2.3. Modification of MWCNTs

2.3.1. The introduction of carboxyl groups on MWCNTs1.0 g MWCNTs were added into 200 ml 68 wt% nitric acid, and this

mixture was sonicated in a water bath for 10 min. It reacted at 70 �Cfor 24 h to form the carboxylic acid groups on the MWCNTs surface.After the mixture was cooled to room temperature, it was dilutedwith deionized water and then vacuum-filtered through 0.45 mm PPmicroporous membrane, and washed with more deionized water tillPH¼ 7. The filter cake was dried in vacuo at 60 �C for at least 24 h toyield COOH-MWCNTs.

2.3.2. The introduction of benzene rings on MWCNTs500 mg COOH-MWCNTs, 10 g DDM, 10 g DCC and 100 ml DMAC

were mixed in a 250 ml flask. After sonicated in a water bath for15 min, the system refluxed at 120 �C for 48 h, to introduce benzenerings groups on the MWCNTs surface. The mixture was washed byethanol and vacuum-filtered as mentioned above, dried, to yieldDDM-MWCNTs.

2.3.3. The introduction of boric acid on MWCNTs300 mg DDM-MWCNTs were mixed with 2.5 g boric acid, 10 g

DCC, 20 ml toluene, and 50 ml DMAC in a 250 ml flask. The mixturewas sonicated in a water bath for 15 min. After the system washeated to 90 �C, 5–10 drops of ammonia were added. The mixturewas washed with hot ethanol (60 �C) 4 h later. Finally, m-MWCNTswere obtained after vacuum-filtering and drying process.

2.4. Preparation of m-MWCNTs/BPR nanocompositesby in-situ polymerization

30 g phenol, together with the m-MWCNTs contents rangingfrom 0.25 to 1.0 wt%, was placed in a 250 ml three-necked roundbottom flask, and the mixture was sonicated in a water bath for15 min 1.6 g 40 wt% NaOH as catalyst and 31 g formaldehydewere added into the flask. The mixture was first stirred andheated to 70 �C, then held at 70 �C for 1 h, followed with waterremoval using a vacuum pump. In the second step, after 5.3 gboric acid was added to the system, the system was heated andkept between 102 and 110 �C for 40 min. Then the waterproduced was removed under vacuum, and the m-MWCNTs/BPRnanocomposites were obtained.

2.5. Characterizations

2.5.1. FT-IRInfrared spectra were recorded on a Bruker Equinoxss Four-

iertransform infrared spectrometer using a heated attenuatedtotal reflection (ATR). IR spectra were obtained using a resolution of2.0 cm�1 and 32 scans.

2.5.2. X-ray photoelectron spectroscopyXPS experiments were carried out on an RBD upgraded PHI-5000C

ESCA system (Perkin Elmer) with Mg Ka radiation (hn ¼ 1253.6 eV).The whole spectra (0–1100 eV) and the narrow spectra of all theelements with much high resolution were both recorded by using RBD147 interface (RBD Enterprises, USA) through the AugerScan 3.21software. Binding energies were calibrated by using the containmentcarbon (C1s¼ 284.6 eV). The data analysis was carried out by using theRBD AugerScan 3.21 software provided by RBD Enterprises.

2.5.3. TGAThermogravimetric analysis techniques have been used to

evaluate the performance of MWCNTs, m-MWCNTs, BPR andM-MWCNTs/BPR nanocomposites. It was carried out on the amountof 3 mg of samples, which were placed in alumina crucibles. Thesamples were heated from room temperature to 800 �C at a heatingrate of 20 �C min�1, under N2 atmosphere. The BPR and M-MWCNTs/BPR nanocomposites were cured at 180 �C for 4 h and then groundinto fine powder. All samples were dried at 60 �C under vacuum for4 h before testing.

2.5.4. Differential scanning calorimeter (DSC)The curing process of BPR and m-MWCNTs/BPR were investigated

by a differential scanning calorimeter (TA DSC Q100). Isothermal DSC:165 �C, 20 min, heating rate equals 100 �C min�1. Non-isothermal DSC:The heating rate varied from 5 to 20 �C min�1 and the range of thetemperature was from room temperature to 300 �C. The measure-ments were conducted with 2 w 3 mg samples in DSC sample pans.

3. Results and discussion

3.1. Characterizations of BPR

Fig. 1 illustrates the FT-IR absorption spectrum of synthesizedBPR. Absorbance at 1382 cm�1 should belong to borate linkage B-O. Inthe fingerprint region, peaks at 810 cm�1 and 752 cm�1 are desig-nated as the absorption of para-substituting and ortho-substitutingon benzene ring, respectively. It is obvious that the peak score of752 cm�1 is much larger than that of 810 cm�1. It means that the mainsubstituting pattern of synthesized BPR is ortho-substituting. Asa result, excellent curing property and processability of synthesizedBPR’s can be predicted.

The elemental compositions of BPR are investigated by XPS(Figs. 2 and 3). The main components in the resin are carbon,oxygen, and boron. Three characteristic peaks corresponding toB1s, C1s and O1s at approximately 193.4, 287.6 and 533.8 eV, areobserved. The elemental compositions are calculated by peak areascores and atom sensitivity factors (Table 1).

The boron atomic ratio with carbon equals 3.1%, while thetheoretical result shall be 3.7%. It indicates that the reaction extentof boric acid reaches 83.8%.

3.2. Effect of MWCNTs modification

The estimated structure of the m-MWCNTs is shown in Scheme 1.Benzene rings are introduced for the purpose of compatibility withthe main chains of BPR molecules. Also, in order to initiate the

Fig. 1. FT-IR spectrum of synthesized BPR.

L. Liu, Z. Ye / Polymer Degradation and Stability 94 (2009) 1972–19781974

interaction between m-MWCNTs and BPR, boric acid is used tomodify the MWCNTs.

The m-MWCNTs have shown in the FT-IR spectra (Fig. 4) thetypical borate linkage B–O stretching absorptions at 1380 cm�1

exactly, as well as peaks in the 1500–1600 cm�1 region due tobenzene skeleton. This result indicates that the expected groups areeffectively introduced on the surface of the m-MWCNTs via thechemical treatment.

TGA was also conducted to identify the modification result ofthe m-MWCNTs. As shown in Fig. 5, the weight loss of MWCNTssums up to 1.0% at 800 �C, which is basically due to the impuritiesand their own structure defects. While we notice the weight lossof m-MWCNTs’ at the same temperature is sharply increased to9.4%. This increase could be attributed to the grafted groups on thesurface of carbon nanotubes, which reflects that the expectedm-MWCNTs are gained.

The elemental compositions of the carbon nanotubes werecharacterized by XPS. The XPS survey spectra of MWCNTs and them-MWCNTs are shown in Fig. 6(a). The increase of oxygen elementcontent can be easily seen from the peaks at 533.8 eV. The high-resolution C1s XPS spectra of MWCNTs and the m-MWCNTs are

Fig. 2. XPS survey spectrum of the synthesized BPR.

Fig. 3. High-resolution XPS spectra of the synthesized BPR: (a) C 1s (b) O 1s and (c) B 1s.

Table 1The elemental compositions of synthesized BPR.

Bindingenergy (eV)

Height Area Atom sensitivityfactor

Atomic ratiowith C (%)

C1s 287.6 29 055 88 599 0.25 100O1s 533.8 22 776 73 465 0.66 9.7B1s 193.4 496 1417 0.13 3.1

Scheme 1. Schematic showing possible structure of m-MWCNTs.

L. Liu, Z. Ye / Polymer Degradation and Stability 94 (2009) 1972–1978 1975

shown in Fig. 6(b). Compared with that of MWCNTs, the C1s spec-trum of the m-MWCNTs shifts to higher binding energy. This slightshifting of the characteristic peaks to higher binding energy for them-MWCNTs is attributed to the presence of phenyl and carbonylgroups on the surface of m-MWCNTs. Also, the high-resolution B1sXPS spectrum (Fig. 6(c)) of the m-MWCNTs demonstrates the pres-ence of boron.

3.3. Effect of m-MWCNTs on BPR’s curing reaction

Isothermal DSC measurements were conducted to determine thekinetic parameters for the curing reactions of BPR and m-MWCNTs/BPR nanocomposites. In general, the curing kinetics of resins can becategorized as n-th order or auto-catalyzed. Reactions obeying then-th order kinetics will have a maximum reaction rate at time t z 0[22], while the maximum reaction rate of an auto-catalyzed reactionappears at 30–40% of the curing reaction and the formation ofintermediate species which initiate and accelerate the reaction [23].As shown in Fig. 7, It is obvious that the maximum reaction rate of all

Fig. 4. FT-IR spectrum of m-MWCNTs.

the three samples emerged at time t z 0. Accordingly, we concludedthat the curing kinetics of three samples should accord with the n-thorder. What’s more, we could also speculate that the addition of m-MWCNTs did not change the cure reaction mechanism of the BPRresin system.

To investigate the curing kinetics further, non-isothermal DSCmeasurements were also carried out (Fig. 8).

The n-th order reaction obeys the Kissinger equation [24]:

D

�ln

b

Tp

��D

�1Tp

�¼ �Ea

R(1)

Where bdheating rate(�C min�1); Tpdpeak temperature(K);Eadthe apparent activation energy (kJ/mol); Rdideal gas constant,8.314 J/mol K.

Table 2 demonstrates the effects of m-MWCNTs on the curinginitial temperature Ti, the curing extrapolated onset temperature To,the curing peak temperature Tp and the curing reaction heat 6H ofBPR and m-MWCNTs/BPR nanocomposites. At the same heatingrate, Ti and To increase with the addition of m-MWCNTs, while Tp

decreases. It shows that low content of m-MWCNTs may acceleratethe curing reaction of BPR. One possible reason for this may be thatcarbon nanotubes possess effective thermal conductivity.

Fig. 5. TGA thermograms of the MWCNTs and m-MWCNTs.

Fig. 6. XPS survey spectrum of the MWCNTs and m-MWCNTs (a); High-resolution C1sXPS spectra of the MWCNTs and m-MWCNTs (b); High-resolution B1s XPS spectra ofthe m-MWCNTs (c).

Fig. 7. Isothermal DSC thermograms at 165 �C for neat BPR, 0.25 wt% m-MWCNTs/BPRand 0.5 wt% m-MWCNTs/BPR nanocomposites.

Fig. 8. Dynamic DSC thermograms for neat BPR (a) 0.25 wt% m-MWCNTs/BPR (b) and0.5 wt% m-MWCNTs/BPR (c) nanocomposites.

L. Liu, Z. Ye / Polymer Degradation and Stability 94 (2009) 1972–19781976

In the actual production, resins cure under a constant temper-ature, in other words, the heating rate equals 0 �C min�1. Straightlines are fitted according to Tp under different heating rates. Theintercepts of the lines are gained as the actual curing peaktemperatures. An obvious trend can be observed in Fig. 9. The Tp atheating rate ¼ 0 declines with the addition of m-MWCNTs.

Fig. 10 shows the plot of – ln b=Tp vs. 1000=Tp for the BPR andthe m-MWCNTs/BPR nanocomposites. Lines are fitted to calculateEa. According to Kissinger equation, Ea/R equals the slope of thelines, which equals19.54, 17.68 and 15.57 respectively. As a result,The Ea value of BPR, 0.25 wt% m-MWCNTs/BPR and 0.5 wt%m-MWCNTs/BPR nanocomposites were 162.4,147.0 and 129.5 kJ/mol.Namely, the addition of m-MWCNTs promotes the curing process ofthe nanocomposites. In addition, it can be deduced that the Ea valuescalculated from the Kissinger equation exhibited good reliabilityon describing the curing kinetics of the BPR and m-MWCNTs/BPRnanocomposites, because the correlation coefficient (r2) were allgreater than 0.98.

At the moment, there is no final conclusion about the effect ofcarbon nanotubes on matrix’s curing process. Researchers whobelieve the negative effect claim that MWCNTs can hinder themobility of the reactive species, disrupting the reaction stoichiometryon the local scale [17]. As for this work, the content of m-MWCNTs isrelatively low, resulting in faint steric effect, which increases theTi and To. But the dispersion of m-MWCNTs in the BPR matrix helps totransmit heat, which decreases the Tp. Moreover, the functionalgroups on the carbon nanotubes were designed to participate in thecuring reaction.

Table 2The curing behaviors of BPR with different amounts of m-MWCNTs.

m-MWCNTscontent (wt%)

b(oC min�1)

Samplequality (mg)

Ti (oC) To (oC) Tp (oC) DH(J g�1)

0 5 2.89 118.7 131.5 151.3 74.110 2.89 124.6 137.6 157.7 71.015 2.85 126.5 138.0 161.3 77.120 2.78 135.2 141.2 163.9 71.0

0.25 5 2.92 125.0 137.3 147.4 92.510 2.83 129.0 138.0 154.4 79.815 2.68 130.6 143.1 158.5 68.720 2.85 132.6 146.4 160.9 83.4

0.50 5 2.83 129.2 138.8 145.2 85.910 2.78 135.0 140.4 152.8 78.615 2.82 137.5 145.5 157.9 84.120 2.68 140.0 149.0 160.1 71.3

Fig. 10. Relations between ln b=T2p and 1000=Tp of BPR (a) and 0.25 wt% m-MWCNTs/

BPR (b) and 0.5 wt% m-MWCNTs/BPR (c) nanocomposites.

Fig. 11. The fitting lines of Crane equation of BPR (a) and 0.25 wt% m-MWCNTs/BPR(b) and 0.5 wt% m-MWCNTs/BPR (c) nanocomposites.

L. Liu, Z. Ye / Polymer Degradation and Stability 94 (2009) 1972–1978 1977

As Ea values were gained, the curing reaction orders can also becalculated by Crane equation [25]:

dln b

d�1=Tp

� ¼ ��

Ea

nRþ 2Tp

�(2)

Where, n-reaction order.When Ea/nR >> 2Tp, Tp could be treated as zero, subsequently

Ea/nR equaled the slope of ln b vs. 1000/Tp. As shown in Fig. 11, theslopes of the lines equal �20427, �18557 and �16348 respectively.The reaction orders could be obtained as 0.956, 0.952 and 0.953 forBPR, 0.25 wt% m-MWCNTs/BPR and 0.5 wt% m-MWCNTs/BPRnanocomposites. With the addition of m-MWCNTs, the reactionorder decreased, but negligible.

3.4. Thermal behavior

The effect of the MWCNTs on the thermal stability of BPR wasstudied using thermogravimetric analysis (TGA). TGA curves forneat BPR and m-MWCNTs/BPR nanocomposites at a heating rate of20 �C/min under an inert atmosphere are presented in Fig. 12. Theresults revealed that thermal degradation of BPR and m-MWCNTs/BPRnanocomposites takes place through a one-step process. The thermaldecomposition temperatures (Td), defined as the temperature at 5%weight loss, and residual weights of the samples at 800 �C aresummarized in Table 3. The Td of BPR is about 434.0 �C, while thistemperature is shifted to higher values in nanocomposites containingm-MWCNTs. From these data, it is clear that the addition of

Fig. 9. Relations between Tp and heating rate of BPR (a) and 0.25 wt% m-MWCNTs/BPR(b) and 0.5 wt% m-MWCNTs/BPR (c) nanocomposites. Fig. 12. TGA curves of BPR with different amounts of m-MWCNTs.

Table 3Thermal decomposition temperature Td and char yield of the nanocomposites at800 �C.

Samples Td (�C) Char yield at 800 �C (%)

BPR 434.0 66.00.25 wt% m-MWCNTs/BPR 442.2 68.70.50 wt% m-MWCNTs/BPR 456.4 70.91.00 wt% m-MWCNTs/BPR 470.7 72.2

L. Liu, Z. Ye / Polymer Degradation and Stability 94 (2009) 1972–19781978

MWCNTs leads to a remarkable increase of BPR thermal stability.Analogous behavior in terms of thermal stability in inert atmo-sphere of MWCNTs/polymer nanocomposites containing carbonnanotubes has been reported in the literatures [26,27]. Thestabilization effect of m-MWCNTs is mainly attributed to goodmatrix–nanotubes interaction, nice thermal conductivity of thenanotubes, as well as their barrier effect. Likewise, the char yieldat 800 �C of the nanocomposites appears the same tendency. Withincreasing amount of m-MWCNTs loading from 0.0 to 1.0 wt%, thechar yield increases from 66.0% to 72.2%.

4. Conclusion

(1) Multi-walled carbon nanotubes were functionalized successfullywith DDM and boric acid. The m-MWCNTs/BPR nanocompositeswere obtained through a in-situ reaction. The phenyl groups onthe m-MWCNTs and possible reaction between BPR and boricacid groups enhanced the m-MWCNTs/BPR interfacial interac-tion and improved their compatibility, and this improved thethermal stability of the m-MWCNTs/BPR nanocomposites.

(2) This work showed that low levels of m-MWCNTs mightincrease the initial temperature of curing reaction. On the otherhand, the peak temperature of curing reaction decreased withthe addition of m-MWCNTs. The curing apparent activationenergy was found to decrease with the addition of m-MWCNTs.But the reaction order seemed to undertake no changes.

(3) The TGA results showed that the incorporation of 1.0 wt% of them-MWCNTs into the BPR matrix enhanced the thermal stabilityof the m-MWCNTs/BPR nanocomposites and this enhancingeffect got more pronounced when more m-MWCNTs wereused. When the content of m-MWCNTs reached 1.0 wt% of BPR,Td and the char yield increased by 36.7 �C and 6.2% respectively.

Acknowledgements

The authors sincerely thank Prof. Charles Baldwin and Ms. M.N.You for their assistance in English writing and beneficial discussion.

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