EB treatment of carbon nanotube-reinforced polymer composites

Radiation Physics and Chemistry 81 (2012) 1383–1388

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EB treatment of carbon nanotube-reinforced polymer composites G. Szebe´nyi a,n, G. Romha´ny a, B. Vajna b, T. Czvikovszky a a b

+ Department of Polymer Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Muegyetem rkp. 3, Hungary + Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, H-1111 Budapest, Muegyetem rkp. 3, Hungary

a r t i c l e i n f o

abstract

Article history: Received 3 October 2011 Accepted 8 November 2011 Available online 25 November 2011

A small amount — less than 0.5% — carbon nanotube reinforcement may improve the mechanical properties of epoxy based composite materials significantly. The basic technical problem on one side is the dispersion of the nanotubes into the viscous matrix resin, namely, the fine powder-like — less than 100 nanometer diameter — nanotubes are prone to form aggregates. On the other side, the good connection between the nanofiber and matrix, which is determining the success of the reinforcement, requires some efficient adhesion promoting treatment. The goal of our research was to give one such treatment capable of industrial size application. A two step curing epoxy/vinylester resin process technology has been developed where the epoxy component has been cured conventionally, while the vinylester has been cured by electron treatment afterwards. The sufficient irradiation dose has been selected according to Raman spectroscopy characterization. Using the developed hybrid resin system hybrid composites containing carbon fibers and multiwalled carbon nanotubes have been prepared. The effect of the electron beam induced curing of the vinylester resin on the mechanical properties of the composites has been characterized by three point bending and interlaminar shear tests, which showed clearly the superiority of the developed resin system. The results of the mechanical tests have been supported by AFM studies of the samples, which showed that the difference in the viscoelastic properties of the matrix constituents decreased significantly by the electron beam treatment. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Nanocomposite Carbon nanotube EB irradiation

1. Introduction The field of nanocomposites is a novel ever growing area in material science. After their discovery in 1991 (Iijima, 2001), carbon nanotubes (CNTs) became one of the most intensively investigated composite reinforcement material candidates (Gryshchuk et al, 2006; Thostenson et al, 2001). Although the first theoretical studies and research articles provided promising results, some key problems surfaced. The two most critical problems lowering the reinforcing potential of CNTs are their troublesome dispersability and their weak interfacial adhesion towards polymer matrices (Ma et al, 2010). Proper dispersion can already be achieved by high yield mixing methods, like three roll milling or other high shear technologies (Gojny et al, 2005; Thostenson and Chou, 2006). To improve the interfacial adhesion between CNTs and polymers the main stream of research focuses on covalent chemical functionalization, the bonding of polar side-groups to the nanotube walls by the chemical treatment (Gabriel et al, 2006; Kuzmany et al, 2004; Wu and Liu, 2010). Although good laboratory results could be obtained using plasma (Valentini et al, 2005), microwave (Wang et al, 2005)

n

Corresponding author. Tel.: þ36 1 463 1466; fax: þ36 1 463 1527. E-mail address: [email protected] (G. Romha´ny).

0969-806X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.11.015

and mechanical milling (Ko´nya et al, 2002; Koo´s et al, 2003) induced high yield functionalization processes, the key issues, which have to be mentioned are low yield and the use of excessive amounts of environmentally harmful and dangerous materials. Electron beam (EB) irradiation is already a well known technique in the field of polymer matrix composite manufacture (Chappas et al 1999; Singh, 2001; Zsigmond et al, 2003). EB irradiation can not only be used for the initiation of the curing of a thermoset matrix, but also to improve the interaction between the reinforcing fibers and the matrix. Giovedi et al. Giovedi et al. (2005) and Pino et al. Pino et al. (2007) have irradiated the reinforcing carbon fibers by EB, reported, that the EB irradiation of impregnated fiber rowing has changed their failure mode and demonstrated that thanks to the modification of the carbon fiber surface the tensile strength of the composite improved significantly. Thomas et al. investigated the effect of EB induced curing of an acrylic resin combined with difunctional coupling agents (Thomas et al., 1994). According to their results the interlaminar shear strength and fiber pull-out strength of the system improved substantially, indicating that the interaction between the reinforcing material and the matrix can be improved with a proper coupling agent. A few articles have been already published investigating the modification of CNTs by different types of irradiations (Banhart, 1999; Krasheninnikov and Nordlund, 2010; Mølhave et al, 2007; Ritter et al, 2006, Safibonab

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et al, 2011). In some of the cases the authors focused on the structure changes caused by the electron beam (EB) irradiation during TEM investigations. Researchers have cut, welded and transformed CNTs using a TEM as EB source (Zou et al, 2002; Yang et al, 2006). Chen et al. have reported the swelling of CNTs in the presence of carbohydrates, which bonded to their surface (Chen et al, 2001). EB irradiation has been evaluated as a method for the sidewall functionalization of CNTs by maleic anhydride (Lu et al, 2010), polyamide (Yu et al, 2007), several vinyl monomers (Yang et al, 2010). The direct electron irradiation of the CNT containing polymer composites (Khalid et al., 2010; Martı´nez-Morlanes et al, 2011) can be a possible alternative to the low yield chemical functionalization processes. During the EB irradiation the bonds in the irradiated materials (both the CNTs and the matrix) can be broken and after recombination, some bonds between the CNTs and the matrix can be formed. With the presence of an active component, containing unsaturated bonds, like uncured vinylester resin, the probability of producing such links between the CNTs and the matrix can be raised. In our work we have developed a two step curing epoxy/ vinylester (EP/VE) hybrid resin system, in which the EP component has been cured conventionally and the curing of the VE component has been initiated by high energy EB irradiation. Our research hypothesis was, that such matrix makes the formation of bonds between CNTs and the matrix possible, thereby improving the interfacial adhesion between the reinforcement material and the matrix, and through this the mechanical properties of the prepared hybrid nanocomposites.

The two component VE/EP hybrid resins have been mixed using a conventional overhead stirrer for 2 h. Zoltek PX35FBUD0300 (Zoltek, Hungary) unidirectional carbon fabric was used as microsized reinforcement. The fabric consisted of Panex 35 50 k rovings and had a surface weight of 309 g/m2. Two adhesion promoter agents, Byk Anti-Terra-U and Byk Disperbyk 2050 have been used as reference materials in 1:1 adhesion promoter-nanoparticle weight ratio. MWCNT/CF/EP and MWCNT/CF/EP/VE hybrid composite specimens with different vinylester contents have been prepared by hand lamination assisted by the vacuum bag technology. For the bending and interlaminar shear tests 6 layers of carbon fabric with 01 orientation has been used. After 48 h room temperature the specimens have been cured in a Heraeus UT20 drying oven at 80 1C for 4 h. Electron irradiation has been performed at FEMA Ltd. using a LUE-8-5V 8 MeV maximal electron energy, LINAC type, high frequency electron accelerator at athmospheric conditions. The dose rate was 10 Gy/s to minimize the degradation of the polymeric resins. The Raman spectra have been recorded using a Jobin Yvon LabRam Raman microscope with a doubledfrequency Nd:YAG and a diode laser. The atomic force microscopic (AFM) tests have been performed using a TA Instruments mTA 2990 Micro-Thermal Analyzer AFM with Microscopes SFM 1650-00 silicone tip probes in noncontact and tapping operation. Three point bending, and interlaminar shear tests were carried out according to EN ISO 14125 and ASTM D3846-94, respectively, using a Zwick Z020 computer controlled loading frame.

2. Experimental 3. Results and discussion For our research AH-12 low viscosity epoxy resin was used (PþM Polimer Ke´mia, Hungary) with T-58 curing agent (PþM Polimer Ke´mia, Hungary) as matrix. The recommended mixing weight ratio was 100:40. Viapal VUP 4652/67 (Cytec, USA) epoxy novolac based vinylester resin has been used as reactive coupling agent. Baytubess C 150 HP (Bayer, Germany), multiwalled carbon nanotubes were used as nanosized reinforcement. The nanotubes were produced by catalytic CVD method, which resulted in loose aggregates, no functionalization has been used. The nanotubes were 13–16 nm in outer mean diameter, above 1 mm in length, above 99% carbon purity content and free of amorphous carbon. The MWCNTs were mixed to the matrix by masterbatch mixing in 0.3 wt% proportions to the epoxy or in case of coupling agent containing samples, to the vinylester component of the resin.

In the first step of the research the necessary irradiation dose had to be declared for the full curing of the vinylester component of the hybrid resins. Cast resin specimens have been prepared with 1, 5 and 10 wt% vinylester content and irradiated by 25, 50 and 100 kGy dose EB irradiation at 10 Gy/s dose rate. The specimens have been investigated by Raman spectroscopy. The Raman spectra of the specimens before and after the EB irradiation can be seen in Figs. 1 and 2, respectively. In the spectra of the untreated, VE containing samples, the intensity peaks corresponding to the unsaturated double bonds of the neat VE content can be easily located at 1630 cm  1 Raman shift, which indicates that the VE component remained reactive after the conventional curing of the EP component of the resin.

Fig. 1. Raman spectra of the specimens before EB irradiation (from top to bottom: 10, 5, 1 and 0 wt% VE content EP samples).

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Fig. 2. Raman spectra of the 10 wt% VE containing EP specimens after EB irradiation (from top to bottom: 0, 25, 50 and 100 kGy irradiation dose).

The relative intensity of the peak is increasing with the increasing VE content. This peak has been selected to follow the curing process of the VE component of the resin. In the spectra of the irradiated samples the peak at 1630 cm  1 Raman shift almost completely disappeared even at the lowest, 25 kGy irradiation dose. The relative intensity did not change significantly with the increasing irradiation dose, so we can assume that the curing was complete even in this case. To avoid unnecessary degradation, the lowest, 25 kGy dose has been selected for the preparation of the composites. To provide a sufficient amount of reactive component for the EB irradiation of the composites, the specimens for the further tests have been prepared with 10, 30 and 50 wt% VE contents. To characterize the effect of the EB treatment on the mechanical properties of the prepared composites, five kinds of composite specimens have been prepared differing only in matrix material and treatment: neat epoxy matrix composites, composites with uncured (EPþVEþNT), conventionally cured (EPþ VEþAKT) and EB cured (EPþVEþNTþEB) vinylester containing epoxy matrix. A conventional composite, free of MWCNTs has been prepared and irradiated with EB (EPþVEþEB) as a reference, to be able to characterize the effect of EB on the interaction of the matrix and the nanotubes. Also two conventional adhesion promoting agents have been also tested (EPþNTþATU, EPþNTþ DB) to compare their performance to the EB cured vinylester/epoxy system. Three point bending and interlaminar shear tests have been performed on the composites. The results of the three point bending tests can be seen in Figs. 3 and 4. In both the bending strength and the modulus the beneficial effect of the EB curing of the VE component of the hybrid resin system can be clearly observed. In case of the EB curing bonds can not only be formed to form the crosslinked molecule of the VE, but also between the matrix molecules, and between the reinforcing nanoparticles and fibers. In case of the conventional curing the VE molecules can only connect to each other, the interpenetrating network structure is only held together loosely by the entanglement of the two thermoset molecules, in case of the EB cured system, primary bonds can be formed between the two molecules enhancing the mechanical properties and stability. In case of the EB cured system mainly the VE, but to some extent also the EP component of the matrix can bond to the dispersed MWCNTs and also to the carbon fibers. With the increasing VE content the number of unsaturated bonds, mainly providing these crosslinks also rises. This can be the cause of the increasing

Fig. 3. Bending strength of the composites and hybrid composites.

Fig. 4. Bending modulus of elasticity of the composites and hybrid composites.

difference between the properties of the MWCNT containing hybrid composite (EPþVEþNT þEB) and the composite without MWCNTs (EPþVEþ EB). The decrease of both the bending strength and the modulus can be observed with the increase of VE content in case of the uncured and conventionally cured VE containing composites. This can be explained by the formation of a tougher, less stiff VE phase in the EP resin, which weakens the structure. This less stiff phase reaches the EP phase in terms of mechanical properties with the formation of the cross-links in case of the EB treated samples, so it no longer functions as a weak point in the structure. In case of the MWCNT containing EB treated sample both the bending strength and modulus values

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increased with the increasing VE content, which can be subjected to the improved interfacial adhesion between the matrix and the nanoparticles. The samples prepared using the conventional adhesion promoters proved to be inferior to the samples prepared using the EB cured hybrid resin. The results of the interlaminar shear tests showed similar results (Fig. 5), same phenomena, as the bending results. The interlaminar shear strength steadily decreased with the increasing VE content in case of both the uncured and the conventionally cured VE containing samples. The properties of the sample not containing MWCNTs and treated with EB remained the

Fig. 5. Interlaminar shear strength of the composites and hybrid composites.

same with the change in VE content, while the interlaminar shear strength of the hybrid composite treated with EB increased with the VE content even more than in case of the bending properties. This can be because that the interlaminar, mainly matrix dominated properties are more sensitive to the adhesion between the components. The adhesion did not only improve between the two resin components, but also between them and the carbon fibers and the MWCNTs thanks to the EB irradiation. The conventional adhesion promoters did not change the properties of the reference material significantly, so in this case also only the developed hybrid matrix system, containing active coupling agent could effectively improve the interfacial adhesion. To further evaluate the structure of the prepared samples, AFM studies have been carried out. The AFM images of the tested specimens can be seen in Fig. 6. In case of the neat epoxy matrix composite and the uncured VE containing epoxy matrix composite, only one phase could be observed, the inhomogenities observed are only caused by surface contaminants and imperfections. In case of the cured VE containing samples the two matrices with different moduli of elasticity can be distinguished. Higher resolution images of these samples can be seen in Fig. 7. In the higher resolution images smooth phase composition can be observed, which is much more detailed in case of the EB cured sample. The total phase range measured in case of the EB cured VE containing sample is much lower compared to the conventionally cured VE containing sample. The phase shift value is directly related to the viscoelastic properties of the tested materials. The wide range

Fig. 6. AFM phase images of (a) neat epoxy matrix, (b) uncured, (c) conventionally cured, (d) EB cured 50 wt% VE containing epoxy matrix 0.3 wt% MWCNT content hybrid composite samples.

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Fig. 7. AFM phase images of (a) conventionally cured and (b) EB cured 50 wt% VE containing epoxy matrix 0.3 wt% MWCNT content hybrid composite samples.

in case of the conventionally cured VE containing sample indicates larger difference between the elastic moduli and the damping properties of two components (a tough VE and a brittle epoxy has been used). In case of the EB cured VE containing sample, the difference is much smaller, which was caused by the EB irradiation initiated increase in crosslink density of the materials and on the other hand by the EB irradiation initiated crosslinks formed between the two phases (some of the dissolved bonds could be recombined as bonds connecting the two phases). This result also supports that not only the initiation of the curing of the VE component has been successful, but also the interfacial adhesion between the two phases could be improved.

4. Conclusions According to our test results it can be declared that the application of the developed two-step curing VE/EP hybrid resin system bears a high potential as a matrix material used in nanocomposites. According to the Raman spectrometry results, the electron irradiation initiated curing of the vinylester component of the hybrid matrices has been successful even at 25 kGy. The relative intensity of the peaks in the Raman spectra of the epoxy resin did not change, which indicates no degradation. According to the mechanical test results, the addition of the EB cured vinylester had significant positive effect on the mechanical properties of the hybrid composites. The AFM test showed significantly lower total phase range difference in case of the EB cured system, which indicates that the improvement of the mechanical properties was probably caused by the improvement of interaction between the nanotubes and the matrix.

Acknowledgments The authors would like to sincerely thank Bayer Material Science AG. and Bayer Hungaria Ltd. for the donation of the + Baytubess, Dr. Ma´ria Body and FEMA Ltd. for the support, consultation and the EB irradiation. This work was supported by the Hungarian Scientific Research Fund (OTKA F67897 and NI62729) and Ja´nos Bolyai Research Scholarship of the Hungarian Academy of Sciences. This work is connected to the scientific program of the ‘‘Development of quality-oriented and harmonized RþDþ I strategy and functional model at BME’’ project. This project is supported by the New Sze´chenyi Plan (Project ID: TA´MOP-4.2.1/B-09/1/KMR-2010-0002).

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