12 Nanostructured Polymer Composites with Modified

12 Nanostructured Polymer Composites with Modified Carbon Nanotubes A.P. Kharitonov1,2, A.G. Tkachev2, A.N. Blohin2, I.V. Burakova2*, A.E. Burakov2, A.E. Kucherova2 and A.A. Maksimkin3 Branch of the Talrose Institute for Energy Problems of Chemical Physics of the Russian Academy of Sciences, Chernogolovka, Moscow, Russia 2 Department of Technologies and Equipment for Nanoproduction, Tambov State Technical University, Tambov, Russia 3 National University of Science and Technology ‘MISIS’, Moscow, Russia

1

Abstract

This investigation describes the influence of the modified carbon nanotubes (CNTs) on polymer composites properties. Epoxy resin (diglycidyl ether bisphenol-A type) polymer materials with added pristine and fluorinated CNTs were studied by Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. Flexural strength and composites tensile were measured. CNTs fluorination markedly (by a factor of 2.26) increased its specific surface. Fluorination did not influence CNTs thermal stability below 260 °C and did not worsen thermal stability of filled composites. Flexural strength of composite filled with 0.2 weight % fluorinated at 150 °C CNTs was increased to 199.7 ± 4.8 MPa (+58% as compared with unfilled composite). Insertion of 0.1 weight % of CNTs fluorinated at 150 °C into polymer resulted in the composite tensile strength increase to 89.6 ± 4.1 MPa (35% increase as compared with unfilled composites). Obtained reinforcement values exceeded all of the literature data reported for composites based on epoxy resins similar to that used in the current work. Pristine CNTs were less effective in composite tensile and flexural strength improvement as compared with fluorinated CNTs. Insertion of fluorinated CNTs into a polymer increased glassy temperature and did not influence the composites’ thermal stability. The reinforced composites can be applied in aviation, automotive, wind turbine propeller blades, etc. Keywords: Metal-oxide catalyst, chemical vapor deposition, carbon nanotubes, polymers, particle-reinforced composites, mechanical properties, direct fluorination

*Corresponding author: [email protected] Vijay Kumar Thakur, Manju Kumari Thakur and Michael R. Kessler (eds.), Handbook of Composites from Renewable Materials, Volume 7, (381–408) © 2017 Scrivener Publishing LLC

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382 Handbook of Composites from Renewable Materials-Volume 7

12.1 Introduction 12.1.1 Polymer Materials and Their Application Commonly used polymers have many advantages, such as low cost, processability, but they have also a lot of disadvantages (often poor adhesion, poor printability and b arrier properties, low chemical resistance, etc.). It is possible to fabricate the whole article from specialty polymers, e.g., fluorine-containing polymers, which have improved commercial properties. Fluorinated polymers have a set of unique properties such as enhanced chemical stability, thermal stability, and good barrier properties (Lagow & Margrave, 1979; Kharitonov, 2007, 2008; Anand et al., 1994; Kharitonov et al., 2005; Prorokova et al., 2015). However, practical use of specially synthesized polymers, such as fluorine-containing polymers, is restricted due to their high cost and complexity of synthesis (Kumar Thakur & Kessler, 2015). But very often application properties of polymer goods are defined mainly by their surface properties (Kumar Thakur et al., 2014; Kumar Thakur & Kumari Thakur, 2014). Hence, it is not necessary to fabricate articles from fluoropolymers but simpler, cheaper and more convenient to apply a surface treatment of articles made from commonly used polymers. In this case, the direct fluorination can be effectively used (Kharitonov et al., 2005, 2007; Carstens et al., 2000; Taege & Ferrier, 2006). Direct fluorination of polymers is a heterogeneous reaction of gaseous F2 and its mixtures with a polymer surface. This is a method of the surface modification: for majority of glassy polymers only upper surface layer is modified (~0.01–10 μm in thickness), but the bulk properties remain unchanged. Since fluorination is one of the most effective chemical methods to modify and control physicochemical properties of polymers over a wide range, this process has become an important tool of great interest. Direct fluorination has many advantages when used in industry. Due to the high exothermicity of the main elementary stages fluorination proceeds spontaneously at room temperature with sufficient for industrial applications rate. Direct fluorination is a dry technology. Polymer articles of any shape can be treated. There are safe and reliable methods to neutralize (by converting into the solid phase) unused F2 and the end-product HF. These features of the direct fluorination initiated its wide industrial utilization for enhancement of the barrier properties of automotive polymer fuel tanks and vessels for storage of toxic and volatile liquids (Tressaud et al., 2007). Gas separation properties of polymer membranes can be also highly enhanced by direct fluorination (Kharitonov et al., 2011; Kharitonov, 2012; http://www.binep. ac.ru). Adhesion properties of polymer articles of any shape can be substantially improved (Kharitonov et al., 2015). Also coefficient of friction can be reduced (Murali et al., 2012; Peyroux et al., 2015). Direct fluorinated was very effective in improvement of antibacterial properties of nonwoven polypropylene fabric. A one-step ‘dry’ method of the direct fluorination was applied to highly improve antibacterial properties of polypropylene nonwoven fabric. A treatment of polypropylene nonwoven fabric with F2/N2 mixture highly improved the barrier antibacterial properties with respect to test bacteria—Gram-positive Staphylococcus aureus (total reproduction suppression) and partially for Gram-negative Escherichia coli. A treatment with F2/N2/O2 mixture totally suppressed Candida albicans microfungus

Nanostructured Polymer Composites with Modified Carbon Nanotubes 383 reproduction. Polypropylene nonwoven fabric tensile strength was slightly increased after fluorination. Polymeric membranes also can be used for the separation of gas mixtures such as He–CH4, H2–CH4, CO2–CH4, and H2–N2. There is, however, a common problem in a gas separation when a polymeric membrane is used: membranes with high gas permeability often have low gas separation factor and on the contrary membranes with high separation factor have low permeability (Figure 12.1) (Robeson, 2008). The direct fluorination can be effectively used to improve gas separation properties of polymer membranes when the gas mixture consists of gases with markedly different gas kinetic diameters. In this case substantial increase of separation selectivity (up to several tens times for the case of He/CH4 mixture) is accompanied with a relatively small decrease (or no change) of permeability of a gas with smaller gas kinetic diameter (He, H2 etc.). Direct fluorination of polymers is a heterogeneous reaction of gaseous F2 mixtures with a polymer surface. This is a method of the surface modification: only upper surface layer is modified (~0.01 to several microns in thickness), but the bulk properties (e.g., tensile strength) remain unchanged. The direct fluorination proceeds spontaneously at room temperature with sufficient for industrial applications rate. Fluorination results in a substitution of H-atoms for F-atoms, saturation of double (conjugated) bonds with fluorine, disruption of majority of C–N and C–Si bonds followed with formation of C–F bonds (Kharitonov, 2012; http: //www.airproducts.org). The chemical composition of fluorinated layer depends on composition and pressure of fluorinating mixture and treatment duration. Treatment at mild fluorination conditions does not cause disruption of C–C bonds in the main polymer chain. Direct fluorination is a dry technology. Polymer hollow fibers, fabricated membrane modules and composite membranes can be treated. For the case of hollow fibers and composite membranes only the dense separation layer can be fluorinated and the porous

4

Lg (a(He/CH4))

3 2 1 0 –1

–1

0

1 2 3 Lg(PHe), barrer

4

5

Figure 12.1 Separation selectivity α for the He/CH4 mixture versus permeability of He for various polymer membranes in logarithmic scale. Filled points—literature data (Robeson, 2008). Empty triangle, square and diamond correspond to pristine polyamide Matrimid® 5218 hollow fiber module (points correspond to different treatment conditions), PVTMS (Kharitonov, 2007, 2008) and PTMSP (Langsam et al., 1988) flat membrane. Filled symbols represent transport properties of fluorine-treated membranes.

384 Handbook of Composites from Renewable Materials-Volume 7 support will remain untouched so the tensile strength of membrane element will not be decreased. There are safe and reliable methods to neutralize (by converting into the solid phase) unused F2 and the end-product HF (Kharitonov, 2012). The direct fluorination was used to enhance gas separation properties of several polymer membranes (both homogeneous and composite) and hollow fiber modules: polyimide (PI), polyvinyltrimethylsilane (PVTMS), poly(1-trimethylsilylpropyne) (PTMSP), poly(phenylene oxide), polysulfone, poly(4-methyl-pentene), polycarbonatesiloxane, etc. (Langsam et al., 1988; Le Roux et al., 1994; Amirkhanov et al., 1998; Kharitonov, 2007, 2008). Figure 12.1 illustrates the influence of direct fluorination on separation selectivity for He/CH4 mixture. As it is evidenced in Figure 12.1, direct fluorination results in a very remarkable increase (by a factor of several tens times or more than hundred times) of separation selectivity. The permeability of He and H2 is not practically changed after fluorination. Hence the direct fluorination of PVTMS and PI Matrimid 5218 provides the possibility to ‘overjump’ the Robeson boundary (straight line in Figure 12.1). The direct fluorination can substantially improve the separation selectivity of CO2/CH4, He/N2, and He/CH4 mixtures (Figure 12.2) (Kharitonov, 2012). The investigations of reinforcement of polymer composites by insertion of fluorinated high modulus polymer fibers were carried out by authors. The possible commercial applications of the enhancement of adhesion are as follows: enhancement of adhesion of polymer fibers and fabric to rubber, improvement of dye ability of polymer goods, reinforcement of polymer composites, and increased resistance to delamination in coated flexible films (Anand et al., 1994). Direct fluorination of fibrous plastics was used to strengthen and reinforce composite materials, such as cementitious and metal components (Carstens et al., 2000; Carstens et al., 1998; Carstens et al., 1999). Adhesive bonding of various polymers, such as polypropylene, polybutyleneterephthalate and its

1200

Separation selectivity

1000 800 600 He/CH4

400

He/N2

200 CO2/CH4 0

Figure 12.2 Influence of treatment conditions of PVTMS flat membrane on the separation selectivity of CO2/CH4, He/N2, and He/CH4 mixtures. Treatment condition (from left to right in each group at the plot): virgin PVTMS, treatment with 2%F2 + 98%He mixture, treatment with 33%F2 + 67%He mixture, treatment with 2%F2 + 98%He mixture followed by a grafting of acrylonitrile, and treatment with 60%F2 + 40%O2 mixture.

Nanostructured Polymer Composites with Modified Carbon Nanotubes 385 blends, polyetheretherketone (Carstens et al., 1998; Green et al., 2002; Kruse et al., 1995) can be improved by fluorination. Oxyfluorination (i.e. treatment with F2–O2 mixtures) results in a better (as compared with treatment without oxygen) adhesion of polypropylene and such an effect is long lasting (Taege & Ferrier, 2006; Milker & Koch, 1989). The influence of fluorination of one of the components of polymer-fiber composite can be demonstrated by the following example (Mukherjee et al., 2006). Short-fiber reinforced polymeric composites are important due to the advantages in outstanding mechanical properties, low cost and processing. P-phenylene terepthalamide (i.e., KevlarÒ), is well-known synthetic polymeric fiber for high-performance composite applications due to its high specific strength, high modulus, high thermal and chemical resistance, and low electrical conductivity when compared to metallic or carbon glass fibers. However, a poor interfacial adhesion affects the chemical and thermal properties of the composites. Thermal and mechanical properties of composites fabricated from Kevlar fiber, modified by direct fluorination and oxyfluorination, and ethylene–propylene (EP) co-polymer were studied in Mukherjee et al. (2006) (Table 12.1). Kevlar fibers were fluorinated and oxy-fluorinated by 5%F2+95%He mixture (for fluorination) and 5%F2+1%O2+90%He+4%N2 (for oxyfluorination), respectively, under 0.8 bar pressure for 30 min at 17 °C. Ethylene–polypropylene (in 100%) was mixed with 1.43% of original, fluorinated, and oxyfluorinated Kevlar fibers differently in Brabender mixer with 60 rpm at 200 °C for 10 min. Then, the mixtures were cured in hydraulic press at 200 °C and at 10 MPa pressure for 10 min. It was shown that the thermal and mechanical Table 12.1 Influence of fluorination on polymer composite properties.

Composite

First Weight decomposition loss at temperature first step (°C) (%)

Tensile strength (MPa)

Tensile modulus (GPa)

Virgin EP

233.0

69.2

27

0.36

EP+Kevlar

245.0

32

20

0.40

EP+fluorinated Kevlar

254.4

39

30

0.56

269.0 (+36 °C as compared with virgin EP)

54

EP+oxyfluorinated Kevlar

Elongation at break (%)

33 (+22%) 0.68 (+89% as compared with virgin EP)

Virgin LDPE

8.8

90.4

LDPE + virgin UHMWPE fibers

11.2

18.2

21.2 Increase by a factor of 2.4

8.1 Decrease by a factor of 11

LDPE + 10% of fluorinated ultra-highmolecular-weight polyethylene (UHMWPE) fibers

386 Handbook of Composites from Renewable Materials-Volume 7 properties of composite material based on EP copolymer reinforced with KevlarÒ fibers can be markedly enhanced under oxyfluorination of KevlarÒ fibers. Addition of only 1.4 weight % of oxyfluorinated KevlarÒ fibers to EP copolymer results in increase of the first decomposition temperature of the composite material by 36 °C, tensile strength by 22%, and tensile modulus by 89%. More pronounced effect was obtained for the case of a composite on the base of LDPE and 10 % (wt) of ultra-high-molecular-weight polyethylene fibers. Fluorination of fibers resulted in increase of the composite tensile strength by a factor of ~2 and tensile modulus by a factor of ~1.5 (Table 12.1) (Maity et al., 2008). We have studied the influence of the carbon nanotubes (CNTs) and multilayered graphene direct fluorination on polymer composites mechanical properties (Kharitonov et al., 2015). CNTs ‘Taunit-M’ and multilayered graphene were obtained from ‘Nanotechcenter Ltd.’ (Tambov, Russia). The UHMWPE were used as a polymer matrix. In many cases, low polarity of the carbon nanomaterials (CNMs) surface and also polymer matrix adhesion between fillers and polymer matrix is too low to provide marked reinforcement (Wu & Chou, 2012; Paul & Robeson, 2008; Potts et al., 2011). Moreover, CNTs tend to agglomerate in a polymer matrix (Liu & Wagner, 2005). To increase surface energy and adhesion properties fillers should be modified. A wide variety of modification methods are applied to modify CNMs: plasma/plasma chemical treatment, monomers grafting, acid treatment, etc. Review of the influence of those methods on the reinforcing properties of various fillers will be presented below. One of the most prospective methods is the direct fluorination, i.e., treatment with gaseous fluorine at elevated temperature (Mickelson et al., 1999; Krestinin et al., 2009; Zhang et al., 2010; Ahmad et al., 2013; Shulga et al., 2011). For the case of thermoplastic polymers insertion of fluorinated single-walled CNTs (SWCNTs) into polymer matrix (1, 0.5 and 10 weight % of SWCNTs in polyethyleneoxide, polyamide-6 and polypropylene respectively) resulted in increase of the tensile strength by a factor of 3, 3.3 and 2.7 respectively (Geng et al., 2002; Rangari et al., 2008; McIntosh et al., 2006). Module was highly increased also. The effect of nonfluorinated SWCNTs was much less pronounced. In (Davis et al., 2010) bisphenol A epichlorohydrin-based epoxy resin (similar to that used in our research) was reinforced by insertion of 0.5 weight % of SWCNTs, double-, and multiwalled CNTs (MWCNTs) mixture in a laminate composed from epoxy resin and carbon fabric. Reinforcement has been resulted in an increase of the tensile strength and module by 18% and 24%, respectively. In our case, CNTs were fluorinated in gaseous fluorine atmosphere at temperatures 150 and 250 °C and fluorine pressure 0.7–0.9 atm. Multilayered graphene was treated at 350 °C and fluorine pressure 0.5–0.6 atm. We have shown that fluorination did not influence thermal stability of CNTs below 300 °C. Insertion of fluorinated CNMs into UHMWPE did not worsen thermal stability of filled composites. Fluorinated CNTs and multilayered graphene were more efficient in reinforcement (both tensile strength and flexural strength) of polymers as compared with virgin (untreated) CNTs. Fluorinated at 250 °C CNTs did not result in composite reinforcement. The best reinforcement was obtained when CNTs were fluorinated at 150 °C. Multistage process including insertion of fluorinated CNTs in UHMWPE matrix followed by a hot pressing and orientation pulling at room temperature was applied (Kharitonov et al., 2015). It was shown that fluorinated CNTs are more efficient in reinforcement than

Nanostructured Polymer Composites with Modified Carbon Nanotubes 387 pristine CNTs. Below 250 °C fluorinated CNTs did not influence composite thermal stability. Scanning electron microscopy (SEM) study confirmed that fluorinated CNTs can act as crystallization centers and fluorinated CNTs exhibit improved adhesion to polymer matrix as compared with pristine CNTs. Composites with inserted fluorinated CNTs have a block structure with high concentration of nanofibrils. The applied procedure resulted in an increase of the composite tensile strength from 21 to 132 MPa or by a factor of 6.3 as compared with pristine bulk UHMWPE. Previously reported in available literature tensile strength values of reinforced bulk UHMWPE did not exceed 40 MPa. Results of the research may permit the articles weight decrease without strength loss or the strength increase accompanied with no weight increase. Reinforced composites can be applied for several industries: aviation, automotive, wind turbine propeller blades, for producing yachts and boats, etc.

12.1.2 Carbon Nanotubes Application and Their Main Properties As mentioned above CNMs especially SWCNTs and MWCNTs are widely used as fillers to reinforce polymer composites (Davis et al., 2010). CNMs discovered at the end of the 20th century have found practical applications in electronics, construction industry, chemical, and other industries. A unique combination of the properties of CNTs, such as small size, large surface area, and chemical and thermal stability, as well as the ability to participate in a variety of chemical transformations, high strength, good emission characteristics of semiconducting nanotubes, and the high electrical conductivity of CNTs with metal type conductivity, are of f undamental interest and present broad prospects for use in the technical and technological purposes in the innovation fields of science and industry (Badamshina et al., 2010). To understand the structure and main properties of the CNTs, we consider CNTs under trade mark ‘Taunit’ [‘NanoTechCenter’ Ltd. (Tambov, Russia)]. These products represent MWCNTs—quasi-one-dimensional, nanoscale, polycrystalline, filamentous graphite cylindrical formations with internal channels. Their characteristics are given in Table 12.2, and scanning electron micrographs of their surfaces are presented in Figure 12.3. The distance between sheets is about 0.34 nm. Table 12.2 Principal characteristics of the industrially produced ‘Taunit’ nanoproducts. CNTs Parameter

Taunit

Taunit-M

Taunit-MD

Taunit-4

External diameter, nm

20–70

8–15

30–80

4–8

Internal diameter, nm

5–10

4–8

10–20

1–2

≥2

≥2

≥20

≥100

Total amount of impurities, %: initial (after purification)

≤5 (≤1)

≤5 (≤1)

≤5 (≤1)

≤5 (≤1)

Bulk density, g cm–3

0.4–0.6

0.03–0.05

0.03–0.05

0.03–0.05

Specific surface area, m g

≥120–130

≥300–320

180–200

650

Thermal stability in air, °C

≤600

≤600

≤600

≤600

Length, µm

2

–1

Storage conditions are not controlled; purification method: coarse acid cleaning; nanocarbon content: ≥95 %.


(a)

(b)

(c)

(d)

Figure 12.3 Scanning electron micrographs of the CNTs ‘Taunit’ (a), ‘Taunit-M’ (b), ‘Taunit-MD’ (c), and ‘Taunit-4’ (d).

CNTs are unique nanostructures which are known to have remarkable electronic, thermal, optical, mechanical, etc. properties. These characteristics have sparked great interest in their possible uses for nanoelectronic and nanomechanical devices. CNTs are predicted to have high stiffness and axial strength as a result of the carbon–carbon sp2 bonding. Studies exploring the elastic response, inelastic behavior and buckling yield strength and fracture need to be conducted to find practical uses of the nanotubes (Paradise & Goswami, 2007). The mechanical properties of a solid must ultimately depend on the strength of its interatomic bonds. Experimental and theoretical results have shown an elastic modulus of greater than 1 TPa (that of a diamond is 1.2 TPa) and have reported strengths 10–100 times higher than the strongest steel at a fraction of the weight. It has been predicted that CNTs have the highest Young’s modulus of all different types of composite tubes. In general, the strength of the chemical bonds determines the actual value of Young’s modulus and smaller diameters result in a smaller Young’s modulus (Paradise & Goswami, 2007). However, in tests conducted on CNTs show that little dependence exists on the diameter of the tube with Young’s modulus, which does help to hypothesize that CNTs do possess the highest Young’s modulus. Experiments conducted have resulted in tensile strengths in the range from 11 to 63 GPa, with dependence on the outer shell diameter, which is not far from the theoretical yield strength of 100 GPa (Paradise & Goswami, 2007). Due to high in-plane tensile strength of graphite, both single and MWCNTs, are expected to have large bending constants since they mostly depend on Young’s modulus. The nanotube has been found to be very flexible. It can be elongated, twisted, flattened, or bent into circles before fracturing. Simulations conducted by Bernholc and colleagues indicate it can regain their original shape. Their ‘kink-like’ ridges allow the

Nanostructured Polymer Composites with Modified Carbon Nanotubes 389 structure to relax elastically while under compression, unlike carbon fibers which fracture easily. The unique elastic and inelastic properties have brought about more studies on the durability of CNTs. For SWCNTs simulations of deformations showed that each shape change corresponded directly to an abrupt release in energy and a singularity in the stress/strain curve. The nanotubes were found to have an extremely large breaking strain which decreased with temperature (Paradise & Goswami, 2007). For MWCNTs, the properties were a little more complicated to calculate. An empirical lattice dynamics model was used, which showed that MWCNTs were insensitive to parameters such as the chirality, tube radius, and the number of layers. Thermal properties including specific heat and thermal conductivity of CNTs are determined primarily by the phonons. A phonon is a quantum acoustic energy similar to the photon. Phonons are a result of lattice vibrations observed in the Raman spectra. Especially at low temperatures the phonon contribution to these quantities dominates and is due to the acoustic phonons (Paradise & Goswami, 2007). The measurements of thermoelectric power of nanotube systems give direct information for the type of carriers and conductivity mechanisms. Theoretical and experimental results show superior electrical properties of CNTs. They can produce electric current carrying capacity 1000 times higher than copper wires. The electronic capabilities possessed by CNTs are seen to arise predominately from interlayer interactions, rather than from interlayer interactions between multilayers within a single CNT or between different nanotubes (Paradise & Goswami, 2007). These optical properties have proved to be especially unique with capabilities of acting as either a metallic or semiconductor, which depends on tubule diameter and chiral angle. Metallic conduction can be achieved without introduction of doping effects. For semiconducting nanotubes the band gaps have been found to be proportional to a fraction of the diameter and without relation to the tubule chirality (Paradise & Goswami, 2007). The unique porosity of SWCNTs has prompted considerable interest in their gas and liquid adsorption properties (Smith et al., 2003; Zare et al., 2015). At present time, new materials with enhanced properties are necessary for hydrogen storage. A lot of reviewers on this topic were published. It was noted that significant amounts of hydrogen can be absorbed by SWCNT. Hydrogen adsorption high values of were detected at low temperature, room temperature and even higher temperatures. In contrast, most theoretical calculations of hydrogen adsorption have given to considerably lower estimates. These results may indicate that physisorption on pure nanotubes may not be an effective method of storing hydrogen (http://www.seca.doe.gov). The most promising method of preparing MWCNTs is the chemical vapor deposition (CVD) (Rakov, 2007). The catalyst is a key factor in controlling the quality of CNTs. Compared to the other methods of preparing the catalyst, i.e., precipitation, coating, thermal decomposition, and mechanical mixing, the use of sol–gel technology allows significantly to decrease the temperature of the formation of nanomaterials and provides the high chemical homogeneity of the systems (Zhabrev et al., 2005). The variations in the characteristics of the catalyst particles, including the size, chemical and phase composition, and structure; the carrier composition; and the parameters of the synthesis allows to obtain CNTs of a given quality, i.e., with a certain diameter, length, orientation of the graphene layers, and degree of purity.

390 Handbook of Composites from Renewable Materials-Volume 7 As a result of the CVD process of organic substances, the elemental carbon is eventually formed. This method allows to obtain not only the CNTs, but also certain structures on the carriers and composite materials (Rakov, 2007). The purpose of the present work is to create highly efficient modifiers (CNMs) for preparation reinforcement polymer materials and the assessment of the impact of the morphology and exploitation characteristics of the modifiers on the composites properties.

12.2 Experimental Methods 12.2.1 Investigation of the CNTs Synthesis There are many types of the CNTs, which differ from each other morphology, diameter, length, and orientation of graphite layers. In all these cases, the chemical composition of the catalyst and operation conditions of the CNTs synthesis has a key role. During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination (Rakov, 2007). When investigating the influence of the chemical composition of the catalyst on the CVD process, the following catalytic systems were considered: Ni–Co–MgO, Ni–Y– MgO, Ni–Mo–MgO, Ni–Fe–Co–MgO, Ni–MgO, Fe–Co–MgO, Fe–Mo–Al2O3, and Mo–Co. In the course of the synthesis, the formed gel is an organic polymer, the structure of which contains metal ions as a result of chemical reactions. The α-hydroxy acids, which contain both hydroxyl and carboxyl (–OH and –COOH) groups together, were used as an organic precursor. A characteristic feature of these α-hydroxy acids is the ability to form chelate complexes with metal ions (Kostromina et al., 1990; Roberts & Caserio, 1978; Hao et al., 2003). Nickel, yttrium, cobalt, molybdenum, iron, aluminum and magnesium nitrates, citric acid, and ethylene glycol were chosen as precursors. The process of gel preparation included the following steps: 1. The chemical interactions of citric acid and metal nitrates to form chelates. 2. The formation of oligomers as a result of the interaction of chelates with ethylene glycol at elevated temperatures, since the removal of water from the reaction medium by evaporation is necessary for the formation of polymer products (provided the excess of ethylene glycol). 3. The polymerization of oligomers into a polymer that has the form of a viscous bulk resin (gel). The process takes place at a temperature of 180–200 °C in an SPT-200 Vacuum Drier (maximum temperature of 250 °C, power of 3.5 kW, voltage of 220 V). The resulting gel was subjected to thermal treatment in a muffle furnace (maximum temperature of 1100 °C, power of 3.6 kW, voltage of 220 V) at 500–550 °C. The final product consisted of a powder of catalyst (Ni, Co, Fe) oxide mixtures, carrier (Mg, Al), and promoter (Y, Mo). The obtained catalyst was ground and sieved to fractions of 100, 80, 71, 64, 56, and 40 μm.

Nanostructured Polymer Composites with Modified Carbon Nanotubes 391 Nanotubes were synthesized by the CVD process in a batch reactor. The productivity of the reactor is 2000 kg per year (under single-shift operation regime). Catalyst loading and CNTs unloading were carried out automatically (www.nanotc.ru). The process was performed at atmospheric pressure and at a temperature of 650 °C. The time of CVD process is 40 min. The catalyst productivity was determined by the specific yield of CNTs, which was calculated as the ratio of the product mass obtained after pyrolysis to the catalyst mass (gC/gcat). In this work, we have investigated the structure and dispersion characteristics of the catalysts. The measurement of the specific surface area of the catalysts was conducted on surface analyzer SORBTOMETR-M, gas–adsorbate–nitrogen (State Standard 23401-90. Metallic Powders: Catalysts and Carriers). The structure parameters characterize the porous structure of the catalysts; the larger the specific surface area and pore volume of the catalyst, the higher the activity. In this case, the high specific surface and pore volume of the catalyst promote the efficient growth of high-quality CNTs, i.e., with the lowest content of amorphous carbon and catalyst particles. The Ni–Co–MgO catalytic systems have higher specific surface areas (61 and 133 m2/g), higher values of the specific pore volume (0.029 and 0.062 cm3/g) and larger surface areas of mesopores (54 and 138 m2/g) than catalysts of other chemical natures studied in this work. The dispersity of the catalyst was determined on a Microsizer 201C laser particle analyzer (measurement range of particle size was 0.2–600 μm). The surface area of the catalyst increases with decreasing particle size. The higher mass fraction of the catalyst particles is in the size range of 37.7–43.3 μm. Thus, it can be concluded that, in the CVD process, separation into fractions in order to increase the specific yield of CNTs is not worthwhile. The influence of the chemical composition of the catalyst (components ratio) on its main characteristics are shown in Table 12.3. Figure 12.4 gives the SEM images of the catalysts structure. The size of the catalyst is in the range of 10–25 nm. It can be seen from Table 12.3 that the chemical nature of the catalyst influences its characteristics and activity, as well as the structure of the synthesized CNTs. An investigation of the CNTs structure was performed using a Neon 40 Carl Zeiss scanning electron microscope (Tambov State University, EIC ‘Nanotechnologies and Nanomaterials’) Table 12.3 Summary table of basic characteristics of the catalysts under study. Catalyst chemical composition Ni–Co–MgO Ni–Y–MgO Ni–Mo–MgO Ni–Fe–Co–MgO Ni–MgO Fe–Co–MgO Fe–Mo–Al2O3 Mo–Co

Yields, Surface area, Particle size, Bulk density, Real density, mcm kg/m3 kg/m3 g/g m2/g 38.4 26.5 20.9 15.4 1.96 1.83 1.57 2.21

61 46 50 41 34 62 23 3.2

49.7–57.1 39.9–47.1 39.1–47.1 39.1–47.1 33.8–39.9 39.1–47.1 24.3–28.7 29.8–36.4

640 410 800 590 390 370 410 360

3306 2749 2794 2059 2136 2012 2157 2037


Figure 12.4 SEM images of the Ni–Co/MgO catalyst structure.

Figure 12.5 CNTs structure on (wt %) 50Ni–10Co–40MgO catalyst.

(Figures 12.5, 12.6, and 12.8), as well as methods of high-resolution electron microscopy (HREM) (Institute of Crystallography, Russian Academy of Sciences) (Figure 12.7). Based on the obtained experimental data, one can formulate the following conclusions: On a catalyst with a composition of 50Ni–10Co–40MgO, in the absence of amorphous carbon, a specific yield of CNTs 38.4 gC/gcat is observed. Using this catalyst, the diameter of CNTs is 10–30 nm, and the size of catalyst particles is 25 nm (Figure 12.5). Changing the content of Co in either direction leads to changes in the structure and a decrease in the specific yield of CNTs. The Ni–Mo–MgO catalytic system with a Mo concentration of about 1% represents CNTs in 7– 30 nm diameter and 1–2 μm length (Figure 12.6). The absence of amorphous carbon and catalyst particles was found. A further increase in the concentration

Nanostructured Polymer Composites with Modified Carbon Nanotubes 393

Figure 12.6 CNTs structure on (wt%) 59Ni–1Mo–40MgO catalyst.

30 HM

Figure 12.7 HREM image of several CNTs and CNFs with conical walls grown on catalyst with a composition (wt%) of 59Ni–1Mo–40MgO.

of Mo leads to the lack of uniform distribution of the catalyst particles on the carrier, thus decreasing the yield and deteriorating the CNTs quality. According to HREM (Figure 12.7), CNM obtained on the studied catalyst compositions represent CNTs with conical walls and carbon nanofibers (CNFs) 10–100 nm diameter and 20–100 nm length. In the channels of some fibers, jumpers are visible. The internal channel of CNTs is no more than 20% of the outside diameter, while the walls of some tubes are uneven and have a lot of internal defects. The use of Y as a promoter with a concentration of about 8% in the Ni–Y–MgO system allows to obtain good quality CNTs (Figure 12.8). The diameter of the nanotubes is 25–50 nm, and the length is ~1–2 μm. According to the pictures, the material does not contain amorphous carbon and catalyst particles. However, the specific


Figure 12.8 CNTs structure on (wt %) 55Ni–5Y–40MgO catalyst.

yield of CNTs is lower than in the composition of 50Ni–10Co–40MgO, which may be associated with a decrease in the specific surface area of the Ni–Y–MgO catalytic system. The Fe–Co–MgO and Ni–MgO catalytic systems did not show any catalytic activity during the experiments. Furthermore, CNMs obtained on the catalysts in Mo–Co and Fe–Mo–Al2O3 systems are single CNTs synthesized on the agglomerates of sintered catalyst particles. The application of the catalyst composition Ni–Fe–Co–MgO allows to obtain CNTs and CNF with uneven walls and a variety of external defects. The catalyst that gives the specific yield of the order of 20–50 gC/gcat allows CNTs of the necessary quality to be obtained; the orientation of graphene layers is conical, amorphous carbon is absent, the diameter is 5–30 nm, and the length is 1–2 μm. Due to the presence of the terminal carbon atoms, conical CNTs are most prone to the addition of various functional groups. Consequently, nanotubes grown on the catalysts under study, in particular the compositions Ni–Co–MgO, Ni–Y–MgO and Ni–Mo–MgO, have increased adsorption characteristics. During the course of the work, it was found that the use of sol–gel method allows to obtain the efficient catalyst compositions for the CNTs synthesis. Here, it was noted that the maximum yield of nanotubes (38.4 gC/gcat) is provided by the catalyst composition Ni–Co–MgO, including a Co concentration of up to 10%, a specific surface area of 61 m2/g, a maximum in the particle size distribution in the range of 49.7–57.1 μm, a bulk density of 640 kg/m3, and a real density of 3306 kg/m3. The composition of this catalyst allows to achieve a high yield and the required quality of CNTs in comparison to other compositions. Here, small impurities of the catalyst particles and the absence of amorphous carbon were observed, thereby reducing the production costs of material purification, and the decrease in its cost will occur. Catalysts with the given characteristics are recommended for use in the preparation of CNTs on an industrial scale.


12.2.2 CNTs Treatment CNTs (obtained on the Ni–Co–MgO catalyst) fluorination has been carried out in a closed reaction vessel at 0.8–0.9 bar fluorine pressure. That pressure was chosen for the following reason. Increase of fluorine pressure resulted in fluorination rate acceleration but at higher than 0.8–0.9 bar pressure local ignitions took place because the heat released in the reaction cannot be quickly dissipated by gas phase or reactor walls. Also previously we have found that at treatment temperature around 150–200 °C the CNTs structure was not disturbed and rather high degree of fluorination (CF0.2 to CF0.4) could be reached (Krestinin et al., 2009). At the temperature above 250 °C some destruction of fluorinated CNTs was observed (Shulga et al., 2011). Treatment duration was varied over 10 min to 2 h. It was shown that fluorination degree at 2 h treatment markedly exceeded one at 150 °C so 2 h treatment duration was chosen. Fluorine was inserted into heated reactor and its pressure was maintained constant within accuracy 5%. Pristine CNTs were designated as ‘TM’. Index ‘F’ indicates that the CNTs were fluorinated. Samples TM-F26, TM-F28, and TM-F30 were treated at temperatures 250, 150, and 100 °C, respectively. Treatment duration was equal to 2 h (TM-F26, TM-F28) and 10 min (TM-F30).

12.2.3 Composites Fabrication Epoxy resin ED-22 (State Standard 10587-84) based on diglycidyl ether of bisphenol-A type and ‘Polyam-B10’ hardener were used to fabricate composites. Fluorine had less than 0.1 volume % of admixtures (mainly O2). Sample for testing were fabricated in ‘Lepta-550-40’ silicon compound casting molds. Composites were mixed in ‘EXAKT 80E’ three-roll mill and undergone sonication. Epoxy resin was mixed with hardener ‘Polyam-B10’, evacuated, transferred to casting molds, evacuated and was cured inside heated thermostate during 5 h. The composites composition was designated as ER+ (percentage of filler) where ‘ER’ means pristine epoxy composition (epoxy resin + hardener).

12.2.4 Testing Procedures To measure IR spectra of KBr pellets containing CNTs FTIR spectrometer FT-02 (Lumex Ltd, Saint Petersburg, Russia) was used. 1000 scans at 4 cm–1 resolution were collected to measure 1 spectrum over 4000–400 cm–1 spectral range. Influence of fluorination on the CNTs surface chemical composition was studied by XPS spectro meter PHI 5500 ESCA (Perkin Elmer). Texture of composite cuts was studied by scanning electron microscope JEOL JSM-6610LV at accelerating potential 20 kV. To avoid charge accumulation polymer surface was coated with Pt layer 1–2 nm in thickness. Thermogravimetric analysis (TGA) Instruments Q600 was used to study thermal stability in air flow. Heating rate was equal to 10 °C per min over room temperature to 1000 °C range. Differential scanning calorimetry (DSC) was performed by calorimeters NETZSCH DSC204F1 and STA 449F3 Jupiter (Netzsch) in Ar atmosphere with heating rate 10 °C per min over 35–420 °C range. Mechanical properties of composites were investigated by testing machine ‘Testometric M350-5AT’ at 50 mm/min (tensile strength) and 20 mm/min (flexural strength). Specific surface was measured by

396 Handbook of Composites from Renewable Materials-Volume 7 sorption meter SORBTOMETR-M (Russia). BET method (State Standard 2340-90) and multipoint method STSA (ASTM D5816) were applied.

12.3 Results and Discussion 12.3.1 FTIR Spectroscopy IR spectra of virgin and fluorinated CNTs are shown in the Figure 12.9. Baseline was corrected by application of multi-point correction method from ‘GRAMS-32’ software. All the spectra in Figure 12.9 were corrected with respect to KBr absorption. Fluorination of the CNTs resulted in a formation of a wide diffuse band over 1300–900 cm–1 with maximum at 1200 cm–1 which is due to absorption of covalent C–Fx bonds (Krestinin et al., 2009; Zhang et al., 2010; Lee et al., 2003) and in an increase of the 1720–1700 cm–1 band which can be attributed to C=O groups formation due to an oxygen admixture in used fluorine (Socrates et al., 2001). Intensity of that band is increased with treatment temperature which can be due to increase of the structure defect amount.

12.3.2 Influence of Fluorination on the CNTs Specific Surface CNTs fluorination resulted in a marked increase of CNTs specific surface. Pristine CNTs have specific surface equal to 105 m2/g. CNTs treatment at 150 and 250 °C resulted in a specific surface increase to 238 (increased by a factor of 2.26 as compared with pristine CNTs) and 166 m2/g values. Increase of the CNTs specific surface could enhance the reinforcing effect of CNTs which will be used as composites fillers.

12.3.3 X-Ray Photoelectron Spectroscopy Study X-ray photoelectron spectroscopy (XPS) spectra of pristine and fluorinated CNTs are shown in the Figure 12.10. Several bands can be separated (mixture of Gauss and C-Fx 0.25

C=O

Absorbance

TM-F26 0.20 TM-F30 TM-F28 0.15 TM 0.10 2000

1800

1600

1400

1200

1000

800

V, cm–1

Figure 12.9 Spectra of virgin (TM) and fluorinated (TM-F26, TM-F28, TM-F30) CNTs. Treatment conditions are indicated in the point 2.2 (Kharitonov et al., 2015).

Counts/s


4 300

295

1 3

290

2 285

280

Binding energy, eV

Figure 12.10 XPS spectra of pristine (solid line) and fluorinated at 150 °C (1, dotted line) CNTs. 2, 3, and 4—fitted bands for fluorinated CNTs. The resulting fitting spectra are not presented at the figure 2 because it coincides with the experimentally measured spectrum (dotted line) within accuracy of the line width (Kharitonov et al., 2015).

Lorentz functions was applied for fitting) and assigned (Lee et al., 2009; Crassous et al., 2009). The resulting fitting line is not presented at the Figure 12.10 because it coincides with the experimentally measured spectrum (dotted line) within accuracy of the line width. Areas of peaks 1, 2, 3 and 4 correspond to 74:4:7:15 sequence. Band 1 corresponds to C atoms surrounded only by C atoms without attached F atoms. Bands 2 relates probably to C atom without attached F atom surrounded by three (sp2 hybridization) or four C atoms (sp3 hybridization) with attached F atom. Band 3 corresponds to C atom with attached F atom surrounded with two C atoms with attached F atoms. Band 4 corresponds to C atom with attached F atom surrounded with three C atoms with attached F atoms. Chemical composition C:O:F of CNTs are as follows: 98:2:0 for the pristine CNTs and 78:1.3:20.7 for fluorinated at 150 °C CNTs (TM-F28).

12.3.4 TGA of Virgin and Fluorinated CNTs TGA results for virgin and fluorinated CNTs measured in air are shown in the Figure 12.11. CNTs fluorinated at 250 °C are much less stable as compared with CNTs fluorinated at 100–150 °C. Pristine CNTs are more stable than fluorinated ones because C–C triple bond energy in CNTs markedly exceeds C–F bond energy. Below 250–300 °C thermal stability of pristine and fluorinated CNTs are close each to other. Unfilled composites started to degrade at 250 °C so fluorination should not influence thermal stability of filled composites below 250 °C.

12.3.5 SEM Data of Composites Fracture Epoxy resin is rather fragile polymer material so its destruction is accompanied with localization of stresses on its surface or inside its bulk. The stress is localized in defects region and stress increase results in a formation and rise of cracks. The texture of cured

398 Handbook of Composites from Renewable Materials-Volume 7 1.0

TM

0.8

TM-F28

TM-F26

TM-F27

m/m°

0.6 TM-F29

0.4

TM-F30 0.2 0.0 0

100

200

300

400 500 t, °C

600

700

800

900

Figure 12.11 TGA of virgin CNTs and fluorinated CNTs (TM-F26–TM-F30) (Kharitonov et al., 2015).

(a)

(b)

Figure 12.12 Cut of the destructed unfilled composite. Scale from left to right: 500 and 5 µm.

(a)

(b)

Figure 12.13 Cut of the unfilled composite destructed via flow mechanism. Scale from left to right: 100 and 10 µm (Kharitonov et al., 2015).

unfilled epoxy resin is rather flat (Figures 12.12 and 12.13). The sample destruction via fragile mechanism was initiated by cracks formation (Figure 12.12, left top corner of the left photo) which have flake-like structure (Figure 12.12). Except destruction via fragile mechanism there exist local regions destructed via flow mechanism (Figure 12.13) which can be due to a lowered concentration of polymer chains cross-links and higher plasticity of cured epoxy resin.

Nanostructured Polymer Composites with Modified Carbon Nanotubes 399 Destruction mechanism of ER+0.1%TM composite (epoxy resin matrix with 0.1 weight % of virgin CNTs) is similar to one of unfilled composite (Figure 12.14). Flakelike relief (which is probably due to CNTs covered with polymer matrix material) was observed on the flakes surface contrary to unfilled composites. CNTs directed outside of the composite bulk were not observed. It might be due to rather good adhesion between CNTs and polymer matrix. Destruction via flow mechanism is illustrated in Figure 12.15. In some places finger-like structures arose due to a polymer bulk pulling. Composite plasticization can be explained if CNTs are considered as a solid-phages hardener (Yang et al., 2012). In this case amount of cross-links at CNTs–polymer matrix boundary layer will be reduced and that region will possess higher mobility. For the case of composite containing 0.5 weight % of virgin CNTs the prevailing mechanism of destruction became plastic destruction (via flow mechanism), as shown in Figure 12.16. Flakes boundaries became less sharp and their size tends to decrease. Finger-like structures (similar to Figure 12.15) were well visualized. At high-resolution CNTs agglomerates (5–7 μm in size) became visible. They are placed above composite surface. It means that CNTs were removed from the bulk as a whole without their destruction. Hence at least part of virgin CNTs did not take part in composite reinforcement. Composite cut with inserted 0.1% fluorinated CNTs (TM-F28) is to some extent similar to composite with 0.5% of virgin CNTs but finger-like structures were not

(a)

(b)

Figure 12.14 Cut of the polymer composite containing 0.1 weight % of virgin CNTs. Destruction via fragile mechanism. Scale from left to right: 50 and 5 µm.

(a)

(b)

Figure 12.15 Cut of the polymer composite containing 0.1 weight % of virgin CNTs. Destruction via flow mechanism. Scale from left to right: 50 and 1 µm (Kharitonov et al., 2015).


(a)

(b)

Figure 12.16 Cut of the polymer composite containing 0.5 weight % of virgin CNTs. Scale from left to right: 500 and 1 µm (Kharitonov et al., 2015).

(a)

(b)

Figure 12.17 Cut of the polymer composite containing 0.1 weight % fluorinated CNTs (TM-F28). Scale from left to right: 500 and 5 µm (Kharitonov et al., 2015).

(a)

(b)

Figure 12.18 Cut of the polymer composite containing 0.5 weight % of fluorinated CNTs (TMF-28). Scale from left to right: 50 and 5 µm (Kharitonov et al., 2015).

detected (Figure 12.17). It can be due to CNTs surface functionalization which resulted in number of cross-linking between CNTs and epoxy bulk increase as compared to composites filled with virgin CNTs. So, plasticity of the epoxy bulk can be decreased as compared with epoxy filled with virgin CNTs. Surface of the composite cut with inserted 0.5% of fluorinated CNTs (TM-F28) is shown in the Figure 12.18. The surface has the most roughness as compared with other studied composites. ‘Flakes’ are of large size and longitudinal shape. Finger-like structures were not observed. CNTs outside of the composite bulk were not detected. It means that fluorinated CNTs exhibit good adhesion with epoxy resin and during destruction are not removed from the polymer bulk.


12.3.6 TGA and DSC of Composites TGA data of composites are shown in the Figure 12.19. At temperature below 260 °C CNTs additions does not influence (and does not worsen) composite thermal stability. Composites glass transition temperature can be calculated from the DSC data shown in the Figure 12.20. For all the composites transition over glassy temperature takes place not at a point but over some temperature interval. So, the middle of a flexure on the DSC curves was assigned to a glassy temperature (Höhne et al., 2003). For all the composites (except ER+0.1%TM) insertion of CNTs resulted in glassy temperature increase. Unfilled epoxy resin has a glass temperature 110.9 °C, composites ER+0.1%TM and 0.5%TM—108.7 and 125 °C glass temperature. Composites

1.0 3

m/m0

0.8

2

0.6 0.4

1

0.2 0.0

0

100

200

300

400

500

600

t, °C

Figure 12.19 TGA of composites in air environment. 1 (solid line)—unfilled composite, 2 (dotted line) and 3 (dashed line) —composite filled with 0.1 weight % of virgin CNTs and with 0.1 weight % fluorinated at 150 °C CNTs respectively (Kharitonov et al., 2015).

1000 ER unfilled DSC respose, mWt/mg

800

600

ER + 0.1%TM

400

ER + 0.5%TM

200 ER + 0.1%TM-F28 0 80

90

100

110 T, °C

Figure 12.20 DSC data of composites (Kharitonov et al., 2015).

120

130

140

402 Handbook of Composites from Renewable Materials-Volume 7 filled with fluorinated CNTs exhibited 113.0 °C (ER+0.1%TM-F28) and 121.3 °C (ER+0.5%TM-F28). According to Yang et al. (2012), CNTs insertion into epoxy matrix resulted in chemical conversion degree increase and improved net of chemical crosslinks formation. It is evident that filled composites will possess increased glassy temperature as compared with unfilled ones.

12.3.7 Mechanical Properties of Composites 12.3.7.1 Tensile Strength Tensile strength of composites is shown in Figure 12.21 together with available literature data for the composites based on the epoxy resin Bisphenol-A type (similar to that used in the current research) are shown. In Liu and Wagner (2005), Yang et al. (2012), Mirmohseni and Zavareh (2010), Rana et al. (2012), Nie and Hubert (2011), Rana et al. (2011), Chaos-Moran et al. (2011), Rana et al. (2010), Ahn et al. (2008), Zhou et al. (2008), Kim et al. (2006), Gojny et al. (2005), Villoria et al. (2006), Ci and Bai (2006), the following fillers were used: CNTs and nanofibers (pristine, silanizied, carboxilated, aminated, plasma-modified, treated with acids), soot, graphite, organic clay, and ABS copolymer. In our research insertion of 0.01% of both pristine and fluorinated CNTs resulted in a tensile strength decrease. Composites with 0.1% of fluorinated at 150 °C CNTs (TM type) exhibited maximum tensile strength—89.6±4.1 MPa (for the unfilled epoxy tensile strength was equal to 66.1±2.4 MPa) and exceeded one for unfilled composite by 35%. Increase of the fluorinated CNTs content to 0.5% resulted in a tensile strength drop to 60.8±3.6 MPa. Increase of CNTs fluorinating temperature to 250 °C did not result in reinforcement of composites. For the case of pristine CNTs maximum reinforcement effect was observed for 0.5% CNTs: tensile strength 84.0±2.3 MPa, which was smaller than one for the case of 0.1% fluorinated CNTs addition. Maximum tensile strength value for reinforced composites based on bisphenol-A epoxy resin known from literature data is equal to 90 ± 6.8 MPa (Ahn et al., 2008). That value was obtained for

Tensile strength, MPa

100 80 60 40 20 0

0.0

0.2

0.4

0.6

2.0

4.0

6.0

8.0

CNTs content, weight %

Figure 12.21 Tensile strength (MPa) versus CNTs content in epoxy resin (weight %). Triangles—pristine CNTs addition, squares—TMF-28 addition (CNTs fluorinated at 150 °C), crosses—TM-F26 addition (CNTs fluorinated at 250 °C). Open circles—literature data (Liu & Wagner, 2005) for Bisphenol-A based composites similar to one used in the current research (Kharitonov et al., 2015).


Flextural strength, MPa

200 150 100 50 0 0.0

0.2

0.4 0.6 10.0 20.0 30.0 40.0 50.0 60.0 CNTs content, weight %

Figure 12.22 Flexural strength (MPa) versus CNTs content in epoxy resin (weight %). Triangles—pristine CNTs addition, squares—TM-F28 addition (CNTs fluorinated at 150 °C), crosses—TM F26 addition (CNTs fluorinated at 250 °C). Open circles—literature data (Ma et al., 2010, 2007; Patton et al., 1999) for Bisphenol-A based composites similar to one used in the current research (Kharitonov et al., 2015).

6.4% CNFs content. We have reached the same reinforcement using 64 times smaller filler amount (i.e., 0.1%). Modulus value 1644±76 MPa was reached for the case of 0.1% fluorinated CNTs addition (+33% as compared with unfilled composite).

12.3.7.2 Flexural Strength Influence of fluorinated CNTs addition on composite flexural strength is shown in the Figure 12.22 together with literature data. According to literature review data (Ma et al., 2010, 2007; Patton et al. 1999; Xu et al., 2004; Lau et al., 2005; Khalil et al., 2010) maximum reported in literature flexural strength value for composites based on bisphenol-A epoxy resin is equal to 139.7±1.98 MPa (Khalil et al., 2010). Figure 12.22 presents the experimental results on the composites flexural strength. Maximum flexural strength value 199.7±4.8 MPa (+58% increase with respect to unfilled composite) was obtained in our research for composite with 0.2 weight % CNTs fluorinated at 150 °C. Obtained value exceeds maximum reported in literature value by 43% for bisphenol-A composites similar to those used in our research. But obtained in (Khalil et al., 2010) reinforcement was reached for 5 weight % filler concentration as compared with 0.2% in our research. Module of composite with 0.2% fluorinated at 150 °C CNTs was equal to 3603 ± 86 MPa and exceeded module for unfilled composite by 13%. Pristine CNTs are less effective in reinforcement: maximum flexural strength was obtained for epoxy composite contained 0.5 weight % pristine CNTs: 152.3±8.1 MPa (module 3217±170 MPa).

12.8 Conclusion Both pristine and fluorinated CNTs can be effectively used to reinforce epoxy-based composites when their concentration in composite is around 0.1–0.2 weight %. Both tensile and flexural strength and modulus were markedly improved. Fluorinated CNTs are more effective as fillers as compared to pristine ones. It was confirmed by mechanical tests and electron microscopy study. Fluorination resulted in an increase of the CNTs specific surface. Fluorinated CNTs surface became more polar and the

404 Handbook of Composites from Renewable Materials-Volume 7 specific surface was increased so the adhesion between CNTs and matrix (epoxy resin) was improved and mechanical properties were simultaneously improved. Application of CNTs fluorinated at 150 °C provides higher increase of both tensile and flexural strength than CNTs fluorinated at 250 °C. Probably treatment at 250 °C resulted in a surface damage of CNTs. Obtained in the current research tensile and flexural strength of based on Bisphenol-A epoxy composites reinforced with fluorinated CNTs markedly exceeded all the available literature data for epoxy composites when the CNTs concentration is below 5 weight %. Fluorinated CNTs as fillers did not worsen composites thermal stability and resulted in a slight glassy temperature increase.

Acknowledgments The research has been partially supported by the grant from the Russian Science Foundation #15-13-10038 and State Contract #16.711.2014/K from the Ministry of Education and Sciences of Russian Federation.

References Abdolreza, M., Siamak, Z., Epoxy/acrylonitrile-butadiene-styrene copolymer/clay ternary nanocomposite as impact toughened epoxy. J. Polym. Res., 17(1), pp. 191–201, 2010. Ahmad, Y., Dubois, M., Guérin, K., Hamwi, A., Fawal, Z., Kharitonov, A.P., Generalov, A.V., Klyushin, A.Yu., Simonov, K.A., Vinogradov, N.A. et al., NMR and NEXAFS study of various graphite fluorides. J. Phys. Chem. C, 117(26), pp. 13564–13572, 2013. Ahn, S-N., Lee, H-J., Kim, B-J. et al., Epoxy/amine-functionalized short-length vapor-grown carbon nanofiber composites. J. Polym. Sci. A Polym. Chem., 46(22), pp. 7473–7482, 2008. Amirkhanov, D.M., Kotenko, A.A., Tul’skii, M.N., Technological characteristics of the manufacture and use of fluorine-modified Graviton hollow gas-separation fibres. Fibre Chem., 30, pp. 318–324, 1998. Anand, M.; Hobbs, J.P.; Brass, I.J., Surface fluorination of polymers, in: Organofluorine Chemistry: Principles and Commercial Applications, R.E. Banks, B.E. Smart, J.C. Tatlow (Ed.), pp. 469–481, Plenum, New York, 1994. Badamshina, E.R., Gafurova, M.P., Estrin, Ya.I., Modification of carbon nanotubes and synthesis of polymeric composites involving the nanotubes. Usp. Khim., 79(11), pp. 1027–1064, 2010. Carstens, P.A.B., Boyazis, G.A.B.M. Process for the production of plastic components for containing and/or transporting fluids. US Patent 5792528, 1998. Carstens, P.A.B., Boyazis, G.A.B.M., De Beer, J.A. Method for the production of composites. US Patent 5900321, 1999. Carstens, P.A.B. Production of composites. US Patent 5744257, 1998. Carstens, P.A.B., Marais, S.A., Thompson, C.J., Improved and novel surface fluorinated products. J. Fluorine Chem., 104, pp. 97–107, 2000. Chaos-Moran, R., Salazar, A., Urena, A., Mechanical analysis of carbon nanofiber/epoxy resin composites. Polym. Compos., 32(10), pp. 1640–1651, 2011. Ci, L., Bai, J., The reinforcement role of carbon nanotubes in epoxy composites with different matrix stiffness. Comp. Sci. Technol., 66(3–4), pp. 599–603, 2006.

Nanostructured Polymer Composites with Modified Carbon Nanotubes 405 Crassous, I., Groult, H., Lantelme, F., Devilliers, D., Tressaud, A., Labrugere, C., Dubois, M. et al., Study of the fluorination of carbon anode in molten KF-2HF by XPS and NMR investigations. J. Fluor. Chem., 130(12), pp. 1080–1108, 2009. Davis, D.C., Wilkerson, J.W., Zhu, J., Ayewah, D.O.O., Improvements in mechanical properties of a carbon fiber epoxy composite using nanotube science and technology. Compos. Struct., 92(11), pp. 2653–2662, 2010. Geng, H., Rosen, R., Zheng, B., Shimoda, H., Fleming, L., Liu, J., Zhou, O., Fabrication and properties of composites of poly(ethylene oxide) and functionalized carbon nanotubes. Adv. Mater., 14(19), pp. 1387–1390, 2002. Gojny, F.H., Wichmann, M.H.G., Fiedler, B., Schulte, K., Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites—a comparative study. Comp. Sci. Technol., 65(15–16), pp. 2300–2313, 2005. Green, M.D., Guild, F.J., Adams, R.D., Characterisation and comparison of industrially pretreated homopolymer polypropylene. J. Adh. Adh., 22, pp. 81–90, 2002. Hao, Y., Qunfeng, Z., Fei, W., Weizhong, Q., Guohua, L., Agglomerated CNTs synthesized in a fluidized bed reactor: agglomerate structure and formation mechanism. Carbon, 41, pp. 2855–2863, 2003. Höhne, G.W.H., Hemminger, W.F., Flammersheim, H-J., Differential Scanning Calorimetry, Springer-Verlag, Berlin, Heidelberg, 298, 2003. Khalil, H.P.S.A., Foroozian, P., Bakare, I.O., Akil, H.M., Noor, A.M., Exploring biomass based carbon black as fille in epoxy composite: flexural and thermal properties. Mater. Des., 31(7), pp. 3419–3425, 2010. Kharitonov, A.P., Simbirtseva, G.V., Bouznik, V.M., Chepezubov, M.G., Dubois, M., Guerin, K., Hamwi, A., Kharbache, H., Masin, F., Modification of ultra-high-molecular weight polyethylene by various fluorinating routes. J. Polym. Sci. A Polym. Chem., 49, pp. 3559–3573, 2011. Kharitonov, A.P., Direct Fluorination of Polymers, Nova Science Publishers, Inc., New York, pp. 35–103, 2008. Kharitonov, A.P., Direct fluorination of polymers—from fundamental research to industrial applications, in: Fluorine Chemistry Research Advances, I.V. Gardiner, (Ed.), pp. 35–103, Nova Science Publishers, Inc., New York, 2007. Kharitonov, А.P., Taege, R., Ferrier, G., Teplyakov, V.V., Syrtsova, D.A., Koops, G.H., Direct fluorination—useful tool to enhance commercial properties of polymer articles. J. Fluor. Chem., 126(2), pp. 251–263, 2005. Kharitonov, A.P., Simbirtseva, G.V., Nazarov, V.G., Stolyarov, V.P., Dubois, M., Peyroux, J., Enhanced anti-graffiti or adhesion properties of polymers using versatile combination of fluorination and polymer grafting. Prog. Org. Coat., 88, pp. 127–136, 2015. Kharitonov, A.P., Gas separation properties enhancement by direct fluorination of polymer membranes, in: Encyclopedia of Membranes, E. Drioli, L. Giorno (Ed.), Springer, Berlin, 2012. Kharitonov, A.P., Direct fluorination of polymers—from fundamental research to industrial applications, in: Fluorine Chemistry Research Advances, I.V. Gardiner (Ed.), pp. 35–103, Nova Science Publishers, New York, 2007. Kharitonov, A.P., Direct fluorination of polymers, in: Fluorine Chemistry Research Advances, I.V. Gardiner (Ed.), Nova Science Publishers, New York, 2008. Kharitonov, A.P., Simbirtseva, G.V., Tkachev, A.G., Blohin, A.N., Dyachkova, T.P., Maksimkin, A.A., Chukov, D.I., Reinforcement of epoxy resin composites with fluorinated carbon nanotubes. Compos. Sci. Technol., 107, pp. 162–168, 2015. Kharitonov, A.P., Maksimkin, A.V., Mostovaya, K.S., Kaloshkin, S.D., Gorshenkov, M.V., D’yachkova, T.P., Tkachev, A.G., Alekseiko, L.N., Reinforcement of bulk ultrahigh molecular weight polyethylene by fluorinated carbon nanotubes insertion followed by hot pressing and orientation stretching. Compos. Sci. Technol., 120, pp. 26–31, 2015.

406 Handbook of Composites from Renewable Materials-Volume 7 Kim, J.A., Seong, D.G., Kang, T.J., Youn, J.R., Effects of surface modification on rheological and mechanical properties of CNT/epoxy composites. Carbon, 44(10), pp. 1898–1905, 2006. Kostromina, N.A., Kumok, V.N., Skorik, N.A., Chemistry of Coordination Compounds, Vysshaya Shkola publisher, Moscow, 432, 1990. Krestinin, A.V., Kharitonov, A.P., Shul’ga, Yu.M., Zhigalina, O.M., Knerel’man, E.I., Dubois, M. et al., Fabrication and characterization of fluorinated single-walled carbon nanotubes. Nanotech. Russia, 4(1), pp. 60–78, 2009. Kruse, A., Kruger, G., Baalmann, A., Hennemann, O.D., Surface pretreatment of plastics for adhesion bonding. J. Adhes. Sci. Technol., 9, pp. 1611–1621, 1995. Kumar Thakur, V., Kessler R., Michael, Self-healing polymer nanocomposite materials: a review. Polymers, 69, pp. 369–383, 2015. Kumar Thakur, V., Kumari Thakur, M., Raghavan, P., Kessler R., Michael, Progress in green polymer composites from lignin for multifunctional applications: a review. ACS Sust. Chem. Eng., 2(5), pp. 1072–1092, 2014. Kumar Thakur, V., Kumari Thakur, M., Processing and characterization of natural cellulose fibers/thermoset polymer composites. Carbohydr. Polym., 109, pp. 102–117, 2014. Lagow, R.J., Margrave, J.L., Direct fluorination: a new approach to fluorine chemistry. Prog. Inorg. Chem., 26, pp. 162–210, 1979. Langsam, M, Anand, M, Karwacki, E.J., Chemical surface modification of poly[1-(trimethylsilyl) propyne] for gas separation membranes. Gas Sep. Purif., 2, pp. 162–170, 1988. Lau, K-T., Lu, M., Lam, C-K., Cheung, H-Y., Sheng, F-L., Li, H-L., Thermal and mechanical properties of single-walled carbon nanotube bundle-reinforced epoxy nanocomposites: the role of solvent for nanotube dispersion. Compos. Sci. Technol., 65(5), pp. 719–725, 2005. Lee, J-M., Kim, S.J., Kim, J.W., Kang, P.H., Nho, Y.C., Lee, Y-S., A high resolution XPS study of sidewall functionalized MWCNTs by fluorination. J. Ind. Engng. Chem., 15(1), pp. 66–71, 2009. Lee, Y.S., Cho, T.H., Lee, B.K., Rho, J.S., An, K.H., Lee, Y.H., Surface properties of fluorinated single-walled carbon nanotubes. J. Fluor. Chem., 120, pp. 99–104, 2003. Le Roux, J.D., Paul, D.R., Kampa, J., Lagow, R.J., Modification of asymmetric polysulfone membranes by mild surface fluorination. J. Membr. Sci., 94, pp. 121–141, 1994. Liu, L.Q., Wagner, H.D., Rubbery and glassy epoxy resins reinforced with carbon nanotubes. Sep. Sci. Technol., 65(11–12), pp. 1861–1868, 2005. Ltd. ‘NanoTechCenter’, 2006. http://www.nanotc.ru Ma, P.-C., Mo, S.-Y., Tang, B.-Z., Kim, J.-K., Flexural, compression, chemical resistance, and morphology studies on granite powder-filled epoxy and acrylonitrile butadiene styrenetoughened epoxy matrices. J. Appl. Polym. Sci., 104(1), pp. 171–177, 2007. Ma, P-C., Mo, S-Y., Tang, B-Z., Kim, J-K., Dispersion, interfacial interaction and re-agglomeration of functionalized carbon nanotubes in epoxy composites. Carbon, 48(6), pp. 1824–1834, 2010. Maity, J., Jacob, C., Das, C.K., Alam, S., Singh, R.P., Homocomposites of chopped fluorinated polyethylene fiber with low density polyethylene matrix. Mater. Sci. Eng. A, 479, pp. 125–135, 2008. McIntosh, D., Khabashesku, V.N., Barrera, E.V., Nanocomposite fiber systems processed from fluorinated single-walled carbon nanotubes and a polypropylene matrix. Chem. Mater., 18(19), pp. 4561–4569, 2006. Mickelson, E.T., Chiang, I.W., Zimmerman, J.L. et al., Solvation of fluorinated single-wall c arbon nanotubes in alcohol solvents. J. Phys. Chem. B, 103(21), pp. 4318, 1999. Milker, R., Koch, A., Fluor makes plastics pliable. J. Adhes., 33, pp. 33–39, 1989. Mukherjee, M., Das, C.K., Kharitonov, A.P., Fluorinated and oxyfluorinated short Kevlar fiberreinforced ethylene propylene polymer. Polym. Compos., 27, 205, 2000.

Nanostructured Polymer Composites with Modified Carbon Nanotubes 407 Murali, R.S., Sankarshana, T., Sridhar, S., Kharitonov, A.P., Fluorinated polymer membranes for separation of industrial gas mixtures. J. Polym. Mater., 29, pp. 317–330, 2012. Nie, Y., Hubert, T., Effect of carbon nanofiber (CNF) silanization on the properties of CNF/ epoxy nanocomposites. Polym. Int., 60(11), pp. 1574–1580, 2011. Paradise, M., Goswami, T., Carbon nanotubes—production and industrial applications. Mater. Des., 28(5), pp. 1477–1489, 2007. Patton, R.D., Pittman, C.U., Wang, L., Hill, J.R., Vapor grown carbon fiber composites with epoxy and poly(phenylene sulfide) matrices. Compos. A, 30(9), pp. 1081–1091, 1999. Paul, D.R., Robeson, L.M., Polymer nanotechnology: nanocomposites. Polym. J., 49(15), pp. 3187–3204, 2008. Peyroux, J., Dubois, M., Tomasella, E., Brezet, L., Kharitonov, A.P., Flahaut, D., Enhancement of surface properties on low density polyethylene packaging films using various fluorination routes. Eur. Polym. J., 66, pp. 18–32, 2015. Robeson, L.M., Upper bound revisited. J. Membr. Sci., 320, pp. 390–400, 2008. Potts, J.R., Dreyer, D.R., Bielawski, C.W., Ruof, R.S., Graphene-based polymer nanocomposites. Polym. J., 52 (1), pp. 5–25, 2011. Prorokova, N.P., Istratkin, V.A., Kumeeva, T.Yu., Vavilova, S.Yu., Kharitonov, A.P., Bouznik, V.M., Improvement of polypropylene nonwoven fabric antibacterial properties by direct fluorination. RSC Adv., 88, pp. 127–136, 2015. Rana, S., Alagirusamy, R., Fangueiro, R. et al., Effect of carbon nanofiber functionalization on the in plane mechanical properties of carbon/epoxy multiscale composites. J. Appl. Pol. Sci., 125(3), pp. 1951–1958, 2012. Rana, S., Alagirusamy, R., Joshi, M., Effect of carbon nanofiber dispersion on the tensile properties of epoxy nanocomposites. J. Compos. Mater., 45, pp. 2247–2256, 2011. Rana, S., Alagirusamy, R., Joshi, M., Mechanical behavior of carbon nanofibre-reinforced epoxy composites. J. Appl. Polym. Sci., 118(4), pp. 2276–2283, 2010. Rakov, E.G., Preparation of thin carbon nanotubes by catalytic pyrolysis on a support. Usp. Khim., 76(1), pp. 3–26, 2007. Rangari, V.K., Yousuf, M., Jeelani, S., Pulikkathara. M.X., Khabashesku, V.N., Alignment of carbon nanotubes and reinforcing effects in nylon-6 polymer composite fibers. Nanotechnology, 19(24), pp. 245703, 2008. Roberts, J., Caserio, M., Basic Principles of Organic Chemistry, Mir publisher, Moscow, 842, 1978. Shulga, Y.M., Martynenko, V.M., Krestinin, A.V., Kharitonov, A.P., Davidova, G.I., Knerelman, E.I., Krastev, V.I., Schur, D.V., Mass-spectrometric investigation of gases evolved by fluorinated single-wall carbon nanotubes during heating. Int. J. Hydrogen Energy, 36, pp. 1349–1354, 2011. Smith, M. R., Bittner, E. W., Shi, W., Johnson, J. K., & Bockrath, B.C., Chemical activation of single-walled carbon nanotubes for hydrogen adsorption. J. Phys. Chem. B, 107(16), pp. 3752–3760, 2003. Socrates, G., Infrared Characteristic Group Frequencies, John Wiley & Sons, Chichester, p. 366, 2001. Taege, R., Ferrier, G., Kharitonov, A.P. Fluoration en surface des matériaux plastiques. EP Patent 1865017, 2007. Taege, R., Ferrier, G., Kharitonov, A.P. Process for reducing the permeability of plastics materials. EP Patent 1609815, 2005. Taege, R., Ferrier, G., Reliable, robust and safe: how surface oxyfluorination improves adhesion of coatings to plastics. Eur. Coat. J., 5–6, pp. 36–40, 2006. Tressaud, A., Durand, E., Labrugere C., Kharitonov, A.P., Kharitonova, L.N., Modification of surface properties of carbon-based and polymeric materials through fluorination routes: from fundamental research to industrial applications. J. Fluor. Chem., 128, pp. 378–391, 2007.

408 Handbook of Composites from Renewable Materials-Volume 7 Villoria, R.G., Miravete, A., Cuartero, J., Chiminelli, A., Tolosana, N., Mechanical properties of SWNT/epoxy composites using two different curing cycles. Compos. B, 37(4–5), pp. 273–277, 2006. Wu, A.S., Chou, T-W., Carbon nanotube fibers for advanced composite. Mater. Today, 15(7–8), pp. 302–310, 2012. Xu, J., Donohoe, J.P., Pittman, C.U., Preparation, electrical and mechanical properties of vapor grown carbon fiber (VGCF)/vinyl ester composites. Compos. A, 35(6), pp. 693–701, 2004. Yang, Z., McElrath, K., Bahr, J., D’Souza, N.A., Effect of matrix glass transition on reinforcement efficiency of epoxy-matrix composites with single walled and multi-walled carbon nanotubes, carbon nanofibers and graphite. Compos. B, 43(4), pp. 2079–2089, 2012. Zare, K., Gupta, V.K., Moradi, O., Makhlouf, A.S.H., Sillanpää, M., Nadagouda, M.N. & Kazemi, M., A comparative study on the basis of adsorption capacity between CNTs and activated carbon as adsorbents for removal of noxious synthetic dyes: a review. J. Nanostr. Chem., 5(2), pp. 227–236, 2015. Zhabrev, V.A, Moshnikov, V.A., Tairov, Yu.M., Fedotov, A.A., Shilova, O.A., The Sol–Gel Technology, St. Petersburg, Saint Petersburg Electrotechnical University “LETI”, 160, 2004. Zhang, W., Dubois, M., Guérin, K., Bonnet, P., Kharbache, H., Masin, F., Kharitonov, A.P., Hamwi, A., Effect of curvature on C-F bonding in fluorinated carbons: from fullerene and derivatives to graphite. Phys. Chem. Chem. Phys., 12, pp. 1388–1398, 2010. Zhang, W., Spinelle, L., Dubois, M., Guérin, K., Kharbache, H., Masin, F., Kharitonov, A.P., Hamwi, A., Brunet, J., Varenne, C. et al., New synthesis methods for fluorinated carbon nanofibres and applications. J. Fluor. Chem., 31(6), pp. 676–683, 2010. Zhou, Y., Pervin, F., Jeelani, S. et al., Improvement in mechanical properties of carbon fabricepoxy composite using carbon nanofibers. J. Mater. Process. Technol., 198(1–2), pp. 445–453, 2008.