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Effect of nano-silica and boron carbide on adhesion strength of high temperature adhesive based on phenolic resin for graphite bonding Seyyed Arash Haddadi, Mohammad Mahdavian-Ahadi, and Farhang Abbasi Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 2, 2014

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Industrial & Engineering Chemistry Research

Effect of nano-silica and boron carbide on adhesion strength of high temperature adhesive based on phenolic resin for graphite bonding Seyyed Arash Haddadi1, 2, Mohammad Mahdavian-Ahadi3,*, Farhang Abbasi1, 2 1

2

3

Institute of Polymeric Material, Sahand University of Technology, Tabriz, Iran

Polymer Engineering Departments, Sahand University of Technology, Tabriz, Iran

Surface Coating and Corrosion Department, Institute for Color Science and Technology, Tehran, Iran

*

Corresponding author - Email: [email protected]; Fax: +982122969771; Tel: +982122947537 1 Environment ACS Paragon Plus


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Abstract In this study, the effect of nano-silica and boron carbide on adhesion strength of a phenolic resin for graphite bonding was studied. The adhered specimens were cured at 250 ºC and then heat-treated to examine the thermal resistance in the range of 200-1000 ºC. Then, the adhesion strength of specimens was examined by tensile method. Chemical structure of bonding and possible carbonization was examined using Fourier transform infrared spectroscopy (FTIR). Thermo-gravimetric analysis (TGA) was employed to determine char yield and X-ray diffraction (XRD) to study crystalline phases at different temperatures. The elemental analysis and morphology of the adhesive bond was investigated using energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM), respectively. The results showed that sintering of reformed boron carbide and amorphous carbon above 800 ºC resulted in a significant increasing in adhesion strength at 1000 ºC. Keywords: Phenolic resin; Adhesion strength; Tensile; Carbonization

2 ACS Paragon Plus Environment

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1. Introduction Graphite is wildly used in the aerospace, metallurgy, pipeline, chemical, petrochemical, oil and gas industrial applications due to its good physical properties as well as chemical and thermal stability 1,2. Graphite materials usually have a large size and complex shape, so the use of mechanical methods such as riveting and bolting to join them may cause stress concentration3–6. Therefore, using the adhesive with high adhesion strength and good chemical and thermal stability is of particular interest. Organic and inorganic adhesives could be used for high temperature applications. Inorganic adhesives are heat resistant, but they are brittle which has restricted their use in bonding of structural components. Organic adhesives possess excellent bonding strength. The organic components begin to decompose up to 300 ºC; however, the residues derived from them may be converted into carbonaceous char at high temperature up to 1000 ºC which possess excellent thermo-physical properties 7–11. It is shown that some organic resins including phenol formaldehyde, epoxy, furan and pitch could be employed as the matrix of high temperature adhesives

11–13

. Among the mentioned

organic resins, phenolic resin has been proved to possess many good properties including heat resistance, corrosion resistance and the excellent adhesive strength

14,15

. However, the application of

pure phenolic resin in high temperature adhesives was also restricted due to the thermal degradation and failure

16,17

. At high temperatures, the drastic pyrolysis leads to the disintegration of polymeric

structure and subsequently resulting in the failure of resin and composite structure

8,11,18

. Therefore,

the phenolic resin should be modified for high temperature applications. Inclusion of some heat resistant chemical group such as B-O in the structure of phenolic resin chemical groups such as phenylmaleimide

23–26

19–22

, the use of more stable

and addition of multi-wall carbon nanotubes

27

and

boron carbide (B4C) 6,9,10,28–31 into the phenolic resin matrix has been shown to be effective to improve its thermal stability. In addition, inclusion of fumed silica with the optimal weight percentage of 47 wt. % is shown to be effective on the adhesive bond strength

10

. In this weight percentage, it is

reported that 387 % increase in bonding strength in comparison with pure resin was obtained at 800 ºC. Furthermore, addition of 50 wt. % of B4C induced a 725 % increase in bonding strength at 800 ºC



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and addition of the B4C and SiO2 in phenolic resin matrix lead to a 1050 % increase of the joint strength in comparison with pure phenolic resin at 800 ºC. It seems to be impossible to add the 47 wt. % of fumed silica in phenolic matrix because of its high surface area which restricts its application at high concentrations. Such a concentration is still high for some micro-fillers. This work intends to study the effect of inclusion of 3 wt. % SiO2 nanoparticles and 1 wt. % B4C in the phenolic resin on adhesion strength for graphite bonding. 2. Experimental 2.1. Raw materials The resole type phenolic resin with density 1.2 g/cm3 was obtained from Resitan resin factory (Iran). The viscosity of resin at 20 ºC temperature was 600-800 Pa.s. The solid content of resin was 75±1%. Nano silica was obtained from US-Nano Company and boron carbide from Aldrich with the properties tabulated in Table 1. The graphite was used as the substrate. The density and tensile strength of graphite were 2.15 g/cm3 and 14 MPa, respectively. Table 1. The characteristic parameters of the micro- and nano-fillers Material powder

Density (g/cm3)

Particle size

Purity (%)

SiO2

0.11

20-30 nm

95-98

B4 C

2.53

10 µm

98

2.2. The preparation of adhesive and graphite specimens The adhesive was prepared by dispersion of 3 wt. % nano-SiO2 and 1 wt. % B4C using a high speed disperser (6000 rpm) into the phenolic resin matrix. The ice bath was used to prevent heating of adhesive material during mixing. Graphite plate after cleaning by acetone was cut into the samples with dimensions of 30×20×10 and 10×20×10 mm3. The configuration of the samples to prepare lap test specimens is shown in Figure 1a. An industrial phenolic base adhesive (grade ARMC-669, part number AR001845 supplied from 4 ACS Paragon Plus Environment

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Aremco products, Inc.) was used to fix the small samples on the big samples. Then, the graphite sample ensembles were cured at 250 ºC for 1 h. After that, the surface of graphite sample ensembles to be bonded was cleaned by acetone. Then, the adhesives were brushed on the surface and the two graphite sample ensembles were bonded together at room temperature for 1 day and then at 250 ºC for 1 h. The bonding area was 95 ± 5 mm2. Then, the assembled specimens were treated at different temperature ranging from 200 to 1000 ºC in tubular furnace TF5/40-1250 (Azar Furnaces, Iran) at a constant rate of 5 ºC/min and maintained at each temperature for 90 min. Then, the specimens were cooled down to room temperature at a constant rate of 0.5 ºC/min. After cooling, the specimens were fixed on jaw fullers and tensile tests were performed on graphite specimens according to Figure 1b. Figure 1. Schematic representation of (a) configuration of specimens and (b) bonding strength measurement using lap test. 2.3. Testing and analysis The bonding strength of graphite joints treated at different temperatures was tested using universal tensile machine Roell-Z010 (Zwick, Germany) at room temperature with a loading rate of 1 mm/min under shear condition. The thermogravimetric analysis (TGA) was carried out under nitrogen atmosphere and air using the Thermal Gravimetric (TG) Analyzer L-801I (LINSEIS, Germany) at the range of 25 to 800 ºC with heating rate of 10 ºC/min. The adhesive materials after heat treatment were analyzed by Fourier transforms infrared (FT-IR) spectrometer model Tensor 27 (Bruker) and X-ray diffraction (XRD) analyzer model D8-Avance (Bruker). A scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectrometer (EDS) model Cam Scan MV2200 (Vega Tescan, Czech Republic) was employed to investigate the structure and morphology of the adhesive materials heat treated at different temperatures. To measure the rheological characteristics of compounded and blank phenolic resin a rheometric mechanical spectrometer (RMS) model MCR301 (Anton Paar, Austria) was used. All the measurements were carried out in parallel plate geometry within frequency range of 0.04-625 s-1. The



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diameter of both upper and lower plates was 25 mm and the gap between two parallel plates was 1 mm. The rheological measurements were carried out at 23 ºC under a nitrogen atmosphere. 3. Results and discussion 3.1. The bonding strength results Figure 2 shows the results of bonding strength of graphite specimens bonded by the blank phenolic resin and the phenolic resin containing nano-SiO2 and B4C heat-treated at temperatures ranging from 200 to 1000 ºC. The adhesion strength of specimens was calculated by measuring the highest

stress (force per jointed surface) in the stress-strain plot where the adhesive joint failed. However, for some specimens fracture occurred through graphite itself. For these specimens, adhesion strength of the joint was considered to be equal to the cohesive strength of the graphite.

Therefore, the tensile strength for graphite (14 MPa) was calculated by measuring the highest stress (force per cross section area of graphite) in the stress-strain plot where the cohesive failure of graphite occurred. Figure 2. The bonding strength of graphite components bonded by: (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2 and B4C at different temperatures. According to Figure 2a, when the treatment temperature was 200 ºC, the blank phenolic resin showed a maximum bonding strength which could be due to the outstanding wettability and good properties of the resin 10. The bonding strength decreased markedly from 4 to 0.8 MPa with increasing of the heattreatment temperature from 200 to 800 ºC. At temperature ranging 400-600 ºC, the volatile components such as water, free phenol, CO2 can be released leading to a large amount of volume shrinkage resulting in many defects in adhesive's structure in the form of voids and cracks

30,32,33

.

Consequently, internal stress was formed in adhesive's structure 13,34. At 800 ºC, carbonization of resin was completed and the resin's structure was destroyed and converted to amorphous carbon. With increasing of temperature from 800 to 1000 ºC, the residue amorphous carbon was sintered and the


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bonding strength increased to 2.2 MPa. Consequently, the blank phenolic resin cannot be used for high temperature applications. The bonding strength of graphite specimens bonded by the phenolic resin containing nano-SiO2 and B4C at different temperatures is also shown in Figure 2b. It can be found that the bonding strength of the phenolic resin containing nano-SiO2 and B4C at all temperatures is higher than that of the blank phenolic resin. The bonding strength is considerably high at 200 ºC and 400 ºC. It seems that the existence of SiO2 and B4C as fillers in resin's structure reduced the volatile content 10, which results in lower shrinkage and internal stresses. Above 400 ºC, adhesive's matrix was involved in the carbonization reaction and the majority of volatile component was released as the temperature increased. It is shown that conversion of B4C to B2O3 with low melting point (450 °C) can overcome volume shrinkage in adhesive's matrix

30

providing 250 % volume increase. The melted B2O3 have a

good wettability and chemical properties. So a kind of chemical interaction between graphite surface and B2O3 could be formed according to Figure 3 30,35. Figure 3. Formation of chemical bonding between B2O3 and graphite substrate 30,35. Formation of chemical interaction can support the higher bonding strength at temperature range of 400-800 ºC compared with the blank phenolic resin. At 800 ºC, the network of resin was destroyed and resin was converted to amorphous carbon 10,30. Considerable increase of bonding strength at 1000 ºC is related to reformation of B4C discussed in section 3.2, 3.4 and 3.5. The outstanding bonding strength of the phenolic resin containing nano-SiO2 and B4C especially above 800 ºC reveals that it could be used in high temperature applications. It should be mentioned that the results reported

here are for the specimens heat treated and then were cooled to room temperature. During the cooling step two major processes could occur which makes the properties of the cooled adhesive material different from that at elevated temperatures. Molten species (e.g. B2O3) solidify during cooling. The solidification of molten species could enhance adhesion strength of the cooled adhesive material compared to that at elevated temperatures. In addition,



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thermal stress may develop due to the difference between the thermal expansion coefficients of the adhesive and graphite. 3.2. FTIR results Fourier transform infrared spectroscopy was used to investigate the structural evolution of the phenolic resin containing fillers at different heat-treatments. The results are shown in Figure 4. Figure 4. The FTIR spectrum of (a) B4C, (b) B4C at 700 ºC, (c) SiO2, the phenolic resin containing nano-SiO2 and B4C at different temperatures including (d) 200 ºC, (e) 600 ºC, (f) 1000 ºC. The spectrum of B4C without heat treatment is shown in Figure 4a. The weak absorption band occurring at 685 cm-1 is assigned to the B-B bond 36. The characteristics peaks at 830 and 1116 cm-1 are assigned to the B-B-B-C and B-C bonds, respectively

36,37

. Figure 4b shows the spectrum of B4C

after heat treatment at 700 ºC. The characteristic peaks of B4C were disappeared for the heat treated B4C. The peaks at 720, 930-950 and 2503 cm-1 are assigned to [BO3], [BO4] and B-H bonds, respectively 37,38. The strong absorption band at 1437 cm-1 is attributed to the stretching of B-O bond 38,39

. Therefore, it seems that heat treatment resulted in the conversion of B4C to B2O3. This finding is

in accordance with the previous reports on conversion of B4C to B2O3 above 550 ºC. It is shown that at 550 ºC, B4C reacts with O2 and other oxygen-containing components such as CO2, H2O and CO resulted from pyrolysis of the resin material leading to formation of B2O3 as follows 6,10,28,30:

(1) (2) (3) Figure 4c shows the characteristic absorption bands of SiO2. The peaks at 489 and 1108 cm-1 are assigned to the asymmetric stretching of Si-O-Si and the peak at 790 cm-1 is assigned to the network of Si-O-Si 40,41. The spectrum of the phenolic resin containing nano-SiO2 and B4C heat-treated at 200 ºC is shown in Figure 4d. The wide and sharp peaks at 3444 cm-1 are assigned to the hydroxide groups


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of phenol and water, respectively39. The peaks appearing at 1472 and 1630 cm-1 can be attributed to the stretching of C-H bond in benzene rings 32,41. The peaks at 1855 and 1924 cm-1 can be assigned to the C-H bond in aliphatic groups such as methylene linkages

6,33

. The existence of fillers in resin

restricts the release of volatile component (such as phenol, acetone, xylene, etc.) into the surrounding environment. As a result, lots of small peaks can be observed ranging from 500 to 1150 cm-1 which can be attributed to the volatile components

6,11

. The spectrum of the phenolic resin containing nano-

SiO2 and B4C heat-treated at 600 ºC is shown in Figure 4e. The peaks at 466 and 1111 cm-1 are assigned to the SiO2. The peaks of the formed B2O3 and stretching C-H bond are overlapped that can be observed at 1467 cm-1. The peaks of CO and CO2 can be observed at 2310 and 2381 cm-1

39,41

.

Because of difference between electron-negativity values of boron and silicon atoms, there could be an electron attraction between SiO2 and B2O3, so the B-O-Si can be formed. As a result, the new peak that can be observed at 930 cm-1 is assigned to the borosilicate bond 6. Figure 4f shows the FTIR spectrum of the phenolic resin containing nano-SiO2 and B4C heat-treated at 1000 ºC. The peaks at 687, 802 and 914 cm-1 are assigned to the B4C

36,37

. This result is in

accordance with the previous report of reformation of B4C above 800 ºC due to the reaction of the amorphous carbon from carbonization of resin with B2O3. It is shown that above 800 ºC, the amorphous carbon could react with B2O3. Consequently, the B4C could be reformed as follows 36: (4) (5)

The peaks related to reformed B4C (Figure 4f) are somehow different from the pure B4C (Figure 4f) which could be related to its sintering with the amorphous carbon. 3.3. TGA results The TGA curves of the blank phenolic resin and the phenolic resin containing nano-SiO2 and B4C are shown in Figure 5.



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Figure 5. TGA curves of (a) the blank phenolic resin under N2 atmosphere, (b) the phenolic resin containing nano-SiO2 and B4C under N2 atmosphere, (c) the phenolic resin containing nano-SiO2 and B4C under air atmosphere. Figure 5a shows the TGA pattern of the blank phenolic resin. It can be seen that the content of carbon char is 61.4 % after being carbonized and decomposed at 800 ºC. The maximum reducing of weight was happened at temperature ranging of 370-560 ºC. In this range, all of volatile component was released and the blank phenolic resin converted to the carbon char. Figure 5b shows the TGA pattern of the phenolic resin containing nano-SiO2 and B4C under N2 atmosphere. Although the contents of fillers in resin was 4 %, but the content of carbon char was 75.6 % that was 14.2 % higher than that for the blank phenolic resin. The thermal decomposition of resin was delayed considerably because of the existence of fillers in the resin. In the presence of fillers, the volatile products of pyrolysis can adsorb onto the surface of the fillers and react with the B4C, which results in a considerable decrease in the weight loss during heat treatment (see section 3.2). Figure 5c shows the TGA pattern of the phenolic resin containing nano-SiO2 and B4C under air atmosphere. The result shows a considerably higher char yield in air atmosphere compared to the N2 atmosphere. In air atmosphere, there are enough O2 for conversion of all B4C to the B2O3 as follows:

(6) When the B4C converted to B2O3 each mole of B4C produce 2 moles B2O3 and 1 mole carbon. In addition, the molecular weight of B2O3 is much higher than B4C. Therefore, as expected, the char yield of the phenolic resin containing nano-SiO2 and B4C was much higher in presence of O2 than that in N2 atmosphere. 3.4. XRD results XRD patterns of the phenolic resin containing nano-SiO2 and B4C treated at different temperatures are shown in Figure 6.


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Figure 6. X-ray diffraction patterns of the phenolic resin containing nano-SiO2 and B4C heat-treated for 1 h in air at different temperatures (a) 200 ºC, (b) 600 ºC, (c) 1000 ºC. Figure 6a shows the X-ray diffraction pattern of filled resin at 200 ºC. The XRD results at 200 ºC indicates that the main crystalline phase is B4C; however, there are some minor phases including B11.72C3.28, B12C3 and B13C2 36,42. X-ray diffraction pattern of modified resin treated at 600 ºC is shown in Figure 6b. The B4C, B2O3, SiO2 and borosilicate could be identified from the spectrum 36,38. Figure 6c shows the X-ray diffraction pattern of modified resin at 1000 ºC. The identified phases were B4C reformed at 1000 ºC and SiO2. At 1000 ºC, the resin was carbonized and the all of volatile components were released. Therefore, the content of SiO2 in resin increased and the uniform network of organic resin was destroyed. Figure 7 shows the characteristic peak of B2O3 at different heattreatment temperature. Figure 7. X-ray diffraction pattern of B2O3 in adhesive at different temperatures. The characteristic peak of B2O3 appears at 2θ = 28 which could be observed at 600 ºC

38

. It is

mentioned that the B2O3 reacts with carbon residue above 800 ºC and B4C is reformed so the characteristic peak of B2O3 could not be observed at 1000 ºC. This result confirms the FTIR results where reformation of B4C was observed at 1000 ºC (see section 3.2). As a result, the bonding strength increase reaching to 14 MPa at 1000 ºC (shown in section 3.1) could be caused by sintering of the residue amorphous carbon and reformed B4C. 3.5. SEM-EDS results The structural morphology evaluation of adhesives on graphite substrate was investigated using SEMEDS. Figure 8 shows the voids, cracks and micro-cavities on graphite surface. Figure 8. SEM image of graphite surface. The existence of these micro-cavities increases the contact area between adhesive and substrate and that is one of the good reasons for high bonding strength of the phenolic adhesive.



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Figure 9. The micrographs of graphite joint bonded by the phenolic resin containing nano-SiO2 and B4C heat-treated at 300 ºC (a) 200×, (b) 5000×, (c) EDS spectrum of B4C particle. Figure 9a shows the morphology of graphite substrate bonded by the phenolic resin containing nanoSiO2 and B4C heat-treated at 300 ºC. There are several cracks on adhesive's surface, which could be attributed to the release of the volatile components at 300 ºC. The dispersion of SiO2 particles in resin is shown in Figure 9b. It can be observed that SiO2 particles were dispersed uniformly and there is no shrinkage in adhesive's structure. The micro-particles detected in Figure 9a are composed of B4C confirmed by EDS. Figure 9c and Table 2 show the results of EDS spectrum of B4C particle. Table 2. The EDS analysis results of micro-grain shown in Figure 9c. Element

Wt %

At %

B

67.3

72.81

C

27.9

23.8

O

4.8

3.39

The boron and carbon with high concentration exist in the grain shown in Figure 9a, which shows that B4C was not oxidized at 300 ºC. The morphology and the EDS spectrum of the adhesive film heat treated at 900 ºC are shown in Figure 10a-c. The composition of the adhesive film heat-treated at 900 ºC obtained from EDS is indicated in Table 3. Figure 10. (a) the morphology and (b) EDS spectrum of the small particles and (c) EDS spectrum of large particles formed in the adhesive film heat treated at 900 ºC.


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Table 3. The EDS analysis results of the small particles shown in Figure 10a. Element

Wt %

At %

B

52.85

60.07

C

11.46

11.32

O

35.69

28.61

The results of EDS indicate the carbon concentration reduction and oxygen concentration increment at 900 ºC compared to 300 ºC. So, the small particles shown in Figure 10a are assigned to the B2O3. The composition of the large particles shown in Figure 10a is listed in Table 4. The higher carbon/oxygen ratio for the large particles is an indication of reformation of B4C, which was postulated from the FTIR and XRD results. Table 4. The EDS analysis results of the large particles shown in Figure 10a. Element

Wt %

At %

B

58.47

61.3

C

31.34

25.33

O

10.19

13.37

3.6. RMS results The effect of SiO2 and B4C on rheological behavior of phenolic resin was investigated using the RMS method. Figure 11 shows the complex viscosity of pure and filled phenolic resin as functions of angular frequency. Figure 11. Complex viscosity (η*) versus angular frequency at 23 ºC for (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2, (c) the phenolic resin containing nano-SiO2 and B4C. Polymeric materials generally show power law and pseudo plastic behaviors

43,44

. It means that with

increase of the angular frequency (shear-rate), the complex viscosity reduces and storage modulus increases due to the sliding of the polymer chains over each other because of increase in the shear 13 ACS Paragon Plus Environment


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rate. Figure 11b shows that that the complex viscosity of the phenolic resin containing nano-SiO2 is higher than the blank phenolic resin (Figure 11a) at all of frequency rang due to the presence of SiO2. The surface of SiO2 has considerable amount of hydroxide group

45

; therefore, the H-bonding force

can be formed between resin chains and surface hydroxide groups on the SiO2. Such an interaction is schematically shown in Figure 12 30,46. Figure 12. Formation of H-bonding between resin chains and SiO2 particle. Formation of this H-bonding interaction leads to formation of a physical framework in resin structure, which is responsible for the increase in the complex viscosity in all of frequency rang. Similar rheological behavior has been observed where fumed silica was included in the polyurethane adhesive 47,48

.

Results of the storage modulus of pure and filled phenolic resin as functions of angular frequency are shown in Figure 13. Figure 13. Storage modulus versus angular frequency at 23 ºC for (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2, (c) the phenolic resin containing nano-SiO2 and B4C. Storage modulus at low frequency could be an indication of chemical interaction between different components. By increase of the angular frequency, the effect of interfacial tension and interaction between particles is reduced and the effect of molecular weight of resin will be more effective. As a result, the storage modulus of the phenolic resin containing nano-SiO2 adhesive is much higher than the blank phenolic resin at low angular frequencies. These results is similar to rheological behavior observed for the epoxy resin filled with fumed silica 45,49. Figure 14 compares the results of damping factor for pure and the filled phenolic resin as functions of angular frequency. Figure 14. Damping factor (tan δ) versus angular frequency at 23 ºC for (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2, (c) the phenolic resin containing nano-SiO2 and B4C.


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Damping factor of the filled resin is lower than that for pure resin in all frequency ranges, which shows that fillers prevent sliding of resin chains and moving past one another. The complex viscosity, storage modulus and damping factor of the phenolic resin containing nanoSiO2 and B4C, compared to that of the phenolic resin containing only nano-SiO2 reveals that the presence of B4C has no considerable effect on the rheological behavior of the formulated adhesive and the formation of hydrogen bonding between nano-SiO2 and phenolic resin caused the different rheological behavior for the modified resin in comparison with the pure resin. So the addition of 3 wt. % of nano-SiO2 in phenolic resin increased the viscosity, imparted pseudo-plasticity to the adhesive which makes it more convenient for industrial applications. 4. Conclusions The resole type phenolic resin was used as a matrix for high temperature adhesive. Pure resin adhesive did not exhibit satisfactory bonding strength during treatment at different temperatures. Increasing the temperature led to carbonization and disintegration of organic resin which resulted in the raise of internal stress, the destruction of the adhesive framework and decreasing of bonding strength. The adhesive formulated by nano-SiO2 and B4C exhibited satisfactory bonding strength at all temperature ranges. The formation of H-bonding between phenolic resin and surface of nano-SiO2 improved the rheological properties of modified resin. In addition, inclusion of nano-SiO2 and B4C reduced the shrinkage and stress concentration during heat treatment. At low temperatures, up to 400, ºC, the B4C additive only reduced the content of volatiles components in the resin. Above 600 ºC, the B4C was converted to B2O3. Formation of B2O3 caused a 250 % volume increase. In addition, formation of chemical interaction between B2O3 with nano-SiO2 and graphite substrates led to satisfactory bonding strength at 1000 ºC where fracture occurred through graphite itself. Consequently, resin containing nano-SiO2 and B4C fillers can be used effectively as a high temperature adhesive for graphite bonding.



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References (1)

Pierson, H. O. Handbook of Carbon, Graphite, Diamond, and Fullerenes: Properties, Processing, and Applications; Noyes Publications Park Ridge, NJ, 1993.

(2)

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Preghenella, M.; Pegoretti, A.; Migliaresi, C. Thermo-Mechanical Characterization of Fumed Silica-Epoxy Nanocomposites. Polymer (Guildf). 2005, 46, 12065. 19 ACS Paragon Plus Environment


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For Table of Contents Only


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Figure 1. Schematic representation of (a) configuration of specimens and (b) bonding strength measurement using lap test. 242x154mm (96 x 96 DPI)

ACS Paragon Plus Environment


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Figure 2. The bonding strength of graphite components bonded by: (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2 and B4C at different temperatures. 130x94mm (96 x 96 DPI)


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Figure 3. Formation of chemical bonding between B2O3 and graphite substrate. 203x186mm (96 x 96 DPI)



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Figure 4. The FTIR spectrum of (a) B4C, (b) B4C at 700 ºC, (c) SiO2, the phenolic resin containing nanoSiO2 and B4C at different temperatures including (d) 200 ºC, (e) 600 ºC, (f) 1000 ºC. 184x110mm (96 x 96 DPI)


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Figure 5. TGA Curves of (a) the blank phenolic resin under N2 atmosphere, (b) the phenolic resin containing nano-SiO2 and B4C under N2 atmosphere, (c) the phenolic resin containing nano-SiO2 and B4C under air atmosphere. 129x86mm (96 x 96 DPI)



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Figure 6. X-ray diffraction patterns of the phenolic resin containing nano-SiO2 and B4C heat-treated for 1 h in air at different temperatures (a) 200 ºC, (b) 600 ºC, (c) 1000 ºC. 178x189mm (96 x 96 DPI)


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Figure 7. X-ray diffraction pattern of B2O3 in adhesive at different temperatures. 242x146mm (96 x 96 DPI)



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Figure 8. SEM image of graphite surface. 138x141mm (96 x 96 DPI)


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Figure 9. The micrographs of graphite joint bonded by the phenolic resin containing nano-SiO2 and B4C heat-treated at 300 ºC (a) 200×, (b) 5000×, (c) EDS spectrum of B4C particle. 259x286mm (96 x 96 DPI)



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Figure 10. (a) the morphology and (b) EDS spectrum of the small spheres and (c) EDS spectrum of large particles formed in the adhesive film heat treated at 900 ºC. 226x288mm (96 x 96 DPI)


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Figure 11. Complex viscosity (η*) versus angular frequency at 23 ºC for (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2, (c) the phenolic resin containing nano-SiO2 and B4C. 123x75mm (96 x 96 DPI)



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Figure 12. Formation of H-bonding between resin chains and SiO2 particle. 189x134mm (96 x 96 DPI)


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Figure 13. Storage modulus versus angular frequency at 23 ºC for (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2, (c) the phenolic resin containing nano-SiO2 and B4C. 127x80mm (96 x 96 DPI)



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Figure 14. Damping factor (tan δ) versus angular frequency at 23 ºC for (a) the blank phenolic resin, (b) the phenolic resin containing nano-SiO2, (c) the phenolic resin containing nano-SiO2 and B4C. 121x76mm (96 x 96 DPI)


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