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Development of novel TGDDM epoxy nanocomposites for aerospace and high performance applications – Study of their thermal and electrical behaviour
⁎Corresponding author. Tel.: +91 44 24793886; fax: +91 44 22603743. shreemeenakshik@gmail.com (K. Shree Meenakshi)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The present work focuses on a study of the thermal and electrical behaviour of pure N,N′-tetraglycidyl diaminodiphenylmethane (TGDDM) for use in aerospace, high performance applications. The synthesis of the tetraglycidyl epoxies was done and they were characterized by FT-IR (Fourier transform infrared spectra) and Nuclear magnetic resonance spectra (1H NMR and 13C NMR). Nanoclay and POSS-amine nanoreinforcements denoted as N1 and N2 were incorporated into the synthesized epoxy resins. Curing was done with diaminodiphenylmethane (DDM) and bis(3-aminophenyl)phenylphosphine oxide (BAPPO) curing agents denoted as X and Y, respectively. The thermal behaviour of the tetraglycidyl resins and their corresponding nanocomposites was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The electrical behaviour namely dielectric strength, comparative tracking index (CTI), volume resistivity, surface resistivity and arc resistance of the nanocomposites was also studied and the interesting results obtained are discussed.
Keywords
Epoxy resin
Thermal stability
Electrical behaviour
Thermogravimetric analysis
Differential scanning calorimetry
1 Introduction
Epoxy resins have been widely used for engineering components, adhesives, electric, electronic industries and also as a matrix for fibre-reinforced composites owing to their exceptional combination of properties such as excellent toughness, adhesion and chemical resistance (Shree Meenakshi et al., 2011; Clayton, 1988; Kornmann et al., 2002; Potter, 1970; May and Tanaka, 1973; Bouer, 1979; Lee and Neville, 1967; Mark and Gaylord, 1969; Yee and Pearson, 1986; Kinloch et al., 1983). However, they do not possess adequate thermal and mechanical properties to meet the requirements of high performance structural products. Hence, modification of epoxy resins using suitable modifiers such as phosphorus, sulphone, silicone, polyhedral oligomeric silsesquioxanes (POSS) and nanoclay is mandatory.
Demands for epoxy resins are extremely strong because of their wide application as adhesives, coatings and as advanced composites in aerospace and electronic industries (Potter, 1970; Hergenrother et al., 2005). Incorporation of sulfone unit into the epoxy resin makes the resin become transparent, helps the resin withstand exposure to water, chemicals, increases the resistance of the resin to thermal oxidation and leads to easy processing. Polyhedral oligomeric silsesquioxanes (POSS) reagents are emerging as a new chemical technology for the nano-reinforced organic–inorganic hybrids and the polymers incorporating POSS monomers are becoming the focus for many studies due to the simplicity in processing and the excellent mechanical properties, thermal stability and flame retardation (Choi et al., 2001). POSS are a family of nanoscale inorganic cage structures containing a robust silicon/oxygen framework that are intermediate between silica (SiO2) and silicone (R2SiO). POSS can be easily incorporated into common plastics by means of co-polymerization, blending or grafting. Incorporation of POSS into polymers like acrylics, styryls, epoxy and polyethylene has led to enhancements in thermal stability, mechanical properties, glass transition, degradation temperatures, oxygen permeability, reduced flammability and heat evolution as well as modified mechanical properties relative to conventional organic systems (Li et al., 2001).
Although the organic-clay complexes have been recognized for a long time, the interest in studying these layered silicate materials as nanoscale-reinforcing agent for polymeric materials has only been developed recently. The introduction of as low as 1–5 wt. % of montmorillonite (MMT) into polymer matrix is well known to result in significant improvement in mechanical strength due to nanometric dimensions and high aspect ratio (Kornmann et al., 2002). The nanoscopic phase distribution as well as synergism between polymer and the layered silicate results in additional properties, such as flame retardation, enhanced barrier properties and ablation resistance (Gacitua, 2005). Layered silicates are proven to possess excellent barrier resistance against the movement of water or gas molecules and other chemicals due to their high aspect ratio. The present work focuses on a study of the thermal and electrical behaviour of TGDDM for use in high performance applications.
2 Experimental
2.1 Materials
All chemicals are reagent grade and were used without further purification. 4,4′-Diamino diphenyl methane (DDM) was obtained from Huntsman, USA. Epichlorohydrin and sodium hydroxide were obtained from SD Fine chemicals, India. Triphenylphosphine oxide was obtained from Alfa-Aaser, Germany. Aminopropyltrimethoxysilane (γ-APS), stannous chloride and sulfuric acid were obtained from Merck (Germany). Tetrahydrofuran (THF) and benzene were obtained from Sisco research laboratories, India. Nanoclay was obtained from Nanocor, USA. Hydrochloric acid was obtained from Hi-pure, India.
2.2 Synthesis of N,N′-tetraglycidyl diaminodiphenylmethane (TGDDM)
Epichlorohydrin (6.25 mol) was taken in a 1 l three necked round bottomed flask provided with a mechanical stirrer, nitrogen atmosphere and a water condenser. This was heated to 50 °C in an oil bath. Then 1.5 mol of DDM was added and stirred continuously for 4 h at about 50–55 °C temperature under nitrogen atmosphere. Chlorohydrin, the intermediate product was formed and the excess epichlorohydrin used was distilled off under vacuum. Then 2.84 mol of 40% NaOH solution was added drop-wise for 1 h at 54 °C. The reaction was continued at this temperature for a further period of 1 h. The resulting solution was extracted with chloroform. The organic layer was collected and concentrated at a reduced pressure to get the light brown coloured liquid epoxy product (Shih and Ma, 1998; Jagdeesh and Shashikiran, 2004; Liu et al., 1997). The synthesis of TGDDM is shown in Scheme 1.
Synthesis of TGDDM.
2.3 Synthesis of phosphorus containing diamine
2.3.1 Synthesis of Bis(3-aminophenyl)phenylphosphine oxide (BAPPO)
Triphenylphosphine oxide (0.1 M) was taken in a 500 ml round bottomed flask equipped with a stirrer, nitrogen atmosphere and 200 ml of 96% sulfuric acid was added to it. The reactant was dissolved and the reaction system was cooled to −5 °C with an ice/salt bath. A solution (0.4 M) of fuming nitric acid in 100 ml of sulphuric acid was added drop wise over a period of two hours. The reaction system was kept at room temperature for another eight hours. The reaction mixture was hydrolysed over ice. After the melting of ice, the mixture was extracted with chloroform and washed with aqueous sodium bicarbonate solution until neutral pH. The solvent was removed. The solid product bis(3-nitrophenol)phenylphosphine oxide (BNPPO) was recrystallized from absolute ethanol.
A calculated amount of BNPPO was taken in a 1000 ml round bottomed flask with 180 g of anhydrous powder tin (II) chloride. A solution of 200 ml fuming hydrochloric acid in 400 ml ethanol was introduced in to the flask. The reaction mixture was stirred at room temperature for 5 h. The solution was concentrated and then neutralized by 25% of aqueous sodium hydroxide solution. The obtained solution was extracted with chloroform. The organic layer was collected and concentrated under reduced pressure. The obtained solid was recrystallized from dichloromethane to give a pure product of bis(3-aminophenyl)phenylphosphine oxide (BAPPO). The synthesis of BAPPO (Y) is depicted in Scheme 2.
Synthesis of bis(3-aminophenyl)phenylphosphine oxide (BAPPO).
2.4 Synthesis of amine functionalized POSS (POSS-NH2)
The synthesis of POSS-amine was carried out as per the reported procedure (Liu and Zheng, 2005). Stoichiometric amount of POSS-triol (1.0 mol) dissolved in dry toluene was mixed with aminopropyl triethoxysilane (1.0 mol) in a 50 ml round bottomed flask and refluxed for eight hours at 90 °C. The filtrate was then subjected to solvent evaporation to obtain the desired product, which was confirmed by FT-IR and NMR spectra, respectively. The synthesis is shown in the Scheme 3.
Synthesis of amine functionalized POSS (POSS-NH2).
2.5 Fabrication of resin laminates
In order to study the properties of the epoxy resins, neat resin laminates were prepared by curing the six different synthesized tetraglycidyl epoxy resins by X and Y curing agents as shown in Table 1.
Type of epoxy
Matrix name
Nanoreinforcement
Curing agent
TGDDM
AX
–
X
AXN1
N1
X
AXN2
N2
X
AY
–
Y
AYN1
N1
Y
AYN2
N2
Y
For the fabrication of nanocomposites, the N1 (Nanocor 1.30E) was dried at 24 h at 50 °C under vacuum. The epoxy resin was mixed mechanically in a reaction vessel with the nanoclay at 50 °C for 3 h. Then it was further mixed in an ultrasonic bath for 30 min to disperse the clay in the resin. Later the mixture was cooled to room temperature in 30 min. The curing agent was then added. After mixing mechanically for 10 min, the mixture was degassed by a vacuum pump to remove the air bubbles and poured into moulds. The nanocomposites were cured for 3 h at 120 °C and post-cured for 2 h at 200 °C. After that the resin plaque was cooled to room temperature naturally. The nanoreinforcement N2 was mixed with the epoxy resin and then cured. Blends of epoxy, N2 and curing agent were molten at 100 °C for 25 min and then poured into a mould coated with a release agent on the inner walls of the mould that was preheated to 120 °C. The curing cycle was 180 °C for 3 h and 220 °C for 2 h. After that the resin plaque was cooled to room temperature naturally, and it was cut into specimens of required dimensions, required for different testing and evaluation studies.
2.6 Test methods
The FT-IR spectra were recorded on a Perkin-Elmer 781 infrared spectrometer. 1H NMR spectra was run on a Bruker 400 MHz spectrometer using CDCl3 and DMSO as solvent and tetramethylsilane as the internal standard proton decouple. Thermo gravimetric analysis of the polymeric matrices is carried out in TGA-Thermal Analyst Perkin Elmer (TA instrument USA) at a heating rate of 10 °C per minute in an inert atmosphere to determine thermal degradation temperature and percentage weight loss. Dielectric strength was determined as per ASTM D149 in a dielectric strength tester and expressed in kV/mm. Arc resistance was measured as per ASTM D495 on a test specimen of thickness 3.17 mm. In order to assess the performance of the insulating material, the resistance offered by the polymeric material to electrical current along a 1 cm2 surface or through the thickness was determined using a resistivity metre as per ASTM D257. In the comparative tracking index test, the specimen was exposed to either fifty or hundred drops of an aqueous contaminant solution of ammonium chloride and a wetting agent that produced tracking on the surface of the specimen. It was measured as per ASTM D3638.
3 Results and discussion
3.1 Spectroscopy analysis
3.1.1 FT-IR spectroscopy analysis
The FT-IR spectrum of TGDDM is shown in Fig. 1. The peaks corresponding to the –CH2 group of TGDDM appear at 2900 cm−1. The peaks at 1600 cm−1 correspond to the presence of aromatic group. Furthermore, absorption peaks which appear at 900–910 cm−1 confirmed the presence of the epoxy group in the prepared resin.
FT-IR spectra of TGDDM.
The FT-IR spectrum of bis(3-aminophenyl)phenylphosphine oxide (BAPPO) is shown in Fig. 2. The characteristic peaks that appear at 3499 and 3393 cm−1 confirm the presence of the Ph–NH2 group. The absorption band for P–O–Ph stretching appears at 1093 cm−1. The peak appearing at 1277 cm−1 confirms the presence of P⚌O group. Other FT-IR absorption band at 1498 cm−1 is attributed to the P–Ph stretching. The FT-IR spectrum of POSS-NH2 is depicted in Fig. 3.The peaks corresponding to the –NH2 and –CH2 groups of POSS-NH2 appear at 3500 and 2900 cm−1, respectively. Furthermore, absorption peaks that appear between 1200, 1300 and at 1500 cm−1 confirm the presence of Si–O–Si linkage and –Si–CH2, respectively.
FT-IR spectra of BAPPO.
FT-IR spectra of POSS-amine.
3.1.2 1H and 13C NMR spectrum analysis
the 1H NMR spectrum of TGDDM is shown in Fig. 4. The signals between 6.5–8.0 ppm maybe due to the aromatic protons. The remaining oxirane and methylene protons adjacent to oxirane appear at 3.0–4.0 ppm. the 13C NMR spectrum of TGDDM is shown in Fig. 5.
1H NMR spectra of TGDDM.
13C NMR spectra of TGDDM.
The signal at 50 ppm may be due to the presence of the –CH– of the epoxy group. The signal which appears at 44 ppm may be due to the –OCH2– of the epoxy group. The signal at about 65 ppm may be due to the presence of –N–CH2– carbons. The remaining signal that appears at around 120–140 ppm may be due to aromatic carbons. the 1H NMR spectrum of BAPPO is shown in Fig. 6. The resonance signal at 3.45 ppm confirms the presence of –NH2 group. The signal at 6.62–7.13 ppm may be due to the aromatic amine proton (C6H4–NH2). The remaining signal that appears at 7.4–7.9 ppm is due to aromatic protons.
1H NMR spectra of BAPPO.
the 13C NMR spectrum of BAPPO is shown in Fig. 7. The signal at 40 ppm is due to the presence of the –C–P group. The signal at about 30 ppm may be due to the –CH–NH2– group. The remaining signals appearing at around 120–150 ppm are due to aromatic carbons. The single signal in the 31P spectra at around 30 ppm as seen in Fig. 8 confirms the structure of BAPPO. the 1H NMR spectrum of POSS-amine is shown in Fig. 9. The signal at 0.64 ppm is due to the Si–CH2 protons. The signal at 1.4 ppm corresponds to C–NH2 protons.
13C NMR spectra of BAPPO.
31P NMR spectra of BAPPO.
1H NMR spectra of POSS-amine.
3.2 Discussion on thermal stability
3.2.1 Thermogravimetric analysis (TGA)
The thermal analyses of the resins were studied by thermogravimetric analysis and from the data the thermal degradation temperature of the resins could be found. The char yield and LOI of the resins were found by TGA. The data obtained from the TGA studies are shown in Table 2 and the Figs. 10 and 11.
Resin system
Initial decomposition temperature ( °C)
Char yield (%)
LOI
AX
290
20
26
AXN1
300
23
28
AYN2
225
35
33
AY
190
28
30
AYN1
210
31
32
AYN2
225
35
33
TGA of AX based systems.
TGA of AY based systems.
For example, the initial decomposition temperature of the AX system was 290 °C (Fig. 10). The Y cured systems (AY, AYN1 and AYN2) showed a similar double decomposition pattern (Fig. 11). In contrast to this observation, the X cured resin systems (AX, AXN1 and AYN1) showed a single decomposition pattern. The initial degradation temperature of the Y cured system was lower than that of the X cured systems, which was due to the decomposition of P–O–C bond at lower temperatures. It was interesting to note that the char yield and LOI of the tetraglycidyl systems cured with Y were found to be higher than those of X cured systems. For example as seen from Table 2, the char yield of AX was 20% and LOI was 26, whereas the char yield of AY was 28% (Fig. 11) and LOI was 30. The higher char yield observed for Y cured systems may be due to the formation of a protective char layer formed as a result of the degradation that occurred at a lower temperature, thereby protecting the underlying matrix from further degradation.
The addition of nanoreinforcements produced a significant improvement on the thermal stability, char yield and LOI of all tetra functional epoxy resin systems. For example the initial decomposition temperature of AX system was 290 °C while the initial decomposition temperatures of AXN1 and AXN2 systems were significantly improved to 300 and 320 °C, respectively, as seen from the Table 2 and Fig. 10. Similarly the char yield for the AX system was 20% and that for the AXN1 and AXN2 systems was 23% and 26%, respectively, leading to an enhanced LOI values. Incorporation of POSS-amine significantly enhanced the thermal stability of the epoxy resin due to the formation of an inert silica layer on the surface of materials when decomposition takes place and prevented further oxidation of the inner part of the epoxy matrices. POSS molecules, having silica like Si–O–Si structure, led to higher inorganic components in the cured materials resulting in higher char yields (Zhang et al., 2007). This Si–O–Si linkage prevented the underlying polymeric matrix from further degradation. On the other hand, the clay nanolayers acted as barriers by preventing the evolution of volatile degradation products from the epoxy matrices (Leszczyńska et al., 2005). Between the two nanoreinforcements, the best results of thermal stability and flame retardancy were given by POSS-amine reinforced systems namely AYN2.
3.3 Discussion on electrical properties
The results of the various electrical studies are shown in Table 3.
As seen from the Table.3, the dielectric property of the neat AX system was 19.7 kV/mm, whereas the Y cured systems were found to show better electrical properties than the X cured systems. For example the dielectric property of the X cured system AX was 19.7 kV/mm while that of the Y cured system AY was 20.6 kV/mm. This could be due to the presence of rigid aromatic groups of Y which might have enhanced the hydrophobic behaviour and may also be due to the formation of insulative char layer that contributed to better electrical properties. The incorporation of nanoreinforcements (nanoclay and POSS-amine) was found to enhance the electrical properties significantly. The nano-reinforced epoxy composites yielded breakdown strength values as high as that of the base resin indicating their insulative behaviour required for electrical applications. For example surface resistivity of AX system was 1.13 × 1013 Ohm whereas for the system AXN1 and AXN2 the surface resistivity was enhanced to 3.62 × 1013 and 5.76 × 1013 Ohm, respectively. Similar observation was noticed for the dielectric strength and arc resistance measurements of nanocomposites. This could possibly be due to the presence of the siloxane linkages present in the nanoreinforcements that may be responsible for the enhanced insulative properties.
Systems
Dielectric strength (kV/mm)
Surface resistivity (Ohm)
Volume resistivity (Ohm)
Arc resistance (s)
Comparative tracking index (Volts)
AX
19.7
1.13 × 1013
>1014
100
>600
AXN1
21.0
3.62 × 1013
>1014
111
>600
AXN2
23.2
5.76 × 1013
>1014
121
>600
AY
20.6
2.01 × 1013
>1014
110
>600
AY N1
22.8
4.46 × 1013
>1014
119
>600
AY N2
24.3
6.49 × 1013
>1014
128
>600
4 Conclusion
The present work deals with the development and characterization of high functionality epoxy resin namely TGDDM. These epoxies were cured with DDM and BAPPO, respectively, with and without the incorporation of nanoclay (Nanomer 1.30 E) and nanoreinforcement (POSS-amine) to get matrix materials to be utilized for high performance applications. The developed materials were characterized by spectral studies to ascertain their structures. The polymerization of these resins was carried out in the presence and absence of nanoclay and nanoreinforcement to get nanocomposites with improved properties ideally suited for advanced engineering applications. The thermal studies of these materials were carried out by means of TGA and the results obtained from this study clearly indicate that BAPPO (Y) cured tetra epoxies, irrespective of nanoclay and nanoreinforcement showed better char yield and LOI values in spite of their degradation at a lower temperature. The incorporation of nanoclay and nanoreinforcement to the epoxy improved the IDT, char yield and LOI values.
The Y cured systems were found to show better electrical properties than that of X cured systems. This could be due to the presence of rigid aromatic groups which enhanced the hydrophobic behaviour and also due to the formation of insulative char layer that contributed to improved electrical properties. The addition of nanoreinforcements (nanoclay and POSS-amine) was found to significantly enhance the electrical properties. For example, the arc resistance value of the neat system AX was found to be 100 s, while for the nanocomposites AXN1 and AXN2 the values were enhanced to 111 and 121 s, respectively. This could be accounted due to the presence of the siloxane linkages present in the nanoreinforcements that enhanced the insulative properties. From the data obtained from different studies, it can be concluded that the novel organic–inorganic nano hybrid composites synthesized in the present study, having improved mechanical, thermal properties being self extinguishable, heat resistant at the same time with excellent dielectric properties can very well be used for automotive, electronics and advanced aerospace application for improved performance and longevity than the materials that are currently in use.
Acknowledgements
The authors are grateful to the Council for Scientific and Industrial Research (CSIR) and Department of Science and Technology (DST) for having provided financial assistance to carry out this work and thankful to Anna University, Chennai, India for providing necessary facilities for this work.
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