Surface Modified Clay Reinforced Silicon Incorporated Epoxy Hybrid Nanocomposites: Thermal,Mechanical,and Morphological Properties

Abstract

The silicon-containing epoxy/clay nanocomposites were developed by incorporating the surface-modified MMT clay upto 7wt% into Si-epoxy resin. The surface of the montmorillonite (MMT) clay was modified with two surface modifiers namely cetyltrimethylammonium bromide (CTAB) and 3-aminopropyltriethoxysilane (γ-APS). The surface modified clay reinforced Si-epoxy composites were developed in the form of castings, and were characterized for their thermal and mechanical properties. Thermal behaviour of the composites was characterized by differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Mechanical properties were studied as per ASTM standards. Data result from the different studies, it is inferred that the surface modified clay reinforced Si-epoxy composites exhibit lower T_g than that of neat epoxy matrix (127°C <165°C). The decomposition temperature for 60% weight loss of clay reinforced Si-epoxy composites is 674–823°C which is higher when compared to that of neat epoxy matrix. For 5wt% clay reinforced Si-epoxy composites, the values of tensile, flexural and impact strength are increased to 26%, 21% and 29% respectively. The storage modulus (E’) is increased from 5932 to 6308 MPa for clay reinforced Si-epoxy resin. XRD analysis confirmed the well-dispersed exfoliated nanocomposites structure.

Keywords

Si-epoxy MMT clay thermal stability mechanical properties storage modulus char yield flame retardance nanocomposites

1 Introduction

Epoxy resins are thermosets exhibiting an attractive combination of thermal, mechanical, chemical and environmental stability. Owing to these characteristic properties, they are widely applied as coatings, adhesives and matrices for composites [1,2]. However, epoxies possess some limitations, such as brittleness and limited thermomechanical properties which restricts their utility for high performance applications. Thus the need for improvement of properties of the epoxy resin is essential [2,3]. The characteristic properties of the epoxy resin can be tailored by introducing structurally designed chemical modifiers into epoxy resin skeleton along with the reinforcement of nano materials [4–7]. The introduction of silicon containing compounds into epoxy resin was attractive for its significant improvement on thermal stability, flame retardancy and electrical properties of the epoxy resin [7,8–11]. The excellent thermal stability and high flammability of silicon containing epoxy compounds satisfy the requirements for application in advanced electronics. Other properties like high resistance to thermal oxidation, low toxicity and low surface energy were also observed for organosilicon compounds and polymers [12–14]. On heating, the low surface energy of silicon renders it to migrate to the surface of epoxy resin and to form a protective layer with high heat resistance to avoid polymer residue from thermal degradation. The epoxy resin with improved thermal properties and high flame retardancy by incorporating silicon and phosphorus simultaneously into the epoxy resin was reported [15–18].

Nanocomposites using organophilic layered silicates as nano-reinforcements, offer new opportunities for the modification of micromechanics of thermoset epoxy resin. Since the dispersed phase in nanocomposites is in nano scale level the structural aspect of the dispersed phase has a large specific surface area and strong interaction with the epoxy matrix. The two dimensional and the high aspect ratios of the nano scale clay layers can exhibit lighter weight dimensional stability and a high degree of strength, stiffness, heat resistance and barrier properties with far less clay loading than that used in conventional filled polymer composites. In addition, the effect of MMT nanoclay concentration and its surface modification on the mechanical behavior of MMT/epoxy nanocomposites was studied by many researchers [19–24]. The elastic modulus of the MMT/epoxy nanocomposites increased, but tensile strength decreased with increasing MMT concentration [25,26]. Nano graphene-alumina reinforced epoxy composites for high thermally conductive and electrically insulating epoxy composites. Polyetheramine modified organoclays polymer composites enhanced their rheological properties for the application in LED packing [27,28].

In this study, the commercially available DGEBA epoxy (LY 556) was reacted with diphenylsilandiol (DPSD) through the addition reaction between the oxirane ring and silanol group. The resulting Si-epoxy resin was further incorporated with surface modified MMT nanoclay in order to study the effect of surface modifiers namely 3-aminopropyltriethoxysilane (γ-APS) and cetyltrimethylammonium bromide (CTAB) and clay loading on thermal and mechanical properties was investigated. The mechanical properties such as tensile strength, flexural strength, impact strength, storage modulus and thermal properties like glass transition temperature (T_g), thermal stability, char yield and flame retardancy were studied. XRD analysis was performed to determine the increase of d-spacing between the surface modified clay layers. The morphology behaviour was studied by SEM analysis on fractured surfaces of tensile specimens of Si-epoxy/clay nanocomposites.

2 Experimental

2.1 Materials

The commercially available DGEBA epoxy resin (LY 556) having epoxy equivalent weight about 180–190 g/eq and 4,4'-diaminodiphenylmethane (DDM), curing agent were used as obtained from Ciba-Geigy Ltd, India. Diphenylsilanediol (DPSD) montmorillonite (MMT) clay and 3-aminopropyltriethoxysilane (γ-APS) were procured from Aldrich Chemicals, USA. Cetyltrimethylammonium bromide (CH₃ (CH₂)₁₅ N(CH₃)₃ Br) (CTAB) was obtained from SRL, India. Tin (II) chloride was purchased from Merck, Germany. The chemical structures of DGEBA, DPSD, DDM, γ-APS and CTAB are presented in Figure 1 .

Figure 1

The chemical structures of DGEBA, diphenylsilanediol, DDM and clay modifiers

2.2 Synthesis of Silicon-Containing Epoxy (Si-epoxy) Resin

Silicon-containing epoxy resin was prepared by reacting DGEBA epoxy resin with diphenylsilanediol (DPSD) with 500 ppm tin(II) chloride as a catalyst. DGEBA-DPSD epoxy resin was synthesized as per the procedure reported elsewhere [18]. 18.9 g of DGEBA epoxy, 7.9 g of DPSD and 0.013 g of tin(II) chloride were taken in a 50 ml round-bottomed flask. The reaction mixture was stirred at 140°C for 2 h (Scheme 1). A transparent product with 100% yield was obtained.

Scheme 1

Preparation of the silicon-containing DGEBA epoxy resin

2.3 Preparation of Cured Silicon Containing Epoxy (Si-epoxy) Resins

The fixed amount of silicon-containing epoxy resin (100 g), and a stoichiometric amount of 4,4'-diaminodiphenylmethane (27.2 g), with respect to epoxy resin were mixed at 90°C for 10 min with constant stirring. The hybrid product was then degassed to remove the entrapped air and was then transferred into a preheated mould kept at 140°C for 1 h and post-cured at 190°C for 2 h.

3 Preparation of Surface Modified Mmt Clay

3.1 Synthesis of 3-Aminopropyltriethoxysilane Modified MMT Clay

One gram of unmodified MMT clay was dispersed in 100 ml of distilled water at 25°C, using a mechanical stirrer. 1 g of 3-aminopropyltriethoxysilane (γ-APS) dissolved in 100 ml of distilled water was added to the clay dispersion under constant stirring, and stirring was continued for 30 min. Then, it was filtered off and dried at 60°C under vacuum [19].

3.2 Synthesis of CTAB Modified MMT Clay

For the preparation of nanocomposites, hydrophilic nature of clay was changed into organophilic using cetyltrimethylammonium bromide (CH₃ (CH₂)₁₅ N (CH₃)₃ Br). 15 g of the purified Na-montmorillonite clay was dispersed into 1200 ml of distilled water at 80°C. Cetyltrimethylammonium bromide, 5.7 g, in 300 ml distilled water was poured in the hot montmorillonite/water solution and stirred vigorously for 1 h at 80°C. A white precipitate formed, was isolated by filtration and washed several times with hot water/ethanol (1:1) mixture until no chloride was detected in the filtrate by adding one drop of 0.1 M AgNO₃ solution. The cetyltrimethylammonium ion exchanged montmorillonite was then dried for about 15 days at 75°C, ground with a mortar and pestle, and then the <50 μm fraction was collected. The organophilic nanoclay was stored in a desiccator [20] for further use.

3.3 Preparation of Organophilic MMT Clay Filled Si-epoxy Hybrid Nanocomposites

Prior to curing, the silicon containing epoxy resin (100 g) was mixed mechanically in a reaction kettle with the desired amount of organophilic nanoclay (1, 3, 5 and 7%) at 70°C for 24 h. Then, it was further mixed in an ultrasonic bath for 30 min to disperse the clay effectively in the resin. A stoichiometric amount of the curing agent 4,4'-diaminodiphenylmethane (27.2 g) corresponding to epoxy equivalents was added. The product was subjected to vacuum to remove the entrapped air and then cast and cured at 120°C for 3 h. The castings were then post-cured at 200°C for 2 h and finally removed from the mold and characterized.

3.4 Instrumental Measurements

The infrared spectra for the samples were recorded on a Perkin-Elmer (Model RX1) FT-IR spectrometer, with KBr pellets for solid samples. ¹H NMR spectra were measured with a JEOL NMR (500 MHz) spectrometer using deuterated chloroform (CDCl₃) as a solvent.

The tensile strength was determined using INSTRON (Model 6025 UK) as per ASTM D 3039 at 10 mm/min cross-head speed using specimen with a width of 25 mm, length of 200 mm and thickness of 3 mm. The flexural strength was measured (INSTRON, Model 6025 UK) as per ASTM D 790 using specimen with dimensions 3 mm in depth, 10 mm in width and 90 mm in length at 10 mm/min cross-head speed. The unnotched Izod impact strength of each sample was studied as per ASTM D 256. Five specimens were tested for each sample. Hardness was measured using Durometer-Type D as per ASTM D 2240. Specimens with 3 mm thickness. Hardness was determined at five different positions on the specimen atleast 6 mm apart and arithmetic mean was taken.

Differential scanning calorimetry (DSC) thermograms were determined using a Netzsch-Gerateban Gmbh (DSC 200 PC) in the temperature range between 50°C and 250°C at a heating rate of 10°C min⁻¹ under nitrogen atmosphere. Thermogravimetric analysis (TGA) was carried out using a Netzsch – Gerateban GmbH (DSTA 409 PC) thermogravimetric analyzer at a heating rate of 10°C min⁻¹ under nitrogen atmosphere.

Dynamic mechanical behavior of the samples was measured using a DMA Netzsch 242 at a heating rate of 10°C per min from 30°C to 300°C. The sample dimension was 55 × 10 × 3 mm. The heat deflection temperature (HDT) of the samples was tested as per ASTM D 648-72. The sample dimension was 120 × 13 × 5 mm. Coefficient of thermal expansion of the samples was determined using METTLER TMA 2940 in the temperature range between 30°C and 400°C at heating rate of 10 K/min with a force of 0.05 N. The sample dimension was 5 × 5 × 1 mm.

Arc resistance of the nanocomposite samples was measured as per ASTM D 495 on test specimen of thickness 3.2 mm. A high voltage arc (12.5 kV) was struck between two electrodes placed on the test specimen, the initial current being a few milliamperes (10 mA). Water absorption behavior was determined by swelling the samples in distilled water for 48 hours at 25°C. The sample dimension was 10 × 10 × 3 mm. The absorption of the water molecule was measured based on the gain in weight.

The surface morphology of fractured surface of the tensile samples was examined using scanning electron microscope (SEM; JEOL JSM Model 6360). Auto Fine Coater (JEOL JFC-1600); platinum sputtered on the fractured surface of the samples at vacuum pressure 8 Pa / 20 mA for 95 seconds. The fractured surface of the samples was coated with platinum before scanning. A JEOL JEM-3010 analytical transmission electron microscope, operating at 300 kV with a measured point-to-point resolution of 0.23 nm, was used to characterize the phase morphology in the polymers.

TEM analyses were made using powdered nanocomposites samples and spreading them over carbon-coated Cu TEM grids. X-ray diffraction measurement was performed on a Rich Seifert (Model 3000) X-ray powder diffractometer by monitoring the diffraction angle 2θ from 0.5° to 40° as low angle. The diffractometer was equipped with a Copper target (λ = 1.54 A°) radiation using Guinier type camera employed as a focusing geometry and a solid-state detector. Curved Nickel crystal used as a monochromator. The step width (scanning speed) used was 2θ = 0.04 deg/min. Bragg's law (λ = 2dsin θ) was used to calculate the d-spacing.

4 Results and Discussion

4.1 Spectral Analysis

The formation of Si-epoxy was confirmed by FTIR and ¹H NMR analysis.

FT-IR (KBr, cm⁻¹): 1325 (Si–O–CH₂), 1583 (Si–Ph), 1249 (C–O–C), 1121 (Si–O) and 915 (oxirane ring).

¹H NMR (CDCl₃, ppm): δ = 1.78 ppm (–C(CH₃)₂–); δ = 2.83 and 2.98 ppm (oxirane ring, –CH(O)CH₂); δ = 3.43 ppm (oxirane ring, –CH(O)CH₂); δ = 3.81 – 4.31 ppm (other protons in cyclic and opened oxirane ring), and δ = 6.92 −7.78 ppm (aromatic protons).

The nucleophilic addition reaction between diphenylsilanediol (DPSD) and the oxirane groups of epoxy was observed by FTIR spectra. The disappearance of Si–OH absorption peak at 2900 cm⁻¹ and the appearance of Si–O–C (aliphatic carbon) peak at 1325 cm⁻¹ demonstrated the occurrence of the reaction between Si–OH and oxirane ring. The appearance of band at represents 1583 cm⁻¹ Si–Ph was observed for all the resultant products, further to indicate the incorporation of silyl groups into epoxy resin. ¹H NMR analysis also confirmed the formation of silicon-containing epoxy resin (Si-epoxy).

5 Mechanical Properties

5.1 Tensile Strength and Flexural Strength

The values of tensile, flexural and impact strength obtained for neat epoxy, Si-epoxy and modified clay reinforced Si-epoxy are presented in Table 1 . The incorporation of unmodified clay upto 7wt% into the Si-epoxy resin decreased the tensile strength and flexural strength, with increasing clay concentrations due to the agglomeration of MMT clay and phase separation between silicon-epoxy and clay layers [19]. The neat clay layers debonded from the Si-epoxy matrix and created micro-voids around the clay as a result of lower interfacial strength between unmodified clay and Si-epoxy matrix.

Table 1
Mechanical properties of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Tensile Strength (MPa) Flexural Strength (MPa) Unnotched Izod Impact Strength J/m CTE ppm/K Hardness Shore D

E 63.5±6 105.7±5 97.4±3 76.2 43.0

SE 61.7±4 99.6±4 104.5±3 78.1 40.8

SE_A1 59.4±7 96.8±.6 108.4±5 77.2 42.1

SE_A3 57.4±3 91.3±2 112.9±4 73.6 44.0

SE_A5 52.7±2 88.5±7 115.7±6 70.3 46.3

SE_A7 50.5±5 85.6±5 117.8±5 67.8 47.8

SE_B1 67.8±6 112.7±4 111.5±5 78.9 41.4

SE_B3 73.7±3 117.2±3 119.6±4 75.3 42.6

SE_B5 76.2±4 123.6±4 124.5±3 73.1 44.7

SE_B7 78.3±2 126.4±2 128.3±3 71.7 46.5

SE_C1 71.4±3 119.5±3 124.7±4 81.6 41.2

SE_C3 77.3±4 127.6±5 131.3±7 80.5 42.0

SE_C5 82.6±5 132.6±3 137.6±6 77.4 43.8

SE_C7 85.5±7 136.5±2 141.4±5 75.8 45.9

Si-epoxy/clay w/w	Tensile Strength (MPa)	Flexural Strength (MPa)	Unnotched Izod Impact Strength J/m	CTE ppm/K	Hardness Shore D
E	63.5±6	105.7±5	97.4±3	76.2	43.0
SE	61.7±4	99.6±4	104.5±3	78.1	40.8
SE_A1	59.4±7	96.8±.6	108.4±5	77.2	42.1
SE_A3	57.4±3	91.3±2	112.9±4	73.6	44.0
SE_A5	52.7±2	88.5±7	115.7±6	70.3	46.3
SE_A7	50.5±5	85.6±5	117.8±5	67.8	47.8

SE_B1	67.8±6	112.7±4	111.5±5	78.9	41.4
SE_B3	73.7±3	117.2±3	119.6±4	75.3	42.6
SE_B5	76.2±4	123.6±4	124.5±3	73.1	44.7
SE_B7	78.3±2	126.4±2	128.3±3	71.7	46.5
SE_C1	71.4±3	119.5±3	124.7±4	81.6	41.2
SE_C3	77.3±4	127.6±5	131.3±7	80.5	42.0
SE_C5	82.6±5	132.6±3	137.6±6	77.4	43.8
SE_C7	85.5±7	136.5±2	141.4±5	75.8	45.9

The incorporation of CTAB modified clay reinforced Si-epoxy composites enhanced the values of tensile strength and flexural strength (20% and 16%) with increasing clay concentrations upto 7wt% due to strong interfacial interaction arises between organophilic Si-epoxy resin and organo modified clay layers. The introduction of γ-APS modified clay upto 7wt% into Si-epoxy increased the values of tensile and flexural strength significantly (31% and 25%).

The incorporation of 1, 3, 5 and 7wt% of unmodified, CTAB modified and γ-APS modified clay reinforced Si-epoxy composites enhanced the impact strength according to the nature and percentage the increase in the clay content. This is due to its flexible molecular nature of silane ( Table 1 ). Further, the increase in the values of impact strength of both CTAB and γ-APS modified clay into both Si-epoxy resin is due to the flexibility imparted by the plasticization effect of organoclay ( Table 1 ).

5.2 Impact Strength and Hardness

The unfilled Si-epoxy resin possesses the higher value of impact strength than that of neat epoxy resin (104.5>97.4 J/m) ( Table 1 ). The values of impact strength of silane modified clay (upto 7wt%) reinforced Si-epoxy matrix is much higher when compared to that of CTAB modified clay reinforced Si-epoxy composites (33%>21%). This is due to the absorption of energy by the Si-epoxy/clay nanocomposites. When a load is applied to the Si-epoxy/clay nanocomposites, the applied load is transferred to the clay dispersed throughout the Si-epoxy resin matrix system. In this case, the clay strengthens nanocomposites by resisting the applied load and inhibits the deformation of nanocomposites [19]. The inhibiting effect on deformation of nanocomposites increases with the increase in the clay content.

The values of hardness of a neat epoxy and the Si-epoxy systems were found to be 43 and 40.8 respectively ( Table 1 ). The values of hardness for the Si-epoxy systems were increased with the incorporation of increased percentage clay content. The values of hardness for 5wt% neat, CTAB and γ-APS modified clay reinforced Si-epoxy systems are 46.3, 44.7 and 43.8 respectively, due to that the clay dispersed within the epoxy prevents particles on the surface of Si-epoxy from being damaged by an external force and filling up the microcracks existing inside the epoxy. This restricts the surface of Si-epoxy system further to crack. The value of hardness is lower for γ-APS modified Si-epoxy system than that of neat and CTAB modified clay reinforced Si-epoxy systems. This occurs due to the clay was intercalated within the Si-epoxy matrix and the adhesion strength between clay and Si-epoxy matrix was increased by the γ-APS modification which inturn imparts flexibility.

6 Thermal Properties

6.1 Heat Distortion Temperature

Heat distortion temperature is determined to assess the thermo-mechanical behaviour of matrix systems. HDT values for neat epoxy system, silicon incorporated epoxy and surface modified clay reinforced silicon incorporated epoxy composites are presented in Table 2 . The values of HDT for neat DGEBA and SE matrix systems are 154°C and 135°C respectively. The values of HDT for 5wt% unfilled, CTAB and γ-APS modified clay reinforced Si-epoxy matrix systems are 150°C, 130°C and 125°C respectively ( Table 2 ). The γ-APS modified clay reinforced Si-epoxy composites possess the lower values of HDT when compared to that of DGEBA epoxy matrix systems due to the presence of flexible –Si–O–C– and –Si–O–Si– linkages in the backbone of SE and γ-APS, respectively.

Table 2
Thermal and water absorption behaviour of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Heat distortion temperature (°C) Glass transition temperature (°C)
Water absorption (%) 25°C, 48 h

DSC DMA

E 154 162 165 0.13

SE 135 146 148 0.11

SE_A1 143 148 150 0.10

SE_A3 147 151 153 0.09

SE_A5 150 154 155 0.08

SE_A7 145 147 152 0.07

SE_B1 139 141 146 0.10

SE_B3 134 139 143 0.09

SE_B5 130 136 139 0.07

SE_B7 128 132 137 0.06

SE_C1 135 139 142 0.08

SE_C3 127 137 136 0.06

SE_C5 125 134 132 0.05

SE_C7 119 125 127 0.04

Si-epoxy/clay w/w	Heat distortion temperature (°C)	Glass transition temperature (°C)	Water absorption (%) 25°C, 48 h
E	154	162	165	0.13
SE	135	146	148	0.11
SE_A1	143	148	150	0.10
SE_A3	147	151	153	0.09
SE_A5	150	154	155	0.08
SE_A7	145	147	152	0.07
SE_B1	139	141	146	0.10
SE_B3	134	139	143	0.09
SE_B5	130	136	139	0.07
SE_B7	128	132	137	0.06
SE_C1	135	139	142	0.08
SE_C3	127	137	136	0.06
SE_C5	125	134	132	0.05
SE_C7	119	125	127	0.04

Nomenclature of epoxy and modified epoxy systems

E - Unmodified DGEBA epoxy resin

SE - Si-epoxy resin

SE_A - unmodified clay filled Si-epoxy resin

SE_B - CTAB modified clay filled Si-epoxy resin

SE_C - γ-APS modified clay filled Si-epoxy resin

1, 3, 5 and 7 - clay content (wt. %)

6.2 Glass Transition Temperatures and Curing Behavior

Table 2 shows the data of DSC thermograms of neat Si-epoxy resin and the clay reinforced Si-epoxy nanocomposites. The glass transition temperature and curing behavior of neat Si-epoxy and clay reinforced Si-epoxy resin were examined to study the effect of clay surface modification and clay loading on Si-epoxy resin. The curing behaviour of Si-epoxy resin with curing agent DDM has already been reported by C. S. Wu et al [18]. The introduction of silicon into DGEBA epoxy resin increases the reactivity of the epoxy ring towards DDM [18]. The high electronegativity of silicon reduces the electron density of the oxirane ring, which increases its ring opening process. Glass transition temperature (T_g) of the cured Si-epoxy resin was determined from DSC measurements. The incorporation of silyl groups into the epoxy resin lowered the T_g. The value of T_g for Si-epoxy is decreased when compared to that of neat epoxy. This is due to the presence of flexible linkage of Si–O–Ph in the backbone of the Si-epoxy resin. Further, the incorporation of silicon group into epoxy resin would result in epoxy compounds with two aryl groups as bulky pendants. The bulky pendant groups creates more free volume due to the crowding and reduce the crosslinking density of the cured matrices and thus lowers the value of T_g [18].

The incorporation of unmodified clay into Si-epoxy resin increases the glass transition temperature from 148°C to 155°C with increasing the clay content upto 5wt%. The incorporation of 7wt% unmodified clay into Si- epoxy resin reduces the T_g to 152°C. The addition of CTAB modified clay into Si-epoxy resin decreases the T_g from 146°C to 137°C with the increment in the clay content upto 7wt% due to the self-polymerization and plasticization effect of Si-epoxy resin induced by the CTAB. Similarly the incorporation of γ-APS modified clay into Si-epoxy resin decreases the glass transition temperature from 142°C to 127°C with increasing the clay content upto 7wt%. The effect of silane modification of clay also increases with increasing clay loading into Si-epoxy resin. This is due to the increase in the exfoliation of silane modified clay and thereby increasing the distance between the clay layers. In other words, the amount of insertion of Si-epoxy resin increases by widening of space between the clay layers. The values of HDT for Si- epoxy resin and clay reinforced Si-epoxy resin matrix systems follow the similar trend as T_g ( Table 2 ).

6.3 Thermal Stability

The thermal degradation temperatures and the char yield of the cured Si-epoxy resin and clay reinforced Si-epoxy resin systems were studied by TGA. The initial decomposition temperature of Si-epoxy resin is higher than that of neat epoxy resin (294°C>289°C) due to the incorporation of silyl group into the epoxy which delays the degradation temperature of the Si-epoxy resin ( Table 3 ). The oxydiphenylsilane group (O–SiPh₂–O) exhibited a high thermal stability over 400°C [29–31]. The incorporation of modified clay into Si-epoxy resin delays the degradation temperature upto 820°C. The CTAB modified clay filled Si-epoxy resin enhanced the initial decomposition temperature from 304°C to 321°C upto 5wt% of clay addition. Similarly, the γ-APS modified clay reinforced Si-epoxy resin increased the initial decomposition temperature from 327°C to 349°C. The char formation increased with the increasing clay content into Si-epoxy resin [32–34]. The weight loss was preventing from silicon migration to char surface (due to the low surface energy of silicon) and formation of a silicon-protecting layer to char (accompanied with the anti-oxidant property and high thermal stability of silicon) [15,16]. Since the char yield has been correlated to denote the flame retardancy of the polymer [33,36], the clay filled Si-epoxy nanocomposites prepared in this work are expected to possess good flame retardant property. A polymer producing high char yield induces to high flame retardance has reported [35–37]. In the case of clay reinforced Si-epoxy resin, the char yield increases from 33 to 39%. For silane modified clay reinforced Si-epoxy resin, the char yield increases from 35 to 38% which is higher than that of CTAB modified clay filled Si-epoxy resin ( Table 3 ). Thus, the incorporation of silane modified clay into Si-epoxy resin increases the char yield which also accounts for the enhancement in flame retardant behaviour.

Table 3
TGA data of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Initial decomposition temperature(°C) Temp. at characteristic weight loss (°C)
Char yield (%) 850°C LOI Values^*

20% 40% 60%

E 352 371 390 415 13.8 23.0

SE 296 366 397 464 24.2 27.20

SE_A1 298 402 455 711 35.2 31.58

SE_A3 305 410 462 790 36.8 32.22

SE_A5 313 414 459 763 37.5 32.50

SE_A7 319 390 431 752 36.2 31.98

SE_B1 304 402 461 681 33.9 31.06

SE_B3 315 417 480 693 34.8 31.42

SE_B5 321 434 489 702 35.7 31.78

SE_B7 285 389 506 674 34.2 31.18

SE_C1 327 426 543 787 34.8 31.42

SE_C3 336 438 565 814 36.5 32.10

SE_C5 349 457 593 823 38.2 32.78

SE_C7 315 434 572 781 39.1 33.14

Si-epoxy/clay w/w	Initial decomposition temperature(°C)	Temp. at characteristic weight loss (°C)	Char yield (%) 850°C	LOI Values^*
E	352	371	390	415	13.8	23.0
SE	296	366	397	464	24.2	27.20
SE_A1	298	402	455	711	35.2	31.58
SE_A3	305	410	462	790	36.8	32.22
SE_A5	313	414	459	763	37.5	32.50
SE_A7	319	390	431	752	36.2	31.98
SE_B1	304	402	461	681	33.9	31.06
SE_B3	315	417	480	693	34.8	31.42
SE_B5	321	434	489	702	35.7	31.78
SE_B7	285	389	506	674	34.2	31.18
SE_C1	327	426	543	787	34.8	31.42
SE_C3	336	438	565	814	36.5	32.10
SE_C5	349	457	593	823	38.2	32.78
SE_C7	315	434	572	781	39.1	33.14

Calculated from Krevelen's equation using Char Yield [35,36]

6.4 Flame Retardancy

The flame retardant behavior of the unfilled Si-epoxy resin and clay reinforced Si-epoxy composites was evaluated from the char yield by measuring their LOI values ( Table 3 ) using Krevelen's equation (1) [38,39].

LOI = 17.5 + 0.4 σ

(1)

where σ is polymer's char yield.

The LOI values obtained from the above equation (1) are in good agreement with improved flame retardance. The LOI value of Si-epoxy is 27.2. The clay reinforced Si-epoxy composites possess the higher values of LOI (31.06–33.14) compared to that of neat epoxy resin. The silane modified clay reinforced Si-epoxy composites are also exhibits higher values of LOI ( Table 3 ). This further supports that unmodified Si-epoxy resin and silane modified clay reinforced Si-epoxy composites possess higher flame retardancy. The char protecting effect of silicon exhibited the synergistic effect on LOI enhancement [39–41].

7 Thermo-Mechanical Properties

7.1 Coefficient of Thermal Expansion

The coefficients of thermal expansion (CTE) of neat epoxy systems, neat Si-epoxy, CTAB modified and γ-APS modified clay reinforced epoxy systems were measured in the temperature range between 30°C and 400°C and the values are presented in Table 1 . It is noted that the values of CTE of neat DGEBA, SE systems are 76.2 and 78.1. The values of CTE for 5wt% unmodified, CTAB and γ-APS modified clay incorporated epoxy composites are 70.3, 73.1 and 77.4 respectively ( Table 1 ). The γ-APS modified Si-epoxy system possesses the highest value of CTE due to the flexible behaviour of siloxane linkages. The lowest value of CTE for neat DGEBA epoxy system is due to high rigidity.

7.2 Dynamic Mechanical Analysis

Glass transition temperature (T_g) of the cured Si-epoxy resin was determined by DMA measurement. Table 4 shows the values of tan δ with respect to temperature for the Si-epoxy matrices cured with DDM. The value of T_g was taken as a maximum value of the tan δ curve. The T_g of the Si-epoxy cured with DDM is 148°C, whereas for the cured DGEBA/DDM, the T_g is 165°C. The results can be interpreted in terms of the Si-epoxy which relatively possess higher molecular weight and thus reduced the cross-linking density of the Si-epoxy network, resulting in decreasing the T_g in the Si-epoxy/DDM system. The lower T_g of the Si-epoxy/DDM system is resulted from the flexible linkage of Si–O–C in the backbone of the epoxy resin [18]. The storage modulus E'MPa of neat Si-epoxy resin and clay reinforced Si-epoxy resin is 5932 and 6308 respectively. Similarly, the elasticity of Si-epoxy resin is higher when compared to that of neat epoxy resin (5932>5104 E'MPa).

Table 4
DMA and Morphology of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Storage modulus E'/ MPa Tan δ SEM morphology observations

E 5104 1.38 homogeneous

SE 5932 1.54 homogeneous

SE_A5 5797 1.56 heterogeneous

SE_B5 6265 1.59 homogeneous

SE_C5 6308 1.63 homogeneous

Si-epoxy/clay w/w	Storage modulus E'/ MPa	Tan δ	SEM morphology observations
E	5104	1.38	homogeneous
SE	5932	1.54	homogeneous
SE_A5	5797	1.56	heterogeneous
SE_B5	6265	1.59	homogeneous
SE_C5	6308	1.63	homogeneous

7.3 Water Absorption Studies

The introduction of silyl group into epoxy resin decreased the percentage water uptake ( Table 1 ) due to its inherent hydrophobic nature. Further, the incorporation of surface modified clay into Si-epoxy systems also reduced the percentage water uptake due to the fact that the reduction in permeability behavior influenced by the formation of exfoliated nanocomposites structure.

8 Electrical Studies

Table 5 represent the values of arc resistance, dielectric constant and loss of the unmodified and modified systems. The introduction of clay and silane content increased the arc resistance due to hydrophobic nature. The dielectric constant and loss decreased the weight percentage of silane and clay content because of the formation of inter-crosslinked network and inorganic skeleton.

Table 5
Electrical behavior of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Dry Arc Resistance (seconds) Dielectric Constant (ε’) (1 MHz) Dielectric loss (ε”) (1 MHz)

E 68 3.37 0.168

SE 76 2.94 0.141

SE_A5 79 2.78 0.126

SE_B5 82 2.56 0.115

SE_C5 86 2.19 0.103

Si-epoxy/clay w/w	Dry Arc Resistance (seconds)	Dielectric Constant (ε’) (1 MHz)	Dielectric loss (ε”) (1 MHz)
E	68	3.37	0.168
SE	76	2.94	0.141
SE_A5	79	2.78	0.126
SE_B5	82	2.56	0.115
SE_C5	86	2.19	0.103

9 Morphology

9.1 X-Ray Diffractions

X-ray diffraction patterns of the unmodified clay and the surface modified clay with cetyltrimethylammonium bromide and 3-aminopropyltriethoxysilane which show the d-spacing Figure 2a–c and Figure 3a–c . As shown in Figure 2a , for unmodified clay, a sharp peak appeared at around 2θ = 7.2°, in which the d-spacing was calculated to be ∼12.3 A°. Figure 2b shows the CTAB modified clay reinforced Si-epoxy composites, in which the peak appeared at 5.2°. The d-spacing was found to be ∼17.0 A°. The 3-aminopropyltriethoxysilane modified clay Figure 2c , however, shifts the peak to the lower 2θ value at around 4.4°, which corresponds to a d-spacing value of ∼20.1 A°. Therefore, the d-spacing of 3-aminopropyltriethoxysilane modified clay is larger by about 8.0 A° than that of the unmodified clay. The diffraction patterns of 3% unmodified clay reinforced Si-epoxy, 3% CTAB modified clay reinforced Si-epoxy and 3% γ-APS modified clay reinforced Si-epoxy are shown in Figure 3a–c . The disappearance of peaks between 0–10° indicates the presence of exfoliated nanocomposites structure.

Figure 2

XRD pattern of a) unmodified MMT clay, b) organo modified MMT clay and c) γ-APS modified MMT clay

Figure 3

XRD pattern of a) Si-Epoxy/unmodified MMT clay (100:3); b) Si-Epoxy/organo modified MMT clay (100:3) and c) Si-Epoxy/γ-APS modified MMT clay (100:3)

9.2 Scanning Electron Microscopy

The SEM micrographs of fractured surfaces of the unmodified epoxy, Si-epoxy and clay reinforced Si-epoxy systems are presented in Figures 4a–e. Figure 4a indicates smooth, glassy and homogeneous microstructure. This supports brittle nature and inferior impact strength of neat epoxy system. The micrograph of the fractured surface of Si-epoxy system also exhibits a homogeneous morphology ( Figure 4b ). The appearing of spherical domains in the micrographs ( Figure 4c ) indicates the presence of O–Si–O moiety in the Si-epoxy resin. The heterogeneous morphology observed for Si-epoxy system filled with unmodified clay confirms the existence of phase separation between the two components due to the existence of incompatibility between clay and the resin. Figures 4d and 4e show the homogeneous morphologies of the CTAB and γ-APS modified clay reinforced hybrid Si-epoxy nanocomposites. The surface modified clay layers occupy the full volumes of the polymer matrix and they are dispersed over the whole of the matrix, which confirm the exfoliation of clay with Si-epoxy matrix system.

Figure 4

SEM photographs of a) unmodified epoxy, b) unfilled Si-Epoxy, c) Si-Epoxy with 3% unmodified MMT clay, d) Si-Epoxy with 3% organo modified MMT clay and e) Si-Epoxy with 3% γ-APS modified MMT clay

9.3 Transmission Electron Microscopy

The TEM micrographs of unmodified clay reinforced Si-epoxy system, CTAB modified clay reinforced Si-epoxy composites and γ-APS modified clay reinforced Si-epoxy composites are shown in Figures 5a–c. The unmodified clay filled Si-epoxy system ( Figure 5a ) shows the more dark points which confirm the phase separation between two components. The heterogeneous phase morphology indicates the intercalated clay stacks dispersed in polymer matrix. There is no clear intercalated morphology and aggregated clay particles were detected. However, the presence of an intercalated clay structure could not be fully excluded, as both morphologies usually possess inconsistencies among in unmodified clay filled Si-epoxy system. Figures 5b and 5c show the homogeneous morphologies of the CTAB and γ-APS modified clay reinforced hybrid Si-epoxy nanocomposites. The efficient adhesion arises due to the influence of intermolecular specific interactions between cetyltrimethylammonium ions and 3-aminopropyltriethoxysilane modified clay and Si-epoxy matrix systems and hence leads to the formation exfoliated nanocomposites.

Figure 5

TEM photographs of a) Si-Epoxy with 3% unmodified MMT clay, b) Si-Epoxy with 3% organo modified MMT clay and c) Si-Epoxy with 3% γ-APS modified MMT clay

10 Conclusions

In summary, the silicon incorporated DGEBA epoxy resin was synthesized and reinforced with surface modified clay to investigate the glass transition temperature, thermal stability, and mechanical properties. The silane modified clay reinforced Si-epoxy resin enhanced the thermal stability upto 820°C. The addition of surface modified clay into Si-epoxy with different ratios further enhanced the LOI values. The cured Si-epoxy resin system showed a lower glass transition temperature and higher mechanical properties than those of the unmodified epoxy resin. The results could be attributed to the introduction of soft silicon segment into the main chain of the epoxy resin, resulting in increasing the flexibility in the Si-epoxy system. The incorporation of γ-APS modified clay into Si-epoxy resin significantly enhanced the mechanical properties of Si-epoxy resin system. The disappearance of peaks between 0–10° indicated the presence of exfoliated nanocomposite structures. The silane and CTAB modified clay reinforced Si-epoxy resin exhibited the homogeneous morphology. These surface modified clay reinforced Si-epoxy matrix systems could be used to fabricate advanced nanocomposite components of improved toughness with better thermomechanical behaviour for engineering applications.

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Surface Modified Clay Reinforced Silicon Incorporated Epoxy Hybrid Nanocomposites: Thermal,Mechanical,and Morphological Properties

Abstract

Keywords

1 Introduction

2 Experimental

2.1 Materials

3 Preparation of Surface Modified Mmt Clay

3.1 Synthesis of 3-Aminopropyltriethoxysilane Modified MMT Clay

3.2 Synthesis of CTAB Modified MMT Clay

3.3 Preparation of Organophilic MMT Clay Filled Si-epoxy Hybrid Nanocomposites

3.4 Instrumental Measurements

4 Results and Discussion

4.1 Spectral Analysis

5 Mechanical Properties

5.1 Tensile Strength and Flexural Strength

6 Thermal Properties

6.1 Heat Distortion Temperature

6.3 Thermal Stability

7.1 Coefficient of Thermal Expansion

7.2 Dynamic Mechanical Analysis

Table 4 DMA and Morphology of Si-epoxy/clay nanocomposites Si-epoxy/clay w/w Storage modulus E'/ MPa Tan δ SEM morphology observations E 5104 1.38 homogeneous SE 5932 1.54 homogeneous SEA5 5797 1.56 heterogeneous SEB5 6265 1.59 homogeneous SEC5 6308 1.63 homogeneous

8 Electrical Studies

Table 5 Electrical behavior of Si-epoxy/clay nanocomposites Si-epoxy/clay w/w Dry Arc Resistance (seconds) Dielectric Constant (ε’) (1 MHz) Dielectric loss (ε”) (1 MHz) E 68 3.37 0.168 SE 76 2.94 0.141 SEA5 79 2.78 0.126 SEB5 82 2.56 0.115 SEC5 86 2.19 0.103

9.1 X-Ray Diffractions

References

Table 4
DMA and Morphology of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Storage modulus E'/ MPa Tan δ SEM morphology observations

E 5104 1.38 homogeneous

SE 5932 1.54 homogeneous

SE_A5 5797 1.56 heterogeneous

SE_B5 6265 1.59 homogeneous

SE_C5 6308 1.63 homogeneous

Table 5
Electrical behavior of Si-epoxy/clay nanocomposites

Si-epoxy/clay w/w Dry Arc Resistance (seconds) Dielectric Constant (ε’) (1 MHz) Dielectric loss (ε”) (1 MHz)

E 68 3.37 0.168

SE 76 2.94 0.141

SE_A5 79 2.78 0.126

SE_B5 82 2.56 0.115

SE_C5 86 2.19 0.103