Thermal and flexural properties of room-temperature cured PMMA grafted natural rubber toughened epoxy layered silicate nanocomposite

Int. J. Nano and Biomaterials, Vol. 5, No. 1, 2014 45

Copyright © 2014 Inderscience Enterprises Ltd.

Thermal and flexural properties of room-temperature cured PMMA grafted natural rubber toughened epoxy layered silicate nanocomposite

Nor Yuliana Yuhana* Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43000 Bangi, Selangor, Malaysia Fax: +603-8921-6148 E-mail: [email protected] *Corresponding author

Sahrim Hj. Ahmad Department of Material Science, Universiti Kebangsaan Malaysia, 43000 Bangi, Selangor, Malaysia E-mail: [email protected]

Mahmood Mebrabzadeh Poly M Inc., 2152 Etienne Montgolfier, St. Laurent Montreal, QC, H4R 3H7, Canada E-mail: [email protected]

Abdul Razak Shamsul Bahri Agrotechnology and Food Science Faculty, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia E-mail: [email protected]

Abstract: Thermal and flexural properties of ternary system containing epoxy, poly(methyl methacrylate) (PMMA) grafted natural rubber, and organic chemically modified montmorillonite clay (Cloisite 30B) were studied. The results of TEM, SEM, XRD, FTIR, DSC, TGA and flexural tests were discussed. The rates of glass transition and decomposition process were introduced as relevant parameters to evaluate glass transition and thermal stability. It was observed that the addition of 5 phr Cloisite 30B in epoxy contributed to significant improvement in thermal properties and flexural modulus of nanocomposites. The intercalated structure of silicate clay may help to introduce tortuous path for volatiles to escape the epoxy network, hence retarding the decomposition process. The high char yield content for epoxy/clay and rubber toughened epoxy/clay nanocomposite show that they can be used as adhesive and coating materials with good thermal stability properties.

46 N.Y. Yuhana et al.

Keywords: epoxy; nanocomposites; rubber; thermal stability; flexural properties.

Reference to this paper should be made as follows: Yuhana, N.Y. Ahmad, S.H., Mebrabzadeh, M. and Shamsul Bahri, A.R. (2014) ‘Thermal and flexural properties of room-temperature cured PMMA grafted natural rubber toughened epoxy layered silicate nanocomposite’, Int. J. Nano and Biomaterials, Vol. 5, No. 1, pp.45–58.

Biographical notes: Nor Yuliana Yuhana is a Lecturer in the Department of Chemical and Process Engineering at Universiti Kebangsaan Malaysia. She is currently working on epoxy ternary nanocomposite systems. Her interests are in processing-structure-property relationship of polymer blends and composites, phase separation in polymer blends, and also in the barrier and thermal properties of polymer. Besides research, she is a travel lover.

Sahrim Hj. Ahmad is a Professor in the Material Science Program at Universiti Kebangsaan Malaysia. His current research areas are in polymer composites, magnetic materials, natural fibre reinforced composites and polyurethane from natural polyol.

Mahmood Mehrabzadeh is Founder and President of Poly M Inc. in Montreal, Canada. He has vast experience in polymer research and works closely with the plastics, rubbers and composite companies to solve their technical, material, quality and productivity issues.

Abdul Razak Shamsul Bahri is a Lecturer in the Agrotechnology and Food Science Faculty at Universiti Malaysia Terengganu. His strong technical expertise includes the microscopy works on rubber latex, polymer blends and composites.

This paper is a revised and expanded version of a paper entitled ‘Thermal stability and morphology of room-temperature cured PMMA grafted natural rubber toughened epoxy layered silicate nanocomposite’ presented at the 2nd International Conference on Process Engineering and Advance Materials (ICPEAM12), Kuala Lumpur Convention Center, 12–14 June 2012.

1 Introduction

Epoxy resins are widely used as coating, adhesive and composite matrices in the structural, building and transportation industries due to their low cost, ease of processing, excellent adhesion and good mechanical, thermal and barrier properties. The reaction of the epoxy monomer with a variety of curing agents produces a wide range of product properties. Under optimum conditions of mixing with the curing agent, the cured epoxy material exhibits a three dimensional network structure (Kaelbe, 1973). Normally, the resulting structure leads to brittleness, due to the high crosslink density.

The common approach to deal with resin brittleness involves the incorporation of fibres, rubber and thermoplastic polymers, micro or nano fillers and polyorganic siloxane to improve the fracture toughness (Zhang, 2003; Becker and Simon, 2005). The addition of rubber has been the most successful commercially. Ratna and Banthia (2004) reviewed the use of liquid and preformed rubber particles. Liquid synthetic

Thermal and flexural properties of room-temperature cured PMMA 47

rubbery components have been used, such as carboxyl terminated butadiene-acrylonitrile (CTBN) (Ramos et al., 2005; McEwan et al., 1999; Russell and Chartoff, 2005), carboxyl-randomised butadiene-acrylonitrile (CRBN), acrylonitrile-butadiene rubbers (NBR) (Frounchi et al., 2000), and hydroxyl-terminated butadiene-acrylonitrile (HTBN) rubber (Minfeng et al., 2008), and hydroxyl-terminated polybutadiene (HTPB) (Ramos et al., 2005; McEwan et al., 1999; Russell and Chartoff, 2005; Minfeng et al., 2008; Thomas et al., 2008). Chemically modified natural rubber, such as PMMA-grafted natural rubber and epoxidised liquid natural rubber have been also studied and used (Zainol et al., 2006; Rezaifard et al., 1994).

The incorporation of nano-fillers in polymer matrices to improve thermal, mechanical, barrier, electrical, and optical properties has gained interest recently. The most common types of polymer nanocomposites incorporate silicate, carbon or metal oxide nanoparticles. In polymer nanocomposites, a small amount of silicate nanoparticles (less than 5 wt. %) is commonly used. The silicate types used include natural silicate (montmorillonite, hectorite, etc.) and synthetic silicate (laponite, magadiite and flourohectorite). However, the most commonly used and studied nanoclay is montmorillonite (MMT), a layered aluminosilicate (Azeez et al., 2013). The interesting feature of the layered silicates is their high aspect ratio. Also, it is available in the market as organically modified clay. This facilitates the dispersion and exfoliation of the layers in a polymer matrix, at the nanometer scale.

It is often stated that the layered silicate can increase the glass transition temperature (Tg) by restricting the epoxy chain mobility. They can also reduce the Tg, due to the presence of hydroxyl group in unmodified mica and acidic onium ion in organically modified silicate, which both, can act as catalyst for epoxy homopolymerisation (Becker et al., 2003; Froehlich et al., 2004). If this happens, the unreacted curing agent will plasticise the matrix. Kinloch et al. concluded that intercalated structure of silicate can increase the Tg (Kinloch and Taylor, 2003). Becker found that increasing organoclay concentration steadily decreases the Tg (Becker et al., 2003). Also, the glass transition temperature of the final nanocomposites depends on the montmorillonite modifier structure (Gârea et al., 2010).

The improvement in thermal stability of nanocomposites is often mentioned to be due to the torturous path for volatile decomposition products. The thermal stability of epoxy increased with the contents of inorganic components, owing to the increased char (Chiang et al., 2007). The char layer can protect unburned structure materials during fire. The decrease in thermal stability may be attributed to the catalytic activity of organoclay on epoxy decomposition (Camino et al., 2005). In the case of epoxy/rubber blend, the lower Tg may be due to the good compatibility between rubber and epoxy that would cause rubber to remain in epoxy matrix and plasticised the system. The increase in Tg may be explained by phase separation of dissolved rubber (Lee et al., 1989; Balakrishnan et al., 2005).

The flexural modulus is generally improved by incorporation of layered silicate, due to shear deformation and stress transfer to the layered particles. The decrease in flexural strength and failure strain, however, could be due to an inhomogeneous and reduced network density in the presence of fillers. Meanwhile, the incorporation of dispersed rubbery particles in a layered silicate/epoxy nanocomposite has received significant attention (Liu et al., 2004; Froehlich et al., 2004; Balakrishnan and Raghavan, 2004; Balakrishnan et al., 2005; Ratna et al., 2003; Becker and Simon, 2005; Lee et al., 2010; Marouf et al., 2009). It was found that the flexural strength and modulus of rubber


toughened epoxy silicate nanocomposites were higher, compared to the neat epoxy (Balakrishnan et al., 2005).

The current research attempts to study the effect of both liquid natural rubber and layered silicates (i.e., montmorillonite) in epoxy on the properties of the ternary system. The DGEBA type epoxy is toughened with PMMA-grafted natural rubber latex and nanoclay (Cloisite 30B). The direct use of the chemically modified latex is favourable, since it is cheaper, easy to blend and eco-friendly, compared to the use of dried chemically modified rubber that requires volatile organic compounds such as toluene, methylethylketone or dichloromethane for dissolution, prior to mixing with epoxy. The use of dried PMMA-grafted natural rubber to toughen epoxy was studied by Zainol et al. (2006) and Rezaifard et al. (1994). In this paper, we report the thermal and flexural properties of the room temperature cured samples. The detailed morphological studies were briefly discussed elsewhere (Yuhana et al., 2012).

2 Experimental

2.1 Materials

Diglycidyl ether bisphenol A (DGEBA) epoxy resin, Epikote 828, was obtained from Hexion Specialty Chemicals, Korea. The epoxide equivalent weight was 187 g/mol. The liquid curing agent, Baxxodur TM EC301 (formerly known as Polyetheramine D230), was supplied by BASF. The composition is alpha-(2-Aminomethylethyl)-omega-(2-aminomethylethoxy)-poly(oxy(methyl-1,2-ethanediyl)). PMMA-grafted natural rubber Megatex 30 was obtained from Green HPSP (M) Sdn. Bhd. The organically modified montmorillonite utilised was Cloisite 30B, purchased from Southern Clay Products, USA. The composition of the modifier is bis (hydroxyl ethyl) methyl tallow alkyl ammonium salts with montmorillonite.

2.2 Sample preparation by mechanical stirring

The compositions of the four prepared systems are demonstrated in Table 1. One hundred grams of bisphenol A diglycidyl ether were added to a stainless steel cup. It was heated to about 55°C by using a hot plate. This was followed by the addition of Cloisite 30B or latex, and mechanical stirring at about 50 rpm for about 1 hr. A stoichiometric amount of the liquid curing agent was then added and stirred for about 10 minutes. For the ternary system, the natural rubber in latex was added after 30 minutes of mixing Cloisite 30B with the epoxy, and the mixture was stirred for another 30 min at 50–70 rpm. The total mixing duration of the ternary system was about 1 hr. The mixture was then cured at room temperature for a week.

2.3 Fourier transform infrared spectroscopy (FTIR)

Infrared analysis of the cured epoxy films was carried out using a Nicolet 6700 by Thermo Scientific. In this report, spectra are presented in the range between 4,000 cm–1 and 800 cm–1.


Table 1 Composition of prepared samples

Weight of component [gram]

Epikote 828 Megatex 30 Cloisite 30B

Epoxy 100 0 0

Epoxy-5 MG30 100 5 0

Epoxy-530B 100 0 5

Epoxy-5MG30-530B 100 5 5

2.4 Differential scanning calorimetry analysis

The glass transition temperature of cured samples were determined by using Mettler Toledo DSC 822e. The sample was heated at 10°C min–1 from 25 to 170°C. The sample was also maintained at that temperature for approximately 2 min, next it was cooled to –20°C at the rate of 20°C min–1 to remove any thermal history. The sample was then re-scanned using a modulated heating programme as described above, twice.

The studied parameters are the onset of glass transition (Tg), the inflection slope (mg) and the specific heat capacity increment (Δcgp). These parameters were estimated by wielding STARe software.

2.5 Thermalgravimetric analysis

The thermal stability of cured samples was studied by using Mettler Toledo TGA 851e. Specimens comprising a mass between 15 mg and 20 mg were placed in 40 uL alumina crucible, and the mass was heated from 25 to 500°C at a rate of 10°C /min, under nitrogen flow.

The decomposition of epoxy is divided into two steps, namely volatilisation of low molecular weight compound and epoxy decomposition which are estimated to take place between 50–250°C, and 250–500°C respectively. The measured parameters during the two processes are the onset temperature (Tv or Td), temperature at maximum rate (Tmaxv or Tmaxd), inflection slope value (mv or md) and volatiles and char mass percentage (wv or wd). The TGA thermograms and derivative (DTG) curves of cured epoxy and modified epoxies were analysed by utilising STARe software.

2.6 Flexural test

Flexural tests under four-point bend configuration were performed by using Instron 5567 universal testing machine, according to ASTM D790. The cross head speed was 1 mm/min and all the tests were performed at room temperature. The dimensions of the samples were about 60 × 25 × 3 mm3 and the span to thickness ratio was set at L/D = 16:1 in all cases.


3 Result and discussions

3.1 FTIR

The FTIR spectra of cured samples, epoxy resin and curing agent liquid are compared in Figures 1(a) and (b). Two peaks at 830 and 911 cm–1 corresponding to the epoxide ring can be observed in the spectrum of epoxy resin liquid. It appears that cured sample of epoxy-rubber blend and epoxy nanocomposite contained the unreacted epoxide functional groups, due to peak appearance at 830 cm–1. The primary amine presence in the curing agent, polyetheramine could be observed by the presence of two peaks between 3,300 and 3,500 cm–1. The peaks however vanished for all cured modified and unmodified epoxy systems. The epoxy-rubber blend and epoxy nanocomposites, however, showed a weak peak at 3,400 cm–1, that can indicate the presence of secondary amine. No epoxide ring, primary and secondary amine peaks were observed in the spectrum of cured epoxy and ternary system.

Figure 1 The FTIR spectra of (a) epoxy liquid resin, Epikote 828 and (b) curing agent liquid Polyetheramine D230, and cured samples

(a)

(b)


3.2 Differential scanning calorimetry analysis

The graphs of DSC analysis on the four samples are given in Figure 2. It is obvious that there is no distinct exothermic heat, upon first heating step. It can be concluded that all samples were fully cured. However, the FTIR results showed traces of epoxide in both epoxy-rubber blend and epoxy nanocomposite systems, as shown in Figure 1.

Figure 2 DSC graph of cured (a) epoxy (b) epoxy-5MG30 (c) epoxy-530B and (d) epoxy-5MG30-530B

Upon cooling to –20°C, it can be observed the peaks of exothermic with ‘stages’. These peaks can indicate different structures of amorphous and crystalline formation, suggesting that all samples are polymorphism.

Figure 3 The DSC analysis shows the glass transition temperature with enthalpy of relaxation (a) during the first heating step and (b) during second heating step

Temperature (°C)

(a)

Notes: 1: Epoxy, 2: Epoxy-5MG30, 3: Epoxy-530B and 4: Epoxy-5MG30-530B.


Figure 3 The DSC analysis shows the glass transition temperature with enthalpy of relaxation (a) during the first heating step and (b) during second heating step (continued)

Temperature (°C)

(b)

Notes: 1: Epoxy, 2: Epoxy-5MG30, 3: Epoxy-530B and 4: Epoxy-5MG30-530B.

The glass transitions of cured unmodified and modified epoxies were measured and are illustrated in Figures 3(a)–3(b). The studied parameters are the onset of glass transition (Tg), the inflection slope (mg) and the specific heat capacity increment (Δcgp), are given in Table 2. Table 2 The DSC analysis during first, second and third heating step

First heating Second heating Third heating

Onset of glass transition temperature, Tg [°C]

Epoxy 62 81 83

Epoxy- 5MG30 63 80 84

Epoxy-530B 64 81 87

Epoxy-5MG30-530B 62 81 85

Inflection slope of glass transition, mg [mW/min]

Epoxy 4.9 0.7 0.7

Epoxy- 5MG30 4.1 0.5 0.5

Epoxy-530B 2.5 0.5 0.7

Epoxy-5MG30-530B 3.4 0.5 0.5

The specific heat increment, Δcpg at glass transition [J/gK]

Epoxy 0.45 0.34 0.45

Epoxy- 5MG30 0.73 0.50 0.47

Epoxy-530B 0.72 0.45 0.26

Epoxy-5MG30-530B 0.41 0.53 0.38

It can be seen from Figure 3(a) that the Tgs starts at about 60°C, for all samples with enthalpy of relaxation during the first heating. The slightly higher values for the modified


epoxies could be due to the ability of rubber and intercalated silicate to restrict the epoxy network mobility. The Tgs were increased by 20°C, with the absence of relaxation enthalpy for all samples in the second heating step, as shown in Figure 3 (b). This may be attributed to the absence of volatiles in all samples, which may act as a plasticiser. During the third heating step, the nanocomposite shows the highest Tg, followed by ternary system, as shown in Table 2. The higher value Tgs could be related to restricted segmental motions near the interface between the epoxy chain and Cloisite 30B. The presence of rubber, however, does not give significant effect to the Tg of the epoxy/rubber blend. This may be ascribable to the presence of nano sized rubber, which can be considered as dissolved rubber as illustrated in Figure 2(b). The ternary systems do not show significant change in Tg. Similar trend was obtained by Balakrishnan et al. (2005).

The inflection slope (mg) of DSC graph for glass transition process was unlikely being reported in any literatures. The lower value of mg indicates the material is more stable upon heating process. The first heating step show the highest value of mg, compared to second and third heating steps, as given in Table 2. This may indicate the high content of volatiles before heating process. The low value of mgs for modified epoxies may suggest that the presence of rubber and silicate can disrupt the glass transition process by restricting the epoxy network mobility. During the third heating step, the mg for epoxy containing 5 phr Cloisite 30B is increased by 0.2 mW/min compared to the one in the second heating step. There is a possibility that the chemical modifier in Cloisite 30B has been decomposed during the third heating step, and can act as plasticiser to fasten the glass transition process.

The specific heat increment value (Δcgp) can represent the increase in rubbery nature of epoxy chain before and after glass transition process. During the first heating step, the values are higher for epoxy nanocomposite and epoxy-rubber blend. This can be due to the presence of unreacted epoxy and amine in both samples that could act as plasticisers upon heating, as being proven by FTIR results. Rubber particles and the chemical modifier of Cloisite 30B could also act as plasticisers to ease the chain mobility of epoxy. At the second heating step, it can be seen that all modified epoxies have higher value in Δcgp, compared with the neat epoxy, as given in Table 2. Nevertheless, during the third heating step, the value of Δcgp is the lowest for epoxy containing 5 phr Cloisite 30B. This can be attributed to rigidity of the epoxy nancomposite material.

3.3 Thermalgravimetric analysis

Figure 4 shows the TGA thermograms and derivative (DTG) curves of cured epoxy and modified epoxies.

The volatilisation of low molecular weight molecules and polymer chain decomposition weight loss were measured by TGA and are reported in Table 3. The studied parameters during the two processes, are the onset temperature (Tv or Td), temperature at maximum rate (Tmaxv or Tmaxd), inflection slope value (mv or md), volatiles and char mass percentage (wv or wd), during volatilisation and polymer chain decomposition respectively.

Table 3 shows a relatively higher Tv and Tmaxv for epoxy-Cloisite 30B compared with the other cured samples. The onset temperature of volatilisation, Tv for epoxy containing 5 phr Cloisite 30 was about 40°C higher than that of neat epoxy. This can be due to the existence of intercalated silicate which could resist volatiles to escape from epoxy networking by introducing tortuous path, as shown in Figure 2(a).


Figure 4 The graph of weight loss thermograms and the derivative (DTG) curves from TGA analysis on cured samples

Table 3 TGA analysis result on volatilisation and polymer decomposition process

Onset temperature

(°C)

Temperatureat maximum

rate (°C)

Inflection slope value

(%/min)

Mass percentage of volatiles,

wv or char, wd (%) Parameters

Tv Td Tmaxv Tmaxd mv (× 10–3) md wv wd

Epoxy 99 364 150 382 0.96 0.26 1.13 10.52

Epoxy-5MG30 99 362 156 381 1.32 0.24 1.75 9.48

Epoxy-530B 141 360 174 379 1.66 0.23 1.68 11.91

Epoxy-5MG30-530B 104 359 134 379 1.08 0.22 1.33 12.42

Chin et al. (2007) found that the onset of mass loss started at a lower temperature, could be attributed to the presence of low molecular weight compounds that are volatile and easily removed. These species may be related to the accelerator and/or to the differences in curing agent formulation. The improvement in thermal stability of nanocomposites is often mentioned to be due to the torturous path for volatile decomposition products. Chena et al. (2006) suggested that the presence of inorganic nanofiller can produce more thermally-stable material, as it reduces chemical bond movements of the organic component.

The epoxy-Cloisite30B possesses the highest mv, showing the highest volatilisation rate. This can be attributed to the presence of chemical modifier in Cloisite 30B that degrades around 200°C. Also, as being proven by FTIR results, the sample contains unreacted epoxy. In the presence of MG30 rubber, there is a possibility that chemical interaction or reaction occurs between the Cloisite 30B chemical modifier and rubber, hence resulted the low value of mv.

The volatiles content is high for epoxy/rubber blend and epoxy/Cloisite 30B, which can be due to the presence of unreacted epoxy. This was confirmed by FTIR analysis, as shown in Figure 1. The wv however decreased for ternary system since the presence of Cloisite 30B could resist the volatilisation process.


The value of md is low for all modified epoxies, show that the Cloisite 30B and rubber could reduce the polymer chain decomposition, by introducing torturous path for volatile decomposition products. However, they have slightly low Td and Tmaxd, in comparison with the neat epoxy. This can be attributed to the possibility of catalytic activity of the modifiers (MG30 rubber and Cloisite 30B) on the epoxy decomposition process. The high char yield for epoxy/Cloisite30B, and toughened epoxy/Cloisite 30B nanocomposites can indicate good thermal stability of the samples. According to Chiang et al. (2007), the thermal stability of epoxy increased with the contents of inorganic components, owing to the increased char.

3.4 Flexural properties

The stress-strain behaviour of the modified epoxies is shown in Figure 5. Clearly, the addition of rubber and Cloisite 30B influence the stress-strain curve. The parameters measured from the curve are maximum flexural stress, failure strain and flexural modulus, as given in Figures 6 and 7.

It can be observed from Figure 6 that the flexural strength and failure strain of all modified epoxies are lower than the neat epoxy. This can be due to the weak interfacial interaction between the epoxy matrix and the modifiers (MG30 rubber and Cloisite 30B) and the inhomogeneous network density. Interfacial adhesion between rubber particles and the epoxy is weaker in the presence of Cloisite 30B, as will be shown in later.

Figure 5 Stress-strain curve of cured samples

Figure 6 The flexural strength and failure strain of cured samples


Figure 7 The flexural modulus of cured samples

Nanocomposites show the highest flexural modulus, as shown in Figure 7, which indicates the rigidity of the samples. The increase in flexural modulus is mainly attributed to the inherent stiffness of Cloisite 30B layers and the restriction of chain mobility. We found that the modulus was not affected significantly by the presence of rubber, in the ternary system.

4 Conclusions

The modified epoxies possess good thermal stability by having lower volatilisation and polymer chain decomposition rate at the increased temperatures. Furthermore, the high char yield content for epoxy/Cloisite 30B and rubber toughened epoxy/Cloisite 30B nanocomposite show that they could be used as adhesive and coating materials with good thermal stability properties. The onset temperature of volatilisation in the presence of Cloisite 30B is increased by 40°C than the neat epoxy, suggested Cloisite 30B as a good barrier agent. However, the presence of chemical modifier in Cloisite 30B and rubber particles in epoxy can affect the thermal stability of the blend by acting as plasticiser. It seems that the presence of Cloisite 30B aggregates, as being observed in our previous study does not reduce the thermal stability and the glass transition temperature significantly. The intercalated structure of silicate can help to introduce tortuous path for volatiles to escape the epoxy network, hence retarding the decomposition process. As expected, the presence of Cloisite 30B increases the flexural modulus of epoxy/silicate nanocomposite and rubber toughened epoxy/silicate nanocomposite. The SEM images of the fractured samples proved the existence of multiple fracture paths for the two samples. The poor adhesion between the rubber and epoxy phases in ternary blend was observed by using TEM. This is one of the possible reasons for low flexural strength and failure strain of the ternary blend, compared to neat epoxy. Significant improvement in thermal and flexural properties is expected at higher curing temperature and better mixing technique.

Acknowledgements

The authors would like to acknowledge help from Mr. Md. Yaakub Yasin, Mr. Wan Mohd Nazir Bin Wan Taha and Mr. Mohd. Razif Maafol for their kind assistance. Also, the authors wish to acknowledge Universiti Kebangsaan Malaysia, Malaysia Public Service Department (PSD) and the Ministry of Science, Technology and


Innovation (MOSTI) for SLAI scholarship and Science Fund Grant (03-01-02-SF0059) to conduct the research.

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Documents

Thermal and flexural properties of room-temperature cured PMMA grafted natural rubber toughened epoxy layered silicate nanocomposite