1 Introduction

Epoxy thermosetting polymers are extensively used as matrix phase in high performance applications in aerospace and automobile industries. Epoxy resins are inherently brittle and poor resistance to crack growth due to high cross-linking density results in limiting their usage as structural materials. The mechanical properties of epoxy matrices has been improved by adding nano materials and polymeric fillers like thermoplastics, rubber particles, carbon nanotubes, nano silica, nanoclay and grapheme [1,2,3,4,5,6]. Zhang et al. [7] observed 30 to 40% enhancement in tensile strength and flexural modulus by adding silica as reinforcement in epoxy resin. Bakar et al. [8] investigated the effect of reactive polyster and kaolin on the mechanical properties of epoxy resin. They showed that significant enhancement in impact and flexural strength due to addition of kaolin clay and polyster in epoxy. Abhishek K. Pathaka et al. [9] investigated the mechanical properties of hybrid composites by adding carbon fibre/grapheme oxide into brittle epoxy resin. They observed that, flexural strength amplified by 66%, flexural modulus by 72%, while interlaminar shear strength improved by 25% in polymer hybrid composites.

Previous studies carried out on PMMA and epoxy resin systems have concerned mainly their reaction kinetics and miscibility. The miscibility, curing, dynamic properties, and morphology of composites based on PMMA and epoxy was studied by Cook et al. [10]. It was seen that the inclusion of PMMA caused a slight decrease in curing rate because of dilution effect. Moreover, evidence of phase separation of the components into PMMA loaded domain and an epoxy-rich matrix was provided by dynamic mechanical thermal analysis and confirmed with electron microscopy studies. And also, the unification of little measures of PMMA with the epoxy resin appeared to source just a modest sacrifice in thermal resistance.

The halloysite nanoclay was found over 100 years prior, however was used for applications just in the last 10–20 years when it accessible in unadulterated structure. Almost every natural kaolin deposit has a fraction of halloysite, which is often referred to as rolled kaolin, but there are only a few deposits where halloysite is presented in almost pure form and contains 90–95 wt% of tubular structure. HNT has the advantage of using existing processing techniques, similar to other clays, however it has a special extended rod-like shape with axis ratio of 20:1 and an inner lumen which may contain distinctive functional chemicals for sustained release. Besides, HNT dispersion to single particles is much simpler because these clay nanotubes are not stacked together as platy kaolin, montmorillonite, or bentonite particles [11]. The high aspect ratio of HNTs helps reinforce polymers in composites by optimising the load transfer from the matrix to the nanotubes [12].

In our previous research work, we investigated the effect of PMMA on mechanical properties of epoxy [13] and effect of MHNT on mechanical and thermal properties of epoxy alone [14]. But the research in addition of MHNT’s in the thermoplastic toughened epoxy is not dealt. More ever, mechanical properties were also not evaluated on above mentioned hybrid nanocomposites. The fundamental reason for this study is to estimate the influence of PMMA as thermoplastic phase and MHNT’s as nano reinforcement on mechanical and thermal properties of hybrid nanocomposites.

2 Experimental details

2.1 Materials used

For fabrication of epoxy based hybrid nanocomposites, Halloysite nanoclay (kaolin clay) (HNT’s) and Poly-methyle-metacrylate (PMMA) was used as reinforcement. HNT’s with average diameter 30–70 nm, length 1–3 µm and (3-Aminopropyl)trimethoxysilane as modifier for HNT supplied from Sigma Aldrich, India. Epoxy resin (LAPOX L-12) Bisphenol-A-Diglycidyl-Ether (DGEBA) with an epoxide equivalent weight of 185 g/eq formed the foundation of all the materials examined in the present work. Hardner diamino-diphenyl-methane (DDM) with commercial designation K-6 is selected for the present work. The LAPOX L-12 epoxy resin and the corresponding curing agent K6 are of commercial grade are supplied from Atul Ltd. Gujarat, India. PMMA of commercial grade in the form of granules with glass transition temperature of 105 °C was purchased from chemical house of Bangalore, India. Acetone (CH3)2CO, used as PMMA solvent, was purchased from Nice Chemicals (P) Ltd. Kerala, India.

2.2 Preparation of hybrid nanocomposites

Modification of Halloysite nano clay has been done to reduce agglomeration, using 3-aminopropyltriethoxysilane in the presence of distilled water as solvent media. Hybrid nanocomposites were prepared with addition of MHNT’s at varying amounts (1, 2, 3 and 4 wt%) and fixed amount of 6 wt% of dissolved-PMMA to epoxy resin matrix, 6 wt% of PMMA is selected because at this wt% superior mechanical properties were observed. Epoxy resin and MHNT’s were blended by mechanical stirring for 0.5 h followed by ultra-sonication for 0.5 h at 50 °C [15]. The dissolved-PMMA was then added to each blend and then stirred for 20 min at 60 °C to remove solvent. The blend is cooled to room temperature and hardener was then added and stirred for 5 min. The blend was poured immediately into the molds prepared according to ASTM standards. The molds were kept in autoclave for curing at 85 °C for 8 h. Flow chart of preparation of samples is shown in Fig. 1.

Fig. 1
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Flow chart for preparation of hybrid nanocomposites

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2.3 Characterization of epoxy hybrid nanocomposites

XRD analyses were performed by using Philips X’pert-PRO X-ray diffractometer with graphite monochromatized CuKα anode (λ = 1.5418 Å) radiation at the scan rate of 2° min−1 and 2θ ranging from 10° to 80°. SEM analysis was carried out on fractured surfaces of the samples using field emission scanning electron microscopy (VEGA3 TESCAN) and all samples were coated with a thin layer of sputtered gold before analysis. Fourier Transform Infrared Spectrophotometer (FT-IR) of SHIMADZU (Model: IRAffinity-1S) is used to take FT-IR spectra in the range from 400 to 4000 cm−1 in transmission measurement mode. The instrument resolution is set for 4 cm−1 and with 45 numbers of scans. TGA was done on 15–16 mg samples on a Perkin Elmer Simultaneous Thermal Analyzer (STA) 8000 instrument with alumina sample pan in a 20 ml/min nitrogen flow, heating from 50 to 800 °C with a heating rate 20 °C/min.

2.4 Mechanical properties

Tensile test were conducted as per ASTM D638 standards (specimen dimension 50 mm long, 13 mm width and 5 mm thickness) at a constant rate of traverse of the moving grip of 1 mm/min. Three point bending test specimens are tested according to ASTM D790 (standard dimension of 125 mm × 12.7 mm × 3.2 mm) at a constant rate of traverse of the moving grip of 5.2 mm/min and support span of 100 mm. Fracture toughness specimens are tested according to ASTM E399 (standard dimension of 48 mm long, 10 mm width and 5 mm thickness) at a rated speed of 2 mm/min, with distance between supports is 40 mm. Five specimens of each composition are tested using a UTM AUTOGRAPH AG-IS 10 KN.

3 Results and discussion

3.1 FT-IR of epoxy hybrid nanocomposites

Figure 2 shows FT-IR spectrum of neat epoxy, the peaks at 913 and 761 cm−1 in the FTIR spectra were allocated to the characteristic absorptions of epoxy group and peaks with wavelength 3700 cm−1 are characterized for hydroxyl groups of epoxy resin or secondary amines [16]. From Fig. 3 it can be seen that as amount of MHNT’s was increased, the height of the epoxy hydroxyl groups decreased (3700 cm−1), while peaks related to epoxy groups remain unchanged at 913 and 761 cm−1[17, 18]. The peaks at 1790 cm−1 depicts the spectrum of PMMA functionalized MHNT’s, mainly due to the C=O band of –COOCH3 groups [19].

Fig. 2
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FT-IR spectra of neat epoxy

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Fig. 3
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FT-IR spectra of hybrid nanocomposites

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3.2 SEM of hybrid nanocomposites

The Fig. 4 shows SEM images of fractured surface of tensile samples at most favorable content of MHNT, PMMA and hardener in epoxy. Cross-linking density plays important role in epoxy materials, to attain better mechanical properties. As seen in Fig. 4b, c, fine dispersion of MHNT’s and PMMA as thermoplastic phase with little agglomeration is occurred (up to 4 wt% MHNT). It can also be observed that phase separation of nano and micro particles in epoxy resin matrix [20]. And also, in 2 wt% MHNT’s crack deviation mechanism was less substantial, as shown in Fig. 4b and c. In addition, high energy can be dissipated in epoxy hybrid nanocomposites up to 4 wt% MHNT, and higher strength can be obtained. More ever, as the crack encountered the modified MHNT’s in epoxy matrix, it should pass by either the MHNT or break through it; hence, much more energy would be consumed. As shown in Fig. 4d, crack blunting/branching, pull-out mechanism and crack pinning can be observed in fracture surface of epoxy hybrid nanocomposites included with 5 wt% of MHNT’s and 6 wt% PMMA [21]. As clay content increases, clay particles form larger agglomarates and joins PMMA domains which may act as defect and instigate failure.

Fig. 4
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SEM images of (a) Neat epoxy; b Epoxy/2 wt% MHNT/6 wt% PMMA (c) Epoxy/4 wt% MHNT/6 wt% PMMA (d) Epoxy/5 wt% MHNT/6 wt% PMMA

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3.3 XRD of hybrid nanocomposites

So as to distinguish the morphology framed during the hybrid nanocomposites preparation process, XRD spectra were generated, for neat epoxy and hybrid nanocomposites. Figure 5 shows XRD patterns of neat epoxy resin whose peaks show at 2θ = 20° indicating the amorphous nature. Therefore the cured epoxy is non-crystalline. Figure 6 shows XRD pattern of Epoxy/MHNT/PMMA hybrid nanocomposites. It has been found that the XRD pattern of hybrid nanocomposites, diffraction peak is shifted to 2θ = 18°. The shift of the diffraction peaks towards lesser angles indicates that all the prepared nanocomposites are portrayed by an intercalated structure, with an expansion of the interlamellar d-spacing with reverence to the original HNT. The hybrid nanocomposites didn’t highlight any peak, regardless of MHNT content, in the XRD range of 2° to 10°. This means the polymer had the capacity to enter between the MHNT platelets bringing about conceivable exfoliated or tangled intercalated morphology. Nonetheless, with increasing the MHNT content to 5 wt%, a weak scattering peak showed up with a slight substantial d-spacing. The low dispersion at high concentration of clay might be because of agglomeration of clay platelets within hybrid nanocomposites [22].

Fig. 5
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XRD patterns of pure epoxy resin specimen

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Fig. 6
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XRD patterns of hybrid nanocomposites

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And also, it can be noted that PMMA domains don’t enter into the clay galleries and the interlayer spacing doesn’t alter with the inclusion of PMMA. Since PMMA has a much better molecular weight, it is required to diffuse less in between the clay layers [23].

3.4 Thermal stability of hybrid nanocomposites

TGA was carried out to study the thermal stability of the hybrid nanocomposites. Thermogravimetric graphs of the hybrid nanocomposites with 6 wt% PMMA and different amount of MHNT’s and epoxy are shown in Fig. 7. Hybrid nanocomposites with 6 wt% of PMMA and 1–2 wt% of MHNT demonstrated a more gradual weight reduction than the composites contain higher wt% MHNT. The reality that the PMMA is more exceptionally cross-linked than the epoxy might be the reason for the more gradual weight reduction of the composites at 350–450 °C. An initial weight reduction of all the composites at low temperature was similar because of evaporation of moisture and un-reacted epoxy resin [24, 25].

Fig. 7
figure 7

TGA thermograms of neat epoxy and hybrid nanocomposites

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The 40% weight loss for neat epoxy, epoxy/PMMA/MHNT hybrid nanocomposites were occurred at the temperature of 394, 377, 388, 402 and 411 °C correspondingly. Likewise it was seen that the 60 wt% reduction of the composites occurred at the temperature of 421, 404, 423, 477 and 508 °C respectively. The inclusion of PMMA into the epoxy initially degrades at lesser temperature but the modified MHNT’s into the matrix steadily improved the thermal stability of the nanocomposites. This is due to the efficient interaction of the MHNT with epoxy and the epoxy makes the composites in a state of three dimensional cross linked network structures [26].

3.5 Mechanical properties of hybrid nanocomposites

Clay-PMMA-epoxy hybrid nanocomposites consist of modified clay nanoparticles and epoxy resin with low temperature solvent based PMMA solution were successfully fabricated through a simple sol–gel process and solvent evaporation. In order to evaluate the mechanical properties of hybrid nanocomposites we conducted tensile test, flexural stress, fracture toughness test and impact test according to ASTM standards. It was observed mechanical properties of hybrid nanocomposites increased with increase in MHNT’s up to 4 wt% as shown in Figs. 8, 9, 10, 11 and 12. All the values indicate average value of five specimens in each composition.

Fig. 8
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Tensile strength of neat epoxy and hybrid nanocomposites

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Fig. 9
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Tensile Modulus of neat epoxy and hybrid nanocomposites

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Fig. 10
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Flexural strength of neat epoxy and hybrid nanocomposites

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Fig. 11
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Flexural Modulus of neat epoxy and hybrid nanocomposites

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Fig. 12
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Fracture toughness of neat epoxy and hybrid nanocomposites

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Tensile strength of pure epoxy and epoxy/MHNT/PMMA hybrid nanocomposites are presented in Fig. 8 with error bars corresponding to standard deviation. When PMMA polymer is added by 6 wt% into epoxy, tensile strength was decreased by 3.81%. Now, by having the combination of epoxy, PMMA and HNT’s we observed increase in tensile strength up to 4 wt% of MHNT’s. The tensile strength is increased by 3.65%, 10%, 13.4% and 5.48% by adding 2, 3, 4 and 5 wt% of MHNT’s respectively and 6 wt% of PMMA into neat epoxy. Indeed, for hybrid nanocomposites introducing PMMA and MHNT’s, the strength increased in response of MHNT concentration.

It can be observed that the young’s modulus of hybrid nanocomposites is increased by 10.14%, 18.8%, 22.7% and 12.07% by adding 2, 3, 4 and 5 wt% of MHNT’s respectively and 6 wt% of PMMA into neat epoxy. Therefore for hybrid nanocomposites, tensile strength enhancement is directly proportional to amount of MHNT’s and the effect of PMMA has only influence on tensile modulus. Higher interfacial properties of MHNT also have contributed to enhancement of tensile modulus [27].

Improvement of tensile modulus and strength and preservation of ductility indicate that the strengthening effect of the MHNT’s and the ductility improvement of PMMA are concurrently functioning in the hybrid nanocomposites.

Flexural strength is another significant directory to assess the mechanical properties of hybrid epoxy nanocomposites. The flexural strength at fracture measured under three-point bending of epoxy monomer modified with MHNT’s and PMMA is shown in Fig. 10. Flexural strength increases with increasing the MHNT’s loading, reducing of the increasing trend after 4 wt% of MHNT’s. The flexural strengths of hybrid nanocomposites with 6% PMMA and 4% MHNT’s are higher by 8% compared to neat epoxy. Similarly, as shown in Fig. 11 the flexural modulus increases by 1.3%, 9%, 13% and 11% for 2, 3, 4 and 5 wt% of MHNT’s and 6 wt% of PMMA in epoxy system. Flexible chains in the PMMA structure would diminish the rigidity of the epoxy chain and consequence in the decline of the flexural strength. Alternatively, plenty amount of active hydroxyl groups in PMMA would contribute to improve the cross-connecting density and then advance the mechanical properties [28]. On the whole, the MHNT’s together with PMMA can improve the flexural strength and flexural modulus of hybrid nanocomposites.

Fracture toughness of the hybrid nanocomposites in terms of the critical stress intensity factor was measured at normal temperature. Figure 12 displays fracture toughness of SENB specimens of neat epoxy and MHNT’s/PMMA/Epoxy hybrid nanocomposites. The values of neat epoxy fracture toughness and impact strength were calculated to be 0.73 MPa√m and 18 kJ/m2 respectively, which are usual for an unmodified, highly cross-connected brittle epoxy. As depicted in Fig. 12, neat epoxy shows less fracture toughness value, due to unstable brittle crack growth behavior where the crack propagates by the characteristic slip mechanism. Inclusion of PMMA in epoxy matrix greatly improved fracture toughness and impact strength relative to the neat epoxy, as has been clarified previously. The sample which consists of 4 wt% of HNT’s and 6 wt% of PMMA has highest value which indicates a stable crack growth characteristic. Increases of approximately 95% were observed in fracture toughness by adding of 4 wt% of HNT’s and 6 wt% of PMMA. It can be observed that after 4 wt% of modified HNT, composites exhibit lower fracture toughness and sudden failure. This sudden failure mechanism occurs since higher HNT wt% leads to brittle the composites. Hybrid nanocomposites fracture behavior is dependent on the HNT’s and PMMA compositions.

It is understood that during the fracture process, the stress field ahead of the crack tip caused the PMMA core to cavitate. Proof of cavitation will be shown in SEM analysis. These cavitated particles begin and assist subsequent matrix shear yielding [29].

In hybrid nanocomposites, on the other hand, adding of MHNT’s and PMMA did not influence any substantial change to epoxy impact strength. Although the MHNT content was raised from 1 to 5 wt% the impact strength remained almost constant relative to the neat epoxy. Kinloch and Taylor [30] studied the mechanical and fracture properties of Nanomer modified epoxy specimens. Analogous to present findings, these researchers also observed only a little effect of nanoclay in impact strength of epoxy.

4 Conclusion

Epoxy/PMMA/HNT hybrid nanocomposites were prepared by adding 2, 3, 4, and 5 wt% of HNT’s and 6 wt% of PMMA into neat epoxy. Effects of HNT’s and PMMA on mechanical and thermal properties of hybrid nanocomposites were investigated.

The tensile strength is increased by 3.65%, 10%, 13.4% and 5.48% by adding 2, 3, 4 and 5 wt% of MHNT’s respectively and 6 wt% of PMMA into neat epoxy. The young’s modulus of hybrid composite is increased by 10.14%, 18.8%, 22.7% and 12.07% by adding 2, 3, 4 and 5 wt% of HNT’s respectively and 6 wt% of PMMA into neat epoxy. The flexural strengths of hybrid nanocomposites with 6% PMMA and 4% HNT’s are higher by 8% compared to neat epoxy. The flexural modulus increases by 1.3%, 9, 13% and 11% for 2, 3, 4 and 5 wt% of HNT’s and 6 wt% of PMMA in epoxy system. The sample which consists of 4 wt% of HNT’s and 6 wt% of PMMA has highest value of fracture toughness which indicates a stable crack growth characteristic. Increases of approximately 95% were observed in fracture toughness by adding of 4 wt% of HNT’s and 6 wt% of PMMA. Higher thermal stability of hybrid nanocomposites is noticed at more than 4 wt% of clay is added with the combination of 6 wt% PMMA.