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Understanding the effect of nano-Al2O3 addition upon the properties of epoxy-based hybrid composites
Materials Science and Engineering A 517 (2009) 185–190
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Understanding the effect of nano-Al2 O3 addition upon the properties of epoxy-based hybrid composites Abdollah Omrani ∗ , Abbas A. Rostami Faculty of Chemistry, University of Mazandaran, P.O. Box 453, Babolsar, Iran
a r t i c l e
i n f o
Article history: Received 26 December 2008 Received in revised form 17 March 2009 Accepted 26 March 2009 Keywords: Particle-reinforcement Thermal properties Surface analysis Thermosetting resin
a b s t r a c t With the recent advances in nanoscience, there is promoting interest in nano-sized reinforced polymers for high technology applications. Evaluations on the curing reaction of diglycidyl ether of bisphenol A (DGEBA) cured with 4,4 -diaminodiphenylmethane (DDM) in the presence of nano-alumina have been carried out using DSC and In situ FT-IR measurements. The results from calorimetry measurements showed that the loading level of 2 is the optimum alumina concentrations for DDM-based composite. Different kinetic models including nth order, the Kamal autocatalytic, and the modified Avrami equations have been tested and the results of kinetic analysis are presented just for the last model owing to the better fitting of the experimental data. The viscoelastic behavior of the cured composites is determined using DMA showing that the storage modulus and the Tg of the composites are improved with the addition of nano-filler concentration. Results from SEM analyses revealed that the filler encourages the development of large particles in the resin matrix. Mechanical property measurement of the composites indicated that high flexural modulus has been attained with increasing concentration of nano-Al2 O3 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resins are thermosetting materials that require a cure treatment to attain suitable physical and mechanical properties for industrial applications. Epoxy resin based composites are used as laminates for printed circuit boards, aerospace, ballistic and engineering hardware components, automotive parts, electrical components, and rehabilitation products, etc. However, some of its inferior characteristics such as impact strength, weather resistance and thermal stability restrict the use in high performance applications [1,2]. The properties of cured materials depend on a combination of several factors including the structure of epoxy resin, type and concentration of curing agent, and the other specific agents such as accelerators, catalysts, additives, and modifiers. Also, several works have been addressed to modify epoxy resin toughness using rubber materials [3–5]. However, the toughening improvement was achieved at the cost of a decreased modulus and glass transition temperature. Over the past decades, the utility of inorganic nanoparticles as additives to enhance polymer performance has been established [6–10]. Also, an inorganic nanoparticle is an excellent toughening and reinforcing materials to develop new nanocomposites [11,12]. Elsewhere, it has been reported that dispersion of rigid inorganic filler into epoxy resin is important
∗ Corresponding author. Tel.: +98 112 5242025; fax: +98 112 5242002. E-mail address: [email protected] (A. Omrani). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.076
for the improvement in strength, modulus, and fracture toughness [13–16]. In these particular filled systems, the interfacial bonding or adhesion between the filler and the resin matrix has a great effect on the mechanical properties [17–19]. It is also well known that the structure and properties of an organic polymer are considerably different from those of inorganic nanoparticles from the specific gravity, dielectric properties, and heat resistance point of views. Therefore, much more attention has been made to produce new nanocomposites (hybrid materials) with a combination of the properties of organic and inorganic components. More recently works have been focused to prepare polymer-layered silicate nanocomposites. Various methods have been utilized to generate materials, such as hybrid materials, which is beyond the scope of discussion in the present work. However, there are a few studies on the formation of nanocomposites by directly dispersion of nanoparticles into the polymer matrix. The comprehensive properties of nanocomposites depend not only on the properties of the components but also on the morphology of the microphase structure [20,21]. The present work focuses on the performance of composites containing alumina nanoparticles and to understand what role these inorganic fillers play on the nanometer scale. The first step was aimed to finding out the effect of nanoalumina on diglycidyl ether of bisphenol A (DGEBA) cure. In the second step, the best possible viscoelastic and mechanical responses as a function of the nanoparticle content have been investigated. Recently, a comprehensive study on the dynamic mechanical behavior of
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DGEBA/cyclohexylamine/nanoalumina system has appeared in literature [22].
2. Experimental 2.1. Materials Alumina particles with an average size of 50 nm were purchased from Aldrich. The epoxy used was DGEBA purchased from Sigma–Aldrich, Canada with an epoxy equivalent weight of 185 g/eq. The hardener was 4,4 -diaminodiphenylmethane (DDM) provided by Sigma–Aldrich Canada. The chemicals were used, as received, without further purification. 2.2. Preparation of the specimens Epoxy resin and the alumina nanoparticles were mixed and then stirred for about 10 min to produce homogeneous blends. The epoxy/alumina blends were heated to 90 ◦ C and then a stoichiometry ratio of DDM, i.e. 27 phr, was added and mixed uniformly. The resulted mixture was kept under liquid nitrogen whilst being undergoing measurements. For the DSC measurements about 4–5 mg of the freshly prepared composites were used. Samples for the DMA and flexural tests were prepared by pouring the epoxy composites into a preheated Teflon mold and finally cured for 2 h into a vacuum oven at 100 ◦ C. The cured composites specimens were taken out of the oven and then cooled at room temperature and cut in the suitable size for the tests. 2.3. Characterization DSC measurements were carried out by means of a TA Q100 calorimeter under a nitrogen atmosphere. The samples were heated in the temperature interval of −30 and 300 ◦ C at a heating rate of 20 ◦ C/min and according to a heating/cooling/heating treatment. Following the first step, the cured samples were cooled to −30 ◦ C to minimize the enthalpy relaxation in the second heating scan. Finally, the samples were reheated to 250 ◦ C at 20 ◦ C/min to determine the glass transition temperature (Tg ) along with middle point method using TA software and to confirm the absence of any residual heat peak. DMA of both the neat epoxy and nanoalumina/epoxy/amine composites were carried out on a model DMTA V Dynamic mechanical analyzer (Rheometric Scientific) to estimate their thermomechanical properties. All samples were tested using a sample size of (15 mm × 5 mm × 1 mm) under single cantilever mode. The temperature range varied from 35 ◦ C to above the Tg of fully cured materials at a heating speed of 5 ◦ C/min at frequency of 1 Hz. A scanning electron microscope (SEM 1530) from LEO was utilized to observe the dispersion of nanoalumina on the fracture surfaces of the cured composites. The fracture surface was gold-coated prior to SEM studies to avoid charging and were examined at 15 kV accelerating voltage. The flexural properties of the epoxy systems and its composites are determined in a threepoint bending mode on samples with approximately dimensions of 25 mm × 5 mm × 1 mm using a MiniMat 2000 mechanical testing machine. Five tests were performed for each sample according to ASTM D-790-93 method and a compression speed of 2 mm/min was utilized. In situ FT-IR experiments were conducted on the samples between two KBr pellets inside the Bruker Tensor 27 spectrometer. Each spectrum was scanned 32 times at a resolution of 4 cm−1 from 400 to 4000 cm−1 . For each epoxy composition, four isothermal temperatures were used based on the data provided by DSC thermograms. Spectra were recorded and analyzed by OPUS software.
Table 1 Thermal properties of the cured composites obtained using DSC measurements. System
Al2 O3 (phr)
Tmax (◦ C)
H (J/g-sample)
H (J/g-epoxy)
DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM
0 0.5 2 5
182 183 181 180
407 426 431 416
517 543 556 549
3. Results and discussion 3.1. Results from the DSC experiment The curing reaction of the epoxy compositions has been investigated by means of DSC and the results are shown in Table 1. A small decrease in the peak temperature of the curing exotherms with increasing concentration of alumina particle has been observed. This effect was not significant for the composite involved low level of loading 0.5 phr. However, the peak maximum is shifted towards the lowest temperature for the composite having 5 phr of the nanofiller. Table 1 shows some key data about the effect of the used nanofiller on DGEBA cure which was obtained from the analysis of DSC thermograms. As can be seen in Table 1, the reaction heat of DGEBA/DDM system included 0.5 and 2 phr of the filler is higher than that of the neat resin and then decreases with increasing nanoalumina concentration. This means that high level of nanoalumina loading has a converse effect on the polymer network formation. It could be described by topological restrictions produced during the formation of epoxy network. However, H is the highest for the composition having 2 phr of nanoalumina. It is observed that the glass transition temperature of uncured materials (Tg,0 ) lies approximately between −8 and −10 ◦ C with increasing of the filler concentration. From the DSC thermograms, it is observed that the Tg values of composites are enhanced when compared with that of unmodified epoxy system. The Tg of the polymer composites is depicted in Fig. 1. The highest observed Tg of the DGEBA/DDM system incorporated with 2 phr of nanoalumina could be attributed to its highest reaction heat (see Table 1). It is noteworthy that the change in Tg for nano-sized filled composites has been controversial. Some authors have found that Tg of nanocomposites increases as a function of the filler loading, whereas others have observed an opposite trend. However, the Tg of an epoxy based composite may be changed due
Fig. 1. Dependence of the glass transition temperature of the fully cured epoxy on nanoalumina content.
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to network tacticity, molecular weight, cross-linking density, and amount of reaction residue, etc.
Table 2 Reaction rate constants, reaction orders, and activation energies calculated from FT-IR data.
3.2. Cure kinetics
System
Al2 O3 (phr)
Temperature (◦ C)
n
k (s−1 )
DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM
0 0 0 0 0.5 0.5 0.5 0.5 2 2 2 2 5 5 5 5
90 100 110 120 90 100 110 120 90 100 110 120 90 100 110 120
0.89 0.94 0.99 1.03 1.01 0.85 0.76 0.77 0.86 0.81 0.82 0.79 0.85 0.83 0.80 0.87
0.0418 0.0470 0.0509 0.0568 0.0460 0.0650 0.1160 0.1283 0.0506 0.0667 0.0786 0.0984 0.0554 0.0724 0.0871 0.0886
Kinetics of the studied cure reaction was investigated under isothermal conditions by analyzing the spectra obtained by means of FT-IR instrument. The method was based on probing reduce in the peak area of the epoxy group vibration at 915 cm−1 with time. The degree of conversion is calculated by normalizing epoxy group vibration with the absorbance bond at 1509 cm−1 as the reference peak [23,24]. The DDM-based composites were cured at 90, 100, 110, and 120 ◦ C. It is clear that the decrease in peak area of the epoxy group with time is an indicator of curing. From the IR spectra, the epoxy group conversion with time has been calculated. It is claimed that the modified Avrami expression provides an estimation of the kinetic parameters since the model is extensively used for thermosetting polymers [25–27]. Using the modified Avrami equation [28–31] (Eq. (1)), 1 − ˛t = exp(−kt n )
(1)
and its double logarithmic form (Eq. (2)), the kinetic parameters of reaction order and rate constant for the both systems have been obtained. ln[−ln(1 − ˛t )] = ln k + n ln t
(2)
where k is the rate constant, ˛t is the conversion degree at time t, and n is the reaction order. For nth-order reactions, based on Eq. (2), a plot of ln[−ln(1 − ˛t )] against ln t (min) should yield a linear relationship. The slope and the intercept are n and ln k, respectively. The Eq. (2) is used to fit the all experimental data at each isothermal temperature. Further, a line can be obtained between the reciprocal of temperature (1/T) and (ln k)/n in the following form: (ln k) Ea = ln A − n RT
(3)
The slope and the intercept lead to the activation energy Ea and frequency factor A, respectively. In the present study, Eq. (3) has been used to estimate the value of activation energy. Representative plots of ln[−ln(1 − ˛t )] versus ln t (min) for the composite at 0.5 phr of nanoalumina concentration is shown in Fig. 2 whose slope and intercept are applied to the estimation of k and n, which can be used to calculate activation energy at different content of nanoalumina.
Fig. 2. Representative plots obtained using the Avrami equation to calculate the kinetics parameters for the DGEBA/DDM/nanoalumina (0.5 phr) composite at different isothermal temperatures.
Ea (kJ/mol) 56.3
26.1
20.6
22.5
The curves in Fig. 2 show that the used kinetic model presents a satisfactory description of the studied cure reactions. Fig. 2 indicates that good linearity existed during the curing process. Results of the kinetic analysis are listed in Table 2. As it is seen in the Table 2, the curing rate constants are appeared to be influenced by the isothermal temperatures and the filler concentration. The values of k have a shift toward higher values with increasing curing temperature at the same content of nanoalumina as the curing rate constant depends on the temperature. The exponent n of DGEBA/DDM based composites varied between 0.76 and 1.01 which indicates the higher nucleation ability of the DGEBA/DDM matrix. Table 2 reveals that Ea decreased corresponding to the addition of nanoalumina concentration demonstrating the catalytic effect of the used filler. The Ea for the pure epoxy composition (without filler) is determined to be 56 kJ/mol while the corresponding values for the incorporated systems lies between 20 and 26 kJ/mol. However, the effect of nanofiller on the activation energy of DDM-based composites is significant in terms of the type of epoxy resin and curing conditions used. This may be interpreted that the cure reaction with DDM was obviously affected further by the existence of nanoalumina. 3.3. Results from the DMTA experiment Dynamic mechanical tests over a wide temperature range are very sensitive to the physical and chemical structure of polymers and composites. They allow studying secondary transition of Tg and yield information about the morphology of polymers. The curves of storage modulus and tan ı of the neat epoxy system and its composites having different levels of filler loading are shown in Fig. 3. The main mechanical relaxation is modified by the presence of the nanofiller. As is shown in Fig. 3, it may be confirmed that the amplitude of storage modulus curve in the rubbery plateau region increases with increasing the filler loading up to 2 phr. As is expected, this is due to the reinforcement of the filler as well. The complex moduli for all composites containing fillers are pushed to a higher level relative to neat epoxy system as the filler content increases. It is also observed that the storage modulus, which corresponds to the same temperature in the glass transition region, increased with increasing nanoalumina concentration. The onset of the considerable composites decay is also shifted toward slightly higher temperatures. Further decay in the storage modulus of DDM based composites can be observed at around 170 ◦ C. The composite containing 2 phr nanofiller caused the increase in modulus whereas the composites containing more filler led to the decrease in modulus. However, the composites containing alumina nanoparticles have higher modulus than that neat epoxy system. This means that
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Fig. 4. Scanning electron micrographs of fracture surface for neat epoxy system at (a) low magnification (10K×) (b) high magnification (50K×).
Fig. 3. DMTA profiles of storage modulus and tan ı for the neat epoxy and its composite.
there is strong adhesion between the alumina particles and epoxy resin. At a high concentration of 5 phr nanoalumina, it is considered that the filler is dispersed into the epoxy matrix irregularly and the void of the cross-linking is increased. Then the segmental mobility rises, which leads to the decrease of the storage modulus at the same temperature. In general, a good explanation for the dependence of the modulus of epoxy/nanoalumina composites on the alumina content could be done in terms of particle-particle contacts and agglomeration, which differ below and above the glass transition temperature. Also, the temperature of the maximum of tan ı peak is increased as the level of nanoalumina increased up to 2 phr and it will correspond with the apex of loss modulus curves. This relaxation process is associated to the Tg phenomenon. It has been earlier confirmed by DSC measurement that the Tg is increased on the filler content up to 2 phr (see Fig. 1). Further, below Tg , the reinforcement in terms of storage modulus is high. The increase of modulus for 2 phr filler is about 35%. This is not surprising due to the amount of the added filler. However, the main important factors that can affect Tg are degree of particle dispersion and curing conditions. The degree of particle dispersion includes size, homogeneity, orientation, and spacing between particles, whereas curing conditions include curing speed and degree of cross-linking. The improvement in the Tg observed in this study may arise from some of these factors. 3.4. Fracture surface morphology of the composites The fracture surface for the neat epoxy matrix is depicted in Fig. 4 which reveals a brittle behavior characterized by a large
smooth area, ribbons and fracture steps in the direction of crack propagation which accounts for its poor toughness. The SEM photographs obtained for fracture studies of composites specimen after three-point bending tests are presented in Fig. 5. In contrary to the pure polymer, composite surfaces are more roughly structured. The images show that the alumina particles are dispersed into the epoxy matrix as well as at low level of loadings. However, the particles are not homogeneously dispersed at high concentration of 5 phr. It could be described according to the cohesion of the alumina particles and the viscosity of the epoxy resin that might be hindered the uniform dispersion at high loading level. In the magnified images of the particle-matrix parts (Fig. 5c), the epoxy resin besieges the particles at low level of nanoalumina loadings as well. Also, Fig. 5c shows some defect regions possibly in the direction of crack propagation indicated by a large number of cavities on fracture surface. Their special shape originates in the unification of the propagating primary crack front with secondary cracks initiated at local inhomogeneities in front of the primary crack, e.g. particles or agglomerates. At high level of loading 5 phr, the SEM image (Fig. 5d) is extensively rugged, which indicates that cracks travel at high speed, allowing less matrix deformation because time for yielding is limited. Impeded matrix deformation reduces flexural strength since a large amount of energy may be consumed at the particle/matrix interface which plays a key role to the deformation behavior. SEM photographs in Fig. 5d and f depicts a fracture surface where shear yielding, particle pull-out and also crack pinning can be observed. However, it is clear in Fig. 5 that there is alumina particles embedded in the resin matrix at low concentrations of nanoalumina. Obviously, the polymer adheres well to the particle and demonstrates sufficient bonding. Deformed structures and small cavities are also detected at the interface. At high magnifications, the particle shows signs of being pulled out of the matrix and the nanoalumina particles occur difficult to be broken by propagating cracks due to their high hardness.
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Fig. 5. Scanning electron micrographs of fracture surface for samples having 0.5 and 5 phr nanoalumina at low (1 and 10K×) and at high magnifications (50K×).
3.5. Flexural properties The mechanical properties of the prepared composites are examined using the three-point bending tests according to ASTM D790 method. The results showed that the mechanical properties of the composites were greatly promoted compared to that of unmod-
ified epoxy system. The relationship between the flexural modulus and the addition of nanoalumina is shown in Fig. 6. The flexural modulus of the composite was remarkably increased when 0.5 phr of the nanoalumina is added. The maximum flexural modulus was achieved when the addition of the filler was 2 phr. When the addition of nanoalumina was over 2 phr, the flexural modulus of the composite was decreased. The maximum flexural modulus of the DDM-based composites was 4011 MPa, which was increased up to about 23% relative to that of the pure epoxy resin. These results indicated that the mechanical properties of the composites were considerably improved by dispersion of nano-Al2 O3 particles in the epoxy matrix. 4. Conclusions
Fig. 6. Changes in flexural modulus with nanoalumina concentration.
Epoxy nanocomposites reinforced with nanometric alumina particles were prepared. Such composites are experimentally characterized by means of thermal, viscoeastic, microscopy, and mechanical testing. It is demonstrated by calorimetry measurements that incorporating alumina nanoparticles can distinctly increase the reaction heat and the glass transition temperature of DGEBA cured with DDM. SEM analysis and tensile tests carried out on the composites indicated the absence of particle aggregation at low nanofiller content and reinforcing effect in terms of increased elastic modulus. The relation between the nanofiller content and the cure kinetic parameters are described analytically. The results
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showed that the constant rate is increased with isothermal curing temperature for the pure epoxy and its composites. It has been concluded that the decreased mechanical properties are due to weak inter-phase bonding between the resin and nanofiller. Finally, modified composite having 2 phr nano-Al2 O3 showed the best balance of thermal, mechanical, and viscoelastic properties. References [1] [2] [3] [4] [5] [6] [7] [8]
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Understanding the effect of nano-Al2 O3 addition upon the properties of epoxy-based hybrid composites Abdollah Omrani ∗ , Abbas A. Rostami Faculty of Chemistry, University of Mazandaran, P.O. Box 453, Babolsar, Iran
a r t i c l e
i n f o
Article history: Received 26 December 2008 Received in revised form 17 March 2009 Accepted 26 March 2009 Keywords: Particle-reinforcement Thermal properties Surface analysis Thermosetting resin
a b s t r a c t With the recent advances in nanoscience, there is promoting interest in nano-sized reinforced polymers for high technology applications. Evaluations on the curing reaction of diglycidyl ether of bisphenol A (DGEBA) cured with 4,4 -diaminodiphenylmethane (DDM) in the presence of nano-alumina have been carried out using DSC and In situ FT-IR measurements. The results from calorimetry measurements showed that the loading level of 2 is the optimum alumina concentrations for DDM-based composite. Different kinetic models including nth order, the Kamal autocatalytic, and the modified Avrami equations have been tested and the results of kinetic analysis are presented just for the last model owing to the better fitting of the experimental data. The viscoelastic behavior of the cured composites is determined using DMA showing that the storage modulus and the Tg of the composites are improved with the addition of nano-filler concentration. Results from SEM analyses revealed that the filler encourages the development of large particles in the resin matrix. Mechanical property measurement of the composites indicated that high flexural modulus has been attained with increasing concentration of nano-Al2 O3 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction Epoxy resins are thermosetting materials that require a cure treatment to attain suitable physical and mechanical properties for industrial applications. Epoxy resin based composites are used as laminates for printed circuit boards, aerospace, ballistic and engineering hardware components, automotive parts, electrical components, and rehabilitation products, etc. However, some of its inferior characteristics such as impact strength, weather resistance and thermal stability restrict the use in high performance applications [1,2]. The properties of cured materials depend on a combination of several factors including the structure of epoxy resin, type and concentration of curing agent, and the other specific agents such as accelerators, catalysts, additives, and modifiers. Also, several works have been addressed to modify epoxy resin toughness using rubber materials [3–5]. However, the toughening improvement was achieved at the cost of a decreased modulus and glass transition temperature. Over the past decades, the utility of inorganic nanoparticles as additives to enhance polymer performance has been established [6–10]. Also, an inorganic nanoparticle is an excellent toughening and reinforcing materials to develop new nanocomposites [11,12]. Elsewhere, it has been reported that dispersion of rigid inorganic filler into epoxy resin is important
∗ Corresponding author. Tel.: +98 112 5242025; fax: +98 112 5242002. E-mail address: [email protected] (A. Omrani). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.076
for the improvement in strength, modulus, and fracture toughness [13–16]. In these particular filled systems, the interfacial bonding or adhesion between the filler and the resin matrix has a great effect on the mechanical properties [17–19]. It is also well known that the structure and properties of an organic polymer are considerably different from those of inorganic nanoparticles from the specific gravity, dielectric properties, and heat resistance point of views. Therefore, much more attention has been made to produce new nanocomposites (hybrid materials) with a combination of the properties of organic and inorganic components. More recently works have been focused to prepare polymer-layered silicate nanocomposites. Various methods have been utilized to generate materials, such as hybrid materials, which is beyond the scope of discussion in the present work. However, there are a few studies on the formation of nanocomposites by directly dispersion of nanoparticles into the polymer matrix. The comprehensive properties of nanocomposites depend not only on the properties of the components but also on the morphology of the microphase structure [20,21]. The present work focuses on the performance of composites containing alumina nanoparticles and to understand what role these inorganic fillers play on the nanometer scale. The first step was aimed to finding out the effect of nanoalumina on diglycidyl ether of bisphenol A (DGEBA) cure. In the second step, the best possible viscoelastic and mechanical responses as a function of the nanoparticle content have been investigated. Recently, a comprehensive study on the dynamic mechanical behavior of
186
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DGEBA/cyclohexylamine/nanoalumina system has appeared in literature [22].
2. Experimental 2.1. Materials Alumina particles with an average size of 50 nm were purchased from Aldrich. The epoxy used was DGEBA purchased from Sigma–Aldrich, Canada with an epoxy equivalent weight of 185 g/eq. The hardener was 4,4 -diaminodiphenylmethane (DDM) provided by Sigma–Aldrich Canada. The chemicals were used, as received, without further purification. 2.2. Preparation of the specimens Epoxy resin and the alumina nanoparticles were mixed and then stirred for about 10 min to produce homogeneous blends. The epoxy/alumina blends were heated to 90 ◦ C and then a stoichiometry ratio of DDM, i.e. 27 phr, was added and mixed uniformly. The resulted mixture was kept under liquid nitrogen whilst being undergoing measurements. For the DSC measurements about 4–5 mg of the freshly prepared composites were used. Samples for the DMA and flexural tests were prepared by pouring the epoxy composites into a preheated Teflon mold and finally cured for 2 h into a vacuum oven at 100 ◦ C. The cured composites specimens were taken out of the oven and then cooled at room temperature and cut in the suitable size for the tests. 2.3. Characterization DSC measurements were carried out by means of a TA Q100 calorimeter under a nitrogen atmosphere. The samples were heated in the temperature interval of −30 and 300 ◦ C at a heating rate of 20 ◦ C/min and according to a heating/cooling/heating treatment. Following the first step, the cured samples were cooled to −30 ◦ C to minimize the enthalpy relaxation in the second heating scan. Finally, the samples were reheated to 250 ◦ C at 20 ◦ C/min to determine the glass transition temperature (Tg ) along with middle point method using TA software and to confirm the absence of any residual heat peak. DMA of both the neat epoxy and nanoalumina/epoxy/amine composites were carried out on a model DMTA V Dynamic mechanical analyzer (Rheometric Scientific) to estimate their thermomechanical properties. All samples were tested using a sample size of (15 mm × 5 mm × 1 mm) under single cantilever mode. The temperature range varied from 35 ◦ C to above the Tg of fully cured materials at a heating speed of 5 ◦ C/min at frequency of 1 Hz. A scanning electron microscope (SEM 1530) from LEO was utilized to observe the dispersion of nanoalumina on the fracture surfaces of the cured composites. The fracture surface was gold-coated prior to SEM studies to avoid charging and were examined at 15 kV accelerating voltage. The flexural properties of the epoxy systems and its composites are determined in a threepoint bending mode on samples with approximately dimensions of 25 mm × 5 mm × 1 mm using a MiniMat 2000 mechanical testing machine. Five tests were performed for each sample according to ASTM D-790-93 method and a compression speed of 2 mm/min was utilized. In situ FT-IR experiments were conducted on the samples between two KBr pellets inside the Bruker Tensor 27 spectrometer. Each spectrum was scanned 32 times at a resolution of 4 cm−1 from 400 to 4000 cm−1 . For each epoxy composition, four isothermal temperatures were used based on the data provided by DSC thermograms. Spectra were recorded and analyzed by OPUS software.
Table 1 Thermal properties of the cured composites obtained using DSC measurements. System
Al2 O3 (phr)
Tmax (◦ C)
H (J/g-sample)
H (J/g-epoxy)
DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM
0 0.5 2 5
182 183 181 180
407 426 431 416
517 543 556 549
3. Results and discussion 3.1. Results from the DSC experiment The curing reaction of the epoxy compositions has been investigated by means of DSC and the results are shown in Table 1. A small decrease in the peak temperature of the curing exotherms with increasing concentration of alumina particle has been observed. This effect was not significant for the composite involved low level of loading 0.5 phr. However, the peak maximum is shifted towards the lowest temperature for the composite having 5 phr of the nanofiller. Table 1 shows some key data about the effect of the used nanofiller on DGEBA cure which was obtained from the analysis of DSC thermograms. As can be seen in Table 1, the reaction heat of DGEBA/DDM system included 0.5 and 2 phr of the filler is higher than that of the neat resin and then decreases with increasing nanoalumina concentration. This means that high level of nanoalumina loading has a converse effect on the polymer network formation. It could be described by topological restrictions produced during the formation of epoxy network. However, H is the highest for the composition having 2 phr of nanoalumina. It is observed that the glass transition temperature of uncured materials (Tg,0 ) lies approximately between −8 and −10 ◦ C with increasing of the filler concentration. From the DSC thermograms, it is observed that the Tg values of composites are enhanced when compared with that of unmodified epoxy system. The Tg of the polymer composites is depicted in Fig. 1. The highest observed Tg of the DGEBA/DDM system incorporated with 2 phr of nanoalumina could be attributed to its highest reaction heat (see Table 1). It is noteworthy that the change in Tg for nano-sized filled composites has been controversial. Some authors have found that Tg of nanocomposites increases as a function of the filler loading, whereas others have observed an opposite trend. However, the Tg of an epoxy based composite may be changed due
Fig. 1. Dependence of the glass transition temperature of the fully cured epoxy on nanoalumina content.
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to network tacticity, molecular weight, cross-linking density, and amount of reaction residue, etc.
Table 2 Reaction rate constants, reaction orders, and activation energies calculated from FT-IR data.
3.2. Cure kinetics
System
Al2 O3 (phr)
Temperature (◦ C)
n
k (s−1 )
DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM DGEBA/DDM
0 0 0 0 0.5 0.5 0.5 0.5 2 2 2 2 5 5 5 5
90 100 110 120 90 100 110 120 90 100 110 120 90 100 110 120
0.89 0.94 0.99 1.03 1.01 0.85 0.76 0.77 0.86 0.81 0.82 0.79 0.85 0.83 0.80 0.87
0.0418 0.0470 0.0509 0.0568 0.0460 0.0650 0.1160 0.1283 0.0506 0.0667 0.0786 0.0984 0.0554 0.0724 0.0871 0.0886
Kinetics of the studied cure reaction was investigated under isothermal conditions by analyzing the spectra obtained by means of FT-IR instrument. The method was based on probing reduce in the peak area of the epoxy group vibration at 915 cm−1 with time. The degree of conversion is calculated by normalizing epoxy group vibration with the absorbance bond at 1509 cm−1 as the reference peak [23,24]. The DDM-based composites were cured at 90, 100, 110, and 120 ◦ C. It is clear that the decrease in peak area of the epoxy group with time is an indicator of curing. From the IR spectra, the epoxy group conversion with time has been calculated. It is claimed that the modified Avrami expression provides an estimation of the kinetic parameters since the model is extensively used for thermosetting polymers [25–27]. Using the modified Avrami equation [28–31] (Eq. (1)), 1 − ˛t = exp(−kt n )
(1)
and its double logarithmic form (Eq. (2)), the kinetic parameters of reaction order and rate constant for the both systems have been obtained. ln[−ln(1 − ˛t )] = ln k + n ln t
(2)
where k is the rate constant, ˛t is the conversion degree at time t, and n is the reaction order. For nth-order reactions, based on Eq. (2), a plot of ln[−ln(1 − ˛t )] against ln t (min) should yield a linear relationship. The slope and the intercept are n and ln k, respectively. The Eq. (2) is used to fit the all experimental data at each isothermal temperature. Further, a line can be obtained between the reciprocal of temperature (1/T) and (ln k)/n in the following form: (ln k) Ea = ln A − n RT
(3)
The slope and the intercept lead to the activation energy Ea and frequency factor A, respectively. In the present study, Eq. (3) has been used to estimate the value of activation energy. Representative plots of ln[−ln(1 − ˛t )] versus ln t (min) for the composite at 0.5 phr of nanoalumina concentration is shown in Fig. 2 whose slope and intercept are applied to the estimation of k and n, which can be used to calculate activation energy at different content of nanoalumina.
Fig. 2. Representative plots obtained using the Avrami equation to calculate the kinetics parameters for the DGEBA/DDM/nanoalumina (0.5 phr) composite at different isothermal temperatures.
Ea (kJ/mol) 56.3
26.1
20.6
22.5
The curves in Fig. 2 show that the used kinetic model presents a satisfactory description of the studied cure reactions. Fig. 2 indicates that good linearity existed during the curing process. Results of the kinetic analysis are listed in Table 2. As it is seen in the Table 2, the curing rate constants are appeared to be influenced by the isothermal temperatures and the filler concentration. The values of k have a shift toward higher values with increasing curing temperature at the same content of nanoalumina as the curing rate constant depends on the temperature. The exponent n of DGEBA/DDM based composites varied between 0.76 and 1.01 which indicates the higher nucleation ability of the DGEBA/DDM matrix. Table 2 reveals that Ea decreased corresponding to the addition of nanoalumina concentration demonstrating the catalytic effect of the used filler. The Ea for the pure epoxy composition (without filler) is determined to be 56 kJ/mol while the corresponding values for the incorporated systems lies between 20 and 26 kJ/mol. However, the effect of nanofiller on the activation energy of DDM-based composites is significant in terms of the type of epoxy resin and curing conditions used. This may be interpreted that the cure reaction with DDM was obviously affected further by the existence of nanoalumina. 3.3. Results from the DMTA experiment Dynamic mechanical tests over a wide temperature range are very sensitive to the physical and chemical structure of polymers and composites. They allow studying secondary transition of Tg and yield information about the morphology of polymers. The curves of storage modulus and tan ı of the neat epoxy system and its composites having different levels of filler loading are shown in Fig. 3. The main mechanical relaxation is modified by the presence of the nanofiller. As is shown in Fig. 3, it may be confirmed that the amplitude of storage modulus curve in the rubbery plateau region increases with increasing the filler loading up to 2 phr. As is expected, this is due to the reinforcement of the filler as well. The complex moduli for all composites containing fillers are pushed to a higher level relative to neat epoxy system as the filler content increases. It is also observed that the storage modulus, which corresponds to the same temperature in the glass transition region, increased with increasing nanoalumina concentration. The onset of the considerable composites decay is also shifted toward slightly higher temperatures. Further decay in the storage modulus of DDM based composites can be observed at around 170 ◦ C. The composite containing 2 phr nanofiller caused the increase in modulus whereas the composites containing more filler led to the decrease in modulus. However, the composites containing alumina nanoparticles have higher modulus than that neat epoxy system. This means that
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Fig. 4. Scanning electron micrographs of fracture surface for neat epoxy system at (a) low magnification (10K×) (b) high magnification (50K×).
Fig. 3. DMTA profiles of storage modulus and tan ı for the neat epoxy and its composite.
there is strong adhesion between the alumina particles and epoxy resin. At a high concentration of 5 phr nanoalumina, it is considered that the filler is dispersed into the epoxy matrix irregularly and the void of the cross-linking is increased. Then the segmental mobility rises, which leads to the decrease of the storage modulus at the same temperature. In general, a good explanation for the dependence of the modulus of epoxy/nanoalumina composites on the alumina content could be done in terms of particle-particle contacts and agglomeration, which differ below and above the glass transition temperature. Also, the temperature of the maximum of tan ı peak is increased as the level of nanoalumina increased up to 2 phr and it will correspond with the apex of loss modulus curves. This relaxation process is associated to the Tg phenomenon. It has been earlier confirmed by DSC measurement that the Tg is increased on the filler content up to 2 phr (see Fig. 1). Further, below Tg , the reinforcement in terms of storage modulus is high. The increase of modulus for 2 phr filler is about 35%. This is not surprising due to the amount of the added filler. However, the main important factors that can affect Tg are degree of particle dispersion and curing conditions. The degree of particle dispersion includes size, homogeneity, orientation, and spacing between particles, whereas curing conditions include curing speed and degree of cross-linking. The improvement in the Tg observed in this study may arise from some of these factors. 3.4. Fracture surface morphology of the composites The fracture surface for the neat epoxy matrix is depicted in Fig. 4 which reveals a brittle behavior characterized by a large
smooth area, ribbons and fracture steps in the direction of crack propagation which accounts for its poor toughness. The SEM photographs obtained for fracture studies of composites specimen after three-point bending tests are presented in Fig. 5. In contrary to the pure polymer, composite surfaces are more roughly structured. The images show that the alumina particles are dispersed into the epoxy matrix as well as at low level of loadings. However, the particles are not homogeneously dispersed at high concentration of 5 phr. It could be described according to the cohesion of the alumina particles and the viscosity of the epoxy resin that might be hindered the uniform dispersion at high loading level. In the magnified images of the particle-matrix parts (Fig. 5c), the epoxy resin besieges the particles at low level of nanoalumina loadings as well. Also, Fig. 5c shows some defect regions possibly in the direction of crack propagation indicated by a large number of cavities on fracture surface. Their special shape originates in the unification of the propagating primary crack front with secondary cracks initiated at local inhomogeneities in front of the primary crack, e.g. particles or agglomerates. At high level of loading 5 phr, the SEM image (Fig. 5d) is extensively rugged, which indicates that cracks travel at high speed, allowing less matrix deformation because time for yielding is limited. Impeded matrix deformation reduces flexural strength since a large amount of energy may be consumed at the particle/matrix interface which plays a key role to the deformation behavior. SEM photographs in Fig. 5d and f depicts a fracture surface where shear yielding, particle pull-out and also crack pinning can be observed. However, it is clear in Fig. 5 that there is alumina particles embedded in the resin matrix at low concentrations of nanoalumina. Obviously, the polymer adheres well to the particle and demonstrates sufficient bonding. Deformed structures and small cavities are also detected at the interface. At high magnifications, the particle shows signs of being pulled out of the matrix and the nanoalumina particles occur difficult to be broken by propagating cracks due to their high hardness.
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Fig. 5. Scanning electron micrographs of fracture surface for samples having 0.5 and 5 phr nanoalumina at low (1 and 10K×) and at high magnifications (50K×).
3.5. Flexural properties The mechanical properties of the prepared composites are examined using the three-point bending tests according to ASTM D790 method. The results showed that the mechanical properties of the composites were greatly promoted compared to that of unmod-
ified epoxy system. The relationship between the flexural modulus and the addition of nanoalumina is shown in Fig. 6. The flexural modulus of the composite was remarkably increased when 0.5 phr of the nanoalumina is added. The maximum flexural modulus was achieved when the addition of the filler was 2 phr. When the addition of nanoalumina was over 2 phr, the flexural modulus of the composite was decreased. The maximum flexural modulus of the DDM-based composites was 4011 MPa, which was increased up to about 23% relative to that of the pure epoxy resin. These results indicated that the mechanical properties of the composites were considerably improved by dispersion of nano-Al2 O3 particles in the epoxy matrix. 4. Conclusions
Fig. 6. Changes in flexural modulus with nanoalumina concentration.
Epoxy nanocomposites reinforced with nanometric alumina particles were prepared. Such composites are experimentally characterized by means of thermal, viscoeastic, microscopy, and mechanical testing. It is demonstrated by calorimetry measurements that incorporating alumina nanoparticles can distinctly increase the reaction heat and the glass transition temperature of DGEBA cured with DDM. SEM analysis and tensile tests carried out on the composites indicated the absence of particle aggregation at low nanofiller content and reinforcing effect in terms of increased elastic modulus. The relation between the nanofiller content and the cure kinetic parameters are described analytically. The results
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showed that the constant rate is increased with isothermal curing temperature for the pure epoxy and its composites. It has been concluded that the decreased mechanical properties are due to weak inter-phase bonding between the resin and nanofiller. Finally, modified composite having 2 phr nano-Al2 O3 showed the best balance of thermal, mechanical, and viscoelastic properties. References [1] [2] [3] [4] [5] [6] [7] [8]
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