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Effect of sepiolite filler on mechanical behaviour of a bisphenol A-based epoxy system
Composites: Part B 67 (2014) 400–409
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of sepiolite filler on mechanical behaviour of a bisphenol A-based epoxy system Aldobenedetto Zotti, Anna Borriello ⇑, Alfonso Martone, Vincenza Antonucci, Michele Giordano, Mauro Zarrelli Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, P.le Fermi, 1, 80055 Portici, Naples, Italy
a r t i c l e
i n f o
Article history: Received 17 April 2014 Received in revised form 11 June 2014 Accepted 11 July 2014 Available online 27 July 2014 Keywords: A. Hybrid A. Thermosetting resins B. Thermomechanical Sepiolite
a b s t r a c t Sepiolite/epoxy systems, characterised by an inorganic content from 2% to 10% by weight, were tested by using different experimental techniques in order to assess the effects thermo-mechanical and fracture behaviour. It was found that filler dispersion is independent on the type of sepiolite used, i.e. hydrated and de-hydrated, whereas the addition of hydrated sepiolite strongly increases the viscosity of the final suspension. Mechanical properties have been investigated and discussed. The conservative modulus of the sepiolite/epoxy system increases as much as 21% and 94% respectively, at 35 °C and 200 °C respect to the unfilled polymer, with the incorporation of 10 wt% of sepiolite, both hydrated and dehydrated, with a slightly increasing of the corresponding glass transition temperature. The effect of sepiolite on the coefficient of thermal expansion lead to a variation in both glassy and rubbery region, respectively of about 7–8% and 9%. Fracture toughness analysis revealed that the critical stress intensity factor negligibly changes by increasing the sepiolite content, whereas the critical strain energy release rate (GIC) reaches an noteworthy value around 30% higher than neat epoxy. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resins are the most commercially successful thermosetting materials, especially as structural adhesives, high-performance protective coatings and matrices of composite materials [1,2]. Epoxy resins have many advantages, as low specific weight compared to metals, high mechanical properties and high corrosion resistance. Moreover these materials are brittle, having limited utility in applications that require high fracture toughness or impact resistance. The extensive applications of epoxy resins motivate intense studies having the objectives to prepare organic–inorganic nanocomposites with novel and improved performances. During the last years, the polymer/clay and nanotube based nanocomposites have obtained a great deal of attention [3,4]. In fact, low quantity of clay filler can significantly improve the mechanical and thermal properties of the system. However the entity of the improvement depends strongly on the particle content, dimension, shape, and degree of dispersion. For instance, composites filled with micrometric size particles have mechanical and thermal properties that result inferior compared to composites ⇑ Corresponding author. Tel.: +39 0817758814; fax: +39 0817758850. E-mail address: [email protected] (A. Borriello). http://dx.doi.org/10.1016/j.compositesb.2014.07.017 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.
filled with nanometric size particles [5]. Addition of nanoclay commonly produces an increase of 22% and 36% with 3 and 5 wt% nanoclay loadings, respectively, in longitudinal compressive strengths of off-axis GFRP composites [6] and transverse compressive strength of unidirectional GFRP [7]. The main failure mode was identified as fibre/matrix debonding. The organoclay modified epoxy gave rise to significant improvements in both crack growth resistance and fracture toughness in mode I interlaminar fracture of CFRP composites [8]. In another study [9], both the initiation and propagation values of mode I interlaminar fracture toughness of CFRP composites increased at higher clay concentration. A strong correlation was established between the fracture toughness of clay–epoxy nanocomposite and the CFRP composite interlaminar fracture toughness. The CFRPs containing organoclay brought about 53% and 85% improvement in the interlaminar fracture toughness with the introduction of relatively small amount of filler. Liu et al. [10] have carried out fracture toughness studies in epoxy–clay based nanocomposites. Fracture toughness of this epoxy system has been greatly enhanced with the addition of nanoclays. With the addition of only 4.5 phr of clay, the strain energy release rate of the epoxy was increased 5.8 times from the original value. Bureau et al. [11] have studied fracture analysis
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for PP based polymer at 2 wt% nanoclay content with various types of coupling agents. Although the addition of nanofiller may lead to strong improvement in fracture behaviour, the morphology and the level of dispersion or disaggregation (exfoliation or intercalation in the case of nanoclay) certainly play the major rule on property effects. The toughness mechanisms could be very depended by particles sizes, agglomeration geometry, filler/matrix bonding energy and particle orientation, therefore the level of property (i.e. level enhancement or detrimental effects) needs to be analyzed in conjunction to a more general scenario rather than considering only the type of nanofiller. Among the different clay which are suitable as candidate for nanocomposite production, surely sepiolite can be a valuable nanofiller. Sepiolite is a fibrous magnesium phyllosilicate, characterised by a needle-shaped geometry, whose ideal formula is [Si12Mg8O30(OH)4](H2O)48H2O. Its structure is formed by two layers of silica tetrahedrons linked by a layer of magnesium ions in octahedral coordination. The octahedral sheets are not continuous like in the other phyllosilicates: this structure provides infinite channels along the fibre axis with a cross section of about 1 0.4 nm2 [3,12,13]. The presence of this channels strongly increases the specific surface area of this particles, up to 200– 300 m2/g. Thanks to this large specific surface area, the sepiolite is used in rheological, sorptive and catalytic applications [12]. Sepiolite is also widely used like filler in micro and nanocomposites. For instance, Zheng have demonstrated that the addition of 1 wt% of sepiolite in the epoxy matrix increases the Tg by 50 °C [3]. In this work, sepiolite nano-powder is used as filler in an epoxy system in order to investigate the effect on the thermo-mechanical, mechanical and fracture properties of the final nanocomposite. Moreover, being the considered epoxy resin generally used as a matrix for advanced polymer composites, the induced-sepiolite variation of the rheological behaviour of the neat system was experimentally assessed to establish limiting content of the filler for the later manufacturing process of composite elements. The physical properties of the final systems have been carefully examined by different experimental methods, i.e. thermo mechanical analysis (TMA), Dynamic Mechanical Analysis (DMA), fracture toughness tests, rheometry, optical and electron microscopy in order to investigate the potential enhancement of mechanical property. It was found that the achieved dispersion of the filler lead to an overall increasing of mechanical behaviour compared with neat hosting system, not only in term of bending modulus and coefficient of thermal expansion but interestingly in term of fracture energy. The sepiolite was used as received (hydrated form) and after a pre-treatment to eliminate absorbed water (dehydrated form).
2. Experimental section 2.1. Materials In this work, the matrix used is a bisphenol-A/epichlorohydrin derived liquid epoxy resin (EPON™ Resin 828) supplied by Hexion characterised by an equivalent weight (EEW) of 188 g/mol and a room temperature (25 °C) viscosity of about 10–15 Pa s [14]. The used curing agent was a Triethylene Tetramine (TETA), commercially known as EPIKURE3234, which is a polyamine with an amine hydrogen equivalent weight (AHEW) of 1.41–1.46 mg/g [15]. This curing agent provides low viscosity with high performance properties after a relatively low temperature cure. Sepiolite was gently provided by Mi.Mac. (Capua, Italy) and it was used in different concentrations as inorganic filler in two forms: hydrated (idra) and dehydrated (anidra). Pre-treatment of pristine sepiolite was carried out in order to remove absorbed moisture during
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micronization and handling. The dehydration pre-stage was carried out by holding the powder filler at isothermal temperature (100 °C) in oven for 3 h under vacuum. After the pre-stage, the inorganic filler was stored in an desiccator. The criteria to identify the correct procedure for the dehydration of sepiolite filler was experimentally determined by thermal ad thermo-gravimetrical analysis showing a substantial absence of endothermic desorption peak (DSC) and weight reduction around 100 °C (TGA). The mixing of epoxy resin and sepiolite was performed by sonication using a Misonix S3000 sonicator, set at 30 W power for 1 h. An oil bath was employed to hold the mixture at 50 °C. After sonication, few grams of the mixture (EPON828 and sepiolite) were stored for the rheological analysis, while the remaining was degassed in a planetary mixer (Thinky Mixer ARV-310) at the velocity of 2000 rpm under vacuum for 5 min. The curing agent (EPIKURE3234) was then added to the degased mixture, with 13 PHR (Part for Hundred of Resin). The mixing of sepiolite/epoxy mixture and the EPIKURE3234 was performed by using the planetary mixer at the velocity of 2000 rpm under vacuum for 1 min and then poured into a steel, previously coated with a release agent (FREKOTE 70). The same curing cycle was assumed for all the manufactured sample plates, i.e. 1 h @70 °C followed by a post-curing stage of 2 h @120 °C. The cured plates were cooled down slowly in the curing oven over the night to room temperature, removed from the mould and cut to prescribed sample dimensions for the programmed mechanical tests. 2.2. Characterisation 2.2.1. Rheology The rheological characterisation of the EPON 828/sepiolite suspension was carried out by MRC-301 rheometer (Anton Paar), using parallel plates PP25 (25 mm diameter) with a 1 mm gap. In the case of the unreacted epoxy mixture, two kind of tests were performed, namely, amplitude sweep and frequency sweep in order to limit the linear viscoelastic region of the suspension and also to assess the effect of the filler over the rheological behaviour of neat material. All mentioned rheological tests were executed at a 25 °C. For the amplitude sweep tests the deformation was varied within the percentage range [102, 102] with a fixed frequency 1 Hz. In the case of frequency sweep, all the tests were carried out at fixed formation percentage and variable frequency within the rage 0.1– 100 rad/s. Rheological tests were also executed on reactive suspension (EPON 828/sepiolite and curing agent), by using an MRC-301 Rheometer in temperature ramp mode. The heating rate was set to 3 °C/min from ambient temperature at frequency (1 Hz) and deformation (c = 0.5%). The tests were stopped when viscosity exceeded the value of 106 Pa s. 2.2.2. SEM and optical microscopy SEM and optical micrographs of additive powder and nanocomposites were taken respectively by an 1450VP LEO SEM microscopy and an optical Olimpus BX51 Instruments equipped with different magnitude oculars. Cross-linked samples for SEM analysis were cut by ultra-microtome with a diamond knife in thin layer or by cryogenic rupture to prepare fracture surface. Samples observed by optical microscopy were prepared by operating a precise sawing machine at high speed rotation. 2.2.3. Thermo mechanical analysis The thermo-mechanical behaviour of solid cured mixture (EPON828/sepiolite and curing agent) was investigated by TMA 60 (Shimadzu) set with a macro probe and programming an heating ramp of 5 °C/min. The nominal dimensions of the TMA samples
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were 10 3 3 mm according ASTM E831-03 standard . Thermal Expansion Coefficient (CTE), respectively, in the glassy phase (CTEg) and rubbery stage (CTEr) were measured and results are reported hereafter. 2.2.4. Dynamic Mechanical Analysis Dynamic Mechanical Analysis (DMA) was performed by Q800 DMA (TA Instruments) at fixed frequency of 1 Hz and heating ramp of 3 °C/min. The testing configuration was the single cantilever mode with sample nominal dimension of 30 10 2.5 mm; 50 lm amplitude was considered for all the performed tests. 2.2.5. Fracture test Mode-I fracture tests were conducted using single edge notched beam (SENB) specimen. The test specimen was chosen according to the ASTM D5045-99 standard test method for plane strain fracture toughness and strain energy release rate of plastic materials. To ensure the plane strain conditions, the following specimen dimensions were chosen: B = 3 mm, W = 6 mm and S = 24 mm. According to the ASTM standard, the crack length should be selected such that 0.45 < a/W < 0.55. Results are considered valid according to the standard if the following size criteria are satisfied:
where KQ is the conditional or trial KIC value, and ry is the yield stress of the material for the temperature and loading rate of the test. The fracture tests were performed using an Instron 3360 dynamometer equipped with 1 kN load cell and at a displacement rate of 10 mm/min. KIC values were calculated according to the standard as:
PQ
reduced number of sepiolite needle aggregation having sub or micrometric size. 3.2. Rheological analysis
B; a; ðW aÞ > 2:5ðK Q =ry Þ2 ;
K IC ¼ f ða=WÞ
Fig. 1. Magnification of sepiolite particle that shows the needles sizes.
BW 1=2
with
h 2 i a 1=2 1:99 Wa 1 Wa 2:15 3:93 Wa þ 2:7 Wa F ¼6 3=2 W W 1þ2 a 1 a a
W
W
where PQ is the load at failure, a the crack length, while W and B are the specimen width and thickness, respectively. Critical strain energy release rates (GIC) were calculated from the stress intensity values according to the cited ASTM standard. 3. Results and discussion 3.1. Morphology of suspensions Fig. 1 shows an higher magnification of a sepiolite particle with characteristic needle-shaped geometry and nominal dimension [12,18]. The needle-shaped sepiolite particle is characterised by nominal dimension of tenths of nanometer and a variable length of few microns. The final aggregation of sepiolite within the hosting matrix is a characteristic round-shaped particles of nominal dimension less than 10 lm. In Fig. 2, the micro-aggregations of sepiolite are shown for a cryogenic fractured surface. Fig. 3 reports SEM micrographic of loaded samples along with a zooming-in of the same sample which shows the characteristic needle geometry of the sepiolite completely encapsulated within the hosting epoxy system (Fig. 3). The high shear mixing technique used in this work (i.e. tip sonication) assures an uniform dispersion of the inorganic filler within the epoxy for all the considered weight percentages. Optical micrograph taken from neat and sepiolite loaded samples (Fig. 4) have highlighted that the dispersion of the inorganic filler is highly exfoliated to reach in some case a
Fig. 5 shows the amplitude sweep test curves obtained for the neat system and for epoxy suspensions containing different percentages of sepiolite, respectively hydrated and dehydrated. The addition of a small amount of sepiolite induces only a slightly increase of the loss modulus compared to the neat resin with a negligible difference between the two cases, i.e. idra and anidra. At higher content (5 wt% and 10 wt%) and under low strain, not only the loss modulus of the suspension increases, as expected, but also the variation between idra and anidra sepiolite loading increases. The effect of sepiolite content can also be appreciated considering that, at low content (2 wt%), a linear viscoelastic behaviour within the whole analysed frequency range is recorded. Whereas, increasing the sepiolite content, a non-linear viscoelastic behaviour occurs to smaller strains. For dynamic experiment a fix value of strain was chosen assuring that all suspensions would report a linear viscoelastic regime. As reported on the curves (Fig. 5), by choosing a strain percentage of 0.5%, it is sure that all prepared suspensions will respond in their linear viscoelastic regime. Fig. 6 report the flow curves for neat epoxy and suspensions containing different percentages of sepiolite both idra and anidra. The recorded level of viscosity strongly depends on the typology of sepiolite loaded, in fact, for the 10 wt% loading of pristine (idra) sepiolite and under low shear, the viscosity abruptly varies of two order of magnitude, i.e. from 14 Pa s to 1156 Pa s. In contrast, by loading a similar amount of pre-treated (anidra) sepiolite at same strain percentage, the viscosity reaches 323 Pa s. This effect could be probability due to the interaction between the water molecules, adsorbed on the sepiolite surface, and the –OH groups of the epoxy monomer, which lead to ‘‘thickening’’ effect of the final suspension. The different behaviour in term of viscosity according to the type of loaded sepiolite, may represent a ‘‘working’’ feature in real industrial processing technology. In fact, whether the matrix will be used for infusion or liquid process to manufacture composite element and/or semi-finished product such as prepreg material, the potentiality of tuning the suspension viscosity by pre-treating the sepiolite content, i.e. controlling the amount of water adsorbed, being constant its content, could be an extremely valuable technological methodology for process optimisation. In all cases, as expected, the viscosity increases with filler content. Nevertheless, the epoxy monomers exhibits a Newtonian
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Fig. 2. Particle size of sepiolite in cured 2 wt% epoxy.
Fig. 3. SEM micrograph for 5 wt% sepiolite content sample.
Fig. 4. Optical microscopy (20 and 50) of neat and filled.
behaviour within the whole considered frequency range, whereas, the epoxy/sepiolite suspensions exhibit the same behaviour only at lower percentages of filler (2 wt%). At higher sepiolite content (i.e. 5 wt% and 10 wt%) a pronounced shear thinning effect (i.e. high
viscosity progressively decreases with shear rate/angular frequency increasing) is monitored. The sepiolite induced shear thinning effect results significantly different depending on the type of loaded sepiolite, respectively pristine (idra) or pre-treated (anidra).
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Fig. 5. Loss modulus (G00 ) vs. strain for the EPON828 monomer and 2, 5 and 10 wt% epoxy suspensions of sepiolite: (A) hydrated and (B) dehydrated.
In particular, the addition of 10 wt% idra sepiolite decreases the viscosity from 1155 Pa s to 69 Pa s respectively from 0.1 Hz to 100 Hz; over the same frequency range, but in the case of anidra filler, the viscosity drops from 323 Pa s to around 113 Pa s. A reasonable explanation for this behaviour could be that initially, after disaggregation of the pristine agglomeration, sepiolite needles are highly dispersed in the hosting matrix [16]. Thus, the interaction between needle-shaped nanoparticles and epoxy monomers induces percolation of the sepiolite filler in a threedimensional network: at low shear rate, the formed networking architecture is not altered with a consequent high viscosity. At higher shear rate, the breaking of the percolated network occurs, thus the system viscosity decreases. A further mechanism takes place if the geometrical configuration of sepiolite is considered. In fact, at higher shear rate, the needle-shaped sepiolite filler will tend to align along the flow field vectors and thus decreasing the viscosity. In dynamic experiments, two different types of behaviour could be highlighted depending on the sepiolite content. At 2 wt%, 5 wt% and 10 wt% sepiolite percentage (Fig. 7), the curve of loss modulus results higher than the corresponding elastic modulus over the whole examined strain range revealing a liquid-like behaviour. When the level of sepiolite is higher (20 wt%), under lower strain, the system shows an elastic modulus higher than the corresponding viscous part, whereas after a threshold strain the curves cross each other. The crossover point identifies a transition region from
a pseudo solid-like to a liquid-like behaviour likely related to a forming physical gel structure induced by the extremely high content of nanoloading (i.e. 20 wt%, Fig. 7). It remains obvious that the amount of sepiolite or in general nanoloading which characterises the formation of the physical gel network and therefore induce this later effect is related not only to the amount of nano-loading but also to the dispersion technique employed to prepare the suspension. In fact, the critical percentage of sepiolite at which this physical gel is formed, could be as lower as 8.4 wt% processed by using ‘conventional’ technique (i.e. mechanical stirring) or 20 wt% such as in our case by using high shear mixing (i.e. sonication). This observation leads to the conclusion that the level of dispersion in term of nominal dimension of the filler disaggregation represents a fundamental feature for the network formation. Temperature-ramp tests were performed on reactive mixture (i.e. sepiolite/EPON828 suspension added with curing agent) in order to analyse gel formation of the system. In general, the rheological results are depending on the measuring methods used. In fact, while in oscillation, the gel formation is not disturbed, in rotation the forming gel network is partly disrupted by the applied shear rate. For this reason different methodologies could be accounted for the determination of gelation time. According to the ASTM 4473-90, gelation can be identified on the time scale as the time corresponding to the maximum in tan d curve, in isothermal tests. Some researchers [17] have expressed reservations about this specific method, especially in the case of highly filled epoxy resins. In temperature ramped experiments, the viscosity sharply arises at gelation, due to inherent built-up of the 3D structure which limits the flow capability. The gelation point can be identified, at fixed heating rate, as the temperature at which viscosity arises above 104 Pa s. Fig. 8 reports the viscosity evolution during a dynamic temperature ramp and the corresponding conversion profile. The actual conversion reached by the system at ‘‘sharp onset’’, i.e. when he viscosity is higher than 104 Pa s, represents the gelation degree of cure 4 g ¼10 Pa s . agel Fig. 9 shows the viscosity vs. temperature curves for reactive suspensions containing different amounts of both idra and anidra sepiolite along with the set level to identify the ‘‘gelation conversion’’. If the temperature ramp is constant (i.e. 3 °C/min), the gelation temperature changes with the sepiolite contents. Particularly in the case of 10 wt%, the system undergoes the gel transition at temperature lower than the corresponding neat epoxy monomer. Moreover, the typology of sepiolite (i.e. idra and anidra) induces a different variation of the temperature corresponding to the
Fig. 6. Complex viscosity vs. frequency for the neat and sepiolite/epoxy suspensions of sepiolite: (A) hydrated and (B) dehydrated.
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Fig. 7. Amplitude sweep tests at different idra sepiolite content.
whereas in the case of anidra filler, this variation is much more attenuated. 3.3. Mechanical behaviour
Fig. 8. Comparison of dynamic conversion and viscosity at gelation.
viscosity arising, from 85.5 °C to 80.7 °C and 83.4 °C, respectively, for idra and anidra sepiolite loading. The temperature at which the tri-dimensional network is formed and thus viscosity arises, results not significantly changed at low sepiolite content specially in the case of idra sepiolite filler. However, at the same content, a slightly variation could be appreciated when anidra sepiolite is loaded. An opposite behaviour is revealed at higher content of sepiolite, in fact in the case of idra sepiolite, the remarkable variation of ‘‘gelation temperature’’ could be reasonably related with the higher amount of water molecules,
Temperature ramp tests by using TMA technique were performed on neat and sepiolite (idra and anidra) loaded samples. The linear coefficient of thermal expansion (CTE) was measured below (CTEg) and above (CTEr) glass transition temperature and results are reported in Table 1. The CTE mismatch between matrix and fibres is among the main controlling factors for the arising of residual stresses within composite materials, thus the effect of sepiolite at different percentages surely represents a fundamental information for the correct design of specific element or component. CTE values under and above Tg were calculated by the slope of the linear regression line within the glassy and rubber region. It was found that, at low sepiolite content, CTE of the system does not change significantly compared with the neat epoxy. However, according to literatures [19,20], the expansion behaviour changes of about 7–8% and 9 % respectively below and above glass transition in the case of 10 wt% sepiolite loading. Dynamic Mechanical Analyser allows the measurement of epoxy modulus as function of the temperature. The main results in term of modulus @35 °C and @200 °C along with glass transition temperature as obtained by DMA tests on neat epoxy and sepiolite loaded nanocomposites are reported in Table 2. The addition of sepiolite lead to an overall increase of the flexural modulus from the initial value of 2240 MPa, for the neat epoxy
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Fig. 9. Complex viscosity vs. temperature for the EPON828 monomer and for suspensions with 2 wt%, 5 wt% and 10 wt% of pristine sepiolite: (A) hydrated and (B) dehydrated.
Table 1 TMA results for the EPON828/sepiolite systems. Sample
ag 106 (K1)
ar 106 (K1)
Dag (%)
Dar (%)
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
69.2 ± 1.1 68.5 ± 2.2 69.2 ± 2.1 67.6 ± 1.3 67.3 ± 1.2 63.7 ± 3.0 64.3 ± 3.0
187.3 ± 2.7 182.4 ± 2.8 181.9 ± 4.1 179.3 ± 1.3 176.1 ± 1.0 169.5 ± 4.9 170.5 ± 0.8
– 1.0 0 1.9 2.7 7.9 7.1
– 2.6 2.9 4.3 5.7 9.5 8.9
system, to a maximum of 2737 MPa in the case of 10% sepiolite by weight. Increasing the content of sepiolite, according to the dispersion level achieved by using high shear mixing sonication, the value of the bending modulus is gradually increased. The conservative modulus at 35 °C changes significantly of about 22.1% and 21.6% respect to the neat epoxy system, respectively, for idra and anidra sepiolite filler. Interesting results were obtained for the bending modulus at 200 °C. The maximum variation achieved is of about 94.6% as the measured value changes from 35.45 MPa to 58.4 MPa or 65.2 MPa, with 10 wt% of sepiolite, respectively, dehydrated and hydrated. Fig. 10 shows the conservative modulus as function of temperature for the sample with hydrated sepiolite (A) and dehydrated sepiolite (B). It can be seen that there are not significantly difference between the modules of sample with hydrated and dehydrated sepiolite. Increased glass transition temperatures have been reported for some nanocomposite systems [5,21,22], while in others a constant [4,12] or slightly decreased [23] of Tg have been found. The presence of sepiolite has little effect on the value of the glass transition temperature, with not more than 6 °C as in the case of 10 wt% of sepiolite. The level of dispersion achieved by high shear mixing may represents a key factor for the reinforcement efficiency related to
loaded nanoclay. Coleman et al. [14] has introduced a reinforcement efficiency parameter for carbon nanotube nancomposite which could be suitably applied for our nanocomposite samples. Considering the variation of measured modulus for the neat and for the sepiolite loaded epoxy for unit volume filler, the level of dispersion could be parameterised and therefore rationally monitored and discussed. Considering that the density of the fully cured resin and the sepiolite is respectively 1.16 g/cm3 [15] and 2.10 g/cm3 [12] the sepiolite volume fraction corresponding to the used weight percentage could be computed and the reinforcement efficiency can be evaluated. Table 3 reports the computed values of reinforcement efficiency according to Coleman’s parameter for the nanocomposite samples loaded respectively with idra and anidra sepiolite filler at different percentages. It is important to note that the type of sepiolite (i.e. idra or anidra) induces a negligible variation of the reinforcement efficiency as the computed values, for the considered nanofiller content, is very closed. Moreover, the efficiency slightly reduces at higher filler content as the dispersion procedure make up more sepiolite agglomerations thus reducing the stress transfer. Fracture tests were performed using a three point bending test according to the corresponding ASTM D5045 standard for polymer system. Typical linear load–displacement curves have been obtained for the neat epoxy and its nanocomposites, both showing a brittle fracture surface (see Table 4). The stress intensity factor represents the constant of proportionality between the applied stress and square root of the distance (and angle displacement if cylindrical coordinate is assume around the crack tip) from the crack and it is used to determine the fracture toughness of most materials. Because the dependence of the stresses on the coordinate variables remain the same for different types of cracks and shaped bodies, the stress intensity factor is a single parameter characterisation of the crack tip stress field. The value of the stress intensity factor at which unstable crack propagation occurs is called the fracture toughness or critical stress
Table 2 Results of DMA tests for the EPON828/sepiolite systems. Sample
Tg (°C)
E0 @35 °C (MPa)
E0 @200 °C (MPa)
DE0 @35 °C (%)
DE0 @200 °C (%)
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
130.9 132.9 132.4 133.7 133.2 134.8 136.4
2240 ± 24 2440 ± 27 2435 ± 20 2565 ± 54 2524 ± 11 2723 ± 31 2737 ± 20
35.4 ± 0.5 38.4 ± 0.5 39.1 ± 1.2 50.9 ± 1.1 50.7 ± 0.4 58.4 ± 1.2 65.2 ± 2.9
– 8.9 8.7 9.9 12.7 21.6 22.1
– 8.3 10.3 43.8 43.2 64.9 94.6
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Fig. 10. Conservative modulus (E0 ) vs. Temperature for neat and 2, 5 and 10 wt% sepiolite/epoxy: (A) hydrated and (B) dehydrated.
Table 3 Reinforcement efficiency according to mechanical characterisation. Sample
Sepiolite content
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
dE/dV (GPa)
wt%
vol%
0 2
0 1.12
5
2.83
10
5.78
0 17.45 17.86 10.05 11.50 8.59 8.35
Table 4 Results of fracture toughness tests for the neat and sepiolite/epoxy systems. Fig. 12. Metallised fracture surface of neat epoxy sample after fracture test. Sample
KIC (MPa m1/2)
St. dev. (%)
GIC (kJ/m2)
St. dev. (%)
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
1.24 1.07 1.01 1.09 1.12 0.99 1.17
4.12 5.3 6.1 4.2 5.3 3.7 6.3
1.04 1.0 0.93 1.11 0.96 1.33 1.35
0.3 5.2 7.9 7.8 8.1 6.4 6.2
intensity factor, KIC. Maximum loads are used to calculate the fracture toughness of the samples along with the initial crack length. Results show that the value for the EPON828 (neat epoxy) was found equal to 1.24 MPa m1/2 in line with data sheet provided by supplier; this value negligibly change with sepiolite content according to error ranges (see Fig. 11). The main hypothesis we can suggest for the negligible variation of the KIC is mainly related
Fig. 11. Critical stress intensity factor, KIC, and critical strain energy release rate, GIC, vs. sepiolite %.
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Fig. 13. SEM of fractured surfaces of epoxy loaded with 2% sepiolite (A) anidra, (B) idra; 5% sepiolite (C) anidra, (E) idra; 10% sepiolite (D) anidra, and (F) idra.
to the brittleness of the matrix due to the presence of nanofiller. For all samples, the fracture is brittle (see Figs. 12 and 13) and the presence of sepiolite, in any percentages, does not affect the stress distribution surrounding the crack tip. A different trend was recorded in the case of strain energy release rate, GIC. The computed values of GIC, according to the standard, represents the released energy, under plain strain conditions, per unit area of propagating crack. Irrespective of the type of sepiolite content (i.e. idra or anidra), GIC values increase with increasing percentage of nanofiller (see Fig. 11). These results can be suitably discussed analysing the SEM fracture surface for neat epoxy and corresponding nanocomposites. Morphological analysis was performed with a Scanning Electron Microscope (SEM). The fracture surface of the neat epoxy was very smooth, characterised by extended fracture surface (dotted curve) grooved by tiny river line markings (as indicated the arrows in Fig. 12). This fractography is typical for brittle polymers, revealing
that the resistance to crack propagation is fast and catastrophic. Comparing the SEM fractography images taken on nanofilled samples, it results that rupture, although still brittle, presents a high degree of surface corrugation and level of indentations, suggesting that the crack propagates along different directions. Thus the energy released to growth the crack increases due to the number of surface to create. Values of plain released energy rate varies from 1.04 kJ/m2 for the neat epoxy to 1.33 kJ/m (anidra sepiolite) and 1.35 kJ/m2 (idra sepiolite) with increment of more than 30% in both cases.
4. Conclusions In this work, the influence of the pristine sepiolite (both hydrated and dehydrated) in an epoxy resin has been investigated and discussed. Rheological measurements have showed that shear
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thinning effect induced by sepiolite loadings results much more attenuated in the case of dehydrated nanofiller; while in all cases, the viscosity level increases. Thermo-mechanical property of the neat epoxy and corresponding sepiolite/epoxy nanocomposites was investigated and it was found that the presence of sepiolite lead to remarkable effects on coefficient of thermal expansion, with a final reduction of about 7% and 8% of the glassy coefficient in the sample with 10 wt% of sepiolite, respectively hydrated and dehydrated. DMA shows a slight increase of the Tg (about 6°) compared to the neat sample in line with literature data. Moreover, the addition of sepiolite strongly increase the elastic modulus of the system: particularly, the samples with 10 wt% of filler show an increasing of the modulus at 35 °C of 22.1% and 21.6%, respectively for hydrated and dehydrated sepiolite. A much remarkable results was achieved if modulus at higher temperature (above glass transition temperature) is considered. In this case the variation is up to 94% as for 10 wt% of idra sepiolite. Fracture test show that the surface for the neat and for the sepiolite loaded sample is brittle with a substantially invariance of the stress intensity factor (KIC). This results suggest that the stress transfer with presence of sepiolite is not altered. Although the fracture surface indicate a brittle surface, the nanocomposite rupture surface indicates that more indentations and lamellar rupture bands are present when sepiolite is loaded and therefore, as expected, the strain released energy rate increases. Acknowledgment The authors thank Mr. F. Docimo for his help in sample preparation. References [1] Ellis B. Chemistry and technology of epoxy resins. London: Chapman & Hall; 1993. [2] Pascault JP, Sautereau H, Verdu J, Williams RJJ. Thermosetting polymers. New York: Marcel Dekker; 2002. [3] Zheng Y, Zheng Y. Study on sepiolite-reinforced polymeric nanocomposites. J Appl Polym Sci 2006;99(5):2163–6. [4] Glaskova T, Aniskevich K, Borisova A. Modeling of creep for multiwall carbon nanotube/epoxy nanocomposite. J Appl Polym Sci 2013;129(6):3314–24.
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[5] Meo M, Rossi M. Tensile failure prediction of single wall carbon nanotube. Eng Fract Mech 2006;73(17):2589–99. [6] Subramaniyan Arun K, Sun CT. Enhancing compressive strength of unidirectional polymeric composites using nanoclay. Compos A Appl Sci Manuf 2006;37(12):2257–68. [7] Tsai JL, Kuo JC, Hsu SM. Organoclay effect on transverse compressive strength of glass/epoxy nanocomposites. J Mater Sci 2006;41(22):7406–12. [8] Becker O, Varley RJ, Simon GP. Use of layered silicates to supplementarily toughen high performance epoxy–carbon fiber composites. J Mater Sci Lett 2003;22(20):1411–4. [9] Lahiri J, Srinivas K, Siddiqui AO, Vavilov VP. IR thermographic inspection of filament wound CFRP shell samples. In: Proceedings of SPIE; 2007, 6541, XXIX, 65410S. [10] Lui WP, Hoa SV, Pugh M. Fracture toughness and water uptake of highperformance epoxy/nanoclay nanocomposites. Compos Sci Technol 2005;65(15–16):2364–73. [11] Bureau MN, Perrin-Sarazin MT, Perrin-Sarazin TT, Perrin-Sarazin F. Essential work of fracture and failure mechanisms of polypropylene–clay nanocomposites. Eng Fract Mech 2006;73(16):2360–74. [12] Yu Y, Qi S, Zhan J, Wu Z, Yang X, Wu D. Polyimide/sepiolite nanocomposite films: preparation, morphology and properties. Mater Res Bull 2010;46(10):1593–9. [13] Suárez M, García-Romero E. Variability of the surface properties of sepiolite. Appl Clay Sci 2011;67(68):72–82. [14] Coleman JN, Khan U, Blau WJ, Gun YK. Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites. Carbon 2006;44(9):1624–52. [15] Technical Data Sheet EPIKURE™ Curing Agent 3200, 3223, 3234 & 3245. Momentive, Houston; 2005. [16] Buonocore GG, Schiavo L, Attianese I, Borriello A. Hyperbranched polymers as modifiers of epoxy adhesives. Compos B Eng 2013;53:187–92. [17] Peters GWM, Spoelstra AB, Meuwissen MHH, Corbey R, Meijer HEH. Rheology and rheometry for highly filled reactive materials. Top Appl Mech 1993:331–8. [18] Gallan E. Properties and applications of palygorskite–sepiolite clays. Clay Miner 1996;31:443–53. [19] Lee JH, Rhee KY, Park SJ. Effects of silane modification and temperature on tensile and fractural behaviors of carbo nanotube/epoxy nanocomposites. J Nanosci Nanotechnol 2011;11(1):275–80. [20] Martone A, Formicola C, Piscitelli F, Lavorgna M, Zarrelli M, Antonucci V. Thermo-mechanical characterization of epoxy nanocomposites with different carbon nanotube distributions obtained by solvent aided and direct mixing. Express Polym Lett 2012;6(7):520–31. [21] Verge P, Fouquet T, Barrère C, Toniazzo V, Ruch D, Bomfima JAS. Organomodification of sepiolite clay using bio-sourced surfactants: compatibilization and dispersion into epoxy thermosets for properties enhancement. Compos Sci Technol 2013;79:126–32. [22] Yasmin A, Daniel IM. Mechanical and thermal properties of graphite platelet/ epoxy composites. Polymer 2004;45(24):8211–9. [23] Ye Y, Chen H, Wu J, Ye L. High impact strength epoxy nanocomposites with natural nanotubes. Polymer 2007;48(21):6426–33.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of sepiolite filler on mechanical behaviour of a bisphenol A-based epoxy system Aldobenedetto Zotti, Anna Borriello ⇑, Alfonso Martone, Vincenza Antonucci, Michele Giordano, Mauro Zarrelli Institute for Polymers, Composites and Biomaterials, National Research Council of Italy, P.le Fermi, 1, 80055 Portici, Naples, Italy
a r t i c l e
i n f o
Article history: Received 17 April 2014 Received in revised form 11 June 2014 Accepted 11 July 2014 Available online 27 July 2014 Keywords: A. Hybrid A. Thermosetting resins B. Thermomechanical Sepiolite
a b s t r a c t Sepiolite/epoxy systems, characterised by an inorganic content from 2% to 10% by weight, were tested by using different experimental techniques in order to assess the effects thermo-mechanical and fracture behaviour. It was found that filler dispersion is independent on the type of sepiolite used, i.e. hydrated and de-hydrated, whereas the addition of hydrated sepiolite strongly increases the viscosity of the final suspension. Mechanical properties have been investigated and discussed. The conservative modulus of the sepiolite/epoxy system increases as much as 21% and 94% respectively, at 35 °C and 200 °C respect to the unfilled polymer, with the incorporation of 10 wt% of sepiolite, both hydrated and dehydrated, with a slightly increasing of the corresponding glass transition temperature. The effect of sepiolite on the coefficient of thermal expansion lead to a variation in both glassy and rubbery region, respectively of about 7–8% and 9%. Fracture toughness analysis revealed that the critical stress intensity factor negligibly changes by increasing the sepiolite content, whereas the critical strain energy release rate (GIC) reaches an noteworthy value around 30% higher than neat epoxy. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resins are the most commercially successful thermosetting materials, especially as structural adhesives, high-performance protective coatings and matrices of composite materials [1,2]. Epoxy resins have many advantages, as low specific weight compared to metals, high mechanical properties and high corrosion resistance. Moreover these materials are brittle, having limited utility in applications that require high fracture toughness or impact resistance. The extensive applications of epoxy resins motivate intense studies having the objectives to prepare organic–inorganic nanocomposites with novel and improved performances. During the last years, the polymer/clay and nanotube based nanocomposites have obtained a great deal of attention [3,4]. In fact, low quantity of clay filler can significantly improve the mechanical and thermal properties of the system. However the entity of the improvement depends strongly on the particle content, dimension, shape, and degree of dispersion. For instance, composites filled with micrometric size particles have mechanical and thermal properties that result inferior compared to composites ⇑ Corresponding author. Tel.: +39 0817758814; fax: +39 0817758850. E-mail address: [email protected] (A. Borriello). http://dx.doi.org/10.1016/j.compositesb.2014.07.017 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.
filled with nanometric size particles [5]. Addition of nanoclay commonly produces an increase of 22% and 36% with 3 and 5 wt% nanoclay loadings, respectively, in longitudinal compressive strengths of off-axis GFRP composites [6] and transverse compressive strength of unidirectional GFRP [7]. The main failure mode was identified as fibre/matrix debonding. The organoclay modified epoxy gave rise to significant improvements in both crack growth resistance and fracture toughness in mode I interlaminar fracture of CFRP composites [8]. In another study [9], both the initiation and propagation values of mode I interlaminar fracture toughness of CFRP composites increased at higher clay concentration. A strong correlation was established between the fracture toughness of clay–epoxy nanocomposite and the CFRP composite interlaminar fracture toughness. The CFRPs containing organoclay brought about 53% and 85% improvement in the interlaminar fracture toughness with the introduction of relatively small amount of filler. Liu et al. [10] have carried out fracture toughness studies in epoxy–clay based nanocomposites. Fracture toughness of this epoxy system has been greatly enhanced with the addition of nanoclays. With the addition of only 4.5 phr of clay, the strain energy release rate of the epoxy was increased 5.8 times from the original value. Bureau et al. [11] have studied fracture analysis
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for PP based polymer at 2 wt% nanoclay content with various types of coupling agents. Although the addition of nanofiller may lead to strong improvement in fracture behaviour, the morphology and the level of dispersion or disaggregation (exfoliation or intercalation in the case of nanoclay) certainly play the major rule on property effects. The toughness mechanisms could be very depended by particles sizes, agglomeration geometry, filler/matrix bonding energy and particle orientation, therefore the level of property (i.e. level enhancement or detrimental effects) needs to be analyzed in conjunction to a more general scenario rather than considering only the type of nanofiller. Among the different clay which are suitable as candidate for nanocomposite production, surely sepiolite can be a valuable nanofiller. Sepiolite is a fibrous magnesium phyllosilicate, characterised by a needle-shaped geometry, whose ideal formula is [Si12Mg8O30(OH)4](H2O)48H2O. Its structure is formed by two layers of silica tetrahedrons linked by a layer of magnesium ions in octahedral coordination. The octahedral sheets are not continuous like in the other phyllosilicates: this structure provides infinite channels along the fibre axis with a cross section of about 1 0.4 nm2 [3,12,13]. The presence of this channels strongly increases the specific surface area of this particles, up to 200– 300 m2/g. Thanks to this large specific surface area, the sepiolite is used in rheological, sorptive and catalytic applications [12]. Sepiolite is also widely used like filler in micro and nanocomposites. For instance, Zheng have demonstrated that the addition of 1 wt% of sepiolite in the epoxy matrix increases the Tg by 50 °C [3]. In this work, sepiolite nano-powder is used as filler in an epoxy system in order to investigate the effect on the thermo-mechanical, mechanical and fracture properties of the final nanocomposite. Moreover, being the considered epoxy resin generally used as a matrix for advanced polymer composites, the induced-sepiolite variation of the rheological behaviour of the neat system was experimentally assessed to establish limiting content of the filler for the later manufacturing process of composite elements. The physical properties of the final systems have been carefully examined by different experimental methods, i.e. thermo mechanical analysis (TMA), Dynamic Mechanical Analysis (DMA), fracture toughness tests, rheometry, optical and electron microscopy in order to investigate the potential enhancement of mechanical property. It was found that the achieved dispersion of the filler lead to an overall increasing of mechanical behaviour compared with neat hosting system, not only in term of bending modulus and coefficient of thermal expansion but interestingly in term of fracture energy. The sepiolite was used as received (hydrated form) and after a pre-treatment to eliminate absorbed water (dehydrated form).
2. Experimental section 2.1. Materials In this work, the matrix used is a bisphenol-A/epichlorohydrin derived liquid epoxy resin (EPON™ Resin 828) supplied by Hexion characterised by an equivalent weight (EEW) of 188 g/mol and a room temperature (25 °C) viscosity of about 10–15 Pa s [14]. The used curing agent was a Triethylene Tetramine (TETA), commercially known as EPIKURE3234, which is a polyamine with an amine hydrogen equivalent weight (AHEW) of 1.41–1.46 mg/g [15]. This curing agent provides low viscosity with high performance properties after a relatively low temperature cure. Sepiolite was gently provided by Mi.Mac. (Capua, Italy) and it was used in different concentrations as inorganic filler in two forms: hydrated (idra) and dehydrated (anidra). Pre-treatment of pristine sepiolite was carried out in order to remove absorbed moisture during
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micronization and handling. The dehydration pre-stage was carried out by holding the powder filler at isothermal temperature (100 °C) in oven for 3 h under vacuum. After the pre-stage, the inorganic filler was stored in an desiccator. The criteria to identify the correct procedure for the dehydration of sepiolite filler was experimentally determined by thermal ad thermo-gravimetrical analysis showing a substantial absence of endothermic desorption peak (DSC) and weight reduction around 100 °C (TGA). The mixing of epoxy resin and sepiolite was performed by sonication using a Misonix S3000 sonicator, set at 30 W power for 1 h. An oil bath was employed to hold the mixture at 50 °C. After sonication, few grams of the mixture (EPON828 and sepiolite) were stored for the rheological analysis, while the remaining was degassed in a planetary mixer (Thinky Mixer ARV-310) at the velocity of 2000 rpm under vacuum for 5 min. The curing agent (EPIKURE3234) was then added to the degased mixture, with 13 PHR (Part for Hundred of Resin). The mixing of sepiolite/epoxy mixture and the EPIKURE3234 was performed by using the planetary mixer at the velocity of 2000 rpm under vacuum for 1 min and then poured into a steel, previously coated with a release agent (FREKOTE 70). The same curing cycle was assumed for all the manufactured sample plates, i.e. 1 h @70 °C followed by a post-curing stage of 2 h @120 °C. The cured plates were cooled down slowly in the curing oven over the night to room temperature, removed from the mould and cut to prescribed sample dimensions for the programmed mechanical tests. 2.2. Characterisation 2.2.1. Rheology The rheological characterisation of the EPON 828/sepiolite suspension was carried out by MRC-301 rheometer (Anton Paar), using parallel plates PP25 (25 mm diameter) with a 1 mm gap. In the case of the unreacted epoxy mixture, two kind of tests were performed, namely, amplitude sweep and frequency sweep in order to limit the linear viscoelastic region of the suspension and also to assess the effect of the filler over the rheological behaviour of neat material. All mentioned rheological tests were executed at a 25 °C. For the amplitude sweep tests the deformation was varied within the percentage range [102, 102] with a fixed frequency 1 Hz. In the case of frequency sweep, all the tests were carried out at fixed formation percentage and variable frequency within the rage 0.1– 100 rad/s. Rheological tests were also executed on reactive suspension (EPON 828/sepiolite and curing agent), by using an MRC-301 Rheometer in temperature ramp mode. The heating rate was set to 3 °C/min from ambient temperature at frequency (1 Hz) and deformation (c = 0.5%). The tests were stopped when viscosity exceeded the value of 106 Pa s. 2.2.2. SEM and optical microscopy SEM and optical micrographs of additive powder and nanocomposites were taken respectively by an 1450VP LEO SEM microscopy and an optical Olimpus BX51 Instruments equipped with different magnitude oculars. Cross-linked samples for SEM analysis were cut by ultra-microtome with a diamond knife in thin layer or by cryogenic rupture to prepare fracture surface. Samples observed by optical microscopy were prepared by operating a precise sawing machine at high speed rotation. 2.2.3. Thermo mechanical analysis The thermo-mechanical behaviour of solid cured mixture (EPON828/sepiolite and curing agent) was investigated by TMA 60 (Shimadzu) set with a macro probe and programming an heating ramp of 5 °C/min. The nominal dimensions of the TMA samples
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were 10 3 3 mm according ASTM E831-03 standard . Thermal Expansion Coefficient (CTE), respectively, in the glassy phase (CTEg) and rubbery stage (CTEr) were measured and results are reported hereafter. 2.2.4. Dynamic Mechanical Analysis Dynamic Mechanical Analysis (DMA) was performed by Q800 DMA (TA Instruments) at fixed frequency of 1 Hz and heating ramp of 3 °C/min. The testing configuration was the single cantilever mode with sample nominal dimension of 30 10 2.5 mm; 50 lm amplitude was considered for all the performed tests. 2.2.5. Fracture test Mode-I fracture tests were conducted using single edge notched beam (SENB) specimen. The test specimen was chosen according to the ASTM D5045-99 standard test method for plane strain fracture toughness and strain energy release rate of plastic materials. To ensure the plane strain conditions, the following specimen dimensions were chosen: B = 3 mm, W = 6 mm and S = 24 mm. According to the ASTM standard, the crack length should be selected such that 0.45 < a/W < 0.55. Results are considered valid according to the standard if the following size criteria are satisfied:
where KQ is the conditional or trial KIC value, and ry is the yield stress of the material for the temperature and loading rate of the test. The fracture tests were performed using an Instron 3360 dynamometer equipped with 1 kN load cell and at a displacement rate of 10 mm/min. KIC values were calculated according to the standard as:
PQ
reduced number of sepiolite needle aggregation having sub or micrometric size. 3.2. Rheological analysis
B; a; ðW aÞ > 2:5ðK Q =ry Þ2 ;
K IC ¼ f ða=WÞ
Fig. 1. Magnification of sepiolite particle that shows the needles sizes.
BW 1=2
with
h 2 i a 1=2 1:99 Wa 1 Wa 2:15 3:93 Wa þ 2:7 Wa F ¼6 3=2 W W 1þ2 a 1 a a
W
W
where PQ is the load at failure, a the crack length, while W and B are the specimen width and thickness, respectively. Critical strain energy release rates (GIC) were calculated from the stress intensity values according to the cited ASTM standard. 3. Results and discussion 3.1. Morphology of suspensions Fig. 1 shows an higher magnification of a sepiolite particle with characteristic needle-shaped geometry and nominal dimension [12,18]. The needle-shaped sepiolite particle is characterised by nominal dimension of tenths of nanometer and a variable length of few microns. The final aggregation of sepiolite within the hosting matrix is a characteristic round-shaped particles of nominal dimension less than 10 lm. In Fig. 2, the micro-aggregations of sepiolite are shown for a cryogenic fractured surface. Fig. 3 reports SEM micrographic of loaded samples along with a zooming-in of the same sample which shows the characteristic needle geometry of the sepiolite completely encapsulated within the hosting epoxy system (Fig. 3). The high shear mixing technique used in this work (i.e. tip sonication) assures an uniform dispersion of the inorganic filler within the epoxy for all the considered weight percentages. Optical micrograph taken from neat and sepiolite loaded samples (Fig. 4) have highlighted that the dispersion of the inorganic filler is highly exfoliated to reach in some case a
Fig. 5 shows the amplitude sweep test curves obtained for the neat system and for epoxy suspensions containing different percentages of sepiolite, respectively hydrated and dehydrated. The addition of a small amount of sepiolite induces only a slightly increase of the loss modulus compared to the neat resin with a negligible difference between the two cases, i.e. idra and anidra. At higher content (5 wt% and 10 wt%) and under low strain, not only the loss modulus of the suspension increases, as expected, but also the variation between idra and anidra sepiolite loading increases. The effect of sepiolite content can also be appreciated considering that, at low content (2 wt%), a linear viscoelastic behaviour within the whole analysed frequency range is recorded. Whereas, increasing the sepiolite content, a non-linear viscoelastic behaviour occurs to smaller strains. For dynamic experiment a fix value of strain was chosen assuring that all suspensions would report a linear viscoelastic regime. As reported on the curves (Fig. 5), by choosing a strain percentage of 0.5%, it is sure that all prepared suspensions will respond in their linear viscoelastic regime. Fig. 6 report the flow curves for neat epoxy and suspensions containing different percentages of sepiolite both idra and anidra. The recorded level of viscosity strongly depends on the typology of sepiolite loaded, in fact, for the 10 wt% loading of pristine (idra) sepiolite and under low shear, the viscosity abruptly varies of two order of magnitude, i.e. from 14 Pa s to 1156 Pa s. In contrast, by loading a similar amount of pre-treated (anidra) sepiolite at same strain percentage, the viscosity reaches 323 Pa s. This effect could be probability due to the interaction between the water molecules, adsorbed on the sepiolite surface, and the –OH groups of the epoxy monomer, which lead to ‘‘thickening’’ effect of the final suspension. The different behaviour in term of viscosity according to the type of loaded sepiolite, may represent a ‘‘working’’ feature in real industrial processing technology. In fact, whether the matrix will be used for infusion or liquid process to manufacture composite element and/or semi-finished product such as prepreg material, the potentiality of tuning the suspension viscosity by pre-treating the sepiolite content, i.e. controlling the amount of water adsorbed, being constant its content, could be an extremely valuable technological methodology for process optimisation. In all cases, as expected, the viscosity increases with filler content. Nevertheless, the epoxy monomers exhibits a Newtonian
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Fig. 2. Particle size of sepiolite in cured 2 wt% epoxy.
Fig. 3. SEM micrograph for 5 wt% sepiolite content sample.
Fig. 4. Optical microscopy (20 and 50) of neat and filled.
behaviour within the whole considered frequency range, whereas, the epoxy/sepiolite suspensions exhibit the same behaviour only at lower percentages of filler (2 wt%). At higher sepiolite content (i.e. 5 wt% and 10 wt%) a pronounced shear thinning effect (i.e. high
viscosity progressively decreases with shear rate/angular frequency increasing) is monitored. The sepiolite induced shear thinning effect results significantly different depending on the type of loaded sepiolite, respectively pristine (idra) or pre-treated (anidra).
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Fig. 5. Loss modulus (G00 ) vs. strain for the EPON828 monomer and 2, 5 and 10 wt% epoxy suspensions of sepiolite: (A) hydrated and (B) dehydrated.
In particular, the addition of 10 wt% idra sepiolite decreases the viscosity from 1155 Pa s to 69 Pa s respectively from 0.1 Hz to 100 Hz; over the same frequency range, but in the case of anidra filler, the viscosity drops from 323 Pa s to around 113 Pa s. A reasonable explanation for this behaviour could be that initially, after disaggregation of the pristine agglomeration, sepiolite needles are highly dispersed in the hosting matrix [16]. Thus, the interaction between needle-shaped nanoparticles and epoxy monomers induces percolation of the sepiolite filler in a threedimensional network: at low shear rate, the formed networking architecture is not altered with a consequent high viscosity. At higher shear rate, the breaking of the percolated network occurs, thus the system viscosity decreases. A further mechanism takes place if the geometrical configuration of sepiolite is considered. In fact, at higher shear rate, the needle-shaped sepiolite filler will tend to align along the flow field vectors and thus decreasing the viscosity. In dynamic experiments, two different types of behaviour could be highlighted depending on the sepiolite content. At 2 wt%, 5 wt% and 10 wt% sepiolite percentage (Fig. 7), the curve of loss modulus results higher than the corresponding elastic modulus over the whole examined strain range revealing a liquid-like behaviour. When the level of sepiolite is higher (20 wt%), under lower strain, the system shows an elastic modulus higher than the corresponding viscous part, whereas after a threshold strain the curves cross each other. The crossover point identifies a transition region from
a pseudo solid-like to a liquid-like behaviour likely related to a forming physical gel structure induced by the extremely high content of nanoloading (i.e. 20 wt%, Fig. 7). It remains obvious that the amount of sepiolite or in general nanoloading which characterises the formation of the physical gel network and therefore induce this later effect is related not only to the amount of nano-loading but also to the dispersion technique employed to prepare the suspension. In fact, the critical percentage of sepiolite at which this physical gel is formed, could be as lower as 8.4 wt% processed by using ‘conventional’ technique (i.e. mechanical stirring) or 20 wt% such as in our case by using high shear mixing (i.e. sonication). This observation leads to the conclusion that the level of dispersion in term of nominal dimension of the filler disaggregation represents a fundamental feature for the network formation. Temperature-ramp tests were performed on reactive mixture (i.e. sepiolite/EPON828 suspension added with curing agent) in order to analyse gel formation of the system. In general, the rheological results are depending on the measuring methods used. In fact, while in oscillation, the gel formation is not disturbed, in rotation the forming gel network is partly disrupted by the applied shear rate. For this reason different methodologies could be accounted for the determination of gelation time. According to the ASTM 4473-90, gelation can be identified on the time scale as the time corresponding to the maximum in tan d curve, in isothermal tests. Some researchers [17] have expressed reservations about this specific method, especially in the case of highly filled epoxy resins. In temperature ramped experiments, the viscosity sharply arises at gelation, due to inherent built-up of the 3D structure which limits the flow capability. The gelation point can be identified, at fixed heating rate, as the temperature at which viscosity arises above 104 Pa s. Fig. 8 reports the viscosity evolution during a dynamic temperature ramp and the corresponding conversion profile. The actual conversion reached by the system at ‘‘sharp onset’’, i.e. when he viscosity is higher than 104 Pa s, represents the gelation degree of cure 4 g ¼10 Pa s . agel Fig. 9 shows the viscosity vs. temperature curves for reactive suspensions containing different amounts of both idra and anidra sepiolite along with the set level to identify the ‘‘gelation conversion’’. If the temperature ramp is constant (i.e. 3 °C/min), the gelation temperature changes with the sepiolite contents. Particularly in the case of 10 wt%, the system undergoes the gel transition at temperature lower than the corresponding neat epoxy monomer. Moreover, the typology of sepiolite (i.e. idra and anidra) induces a different variation of the temperature corresponding to the
Fig. 6. Complex viscosity vs. frequency for the neat and sepiolite/epoxy suspensions of sepiolite: (A) hydrated and (B) dehydrated.
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Fig. 7. Amplitude sweep tests at different idra sepiolite content.
whereas in the case of anidra filler, this variation is much more attenuated. 3.3. Mechanical behaviour
Fig. 8. Comparison of dynamic conversion and viscosity at gelation.
viscosity arising, from 85.5 °C to 80.7 °C and 83.4 °C, respectively, for idra and anidra sepiolite loading. The temperature at which the tri-dimensional network is formed and thus viscosity arises, results not significantly changed at low sepiolite content specially in the case of idra sepiolite filler. However, at the same content, a slightly variation could be appreciated when anidra sepiolite is loaded. An opposite behaviour is revealed at higher content of sepiolite, in fact in the case of idra sepiolite, the remarkable variation of ‘‘gelation temperature’’ could be reasonably related with the higher amount of water molecules,
Temperature ramp tests by using TMA technique were performed on neat and sepiolite (idra and anidra) loaded samples. The linear coefficient of thermal expansion (CTE) was measured below (CTEg) and above (CTEr) glass transition temperature and results are reported in Table 1. The CTE mismatch between matrix and fibres is among the main controlling factors for the arising of residual stresses within composite materials, thus the effect of sepiolite at different percentages surely represents a fundamental information for the correct design of specific element or component. CTE values under and above Tg were calculated by the slope of the linear regression line within the glassy and rubber region. It was found that, at low sepiolite content, CTE of the system does not change significantly compared with the neat epoxy. However, according to literatures [19,20], the expansion behaviour changes of about 7–8% and 9 % respectively below and above glass transition in the case of 10 wt% sepiolite loading. Dynamic Mechanical Analyser allows the measurement of epoxy modulus as function of the temperature. The main results in term of modulus @35 °C and @200 °C along with glass transition temperature as obtained by DMA tests on neat epoxy and sepiolite loaded nanocomposites are reported in Table 2. The addition of sepiolite lead to an overall increase of the flexural modulus from the initial value of 2240 MPa, for the neat epoxy
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Fig. 9. Complex viscosity vs. temperature for the EPON828 monomer and for suspensions with 2 wt%, 5 wt% and 10 wt% of pristine sepiolite: (A) hydrated and (B) dehydrated.
Table 1 TMA results for the EPON828/sepiolite systems. Sample
ag 106 (K1)
ar 106 (K1)
Dag (%)
Dar (%)
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
69.2 ± 1.1 68.5 ± 2.2 69.2 ± 2.1 67.6 ± 1.3 67.3 ± 1.2 63.7 ± 3.0 64.3 ± 3.0
187.3 ± 2.7 182.4 ± 2.8 181.9 ± 4.1 179.3 ± 1.3 176.1 ± 1.0 169.5 ± 4.9 170.5 ± 0.8
– 1.0 0 1.9 2.7 7.9 7.1
– 2.6 2.9 4.3 5.7 9.5 8.9
system, to a maximum of 2737 MPa in the case of 10% sepiolite by weight. Increasing the content of sepiolite, according to the dispersion level achieved by using high shear mixing sonication, the value of the bending modulus is gradually increased. The conservative modulus at 35 °C changes significantly of about 22.1% and 21.6% respect to the neat epoxy system, respectively, for idra and anidra sepiolite filler. Interesting results were obtained for the bending modulus at 200 °C. The maximum variation achieved is of about 94.6% as the measured value changes from 35.45 MPa to 58.4 MPa or 65.2 MPa, with 10 wt% of sepiolite, respectively, dehydrated and hydrated. Fig. 10 shows the conservative modulus as function of temperature for the sample with hydrated sepiolite (A) and dehydrated sepiolite (B). It can be seen that there are not significantly difference between the modules of sample with hydrated and dehydrated sepiolite. Increased glass transition temperatures have been reported for some nanocomposite systems [5,21,22], while in others a constant [4,12] or slightly decreased [23] of Tg have been found. The presence of sepiolite has little effect on the value of the glass transition temperature, with not more than 6 °C as in the case of 10 wt% of sepiolite. The level of dispersion achieved by high shear mixing may represents a key factor for the reinforcement efficiency related to
loaded nanoclay. Coleman et al. [14] has introduced a reinforcement efficiency parameter for carbon nanotube nancomposite which could be suitably applied for our nanocomposite samples. Considering the variation of measured modulus for the neat and for the sepiolite loaded epoxy for unit volume filler, the level of dispersion could be parameterised and therefore rationally monitored and discussed. Considering that the density of the fully cured resin and the sepiolite is respectively 1.16 g/cm3 [15] and 2.10 g/cm3 [12] the sepiolite volume fraction corresponding to the used weight percentage could be computed and the reinforcement efficiency can be evaluated. Table 3 reports the computed values of reinforcement efficiency according to Coleman’s parameter for the nanocomposite samples loaded respectively with idra and anidra sepiolite filler at different percentages. It is important to note that the type of sepiolite (i.e. idra or anidra) induces a negligible variation of the reinforcement efficiency as the computed values, for the considered nanofiller content, is very closed. Moreover, the efficiency slightly reduces at higher filler content as the dispersion procedure make up more sepiolite agglomerations thus reducing the stress transfer. Fracture tests were performed using a three point bending test according to the corresponding ASTM D5045 standard for polymer system. Typical linear load–displacement curves have been obtained for the neat epoxy and its nanocomposites, both showing a brittle fracture surface (see Table 4). The stress intensity factor represents the constant of proportionality between the applied stress and square root of the distance (and angle displacement if cylindrical coordinate is assume around the crack tip) from the crack and it is used to determine the fracture toughness of most materials. Because the dependence of the stresses on the coordinate variables remain the same for different types of cracks and shaped bodies, the stress intensity factor is a single parameter characterisation of the crack tip stress field. The value of the stress intensity factor at which unstable crack propagation occurs is called the fracture toughness or critical stress
Table 2 Results of DMA tests for the EPON828/sepiolite systems. Sample
Tg (°C)
E0 @35 °C (MPa)
E0 @200 °C (MPa)
DE0 @35 °C (%)
DE0 @200 °C (%)
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
130.9 132.9 132.4 133.7 133.2 134.8 136.4
2240 ± 24 2440 ± 27 2435 ± 20 2565 ± 54 2524 ± 11 2723 ± 31 2737 ± 20
35.4 ± 0.5 38.4 ± 0.5 39.1 ± 1.2 50.9 ± 1.1 50.7 ± 0.4 58.4 ± 1.2 65.2 ± 2.9
– 8.9 8.7 9.9 12.7 21.6 22.1
– 8.3 10.3 43.8 43.2 64.9 94.6
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Fig. 10. Conservative modulus (E0 ) vs. Temperature for neat and 2, 5 and 10 wt% sepiolite/epoxy: (A) hydrated and (B) dehydrated.
Table 3 Reinforcement efficiency according to mechanical characterisation. Sample
Sepiolite content
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
dE/dV (GPa)
wt%
vol%
0 2
0 1.12
5
2.83
10
5.78
0 17.45 17.86 10.05 11.50 8.59 8.35
Table 4 Results of fracture toughness tests for the neat and sepiolite/epoxy systems. Fig. 12. Metallised fracture surface of neat epoxy sample after fracture test. Sample
KIC (MPa m1/2)
St. dev. (%)
GIC (kJ/m2)
St. dev. (%)
Neat 2% SEP ANIDRA 2% SEP IDRA 5% SEP ANIDRA 5% SEP IDRA 10% SEP ANIDRA 10% SEP IDRA
1.24 1.07 1.01 1.09 1.12 0.99 1.17
4.12 5.3 6.1 4.2 5.3 3.7 6.3
1.04 1.0 0.93 1.11 0.96 1.33 1.35
0.3 5.2 7.9 7.8 8.1 6.4 6.2
intensity factor, KIC. Maximum loads are used to calculate the fracture toughness of the samples along with the initial crack length. Results show that the value for the EPON828 (neat epoxy) was found equal to 1.24 MPa m1/2 in line with data sheet provided by supplier; this value negligibly change with sepiolite content according to error ranges (see Fig. 11). The main hypothesis we can suggest for the negligible variation of the KIC is mainly related
Fig. 11. Critical stress intensity factor, KIC, and critical strain energy release rate, GIC, vs. sepiolite %.
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Fig. 13. SEM of fractured surfaces of epoxy loaded with 2% sepiolite (A) anidra, (B) idra; 5% sepiolite (C) anidra, (E) idra; 10% sepiolite (D) anidra, and (F) idra.
to the brittleness of the matrix due to the presence of nanofiller. For all samples, the fracture is brittle (see Figs. 12 and 13) and the presence of sepiolite, in any percentages, does not affect the stress distribution surrounding the crack tip. A different trend was recorded in the case of strain energy release rate, GIC. The computed values of GIC, according to the standard, represents the released energy, under plain strain conditions, per unit area of propagating crack. Irrespective of the type of sepiolite content (i.e. idra or anidra), GIC values increase with increasing percentage of nanofiller (see Fig. 11). These results can be suitably discussed analysing the SEM fracture surface for neat epoxy and corresponding nanocomposites. Morphological analysis was performed with a Scanning Electron Microscope (SEM). The fracture surface of the neat epoxy was very smooth, characterised by extended fracture surface (dotted curve) grooved by tiny river line markings (as indicated the arrows in Fig. 12). This fractography is typical for brittle polymers, revealing
that the resistance to crack propagation is fast and catastrophic. Comparing the SEM fractography images taken on nanofilled samples, it results that rupture, although still brittle, presents a high degree of surface corrugation and level of indentations, suggesting that the crack propagates along different directions. Thus the energy released to growth the crack increases due to the number of surface to create. Values of plain released energy rate varies from 1.04 kJ/m2 for the neat epoxy to 1.33 kJ/m (anidra sepiolite) and 1.35 kJ/m2 (idra sepiolite) with increment of more than 30% in both cases.
4. Conclusions In this work, the influence of the pristine sepiolite (both hydrated and dehydrated) in an epoxy resin has been investigated and discussed. Rheological measurements have showed that shear
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thinning effect induced by sepiolite loadings results much more attenuated in the case of dehydrated nanofiller; while in all cases, the viscosity level increases. Thermo-mechanical property of the neat epoxy and corresponding sepiolite/epoxy nanocomposites was investigated and it was found that the presence of sepiolite lead to remarkable effects on coefficient of thermal expansion, with a final reduction of about 7% and 8% of the glassy coefficient in the sample with 10 wt% of sepiolite, respectively hydrated and dehydrated. DMA shows a slight increase of the Tg (about 6°) compared to the neat sample in line with literature data. Moreover, the addition of sepiolite strongly increase the elastic modulus of the system: particularly, the samples with 10 wt% of filler show an increasing of the modulus at 35 °C of 22.1% and 21.6%, respectively for hydrated and dehydrated sepiolite. A much remarkable results was achieved if modulus at higher temperature (above glass transition temperature) is considered. In this case the variation is up to 94% as for 10 wt% of idra sepiolite. Fracture test show that the surface for the neat and for the sepiolite loaded sample is brittle with a substantially invariance of the stress intensity factor (KIC). This results suggest that the stress transfer with presence of sepiolite is not altered. Although the fracture surface indicate a brittle surface, the nanocomposite rupture surface indicates that more indentations and lamellar rupture bands are present when sepiolite is loaded and therefore, as expected, the strain released energy rate increases. Acknowledgment The authors thank Mr. F. Docimo for his help in sample preparation. References [1] Ellis B. Chemistry and technology of epoxy resins. London: Chapman & Hall; 1993. [2] Pascault JP, Sautereau H, Verdu J, Williams RJJ. Thermosetting polymers. New York: Marcel Dekker; 2002. [3] Zheng Y, Zheng Y. Study on sepiolite-reinforced polymeric nanocomposites. J Appl Polym Sci 2006;99(5):2163–6. [4] Glaskova T, Aniskevich K, Borisova A. Modeling of creep for multiwall carbon nanotube/epoxy nanocomposite. J Appl Polym Sci 2013;129(6):3314–24.
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