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Understanding the effect of particle surface free energy on the structural and mechanical properties of clay-laden rigid polyurethane foams
European Polymer Journal 60 (2014) 135–144
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Understanding the effect of particle surface free energy on the structural and mechanical properties of clay-laden rigid polyurethane foams Rob Van Hooghten a, Sarah Gyssels a, Sergio Estravis b, Miguel Angel Rodriguez-Perez b, Paula Moldenaers a,⇑ a b
Department of Chemical Engineering, KU Leuven, University of Leuven, Willem de Croylaan 46, B-3001 Heverlee, Belgium Cellular Materials Laboratory (CellMat), Condensed Matter Physics Department, Paseo de Belen 7, Campus Miguel Delibes, Valladolid, Spain
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 11 August 2014 Accepted 29 August 2014 Available online 16 September 2014 Keywords: Polyurethane Foam Nanoclay Surface free energy Shear modulus
a b s t r a c t The surface free energy of nanoclays is shown to be a key parameter in controlling the localization of the particles in the polyurethane matrix, and the structural and mechanical properties of nanoclay laden rigid polyurethane foams. The surface free energy determines the dispersion quality of the nanoclays before foaming, which in turn determines the available surface area for heterogeneous nucleation during foaming. It has been found that particles showing the best dispersion quality do not necessarily lead to the best mechanical properties of the foams. On the contrary, the nanoclays which are likely to get trapped at the polyurethane-air interface during foaming, yield improved mechanical properties as compared to the neat polyurethane foam, despite their less favorable dispersion. The interfacial adsorption stabilizes the cell wall during cell growth, leading to smaller cells after rigidification, as compared to the nonadsorbing nanoclays. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polymer foams form a class of lightweight materials with typical applications in insulation, cushioning and packaging. High porosity foams with interconnected pores are also used as tissue engineering scaffolds. Applications as a structural material are however limited due to their low mechanical strength and low dimensional stability [1]. The addition of nanoparticles to polymers and polymer foams offers an efficient way to increase their mechanical, thermal, barrier and flame retardant properties [2]. Especially nanoclays have been studied in polymer nanocomposities and foamed nanocomposites as they are readily available with different organic modifiers ⇑ Corresponding author. E-mail address: [email protected] (P. Moldenaers). http://dx.doi.org/10.1016/j.eurpolymj.2014.08.029 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.
and have large intercalation and exfoliation possibilities. The large aspect ratio of the individual clay platelets implies a large specific surface area to interact with the matrix material. Hence, only small amounts of nanoclays need to be added to improve the properties, at least if they are dispersed properly. Polyurethane (PU) foams, with or without nanoparticles, have a large diversity in chemistry, density and applications. In general, a PU foam consists of five main components: polyol, isocyanate, blowing agent, catalysts and surfactants. PU foams are prepared by blending the polyol with the surfactants, the blowing agent and the catalysts. In a subsequent step, the isocyanate is added after which foaming initiates. Foaming proceeds by two chemical reactions that occur simultaneously. The isocyanate reacts with the blowing agent in a first reaction. When water is used as a blowing agent, CO2 is generated which
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blows the foam. The cross linking reaction between the polyol and the isocyanate solidifies the foam and stabilizes the foam morphology. The final foam properties such as density and cell size are a strong function of the balance between the kinetics of these two reactions. Depending on the cross linking degree, PU foams are divided into flexible and rigid PU foams. Flexible PU foams have a low cross linking degree and usually have a high open cell content. These soft foams are preferentially used in cushioning applications. The rigid PU foams, with a higher degree of cross linking, are mostly closed cell foams and are used in insulation applications. The addition of nanoclays to PU foams is known to directly influence foam morphology, usually leading to smaller cell sizes [3–5]. The nanoclays act as a heterogeneous nucleation agent [6,7] or as a catalyst modifying the reaction kinetics [8]. The mechanical properties, often measured as compressional properties, are indirectly affected. However, the effect of the presence of nanoclays are not yet fully understood as opposite results are reported for flexible and rigid PU foams reinforced with montmorillonite based clays. Since rigid PU foam intrinsically have a higher mechanical strength, it appeared in general more difficult to increase the properties of rigid PU foams than those of flexible ones. Krishnamurthi et al. [9] incorporated 5 wt% clay in a flexible PU foam and observed an increase of the compressional strength, the same property decreased for a rigid PU foam. Similarly, Cao et al. [3] observed an increase in the compressive properties of flexible PU foams upon the addition of Cloisite 30B nanoclay, whereas the compressive properties reduced for a more rigid PU foam. This observation was explained by the chemistry of the PU foam and the clay. Hydroxyl groups on the clay surface can react with the urethane group and can interfere with the H-bond formation in the matrix, reducing the strength of the matrix. This led to an overall reduction in strength of the foam, despite the beneficial effect of the nanoclays on the cell size. These results on rigid PU foams were confirmed by Widya and Macosko [4]. The mechanical properties of both flexible and rigid PU foams depend strongly on the type of clay used. The compressive strength of a flexible PU foam was seen to increase upon addition of untreated Cloisite-Na+ nanoclay, whereas it diminished upon the addition of the organically modified Cloisite 10A [10]. Harikrishnan et al. [11] studied both flexible and rigid PU foams using four types of organically modified Cloisite clay. In this specific investigation, the mechanical properties were reduced for all flexible PU foams, while only for two rigid PU foams, at a specific clay concentration, an increase was observed. Mondal and Khakhar [8] observed an increase in the compressional modulus parallel to the foaming direction for rigid PU foams, while no effect was seen on the modulus perpendicular to the foaming direction upon addition of Cloisite Na+ and Cloisite 30B. Simultaneously, they observed more elongated cells in the foaming direction. They attributed this and the increase of the mechanical properties to a catalytic effect of the nanoclays accelerating the foaming reaction. Patro et al. [12] also observed an increase of the reaction rates and better mechanical
properties, for vermiculite based clays as compared to montmorillonite based clays. The type of nanoparticles, more specifically their surface free energy, has also been found to strongly affect the properties of polymer nanocomposites and polymer blends. Kamal et al. [13] showed that the properties of polystyrene and high density polyethylene clay nanocomposites depend on the state of dispersion of the clays and the surface free energy of the clays. The same conclusion was drawn by Xia and Song [14], who dispersed four types of clay in a PU matrix and found only for Cloisite 30B exfoliated clay layers. Similar results on other nanocomposite systems have been reported [15–17]. In immiscible polymer blends, the role of surface free energy has been shown to be of prime importance. Particles can adsorb at the interphase between the two polymers and stabilize the morphology against coalescence [18] and other morphological changes if the particle is wetted by both phases [19]. Recently, this result has been extended to polymer foams and to polymers, which are normally unfoamable [20]. Though a few studies have used different types of clay in PU foams and studied their properties, no systematic link with the clay surface free energy and their wetting properties was made to interpret the results. One reason for this is that surface free energies are difficult to measure and several research groups have reported widely different values for montmorillonite based clays [13,15,21–24]. The surface free energies of solids are calculated from contact angle measurements with reference liquids. However, clays are provided in a powdery form and contact angle measurements become cumbersome due to the porosity of the compressed clays. The reported results depend strongly on the method used and the experimental details. For example, reported surface free energies for Cloisite 15A range from 19.3 mN/ m [22] to 42.5 mN/m [21], both measured with the sessile drop technique, and a value of 25.4 mN/m was measured with the capillary rise technique [24]. Similarly, for Cloisite 10A, Kamal et al. [13] reported a value of 45.3 mN/ m, whereas 27.6 mN/m is reported by Bhattacharya et al. [22] and 30.0 mN/m by Burgentzle et al. [24]. Thus, for these two types of clay, which differ strongly in hydrophobicity according to the manufacturer [25], a relatively large range of surface free energies is reported. Hence, there is a clear need to characterize the surface free energies of these clays in a uniform, reproducible and reliable manner in order to link these values to the foam structure and properties. In summary, the goal of the present work is to systematically study the effect of clay surface free energy on the structural and mechanical properties of rigid PU foams. For this purpose, the surface free energy of three different Cloisite nanoclays is measured in an uniform way. A relation between the free surface energy of the clays and the mechanical properties of the PU foams is sought. A simple rheological test is used to measure the shear modulus of the foams. This modulus is used as the representative mechanical property as it is a relevant property in many design applications, but often not reported [26].
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Table 2 Surface free energy components of the reference liquids and a representative polyol according to the approach of Owens–Wendt [28] (Data from [30,31]).
2. Materials and methods 2.1. Materials A rigid closed cell PU foam from Recticel is used here to study the effect of the surface free energy of nanoclay particles. The PU foam is a blend of a polypropylene glycol based polyol and methylene diphenyl diisocyanate, with a nominal foam density (in free foaming) of 150 kg/m3. Three different organoclays, based on natural montmorillonite clays modified with an organic quaternary ammonium salt from Southern Clays, were used as nanofiller. They are in order of increasing hydrophobicity [25]: Cloisite 30B, Cloisite 10A and Cloisite 15A. The nanoclays were dried in a vacuum oven at 80 C for at least 24 h before use. The nanoclays were dispersed in the polyol by solution mixing with tetrahydrofuran (THF, Sigma Aldrich, 99%) before foaming. First, 2.5% by weight of the nanoclays were premixed with THF with a spatula. This mixture was then ultrasonicated for 30 min with an ultrasonic tip (Ikasonic, UP400S) with an amplitude of 50% at a frequency of 70 kHz. The required amount of polyol was subsequently slowly added while mixing with a magnetic stirrer and by ultrasonication during 15 min. The THF was finally removed with a rotary evaporator (Buchi, Rotavapor R215). This procedure did not affect the polyol as was checked with rheology. Foams were prepared by first adding the precise amounts of blowing agent (water) and catalyst. Foaming is initiated by adding the isocyanate while stirring at 1200 rpm (Ika Eurostar Power equipped with a Lenart Disc) during 15 s.
2.2. Clay characterization The surface free energy of the nanoclays is determined from contact angle measurements with three reference liquids: deionized water (Sartorius Arium 611 DI, conductivity = 0.55 lS/cm), diiodomethane (Sigma Aldrich, 99%) and ethanediol (Sigma Aldrich, 99%). The surface free energy of these liquids and their components are summarized in Table 1 according to the approach of van Oss et al. [27]. Table 2 shows the surface free energy of the reference liquids and of a representative polyol according to the approach of Owens–Wendt [28]. The nanoclay powders were compressed into cylindrical disks of 25 mm diameter and 2 mm thickness by compressing at 200 bar in a plate press (Collin P200E) at room temperature. Liquid droplets were deposited on the compressed disk and their volume was stepwise increased. The shape of the droplets was
Table 1 Surface free energy components of the reference liquids and a representative polyol according to the approach of van Oss et al. [27] (Data from [29]). Reference liquid
rl (mN/m)
rLW l
rþl (mN/m)
rl (mN/m)
Deionized water Diiodomethane Ethanediol
72.9 51.2 48.1
22.5 50.9 31.2
28.8 0.47 2.1
10.8 0.04 34.0
Reference liquid
rl (mN/m)
rdl (mN/m)
rpl (mN/m)
Deionized water Diiodomethane Ethanediol Dipropyleneglycol
72.8 50.8 48.8 33.9
22.6 44.1 32.6 29.4
50.2 6.7 16.0 4.5
recorded and analyzed with a contact angle goniometer (CAM200, KSV). The contact angle was determined as the angle between the baseline of the compressed disks and the tangent to the droplets shape at the baseline. At least nine angles were measured for each clay. 2.3. Rheology Rheology is used to study the effect of nanoclay’s surface free energy on the dispersion quality of the nanoclays in the polyol. Rheological measurements were performed on a strain-controlled rheometer (ARES-G2, TA Instruments) equipped with a cone and plate geometry (d = 25 mm, cone angle = 0.02 rad). No sign of wall slip was detected as measurements were independent of the cone angle used. The temperature was controlled with a Peltier element and maintained at 25 C. To erase the mechanical history of the sample and loading effects, a fixed preshear protocol was adopted to obtain a reproducible initial state for the rheological measurements. First, the sample was sheared at 5 s1 during 300 s to break the clay network. The sample was then left at rest during 4000 s to rebuild the clay network. Dynamic strain sweep experiments were performed from 0.1% to 100% at 5 rad/s. The critical strain cc is defined here as the strain where the highest modulus has dropped to 90% of its plateau value. Dynamic frequency sweep experiments were then performed in the linear viscoelastic regime between 0.01 and 100 rad/s. Only a small, negligible difference was observed when performing this experiment from high to low and from low to high frequency, indicating that the 4000 s rebuild time was sufficient to arrive to a reproducible initial state. 2.4. Mechanical properties of the PU foams The shear modulus of the PU foams is measured with dynamic oscillatory measurements on a strain-controlled rheometer (ARES, TA Instruments) equipped with a torsion rectangular geometry. The foams were cut into rectangular bars of 45 10 5 mm3. The dimensions of the sample were determined accurately with a vernier caliper. A torsional deformation is applied to these bars creating a shear deformation in the foams. To probe possible anisotropy of the foams, the samples were cut with the length direction both parallel (k) and perpendicular (?) to the foaming direction. Dynamic strain sweeps experiments at 1 rad/s were performed to determine the linear deformation regime. The shear modulus was then measured by a frequency sweep in the linear regime.
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2.5. Scanning electron microscopy Scanning electron microscopy (SEM, JEOL JSM 820) was used to visualize the cell morphology. Cured foams were cut to obtain a smooth surface and vacuum coated with gold. Image analysis with ImageJ software was performed to measure the cell size and cell anisotropy.
3. Results and discussion 3.1. Surface free energy of nanoclays The determination of the surface free energy of nanoclays is not straightforward. First of all, the surface free energy of a solid material cannot be measured directly as opposed to that of a simple liquid. Instead, the contact angle of the solid material with some reference liquids is measured and in combination with a suitable model, the surface free energy can be estimated. A second complication arises from the fact that the nanoclays are available in a powdery form. Despite the compression of the clay powders at high pressure, the reference liquids may penetrate the porous structure leading to an apparent contact angle, which is smaller than the true equilibrium contact angle [32]. Contact angle measurements on compressed powders with a goniometer are therefore prone to effects of the porosity and surface roughness. Hence, the capillary rise method combined with the Washburn method, is often used to overcome these effects. In this approach, the liquid sorption into the porous structure is in fact used to determine the contact angle [33]. In this method, the porosity of the powder is ‘calibrated’ with a reference liquid which is assumed to be completely wetting. Recently, it has however been shown that this assumption may lead to erroneous results [34]. A second disadvantage is that nonwetting liquids cannot be measured with this technique. In this work, a goniometer is therefore used to measure the contact angle on compressed nanoclay powders. Whenever the volume of a deposited drop decreased due to penetration into the powder, the results were discarded. Table 3 shows the results of the contact angle measurements. The uncertainty is mainly due to surface roughness effects as a result of the compression [35]. Cloisite 10A and 30B show very comparable wetting behavior as the contact angles in Table 3 are very similar. This is in agreement with the manufacturers data that rank these two clays close to each other on the hydrophobicity scale. No statistically relevant difference can be found for the contact angles of diiodomethane and ethanediol. The experimentally measured contact angle of water is slightly lower for Cloisite 10A than for Cloisite 30B. This is not expected based on the molecular structure of the organic modifier, as Cloisite
Table 3 Contact angles of Cloisite 15A, 10A and 30B with the reference liquids. Reference liquid
Cloisite 15A
Cloisite 10A
Cloisite 30B
Deionized water Diiodomethane Ethanediol
82 ± 5 60 ± 5 63 ± 3
68 ± 4 49 ± 3 54 ± 2
723 50 ± 3 54 ± 5
30B has two hydroxyethyl groups attached to the quaternary ammonium, which are expected to be more readily wetted by the water. However, a similar result has also been observed by Pegoretti et al. [36] on compressed nanoclay disks. The more hydrophobic Cloisite 15A particles are clearly wetted the least by the reference liquids. Based on the contact angle data, the unknown solid surface free energy rs and its three unknown components þ rLW s ; rs and rs can be calculated according the approach of van Oss et al. [27]. Data of three reference liquids, in combination with Eqs. (1) and (2), in which h is the contact angle, is sufficient, though a proper selection of reference liquids is required to obtain accurate results. According to Della Volpe et al. [29], the triplet water-diiodomethane-ethanediol indeed leads to a well-conditioned system. The results of the calculation are shown in Table 4.
rl ð1 þ cos hÞ ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffi
LW rLW þ rþs rl þ rs rþl s rl
pffiffiffiffiffiffiffiffiffiffiffiffi
rs ¼ rLW þ 2 rþs rs s
ð1Þ ð2Þ
The results in Table 4 are in line with the available literature data [22–24], although the absolute values differ to some extent and should be handled with care. It is however safe to conclude that, even though a different organic modifier is used and at a different concentration, the surface free energies of Cloisite 10A and 30B are similar. The surface free energy of Cloisite 15A is significantly lower than that for both Cloisite 10A and 30B. This is probably due to the absence of a polar group attached to the quaternary ammonium of the organic modifier in Cloisite 15A. To predict the dispersion quality of the nanoclays in the polyol and the behavior of the PU-laden foams, the wetting coefficient xlv of the nanoclays between the polyol and air is calculated. The wetting coefficient xlv is defined in Eq. (3), in which the subscript l denotes the polyol and the subscript v denotes the surrounding air phase. The measured interfacial tension of the polypropylene based polyol (rl = 32.5 mN/m), with unknown surface free energy components, is close to the interfacial tension of dipropyleneglycol (see Table 2). Hence, dipropyleneglycol is used as a representative polyol to calculate the wetting coefficient. However, only the surface free energy components required in the Owens–Wendt approach are known for this liquid. The surface free energy is thus recalculated with the Owens–Wendt approach (see Eq. (4)) [28], despite its lower accuracy [27]. This method allows determining the polar rps and dispersive rds part of the surface free energy of the solid and hence the total surface free energy of the solid rs (see Eq. (5)) when contact angle data of at least two reference liquids are available. As contact angle measurements are prone to multiple sources of error [37], the
Table 4 Surface free energy of Cloisite 15A, 10A and 30B according to the approach of van Oss, Good and Chaudhury.
rs (mN/m) rLW rs (mN/m) (mN/m) rþ s (mN/m) s Cloisite 15A Cloisite 10A Cloisite 30B
29 34 35
27 32 32
0.3 0.2 0.3
3.9 8.1 6.1
R. Van Hooghten et al. / European Polymer Journal 60 (2014) 135–144 Table 5 Surface free energy and wetting coefficient of Cloisite 15A, 10A and 30B according to the approach of Owens–Wendt.
Cloisite 15A Cloisite 10A Cloisite 30B
rs (mN/m)
rds (mN/m)
rps (mN/m)
x lv
28 35 34
21 22 23
7 13 11
0.8 1 1
data of all three reference liquids is used here and Eqs. (4) and (5) are solved with a least squares procedure with MatlabÒ. The results are summarized in Table 5.
rsl rsv r lv qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi rl ð1 þ cos hÞ ¼ 2 rds rdl þ rps rpl
ð4Þ
rs ¼ rps þ rds
ð5Þ
xlv ¼
ð3Þ
A comparison of the surface free energies obtained with the approaches of van Oss, Good and Chaudhury on the one hand and Owens–Wendt on the other hand leads to the conclusion that both methods give similar results, though the numerical values differ slightly. We can however conclude that the wetting coefficient using the Owens–Wendt approach leads to reasonable and valid results. These values are therefore used to calculate the wetting coefficient of the nanoclays between polyol and air. When xlv < 1, the nanoclays tend to reside in the polyol, whereas when 0 6 jxlv j 6 1, the nanoclays are likely to adsorb at the polyol-air interface. The calculated values of the wetting coefficient in Table 5 show that Cloisite 10A and 30B should behave in a similar manner, whereas Cloisite 15A could behave differently in the polyol and the final PU foam. During foaming the Cloisite 15A particles are expected to get trapped at the interface in the cell walls, whereas Cloisite 10A and 30B are expected to localize in the PU matrix phase. 3.2. Dispersion quality It was recently shown that at constant surface free energy, the dispersion quality of nanoclays in the polyol before foaming can have an influence on the thermal and mechanical properties of the PU foams [7]. In order to understand the effect of the surface free energy of the nanoclays on the mechanical properties of PU foams, the state of dispersion in the polyol needs to be analyzed. Rheology in the linear viscoelastic regime can be used as a tool to asses the dispersion quality in a qualitative [38–41] or more quantitative [42,43] manner. The polyol-clay dispersions were prepared by solution mixing with THF. Due to its polarity, THF is expected to be effective in dispersing the organophilically modified montmorillonites [24]. Furthermore, THF is compatible with the polyol; it has a low viscosity which increases the efficiency of the ultrasonication procedure and it has a relatively low boiling point making it readily removable from the mixture by evaporation. Fig. 1 shows the frequency dependence of the storage modulus G0 and the loss modulus G00 in the linear viscoelas-
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tic regime for different clay concentrations of the three types of clay. The response of the polyol matrix is also shown. The polyol behaves as a simple Newtonian liquid with terminal behavior across the entire measured frequency range, as indicated by the solid lines: G00 has the expected slope of 1 versus x and G0 has a slope of 1.8. This is very close to the expected slope of 2, but due to the large difference between G0 and G00 an accurate measurement of the phase angle is difficult. As the clay concentration increases, both G0 and G00 increase across the entire frequency range and a plateau starts to develop at low frequencies. This is an indication of a liquid-to-solid transition and the formation of a percolating network of clay particles. At high frequencies, the matrix contribution dominates and the response of G00 becomes parallel to the response of the neat matrix material. As the matrix material flows through the small interstitial space at high clay concentrations, it experiences a high local strain leading to an increased dissipation. At concentrations of 7 wt% or higher, the rheological measurements becomes difficult and some wall slip effects are seen, mainly at the highest frequencies probed. The liquid-to-solid transition in Fig. 1 is a typical characteristic of suspensions with attractive interactions between the particles when the volume fraction of particles or when the attractive interaction strength is increased. A universal scaling of the dynamic moduli above the liquid-to-solid transition exists, taking into account both concentration and interaction strength effects [44,45] by using the cross over frequency xc and the cross over modulus Gc as scaling factors. Fig. 2 shows a mastercurve of the frequency dependent dynamic moduli for the three clays at different concentrations (data of Fig. 1). This scaling has also been successfully applied to other polymer–clay nanocomposites [46,47]. It assumes that the elasticity stems from the formation of a weak network, characterized by a frequency independent modulus, which dominates at low frequencies. The dissipative contribution comes from the viscous matrix, which is confined between the particles, increases linearly with x and is independent of the particle volume fraction and attraction strength. In this picture, the viscosity of the matrix dominates at the highest frequencies. This is clearly seen in Fig. 2, where the G00 data at high frequencies coincide with the viscosity data of the polyol matrix. As a consequence, the moduli scale with the viscosity of the matrix liquid. The insert also shows the linear dependence of the scaling factors, confirming the validity of the scaling model. This scaling or time-concentration superposition allows expanding the experimentally attainable frequency range and determining the value of the plateau modulus for low concentration samples which otherwise cannot be measured. Furthermore, the scaling factors themselves can be used to assess the dispersion quality, at least in a qualitative way [43]. The Cloisite 15A samples have the lowest Gc and xc , whereas the Cloisite 30B samples have the highest, indicating that the Cloisite 30B clay particles are, as expected, dispersed the best in the polyol. To evaluate the state of dispersion quantitatively, the plateau values of G0 at low frequencies can be used [42,43]. Fig. 3 shows these plateau values as a function of
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Fig. 1. Frequency dependent response of the dynamic moduli of the polyol-clay nanocomposites with different clay concentrations at T = 25 C and c = 0.1%: (a) Cloisite 15A (b) Cloisite 10A (c) Cloisite 30B. Storage modulus G0 : filled symbols and loss modulus G00 : open symbols.
concentration for the three different clays. Close to the percolation threshold, the elastic properties can be scaled as:
G0 ð/ /p Þm
ð6Þ
with /p the percolation threshold and m a power law exponent [48–50]. The lines in Fig. 3 show this scaling relation. For the case of layered particles, a lower percolation threshold implies a better dispersion. Indeed, when the layered particles get more intercalated or exfoliated, the tactoids or individual platelets occupy a larger hydrodynamic volume and will hence form a percolating network at lower particle concentrations. The power law exponents decrease as a more open structure is obtained when the
clay particles are better dispersed. Table 6 shows the percolation threshold and the power law exponent for the three different clays. The percolation threshold data lead to the following order in dispersion quality: Cloisite 30B > Cloisite 10A > Cloisite 15A. This is in accordance with the surface free energy data and the calculated wetting coefficients, which show that Cloisite 15A is indeed the least compatible with the polyol matrix. The percolation threshold in Table 6 has also been calculated in units of vol%, assuming a clay density of 1.8 g/ cm3, to be able to compare with literature results. The Cloisite 30B samples have a percolation threshold of the order of 1 vol%, which is in agreement with other well dispersed montmorillonite nanocomposites [42,51,52]. With model
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Fig. 2. Mastercurve of the dynamic moduli for different clays at different concentrations as shown in Fig. 1. The solid line represents the viscosity of the polyol matrix. The insert shows the scaling parameters xc and Gc . Storage modulus G0 : filled symbols and loss modulus G00 : open symbols.
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as compared to the aspect ratio of a single platelet of a tactoid (100–200). Rheology however determines a hydrodynamic aspect ratio rather than a geometric one, measured with electron microscopy [55]. This hydrodynamic ratio will hence always be lower than the geometric one for anisotropic particles. Secondly, less exfoliation also leads to a lower average aspect ratio. From this, it can be concluded that the clays are mainly present as intercalated tactoids for the Cloisite 15A and 10A samples. The Cloisite 30B samples however are expected to have a large fraction of exfoliated clay platelets in combination with intercalated tactoids. No large scale aggregates of clay particles are hence expected. Optical micrographs also do not indicate the presence of large scale aggregates. It is thus found that Cloisite 30B clay is the most compatible with the polyol, probably due to presence of the OH-groups on the organic modifier, leading to enhanced exfoliation. 3.3. Mechanical properties of the PU foams
Fig. 3. Low frequency plateau storage modulus of polyol-Cloisite 15A– 10A–30B nanocomposites at different clay concentrations. Data of G0 taken from Fig. 1 at a frequency of 0.016 rad/s. The lines represent the best fit to Eq. (6).
Table 6 Percolation threshold /p and power law exponent m of Eq. (6) fitted to the data of Fig. 3.
/p (wt%) /p (vol%)
m
Cloisite 15A
Cloisite 10A
Cloisite 30B
8.3 4.8 2.37
4.1 2.3 2.35
1.2 0.7 2.31
laponite particles in PEO, Loiseau and Tassin [53] even obtained a critical volume fraction as low as 0.2–0.4 vol%. To analyse the microstructure and the degree of intercalation and exfoliation, Ren and Silva [38] proposed an expression to relate the percolation threshold with the number of silicate layers in a tactoid. This expression was revised by Vermant et al. [42] to derive the average aspect ratio of the tactoids Af :
Af ¼
3/pR ; 4/p
ð7Þ
with /pR the percolation threshold volume fraction for randomly packed overlapping spheres (/pR = 0.30 [54]). Eq. (7) leads to an average tactoid aspect ratio of 5 for the Cloisite 15A samples, 10 for the Cloisite 10A samples and 33 for the Cloisite 30B samples. These values appear to be rather low,
Foams loaded with 3 wt% clay were prepared to study the effect of the surface free energy of the nanoclays on the mechanical properties of rigid PU foams. Fig. 4 shows SEM images of these foams. It is seen that the cellular structure is similar for the neat PU, the Cloisite 15A and the Cloisite 10A samples. These foams show mainly closed cells. The Cloisite 30B samples however have smaller and more isotropic cells. As foams are prepared by free foaming, cells are more elongated in the foaming direction and anisotropy in the foam properties is expected. The cell size and cell anisotropy have been quantified with image analysis and are summarized in Fig. 5. The cell anisotropy is on average 1.3 for all samples, except the Cloisite 30B samples which have more isotropic cells. Moreover, the Cloisite 30B samples have on average the smallest cells. The Cloisite 15A samples also have smaller cell than the neat PU foam, whereas the Cloisite 10A samples have on average larger cells than the neat PU foam. The dynamic shear modulus G is considered to be a key mechanical property [26]; it was measured with a torsion rectangular geometry. This technique has also been successfully applied to measure the shear modulus of anisotropic polymer composite materials [56]. The frequency sweep measurements showed that the foams behave as an elastic solid in the linear regime (data not shown). As no frequency dependency is found, only average data are reported. To exclude density effects on the mechanical properties, the shear modulus is normalized with the square of the density q, as G is predicted to scale with q2 [57]. This relative shear modulus for the different clay samples is shown in Fig. 5 for samples cut parallel (k) and perpendicular (?) to the foaming direction. The error bars in Fig. 5 of the relative shear moduli are determined based on the standard deviations of at least three measurement for each type of foam. A higher modulus in the k direction than in the ? direction is observed for all samples, except for the Cloisite 30B samples. Gibson and Ashby [57] modeled the anisotropy of the shear modulus with the cell anisotropy R for open cell foams (see Eq. (8)), with Gk and G? the shear modulus of the samples cut respectively in the k and ? direction. Though the model cannot be used
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(a) Neat
(b) 15A
(c) 10A
(d) 30B
Fig. 4. SEM images of the neat PU foam and PU foams loaded with 3 wt% clay.
Fig. 5. Relative shear modulus, cell size and cell anisotropy of the PU foams: neat PU and PU loaded with 3 wt% Cloisite 15A, 10A and 30B.
quantitatively, it describes the anisotropy seen in G for the neat PU foam, the Cloisite 15A and 10A samples and it explains why the Cloisite 30B foam behaves more isotropically.
G? 2 ¼ Gk 1 þ R
ð8Þ
The neat PU foam has a relative shear modulus of 1.2 kPa m6/kg2 in the k direction, which, with a nominal density of 150 kg/m3, corresponds to a shear modulus G of 27 MPa. This is already a very high shear modulus for rigid PU foams even without the presence of the nanoclays [58]. From Fig. 5, only the foams prepared with 3 wt% Cloisite 15A show an improvement of Gk , with an increase of 12% for the relative shear modulus, which is a relatively large
increase for clay-laden rigid PU foams. The presence of Cloisite 10A and 30B on the contrary does not affect or slightly decreases Gk . For the Cloisite 30B samples, this is most probably related to a negative effect of the presence of nanoclays on the matrix strength. The organic modifier of the Cloisite 30B clays has hydroxyl groups which can react with the isocyanate. This tethered clay particles may interfere with the hydrogen bond formation in the PU foam, reducing the strength of the PU matrix [3]. Despite the large surface area available for heterogeneous nucleation and the resulting beneficial cellular structure, no real improvement of G is therefore seen. The Cloisite 10A samples on the other hand have larger cells than the neat PU foam, despite the presence of the particles, which act as a nucleation agent. This might be due to a change in foaming kinetics, where the particles may speed up the blowing agent generation or slow down the cross linking reaction [8,12]. Both effects would lead to less stable cells during cell growth and hence larger cells. This cellular structure has a negative influence on the shear modulus. In spite of the lowest dispersion quality in the Cloisite 15A samples, the best mechanical properties are seen for these samples. Moreover these samples have, on average, smaller cells than the neat PU foam. We hypothesize that this is due to the increased cell stability during the growth phase. As shown in Section 3.1, the Cloisite 15A particles are thermodynamically favored to localize at the interface in the cell wall during foaming, thereby ensuring an increased cell wall stability against collapse, coalescence and coarsening, as is also seen in aqueous foams and emulsions [59] and polymer blends [19]. Secondly, the strength of the PU matrix is affected less by the presence the clay
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particles as the total surface area of the clay in contact with the PU matrix is lower due to the interfacial adsorption and the lower dispersion quality. The net effect of the addition of the Cloisite 15A particles is an increase of the relative shear modulus. 4. Conclusions The improvement of the mechanical properties of rigid PU foam by incorporation of organically modified nanoclay particles turned out not to be trivial. Knowledge of the surface free energy characteristics of the nanoclays is crucial in understanding the properties of PU foams. First, the surface free energy determines the degree of dispersion of the nanoclays. As expected, nanoclays with a high compatibility with the matrix material show a higher degree of exfoliation. This improves in turn the nucleation rate during foaming leading to more and smaller cells. However, a better dispersion does not imply better mechanical properties of the final foam. The addition of nanoclay can lead to a reduction of the shear modulus of the PU matrix material, which cancels out the beneficial effects of these nanoclays. Nanoclay particles with the appropriate surface free energy, which are expected to get trapped at the interface, show less exfoliation during dispersion. Nevertheless, these particles can stabilize the cells during cell growth, leading to a better cellular structure, while having a smaller effect on the matrix properties. The net effect can lead to an improvement in the mechanical properties, even though the particle dispersion is less optimal. Hence, the surface free energy of nanofillers incorporated into rigid PU foams is a key parameter in rationally designing foamed materials as it determines the dispersion quality and the localization of these nanofillers and the resulting mechanical properties. Acknowledgements R.V.H. is grateful for financial support from the European Union Seventh Framework Program (FP7/20072013) project NANCORE under Grant No. NMP214148. Financial assistance from MINECO (MAT2009-14001-C0201 and MAT 2012-34901) and FPU grant AP-2008-03602 (S. Estravis) is gratefully acknowledged. Recticel is acknowledged for providing the PU foam. References [1] Lee LJ, Zeng C, Cao X, Han X, Shen J, Xu G. Polymer nanocomposite foams. Compos Sci Technol 2005;65:2344–63. [2] Ibeh CC, Bubacz M. Current trends in nanocomposite foams. J Cell Plast 2008;44:493–515. [3] Cao X, Lee LJ, Widya T, Macosko CW. Polyurethane/clay nanocomposites foams: processing, structure and properties. Polymer 2005;46:775–83. [4] Widya T, Macosko CW. Nanoclay-modified rigid polyurethane foam. J Macromol Sci Phy 2005;44:897–908. [5] Pardo-Alonso S, Solorzano E, Estravis S, Rodriguez-Perez MA, De Saja JA. In-situ evidences of nanoparticle nucleation effect in polyurethane-nanoclay foamed systems. Soft Matter 2012;8: 11262–70. [6] Pardo-Alonso S, Solorzano E, Rodriguez-Perez MA. Time-resolved Xray imaging of nanofiller-polyurethane reactive foam systems. Colloid Surface A 2013;438:119–25.
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Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Understanding the effect of particle surface free energy on the structural and mechanical properties of clay-laden rigid polyurethane foams Rob Van Hooghten a, Sarah Gyssels a, Sergio Estravis b, Miguel Angel Rodriguez-Perez b, Paula Moldenaers a,⇑ a b
Department of Chemical Engineering, KU Leuven, University of Leuven, Willem de Croylaan 46, B-3001 Heverlee, Belgium Cellular Materials Laboratory (CellMat), Condensed Matter Physics Department, Paseo de Belen 7, Campus Miguel Delibes, Valladolid, Spain
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 11 August 2014 Accepted 29 August 2014 Available online 16 September 2014 Keywords: Polyurethane Foam Nanoclay Surface free energy Shear modulus
a b s t r a c t The surface free energy of nanoclays is shown to be a key parameter in controlling the localization of the particles in the polyurethane matrix, and the structural and mechanical properties of nanoclay laden rigid polyurethane foams. The surface free energy determines the dispersion quality of the nanoclays before foaming, which in turn determines the available surface area for heterogeneous nucleation during foaming. It has been found that particles showing the best dispersion quality do not necessarily lead to the best mechanical properties of the foams. On the contrary, the nanoclays which are likely to get trapped at the polyurethane-air interface during foaming, yield improved mechanical properties as compared to the neat polyurethane foam, despite their less favorable dispersion. The interfacial adsorption stabilizes the cell wall during cell growth, leading to smaller cells after rigidification, as compared to the nonadsorbing nanoclays. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polymer foams form a class of lightweight materials with typical applications in insulation, cushioning and packaging. High porosity foams with interconnected pores are also used as tissue engineering scaffolds. Applications as a structural material are however limited due to their low mechanical strength and low dimensional stability [1]. The addition of nanoparticles to polymers and polymer foams offers an efficient way to increase their mechanical, thermal, barrier and flame retardant properties [2]. Especially nanoclays have been studied in polymer nanocomposities and foamed nanocomposites as they are readily available with different organic modifiers ⇑ Corresponding author. E-mail address: [email protected] (P. Moldenaers). http://dx.doi.org/10.1016/j.eurpolymj.2014.08.029 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.
and have large intercalation and exfoliation possibilities. The large aspect ratio of the individual clay platelets implies a large specific surface area to interact with the matrix material. Hence, only small amounts of nanoclays need to be added to improve the properties, at least if they are dispersed properly. Polyurethane (PU) foams, with or without nanoparticles, have a large diversity in chemistry, density and applications. In general, a PU foam consists of five main components: polyol, isocyanate, blowing agent, catalysts and surfactants. PU foams are prepared by blending the polyol with the surfactants, the blowing agent and the catalysts. In a subsequent step, the isocyanate is added after which foaming initiates. Foaming proceeds by two chemical reactions that occur simultaneously. The isocyanate reacts with the blowing agent in a first reaction. When water is used as a blowing agent, CO2 is generated which
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blows the foam. The cross linking reaction between the polyol and the isocyanate solidifies the foam and stabilizes the foam morphology. The final foam properties such as density and cell size are a strong function of the balance between the kinetics of these two reactions. Depending on the cross linking degree, PU foams are divided into flexible and rigid PU foams. Flexible PU foams have a low cross linking degree and usually have a high open cell content. These soft foams are preferentially used in cushioning applications. The rigid PU foams, with a higher degree of cross linking, are mostly closed cell foams and are used in insulation applications. The addition of nanoclays to PU foams is known to directly influence foam morphology, usually leading to smaller cell sizes [3–5]. The nanoclays act as a heterogeneous nucleation agent [6,7] or as a catalyst modifying the reaction kinetics [8]. The mechanical properties, often measured as compressional properties, are indirectly affected. However, the effect of the presence of nanoclays are not yet fully understood as opposite results are reported for flexible and rigid PU foams reinforced with montmorillonite based clays. Since rigid PU foam intrinsically have a higher mechanical strength, it appeared in general more difficult to increase the properties of rigid PU foams than those of flexible ones. Krishnamurthi et al. [9] incorporated 5 wt% clay in a flexible PU foam and observed an increase of the compressional strength, the same property decreased for a rigid PU foam. Similarly, Cao et al. [3] observed an increase in the compressive properties of flexible PU foams upon the addition of Cloisite 30B nanoclay, whereas the compressive properties reduced for a more rigid PU foam. This observation was explained by the chemistry of the PU foam and the clay. Hydroxyl groups on the clay surface can react with the urethane group and can interfere with the H-bond formation in the matrix, reducing the strength of the matrix. This led to an overall reduction in strength of the foam, despite the beneficial effect of the nanoclays on the cell size. These results on rigid PU foams were confirmed by Widya and Macosko [4]. The mechanical properties of both flexible and rigid PU foams depend strongly on the type of clay used. The compressive strength of a flexible PU foam was seen to increase upon addition of untreated Cloisite-Na+ nanoclay, whereas it diminished upon the addition of the organically modified Cloisite 10A [10]. Harikrishnan et al. [11] studied both flexible and rigid PU foams using four types of organically modified Cloisite clay. In this specific investigation, the mechanical properties were reduced for all flexible PU foams, while only for two rigid PU foams, at a specific clay concentration, an increase was observed. Mondal and Khakhar [8] observed an increase in the compressional modulus parallel to the foaming direction for rigid PU foams, while no effect was seen on the modulus perpendicular to the foaming direction upon addition of Cloisite Na+ and Cloisite 30B. Simultaneously, they observed more elongated cells in the foaming direction. They attributed this and the increase of the mechanical properties to a catalytic effect of the nanoclays accelerating the foaming reaction. Patro et al. [12] also observed an increase of the reaction rates and better mechanical
properties, for vermiculite based clays as compared to montmorillonite based clays. The type of nanoparticles, more specifically their surface free energy, has also been found to strongly affect the properties of polymer nanocomposites and polymer blends. Kamal et al. [13] showed that the properties of polystyrene and high density polyethylene clay nanocomposites depend on the state of dispersion of the clays and the surface free energy of the clays. The same conclusion was drawn by Xia and Song [14], who dispersed four types of clay in a PU matrix and found only for Cloisite 30B exfoliated clay layers. Similar results on other nanocomposite systems have been reported [15–17]. In immiscible polymer blends, the role of surface free energy has been shown to be of prime importance. Particles can adsorb at the interphase between the two polymers and stabilize the morphology against coalescence [18] and other morphological changes if the particle is wetted by both phases [19]. Recently, this result has been extended to polymer foams and to polymers, which are normally unfoamable [20]. Though a few studies have used different types of clay in PU foams and studied their properties, no systematic link with the clay surface free energy and their wetting properties was made to interpret the results. One reason for this is that surface free energies are difficult to measure and several research groups have reported widely different values for montmorillonite based clays [13,15,21–24]. The surface free energies of solids are calculated from contact angle measurements with reference liquids. However, clays are provided in a powdery form and contact angle measurements become cumbersome due to the porosity of the compressed clays. The reported results depend strongly on the method used and the experimental details. For example, reported surface free energies for Cloisite 15A range from 19.3 mN/ m [22] to 42.5 mN/m [21], both measured with the sessile drop technique, and a value of 25.4 mN/m was measured with the capillary rise technique [24]. Similarly, for Cloisite 10A, Kamal et al. [13] reported a value of 45.3 mN/ m, whereas 27.6 mN/m is reported by Bhattacharya et al. [22] and 30.0 mN/m by Burgentzle et al. [24]. Thus, for these two types of clay, which differ strongly in hydrophobicity according to the manufacturer [25], a relatively large range of surface free energies is reported. Hence, there is a clear need to characterize the surface free energies of these clays in a uniform, reproducible and reliable manner in order to link these values to the foam structure and properties. In summary, the goal of the present work is to systematically study the effect of clay surface free energy on the structural and mechanical properties of rigid PU foams. For this purpose, the surface free energy of three different Cloisite nanoclays is measured in an uniform way. A relation between the free surface energy of the clays and the mechanical properties of the PU foams is sought. A simple rheological test is used to measure the shear modulus of the foams. This modulus is used as the representative mechanical property as it is a relevant property in many design applications, but often not reported [26].
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Table 2 Surface free energy components of the reference liquids and a representative polyol according to the approach of Owens–Wendt [28] (Data from [30,31]).
2. Materials and methods 2.1. Materials A rigid closed cell PU foam from Recticel is used here to study the effect of the surface free energy of nanoclay particles. The PU foam is a blend of a polypropylene glycol based polyol and methylene diphenyl diisocyanate, with a nominal foam density (in free foaming) of 150 kg/m3. Three different organoclays, based on natural montmorillonite clays modified with an organic quaternary ammonium salt from Southern Clays, were used as nanofiller. They are in order of increasing hydrophobicity [25]: Cloisite 30B, Cloisite 10A and Cloisite 15A. The nanoclays were dried in a vacuum oven at 80 C for at least 24 h before use. The nanoclays were dispersed in the polyol by solution mixing with tetrahydrofuran (THF, Sigma Aldrich, 99%) before foaming. First, 2.5% by weight of the nanoclays were premixed with THF with a spatula. This mixture was then ultrasonicated for 30 min with an ultrasonic tip (Ikasonic, UP400S) with an amplitude of 50% at a frequency of 70 kHz. The required amount of polyol was subsequently slowly added while mixing with a magnetic stirrer and by ultrasonication during 15 min. The THF was finally removed with a rotary evaporator (Buchi, Rotavapor R215). This procedure did not affect the polyol as was checked with rheology. Foams were prepared by first adding the precise amounts of blowing agent (water) and catalyst. Foaming is initiated by adding the isocyanate while stirring at 1200 rpm (Ika Eurostar Power equipped with a Lenart Disc) during 15 s.
2.2. Clay characterization The surface free energy of the nanoclays is determined from contact angle measurements with three reference liquids: deionized water (Sartorius Arium 611 DI, conductivity = 0.55 lS/cm), diiodomethane (Sigma Aldrich, 99%) and ethanediol (Sigma Aldrich, 99%). The surface free energy of these liquids and their components are summarized in Table 1 according to the approach of van Oss et al. [27]. Table 2 shows the surface free energy of the reference liquids and of a representative polyol according to the approach of Owens–Wendt [28]. The nanoclay powders were compressed into cylindrical disks of 25 mm diameter and 2 mm thickness by compressing at 200 bar in a plate press (Collin P200E) at room temperature. Liquid droplets were deposited on the compressed disk and their volume was stepwise increased. The shape of the droplets was
Table 1 Surface free energy components of the reference liquids and a representative polyol according to the approach of van Oss et al. [27] (Data from [29]). Reference liquid
rl (mN/m)
rLW l
rþl (mN/m)
rl (mN/m)
Deionized water Diiodomethane Ethanediol
72.9 51.2 48.1
22.5 50.9 31.2
28.8 0.47 2.1
10.8 0.04 34.0
Reference liquid
rl (mN/m)
rdl (mN/m)
rpl (mN/m)
Deionized water Diiodomethane Ethanediol Dipropyleneglycol
72.8 50.8 48.8 33.9
22.6 44.1 32.6 29.4
50.2 6.7 16.0 4.5
recorded and analyzed with a contact angle goniometer (CAM200, KSV). The contact angle was determined as the angle between the baseline of the compressed disks and the tangent to the droplets shape at the baseline. At least nine angles were measured for each clay. 2.3. Rheology Rheology is used to study the effect of nanoclay’s surface free energy on the dispersion quality of the nanoclays in the polyol. Rheological measurements were performed on a strain-controlled rheometer (ARES-G2, TA Instruments) equipped with a cone and plate geometry (d = 25 mm, cone angle = 0.02 rad). No sign of wall slip was detected as measurements were independent of the cone angle used. The temperature was controlled with a Peltier element and maintained at 25 C. To erase the mechanical history of the sample and loading effects, a fixed preshear protocol was adopted to obtain a reproducible initial state for the rheological measurements. First, the sample was sheared at 5 s1 during 300 s to break the clay network. The sample was then left at rest during 4000 s to rebuild the clay network. Dynamic strain sweep experiments were performed from 0.1% to 100% at 5 rad/s. The critical strain cc is defined here as the strain where the highest modulus has dropped to 90% of its plateau value. Dynamic frequency sweep experiments were then performed in the linear viscoelastic regime between 0.01 and 100 rad/s. Only a small, negligible difference was observed when performing this experiment from high to low and from low to high frequency, indicating that the 4000 s rebuild time was sufficient to arrive to a reproducible initial state. 2.4. Mechanical properties of the PU foams The shear modulus of the PU foams is measured with dynamic oscillatory measurements on a strain-controlled rheometer (ARES, TA Instruments) equipped with a torsion rectangular geometry. The foams were cut into rectangular bars of 45 10 5 mm3. The dimensions of the sample were determined accurately with a vernier caliper. A torsional deformation is applied to these bars creating a shear deformation in the foams. To probe possible anisotropy of the foams, the samples were cut with the length direction both parallel (k) and perpendicular (?) to the foaming direction. Dynamic strain sweeps experiments at 1 rad/s were performed to determine the linear deformation regime. The shear modulus was then measured by a frequency sweep in the linear regime.
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2.5. Scanning electron microscopy Scanning electron microscopy (SEM, JEOL JSM 820) was used to visualize the cell morphology. Cured foams were cut to obtain a smooth surface and vacuum coated with gold. Image analysis with ImageJ software was performed to measure the cell size and cell anisotropy.
3. Results and discussion 3.1. Surface free energy of nanoclays The determination of the surface free energy of nanoclays is not straightforward. First of all, the surface free energy of a solid material cannot be measured directly as opposed to that of a simple liquid. Instead, the contact angle of the solid material with some reference liquids is measured and in combination with a suitable model, the surface free energy can be estimated. A second complication arises from the fact that the nanoclays are available in a powdery form. Despite the compression of the clay powders at high pressure, the reference liquids may penetrate the porous structure leading to an apparent contact angle, which is smaller than the true equilibrium contact angle [32]. Contact angle measurements on compressed powders with a goniometer are therefore prone to effects of the porosity and surface roughness. Hence, the capillary rise method combined with the Washburn method, is often used to overcome these effects. In this approach, the liquid sorption into the porous structure is in fact used to determine the contact angle [33]. In this method, the porosity of the powder is ‘calibrated’ with a reference liquid which is assumed to be completely wetting. Recently, it has however been shown that this assumption may lead to erroneous results [34]. A second disadvantage is that nonwetting liquids cannot be measured with this technique. In this work, a goniometer is therefore used to measure the contact angle on compressed nanoclay powders. Whenever the volume of a deposited drop decreased due to penetration into the powder, the results were discarded. Table 3 shows the results of the contact angle measurements. The uncertainty is mainly due to surface roughness effects as a result of the compression [35]. Cloisite 10A and 30B show very comparable wetting behavior as the contact angles in Table 3 are very similar. This is in agreement with the manufacturers data that rank these two clays close to each other on the hydrophobicity scale. No statistically relevant difference can be found for the contact angles of diiodomethane and ethanediol. The experimentally measured contact angle of water is slightly lower for Cloisite 10A than for Cloisite 30B. This is not expected based on the molecular structure of the organic modifier, as Cloisite
Table 3 Contact angles of Cloisite 15A, 10A and 30B with the reference liquids. Reference liquid
Cloisite 15A
Cloisite 10A
Cloisite 30B
Deionized water Diiodomethane Ethanediol
82 ± 5 60 ± 5 63 ± 3
68 ± 4 49 ± 3 54 ± 2
723 50 ± 3 54 ± 5
30B has two hydroxyethyl groups attached to the quaternary ammonium, which are expected to be more readily wetted by the water. However, a similar result has also been observed by Pegoretti et al. [36] on compressed nanoclay disks. The more hydrophobic Cloisite 15A particles are clearly wetted the least by the reference liquids. Based on the contact angle data, the unknown solid surface free energy rs and its three unknown components þ rLW s ; rs and rs can be calculated according the approach of van Oss et al. [27]. Data of three reference liquids, in combination with Eqs. (1) and (2), in which h is the contact angle, is sufficient, though a proper selection of reference liquids is required to obtain accurate results. According to Della Volpe et al. [29], the triplet water-diiodomethane-ethanediol indeed leads to a well-conditioned system. The results of the calculation are shown in Table 4.
rl ð1 þ cos hÞ ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffi
LW rLW þ rþs rl þ rs rþl s rl
pffiffiffiffiffiffiffiffiffiffiffiffi
rs ¼ rLW þ 2 rþs rs s
ð1Þ ð2Þ
The results in Table 4 are in line with the available literature data [22–24], although the absolute values differ to some extent and should be handled with care. It is however safe to conclude that, even though a different organic modifier is used and at a different concentration, the surface free energies of Cloisite 10A and 30B are similar. The surface free energy of Cloisite 15A is significantly lower than that for both Cloisite 10A and 30B. This is probably due to the absence of a polar group attached to the quaternary ammonium of the organic modifier in Cloisite 15A. To predict the dispersion quality of the nanoclays in the polyol and the behavior of the PU-laden foams, the wetting coefficient xlv of the nanoclays between the polyol and air is calculated. The wetting coefficient xlv is defined in Eq. (3), in which the subscript l denotes the polyol and the subscript v denotes the surrounding air phase. The measured interfacial tension of the polypropylene based polyol (rl = 32.5 mN/m), with unknown surface free energy components, is close to the interfacial tension of dipropyleneglycol (see Table 2). Hence, dipropyleneglycol is used as a representative polyol to calculate the wetting coefficient. However, only the surface free energy components required in the Owens–Wendt approach are known for this liquid. The surface free energy is thus recalculated with the Owens–Wendt approach (see Eq. (4)) [28], despite its lower accuracy [27]. This method allows determining the polar rps and dispersive rds part of the surface free energy of the solid and hence the total surface free energy of the solid rs (see Eq. (5)) when contact angle data of at least two reference liquids are available. As contact angle measurements are prone to multiple sources of error [37], the
Table 4 Surface free energy of Cloisite 15A, 10A and 30B according to the approach of van Oss, Good and Chaudhury.
rs (mN/m) rLW rs (mN/m) (mN/m) rþ s (mN/m) s Cloisite 15A Cloisite 10A Cloisite 30B
29 34 35
27 32 32
0.3 0.2 0.3
3.9 8.1 6.1
R. Van Hooghten et al. / European Polymer Journal 60 (2014) 135–144 Table 5 Surface free energy and wetting coefficient of Cloisite 15A, 10A and 30B according to the approach of Owens–Wendt.
Cloisite 15A Cloisite 10A Cloisite 30B
rs (mN/m)
rds (mN/m)
rps (mN/m)
x lv
28 35 34
21 22 23
7 13 11
0.8 1 1
data of all three reference liquids is used here and Eqs. (4) and (5) are solved with a least squares procedure with MatlabÒ. The results are summarized in Table 5.
rsl rsv r lv qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi rl ð1 þ cos hÞ ¼ 2 rds rdl þ rps rpl
ð4Þ
rs ¼ rps þ rds
ð5Þ
xlv ¼
ð3Þ
A comparison of the surface free energies obtained with the approaches of van Oss, Good and Chaudhury on the one hand and Owens–Wendt on the other hand leads to the conclusion that both methods give similar results, though the numerical values differ slightly. We can however conclude that the wetting coefficient using the Owens–Wendt approach leads to reasonable and valid results. These values are therefore used to calculate the wetting coefficient of the nanoclays between polyol and air. When xlv < 1, the nanoclays tend to reside in the polyol, whereas when 0 6 jxlv j 6 1, the nanoclays are likely to adsorb at the polyol-air interface. The calculated values of the wetting coefficient in Table 5 show that Cloisite 10A and 30B should behave in a similar manner, whereas Cloisite 15A could behave differently in the polyol and the final PU foam. During foaming the Cloisite 15A particles are expected to get trapped at the interface in the cell walls, whereas Cloisite 10A and 30B are expected to localize in the PU matrix phase. 3.2. Dispersion quality It was recently shown that at constant surface free energy, the dispersion quality of nanoclays in the polyol before foaming can have an influence on the thermal and mechanical properties of the PU foams [7]. In order to understand the effect of the surface free energy of the nanoclays on the mechanical properties of PU foams, the state of dispersion in the polyol needs to be analyzed. Rheology in the linear viscoelastic regime can be used as a tool to asses the dispersion quality in a qualitative [38–41] or more quantitative [42,43] manner. The polyol-clay dispersions were prepared by solution mixing with THF. Due to its polarity, THF is expected to be effective in dispersing the organophilically modified montmorillonites [24]. Furthermore, THF is compatible with the polyol; it has a low viscosity which increases the efficiency of the ultrasonication procedure and it has a relatively low boiling point making it readily removable from the mixture by evaporation. Fig. 1 shows the frequency dependence of the storage modulus G0 and the loss modulus G00 in the linear viscoelas-
139
tic regime for different clay concentrations of the three types of clay. The response of the polyol matrix is also shown. The polyol behaves as a simple Newtonian liquid with terminal behavior across the entire measured frequency range, as indicated by the solid lines: G00 has the expected slope of 1 versus x and G0 has a slope of 1.8. This is very close to the expected slope of 2, but due to the large difference between G0 and G00 an accurate measurement of the phase angle is difficult. As the clay concentration increases, both G0 and G00 increase across the entire frequency range and a plateau starts to develop at low frequencies. This is an indication of a liquid-to-solid transition and the formation of a percolating network of clay particles. At high frequencies, the matrix contribution dominates and the response of G00 becomes parallel to the response of the neat matrix material. As the matrix material flows through the small interstitial space at high clay concentrations, it experiences a high local strain leading to an increased dissipation. At concentrations of 7 wt% or higher, the rheological measurements becomes difficult and some wall slip effects are seen, mainly at the highest frequencies probed. The liquid-to-solid transition in Fig. 1 is a typical characteristic of suspensions with attractive interactions between the particles when the volume fraction of particles or when the attractive interaction strength is increased. A universal scaling of the dynamic moduli above the liquid-to-solid transition exists, taking into account both concentration and interaction strength effects [44,45] by using the cross over frequency xc and the cross over modulus Gc as scaling factors. Fig. 2 shows a mastercurve of the frequency dependent dynamic moduli for the three clays at different concentrations (data of Fig. 1). This scaling has also been successfully applied to other polymer–clay nanocomposites [46,47]. It assumes that the elasticity stems from the formation of a weak network, characterized by a frequency independent modulus, which dominates at low frequencies. The dissipative contribution comes from the viscous matrix, which is confined between the particles, increases linearly with x and is independent of the particle volume fraction and attraction strength. In this picture, the viscosity of the matrix dominates at the highest frequencies. This is clearly seen in Fig. 2, where the G00 data at high frequencies coincide with the viscosity data of the polyol matrix. As a consequence, the moduli scale with the viscosity of the matrix liquid. The insert also shows the linear dependence of the scaling factors, confirming the validity of the scaling model. This scaling or time-concentration superposition allows expanding the experimentally attainable frequency range and determining the value of the plateau modulus for low concentration samples which otherwise cannot be measured. Furthermore, the scaling factors themselves can be used to assess the dispersion quality, at least in a qualitative way [43]. The Cloisite 15A samples have the lowest Gc and xc , whereas the Cloisite 30B samples have the highest, indicating that the Cloisite 30B clay particles are, as expected, dispersed the best in the polyol. To evaluate the state of dispersion quantitatively, the plateau values of G0 at low frequencies can be used [42,43]. Fig. 3 shows these plateau values as a function of
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Fig. 1. Frequency dependent response of the dynamic moduli of the polyol-clay nanocomposites with different clay concentrations at T = 25 C and c = 0.1%: (a) Cloisite 15A (b) Cloisite 10A (c) Cloisite 30B. Storage modulus G0 : filled symbols and loss modulus G00 : open symbols.
concentration for the three different clays. Close to the percolation threshold, the elastic properties can be scaled as:
G0 ð/ /p Þm
ð6Þ
with /p the percolation threshold and m a power law exponent [48–50]. The lines in Fig. 3 show this scaling relation. For the case of layered particles, a lower percolation threshold implies a better dispersion. Indeed, when the layered particles get more intercalated or exfoliated, the tactoids or individual platelets occupy a larger hydrodynamic volume and will hence form a percolating network at lower particle concentrations. The power law exponents decrease as a more open structure is obtained when the
clay particles are better dispersed. Table 6 shows the percolation threshold and the power law exponent for the three different clays. The percolation threshold data lead to the following order in dispersion quality: Cloisite 30B > Cloisite 10A > Cloisite 15A. This is in accordance with the surface free energy data and the calculated wetting coefficients, which show that Cloisite 15A is indeed the least compatible with the polyol matrix. The percolation threshold in Table 6 has also been calculated in units of vol%, assuming a clay density of 1.8 g/ cm3, to be able to compare with literature results. The Cloisite 30B samples have a percolation threshold of the order of 1 vol%, which is in agreement with other well dispersed montmorillonite nanocomposites [42,51,52]. With model
R. Van Hooghten et al. / European Polymer Journal 60 (2014) 135–144
Fig. 2. Mastercurve of the dynamic moduli for different clays at different concentrations as shown in Fig. 1. The solid line represents the viscosity of the polyol matrix. The insert shows the scaling parameters xc and Gc . Storage modulus G0 : filled symbols and loss modulus G00 : open symbols.
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as compared to the aspect ratio of a single platelet of a tactoid (100–200). Rheology however determines a hydrodynamic aspect ratio rather than a geometric one, measured with electron microscopy [55]. This hydrodynamic ratio will hence always be lower than the geometric one for anisotropic particles. Secondly, less exfoliation also leads to a lower average aspect ratio. From this, it can be concluded that the clays are mainly present as intercalated tactoids for the Cloisite 15A and 10A samples. The Cloisite 30B samples however are expected to have a large fraction of exfoliated clay platelets in combination with intercalated tactoids. No large scale aggregates of clay particles are hence expected. Optical micrographs also do not indicate the presence of large scale aggregates. It is thus found that Cloisite 30B clay is the most compatible with the polyol, probably due to presence of the OH-groups on the organic modifier, leading to enhanced exfoliation. 3.3. Mechanical properties of the PU foams
Fig. 3. Low frequency plateau storage modulus of polyol-Cloisite 15A– 10A–30B nanocomposites at different clay concentrations. Data of G0 taken from Fig. 1 at a frequency of 0.016 rad/s. The lines represent the best fit to Eq. (6).
Table 6 Percolation threshold /p and power law exponent m of Eq. (6) fitted to the data of Fig. 3.
/p (wt%) /p (vol%)
m
Cloisite 15A
Cloisite 10A
Cloisite 30B
8.3 4.8 2.37
4.1 2.3 2.35
1.2 0.7 2.31
laponite particles in PEO, Loiseau and Tassin [53] even obtained a critical volume fraction as low as 0.2–0.4 vol%. To analyse the microstructure and the degree of intercalation and exfoliation, Ren and Silva [38] proposed an expression to relate the percolation threshold with the number of silicate layers in a tactoid. This expression was revised by Vermant et al. [42] to derive the average aspect ratio of the tactoids Af :
Af ¼
3/pR ; 4/p
ð7Þ
with /pR the percolation threshold volume fraction for randomly packed overlapping spheres (/pR = 0.30 [54]). Eq. (7) leads to an average tactoid aspect ratio of 5 for the Cloisite 15A samples, 10 for the Cloisite 10A samples and 33 for the Cloisite 30B samples. These values appear to be rather low,
Foams loaded with 3 wt% clay were prepared to study the effect of the surface free energy of the nanoclays on the mechanical properties of rigid PU foams. Fig. 4 shows SEM images of these foams. It is seen that the cellular structure is similar for the neat PU, the Cloisite 15A and the Cloisite 10A samples. These foams show mainly closed cells. The Cloisite 30B samples however have smaller and more isotropic cells. As foams are prepared by free foaming, cells are more elongated in the foaming direction and anisotropy in the foam properties is expected. The cell size and cell anisotropy have been quantified with image analysis and are summarized in Fig. 5. The cell anisotropy is on average 1.3 for all samples, except the Cloisite 30B samples which have more isotropic cells. Moreover, the Cloisite 30B samples have on average the smallest cells. The Cloisite 15A samples also have smaller cell than the neat PU foam, whereas the Cloisite 10A samples have on average larger cells than the neat PU foam. The dynamic shear modulus G is considered to be a key mechanical property [26]; it was measured with a torsion rectangular geometry. This technique has also been successfully applied to measure the shear modulus of anisotropic polymer composite materials [56]. The frequency sweep measurements showed that the foams behave as an elastic solid in the linear regime (data not shown). As no frequency dependency is found, only average data are reported. To exclude density effects on the mechanical properties, the shear modulus is normalized with the square of the density q, as G is predicted to scale with q2 [57]. This relative shear modulus for the different clay samples is shown in Fig. 5 for samples cut parallel (k) and perpendicular (?) to the foaming direction. The error bars in Fig. 5 of the relative shear moduli are determined based on the standard deviations of at least three measurement for each type of foam. A higher modulus in the k direction than in the ? direction is observed for all samples, except for the Cloisite 30B samples. Gibson and Ashby [57] modeled the anisotropy of the shear modulus with the cell anisotropy R for open cell foams (see Eq. (8)), with Gk and G? the shear modulus of the samples cut respectively in the k and ? direction. Though the model cannot be used
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(a) Neat
(b) 15A
(c) 10A
(d) 30B
Fig. 4. SEM images of the neat PU foam and PU foams loaded with 3 wt% clay.
Fig. 5. Relative shear modulus, cell size and cell anisotropy of the PU foams: neat PU and PU loaded with 3 wt% Cloisite 15A, 10A and 30B.
quantitatively, it describes the anisotropy seen in G for the neat PU foam, the Cloisite 15A and 10A samples and it explains why the Cloisite 30B foam behaves more isotropically.
G? 2 ¼ Gk 1 þ R
ð8Þ
The neat PU foam has a relative shear modulus of 1.2 kPa m6/kg2 in the k direction, which, with a nominal density of 150 kg/m3, corresponds to a shear modulus G of 27 MPa. This is already a very high shear modulus for rigid PU foams even without the presence of the nanoclays [58]. From Fig. 5, only the foams prepared with 3 wt% Cloisite 15A show an improvement of Gk , with an increase of 12% for the relative shear modulus, which is a relatively large
increase for clay-laden rigid PU foams. The presence of Cloisite 10A and 30B on the contrary does not affect or slightly decreases Gk . For the Cloisite 30B samples, this is most probably related to a negative effect of the presence of nanoclays on the matrix strength. The organic modifier of the Cloisite 30B clays has hydroxyl groups which can react with the isocyanate. This tethered clay particles may interfere with the hydrogen bond formation in the PU foam, reducing the strength of the PU matrix [3]. Despite the large surface area available for heterogeneous nucleation and the resulting beneficial cellular structure, no real improvement of G is therefore seen. The Cloisite 10A samples on the other hand have larger cells than the neat PU foam, despite the presence of the particles, which act as a nucleation agent. This might be due to a change in foaming kinetics, where the particles may speed up the blowing agent generation or slow down the cross linking reaction [8,12]. Both effects would lead to less stable cells during cell growth and hence larger cells. This cellular structure has a negative influence on the shear modulus. In spite of the lowest dispersion quality in the Cloisite 15A samples, the best mechanical properties are seen for these samples. Moreover these samples have, on average, smaller cells than the neat PU foam. We hypothesize that this is due to the increased cell stability during the growth phase. As shown in Section 3.1, the Cloisite 15A particles are thermodynamically favored to localize at the interface in the cell wall during foaming, thereby ensuring an increased cell wall stability against collapse, coalescence and coarsening, as is also seen in aqueous foams and emulsions [59] and polymer blends [19]. Secondly, the strength of the PU matrix is affected less by the presence the clay
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particles as the total surface area of the clay in contact with the PU matrix is lower due to the interfacial adsorption and the lower dispersion quality. The net effect of the addition of the Cloisite 15A particles is an increase of the relative shear modulus. 4. Conclusions The improvement of the mechanical properties of rigid PU foam by incorporation of organically modified nanoclay particles turned out not to be trivial. Knowledge of the surface free energy characteristics of the nanoclays is crucial in understanding the properties of PU foams. First, the surface free energy determines the degree of dispersion of the nanoclays. As expected, nanoclays with a high compatibility with the matrix material show a higher degree of exfoliation. This improves in turn the nucleation rate during foaming leading to more and smaller cells. However, a better dispersion does not imply better mechanical properties of the final foam. The addition of nanoclay can lead to a reduction of the shear modulus of the PU matrix material, which cancels out the beneficial effects of these nanoclays. Nanoclay particles with the appropriate surface free energy, which are expected to get trapped at the interface, show less exfoliation during dispersion. Nevertheless, these particles can stabilize the cells during cell growth, leading to a better cellular structure, while having a smaller effect on the matrix properties. The net effect can lead to an improvement in the mechanical properties, even though the particle dispersion is less optimal. Hence, the surface free energy of nanofillers incorporated into rigid PU foams is a key parameter in rationally designing foamed materials as it determines the dispersion quality and the localization of these nanofillers and the resulting mechanical properties. Acknowledgements R.V.H. is grateful for financial support from the European Union Seventh Framework Program (FP7/20072013) project NANCORE under Grant No. NMP214148. Financial assistance from MINECO (MAT2009-14001-C0201 and MAT 2012-34901) and FPU grant AP-2008-03602 (S. Estravis) is gratefully acknowledged. Recticel is acknowledged for providing the PU foam. References [1] Lee LJ, Zeng C, Cao X, Han X, Shen J, Xu G. Polymer nanocomposite foams. Compos Sci Technol 2005;65:2344–63. [2] Ibeh CC, Bubacz M. Current trends in nanocomposite foams. J Cell Plast 2008;44:493–515. [3] Cao X, Lee LJ, Widya T, Macosko CW. Polyurethane/clay nanocomposites foams: processing, structure and properties. Polymer 2005;46:775–83. [4] Widya T, Macosko CW. Nanoclay-modified rigid polyurethane foam. J Macromol Sci Phy 2005;44:897–908. [5] Pardo-Alonso S, Solorzano E, Estravis S, Rodriguez-Perez MA, De Saja JA. In-situ evidences of nanoparticle nucleation effect in polyurethane-nanoclay foamed systems. Soft Matter 2012;8: 11262–70. [6] Pardo-Alonso S, Solorzano E, Rodriguez-Perez MA. Time-resolved Xray imaging of nanofiller-polyurethane reactive foam systems. Colloid Surface A 2013;438:119–25.
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