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Thermal properties of epoxy resin nanocomposites based on hydrotalcites
Polymer Degradation and Stability 96 (2011) 164e169
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Thermal properties of epoxy resin nanocomposites based on hydrotalcites A. Frache a, *, O. Monticelli b, M. Nocchetti c, G. Tartaglione a, U. Costantino c a
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, sede di Alessandria, Via T. Michel, 5, 15121 Alessandria, Italy Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso, 31 16146 Genova, Italy c CEMIN-Centro di Eccellenza Materiali Innovativi Nanostrutturati, Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2010 Received in revised form 3 September 2010 Accepted 21 October 2010 Available online 11 November 2010
Epoxy resin nanocomposites containing homeemade hydrotalcites (HTlc) have been prepared and their properties have been studied and compared with those of montmorillonite (MMT)-type layered silicatesbased nanocomposites. Nanofiller dispersion in the polymer matrix has been evaluated by transmission (TEM) electron microscopy and wide angle X-ray diffraction (WAXD), while nanocomposite thermal properties have been studied in detail by thermogravimetric analysis (TGA/DTG) and cone calorimeter tests. The morphological studies have shown that the compatibilisation of the above two type of nanofillers allowed us to obtain nanostructured materials. As far as thermal properties are concerned, nanocomposites based on HTlc are found to decompose, both in air and nitrogen, following a trend similar to that of the neat polymer matrix, while in the case of the nanocomposite based on the organophilic MMT a slight improvement was found in air. Conversely, cone calorimetric tests have demonstrated that only the organophilic hydrotalcite was capable of decreasing the peak of the heat release rate in a relevant way. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Epoxy resin Montmorillonite Hydrotalcite Thermal stability Cone calorimeter
1. Introduction Epoxy resins (ER) show excellent thermal and environment resistance which makes them particularly suitable for electrical and electronic applications [1,2]. However, in case of overheating, the above applications involve risk of fire, leading to volatile combustible products. Traditionally, in order to improve flame retardancy of epoxy resins, halogenated additives are employed [3e6]. Unfortunately, recent studies evidenced health and environmental hazards related to the use of these molecules, which could release, during combustion, corrosive, toxic and super-toxic compounds, such as halogenated dibenzodioxins and dibenzofurans [7]. It is relevant to underline that the enactment of the directive 2002/95/ EG proscribes the use of many of these well proven halogenated additives. On this basis, extensive research work, from both academic and industrial groups, has been accomplished in order to develop novel halogen-free flame retardants, capable to replace the halogen containing additives by providing sufficient flame retardancy [8e12]. Indeed, nowadays, a completely new fire retardant strategy is represented by polymereclay nanocomposites (PCN) [13e16]. In
* Corresponding author. Fax: þ39 131229399. E-mail address: [email protected] (A. Frache). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.10.006
the last two decades, these novel systems have attracted great interest because they often exhibit remarkable improvements of mechanical, thermal and other physicochemical properties as compared to either the neat polymer or conventional composites [17e21]. As far as nanocomposite thermal stability is concerned, the effect of clay has been found to depend essentially on layer distribution in the polymeric matrix, namely the exfoliation has turned out to be the ideal morphology to gain [22]. Nevertheless, a sheared conclusion on the optimal morphology to improve the combustion behaviour of polymereclay composites has not reported so far. Nanocomposites are commonly based on natural layered clay minerals such as montmorillonite (MMT), which can be made organophilic by the exchange of pristine cations with alkyl (aryl) ammonium cations. More recently, together with MMT, different types of nanofillers have been exploited to prepare nanostructured materials. Among these, hydrotalcite-like compounds (HTlc) have been gained increasing interest [23e31]. Indeed, HTlc have a layered double hydroxides structure consisting of layers constituted of M(II) and M(III) hydroxide octahedrons interconnected at the edges. The positively charge of HTlc layers is balanced by anions located into the interlayer galleries. Thus, as the main difference with respect to MMT is the ion balancing charge, they are also referred as anionic clay in analogy with MMT cationic clays. Although HTlc are natural, being their preparation cheap and easy,
A. Frache et al. / Polymer Degradation and Stability 96 (2011) 164e169
generally they are synthesized, in order to finely tune their final features by controlling the preparation parameters. In this work, the characteristics and the thermal properties of nanocomposites, based on epoxy resins and containing both MMT and synthetic HTl-type clays, have been evaluated and compared. While the former layered silicates were commercial, hydrotalcites were homeemade. The composites have been prepared by in situ polymerization method, using both unmodified and modified nanofillers. 2. Experimental 2.1. Materials
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The stoichiometric amount of amine curing agent (27.3 phr) was added to the DGEBA/filler mixtures at 80 C and the mixture was sonicated for 5 min, a time sufficient for complete melting and homogeneous dispersion of DDM. The samples were poured in an aluminium mould and outgassed for 40 min at 40 C in a vacuum oven. Curing was performed in 120 min at 80 C at room pressure to produce the nanocomposites containing 5 wt.% organoclay. Post curing was performed at 140 C for 90 min in vacuum to limit oxidative effects on the polymeric matrix. Unfilled resin was prepared as reference. 2.4. Characterization
The polymer matrix was prepared by using a prepolymer, namely diglycidyl ether of bisphenol-A [DGEBA] (Sigma Aldrich), which was cured by 4,40 -diaminodiphenylmethane [DDM] (Sigma Aldrich). As shown in Table 1, two different kinds of layered nanofillers were used, namely commercial MMT (DelliteÒNa, DelliteÒ26C), kindly purchased by Laviosa Chimica Mineraria, and laboratory made HTlc, both unmodified and organically modified. 2.2. Synthesis and compatibilisation of HTlc fillers The synthetic hydrotalcites [Mg0.67Al0.33(OH)2][CO3]0.165$0.5H2O were prepared by urea method [31]. The corresponding chloride forms were obtained by titrating the carbonate forms, dispersed in a 1 mol/dm3 NaCl solution (1g/50 ml), with a 0.1 mol/dm3 HCl by means of an automatic titrator operating at pH stat mode and pH value of 5.0. To perform the Cl/NO 3 exchange, 1g of the chloride form was suspended in 45 ml of a CO2free aqueous solution 0.5 mol/dm3 of NaNO3 for 24 h. The recovered solid was washed three times with CO2-free de-ionized water and finally dried over a saturated NaCl solution (relative humidity, R.H., of 75%). The intercalation of stearate anions was achieved by suspending, for three days at room temperature, the HTlc in nitrate form in a 0.05 mol/dm3 sodium stearate solution H2O/methanol 1/1 (v/v) (1 g/76 ml). The solid, separated from the solution by centrifugation, was washed two times with H2O/methanol 1/1 (v/v) solution and finally dried over a saturated NaCl solution (R.H. 75%). The Mg and Al content of the hydrotalcite samples was determined by complexometric titration with standard EDTA solutions after dissolution of a weighed amount of the sample (ca. 200 mg) in a few drops of concentrated HCl and successive dilution with water to 100 ml.
Nanocomposites morphology was investigated by means of wide angle X-ray diffractometry (WAXD) and by transmission electron microscopes (TEM). The X-ray diffractometer was a X’TRA 48 Thermo ARL with CuKa radiation (a ¼ 0.15406 nm) operating at 45 kV and 40 mA. Radial scans were recorded in the reflection scanning mode from 1 to 15 C step-size 0.02 at 1 /min scanning rate. TEM measurements were performed using a high-resolution transmission electron microscope (JEOL 2010). Ultrathin sections (about 100 nm thick) of material as received after polymerization were cut by a Power TOME X microtome equipped with a diamond knife and placed on a 200-mesh copper grid. In order to obtain a quantitative measurement of clay dispersion in the polymer matrix for the intercalated nanocomposites, at least 25 images for each sample were examined. Thermogravimetric analysis (TGA) were performed on 10 mg samples by heating from 50 to 800 C at 20 C/min under nitrogen and oxygen flow (30 ml/min) on Al2O3 sample pan using a TA Instrument thermobalance TGA Q500. Combustion studies were performed using an oxygen consumption calorimeter (Fire Testing Technology Limited FFT Cone Calorimeter model). Heat flux: 50 kW/m2, Sample: 75 4 mm and 7 mm thick. Heat Release Rate measurements were carried out in triplicate. Experimental error was evaluated to be 5e10%. DSC measurements, which were carried out in order to define the resin and composite systems crosslinking degree, were performed under a continuous nitrogen purge on a Mettler calorimetric apparatus, mod. TC10A. Data were gathered using a scan rate of 10 C/min from 0 to 300 C. The results obtained demonstrated that by applying the curing conditions previously reported, it was possible to obtained complete crosslinked samples. 3. Results and discussion
2.3. Synthesis of epoxy/clay systems 3.1. Morphological characterization Homogeneous mixtures, DGEBA/nanofiller, were obtained by dispersing finely milled powders (30 O 50 mm) of nanolayered fillers (6.7 phr) in DGEBA. In order to favour epoxy intercalation in the filler interlayer galleries, the mixtures were sonicated at 80 C for 1 h using an ultrasonic-bath AC14 (EMME GI) which has a generator with a 600 W output and a 25 (2) KHz convector. Table 1 Characteristics of the filler used. filler
code
organic modifier
Na-MMT (DelliteÒ) OMMT (DelliteÒ)
DelHPS Del26C
HTlc Mg/Al HTlc Mg/Al
MgAlCl MgAlStea
e methyl tallow bis(2-hydroxyethyl) ammonium Chlorine Stearate
The epoxy composites, based on MMT and HTlc, were prepared by in situ polymerization, by applying a two-step procedure. Indeed, preliminarily the filler underwent a swelling process by the intercalation of the prepolymer, DGEBA, under sonication, while the second step of the method involved the curing of the intercalated prepolymer by a crosslinker, namely DDM. The dispersion of the filler into the polymeric matrix of the composites has been studied by WAXD, analysing the sample both after the swelling of the filler in DGEBA and curing. It is worth underlining that the final morphology of the composites depends on the difference between intra-gallery and extra-gallery polymerization. Generally, if the intra-gallery polymerization is faster, the expansion of the interlayer distances occurs, while the constriction takes places in the opposite case.
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Table 2 summarizes WAXD results, obtained by studying all the samples after each steps of the procedure. Namely, the first column reports the values of the basal spacing (d) of the neat nanofillers while the second and third columns list the d values of the mixture, DGEBA/clay, and of the final composites, respectively. Considering the samples based on DelliteÒ-type clay, it came out that the interlayer distance of Na-MMT does not change both in the system clay/DGEBA after swelling and in the resultant composite, thus demonstrating that the intercalation of the prepolymer and consequently its polymerization does not occur inside the above layered silicate galleries. Conversely, the organically modified clay, namely Del26C, shows an increase of the interlayer distance upon mixing with DGEBA, indicating an intercalation of the prepolymer inside the clay galleries and a resulting formation of an intercalated nanocomposite. Moreover, the basal spacing of Del26C, after swelling and curing, are very similar, thus indicating that the final level of prepolymer and polymer intercalation does not depend on the modifier structure and on the starting value of the clay interlayer distance. In order to further validate and complement the results obtained by means of WAXD, TEM measurements were carried out on the composites prepared. TEM micrograph of the nanocomposite ER-Del26C, based on the clay Del26C, is shown in Fig. 1. Analysing the above micrograph, it is possible to identify some clay layers which show individual dispersion of delaminated sheets, as well as regions where a more regular stacking arrangement is maintained. Although in the latter regions the clay layers are organized in a parallel way, some deformations in this arrangement are visible. Furthermore, in agreement with WAXD results, the basal spacing of Del26C (d ¼ 1.75 nm), turns out to be much higher in the composite (d ¼ 3.56 nm). In Table 2, the d values of the neat HTlc and those of the clays after swelling and curing are also shown. As described for MMT-Na, also in the case of HTlc containing chloride ions (MgAlCl), the very low interlayer distance of the above clay does not increase by the swelling process, thus preventing the final formation of a nanostructured material. Clearly, the hydrophilicity of MgAlCl as well as the limited space inside its galleries make the diffusion of the prepolymer difficult to occur. The hydrotalcite studied, containing stearate anions, shows a behaviour similar to that of the organically modified MMT-type layered silicate. Indeed, as demonstrated by the increase of organically modified HTlc interlayer distance, both during its swelling in DGEBA and after curing, the organophilicity and the considerable interlayer distance of this system turns out to facilitate the diffusion of DGEBA and, consequently its polymerization, inside the clay galleries. Moreover, although hydrotalcite dispersion has been found not to be extremely uniform, some zones of the above sample turn out to be characterized by intercalated structures, as shown in the TEM micrograph reported in Fig. 2.
Fig. 1. TEM micrograph of the sample ER-Del26C based on Del26C.
both in nitrogen and air. Fig. 3 shows TGA curves of ER and of the composites based on hydrotalcite. Considering the thermal decomposition under inert atmosphere (Fig. 3a), it comes out that epoxy resin thermally degrades through one single step process with a DTG peak at 364 C and leaving a residue of 14%. The well known degradation process of the above polymer involves water elimination followed by homolytic scission of the crosslinked structure giving volatile fragments. Volatilization is however limited by fragments recombination and rearrangements such as cyclisation that produces a relatively stable charred structure. The curves related to ER/HTlc composites show a maximum decomposition rate at a temperature similar to that of the neat polymer matrix. As far as the beginning of the decomposition is concerned, it has been found to occur at ca. 350 C for all the samples, namely 10 C below the Tonset of the polymer matrix. In air, the epoxy resin degrades in two steps with the maximum decomposition rate at ca. 375 C and 560 C (Fig. 3b). While ER/ MgAlCl anticipates the first step of its decomposition, ER/MgAlStea shows a thermal behaviour very similar to that of the epoxy resin.
3.2. TGA measurements Thermal properties of the neat epoxy resin and of the composites prepared have been investigated by TGA measurements Table 2 Interlayer distance (d001) of the neat clays, the clay/DGEBA systems and composites. sample code
ER/DelHPS ER/Del26C ER/MgAlCl ER/MgAlStea
d001 (nm) neat clay
mixture (clay/DGEBA)
composite
1.21 1.75 0.77 3.05
1.23 3.46 0.77 5.20
1.23 3.56 0.76 5.50 Fig. 2. TEM micrograph of the sample ER/MgAlStea based on MgAlStea.
A. Frache et al. / Polymer Degradation and Stability 96 (2011) 164e169
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Fig. 3. (a) TG and DTG curves in inert atmosphere of: ER, ER/MgAlCl and ER/MgAlStea; (b) TG and DTG curves in air of: ER, ER/MgAlCl and ER/MgAlStea.
TGA curves, in nitrogen and air, of the composites based on MMT-type clay are reported in Fig. 4. In nitrogen (Fig. 4a), the weight loss of the composites starts at lower temperature, being the Tonset of ER/Del26C 10 C and that of ER/HPS 70 C lower than Tonset of the polymer matrix, while DTG peak is similar for all the samples. Comparing the thermal properties of the composites with that of the epoxy resin in air (Fig. 4b), it comes out that the first degradation step shows a very similar trend to that described in inert atmosphere. However, in the case of ER/Del 26C above 500 C, the degradation process turns out to be delayed. Indeed, the fine organophilic MMT dispersion with respect to that of DelHPS seems to promote a delay of the char thermal oxidation. 3.3. Cone calorimeter tests Flame retardant properties of the neat resin and of the composites have been investigated by cone calorimeter, technique which allows to measure heat release and other reaction-to-fire properties in bench scale. Indeed, our attention has been focused on the peak of heat release rate (PHRR) which is the most relevant parameter controlling flame propagation in fires. Fig. 5 shows cone calorimeter curves of the epoxy resin and of the composites prepared while Table 3 summarizes the results obtained by these measurements, comparing the behaviour of the neat polymer matrix with that of the composites.
Fig. 4. (a) TG and DTG curves in inert atmosphere of: ER, ER/DelHPS and ER/Del26C; (b) TG and DTG curves in air of: ER, ER/DelHPS and ER/Del26C.
By considering the above Figures and Table, it comes out that the samples based on the pristine inorganic fillers, namely DelHPS and MgAlCl, are characterized by a slight higher PHRR than that of the neat epoxy resin. In the case of the composite ER/Del 26C, the above parameter further increases, being the increment of PHRR 20%. Conversely to the above described composites, the samples containing the organically modified hydrotalcite has shown a relevant decrease of PHRR (ca. 30%). Moreover, all the composite samples show a slight higher time to ignition (TTI) than the polymer matrix. Indeed, the better combustion behaviour of ER/MgAlStea with respect to that of the other composites prepared can be explained by taking into account the peculiar behaviour of hydrotalcite during combustion. In order to explain in detail the results, the combustion residues, obtained after cone calorimeter tests, were characterized by WAXD and TEM analysis.
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ER ER/DelHPS ER/Del26C
800 700
2
HRR (KW/m )
600 500 400
Intensity (a.u.)
a
300 200
10
100
0
100
200
300
400
500
Time (s)
ER ER/MgAlCl ER/MgAlStea
700 600 2
40
50
60
70
80
Fig. 6. X-ray diffraction pattern of the sample ER/MgAlStea after cone calorimeter test.
900 800
HRR (KW/m )
30
2θ(°)
0
b
20
500 400 300
the disappearance of the original hydrotalcite layered structure occurs. Together with morphological characterization, EDS analysis allows to evaluate the inorganic phase chemical composition (Fig. 7b) Indeed, the inorganic particles of the sample ER/MgAlStea have been found to be characterized by an uniform composition, containing very similar concentration of Mg and Al. On the basis of the results obtained, we can conclude that, conversely to the MMT-type layered silicates, hydrotalcites are capable to ameliorate the combustion behaviour of the polymer matrix in the scenario of cone calorimeter combustion.
200 100 0 0
100
200
300
400
500
Time (s) Fig. 5. (a) Cone calorimeter curves of: ER, ER/DelHPS and ER/Del26C; (b) ER, ER/ MgAlCl and ER/MgAlStea.
As already reported, both the above techniques show that the composites containing MMT-type layered silicates maintain a quite high crystalline layered structure (results not shown). In contrast, WAXD profiles of the hydrotalcite-based composites indicate that the combustion residues consist mainly of oxides-type compounds (Fig. 6). Indeed, the main reflections observed in the 2q range from 30 to 70 are typical of MgO (ICDD-PDF2 no. 00-0450946) or ZnO (ICDD-PDF2 no 01-075-0576). Moreover, as shown in Fig. 7a, TEM micrographs of the samples ER/MgAlStea, which underwent the combustion treatment, evidences the formation of globular inorganic structures, namely
Table 3 Cone calorimeter results of ER and of the composites. sample code
ER ER/Del HPS ER/Del 26C ER/MgAlCl ER/MgAlStea *D% in comparison to ER
TTI
PHRR
(s)
(kW/m2)
D%*
Residue (%)
53 62 64 63 63
712 768 857 792 491
þ8 þ20 þ11 31
9.5 16.0 11.5 14.5 12.5
Fig. 7. (a) TEM micrograph and (b) EDS analysis of the sample ER/MgAlStea after cone calorimeter test.
A. Frache et al. / Polymer Degradation and Stability 96 (2011) 164e169
Nevertheless, the above material is effective only when the polymer is intercalated into its structure. 4. Conclusions Nanocomposites based on epoxy resin and homeemade hydrotalcites have been prepared by in situ polymerization and their properties have been studied and compared with those of classical montmorillonite-type layered silicates-based nanocomposites. As far as filler dispersion is concerned, it has been verified that it is possibile to obtain nanostructured materials, characterized by the resin intercalation into HTlc galleries, by using a proper exchange of the pristine hydrotalcite with stearate anions. TGA measurements and cone calorimeter tests have demonstrated a peculiar thermal behaviour of the hydrotalcite-based nanocomposites. Indeed, the above materials, have turned out to decompose, both in air and nitrogen, following a trend similar to that of the neat polymer matrix. Nevertheless, cone calorimetric tests have proved that, among all the samples studied, only the organophilic hydrotalcite is capable of ameliorating the combustion behaviour of the polymer matrix in cone calorimeter combustion although characterized by a filler dispersion similar to that of the organically modified MMT-based nanocomposite. Acknowledgments The precious help of Dr. Marcella Pani and Mr. Claudio Uliana in WAXD and TEM measurements is gratefully acknowledged. Prof. Giovanni Camino is gratefully acknowledged for helpful discussions on various aspects of the present work. References [1] Potter WG. Epoxy resins. New York: Springer; 1970. [2] Kinjo N, Ogata K, Keneda A. Epoxy molding compounds as encapsulation materials for microelectronic devices. Adv Polym Sci 1989;88:1e48. [3] Lewin M, Atlas SM, Pearce EM. Flame retardant polymeric materials. New York: Plenum Press; 1975. [4] Mikroyannidis JA, Kourtides DA. Curing of epoxy resins with 1-[di(2-chloroethoxyphosphinyl) methyl]-2,4-and -2,6-diaminobenzene. J Appl Polym Sci 1984;29:197e209. [5] Yang CP, Hsiao SH. Flameproofed polyesters prepared by direct polycondensation of aromatic dicarboxylic acids and brominated bisphenols with tosyl chloride and N, N0 -dimethylformamide in pyridine. J Appl Polym Sci 1988;36:1221e32. [6] Camino G, Costa L. Performance and mechanisms of fire retardants in polymersda review. Polym Degrad Stab 1988;20:271e94. [7] Dumler R, Thoma H. Thermal formation of polybrominated dibenzodioxins (PBDD) and dibenzofurans (PBDF) from bromine containing flame retardants. Chemosphere 1989;19:305e8. [8] Levchik SV, Camino G, Luda MP, Costa L, Muller G, Costes B, et al. Epoxy resins cured with aminophenylmethylphosphine oxide 1: combustion performance. Polym Adv Tech 1996;7:823e30. [9] Zhao C-S, Huang F-Li, Xiong W-C, Wang Y-Z. A novel halogen-free flame retardant for glass-fiber-reinforced poly(ethylene terephthalate). Polym Degrad Stab 2008;93:1188e93.
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[10] Mauerer O. New reactive, halogen-free flame retardant system for epoxy resins. Polym Degrad Stab 2005;88:70e3. [11] Toldy A, Szabó A, Novák Cs, Madarász J, Tóth A, Marosi Gy. Intrinsically flame retardant epoxy resin e fire performance and background e Part II. Polym Degrad Stab 2008;93:2007e13. [12] Levchik SV, Camino G, Luda MP, Costa L, Muller G, Costes B. Epoxy resins cured with aminophenylmethylphosphine oxidedII. Mechanism of thermal decomposition. Polym Degrad Stab 1998;60:169e83. [13] Gautam D, Niranjan K. Vegetable oil-based flame retardant epoxy/clay nanocomposites. Polym Degrad Stab 2009;94:1948e54. [14] Laoutid F, Bonnaud L, Alexandre M, Lopez-Cuesta J-M, Dubois Ph. New prospects in flame retardant polymer materials: from fundamentals to nanocomposites. Mater Sci Eng R 2009;63:100e25. [15] Huang G, Gao J, Li Y, Han L, Wang X. Functionalizing nano-montmorillonites by modified with intumescent flame retardant: preparation and application in polyurethane. Polym Degrad Stab 2010;95:245e53. [16] Suguna Lakshmi M, Narmadha B, Reddy BSR. Enhanced thermal stability and structural characteristics of different MMT-clay/epoxy-nanocomposite materials. Polym Degrad Stab 2008;93:201e13. [17] Yuanqiao Rao, Pochan John M. Mechanics of polymer-clay nanocomposites. Macromolecules 2007;40:290e6. [18] Berta M, Lindsay C, Pans G, Camino G. Effect of chemical structure on combustion and thermal behaviour of polyurethane elastomer layered silicate nanocomposites. Polym Degrad Stab 2006;91:1179e91. [19] Bertini F, Canetti M, Audisio G, Costa G, Falqui L. Characterization and thermal degradation of polypropyleneemontmorillonite nanocomposites. Polym Degrad Stab 2006;91:600e5. [20] Sahoo PK, Samal R. Fire retardancy and biodegradability of poly(methyl methacrylate)/montmorillonite nanocomposite. Polym Degrad Stab 2007;92:1700e7. [21] Monticelli O, Musina Z, Frache A, Bellucci F, Camino G, Russo S. Influence of compatibilizer degradation on formation and properties of PA6/organoclay nanocomposites. Polym Degrad Stab 2007;92:370e8. [22] Costantino U, Gallipoli A, Nocchetti M, Camino G, Bellucci F, Frache A. New nanocomposites constituted of polyethylene and organically modified ZnAlhydrotalcites. Polym Degrad Stab 2005;90:370e8. [23] Sorrentino A, Gorrasi G, Tortora M, Vittoria V, Costantino U, Marmottini F, et al. Incorporation of MgeAl hydrotalcite into a biodegradable poly(3-caprolactone) by high energy ball milling. Polymer 2005;46:1601e8. [24] Zhang Y-T, Fan L-H, Zhi T-T, Zhang L, Huang H, Chen H-L. Synthesis and characterization of poly(acrylic acid-co-acrylamide)/hydrotalcite nanocomposite hydrogels for carbonic anhydrase immobilization. J Polym Sci Part A: Polym Chem 2009;47:3232e40. [25] Lee W-F, Chen Y-C. Effect of hydrotalcite on the physical properties and drugrelease behaviour of nanocomposite hydrogels based on poly[acrylic acid-copoly(ethylene glycol) methyl ether acrylate] gels. J Appl Polym Sci 2004;94:692e9. [26] Ardanuy M, Velasco JI, Maspoch ML, Haurie L, Fernández AI. Influence of EMAA compatibilizer on the structure and properties of HDPE/hydrotalcite nanocomposites prepared by melt mixing. J Appl Polym Sci 2009;113:950e8. [27] Costantino U, Nocchetti M, Sisani M, Vivani R. Recent progress in the synthesis and application of organically modified hydrotalcites. Z Kristallogr 2009;224:273e81. [28] Yan-wu Z, Jun-qing J. Preparation of low-molecular-weight polyamide 6/hydrotalcite intercalated nanocomposites via insitu polymerization. Express Polym Lett 2007;1:98e103. [29] Zammarano M, Franceschi M, Bellayer S, Gilman JW, Meriani S. Preparation and flame resistance properties of revolutionary self-extinguishing epoxy nanocomposites based on layered double hydroxides. Polymer 2005;46:9314e28. [30] Hornsby P. Fire retardant fillers. In: Wilkie ChA, Morgan AB, editors. Fire retardancy of polymeric materials. 2nd ed. CRC Press; 2010. p. 180. [31] Costantino U, Marmottini F, Nocchetti M, Vivani R. New synthetic Routes to hydrotalcite-like compounds, Characterisation and properties of the obtained materials. Eur J Inorg Chem; 1998:1439e46.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Thermal properties of epoxy resin nanocomposites based on hydrotalcites A. Frache a, *, O. Monticelli b, M. Nocchetti c, G. Tartaglione a, U. Costantino c a
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, sede di Alessandria, Via T. Michel, 5, 15121 Alessandria, Italy Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso, 31 16146 Genova, Italy c CEMIN-Centro di Eccellenza Materiali Innovativi Nanostrutturati, Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2010 Received in revised form 3 September 2010 Accepted 21 October 2010 Available online 11 November 2010
Epoxy resin nanocomposites containing homeemade hydrotalcites (HTlc) have been prepared and their properties have been studied and compared with those of montmorillonite (MMT)-type layered silicatesbased nanocomposites. Nanofiller dispersion in the polymer matrix has been evaluated by transmission (TEM) electron microscopy and wide angle X-ray diffraction (WAXD), while nanocomposite thermal properties have been studied in detail by thermogravimetric analysis (TGA/DTG) and cone calorimeter tests. The morphological studies have shown that the compatibilisation of the above two type of nanofillers allowed us to obtain nanostructured materials. As far as thermal properties are concerned, nanocomposites based on HTlc are found to decompose, both in air and nitrogen, following a trend similar to that of the neat polymer matrix, while in the case of the nanocomposite based on the organophilic MMT a slight improvement was found in air. Conversely, cone calorimetric tests have demonstrated that only the organophilic hydrotalcite was capable of decreasing the peak of the heat release rate in a relevant way. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Epoxy resin Montmorillonite Hydrotalcite Thermal stability Cone calorimeter
1. Introduction Epoxy resins (ER) show excellent thermal and environment resistance which makes them particularly suitable for electrical and electronic applications [1,2]. However, in case of overheating, the above applications involve risk of fire, leading to volatile combustible products. Traditionally, in order to improve flame retardancy of epoxy resins, halogenated additives are employed [3e6]. Unfortunately, recent studies evidenced health and environmental hazards related to the use of these molecules, which could release, during combustion, corrosive, toxic and super-toxic compounds, such as halogenated dibenzodioxins and dibenzofurans [7]. It is relevant to underline that the enactment of the directive 2002/95/ EG proscribes the use of many of these well proven halogenated additives. On this basis, extensive research work, from both academic and industrial groups, has been accomplished in order to develop novel halogen-free flame retardants, capable to replace the halogen containing additives by providing sufficient flame retardancy [8e12]. Indeed, nowadays, a completely new fire retardant strategy is represented by polymereclay nanocomposites (PCN) [13e16]. In
* Corresponding author. Fax: þ39 131229399. E-mail address: [email protected] (A. Frache). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.10.006
the last two decades, these novel systems have attracted great interest because they often exhibit remarkable improvements of mechanical, thermal and other physicochemical properties as compared to either the neat polymer or conventional composites [17e21]. As far as nanocomposite thermal stability is concerned, the effect of clay has been found to depend essentially on layer distribution in the polymeric matrix, namely the exfoliation has turned out to be the ideal morphology to gain [22]. Nevertheless, a sheared conclusion on the optimal morphology to improve the combustion behaviour of polymereclay composites has not reported so far. Nanocomposites are commonly based on natural layered clay minerals such as montmorillonite (MMT), which can be made organophilic by the exchange of pristine cations with alkyl (aryl) ammonium cations. More recently, together with MMT, different types of nanofillers have been exploited to prepare nanostructured materials. Among these, hydrotalcite-like compounds (HTlc) have been gained increasing interest [23e31]. Indeed, HTlc have a layered double hydroxides structure consisting of layers constituted of M(II) and M(III) hydroxide octahedrons interconnected at the edges. The positively charge of HTlc layers is balanced by anions located into the interlayer galleries. Thus, as the main difference with respect to MMT is the ion balancing charge, they are also referred as anionic clay in analogy with MMT cationic clays. Although HTlc are natural, being their preparation cheap and easy,
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generally they are synthesized, in order to finely tune their final features by controlling the preparation parameters. In this work, the characteristics and the thermal properties of nanocomposites, based on epoxy resins and containing both MMT and synthetic HTl-type clays, have been evaluated and compared. While the former layered silicates were commercial, hydrotalcites were homeemade. The composites have been prepared by in situ polymerization method, using both unmodified and modified nanofillers. 2. Experimental 2.1. Materials
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The stoichiometric amount of amine curing agent (27.3 phr) was added to the DGEBA/filler mixtures at 80 C and the mixture was sonicated for 5 min, a time sufficient for complete melting and homogeneous dispersion of DDM. The samples were poured in an aluminium mould and outgassed for 40 min at 40 C in a vacuum oven. Curing was performed in 120 min at 80 C at room pressure to produce the nanocomposites containing 5 wt.% organoclay. Post curing was performed at 140 C for 90 min in vacuum to limit oxidative effects on the polymeric matrix. Unfilled resin was prepared as reference. 2.4. Characterization
The polymer matrix was prepared by using a prepolymer, namely diglycidyl ether of bisphenol-A [DGEBA] (Sigma Aldrich), which was cured by 4,40 -diaminodiphenylmethane [DDM] (Sigma Aldrich). As shown in Table 1, two different kinds of layered nanofillers were used, namely commercial MMT (DelliteÒNa, DelliteÒ26C), kindly purchased by Laviosa Chimica Mineraria, and laboratory made HTlc, both unmodified and organically modified. 2.2. Synthesis and compatibilisation of HTlc fillers The synthetic hydrotalcites [Mg0.67Al0.33(OH)2][CO3]0.165$0.5H2O were prepared by urea method [31]. The corresponding chloride forms were obtained by titrating the carbonate forms, dispersed in a 1 mol/dm3 NaCl solution (1g/50 ml), with a 0.1 mol/dm3 HCl by means of an automatic titrator operating at pH stat mode and pH value of 5.0. To perform the Cl/NO 3 exchange, 1g of the chloride form was suspended in 45 ml of a CO2free aqueous solution 0.5 mol/dm3 of NaNO3 for 24 h. The recovered solid was washed three times with CO2-free de-ionized water and finally dried over a saturated NaCl solution (relative humidity, R.H., of 75%). The intercalation of stearate anions was achieved by suspending, for three days at room temperature, the HTlc in nitrate form in a 0.05 mol/dm3 sodium stearate solution H2O/methanol 1/1 (v/v) (1 g/76 ml). The solid, separated from the solution by centrifugation, was washed two times with H2O/methanol 1/1 (v/v) solution and finally dried over a saturated NaCl solution (R.H. 75%). The Mg and Al content of the hydrotalcite samples was determined by complexometric titration with standard EDTA solutions after dissolution of a weighed amount of the sample (ca. 200 mg) in a few drops of concentrated HCl and successive dilution with water to 100 ml.
Nanocomposites morphology was investigated by means of wide angle X-ray diffractometry (WAXD) and by transmission electron microscopes (TEM). The X-ray diffractometer was a X’TRA 48 Thermo ARL with CuKa radiation (a ¼ 0.15406 nm) operating at 45 kV and 40 mA. Radial scans were recorded in the reflection scanning mode from 1 to 15 C step-size 0.02 at 1 /min scanning rate. TEM measurements were performed using a high-resolution transmission electron microscope (JEOL 2010). Ultrathin sections (about 100 nm thick) of material as received after polymerization were cut by a Power TOME X microtome equipped with a diamond knife and placed on a 200-mesh copper grid. In order to obtain a quantitative measurement of clay dispersion in the polymer matrix for the intercalated nanocomposites, at least 25 images for each sample were examined. Thermogravimetric analysis (TGA) were performed on 10 mg samples by heating from 50 to 800 C at 20 C/min under nitrogen and oxygen flow (30 ml/min) on Al2O3 sample pan using a TA Instrument thermobalance TGA Q500. Combustion studies were performed using an oxygen consumption calorimeter (Fire Testing Technology Limited FFT Cone Calorimeter model). Heat flux: 50 kW/m2, Sample: 75 4 mm and 7 mm thick. Heat Release Rate measurements were carried out in triplicate. Experimental error was evaluated to be 5e10%. DSC measurements, which were carried out in order to define the resin and composite systems crosslinking degree, were performed under a continuous nitrogen purge on a Mettler calorimetric apparatus, mod. TC10A. Data were gathered using a scan rate of 10 C/min from 0 to 300 C. The results obtained demonstrated that by applying the curing conditions previously reported, it was possible to obtained complete crosslinked samples. 3. Results and discussion
2.3. Synthesis of epoxy/clay systems 3.1. Morphological characterization Homogeneous mixtures, DGEBA/nanofiller, were obtained by dispersing finely milled powders (30 O 50 mm) of nanolayered fillers (6.7 phr) in DGEBA. In order to favour epoxy intercalation in the filler interlayer galleries, the mixtures were sonicated at 80 C for 1 h using an ultrasonic-bath AC14 (EMME GI) which has a generator with a 600 W output and a 25 (2) KHz convector. Table 1 Characteristics of the filler used. filler
code
organic modifier
Na-MMT (DelliteÒ) OMMT (DelliteÒ)
DelHPS Del26C
HTlc Mg/Al HTlc Mg/Al
MgAlCl MgAlStea
e methyl tallow bis(2-hydroxyethyl) ammonium Chlorine Stearate
The epoxy composites, based on MMT and HTlc, were prepared by in situ polymerization, by applying a two-step procedure. Indeed, preliminarily the filler underwent a swelling process by the intercalation of the prepolymer, DGEBA, under sonication, while the second step of the method involved the curing of the intercalated prepolymer by a crosslinker, namely DDM. The dispersion of the filler into the polymeric matrix of the composites has been studied by WAXD, analysing the sample both after the swelling of the filler in DGEBA and curing. It is worth underlining that the final morphology of the composites depends on the difference between intra-gallery and extra-gallery polymerization. Generally, if the intra-gallery polymerization is faster, the expansion of the interlayer distances occurs, while the constriction takes places in the opposite case.
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Table 2 summarizes WAXD results, obtained by studying all the samples after each steps of the procedure. Namely, the first column reports the values of the basal spacing (d) of the neat nanofillers while the second and third columns list the d values of the mixture, DGEBA/clay, and of the final composites, respectively. Considering the samples based on DelliteÒ-type clay, it came out that the interlayer distance of Na-MMT does not change both in the system clay/DGEBA after swelling and in the resultant composite, thus demonstrating that the intercalation of the prepolymer and consequently its polymerization does not occur inside the above layered silicate galleries. Conversely, the organically modified clay, namely Del26C, shows an increase of the interlayer distance upon mixing with DGEBA, indicating an intercalation of the prepolymer inside the clay galleries and a resulting formation of an intercalated nanocomposite. Moreover, the basal spacing of Del26C, after swelling and curing, are very similar, thus indicating that the final level of prepolymer and polymer intercalation does not depend on the modifier structure and on the starting value of the clay interlayer distance. In order to further validate and complement the results obtained by means of WAXD, TEM measurements were carried out on the composites prepared. TEM micrograph of the nanocomposite ER-Del26C, based on the clay Del26C, is shown in Fig. 1. Analysing the above micrograph, it is possible to identify some clay layers which show individual dispersion of delaminated sheets, as well as regions where a more regular stacking arrangement is maintained. Although in the latter regions the clay layers are organized in a parallel way, some deformations in this arrangement are visible. Furthermore, in agreement with WAXD results, the basal spacing of Del26C (d ¼ 1.75 nm), turns out to be much higher in the composite (d ¼ 3.56 nm). In Table 2, the d values of the neat HTlc and those of the clays after swelling and curing are also shown. As described for MMT-Na, also in the case of HTlc containing chloride ions (MgAlCl), the very low interlayer distance of the above clay does not increase by the swelling process, thus preventing the final formation of a nanostructured material. Clearly, the hydrophilicity of MgAlCl as well as the limited space inside its galleries make the diffusion of the prepolymer difficult to occur. The hydrotalcite studied, containing stearate anions, shows a behaviour similar to that of the organically modified MMT-type layered silicate. Indeed, as demonstrated by the increase of organically modified HTlc interlayer distance, both during its swelling in DGEBA and after curing, the organophilicity and the considerable interlayer distance of this system turns out to facilitate the diffusion of DGEBA and, consequently its polymerization, inside the clay galleries. Moreover, although hydrotalcite dispersion has been found not to be extremely uniform, some zones of the above sample turn out to be characterized by intercalated structures, as shown in the TEM micrograph reported in Fig. 2.
Fig. 1. TEM micrograph of the sample ER-Del26C based on Del26C.
both in nitrogen and air. Fig. 3 shows TGA curves of ER and of the composites based on hydrotalcite. Considering the thermal decomposition under inert atmosphere (Fig. 3a), it comes out that epoxy resin thermally degrades through one single step process with a DTG peak at 364 C and leaving a residue of 14%. The well known degradation process of the above polymer involves water elimination followed by homolytic scission of the crosslinked structure giving volatile fragments. Volatilization is however limited by fragments recombination and rearrangements such as cyclisation that produces a relatively stable charred structure. The curves related to ER/HTlc composites show a maximum decomposition rate at a temperature similar to that of the neat polymer matrix. As far as the beginning of the decomposition is concerned, it has been found to occur at ca. 350 C for all the samples, namely 10 C below the Tonset of the polymer matrix. In air, the epoxy resin degrades in two steps with the maximum decomposition rate at ca. 375 C and 560 C (Fig. 3b). While ER/ MgAlCl anticipates the first step of its decomposition, ER/MgAlStea shows a thermal behaviour very similar to that of the epoxy resin.
3.2. TGA measurements Thermal properties of the neat epoxy resin and of the composites prepared have been investigated by TGA measurements Table 2 Interlayer distance (d001) of the neat clays, the clay/DGEBA systems and composites. sample code
ER/DelHPS ER/Del26C ER/MgAlCl ER/MgAlStea
d001 (nm) neat clay
mixture (clay/DGEBA)
composite
1.21 1.75 0.77 3.05
1.23 3.46 0.77 5.20
1.23 3.56 0.76 5.50 Fig. 2. TEM micrograph of the sample ER/MgAlStea based on MgAlStea.
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Fig. 3. (a) TG and DTG curves in inert atmosphere of: ER, ER/MgAlCl and ER/MgAlStea; (b) TG and DTG curves in air of: ER, ER/MgAlCl and ER/MgAlStea.
TGA curves, in nitrogen and air, of the composites based on MMT-type clay are reported in Fig. 4. In nitrogen (Fig. 4a), the weight loss of the composites starts at lower temperature, being the Tonset of ER/Del26C 10 C and that of ER/HPS 70 C lower than Tonset of the polymer matrix, while DTG peak is similar for all the samples. Comparing the thermal properties of the composites with that of the epoxy resin in air (Fig. 4b), it comes out that the first degradation step shows a very similar trend to that described in inert atmosphere. However, in the case of ER/Del 26C above 500 C, the degradation process turns out to be delayed. Indeed, the fine organophilic MMT dispersion with respect to that of DelHPS seems to promote a delay of the char thermal oxidation. 3.3. Cone calorimeter tests Flame retardant properties of the neat resin and of the composites have been investigated by cone calorimeter, technique which allows to measure heat release and other reaction-to-fire properties in bench scale. Indeed, our attention has been focused on the peak of heat release rate (PHRR) which is the most relevant parameter controlling flame propagation in fires. Fig. 5 shows cone calorimeter curves of the epoxy resin and of the composites prepared while Table 3 summarizes the results obtained by these measurements, comparing the behaviour of the neat polymer matrix with that of the composites.
Fig. 4. (a) TG and DTG curves in inert atmosphere of: ER, ER/DelHPS and ER/Del26C; (b) TG and DTG curves in air of: ER, ER/DelHPS and ER/Del26C.
By considering the above Figures and Table, it comes out that the samples based on the pristine inorganic fillers, namely DelHPS and MgAlCl, are characterized by a slight higher PHRR than that of the neat epoxy resin. In the case of the composite ER/Del 26C, the above parameter further increases, being the increment of PHRR 20%. Conversely to the above described composites, the samples containing the organically modified hydrotalcite has shown a relevant decrease of PHRR (ca. 30%). Moreover, all the composite samples show a slight higher time to ignition (TTI) than the polymer matrix. Indeed, the better combustion behaviour of ER/MgAlStea with respect to that of the other composites prepared can be explained by taking into account the peculiar behaviour of hydrotalcite during combustion. In order to explain in detail the results, the combustion residues, obtained after cone calorimeter tests, were characterized by WAXD and TEM analysis.
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ER ER/DelHPS ER/Del26C
800 700
2
HRR (KW/m )
600 500 400
Intensity (a.u.)
a
300 200
10
100
0
100
200
300
400
500
Time (s)
ER ER/MgAlCl ER/MgAlStea
700 600 2
40
50
60
70
80
Fig. 6. X-ray diffraction pattern of the sample ER/MgAlStea after cone calorimeter test.
900 800
HRR (KW/m )
30
2θ(°)
0
b
20
500 400 300
the disappearance of the original hydrotalcite layered structure occurs. Together with morphological characterization, EDS analysis allows to evaluate the inorganic phase chemical composition (Fig. 7b) Indeed, the inorganic particles of the sample ER/MgAlStea have been found to be characterized by an uniform composition, containing very similar concentration of Mg and Al. On the basis of the results obtained, we can conclude that, conversely to the MMT-type layered silicates, hydrotalcites are capable to ameliorate the combustion behaviour of the polymer matrix in the scenario of cone calorimeter combustion.
200 100 0 0
100
200
300
400
500
Time (s) Fig. 5. (a) Cone calorimeter curves of: ER, ER/DelHPS and ER/Del26C; (b) ER, ER/ MgAlCl and ER/MgAlStea.
As already reported, both the above techniques show that the composites containing MMT-type layered silicates maintain a quite high crystalline layered structure (results not shown). In contrast, WAXD profiles of the hydrotalcite-based composites indicate that the combustion residues consist mainly of oxides-type compounds (Fig. 6). Indeed, the main reflections observed in the 2q range from 30 to 70 are typical of MgO (ICDD-PDF2 no. 00-0450946) or ZnO (ICDD-PDF2 no 01-075-0576). Moreover, as shown in Fig. 7a, TEM micrographs of the samples ER/MgAlStea, which underwent the combustion treatment, evidences the formation of globular inorganic structures, namely
Table 3 Cone calorimeter results of ER and of the composites. sample code
ER ER/Del HPS ER/Del 26C ER/MgAlCl ER/MgAlStea *D% in comparison to ER
TTI
PHRR
(s)
(kW/m2)
D%*
Residue (%)
53 62 64 63 63
712 768 857 792 491
þ8 þ20 þ11 31
9.5 16.0 11.5 14.5 12.5
Fig. 7. (a) TEM micrograph and (b) EDS analysis of the sample ER/MgAlStea after cone calorimeter test.
A. Frache et al. / Polymer Degradation and Stability 96 (2011) 164e169
Nevertheless, the above material is effective only when the polymer is intercalated into its structure. 4. Conclusions Nanocomposites based on epoxy resin and homeemade hydrotalcites have been prepared by in situ polymerization and their properties have been studied and compared with those of classical montmorillonite-type layered silicates-based nanocomposites. As far as filler dispersion is concerned, it has been verified that it is possibile to obtain nanostructured materials, characterized by the resin intercalation into HTlc galleries, by using a proper exchange of the pristine hydrotalcite with stearate anions. TGA measurements and cone calorimeter tests have demonstrated a peculiar thermal behaviour of the hydrotalcite-based nanocomposites. Indeed, the above materials, have turned out to decompose, both in air and nitrogen, following a trend similar to that of the neat polymer matrix. Nevertheless, cone calorimetric tests have proved that, among all the samples studied, only the organophilic hydrotalcite is capable of ameliorating the combustion behaviour of the polymer matrix in cone calorimeter combustion although characterized by a filler dispersion similar to that of the organically modified MMT-based nanocomposite. Acknowledgments The precious help of Dr. Marcella Pani and Mr. Claudio Uliana in WAXD and TEM measurements is gratefully acknowledged. Prof. Giovanni Camino is gratefully acknowledged for helpful discussions on various aspects of the present work. References [1] Potter WG. Epoxy resins. New York: Springer; 1970. [2] Kinjo N, Ogata K, Keneda A. Epoxy molding compounds as encapsulation materials for microelectronic devices. Adv Polym Sci 1989;88:1e48. [3] Lewin M, Atlas SM, Pearce EM. Flame retardant polymeric materials. New York: Plenum Press; 1975. [4] Mikroyannidis JA, Kourtides DA. Curing of epoxy resins with 1-[di(2-chloroethoxyphosphinyl) methyl]-2,4-and -2,6-diaminobenzene. J Appl Polym Sci 1984;29:197e209. [5] Yang CP, Hsiao SH. Flameproofed polyesters prepared by direct polycondensation of aromatic dicarboxylic acids and brominated bisphenols with tosyl chloride and N, N0 -dimethylformamide in pyridine. J Appl Polym Sci 1988;36:1221e32. [6] Camino G, Costa L. Performance and mechanisms of fire retardants in polymersda review. Polym Degrad Stab 1988;20:271e94. [7] Dumler R, Thoma H. Thermal formation of polybrominated dibenzodioxins (PBDD) and dibenzofurans (PBDF) from bromine containing flame retardants. Chemosphere 1989;19:305e8. [8] Levchik SV, Camino G, Luda MP, Costa L, Muller G, Costes B, et al. Epoxy resins cured with aminophenylmethylphosphine oxide 1: combustion performance. Polym Adv Tech 1996;7:823e30. [9] Zhao C-S, Huang F-Li, Xiong W-C, Wang Y-Z. A novel halogen-free flame retardant for glass-fiber-reinforced poly(ethylene terephthalate). Polym Degrad Stab 2008;93:1188e93.
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