Ind. Eng. Chem. Res. 2005, 44, 8573-8579
8573
Influence of the Epoxy Structure on the Physical Properties of Epoxy Resin Nanocomposites S. McIntyre, I. Kaltzakorta, J. J. Liggat, R. A. Pethrick,* and I. Rhoney Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, U.K.
The preparation and physical properties of a series of nanocomposites based on dispersions of Montmorillonite clays in thermoset epoxy resins are reported. The effects of the variation of the concentration of the clay and the influence of a change of the functionality of the epoxy compounds and the amine curing agent are reported. The effects of the method of dispersion of the clay are studied, and it was found that ultrasound provides an effective aid to dispersion of the clay platelets. In general, the addition of clay platelets leads to an increase in the glass-rubber transition, but in the case of a highly cross-linked system, the reverse effect was observed. The effects observed are discussed in the context of the way in which the chemical structure of the monomers influence the dispersion process and the structure of the final resin system. Introduction In recent years, a number of publications have discussed the advantages that can be achieved from the incorporation of organically modified clay into polymer matrixes.1-5 The nature of the dispersion that was achieved appears to be an issue of some contention. The X-ray diffraction (XRD) data presented by certain authors6,7 indicate that they were unable to achieve complete exfoliation. The preferred structure observed possessed an increased intergallery spacing compared with that of the organically modified clay used. Little enhancement of the physical properties was achieved. The dicyandiamide- and benzyldimethylamine-cured materials exhibited increased intergallery spacing with increased values of the low- and high-temperature moduli but no increase in the values of Tg.6 Although the advantages of these materials have been demonstrated, the issue of achieving the exfoliated state has received relatively minor attention. In the case of a thermoplastic where the viscosities used during processing are relatively high, shear was able to achieve an acceptable degree of exfoliation.8 However, in the case of thermoset systems where the viscosities of the initial monomer system used are relatively low, the problems of exfoliation are real. In the case of polyurethanes, we have demonstrated the usefulness of ultrasonic irradiation to facilitate the exfoliation process.10 In this case, we have shown that a truly exfoliated system will exhibit extensive platelet-to-platelet interactions, building a house of cards structure that will increase the viscosity of the media by several orders of magnitude. In fact, the increases were so large that it was necessary to use coupling agents to inhibit platelet contact and reduce the viscosity to acceptable values. The quality of the exfoliated system achieved using this approach was demonstrated using X-ray scattering and electron microscopy of the final cured material. A number of papers have been published on the effects of the addition of nanoclay particles to epoxy resins.10-14 Diglycidyl ether of bisphenol A (DGEBA), triglycidyl* To whom correspondence should be addressed. Tel.: +44 (0)141 548 2260. Fax: +44 (0)141 548 4822. E-mail:
[email protected].
Figure 1. Viscosity vs shear rate illustrating changes in the flow curves with increased Cloisite 30B loading in the epoxy monomer: (O) 0 wt %; (9) 2 wt %; (]) 4 wt %; (×) 6 wt %.
Figure 2. XRD traces for TETA/DGEBA samples with varying levels of clay contents: (A) Cloisite 30B powder; (B) 1.0% Cloisite 30B; (C) 2.0% Cloisite 30B; (D) 3.0% Cloisite 30B; (E) 4.0% Cloisite 30B; (F) 5.0% Cloisite 30B.
p-aminophenol (TGAP), and tetraglycidyldiaminodiphenylmethane (TGDDM) mixed with an octadecylammonium ion modified organoclay and cured with diethyltoluene diamine (DETDA)15 have been investigated. These systems all exhibited enhanced mechanical properties with the addition of clay platelets. In this paper, we focus on two aspects; first, the quality of the
10.1021/ie048835w CCC: $30.25 © 2005 American Chemical Society Published on Web 05/06/2005
8574
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005
Figure 3. DMTA traces as a function of the concentration of added clay for the TETA/DGEBA system: (a) (9) no Cloisite 30B; (b) 1.0% Cloisite 30B; (2) 2.0% Cloisite 30B; (1) 3.0% Cloisite 30B; ([) 4.0% Cloisite 30B + 5.0% Cloisite 30B; (b) E′ bending at 25 °C; (c) E′ bending at 160°C; (d) Tg vs % Cloisite 30B.
dispersion that can be achieved using ultrasound to assist the dispersion process and, second, the effect of a change of the chemical structure on the physical property changes that are observed for the nanoclay systems. Experimental Section Materials. The epoxy resins diglycidyl ether of bisphenol A (DGEBA) and tetraglycidyldiaminodiphenyl-
methane (TGDDM) were supplied by Ciba Specialty Chemicals (Araldite resin, MY 750), and the hardener 4,4-diaminodiphenylsulfone (DDS), triethylenetetramine (TETA), and 4,4-diaminodiphenylmethane (DDM) were supplied by Aldrich Chemicals. The epoxy resin nanocomposites were prepared using a stoichiometric ratio of 100:17.7 (w/w) epoxy resin for DDS, 100:14.2 (w/w) epoxy resin/hardener for DDM, and 100:31.5 (w/ w) epoxy resin/hardener for TGDDM with DDS. The organically modified montmorillonite clays (OMMT),
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8575
analyzer MKIII operating at a frequency of 1 Hz, a strain of ×4, and a scanning rate of 3 °C/min was used to analyze samples cut from the cured materials. Measurements were performed from -100 to +200 °C and the resulting changes in tan δ and E′ plotted. XRD Measurements. A Siemens X-ray diffractometer with a Cu KR (λ ) 1.54 Å) radiation source and a curved graphite crystal monochromator was used to analyze the samples. XRD experiments were performed directly on the organically modified montmorillonite (OMTS) powders and on the epoxy/OMTS-cured samples. Thermal Gravimetric Analysis (TGA). TGA measurements were performed using a Shimadzu TGA-51. A sample of approximately 20 mg was used for the measurements, which were performed between ambient and 800 °C with a heating rate of 10 °C/min and a hold time of 20 min at 800 °C. Figure 4. TGA traces for the TETA/DGEBA system with various levels of clay particles: (9) no Cloisite 30B; (b) 2.0% Cloisite 30B; (2) 5.0% Cloisite 30B.
Figure 5. XRD traces for DDM/DGEBA systems as a function of the concentration of clay: (A) Cloisite 30B powder; (B) 2.0% Cloisite 30B; (C) 4.0% Cloisite 30B; (D) 6.0% Cloisite 30B; (E) 8.0% Cloisite 30B; (F) 10.0% Cloisite 30B; (G) 12.0% Cloisite 30B. Table 1. Calculated Modulus of the Filled Epoxy Resin Systems clay concn (wt %)
E′ experimental (×109)
E′ calculated (×109)
0 1 2 3 4 5
1.57 1.83 1.86 1.83 2.20 2.12
1.57 1.99 2.41 2.80 3.22 3.62
Cloisite 30b, were supplied and characterized by Southern Clay. Sample Preparation. A known amount of OMMT was added to 50 g of DGEBA at 80 °C, stirred, and then sonicated with stirring for 15 min using a Cole-Palmer Ultrasonic Processor (model CPX 750) coupled to 6.35mm tapered probe. The resulting suspension was maintained at 80 °C and sonicated with stirring for another three periods of 15 min for 24 h before addition of the required amounts of DDS and DDM. The mixture was sonicated with stirring for 10 min until the hardener dissolved. The solution was now degassed, poured into a preheated mould, and cured for a preset time and temperature. Dynamic Mechanical Thermal Analysis (DMTA). A Polymer Laboratories dynamic mechanical thermal
Results and Discussion Sonication. Previous studies using poly(propylene glycol)10 had indicated that to achieve efficient exfoliation of the organically modified clays, it was advantageous to use sonication. The rheology of the dispersion provides a clear indication of the extent to which a good dispersion has been achieved. The general characteristics are that the viscosity has been increased by an order of magnitude or greater and that very significant shear thinning is observed. Shear thinning is typical of a viscous polymer system and is associated with relaxation of the polymers. However, in the case of a solution of clay in monomer, the shear thinning is associated with disruption of the “house of cards” structure created by edge-to-face interactions of the clay platelets. The data presented in Figure 1 indicate that the epoxy resins are exhibiting the same type of behavior as that observed for poly(propylene glycol) and efficient exfoliation is achieved by the use of sonication. All of the samples of epoxy resins discussed in this paper were prepared by sonication of the resin for an appropriate period of time to ensure efficient exfoliation prior to the cure process being undertaken. TETA with DGEBA. Stoichiometric amounts of the curing agent and the level of clay were changed between 0 and 5 wt %. The samples were first cured for 6 h at 100 °C and then postcured at 160 °C for a further 4 h (Figure 2). XRD traces were measured for the samples. The peak associated with the Cloisite 30B is absent in the diffraction traces of the epoxy samples, indicating a high degree of exfoliation. DMTA measurements were performed on samples cut from the cured slabs of material. The data presented in Figure 3a represent the average data obtained from a series of DMTA experiments carried out on samples from each material. It is apparent that the addition of the clay platelets leads to an increase in the ambient temperature modulus consistent with the reinforcing action of the addition of the clay as a filler (Figure 3b). The higher temperature modulus (Figure 3c) indicates the reinforcing action of the clay platelets and effectively raises the “cross-link” density, and hence the hightemperature rubbery modulus, as the concentration of clay is increased. The glass transition temperature, Tg, is also observed to increase in an approximately linear fashion with the increased addition of the clay particles. The magnitude of the tan δ peak decreases with an increase in the concentration of clay and is consistent with the predictions of the law of mixing16 to give an
8576
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005
Figure 6. DMTA traces for DDM/DGEBA as a function of the clay concentration: (a) (2) no Cloisite 30B; (9) 2.0% Cloisite 30B; (b) 4.0% Cloisite 30B; (1) 8.0% Cloisite 30B; (b) (2) no Cloisite 30B; (9) 2.0% Cloisite 30B; (b) 4.0% Cloisite 30B; (1) 8.0% Cloisite 30B; (c) Tg; (d) E′ bending at 50 °C; (e) E′ bending at 180 °C.
approximation of the degree of interaction between the filler and polymer matrix; thus
tan δc ) tan δp(1 - Bφf) where subscripts c, p, and f refer to the composite, polymer, and filler, respectively. The magnitude of factor B gives a measure of the strength of the coupling made between the filler and polymer matrix and is reflected in the decrease of tan δ (max). An approximation for the maximum possible modulus, E′, of any epoxy/nanoclay sample can be
calculated from the “rule of mixtures”;17 thus
E′comp ) E′clayφclay + E′epoxyφepoxy where E′comp is the modulus of the composite, E′clay is the modulus of clay ) 0.7 × 1011 Pa s, φclay is the volume fraction of clay, E′epoxy is the modulus of epoxy ) 1.57 × 109 Pa s, and φepoxy is the volume fraction of epoxy. The results of the calculation are presented in Table 1. The enhancement is lower than would be predicted at higher concentrations and can, in part, be attributed
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8577
Figure 7. XRD traces for DDS/DGEBA as a function of the clay concentration: (A) Cloisite 30B powder; (B) 2.0% Cloisite 30B; (C) 4.0% Cloisite 30B; (D) 6.0% Cloisite 30B; (E) 8.0% Cloisite 30B; (F) 10.0% Cloisite 30B.
to partial exfoliation of the clay, leading to the enhancement being lower than expected. TGA measurements of the filled samples (Figure 4) indicated that the degradation characteristics of the materials were not significantly influenced by the presence of organically modified clay. It has previously
been pointed out that the quaternary organics used to disperse the clay can have a destabilizing effect on the polymer matrix, but this is not observed in this case.18 DGEBA/DDM Nanocomposites. The DDM hardener is less flexible and, as a result, produces a higher Tg matrix than TETA-cured materials. It is therefore anticipated that this hardener might behave in a manner different from that of the flexible TETA. Samples were prepared with varying levels of clay using a method similar to that used for TETA except that the cure process involved curing at 180 °C for 16 h. A range of materials of 0-12% Cloisite 30b were prepared; however, the XRD indicated that it was only possible to achieve exfoliation up to 6% in this system (Figure 5). Above 6%, a peak is observed at low angles, which corresponds to a spacing of about 40 Å, which indicates that the polymer resin has become inserted between the clay platelets but they are exhibiting a degree of order and are not randomly oriented as in a truly exfoliated structure. It has been noted by Vaia2 that, at high concentrations, clay platelets should order into a pseudonematic phase, which would be predicted to exhibit XRD peaks at low angles. DMTA measurements showed an increase in Tg with an increase in the concentration of Cloisite 30B, from 115 to 140 °C at 12% Cloisite, although at the higher concentrations, the clay platelets are exhibiting an intercalated structure (Figure 6). The low-temperature
Figure 8. DMTA traces for the DDS/DGEBA system as a function of the clay concentration: (a) (9) no Cloisite 30B; (b) 2.0% Cloisite 30B; (1) 6.0% Cloisite 30B; ([) 10.0% Cloisite 30B; (b) E′ bending at 100 °C; (c) E′ bending at 220 °C; (d) Tg for a system cured at 200 °C for 6 h.
8578
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005
Figure 9. XRD studies of TGDDM/DDS as a function of the clay platelet concentration: (A) Cloisite 30B powder; (B) 2.5% Cloisite 30B; (C) 5.0% Cloisite 30B; (D) 7.5% Cloisite 30B; (E) 10.0% Cloisite 30B; (F) 15.0% Cloisite 30B.
modulus initially increases linearly with added clay, but deviations are observed once the structure starts to exhibit order and a nematic phase is formed. The glass transition temperature shows a linear increase with concentration; however, the trends are less convincing for the highest concentrations. TGA studies were essentially very similar to those reported for the TETA/DGEBA system. DGEBA/DDS Nanocomposite. A range of materials of 0-12% Cloisite 30B were prepared, and exfoliation
was observed up to 10% in this system (Figure 7). It is clear that the subtle change of moving from DDM to DDS has influenced the ability to achieve an exfoliated state. There is, however, a suspicion of a peak developing at a low angle for the highest concentrations of clay. DMTA data showed an increase in Tg with an increase in the concentration of Cloisite 30b, from 142 °C at 0% Cloisite 30B to 185 °C at 10% Cloisite 30B, although at the higher clay concentrations, the platelets were beginning to exhibit intercalation (Figure 8). At low temperatures, there is a small increase in the value of E′ with an increase in the concentration of clay; however, above Tg, the values of E′ increased by a factor of 5, from 1.2 × 107 to 6.0 × 107 over the concentration range. The TGA analysis once more indicated that the clay was not having a significant effect on the thermal stability of the resin. TGDDM/DDS Nanocomposite. For really high temperature resin applications, DGEBA is often replaced by TGDDM. The functionality of DGEBA is 2, whereas the functionality of TGDDM is 4. As a result, this latter resin tends to produce a much more highly cross-linked resin with a corresponding higher Tg than those obtained with DGEBA. A range of materials with concentrations of Cloisite 30B between 0 and 15 wt % were prepared. XRD studies (Figure 9) indicate that up to 10 wt % the clay platelets are in an exfoliated state. Once more it is observed that at high concentrations the platelets adopt an intercalated structure. There is, however, some indication that some of the clay is in a
Figure 10. DMTA analysis of the TGDDM/DDS system as a function of the clay concentration: (a) (1) no Cloisite 30B; (9) 2.5% Cloisite 30B; (b) 5.0% Cloisite 30B; (2) 10.0% Cloisite 30B; (b) Tg vs % Cloisite 30B; (c) E′ bending at 100 °C vs % Cloisite 30B; (d) E′ bending at 280 °C vs % Cloisite 30B.
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8579
state that resembles a slightly expanded original spacing of the platelets. DMTA measurements (Figure 10) show a decrease in Tg with an increase in the concentration of Cloisite 30B, from 256 to 245 °C at 15 wt % Cloisite. The value of the modulus E′ at low temperature shows the expected small increase with an increase in the concentration. Above Tg, this system, in contrast to the others studied, showed only a slight increase with an increase in the concentration from 4.8 × 107 to 7.3 × 107 of E′ over the concentration range. TGDDM/DDS is a very highly cross-linked resin, and DMTA implies that the clay is disrupting the cross-linking process and leading to a more open, lower glass transition temperature structure. This is interesting in contrast to the other systems, where the clay particles are enhancing the effective cross-link density of the matrix. TGA analysis indicated that the clay had not in any way influenced the stability of the resin and the traces were essentially indistinguishable from those of the resin without the clay. Conclusion The properties of the nanocomposites based on organically modified clay platelets depend on the technique used to achieve the dispersion and on the cure temperature of the system. The data presented in this paper were obtained using sonication to assist the dispersion process, and the XRD data support the contention that exfoliation has been achieved in most cases. It is, however, noticeable that at high concentrations of clay intercalation rather exfoliation appears to predominate. By selection of an optimum cure temperature, it is possible to achieve a reasonable degree of exfoliation over a large concentration range. In most cases, the interaction of the clay with the resin leads to a reinforcing action with observation of an increase in the ambient values of the elastic modulus and the value observed above the Tg of the resin. However, in the case of the very highly cross-linked TGDDM/DDS resin, the interactions with the clay suppress the formation of the completely cross-linked matrix characteristics of the property enhancements associated with this system and a lowering of the Tg is observed. It was also observed, as has been reported previously,19 that the thermal stabilities of the composites with epoxy resins are not affected by the type of intercalatent. It has, however, been reported that the clay increases the tensile strength and elongation.19 Acknowledgment S.M. acknowledges the support of Cytec and the EPSRC in the provision of a CASE Industrial Studentship. I.R. acknowledges the support of EPSRC by a Research Fellowship for the period of this study. Literature Cited (1) Beall, G. W. In Polymer Clay Nanocomposites; Pinnavaia, T. J., Beall, G. W., Eds.; John Wiley & Sons Ltd.: New York, 2000; p 267.
(2) Vaia, R. A. In Polymer Clay Nanocomposites; Pinnavaia, T. J., Beall, G. W., Eds.; John Wiley & Sons Ltd.: New York, 2000; p 244. (3) Wang, Y.; Shen, S. Y.; Gai, G. S.; Fu, C. S. Preparation and major properties of montmorillonite/epoxy nanocomposites. Key Eng. Mater. 2003, 249, 413. (4) Myskova, M. Z.; Zelenka, J.; Spacek, N.; Socha, F. Properties of epoxy systems with clay nanoparticles. Macromol. Symp. 2003, 200, 291. (5) Brown, S.; Lindsay, C. Model PU Cast Elastomer Nanocomposites; Huntsman Polyurethanes: Salt Lake City, UT, 1999. (6) Noh, J. Y.; Kim, J. Mechanical and thermal properties of epoxy/organically modified mica type silicate (OMTS) nanocomposites. Polymer (Korea) 2001, 25, 691. (7) Chen, C. G.; Curliss, D. Preparation, characterization, and nanostructural evolution of epoxy nanocomposites. J. Appl. Polym. Sci. 2003, 90, 2276. (8) Vaia, R. A.; Ishii, H.; Gianelis, E. P. Synthesis and properties of 2-dimensional nanostructures by direct intercalation of polymer melts in layered silicates. Chem. Mater. 1993, 5, 1694. (9) Lee, D. C.; Jang, L. W. Characterization of epoxy-clay hybrid composite prepared by emulsion polymerization. J. Appl. Polym. Sci. 1998, 68, 1997. (10) Rhoney, I.; Brown, S.; Hudson, N. E.; Pethrick, R. A. Influence of processing method on the exfoliation process for organically modified clay systems. I. Polyurethanes. J. Appl. Polym. Sci. 2004, 91, 1335. (11) Kornmann, X.; Lindberg, H.; Berland L. A. Synthesis of epoxy-clay nanocomposites. Influence of the nature of the curing agent on structure. Polymer 2001, 42, 4493. (12) Chin, I. J.; Thurn-Albrecht, T.; Kim, H.; Russel, T. P.; Wang, J. On exfoliation of montmorillonite in epoxy. Polymer 2001, 42, 5947. (13) Guo, B. C.; Ha, D. M.; Cai, C. G. Effects of organomontmorillonite dispersion on thermal stability of epoxy resin nanocomposites. Eur. Polym. J. 2004, 40, 1743. (14) Becker, O.; Varley, R. J.; Simon, G. P. Thermal stability and water uptake of high performance epoxy layered silicate nanocomposites. Eur. Polym. J. 2004, 40, 187. (15) Becker, O.; Simon, G. P.; Varley, R. J.: Halley, P. J. Layered silicate nanocomposites based on various high-functionality epoxy resins: The influence of an organoclay on resin cure. Polym. Eng. Sci. 2003, 43, 850. (16) Guest, M. J.; Daly, J. H. Practical Aspects of Solid State Mechanical Spectroscopy for Polymers. Polymer Yearbook; 1993; Vol. 10, pp 141 and 197. (17) Nielsen, L. E.; Landel, R. F. Mechanical Properties of Polymers and Composites, 2nd ed.; Marcel Dekker: New York, 1994; Chapter 7. (18) Akelah, A.; Kelly, P.; Qutubuddin, S.: Moet, A. Synthesis and characterization of epoxyphilic montmorillonites. Clay Miner. 1994, 29, 169. (19) Kang, J. H.; Lyu, S. G.; Choi, H. K.; Sur, G. S. Effect of intercalant on the synthesis and properties of epoxy nanocomposites. Polymer (Korea) 2001, 25, 414.
Received for review December 2, 2004 Revised manuscript received March 9, 2005 Accepted March 10, 2005 IE048835W
