Thermomechanical and barrier properties of UV-cured epoxy/O-montmorillonite nanocomposites.
Naturally available layered silicates (montmorillonites) have attracted a great interest in the research field of polymeric nanocomposites because of their reinforcing effect on the host polymer matrices. The layered structure is about 1 nm thick and has an aspect ratio from 100 to 1500. These characteristics contribute to enhance the thermal, mechanical, and barrier properties and the flame retardancy of the polymer matrix [1-3].
The structure of the unit layer of montmorillonite consists of an octahedral sheet of alumina or magnesia between two tetrahedral silica sheets. Because of the replacement of silicon by aluminum or of aluminum by magnesium ions, the layers become negatively charged and attract cations such as sodium, potassium, and calcium. As the hydration of the inorganic cations by water molecules that enter the interlayer region makes the nanoclays hydrophilic, their dispersion into polymer matrices may become more difficult. For this reason, it is crucial to modify the structure of the nanoclay to enhance the dispersibility within the organic matrix.
Usually, montmorillonites are modified by ions substitution reactions with cationic surfactants that give rise to more organophilic surfaces. Such modification allows polymer chains to enter the intergallery regions and provide intercalated or exfoliated structures. Intercalated morphologies are obtained when there is a limited inclusion of the polymer chains between the nanoclay layers. When the nanoclay platelets are well separated and individually dispersed in the continuous polymer matrix, exfoliated structures are achieved; thus, it is possible to obtain the best effect of the nanofillers on the final properties of the nanocomposites [4, 5].
The nanocomposites can be obtained by in situ polymerization, solvent casting, or melt intercalation [6-8]. Among the methods used for the in situ polymerization, the ultraviolet (UV) curing technique is fast, effective, and environmentally friendly [5, 9]. The reaction mechanism can be radical or cationic depending on the type of monomer used. The presence of an appropriate photoinitiator allows the formation of radical or cationic species that, through a propagating mechanism, allow the building up of the polymeric thermoset matrix. Compared with the radical photopolymerization, the cationic is not inhibited by oxygen, so as it does not require an inert atmosphere; furthermore, it provides good adhesion properties, low shrinkage, and the starting monomers are less toxic than those used in the radical photopolymerization. The process starts with the photoexcitation of the reactive groups of the initiator that interact with the monomer, propagating the growth of the polymer chains through an addition mechanism [10-12].
In this work, UV-curable epoxy nanocomposites were prepared by adding modified montmorillonites to an epoxy resin. Cloisite [Na.sup.+] and Cloisite 30B were modified with different types of organic surfactants (dodecylsuccinic anhydride, octadecylamine, octadecyl alcohol, and octadecanoic acid). The effect of such treatment on the basal spacing of the nanoclays was investigated by X-ray diffractometry (XRD) analysis.
Bis-(3,4-epoxycyclohexyl)adipate was used as epoxy monomer. It was added to 5 wt% of the modified nanoclays, then UV-cured in the presence of triphenylsulfonium-hexa-fluoroantimonate as photoinitiator. The photopolymerization kinetics was evaluated by real-time Fourier transform infrared (FTIR) spectroscopy. The morphology of the obtained nanocomposites was investigated through XRD and transmission electron microscopy (TEM) analyses. Thermogravimetric (TGA) and dynamic-mechanical (DMTA) analyses were carried out to study the thermal properties of the nanocomposites. In addition, the gas barrier properties of nanocomposite films coated on a polyethyleneterephtalate (PET) substrate were evaluated by measuring their oxygen permeability.
Cloisite [Na.sup.+] (Clo-Na, cationic exchange capacity (CEC) = 90 meq/100 g) and Cloisite 30B (Clo-30B) were purchased from Southern Clay Product (USA). ETHO-QUAD 0/12 (ETH) (9-octadecen-1-amino, N,N-bis(2-hydroxyethyl)-N-methyl-chloride, Akzo Nobel) was an ionic liquid used to modify Clo-Na. Its structure is reported in Fig. 1, together with Clo-30B intercalant. Dodecylsuc-cinic anhydride (DSA), octadecylamine (ODA), octadecyl alcohol (ODOH), and octadecanoic acid (AcOD) were used for the organo-nanoclays modification. They were Aldrich products, used as received.
[FIGURE 1 OMITTED]
Bis-(3,4-epoxycyclohexyl)adipate (UVR 6128 from Dow Chemicals) was used as epoxy resin. Its structure is reported in Fig. 2. Triphenylsulfonium-hexafluoroantimo-nate (UVI 6976) was supplied from Dow Chemicals and used as photoinitiator (see Fig. 2); it is a solution in propylene carbonate (50% w/w) and it was added at a 5 wt% concentration. The photopolymerization reaction scheme is shown in Fig. 3.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
PET films (3M products, thickness 12 [micro]m) were used as standard substrate for the evaluation of the barrier properties of the nanocomposite coatings toward oxygen.
Modification of Clo-Na. A dispersion of Clo-Na in deionized water (1 wt%) was prepared. It was left at 60[degrees]C for 5 min, then ETH was added at a concentration that exceeded 20% of the CEC of Clo-Na. This dispersion was kept at 60[degrees]C under vigorous stirring for 24 h. Subsequently, it was filtered with a G3 Duran filter. Ag[NO.sub.3] was added to the filtered solution to check the disappearance of chloride ions in the modified Cloisite [Na.sup.+] (Clo-ETH).
Modification of Clo-30B and Clo-ETH. Five grams of Clo-30B or Clo-ETH was dispersed into 200 ml of 2,2-dimethoxyethane at 75[degrees]C under vigorous stirring for 5 min. A stoichiometric amount of each modifier with respect to the--OH groups present in the modified nanoclays was added, and the dispersion was kept for 2 h at 75[degrees]C. It was then filtered, washed with fresh solvent, and dried in oven at 70[degrees]C until constant weight.
Preparation of the UV-Cured Nanocomposites. The modified nanoclays were dispersed into the liquid epoxy monomer at a concentration of 5 wt%. The dispersions were left in an ultrasonic bath (mod. Branson 1210) for 2 h at room temperature. The dispersions were then added to 5 wt% of photoinitiator and coated on polypropylene substrates (film thickness, 90 [micro]m). They were subsequently cured with a medium vapor pressure Hg UV lamp (Helios Italquartz, Milano, Italy) with a radiation intensity on the surface of the samples of 20 mW/[cm.sup.2]. The samples were UV-cured in air atmosphere.
Characterization Techniques. The structure of the modified nanoclays and the UV-cured nanocomposites was studied through XRD analysis using a Philips X'Pert-MPD diffractometer ([CuK.sub.[alpha]] radiation). Two 2[theta] ranges were analyzed; 2 < 2[theta] < 30[degrees] with a 0.02 [delta]2[theta] step and 2 s step time; 0.7 < 2[theta] < 10[degrees], through wide-angle X-ray scattering analysis, with 0.01 [delta][theta] step and 10 s step time.
The morphology and distribution of the modified nanoclays were directly observed with a Philips CM-12 TEM at an accelerating voltage of 100 kV. After being trimmed to shape by a Leica EM TRIM, ultrathin sections (less than 100 nm) were cut at room temperature using a Leica Ultracut UCT microtome equipped with a diamond knife. Finally, ultrathin sections were collected in a trough filled with water and lifted onto formvar-coated copper grids. TEM images were analyzed to evaluate the percentage of silicate exfoliation by enumerating the layers in stacks (intercalated tactoids), the exfoliated layers and the disordered layers in a series of around 10 images per sample. Even though the total area covered by such number of images is very small to carry high statistical significance, in this case they were sufficient to point out significant differences between the nanocomposites investigated.
The photopolymerization kinetics was monitored by means of real-time FTIR spectroscopy, using a Thermo-Nicolet 5700 instrument. The liquid dispersions were placed on a silicon wafer, and the samples were exposed simultaneously to the UV light to induce the photopolymerization and to the IR beam to analyze the extent of the reaction. The photopolymerization was induced by a medium pressure Hg lamp equipped with an optical waveguide (light intensity on the surface of the sample of about 5 mW/[cm.sup.2]). The decrease of the epoxy absorption peak, in the 760-780 [cm.sup.-1] region, was continuously monitored, and at the end of the analysis, conversion versus irradiation time profiles were determined. The polymerization reactions were performed at room temperature and 25-30% relative humidity (RH).
The extent of the polymerization was evaluated by measuring the gel content or gel % of the cured films measuring the weight loss after 24 h extraction with chloroform at room temperature, according to the standard test method ASTM D 2765.
DMTA analyses were performed on a MKIII Rheometric Scientific Instrument at 1 Hz frequency in the tensile configuration from room temperature up to 200[degrees]C with a heating rate of 5[degrees]C/min and a specimen size of 20 mm X 8 mm X 0.3 mm.
TGA analyses were performed on a Mettler TGA-SDTA 851 instrument in the temperature range 25-750[degrees]C with a heating rate of 10[degrees]C/min and an air flow rate of 60 [cm.sup.3]/min. The mass of the sample placed in the alumina crucible was of about 12 mg.
The barrier properties toward oxygen were evaluated by using a MultiPerm permeometer (ExtraSolution, Pisa, Italy). The analysis was carried out at a [rho]O2 = 1.4805 atm, 25[degrees]C and 0% RH. The neat UV-cured resin and the nanocomposite films (average thickness: 20 [micro]m) were coated on 12-[micro]m PET films used as standard substrates.
RESULTS AND DISCUSSION
Morphology of the Nanoclays and the Nanocomposites
In Table 1, the [d.sub.001] values (together with the corresponding 2[theta] angles) for Clo-Na and Clo-ETH are collected. The modification of Clo-Na with ETH (Clo-ETH) increases the basal spacing of the nanoclay of about 5 [Angstrom], reaching a [d.sub.001] value of 17.3 [Angstrom]. The interaction between Clo-ETH and the different organic compatibilizers gives rise to a final spacing of 34.2 [Angstrom] with DSA and around 18[Angstrom] for all the other modifiers. This can be explained considering the formation of emi-ester products by reaction of DSA with the hydroxyl groups of Clo-ETH, whereas the other modifiers interact only through polar H-bonds. A similar behavior was observed when maleinized liquid polybutadienes were reacted with nanoclays containing hydroxyethyl groups .
TABLE 1. [d.sub.001] ([Angstrom]) and 2[theta] values of the modified nanoclays and nanocomposites. (a) With DSA With ODA Clo-Na 12.0 -- -- 2[theta] = 7.35 Clo-ETH 17.3 34.2 17.4 (5.3) (22.2) (5.4) 2[theta] = 5.09 2[theta] = 2.58 2[theta] = 5.06 UV6128+Clo-Na 14.3 -- -- (2.3) 2[theta] = 6.17 UV6128+Clo-ETH 25.7 35.6 31.2 (13.7) (23.6) (19.2) 2[theta] = 3.38 2[theta] = 2.48 2[theta] = 2.83 Clo-30B 18.4 38.2 40.3 2[theta] = 4.8 (19.8) (21.9) 2[theta] = 2.31 2[theta] = 2.19 UV6128+Clo-30B 37.9 42.6 40.7 (19.5) (24.2) (22.3) 2[theta] = 2.33 2[theta] = 2.07 2[theta] = 2.17 With ODOH With AcOD Clo-Na -- -- Clo-ETH 17.5 18.6 (5.5) (6.6) 2[theta] = 5.04 2[theta] = 4.75 UV6128+Clo-Na -- -- UV6128+Clo-ETH 31.6 34.6 (19.6) (22.6) 2[theta] = 2.81 2[theta] = 2.55 Clo-30B 42.6 39.2 (24.2) (20.8) 2[theta] = 2.07 2[theta] = 2.25 UV6128+Clo-30B 40.8 39.6 (22.4) (21.2) 2[theta] = 2.16 2[theta] = 2.23 (a) In parenthesis, the [[DELTA]d.sub.001] differences calculated with respect to precursor Clo-Na or Clo-30B nanoclay are given.
Table 1 also reports the [d.sub.001] values for Clo-30B modified with the same organic compatibilizers. A strong increase of the basal spacing is achieved, giving rise to a final spacing of about 39 [Angstrom] for all the compatibilizers. Therefore, for Clo-30B, unlike Clo-ETH-based systems, a strong interaction through H-bonds between the nanoclay and the organic compatibilizers occurs, which reaches the same intensity as that observed using DSA as compatibilizer.
These results can be attributed to the different structure of the ammonium ion of Clo-ETH with respect to Clo-30B.
The modified nanoclays were then dispersed in the liquid epoxy resin UVR 6128 (5 wt% of nanoclay). After 2-h treatment with an ultrasonic bath at room temperature, the dispersions were analyzed through XRD analysis; then, the analysis was repeated on the UV-cured nanocomposites.
In any case, no difference was observed for the basal spacing values of the liquid dispersions and the UV-cured nanocomposites. Thus, only the interlamellar distances of the UV-cured nanocomposites are listed in Table 1. A similar behavior was previously observed for UV-cured epoxy nanocomposites .
Considering the results collected in Table 1, no interaction between Clo-Na and the epoxy resin is evident. On the contrary, Clo-ETH and Clo-30B clearly increase their [d.sub.001] values when dispersed in the epoxy resin, indicating that interactions occur between the modified clays and the resin.
The systems based on Clo-ETH modified with DSA, ODA, ODOH, and AcOD increase their basal spacing in the presence of the epoxy resin, giving rise practically to the same values (about 34 [Angstrom]).
In the case of systems based on Clo-30B, practically the same [d.sub.001] values (about 39 [Angstrom]) are achieved for Clo-30B, pure and modified with all the compatibilizers. Figures 4 and 5 report the XRD patterns of some nanoclays and the nanocomposites listed in Table 1.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The results obtained from XRD analysis were further confirmed by means of TEM microscopy, which gives a direct observation of the morphology of the nanoclays in the epoxy matrix. Figures 6 and 7 show two typical pictures of the UV-cured nanocomposites based on Clo-30B and Clo-30B modified with DSA, respectively, considered as representative of the series. TEM analysis showed a behavior that is common for most polymer/montmorillonite nanocomposites independently on the preparation procedure, in which larger layers evolve to intercalated tactoids, whereas the smaller--in lateral size--layers tend to exfoliate. As a consequence, the investigated nanocomposites may be best described as intercalated/exfoliated systems.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
In addition, the coexistence of intercalated tactoids and exfoliated/disordered layers is more evident in the presence of the modified Cloisite (e.g., Fig. 6a vs. 7a). A semiquantitative analysis of TEM images could be used to roughly evaluate the percentage of exfoliation. Taking into account the number of layers in intercalated tactoids and those separated or nonparallel, the percentage of exfoliated/disordered layers appears to increase from around 10% for Clo-ETH- and Clo-30B-based nanocomposites to 30-40% for Clo-30B and Clo-ETH modified-based nanocomposites.
The photopolymerization reaction was monitored through real-time FTIR spectroscopy. The obtained conversion versus time profiles are plotted in Figs. 8 and 9. It can be seen that the polymerization proceeds rapidly and reaches its maximum value within 2 min. Furthermore, the presence of the nanoclays does not practically affect the photopolymerization kinetics; practically, all the curves exhibit the same slope (i.e., rate of photopolymerization) and final conversion of the epoxy groups (about 90%).
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Properties of the UV-Cured Nanocomposites
Tables 2 and 3 present some properties of the UV-cured nanocomposites. All the systems exhibit high insoluble fractions (gel %), according to the high final conversions of the epoxy groups measured by real-time FTIR spectroscopy. Figures 10 and 11 plot some typical DMTA spectra of the nanocomposites, compared with that of the pure UV-cured epoxy system. The [T.sub.g] values, assumed as the maximum of tan [delta] peak, are listed in Table 2. It can be noted that the glass transition temperature is strongly affected by the presence of the nanoclays, that is, the [T.sub.g] increases by increasing the level of intercalation/exfoliation as revealed by TEM analyses. In fact nanocomposites containing Clo-Na give no increase of [T.sub.g]; the nanocomposites containing Clo-ETH and Clo-30B evidence an intermediate value; the systems based on modified Clo-ETH and Clo-30B give the highest [T.sub.g] values. These results can be attributed to the interactions between the clay lamellae and the polymer chains. In fact, the mobility of the polymer segments present on the surface and inside the clays galleries is lowered by the interfacial interactions with the clay. Similar results were obtained in several articles [9, 13, 14].
TABLE 2. Gel %, [T.sub.g] values, and [O.sub.2] permeability of the nanocomposites. Sample Gel (%) [T.sub.g] DMTA [O.sub.2] permeability ([degrees]C) ([cm.sup.3] mm/day atm) Pure UV6128 96 104 0.194 UVR 6128+Clo-Na 94 104 0.189 UVR 6128+Clo-ETH 96 110 0.185 UVR 6128+Clo-ETH+DSA 97 114 0.167 UVR 6128+Clo-ETH+ODA 95 116 0.173 UVR 6128+Clo-ETH+ODOH 96 114 0.177 UVR 6128+Clo-ETH+AcOD 95 114 0.176 UVR 6128+Clo-30B 95 109 0.170 UVR 6128+Clo-30B+DSA 95 113 0.150 UVR 6128+Clo-30B+ODA 95 112 0.168 UVR 6128+Clo-30B+ODOH 96 114 0.167 UVR6128+Clo-30B+AcOD 95 114 0.173 TABLE 3. Thermal stability (in air) of the nanocomposites. Sample [T.sub.10] [T.sub.50] ([degrees]C) Char (wt%) ([degrees]C) Pure UV6128 282 391 0 UV6128+Clo-Na 285 390 4.8 UV6128+Clo-ETH 303 392 4.5 UV6128+Clo-ETH+DSA 295 390 3.8 UV6128+Clo-ETH+ODA 290 392 3.7 UV6128+Clo-ETH+ODOH 303 389 3.9 UV6128+Clo-ETH+AcOD 306 389 3.8 UV6128+Clo-30B 289 393 4.0 UV6128+Clo-30B+DSA 292 393 3.5 UV6128+Clo-30B+ODA 293 394 3.6 UV6128+Clo-30B+ODOH 294 394 3.5 UV6128+Clo-30B+AcOD 294 393 3.5
In Figs. 10 and 11, it is worthy to note that E' (storage modulus) values, especially in the rubbery plateau, increase in the presence of intercalated/exfoliated nanoclays.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
The thermal stability of the nanocomposites was evaluated by means of TGA analysis performed in air. In Table 3, the temperatures of initial degradation [T.sub.10] and corresponding to 50% weight loss (T.sub.50) are reported, together with the char content. [T.sub.10] values show an increase of the thermal stability of the nanocomposites. The detailed mechanism related to the enhancement of the thermal and thcrmooxidative properties of the nanocomposites has not been fully understood yet. However, it has been suggested that the improvement of the thermal stability is mainly due to the formation of char on the nanocomposites surface that reduces the diffusion of volatile product of the polymer degradation so as a lower weight loss is achieved. In addition, the presence of the char reduces the diffusion of the atmospheric oxygen that is responsible for the thermooxidative degradation as well as the diffusion of heat. This double insulation and barrier effect of the nano-clays may favor the formation of less volatile products from the secondary reactions of the radicals, which contribute to the formation of char and improve the thermal stability of the polymer [15-17].
Table 2 also presents the permeability values toward oxygen. The obtained data evidence that the presence of modified nanoclays in the epoxy matrix improves the barrier properties of the coating. In particular, the data collected in Table 2 indicate that lower oxygen permeability values are achieved for the nanocomposites, which, on the basis of TEM analysis, exhibit higher degree of exfoliation. In particular, the systems based on Clo-ETH and Clo-30B modified with the organic compatibilizers generally show a decrease of permeability of about 15% with respect to the pure UV-cured resin. These results can be mainly attributed to the increase of the tortuosity of the diffusion path, which is related to the degree of dispersion of the clay nanoplatelets .
Two types of commercially available montmorillonites, Cloisite [Na.sup.+] and Cloisite 30B, were modified with organic compatibilizers (dodecylsuccinic anhydride, octadecylamine, octadecyl alcohol, and octadecanoic acid) to improve their basal spacing and enhance the degree of dispersion in the polymer matrix. Cloisite [Na.sup.+] was previously exchanged with a cationic surfactant. The modified nanoclays were then dispersed into an epoxy resin at 5 wt% and subsequently photopolymerized.
The presence of the modified nanoclays did not affect the photopolymerization kinetics of the epoxy resin, and high insoluble fractions (gel %) were achieved.
XRD and TEM analyses showed that intercalation and partial exfoliation of the nanocomposites occur.
The [T.sub.g] values of the cured products, as measured by DMTA analysis, revealed an increase of the glass transition temperatures for the nanocomposites, because of the strong interactions between the intercalated/exfoliated platelets and the polymer matrix. The thermal stability, in terms of temperature of initial degradation, and the oxygen barrier properties were improved as well.
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Roberta Peila, (1) Giulio Malucelli, (1)Massimo Lazzari, (2) Aldo Priola (1)
(1) Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
(2) Departamento de Quimica Fisica, Facultad de Quimica, Universidad de Santiago de Compostela, Campus Universitario Sur, 15782 Santiago de Compostela, Spain
Correspondence to: Giulio Malucelli; e-mail: firstname.lastname@example.org
Contract grant sponsor: Regional Project (Piedmont); contract grant number: D26 2004.
Published online in Wiley InterScience (www.interscience.wiley.com).
[C] 2010 Society of Plastics Engineers
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|Author:||Peila, Roberta; Malucelli, Giulio; Lazzari, Massimo; Priola, Aldo|
|Publication:||Polymer Engineering and Science|
|Date:||Jul 1, 2010|
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