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LDH nanocomposite used for UV curing coatings
Progress in Organic Coatings 65 (2009) 450–456
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and properties of polymer/LDH nanocomposite used for UV curing coatings Shichang Lv, Wei Zhou, Hui Miao, Wenfang Shi ∗ Joint Laboratory of Polymer Thin Films and Solution, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
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Article history: Received 28 February 2008 Received in revised form 5 March 2009 Accepted 9 April 2009 Keywords: Layered double hydroxide (LDH) UV curing coating Nanocomposite
a b s t r a c t The thermal and mechanical properties of UV curing coatings consisted of urethane acrylates as an oligomer and a diacrylate monomer were reinforced by using layered double hydroxide (LDH). The LDH was organically modified by an ion-exchange process, in which the nitrate anions were replaced by long alkyl sulfate anions. With organic modification, the d-spacing of inorganic LDH layers was greatly enlarged to 2.75 from 0.78 nm, leading the LDH to be organophilic. During in situ photopolymerization process, the d-spacing of LDH layers was further enlarged to 4.29 nm, indicating the intercalation of polymer chains. For comparison, polymer/LDH nanocomposite filled with un-modified LDH was also prepared, and showed less enhancement in thermal and mechanical properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, organic–inorganic nanocomposites have drawn increased attention because of their distinct characteristics, in particular superior mechanical and barrier properties, as well as improved thermal stability and optical properties. Moreover, there has been considerable interest in the preparation of nanocomposites based on layered materials as guests and polymers as hosts [1–3]. These layered materials include layered silicates [4], manganese oxides [5], titanates [6] and layered phosphates [7]. The majority of previous studies have been focused on the layered silicates (clay), most of which are natural clay minerals. Their interlayer ions are cations, such as Na+ , K+ and Ca2+ . To make the clay organophilic, these alkalino cations could be replaced by alkylammonium cations. Since the first work done by researchers at Toyota Research Center on exfoliated nylon/clay nanocomposites [8], many kinds of polymers have been introduced for the preparation of polymer/clay nanocomposites, such as epoxy [9], polystyrene [10], polyimide [11], and polymethylmethacrylate [12]. However, little work has been done on polymer/layered double hydroxide (LDH) nanocomposites. LDHs, also known as anionic clays, are a large family of lamellar hydroxides. They may be represented by the ideal formula MII x MIII 1−x (OH)2 Ax/n n− ·mH2 O, where MII and MIII represent di- and tri-valent metal cations within the brucite-like layers, such as Mg2+ and Al3+ , respectively. An− is an interlayer anion, such as NO3 − , CO3 2− , and SO4 2− . Compared with the above men-
∗ Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi). 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.04.001
tioned cationic clays, these anions can be replaced by long alkyl chain anions during the modification process. The strong interaction between LDH layers and the presence of polar hydroxyl groups on the layer surface impede the non-polar organic molecules or polymer chains to enter into the galleries. In order to overcome the incompatibility between the organic phase and inorganic LDH fillers, the method frequently used is to replace the anions with hydrophobic tail surfactants in virtue of the ion-exchange ability of LDH. Two approaches have mainly been used for preparing polymer/LDH nanocomposites. One is the direct intercalation of polymer chains such as polystyrene, poly(ethylene oxide) and polyamide 6, to LDH layers [13–15]. The other is in situ polymerization of the monomers such as methylmethacrylate, epoxy and styrene, between LDH layers [16–18]. While a lot of studies have been done with thermally initiated polymer systems, less work is done for the preparation of UV-curable polymer nanocomposites by using UV irradiation. UV curing technology is increasingly applied in various scientific and industrial fields, because of its advantages, such as solvent-free, low energy consumption, and short processing time. Zahouily et al. prepared the UV-curable urethane polymer/clay nanocomposites based on layered organo-modified clay for the first time [19]. The X-ray diffraction method was used to confirm the presence of exfoliated nanometer-thick clay platelets. Uhl et al. reported a variety of UV-curable epoxy polymer/clay nanocomposites by using commercial clays and monomers as well as oligomers [20]. To our knowledge, the preparation of UV-curable polymer/LDH nanocomposites has not been reported. In the present work, MgAl-LDH is first synthesized, and then the long alkyl chain of dodecyl sulfate is intercalated into MgAl-LDH galleries through the ion-exchange. The modified MgAl(DS)-LDH becomes more
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organophilic, and the layer spacing distance extends greatly, which facilitates the intercalation and dispersion of organic molecules or polymer chains. Finally, the obtained LDH is incorporated to a urethane acrylate resin to prepare the UV-curable polymer/LDH nanocomposite. The enhanced thermal and mechanical properties of the nanocomposite are investigated in detail. For comparison, the polymer/un-modified LDH nanocomposite is also prepared. 2. Experimental 2.1. Materials Al(NO3 )3 ·9H2 O and Mg(NO3 )2 ·H2 O (analytical pure) were supplied by Shanghai Zhenxing Chemicals No. 1 Plant. Sodium dodecyl sulfate (SDS) and NaOH (analytical pure) were purchased from China National Medicine Group (Shanghai Chemical Reagents Co.). EB8402, an aliphatic urethane diacrylate with an unsaturation concentration of 2.0 mmol g−1 and a molar mass of 1000 g mol−1 , and 1,6-hexamethyldiol diacrylate (HDDA) were supplied by Cytec Industries Inc. (USA). 1-Hydroxy-cyclohexyl-phenyl ketone (Runtecure 1104), used as a photoinitiator, was supplied by Runtec chemicals Co. (Changzhou, China). All chemicals were used as received without further purification. 2.2. Preparation of MgAl-LDH and MgAl(DS)-LDH MgAl-LDH was prepared by the coprecipitation method previously described by Miyata [21]. A solution mixture of Mg(NO3 )2 ·H2 O (0.75 M) and Al(NO3 )3 ·9H2 O (0.25 M) was slowly added into 150 mL of decarbonated water under N2 atmosphere at a constant pH = 10.0 adjusted simultaneously by drop-wise addition of a 1.0 M NaOH aqueous solution. After the formed suspension was aged at 70 ◦ C for 24 h, the resultant precipitate was filtered, thoroughly washed with distilled water and then washed three times with acetone. The wet product was added to acetone to form the MgAl-LDH acetone suspension. The surfactant-intercalated MgAl(DS)-LDH was prepared by the ion-exchange approach of MgAl-LDH water suspension with 0.1 M SDS aqueous solution at 80 ◦ C for 3 days. The resultant precipitate was treated as that for MgAl-LDH, and added to acetone to form the MgAl(DS)-LDH acetone suspension.
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750 spectrometer with a KBr disk or thin film. The transmission electron microscope (TEM) images were obtained with JEOL-2011, operated at an acceleration voltage of 200 kV. The samples were ultramicrotomed with a diamond knife on a LKB Pyramitome to give 60-nm thick slices. The thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50H thermoanalyzer. In each case, a 10-mg sample was examined under a N2 flow rate of 6 × 10−5 m3 min−1 at a heating rate of 10 ◦ C min−1 from room temperature to 700 ◦ C. The photopolymerization rate was monitored in air by a CDR-1 differential scanning calorimeter (DSC) (Shanghai Balance Instrument Co., Shanghai, China) equipped with a UV spot cure system BHG-250 (Mejiro Precision Co., Japan). The incident light intensity at the sample pan was measured to be 2.4 mW cm−2 with a UV power meter. The unsaturation conversion (Pt ) was calculated by the formula, Pt = Ht /H∞ , where Ht is the heat effect within t s, H∞ is the heat effect of 100% unsaturation conversion. The DSC curves were normalized by the weight (g) of samples. The photo polymerization rate is defined by mmolC C g−1 s−1 , namely, the variation of unsaturation concentration (mmolC C g−1 ) per second. For calculating the polymerization rate and H∞ , the value, H0 = 86 J mmol−1 , for the heat of polymerization per acrylate unsaturation was taken. The tensile storage modulus (E ) and tensile loss factors (tan ı) of UV-cured films were measured by a dynamic mechanical thermal analyzer (Diamond DMA, PE Co., USA) at a frequency of 10 Hz and a heating rate of 10 ◦ C min−1 in the range of −100 to 200 ◦ C with 25 mm × 5 mm × 1 mm specimens. The crosslink density (e ) as the molar number of elastically effective network chain per cube centimeter of the film, was calculated from the storage modulus in the rubbery plateau region according to: e = (E /3RT), where E is the elastic storage modulus, R is the ideal gas constant, and T is the temperature in K. The mechanical properties were measured with an Instron Universal tester (model 1185, Japan) at 25 ◦ C with a crosshead speed of 25 mm min−1 . The dumbbell shaped specimens were prepared according to ASTM D412-87. Six samples were analyzed to obtain an average value and standard deviation. The abrasion resistance was measured with a QMX abrasion apparatus (Tianjin Exp. Apparatus Co., China) in accordance with the corresponding State Standard Testing Method (GB 173193). The pendulum hardness of the cured films was determined using a QBY pendulum apparatus (Tianjin Instrument Co., China). The pencil hardness of the cured films was determined using a QHQ-A pencil hardness apparatus (Tianjin Instrument Co., China).
2.3. Preparation of UV-cured nanocomposite film The formulation utilized in this study consisted of a 7:3 (w/w) mixture of EB8402 as an oligomer to HDDA as a monomer, and the LDH loadings of 1, 3, 5 wt%, respectively. A desired amount of MgAlLDH or MgAl(DS)-LDH acetone suspension was first dispersed in the EB8402/HDDA resin. After stirring vigorously at the speed of 1000 rad/min for 24 h and sonicating for 6 h to achieve the complete dispersion, acetone was removed under vacuum. Then, 1.5 wt% of photoinitiator (Runtecure 1104), based on the weight of formulation, was added, sealed in dark and stirred in N2 atmosphere at room temperature for 2 h to prevent unintentional cure. The final resin was applied to glass substrates to get films, and then exposed to a medium pressure mercury lamp (1 kW, Fusion UV systems, USA) with conveyer speed of 2.0 m min−1 . The incident light intensity on the sample was measured to be 30 mW cm−2 with a UV power meter. 2.4. Characterization The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu K␣ tube and Ni filter ( = 0.1542 nm). The Fourier transfer infrared (FTIR) spectra were recorded using a Nicolet MAGNA-IR
3. Results and discussion 3.1. Characterization of MgAl-LDH and MgAl(DS)-LDH The evidence for the preparation of MgAl-LDH and MgAl(DS)LDH comes from the FTIR, XRD and TGA studies. Fig. 1 shows the FTIR spectra of MgAl-LDH and MgAl(DS)-LDH. In the spectrum of MgAl-LDH (shown in Fig. 1a), the broad and strong band in the range of 3600–3200 cm−1 centered at 3472 cm−1 is due to the O–H stretching vibrations of surface and interlayer water molecule. The band observed near 1637 cm−1 is assigned to the bending vibration of water molecule. A strong absorption band at 1385 cm−1 is due to the presence of nitrate. The spectrum of MgAl(DS)-LDH (shown in Fig. 1b) displays the characteristic absorption bands of –CH2 or –CH3 in the aliphatic chains of DS at 2850, 2920, and 2956 cm−1 . Two strong absorption peaks at 1223 and 1469 cm−1 can be assigned to the stretching vibrations of sulfate and methylene, respectively. These FTIR assignments demonstrate that the DS chains had been intercalated into the galleries of MgAl-LDH. The powder X-ray diffraction patterns for MgAl-LDH and MgAl(DS)-LDH are shown in Fig. 2. Fig. 2a shows the typical XRD pattern of MgAl-LDH as a pure hydrotalcite. The first diffraction
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Fig. 1. FTIR spectra of MgAl-LDH (a) and MgAl(DS)-LDH (b).
Fig. 3. TG/DTG curves of MgAl-LDH and MgAl(DS)-LDH.
Fig. 2. XRD patterns of MgAl-LDH (a) and MgAl(DS)-LDH (b).
peak at 2 = 11.48◦ corresponds to a d-spacing of 0.78 nm, which is consistent with the value reported by Chibwe and Jones [22]. After the ion-exchange with SDS solution, the first diffraction peak of MgAl(DS)-LDH moved to the lower angle position (2 = 3.21◦ ), which corresponding to a d-spacing of 2.75 nm (shown in Fig. 2b). The enlarged d-spacing of MgAl(DS)-LDH indicates the successful intercalation of DS chains to the layers of MgAl-LDH, thus making LDH organophilic and facilitating the intercalation and dispersion of organic materials. The thermogravimetric analysis (TGA) in N2 was used to determine the content of organic modifier and other volatile materials (e.g. water) in the sample. As shown in Fig. 3, the decomposition of MgAl-LDH is divided into two steps. In the first one, at around 177 ◦ C, a weight loss of 11.3% is due to the loss of physically adsorbed and interlayer water. In the second one, at around 331 ◦ C, a weight loss of 24.9% can be ascribed to the dehydroxylation of the LDH layer and the elimination of NO3 − . The total weight loss for the region (35–600 ◦ C) is 38.2%. In the DTG curve of MgAl(DS)-LDH, there are three peaks at around 70, 226 and 392 ◦ C, with three corresponding steps of weight loss. In the first step, the weight loss from room temperature to 150 ◦ C can be attributed to the loss of the absorbed water [23,24]. After ion-exchange, LDH becomes organophilic and the hydrogen bond interaction between water molecules decreases,
resulting in a reduction of the decomposition temperature. In the second step, at around 226 ◦ C, a weight loss of 33.4% can be ascribed to the degradation of DS chain in the LDH layer. The third step, around 392 ◦ C, is due to the dehydroxylation of LDH sheet, which is hindered by the second step. As a result, the decomposition temperature increases. The total weight loss for the same region is 54.9%, which is 16.7% higher than that for MgAl-LDH. These results are in agreement with the FTIR and XRD measurements. 3.2. Morphology and dispersion of MgAl-LDH and MgAl(DS)-LDH in nanocomposites The XRD patterns of polymer/MgAl-LDH and polymer/ MgAl(DS)-LDH nanocomposites at 5 wt% LDH loading are presented in Fig. 4. It should be noted that the absence of XRD diffraction peaks does not necessarily mean the occurrence of exfoliation, as the observed absence of diffraction peak could be due to the lack of sensitivity at the lower amount of LDH loading or random orientation of LDH layers. For the polymer/MgAl-LDH nanocomposite, there is a diffraction peak observed at 10.84◦ corresponding to the d-spacing of 0.82 nm, which is slightly larger than that of MgAl-LDH (0.78 nm). This indicates that relatively few polymer chains had been intercalated into the LDH layers during the photopolymerization process. However, for the cured polymer/MgAl(DS)-LDH nanocomposite, the d-spacing value was enlarged from 2.75 nm of MgAl(DS)-LDH to 4.29 nm, indicating a higher number of polymer chains had been intercalated into the
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Fig. 4. XRD patterns of polymer/MgAl-LDH (a) and polymer/MgAl(DS)-LDH (b) nanocomposites at 5 wt% LDH loading.
LDH layers as compared to polymer/MgAl-LDH nanocomposite. During the ion-exchange process, the long alkyl DS chains were intercalated into the LDH layer, converting hydrophilic MgAlLDH to more hydrophobic MgAl(DS)-LDH. This process not only enlarged the d-spacing between the LDH layers, but also improved the interfacial properties between inorganic and organic phases, facilitating the intercalation and diffusion of acrylic molecule into the interlayer space. Fig. 5 shows the TEM micrographs of polymer/MgAl-LDH and polymer/MgAl(DS)-LDH nanocomposites at 5 wt% LDH loading. The dark lines are the intersections of LDH platelets. It is well known that TEM photograph shows the local microscopic conformation. For the polymer/MgAl-LDH nanocomposite (Fig. 5a), the MgAlLDH particles stacked tightly in a lamellar structure, and had an intercalated structure. However, for the polymer/MgAl(DS)-LDH nanocomposite, it can be seen that MgAl(DS)-LDH platelets showed well dispersion in the polymer matrix (Fig. 5b), because the LDH was previously intercalated by organophilic DS chains. 3.3. Properties of UV-cured polymer/LDH nanocomposite film The properties of a UV-cured film depend on not only the resin composition but also the photopolymerization kinetics. In order to investigate the effect of LDH loading on the photopolymeriza-
Fig. 5. TEM micrographs of polymer/MgAl-LDH (a) and polymer/MgAl(DS)-LDH (b) nanocomposites at 5 wt% LDH loading.
Fig. 6. Photopolymerization rates of pure polymer and polymer/LDH nanocomposites.
tion kinetics, the photopolymerization rate at the peak maximum (Rpmax ) and the final conversion (Pf ) of double bond were measured, as shown in Figs. 6 and 7, respectively. For polymer/MgAl(DS)LDH nanocomposites, with increasing the LDH content, the Rpmax decreases slightly and the longer irradiation time is needed to reach to Rpmax . This can be attributed to the lower concentration of double bond, and higher viscosity compared with the pure resin. Moreover, the final unsaturation conversion also decreases with increasing the LDH content, but leveling to acceptable values. This indicates the better compatibility between MgAl(DS)-LDH and the organic phases. However, for polymer/MgAl-LDH, not only Rpmax but also final Pf decrease distinctly with the incorporation of inorganic component. For the nanocomposite with high MgAl-LDH loading (such as 5%), a very low unsaturation conversion of less than 50% is obtained. This can be due to the poor compatibility between acrylate resin and inorganic MgAl-LDH, the inorganic LDH layer restricts the acrylate molecules from further photopolymerization. Fig. 8 shows the TGA curves of UV-cured polymer/MgAl-LDH and polymer/MgAl(DS)-LDH nanocomposites with different LDH loadings. When 30% weight loss is selected as a point of comparison, the decomposition temperatures for pure polymer, and polymer/MgAl-LDH nanocomposite with 1, 3, and 5 wt% LDH loading are determined to be 353, 338, 331 and 327 ◦ C, respectively. However, polymer/MgAl(DS)-LDH nanocomposites with the same LDH loadings, the decomposition temperatures increase with
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Fig. 7. Unsaturation conversion of pure polymer and polymer/LDH nanocomposites.
increasing MgAl(DS)-LDH content up to 357, 361, and 366 ◦ C, respectively. As the LDH content increases from 1 to 5 wt%, the increase of the char residue is also obtained. However, MgAl(DS)LDH (5.9% residue for 5% loading) can promote the charring process more effectively than MgAl-LDH (2.9% residue for 5% loading) during the degradation process. It can be clearly found from the figure that the thermal stability of the cured polymer/LDH nanocomposite films was improved by the incorporation of LDH to the polymer matrix. The thermal stability between 300 and 500 ◦ C was lower for the incorporation of MgAl-LDH, but enhanced for MgAl(DS)-LDH. This can be explained that the addition of MgAlLDH can largely reduce the real cross-density of the polymer matrix due to its poor compatibility with organic phase. However, for MgAl(DS)-LDH, the LDH platelets dispersed very well in the polymer matrix, and hindered the thermal degradation of the polymer chains. Dynamic mechanical thermal analysis (DMTA) was utilized to investigate the viscoelastic properties in an effort to further examine the microstructures of cured films. The storage modulus (E ) and tan ı curves of the pure polymer and its corresponding nanocomposites with 5 wt% MgAl-LDH and MgAl(DS)-LDH loadings are shown in Fig. 9. The glass transition temperature (Tg ) is defined as the peak temperature of tan ı curve. At the rubbery plateau (T = 110 ◦ C), the E /Tg values for MgAl-LDH and MgAl(DS)LDH nanocomposites were measured to be 66.1 MPa/61.9 ◦ C and
Fig. 8. TGA curves of pure polymer and polymer/LDH nanocomposites under N2 flow.
Fig. 9. DMTA curves of pure polymer, polymer/MgAl-LDH and polymer/MgAl(DS)LDH nanocomposites at 5 wt% LDH loading.
83.2 MPa/65.6 ◦ C compared with 53.7 MPa/58.0 ◦ C for that of the pure polymer. Table 1 reports the data obtained from all DMTA tests. As listed in Table 1, for the nanocomposite with MgAl-LDH or MgAl(DS)-LDH addition, the increases in E , Tg and crosslink density (XLD) are observed with increasing LDH content, even though
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Table 1 Rubbery storage modulus, Tg and crosslink density (XLD) from DMTA. Sample
Rubbery storage modulus, E (MPa)
Tg (◦ C)
XLD (mol/cm3 )
Pure polymer 1 wt% MgAl-LDH 3 wt% MgAl-LDH 5 wt% MgAl-LDH 1 wt% MgAl(DS)-LDH 3 wt% MgAl(DS)-LDH 5 wt% MgAl(DS)-LDH
53.7 58.7 66.5 66.1 68.3 74.1 83.2
58.0 58.8 60.6 61.9 60.3 62.7 65.6
5.62 × 10−3 6.14 × 10−3 6.96 × 10−3 6.92 × 10−3 7.15 × 10−3 7.75 × 10−3 8.71 × 10−3
Table 2 Properties of the pure polymer and polymer/LDH nanocomposites. Sample
Pure polymer 1 wt% MgAl-LDH 3 wt% MgAl-LDH 5 wt% MgAl-LDH 1 wt% MgAl(DS)-LDH 3 wt% MgAl(DS)-LDH 5 wt% MgAl(DS)-LDH
Tensile strength (MPa)
44.6 46.1 49.8 50.3 47.0 51.9 55.7
Elongation at break (%)
Abrasion resistance (mg)
25.7 22.0 19.2 16.3 22.6 17.1 13.7
Rpmax and Pf values for these films were lower than that for the pure polymer film. These results can be attributed to the incorporation of LDH. It is well known that polymer chains can physically aggregate onto particulate surface, resulting in an increase in the effective degree of crosslinking [25]. In this system, the LDH layers acted as crosslinkers in the polymer chain networks. Based on our results, MgAl(DS)-LDH shows a bigger rise in the effective degree of crosslinking, which is consistent with the results obtained from the TEM observation that MgAl(DS)-LDH dispersed better than MgAlLDH in the polymer matrix. The influence of LDH content on the other physical properties of polymer/LDH nanocomposites is listed in Table 2. The tensile strength of the nanocomposites increases with increasing LDH loading, which is in agreement with the results obtained by DMTA. The increase in E is also observed in the presence of LDH. However, the percent elongation decreases compared with the pure polymer, as the polymer chains in nanocomposites are restricted by the LDH layers, resulting in the decreased degree of freedom. On the other hand, as the LDH component increases, the enhancement of abrasion resistance is obtained due to the interaction between the LDH layer and the polymer matrix. The pendulum and pencil hardness of the cured nanocomposites were used to determine the hardness of the nanocomposite films. As listed in Table 2, the pendulum and pencil hardness of the UV-cured pure polymer films are 88 s and 2H, respectively. With the addition of 1, 3, 5 wt% MgAl-LDH/MgAl(DS)-LDH in the cured films, the values go up to 91/92, 94/98, 96/103 s and 2H/3H, 2H/3H, 3H/4H, respectively. Compared with the pure polymer film, the pendulum and pencil hardness of the cured nanocomposites increase with increasing the LDH loading in the nanocomposite film. Moreover, the polymer/MgAl(DS)-LDH nanocomposite shows better physical properties than polymer/MgAl-LDH nanocomposite. These results can be explained by the better compatibility of MgAl(DS)-LDH with the polymer matrix and the higher effective crosslink density of the polymer/MgAl(DS)-LDH nanocomposite. 4. Conclusion In conclusion, the preparations and properties of UV-curable urethane acrylate nanocomposites based on MgAl-LDH and MgAl(DS)-LDH were studied. After ion-exchange, the hydrophobic
11.2 10.4 9.2 8.8 10.2 8.7 7.5
Hardness Pendulum (s)
Pencil (H)
88 91 94 96 92 98 103
2 2 2 3 3 3 4
MgAl(DS)-LDH nanoparticles were well dispersed in the polymer matrix and showed good compatibility. The LDH platelets played the role of crosslinkers during the photopolymerization and restricted the freedom of the polymer chains. Therefore, the polymer/MgAl(DS)-LDH nanocomposite possessed remarkable enhanced thermal and mechanical properties as compared to the pure polymer. On the contrary, the MgAl-LDH nanoparticles acted as conventional inorganic nano-fillers in the composite because of their poor compatibility with the polymer matrix. The higher Tg and mechanical properties were resulted from simple addition of more hard material and less soft material, resulting in the limited improvement in the thermal and mechanical properties. This study indicated that, even though intercalated structures were formed during the photopolymerization, the enhancement in properties could still be obtained. Thus, the use of LDHs as nano-reinforcement fillers is a feasible approach to improve the properties of UV-cured nanocomposites. Acknowledgment The financial support of National Natural Science Foundation of China (No. 50633010) is gratefully acknowledged. References [1] T. Lan, T.J. Pinnavaia, Chem. Mater. 6 (1994) 2216. [2] K. Wang, L. Chen, J.S. Wu, M.L. Toh, C.B. He, A.F. Yee, Macromolecules 38 (2005) 788. [3] F.N. Cao, S.C. Jana, Polymer 48 (2007) 3790. [4] J. Ma, Z.Z. Yu, Q.X. Zhang, X.L. Xie, Y.W. Mai, I. Luck, Chem. Mater. 16 (2004) 757. [5] A.H. Gemeay, I.A. Mansour, R.G. El-Sharkawy, A.B. Zaki, Eur. Polym. J. 41 (2005) 2575. [6] D.Y. Tang, L.S. Qiang, Z. Jin, W.M. Cai, J. Appl. Polym. Sci. 84 (2001) 709. [7] L.Y. Sun, W.J. Boo, D.Z. Sun, A. Clearfield, H.J. Sue, Chem. Mater. 19 (2007) 1749. [8] Y. Fukushima, S. Inagaki, J. Inclus. Phenom. 5 (1987) 473. [9] X. Kornmann, H. Lindberg, L.A. Berglund, Polymer 42 (2001) 1303. [10] X. Fu, S. Qutubuddin, Polymer 42 (2001) 807. [11] H.L. Tyan, Y.C. Liu, K.H. Wei, Chem. Mater. 11 (1999) 1942. [12] D.C. Lee, L.W. Jang, J. Appl. Polym. Sci. 61 (1996) 1117. [13] L.Z. Qiu, W. Chen, B.J. Qu, Polym. Degrad. Stabil. 87 (2005) 433. [14] C.S. Liao, W.B. Ye, J. Polym. Res. 10 (2003) 241. [15] M. Zammarano, S. Bellayer, J.W. Gilman, M. Franceschi, F.L. Beyer, R.H. Harris, S. Meriani, Polymer 47 (2006) 652.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and properties of polymer/LDH nanocomposite used for UV curing coatings Shichang Lv, Wei Zhou, Hui Miao, Wenfang Shi ∗ Joint Laboratory of Polymer Thin Films and Solution, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
a r t i c l e
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Article history: Received 28 February 2008 Received in revised form 5 March 2009 Accepted 9 April 2009 Keywords: Layered double hydroxide (LDH) UV curing coating Nanocomposite
a b s t r a c t The thermal and mechanical properties of UV curing coatings consisted of urethane acrylates as an oligomer and a diacrylate monomer were reinforced by using layered double hydroxide (LDH). The LDH was organically modified by an ion-exchange process, in which the nitrate anions were replaced by long alkyl sulfate anions. With organic modification, the d-spacing of inorganic LDH layers was greatly enlarged to 2.75 from 0.78 nm, leading the LDH to be organophilic. During in situ photopolymerization process, the d-spacing of LDH layers was further enlarged to 4.29 nm, indicating the intercalation of polymer chains. For comparison, polymer/LDH nanocomposite filled with un-modified LDH was also prepared, and showed less enhancement in thermal and mechanical properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, organic–inorganic nanocomposites have drawn increased attention because of their distinct characteristics, in particular superior mechanical and barrier properties, as well as improved thermal stability and optical properties. Moreover, there has been considerable interest in the preparation of nanocomposites based on layered materials as guests and polymers as hosts [1–3]. These layered materials include layered silicates [4], manganese oxides [5], titanates [6] and layered phosphates [7]. The majority of previous studies have been focused on the layered silicates (clay), most of which are natural clay minerals. Their interlayer ions are cations, such as Na+ , K+ and Ca2+ . To make the clay organophilic, these alkalino cations could be replaced by alkylammonium cations. Since the first work done by researchers at Toyota Research Center on exfoliated nylon/clay nanocomposites [8], many kinds of polymers have been introduced for the preparation of polymer/clay nanocomposites, such as epoxy [9], polystyrene [10], polyimide [11], and polymethylmethacrylate [12]. However, little work has been done on polymer/layered double hydroxide (LDH) nanocomposites. LDHs, also known as anionic clays, are a large family of lamellar hydroxides. They may be represented by the ideal formula MII x MIII 1−x (OH)2 Ax/n n− ·mH2 O, where MII and MIII represent di- and tri-valent metal cations within the brucite-like layers, such as Mg2+ and Al3+ , respectively. An− is an interlayer anion, such as NO3 − , CO3 2− , and SO4 2− . Compared with the above men-
∗ Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi). 0300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2009.04.001
tioned cationic clays, these anions can be replaced by long alkyl chain anions during the modification process. The strong interaction between LDH layers and the presence of polar hydroxyl groups on the layer surface impede the non-polar organic molecules or polymer chains to enter into the galleries. In order to overcome the incompatibility between the organic phase and inorganic LDH fillers, the method frequently used is to replace the anions with hydrophobic tail surfactants in virtue of the ion-exchange ability of LDH. Two approaches have mainly been used for preparing polymer/LDH nanocomposites. One is the direct intercalation of polymer chains such as polystyrene, poly(ethylene oxide) and polyamide 6, to LDH layers [13–15]. The other is in situ polymerization of the monomers such as methylmethacrylate, epoxy and styrene, between LDH layers [16–18]. While a lot of studies have been done with thermally initiated polymer systems, less work is done for the preparation of UV-curable polymer nanocomposites by using UV irradiation. UV curing technology is increasingly applied in various scientific and industrial fields, because of its advantages, such as solvent-free, low energy consumption, and short processing time. Zahouily et al. prepared the UV-curable urethane polymer/clay nanocomposites based on layered organo-modified clay for the first time [19]. The X-ray diffraction method was used to confirm the presence of exfoliated nanometer-thick clay platelets. Uhl et al. reported a variety of UV-curable epoxy polymer/clay nanocomposites by using commercial clays and monomers as well as oligomers [20]. To our knowledge, the preparation of UV-curable polymer/LDH nanocomposites has not been reported. In the present work, MgAl-LDH is first synthesized, and then the long alkyl chain of dodecyl sulfate is intercalated into MgAl-LDH galleries through the ion-exchange. The modified MgAl(DS)-LDH becomes more
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organophilic, and the layer spacing distance extends greatly, which facilitates the intercalation and dispersion of organic molecules or polymer chains. Finally, the obtained LDH is incorporated to a urethane acrylate resin to prepare the UV-curable polymer/LDH nanocomposite. The enhanced thermal and mechanical properties of the nanocomposite are investigated in detail. For comparison, the polymer/un-modified LDH nanocomposite is also prepared. 2. Experimental 2.1. Materials Al(NO3 )3 ·9H2 O and Mg(NO3 )2 ·H2 O (analytical pure) were supplied by Shanghai Zhenxing Chemicals No. 1 Plant. Sodium dodecyl sulfate (SDS) and NaOH (analytical pure) were purchased from China National Medicine Group (Shanghai Chemical Reagents Co.). EB8402, an aliphatic urethane diacrylate with an unsaturation concentration of 2.0 mmol g−1 and a molar mass of 1000 g mol−1 , and 1,6-hexamethyldiol diacrylate (HDDA) were supplied by Cytec Industries Inc. (USA). 1-Hydroxy-cyclohexyl-phenyl ketone (Runtecure 1104), used as a photoinitiator, was supplied by Runtec chemicals Co. (Changzhou, China). All chemicals were used as received without further purification. 2.2. Preparation of MgAl-LDH and MgAl(DS)-LDH MgAl-LDH was prepared by the coprecipitation method previously described by Miyata [21]. A solution mixture of Mg(NO3 )2 ·H2 O (0.75 M) and Al(NO3 )3 ·9H2 O (0.25 M) was slowly added into 150 mL of decarbonated water under N2 atmosphere at a constant pH = 10.0 adjusted simultaneously by drop-wise addition of a 1.0 M NaOH aqueous solution. After the formed suspension was aged at 70 ◦ C for 24 h, the resultant precipitate was filtered, thoroughly washed with distilled water and then washed three times with acetone. The wet product was added to acetone to form the MgAl-LDH acetone suspension. The surfactant-intercalated MgAl(DS)-LDH was prepared by the ion-exchange approach of MgAl-LDH water suspension with 0.1 M SDS aqueous solution at 80 ◦ C for 3 days. The resultant precipitate was treated as that for MgAl-LDH, and added to acetone to form the MgAl(DS)-LDH acetone suspension.
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750 spectrometer with a KBr disk or thin film. The transmission electron microscope (TEM) images were obtained with JEOL-2011, operated at an acceleration voltage of 200 kV. The samples were ultramicrotomed with a diamond knife on a LKB Pyramitome to give 60-nm thick slices. The thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50H thermoanalyzer. In each case, a 10-mg sample was examined under a N2 flow rate of 6 × 10−5 m3 min−1 at a heating rate of 10 ◦ C min−1 from room temperature to 700 ◦ C. The photopolymerization rate was monitored in air by a CDR-1 differential scanning calorimeter (DSC) (Shanghai Balance Instrument Co., Shanghai, China) equipped with a UV spot cure system BHG-250 (Mejiro Precision Co., Japan). The incident light intensity at the sample pan was measured to be 2.4 mW cm−2 with a UV power meter. The unsaturation conversion (Pt ) was calculated by the formula, Pt = Ht /H∞ , where Ht is the heat effect within t s, H∞ is the heat effect of 100% unsaturation conversion. The DSC curves were normalized by the weight (g) of samples. The photo polymerization rate is defined by mmolC C g−1 s−1 , namely, the variation of unsaturation concentration (mmolC C g−1 ) per second. For calculating the polymerization rate and H∞ , the value, H0 = 86 J mmol−1 , for the heat of polymerization per acrylate unsaturation was taken. The tensile storage modulus (E ) and tensile loss factors (tan ı) of UV-cured films were measured by a dynamic mechanical thermal analyzer (Diamond DMA, PE Co., USA) at a frequency of 10 Hz and a heating rate of 10 ◦ C min−1 in the range of −100 to 200 ◦ C with 25 mm × 5 mm × 1 mm specimens. The crosslink density (e ) as the molar number of elastically effective network chain per cube centimeter of the film, was calculated from the storage modulus in the rubbery plateau region according to: e = (E /3RT), where E is the elastic storage modulus, R is the ideal gas constant, and T is the temperature in K. The mechanical properties were measured with an Instron Universal tester (model 1185, Japan) at 25 ◦ C with a crosshead speed of 25 mm min−1 . The dumbbell shaped specimens were prepared according to ASTM D412-87. Six samples were analyzed to obtain an average value and standard deviation. The abrasion resistance was measured with a QMX abrasion apparatus (Tianjin Exp. Apparatus Co., China) in accordance with the corresponding State Standard Testing Method (GB 173193). The pendulum hardness of the cured films was determined using a QBY pendulum apparatus (Tianjin Instrument Co., China). The pencil hardness of the cured films was determined using a QHQ-A pencil hardness apparatus (Tianjin Instrument Co., China).
2.3. Preparation of UV-cured nanocomposite film The formulation utilized in this study consisted of a 7:3 (w/w) mixture of EB8402 as an oligomer to HDDA as a monomer, and the LDH loadings of 1, 3, 5 wt%, respectively. A desired amount of MgAlLDH or MgAl(DS)-LDH acetone suspension was first dispersed in the EB8402/HDDA resin. After stirring vigorously at the speed of 1000 rad/min for 24 h and sonicating for 6 h to achieve the complete dispersion, acetone was removed under vacuum. Then, 1.5 wt% of photoinitiator (Runtecure 1104), based on the weight of formulation, was added, sealed in dark and stirred in N2 atmosphere at room temperature for 2 h to prevent unintentional cure. The final resin was applied to glass substrates to get films, and then exposed to a medium pressure mercury lamp (1 kW, Fusion UV systems, USA) with conveyer speed of 2.0 m min−1 . The incident light intensity on the sample was measured to be 30 mW cm−2 with a UV power meter. 2.4. Characterization The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu K␣ tube and Ni filter ( = 0.1542 nm). The Fourier transfer infrared (FTIR) spectra were recorded using a Nicolet MAGNA-IR
3. Results and discussion 3.1. Characterization of MgAl-LDH and MgAl(DS)-LDH The evidence for the preparation of MgAl-LDH and MgAl(DS)LDH comes from the FTIR, XRD and TGA studies. Fig. 1 shows the FTIR spectra of MgAl-LDH and MgAl(DS)-LDH. In the spectrum of MgAl-LDH (shown in Fig. 1a), the broad and strong band in the range of 3600–3200 cm−1 centered at 3472 cm−1 is due to the O–H stretching vibrations of surface and interlayer water molecule. The band observed near 1637 cm−1 is assigned to the bending vibration of water molecule. A strong absorption band at 1385 cm−1 is due to the presence of nitrate. The spectrum of MgAl(DS)-LDH (shown in Fig. 1b) displays the characteristic absorption bands of –CH2 or –CH3 in the aliphatic chains of DS at 2850, 2920, and 2956 cm−1 . Two strong absorption peaks at 1223 and 1469 cm−1 can be assigned to the stretching vibrations of sulfate and methylene, respectively. These FTIR assignments demonstrate that the DS chains had been intercalated into the galleries of MgAl-LDH. The powder X-ray diffraction patterns for MgAl-LDH and MgAl(DS)-LDH are shown in Fig. 2. Fig. 2a shows the typical XRD pattern of MgAl-LDH as a pure hydrotalcite. The first diffraction
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Fig. 1. FTIR spectra of MgAl-LDH (a) and MgAl(DS)-LDH (b).
Fig. 3. TG/DTG curves of MgAl-LDH and MgAl(DS)-LDH.
Fig. 2. XRD patterns of MgAl-LDH (a) and MgAl(DS)-LDH (b).
peak at 2 = 11.48◦ corresponds to a d-spacing of 0.78 nm, which is consistent with the value reported by Chibwe and Jones [22]. After the ion-exchange with SDS solution, the first diffraction peak of MgAl(DS)-LDH moved to the lower angle position (2 = 3.21◦ ), which corresponding to a d-spacing of 2.75 nm (shown in Fig. 2b). The enlarged d-spacing of MgAl(DS)-LDH indicates the successful intercalation of DS chains to the layers of MgAl-LDH, thus making LDH organophilic and facilitating the intercalation and dispersion of organic materials. The thermogravimetric analysis (TGA) in N2 was used to determine the content of organic modifier and other volatile materials (e.g. water) in the sample. As shown in Fig. 3, the decomposition of MgAl-LDH is divided into two steps. In the first one, at around 177 ◦ C, a weight loss of 11.3% is due to the loss of physically adsorbed and interlayer water. In the second one, at around 331 ◦ C, a weight loss of 24.9% can be ascribed to the dehydroxylation of the LDH layer and the elimination of NO3 − . The total weight loss for the region (35–600 ◦ C) is 38.2%. In the DTG curve of MgAl(DS)-LDH, there are three peaks at around 70, 226 and 392 ◦ C, with three corresponding steps of weight loss. In the first step, the weight loss from room temperature to 150 ◦ C can be attributed to the loss of the absorbed water [23,24]. After ion-exchange, LDH becomes organophilic and the hydrogen bond interaction between water molecules decreases,
resulting in a reduction of the decomposition temperature. In the second step, at around 226 ◦ C, a weight loss of 33.4% can be ascribed to the degradation of DS chain in the LDH layer. The third step, around 392 ◦ C, is due to the dehydroxylation of LDH sheet, which is hindered by the second step. As a result, the decomposition temperature increases. The total weight loss for the same region is 54.9%, which is 16.7% higher than that for MgAl-LDH. These results are in agreement with the FTIR and XRD measurements. 3.2. Morphology and dispersion of MgAl-LDH and MgAl(DS)-LDH in nanocomposites The XRD patterns of polymer/MgAl-LDH and polymer/ MgAl(DS)-LDH nanocomposites at 5 wt% LDH loading are presented in Fig. 4. It should be noted that the absence of XRD diffraction peaks does not necessarily mean the occurrence of exfoliation, as the observed absence of diffraction peak could be due to the lack of sensitivity at the lower amount of LDH loading or random orientation of LDH layers. For the polymer/MgAl-LDH nanocomposite, there is a diffraction peak observed at 10.84◦ corresponding to the d-spacing of 0.82 nm, which is slightly larger than that of MgAl-LDH (0.78 nm). This indicates that relatively few polymer chains had been intercalated into the LDH layers during the photopolymerization process. However, for the cured polymer/MgAl(DS)-LDH nanocomposite, the d-spacing value was enlarged from 2.75 nm of MgAl(DS)-LDH to 4.29 nm, indicating a higher number of polymer chains had been intercalated into the
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Fig. 4. XRD patterns of polymer/MgAl-LDH (a) and polymer/MgAl(DS)-LDH (b) nanocomposites at 5 wt% LDH loading.
LDH layers as compared to polymer/MgAl-LDH nanocomposite. During the ion-exchange process, the long alkyl DS chains were intercalated into the LDH layer, converting hydrophilic MgAlLDH to more hydrophobic MgAl(DS)-LDH. This process not only enlarged the d-spacing between the LDH layers, but also improved the interfacial properties between inorganic and organic phases, facilitating the intercalation and diffusion of acrylic molecule into the interlayer space. Fig. 5 shows the TEM micrographs of polymer/MgAl-LDH and polymer/MgAl(DS)-LDH nanocomposites at 5 wt% LDH loading. The dark lines are the intersections of LDH platelets. It is well known that TEM photograph shows the local microscopic conformation. For the polymer/MgAl-LDH nanocomposite (Fig. 5a), the MgAlLDH particles stacked tightly in a lamellar structure, and had an intercalated structure. However, for the polymer/MgAl(DS)-LDH nanocomposite, it can be seen that MgAl(DS)-LDH platelets showed well dispersion in the polymer matrix (Fig. 5b), because the LDH was previously intercalated by organophilic DS chains. 3.3. Properties of UV-cured polymer/LDH nanocomposite film The properties of a UV-cured film depend on not only the resin composition but also the photopolymerization kinetics. In order to investigate the effect of LDH loading on the photopolymeriza-
Fig. 5. TEM micrographs of polymer/MgAl-LDH (a) and polymer/MgAl(DS)-LDH (b) nanocomposites at 5 wt% LDH loading.
Fig. 6. Photopolymerization rates of pure polymer and polymer/LDH nanocomposites.
tion kinetics, the photopolymerization rate at the peak maximum (Rpmax ) and the final conversion (Pf ) of double bond were measured, as shown in Figs. 6 and 7, respectively. For polymer/MgAl(DS)LDH nanocomposites, with increasing the LDH content, the Rpmax decreases slightly and the longer irradiation time is needed to reach to Rpmax . This can be attributed to the lower concentration of double bond, and higher viscosity compared with the pure resin. Moreover, the final unsaturation conversion also decreases with increasing the LDH content, but leveling to acceptable values. This indicates the better compatibility between MgAl(DS)-LDH and the organic phases. However, for polymer/MgAl-LDH, not only Rpmax but also final Pf decrease distinctly with the incorporation of inorganic component. For the nanocomposite with high MgAl-LDH loading (such as 5%), a very low unsaturation conversion of less than 50% is obtained. This can be due to the poor compatibility between acrylate resin and inorganic MgAl-LDH, the inorganic LDH layer restricts the acrylate molecules from further photopolymerization. Fig. 8 shows the TGA curves of UV-cured polymer/MgAl-LDH and polymer/MgAl(DS)-LDH nanocomposites with different LDH loadings. When 30% weight loss is selected as a point of comparison, the decomposition temperatures for pure polymer, and polymer/MgAl-LDH nanocomposite with 1, 3, and 5 wt% LDH loading are determined to be 353, 338, 331 and 327 ◦ C, respectively. However, polymer/MgAl(DS)-LDH nanocomposites with the same LDH loadings, the decomposition temperatures increase with
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Fig. 7. Unsaturation conversion of pure polymer and polymer/LDH nanocomposites.
increasing MgAl(DS)-LDH content up to 357, 361, and 366 ◦ C, respectively. As the LDH content increases from 1 to 5 wt%, the increase of the char residue is also obtained. However, MgAl(DS)LDH (5.9% residue for 5% loading) can promote the charring process more effectively than MgAl-LDH (2.9% residue for 5% loading) during the degradation process. It can be clearly found from the figure that the thermal stability of the cured polymer/LDH nanocomposite films was improved by the incorporation of LDH to the polymer matrix. The thermal stability between 300 and 500 ◦ C was lower for the incorporation of MgAl-LDH, but enhanced for MgAl(DS)-LDH. This can be explained that the addition of MgAlLDH can largely reduce the real cross-density of the polymer matrix due to its poor compatibility with organic phase. However, for MgAl(DS)-LDH, the LDH platelets dispersed very well in the polymer matrix, and hindered the thermal degradation of the polymer chains. Dynamic mechanical thermal analysis (DMTA) was utilized to investigate the viscoelastic properties in an effort to further examine the microstructures of cured films. The storage modulus (E ) and tan ı curves of the pure polymer and its corresponding nanocomposites with 5 wt% MgAl-LDH and MgAl(DS)-LDH loadings are shown in Fig. 9. The glass transition temperature (Tg ) is defined as the peak temperature of tan ı curve. At the rubbery plateau (T = 110 ◦ C), the E /Tg values for MgAl-LDH and MgAl(DS)LDH nanocomposites were measured to be 66.1 MPa/61.9 ◦ C and
Fig. 8. TGA curves of pure polymer and polymer/LDH nanocomposites under N2 flow.
Fig. 9. DMTA curves of pure polymer, polymer/MgAl-LDH and polymer/MgAl(DS)LDH nanocomposites at 5 wt% LDH loading.
83.2 MPa/65.6 ◦ C compared with 53.7 MPa/58.0 ◦ C for that of the pure polymer. Table 1 reports the data obtained from all DMTA tests. As listed in Table 1, for the nanocomposite with MgAl-LDH or MgAl(DS)-LDH addition, the increases in E , Tg and crosslink density (XLD) are observed with increasing LDH content, even though
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Table 1 Rubbery storage modulus, Tg and crosslink density (XLD) from DMTA. Sample
Rubbery storage modulus, E (MPa)
Tg (◦ C)
XLD (mol/cm3 )
Pure polymer 1 wt% MgAl-LDH 3 wt% MgAl-LDH 5 wt% MgAl-LDH 1 wt% MgAl(DS)-LDH 3 wt% MgAl(DS)-LDH 5 wt% MgAl(DS)-LDH
53.7 58.7 66.5 66.1 68.3 74.1 83.2
58.0 58.8 60.6 61.9 60.3 62.7 65.6
5.62 × 10−3 6.14 × 10−3 6.96 × 10−3 6.92 × 10−3 7.15 × 10−3 7.75 × 10−3 8.71 × 10−3
Table 2 Properties of the pure polymer and polymer/LDH nanocomposites. Sample
Pure polymer 1 wt% MgAl-LDH 3 wt% MgAl-LDH 5 wt% MgAl-LDH 1 wt% MgAl(DS)-LDH 3 wt% MgAl(DS)-LDH 5 wt% MgAl(DS)-LDH
Tensile strength (MPa)
44.6 46.1 49.8 50.3 47.0 51.9 55.7
Elongation at break (%)
Abrasion resistance (mg)
25.7 22.0 19.2 16.3 22.6 17.1 13.7
Rpmax and Pf values for these films were lower than that for the pure polymer film. These results can be attributed to the incorporation of LDH. It is well known that polymer chains can physically aggregate onto particulate surface, resulting in an increase in the effective degree of crosslinking [25]. In this system, the LDH layers acted as crosslinkers in the polymer chain networks. Based on our results, MgAl(DS)-LDH shows a bigger rise in the effective degree of crosslinking, which is consistent with the results obtained from the TEM observation that MgAl(DS)-LDH dispersed better than MgAlLDH in the polymer matrix. The influence of LDH content on the other physical properties of polymer/LDH nanocomposites is listed in Table 2. The tensile strength of the nanocomposites increases with increasing LDH loading, which is in agreement with the results obtained by DMTA. The increase in E is also observed in the presence of LDH. However, the percent elongation decreases compared with the pure polymer, as the polymer chains in nanocomposites are restricted by the LDH layers, resulting in the decreased degree of freedom. On the other hand, as the LDH component increases, the enhancement of abrasion resistance is obtained due to the interaction between the LDH layer and the polymer matrix. The pendulum and pencil hardness of the cured nanocomposites were used to determine the hardness of the nanocomposite films. As listed in Table 2, the pendulum and pencil hardness of the UV-cured pure polymer films are 88 s and 2H, respectively. With the addition of 1, 3, 5 wt% MgAl-LDH/MgAl(DS)-LDH in the cured films, the values go up to 91/92, 94/98, 96/103 s and 2H/3H, 2H/3H, 3H/4H, respectively. Compared with the pure polymer film, the pendulum and pencil hardness of the cured nanocomposites increase with increasing the LDH loading in the nanocomposite film. Moreover, the polymer/MgAl(DS)-LDH nanocomposite shows better physical properties than polymer/MgAl-LDH nanocomposite. These results can be explained by the better compatibility of MgAl(DS)-LDH with the polymer matrix and the higher effective crosslink density of the polymer/MgAl(DS)-LDH nanocomposite. 4. Conclusion In conclusion, the preparations and properties of UV-curable urethane acrylate nanocomposites based on MgAl-LDH and MgAl(DS)-LDH were studied. After ion-exchange, the hydrophobic
11.2 10.4 9.2 8.8 10.2 8.7 7.5
Hardness Pendulum (s)
Pencil (H)
88 91 94 96 92 98 103
2 2 2 3 3 3 4
MgAl(DS)-LDH nanoparticles were well dispersed in the polymer matrix and showed good compatibility. The LDH platelets played the role of crosslinkers during the photopolymerization and restricted the freedom of the polymer chains. Therefore, the polymer/MgAl(DS)-LDH nanocomposite possessed remarkable enhanced thermal and mechanical properties as compared to the pure polymer. On the contrary, the MgAl-LDH nanoparticles acted as conventional inorganic nano-fillers in the composite because of their poor compatibility with the polymer matrix. The higher Tg and mechanical properties were resulted from simple addition of more hard material and less soft material, resulting in the limited improvement in the thermal and mechanical properties. This study indicated that, even though intercalated structures were formed during the photopolymerization, the enhancement in properties could still be obtained. Thus, the use of LDHs as nano-reinforcement fillers is a feasible approach to improve the properties of UV-cured nanocomposites. Acknowledgment The financial support of National Natural Science Foundation of China (No. 50633010) is gratefully acknowledged. References [1] T. Lan, T.J. Pinnavaia, Chem. Mater. 6 (1994) 2216. [2] K. Wang, L. Chen, J.S. Wu, M.L. Toh, C.B. He, A.F. Yee, Macromolecules 38 (2005) 788. [3] F.N. Cao, S.C. Jana, Polymer 48 (2007) 3790. [4] J. Ma, Z.Z. Yu, Q.X. Zhang, X.L. Xie, Y.W. Mai, I. Luck, Chem. Mater. 16 (2004) 757. [5] A.H. Gemeay, I.A. Mansour, R.G. El-Sharkawy, A.B. Zaki, Eur. Polym. J. 41 (2005) 2575. [6] D.Y. Tang, L.S. Qiang, Z. Jin, W.M. Cai, J. Appl. Polym. Sci. 84 (2001) 709. [7] L.Y. Sun, W.J. Boo, D.Z. Sun, A. Clearfield, H.J. Sue, Chem. Mater. 19 (2007) 1749. [8] Y. Fukushima, S. Inagaki, J. Inclus. Phenom. 5 (1987) 473. [9] X. Kornmann, H. Lindberg, L.A. Berglund, Polymer 42 (2001) 1303. [10] X. Fu, S. Qutubuddin, Polymer 42 (2001) 807. [11] H.L. Tyan, Y.C. Liu, K.H. Wei, Chem. Mater. 11 (1999) 1942. [12] D.C. Lee, L.W. Jang, J. Appl. Polym. Sci. 61 (1996) 1117. [13] L.Z. Qiu, W. Chen, B.J. Qu, Polym. Degrad. Stabil. 87 (2005) 433. [14] C.S. Liao, W.B. Ye, J. Polym. Res. 10 (2003) 241. [15] M. Zammarano, S. Bellayer, J.W. Gilman, M. Franceschi, F.L. Beyer, R.H. Harris, S. Meriani, Polymer 47 (2006) 652.
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