ARTICLE pubs.acs.org/IECR
Fabrication and Characterization of TiO2 NanotubeEpoxy Nanocomposites Yu Wu, Lei Song, and Yuan Hu* State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China ABSTRACT: A systematic study has been conducted to investigate the polymer matrix properties resulting from the introduction of TiO2 nanotubes (15% by weight) that had been surface-modified with phenyl dichlorophosphate (PDCP), into an epoxy resin. The thermal, mechanical, and combustion properties of the nanocomposites and the neat resin were measured by thermogravimetric analysis, dynamic mechanical thermal analysis, and microscale combustion calorimetry. The nanofiller infusion improved the thermal and mechanical properties of the epoxy resin. The nanocomposites resulted in an increase in the storage modulus and glass transition temperature compared to those of the neat epoxy resin. In addition, the combustion characteristics of the epoxy polymer were effectively improved by addition of the surface-modified nanotubes. Thermogravimetric analysis/infrared spectrometry and direct pyrolysis mass spectrometry confirmed that the TiO2 nanotubes were able to improve the thermal stability and combustion behavior of the polymer matrix because the TNTs retard the release of inflammable gas and change the degradation pathway.
1. INTRODUCTION High-performance polymer composite materials have wide applications under difficult working conditions. The materials must provide unique mechanical and thermal properties combined with a low specific weight and thermal properties combined with a high resistance and low specific weight to degradation in order to ensure safety and economic efficiency. Epoxy resins, a thermoset polymer matrix used for technical applications, already exhibits some of the required properties. Epoxy resin is noted for its good mechanical properties, excellent thermal and chemical resistance, and high dimensional stability. Thus, epoxy has been widely used in advanced composites, adhesives, surface coatings, and encapsulating materials.14 However, in the case of overheating, the above applications involve risk of fire, leading to volatile combustible products. TiO2 is extensively used in industry as additives in plastics, catalysts and catalyst supports, photochemical degradation of toxic chemicals, and so on. Therefore, epoxyTiO2 nanocomposites might have wide applications, particularly for organic photovoltaics, fire retardant composites, and so on. Moreover, the influence of TiO2 on the mechanical and thermal properties of epoxies has been characterized.5,6 However, TiO2 particles have limited specific surface areas, thus limiting their applications involving adsorption phenomena, such as catalysis. Recently, nanotubes of titania have been synthesized through a relatively simple alkaline hydrothermal method, which represents an alternative to increase the specific subsurface area.7 Nanotubes exhibit large internal and external surface areas, along with the surface in the vertex and in the interlayer regions that compose the nanotube walls.8 In this work, TiO2 nanotubes (TNTs) were modified with phenyl dichlorophosphate (PDCP) to introduce phosphorus and highly reactive hydroxyl groups. Then, TNTepoxy nanocomposites were prepared. The thermal stability, combustion properties, and mechanical performance of the nanofilled polymer matrix were evaluated as a function of the nanotube content (15 wt %). The degradation of the epoxy nanocomposite and r 2011 American Chemical Society
some changes were found by thermogravimetric analysis/Fourier transform infrared (TGA/FTIR) spectrometry. The possibility that the presence of the TNTs caused some chemical changes in the degradation pathway of epoxy nanocomposites was explored using direct pyrolysis mass spectrometry (DP-MS). The expected overall properties of the nanocomposites could open the way toward new applications of high-performance polymers, leading to innovative product development in many applications.
2. EXPERIMENTAL SECTION 2.1. Materials. Titanium dioxide (TiO2, anatase), sodium hydroxide (NaOH), hydrochloric acid (HCl, 3638%), phenyl dichlorophosphate (PDCP, distilled), tetrahydrofuran (THF), and 4,40 -methylenedianiline are purchased from Shanghai Chemicals No. 1 Plant. Epoxy resin (diglycidyl ether of bisphenol A, DGEBA, commercial name E-44) was supplied by Hefei Jiangfeng Chemical Industry Co. Ltd. (Anhui, China). 2.2. Preparation of EpoxyTNT Nanocomposites. TNTs were prepared by a hydrothermal process from commercial titanium dioxide powder.8 Surface modification of TNTs was performed by mixing 10 mL of PDCP with 1 g of TNTs. After being shaken for a short time, the mixture was moved into a three-necked round-bottom flask and heated to 85 °C for about 5 h under stirring. Then, excess water was slowly added to the flask at 10 °C. Thereafter, the product was filtered and washed with tetrahydrofuran (THF) on a Soxhlet extractor for over 36 h. Finally, the particles were dried at 80 °C in an oven. The modified TNTs were first dispersed in THF using bath sonication for 30 min to obtain a TNT suspension solution. The suspension was moved to a three-necked round-bottom flask and Received: July 29, 2011 Accepted: September 23, 2011 Revised: September 15, 2011 Published: September 23, 2011 11988
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Scheme 1. Synthetic Route of EpoxyTNT Nanocomposites
Figure 1. TEM images of (a) TNTs and (b) TNTPDCP.
reacted with the epoxy resin at 65 °C for about 5 h under reflux. Then, the mixture was placed in a vacuum oven at 80 °C for 1 h (Scheme 1). An appropriate amount of the curing agent 4,40 methylenedianiline was then added to the mixture. After being mixed well with the curing agent, the mixture was poured quickly into a preheated mold and placed into a vacuum oven to remove the air bubbles. All of the samples were cured at 100 °C for 2 h and then postcured at 150 °C for 2 h. The nanocomposites contained different weight fractions (1, 3, and 5 wt %) of modified TNTs. Neat epoxy samples were also prepared following the same procedure for comparison purposes. 2.3. Measurements and Characterization. Fourier transform infrared (FTIR) spectrometry (Nicolet 6700 FTIR spectrophotometer) was employed to characterize the samples using thin KBr disks. Transmission mode was used, and the wavenumber range was set from 4000 to 500 cm1. Thermogravimetric analysis (TGA) was carried out on a TGA Q5000 IR thermogravimetric analyzer (TA Instruments) using a heating rate of 20 °C/min in air or nitrogen atmosphere (flow rate of 100 mL/min). The gel content was determined by extracting the sample (with weight m1) in boiling THF for 48 h. The solvent was renewed after the first 24 h. After being dried, the insoluble residue (m2) was weighed. The gel content was calculated as m2/m1 100%. Three samples were analyzed to determine the average gel content for a given set of irradiation conditions. This method of measuring gel content has been demonstrated to give reproducible results. Transmission electron microscopy (TEM) images were obtained on a Jeol JEM-100SX transmission electron microscope with an acceleration voltage of 100 kV. The TEM specimens were
cut at room temperature using an ultramicrotome (Ultracut-1) with a diamond knife from an epoxy block in which the films of the nanocomposite were embedded. Thin specimens, 5080 nm, were collected in a trough filled with water and placed on 200mesh copper grids. The combustion properties of the neat epoxy and epoxy TNT nanocomposites were investigated by microscale combustion calorimetry (MCC) using a GOVMARK MCC-2 microscale combustion colorimeter. Dynamic mechanical thermal analysis (DMA) was performed on a Rheometric Q800 DMA apparatus (TA Instruments) using a heating rate of 5 °C/min from 25 to 200 °C, at a frequency of 1 Hz in the tensile configuration. Thermogravimetric analysis/Fourier transform infrared (TGA/ FTIR) spectrometry of the sample was performed using the TGA Q5000 IR thermogravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of sample was placed in an alumina crucible and heated from 30 to 700 °C at a heating rate of 20 °C/min (nitrogen atmosphere, flow rate of 55 mL/min). Direct pyrolysis mass spectrometry (DP-MS) analysis was carried out with a Micromass GCT-MS spectrometer using a standard direct insertion probe for solid polymer materials, at a heating rate of 15 °C/min in the range of 30600 °C. The mass data were continuously acquired at a scan rate of 10 scans/s. Electron impact (EI) was used for the mass spectra at 70 eV over a mass range of m/z 101000. All pyrolysis products were identified using the software of the mass spectrometer or by comparison with mass spectra in the literature.
3. RESULTS AND DISCUSSION As presented in Scheme 1, the reactions of the TNTs included two steps. The first step was the immobilization of PDCP onto the TNT surface, producing TNTPDCP. In the second step, the DGEBA was applied to the TNT surface using PDCP as a chain-transfer agent. 3.1. Characterization of TNT Reaction with PDCP and DGEBA. TEM images of the TNTs (Figure 1a) show that the
TNTs had multilayered walls and diameters of about 10 nm. The lengths of these TNTs ranged from 50 to 200 nm. In contrast, Figure 1b shows a TEM image of the TNTPDCP. Obviously, the individual TNTPDCP structures were much shorter than the pure TNTs. The majority of these nanotubes exceeded about 50 nm in length. FTIR spectroscopy is an important technique for studying functional groups and polymers attached to the TNT surface. Figure 2a shows FTIR spectra of unmodified TNTs, TNTPDCP, and TNTPDCPDGEBA. In the FTIR spectrum of unmodified TNTs, the peak around 3410 cm1 is assigned to the stretching vibration of OH groups. In the spectrum of TNTPDCP, characteristic absorptions at 750 and 700 cm1 are assigned 11989
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Figure 2. (a) FTIR spectra and (b) TGA curves of unmodified TNTs, TNTPDCP, and TNTPDCPDGEBA.
Figure 3. TEM micrographs of samples (a,b) EPTP1 and (c,d) EPTP5. Figure 4. Heat release rate curves of various samples.
to C6H5 groups, and that at 1050 cm1 is assigned to the PO groups of PDCP. In the spectrum of TNTPDCPDGEBA, the two characteristic absorption peaks at 850 and 2970 cm1 are mainly due to the vibrations of p-substituted phenyl and CH3, respectively. The TGA results provide further evidence regarding the content and species of the organic moieties grafted onto the TNTs, because the organic and TNT parts have distinct thermal stabilities. As shown in Figure 2b, the sample of unmodified TNTs had a weight loss of 10% before 350 °C and remained steady without any significant weight loss between 350 to 700 °C. The weight loss can be assigned to the loss of hydroxyl groups or absorbed gases on the TNT surface. The TNTPDCP and TNTPDCPDGEBA samples had weight losses of 21% and 33%, respectively. Taking the weight loss of the unmodified TNTs into account, the contents of PDCP and DGEBA grafted onto the TNT surface were 10 and 12 wt %, respectively. Both TGA and FTIR data provided evidence for the presence of PDCP and DGEBA on the TNT surface. 3.2. Dispersion of nanotubes in the composites. The dispersion of functionalized TNTs in the polymeric matrix of the composites was investigated by TEM of thin microtomed slices of the composites. TEM images of samples EPTP1 and EPTP5 are shown in Figure 3, revealing that the TNTs were dispersed homogeneously throughout the composite, with strong adhesion to the matrix, even in EPTP5, which had the highest content of TNTs (5 wt %).
Table 1. Thermogravimetric Analysis and MCC Data of the Samples content of TNTs
residue at
PHRR
THRa
(wt %)
550 °C (%)
(W/g)
(kJ/g)
EP
0
15
478.0
20.7
EPTP1 EPTP3
1 3
21 23
322.6 234.0
19.7 18.1
EPTP5
5
27
190.7
14.8
sample
a
Total heat release.
3.3. Combustion of EpoxyTNT Nanocomposites. MCC has become an effective bench-scale method for investigating the combustion properties of polymer materials.911 It uses oxygen consumption calorimetry to measure the rate and amount of heat. The heat is produced by the complete combustion of the fuel gases generated during controlled pyrolysis of a milligramsized sample. MCC results are presented in Figure 4 and Table 1. As the TNT content increased, the peak heat release rate (PHRR) and total heat release (THR) of the nanocomposites decreased. The PHRR of EPTP5 gave the highest reduction, 60%. This can be explained by the following two aspects: First, the TNTs had a high surface area for their tubular structure. The degradation of polymer was retarded because of the dispersion of 11990
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Figure 5. (a) TGA and (b) DTG curves obtained for various samples in a nitrogen atmosphere.
Figure 6. Behavior with temperature of the (a) storage and (b) loss moduli of various samples.
the TNTs, which reduced the rate of release of inflammable gases by adsorption of the pyrolysis products. Second, one nanotube could react with many PDCP molecules. Hence, the surface-modified TNTs in the epoxy system might exist as a cross-linking agent. The cross-linking density of the epoxy nanocomposite may be higher than that of the neat epoxy resin system. Thermoset polymers having higher cross-linking densities exhibit higher thermal stabilities.12 3.4. Thermal Stability of Nanocomposites. The thermal stability of the synthesized samples was examined by nonisothermal thermogravimetry. The TGA and differential thermogravimetric (DTG) data obtained for the neat epoxy and epoxyTNT nanocomposites in a nitrogen atmosphere are presented in Figure 5 and Table 1. The TGA and DTG graphs show that, as the load of TNTs increased, the thermal stability of the epoxy resin improved. The weight losses of the nanocomposites started at lower temperatures, whereas the DTG peak was similar for all the samples. However, the degradation rate decreased with increasing loading of TNTs. The TGA measurements show that the neat resin had 15% char content at 550 °C. However, for sample EPTP1, about 21% of the weight remained at 550 °C, giving a ∼6% higher weight retention than that of the neat epoxy at that high temperature. The TNTs were dispersed uniformly and developed a barrier to heat and decomposition products in the epoxy matrix because of the ceramic nature of the tubes. The retardant effects of the composites on heat and decomposition products in the epoxy matrix were strengthened as the TNT loading increased. 3.5. Dynamic Mechanical Thermal Properties. Dynamic mechanical tests over a wide range of temperatures were
Figure 7. Evolution of the bands at 3010, 2960, and 2180 cm1 as a function of degradation time for samples EP and EPTP5.
performed to investigate the physical and chemical structural changes of the polymers and nanocomposites. The glass transitions or secondary transitions and yield information about the morphologies of the polymers were determined. The results of dynamic tests conducted on the nanocomposites and the neat epoxy are presented in Figure 6 in the temperature range from room temperature to 200 °C. The storage modulus increased with TNT loading. The addition of 5 wt % TNTs yielded a 130% increase in the storage modulus at 30 °C and a 420% increase at 11991
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Figure 8. Pyrolysis mass spectra recorded at three times (ac) for EP and (df) EPTP5.
100 °C. This behavior can be explained in terms of interfacial interactions between the TNTs and the epoxy, as the improved interfacial interactions due to PDCP functionalization of the TNTs reduced the mobility of the local matrix material around the TNTs, thereby increasing the thermal stability. According to the DMA results, as the TNT loading increased, the glass transition temperatures (Tg) of the materials increased. The increase in Tg can be attributed to a loss in the mobility of the chain segments of the epoxy systems resulting from the nanotubematrix interactions. Impeded chain mobility was possible because the nanotubes were well dispersed in the matrix. The nanotube surface-to-surface distance should then be relatively
small, and chain segment movement should be restricted. Good adhesion of the nanotubes with the surrounding polymer matrix would additionally benefit the dynamic modulus by hindering molecular motion to some extent. The nanotubes incorporated into the polymer would act as additional virtual network nodes. In most cases, the cross-linking density is a key factor of controlling Tg for normal thermoset polymer systems. The modified TNTs chemically reacted with the epoxy amine system, so that interfacial layers formed during the curing process exhibited significantly different segmental dynamics from that of the bulk resin. Therefore, epoxyTNT nanocomposite exhibited a higher Tg values than the neat resin system. 11992
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Table 2. Structural Assignments in the DP-MS of EP and EPTP5
3.6. Effect of Modified TNTs on Gel Content. Gel content measurements are usually carried out to determine the crosslinking degree of cross-linked samples, with higher gel contents being due to greater cross-linking reactions. The effect of modified TNTs on the gel content of the epoxy system was studied. The results demonstrated that, as the concentration of modified TNTs increased from 0 to 5 wt %, the gel content increased from 92% to 95%. 3.7. TGA/FTIR Spectrometry of EP and EPTNT Composites. TGA/FTIR spectrometry was used to analyze the gas products during the thermal degradation process. Some small molecular gaseous decomposition products of the sample with no TNTs (EP) were identified unambiguously, including methane, CO2, CO, and water.13 The peaks at 29802900 cm1 indicate the appearance of saturated hydrocarbons. The peak at 2180 cm1 indicates the appearance of CO. To further understand the change in the pyrolysis products, the absorbance of the pyrolysis products for samples EP and EPTP5 is presented as a function of time in Figure 7. It can be seen that the absorbance intensity of pyrolysis products for EPTP5 was lower than that for EP. Consequently, the addition of TNTs reduced the release of combustible gases and the weight loss, which corresponds well with the thermal analysis results. Comparative vapor-phase FTIR spectra clearly showed some difference in the composition of
degradation products of EP and EPTP5, suggesting a change in the degradation pathway. The first distinct difference in the IR spectra of EP and EPTP5 was found in the region near 3000 cm1. The spectra of the EP degradation products displayed two peaks at 2970 cm1 (—CH3) and 2930 cm1 (—CH2) with a shoulder at 3010 cm1 (dC—H). In virgin EP, the peaks at 2970 and 3010 cm1 had very similar shapes, with maxima occurring at practically the same time. However, the FTIR spectra of EPTP5 showed some changes with TNT content, in terms of both relative strength and type. This means that more vinyl groups were generated in EP than in EPTP5. The second difference in the IR spectra was found in the region near 2180 cm1. In the presence of TNT content (i.e., for sample EPTP5), almost no CO was generated. 3.8. DP-MS Analysis of EP and EPTP5. The direct pyrolysis mass spectra of polymers are usually very complex, as thermal degradation products further dissociate in the mass spectrometer during ionization. In DP-MS, secondary and condensation reactions are avoided, and the detection of high-mass pyrolyzates and unstable thermal degradation products is possible. The mass spectra recorded at the initial and peak stages are shown for samples EP and EPTP5 in Figure 8, and the products indentified are listed in Table 2.13 In the case of EP, at about 225 °C, two main peaks were observed, C21H25O3 (m/z 325) and C18H21O2 11993
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Scheme 2. Schematic Outline of the Degradation Processes of (a) EP and (b) EPTP Composites
(m/z 269). Because of the presence of the TNTs in EPTP5, the main peaks changed to C2H6O5P (m/z 141), PO4H4 (m/z 99), and so on. In the mass spectra at about 330 and 390 °C, the same main peak of C14H13O2 (m/z 213) was shown for EP and EPTP5. However, the amounts of large molecular-ion fragments (m/z > 213) were less for EPTP5 than for EP. Oversimplified fragments of EP and EPTP5 are shown in Scheme 2, with the data shown collectively in Figure 8. It can be seen that the degradation of EPTP5 can be divided into two steps. At about 220 °C, the degradation was mainly due to phosphate groups in the EPTP5. At higher temperatures, alkyl chains and some aromatic molecules of the epoxy resin matrix also decomposed. Therefore, on the basis of the results for volatile pyrolysis products from TGA/FTIR spectrometry and DP-MS, a possible route for the degradation process of epoxyTNT nanocomposites was postulated and is summarized for the main decomposition pathway for EPTP nanocomposites in Scheme 2. The FTIR and MS measurements confirmed the formation of phosphoric acid and phosphate at the beginning. When the temperature was
raised, the pyrolysis products of m/z 213, saturated hydrocarbons and unsaturated alkane groups, were generated by decomposition of the epoxy resin backbone. However, some of the compounds containing aromatic rings were adsorbed on the solid phase by TNTs instead of cracking or charring. Therefore, few macromolecules were produced from EPTP5.
4. CONCLUSIONS TiO2 nanotubes were surface-modified with PDCP and dispersed homogeneously throughout the epoxy resin system. The resin properties were influenced based on the percentage of nanotubes added. Nanotube infusion changed the morphology of the resin systems, increasing the glass transition temperature (Tg) of the bulk matrix and the mechanical and thermal properties of the nanocomposites. MCC data showed that the peak heat release rate and total heat release of the epoxyTNT nanocomposites were significantly reduced compared with those of pure EP. Moreover, TGA/FTIR spectrometry and DP-MS confirmed 11994
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that the TNTs were able to improve the thermal stability and combustion behavior of the polymer matrix because the TNTs retarded the release of inflammable gases and changed the degradation pathway.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: +86-551-3601664.
’ ACKNOWLEDGMENT The work was financially supported by the Program for Specialized Research Fund for the National Natural Science Foundation of China (No. 51036007), the joint fund of NSFC and CAAC (No. 61079015), and the China Postdoctoral Science Foundation (20080430101). ’ REFERENCES (1) Ye, Y. P.; Chen, H. B.; Wu, J. S.; Ye, L. High impact strength epoxy nanocomposites with natural nanotubes. Polymer 2007, 48, 6426–6433. (2) Dzunuzovic, E.; Vodnik, V.; Jeremic, K.; Nedeljkovic, J. M. Thermal properties of PS/TiO2 nanocomposites obtained by in situ bulk radical polymerization of styrene. Mater. Lett. 2009, 63, 908–910. (3) Kim, D. J.; Kang, P. H.; Nho, Y. C. Characterization of mechanical properties of γAl2O3 dispersed epoxy resin cured by γ-ray radiation. J. Appl. Polym. Sci. 2004, 91, 1898–1903. (4) Deng, S. Q.; Ye, L.; Friedrich, K. Fracture behaviors of epoxy nanocomposites with nano-silica at low and elevated temperatures. J. Mater. Sci. 2007, 42, 2766–2774. (5) Chatterjee, A.; Islam, M. S. Fabrication and characterization of TiO2epoxy nanocomposite. Mater. Sci. Eng. A 2008, 4876, 574–585. (6) Xiong, L.; Liang, H. B.; Wang, R. M.; Pang, Y. The effect of surface modification of TiO2 with diblock copolymers on the properties of epoxy nanocomposites. Polym. Plast. Technol. 2010, 49, 1483–1488. (7) Du, G. H.; Che, R. C.; Yuan, Z. Y.; Peng, L. M. Preparation and structure analysis of titanium oxide nanotubes. Appl. Phys. Lett. 2001, 79, 3702. (8) Tenne, R. Materials physics: Doping control for nanotubes. Nature 2004, 431, 640. (9) Mosurkal, R.; Samuelson, L. A.; Smith, K. D.; Westmoreland, P. R.; Parmar, V. S.; Yan, F.; Kumar, J.; Watterson, A. C. Nanocomposites of TiO2 and siloxane copolymers as environmentally safe flameretardant materials. J. Macromol. Sci. A 2008, 45, 943. (10) Tripathy, A. R.; Farris, R. J.; MacKnight, W. J. Novel fire resistant matrixes for composites from cyclic poly(butylene terephthalate) oligomers. Polym. Eng. Sci. 2007, 47, 1536. (11) Zhang, H.; Westmoreland, P. R.; Farris, R. J.; Coughlin, E. B.; Plichta, A.; Brzozowski, Z. K. Thermal decomposition and flammability of fire-resistant, UV/visible-sensitive polyarylates, copolymers and blends. Polymer 2002, 43, 5463. (12) Miyagawa, H.; Drzal, L. T. Thermo-physical and impact properties of epoxy nanocompisites reinforced by single-wall carbon nanotubes. Polymer 2004, 45, 5163. (13) Wang, Q. F.; Shi, W. F. Kinetics study of thermal decomposition of epoxy resins containing flame retardant components. Polym. Degrad. Stabil. 2006, 91, 1747.
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