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Applying the flame retardant LDH as a Trojan horse for molecular flame retardants
Applied Clay Science 114 (2015) 603–608
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Applying the flame retardant LDH as a Trojan horse for molecular flame retardants Andreas Edenharter, Josef Breu ⁎ Lehrstuhl für Anorganische Chemie I, Universität Bayreuth, 95440 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 29 April 2015 Received in revised form 3 July 2015 Accepted 6 July 2015 Available online 16 July 2015 Dedicated to Prof. Dr. Christian Robl on the occasion of his 60th birthday. Keywords: LDH Anionic clay Hydration Topotactic anion exchange Flame retardancy
a b s t r a c t The direct synthesis of Mg2Al–Cl LDH (Cl-LDH) has been accomplished by combining the urea method with the addition of NH4Cl as a buffer. It has been shown, that the buffer limits the maximum pH reached during urea hydrolysis, which in turn limits the concentration of carbonate in the reaction mixture and allows for recovery of Cl-LDH. By anion exchange into Cl-LDH phenyl phosphate (PP) was incorporated as a model flame retardant. The mechanism of this anion exchange could be shown to be topotactic rather than of dissolution/ re-precipitation type. Despite hydration of PP-LDH up to a d-value of 15.8 Å, no exfoliation could be observed upon applying high shear forces. With a filler content of 5 wt.%, PP-LDH could be shown to significantly push the flame retardant properties of polystyrene as compared to ordinary CO3-LDH as indicated by a doubling of the reduction of the peak of heat release rate (22% and 47%, respectively). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDH) possess an anisotropic, platy morphology and a considerable anion exchange capacity that renders them a versatile carrier for anionic functional molecules. The potential of LDH as efficient synergetic flame retardants for polymer composites has been compellingly demonstrated (Zammarano et al., 2005; Wang et al., 2005; Zammarano et al., 2006; Nyambo et al., 2008; Diar-Bakerly et al., 2012; Matusinovic and Wilkie, 2012). Although the dispute regarding the retardancy mechanism continues (Nyambo et al., 2008; Zhao et al., 2008; Gao et al., 2014) it has been shown that the aspect ratio (ratio of lateral extension to thickness of the platelets) is a crucial factor for this application (Diar-Bakerly et al., 2012). The method of choice to optimize aspect ratio is the so-called urea method where the required pH-rise is accomplished by urea hydrolysis (Costantino et al., 1998). Urea hydrolysis, however, produces significant carbonate levels in the solution that then in turn become the preferred interlayer anion. Due to the pronounced thermodynamic stability of CO23 −-LDH it is then unfortunately troublesome to functionalize the material by ion-exchange (Costa et al., 2005; Iyi and Sasaki, 2008a,b). Coprecipitation allows direct access to for instance NO− 3 -LDH or Cl−-LDH that can easily be functionalized by ion-exchange but ⁎ Corresponding author. E-mail address: [email protected] (J. Breu).
http://dx.doi.org/10.1016/j.clay.2015.07.013 0169-1317/© 2015 Elsevier B.V. All rights reserved.
coprecipitation delivers rather small aspect ratios. More recently, Schwieger et al. reported a modification of urea hydrolysis that gives direct access to Zn2AlNO3-LDH (Inayat et al., 2011). This approach for the first time allows combining large aspect ratio with easy anion exchange for LDH. In this work, we employed this modified urea hydrolysis method to synthesize Mg2AlCl-LDH (Cl-LDH) which was then ion-exchanged with a functional molecule, phenyl phosphate (PP), a model compound for P-containing flame retardants. The topotactic character of this ionexchange was established by looking at intermediate, partially exchanged stages, the hydration of PP-LDH was studied, and the potential of the PP-LDH-filler for flame retardancy was tested by cone calorimetry of the polystyrene-nanocomposite. 2. Experimental 2.1. Materials Water used in this work was distilled twice and saturated with Argon to exclude ubiquitous CO2. Ethanol was of analytical grade, THF of technical grade with a purity of 99.5%. Urea, MgCl2·6H2O and AlCl3·H2O was purchased from Grüssing GmbH and sodium phenyl phosphate dihydrate from Sigma-Aldrich. Polystyrene 158 K was purchased from BASF. All chemicals were used as received without purification unless stated otherwise.
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2.2. Synthesis
2.6. Characterization
For the synthesis of Mg2AlCl-LDH (Cl-LDH), an aqueous solution of urea (1.33 M), the chloride salts of Mg2+ (0.33 M) and Al3+ (0.17 M) with the molar ratio of 2/1 and NH4Cl (1 M) was added into a glass reactor (1300 mL) equipped with various on-line sensors (pH, React-IR). The solution was refluxed at 100 °C for 48 h and the parameters were recorded and controlled by a Mettler Toledo LabMax. The white precipitate was washed by repeated centrifugation at 8000 rpm and redispersing in ethanol until a test with AgNO3 of the supernatant for Cl− was negative. Subsequently, the product was dried at 60 °C overnight.
The Mg and Al content of the samples was determined by flame atomic absorption spectroscopy (F-AAS) with a Varian AA100. The Cl content was determined by ion chromatography. Powder X-ray diffraction (PXRD) measurements at different relative humidities were obtained on a Bragg–Brentano-type diffractometer (Panalytical XPERT-PRO) using nickel filtered Cu-Kα radiation and an X'Celerator Scientific RTMS detector. The instrument was equipped with an Anton Paar temperature–humidity chamber connected to a VTI Corp. RH200 humidity generator. Infrared spectroscopy was conducted in ATR mode on a Jasco FT/IR-6100. Cone calorimetry was performed on a Fire Testing Technology iCone Calorimeter using 100 × 100 × 4 mm3 plaques of the samples in horizontal geometry at 35 kW/m2 heat flux. Testing was done according to ASTM-E1354.
2.3. Anion exchange For a partial anion exchange, 75 mg of Cl-LDH was suspended in water by shaking for 1 h. The dispersion was transferred to a 250 mL three-neck flask equipped with a dropping funnel and an Argon inlet. An aqueous solution of sodium phenyl phosphate (0.03 M) was prepared. The volume of this solution necessary for a partial anion exchange was calculated as a fraction of the nominal anion exchange capacity (AEC) of 4.0 meq/g for the Mg2AlCl-LDH. For example, the reaction with 40% AEC required 60 μmol of phenyl phosphate (PP). Accordingly, 2 mL of this solution was added dropwise to the LDH dispersion under stirring and in an argon atmosphere. The product (PP-LDH) was washed with water (20 mL) thrice and then freeze dried.
2.4. Mechanical treatment of PP-LDH For milling a stirred media mill was applied (Retsch LabStar LS1). The setup and procedure used are described in detail elsewhere (Ziadeh et al., 2012). An agitation speed of 2000 rpm and yttrium stabilized ZrO2 beads (SiLi beads) with a diameter of 1.4–1.6 mm was used. An aqueous dispersion of approx. 5 wt.% of PP-LDH was circulated through the milling chamber for 30 min and then freeze dried immediately. Alternatively, a microfluidizer (Microfluidics Inc. type M-110Y) equipped with an H30Z (200 μm, ceramics) and an H10Z (100 μ, diamond) (pre-)interaction chamber was used to generate high shear forces. Here, an aqueous dispersion of approx. 1 wt.% of PP-LDH was pressed through the interaction chambers at 1 kbar 15 times and then freeze dried immediately.
2.5. Preparation of polystyrene/PP-LDH nano composites PS nanocomposites were prepared by adding appropriate amounts of dispersion of PP-LDH in THF to a solution (25 wt.%) of PS in THF to yield 5°wt.% of PP-LDH in the final nanocomposite. The dispersions were first mixed for 30 min using an overhead stirrer (500 rpm, RT). Afterwards, the dispersion was further homogenized applying a threeroll mill (Exakt-80E) placed in an exhaust hood. All samples were passed through the mill 6 times. The roller distance was gradually lowered from initially 25 μm in the first cycle to 5 μm in the final cycle. Despite cooling at − 23 °C a good share of THF evaporated in the course of three-roll milling (1 h) and highly viscous dispersions were obtained which were pre-dried in a vacuum oven (80 °C, 600 mbar, 3 h). For better processability, all samples were then ground into a powder in a planetary ball mill (Retsch PM 100, 5 min, 350 rpm) and dried again in a vacuum oven (110 °C, 300 mbar, 24 h) to remove remaining traces of THF. Moreover, a blank sample of the virgin polymer was prepared following the same described procedure. Finally, the dried powders were hot pressed at 180 °C for 5 min to obtain plaques with dimensions of 100 × 100 × 4 mm3 for cone calorimetric analysis.
3. Results and discussion 3.1. Synthesis of Mg2AlCl-LDH (Cl-LDH) 2− was The variation of the concentrations of OH−, HCO− 3 and CO3 followed during synthesis by on-line sensors (Fig. 1). NH4Cl serves as reservoir for both, chloride ions needed as interlayer ions and OH−. The evolution of the pH during hydrolysis in the presence of NH4Cl is distinctly modified as compared to ordinary urea hydrolysis. In the absence of NH4Cl early in the reaction (200 min.) a pronounced maximum of the pH is observed with a peak at around pH = 6.3. In the presence of NH4Cl only a very shallow maximum at around pH = 6.1 is observed at a much later stage of the reaction (430 min). At the final stages pH values converge to 6.2 and 6.7 with and without NH4Cl, respectively. As indicated by a similar variation of the intensity of the symmetrical C_O stretching vibration, the kinetics of the urea-hydrolysis are little affected by the presence of NH4Cl. Contrary to this, due to the different pH of the solution, the concentration of the carbonate ion is significantly lowered by the presence of NH4Cl. This pronounced reduction of the activity most likely allows Cl− that is present at a concentration CO2− 3 of 2.16 mol/L to win the competition for the interlayer position despite for this position. the higher affinity of CO2− 3 As expected, the distinctly different evolution of the pH also influences the partition of the two cations between solid product and solution. The solubility of initially precipitated amorphous Al(OH)3 depends crucially on the pH. The higher the pH, the higher the concentration of [Al(OH)4]−. The availability of [Al(OH)4]− in solution in turn determines the composition of the octahedral sheet of the LDH formed. The concentration of Al3 + in solution is in any case below detection limit while the concentration of Mg2+ is significantly increased by the presence of NH4Cl (Table 1). The Mg-level in solution is determined by the solubility product of the LDH which in turn depends on the pH of the solution. The precipitate obtained after 48 h is significantly
Fig. 1. Variation over reaction time of pH (solid lines) and intensities of the FTIR signals for carbonate (solid triangles) and urea (empty circles) with (red) and without (black) NH4Cl.
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Table 1 Cation composition and yield at the end of LDH synthesis with and without NH4Cl.
Mg2+ conc. in supernatant [mmol/L] Al3+ conc. in supernatant [mmol/L] Mg/Al ratio in precipitate Space yield [g/L] Cl− content [wt.%]
NH4Cl
No NH4Cl
64 bLOD 1.43 20.6 13.4
25 bLOD 1.94 24.5 –
Al-richer in the presence of NH4Cl. The composition of the precipitate is, however, not necessarily identical with the composition of the LDH-phase whenever amorphous Al(OH)3 is not completely transformed into LDH. The FTIR spectra (Fig. 2) of the final precipitates clearly indicate the absence of the asymmetric valence vibration of carbonate (1350 cm− 1) only when the NH4Cl buffer is applied during synthesis. Assuming charge neutrality and full occupation of the octahedral positions, the following formula for Cl-LDH may be derived from the concentration of chloride (13.4 wt.%) as determined by ion chromatography: Mg0.65Al0.35Cl0.28. This formula indicates a Mg/Alratio of 1.83 which is significantly higher than the 1.43 found by AAS-analysis of the precipitate. This would indeed suggest that the precipitate contains some amorphous Al(OH)3 alongside the main Cl-LDH phase. Thus, the use of a NH4Cl buffer allows for recovery of Mg 2Al-Cl LDH but with the drawback of both a somewhat lower yield and potentially some amorphous side phase. Anyhow, PXRD patterns (Fig. 3) show that for both CO3-LDH and Cl-LDH obtained by urea hydrolysis no other crystalline phases are present.
Fig. 2. ATR-FTIR spectra of the final precipitates obtained with (red) and without (black) NH4Cl.
Fig. 3. PXRD patterns of Mg2Al–CO3 (black) and Mg2Al–Cl LDH (red).
Fig. 4. Evolution of PXRD patterns with progressing anion exchange.
3.2. Anion exchange with phenyl phosphate The larger phosphorous containing di-anion phenyl phosphate (PP) readily replaces the monovalent Cl−. To prove the topotactic nature of the anion exchange rather than a dissolution/re-precipitation mechanism, partial exchange with increasing amounts of PP was performed. The electrostatic attraction between positive octahedral sheet and interlayer anions will in any case assure that anions of different size like PP and Cl− will unequivocally segregate into different interlayers. This way interstratified materials will be produced by partial ion exchange that at any given degree of ion exchange will assure the minimum average basal spacing going along with a maximum of Coulomb attraction. Random interstratifications of an increasing number of PP-interlayers in Cl-LDH-tactoids will, according to Mering's rules, translate into gradual shifts of the basal spacing to higher dvalues (Figs. 4 and 5a). Moreover, random interstratification will render the 00l-series non-rational as indicated by the coefficient of variation (Fig. 5b) and will contribute significantly to peak broadening as indicated by the full width at half maximum (FWHM) of the 001-reflection (Fig. 5b). Please note, the rhombohedral symmetry is lost upon ion exchange to PP-LDH, which is why the Miller indices 001 were used to refer to the basal reflection. FWHM and coefficient of variation (CV) values are low for the starting Cl-LDH, go through maxima for intermediate amounts of PP added and finally decrease again when approaching the PP-end. This is just what is to be expected for random interstratifications, proving a topotactic reaction mechanism. At 40% two maxima are observed, however, none of them corresponds to the basal spacing of the end-members (Cl-LDH and PP-LDH). These two phases rather both correspond to interstratified phases but with different contributions of the two types of interlayer heights. Completion of ion-exchange is indicated by regaining a rational 00l-series with a unique basal spacing (8.7 Å) of PP-interlayers, low CV and FWHM values.
Fig. 5. Variation of apparent d-spacing, FWHM, and CV with progressing anion exchange.
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Fig. 6. PXRD patterns of PP-LDH at increasing relative humidities recorded in-situ in a humidity chamber with 60 min equilibration time after each step.
Fig. 7. Variation of apparent d-spacing of PP-LDH with increasing RH. Data was taken from reflections assigned to the 0-hydrate (black square), 1-hydrate (red dot), 2-hydrate (blue triangle).
3.3. Hydration behavior PP-LDH eagerly hydrates when exposed to humid air. Discrete crystalline swelling steps are observed as a function of increasing relative humidity (RH) (Fig. 6). The basal spacing increases in two steps from 8.7 Å (0-hydrate) in the dry state to 11.2 Å (1-hydrate) and finally 15.8 Å (2-hydrate) (Fig. 7). The first step is well defined with 20% RH being in the middle of the relatively sharp transition. The comparatively broad basal reflections at this very water vapor activity of transition result from a random interstratifications of yet non-hydrated interlayers and already hydrated interlayers. Aside from the transition zone, all tactoids nevertheless hydrate uniformly at a well defined water vapor activity. Hydration enthalpy has contributions of the hydration of hydroxyl groups of the octahedral sheets and of the hydration of the interlayer ions. This uniform intracrystalline reactivity would consequently indicate that either the first hydration is dominated by the hydration
Fig. 8. Variation of FWHM of PP-LDH with increasing RH. Data was taken from reflections assigned to the 0-hydrate (black square), 1-hydrate (red dot), 2-hydrate (blue triangle).
Fig. 9. Variation of CV of PP-LDH with increasing RH. Data was taken from reflections assigned to the 0-hydrate (black square), 1-hydrate (red dot), 2-hydrate (blue triangle).
of the hydroxyl groups or, and this appears more likely, that the concentration of anions is similar at all length scales. Contrary to the spontaneous transition from 0- to 1-hydrate, the transition from 1-hydrate to 2-hydrate is for whatever reason retarded. 1-hydrate and 2-hydrate exist concomitantly over a broad range of RH (Fig. 6). As indicated by the FWHM (Fig. 8) and CV (Fig. 9) of the two distinct basal reflections in this range of coexistence, the 1-hydrate tactoids show very little interstratifications while the newly formed 2-hydrate tactoids show significant interstratifications. The basal spacing of the latter gradually shifts from 15.1 Å to its final value of 15.8 Å. The shift again may be explained by a certain degree of random interstratification of 1-hydrate interlayers in mostly 2-hydrate tactoids and the degree of these interstratifications diminishes with increasing RH. The reflection of 2-hydrate tactoids starts appearing at 50% RH and the intensity of the basal reflections increase further with increasing RH, while the intensities of hydrate 1 decrease until it completely vanishes at 80% RH. Most likely the coexistence is related to the kinetics of intercalation. During swelling to the 1-hydrate and then on to the 2-hydrate the octahedral sheets become elastically deformed and a considerable strain develops at the reaction front. Based on results obtained for intercalation reactions of kaolinites, Weiss et al. argued that depending on the lateral dimensions of the tactoids, two different intercalation mechanisms would be followed (Weiss et al., 1970; Stöcker et al., 2008, 2010): The type of mechanism is moreover determined by the ratio of the crystal diameter to the cooperative elastic action length (a0), which is the longer, the better the crystallinity. Expansion of the interlamellar space in tactoids with diameters smaller than the cooperative elastic action length can only occur via the “one-sided mechanism” also called the “wedge mechanism”. Since through the elastic deformation all edges immediately sense the start of the widening of the interlayer gallery, nucleation on the opposite edges is stalled in this case. Contrary to this, large tactoids with a diameter larger than the cooperative elastic action length may, however, intercalate via the so
Fig. 10. Particle size distribution of PP-LDH in water measured by laser light scattering.
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Table 3 Cone data.
Fig. 11. PXRD patterns of PP-LDH before (black) and after (red) treatment in a microfluidizer and before (blue) and after (pink) milling.
Table 2 FWHM of PP-LDH before and after mechanical treatment. FWHM [°] Milling: 0 min Milling: 30 min Microfluidizer: 0 cycles Microfluidizer: 15 cycles
1.57 1.13 1.47 1.37
called “ring mechanism”. This is feasible because the lateral diameter is longer than a0, opposite edges thus do not sense the beginning of elastic deformation and therefore the reaction can proceed from all the edges concomitantly. In the line of Weiss hypothesis we speculate, that the broad particle size distribution obtained by the urea hydrolysis (Fig. 10) stretches across the range where a change of the type of intercalation mechanism might be expected for LDH. Consequently, the kinetics of tactoids of various sizes might differ significantly as observed for the 1-hydrate to 2-hydrate transition. 3.4. Exfoliation by shearing The difference in d-values between the 0-hydrate to the 2-hydrate is 7.1 Å. To the best of our knowledge, this represents the largest expansion of the interlayer by hydration reported for LDH. With a total interlayer distance (gallery height) of approx. 11 Å (15.8 Å–4.8 Å) in the fully hydrated state, attractive forces between the layers could be weakened sufficiently to render the tactoids shear labile, allowing for exfoliation in water (Möller et al., 2010; Ziadeh et al., 2012). To test for potential exfoliation, two well established methods, a ball mill and a microfluidizer were utilized to apply high shear forces. To minimize a potential contribution of interstratifications of differently hydrated interlayers, the samples were dried to the 0-hydrate state and the FWHM of the 001reflection is used to gauge, whether or not exfoliation has occurred
Fig. 12. Heat release rate for pristine polystyrene (blue) and nanocomposites filled with CO3-LDH (black) or PP-LDH (red).
Formulation
PHRR [kW/m2]
THE [MJ/m2]
tig [s]
Burn out time [s]
EHC [MJ/m2 g]
PS CO3-LDH PP-LDH
865 ± 28 673 ± 22 457 ± 15
137 ± 3 135 ± 2 136 ± 2
81 ± 3 59 ± 2 65 ± 2
550 ± 15 612 ± 32 695 ± 35
3.4 ± 0.1 3.4 ± 0.1 3.5 ± 0.1
(Fig. 11 and Table 2). Unfortunately, for both tested shearing methods, we failed to increase FWHMs indicating that efficient exfoliation is impossible. Apparently, the charge density of the LDH (4.2 charges/nm2 for Mg/Al = 2/1) is too high and Coulomb attraction remains too strong. This is in contrast to observations made for smectites with lower charge density for which with the same methods efficient exfoliation could be achieved (Möller et al., 2010; Ziadeh et al., 2012). 3.5. Flame retardancy The PP-LDH and CO3-LDH as obtained from the urea hydrolysis method had specific surface areas of 18 and 45 m2/g, respectively. Surface areas together with the FWHM of 001-reflections and the average lateral extension (Fig. 10) as determined by laser scattering allow to estimate (Ziadeh et al., 2012) similar aspect ratios of 10 for both LDH. Both diameter and aspect ratio of CO3-LDH agree within experimental error with published values (Diar-Bakerly et al., 2012). As stated in the introduction, here we took advantage of the anion exchange capability of Cl-LDH to introduce phenyl phosphate as a model compound for P-containing flame retardants into the nanocomposites to improve the flame retardancy potential of the synergistic LDH-filler beyond what has been reported for CO3-LDH. The flame retardant properties of PP-LDH filler in a polystyrene matrix were tested by cone calorimetry and compared to the effect of an ordinary CO3-LDH with comparable aspect ratio and specific surface (Fig. 12). Cone calorimetry is the most versatile analysis to test the fire properties of polymer nanocomposites. Information is collected about the time to ignition (tig), the heat release rate (HRR), the peak of heat release rate (PHRR), the total heat release (THR), and burn out time in a developing fire scenario. For good flame retardants, as known from literature, it is favorable to increase tig, decrease PHRR and THR, and increase the burn out time. Table 3 summarizes the cone data for PP-LDH and CO3-LDH which show similar aspect ratios. Therefore, differences observed may mainly be attributed to the functional PP molecule introduced as interlayer anion. Heat release rate curves obtained at a heat flux of 35 kW/m2 are shown in Fig. 12. As typical for LDH composites (Zammarano et al., 2005; Diar-Bakerly et al., 2012; Matusinovic and Wilkie, 2012; Matusinovic et al., 2013), for both fillers the time to ignition was found to be shorter than for the pristine non-filled polymer. Quite
Fig. 13. Residue of the cone test for Polystyrene/PP-LDH nanocomposite.
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pleasing a reduction of the peak of heat release rate of 47% with PP-LDH as compared to only 22% with CO3-LDH, unequivocally proves the positive influence of the interlayer anion on the flame retardant effect of PP-LDH. Furthermore, there is practically no change in the effective heat of combustion (EHC, Table 3), indicating a condensed phase mechanism of flame retardancy (Schartel et al., 2006). This is in line with previous investigations by Braun et al. (2006) showing that phosphates primarily act in the condensed phase. Moreover, the heat release of the PP-LDH nanocomposite was spread over a wider range of time resulting in a higher burn out time (from 550 to 695 s) that indicates a slower transfer of mass and heat during the combustion of the polymer. The longer burning time at lower HRR may be related to the formation of a thin layer of char and residues of metal oxides that insulate the polymer from heat radiation (Fig. 13). 4. Conclusion For the direct synthesis of Mg2Al-Cl LDH by the urea hydrolysis method, addition of NH4Cl not only assures a high Cl− background but additionally its buffer capacity controls the pH and this way in turn concentration during synthesis is kept low. Most likely due the CO2− 3 to its high charge density, PP-LDH despite a significant degree of swelling cannot be rendered shear labile to allow for exfoliation by applying high shear forces. LDH exhibit a comparatively large anion exchange capacity that can be taken advantage of to push the flame retardancy of platy nanofillers. With phenyl phosphate applied as a model compound for P-containing flame retardants this potential could be proven unequivocally. The cone calorimetry data underline the potential of large aspect ratio LDH acting as Trojan horses for molecular flame retardants. We expect even more pronounced effects when delivering even more sophisticated molecular flame retardants via the layered flame retardant. Acknowledgments The authors acknowledge the Deutsche Forschungsgemeinschaft (SFB 840) for financial support. References Braun, U., Balabanovich, A.I., Schartel, B., Knoll, U., Artner, J., Ciesielski, M., Doring, M., Perez, R., Sandler, J.K.W., Altstadt, V., Hoffmann, T., Pospiech, D., 2006. Influence of the oxidation state of phosphorus on the decomposition and fire behaviour of flame-retarded epoxy resin composites. Polymer 47, 8495–8508.
Costa, F.R., Abdel-Goad, M., Wagenknecht, U., Heinrich, G., 2005. Nanocomposites based on polyethylene and Mg–Al layered double hydroxide. I. Synthesis and characterization. Polymer 46, 4447–4453. Costantino, U., Marmottini, F., Nocchetti, M., Vivani, R., 1998. New synthetic routes to hydrotalcite-like compounds — characterisation and properties of the obtained materials. Eur. J. Inorg. Chem. 1439–1446. Diar-Bakerly, B., Beyer, G., Schobert, R., Breu, J., 2012. Significance of aspect ratio on efficiency of layered double hydroxide flame retardants. In: Morgan, A.B., Wilkie, C.A., Nelson, G.L. (Eds.), Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science. American Chemical Society, pp. 407–425. Gao, Y.S., Wu, J.W., Wang, Q., Wilkie, C.A., O'Hare, D., 2014. Flame retardant polymer/ layered double hydroxide nanocomposites. J. Mater. Chem. A 2, 10996–11016. Inayat, A., Klumpp, M., Schwieger, W., 2011. The urea method for the direct synthesis of ZnAl layered double hydroxides with nitrate as the interlayer anion. Appl. Clay Sci. 51, 452–459. Iyi, N., Sasaki, T., 2008a. Decarbonation of MgAl-LDHs (layered double hydroxides) using acetate-buffer/NaCl mixed solution. J. Colloid Interface Sci. 322, 237–245. Iyi, N., Sasaki, T., 2008b. Deintercalation of carbonate ions and anion exchange of an Al-rich MgAl-LDH (layered double hydroxide). Appl. Clay Sci. 42, 246–251. Matusinovic, Z., Wilkie, C.A., 2012. Fire retardancy and morphology of layered double hydroxide nanocomposites: a review. J. Mater. Chem. 22, 18701–18704. Matusinovic, Z., Feng, J.X., Wilkie, C.A., 2013. The role of dispersion of LDH in fire retardancy: the effect of different divalent metals in benzoic acid modified LDH on dispersion and fire retardant properties of polystyrene- and poly(methyl-methacrylate)-LDH-B nanocomposites. Polym. Degrad. Stab. 98, 1515–1525. Möller, M.W., Handge, U.A., Kunz, D.A., Lunkenbein, T., Altstädt, V., Breu, J., 2010. Tailoring shear-stiff, mica-like Nanoplatelets. ACS Nano 4, 717–724. Nyambo, C., Songtipya, P., Manias, E., Jimenez-Gasco, M.M., Wilkie, C.A., 2008. Effect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fireretardancy of PMMA and PS. J. Mater. Chem. 18, 4827–4838. Schartel, B., Knoll, U., Hartwig, A., Pütz, D., 2006. Phosphonium-modified layered silicate epoxy resins nanocomposites and their combinations with ATH and organophosphorus fire retardants. Polym. Adv. Technol. 17, 281–293. Stöcker, M., Seidl, W., Seyfarth, L., Senker, J., Breu, J., 2008. Realisation of truly microporous pillared clays. Chem. Commun. 629–631. Stöcker, M., Seyfarth, L., Hirsemann, D., Senker, J., Breu, J., 2010. Microporous PILCs — synthesis, pillaring mechanism and selective cation exchange. Appl. Clay Sci. 48, 146–153. Wang, Z.Y., Han, E.H., Ke, W., 2005. Influence of nano-LDHs on char formation and fireresistant properties of flame-retardant coating. Prog. Org. Coat. 53, 29–37. Weiss, A., Becker, H.O., Orth, H., Mai, G., Lechner, H., Range, K.-J., 1970. Particle size effects and reaction mechanism of the intercalation into kaolinite. In: Heller-Kallai, L. (Ed.), Proc. Internat. Clay Conf. Tokyo 1969 II. Israel Univ. Press, Jerusalem, pp. 180–186. Zammarano, M., Franceschi, M., Bellayer, S., Gilman, J.W., Meriani, S., 2005. Preparation and flame resistance properties of revolutionary self-extinguishing epoxy nanocomposites based on layered double hydroxides. Polymer 46, 9314–9328. Zammarano, M., Bellayer, S., Gilman, J.W., Franceschi, M., Beyer, F.L., Harris, R.H., Meriani, S., 2006. Delamination of organo-modified layered double hydroxides in polyamide 6 by melt processing. Polymer 47, 652–662. Zhao, C.X., Liu, Y., Wang, D.Y., Wang, D.L., Wang, Y.Z., 2008. Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly(vinyl alcohol). Polym. Degrad. Stab. 93, 1323–1331. Ziadeh, M., Chwalka, B., Kalo, H., Schütz, M.R., Breu, J., 2012. A simple approach for producing high aspect ratio fluorohectorite nanoplatelets utilizing a stirred media mill (ball mill). Clay Miner. 47, 341–353.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Applying the flame retardant LDH as a Trojan horse for molecular flame retardants Andreas Edenharter, Josef Breu ⁎ Lehrstuhl für Anorganische Chemie I, Universität Bayreuth, 95440 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 29 April 2015 Received in revised form 3 July 2015 Accepted 6 July 2015 Available online 16 July 2015 Dedicated to Prof. Dr. Christian Robl on the occasion of his 60th birthday. Keywords: LDH Anionic clay Hydration Topotactic anion exchange Flame retardancy
a b s t r a c t The direct synthesis of Mg2Al–Cl LDH (Cl-LDH) has been accomplished by combining the urea method with the addition of NH4Cl as a buffer. It has been shown, that the buffer limits the maximum pH reached during urea hydrolysis, which in turn limits the concentration of carbonate in the reaction mixture and allows for recovery of Cl-LDH. By anion exchange into Cl-LDH phenyl phosphate (PP) was incorporated as a model flame retardant. The mechanism of this anion exchange could be shown to be topotactic rather than of dissolution/ re-precipitation type. Despite hydration of PP-LDH up to a d-value of 15.8 Å, no exfoliation could be observed upon applying high shear forces. With a filler content of 5 wt.%, PP-LDH could be shown to significantly push the flame retardant properties of polystyrene as compared to ordinary CO3-LDH as indicated by a doubling of the reduction of the peak of heat release rate (22% and 47%, respectively). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDH) possess an anisotropic, platy morphology and a considerable anion exchange capacity that renders them a versatile carrier for anionic functional molecules. The potential of LDH as efficient synergetic flame retardants for polymer composites has been compellingly demonstrated (Zammarano et al., 2005; Wang et al., 2005; Zammarano et al., 2006; Nyambo et al., 2008; Diar-Bakerly et al., 2012; Matusinovic and Wilkie, 2012). Although the dispute regarding the retardancy mechanism continues (Nyambo et al., 2008; Zhao et al., 2008; Gao et al., 2014) it has been shown that the aspect ratio (ratio of lateral extension to thickness of the platelets) is a crucial factor for this application (Diar-Bakerly et al., 2012). The method of choice to optimize aspect ratio is the so-called urea method where the required pH-rise is accomplished by urea hydrolysis (Costantino et al., 1998). Urea hydrolysis, however, produces significant carbonate levels in the solution that then in turn become the preferred interlayer anion. Due to the pronounced thermodynamic stability of CO23 −-LDH it is then unfortunately troublesome to functionalize the material by ion-exchange (Costa et al., 2005; Iyi and Sasaki, 2008a,b). Coprecipitation allows direct access to for instance NO− 3 -LDH or Cl−-LDH that can easily be functionalized by ion-exchange but ⁎ Corresponding author. E-mail address: [email protected] (J. Breu).
http://dx.doi.org/10.1016/j.clay.2015.07.013 0169-1317/© 2015 Elsevier B.V. All rights reserved.
coprecipitation delivers rather small aspect ratios. More recently, Schwieger et al. reported a modification of urea hydrolysis that gives direct access to Zn2AlNO3-LDH (Inayat et al., 2011). This approach for the first time allows combining large aspect ratio with easy anion exchange for LDH. In this work, we employed this modified urea hydrolysis method to synthesize Mg2AlCl-LDH (Cl-LDH) which was then ion-exchanged with a functional molecule, phenyl phosphate (PP), a model compound for P-containing flame retardants. The topotactic character of this ionexchange was established by looking at intermediate, partially exchanged stages, the hydration of PP-LDH was studied, and the potential of the PP-LDH-filler for flame retardancy was tested by cone calorimetry of the polystyrene-nanocomposite. 2. Experimental 2.1. Materials Water used in this work was distilled twice and saturated with Argon to exclude ubiquitous CO2. Ethanol was of analytical grade, THF of technical grade with a purity of 99.5%. Urea, MgCl2·6H2O and AlCl3·H2O was purchased from Grüssing GmbH and sodium phenyl phosphate dihydrate from Sigma-Aldrich. Polystyrene 158 K was purchased from BASF. All chemicals were used as received without purification unless stated otherwise.
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2.2. Synthesis
2.6. Characterization
For the synthesis of Mg2AlCl-LDH (Cl-LDH), an aqueous solution of urea (1.33 M), the chloride salts of Mg2+ (0.33 M) and Al3+ (0.17 M) with the molar ratio of 2/1 and NH4Cl (1 M) was added into a glass reactor (1300 mL) equipped with various on-line sensors (pH, React-IR). The solution was refluxed at 100 °C for 48 h and the parameters were recorded and controlled by a Mettler Toledo LabMax. The white precipitate was washed by repeated centrifugation at 8000 rpm and redispersing in ethanol until a test with AgNO3 of the supernatant for Cl− was negative. Subsequently, the product was dried at 60 °C overnight.
The Mg and Al content of the samples was determined by flame atomic absorption spectroscopy (F-AAS) with a Varian AA100. The Cl content was determined by ion chromatography. Powder X-ray diffraction (PXRD) measurements at different relative humidities were obtained on a Bragg–Brentano-type diffractometer (Panalytical XPERT-PRO) using nickel filtered Cu-Kα radiation and an X'Celerator Scientific RTMS detector. The instrument was equipped with an Anton Paar temperature–humidity chamber connected to a VTI Corp. RH200 humidity generator. Infrared spectroscopy was conducted in ATR mode on a Jasco FT/IR-6100. Cone calorimetry was performed on a Fire Testing Technology iCone Calorimeter using 100 × 100 × 4 mm3 plaques of the samples in horizontal geometry at 35 kW/m2 heat flux. Testing was done according to ASTM-E1354.
2.3. Anion exchange For a partial anion exchange, 75 mg of Cl-LDH was suspended in water by shaking for 1 h. The dispersion was transferred to a 250 mL three-neck flask equipped with a dropping funnel and an Argon inlet. An aqueous solution of sodium phenyl phosphate (0.03 M) was prepared. The volume of this solution necessary for a partial anion exchange was calculated as a fraction of the nominal anion exchange capacity (AEC) of 4.0 meq/g for the Mg2AlCl-LDH. For example, the reaction with 40% AEC required 60 μmol of phenyl phosphate (PP). Accordingly, 2 mL of this solution was added dropwise to the LDH dispersion under stirring and in an argon atmosphere. The product (PP-LDH) was washed with water (20 mL) thrice and then freeze dried.
2.4. Mechanical treatment of PP-LDH For milling a stirred media mill was applied (Retsch LabStar LS1). The setup and procedure used are described in detail elsewhere (Ziadeh et al., 2012). An agitation speed of 2000 rpm and yttrium stabilized ZrO2 beads (SiLi beads) with a diameter of 1.4–1.6 mm was used. An aqueous dispersion of approx. 5 wt.% of PP-LDH was circulated through the milling chamber for 30 min and then freeze dried immediately. Alternatively, a microfluidizer (Microfluidics Inc. type M-110Y) equipped with an H30Z (200 μm, ceramics) and an H10Z (100 μ, diamond) (pre-)interaction chamber was used to generate high shear forces. Here, an aqueous dispersion of approx. 1 wt.% of PP-LDH was pressed through the interaction chambers at 1 kbar 15 times and then freeze dried immediately.
2.5. Preparation of polystyrene/PP-LDH nano composites PS nanocomposites were prepared by adding appropriate amounts of dispersion of PP-LDH in THF to a solution (25 wt.%) of PS in THF to yield 5°wt.% of PP-LDH in the final nanocomposite. The dispersions were first mixed for 30 min using an overhead stirrer (500 rpm, RT). Afterwards, the dispersion was further homogenized applying a threeroll mill (Exakt-80E) placed in an exhaust hood. All samples were passed through the mill 6 times. The roller distance was gradually lowered from initially 25 μm in the first cycle to 5 μm in the final cycle. Despite cooling at − 23 °C a good share of THF evaporated in the course of three-roll milling (1 h) and highly viscous dispersions were obtained which were pre-dried in a vacuum oven (80 °C, 600 mbar, 3 h). For better processability, all samples were then ground into a powder in a planetary ball mill (Retsch PM 100, 5 min, 350 rpm) and dried again in a vacuum oven (110 °C, 300 mbar, 24 h) to remove remaining traces of THF. Moreover, a blank sample of the virgin polymer was prepared following the same described procedure. Finally, the dried powders were hot pressed at 180 °C for 5 min to obtain plaques with dimensions of 100 × 100 × 4 mm3 for cone calorimetric analysis.
3. Results and discussion 3.1. Synthesis of Mg2AlCl-LDH (Cl-LDH) 2− was The variation of the concentrations of OH−, HCO− 3 and CO3 followed during synthesis by on-line sensors (Fig. 1). NH4Cl serves as reservoir for both, chloride ions needed as interlayer ions and OH−. The evolution of the pH during hydrolysis in the presence of NH4Cl is distinctly modified as compared to ordinary urea hydrolysis. In the absence of NH4Cl early in the reaction (200 min.) a pronounced maximum of the pH is observed with a peak at around pH = 6.3. In the presence of NH4Cl only a very shallow maximum at around pH = 6.1 is observed at a much later stage of the reaction (430 min). At the final stages pH values converge to 6.2 and 6.7 with and without NH4Cl, respectively. As indicated by a similar variation of the intensity of the symmetrical C_O stretching vibration, the kinetics of the urea-hydrolysis are little affected by the presence of NH4Cl. Contrary to this, due to the different pH of the solution, the concentration of the carbonate ion is significantly lowered by the presence of NH4Cl. This pronounced reduction of the activity most likely allows Cl− that is present at a concentration CO2− 3 of 2.16 mol/L to win the competition for the interlayer position despite for this position. the higher affinity of CO2− 3 As expected, the distinctly different evolution of the pH also influences the partition of the two cations between solid product and solution. The solubility of initially precipitated amorphous Al(OH)3 depends crucially on the pH. The higher the pH, the higher the concentration of [Al(OH)4]−. The availability of [Al(OH)4]− in solution in turn determines the composition of the octahedral sheet of the LDH formed. The concentration of Al3 + in solution is in any case below detection limit while the concentration of Mg2+ is significantly increased by the presence of NH4Cl (Table 1). The Mg-level in solution is determined by the solubility product of the LDH which in turn depends on the pH of the solution. The precipitate obtained after 48 h is significantly
Fig. 1. Variation over reaction time of pH (solid lines) and intensities of the FTIR signals for carbonate (solid triangles) and urea (empty circles) with (red) and without (black) NH4Cl.
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Table 1 Cation composition and yield at the end of LDH synthesis with and without NH4Cl.
Mg2+ conc. in supernatant [mmol/L] Al3+ conc. in supernatant [mmol/L] Mg/Al ratio in precipitate Space yield [g/L] Cl− content [wt.%]
NH4Cl
No NH4Cl
64 bLOD 1.43 20.6 13.4
25 bLOD 1.94 24.5 –
Al-richer in the presence of NH4Cl. The composition of the precipitate is, however, not necessarily identical with the composition of the LDH-phase whenever amorphous Al(OH)3 is not completely transformed into LDH. The FTIR spectra (Fig. 2) of the final precipitates clearly indicate the absence of the asymmetric valence vibration of carbonate (1350 cm− 1) only when the NH4Cl buffer is applied during synthesis. Assuming charge neutrality and full occupation of the octahedral positions, the following formula for Cl-LDH may be derived from the concentration of chloride (13.4 wt.%) as determined by ion chromatography: Mg0.65Al0.35Cl0.28. This formula indicates a Mg/Alratio of 1.83 which is significantly higher than the 1.43 found by AAS-analysis of the precipitate. This would indeed suggest that the precipitate contains some amorphous Al(OH)3 alongside the main Cl-LDH phase. Thus, the use of a NH4Cl buffer allows for recovery of Mg 2Al-Cl LDH but with the drawback of both a somewhat lower yield and potentially some amorphous side phase. Anyhow, PXRD patterns (Fig. 3) show that for both CO3-LDH and Cl-LDH obtained by urea hydrolysis no other crystalline phases are present.
Fig. 2. ATR-FTIR spectra of the final precipitates obtained with (red) and without (black) NH4Cl.
Fig. 3. PXRD patterns of Mg2Al–CO3 (black) and Mg2Al–Cl LDH (red).
Fig. 4. Evolution of PXRD patterns with progressing anion exchange.
3.2. Anion exchange with phenyl phosphate The larger phosphorous containing di-anion phenyl phosphate (PP) readily replaces the monovalent Cl−. To prove the topotactic nature of the anion exchange rather than a dissolution/re-precipitation mechanism, partial exchange with increasing amounts of PP was performed. The electrostatic attraction between positive octahedral sheet and interlayer anions will in any case assure that anions of different size like PP and Cl− will unequivocally segregate into different interlayers. This way interstratified materials will be produced by partial ion exchange that at any given degree of ion exchange will assure the minimum average basal spacing going along with a maximum of Coulomb attraction. Random interstratifications of an increasing number of PP-interlayers in Cl-LDH-tactoids will, according to Mering's rules, translate into gradual shifts of the basal spacing to higher dvalues (Figs. 4 and 5a). Moreover, random interstratification will render the 00l-series non-rational as indicated by the coefficient of variation (Fig. 5b) and will contribute significantly to peak broadening as indicated by the full width at half maximum (FWHM) of the 001-reflection (Fig. 5b). Please note, the rhombohedral symmetry is lost upon ion exchange to PP-LDH, which is why the Miller indices 001 were used to refer to the basal reflection. FWHM and coefficient of variation (CV) values are low for the starting Cl-LDH, go through maxima for intermediate amounts of PP added and finally decrease again when approaching the PP-end. This is just what is to be expected for random interstratifications, proving a topotactic reaction mechanism. At 40% two maxima are observed, however, none of them corresponds to the basal spacing of the end-members (Cl-LDH and PP-LDH). These two phases rather both correspond to interstratified phases but with different contributions of the two types of interlayer heights. Completion of ion-exchange is indicated by regaining a rational 00l-series with a unique basal spacing (8.7 Å) of PP-interlayers, low CV and FWHM values.
Fig. 5. Variation of apparent d-spacing, FWHM, and CV with progressing anion exchange.
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Fig. 6. PXRD patterns of PP-LDH at increasing relative humidities recorded in-situ in a humidity chamber with 60 min equilibration time after each step.
Fig. 7. Variation of apparent d-spacing of PP-LDH with increasing RH. Data was taken from reflections assigned to the 0-hydrate (black square), 1-hydrate (red dot), 2-hydrate (blue triangle).
3.3. Hydration behavior PP-LDH eagerly hydrates when exposed to humid air. Discrete crystalline swelling steps are observed as a function of increasing relative humidity (RH) (Fig. 6). The basal spacing increases in two steps from 8.7 Å (0-hydrate) in the dry state to 11.2 Å (1-hydrate) and finally 15.8 Å (2-hydrate) (Fig. 7). The first step is well defined with 20% RH being in the middle of the relatively sharp transition. The comparatively broad basal reflections at this very water vapor activity of transition result from a random interstratifications of yet non-hydrated interlayers and already hydrated interlayers. Aside from the transition zone, all tactoids nevertheless hydrate uniformly at a well defined water vapor activity. Hydration enthalpy has contributions of the hydration of hydroxyl groups of the octahedral sheets and of the hydration of the interlayer ions. This uniform intracrystalline reactivity would consequently indicate that either the first hydration is dominated by the hydration
Fig. 8. Variation of FWHM of PP-LDH with increasing RH. Data was taken from reflections assigned to the 0-hydrate (black square), 1-hydrate (red dot), 2-hydrate (blue triangle).
Fig. 9. Variation of CV of PP-LDH with increasing RH. Data was taken from reflections assigned to the 0-hydrate (black square), 1-hydrate (red dot), 2-hydrate (blue triangle).
of the hydroxyl groups or, and this appears more likely, that the concentration of anions is similar at all length scales. Contrary to the spontaneous transition from 0- to 1-hydrate, the transition from 1-hydrate to 2-hydrate is for whatever reason retarded. 1-hydrate and 2-hydrate exist concomitantly over a broad range of RH (Fig. 6). As indicated by the FWHM (Fig. 8) and CV (Fig. 9) of the two distinct basal reflections in this range of coexistence, the 1-hydrate tactoids show very little interstratifications while the newly formed 2-hydrate tactoids show significant interstratifications. The basal spacing of the latter gradually shifts from 15.1 Å to its final value of 15.8 Å. The shift again may be explained by a certain degree of random interstratification of 1-hydrate interlayers in mostly 2-hydrate tactoids and the degree of these interstratifications diminishes with increasing RH. The reflection of 2-hydrate tactoids starts appearing at 50% RH and the intensity of the basal reflections increase further with increasing RH, while the intensities of hydrate 1 decrease until it completely vanishes at 80% RH. Most likely the coexistence is related to the kinetics of intercalation. During swelling to the 1-hydrate and then on to the 2-hydrate the octahedral sheets become elastically deformed and a considerable strain develops at the reaction front. Based on results obtained for intercalation reactions of kaolinites, Weiss et al. argued that depending on the lateral dimensions of the tactoids, two different intercalation mechanisms would be followed (Weiss et al., 1970; Stöcker et al., 2008, 2010): The type of mechanism is moreover determined by the ratio of the crystal diameter to the cooperative elastic action length (a0), which is the longer, the better the crystallinity. Expansion of the interlamellar space in tactoids with diameters smaller than the cooperative elastic action length can only occur via the “one-sided mechanism” also called the “wedge mechanism”. Since through the elastic deformation all edges immediately sense the start of the widening of the interlayer gallery, nucleation on the opposite edges is stalled in this case. Contrary to this, large tactoids with a diameter larger than the cooperative elastic action length may, however, intercalate via the so
Fig. 10. Particle size distribution of PP-LDH in water measured by laser light scattering.
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Table 3 Cone data.
Fig. 11. PXRD patterns of PP-LDH before (black) and after (red) treatment in a microfluidizer and before (blue) and after (pink) milling.
Table 2 FWHM of PP-LDH before and after mechanical treatment. FWHM [°] Milling: 0 min Milling: 30 min Microfluidizer: 0 cycles Microfluidizer: 15 cycles
1.57 1.13 1.47 1.37
called “ring mechanism”. This is feasible because the lateral diameter is longer than a0, opposite edges thus do not sense the beginning of elastic deformation and therefore the reaction can proceed from all the edges concomitantly. In the line of Weiss hypothesis we speculate, that the broad particle size distribution obtained by the urea hydrolysis (Fig. 10) stretches across the range where a change of the type of intercalation mechanism might be expected for LDH. Consequently, the kinetics of tactoids of various sizes might differ significantly as observed for the 1-hydrate to 2-hydrate transition. 3.4. Exfoliation by shearing The difference in d-values between the 0-hydrate to the 2-hydrate is 7.1 Å. To the best of our knowledge, this represents the largest expansion of the interlayer by hydration reported for LDH. With a total interlayer distance (gallery height) of approx. 11 Å (15.8 Å–4.8 Å) in the fully hydrated state, attractive forces between the layers could be weakened sufficiently to render the tactoids shear labile, allowing for exfoliation in water (Möller et al., 2010; Ziadeh et al., 2012). To test for potential exfoliation, two well established methods, a ball mill and a microfluidizer were utilized to apply high shear forces. To minimize a potential contribution of interstratifications of differently hydrated interlayers, the samples were dried to the 0-hydrate state and the FWHM of the 001reflection is used to gauge, whether or not exfoliation has occurred
Fig. 12. Heat release rate for pristine polystyrene (blue) and nanocomposites filled with CO3-LDH (black) or PP-LDH (red).
Formulation
PHRR [kW/m2]
THE [MJ/m2]
tig [s]
Burn out time [s]
EHC [MJ/m2 g]
PS CO3-LDH PP-LDH
865 ± 28 673 ± 22 457 ± 15
137 ± 3 135 ± 2 136 ± 2
81 ± 3 59 ± 2 65 ± 2
550 ± 15 612 ± 32 695 ± 35
3.4 ± 0.1 3.4 ± 0.1 3.5 ± 0.1
(Fig. 11 and Table 2). Unfortunately, for both tested shearing methods, we failed to increase FWHMs indicating that efficient exfoliation is impossible. Apparently, the charge density of the LDH (4.2 charges/nm2 for Mg/Al = 2/1) is too high and Coulomb attraction remains too strong. This is in contrast to observations made for smectites with lower charge density for which with the same methods efficient exfoliation could be achieved (Möller et al., 2010; Ziadeh et al., 2012). 3.5. Flame retardancy The PP-LDH and CO3-LDH as obtained from the urea hydrolysis method had specific surface areas of 18 and 45 m2/g, respectively. Surface areas together with the FWHM of 001-reflections and the average lateral extension (Fig. 10) as determined by laser scattering allow to estimate (Ziadeh et al., 2012) similar aspect ratios of 10 for both LDH. Both diameter and aspect ratio of CO3-LDH agree within experimental error with published values (Diar-Bakerly et al., 2012). As stated in the introduction, here we took advantage of the anion exchange capability of Cl-LDH to introduce phenyl phosphate as a model compound for P-containing flame retardants into the nanocomposites to improve the flame retardancy potential of the synergistic LDH-filler beyond what has been reported for CO3-LDH. The flame retardant properties of PP-LDH filler in a polystyrene matrix were tested by cone calorimetry and compared to the effect of an ordinary CO3-LDH with comparable aspect ratio and specific surface (Fig. 12). Cone calorimetry is the most versatile analysis to test the fire properties of polymer nanocomposites. Information is collected about the time to ignition (tig), the heat release rate (HRR), the peak of heat release rate (PHRR), the total heat release (THR), and burn out time in a developing fire scenario. For good flame retardants, as known from literature, it is favorable to increase tig, decrease PHRR and THR, and increase the burn out time. Table 3 summarizes the cone data for PP-LDH and CO3-LDH which show similar aspect ratios. Therefore, differences observed may mainly be attributed to the functional PP molecule introduced as interlayer anion. Heat release rate curves obtained at a heat flux of 35 kW/m2 are shown in Fig. 12. As typical for LDH composites (Zammarano et al., 2005; Diar-Bakerly et al., 2012; Matusinovic and Wilkie, 2012; Matusinovic et al., 2013), for both fillers the time to ignition was found to be shorter than for the pristine non-filled polymer. Quite
Fig. 13. Residue of the cone test for Polystyrene/PP-LDH nanocomposite.
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pleasing a reduction of the peak of heat release rate of 47% with PP-LDH as compared to only 22% with CO3-LDH, unequivocally proves the positive influence of the interlayer anion on the flame retardant effect of PP-LDH. Furthermore, there is practically no change in the effective heat of combustion (EHC, Table 3), indicating a condensed phase mechanism of flame retardancy (Schartel et al., 2006). This is in line with previous investigations by Braun et al. (2006) showing that phosphates primarily act in the condensed phase. Moreover, the heat release of the PP-LDH nanocomposite was spread over a wider range of time resulting in a higher burn out time (from 550 to 695 s) that indicates a slower transfer of mass and heat during the combustion of the polymer. The longer burning time at lower HRR may be related to the formation of a thin layer of char and residues of metal oxides that insulate the polymer from heat radiation (Fig. 13). 4. Conclusion For the direct synthesis of Mg2Al-Cl LDH by the urea hydrolysis method, addition of NH4Cl not only assures a high Cl− background but additionally its buffer capacity controls the pH and this way in turn concentration during synthesis is kept low. Most likely due the CO2− 3 to its high charge density, PP-LDH despite a significant degree of swelling cannot be rendered shear labile to allow for exfoliation by applying high shear forces. LDH exhibit a comparatively large anion exchange capacity that can be taken advantage of to push the flame retardancy of platy nanofillers. With phenyl phosphate applied as a model compound for P-containing flame retardants this potential could be proven unequivocally. The cone calorimetry data underline the potential of large aspect ratio LDH acting as Trojan horses for molecular flame retardants. We expect even more pronounced effects when delivering even more sophisticated molecular flame retardants via the layered flame retardant. Acknowledgments The authors acknowledge the Deutsche Forschungsgemeinschaft (SFB 840) for financial support. References Braun, U., Balabanovich, A.I., Schartel, B., Knoll, U., Artner, J., Ciesielski, M., Doring, M., Perez, R., Sandler, J.K.W., Altstadt, V., Hoffmann, T., Pospiech, D., 2006. Influence of the oxidation state of phosphorus on the decomposition and fire behaviour of flame-retarded epoxy resin composites. Polymer 47, 8495–8508.
Costa, F.R., Abdel-Goad, M., Wagenknecht, U., Heinrich, G., 2005. Nanocomposites based on polyethylene and Mg–Al layered double hydroxide. I. Synthesis and characterization. Polymer 46, 4447–4453. Costantino, U., Marmottini, F., Nocchetti, M., Vivani, R., 1998. New synthetic routes to hydrotalcite-like compounds — characterisation and properties of the obtained materials. Eur. J. Inorg. Chem. 1439–1446. Diar-Bakerly, B., Beyer, G., Schobert, R., Breu, J., 2012. Significance of aspect ratio on efficiency of layered double hydroxide flame retardants. In: Morgan, A.B., Wilkie, C.A., Nelson, G.L. (Eds.), Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science. American Chemical Society, pp. 407–425. Gao, Y.S., Wu, J.W., Wang, Q., Wilkie, C.A., O'Hare, D., 2014. Flame retardant polymer/ layered double hydroxide nanocomposites. J. Mater. Chem. A 2, 10996–11016. Inayat, A., Klumpp, M., Schwieger, W., 2011. The urea method for the direct synthesis of ZnAl layered double hydroxides with nitrate as the interlayer anion. Appl. Clay Sci. 51, 452–459. Iyi, N., Sasaki, T., 2008a. Decarbonation of MgAl-LDHs (layered double hydroxides) using acetate-buffer/NaCl mixed solution. J. Colloid Interface Sci. 322, 237–245. Iyi, N., Sasaki, T., 2008b. Deintercalation of carbonate ions and anion exchange of an Al-rich MgAl-LDH (layered double hydroxide). Appl. Clay Sci. 42, 246–251. Matusinovic, Z., Wilkie, C.A., 2012. Fire retardancy and morphology of layered double hydroxide nanocomposites: a review. J. Mater. Chem. 22, 18701–18704. Matusinovic, Z., Feng, J.X., Wilkie, C.A., 2013. The role of dispersion of LDH in fire retardancy: the effect of different divalent metals in benzoic acid modified LDH on dispersion and fire retardant properties of polystyrene- and poly(methyl-methacrylate)-LDH-B nanocomposites. Polym. Degrad. Stab. 98, 1515–1525. Möller, M.W., Handge, U.A., Kunz, D.A., Lunkenbein, T., Altstädt, V., Breu, J., 2010. Tailoring shear-stiff, mica-like Nanoplatelets. ACS Nano 4, 717–724. Nyambo, C., Songtipya, P., Manias, E., Jimenez-Gasco, M.M., Wilkie, C.A., 2008. Effect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fireretardancy of PMMA and PS. J. Mater. Chem. 18, 4827–4838. Schartel, B., Knoll, U., Hartwig, A., Pütz, D., 2006. Phosphonium-modified layered silicate epoxy resins nanocomposites and their combinations with ATH and organophosphorus fire retardants. Polym. Adv. Technol. 17, 281–293. Stöcker, M., Seidl, W., Seyfarth, L., Senker, J., Breu, J., 2008. Realisation of truly microporous pillared clays. Chem. Commun. 629–631. Stöcker, M., Seyfarth, L., Hirsemann, D., Senker, J., Breu, J., 2010. Microporous PILCs — synthesis, pillaring mechanism and selective cation exchange. Appl. Clay Sci. 48, 146–153. Wang, Z.Y., Han, E.H., Ke, W., 2005. Influence of nano-LDHs on char formation and fireresistant properties of flame-retardant coating. Prog. Org. Coat. 53, 29–37. Weiss, A., Becker, H.O., Orth, H., Mai, G., Lechner, H., Range, K.-J., 1970. Particle size effects and reaction mechanism of the intercalation into kaolinite. In: Heller-Kallai, L. (Ed.), Proc. Internat. Clay Conf. Tokyo 1969 II. Israel Univ. Press, Jerusalem, pp. 180–186. Zammarano, M., Franceschi, M., Bellayer, S., Gilman, J.W., Meriani, S., 2005. Preparation and flame resistance properties of revolutionary self-extinguishing epoxy nanocomposites based on layered double hydroxides. Polymer 46, 9314–9328. Zammarano, M., Bellayer, S., Gilman, J.W., Franceschi, M., Beyer, F.L., Harris, R.H., Meriani, S., 2006. Delamination of organo-modified layered double hydroxides in polyamide 6 by melt processing. Polymer 47, 652–662. Zhao, C.X., Liu, Y., Wang, D.Y., Wang, D.L., Wang, Y.Z., 2008. Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly(vinyl alcohol). Polym. Degrad. Stab. 93, 1323–1331. Ziadeh, M., Chwalka, B., Kalo, H., Schütz, M.R., Breu, J., 2012. A simple approach for producing high aspect ratio fluorohectorite nanoplatelets utilizing a stirred media mill (ball mill). Clay Miner. 47, 341–353.