Effect of Rare Earth Ions on the Properties of Composites Composed of Ethylene Vinyl Acetate Copolymer and Layered Double Hydroxides

Background The study on the rare earth (RE)-doped layered double hydroxides (LDHs) has received considerable attention due to their potential applications in catalysts. However, the use of RE-doped LDHs as polymer halogen-free flame retardants was seldom investigated. Furthermore, the effect of rare earth elements on the hydrophobicity of LDHs materials and the compatibility of LDHs/polymer composite has seldom been reported. Methodology/Principal Findings The stearate sodium surface modified Ni-containing LDHs and RE-doped Ni-containing LDHs were rapidly synthesized by a coprecipitation method coupled with the microwave hydrothermal treatment. The influences of trace amounts of rare earth ions La, Ce and Nd on the amount of water molecules, the crystallinity, the morphology, the hydrophobicity of modified Ni-containing LDHs and the adsorption of modifier in the surface of LDHs were investigated by TGA, XRD, TEM, contact angle and IR, respectively. Moreover, the effects of the rare earth ions on the interfacial compatibility, the flame retardancy and the mechanical properties of ethylene vinyl acetate copolymer (EVA)/LDHs composites were also explored in detail. Conclusions/Significance S-Ni0.1MgAl-La displayed more uniform dispersion and better interfacial compatibility in EVA matrix compared with other LDHs. Furthermore, the S-Ni0.1MgAl-La/EVA composite showed the best fire retardancy and mechanical properties in all composites.

Ethylene vinyl acetate (EVA) copolymer with different vinyl acetate (VA) contents are widely used in the wire, cable, wrapper, adhesive and drug industry [19][20][21][22]. However, EVA resins are particularly flammable, and its subsequent combustion gives off large volumes of toxic smoke. A feasible solution to this problem is the addition of flame retardant which may improve the fire safety of EVA. Therefore, as a promising non-halogenated additive, LDHs have been applied in EVA for having flame retardancy [23][24][25][26].
It is well known that good interfacial compatibility between inorganic LDHs and polymer is a very important factor for the improvement of properties of LDHs/Polymer composite. To the best of our knowledge, to improve the dispersion and compatibility of inorganic LDHs phase with polymer matrix, most studies concentrate on the organo-modified treatment of LDHs so far [15,16,27]. The reason is because the organo-modifiers make the organic-inorganic hybrid LDHs more hydrophobic. However, although organo-modified treatment can enhance the hydrophobicity of LDHs, the use of large amounts of organic modifiers is not only detrimental to the flame retardancy and the mechanical properties of LDHs/polymer composites due to the existence of large amounts of carbon, but also being not environment-friendly. Therefore, the addition of a small amount of organic modifier combined with other effective way to improve the hydrophobicity of LDHs are feasible.
Rare earth (RE) has been the object of considerable scientific and technological interest due to their distinctive optical [28][29][30], electronic [31,32], magnetic [33,34], anticorrosive [35,36] and catalytic properties [37]. Many of them, especially La and Ce, were used in LDHs materials to enhance the catalytic efficiency [38][39][40]. However, the effects of rare earth elements on the hydrophobicity of LDHs materials and the compatibility of LDHs/polymer composite have been relatively seldom reported.
In our previous report, Ni-containing MgAl-LDHs showed obviously higher flame retardant efficiency in comparison with alone MgAl-LDHs in the EVA matrix [41]. Therefore, for comparison purpose, a small amounts of stearate sodium surface modified Ni-containing LDHs and RE (La, Ce, Nd)-doped Nicontaining LDHs were synthesized by a coprecipitation method coupled with the microwave hydrothermal treatment in the present paper. The main aim of this study focuses on the effects of doped trace amounts of rare earth ions on the amount of water molecules, the crystallinity, the morphology, the hydrophobicity of RE-doped modified Ni-containing LDHs and the adsorption of modifier in the surface of LDHs. Moreover, the effects of interfacial compatibility between LDHs and EVA on the flame retardancy and the mechanical properties of EVA/LDHs composites also were investigated in detail.

Results and Discussion
Thermal Analysis of LDHs Figure 1 illustrates the TGA curves of S-Ni 0.1 MgAl and REdoped modified LDHs S-Ni 0.1 MgAl-La, S-Ni 0.1 MgAl-Ce and S-Ni 0.1 MgAl-Nd, respectively. As shown in Figure 1, on heating, all LDHs mainly underwent two stages decomposition. The first stage corresponds to the loss of physical absorbed water and interlayer water in the range of 50 to 230uC [42]. The mass losses of water molecules for S-Ni 0.1 MgAl, S-Ni 0.1 MgAl-La, S-Ni 0.1 MgAl-Ce and S-Ni 0.1 MgAl-Nd are 14.2%, 15.0%, 14.5% and 16.0%, respectively. This result indicates that S-Ni 0.1 MgAl-Nd has the most water molecules in all of the above LDHs. The second stage in the range of 230 to 800uC is associated with the dehydroxylation of the metal hydroxide layers, the degradation of carbonates and stearate in LDHs [43]. This stage relates to the mass losses which are 30.3%, 29.0%, 28.9% and 29.0% for S-Ni 0.1 MgAl, S-Ni 0.1 MgAl-La, S-Ni 0.1 MgAl-Ce and S-Ni 0.1 MgAl-Nd, respectively. The above thermal analysis data point out that the addition of the rare earth ions causes different contents of water molecules and stearate in LDHs.  110) and (113) characteristic reflections of the LDHs structure, which can be indexed in a 3R polytype [44]. These reflection positions are in good agreement with those of the reported unmodified LDHs [44,45]. This result indicates that the stearate was uptaken on the surface of LDHs rather than intercalated into the interlayer.   63u) becomes distinguishable, suggesting a well ordering within the brucite-type layers [47]. However, by carefully observing, it can be found that the addition of rare earth ions reduces the reflection intensity of LDHs (see Figure 2), especially for S-Ni 0.1 MgAl-Nd in comparison with S-Ni 0.1 MgAl. This result indicates that S-Ni 0.1 MgAl-Nd has the lowest crystallinity in all of the above LDHs [48].

Powder XRD Characterization of LDHs
From the values of d 003 , d 006 and d 110 of the reflections in the XRD patterns, the lattice parameters a and c can be calculated. Furthermore, the d 003 , d 006 , d 110 , the lattice parameters a and c of all LDHs are listed in Table 1. The parameter a depends on the chemical composition of the metal cations in the layers. It corresponds to the average closest metal-metal distance within a layer, and is calculated as twice the position of the plane (1 1 0). As seen from Table 1, the a values of S-Ni 0.1 MgAl-La, S-Ni 0.1 MgAl-Ce and S-Ni 0.1 MgAl-Nd are larger than that of S-Ni 0.1 MgAl due to the addition of rare earth ions. This result confirms that the rare earth cations La, Ce and Nd were introduced to the brucite-type layers. The possible reason is that the radii of rare earth ions are much larger than that of Al 3+ . It leads to a tiny deformation of the structure of RE-doped Ni-containing LDHs compared with S-Ni 0.1 MgAl. The lattice parameter c, which is the thickness of crystal cell, can be calculated based on the following equation [49].
The c values of S-Ni 0.1 MgAl-La, S-Ni 0.1 MgAl-Ce and S-Ni 0.1 MgAl-Nd are slightly larger than that of S-Ni 0.1 MgAl. Namely, the interlayer region of S-Ni 0.1 MgAl is smaller than those of the RE-doped modified LDHs. An important reason is due to the effect of the amount of water molecules in the interlayer of LDHs, as described by Wypych et al. [50]. As expected, S-Ni 0.1 MgAl has the least water. On the contrary, the RE-doped LDHs have more water. Especially, S-Ni 0.1 MgAl-Nd has the most water based on the TGA analysis.
The crystallite sizes of all of the above LDHs can be calculated by using Debye-Scherrer formula [51].
(2)Where L is the crystallite size of the LDHs, l is the wavelength of the radiation used, B(h) is the full width at half maximum and h is the Bragg diffraction angle. Table 1 lists the results of the crystallite sizes of all LDHs L 003 in the direction c and L 110 in the direction a. These results indicate that all of the above LDHs show the nanometer crystallite size.  21 and 3000 cm 21 due to the OH stretching vibration of layer hydroxyl groups and interlayer water molecules. The bands at near 1635 cm 21 are originated by the bending mode of interlayer water molecules. The sharp and intense bands at 1365 cm 21 are due to the antisymmetric stretching mode of interlayer carbonate [52]. Furthermore, the weak bands at 1483 cm 21 are assigned to the bridged-bidentate complexation of interlayer in the above four LDHs [53,54]. At the same time, the two bands around 867 cm 21 and 664 cm 21 are characteristic for the n 2 (out-of-plane deformation) and the n 4 (in-plane bending) of interlayer carbonate [55]. In the low wave number region between 400 cm 21 and 1000 cm 21 , a series of bands at 776, 562 and 422 cm 21 are ascribed to condensed groups, the translation and deformation of M-OH in the brucite-like layers [56]. In addition, all LDHs also display two weak bands at 2920 cm 21

Combustion Behavior of Composites
The fire and smoke properties of composites with 20 wt.% loading of LDHs were evaluated using the cone calorimeter. Figure 9 A shows the heat release rate (HRR) for the EVA and its composites. It can be seen that the peak value of HRR (pk-HRR) for the pristine EVA is 1247 kW/m 2 , while the pk-HRR for S-Ni 0.    Therefore, FPI values also confirm that S-Ni 0.1 MgAl-La/EVA has the highest flame retardant efficiency. In other word, the lower FPI values mean the better fire safe materials [24]. In addition, as can be seen from the total heat release (THR) curves in Figure 9 B, the THR values of composites are smaller than that of the pristine EVA except S-Ni 0.1 MgAl-Nd/EVA. The above all typical parameters obtained from the cone calorimeter experiment

Preparation of the Surface Modified Ni-containing LDHs and RE-doped Ni-containing LDHs
All chemicals used in the preparation were analytical grade without further purification. EVA (VA-28%) was purchased from Samsung Co. in Korea. Deionized water was made by Milli-Q academic water purification system in our Lab.
The synthesis of stearate sodium surface modified Ni-containing LDHs was implemented by a coprecipitation method coupled with the microwave hydrothermal treatment. In a four-necked flask (l L), a mixed aqueous solution containing 6 ml 1 M Ni(NO 3 ) 2 , 174 ml 1 M Mg(NO 3 ) 2 and 60 ml 1 M Al(NO 3 ) 3 with a Ni 2+ :Mg 2+ :Al 3+ molar ratio of 0.1:2.9:1 was dropwise added to 100 ml of deionized water at 70uC under continuous magnetic stirring, while the pH was adjusted to the range of 8-9 by adding NaOH-Na 2 CO 3 mixed solution (0.6 M NaOH and 0.45 M Na 2 CO 3 ). After the titration, a heavy suspension gel was obtained and transfered to the beaker (l L). Then, the beaker with suspension gel was put in a microwave oven (XH-300A, the maximum power of 1000 W and a frequency of 2.45 GHz) and crystallized at 70uC for 30 min. The temperature was controlled by a temperature feedback monitoring system with dual IR sensors. The obtained precipitate was washed by the deionized water to pH 7, and then filtered. The synthesized sample was dried under air atmosphere at 70uC.
Preparation of the modified RE-doped Ni-containing LDHs. In a four-necked flask (l L), a mixed aqueous solution containing 6 ml 1 M Ni(NO 3 ) 2 , 174 ml 1 M Mg(NO 3 ) 2 , 30 ml 0.1 M RE(NO 3 ) 3 (RE = La, Ce, Nd) and 57 ml 1 M Al(NO 3 ) 3 with a Ni 2+ :Mg 2+ :RE 3+ :Al 3+ molar ratio of 0.1:2.9:0.05:0.95 was dropwise added to 100 ml of deionized water at 70uC under continuous magnetic stirring, while the pH was adjusted to the range of 8-9 by adding NaOH-Na 2 CO 3 mixed solution (0.6 M NaOH and 0.45 M Na 2 CO 3 ). All the following synthetic processes are in accordance with the synthesis of stearate sodium surface modified Nicontaining LDHs. 0.15 g stearate sodium and the above synthesized LDHs 10 g were dispersed into 200 ml deionized water. The mixture was vigorously stirred at 70uC with the microwave irradiation for 30 min. Then the precipitate was washed to pH 7 with the 70uC hot water to eliminate the excessive stearate sodium, and then filtered. The surface modified LDHs were dried in an oven for 4 h at 70uC. The resulting surface modified Ni-containing LDHs and RE-doped Ni-containing LDHs were designated as S-Ni 0.1 MgAl, S-Ni 0.1 MgAl-La, S-Ni 0.1 MgAl-Ce and S-Ni 0.1 MgAl-Nd, respectively.

Preparation of LDHs/EVA Composites
The LDHs/EVA composites were prepared via melt blending at 150uC in an RM-200A torque rheometer for 10 min with a rotor speed of 60 rpm. In order to obtain the optimum performance of composites, several formulations with the additive amount of LDHs from small to large rations (2 wt.%, 5 wt.%, 10 wt.% and 20 wt.% mass fraction of LDHs content) were systematically investigated. The composites are named as S-Ni 0.1 MgAl/EVA, S-Ni 0.1 MgAl-La/EVA, S-Ni 0.1 MgAl-Ce/EVA and S-Ni 0.1 MgAl-Nd/EVA, respectively.

Characterization Techniques
Thermal analysis were carried out with a thermogravimetric analyzer (TGA pyris 1 from Perkin Elmer) using a constant heating rate of 10uC/min under nitrogen atmosphere from 50 to 800uC. X-ray diffraction (XRD) experiments were performed by using a D/MAX 2200 diffractometer with l = 1.5406 Å for angle 2h = 5-75u. Data were collected at the rate of 4 u/min and step of 0.02 u with Cu Ka irradiation operated at 40 kV and 45 mA. Infrared (IR) spectra of samples were collected using Nicolet FTIR360 spectrometer (KBr pellet method, 4 cm 21 resolution). Contact angles (CA) were measured using JC2000A contact angle measurer. All LDHs were pressed into disks with a diameter of 11 mm and a thickness of 0.4 mm. Each disk was dripped by 3 mL of deionized water. The observation of all water drops maintains for 30 sec. The measurement for each sample repeats five times.