Structures of LDHs Intercalated with Ammonia and the Thermal Stability for Poly(vinyl chloride)

MgAl-NH3 LDHs (layered double hydroxides), ZnAl-LDHs and ZnAl-NH3 LDHs with different molar ratio of Al/ammonia were synthesized by co-precipitation method. The synthetic LDHs samples were characterized by XRD, FT-IR, TGA-DTG and SEM techniques and tested by Congo red method. In addition, after PVC (Poly vinyl chloride) and DOP (Dioctylphthalate) were added to LDHs, the thermal stability for PVC was characterized by TGA-DTG. The results indicated that LDHs intercalated with ammonia improved thermal stability for PVC significantly, MgAl-NH3 LDHs (Mg2+:Al3+: NH3·H2O = 3:1:0.5) and ZnAl-NH3 LDHs (Zn2+:Al3+:NH3·H2O=2:1:1.5) showed the best thermal stability for PVC, interlayered space of LDHs became larger by intercalating ammonia.


Introduction
Hydrotalcite (called Layered Double Hydroxides, abbreviated as LDHs) is a kind of synthetic inorganic anionic laminar compound [1].The interchangeable formula for the chemical composition of LDHs is: , where M 2+ represents a kind of divalent metal cation, A n− represents a kind of interlayered n-valent inorganic (organic) anion [2] [3], and M 3+ represents a kind of trivalent metal cation.The structure of LDHs is shown in Figure 1 [4].Different kinds of ions can be intercalated into the layers of hydrotalcite according to actual needs [5].Due to the ion-exchange properties, adjustability of the laminates spacing, adjustability of LDHs, hydrotalcite has different selective adsorption properties and catalytic properties [6].LDHs had to be modified so as to improve the thermal stability for PVC (Poly vinyl chloride).
In this research, MgAl-NH 3 LDHs, ZnAl-LDHs and ZnAl-NH 3 LDHs with different molar ratio of Al/ammonia were synthesized by co-precipitation method.The synthetic LDHs samples were characterized by XRD, FT-IR, TGA-DTG, and SEM techniques and tested by Congo red method.In addition, after PVC and DOP (Dioctylphthalate) were added to LDHs, the thermal stability for PVC was characterized by TGA-DTG.

Synthesis of LDHs
All of the LDHs samples were synthesized by co-precipitation method [7].
MgAl /OH − = 1.6) were added simultaneously to a three-necked round-bottom flask containing 50 mL deionized water.The reaction temperature was controlled at 90˚C, and stirring speed was controlled at 400 r/min.The reaction solution was obtained under another 2 -3 hours after the acid and alkaline solution were dropped perfectly.Then the reaction solution were aged for 18 hours at 65˚C, filtered in vacuum to neutrality with de-ionized water and dried for 15 hours at 80˚C in vacuum.Finally the samples obtained were ground, weighed and bagged.

Characterization and Property Tests
In the light of ISO 182/1-1990, Congo Red tests was applied to detect the thermal stability on LDHs for PVC.The mass ratio of PVC/DOP/LDHs was 100:5:5 and the oil bath temperature was kept at 190˚C for at least 1 hour.The discoloration time of Congo Red dipstick required was the heat stabilizers time on LDHs for PVC.
X-ray diffraction (XRD) results for LDHs samples were recorded using a PAN alytical X Per PRO diffractometer, Cu Kα, 40 kV, 4 mA.2θ value range from 5˚ to 80˚, and the rate was kept at 4˚•min −1 .
Fourier Transform Infrared (FT-IR) spectrum was obtained with a Nicolet AVATAR 380 spectrometer, 400 -4000 cm −1 in air.The synthetic LDHs samples were mixed with KBr (0.200 g, p.a.) to form flake of cm −1 .
Scanning electron microscope (SEM) pictures were received by a S-3400N instrument produced by Japan's Hitachi corporation.

Congo Red Tests
The Congo Red test results were shown in Table 1.Sample 2 (Mg 2+ :Al 3+ :NH 3 •H 2 O = 3:1:0.5)and Sample 8 (Zn 2+ :Al 3+ : NH 3 •H 2 O = 2:1:1.5)owned the best thermal stability time for PVC within the same series of LDHs samples.Thermal stabilization time of Sample 2, Sample 5, Sample 8 for PVC were respectively 43.0 min, 17.5 min, 41.0 min.In addition, thermal stabilization time of PVC/DOP without LDHs was 8.0 min.It was obvious that LDHs intercalated with ammonia improved the thermal stability time for PVC compared to non-intercalated LDHs.This was due to the effective reaction of ammonia with HCl, which reduced the concentration of HCl and weakened the catalytic effect for PVC degradation.The reaction equation was as below:

Powder X-Ray Diffraction
As was shown in XRD (Figure 2) spectrum, the three samples all had characteristic diffraction peaks for LDHs.D (003) was relevant to the radius of anion in the intermediate layer and the interactions between anion and cation on the layers, where d(003) represented the layer-layer spacing of the hydrotalcite-like compounds.If the charge density of anion was low, the interactions between anions and positively charged layers was weak, and then the layer spacing was large.d(110) reflected the atomic arrangement density of the (110) crystal plane [8].

Infrared Spectroscopy
As was shown in IR spectra (Figure 3), the effective intercalation of ammonia can also be confirmed.The broad   CO − group of LDHs appeared at 1320 -1380 cm −1 , significantly shifting to the lower wave number, which attests carbonate ion inserted between layers of LDHs are not really disengaged ions and have strongly interacted with interlaminar H 2 O molecules by hydrogen bonding.Other absorption peak appearing near 700 cm −1 are assigned to the bending and stretching vibration modes for metal-oxygen bonds [10].

TGA and DTG Analysis
As was shown in TGA and DTG curves (Figure 4 and Figure 5), there existed two weight loss stage.The first weight loss stage (T < 250˚C) corresponded to the release of crystal water between the layers and adsorbed water on the surface of LDHs samples.The second weight loss stage corresponded to the release of 2 3 CO − in the form of CO 2 between the layers, and during the second stage the reduced quality also included dehydration of most hydroxy group on the laminates.All the hydroxy group were removed at higher temperatures, and thus the laminate structure of LDHs samples were completely destroyed, generating mixed oxides finally [11] [12].As was shown, when the temperature was below 300˚C, the weight loss for the three samples were different, and as temperature growed, Sample 5 showed the best thermal stability.Intercalating ammonia reduced the thermal stability for LDHs.

SEM Analysis
SEM pictures shown in Figure 6 exhibited the particle morphologies of all the three synthesized LDHs samples.Sample 2 (Mg 2+ :Al 3+ :NH 3 •H 2 O = 3:1:0.5),Sample 5 (Zn 2+ :Al 3+ = 2:1) and Sample 8 (Zn 2+ :Al 3+ :NH 3 •H 2 O = 2:1:1.5)all exhibited the layered structures and the crystals dispersed on the surface of the particles, but the crystal sizes were different and tended to agglomerate into a big block.It can obviously be seen that intercalating ammonia had weaken the grain dispersion and the layer space became larger, the particle sizes became larger, the surface area became larger with the tendency to agglomerate.The SEM analysis further validated the effect of intercalated ammonia for the crystal structures of LDHs.

The Thermal Stability for PVC
TGA and DTG curves in N 2 for LDHs/PVC/DOP were shown in Figure 7 and Figure 8.Compared with PVC/DOP, it was obvious that adding LDHs improved the thermal stability for PVC.TGA curves can be divided into two well-differentiated main regions.The first region (ranging from 100˚C to 450˚C) was related to the formation of conjugated polyene sequences and the dehydrochlorination.Released HCl will speed up the degradation for PVC.CO − as well as the hydroxyl groups and other interlayer anions such as ammonia will be removed from the brucite-like layers of LDHs in this temperature range [13].The second region (over 450˚C) attributed to formation of residual chars and decomposition of polyene backones.

Conclusion
Three kinds of LDHs samples all have typical diffraction peaks.Thermal stabilization time of MgAl-NH 3 LDHs,

Figure 3 .
Figure 3. IR spectra.2: MgAl-NH 3 LDHs; 5: ZnAl-LDHs; 8: ZnAl-NH 3 LDHs.bandvisibly appearing at 3430 -3580 cm −1 may be due to O-H groups of the intercalated H 2 O and stretching vibrations of OH − .The weak band appearing at about 1600 cm −1 is attributed to the bending mode of H 2 O molecules.Weak peak appearing at 2118 cm −1 in Sample 8 (Zn 2+ :Al 3+ :NH 3 •H 2 O = 2:1:1.5)is associated with the stretching vibration of N-H bond.Compared with carbonate ion in free state (appearing at 1415 cm −1 ), the peak resulted from the asymmetric stretching vibration for C-O bond in 2 3

Table 1 .
Congo Red test results.