Synthesis and Characterization of Zn-Ca-Mg-Al Hydrotacite-like

Compounds and Its Application in the PVC

LI Long-fenga, GAO Yuanb, ZHANG Mao-linc

School of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, China alfli9469@163.com,

bgaoyuan0205@126.com,

cmlzhang1268@163.com (corresponding author)

Keywords: Hydrotalcite-like Compounds, Hydrothermal Method, Synthesis, polyvinyl chloride.

Abstract. Ca-Mg-Al hydrotalcite-like compounds (CaMgAl-HTLcs) were synthesized by a

hydrothermal method, and characterized by X-ray powder diffraction (XRD), Fourier transform

infrared spectroscopy (FT-IR) and differential thermal analysis (DTA) techniques. The effects of the

medium pH value, the molar ratio of the raw materials, the reaction temperature and the reaction time

on the structure of CaMgAl-HTLcs were studied. The results showed that increasing treatment

temperature and reaction time could improve the crystallinity and monodispersity of hydrotalcite-like

compound particles. And well-defined CaMgAl-HTLcs could be prepared at a pH value of 10~11

with n(Zn+Mg+Ca):n(Al) =2. The products synthesized were applied to PVC to improve the thermal

stability of PVC.

Introduction

Because of its unique layered structure, variability and adjustability of interlayer elements, and

exchangeability of interlayer anions, more and more attention had been paid to hydrotalcite-like

compounds in recent years [1]. A number of binary, ternary or quaternary hydrotalcite-like

compounds could be synthesized via isomorphic substitution of Al or Mg with divalent or trivalent

transition metal cations with a similar radius, and it was reported in literature that hydrotalcite-like

materials with catalytic properties had been synthesized by introducing various kinds of transition

metals or other active metal ions into the brucite-like layer of Mg and Al hydrotalcite [2]. The

commonly used methods of preparation of hydrotalcite-like compounds at present includes:

co-precipitation method, sol-gel method, hydrothermal method, roasting and recovery method as well

as ion exchange method [3]. Among these methods, the hydrothermal method had proved to be a

promising approach for the preparation of hydrotalcite-like compounds with a regular and uniform

morphology. However, a few documents focused on hydrothermal preparation of hydrotalcite-like

compounds, especially Ca-rich hydrotalcite-like compounds. Ca-rich hydrotalcite-like compounds

had the long-term thermal stability and the fire-retardant features, and thus can be employed in PVC,

greatly improving the thermal stability of PVC [4]. At present, CaMgAl hydrotalcite-like compounds

could be prepared by co-precipitation method [5].

Different from co-precipitation method mentioned above, we employed hydrothermal process to

synthesize ZnCaMgAl hydrotalcite-like compounds in our present study. Our aim was to provide an

effective method for the preparation of ZnCaMgAl hydrotalcite-like compounds. Meanwhile, to

optimize the procedure, we investigated the impacts of reaction temperature, reaction time and raw

material ratio on the preparation of ZnCaMgAl hydrotalcite-like compounds. At the same time, the

effects of products synthesized on the thermal stability of PVC were also investigated.

Applied Mechanics and Materials Vols. 66-68 (2011) pp 65-69Online available since 2011/Jul/04 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.66-68.65

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Experimental

Synthesis procedure. All of the reagents used in this experiment were analytical pure grade and were

used without further purification. The synthetic procedures are as follows: 100ml mixed salt solution

was prepared by using Ca (NO3)2, Mg(NO3)2, Zn(NO3)2 and Al(NO3)3 in a certain molar ratio.

Subsequently, a certain amount of NaOH solution and Na2CO3 solution were dropped into the mixed

salt solution with vigorous stirring by c(OH-) = 2(c(M

2+)+c(M

3+)) and c(CO3

2-) = 0.5c(M

3+), and the

pH value of the mixed reaction solution was in the range from 10 to 11. Then the above reaction

mixture was taken in Teflon-lined stainless-steel autoclave, heated to a certain temperature and kept

under the temperature for a certain time to perform hydrothermal reaction. After the completion of the

reaction, the product was separated from the reaction solution, washed with water until neutral and

dried under vacuum at 65°C to obtain the Zn-Ca-Mg-Al hydrotacite-like products.

Characterization. A Bruker D8 X-ray diffractometer with Cu Kα radiation(λ= 0.15418 nm), the

accelerating voltage of 40 kV, emission current of 40 mA and the scanning speed of 8°/min were used

to determine the phase structures of the product. Fourier transform infrared spectroscopy analysis was

performed on NICOLET6700 spectrometer using KBr pellet transmission measurements. Differential

thermal analysis was performed on DTG-60H thermal analyzer.

Results and discussion

Fig. 1 showed the XRD patterns of the sample prepared at 80,100,120, 140°C, respectively. As we

could see from Fig. 1, all samples appeared strong diffraction peaks at low angles (2θ = 11°, 24°, and

35° corresponding to the reflection planes of 003,006, and 009, respectively), and appeared weak

diffraction peaks at high angles (2θ = 39°, 47°, 61° corresponding to the reflection planes of 105, 108,

and 110). Compared to the JCPDS standard pattern, the synthetic samples had hydrotalcite-like

structure. In addition, the characteristic diffraction peaks became stronger and narrower with the

increase of reaction temperature and the prolonging of the calcination time (Fig. 1), indicating that

crystalline structure of the samples became more perfect.

10 20 30 40 50 60 70

a=80℃; b=100℃; c=120℃; d=140℃

d

c

b

a

2 Theta (degree)

Fig. 1 XRD patterns of the samples synthesized at different hydrothermal reaction temperatures for 10 h

(n(Zn+Ca+Mg):n(Al)=2, n( Zn): n(Ca): n(Mg)=1:1:1)

66 Mechanical, Materials and Manufacturing Engineering

XRD patterns of the samples synthesized at 100°C for different hydrothermal reaction times are

showed in Fig. 2. We can see that the characteristic diffraction peaks were stronger with prolonging

reaction time, suggesting that the prolonged reaction time improved the crystallinity of the products.

Fig. 3 was the XRD patterns and the crystal structure parameters of samples prepared by different

molar ratios of raw materials. Fig. 3 showed that the diffraction peaks were gradually broadened and

weakened with an increase of aluminum content, which implying that the structure regularity

worsened and the crystallinity decreased. These may be attributed to trivalent metal cation in layered

structure having a relatively large polarization, which resulted in difficulties in directional alignment.

Correspondingly, Fig. 3 also showed that the interlayer spacing broadened with an increase of Zn and

Ca content. This may be explained from the fact that the radius of Zn2+

or Ca2+

was larger than that of

Mg2+

, and thus the interaction between interlayer anions and cations in layers decreased after Mg2+

in

layers was partially replaced with Zn2+

and Ca2+

, resulting in that the interaction between interlayer

anions and Ca2+

in layers is weaker than between interlayer anions and Mg2+

in layers. Besides, from

Fig. 3, we can observe that the interlayer spacing increased slightly with the increase in n (Ca): n (Zn)

ratios under the condition of n(Zn+Mg+Ca): n(Al) = 2. Accordingly, we took samples with the molar

ratio of n(Zn+Mg+Ca): n(Al) equal to 2 for the FT-IR. Fig. 4 displayed the FT-IR curve of sample

prepared under the conditions of n(Zn+Ca+Mg):n(Al)=2, n(Zn):n(Ca):n(Mg)=1:1:1. The broad

absorption of 3600~2700cm-1

was stretching vibration of –OH in layer structures, which shifted

towards low wave number in comparison with –OH of the free state, implying the presence of the

hydrogen bonds between interlayer water and layer –OH. In addition, the absorption peak near ~1640

cm-1

was the bending vibration peak of –OH in water, while the absorption peak near 1350 cm-1

was

caused by asymmetric stretching vibration of C-O, which shifted towards low wave number in

comparison with C-O in the free state, indicating that the interlayer CO32-

was not really free, and

there were strong hydrogen bonds between the interlayer water molecules with one another. Finally,

the broad absorption peak of 660-880cm-1

was caused by C-O out-plane stretching vibration and

in-plane bending vibration. The absorption peaks appearing at 400-450cm-1

were mainly vibration

absorption peaks of M-O, M-O-M and O-M-M.

10 20 30 40 50 60 70

a=5h; b=10h; c=15h; d=20h; e=25h

e

d

c

b

a

2 Theta (degree)

Fig. 2 XRD patterns of the samples synthesized at 100°C for different hydrothermal reaction times

(n(Zn+Ca+Mg) :n(Al)=2, n( Zn): n(Ca): n(Mg)=1:1:1)

Applied Mechanics and Materials Vols. 66-68 67

10 20 30 40 50 60 70

samples d(003) d(110)

a 7.5815 1.5244

b 7.6752 1.5367

c 7.7041 1.5392

c

b

a

2 Theta (degree)

Fig. 3 XRD patterns and the crystal structure parameters of samples prepared by different molar ratios of raw

materials (a. n (Zn): n(Ca): n(Mg)=1:1:1; b. n( Zn): n(Ca): n(Mg)=1:2:1; c. n(Zn): n(Ca):n(Mg)=2:1:1)

4000 3000 2000 1000

0

20

40

60

80

100

Transmission (100%)

Wave number(cm-1)

Fig. 4 FT-IR curve of sample prepared under the conditions of n(Zn+Ca+Mg):n(Al)=2, n( Zn):n(Ca):n(Mg)=1:1:1

The effect of the hydrotacite-like on the thermal stability of PVC was tested, and the experimental

results were shown in Tab.1. We found that hydrotacite-like compounds could improve the thermal

stability of PVC, and the thermal stability of Zn-Ca-Mg-Al hydrotacite-like compound is better than

that of the Mg-Al hydrotacite-like compound and the Ca-Mg-Al hydrotacite-like compound. This

may be explained from the fact that Ca has long-term thermal stability and Zn compound has an effect

on the formation of carbon film.

68 Mechanical, Materials and Manufacturing Engineering

Tab. 1 The effects of the hydrotacite-like compounds on the thermal stability of PVC

Samples The heating time at 140℃ under air (min)

30 40 50 60 70 80 90 100 110

Pure PVC Black

Mg-Al-

hydrotacite-like

+PVC

Buff Buff Yellow Jaffa

orange

Sandy

beige Brown Black

Ca-Mg-Al-

hydrotacite-like

+PVC

Whit

e Buff Buff Buff

Yello

w

Sandy

beige Brown Black

Zn-Ca-Mg-Al-

hydrotacite-like

+PVC

Whit

e White White Buff Buff

Yello

w

Sandy

beige Brown Black

Conclusions

Zn-Ca-Mg-Al hydrotalcite-like compounds (ZnCaMgAl-HTLcs) were synthesized by a hydrothermal

method, and some impact factors on the preparation were examined. The results showed that the

molar ratio of the raw materials, the reaction temperature and the reaction time had significantly

affected on the structure of ZnCaMgAl-HTLcs, with increasing treatment temperature and prolonging

reaction time at a pH value of 10~11 improving the crystallinity and monodispersity of

hydrotalcite-like compounds. Through the studies on the thermal stability of PVC, it was found that

hydrotacite-like compounds could improve the thermal stability of PVC, and the thermal stability of

Zn-Ca-Mg-Al hydrotacite-like compound is better than that of the Mg-Al hydrotacite-like compound

and the Ca-Mg-Al hydrotacite-like compound.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Anhui Provincial

Education Department (KJ2010A302).

References

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[2] K. Kaneda, K. Kamaguchi and K. Mori, Catal. Surv. Jpn. Vol. 4 (2000), p.31

[3] M. A. Aramendia, V. Borau, C. Jimenez, J. Solid State Chem. Vol. 168 (2002), p.156

[4] H. Y. Zhang, X. Y. Jing, Chin. J. Inorg. Chem. Vol.18 (2002), p.185 (in Chinese)

[5] Z. H. Yang, W. W. Chi, S. Yi, S. W. Wang, Z. Zhi, Plastic Additives (2008), p.18 (in Chinese)

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