Lateks Nanocomposite

7/29/2019 Lateks Nanocomposite

1/8

Preparation of natural rubber/ pristine clay nanocomposites by co-coagulating

natural rubber latex and clay aqueous suspension

Mahdi Abdollahi[*]

, Ali Rahmatpour, Homayoon Hossein Khanli, Jamal Aalaie

Division of Polymer Science and Technology, Research Institute of Petroleum Industry (RIPI),

P. O. Box 18745-4163, Tehran, Iran

Abstract

Natural rubber (NR)/ sodium- montmorillonite (Na-MMT) nanocomposites were prepared by co-

coagulating NR latex and various amounts of Na-MMT aqueous suspension. Tapping mode AFM

and XRD were applied to characterize the microstructure of the nomocomposites. It was observed

that fully exfoliated structure could be obtained by this method only when the low loading of layered

silicates is used. With increasing Na-MMT content, both non- exfoliated (stacked layers) and

exfoliated structures can be observed simultaneously in the nanocomposites. The vulcanized

nanocomposites were subjected to the mechanical test. The results showed that nanocomposites

present initial moduli and tensile strength greater than NR. Furthermore, initial moduli increased

with increasing the Na-MMT loading, indicating the reinforcement effect of Na-MMT on the

mechanical properties of NR/ Na-MMT nanocomposites. Compared to the pure NR, the

nanocomposite exhibits a higher glass transition temperature and lower tan peak value. TGA

results indicated an improvement in main and end decomposition temperature but there is no effect

on the suppression of the initial decomposition.

Keywords: Natural rubber latex, pristine clay, nanocomposite, latex compounding method,microstructure

Introduction

Polymer- clay nanocomposites have attracted the attention of many researchers and experimental

results are presented in a large number of recent papers and patents because of the outstanding

mechanical properties and low gas permeabilites that are achieved in many cases. The advantages o

nanocomposites containing single silicate layers uniformly dispersed in a polymer matrix were first

demonstrated by researches at Toyota in Japan who made nylon-6 nanocomposite [1-3].Commercial clay has also been used as cheap and non-reinforcing filler in rubber industry for many

years. Success of polymer/ layered silicate nanocomposites based on the plastics greatly improves

and impels the study on the polymer/ layered silicate based on the rubber matrices. Some rubber/

silicate nanocomposites with excellent properties and unique phase structure have been prepared by

different methods, such as polyurethane [4], acrylonitrile- butadiene rubber [5], ethylene- propylene-

diene monomer [6,7], natural rubber (NR) [8], butadiene rubber and styrene-butadiene rubber [9,

10]. In general, the methods for the preparation of rubber- clay nanocomposites include in situ

polymerization, solution and melt intercalation, wherein organic modified silicate must be employed.

However, co-coagulating rubber latex and layered silicate aqueous suspension (latex compounding

method) is an easy method to prepare layered silicate/ rubber nanocomposites wherein pristine clay

1


2/8

is utilized instead of organic clay [1113].

NR is an important and widely used rubber. Karger-Kocsis et al [1416] showed tha t the

organophilic modification of the clay is not always necessary. They prepared nanocomposites by

drying clayNR latex dispersion and vulcanizating the rubber. Using this method with natural

sodium bentonite and synthetic sodium fluorohectorite, vulcanized NR nanocomposites were

obtained with great increase in modulus and tensile strength. Besides the Karger-Kocsis study, there

are few reports in the literature on NR/ clay (rectorite or Na-MMT) nanocomposites made by latex

compounding method [17-19].

The dispersion of the silicate layers is one of the key issues in preparing polymer/ layered silicate

nanocomposites, which exhibit interesting technological properties. To characterize the dispersion,

transmission electron microscopy (TEM) is the most commonly used technique, because of its very

good resolution capacity. Nevertheless, TEM experiments require the preparation of microtome

sections, even cryomicrotome sections or sample modifications in the case of electrometric samples.

Sample modifications, such as embedding in a rigid matrix, in order to perform the microtomesection at room temperature, may induce changes in the original silicate dispersion.

Recently, interest has focused on atomic force microscopy (AFM) imaging of composite materials

[20, 21], a technique which does hot require any specific preparation of the samples, because it is

easy to use and non-destructive. Specifically, we use tapping mode AFM with phase imaging, which

is one of the latest developments in scanning probe microscopy.

In this paper, NR/ clay nanocomposites were prepared by co-coagulating rubber latex and sodium-

montmorillonite aqueous suspension and then their structures and properties were studied in detail,

giving special attention to the clay- rubber interface. Structures of this type of nanocomposites wereinvestigated at first time by tapping mode AFM.

Experimental

Materials

Cloisite Na+ (natural sodium- montmorillonite) with cation exchange capacity of 92 meq/100 gr o

clay was provided by Southern Clay Products. Centrifuged NR with 61.7% dry rubber content

(stabilized with high amount of ammonia) was supplied by Rubber Research Institute of India.

Preparation of NR/ Na-MMT nanocomposites

Na-MMT was dispersed in deionized water with vigorous stirring (24000 rpm) by a special type o

mechanical stirrer (Polytron, Switzerland) at a concentration of 2% and an aqueous suspension o

layered silicate was obtained. Then, a given amount of NR latex was added into the aqueous

suspension and stirred for 30 min. Finally, the mixture was co-coagulated by cation- type

coagulating agent (dilute solution of sulfuric acid, 2%), washed with water several times until its pH

was about 7, and then dried at 80 oC for 24 h. The NR/ Na-MMT nanocomposites were obtained.

To obtain vulcanizates, the above-mentioned nanocomposites were mixed with ingredients according

to the recipe in Table 1 [18] in a 6-inch two-roll mill, then, vulcanized at 143 oC in a hot press for

the optimum cure time (t90) determined by a rheometer.

2


3/8


4/8

molding (hot pressing). The image reveals the individual silicate sheets, which confirm the

exfoliated structure that was indicated by XRD analysis in Fig. 2 (see next section). It should be

noted that the almost similar phase images were observed for nanocomposites prepared with 2 and 6

phr of Na-MMT. Fig. 1b shows the phase image of the nanocomposite prepared using 10 phr Na-

MMT, after the sample had been subjected to the hot pressing. This image clearly reveal that a

hybrid structure of monolayered and multilayered silicates can be formed simultaneously in the

nanocomposite with high Na-MMT loading, consistent with exfoliated and non- exfoliated hybrid

structure in accord with the XRD analysis in Fig. 2 (see next section). The AFM images shown in

Fig. 1 are similar in appearance to recently published TEM micrographs of NR/ clay nanocomposites

[14, 17-19].

Fig. 1. AFM phase images of NR/ Na-MMT nanocomposites prepared by co-coagulating NR latex and

Na-MMT aqueous suspension: (a): 3.3 phr Na-MMT and (b): 10 phr Na-MMT.

AFM image allow a qualitative understanding of the structure of the nanocomposite through direct

observation. On the other hand, XRD is a conventional method to determine the interlayer spacing o

clay layers in the original clay and in the polymer/ clay nanocomposite. The XRD patterns of the

NR/ Na-MMT nanocomposites and Na-MMT powder are presented in Fig. 2. From Fig. 2, the

measured d001 basal spacing of sodium montmorillonite is 1.4 nm, whereas the nanocomposites are

slightly lower than 1.4 nm. With the Na-MMT content increasing from 2 phr to 10 phr, NR/ Na-

MMT nanocomposites show almost the same diffraction peak except for the increase of intensity.

It is well-known that if intercalation of rubber macromolecules into the interlayer occurs, XRD peak

will shift to a smaller angle for the intercalated nanocomposites, and if the Na-MMT layers are

exfoliated completely, there will be no diffraction peaks observed because of the disorder of sheets

or the larger space of the layers beyond the XRD resolution. However, in this study, the d 001basal

spacing of the nanocomposites decreased slightly, which would be too small for intercalation o

rubber molecules into the space of Na-MMT. The possible explanation is that the slight decrease o

the d001

basal spacings is caused by the introduction of the H+ cations of flocculant different from

the original ions (Na+) existing in the gallery of the Na-MMT through a cation exchange reaction

during the process of co-coagulation. This nanodispersed structure without macromolecules

intercalated into intergallery is completely different from the well-known intercalated structure and

exfoliated structure, which is apparently due to the unique preparation method. This will be

explained in the later section.On the other hand, the XRD peak intensities increase with increasing Na-MMT content from 2 phr to

10 phr, which is mainly related to the increase in the number of stacking Na-MMT layer aggregates.

b

a

4


5/8

Simultaneously, it is believed that the exfoliated individual layers also increase with the increase o

Na-MMT content.

Fig. 2. XRD patterns for the pure Na-MMT and NR/ Na-MMT nanocomposites

Based on the above results, the dispersion phase of the nanocomposites prepared by co-coagulating

method involves individual layer and nanoscale orderly stacking layers without polymer inserted. To

explain the formation mechanism of this special dispersion structure, the preparation process is

described as follows.As shown in Fig. 3, in the first step, Na-MMT is dispersed in water. It has been shown that with the

water content increasing, the d001

diffraction peaks of MMT gradually shift to small angles, which

means the interlayer space is gradually enlarged by the introduction of water. When the water

content is more than 90%, no diffraction peak appears, indicating that MMT can be separated into

the individual layers in the dilute aqueous solution [17-19].

Fig. 3. Schematic illustration of the mixing and co-coagulating processes [18]

The second step is to mix the NR latex with the dilute Na-MMT aqueous suspension. Since both the

surface of rubber latex particles and MMT layers are hydrophilic, the introduction of rubber latex

does not cause the aggregation of silicate layers. A uniform and stable mixture is obtained.The third step is to co-coagulate the mixture by the addition of a flocculant. After adding a

flocculant, the flocculant coagulated both the rubber latex and MMT layers, but the rubber

macromolecules did not intercalate into the galleries of clay. This mainly results from the

competition between flocculation of rubber latex particles and agglomeration of silicate layers on

addition of flocculant. Since rubber latex particles are composed of several rubber macromolecules,

the existence of latex particles between the galleries of silicate layers in the water medium should

result in a complete separation (exfoliation) of silicate sheets. However, cations in flocculant

probably cause the separated MMT layers to reaggregate so that the rubber latex particles between

the MMT layers may be expelled. As a result, there are some non-exfoliated (stacking) layers in the

nanocomposites.

5


6/8

The last stage is to dry NR/ Na-MMT nanocomposites.

Dynamic mechanical and thermal properties of NR/ Na-MMT nanocomposites

Dynamic mechanical properties are measured to examine the degree of fillermatrix interaction o

NR/ Na-MMT nanocomposites. The loss factor ( ) and the storage modulus ( ) of the pure

NR and some of the NR/ Na-MMT nanocomposites versus temperature are depicted in Figs. 4 and 5,

respectively. The glass transition temperature (Tg) for pure NR is obtained at 43.8oC, while for the

nanocomposites with 3.3 and 6 phr Na-MMT it increases to 40.1 and 39.0 oC respectively. The

NR/ Na-MMT nanocomposites show a higher glass transition temperature and lower peak

value.

As shown in Fig. 4, the nanocomposites exhibits a strong enhancement of the modulus over the

temperature range investigated, indicating the elastic responses of pure NR towards deformation are

strongly influenced by the presence of nanodispersed MMT layers. This behavior also reflects the

strong confinement of MMT layers on the rubber molecules. Below Tg, the improvement of modulusis slighter than that above Tg, which is easy to be understood based on the mechanism of stress

transfer of composites.

Fig. 4. Storage modulus versus temperature for pure Na-MMT and NR/ clay nanocomposites

Fig. 5. versus temperature for pure NR and NR/ Na-MMT nanocomposites

Thermal decomposition of nanocomposites was investigated by thermogravimetry analysis as shown

in Fig. 6. Results show that the main decomposition and end decomposition temperature shift to the

higher values with increasing MMT loading; however, the presence of MMT does not affect the low

temperature decomposition peak. The similar results have been observed for poly(ethyl acrylate)/

poly(methyl methacrylate) latex blends compounded with MMT [21]. This phenomenon is still

investigating.

tan E

tan

tan

6


7/8

Fig. 6. TGA curves of pure NR and NR/Na-MMT nanocomposites

Stress-strain behavior of NR/ Na-MMT nanocomposites

Figure 7 represents the dependence of the stress- strain characteristics of NR/ Na-MMT

nanocomposites on the Na-MMT content. For pure NR, it demonstrates a typical stress induced

crystallization behavior, i.e., in the lower strain region, the modulus of rubber is low and slowly

increases with the increase of strain, and after the strain approaches a certain value, the stress will

sharply increase within a small strain range due to the occurrence and subsequent repair development

of tensile crystallization. After introduction of Na-MMT, the modulus of NR within the strain before

tensile crystallization happening is significantly improved because of mark reinforcement onanodispersed Na-MMT. The magnitude of improvement rises with increasing Na-MMT content.

Correspondingly, the stress transition caused by tensile crystallization gradually weakens and even

disappears, which is resulted from both the reinforcement and the hindrance of Na-MMT layers to

the tensile crystallization. The tensile strength of NR is also effectively improved by nanodispersed

Na-MMT. However, when the loading amount of Na-MMT overpasses 6 phr, the tensile strength o

NR no longer rises and elongation at break significantly decreases. There are two reasons

responsible for it. Firstly, the introduction of more Na-MMT layers strongly impedes the stress-

induced crystallization of pure NR. Secondly, higher Na-MMT content easily causes the increase othe dispersion phase size of the nanocomposites prepared by co-coagulating methods [13, 18].

Fig. 7. Stress- strain curves for pure NR and NR/ Na-MMT nanocomposites

Conclusion

NR/ Na-MMT nanocomposite were prepared by co-coaglating NR latex and Na-MMT aqueous

suspension. The microstructures of the nomocomposites were investigated by tapping mode atomic

force microscopy and X-ray diffraction. It was observed that fully exfoliated structure could be

obtained by this method only when the low loading of layered silicates is used. Compared to pure

NR, the nanocomposites exhibit a higher glass transition temperature and lower peak value

and slightly broader glass transition region. Also, the nanocomposites have a unique stress-strain

behavior and higher modulus at the same stress, which is resulted from both the reinforcement and

hindrance of Na-MMT layer to the tensile crystallization of NR. TGA results indicated an

tan

7


8/8

improvement in main and end decomposition temperature but there is no effect on the suppression o

the initial decomposition.

References

1) Usuki, A. Kojima, Y., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T., Kamigaito,

O. (1993) J. Mater. Res., 8(5):117984.

2) Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T., Kamigaito,

O. (1993) J. Mater. Res., 8(5):11859.

3) Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Kurauchi, T., Kamigaito, O. (1993) J.

Polym. Sci., Part A: Polym. Chem., 31(7):17558.4) Zilg, C., Thomann, R., Mulhaupt, R., Finter, J. (1999) Adv. Mater., 11: 4952.

5) Markus, G., Wolfram, G., Peter, R., Rolf, M. (2002) Rubber Chem. Tech., 74: 22135.

6) Ma, J., Xu, J., Ren, J.H., Yu, Z.Z., Mai, Y.W. (2003) Polymer, 44:461924.

7) Usuki, A., Tukigase, S., Kato, M. (2002) Polymer, 43:21859.

8) Arroyo, M., Lopen-Manchado, M.A., Herrero, B. (2003) Polymer, 44:244753.

9) Song, M., Wong, C.W., Jin, J., Ansarifar, A., Zhang, Z.Y., Richardson, M. (2005) Polym.

Int., 54: 560568.

10) Wang, Y., Zhang, H., Wu, Y., Yang, J., Zhang, L. (2005) J. Appl. Polym. Sci., 96: 324328.11) Zhang, L.Q., Wang, Y.Z., Wang, Y.Q., Sui, Y., Yu, D. (2000) J. Appl. Polym. Sci., 78:1873

8.

12) Wang, Y.Z., Zhang, L.Q., Tang, C.H., Yu, D. (2000) J. Appl. Polym. Sci., 78:187983.

13) Wu, Y.P., Zhang, L.Q., Wang, Y.Q., et al. (2001) J. Appl. Polym. Sci., 82: 28426.

14) Varghese, S., Karger-Kocsis, J. (2003) Polymer, 44: 49217.

15) Karger-Kocsis, J., Wu, CM. (2004) Polym. Eng. Sci., 44(6): 108393.

16) Varghese, S., Karger-Kocsis, J. (2004) J. Appl. Polym. Sci., 92(1): 54351.

17) Wu, Y.P., Wang, Y.Q., Zhang, H.F., Wang, Y.Z., Yu, D.S., Zhang, L.Q., Yang, J. (2005)

Composites Sci. Tech., 65: 1195-1202.

18) Wang, Y., Zhang, H., Wu, Y., Yang, J., Zhang, L. (2005) Eur. Polym. J., 14: 2776-2783.

19) Valadares, L.F., Leite, C.A., Galembeck, F. (2006) Polymer, 47: 672-678.

20) Huang, X., Brittain, W.J. (2001) Macromolecules, 34: 3255-3260.

21) Xu, Y., Brittain, W.J., Vaia, R.A., Pric,e G. (2006) Polymer, 47: 4564-4570.

[*]To whom all correspondence should be addressed: Tel: +9821 88937006; E-mail: [email protected]

Lateks Nanocomposite

Documents

Transcript of Lateks Nanocomposite