Morphology and Characterization of Clay-Reinforced EPDM Nano Composites

Morphology and Characterization ofClay-reinforced EPDM Nanocomposites

SEYED JAVAD AHMADI, YU DONG HUANG* AND WEI LI

Department of Applied Chemistry, Faculty of Science

Polymer Materials and Engineering Division

Harbin Institute of Technology

Harbin 150001, People’s Republic of China

(Received January 12, 2003)(Accepted July 17, 2004)

ABSTRACT: The clay-reinforced ethylene propylene diene terpolymer (EPDM)nanocomposites with organo-montmorillonite (OMMT) and EPDM–clay conven-tional composites with pristine MMT are compared in terms of their morphology,mechanical properties, and solvent resistance. The maleic anhydride grafted EPDM(MAH-g-EPDM) is used as the compatiblizer for preparing clay-reinforced EPDMnanocomposites via a melt intercalation process. The formation of exfoliated nano-composites is confirmed by X-ray diffraction (XRD) and transmission electronmicroscopy (TEM). Furthermore, the morphological change during the mixing timesof vulcanization process through exerting shearing force on clay particles isdiscussed. The mechanical properties and solvent resistance of clay-reinforcedEPDM nanocomposites are examined as a function of the organoclay content in thematrix of the polymer, the result shows remarkable improvement relative to that ofconventional composites.

KEY WORDS: clay-reinforced EPDM nanocomposites, exfoliation, organoclay,melt intercalation, morphology.

INTRODUCTION

ADDITION OF INORGANIC fillers to a polymer matrix has been demonstrated to be aneffective method to achieve reinforcement of the polymer hybrid materials consisting

of organic and inorganic components. Nanocomposites are formed when phase mixingoccurs on a nanometer length scale. Due to the improved phase morphology andinterfacial properties, nanocomposites exhibit mechanical, thermal, barrier, and chemicalproperties superior to conventional composites [1–7]. Montmorillonite is the mostcommonly used clay to prepare nanocomposites, but the lack of affinity betweenhydrophilic layered silicates and hydrophobic polymer makes them difficult to be miscible

*Author to whom correspondence should be addressed. E-mail: [email protected]

Journal of COMPOSITE MATERIALS, Vol. 39, No. 8/2005 745

0021-9983/05/08 0745–10 $10.00/0 DOI: 10.1177/0021998305048154� 2005 Sage Publications

at the nanoscale level. Thus, chemical modification of layered silicates is important in thepreparation of nanocomposites. This can be done by an ion-exchange reaction throughions that already exist in clay such as Kþ, Naþ, Caþ, Mgþ, and organic cations such asalkyl ammonium ions to form the organosilicate [8]. The organosilicate can be brokendown into their nanoscale building blocks and uniformly dispersed in the polymer matrix.When the clay platelets are thoroughly dispersed in the polymer matrix, the nano-composites are ‘exfoliated.’

EPDM is an unsaturated polyolefin rubber with a wide range of applications. However,it is incompatible with polar organophilic clay to prepare products having desiredproperties because it does not include any polar group in its backbone. Recently, Kato etal. [9–12] have reported a new approach to prepare nonpolar polymer clay hybrids byusing a functional oligomer. Till now, there are few studies on the formation, morphology,and properties of EPDM–clay nanocomposites. Usuki et al. [13] and Chang et al. [14] havereported the preparation and mechanical properties of EPDM–clay nanocomposites,prepared via the vulcanization process by using some special vulcanization acceleratorsand the melt compounding process with a liquid low molecular weight EPDM, respectively.

In this paper, we compare the clay-reinforced EPDM nanocomposites with EPDM–clay conventional composites prepared through the melt blending method by usingmaleic anhydride grafted EPDM (MAH-g-EPDM) as a compatibilizer, in terms of theirmorphology, mechanical properties, and solvent resistance. The morphological changecaused by shearing force during the mixing times of vulcanization process is also discussed.

EXPERIMENTAL

Materials

Pure sodium montmorillonite (Kunipia-F) with a cation-exchange capacity (CEC)of 119meq/100 g was supplied by Kunimine Mining Ind. Co. (Tokyo, Japan).Octadecylamine purchased from Fluka was used as the organic modifier for MMT.MAH-g-EPDM (0.8wt%) oligomer from Huzhou Genius Engineering Plastics Co. Ltd(Huzhou, China) was used. The EPDM (J-3062E) ENB type was obtained from JilinChemical Ind. Co., Ltd (Jilin, China). All chemicals were used without furtherpurification.

Preparation of Organophilic Clay and Clay-reinforced EPDM Nanocomposites

The organophilic clay was prepared via ion-exchange reaction in water by usingoctadecylamine as the reference [11]. In order to prepare clay-reinforced EPDM nano-composites, different amounts of organoclay powder were premixed by shaking with100 phr (part per hundred of rubber by weight) of EPDM and 20 phr of MAH-g-EPDM ina bag. This mixture was melt blended together in a twin-screw blender, model RM-200,with a chamber size of about 40 cm3 at 150�C. The rotational speed of the screw was90 rpm and mixing time was 15min for all cases. To compare the properties of nano-composites with that of the conventional composites, the EPDM–clay composites withpristine MMT were prepared under the above-mentioned conditions. Then, the EPDMhybrids (100 phr) were sequentially mixed with zinc oxide (5 phr), stearic acid (1 phr),

746 S. J. AHMADI ET AL.

vulcanization accelerator [M (2-Mercapto benzothiazole, 0.5 phr) and TMTD (tetramethylthiuram disulfide, 1.5 phr)] and sulfur (1.5 phr) by using a roll mill, model SK-160B, at atemperature about 60�C. The vulcanization was carried out in standard hot press at 150�Cfor 30min to yield rubber sheets (340� 150� 2mm3).

To study the effect of MAH-g-EPDM as a compatibilizer, the composites of EPDM andorganoclay without MAH-g-EPDM were also prepared.

Measurement

The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet spectro-photometer model Nexus-670 in the range of 400–4000 cm�1. The KBr discs were usedfor powder samples. Dispersibility of the organoclay in the polymer matrix was evaluatedby XRD and TEM. XRD patterns were recorded by a Phillips X’Pert X-ray generatorwith Cu K� radiation at 40 kV and 40mA. The diffractograms were scanned in 2� rangesfrom 1 to 9� at a rate of 2.4�/min. The basal spacing of the silicate layers, d, was calcu-lated according to Bragg’s equation, �¼ 2dsin�. A transmission electron micrograph wasobtained with a transmission electron microscope (H-800, Hitachi Co.) using anacceleration voltage of 200 kV. The tensile measuring test was carried out with universaltensile tester (Model DSC-5000, Shimadzu Co.) at 25�C with a head speed of 500mm/min.The shore hardness was measured by using a Shore-A hardness instrument (LX-A,Shanghai Liuling Instrument Factory, China). All measurements were taken three timesand the result values were averaged. The solvent resistance of EPDM–clay hybrids wasmeasured as follows: The samples with similar shape (20� 10� 2mm3) were entirelyimmersed in a container of pure xylene (xylene is a good solvent for EPDM) maintained atabout 25�C for 24 h. The percentage of the change in mass was calculated by using theformula:

Change in mass% ¼M �M0

M0� 100

where, M0 represents the dried weight of the specimen and M the weight of the specimenafter immersion.

RESULTS AND DISCUSSION

Morphology of Clay-reinforced EPDM Nanocomposites

IR, XRD, and TEM were used to investigate the morphology of clay-reinforced EPDMnanocomposites. Figure 1 is the IR spectra of pristine MMT and organo-MMT. Theabsorption band at 1090 cm�1 was characteristic of Naþ-MMT (Figure 1(a)).

After the treatment, organo-MMT exhibited the characteristic bands of C–H stretchingat 2921 cm�1 and 2850 cm�1 as well as the peak at 1469 and 724 cm�1 related to CH2

(Figure 1(b)). So it can be concluded that the alkyl ammonium has been exchangedwith cations of the MMT interlayer. Further evidence of ion-exchange reaction andintercalation of alkyl ammonium chains between the nanolayers was supported by XRDpatterns. The X-ray diffraction patterns are shown in Figure 2. The peak of organo-MMT

Morphology and Characterization of EPDM Nanocomposites 747

(Figure 2(b)) is shifted to lower angles compared with Naþ-MMT (Figure 2(a)). Thus,the interlayer distance of Naþ-MMT was extended after organic modification withthe octadecylamine. This clearly indicates the intercalation of alkyl ammonium chainsbetween the silicate layers.

Figure 2(c) shows the XRD pattern of melt mixing of organoclay with EPDM (EPDM–OMMT). It is clear that, the d-spacing of EPDM–OMMT is nearly the same as thatof OMMT (Figure 2(b)). It indicates that the EPDM–OMMT has deintercalated themorphology, which may result from immiscibility between EPDM and organoclaybecause EPDM does not include any polar group in its backbone.

Figure 2(d) shows the XRD patterns of composites of MAH-g-EPDM with organo-MMT (MAH-g-EPDM–OMMT). After melt compounding, in the case of MAH-g-EPDM–OMMT, the characteristic peak is widening and shifting to angles smaller thanthat of organoclay. This indicates that the molecules of MAH-g-EPDM are intercalatedinto the layered silicate of clay, expanding the basal spacing of organoclay. The stronghydrogen bonding between the maleic anhydride group and the oxygen group of the

Figure 1. IR spectra of MMT and OMMT.

Figure 2. XRD patterns of (a) pure clay (MMT); (b) organoclay (OMMT); (c) EPDMþMMT; and(d) OMMTþMAH-g-EPDM.


silicate as well as the shear force exerted on OMMT during melting compound canproduce the driving force necessary for the intercalation [9]. The interlayer spacing of theclay as well as its compatibility with polymer are increased, so the clay galleries couldeasily be intercalated with the EPDM.

The XRD results of EPDM–clay nanocomposites with different clay contents, (a)before and (b) after vulcanization process, are shown in Figure 3. Before the vulcanizationprocess for some nanocomposites with low clay content (2–10 phr), an exfoliated structurewas obtained as verified by the absence of any distinct diffraction peak, as well as by TEMobservations. For some composites with high clay content (15 phr), there is a weak peakaround 2.5�, suggesting that some portion of the organoclay is aggregated, but it canbe seen that after the vulcanization process, the weak peak of the high clay content hasdisappeared, which means the dispersion of organo-MMT in the EPDM matrix issignificantly improved and the basal spacing of clay layers has increased. A homogenizeddispersion of MMT in the EPDM matrix was obtained by applying high shear stressduring compounding by roll mill, which causes the agglomerates of organoclay to becomesmaller and the intercalated structure to be transferred into an exfoliated structure. Thisis caused by the shear-induced diffusion of polymer chains into the agglomerates andthe diffusion of polymer chains within the silicate galleries.

In general, the organophilic modification is accompanied by increasing interlayerdistances. Then, the energy gained through favorable interaction between the com-patiblizer (MAH-g-EPDM) and the organo-silicate layer, while exerting a shearing force,leads to obtaining exfoliation structure. It is to be emphasized that the coexistence of thethree parameters, clay modification, use of a suitable compatiblizer, and exerting ashearing force were necessary for achieving a high degree of exfoliation structure inEPDM–clay nanocomposites.

Figure 4 shows the XRD patterns of a vulcanized EPDM–clay conventional composite.It contains two peaks at 7.1� corresponding to a basal spacing of 1.24 nm and 6.6�

corresponding to a basal spacing of 1.33 nm. The peak at 7.1� (1.24 nm) is related tosilicate layers of MMT in EPDM clay composites (Figure 2(a)) and indicates that thegallery distance of clay is not changed. Thus, the EPDM chains does not intercalate intothe gallery of MMT. The second peak that appears at 2�¼ 6.6� (1.33 nm) which can alsobe seen in the vulcanized EPDM and vulcanized mixture of EPDMþMAH-g-EPDM(Figure 5), maybe caused by a high content of ethylene in the EPDM that we used in this

Figure 3. XRD patterns of EPDM–organoclay nanocomposites with different clay content: (a) before and(b) after vulcanization process.


study (about 68.5–74.5%). However, in the EPDM nanocomposites, this peak does notexist (Figure 3). The strong interaction between the organoclay and the molecules ofpolymer can change the morphology of nanocomposites.

Transmission electron microscopy was also used to visually evaluate the morphologyand dispersion of organoclay in the polymer matrix. Figure 6(a) determines that everylayer of silicate is well dispersed in the EPDM matrix and a high degree of exfoliation ofnanometer-size clay particles is obtained; the dark lines represent a cross section of theclay layers. But in Figure 6(b), for EPDM composites prepared by pristine MMT, theagglomerated particles are detected. From the above result, it can be concluded thatthe modification of MMT with an organic modifier and MAH-g-EPDM as compatiblizer,as well as the high shear stress are all important factors influencing the morphologydevelopment of EPDM–organoclay nanocomposites.

Influence of Clay Dispersion on Mechanical Properties

The mechanical properties tests were performed on EPDM–clay hybrids under differentMMT loadings and the results are shown in Figures 7–9. Figure 7 shows that below 5 phrOMMT, the tensile strength increases with increasing clay content but decreases a little bitabove 5 phr.

The EPDM–organoclay nanocomposites have higher values of tensile strengthwhen compared to the macrocomposites with the pristine clay and unfilled EPDM.

Figure 5. XRD patterns of: (a) vulcanized EPDM and (b) vulcanized EPDMþMAH-g-EPDM.

Figure 4. XRD patterns of vulcanized EPDM–clay composites with different clay content.


For example, the tensile strength of nanocomposites with 5 phr organoclay is about 175%higher than that of the conventional composites with the same value of clay contentand also 158% bigger than that of unfilled EPDM.

The tensile modulus of EPDM nanocomposites also is higher than that of EPDMcomposites and unfilled EPDM (Figure 8). For instance, by the addition of 15 phr claycontent, the tensile modulus of nanocomposites is increased to about 60% higher thanthat of EPDM macrocomposites and 97% higher than that of EPDM without clay. Theenhancement in tensile strength and tensile modulus is directly attributed to the dispersionof nanosilicate layers in the EPDM matrix and strong interaction between EPDM andorganoclay. The lower tensile strength above 5 phr OMMT can be attributed to theinevitable aggregation of the silicate layers in high clay content.

A similar result is observed for the shore hardness of EPDM–clay hybrids (Figure 9).The shore hardness of EPDM–organoclay nanocomposites increases with increasein clay content. Improvement in hardness with increase in clay content for EPDM

Figure 6. Transmission electron micrographs of: (a) EPDM–organoclay nanocomposites and (b) EPDM–claycomposites.


Figure 7. Effect of filler loading on tensile strength of: (a) EPDM–organoclay nanocomposites and (b) EPDM–clay composites.

Figure 9. Effect of filler loading on shore hardness for: (a) EPDM–organoclay nanocomposites, and(b) EPDM–clay composites.

Figure 8. Effect of filler loading on tensile modulus of: (a) EPDM–organoclay nanocomposites and (b) EPDM–clay composites.


nanocomposites is observed to be greater than that of the conventional EPDM. Theuniformly distributed exfoliation platelets improved the stiffness of EPDM. The presenceof these stiff clay platelets and entanglement of polymer chains made the EPDMnanocomposites harder.

Solvent Resistance Property of Clay-reinforced EPDM Nanocomposites

Figure 10 shows the results of the solvent uptake measurements of unfilled-EPDM,EPDM–organoclay nanocomposites, and EPDM–clay composites. The nanocompositesexhibited not only superior mechanical properties but also exceptional solvent resistance;the solvent uptake ratio of the EPDM nanocomposites was lower than that of the unfilled-EPDM and the conventional EPDM composites. For example, the solvent uptake ratiowas decreased from 358.8% for unfilled-EPDM and 351.2% for EPDM composites (claycontent of 5 phr clay) to 180.6% for EPDM nanocomposites (clay content same as thatof EPDM composites). This phenomenon can be explained by the fact that the largeaspect ratio of organo-MMT layers possesses excellent barrier properties, and especiallythe exfoliated structure of MMT layers can maximize the available surface area of thereinforcing phase [15].

CONCLUSIONS

EPDM nanocomposites and EPDM conventional composites were prepared throughthe melt blending method. Their morphologies were demonstrated by XRD, IR and TEM,the results show that the silicate layers of nanocomposites were exfoliated and disperseduniformly in the polymer matrix. The nanocomposites had superior mechanical propertiesand solvent resistance when compared to that of the conventional composites. Theseenhancements are attributed to the more uniformly dispersed nanoparticles of organoclayin the polymer matrix.

Figure 10. Solvent resistance of unfilled-EPDM, EPDM–organoclay nanocomposites, and EPDM–claycomposites with different clay content in pure xylene after 24 h at 25�C.


ACKNOWLEDGMENTS

The authors wish to thank the Harbin University of Science and Technology forproviding the twin-screw blender instrument and also to thank Heilongjiang PlasticsEngineering Institute of Technology for their technical help in the tensile propertiesmeasurements.

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Morphology and Characterization of Clay-Reinforced EPDM Nano Composites

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