A study on melt processing and thermal properties of fluoroelastomer nanocomposites

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 1431–1443 www.elsevier.com/locate/compscitech

A study on melt processing and thermal properties of fluoroelastomer nanocomposites M. Abdul Kader

a,b

, Min-Young Lyu c, Changwoon Nah

a,*

a

c

Department of Polymer Science and Technology, Chonbuk National University, 664-14, Duckjin-dong, Duckjin-gu, Jeonju, Chonbuk 561-756, Republic of Korea b Department of Polymer Technology, Crescent Engineering College, Vandalur, Chennai 600 048, India Institute of Precision Machinery Technology, Seoul National University of Technology, Seoul 139-743, Republic of Korea Received 12 October 2004; received in revised form 26 August 2005; accepted 6 September 2005 Available online 20 October 2005

Abstract Fluoroelastomer (FKM)/layered clay nanocomposites were prepared by melt mixing and their clay dispersion, morphological, rheological and thermal properties were characterized. Wide angle X-ray diffraction (XRD) and transmission electron microscopic (TEM) studies revealed the presence of intercalated/exfoliated clay layers with good dispersion at lower levels of filler loading. Both the pure FKM and FKM/clay nanocomposites showed shear thinning behavior and temperature dependency on shear stress. The apparent shear viscosity of the FKM/organo clay (O-MMT) nanocomposites was lower than that of the pure polymer at all shear rates and temperatures. The activation energy of the melt flow process decreased with increasing shear rate for all of the systems studied. However, the FKM/O-MMT hybrids exhibited higher activation energy compared to other filled systems. The nanocomposites exhibited reduced equilibrium die-swell with a smooth extrudate appearance. A comparison of the flow properties of the nanocomposites with the conventional composites revealed that the nanocomposites exhibited improved processability. The addition of layered clay increased the glass transition temperature of FKM. Although, the addition of untreated clay improved the thermal stability of FKM, organo clay reduced degradation initiation temperature due to the decomposition of organic component of modified clay.  2005 Elsevier Ltd. All rights reserved. Keywords: Fluoroelastomer; Morphology; Rheology; Die-swell; Thermal properties

1. Introduction Fluoroelastomers have been widely used in automotive parts due to their superior thermal and fluid resistance characteristics [1]. However, modern automotive operating environments require high performance materials having versatile properties. Polymer nanocomposites are generally regarded as a potential substitute for conventional microcomposites when such enhanced properties are required. Polymer-layered silicate clay nanocomposites show much improved mechanical, thermal and barrier properties even at a lower clay content [2]. Among the many clay materials

*

Corresponding author. Tel.: +82632704281; fax: +82632702341. E-mail address: [email protected] (C. Nah).

0266-3538/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.09.001

used in such polymers, montmorillonite, which has a large aspect ratio and size, has been recognized as a potential candidate for improving their physico-mechanical properties. A detailed description of the crystal structure, stacking of layers, and exchangeability of the interlayer cations of montmorillonite can be found in the literature [3–5]. Recently, many review articles on polymer/clay nanocomposites covering various aspects of this field were published [4,6–10]. Melt intercalation is considered to be a promising method of fabricating polymer/clay nanocomposites due its inherent advantage of allowing the use of existing processing equipment and because it is environmentally less damaging than other methods [11]. Among the various types of polymer clay nanocomposites [12], rubber nanocomposites constitute only a minor proportion, and few representative systems with rubber as a matrix, such

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as natural rubber [13,14], styrene–butadiene rubber [15], acrylonitrile–butadiene rubber [16,17], ethylene–propylene–termonomer (EPDM) [18], vinyl acetate copolymers [19], polyurethane [21], polyepichlorohydrin [22] and fluoroelastomer [23] etc. have been reported in the literature [20]. The melt processing of nanocomposites requires information on their rheological properties. It is thought that the rheological properties of these materials are sensitive to many factors, such as the processing conditions, the polymer–filler interaction and the structure and morphology of the system [24]. Recent publications on the rheology of polymer-layered silicate nanocomposites provide several notable references in this field [25–28]. Among the studies of the rheological and viscoelastic properties of polymer/ layered clay nanocomposites which were performed using a capillary rheometer, very few dealt with the melt rheological behavior at very high shear rates [29–31]. Also, there are very few studies on the effect of the melt rheology on the extrudate characteristics of polymer/clay nanocomposites. The present paper deals with the melt rheology of FKM/ layered clay nanocomposites, based on the measurements made using a capillary rheometer (Monsanto processability tester) in the range of shear rates normally experienced in processing operations such as injection molding and extrusion. The influence of high shear rates and temperature on the melt viscosity, activation energy, extrudate die-swell and extrudate appearance were examined. Finally, the thermal characteristics and thermal stability of the clay-filled FKM nanocomposites were also evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. 2. Experimental 2.1. Materials Fluoroelastomer [FKM, VITON A-500, specific gravity: 1.82, Mooney Viscosity (ML 1+10 at 121 C :50)] was supplied by DuPont Dow elastomers, Wilmington, USA. Montmorillonite (Na-MMT) with a cation-exchange capacity (CEC) of 119 meq/100 g was provided by Kunimine Industries Co Ltd., Japan. The O-MMT (Organo-MMT, quaternary ammonium salt of dimethyl hydrogenated tallow: Cloisite 15A) was supplied by Southern Clay Products, Inc., TX, USA. Semi reinforcing carbon black

(N550) and silica (Ultasil VN3) were obtained from Kumho Petrochemical Ltd. and Degussa, GmbH, Germany, respectively (see Table 1). 2.2. Preparation of rubber nanocomposites Two different clays (Na-MMT and O-MMT) carbon black and silica were used as fillers for studying the rheological properties. The clays were added to FKM at 2.5, 5, 10, 15 and 20 wt% of rubber. The carbon black and silica were added at fixed proportion of 10 wt%. The compounds were designated as FM for FKM/Na-MMT, FOM for FKM/O-MMT, FC for FKM/carbon black and FS for FKM/silica compounds. The number in the compound code indicates the amount of filler (in weight percent). The rubber compounds were prepared in an open two-roll mill at room temperature at a friction ratio of 1:1.1 with a nip gap of about 1 mm. The mixing time of 15 min was kept uniformly for all the compounds. 2.3. Characterization 2.3.1. X-ray diffraction pattern To establish the inter-layered spacing of Na-MMT, organoclay and their composites, wide angle X-ray diffraction (XRD) data between 2 and 30 of the 2h value were obtained from the scattering patterns taken with 40 kV, 40 mA Cu Ka radiation using an X-ray diffractometer (Rigaku 2500PC, Japan) with a radiation wavelength of 0.154 nm at room temperature. 2.3.2. Transmission electron microscopy (TEM) TEM images were taken from cryogenically microtomed ultra thin sections of nanocomposite samples using a ZEISS EFTEM (model: EM912 OMEGA H-800) operating at 120 kV. 2.4. Measurement of flow properties 2.4.1. Viscosity measurements The flow properties of pristine and filled FKM were measured by a Monsanto processability tester (MPT). The capillary used in our investigation had a length to diameter (L/D: 22.9/0.762 mm) ratio of 30:1 with compound entrance angles of 45 and 60. The samples were

Table 1 Formulation of FKM rubber compounds for rheological and thermal characterization Materials (wt%)

FKM

FOM2.5

FOM5

FOM10

FOM15

FOM20

FM2.5

FM5

FM10

FM15

FC10

FS10

VITON A500 Na-MMTa O-MMTb Carbon black (N550) Silica

100

97.5

95

90

85

80

– – –

2.5 – –

5 – –

10 – –

15 – –

20 – –

97.5 2.5 – – –

95 5 – – –

90 10 – – –

85 15 – – –

90 – – 10 –

90 – – – 10

a b

Sodium-montmorillonite. Organo modified MMT (Cloisite 15A).

M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

preheated for 5 min in order to obtain a uniform temperature and the rheological studies were carried out at three different temperatures, viz. 130, 150 and 170 C and at four different shear rates of 139, 278, 1390 and 2780 s1. The wall shear stress (sw), apparent wall shear rate (cw,a) and apparent viscosity (ga) were obtained from the rheological relationship [32]. Due to the high L/D ratio of the capillary, the Bagley correction for shear stress is negligible and, hence, the values obtained for the shear stress are taken as the true shear stress values [33]. 2.4.2. Die swell measurement The running die-swell values could not be obtained due to the large variation in the diameter of the extrudate during extrusion. However, the equilibrium die swell of the extrudates was calculated by obtaining the diameter of the extrudate using the relationship indicated below. A fixed length of a uniform portion of the extrudate was cut and weighed accurately to determine the volume of the extrudate  2 pd e Volume of extrudate; ðV Þ ¼ le ; d 2e ¼ 4V =ple . 4

ga ¼ B expðEc_ =RT Þ ;

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ð6Þ

where ga is the viscosity at a particular shear rate; B the pre-exponential component; R the gas constant and T the absolute temperature. 2.5. Thermal analysis Thermogravimetric analysis (TGA) of the samples was performed using a thermogravimetric analyzer (TA instruments, model No. 2050) attached to an automatic programmer from ambient temperature to 700 C at a programmed heating rate of 20 C/min in a nitrogen atmosphere. A sample weight of approximately 15 mg was used for the measurement. The temperature at which degradation is initiated (T5%), the temperature at which 50% loss in weight occurs (T50) and the maximum weight loss (Tmax) were evaluated for each sample. 3. Results and discussion 3.1. Characterization of structure and morphology

ð1Þ Density; q ¼ W =V ;

V ¼ w=q;

therefore  1=2 4W de ¼ ; ple q

ð2Þ

where le, de and W are the length, diameter and weight of the extrudate, respectively. The swelling index (a) or equilibrium die swell was determined by using the following relation: a¼

Diameter of extrudate ðd e Þ . Diameter of capillary ðd c Þ

ð3Þ

2.4.3. Maximum recoverable deformation The maximum recoverable deformation, cm, was calculated from the following equation [34]:  4 1=2 a þ 2a2  3 cm ¼ ; ð4Þ 2C where C¼

3n þ 1 ; 4ðn þ 1Þ

ð5Þ

where n is the flow behaviour index determined from the slope of the curve obtained by plotting log sw versus log c_ w;a . 2.4.4. Activation energy of melt flow The activation energy of melt flow at constant shear rate, Ec_ , was obtained from the Arrhenius–Frenkel–Eyring equation [33]

Fig. 1(a) shows the XRD patterns of the pristine NaMMT and FKM/Na-MMT nanocomposites containing different levels of Na-MMT. The Na-MMT gave a major peak at about 2h = 7.25 corresponding to the d0 0 1 plane with a d spacing of 1.28 nm. Compared with the pristine Na-MMT, the FKM/Na-MMT hybrid with 5 wt% NaMMT exhibited a small and broad diffraction peak with an increase in basal spacing (2h = 6.25, d = 1.41 nm), indicating the presence of an intercalated structure. The high shearing of FKM (having high viscosity) during two roll mixing coupled with small amount of interaction between Na-MMT and polar FKM might have resulted in small expansion in clay layers leading to the formation of intercalated structure. However, at higher loading of NaMMT (20 wt%), this peak became more visible and there was a slight shift in peak position towards higher 2h value. This may be possibility due to the formation of clay aggregates which could not be broken because of higher proportion of clay. On the other hand, O-MMT showed an increase in the average basal spacing of Na-MMT from 1.22 to 3.15 nm (2h = 2.80, Fig. 1(b)). The increase in d spacing suggested that the organic modifier entered into the gallery of the MMT, which is desirable for making intercalated and exfoliated polymer/clay hybrids. It was also seen from the X-ray diffraction pattern that OMMT showed a residual small peak corresponding to unintercalated MMT at its original position of 2h = 7.25. The FKM/O-MMT hybrid with 5% clay loading showed a weak peak at 2h = 2.28 (d spacing: 3.87 nm). The effect of O-MMT loading was similar to that of MMT in that the increase in the loading increased the peak height. This reduction in the peak height and shift in the peak position of FKM/O-MMT demonstrated the

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M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443 N a-MMT FKM+ 5 w t % N a-MMT FKM+ 10 wt % N a-MMT FKM+ 20 wt % N a-MMT

Intensity (a.u)

1234-

1 4 3 2

0

5

10

20

25

30

2θ (Degree)

a

Intensity (a.u)

15

1- O-MMT 2- FKM+ 5 wt % O-MMT 3- FKM+10 wt % O-MMT 4- FKM+20 wt % O-MMT

1

peak due to unintercalated MMT

4

3 2

1.5

b

3.0

4.5

6.0

7.5

9.0

2θ (degree)

Fig. 1. Wide angle X-ray diffraction pattern for (a) Na-MMT and FKM/ Na-MMT nanocomposites with varying proportions of Na-MMT and (b) O-MMT and FKM/O-MMT nanocomposites with different O-MMT loadings.

presence of intercalated and partially exfoliated clay layers. In addition to this, the peak position of residual MMT was also shifted to 2h = 4.38 (d spacing: 2.16 nm), as shown in Fig. 1(b). It was inferred from this that the FKM/layered clay hybrids showed intercalated and partially exfoliated clay layers at the lower clay loading. At the same, these hybrids showed the presence of aggregate at higher loading of O-MMT due to the reason explained for FKM/Na-MMT composite. It is known that the X-ray measurements cannot always discriminate between disordered intercalates and a partially or fully exfoliated morphology [35]. So, the extent of intercalation/exfoliation can only be detected using TEM, which characterizes the nanostructural features of the polymer/clay hybrids. Fig. 2(a) shows the TEM microphotograph of FKM/ Na-MMT nanocomposite. The dark lines correspond to the silicate layers in the FKM matrix. The clay layers are dispersed in the polymer matrix having an intercalated structure with stacks of clay platelets having only few

Fig. 2. High magnification TEM image of (a) FKM/Na-MMT intercalated nanocomposite (mass fraction of Na-MMT = 5 wt%) and (b) FKM/ O-MMT intercalated/exfoliated nanocomposite (mass fraction of OMMT = 20 wt%).

layers of clay. On the other hand, Fig. 2(b) shows a TEM microphotograph of 20% FKM/O-MMT having a random orientation of clay layers and few stacks of clay platelets along with exfoliated single platelets. These smaller stacks gave rise to a peak in the XRD pattern. It was inferred from the XRD data that the O-MMT was almost dispersed in the FKM matrix at lower loading of less than 5%. However, at a considerably higher loading, O-MMT formed small aggregates with few platelets of clay layers. The interlayer thickness calculated from the TEM data agreed closely with the data obtained from the XRD.

M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

(Pa)

5

7x10

5

6x10

5

wt % of O-MMT 0 2.5 5 10 15 20

w

8x10

5x10

5

4x10

5

10

2

10

3

-1

Apparent shear rate (s ) 5

b

8x10

5

7x10

5

(Pa)

6x10

w

wt % of Na-MMT 0 2.5 5 10 15

5

5x10

5

4x10

2

10

3

10

-1

Apparent shear rate (s ) c

8x10

5

7x10

5

6x10

5

5x10

5

4x10

5

(Pa)

3.2.1. Effect of shear rate on shear stress The rheological properties of the filled polymers mostly depend on the viscoelastic characteristics of the polymer. However, the addition of filler can alter these characteristics through polymer–filler interaction, which depend on many factors including the nature, size and shape of the filler. Moreover, the rheological behavior of the filled polymer can also be altered by the processing conditions [36]. The reinforcement mechanism for rubbery polymeric material is still controversial. A comprehensive review on the mechanism of reinforcement for the viscoelastic material dealing with filler agglomeration and network formation has been published [37]. The rheological behavior of polymer nanocomposite reinforced with nanofiller has received considerable attention due to the influence of many process parameters which ultimately affect viscoelastic property. The present work is focused on the influence of nanofiller on the rheological properties of FKM under very high shear rate comparable to actual processing condition of rubber by injection molding or extrusion. Fig. 3(a) depicts the flow curves of the FKM/O-MMT hybrids at 130 C with different loadings of O-MMT. The neat FKM and its nanocomposites showed non-Newtonian (pseudo plastic or shear thinning) behavior and followed the power law model. A comparison of flow behavior of the composites showed that O-MMT had a significant effect on flow curves, particularly at lower shear rate. The FKM/OMMT nanocomposites showed considerably low shear stress compared to pure FKM even at 5 wt% of O-MMT loading. This reduction in the shear stress might be partly due to the breakdown of clay platelets under high shear and subsequent dispersion and exfoliation. These clay platelets on the nanometer scale can be easily aligned during shear flow, particularly for the matrices imparting high shear stress. This flow-induced orientation of the clay layers can lead to the reduction in shear stress. Moreover, the presence of long chain organic modifier in the clay can impart some plasticizing action to the clay layers. However, upon increasing the shear rate, all the curves converge to nearly same shear stress values. The rheological response of polymer nanocomposites with well dispersed, exfoliated clay platelets depends on the shear rate, stress and time. The ability of clay platelets to interact with matrix depends on the degree of dispersion, aspect ratio, concentration and orientation. The effect is particularly large at low shear rate. Thus, at low shear rate, the shear flow is dominated by the stack of clay platelets and their degree of dispersion and at the higher shear rate the rheological properties are dominated by the viscoelastic characteristic of the matrix polymer. From our experiments we found that the decrease in shear stress of the nanocomposite continued up to 10 wt% clay loading. At higher loading, there was an increasing in shear stress at lower shear rate due to the formation of filler aggregates which resisted the shear flow to some extent depending the amount of

a

w

3.2. Rheological behavior

1435

10

FKM FKM+O-MMT FKM+Na-MMT FKM+carbon black FKM+silica

2

10

3

-1

Apparent shear rate (s ) Fig. 3. Wall shear stress vs. apparent shear rate for FKM and its nano and conventional composites at 130 C. Influence of (a) O-MMT loadings; (b) Na-MMT loadings and (c) comparison of nanocomposites with conventional composites at same filler loading (10 wt%).

filler aggregation. This fact was confirmed by the XRD and TEM results. The effect of Na-MMT on the rheological behavior of FKM/Na-MMT systems is represented in Fig. 3(b).

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M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

Although there was a slight decrease in the shear stress at a lower loading (5 wt%), the addition of Na-MMT did not change the flow properties of FKM considerably. Interestingly, at higher loadings, there was an increase in shear stress similar to the behavior of conventional composite. This might be due to presence of clay aggregates. It has been inferred from the above results, the rheological properties of FKM/Na-MMT composite were mostly governed by the viscoelastic properties of the matrix. A comparison of the processability of conventional composites such as carbon black and silica-filled FKM with nanocomposites at 10 wt% loading (Fig. 3(c)) revealed that the FKM/OMMT system showed much lower shear stress at low shear rates, which eventually approached to that of FKM at higher shear rate. The carbon black and silica-filled composites showed an increase in shear stress due to the polymer–filler interaction characteristic of filler reinforcement. A reduction in shear stress of about 80–120 kPa was observed for FKM/O-MMT nanocomposite, which is significant for processing of polymer. Table 2 enlists the flow behavior index, n, and the consistency index, k, values for nano and conventional composites, which were obtained from the slope and intercept of the flow curves, log sw versus c_ w;a , respectively. The n values for FKM/O-MMT systems increases with increasing O-MMT loading, indicating a shift towards Newtonian fluid behavior upon increasing the filler content. However, addition of other fillers (Na-MMT, carbon black and silica) at a same filler loading (10 wt%) did not alter the n value of pure FKM considerably. On the other hand, the consistency index, representing the ease of flow, decreased with increasing O-MMT loading. Again, the other systems exhibited only a small variation in k from that of pure FKM. A linear relationship between the temperature and the consistency index was observed for the FKM/O-MMT hybrid systems. This linear can be expressed by the following equation [38]: log k ¼ a þ ðb  T Þ;

ð7Þ

where T is the temperature of measurement and a and b are the parameters. The calculated value of the parameter, b, which is related to the influence of temperature, ranged from 0.0160 (for pristine FKM) to 0.008 (for 20% FKM/O-MMT). A reduction in this value means that the temperature has a greater influence on the consistency index. All of the above observations showed that the temperature can have a marked influence on the processability of the FKM/O-MMT nanocomposites. 3.2.2. Effect of shear on viscosity Fig. 4(a) shows a plot of the apparent shear viscosity versus the apparent shear rate at 150 C for the FKM and the FKM/O-MMT nanocomposites with various OMMT loadings. The shear viscosity of the nanocomposites was lower than that of FKM over the entire shear rate range. The decrease in viscosity with increasing shear rate reflected the typical shear thinning behavior of polymer melts. The low viscosity of the nanocomposites suggested that they would exhibit improved melt processability over a wide range of shear rates. The reduction in viscosity might arise from polymer chain slippage over the clay platelets and the orientation of the clay platelets in the direction of the applied stress. It was observed that those polymers which show a high shear stress requirement can impart alignment of nanometer scale clay platelets during shear flow [29]. Additionally, the presence of the long chain organic modifier in O-MMT acted like a plasticizer. This could lead to a higher amount of exfoliation, especially at lower loadings of O-MMT. The ability of shear to orient the anisotropic silicate layers along the direction of flow has been previously demonstrated by viscoelastic measurements [24,25]. While exhibiting a considerable reduction in shear viscosity at lower shear rates, the nanocomposites showed comparable viscosity to that of pure FKM at higher shear rates. The strong shear thinning character of FKM/O-MMT nanocomposite at low shear rates can be attributed to the formation of clay platelets from the clay tactoids during the shear flow. These smaller size clay platelet can easily align along the flow direction. While

Table 2 Flow behavior index, n, and consistency index, k, for FKM and its nano- and microcomposites at three different temperatures Samples

FKM FKM + 2.5 wt% O-MMT FKM + 5 wt% O-MMT FKM + 7.5 wt% O-MMT FKM + 10 wt% O-MMT FKM + 15 wt% O-MMT FKM + 20 wt% O-MMT FKM + 2.5 wt% Na-MMT FKM + 5 wt% Na-MMT FKM + 15 wt% Na-MMT FKM + 20 wt% Na-MMT FKM + 10 wt% carbon black FKM + 10 wt% silica

k (·104 Pa sn)

n 130 C

150 C

170 C

130 C

150 C

170 C

0.08 0.10 0.11 0.15 0.16 0.16 0.15 0.08 0.08 0.08 0.07 0.10 0.09

0.12 0.2 0.19 0.20 0.20 0.20 0.18 0.12 0.14 0.13 0.12 0.14 0.11

0.23 0.27 0.28 0.22 0.25 0.22 0.21 0.25 0.26 0.24 0.22 0.22 0.18

35.43 31.42 29.30 20.55 19.35 18.99 20.91 35.38 36.24 37.27 39.33 34.22 37.68

26.38 13.62 14.24 12.21 12.13 11.53 13.66 25.20 22.03 24.78 27.78 24.74 32.60

10.68 7.11 7.00 9.53 7.51 8.40 9.31 9.47 8.60 10.26 12.24 12.95 17.59

M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

3

10

2

η a (Pa.S)

10

wt % of O-MMT 0 5 10 15 20

10

2

10

3

-1

Apparent shear rate (s )

a

wt % of Na-MMT 0 2.5 5 10 15

3

η a (Pa.S)

10

2

10

2

10

3

10

-1

Apparent shear rate (s )

b

FKM FKM+carbon black FKM+silica FKM+Na-MMT FKM+O-MMT 3

10

2

η a (Pa.S)

10

10

c

2

10

3

-1

Apparent shear rate (s )

Fig. 4. Shear rate dependence of apparent shear viscosity at 150 C for FKM and its hybrids (a) FKM/O-MMT nanocomposites with different OMMT loadings (b) FKM/Na-MMT with varied Na-MMT loadings and (c) comparison of nanocomposites with conventional composites at same filler loading (10 wt%).

the viscosity reduction and shear thinning behavior of the FKM/O-MMT nanocomposite were governed by the concentration and size of the filler aggregates, the flow

1437

behavior at high shear rates was mostly dominated by the viscoelastic properties of the matrix polymer. Similar results were reported for the nylon-6/organo modified MMT nanocomposite [29] and the polyamide-12/layered silicate nanocomposite [30]. The effect of Na-MMT on the shear viscosity of FKM is shown in Fig. 4(b). The addition of Na-MMT did not change the shear viscosity of FKM considerably. Although the viscosity of 5 wt% Na-MMT filled FKM was slightly reduced, at all measured temperatures, it rose above that of FKM at higher loadings. Again, these viscosity curves merged together at a high shear rate. These results suggest that Na-MMT has minimal influence on the rheological properties of the nanocomposites especially at higher loadings. Fig. 4(c) compares the variation in viscosity as a function of the shear rate for the nanocomposites with that of conventional composites. As expected, carbon black and silica imparted a higher viscosity to FKM through polymer–filler and filler–filler interactions. The difference in viscosity between the conventional composites and the organo clay nanocomposite was quite substantial. This would imply that the nanocomposite could be more easily melt processed. The magnitude of the viscosity reduction for the FKM/ O-MMT nanocomposites at 130 C in comparison with pure FKM is represented in Fig. 5(a). The relative viscosity was calculated from the difference in the viscosity values between pure FKM and its O-MMT filled systems. It can be seen that there was a significant reduction in the viscosity of the nanocomposites at low shear rates. This reduction continued up to a loading of around 10% only to increase again for highly filled systems. It was inferred that at higher loadings of O-MMT there was some filler aggregation in the form of clay stacks, which resisted the shear flow. The XRD as well as the TEM results also confirmed the formation of clay aggregates having few clay platelets. Interestingly, the difference in viscosity at high shear rates was very small. The above result suggests that, at high shear rates, the orientation of the polymer molecules along the flow direction and wall slip play a major role in reducing the viscosity, whereas the orientation of the clay layers was predominant factor at lower shear rates. A more detailed understanding of the effect of the shear rates and the temperatures on shear viscosity of nanocomposites with different O-MMT loadings can be obtained from Fig. 5(b). Increase in both temperature and shear rate reduced the viscosity of the systems considerably for both the pure and filled FKM. The effect of the temperature on viscosity was more prominent at lower shear rates than that at higher shear rates. 3.2.3. Activation energy of melt flow In order to evaluate the influence of temperature on viscosity, the activation energy ðEc_ Þ of melt flow was calculated for all systems from the slope of the plot of log ga versus 1/T as given by the Arrhenius–Frenkel–Eyring equation [33]. The Ec_ values for pure FKM and FKM/

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M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

a

a

25

wt % of O-MMT 0 5 10 15 20

Activation energy, Eγ (kJ/mole)

Relative viscosity

1.0

0.9

-1

Shear rate (s ) 139 278 1390 2780

0.8

20

15

10

5

0

0

5

10

15

500

20

1000

4000

o

130 C o 150 C o 170 C

3000

ηa (Pa.s)

139 s

-1

2000

278 s

-1

1000 -1

1390 s -1 2780 s 0

5

10

15

2000

2500

3000

-1

20

25

b

FKM FKM+O-MMT FKM+Na-MMT FKM+carbon black FKM+silica

20

Activation energy, Eγ (kJ/mole)

b

1500

Apparent shear rate (s )

O-MMT loading (wt %)

16

12

8

4

0

500

1000

1500

2000

2500

3000

-1

Apparent shear rate (s )

O-MMT loading (wt %) Fig. 5. Effect of O-MMT loading on (a) relative viscosity at 130 C and (b) apparent shear viscosity at different shear rates and temperatures.

O-MMT were in the range of 3–19 and 5–22 kJ/mol, respectively. Fig. 6(a) depicts the dependence of the activation energy on the rate of shear for the FKM/O-MMT nanocomposites with varying filler loadings. The nanocomposites exhibited higher activation energies at the entire shear rates employed, indicating that there was a strong interaction between FKM and O-MMT. The activation energy is related to the magnitude of chain entanglements in the polymer. The degree of entanglement of the polymer chains was greatly reduced by the intercalated/exfoliated structure and this reduction in the ability of chains to become entangled leads to a greater energy requirement in the form of higher activation energy. Upon increasing the shear rate, considerable reduction in the activation energy of the pristine FKM was observed, whereas the magnitude of this reduction decreased as the proportion of O-MMT was increased in the nanocomposites. This reduction in activation energy at high shear rates might arise from the flow-induced orientation of the molecular segments in the direction of applied stress and increased wall slippage. The small difference in Ec_ for the FKM/O-MMT nanocomposites between the lower and

Fig. 6. Activation energy of melt flow as a function of shear rate for (a) FKM/O-MMT with different O-MMT loadings and (b) FKM and its nano- and microcomposites at 10 wt% of filler loading.

higher shear rates was due to the extensive orientation of clay platelets even at lower shear rates. Increasing the shear rate had little influence on the intercalated/exfoliated clay layers which were already aligned at a lower shear rate. Moreover, increasing the O-MMT loading decreased the activation energies between low and high shear rates. On the basis of the Arrhenius equation [39], the dependence of viscosity on temperature can be assessed by the following equation: 1 1 R  ¼ lnðg1  g2 Þ ; T1 T2 E

ð8Þ

where g1 and g2 are the viscosities at temperatures T1 and T2, respectively. Thus, from the above equation, it can be inferred that a system with higher activation energy of melt flow requires a smaller change in temperature for the same degree of viscosity reduction. This implies that the FKM/O-MMT nanocomposites should be able to be processed at a considerably lower temperature. The effect of loading of Na-MMT on Ec_ at all shear rates showed only a minor variation from that observed for pure FKM. Increasing the Na-MMT loading decreased Ec_ at lower shear rates.

M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

a

o

Temperature : 130 C 1.3

-1

shear rate (s ) 139 278 1390 2780

1.2

α

However, at higher shear rates there was no change for any amount of filler loadings. This might be due to the formation of filler aggregate that had a negligible contribution towards chain alignment and entanglement. Fig. 6(b) compares the activation energies of the carbon black and silica filled FKM with FKM/clay nanocomposites. The conventional composites showed a small reduction in their activation energy due to the increased viscosity caused by polymer–filler interaction and the dispersing action of the filler in the polymer matrix.

1439

1.1

1.0

0.9 0

5

10

15

20

O-MMT loading (wt %) b 1.25

Temperature :130 C

FKM FKM+Na-MMT FKM+ O-MMT FKM+ carbon black FKM+silica

500

2000

o

1.20

α

1.15

1.10

1.05

1.00 1000

1500

2500

3000

-1

Apparent shear rate (s ) c

1.3

Shear rate : 2780 s

Wt % of clay loading 5 10 5 10

-1

15 (O-MMT) 15 (Na-MMT)

1.2

α

3.2.4. Equilibrium die-swell The melt elasticity of polymers leads to undesirable phenomena such as extrudate swell (die-swell), melt fracture, shark skin, etc. when they flow through a capillary. The die-swell behavior is an important elastic characteristic of polymer melts in processing operations such as extrusion and injection molding. The polymer molecules become oriented in the direction of flow under the action of shear when subjected to flow through a capillary. On emerging from the capillary exit, due to the elastic nature of the oriented molecules, they tend to undergo recoiling. As a result, lateral expansion takes place which leads to extrudate swell [39]. This phenomenon is influenced by many factors such as the processing conditions, the shape and dimension of the capillary, the nature of the polymer and the type and amount of filler, etc. Fig. 7(a) represents the effect of O-MMT loading on the equilibrium die-swell (swelling index) at different shear rates. The pristine FKM displayed a high swelling index at all shear rates. For the filled systems, there was a decreasing trend in die-swell with increasing O-MMT loading irrespective of the shear rate. Interestingly, this trend was steeper at low shear rates than at high shear rates. The dependence of the die-swell on the shear rate indicated that two opposite trends at low and high filler loadings, as can be seen from Fig. 7(a). There was a crossover point in these trends at an O-MMT loading of about 7–8 wt%. This implies that a lower die-swell could be obtained by adjusting the shear rate and the loading of O-MMT. In order to obtain lower die-swell values, the nanocomposites can be processed either at a higher shear rate and a lower filler loading or at a lower shear rate and a higher filler loading. The extremely low level of die-swell observed in the case of the highly-filled nanocomposites (a < 1) at lower shear rates was attributed to the drooling or sagging of the extrudate due to the large reduction in viscosity, which reduced the extrudate diameter. The low level of the die-swell of filled system can be attributed to the polymer–filler interaction, the reduction in polymer content per unit volume, the decrease in elastic nature and the variation in viscosity. The influence of shear rate on the die-swell of FKM and its 10 wt% filled composites is shown in Fig. 7(b). Increasing the shear rate decreased the die-swell marginally up to 1390 s1 for all of the systems except for FOM10 and then caused to increase slightly thereafter, whereas FOM10 showed a continuous increase with increasing shear rate

1.1

1.0 130

140

150

160

170

o

Temperature ( C) Fig. 7. Equilibrium die-swell (swelling index, a) (a) as a function of OMMT loading at four different shear rates; (b) as a function of shear rate for various composites containing 10 wt% of filler and (c) as a function of temperature for nanocomposites with three different filler loadings.

from the very beginning. As previously mentioned, this trend was followed only for the FKM/O-MMT hybrids with higher clay loadings. However, our experiments showed that FOM5 demonstrated die-swell behavior

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M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

comparable to that of the other composites. The effect of temperature on the die-swell at high shear rate (2780 s1) for FKM/Na-MMT and FKM/O-MMT is depicted in Fig. 7(c). The die-swell for FKM at high shear rates could not be measured, as it underwent excessive melt fracture, resulting in a non-uniform surface which was unsuitable for equilibrium die-swell measurement. For the filled system, a gradual reduction in die-swell was observed upon increasing the temperature. This decrease in die-swell with increasing temperature might be due to increased polymer chain mobility and a reduction in the stored elastic energy. A comparison of the die-swell values of the Na-MMT and O-MMT filled systems at the same level of filler loading indicated that FKM/O-MMT exhibited higher values at all temperatures. This might be due to the fast disentanglement of the polymer chains and the slight change in orientation direction of the silicate layers at the exit of the capillary, caused by the reduction in the viscosity of the O-MMT filled system. 3.2.5. Maximum recoverable deformation (cm) Polymers do not follow HookÕs law at higher level of strain. However, it is quite possible to correlate cm with the available experimental parameters, such as the equilibrium die swell (a), viscosity (ga), filler characteristics and their loading, etc. The value of cm was calculated using Eq. (4) for different systems at various shear rates, temperatures and filler loadings. Fig. 8 shows a plot of the die swell against cm, which is linear and independent of all the above parameters. The following relationship between a and cm can be obtained as a ¼ 3:643cm  3:612.

ð9Þ

3.2.6. Extrudate appearance When the polymer is extruded from a capillary at high shear rate the flow instabilities are quite prominent. This

gives rise to melt fracture, extrudate distortion and shark skin, etc. The presence of stick and slip phenomena in a viscoelastic material generates uneven flow characteristics at the exit of the die. Additives such as fillers and plasticizers can influence the flow instabilities and reduce such undesirable effects. In our experiments, the influence of the layered clays on the appearance of the extrudate was examined at different filler levels, shear rates and temperatures. Magnified photographs of the extrudates are shown in Fig. 9. Pure FKM showed extrudate distortion in the form of saw-tooth waves at all measured temperatures and shear rates. At low shear rates the surface of the extrudates appeared to be smooth. When the shear rate was increased above a certain critical level, the extrudate showed periodical patterns on the surface. The changes in the velocity distribution at the exit of the capillary and extensive stick–slip phenomena at the capillary wall at shear rates above a certain critical level can cause the formation of melt fracture in the extrudate. The surface smoothness of the FKM/OMMT nanocomposites was greatly improved when a small amount of O-MMT was added and the surface became smoother as the filler loading increased (Fig. 9(a)). The presence of an organic modifier, which acts like a plasticizer, causing more wall slippage and decreasing the melt elasticity, was responsible for the reduction in the melt fracture of these nanocomposites. Further, increasing the temperature reduced the melt fracture of all of the systems due to (i) reduced melt viscosity (ii) a more swift stress relaxation process (iii) an decreased residual elastic strain and (iv) increased critical extrusion rate of onset of unstable flow [40]. Fig. 9(b) compares the texture of the extrudates of the nano- and microcomposites with 10 wt% filler loading at 170 C. FKM/O-MMT showed a smoother surface than any other system at all shear rates, followed by the carbon black and Na-MMT filled systems. In general, the FKM/O-MMT nanocomposites demonstrated good extrudate appearance irrespective of the processing conditions. 3.3. Thermal characteristics

1.4

α

1.2

α = 3.643 γ - 3.612 m

1.0

0.0

0.2

0.4

0.6

γm

0.8

1.0

1.2

Fig. 8. Die-swell as a function of maximum recoverable deformation of pristine FKM and its nanocomposites.

Fig. 10 and Table 3 show the thermogravimetric (TGA and DTG) curves and thermal degradation data, respectively, for FKM and its nanocomposites. The initiation of degradation (T5%) of pristine FKM was found to occur at around 500 C and maximum degradation occurred at 527 C. The mechanism of degradation of FKM through dehydrofluorination and other chain scission process is discussed by Banik et al. [41]. The incorporation of 10 wt% Na-MMT into the FKM increased both the initiation and maximum degradation temperatures to 508 and 539 C, respectively. It is generally believed that the inclusion of inorganic components into organic materials can improve their thermal stability [42]. The observed increase in the thermal stability of FKM/Na-MMT may be due to the high thermal stability of clay and the interaction between the clay layers and the polymer matrix through

M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443

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Fig. 9. Magnified photographs (1.5·) of extrudates (a) FKM and FKM/O-MMT nanocomposites with different O-MMT loadings at 150 C (i) 0 wt%; (ii) 5 wt%; (iii) 10 wt%; (iv) 15 wt% and (v) 20 wt%. (b) FKM and it nano- and microcomposites with 10 wt% of filler loading at 170 C: (i) FKM; (ii) FKM + Na-MMT; (iii) FKM + O-MMT; (iv) FKM + carbon black and (v) FKM + silica. The numbers, 1–4, indicate the shear rates at 139, 278, 1390 and 2780 s1, respectively.

intercalation/exfoliation. These intercalated polymer chains were covered by clay layers which prevents direct exposure of these chains to thermal influence. On the other hand, the incorporation of O-MMT into FKM reduced the thermal stability of the nanocomposite by lowering the temperature at which the degradation was initiated (T5% = 414 C and T10% = 450 C). Similar results of reduction in thermal stability of O-MMT filled ethylene vinyl acetate and epoxy nanocomposites are reported in the literature [43,44]. Fur-

ther, the TGA curve of FKM/O-MMT nanocomposite showed two step decompositions. The first degradation step, corresponding to the emission of decomposition products such as long chain tertiary amine, short and branched olefins and aldehyde from the organic moiety of O-MMT occurred at lower a temperature (Tmax1 = 459 C). The second degradation step, which was related to chain scission and the decomposition of FKM, occurred at a higher temperature. In the nanocomposite, the degradation product of

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M.A. Kader et al. / Composites Science and Technology 66 (2006) 1431–1443 100

o

50

Weight (%)

Derivative weight (%/ C)

(a) (b) (c)

4 0

2

0 0

100

200

300

400

500

600

700

o

Temperature ( C) Fig. 10. Thermogravimetric curves (TGA and DTG) of FKM and its nanocomposites at 10 wt% loading: (a) FKM, (b) FKM + Na-MMT and (c) FKM + O-MMT.

Table 3 Thermal characteristics of FKM and FKM/layered clay nanocomposites Samples

T5% (C)

T10% (C)

T50% (C)

Tmax (C)

FKM FM10 FOM10

499.2 507.3 413.9

506 515 450

525 537 505

527 539 459a 505b

a b

Tmax for organic modifier in O-MMT. Tmax for FKM.

the first step might have catalyzed the degradation of FKM by shifting the temperature towards the lower region. Xie et al. [45] reported that O-MMT could undergo multi step degradation in the temperature range of 120–450 C. However, this did not occur in our system, probably due to the presence of polymer matrix, which protected the O-MMT from direct exposure to the degradation environment.

4. Conclusions Fluoroelastomer/layered clay based nanocomposites were prepared by a melt mixing process and their dispersion, morphology, rheological and thermal properties were characterized. The XRD pattern of the nanocomposites revealed an increase of d001 spacing corresponding to the clay layers and a reduced peak height, indicating the presence of an intercalated/exfoliated clay structure. The TEM image augmented the findings of the XRD analysis by showing the presence of well dispersed clay platelets with partially exfoliated morphology. The rheological behavior of the FKM/O-MMT nanocomposites demonstrated shear thinning behavior with a shift towards Newtonian fluid behavior at higher loadings. The shear viscosity of the FKM/O-MMT nanocomposites decreased

with increasing filler loading. Unmodified MMT showed marginal influence on the shear viscosity. The calculated activation energy of melt flow indicated that the OMMT filled system should be easy to process. The FKM/O-MMT hybrids exhibited improved processability because of their reduced equilibrium die-swell and better extrudate appearance. A linear relationship was observed between the maximum recoverable deformation and the extrudate die-swell. The nanocomposites showed increased glass transition temperatures due to the restricted segmental motion of the intercalated polymer chains. Although unmodified clay improved the thermal stability of FKM, O-MMT caused the initiation of degradation to be pushed towards the lower temperature region, due to the decomposition of the organic modifier present in O-MMT. Acknowledgments This research was supported by program for cultivating graduate students in regional strategic industry (2003) and by the grant of Post-Doc. Program, Chonbuk National University (2003). References [1] Schroeder H. In: Morton M, editor. Rubber technology. New York: Van Nostrand Reinhold Co.; 1987. p. 423–30. [2] Okada A, Usuki A, Kurauchi T, Kamigaito O. In: Mark JE, Lee CYC, Bianconi PA, editors. Hybrid organic–inorganic composites. ACS symposium series; 1995. [3] Theng BKG. Formation and properties of clay–polymer composites – Development in soil science. Amsterdam: Elsevier; 1979. [4] Alexandre M, Dubois P. Mater Sci Eng 2000;28:1–63. [5] Vaia RA, Teukolsky RK, Giannelis EP. Chem Mater 1994;6:1017–22. [6] Ray SS, Okamoto M. Prog Polym Sci 2003;28:1539–641. [7] Fisher H. Mater Sci Eng 2003;23:763–72. [8] Lebaron PC, Wang Z, Pinnavaia TJ. Appl Clay Sci 1999;15:11–29. [9] Utracki LA. Clay containing polymeric nanocomposites, vol. 1. UK: Rapra Technology Limited; 2004. [10] Karger-Kocsis J, Wu C-M. Polym Eng Sci 2004;44:1083–93. [11] Varghese S, Korger-Kocsis J, Gatos KG. Polymer 2003;44:3977–83. [12] Sur GS, Sun HL, Lyu SG, Mark JE. Polymer 2001;42:9783–9. [13] Joly S, Garnaud G, Ollitrault R, Bokobza L. Chem Mater 2002;14:4202–8. [14] Arroyo M, Lo´pez-Manchado MA, Herrero B. Polymer 2003;44:2447–53. [15] Mousa A, Karger-Kocsis J. Macromol Mater Eng 2001;286:260–6. [16] Kojima Y, Fukumori K, Usuki A, Okada A, Kurauchi T. J Mater Sci Lett 1993;12:889–90. [17] Nah C, Ryu HJ, Kim WD, Chang YW. Polym Int 2003;52:1359–64. [18] Usuki A, Tukigase A, Kato M. Polymer 2002;43:2185–9. [19] Artzi N, Nir Y, Narkis M, Siegmann AJ. Polym Sci Part B 2002;40:1741–53. [20] LeBaron PC, Pinnavaia TJ. Chem Mater 2001;13:3760–5. [21] Yao KJ, Song M, Hourston DJ, Luo DZ. Polymer 2002;43:1017– 20. [22] Lim SK, Kim JW, Chin IJ, Choi HJ. J Appl Polym Sci 2002;86:3735–9. [23] Kader MA, Nah C. Polymer 2004;45:2237–47. [24] Krishnamoorti R, Silva AS. In: Pinnavaia TJ, Beal GW, editors. Polymer clay nanocomposites. New York: Wiley; 2000. p. 315–43.

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