Fabrication, thermal and electrical properties of ...

Materials Chemistry and Physics 128 (2011) 114–120

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Fabrication, thermal and electrical properties of polyphenylene sulphide/copper composites R.K. Goyal a,∗ , K.R. Kambale a , S.S. Nene a , B.S. Selukar b , S. Arbuj c , U.P. Mulik c a b c

Department of Metallurgy and Materials Science, College of Engineering, Pune 411 005, India Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411008, India Centre for Materials for Electronics Technology, Panchwati, Off Pashan Road, Pune 411008, India

a r t i c l e

i n f o

Article history: Received 28 October 2010 Received in revised form 14 February 2011 Accepted 18 February 2011 Keywords: Electrical conductivity Dielectric property Hardness Thermal expansion

a b s t r a c t The thermal and electrical properties of high performance poly(phenylene sulphide) (PPS) composites reinforced up to 31 vol% Cu particles were investigated to be used as materials for electronic applications. The thermal stability and char yield of the composites increased significantly. Both pre- and post- glass transition coefficient of thermal expansion (CTE) of composites decreased significantly. The microhardness was increased by more than 50% compared to pure PPS matrix. Microhardness and CTE of composites correlated well with the rule of mixtures. A percolation threshold about 6 vol% Cu was obtained. The electrical conductivity was increased by about eight orders of magnitude for 18 vol% composite. Dielectric constant and dissipation factor of composites at 1 MHz was increased by about 6-fold and 70-fold compared to matrix, respectively. They decreased gradually with increasing frequency up to 1 MHz and thereafter, there was insignificant change. The scanning electron microscope showed almost uniform distribution of Cu particles in the matrix. Owing to better dimensional stability and good electrical properties, these composites are very promising for electronic applications. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, metal filled polymer composites are widely studied for the applications in electromagnetic interference (EMI) shielding of computers and electronic instruments, as heat sinks in electronic circuits, passive components and pressure sensors etc. [1–5]. The interest in this study has arisen from the fact that the various properties such as thermal, mechanical and electrical properties of such composites are tailored by varying the loading of filler particles. Additionally, these composites offer several advantages over pure metals such as low cost, flexibility, reduced weight, good toughness and corrosion resistance, and ability to form complex parts due to easy processing of polymers [6]. Owing to high electrical conductivity (5.9 × 105 S cm−1 ), high thermal conductivity (400 W (m−1 K−1 )) and good mechanical properties, copper (Cu) has been widely used as conductive filler in polymer matrix for various applications particularly for EMI shielding application. Nevertheless, the cost of Cu is less than that of Ni and Ag, which are also used as conducting filler for EMI shielding applications. However, it has density of 8.9 g cm−3 and is sensitive to corrosion. In view of these advantages and drawbacks, Cu-filled composites are widely studied. In case of Cu-filled low

∗ Corresponding author. Tel.: +91 20 25507275; fax: +91 20 25507299. E-mail address: [email protected] (R.K. Goyal). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.02.065

density polyethylene (LDPE) composites, percolation threshold was obtained at 10 vol% Cu by Alvarez et al. [6]. They anticipated percolation threshold in the range between 15 and 30 vol% Cu. However, significant reduction in percolation threshold value achieved was attributed to submicron sized Cu particles and hot pressing technique used for composite preparation. This might have caused due to favourable penetration of Cu in the polyethylene (PE) matrix. Luyt et al. reported percolation threshold of 19 vol% Cu for LDPE and linear low density polyethylene (LLDPE) composites [7]. However, at a constant volume fraction of filler, the electrical conductivity of LDPE/Cu composites was higher than that of LLDPE/Cu composites, and it can be attributed to the more amorphous nature of LDPE, which allows higher mobility of Cu particles during processing. Boudenne et al. [8] studied polypropylene (PP)/Cu composite filled with Cu particles with 10–100 ␮m and 100–1000 ␮m sizes and found percolation thresholds of 6 vol% and 11 vol%, respectively. Similarly, the percolation threshold for Cu (∼5 ␮m) filled epoxy composites was 25 vol% [4]. In epoxy/Cu system, despite lower sized Cu particles the percolation threshold is higher than PP/Cu system. Bagwell et al. [9] studied Cu fiber reinforced epoxy composites and found significant improvement in EMI shielding effectiveness of about 45 dB along with improvement in fracture and impact toughness of composite. Furthermore, the thermal conductivity of Cu filled polymer composites can also be improved significantly [10]. Thus, it can be seen that Cu filled polymer composites have multifunctional properties depending upon several

R.K. Goyal et al. / Materials Chemistry and Physics 128 (2011) 114–120

parameters such as volume fraction, size and shape of filler, degree of filler dispersion, interfacial adhesion between filler and matrix, and fabrication methods etc. It is also true that for aerospace sector environment conditions are severe and there is need to study high performance polymers based composites. One such polymer is poly(phenylene sulphide) (PPS), a thermoplastic semi-crystalline polymer with 65% crystallinity and good fire retardancy. PPS has high melting point (285–295 ◦ C), high degradation temperature (>450 ◦ C) and good mechanical properties [11]. Owing to these properties, PPS has been reinforced with several types of fillers like carbon nanotube (CNT) [12,13], expanded graphite [14], CaCO3 [15], Clay [16], TiO2 [17], Al2 O3 [18], and CuO [19] for various applications. For example, addition of small amount of CuO [19] or Ag2 S [20] in PPS matrix decreased significantly wear rate of PPS composites. The percolation threshold for untreated CNT filled PPS composites vary from 1 to 5 wt% depending upon the degree of dispersion and processing route [21]. Nevertheless, the cost of CNT is high and hence, low cost filler filled high performance composites need to be studied. To the best of our knowledge, until now, no results have been published on coefficient of thermal expansion (CTE), microhardness, thermal stability, electrical conductivity and dielectric properties of copper (Cu) reinforced PPS composites. Therefore, the objectives of this study are to decrease CTE, and increase microhardness, thermal stability and electrical properties of PPS/Cu composites prepared by mechanical mixing followed by hot pressing. The data will be useful for EMI shielding, electrostatic dissipation and electrostatic painting applications. Finally, the results of the PPS/Cu composites are presented and compared with data reported in the open literature.

115

Fig. 1. Differential particle size distribution of Cu powder.

Table 1 Properties of PPS and Cu. Material

PPS [exp.]

Cu [24,35]

Density (g cm−3 ) CTE (×10−6 /◦ C)

1.35 51.6 (Tg ) – 24 1.5 × 10−16 S cm−1

8.9 17

Thermal conductivity (W (m−1 K−1 )) Vickers microhardness (kg mm−2 ) Electrical conductivity (S cm−1 )

398 67 [24] 5.9 × 105

rough surfaces. The sizes of each isolated Cu particles are in agreement with the sizes determined from laser particle size analyzer. Table 1 shows properties of PPS and Cu material.

2. Experimental 2.1. Materials The commercial PPS powder purchased from Aldrich Chemical Company (Mn : 10000, CAS: 25212-74-2) was used as a matrix. The reinforcement used in the present study was sub-micron sized electrolytic grade copper powder purchased from Aldrich Chemical Company. The differential particle size distribution of Cu powder was determined on a GALAI CIS-1 LASER particle size analyzer using the standard test method as prescribed by manufacturer. For this, few mg quantity of Cu powder was suspended in an ethanol solvent using ultrasonic bath for 10 min. Suspended Cu/ethanol solution was filled in a standard sample holder and kept inside the instrument. The suspended sample is scanned with a laser beam using a rotating wedge prism. Light scattering occurs when particles in a sample are bathed in the oscillating laser beam. As shown in Fig. 1, its size ranges from 0.4 ␮m to 1.8 ␮m with mean particle size of 0.91 ␮m. Fig. 2 shows the typical SEM micrographs of Cu powder. Fig. 2b (including inset) is the SEM images at higher magnifications. It can be seen from Fig. 2 that the Cu particles are in aggregate form having dendrites like shape. The Cu particles were found in aggregates because SEM analysis was carried out directly on dry Cu powder. The sizes of each dendrite vary from few to several tens of microns. It also seems that each dendrite consists of several isolated spherical particles which are bounded together. Moreover, isolated particles have

2.2. Preparation of PPS/Cu composites Various compositions of composites based on PPS and Cu were prepared using mechanical mixing followed by hot pressing. The well dried PPS and Cu powders were mixed mechanically using mortar and pestle for 2 h. Then, the mixture was again dried in a vacuum oven at 100 ◦ C and finally it was hot pressed using a 15 T compression molding machine. A mould release agent was used to avoid sticking of PPS melt to the die surface. An appropriate quantity of dried and mixed PPS/Cu power was filled in a tool steel die to fabricate sample with diameter of 13 mm and thickness of 3.5 mm. Then the die was kept inside the heater to increase the temperature with an average heating rate of 7 ◦ C min−1 to a maximum temperature of 325 ◦ C (>Tm ). The pressure was gradually increased to 45 MPa and maintained constant at 325 ◦ C for 20 min. Then, the sample was cooled with an average cooling rate of 3.5 ◦ C min−1 to 65 ◦ C inside the mould. Finally the samples were ejected out of a die cavity. Five different compositions containing 0, 30, 45, 60, and 75 wt% Cu in PPS matrix were fabricated and coded as C-0, C-30, C-45, C-60 and C-75, respectively.

Fig. 2. SEM images of pure Cu powder at (a) 100× and (b) 1000× (inset at 10,000×).

116


Table 2 The compositions, experimental and theoretical density of PPS/Cu composites. Sample code

Cu in PPS matrix by

C-0 C-30 C-45 C-60 C-75

wt%

vol%

0 30 45 60 75

0 6.1 11.0 18.5 31.2

Theoretical density (g cm−3 )

Experimental density (g cm−3 )

Porosity (%)

1.3500 1.8111 2.1842 2.7508 3.7148

1.3500 1.7959 2.1796 2.6253 3.4156

0 0.84 0.22 4.56 8.00

2.3. Characterization Since the properties of composite depend upon the volume fraction of reinforcing particles, volume fraction of Cu particles for a given weight fraction was determined using Eq. (1) and specified in Table 2. Vf =

Wf

(1)

Wf + Wm (f /m )

where Vf is the volume fraction, Wf is the weight fraction, and f is the density of Cu particles. Wm and m is the weight fraction and density of PPS matrix, respectively. 2.4. Density Density is an important parameter, required to obtain an idea about the soundness of the samples. Theoretical density of the composites was calculated by the rule of mixtures (ROM) using the density of Cu 8.9 g cm−3 and of PPS 1.35 g cm−3 (of compacted sample). For theoretical density, it was assumed that there were no voids and no loss of constituents during processing. The ROM can be expressed as: th = m (1 − Vf ) + f Vf

(2)

where th is the theoretical density of the composite. The experimental density (ex ) of the composites was determined by the Archimedes method using Eq. (3).

ex =

Wair Wair − Walcohol

was evaluated by the relation ε = Ct/ε0 S, where S is the surface area and t is the thickness of the dielectric material. The ε0 is the permittivity of the free space (8.854 × 10−12 F m−1 ). The dielectric loss tangent which is the ratio of dielectric loss to dielectric constant is obtained directly from the instrument. Silver conductive paste was applied on both sides of the sample to make good electrical contact with the electrodes.

alcohol

(3)

where Wair and Walcohol is the weight of the sample in air and alcohol medium, respectively. The alcohol is the density of the alcohol used. 2.5. Morphology of composites Scanning electron microscope (SEM, Stereoscan-440) was used to examine dispersion of Cu particles in the PPS matrix. For this, a small cross section of samples was mounted in a cold setting polymer resin, polished on successive emery papers and subsequently lapped to remove all scratches. The polished sample surface was coated with a thin layer of platinum to minimize sample charging. 2.6. Vickers microhardness The microhardness of well polished samples was measured using Vickers hardness tester (Future Tech Corp FM-700, Tokyo, Japan) at a constant load of 100 g and dwell time of 15 s. Average values of six readings were reported as the microhardness of samples. 2.7. Thermogravimetric analysis The thermal stability was determined by heating up to 900 ◦ C in nitrogen atmosphere at a heating rate of 10 ◦ C min−1 using TGA (Mettler Toledo TGA/SDTA 851e , Switzerland). The effect of Cu particles on the degradation temperatures (Td ), corresponding to 10 and 20% weight loss, was determined. The % char yield was also determined at temperature of 800 ◦ C. 2.8. Coefficient of thermal expansion (CTE) The linear out-of-plane (through-thickness direction) CTE of the samples was determined using Perkin-Elmer DMA 7e in thermomechanical analyzer mode. A 100 mN force was applied to make the probe in good contact with sample. Before measuring CTE, samples were annealed at 180 ◦ C for 2 h in a vacuum oven to relieve stresses, if any. Then, annealed sample was held under pressure for 5 min and heated to 180 ◦ C at a heating rate of 5 ◦ C min−1 in argon atmosphere. The CTE values were calculated from the slope taken over specific temperature ranges of 40–80 ◦ C (i.e., below glass transition) and 130–170 ◦ C (above glass transition). 2.9. Dielectric properties The dielectric constant was obtained from the measurement of capacitance (C) using Wayne Kerr Electronics precision impedance analyzer [6515B, UK] at frequencies varying from 100 kHz to 15 MHz at 30 ◦ C. The dielectric constant (ε)

2.10. Electrical conductivity Samples were coated with a thin layer of silver paste. The volume resistance of samples was determined by using high resistance meter (Keithley 6517B). However, when the volume resistance is below 106 , an Agilent 6½ digital multi meter (Agilent 34401A) was used. The volume resistivity was measured by the relation = R (A/L), where is resistivity in cm−1 , R is resistance in ohms, L is the thickness in cm and A is the cross sectional area (cm2 ) of sample. The electrical conductivity was reported as the reciprocal of the volume resistivity.

3. Results and discussion 3.1. Density of PPS/Cu composites Table 2 shows the experimental and theoretical density for the composites. It can be seen that the composites density increased with Cu loading due to the higher density of Cu (8.9 g cm−3 ) than that of pure PPS (1.35 g cm−3 ). The experimental density is in good agreement with the theoretical density up to 11 vol% Cu and thereafter, on further increasing Cu particles in the matrix the experimental density decreases. This is due to the increased porosity which was estimated from an Eq. (4) and shown in Table 2. % porosity =

− ex th th

× 100

(4)

It is observed that percent porosity increased particularly at higher loading of Cu. This is due to the fact that when Cu particle loading is increased the inter-particle distance decreases which in turn results in increased agglomeration of Cu particles. The penetration of molten polymer becomes difficult due to agglomerates leading to porosity. The porosity of the C-75 composite is about 8%. 3.2. Morphology of PPS/Cu composites Fig. 3 shows the SEM images of C-30, C-45, and C-75 composites at 200× and 1000× magnifications. It can be seen that the Cu particles are almost well distributed in the matrix. Fig. 4c and e shows SEM images of C-45 and C-75 composites at 1000× magnification, respectively. It can be clearly seen from these images that Cu particles are well coated with PPS matrix and there is no distinct boundary between the Cu particles and the PPS matrix. In addition, the size of Cu aggregates is much smaller than the size observed in pure Cu powder (Fig. 2). It indicates that mechanical mixing has resulted in uniform dispersion of constituents. Moreover, it seems that Cu particles tend to form 3-dimensional network in C-45 composite (Fig. 3c) and complete, in C-75 composite (Fig. 3e). Fig. 4e also shows the presence of porosity in the vicinity of Cu aggregates which might be the reason for decreased density at higher Cu content.


117

Fig. 3. SEM images of PPS composites containing (a) 30 wt% Cu, (b and c) 45 wt% Cu, and (d and e) 75 wt% Cu. Scale bar 100 ␮m for images (a, b, and d) and 20 ␮m for images (c and e).

3.3. Microhardness of PPS/Cu composites

50

Vickers hardness (kg/mm 2)

Exp. microhardness

R2 = 0.9672

ROM 40

30

20

10

0

10

20

30

Volume % Cu in PPS matrix Fig. 4. Hardness as a function of vol% of Cu in PPS matrix.

Fig. 4 shows experimental and theoretical microhardness of composites as a function of Cu content. The theoretical values were predicted from the rule of mixtures (ROM). The ROM for microhardness can be expressed as; Hc = Hm (1 − Vf ) + Hf Vf , where Hc , Hm , and Hf represent microhardness of composite, matrix and Cu filler, respectively, and Vf is the volume fraction of the filler. The microhardness of pure PPS is 24 kg mm−2 . It can be seen that as Cu content increases microhardness of composites increases with respect to pure PPS matrix. It is to be noted that microhardness of pure Cu varies from 39 kg mm−2 for annealed Cu, to 58.9 kg mm−2 for sintered Cu [22] and to 67 kg mm−2 for electrolytic Cu sample [23]. Since, we have used electrolytic grade Cu for the present study, the microhardness of 67 kg mm−2 was considered for ROM. It can be seen from Fig. 4 that experimental data fits well the ROM. The microhardness of C-75 (31 vol% Cu) composite is increased to 36.7 kg mm−2 which is very close to the microhardness of annealed Cu. That is about 50% improvement in microhardness was observed. The significant improvement may be attributed to higher micro-

118


(a)

Exp. CTE (> Tg)

(b)

0.004

(c)

ROM (< Tg)

100

(b)

ROM (> Tg)

0.002

(c) 0.006

Exp. CTE (< Tg)

(a)

0 30

50

70

90

110

Temperature ( C)

0.004

(a): Pure PPS

-6 0

0.008

Thermal strain

Thermal strain

0.01

120

0.006

CTE (x 10 / C)

0.012

80

60

(b): 18.5 vol% Cu 0.002 0 30

40

(c): 31.2 vol% Cu

80

130

20

180

Temperature (ºC)

hardness of Cu than that of the matrix. Misra et al. [24,25] have reported that there is enhanced crystal nucleation and different local polymer chain conformation in the regions surrounding the reinforcing particles than that of away from the particles. Moreover, uniform dispersion of Cu particles in the matrix can well resist penetration of Vickers indenter resulting in significantly increased microhardness of composites. Nevertheless, the microhardness can be further increased by reducing the porosity which is at present 8%. Moreover, porosities allow easy penetration of indenter and results in decreased microhardness compared to that of porosityfree samples [26]. 3.4. Coefficient of thermal expansion (CTE) of PPS/Cu composites

5

10

15

20

25

30

35

Volume % Cu in PPS matrix Fig. 6. Experimental and theoretical linear coefficient of thermal expansion (CTE) of composites.

120

100

Residual weight (%)

Fig. 5. Thermal strain (L/L) of composites as a function of temperature (30–180 ◦ C).

0

PPS/18.5 vol% Cu

80 PPS/6.1 vol% Cu

60 Pure PPS

40

20

The thermal strain, i.e., the ratio of change in length to the original length of sample, was obtained during the second heating of samples from 30 to 180 ◦ C. Fig. 5 shows the thermal strain (L/L0 ) of pure PPS and its composites measured along out-of-plane (thickness direction) direction. The meeting point of the extrapolated tangents drawn over the linear curves (i.e., between 40–80 ◦ C and 130–170 ◦ C) of thermal strain versus temperature was assumed as the glass transition temperature (Tg ) of PPS. The Tg obtained by this method for pure PPS was 85 ◦ C which is close to the reported value. It can be seen from Fig. 5 that thermal strain of PPS increased significantly above Tg . The lower thermal strain of PPS below Tg is due to the fact that below Tg the free volume is too low to move molecules or segments. However, this is sufficient to permit bond vibrations which results in lower thermal strain as compared to that of above Tg . In order to cause movement of molecules or segments of molecules from place to place, there should be free volume into which these molecules or segments may move. Above Tg , there is sufficient energy for molecular movement and the free volume increases sharply with an increase in temperature and thus, results in higher thermal strain. Fig. 5 also shows that the thermal strain of composites is significantly decreased due to the restriction to the polymer chain movement by Cu particles with increasing temperature. Fig. 6 shows correlation between the experimental CTEs measured before and after Tg of PPS and values predicted from the ROM as a function of Cu content. The ROM can be expressed as; ˛c = ˛m (1 − Vf ) + ˛f Vf , where ˛c , ˛m , and ˛f represent the linear CTE of composite, matrix and Cu filler, respectively. The pre-Tg CTE of pure PPS is 51.6 × 10−6 /◦ C which is very close to the reported CTE of 49 × 10−6 /◦ C [27]. The post-Tg CTE of pure PPS is 97.9 × 10−6 /◦ C. It can be clearly seen from Fig. 6 that both CTEs decreased with increasing Cu content and validated fully the values predicted from the ROM. The pre-Tg and post-Tg CTE of C-75 composite is about

0 50

150

250

350

450

550

650

750

850

Temperature (ºC) Fig. 7. TGA of PPS/Cu composites determined in nitrogen atmosphere at 10 ◦ C min−1 .

39.5 × 10−6 /◦ C and 72.7 × 10−6 /◦ C, which is about 23% and 26% lower than that of the pure PPS, respectively. It indicates that the dimensional stability of the composites is improved. The appreciable improvement in dimensional stability of composites may be attributed to the decrease in volume fraction of the PPS and the lower intrinsic CTE of Cu (17 × 10−6 /◦ C) than pure PPS matrix [28–30]. 3.5. Thermo gravimetric analysis Fig. 7 shows the thermal decomposition behavior of PPS and its composites under nitrogen atmosphere. The degradation temperatures for 10% (T10 ) and 20% (T20 ) weight loss for composites are shown in Table 3. The T10 was increased from 492.1 ◦ C for pure PPS to 510.2 ◦ C for C-30 composite. Similarly, T20 was increased from 523.6 ◦ C for pure PPS to 535.5 ◦ C for C-30 and to 582.2 ◦ C for C-60 Table 3 Thermal (TGA) properties of PPS/Cu composites. Sample code

C-0 C-30 C-60

Decomposition temperature (◦ C) T10

T20

492.1 510.2 510.5

523.6 535.5 582.2

Char yield at 800 ◦ C (%) 39.9 58.7 76.4


50

1.E-05

a

C-0 C-30

40

Dielectric constant

1.E-07

Conductivity (S/cm)

119

1.E-09 1.E-11 1.E-13

C-45 C-60 C-75

30

20

10

1.E-15 1.E-17

0

10

20

0 100

30

1000

Volume % Cu in PPS matrix 25

3.6. Electrical conductivity Fig. 8 shows the volume electrical conductivity of composites as a function of Cu content. The conductivity of pure PPS is about 1.5 × 10−16 S cm−1 which is similar to the value reported by Yu et al. [12]. The conductivity increased to 2.9 × 10−12 , 1.4 × 10−8 and 4.2 × 10−7 S cm−1 for C-30, C-45 and C-60 composites, respectively. It can be seen clearly from Fig. 8 that percolation threshold is about 6 vol% (30 wt%). In other words, there is almost four orders of magnitude improvement in conductivity of C-30 which is comparable with those mentioned in literature [6–8]. A further increase in conductivity is achieved as the Cu content is increased to 60 wt% (C-60). The significant improvement in conductivity results in due to an infinite conductive cluster in the matrix and thus, composite becomes conducting. However, on further increasing Cu content the conductivity decreased. For example, C-75 composite showed conductivity of about 2.3 × 10−7 S cm−1 which is lower than that of C-60 composite and is attributed to the presence of porosities [27] which are the scattering centers for electrons and hence, electron mobility is decreased. In this study, lower percolation threshold of 6 vol% can be attributed to the lower mean particle size, i.e., 0.72 ␮m of Cu. This is better than the reported values of 11 vol% Cu [4], 19 vol% Cu (38 ␮m) [6] and 25 vol% Cu [7] for PP, LDPE, and epoxy based composites, respectively. Moreover, the electrical conductivity of PPS/Cu composites is better than PMMA/Al composites which showed conductivity only in the range of 20–40 vol% Al [32]. The better results in present work may be attributed to hot pressing which produces favourable penetration of conducting fillers in polymer matrix and thus, lowering the percolation threshold. This is in good agreement with the electrical properties of submicron sized Cu filled PE composites fabricated by hot pressing [9]. However, if the composites are fabricated using hot pressing without complete melting of polymer, the uniform dispersion of conducting filler in matrix may not be achieved but un-melted polymer particles will be surrounded by fillers. This might be the reason for higher percolation threshold in PMMA/Al composites [32]. To control the polymer viscosity during solvent mixing, generally solvents are added for the better distribution of filler in matrix. However, due to environment concerns, addition of solvents is generally not preferred by indus-

100000

b

Fig. 8. Electrical conductivity of composites as a function of vol% Cu in PPS matrix.

20

Dlelectric constamt

composite. Similar to this, an improvement in thermal stability was reported for copper filled LDPE composites [31]. The char yield, i.e., residual weight, determined at 800 ◦ C was increased from 39.9% for pure PPS to 58.7% for C-30 and to 76.44% for C-60 composites. An improvement in char yield may be attributed to higher thermal stability of Cu than that of PPS.

10000

Frequency (in KHz), Lag scale

15

10

5

0

0

5

10

15

20

25

30

35

Volume % Cu in PPS matrix Fig. 9. Dielectric constant of composites versus; (a) frequencies and (b) vol% Cu.

tries. Hence, mixing of polymer and filler using a simple mechanical mixing method (i.e., mortar/pestle) followed by hot pressing which results into lower percolation threshold would be beneficial for the industries. The measured conductivities of about 10−8 –10−7 S cm−1 for present samples are useful for electrostatic discharge or EMI shielding applications. 3.7. Dielectric properties Fig. 9a shows the dielectric constants for the pure PPS and its composites measured at frequencies varying from 100 kHz to 15 MHz. The dielectric constants for C-0, C-30 and C-45 composites are almost frequency independent. However, dielectric constant of C-60 and C-75 composites decreased gradually with increasing frequency up to 1 MHz and thereafter their dependence seems to be insignificant on frequencies. The significantly higher dielectric constants at lower frequencies are believed to be due to the clusters of the Cu particles in the composite [33]. Fig. 9b shows the dielectric constants of composites measured at 1 MHz as a function of Cu content. The dielectric constant of PPS (1 MHz) is 3.25, which is in agreement with the typical literature value [28]. It can be seen from Fig. 9b that dielectric constants of composites increased with increasing Cu content. The dielectric constant of C-30, C-45, and C-60 composites at 1 MHz is 5.4, 8.3, and 19.5, respectively. The increased dielectric constants are primarily due to the polarization associated with the charges induced on Cu particle surfaces and at the interfaces. The improvement in dielectric constant for C-60 (18.5 vol% Cu) is significant compared to that of LDPE/Cu (8–10 ␮m) composite [34]. The dielectric constant (1 MHz) for LDPE/20 vol% Cu composite increased by about 2.5-fold in contrast to about 6-fold for PPS/Cu composite with approximately same volume fraction. The

120


0.8

a

Dissipation factor

0.7

C-0

0.6

C-30

0.5

C-45

0.4

C-60 C-75

0.3 0.2 0.1 0 100

1000

10000

100000

Frequency (KHz), Log scale

The microhardness increased by about 50% compared to pure PPS. It is interesting that both microhardness and CTE validated the rule of mixtures. A percolation threshold at about 6 vol% Cu was obtained and the electrical conductivity was increased by about eight orders of magnitude for 18 vol% composite. Above 18 vol% Cu, improvement in electrical conductivity was not observed due to the presence of porosity as confirmed from SEM. The dielectric constant and dissipation factor were studied as a function of frequency. Dielectric constant and dissipation factor measured at 1 MHz of the composites containing 31 vol% Cu is 19 and 0.28, respectively. SEM showed uniform dispersion of copper particles in the matrix. The improvement of the properties of composites was demonstrated and ascribed to the excellent intrinsic properties of Cu. This study indicates that novel PPS/Cu composites may prove to be the futuristic high performance materials for electronic applications.

0.4

Dissipation factor

b

Acknowledgements Authors particularly RKG wish to acknowledge financial support from University Grants Commission (UGC), India. Professor A.N. Tiwari, IIT Bombay is thanked for fruitful discussions.

0.3

0.2

References

0.1

0

0

5

10

15

20

25

30

35

Volume % Cu in PPS matrix Fig. 10. Dissipation factor of composites versus; (a) frequencies and (b) vol% Cu.

higher dielectric constant for PPS/Cu composites may be attributed to the presence of thin oxide layer on Cu particles. A high dielectric constant would be beneficial for flexible capacitors and EMI shielding applications [35]. However, the dielectric constant of C75 (31 vol% Cu) composite is slightly lower than that of C-60. It may be attributed to the presence of porosity of about 8% because the dielectric constant of air is 1. Fig. 10a shows the dissipation factor for the pure PPS and its composites measured at frequencies varying from 100 kHz to 15 MHz. The dissipation factors of composites except C-30 decreased gradually with increasing frequency up to 10 MHz. However, there is not much difference in dissipation factor measured at 10 MHz and 15 MHz. Fig. 10b shows the dissipation factors for composites measured at 1 MHz as a function of Cu content. The dissipation factor increased with increased Cu content in the PPS matrix. The dissipation factor of pure PPS measured at 1 MHz is 0.004. It increased to 0.0075 for C-30, 0.22 for C-60, and to 0.28 for C-75. The high dissipation factor is good for EMI shielding applications. 4. Conclusions The thermal, mechanical, electrical conductivity and dielectric properties of high performance Cu filled PPS matrix composites fabricated by hot pressing have been discussed. Both pre-Tg and post-Tg linear CTEs of the composites were reduced significantly compared to the pure PPS due to the constraint on the mobility of polymer chains by the copper particles with increasing temperature. The thermal stability of composites increased significantly.

[1] X. Luo, D.D.L. Chung, Compos. B: Eng. 30 (1999) 227–231. [2] N. Das, Y. Liu, K. Yang, W. Peng, S. Maiti, H. Wang, Polym. Eng. Sci. 49 (2009) 1627–1634. [3] S. Kumar, T. Rath, R.N. Mahaling, C.S. Reddy, C.K. Das, K.N. Pandey, R.B. Srivastava, S.B. Yadaw, Mater. Sci. Eng. B 141 (2007) 61–70. [4] Y. Ishigure, S. Iijima, H. Ito, T. Ota, H. Unuma, M. Takahashi, Y. Hikichi, H. Suzuki, J. Mater. Sci. 34 (1999) 2979–2985. [5] D.D.L. Chung, J. Mater. Eng. Perform. 9 (2000) 350–354. [6] M.P. Alvarez, V.H. Poblete, M.E. Pilleux, V.M. Fuenzalida, J. Appl. Polym. Sci. 99 (2006) 3005–3008. [7] A.S. Luyt, J.A. Molefi, H. Krump, Polym. Degrad. Stab. 91 (2006) 1629–1636. [8] A. Boudenne, L. Ibos, M. Fois, J.C. Majeste, E. Gehin, Compos. A: Appl. Sci. Manf. 36 (2005) 1545–1554. [9] R.M. Bagwell, M. Joseph, R.C. Wetherhold, Compos. Sci. Technol. 66 (2006) 522–530. [10] I.H. Tavman, Powder Technol. 91 (1997) 63–67. [11] Polymer Data Handbook, Oxford University Press, London, 1999. [12] S. Yu, W.M. Wong, Y.K. Juay, M.S. Yong, Simtech Tech. Rep. 8 (2007) 71–75. [13] J. Yang, T. Xu, A. Lub, Q. Zhang, H. Tan, Q. Fu, Compos. Sci. Technol. 69 (2009) 147–153. [14] Y.F. Zhao, M. Xiao, S.J. Wang, X.C. Ge, Y.Z. Meng, Compos. Sci. Technol. 67 (2007) 2528–2534. [15] X. Wang, W. Tong, W. Li, H. Huang, J. Yang, G. Li, Polym. Bull. 57 (2006) 953–962. [16] H. Zou, W. Xu, Q. Zhang, Q. Fu, J. Appl. Polym. Sci. 99 (2006) 1724–1731. [17] Z. Jiang, L.A. Gyurova, A.K. Schlarb, K. Friedrich, Z. Zhang, Compos. Sci. Technol. 68 (2008) 734–742. [18] M.H. Choa, S. Bahadur, A.K. Pogosian, Wear 258 (2005) 1825–1835. [19] S. Bahadur, C. Sunkara, Wear 258 (2005) 1411–1421. [20] C.J. Schwartz, S. Bahadur, Wear 251 (2001) 1532–1540. [21] C.S. Zhang, Q.Q. Ni, S.Y. Fu, K. Kurashiki, Compos. Sci. Technol. 67 (2007) 2973–2980. [22] F. Shehata, A. Fathy, M. Abdelhameed, S.F. Moustafa, Mater. Des. 30 (2009) 2756–2762. [23] V. Rajkovic, D. Bozic, M.T. Jovanovic, J. Mater. Process. Technol. 200 (2008) 106–114. [24] A. Dasari, J. Rohrmann, R.D.K. Misra, Mater. Sci. Eng. A 364 (2004) 357–369. [25] R. Hadal, A. Dasari, J. Rohrmann, R.D.K. Misra, Mater. Sci. Eng. A 380 (2004) 326–339. [26] S.K. Bhattacharya, Polymer 20 (1979) 1166–1167. [27] H.-G. Elias, An Introduction to Plastics, 2nd ed., Wiley-VCH GmbH & Co. KGaA, Weinheim, 2003. [28] S.N. Maiti, K. Ghosh, J. Appl. Polym. Sci. 52 (1994) 1091–1103. [29] L. Li, D.D.L. Chung, Compos 22 (1991) 211–218. [30] R.K. Goyal, A.N. Tiwari, U.P. Mulik, Y.S. Negi, J. Phys. D: Appl. Phys. 41 (2008) 1–7. [31] X. Xia, S. Cai, C. Xie, Mater. Chem. Phys. 95 (2006) 122–129. [32] V. Singh, A.N. Tiwari, A.R. Kulkarni, Mater. Sci. Eng. B 41 (1996) 310–313. [33] H.S. Cokturk, T.J. Fiske, D.M. Kalyon, J. Appl. Polym. Sci. 50 (1993) 1891–1901. [34] Z.-M. Dang, Y.-H. Zhang, S.-C. Tjong, Synthetic Met. 146 (2004) 79–84. [35] D.D.L. Chung, Appl. Therm. Eng. 21 (2001) 1593–1605.