Conductive Composites Based on Metallocene Isotactic Poly(propylene): Preparation and Properties

Abstract

Summary: The research deals with the preparation and the further comprehensive characterization of metallocene polypropylene-based composite materials by incorporation of carbon black nanoparticles. Composites containing up to 10 wt% of carbon black were prepared by direct melt mixing in a single screw extruder Brabender Extrusiograph type 30/25D with attached static mixer at melt temperature of 200 °C and a screw speed of 30 rpm, according to a two-step process. Some composites were treated with 3 wt% maleic anhydride grafted polypropylene (MAH-PP). The rheological behaviour of the miPP nanocomposites was determined by cone/plate rheological measurements at 180 °C. The composites were characterized by SEM for morphological details and uniaxial stress-strain measurements for determining the mechanical parameters. Electric conductivity of injection molded plates from these composites was investigated. The different miPPs studied are ranked in an ascending order according to their increasing molecular weight concerning the magnitude of their rheological parameters. The maleic anhydride compatibilizer leads to lower viscosity values even at high shear gradients and to better homogenization of the nanofiller in the polymer matrix. The processing conditions, carbon black concentration and viscosity of the virgin polymer have an impact on the final conductivity of the miPP/carbon black composites.

Introduction

The manufacture of polyolefins by metallocene catalysts has brought novel aspects in polymer industry. Since the first patent for a metallocene catalyst in 1980, there has been a constant dramatic rise in the volume of research on metallocene technologies and metallocene polyolefins.1, 2 Metallocene-based poly(propylene) (mPP) has many unique properties that are unattainable with the conventional catalyst systems as narrower molecular weight distribution (MWD), lower melt elasticity, lower melting and crystallization temperature and lower heat of fusion (hence less energy needed to change the polymer from the solid to the molten state).3–5 The rheological properties of metallocene derived iPP in comparison with Ziegler-Natta derived iPP were investigated by Fujiyama and Inata.6 These authors compared their samples with similar MFI and found that die swell, the entrance pressure loss, end correction coefficient and the critical shear rate at which a melt fracture begins to occur are lower.

These properties are beneficial for the extrusion processes and affect both end use applications and fabrication procedures. Therefore metallocene isotactic polypropylene has been recently promoted for a variety of applications. From a practical point of view it is very important to combine a metallocene polymer - polypropylene, and nanosized filler – carbon black. In such a way different types of cost-effective volume composites can be produced that can impart electrical conductivity to polymer products forming conductive networks among carbon filler particles at low filler loadings. The goal of the present study is to evaluate the rheological behaviour, morphology, electrical conductivity and mechanical properties of metallocene-based polypropylene-carbon black composites.

Experimental Part

Materials

Three grades of commercial metallocene isotactic polypropylenes, products of Basell-Polyolefins, were applied: HM562P, HP562S and HM648T, designated as mPP-P, mPP-S and mPP-T, respectively. The molecular characteristics and data for the structural defects of the polymers are listed in Table 1. Ketjen black EC-300J (product of Akzo Nobel Polymer Chemicals, China) was selected as carbonic nanofiller (labeled as KB) with specific surface area of 803 m²/g, pore volume about 325 ml/100g and size of egg-shell shaped carbonic particles within the range of 30–100 nm. Maleic anhydride grafted polypropylene Licomont AR 504 (MAH-PP), product of Clariant GmbH, Germany, was used to compatibilize the PP/carbon black composites in concentrations of 3 wt% in order to achieve a better dispersion in the mPP matrix.

Table 1. Molecular characteristics and structural defects.

Sample	Molecular characteristics			Structural defects
	MFI g/10min	Mw	MWD	Isotacticity	Regio 2,1-1	Regio 2,1-2	%mmmm
mPP-P	15.1	268680	2.60	98.84	0.58	0.58	92.75
mPP-S	28.8	206385	2.37	98.45	0.77	0.77	91.27
mPP-T	60.2	–	–	98.45	0.77	0.77	90.86

Mixing

The nanocomposites were produced by direct melt compounding in a single screw extruder, Brabender Extrusiograph 30/25D, according to a two step process. The extruder was equipped with a static mixer and screw with 4 to 1 compression ratio and mixing zone for effective and homogenous mixing. First, PP-composites with 10 wt% of carbon black were prepared at melt temperature of 200 °C and a screw speed of 30 rpm and second, these compositions were diluted under the same conditions to different carbon black concentrations. The test PP/KB compositions labeling is shown in Table 2.

Table 2. Sample designation.

Sample	Ketjen black content (KB), wt%
	0	2	5	7	9
mPP-P	P0	P2KB	P5KB	P7KB	P9KB
mPP-P-MA	–	P2KBM	P5KBM	–	–
mPP-S	S0	S2KBS	S5KBS	S7KB	S9KB
mPP-T	T0	T2KBT	T5KBT	T7KB	T9KB

Characterization of Composites

The rheological characterization of the samples was determined using a rotational viscometer (Rheotron Brabender, Germany) in two modes – steady state shear measurement (shear rates of 0.02–20 s⁻¹) and dynamic oscillatory measurement (angular frequency sweep from 0.20–75 s⁻¹) in the linear viscoelastic region at temperature of 180 °C, according to a methodology described in.7

SEM studies were carried out with a JEOL 5510 scanning electron microscope in a regime of secondary electrons and accelerating voltage of 10 kV. The examined surfaces were obtained by cooling samples for 5 minutes in liquid nitrogen, fracturing and coating them with 2 nm of Au in argon atmosphere.

The DC electrical resistivity was measured at ambient temperature 25 °C and humidity 50% by two different methods depending on the sample resistance. Plates with dimensions of 100 × 80 × 2 mm were injection molded from the composites. The volume resistivity was measured in a test fixture by means of high-resistance meter Keithley 6517B (Keithley Instruments Inc., USA) at a field strength 50 kV/m. When the resistivity of the samples was lower than 10⁷ Ω, it was measured by the four-probe van der Pauw technique. In this case the test samples were discs with diameter of about 14 mm punched from the plates. Silver paste was used to ensure good contact of the sample with the copper electrodes.

The mechanical parameters of the nanocomposites (Young's modulus, E, tensile strength, σ_t, and elongation, ε, at maximal stress value) were determined from stress-strain curves obtained by means of a Tiratest 2300 testing machine at a crosshead speed of 10 mm/min and room temperature. The dumb-bell shaped specimens with gauge dimensions of 40 mm in length and 5 mm in width were punched out from extruded 1.5 mm thick sheets.

Results and Discussion

Rheology

Figure 1 presents the rheological characterization of the neat polymers under steady state and dynamic mode testing. First, the three grades of polymers exhibit viscosity values, which are in agreement with their Mw and MWD data and they are ranked in descending order in the following manner: mPP-P > mPP-S > mPP-T. From the comparison of the parameters for both modes in the low range of angular frequency, shear rate, respectively (ω, ) – 0.1–0.3 s⁻¹, it may be assumed that the Cox-Merz rule is approximately valid for the dynamic and shear viscosity (η′, η), although the steady state values are somewhat higher in the whole ω and sweep range. The reduced shear thinning is obvious and it acquires more pronounced expression only for the higher angular frequency range (10 to 75 s⁻¹) of the oscillatory mode. The slope n of the shear stress τ and m of the storage modulus G′ is approximately the same and equal to 1. Only the lower part of the storage modulus relation on angular frequency is characterized by m = 2.

Figure 1. Rheological behaviour of three types of neat mPP in steady state (η, τ) and oscillatory mode (η′, G′) depending on shear rate () and angular frequency (ω).

In the lower shear rate and angular frequency range the matrix and the filled systems may be described by the power law relationships:7

• 

In the higher shear rate and angular frequency range the matrix and the filled systems conform to the following power law dependences:

• 

The rheological behaviour of the nanofilled compositions is shown in Figure 2 for the case of mPP-S-KB. First, the character of shear-thinning of the nanocompositions has not been significantly changed, except for certain reduction of the rheological parameters with increasing the nanofiller content. We have observed similar behaviour in isotactic Ziegler-Natta polypropylene compositions filled with MWCNTs, where a minimum of viscosity is observed at about 0.5 wt%.8 This reduction is attributed to the introduced polypropylene and carbon nanofibers with the masterbatch, i.e. to changes in the MWD and polydispersity of the composites. A similar effect is observed in micro- and macro-dispersions but at higher filler contents and is known as the “binary filler effect on viscosity reduction”.

Figure 2. Dynamic viscosity (η′), storage (G′) and loss (G″) moduli vs. angular frequency (ω) of mPP-S-KB composites.

Polydispersity plays a major role in the viscous properties formation and this holds true for nanodispersions too, but due to the filler nanoscale the effects are shifted towards much lower concentrations.7, 8 The observed viscosity reduction in conventional iPP is much more expressed in metallocene polypropylene due to its narrower MWD, needing sometimes mixing of two types of mPP to broaden the MWD.2, 5 The added nanofiller plays a similar role for enhancing viscosity reduction and here the minimum is observed at about 2–4 wt%, as seen in Figure 3.

Figure 3. Effect of KB concentration and angular frequency ω on the dynamic viscosity (η′) of different mPP-KB composites.

The dependence of all rheological parameters on shear rate and angular velocity ω (Figure 2 and Figure 3) is better expressed for low gradients. The differences between the different polymers (Figure 1, Figure 2), concentrations and additives (Figure 2 and Figure 3) become smaller with the growth of and ω.

The effect of maleic anhydride, although studied here only for the mPP-P compositions, is expressed in certain reduction of the rheological parameters (Figure 3). The compatibilizer usually concentrates at interfaces during blending, and helps for reducing interfacial tension and strengthening interface adhesion, thus adjusting processing conditions and miscibility between phases and producing a fine dispersion of phases with small particle size.

The plot of G′ vs. G″ is considered to give useful information for the prevalence of viscous or elastic properties in the composite melt behaviour (Figure 4). This relation shows that the equilibrium between viscous and elastic properties is reached at high deformation gradients for both the matrix and the filled polymers.

Figure 4. Storage modulus G′ vs. loss modulus G″ for mPP-KB composites.

SEM

Generally, the SEM observations show homogenous dispersion of carbon black particles within the polymer matrix in the whole concentration range due to the two step extrusion process. The SEM micrographs (Figure 5) show that the treated with MAH-PP composition P5KBM (a) in comparison to P5KB (b) has a denser and uniform structure and the nanofiller is better dispersed.

Figure 5. SEM micrographs of 5 wt% mPP composites: P5KBM (a) and P5KB (b).

Electrical Properties

The volume resistivity of mPP composites as a function of KB concentration is shown in Figure 6. It should be noted that the volume resistivity gradually decreases with increasing of KB content and at low filler loadings the composites are still insulating. At concentrations above 7 wt% KB, the volume resistivity of mPP-S and mPP-T compositions drops significantly by several orders of magnitude, reaching the percolation threshold. The curve of mPP-P/KB composites shows a percolation threshold at 9 wt% of KB. The reason for this are the higher melt viscosity of mPP-P and hence the worse dispersion of the filler particles and presence of larger agglomerates in the matrix.

Figure 6. Volume resistivity of the mPP composites vs. carbon black concentration.

Mechanical Properties

Table 3 reports the values of different mechanical parameters obtained from stress-strain experiments. The behavior observed depends on the molecular weight of the mPP used. Rigidity diminishes with addition of KB nanoparticles in the nanocomposites derived from the mPP-P whereas a significant increase is seen when the other mPP are used. This feature seems to indicate that polymer/nanofiller interactions at interfaces are hindered if molecular weight is too high and stress cannot be properly transferred. The incorporation of a stiffer component (KB nanoparticles) reduces elongation at breaking point and, consequently, tensile strength. The tensile strength of P5KBM composition (30.9 MPa) is relatively higher than P5KB, which is attributed to the enhanced interaction at the interface and better distribution of the filler (Figure 5).

Table 3. Mechanical parameters of mPP composites.

mPP/KB ratio	Values of mechanical characteristics
	E, GPa			σt, MPa			ε, %
	mPP-P	mPP-S	mPP-T	mPP-P	mPP-S	mPP-T	mPP-P	mPP-S	mPP-T
100/0	1.37	1.31	1.20	30.6	29.8	29.8	6.8	6.8	7.9
98/2	1.28	1.33	1.39	30.4	30.4	30.4	6.7	6.4	6.6
95/5	1.37	1.39	1.45	29.4	29.7	29.5	6.0	5.5	5.6
93/7	1.26	1.38	1.51	29.4	27.4	28.7	5.7	4.5	4.7
91/9	1.26	1.41	–	27.4	28.5	–	4.7	3.9	–

Conclusion

Metallocene iPP/KB nanocomposites were produced by direct melt compounding in a single screw extruder with attached static mixer according to a two step process at different levels of loading and controlling the particle distribution. The rheological behaviour of three types of mPPs is in good agreement with the data for their molecular weight and MWD. The flow behaviour of the matrix and filled nanocompositions can be described by a power law relationship with values of the power exponents n ∼ 1 and m ∼ 2 or 1 < m < 2. The mPP/KB nanocompositions exhibit a minimum in viscosity in the low concentration range, which is attributed to changes in the mPP polydispersity due to the introduced KB. SEM images show homogenous dispersion of carbon black particles within polymer matrix in the whole concentration range. The maleic anhydride compatibilizer enhances better structure formation because it concentrates at interfaces during blending, and helps for reducing the interfacial tension and strengthening the interface adhesion, thus contributing to miscibility between phases and producing a fine dispersion of phases with small particle size. The resulting structure is more dense and uniform and the nanofiller is better dispersed. The melt viscosity has a large influence on the final electroconductive properties. The mPP-T polymer with the lowest viscosity has the lowest percolation threshold. The mechanical parameters evaluated (Young's modulus, tensile strength, elongation at break) are also nearly independent of KB incorporation.

Acknowledgements

The authors are grateful for the financial support of the Bulgarian Ministry of Education and Science (Projects D01-469 and DO 02-202 and Exchange Collaboration Program CSIC/BAS (projects 2007BG0007). The authors would like to thank LiondelBasell Ltd., and Ultrapolymers Bulgaria Ltd. for submitting the metallocene isotactic polypropylenes.