Electrically Conductive Polymer Composites and Blends*

BIING-LIN LEE

Research and Development Center The BFGoodrich Company

Brecksville. Ohio 441 41

Increasing utilization of the electrical properties of polymeric blends and composites has prompted our renewed interest in developing a general working relationship which can explain the electrical properties of polymer composites and blends in terms of processing characteristics, morphology, and composi- tions. Here, we restrict our attention to the following two-component systems: (1) two component systems with conductive particulate inclusions (e.g. carbon black) embedded in a continuous polymeric matrix, and (2) two component polymer blend systems with one conductive polymer (e.g., polyether copolymer) dispersed in another continuous polymeric matrix. The following processing aspects related to the electrical property of particulate filled composites are discussed: (1) critical concentration of rigid conductive fillers, 4c, and (2) redistri- bution of conductive fillers upon processing. An equation based on the crowding factor of concentrated suspension rheology and Janzen's particle contacts perco- lation is proposed to describe the relationship between 4,, and the maximum packing fraction of conductive fillers. The relationship is used to explain the influence of particle morphology on conductivity, and the conductivity difference in the high shear and the low shear region of a processed polymer composite part. Furthermore, some qualitative guidelines for blending a low conductivity polyether copolymer to achieve an overall balance of antistatic and mechanical properties of polymer blends are also discussed.

INTRODUCTION

he ability of polymers to act as electrical insula- T tors is the basis for their manifold applications in the electrical and electronic areas. But there are many cases when the electrical conductivity of poly- meric materials is required, such as the dissipation of electrostatic charge from rubber and plastic parts, and the shielding of plastic boxes from the effects of electromagnetic waves ( 1). Consequently, material engineers have long sought to combine the versatil- ity of polymers with the electrical properties of met- als. The advantages relate not only to the ability to produce electrically conductive materials, but also the ability to modify their electrical characteristics with wide limits.

The method currently used to increase the electri- cal conductivity of polymers is to fill them with specific conductive additives, such as metallic pow-

composites have been reported (5, 6, 13). In the case of metal filled systems, Frenkel proposed Ihe elec- tron tunneling theory to describe the contact resis- tance between two metallic bodies ( 1 3 ) . In the case of carbon black filled composites, Sheng, Sichel, and Gittleman have shown that tunneling activated by thermal fluctuation of the electric potential is the dominant mechanism under certain conditions (5, 6).

In this paper, we will discuss the relationship of the particle morphology to the critical volume frac- tion of conductive fillers required to achieve a semi- conductor for particulate filled composites. We will then discuss the influence of mixing and/or process- ing on the electrical properties of polymeric compos- ites. We will also discuss the antistatic properties of polymer blends containing a low conductivity polyether copolymer.

ders (2), metallic fibers (3), carbon black (4-7), ionic conductive polymers (8- lo), and intrinsically con- ductive polymeric powders, e.g. polypyrrole ( 1 1, 12). Some mechanisms of electrical conduction in filled

ELECTRICALLY CONDUCTIVE POLYMER COMPOSITES

The behavior of composites made with an insula- tor polymer and conductive filler is quite interest- ing. First of all, a polymer composite could change from an insulator to a (semi)conductor over a very

- __ *Presenied in part at the 61st Annual Meeting of the Society of Rheol- ogy. Montreal, Canada Oct 21-26. 1989

36 POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, VOl. 32, NO. 1

Electrically Conductive Polymer Composites and Blends

narrow range of filler concentrations. At low filler concentrations, the composite remains an effective insulator. At a “critical volume concentration,” the conductivity of the composite starts to increase sharply to a level at which the composite can con- duct electricity. An additional increase in filler con- tent has a much smaller effect: a plateau is attained.

From the compounding and processing points of view, it is very important that the relationship of the minimum concentration of fillers +, required to achieve a semiconductor could be determined a pri- ori. Namely, we are looking for the general relation- ship of +, as a function of the morphology of parti- cles, e.g., maximum packing fraction of fillers (or structure of fillers), and how it is influenced by processing and mixing.

In fact, in an insulating matrix containing ran- domly dispersed conductive particles, probability calculations predict a sharp change (indeed, a power law dependence) in conductance when the volume of the conductive particles reaches a critical volume called the percolation threshold (14). This theoreti- cal volume depends on the hypothetical shape and distribution of the particles (14). According to the formation of interparticle contacts (15, 16), the re- quired critical concentration of fillers is dependent on the packing mode of the conductive particles. Generally, it predicts that the critical volume frac- tion, +,, decreases as the maximum packing frac- tion of powder, + M , increases. It does not suggest an explanation for observed threshold loadings of silver spheres loading approaching 30% (2, 17).

Janzen studied the effects of the structure of fillers on the percolation threshold, with emphasis on the number of contacts that a particle makes with its neighbors, rather than the bond or site percolation probability. He proposed the following equation to relate +, to some properties of carbon black in the systems of carbon black filled polymers (17):

1 1 +4pv +,= ~

where p = density of carbon black v = Dibutylphthalate (DBP) value of carbon

black sample in cm3/g measured on a compressed sample according to ASTM

=specific void space of random dense

Equation 1 describes a simple relation of the 4, to the apparent change of morphology of filler parti- cles, v. It predicts lower +, from higher v. That is, +, decreases as +m decreases. It expresses semi- quantitative rationalization of polymer-embedded metal spheres and rubbers containing carbon black (17). However, it under-estimates some known be- haviors of rubbers and plastics containing carbon black.

In the following, we use Janzen’s argument of particle network and propose a relationship which

D3493-76

packed filler

could relate +, to +, of some known behavior of filled conductive polymer composites. We further use +m, the maximum packing fraction of fillers, as a gross parameter to describe the morphology of particles.

According to Janzen (17). the essential plan is to obtain +, as the solution of a particle-loaded com- posite system:

where +, is the critical volume fraction of particles for particle network formation.

Cp is the critical mean number of contacts per particle at which the probability of an infinite parti- cle-particle chain formation first becomes non-zero. According to Gurland (2) and Janzen (17). a value of 1.5 for Cp might be used.

Z is the maximum number of possible contacts per particle (i.e. coordination number). It is related to the structure of packing (see Table 1) (18). The coordination number (2) increases sharply as the maximum packing fraction ( +m) of fillers increases.

The next step is to find an appropriate formula for f(+,). We may assume f(+,) to be of the following form:

(3)

+m

The form M is similar to that of Mooney’s approach on concentrated suspension rheology (19). Table 2 shows the value of +,,, for several grades of carbon black and some fillers of different morphology (20). We also show in Table 2 that Brabender shear mix- ing of dry carbon black causes an increase in the

Table 1. Coordination Number of Equal Size Spheres in Regular Arrangement (18).

4 m Z (Maximum Packing (Coordination

Fraction) Number)

Tetrahedral 0.34 Cubic 0.52 Tetragonal 0.60 Rhombohedra1 0.70 Hexagonal 0.74

4 6 8

10 12

Table 2. Some Typical Values of Maximum Packing Fraction of Fillers b,,, (20).

Particles

Sphere Ground spheres Ground table salt Carbon black N330 (as-received) Carbon black N330 (Brabender sheared) Carbon black N229 Ketjen black

Type of Packing 4ln Random close packing 0.637 Random packing 0.458 Random 0.60 Random 0.25

Random > 0.30

Random Random

0.22 0.1 1

POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 7992, VOl. 32, NO. 1 37

Biing-Lin Lee

value of 9,. This is due to the breakdown of the carbon black structure under shear. In mixing pro- cess that is necessary to produce a good dispersion of carbon black generally leads to some breakdown of carbon black aggregates. This also causes a de- crease of shear viscosity and modulus (20, 21). We further assume L is of the form, which is related to 4,:

(4)

Please note that Janzen ( 17) assumed the following relation to obtain Eq 1

( 5 )

When Eqs 3 and 4 are substituted into Eq 2 and the equation is solved for +c, the result is:

Now we will elaborate on the effects of particle mor- phology and mixing on the conductivity of compos- ites according to Eq 6.

1. In Fig. 1, we plot the +c of a variety of fillers according to Eq 6, in which we use &,, as a gross parameter to describe the morphology of parti- cles. I t also predicts that q5c increases as 4, in- creases. In curve A of Fig. 1 (solid line), we use the relationship described in Table 1 to estimate 4,. In curve B (dashed line), the random densely-packed mono-disperse spheres are dis- cussed, in which we use Z = 6 (22). The esti- mated 4c for mono-disperse conductive spheres is predicted to be in the range of 0.30 to 0.37. This is reasonably in agreement with the results reported by Gurland for a system of silver spheres in Bakelite (2). Gurland’s data showed that c $ ~ = 0.30 is close to the onset of a sharp increase in conductivity, while dc = 0.38 approaches the con- centration above which the conductivity would just gradually increase. In Fig. 1 , we also show the relationship of 4, and $c of Ketjen black loaded in a melt-mixed PVC matrix, and N 3 3 0 carbon black in SBR and the natural rubber ma- trix (23). Despite a wide range of different mor- phology of fillers (i.e. black with high porosity, black with high structure, and conductive silver sphere), the results appear to be quite in agree- ment with the relation described by Eq 6.

2. It has been known that the electrical conductivity of carbon black reinforced polymeric composites is determined by the properties of the particular grade as well as the loading of black used. Ac- cording to our analysis, the maximum packing fraction of fillers, which is related to the structure and porosity of black, is an even more significant factor. Other properties being equal, Eq 6 implies increasing conductivity with increasing structure

0.4

C 0 ._ c

0.3 c a, c s L a,

L L - - ._ - B ._ 0.2

k? 6

0.1

0

Eq 1 -..-.. - A (SeeTextforA&B) _ _ _ _ _ . ,Eq(6)

0 Kraus, Svetlik, 1956; N330/SBR

Gurland, 1966; Silver Sphere in Bakelite

, 0 Kraus. Svetlik. 1956; N330iNR

1 I I I I I 1 I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Om, Maximum Packing Fraction

Fig. 1 . Critical jllter concentration, @=, versus the maxi- m u m packing fraction of fillers, @m.

of black. This is in agreement with the experi- mental results in rubber compounding (4,7, 17).

3. The electrical conductivity of polymer compos- ites is strongly dependent on mixing. Figure 2 shows the conductivity of two different composi- tions of Ketjen black loaded PVC composites as a function of mixing conditions (24). The com- posites were made by melt mixing using a Brabender internal mixer at 190°C followed by compression molding. The composites formed under different mixing times from identical start- ing materials showed different conductivities. A composite with longer melt shear mixing exhib- ited a lower conductivity (i.e., nigher resistivity). The composites formed by direct compression molding of the powder mixture (i.e. with a mini- mum shear mixing) exhibited a high conductiv- ity. This phenomenon may be due to dispersive mixing. Generally, the carbon black structure could be broken down due to intensive shear mixing. This, in turn, increases 4m (see Table 2) and the interaggrate separation, and conse- quently, decreases the chance of conductive net- work formation. In rubber compounding, it has been reported that the conductivity of a carbon black-rubber composite increases rapidly during the very first stages of mixing, as carbon black is incorporated and conductive pathways are

3a POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, Vol. 32, NO. 1

Electrically Conductive Polymer Composites and Blends

established between the islands of rubber-filled pellet fragments; the conductivity then decreases gradually during the later states of dispersive mixing (25-30). These results also imply that, technologically, defined values of conductivity of filled system may be achieved via compression molding of polymer chips and/or particulates coated with a conductive filler. Indeed, electro- static dissipative floor tiles are made this way (see Fig. 3).

4. The conductivity of injection molded-composites of carbon-black thermoplastics is lower than that of the corresponding compression molded sam- ples (31). This is presumably due to the break-

= 0 Direct Powder Compression Forming

0 Brabender Melt Mixing at 190°C

1 O 0 I 20 PHR Black

I - /

Brabender Mixing Time (min)

Fig. 2. Electrical resistivity of Ketjen black-PVC compos- ites as a function of mixing conditions.

polymer compounding -+ pelletizing -+ crushing

+ surface coating of compression molding conductive fillers

--+ surface finishing

polymer matrix

/

conductive path

Fig. 3. A schematic procedure for making electrostatic dissipative tiles.

down of the black aggregate structure under high shear and also due to the existence of a polymer- rich skin layer. Since injection molded parts nor- mally show a layered structure (i.e. skin and core structure), the conductivity distribution in injec- tion molded composites is expected to be more complicated. Accordingly, we sectioned the skin of an injection molded Ketjen black loaded PVC composite and measured its conductivity. We ob- served that the skin layer, i.e., high shear zones, had a substantially lower conductivity than did the core region.

5. The conductivity of filled polymer systems can be improved by the addition of a second polymer of lower viscosity. Table 3 shows our results on the effects of adding a thermotropic liquid crystal polymer to carbon black-PVC composites. The composition was melt-mixed using a Brabender internal mixer at 190°C for 5 min followed by compression molding. In the presence of Klucel H (a thermotropic liquid crystal polymer), the con- ductivity of carbon black loaded PVC composites (CPD D) was improved even with a lesser concen- tration of carbon black. The reason for this is that the thermotropic liquid crystalline polymer re- duces the melt viscosity of the PVC matrix, and consequently, reduces the chance of the break- down of the carbon black structure. Similarly, the conductivity of Ketjen black loaded PVC com- posites can be improved by increasing the melt mixing temperature (from 190°C to 200°C) or by using a resin with a low molecular weight to reduce the melt viscosity.

LOW CONDUCTIVITY POLYMER BLENDS

Permanent antistatic property is becoming a more and more frequently required attribute of plastic and rubber materials. Packaging for electronic com- ponents is one of many examples. In many applica- tions, the values of conductivity achieved with car- bon black with a concentration higher than +c (in the plateau region) are too high, since special grounding measures often have to be taken for the sake of safety.

To set definite values of resistivity between lo6 to 10" ohmlsquare as required for a number of anti- static application, several ways are possible: 1) the compounding of the most accurate concentration of conductive fillers in the region of critical volume concentration as discussed in the previous section,

Table 3. Effect of Klucel H, a Thermotropic Liquid Crystal Polymer, on the Conductivity of Carbon Black Loaded

PVC Composites.

CpdA CpdB CpdC CpdD

PVC compound* 100 100 100 100 Ketjen black 10 20 30 14 Klucel H 0 0 0 4 Resistivity (ohm-cm) 60 3.0 0.6 3.0

*The PVC compound contained thermal stabilization, lubricant, and impact modifier.

POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, VOI. 32, NO. 1 39

Biing-Lin Lee

2) the use of a lesser conductive filler in the plateau of the conductivity versus concentration curve, and 3) the use of conductive alloying polymeric additives with a resistivity in the range of lo5 and 10l2 ohmlsquare.

The first method can be applied by the controlled addition of conductive carbon black. The surface resistivity can be compounded in the range of 10' to 10" ohmlsquare, which is the range required to guarantee a satisfactory insulation between the con- tacts, while maintaining a resistivity low enough for electrostatic dissipation. However, some of the dis- advantages of carbon black are sloughing, contami- nation of clean room environment, and embrittling of thermoplastics. Another drawback black is that it can only provide one color: black (32).

The second method to obtain a defined low con- ductivity is to use fillers that have a lower conduc- tivity than the usual conductive carbon black. How- ever, they do have a tendency to increase the melt viscosity for processing and to decrease physical properties such as impact resistance (33). In this case, an appropriate chosen blend of polymers as the matrix material is preferred, since filler addi- tives can form networks aligning between the poly- meric interface.

A new method to obtain a defined low conductiv- ity is to use a high molecular weight polyether copolymer melt-blended with an insulating pdymer (8 - 10). The copolymer, hereafter referred to as ESD polymer, is intrinsically conductive with a resistiv- ity of 10" ohmlsquare. When the conductive poly- mer networks form, the resistivity values of the blends are in the range lolo to 10l2 ohmlsquare to be effective to impart a permanent anti-static prop- erty (9, 10) (see Fig. 4). The static dissipative per- manence is also independent of humidity. This poly- mer blend approach is highly suitable for a precise adjustment of conductivity values in the required conductivity region for antistatic applications. The overall balance of electrical and mechanical proper- ties of ESD blends can further be achieved by an appropriate choice of matrix polymers, blend com- positions, rheological properties of the individual polymers, and processing conditions (34,35).

10'8 I I 4 Styrene Maleic Anhydride

1018 -'..... -- 0- Polystyrene

-I).. High Impact Polystyrene

1011 I I I I I I I I I 0 10 20 30 40 50

Weight % ESD Polymer

Fig. 4. Surface resistiuity of thermoplastic resins as a function of the leuel of polyether copolymer.

MECHANICAL PROPERTIES

The change in certain properties due to the incor- poration of rigid conductive fillers into polymeric materials has an effect on their other properties. Although carbon black is used as a reinforcement agent for rubber compounds, the addition of carbon black with the goal of achieving appropriate conduc- tivity to a rigid polymer matrix (e.g., ABS, PVC) usually causes an increase in stiffness. The tensile modulus increases with an increase in the carbon black concentration. The breaking strength and the elongation at break decrease markedly with the de- gree of loading. The impact resistance is reduced significantly with an increase in the amount of car- bon black.

Carbon black also has a negative effect on the processability of a compound. The melt viscosity increases with an increasing level of carbon black. The isoviscous temperature or flow temperature (Tf) of Ketjen black loaded PVC melts as measured using a constant pressure, non-isothermal rheometer, for example, increases with black loading (see Fig. 5) (36). Also, the decline of the ability to flow due to the addition of carbon black extends over the entire range of shear rates.

The addition of a high molecular weight polyether copolymer, however, may increase the melt viscos- ity to a lesser degree than identical amounts of carbon black. Figure 6, for example, shows the vis- cosity of PETG and its blend with 20%, by weight, of a polyether copolymer (10). Please note that the magnitude of the viscosity of this ESD blend is al- most the same as that of the PETG polymer matrix.

I I I l l 1 I 1 I I I I I I I I I I I

Temperature ("C)

Fig. 5. The constant pressure, non-isothermal rheometer extrusion of a series of PVC composites with dijfeerent loading of Ketjen black (amount = 1 gram; applied load: 1000 lb; rate of heating: 20"Clmin).

40 POLYMER ENGINEERING AND SCIENCE, MID-JANUARY 1992, Vol. 32, NO. 1

Electrically Conductive Polymer Composites and Blends

t

105 i

---- Blend I.

103 I I 1 1 1 1 1 1 1 I I I I I1 I l l I I I I I I I I

100 101 102 103 w [radis]

Fig. 6. Dynamic viscosity of PETG, polyether copolymer and their blend (blend ratio = 80/20) at 210°C.

Table 4. Some Properties of Blends of ESD Polyether CopolymerlPETG (10).

Composition A B C D

Polyether copolymer 0 100 20 30 PETG 100 0 80 70

(ohmkquare) Static decay (SEC)* @15% relative humidity 5000 volt to 500 volt Insulator No test 0.47 0.36 500 volt to 50 volt Insulator No test 1.32 1.02 Notched lzod impact 0.5 (ft-lbiin) @room temp.

Surface resistivity > 1014 l o i o i o q 2 10”

1.30 5.8 -

*FTMS l O l C Method 4046.

In most cases, depending on the polymer matrix, the high molecular weight ESD polyether copolymer even decreases the melt viscosity of blends for im- proved processability (9, 10).

For blends containing ESD polyether copolymer, the extent of changes in physical/mechanical prop- erties depends on the matrix material (9, 10). Table 4 shows some physical properties of blends of PETG with ESD polyether copolymer (10). In addition to the improvement of the electrical property of PETG, the polyether had a minimal negative effect on the viscosity and impact strength. The mutual effects between the polyether copolymer and polymer ma- trix are very complex. Based on Ratner’s theoretical treatment of polymer solid electrolytes (37) and working knowledge of the rheology and morphology of polymer blends, qualitative guidelines for using polyether copolymer to optimize a polymeric com- pound for anti-static applications can be formed. This experience is valid particularly for electrostatic dissipative (ESD) polymer blend compositions be- cause high concentrations of polyether copolymers affect other mechanical properties (9, 10, 34, 35).

1. The conductivity is inhibited in a polymer matrix that shows a strong interaction with the polyether copolymer.

2. The electrical properties of blends containing polyether copolymer are favored in a polymer matrix with a high permittivity.

3. The optimum balance between conductivity and mechanical strength can be achieved with the addition of other ingredients, such as rigid fillers.

CONCLUSIONS

The classical method to make polymers electri- cally conductive is to introduce a specific rigid con- ductive filler to the nonconductive polymer matrix. However, it is difficult to achieve a clearly defined conductivity using extrusion or injection molding processes. The reason for that is the changes of conductive paths due to variations in the concentra- tions of the fillers, changes in the morphology of the fillers, and the hetero-distribution of the fillers. An equation based on particle contact percolation and Mooney’s crowding factor on concentrated suspen- sion rheology is described to explain these compli- cated phenomena.

Polymer blends using a high molecular weight polyether copolymer appear to offer a viable alterna- tive for making plastics and rubbers static dissipa- tive. In addition to imparting electrical properties of polymers for ESD applications, polyether copolymer had a minimal negative effect on melt viscosity.

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