Method of manufacturing an electrically conductive thermoplastic material

A method of manufacturing a highly electrically conductive plastic material from thermoplastic resin and carbon fill through a two-screw mixer having a screw length of more than twenty times the screw diameter involves continuously inserting a thermoplastic resin into a first zone of the two-screw mixer and compacting and preheating the thermoplastic resin. The resin is then passed into a second zone where it is kneaded and plastified. A particulate carbon fill is continuously inserted into a third zone with the fill constituting 30 to 50% by weight of the mixture and being dispersed in said resin within the third zone. The speed of rotation of the two screw mixer lies in the range between 150 to 250 RPM and the temperatures of the zones lie within the range of 165.degree. C. to 300.degree. C. After passage through the various zones, the mixture is removed via an outlet of an extrusion head. A copolymer propylene and ethylene forms the thermoplastic resin in the carbon black and the carbon black constitutes the particular carbon fill. A fill of not more than 10% by weight carbon fibers may be inserted at one of the zones. Further, a fourth zone may follow the third zone with the method further including the step of degasing the mixture at a pressure lying in the range of atmospheric pressure to a few millibars at the fourth zone.

The present invention relates to a method of manufacturing a highly 
electrically conductive thermoplastic material usable, for example in 
conductive seals, in bipolar elements for fuel cells, in other 
electrochemical devices, etc. 
Such a material must present several characteristics simultaneously: 
electrical resistivity which is as low as possible and in the range a few 
ohm-centimeters to a few tenths of an ohm-centimeter, for example; 
good uniformity of composition and of electrical and mechanical 
characteristics in particular; 
the absence of additives which are harmful to catalysts that may be used in 
the electrochemical devices concerned; 
sufficient chemical inertness relative to the fluids flowing through such 
electrochemical devices at their operating temperatures; 
very low cost price; 
suitability for cheap industrial fabrication by the operations of: 
extrusion, calendering, hot pressing, injection, etc. for mass producing 
parts having the general shape of large surface area thin plates: 
thickness of about one millimeter; surface area of several tens of square 
decimeters up to one square meter; 
mechanical characteristics such that the plates made in this way with this 
material are not fragile and have sufficient bending strength when hot and 
when cold to be compatible with the assembly methods used for 
electrochemical and other devices in which they may constitute component 
parts; 
adequate sealing properties at the above-mentioned thicknesses relative 
both to gases and to liquids; and 
long-term stability of these characteristics compatible with the intended 
use. 
It is possible to obtain a material which has suitable resistivity by 
incorporating a metal fill in a thermoplastic material. However, the cost 
of such a material is very high and in addition the metal fill cannot be 
chemically inert relative to the surrounding medium. It is thus more 
advantageous for these reasons to use carbon fills, which are much less 
expensive and which are generally less reactive. 
Numerous electrically conductive thermoplastic materials containing carbon 
fills have already been made, and sometimes they have even been sold, as 
can be seen from the following articles: 
Electrical Conduction Mechanism in carbon filled polymers (IEEE 
Transactions May/june 1971; pages 913 to 916); 
Modern Plastics International: March 1976; pages 45 to 47; 
JEE: November 1978; pages 42 to 45; 
Modern Plastics International: August 1983; pages 38 to 40; 
Research and Development: May 1984; pages 118 to 123; 
Adhesive Age: June 1984; pages 17 to 20. 
None of the materials described in these articles has all of the 
above-mentioned characteristics. 
Table I below summarizes the electrical resistivity of currently known 
commercially available materials, together with a few details concerning 
their manufacture, their degree of carbon filling, and their melt flow 
index which determines whether they are suitable for cheaply manufacturing 
thin parts of large surface area. 
TABLE I 
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METHOD RESIS- CONCEN- MELT 
Type of TIVITY TRATION INDEX 
REFERENCE machine (.OMEGA.cm) 
(%) (g/10 mn) 
______________________________________ 
DARLING Co. 
Open mixer 
25.4 64 
DARLING Co. 
" 7.6 70 
DARLING Co. 
" 5.1 76 
Esso Research 57 39 (1) 
Esso Research 17 32 (1) 
LNP 5 to 20 40 (2) 
UNIROYAL 150 
(TPR) 
CAPREZ 9.3 30 14.4 at 230.degree. 
and 21.6 kg 
CAPREZ CP 6 4.5 at 230.degree. 
and 21.6 kg 
ABBEY 100 8 45 
DUPONT DE 30 
NEMOURS 
(NDX4769) 
TECKNIT 860 10 
TECKNIT 861 5 
CABELEC 0.7 50 
CABELEC 0.9 47 
______________________________________ 
(1) Vulcan XC72 carbon black 
(2) Carbon fibers 
It can be see that the low electrical resistivity obtained by conventional 
methods (either using an open mixer or an internal, Banbury type mixer) 
are obtained at the price of a very high degree of carbon fill. 
The materials obtained in this way are not usable for cheaply injecting 
thin parts of large surface area because they do not flow sufficiently 
when hot, or because the resulting objects are brittle. 
U.S. Pat. No. 4 124 747 also describes a discontinuous method consisting in 
using a preheated Banbury type mixer to mix finely divided carbon into a 
propylene-ethylene thermoplastic copolymer, with there being about 30% 
carbon by weight. The preheat temperature is about 100.degree. C.; mixing 
takes place for a duration of 3 to 5 minutes. 
The thermoplastic material obtained in this way may be extruded in the form 
of sheets having a thickness in the range 150 microns to 500 microns and 
having a resistivity of a few ohm centimeters. However, it is practically 
impossible to use for obtaining thin parts of large surface area by 
injection because of the above-mentioned reasons. 
Further, the applicant has performed a number of tests in order to improve 
this situation by using open or closed (Banbury type) mixers and also by 
prior mixing the two components in the powder state in an ultrafast mixer 
followed by plastification of the mixture in an extruding machine. The 
main results of these tests are summarized below in Table II. 
TABLE II 
______________________________________ 
METHOD RESIS- CONCEN- MELT 
Type of TIVITY TRATION INDEX 
REFERENCE machine (.OMEGA.cm) 
(%) (g/10 min) 
______________________________________ 
TEST No. 1 
Open mixer 
0.85 47 
TEST No. 2 
Open mixer 
0.87 47 
TEST No. 3 
Banbury 0.43 47 0 to 230.degree. 
and 21 kg 
TEST No. 4 
Banbury 0.80 47 
TEST No. 5 
Banbury 1 47 1 to 230.degree. 
and 21.6 kg 
______________________________________ 
In conclusion, it appears that the various methods used up until now do not 
provide a material which is sufficiently electrically conductive without 
using concentrations of carbon which give rise to much too low a melt 
index to be able to inject thin parts of large surface area. 
Preferred implementations of the present invention provide a method of 
manufacturing a conductive thermoplastic material whose resistivity is 
less than that of the earlier materials, i.e. is a few tenths of an 
ohm-centimeter, and whose fluidity when hot is sufficient for industrial 
application of the above-mentioned operations in order to obtain very thin 
conductive sheets which are not very fragile. 
The present invention provides a method of continuously manufacturing a 
thermoplastic material comprising a mixture of thermoplastic resin and a 
carbon fill, characterized by the fact that a two-screw mixer is used, the 
screw length being more than twenty times the screw diameter, said 
thermoplastic resin being continuously inserted into a first zone of said 
two-screw mixer where it is compacted and preheated, the said resin then 
passing into a second zone of said two-screw mixer where it is kneaded and 
plastified, a particulate carbon fill being continuously introduced into a 
third zone of said two-screw mixer, the fill constituting 30% to 50% by 
weight of the mixture and being dispersed in said third zone, the speed of 
rotation of said two-screw mixer lying in the range 150 to 250 rpm, the 
temperatures of said zones lying in the range between 165.degree. C. to 
300.degree. C., and the said mixture then being removed via the outlet 
from an extrusion head. 
The carbon fill preferably consitutes 35% to 45% by weight of the mixture. 
Preferably, a fill of carbon fibers is also introduced into the mixture 
either together with the fill of particles or at another moment, the total 
quantity of carbon fiber fill not exceeding 10% by weight of the mixture. 
Advantageously, the two-screw mixer includes a fourth zone following said 
third zone and intended to degas said mixture at a pressure lying between 
atmospheric pressure and a few millibars. 
In an entirely unexpected manner, a material is obtained whose electrical 
and mechanical characteristics are most remarkable for a carbon particle 
fill: the electrical resistivity is an order of magnitude less than that 
obtained for a material of the same composition made by conventional 
methods, and the mechanical characteristics of thin plates of large area 
obtained therefrom, for example, are considerably improved. 
Also unexpectedly, the electrical conductivity of the above material is 
further improved by a factor of several times if the carbon particle fill 
includes a few percent of a carbon fiber fill. 
The thermoplastic resin may be a copolymer of ethylene and propylene, and 
the carbon fill may be selected from various types of carbon black. The 
carbon fill should be highly conductive and as cheap as possible while 
still being easy to incorporate in the resin. The lower the density of the 
fill, the easier it is to disperse, but the more difficult it turns out to 
incorporate in the resin. A compromise has to be achieved between a low 
fill ratio using finely divided carbon black and a higher fill ratio using 
a denser carbon black which is easier to incorporate. 
The above-mentioned results are easily obtained if the continuous feeds of 
thermoplastic resin and carbon fill are therefore performed by means of 
weighing dispensers capable of ensuring that the raw material feed rates 
do not depart from their nominal settings by more than .+-.1%.

FIG. 1 shows very diagrammatically a mixer 11 suitable for manufacturing 
material in accordance with the invention. The mixer is a two-screw type 
mixer as sold under the trademark Werner und Pfleiderer, with a screw 
length which is more than twenty times the screw diameter. By way of 
example, the mixer may be a ZSK 30 or a ZSK 57 type mixer which 
respectively have diameters of 30 mm and 57 mm. 
The two screws are encased in a series of ovens referenced 1 to 10 which 
are regulated, as is explained below, to temperatures in the range 
165.degree. C. to 300.degree. C. The screws are driven by a motor which is 
diagrammatically represented by a box 20, and its speed of rotation lies 
in the range 150 rpm to 250 rpm. 
A copolymer of propylene and ethylene 22 is accurately measured out to 
within .+-.1% by a weighing dispenser 21 and is introduced via a funnel 25 
into a first zone of the two screws encased by the oven 1. At this stage 
the resin is compacted and preheated, but any gelification must be 
avoided. In succeeding ovens 2 and 3, the mixing and kneading continues 
and the resin is plastified, with the oven 20 being adjusted to 
220.degree. C. and the oven 3 to 295.degree. C. The oven 4 is likewise 
adjusted to 295.degree. C. and the carbon fill which is constituted by 
carbon black is introduced at this point. The fill is accurately measured 
by a weighing dispenser 23. Carbon black 24 is thus inserted into the 
resin and is dispersed therein. 
In order to obtain a uniform conductivity and melt index in the product, it 
is essential for the degree of carbon fill to be uniform and accurate. The 
product must be uniformly conductive and must have a uniform melt index in 
order to be capable of rapid injection since this process requires raw 
materials having highly uniform characteristics. 
As already mentioned, no other technique for manufacturing conductive 
mixtures enables the following desirable characteristics to be obtained 
simultaneously: the conductive fill is thoroughly dispersed to a uniform 
concentration, thereby achieving high and uniform electrical conductivity; 
the hot melt index is satisfactory and constant and compatible with the 
implementation of the intended application. 
Mixing and dispersal continue along the two screws through ovens 5 to 10, 
with the oven 5 being adjusted to 295.degree. C. and ovens 6 to 10 being 
adjusted to 200.degree. C. The mixture is degased at oven 9 which may be 
maintained at a pressure of 55 mBar, for example. 
The resulting mixture passes through an extrusion head 30 which is adjusted 
to a temperature of about 250.degree. C. Extrusion takes place through a 
die having ten holes each having a diameter of a few millimeters, with the 
material and the outlet from the die being at a temperature of about 
270.degree. C. The rods made in this manner are passed through a trough of 
water and are then inserted into a granulator. 
Material may be obtained at a rate of about 55 kg per hour when the two 
screws are rotated at 250 revolutions per minute (rpm). 
FIG. 2 shows the surprising electrical characteristics of the material 
obtained by the method in accordance with the invention in comparison with 
the electrical characteristics of commercially available materials and of 
the material produced by the prior art discontinuous Banbury method. In 
all cases, the mixture is based on the same materials. The carbon black is 
Vulcan XC 72 black sold under the trademark Cabot. Curve A shows the value 
of the resistivity of the prior art material and curve B shows the 
resistivity of material in accordance with the invention. Thus, for a 
carbon black concentration of about 35%, the resistivity of the prior art 
material is 6 .OMEGA.cm, whereas the resistivity of the mixture in 
accordance with the invention is only 0.85 lcm. A prior art material of 
the same resistivity can be obtained, but this requires a carbon black 
concentration of about 50%. Such a mixture is so viscous that it is 
unusable in an injection process. Further, thin plates made from such 
material by a hot pressing technique would be very fragile. 
Turning to FIG. 3, a portion of the FIG. 2 curve B is reproduced on a 
larger scale, together with a curve D showing the melt index (i) as a 
function of the concentration C(%) of carbon black. 
Point E represents the resistivity of a prior art mixture at a very high 
concentration of 47% carbon together with a hot melt index which is 
incompatible with making thin conductive parts of large surface area by 
injection. 
Vulcan carbon black may be replaced by other particulate carbon fills. 
Thus, Ketjen black sold under the trademark Akzo can be used with the same 
method to achieve the same resistivity as with Vulcan, but with a 
concentration of 25% instead of 38%. 
The completely surprising results obtained by implementing the method of 
the invention make it possible to inject products of the bipolar element 
type for a fuel cell using mass production machinery. 
In addition to their high conductivity, these elements possess numerous 
other qualities, in particular: 
hardness: 60 to 70 on the Shore scale; 
flexibility: an element having the thickness of 1.5 mm may be bent without 
damage into a circular arc having a diameter of 150 mm; 
tensile strength: the same element may be subjected to an ultimate tensile 
stress of 0.5 da N/mm.sup.2 to 3.5 da N/mm.sup.2 for an elongation of 4% 
to 7.5%; and 
the material is thermally and chemically inert. 
In accordance with an improvement of the method in accordance with the 
invention, the above-described mixture which already contains 38% Vulcan 
XC 72 carbon black had various carbon fiber fills added thereto at rates 
of 3% to 9% by weight. It was observed in a surprising manner that the 
electrical conductivity was multiplied thereby by a factor of 3 to 6. 
Naturally, the invention is not limited to the implementation which has 
been described, in particular concerning the type of two-screw mixer, the 
temperatures to which the various ovens are adjusted, or the nature of the 
thermoplastic resin or of the carbon fill.