Process for making an electroconductive polymer composition

A process for making an electroconductive polymer composition containing a matrix composed of a mixture of at least two polymers which are incompatible with each other, and an electroconductivity-imparting filler distributed predominantly in the polymer having a higher affinity with the filler. The plastic composition exhibits a higher conductivity than that obtained by blending each of the polymers as the matrix with the same filler.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to an electroconductive polymer composition. More 
particularly, it relates to an electroconductive polymer composition for 
preventing electromagnetic interference (hereinafter referred to as "EMI") 
and an accumulation of static electricity. 
(2) Description of the Related Art 
With the development of electronic appliances such as a computer, the 
prevention of EMI and an accumulation of static electricity have become 
important, and various processes therefor have been proposed. Typical 
examples of known EMI prevention processes are a surface-treating process 
in which only the surface of a non-electroconductive material is treated, 
for example, coated with an electroconductive substance; an internal 
treating process in which an electroconductive filler is dispersed in or 
laminated on a non-electroconductive material; and a metal process in 
which a metal having a high electroconductivity is used. 
Note, an electroconductive polymer composition comprising a thermoplastic 
resin and an electroconductive filler dispersed in the resin is known. 
Of these conventional techniques, many reports have been made on an 
electroconductive polymer composition comprising an electroconductive 
filler dispersed in a polymer. 
In the usual process for the preparation of an electroconductive polymer 
article, a filler is first mixed and kneaded with a matrix resin and the 
mixture is shaped by injection molding or the like. A banbury mixer, a 
mixing roll, a twin-screw extruder or the like is used for the kneading. 
If the amount of the filler incorporated is increased, to improve the 
electroconductivity, the melt fluidity is reduced and the molding and 
processing become difficult. Furthermore, even if molding is possible, 
because of a short shot or the like, a satisfactory shaped article cannot 
be obtained, and often the resulting shaped article has poor mechanical 
properties, such as a poor impact strength. 
The incorporation of excessive amounts of some fillers is not advantageous 
from the economical viewpoint. 
The scatter of the electroconductivity of products is large, due to the 
processing conditions, and in practical operation, it is often found that 
the electroconductivity of the products differs. Moreover, in some cases, 
even if a good electroconductivity is obtained just after the preparation, 
the electroconductivity is gradually lost with the lapse of time. 
The present inventors collectively reported on the series of investigations 
they made, in the "Journal of the Adhesion Society of Japan, Vol. 23, No. 
3, pages 103-111 (1987)", and in this report showed that the interfacial 
affinity between the polymer and the filler is an important factor having 
an influence on the manifestation of the electroconductivity, the behavior 
of the electroconductivity in various resin matrices can be elucidated 
based on the difference in the dispersion state of the filler, and the 
volume fraction (hereinafter referred to as "Vf") of the filler producing 
the electroconductivity by a formation of conducting paths differs greatly 
according to the kind of resin matrix. Furthermore, by changing the 
dispersion state of fine particles of carbon black by utilizing a thermal 
relaxation of polymers, it was found that the electroconductivity was 
improved by an aggregation of the filler. 
When preparing an electroconductive polymer composition by dispersing an 
electroconductive filler into a polymer, if an attempt is made to improve 
the electroconductivity by increasing the amount of the electroconductive 
filler, a deterioration of properties other than the electroconductivity, 
such as the processability and mechanical properties, and an increase of 
the costs, cannot be avoided. 
SUMMARY OF THE INVENTION 
In view of the above, the primary object of the present invention is to 
provide an electroconductive polymer composition, which contains an 
electroconductive filler in substantially the same amount as that 
customarily adopted but exhibits an electroconductivity superior to the 
customarily obtained electroconductivity or an electroconductive polymer 
composition, and which contains an electroconductive filler in an amount 
smaller than that customarily adopted but exhibits an electroconductivity 
comparable or superior to the customarily obtained electroconductivity. 
During investigations by the present inventors into particle-dispersed 
polymeric materials, it was found that the Vf and dispersion state of the 
filler have a great influence on the physical properties of the obtained 
composite material. Vf is a factor having a great influence on the 
electroconductivity, but as pointed out hereinbefore, an increase of Vf is 
industrially disadvantageous. Accordingly, the present inventors attempted 
to solve the above-mentioned problem mainly by controlling the dispersion 
state, and not by controlling the amount of the filler, and thus created 
the present invention. 
In accordance with the present invention, there is provided an 
electroconductive polymer composition comprising a matrix composed of a 
mixture of at least two polymers which are incompatible with each other, 
and an electroconductivity-imparting filler distributed predominantly in 
the polymer having a higher affinity with the filler, wherein said mixture 
of polymers has a composition such that the extraction ratio of the 
polymer having a higher affinity with the filler is at least 0.3. The 
extraction ratio is expressed by the following formula: 
EQU Extraction ratio=A/B 
wherein A is the amount of the polymer having a higher affinity with the 
filler, which has been extracted from the polymer mixture by a solvent 
capable of dissolving ony the polymer having a higher affinity with the 
filler, and B is the amount of the polymer having a higher affinity with 
the filler in the polymer mixture. By producing the mixing state in which 
the phase of the polymer having the filler predominantly distributed 
therein is continuous or substantially continuous, the electroconductivity 
is manifested at a high efficiency and a product having a reduced scatter 
of the electroconductivity can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
All polymers customarily used can be used as the polymer in the present 
invention. For example, a matrix can be formed by combining two polymers 
selected from polyolefins such as polyethylene and polypropylene, 
polyamides such as nylon 6 and nylon 66, polyesters such as polyethylene 
terephthalate and polybutylene terephthalate, acrylic resins such as 
polymethyl methacrylate, styrene resins such as polystyrene and 
poly-.alpha.-methylstyrene, polycarbonates, polyketones, polyvinyl 
chloride, saponified ethylene/vinyl acetate copolymers, ethylene/vinyl 
alcohol copolymers, polyvinyl acetate polyoxymethylene, polyphenylsulfone, 
and polyphenylene oxide. Note, the two combined polymers must be 
incompatible with each other. If the two polymers are completely 
compatible with each other, the mixture forms substantially one phase, and 
therefore, the effect of distributing the filler predominantly in one 
phase and substantially increasing the Vf in this phase, as intended by 
the present invention, cannot be obtained. Accordingly, in the combination 
of two polymers which are inherently incompatible with each other, if the 
mixture appears to be one phase, the intended effect can not be obtained. 
Any method of analyzing polymers can be used for determining an appropriate 
mixing ratio, for example, the differential scanning calorimetric 
measurement (hereinafter referred to as "DSC") can be utilized. 
In the case of a combination of crystalline polymers, if the mixing ratio 
is such that crystallization peaks attributed to the respective polymers 
occur, a product having a high electroconductivity and a good 
electroconductivity stability can be obtained. For example, in the case of 
a combination of polyethylene/polypropylene, if the 
polyethylene/polypropylene ratio is 60/40 or higher, it becomes difficult 
to clearly discriminate two peaks, and the effect intended by the present 
invention is not obtained in some samples or the variability of the 
product characteristics is increased. In the case of a combination of 
amorphous polymers, second-order transitions can be adopted instead of the 
crystallization peaks, for determining the mixing ratio. These 
second-order transitions can be detected as stepwise signals in DSC, but 
if the detection is difficult, peaks observed at the dynamic 
viscoelasticity measurement can be adopted. If the change of the peak 
shift as observed by DSC or the dynamic viscoelasticity measurement where 
the electroconductive filler is incorporated in a mixture of polymers is 
larger than the change of the peak shift as observed where the filler is 
incorporated in each of the polymers, then the filler is predominantly 
distributed in the polymer having a higher affinity with the filler, and 
thus the electroconductivity-imparting effect is more prominently 
manifested. 
Where kneading is carried out, aggregation is more remarkably advanced and, 
therefore, the effect becomes more prominent than where melting and 
solidification are carried out after dry blending. 
The electroconductivity of the product is determined by various factors, 
and to ensure that the effect is manifested, the mixing state must be such 
that the filler-aggregated phase is completely continuous, or even if not 
completely continuous, two adjacent parts of the filler-aggregated phase 
are separated by a very thin part of the other polymer phase having a 
lower filler concentration, to an extent such that electroconductivity is 
manifested, where a good electroconductivity is manifested at a high 
efficiency and a stable product having a small scatter of the 
electroconductivity can be obtained. 
The method of selectively extracting the filler-aggregated phase is 
preferably adopted as the method for evaluating the mixing state. If the 
ratio (A/B) of the polymer having a higher affinity with the filler, which 
has been extracted from the polymer mixture by a solvent capable of 
dissolving only the polymer having a higher affinity with the filler, to 
the amount (B) of the polymer having a higher affinity with the filler in 
the polymer mixture, is at least 0.3, the electroconductivity-imparting 
effect is stably obtained in the mixed matrix. The larger the value of 
this ratio, the higher the continuity, and the smaller the value of this 
ratio, the larger the proportion of the discontinuous part included in the 
non-aggregated phase. Theoretically speaking, even in the case of a 
smaller value, electroconductivity is manifested if a conducting path is 
formed, but practically, if the value of the above-mentioned ratio is 
below the above range, the degree of formation of conducting paths is low 
and the scatter of the electroconductivity in the product is large. 
The filler to be used in the present invention will now be described. 
Since the filler is used for imparting electroconductivity to a 
non-electroconductive plastic material, obviously the filler must be 
electroconductive. The filler is preferably a carbonaceous or metallic 
filler, or a filler, the surface of which is coated with carbon or metal. 
Further, the filler is in the form of powder, flakes or short fibers. 
Preferably the absolute size of the filler is as small as possible, because 
a fine filler is readily aggregated due to the thermal motion of the 
molecule chain of the polymer, and the electroconductivity-imparting 
effect is more easily manifested. Accordingly, preferably carbon black or 
an ultrafine fiber having an average diameter not larger than 1 .mu.m, 
such as a vapor grown carbon fiber, or a potassium titanate whisker or SiC 
whisker which is subjected to the electroconductive treatment, namely, 
surface-coated with metal, is used. A vapor grown carbon fiber, for 
example, a fiber obtained by introducing a solution of a hydrocarbon and 
an organic transition metal compound in an atmosphere maintained at a high 
temperature such as 900.degree. to 1,300.degree. C. by using hydrogen as a 
carrier gas and having a fiber diameter of 0.1 to 1.0 .mu.m, is apt to 
aggregate, and if this carbon fiber is used, the 
electroconductivity-imparting effect is prominently manifested, as 
apparent from the examples given hereinafter. Even if various carbon 
fibers or metal fibers customarily used as electroconductive fillers are 
used, composite products having a higher electroconductivity than that of 
a conventional product obtained by using one polymer as the matrix can be 
obtained. 
The amount of the electroconductive filler is preferably from 3 to 50% by 
weight, more preferably 5 to 30% by weight, based on the electroconductive 
polymer composition. If the amount of the filler is too large the melt 
fluidity is reduced and molding and processing become difficult. 
Furthermore, even if molding is possible, because of a short shot or the 
like, a satisfactory shaped article cannot be obtained, and often the 
resulting shaped article has poor mechanical properties, such as a poor 
impact strength. 
The invention will be described by the following examples. 
EXAMPLES 1 THROUGH 12 
Examples 1 through 12 were carried out under the conditions shown in Tables 
1-1 and 1-2. 
The kneading method, the materials used, and the methods of measuring 
physical properties were as described below. 
Kneading Method 
When using a mixing roller or a laboratory mixer, only the resins were 
first blended and kneaded, and the filler was added to the kneaded 
mixture. Then, kneading was conducted for 10 minutes from the point of 
completion of the addition. 
Molding Method 
The kneaded sample was set in a predetermined mold, and the sample was 
melted and deaerated, pressed for 1 minute after deaeration, and then 
cooled to obtain a molded sample. 
Matrix 
HDPE: high-density polyethylene 
PP: polypropylene 
PMMA: polymethyl metacrylate 
PS: polystyrene 
6: polyhexamethylene adipamide 
Filler 
VGCF: ultrafine fiber obtained by introducing a solution of a hydrocarbon 
and an organic transition metal compound in an atmosphere at 1,200.degree. 
C. by using hydrogen as a carrier gas, which had a fiber diameter of 0.1 
to 1.0 .mu.m (average fiber diameter was 0.3 .mu.m) 
Potassium Titanate Whisker 
Dentall BK-100 (supplied Otsuka Chemical) subjected to the 
electroconductive treatment, which had a fiber diameter of 0.2 to 0.5 
.mu.m and a fiber length of 10 to 20 .mu.m. 
Method of Measurement of Specific Electric Resistance 
In Examples 1 through 4 and 7 through 12, both surfaces of a sample having 
a thickness of about 0.5 mm were coated with a silver paste, and after 
drying, the specific resistance in the thickness direction was measured. 
In the case of a sample having a low electroconductivity, the measurement 
was carried out by using a vibrating reed electrometer (Model TR84M 
supplied by Takeda Riken), and in the case of a sample having a high 
electroconductivity, the measurement was carried out by using a Toa 
Digital Meter Model DMM-120A. 
In Examples 5, 6 and 12, the measurement was carried out by the 
four-terminal method using a Loresta supplied by Mitsubishi Petrochemical. 
Judgement of Filler-Predominantly-Distributed Phase 
The filler-predominantly-distributed phase was checked by observation under 
an SEM (scanning electron microscope) or TEM (transmission electron 
microscope). 
Extraction 
A sample having a thickness of about 0.5 mm was extracted by using a 
Soxhlet extractor. 
DSC 
The measurement was carried out by using Instruments 910 DSC supplied by du 
Pont. 
Dynamic Viscoelasticity 
The measurement was carried out by using a rheolograph supplied by Toyo 
Seiki. 
The results of the measurement of the specific electric resistance are 
shown in Table 2. 
In each of the polymer combinations, an effect of drastically reducing the 
specific electric resistance was obtained by blending. 
When only the filler-predominantly-distributed phase confirmed by the SEM 
or TEM was extracted, it was found that, if the extraction ratio was at 
least 0.3, the effect was obtained by blending (see Table 3). 
DSC 
The crystallization of samples molded in the same manner as described in 
Example 2, except that the filler was not added, was examined by DSC while 
the samples were cooled at a rate of 0.5.degree. C./min. 
More specifically, the temperature of each sample was rapidly elevated from 
room temperature to 220.degree. C. annealing was conducted for 2 minutes, 
and the sample was cooled to 160.degree. C. at a rate of 10.degree. C./min 
and then cooled at a rate of 0.5.degree. C./min. The measurement was 
conducted during this cooling process. The measurement results are shown 
in FIG. 1, wherein curves (a), (b), (c), (d), (e) and (f) correspond to 
the compositions of HDPE/PP=100/0, 80/20, 60/40, 40/60, 20/80, and 0/100, 
respectively. 
When the data in Table 2 is compared with FIG. 1, it is seen that, in a 
composition where crystallization peaks attributed to two polymers are 
clearly observed an effect of reducing the specific electric resistance by 
blending is manifested. 
Measurement of Dynamic Viscoelasticity 
In Example 7, the difference of the Tg temperature (Tg shift) between 
samples having different polymer compositions and comparative samples 
different from the samples in that the filler was not incorporated, was 
measured (see Table 4). 
The measurement was carried out at a frequency of 10 Hz and a 
temperature-elevating rate of 3.degree. C./min within a temperature range 
of from room temperature to 110.degree. C. 
When Table 4 is compared with Table 1, it is seen that the effect of 
reducing the specific electric resistance by blending increases in 
proportion to the Tg shift of PMMA of the filler-predominantly-distributed 
phase. 
TABLE 1 
__________________________________________________________________________ 
Sample-Preparation Conditions 
Amount Kneading Molding 
of filler 
Kneading 
temperature 
Molding 
temperature 
Matrix Filler (phr) 
apparatus 
(.degree.C.) 
apparatus 
(.degree.C.)*.sup.12 
__________________________________________________________________________ 
Example 1 
HDPE*.sup.1 /PP*.sup.2 
Carbon black*.sup.8 
15 Mixing roller 
180-185 
Press 
200 
Example 2 
HDPE*.sup.3 /PP*.sup.4 
" 10 " 180-185 
" 200 
Example 3 
" VGCF*.sup.9 
10 " 180-185 
" 200 
Example 4 
" Potassium 
10 " 180-185 
" 200 
titanate 
whisker*.sup.10 
Example 5 
HDPE*.sup.5 /PP*.sup.6 
VGCF*.sup.9 
25 Labo plasto 
230 " 180 
mill*.sup.11 
Example 6 
HDPE*.sup.7 /PP*.sup.6 
" 25 Labo plasto 
230 " 180 
mill*.sup.11 
Example 7 
PMMA*.sup.13 /PP*.sup.4 
Carbon black*.sup.8 
10 Mixing roller 
180-185 
Press 
185 
Example 8 
" VGCF*.sup.9 
10 " 180-185 
" 185 
Example 9 
" Potassium 
10 " 180-185 
" 185 
titanate 
whisker*.sup.10 
Example 10 
PMMA*.sup.13 /PS*.sup.14 
Carbon black*.sup.8 
10 " 180-185 
" 185 
Example 11 
" VGCF*.sup.9 
10 " 180-185 
" 185 
Example 12 
6*.sup.15 /PP*.sup.16 
" 25 " 230 " 200 
__________________________________________________________________________ 
Note 
*.sup.1 High-density polyethylene supplied by Mitsubishi Petrochemical. 
*.sup.2 Polypropylene supplied by Mitsubishi Petrochemical. 
*.sup.3 High-density polyethylene, Sholex S5008 supplied by Showa Denko, 
MI = 0.65. 
*.sup.4 Polypropylene, Shoallomer FA110 supplied by Showa Denko, MFI = 
1.2. 
*.sup.5 High-density polyethylene, Sholex F6200V supplied by Showa Denko, 
MI = 20. 
*.sup.6 Polypropylene, Shoallomer MA510 supplied Showa Denko, MFI = 12. 
*.sup.7 High-density polyethylene, Sholex 5012 M supplied by Showa Denko, 
MI = 1.2. 
*.sup.8 Carbon black, Seast 300 supplied by Tokai Carbon. 
*.sup.9 VGCF, vapor grown carbon fiber. 
*.sup.10 Dentall BK100 supplied by Otsuka Chemical. 
*.sup.11 Supplied by Toyo Boldwin. 
*.sup.12 At the indicated temperatures, each molten sample was deaerated 
and then maintained under a pressure for one minute. 
*.sup.13 Polymethyl methacrylate, MF supplied by Mitsubishi Rayon. 
*.sup.14 Polystyrene supplied by Yoneyama Chemical, MW = 150,000-170,000. 
*.sup.15 Polyhexamethylene adipamide, Technyl A216 supplied by Showa 
Denko. 
TABLE 2 
______________________________________ 
Specific Electric Resistance 
Specific electric resistance (.OMEGA. .multidot. cm.sup.-1) 
______________________________________ 
Example 1 
HDPE 100% HDPE/PP = 20/80 PP 100% 
10.sup.17 10.sup.5 10.sup.18 
Example 2 
HDPE 100% HDPE/PP PP 100% 
10.sup.18 20/80 40/60 60/40 80/20 
10.sup.18 
10.sup.7 10.sup.9 10.sup.18 10.sup.18 
Example 3 
HDPE 100% HDPE/PP = 20/80 PP 100% 
10.sup.18 10.sup.4 10.sup.18 
Example 4 
HDPE 100% HDPE/PP = 20/80 PP 100% 
10.sup.18 5 .times. 10.sup.6 
10.sup.18 
Example 5 
HDPE 100% HDPE/PP = 40/60 PP 100% 
4 .times. 10.sup.0 
7 .times. 10.sup.-1 
2 .times. 10.sup.1 
Example 6 
HDPE 100% HDPE/PP = 40/60 PP 100% 
3 .times. 10.sup.1 
10.sup.0 2 .times. 10.sup.1 
Example 7 
PMMA 100% PMMA/PP PP 100% 
10.sup.18 20/80 40/60 60/40 
10.sup.18 
2 .times. 10.sup.6 10.sup.5 10.sup.8 
Example 8 
PMMA 100% PMMA/PP PP 100% 
10.sup.18 20/80 40/60 60/40 
10.sup.18 
5 .times. 10.sup.4 2 .times. 10.sup.3 7 .times. 
10.sup.5 
Example 9 
PMMA 100% PMMA/PP PP 100% 
10.sup.18 20/80 40/60 60/40 
10.sup.18 
7 .times. 10.sup.6 5 .times. 10.sup.5 10.sup.8 
Example 10 
PMMA 100% PMMA/PS = 60/40 PS 100% 
10.sup.18 10.sup.5 5 .times. 10.sup.16 
Example 11 
PMMA 100% PMMA/PS = 60/40 PS 100% 
10.sup.18 8 .times. 10.sup.3 
10.sup.16 
Example 12 
6 100% 6/PP = 40/60 PP 100% 
10.sup.3 2 .times. 10.sup.0 
2 .times. 10.sup.1 
______________________________________ 
TABLE 3 
__________________________________________________________________________ 
Extraction Ratio of Filler-Predominantly-Distributed Phase 
Extraction 
Filler- Ratio of Filler- 
Predominantly Polymer Predominantly 
Distributed composition 
Distributed 
Phase Solvent 
ratio Polymer 
__________________________________________________________________________ 
Example 7 
PMMA Chloroform 
PMMA/PP = 20/80 
0.54 
PMMA/PP = 40/60 
0.92 
PMMA/PP = 60/40 
0.92 
Example 8 
PMMA Chloroform 
PMMA/PP = 20/80 
0.35 
PMMA/PP = 40/60 
0.85 
PMMA/PP = 60/40 
0.90 
Example 9 
PMMA Chloroform 
PMMA/PP = 20/80 
0.40 
PMMA/PP = 40/60 
0.88 
PMMA/PP = 60/40 
0.88 
Example 10 
PMMA Acetic acid 
PMMA/PS = 60/40 
0.89 
Example 11 
PMMA Acetic acid 
PMMA/PS = 60/40 
0.93 
Example 12 
6 Formic acid 
6/PP = 40/60 
0.95 
__________________________________________________________________________ 
TABLE 4 
______________________________________ 
Tg and Tg Shift in Example 7 
(PMMA/PP system) 
Amount of 
Matrix carbon black PMMA Tg Tg shift 
PMMA/PP (phr) (.degree.C.) 
(.degree.C.) 
______________________________________ 
20/80 10 102 4.2 
-- 97.8 
40/60 10 100 4.3 
-- 95.7 
60/40 10 97.8 2.4 
-- 95.4 
100/0 10 95.1 0.9 
-- 94.2 
______________________________________ 
As apparent from the results of the foregoing examples, if a mixture of at 
least two polymers which are in compatible with each other is prepared, 
and electroconductive filler is predominantly distributed in the polymer 
having a higher affinity with the filler, an electroconductive polymer 
composition having a higher conductivity than that obtained by blending 
each of the polymers as the matrix with the same filler can be obtained. 
If the solvent extraction method, the differential scanning calorimetric 
measurement method or the dynamic viscoelasticity measurement method is 
adopted, to obtain an appropriate mixing ratio, a preferred range of the 
mixing ratio can be easily determined.