Process for producing self-restoring over-current protective device by grafting method

A process for producing a self-restoring overcurrent protective device by the grafting method, wherein an organic peroxide is added to colloidal graphite, at least one kind of carbon black selected from among acetylene black, Ketjen black and furnace black having a high structure, and at least one kind of crystalline polyner substance while heating and milling the latter three components, and the heated mixture having a high viscosity is forcibly milled, whereby the organic peroxide is reacted with the polymer substance to give unpaired electrons to the polymer substance to thereby form polymer radicals. Subsequently, the formed polymer radicals are preferentially grafted onto the above-mentioned graphite and carbon black to form a milled mass wherein the grafting products are homogeneously dispersed in the above-mentioned polymer substance. The milled mass is molded into a predetermined shape while it still retains thermoplasticity. Subsequently, the above-mentioned organic peroxide not involved in the formation of the above-mentioned polymer radicals is thermally decomposed to crosslink the above-mentioned grafting products and polymer substance, whereby a molding having a three-dimensional network structure is obtained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing a self-restoring 
overcurrent protective device which undergoes heat buildup upon flowing 
therethrough of an overcurrent to increase the resistance thereof to 
thereby limit the current and is reversibly self-restored stably to an 
original state thereof upon returning of a circuit to a normal state 
thereof by utilizing the phenomenon of the same as a positive temperature 
coefficient thermistor (hereinafter referred to briefly as the "PTC"). 
2. Description of the Prior Art 
Conventional processes for producing a resistor or a device which utilizes 
the PTC characteristics thereof include the following ones: 
(i) a process for producing a resistor having PTC characteristics which 
comprises sintering a semiconductor of a barium titanate type at a high 
temperature to form a device; 
(ii) a process for producing a relatively low resistance PTC device 
comprising a polymer substance and carbon black incorporated thereinto, an 
example of which is a process for producing a device having PTC 
characteristics as disclosed in U.S. Pat. No. 4,237,441 which comprises 
molding a mixture of a crystalline polymer and carbon black into a 
predetermined shape with an extruder and irradiating the resulting molding 
with a radiation such as electron beams to crosslink the crystalline 
polymer between the molecules thereof to form a network structure, thereby 
improving the molding in the thermal deformation thereof; 
(iii) a process for producing a resistor having PTC characteristics as 
disclosed in, for example, Japanese Patent Laid-Open No. 8,443/1981 which 
comprises molding a mixture of a rubbery substance, carbon black, 
graphite, an organic peroxide, and the like into a predetermined shape and 
heating the resulting molding to decompose for the first time the organic 
peroxide whereupon a network structure is given to the rubbery substance 
to improve the molding in the thermal deformation thereof; and 
(iv) a process for producing a resistor having PTC characteristics as 
disclosed in, for example, Japanese Patent Publication No. 36,876/1976 
which comprises graft-copolymerizing a vinyl monomer onto carbon black in 
a solvent, adding a cross-linking agent to the resulting kneaded mass, and 
heating the resulting mixture to give a network structure thereto for 
attaining an improvement in the thermal resistance thereof. 
The above-mentioned conventional process (i) comprising sintering a 
semiconductor of a barium titanate type at a high temperature involves 
problems that, since the resulting device has a high volume resistivity, 
the voltage drop of a circuit at a steady-state current is large, that, 
when the temperature of the device is further raised after the 
manifestation of PTC characteristics, the device turns into a negative 
temperature coefficient thermistor (hereinafter referred to briefly as the 
"NTC") so that the current-limiting function thereof is drastically 
reduced, and that scattering of resistance values is liable to occur due 
to the deformation of the device caused by sinter molding at a high 
temperature. 
The above-mentioned conventional processes (ii) and (iii) comprising 
crosslinking a crystalline polymer substance admixed with carbon black or 
a rubbery substance admixed with carbon black and graphite provide a 
thermally stable PTC material as a heater which acts as an overcurrent 
protective device, but involve a problem that part of carbon black 
particles or part of carbon black and graphite particles move due to 
segment expansion and contraction in a crosslinked network structure 
during the course of repeated current-limiting actions of the PTC device 
as the overcurrent protective device to lower the reproducibility of PTC 
characteristics and resistance value between the repeated actions and 
particularly to largely vary the resistance value therebetween. 
The above-mentioned conventional process (iv) comprising 
graft-copolymerizing a vinyl monomer onto carbon black in a solvent 
involves a problem that the compatibility of the resulting crystalline 
polymer substance with the solvent during the course of graft 
copolymerization is so problematic because of the use of the solvent in 
the graft copolymerization that polyethylene and polypropylene which are 
crystalline polymer substances effective in manifestation of PTC 
characteristics cannot be employed. 
It is known to use an organic peroxide, such as dicumyl peroxide, as a 
network-forming agent for an ethylene-propylene rubber and the like. Where 
such an organic peroxide is added to a rubber and they are roll-milled, 
roll milling is conducted at a comparatively low temperature, for example, 
around 50.degree. C., for the purpose of preventing gelation (network 
formation) during the course of milling. In an unavoidable case, 
particularly in the case of using a crystalline substance such as 
polyethylene, a method like one in which addition of the organic peroxide 
is completed in a comparatively short time is employed with consideration 
given to an indication of the thermal decomposition rate of the organic 
peroxide, namely the half-life thereof. This is done for the purpose of 
suppressing the decomposition of the organic peroxide as much as possible 
during the course of milling. 
Accordingly, milling of a polymer substance with an organic peroxide at or 
above the thermal decomposition temperature thereof to allow both to react 
with each other during the course of milling has heretofore been avoided 
as much as possible. 
Meanwhile the inventors of the present invention have found an interesting 
fact that, when an adequate amount of an organic peroxide is added while 
milling a crystalline polymer substance in the presence of graphite and 
carbon black, the organic peroxide does not serve as a crosslinking agent 
for the polymer but, instead, acts as a grafting agent to enable the 
polymer to be grafted onto the surfaces of graphite and carbon black 
particles even during the course of milling at or above the thermal 
decomposition temperature of the organic peroxide. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for producing a 
self-restoring overcurrent protective device having a low resistance value 
and PTC characteristics particularly with a good reproducibility of PTC 
characteristics and resistance value in repeated current-limiting actions 
according to the grafting method. 
In accordance with an embodiment of the process for producing a 
self-restoring overcurrent protective device by the grafting method 
according to the present invention, in the first step thereof, an organic 
peroxide is added to colloidal graphite, at least one kind of carbon black 
selected from among acetylene black, Ketjen black and furnace black having 
a high structure, and at least one kind of crystalline polymer substance 
while heating and milling the latter three components, and the heated 
mixture having a high viscosity is forcibly milled, whereby the organic 
peroxide is reacted with the polymer substance to give unpaired electrons 
to the polymer substance to thereby form polymer radicals. Subsequently, 
the formed polymer radicals are preferentially grafted onto the 
abovementioned graphite and carbon black to form a milled mass wherein the 
grafting products are homogeneously dispersed in the above-mentioned 
polymer substance. The milled mass is molded into a predetermined shape 
while it still retains thermoplasticity. Subsequently, in the second step, 
the above-mentioned organic peroxide not involved in the formation of the 
above-mentioned polymer radicals is thermally decomposed to crosslink the 
above-mentioned grafting products and polymer substance, whereby a molding 
having a three-dimensional network structure is obtained. 
In accordance with another embodiment of the present invention, a first 
organic peroxide for grafting a polymer substance onto graphite and carbon 
black and a second organic peroxide for crosslinking added in respective 
different steps in the above-mentioned embodiment. 
In accordance with a further embodiment of the present invention, after the 
milled mass has been molded, the molded mass is irradiated with radial ray 
to crosslink the grafting products and polymer substance, whereby a 
molding having a three-dimensional network structure is obtained in the 
first embodiment. 
According to the present invention, since colloidal graphite and carbon 
black are added to a crystalline polymer substance and part of the 
crystalline polymer substance is grafted onto the surfaces of the graphite 
and carbon black particles in the presence of an organic peroxide while 
heating and milling them with a mixing roll mill or the like, a solution 
is worked out for a problem that grafting of the polymer substance onto 
graphite or carbon black particles alone increases the resistance value of 
the resulting device though it improves the dispersibility of the 
particles in the polymer substance, and a stable PTC device which has a 
low resistance and can resist repeated current-limiting actions is 
obtained. 
Other objects and features of the present invention will be apparent while 
illustrating the invention with reference to the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to the description of Examples of the present invention, description 
will be made of (i) colloidal graphite, carbon black having a high 
structure, a crystalline polymer substance, an organic peroxide, and the 
like; (ii) the mechanism of a graft reaction and that of network 
formation; (iii) an experiment conducted for confirming that a polymer 
substance is grafted onto graphite particles; and (iv) an experiment 
conducted for confirming the suitable amount of an organic peroxide 
consumed as a grafting agent. 
(i) Description will be made of colloidal graphite, carbon black having a 
high structure, a crystalline polymer substance, and an organic peroxide. 
Colloidal graphite is a powder prepared by pulverizing graphite into fine 
particles with a mechanical means and has oxygen-containing groups on the 
surfaces of the particles thereof. 
Furnace black having a high structure is composed of aggregates consisting 
of a large number of fine particles connected to each other in a 
chain-like form, and is desired to have an oil absorption of 1 ml/g or 
more as an indication of the structure. 
Carbon black is mixed with graphite since the mixture can impart a lower 
resistance value to the resulting device than graphite alone. The mixing 
ratio of graphite to carbon black is desirably in the range of 1:9 to 8:2 
by weight. One or two kinds of carbon blacks are selected from among 
furnace black, acetylene black, and Ketjen black depending on the 
resistance value required of the resulting device. The resistance value of 
the device can be reduced in the order of furnace black, acetylene black, 
and Ketjen black. 
Although the mixing ratio of the amount of graphite and carbon black 
particles to that of a crystalline polymer substance can be varied 
depending on the desired resistance value of the resulting device, a 
device having desirable physical strengths and a low resistance value can 
be obtained when the above-mentioned mixing ratio is in the range of 6:4 
to 3:7 by weight. 
The crystalline polymer substance is a polymer having a melting point of, 
for example, 90.degree. to 180.degree. C. and desirably containing 
hydrogen atoms or methylene groups bonded to tertiary carbon atoms in its 
structure, examples of which include low-density polyethylene, 
medium-density polyethylene, high-density polyethylene, polypropylene, and 
polyesters. The crystalline polymer substance is used for the purpose of 
notably manifesting the PTC characteristics of the resulting device around 
the melting point of the polymer substance. 
The organic peroxide is a peroxide involving no danger of explosion during 
the course of milling therewith the graphite particles and the crystalline 
polymer substance at a temperature ranging from 90.degree. C. to 
200.degree. C., allowing the milling operation to be comparatively easily 
effected, and having a capability of reacting with the crystalline polymer 
substance to give unpaired electrons to the polymer substance, examples of 
which include bis (.alpha., .alpha.'-dimethylbenzyl)-peroxide (dicumyl 
peroxide, hereinafter referred to briefly as "Di-Cup") and 
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 (hereinafter referred to 
briefly as "TBPH-3"). Besides, use may be made of 
2,5-dimethyl-2,5-di(t-butylperoxy)-hexane, 
1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, or the like. 
The term "suitable amount of the organic peroxide" means an amount thereof 
involving no fear of gelation due to network formation during the course 
of milling therewith the graphite particles, the carbon black particles 
and the crystalline polymer substance, and still being enough to enable 
part of the polymer substance to be grafted onto the surfaces of the 
graphite particles and the carbon black particles. This amount is 
determined depending on the amount of polymer radicals to be formed which 
are to be preferentially captured by the surfaces of the graphite 
particles and the carbon black particles during the course of milling. The 
suitable amount can be determined by rough calculation from the number of 
radical-capturing sites on the surfaces of these particles, which is 
regarded as 10.sup.20 /g, and the molecular weight of an organic peroxide 
to be used. For example, in the case of Di-Cup, since the molecular weight 
thereof is 270, a suitable amount thereof is about 5 g per 100 g of 
particles (of graphite and carbon black) on the assumption that the 
proportion of Di-Cup to effectively act on the crystalline polymer 
substance is about 80% with consideration given to the secondary 
decomposition thereof to form acetophenone and the like. Moreover, since 
the half-life of Di-Cup is known to be 1 minute at 171.degree. C., the 
organic peroxide remaining in milling serves to form a network in the 
resulting device. Furthermore, it is desirable in network formation to add 
an organic peroxide which decomposes at a temperature higher than the 
decomposition temperature of Di-Cup. An example of such an organic 
peroxide is TBPH-3 which is known to have a half-life of 1 minute at 
193.degree. C. Addition of TBPH-3 to a milled mass as mentioned 
hereinbefore is desirable because it provides little function and effect 
of TBPH-3 as a network-forming agent by virtue of its low thermal 
decomposition rate even when the resulting mixture is further milled at a 
temperature around 140.degree. C. In other words, gelation of the 
above-mentioned milled mass can be completely suppressed. 
Although the conditions of formation of a three-dimensional network 
structure to be given to a device depend on the thermal stability of an 
organic peroxide to be used, it is generally desirable that they involve a 
heat treatment temperature of 160.degree. to 200.degree. C. and a heat 
treatment time of 5 to 60 minutes. 
In case of irradiating radial ray, it is desirable to use .gamma. ray of 
5-40 Mrad. 
(ii) Description will be made of the mechanism of a graft reaction and that 
of network formation by taking as an example a case where polyethylene is 
used as the crystalline polymer while Di-Cup is used as the organic 
peroxide. 
Firstly, in the step of heating and milling, Di-Cup decomposed according to 
the following formula: 
##STR1## 
Subsequently, part of hydrogen atoms present in the main chain of a 
polyethylene molecule are abstracted by RO. to form polyethylene radicals 
(P.): 
##STR2## 
(Note) This reaction formula shows abstraction of a hydrogen atom which 
occurs at a branched site of a polyethylene molecule. 
If RO. is bonded to a phenoxy radical or the like present on the surface of 
a graphite particle or a carbon black particle, a peroxide is formed. 
Since this peroxide is unstable, however, a graft reaction involving 
bonding of RO. to P. and represented by the following formula 
preferentially proceeds: 
##STR3## 
In the formula, CB indicates a graphite or carbon black particle. 
Among others, grafting proceeds in the presence of graphite and carbon 
black particles in advance of a crosslinking reaction of P. with another 
P.. Thus, grafting can be preferentially allowed to proceed while 
suppressing network formation by the reaction between molecules of the 
polymer substance when the amount of an organic peroxide capable of 
forming P necessary for grafting is determined beforehand on the basis of 
the molecular weight of the organic peroxide and with consideration given 
to the number of reactive sites present on the surfaces of graphite and 
carbon black particles (approximately 10.sup.20 /g; see Kumakazu Okita, 
"Carbon Black no Graft-ka (Grafting onto carbon Black)" published by K.K. 
Rubber Digest) and the milling temperature is controlled with 
consideration given to the half-life of the organic peroxide resulting 
from the thermal decomposition thereof. In this way, graphite particles 
and carbon black particles can be homogeneously dispersed in the polymer 
substance with the progress of grafting while retaining the 
thermoplasticity inherent in the polymer substance. This provides a 
feature that the resistance value of the resulting device is uniform 
everywhere therein. 
After the grafting, the milled mass is formed into a suitable device and 
then exposed to a temperature condition capable of decomposing the whole 
of the organic peroxide to provide a three-dimensional network structure 
inside the resulting device. Alternatively, the same organic peroxide as 
that first added or a second new organic peroxide of a different kind is 
further added in an amount necessary for providing a denser network 
structure of the crystalline polymer substance to the milled mass, and the 
resulting mixture is milled, formed into a device, and then heated while 
keeping the above-mentioned shape to complete the crosslinking reaction. 
The second organic peroxide to be newly added is desirably a compound 
having a relatively high decomposition temperature, a suitable example of 
which is TBPH-3. 
FIG. 1 is a model diagram of a graft polymer comprising polymer molecules 
grafted onto graphite and carbon black in the obtained device, wherein 
graft polymer molecules 3 extend in a chain-like form from graphite 
particles 1 and carbon black particles 2 and are connected to each other 
at crosslinkage sites 4 to form a network structure. 
(iii) Description will be made of an experiment conducted for confirming 
that a polymer substance is grafted onto graphite particles. 
If isolation of a grafting product is possible, it can be confirmed by 
examining the thermally decomposed matter of polymer combined with the 
surfaces of particles thereof according to gas chromatography that a 
crystalline polymer substance such as polyethylene or polypropylene is 
grafted onto the surfaces of graphite particles by using an organic 
peroxide such as Di-Cup during the course of milling with a heated roll 
mill. Since no suitable solvent was available, however, grafting was 
indirectly confirmed by using a homopolymer of 2-ethylhexyl methacrylate 
(hereinafter referred to briefly as "P-OMA") having hydrogen atoms bonded 
to a methylene chain and tertiary carbon atoms in its structure as a 
polymer substance having a structure analogous to those of polyethylene, 
polypropylene, and the like. Confirmation was also made of the phenomenon 
that polymer radicals were grafted onto the surfaces of graphite particles 
and the fact that the grafting sites of graphite particles have a 
structure of a phenoxy radical. 
Confirmation of grafting of P-OMA was made according to the following 
method. 
P-OMA synthesized by another method was milled together with graphite with 
a heated roll mill at a blending ratio as shown in Table 1. 
TABLE 1 
______________________________________ 
Amounts of Materials in Milled Mass [g] 
Sample Graphite P-OMA Di-Cup 
______________________________________ 
A 20 30 0.8 
B 20 30 0 
C 0 30 0.8 
______________________________________ 
Subsequently, the milled mass was dispersed in methyl isobutyl ketone 
(hereinafter referred to briefly as "MIBK") and the resulting dispersion 
was subjected to centrifugal separation at 4,000 rpm for one hour, 
followed by observation of the state of the supernatant liquid. Sample A 
alone gave a black supernatant liquid, while Samples B and C gave 
transparent supernatant liquids. The black supernatant liquid resulted 
from grafting of P-OMA onto the surfaces of the graphite particles. 
Specifically, a radical (RO.) formed by thermal decomposition of Di-Cup 
abstracts a hydrogen atom bonded to a tertiary carbon atom of P-OMA to 
form a polymer radical (P.), which then reacts with a phenoxy radical 
present on the surface of a graphite particle to graft thereonto. 
Graphite particles onto which P-OMA is grafted in the above-mentioned way 
are improved in dispersibility in MIBK so that they are less liable to 
precipitate by centrifugal separation. This substantiates that, when 
graphite and P-OMA are milled together with Di-Cup with a heated roll 
mill, P-OMA is grafted onto the surfaces of graphite particles. 
By isolating a grafting product from the milled mass and examining the 
thermally decomposed matter thereof according to gas chromatography, it 
was confirmed that the grafting product was P-OMA. 
Another experiment will be described to substantiate grafting of polymer 
radicals onto the surface of graphite particles. 
It has heretofore been said that the surfaces of graphite particles are 
inactive. However, the inventors of the present invention have found that 
active free radicals, namely unpaired electrons, are present on the 
surfaces of graphite particles and these unpaired electrons easily react 
with polymer radicals to bring about grafting. For example, 1 g of 
graphite particles and 20 cc of styrene were well stirred at 90.degree. C. 
for 20 hours to effect a reaction, and the reaction product was dispersed 
in 60 cc of toluene. For comparison, 20 cc of styrene alone was 
polymerized at 90.degree. C. for 20 hours and the polymerization product 
was mixed with 1 g of graphite particles and 60 cc of toluene to prepare a 
dispersion. 
The two dispersions were allowed to stand for 2 days and were observed. All 
graphite particles precipitated in the dispersion prepared by merely 
mixing the materials, while the supernatant liquid of the dispersion 
prepared by reacting graphite particles with styrene assumed the color of 
graphite though there was a precipitate. 
The fact that the supernatant liquid assumed the color of graphite 
substantiates that polystyrene radicals reacted with unpaired electrons 
present on the surfaces of graphite particles to bring about grafting. 
From the above results, it can be easily presumed that, when graphite 
particles and a crystalline polymer substance such as polyethylene or 
polypropylene are milled in the presence of an organic peroxide with a 
heated roll mill, polymer radicals are grafted onto the surfaces of the 
graphite particles. 
An experiment was conducted to confirm that the main oxygen-containing 
groups present on the surfaces of graphite particles are phenoxy radicals 
Use was made of a difference in reactivity between benzoyl peroxide 
(hereinafter referred to briefly as "Bz.sub.2 O.sub.2 ") and .alpha., 
.alpha.'-azobisisobutyronitrile (hereinafter referred to briefly as 
"AIBN") as polymerization initiators for effecting a reaction of graphite 
with polystyrene. If phenoxy radicals are present on the surfaces of 
graphite particles, two kinds of reactions, namely bonding of phenoxy 
radicals to 2-cyano-2-propyl radicals formed by thermal decomposition of 
AIBN and bonding of phenoxy radicals to polystyrene radicals, must compete 
with each other in a reaction system using AIBN. In this case, if phenoxy 
radicals are bonded to 2-cyano-2-propyl radicals, the dispersibility of 
the reaction product in solvents is poor. In contrast, where Bz.sub.2 
O.sub.2 is used, only grafting of polystyrene proceeds so that a colloidal 
dispersion of the reaction product is more stable. In this experiment, two 
reaction systems, namely one composed of 1 g of graphite particles, 20 cc 
of styrene and 0.3 g of Bz.sub.2 O.sub.2, and one composed of 1 g of 
graphite particles, 20 cc of styrene and 0.2 g of AIBN, were each stirred 
at 80.degree. C. for one hour to effect respective reactions for 
comparison, Bz.sub.2 O.sub.2 and AIBN were used in substantially the same 
molar amount. The two reaction products were each dispersed in 40 cc of 
toluene and allowed to stand at room temperature for 5 days. Colloidal 
particles in the dispersion of the reaction product prepared using 
Bz.sub.2 O.sub.2 were more stable than those in the dispersion of the 
reaction product prepared using AIBN. The above results may be understood 
to prove the presence of phenoxy radicals on the surfaces of graphite 
particles. 
Reference materials relevant to the above experiments include: 
1 "Amimekozo o motsu Carbon Black Graft Polymer (Carbon Black Graft 
Polymer Having Network Structure)" (Okita et al., Journal of the Society 
of Rubber Industry, Japan, Vol. 44, No. 1, pp. 63 to 68, 1971); 
2 "Carbon Black Graft Polymer no Denkiteki Seishitsu (Electrical 
Properties of Carbon Black Graft Polymer)" (Tsubata et al., 
Niigata-daigaku Kogaku-bu Kenkyu Hokoku, No. 15 pp. 71 to 81, 1966); 
3 "Carbon Black Graft Polymer (2)" (Okita, Polymer no Tomo, Vol. 2, 
(10), pp. 10 to 17, 1965); and 
4 "Carbon Black Graft polymer (3)" (Okita, Polymer no Tomo, Vol. 2, 
(11), pp. 8 to 17, 1965). 
(iv) Description will be made of an experiment conducted for confirming the 
suitable amount of an organic peroxide consumed as a grafting agent. 
The organic peroxide has the roles of a grafting agent and a crosslinking 
agent for a crystalline polymer substance. However, the organic peroxide 
is consumed as the grafting agent in the presence of graphite and carbon 
black particles because of a preferential reaction of grafting of polymer 
radicals onto the surfaces of the above-mentioned particles, while the 
polymer is crosslinked with any surplus of the organic peroxide. 
Accordingly, the suitable amount of an organic peroxide as the grafting 
agent can be found by examining the extents of gelation, due to network 
formation, of milled masses respectively containing appropriately varied 
amounts of the organic peroxide during the course of milling. Further, 
whether or not any amount of the organic peroxide remains in a milled mass 
can be confirmed by the state of molding of the milled mass. 
Materials and amounts of blending thereof used in the experiment are shown 
in Tables 2 and 3, respectively. 
TABLE 2 
______________________________________ 
Materials 
Function and name 
Maker 
(abbreviation) (grade) Remarks 
______________________________________ 
natural graphite 
Nippon Kokuen 
particle 
powder Kogyo K.K. size: 6 .mu.m 
Conductive 
(graphite) (ACP-1000) 
particles 
furnace black 
Cabot, U.S.A. 
oil absorp- 
(CB) (Vulcan XC-72) 
tion: 
1.78 ml/g 
Mitsui Petro- 
Crystalline 
polyethylene chemical Indus- 
M.P.; 131.degree. C. 
polymer (PE) tries, Ltd. 
(Hi-Zex 1300J) 
Grafting 
dicumyl peroxide 
Nippon Oil and 
half-life: 
agent and 
(Di-Cup) Fats Co., Ltd. 
1 min. 
crosslink- (Percumyl D) (171.degree. C.) 
ing agent 
______________________________________ 
TABLE 3 
______________________________________ 
Blending of Materials (g) 
Sample CB Graphite PE Di-Cup 
______________________________________ 
I 40 60 100 0 
II 40 60 100 2 
III 40 60 100 4 
IV 40 60 100 6 
V 40 60 100 8 
______________________________________ 
A test mixing roll mill was used as a milling apparatus. 
size of rolls: 150 mm .phi..times.300 mm 
rotation of rolls: 
front roll: 20 rpm 
back roll: 25 rpm 
heating system: Dowtherm oil vapor 
roll spacing during milling: about 0.5 mm 
The procedure of milling is as follows. 
1 roll surface temperature is set at about 140.degree. C. 
2 A predetermined amount of high-density polyethylene is placed on the 
rolls. It is molten into sticky matter and wound around the rolls. 
3 A predetermined amount of graphite particles are placed on the rolls. 
A turnover operation with a metallic spatula is continued for 5 minutes. 
4 A predetermined amount of carbon black is placed on the rolls. The 
turnover operation with the metallic spatula is continued for about 15 
minutes. 
5 A predetermined amount of Di-Cup is incorporated into a milled mass 
over about one minute while continuing the turnover operation with the 
metallic, spatula 
6 The state of the milled mass is observed while continuing the turnover 
operation with the metallic spatula. 
Since a milled mass after placing a predetermined amount of CB on the rolls 
and continuing the turnover operation with the metallic spatula for about 
15 minutes in the step 4 of the procedure of milling has a sufficient 
thermoplasticity and assumes a slightly sticky state, it sometimes happens 
that it sticks to the surfaces of the rolls so that the turnover operation 
cannot smoothly be carried out. With this state of the milled mass as a 
standard, the variation in the state of the milled mass is observed while 
continuing the turnover operation. The obtained results will be shown 
hereinbelow. Milling was terminated after 30 minutes from the beginning 
thereof, except for the case of Sample No. V where milling was terminated 
after 10 minutes because it became leather-like after 10 minutes. 
Sample No. I (Di-Cup: 0 g): without incorporation of Di-Cup, milling was 
continued for 30 minutes. The state of a milled mass did not change at 
all. 
Sample No. II (Di-Cup: 2 g): about 5 minutes after incorporation of the 
predetermined amount of Di-Cup, a milled mass began to become slightly 
hard so that the turnover operation became easy. Thereafter, no change in 
the state of the milled mass was recognized. 
Sample No. III (Di-Cup: 4 g): about 5 minutes after incorporation of the 
predetermined amount of Di-Cup, a milled mass began to become slightly 
hard so that the turnover operation became easy. 9 minutes thereafter, the 
milled mass began to become leather-like. Milling was continued. 
Sample No. IV (Di-Cup: 6 g): about 4 minutes after incorporation of the 
predetermined amount of Di-Cup, a milled mass began to become slightly 
hard so that the turnover operation became easy. 8 minutes thereafter, the 
milled mass began to become leather-like. Milling was continued. The 
milled mass was more leather-like than that of Sample No. III. 
Sample No. V (Di-Cup: 8 g): about 4 minutes after incorporation of the 
predetermined amount of Di-Cup, a milled mass began to become 
leather-like. 6 minutes thereafter, the milled mass was hardened to become 
completely leather-like so that milling was terminated. 
From the above results, it can be understood that, when up to 4 g of Di-Cup 
is used, a milled mass does not become leather-like. This suggests that 
the reaction of polymer radicals onto the surfaces of electrically 
conductive particles is preferential to the reaction of network formation 
of the polymer. If the crosslinking reaction of the polymer proceeded 
simultaneously, the milled mass must have turned into a leather-like mass 
because of network formation. When the amount of Di-Cup is 4 g or larger, 
a milled mass becomes leather-like during milling. This suggests that, 
once the preferential reaction of polymer radicals onto the surfaces of 
electrically conductive particles is completed, a surplus of Di-Cup serves 
as the crosslinking agent for the polymer to promote network formation in 
a milled mass. 
From the above, if may be presumed that the necessary amount of Di-Cup as 
the grafting agent is about 4 g per 100 g of electrically conductive 
particles. This value is close to the theoretically calculated value 
mentioned hereinbefore. 
Description will be made of the state of molding of a milled mass and the 
residual organic peroxide. A milled mass was crushed, with a crusher, into 
chips having a size of about 1 to 5 mm, which was used as a molding 
material. Molding was conducted with a 26 t compression molding machine 
provided with a mold. The procedure of molding comprises the following 
steps 1 to 5 . 
1 about 5 g of a molding material is weighed, 
2 the molding machine is provided with the mold, which is then heated to 
a temperature of 180.degree. C., 
3 the molding material is placed into the mold, pressed with the molding 
machine (50 kg/cm.sup.2), and kept in the mold for 5 minutes, 
4 immediately thereafter, the mold is taken out of the molding machine 
and opened, and 
5 the state of molding is observed. 
The results of observation of the state of molding were as follows. In the 
cases of Samples Nos. I and II, moldings were not hardened, thus proving 
that molding was impossible. In the case of Sample No. III, a molding was 
slightly hardened but the shape of the molding was not desirable. In the 
cases of Samples Nos. IV and V, moldings were hardened, thus proving that 
molding was possible. 
From the above results, it may be presumed that the milled masses of 
Samples Nos. IV and V contained the residual organic peroxide, which 
contributed to crosslinking of the polymer to form a three-dimensional 
network structure during milling. 
Examples will now be described. 
EXAMPLES 1 
First and second organic peroxides were separately added as a grafting 
agent and a crosslinking agent, respectively, during the course of milling 
with a heated roll mill to graft polyethylene onto the surfaces of carbon 
black and graphite particles and further crosslink the grafted 
polyethylene and the ungrafted polyethylene between molecules thereof to 
thereby form a network. 
40 g of furnace black (Vulcan XC 72) and 60 g of graphite (natural graphite 
ACP-1000) were added to 100 g of polyethylene (melting point: 131.degree. 
C.), to which 3 g of Di-Cup (Percumyl D) was then further added as a 
grafting agent (first organic peroxide). They were heated and milled with 
the heated roll mill (grafting). Subsequently, 5 g of TBPH-3 (Perhexyne 
25B-40, concentration: 40%) was added as a crosslinking agent (second 
organic peroxide) to the resulting milled mass, followed by further 
milling. The milled mass was formed into a predetermined device shape and 
heat-treated at 200.degree. C. for 15 minutes (crosslinking) to obtain a 
device. 
It is possible in the step of initial milling to add a suitable amount of a 
first organic peroxide such as Di-Cup (Percumyl D) and a suitable amount 
of a second organic peroxide such as TBPH-3 (Perhexyne 25B-40) 
simultaneously. 
The initial volume resistivity of the device 5 obtained in Example 1 which 
was provided with terminals 6 as shown in FIG. 2 was measured and found to 
be 2.84 .OMEGA.cm. In order to examine the stability of the electric 
resistance value of the device, a temperature cycle test (one cycle: at 
150.degree. C. for 15 minutes and a 25.degree. C. for 15 minutes) was 
conducted. The rate of change in the electric resistance value relative to 
the initial value was -4.6% after the 5th cycle and -3.6% after the 10th 
cycle, thus proving that the device had a stable electric resistance. 
COMATIVE EXAMPLE 1 
A device was produced in substantially the same manner as that of Example 1 
except that 3 g of Di-Cup alone was added as a grafting agent without 
addition of any crosslinking agent. The initial volume resistivity of the 
obtained device was 1.93 .OMEGA.cm. The rate of change in the electric 
resistance value as measured according to the same temperature cycle test 
as that of Example 1 was 7.6% after the 5th cycle and 12.9% after the 10th 
cycle. 
COMATIVE EXAMPLE 2 
A device was produced in substantially the same manner as that of Example 1 
except that neither grafting agent nor crosslinking agent was added and 
that the heat treatment at 200.degree. for 15 minutes was dispensed with 
to avoid deformation of the device. The initial volume resistivity of the 
obtained device was 0.30 .OMEGA.cm. The rate of change in the electric 
resistance value as measured according to the same temperature cycle test 
as that of Example 1 was 43.2% after the 5th cycle and 61.2% after the 
10th cycle. 
COMATIVE EXAMPLE 3 
A device was produced in substantially the same manner as that of Example 1 
except that graphite alone was used without use of carbon black and that 4 
g of Di-Cup (Percumyl D) alone was added as the organic peroxide in the 
step of initial milling. The initial volume resistivity of the obtained 
device was 3.10.times.10.sup.3 .OMEGA.cm. The rate of change in the 
electric resistance value as measured according to the same temperature 
cycle test as that of Example 1 was -67.3% after the 5th cycle and -84.8% 
after the 10th cycle. 
FIG. 3 shows characteristic curves the temperature versus the rate of 
change in resistance value of the devices obtained in Example 1 and 
Comparative Examples 1 and 2. It can be understood from these 
characteristic curves that, after the manifestation of PTC 
characteristics, the device of Comparative Example 2 showed notable NTC 
characteristics and that of Comparative Example 1 showed slightly 
suppressed NTC characteristics while that of Example 1 showed largely 
suppressed NTC characteristics. 
FIG. 5 shows the static voltage versus current characteristic curve of the 
device of Example 1 as measured by connecting the device 5 in series to a 
load 7 and applying a voltage V from a power source 8 to the device 5 as 
shown in FIG. 4. In the curve A of FIG. 5, the operating point settles at 
a point a where a steady state is attained without current limitation. 
This state corresponds to that attained when a rated current flows through 
a metallic fuse. When the voltage of the power source is changed from 
V.sub.1 to V.sub.2, a load line B is replaced by a load line C, whereupon 
the operating point shifts from the point a to a point b. A current 
I.sub.2 flows through the device 5 and the temperature of the device 5 is 
raised by heat buildup thereof due to Joule's heat, with the result that 
the operating point shifts from the point b to a point d with some time 
lag and the current is finally limited to I'.sub.2. Where the voltage of 
the power source is constant and the load is changed, the load line B is 
replaced by a load line D and the operating point shifts from the point a 
to a point b. As a result of heat buildup of the device 5, the operating 
point shifts from the point b to a point e with some time lag and the 
current is limited to I".sub.2. 
Thus, when an overcurrent flows through a circuit as a result of any change 
in the power source or the load, the current value can be limited to a 
rated current or below though the limited current value varies depending 
on conditions. When the current returns to the rated state, the operating 
point returns to the point a again. Thus, the device can be repeatedly 
used as an overcurrent protective device. Accordingly, utilization of 
these characteristics enables the use of the device as a self-restoring 
overcurrent protective device. 
FIG. 6 shows the dynamic time versus current characteristic curve of the 
device 5, which shows a variation of current with time during the course 
of limitation of the current from I.sub.2 to I".sub.2 with shift of the 
operating point from the point b to the point e in FIG. 5. The time 
t.sub.L spent during limitation of the current from I.sub.2 to I".sub.2 is 
a current-limiting time. 
EXAMPLE 2 
This is an example wherein two kinds of carbon blacks were used. 
20 g of furnace black (Vulcan XC 72), 20 g of acetylene black (Denka 
Black); and 60 g of graphite (ACP-1000) were added to 100 g of 
polyethylene (1300J), to which 3 g of Di-Cup (Percumyl D) and 5 g of 
TBPH-3 (Perhexyne 25B-40) were then further added as a grafting agent for 
effecting grafting onto the surfaces of the above-mentioned particles and 
a crosslinking agent, respectively. A device was produced in substantially 
the same manner as that of Example 1 except for the above-mentioned 
materials. The initial volume resistivity of the obtained device was 1.68 
.OMEGA.cm. The rate of change in the electric resistance value as measured 
according to the same temperature cycle test as that of Example 1 was 4.5% 
after the 5th cycle and 5.0% after the 10th cycle. 
EXAMPLE 3 
This is an example wherein artificial graphite was used. 
60 g of furnace black (Vulcan XC 72) and 40 g of graphite (artificial 
graphite GA-5) were added to 150 g of polyethylene (1300J), to which 3 g 
of Di-Cup (Percumyl D) and 5 g of TBPH-3 (Perhexyne 25B-40) were then 
further added as a grafting agent for effecting grafting onto the surfaces 
of the above-mentioned particles and a crosslinking agent, respectively. A 
device was produced in substantially the same manner as that of Example 1 
except for the above-mentioned materials. The initial volume resistivity 
of the obtained device was 3.78 .OMEGA.cm. The rate of change in the 
electric resistance value as measured according to the same temperature 
cycle test as that of Example 1 was 8.9% after the 5th cycle and 14.2% 
after the 10th cycle. 
EXAMPLE 4 
This is an example wherein two kinds of polymers were mixed together. 
50 g of furnace black (Vulcan XC 72) and 50 g of graphite (ACP-1000) were 
added to 80 g of polyethylene (1300J) and 40 g of polypropylene (J900P, 
melting point: about 160.degree. C.), to which 3 g of Di-Cup (Percumyl D) 
and 5 g of TBPH-3 (Perhexyne 55B-40) were then further added as a grafting 
agent for effecting grafting onto the surfaces of the above-mentioned 
particles and a crosslinking agent, respectively. A device was produced in 
substantially the same manner as that of Example 1 except for the 
above-mentioned materials. The initial volume resistivity of the obtained 
device was 4.06 .OMEGA.cm. The rate of change in the electric resistance 
value as measured according to the same temperature cycle test as that of 
Example 1 was -13.4% after the 5th cycle and -18.7% after the 10th cycle. 
EXAMPLE 5 
This is an example wherein Ketjen black was used as carbon black. 
20 g of Ketjen black (EC) and 80 g of graphite (ACP-1000) were added to 100 
g of polyethylene (1300J), to which 3 g of Di-Cup (Percumyl D) and 5 g of 
TBPH-3 (Perhexyne 25B-40) were then further added as a grafting agent for 
effecting grafting onto the surfaces of the above-mentioned particles and 
a crosslinking agent, respectively. A device was produced in substantially 
the same manner as that of Example 1 except for the above-mentioned 
materials. The initial volume resistivity of the obtained device was 1.60 
.OMEGA.cm. The rate of change in the electric resistance value as measured 
according to the same temperature cycle test as that of Example 1 was 
14.2% after the 5th cycle and 18.6% after the 10th cycle. 
EXAMPLE 6 
This is an example wherein a polyester was used as a crystalline polymer 
substance. 
40 g of furnace black (Vulcan XC 72) and 50 g of graphite (ACP-1000) were 
added to 100 g of a polyester (polyhexamethylene terephthalate, melting 
point: 146.degree. C.), to which 2 g of Di-Cup (Percumyl D) and 5 g of 
TBPH-3 (Perhexyne 25B-40) were then further added as a grafting agent for 
effecting grafting onto the surfaces of the above-mentioned particles and 
a crosslinking agent, respectively. A device was produced in substantially 
the same manner as that of Example 1 except for the above-mentioned 
materials. The initial volume resistivity of the obtained device was 2.44 
.OMEGA.cm. The rate of change in the electric resistance value as measured 
according to the same temperature cycle test as that of Example 1 was 9.1% 
after the 5th cycle and 5.4% after the 10th cycle. 
EXAMPLE 7 
This is an example wherein an organic peroxide was initially added as both 
of a grafting agent and a crosslinking agent without later addition of any 
organic peroxide. 
40 g of furnace black (Vulcan XC 72) and 60 g of graphite (ACP-1000) were 
added to 100 g of polyethylene (1300J), to which 6 g of Di-Cup (Percumyl 
D) was then further added as a grafting agent for effecting grafting onto 
the surfaces of the above-mentioned particles and a crosslinking agent for 
the polymer. They were heated and milled with a heated roll mill without 
further addition of any crosslinking agent, then formed into a device 
shape, and subsequently heat-treated at 200.degree. C. for 15 minutes to 
produce a device. The initial volume resistivity of the obtained device 
was 5.24 .OMEGA.cm. The rate of change in the electric resistance value as 
measured according to the same temperature cycle test as that in Example 1 
was 12.2% after the 5th cycle and 19.8% after the 10th cycle. 
EXAMPLE 8 
This is an example wherein two kinds of carbon blacks and two kinds of 
polymers were mixed together. 
60 g of graphite (natural graphite ACP-100), 20 g of furnace black (Vulcan 
XC 72), and 20 g of Ketjen black (EC) were added to 120 g of polyethylene 
(1300J) and 30 g of polypropylene (J900P), to which 3 g of Di-Cup 
(Percumyl D) and 5 g of TBPH-3 (perhexyne 25B-40) were then further added 
as a grafting agent for effecting grafting onto the surfaces of the 
above-mentioned particles and a crosslinking agent, respectively. 
A device was produced in substantially the same manner as that of Example 1 
except for the abovementioned materials. The initial volume resistivity of 
the obtained device was 5.78 .OMEGA.cm. The rate of change in the electric 
resistance value as measured according to the same temperature cycle test 
as that in Example 1 was 7.4% after the 5th cycle and 8.3% after the 10th 
cycle. 
EXAMPLE 9 
This is an example wherein three kinds of carbon blacks and two kinds of 
polymers were mixed together. 
10 g of furnace black (Vulcan XC 72), 10 g of acetylene black (Denka 
Black), 20 g of Ketjen black (EC), and 60 g of graphite (natural graphite 
ACP-1000) were added to 120 g of polyethylene (1300J) and 30 g of 
polypropylene (J900P), to which 3 g of Di-Cup (Percumyl D) and 5 g of 
TBPH-3 (Perhexyne 25B-40) were then further added as a grafting agent for 
effecting grafting onto the surfaces of the above-mentioned particles and 
a crosslinking agent, respectively. A device was produced in substantially 
the same manner as that of Example 1 except for the above-mentioned 
materials. The initial volume resistivity of the obtained device was 2.99 
.OMEGA.cm. The rate of change in the electric resistance value as measured 
according to the same temperature cycle test as that of Example 1 was 8.9% 
after the 5th cycle and 9.0% after the 10th cycle. 
The blending ratios, the results of measurement of the initial volume 
resistivities, the rates of change in resistance value and the like, and 
the properties and the like of the materials used in Examples 1 to 9 and 
Comparative Examples 1 to 3 are summarized in Tables 4 to 6. 
TABLE 4 
__________________________________________________________________________ 
Amounts of Materials [g] 
A grade name is shown in ( ). 
Organic peroxide 
Graphite TBPH 
artifi- 
Carbon black Crystalline polymer (Perhexyne 
natural cial furnace black Ketjen 
poly- 
poly- Di-Cup 25B-40) 
graphite graphite 
(Vulcan 
acetylene black 
black 
ethylene 
propylene 
poly- 
(Percumyl 
[crosslinking 
(ACP-1000) 
(GA-5) 
XC 72) (Denka Black) 
(EC) 
(1300J) 
(J900P) 
ester 
[grafting 
agent] 
__________________________________________________________________________ 
Ex. 1 
60 -- 40 -- -- 100 -- -- 3 5 
Ex. 2 
60 -- 20 20 -- 100 -- -- 3 5 
Ex. 3 
-- 40 60 -- -- 150 -- -- 3 5 
Ex. 4 
50 -- 50 -- -- 80 40 -- 3 5 
Ex. 5 
80 -- -- -- 20 100 -- -- 3 5 
Ex. 6 
50 -- 40 -- -- -- -- 100 
2 5 
Ex. 7 
60 -- 40 -- -- 100 -- -- 6 
(Percumyl D) 
Ex. 8 
60 -- 20 -- 20 120 30 -- 3 5 
Ex. 9 
60 -- 10 10 20 100 20 -- 3 5 
Comp. 
60 -- 40 -- -- 100 -- -- 3 -- 
Ex. 1 
Comp. 
60 -- 40 -- -- 100 -- -- -- -- 
Ex. 2 
Comp. 
100 -- -- -- -- 100 -- -- 4 
Ex. 3 (Percumyl 
__________________________________________________________________________ 
D) 
TABLE 5 
__________________________________________________________________________ 
Results of Ex. and Comp. Ex. 
Rate of change in 
Initial resistance by temp. 
volume cycle test 
resistivity 
(after 5th cycle) 
(after 10th cycle) 
PTC 
[.OMEGA. cm] 
[%] [%] characteristics 
Remarks 
__________________________________________________________________________ 
Ex. 1 
2.84 -4.6 -3.6 shown This Example should be compared in 
characteristics 
with Comp. Ex. 1 and 2. 
Ex. 2 
1.68 4.5 5.0 shown Example wherein furnance black was 
mixed with 
acetylene black. 
Ex. 3 
3.78 8.9 14.2 shown Example wherein artificial graphite 
was used. 
Ex. 4 
4.06 -13.4 -18.7 shown Example wherein two kinds of polymers, 
polyethylene 
and polypropylene, were mixed 
together. 
Ex. 5 
1.60 14.2 18.6 shown Example wherein Ketjen black used. 
Ex. 6 
2.44 9.1 5.4 shown Example wherein a polyester was used. 
Ex. 7 
5.24 12.2 19.8 shown Example wherein a grafting agent and a 
crosslinking 
agent were initially added 
simultaneously. 
Ex. 8 
5.78 7.4 8.3 shown Example wherein two kinds of polymers, 
polyethylene 
and polypropylene, were used in 
combination in a 
system wherein furnace black was 
blended with 
Ketjen black. 
Ex. 9 
2.99 8.9 9.0 shown Example wherein two kinds of polymers, 
polyethylene and 
polypropylene, were used in 
combination in a system 
wherein furnace black was blended with 
Ketjen black and 
acetylene black. 
Comp. 
1.93 7.6 12.9 shown This Example should be compared with 
Ex. 1. 
Ex. 1 
Comp. 
0.30 43.2 61.2 shown This Example should be compared with 
Ex. 1. 
Ex. 2 
Comp. 
31.0 .times. 10.sup.3 
-67.3 -84.8 shown Comparative Example wherein graphite 
alone was used 
Ex. 3 without use of carbon 
__________________________________________________________________________ 
black. 
TABLE 6 
__________________________________________________________________________ 
Materials 
Division 
Kind Grade name (chemical name) 
Maker Properties 
__________________________________________________________________________ 
Graphite 
natural ACP-1000 Nippon Kokuen 
particle size: 6 .mu.m 
graphite Kogyo K.K. 
artificial 
GA-5 Nippon Carbon 
particle size: 40 .mu.m 
graphite Co., Ltd. 
KS-15 Ronza, Switzerland 
particle size: 8 .mu.m 
Carbon 
furnace Vulcan XC 72 Cabot, U.S.A. 
oil absorption: 1.78 ml/g 
black black 
AG-300 Asahi Carbon K.K. 
oil absorption: 3 ml/g 
acetylene 
Denka Black Denki Kagaku 
oil absorption: 1.15 ml/g 
black Kogyo K.K. 
Ketjen black 
EC Nippon EC K.K. 
oil absorption: 3.5 ml/g 
Crystalline 
polyethylene 
1300J Mitsui Petro- 
m.p.: 131.degree. C. 
polymer chemical 
Industries, Ltd. 
polypropylene 
J900P Mitsui Petro- 
m.p.: 160.degree. C. 
chemical 
Industries, Ltd. 
polyester 
(polyhexamethylene 
-- m.p.: 146.degree. C. 
terephthalate) 
Organic 
Di-Cup Percumyl D Nippon Oil and 
half-life: 171.degree. C., 1 min. 
peroxide Fats Co., Ltd. 
TBPH-3 Perhexyne 25B-40 
Nippon Oil and 
half-life: 193.degree. C., 1 min. 
Fats Co., Ltd. 
TBPH-3 content: 40% 
__________________________________________________________________________ 
In Table 5, the rate of change in the resistance value of the device of 
Comparative Example 1 is better than those of the devices of some 
Examples. As shown in FIG. 3, however, the device of Comparative Example 1 
shows the NTC phenomenon when the temperature thereof is further raised 
after the manifestation of the PTC phenomenon. This results in a drastic 
reduction in the current-limiting characteristics of the device as an 
overcurrent protective device. The device of Comparative Example 2 also 
shows the same phenomenon. 
The low initial volume resistivity of the device of Comparative Example 2 
as shown in Table 5 is due to the presence of aggregates of particles in 
the crystalline polymer substance which resulted from the poor 
dispersibility of the graphite and the carbon black attributed to the fact 
that no organic peroxide was used so that the polymer substance was not 
grafted onto the graphite and the carbon black. This is substantiated by 
the rate of change in the initial resistance value as measured according 
to the temperature cycle test. 
In Table 5, the device of Comparative Example 3 has a very high initial 
volume resistivity, which is attributed to the fact that no carbon black 
was blended. Further, the device of Comparative Example 3 shows a very 
high rate of change in the resistance value. 
Effects of the Invention 
Where a crystalline polymer substance is milled in the presence of graphite 
and carbon black particles using an organic peroxide as a reaction 
catalyst, the milling time is shortened to suppress the thermal 
decomposition of the organic peroxide for preventing the crosslinking of 
the polymer substance due to the decomposition of the organic peroxide 
according to the conventional process, whereas, according to the process 
of the present invention, a suitable amount of the organic peroxide is 
determined and heated together with the other materials at or above the 
thermal decomposition temperature thereof while sufficiently milling them 
to graft part of the polymer substance onto the surfaces of the particles, 
whereby the compatibility of the particles with the polymer substance can 
be improved. In the latter case, therefore, the carbon black is broken 
into primary particles and homogeneously dispersed in the polymer 
substance. 
Accordingly, a device having a significantly reduced scattering of 
resistance value can be obtained as an overcurrent protective device. 
Further, in the process of the present invention, the polymer substance is 
crosslinked between the molecules thereof after the completion of milling 
to form a three-dimensional network structure involving the graphite and 
carbon black particles therein, which enables the order of the 
electrically conductive particles, even after repeated manifestation of 
the PTC phenomenon (current-limiting actions), to return to the original 
state to provide an effect of restoring the resistance value stably to the 
original value. Moreover, the network structure serves to retain the shape 
of the device even in a temperature range where the crystalline polymer is 
molten, and to provide an effect of suppressing the NTC phenomenon after 
the manifestation of the PT phenomenon.