Note: Descriptions are shown in the official language in which they were submitted.
i3
This invention relates to conductive polymer compositions and devices
containing them, especially circuit protection devices.
Conductive and semi-conductive compositions comprising carbon black
dispersed in a polymer are known. They may have room temperature resistivities
ranging from less than 1 ohm.cm to 108 ohm.cm or more, and may exhibit positive
temperature coefficient (PTC) behavior, zero temperature coefficient (ZTC or
constant wattage) behavior or negative temperature coefficient (NTC) behavior.
Reference may be made, for example, to United States Patent Nos. 2,978,665,
3,243,753, 3,351,882, 3,823,217, 3,861,029, 3,950,604, 4,017,715, 4,177,376
and 4,246,468, to German OLS Nos. 2,413,475, 2,746,602, 2,755,076 and 2,821,570
to J. Applied Polymer Science 19, 813-815 (1975), Klason and KubatJ and to
Polymer Engineering and Science 18 649-653 (1978), Narkis et al. Recent advances
in this field are described in German OLS Nos. 2,948,350, 2,948,281, 2,949,173
and 3,002,721, in the applications corresponding to Canadian Serial Nos. 352,414
(MP0295), 352,413 (MP0295), 358,374 (MP0299) and 363,205 (MP0701), and in the
applications filed contemporaneously with this application corresponding to
Canadian Serial Nos. 375,886 (MP0712), 375,795 (MP0713), 375,856 (MP0714) and
375,877 (MP0720), and United States Patent Nos. 4,352,083 (MP0724) and 4,317,027
(MP0725).
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~ 7~3 Mp0715
In recent research into the use of circuit
protection devices containing PTC conductive polymer
elements, it was observed that previously proposed devices
failed to give repeated and effective protection against
fault conditions in which the device was subjected to a
combination of high current and high voltage. We have
found that the performance, under conditions of high
electrical stress, of conductive polymer compositions
containing carbon black or graphite as the sole conductive
filler, can be markedly improved by adding tn such compo-
sitions one or more of the additives which have previously
been used to improve the tracking resistance of polymeric
insulating compositions. Although it is not entirely
clear precisely why such additives have this vaLuable
effect, they are referred to herein as arc-controlling
additives. It is thought that their efficacy is probably
due, at least in part, to their ability to extinguish arcs
after they have been formed, but the additives may also
act to reduce the susceptibility of the composition to
form arcs in the first place. In any event~ it is to be
noted that the prior use of these additives, which has
been to extinguish arcs on the contaminated surface of an
electrical insulator, involves a very different situation
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MP0715
from the present one, where the additives are effective
in controlling arcs within a mass of conductive polymer
(as well as at the surface thereof).
In one aspect, the invention provides an electrical
device which comprises
(a) a PTC element composed of a
conductive polymer composition which exhibits PTC
behavior, which has a resistivity at 23C of less than
106ohm. cm, and which comprises:
(1) 20 to 91% by volume of an organic polymer
component;
(2) 4 to 65% by volume of a conductive filler
component which is dispersed in said polymer
component and which consists essentially of
carbon black and/or graphite; and
(3) an arc-controlling additive which is a hydrated
inorganic particulate material and which is
dispersed in the polymer component; and
(b) at least two electrodes which can be connected
to a source of electrical power and which when so
connected cause current to flow through the
PTC element.
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7 ~
~P07~5
The PTC compositions defined above are particularly
useful in circuit protection devices as described in
the comtemporaneously filed applications corresponding
to Canadian Serial No. 375,795, and United States
Patent Nos. 4,352,083 and 4,317,027 (MP0713, MP0724 and
~pO725).
The compositions can be any of the PTC compositions
disclosed in the prior art and the earlier applications
referred to above modified by the inclusion of at least
one arc-controlling additive as defined. The invention
is especially valuable in relation to circuit protection
devices which usually have a resistance below 50 ohms,
preferably 0.1 to 25 ohms, at 23C as described in
copending Canadian Patent Application No. 375,795, and
in which the PTC composition has low resistivity at
23C, e~g. below 20 ohm. cm, preferably below 10 ohm.
cm, especially below 2 ohm. cm.
The preferred arc-controlling additive for use in the
present invention is alumina trihydrate, A12033H20.
Another additive that can be used is magnesia hydrate.
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~ 76453 MP0715
The conductive filler and the arc-controlling
additive preferably have a total surface area of at
least 1,800, especially at least 3,000, particularly at
least 4,000, m /100 cc of composition, with higher
values, eg. at least 8,000 m2/100 cc, at leas-t 10,0~0
m2/100 cc and at least 12,000 m2/100 cc being
particularly preferred.
The composition should contain an effective
amount of the arc-controlling additive, typically 5 ~o
65%, preferably 10 to 35~, by volume of the composition.
The composition can also contain further additives
which are known to enhance the effectiveness of anti-
tracking additives in insulating compositions.
Examples of such additives include the phosphorus-
containiny compounds disclosed in British Patent No.1,575,465 and the oxides of elements of the transition
series, lanthanide series or non-transuranic actinicle
series disclosed in British Patents ~os. 1,337,951 and
1,337,952.
.~
~ 53 MP0715
The conductive filler in the composition prefer-
ably consists essentially of at least one carbon black.
The carbon black is selected ~ith a view to the elec-
trical characteristics desired in the composition, as
taught by the various patents and applications referred
to above. Thus for low resistivity PTC composltions,
the carbon black preferably has a particle size, D,
which is from 20 to 150 millimicrons and a surface
area, S in m /g such that S/D is not more than
10.
~hen usin~ such a carbon black, preferably the quantity
S (volume of conductive filler + volume of arc-controlling additive)
D volume of p~lymer comp~nent
is less than 1.
The polymer component in the composition, which
15 may comprise one or more polymers, preferably has a
crystallinity of at least 1%, especially at least 5%,
particularly at least 10~. Preferably the polymer
component consists essentially of one or more crystal-
line polymers selected from polyolefins and copolymers
20 of at least one olefin and at least one polar comonomer
copolymerisable therewith, eg. polyethylene or poly-
propylene. Other suitable polymers are referred to in
the patents and applications referred to above.
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~ 3 ~iP0715
The composition may be substantially free of
cross-linking or may be cross-linked, eg. to a gel
fraction of at least 0.4 or 0.6. For some purposes,
compositions free of cross-linking are preferred,
because the presence of cross-linking tends to increase
the likelihood of formation of carbonaceous conductive
paths when arcing takes place.
The composition can be prepared by dispersing
the carbon black or graphite, the arc-controlling
additive and any other additives in the polymer com-
ponent in any suitable way. ~he composition can be
shaped by molding or extrusion or another melt-shaping
technique into an element of the desired shape, any
cross-linking thereof being carried out after such
shaping.
Conductive polymer compositions comprising
a crystalline polymer component, at least 4~ by volume
of carbon black and at least 4~ by volume of a non-
conductive particulate filler, the total surface
area of the carbon black and filler being at least 1800
m /100 cc of composition, are described and claimed in
the comptemporaneously filed application corresponding
to Canadian Serial Nos. 375,886 and 375,856 (~P0712 and
MP0714).
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MP0715
The invention is illustratad by the ~ollowing
Examples.
Example 1
The ingredients and amo~nts thereof given in
the Table 1 below were used in this Example.
TABEE 1
MASTER8ATCH FINAL MIX
g wt~, v o l O 9 w t ,o vol~
Carbon ~lack 144446.9 32.2 1143.933.826.9
¦Furnex N765)*
Polyethylene 157251.1 65.4 1246.336.854.7
(Marlex 60û:3)*
Filler - - - 94828.016.5
(Hydral 705)*
Antioxidant 622.0 2.3 48.81.41.9
..
NOTES: -
Furnex N765*(available from City Services Co) has a particle `
size (D) of 60 millimicr~ns, a density of 1.8 g/cc, and a
surface area (s) of 32 m /9.
Marlex 6003*is a high density polyethylene with a rrlelt index
of 0.3 whic17 iS available from PhiLlips Petroleum Co.
The antioxidant used was an oLigomer of 4,4-thio bis (3-methyl-
6-t-butyl phenol) with an average degree of polymerization of
3-4, as described in U.S. Patent No. 3,986,981.
Hydral 705*is alurnina trihydrate.
*Tracle Mark
-10-
6~53
The ingredients for the master batch were dry blended and then mixed
for 8 minutes in a Banbury* mixer turning at high gear. The mixture was dumped,
cooled and granulated. The final mix was prepared by dry blending 948 g of the
hydral 705* with 2439 g. o~ the master batch mixture, and then mixing the dry
blend for 4-5 minutes in a Banbury* mixer turning at high gear. The mixture
was dumped, cooled, granulated and dried ~at 70C. 1 Torr for 16 hours).
The granulated final mix was melt extruded in the form of a strip
about 1.27 cm wide and about 0.27 cm thick, using a cross-head die, around a
pair of pre-heated 20 AWG (diameter 0.095 cm) l9/32 stranded nickel-plated copper
wires whose centers were 0.6 cm apart. The extruded product was cut into 2.5
cm lengths, and the polymeric composition removed from half of each length to
produce a circuit control device as shown in Figure 4 of the contemporaneously
filed application corresponding to United States Patent No. 4,352,083 (MP0724).
Examples 2-4
The ingreidents used in these Examples and the amounts thereof are
shown in Table 2 below. The antioxidant is as specified in Table 1. Sterling NS*
and Sterling
*Trade Mark
- 11-
~L~'7~i3
SO* are available from Cabot, Hydral 705* from Alcan,
Maglite D* from Merck, and Kadox 15* from Gulf and Western,
and they have the following properties
Material Particle Size DensitySurface Area
millimicrons g/cc m2/g
Sterling NS* Carbon black (N774~ 75 1.8 25
Sterling SO* Carbon black (N550~ 41 1.8 42
~Iydral 705* A1203 3~l200.5-2,000 2.42 12 - 15
Maglite D* MgO ~44 3.32
Kadox 15* 130 5.52-6.52 8.5
In Example 2, the Master Batch ingredients were
blended in a pre-heated Banbury* mixer, and the mixture
dumped, cooled and granulated. 67 g. of the granulated
mixture was banded on a 7.6 cm electric roll mill, and
the Hydral* was added in portions to give a uniform mixture;
mixing was continued for several more minutes and the
mixture was then removed from the mill, cooled, granulated
and compression-molded into slabs.
In Example 3, the Master Batch ingredients were
blended in a pre-heated Banbury* mixer, and the mixture
dumped, cooled and granulated. 67 g. of the granulated
mixture was b.lllded on a 3 inch electric roll mlll, and
the ~Iydral* was added ln portions to glvc a unlform mixture;
mixing was continued Eor several more minutes and the
*Trade Ma-rk
,
~'76453
MP0715
mixture was then removed from the mill, cooled, granulated
and co~pression-molded into slabs.
In Example 4 the procedure described For Example
2 was followed, using the different ingredients shown in
Table 2, except that 50 9. of the granulated Master Batch
was used and 50 g. of the filler (Maglite D)*added to it.
*Trade Mark
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