Note: Descriptions are shown in the official language in which they were submitted.
-`` 115~413
-- 2 --
This invention relates to electrical heaters comprising
a heating element which is composed of a polymeric matrix
having carbon black dispersed therein.
The electrical heating strips of th~ present invention
comprises a heating element composed of a carbon black-loaded
polymeric composition and a pair of elongate electro~es which
are electrically connected by said heating element, the heating
element being composed of a polymeric matrix having dispersed
therein
(i) a first carbon black which is a relatively
conductive filler (as hereinafter defined) and
(ii) a second carbon black which is a relatively less
conductive filler (as hereinafter defined).
The term "relatively conductive filler" is used herein
to denote a filler which, when dispersed in polyethylene
having a density of 0.916 g/cm3 and a melt flow index of 2,
exhibits a curve of impedance versus loading which falls
steeply in the range of specific impedance values from 10
to 10 ohm.cm and reaches a specific impedance of less than
5 x 106 ohm.cm, preferably less than 10 ohm.cm, at a loading
of 60 parts by weight per 100 parts by weight of polyethylene,
the specific impedances being measured at 50 Hz with a field
strength of 4 kV/cm. The term "relatively less conductive
,' , ~
,. ._ .. . . .
13
-- 3
filler" is used herein to denote a filler which, when
dispersed in polyethylene having a density of 0.916 g/cm
and a melt flow index of 2, exhibits a curve of impedance
versus loading having a relatively shallow qlope with no
steep sections and which has a specific impedance greater
than 109 ohm.cm, and preferably less than 101 ohm.cm, at a
loading of 60 parts by weight per 100 parts by weight of
polyethylene, the specific impedances bein~ measured at 50 Hz
with a field strength of 4 kV/cm.
The specific impedances referred to in this specification
are measured by the procedure given in Example 9 (i.e. by
mixing the ingredients in a Banbury mixer, pres~ing the
mixture into a slab, and measuring the impedance of the un-
annealed slab at room temperature at 50 Hz and a field
strength of 4 kV/cm).
The polymeric matrix in the heating strips of the
present invention preferably comprises a thermoplastic resin,
especially a polyolefin, e.g. polyethylene or polypropylene~
an olefin copolymer, e.g. an ethylene/ethyl acrylate or
ethylene/ethyl methacrylate polymer, a halogen-substituted
olefin copolymer, e.g. an ethylene/tetrafluoroethylene
ccpolymer or polyvinylidene chloride, or a mixture of two or
more of these.
The first carbon black (which is a relatively conductive
filler) may have an average particle size of 10 to 100 milli-
microns, preferably 20 to 60 millimicrons, and an average
specific surface area greater than 30 m /g, preferably greater
1~514~3
-- 4 --
than 100 m2/g, as measured by the method of ASTM D3037-76.
Suitable carbon blacks can be chosen from carbon blacks which
are commercially available as Types HAF, SRF, EPC, FEF and
ECF, e.g. those manufactured by Cabot under the trade names
Vulcan XC72, Vulcan P, and Vulcan 3, by Columbian under the
trade names Statex 160, Statex 125 and Conductex g~0, by AKZ0
Chemie under the trade name Ketjen Black EC, ar.d by Degussa
under the trade names Corax L and Corax P. ["Vulcan", "Statex",
"Ketjen" and "Corax" are trade marks~.
me second carbon black ~which is a relatively lesq
conductive filler) is preferably one of the so-called low
structure carbon blacks. Suitable carbon blacks include
commercially availab~e Thermal blacks, e.g. those manufactured
by Vanderbilt under the trade names Thermax and P-33, by
Sevalco under the trade name Sevacarb MT, and by Columbian
under the trade name Statex MT. ~"Thermax" is a trade mark].
The second carbon black may have an average particle size of
at least 30 millimicrons and an average specific surface
area of less then 20 m2/g.
By use of suitable amounts of the first and second
carbon blacks, it is possible to prepare conductive composi-
tions having a desired level of specific impedance without
undue sensitivity to small changes in the loadings of the
carbon blacks. The ratio by weight of the second carbon
black to the first carbon black is preferably at least 1:1,
e.g., 2:1 to 8:1 or 3:1 to 6:1. The amount of the first
d~ r/~
.. .. . ..... . , . . . _ . ~ ..... ~ ... . . . . ..
~31 5~13
-- 5
carbon black may be at least 90/0, e.g. 10 to 25%, by weight,
based on the weight of the polymeric matrix. The amount of
the second carbon black may be at least 40% by weight, based
on the weight of the pol~neric matrix.
The following Examples, in which parts are by weigh~,
illustrate compositions suitable for use in the heating
element according to the invention.
Example 1
Three sets of compositions were made by blending 62.5
parts of chlorinated polyethylene (CPE 3614 manufactured by
Dow Corning), 37.5 parts of low density polyethylene (DYNE 3
manufactured by BXL Bakelite), various amounts of one or
both of Vulcan P (a carbon blac~ manufactured by Cabo~ which
is a relatively conductive filler) and Thermax MT (a carbon
black manufactured by Vanderbilt which is a relatively less
conductive filler), and small amounts of conventional anti-
oxidants and stabilizers. The ingredients were comp3unded ina Bridge Banbury internal mixer with a charge weight of 1.2 kg,
the mixture was pressed 150C into plaque samples 2 mm. thick
and the samples cooled to room temperature. Circular electrodes
were painted on both sides of each sample which was placed
between brass electrodes conforming to BS 2782(201). The
specific impedance of the composition was calculated from
measurement of both voltage and current at 50 Hz and the known
sample dimensions, using a field strength of 4 kV!cm. In the
q~/e,"~r/~ , ~
~51413
-- 6 --
first set of compositions, Vulcan P was the sole carbon
black, in amount 10, 20, 30, 40, 50 or 60 parts. 'n the
second set of compositions, Thermax MT was the sole carbon
black, in amount 10, 20, 30, 40, 50, 60, 70, 80, 90, ~0~,
110 or 120 parts. In the third set of compositions, ~7ulcan P
and Thermax MT were used together, the ratio by weight in
each composition of Vulcan P to Thermax MT being 1:4 and the
total amount of both carbon blacks being 10, 20, 30, ~0, 50,
60, 70, 80 or 90 parts.
/ me results obtained are shown graphically in Figure 1,
Curves A, B and C showing the specific impedances of the
first, second and third sets of compositions respectively.
Example 2
The procedure described in Example 1 was used to
determine the specific impedance of compositions (1) to (8)
containing the ingredients and amounts thereof shown in the
Table below and in addition (same for each composition) 2.5
parts of a conventional polymer stabiliser (Agerite Resin D),
4.375 parts of dibasic lead phthalate, 1.875 parts of tetra-
basic lead fumarate and 1.875 parts of triallyl cyanurate.
The various ingredients listed by their trade names in the
Table are identified above or as set out below.
DPD 6169 is an ethylene/ethyl acrylate poly~er ~anu-
factured by Union Carbide and having a melt flow index of 6
and a density of 0,931, Royalene 611 is an ethylene/
.
:~ tr~o~<
.. ... . . .. . .. . . . .. . .....
~151413
-- 7 -
propylene/non-conjugated diene terpolymer manufactured by
Uniroyal. Philips GPF is a furnace black which is a re-
latively conductive filler, manufactured by Philips
Petroleum Co. ["Philips" is a trade mark.~. The specific
impedances of the various compositions are also given in the
Table.
Composition ~o. (1) (2) (3) (4) (5) (6) (7) (8)
-
DYNH 3 37.5 - - 37.537.5 37.5 37.5 37.5
CPE 3614 - 100 - 62.562.5 62.5 62.5 62.5
DPD 6169 ~ - - 100 - - - - -
.,~, .
Royalene 611 62.5
Vulcan P 14 14 14 20 16.6 10
Phillips GPF - - - - - - 16 20 -
Thermax MT 56 56 56 40 33.3 60 64 80
Specific Impedance 5.6 3.2 1.6 0.38 2.2 7.6 6.3 0.32
0-8
(ohm.cm)
D
" ' ',:
