Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to conductive polymer
composition~, their preparation, and devices comprising them.
It is known that polymers, including crystalline
polymers, can be made electrically conductive by dispersing
therein suitable amounts of finely divided fillers. Some
conductive polymers exhibit what is known as PTC (positive
temperature coefficient) behavior. The term "PTC" has been
used in various different ways in the past, but in this
specification, the terms "composition exhibiting PTC
behavior" and l'PTC composition" are used to denote a
composition having at least one temperature range which is
within the limits of -100C and about 250C; at the beginning
of which the composition has a resistivity below about 105
ohm. cm.; and in which the composition has an R14 value of at
least 2.5 or an Rloo value of at least 10 (and preferably
both), and preferably has an R30 value of at least 6, where
R14 is the ratio of the resistivities at the end and the
beginning of a 14C range, Rloo is the ratio of the
resistivities at the end and the beginning of a 100C range,
and R30 is the ratio of the resistivities at the end and the
beginning of a 30C range. The term "PTC element" is used
herein to denote an element composed of a PTC composition as
defined above. .~ plot of the log of the resistance of a PTC
element, measured between two electrodes in contact with the
element, against temperature, will often show a sharp change
in slope over a part of the critical temperature range, and
in such cases, the term "switching temperature" (usually
abbreviated to Ts) is used herein to denote the temperature
at the intersection point of extensions of the substantially
straight portions of such a plot which lie either side of the
portion showing the sharp change in slope. The PTC
~:
~ - - - .,,
~.
composition in such a PTC element is described herein as
having "a useful Ts''. Ts is preferably between 0 and
175C, e.g. 50C to 120C.
PTC compositions and electrical devices, especially
heaters, which contain PTC elements, have been described in a
number of publications. Reference may be made for example to
U.S. Patents Nos. 2,978,665, 3,243,753, 3,351,882, 3,412,358,
3,413,442, 3,591,526, 3,673,121, 3,793,7~6, 3,823,217,
3,858,144, 3,861,029, 3,914,363 and 4,017,715, British Patent
No. 1,409,695, Brit. J. Appl. Phys. Series 2, 2 569-576
(1969, Carley Read and Stow), Kautschuk und Gummi II WT, 138-
148 (1958, de Meij), Poiymer Engineering and Science, Nov.
1973, 13, No. 6, 462-468 (J. Meyer), U.S. Patent Office
- Defensive Publication No. T905,001, German
Offenlegungschriften Nos. 2,543,314.1, 2,543,338.9,
2,543,346.9, 2,634,478.5, 2,634,931.5, 2,634,932.6,
2,634,999.5, 2,635,000.5, and 2,655,543.1, and German
Gebrauchsmuster 7,527,288.
Particularly useful known PTC compositions comprise
~a thermoplastic crystalline polymer with carbon black ',
dispersed therein, and such compositions have been widely
- used in self-regula-ting strip heaters. The polymers which
have been used include polyolefins, e.g. polyethylene, and
copolymexs of olefins and polar comonomers, e.g.
'.` :~ ' ' '
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~ '
. ' ~
.~ .
-- 2 --
~' ' ' '
; ' ' ` ' ' ' ' : -
. ~ . ~ , ,
~ 3~
ethylene/ethyl acrylate copolymers. ',uch compositions show a
rapid increase in resistance over a range which begins at the
softening point of the polymer and have a useful Ts at or
near the crystalline melting point of the polymer; the
greater the crystallinity of the polymer, the smaller the
temperature range over which the resistance increase takes
place. Generally, the composition is cross-linked,
preferably by irradiation at room temperature, to improve its
stability at temperatures above Ts.
Carbon blacks vary widely in their ability to
impart conductivity to polymers with which they are mixed,
and mixtures of`polymers and carbon blacks generally have
poor physical properties when the proportion of carbon black
becomes too high, e.g~ above 30% to 50%, dependiny on the
polymer (percentages are by weight throughout this
specification). Not surprisingly, therefore, only a very
limited number of carbon blacks have been used or
recommended for use in conductive polymer compositions, i~e.
compositions whose utility depends upon their electrical
characteristics, especially when the conductive polymer
forms part of a circuit through which current must flow.
The carbon blacks in question are, in qeneral,those which
have been recognised to have the ability to impart high
conductivity, for example acetylene blacks and various
~i furnace blacks, such as Vulcan XC-7~ and Vulcan SC (both
sold by Cabot corporation), which are characterised by nigh
surface area (as measured by nitrogen absorption) and high
structure (as measured by dibutyl phthalate absorption).
The latter two parameters and the particle size are often
; 30 used to characterise carbon blacks, and for details of how
they are measured, reference should be made to "Analysis of
--3--
~ ~r O ~
- ' ' ` ..
Carbon Black" by Schubert, Ford and Lyon, Vol. 8,
Encyclopedia of Industrial Chernical Analysis (1969), 179,
published by John Wiley & Son, New York. For details of the
nomenclature used in the carbon black industry, reference
should be made to ASTM standard D 1765-67. Another
characterising property of a carbon black is its d-spacing
(the average distance in pico-meters between adjacent
graphitic planes in the carbon black); thus acetylene black
has a substantially smaller d-spacing (less than 360,
typically about 355) than other carbon blacks. The d-
spacings given herein are measured by electron microscopy.
For further details reference may be made to "Carbon Black"
by Donnet and Voet, published by Marcel Dekker Inc., New
York (1976).
The conductivity of conductive polymers containing
carbon black can be increased by annealing, as described in
U.S. Patents Nos. 3,861,029 and 3,914,363~ By making use of
; this annealing procedure, it is possible to prepare PTC
compositions which contain less than 15~ of carbon black but
which have satisfactory initial conductivity, ror example
~or use in strip heaters.
, : :
A serious problem that arises with conductive
polymers, particularly those exhibiting PTC behavior, is
lack of voltage stability, i.e. a tendency for the
resistivity to rise irreversibly when the composition is
subjected to voltages greater than about 110 volts, e.g. 220
or 480 volts AC, generally at a rate which is dependent on
- ~
the voltage. This problem is particularly serious with
heating devices, because the rise in resistance results in a
:
:
~ ~ _a_
corresponding loss in power output. Although voltage
instability is a serious problem, it appears not to have
been recognized as such in the prior art. German
Offenlegungsch~ift No. 2,634,931 is concerned with
improving the voltage stability of PTC compositions comprising
carbon black dispersed in a polymer containing fluorine,
e.g. polyvinylidene fluoride, by cross-linking the
composition with an uns~turated monomer. However, this
expedient does not yield improved voltage stability with
other polymers.
We have now discovered that improved voltage
stability is possessed by a conductive polymer composition
which comprises
(a) at least one crystalline copolymer which
consists essentially of units derived from
at least one olefin and at least 10 weight
%, based on the copolymer, of-units derived
from at least one olefinically unsaturated
comonomer containing a polar group, and
(b) dispersed in said copolymer, a conductive
carbon black having a particle size greater
than 18 milllmicrons, a d-spacing greater
than 360, and a surface area (A) which is
less than
S
1.2S + e50
where S is the DBP adsorption of the carbon
black.
In one aspect, the present invéntion provides a
conductive polymer composition which comprises
,
.
~, _ 5 _
- .. . .
. , - ~ ' .
3f~
(a) at least one crystallir1e copolymer as defined
above, and
(b) a conductive carbon black as defined above;
subject to the provisos that
(l) if the crystalline copolymer (i) has a melt
index of more than 20 and (ii) is
substantially the only polymeric component in
the composition, the composition has a
resistivity of more than 80 ohm. cm. at 25C;
and
(2) if (i) the crystalline copolymer has a melt
index of more than 20, (ii) the composition
comprises 65 to 85 % by weight of
polyethylene, (iii) the content (L) of carbon
lS black is less than 15~ by weight of the
composition, and (iv) the resistivity (R) of
; the composition at 25C in ohm~ cm. is such
that
2L ~ 5 logl0 R < 451
the composition has a gel fraction of a-t
least 0.6.
The Melt Indexes referred to herein are expressed
in g/lO min.
The compositions of the invention may contain
~25 other polymers, pr~eferably crystalline polymers, in addition
to the crystalline copolymer as defined above. Preferably
the carbon-black-containing copolymer is dispersed in a
second polymer which serves as a matrix therefor, i.e. which
: i
forms a continuous phase in the composition. The other
~ polymer is pre~erably substantially free of carbon black but
; may contain a relatively small proportion of carbon black,
~: : :: :
- : :
~; . ,
~ ~ -6-
e.g. by mi~ration from -the copolymer, such that the resistance/
temperature characteris-tics of the composition are dominated
by the carbon-black-containing copolymer.
The compositions of the invention preferably
exhibit useful PTC behavior, and will then generally ha-ve a
usefuI Ts, which is preferably from 0 to 120C. The
compositions are preferably cross-linked, and it is often
preferred that the gel fraction of the polymeric component of
the composition is at least 0.6. Generally it is desirable
that the composition should have a resistivity at 25C of at
least 80 ohm. cm.
The invention is illustrated in the accompanying
drawings, in which the Figure shows the relationahip between
the surface area and the DBP absorption of the class of carbon
blacks defined above, the continuous line corresponding to
the relationship A=1.2S ~ e S50, and, more especially, of
the specific carbon blacks used in the Examples, lying to the
left of the continuous line, and Comparative ~xamples, lying
to the right, given below, with the exception of Shawinigan
Black, used in comparative Example 12, which has a d-spacing
of about 355 and is excluded from the scope of the invention
by this feature rather.than the relationship between A and S,
and Ketjen black EC, used in Example 23, for which the values
of A and S are.too high to be shown.
The copolymer (a) should be a crystalline copo-
lymer which consists essentially of units derived from at
least one olefin, preferably ethylene and at least 10 ~ by
weight, based on the weight of the copolymer, of units derived
from at least one olefinically unsaturated comonomer containing
a polar group, preferably an acrylate ester, e.g. methyl
acrylate or ethyl acrylate, or vinyl acetate, or acrylic
: or methacrylic acid. The term "crystalline" is used herein
to mean that the polymer has a crystallinity of at least
1%, preferably at least 3%, especially at least 10%. In-
creasing polar comonomer content leads to. reduced crystalli-
nity,-~nd the polar comonomer conten~ is preferabl~ not more~
than.30%. The ~el* Ind@x of the copolymer is.prefe~ably less
tha~ ~0, especially less th`an `10. The hi~her the ~lelt Index,
the more desirable it is that the composition should be cross-
.. , .. .: .
linked to a relatively high level, especially when the
composition is prepared by a process in which annealing is
used to decrease the resistivity of the composition. Thus
the composition should preferably have a gel fraction of at
least 0O6 when the copolymer has a melt index of more than 20
and the composition has been annealed so that
gl0 R < 45
where L is the content of carbon black in percent by weight,
based on the weight of the composition; and R is the
resistivity of the composition at 25C in ohm. cm.
When the composition comprises a polymer which
serves as a matrix for the carbon-black-containing copolymer,
i.e. for the dispersion of the carbon black in the copolymer,
then the matrix polymer preferably has a higher softening
point than the copolymer. Preferably the matrix polymer has
limited compatibility for the copolymer, so that migration of
the carbon black into the matrix polymer is minimised.
Particularly suitable matrix polymers are crystalline
polymers which consist essentially of units derived from one
or more olefins, e.g. high, medium or low density
polyethylene. Other polymers which can be used are
crystalline polymers which comprise 50 to 100%, preferably 80
to 100%, by weight of -CN2CF2- or -CH2C~Cl- units, and in
compositions wnich are n~t annealed, polymers which contain
~ 25 at least 50~, preferably at least 80%, by weight of units
`~ derived~from one or more olefins, together with suitable
comonomers.
~Suitable blacks for use in the invention include
blacks selected from furnace blacks, thermal blacks and
channel blacks. The content of carbon black is preferably 5
-8- ~
. . .
to 25 % by weight of the composition. The content may be
relatively low, e.g. not more than 12 or 15~, in which case
it is preferred that the composition should be annealed,
prior to any cross-linking, at a temperature above the
melting point of the copolymer, and preferably above the
melting point of the highest-melting polymer in the
composition, so as to decrease its resistivity. Typically
the composition will be annealed so that
2L ~ 5 log10 R < 45,
where L and R are as defined above.
Alternatively, the content of carbon black may be relatively
high, e.g. above 15%, in which case annealing prior to cross-
linking may be unnecessary, or may be for a limited time such
that, at the end of the annealing,
2L + 5 log10 R > 45.
In such compositions the particle size of the carbon black is
preferably greater than 30 millimicrons~ It is of~en
advantageous, whether or not the composi-tion has been
annealed before cross-linking, to heat the cross-linked
composition for a short period at a temperature ahove its
melting point.
Cross-linklng of the compositions is carried out
after they have shaped, e~g. by melt extrusion, and can be
effected by any of the methods well known in the art,
preferably with the aid of ionising radiation or an organic
peroxide. Preferably the composition is cross-linked at
least to an extent equal to that induced by exposure to
ionising radiation to a dosage of at least 0.75 M megarads,
where M is the Melt Index of the copolymer, e.g. to a gel
fraction of at least 0.6.
_~9_
~,~,e~
The compositions of the invention may contain
other ingredierlts which are conventional in the art, e.g.
antioxidants, flame retardants, inorganic fillers, thermal
stabilisers, processing aids and cross-linking agents or the
residues of such ingredients after processing. The addition
of a prorad (an unsaturated compound which assists radiation
cross~linking) is often useful in improving stability,
especially in unannealed products; suitable amounts of pro-
rad are less than 10%, preferably 3 to 6%.
The compositions of the invention in which the
only polymeric component is the copolymer (a) can be made by
blending the ingredients in conventional mixing equipmen4 at
a temperature above the melting point of the copolymer,
followed by annealing and cross-linking as desired.
Alternatively, a master batch containing the carbon black
and part of the copolymer can first be prepared, and the
master batch then blended with the remainder of the
copolymer. Similarly, when the composition contains a
matrix polymer in which the carbon-black-containing
copolymer is distributed, such compositions are made by
blending the matrix polymer and a master batch of the carbon
black in the copolymer, followed by annealing and cross-
linking as desired~ The master batch preferably contains 20
to 50%, e.g. 30 to 50~ of the carbon black.
~ ~ 25 The invention includes electrical devices
; comprising an element composed of a composition of the
invention ancl at least two electrodes adapted to be
connected to a source of electrical power so as to cause
current to flow through the elementO One class of such
devices comprise a pair of laminar electrodes having a said
element in the form of a lamina therebetween. Another class
--1 0--
of such devices comprise an elongate element of a
composition of the invention; at least two longitudinally
~ ~/c~ /
~; extending electrodes embedded in said element ~r~l~e to
each other; and an outer layer of a protective and
insulating composition.
The invention is illustrated by the following
Examples.
EXA
In the examples which follow, the test samples
were prepared in accordance with the procedure described
below unless otherwise stated. The ingredients for the
master batches were milled together on a 2 roll mill, 10 to
30C above the melting point of the polymer. When used,
additives were added before the carbon black. The preferred
range of carbon black concentration in the master batch is
30 to 50~ and most of the mixes prepared were in this range,
although for some compositions loadings as low as 20 or as
high as 70% were used. The carbon black master batch was
milled for five minutes, then removed from the mill and
either cooled to room temperature for subsequent use, or
immediately mixed with the matrix polymer to form the final
blend. For t:he~preparation of the final blend, the desired
amount of master batch was fluxed on a 2 roll mill at a
temperature 10-30C higher than the melting temperature of
the highest melting polymer in the final blend. The
remaining constituents, including the other polymer(s), were
immediately added to the master batch and the mixture
blended for five minutes. The amount of master batch was
chosen to yield a resistance of about 10 kilo ohm. in the
~Q~
test samples. The final blends were hydraulically pressed
into 15 x 15 x 0.06 cm. sheets at 2,800 kg/cm2 and a
temperature of at least 175C. Samples 2.5 x 3.75 cm. were
cut from the slabs and 0.6 cm. strips~of conductive silver
paint were coated on each end of the longest dimension to
define a test area 2.5 x 2.5 cm.
Where indicated, prior to crosslinking, the above
samples were annealed at 150 to 160 (200 for
polypropylene) cyclically for up to two hours periods
followed by cooling to room temperature until a minim~m
resistance level was reached. (Usually, two or three
annealing cycles sufficed). Usually the samples were
crosslinked by radiation; the doses used ranged from 6 to 50
Mrads, with most samples receiving 12 Mrads.
Voltage stability was assessed by measuring the
room temperature resistance of the sample before (Ri) and
after (Rf) the sample had been subjected to a period of
operation at high voltage stress. In most instances this
involved operating the heater for 72 hours at 480 volts in
ambiant air, then disconnecting from the source of
electrical power and cooling to room temperature before
. .
remeasurement. The voltage stability is expressed as the
ratio of initial resistance to final resistance (Ri/Rf).
~'
,
:
:
; :
-12-
'
. . .
EXAMPLE l
It should be noted that the proportion of master batch (and
hence of carbon) required to achieve the desired resistance level of lO
~! kilo ohms is somewhat dependent on the processing conditions and on thetype of carbon black. To illustrate this, blends containing Sterling
S0, Vulcan XC-7~ and Black Pearls 880 were prepared as described above
0.~5
and also using a/kg Banbury mixer in place of the two roll mill, the
temperatures and times being the same in each experiment. The master
batch polymer was an ethylene (18%) ethyl acrylate copolymer (DPD6169)
and the matrix polymer was a low density polyethylene (Alathon 34). The
concentration of carbon in the master batch in each case was 36%. Table
I shows the percentage of master batch in the final blend (% MB) and the
percentage of carbon black in the final blend (% ce ) .
TABLE I
Carbon Black Two Roll Mill Banbury Mixer
%MB %CB %MB %CB
; Sterling S0 50 18 60 22
Vulcan XC-72 40 l4.4 50 l8
" ~ Black Pearls 880 40 l4.4 40 l4.4
EXAMPLE 2
; A variet.y of carbon blacks were blended with DPD 6l69 to pro-
vide master batches which were then mixed with Alathon 34 as the matrix
to achieve a resistance level
polymer in the amount needed/in the final product of lO kilo ohms. All
the samples were irradiated to a dosage of 12 Mrads, and most were an-
nealed prior to irradiation.
The carbon blacks employed are identified in Table II below,giving the~trade ia~e, the ASTM code, the particle size in miliimicrons
(D), the surface anea as measured by ~irrosen absorption in M 2/9 (A)
.
and the dibutylphthalate absorption in CC/lOOg(s). Table II
also shows the percentage of carbon black in the different
samples, and the results of stability tests on these sarnples.
In Table II, the samples marked C are comparative Examples.
TABLE I I
Carbon Bl ack
Annealed Unannealed
Samples Sarnples
ASTM D A S % %
Trade Mark Code Carbon Ri/ Carbon Ri/
Black Rf Black Rf
1. Sterling NS N7747527 70 15.1 0.76
2. Philblack N765N76560 30 10211.1 0.56
3. Furnex N765 N765~030 107 9.70.4
4. Sterling N765N76560 30 1169.4 0.58 16.2 0.76
5 . Sterling V N6605035 91 10.8 0.7
6. Sterling VH N6506036 122 9.40.49
7. Statex N550 N5504240 122 7.90.83
8. Sterling So-lN5394242 109 10.8 0.55
9. Sterling SO N5504242 120 9.70.6 18 0.63
10. Philblack N550N55042 44 1189 4 0.65
11. Regal 99N440 364660 19.1 0.35
C 12. Shawnigan Black --42 64 - 15.1 0.004
13. Vulcan KN351 2870124 10.~ C.47
14. Vulcan 3N330 2780103 10.1 0.48
15. Vulcan 3H N3472690 124 7.90.38
' C 16. Regal 330 N3272594 70 16.2 0.19
17. Vulcan 6H N24221124 128 10.1 0.38
C 18. Vulcan CN293 23145100 11.9 0.29 16.2 *
C 19. V~lcan SC N29422203 106 10.1 0.24
C 20. Black Pearls 880 --16 220 110 14.4 *
~ .
;C.21. Vulcan XC-72N472 35 254178 10.8 0.23
C 22. Black Pearls 74 --17 320 10910.8
23. Ket jen black EC --30 1000 340 5.3 0.52
'
~ Sample has such poor voltage stability that it burns.
; ' - 14-
~ .
~t
.~ .
EXAMPLE 3
Tests similar to thGse described in Example 2 were
carried out using different polymers in place of the DPD 6169
and/or the Alathon 34. The tests are summarized in Table
III below.
TA3LE III
Copolymer Trade Mark Polymer in Trade Mark
in master batch and Melt Final i31end Melt Index Remarks
Index (M.I.) (M.I.)
_
Ethylene (18%) ethyl DPDA 61 81 Polyethylene Alathon 3~ Very similar
acrylate M.I. 2.2 0.93 density M.I.-3 results to
those of
Table II
Ethylene-(18%) ethyl DPDA 916g as above as above Similar
acrylate M.I. 20 results to
those of
- Table~
Ethylene-(6.6%) DPD 7365 as above as above Voltage stab-
ethyl acrylate M.I. 8 ility very
poor with most
carbon blacks
Ethylene-(5.5%1 DPD 7870 as above as above VoltagQ stab-
ethyl acrylate M.I. ility very
poor with most
carbon blacXa
i Ethylene-(18%) vinyl Alathon 3175 as above as above Very similar
acetate M.I. 8 results to
those of
- Table II
Ethylene-(28%) vinyl Alathon 3172 as above as above Very similar
acetate M.I. 6 results to
Table II
Ethylene-(30%) Vistalon 702 as above as above Voltage stab-
propylene Mooney Visc. 30 with most
carbon blacks
Polyethylene DYNH Polyethylene Alathon 7030
0.93 density M.I. 2 O.g6 density M.I. 3
Ethylene-~18%) ethyl DPD 6169 Ethylene-(6.6%) DPD 7365 Results similar
- acrylate M.I. 6 ethyl acrylate M.I. 8 to Table II
s~ightly diff-
erent preferrad
. range
E Polyethylene DYNH Polypropylene Profax 8263 Voltage stab-
0.93 density M.I. 2 (High Impace) ility very poor
with most
carbon blacks
Ethylene-(18%) ethyl DPD 6169 Vinylidene di Kynar 7201 ~esults similar
acrylate M.I. 6 Fluoride copolymer M.I.33 to Table II
as above as above none - Results very
similar to
- lS -
'
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