Note: Descriptions are shown in the official language in which they were submitted.
rlo~
This invention relates to polymeric compositions
having electrical resistances with a positive temperature
coefficient (PTC), which are referred to herein as PTC
materials.
Many electrical heating appliances in recent years
have been self-regulating heating systems which utilize
materials exhibiting certain types of PTC characteristics,
distinguished by the property that, upon attaining a
certain temperature, a substantial rise in resistance occurs.
Prior art heaters utilizing PTC materials generally exhibit
more or less sharp rises in resistance within a narrow
temperature range, but below that temperature range exhibit
only relatively small changes in resistance with temperature.
The temperature at which the resistance commences to
increase sharply is often designated the switching or
anomaly temperature (Ts) since on reaching that temperature
the heater exhibits an anomalous change in resistance
and tends to switch off. Unfortunately, such switch-off
occurs at relatively low power densities with prior art
PTC elements. Self-regulating heaters utilizing PTC
materials have advantages over conventional heating
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107~()9f~
apparatus in that they generally eliminate the need for
thermostats, fuses or in-line electrical resistors.
The most widely used PTC material has been doped
barium titanate which has been utilized for self-regulating
ceramic heaters employed in such applications as food
warming trays and other small portable heating appliances.
Although such ceramic PTC materials are in common use for
heating applications, their rigidity has severely limited
the type of applications for which they can be used. PTC
materials comprising electrically conductive polymeric
compositions are also known and certain types have been
shown to possess the special characteristics described above.
However, in the past, use of such polymeric PTC materials
has been relatively limited, primarily because of their low
heating capacity. Such materials generally comprise one
or more conductive fillers, for example, carbon black or
powdered metal, dispersed in a crystalline thermoplastic
polymer. PTC compositions prepared from highly crystalline
polymers generally exhibit a steep rise in resistance
commencing a few degrees below their crystalline melting
point similar to the behaviour of their ceramic counter-
parts at the Curie temperature (the TS for ceramics). PTC
compositions derived from homopolymers and copolymers of
lower crystallinity, for example, a crystallinity of less than
about 50%, exhibit a somewhat less steep increase in resistance
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which commences at a less well defined temperature range
often considerably below the crystalline melting point. In
the extreme case some polymers of` low crystallinity yield
resistance temperature curves which are more or less simply
concave (from above) with no definable point at which a
steep rise begins. Other types of thermoplastic polymers
yield resistance temperature curves which increase fairly
smoothly and more or less steeply but continuously with
temperature.
The present invention will be better under-
stood by way of reference to the accompanying drawings in
which:
Figure 1 illustrates characteristic curves
for various different types of PTC compositions, and
Figures 2 to 14 illustrate the variation in
resistance with temperature (R Vs T) for some compositions
exemplified herein.
In Figure 1 curve 1 illustrates the sharp
increase in temperature (hereinafter known as type I be-
haviour) characteristic of (inter alia) barium titanate
and polymers having very high crystallinity, curve II shows
the more gradual increase beginning at lower temperatures
(relative to the polymer melting point) hereinafter known
as type II behaviour characteristic of most medium to
high crystallinity polymers, curveIII (Type 3 behaviour)
illustrates the concave (from above) characteristic of
many very low crystallinity polymers while curve IV illus-
trates the large increase in resistance without any region
of more or less constant resistance (at least in the range
of commercial interest) seen with some materials (Type IV
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behaviour). Curve V illustrates the gently increasing
resistance temperature characteristic shown by many
"normal" electrical resistors (Type V behaviour). ~lthough
the above types of behaviour have been illustrated by
reference to specific types of material it will be realized
by those skilled in the art that the type of behaviour is
very significantly influenced by the type and amount of
conductive filler present and,
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particularly in the case of carbon black filler, its particle
size, surface characteristics, tendency to agglomerate and
the shape of the particles or particle agglomerates (i.e.
its tendency to structure).
It should be noted that although the prior art
references describe compositions that purportedly manifest
Type I behaviour, experiment shows that such prior art
compositions in fact usually manifest Type II to Type IV
behaviour, these Types being unrecognized by the prior art.
Additionally, even those prior art materials which do have
a distinct anomaly point, i.e., undergo a sharp increase in
resistance at Ts, show a fall-off, i.e., decrease in
resistance if the temperature of the PTC element increases
significantly above TS which can occur particularly when
high power densities are present in the element.
Kohler, in United States Patent 3,243,753, discloses
carbon filled polyethylene wherein the conductive carbon
particles are in substantial contact with one another.
Kohler describes a product containing by weight 40% polyethylene
and 60% carbon particles so as to give a resistance at room
temperature of about 0.4 ohm/cm. Asis typical of the alleged
performance of the prior art meterials, Kohler's PTC product
is purportedly characterized by a relatively flat curve of
electrical resistance versus temperature below the switching
temperature followed by a sharp rise in resistivity of at
least 250% over a 14 C range (i.e., approaching Type I
behaviour). The mechanism suggested by Kohler for the
sharp rise in resistivity is that such change is a function
10'~ 6
of the difference in thermal expansion of the materials,
i.e. polyethylene and particulate carbon. It is suggested
that the composition's high level of conductive filler
forms a conductive network through the polyethylene polymer
matrix, thereby giving an initial constant resistivity at
lower temperatures. However, at about its crystalline
melt point, the polyethylene matrix rapidly expands, such
expansion causing a breakup of many of the conductive
networks, which in turn results in a sharp increase in the
resistance of the composition.
Other theories proposed to account for the PTC
phenomenon in conductive particle filled polymer compositions
include complex mechanisms based upon electron tunnelling
through inter grain gaps between particles of conductive
filler or some mechanism based upon a phase change from
crystalline to amorphous regions in the polymer matrix.
A background discussion of a number of proposed alternative
mechanisms for the PTC phenomenon is found in "Glass
Transition Temperatures as a Guide to the Selection of
Polymers Suitable for PTC Materials", J. Meyer, PolYmer
Enqineerinq and Science, November 1973, Vol. 13, No. 6.
Of significance is the fact that the PTC polymeric
materials of the prior art contemplate compositions which
exhibit a TS at or below the melting point of a thermoplastic
component.
As mentioned above, Kohler discloses carbon black
dispersed in a polyethylene or polypropylene polymeric matrix,
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the polyolefin having been polymerized in situ, such
materials exhibiting PTC characteristics at the melting
temperature of the po]ymers. Likewise, Kohler discloses
carbon particles dispersed in polyethylene in which the
composition may be crosslinked, or may contain a thermo-
setting resin to add strength or rigidity to the system.
However, TS still remains at about the crystalline melting
point of the thermoplastic polyethylene, i.e., 120 C.
United States Patent No. 3,825,217 to Kampe discloses
a wide range of crystalline polymers which exhibit PTC
characteristics. These include polyolefins, for example,
low,medium, and high density polyethylenes and polypropylene,
polybutene-l, poly (dodecamethylene pyromellitimide) and
ethylenepropylene copolymers. It is also suggested that
blends of crystalline polymers, for example, a polyethylene
with an ethylene-ethyl acrylate copolymer may be employed for
the purpose of varying the physical properties of the final
product. Also disclosed by Kampe is a process of thermal
cycling above and below the melting temperature of the polymers
to achieve a lower level of resistance. Similarly,
Kawashima et al, United States Patent 3,591,526 discloses
polymer blends containing carbon black exhibiting PTC
characteristics. However, again the thermoplastic material
dictates TS which occurs at about its crystalline melting
point, while the second material is functioning merely as a
carrier for the carbon black loaded thermoplastic.
Finally, United States Patent 3,793,716 to Smith-
Johannsen discloses conductive particle-polymer blends
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exhihiting PTC characteristics in which a crystalline polymer
hav;ng dispersed carbon black therein is dissolved in a
suit~ble solvent above the polymer melting point which solvent
is then evaporated to afford a composition manifesting a
decrease in room temperature resistivity for a given level
of conductive filler. Again TS is at or below the melting
point of the polymer matrix, and the process of heating the
polymer above the melting temperature is directed at decreasing
resistance and/or maintaining constant resistance at
ambient temperatures.
Current self-regulating thermal devices utilizing
a PTC material contemplate, as above indicated, but do not
in fact provide extremely steep (Type I) R = f (T) curves so
that above a certain temperature the device will effectively
shut off, w~ile below that temperature a relatively constant
wattage output at constant voltage is achieved. At temperatures
below TS the resistance is at a relatively low and constant
level and thus the current flow is relatively high for any
given applied voltage. The energy generated by this current
flow is dissipated as heat, thereby warming up the PTC
material. The resistance stays at this relatively low level
until about Ts, at which point a rapid increase in resistance
occurs. With the increase in resistance there is a concomitant
decrease in power, thereby limiting the amount of heat generated
so that when the TS temperature is reached heating is
essentially stopped. Then, upon a lowering of the temperature
of the device below the TS temperature by dissipation of heat
to the surroundings, the resistance drops thereby increasing
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the power output. At a steady state, the heat generated
will balance the heat dissipated. Thus, when an applied
voltage is directed across a Type I PTC heating element,
the Joule heat causes heating of the PTC element up to about
its TS (the rapidity of such heating depending on the type
of PTC element), after which little additional temperature
rise will occur due to the increase in resistance. Because
of the resistance rise, such a PTC heating element will
ordinarily reach a steady state at approximately TS thereby
self-regulating the heat output of the element without
resort to fuses or thermostats.
From the preceding discussion, materials manifesting
Type I behaviour will have advantages over PTC materials
showing other types of behaviour. Types II and III have a
disadvantage in that because of the much less sharp transition
the steady state temperature of the heater is very dependent
on the thermal load placed on it. Such materials also suffer
from a current inrush problem as described in greater detail
below. Type IV PTC materials, because they lack a temperature
range in which the power output is not markedly dependent on
temperature, have so far not been considered as suitable
materials for practical heaters.
Although as mentioned above the prior art recognizes
the considerable advantage of having a heater composition
which possesses a resistance-temperature characteristic of
Type I, many of the allegedly Type I compositions alluded
to in the prior art in fact show behaviour more closely
resembling Type II or Type III behaviour. The optimum
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(Type I) behaviour is shown by only a limited number of
compositions and there has been a long felt need for a means
of modifying compositions showing Type II or III behaviour
which on the basis of physical or other characteristics
would be useful for PTC heating elements so that their
behaviour more closely approaches Type I. Furthermore,
and to their great disadvantage, as heretofore indicated,
many prior art materials although showing a more or less
sharp rise in resistance at TS "turn on" again if the
temperature rises slightly above Ts. If the increase in
resistance at or above TS is not great enough and/or if
resistance drops above the composition's melting point (as
is generally the case with prior art materials) then thermal
runaway and burn-out can occur.
Polymeric PTC compositions have also been suggested
for heat shrinkable articles. For example, Day in United
States Patent Office Defensive Publication T905,001 teaches
the use of a PTC heat shrinkable plastic film. However,
the Day shrinkable film suffers from the rather serious
shortcoming that, since TS is below the crystalline melting
point of the film, very little recovery force can be
generated. Neither Day nor any of the other previously
discussed prior art teachings even address themselves to,
much less solve, certain additional problems inherent in
all prior art PTC heaters. One is the problem of current
inrush. This problem is particularly severe when it is
desired to provide a heater having a TS in excess of about
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100C. While it is feasible to find a polymeric PTCmaterial having a TS as high as 150 C, the resistance of
such material at or ]ust below the TS may be as much as 10
times its resistance at ambient temperature. Since the
PTC heater ordinarily functions at or slightly below its
Ts, its effective heat output is determined by its resistance
at slightly below Ts. Therefore, a PTC heater drawing, for
example 50 amps at 150 C may well draw 500 amps at ambient
temperatures.
When one desires to use a heat recoverable material
comprising a PTC heater further deficiencies of compositions
exhibiting current inrush appear. It is advantageous for
heat recoverable articles to shrink as rapidly as possible.
Obviously a heater having a flat power/temperature characteris-
tic,will heat up more rapidly and uniformly than a heater
having, for example, a power output which drops to one
tenth of its ambient temperature value as its temperature
rises to Ts. The use of a polymer of high crystallinity
as the matrix for the conductive particles minimizes this
aforesaid current inrush problem. Furthermore, such high
crystallinity polymers exhibit a steep increase in resistance
(i.e. have a Ts) about 15 C below their crystalline melting
point. Unfortunately, such polymers still possess considerable
crystallinity at TS and thus not only show little recovery
if themselves converted into a heat recoverable state but
resist recovery of associated heat recoverable members which
may themselves be above their recovery temperature. Obviously,
if one selects heater resistances (i.e. lower resistances)
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so that th~ heater is switched off at a temperature closer
to its peak resistance temperature (T ), which correspond
closely to the actual melting point, the aforementioned
disadvanta~e may be avoided. However, all prior art heaters
show resistances which either decrease sharply or in a very few
instances stay substantially constant as the temperature of the
PTC material is increased above its melting point. Another
shortcoming of prior art PTC polymeric compositions is that as
they are elongated (as is necessary in the normal methods of form-
ing a heat recoverable object) the ratio of the resistance atTp to the resistance at TS decreases dramatically. Thus an
initial ratio of 10 may fall to 105 at 10% elongation and 103
at 25% elongation. These last factors greatly increase the
potential for runaway overheating with prior art heaters when
used in heat shrinkable devices.
It would therefore substantially advance the art to
provide a PTC material which more nearly approaches Type I
behaviour and which does not suffer from severe current inrush.
The present invention is based on the surprising observation
that many of the hereinabove discussed deficiencies of the
prior art may be remedied by the provision of a polymeric, thermo-
plastic electrically conductive composition which exhibits a
sharp rise in resistance just below its melting point but whose
resistance continues to rise as the temperature is increased above
the melting point. Heaters having this characteristic will con-
tinue to control (i.e., will not "turn on" again to any deleteri-
ous extent) even if their temperature rises above the melting point
of the thermoplastic polymer, while prior art heaters would suffer
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thermal runaway and perhaps burn out under these conditions.
By provision of PTC compositions having this
characteristic the present invention allows the manufacture of
heaters which will control at a resistance level even considerably
above the resistance lever at TS with reduced risk of thermal
runaway and burn out. Furthermore, because the resistance
continues to increase above TS and above the melting point, the
heater temperature under power shows very little change under condi-
tions which vary from low to high therm~l loads. Heaters made
from the compositions of the present invention may reach their
operating range in about the same period of time irrespective
of the thermal environment within wide limits and are "demand
insensitive'~. This insensitivity of the heater temperature to
the thermal load enables the manufacturer of heat shrinkable
devices to design products whose behaviour is predictable and
which will not damage any substrate, such as a thermoplastic cable
jacket, onto which the device is recovered.
The present invention provides an electrically
conductive crosslinked polymeric composition comprising a
polymeric material which exhibits significant green strength
before crosslinking and which is elastomeric at room temperature
in its crosslinked state and a thermoplastic polymeric material,
the composition having conductive particles dispersed therein,
the composition showing increasing electrical resistance with
increase in temperature at least in some temperature range above
the melting point of both materials.
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The materials may be segments of one polymer, for
example a graft or block copolymer, or they may be a physical
blend of two polymers. Also, where a copolymer is used, there
may also be present a thermoplastic or elastomeric material, or
both, in physical admixture therewith.
As especially advantageous grafts there may be mentioned
ethylene-propylene grafted with a crystalline polyolefin, for
example, polypropylene, while as block copolymers there may be
mentioned especially ethylene-propylene/polystyrene and polytetra-
methylene oxide/polytetramethylene terephthalate.
As the thermoplastic materials, there may especially
be used, polypropylene, polyvinylidene fluoride, or polyethylene,
while as preferred elastomeric materials there may be mentioned
chlorinated and chlorosulfonated polyethylenes and neoprene.
An especially advantageous composition is an ethylene-
propylene rubber, polypropylene and carbon black, the preferred
weight ranges being 40 to 90%, 5 to 40% and 5 to 35%, respectively.
Advantageously, the composition is a PTC material at
least from ambient temperature to the melting point of the
thermoplastic material.
The present invention also provides a heating element
in which a shaped structure of the material is positioned between
at least a pair of electrodes. The electrodes may be connected
to an appropriate electrical power supply, to energize the heater.
The element may be heat-recoverable.
The invention also provides a composition useful as
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starting material for the crosslinked compositions, which
comprises an uncrosslinked blend of elastomeric and thermo-
plastic polymers, with carbon black, the elastomeric polymer
havin~ sufficient strength at room temperature to support its
own weight without appreciable deformation for an extended
period.
Uncured (uncrosslinked) elastomers are often referred
to as "gum stocks". If mixtures of most gum stocks and a
thermoplastic are equilibrated (heated for a time sufficient to
achieve a preferred molecular configuration and orientation
with respect to each other at that temperature) and then
cooled, the mixture on cooling will equilibrate rapidly to a
different lower temperature molecular configuration. However,
some gum stocks, whether because of very high molecular weight
(which causes entanglements), small regions of crystallinity,
or other characteristics, for example rigid or glassy portions
of the molecules, after being equilibrated to a high temperature
favoured configuration, changefrom this configuration, whether
alone or in admixture with a thermoplastic, only very slowly or
not at all when cooled to room temperature. The resulting
gum stocks show what is often called "green strength". ~his
property is well known in the art, as is a concomitant property,
that the gum stock at room temperature possesses form stability
such that articles prepared from such materials do not distort
and flow to any significant extent even though uncorsslinked.
Such gum stocks, for example, show a significant resistance
to creep and a reluctan~e to coalescewhen in contact in a
granular form when compared with other gum stocks that do not
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107~ 6
possess green strength. Suitable elastomers for the composi-
tions of the present invention are thase possessing significant
green strength and it is believed that this characteristic
enables the composition to be crosslinked to "lock in"
the desired configuration which leads to the observed PTC
behaviour which unexpectedly continues in a temperature range
where a completely amorphous mixture of polymers exist, i.e.,
above the melting point of any component.
In this specification, the term elastomer means a
polymeric material which exhibits elastic deformation under
stress, flexibility, and resilience and is capable of recover-
ing from large strains. The term thermoplastic means a
polymeric material which is incapable of recovering to a
substantial degree from large strains at room te~perature,
while at higher temperatures, above its melting point, it is
capable of being reformed into any desired new shape. The
term melting point means the temperature above which a specific
material becomes elastomeric if crosslinked, or a viscous
fluid if uncrosslinked and includes the softening point of
non-crystalline materials.
Melting points are determined in accordance with the
- ASTM method appropriate to the material. If the non-elastomeric
component, e.g., of a graft or block copolymer, is at least
partially crystalline ASTM D 2117 - 64, which monitors the
disappearance with temperature rise of the birefringence due
to the crystalline material, is suitable. If the component is
non-crystalline, i.e., glassy, a preferred method is D 2236 - 70,
using a torsional pendulum. If neither of these methods is
appropriate, there may be used methods ~ 648 - 72 or D 1525 - 70.
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The term gum stock denotes an uncrosslinked material
which after crosslinking exhibits elastomeric properties.
The term "significant green strength" when applied to either
a gum stock or thermoplastic elastomer means that the material
has significant resistance to creep and a reluctance to
coalesce when brought into contact with itself and exhibits
a tensile st~ess of at least 10 p.s.i. at 20% elongation.
As examples of the many classes of thermoplastic
materials suitable for use in the invention, there may be
mentioned
(i) Polyolefins, for example, polyethylene and
polypropylene.
(ii) Thermoplastic copolymers of olefins, for example,
ethylene or propylene, with each other and with
other copolymerizable ethylenically unsaturated
monomers, for example, vinyl esters, acids or
esters of x, ~-unsaturated organic acids.
(iii) Halogenated vinyl or vinylidene polymers, for
example, those derived from vinyl chloride,
vinylidene chloride, vinyl fluoride, vinylidene
fluoride and copolymers thereof with each other
or with other halogenated, or other, unsaturated
monomers.
(iv) Polyesters both aliphatic and partially or wholly
aromatic for example poly (hexamethylene adipate
or sebacate), poly (ethylene terephthalate) and
poly (tetramethylene terephthalate).
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(v) Polyamides, for example~ Nylon-6, Nylon -6 6,
Nylon -6 10 and the "Versamids" (a condensation
product of dimerized and trimerized unsaturated
fatty acids, in particular linoleic acid with
polyamines - Versamid is a trade mark).
(vi) Miscellaneous polymers such as polystyrene,
polyacrylonitrile, thermoplastic silicone resins,
thermoplastic polyethers, thermoplastic modified
celluloses, and polysulphones.
As the elastomeric component, any gum stock may be
used which exhibits significant 'rgreen strength" as previously
defined. Most commercially available gum stocks for elastomers
possesseither substantial green stre~gth or little if any green
strength. Therefore, those skilled in the art may readily
differentiate from the following list of elastomer gum stocks
those memberspo~sessin~g substantial green strength, for example,
polyisoprene both natural and synthetic, ethylenepropylene random
copolymers, styrenebutadiene random copolymer rubbers, styrene-
acrylonitrilebutadiene terpolymer rubbers with and without added
minor copolymerized amounts of ~, ~-unsaturated carboxylic
acid, polyacrylate rubbers, polyurethane gums, random
copolymers of vinylidene fluoride and, for example, hexafluoro-
propylene, polychloroprene, chlorinated polyethylene, chloro-
sulphonated polyethylene, poly ethers, plasticized polyvinyl
chloride containing more than 21% plasticizer and substantially non-
crystalline random co- or ter-polymers of ethylene with vinyl
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esters or acids and esters of ,~-unsaturated acids.
Thermoplastic-elastomeric copolymers, which are suitable
for use in this invention include both graft and block
copolymers. There may be mentioned, for example:
(i) random copolymers of ethylene and propylene
grafted with polyethylene or polypropylene
side chains.
(ii) slOck copolymers of ~-olefins, for example,
polyethylene or polypropylene, with ethylene/
propylene or ethylene/propylene/diene rubbers,
polystyrene with polybutadiene,
polystyrene with polyisoprene,
polystyrene with ethylene-propylene rubber,
poly vinylcyclohexane with ethylene-propylene
rubber, poly -methylstyrene with polysiloxanes,
poly-carbonates with polysiloxanes, poly
(tetramethylene terephthalate) with
poly (tetramethylene oxide1 and thermoplastic
polyurethane rubbers.
Advantageously, the composition comprises a mixture
containing from about 3.0 up to about 75.0 wt %, preferably
from 4 to 40/O~ of elastomer based on the combined weight of
elastomeric and thermoplastic materials. In block copolymers
possessing both thermoplastic and elastomeric regions the
elastomeric region will preferably comprise from 30 to 70 wt %
of the polymer molecule. The preparation of these block and
graft copolymers or the admixing of the thermoplastic and
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elastomer can be effected by conventional means well known
to the art, such as for example, milling, sanbury blending,
etc. The amounts of particulate conductive filler advantage-
ously range from 4 to 60%, 5 to 5~/~ being preferred.
In general, any type of conductive particulate
material may be used to render these compositions conductive.
Preferred conductive fillers for the polymeric PTC composition
useful in the present invention, in addition to particulate
carbon black, include graphite, metal powders, conductive
metal salts and oxides and boron or phosphorus doped silicon
or germanium.
Those skilled in the art will understand that any
suitable crosslinking method may be used to effect crosslinking
of the admixture of the thermoplastic and the gum stock (or the
block copolymer), provided that both polymer phases are cross-
linked thereby. Suitable methods include chemical crosslinking
agents, for example, peroxides, and preferably ionizing
radiation.
The following Examples illustrate the invention,
Examples 3 and 14 being for comparison, the accompanying
drawings show, in Figures 2 to 14, the variation in resistance
with temperature (R Vs T) for some of the exemplified compositions.
In the examples according to the invention, the elastomers
all exhibited significant green strength.
Unless otherwise noted, all samples for the examples
below were prepared and tested in the following manner, with
the amounts given in percentages by weight.
The polymeric constituents were blended on an
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electrically heated two-roll mill at 200C for five minutes,
after which carbon black was added and the mixture then
blended for a further five minutes.
~ e blended compositions were pressed at 200 C to
slabs approximately 0.063 cm thick. Specimens 2.5 x 3.8 cm
were cut from the slab, and conductive paint was applied in
two 0.63 cm wide strips along opposing edges on both sides
of the slab.
The specimens were annealed to reduce their resistance
to a minimum by heating to 200C for intervals of five minutes,
and then cooling to room temperature. This thermal cycling was
repeated as necessary to obtain a minimum resistance. Generally,
a total annealing time of 15 minute~ at 200C was found to be
adequate. For a more detailed description of annealing to
minimize resistance, see Kampe United States Patent 3,823,217.
The specimens were crosslinked by irradiation at a
dose of 12 megarads. The resistance vs. temperature curves
were plotted by measuring the resistance across the specimen
with an ohmmeter that uses small applied voltages (less than
1 volt) thereby avoiding self-heating of the specimens. The
specimens were heated in an air circulating oven with resistance
measured at selected temperatures.
EXAMPLE 1
TPR-2000 (a graft copolymer of ethylenepropylene
rubber and approximately
20% polypropylene from Uniroyal Corp.) 70
Vulcan~XC-72 (carbon black from Cabot Corp.) 30
* Trade ~!lark
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The elastomer is believed to be grafted with a
substantially crystalline polypropylene. From Figure 2 it
can be seen that the material exhibits a steady rise in
resistance from ambient to the melt temperature of polypro-
pylene (165 C~ after which it exhibits substantially constant
resistance or a slight decrease in resistance to a higher
temperature where the resi.stance commences to rise again,
only falling at a temperature above 220C.
EXAMPLE 2
Kraton G 6521 (an ABA type block copolymer of poly-
styrene and ethylene-propylene
rubber from Shell Chemical Corp.) 75
~C-72 (carbon black) 25
Referring to Figure 3, it can be seen that the
substantially amorphous polystyrene-ethylene-propylene rubber
block copolymer exhibits a relatively sharp rise in resistance
commencing just below the Tg of polystyrene which resistance
continues to rise above the Tg to a peak value at 200C. This
is in sharp contrast with the teachings of the prior art, as
for example J. Meyer "Glass TranSition Temperature as a
Guide to Polymers Suitable for PTC Materials", supra, wherein
it was indicated that substantially amorphous materials, for
example polystyrene or ethylene propylene rubber only exhibit
PTC characteristics up to their Tg.
EXAMPLE 3
Vistalon 404 (ethylene-propylene rubber: E/P ratio of
* Trade Mark
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45.55 from Exxon Chemical Corp.) 65
XC-72 (carbon black) 35
An ethylene-propylene rubber having an ethylene to
propylene ratio of 45 to 55 was found to exhibit no PTC
characteristics. More specifically, the specimen exhibited
relatively high resistances at ambient temperature notwith-
standing a number of thermal cycles. The material then
exhibited a continuous and rapid decrease in resistance upon
heating making it unsuited for self-regulating heating
applications. (Figure 4)
EXAMPLE 4
TPR-l900 (believed to be an EPR-polypropylene graft
copolymer) 60
Profax 6523 (polypropylene) 20
Vulcan XC-72 (carbon black) 20
The EPR-polypropylene graft copolymer was blended with
polypropylene after which it was milled with carbon black as
given in the general procedure. As can be seen from Figure 5,
a relatively uniform increase in resistance occurs up to the
melting point, after which a small decrease in resistance
occurs. However, the material then exhibits a pronounced
PTC well above the melting point.
EXAMPLE 5
Kynar*304 (Polyvinylidene Fluoride from
Pennwalt Corporation) 25
Cyanocril R (Polyethyl acrylate from American
Cyanamid) 50
Vulcan XC 72 25
* Trade Mark
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The resistance of this composition increased with
increasing temperature to a temperature substantially above
the melting point of polyvinylidene fluoride. In the absence
of the elastomer, blends of this thermoplastic with carbon
black show a resistance peak at about 160C and at higher
temperatures show a pronounced drop in resistance.
EXAMPLE 6
TPR 1900 (believed to be an EPR-polypropylene
graft copolymer) 62.5
Vulcan XC-72 (carbon black) 17.5
Profax 6523 (polypropylene) 20
This composition varies only slightly irom Example 4.
However, unlike the previous examples in which the general
procedure was used in this case the carbon black was first
blended with the grafk copolymer and thereafter the polypro-
pylene was blended into the mixture.
It was generally believed, in view of the prior art,
that a mixture of polymeric materials would exhibit R vs. T
characteristics of the polymer in most intimate contact with
the carbon black, i.e., the polymer with which the carbon
black was first blended and any polymer blended thereafter
would not be in such intimate contact with the conductive
particles as to substantially affect the R vs. T curve. Thus,
from the prior art, it would be expected that the composition
and blending sequence of this Example would exhibit the
R vs. T curve of the graft copolymer. However, as can be
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seen by comparing Figure 6 and Figure 5, the polypropylene
appears to have a substantial effect on the R vs. T
characteristics of the blend.
EXAMPLE 7
CPE 3614 (chlorinated polyethylene, containing
36% Cl, from Dow Chemical Corp.) 35
Profax 6823 (polypropylene from Hercules Corp.) 35
Vulcan XC-72 (carbon black) 30
A chlorinated polyethylene elastomeric material,
exhibiting significant green strength was blended with a rigid
thermoplastic, crystalline polypropylene. The blend exhibits
a steadily rising resistance above the melting temperature
of the polypropylene as shown in Figure 7. Thus, where
the elastomeric portion of the composition is sufficiently
structured by itself to exhibit green strength on the order of
that described hereinbefore, physical blending with the
thermoplastic portion of the composition, as opposed to
grafting or block copolymerizing, is able to achieve increasing
resistance characteristics above the melting temperature of
the thermoplastic component.
EXAMPLE 8
CPE 3614 (chlorinated polyethylene) elastomer 35
Kynar 451 (polyvinylidene fluoride from
Pennwalt Corp.) 35
Vulcan XC-72 (carbon black) 30
In a similar experiment to Example 7, the elastomer,
having significant green strength, was blended with a
substantially rigid, crystalline thermoplastic. The resultant
* Trade Mark - 25 -
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composition exhibited a continuous rise in resistance from
ambient temperature all the way through the melt temperature
of the crystalline material until the termination of
measurement at 260 C as illustrated in Figure 8.
EXAMPLE _
CPE 3614 (chlorinated polyethylene) elastomer 35
Diamond PVC-35 (polyvinyl chloride from
Diamond Shamrock Chem. Co.) 32
XC-72 (carbon black) 30
10 Stabilizers 3
In this example, the elastomeric material was mixed
with an amorphous thermoplastic (PVC). As can be seen from
Figure 9, the composition exhibited PTC characteristics
from ambient temperature to 220C, and specifically in the
range above the Tg of PVC ( 80C). The decrease in resistance
in this and certain other samples at temperatures well in
excess of 200C is probably related to thermal or oxidative
degradation.
EXAMPLE 10
Hypalon 45 (chlorosulfonated polyethylene from du Pont)
elastomer 35
Profax 6823 (polypropylene) 35
XC-72 (carbon black) 30
A chlorosulfonated polyethylene elastomer was physically
blended with a crystalline thermoplastic (polypropylene).
The mixture initially exhibited a decrease followed by an
increase in resistance above the melting temperature of the
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polypropylene, as seen in Figure 10.
EXAMPLE 11
Neoprene TRT (chloroprene, du Pont) 35
Profax 6823 (polypropylene~ 35
XC-72 (carbon black) 30
An elastomeric neoprene was physically blended with
polypropylene, such blend continuing t.o exhibit PTC
characteristics above the melting temperature of the poly-
propylene as can be seen from Figure 11.
10 EXAMPLE 12
Valox 310 poly(tetramethylene terephthalate),
from General Electric Corp. 42.5
Hytrel 4055 tblock copolymer of poly tetra-
methylene terephthalate) and polytetramethylene-
oxide, from du Pont) 42.5
XC-72 (carbon black) 15.0
Poly (tetramethylene terephthalate) exhibiting a
crystallinity of greater than 50/O~ was blended with a block
copolymer of the crystalline thermoplastic and noncrystalline
polytetramethyleneoxide elastomeric moieties. The material
exhibited a resistance peak at the melt temperature of the
crystalline material, i.e., 180 C, and thereafter exhibited
a rise in resistance in the amorphous region as shown in
Figure 12.
EXAMPLE 13
Hypalon 45 (chlorosulphonated polyethylene) 35
Kynar 451 (polyvinylidene fluoride) 35
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XC-72 (carbon black) 30
As can be seen from Figure 13, a blend of the
elastomeric Hypalon 45 with the substantially rigid and
crystalline thermoplastic polyvinylidene fluoride exhibited a
rise in resistance up to the melting point of the polyvinylidene
fluoride after which the resistance remains constant until a
temperature well above the melt temperature of the composition
and then increases steadily.
EXAMPLE 14
10 Silastic~r437 (Silicone rubber from Dow Corning Co.) 60
Profax 6523 (polypropylene) 24
Vulcan XC-72 (carbon black) 16
The composition described above is an example of the
blending of a thermoplastic with an elastomer which does not
have green strength. It produces a product not exhibiting
PTC characteristics above the melt temperatures of the poly-
propylene when physically blended as seen in Figure 14.
A similar blend of 45.7 parts Marlex 6003 (a 0.96 density
polyethylene supplied by Phillips Petroleum Corp.) and
20 Silastic 437, 26.3 parts with SRF-NS 28 parts (a carbon black
from Cabot Corp.) exhibited a pronounced negative temperature
coefficient of resistance above the melting point of the
thermoplastic as did a similar irradiated composition contain-
ing Marlex 6003 and carbon black only. Thus these compositions,
which are not in accordance with the instant invention, or
which represent the teachings of the prior art, do not display
the advantageous properties of said invention.
* Trade Mark
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EXAMPLE 15
Kynar 451 30
Viton B 50 (an elastomeric vinylidene
fluoride copolymer from du Pont) 30
Vulcan XC 72 40
The above composition, which is in accordance with
the instant invention, was irradiated to 12 and 24 Mrad. In
both cases, the resistance started to rise rapidly below the
melting point of the thermoplastic, and continued to rise
with further increase in temperature.
EXAMPLES 16-27
A high density polyethylene was blended with various
elastomers and carbon black in accordance with the present
invention, as shown in Table I, and irradiated to a dose
of 6 Mrads. The variation of resistance with temperature
is also indicated on this Table.
TABLE I
Example Elastomer Uncured M. Resist-
No. Resins modulus at lxes ance
20% elongation Parts by weight behaviour
p.s.i. (1) (2) (3) above
melting
point
16 Texin 480 300 58.2 5.8 36 PTC
17 Roylar*E9 300 58.2 5.8 36 PTC
18 Roylar Ed 65 300 50 5 45 MarkedPTC
19 Royalene 502 21 52.7 5.3 42 MarkedPTC
Elvax 250 50 56.4 5.6 38 Marked PTC
21 Neoprene WRT 10 56.4 5.6 38 SlightPTC
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TABLE I (continued)
Example Elastomer Uncured
No. Resins Modulus at Mixes
20% elongation PartS by weight Resistance
p.s.i. (1) (2) (3) behaviour
above
melting
point
22 Neoprene WRT 40 15 45 Marked PTC
23 Nysin 35-8 22 40 15 45 Marked PTC
24 Epsin 5508 30 52.7 5.3 42 Marked PTC
Epsin 5508 40 23 37 PTC
26 CPE 3614 83 56.4 5.6 38 Marked PTC
27 CPE 3614 40 23 37 PTC
*Notes on Table.
Columns 1, 2 and 3 give the parts by weight of Marlex 6002
(polyethylene), elastomer and carbon black (SRF/NS) respectively.
The designations given in ASTM D 1765 - 73a to the carbon
blacks used in the examples are
Vulcan XC-72 - N474
SRF/NS - N774
Trade Mark
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