CA1074096A - Positive temperature coefficient compositions - Google Patents
Positive temperature coefficient compositionsInfo
- Publication number
- CA1074096A CA1074096A CA236,456A CA236456A CA1074096A CA 1074096 A CA1074096 A CA 1074096A CA 236456 A CA236456 A CA 236456A CA 1074096 A CA1074096 A CA 1074096A
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- Prior art keywords
- composition
- elastomeric
- copolymer
- temperature
- thermoplastic
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
- H01C7/02—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material having positive temperature coefficient
- H01C7/027—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material having positive temperature coefficient consisting of conducting or semi-conducting material dispersed in a non-conductive organic material
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L101/00—Compositions of unspecified macromolecular compounds
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
- H01B1/20—Conductive material dispersed in non-conductive organic material
- H01B1/24—Conductive material dispersed in non-conductive organic material the conductive material comprising carbon-silicon compounds, carbon or silicon
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Dispersion Chemistry (AREA)
- Physics & Mathematics (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electromagnetism (AREA)
- Health & Medical Sciences (AREA)
- Ceramic Engineering (AREA)
- Medicinal Chemistry (AREA)
- Polymers & Plastics (AREA)
- Organic Chemistry (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Processes Of Treating Macromolecular Substances (AREA)
- Conductive Materials (AREA)
- Thermistors And Varistors (AREA)
- Resistance Heating (AREA)
Abstract
ABSTRACT
A crosslinked polymeric composition having a positive temperature coefficient of resistance, comprising a first polymeric material exhibiting high green strength and elastomeric behaviour subsequent to cross-linking, and a second polymeric thermoplastic material, the composition having dispersed therein conductive particles, for example, carbon black, the composition exhibiting a rise in resistance with increased temperature at some temperature above the melting point of either material; heating elements made form the cross-linked compositions are also disclosed.
A crosslinked polymeric composition having a positive temperature coefficient of resistance, comprising a first polymeric material exhibiting high green strength and elastomeric behaviour subsequent to cross-linking, and a second polymeric thermoplastic material, the composition having dispersed therein conductive particles, for example, carbon black, the composition exhibiting a rise in resistance with increased temperature at some temperature above the melting point of either material; heating elements made form the cross-linked compositions are also disclosed.
Description
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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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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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10'7~ 6 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, - 4a -B
1074()~6 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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10~()96 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 - ~ -. .. .. ..
10~ 6 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 .
10~ 6 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 10'74(~96 (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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~07405~6 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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10'~ 6 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.
~074U96 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 ~,. . : . .
~ ot7~05,6 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 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.
. , . . : : : . - . : -: ~ -1074()5~6 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 - , ., ... . . .
10~7~
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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4()5~6 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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. ' !6 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.
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.
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.
Vistalon 404 (ethylene-propylene rubber: E/P ratio of * Trade Mark 10'~
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) 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.
Kynar*304 (Polyvinylidene Fluoride from Pennwalt Corporation) 25 Cyanocril R (Polyethyl acrylate from American Cyanamid) 50 Vulcan XC 72 25 * Trade Mark . , ' ,.
10740~6 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.
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 -, . ~
107~U9~
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.
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.
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 -.: . : -.
.. .. , - :
- .
lQ'~4U~6 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.
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 Trade Mark .
:
1074(~Y36 polypropylene, as seen in Figure 10.
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.
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.
Hypalon 45 (chlorosulphonated polyethylene) 35 Kynar 451 (polyvinylidene fluoride) 35 * Trade ~rk ~ ' .
..
: ~ .
10'74()~t~
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.
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 . ~ ' ,~ .
10'7~
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.
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
~Trade Mark ::''' -, ' : :
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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
*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 Trade Mark ' ' '' ' . ~ . ~
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 .
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
~ _ 4 _ ', ~ ' .
10'7~ 6 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, - 4a -B
1074()~6 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, , .
- ' ` - : : , -:
10~()96 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 - ~ -. .. .. ..
10~ 6 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 .
10~ 6 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 10'74(~96 (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 .
-' " . : , :
~07405~6 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) :, , - ~ .
l0~7~as~
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 , . : :-.. : - . .
- . . .
.~ - . . .
10'~ 6 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.
~074U96 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 ~,. . : . .
~ ot7~05,6 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 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.
. , . . : : : . - . : -: ~ -1074()5~6 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 - , ., ... . . .
10~7~
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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~.
:
.
:
:
4()5~6 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 ` . '' ' : ' ' ~' :
' ~ ~
. ' !6 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.
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 .
, .
. . :: : . ~ .
,, , ~
107~
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.
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.
Vistalon 404 (ethylene-propylene rubber: E/P ratio of * Trade Mark 10'~
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) 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.
Kynar*304 (Polyvinylidene Fluoride from Pennwalt Corporation) 25 Cyanocril R (Polyethyl acrylate from American Cyanamid) 50 Vulcan XC 72 25 * Trade Mark . , ' ,.
10740~6 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.
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 -, . ~
107~U9~
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.
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.
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 -.: . : -.
.. .. , - :
- .
lQ'~4U~6 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.
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 Trade Mark .
:
1074(~Y36 polypropylene, as seen in Figure 10.
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.
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.
Hypalon 45 (chlorosulphonated polyethylene) 35 Kynar 451 (polyvinylidene fluoride) 35 * Trade ~rk ~ ' .
..
: ~ .
10'74()~t~
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.
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 . ~ ' ,~ .
10'7~
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.
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
~Trade Mark ::''' -, ' : :
:
: ' . - ~ ' ~ , ' ' :
: , ' : ,, : :
,: ~
.
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
*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 Trade Mark ' ' '' ' . ~ . ~
Claims (22)
1. 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.
2. A composition as claimed in claim 1, wherein the ratio of elastomeric material to thermoplastic material is from 3:97 to 75:25 by weight.
3. A composition as claimed in claim 1, wherein the ratio of elastomeric material to thermoplastic material is from 4:96 to 40:60 by weight.
4. A composition as claimed in claim 1, wherein the elastomeric and thermoplastic materials are segments of a copolymer.
5. A composition as claimed in claim 4, wherein the copolymer is a graft copolymer.
6. A composition as claimed in claim 5, wherein the copolymer is an ethylene-propylene/crystalline polyolefine graft copolymer.
7. A composition as claimed in claim 6, wherein the copolymer is an ethylene-propylene/polypropylene graft copolymer.
8. A composition as claimed in claim 4, wherein the copolymer is a block copolymer.
9. A composition as claimed in claim 8, wherein the copolymer is an ethylene-propylene/polystyrene block copolymer.
10. A composition as claimed in claim 8, wherein the copolymer is polytetramethylene oxide/polytetramethylene terephthalate block copolymer.
11. A composition as claimed in any one of claims 4 to 6, wherein the ratio of elastomeric to thermoplastic materials is from 30:70 to 70:30 by weight.
12. A composition as claimed in claim 1, wherein the elastomeric and thermoplastic materials are a physical blend of polymers.
13. A composition as claimed in claim 12, wherein the elastomeric material is chlorinated polyethylene, chloro-sulfonated polyethylene, or chloroprene.
14. A composition as claimed in claim 12 or claim 13, wherein the thermoplastic material is polypropylene, polyvinyli-dene fluoride or polyethylene.
15. A composition as claimed in any one of claims 1, 4 and 12, wherein the conductive particles are carbon black.
16. A composition as claimed in any one of claims 1 to 3, which comprises a copolymer comprising the elastomeric and thermoplastic materials and in addition the elastomeric material, the thermoplastic material, or both, in the form of a separate polymer.
17. A composition as claimed in claim 1 which comprises ethylene-propylene rubber polypropylene and carbon black.
18. A composition as claimed in claim 17 wherein the ethylene-propylene rubber is from 40 to 90%, the polypropylene from 5 to 40% and the carbon black is from 5 to 35% by weight of the composition.
19. A composition as claimed in any one of claims 1 4, and 12 that has a positive temperature of coefficient of resistance in the range between ambient temperature and the melting point of the thermoplastic material.
20. A self-regulating heating element comprising at least two electrodes having positioned therebetween a shaped structure of a composition as claimed in claim 1.
21. An element as claimed in claim 20, which is heat recoverable,
22. An uncrosslinked composition suitable as a starting material for the manufacture of composition as claimed in claim 1, or of an element as claimed in claim 20 or claim 21 which comprises a physical blend of carbon black, and elastomeric and thermoplastic polymers, and wherein the elastomeric component exhibits sufficient strength in the uncrosslinked state at room temperature to support its own weight without signifieant deformation over an extended time period.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US51003574A | 1974-09-27 | 1974-09-27 | |
| US60163975A | 1975-08-04 | 1975-08-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1074096A true CA1074096A (en) | 1980-03-25 |
Family
ID=27056755
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA236,456A Expired CA1074096A (en) | 1974-09-27 | 1975-09-26 | Positive temperature coefficient compositions |
Country Status (12)
| Country | Link |
|---|---|
| JP (1) | JPS5160236A (en) |
| AU (1) | AU504687B2 (en) |
| BR (1) | BR7506262A (en) |
| CA (1) | CA1074096A (en) |
| CH (1) | CH639230A5 (en) |
| DE (1) | DE2543346C2 (en) |
| ES (1) | ES441299A1 (en) |
| FR (1) | FR2286169A1 (en) |
| GB (1) | GB1528622A (en) |
| IT (1) | IT1042912B (en) |
| NL (1) | NL7511394A (en) |
| SE (1) | SE412976B (en) |
Families Citing this family (20)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4017715A (en) * | 1975-08-04 | 1977-04-12 | Raychem Corporation | Temperature overshoot heater |
| FI64482C (en) * | 1974-09-27 | 1983-11-10 | Raychem Corp | VAERMEAOTERHAEMTBAR ANORDNING OCH ANORDNING AV DENSAMMA FOER EN KABELSKARV |
| FR2320678A1 (en) * | 1975-08-04 | 1977-03-04 | Raychem Corp | Thermal shrink fit sleeve or cover - has high electrical resistance and contains web of interwoven conductors connectable to current source for local heating |
| US4095044A (en) * | 1976-10-26 | 1978-06-13 | Raychem Corporation | Multiple cable adapter and splice case including the same |
| EP0001307A1 (en) * | 1977-09-19 | 1979-04-04 | Shell Internationale Researchmaatschappij B.V. | A semi-conductive composition and an electric wire or cable coated with such a composition |
| JPS5835536B2 (en) * | 1978-05-22 | 1983-08-03 | 大日本インキ化学工業株式会社 | resin composition |
| FR2440104A1 (en) * | 1978-10-27 | 1980-05-23 | Raychem Sa Nv | HEAT SHRINKABLE FITTINGS IN PARTICULAR FOR CABLES, METHOD FOR THEIR IMPLEMENTATION AND ASSEMBLY COMPRISING SAME |
| JPS5784586A (en) * | 1980-11-13 | 1982-05-26 | Hitachi Cable | Self-temperature controllable heater |
| JPS5917935B2 (en) * | 1981-08-25 | 1984-04-24 | 住友電気工業株式会社 | heat shrink tube |
| DE3245589A1 (en) * | 1982-12-09 | 1984-06-14 | Hoechst Ag, 6230 Frankfurt | UNCROSSLINKABLE, ELECTRICALLY CONDUCTIVE MOLDING MATERIALS BASED ON THERMOPLASTIC PLASTICS AND CARBON |
| JPS59214103A (en) * | 1983-05-19 | 1984-12-04 | 株式会社フジクラ | Mixture for electromagnetic wave absorber |
| JPS60188439A (en) * | 1984-03-09 | 1985-09-25 | Meidensha Electric Mfg Co Ltd | Electroconductive composite material |
| US4724417A (en) * | 1985-03-14 | 1988-02-09 | Raychem Corporation | Electrical devices comprising cross-linked conductive polymers |
| JPS6265401A (en) * | 1985-09-18 | 1987-03-24 | 安田 繁之 | Regulating method for ordinary heating temperature in thermosensitive electric resistance compositiion |
| DE3635286A1 (en) * | 1986-10-16 | 1988-04-21 | Lentia Gmbh | Surface heating element and process for producing it |
| JPH028258A (en) * | 1988-06-28 | 1990-01-11 | Matsushita Electric Ind Co Ltd | Self-temperature control heating element composition |
| CN1086929A (en) * | 1992-09-04 | 1994-05-18 | 单一检索有限公司 | Flexible plastic electrode and manufacturing method thereof |
| JP3684867B2 (en) * | 1998-10-07 | 2005-08-17 | Ｎｏｋ株式会社 | PTC composition and planar heating element |
| DE10021803B4 (en) * | 2000-05-04 | 2006-06-22 | Franz Koppe | Heating mat and method of making and using same |
| JP7024143B2 (en) * | 2019-04-23 | 2022-02-22 | 株式会社クラレ | Thermoplastic liquid crystal polymer films, laminates, and molded bodies, and methods for manufacturing them. |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3243753A (en) * | 1962-11-13 | 1966-03-29 | Kohler Fred | Resistance element |
| US3591526A (en) * | 1968-01-25 | 1971-07-06 | Polyelectric Corp | Method of manufacturing a temperature sensitive,electrical resistor material |
| US3793716A (en) * | 1972-09-08 | 1974-02-26 | Raychem Corp | Method of making self limiting heat elements |
| US3849333A (en) * | 1972-09-26 | 1974-11-19 | Union Carbide Corp | Semi-conducting polymer system comprising a copolymer of ethylene-ethylarcralate or vinyl acetate,ethylene-propylene-termonomer and carbon black |
| US3823217A (en) * | 1973-01-18 | 1974-07-09 | Raychem Corp | Resistivity variance reduction |
-
1975
- 1975-09-26 FR FR7529585A patent/FR2286169A1/en active Granted
- 1975-09-26 CA CA236,456A patent/CA1074096A/en not_active Expired
- 1975-09-26 ES ES441299A patent/ES441299A1/en not_active Expired
- 1975-09-26 NL NL7511394A patent/NL7511394A/en not_active Application Discontinuation
- 1975-09-26 BR BR7506262*A patent/BR7506262A/en unknown
- 1975-09-26 AU AU85230/75A patent/AU504687B2/en not_active Expired
- 1975-09-26 GB GB39518/75A patent/GB1528622A/en not_active Expired
- 1975-09-26 IT IT27705/75A patent/IT1042912B/en active
- 1975-09-26 JP JP50116270A patent/JPS5160236A/en active Granted
- 1975-09-26 SE SE7510846A patent/SE412976B/en unknown
- 1975-09-29 CH CH1261775A patent/CH639230A5/en not_active IP Right Cessation
- 1975-09-29 DE DE2543346A patent/DE2543346C2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| SE412976B (en) | 1980-03-24 |
| IT1042912B (en) | 1980-01-30 |
| GB1528622A (en) | 1978-10-18 |
| CH639230A5 (en) | 1983-10-31 |
| JPS5160236A (en) | 1976-05-26 |
| JPS6135223B2 (en) | 1986-08-12 |
| FR2286169B1 (en) | 1979-05-04 |
| DE2543346C2 (en) | 1985-09-12 |
| ES441299A1 (en) | 1977-07-01 |
| SE7510846L (en) | 1976-03-29 |
| AU8523075A (en) | 1977-03-31 |
| AU504687B2 (en) | 1979-10-25 |
| NL7511394A (en) | 1976-03-30 |
| DE2543346A1 (en) | 1976-04-15 |
| BR7506262A (en) | 1976-08-03 |
| FR2286169A1 (en) | 1976-04-23 |
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