CA1095962A

CA1095962A - Self heating article with fabric electrodes

Info

Publication number: CA1095962A
Application number: CA258,297A
Authority: CA
Inventors: Stephen H. Diaz; David A. Horsma
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1975-08-04
Filing date: 1976-08-03
Publication date: 1981-02-17
Also published as: BE844849A

Abstract

ABSTRACT
Heat recoverable articles that can be self heating are provided by incorporating at least one pliable, dimensionally deformable fabric electrode into a heat recoverable member, advantageously comprising a conductive polymeric composition.
The or each electrode is incorporated into the member prior to its being deformed into its heat unstable state. As a result of their pliability, the electrodes deform as the heat recoverable member is deformed into its heat unstable state and recover with it to its heat stable configuration. The polymeric compsition is chosen to have an ohmic resistance high enough so that, when the electrodes are connected in series to an electrical power source, the current passing through the composition will cause it to be heated. The heat generated in this way can be used to provide all or part of the heat necessary to occasion recovery of the article or to allow its use as a heater after recovery. If desired, the polymeric composition can be one that exhibits a positive temperature coefficient of resistance thereby regulating the maximum temperature to which the article can be self-heated.

Description

1~9S96i~:
This invention relat~s to heat recoverable articles, particularly self heating, heat recoverable ariicles.
Polymeric heat recovexable articles may be used, for example, in tubular form a~ ~ealing, insulating or protective coating~ for elongate object~ e q. cables and pipes. A tube of heat recoverable material is slipped over the substrate ar~d heated to it~ recovery temperature, when it ~hrinks to a smaller diameter. Since the tube is pliant and elastic at its recovery temperature it shrinks and contact~ the substrate, thereby conforming to its shape.
In many caseQ there is not a free or conveniently accessible end to the elongate object and a heat recoverable sheet material is wrapped about the substrate and its ends joined. In position, the wrap-around sleeve i:3 hea~ed to cauSe its recovery. Examples of wrap-around devicQs for substrates in which there is no access to a free end are given in U.S. Pa~ent No. 3 379 218 issued on 23rd April 1968 to Julian s. Conde, U.S. Patent No. 3 445 336 i~sued on 15th - July 1969 to Roger H. Elli~ and U.S. Patent No. 3 770 556 issued on 6th ~ovember 1973 to Joseph H. Evans and Gare~'n W. Will.
The heat recoverable articles described above may be conveniently obtained from a wide variety of crosslin~ed crystalline polymeric compositions. Suitable ~aterials an~
processes for causing crosslinking, e.g. by ionizing radiation, and for rendering t~e articles heat recoverable are described in U.S. Patent 3 086 242 issued on 23xd April 1963 to Paul ~.
Cook and Richlard W. Muchmore. Chemical crosslinkin~ agents, for example, peroxides, may also be employed.
.

S~;~
To render an article made from such a composition heat recoverable, it i~ dimen.sionally deformed above the crystalline melting point (or range~ and held in the deformed state until the article is cooled below the melting point. In the cooled 5 -~tate the article can be said to be dimensionally unstable since it will exhibit "elastic memory" when, without restraint, it is again heated above the cry~talline melting point, iOeO
it will return to its original dimensions. In its recovered form, the article is frequently said to be dimensionally heat stable.
The recovery of a heat recoverable article may suitably be affected by the application o~ heat to the ar~icle using a torch or heat gun. However, in certain cases, for example, if the zone where ~he application of heat is required is in~
accessible or located in an area where flames are hazardous or prohibited as in mines where large amounts of flamm~bl~ gases may be present, treating by such means is undesirable or aven impossible. In these circumstances, conductive polymers ~ay b2 used as the heat reco~erable material in heat recoverable articles. An article made from a conductive polymer is wired into an electrical circuit as a resistive heating element and when connectea to a power source, for example, a 12 to 24 or e~en 36 volt battexy or an A~C. (e.g., at 115 volts) outlet, current flows through the article. As a result of its ohmic resistance it becomes heated and this heat can be employed as part or al1 of the heat required to cau3e recovery of the article. If heated to too high a temperature, the polymeric material will undergo decomposition. Delicate components shielded by the article may also be damaged, although the extent of ~his damage mav not be apparent by- visible inspection.

1~59~
To avoic' ~cessive temperatura3, ther~ostat~ ox other heat control de~ice3 may be u~ed. However, in m~ny ,insta~css thi~ defeats the purpose of using self h2ated ~ecoverable articles in that bulky and expensive hardware I~US~ b2 employed in places that are frequently virtually inaccessi~le. I~ has previously been proposed to eliminate added hardware by using as the polymeric composition, a composition that exhibits a positive temperature coefficient (PTC) of resistance. rrhese compositions may be referred to as PTC oompositions.
Suitable compositions compris~ an or~anic thermoplasti~
polymer, particularly a crystalline polymer, that contains carbon or another particulate conductor, for exa~ple a finaly divided mPtal. PTC compositions undergo a relatively smal1 increase in resistance as the temperature increases until a certain temperature Ts (which may be a t~mperature range) sometimes reerred to as the ~witching or anomaly temperature is reached. Above Ts a furthar increase in ternpera'ur~ results in a large increase in resistance. rme inc~ease in resistanca ~ may be so sharp that the currant i~ cut down to a level which limits the temperature of the composition to its r~s te~pera-ture. For crystalline polymers, Tg is ai or just below the crystalline melting point or range.
When a PTC compositiorl is emloyed as a component of a self-heating, heat-recoverable article, it can effectively limit the temperature to which the article can be heatad.
Suitable Pr~C compositions are described in British Patent Specification No. 1,528,622,published Octobar 18th, 1978 in the name of Raychem Corporation.

The types of applications of heat-recoverable articles suggested above have previously been limited because in most configurations the heat-recoverable article must be provided with a plurality of electrodes, generally pairs of substantially parallel wires. The article, can therefore, only be de-formed into its heat unstable state by deformation in a direction transverse to the direct;on in which the electrodes are arranged. This feature has significantly limited the useful shapes of self-heatingJ heat recoverable articles.
The present invention provides an article which comprises electrical-ly resistive material, at least a part of which material is in the form of a member which is heat-recoverable, or can be rendered heat-recoverable, or which has been heat-recovered, and which article also comprises at least one pliable fabric electrode, and a second electrode, the electrodes being capable on connection with source of electrical power of causing current to flow through the electrically resistive material.
The second electrode may be incorporated in the electrically resistive material, may be in contact with the heat recoverable member, for example as a substrate or may be incorporated in a second member adjacent to the heat recoverable member. The second electrode may consist of a strip or grid of metal provided that it does not hinder the deformation of the recoverable article between its initial, recoverable and recovered configurations. Advantageously the second electrode is a pliable fabric electrode which is incorporated in the electrically resistive material.
The heat-recoverable member advantageously comprises a polymeric composition having sufficient conductive filler, for example, a particulate carbon black or metal, so that it is capable of conducting an electrical current at a constant voltage, _ 5 ~

S~6~

for example, 12 to 36 volts from a battery or 1~5 volt A~C.
The composition should al~o e~ibit 3ufficient ohmic re-sistance ~o that its heat output is capable of effecting recovery of the heat recoverable member, which may be S several millimetres thick. ~lere are many suitable pol~mers for use in these compositionsv particularly useful are crosslinked crystallin2 polymers, for example, those described in U.S. Patent No. 3 086 242. An article made from such a polymer can be deformed above the crystalline melting point or range of the polymer (hereinafter crystalline m.p.) and maintained in the deformed state until cool in order to render the article heat recoverable.
Preferably the heat recoverable member comprises at least one layer of a conductive polymer as described above.
15The layer or layers may exhibit either constant wattage behaviour or have 2 positive temperature coefficient of resistance. Preferably, a PTC material used in the heat recoverable member exhibits at least a six fold in~.rease in ~, resistance in a 30C range o~ temperature above T3. A
constant watiage material has a resistance which may increase with temperature but which does not have a temperature at which its re~istance increases so rapidly that for most pur-poseY it becomes an insulator~ Within the context o~ this specification, a constant wattage material may be regarded as one whose resistance does not increase by a fac or of more then approximately six in any 30 range of temperature ` between 25C and the melting point of the polymer or a higher temperature if the polymer has been crosslinked to give it structural qtability above the melting point. Pxe-ferably it has a resistivity of at least ~ ohm/square at 2S~C.

~ ~ S~ 6 Z
Constant wattage materi~ls suitable for u5e in the present invention are known in the prior art. In many cases the polymers used in PTC compofiitions can be used in con~tant wattage compos~tions by incorporating in the polymer a higher proportion of conductive filler than is used in PTC compos~~
tions. If the Ts Of compositions exhibiting PTC character is high enough these compositions can be used as constan~ wa.tage materials. The details that characteri2e PTC materials and constant wattage materials useful in the heat recoverable articles of the present invention are described in British Patent Specification ~o. 1,529,354 publis'ned October 18th, 1978 in the name of Raychem Corporation.
Advantageously, the extensible fabric electrode comprise strand material of any suitable conductive material.
Preferably the strands are made from materials that have low resistivity and that are moderate in cost, for example, copper wixe strand, and preferably 28-40 gauge wire strand.
Strands of other metals, including alloys and bico.~ponent metal ~trands, metal and polymer fibre composites, metal plated polymer fibres or conductive carbon fibres may also be used.
The fabric el~ctrode must be sufficiently pliable ~hat, when incorporated into the polymer composition, ~t can be dimensionally deformed to the same extent as the composi-tion when the latter is heated above the crystalline m.p. inorder to render it heat recoverable as described in U.S.
Patent No. 3 086 242. In addition the electrode should not substantially impede the recovery of the h~at recoverable-member when it is heated. Such electrodes are advantageously capable of undergoing at lea~t a 100,6 change ~n dimension, ~09S962 for exa~ple, of length and width, or if tubula~, diameter andjor length based on the heat ~table st~te and pr~ferably at least a 300% change in dim~nsion based on the heat stable state. However any fabric that may be deformed Wit-h the recoveraDle article between its initial, recoverable, and recovered configurations is suitable for use in the invention.
It will be appreciated, therefore, that the suitability of a given material will be readily determined by simple rout.ine experiment in any individual case.
Suitable electrode fabric can be made by techniques well known to the art, e . q . weaving, knitting or b~aiding.
Of these braiding is preferred because, using this 'echnique, electrodes having good compliancy or flexibility are obtained. Preferably the electrodes will readily undergo ~he elongation involved in the deformation process that renders the polymer composition heat recoverable and also Gffer a minimum of resistanc~ to recovery of the heat recoverable member. Advantageously braided electrodes comprisa tubular braids preferably braided about a core of a thermoplastic material. Tubular braid having a high braid angle, relative to the axis of the tube for example, an ang1e yreater than approximately 50 and preferably approximately 75, is particularly adYantageous. A suitable braid is obtained u~ing 16 carriers each of four strands o~ 38 Americ~n wire gauge tinnecl copper wire at a 75 braid angle to form a ; braid about a cylindrical core, which may be tubular, of a conductive or non-conductive thermoplastic material pre-fera~ly a conductive material compatible with the polymer from which the heat recoverable member is made, having an outside diameter of 0.64 cm. Usually, the braided tube is heated above the softening point of the ther~oplastic core and ~lattened, care being taken to avoid stretchirlg the ,~

~9sg~
braid in the course o~ this defor~ation. For some purpo~es, for example, wnere it is desired that the tube be radially ex~ansible rather than longitudinally extensiblel lower braid angles can be employed. The braiding character of radially expansible tube is described in U.S. Patent ~o. 3 253 619 issued on 31st May 1966 to Paul M. Cook and F. Raymond You~g.
The invention will now be described in greater detail with reference to the accompanying drawings, in which Figure 1 is a perspective view of a braided electrode.
Figures 2, 3 and 5 to 7 are perspective views of fi~e different embodiments o~ heat-recoverable articles construct- ?
ed in accordance with the invention and, Figura 4 is a perspective view of an electrode assembly.
Referring to Figure 1, a braide~ electrode 10 comprises a tubular braid 11 wound over a tubular core mem~er 12.
Referring to Figure 2, the heat recoverable ar~icle 13 com-prises a plurality of electrodes 10 after rlattening, embedded in a layer 14 of a PTC polymer cGmposi~ion as hsre inbefore described. The electrodes 10 are conterminous wi~h the PTC layer. Adjacent pairs of electrodes 10 are conn~cted in series to a potential source 15.
In operation, current is conducted between the elec-trodes through the PTC composition. After generation of sufficient heat recovery of the arti.cle 13 is effected~ The PTC layer 14 can deform in either or both of two di~ecti.ons, i.e. in a direction to cause separation of the electrode3 10 and/or in a direction to cause elongation of the article 13 along the long axis of the electrodes 10, to impart heat recoverabili.ty. A conventional rigid electrode allows de-formation only in a direction to cause separa~ion of the l~S~I~Z
electrodes 10. since the separation of the electrod~ 10 increases the path length an~ therefore the resistance of the article 13 thi~ limits the power output o the article 13 in the expanded state.
since the layer 14 of the article 13 compri~s a PTC
composition, the heating will be limit~d when the TS tem-perature of the polymer in the composition i~ reached~ The article is therefore self-heating and c pable of function-ing advantageously a~ a heater after r~covery wheth~r or not tO the self-heating capability i9 employed to cause recovery o~
the article.
Referring to Figure 3, the heat recoverable article 30 comprises two layers of electrodes 10 embedded in a layer ~
o~ a PTC polymer compsition. The electrodes 10 n the upper layer, are connected paxallel with one another and in series with those of the lower layer through the polymer layer" A
layer of electrodes 10 is shown in Figure 4 before incor-poration into the heat recoverable article~ ~ach layer has at both ends a bussing electrode 16. The bus or bus-~ing - 20 electrode~ 16 are fabric electrodes and can be caused to adhere to the electrodes 10 by heatiny their thermopla.5tic - core above the softening point and pressing them together, or by conventional spot weldin~ or soldering techniques.
The ~ussing electrodes 16 are used to distribute-power to the electro~les 10 of one layer. The heat-recoverable article 30 can be deformed along and/or perpendicular to the long axis of the electrode~ 10.
It will be appreciated that the heat-recoverable articles shown in Figures 2 and 3 may comprise a constant wattage pol~meric composition instead of a PTC composition.

~`

~95~6~

Re~erring ~o Figure 5, the hea~ rec~verable arkicls 17 c!OmpriSe~ a set of electrodes 10 embedded in a layer 18 of a constant wattage polymeric compo~ition and a second set o~
elect-odes 10 emb~dded in a layer ~9 of a PTC polymeric composition. In Figure 6 a heat recovexable article employ-ing three layers of polymeric compsition is sho~. 'hwo sets of electlodes 10 are embedded in two layers 21 and 22 o a constant wattage polymeric material, which sandwich a layer 23 o~ a PTC polymeric material.
,0 In a heat recoverable article using thin films of PTC
compositions only in which the current fiow i~ in the plane of the ~ilm, it has been found that e~en at moderate power outputs, the phenomenon of "hotlining" occurs, iIl which only a narrow band of the PTC layer functions as a neater. r~lis problem has been discussed in the aforementloned British Patent Specification No. 1,529,3S4.
The tem~erature T~ at which the resistal~ce of a PTC
material increases sharply is at or below the crystalline m.p. for cry talline polymers. These polymers if in a heat rec~vera~le form, undergo recovery abo~re their c~ystal'ine m.p~ and sufficient mo~ility for efficient recovery re~uires temperatures at least 10C above the crystalline m.p~ A
layer system for example on~ shown in Figure 5 or 6 ~rovides relative resistance between the layer or layers of constan~
wattage material and the layer of PTC mater~al so ~hat the layer of constant wattage material heats first, if 7 ts re~istance is higher than the _esistance of the layex, of the P~C com~osition, so the articlç can be7 neated above TS
by rapidly heating the constant wattage material be~ore thermal conduction r~ises ~he tempexature of the FTC

1 'i -~ ~ S 9 6.~
composition t.o Ts thereby cuttin~ off the current.
A more efficient method ~or overcomin~ ~.he disadvan-tage of "hotlining" is described in German Patent A~plica-tion No. 2 634 932, published Fehruary 1Oth, 1977, in the name of Raychem Corporatinn. ~le art.icle disclo.sed in the application has a structure in which a relatively low xe~
sistance, thermally insulatinq layer of constant wattage matel^ial is disposed between a heating 1ayer of a constant wattage ~aterial and a layer of PTC composition, as sho~,n in Fi~ure 7. Th~ heat recoverable article 24 co~.prises a layer 25 of a constant wattage material of relatively high resis-tance, a thermally insulating layer 26 of relatively low re~
sistance and a layer 27 of a PTC composition of intermediate initial resistance. The layer 26 may b~ made ~rom a foame~
polymeric material so that it has good thermal insulati.ng properties. ~men current is applied, the layer 25 heats up but the layer 27 is thermally shielded by the layer ~6 and its temperature increase will lag behind that of laysr 2$.
~nus the layer 25 can ~e caused to heat wall above the 20 crystalline melting point of the heat xecoverable memher -~
before 'he power is cut off by the temperature of tha PTC
composition reaching Ts. m ermal conductivity effects will allow ~he entire article to eventually be heated above the recovery temperature, i.e., the cryst~lline melting point by an amount adequate to insure efficient recovery. It will be a~preciated that layer 25 may also be OI a PTC material provided that it has a T~ higher than that of layer 27.
All of the layer~ of a multi-layer heat recoverable article need~ not be heat recove~rable, provided that at least one layer is heat-recoverahle and has sufficient "ho7d~out"
strength to retain the other layers in a deformed condition, ~ 12 ~95g6~

or has ~;u~icl~n~: ~cov~y ~r~e to urge the other layarq to~ .rds the `ne~ st~ble conf i~u~ a ~ior~ t:he other lay~r.~; ~eed not '~e heat reco~terable.
In the embodi.ment~ s'l~own the electrodes ~0 are embedded in a layer cor~prising a polymeric compo~ition~ l~7Li~
is advantageously achie~ed by d.isposing the electrocles hetwe~n two polym~r sublayers that are su~seq~lentl~ lamin~ted together, for exampl~ ~ublayers that have been heated abov~
their softening points and bonded together using lam.inati.r rollersO
Although th~ articles s'nown have been relatively planar articles it will be appreciated that ~rticles of re~
gular or irregular conigurations can be constructed. For example, tu~ular article having a plurality of fab~.^ic elec-trodes disposed in the tube and or more layers either parallelto the long ~xis of the tube or pe.rpendicular to t'r.at axis ~n be employed. A particularly preferred art.icle is described in British Patent Speciication ~o. 1 529 353, published October 18th, 1978, in the name o; ~aychem Corpora~ion. Al~ernatively the tubular structure can he disposed about a cylindrical conduit and used af e~ in~
sulation to heat the contents of the conduit to prev2nt fr~æing or salting out oI solid~s.

: - 13

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An article which comprises electrically resistive material, at least a part of which material is in the form of a member which is heat-recoverable, or can be rendered heat-recoverable, or which has been heat-recovered, and which article also comprises at least one pliable fabric electrode, and a second electrode, the electrodes being capable on connection with source of electrical power of causing current to flow through the electrically resistive material.

2. An article as claimed in claim 1 which comprises a pair of pliable fabric electrodes incorporated in the elec-trically resistive material.

3. An article as claimed in claim 1 wherein the fabric electrode comprises a woven, knitted or braided electrode.

4. An article as claimed in claim 1 wherein the fabric electrode comprises a braided electrode.

5. An article as claimed in claim 4 wherein the braided electrode comprises a tubular braid.

6. An article as claimed in claim S wherein the tubular braid is braided about a core of a thermoplastic material.

7. An article as claimed in claim 6 wherein the core is in the form of a tube.

8. An article as claimed in claim 7 wherein the tubular braid has a braid angle relative to the axis of the tube of at least 50°.

9. An article as claimed in claim 8 wherein the braid angle is 75°.

10. An article as claimed in claim 1 wherein the or each fabric electrode is capable of undergoing a change in a dimension relative to the heat stable state, of greater than 100%.

1 . An article as claimed in claim 10 wherein the or each fabric electrode is capable of undergoing a change in a dimension based on the heat stable state of at least 300%.

12. An article as claimed in claim 1 wherein at least one of the electrodes comprises copper wire strand.

13. An article as claimed in claim 12 wherein the copper wire strand has a gauge within the range of from 28 at 40.

14. An article as claimed in claim 1 wherein the heat recoverable member comprises a layer of a polymeric com-position incorporating a particulate conductive filler.

15. An article as claimed in claim 14 wherein the heat recoverable member comprises a layer of a material exhibiting constant wattage behaviour.

16. An article as claimed in claim 14 or claim 15 wherein the heat recoverable member comprises a layer of material having a positive temperature coefficient of resistance.

17. An article as claimed-in claim 14 wherein the heat recoverable member comprises a first layer of material exhibiting constant wattage behaviour and a second layer of material having a positive temperature coefficient of re-sistance, each layer having incorporated therein at least one fabric electrode capable on connection with a source of electrical power of causing current to flow from one layer to the other layer.

18. An article as claimed in claim 14 wherein the heat recoverable member comprises a first layer having a positive coefficient of resistance which layer is positioned between two second layers exhibiting constant wattage behaviour each second layer incorporating at least one fabric electrode capable, on connection with 2 source of electrical power, of causing current to flow from one second layer to the other second layer.

19. An article as claimed in claim 17 wherein a layer of an electrically conductive constant wattage material having a resistance lower than that of the first or second layers is sandwiched between the first and second layers.