CA1071281A - Heat-recoverable laminated heater with ptc layer operating above switching temperature - Google Patents
Heat-recoverable laminated heater with ptc layer operating above switching temperatureInfo
- Publication number
- CA1071281A CA1071281A CA258,295A CA258295A CA1071281A CA 1071281 A CA1071281 A CA 1071281A CA 258295 A CA258295 A CA 258295A CA 1071281 A CA1071281 A CA 1071281A
- Authority
- CA
- Canada
- Prior art keywords
- ptc element
- layer
- layers
- heat
- temperature
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B3/00—Ohmic-resistance heating
- H05B3/10—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor
- H05B3/12—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material
- H05B3/14—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material the material being non-metallic
- H05B3/146—Conductive polymers, e.g. polyethylene, thermoplastics
-
- 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
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S174/00—Electricity: conductors and insulators
- Y10S174/08—Shrinkable tubes
Abstract
ABSTRACT
The invention relates to electrical devices in which a PTC
element is used to control the temperature of the device. In the devices of the invention, a PTC element is associated with a heat-supplying means which supplies a controlled amount of heat to at least part of the PTC element after it has reached its switching temperature. The devices are particularly useful when in the form of laminates, for example, heat-recoverable laminates which will recover when current is passed through them.
The invention relates to electrical devices in which a PTC
element is used to control the temperature of the device. In the devices of the invention, a PTC element is associated with a heat-supplying means which supplies a controlled amount of heat to at least part of the PTC element after it has reached its switching temperature. The devices are particularly useful when in the form of laminates, for example, heat-recoverable laminates which will recover when current is passed through them.
Description
1~7~Z8i 146/254 Ihis invention relates to electrical heating devices which contain PIC elements.
Ihe term "PIC element" is used herein, in the way well understood in the art, to denote an element which exhibits a rapid S increase in electrical resistance at a particular temperature or over a particular te~perature range. Well-known PIC (Positive 13mperature Coefficient) materials include doped barium titanate ceramics twhich are of course rigid) and cross-linked thermoplastic crystalline polymers havin~ an electrically conductive particulate filler, e.g.
~ 10 carbon black or metal particles, dispersed therein. The term ; "switching temperature" (usually abbreviated to Ts) is used to denote the temperature at which the steep rise in resistance takes place and above which the materials effectively become electrical insulators for many purposes. When the change fr comparatively low to 15 ccrparatively high resistance takes place over a temperature range (as is often the case), then Ts can conveniently be designated as the temperature at which extensions of the substantially straight portions ' of the resistance/temperature curve (above and below the range) cross.
'~ ~ Polymeric PIC materials, whose Ts is at or near the crystalline - 20 melting point of the polymer, have been widely used in self-regulating flexible strip heaters which automatically shut off at temperatures above Ts, and thus maintain themselves at temperatures at or near Ts.
Rigid PIC elements have been proposed for use as temperature-sensitive switches and auxiliary heaters in devices such as coffee pots - see for example U.S. Patents Nos. 3,551,644 and 3,375,744. In these devices the PIC elemen~ is connected in series with a conventional electrical resistance heater and placed in thermal contact with a body (e.g. of liquid) to be heated by the conventional heater and the P~C
:
~k ~712~1 element. The PTC element shuts off the current through itself and the con-ventional heater when it reaches its switching temperature Ts, and thus controls the maximum temperature of the body being heated. The temperature of the body being heated rises to or near T and then, depending on the thermal and electrical characteristics of the device, stabilises at a temper-ature below T or cycles (with constant or decreasing amplitude) about an average temperaturc below T , the maximum temperature also being below Ts.
The rigid PTC elements used in these devices may comprise two or more PTC
components which are connected in parallel, the components having different switching temperatures and therefore causing stepwise modification of the electrical characteristics of *he device as the temperature of the body being heated increases. The PTC component having the highest T controls the maximum temperature of the body being heated. In all these devices, the maximum temperature reached by the PTC element is its switching temperature Ts or, where the PTC element contains PTC components connected in parallel and having different switching temperatures, the highest of those switching temperatures.
The present invention is concerned with devices in which at least part of a PTC element is heated in a controlled fashion to a temperature in excess of Ts.
Thus in its first aspect, the invention provides an electrical device which comprises:
(1) a PTC element having a switching temperature Ts;
Ihe term "PIC element" is used herein, in the way well understood in the art, to denote an element which exhibits a rapid S increase in electrical resistance at a particular temperature or over a particular te~perature range. Well-known PIC (Positive 13mperature Coefficient) materials include doped barium titanate ceramics twhich are of course rigid) and cross-linked thermoplastic crystalline polymers havin~ an electrically conductive particulate filler, e.g.
~ 10 carbon black or metal particles, dispersed therein. The term ; "switching temperature" (usually abbreviated to Ts) is used to denote the temperature at which the steep rise in resistance takes place and above which the materials effectively become electrical insulators for many purposes. When the change fr comparatively low to 15 ccrparatively high resistance takes place over a temperature range (as is often the case), then Ts can conveniently be designated as the temperature at which extensions of the substantially straight portions ' of the resistance/temperature curve (above and below the range) cross.
'~ ~ Polymeric PIC materials, whose Ts is at or near the crystalline - 20 melting point of the polymer, have been widely used in self-regulating flexible strip heaters which automatically shut off at temperatures above Ts, and thus maintain themselves at temperatures at or near Ts.
Rigid PIC elements have been proposed for use as temperature-sensitive switches and auxiliary heaters in devices such as coffee pots - see for example U.S. Patents Nos. 3,551,644 and 3,375,744. In these devices the PIC elemen~ is connected in series with a conventional electrical resistance heater and placed in thermal contact with a body (e.g. of liquid) to be heated by the conventional heater and the P~C
:
~k ~712~1 element. The PTC element shuts off the current through itself and the con-ventional heater when it reaches its switching temperature Ts, and thus controls the maximum temperature of the body being heated. The temperature of the body being heated rises to or near T and then, depending on the thermal and electrical characteristics of the device, stabilises at a temper-ature below T or cycles (with constant or decreasing amplitude) about an average temperaturc below T , the maximum temperature also being below Ts.
The rigid PTC elements used in these devices may comprise two or more PTC
components which are connected in parallel, the components having different switching temperatures and therefore causing stepwise modification of the electrical characteristics of *he device as the temperature of the body being heated increases. The PTC component having the highest T controls the maximum temperature of the body being heated. In all these devices, the maximum temperature reached by the PTC element is its switching temperature Ts or, where the PTC element contains PTC components connected in parallel and having different switching temperatures, the highest of those switching temperatures.
The present invention is concerned with devices in which at least part of a PTC element is heated in a controlled fashion to a temperature in excess of Ts.
Thus in its first aspect, the invention provides an electrical device which comprises:
(1) a PTC element having a switching temperature Ts;
(2) means for connecting the PTC element (1) to a source of electrical ;
power so that electrical current passes therethrough, the PTC element (1) determining the maximum temperature reached by the device when the electrical current passes therethrough, and
power so that electrical current passes therethrough, the PTC element (1) determining the maximum temperature reached by the device when the electrical current passes therethrough, and
(3) heat-supplying means which, when the device has been connected to a source of electrical power for a time such that at least part of the PTC
element has reached a temperature equal to Ts, will supply heat to that part of the PTC element at a rate which is sufficient to heat that part above Ts and which is dependent on the size of the current selected from the group ,-- .
1~
.
- \
~712Bl consisting of ~a) the current which is passing through another part of the PTC element (1) at the time and (b) the current which was passing through -that part of the PTC element (1) at a predetermine~ earlier time.
The heat-supply means (3) generally comprises at least one heater whose output of heat at any given moment is dependent on the size of the current passing through the PTC element (1) at the same moment, for example, an electrical resistance heater which is connected in series with the PTC
element (1). Suitable resistance heaters include those having a uniform volume resistivity ~hich is substantially constant at temperatures up to and substantially in excess of Ts, and also PTC elements having a switching temperature higher than the switching temperature T of the PTC element (1).
As explained below, the use of such PTC elements as resistance heaters pro-vides additional control over the temperature/time characteristics of the PTC element (1). Preferably, the resistance heaters employed have a resist-ance which does not increase by a factor of more than six over any 30C
segment between 25 C and Ts, and such heaters are referred to herein as constant wattage heaters.
.~
. ' , -, When using resistance heaters, it is of course essential to ensure that at least part of the heater reaches a temperature greater than Ts while at least part of the PIC element is still at a temperature below Ts; and to ensure that at least part of the heat put out by the heater while it is at a temperature greater than Ts either is supplied to the PqC element after a time delay (during which the PIC element may have reached a temperature f TS or above), and/or to a part of the PIC element which is already at a tenperature of Ts or above. A time delay can be achieved b~ interposing a thermal delay element having substantial thermal capacity between the PIC element and the heater. The thermal delay element may be an electrical insulator, e.g. a foamed polymer, or a conductor, e.g. a foamed polymer containing a fibrous and/or particulate metal filler; the thermal delay element can also comprise a material that can isothermzlly absorb heat, e.g. by undergoing a phase change such as melting, preferably at a temperature above Ts. Supply of the heat to a part of the PIC
element which is already at a temperature of Ts or above can be achieved by one or a ccmbination of various measures, described in ` detail hereinafter, which ensure that there is a non-uniform current flow through the PIC element.
Ihe devices can also comprise, in order to increase the amount of heat available to heat the PIC element after it has reached Ts, a thermal storage element having substantial thermal capacity which is in therm21 contact with a said heater, but is not interposed between a said heater and the PqC element.
Preferred devices are laminates including a plurality of contacting layers, each of which is a PIC element, an electrical resistance heater connected in series with the PIC element, a thermal delay element or a thermal storage element. Preferably each of said layers is an electrical conductor at temperatures at least as high as Ts~ In such laminates, each of the layers may be of constant thickness (the thicknesses of the different layers being the same or different), or one or more of the layers may be of nonruniform thickness~
When each of the layers is of constant thickness and the laminate includes electrodes, e.g. in the form of metal foil, paint or plate,or wire mesh, which ensure a uniform flcw of electricity through all parts of the PIC element, then the laminate must include a thermal delay element interposed between the PlC element and the heater layer. Preferred laminates of this type include laminates comprising (a) a first electrode layer:
(b) a constant wattage heater layer, preferably a layer composed of a PIC material having a switchLns temperature higher than Ts;
(c) a thermal delay layer;
(d) a said PIC element layer having a volume resistivity at temperatures below TS which is less than the volume resistivity o~ the heater layer (b); and (e) a second electrode layer;
and laminates of this type which include either an electrically conductive thermal storage layer between the first electrode layer (a) and the heater layer (b), the StOrage layer having a volume resistivity at temperatures below Ts which is less than the volume resistivity of the heater layer (b), or a thermal storage layer on the side of the first electrode layer (a) which is remote from the heater layer (b).
~071281 146/254 On the other hand, when one or more of the PIC and heater - layers is of non-uniform thickness, iK~ when each of the~ayers is of constant thickness but the electrodes are placed so that there is a non-uniform flow of electricity through the PIC element, a thermal delay element is not essential. Preferred devices of this type comprise a PIC layer sandwiched between tw~ heater la~ers, and preferably in physical contact with at least one of said heater layers. When each of the layers is of constant thickness, then there is preferably one strip electrode contacting each of the heater layers, the electrodes being on the same side of the device or in diagonally opFosite corner portions thereof.
It is often preferred, for example in order that the laminates should be flexible, that each of the layers should be comFosed of a polymeric material. Layers having appropriate resistivities (and, if required, PIC characteristics) can be prepared (as is well known in the art) from blends of polymers with different amounts of various conductive fillers, e.g. carbon blacks. The polymer (or mixture of polymers) can be the same or different in the different layers. When the polymer is crystalline, as is generally preferred, it is usually desirable that it should also be cros s linked. Ihe total thickness of such laminates may be, for example 0.25 cm or more, e.g. up to 0.75 or 1 cm.
~071281 146/254 he Pq~ element (l), and each of any other PIC elements present as electrical resistance heaters, thermal delay elements or thermal storage elements, preferably comprises an organic polymer, especially a cross-linked thermoplastic crystalline polymer, having a particulate electrically conductive filler, preferably a carbon black, dispersed therein. Preferably the PIC element (l) exhibits at least a six-fold increase in resistance between Ts and Ts+ 30C. Suitable constant wattage heaters can be composed of similar dispersions containing greater proportions of the filler.
' Particularly preferred devices are heat-recoverable laminates which comprise at least one polymeric layer, preferably the PIC element (1) layer, which will recover to a different configuration on heating to a temperature of Ts or above, say Ts + 80C. 5~chniques for making heat-recoverable polymeric articles are well known and are described for example in U.S. Patent No. 3,086,242 (Cook~. q~pically a shaped article of a cross-linked polymeric composition is deformed at a temperature above its crystalline melting point and then cooled in the deformed state. On being re-heated without restraint to a temPerature above the crystalline melting point, the article reco~ers to its original form. Heat-recoverable articles can be prepared from blends of polymers with electrically conductive fillers, which blends are conductive but do not exhibit PIC behavior, and such articles can be recovered by passing current through them. However, this method is highly unreliable because excessive heating of the article is very difficult to avoid. PIC articles based on cross-linked crystalline polymers can be rendered heat-recoverable. However, Ts for such articles is just below the crystalline melting point of the polymer and heating to a temperature at least 10C above the crystalline melting point is desirable for recovering the article. Such articles therefore cannot be recovered satisfactorily merely by passing an 1071Z~1 electric current through the heat-recoverable article. It is a particular advantage of the present invention that it overcomes this difficulty and thus provides an alternative to the methods of heating which are currently employed, for example the use of an open flame, S which is undesirable in some situations.
Other preferred laminates, which may or may not be heat-recoverable, include an external layer of a heat-activated adhesive.
qhe invention also includes a process for heating at least part of a PIC element (as hereinbefore defined) to a tenperature above its Ts, by connecting a source of e~ectrical power to the PlC
element (l) of an electrical device as described above so that electric current passes through the PIC element (1) at least until at least part of the element reaches a temperature equal to Ts~
Preferably the PIC element is heated to a temperature at least 5C, especially at least 10C, but not more than 80C above its Ts. If the PIC element remains connected to the source of power after the element has reached Ts and current has therefore substantially ceased to flcw, then the variation of the temperature of the P~C
~ ar~ ,ern~
"~; element with time depends upon the electricallcharacteristics of the fl~ ermal device and upon itslenvironment. Preferably the temperature of the PIC element reaches a maximum and then falls smoothly to a constant value which may be at or near Ts. However, it is also possible for the temperature to reach a maximum and then to oscillate (with constant or diminishing amplitude) about a mean value which may be at or near Ts.
1~71Z~l In carrying out the process of the present invention, any convenient power source which is appropriate to the particular device may be employed, e.g. a voltage of less than 40 volts, e.g. 12-36 volts from a battery, or 115 volt AC. When the device is heat-recoverable, the power source and the resistances of the various layers must of course be such that the power output is sufficient to cause recovery.
Ihe volume resistivities, thicknesses and shapes of the different layers and the positioning of the electrodes will have a profound influence on the preferred current path through the device at different temperatures, and hence the power density (and therefore heat generated) at any particular point at any particular time. Ihe operating characteristics of the device will also depend upon the thermal conductance and the thermal mass of the layers and the rates at which they lose heat to the environment, and on the power source employed. ~owever, those skilled in the art will have no difficulty, having regard to the disclosure herein, in obtain mg the benefits and objects of the invention.
Ihe invention is illustrated in the accompanying drawings, in which Figures 1-12 are isometric views of laminated devices of the invention. It is to be understood that when, in describing the various Figures, reference is made to possible modifications of the devices described, those modifications can also be employed, where appropriate, with other devices according to the invention. In describing the Figures, a constant wattage heater layer is referred to as a CW layer, the PIC element (1) layer as the PIC layer, a thermal delay layer as a lD layer and a thermal storage layer as a IS layer.
_g_ 10712~
Referring now to Figure 1, laminate 10 has metallic me~h electrodeq 14 and 15, CW layer 11, TD layer 13 and PTC layer 12. The reJistivity of CW layer 11 i~ greater than that of TD layer 13 and TC layer 12. The resistivity of TD layer 13 i~ preferably equal to or le88 than that of PTC layer 12. The electrodes could alternatively be layers of metallic plate or paint, or a plurality of ~trip or wire electrode~. When electrode~ 14 and 15 are connected to power source 16, current flow~ uniformly between the electrodes, and layer 11 heat~ more rapidly than layer 13 or layer 12~ When the junction of layer~ 12 and 13 reache~ T8, the current i8 shut off, (i.e. reduced to a very low level), but ~ince layer 11 i9 at a temperature above T~, heat is ~upplied to PTC layer 12 at a rate which i~ dependent on the temperature of layer 11, which i3 in turn dependent on the current which wa~ passing through the laminate at some earlier time. The rate i~ alao dependent on the characteri3tics of the TD layer. (It i8 of course to be understood that the word "dependent"
ia u~ed in thia apecification in a broad sense and does not mean that there i a direct or simple relation hip between the two quantitiea which are dependant upon each other.2 The maximum temperature to which the PTC layer 12 will be heated will lie between T8 and the maximum temperature of CW layer 11.
In a modification of the device of Figure 1, one or both of CW layer 11 and TD layer 13 is compoaed of a PTC material having a switching temperature greater than T8 of the layer 12. In thic modification, the switching temperature of layer 11 or 13 will limit the maximum te~perature achieved by PTC layer 12.
~ ' lG71281 Referring now to Figure 2, laminate 17 has an upper CW layer and a lower PIC layer~, with an intermediate ID layerJwhich is an electrical insulator. Electrodes 21, 22, 23, and 24 connect the CW
and PIC layers in series with a power supply 25. ~his device operates in the same way as that shown in Figure 1.
Referring now to Figure 3, laminate 26 has electrodes 30 and 31, CW layer 27, ID layer 29 and P~C layer 28 as in Figure 1, and in /~f~`o~
a~ Lt~s electrically conduc~ing IS layer 32, which has high thermal conductivity and thermal mass. Ihis device operates in the same way as that shown in Figure 1 except that the ~S layer 32 increases the maximum temperature reached by PqC layer 29 and/or the time for which PIC layer 29 remains substantially above Ts. In a modification of this device qS layer 32 is an electrical insulator and electrode 30 is placed between layers 27 and 32. IS layer 32 may comprise a crystalline polymer whose melting point is between Ts f layer 28 and the maximum temperature reached by layer 27. Melting of the polymer in layer 32 stores heat which is released after the current has shut off.
Referring now to Figure 4, laminate 33 has CW layers 34 and 35, lD layer 37 and PIC layer 36, and strip electrodes 38 and 39 connected to a power source. Ihe resistivity of CW layer 34 is higher than that of CW layer 35, and both are much less than that of ID layer 37 and PIC layer 36. Ihe resistivity of the ~D layer 37 is preferably equal to or less than that of PTC layer 36. When the electrodes are connected to the power source, the current path is predominantly in the plane of layers 34 and 35 and normal to the plane of layers 36 and 37, and layer 34 heats faster than any of the other layers. Ihe PlC
layer is thus heated to a temperature greater than Ts, as in the -' 107~28iL
devices of Figures 1 to 3.
Referring now to Figure 5, this is similar to Figure 4 except that the laminate includes a second ~D
layer 41, and in consequence CW layer~ 34 and 35 can have the same resi~tivity.
In a modification of the laminate of Figure 5, the strip electrodes 38 and 39 are replaced by mesh electrodes over the outer faces of layer 34 and 35, and the resistivity of each of layers 34 and 35 is greater than that of the other layer~, since in all the layer~ the current flow i8 normal to the plane of the layer.
Referring now to Figure 6, laminate 42 has CW
layer~ 43 and 44, P~C layer~ 45 and 4~ and TD layer 47.
PTC layer 45 has a T~ higher than PTC layer 46. Electrodes 48 and 49 are disposed diagonally in layers 43 and 44.
When èlectrically powered by current source 50, layer 43 heat~ fir~t, layer 45 accordingly reaching it~ T8, when the current i~ ~hut off, before layer 46 reache~ it~
T8, ~ though it is lower. The heat stored in layer 43 and 45 i3 transmitted to PTC layers 46 which reache~ a temperature between its own T8 and the T~ of layer 45 before reaching a ~teady state at about its own T8.
In modifications of the laminates shown in Figures 1 to 6, one or both of the monolithic electrodes shown can be replaced by a plurality of electrodes. ~owever, the position, ~pacing and number of electrodes may alter the characteristics of the laminate, as previously mentioned and a~ further de~cribed below.
Referring now to Figures 7 and 8,1aminates 51 and 58 ccrprise CW layers 52 and 53, ID layer 54 and PIC layer 55. Ihe volume resistivities of CW layers 52 and 53 are the same. Strip electrodes 56 are embedded in layers 52 and 53. When the electrodes . are connected to power sourcel layer 52 heats more rapidly than the other layers, in Figure 7 because the electrodes are further apart than in layer 53, and in Figure 8 because layer 52 is thiM er than layer 53.
- Referring now to Figure 9, laminate 59 illustrated comprises CW layers 60 and 61, PIC layer 62, and strip electrodes 63 and 64.
The resistivities of CW layers 60 and 61 are the same. When the electrodes are connected to power source 65, the flcw of current between the electrodes is non-uniform, and the edge portions of the laminate heat first. When the edge portions of PIC layer 62 reaches Ts, the current is forced to flcw through a center portion of layer 62 whose width diminishes, and while it does so, the heat generated in layers 60 and 61 heats the gradually widening edge portions of PIC
layer 62 to tenFeratures above Ts~
In modifications of the laminate shown in Figure 9, one or both of the CW layers are PIC layers having a Ts above that of layer 62. The laminate can also be modified to include a ~D or T~ layer as previously described.
Referring now to Figure 10, laminate 66 comprises CW layers 67 and 68, PIC layer 69 and strip electrodes 70 and ;~. When the electrodes are connected to power source 72, current initially flows directly between the electrodes, heating the left hand side of the laminate. However, when the left hand edge portion of PTC layer 69 ~07~Z81 reaches Ts, the current is forced into an increasingly more roundabout route as re and more of PIC layer 69 is shut off. The heat generated in layers 67 and 68 heats the shut-off part of layer 69 to a temperature above Ts. :
Referring ncw to Figure 11, laminate 73 comprises CW layers 74 and 75 which increase in thickness from left to right, PIC layer 76 and mesh electrodes 77 and 78 connected to power source 79. Because of the lesser thickness of layers 74 and 75 at the left hand edge, the left hand edge will heat more rapidly, and the PIC layer shuts off progressively from left to right, in a way similar to the laminate of Figure 10.
Referring ncw to Figure 12, laminate 80 comprises CW layers 81 and 82, PIC layer 83 which is thicker in the center than at the sides, and mesh electrodes 84 and 85. When the electrodes are connected to power source 86, the edge portions of the PIC layer 83 are heated more rapidly than the center portion. The side portions therefore reach Ts and are shut off first.
In difications of the laminate of Figure 12, PlC layer 83 is thicker at the sides than at the center, or be wedge-shaped, or has a thickness which varies uniformly or non-uniformly in some other way.
In modifications of the laminates of Figures 11 and 12, the thicknesses of the CW layers change at the same or different rates, uniformly or non-uniformly.
In difications of the laminates of Figures 10 to 12, one - 107128~
or both of the Cw layers are PTC layers having a Ts greater than that of the intermediate PTC layer.
The laminates shown in the drawings ha~e a planar configuration for ease of illustration, but they can, of course, have a non-planar configur-ation, e.g. a tubular shape, which may be regular or irregular.
As previously noted, the laminates of the present invention are particularly useful when they are heat-recoverable articles, when the higher temperatures reached by the PTC element ~1) cause recovery. The lower steady-state temperature might, for example~ be useful to promote flow of an adhesive coating on the heat-recoverable article after its recovery. The devices may also be employed in other situations where it is useful to gener-ate an initial high temperature and to maintain a subsequent low temperature, for example to initiate a reaction, e.g. by decomposing a peroxide, and then to maintain it. The devices are particularly useful when in the form of heat-recoverable articles as claimed in British patent specification Nos.
1,529,353, 1,529,355 and 1,529,356 published October 18, 1978 in the name of Raychem Corporation.
The devices of the present invention can include conventional electrodes as previously indicated, but particularly when the devices are heat-recoverable, it is preferred that at least one of the electrodes is a ' ' :
:
10712~
tubular electrode as disclosed in Canadian patent application serial No.
258,297 filed August 3, 1976 in the names of David A. Horsma and Stephen H.
Diaz. Conventional PTC compositions can be used in the present invention, but there can also be used PTC compositions as disclosed in British patent specification No. 1,528,623 in the name of Raychem Corporation, published October 18, 1978 and in copending Canadian patent applications Nos. 258,294 in the name of Wendell W. Moyer and No. 258,296 in the names of Bernard J.
Lyons and Young J. Kim filed August 3, 1976. Further information about PTC
compositions and their use will be found in British patent specification No.
1,529,354 in the name of Raychem Corporation, published October 18, 1978.
.~ ' , : , :
1(~7128i The invention is further illustrated by the following Examplea, in which percentages are by weight.
Example 1 A laminate generally âJ shown in Figure 7 was propared. CW layer~ 52 and 53 were each 0.12 cm thick and compo~ed of a blend of 40X carbon black and 60~
ethylene/vinyl acetate copolyme~ (Elvax 260). TD Iayer 54 was 0.24 cm thick and composed of a blend of 20%
carbon black and 80% of a blend of about equal parts of a crystalline polypropylene and an ethylenepropylene rubber (Uniroyal TPR-2000). PTC layer 55 had a T8 of about 120C, was 0.12 cm thick and was composed of a blend of 40% carbon black (Sterling SRF-~S) and 6~%
high den~ity polyethylene~(Marlex 6003). The electrodes were 2.54 cm apart in layer 52 and 1.27 cm apart in layer 53. The electrodes were connected to a 36 volt DC source.
After about 60 seconds the current dropped sharply and the temperature of the outer surfaces of layers 52 and 53 were about 150C and 110C respectively. After a total time of 120 seconds both ~urfaces were at about 120C.
* Trade Mark 1071~1 146/254 Example 2 A laminate 2.54 cm wide and generally as shown in Figure 9 was prepared. CW layers 60 and 61 were each 0.1 cm thick and composed of a conductive silicone rubber (Union Carbide K 1516). PIC layer 62 --had a Ts of about 120C, was 0.025 cm thick and was com~osed of a blend of 40% carbon black (Sterling SRF-NS) and 60% high density polyethylene (Marlex 6003). Ihe laminate was placed on a table, without insulation, and the electrodes were connected to a 36 volt DC
source; each surface reached a maximum temperature of about 132C
after about 12 seconds, and after about 30 seconds dropped to a temperature of about 100C which remained constant. Insl~lation was then placed on both surfaces of the laminate and the electrodes connected in turn to 12, 24, and 36 volt DC sources. The surfaces of the laminate reached maximum temperatures of 138, 169 and 202C
; 15 respectively and dropped to a steady state temFerature of about 110C.
~rc/Je rna~ks 10~2~1 Example 3 A laminate generally as described in Figure 1 was prepared, using copper foil for the electrodes 14 and 15. Each of the layers was 0.15 cm thick. CW layer 11 was composed of a blend of 25% carbon black, 37.5% of an ethylene-propylene diene modified ~EPDM) rubber ("Nordel" 1440) ànd 37.5% of a conductive silicone rubber ~"Silastic" 350). TD layer 13 was composed of a blend of 35% carbon black and 65% of the same EPDM rubber. PTC
layer 12 was composed of a blend of 45% carbon black (Sterling SRF-NS) and 55% high density polyethylene (Marlex 6003) and had a Ts of about 120C.
The electrodes were connected to a 25 volt AC power source. The bottom surface of the laminate reached a temperature of about 140C in about 4 minutes and then dropped to a steady state temperature of about 115C.
~-f~Je ~R~ks .` 19 -
element has reached a temperature equal to Ts, will supply heat to that part of the PTC element at a rate which is sufficient to heat that part above Ts and which is dependent on the size of the current selected from the group ,-- .
1~
.
- \
~712Bl consisting of ~a) the current which is passing through another part of the PTC element (1) at the time and (b) the current which was passing through -that part of the PTC element (1) at a predetermine~ earlier time.
The heat-supply means (3) generally comprises at least one heater whose output of heat at any given moment is dependent on the size of the current passing through the PTC element (1) at the same moment, for example, an electrical resistance heater which is connected in series with the PTC
element (1). Suitable resistance heaters include those having a uniform volume resistivity ~hich is substantially constant at temperatures up to and substantially in excess of Ts, and also PTC elements having a switching temperature higher than the switching temperature T of the PTC element (1).
As explained below, the use of such PTC elements as resistance heaters pro-vides additional control over the temperature/time characteristics of the PTC element (1). Preferably, the resistance heaters employed have a resist-ance which does not increase by a factor of more than six over any 30C
segment between 25 C and Ts, and such heaters are referred to herein as constant wattage heaters.
.~
. ' , -, When using resistance heaters, it is of course essential to ensure that at least part of the heater reaches a temperature greater than Ts while at least part of the PIC element is still at a temperature below Ts; and to ensure that at least part of the heat put out by the heater while it is at a temperature greater than Ts either is supplied to the PqC element after a time delay (during which the PIC element may have reached a temperature f TS or above), and/or to a part of the PIC element which is already at a tenperature of Ts or above. A time delay can be achieved b~ interposing a thermal delay element having substantial thermal capacity between the PIC element and the heater. The thermal delay element may be an electrical insulator, e.g. a foamed polymer, or a conductor, e.g. a foamed polymer containing a fibrous and/or particulate metal filler; the thermal delay element can also comprise a material that can isothermzlly absorb heat, e.g. by undergoing a phase change such as melting, preferably at a temperature above Ts. Supply of the heat to a part of the PIC
element which is already at a temperature of Ts or above can be achieved by one or a ccmbination of various measures, described in ` detail hereinafter, which ensure that there is a non-uniform current flow through the PIC element.
Ihe devices can also comprise, in order to increase the amount of heat available to heat the PIC element after it has reached Ts, a thermal storage element having substantial thermal capacity which is in therm21 contact with a said heater, but is not interposed between a said heater and the PqC element.
Preferred devices are laminates including a plurality of contacting layers, each of which is a PIC element, an electrical resistance heater connected in series with the PIC element, a thermal delay element or a thermal storage element. Preferably each of said layers is an electrical conductor at temperatures at least as high as Ts~ In such laminates, each of the layers may be of constant thickness (the thicknesses of the different layers being the same or different), or one or more of the layers may be of nonruniform thickness~
When each of the layers is of constant thickness and the laminate includes electrodes, e.g. in the form of metal foil, paint or plate,or wire mesh, which ensure a uniform flcw of electricity through all parts of the PIC element, then the laminate must include a thermal delay element interposed between the PlC element and the heater layer. Preferred laminates of this type include laminates comprising (a) a first electrode layer:
(b) a constant wattage heater layer, preferably a layer composed of a PIC material having a switchLns temperature higher than Ts;
(c) a thermal delay layer;
(d) a said PIC element layer having a volume resistivity at temperatures below TS which is less than the volume resistivity o~ the heater layer (b); and (e) a second electrode layer;
and laminates of this type which include either an electrically conductive thermal storage layer between the first electrode layer (a) and the heater layer (b), the StOrage layer having a volume resistivity at temperatures below Ts which is less than the volume resistivity of the heater layer (b), or a thermal storage layer on the side of the first electrode layer (a) which is remote from the heater layer (b).
~071281 146/254 On the other hand, when one or more of the PIC and heater - layers is of non-uniform thickness, iK~ when each of the~ayers is of constant thickness but the electrodes are placed so that there is a non-uniform flow of electricity through the PIC element, a thermal delay element is not essential. Preferred devices of this type comprise a PIC layer sandwiched between tw~ heater la~ers, and preferably in physical contact with at least one of said heater layers. When each of the layers is of constant thickness, then there is preferably one strip electrode contacting each of the heater layers, the electrodes being on the same side of the device or in diagonally opFosite corner portions thereof.
It is often preferred, for example in order that the laminates should be flexible, that each of the layers should be comFosed of a polymeric material. Layers having appropriate resistivities (and, if required, PIC characteristics) can be prepared (as is well known in the art) from blends of polymers with different amounts of various conductive fillers, e.g. carbon blacks. The polymer (or mixture of polymers) can be the same or different in the different layers. When the polymer is crystalline, as is generally preferred, it is usually desirable that it should also be cros s linked. Ihe total thickness of such laminates may be, for example 0.25 cm or more, e.g. up to 0.75 or 1 cm.
~071281 146/254 he Pq~ element (l), and each of any other PIC elements present as electrical resistance heaters, thermal delay elements or thermal storage elements, preferably comprises an organic polymer, especially a cross-linked thermoplastic crystalline polymer, having a particulate electrically conductive filler, preferably a carbon black, dispersed therein. Preferably the PIC element (l) exhibits at least a six-fold increase in resistance between Ts and Ts+ 30C. Suitable constant wattage heaters can be composed of similar dispersions containing greater proportions of the filler.
' Particularly preferred devices are heat-recoverable laminates which comprise at least one polymeric layer, preferably the PIC element (1) layer, which will recover to a different configuration on heating to a temperature of Ts or above, say Ts + 80C. 5~chniques for making heat-recoverable polymeric articles are well known and are described for example in U.S. Patent No. 3,086,242 (Cook~. q~pically a shaped article of a cross-linked polymeric composition is deformed at a temperature above its crystalline melting point and then cooled in the deformed state. On being re-heated without restraint to a temPerature above the crystalline melting point, the article reco~ers to its original form. Heat-recoverable articles can be prepared from blends of polymers with electrically conductive fillers, which blends are conductive but do not exhibit PIC behavior, and such articles can be recovered by passing current through them. However, this method is highly unreliable because excessive heating of the article is very difficult to avoid. PIC articles based on cross-linked crystalline polymers can be rendered heat-recoverable. However, Ts for such articles is just below the crystalline melting point of the polymer and heating to a temperature at least 10C above the crystalline melting point is desirable for recovering the article. Such articles therefore cannot be recovered satisfactorily merely by passing an 1071Z~1 electric current through the heat-recoverable article. It is a particular advantage of the present invention that it overcomes this difficulty and thus provides an alternative to the methods of heating which are currently employed, for example the use of an open flame, S which is undesirable in some situations.
Other preferred laminates, which may or may not be heat-recoverable, include an external layer of a heat-activated adhesive.
qhe invention also includes a process for heating at least part of a PIC element (as hereinbefore defined) to a tenperature above its Ts, by connecting a source of e~ectrical power to the PlC
element (l) of an electrical device as described above so that electric current passes through the PIC element (1) at least until at least part of the element reaches a temperature equal to Ts~
Preferably the PIC element is heated to a temperature at least 5C, especially at least 10C, but not more than 80C above its Ts. If the PIC element remains connected to the source of power after the element has reached Ts and current has therefore substantially ceased to flcw, then the variation of the temperature of the P~C
~ ar~ ,ern~
"~; element with time depends upon the electricallcharacteristics of the fl~ ermal device and upon itslenvironment. Preferably the temperature of the PIC element reaches a maximum and then falls smoothly to a constant value which may be at or near Ts. However, it is also possible for the temperature to reach a maximum and then to oscillate (with constant or diminishing amplitude) about a mean value which may be at or near Ts.
1~71Z~l In carrying out the process of the present invention, any convenient power source which is appropriate to the particular device may be employed, e.g. a voltage of less than 40 volts, e.g. 12-36 volts from a battery, or 115 volt AC. When the device is heat-recoverable, the power source and the resistances of the various layers must of course be such that the power output is sufficient to cause recovery.
Ihe volume resistivities, thicknesses and shapes of the different layers and the positioning of the electrodes will have a profound influence on the preferred current path through the device at different temperatures, and hence the power density (and therefore heat generated) at any particular point at any particular time. Ihe operating characteristics of the device will also depend upon the thermal conductance and the thermal mass of the layers and the rates at which they lose heat to the environment, and on the power source employed. ~owever, those skilled in the art will have no difficulty, having regard to the disclosure herein, in obtain mg the benefits and objects of the invention.
Ihe invention is illustrated in the accompanying drawings, in which Figures 1-12 are isometric views of laminated devices of the invention. It is to be understood that when, in describing the various Figures, reference is made to possible modifications of the devices described, those modifications can also be employed, where appropriate, with other devices according to the invention. In describing the Figures, a constant wattage heater layer is referred to as a CW layer, the PIC element (1) layer as the PIC layer, a thermal delay layer as a lD layer and a thermal storage layer as a IS layer.
_g_ 10712~
Referring now to Figure 1, laminate 10 has metallic me~h electrodeq 14 and 15, CW layer 11, TD layer 13 and PTC layer 12. The reJistivity of CW layer 11 i~ greater than that of TD layer 13 and TC layer 12. The resistivity of TD layer 13 i~ preferably equal to or le88 than that of PTC layer 12. The electrodes could alternatively be layers of metallic plate or paint, or a plurality of ~trip or wire electrode~. When electrode~ 14 and 15 are connected to power source 16, current flow~ uniformly between the electrodes, and layer 11 heat~ more rapidly than layer 13 or layer 12~ When the junction of layer~ 12 and 13 reache~ T8, the current i8 shut off, (i.e. reduced to a very low level), but ~ince layer 11 i9 at a temperature above T~, heat is ~upplied to PTC layer 12 at a rate which i~ dependent on the temperature of layer 11, which i3 in turn dependent on the current which wa~ passing through the laminate at some earlier time. The rate i~ alao dependent on the characteri3tics of the TD layer. (It i8 of course to be understood that the word "dependent"
ia u~ed in thia apecification in a broad sense and does not mean that there i a direct or simple relation hip between the two quantitiea which are dependant upon each other.2 The maximum temperature to which the PTC layer 12 will be heated will lie between T8 and the maximum temperature of CW layer 11.
In a modification of the device of Figure 1, one or both of CW layer 11 and TD layer 13 is compoaed of a PTC material having a switching temperature greater than T8 of the layer 12. In thic modification, the switching temperature of layer 11 or 13 will limit the maximum te~perature achieved by PTC layer 12.
~ ' lG71281 Referring now to Figure 2, laminate 17 has an upper CW layer and a lower PIC layer~, with an intermediate ID layerJwhich is an electrical insulator. Electrodes 21, 22, 23, and 24 connect the CW
and PIC layers in series with a power supply 25. ~his device operates in the same way as that shown in Figure 1.
Referring now to Figure 3, laminate 26 has electrodes 30 and 31, CW layer 27, ID layer 29 and P~C layer 28 as in Figure 1, and in /~f~`o~
a~ Lt~s electrically conduc~ing IS layer 32, which has high thermal conductivity and thermal mass. Ihis device operates in the same way as that shown in Figure 1 except that the ~S layer 32 increases the maximum temperature reached by PqC layer 29 and/or the time for which PIC layer 29 remains substantially above Ts. In a modification of this device qS layer 32 is an electrical insulator and electrode 30 is placed between layers 27 and 32. IS layer 32 may comprise a crystalline polymer whose melting point is between Ts f layer 28 and the maximum temperature reached by layer 27. Melting of the polymer in layer 32 stores heat which is released after the current has shut off.
Referring now to Figure 4, laminate 33 has CW layers 34 and 35, lD layer 37 and PIC layer 36, and strip electrodes 38 and 39 connected to a power source. Ihe resistivity of CW layer 34 is higher than that of CW layer 35, and both are much less than that of ID layer 37 and PIC layer 36. Ihe resistivity of the ~D layer 37 is preferably equal to or less than that of PTC layer 36. When the electrodes are connected to the power source, the current path is predominantly in the plane of layers 34 and 35 and normal to the plane of layers 36 and 37, and layer 34 heats faster than any of the other layers. Ihe PlC
layer is thus heated to a temperature greater than Ts, as in the -' 107~28iL
devices of Figures 1 to 3.
Referring now to Figure 5, this is similar to Figure 4 except that the laminate includes a second ~D
layer 41, and in consequence CW layer~ 34 and 35 can have the same resi~tivity.
In a modification of the laminate of Figure 5, the strip electrodes 38 and 39 are replaced by mesh electrodes over the outer faces of layer 34 and 35, and the resistivity of each of layers 34 and 35 is greater than that of the other layer~, since in all the layer~ the current flow i8 normal to the plane of the layer.
Referring now to Figure 6, laminate 42 has CW
layer~ 43 and 44, P~C layer~ 45 and 4~ and TD layer 47.
PTC layer 45 has a T~ higher than PTC layer 46. Electrodes 48 and 49 are disposed diagonally in layers 43 and 44.
When èlectrically powered by current source 50, layer 43 heat~ fir~t, layer 45 accordingly reaching it~ T8, when the current i~ ~hut off, before layer 46 reache~ it~
T8, ~ though it is lower. The heat stored in layer 43 and 45 i3 transmitted to PTC layers 46 which reache~ a temperature between its own T8 and the T~ of layer 45 before reaching a ~teady state at about its own T8.
In modifications of the laminates shown in Figures 1 to 6, one or both of the monolithic electrodes shown can be replaced by a plurality of electrodes. ~owever, the position, ~pacing and number of electrodes may alter the characteristics of the laminate, as previously mentioned and a~ further de~cribed below.
Referring now to Figures 7 and 8,1aminates 51 and 58 ccrprise CW layers 52 and 53, ID layer 54 and PIC layer 55. Ihe volume resistivities of CW layers 52 and 53 are the same. Strip electrodes 56 are embedded in layers 52 and 53. When the electrodes . are connected to power sourcel layer 52 heats more rapidly than the other layers, in Figure 7 because the electrodes are further apart than in layer 53, and in Figure 8 because layer 52 is thiM er than layer 53.
- Referring now to Figure 9, laminate 59 illustrated comprises CW layers 60 and 61, PIC layer 62, and strip electrodes 63 and 64.
The resistivities of CW layers 60 and 61 are the same. When the electrodes are connected to power source 65, the flcw of current between the electrodes is non-uniform, and the edge portions of the laminate heat first. When the edge portions of PIC layer 62 reaches Ts, the current is forced to flcw through a center portion of layer 62 whose width diminishes, and while it does so, the heat generated in layers 60 and 61 heats the gradually widening edge portions of PIC
layer 62 to tenFeratures above Ts~
In modifications of the laminate shown in Figure 9, one or both of the CW layers are PIC layers having a Ts above that of layer 62. The laminate can also be modified to include a ~D or T~ layer as previously described.
Referring now to Figure 10, laminate 66 comprises CW layers 67 and 68, PIC layer 69 and strip electrodes 70 and ;~. When the electrodes are connected to power source 72, current initially flows directly between the electrodes, heating the left hand side of the laminate. However, when the left hand edge portion of PTC layer 69 ~07~Z81 reaches Ts, the current is forced into an increasingly more roundabout route as re and more of PIC layer 69 is shut off. The heat generated in layers 67 and 68 heats the shut-off part of layer 69 to a temperature above Ts. :
Referring ncw to Figure 11, laminate 73 comprises CW layers 74 and 75 which increase in thickness from left to right, PIC layer 76 and mesh electrodes 77 and 78 connected to power source 79. Because of the lesser thickness of layers 74 and 75 at the left hand edge, the left hand edge will heat more rapidly, and the PIC layer shuts off progressively from left to right, in a way similar to the laminate of Figure 10.
Referring ncw to Figure 12, laminate 80 comprises CW layers 81 and 82, PIC layer 83 which is thicker in the center than at the sides, and mesh electrodes 84 and 85. When the electrodes are connected to power source 86, the edge portions of the PIC layer 83 are heated more rapidly than the center portion. The side portions therefore reach Ts and are shut off first.
In difications of the laminate of Figure 12, PlC layer 83 is thicker at the sides than at the center, or be wedge-shaped, or has a thickness which varies uniformly or non-uniformly in some other way.
In modifications of the laminates of Figures 11 and 12, the thicknesses of the CW layers change at the same or different rates, uniformly or non-uniformly.
In difications of the laminates of Figures 10 to 12, one - 107128~
or both of the Cw layers are PTC layers having a Ts greater than that of the intermediate PTC layer.
The laminates shown in the drawings ha~e a planar configuration for ease of illustration, but they can, of course, have a non-planar configur-ation, e.g. a tubular shape, which may be regular or irregular.
As previously noted, the laminates of the present invention are particularly useful when they are heat-recoverable articles, when the higher temperatures reached by the PTC element ~1) cause recovery. The lower steady-state temperature might, for example~ be useful to promote flow of an adhesive coating on the heat-recoverable article after its recovery. The devices may also be employed in other situations where it is useful to gener-ate an initial high temperature and to maintain a subsequent low temperature, for example to initiate a reaction, e.g. by decomposing a peroxide, and then to maintain it. The devices are particularly useful when in the form of heat-recoverable articles as claimed in British patent specification Nos.
1,529,353, 1,529,355 and 1,529,356 published October 18, 1978 in the name of Raychem Corporation.
The devices of the present invention can include conventional electrodes as previously indicated, but particularly when the devices are heat-recoverable, it is preferred that at least one of the electrodes is a ' ' :
:
10712~
tubular electrode as disclosed in Canadian patent application serial No.
258,297 filed August 3, 1976 in the names of David A. Horsma and Stephen H.
Diaz. Conventional PTC compositions can be used in the present invention, but there can also be used PTC compositions as disclosed in British patent specification No. 1,528,623 in the name of Raychem Corporation, published October 18, 1978 and in copending Canadian patent applications Nos. 258,294 in the name of Wendell W. Moyer and No. 258,296 in the names of Bernard J.
Lyons and Young J. Kim filed August 3, 1976. Further information about PTC
compositions and their use will be found in British patent specification No.
1,529,354 in the name of Raychem Corporation, published October 18, 1978.
.~ ' , : , :
1(~7128i The invention is further illustrated by the following Examplea, in which percentages are by weight.
Example 1 A laminate generally âJ shown in Figure 7 was propared. CW layer~ 52 and 53 were each 0.12 cm thick and compo~ed of a blend of 40X carbon black and 60~
ethylene/vinyl acetate copolyme~ (Elvax 260). TD Iayer 54 was 0.24 cm thick and composed of a blend of 20%
carbon black and 80% of a blend of about equal parts of a crystalline polypropylene and an ethylenepropylene rubber (Uniroyal TPR-2000). PTC layer 55 had a T8 of about 120C, was 0.12 cm thick and was composed of a blend of 40% carbon black (Sterling SRF-~S) and 6~%
high den~ity polyethylene~(Marlex 6003). The electrodes were 2.54 cm apart in layer 52 and 1.27 cm apart in layer 53. The electrodes were connected to a 36 volt DC source.
After about 60 seconds the current dropped sharply and the temperature of the outer surfaces of layers 52 and 53 were about 150C and 110C respectively. After a total time of 120 seconds both ~urfaces were at about 120C.
* Trade Mark 1071~1 146/254 Example 2 A laminate 2.54 cm wide and generally as shown in Figure 9 was prepared. CW layers 60 and 61 were each 0.1 cm thick and composed of a conductive silicone rubber (Union Carbide K 1516). PIC layer 62 --had a Ts of about 120C, was 0.025 cm thick and was com~osed of a blend of 40% carbon black (Sterling SRF-NS) and 60% high density polyethylene (Marlex 6003). Ihe laminate was placed on a table, without insulation, and the electrodes were connected to a 36 volt DC
source; each surface reached a maximum temperature of about 132C
after about 12 seconds, and after about 30 seconds dropped to a temperature of about 100C which remained constant. Insl~lation was then placed on both surfaces of the laminate and the electrodes connected in turn to 12, 24, and 36 volt DC sources. The surfaces of the laminate reached maximum temperatures of 138, 169 and 202C
; 15 respectively and dropped to a steady state temFerature of about 110C.
~rc/Je rna~ks 10~2~1 Example 3 A laminate generally as described in Figure 1 was prepared, using copper foil for the electrodes 14 and 15. Each of the layers was 0.15 cm thick. CW layer 11 was composed of a blend of 25% carbon black, 37.5% of an ethylene-propylene diene modified ~EPDM) rubber ("Nordel" 1440) ànd 37.5% of a conductive silicone rubber ~"Silastic" 350). TD layer 13 was composed of a blend of 35% carbon black and 65% of the same EPDM rubber. PTC
layer 12 was composed of a blend of 45% carbon black (Sterling SRF-NS) and 55% high density polyethylene (Marlex 6003) and had a Ts of about 120C.
The electrodes were connected to a 25 volt AC power source. The bottom surface of the laminate reached a temperature of about 140C in about 4 minutes and then dropped to a steady state temperature of about 115C.
~-f~Je ~R~ks .` 19 -
Claims (17)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electrical device which comprises:
(1) a PTC element having a switching temperature Ts;
(2) means for connecting the PTC element (1) to a source of electrical power so that electrical current passes therethrough, the PTC element (1) determining the maximum temperature reached by the device when the electrical current passes therethrough, and (3) heat-supplying means which, when the device has been connected to a source of electrical power for a time such that at least part of the PTC
element has reached a temperature equal to Ts, will supply heat to that part of the PTC element at a rate which is sufficient to heat that part above T
and which is dependent on the size of the current selected from the group consisting of (a) the current which is passing through another part of the PTC element (1) at the time and (b) the current which was passing through that part of the PTC element (1) at a predetermined earlier time.
(1) a PTC element having a switching temperature Ts;
(2) means for connecting the PTC element (1) to a source of electrical power so that electrical current passes therethrough, the PTC element (1) determining the maximum temperature reached by the device when the electrical current passes therethrough, and (3) heat-supplying means which, when the device has been connected to a source of electrical power for a time such that at least part of the PTC
element has reached a temperature equal to Ts, will supply heat to that part of the PTC element at a rate which is sufficient to heat that part above T
and which is dependent on the size of the current selected from the group consisting of (a) the current which is passing through another part of the PTC element (1) at the time and (b) the current which was passing through that part of the PTC element (1) at a predetermined earlier time.
2. A device according to Claim 1 wherein the heat-supply means (3) comprises at least one constant wattage electrical resistance heater which is connected in series with the PTC element (1).
3. A device according to Claim 1 wherein the heat-supply means (3) comprises at least one electrical resistance heater which is a PTC element having a switching temperature higher than the switching temperature Ts of the PTC element (1).
4. A device according to Claim 2 wherein the heat supply means also comprises at least one thermal delay element which is interposed between the PTC element (1) and the resistance heater.
5. A device according to claim 2, 3 or 4 characterised in that it includes at least one thermal storage element which is in thermal contact with a said heater on the side thereof which is remote from the said PTC element (1).
6. A device according to Claim 2 which comprises a plurality of contacting layers, one of which is a said PTC element (1), and at least one of which is a said electrical resistance heater.
7. A device according to Claim 6 wherein each of said layers is of constant thickness, wherein at least one of said layers is a thermal delay element which is interposed between PTC element (l) and a said resistance heater, and which includes electrodes so placed that, when the device is connected to a source of electrical power through the electrodes, all parts of the PTC element (1) are heated at the same rate.
8. A device according to Claim 7 which comprises (a) a first electrode layer;
(b) a constant wattage heater layer;
(c) a thermal delay layer;
(d) a said PTC element layer having a resistance at temperatures below Ts which is less than the resistance of the heater layer (b); and (e) a second electrode layer.
(b) a constant wattage heater layer;
(c) a thermal delay layer;
(d) a said PTC element layer having a resistance at temperatures below Ts which is less than the resistance of the heater layer (b); and (e) a second electrode layer.
9. A device according to Claim 6 wherein at least one of the said layers is of non-uniform thickness.
10. A device according to claim 9 which comprises a PTC layer sandwiched between two heater layers.
11. A device according to Claim 6 wherein each of said layers is of constant thickness and which includes discrete electrodes so placed that, when the device is connected to a source of electrical power through the electrodes, different parts of the PTC element are heated at different rates.
12. A device according to Claim 11 characterised in that it comprises a PTC layer sandwiched between two heater layers.
13. A device according to Claim 7, 9 or 11 wherein each of the elec-trically conductive layers comprises an organic polymer having a particulate electrically conductive filler dispersed therein.
14. A device according to Claim 7, 9 or 11 which is heat-recoverable.
15. A process for heating at least part of a PTC element to a temper-ature above its Ts, which comprises connecting a source of electrical power to the PTC element (1) of an electrical device as claimed in Claim 7, 9 or 11.
16. A process for heating a PTC element to a temperature above its Ts, and at least part thereof to a temperature at least 5°C above its Ts, which comprises connecting a source of electrical power to the PTC element (1) of an electrical device as claimed in Claim 7, 9 or 11.
17. A process for heating a PTC element to above its Ts, and of heating at least part of the PTC element to a temperature which is at least 10°C
above its Ts and no part of the PTC element to a temperature which is more than 80°C above its Ts, which comprises connecting a source of electrical power to the PTC element (1) of an electrical device as claimed in Claim 7, 9 or 11.
above its Ts and no part of the PTC element to a temperature which is more than 80°C above its Ts, which comprises connecting a source of electrical power to the PTC element (1) of an electrical device as claimed in Claim 7, 9 or 11.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/601,427 US4017715A (en) | 1975-08-04 | 1975-08-04 | Temperature overshoot heater |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1071281A true CA1071281A (en) | 1980-02-05 |
Family
ID=24407447
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA258,295A Expired CA1071281A (en) | 1975-08-04 | 1976-08-03 | Heat-recoverable laminated heater with ptc layer operating above switching temperature |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4017715A (en) |
| AU (1) | AU511873B2 (en) |
| BE (1) | BE844848A (en) |
| CA (1) | CA1071281A (en) |
| DE (1) | DE2634932C2 (en) |
| GB (1) | GB1562085A (en) |
| IT (1) | IT1065401B (en) |
Families Citing this family (93)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4330703A (en) * | 1975-08-04 | 1982-05-18 | Raychem Corporation | Layered self-regulating heating article |
| US4177376A (en) * | 1974-09-27 | 1979-12-04 | Raychem Corporation | Layered self-regulating heating article |
| US4188276A (en) * | 1975-08-04 | 1980-02-12 | Raychem Corporation | Voltage stable positive temperature coefficient of resistance crosslinked compositions |
| US4421582A (en) * | 1975-08-04 | 1983-12-20 | Raychem Corporation | Self-heating article with deformable electrodes |
| US4177446A (en) * | 1975-12-08 | 1979-12-04 | Raychem Corporation | Heating elements comprising conductive polymers capable of dimensional change |
| US4117312A (en) * | 1976-07-22 | 1978-09-26 | Thermon Manufacturing Company | Self-limiting temperature electrical heating cable |
| US4388607A (en) * | 1976-12-16 | 1983-06-14 | Raychem Corporation | Conductive polymer compositions, and to devices comprising such compositions |
| US4314145A (en) * | 1978-01-30 | 1982-02-02 | Raychem Corporation | Electrical devices containing PTC elements |
| US4246468A (en) * | 1978-01-30 | 1981-01-20 | Raychem Corporation | Electrical devices containing PTC elements |
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-
1975
- 1975-08-04 US US05/601,427 patent/US4017715A/en not_active Expired - Lifetime
-
1976
- 1976-08-03 BE BE169528A patent/BE844848A/en not_active IP Right Cessation
- 1976-08-03 GB GB32275/76A patent/GB1562085A/en not_active Expired
- 1976-08-03 CA CA258,295A patent/CA1071281A/en not_active Expired
- 1976-08-04 DE DE2634932A patent/DE2634932C2/en not_active Expired
- 1976-08-04 AU AU16552/76A patent/AU511873B2/en not_active Expired
- 1976-08-04 IT IT26017/76A patent/IT1065401B/en active
Also Published As
| Publication number | Publication date |
|---|---|
| AU1655276A (en) | 1978-02-09 |
| US4017715A (en) | 1977-04-12 |
| AU511873B2 (en) | 1980-09-11 |
| IT1065401B (en) | 1985-02-25 |
| DE2634932C2 (en) | 1986-04-03 |
| DE2634932A1 (en) | 1977-02-10 |
| GB1562085A (en) | 1980-03-05 |
| BE844848A (en) | 1977-02-03 |
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