FI63848B - SKIKTAT ELEKTRISKT MOTSTAONDSELEMENT SAMT ANVAENDNING AV DETSAMMA FOER OEVERDRAGNING AV EN UNDERLAGSYTA - Google Patents

Description

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JSBa lBJ (11) UTLÄGGININOSSKAIFT OOÖ4Ö 2¾¾ C (45) r'trr!, ·· 1) CC 11J3 · * - - - · ij ^ '(51) Kv.ik.3 / int.ci.3 H 05 B 3 / Ί0 FINLAND —FINLAND (21) P »t.nttlh.k« mu · —Ptt «nttn.8kn» n | 783067 (22) Haketnlspitvt - AiwSfcnlnpdaf 09-10.78 * * (23) Alkupttvt — GlMghttadag 23.09.75 (41) Tulhit JulklMktl - BIWH offwitHf 09.10.78 PManttt · I * Registry Board, ... _,. .....

_ '(44) Date of issue of the declaration. -

Patent · och registerstyrelsen An * ek »n utiagd och utUkrifaMt pubiicorad 29.0U. 83 (32) (33) (31) Pyy4 «« y etuolkeu * - ** | * r <l priortut 27.09.7 ΙΟ **. 08.75 USA (US) 510036, 601638 (71) Raychem Corporation, 300 Constitution Drive, Menlo Park, California 91 * 025, USA (US) (72) David August Horsma, Palo Alto, California, Bernard John Lyons,

Atherton, California, Robert Smith-Johannsen, Portola Valley,

California, USA (US) (71 *) Berggren Oy Ab (51 *) Laminated electrical resistor element and its use to cover the substrate surface - Skiktat elektriskt motst & ndselement samt användning av detsamma för överdragning av en underlagsyta (62) Divided by application 752667 - Avdelad frUn ans _

The present invention relates in particular to a layered electrical resistance element according to the preamble of claim 1 for electric heating.

An improvement in electric heaters in recent years has been the introduction of self-regulating heating systems using materials with certain types of PTC properties, namely that when a certain temperature is reached, a significant increase in resistance occurs. Heaters using PTC materials are reported to have more or less sharp resistance rises in the narrow temperature range, but below this temperature range they have only relatively small changes in resistance with temperature. The temperature at which the resistance begins to rise sharply is often defined as the switching or anomaly temperature (T), because when this temperature is reached, the heater exhibits an irregular change in resistance and, for practical reasons, it switches off. Self-regulating heaters using PTC materials have the advantage over conventional heaters that they generally eliminate the need for separate thermostats, fuses, or in-line electrical resistors.

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The most commonly used PTC material has been lacquered barium titanate, which has been used to self-adjust ceramic heaters used in applications such as food warmers and other small portable heaters. Although such ceramic PTC materials are commonly used in heating applications, their rigidity severely limits the field of application in which they can be used. PTC materials containing electrically conductive polymer blends are also known, some of which are claimed to have the Special Properties described above. However, the use of such polymeric PTC materials has been relatively limited mainly due to their low heating capacity. Such materials generally contain one or more conductive fillers, e.g., carbon black or powdered metal dispersed in a crystalline thermoplastic polymer. PTC alloys made from highly crystalline polymers generally have a steep rise in resistance that begins some degrees below the melting point of their crystals in the same manner as their ceramic counterparts behave at the Curie temperature (for T ceramics). PTC blends derived from homopolymers and copolymers with lower crystallinity, e.g., less than about 50%, have somewhat less sharp increases in resistance that begin at a less defined temperature in a range that is often well below the melting point of the polymer crystals. In the extreme case, some polymers with low crystallinity give resistance-temperature curves that are more or less concave (from above). Other types of thermoplastic polymers give resistors that grow fairly evenly and more or less sharply, but constantly with temperature. Figure 1 of the accompanying drawings shows characteristic curves for the various types of PTC alloys mentioned above. In Figure 1, curve I shows a sharp really momentary increase in resistance (hereinafter referred to as type I behavior), which is · generally characteristic of, inter alia, polymers with high crystallinity; curve II shows a slower growth at lower temperatures (relative to the melting point of the polymer), hereinafter referred to as the type II behavior, which is generally characteristic of polymers with lower crystallinity. Curve III depicts a concave (top) curve characteristic (type III behavior) of many polymers with very low crystallinity, while curve IV depicts a large increase in resistance with no more or less constant resistance range (type IV behavior) at least at a commercially interesting temperature for some materials. Curve V shows the slightly rising resistance-temperature curve of resistance 63848 (type V behavior) shown by many "normal" electrical resistors. Although the above types of behavior have been described with reference primarily to different types of polymeric material, those skilled in the art will appreciate that the particular type of behavior exhibited by the substance is also highly dependent on the type and amount of conductive filler and its particle size, agglomeration, agglomeration, and the shape of the particle agglomerates (i.e., the tendency to form its structure).

It should be noted that the preferred PTC blends previously disclosed in the art are all claimed to exhibit substantially type I behavior. In fact, the behavior of types II-IV has not been particularly known in the art in the past, despite the fact that many PTC alloys previously disclosed in the art do not in fact have type I but rather type II, III or IV behavior.

In the case of type I resistance-temperature characteristics, the increase in resistance above the Ts point is rapid so that the point Ts can be considered as the temperature at which the device switches off. However, for type II or III PTC materials, the change from resistance that is relatively stable with increasing temperature to resistance that rises sharply with temperature is much less determinable and the anomaly temperature or Tg is often not the exact temperature. In this specification, although the device may be described as self-interrupting with a particular Ts value, those skilled in the art will appreciate that in many practical cases it is more appropriate to understand the T value as the lowest temperature in the temperature range at which the device turns off. as a relatively narrow temperature range rather than a certain temperature.

Previously disclosed self-regulating heating devices using PTC material are shown to have very steep (type I) R = f (T) curves so that above a certain temperature the device actually closes, while below this temperature a relative constant wattage is achieved at constant voltage. At temperatures below the T value, the resistance is relatively small and at a constant level and thus the current flux is relatively high at any voltage used. The energy generated by this current flow is lost to heat, i. an electrical resistor generates heat and heats the FTC material. As the temperature rises, the resistance remains relatively low * 63848, up to approximately the T temperature, at which point a rapid increase in resistance occurs. As the resistance increases, a simultaneous decrease in power occurs, which limits the amount of heat generated so that when Ts is reached, heating essentially ceases. After the device temperature has dropped below the Ts ~ point as a heat loss to the environment, the resistance decreases, increasing the power output.

The heat generated in the steady state is substantially in equilibrium with the heat lost. Thus, when the applied voltage is directed through the PTC heating element, the Joule heat causes the PTC element to heat up to approximately its T point, this

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depending on the voltage used and the type of PTC element, after which only a small temperature rise occurs due to the increase in resistance. Due to the increase in resistance, the PTC heating element usually reaches a steady state at approximately the Ts point, thereby controlling the heat production of the element itself without recourse to fuses or thermostats. The advantages of such a self-contained thermal control element are obvious in many applications.

In U.S. Patent 3,243,753, Kohler discloses a carbon black-filled polyethylene in which the conductive carbon black particles are in substantial contact with each other. Kohler describes a product containing 40% polyethylene and 60% carbon black particles to give a resistance of about 0.4 ohms / cm at room temperature. As is typical of the proven properties of prior art materials, Köhler's PTC product is described as having a relatively low electrical resistance curve as a function of temperature below the switching temperature, followed by a sharp rise in resistivity of at least 250% in the 14 ° C range. The mechanism proposed by Kohler for a sharp rise in resistivity is that then the change is in the materials, i.e. a function of the thermal expansion difference between polyethylene and particulate soot. It is believed that the large amount of conductive filler in the mixture forms a conductive network through the polyethylene polymer matrix, thus initially providing unchanged resistivity at lower temperatures. However, at approximately the melting point of its crystals, the polyethylene matrix expands rapidly and this expansion causes the rupture of many conductive networks, which in turn leads to a rapid increase in the resistance of the mixture.

Other theories proposed to explain the PTC phenomenon in conductive particle-filled polymer blends include complex mechanisms based on the tunneling of electrons between intergranular interstitial filler particles or some mechanisms based on phase change in amorphous crystalline regions. Background discussion for numerous proposed alternative mechanisms of PTC phenomenon can be found in the article "Glass Transition Temperature as a Guide to the Selection of Polymers Suitable for PTC Materials", J. Meyer, Polymer Engineering and Science, November, 1973> 1_3, No. 6. In U.S. Patent No. 3,673,121, Meyer suggests that, based on phase change theory, to achieve a sharply rising resistance PTC curve with a sharp boundary (type I), the polymer matrix should contain a crystalline polymer with a narrow molecular weight distribution. Kawashima et al. in U.S. Patent 3,591,526 disclose a PTC casting composition in which conductive particles such as carbon black are first dispersed in a thermoplastic material and then this dispersed mixture is mixed with a casting resin. Kawashima et al. also emphasize the desirability of a very steep temperature resistance curve (i.e., R = f (T)) curve with a T point between n.

o 100-130 ° C.

Due to their flexibility, relatively low cost, and ease of installation, PTC strip heaters containing conductive particles dispersed in a crystalline polymer have recently found widespread use as pipe wire heaters in industrial piping and similar applications. For example, such polymeric PTC heaters, due to their self-regulating features, have been used to cover pipes in chemical plants to protect them from freezing or to maintain a constant temperature, which in turn allows aqueous or other solutions to flow through the pipes without "salt separation".

In such applications, the heaters ideally reach and maintain a temperature at which the energy wasted from heat transfer to the environment corresponds to the energy received from the stream. Such heaters usually consist of a relatively narrow and thin strip or strip of carbon-filled polymeric material with electrodes (such as embedded copper wires) at opposite edges along the longitudinal axis of the strip. Thus, it is generally thought that an electrical voltage gradient travels along the longitudinal axis of the strip and a plane perpendicular thereto, with the voltage applied between the opposite electrodes causing the entire strip 6,63848 to heat up, usually to approximately its T point b.

It will be apparent from the foregoing description that Type I materials have significant advantages over the other types of PTC materials listed above in most applications. Types II and III have the disadvantage that, due to a much less sharp change, the constant temperature of the heater is more dependent on the thermal load applied to it. Such alloys also suffer from the current impulse problem described in more detail below. Type IV and V materials, due to their lack of a useful temperature range in which energy production changes from temperature-independent to temperature-dependent, have so far not been considered suitable materials for practical heaters under normal conditions.

In the applications described above and others, there is a need for flexible strip heaters with much higher energy production densities and / or higher operating temperatures than previously thought in the art. It does not seem possible to use heated heaters, in particular strip heaters made from prior art mixtures and in accordance with prior art structures, with higher energy productions, i.e. at higher wattage levels n (above about 0.23 W / cm) and / or at higher temperatures (above about 100 ° C). The actual amount of watts produced by prior art heaters is much less than would be expected by calculating the heater area and heat transfer analysis, apparently due to the fact that heat is produced in a very thin strip along the longitudinal axis of the strip between the two electrodes. Such a phenomenon is referred to herein as the hotline. This hotline results in insufficient and non-uniform heating properties and renders the entire heating device unusable for most of the heating cycle in applications where high wattages, especially at temperatures above 100 ° C, are desirable. More specifically, since heat production is limited to a narrow strip or line across the flow path, the high resistance of this line prevents current from flowing across the line, effectively causing the entire heater to close until the hot line temperature drops below the T value again.

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It has now been found that this hotline state occurs in most if not all prior art polymeric PTC strip heaters where the voltage is on and current 7 63848 flows perpendicular to the longitudinal direction across the strip, the extent of such state generally depending on the amount of voltage used and the polymer thermal conductivity and the amount of non-uniform heat loss. The hot line along the longitudinal axis of the strip between the electrodes effectively closes the heating device despite the fact that only a small part of the surface area of the film, i. the hot line has reached the Ts value. In many cases, this destroys the heater, or at least makes it so inefficient that it turns out to have the very low heating capacity that has been found to be commonly associated with prior art PTC polymer strip heaters.

From the above description, it is clear that the removal of the hot line is important for the efficient operation of the PTC self-regulating heater, especially one with high energy production and / or high operating temperature.

It would also be most advantageous if a PTC self-adjusting heater could be made with a heating surface shape other than a relatively long, narrow strip, e.g. a square or circular heating pad. It would also be desirable to have a PTC self-regulating heater that could be made into relatively complex three-dimensional shapes, e.g., one that is capable of making effective contact with substantially the entire outer surface of the chemical process vessel. Unfortunately, the tendency for a hot line is particularly prevalent when the distance of the current path, i. the distance between the electrodes is large compared to the cross-sectional area per unit length of PTC material through which the current must pass. For example, in the case of a heating strip with electrodes at the edges of the strip, a wide short strip has a greater tendency to a hot line than a narrow strip of the same length and thickness. Similarly, when the length and width are the same, the thinner the strip, the greater the tendency for the hot line. Increasing the length of the strip while keeping the width and thickness constant does not have a significant effect on the tendency to form a hotline. The problem of hotline formation has apparently not been sufficiently understood in the past, and with certainty no composition or structural proposal has been made to reduce it.

Polymeric PTC blends have also been proposed for heat shrinkable products. For example, Day describes in Figure II.S. Patent Office Defensive Publication T 905 00] use of a heat-shrinkable FTC film. However, Day's cut film suffers from a rather serious drawback that since T is not higher than the melting point of the crystals in the film, only a very small recovery force can be generated. Buiting et al. propose in U.S. Patent 3 ^ 13 ^ 2 heater structures involving lamination of a polymer layer between silver electrodes. A significant disadvantage of the structure of Buiting et al. Is its inflexibility. Moreover, neither the previous description of the art described by Buiting et al. Nor any other described above suggests, much less solves, certain additional problems that are characteristic of all prior art PTC heaters in the art.

First, there is the problem of current impulse. This. the problem is particularly difficult when it is desired to provide a heater with a T i of more than about 100 ° C.

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In many applications, self-regulating heaters with a Tg of 200 ° C or even higher could be advantageously used. Unfortunately, as mentioned above, the previously proposed PTC heater structures are substantially unsuitable for such high T applications.

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For materials with Ts values well above 100 ° C, the resistance of such a material at or just below the Ts temperature can be up to 10 times its resistance at ambient temperature. Since the PTC heater usually operates at or slightly below the Ts temperature, its effective heat production is determined by its resistance slightly below the Ts temperature. Therefore, a PTC heater consuming e.g. 15 amps at 200 ° C could easily consume 150 amps at ambient temperature. Such a heater system would unnecessarily require additional current carrying capacity over what is required for steady state operation or alternatively requires the installation of a complex and usually fragile or expensive control circuit to prevent an initial 150 amp impulse from burning the heater or its conductors when the heater is first connected.

Referring to Figure 2 of the accompanying drawings, which is a resistance-temperature-curve, the preferred type of heater characteristic curve (curve ABC) has an ideal constant island response (indicated by a straight line AB) up to point T, and a resistance rising very rapidly (indicated by a straight line). BC) above the Tg point. Thus, the operating range, let's say its maximum value for approximately 0 current consumed, is that represented by the dashed lines intersecting the resistance-temperature curve at points B and D. The energy production of an ideal heater is not affected by temperature changes below the 9 63848 temperature, but by changes in its size. in a very narrow temperature range above the T point o. Unfortunately, as described above, very few if any PTC material actually has this ideal property. The nearest curve, which can usually be obtained with a practical heater, is shown by the lines AB'C '. If the maximum allowable power taken out of the electrical circuit is obtained by resisting at point A, then the operating range for self-limiting or "control" is obtained by the part of line B'C "between the dashed lines. It is obvious that the heater temperature varies much more under" controllable "conditions. in the latter case and the available power range in the "regulated" range is smaller than in the ideal case, if a power range corresponding to the ideal range is desired, a resistance characteristic curve such as line A'B "C" is required.

Referring again to Figure 2, the curve AEF shows part of the resistance characteristic curve of the type II PTC material. If, as in the previous case, the operating power range is determined by intermittent resistance pins, it can be easily estimated that the temperature of the heater varies quite widely in use depending on the thermal load.

Although, as mentioned above, the considerable advantage of using a heater mixture with a type I resistance-temperature curve was previously understood in the art, many of the compositions previously referred to in the art exhibit behavior more closely resembling type II or even type III behavior. The optimal (type I) characteristic is only with a limited range of mixtures and there has long been a recognized need for a means of modifying mixtures with type II or III behavior so that the behavior becomes type I or at least better approaches it.

An additional problem characteristic of prior art PTC strip heaters is that when it is desired to heat an irregularly shaped substrate, the heater must be wrapped around the substrate, resulting in certain parts of the strip completely or partially covering other parts. This overlap may cause irregular heating.

Thus, it is apparent that while a large number of PTC alloys and structures are already known in the art, all such alloys and structures; and, in fact, all of their obvious combinations have serious shortcomings that severely limit the use of 63848 10 self-adjusting PTC heating elements.

According to the present invention, there is provided a layered resistance element of the above type, characterized in that the second layer is an electrical resistive material having a substantially constant resistance (CW layer) at least below the anomaly temperature of the first layer, that at least a portion of the first layer is in contact with at least a portion. from the surface of the second layer down to the interface which provides direct electrical and thermal contact between them, and that the element is heat recoverable.

An embodiment of the invention is characterized in that it is a self-directing heating element comprising a laminate comprising said first and second layers, and that at least one pair of electrodes is arranged so that when there is a potential difference between the electrodes, a current flows between the electrodes and at least the second layer. through part of the floor.

It is recommended that the length of the current path through the PTC layer does not exceed its thickness (measured perpendicular to the line between the electrodes) by more than 50% and preferably not more than 20%. It is further preferred that the PTC layer has two substantially planar surfaces, which may be parallel and both of which are at least partially in contact with the surface of the CW layer.

In one embodiment, the conductivity of the CW layer or layers is selected so that the material, while sufficiently resistive to generate heat when connected to a suitable electrical source, is sufficiently conductive to also function as an electrode material.

Alternatively, the electrode may be metal, which may be embedded or in contact with either the surface of the PTC layer or the CW layer, or in contact with either (i.e., a surface remote from the interface) or both surfaces at the interface between them. The electrode may be in the form of a fabric, braid, lattice (e.g., a series of parallel electrodes or a screen or mesh) and a wire, tape or film. It can also be fiber. When the element is to be placed on a conductive substrate such as a metal tube, the substrate trol -S ♦ - *; # »mi'ri <lA6 * aa o> -> ^ l / f ^ ^ 63848

The element can be coated on one or more or all sides with an insulating layer. Alternatively or in addition, a thermally activated adhesive or sealant layer may be applied to at least one surface. In some embodiments, the CW layer may serve this purpose.

The material and structure of the layers contained in the article as well as the electrodes are described in detail in basic application 752667, which is incorporated herein by reference.

The present invention also comprises a method of making the layered article in question, and the method of the invention is characterized in that the substrate material is coated with a thermally recoverable element comprising at least two electrically resistive layers, the first layer having a positive resistance temperature coefficient (PTC) and an anomaly temperature above it is substantially non-conductive, and the second layer has a substantially constant resistance CW at least below the anomaly temperature of the first layer, and that the element is heated to effect recovery.

The shape and position ratio of PTC and CW layers and electrodes are subject to certain restrictions and the following requirements must be met: 1. At any temperature, at least a portion of the current flow between electrodes of opposite polarity passes through at least one portion of the PTC layer and also at least one CW through part of the floor. '- ·· 2. There is both electrical and thermal contact (and hence coupling) between the PTC and CW layers. The electrical and thermal gradients may be parallel or non-parallel to each other.

As discussed in more detail below, certain elements made in accordance with this invention have a higher anomaly temperature than the intrinsic Tg point of the PTC layer itself. The T point of an element is called the effective T point.

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It is preferred that the thermal and electrical gradients in the PTC layer are substantially along the same line or axis at the Ts ~ point of the PTC layer or above it or the effective Tg point, if the latter is higher.

3. At or above the Tg point or the effective Ts point, if the latter is the higher maximum current flow path, it is the path with the minimum current path length through the PTC layer or layers, although this results in a longer current path length in the CW layer or layers. through the layers.

In certain cases, the structure of the element is preferably such that the shortest current path in the current direction through the PTC layer does not exceed the maximum thickness of the PTC layer in a plane perpendicular to the plane connecting the electrodes and perpendicular to the current flow by more than about 50% and preferably more than n. 20%.

The same thickness, as used herein, is intended to mean the dimension between the surface (inner and outer surface) of any two PTC layers, which is the dimension of the smallest dimension. In most heater structures of the present invention, the flow through the PTC material at or above the T point is predominantly perpendicular to the PTC

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and against the interface between the CW layer.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

Figures 1 and 2, which have already been described, illustrate the resistance-temperature properties of different PTC materials; Figures 3-5 are perspective views of prior art structures # using PTC alloys; Figures 6-12, 13b and 15-34 are perspective views of various elements made in accordance with the invention or serve to illustrate and explain them; 13 63848 Fig. 13a is a cross-sectional view of the embodiment shown in Fig. 13b, while Fig. 14 is a cross-sectional view of the embodiment shown in Fig. 15; Fig. 35 shows an embodiment in which, in fact, the tip electrodes are arranged at certain intervals along the length of the element; Figures 36 and 37 show the power-temperature dependence for the products described in certain examples.

Referring now in more detail to Figures 3-5, various prior art structures using PTC alloys are shown. Figure 3 shows a strip heater similar to that disclosed in U.S. Patent No. 3,413,442, in which thin silver films 1 and 3 are placed on either side of the PTC material 2. This is not in accordance with the present invention, although a laminated structure is shown, as the material adjoining the PTC layer is so conductive that it does not itself act as a heater.

Figure 4 shows a strip heater according to U.S. Patent 3,243,753, in which both edges of the PTC material 6 have conductive gate electrodes 5 and 7.

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Figure 5 shows a previously proposed strip heater in which a PTC material 10 having a dumbbell-shaped cross-section has conductive wire electrodes 8 and 9 disposed along its length.

Turning now to the structures constructed in accordance with the present invention, Figure 6 shows a PTC layer 11 bounded or partially bounded by a CW heating layer 12. A wattage electrode 13 has a gate electrode 13 on its unchanged wattage layer, while a second gate electrode 14 abuts the furthest surface of the PTC layer. of the unchanged layer 12.

In Fig. 7, a large number of strip electrodes 16 connected in parallel are embedded in the CW layer 15. · The counter electrode 18 is a continuous film applied to the outer surface of the PTC material 17.

Figure 8 shows a further variant in which the electrodes 20 and 22 are strip electrodes (with the electrodes 20 connected in parallel or in series and the electrodes 22 as well), the electrodes 20 being sandwiched between the PTC layer 21 and the CW layer 19. In this structure, a low resistance CW layer is desirable because the gradient potential along the interface between the layers 21 and 19 decreases.

Fig. 9 shows a structure similar to Fig. 6 with the gate electrode 23 on the CW layer 24, which in turn is bounded by the PTC layer 25. However, the second electrode is a gate electrode deposited inside the PTC layer.

Referring to Fig. 10, a first set of electrodes 28 is embedded in the CW layer 27, while a second set of electrodes 30 is embedded in the PTC layer 29.

It is to be understood that the various embodiments shown in Figures 6-10 may be used in accordance with this invention in any combination. More specifically, the gate electrodes shown in Figures 6 and 9, the film electrodes shown in Figure 7, or the band electrodes shown in Figure 8 may be used in any of these embodiments, and a combination of two or more different types of electrodes may be used in a given structure. The first electrode can be placed on top of the CW layer, embedded in the CW layer or placed between the CW layer and the PTC layer. The second electrode can be placed on the opposite side of the PTC layers on top of, inside or between the second CW layer or under the PTC layer. or inside it.

Figure 11 shows strip electrodes 32 and 34 embedded in two CW layers 31 and 35 with the PTC layer 33 sandwiched between the electrode di-CW layers. Of course, as discussed above, the electrode may have a lattice, membrane or other structure.

Figure 12 shows a special embodiment of the present invention which has been found to be useful for increasing the T value. As stated above,

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by staggering the electrodes so that the current path has a component across the layers instead of being perpendicular through, the effective Ts value can be increased. Thus, in Figure 12, the strip electrodes 37 are staggered between geometric perpendicular projections of the strip electrodes 39, with the electrode arrays 37 and 39 embedded in the CW layers 36 and 40 and the PTC layer 38 interposed therebetween.

Figures 13a and 13b are a cross-sectional and perspective view of a preferred embodiment. A large number of wire electrodes 42 connected in series are embedded in the CW layer 41 and similarly a large number 45 in the layer 44, the PTC layer 43 being sandwiched between the layers 41 and 44. Preferably, the wires 42 are all substantially in one direction with the wires 45 in a second direction substantially perpendicular to the first. Furthermore, the entire layer structure may have a disc shape, which is particularly well suited for a variety of heating applications.

Referring to Figures 14 and 15, they show a layered structure which is particularly suitable for the production of heat-recoverable encapsulation products, as fully disclosed in Finnish Patent Application No. 752666. For this purpose, the layers are generally a flexible polymeric material with any or all layers made heat-recoverable. For a more detailed description of the heat recovery elements and their applications, see the above-mentioned application. If the element is to be used to seal an electrical connection using the layer combination of the present invention, an outer layer 46 is provided, which may be an insulating material, which may or may not be heat recoverable. Next, the laminate has a CW material 16 63848 embedded with electrodes 48, which may be braided, sawn or helical in shape, and connected in series with a power supply. Next, a PTC material layer 49, with a second set of electrodes 51 embedded in the second CW layer 50. A second layer 53 of insulating material, which may be thermally recoverable, is disposed adjacent to the heating layers and the outer surface of this layer 53 has an adhesive layer 54 thermally activated. -mentillä.

Referring now to Figures 16-34, the paste-shaped electrodes are denoted by reference numerals 55 and 56, the CW layers are denoted by 57 and 58, the PTC layers by 59 and 60, and the conductive substrate, e.g., a tube by 61.

Figure 16 represents an embodiment in which the dimensions (e.g., thickness) of a particular layer and, consequently, the relative thicknesses of the CW and PTO layers are varied locally to change the power generation density and / or the effective Tg value. Figure 17 represents an embodiment in which the PTC and / or CW layer contains different mixtures at different locations to change the wattage and / or the effective Tg.

Fig. 18 is a cross-sectional view of an embodiment in which a substrate, for example a metal tube, is part of an electrical circuit, i.e. it forms one of the electrodes. Fig. 19 represents an embodiment in which the private layers are successively wrapped around an object that needs to be heated to form a layered heater in situ. The layers may be adhered together by either external heating or electrical conduction, or the layers may be formed of materials that adhere together at the temperature at which the article is used. This is an example of an embodiment in which it may be particularly useful for the substrate to form part of an electrical circuit. Figures 20-26 show another group of embodiments. In the construction shown in Fig. 20, the electrode 56 may also have a concentric layer 60 of PTC material as shown for the electrode 55. The structures shown in Figures 23-25 are examples of heaters where conduction below the effective Ts point (depending on the relative resistivities of the PTC and CW layers) can occur primarily across the PTC layer between the electrodes. However, when the PTC layer heats above its T3 temperature, conduction occurs mainly or almost entirely from the second electrode π 6384 8 ta through the thickness of the PTC layer through the shortest possible path from this electrode to the wattage unchanged layer and then through the wattage unchanged layer to the second electrode (again possibly through the minimum thickness of the intervening PTC layer).

It is noted that the “main” current flow referred to here is related to the path along which the largest current “flux” flows, although in theory this path is not always exactly the shortest path in the PTC layer, because even at the T point or even above it, the remainder of the PTC material carries some part of the current, this part can be disregarded for practical purposes, e.g. in a structure such as Figure 2b, as shown in the drawing the current flows for practical reasons mainly perpendicularly up and down through the PTC layer 59 and along layers 57 and 58, although there must be a very small component per second electrode in the path of the main current in the PTC layer, this is small enough to be disregarded for practical purposes.

In the construction alternative shown in Fig. 25, the layer 59 may be omitted and the electrode 56 placed in contact with the layers 57 and 58 separated from the electrode 55.

Figures 26 and 27 illustrate embodiments in which the PTC layer is only partially limited to the CW layer. We have found that when the portion of the total CW surface area that is in contact with the PTC surface area is reduced, the ambient temperature at which the heater limits power output at a given operating voltage also decreases.

Fig. 28 shows another modification of the embodiment shown in Fig. 21, and this modification of Fig. 28 may have one CW layer 57 located where layer 59 is depicted and a pair of PTC layers 59 and 60 replacing the CW layers 57 and 58 shown. .

Figures 29 and 30 show further modifications of a layered basic heater having the same general shape and mode of operation as in Figures 23-25.

Figures 31 and 32 show other embodiments of the embodiment shown in Figure 12, in which the effective Tg of the heater may advantageously be different from the value T1 of the PTC material alone, as described above.

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Figures 33 and 31 * show how useful layered heaters can be formed by combining extruded coated yarns with coatings having PTC or CW properties.

Referring now to Figure 35, there is shown another element made in accordance with the present invention in which conductors 55 and 56, which are of different poles in use, have a concentric insulating layer 62 around them. Reference numeral 59 represents PTC material and 57 CVJ material. . The layer 62 is discontinuous on the surface of the conductor such that, as shown in a substantially linear elongate element, the insulating segments are periodically removed along the length of the conductor. As can be seen, where the insulation is removed, the conductor is in direct conductive contact with the CW material. Such contact areas are not opposite each other at the electrode but are in fact obliquely opposite along the longitudinal axis of the element. The advantage of this embodiment is that the current flow between electrodes of opposite polarity does not necessarily take place only across the width of the element, i. distance X, but in fact the current must travel a distance Y so that the current path runs down part of the length of the element. A long current path is desirable in the sense that it allows the use of low resistance CW material (which allows the use of higher voltages) without showing an tendency to burn. It is natural that alternative structures that ensure that the current flows at least partially downwards along the length of the element are easy to fabricate. For example, in a structure in which a PTC layer is sandwiched between two CW layers with strip electrodes disposed on the outer surface of the CW layers; the discontinuous insulating layer may be interposed between each of the wattage-constant layers and the electrode placed thereon. Or, when the continuous insulating layer is placed on the outer surface, the electrodes may alternatively pass through the insulating layer and contact the CW layer.

The following examples illustrate the invention: the elements made according to the present invention can be manufactured in many different ways known in the art. For polymer heaters, the private layers can be extruded separately and then laminated, bonded, or otherwise bonded together, and the electrodes embedded therein during extrusion or lamination as desired. The layers may otherwise be made by calendering or coextrusion and the electrodes embedded in them as above in any suitable manner; 19 63848 at the stage of the operation. A preferred method of making a particular embodiment of a heater according to the present invention is described in the above-mentioned Finnish Patent Application No. 752666.

Methods for preparing non-polymeric conductive compositions suitable for use in this invention, e.g., ceramic compositions or soot-filled asbestos paper, are well known in the art. The layers can be attached to the other layers by bonding, welding, gluing, or other well-known methods for maintaining or maintaining conductive contact between the layers.

Example 1

A laminate as generally shown in Fig. 1k was prepared, with a PTC layer similar to that of the mixture 2 shown in Example 5 and a wattage-like layer similar to that shown in Example 3, the insulating layer consisting of a mixture of polyethylene and low-structure, poorly conductive carbon black. The adhesive layer was a hot melt adhesive having a ring and ball softening temperature of .0 ° C. The laminate was irradiated to effect crosslinking prior to coating with adhesive, hot stretched perpendicular to the helical wire electrodes, and cooled. The stretched film was wrapped around a polyethylene sheathed telephone cable and the opposite ends were tied together. When the electro-wires were connected to a 12-volt lead-acid battery, the laminate shrank evenly and uniformly over the telephone cable.

Example 2

A 2.5 x 15.2 x 0.05 cm strip with copper electrodes attached to opposite edges along its length and consisting of 70 5? Medium density polyethylene, 18 5? Ethylene / ethyl acrylate copolymer and 12% Cabot Corp.'s XC72 carbon black was heat treated at 150 ° C under vacuum for 16 hours and then irradiated to a dose of 20 Mrad and coated with a temperature indicating paint (Templace 76 ° C paint). The electrodes were connected to a 110 volt AC power source. In less than a minute, the white paint had melted into a narrow area about 2.5 ^ / 25 ^ cm wide and roughly equidistant between the electrodes, the so-called The mating temperature in the middle of the hot line was estimated to be close to 85 ° C, which is just above the T point of this special mixture. Areas that were only 0.5

O

cm from the hot line, were below 50 ° C. In this state, the element synthesized substantially all of its power from the hotline region. In a similar experiment where an element was isolated, placed in water, and connected to a power supply, a similar "hot line" was observed.

The mixture of this example was then made into a laminated core sandwiched between CW layers of carbon black-filled silicone rubber, with each CW layer introducing 20 AWG (ca. 0.081 cm in diameter) of multi-stranded copper wire into its center. The element was uniformly heated to a uniform surface temperature of about 65 ° C in air with a core temperature of about 80 ° C. Thus, deposition of the PTC layer between the unchanged wattage layers removed the hot line from this PTC alloy.

Example 3

A series of laminated heaters were prepared using a wattage-free layer consisting of 35 parts of ethylene-propylene rubber, 30 parts of ethylene-vinyl acetate copolymer and 35 parts of carbon black, and a PTC core mixture described in Table I below, in which the carbon black was dispersed in polypropylene before TPR 1900 rubber was mixed in.

TABLE I

Sample No. 1 2 3 4 5 6 TPR 1900 (thermoplastic 72.5 70.0 68.75 67.5 66.25 65.0 ethylene-propylene rubber, manufactured by Uniroyal Corporation)

Profax 6524 (polypropylene- 16.5 18.0 18.75 19.5 20.25 to 21.0, manufactured by Hercules

Corporation) XC72 (Cabot Corp, - 11.0 12.0 '12.5 13.0 13.5 1 * 1.0 company carbon black) CW and PTC materials were hydraulically pressed at 200 ° C with dimensions of 15.2 x 15.2 x 0.05 cm tiles for one minute and heater structures containing a PTC layer sandwiched between two CW layers were laminated at 200 ° C for two minutes and then heat treated at 200 ° C for 10 minutes and irradiated. 2.5 x 3.75 cm heater segments were cut from each sample and 2.5 x 0.635 cm conductive silver paint electrodes were painted bounded diagonally on the opposite 2.5 edge of the CW layers, resulting in a similar heater structure as in Figure 12. The effect of the change in the mixture on the ratio between the starting impulse and the operating current and on the self-regulating temperature can be seen from the starting impulse ratio and the Tg temperature in Table II below:

TABLE II

Laminate-

Soot control room temperature

Mixture in the core,% at temperature (ohm) ratio * (C> c) xx

Plain, “ce oc PTC core 12 · 5 '8 85 1 11 21 000 8 90 2 12 260 5 105 5 12.5 245 4.4 125 4 13 230 3.9 165 5 13.5 220 3.7 185 6 14 '205 v}' Defined as the ratio of Ts-temperature resistance to room temperature resistance.

xx) melting point of pTC material about 165 ° C.

As can be seen, a slight change in the composition of the PTC material when the CW material is kept unchanged can significantly change the Ts ~ point and the starting impulse ratio when used in a heater assembled according to the invention. In particular, the Ts point can be changed above the melting point of the PTC material. In addition, when a PTC material with a Ts of 85 ° C and containing 12.5% carbon black was deposited between the CW layers, the effective Tg rose to 125 ° 0, with the latter resistance temperature curve represented by the starting impulse ratio being much closer to the type I behavior (with by definition there is a starting impulse ratio of 1).

Example 4

A 0.063 cm thick plate of PTC material having the composition described in Example 2 was laminated between two 0.063 cm thick CW layers having the same composition as the CW layers of Example 3. The laminate was heat treated at 150 ° C for 16 hours and then irradiated to a dose of about 10 megarads. A 2.5 cm square piece of laminate cut with conductive silver paint over the entire outer surfaces of the CW layers, i. a similar basic structure to that in Fig. 11 was found to have a Ts value of 70 ° C. A similar sample to which two 2.5 x 0.63 emi strip electrodes were attached to the diagonally opposite planar surfaces of the unattended layer (one for each layer) (i.e., similar to Figure 12) was found to have T value above 90 ° C. It is therefore

O

that the placement of the electrodes can significantly alter the Tg values of the structures of this invention.

Example 5 PTC blends having the composition and properties shown in Table III were prepared by roller mixing, then hydraulically pressed into 0.025 cm thick slabs and irradiated to effect crosslinking. Layered heaters were prepared by depositing a PTC layer between two CW layers with a resistance of 7 ohm-cm and made of conductive silicone rubber (R 1515) »which was either 0.025 or 0.10 cm thick.

TABLE III

Sample Marlex 6003 Sterling SRPNS Dose 0.025 cm membrane%% Mrad response ohm-cm 5-1 58 42 12 1.5 5-2 61 39 12 20 5-3 65 35 12 200

Electrodes of 2.5 x 0.63 cm were applied to the outer surfaces of the heater segments as in Example 4. The heater was then placed on and in good thermal contact with a stainless steel block equipped with a thermometer and mounted on a heat controlled hot plate to vary the temperature of the block. The heater was connected to a voltage source of such a magnitude that it generated about 0.31 W / cm 2 at approximately room temperature. The power production of the heater was controlled when the temperature of the metal trap rose. See Figure 36 for results.

Figure 37 shows how the power / temperature curve for a heater assembled from a 0.25 cm layer of 5-2 alloy and an unirradiated 0.025 cm layer of unchanged watt silicone varies with the electrode structure. Non-irradiated silicone inverted layers were chosen because their resistance changes very little with temperature and thus the observed changes can be attributed to geometric effects and changes in the resistance of the PTC layer. Three shapes were compared: A) where the electrodes covered the entire top and bottom surface of the specimen (i.e. similar to 23 63848 as in Figure 6 except that two layers of CW were used and the electrodes were silver paint, not mesh), B) where the opposite silver paint electrodes were 0 , 63 cm x 2.5 cm were placed across the top and bottom surfaces (two on each side, with the electrodes 2.5 cm apart on each side) and C) with one upper and one lower electrode 0.63 x 2.5 cm varied at 2.5 cm intervals in a staggered structure. The power density / temperature dependences shown in Fig. 37 for these three structures indicate that the power / temperature curve can be changed decisively and unexpectedly by changes in the electrode structure. For many purposes, the power curve shown in section C is recommended, and Figure 37 shows that with selected alloys and resistors, this can be achieved with an alternating or laterally thinned electrode structure. However, even when the electrodes cover the entire upper and lower surfaces of the CW layer, a type C curve can be obtained by appropriate selection of PTC and CW layer resistivity, as shown in Fig. 36, showing that to obtain a type C curve at room temperature. must be less than the resistance of the CW layer. However, alternately, with laterally thinned electrodes, type C power curves are obtained by selecting a PTC layer with a higher resistivity than the CW layers.

Example 6

Heaters were assembled according to the structure A of Example 5 and from the same mixtures as in Example 5. However, in certain test pieces, as shown below, the CW layer was 0.10 cm thick. The heaters were tested mounted on a stainless steel block as described in Example 5. The temperature of the block at which the power generated by the heater began to drop is shown in Table IV. The results show that by varying the relative resistances of the PTC and CW layers, the drop temperature and thus the T point can be varied quite significantly.

S

extent.

24 63848

TABLE IV

Heater CVJ layer Power drop Power at 23.9 ° C PTC core thickness, cm temperature, ° C Power at 85 ° C 5-1 0.025 124 1.31 0.1 127 1.15 5-2 0.025 HO 1.06 0.1 113 1.06 5-3 0.025 77 1.27 0.1 80 1.30 5-2 * 0.025 93 ** 0.1 80 The PTC layer covers 1/3 of the CW layer ** The PTC layer covers 1/6 of the CW layer

A special advantage of thicker, i.e. one of the higher resistance CW layers is that the variations of the resistance in the PTC layer do not affect the power production so much, i. there is less heat-space variation in power generation. In this way, a highly crystalline, high molecular weight polymer with highly structured carbon black can be used for the PTC layer (such combinations give the desired behavior, approximately type I, but show the extreme sensitivity of the resulting resistance to treatment and thermal history). By combining such blends with CW layers with much higher resistivity, which can be made from blends of low or amorphous polymers and medium or high structure nipples (giving resistances that are less sensitive to treatment or thermal history), a heater with a high greater coherence, repeatability, and functional usability than has hitherto been available.

As mentioned above, an important feature of a functioning heater is the relationship between the room temperature resistance and the desired operating temperature resistance. This ratio is proportional to, but not identical to, the start-to-impulse ratio. In addition, lower values of this resistance ratio also indicate a better approach to the type I resistance characteristic curve. For the heaters described in this example, the operating range near 85 ° C is considered optimal. To obtain low ratios, specific resistivity ratios (at 24 ° C) between the PTC and CW layers between about 0.1: 1 and 20: 1 (with the exact ratio depending on the relative thickness of the layers) are recommended, with ratios between 1 and 10 being particularly preferred.

25 63848

Example 7

PTC materials having the compositions given in Table V were prepared as described in the previous example. 0.05 cm thick tiles of the blends were laminated between two 0.05 cm tiles of a blend of 20% Black Pearls Soot in a Silastic 437 ′ ′ pulp (resistivity 400 ohm-cm) and the laminates were then irradiated with 12 Mrad: ionizing radiation to cause crosslinking through them.

TABLE V

Sample Marlex 6003 SRF-NS PTC Layer Resis- Power Curve Type No. (%) (%) Density, ohm-cm (Figure 35)

7-1 58 42 100 B

7-2 60 40 240 C, but some deviation near room temperature 7-3 62 38 400 Very good type C This example shows how the shape of the power curve can be changed by selecting the appropriate resistivity ratios for the PTC and CW layers.

The power-temperature dependence is, of course, consistent with the temperature-resistance relationship with the formula P = I R or P =.

T? The curve called C is close to the expected ideal with a heater with a type I resistance temperature curve.

Example 8

Two 30 cm long thin strip heater parts made in accordance with U.S. Patent No. 3,861,029 and having a PTC core composition similar to that used in Example 1 and shaped as shown in Figure 5 (0.8 cm wide) were attached. to an aluminum block maintained at 18 ° C with circulating water. The other side of each heater body was painted with a temperature indicating paint. The voltage applied to the pieces was varied to slowly increase their power output. The resistance of the second body was 488 ohm / m. This body could be operated with a power of up to about 5 * 48 W / m without the formation of a hotline, but with the core operating below its T3 temperature. At a power output of about 6.1 W / m, at which power level the core warmed to its Tg point, a hotline formed. The second heater body, with a resistance of about 8080 ohms / m, could similarly be operated at a power of about 4.88 W / m without the formation of a hotline, but the hotline was formed when operating at a power of more than about 6.1 W / m . Attempts to use 26 6 3 8 4 8 both heaters at higher voltage levels resulted in simultaneous current drops so that under the experimental conditions these heaters did not consume more than about 9.3 W / m and their maximum power under these conditions was about 0.15 W / cm2. Thus, attempts to use a strip heater at power levels higher than about 0.08 W / cm2 resulted in the formation of a hotline.

Example 9

A layered heater was prepared in which the composition of the PTC layer (0.075 cm thick) was 47% Marlex 6003 polyethylene, 5% Epsyn 5508 (modified ethylene-propylene diene rubber) and 48% Sterling SRF-NS (carbon black). Two 0.15 cm thick CW layers of 6OJK Elvaz 250 (ethylene-vinyl acetate copolymer) and 40% Cabot XC 72 (carbon black) embedded 0.95 cm wide and 0.95 cm thick. spaced apart flattened braided wire electrodes (three in total on both CW layers) were applied to both sides of the PTC layer so that the electrodes were opposite each other, i. in the same manner as in Fig. 11 except that the electrodes were braided instead of strips. The dimensions of the heater were 7.5 x 15 cm with the electrodes running along the long side and the electrodes of opposite polarity extending over the polymer layers at opposite ends of the heater. The layers were gently laminated together and the element was then held at 200 ° C for 10 minutes to remove any tension, then cooled and irradiated to a 12 Mrad dose using cobalt ^ ° gamma rays with the element sealed in a nitrogen containing container. The heater was sandwiched between 0.025 cm thick insulating layers of low density polyethylene and pressed firmly into a cooled aluminum block as in the previous example and a temperature indicating paint was applied to the top surface of the heater. Electrodes of opposite polarity were connected to a 12 V battery. The heater consumed more than 70 A when heating, i.e. more than 5.4 W / cm. Within a few minutes, the heater was stabilized to a current of more than 20 A, i.e. more than 15.5 W / cm2. Eventually, the aluminum block began to heat despite the cooling used and the PTC layer of the heater warmed to its Tg point (ca. 120 ° C). During this last stage, the temperature-indicating paint melted, starting in the middle and proceeding rapidly and evenly to the edges. In this final state, the heater kept itself at a temperature very close to its Ts point and consumed about 10 amps, i.e. its heat output was about 7.1 W / cm2 when the aluminum block was replaced with a plate of thermally insulating material. The current dropped much below one ampere, i.e. with a heater temperature below 0.67 W / cm2, the temperature of the heater still being very close to the T point and the entire surface of the heater being approximately at this temperature. Thus, it is apparent that the heater of this invention can operate at high power outputs at temperatures well above 100 ° C without the formation of a hotline.

It is noted that the reference in this context to a PTC layer that becomes or becomes substantially non-conductive is proportional to the electrical properties of the CW layer. It is not appropriate to give absolute values to such properties, as they depend, among other factors, on the relative structures of the different layers, but in the simple laminate shown in Figure 23, for example, as soon as the PTC layer exceeds its anomaly temperature in the part of the laminate where the two layers are electrically parallel. It is preferred that when the two layer types are electrically parallel, the amount of current flowing through the CW layer is at least 10 and preferably 25 times the current flowing through the PTC layer above its anomaly temperature, although in certain cases, e.g. in the vicinity, lower ratios such as 5 or less may be sufficient.

Claims (9)

A layered electrical resistance element which, when it reaches an elevated temperature, substantially interrupts the current through the element, in particular a self-regulating heating element, which comprises a first, electrically resistant layer (11, 21, 25, 33, 38, 43). 45, 49, 60) with a positive temperature coefficient of resistance (PTC layer) and with an anomaly temperature (Tg) above which it is substantially nonconductive, and at least a second layer (12, 15, 19, 24, 31, 35, 36, 40, 41, 44, 47, 50, 57, 58), characterized in that the second layer consists of electrical resistance material having substantially constant resistance (CW layer), at least below the anomaly temperature (T) of the first layer, that at least some of the of the first layer surface 5 in contact with at least a portion of the second layer surface along an interface providing a direct electrical and thermal connection therebetween, and the element being heat-returnable. Element according to claim 1, characterized in that it consists of a self-regulating heating element comprising a laminate containing at least the first and second layers, and at least a pair of electrodes (13, 14, 16, 18, 20, 22). 23, 26, 29, 30, 32, 34, 37, 39, 42, 45, 48, 51, 55, 56) are adjusted so that when there is a potential difference between the electrodes at normal temperature a current is preferred between the electrodes by part of the first layer and at least part of the second layer. Element according to claim 1, characterized in that the first layer is connectable to an electric power source such that electric current passes through at least part of the first layer and the second layer, the layers being arranged so that at higher temperatures there ( A) the resistance of the first layer exceeds the resistance of the second layer or (B) the anomaly temperature of the first layer, the current mainly follows a path whose length through the first layer does not in any case exceed the thickness of the second layer by more than 50%. Element according to claims 1-3, characterized in that it comprises a layer (33, 43, 49) of PTC material interposed between two layers (31, 35, 36, 40, 41, 44) of CW material .