EP0209224B1 - Eléments chauffants en forme de feuille - Google Patents

Eléments chauffants en forme de feuille Download PDF

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Publication number
EP0209224B1
EP0209224B1 EP86303722A EP86303722A EP0209224B1 EP 0209224 B1 EP0209224 B1 EP 0209224B1 EP 86303722 A EP86303722 A EP 86303722A EP 86303722 A EP86303722 A EP 86303722A EP 0209224 B1 EP0209224 B1 EP 0209224B1
Authority
EP
European Patent Office
Prior art keywords
electrodes
resistive element
insulating layer
heater according
inch
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 - Lifetime
Application number
EP86303722A
Other languages
German (de)
English (en)
Other versions
EP0209224A2 (fr
EP0209224A3 (en
Inventor
Neville Sam Batliwalla
Jeff Shafe
Ravinder K. Oswal
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Raychem Corp
Original Assignee
Raychem Corp
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Raychem Corp filed Critical Raychem Corp
Priority to AT86303722T priority Critical patent/ATE79209T1/de
Publication of EP0209224A2 publication Critical patent/EP0209224A2/fr
Publication of EP0209224A3 publication Critical patent/EP0209224A3/en
Application granted granted Critical
Publication of EP0209224B1 publication Critical patent/EP0209224B1/fr
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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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
    • H05B3/02—Details
    • H05B3/06—Heater elements structurally combined with coupling elements or holders
    • 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
    • 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
    • H05B3/20—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater
    • H05B3/34—Heating elements having extended surface area substantially in a two-dimensional plane, e.g. plate-heater flexible, e.g. heating nets or webs
    • 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
    • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
    • H05B2203/002—Heaters using a particular layout for the resistive material or resistive elements
    • H05B2203/006—Heaters using a particular layout for the resistive material or resistive elements using interdigitated electrodes
    • 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
    • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
    • H05B2203/011—Heaters using laterally extending conductive material as connecting means
    • 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
    • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
    • H05B2203/017—Manufacturing methods or apparatus for heaters
    • 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
    • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
    • H05B2203/02—Heaters using heating elements having a positive temperature coefficient

Definitions

  • This invention relates to sheet heaters.
  • Sheet heaters typically comprise a laminar resistive element and two or more electrodes.
  • the resistive element may be composed of a conductive polymer, i.e. a mixture of a conductive filler and an organic polymer (this term being used to include polysiloxanes), the filler being dispersed in, or otherwise held together by, the organic polymer.
  • the resistive element may exhibit PTC behavior, thus rendering the heater self-regulating.
  • the electrodes are positioned on one face of the resistive element, e.g. by printing a conductive ink onto the heating element. Particularly relevant in this regard are EP-A-158 410, 175,550 and 176 284. Other documents describing conductive polymer compositions and devices comprising them include U.S.
  • DE-A-2 006 165 relates to a flexible heating plate having an applied solid insulation layer of PVC material.
  • US-A-4 250 398 relates to a composite laminated heating article having two insulating layers with a conductive layer therebetween.
  • insulation which comprises (a) a first layer which is bonded to the electrodes and the resistive element and is composed of a cured polymeric composition having a relatively low tensile strength so that it can accommodate to the stresses imposed by flexing and/or thermal cycling, and (b) a second, outer, layer which may be a conventional insulating material except that it is not bonded, or is at most lightly bonded, to the first layer.
  • the first layer (which is often referred to herein as the dielectric layer) is formed by applying to the electrode-bearing surface a composition which is liquid when it is applied and which is cured in situ so that it is intimately bonded to at least part of the electrodes and preferably also to at least part of the resistive element, and especially to the surface as a whole.
  • the applied dielectric layer provides improved electrical properties, in particular improved electrical safety, which eliminate, or at least reduce, the possibility of sparking and burning if one of the electrodes is broken.
  • an electrical sheet heater which comprises:
  • the dielectric layer covers at least a part of the electrodes, and preferably the whole of the electrodes except for those parts which are connected to electrical leads (for connecting the heater to a power supply) or to connection members which connect spaced-apart electrodes or which provide a bus along an edge or central portion of an electrode, eg. an electrode having arms which extend from the connection member. If such connection members are present, the dielectric layer should be formed after the connection members have been applied, or should not cover the parts of the electrodes to which the connection members will later be applied.
  • the dielectric layer preferably also covers at least part, and preferably all, of the exposed part of the electrode-bearing surface(s) of the resistive element, particularly those parts which are adjacent the electrodes.
  • the polymeric component of the conductive polymer comprises, eg. contains at least 75% by weight of, a polymer of low surface energy, for example less than 40, particularly less than 35, especially less than 30, dynes/cm, since it is difficult to bond to such conductive polymers by the melt-bonding and adhesive techniques which are conventionally used for applying insulation.
  • the dielectric layer is formed by applying a suitable liquid composition, ie. one containing a curable organic polymer or suitable precursor(s) for such a polymer, and then solidifying and curing the composition in situ , if necessary after removing any solvent present in the liquid composition.
  • a suitable liquid composition ie. one containing a curable organic polymer or suitable precursor(s) for such a polymer
  • the composition is preferably applied, solidified and cured at a temperature which avoids possible damage to the resistive element and/or electrodes. Curing can result merely from the selection of appropriate ingredients for the liquid composition or can depend on, or be accelerated by, an external stimulus such as moisture, heat, and/or irradiation.
  • Particularly suitable materials are two-part silicone systems, one part containing the monomer and the other containing the catalyst, for example Sylgard 577 as supplied by Dow-Corning. Flexible two-part epoxy systems can also be used.
  • the peel strength of the bond between the cured material and the electrodes is at least 1, particularly at least 2, especially at least 3, pounds per linear inch at 20°C, such that when the dielectric layer is peeled away from the electrodes, it fails cohesively (ie. the layer is torn apart, leaving the interface substantially unchanged) rather than adhesively (ie. the layer separates from the substrate along the interface).
  • the cured material should have a tensile strength at 23°C of at most 4,000 psi, preferably at most 3,000 psi, particularly at most 2,000 psi, especially at most 1,000 psi, so that it can conform to changes in the dimensions of the electrodes and the resistive element as a result of flexing and/or differences in thermal expansion and contraction.
  • the thickness of the dielectric layer is preferably at least 0.002 inch, particularly at least 0.004 inch, especially at least 0.006 inch, in order to ensure that it is free from pinholes.
  • the dielectric layer preferably has a dielectric strength of at least 1,000 volts per inch at all temperatures likely to be encountered during use of the heater, eg. at all temperatures from room tempeature to the switching temperature of a PTC conductive polymer composition in a self-regulating heater.
  • the second insulating layer is preferably not bonded to the dielectric layer or to any parts of the electrodes and the electrode-carrying surface of the resistive element which are not covered by the dielectric layer. If there is any such bonding, it is preferably such that if the second insulating layer is peeled away from the resistive element, the dielectric layer remains in place, the peeling taking place through cohesive failure of the dielectric layer or through adhesive failure at the interface between the dielectric layer and the second insulating layer (ie. the peel strength of the bond between the dielectric and second layers is less than the peel strength of the bond between the dielectric layer and the electrodes).
  • the second insulating layer is preferably a polymeric material, eg. a preformed flexible sheet thereof, which has satisfactory resistance to physical stresses and to chemicals.
  • An important reason for the presence of the second insulating layer is that the compositions suitable for use in the dielectric layer have poor resistance to physical stresses and/or to chemicals.
  • the heater will normally include a third insulating layer which covers the non-electrode-carrying surface of the resistive element (and may be bonded thereto) and which is bonded to the second insulating layer along the edges thereof.
  • the heaters according to the invention are preferably flexible, by which is meant that at 23°C, and preferably at -20°C, they can be wrapped around a 4 inch diameter mandrel without damage.
  • the conductive polymer preferably has a resistivity at 23°C of at least 0.5 ohm.cm, particularly 0.5 to 100,000 ohm.cm. It is preferably cross-linked, particularly by radiation, eg. electron beam or gamma radiation, eg. to a uniform dose of at least 5 Mrads, preferably at least 12 Mrads.
  • radiation eg. electron beam or gamma radiation
  • the invention is particularly useful when the electrodes have been formed by printing, particularly silk screen printing, a conductive ink onto the resistive element, or by a like technique which results in somewhat fragile electrodes, eg. electrodes formed by the use of polymer thick film technology, or by sputtering, or by a process comprising an etching step, because application of the dielectric has little or no effect on the electrodes or the resistive element/electrode interface.
  • the electrodes preferably comprise a conductive polymer, for example in the form of an ink, in which the conductive filler consists of or contains a metal, preferably silver, or a mixture of silver and graphite.
  • the electrodes preferably have a resistivity in the range 2.5 x 10 ⁇ 4 to 1 x 10 ⁇ 3 ohm.cm.
  • a contact layer having a resistivity between the resistivities of the electrodes and the resistive element, as described in European Application No. 176,284.
  • the electrodes are preferably positioned on the same surface of the resistive element, so that current passing between them flows mainly parallel to the surface, but they can be on both surfaces. It is particularly preferred that the electrodes be interdigitated as disclosed in European Application No. 185,410.
  • the heater When the heater requires a ground plane, eg. if it is to be used in hazardous location, it preferably includes a laminar metallic element which functions as a ground plane and which is not bonded directly or indirectly to the resistive element or the insulating elements of the heater.
  • the novel heaters are substantially safer than identical heaters without the dielectric layer.
  • the dielectric layer both increases the force rquired to damage an electrode, and reduces the dangers resulting from damage to the electrodes.
  • arcing, sparking and subsequent burning of the resistive element can occur.
  • the dielectric layer even though a break in the electrodes can result in arcing across the break, it does not lead to sparking and subsequent burning.
  • the absence of sparking and burning may be due to the fact that the dielectric layer prevents, or at least minimizes, access of oxygen to the break in the electrode, so that sparking and burning cannot be sustained.
  • the material of the dielectric layer preferably has a high resistance to tracking, so that it helps to extinguish any continued sparking. Also the dielectric layer prevents water or any other electrolyte contacting and bridging the electrodes, and therefore avoids the possibility of short circuits between the electrodes and the problems of consequent sparking and burning of the resistive element. In this respect the invention is particularly useful when adjacent electrodes are less than 1 inch apart, and easily short-circuited.
  • the Figures illustrate a heater which comprises a heating element comprising a laminar conductive polymer resistive element 2 having printed on the top surface thereof inter-digitated electrodes 4 and 6.
  • a dielectric layer 8 overlies the interdigitating portions of the electrodes, but does not extend to the longitudinal margins of the electrodes.
  • the dielectric layer 8 comprises a polysiloxane obtained by curing a liquid two-part system applied over the element 2 and electrodes 4 and 6, and then heated to 275°F for 10 minutes.
  • the dielectric layer 8 is intimately bonded to the underlying element 2 and the electrodes 4 and 6.
  • Bus bars 10 and 12 composed of metal mesh, are folded around uncovered marginal portions of the element 2 and the electrodes 4 and 6 respectively.
  • An insulating jacket (shown in Figure 1 only) is formed around the heating element, and bus bars by a polymeric bottom sheet 14 and a polymeric top sheet 16.
  • Sheet 14 is secured to the bottom of the heating element 2, and to the edge portions of the top sheet by a substantially continuous layer of adhesive 17 (as shown), or by melt bonding (not shown).
  • the top sheet 16 is adjacent to but not secured to the bus bars 10 and 12, the dielectric 8, the electrodes 4 and 6, or the resistive element 2.
  • the electrodes have width t and length l and are separated by a distance d
  • the bus bars have width x
  • the dielectric layer a length y parallel to the length of the electrodes. Typical values for these variables are
  • the invention is further illustrated by the following Example.
  • a heater as illustrated in Figures 1 and 2 was made in the following way.
  • the sheet was irradiated to a dose of 14 Mrads (7 Mrads each side) thus cross-linking the polymer.
  • An electrode pattern as illustrated in Figure 2 was deposited on the strips by screen printing a layer comprising a graphite-and silver-containing composition, having a resistivity of 1.3 ⁇ 10 ⁇ 2 ohm.cm, followed by drying. The distance (d) between adjacent electrodes was 0.25 inch; the width (t) of each electrodes was 0.0625 inch, and the length (l) of each electrode was 5.4 inch. Then the sheet was heated to 175°F for 1 hour and slit into strips 7.25 inches wide.
  • a layer 8 to 10 mils thick of a curable two part silicone liquid (Sylgard 577, sold by Dow Corning ) was then applied to the strips and the strips were placed in an oven at 275°F for 5 to 10 minutes to cure the silicone.
  • Bus bars of nickel-coated copper expanded metal were folded around the edges of the electrode-bearing strip.
  • the resulting assembly was laminated between (A) a bottom sheet of ethylene-chlorotrifluoroethylene copolymer (“Halar”) 8.5 inch wide and 0.020 inch thick, coated on the whole of its top surface with a layer 0.002 inch thick of a silicone adhesive sold by Adhesives Research Corporation under the trade name "Arclad”, and (B) a top sheet of ethylene-chlorotrifluoroethylene copolymer (“Halar”) 8.5 inch wide and 0.010 inch thick, which was coated on 0.5 inch wide edge portions of its bottom surface with a layer 0.002 inch thick of the same adhesive.
  • Halar ethylene-chlorotrifluoroethylene copolymer

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  • Resistance Heating (AREA)
  • Surface Heating Bodies (AREA)
  • Magnetic Heads (AREA)

Claims (9)

  1. Elément électrique chauffant en feuille, qui comprend :
    (1) un élément résistant laminaire (2) qui (a) est constitué d'une composition polymérique conductrice qui présente un comportement CTP et (b) comprend une première surface laminaire et une seconde surface laminaire ;
    (2) deux ou plus de deux électrodes enchevêtrées (4, 6) qui sont fixées à la première surface de l'élément (2) et qui sont positionnées de manière à laisser à découvert une partie de cette première surface ;
    (3) une première couche isolante (8) qui (a) est positionnée sur et entre directement en contact avec les électrodes (4, 6) et la première surface à découvert de l'élément résistant (2), et (b) comprend une composition polymérique organique qui est appliquée sous forme liquide et qui, à l'état durci, possède une résistance à la traction inférieure à 275,79 bars (4000 lb/in²) à 23°C ; et
    (4) une seconde couche isolante (16) qui est positionnée sur les électrodes (4, 6), l'élément résistant (2) et la première couche isolante (8) ;
    dans lequel n'importe quelle liaison entre les première (8) et seconde (16) couches isolantes est telle que, si la seconde couche isolante (16) est arrachée de l'élément résistant (2), la résistance à la rupture de la liaison entre les première (8) et seconde (16) couches isolantes est inférieure à la résistance à la rupture de la liaison entre la première couche isolante (8) et les électrodes (4, 6).
  2. Elément chauffant suivant la revendication 1, dans lequel la seconde couche isolante n'est pas liée à la première couche isolante.
  3. Elément chauffant suivant la revendication 1 ou 2, dans lequel la première couche isolante possède une épaisseur de 0,0508 à 0,0762 mm (0,002 à 0,003 inch), de préférence de 0,1016 à 0,508 mm (0,004 à 0,020 inch), et est constituée d'une matière ayant une résistance à la traction inférieure à 206,843 bars (3000 lb/in²) à 23°C, de préférence inférieure à 137,895 bars (2000 lb/in²) à 23°C.
  4. Elément chauffant suivant la revendication 3, dans lequel la première couche isolante possède une épaisseur de 0,1524 à 0,3048 mm (0,006 à 0,012 inch) et est constituée d'une matière ayant une résistance à la traction inférieure à 68,948 bars (1000 lb/in²) à 23°C.
  5. Elément chauffant suivant l'une quelconque des revendications précédentes, dans lequel la première couche isolante est constituée d'un polysiloxane.
  6. Elément chauffant suivant l'une quelconque des revendications précédentes, dans lequel les électrodes sont positionnées sur la même face de l'élément résistant de sorte que, lorsqu'un courant passe entre ces électrodes, une proportion notable du courant soit parallèle aux faces de l'élément résistant.
  7. Elément chauffant suivant la revendication 6, qui comprend des électrodes enchevêtrées qui sont positionnées sur la même surface d'un élément résistant, et l'élément résistant est constitué d'une composition polymérique conductrice présentant un comportement CTP.
  8. Elément chauffant suivant la revendication 7, dans lequel la composition polymérique conductrice a été extrudée à l'état fondu et possède une résistivité de 0,5 à 100 000 ohms.cm à 23°C.
  9. Elément chauffant suivant l'une quelconque des revendications précédentes, dans lequel les électrodes sont formées par un procédé qui consiste à imprimer une encre conductrice sur l'élément résistant.
EP86303722A 1985-05-17 1986-05-15 Eléments chauffants en forme de feuille Expired - Lifetime EP0209224B1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT86303722T ATE79209T1 (de) 1985-05-17 1986-05-15 Folienheizelemente.

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US73540985A 1985-05-17 1985-05-17
US735409 1985-05-17

Publications (3)

Publication Number Publication Date
EP0209224A2 EP0209224A2 (fr) 1987-01-21
EP0209224A3 EP0209224A3 (en) 1988-03-30
EP0209224B1 true EP0209224B1 (fr) 1992-08-05

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ID=24955673

Family Applications (1)

Application Number Title Priority Date Filing Date
EP86303722A Expired - Lifetime EP0209224B1 (fr) 1985-05-17 1986-05-15 Eléments chauffants en forme de feuille

Country Status (6)

Country Link
EP (1) EP0209224B1 (fr)
JP (1) JPS61267287A (fr)
AT (1) ATE79209T1 (fr)
CA (1) CA1262468A (fr)
DE (1) DE3686296T2 (fr)
IN (1) IN167714B (fr)

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2228653B (en) * 1989-01-25 1992-03-04 Thermaflex Ltd Flexible heating element
DE19823495B4 (de) * 1998-05-26 2006-06-22 Latec Ag Flexibles Flächenheizelement
CN102067719B (zh) * 2008-04-22 2014-07-16 达泰克涂料股份公司 厚膜高温热塑性绝缘加热元件
US8575523B2 (en) * 2008-04-25 2013-11-05 Innovative Heating Technologies Inc Planar heating element for underfloor heating

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4250398A (en) * 1978-03-03 1981-02-10 Delphic Research Laboratories, Inc. Solid state electrically conductive laminate
EP0158410A1 (fr) * 1984-01-23 1985-10-16 RAYCHEM CORPORATION (a Delaware corporation) Dispositifs lamellés avec polymère conducteur

Family Cites Families (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2006165A1 (de) * 1969-02-11 1971-05-06 Renfrew Electronics Ltd Flexible Heizplatte
GB1604735A (en) * 1978-04-14 1981-12-16 Raychem Corp Ptc compositions and devices comprising them
US4134004A (en) * 1977-07-18 1979-01-09 American Can Company Electrically heated pizza package
US4400614A (en) * 1980-05-19 1983-08-23 Raychem Corporation PTC Devices and their preparation
US4532164A (en) * 1983-09-15 1985-07-30 Raychem Corporation Heat-shrinkable article
US4535113A (en) * 1984-03-13 1985-08-13 Union Carbide Corporation Olefin polymer compositions containing silicone additives and the use thereof in the production of film material
EP0175550A1 (fr) * 1984-09-14 1986-03-26 RAYCHEM CORPORATION (a California corporation) Elément chauffant en feuilles avec isolant séparé
DE3585761D1 (de) * 1984-09-14 1992-05-07 Raychem Corp Elektrischer kontakt zwischen elementen mit verschiedenem spezifischen widerstand.
DE8433753U1 (de) * 1984-11-17 1985-05-09 Witte & Sutor Gmbh, 7157 Murrhardt Elektrisch beheizte schreibunterlage

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4250398A (en) * 1978-03-03 1981-02-10 Delphic Research Laboratories, Inc. Solid state electrically conductive laminate
EP0158410A1 (fr) * 1984-01-23 1985-10-16 RAYCHEM CORPORATION (a Delaware corporation) Dispositifs lamellés avec polymère conducteur

Also Published As

Publication number Publication date
JPS61267287A (ja) 1986-11-26
EP0209224A2 (fr) 1987-01-21
DE3686296D1 (de) 1992-09-10
IN167714B (fr) 1990-12-08
DE3686296T2 (de) 1992-12-17
ATE79209T1 (de) 1992-08-15
EP0209224A3 (en) 1988-03-30
CA1262468A (fr) 1989-10-24

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