CA1155721A

CA1155721A - Process for increasing the electric power of heating elements consisting of metallized textile sheets by a subsequent surface treatment

Info

Publication number: CA1155721A
Application number: CA000355188A
Authority: CA
Inventors: Harold Ebneth; Gerhard D. Wolf
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1979-07-04
Filing date: 1980-07-02
Publication date: 1983-10-25
Also published as: ATE3488T1; DE2926941A1; EP0023554B1; JPS5613690A; EP0023554A1; DE3063299D1

Abstract

Process for increasing the electric power of heating elements consisting of metallized textile sheets by a subsequent surface treatment Abstract of the Disclosure The invention relates to a process for increasing the electrical power of heating elements consisting of metallized textile sheets, comprising subjecting the metallized textile sheets to a surface treatment with a coating material.

Le A 19 760

Description

Process for increasing the electric power of heating elements consisting of metallized textile sheets by a subsequent surface treatment -In numerous technical fields of application, e.g. in hydroculture, aquaristics, aircraft constructions and the construction of motor vehicles, there is a demand for light-weight, flexible heating elements of large surface area which operate on a low voltage ( ~50 Volt) and virtually without inertia, are safe to handle and as far as possible preserve their elastic textile character.
Heating elements in the form of foils or of weaves coated with carbon or graphite have been disclosed, e.g.
by R. Aigner and P. Haasemann in Kunststoffe 63 (1973) 11, 769 - 771. Such heating elements are conventionally constructed of, for example, 5- to 7-stranded resistance wires and heated with current at different voltages. These structures have the disadvantage that, in the event of breakage of the metal wires or bending of the foils or of glass fabrics coated with sintered carbon material (for example due to repeated bending stresses or damage in transport), the function of the elements is impaired and the heating blankets or mats manufactured from them cannot be repaired.
It is for this reason that an electrically conductive textile sheet, which has the elastic character of a textile, e.g. a woven or knitted fabric or a non-woven mat or paper, is preferable for certain special purposes to rigid structures or foils since it can be subjected without damage to treatments such as bending, folding, rolling, compression or stretching.
A method of manufacturing a textile sheet which is electrically conductive, i.e. covered with a metal coating, has been described in German Offenlegungsschrift No.

2,743,7~. The possibility of using these structures as large area heating elements is mentioned in this Offenlegungs-schrift.
It has now surprisingly been found that, for a given electric voltage, the electrica' power and hence also the Le A 19 760.,:

surface temperature obtained in such large areaheating elements can be considerably increased if the elements are coated, laminated, impregnated, bonded, pressed or calendered in conventional manner with a polymer before they are used, e.g. as heating elements.
This invention therefore provides a process for increasing the electrical power of heating elements consisting of metallized textile sheets, comprising sub-jecting the metallized textile sheets to a surface treat-ment with a coating material mhe textile sheet ~hich is provided with such a metalcoating, preferably of nic~el, is preferably a woven or knitted fabric or non-woven ma.. The metal coating preferab~y has a thickness of at least 0.05~um. Thicknesses of from 0.2 to 0.8 ~um are generally particularly advantageous. Metallization of the textile sheets is preferably carried out by the process described in German Offenlegungsschrift No. 2,747,768 but may also be carried out by other methods.
The process of surface-coating according to the invention provides the further advantage of protecting the thin metal layer against mechanical or chemical damage.
The electrical conductivity of such surface heating elements depends, as is well known, on the thickness of the layer of metal on the fibres or threads, and increases with increasing thickness of the layer owing to the increase in the cross-sectional area of the metal layer.
A layer of nickel only 0.05 ~m in thickness has sufficient conductivity for ~cst heating purposes. By using thic~er layers of metal, measured in g of metal per unit surface area or in ~m of thickness o~ the layer OI
deposited metal, the current flow at a given voltage can be in^reased, thereby increasing the heating power. A layer of nic~el about-0.5 ,um in thickness is sufficient, for example, to produce a surface temperature of 250C at Le A 19 760 1 15~72 1 36 volt/13.5 A in a spun fibre fabric (400 x 330 mm) of aromatic polyamides, using either alternating or direct current.
The increase in heating power of metallized sheets which is an object of this invention is obtained by, for example, impregnating such a sheet with a polyester urethane adduct based on tolylene diisocyanate with the addition of poly-isocyanate or by laminating and gelling the sheet with a soft PVC on a laminating machine o~ by a process Gf vacuum-forming using plates or foils of thermoplastic r~sins (e.g.ABS polymers, polyolefines, polycarbonates and the like) or simply by coating the sheet with a conventional rubberizing or coating material.
Examples of practical applications of the invention lS include seat covers laminated with soft PVC which can be heated by a low voltage current, or heating mats, e.g. for aquariums or hydrocultures, or heatable rubberized textiles, e.g. for use in diving suits or in kidney belts for motor cyclists, etc. The invention is also applicable, for example, to heat~ng mattresses for hospital use and covers for operating tables.
One particular method of application is that of the direct vacuum process on deep drawing machines or other forming machines commonly u~ed in the plastics processing industry. The metallized textile sheet is in such cases firmly bonded to the thermoplastic material either by a mechanical-physical bond or with the aid of an adhesive and becomes permanently incorporated between the two plates or foils. It is particularly with this method that an up to 4-fold increase in current flow measured in Amperes can be obtained, depending on the particular variation employed.
The protective layers may also be applied by means of presses, calenders, rollers or other conventional apparatus and they result in a reduction in the electric Le A 19 760 resistance combined with an increased wattage.
Example 1 A fabric woven from commercial polyacrylonitrile spun fibres and covered with 25 g/m2 of nickel and a knitted fabric of porous polyacrylonitrile fibres according to German Offenlegungsschrift No. 2,554,124 covered with 32 g/m of nickel were impregnated with a solution of a polyurea-polyurethane in a 7:3 mixture of toluene and iso-propanol and dried in a vacuum at 40C. Coatings of various thicknesses were obtained on the metallized sheets by using different concentrations of solution (2.5; 5; 10 and 20~). The electrical resistance of the metallized textile sheets was considerably reduced by the coating of polyurea polyurethane, in some cases to less than half the original value ~see Table 1).

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115~721 The textile character is preserved if only a small quantity of coating substance (e.g. from 2.5% solution) is applied. As the thickness of the coating increases, the handle becomes progressively rougher and the sheets become progressively more rigid. The abrasion resistance of nickel is improved and finally no abrasion is found (measurements using a Crcck meter, Note 5, DIN 54 021).
Example 2 A clear ~U foil 50 /u in thickness was applied by press-moulding at 140C to both sides of the woven poly-acrylonitrile fa~ric and the porous knitted polyacrylo-nitrile fabric used in Example 1. This treatment reduced the electrical resistances to very lcw values compared to those of the untreated samples (see Table 2). Virtually no abrasion of nickel was detected with the Crock meter.
Table 2 Electrical resistance before and after pressure moulding in Ohm (~) before after Woven fabric of poly- 62.0 (direction of we~t) 34.2 acrylonitrile fibres 22.5 (direction of warp) 6.3 Porous knitted fabric 9.4 (in the longitudinal 3.5 direction of tne of polyacrylonitrile stitches) fibres 37.5 (transversely to the 6.3 stitches) Example 3 A fabric woven from cotton/polyester fibre yarn measuring 43 cm x 75 c~. wover in cloth weave from 50% cotton fibres and 50% polyester staple fibres was covered w7th a coating of nickel about 0.7 ~m in thickness and its surface temperature was measured at various ~Joltages:
A surface temperature of 25C was measured at 12 V/1.3 A, a surface temperature of 40~C at 24 V/2.6 A and a surface temperature of 51C at 36 V/4~2 A.
Le A 1~ 760 The sample was subsequently laminated with a soft PVC coating and the surface temperature was again measured:
A surface temperature of 31C was found at 12 V/2 A, 61C at 24 V/3.7 A and 74C at 36 V/6 A.
It is immaterial to the results whether alternating or direct current is used.
Example 4 A coarse weave fabric of commercial polyacrylonitrile fibres covered with 45 g/m2 of nickel was found to have a surface temperature of 29C at 24 V/0.6 A.
After the fabric had been laminated on both sides with a soft PVC combination by the reversal process, its surface temperature was found to be 45C at 24 V/2.1 A.
Example 5 A woven polyacrylonitrile fabric measuring 20 x 20 cm and having a nickel content of 9.43~ by weight and an electrical resistance R of 15.7 Ohm in the direction of the warp and 32.4 Ohm in the direction of the weft was found to have a surface temperature of 21C at 12 V/0.05 A, a surface temperature of 24C at 24 V/0.1 A, and a surface temperature of 28C at 30 V/0.2 A.
This sample was then covered on both sides with a polyethylene foil 0.1 mm in thickness by the application of a pressure of 100 kp at 115C for 2 minutes. The following results were obtained:
a surface temperature of 37C at 12 V/0.6 A, a surface temperature of 48C at 24 V/1.2 A, and a surface temperature of 68C at 30 V/1.5 A.
A similar sample was covered on both sides with a soft PVC foil 0.3 mm in thickness by the application of a pressure of 100 kp at 115C for 2 minutes.
The sample was found to have a surface temperature of 29C at 12 V/0.4 A, Le A 19 760 38C at 24 V/0.8 A, and 58C at 30 V/1.2 A.
Example 6 An A3S graft polymer plate 3mm in thickness and measuring 400 x 300 mm was deep drawn in a vacuum-forming machine after it had been pre-dried for 2 minutes. The plate was heated above its softening point in an infrared heater (temperature of heater 280C) and then deep drawn, using a vacuum of 70 mm. The area of the deep drawn surface was then 310 x 210 mm.
Small holes were bored in the depressed areas of the deep drawn plate so that the vacuum of the machine could be employed in a second deep drawing operation. A fabric woven from a 50:50 mixture of polyester and cotton fibres and covered with nickel was then placed in the depression.
The fabric contained 29.5 % by weight of nickel and had a resistance per unit square area of 4.3 Ohm in the direction of the weft and 18.2 Ohm in the direction of the warp.
The section of fabric measured 300 x 200 mm. An adhesive was applied to the top edge of the dish.
A second ABS graft polymer plate was then heated above its softening point under the infrared heater and deep drawn into the prepared dish by the vacuum of the machine.
The nickel coated fabric and heated ABS foil were firmly bonded to the top edge of the first, previously formed ABS dish, which was coated with adhesive.
The resistance per unit square area in the direction of the weft was found to be reduced to 2.6 Ohm after the deep drawing process and cooling.
The plastics dish of composite material which can be heated at a low voltage was found to heat 3 litres of water to 44C within a short time at 12 Vt4.5 A.
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Claims

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

1. A process for increasing the electrical power of heating elements consisting of metallized textile sheets, comprising subjecting the metallized textile sheets to a surface treatment with a coating material.

2. A process according to claim 1, wherein said surface treatment is selected from the group consisting of laminating, impregnating, bonding with adhesive, press moulding, calendering or coating with a polymer.

3. A process according to claim 1, wherein said metallized textile sheet is a woven or knitted fabric or non-woven mat.

4. A process according to claim 1, wherein said textile sheet has a metal coating with a total thickness of at least 0.05 µm.