Note: Descriptions are shown in the official language in which they were submitted.
1155721
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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
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1 155721
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
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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
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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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1155721
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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.
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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,
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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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