Seat heater

A flexible heater in which a heating element in the form of a tape is wrapped around a support member in the form of a flat sheet. The tape, which exhibits flexibility and toughness, has a composition composed of conductive sintered ultrahigh molecular weight polyethylene. The flexible heater is suitable for heating an upholstered seat, e.g. an automobile seat.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to flexible heaters which are suitable for heating 
seats in automobiles and other vehicles. 
2. Introduction to the Invention 
In cold climates, it is desirable to heat not only the air in the passenger 
compartment of an automobile or similar vehicle, but also the seats in 
which people are sitting. Until now, car seats have been heated, if at 
all, by means of series-connected heating wires. The known heaters, 
however, suffer from a variety of problems. These include failure due to 
intermittent flexing of the wires as the seat is occupied, the requirement 
for high power output to provide a minimum comfort level, the slow rate of 
heating due to the low ratio of heater coverage to seat area, and the 
partial penetration of the wires through the leather or fabric covering 
the seat leading to a "show through effect". 
Attempts to correct some of these problems have been made. For example, 
Damron U.S. Pat. No. 3,781,526 discloses a sheet heater suitable for 
heating a stadium seat. The heater comprises an electrically conductive 
paper; interdigitated electrodes are positioned at the edges of the paper. 
Japanese Patent Publication No. 1-164,620/1989 (Toyoda Boshoku KK; Tokai 
Senko KK) discloses a durable, flexible sheet heater for heating vehicle 
seats. The heater comprises a fabric layer on which a conductive metal 
layer is electroplated. The resulting heater is attached to the seat 
cushion. Neither solution has solved all the problems. 
SUMMARY OF THE INVENTION 
We have now found that a thin, flexible heating element in the form of a 
tape can be used to provide efficient, reliable heat. Thus in a first 
aspect, the invention provides a flexible heater which comprises 
(1) a support member which is in the form of a flat sheet; and 
(2) a heating element which is in the form of a tape which has a ratio of 
external surface area to polymer volume of at least 20 inch.sup.-1, which 
is wrapped around the support member, and which comprises 
(a) a resistive element which is composed of 
(i) particles of ultrahigh molecular weight polyethylene having a molecular 
weight of at least 3 million, which particles have been sintered without 
completely losing their identity, and 
(ii) a particulate conductive filler which is present substantially only at 
or near the boundaries of the coalesced particles; and 
(b) elongate electrodes which are secured to opposite margins of the 
resistive element and which can be connected to a source of electrical 
power to cause current to pass through the resistive element. 
In a second aspect, the invention provides a shaped article, e.g. a seat 
back, which comprises 
(1) a resilient foam of a polymeric material, and 
(2) at least partially embedded in the foam, a heating element which is in 
the form of a tape which has a ratio of external surface area to polymer 
volume of at least 20 inch.sup.-1 and which comprises 
(a) a resistive element which is composed of 
(i) particles of ultrahigh molecular weight polyethylene having a molecular 
weight of at least 3 million, which particles have been sintered without 
completely losing their identity, and 
(ii) a particulate conductive filler which is present substantially only at 
or near the boundaries of the coalesced particles; and 
(b) elongate electrodes which are secured to opposite margins of the 
resistive element and which can be connected to a source of electrical 
power to cause current to pass through the resistive element. 
In a third aspect, the invention provides an upholstered seat which 
comprises a resilient seat member which is covered by a seat cover, a 
resilient back member which is covered by a back cover, and a flexible 
heater which lies between the resilient back member and the back cover, or 
between the resilient seat member and the seat cover, or both, the 
flexible heater comprising 
(1) a support member which is in the form of a flat sheet; and 
(2) a heating element which is in the form of a tape which has a ratio of 
external surface area to polymer volume of at least 20 inch.sup.-1, which 
is wrapped around the support member, and which comprises 
(a) a resistive element which is composed of 
(i) particles of ultrahigh molecular weight polyethylene having a molecular 
weight of at least 3 million, which particles have been sintered without 
completely losing their identity, and 
(ii) a particulate conductive filler which is present substantially only at 
or near the boundaries of the coalesced particles; and 
(b) elongate electrodes which are secured to opposite margins of the 
resistive element and which can be connected to a source of electrical 
power to cause current to pass through the resistive element.

DETAILED DESCRIPTION OF THE INVENTION 
In this invention, the heating element is in the form of a tape which 
comprises a resistive element and elongate electrodes. The resistive 
element comprises a conductive polymer composition composed of a polymer 
matrix, and, dispersed, or otherwise distributed in the matrix, a 
particulate conductive filler. The polymeric component is preferably a 
crystalline organic polymer or blend comprising at least one crystalline 
organic polymer. Particularly preferred is ultrahigh molecular weight 
polyethylene (UHMWPE), a polymer which has a molecular weight greater than 
about 1.5 million, particularly greater than about 3 million, and 
especially as high as about 4 to 6 million, and which maintains a 
relatively high viscosity above its melting point. The conductive filler 
may be carbon black, graphite, metal, metal oxide, or a combination of 
these, or a particulate conductive filler which itself comprises an 
organic polymer with a particulate conductive filler dispersed in it. Such 
composite particulate conductive polymers are disclosed in copending, 
commonly assigned U.S. application Ser. No. 07/75,929, filed Jul. 21, 1987 
(Barma et al), the disclosure of which is incorporated herein by 
reference. The conductive polymer element may also comprise antioxidants, 
inert fillers, chemical crosslinking agents (often referred to as 
prorads), stabilizers, dispersing agents, or other components. Dispersion 
of the conductive filler and other components is preferably achieved by 
dry-blending of powders. The resulting mixture can then be shaped, 
preferably by sintering. Thus the preferred resistive element comprises a 
matrix consisting essentially of organic polymer particles, preferably 
ultrahigh molecular weight polyethylene, which have been sintered together 
so that the particles have coalesced without completely losing their 
identity, and a particulate conductive filler, preferably carbon black, 
which is dispersed in the matrix but which is present substantially only 
at or near the boundaries of the coalesced particles. The preferred 
compositions have a resistivity of less than 1000 ohm-cm, preferably less 
than 100 ohm-cm, particularly less than 10 ohm-cm, e.g. from 0.5 to 10 
ohm-cm. Examples of such compositions and devices comprising them may be 
found in U.S. Pat. Nos. 4,775,501 (Rosenzweig et al), 4,853,165 
(Rosenzweig et al), International Application Nos. PTC/US88/00592 (McMills 
et al, filed Feb. 24, 1988, published as No. W088/06517 on Sep. 7, 1988) 
and PCT/US89/02738 (McMills et al, filed Jun. 22, 1989), and copending, 
commonly assigned application Ser. Nos. 07/194,780 (Rosenzweig et al, 
filed May 17, 1988 now U.S. Pat. No. 4,921,648, 07/250,024 (McMills et al, 
filed Sep. 26, 1988), 07/299,915 (McMills et al, filed Oct. 21, 1988), 
07/394,288 (McMills, filed Aug. 15, 1989) now U.S. Pat. No. 4,938,820, 
07/407,595 (McMills et al, filed Sep. 15, 1989) 07/428,487 (McMills et al, 
filed Oct. 31, 1989), now abandoned in favor of a continuation 
application, Ser. No. 07/547,300 (filed Oct. 12, 1990), 07/435,854 
(Rosenzweig et al, filed Nov. 13, , 07/462,893 (Soni et al, filed Jan. 3, 
1990), the disclosures of which are incorporated herein by reference. 
The compositions used in this invention generally exhibit ZTC (zero 
temperature coefficient of resistance) behavior, i.e. they have a 
resistivity which changes by less than 6 times, preferably by less than 2 
times, in any 30.degree. C. temperature range within the operating range 
of the heater. For some applications, however, compositions which exhibit 
PTC (positive temperature coefficient of resistance) behavior may be used. 
In this specification, the term "PTC" is used to mean a material or device 
which has an R.sub.14 value of at least 2.5 and/or an R.sub.100 value of 
at least 10, and particularly preferred that it should have an R.sub.30 
value of at least 6, where R.sub.14 is the ratio of the resistivities at 
the end and the beginning of a 14.degree. C. range, R.sub.100 is the ratio 
of the resistivities at the end and the beginning of a 100.degree. C. 
range, and R.sub.30 is the ratio of the resistivities at the end and the 
beginning of a 30.degree. C. range. 
The resistive element can be configured into a tape by any suitable means, 
although for preferred compositions comprising ultrahigh molecular weight 
polyethylene, skiving from a ram-extruded rod or tube is preferred. The 
tape may be crosslinked by chemical means or by irradiation. In this 
specification the term "tape" is used to mean any configuration of the 
resistive element in which the resistive element is in the form of a 
laminar element having a relatively wide and thin cross-section. There is 
a sufficiently high ratio of external surface area of the tape from which 
heat is dissipated to polymer volume in the heat-producing region to 
enable it to withstand a minimum of about 50 watts/cm.sup.3 and/or about 7 
watts/in.sup.2 when the tape is in contact with a solid substrate. 
Although the tape normally has a rectangular cross-section, other 
cross-sectional shapes, e.g. oval or dog-bone, may be appropriate for 
various applications, as long as the resistive element has a ratio of 
width to thickness of at least 8, preferably at least 20, particularly at 
least 50, especially at least 100, e.g. 100 to 160. The ratio of the 
external surface area to the polymer volume is at least 20 inch.sup.-1, 
preferably at least 40 inch.sup.-1, Particularly at least 40 to 100 
inch.sup.-1, e.g. 55 to 75 inch.sup.-1. In calculating this ratio, the 
surface area of both sides of the tape is used. The useful tape has a 
thickness of 0.005 to 0.150 inch (0.013 to 0.381 cm), preferably 0.005 to 
0.075 inch (0.013 to 0.191 cm), particularly 0.005 to 0.050 inch (0.013 to 
0.127 cm), e.g. about 0.010 to 0.030 inch (0.025 to 0.076 cm), a thickness 
which allows the tape to exhibit excellent toughness and flexibility. The 
width of the tape, as measured between the electrodes, is 0.5 to 2 inches 
(1.27 to 5.08 cm), preferably 0.75 to 1.75 inches (1.91 to 4.44 cm), 
particularly 1.0 to 1.5 inches (2.54 to 3.81 cm). Generally the tape is of 
uniform width and thickness, but can be of non-uniform width and/or 
non-uniform thickness, e.g. corrugated, ribbed, or grooved. 
The heating element also comprises elongate electrodes which are secured to 
opposite edge portions, i.e. margins, of the resistive element and which 
can be connected to a source of electrical power to cause current to pass 
through the resistive element. While most heating elements are designed 
with two electrodes, there may be any number depending on the power source 
and electrical configuration. The electrodes may be partially or 
completely embedded in the conductive polymer element, or they may be 
attached to one surface or opposite surfaces of the resistive element, 
preferably on the same surface. In this embodiment, substantially all of 
the current flows in the plane of the laminar element and little or none 
of the heated portion of the laminar element is covered by the electrodes 
so that heat is generated in the section between the electrodes. The 
electrodes may comprise any convenient material, e.g. a flexible wire, a 
conductive ink, a metal foil such as electrodeposited copper or nickel, or 
a combination of these, e.g. a metal foil attached to the resistive 
element by means of a conductive silver ink. In a preferred embodiment, 
the electrodes comprise a metal layer, e.g. a metal braid or apertured 
metal foil, surrounding a core of adhesive, particularly conductive 
adhesive. If the electrode is heated, e.g. from an external source or 
through I.sup.2 R heating, while in contact with the conductive polymer 
resistive element, the adhesive will melt and flow through the interstices 
of the metal layer to contact and bond to the resistive element. In some 
cases, where excellent flexibility or very low contact resistance is 
required, it is desirable to attach the adhesive to an intermediate layer 
such as a layer of silver paint, a conductive epoxy, or a resilient, 
deformable conductive material. Electrical leads may be attached to each 
electrode to connect them to a power source. In an automobile or other 
vehicle, the power source is commonly the battery, although another power 
supply may be used. 
The heating element may optionally be covered with an insulating jacket 
layer in order to provide electrical insulation and environmental 
protection. 
At least any surfaces of the support member which are contacted by the 
electrodes or heating element are composed of electrically insulating 
material. Preferably the support member is in the form of a flat sheet of 
electrically insulating material. Suitable materials include woven or 
nonwoven fabrics, e.g. felt, fiberglass, or nylon cloth, polymeric sheets, 
e.g. foam or polymer-impregnated fabrics, and cardboard or other 
reinforced paper. If the support member comprises a polymer it is 
preferred that the melting point of the polymer be greater than the 
temperature reached during normal operation of the heating element. The 
support member may be of any desired shape depending on the application 
and frequently it is preferred that the shape conform to the area to be 
heated. A suitable support member may have any thickness, although for 
flexibility, a thickness of less than about 0.500 inch (1.27 cm), 
preferably less than 0.250 inch (0.635 cm), particularly less than 0.100 
inch (0.254 cm), e.g. 0.020 to 0.070 inch (0.051 to 0.178 cm) is 
preferred. The heating element is mounted on, wrapped around, or otherwise 
in contact with the support member. In a preferred embodiment, the tape is 
wrapped around the support member, i.e. laid out in a folded zigzag 
pattern with the support member separating the folds of the tape. In this 
design, the pitch of the tape, i.e. the distance between every two 
adjacent folds, is dependent on the thickness, width, and flexibility of 
the tape, as well as the desired power density. It has been found, for 
example, for a tape with a width of one inch, a pitch of 5 to 6 inches 
(12.7 to 15.2 cm) is suitable for a support member with dimensions of 
approximately 6 by 10 inches (15.2.times.25.4 cm). The pitch would 
normally be greater for a tape with less flexibility. A balance of useful 
heat output and flexibility is achieved in many applications when the area 
of coverage on the support member by the heating tape is about 50 to 75%. 
For optimum heat transfer, the tape is positioned on the support member 
with the electrodes facing away from the support member. This is 
particularly important when there is no insulating jacket on the tape in 
order to prevent electrical contact of the wires at any cross-over points 
of the heater, e.g. at the edges of the support member. The tape may be 
attached to the support member by any suitable means, e.g. stitched, 
stapled, or glued. For ease of fabrication a spray-on adhesive may be 
preferred. If metallic staples are used, it is necessary to avoid 
disturbing the electrical connections and avoid shorting to the 
electrodes. The flexible heater may be covered with an insulating jacket. 
It is preferred that the jacket, as well as the support member, be 
permeable to moisture, in order to allow any moisture, e.g. perspiration, 
to pass through the seat. 
A plurality of individual flexible heaters can be attached to or sandwiched 
between a substrate or substrates if more than one distinct area must be 
heated or if the size of one flexible heater is insufficient to heat the 
entire area. When the heater is designed to heat people sitting in a seat, 
individual flexible heaters can be positioned only in those areas likely 
to be in contact with the person, thus reducing power requirements for the 
heater. Like the support member, the substrate may be in the form of a 
sheet. The flexible heater can be glued, stapled, sewn, or otherwise 
attached, to the substrate. The individual flexible heaters can then be 
electrically connected by soldering, crimping, or other attachment 
methods, or else can be individually powered. It may be desirable to 
supply separate power to each heater if, for example, one section of the 
heater must be constantly heated, but other sections require heat only 
intermittently. 
As an alternative to being wrapped around a support member, the heating 
element may be at least partially embedded in a resilient polymeric foam. 
If, for example, the heating element is positioned in a desired 
configuration in a mold, a foamable polymeric composition could be poured 
into the mold. Upon curing, the heating element would be correctly fixed 
and the shaped, molded article could be incorporated directly into a seat 
or other element to be heated. 
In a particularly preferred form the flexible heater is part of an 
upholstered seat for use, for example, in an automobile, boat, plane, 
snowmobile, or other vehicle. The seat comprises a back portion and a seat 
portion, the back portion constructed of a resilient back member which is 
covered by a back cover, and the seat portion constructed of a resilient 
seat member which is covered by a seat cover. In general, the back cover 
and the seat cover are the sections in contact with the passenger or 
person sitting. They may be made of leather, vinyl, cloth fabric, or some 
combination of these. The flexible heater may be positioned between the 
resilient seat member and the seat cover, between the resilient back 
member and the back cover, or both. For ease of construction, individual 
flexible heaters may be used in the back portion and the seat portion, but 
one flexible heater alone may be suitable for both portions. For many 
automotive applications, sufficient passenger comfort is provided by 
positioning a flexible heater in the back portion alone. Under these 
circumstances, it is preferred that the total surface area of the 
resistive element is 50 to 100 inch.sup.2 (323 to 645 cm.sup.2). In an 
automobile seat, the power source for the flexible heater is usually the 
car battery, and the heater is normally connected by a switch to an 
electrical lead connected to the battery. For the convenience of the 
passenger, a control unit which allows control of the amount of heat 
produced by the flexible heater is generally mounted next to the seat. A 
thermostat may also be used. 
The precise width and thickness requirements of the heating element for a 
given application are determined by the available voltage and the desired 
power density of the tape. This power density, in turn, is dependent on 
the highest permissible temperature. Because the area of coverage on the 
support member by the heating tape (as determined by measuring both 
laminar surfaces of the support member) is about 15 to 40% for most 
applications, i.e. substantially greater than the coverage on conventional 
wire heaters, the heater can operate at a lower temperature, providing 
improved efficiency and safety. 
The invention is illustrated by the drawing in which FIG. 1 shows, in 
perspective, a heating element 1 in the form of a tape. Two elongate 
electrodes 5,7 are positioned on one surface of the resistive element 3 
near the edge. No electrically insulating jacket layer over the heating 
element is shown, but for some applications, this would be desirable. 
FIG. 2 shows a plan view of a flexible heater 9 of the invention. In this 
embodiment, a heating element 1 is wrapped in a zigzag manner around a 
support member 11. The electrodes 5,7 of the heating element 1 are 
positioned away from the support member 11 in order to avoid electrically 
shorting out. FIG. 3 is a cross-sectional view along line 3--3 of FIG. 2 
and shows sections of the zigzagged heating element 1 separated by the 
support member 11. 
FIG. 4 is a plan view of an embodiment of the invention comprising a 
flexible heater 13 which is suitable for heating a substrate, e.g. an 
automotive seat back or automotive seat base. In this design, two flexible 
heaters 9 are positioned between two sheets of a felt cover 15, i.e. a 
substrate. An electrical lead 17, suitable for connection to a source of 
electrical power, e.g. a battery, connects the two flexible heaters 9. 
FIG. 5 is a cross-sectional view along line 5--5 of FIG. 4 and shows the 
two flexible heaters 9 sandwiched between the felt cover sheets 15. 
FIG. 6 is a partially cut-away perspective view of a shaped article 19 of 
the invention. In this embodiment, the heating element 1 is embedded in a 
resilient polymeric foam 21. 
FIG. 7 is a partially cut-away perspective view of an upholstered seat 23 
of the invention. In this embodiment, two flexible heaters 13 are 
positioned to heat the seat, one on the base of the seat between the 
resilient seat member 25 and the seat cover 27, and one on the back of the 
seat between the resilient back member 29 and the back cover 31. 
The invention is illustrated by the following example. 
EXAMPLE 
A conductive polymer composition was prepared by dry-blending in a high 
speed blender 95 parts by volume of ultra high molecular weight 
polyethylene powder, UHMWPE (Hostalen.TM. GUR-413, available form American 
Hoechst), having a molecular weight of about 4.0 million and an average 
particle size of about 0.1 mm, and 5 parts by volume of carbon black 
(Ketjenblack.TM. EC 300 DJ, available from Akzo Chemie). The mixture was 
extruded through a ram extruder heated to 200 to 225.degree. C. at a rate 
of 5 feet/hour (1.52 m/hour) and a pressure of 3000 psi (2.07 MPa) to 
produce a sintered tube with an outer diameter of 8 inches (20.3 cm) and 
an inner diameter of 5.25 inches (13.3 cm). After cutting into 6 inch 
(15.2 cm) lengths, the tube was skived to produce a 0.010 inch by 6.0 inch 
(0.025 by 15.2 cm) element. This element was slit into four equal strips, 
each with a width of 1.5 inches (3.81 cm). 
A conductive adhesive composition was prepared by mixing 89.5% by weight 
acrylic grafted polyolefin resin (Polybond.TM. 1016, available from 
Polymer Industries), 9.5% by weight carbon black (Ketjenblack.TM. EC 600, 
available from Akzo Chemie), and 1% antioxidant in a Banbury.TM. mixer. 
The mixture was pelletized and the pellets were then extruded to produce a 
solid rod with a diameter of 0.025 inch (0.064 cm). Electrodes were 
prepared by flattening 30 AWG silver-coated copper wire to give a 
cross-section 0.003 by 0.013 inch (0.008 by 0.033 cm), and then braiding 
twelve flattened wires around the conductive adhesive core. 
A laminar heating element as shown in FIG. 1 was prepared by attaching two 
electrodes to the surface of a conductive polymer strip. The electrodes 
were positioned 1 inch (2.5 cm) apart on the surface of the conductive 
polymer strip and were pressed against the strip while passing a current 
of 25A per electrode through each electrode. As the electrodes heated, the 
adhesive melted and swelled through the interstices of the braided wires, 
thus attaching them to the polymer strip. 
A heater cell was prepared by attaching a 20 inch- (50.8 cm-) long strip of 
heating element to a piece of felt measuring approximately 
0.030.times.6.times.10 inches (0.076.times.15.2.times.25.4 cm) by means of 
a pressure sensitive adhesive. The heating element was positioned as is 
shown in Fioure 2, by folding the heating element in a zigzag pattern with 
a pitch of about 6 inches (15.2 cm) over the edge of the shorter end of 
the felt. The side of the heater with the electrodes was positioned away 
from the felt. The heating element covered approximately 35% of the total 
area of the heater cell. The electrodes of a first heating cell were 
soldered to the electrodes of a second heating cell and the two heating 
cells were then sandwiched between and attached with a pressure sensitive 
adhesive to two pieces of felt cut as shown in FIG. 4. The resulting 
heater had dimensions of approximately 0.080.times.14.5.times.21.5 inches 
(0.203.times.36.8.times.54.6 cm). An electrical lead was soldered to the 
electrodes of the first heating cell to provide electrical connection to a 
power source. When powered at 12 volts, the heater supplied about 24 watts 
of power. 
Although the specific embodiments disclosed in this specification have been 
directed to automobile or vehicle seats, it is to be understood that 
heaters of the invention can be used to heat any type of surface, e.g. 
home or office furniture.