GB2196818A - Heating pad - Google Patents

Abstract

A flexible heating pad comprising a sandwich composed of outer flexible insulating layers 16 of elastomeric material bonded to or united to, and sandwiching a conductive layer 18 of elastomeric material. Embedded within the pad and extending lengthwise thereof is a pair of metallic conductors 20 spaced apart by the conductive elastomer. Preferably these metallic conductors are in the form of two parallel wires spaced on either side of the longitudinal axis of the pad. Also it is preferred that a number of parallel conductor wires or strips, also parallel to the two parallel wires be embedded within the conductor layer so as to avoid the incidence of hots spots in the conductive elastomer. <IMAGE>

Description

SPECIFICATION Electrical heaters This invention relates to electrical heaters and in particular those which are in the form of a pad or sheet of material and so of almost two-dimensional appearance.

Such heating elements are known and have been formed by, for example, embedding a resistance wire or a metal foil in a thin sheet of insulating elastomer. By way of example, the elastomer had been a silicone elastomer capable of withstanding the high temperatures produced. Such heating elements find uses in the underfloor heating of rooms and as heating elements in hot trays on which food can be kept warm.

Existing heating elements of this type however have to be formed of a predetermined size since the heating element itself is a discrete length of electrical resistance wire or pattern of foil and obviously the continuity of this cannot be disrupted.

It is an object to the present invention therefore to provide heating pads which can be made of an indefinite length but can thereafter be cut to a suitable length by the user so as to suit the particular end use.

According to the invention, a heating pad comprises a sandwich composed of outer flexible insulating layers of elastomeric material bonded or united to, and sandwiching a conductive layer of elastomeric material and, embedded in the pad and extending lengthwise of the pad, a pair of metallic conductors which are spaced apart by the conductive elastomer so that, when a potential is applied across the two conductors, a current passes through the conductive elastomeric layer to provide the necessary heating output.

Such a pad can be of indefinite length and thereafter cut at any desired point along the length to give a particular chosen end user length. The cutting will of course sever the electrical conductors but the electrical continuity of the resistance heating path will -not in any way be disrupted thereby.

Also, because the conductors will generally be spaced a consistent distance apart lengthwise of the pad, the heat output of the final pad for a given voltage will be proportional to the length. Therefore changing the length by cutting the pad will not in any way affect the recommended electrical voltage to be applied since, although the resistance across the two conductors will vary inversely of the length and the current will be greater the greater the length, the current per unit length will stay consistent. Therefore, provided the end user applies the correct voltage to the final pad, there will be no risk of overheating or subjecting any pad produced by the end user to unsafe electrical conditions.

One particular use for a pad according to the invention can be the trace heating or warming of water pipes to prevent freezing.

Thus the pad can be formed as a long strip and the strip helically coiled around and along a pipe and the length of pad cut to the desired final length.

According to one embodiment of the invention, the electrical conductors are in the form of two parallel wires spaced on either side of the longitudinal axis of the pad as formed.

These wires are embedded in the layer of conductive elastomeric material and so in contact with it and apply the input voltage across the material of the conductive elastomer.

In such an embodiment, we have found according to one preferred feature of the invention that the life of such a pad can be substantially improved by embedding within that conductive layer a number of parallel conductive wires or strips which are also parallel to the two input wires. These conductive strips can be in the form of thin wires or the like and each is separate from the other and merely connected to one another by being embedded in the conductive elastomer. We have found that by spacing these conductive wires or strips substantially evenly across the voltage drop between the input wires, this ensures a correct spreading of the voltage applied throughout the mass of the conductive elastomer and reduces the incidence of hot spots or the like where a short circuit is likely to occur with a resulting chance.of damage to the pad and burning out of the pad.

According to another embodiment of the invention however, two metallic webs are sandwiched within the overall sandwich and the webs are positioned on either side of the layer of conductive elastomer. These webs are then used to apply the input voltage across the elastomer and in such an embodiment the conductive path passes across the relatively thin dimension of the conductive elastomer layer in contrast with the embodiment where a pair of wires are embedded on opposite side edges of the elastomer in which case the conductive path passes across the transverse width of the conductive elastomer layer.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of one form of heating pad according to the invention; Figure 2 is a section through the pad taken on the line X-X of Fig. 1; Figure 3 is a diagram of a modified heating pad; Figure 4 is a section taken on line X-X of Fig. 1 through an alternative form of heating pad according to the invention similar to that of Fig. 3; and Figure 5 is a section taken on the line X-X of Fig. 1 through a further form of heating pad according to the invention.

The heating pad 10 according to the inven tion and shown in Fig. 1 is in the form of a sheet of material of length L and width Y. The sheet is flexible and has been made of elastomeric material. A pair of input electrical leads 12 and 14 are provided across which the input voltage is applied.

The sheet 10 has been made up of a number of layers of material as will be described but the layers are all united and bonded to one another so that in effect the overall sheet has the appearance ahd feel of a single sheet of elastomeric material.

Referring to Fig. 2, the sheet 10 is made up of two outer layers of insulating elastomeric material 1 6 and sandwiched between these two layers and wholly enclosed by them is an intermediate conductive layer 18, again of elastomeric material. At either side of the transverse width Y of the sheet are embedded uninsulated metal wires 20. These wires are connected to the input leads 12 and 14 and are embedded in the edges of the conductive layer 18. They are therefore enclosed by the outer layers 16 and insulated thereby.

Therefore, when a voltage is applied across the input leads 12 and 14, current passes from one wire 20 to the other through the mass of the conductive elastomeric layer 18 to provide the required heat output.

Preferably the elastomeric material of the layers 16 is a silicone rubber which is capable of withstanding the operating temperatures best. However, the insulating layer could be a cloth layer or an elastomeric layer made of polyvinyl chloride, polyester rubber, nylon or paper. The thickness of the layers 16 can be varied but will be chosen according to the dielectric strength of the material to provide the necessary insulating properties.

Preferably the conductive layer is also made of a silicone rubber since this will then bond well with the layers 16 when they are also made of silicone rubber. The conductive properties of the elastomer can be imparted by including in the rubber a finely ground or divided powder of conducting material such as carbon black, graphite or metallic particles such as silver. The amount of conductive filler will vary the resistivity of the conductive layer 18 and this is chosen according to the desired heat output and input voltage.

Silicone electronic materials are available in a conductive form containing a conductive filler and in an insulating form and, by mixing such materials, a final conductive material of a desired resistivity for a particular usage can easily be prepared.

An example of a suitable elastomeric material which can be used for the layers 16 and 18 is the silicone rubber compound, methyl phenyl vinyl siloxane gum (Toray SRX-539U-6759U). This can be cured by dicumyl peroxide and contains 40% precipitated calcium carbonate.

Fig. 3 shows a conductive pad 30 which is similar to the conductive pad 10 but has been made in the form of an elongated strip where the length L in Fig. 1 can be of an indefinite length.

As can be seen, it is composed of the three layers 16 and 18 which are diagrammatically shown successively peeled back in Fig. 3 to reveal these three layers 16 and 18 and the pair of conductor wires 20 are sandwiched between two metallic webs 42 on either side of it. In turn the webs 42 are sandwiched between the respective outer insulating layer 16 and the layer 18. These webs are separate from one another and each is respectively connected to one of the input leads 12 or 14.

In this embodiment of heating pad 40 as shown in Fig. 5, the current passes across the narrow width of the layer 18 but the metallic webs ensure that the current is spread evenly throughout the conductive layer and so prevent preferantial routes for the electrical current which might form hot spots and burn out the material of the layer 18.

Claims (6)

1. A heating pad comprising a sandwich composed of outer flexible insulating layers of elastomeric material bonded or united to, and sandwiching a conductive layer of elastomeric material and, embedded in the pad and extending lengthwise of the pad, a pair of metallic conductors which are spaced apart by the conductive elastomer so that, when a potential is applied across the two conductors, a current passes through the conductive elastomeric layer to provide the necessary heating output.

2. A pad as claimed in Claim 1 in which the conductors are spaced a constant distance apart width-wise of the pad.

3. A pad as claimed in Claim 1 or Claim 2 in which the electrical conductors are in the form of two parallel wires spaced on either side of the longitudinal axis of the pad, these wires being embedded in the layer of conductive elastomeric material and so in contact with it.

4. A pad as claimed in Claim 3 in which a number of parallel conductive wires or strips which are substantially evenly spaced and parallel to the two input wires are embedded within the conductive layer.

5. A pad as claimed in Claim 1 in which two metallic webs are sandwiched within the overall sandwich and the webs are sandwiched within the overall sandwich and the webs are positioned on either side of the layer of conductive elastomer, these webs being used to apply the input voltage across the elastomer so that the conductive path passes across the relatively thin dimension of the conductive elastomer layer.

6. A heating pad substantially as herein described with reference to Figs. 1 and 2, Fig.

3, Fig. 4 or Fig. 5, of the accompanying drawings.