WO1993024300A1

WO1993024300A1 - Method of making an electroded laminated article

Info

Publication number: WO1993024300A1
Application number: PCT/GB1993/001046
Authority: WO
Inventors: Willy Maurice Van Esch
Original assignee: N.V. Raychem S.A.; Raychem Limited
Priority date: 1992-05-22
Filing date: 1993-05-21
Publication date: 1993-12-09
Also published as: GB9210954D0

Abstract

A method is described for making a laminated article which is to be electrically powered to heat the article via one or more electrodes (10) sandwiched between first and second layers of the article. The method involves bonding an electrode to an outer surface of the first layer (6, 8) then pulling the electrode and the adjacent region of first layer away from the remainder of the article, and inverting the removed electrode and first layer section to achieve the sandwiched construction.

Description

Method of making an electroded laminated article

This invention relates to a method of making a laminated article that has an electrode sandwiched between a first layer and a second layer of the article.

It is known to make laminated articles that include an electrically powered heating layer. The article may itself function as a heater, or the heating layer may be provided to supply heat to the article itself for some reason, for example, where the article is a heat-recoverable article, to effect heat recovery of the article.

A heat recoverable article is one whose dimensional configuration can be made to change substantially when subjected to heat treatment. A heat recoverable article will usually recover towards an original shape from which it has previously been deformed but the term as used herein also includes articles which adopt a new configuration when subjected to heat treatment, even if it has not previously been deformed.

One example of a heat recoverable article including an electrically powered heating layer is described in EP-0245067 A (N V Raychem S A B129). This comprises a layer of heat recoverable polymeric material for forming a bond between the recoverable material and an object, and an aperture laminar metal heating element (e.g. in the form of a mesh) built in or between the layers. When heat is applied by the metal element it causes the recoverable material to deform the heating element and force the bonding material against the object. The disclosure of EP 0245067 is incorporated herein by reference.

In an article, such as that described in EP 0245067 including a metal heating element, it is convenient to use electrodes to connect the element to a power source. A typical electrode, described in EP-0245067 is in the form of a piece of a foil or a braid. The electrode must be fixed to the metal heating element in some way. One way of doing this is to expose part of the heating element after it has been laminated with the layers of heat recoverable material and of bonding material, by removal of a portion of one of those layers e.g. by scraping, or solvent. This method, which is described in EP- 245067, is successful, but must be carried out carefully to avoid damaging the heating element. It also desirably requires re-insulation of the heating element in the exposed region to avoid danger to the installer of the heat recoverable article when it is powered in the field. Another way of connecting the electrode to the heating element is to laminate it between the heating element, and either the bonding layer or the recoverable layer as part of the lamination step, and arranging for the electrode to project beyond the edges of the other layers so it can easily be connected to a power source. While this again is a successful process, it requires the lamination procedure to be set up to produce finished articles of a predetermined size. In contrast, if the electroding stage is the final stage it means that large sized laminated articles can be made on a single machine run, cut to desired field installation sizes at a later stage, and then electroded as appropriate. Thus inventory is reduced and machine efficiency increased.

We have discovered a new method of making a laminated article including an elongate electrode sandwiched between two layers of the article, where the electrode is applied as the last step in a lamination process, and therefore avoids the problem of having to make laminated articles of predetermined size. The method can be used inter alia to electrode the article described in EP-0245067.

A first aspect of the invention provides a method of making a laminated article that has an elongate electrode sandwiched between a first layer and a second layer of the article, the method comprising:

(a) bonding the first layer to the second layer;

(b) bonding the electrode to the free surface of the first layer such that the bond between the electrode and the first layer is stronger than the bond between the first layer and the second layer;

(c) applying a stress to pull the electrode and that portion of the first layer that is in contact with the electrode away from the second layer; (d) inverting the electrode and that portion of the first layer bonded thereto, and then bonding it again to the second layer so that the electrode is sandwiched between the first and the second layer.

One or more electrodes may be applied in this way. Typically two electrodes are applied, one of which is connected in use to a positive terminal of a power supply, and the other of which is connected to a negative terminal of a power supply.

Generally the surface area of the electrode in contact with the first layer is much smaller than the overall surface area of the first layer, e.g. the elongate electrode extends along one strip of the rectilinear sheet first layer. Therefore only that portion of the first layer in contact with (or in the immediate vicinity of) the electrode is pulled away from the second layer, and over the remaining surface area the first layer remains bonded to the second layer.

The terms first layer and second layer do not preclude each of them comprising two or more layers, that may be for example laminated together.

One of the first and second layers preferably comprises a heat recoverable layer. The other of the first and second layers preferably comprises a layer of a bonding material for forming a bond between the heat recoverable layer and an object to be covered. The layer of bonding material may be of the type described in EP-0245067. Details of suitable materials are described later in this specification.

Preferably one of the first and second layers also comprises an electrically conductive layer. This is preferably of the type described in EP- 0245067, i.e. a laminar metal heating element having a plurality of apertures through its thickness to permit the element to be deformed in its plane by a change in shape of the apertures, e.g. a metal mesh or brass, aluminium, copper, steel, bronze or nickel. The electrically conductive layer is positioned on that surface of the first or second layer so that in the finished article it is positioned between at least part of the heat recoverable layer and at least part of the layer of bonding material (although not necessarily in direct contact with either), and is in electrical contact with the electrode. In addition a further layer, which we shall call a connection layer may be included. This is preferably adjacent to the electrically conductive layer over at least part of the surface of the electrically conductive layer in the finished laminated article. Where the electrically conductive layer is to promote bonding between the layers of material between which the aperture conductive layer is positioned. This therefore helps to maintain the position of the conductive layer during installation of the product. A suitable connection layer is described in EP-0245067 where it is similarly described as a connection layer. More details about his composition are given later.

In a particularly preferred method according to the invention, the said first layer preferably comprises a bonding layer laminated to a connection layer, and the second layer preferably comprises an apertured copper mesh heating conductive layer bonded to a heat recoverable polymeric layer. According to the method the layers are preferably initially bonded to each other so that they appear in the following sequential order: heat recoverable layer, mesh heating layer, connection layer, bonding layer. To achieve this sequence the layers may be relatively bonded to each other in any order timewise, or subsequently simultaneously. In one embodiment the connection layer is first bonded to the connection layer, then the mesh bonded to the previously bonded connection layer, and then the heat recoverable layer bonded over the mesh. Heat is preferably applied to form the bonded laminate structure. The heat is preferably applied by infra-red heaters. Pressure may also be applied.

The electrode may also be simultaneously applied or may be subsequently applied to the free surface of the bonding layer so it is bonded thereto. This is preferably achieved by electrical heating and pressure, as described in more detail below. Then the electrode is pulled away from the layers, taking the underlying region of bonding layer and connection layer with it. The removed piece is then turned over (i.e. inverted) and replaced in the position from which is was removed. The result is therefore that in the region the order is connection layer, bonding layer, electrode, conductive mesh, heat recoverable layer. This means that immediately beneath the electrode the innermost layer (if the article is to be used to cover a substrate) is not the bonding layer. For most applications this is acceptable since on heating to effect recovery and melt the bonding layer, the bonding layer flows, from the area of the article on either side of the removed/ inverted /replaced electrode region, to cover the region of the inner surface where bonding material is missing. If desired, additional bonding material can be applied to cover that region.

Turning now to the materials used. Polymeric materials that may be used for the heat recoverable layer, and for fusion bonding layer include, for example polyethylene, polypropylene, polybutene, copolymers of ethylene, propylene, butene and hexene, copolymers of ethylene with ethyl acrylate, vinyl acetate, acrylic acid, methacrylic acid, acrylic esters or methacrylic esters in which polyethylene predominates, blends of these polymers, and blends of these polymers with elastomers.

A particularly preferred material for the layer of heat-recoverable material comprises polyethylene especially a high density polyethylene. Particularly preferred materials for the layer of fusion bonding material include polyethylene, ethylene vinyl-acetate copolymer, and ethylene ethyl- aαylate copolymer. For the connection layer a softenable polymer, for example the ethylene-ethyl acrylate copolymer sold under the trademark DPD 6181 can be used. The connection layer may be lightly cross-linked to minimise risk of delamination when heated to effect recovery.

In order to achieve the desired bonding between the layers of the laminate including the conductive layer, and also bonding to the electrode, heat and pressure are preferably applied. In order to bond the component layers of the first and second layer together, and the first and second layers to each other, heat is preferably applied by infra red heaters. Pressure may be applied by platens, e.g. with pressures of from 5 to 20 bar. In order to bond the electrode to the layers, pressure in the range 5 to 20 bar, preferably 8 to 12 bar is preferably applied using a platen press. The pressure is preferably applied for a minimum of 20 seconds. While this pressure is applied, heat is preferably also supplied. This may conveniently be done by connecting the electrodes to a power source and supplying a current through the electrodes in the range 200-300 amps, preferably about 240 to 250 amps at 60 volts for from 7 to 13 seconds, preferably about 10 seconds. This applies a watt density of about 1500W/cm2. The pressed /heated laminate is then preferably allowed to cool for a period before the electrode and that portion of the first layer in contact with the electrode is pulled away from the second layer. The laminate may simply be allowed to cool for the period, or may be actively cooled, e.g. by a water cooling system. Preferably the cooling period is at least 7 seconds and at most 15 seconds, preferably about 10 seconds.

The electrode may take any suitable form. For example it may be in the form of a solid bus bar. Preferably the electrode is in the form of a metallic braid, e.g. comprising copper or brass. Preferably the electrode is flexible.

Reference is made in the specification to "the" electrode, since the method is applicable to applying one or more electrodes in a laminate structure. It will be appreciated that in general there will be more than one electrode, typically a pair (connected to positive and negative terminals of a power source) or pairs of electrodes.

In a preferred embodiment the electrode comprises a braided material filled with a polymeric material. Any of the materials suggested for the bonding material are suitable for use as the filling material for the braid. The advantage of using a filling material is to enhance water or moisture impermeability which might impair electrical behaviour, or be otherwise disadvantageous.

Embodiments of the present invention will now be discussed, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a perspective view of an article at an intermediate stage in the process of the present invention;

Figure 2a is a cross-sectional view through part of the article of Figure 1; and Figure 2b is a cross-sectional view through the same part of the article at a subsequent stage in the process;

Figure 3 is a cross-sectional view through the same part of the article as shown in Figure 2, at the end of the process of the present invention;

Figure 4 is a perspective view of the article that is partly shown in Figure 3; and Figure 5 is a cross-sectional view showing an application of the article shown in Figures 1-4.

Referring now to the drawings, Figures 1 and 2 show a laminate structure of several generally rectangular layers. A heat recoverable polyethylene backing layer 2 is bonded to and through an electrically conductive copper mesh 4. Bonded on the other side of, and through the mesh 4 is a connection layer 6 comprising a lightly beamed EEA, and to that is bonded a bonding layer 8 comprising a non-cross-linked EEA.

The heat recoverable backing layer is recoverable by shrinkage in the direction indicated in Figure 1 by arrow R. Two electrodes 10 extend along opposite edges of the bonding layer in a direction substantially perpendicular to the recovery direction R. The individual layers 2, 4, 6 and 8 of the laminate, and the electrodes 10 are subjected to a temperature of 80-200°C and a pressure of 20000N/ cm² to form the initial laminate structure as shown in Figures 1 and 2. The electrodes 10 project at their far ends 12 beyond the edges of the laminate structure.

In the next stage of the process as shown in Figure 2b each electrode 10 is pulled away from the laminate. The bond between the electrode 10 and the bonding material 6 and connection layer 8 is greater than the bond between the connection layer 6 and the mesh 4. Therefore the region of layers 6 and 5 in contact with the electrodes are also pulled away with the electrode (see Figure 2b).

Finally the withdrawn portion (electrode 10, and layers 6 and 8 is inverted and replaced inverted in the region from which it is withdrawn. Again it is subjected to pressure and temperature, similar to that already described, to bond the electrode in place between the polymeric layers 6 and 8 and the mesh 4. The resulting structure as shown in Figures 3 and 4 comprises electrodes 10 in contact with mesh layer 4 and projecting beyond the edges of the rectangular laminate for easy connection to a power source.

Figure 5 is a cross-sectional view showing the article of Figure 4, designated by the general reference numeral 20, wrapped around a joint between district heating pipes 22 and a central casing 24. Each pipe 22 comprises a central steel pipe 26, foam insulation 28 and a polyethylene jacket 30. The electrodes 10 project beyond the edges of the article 20 and when connected to a source of electrical power cause the article 20 to heat and hence to shrink into contact with the pipes 22 and casing 24 to form a joint. The pre- shrunk configuration of the article 20 is shown on the left of Figure 5, and the shrunk configuration 20' is shown on the right of the Figure 5. Heating is supplied via the mesh 4. As mentioned above the connection layer 8 supports the mesh on heating and impedes movement or "floating" of the mesh during the shrinkable process. Although the connection layer is not in contact with the mesh in the region immediately below the electrode, it is in contact over the remainder of the article, so that function of the connection layer is only slightly impaired. Also the bonding layer 6 is not on the inner surface immediately below the electrode 10. However on heating and shrinking the bonding material from the adjacent regions of the inner surface melts and flows to form a good bond.

Claims

1. A method of making a laminated article that has an elongate electrode sandwiched between a first layer and a second layer of the article, the method comprising:

(a) bonding the first layer to the second layer;

(c) applying a stress to pull the electrode and that portion of the first layer that is in contact with the electrode away from the second layer;

(d) inverting the electrode and that portion of the first layer bonded thereto, and then bonding it again to the second layer so that the electrode is sandwiched between the first and the second layer.

2. A method according to claim 1, wherein the second layer comprises a heat recoverable layer, and the first layer comprises a bonding material for forming a bond between the heat recoverable layer and an object to be covered.

3. A method according to claim 1 or 2, wherein one of the first and second layers, preferably the second layer comprises an electrically conductive layer.

4. A method according to claim 3 comprising a connection layer adjacent to the electrically conductive layer.

5. A method according to any preceding claim, wherein the first layer comprises a bonding laver laminated to a connection layer, and the second layer comprises an apertured copper mesh heating conductive layer bonded to a heat recoverable polymeric layer.

6. A method according to any preceding claim, wherein the component layers are bonded to each other in any order or substantially simultaneously.

7. A method according to any preceding claim, wherein heat and pressure are applied to bond the first and second layer to each other or components of the first and second layer to each other or to bond the electrode to the first layer.

8. A method according to claim 7 wherein the heat to bond the first and second layer to each other or components of the first and second layer to each other is provided by an infra-red heater.

9. A method according to claim 7 or 8, wherein the heat to bond the electrode is supplied electrically via the electrode.

10. A method according to any preceding claim, wherein the laminate is cooled for a period after bonding prior to step (c) of pulling the electrode away from the second layer.