Technical area The present invention relates to a covering membrane sheet adapted to be joined to at least one other covering membrane sheet for covering a surface and comprising a sheet of elastomeric material and, attached to an edge area of the sheet of elastomeric material, a strip of thermoplastic film of a material being adhesively compatible with said elastomeric material. The invention further relates to a method of manufacturing a covering membrane sheet of the above mentioned type. The invention also relates to a method of forming a laminate seam using two covering membrane sheets of the above mentioned type.

Background art Polymeric covering membrane sheets are used as a single ply covering membrane for covering roofs. Such membranes are generally applied to the roof surface in a vulcanised or cured state. Because of outstanding weathering resistance and flexibility cured elastomeric roof sheeting has rapidly gained acceptance. Polymeric covering membrane sheets may also be used as geo membranes for forming waterproof bottoms of ponds, landfills, tanks etc. A problem in relation to the use of elastomers in roof sheeting or geo membrane sheeting is the lack of adhesive affinity between the individual sheets of the sheeting material. This is a serious problem because in applying the sheets to a surface it is usually necessary to splice the sheets together. This splice or seem is usually subjected to both short term and long term stresses, such as those caused by roof movement, heavy winds, freeze-thaw cycling and thermal cycling. Such stresses may manifest themselves in shear forces and peal forces, i.e. the seam peels back under severe stress conditions or may cause a partially open seam (often referred to as a fish-mouth condition) under less severe conditions . In view of the foregoing problem it has been necesarry to utilise an adhesive to bond the elastomer sheets together. As will be evident from the above discussion, an adhesive for bonding elastomer covering sheets together must meet a number of requirements, which are extremely difficult to satisfy. Thus the adhesive must prove sufficient seam peel and sheer strength to permit the splice formed by bonding the elastomer covering sheets together to resist both the short term and the long term stresses as discussed hereinabove. Moreover, the adhesive must be resistant to oxidation, hydrolysis and chemical attack from pooled water. US 4,767,658 to Lorenz describes rubber sheets having integral thermoplastic adhesive strips moulded along the lateral edges of a sheet. A first strip is attached to the upper surface of the sheet, at a first edge thereof, and a second strip is attached to the lower surface of the sheet, at a second edge thereof, the second edge being opposite to said first edge. By positioning the second strip of a first sheet on top of the first strip of a second sheet and applying heat and pressure to said strips a heat joined splice may be obtained. A problem of the splice obtained according to US 4,767,658 is that the free end of the first sheet, i.e. the portion of the first sheet being located adjacent to the second edge of that sheet, is exposed to both weather and mechanical loads, such as persons walking on the roof. There is thus a certain risk that the wind or a shoe getting a "grip" on the free edge may destroy the splice and cause a partially open seam. One solution, which is not entirely satisfying in solving the problem, to the above mentioned problem is indicated in US 4,910,059, which shows a roofing membrane which at its end has been bevelled to reduce the risk of a shoe or a strong wind getting a grip on the free edge and causing stress or delamination of the seam.

Summary of the invention A purpose of the present invention is to provide a covering membrane sheet which in a simple manner provides a limited risk that a shoe, a wind or another object may get a grip on a free edge of the covering membrane sheet when joined to another membrane sheet of the same kind. This purpose is achieved by a covering membrane sheet according to the preamble and characterised in that the strip of thermoplastic film is incorporated into the sheet of elastomeric material so that the outward surfaces of said strip are substantially flushed with the corresponding outward surfaces of said sheet. An advantage of this covering membrane sheet is that, when joined to another covering membrane sheet, the free edge will be well protected against mechanical damage, such as from wind and persons walking on the membrane, and will be very firmly attached to the other covering membrane which minimises the risk of a delamination of a seam. Another advantage of this covering membrane sheet is that it reduces the amount of heat required to join the membrane sheet with another membrane sheet. This is due to the fact that the sheet of elastomeric material is thinner in the area of the strip and thus heat may travel quicker through the sheet of elastomeric material and to the strip and melt it such that it welds together with the strip of the other covering membrane sheet. Yet another advantage is that the covering membrane sheet according to the invention is more elastic than those of the prior art, for instance compared to that of US 4,767,658. The reason for this is that the attachment between the strip and the sheet of elastomeric material is located closer to the centre of the sheet of elastomeric material in the invention. Thus the strains that occur in the attachment between the strip and the sheet of elastomeric material when bending the covering membrane sheet are much lower than in the prior art. This makes the covering membrane sheet of the invention much easier to handle and to shape to the desired shape compared to the membranes of the prior art. A further advantage of this covering membrane is that it will be easy to wind up on a transportation roll since there is no difference in thickness between the edge area and the area outside the edge area. Thus the wrinkles that may occur when winding the prior art roofing membrane of US 4,767,658 on a roll are avoided. Still another advantage is that the inventive covering membrane is very easy to apply to a surface, such as a roof or a ground surface, without causing the "waviness" that may occur in the prior art roofing membrane of US 4,767,658. According to a preferred embodiment the strip is welded to the sheet of elastomeric material at two interfaces being substantially perpendicular to each other. An advantage of this embodiment is that the strip is very firmly attached to the sheet of elastomeric material. Thereby the mechanical resistance is very good against forces acting to tear away the strip, both in a direction parallel to the plane of the covering membrane sheet and in a direction perpendicular to the plane of the covering membrane sheet . Preferably the strip is attached to a notch formed in said sheet. The notch provides a suitable room for attaching the strip to the sheet in such manner that the outward surfaces of the strip are substantially flushed with the corresponding outward surfaces of the sheet. Further the strip is preferably welded to two interfaces being substantially perpendicular to each other and being formed by the walls of the notch. Preferably the thickness of the strip of thermoplastic film is substantially the same as the depth of the notch and the width of the strip of thermoplastic film is substantially the same as the width of the notch. Another purpose of the present invention is to provide an efficient method of manufacturing a covering membrane sheet which provides a limited risk that a shoe, a wind or another object may get a grip on a free edge of the covering membrane sheet when joined to another covering membrane of the same kind. This purpose is achieved by a method according to the preamble and characterised in the following steps: attaching a strip of thermoplastic film to the edge area of one sheet of elastomeric material, which is adhesively compatible with the thermoplastic film material, while said elastomeric material at said edge area is in a substantially uncured state, so as to form a pre-cured edge assembly in which the strip of thermoplastic film is incorporated into the sheet of elastomeric material so that the outward surfaces of said strip are substantially flushed with the corresponding outward surfaces of said sheet of elastomeric material, and curing said elastomeric material of said pre-cured edge assembly at a temperature over the peak melt temperature of the thermoplastic film so as to form a covering membrane sheet having a cured edge assembly. The strip of thermoplastic film is thus incorporated into the sheet of elastomeric material so that the outward surfaces of said film are substantially flushed with the outward surfaces of said sheet. An advantage of this method is that it is efficient and produces a covering membrane sheet that can be joined by a seam to another covering membrane sheet in such manner that the risk of damage to the seam is very small. A further advantage of this method is that, since the outward surfaces of the strip are substantially flushed with the outward surfaces of the sheet, the risk that the covering membrane sheet is exposed to vibrations, shear forces and variations in width when being transported on rollers is reduced as compared to the prior art, such as described in US 4,767,658. Thus the inventive manufacturing method is much less prone to causing operational problems. During the step of curing said elastomeric material of said pre-cured edge assembly, the strip is preferably welded to the sheet at two interfaces being substantially perpendicular to each other. This provides a very firm attachment of the strip to the sheet. Another object of the invention is to provide an efficient way of forming a laminate seam, which is robust towards mechanical damage. This object is achieved by a method of forming a laminate seam, using two covering membrane sheets as described above or made according to the above described method of forming a covering membrane sheet, the method of forming the laminate seam comprising the steps of: bringing the strip of thermoplastic film of one covering membrane sheet into overlapping contact with the strip of thermoplastic film of the other covering membrane sheet, and applying heat and pressure at the overlapping edge area so as to weld together the strips of thermoplastic film of each covering membrane sheet. An advantage of this method is that it provides for a simple attachment of covering membrane sheets to each other and results in a laminate seam which is very robust against mechanical damage. A further advantage is that the problems of "waviness" of the prior art are avoided.

Brief description of the drawing The invention will now be described in more detail with reference to the appended drawing. The drawing figure is a vertical cross-section of two elastomeric roofing membranes and thus shows a first roofing membrane sheet and a second roofing membrane sheet according to one aspect of the invention.

Description of preferred embodiments The figure shows two layers of roofing membrane sheets 10 and 20 that are depicted with the edge area 30 of sheet 10 being overlapped by the edge area 40 of sheet 20. Incorporated into sheet 10 is a strip 50 of thermoplastic film. Strip 50 is attached to and welded with sheet 10 at interfaces 70 and 80. Similarly a strip 60 of thermoplastic film is incorporated into sheet 20. Strip 60 is attached to and welded with sheet 20 at interfaces 90 and 100. As is apparent from the figure the interfaces 70 and 90, respectively, are perpendicular to the interfaces 80 and 100, respectively. Thus the strip 50 and 60, respectively, will be fixed to the respective sheet in such manner, i.e. welded to two interfaces being perpendicular to each other, that it has a very high resistance to forces both in a vertical and horizontal direction, as seen in the figure. As seen in the drawing figure, the outward surfaces of strip 50 and 60 are substantially flush with the outward surfaces of sheet 10 and 20, respectively. This arrangement provides the advantage that the laminate seam will not be thicker than the sum of the joined sheets. In an alternative embodiment the combined thickness of the thermoplastic strip and the elastomer sheet at the edge area may even be less than the thickness of the same elastomer sheet outside the edge area, providing for a laminate seam thickness smaller than the combined thickness of the adjoined elastomer sheets. A method of providing an elastomeric laminate seam between two covering membrane sheets comprises the steps of: attaching a strip 50; 60 of thermoplastic film to the edge area 30; 40 of one sheet of elastomeric material, which is adhesively compatible with the thermoplastic film material, while said elastomeric material at said edge area is in a substantially uncured state, so as to form a pre-cured edge assembly; curing said elastomeric material of said pre-cured edge assembly at a temperature over the peak melt temperature of the thermoplastic film so as to form a cured edge assembly; - bringing the cured edge assembly into overlapping contact with a similar cured edge assembly, with the strips 50; 60 of the thermoplastic films of each respective assembly facing each other; applying heat and pressure at the overlapping edge areas 30; 40 of the cured edge assemblies so as to weld together the strips 50; 60 of the thermoplastic films of each respective assembly. An elastomeric laminate seam between two covering membrane sheets obtained according the above method has been found to have a surprisingly high strength. The reason behind this is not fully known. The present invention also relates to a covering membrane sheet, such as the roofing membrane sheets 10 and 20, joinable by an elastomeric seam of the type described above, the covering membrane sheet being obtainable by: - attaching a strip 50; 60 of thermoplastic film to the edge area 30; 40 of one sheet of elastomeric material, which is adhesively compatible with the thermoplastic film material, while said elastomeric material at said edge area 30; 40 is in a substantially uncured state, so as to form a pre- cured edge assembly; and - curing said elastomeric material of said pre-cured edge assembly at a temperature over the peak melt temperature of the thermoplastic film.

When used for roofing purposes, a first roofing membrane sheet 10 is unrolled over the roof substructure in an otherwise conventional fashion, with the strip 50 of thermoplastic material of the cured edge assembly facing upwards, i.e. away from the roof substructure. A second roofing membrane sheet 20 is then applied onto the roof with its strip 60 of thermoplastic film of the cured edge assembly facing downwards, i.e. towards the roof substructure. The strips 50, 60 of thermoplastic film of the first and the second roofing membrane sheets 10, 20 are arranged to overlap each other, as shown in the figure. Next heat and some pressure are applied to the overlapped edges of the two roofing membrane sheets 10, 20 to form a seam. The overlapping edge areas 30, 40 of the cured edge assemblies may, for instance, be heated by way of hot air welding, and the pressure applied may be hand pressure, in the step of welding together the strips of thermoplastic films of each respective assembly. At the location of the seam, temperature is conveniently applied from, for instance, but not limited to, about 450 to 5500C using a hand-held heating gun and some pressure. Generally, the seam area should be heated to slightly above the melt temperature of the thermoplastic film. Pressure can vary widely, from, for instance, but not limited to, a minimum of about 0.1 bar to as high as 4 bar, typically as long as it is adequate to provide an acceptable, water tight seam. Numerous techniques which utilise heat and some pressure can be used to produce a fully acceptable seam as are known to those skilled in the art. A preferred example of such a technique is to use a heater commonly used for splicing plastic sheets. An example is to use a Leister Varimat hot air automatic welding machine which is available from LEISTER Process Technologies, Sarnen, CH. The Leister Varimat is commonly used for splicing TPO roofing membranes but could preferably be used to splice together the strips of thermoplastic film incorporated in the covering membrane sheets of the present invention. The elastomeric sheeting materials used in the present invention for forming the inventive covering membrane sheets may be any elastomer or elastomer blend with properties suitable for cover sheeting. A non- exhaustive list of such materials may comprise EPDM, EPR or other olefin type polymers. Preferred elastomeric sheeting materials are EPDM type polymers. The thermoplastic film used in the present invention may be any thermoplastic material that is adhesively compatible with the elastomeric sheeting material used. By "adhesively compatible materials" it is meant materials which, at least- upon activation, will seal together at their interface. Preferred thermoplastic materials are thermoplastic olefins (TPO' s), in particular blends of polypropylene copolymers and low density polyethylene. The elastomeric material of the pre-cured edge assembly is preferably cured at a temperature that is about 14O0C over the peak melt temperature of the thermoplastic film. In order to demonstrate practice of the present invention, seams were formed by bonding together layers of elastomeric sheeting material and thermoplastic films. These seems were prepared and subjected to both peel and shear adhesion tests. The elastomeric sheeting compositions employed for these tests were based on EPDM. The thermoplastic films compositions employed for the tests were based on TPO. The compositions exemplified are submitted for the purpose of illustrating the nature of the present invention and are not to be considered as a limitation on the scope thereof. The obtained seems were tested in accordance with Standard Methods EN 12316-2 ("Flexible sheets for waterproofing -Determination of peel resistance of joints - Plastic and rubber sheets for roof waterproofing") and EN 12317-2 ("Flexible sheets for waterproofing - Determination of shear resistance of joints - Plastic and rubber sheets for roof waterproofing") . The test results following EN 12316-2 (peel) was > 3 N/mm2, and according to EN 12317-2 (shear) > 7 N/mm2. The covering membranes described above with reference to the figure are roofing membrane sheets. The covering membrane sheets according to the invention are also useful in geo membrane applications. A geo membrane sheet according to the invention has the same principal design and function as the roofing membrane sheet 10, 20 described above. Examples of technical areas in which the geo membrane sheets according to the invention are useful include ponds, waste and waste water treatment, and agricultural applications. The geo membrane sheets could be attached to each other by forming seams according to the principles described above to form a geo membrane sheeting which could used for covering a portion of ground forming the bottom of a pond for the purpose of avoiding leakage of water from the pond. Such a pond could be a pond in a park or recreational area but also a pond for containing an industrial water. In the technical area of water treatment the geo membrane sheets could be used for forming a waterproof bottom for flotation tanks and sedimentation ponds. In the area of waste treatment the geo membranes could be used for forming a waterproof bottom for a controlled landfill for solid waste to prevent leaching water from reaching the environment. In the agricultural area the geo membrane sheets could be used for forming waterproof bottoms for manure ponds. It will be appreciated that also in the geo membrane sheeting area it is important to avoid mechanical damage to seams between adjacent covering membrane sheets and thus the invention is very useful also in this application. It is to be understood that the invention is not limited to the specific types of thermoplastic films or elastomers exemplified herein or by the disclosure of other specific polymers provided herein, the examples having been provided merely to demonstrate the practice of the subject invention. Those skilled in the art may readily select other thermoplastic films, or other elastomers.