SE1450978A1

SE1450978A1 - Method for producing a sandwich panel

Info

Publication number: SE1450978A1
Application number: SE1450978A
Authority: SE
Inventors: Bengt Andersson
Original assignee: Composite Technologies Sweden Ab
Priority date: 2014-08-22
Filing date: 2014-08-22
Publication date: 2016-02-23

Abstract

A method for producing a sandwich panel (1) comprising the steps of: - providing a web-like or sheet-like semi-finished product capable of forming, by heat treatment, a foam-like composite material, wherein this semi-finished product is formed by a non-woven web-like or sheet-like fibrous material impregnated with a mixture comprising unexpanded thermoplastic particles and an inorganic silicate binder or a binder in the form of curable resin; - superposing several layers of said semi-finished product; - hot pressing the superposed layers to form a plate; - cutting the plate along parallel spaced cutting planes perpendicular to the top and bottom faces of the plate to form several elongated strips (8); - rotating the strips 90° about their longitudinal axis and arranging them side by side to form a core (9) for the sandwich panel; and - bonding a top skin (11) and a bottom skin (12) to the core (9).(Fig 5)

Description

Method for producing a sandwich panel FIELD OF THE INVENTION AND PRIOR ART This invention relates to a method for producing a sandwich panel with a core comprising a foam-like composite material.

A foam-like composite material containing a fibrous material, resin and expanded thermoplastic particles, so-called microspheres, is previously known from EP O 041 054 A2. Thefoam-like composite material according to EP O 041 054 A2 isformed by a web-like fibrous material impregnated with a mixture of curable resin and unexpanded or expandedthermoplastic particles. Several layers of this material may besuperposed and bonded together in order to form a core for use in a sandwich structure.

DE 10 2007 037 137 B4 discloses a sandwich panel consistingof a core and outer skins bonded to the core, wherein the core isformed of two superposed layers which are bonded together.Each one of said layers comprises a glass fibre plate, aninorganic silicate binder and a filler in the form of expandedparticles. The sandwich panel according to DE1O 2007 037137 B4 may for instance be used in the construction of ventilation ducts.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method forproducing a sandwich panel of the above-mentioned type with acore foam-like material and with comprising composite properties that are improved in at least some aspect.

According to the invention, this object is achieved by means of a method having the features defined in claim 1.

The method according to the present invention comprises thesteps of: a) providing a web-like or sheet-like semi-finished productcapable of forming, by heat treatment, a foam-like compositematerial, wherein this semi-finished product is formed by a non-woven web-like or sheet-like fibrous material impregnated with amixture comprising unexpanded thermoplastic particles and aninorganic silicate binder or a binder in the form of curable resin;b) superposing several layers of said semi-finished product,optionally with one or more layers of one or more othermaterials; c) subjecting the superposed layers to hot pressing so thathardening or curing of the binder and expansion of theunexpanded thermoplastic particles in said layers take placeand so that the superposed layers are bonded together to form a plate with a top face and an opposite bottom face; d) cutting the plate along parallel spaced cutting planesperpendicular to the top and bottom faces of the plate to formseveral elongated strips; e) rotating the strips 90° about their longitudinal axis andarranging them side by side to form a core for the sandwichpanel, the core having a top face and a bottom face; and f) bonding a top skin to the top face of the core and a bottomskin to the bottom face of the core by means of an adhesive to form the sandwich panel.

The fibres included in a non-woven web-like or sheet-like fibrousmaterial essentially extend in the xy-plane, i.e. in the extensionplane of the fibrous web or sheet. Thus, in a sandwich panelhaving a core of the type disclosed in EP O 041 054 A2 andDE 10 2007 037 137 B4, the fibres of the core extend essentiallyin the xy-plane of the sandwich panel, i.e. essentially in parallelwith the Consequently, the individual fibres of the core only have a very top and bottom skins of the sandwich panel. limited extension in the z-direction of such a sandwich panel,i.e. in the thickness direction of the sandwich panel. However, ina sandwich panel formed by the method according to thepresent invention, the orientation of the fibrous material of thecore has been modified so that the fibres included in the foam-like composite material will extend essentially in the z-directionof the compression strength and shearing strength of the sandwich sandwich panel. Hereby, the tensile strength, panel are improved. Furthermore, owing to the fact that the coreof a sandwich panel formed by the method according to the present invention is not layered in the z-direction of the sandwich panel, the core will have reduced tendency todelamination when the sandwich panel is subjected to bending under the effect of compressive forces.

An embodiment of the invention is characterized in: - that it comprises the steps of:a1) providing a first web-like or sheet-like semi-finishedproduct capable of forming, by heat treatment, a foam-likecomposite material of a first density, wherein this firstsemi-finished product is formed by a non-woven web-likeor sheet-like fibrous material impregnated with a mixturecomprising unexpanded thermoplastic particles and aninorganic silicate binder or a binder in the form of curableresin; anda2) providing a second web-like or sheet-like semi-finishedproduct capable of forming, by heat treatment, a foam-likecomposite material of a second density, which is lowerthan said first density, wherein this second semi-finishedproduct is formed by a non-woven web-like or sheet-likefibrous material impregnated with a mixture comprisingunexpanded thermoplastic particles and an inorganicsilicate binder or a binder in the form of curable resin; and - that, in step b, several first layers of said first semi-finished product are superposed with several second layers of said second semi-finished product and optionally with one or more layers of one or more other materials.

Hereby, areas of foam-like composite material with higher density and areas of foam-like composite material with lower density will be alternately arranged in the core of the produced sandwich panel, wherein each one of these areas extends allthe way between the top skin and the bottom skin of thesandwich panel. The areas of higher density contribute to hightensile strength, compression strength and shearing strength ofthe sandwich panel, whereas the areas of lower densitycontribute to a low weight of the sandwich panel. Thus, with thissolution, the overall weight of the sandwich panel may bereduced while still maintaining an excellent strength of the sandwich panel.

According to another embodiment of the invention, the layers of said semi-finished product or products are, in step b,superposed with one or more reinforcement layers of a materialwhich has a higher tensile strength than the foam-like compositematerial formed by said semi-finished product or products.Hereby, areas of reinforcing material with higher tensile strengthwill be provided in the core of the produced sandwich panelbetween the areas of foam-like composite material, whereineach one of these areas of reinforcing material extends all theway between the top skin and the bottom skin of the sandwichpanel. The areas of reinforcing material contribute to a furtherimprovement of the strength of the sandwich panel. The areas ofreinforcing material are kept in place in the core and stabilizedby the areas of foam-like composite material. Thus, the areas offoam-like composite material counteract bending anddeformation of the areas of reinforcing material and therebyprotect the latter areas from cracking when subjected to bending under the effect of compressive forces and/or shearing forces.

According to another embodiment of the invention, eachreinforcement layer is placed between two of said first layers instep b and bonded to these two first layers in step c. Hereby,each area of reinforcing material will be fixed between two areasof foam-like composite material with higher density in the core ofthe produced sandwich panel.

According to another embodiment of the invention, eachreinforcement layer is a layer of a web-like or sheet-like semi-finished product formed by a woven fibrous material, preferablyin the form of fibreglass fabric or basalt fabric, impregnated withan inorganic silicate binder or a binder in the form of curableresin or with a mixture comprising unexpanded thermoplasticparticles and an inorganic silicate binder or a binder in the formof curable resin. Hereby, the reinforcement layers may beproduced in a similar manner as the other layers and a goodbond may be achieved between the reinforcement layers and the other layers in a simple manner.

Further advantages as well as advantageous features of themethod according to the invention will appear from the following description and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will in the following be more closely described bymeans of embodiment examples, with reference to the appended drawings. lt is shown in: Fig 1-4 schematic illustrations of different steps in a method Fig 5 Fig 6 Fig 7 Fig 8 Fig 9 Fig 10 according to a first embodiment of the present invention for producing a sandwich panel, a schematic perspective view of a sandwich panel formed by a method according to the invention, a schematic perspective view of different material layers used in a method according to a second embodiment of the invention, a schematic section through a part of a sandwich panel formed by the method according to said second embodiment, a schematic perspective view of different material layers used in a method according to a third embodiment of the invention, a schematic section through a part of a sandwich panel formed by the method according to said third embodiment, a schematic perspective view of different material layers used in a method according to a fourth embodiment of the invention, and Fig 11 a schematic section through a part of a sandwich panel formed by the method according to said fourth embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Different steps in a method according to the present inventionfor producing a sandwich panel 1 are schematically illustrated inFigs 1-4. The produced sandwich panel 1 may for instance beused as a floor panel, such as a floor panel for railway vehicles or ships.

A web-like or sheet-like semi-finished product capable offorming, by heat treatment, a foam-like composite material isprovided in a first step. This semi-finished product is formed bya non-woven web-like or sheet-like fibrous material impregnatedwith a mixture comprising unexpanded thermoplastic particlesand an inorganic silicate binder or a binder in the form of curable resin.

According to the invention, several layers 2 of said semi-finishedproduct are superposed, optionally with one or more layers 3, 4of one or more other materials, whereupon the superposedlayers 2, 3, 4 are subjected to hot pressing. During the hotpressing, the superposed layers 2, 3, 4 are subjected to heatand pressure so that hardening or curing of the binder andexpansion of the unexpanded thermoplastic particles in saidlayers 2, 3, 4 take place and so that the superposed layers 2, 3, 4 are bonded together to form a plate 5 with a top face 6a and an opposite bottom face 6b. The plate 5 suitably has a thicknessof 10-50 mm. The hot pressing may be carried out in a pressdevice in the form of a hot press or the similar. As analternative, the layers 2, 3, 4 are positioned in a vacuum press,which is then subjected to heating. The temperature in the pressdevice may be between 100 and 200°C, preferably 120-160°C, during the hot pressing operation.

After the hot pressing, the formed plate 5 is cut, for instance bywater jet cutting or in any other suitable manner, along parallelspaced cutting planes 7 perpendicular to the top and bottomfaces 6a, 6b of the plate to form several elongated strips 8, asillustrated in Fig 3. The intended cutting planes 7 are illustratedby broken lines in Fig 3. The strips 8 are then rotated 90° abouttheir longitudinal axis and arranged side by side to form a core 9for the sandwich panel 1. Finally, a top skin 11 is bonded to thetop face 10a of the core 9 and a bottom skin 12 is bonded to thebottom face 10b of the core 9 by means of an adhesive to formthe sandwich panel 1. Adhesive is with advantage also appliedbetween the strips 8 of the core 9 in order to bond the stripstogether. The adhesive is for instance an epoxy adhesive, a polyurethane adhesive or a hot-melt adhesive.

The width of each strip 8 in the core 9 corresponds to thethickness of the plate 5 from which the strips are cut, and theheight of each strip 8 in the core 9 and thereby the height of thecore 9 corresponds to the distance between the cutting planes7.

The top and bottom skins 11, 12 may for instance consist ofmetal sheets, preferably of aluminium or steel, or sheets ofglass fibre reinforced plastic or sheets of any other suitable material. ln the embodiment illustrated in Figs 1-4, the above-mentioned plate 5 is formed by layers 2 of one and the same type. ln the embodiment illustrated in Figs 10 and 11, several layers 2of the above-mentioned semi-finished product is, in order toform the plate 5, superposed with reinforcement layers 4 of amaterial which has a higher tensile strength than the foam-likecomposite materials formed by said semi-finished product. Eachreinforcement layer 4 is placed between two layers 2 of saidsemi-finished product and bonded to these two layers 2. ln thisthe strips 8 included in the core 9 will case, comprise longitudinal areas 14 of foam-like composite material andlongitudinal areas 16 of reinforcing material with higher tensilestrength, wherein each one of these areas 14, 16 extends all theway between the top skin 11 and the bottom skin 12 in thethickness direction of the sandwich panel 1, as illustrated in Fig 11. ln the embodiment illustrated in Figs 6 and 7, the plate 5 is formed by layers 2, 3 of two different types and in theembodiment illustrated in Figs 8 and 9, the plate 5 is formed bylayers 2, 3, 4 of three different types. ln these cases, the method comprises the initial steps of: 11 - providing a first web-like or sheet-like semi-finished productcapable of forming, by heat treatment, a foam-like compositematerial of a first density, wherein this first semi-finishedproduct is formed by a non-woven web-like or sheet-like fibrousmaterial impregnated with a mixture comprising unexpandedthermoplastic particles and an inorganic silicate binder or abinder in the form of curable resin; and - providing a second web-like or sheet-like semi-finished productcapable of forming, by heat treatment, a foam-like compositematerial of a second density, which is lower than said firstdensity, wherein this second semi-finished product is formed bya non-woven web-like or sheet-like fibrous material impregnatedwith a mixture comprising unexpanded thermoplastic particlesand an inorganic silicate binder or a binder in the form of curable resin.

The first and second semi-finished products preferably comprisethe same type of binder and the same type of thermoplastic particles. ln the embodiment illustrated in Figs 6 and 7, several first layers2 of said first semi-finished product are superposed with severalsecond layers 3 of said second semi-finished product in order toform the plate 5. The first and second layers 2, 3 may bealternately arranged, as illustrated in Fig 6, or arranged inalternating groups where each group comprises two or morelayers of the first semi-finished product or two or more layers ofthe second semi-finished product. ln this case, the strips 8 included in the core 9 will comprise longitudinal areas 14 of 12 foam-like composite material with higher density andlongitudinal areas 15 of foam-like composite material with lowerdensity, wherein each one of these areas 14, 15 extends all theway between the top skin 11 and the bottom skin 12 in thethickness direction of the sandwich panel 1, as illustrated in Fig 7. ln the embodiment illustrated in Figs 8 and 9, several first layers2 of said first semi-finished product are, in order to form theplate 5, superposed with several second layers 3 of said secondsemi-finished product and with reinforcement layers 4 of amaterial which has a higher tensile strength than the foam-likecomposite materials formed by the first and second semi-finished products. Each reinforcement layer 4 is with advantageplaced between two of said first layers 2 and bonded to thesetwo first layers 2. ln this case, the strips 8 included in the core 9will comprise longitudinal areas 14 of foam-like compositematerial with higher density, longitudinal areas 15 of foam-likecomposite material with lower density and longitudinal areas 16of reinforcing material with higher tensile strength, wherein eachone of these areas 14, 15, 16 extends all the way between thetop skin 11 and the bottom skin 12 in the thickness direction of the sandwich panel 1, as illustrated in Fig 9.

The first layers 2, second layers 3 and reinforcement layers 4that the layers comprise a first group 13a of layers, a are preferably superposed in such a manner superposed second group 13b of layers directly on top of the first group 13a 13 of layers and a third group 13c of layers directly on top of thesecond group 13b of layers, wherein: - each one of said first and third groups 13a, 13c of layerscomprises a reinforcement layer 4 with one or more first layers 2on a first side of the reinforcement layer 4 and one or more firstlayers 2 on the opposite side of the reinforcement layer 4, and - said second group 13b of layers comprises several secondlayers 3. ln the example illustrated in Fig 8, said second group 13b oflayers comprises three layers 3, whereas each one of said firstand third groups 13a, 13c of layers has one single layer 2 on each side of the reinforcement layer 4.

The density of the foam-like composite material formed by the first semi-finished product is at least two times higher,preferably at least three times higher, than the density of thefoam-like composite material formed by the second semi-finished product. The density of the foam-like composite material formed by the first semi-finished product is withadvantage 300-800 kg/m3, and the density of the foam-likecomposite material formed by the second semi-finished productis with advantage 50-200 kg/m3. The desired difference indensity between the foam-like composite materials formed bythe first and second semi-finished products is mainly achievedby different amounts of unexpanded thermoplastic particles in the first and second semi-finished products.

The above-mentioned non-woven web-like or sheet-like fibrous material may comprise organic or inorganic fibres, such as for 14 instance glass fibres, mineral fibres, cellulose fibres, carbonfibres, aramid fibres, natural fibres, Kevlar fibres or polyesterfibres. lt is important that the web-like or sheet-like fibrousmaterial has a sufficient porosity so as to be capable of beingimpregnated with the impregnating mixture in a satisfactorymanner. The semi-finished product suitably comprises 10-70% by weight, preferably 15-60% by weight, of fibres.

When the impregnating mixture comprises a curable resin, thisresin may be solvent-based resin or resin which is solvent freeor only has a low solvent content but is liquid at roomtemperature or at a slightly increased temperature. The curableresins which preferably come in question are so-calledformaldehyde-based resins with carbamide, phenol, resorcinolor melamine, or with a mixture of some of these materials.However, also other types of curable resins may be used, suchas poly-condensated resins, for instance polyimide, poly-addedresins, for and instance polyurethane, radical-polymerized resins, for instance polyester. Also acrylic resins may be used.

The thermoplastic particles included in the impregnating mixturehave shells which can be formed of polymers or co-polymers,such as for instance co-polymers of vinyl chloride and vinylidenechloride, co-polymers of vinyl chloride and acrylonitrile, co-polymers of vinylidene chloride and acrylonitrile, co-polymers ofstyrene and acrylonitrile, co-polymers of methylmethacrylatecontaining up to about 20% by weight of styrene, co-polymers ofmethylmethacrylate containing up to about 50% by weight of combined monomers of ethylmethacrylat, or co-polymers of methylmethacrylate containing up to about 70% by weight ofortho-chlorostyrene. The particle size of the unexpandedspheres, and accordingly of the expanded spheres, may varywithin wide limits, and is chosen based on the desired propertiesfor the final product. The particle size of the unexpandedthermoplastic particles may for instance be 1 pm to 1 mm,preferably 2 um to 0.5 mm, and particularly preferred 5 um to 50um. During the expansion, the diameter of the thermoplasticparticles increases by a factor of about 1-10. The unexpandedthermoplastic particles contain volatile, liquid expanding agentswhich are evaporated in connection with the supply of heat.These expanding agents may be freons, hydrocarbons, such asn-pentane, i-pentane, neopentane, butane, i-butane or otherexpanding agents which are conventionally used inthermoplastic particles of the kind here mentioned. 5-30% byweight of the thermoplastic particle may suitably be constituted by expanding agent.

The impregnating mixture may, if so desired or required, beprovided with different additives, such as stabilizers, couplers,fillers, fire-retarding additives and/or pigments. Particles orspheres of organic or inorganic material, for instance glass, may also be added to the impregnating mixture.

The web-like or sheet-like fibrous material is impregnated withthe impregnating mixture in a conventional manner, for instanceby immersing the web-like or sheet-like material into a bathcontaining the mixture or by spraying the mixture onto the web- like or sheet-like material. The degree of impregnation of the 16 impregnated web-like or sheet-like material may for instance be adjusted by pressing with the aid of rolls.

The above-mentioned semi-finished products may for instancebe produced in any of the manners described in closer detail inEP 0 041 054 A2, EP 1 047 553 B1 or WO 2007/114786 A1, the contents of which being incorporated herein by reference.

The reinforcement layers 4 are for instance formed of metalsheets, preferably of aluminium, or sheets of laminated plasticor glass fibre reinforced plastic. However, each reinforcementlayer 4 is preferably a layer of a web-like or sheet-like semi-finished impregnated with an inorganic silicate binder or a binder in the product formed by a woven fibrous materialform of curable resin or with a mixture comprising unexpandedthermoplastic particles and an inorganic silicate binder or abinder in the form of curable resin. The woven fibrous materialused for this semi-finished product is with advantage a materialwith a weight of 50-2000 g/m2. Fibreglass fabric or basalt fabricis with advantage used as woven fibrous material for this semi-finished This comprises the same type of binder and the same type of product. semi-finished product preferablythermoplastic particles as the other semi-finished products used for the production of the core 9.

The invention is of course not in any way limited to the preferredembodiments described above. On the contrary, several possi- bilities to modifications thereof should be apparent to a person 17 skilled in the art without thereby deviating from the basic idea of the invention as defined in the appended claims.

Claims

1. A method for producing a sandwich panel (1) comprising thesteps of: a) providing a web-like or sheet-like semi-finished productcapable of forming, by heat treatment, a foam-like compositematerial, wherein this semi-finished product is formed by a non-woven web-like or sheet-like fibrous material impregnated with amixture comprising unexpanded thermoplastic particles and aninorganic silicate binder or a binder in the form of curable resin;b) superposing several layers (2) of said semi-finished product,optionally with one or more layers (3, 4) of one or more othermaterials; c) subjecting the superposed layers (2; 2, 3; 2, 4; 2, 3, 4) to hotpressing so that hardening or curing of the binder and expansionof the unexpanded thermoplastic particles in said layers (2; 2, 3;2, 4; 2, 3, 4) take place and so that the superposed layers (2; 2,3; 2, 4; 2, 3, 4) are bonded together to form a plate (5) with atop face (6a) and an opposite bottom face (6b); d) cutting the plate (5) along parallel spaced cutting planes (7)perpendicular to the top and bottom faces (6a, 6b) of the plateto form several elongated strips (8); e) rotating the strips (8) 90° about their longitudinal axis andarranging them side by side to form a core (9) for the sandwichpanel, the core (9) having a top face (10a) and a bottom face(10b);and f) bonding a top skin (11) to the top face (10a) of the core (9)and a bottom skin (12) to the bottom face (10b) of the core (9) by means of an adhesive to form the sandwich panel (1). 19

2. A method according to claim 1, characterized in that, in stepb, the layers (2) of said semi-finished product are superposedwith one or more reinforcement layers (4) of a material whichhas a higher tensile strength than the foam-like composite material formed by said semi-finished product.

3. A method according to claim 1, characterized in: - that it comprises the steps of:a1) providing a first web-like or sheet-like semi-finishedproduct capable of forming, by heat treatment, a foam-likecomposite material of a first density, wherein this firstsemi-finished product is formed by a non-woven web-likeor sheet-like fibrous material impregnated with a mixturecomprising unexpanded thermoplastic particles and aninorganic silicate binder or a binder in the form of curableresin; anda2) providing a second web-like or sheet-like semi-finishedproduct capable of forming, by heat treatment, a foam-likecomposite material of a second density, which is lowerthan said first density, wherein this second semi-finishedproduct is formed by a non-woven web-like or sheet-likefibrous material impregnated with a mixture comprisingunexpanded thermoplastic particles and an inorganicsilicate binder or a binder in the form of curable resin; and - that, in step b, several first layers (2) of said first semi-finished product are superposed with several second layers (3) of said second semi-finished product and optionally with one or more layers (4) of one or more other materials.

4. A method according to claim 3, characterized in that saidfirst density is at least two times higher than said second densüy.

5. A method according to claim 3, characterized in that saidfirst density is at least three times higher than said seconddensüy.

6. A method according to any of claims 3-5 characterized inthat said first density is 300-800 kg/m3, and that said seconddensity is 50-200 kg/m3.

7. A method according to any of claims 3-6, characterized inthat, superposed with one or more reinforcement |ayers (4) of a in step b, said first and second |ayers (2, 3) arematerial which has a higher tensile strength than the foam-likecomposite materials formed by the semi-finished products of the first and second |ayers (2, 3).

8. A method according to claim 7, characterized in that eachreinforcement layer (4) is placed between two of said first |ayers (2) in step b and bonded to these two first |ayers (2) in step c.

9. A method according to claim 8, characterized in that saidfirst |ayers (2), second |ayers (3) and reinforcement |ayers (4)are superposed in step b in such a manner that the superposed|ayers comprise a first group (13a) of |ayers, a second group(13b) of |ayers on top of the first group (13a) of |ayers and a 21 third group (13c) of layers on top of the second group (13b) oflayers, wherein:- each one of said first and third groups (13a, 13c) oflayers comprises a reinforcement layer (4) with one ormore first layers (2) on a first side of the reinforcementlayer (4) and one or more first layers (2) on the oppositeside of the reinforcement layer (4), and- said second group (13b) of layers comprises several second layers (3).

10. A method according to any of claims 2, 7, 8 or 9,characterized in that each reinforcement layer (4) is a layer of aweb-like or sheet-like semi-finished product formed by a wovenfibrous material impregnated with an inorganic silicate binder ora binder in the form of curable resin or with a mixture comprisingunexpanded thermoplastic particles and an inorganic silicate binder or a binder in the form of curable resin.

11. A method according to claim 10, characterized in that said woven fibrous material is a fibreglass fabric or basalt fabric. 12. A method according to any of claims 2, 7, 8 or 9,characterized in that each reinforcement layer (4) is a metalsheet, preferably of aluminium, or a sheet of laminated plastic or glass fibre reinforced plastic.

12. A method according to any of claims 1-11, characterized in that metal sheets, preferably of aluminium or steel, or sheets of 22 glass fibre reinforced plastic are used as top and bottom skins(11, 12).

13. A method according to any of claims 1-12, characterized inthat the strips (8) of said core (9) are bonded together by means of an adhesive applied between the strips.