GB2104839A

GB2104839A - Providing cellular core structures with fillets of adhesive

Info

Publication number: GB2104839A
Application number: GB08126430A
Authority: GB
Inventors: Andrew Stephen Hullah
Original assignee: Ciba Geigy AG
Current assignee: Novartis AG
Priority date: 1981-08-29
Filing date: 1981-08-29
Publication date: 1983-03-16

Abstract

Fillets of adhesive are formed on the edge sections of a honeycomb core by contacting a film adhesive with the edge sections and heating the assembly at an angle of 75 DEG or less with the horizontal until the adhesive melts sufficiently for it to adhere to the core, but insufficiently for it to flow, then, with the core at an angle of 60 DEG to 90 DEG to the horizontal plane, heating it further to such a temperature and for such a period that the adhesive film flows around the cell edges and forms fillets. Honeycomb treated in this way may then be faced with porous sheets to form porous constructional elements.

Description

SPECIFICATION Providing cellular core structures with fillets of adhesive This invention relates to cellular core structures, and in particular to a method of providing cellular core structures with fillets of adhesive, to core structures so provided with fillets, and to assemblies of such core structures and facing panels.

Constructional elements in which lightweight core is bonded to high-strength facing panels to constitute a sandwich structure are well known. One of the more widely used forms of core for such sandwich structures is a honeycomb material made from strips of, for example, a metal such as stainless steel, titanium, or aluminium, resin-impregnated glasscloth, resin-impregnated cellulosic or synthetic paper, or a plastics material. It is essential that there should be a very strong but lightweight adhesive bond between the core and the facing panels. Hitherto this has usually been obtained by placing a film adhesive, i.e., an essentially solid film of an adhesive, between the core and the facing panels and then heating the assembly under pressure. Sometimes a liquid adhesive is also applied to the core and/or the facing panels.If a fillet of adhesive is formed at the junctions of the facing panels with the core material the bond strength can be increased, because the bonded area then extends beyond the edges of the core material, up the walls of the cells and partially onto the inner face of the panels.

Often, however, when a film adhesive is employed the whole of the inside of the facing panel, and not merely those areas of the panel in immediate contact with the edge sections, is coated by the adhesive. In some applications, such as sound-absorption structures, it is preferable, or even essential, that the facing panel be porous or at least permeable, and this is not the case if the whole of the inner face of the panel has been substantially coated with an adhesive film.

It has been proposed to apply a liquid heat-curable adhesive to the edge sections of the cells and allow or cause the adhesive to form a hard, non-tacky deposit of the uncured adhesive on these edge sections. A limitation of this method is that many liquid adhesive compositions. although otherwise eminently suitable for bonding honeycomb core to facing panels, are difficult to use because the liquid adhesive forms "strips" of solidying adhesive, or because the adhesive bridges the edge sections of the cells, or again because the liquid adhesive sometimes does not flow sufficiently and the fillets formed are too flat.

In British Patent Specification No. 1 273 771 there is described a method for the production of fillets of adhesive on the edge sections of a cellular core structure in which a film adhesive is weakened inside the areas of immediate contact with the cell walls of the core, the film and core are then heated and the film melts and contracts to form the fillets on the edge sections. This method gives an excellent product, but suffers from the disadvantage of being laborious to carry out and hence of being expensive.Whilst this is not a serious drawback if the cost of forming the fillets is small compared with the cost of the final panel, for example in the manufacture of high-performance panels such as are required for the aerospace industry, this method is not really a practical solution to the problem of forming fillets if lower-cost panels are required, for example by the building industry.

It has now been found that fillets of adhesive can be produced on the edge sections of a honeycomb core structure by a process in which a film adhesive is lightly affixed onto the cellular core by heating it so that it becomes tacky in contact with the core whilst the core is in a substantially horizontal plane, followed by further heating with the core in a substantially vertical plane. It is surprising that this process results in the adhesive forming fillets around the cell edges, since it might be expected that heating a lightly-adhering film adhesive in a vertical plane would cause it to flow from the cell edges, as happens if such a combination of cellular core and film adhesive is heated in a horizontal plane. A particular advantage of this process is that it allows fillets of adhesive to be formed simultaneously on both faces of the honeycomb core.With the method of forming fillets described in the British Patent Specification mentioned above it is found impracticable to form fillets simultaneously on both faces of the core.

The present invention accordingly provides a method of forming fillets of adhesive on the edge sections of cells in a honeycomb core which comprises (a) placing a film adhesive in contact with the upper edge sections of said cells, the said core making an angle of 750 or less with the horizontal, and heating the film adhesive until it melts sufficiently for it to adhere to the core but insufficiently for it to flow, then, (b) with the said core positioned at an angle between 600 and 900 to the horizontal plane, heating it further to such a temperature and for such a period that the adhesive film flows around the cell edges and forms fillets thereon.

The present invention also provides honeycomb core provided with fillets of adhesive produced by the method of this invention and assemblies prepared by bonding, by means of the fillets of adhesive, one or more facing panels (preferably at least one of which is porous) to the said honeycomb core.

It will readily be appreciated that if, during the process of this invention, the honeycomb core is held at an angle of 600 to 750 to the horizontal plane, it will be at a satisfactory angle for stages (a) and (b), i.e., the adhering stage and the flowing stage, and that there is no need to reposition the core between these stages. Whilst such an arrangement gives a satisfactory product, it has been found that a greater proportion of the resin forms into effective fillets around the cell edges in stage (b) if the core is held substantially in, or nearly in, a vertical position, especially at an angle of 800 to 900 to the horizontal plane. The angle at which the core is held during the initial stage (a) is not critical, as long as good contact between the film and the core is maintained.For this reason we prefer to carry out this initial heating with the core substantially in the horizontal plane, or at an angle up to about 200 from the horizontal plane.

The honeycomb core cells contacting the film adhesive may be uniform or non-uniform, regular or irregular. Usually, however, they will be both uniform and regular, i.e., symmetrical about an axis of rotation, such as in those types of honeycomb core known as square, egg-box, rectangular, hexagonal, reinforced hexagonal, tubular, and sinusoidal. Hexagonal core is a particularly convenient form. The cell size of this core should not be so large that an appreciable amount of the film adhesive is wasted by forming a few, large fillets. Further, the cell size should not be so small that the film does not readily form fillets and tend to bridge over cell openings. With the available commercial film adhesives we have found that a cell size of 5 to 25 mm, measured between two parallel sides, gives the most satisfactory products.

The honeycomb core may be made from a metal, such as aluminium or stainless steel, resinimpregnated glasscloth, or resin-impregnated cellulosic or synthetic (e.g., nylon) paper. Honeycomb made from a metal is preferred.

The assembly of core and film adhesive is usually heated by being placed in an oven at the desired temperature but other methods of heating, for example radiant heating, may be used. The temperature to which it is necessary to heat the film adhesive, and the duration of such heating, in order to carry out the instant process will depend upon the nature of the film adhesive. It must be heated above the softening point of the film for stage (a) and above the melting point for stage (b).

However, the temperature must not be so high nor the duration of heating so long that premature curing of any thermosettable resin present in the film adhesive takes place, since the fillets of adhesive on the core must remain in a heat-curable state in order that the facing sheets may be successfully bonded to the core when required. Usually the temperature at which heating is effected is within the range 100 to 1 500C, especially 1 15 to 1400 C. Successful adhesion between the film and the core is usually achieved within 1 minute and successful formation of fillets after a further 2 to 5 minutes.

To form fillets of adhesive simultaneously on both faces of the honeycomb core, the core is inverted between stages (a) and (b), a second film adhesive is placed on the edge sections which are now uppermost of the cells, stage (a) is repeated with the second film adhesive, then stage (b) is carried out such that both the adhesive films flow around the respective cell edges and form fillets thereon.

The film adhesive may comprise a thermoplastic material, but preferably it essentially consists of a thermosettable plastics material, the fillets of adhesive being formed under conditions such that the adhesive remains thermosettable.

The core bearing the fillets of adhesive may be stored as required, subject to the "shelf-life" of the adhesive not being exceeded. Conventional methods, such as heating in a press, may be used to bond this core to facing panels.

Particularly preferred film adhesives are those wherein the thermosettable plastics material comprises an epoxide resin (i.e., a substance containing on average more than one 1,2-epoxide group per molecule), such as a polyglycidyl ether of a phenol or an alcohol, or a poly-(N-glycidyl) derivative of an aromatic amine, and an effective amount of a curing agent therefor.

Advantageously these thermosetting film adhesives further contain a thermoplastics polymer such as a phenoxy resin, a polysulphone, a nylon, a copolymer of an olefin hydrocarbon with an olefin ester, or a nitrile-butadiene rubber.

The phenoxy resins which may be employed are linear polyarylene polyhydroxy polyethers, substantially free from 1 ,2-epoxide groups and having an average molecular weight of at least 10,000.

They are generally copolymers of a dihydric phenol with either a diglycidyl ether of a dihydric phenol or with epichlorohydrin, and they contain recurring units of the structure

where R denotes the residue of a dihydric phenol after removal of the two phenolic hydroxyl groups. To prepare such resins a diglycidyl ether of, for example, bis-(p-hydroxyphenyl)methane or 2,2-bis(p hydroxyphenyl)propane may be copolymerised with the same phenol or with a different dihydric phenol such as bis(p-hydroxyphenyl) sulphone. Preferably, however, a dihydric phenol, especially 2,2 bis(p-hydroxyphenyl)propane, is copolymerised with epichlorohydrin.

Particularly preferred phenoxy resins are of the formula

where m has an average value in the range 40 to 160.

The polysulphones resins which may be employed are linear polymers having an average molecular weight of at least 10,000 and containing the repeating unit: -ASO2- where A denotes a divalent aromatic group, which may be interrupted by ether oxygen and/or by diva lent aliphatic groups. Especially suitable polysulphones are those obtained by reaction of a di-alkali metal salt of a dihydric phenol, such as the disodium salt of 2,2-bis(p-hydroxyphenyl)propane, with a bis(monochloroaryl) sulphone such as bis(p-chlorophenyl) sulphone.

Particularly preferred polysulphone resins are of the formula:

where n has an average value in the range 50 to 120.

Copolymers of olefin hydrocarbons and olefin esters which may be employed may have, as the olefin hydrocarbon component, a polyolefin, for example butadiene, but preferably it is a mono-olefin, such as styrene or methylstyrene; further preferred are lower olefins containing not more than 4 carbon atoms, such as ethylene, propene, but-l-ene, or 2-methylpropene, ethylene being particularly preferred.

The olefin ester component of the copolymer may be a diene, such as an alkenyl ester of an alkenoic acid; usually, mono-olefinic esters are more suitable, such as alkyl esters of alkenoic acids, in particular those consisting only of carbon, hydrogen, and ester oxygen atoms, and containing not more than 8 carbon atoms, especially an alkyl acrylate or methacrylate such as ethyl acrylate, n-butyl acrylate, or methyl methacrylate. Preferred mono-olefin esters are alkenyl esters of alkanoic acids, particularly those consisting only of carbon, hydrogen, and ester oxygen atoms, and containing not more than 8 carbon atoms, especially a vinyl alkanoate such as vinyl formate, vinyl acetate, or vinyl propionate.

The copolymer may contain a minor amount, not more than 5% by weight calculated on the olefin hydrocarbon and olefin ester components, of an a-ethylenically unsaturated carboxylic acid.

Particularly suitable copolymers contain more than 50%, and preferably from 55 to 85%, by weight, of the olefin hydrocarbon component. Those most preferred contain 25 to 45% by weight of ethylene units.

Nylons which may be used are commercially available aromatic or aliphatic polyamides. The nitrile-butadiene rubbers which may be used are preferably those prepared by the polymerisation of 20 to 50% by weight of acrylonitrile and 80 to 50% by weight of butadiene.

Usually from about 1 to 100, or even from about 1 to 150, but especially from 10 to 80, parts by weight of the thermoplastic polymer are employed per 100 parts by weight of the epoxide resin.

Other types of adhesive film may, of course, be used, the criterion of usefulness being whether the adhesive film is sufficiently "self-filleting" under the conditions employed. The ability to form fillets is governed by, amongst other factors, the viscosity and surface tension of the film adhesive in the liquid state, and this in turn depends on components of the adhesive, the presence of fillers, etc.

Whether a particular film adhesive is suitable for use in particular circumstances can readily be determined by routine experiment.

The following Examples illustrate the invention; all parts are by weight.

"Epoxide resin I" denotes a liquid polyglycidyl ether prepared in a known manner from 2,2-bis(4hydroxyphenyl)propane and epichlorohydrin in the presence of alkali and having a 1,2-epoxide content of 5.16 equivalents per kilogram.

"Epoxide resin II" denotes a solid polyglycidyl ether prepared from 2,2-bis(4hydroxyphenyl)propane and epichlorohydrin, having a 1,2-epoxide content of 0.25 equivalent per kilogram.

Example 1 Using known methods, a film adhesive 0.25 mm in thickness was prepared containing Epoxide resin 1 55 parts Epoxide resin 11 45 parts aluminium powder (passing 200 mesh) 25 parts dicyandiamide 6 parts This adhesive was draped over an aluminium honeycomb core having hexagonal cells measuring 12.7 mm between parallel sides. The assembly was arranged in a horizontal plane and placed into an oven at 1 250C in order to make the film lightly adhere to the honeycomb. After 30 seconds the honeycomb was turned until it was in the vertical plane and left in the oven at the same temperature for 6 2 minutes. The honeycomb was then removed from the oven and cooled.It was observed that the film adhesive had formed fillets on the edge sections of the honeycomb cells, and that there were no adhesive 'bridges' blocking the open cell structure.

Porous panels, comprising a woven fabric, were bonded onto the core by heating for 1 hour at 1 700 C, under a pressure of 34 kPa, to cure the adhesive.

Example 2 Using known methods, a film adhesive 0.25 mm in thickness was prepared containing Epoxide resin 1 65 parts Epoxide resin 11 35 parts zinc powder (passing 200 mesh) 30 parts dicyandiamide 6.9 parts This adhesive film was draped over a honeycomb core made from an aromatic polyamide paper impregnated with a phenolic resin and having hexagonal cells measuring 25 mm between parallel sides. With the assembly in a horizontal plane it was placed into an oven at 1 250C in order to make the adhesive film lightly adhere to the honeycomb. After 30 seconds the assembly was turned to an angle of 800 to the horizontal plane, and left in the oven for 3T minutes. The honeycomb was then removed from the oven and cooled.

It was observed that the film adhesive had formed fillets on the edge sections of the honeycomb and there was no adhesive remaining that blocked the open cell structure.

Example 3 A film adhesive as described in Example 1 was draped over an aluminium honeycomb, also as described in Example 1, then heated in an oven at 1 250C for 30 seconds. The assembly was removed from the oven and cooled. It was then turned through 1 800 so that the film adhesive was adhering to the underside of the honeycomb, and a layer of film adhesive draped over the upper surface of the assembly. This was heated in the oven for 30 seconds in the horizontal plane, then turned until it was vertical. It was left in the oven for 6 minutes, then removed and cooled. The sheets of film adhesive had formed fillets on the cell edges on both sides of the honeycomb without blocking the open cell structure.

Claims

Claims

1. Method of forming fillets of adhesive on the edge sections of cells in a honeycomb core which comprises (a) placing a film adhesive in contact with the upper edge sections of said cells, the said core making an angle of 750 or less with the horizontal, and heating the film adhesive until it melts sufficiently for it to adhere to the core, but insufficiently for it to flow, then, (b) with the said core positioned at an angle between 600 and 900 to the horizontal plane, heating it further to such a temperature and for such a period that the adhesive film flows around the cell edges and forms fillets thereon.

2. Method according to claim 1 in which the initial heating (a) is effected with the honeycomb core substantially in the horizontal plane or at an angle up to about 200 from the horizontal plane.

3. Method according to claim 1 or 2 in which the further heating (b) is effected with the honeycomb core held substantially at an angle of 800 to 900 to the horizontal plane.

4. Method according to any previous claim in which the heating in steps (a) and (b) is effected within the range 1000 to 1 500C.

5. Method according to claim 4 in which the heating is effected within the range 11 50 to 1 400C.

6. Method according to any previous claim in which adhesion between the film and the core is achieved within 1 minute and formation of fillets of adhesive after a further 2 to 5 minutes.

7. Method according to any previous claim in which fillets of adhesive are formed simultaneously on both faces of the honeycomb core by, between stages (a) and (b), inverting the core, placing a second film adhesive on the edge sections which are now uppermost of the cells, repeating step (a) with the second film adhesive, then carrying out step (b) such that both the adhesive films flow around the respective cell edges and form fillets thereon.

8. Method according to any previous claim in which the honeycomb core has hexagonal cells.

9. Method according to any previous claim in which the honeycomb core is metallic.

10. Method according to any previous claim wherein the film adhesive essentially consists of a thermosettable plastics material, the fillets of adhesive being formed under conditions such that the adhesive remains thermosettable.

11. Method according to claim 10 wherein the thermosettable plastics material comprises an epoxide resin.

13. Method according to claim 10 or 11 in which the film adhesive also contains a thermoplastics polymer.

14. Method according to claim 13 in which the film adhesive contains an epoxide resin and, as thermoplastics polymer, a phenoxy resin, a polysulphone, a nylon, a copolymer of an olefin hydrocarbon with an olefin ester, or a nitrile-butadiene rubber.

15. Method according to claim 1, substantially as described in any of the Examples.

16. Honeycomb core provided with fillets of adhesive produced by the method claimed in any previous claim.

17. Assemblies prepared by bonding, by means of the fillets of adhesive, one or more facing panels to honeycomb core as claimed in claim 16.

18. Assemblies according to claim 1 7 in which at least one of the facing panels is porous.