This invention relates to lightweight composite panels.

Lightweight composite panels find application in numerous fields, particularly for forming screens, display boards and as infill panels in larger structures.

The panels generally consist of two surface skins, which may be the same or different, and which have between them a low bulk density filling. This may be, for example, a foam or a cellular structure, and one particular form of cellular structure, viz. a honeycomb structure has found widespread application. Composite panels with a honeycomb central structure are highly resistant to bending, thus giving rigidity to panels made in that way. An alternative cellular structure of value is a foam plastics material, for example a rigid polystyrene, polyurethane or other rigid polymer foam.

One of the major problems encountered in relation to such lightweight composite panels is imparting to them adequate fire retardency and surface spread of flame resistance. The lightweight foam or cellular core may be inflammable, and the outer skins must act to protect the core. Another problem is the tendency of such panels to be sensitive to ambient conditions, particularly changes in humidity,

which can lead to difficulties such as warping or bending in practice. Flat surfaces are needed for aesthetic acceptance and for ease of subsequent processing, for example covering or printing.

European Patent Specification 0068873 discloses further details relating to the manufacture of composite panels of this type and proposes a way of overcoming the tendency to moisture or humidity sensitivity by using for the outer facing layers of the panels paperboard having on each side a water impervious layer consisting of an adherent polymer coating which has been directly applied to the paperboard in a water- and solvent-free manner.

While undoubtedly effective to reduce or eliminate problems due to humidity changes, the manufacture of composite panels in accordance with the invention described in European Patent Specification 0068873 is difficult, not least because of the inert nature of the surface of the water impervious layer. The specification suggests the use of corona discharge treatment to overcome these problems, but that treatment adds to the expense, and the effects of such treatment fade with time, so it is necessary to carry out subsequent processing steps rapidly following the corona discharge treatment. This is disadvantageous from a manufacturing point of view.

We have now found a way of making improved composite panels of the type noted above which can be carried out simply and economically and which avoids the humidity sensitivity problems noted above and at the same time increases the fire resistance of the panels.

According to the present invention, there is provided a composite panel consisting of a cellular core material having adhered each side thereof a facing material,

wherein the facing material consists of paperboard or cardboard to each side of which has been laminated a layer of metal foil and wherein the layer of metal foil adhered

V to the core is provided with a primer coat. »

5

We have found that working in this way it is not necessary to use the solvent- and water-free applied polymer coatings suggested in the European specification referred to above. Rather, it is possible to use as the facing

10 material paperboard or cardboard to which metal foil, usually aluminium foil, has been laminated on both sides using standard water- or solvent-based laminating adhesives. In particular, standard latex or emulsion-type adhesives may be used at extremely low application rates,

15 for example as low as 2 gs .

The core for lightweight panels in accordance with the present invention may be a core of honeycomb material or may be a rigid ^' polymeric foam material. If honeycomb is

20 used, this may be an expandable card or paper honeycomb having hexagonal cells or it may be, e.g. a cellular card fill consisting of alternate straight and corrugated strips of card or paper. In both these types of core, the cells run perpendicular to the plane of the panel. To

25 assist stability, especially in the case of expandable honeycomb fill, the core may itself be faced with paper or light card, e.g. kraft paper or chipboard of basis weight 120-150 gsm. Such a paper facing can be advantageous in certain circumstances, for example, allowing a greater

30 range of cell sizes to be used whilst still supporting the faces adequately and secondly by facilitating the cutting of the core sections to precise size to fit within the cavity of the panel frame, thereby eliminating the risk of small voids occurring which can cause the depressions or

35 blemishes on the finished panel surface.

Alternatively, the core may be made up of alternate layers of paper or card and of corrugated paper or card. The paper or card layers may be of basis weight 120 to 300 gsm and the corrugated paper or card of basis weight 100 to 125 gsm.

In all cases, the paper, card or foam material of the core may be treated with or include additives tending to reduce its combustibility. If the central core of the panels is, for example, a foam polystyrene or foam polyurethane, preferably (and essentially for certain applications) fire retardent (self-extinguishing) grades of such foam materials are used. One or both outer faces of the panels may also be coated with a fire retardant coating of known type further to reduce the combustibility of the panels.

The adhesion of the core material to the facing material is promoted by the use, on the side of the facing material adjacent to the core, of a primer material. A variety of materials is usable as primer, the criteria for selection being that the primer must adhere well to the surface of the metal foil on the one hand and be well receptive to adhesives with which the core material is to be adhesed on the other. Particularly preferred primers are nitrocellulose based lacquers. These are available in commerce in the printing industry.

The core material may be bonded to the primed face of the facing material using conventional composite panel adhesives, for example those based on urea formaldehyde resins or polyvinyl acetates.

The following examples will serve to illustrate the invention:

Example 1

As base material for the facing layers, a commercially available white lined chipboard, 800 microns thick and having a basis weight of 566 grams per square metre was taken. This was laminated on both sides with standard 9 micron aluminium foil using a casein latex adhesive. The adhesive was applied at an application rate of 2 grams per square metre when dried down.

Subsequent to laminating, one side of the aluminium foil lined chipboard sheets was coated with a gravure print grade of nitrocellulose lacquer at a coating weight (dried down) of 1.5 grams per square metre. The other side was coated with a white flame retardant coating at a rate giving a dried down coating weight of 1.5 grams per square metre.

Sheets of this facing material were then cut and adhered using a conventional ureaformaldehyde adhesive to expanded card honeycomb to form composite panels. The thickness of the expanded honeycomb layer varied between 10 and 30 mm depending upon the particular honeycomb used.

Panels so manufactured were subjected to evaluation to determine their dimensional stability under conditions of altering relative humidity and were also subjected to surface spread of flame tests in accordance with BS 476, Part 7, 1987 to determine their suitability for use as display panels for exhibition purposes. Satisfactory results were achieved on both counts. In particular, the maximium travel of flame damage from the point of flame impingement never exceeded 50 mm, well below the maximum permitted travel of 165 mm for certification to Class I, BS 476, Part 7 1987.

Example 2

Sheets of facing material prepared as in Example 1 were cut and adhered using a conventional urea formaldehyde adhesive to sheets of rigid expanded polystyrene

(self-extinguishing grade), 13 mm thick, to form composite panels.

Panels so manufactured were subjected to evaluation to determine their dimensional stability under conditions of altering relative humidity and were also subjected to surface spread of flame tests in accordance with BS 476, Part 7 1987 to determine their suitability for use as display panels for exhibition purposes. Satisfactory results were achieved on both counts. As in the case of the panels of Example 1, the maximum travel of flame damage from the point of flame impingement when the BS 476 test was carried out never exceeded 50 mm.

Example 3

Sheets of facing material prepared as in Example 1 were cut and adhered using a conventional urea formaldehyde adhesive to sheets of multilayer corrugated cardboard to form composite panels.

The multilayer corrugated cardboard consisted of a centre board (115 or 117 gsm chip liner) with a B or C flute corrugated semichemical board (110 or 112 gsm) adhered either side, with a kraft exterior skin (125, 150, 200 or 300 gsm) adhered to each of the corrugated boards. The precise combination of basis weights could be chosen to provide cores of overall thickness 5.5 to 8 mm, the choice depending on the final panel thickness, weight and strength desired.

Panels so manufactured were subjected to evaluation to determine their dimensional stability under conditions of altering relative humidity and were also subjected to surface spread of flame tests in accordance with BS 476, Part 7 1987 to determine their suitability for use as display panels for exhibition purposes. Satisfactory results were achieved on both counts.

Example 4

Expanded hexagonal paper honeycomb (cell size 13-15 mm, paper basis weight 130 gsm) of thickness (cell length) 9 mm was faced each side with a facing of 130 gsm chipboard, using a standard polyvinyl acetate adhesive. To each side of this core was adhered, using a standard ureaformaldehyde based adhesive, the printed side of sheets of facing material as in Example 1.

Panels having similar satisfactory properties with respect to insensitivity to changes in humidity and fire resistance were obtained.