Internal timber framed fire doors are known in which the door interior is filled with layers of plasterboard or chipboard. Such doors provide only limited fire protection of, for example, up to half an hour, and rapidly become unstable in the presence of fire.

Our co-pending application GB 2357111A discloses a method of producing a twin-walled structure, e. g. a fire door or partition wall, which comprises the steps of constructing an open-topped frame comprising a base bordered by upstanding sides which respectively define one wall and side edges in the final structure, filling the frame with a solution of polyurethane foam comprising a mix of isocyanate, particulate exfoliating graphite and/or sodium silicate and resin to a level at or just below the upper surfaces of the upstanding sides, applying to the surface of the fluid mix a releasing agent or member, e. g. paper, closing the open top of the frame, by means of a closure member and securing the closure member in place, allowing the mix to cure to produce a rigid polyurethane foam, removing the closure member and securing to the side edges and/or the solid foam a cover/sheet which defines in the final product the other wall of the structure. Blocks of wood or other suitable material may be placed within the frame to coincide with fittings such as locking mechanisms, handles, etc.

Our co-pending application GB 2287738 discloses a fire door includes a timber frame whose inwardly facing sides are rebated to receive and retain side edges of a sheet of a fire resistant mortar comprising a calcium aluminate cement and an inorganic fibre material.

The fire resistant mortar completely spans the interior of the timber frame. The mortar which can be cast in-situ can be sandwiched between boards of thermally insulating fire resistant material which can be coated with intumescent material.

Our co-pending application GB 2254609 discloses an intumescent fire seal which comprises ceramic fibres mixed with exfoliating graphite and/or sodium silicate in granular and/or powder and/or chip form. The fire seal may comprise a non-woven mat or sheet, a block or other shaped product and may be enveloped wholly or partially in metallic foil or the like. Alternatively, the fire seal may comprise a liquid or paste which can be applied directly to a surface to be fire proofed. Glass fibres may be included in the seals, which are made by forming an aqueous slurry of ceramic fibres and a binder and adding to the slurry said graphite and/or sodium silicate, by affixing into the slurry or dispersing on the surface of the slurry.

EP 0949313A discloses a flexible intumescent material which comprises inorganic fibres and flexible organic fibres together forming a predominantly fibrous matrix, an elastomeric binder and an intumescent substance and which has been compressed at a moisture content of less than 5% by weight. The intumescent material may be produced by wet laying an aqueous slurry of inorganic fibres, flexible organic fibres, the intumescent substance and the elastomeric binder onto a water-pervious support, withdrawing water from the slurry to form a sheet, drying the sheet to a moisture content of less than 5% by weight and then compressing the sheet material.

The present invention sets out to provide a timber framed fire door capable of providing greatly increased fire protection and typically of the order of two or three hours.

According to the present invention, there is provided a fire door including a timber frame which borders a thermally insulating slab comprises a flexible intumescent containing foam or sponge material to each exposed surface of which is adhered a sheet of paper, card or board including or containing a material which exfoliates in the presence of a fire or excessive heat, the exposed frame surfaces being coated or covered with an intumescent material and the foam or sponge material comprises a preformed cellular fire-retardant sponge or foam impregnated with an aqueous surfactant stabilised polymer dispersion including a blend of inert and fire-retardant fillers and/or fibres.

The thermally insulating slab may comprise high density rock or glass wool or a flexible intumescent containing foam or sponge material.

The exfoliating paper, card or board may comprise a blend including ceramic fibres. The slab or rockwool or glass wool may be coated with a layer of intumescent containing solution.

The ceramic fibre may comprise an aluminar silicate fibre.

In a preferred arrangement, at least one surface of the sponge or foam is coated firstly with a primer solution including a sealant and secondly with a flexible coating of a fire-retardant material.

All surfaces of the sponge or foam may be coated.

The primer solution may comprise an aqueous solution of a vinyl acetate copolymer system. The system may comprise an acrylic copolymer system which includes ammonium polyphosphate.

The fire-retardant coating may comprise a chlorinated vinyl copolymer system which preferably includes a quantity of ammonium polyphosphate.

The foam or sponge may comprise polyurethane or alternatively a silicone coated with a silane.

The primer solution may be allowed to dry before the fire-retardant material is applied. One or both coatings may be applied by brushing, rolling or spraying.

The intumescent material may comprise an intumescent substance such as an alkaline metal silicate, e. g. a sodium silicate, or other intumescent substances such as exfoliating graphite. The intumescent material typically also contains a binding agent, most usually a polymeric binding substance such as polyacrylate or derivative thereof, or a polyvinylacetate.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which the sole Figure is a cross-section taken through a fire door in accordance with the invention.

The door illustrated comprises a hardwood or high density soft wood frame 1 typically of 52 or 44mm thickness. The interior of the door comprises outer sheets of thermally insulating heat resistant material 2,3 and an inner slab 4 of thermally insulating rockwool, glass wool or flexible intumescent containing expanded foam or sponge. The sheets 2,3 are preferably ceramic sheets typically of 2 to 3mm thickness and the slab is typically of 50 or 40mm thickness for a 52 or 44mm thick door frame respectively. Typically, the sheets 2,3 comprise sheets of intumescent paper. The sheets are secured to the exposed faces of the slab 4 by adhesive, preferably an intumescent containing adhesive.

If a sponge slab is employed, it preferably comprises a preformed flexible cellular fire-retardant sponge impregnated with an aqueous surfactant stabilised polymer dispersion including a blend of inert and fire- retardant fillers and/or fibres.

The surfaces of the sponge slab may be coated with one or more coatings of a chlorinated vinyl copolymer system which includes a quantity of ammonium polyphosphate. The viscosity of this fire-retardant coating is more akin to an emulsion or paste.

The coatings may be applied by means of a brush, roller or spray.

The primer coating not only seals the open pores of the sponge but also provides a primed surface to which the fire-retardant coating can adhere.

The coated sponge slab retains its flexibility and the bond between the respective coatings is sufficient to prevent cracking or other damage to the surface during handling of the block.

For assembly, the slab 4 of the rockwool, glass wool or expanded foam or sponge is positioned within the hardwood frame and defines a close fit within the frame internal boundary. The sheets 2,3 of thermally insulating material are then adhered to each exposed surface of the slab.

Decorative panels may be secured to the coated exposed faces of the ceramic sheets 2,3 and, to complete the door, edging trim may be applied to the upright stile and head of the door. Typically the edging trim comprises a surface mounted strip of intumescent material from which protrudes a brush seal.

Fire doors in accordance with the invention are capable of withstanding the passage of fire and smoke for periods, for example, of up to three hours.

It will be appreciated that the fire door described above is merely exemplary of fire doors in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention as set out in the appended claims.