CA1142796A

CA1142796A - Fire-resistant composite wood structure particularly adapted for use in fire doors

Info

Publication number: CA1142796A
Application number: CA000333244A
Authority: CA
Inventors: Edwin N. Naslund; Ian Macdonald
Original assignee: CAL-WOOD DOOR
Current assignee: CAL-WOOD DOOR
Priority date: 1978-08-04
Filing date: 1979-08-06
Publication date: 1983-03-15
Also published as: GB2029318B; GB2029318A

Abstract

ABSTRACT OF THE DISCLOSURE
A composite wood material, specially adapted for use as part of a door with a long fire rating, comprises a board material made of wood chips, ground bark and fire-retardant chemical and one or more sheets of material of pressed wood fibers with a fire retardant added thereto, designed to optimize resistance to penetration by fire, increase screw holding ability, resist splitting and mini-mize cost. The board and sheet materials are produced from slurries which are moulded, compressed and dried then they are bonded together to form the composite material.
The material is then applied to the edges of a door core.

Description

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BACI~GROI,'ND OF Tl~l~ lNVEr~irrION
This invention relates to composite wood materials ¦ especially adapted for use in a solid bcard, such as a door, ¦ and more specifically to a fire rated door formed of a solid ¦ core and wooden strips attached around its edges.
l Typical present solid flush door construction includes ¦ three basic components: a core, wood edges attached around the ¦ core (the vertical edges being referred to as stiles and the hor: _ I zontal edgcs as rails), and thin facing material covering both ¦ sides of the door for its appearance. A principal factor taken ¦ into account in choosing the materials for the core and the ¦ wooden edges, and also for determining the thickness of the ¦ wooden edges, is the fire rating desired for the door. Building codes require that doors to be installed in certain building positions need to have a particular fire rating that is measured in time, such as a 20-minute door, or a 4S-minute, one-hour or one and one-half hour ;ioor. Doors are given a fire .. . ., 1.

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rating in accordance with a standard test specifica~ion.
leading test is AST~ 152(1976). Others are UL IO(b)(1974), NFP~ 25 (1972) and UBC 43-2(1973), al] similar to the ASTM test.
In cc.nducting such tests, doors are mounted in an opcning of a fireproof wall and thell exposed on one side to a predetermilled time-temperature rise function. ~he time that a door can withstand the heat before it is penetrated by burning determines its fire rating.
Fire door core materials commonly used at the present time include untreated wood or particle board for doors of a low fire rating, such as 20 minutes, or a particle board treated with a fire retardant, or a mineral core for doors of the higher fire rating, such as 45 minutes or more. A mineral door core is presently commercially available from the Gypsum Division of the ~.eorgia-Pacific Corporation for use in doors rated up to one and one-half hours.
~ 1Ood stiles and rails (edges) are held tightly against the core edges, usually by the door manufacturer, in order to provide edges that will hold wood screws used by the ourchaser of the door to mount normal hardware thereon, such as hinges and door latching mechanisms. Presently available doors with ratings of 45 minutes or greater utilize solid wood stiles and rails that have been treated with a fire retardant, often in a salt form. Hemlock and maple are popularly utilized wood species for door stiles and rails. As the desired fire rating of the door goes up to 45 minutes or more, the stiles and rails must be made very narrow. The reason for this is that such fire retardant treated solid wood material cannot withstand the heat of a standard fire test for such long periods of ~ime without being 3~ penetrated by burning. Therefore, the stiles and top rail are made to be as narrow as the door stoo on a frame on which the .
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~oor is to be mounted during the fire test. The standard fire tests identified above test for fire penetration during the test period of only the door portion between door stops.
That is, fire penetration of the door edges behind the door stops does not disqualify the door; it passes the test anyway because no fire penetration is visible. The door core is made to overlap the door stops. Typical dimensions for a one and one-half hour rated door are stiles of 5/8 inch wide, a top rail of 1/2 inch wide and a bottom rail of 1-1/2 inches wide before installation.
Such a narrow stile, necessitated by the desired fire rating of the door, has low resistance to splitting along its grain and a low ability to hold wood screws. The core material provides no screw holding power. Commercially available doors of all types are listed in manufacturers' product catalogs accumulated in 'ISweet's Catalog File:
Architectural Products for General Building", Volume 5, Section 8.3 (1977), published by the McGraw-Hill Information Systems Company and widely used by architects.
According to the present invention there is provided a solid board characterized by high resistance to fire penetration and including a plurality of layers of sheet material of substantially the same width and length which are firmly adhered to one another, the sheet material being formed from a slurry mixture of individual wood fibers and a fire retardant chemical through the use of heat pressure.
In a specific application of the present invention there is provided a door, wherein the door edges, particularly the stiles, are made of laminated strips of solid pressed wood fibrous sheet material having a fire retardant additive therein.
~he stiles are installed in the door with the surfaces between,the lamunated sheets of material held parallel to its core edge surfaces ïn order to maxi~ize screw holding ability andsplit resistance. I'he laminations of the /t~ -3-

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j ils, on thc otl~er hand, rnay be ~n-iellted eitl~er parallcl or perpendicular to the core edge del)el-lding on the direction of screw attachments.
The principal advantage of sucll door construction is that for a given door core material, the stiles ancl rails (e~yes) ¦
m~y be made thicker than existing solid wood edges so that hard-ware may be more securely a-ttached, even after allowing for some trirr~ing of the edge thickness on the site by the person in-stalling the door, all while rnaintaining a hicJlI fire ratinu of the¦
overall door construction of 45 millutes or more. Sheet mater.al of the type utilized for the edges of the improved door con-struction is commercially available, its intended purpose beilly to prevent flame spread along the surface of the material when installed as wall panelinc3 in buildings and mobile horr~es. But it has been found that such material has improved fire penetration characteristics as well and is advantageously utilized for fire door edges. Although there is presently considerable development effort going into improving fire door cores, no attention has been directed toward improving the wood door edges because, it is believed, that presently used fire retardant salt treated solid wood edges is all that can be done with the prospect of any sisnificant improvement not existing.
It has also been found that the screw holding power of the improved door edges is extremely good relative to that of the fire retardant salt treated solid wood, even though the holding power of a single sheet of such pressed wood material is inferior.
Salt treated wood l-as a Eurtller disadvalltaye o~ havlllg density variations which make its screw holding ability and salt fire retardant content vary throughout the wood. Thus, door hardware is securely attached to uniform density door edges made of the material according to the present invention.
A further improvement for many applications including door construction is a composite wood structure that combines one or more strips of pressed wood fibrous sheets discussed above with a thicker layer of wood material formed from a mixture of wood chips, ground bark and fire-retardant chemi-cals. Such a composite structure maintains the advantage dis-cussed above and has a further advantage of permitting thicker wood door edges while at the same time increasing the resistance of the door edge to fire penetration, all at a reasonable cost.
In a specific embodiment of thé present invention there is provided a fire-resistant wood based structural material comprising: a first layer of material being formed from a slurry mixture of wood chips, ground bark and a fire-retardant chemical that is formed into a board by pressure and drying, and a second layer of material being formed from a slurry mixture of individual wood fibers and a fire-retardant chemical that is formed into a board by heat and pressure, the first and second material layers being firmly adhered to each other in their dry states, thereby forming a composite struc-tural material.
An embodi~ent of the present invention may also provide a fire-door having a core and bonded to that edge of the core forming a stile which in use is hinged to a door jamb, a strip of fire-retardant wood based structural material such as that defined hereinabove.
Furthermore the present invention provides a method for producing the fire-retardant wood based structural material, that method comprising the steps of forming a slurry of wood chips and a fire-retardant chemical, pouring the slurry into a mould cavity, placing a board on top of the slurry within Pg/~

'7~tj the sides of the mould cavity and pressing downwards to drive out exeess water and compressing the material to form the first board, leaving the compressed material to dry~ removincJ
the dried material forming the board from the mould eavity;
euring the dried board in an oven; Eorming a second board by eompressing and heating a slurry of individual wood fibres and a fire retardant ehemieal; and bonding the second board to one faee of the first board~
BRIEF DESCRIPTIO~ OEl TIIE DRAWINGS
Figure 1 shows a door, its faeing par-tially cu-t away, having a eonstruetion according to the present inven-tion;
Figure 2 is a cross-sectional view of the door of Figure 1, taken at section 2-2 thereof;
Figure 3 illustrates the steps of manufacturing the edges of the door of Fi.gures 1 and 2;
Figure 4 is a eross-seetional view of a variation of the door of Figure 1 taken at seetion 2-2 thereof;
Figure 5 illustrates the steps of manufacturing the door edge of Figure 4;

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, Z'79ti ]igure 6 is a crosc;-sectional view of an opti()l;l1 ¦door cross-bar, according to two specific forms of the pr-esent ¦invention, as viewcd at section 6-~, of ~iaure 1; aod l Figure 7 is a horizontal cross-section throuf3h a ¦door having a construction tllat varies in certain par~iclllars ¦from those~ illustrated in other ~igures.

¦ DESCRIPTION OF A PREFEP~RED E~sODI~N'r l A preferred structure having maximum advantage that ¦uiilizes the various aspects of the present invention is a fire 1~ ¦door such as illustrated in Figures 1 and 2. ~uch a fire door ¦comprises three main components. ~ost o' the door is former~ of la core material 11 in a rectangular shape, the first component.
¦of course, for unusual shaped doors, the core material 11 would ¦take on some other shape. The core 11 is usually of a uniform ¦thickness, although it does not have to be for special effects, and has its major opposing surface areas terminate in edges ¦which are themselves planar and perpendicular to the major surface planes of the core 11.
¦ The core 11 may be one continuous, homogeneous piece ¦throughout, or it may consist of a plurality of piecesr as small ¦as one foot or so square, arranged to fill the entire core ¦area within the door. A preferred core material for a hiyh ¦fire rating door is a prefor~ed homogeneous mineral slab made up ¦of a combination of fiber glass, gypsum, calcium silicate and ¦other fire resistive materials. Such a core is one manufact~red ¦by the Georaia-Pacific Company and referenced herein before.

Z~9~j ~lternatively, ~h~ core may be formcd of a com})inatioll of wood particles, fire rcsistive or retarclant adclitives, and adhesive, preformed into slabs. ~lineral cores or fire-retardant treated particle board are generally used for the hiaher fire rating doors, such as ~5 minute alld one-hour doors, while untreated wood product cores are gener~lly used for fire doors of lesser ratirgs, such as those havina a 20 minute fire rating. Of course, other core materials are suitable so long as they are solid and have equivalent fire resistive characteristics.
The second main dcGr component is its edges. Wood edges are attached by an apiropriate adhesive to tlle four edges of the core 11 in the form of stiles 13 and 15, top rail 17 and bottom rail 19. Solid wood edges are generally used in existing doors to permit trimming the sides of the door on the construction site during the installation of the door and also to provide a material for holding door hardware by means of screws. ~ut the material utilized in the door of Figures 1 and 2 is different than that of present solid wood, fire retardant impregnated door edges. The s-tiles 13 and 15 and the top rail 17 are preferably constructed, according to an improvement of the present invention, of a plurality of laminated sheets of uniformiy thick pressed wood fiber material having a non-salt fire retardant added thereto during its manufacture.

Wood fiber sheets are formed, as is generally known, by subjecting wood chips to either pressurized steam or a chemical bath to break the wood down into its individual fibers in the form of a wet slurry. This wet slurry is then reformed by spreadirlg onto an open screened surface mat where it is subjected to pressure and heat. ~ natural chel,lical component of wood then flo to hold the wood fibero to3ether ln its new form. A fire Il ~ 7 Il . I

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I ~Z796 ¦retardant material is conveniently added during the rnanufacturing ¦process while the fibers arc still in a wet slurry in a manncr to ~result in the material being dispersed throughout the resulting ¦wood product substantially uniformly. The fire retarclant material ~may alternately be impregnated into the individual fibers them-¦selves. ~n aluminum compound, such as alumina, aluminum hydroxidc ¦or aluminum silicate is a common fire retardant. P,oron compounds ¦are also kno~n fire retardant:s and can be utili~ed.
¦ A commercially available wood fiber board that is ~satisfactory for this application is one sold under a "~lame Test"
¦panel brand by the r~asonite Cornoration. This material is obtained in wall panel sheets of typical thickness of n.245 inch, ¦with a specific gravity typically of 1.10, and includes an ¦aluminum compound as a fire retardant in the proportion of ¦approximately 35% of its weight. Although this material is ¦designed to prevent flame spread along the surface of wood panelin ¦in order to meet new mobile home fire retardant standards, ¦it has also been found to be a good material to preven-t fire ¦penetration.

~ The stiles 13 and 15 and top rail 17 are preferably ¦made with such commercially available mater al in a manner ¦illustrated in Figure 3. A number of shee-ts of the wood pressed ¦fiber material, such as the sheet 21, are glued toyether to ¦form a composite structure 23. F.acll of the sheets is of sub-¦stantially a uniform thickness, is flat and is of a uniform ¦density. They are sanded or planned on each siclc to provide ¦smooth surfaces to receive adhesive. Up to sevel-l layers of ¦nominally 1/~ inch t]lick sheet m~terial are glued together ¦depending upon i-ow wide the door edges are to be, five sheets ,0 ¦being illustrated herein for a typical fire door application.

The edges 13, 15 and 17 of Iigure 1 are thus approximatcly 1-1/4 inch ~ide. This is in excess of the wiclth of the typical door jamb stop for which thc door is designed to be used.
In forming the composite pressecl wood materia] 23, every other layer receives an application of wet ylue on each side. The altcrnate pieces are dry. The la~crs arc thell built up by alternately laying down dry and wet ylued pieces until the desired numbcr of layers are in the uncurcd board. llle combination is then subjected in a press to pressure for a time until the glue is cured. After the adhesive cu-e bccomes complete the individual cured laminated boards 23 (Figure 3), are cut into narrow strips, such as the strip 25, for installation as a stile or rail (edge) as part of a finished door.
The edge material 25 (Figure 3! is installed on ~he i5 edge of the core 11 of the door of the type illustrated in Figure 1 in a manner that rhe glued together surfaces are parallel to the edge surface of the core 11 to which it may be attached by an appropriate adhesive. This orientation provides the maximum screw holding surface at the edge of the finished door. The 2Q fire retardant capabilities of the wood edges made according to this technique depend upon the thickness of the edge, usually the same as the uniform thickness of the core material 11, and the amount of and type of fire retardant that has been added to the pressed fiberboard during its manufacture. A door is gi~en a fire rating according to standard tests by subjecting the door t~
a flame according to a predetermined time-temperature curve for a predetermined time period, such as 45 minutes, 1 hour or 1-1/2 hours. If there are holes either at the core or arr)und the edge material, the door has failed the test. ~ 45 minute or 1 hour rating according to such tests has been obtained with the aforementioned Masonite material that is formed of five 31.142 ~96 ¦ laye r in width be Eor e tr imming dur ing thc m~ nu f ~ c tur ing proce s se s and a thickness of 1-1/2 inches to match thl' mineral core ;thickness. The use of an aluminum compouncl as a fire retardanL
¦in an amount in excess of about 30~ of the weight of pressed ¦fibrous material having an overall specific gravity of about l.10 appears to be satisfactory for such doors.
The bottom rail 19 may be of the same laminated material ¦but it is not as critical since the bottom of a door does not ¦receive the same intense heat either in the fire tests or in an ¦actual fire as does the top rail 17 or the upper portions of the ¦stiles 13 and 15. If the laminated sheet pressed wood fiber ¦material is not utilized for the bottom rail 19, a standard solid ¦wood with a fire retardant impregnated therein is utilized.
¦ The third ma~or component of the door being descri~ed ¦ is a facing material illustrated as face sheets 27 and 29. These ¦ sheets are attached to the core 11 and the continuous surface ¦ thereof formed by the edges 13, 15, 17 and 19 by an adhesive ¦ under pressure. The face sheets 27 and 29 are typically only ¦ 1/8 inch thick to form a composite door having an overall thick-O ¦ ness of 1-3/4 inch when used with typical edge and core thick-¦ nesses of 1-1/2 inch. The facing sheets 27 and 29 provide an ¦overall covering of the door faces f~ good aesthetics and for ¦securing core components in a sandwich construction.
¦ Pressed fiberboard characteristically resists splittin~
¦because it does not have a grain; rather, the wood fibers are ¦oriented in a random manner rather than being aligned to form la grain as is the case in natural solid wood. But since the ¦fiberboard can be made to have a density greater than that of ¦wood, there is the advantage, if the specific gravity of fiber-board is in excess of 0.80, that the laminated formed door edges ~1~2796 llave a screw holding power significantly in excess of tha~ o~ordinary solid wood that is rpresently bejng usc(l ~or door c(l(~(s.
~ eferrin~ to Figure 2, ,a full mortise hinge 31 is schclnatically illustratcd llaving onc lcaf 33 at~acllc(l to a door edge according to the present invention with screws showr. in dottcd outline in accordance with normal tcchniqucs. ~ sccon(l leaf 35 of the hinge 31 is attached by screws shown in dotted outline to a door jamb that is part oE a wall 37, thc door ~amb including a door stop 39 as well. The stile 15 is also shown schcmatically with a standard lock front 41 (part of a ~ l lock set, the remaining elements not being shown), held to the stile 15 by a screw shown in dotted outline. .~ wall 43 includcs a strike plate 45 attached to the door jamb for accepting a lock bolt 47. The door jamb on the wall 43 also has a door stop 49.
Because the stiles 13 and 15 can be made thicker with tlle improvec construction of the present invention than are stiles of presently available fire doors of long fire rating, there is plenty of edge wood to hold adequately sized wood screws along with the hardware attached to the door, even after the door is trimmed somewhat 2n and the hardware mortised (recessed) into the wood edge according to common practice. Other hardware may be attached as desired.
The reason why the stiles and top rail of present doors are of a long fire rating, such as 45 minute or one-hour doors, are made so thin, generally 3,~4 inch or less, can be seen from Figure 2. In order to pass the standard fire rating tests conducted with a door hung in ,a typical manner, the woocl cdges, ~ccausc thcy cannot rcsist penctra~ioll by ~hc hea~ and fire 'or the rated length of time, must be hidden behind the door stops 39 and 49. That is, the core matcrial 11 that has the required fire rating is caused to overlap the door stops by makiny the wood edges extremely thin. The door constructcd according to the l~Z7~6 ~pre',c t invention on the ol:her hand nec~d not rely upon tle ~ioor stops for fire retardant p~opertic.s and tllus tlle core matcrial ll does not need to extencl so far as to ovcrl.ap the door ~ambs. Tlle . thicker stilcs and rails permit morc matcrial for securc woo~
screw holding and give grcater flcxibility to those installin(j the doors by havin~ excess matcrial w}~ich may bc rcmovcd from thc edge for exact on-site fitting of the door to a particular a cation.
Referring to Figure 4 a modified door structure .lO is illustrated at the same seetional view as given in Figure 2. The referenee numbers of Figure 4 are the same as eorresponding numbers of the previously described door of Figures l and 2 eY.eept that a prime ( ) is added. The main differenee is t~.at a hinge carrying stile 13 of Figure 4 is of a somewhat different eonstruetion than the stile 13 of Figures l and 2.
The stile 13 of Figure 4 is made of a board 53 that is rectangular in cross-seetion and adllered to t~.e edge .
of the eore material 11 . The board 53 is eonstructed from a slurry of wood ehips and ground up wood bark along with fire-retardant ehemieals as deseribed. in detail hereinafter.
Attached to a faee of the board 53 opposite to that adhered to the eore ll are two layers 55 and 56 o pressed fiber board sheet material of a type described hereinabove with respeet to the door embodiments of Figures 1-3. The partieular combination of materials in the arrangement shown in Figure 4 provi(les a door sti.le that has extremely hi~h screw ho~cling eapability is thiek to accept ~.ong serews or allow for easy trim~ing and is extremely resistant to penetration ~y fire. The stile structure of Figure 4 has satisfaetorily been employed in a door that has passed the one and one-half hour fire rating 27~6 test. Although the hinge carrying stile i5 most advantageously constructed as shown in Figure 4 for a long fire rating door, the opposite stile and rails may also be so constructed.
Various arrangements of the board 53 and sheet material 55 and 56 can be employed other than that specifically shown in Figure 4, such as utilizing only a single one of the sheets 55 or 56 on the outside of the door edge, utilizing more than two such sheets, or otherwise facing the board 53 with such sheets on its other edges.
Referring to Figures 5(a) and 5(b), the construction of the stile of Figure 4 will be described. A block of material 61 is formed to have a resulting thickness equal to that desired for the board 53 between the edge of the core 11' and the layer 55. Typically, this might be one-half inchO
The width of the board 61 is made to be some convenient multiple of the thickness of the door core 11' and may be in the range of one foot wide or so. Its length is made somewhat longer than the length of the doors for which the resulting material is to be used, and may be conveniently made to be seven feet long. The board 61 is made from a slurry mixture of wood chips and fire-retardant chemicals, according to -the following formulation with proportional parts given:
1) 159 grams or about 47% by weight of a "salt" solution made of approximately 1.327 parts of magnesium chloride hexahydrate to one part of water.
2) 100 grams or about 29~ by weight of magnesium oxide.

3) 20 grams or about 6% by weight of pine wood flakes (sawdust) of approximately No. 6 mesh screen size (approximately 1/8 inch diameter).

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4) 60 grams or about 18% by weight of fir bark ground into needle shaped particles of the following size mixture:
a) No. 65 mesh screen, 45% plus or minus 10%.
b) No. 100 mesh screen, 35% plus or minus 10%.
c) No. 200 mesh screen, 20% plus or minus 10%.
The above materials are mixed for a few minutes until thoroughly mixed and at the proper consistency. Mixing must be stopped before the resulting solution becomes too fluid for subsequent forming. After mixing, the liquid material is poured into a form having a cavity the shape of the desired resulting board 65 but with sides that are considerably higher. The cavity is initially filled about halfway and pressed down to make sure that the material fits into all of the corners and otherwise fills the bottom of the cavity mold. Several fibers 63 are then laid along the length of the cavity on top of the material, these fibers preferably being commerically available fiberglass. The purpose of the fibers 63 is to increase flexural strength to the resulting board 61 so that it can be carried and handled without the board breaking. After the fibers 63 are in place, the mold is filled up with more of the wood chip and fire-retardant chemical solution.
A board is then placed on top of the solution within the sides of the cavity mold and is pressed downward on to the solution to drive out excess water and compress the material to form the board 61 of the desired height as shown in Figure 5(a). The board is then permitted to dry, first within the mold cavity and then for a time after being removed. The board 61 is dried in its last step by placing in a curing oven for several days.

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Oncc tlle board 6] is ~omp1eted and dried i~ this manner, three layers 551 56 and 57 of the fire-retardant fiber-board sheets discussed with respect to the embodimellts of Figures 1-3 are then glued onto one side of the board 61 as shown in Figure 5(b). The resulting structure is then cut with normal woodworking tools along the dotted lines of Figure 5~b) to provide the composite stile 13' that is shown in Figure 4.
Substantially all of the layer 57 is trimmed from the rouyh constructed door before shipment. Of course for other specific combinations and arrangements of the board 61 and fire-retar~ant fiberboard, the desired materials are glued together in the different configurations.
In some applications, it is desired that a door have a wooden strip across its width, such as the strip 51 shown in dotted outline in Figure 1. Commercially available door core materials generally have little mechanical strength, so such a cross-piece 51 can be utilized to add strength to such a door if it is necessary for a particular door instal-lation. Also, additional pieces of wood (not shown) can be ~0 included in place of standard core material within the door for very specific purposes, such as wood blocks adjacent the stiles in which door hardware may be installed.
Of course, such added wood members within the door core must have a sufficient resistance to fire penetration in order to qualify the door for a desired fire rating time.
Therefore, one of the two composite structures described above for the door stiles may be utilized. Figure 6(a) shows the structure of the cross-member 51 to be made of a plurality of fire-resistant fiberboard sheets glued together in a manner discussed with respect to Figure 3. Alternatively, such Z~36 ¦ iberboard shccts may be combined with ~ core matcri~1 made from a board 61 as described with respect to Figure 5(a), ¦ such a combination being shown as cross-bar 51' in Figure 6(Ji).
¦ ~eferring to Figure 7, a cross-scctional vicw of thc ¦ door according to another construction is shown wherein a core ¦ 65 is made entirely in accorclancc with the proccss dcscribcd .
¦ with respect to Figure 5(a). Sheets 67 and 69 are glued to ¦opposite sides of the core 65 and may be made from the fireproof ¦ fiberboard material described with respect to the embodiments ¦of Figures 1-3. In any event, the sheet material 67 and 69 ¦will be formed.with the appropriate decorative finish on the ¦outside thereof. Around the edges of the core 65 are attached ¦by gluing two layers of the fireproof fiberboard material, ¦thus forming stiles 71 and 73 in the embodiment of the door lS ¦ shown in Figure 7.
¦ ~lthough the various aspects of the present invention ¦ have been described with respect to particular composite wood ¦ material and fire door construction as preferred embodiments, ¦ it will be understood that the invention is entitled to ¦ protection within the full scope of the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fire-retardant wood based structural material comprising a first layer of material being formed from a slurry mixture of wood chips, ground bark and a fire-retardant chemical that is formed into a board by pressure and drying, and a second layer of material being formed from a slurry mixture of individual wood fibres and a fire-retardant chemical that is formed into a board by heat and pressure, the first and second material layers being firmly adhered to each other in their dry states, thereby forming said composite structural material.

2. The structural material as claimed in claim 1, further including a third layer of material of the same type and substantially the same thickness as the second layer, the third layer being firmly adhered to the second layer on a side opposite from that adhered to the first layer.

3. A structural material as claimed in claim 1 or claim 2 wherein said first layer includes elongate fibres running the length of the board.

4. A structural material as claimed in claim 1 or claim 2 wherein said first layer includes elongate glass fibres running the length of the board.

5. A structural material as claimed in claim 1 or claim 2 wherein said slurry used in forming said first layer comprises;
a) a solution of magnesium chloride hexahydrate and water;
b) magnesium oxide, c) wood chips in the form of wood flakes, and d) ground fir bark.

6. A structural material as claimed in claim 1 or claim 2 wherein said slurry used in forming said first layer comprises:
a) about 47% by weight of a 'salt' solution made of 1.327 parts of magnesium chloride hexahydrate to one part of water, b) about 29% by weight of magnesium oxide, c) about 6% by weight of pine wood flakes of No. 6 mesh screen size (1/8 inch diameter); and d) about 18% by weight of fir bark ground into needle shaped particles of the following size;
i) No. 65 mesh screen, 45% plus or minus 10%
ii) No. 100 mesh screen, 35% plus or minus 10%
iii) No. 200 mesh screen, 20% plus or minus 10%.

7. A structural material as claimed in claim 1 or claim 2 wherein the thickness of the first layer is several times that of the second layer.

8. A method of making a fire-retardant wood based structural material comprising the steps of forming a slurry of wood chips, ground bark and a fire-retardant chemical, pouring the slurry into a mould cavity, placing a board on top of the slurry within the sides of the mould cavity and pressing downwards to drive out excess water and compressing the material to form the first board, leaving said compressed material to dry, removing the dried material forming the board from said mould cavity; curing the dried board in an oven; forming a second board by compressing and heating a slurry of individual wood fibres and a fire-retardant chemical; and bonding the second board to one face of the first board.

9. A method as claimed in claim 8 including pouring part of said slurry of wood chips, ground bark and a fire-retardant chemical into said mould cavity, laying a plurality of fibres along the major length of the cavity and then pouring onto said mould cavity over said fibres the remainder of said slurry.

10. A method as claimed in claim 9 wherein said fibres are glass fibres.

11. A fire resistant door having a core and bonded to that edge of the core forming a stile which in use is hinged to a door jamb, a strip of fire-retardant wood based structural material comprising a first layer of material being formed from a slurry mixture of wood chips, ground bark and a fire-retardant chemical that is formed into a board by pressure and drying, and a second layer of material being formed from a slurry mixture of individual wood fibres and a fire-retardant chemical that is formed into a board by heat and pressure, the first and second material layers being firmly adhered to each other in their dry states, thereby forming a composite structural material.

12. A door as claimed in claim 11, wherein the second layer is thinner than the first layer which is bonded to said edge.

13. A door as claimed in claim 11, wherein said strip is substantially the same width as the width of said edge

14. A door as claimed in claim 12 wherein said strip is substantially the same width as the width of said edge.

15. A door as claimed in claim 11, 12 or 13 in which said strip is bonded to the edges of the stile and the top of the door.

16. A door as claimed in claim 14, in which said strip is bonded to the edges of the stile and the top of the door.

17. A door as claimed in claim 11 in which said core is of sheet material having substantially uniform thickness and edges which are planar and perpendicular to major plane surfaces of the core, said edges having bonded thereto said strip.

18. A door as claimed in claim 17, wherein the thickness of the core material and its composition, and the thickness of the strip at the edges of the core material along with its fire-retardant characteristics are selected to have a fire rating of one and one-half hours or more.

19. A door as claimed in claim 11,which is adapted for mounting with the strip on the edge of the stile for hingedly attached to the door jamb, the jamb including a door stop positioned to be held against the strip when the door is closed, the door stop having a thickness that is significantly thinner than the combined thickness of the sheets of the strip taken in a direction away from the door edge.

20. A door as claimed in claim 12, in which said core is of sheet material having substantially uniform thickness and edges which are planar and perpendicular to major plane sur-faces of the core, said edges having bonded thereto said strip.

21. A door as claimed in claim 20 which additionally com-prises a second strip extending across the width of the core dividing it into separate upper and lower portions, the second strip having a centre board with the same characteristics as said first layer with boards adhered on either side thereof to form a smooth surface with opposing surfaces of the core material and that have the same characteristics as the second layer of the first mentioned strip.

22. A door as claimed in claim 20, wherein the thickness of the core material and its composition, and the thickness of the strip at the edges of the core material along with its fire-retardant characteristics are selected to have a fire rating of one and one-half hours or more.

23. A door as claimed in claim 22, which additionally com-prises a second strip extending across the width of the core dividing it into separate upper and lower portions, the second strip having a centre board with the same characteristics as said first layer with boards adhered on either side thereof to form a smooth surface with opposing surfaces of the core material and that have the same characteristics as the second layer of the first mentioned strip.

24. A door as claimed in any one of claims 17, 18 or 21 which additionally comprises facing sheet material attached to the major planar surfaces of the core material as extended by the strip, thereby to form continuous surfaces on each side of the door.

25. A door as claimed in claim 23 which additionally comprises facing sheet material attached to the major planar surfaces of the core material as extended by the strip, thereby to form continuous surfaces on each side of the door.

26. A door as claimed in any one of claims 17, 18, or 21, wherein the core material includes a solid mineral material.

27. A door as claimed in claim 23, wherein the core mater-ial includes a solid mineral material.

28. A door as claimed in any one of claims 17, 18 or 21 which additionally comprises facing sheet material attached to the major planar surfaces of the core material as extended by the strip, thereby to form continuous surfaces on each side of the door, the core material including a solid mineral material.

29. A door as claimed in claim 23, which additionally com-prises facing sheet material attached to the major planar sur-faces of the core material as extended by the strip, thereby to form continuous surfaces on each side of the door, the core material including a solid mineral material.