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1. Field of the Invention
The present invention pertains to the art of doors and, more particularly, to the construction of a door having a fire resistant core.
2. Discussion of the Prior Art
Doors for commercial, and even domestic, applications are often rated for fire resistance. More specifically, fire doors are rated based on their ability to resist burning in the case of a fire, with standard ratings being defined as 20, 45, 60 and 90 minutes depending on the length of time a door can withstand a rating temperature, typically in the range of 1700+° F. The higher the minute rating, the better the fire rating. For instance, low rated fire doors are typically made from an organic material, while high rated fire doors are generally made from mineral or metal materials. More specifically, a 20 minute door typically includes a particle or stave board core. For 45, 60 and 90 minute ratings, a wood door generally has a mineral core.
In general, the higher rated fire doors have more costly constructions. It is known in the door industry that a particle board or wood fiber board can achieved a 20 minutes fire rating, but cannot withstand the hose stream test in a 45 minutes fire test. It is also more difficult or impossible when two layers of fiberboard are laminated together with PVA adhesive. In a typical 45 minutes fire test, the first layer will fall in the furnace after 15 to 20 minutes and the burning will progress fast thereafter leaving little structure to hold the brutal effect of the hose stream test.
Fire rated door cores for 45-60-90 minute ratings are generally made of inorganic materials such as gypsum, perlite, vermiculite and calcium silicate. The process manufacturing is usually by casting. These panels have disadvantages of not being able to provide good mechanical strength specially for screws or nails and they are very dusty for any cutting or boring. They are also very fragile and do not provide always good lamination for the door skins. Of course, these materials are more expensive than wood or wood-based products.
As indicated above, a fire rating of at least 45 minutes is usually achieved with inorganic door cores, rather than with a core made of solid wood, wood particle board or wood fiber board. It has also been more difficult to obtain such results with lamination of two layers of particle board or wood fiber board because of delamination of one layer during the fire test. Also, a high level of distortion or cupping is occurring on a large door, specially when the density of the core is exceeding 40 lbs.
Based on the above, it would be a substantial advantage to provide a cost efficient 45 minutes fire door that would offer good screw holding power on the entire core for hardware or other fastening devices and an effective surface for high strength lamination of door skins, while also being light, stable, sound reducing and ecological.
The present invention is directed to producing a laminated fire door including a core containing panels or boards made from an organic material and a fibrous binder or adhesive, wherein the door exhibits at least a 45 minute fire rating. More specifically, a fire door constructed in accordance with the invention includes a core having multiple panels made from an organic material containing flax, having sandwiched therebetween a relatively thick layer of a fibrous binder, preferably a sodium silicate adhesive. In accordance with a preferred form of the invention, the fire door core is formed by using a silicate adhesive, preferably sodium silicate, in the order of 25 mils (0.6 mm) to laminate together two flax boards, each having a thickness in the range of 0.625-0.875 inches (approximately 16-22 mm). In accordance with the most preferred form of the invention, each of the laminated flax boards has a density of between 20 and 45 pounds per cubic foot (320-700 kgs/m3). The laminated flax board panels can then be trimmed to be used as a core in making a specified sized door. With the exposed organic surfaces, outer door skins can be readily and effectively laminated to the core.
With this construction, a fire door can be readily manufactured with a fire rating level of at least 45 minutes out of laminated organic panels containing flax which establish advantageous adhesion qualities for door skins. Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views.
FIG. 1 is a partial sectional view of a portion of a fire door constructed in accordance with the present invention; and
FIG. 2 is a perspective view of a system for producing a fire door core shown in FIG. 1.
With initial reference to FIG. 1, a fire door constructed in accordance with the present invention is generally indicated at 2. As shown, door 2 includes an outer peripheral frame 5, constituted by interconnected rails, an upper one of which is shown at 7, and stiles, one of which is shown at 8; a core 11; an inner door skin 14; and an outer door skin 15. In general, the above construction of door 2, with the exception of the particulars of core 11, is known in the art. Therefore, these aspects of door 2 will not be discussed further here in detail.
The present invention is particularly directed to the construction and method of forming core 11. In general, core 11 constitutes a laminate including a first panel 20, a second panel 21 and an intermediate fibrous binder layer 23. In accordance with the most preferred form of the invention, each of first and second panels 20 and 21 is formed from an organic material, specifically flax. On the other hand, intermediate binder layer 23 is preferably constituted by a sodium silicate adhesive.
In the most preferred form of the invention, each of first and second panels 20, 21 has a thickness in the order of 0.625-0.875 inches (approximately 16-22 mm) and are spaced by intermediate binder layer 23 in the order of 25 mils (0.6 mm). Of course, these distances and dimensions are merely presented in connection with a preferred embodiment of the invention such that they can be readily altered in connection with forming other standard door thicknesses and/or custom designs.
It has been found that, by using flax board, instead of wood, and by using a sodium silicate fibrous adhesive, core 11 remains together during a standard 45 minute burn test. Also advantageous is the fact that the flax board offers a very high insulation coefficient versus particle board or inorganic products because it contains air pockets in its structure, with the air pockets also reducing the burning rate. The sodium silicate adhesive provides a rating of 2000° F. and can contain talc, aluminum silicate of potash and soda, magnesium silicate, cellulose and/or inert fillers, kaolin clay, mica, perlite and/or cristaline silica. It should be noted that the sandwich construction of flax board would not achieve the 45 minutes rating if laminated with PVA, poly-urethane, epoxy or any other organic adhesives. Therefore, it is the combination of flax board and sodium silicate that makes this core so resistant to fire by creating a heat barrier in center of the core. For instance, using only flax board of even 1.5 inches (3.8 cm) thick as the core would not achieve the same results and would fail the fire test. By constructing door cores in accordance with the invention, doors can be assembled easily with standard production lines that are handling the high volume of 20 minutes particle board constructions. The door faces or skins can be readily laminated to the door core without special adhesives because two organic surfaces are being bonded together.
FIG. 2 illustrates a preferred manner of forming core 11 in accordance with the invention. In general, each of first and second panels 20 and 21 are originally received from a stack 33 of corresponding panels arranged on an in-feed scissor lift 35. Each panel 20, 21 is systematically pushed directly through a coating apparatus 48, at which both the first and second panels 20 and 21 are coated with a respective portion of binder layer 23, and then placed upon a conveyor 42. In general, this step can be performed in various ways, including spraying, pouring, painting and the like. In the most preferred embodiment, a binder layer coating 23 of sodium silicate adhesive having a thickness of about 12 mils (0.3 mm) is applied with a roller spreader on exposed surface of each panel 20, 21. Thereafter, each panel 20, 21 is transferred to another scissor table 50 as illustrated in FIG. 2 with respect to a previously coated panel 20 being arranged on top of a stack 55 of cores 11. As illustrated, top panel 20 on stack 55 has an exposed coating of the binder layer 23. Thereafter, after panel 21 is fully coated, panel 21 is flipped from upon conveyor 42 to a position atop panel 20 in stack 55 thereby combining the binder layers 23 to establish an adhesive thickness of approximately 25 mils (0.6 mm). In this manner, each set of panels 20 and 21 are laminated together to form the various cores 11 arranged in stack 55.
In accordance with the most preferred form of the invention, after stack 55 has achieved a desired number of cores 11, e.g., 20-25 cores, stack 55 is shifted upon a conveyor 60 to within a cold press indicated at 65 for a prior stack 70 of cores 11. Each stack 70 remains in cold press 65 until the various binder layers 23 are fully dried or cured, generally in the order of one hour. Thereafter, the stack 70 is removed from cold press 65 upon conveyor 75 such that cores 11 can be used in making a requisite number of doors 2.
With the above arrangement, it has been found that a portion of the sodium silicate adhesive penetrates the air pockets in the flax board, becomes tacky and sticky, and dries in a cold press to form a hard and glassy bond to weld the two panels together in order to avoid separation or delamination during a 45 minute fire test. Core 11 can then be trimmed on any or all of its four sides for use in making a specified sized door 2, such as with the addition of frame 5 and inner and outer door skins 14 and 15.
With this arrangement, it has been found that the flax-based core 11 achieves at least a fire rating level of 45 minutes. Therefore, a relatively inexpensive door 2 can be formed in accordance with the present invention which exhibits low weight, high mechanical strength, exceptional bonding for outer door skins, good screw holding power, the ability to be readily cut to various sizes, easy machining characteristics, and low dust generation during machining as compared to more expensive mineral cores.
Although described with reference to a preferred embodiment of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof.