FLAME RETARDANT PLYWOOD PANEL

Abstract: A flame retardant plywood panel has one or More structural core layers (12) sandwiched between front and back veneer layers (14, 16). In the preferred embodiment, the veneer layers are coated with an unfired ceramic material (18, 20) which hardens when exposed to a flame and prevents the flame from igniting the core and veneer layers. Perforations are preferably provided in the back of the panel for the ventilation of steam and other gases generated in the panel by exposure to high temperature.

Description: FLAME ETARDANT PLYWOOD PANEL Background of the Invention This invention relates to plywood panels used in the construction of walls for buildings, mobile homes, residential homes, offices, trailer homes, boats, and the like. Particularly, the invention relates to the construction of a plywood panel which is flame retardant and which will maintain its structural integrity when exposed to a flame. Typically, plywood wall panels consist of a bonded lamination of several layers of wood comprising one or more structural core layers of a strong, but inexpensive, wood with a thin wood veneer layer bonded to both the back and front sides of the core structure. The veneer layer on the front surface of the panel is usually of an expensive wood which has decorative qualities, in which case the veneer will be coated with a transparent finish which will allow the wood grain to show through. Alternatively, the veneer may be covered with a paper which is printed with a decorative wood grain. The layers of the laminated panel are bonded to one another by means of a glue. Such plywood panels are strong, decorative, easily manufactured, and relatively inexpensive. However, such panels will ignite when subjected to a temperature of 1200 to 1500 degrees Fahrenheit and therefore such panels will be relatively rapidly consumed when exposed to a flame. This characteristic can contribute greatly to the spreading of a fire within the structure in which such panels are used. Heretofore, flame retardant wall panels have generally been constructed from gypsum board. However, gypsum board panels are significantly heavier in weight than plywood panels. The relative heaviness of gypsum board panels is a particular disadvantage in such applications as mobile homes and boats, since more structural framing is required to support a gypsum board wall structure. The added wall structure, coupled with the weight of the gypsum board panels, thus results in a substantially heavier mobile home or boat. Furthermore, heavier panels are disadvantageous in any building application because of the inconvenience involved in moving and handling such panels. Therefore, there has long been sought a wall panel which combines the flame-retardant qualities of gypsum board with the strength, lightness of weight, and economy of manufacture of plywood. Summary of the Invention In the preferred embodiment of the invention, the front and back veneer layers of a plywood panel are coated with a thin layer of a ceramic mixture. When exposed to a flame, this ceramic coating hardens to form a non-combustible shield, which acts as a fire break, allowing heat to pass through and char the veneer and core layers without a-llowing open flame to ignite these wooden layers. A significant feature of the invention is that the wooden layers are merely charred and not consumed, and, therefore, a substantial portion of the original structural integrity of the panel is retained. Perforations are preferably provided through the rear of the panel up to, but not through, the front veneer layer. These perforations provide for the dissipation of heat and steam and other gases which are prodiced within the panel when it is exposed to heat, thereby preventing the blistering and rupturing of the ceramic layers by a pressure buildup of these gases within the panel. In an alternative embodiment of the invention, the veneer layers are bonded to the core layer or layers with a bonding agent that is essentially of the same ceramic composition as the aforementioned veneer coatings. An additional important advantage of this invention is that the plywood panels having the aforementioned flame-retardant qualities also retain the substantial strength, lightness of weight, and ease and economy of manufacture that are characteristic of standard plywood panels. Brief Description of the Drawings Figure 1 is a rear perspective view of a plywood panel constructed in accordance with the present invention; Figure 2 is an enlarged fragmentary sectional view of the preferred embodiment of the invention; and Figure 3 is a view similar to that of Figure 2 of an alternative embodiment of the present invention. Detailed Description of the Invention Referring to the figures, a plywood panel constructed in accordance with the present invention is designated by the numeral 10. As seen in Figures 2 and 3, the panel 10 consists of a lamination of a structural core layer 12, a back veneer layer 14, and a front veneer layer 16. A backing layer 18 and a facing layer 20 of non- combustible material are applied to the outside surfaces of the back veneer 14 and the front veneer 16, respectively. Although several different types of non- combustible materials may be used for the layers 18 and 20, it has been found that a coating of a ceramic mixture wetly applied is most suitable for use as the backing and facing layers. This is s o because such a ceramic composition, besides having the necessary property of being non-combustible, is also inexpensive, easily applied, self-adhering with the underlying veneer, and will maintain its integrity under exposure to high temperatures. A specific example of such a ceramic composition which has been successfully used to make flame-retardant panels in accordance with this invention has the following formula: 5%-20% Clay (China clay or kaolin; 10% preferred) 5%-20% Feldspar (10% preferred) 5%-20% Silica (10% preferred) 10%-40% Aluminum hydrate (20% preferred) 10%-40% Talc (20% preferred) 5%-20% High borate content frit (such as is marketed by Pemco under the trademark Frit P-380; 10% preferred) Approximately 20% Sodium silicate These ingredients are mixed with a relatively small amoun of water to form a slurry which is then applied wet as the coating which constitutes the backing layer 18 and the facing layer 20. The thickness of the layers 18 and 20 is preferably about 3 to 5 mils when dry. Such a thickness may be conveniently obtained by applying a series of relatively thin coatings (approximately 3 mils wet) , each of which is allowed to dry (thinning to a 1 to 2 mil thickness as it dries) before the next coating is applied until the ultimately desired thickness is achieved. When the ceramic composition is dry, the backing layer 18 and the facing layer 20 will thus each consist of a 3 to 5 mil thick layer of unfired ceramic. The back of the panel is punctured with a multiplicity of perforations 22. These perforations 22 should extend from.the ceramic backing layer 18 through all the successive layers up to, but not through, the front veneer layer 16. The perforations 18 may be conveniently provided by running the back of the panel under a thick embosser having pins which are modified to puncture the panel from the back and into * , but not through, the front veneer layer 16. The number, size, and pattern of the perforations 22 is not critical. It is only necessary that enough area of the panel be perforated to perform adequately the venting of heat- produced gases, as will be presently described. When the plywood panels are installed, the perforated back surface will face the rear side of the wall structure while the unperforated front surface will face the front side or interior of the room. When the panel is exposed to high temperature, as from a fire in the room, the unfired ceramic backing layer 18 and facing layer 20 will blister and foam and harden to become a ceramic film. As a result, these layers form a fire break, allowing heat to pass through the ceramic layers and char the veneer and core layers but not allowing open flame to pass through and ignite the interior wooden layers. This results from the fact that the front surface of the front veneer layer 16 is effectively sealed against the entrance of air, and therefore oxygen, by the formation of an unbroken, non- combustible shield by the ceramic facing layer 20 when that layer is exposed to a flame. The perforated back surface of the panel, which faces the supporting wall structure, is not exposed to the flow of air and therefore the perforations 22 will not permit the entrance of sufficient oxygen to cause ignition of the wooden layers. When the panel is exposed to high temperatures, and the charring of the veneer and core layers begins, steam and other gases will be generated within the panel. The perforations 22 allow these gases to escape, thereby preventing pressure buildup from these gases which would rupture the ceramic layers, thereby defeating the fire break function described above. Further, the perforations serve to dissipate heat from the interior of the panel, thereby acting as an additional factor against the ignition of the wooden layers. The size and number of the perforations 22 will control the amount of gaseous matter which is allowed to escape. Therefore, it is necessary to ensure that the perforations are sufficient in quantity and size to prevent excessive buildup of gas pressure within the panel. Such a determination will depend on such factors as the thickness of the panel and the type of material of which it is made. A significant feature of the invention is that although the wooden wall panel layers are charred and tend to deteriorate, the laminating ceramic layers remain substantially intact thereby maintaining the structural integrity of a wall for a period of time substantially longer than untreated plywood wall panel. During the normal use of the plywood panel, the perforations 22 serve the additional function of allowing moisture to enter and leave the panel, thus allowing the panel to "breathe" in response to changes in ambient humidity, and thereby helping to prevent warping of the panel. The preferred embodiment of the invention, as shown in Figure 2, is easily manufactured. The manufacturing process is commenced with a ready-made plywood panel in which the veneer layers 14__and 16 are laminated to the core layer 12 by a glue-type bonding agent applied as shown in Figure 2 as glue layers 24 with one such glue layer between the core layer 12 and the back veneer layer 14 and a second such glue layer between the core layer and the front veneer layer 16. This preassembled plywood panel is then coated on the back of the back veneer layer 14 with the ceramic backing layer 18. At this point the perforations 22 are made by a tick embosser, as previously described, and finally the ceramic facing layer 20 is applied in the manner previously described. Since the ceramic compound is not transparent, the wood grain of the front veneer layer 16 will not show through. Therefore, it is necessary, if a wood grain finish is desired, to cover the facing layer 20 with a paper having a wood grain finish. r-^m Alternatively, a pattern may be printed directly on the facing layer 20. An alternative embodiment of the invention is shown in Figure 3. This embodiment differs from the preferred embodiment in that the same ceramic compound which is used for the backing layer 18 and the facing layer 20, is also used as the bonding agent for laminating the front veneer layer 16 and the back veneer layer 14 to the core layer 12. Thus, as shown in Figure 3, the veneer layers are joined to the core layer by ceramic bonding layers 26, one such bonding layer between the core layer 12 and the back veneer layer 14 and the second such layer between the core layer and the front veneer layer 16. Such construction necessitates a slightly different method of manufacture from that of the preferred embodiment. With this method, the core layer 12 is first coated front and back with the previously described ceramic compound to form the bonding layers 26. It has been found that a bonding layer thickness of approximately 2 mils gives good results. While the layers 26 are still wet, the veneer layers 16 and 14 are applied, and when the ceramic compound in the bonding layers 26 dries- the veneer layers are bonded to the core layer. The result is a plywood panel similar in all respects to the ready-made panel used in the preferred embodiment except for the nature of the bonding agent between the laminated wood layers. From this point, the method of manufacture is identical to that of the preferrrd embodiment. The backing layer 18 is applied, perforations 22 are formed by a tick embosser, and the face layer 20 is finally applied. As in the preferred embodiment, * the face layer 20 may be directly printed or it may be covered with a printed paper. As can be seen from a comparison of the two embodiments, the preferred embodiment offers the advantage of being adaptable for use with any ready-made plywood panel, and it is therefore relatively easily and inexpensively produced. The alternative embodiment offers the advantage of having the two additional ceramic layers 26, which, when exposed to high temperatur will harden and act as additional fire breaks, much as the backing layer 18 and the facing layer 20. These layers 26 will therefore act to enhance the flame retardant effect in the panel while, at the same time, offering a somewhat greater degree of structural integrit . Although the invention has been shown with a plywood panel having a single core layer, it is to be understood that either embodiment is readily adaptable to a panel having multiple core layers. In such a case, the core layers may be laminated to each other either by glue, as in the preferred embodiment of the invention, or by the ceramic compound, as in the alternative embodiment. A specific example of a plywood wall panel embodying the preferred embodiment of the present invention was constructed as follows: A ready-made panel was provided which was comprised of a 1/10-inch thick structural core layer of Lauan core stock with front and back veneer layers of 1/26-inch thick Lauan (A Grade on front, matching B Grade on back) . A ceramic slurry was prepared using the preferred proportions of the ingredients specified above. This slurry was then roller coated on the rear surface of the back veneer layer to a wet thickness of approximately 3 mils. This coating was allowed to dry down to a dry thickness of 1 to 2 mils, and then a second and third such coating were similarly applied and allowed to dry, resulting in total dry thickness for the ceramic backing layer of about 3-1/2 to 4 mils. A multiplicity of holes were then punched into the back of the panel up to, but not through, the front veneer layer. The holes were approximately 1/16-inch in diameter and spaced about 1 inch on center. Finally, the ceramic facing layer was applied, in a like manner and thickness as the backing layer. The panel so constructed was then tested for its flame-retardant qualities as follows: A test section of 14 inches by 72 inches was cut from the panel and laid across the open top section of a tunnel kiln so that the ceramic facing layer was exposed to and impinged by the flame in the kiln. A draft was then induced in the kiln to draw the flame down the length of the panel. The panel was exposed to the open flame in this manner for a period of 13 minutes and then removed from the flame. The panel showed no signs of ignition as a result of this exposure.

Claims: .WHAT IS CLAIMED IS: 1. A wall panel having means forming a structural layer (12, 14, 16) having front and back faces, characterized by: non-combustible means (18, 20) applied to and substantially covering at least one of said faces, for preventing an open flame from passing through the covered face of said panel, while heat from said flame is allowed to pass through said non- combustible means to char said structural layer in a controlled manner without ignition. 2. The wall panel of Claim 1, further characterized by the fact that said non-combustible means is a non- combustible material that hardens to provide a substantially air-tight fire break for said panel when said panel is exposed to a flame. 3. The wall panel of Claim 1 or 2, further characterized by the fact that said non-combustible means comprises a coating of unfired ceramic. 4. The wall panel of Claim 1, 2 or 3, further characterized by the fact that said non-combustible means is applied to both said front and back faces, forming a fronting layer and a backing layer of non- combustible material. 5. The wall panel of Claim 4, further characterized by the fact that said structural layer and " said backing layer are provided with plural perforations (22) . 6. The wall panel of Claim 1, 2, 3, 4 or 5, further characterized by the fact that said structural layer comprises: a core layer (12) ; a front veneer layer (16) bonded * to one side of said core layer; and a back veneer layer (14) bonded to the other side of said core layer. O «- .N 7. The wall panel of Claim 6, further characterized by the fact that said front and back veneer layers are bonded to said core layer by layers (24) of said non- combustible means. 8. The wall panel of Claim 1, 2, 3, 4, 5, 6, or 7 further characterized by the fact that said non- combustible means preserves a substantial amount of structural integrity when said panel is exposed to a flame, thereby preserving a substantial portion of the structural strength of said panel when said structural layer is charred. 9. The wall panel of Claim 1, 2, 3, 4, 5, 6, 7 or 8 further characterized by the fact that said non- combustible means is substantially formed of a mixture comprising: approximately 5 percent to 20 percent clay; approximately 5 percent to 20 percent feldspar; approximately 10 percent to 40 percent aluminum hydrate; approximately 10 percent to 40 percent talc; approximately 5 percent to 20 percent high borate content frit; and approximately 20 percent sodium silicate. 10. The wall panel of Claim 1, 2, 3, 4, 5, 6, 7, 8 or 9, further characterized, by the fact that said non-combustible means is substantially formed of a mixture comprising: approximately 10 percent clay; approximately 10 percent feldspar; approximately 10 percent silica; approximately 20 percent talc; approximately 20 percent aluminum hydrate; approximately 10 percent high borate content frit; and approximately 20 percent sodium silicate. 11. A method for flame-proofing a wall panel having a structural core layer to which are bonded a front veneer layer and a back veneer layer, characterized by: applying a first layer of non-combustible material to said back veneer layer and a second layer of non-combustible material to said front veneer layer. 12. A method of Claim 11, further characterized by the fact that said non-combustible material is an unfired ceramic. 13. The method of Claim 11 or 12, further characterized by the fact that said non-combustible •>'. material is substantially formed of a mixture comprising: approximately 5 percent to 20 percent clay; approximately 5 percent to 20 percent feldspar; approximately 5 percent to 20 percent silica; approximately 10 percent to 40 percent aluminum hydrate; approximately 10 percent to 40 percent talc; approximately 5 percent to 20 percent high borate content frit; and approximately 20 percent sodium silicate. 14. The method of Claim 11, 12 or 13, further characterized by the fact that said non-combustible material is substantially formed of a mixture comprising: approximately 10 percent clay; approximately 10 percent feldspar; approximately 10 percent silica; • approximately 20 percent talc; approximately 20 percent aluminum hydrate; approximately 10 percent high borate content frit; and approximately 20 percent sodium silicate.

Inventors: MCLAREN J
Applicants: MCLAREN J
Application year: 1980
Main classification: B05D3
IPC codes: C04B28, E04B1