Patent Publication Number: US-6713542-B2

Title: Method for producing a flame resistant cellulosic sheet material

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for producing flame retardant cellulosic materials and more particularly to a method for producing a flame retardant particle board. 
     BACKGROUND FOR THE INVENTION 
     Cellulosic materials such as paper, wood, cotton and the like are commonly used for household and commercial applications. Such materials are highly flammable and when ignited the fire spreads rapidly. Nevertheless, such materials are commonly used. Accordingly, a number of methods for treating such materials to reduce their flammability have been developed. However, such methods may be inadequate for today&#39;s standards, may prove too costly for the commercial market, provide too little protection against fire and smoke and result in products with other shortcomings. 
     The use of cellulosic products which are treated with fire retardant materials frequently involve human beings, who incorporate such products in a confined space as for example in a home, kitchen or other room. Therefore the products must provide a safe environment and at the same time provide a pleasing appearance particularly when the products are made of particle board. Therefore the use of volatile or harmful solvents should be minimized and preferably avoided. 
     It is also important to eliminate volatile or harmful solvents which could cause problems during a process for treating cellulosic material with a flame retardant agent. Further, it is important that any treatment minimizes the likelihood of chemical exuding from the processed material. It is also important that any chemical treatment should not produce or release toxic substances onto the surface of a product. 
     Conventional methods for treating cellulosic materials such as particle board and the like typically include the application of salts of phosphoric acid, boric acid etc. Other methods use various organic solvent systems. Both of such methods may be economical, but often result in crystallization of salts on the surface of a wood product. In addition, such methods may result in migration of undesirable chemicals to the surface of products made thereby. The solvents used are often volatile during the application of the flame retardants and exude from the finished products. 
     It has also been recognized that ammonium phosphates are among the more effective inorganic fire retardants which are commonly used to treat cellulosic materials. Such phosphates are typically present as diammonium phosphate, monoammonium phosphate or simple or complex mixtures of such phosphates. Fire retardants of this type are prepared by reacting aqueous phosphorous acid with an alkaline oxide such as ethylene oxide. 
     A more recent approach to provide a non-blooming fire retardant composition is disclosed in the U.S. Pat. No. 4,539,045 of Wagner. As disclosed therein, an ammonium phosphate-containing fire retardant includes an effective amount of boric acid or alkali metal borate. The ammonium phosphate-containing fire retardant comprises a reaction mixture of aqueous phosphoric acid and an alkylene oxide. Boric acid or alkali metal borate is present in an amount of from about 2.5 to about 12 percent by weight of solids. 
     It is now believed that there may be a large commercial demand for an improved fire retardant cellulosic products in accordance with the present invention. It is believed that this demand extends to a fire retardant particle board which has improved heat and flame resistance. A demand for an improved fire retardant cellulosic material is further supported by the products&#39; enhanced properties. For example, such products do not include toxic substances and produce minimal smoke when exposed to heat and/or open flame. Accordingly, there is no harmful vapor or toxic substance to deal with during the manufacturing process. 
     In addition, products in accordance with the present invention do not exude toxic substances during or after manufacture or when exposed to high temperatures and/or open flame. Further the chemicals incorporated do not migrate to the surfaces or result in unsightly blemishes. Finally it is believed that the products as disclosed herein can be manufactured in a cost effective manner and brought to market at a competitive price. 
     BRIEF SUMMARY OF THE INVENTION 
     In essence, the present invention contemplates a method for producing flame resistant or fire retardant cellulosic materials and more particularly to a method for producing a fire retardant particle board with improved resistance to flame and heat. The method includes the step of providing an aqueous solution of monoammonium phosphate and diammonium phosphate. The solution contains between about 0.325 and about 1.0 moles per liter of monoammonium phosphate and between about 0.325 and 1.0 moles per liter of diammonium phosphate. A metal hydroxide, preferably aluminum hydroxide, is added to the aqueous solution in an amount of between about 0.325 and 1.0 moles per liter to thereby provide a modified solution. 
     The method also includes the step of providing a mass of cellulosic material, preferably wood chips or fibers of the type used in producing a conventional particle board. About 450 to about 550 grams and preferably about 500 grams of cellulosic material per liter is added to the previously mentioned solution. After adding the cellulosic material to the modified solution, the mix is stirred or agitated until the cellulosic material is thoroughly saturated. In practice vigorous stirring, i.e. a conventional stirrer at medium speed, is continued until the mix forms a uniform dark brown color. To ensure thorough saturation, the stirring or other agitation may continue for about one minute after the uniform dark color is reached. 
     The mix is then dried in a conventional manner at a temperature of between about 105° C. to about 120° C. The higher temperatures provide more rapid drying, but care should be used to avoid overheating or carbonizing the heated material. 
     The method also includes the step of providing a powdered glue which is selected from the group of urea formaldehyde resin, phenolic resin or mixtures thereof. In a preferred embodiment of the invention, the powdered glue is dissolved in water and then mixed with an aqueous solution of diammonium phosphate and monoammonium phosphate for a period of 10 to 15 minutes. The treated glue should contain between about 54 to about 66 percent by weight water, between about 33 to about 39 percent by weight glue, between about 1.5 and about 2.5 weight percent monoammonium phosphate and between about 1.5 and 2.5 weight percent diammonium phosphate. 
     The dried saturated cellulosic material is then mixed with the modified glue composition. 
     Finally the pulverized mass is formed into a mat and placed on a suitable press where it is subjected to heat and pressure to consolidate the particles and thereby form a particle board with enhanced heat and flame resistance. 
     The invention will now be described in connection with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical representation showing the composition of a fire retardant composition or solution as used in the production of flame retardant particle boards in accordance with the present invention; 
     FIG. 2 is a pie chart illustrating the compositions of one preferred solution from FIG. 3; 
     FIG. 3 is a pie chart illustrating a preferred glue composition as used in the production of a fire retardant particle board in accordance with the present invention; 
     FIGS. 4 a, b, c  are flowcharts illustrating the process steps for imparting improved flame and/or fire retardancy to particle board or the like in accordance with the present invention; 
     FIG. 5 is graphical representation of the final composition of a fire retardant particle board which was made in accordance with the present invention and shows the composition in weight percent at various molar concentrations of monoammonium phosphate and diammonium phosphate; and 
     FIG. 6 is a pie chart illustrating the preferred composition of a fire retardant particle board from FIG. 1 at 0.4M MAP/DAP. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     The widespread use of cellulosic particle board and increased concern for safety has led to a demand for an improved flame retardant fiber board. For the purpose of the present invention, the term cellulosic particle board encompasses boards such as hard boards, particle board, fiber board, medium density fiber board (MDF), oriented strand board, card board and other pulp products which are made of cellulosic material. Such boards are made from cellulosic materials selected from a group consisting of wood fibers, sawdust, wood particles or chips, bagasse and cellulosic fibers to include pulp, pulp extracts and paper of all grades. 
     As contemplated by the present invention, wood chips or fibers of the type used in manufacturing conventional fiber board may be used in producing an improved particle board as disclosed herein. 
     The first step in manufacturing a cellulosic particle board with enhanced flame and heat retarding characteristics requires the use of a fire retardant solution as defined hereinafter. 
     Fire Retardant Solutions 
     In general the fire retardant composition or solution as disclosed herein comprises an aqueous solution for use in impregnating cellulosic materials and textile fibers. The solution contains monoammonium phosphate and diammonium phosphate in equal molar amounts. For example the molar amounts of monoammonium phosphate and diammonium phosphate range from about 0.3 moles per liter to about 0.95 moles per liter or until maximum dissolution of both chemicals and preferably between about 0.35 moles per liter and 0.55 moles per liter. A metal hydroxide, aluminum, calcium or magnesium and preferably an aluminum hydroxide is added to the solution in an amount of between about 3.75 and about 11.87 grams per liter. 
     In the preferred solutions, the molar concentrations of the monoammonium phosphate (MAP) and diammonium phosphate (DAP) are equal and within the range of 0.35 and 0.55 moles per liter. Examples of compositions for use in the manufacture of cellulosic particle board and treatment of textile fibers are given in the following table. In addition to the examples shown, tests were conducted on sample compositions wherein the aluminum hydroxide was replaced with an equal amount of calcium hydroxide and also with an equal amount of magnesium hydroxide. Tests on those compositions were comparable to those obtained with aluminum hydroxide. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Moles 
                 MAP in Grams 
                 DAP in Grams 
                 Al(OH)3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0.3 
                 34.5 
                 39.6 
                 3.75 
               
               
                   
                 0.35 
                 40.25 
                 46.2 
                 4.375 
               
               
                   
                 0.4 
                 46 
                 52.8 
                 5 
               
               
                   
                 0.45 
                 51.75 
                 59.4 
                 5.625 
               
               
                   
                 0.5 
                 57.5 
                 66 
                 6.25 
               
               
                   
                 0.55 
                 63.25 
                 72.6 
                 6.875 
               
               
                   
                 0.6 
                 69 
                 79.2 
                 7.5 
               
               
                   
                 0.65 
                 74.75 
                 85.8 
                 8.125 
               
               
                   
                 0.7 
                 80.5 
                 92.4 
                 8.75 
               
               
                   
                 0.75 
                 86.25 
                 99 
                 9.375 
               
               
                   
                 0.8 
                 92 
                 105.6 
                 10 
               
               
                   
                 0.85 
                 97.75 
                 112.2 
                 10.625 
               
               
                   
                 0.9 
                 103.5 
                 118.8 
                 11.25 
               
               
                   
                 0.95 
                 109.25 
                 125.4 
                 11.875 
               
               
                   
                   
               
            
           
         
       
     
     The various compositions are also shown in FIG. 1 wherein the concentration of diammonium phosphate and monoammonium phosphate in percent by weight versus molar concentrations are shown. The curve  2  shows the content of the diammonium phosphate in percent by weight while the curve  4  shows the content of the monoammonium phosphate. A curve  6  illustrates the content of aluminum hydroxide which is added to the solution. 
     A representative example of a solution is shown in FIG. 2 by a pie chart which illustrates the percent by weight of each of the ingredients. For example the solution shown in FIG. 2 contains about 91 percent water as indicated by  5 , five percent diammonium phosphate  6 , four percent monoammonium phosphate  7  and less than one percent aluminum hydroxide as indicated by the numeral  8 . A process for making the solution of Table 1 will be described hereinafter in connection with FIG. 4 a.    
     In the manufacture of a particle board as disclosed herein, one liter of one of the above-identified solutions is added to about 500 grams of dry wood chips. The dry wood chips should have a moisture content of about 15 percent of less. The mixture of the wood chips and solution is thoroughly mixed until the wood is completely saturated. For example, the mixture may be stirred with a conventional mixer with shaft or rotating mixer with shaft of the type used with a five-gallon container at medium speed. The mixture is stirred for about 10 to 15 minutes and until the mix reaches a uniform dark brown color. The stirring may be continued for about one minute after displaying the uniform dark color to assure complete saturation. 
     The saturated wood chips are then dried to a uniform light brown or tan color at a temperature of between about 100 to 120° C. for about 45 minutes to about 70 minutes with some stirring. At this stage the wood chips will have a moisture content of about 15 percent. These treated wood chips or dried saturated wood chips are then mixed with a modified glue composition as will be described below. 
     Modified Glue Composition 
     The fire retardant cellulosic materials disclosed herein also include a modified glue composition. That composition is made from a urea formaldehyde resin, phenolic resin or mixtures thereof in the form of a dry powder. Preferred glues include conventional particle board glues such as Kaltliem Type h and Kaurit. In preparing a modified glue composition, about 75 grams of a urea formaldehyde powdered glue are added to about 100 milliliters of water and thoroughly mixed or stirred at about 30-50° C. If a phenolic resin is used the glue water MAP/DAP solution is mixed at about 60° C. The glue, which may be in a brown marble form, is mixed or stirred until all of the glue is dissolved to form an emulsion. Then, about 4 grams of monoammonium phosphate and about 4 grams of diammonium phosphate are dissolved in about 25 milliliters of water in a separate beaker. An additional one gram of aluminum hydroxide may also be added. This ensures that essentially all of the MAP/DAP dissolves in the water. The metal hydroxide may dissolve slightly but most will be undissolved but suspended in the glue water solution. 
     All of the glue water solution is mixed with about 25 milliliters of the water, monoammonium phosphate, diammonium phosphate mix to thereby form the modified glue composition. The monoammonium phosphate and diammonium phosphate are present in about equal if not equal molar concentrations. 
     In a second example of a modified glue composition, about 68 grams of powdered glue is dissolved in about 100 milliliters of water. Then between about 3.6 to about 4.4 grams of monoammonium phosphate and about 3.6 to about 4.4 grams of diammonium phosphate is mixed in a separate beaker with about 25 milliters of water to thereby form a modified glue composition by mixing both solutions in the manner shown in the flow chart FIG. 4 b.    
     One preferred glue composition is illustrated in FIG.  3 . As shown therein, the glue composition includes about 60 percent by weight water as indicated by the wedge  10 , about 36 percent by weight area formaldehyde resin  12 , about 2 percent by weight monoammonium phosphate  14  and about 2 percent by weight diammonium phosphate. As indicated the glue composition contains between about 0%-10% by weight aluminum hydroxide  16 . In some compositions this aluminum hydroxide is totally eliminated. 
     In a further example of a modified glue composition, about 82 grams of powdered glue is dissolved in about 100 milliliters of water. This initial solution is then mixed with a monoammonium phosphate, diammonium phosphate solution in the same manner and amounts as set forth in the aforementioned second example. 
     A number of other glue compositions were made using phenol formaldehyde. Each of those compositions were made by warming about 200 milliters of water to about 60° C. Then about 40, 60, 80, 100 and 120 grams of the powdered glue were each dissolved in about 200 milliters of warmed water with mixing. In a separate beaker about 6 grams of monoammonium phosphate and about 6 grams of diammonium phosphate were dissolved in water and the resulting solution added to each of the powdered glue water solutions mentioned above. Additional experiments were conducted using increments of 8, 10, 12, 14 and 16 grams each of monoammonium phosphate and diammonium phosphate. Each of the treated or modified glues had improved flame retardant properties. 
     Method for Making Fire Retardant Materials 
     A method for making cellulosic sheet material, as for example particle board, with enhance resistance to fire, flame and heat will be described in connection with FIGS. 4 a - 4   c . As illustrated in FIG. 4 c , the method includes the step  20  of providing a mass of wood chips or fibers having a moisture content of 15 percent or less. The particle size of the wood chips may vary widely, but are preferably in the same range of particle sizes as used in manufacturing conventional particle board. Separation of small particles from larger ones may be done to make a conventional sandwich particle board. 
     The method also includes the step  22  of providing an aqueous solution of monoammonium phosphate (MAP) and diammonium phosphate (DAP) wherein the MAP and DAP are present in equal molar lower concentrations as for example from FIG. 4 a . Such solutions which include a small amount of aluminum hydroxide are described above in the section of this specification identified as Fire Retardant Solutions. For example, about one liter of one of these solutions of Table 1 is added to between about 450 to 550 grams and preferably about 500 grams of dried wood chips in step  24 . 
     In step  26 , the wood chips are saturated with the solution from step  22 . This saturation is accomplished by mixing or stirring the wood chips and solution for about 10 to 15 minutes at a temperature within the range of about 20 to 25° C. When the wood chips are thoroughly saturated, the mix will take on a uniform dark brown color. This color change is relatively slow and usually takes place over a period of about several minutes, i.e. after mixing for approximately 10-20 minutes depending on the mixer efficiency. 
     The saturated wood chips from step  26  are then dried in a conventional manner in step  28 . This drying will normally be conducted at a temperature slightly higher than 100° C. (105°-110°) using care to ensure that only the water evaporates. The drying is continued until the dried saturated wood chips have a moisture content of about 15 percent or less. The drying step was accomplished in about 40-75 minutes depending on the relative humidity from temperature. At this point, the treated wood chips referred to herein from time to time as dried saturated cellulosic material will have a light tan color. Drying can also be done using a microwave oven set at low to medium power or by other conventional means. Using a microwave oven requires opening the oven and stirring every 2 to 3 minutes. 
     The method disclosed herein also includes a step  30  of providing a modified glue composition as described above in the section identified as Modified Glue Composition, continued from FIG. 4 b . In that step a urea formaldehyde powdered glue such as one available from BASF and identified as Kaltliem Type H is dissolved in water. This treated or modified glue composition contains between about 54 to about 66 percent by weight water, between about 33 to about 39 percent by weight glue and between about 1.5 and about 2.5 weight percent MAP and about the same amount of DAP. A small amount of aluminum hydroxide, about one percent or more by weight may be added. 
     In step  32  the modified glue composition is mixed with about 500 grams of the dried saturated wood chips, i.e. the wood chips that were saturated with the solution in step  26  and dried in step  28 . This mass of wood chips and modified glue form a mass or mat of treated wood chips. This mass or mat is then place on a conventional platen and subjected to heat and pressure in the same manner, temperature and pressures as used in manufacturing conventional particle board. For example the mat of particles may be subjected to a pressure of about 350 to 400 psi at a temperature of about 110-195° C. and preferably above 120° C. 
     At times a finished particle board made in accordance with the methods disclosed herein may have a thin white film on the surface of the board. This film may be readily removed by wiping with a damp cloth that is either slightly basic or slightly acidic as for example with sodium or potassium hydroxide or hydrochloric or sulfuric acid, respectively. 
     It is also contemplated that a thin laminate as for example with a surface finish such as wood grain may also be added to the particle board in accordance with the present invention. In such application the laminate is dipped in a solution as disclosed herein, dried and glued to the surface of the particle board with a treated glue composition as disclosed herein. 
     Particle Board Composition 
     The cellulosic sheet material or particle board composition as disclosed herein contains between about 60 to about 75 percent by weight wood chips, from about 8 to about 12 percent by weight of a modified glue as described above, between about 6.6 to about 13.6 percent by weight monoammonium phosphate and between about 7.4 and about 15.6 percent by weight diammonium phosphate. The particle board also contains between 0.8 and 1.6 percent by weight aluminum, calcium or magnesium hydroxide. In the composition disclosed herein, the molar concentration of the monoammonium phosphate and diammonium phosphate are about equal and preferably equal. Examples of particle board compositions having enhanced resistance to fire, flame and heat are given in the following Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 % MAP 
                 % DAP 
                 % Al 
                 % Wood Chips 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 5.817028 
                 6.587595 
                 0.717685 
                 75.54581854 
               
               
                   
                 6.557256 
                 7.438966 
                 0.796503 
                 74.09328344 
               
               
                   
                 7.269555 
                 8.258215 
                 0.872347 
                 72.69555103 
               
               
                   
                 7.955478 
                 9.047126 
                 0.945382 
                 71.34957725 
               
               
                   
                 8.616462 
                 9.807356 
                 1.015762 
                 70.0525394 
               
               
                   
                 9.253844 
                 10.54044 
                 1.083629 
                 68.80181637 
               
               
                   
                 9.868866 
                 11.2478 
                 1.149115 
                 67.59497093 
               
               
                   
                 10.46268 
                 11.93078 
                 1.212343 
                 66.42973395 
               
               
                   
                 11.03637 
                 12.59061 
                 1.273428 
                 65.30399007 
               
               
                   
                 11.59095 
                 13.22845 
                 1.332477 
                 64.21576497 
               
               
                   
                 12.12734 
                 13.84538 
                 1.389591 
                 63.16321374 
               
               
                   
                 12.64643 
                 14.44241 
                 1.444862 
                 62.14461051 
               
               
                   
                 13.14904 
                 15.02049 
                 1.498379 
                 61.15833894 
               
               
                   
                 13.63595 
                 15.58051 
                 1.550224 
                 60.20288372 
               
               
                   
                   
               
            
           
         
       
     
     The composition of the particle boards in accordance with the preferred embodiments of the invention are illustrated by FIG.  5 . As shown therein, the curve  22  indicates the percent by weight wood chips while the curve  24  indicates the percent by weight glue. The weight percent of DAP and MAP are illustrated by the curve  26  and  28  respectively. Finally, the curve  30  illustrates the percent by weight aluminum hydroxide. 
     One preferred embodiment of the invention is illustrated by a pie chart in FIG.  6 . As indicated therein the particle board contains about 73 percent by weight wood is shown by the wedge  32  and about 11 percent by weight includes powdered glue as indicated by the numbers  34 . About 8 percent by weight DAP and 7 percent by weight MAP are indicated by the segments  36  and  38  respectively. Finally the numeral  40  shows that 1 percent by weight aluminum hydroxide in the final composition. 
     While the invention has been described in connection with its preferred embodiments, it should be recognized that changes and modifications may be made therein without departing from the scope of the appended claims.