Pretreatment method of producing fire retardant foam
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to a pretreatment method for the production of a fire retardant foam material. More particularly, the present invention relates to a pretreatment method in which solid particles of urea are mixed with one or more of the foam producing reactants prior to combining or at the time of combining such reactants to produce a fire retardant flexible polyurethane foam. 
     In the production of flexible polyurethane foam materials, a polyol such as polypropylene glycol is typically treated with a polyisocyanate in the presence of water and a catalyst. As the polymer forms, water reacts with the isocyanate groups to produce carbon dioxide which causes foaming. In some cases a blowing agent may be employed to produce the foaming effect. 
     In U.S. patent application Ser. No. 412,441 filed Aug. 27, 1982 there is described a method for production of fire retardant foam such as flexible or semi-flexible polyurethane foam in which various materials utilized to provide fire retardancy are incorporated after the foam mass is produced. 
     By the present invention, there is provided an improved method for the production of foam materials having a high degree of fire retardancy, utilizing a pretreatment method in which the materials utilized to promote fire retardancy are incorporated with the reactant materials prior to or during production of the foam mass, thus eliminating the necessity for a second operation. The present invention is an improvement upon current formulations and methods utilized in the foam industry, particularly the urethane industry, in which pretreatment methods are employed which include the use of halogenated phosphorus compounds, antimony oxide and alumina trihydrate. Such prior art formulations have been found to decrease the physical quality of the foam, produce questionable levels of fire retardancy and also to be very expensive to produce. The present invention, on the other hand, results in a highly fire retardant foam which can be produced at low cost and without impairing the physical qualities of the foam. 
     In one embodiment, the urea and the foam producing materials are mixed in a slurry apparatus whereby the urea is pre-mixed with the polyol component of the foam prior to the addition of the isocyanate. In an alternative embodiment, the urea is incorporated into the reacting mass of polyol and isocyanate in solid form at the mixing point, such as by injection into a mixing head with an endless screw conveyor. This latter method is particularly useful in a case where the polyols which are employed are primarily capped with ethylene oxide which tends to dissolve the urea to a point where it can not be processed through typical pumping and metering equipment such as are employed in the slurry method. 
     In the case where polyols which are primarily capped with ethylene oxide are employed, the urea component should be incorporated into the reacting mass in solid form at the mixing point. When an endless screw conveyor is used for this purpose, the urea enters the mixing chamber from the conveyor and is evenly dispersed in the foam mass. The screw conveyor may operate at speeds such as from 2500 to 9000 rpm. In this embodiment, the urea enters the screw chamber from a pressurized container which forces the urea into the chamber. The dispensing mechanism is synchronized by means of a solenoid valve that activates the screw at the same time that the reacting liquids are introduced into the mixing chamber. 
     In adding the urea to the foam producing materials, according to the invention, the urea component may be effectively employed in amounts of from about 30 to 45% by weight of the total foam mass composition, i.e., foam and urea. The minimum desirable level is about 30% by weight of urea and optimum fire retardancy was achieved by the use of about 40 to 45% by weight urea in the composition. Thus, for example, a foam mass prepared with 40% urea exhibited a Flame Spread Index level of less than 25 when treated in accordance with ASTM-E-162 Radiant Panel Test. The same foam mass with no urea yielded a Flame Spread Index of more than 1000. It has been found that the presence of urea in the foam mass does not appreciably affect either the tensile strength, tear value or compression set values of the foam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE is a side elevation of a schematic representation showing one method for producing fire retardant foam materials in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiment of the invention as shown in the FIGURE, there is provided a schematic representation showing a portion of one method for producing fire retardant foam material obtained by mixing urea with the foam producing reactants. The method of the FIGURE is particularly suitable when the polyols which are employed are primarily capped with ethylene oxide. In this embodiment, the urea component is provided in the form of granules within a pressurized container 10 with removable lid portion, having an incoming air line 12 to maintain the container 10 under the desired pressure such as about 20 psi. From the container 10, the urea passes under pressure through line 14 into a screw chamber 16. The screw chamber 16 is provided with a longitudinally extending endless screw conveyor member 18, of helical or other conventional configuration, operated by motor 20. 
     As shown in the FIGURE, the screw chamber 16 communicates with a mixing chamber 22 so that urea granules may enter the mixing chamber 22 where the foam producing reactants are combined. The polyol and polyisocyanate are admitted to the chamber 22 through inlet lines 24 and 26, respectively. A solenoid valve 28 is installed in the line 14 between container 10 and screw chamber 16. The valve 28 is connected with inlet lines 24 and 26 and controls the operation of the screw 18 so that urea passes into the mixing chamber 22 at the same time that the reacting liquids are introduced into the chamber 22. In this manner, the urea is evenly dispersed with the foam producing reactants in the mixing chamber 22 and a screw type conveyor 28 or other suitable means is employed within the chamber 22 to mix the components. 
     Regardless of the procedure utilized to incorporate the urea into the foaming mass, the formulation for the production of polyether slab foam should be adjusted, especially when the type of foam desired is of the conventional type. If the product being manufactured, however, is of the HR type where the primary capped polyol is being utilized, then alterations to the formulation are minimal; the increased kinetic activity of the urea does not interfere with the foaming reaction curve. 
     Among the available formulations are those which incorporate a chlorinated solvent such as methylene chloride and/or an inert, non-reacting plasticizer such as dioctyl phthalate (DOP) into the formulation to prevent undesirable side effects such as shrinkage, closed cells and splitting. Typical formulations following this approach are: 
     EXAMPLE I 
     
         ______________________________________                                    
Component           Amount (parts by weight)                              
______________________________________                                    
Polyol (3000 MW) Propylene Oxide                                          
                    100                                                   
initiated                                                                 
Water               4.2                                                   
Stannous Octoate    .2                                                    
Silicone surfactant 1.0                                                   
Methylene Chloride  22.0                                                  
Urea                100.00                                                
Triethylene diamine .1                                                    
Tolylene Diisocyanate                                                     
                    54.0                                                  
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     EXAMPLE II 
     
         ______________________________________                                    
Component           Amount (parts by weight)                              
______________________________________                                    
Polyol (3000 MW) Propylene Oxide                                          
                    100                                                   
initiated                                                                 
Water               4.2                                                   
DOP                 10.0                                                  
Stannous Octoate    .2                                                    
Silicone Surfactant 1.0                                                   
Methylene Chloride  15.0                                                  
Urea                100.0                                                 
Amine Catalyst Polycat 12                                                 
                    .2                                                    
Tolylene Diisocyanate                                                     
                    54.0                                                  
______________________________________                                    
 
    
     Generally the amount of chlorinated solvent employed is from 3% to 12% by weight of the total foam mass composition. The amount of plasticizer employed is from 1% to 12% by weight of the total foam mass composition. 
     An alternative method in the slurry foam embodiment is to introduce a high molecular weight ethylene oxide (EO) capped polyol containing primary hydroxyl groups, such as a 4500 MW triol (Mobay No. E-3901 is an example) replacing part of the 3000 MW polyol. This allows for better control of the reaction, but it creates a processing limitation in that long term exposure of the slurry method creates increased viscosity and impairs the metering and pumping operations. The high molecular weight polyol is generally employed in an amount of at least 40% by weight of the total polyol component. 
     A formulation that can be used for this method is the following: 
     EXAMPLE III 
     
         ______________________________________                                    
Component           Amount (parts by weight)                              
______________________________________                                    
Polyol (3000 MW) Propylene oxide                                          
                    60                                                    
initiated                                                                 
Polyol (4500 MW) Primary capped                                           
                    40                                                    
with EO                                                                   
Urea                100                                                   
Water               4.2                                                   
Silicone surfactant 1.0                                                   
Stannous Octoate    .2                                                    
Amine catalyst Polycat 12                                                 
                    .4                                                    
Tolylene Diisocyanate                                                     
                    54                                                    
______________________________________                                    
 
    
     The above formulations help to control the reactions so that large quantities of urea may be incorporated into the foam mass utilizing state of the art technology and equipment. Urea affects the initial aspects of the reaction by increasing foaming rate. It acts also as an endothermic agent in the later stages of the reaction thereby delaying gelation and polymer development. 
     An alternative approach to the production of flexible slabstock foam containing large quantities of urea is to utilize the catalytic activity of the urea and an external heat source to control the development of polymer strength. Thus, the following formulation illustrates this method: 
     
         ______________________________________                                    
Component      Amount (parts by weight)                                   
______________________________________                                    
Polyol (3000 MW)                                                          
               100                                                        
Urea           100                                                        
Water          4.2                                                        
Silicone Surfactant                                                       
               1.0                                                        
Tolylene Diisocyanate                                                     
               54                                                         
______________________________________                                    
 
    
     Traces of stannous octoate (0.02) may be introduced. 
     Utilizing a heat source such as an oven that forces air at 200° F. on the expanding mass, foams can be produced without using the conventional amine catalysts. 
     In some cases it has been found that urea, being water soluble, will wash out of the foam mass to a certain extent when the foam material is utilized in an environment in which it is subjected to repeated water washings. To prevent this occurrence, the urea component may be coated with sulfur. The sulfur coating has been found to prevent the urea from dissolving in water, thus effectively maintaining the urea within the foam mass. Coatings of other materials such as shellac, stearic acid or vinyl materials may also be employed, although the use of sulfur is preferred as it has been found to be the most effective of the materials tested in this regard. A particular sulfur coated urea which may be employed is in the form of a time release fertilizer sold by AG Industries Mgf. Corp. of Columbia, Ala. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.