Abstract:
The adduct of hexamethylphosphoramide and a cyclophosphonitrilic chloride,PNCl 2 ) n .sup.. [(CH 3 ) 2  N] 3  PO, useful for enhancing the fire retardancy of epoxy resins and a method for enhancing the fire retardancy of epoxy resins through the combination therewith.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 436,779 filed Jan. 25, 1974 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to epoxy resins containing a fire retardant additve as well as a method by which epoxy resins are rendered fire retardant. Epoxy resins are widely accepted as useful products for a variety of purposes because epoxies are so versatile. By versatile is meant that epoxies vary in molecular weight, viscosity, surface properties and curing properties. Some are high molecular weight, high melting solids while others are low molecular weight liquids. By selecting the proper curing agent they can be made to cure quickly and slowly, at room temperature or at elevated temperatures. Epoxy resins can be used where a soft flexible material is required or where a hard, tough material is required. Epoxies provide excellent electrical insulating properties as well as a high resistance to corrosive media. Perhaps, though, epoxy resins most well known property is that of adhesion. 
     Epoxy resins, however, like many resins are not by themselves resistant to fire. To render them fire retardant, additives must be blended with the epoxy. The desirability of rendering epoxy resins fire resistant is evident. It is especially important that epoxies be rendered resistant to fire when used as an insulating compound to embed electrical equipment. Epoxies are also used as adhesives in equipment that undergo great changes in temperature and pressure where fire is likely to occur. It is essential that the epoxy resin be fire retardant to reduce the likelihood of fire. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an epoxy composition that is fire retardant. 
     Another object of the invention is to provide an adduct that when added to an epoxy resin will enhance the fire retardancy of that epoxy resin. 
     Still another object of the invention is to provide a method of enhancing the fire retardancy of an epoxy resin. 
     And yet another object of the invention is to provide fire retardant additives for epoxy resins which give minimum interference with the properties of the resins. 
     These and other objects are achieved by the inclusion of the adduct of hexamethylphosphoramide and a cyclophosphonitrilic chloride in an epoxy resin. 
     DETAILED DESCRIPTION 
     By &#34;fire retardancy&#34; is meant that the resin is resistant to flame after the igniting flame has been removed. In other words, the fire retardant epoxy resin will not support combustion by itself. When in contact with an open flame, however, it may become charred. 
     The fire retardancy enhancing additives of this invention are best produced by admixing at room temperature for at least about 10 minutes but preferably for 20 minutes in a mole ratio of 1:1 hexamethylphosphoramide, [(CH 3 ) 2  N] 3  PO, and a cyclophosphonitrilic. In order to facilitate the removal of the final product from the reaction vessel, an excess of hexamethylphosphoramide or an inert diluent such as benzene is used, thereby the product is in a slurry upon completion of the reaction. Preferably the cyclophosphonitrilic is either hexachlorocyclotriphosphonitrile, (PNCl 2 ) 3  or octachlorocyclotetraphosphonitrile (PNCl 2 ) 4  or a mixture of the two. 
     The above reaction produces an adduct of the reactants which is a complex of the two molecules. The formulas for the two preferred adducts is (PNCl 2 ) n .[(CH 3 ) 2  N] 3  PO where n = 3 or 4. 
     The types of epoxy resins contemplated to be used are the epoxy resins produced by Shell Oil Company under the trademark EPON. Such resins are of two types. The first type is derived from the diglycidyl ether of Bisphenol-A and may be modified with monoepoxy diluents. These epoxies have the general formula ##SPC1## 
     and are reaction products of epichlorohydrin and bisphenol-A. 
     The second type of epoxy resin is derived from a multiglycidyl derivative of a novolac. An example of such a resin is one having the following formula ##SPC2## 
     reacting phenol and formaldehyde. 
     A listing of the EPON resins to which the adduct of the invention may be added may be found in Handbook of Epoxy Resins, Lee and Neville, 4-66, McGraw-Hill Book Company, 1967. The use herein of EPON resins is merely meant to be illustrative and not a limitation of the specific epoxy resin which may be utilized within the scope of the invention. It is understood that epoxy resins derived from the diglycidyl ether of bisphenol-A or a multiglycidyl derivative of a novolac can be used in the context of the present invention; even though not an EPON resin. 
     The epoxy resin may be cured with any known curing agent. Those proven to be useful as curing agents are those derived from the amines, primary, secondary and tertiary, as well as Lewis Acids and acid anhydrides. Table I is provided as an example of known curing agents for epoxy resins. While it is quite common to cure an epoxy resin at room temperature the curing step is hastened if the resin is heated above room temperature of 70°C. Table I lists preferred temperature range for each curing agent. 
     
                                           TABLE I__________________________________________________________________________                     Concentration                             Curing                     parts/hundred                             temperatureCuring Agent     Chemical Type                     parts of resin                             °F__________________________________________________________________________Diethylentriamine            Polyamine                     10-12    60-300Triethylentetriamine            Polyamine                     11-13    60-300Diethylaminopropylamine            Polyamine                     4-8      77-300N-Amioethylpiperozine            Polyamine                     20-23    80-300M-Phenylenediamine            Polyamine                     13-14   140-400Methylene Dianiline            Polyamine                     28-30   140-400Diaminodiphenyl Sulfone            Polyamine                     20-30   240-400BF.sub.3 Monoethylamine            Lewis Acid                     2-4     240-400            Amino ComplexDodecenylsuccinic Anhydride            Anhydride                     85-95   175-500Phythalic Anhydride            Anhydride                     70-80   200-300Hexalhydrophtalid Anhydride            Anhydride                     75-85   200-400Chlorendic Anhydride            Anhydride                     100-120 200-400Trinellectic Anhydride            Anhydride                     30-40   210-400Tetrahydrophydralic Anhydride            Anhydride                     75-80   200-300__________________________________________________________________________ 
    
     Fire retardance according to the novel aspects of the invention may be achieved in two ways. The first is by mixing hexamethylphosphoramide, a cyclophosphonitrilic chloride, a curing agent and an epoxy resin and heating to cure the mixture. In the second, a stoichiometric amount of hexamethylphosphoramide may be reacted with the cyclophosphonitrilic chloride and then the adduct of such a reaction is added to the epoxy resin along with a curing agent. The mixture is then cured by heating. The latter method is preferred. 
     Generally it has been found that by mixing from 1 to 30% by weight of the adduct with the epoxy resin, the fire retardancy properties of the resin is enhanced. A preferred range is from 5 to 20% by weight of the adduct. 
    
    
     In order to more fully illustrate the advantages and new features of the invention the following examples are presented. 
     EXAMPLE I 
     Hexamethylphosphoramide, 1.0 g and hexachlorocyclotriphosphonitrile (PNCl 2 ) 3 , 0.7 g, and allyl glycidyl ether, 1.0 g, were mixed, then stirred into 7.0 g of a general purpose epoxy casting resin (Bisphenol-A, epichlorohydrin polymer), EPON 828. After complete mixing, methylene dianiline, 3.0 g, was added and this mixture was heated at 90°C for 1 hour. The oxygen index of this resin was substantially higher (0.31) than would be expected from a similar loading of the phosphonitrilic chloride to the epoxy alone. 
     EXAMPLE II 
     Hexamethylphosphoramide, 1.0 g, allyl glycidyl ether, 1.0 g, and hexachlorocyclotriphosphonitrile, 1.0 g, were mixed and heated to 90°C until a clear mixture was formed. After cooling to approximately 50°C the mixture was added to EPON 828, 7.0 g, and methylene dianiline, 3.0 g. The resin was heated to 90°C for 90 minutes then at 120°C for 2 hours. The oxygen index of this resin was greater than 0.30. 
     EXAMPLE III 
     A resin was prepared using the technique described in Example I except that the hexamethylphosphoramide was omitted. The oxygen index was 0.26. 
     EXAMPLE IV 
     A resin was prepared from an epoxy-Novolac type resin (Shell EPON 154), 30.0 g, hexamethylphosphoramide, 3.0 g, hexachlorocyclotriphosphonitrile, 3.0 g, pentaerythritol tetrakis mercaptopropionate, 9.0 g, and methylenedianiline, 3.0 g. This mixture was thoroughly stirred, then heated at 90°C for 2 hours and at 125°C for 4 hours. The cured resin had an oxygen index of 0.29. 
     EXAMPLE V 
     Hexachlorocyclotriphosphonitrile, 11 g, was stirred with hexamethylphosphoramide, 17.0 g, in a reaction vessel and their admixture caused an exotherm that heated the mixture to 40°C. This adduct, a solid that melts at about 40°C, was used with Shell EPON 154 or EPON 828 to produce resins with oxygen index values of 0.32 for the former and 0.34 for the latter. 
     EXAMPLE VI 
     A 6 inch × 6 inch plaque was prepared by saturating six layers of chopped glass mat with a resin consisting of 96 g of EPON 828, 12 g of hexamethylphosphoramide, 12 g of hexachlorocyclotriphosphonitrile, 12 g of allylglycidyl ether and 36 g of methylene dianiline. The composite was held at 50°C for 1 hour, at 100°C for 4 hours and 150°C for 18 hours. The cured laminate has an oxygen index of 0.34. 
     In the Examples the oxygen index was obtained by using the method disclosed under ASTM D 2863-70. By &#34;oxygen index&#34; is meant the minimum oxygen concentration, expressed in volume percent, in a mixture of oxygen and nitrogen that will just support combustion of a material under the conditions of the method. Accordingly, a material having a high oxygen index requires more oxygen to support combustion and is therefore, more fire retardant than one having a low oxygen index. 
     Examples I and II when compared to Example III illustrate that the adduct of hexamethylphosphoramide and hexachlorocyclotriphosphonitrile yields a resin more fire retardant than one containing only hexamethylphosphoramide as a fire retardant agent. Example V illustrates that it is preferable to react hexamethylphosphoramide and the hexachlorocyclotriphosphonitrile to form the adduct and then add that adduct to the epoxy rather than merely adding the reactants to the epoxy resin before they have reacted to form the adduct. 
     It is further to be noted that the oxygen index of the epoxy resins used in the examples is approximately 0.19. It is apparent then that the use of the adduct of the invention significantly increases the fire retardancy of the resin. 
     EXAMPLE VII 
     Hexamethylphosphoramide, 17.95 g and a mixture of hexa-and octa-chlorocyclotriphosphonitrile in a hexa-to-octa ratio of 4:1, 34.8 g in 150 ml of benzene were mixed for two hours at 80°C, then heated at 150°C at 0.1 mm Hg in order to eliminate all excess liquids. A viscous substance remained. This substance was added to EPON 828 and the epoxy cured. The oxygen index of this resin was 0.34. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.