Synergistic flame retardant compositions for polyphenylene ether resins
The present invention relates to synergistic flame retardant compositions for polyphenylene ether resins. More particularly, it relates to synergistic combinations of certain phosphine oxide and phosphonate compounds and to their use as flame retardants for polyphenylene ether resins. 
     In accordance with the present invention, combinations of certain phosphine oxide compounds with certain phosphonate compounds provide flame retardant polyphenylene ether resin compositions at lower concentrations than is obtained by the use of either component of the combination alone. 
     The useful phosphine oxide compounds are represented by Formula I: ##STR1## wherein R 1  and R 2  are each selected from alkyl (6 to 12 carbon atoms, cycloalkyl (5 to 8 carbon atoms), phenyl, halophenyl (1 to 3 halogen atoms especially chlorine or bromine), omega-cyanoalkyl (1 to 4 carbon atoms), and benzyl; and Formula II: ##STR2## wherein each R 3  is selected from cycloalkyl (5 to 8 carbon atoms), and omega-cyanoalkyl (1 to 4 carbon atoms) and wherein X is selected from: 
     (a) 
     
         --CH.sub.2).sub.n 
    
     wherein n is an integer from 1 to 6; 
     (b) the group ##STR3## wherein R&#39; is alkyl of 1 to 4 carbon atoms, and y is an integer from 0 to 4: and 
     (c) the group ##STR4## 
     Preferred species of Formula I include: 
     Tris(2-cyanoethyl)phosphine oxide 
     Tris(benzyl)phosphine oxide 
     Triphenylphosphine oxide 
     Tris(p-chlorophenyl)phosphine oxide 
     Tris(n-octyl)phosphine oxide 
     Benzylbis(2-cyanoethyl)phosphine oxide 
     Benzylbis(cyclohexyl)phosphine oxide 
     Preferred species of Formula II include: 
     Bis[(dicyclohexylphosphinyl)methyl]terephthalate 
     Ethylenebis[bis(2-cyanoethyl)phosphine oxide] 
     2,3,5,6-Tetramethyl-1,4-xylylenebis[bis-(2-cyanoethyl)phosphine oxide] 
     Compounds represented by Formula I are disclosed as flame retardants for polyphenylene ether resins in commonly assigned copending application, U.S. Ser. No. 078,531, filed Sept. 24, 1979; those of Formula II in copending application, U.S. Ser. No. 078,533, filed Sept. 24, 1979, and U.S. Ser. No. 078,534, filed Sept. 24, 1979. 
     The useful phosphonate compounds are represented by Formula III: ##STR5## wherein R 5  is selected from phenyl, benzyl, and alkylbenzyl (1 to 3 alkyl groups each containing 1 to 4 carbon atoms); and Formula IV: ##STR6## wherein Y is xylylene or alkyl substituted xylylene, preferably where the methylene groups are para. 
     Preferred phosphonates of Formula III include: 
     Benzyl neopentylene phosphonate 
     Phenyl neopentylene phosphonate 
     2,4,6-Trimethylbenzyl neopentylene phosphonate 
     Preferred phosphonates of Formula IV include: 
     (p-Phenylenedimethylene)bis[2,2-dimethyltrimethylenephosphonate] 
     (2,5-Dimethyl-1,4-phenylenedimethylene)-bis[2,2-dimethyltrimethylene phosphonate] 
     Compounds represented by Formula III are described as flame retardants for polyphenylene ether resins in commonly assigned, copending application, U.S. Ser. No. 042,832,  filed May 25, 1979; compounds represented by Formula IV are described in copending application U.S. Ser. No. 079,689, filed Sept. 28, 1979, which is a continuation-in-part of U.S. Ser. No. 700,996, filed June 29, 1976, now abandoned. 
     The polyphenylene ether resins, which are flame retarded by the combinations of the invention, may be prepared as described in U.S. Pat. No. 4,024,093, incorporated herein by reference. In particular, the polyphenylene ether resins for which the compounds are useful are rubber-modified polyphenylene ether resins comprising about 10 to 90 parts by weight of polyphenylene ether, preferably 20 to 70 parts by weight, and about 90 to 10 parts by weight, preferably 80 to 30 parts by weight, of a rubber-modified, high-impact polystyrene. Still more preferably, the resins are about 50/50 weight blends of polyphenylene ether and rubber-modified impact polystyrene. Blends of polyphenylene ether resin and rubber-modified, high-impact polystyrene, having greater than 50% polyphenylene ether content, may require somewhat less than 4% by weight of the flame retardant composition of the invention. The resins are blends of the two polymers, which are mutually soluble, to form a homogeneous polymer composition. 
     The flame retardant combinations of the invention are used in the polymer in an effective amount. Generally an effective amount is at least about 4% by weight. The ratio of phosphine oxide compound to phosphonate, by weight, ranges from about 3/1 to 1/3, preferably about 1/1. 
     The flame retardant combination may be added to the polymer by any known method, such as by milling on a two-roll plastic mill or in a Banbury mixer, by dry blending with polymer and extruding, and the like. 
     It is also within the scope of the invention to incorporate such other ingredients into the polymer as plasticizers, dyes, pigments, heat and light stabilizers, antioxidants, antistatic agents, and the like. 
    
    
     The following examples further illustrate the invention. All parts are by weight unless otherwise specified. 
     EXAMPLES 1-5 
     A total of 10 g of a 50/50 blend of polyphenylene ether/rubber-modified impact polystyrene containing the flame retardant was dry blended and extruded through a melt index apparatus (described in ASTM D-1238) at 290°-300° C. The cylindrical extrudate, 3-5 inches in length and 0.25 inch in diameter, is subjected to the following flammability test procedure described in Underwriters&#39; Laboratories Test UL 94, Vertical Test Method 3.10-3.15 (September, 1973). The above polymer sample, clamped in a vertical position, is ignited, with a 3/4-inch blue flame (methane or natural gas) for 10 seconds, and the flame is withdrawn. If the flame is extinguished within 25 seconds, it is ignited again for 10 seconds, and the flame is withdrawn. The flame extinguishment time is again recorded. If the flame extinguishment time does not exceed 5 seconds for either flame application, the sample is rated V-0. If the flame extinguishment time for either the first or second flame application is between 5 and 25 seconds, it is rated V-1. If a sample shows extinguishment time greater than 25 seconds for either flame application, it is rated &#34;free-burning&#34; (FB), and it is considered as failing by this test. 
     Table I shows the data obtained with tris(2-cyanoethyl)phosphine oxide (A) and benzyl neopentylene phosphonate (B). 
     
                       TABLE I                                                     
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Evaluation of Tris(Cyanoethyl)Phosphine Oxide/Benzyl                      
Neopentylene Phosphonate Combination as Flame Retardant in                
Polyphenylene Ether                                                       
                                  Flammability                            
Example Phosphine Oxide                                                   
                      Phosphonate Rating                                  
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1       4% A          --          FB                                      
2       --            4% B        FB                                      
3       2% A          2% B        V-0                                     
4       1% A          3% B        V-1                                     
5       3% A          1% B        V-1                                     
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     Whereas 4% by weight of either compound alone is &#34;free-burning&#34; (inactive), combinations of the two compounds in ratios of 1:3, 2:2 and 3:1, at a total concentration of 4%, are effective flame retardants. 
     EXAMPLES 6-33 
     Following the procedure of Examples 1-5, combinations of phosphine oxides and phosphonates were evaluated as flame retardants. The following compounds were tested: 
     A. Tris(2-cyanoethyl)phosphine oxide 
     B. Benzyl neopentylene phosphonate 
     C. Phenyl neopentylene phosphonate 
     D. (p-Phenylenedimethylene)bis[2,2-dimethyltrimethylene phosphonate] 
     E. (2,5-Dimethyl-1,4-phenylenedimethylene)-bis[2,2-dimethyltrimethylene phosphonate] 
     F. 2,4,6-Trimethylbenzyl neopentylene phosphonate 
     G. Tris(benzyl)phosphine oxide 
     H. Triphenylphosphine oxide 
     J. Tris(p-chlorophenyl)phosphine oxide 
     K. Tris(n-octyl)phosphine oxide 
     L. Benzylbis(cyclohexyl)phosphine oxide 
     M. Bis[(dicyclohexyl phosphinyl)methyl]terephthalate 
     N. Ethylenebis[bis(2-cyanoethyl)phosphine oxide] 
     O. (2,3,5,6-Tetramethyl-1,4-phenylenedimethylene)bis[bis(2-cyanoethyl)phosphine oxide] 
     P. Benzylbis(2-cyanoethyl)phosphine oxide 
     
                       TABLE II                                                    
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                                  Flammability                            
Example Phosphine Oxide                                                   
                      Phosphonate Rating                                  
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6       --            4% C        FB                                      
7       2% A          2% C        V-1                                     
8       --            6% D        FB                                      
9       2% A          2% D        V-1                                     
10      --            4% E        FB                                      
11      2% A          2% E        V-1                                     
12      --            4% F        V-1*                                    
13      2% A          2% F        V-0**                                   
14      4% G          --          FB                                      
15      2% G          2% B        V-1                                     
16      4% H          --          FB                                      
17      2% H          2% B        V-1                                     
18      4% J          --          FB                                      
19      2% J          2% B        V-1                                     
20      4% K          --          FB                                      
21      2% K          2% B        V-1                                     
22      4% L          --          FB                                      
23      2% L          2% B        V-0                                     
24      4% M          --          FB                                      
25      2% M          2% B        V-1                                     
26      4% N          --          FB                                      
27      2% N          2% B        V-1                                     
28      4% O          --          FB                                      
29      2% O          2% B        V-1                                     
30      --            4% C        FB                                      
31      2% G          2% C        V-1                                     
32      4% P          --          FB/V-1                                  
33      2% P          2% C        V-1/V-0                                 
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 *Average time to extinguishment  15 sec.                                 
 **Average time to extinguishment  3 sec.                                 
 
    
     The data illustrate that whereas individual phosphine oxides or phosphonates of the invention are not active flame retardants for the polyphenylene ether resins at a level of about 4%, the combinations of this invention at the same concentration are effective flame retardants.