ABS plastic made transparent and flame retardant with Br2 -Br8 diphenyl ethers
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. application Ser. No. 625,334, filed June 27, 1984 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to improved transparent and flame-retardant compositions and more specifically to transparent, flame-retardant ABS resins. 
     Acrylonitrile-butadiene-styrene (ABS) resins have achieved wide-spread use in the plastics field in a variety of applications for the molding of plastic articles having good impact resistance. One of the early improvements in the formulation of ABS plastics was to blend ABS resins with polyhalogenated aromatic compounds such as polybrominated diphenyl oxides to impart flame retardancy to the composition. Such compounds have frequently been used with an inorganic synergist such as antimony oxide as described in U.S. Pat. Nos. 3,347,822, 4,144,288 and 4,173,561 as well as German Pat. Nos. 2,733,695, 2,933,900 and 3,142,894. 
     ABS resins may generally be characterized as a dispersion of a particulate rubbery impact modifier in a rigid polymeric matrix. As such, ABS resins are normally opaque materials, and the addition of inorganic flame retarding agents and synergists would be expected to contribute toward the opaqueness. There are, however, various applications where the lack of transparency in ABS resins prevents its use. Transparency in ABS systems has heretofore been accomplished by matching the refractive index of the grafted resin component to the impact-modifying rubber substrate, either by incorporating monomers such as methylmethacrylate or methacrylonitrile in the grafted component, or by employing modified rubbery substrates such as acrylate rubber as the impact modifier. Such modifications are known to affect other physical properties including solvent resistance, and the cost and complexity of the resin is increased. A method for improving the transparency of ABS resins without requiring modification of the base resin would be a useful improvement in the compounding art. 
     SUMMARY OF THE INVENTION 
     The addition of an effective amount of at least one polybrominated diphenyl ether to ABS resins not only imparts an expected high degree of flame retardancy properties to the ABS resins, but also renders such resins substantially and unexpectedly transparent. The compositions of the present invention are thus formulated of an ABS resin and at least 25 wt% of a polybrominated diphenyl ether, preferably without inorganic fire retardancy additives, to provide both flame retardancy and transparency. 
     DETAILED DESCRIPTION 
     The polybrominated diphenyl ethers employed preferably contain up to 8 bromine atoms per molecule. Preferred are diphenyl ethers containing 2 to 8 bromine atoms per molecule and still more preferred are diphenyl ethers containing 5-8 bromine atoms per molecule. It will be understood that a single diphenyl ether can be employed, although it is equally possible to employ mixtures of brominated diphenyl ethers as well. Polyphenyl ethers which contain 9 or 10 bromine atoms have been evaluated and found to yield little or no transparency. Without limiting the invention as to theory, it is believed that the higher brominated diphenyl ethers have melting points which are too high relative to acceptable processing temperatures for ABS resins. 
     The amount of the brominated diphenyl ether employed to impart both transparency and flame retardancy is an amount sufficient to render the ABS resin transparent. Generally, the compositions will consist essentially of at least 25%, preferably from 25-40%, of brominated diphenyl ether, based on the weight of ABS and brominated diphenyl ether. It has been found that the higher the bromine content of the diphenyl ethers, the lower is the quantity of brominated diphenyl ether required to achieve transparency. 
     The ABS resins employed in the practice of this invention are generally graft copolymers of a vinyl aromatic monomer and a nitrile monomer grafted onto a backbone which is either a polybutadiene backbone or a backbone formed of a copolymer of butadiene and a vinyl aromatic monomer. The preferred vinyl aromatic monomers employed in the practice of the present invention are styrene and alphamethyl styrene, with styrene being preferred because alphamethyl styrene-based ABS systems tend to exhibit a lesser degree of transparency. The nitrile monomer is preferably acrylonitrile, although methacrylonitrile can also be used. 
     Typically, the ABS resins employed in the practice of the present invention have a butadiene content which ranges from 13.5 to 50% by weight based on the weight of the graft copolymer, the remainder being the vinyl aromatic monomer and the nitrile monomer. The ratio of vinyl aromatic monomer to nitrile monomer is preferably within the range of 1.5:1 to 3:1. 
     As indicated above, it is preferred to formulate the compositions of the present invention without the usual inorganic flame retarding agents such as antimony compounds. It has been found that antimony oxide, for example, has a negative effect on the impact strength of ABS resins and also has a negative effect on optimum transparency of the system. Metal stearates frequently used for lubrication purposes also have a negative effect on optimum transparency. However, conventional dibutyl tin maleate stabilizer, polyoxyethylene ether lubricant and dyes and pigments can be used without substantially affecting transparency. 
     Having described the basic concepts of the invention, reference is now made to the following examples, which are provided by way of illustration and not by way of limitation, of the practice of the invention in the preparation of transparent, flame retardant ABS compositions. Although these compositions include stabilizer and lubricant, all calculations are based only on resin and polybrominated diphenyl ether. 
    
    
     EXAMPLE 1 
     This example illustrates the effect of various levels of polybrominated diphenyl ethers on transparency of an ABS resin. The particular resin employed in the example is Blendex®201 ABS marketed by Borg-Warner Chemicals. 
     The ABS resin was formulated with various amounts of octabromodiphenyl ether, a polybrominated diphenyl ether containing 8 bromine atoms per molecule, by melt-mixing in a Banbury mixer at a stock temperature of 330°-350° F. The cooled resin was pelletized and injected-molded on a Van Dorn 3 oz. injection molding machine to form placs having thicknesses of 1/16&#34;, 1/8&#34; and 1/4&#34;. 
     The results of the test are shown in the following table: 
     
                       TABLE 1                                                     
______________________________________                                    
Br8 Content Transparency                                                  
wt %        @1/16&#34;     @1/8&#34;      @1/4&#34;                                   
______________________________________                                    
0           Very slight                                                   
                       None       None                                    
16.6        Slight     Very slight                                        
                                  None                                    
20          Slight     Very slight                                        
                                  None                                    
23.1        Moderate   Slight     None                                    
25.9        Good       Moderate   Very slight                             
28.6        Good       Moderate   Slight                                  
31.0        Good       Good       Moderate                                
33.3        Good       Good       Moderate                                
______________________________________                                    
 Notes:                                                                   
 The compositions comprise Blendex 201 ABS resin and octabromodiphenyl    
 ether (Br.sub.8).                                                        
 
    
     EXAMPLE 2 
     This example illustrates the transparency of blends consisting of 31% by weight of octabromodiphenyl, a polybrominated diphenyl ether containing 8 bromine atoms per molecule, with various Blendex ABS resins whose butadiene levels range from 13.5 to 50%, the styrene to acrylonitrile ratios ranging from 1.8:1 to 3:1. The ABS resins are not transparent when molded without the bromine-containing additive at 1/16&#34; thickness. The compositions, summarized in Table 2 together with the transparency data, were prepared substantially by the procedure of Example 1. 
     
                       TABLE 2                                                     
______________________________________                                    
         Transparency                                                     
ABS      @1/16&#34;      @1/8&#34;       @1/4&#34;                                    
______________________________________                                    
101      Moderate    Slight      Very slight                              
111      Moderate    Moderate    Slight                                   
133      Slight      Very slight None                                     
211      Good        Good        Moderate                                 
215      Good        Good        Moderate                                 
216      Good        Good        Moderate                                 
310      Moderate    Slight      Very slight                              
601      Good        Moderate    None                                     
608      Good        Good        Moderate                                 
631      Good        Good        Moderate                                 
702      Moderate    Slight      None                                     
703      Slight      Very slight None                                     
______________________________________                                    
 Notes:                                                                   
 Resin blends consist of 69 wt % of the indicated ABS and 31 wt %         
 octabromodiphenyl ether.                                                 
 
    
     EXAMPLE 3 
     Selected blends from Example 2 were injected-molded and tested to determine physical and flammability properties. The results of those tests are shown in the following table: 
     
                                           TABLE 3                                 
__________________________________________________________________________
          Resin                                                           
          211 111                                                         
                 101                                                      
                    215 216 608 631 702 201                               
__________________________________________________________________________
Br.sub.8 wt %                                                             
          31  31 31 31  31  31  31  31  31                                
ABS wt %  69  69 69 69  69  69  69  69  69                                
Izod Impact (IM).sup.1                                                    
          6.0 7.9                                                         
                 8.3                                                      
                    9.2 8.3 2.4 2.3 0.5 7.3                               
Tensile Strength.sup.2                                                    
          6715                                                            
              4790                                                        
                 4900                                                     
                    6523                                                  
                        6290                                              
                            7980                                          
                                7640                                      
                                    7700                                  
                                        6820                              
Elongation.sup.3                                                          
          17  71 70 29  27  13  12  6   21                                
Tensile Modulus.sup.4                                                     
          3.0 2.3                                                         
                 2.4                                                      
                    3.0 2.9 3.6 3.7 4.0 3.0                               
Flexural Modulus.sup.5                                                    
          3.7 2.9                                                         
                 2.8                                                      
                    3.7 3.4 4.3 4.5 4.9 3.7                               
Flexural Strength                                                         
          11880                                                           
              8470                                                        
                 8440                                                     
                    10985                                                 
                        10750                                             
                            13670                                         
                                13600                                     
                                    14860                                 
                                        11490                             
Heat Deflection.sup.6                                                     
          154 151                                                         
                 152                                                      
                    157 155 161 160 166 157                               
UL94 Flamm. (1.8&#34;)                                                        
          V-0 V-1                                                         
                 V-1                                                      
                    V-0 V-0 V-0 V-0 V-0 V-0                               
Ave. Burn Time                                                            
          1.0 7.9                                                         
                 3.3                                                      
                    1.4 1.4 1.1 1.0 1.7 1.1                               
Max. Burn Time                                                            
          1   19 19 2   2   2   1   3   2                                 
1/16&#34; Transp.                                                             
          Good                                                            
              Mod.                                                        
                 Mod.                                                     
                    Good                                                  
                        Good                                              
                            Good                                          
                                Good                                      
                                    Mod.                                  
                                        Good                              
__________________________________________________________________________
 Notes:                                                                   
 Mechanical properties listed above are determined in accordance with ASTM
 D638.                                                                    
 .sup.1 Expressed in ft. lb./inch.                                        
 .sup.2 Expressed in psi.                                                 
 .sup.3 Expressed in %.                                                   
 .sup.4 Expressed in psi × 10.sup.-5.                               
 .sup.5 Expressed in psi × 10.sup.-5.                               
 .sup.6 Expressed in °F.                                           
 
    
     EXAMPLE 4 
     This example illustrates the variability in transparency resulting from the use of different polybrominated diphenyl ethers containing from 2 bromine atoms (Br 2 ) to 10 bromine atoms (Br 10 ) per molecule. Compositions consisting of the indicated polybromodiphenyl ether and Blendex 201 ABS resin were prepared as in Example 1 and molded into test specimens. The transparency data for each are summarized in Table 4. 
     
                       TABLE 4                                                     
______________________________________                                    
           Transparency                                                   
Br.sub.x                                                                  
        wt %     @1/16&#34;     @1/8&#34;    @1/4&#34;                                
______________________________________                                    
Br.sub.2                                                                  
        31.0     Moderate   Very slight                                   
                                     None                                 
Br.sub.4                                                                  
        31.0     Moderate   Very slight                                   
                                     None                                 
Br.sub.5                                                                  
        31.0     Moderate   Slight   None                                 
        39.4     Good       Good     Moderate                             
Br.sub.6                                                                  
        31.0     Good       Good     Moderate                             
Br.sub.9                                                                  
        31.0     Very slight                                              
                            None     None                                 
Br .sub.8 *                                                               
        39.4     Moderate   Slight   None                                 
Br.sub.10 *                                                               
        39.4     None       None     None                                 
______________________________________                                    
 Note:                                                                    
 First six compositions consist of Blendex 201 ABS resin and indicated    
 polybrominated diphenyl ether. Br.sub.8 * and Br.sub.10 * compositions   
 consist of Blendex 133 ABS resin.                                        
 
    
     It will be apparent from the data in Table 4, that polybrominated diphenyl ethers containing from 2 to 8 bromine atoms per molecule are useful in imparting transparency to ABS resins. Nonabromodiphenyl ether (Br 9 ) and decabromodiphenyl ether (Br 10 ) are ineffective, even at levels approaching 40 wt%. 
     It is therefore surprising that transparency will be imparted to ABS by diphenyl ether compounds having from 2 to 8 bromine atoms per molecule, particularly in view of the fact that the corresponding well-known flame retardant diphenyl ether compounds having 9 or 10 bromine atoms per molecule are not effective in this regard. 
     It will thus be apparent that the instant invention is a transparent ABS composition consisting essentially of an ABS resin and a polybrominated diphenyl ether in an amount effective to impart transparency and a method for preparing transparent ABS compositions. Compositions having at least 25 wt% polybrominated diphenyl ether with from 2 to 8, preferably from 5 to 8, bromine atoms per molecule are surprisingly transparent, and are flame retardant. It will be understood that various changes and modifications can be made in the details of procedure, formulation and use without departing from the spirit of the invention, especially as defined in the following claims.