Flame resistant, insulated multi-conductor electric cable
BACKGROUND OF THE INVENTION 
     Common flame retarding agents for imparting resistance to flame or combustion in flammable compositions such as organic polymers, or flame barrier materials for protecting flammable compositions from heat and flame or in general obstructing the spread of fire, typically comprise caustic or corrosive, noxious or otherwise potentially deleterious materials, such as halogens or asbestos. 
     Morover, the possible problems entailed with the use of such materials in all phases of their application or service, including their handling and storage, their presence within products, and their chemical and/or physical behavior in performing their designed function of inhibiting combustion, has become a subject of intensive and extensive concern in recent years, notwithstanding their significant contribution in saving lives and property from distruction or damage by fire. 
     Both the attainment of effective resistance to fire, and the potential problems of conventional flame retarding agents and barrier materials such as halogens and asbestos, have become especially critical factors with respect to insulated electrical wire and cable products because of the possibility of fires in electrical current carrying means, their frequent use in occupied buildings and in close proximity to personnel as well as highly flammable materials and/or costly apparatus such as electric motors, control means and machine tools. 
     SUMMARY OF THE INVENTION 
     This invention comprises an improved, insulated multi-conductor electrical cable product having a high level of resistance to flame and combustion and also a low halogen content with no asbestos or like inorganic fiber. The high degree of resistance to flame or combustion of the improved cable product of this invention, notwithstanding a significant reduction in halogen content and the absence of asbestos, is substantially attributable to the novel combination and arrangement of its components and their respective compositions, all in concert. 
     The flame resistant, insulated multi-conductor cable products of this invention are especially useful for service as power and/or regulating control cables for system controls, motor and machine control units, and related variable purpose or condition apparatus because it can contain substantially any number of individually insulated electrical conductors for a plurality of circuits, and also provide a high degree of resistance to combustion in a fire prone environment with a minimum content of hazardous ingredients. 
     OBJECTS OF THE INVENTION 
     It is a primary object of this invention to provide improved, insulated multi-conductor cable products. 
     It is also an object of this invention to provide insulated, multi-conductor cable products having improved resistance to flame or combustion and the propagation thereof. 
     It is a further object of this invention to provide flame resistant, insulated multi-conductor cable products which contain a low quantity of halogen and are devoid of asbestos and other deleterious agents. 
     It is a still further object of this invention to provide improved flame resistant, insulated multi-conductor cable products having therein substantially reduced amounts of noxious or corrosive flame-resisting agents such as halogens, and which have substantially improved electrical and physical properties and are uniquely suitable for service as control cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing comprises a perspective view of an insulated, multi-conductor cable product comprising the construction and components of this invention. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     The improved flame resistant, insulated multi-conductor electrical cables according to this invention, comprises a combination of assembled components of particular compositions or ingredients which act in concert to provide a composite cable product having acceptable electrical and physical properties and substantially improved resistance to fire or flame and propagation thereof. 
     Referring to the drawing, the composite cable 10 of this invention includes a multiplicity of individually insulated conductors 12, each comprising an electrical conductor 14, of either stranded or solid wire, enclosed within a covering body or coating of dielectric insulation 16. The insulation 16 comprises a cross-link cured, ethylene-containing polymer such as polyethylene. The ethylene-containing polymer compound for the insulation includes minimal amounts, such as about 30 to about 50 parts by weight per 100 parts by weight of polymer, of a conventional bromine-containing flame-retarding agent such as decabromobiphenyl oxide, providing therein about 15 to about 25 percent by weight of halogen content, and antimony oxide, in addition to the usual additives to effect the cure, such as an organic peroxide cross-linking agent, and the usual additives to preserve the chemical and physical properties of the polymer compound, such as conventional antioxidants and stabilizers for the polymer composition. 
     Additional suitable brominated flame retarding agents comprise hexabromobiphenyl, octobromobiphenyl and pentabromotoluene. 
     Ethylene-containing polymers suitable for the instulating compound for this invention comprise polyethylene, copolymers of high ethylene content with other polymerizable materials, and high ethylene content blends of such polymers including copolymer. For example copolymers of major amounts of ethylene with minor amounts of propylene, vinyl acetate, alkyl-acrylate or butene. 
     The ethylene-containing polymers are cross-link cured or thermoset by means of a heat-activated organic peroxide curing agent such as disclosed in U.S. Pat. Nos. 2,888,424; 3,079,370; 3,086,966; and 3,214,422, or by irradiation. 
     A plurality of such individually insulated conductors 12 in a given total quanity, are stranded or grouped together side-by-side in an appropriate, generally parallel arrangement such as illustrated in the drawing, and thus combined with an apt number of filler or spacer units 18. The filler or spacers 18 comprise elongated strips conformed to a suitable cross-sectional shape or shapes to complement the particular configuration of the arrangement of the grouped together, generally cylindrically shaped insulated conductors 12, so as to occupy and substantially fill the interstitial void spaces formed intermediate the generally parallel arranged insulated conductors 12. For instance, with a stranded group of side-by-side generally cylindrical insulated conductors 12 such as illustrated in the drawing, the internal filler strips 20 would be pressed to a generally triangular configuration with concave sides to complement the internal interstices, and external or surface filler strips 22 which occupy the interstices adjoining the outer surface intermediate the parallel insulated conductors 12 would be pressed to a generally triangular configuration with an outer convex side or base and two concave sides to complement the surface interstices. 
     The filler strips 18 perform a many functional role in this invention, including the prevention of distortion or deformation of both the individual insulated conductors 12, and the composite body of their grouped arrangement, provide a substantially smooth or regular and continuous outer surface contour around the composite body, and preclude the presence of any significant voids or open spaces within the composite body of the grouped arrangement of insulated conductors 12. 
     Although it is highly preferred in the practice of this invention that substantially all interstitial voids between the insulated conductors 12 be occupied with filler strips 18 to preclude the presence of any significant void areas, and to minimize any distortion and maximize the surface regularity, it is also within the scope of this invention to apply the filler strips 18 only to the interstices which are most strategic or critical with respect to the primary objective of this invention, effective resistance to flame and its propagation. 
     In accordance with this invention, the filler strips 18, or at least a substantial portion thereof, are composed of styrene-butadiene copolymer rubber containing substantial quantities of hydrated aluminum oxide filler, namely aluminum oxide trihydrate (Al 2  O 3  3H 2  O). The amount of alumina hydrate filler should be substantially greater than the amount of styrene-butadiene copolymer, for example at least about 150 parts by weight of alumina hydrate filler, and preferably about 200 to 250 parts thereof, per 100 parts by weight of the styrene-butadiene copolymer. 
     The styrene-butadiene copolymer, of course, preferably includes the usual additives to enhance its compounding and shaping or molding, and to preserve its designed chemical and physical properties, such as conventional antioxidants and preservatives suitable for styrene-butadiene copolymer. 
     An exemplative formulation for the filler strips of this invention which contribute significantly to the flame resistance of the composite multi-conductor cable product, is as follows in relative parts by weight. 
     
         ______________________________________                                    
                      Parts by Weight                                     
______________________________________                                    
Styrene-Butadiene Copolymer                                               
                        100                                               
Hydrated Alumina        225                                               
Aromatic Petroleum Resin Processing Acid                                  
                        15                                                
(Hercules-Picco Resin 6100-11/2)                                          
Zinc Oxide              5                                                 
Paraffin Wax            6                                                 
Crystalline Hydrocarbon Wax                                               
                        3                                                 
Stearic Acid            1                                                 
Methyl Tuads            3                                                 
______________________________________                                    
 
    
     The side-by-side arranged group of insulated conductors 12 with the interlaid filler strips 18, for the purposes of this invention, can be retained aligned in a straight parallel pattern as illustrated in the drawing, or the conductors 12 and interlaid strips 18 can be axially twisted into a conventional spiral or helical cable construction. 
     A stranded side-by-side grouping of an appropriate number and arrangement of the insulated conductors 12, combined with the hydrated alumina filled, styrene-butadiene rubber filler strips 18 interlaid among the insulated conductors 12 in a pattern to occupy the interstititial voids, is bundled together into a single multi-conductor unit with a wrapping of tape 24. The tape preferably consists of a plastic film such as Mylar polyester (ethylene-glycol terphthalate) of about one to three or four mils thick, and can be wrapped helically, as illustrated, or longitudinally around the grouped arrangement of insulated conductors 12 combined with the filler strips 18. 
     The bundle of tape 24 wrapped insulated conductor 12 and filler strips 18, is enclosed within a durable protective jacket or sheath 26, such as a neoprene (chloroprene) or Hypalon (chlorosulfonated polyethylene) outer jacket. 
     As indicated hereinbefore, the unique combination and arrangement of the components and their compositions of this invention provides a multi-conductor insulated cable having improved resistance to flame and its propagation while containing low, reduced amounts of halogen and no asbestos or similar deleterious ingredients. 
     The improvement and advantages of the novel multi-conductor insulated electrical cable of this invention, in relation to a similarly constructed and comparable multi-conductor cable product as a standard, is demonstrated by the following examples of this invention and the data derived therefrom. 
     In the following examples for the multi-conductor cable 10 of this invention, the dielectric insulation 16 for the conductors 14, comprised cross-link cured polyethylene containing 40 parts by weight of decabromobiphenyl oxide and 14 parts by weight of antimony trioxide. The halogen content therefore was 19% by weight. 
     The filler strips 18 which occupied all interstitial voids as is illustrated in the drawing, were composed of 100 parts by weight of styrene-butadiene copolymer containing 225 parts by weight of aluminum oxide trihydrate filler and of the formulation given above. 
     The wrapping was a Mylar film tape about one mil thick, and the enclosing protective jacket was neoprene about 60 mils thick. 
     The standard comprised a similarly constructed multi-conductor cable which was the same as the cable of the examples of this invention in all aspects except as following. In the standard multi-conductor cable, the dielectric insulation, comparable to insulation 16, was a high halogen content material comprising a blend of equal parts of polyvinyl chloride and chlorinated polyethylene combined with about 20 parts by weight of antimony oxide per 100 parts by weight of the polymer blend. The total halogen content for the insulation of the standard cable was 27.8% by weight, compared with 19% halogen content for the insulation of the cable of the invention. 
     The cable of the standard comprised the same number -- seven -- and arrangement of insulated conductors with all interstitial voids occupied, as is illustrated in the drawing and the same as in the cable of the examples of this invention, with filler strips composed of neoprene (chloroprene), having a halogen content of 14.6% by weight, for the purpose of contributing to the flame resistance of the cable. 
     The wrapping tape used in the cable of the standard consisted of a heat barrier laminate of a felt sheet of asbestos, about nine mils thick, on a mylar film about one mil thick. 
     The enclosing protective jacket of the cable standard was identical to its counterpart for the examples of this invention, a covering of neoprene about 60 mils thick. 
     Several samples of examples of the cable of this invention and of the standard of a comparable cable having a high halogen content for flame resistance and an asbestos heat barrier were tested under identical conditions for a comparison of their respective properties. The evaluations include testing of the individually insulated conductors 12 of the low halogen content insulating composition used in the cable of this invention, and of the high halogen content insulation composition for flame resistance of the standard cable described. 
     The comparative tests and resultant data for each phase of evaluating the individual insulated conductor component and complete composite multi-conductor cable for each of several examples of this invention and also several examples of the standard are given in the following table. 
     
                                           TABLE                                   
__________________________________________________________________________
EXAMPLES OF INVENTION                                                     
               INSULATION            STANDARD                             
Property       I   II III IV V   VI  A   B   C                            
__________________________________________________________________________
Original                                                                  
 Tensile, psi  2522                                                       
                   2436                                                   
                      2405                                                
                          2509                                            
                             2635                                         
                                 2572                                     
                                     2291                                 
                                         3120                             
                                             2887                         
 Elongation, % 387 387                                                    
                      380 383                                             
                             383 383  244                                 
                                          278                             
                                              270                         
 Hot Modulus, psi                                                         
               124.6                                                      
                   104.6                                                  
                      82.7                                                
                          107.8                                           
                             107.8                                        
                                 104.6                                    
                                      206                                 
                                          186                             
                                              182                         
7 Days, 121° C. Oven --- % Ret. Tensile                            
               94  102                                                    
                      89  100                                             
                             94  99   108                                 
                                          109                             
                                              108                         
 % Ret. Elongation                                                        
               95  100                                                    
                      103 100                                             
                             98  102  70  74  72                          
121° C. Heat Distortion, %                                         
               9.84                                                       
                   10.1                                                   
                      8.09                                                
                          8.52                                            
                             10.55                                        
                                 10.78                                    
UL FR-1 Vertical Flame                                                    
 Original      Pass                                                       
                   Pass                                                   
                      Pass                                                
                          Pass                                            
                             Pass                                         
                                 Pass                                     
                                     Pass                                 
                                         Pass                             
                                             Pass                         
 Aged 17 Days, 121° C                                              
               Pass                                                       
                   Pass                                                   
                      Pass                                                
                          Pass                                            
                             Pass                                         
                                 Pass                                     
                                     Pass                                 
                                         Pass                             
                                             Pass                         
 EM-60, 75° C.                                                     
 SIC After 24 Hours                                                       
               --  -- 2.37                                                
                          -- --  --  4.98                                 
                                         4.48                             
                                             4.24                         
 % Change in SIC                                                          
 1-14 Days     --  -- 0.84                                                
                          -- --  --  -3.41                                
                                         -5.58                            
                                             -4.72                        
 7-14 Days     --  -- 0.42                                                
                          -- --  --  0.42                                 
                                         0.48                             
                                             0.50                         
Stability Factor                                                          
               --  -- 0.03                                                
                          -- --  --  0.17                                 
                                         0.09                             
                                             0.12                         
Alt. Stab. Factor                                                         
               --  -- 0.01                                                
                          -- --  --  0.01                                 
                                         0.02                             
                                             0.18                         
Gravemetric Moisture                                                      
Absorption, mg/in.sup.2                                                   
 Oxygen Index  28.89                                                      
                   28.64                                                  
                      28.58                                               
                          28.82                                           
                             28.52                                        
                                 28.58                                    
                                     ( -- -- --                           
                                         -- 30--                          
                                             -- -- --)                    
 % Halogen     ( -- -- -- --  -- --                                       
                        -- 19--                                           
                            -- -- -- -- --  --)                           
                                     (   27.8                             
                                             )                            
 Specific Gravity                                                         
               (        1.27                                              
                            )        (   1.60                             
                                             )                            
__________________________________________________________________________
COMPARISON OF INTERSTICE FILLERS                                          
Property     FILLER OF INVENTION    FILLER OF STANDARD                    
__________________________________________________________________________
Oxygen Index .338                   .43                                   
% Halogen    0                      14.6                                  
Specific Gravity                                                          
             1.55                   1.57                                  
__________________________________________________________________________
             COMPARISON OF COMPLETED CABLE                                
Gms of Halogen/Ft. of                                                     
seven conductor cable                                                     
#12 Awg. 600 V. Cable                                                     
             8.656                  13.769                                
Propagation after IEEE                                                    
383-1974 70,000 BTU/Hrs.                                                  
Flame Test seven conduc-                                                  
tor #12 Awg 600 V. Cable                                                  
             25&#34;                    36&#34;                                   
Cable contains asbestos                                                   
             No                     Yes                                   
__________________________________________________________________________
 
    
     As is evident from the data given in the Table, the similarly designed and electrically performing equivalent, multi-conductor cable product of this invention, containing significantly smaller amounts of halogen, about 40% less, and no asbestos, provides substantially greater resistance to flame and its propagation (25 inches), than that (36 inches) afforded by the high halogen and asbestos barrier containing comparable cable standard. 
     Specifically, the improved flame resistance of the lower halogen and asbestos free multi-cable product of this invention exhibited a reduction in flame propagation about 30% less than the resistance provided by the high halogen and asbestos containing multi-conductor cable standard. 
     Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications are possible and it is desired to cover all modifications falling within the spirit and scope of this invention.