Abstract:
A flame retardant composition is provided which is less expensive to produce than prior compositions and yet which exhibits desirable insulation properties, flame and fire resistance, and which, when ignited, does not emit noxious smoke or corrosive gases. The crosslinkable flame retardant composition includes a copolymer of ethylene and a vinyl ester of a C 2-6  aliphatic carboxylic acid, a hydrated inorganic filler, and a coupling agent.

Description:
This is a continuation of application Ser. No. 08/582,363, filed Jan. 5, 1996, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an improved polymeric composition which is crosslinked to produce flame and heat resistant products for both electrical and nonelectrical uses. More particularly, the invention relates to a crosslinkable flame retardant ethylene vinyl acetate copolymer containing a novel coupling agent and inorganic hydrate filler. 
     Fire resistant compositions are widely used for wire and cable insulation, among other uses. In electrical environments, both insulating and flame resistant properties are essential. That is, the compositions when ignited should not exhibit after glow, should not emit noxious or toxic smoke, and should promote the formation of an inorganic char which is both non-combustible and non-toxic. Additionally, the compositions desirably should not deteriorate over long service times. 
     In some instances, the manufacture of extrudable fire resistant compositions requires the use of halogenated polymers such as chlorinated polyethylene, chlorosulfonated polyethylene, polyvinyl chloride, and chlorobutadiene, or coatings of chlorinated polymers over other polymer compositions. However, in fire situations, such chlorinated compositions evolve toxic hydrogen chloride gas and emit large quantities of noxious smoke. As smoke inhalation is an even greater cause of death in fires than the fire itself, such products are unsuitable. Further, hydrogen chloride gas is a highly corrosive gas which is converted quickly into hydrochloric acid in the presence of any water. Exposure to hydrochloric acid will damage or destroy electrical equipment such as computers, switchboards, printed circuits, and the like. 
     One widely used fire retarding insulation product has been a crosslinkable copolymer of ethylene and vinyl acetate, one or more hydrated inorganic fillers, one or more silane coupling agents, and a crosslinking agent (typically an organic peroxide). Such a product is taught in North and Kuckro, U.S. Pat. Nos. 3,832,326 and 3,922,442. Similar products are also taught in Biggs et al, U.S. Pat. Nos. 4,349,605 and 4,381,362. Compositions of this type are used as fire retardant insulation and jacketing for copper and aluminum conductors and in instrument transformers. The products exhibit good processability and meet SAE J-1128 low tension primary cable standards. 
     One problem with the manufacture of these crosslinkable copolymers has been the need for the use of relatively expensive silane coupling agents. Such silanes are required to provide compatibility between the copolymer and inorganic fillers so that a well-dispersed, homogeneous composition will be produced. 
     Accordingly, there still exists a need in this art for an inexpensive flame and heat resistant polymer composition which also provides electrical insulating properties and which, when ignited, does not emit noxious smoke or corrosive gases. 
     SUMMARY OF THE INVENTION 
     The present invention meets that need by providing a flame retardant composition which is less expensive to produce than prior compositions and yet which exhibits desirable insulation properties, flame and fire resistance, and which, when ignited, does not emit noxious smoke or corrosive gases. In accordance with one aspect of the present invention, a crosslinkable flame retardant composition is provided and includes a copolymer of ethylene and a vinyl ester of a C 2-6  aliphatic carboxylic acid, a hydrated inorganic filler, and a coupling agent. The composition may be crosslinked (vulcanized) to a final form. 
     The coupling agent comprises a compound which includes at least one active carboxylic acid (COOH) group. For example, the coupling agent may be a saturated fatty acid such as stearic acid or palmitic acid, or a dicarboxylic acid such as oxalic acid. The saturated fatty acids are preferred as these compositions are readily available and inexpensive relative to the cost of conventional vinyl silane coupling agents (i.e., on the order of one-tenth the per pound cost). However, the present invention also contemplates the use of these dicarboxylic acid compounds in combination with silane coupling agents to reduce the amount of silane coupling agent required. 
     The preferred hydrated inorganic filler for use in the present invention, comprises calcium borate pentahydrate, although other hydrated compositions and mixtures of hydrated compositions may be utilized. The peneahydrate has the advantage of having five moles of chemically bound water and imparts desirable flame and smoke characteristics to the composition. 
     Chemical crosslinking of the composition may be accomplished in a number of conventional ways. For example, a chemical crosslinking agent may be included in the composition and may comprise any of a number of conventional agents used in the art including organic peroxides. Alternatively, radiation or other non-chemical means may be used to crosslink the composition. 
     The compositions of the present invention find particular use as insulation for building wire, appliance wire, switchboard wire, and automotive wire where the combination of superior electrical properties, flame resistance, and low smoke evolution are desired. Additionally, the compositions of the present invention may also find use in non-electrical environments where flame resistance is desired such as for the fabrication of shower stalls and booths for residential and commercial uses and the manufacture of trays and containers for use in schools, aircraft, factories, and other commercial installations. 
     Accordingly, it is a feature of the present invention to provide a flame retardant composition which is less expensive to produce than prior compositions and yet which exhibits desirable insulation properties, flame and fire resistance, and which, when ignited, does not emit noxious smoke or corrosive gases. This, and other features and advantages of the present invention will become apparent from the following detailed description and accompanying claims. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The novel flame retardant compositions of the present invention are comprised of three basic components: (1) one or more crosslinkable copolymers of ethylene and a vinyl ester of a C 2-6  aliphatic carboxylic acid; (2) a hydrated inorganic filler; and (3) a coupling agent. Preferably, the composition also includes a crosslinking agent, such as an organic peroxide, and an antioxidant. 
     The Copolymer Component 
     The crosslinkable copolymer component of the present invention may be any of the copolymers described in North et al, U.S. Pat. No. 3,832,326, the disclosure of which is incorporated by reference. When using the terms crosslinkable and crosslinking, the normal art recognized meaning of those terms is meant. Further, the composition may include minor proportions of other polymers which are compatible with the copolymer such as polyethylene, copolymers of ethylene and propylene, butene, and the like. A preferred copolymer composition is one of ethylene and vinyl acetate, wherein the vinyl acetate content of the copolymer is between about 9 and 24%, and most preferably about 17%. Lesser percentages of vinyl acetate in the copolymer result in a composition which is more difficult to process and which possesses less flame resistance. Preferably, the copolymers have a melt index of from about 1.0 to 10.0, and most preferably between about 1.0 and 2.0. 
     Crosslinking of the copolymer may be accomplished by any of several known procedures in the art. For example, crosslinking may be induced by the use of chemical crosslinking agents, by exposing the composition to radiation, or by thermal crosslinking. A preferred method for crosslinking the copolymer is the use of organic peroxide crosslinking agents which are well known in the art. Typically, from about 1 to 8 parts by weight crosslinking agent per 100 parts copolymer has been found to be suitable. I have found that one can advantageously incorporate two organic peroxide crosslinking agents (each with a different decomposition temperature) into the composition to provide a faster rate of crosslinking while avoiding scorching of the polymer or premature crosslinking. 
     In general, the higher the degree of crosslinking, the more resistant the copolymer is to moisture and chemical reagents and the less resistant the composition is to abrasion. At lower degrees of crosslinking, there is also some loss of heat resistance as well as an effect on elongation after ageing. The exact degree of crosslinking can be varied to optimize the composition for a particular desired end use. For example, for use as a wire or cable insulation, a crosslinking percentage on the order of about 95% is preferred, with the degree of crosslinking being measurable by art recognized methods. 
     The Coupling Agent Component 
     Prior art compositions have required the use of relatively expensive vinyl and/or alkoxy silane coupling agents to achieve the desired properties of flame and heat resistance coupled with good electrical and mechanical properties. North et al, U.S. Pat. No. 3,832,326, teach that if the silane coupling agent is omitted, an inexplicable lowering of tensile strength and percent retention of elongation after ageing results. I have discovered that these relatively expensive silane coupling agents may be replaced by one or more compounds containing at least one carboxylic acid group in the flame retardant composition without adversely affecting the desired balance of properties of the composition. 
     I have found that such commonly available saturated fatty acids as stearic and palmitic acids, or commonly-available dicarboxylic acids such as oxalic acid can be used to partially or totally replace the prior art silanes. Various commercial grades of such fatty acids are known to contain small amounts of other fatty acids, so that it is certainly within the scope of the present invention to use mixtures and blends of such acids. By fatty acid I mean the class of monobasic acids having the general formula C n  H 2n+1  COOH. The fatty acids also act to disperse the inorganic filler with the copolymer to provide a homogeneous mixture. 
     Generally, I have found that the use of from about 0.25 to about 5 parts by weight coupling agent per 100 parts copolymer are suitable. Because the cost of saturated fatty acids is approximately one-tenth that of commercially available vinyl silanes, it is desirable to use these fatty acids as the coupling agents to replace the silanes either partially or totally. However, in some instances, the combination of the two different coupling agents produce synergistic results in the properties of the crosslinked product. Further, while certain salts of fatty acids have been used in the past in crosslinkable flame retardant compositions as lubricants to improve the stripping properties of wire insulation, I use the acid form of the compositions and have unexpectedly found that they function quite well as coupling agents in the composition. 
     The Hydrated Inorganic Filler Component 
     As the inorganic filler, any of the fillers heretofore used in the art are also suitable for use in the present invention. Thus, hydrated aluminum oxides (Al 2  O 3 .3H 2  O or Al(OH) 3 ), hydrated magnesia, or hydrated calcium silicate may be utilized. Additionally, I have found that hydrated calcium borate (Ca 2  B 6  O 11 .5H 2  O) provides unique properties to the composition which are highly desirable for electrical uses. That is, the use of hydrated calcium borate produces a composition having desirable elongation properties of 200% or greater while still exhibiting good tensile strength. Prior art inorganic fillers such as the hydrated aluminas produce compositions having lower elongation properties and which are more vulnerable to breaking or cracking. 
     The use of hydrated calcium borate as the filler produces a composition which, upon ignition, does not produce any after-glow and which forms an inorganic char which is both nontoxic and noncombustible. Hydrated calcium borate is also insoluble in water. As with other hydrated fillers, the five bound water molecules will be released upon heating endothermically, absorbing the heat needed to propagate combustion. A preferred form of calcium borate is commercially available under the designation colemanite. 
     For the hydrated inorganic filler component, I prefer the filler to be in particulate form, with smaller particle sizes being most preferred. Hydrated alumina is commercially available in particle sizes as small as about 5 μm, while hydrated calcium borate is commercially available in sizes of 100 mesh or less, and preferably 325 mesh or less. 
     The Proportion of Components 
     The amounts of copolymer and filler can be varied within wide proportions. Typically the relative ratio of copolymer to filler will be between from about 1:5 to 2:1. Preferably the percentage of coupling agent in the composition should be between about 0.25 to about 5.0 parts per 100 parts of copolymer. Lesser amounts may be insufficient to provide adequate surface treatment, while larger quantities may have an adverse effect on the physical properties of the composition. When used as an extruded coating for wires and cables, good results may be obtained using from about 80 to 400 or more parts filler (preferably, 80-150 weight parts), 0.25 to 5.0 parts coupling agent, and 100 weight parts copolymer. 
     The compositions of the present invention may be formed in a number of ways. However, in every instance it is necessary that the filler and coupling agent be intimately contacted. For example, a preferred method of filler treatment is to add the coupling agent directly to the copolymer to form a mixture, followed by the addition of filler to that mixture. This can be accomplished in an internal mixer or any other processing device which is capable of producing an intimate mixture of the three components as is well known in the art. Alternatively, the coupling agent may be added directly to the filler, dispersed therein, and the copolymer then added. 
     It will be apparent that additional optional components may be added to the compositions of the present invention. These additives include pigments (such as, for example, titanium dioxide), stabilizers, antioxidants, and lubricants as long as these components do not interfere with the crosslinking reaction or harm other desired properties of the composition. Such additives may be present in small amounts of less than about 10%, and preferably less than about 5%. Suitable antioxidants for use in the practice of the present invention include, for example, trimethyldihydroquinoline and such non-discoloring/nonstaining antioxidants as 4,4&#39;-thiobis(6-tert-butyl-m-cresol), 4,4&#39;-butylidenbis-(6-tert-butyl-m-cresol), and 2,2&#39;-methylene-bis(4-methyl-6-tert butylphenol). 
     In order that the invention may be more readily understood, reference is made to the following examples, which are intended to be illustrative of the invention, but are not intended to be limiting in scope. In each instance, the recited filler and coupling agent were separately mixed together and thereafter were added to and mixed with the copolymer. Care was taken during mixing not to elevate the temperature of the mixture to a point where the peroxide crosslinking catalyst would be prematurely activated. Following mixing, aliquots of the composition were placed in a standard ASTM mold (6.0&#34;×6.0&#34;×0.75&#34;) and then vulcanized. 
     For mixing these compositions, a state-of-the-art high-intensity internal mixer was utilized (such as commercially available Banbury-type mixers). Surprisingly, I have found that pretreatment of the filler with the coupling agent was not required in order to achieve good dispersion of the components. Such pretreatment is taught in Biggs, U.S. Pat. Nos. 4,381,362 and 4,349,605. Rather, I have found that a simple one-step mixing cycle produces adequate dispersion of the components of the composition. That is, I add a charge of polymer to the mixer, immediately followed by a charge of coupling agent. After a few minutes of mixing, I then add the charge of filler as well as any other optional ingredients. Lastly, I add a chemical crosslinking agent after checking the temperature of the mixture to insure that there is no premature vulcanization. 
     Such a one-step process is not only more cost effective than the prior art procedure by eliminating the need for pretreatment of any of the ingredients such as the filler, but also reduces the opportunity for weighing errors and for cross-contamination from other chemicals in the weighing and mixing area. 
     Example 1 
     The following 15 compositions were prepared by mixing the listed ingredients and then curing the compositions (at 350° F. for 20 minutes). Composition 1 represents a prior art composition and composition 8 contains no coupling agent; those compositions are used for comparative purposes; the compositions numbered 2-7 and 9-15 are compositions within the scope of the present invention. All proportions are reported as parts by weight. 
     
         ______________________________________COMPOSITION NO.        1        2        3      4______________________________________UE-630.sup.1 100.00   100.00   100.00 100.00Agerite Resin D.sup.2        2.00     2.00     2.00   2.00Atomite.sup.3        2.00     2.00     2.00   2.00Hydral 710W.sup.4        125.00   --       62.50  125.00Pigment No. 407.sup.5        --       125.00   62.50  --Silane A-172.sup.6        3.00     3.00     3.00   --Stearic Acid.sup.7        --       --       --     3.00VulCup 40KE.sup.8        4.20     4.20     4.20   4.20Total Parts  236.20   236.20   236.20 233.20______________________________________COMPOSITION NO.        5        6        7      8______________________________________UE-630.sup.1 100.00   100.00   100.00 100.00Agerite Resin D.sup.2        2.00     2.00     2.00   2.00Atomite.sup.3        2.00     2.00     2.00   2.00Hydral 710W.sup.4        --       62.50    62.50  125.00Pigment No. 407.sup.5        125.00   62.50    62.50  --Silane A-172.sup.6        --       --       0.50   --Stearic Acid.sup.7        3.00     3.00     1.00   --VulCup 40KE.sup.8        4.20     4.20     4.20   4.20Total Parts  236.20   236.20   234.70 233.20______________________________________COMPOSITION NO.        9        10       11     12______________________________________UE-630.sup.1 100.00   100.00   100.00 100.00Agerite Resin D.sup.2        2.00     2.00     2.00   2.0Atomite.sup.3        2.00     2.00     2.00   2.0Hydral 710W.sup.4        125.00   --       125.00 --Pigment No. 407.sup.5        --       125.00   --     125.0Pigment No. 461.sup.9        --       --       1.5    1.5Silane A-172.sup.6        1.00     1.00     --     --Stearic Acid.sup.7        1.00     1.00     --     --VulCup 40KE.sup.8        4.20     4.20     4.20   4.2Total Parts  235.20   235.20   234.70 234.7______________________________________COMPOSITION NO.       13           14      15______________________________________UE-630.sup.1       100.00       100.00  --Elvax 470.sup.10       --           --      100.00Agerite Resin D.sup.2       2.00         2.00    2.00Atomite.sup.3       2.00         2.00    2.00Hydral 710W.sup.4       125.00       --      125.00Pigment No. 407.sup.5       --           125.00  --Pigment No. 461.sup.6       --           --      --Pigment No. 213.sup.11       --           --      1.50Stearic Acid.sup.7       1.50         1.50    --VulCup 40KE.sup.8       4.20         4.20    4.20Total Parts 234.70       234.70  234.70______________________________________ .sup.1 Ultrathene, a copolymer of ethylene and vinyl acetate (17% VA) fro U.S.I. Chemicals Co. .sup.2 trimethyldihydroquinoline antioxidant .sup.3 calcium carbonate .sup.4 hydrated aluminum oxide (Al.sub.2 O.sub.3.3H.sub.2 O) from ALCOA Industrial Chemicals Division .sup.5 hydrated calcium borate (Ca.sub.2 B.sub.6 O.sub.11.5H.sub.2 O), from American Borate Co. .sup.6 vinyltris(2-methoxyethoxy) silane from Union Carbide .sup.7 from Harwick Chemical Corp. .sup.8 organic peroxide crosslinking agent from Hercules Powder Co. .sup.9 palmitic acid from AcmeHardesty Co. .sup.10 a copolymer of ethylene and vinyl acetate (18% VA) from DuPont (0.7 M.I., 0.94 density) .sup.11 oxalic acid from American International Chemical Co. 
    
     Example 2 
     The physical and electrical properties of the compositions prepared in Example 1 were tested, and the results are shown in Table I below. The flame and smoke behavior of the compositions was also tested, and those results are reported in Table II below. As can be seen, the physical and electrical properties of compositions within the scope of the present invention compare favorably with prior art composition 1. An elongation percentage of 200% or greater is desirable as this is a measure of the ductility of the composition. Compositions 4-7 and 9-15, within the scope of the present invention had % elongation values which are superior to the prior art. As well, the flame and smoke behavior of the compositions of the present invention compare favorably with the prior art. 
     
                                           TABLE I__________________________________________________________________________PHYSICAL AND ELECTRICAL PROPERTIESComposition No.     1  2  3  4  5  6  7  8  9  10 11 12 13 14 15__________________________________________________________________________Tensile Strength,     2996        1618           1872              1772                 1374                    1276                       1372                          1434                             2313                                1330                                   1602                                      1390                                         1915                                            1395                                               1501psi.sup.1Elongation, %.sup.1     171        112           151              480                 449                    429                       355                          101                             200                                320                                   500                                      440                                         486                                            440                                               408100% Modulus.sup.1     2446        1544           1797              784                 585                    612                       1176                          1233                             1924                                788                                   930                                      659                                         826                                            662                                               1214Aged Tensile     3342        1845           2060              1432                 1289                    1135                       1547                          1764                             2630                                1341                                   1614                                      1396                                         1464                                            1375                                               1447Strength, psi.sup.2Aged Elongation, %.sup.2     190        110           140              170                 410                    420                       360                          190                             200                                350                                   117                                      429                                         86 425                                               260Vol. Resistivity,     2.5        2.5           1.9              1.9                 3.9                    1.6                       1.1                          1.8                             1.5                                8.1                                   4.0                                      1.7                                         3.3                                            8.1                                               2.2ohm-cm.sup.3     ×        ×           ×              ×                 ×                    ×                       ×                          ×                             ×                                ×                                   ×                                      ×                                         ×                                            ×                                               ×     10.sup.15        10.sup.15           10.sup.15              10.sup.15                 10.sup.16                    10.sup.16                       10.sup.16                          10.sup.14                             10.sup.15                                10.sup.15                                   10.sup.15                                      10.sup.16                                         10.sup.15                                            10.sup.15                                               10.sup.15Dielec. Strength,     &gt;794        &gt;757           &gt;745              &gt;792                 309                    490                       &gt;921                          &gt;756                             &gt;815                                851                                   756                                      512                                         721                                            315                                               545volts/mil.sup.4Shore A hardness.sup.1     92 93 90 91 87 87 93 92 92 90 92 91 93 93 93Shore D hardness.sup.1     58 54 55 48 39 40 52 53 -- -- 49 42 48 40 48Aged Shore A     94 94 94 95 94 94 93 95 95 94 94 94 94 93 93hardness.sup.1Aged Shore D     58 56 56 57 52 55 55 57 57 56 53 49 53 50 48hardness.sup.1__________________________________________________________________________ .sup.1 ASTM Test Method DC2240-91 and D412-92; die C at 20 inches per minute. .sup.2 ASTM Test Method D573-88; aged 168 hours at 150° C. .sup.3 ASTM Test Method D257-92; Voltmeterammeter method; 500 V DC, 60 seconds; Electrification, temperature 72° F., rel. humidity 51% .sup.4 ASTM Test Method D149-90, Method A, DC modified (ARDL). 
    
     
                                           TABLE II__________________________________________________________________________FLAME AND SMOKE BEHAVIORComposi-tionNo.  1   2   3   4   5   6   7   8   9  10 11  12  13  14  15__________________________________________________________________________Oxygen26.0    24.1        26.0            27.5                25.1                    27.0                        26.5                            28.9                                23.6                                   24.1                                      32.4                                          23.6                                              29.4                                                  26.5                                                      28.9Index.sup.1Maximum7.3 8.0 13.6            10.3                7.6 8.6 17.0                            7.3 7.3                                   9.3                                      8.0 7.78                                              16.7                                                  7.3 12.0SmokeDensity,%.sup.2Smoke5.6 7.1 8.7 7.6 6.4 6.4 10.4                            6.5 4.2                                   5.1                                      6.5 6.0 12.5                                                  6.0 8.2DensityRating,%.sup.2VisibilityGood    Good        Good            Good                Good                    Good                        Good                            Good                                Good                                   Good                                      Good                                          Good                                              Good                                                  Good                                                      Veryof                                                         GoodExit Sign.sup.2BehaviorA   B,C B,C B,C,M                B,C B,C,M                        B,C B,C,M                                A  B,C,                                      C,M B,C B,C B,C A,B,CUpon                                    MIgnition.sup.2SmokeNot Not Light            Not Not Not Grey                            Not Not                                   Black                                      Orange                                          Orange                                              Orange                                                  Orange                                                      Lt.Color.sup.2visible    visible        grey            visible                visible                    visible visible                                visi-                 Grey                                bleType ofOrange    Orange        Orange            Orange                Orange                    Orange                        Orange                            Orange                                Yel-                                   Yel-                                      Not Not Not Not OrangeFlame.sup.2                          low                                   low                                      visible                                          visible                                              visible                                                  visibleType ofRapid    Rapid        Rapid            Rapid                Rapid                    Rapid                        Slow                            Rapid                                Rapid                                   Rapid                                      Slow                                          Medium                                              Rapid                                                  Medium                                                      RapidBurning.sup.2__________________________________________________________________________ .sup.1 ASTM Test Method D2863-91 .sup.2 ASTM Test Method D2843093 A  Flaking embers; describe condition known as &#34;after glow&#34;- B  Bubbling C  Charring M  Melting 
    
     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.