Abstract:
Tracking is reduced in insulators comprising one or more polymers and an anti-tracking filler system comprising (a) a hydrate of alumina having a specific surface area of at least 2 m 2  /g measured by the BET method, and (b) a compound selected from the group consisting of oxides, mixed oxides and mixtures of oxides wherein said compound contains at least one element selected from the group consisting of transition series elements, lanthanide series elements and nontransuranic actinide series elements.

Description:
This is a continuation of then pending application Ser. No. 434,126, filed Jan. 17, 1974 (now abandoned) which in turn was a continuation-in-part of then pending application Ser. No. 81,558, filed Oct. 16, 1970 (now abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to high-voltage insulating materials. 
     While polymeric materials are widely used for insulating a wide variety of electrical apparatus, they are not suitable for high voltage applications in contaminated atmospheres where moisture or fog, together with salts, dust particles and ionic pollution causes leakage currents to flow across the surface of the insulation. This current causes a rise in temperature with consequent moisture evaporation and ultimately dry hand formation. The electrical stress across these dry bands often exceeds the breakdown stress of the air-insulation interface, so that discharge or spark scintillation takes place. The spark temperature is extremely high, often 2000° C. or higher, and the heat produced may be sufficient to cause degradation of the insulation surface with the ultimate formation of carbonaceous spots. These carbonaceous spots usually link up in dendritic fashion and the organic insulation fails by progressive creepage tracking. 
     Over the years many solutions to these problems have been proposed of which perhaps the most effective has been the incorporation of hydrated alumina, preferably the trihydrate, in fairly substantial quantities to for example butyl rubber, epoxy resins, especially of the cycloaliphatic type, and, more recently, to ethylene-propylene rubbers as illustrated in U.S. Pat. Nos. 2,997,526; 2,997,527; and 2,997,528. 
     There have been several suggested modes of operation for the hydrated alumina, but whatever the correct mechanism, it is found in practice that polymeric materials containing large proportions of alumina trihydrate are substantially protected against tracking and usually fail only by progressive surface erosion. The amount of alumina hydrate required to produce the anti-tracking effect is very high, however, and is usually in the region of 50-90% by weight of the entire insulation. Especially in the case of polymers that are shaped by molding or extrusion as well as in the field of heat-recoverable articles, the high filler content is undesirable because the high temperature used and/or the radiation employed in cross-linking cause loss of the hydrated water with accompanying development of porosity and the formation of voids leading ultimately to failure of the insulation. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electrically insulating material which comprises one or more polymers and an anti-tracking filler system comprising, (a) a hydrate of alumina have a specific surface area of at least about 2 m 2  /g, and (b) a compound from the group consisting of oxides, mixed oxides and mixtures of oxides wherein said compound contains at least one element from the transition elements, the lanthanide series of the nontransuranic actinide series, as defined hereinafter. The resulting electrically insulating material has an initial tracking voltage of at least 2.5 kilovolts when tested according to ASTM-D 2303-68  liquid contaminant incline plane test, incorporated herein by reference. 
     By a &#34;transition element&#34; there is herein meant the elements of sub-groups IVa, Va, VIa, VIIa, and Group VIII of Mendeleef periodic table which are not also in the nontransuranic actinide series, e.g., titanium, zirconium, and hafnium, vanadium, niobium, and tantalum; chromium, molybdenum and tungsten; manganese, technetium, and rhenium; and iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. 
     By &#34;lanthanide series&#34; there is herein meant the elements cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. 
     By &#34;nontransuranic actinide series&#34; there is herein meant the elements thorium, protactinium, and uranium. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     All references to &#34;specific surface area&#34; in this specification are to that measured by the Brunauer, Emmett and Teller (BET) nitrogen absorption method assuming the area covered by a nitrogen molecule to be 16.2 A 2 . (The BET method is referred to, for example, in &#34;The Physics and Chemistry of Surfaces&#34; by N. K. Adam published by Dover, and in &#34;Solid Surfaces and the Gas-Solid Interface&#34;, Advances in Chemistry Series Vol. 33) 
     The alumina hydrate is preferably the trihydrate, Al 2  O 3 .3H 2  O, and preferably constitutes a major amount of the anti-tracking filler system. 
     It has surprisingly been found that the use of alumina hydrate of high surface area greatly enhances the properties of the insulating compositions of the present invention. It is preferable for the specific surface area of the alumina hydrate to be at least 3 m 2  /g and advantageously greater than 6 m 2  /g. Especially good results are obtained when the specific surface area lies in the range of from 8 to 20 m 2  /g. Lower surface areas will still yield acceptable results. However, the surface area should not be below about 2.0 m 2  /g to obtain the surprising improvement in insulating properties disclosed herein. 
     The hydrated alumina advantageously has a maximum particle size less than about 4 microns and preferably less than about 2 microns. Since the specific surface area of hydrated alumina is not directly dependent on particle size, it is understood that the invention is not limited to a particular particle size range. 
     The specific surface areas and particle size distributions of two forms of alumina trihydrate suitable for use in the present invention are given below. 
     
         ______________________________________           A     B______________________________________weight % less than             100     1002 micronsweight % less than             99.5    801 micronweight % less than             60      210.5 micronspecific surface  12-15   6-8area m.sup.2 /g______________________________________ 
    
     Types A and B are sold by the Aluminum Company of America as &#34;Hydral 705&#34; and &#34;Hydral 710&#34;, respectively. It is to be noted that the above surface areas are those claimed by Alcoa. Actual surface areas for samples of A have been found to vary from 6 to 21 m 2  /g, averaging about 12 m 2  /g. 
     Hydral 705 gives generally good results when used in the anti-tracking systems of the present invention, especially when the surface area is 10 m 2  /g or higher. 
     Alumina hydrate of the desired specific surface area may be prepared by methods known per se, for example, by dissolving alumina in caustic soda and then reprecipitating it by bubbling carbon dioxide through the solution. According to the pH and the rate of bubbling, the necessary values of which can easily be determined by experiment, alumina hydrate of the desired specific surface area can be obtained. 
     Among oxides, mixed oxides and mixtures suitable as component (b) there may be mentioned, for example, 
     (i) Transition metal oxides, for example, TiO 2 , V 2  O 5 , Cr 2  O 3 , Fe 2  O 3 , NiO 2 , Ni 3  O 4 , Co 2  O 3 , Co 3  O 4 , MoO 3 , WO 3 , Nb 2  O 5 , and mixtures thereof. 
     (ii) Lanthanide series oxides, for example, Pr 6  O 11 , Yb 2  O 3 , Ce 2  O 3 , holmium oxide, erbium oxide, and mixtures thereof. 
     (iii) Nontransuranic actinide series oxides, for example, UO 3 , ThO 2  and mixtures thereof. 
     (iv) Mixed oxides containing one or more transition metals, for example, nickel titanate, nickel molybdate and cobalt silicate. 
     (v) Any of the above oxides, mixed oxides or mixtures carried on an ahydrous alumina support, e.g., nickel cobalt molybdate on alumina and cobalt oxide on alumina CoO.Al 2  O 3  (blue cobalt aluminate). 
     (vi) Any of the above systems doped with small quantities of alkali metal or alkaline earth metal oxides, especially Li 2  O and K 2  O. 
     It is understood that the above listing is illustrative only, and is not intended to be a complete list of all the oxides which are operable in the invention. 
     The component (b), which is believed to react synergistically with the alumina hydrate in reducing tracking, may in some cases be used in quantities down to or below 0.5% by weight based on the total weight of the insulation material, but in general is preferably present in an amount in the range of from 2 to 10%, especially from 3 to 5%. Although amounts higher than 10% may be employed, little additional benefit to the tracking and erosion properties is gained thereby with most oxides. However, with some oxides of the invention e.g., ThO 2 , the oxide is preferably present in quantities of about 15% or more by weight. 
     The particle size and surface area of component (b) are not so important as that of the alumina hydrate but the best results appear to be obtained with materials having a particle size less than 75 microns (BS 200 mesh, U.S. Sieve No. 200) and, for ease of dispersion during preparation of the insulating material, it is preferable to use materials having a particle size less than 45 microns (B.S. 350 mesh, U.S. Sieve No. 325). 
     Compounds of iron, cobalt and chromium are especially preferred as component (b). Particularly good results are obtained when Co 3  O 4  is used as component (b). Ferric oxide, cobaltic oxide and chromic oxide are also very useful. 
     It has also been found that a given weight of some combinations of two or more substances as component (b) is more effective than the same weight of only one of the substances, i.e. a synergistic effect between the substances is noted. An especially good component is a mixture of Fe 2  O 3  and Co 2  O 3 . Insulating materials incorporating a given weight of this mixture show substantially less erosion on testing than materials incorporating an equal amount of either Fe 2  O 3  or Co 2  O 3  alone. 
     One of the problems of the previously proposed anti-tracking compositions in which alumina hydrate is used alone has been the difficulty of obtaining good U.V. stability. Normally, for maximum weather and U.V. protection it is common practice to incorporate small quantities of carbon blacks, usually the channel type. Alternatively, organic U.V. absorbers such as, for example, substituted benzophenones or benzotriazoles are employed which are far less effective than carbon black, although they have the advantage that a variety of colors are obtainable using normal pigments. In the case of polymeric insulating materials for outdoor use, a lifetime exposure running into decades, typically 25-30 years, is required. Some polymeric insulating-materials are not sufficiently U.V. stable to last this length of time and it is therefore necessary to incorporate channel blacks into them. The effect of these channel blacks, even in small concentrations of the order of 0.3% or less, has, however, been catastrophic on the previously proposed insulation materials in that tracking failures have rapidly occurred. 
     One of the surprising features of the present invention is that carbon black fillers can be incorporated into the insulation, in amounts up to about 3% by weight, without failure by progressive creepage tracking occurring such materials failing only by erosion. This is obviously a very important advantage particularly where the base polymers do not have good U.V. resistance. 
     In general, the anti-tracking filler system will constitute from 20-75% of the total weight of the insulating material. However, owing to the synergistic effect between component (b) and the alumina hydrate it is possible to reduce the proportion to below 20% in some cases without loss of anti-tracking properties. This is especially useful in the formation of heat-shrinkable articles from the materials when proportions of from 20 to 35% are preferred to avoid the development of porosity during manufacture and to maintain the necessary elongation, modulus and tensile strength above the crystalline melting point. If the material is to be used in non-heat-shrinkable applications then it is preferred to increase the proportion of alumina hydrate since this effectively reduces erosion rates and also lessens the overall cost of the insulation. The preferred proportion of anti-tracking filler system (a) plus (b) thus falls within the range of from 30 to 60% of the total weight of the insulation in these cases. 
     In general any polymer normally used for high voltage insulation may be used in this invention. Among polymeric materials into which the anti-tracking system may be incorporated there may be mentioned polyolefins and other olefin polymers, obtained from two or more monomers especially terpolymers, polyacrylates, silicone polymers and epoxides, especially cycloaliphatic epoxides. Among epoxide resins of the cycloaliphatic type there may especially be mentioned those sold commercially by CIBA (A.R.L.) limited under the names CY 185 and CY 183. Particularly suitable polymers include polyethylene, ethylene/ethyl acrylate copolymers, ethylene/vinyl acetate copolymers, ethylene/propylene copolymers, ethylene/propylene non-conjugated-diene terpolymers, chlorosulphonated polyethylene, polypropylene, polydimethyl siloxane, diemthyl siloxane/methyl vinyl siloxane copolymers, fluoro silicones, e.g. those derived from 3,3,3-trifluoropropyl siloxane, carborane siloxanes, e.g. &#34;Dexsil&#34; polymers made by Olin Mathieson, polybutyl acrylate butyl/ethyl acrylate copolymers, butyl acrylate/acrylonitrile copolymers, butyl acrylate/glycidyl methacrylate copolymers, polybutene, butyl rubbers, ionomeric polymers, e.g. &#34;Surlyn&#34; materials sold by DuPont, or mixtures of any two or more of the above. 
     The present invention also provides a composition suitable for processing into the insulating material of the invention. 
     The present invention furthermore provides an anti-tracking filler system suitable for use in the insulating materials and compositions of the invention. 
     The present invention also provides an electrical component especially a cable, insulated by the insulating material of the invention. 
     The present invention also provides a shaped article made from an insulating material of the invention. The material may be cross-linked and, if desired, the property of heat-recoverability may be imparted to the article. 
     Among the many uses for the insulating materials of the present invention there may especially be mentioned the production of heat-shrinkable tubing, heat-shrinkable sheds and heat-shrinkable 3-core udders for cable connection and termination, especially at high voltages of up to 33 KV and higher (vide &#34;Heat-shrinkable plastics for termination and jointing of power and auxiliary cables&#34; D. Q. Ash, Ir Bagdzinski and R. J. T. Clabburn PROC.IEE Vol. 117, No. 7, July 1970 pp 1365-1372). These and other shaped parts are especially useful in the termination of high voltage cables to overhead lines, to transformers and to switchgear, especially in outdoor environments. 
     The insulating material of the present invention may also in some cases advantageously be applied to, for example, a termination in situ by application of the basic composition in the form of a lacquer in a suitable solvent, for example, toluene, xylene or carbon tetrachloride. In some cases, especially when the polymer component is a silicone, the composition may itself be sufficiently fluid for in situ application which will harden on standing. 
     It will be appreciated that although the primary purpose of the anti-tracking filler is to prevent creeptracking as outlined above, the materials will also be effective in stabilizing the insulation under arcing conditions, i.e. in cases where a direct arc passes between two parts of an electrical apparatus forming a carbonaceous track along its line. This phenomenon is slightly different from creep-tracking where, for example conducting contaminate and/or surface irregularities cause a leakage current and a dendritic carbonaceous path develops on the surface of the insulation. 
     The insulating material and compositions of the present invention may, if desired, contain other fillers, for example, flame retardants, reinforcing fillers, pigments and mixtures thereof. 
     The anti-tracking system can be incorporated into polymer(s) by any of the commonly used techniques, for example, in a twin-roll mill at elevated temperatures. Similarly the resulting compositions can readily be processed into sheets of material or other shaped articles by any of the usual methods. 
     The following examples illustrate the invention, parts and percentages being by weight unless otherwise stated. The surface area of the alumina trihydrate is 16.0 m 2  /g unless otherwise indicated. 
    
    
     EXAMPLE 1 
     The following formulations were prepared by compounding on a twin-roll mill: 
     
         ______________________________________Sample No.         1     2     3   4   5   6   7______________________________________Dimethyl silicone elasto-              30    30    30  30  30  30  30mer (containing a smallamount of methyl vinylsiloxane)Low density polyethylene              30    30    30  30  30  30  30(MFI 3)Ethylene ethyl acrylate              30    30    30  30  30  30  30copolymer (19% ethylacrylate)Alumina trihydrate 30    60    30  45  60  --  --(Hydral 705)Agerite Resin D    2     2     2   2   2   2   2(polymerized trihydro-quinaline oxidant)sold by VanderbiltCalcined ferric oxide              5     5     --  --  --  5   30Triallyl cyanurate 1     1     1   1   1   1   12,5-dimethyl 2,5-di-t-              1     1     1   1   1   1   1butyl peroxy hexyne-3______________________________________ 
    
     After thoroughly mixing all the ingredients in the order given, plaques 5×2×0.25 inches were pressed at 200° C. for 10 mins. The cross-linked plaques were then tested according to the ASTM D2303-68 liquid contaminant inclined plane test which measures the tracking and erosion resistance of insulating materials. The test method used was the initial tracking voltage test in which the voltage is raised by 0.25 KV every hour and was identical to the ASTM test except that the voltage supply was 50 HZ and no silver paint was used in conjunction with the stainless steel electrodes. The silver paint is not necessary and its absence has no effect on the test result. The samples were used for testing approximately 24 hours after preparation and were stored at room temperature and humidity. They were not soaked in contaminant prior to test. The contaminant which was ammonium chloride had a resistivity of 380 ohm. cm. and contained 0.02% Triton X100 as wetting agent. 
     The results of the tests are given in Table 1. 
     Comparison of the results of the tests on Samples 1 to 7 clearly shows that whereas Samples 6 and 7 containing ferric oxide but no alumina hydrate track rapidly, and Samples 3, 4 and 5 containing alumina hydrate but no ferric oxide track after a relatively short period, Samples 1 and 2 containing both alumina hydrate and ferric oxide in accordance with the present invention are non-tracking and fail only by erosion after 200 and 270 minutes on test, respectively. 
     The results of the tests on Samples 3, 4 and 5 illustrate that in the absence of ferric oxide as component (b), an increased proportion of alumina hydrate does not prevent tracking. 
     The results of the tests on Samples 6 and 7, similarly show that, in the absence of alumina hydrate, an increased proportion of ferric oxide does not prevent tracking. 
     The results of the tests on Samples 1 and 2 show clearly the synergistic anti-tracking effect of a combination of alumina hydrate and ferric oxide. 
     A further experiment was carried out on Sample 1 to evaluate the rate of erosion. In this experiment, the voltage was maintained constant at 2 KV, and the contaminant contained 1% sugar in addition to the ammonium chloride. After 24 hrs. exposure to the scintillation a very small depression about 0.050 in. deep had eroded away, showing the extremely good erosion resistance. By comparison, Sample 3 tracked, under the same conditions after 150 minutes. 
     
                                           TABLE 1__________________________________________________________________________Sample No. 1       2    3    4     5     6     7__________________________________________________________________________Start up voltage KV      3.0     3.0  3.0  3.0   3.0   3.0   3.0Final voltage KV      3.75    4.0  3.25 3.5   3.5   3.0   3.0Tracking voltage KV      non     non  3.25 3.5   --    3.0   --      tracking              trackingTime to track 2&#34;      --      --   69   120   130   8     14(minutes from startof test)Comments   Sample eroded              Sample                   Narrow                        Sample                              Sample                                    Substan-                                          Substantially      through at              eroded                   tracks                        tracks,                              eroded                                    tially                                          immediate      base after              through                   with but then                              rapidly                                    immediate                                          tracking.      200 mins. on              at base                   slight                        tended to                              with some                                    tracking.      test,   after                   tendency                        &#34;clean up&#34;                              tracking              270 mins.                   to den-                        leaving a                              then              on test.                   dritic                        large deep                              &#34;cleaned                   tracking                        erosion                              up&#34;, sub-                   noted.                        crater in                              sequently                        the sample.                              tracking                              again.__________________________________________________________________________ 
    
     EXAMPLE 2 
     Test samples containing the following formulations were prepared as described in Example 1. 
     
         ______________________________________Sample No.            8       9______________________________________Dimethyl silicone elastomer                 30      30(as in Example 1)Low density polythene 30      30(as in Example 1)Ethyleneethyl acrylate copolymer                 30      30(as in Example 1)Alumina trihydrate Hydral 705                 30      30Agerite Resin D (antioxidant)                 2       2Calcined ferric oxide 5       --Kosmos BB (a conducting carbon                 1       1black)Triallyl cyanurate    1       12,5-dimethyl-2,5-di-t-butyl-                 1       1peroxy hexyne-3______________________________________ 
    
     The following results were obtained using the test method described in Example 1. 
     
         ______________________________________Sample No.     8           9______________________________________Start up voltage, kV          1.5         1.5Final voltage  3.75        2.0Tracking voltage          Non-tracking                      2.0Comments.      Sample eroded                      Sample tracked          at bottom elec-                      after 125          trode after 576                      minutes on test.          minutes on test.______________________________________ 
    
     EXAMPLE 3 
     Samples containing the following formulations were prepared and tested as in Example 1. 
     All samples contained 30 parts dimethyl silicone elastomer, 30 parts low density polyethylene, 30 parts ethylene/ethyl acrylate copolymer, 30 parts alumina trihydrate (Type A), 2 parts Agerite Resin D, 1 part triallyl cyanurate and 1 part 2,5-dimethyl-2, 5-di-t-butyl peroxy hexyne-3. 
     In Addition, Samples 10-19 contained 5 parts of the following components, respectively: 
     
         ______________________________________Sample 10.     NiO.sub.2Sample 11.     Ni.sub.3 O.sub.4Sample 12.     Co.sub.2 O.sub.3Sample 13.     Co.sub.3 O.sub.4Sample 14.     Cr.sub.2 O.sub.3Sample 15.     V.sub.2 O.sub.5Sample 16.     TiO.sub.2 (surface area 250 m.sup.2 /g)Sample 17.     MoO.sub.3Sample 18.     WO.sub.3Sample 19.     Nb.sub.2 O.sub.5Sample 20 additionally contained a mixture 5 partsFe.sub.2 O.sub.3 and 5 parts Co.sub.2 O.sub.3.______________________________________ 
    
     The results of the tests are shown in TABLE 2. It will be seen that Samples 13, 14, 16 and 20 gave especially good results and that all the samples show substantially better anti-tracking properties than Samples 3, 4 and 5, which contained only alumina hydrate. 
     
                                           TABLE 2__________________________________________________________________________Sample No. 10     11      12     13     14     15__________________________________________________________________________Start up voltage KV      3.0    3.0     3.0    3.0    3.0    3.0Final voltage KV      3.5    3.75    4.0    5.25   4.25   3.25Tracking voltage KV      non-tracking             non-tracking                     non-tracking                            non-tracking                                   non-tracking                                          non-trackingComments   Eroded Sample eroded                     Eroded Substantially                                   Slow erosion                                          Sample eroded      through at             slowly removed                     through at                            no erosion                                   failed by                                          through at      base after             from test owing                     base after                            even after                                   flaming after                                          base after      165 mins.             to flaming of                     265 mins.                            580 mins.                                   320 mins.                                          106 mins.             surface only             partly eroded.__________________________________________________________________________   Sample No. 16     17     18     19      20__________________________________________________________________________   Start up voltage KV              3.0    3.0    3.0    3.0     3.0   Final voltage KV              5.25   3.75   3.75   3.25    5.0   Tracking voltage KV              non-tracking                     3.75   non-tracking                                   non-tracking                                           non-tracking   Comments   Sample failed                     Sample failed                            Sample ero-                                   Eroded through                                           Very slow              by surface                     by small                            ded through                                   at base after                                           erosion. Sam-              flaming after                     track in mid-                            at base                                   96 mins.                                           ple hardly              588 mins.                     dle of speci-                            after 200      marked when                     men.   min.           test discon-                                           tinued at                                           529 mins.__________________________________________________________________________ 
    
     EXAMPLE 4 
     Samples containing the following formulations were prepared and tested as described in Example 1. 
     The results are shown in Table 3. 
     
         ______________________________________Sample No.       21      22      23    24______________________________________Ethylene ethyl acrylate            200     200     100   100copolymer (as in Example 1)Ethylene-propylene-ethyli-            80      80      80    80dene norbornene terpolymerLow density polythene            --      --      100   100(as in Example 1)Alumina hydrate (Hydral 705)            110     110     110   110Agerite Resin D  5       5       5     5Triallyl cyanurate            3       3       3     32,5-di methyl 2,5-di tert.            2       2       2.5   2.5butyl peroxy hexyne-5Ferric oxide     15      --      15    --______________________________________ 
    
     
         __________________________________________________________________________Sample No. 21     22   23       24__________________________________________________________________________Start up voltage KV      3.0    3.0  3.0      3.0Final voltage KV      4.0    3.25 4.5      3.5Tracking voltage KV      non-tracking             3.25  4.25    3.5Comments   Sample re-             Tracked                  Sample   Failed by      moved at 248             at 76                  failed by                           tracking      mins. due to             mins.                  flaming. at 137 min.      distortion re-                  Erosion and      sulting from                  several      heat of spark                  small tracks                  present. 392 mins.__________________________________________________________________________ 
    
     These results show the improved anti-tracking effects obtained using a system of the present invention in two different types of polymeric insulation. 
     EXAMPLE 5 
     Samples containing the following formulations were prepared and tested as described in Example 1. The results are shown in Table 4. 
     
         ______________________________________Sample No.             25      26______________________________________Low density polythene (as in Example 1)                  200     200Ethylene ethyl acrylate copolymer                  40      40(as in Example 1)Alumina hydrate (Hydral 705)                  100     100Agerite Resin D        2       2Trially cyanurate      2       22,5 di methyl-2,5-di-tert. butyl peroxy                  2.5     2.5hexyne-3Ferric oxide           15      --______________________________________ 
    
     
                       TABLE 4______________________________________Sample No.   25             26______________________________________Start up Voltage kV        3.0            3.0Final Voltage kV        5.75           5.0Comments     Non-tracking. Sample                       Flame failed        removed after 704                       with some        mins. on test, show-                       indication of        ing only some erosion.                       tracking.______________________________________ 
    
     These results show the improved effect obtained using ferric oxide as component (b) in another type of polymeric insulation. 
     EXAMPLE 6 
     Samples containing the following formulations were prepared and tested as described in Example 1, except that the ammonium chloride had a resistivity of 330 ohm cm. (thus making the test slightly more severe). 
     
         ______________________________________Sample No.     27     28     29   30   31   32______________________________________Dimethyl silicone          30     30     30   30   30   30elastomer (containinga small amount ofmethyl vinyl siloxane)Low density polyethylene          30     30     30   30   30   30(MFI 3)Ethylene ethyl acrylate          30     30     30   30   30   30copolymer (19% ethyl.acrylate)Alumina trihydrate          30     30     30   30   30   30(Hydral 705)Agerite Resin D          2      2      2    2    2    2(antioxidant)Calcined ferric oxide          0      1      4    5    71/2 10Triallyl cyanurate          1      1      1    1    1    12,5 dimethyl 2,5 di tert.          1      1      1    1    1    1butyl peroxy hexyne-3______________________________________ 
    
     The results obtained were as follows: 
     
         ______________________________________Sample Number  27     28     29   30   31   32______________________________________Starting voltage KV          1.5    1.5    1.5  1.5  1.5  1.5Finishing voltage          2.5    2.75   3.0  3.25 3.0  3.25Time to track 2&#34;          240    --     --   --   --   --          min.______________________________________ 
    
     Samples 28-32 prepared in accordance with the present invention, were non-tracking and failed only by erosion after 320, 384, 428, 395 and 420 minutes, respectively. These figures illustrate that, where component (b) is ferric oxide, increasing the proportion does not result in increased tracking protection and only has a small effect, if any, on the erosion of the materials. However, with other oxides disclosed as operative in the invention larger proportions of the oxide may be required to obtain the desired anti-tracking properties in the insulation. 
     EXAMPLE 7 
     Three further compositions containing cobalt silicate, cobalt aluminate, and brown iron oxide, respectively, as component (b) were prepared and tested as described in Example 1. 
     
         ______________________________________                      Pts.______________________________________Dimethyl silicone elastomer (containing 0.2 mole %                        30of methyl vinyl siloxane)Low Density polythene (MFI 3.0)                        30Ethylene ethyl acrylate copolymer (18% ethyl acrylate)                        30Alumina trihydrate (surface area 6.3 m.sup.2 /g.)                        30Agerite Resin D              2Component (b) as in Table    5Triallyl cyanurate           12,5 dimethyl 2,5 di tert. butyl peroxy hexyne-3                        0.5______________________________________ 
    
     The starting voltage was 3 KV and the following results were obtained. 
     
                       TABLE 5______________________________________Sample            Final VoltageNo.   Component (b)             KV         Comments______________________________________33    Blue cobalt 5.5        Sample removed at silicate               647 mins. no tracking (about 32%             only slight erosion cobalt)34    Blue cobalt 5.25       Non-tracking, sample aluminate              removed at 569 mins. (32% cobalt)           deeply eroded35    Brown iron  3.75       Sample eroded through oxide (Ref 645T        at base after 180 mins. ex Bayer)______________________________________ 
    
     EXAMPLE 8 
     These experiments illustrate the importance of the surface area of the alumina hydrate. The following formulations were prepared as described in Example 1. 
     
         ______________________________________                 Pts______________________________________Dimethyl silicone elastomer (containing                   300.2 mole % of methyl vinyl siloxane)Low density polythene (M.F.I. 2.0)                   30Ethylene ethyl acrylate copolymer                   30(18% ethyl acrylate)Alumina trihydrate (surface area                   30as in Table 6)Agerite Resin D         2Ferric oxide            5Triallyl cyanurate      2.02,5 dimethyl 2,5 di tert butyl peroxy                   0.5hexyne-3______________________________________ 
    
     Test samples were tested as described in Example 1 but at a constant voltage of 3 KV, so that the time to track or erode at this voltage was determined. The results are presented in Table 6. 
     
                       TABLE 6______________________________________ Surface areaSample of hydrateNo.   m.sup.2 /g   Comments______________________________________36    0.5          Tracked at 24 mins.37    1.8          Tracked at 58 mins.38    2.1          Sample eroded with some small              tracks at 65 mins.39    3.7          Sample eroded through at 181 mins.40    4.8          Sample eroded through at 166 mins.41    5.0          Sample eroded at 195 mins.42    6.3          Sample eroded at 200 mins.43    10.5         Sample eroded at 360 mins.44    12.3         Sample removed at 494 mins.              approximately 1/4 thickness left.45    16           Sample removed at 523 mins              approximately 1/4 thickness left              intact______________________________________ 
    
     The above results show quite clearly that hydrates having a surface area below about 2.0 m 2  /g do not prevent tracking, while, surprisingly, materials containing hydrates of about 2.0 m 2  /g or higher surface area fail by erosion only. Although the mechanism by which alumina hydrates with a surface area above about 2.0 m 2  /g combine synergistically with the disclosed metal oxides is unknown, Table 6 clearly discloses that where the surface area is about 2.0 m 2  /g or over the hydrates confer increasing resistance to erosion as their surface area increases. 
     The surface areas were measured by the BET technique on a Micromeritics Surface Area Analyzer Model 2200 after initially degassing the samples for 40 minutes at 150° C. 
     EXAMPLE 9 
     This Example illustrates compositions based on various other polymers. 
     The following formulations were prepared and tested as described in Example 1. 
     
         ______________________________________Sample No.     46*    47*    48   49   50   51______________________________________Polyethylene (MFI 3.0)          --     --     100  100  --   --Ethylene/propylene          --     --     --   --   100  100copolymer 30%propylene)Polydimethyl Siloxane          100    100    --   --   --   --Aerosil 200 (Fine particle          30     30     --   --   --   --size silica)Alumina trihydrate (Sur-          30     30     20   20   50   50face Area 6.3 m.sup.2 /g)Ferric Oxide   --     5      --   20   --   5Triallyl cyanurate          2      2      2    2    2    22,5-dimethyl-2,5-          1      1      1    1    2.5  2.5ditertiarybutyperoxy-hexane-3______________________________________ *Substantially identical results were obtained using a copolymer of dimethyl siloxane and 0.2 mole % methyl vinyl siloxane in place of pure polydimethyl siloxane. 
    
     The following results were obtained: 
     
                       TABLE 7______________________________________Sample No.   Starting Voltage                Final Voltage                           Comments______________________________________.sup. 46*   1.5 kV       2.25 kV    Complete failure                           by tracking.sup. 47*   1.5 kV       2.75 kV    Failed by erosion48      1.5 kV       2.5 kV     Sample failed by                           several tracks                           and flame failure                           after 243 minutes49      1.5 kV       2.75 kV    Sample failed by                           erosion after 310                           minutes50      1.5 kV       2.5 kV     Sample failed by                           deep tracking                           erosion and flame                           failure after 280                           minutes51      1.5 kV       2.75 kV    Sample failed by                           erosion at 319                           minutes______________________________________ *Substantially identical results were obtained using a copolymer of dimethyl siloxane and 0.2 mole % methyl vinyl siloxane in place of pure polydimethyl siloxane. 
    
     EXAMPLE 10 
     This example illustrates the use of oxides of the lanthanide and actinide series. 
     The following formulations were prepared and tested as in Example 1. 
     
         ______________________________________            52Sample No.       Comparison 53     54   55______________________________________Dimethyl siloxane            30         30     30   30elastomer (containing0.2 mole % methylvinyl siloxane)Low density polythene            30         30     30   30(MFI 3.0)Ethylene/ethylacrylate            30         30     30   30copolymer (as inExample 1)Alumina trihydrate            30         30     30   30(surface area 6.3 m.sup.2 /g.)Agerite Resin D  2          2      2    2UO.sub.3 *       --         5      --   --Pr.sub.6 O.sub.11 *            --         --     5    --Ce.sub.2 O.sub.3 *            --         --     --   5Triallyl cyanurate            1          1      1    12,5-dimethyl-2,5 0.5        0.5    0.5  0.5di-t-butyl peroxyhexyne-3______________________________________ *Purities were 99.9+% 
    
     The ASTM test was commenced at 3.0 kV and the following results were obtained. 
     
                       TABLE 8______________________________________Sample No.    Final Voltage               Comments______________________________________52       3.25 kV    Tracked at 69 minutesComparison53       3.75 kV    Eroded and tracked at 183 minutes54       3.25 kV    Wide erosion sample tracked at               89 minutes55       3.50 kV    Severe erosion and slight               tracking after 160 minutes______________________________________ 
    
     The above figures show that, although in general the results obtained using these oxides are not so good as those obtained using transition metal oxides, there is a definite improvement in anti-tracking properties when compared to the use of alumina trihydrate alone. 
     EXAMPLE 11 
     This example illustrates the use of thorium oxide. 
     The formulations were prepared and tested as in Example 1, except that the ammonium chloride had a resistivity of 385 ohm cms., thus making the test slightly less severe. 
     
         ______________________________________Sample Number    56*   57*     58** 59**/60*______________________________________I.C.I. Silicone Elastomer            30    30      30   30E 322/60L.D. polyethylene, MFI 3.0.            30    30      30   30Ethylene-ethyl acrylate            30    30      30   30copolymer (18% ethylacrylate)Alumina trihydrate            30    30      30   30Thorium oxide, ThO.sub.2            20    60      10   --Agerite Resin D   2     2       2    2Triallyl cyanurate             1     1       1    12,5 dimethyl 2,5 di tert.             1     1       1    1butyl peroxy hexyne-3______________________________________ *Alumina trihydrate surface area was 11.1 m.sup.2 /g. **Alumina trihydrate surface area was 6.44 m.sup.2 /g. 
    
     The ASTM test was commenced at 2.5 KV, and the following results were obtained. 
     
                       TABLE 9______________________________________Sample Number      56         57        58    59/60______________________________________Start up voltage      2.5        2.5       2.5   2.5KVFinal voltage KV      2.75       2.5       2.75  2.75Tracking voltage      2.75       2.5       2.75  2.75KVTime to track 2&#34;      101        28        70    72/75(minutes fromstart of test)Comments:  Started to Tracking  Single                                 Tracking      track at 17                 started   carbon                                 started      mins., but at 14 min.                           track after      cleaned up,          formed                                 10 mins.      to fail by      eroded track.______________________________________ 
    
     The test results in Table 9 demonstrate that thorium oxide performs according to the invention. Best results with thorium oxide are obtained by using alumina hydrate of a fairly high surface area and a moderate amount of the oxide. It is preferable to avoid use of the oxide in large quantities. 
     EXAMPLE 12 
     This example further illustrates the use of oxides and mixtures of oxides in the lanthanide series. 
     The following formulations were prepared and tested as in Example 1, except that the ammonium chloride had a resistivity of 385 ohm cms., thus, making the test slightly less severe. 
     
         ______________________________________Sample Number   61a    62a    63a  64a  65a  66______________________________________I.C.I. Silicone Elastomer           30     30     30   30   30   30E 322/60L.D. Polyethylene, MFI 3.0           30     30     30   30   30   30Ethylene-ethyl acrylate co-           30     30     30   30   30   30polymer (18% ethylacrylate)Alumina trihydrate           30     30     30   30   30   30(surface area-10.2 m.sup.2 /g)Holmium oxide   --     5      --   --   --   --Praeseodymium oxide           --     --     5    --   --   --Pr.sub.6 O.sub.11.sup.1Ytterbium oxide.sup.1           --     --     --   5    --   --Erbium oxide.sup.1           --     --     --   --   5    --Cerium oxide.sup.2           --     --     --   --   --   5Agerite Resin D 2      2      2    2    2    2Triallyl cyanurate           1      1      1    1    1    12,5 di methyl 2,5 di tert.           1      1      1    1    1    1butyl peroxy hexyne-3______________________________________ .sup.1 Purities were 99.9+% .sup.2 Commercial grade sample of 50% cerium oxide with the remainder being oxides of all other lanthanide elements. 
    
     Samples 61b-65b correspond to samples 61a-65a respectively and were prepared from the same components in the same amounts except that the alumina trihydrate had a surface area of 5.8 m 2  /g. 
     The ASTM test was commenced at 2.5 KV. The results are presented in Table 10. 
     
                       TABLE 10______________________________________                   Time toSam-  Final    Tracking track 2&#34;ple   voltage, voltage, (mins. fromNo.   KV       KV       start of test)                            Comments______________________________________61a   3.0      2.5      147      Tracking started(Com-                            at 20 mins. Sev-pari-                            eral tracks wereson)                             evident by 120                            mins. which later                            joined together                            producing final                            failure.62a   3.5      3.25     270      Erosions started at                            82 mins. Later                            large eroded crater                            formed, which pro-                            gressed to top                            electrode with some                            tracking.63a   3.5      3.25     294      Erosion started at                            13 mins. Large                            eroded crater                            formed and ulti-                            mate failure was                            by single carbon                            track from top of                            crater.64a   3.5      3.5      253      Erosion started at                            55 mins. Large                            eroded crater                            formed and failure                            was by tracking                            with very large                            eroded crater at                            bottom electrode.65a   3.25     3.0      239      Erosion started at                            81 mins. Failure                            due to large                            eroded crater with                            single track at                            top.66    3.5      3.25     294      Erosion started                            at 33 mins. Fail-                            ure due to eroded                            crater with                            single carbon                            track at top.61b   2.75     2.5       62      Tracking started(Com-                            after 10 mins.pari-son)62b   3.0      3.0      175      Initially some                            erosion, but this                            turned to a                            single carbon                            track.63b   3.0      3.0      176      Erosion started                            at 15 mins.                            Later several                            carbon tracks                            started even-                            tually going                            up, causing fail-                            ure.64b   3.0      3.0      147      Erosion initially                            but carbon tracks                            formed which                            linked up to one                            single track.65b   3.25     3.0      191      Failure due to                            carbon tracks                            after small                            eroded crater                            formed.______________________________________ 
    
     The test results in Table 10 demonstrate that the lanthanide oxides act synergistically, even at low surface areas, with alumina hydrate, giving increased resistance to tracking over alumina hydrate alone. 
     The test results in Table 10 may be advantageously compared with those presented in Example 8 to demonstrate that the surface area of the alumina trihydrate similarly affects the degree of anti-tracking resistance imparted to electrical insulators by all of the oxides and mixed oxides disclosed as operative in the invention. Thus, as shown in Example 8, with iron oxide, the time for erosion failure increases from 166 through 360 to 523 minutes as the surface area of the alumina hydrate is increased from 3.7 through 10.5 to 16 m 2  /g. Similarly, in the case of ytterbium, the time for erosive failure increases from 147 to 253 minutes as the surface area is increased from 5.8 to 10.2 m 2  /g. 
     In contrast, the effect of surface area variations in the alumina trihydrate when the latter is used alone appears to be minimal, as can be seen from the following Table drawn up from the comparative samples tested and reported elsewhere in this specification. 
     
                       TABLE 10a______________________________________       Surface AreaSample      m.sup.2 /g Time to track 2&#34;______________________________________61b         5.8        6252          6.3        6959          6.44       7261a         10.2       14769          10.9       3060          11.1       753           16         69______________________________________ 
    
     Table 10a, allowing for two apparently anomalous results, indicates that, in the absence of an additional filler in accordance with the present invention, the anti-tracking properties are virtually independent of the surface area of the alumina trihydrate and, implicitly, indicates a synergistic relationship between such fillers and the said surface area. 
     EXAMPLE 13 
     This example compares the effectiveness of a composition employing a metal oxide within the scope of the invention (calcined ferric oxide) with that of a metal oxide outside the scope of the invention (zinc oxide). 
     The following formulations were prepared and tested as described in Example 1. 
     
         ______________________________________Sample No.         67      68      69______________________________________ICI Silicone Elastomer              30      30      30E 322/60LD polyethylene, MFI 3.0              30      30      30Ethylene-ethyl acrylate co-              30      30      30polymer-18% ethyl acrylateAlumina trihydrate 30      30      30(surface area 10.9 m.sup.2 /g)Agerite Resin D    2       2       2Zinc Oxide         --      5       --Calcined ferric oxide              5       --      --Triallyl cyanurate 1       1       12,5 dimethyl 2,5 di tert.              1       1       1butyl peroxy hexyne-3______________________________________ 
    
     The results obtained are reported in Table 11 below. 
     
                       TABLE 11______________________________________Sample No.     67          68          69______________________________________Start up voltage     3.0         3.0         3.0KVFinal voltage     3.25*       3.25        3.0KVTracking volt-     non-tracking                 3.25        3.0age KVTime to track         80          302&#34; (minutesfrom start oftest)Comments: *Sample failed                 Deep eroded Sample     at 119 mins. by                 track between                             started     catching fire                 electrodes. tracking     at edge of sam-                 Tracking com-                             immediately     ple. No track-                 menced within                             voltage ap-     ing occured.                 10 mins. of plied.                 voltage appli-                 cation.______________________________________ 
    
     The above figures clearly show that the use of zinc oxide with alumina hydrate does not prevent tracking failure of the insulating material whereas the use of ferric oxide in the place of zinc oxide in an otherwise identical formulation completely eliminates tracking failure. Although sample 68, containing zinc oxide, took longer to fail by tracking than control sample 69, containing alumina hydrate alone, failure of sample 68 was still a tracking failure. This demonstrates that zinc oxide does not act synergistically with alumina trihydrate to give vastly improved resistance to tracking failure as do the oxides and mixed oxides of the invention. 
     EXAMPLE 14 
     This Example illustrates the use of the insulation materials of the present invention in heat-recoverable articles. 
     Using a formulation as specified in Example 1, Sample 1, a shed of internal diameter 0.49 inches was prepared by moulding at 190° C. for 12 minutes. After moulding, the shed was cooled in water, trimmed and then heated in a glycerine bath at 170° C. for 3 minutes. A PTFE mandrel of diameter 1.1 inches was then forced through the shed and then the mandrel plus shed was cooled in cold water for 5 minutes, whereupon the mandrel was removed. The shed then had an internal diameter of 1.05 inches. It was pore free. It was then reheated with a hot air gun to 170° C. whereupon it shrank and completely recovered to an internal diameter of 0.49 inches. 
     Similar results were obtained with a larger shed of internal diameter 1.05 inches which was expanded to 2.2 inches, and then completely recovered. 
     Using the same formulation, but with the peroxide absent, tubing of the following dimensions was prepared by extrusion. 
     
         ______________________________________Internal diameter           0.475 inchesWall thickness  0.095 inches______________________________________ 
    
     The tubing was irradiated using a 1.5 mev electron beam to a total dose of 12 megarads. The tubing had the following properties. 
     
         ______________________________________         23° C.                  150° C.______________________________________Tensile Strength           101.3 Kg/cm.sup.2                      15.3 Kg/cm.sup.2Elongation at break           416%       293%100% Modulus    --          6.8 Kg/cm.sup.2______________________________________ 
    
     This tubing was then expanded in a Raychem tubing expander of the type described and claimed in British Pat. No. 990,235 at a temperature of 140° C. The tubing obtained had an internal diameter of 1.10 inches. On heating, the tubing immediately recovered to its original internal diameter. 
     The tubing and sheds described above were successfully utilized to form a support insulator of the type described in British Pat. No. 1,292,276. 
     An epoxy polyamide/bisphenol A resin system was used as adhesive.