Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B7/00—Insulated conductors or cables characterised by their form
  • H01B7/17—Protection against damage caused by external factors, e.g. sheaths or armouring
  • H01B7/29—Protection against damage caused by extremes of temperature or by flame
  • H01B7/295—Protection against damage caused by extremes of temperature or by flame using material resistant to flame

Definitions

• This invention is in the field of fire protection materials. Specifically, it relates to fire barrier sheet materials useful in protecting electrical systems during a fire.
• Fire protection in areas with high concentrations of instrument, communication and power transmission wires and cables is very important, especially in such installations as large buildings and power plants.
• Such wires and cables are constructed with electrically insulating coverings which are combustible and can provide a pathway by which fire might spread. Fire protection for such wires and cables is critical when power supply is necessary for the maintenance controls, for example, in aircraft or factories.
• Intumescent sheets for fire protection are disclosed in U.S. Patent 4,273,879. These particular sheets have a significant content of organic materials (binder and char forming resin). During combustion, these organic materials can cause exothermic reactions increasing the temperature of the fire.
• organic materials binder and char forming resin
• Alumina trihydrate (Al 2 O 3 ⁇ 3 H 2 O) has 34.6% chemically combined water of hydration. This water is_ liberated beginning at about 230°C with complete dehydration at about 600°C. This endothermic dehydration is known to have a cooling effect in compositions of which alumina trihydrate is a component. The water vapor given off also dilutes the combustible gases which may be present during a fire to help arrest combustion.
• Alumina trihydrate (ATH) is a known fire retardant filler in the plastics industry.
• Inorganic materials in the form of fibre blankets have been used for the protection of conduits and cable trays.
• One such fibre blanket is prepared from aiumino-silicate glass fibres and marketed under the trademark Kaowool by the Babcock & Wilcox Company.
• Kaowool the trademark Kaowool
• the alumino-silicate fibre blankets are wrapped around cable trays in a thickness of usually two inches or more.
• This blanket insulation has low thermal conductivity and, therefore, not only insulates from the effects of fire but also retains in the conduit the heat generated by the current carried through electrical cables under normal conditions. With the dissipation of normal heat generated by the line resistance thus retarded, the cables in trays or conduit must be derated. That is, the amount of current which they are rated to carry (ampacityor amp capacity) must be decreased.
• DE-AS-2,530,388 discloses fire-resistant, electrically and thermally insulating mouldings composed of a combination of polyester resin, glass or asbestos fibres, and aluminium hydroxide.
• the publication discloses a composition comprising 100 parts by weight polyester resin, 5 to 200 parts by weight aluminium hydroxide, and 5 to 200 parts by weight asbestos fibre. Mouldings formed from this composition are stated to be useful as self-supporting insulating elements in electrical installations, particularly as cell partitions in switchgear. Glass fibre incorporated within the moulding is stated to provide mechanical support.
• the preent invention provides a sheet of an endothermic composition comprising:
• the sheets of this invention have a composition which minimizes the fuel source organic content while still maintaining the necessary strength and physcal integrity required of fire protection application methods.
• the low ratio of organic binder to inorganic material of this invention maximises utilisation of the endothermic and cooling vapour retention aspects of the inorganics (ie retention of the water vapour given off in the interstices of the inorganic fibre). This low ratio minimizes fuel contribution to any fire, and therefore minimizes smoke and harmful gases from combustion.
• the organic binder content is very low, relatively high sheet densities are maintained, eg 0.70­1.0 g/cc as compared to 0.1-0.3 g/cc for fibre blanket type systems.
• the more dense sheet is advantageous because it provides an increased thermal conductivity and therefore, better heat dissipation for cables in normal service.
• the sheet of this invention provides a more compact wrap to protect such items as cables, cable trays and conduits, of particular importance in areas of limited space such as airframe structures.
• the stability of the inorganic materials in the sheet composition will provide superior aging qualities over the fire barrier materials of the prior art which contain larger quantities of organic materials.
• Sheets in accordance with the invention do not rely on inorganic binders (such as clays, colloidal silica or cementitious materials) or on the insulative effect of a fibrous matrix. They rely on the porosity and mechanical entanglement of the fibers, and the materials remain flexible while the useful proportionality of endothermic powders to fiber can be quite high.
• inorganic binders such as clays, colloidal silica or cementitious materials
• the inorganic fiber is chosen from materials which can withstand very high temperatures without significant changes in physical properties, such as refractory alumino-silicate fibers.
• the sheets of this invention are preferably formed by standard paper-making techniques, either hand laid or machine laid on a Fourdrinier or cylinder type paper machine.
• the inorganic fibers used in the protective material of this invention are refractory materials which combine high strength, good thermal resistance and the ability to retain relatively high levels of high density endothermic filler.
• useful inorganic fibers include graphite, silica, alumina-silica, calcium oxide-silica, asbestos, and glass fibers.
• Alumino-silicate fibers are preferred and are available commercially under the trademarks Fiberfrax SK-2600 from the Carborundum Company, Cerafiber from Manville Corporation and Kaowool from Babcock and Wilcox.
• the fiber diameter is usually less than about 6 micrometers, preferably 3 micrometers.
• Fiberfrax a preferred inorganic fiber
• continuous use limit 1260°C melting point 1790°C
• normal packing density 96-192kg/m 3 normal packing density 96-192kg/m 3
• fiber lengths up to 102 mm specific gravity 2.73 and fiber strength of 2.76 x 10 9 N/m 2 .
• the amount of organic binder is preferably 2-6 weight percent of the total, more preferably about 5%.
• Suitable binders can include various polymers and elastomers in latex form, for example, natural rubber latex, styrene-butadiene latices, butadiene acrylonitrile latices, and latices of acrylate and methacrylate polymers and copolymers (e.g., polymethyl acrylate, polyethyl acrylate, and polymethyl methacrylate). It is preferred to use halogen-free polymers to avoid decomposition and release of noxious and corrosive halogen gases during a fire. Acrylic polymers are preferred because of their excellent heat resistance, aging properties, and noncorrosive combustion products.
• the inorganic, endothermic filler raw material is preferably a powder, having a mean particle size less than about 60 micrometers, even more preferably about 8 micrometers. Larger filler particles tend to separate the inorganic fibers during processing, resulting in a sheet of lower tensile strength.
• the weight ratio of endothermic filler to inorganic fibers is in the range of about 1.5 to 3.0.
• Typical fillers would be hydrated metal oxides and borates.
• the filler should be relatively insoluble in water, chemically inert, and should not require a synergist.
• Alumina trihydrate, magnesium hyroxide (hydrated magnesia), and zinc borate possess these properties. Alumina trihydrate is preferred.
• the preferred particle size for the filler is about 8 micrometers. As particle size decreases below that, the dewatering of the slurry in the manufacturing process can be adversely affected. The use of larger particles (greater than 60 micrometers) can reduce-the tensile strength of the sheet.
• the flexible, fibrous, endothermic sheets of this invention are made by mixing the ingredients together with water to form a slurry.
• the latex is coagulated, and the resulting floc suspension flows to a head box and from there onto the Fourdrinier wire screen.
• the dewatered floc will drain readily and knit together to yield a homogeneous mass in which the inorganic fibers are mechanically interlocked and bound together by the polymer binder, and the endothermic filler occupies the interstices between the fibers. Larger flocs are preferred for thicker sheets, to give good drainage necessary in the process.
• the green sheet from the Fourdrinier machine is densified by calendering and dried by passing through heated drying rolls.
• One alternative embodiment of the invention involves adding a backing to the sheet material already described.
• a suitable backing material is aluminum foil having a thickness of about 0.08 mm and a pressure-sensitive adhesive coated on one side. The backing is adhered to the protective sheet by means of the adhesive. Such backing can give an added degree of strength to sheet material which must be bent around sharp corners or small radii.
• the fibrous sheets of this invention may be held in position around conduits and cable trays by being wrapped with ceramic fiber cord, wire cloth or other high temperature resistant material. It is desirable to have the wrapping restrain the sheet, holding it tightly around the cables being protected particularly when exposed to open fire.
• a suitable ceramic fiber cord can be made from the fibers decribed in U.S. Patents 3,709,705,3,795,524 and 4,047,965.
• One commercially available suitable cord is sold under the trademark Nextel brand ceramic fiber cord by Minnesota Mining and Manufacturing Company.
• ASTM test D3286-73 measures the gross calorific value of a fuel in an isothermal-jacket bomb calorimeter. The purpose of these tests is to determine how much the fire barrier material might actually contribute as. fuel to a fire.
• ASTM test E119-78 is a standard fire test for building and construction materials.
• a dilute (about 25 weight percent) aqueous alum solution was then added to the latex-fiber-ATH slurry while it was being mixed, in sufficient amount to reduce the pH to between about 4.7 and 5.1 (preferably about 4.9) which coagulates the latex.
• the latex-fiber-ATH slurry was then pumped at controlled rates to a mixing tank where a flocculant or polyelectrolyte was added in dilute solution at about 50 ml per minute for particle retention and to aid in drainage.
• Suitable polyelectrolytes would be Bufloc 170 from Beckamn Laboratory Inc., used in 0.2 percent solution (50 ml of polyelectrolyte solution per 50 gallons or 189 liters of slurry) or Lufax 295 cationic polyelectrolyte from Rohm & Haas Company.
• the sheet after being dewatered to about 50 weight percent water on the Fourdrinier machine, was calendered at about 0.06 N force to further dewater (less than about 30 percent water) and densify the sheet.
• the wet sheet is dried by passing it through heated drying rolls to reduce the moisture content to less than about three percent. It was then wound on to rolls.
• process parameters the speed at which the sheet is transported through the paper-making machinery and the force used on the calender
• the final compositions of samples 1-10 are shown in Table 2.
• the comparative test data in Table 4 show the calorific value of the control materials to be substantially greater than the composition of this invention in all but one case. These higher values result from the greater organic content. It is preferred to select compositions of the present invention having a calorific vlaue of no more than about 2400 Joules/g, more preferably less than about 2000 Joules/g.
• a metal foil (0.08 mm thick) is laminated to one side of each such layer to provide a flexible yet tough sheet that can be conveniently taped with metal foil tape at abutting sheet edges of the wrap covering.
• a final wrap of high temperature ceramic fiber cords (alumina borosilicate fiber such as Nextel 4/5 cord) or stainless steel wires may be placed around the fire protective sheet, spaced at about 20 cm intervals or in a spiral fashion to ensure the maintainance of complete coverage and prevent the unwinding of the protective sheet, particularly under fire conditions.
• Fire tests in accordance with ASTM E119-78 were conducted on specimens of 2.5 cm diameter conduits and 10.2 by 30.5 cm cable trays wrapped by the above-described procedure.
• the quantity of sheet material used is measured by: (a) the number of layers, (b) the total thickness, and (c) by weight.
• the preferred method measures the weight of protective sheet material per linear distance in kilograms per meter covering the conduit or cable tray.
• the test specimens were heated inside furnaces fueled with natural or propane gas, and the heating rate of the furnace interiors were in conformance with ASTM E119 ⁇ 78.
• the furnace hot zone lengths for the conduit and cable tray tests were 61 cm and 245 cm respectively.
• 315°C is the temperature at which electrical cables are often observed to begin to deteriorate and short circuit. This comparison is shown in Table 5 below.
• the following flame test. was performed on the inventive material and on the controls in the form of putty or mastic.
• the test specimens were cut or formed to 23 by 28 cm rectangles and glued or formed to one side of a metal sheet 0.3 mm thick.
• the test specimen weight and thickness was recorded.
• a thermocouple was attached to the surface in the center of the metal sheet to measure the cold side surface temperature.
• the test sheet was placed vertically and centered in front of a flame source generated by a propane fueled exhaust gas simulator (from Maremont Company) which provided a hot side surface temperature of 1,050 to 1,100°C. The rate of temperature rise on the cold side surface was then recorded.
• Table 6 The results obtained are shown in Table 6 below.
• the inventive sheet sample maintained a much lower temperature for a significantly longer time than either of the control sample mastics.
• the fire barrier sheets used in the study included: the inventive sheet; commercially available intumescent mats comprising acrylic resin, alumina-silica fibers and unexpanded vermiculite (control sample C1); and Kaowool blankets available from Babcock and Wilcox Company:
• the lengths of intumescent mat were 457 mm for the first layer, 572 mm for the second; and 635 mm for the third.
• the lengths of the inventive mat were 394 mm for the first layer,445 mm for the second, and 500 mm for the third.
• the lengths of Kaowool blankets used were 610 mm for the first layer, and 749 mm for the second.
• the Kaowool blankets were applied by completely wrapping around the conduit and overlapping at least 76 mm.
• Scotch 33 tape a black electrical vinyl tape, was wrapped in a helical fashion around the outside of the test specimen to provide a black surface solely to raise the surface emissivity in some tests.
• the length of wrapped conduit under test was about 2.45 meters.
• the conduit tested was a 4 inch (10.2 cm) internal diameter electrical grade schedule 40 steel containing four 500 MCM 600 volt XLPE insulated copper conductor cables.
• the thicknesses of each fire barrier layer were: for the inventive sheets about 5 mm; for the intumescent mat about 5 mm; and for the Kaowool blanket about 25 mm.
• the conductor temperatures measured for bare steel at 190, 300 and 400 amps were 39.7°C, 61.2°C, and 85.1°C respectively and for steel conduit covered with black tape 37.1°C, 54.2°C, and 76.2°C at the same respective currents. Ambient temperature ranged between about 22 and 26°C for these tests. The results are shown in Table 7 below:
• black tape helps to decrease the percent ampacity derating.

Landscapes

• Insulated Conductors (AREA)
• Paper (AREA)
• Fireproofing Substances (AREA)
• Porous Artificial Stone Or Porous Ceramic Products (AREA)
• Nonwoven Fabrics (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Laying Of Electric Cables Or Lines Outside (AREA)

Applications Claiming Priority (2)

Publications (2)

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• 1984
  • 1984-05-24 CA CA000455035A patent/CA1219103A/en not_active Expired
  • 1984-06-04 EP EP19840303739 patent/EP0132936B1/en not_active Expired
  • 1984-06-04 DE DE8484303739T patent/DE3465659D1/de not_active Expired
  • 1984-06-19 JP JP59126311A patent/JPH0772400B2/ja not_active Expired - Lifetime
  • 1984-07-20 ES ES534486A patent/ES8505465A1/es not_active Expired

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