Abstract:
The present invention relates to a fire-resistance composition in particular as a material for a power and/or telecommunications cable. The invention is remarkable in that the composition comprises a polymer together with aluminum oxide in the form of particles having a mean diameter of less than one micrometer.

Description:
RELATED APPLICATION  
       [0001]     This application is related to and claims the benefit of priority from French Patent Application No. 05 52461, filed on Aug. 8, 2005, the entirety of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a composition for a material capable of withstanding extreme temperature conditions.  
         [0003]     A particularly advantageous but non-exclusive application of the invention lies in the field of power and/or telecommunications cables that are to remain operational for a defined length of time when subjected to high temperatures and/or directly to flames.  
       BACKGROUND OF THE INVENTION  
       [0004]     At present, one of the major concerns in the cable industry is improving the behavior and the performance of cables under extreme temperature conditions, particularly those encountered during a fire. Essentially for safety reasons it is necessary to maximize the ability of a cable both to retard flame propagation and also to resist fire. Any significant slowing down of flame propagation leads to a corresponding increase in time for evacuating premises and/or for using suitable fire-extinguisher means. Better resistance to fire makes it possible for a cable to operate for longer since it is damaged more slowly.  
         [0005]     Regardless of whether a cable is electrical or optical, for transporting energy or for transmitting data, it can be said, in outline, to be constituted by at least one conductor element extending inside at least one insulator element. It should be observed that at least one of the insulator elements may also act as protection means and/or that the cable may also have at least one specific protection element constituting a sheath. Unfortunately, it is known that amongst the best insulating and/or protection materials used in cable making, many are also materials that are highly flammable. This applies in particular to polyolefins and copolymers thereof, such as, for example: polyethylene, polypropylene, copolymers of ethylene and vinyl acetate, and copolymers of ethylene and propylene. In any event, in practice, this excessive flammability is completely incompatible with requirements to withstand fire as mentioned above.  
         [0006]     In the field of cable-making, there exist numerous methods of improving the fire behavior of polymers employed as insulating and/or sheathing materials.  
         [0007]     The solution that has been most widely used until now consists in employing halogen compounds in the form of a halogenated by-product dispersed in a polymer matrix, or directly in the form of a halogenated polymer as with polyvinylchloride (PVC) for example. However present regulations are tending to ban future use of substances of that type, essentially because of their potential toxicity and corrosiveness, whether at the time the material is fabricated, or in the event of it being decomposed by fire. This is particularly true when the decomposition in question can occur accidentally during a fire, or also deliberately during incineration. In any event, the recycling of halogenated materials continues to be particularly problematic.  
         [0008]     That is why recourse is being had more and more to fire-retardant fillers that are not halogenated, and in particular to metal hydroxides such as aluminum hydroxide or magnesium hydroxide. Nevertheless, that type of technical solution presents the drawback of requiring large quantities of filler in order to achieve a satisfactory level of effectiveness, whether in terms of ability to retard flame propagation or in terms of resistance to fire. By way of example, the metal hydroxide content may typically reach 100 to 150 parts by weight per 100 parts by weight of polymer resin.  
         [0009]     Any massive incorporation of filler leads to a considerable increase in the viscosity of the composite material. This leads inevitably to a significant reduction in extrusion speed, and consequently to a significant drop in productivity. In the end, that has a negative impact on the cost of the composite material, which is already badly encumbered by the cost price of the non-halogenated fire-retardant filler which is intrinsically high, particularly since the filler needs to be used in large quantity.  
         [0010]     However, independently of this purely economic aspect, the fire-withstanding performance of materials having non-halogenated fire-retardant fillers continue at present to be still insufficient for satisfying all of the conditions of fire tests.  
         [0011]     Phyllosilicates are also known for being usable as non-halogenated fire-retardant fillers. Those inorganic compounds are remarkable in that they are capable of forming nanocomposites with the polymer matrices in which they are dispersed.  
         [0012]     Nevertheless, that type of solution presents the drawback of being particularly expensive, essentially because of the cost of the unavoidable prior treatment that needs to be applied to each phyllosilicate in order to give it a characteristic that is sufficiently organophilic. Such composite materials also present mediocre electrical properties, viscosity that is penalizing for extrusion speeds, and an ability to withstand fire that is in any event always insufficient.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0013]     Thus, the technical problem to be solved by the subject matter of the present invention is to propose a fire-resistant composition, in particular as a material for a power and/or a telecommunications cable, which composition makes it possible to avoid the problems of the prior art, while being inexpensive, and while providing significantly improved properties in terms of withstanding fire.  
         [0014]     According to the present invention, the solution to the technical problem posed consists in that the composition comprises a polymer and aluminum oxide in the form of particles having a mean diameter that is less than one micrometer (μm).  
         [0015]     The term aluminum oxide is used to mean non-hydrated alumina having the formula Al 2 O 3 .  
         [0016]     In other words, the composition of the invention comprises a polymer matrix in which sub-micron alumina is dispersed to act as a fire-retardant filler.  
         [0017]     It should be observed that the term “fire-resistant composition” is used herein very broadly to cover any composition that is for constituting a material capable of slowing down fire propagation and/or of resisting fire.  
         [0018]     In any event, the mean size of the aluminum oxide particles constitutes the essential parameter of the invention in that the ability of the polymer material to withstand fire is directly associated with the grain size of the fire-retardant filler. A particularly pronounced fire-retardant effect is observed once the alumina used presents grain size that is very fine, and in particular when the particles making it up present a mean diameter that is less than one micrometer. It should also be observed that the smaller the size of the alumina oxide particles, the more the fire-retardant effect is remarkable.  
         [0019]     The invention as defined in this way presents the advantage of being capable of providing a polymer material that benefits from improved ability to withstand fire and good mechanical properties, compared with corresponding prior art materials. Such a material is well suited for use in making sheaths for power and/or telecommunications cables. This applies equally well to an insulating covering and to a protective sheath or a layer of cable-filler or “padding” material.  
         [0020]     In a presently preferred embodiment of the invention, the aluminum oxide is constituted by particles presenting a mean diameter that is less than 20 nanometers (nm).  
         [0021]     In particularly advantageous manner, the composition comprises 1% to 80% by weight aluminum oxide, and preferably 2% to 20%.  
         [0022]     According to a feature of the invention, the polymer is selected from a polyethylene, a polypropylene, a copolymer of ethylene and propylene (EPR), an ethylene-propylene-diene terpolymer (EPDM), a copolymer of ethylene and vinyl acetate (EVA), a copolymer of ethylene and methyl acrylate (EMA), a copolymer of ethylene and ethyl acrylate (EEA), a copolymer of ethylene and butyl acrylate (EBA), a copolymer of ethylene and octene, an ethylene-based polymer, a polypropylene-based polymer, an imide polyether, a thermoplastic polyurethane, a polyester, a polyamide, a halogenated polymer, or any mixture thereof.  
         [0023]     According to another feature of the invention, the composition is also provided with at least one associated fire-retardant filler.  
         [0024]     In particularly advantageous manner, each associated fire-retardant filler is selected from compounds containing phosphorous such as organic or inorganic phosphates, compounds containing antimony such as antimony oxide, metallic hydroxides such as aluminum hydroxide and magnesium hydroxide, compounds based on boron such as borates, carbonates of alkaline metals in groups IA and IIA such as the carbonates of calcium, sodium, potassium, or magnesium, and the corresponding hydroxide carbonates, compounds based on tin such as stannates and hydrostannates, melamine and its derivatives such as melamine phosphates, formophenolic resins, phyllosilicates such as sepiolite, attapulgite, montmorilonite, illite, chlorite, kaolinite, micas, and talcs.  
         [0025]     Preferably, the composition includes 1% to 80% by weight of associated fire-retardant filler.  
         [0026]     According to another feature of the invention, the composition is also provided with at least one additive selected from the group comprising lubricants, plasticizers, temperature stabilizers, pigments, antioxidants, and ultraviolet stabilizers.  
         [0027]     The invention also provides any power and/or telecommunications cable having at least one insulating sheath made from a fire-resistant composition as described above. It should naturally be understood that each insulating sheath in question may also perform a protection and/or padding function.  
         [0028]     The invention also provides any power and/or telecommunications cable provided with at least one protective sheath made from a fire-resistant composition as described above. It should be observed at this point that each protective sheath may also perform an insulating and/or padding function.  
         [0029]     Finally, the invention provides any power and/or telecommunications cable provided with at least one padding layer made from a fire-resistant composition as described above. It should be observed that each layer of padding material may also perform an insulating and/or protective function.  
         [0030]     It is important to specify that although such cables are for conveying power and/or transmitting data, they could equally well be electrical and/or optical, depending on whether the conductor elements with which they are provided are of the electrical and/or optical type.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     Other characteristics and advantages of the present invention appear from the following comparative example, said example being given by way of non-limiting illustration.  
       COMPARATIVE EXAMPLE  
       [0032]     Five samples of material were prepared using five different compositions in order to compare their respective performances in terms of withstanding fire. It is specified that the compositions in question were all suitable for being used for making insulating and/or sheathing and/or padding materials for energy and/or telecommunications cables.  
         [0033]     In any event, the polymer was common to all five samples. Specifically it was a copolymer of ethylene and vinyl acetate (EVA). Only the nature and the composition of the mixture of fire-retardant fillers varied from one sample to another. Table 1 gives the differences.  
                                                                           TABLE 1                                       Sample number                1   2   3   4   5                        EVA 28   40%   40%   40%   40%   40%       Magnesium hydroxide   60%   50%   50%   50%   50%       Treated montmorillonite   —   10%   —    5%    5%       Aluminum oxide d50 = 13 nm   —   —   10%    5%   —       Aluminum oxide d50 = 0.5 μm   —   —   —   —    5%                  
 
 Procedure 
 
         [0034]     The compositions were prepared by mixing each fire-retardant filler with an identical quantity of polymer on each occasion, in order to avoid falsifying subsequent comparative analyses; the filler content of the resulting composite remained constant.  
         [0035]     Whatever the precise nature of the composition prepared, the steps of mixing the polymer matrix with the fire-retardant filler were always the same: 
        temperature setpoint of 160° C. throughout the entire duration of mixing;     introducing the polymer into the internal mixer set to rotate at 30 revolutions per minute (rpm);     melting the synthetic polymer at 160° C. for 2 minutes (min) at 30 rpm;     melting at 60 rpm for 2 min;     introducing filler at 30 rpm; and     mixing at 30 rpm for about 10 min. 
 
 Preparing Samples 
       
 
         [0042]     Reference sample 1 was prepared specifically by mixing 100 grams (g) of ethylene and vinyl acetate (EVA) copolymer containing 28% vinyl acetate, a product sold under the trademark Evatane 28-03 by the supplier Arkema, with 150 g of magnesium hydroxide sold under the name Magnifin H10 by the supplier Albemarle. That operation was naturally performed in application of the above-described procedure. Sample 1 is illustrative of a conventional first system providing good ability to withstand fire.  
         [0043]     The same applied for preparing reference sample 2, which specifically comprised a mixture of 100 g of ethylene and vinyl acetate (EVA) copolymer containing 28% vinyl acetate, 125 g of Magnifin H10 magnesium hydroxide, and 25 g of montmorillonite treated with an ammonium alkyl as sold under the name Nanofil by the supplier Sud Chemie. Sample 2 relates to a second system that is well known in the prior art, and that is described in particular in patent document EP 1 033 724.  
         [0044]     Sample 3 comprised a mixture of 100 g of ethylene and vinyl acetate (EVA) copolymer containing 28% vinyl acetate, 125 g of Magnifin H10 magnesium hydroxide, and 25 g of aluminum oxide having a mean diameter d50=13 nm, as sold under the name Aeroxide Alu C by the supplier Degussa. Sample 3 served to evaluate the fire-withstanding performance of a material containing a conventional fire retardant, magnesium hydroxide, and aluminum oxide constituted by particles of very small size.  
         [0045]     Samples 4 and 5 both comprised a mixture of 100 g of ethylene and vinyl acetate (EVA) copolymer containing 28% vinyl acetate, 125 g of Magnifin H10 magnesium hydroxide, and 12.5 g of montmorillonite treated with an ammonium alkyl, and respectively 12.5 g of aluminum oxide having a mean diameter of d50=13 nm, and 12.5 g of aluminum oxide having a mean diameter of d50=0.5 μm, sold under the name Nabalox NO713-10 by the supplier Nabaltec.  
         [0000]     Withstanding Fire  
         [0046]     Fire behavior was evaluated on each occasion using the “épiradiateur” test as specified in French standard NF-P-92-505. To do this the corresponding material needs to be shaped into square plates having a side of 7 centimeters (cm) and a thickness of 3 millimeters (mm). That operation was performed using a hot hydraulic press, in application of the following procedure: 
        melting at 150° C. for 3 min;     applying pressure of 150 bar for 2 min, still at 150° C.; and     cooling in water at 150 bar for 5 min.        
 
         [0050]     Table 2 summarizes fire performance as determined using the “épiradiateur”. Each test had a duration of 5 min during which the time to flaming was evaluated, which time must be as long as possible, and the mean time to self-combustion was also evaluated, which time should be as short as possible.  
                       TABLE 2                               Mean time to       Sample number   Flaming time (s)   self-combustion (s)                   1   110   8.9       2   120   7.7       3   131   8.2       4   161   6.2       5   136   7.3                  
 
         [0051]     It can be seen firstly that reference sample 2 provides better performance than reference sample 1. The flaming time is longer by 10 seconds and the self-combustion time is shorter by more than one second.  
         [0052]     Sample 3 may be compared to sample 2 since they both contain the same conventional fire-retardant filler at identical concentrations, associated with another filler for improving performance in terms of withstanding fire. It can be seen that the time to flaming for sample 3 is longer by more than 11 seconds compared with sample 2. The use of sub-micron aluminum oxide thus achieves a considerable improvement in time to flaming without significantly affecting the self-combustion time.  
         [0053]     The association of sub-micron aluminum oxide with treated montmorillonite and with magnesium hydroxide enables even better performance to be achieved. Samples 4 and 5 show that the time of flaming can be lengthened by 5 seconds to by as many as 30 seconds, while also significantly shortening the self-combustion time, compared with samples 2 and 3.