Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-01-21
2003-04-22
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C174S1100SR
Reexamination Certificate
active
06552112
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cables, in particular for low-voltage electrical energy distribution or for telecommunications, these cables having low-smoke self-extinguishing properties, and to the flame-retardant compositions used therein.
2. Description of the Related Art
Self-eixtinguishing cables can be produced having a flame-retardant coating made from a polymer composition to which fire-resistant properties have been given by adding a suitable additive. Polyolefin-based compositions based, for example, on polyethylene or ethylene/vinyl acetate copolymers, containing an organic halide combined with antimony trioxide as flame-retardant additive can, for example, be used for this purpose. However, halogenated flame-retardant additives have many drawbacks since they partially decompose during processing of the polymer, giving rise to halogenated gases that are toxic to workers and corrode metal parts of the polymer-processing equipment. In addition, when they are placed directly in a flame, their combustion gives rise to large amounts of fumes containing toxic gases. Similar drawbacks are encountered when polyvinylchloride (PVC) supplemented with antimony trioxide is used as base polymer.
Therefore, in recent years the production of self-extinguishing cables has been directed toward halogen-free compositions, using as flame-retardant filler inorganic oxides, preferably in hydrate or hydroxide form, in particular magnesium hydroxide or aluminium hydroxide.
Aluminium hydroxide starts to decompose at a relatively low temperature (about 190° C.), which can result in various drawbacks during extrusion of the polymer composition, with formation of bubbles and defects in the final product. Therefore, the use of aluminium hydroxide as flame retardant is generally limited to polymer materials which do not require high processing temperatures. In contrast, magnesium. hydroxide has a decomposition temperature of about 340° C. and is characterized by greater heat stability and a high decomposition enthalpy. These properties make magnesium hydroxide particularly suitable as flame retardant filler in polymer compositions for coating cables, which require high extrusion temperatures and a small number of morphological defects.
However, the use of magnesium hydroxide as a flame-retardant filler does have certain drawbacks. Firstly, in order to obtain an efficient flame-retardant effect, very large amounts of magnesium hydroxide must be added to the polymer material, generally about 120-250 parts by weight relative to 100 parts by weight of polymer material. Such high levels of filler lead to a reduction in processability and in mechanical and elastic properties of the resulting mixture, in particular as regards impact resistance, elongation and stress at break.
In the U.S. Pat. No. 4,145,404 these drawbacks are attributed to the low affinity of natural magnesium hydroxide, obtained for example by grinding minerals such as brucite, with the polymer material, in particular when the polymer is of low polarity, as in the case of polyolefins.
In the patent EP-780,425 it is pointed out that the presence of different metal impurities, such as iron or manganese salts, in magnesium hydroxide of natural origin causes degradation of the polymer matrix into which the magnesium hydroxide is inserted.
Therefore, research efforts have been directed towards modifying properties of magnesium hydroxide to improve its compatibility with the polymer matrix and its degree of purity. Various synthetic methods have thus been developed in which magnesium hydroxide is produced by adding alkalis to an aqueous solution of a soluble salt thereof and subsequent precipitation. of the hydroxide by heating at high pressure (see for example patent U.S. Pat. No. 4,098,762 or the above-mentioned patents EP-780,425 and U.S. Pat. No. 4,145,404). In this way, a magnesium hydroxide is obtained with a high degree of purity and high structural uniformity with formation of crystallites of flattened hexagonal shape with an average diameter not greater than 2 &mgr;m and a specific surface area, measured by BET method, not greater than 20 m
2
/g.
However, the use of synthetic magnesium hydroxide as flame-retardant filler has a considerable impact on the cost of the finished product, so as to make flame-retardant systems based on magnesium hydroxide non-competitive when compared with the halogen-containing flame-retardant compositions described above.
In certain cases attempts have been made to improve properties of natural magnesium hydroxide using suitable grinding and/or surface treatment processes.
For example, Japanese patent application JP-01-294792 (Kokai) describes a process for the production of magnesium hydroxide, in which natural brucite is wet-ground so as to obtain an average particle diameter of between 2 and 6 &mgr;m, and then surface-treated with a fatty acid ammonium salt, and eventually dried. The resulting magnesium hydroxide would be resistant to efflorescence phenomena caused by carbonation of magnesium hydroxide by atmospheric carbon dioxide. The process of wet-grinding is considered essential to make the particle size of the product more uniform without increasing its lattice distortion coefficient which is thought to be responsible for high resistance to carbonation of natural magnesium hydroxide. The surface treatment is thought to improve dispersibility of the filler in the polymer matrix. The magnesium hydroxide thus obtained is claimed to be useful as a flame-retardant for polyolefin resins. In particular, the examples describe compositions with flame-retardant properties based on ethylene/vinyl acetate (EVA) and ethylene/ethyl acrylate (EEA) copolymers.
Japanese patent application JP-03-231,944 (Kokai) describes polyolefin-based compositions having flame-retardant properties and containing magnesium hydroxide with an average particle diameter of between 3 and 13 &mgr;m and the following particle size distribution: 1-20% by weight of particles with a diameter less than or equal to 1 &mgr;m; 55-98% by weight of particles with a diameter between 1 and 15 &mgr;m; 1-25% by weight of particles with a diameter between 15 and 50 &mgr;m. This particle size distribution is believed to afford higher flame resistance, which would be accompanied by good mechanical strength, flexibility and processability. A magnesium hydroxide with these properties would be obtainable by suitable grinding of natural brucite, followed by sieving or addition of another material of predetermined particle size. According to the description given in the above-mentioned patent application, this type of magnesium hydroxide would be useful as a flame-retardant filler for polyolefins such as polyethylene, olefinic rubbers, polypropylene, polybutene and the like. Particular mention is made of ultra-low-density polyethylene (ULDPE) having a density of 0.860-0.910 g/cm
3
, obtainable by copolymerization of ethylene with an alpha-olefin in the presence of a conventional Ziegler-Natta catalyst based on titanium and/or vanadium compounds.
Lastly, Japanese patent application JP-05-17692 (Kokai) describes polymer compositions having flame-retardant properties and containing natural magnesium hydroxide which has previously been ground and surface-treated with a fatty acid or a fatty acid salt, or alternatively with a silane or a titanate acting as coupling agent. These compositions would be characterized by high resistance to acid attacks. The subsequent Japanese patent application JP-07-161230 (Kokai) describes compositions similar to the above, pointing out that, in order to decrease the hygroscopicity of magnesium hydroxide, the latter must be surface-treated with the same products as mentioned above, in amounts of between 0.5 and 5% by weight relative to the magnesium hydroxide weight. In both of the above-mentioned Japanese patent applications, polyolefins such as polyethylene, ethylene/propylene rubbers, acrylic rubbers and the like are cited as polymeric materials, and flame-retardant com
Albizzati Enrico
Castellani Luca
Grizante Redondo Eduardo
Peruzzotti Franco
Tirelli Diego
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi e Sistemi S.p.A.
Szekely Peter
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