Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
1999-09-02
2001-09-18
Copenheaver, Blaine (Department: 1773)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S372200, C427S398100
Reexamination Certificate
active
06291024
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating for a metal surface and to its application to tubes and to cables; it relates more particularly to a coating including successively, starting at the metal, a layer of polyurethane and a layer of thermoplastic polymer, it being possible for an adhesion binder to be placed between the polyurethane and the thermoplastic polymer.
The objective of the invention is a coating:
which exhibits good adhesiveness, which can be reflected, for example, in the peel strength,
which exhibits good shear strength (that is to say resistance to axial forces),
which exhibits good flexibility and good elasticity, and
which resists corrosion.
The present invention is useful, for example, for coating the external surface of pipes and for coating electrical or telephone cables or metal cables such as stay wires.
BACKGROUND OF THE INVENTION
DE 3 422 920 describes coatings for steel pipes, including successively a layer of epoxy resin, a layer of grafted polypropylene and finally an external layer of a mixture of polypropylene and of a polypropylene/polyethylene block copolymer. The glass transition temperature (Tg) of the epoxy resin is between 80 and 94° C. These coatings are suitable for hot water at 90° C.
Re 30 006 describes coatings for steel pipes, including successively an epoxy resin and a polyethylene modified by grafting or copolymerization with maleic anhydride.
Epoxy resins are not flexible enough and are not perfect for protection against moisture.
EP 185 058 describes telecommunication cables coated with polyurethanes for protection against moisture, but these cables do not have a thermoplastic coating.
DESCRIPTION OF THE INVENTION
By virtue of this shear strength, the coatings of the invention exhibit a very high cohesion; thus, in the case of an electrical cable which must withstand the forces of its own weight or support the weight of connection boxes or other instruments, it is necessary that the forces transmitted through the thermoplastic polymer layer should be capable of being distributed throughout the cable, including within the metal core. It is also useful that, when two electrical cables are connected, the connection box should be able to bear on the external coatings of the two cables to be connected without this resulting in a loss of cohesion within the cable and that the forces such as the tensile forces on the cables can thus be transmitted. It is then possible to make the connection by stripping the cable bare as little as possible. Electrical cables must also withstand being wound; this is why it is important that the coating according to the present invention should exhibit good flexibility.
The same applies in the case of metal cables which can transmit forces via their coating.
The invention is useful for the individually protected strands of civil engineering works which are suspended.
Individually protected strands are known which comprise a number of twisted steel wires surrounded by an external sheath made of flexible plastic, the twisted steel wires leaving interstitial spaces within this sheath which are filled with a protecting material.
These individually protected strands are usually employed for producing bridge stay wires and have been found particularly effective for protecting these stay wires against corrosion.
The protecting material employed in these individually protected strands of the prior art generally consists of wax or of grease, with the result that these individually protected strands cannot efficiently transmit high axial forces from their external sheath towards their twisted steel wires.
This is the reason why such individually protected strands cannot be employed for producing the cables which carry suspension bridges, suspended roofs or other suspended structures, because such carrier cables must take up, by friction, forces which are directed parallel to their axis, forces that are transmitted by cable clamps from which a civil engineering structure is suspended by means of suspenders.
Carrier cables made up of bundles of wires or of bare steel strands are therefore resorted to in suspension bridges or suspended roofs. These carrier cables are surrounded by an external protecting layer which may consist of paint, of bitumen or a tubular sheath, but this protecting layer is interrupted at the clamps which are tightened directly onto the steel.
This configuration has the following serious disadvantages:
the clamps must be tightened very firmly onto the carrier cables, on the one hand on account of the mediocrity of the steel-on-steel friction coefficient and, on the other hand, to limit the relative movements between the steel wires, which give rise to wear and fatigue due to fretting corrosion (also called “fatigue induced by small motions” or “wear induced by small motions”): this intense tightening requires clamps which are very long (for example up to 2 metres) and massive, tightened using many bolts,
fatigue phenomena due to fretting corrosion are never completely avoided, and this in turn results in loosening of the clamps and rupture of the wires constituting the carrier cable, and
chemical corrosion phenomena are extremely frequent.
Prior art FR 2739113 has proposed to coat the strand and no fill the spaces between the metal wires with polybutadiene and then to coat the strand with a polyethylene sheath, a grafted polyethylene being placed to reinforce the bonding between the polybutadiene and the polyethylene sheath.
By virtue of this arrangement the axial forces are transmitted efficiently from the external sheath of the strand as far as its twisted steel wires, both by surface adhesiveness and shape adhesiveness of the polybutadiene to the external sheath and to the twisted steel wires, and by the shear strength of the polybutadiene.
In addition, when such individually protected strands are employed to form the carrier cables of a bridge or other suspended structure, it is no longer necessary to tighten the suspended clamps as forcibly as in the prior art, since the sheaths of the individually protected strands exhibit a good friction coefficient.
In addition, fatigue phenomena due to fretting corrosion are avoided since there is no longer any direct contact between the steel wires of one strand and another.
Finally, a carrier cable made up of strands according to the invention withstands chemical corrosion perfectly.
However, polybutadiene must be vulcanized in order to exhibit good aging resistance and to retain an elastomeric character, to avoid the entry of water into the strands while continuing to ensure mechanical bonding with the polyethylene sheath.
Once vulcanized, polybutadiene is no longer thermoplastic; it must therefore be vulcanized after the strand has been coated, and this is very complicated. The coating of the present invention is much simpler; the polyurethane is formed during the coating of the metal surface, adheres better to the metal surface, is completely hydrophobic and completely fills all the space between the bundles of wires or of bare steel strands. The present invention also relates to the device including these bundles of wires or coated steel strands forming a cable and surrounded by a metal clamp consisting of two substantially hemicylindrical shells which are tightened around the cable by means of bolts; the said clamp is provided with at least one hook.
The present invention is therefore a coated metal surface including successively, starting at the metal: at least one layer of polyurethane and at least one layer of thermoplastic polymer, it being possible for a binder to be placed between the polyurethane and the thermoplastic polymer.
The metal surface may be, for example, an electrical or telephone cable, the external surface of a pipe or a stay wire. Where cables or stay wires are concerned, the metal part may be circular in section or may be a group of cables of circular section, as is common in electrical cables or stay wires.
A stay wire is intended to mean metal cables employed for their tensile strength and generall
Bonnet Evelyne
Deroch Maxime
Flat Jean-Jacques
Copenheaver Blaine
Elf Atochem S.A.
Paulraj Christopher
Smith Gambrell & Russell
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