PROCEDURE TO REPAIR METAL TUBE PLASTIC COATINGS
DESCRIPTIVE MEMORY
The present invention relates to a method for repairing plastic coatings of metal tubes, which allows repairs to be made in a very short time. Metal pipes, usually made of steel, coated with plastic material, are widely used in building pipes that transport fluid materials, such as oil, natural gas or water, even at a great distance. Said pipes must be able to withstand the most diverse environmental conditions and, in certain cases, the high temperatures that remain in the proximity of the pumping stations, for example, or that may be typical of certain fluids extracted from the subsoil. To avoid weak points along the line it is necessary, therefore, that damaged or removed segments of the plastic coating (where the tubes are welded together, for example) be repaired in such a way that the mechanical properties are so homogeneous as possible with those of the original coating. In response to said needs, there are materials and methods known in the art that allow achieving said objective with satisfactory results. However, when the damaged portion of the coating is quite extensive (for example, when the coating has been removed to weld the tubes), complicated repair techniques that take a long time, and sophisticated and costly appliances, such as induction furnaces, are required. . Currently, a particularly fast and efficient repair method has been perfected, which allows limiting the use of the induction furnace to the heating phase that precedes the application of the sizing, and to carry out the other heating phases using a flame protected by inert gas (such as nitrogen, for example). Said process is particularly suitable for repairing polyolefin coatings, preferably polypropylene and / or polyethylene coatings. Therefore, the object of the present invention is a method for repairing coatings of metal tubes made of plastic materials (in particular polyolefin, preferably polypropylene and / or polyethylene coatings), which comprises the following phases of operation:
1) sanding (optionally preceded by removal of greasy and fouling substances) from the exposed portion of the metal tube (i.e., that which is uncoated), 2) application of shells made of a rigid material (such as steel, for example) ) on the edges of the existing coating around the exposed portion, 3) heating of the exposed portion by induction (in particular with induction furnaces) at a temperature ranging from 140 to 270 ° C, preferably from 170 to 240 ° C, 4) application, on the exposed portion, of an epoxy sizing in powder form, and a polyolefin adhesive in powder form, 5) removal of the rigid shields and application of the coating, in the form of a polyolefin tape, by wrapping and heating with a flame protected with inert gas. Preferably, the size of the cuffs used in phase 2) is such that they overlap the existing coating by approximately 10 cm from the edges. Said armor has the purpose of preventing the coating from splitting and rising during the heating of phase 3). In step 4), the powder epoxy sizing and the polyolefin adhesive in powder form are preferably applied using injectors, which are fed from a fluidized bed by a pump that sucks the powders using the Venturi effect. Said injectors can be equipped with a system for electrostatic charge of the powders. Several alternative solutions are possible for phase 5). In particular, step 5) may comprise the following steps: 5i) removal of the rigid shields, 5ii) heating of the exposed portion and the surrounding part of the existing coating by a flame protected with inert gas, 5iii) application of a polyolefin adhesive in powder form on the existing coating by flame spray, 5iv) application, by wrapping , of a polyolefin tape (in particular a polypropylene and / or polyethylene tape) preferably having a thickness of 800 microns, on the exposed portion and on the surrounding area of the existing coating, preferably overlapping the edges of the tape being wrapped in 50 to 70% of its width, 5v) heating of the entire envelope with the aforementioned flame and, optionally, 5vi) cooling of said envelope, for example, with atomized water. In step 5iv), the area of the existing coating, surrounding the exposed portion, which is covered with the tape, preferably consists of at least 1.5 to 2 turns of said tape on both sides of the tube surrounding said exposed portion. Alternatively, phase 5) may comprise the following steps: 5.1) optional application, by flame spraying, of an additional layer of polyolefin adhesive in the form of powder (in case one wishes to increase the thickness of the coating), 5.2 ) application, by wrapping, of a polyolefin tape (in particular a polypropylene and / or polyethylene tape) on the exposed portion and on the cuirasses made of rigid material, preferably with an overlap of approximately 50% of the width of the tape, both on the edges of the tape being wrapped, as well as where said tape is wrapped around the cuirasses, 5.3) removal of said cuirasses, as well as the portions of the tape that overlap the cuirasses, 5.4) heating, by means of a flame protected with inert gas, from the entire envelope, as well as from the area around the existing coating and the newly applied coating, and application of an adhesive of polyolefin in the form of powder over the last area by flame spraying. The purpose of step 5.4) is to form a bond between the existing coating and the newly applied coating. As a further alternative, step 5) may comprise the following steps: 5a) removal of the rigid shell, 5b) application, by flame spraying, of an additional layer of polyolefin adhesive in powder form, preferably having a thickness of about 500 to 600 microns, 5c) application of a polyolefin tape (in particular a polypropylene and / or polyethylene tape) without overlapping it on the existing coating, said tape having been preheated (preferably at 80 to 90 ° C), being so wide as the whole exposed portion, and preferably having a thickness of 1.2 to 1.3 mm, 5d) heating the area where the existing coating meets the newly applied coating, by means of a flame protected with inert gas, and application of a polyolefin adhesive in powder form to said area by spraying with flame. Step 5c) requires a different grading method when applying the primer to the area to be coated without incorporating air bubbles. Step 5d) has the purpose of creating a link between the existing coating and the newly applied coating. In all the previous steps that require flame heating, excluding the particular case of flame spray, the temperature of said heating preferably ranges from 80 to 200 ° C. As mentioned above, the flame used is protected by an inert gas, such as nitrogen, for example. The purpose of such protection is to avoid or reduce the contact between the air and the polymer that is melted by the flame, thus preventing oxidation and degradation of the polymer, which causes a remarkable reduction in the life of the coating, especially when the temperature operating temperature of the coated tube exceeds 80 ° C. This protected flame can be obtained with injectors or sprinklers that have a central nozzle for sprinkling powders, and some concentric circles of nozzles. A combustible gas (such as methane, propane, butane, for example) is fed through one or more circles of nozzles, while the inert gas is fed through one or more concentric circles of nozzles to provide protection. Thus, a protected flame is obtained (that is, capable of preventing the degradation of the polymer with which it comes in contact) by igniting the combustible gas. These injectors or sprinklers are also used to apply the polyolefin adhesive in powder form by the flame spray method. In this case, the surface of the powder particles of the polyolefin adhesive is heated to a temperature ranging from 50 to 240 ° C by flame. Preferably, the adhesive in powder form is made of a polyolefin composition comprising by weight: A) from 30% to 94%, preferably from 50% to 85%, of polypropylene or polyethylene (HDPE, LDPE or LLDPE), or a crystalline random copolymer of propylene / ethylene or of propylene / ethylene / α-olefin of C4-C10;
B) from 0% to 70%, preferably from 5% to 40%, of an ethylene / propylene or ethylene / 1-butene elastomeric polymer; C) from 0.5% to 10%, preferably from 2% to 40%, of polypropylene or polyethylene (HDPE, LDPE or LLDPE) modified with polar groups (in particular, by partial or total grafting of maleic anhydride or isophoronebismalemic acid) in amounts which vary from 0.5 to 10%; D) from 0% to 0.5%, preferably from 0.1 to 0.5%, of a nucleating agent, such as dibenzylidensorbitol or talc, for example; E) from 0% to 10%, preferably from 6% to 10%, of titanium dioxide; said composition having a melt index L (ASTM D 1238 L) ranging from 3 to 150, preferably from 5 to 90 g / 10 minutes if the composition is predominantly based on polypropylene, or a melt index E (AST D 1238) E) which varies from 0.1 to 20, preferably from 0.5 to 15 g / 10 minutes if the composition is predominantly based on polyethylene, and being in powder form with a particle diameter not exceeding 600 microns, with the following distribution of particle size (percentages by weight): not more than 25%, preferably not more than 4%, of the powder having a diameter of the particles ranging from 300 to 450 microns; - not more than 10%, preferably not more than 0.6%, of the powder having a particle diameter greater than 450 micrometers. Preferably, a composition "predominantly based on polypropylene" is a composition wherein the component (A) is made of polypropylene or a random crystalline copolymer of propylene / ethylene or of propylene / ethylene / α-olefin of O, -Cio, while that a "predominantly polyethylene-based" composition is a composition wherein the component (A) is made of polyethylene. Examples of G4-C10 α-olefins optionally present in component (A) are: 1-butene; 1-hexene; 1-octene; and 4-methyl-1-pentene. When present, C -C10 α-olefins vary in amount from 2% to 10% by weight. Examples of the aforementioned polyolefin composition are described in published European patent application n. 603 906. The polyolefin tape mentioned above can be made essentially of a propylene or ethylene homopolymer, or a copolymer of propylene with ethylene and / or higher α-olefins (in particular C 4 -Ce), or a copolymer of ethylene with the above mentioned a-olefins, or a mixture of the aforementioned (co) polymers. Examples of C4-α-olefins -Cs are those given above for the polyolefin adhesive in powder form. Other components that are commonly used in the art, such as fillers or pigments, may be present in the composition of the polyolefin ribbon. In particular, the polyolefin tape can be made of a polyolefin composition comprising: a) from 10 to 60, preferably from 20 to 50 parts by weight of polypropylene homopolymer with an isotatic index greater than 90, particularly ranging from 95 to 98, or a crystalline copolymer of propylene with ethylene and / or an α-olefin of CH2 = CHR, wherein R is an alkyl radical with 2 to 6 carbon atoms, containing more than 85% by weight of propylene and having an index isotatico greater than 85%; b) from 10 to 40 parts by weight of an ethylene-containing polymer fraction, insoluble in xylene at room temperature; c) from 30 to 60, preferably from 30 to 50 parts by weight of an amorphous fraction of ethylene-propylene copolymer optionally containing minor proportions of a diene, soluble in xylene at room temperature, and containing from 40 to 70% by weight of ethylene, said polyolefin composition being entangled by grafting an alkoxysilane alkenyl derivative and by reaction with water, until heat set values are obtained in accordance with CEI 20-31 regulation of less than 175%, preferably less than 120 % at 200 ° C and less than 10% at 23 ° C. The polymer compositions comprising (a) +
(b) + (c) are described in published European patent application n. 400333. Normally, the total content of ethylene poly eated in the aforementioned compositions ranges from 20 to 60% by weight. In fraction (a), the α-olefin of CH2 = CHR, where
R is an alkyl radical with 2 to 6 carbon atoms, preferably selected from 1-butene, 1-pentene or 4-methyl-1-pentene. The diene in fraction (c) is preferably selected from butadiene, 1,4-hexadiene, 1,5-hexadiene and ethylidene-1-norbornene. The molecular weight of the different fractions (which is determined by measuring the intrinsic viscosity in tetrahydronaphthalin at 135 ° C) varies depending on the nature of the components and the total melt index of the composition. Preferably, it falls within the following limits: - from 0.5 to 3 dl / g for fraction (a); - from 2 to 8 dl / g for fraction (b) + (c). The electron microscopy examination of the compositions indicates that the dispersed phase is made of the amorphous ethylene-propylene copolymer, and its average particle size is less than 2 microns. Preferred examples of alkenyl derivatives of alkoxysilanes used to obtain the entanglement of the aforementioned compositions are vinyltriethoxysilane, allyltriethoxysilane and divinyldimethoxysilane. In general, the alkenyl derivatives of the alkoxysilanes are grafted to the composition by extrusion in the presence of peroxides. Examples of the aforementioned polyolefin composition are described in published European patent application n. 619 343. As an alternative to the aforementioned crosslinked polyolefin composition, appropriately interlaced polyethylene (HDPE, LDPE or LLDPE) could be used for the polyolefin ribbon using, for example, the aforementioned grafting technique of an alkoxysilane alkenyl derivative. and reaction with water. According to a preferred embodiment, the polyolefin tape comprises a layer made essentially of the aforementioned polyolefin materials and an additional layer co-extruded, for example, from a polyolefin adhesive. Naturally, during the application of the tape, the adhesive polyolefin layer is brought into contact with the surface to be coated. Preferred examples of the polyolefin adhesive composition that can be used for the additional layer of the aforementioned polyolefin tape tape are those described above for the polyolefin adhesive in powder form. By way of example, repair of the coating of a welded joint between two steel tubes, having a diameter of 508 mm and a thickness of 8 mm, from which the pre-existing polypropylene coating has been removed around the joint, can carried out using the method of the present invention comprising steps 5i) to 5vi), using the following operation times:
phase 1): 60 seconds phases 2 and 3): 60 seconds phase 4): 60 seconds steps 5i), 5ii) and 5iii): 40 seconds step 5iv): 60 seconds step 5v): 40 seconds step 5vi): 40 seconds
thus obtaining a coating, in the area being repaired, with a thickness of approximately 2.5 mm. This shows the speed of operation that can be obtained with the method of the present invention.