WO1992003234A1

WO1992003234A1 - Epoxy/polyolefin coating process

Info

Publication number: WO1992003234A1
Application number: PCT/CA1991/000293
Authority: WO
Inventors: James John William Cox; Toni Alois Pfaff
Original assignee: Du Pont Canada Inc.; Valspar, Inc.
Priority date: 1990-08-20
Filing date: 1991-08-16
Publication date: 1992-03-05
Also published as: CA2089766C; CA2089766A1; GB9018236D0; GB9302474D0; GB2262709A; GB2262709B; BR9106771A

Abstract

A method of coating metallic pipe is disclosed. The method comprises heating the pipe to a temperature of at least 200 C and applying an epoxy resin composition having a thickness of at least 300 microns to the outer surface of the heated pipe. Before the epoxy resin composition has completely cured, but preferably after it has gelled, a coating of a modified polyolefin is applied in a thickness of up to 500 microns. The modified polyolefin is a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms that has been modified by grafting with ethylenically unsaturated organic carboxylic acid or anhydride. An outer coating of a polyolefin is then applied in a thickness of at least 300 microns, the polyolefin being a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms and preferably a homopolymer of ethylene or propylene, or a copolymer of ethylene and a C3?-C10? hydrocarbon alpha-olefin. The resultant coating is resistant to impact damage and cathodic disbondment. The coated pipe may be used in buried pipelines in the petroleum industry.

Description

EPOXY/POLYOLEFIN COATING PROCESS FIELD OF THE INVENTION

This invention relates to the coating of metal objects, and particularly metal pipes, with three-layer specialized epoxy/polyolefin coating compositions.

BACKGROUND OF THE INVENTION

Buried metal pipelines employed to convey liquids e.g. petroleum products including crude oil and petroleum gases, are susceptible to substantial corrosion due to the moist, often acidic or alkaline, environments of the soil in which they are buried. Methods have been devised to reduce pipeline corrosion, but none appear to have adequately solved this potentially very serious problem. Two different types of methods have been used to attempt to protect pipelines from corrosion, and have been used both separately and together. Cathodic protection from corrosion is provided by a procedure involving the application of an electric potential between the metal of the pipe and the material, e.g. earth, that surrounds the pipe. The second method involves placing one or more coatings upon the outer surface of the pipe so as to physically protect the pipe surface from the corrosive environments that may be encountered by buried pipelines.

Epoxy resin coatings have been used extensively for protection of pipe surfaces, and one such composition is shown in Warnken, U.S. Patent 4 009 224. Polyolefin coatings also have been used. Sakayori et al., U.S. Patent Re. 30 006, reissued 1979 May 22, describe the application of a coating of a polyolefin modified by reaction with an unsaturated dicarboxylic organic acid or anhydride to a metal surface bearing a thin, preferably 5-10 micron, primer coating of an uncured epoxy resin to improve adhesion of the polyolefin to the pipe. Japanese Kokai patent SHO 56[1981]-168862 of K. Iwaya et al, published 1981 December 25, discloses the application of a mixture of a modified polyolefin having a melting point above 125^βC and an unmodified polyolefin having a melting point below 125°C to a partially-gelled epoxy primer having a thickness of e.g. 30-40 microns upon a metal surface. Three layer-coated pipe is commercially available in which a thermosetting fusion bonded epoxy powder or thermosetting two-pack liquid epoxy has been applied in a thickness of 70± 20 microns, followed by a terpolymer of ethylene, acrylic ester and maleic anhydride in a thickness of 300-400 microns and then a low or medium density extrusion grade of polyethylene.

SUMMARY OF THE INVENTION

We have now found a process for the manufacture of a metal pipe having a composite pipe coating that exhibits good adhesion to the pipe, is resistant to cathodic disbondment and other modes of coating failure and resistant to physical damage of the type to which pipeline sections may be subjected during construction and installation of a buried pipeline.

In the process, metal pipe of the type used, for example, in the manufacture of petrochemical pipelines is heated to a temperature of at least about 200^βC. A powdered epoxy resin composition comprising an epoxy resin having a softening point of at least about 90°C, and a curing agent therefor, is applied to the hot outer surface of the pipe, the resin composition melting and coalescing upon the hot pipe surface to form a coating having a thickness of at least about 200 microns and desirably in the range of from about 300 to about 800 microns; in a preferred embodiment, the epoxy resin composition has a softening point of at least about 95°C. Before the epoxy resin coating has completely cured, but preferably after it has gelled, an epoxy resin-reactive polyolefin is applied to the hot outer surface of the epoxy resin; the epoxy-resin reactive polyolefin, which is referred to herein as modified polyolefin, is comprised of a polyolefin that has been grafted with an ethylenically unsaturated organic carboxylic acid or anhydride. Finally, an unmodified polyolefin is applied over the layer of modified polyolefin.

The heat capacity of the metal pipe and the temperature to which it is heated prior to application of the epoxy resin composition desirably are such as to provide enough energy to sequentially (a) melt the powdered epoxy resin and cause it to form a continuous coating upon the pipe surface, (b) melt and coalesce the subsequently applied modified polyolefin upon surface of the incompletely cured epoxy resin, and (c) then substantially cure the layer of epoxy resin; an outer layer of polyolefin is also applied. The three-layer composite coating adheres tenaciously to the metal pipe surface and provides the pipe with not only substantial resistance to cathodic disbondment but also with the ability to withstand substantial physical abuse and relatively high operating temperatures, especially when the polyolefin is polypropylene. Accordingly, the present invention provides a method of coating metallic pipe for use in buried pipelines to provide the pipe with resistance to impact damage and to cathodic disbondment, comprising:

(a) heating the pipe to a temperature of at least about 200°C;

(b) applying to the outer surface of the heated pipe a powdered epoxy resin composition comprising an epoxy resin and a curing agent therefor, preferably an epoxy resin composition having a softening point of at least about 90°C, said powdered epoxy resin composition melting and coalescing upon the pipe to form a molten coating having a thickness of at least about 200 microns;

(c) preferably after the epoxy resin composition has gelled upon the pipe surface but in any event before complete curing thereof, applying thereto a modified polyolefin, said modified polyolefin being a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms and which has been grafted with an ethylenically unsaturated organic carboxylic acid or anhydride, the modified polyolefin forming an adherent and protective coating on the epoxy coating and having a thickness in the range of up to 500 microns; and

(d) applying a molten layer of a polyolefin selected from the group consisting of homopolymers or copolymers of hydrocarbon alpha-olefins having 2-10 carbon atoms, especially a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C₃-C₁₀ hydrocarbon alpha-olefins, said layer of polyolefin having a thickness of at least about 300 microns.

The present invention further provides metal pipe suitable for buried pipeline use and bearing an outer composite coating resistant to impact damage and to cathodic disbondment, said coating comprising an inner layer of a cured epoxy resin composition having a thickness of at least about 200 microns, an intermediate coating of a modified polyolefin bonded to the inner layer and having a thickness of up to about 500 microns, and an outer layer in a thickness of at least about 300 microns of a polyolefin that is a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms, especially a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C--C₁₀ hydrocarbon alpha-olefins. The present invention also provides a method of coating metal pipe to be used in the construction of a buried pipeline, comprising

(a) providing a heating station through which said pipe may be axially moved and heated to a temperature of at least about 200"C;

(b) providing a first coating station in line with the heating station and capable of applying a powder coating to the exterior of the heated pipe; (c) providing a second coating station in line with the first coating station for applying a subsequent coating;

(d) providing a third coating station in line with the second coating station for applying a further coating;

(e) applying a powdered epoxy resin composition to the pipe as it passes through the first coating station, the resulting coating melting, coalescing and subsequently gelling upon the surface of the pipe to a thickness of at least about 200 microns;

(f) applying a modified polyolefin, preferably a powdered modified polyolefin, to the pipe section as it passes through the second coating station, the modified polyolefin forming a polyolefin layer having a thickness of up to about 500 microns, said modified polyolefin being a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms and which has been grafted with an ethylenically unsaturated organic carboxylic acid or anhydride; the axial speed of the pipe through said stations and the distance between the first and second coating stations being adjusted to cause the polyolefin to be applied in the second coating station preferably shortly after gelation of the coating of the epoxy resin composition applied in the first coating station but in any event before complete curing thereof; and

(g) at the third coating station, applying a molten layer in a thickness of at least about 300 microns of a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms, especially a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C₃-C₁₀ hydrocarbon alpha-olefins, to the pipe.

DESCRIPTION OF THE DRAWING:

In the embodiments shown in the drawings: Figure 1 is a perspective, schematic view of a portion of a pipeline coating apparatus; and Figure 2 is a broken-away schematic view in partial cross section of a powder coating station employed in the apparatus of Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS COATING MATERIALS

The powdered epoxy resin composition that is employed in the process and pipe of the present invention desirably employs an epoxy resin which is a polyglycidyl ether of a polyhydric phenol having a softening point (Durrans*) of at least about 90°C and preferably from about 90 to about 130^βC, and a curing agent for the epoxy resin. The preferred polyglycidyl ethers are those obtained from the condensation of bisphenol A (2,2'bis(hydroxyphenyl)propane) and epichlorohydrin. Other polyhydric phenols which provide high melting polyglycidyl ethers include the phenol and o-cresol novolaks. Polyglycidyl ethers of the type described are available commercially e.g. from Dow Chemical Canada Inc. under the trade designation DER 663U, from Ciba-Geigy Canada Ltd. under the trade designation GT 7074 and from Shell Canada Products Ltd. under the trade designation Epon^® 2002, or may be made by extending a lower molecular weight epoxy resin with, for example, bisphenol A. The epoxy resins employed in the present invention are high melting solids and are curable at temperatures in the range of from about 180 to about 250^βC. Any of the various known latent curing agents may be employed, and among these may be listed amines e.g. dimethylethanolamine and methylene dianiline, amides e.g. dicyandiamide, especially accelerated dicyandiamide, and phenolic resins.

The epoxy resin compositions used in the invention may include flow control agents e.g. silicones an example of which is Modaflow^® flow agent powder (Monsanto) , pigments e.g. titanium dioxide, iron oxide and carbon black, fillers e.g. talc, calcium carbonate and mica, and other materials for the same purposes and effect as they are used in epoxy coating powders of the prior art. One example is a general purpose pipe coating of Dow Chemical Company that contains DER 642U epoxy resin (32 parts) , DER 672 epoxy resin (32 parts), curing agent (24.5 parts) , barium sulphate (8 parts) , red iron oxide (2 parts) and Modaflow Powder II (1.5 parts). Another example is a pipe coating resin from Shell Chemical Company that contains Epon 2004 epoxy resin (78.2 parts), Epon Curing Agent P-104 (3.1 parts), Epon Resin 2002-FC-1O (5 parts, contains 10% by weight of Modaflow flow control agent), red iron oxide (1.5 parts), barium sulphate (11.7 parts) and Cab-O-Sil^® M-5 silica (0.5 parts) .

The powdered epoxy resin coating compositions may be manufactured by various methods known to the prior art; in a preferred embodiment, the coating compositions may be manufactured using a process in which the ingredients are melt blended, cooled and ground into a powder. Epoxy resin coating compositions of the type described are available commercially, examples of such resins being sold by Valspar, Inc. under the designations D-1003LD and D-1003EG. In general, the epoxy resin compositions of the invention, for reasons which will become apparent below, exhibit gel times of from about 2 to about 20 seconds (and preferably from about 5 to about 10 seconds) at pipe coating temperatures of about 250"C. As used herein, gel time is defined as the time required for an epoxy resin composition to gel i.e. to exhibit a sudden increase in melt viscosity at a predetermined temperature. Gel time is measured by placing the epoxy resin composition on a hot metal surface, especially a hot plate, that is at the predetermined temperature. Using a spatula or other suitable device, a portion of the epoxy resin composition is drawn over the heated surface, to provide a sample having a thickness of approximately 200 microns. A sharp edged object e.g. paper clip, is then moved through the now molten layer of epoxy resin composition until a rapid increase in the melt viscosity of the composition is observed. The time, expressed in seconds, between when the epoxy resin composition is placed in the hot surface and the rapid rise in melt viscosity is the gel time.

It will be understood that the curing phenomenon of epoxy resin compositions involves chemical linking between polymer chains and that this linking (or "cross-linking") mechanism is initiated almost immediately upon application (e.g. by spraying) of the powdered epoxy upon a hot pipe surface and continues as the epoxy resin composition melts, coalesces and gels. After gelation, curing continues for a period of time e.g. about 90 seconds, that depends for instance on the temperature of the pipe, following the application of the powdered resin to the hot metal surface. Curing desirably is essentially complete within about 60 to about 180 seconds following application of the epoxy resin to the hot surface. Degree of cure may be measured by differential scanning calorimetry (DSC) using the procedure of the Canadian Standards Association (CSA) for fusion bonded epoxy resins para. 12.2 (page 28). As an example, the aforementioned epoxy resin composition 1003LD exhibits curing times of 50 seconds at 243"C, 60 seconds at 235^βC and 70 seconds at 232°C.

The modified polyolefins employed in the invention are grafted homopolymers and copolymers of hydrocarbon alpha-olefins having 2-10 carbon atoms. For example, the polymers may be homopolymers or copolymers of ethylene, propylene, butene-1, 4-methyl pentene-1, hexene-1 and octene-1. The preferred polymers are homopolymers and copolymers of ethylene and propylene. The polymers of ethylene may include hompolymers of ethylene and copolymers of ethylene with, for example, butene-1, hexene-1 and/or octene-1. The polymers of propylene may include homopolymers of propylene and copolymers of propylene and ethylene, including so-called random and impact grades of polypropylene. As will be appreciated by persons skilled in the art, such polymers may have a broad range of molecular weights if the polymer is to be applied to the pipe as a powder coating but a more limited range if the polymer is to be applied by extrusion techniques; both procedures of applying the polyolefin are discussed herein. The polymer is modified by grafting with at least one alpha,beta-ethylenically unsaturated carboxylic acid or anhydride, including derivatives of such acids and anhydrides. Examples of the acids and anhydrides, which may be mono-, di- or polycarboxylic acids, are acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, itaconic anhydride, nadic anhydride, maleic anhydride and substituted maleic anhydride e.g. dimethyl maleic anhydride. Examples of the derivatives of the unsaturated acids are salts, amides, imides and esters e.g. mono- and disodium maleate, acrylamide, maleimide and dimethyl fumarate. Methods for the grafting of monomers onto polymers are known in the art, and include solution grafting processes and melt grafting processes. Examples of the latter are described in U.S. Patent 4 612 155 of C.S. Wong and R.A. Zelonka, which issued 1986 September 16, and in published European patent applications Nos. 0 370 735 and 0 370 736 of E.C. Kelusky, both published 1990 May 30. The methods for the grafting of monomers onto polymers involve reacting polymer and monomer, usually in the presence of a catalyst e.g. an organic peroxide. Examples of the latter include bis(tert. alkyl peroxy alkyl) benzene, dicumyl peroxide and acetylenic diperoxy compounds. The amount of grafted monomer is usually in the range of 0.01 to 5% by weight of the polymer; in preferred embodiments, the amount of grafted monomer is in the range of 0.1 to 2% by weight of polymer. The characteristics of the polymer subjected to the grafting reaction will depend on the characteristics required in the grafted polymer that is to be coated over the epoxy resin composition, it being understood that some polymers, especially polypropylene, tend to undergo scission reactions in the presence of organic peroxides i.e. in the grafting process. Additional polymers and/or stabilizing agents e.g. antioxidants for example phenolic antioxidants, UV stabilizers and heat stabilizers, pigments e.g. titanium dioxide and carbon black, extenders e.g. mica and glass, rust proofing agents, fillers e.g. talc, calcium carbonate and mica, slip agents and flame retardants or the like may be added to the polymer either subsequent to the grafting process but prior to extrusion or other recovery of the grafted polymer from the apparatus used in the grafting process or in subsequent steps. For example, ungrafted polymer that is identical to or different from the polymer that has been grafted may be added to the grafted polymer. Toughening agents e.g. elastomers and very low density polyethylenes e.g. with densities below about 0.910 g/cm³, may be added but such agents must be thoroughly dispersed in the polymer matrix. Other materials e.g. metal oxides or hydroxides, may also be added, especially in order to improve the adhesive characteristics of some grafted polymers, e.g. polypropylene. The polyolefin applied as the outer coating is a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms, especially a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C₃-C₁₀ hydrocarbon alpha-olefins ; the hydrocarbon alpha-olefins may be branched or unbranched alpha-olefins. For example, the polyolefin may be a homopolymer of ethylene of, in particular, low or medium density. Alternatively, the polyolefin may be copolymer of ethylene and a hydrocarbon alpha-olefin, especially butene-1, hexene-1 and/or octene-1 having a low or medium density. In other embodiments, the polymer may be a polymer of propylene e.g. a homopolymer of propylene or especially a copolymer of propylene and a minor amount of ethylene.

COATING PROCEDURE

A preferred coating procedure of the invention involves the use of two powder spray stations spaced along the path of travel of a preheated section of metal pipe, the first station being employed to spray the powdered epoxy resin composition upon the pipe surface and the second station being employed to spray the powdered, modified polyolefin upon the incompletely cured epoxy resin surface. Techniques and apparatus for spraying powdered coating materials upon hot pipe surfaces are known in the art, and are described, for example, in J. Protective Coatings and Linings, May 1988, Vol. 5, No. 5 p 26, by S.E. McConkey. In general, the powdered epoxy resin composition is entrained in an air stream and is directed by means of nozzles or jets against the hot pipe surface. A series of nozzles may be provided within a coating station through which lengths of preheated pipe are caused to pass. The nozzles may be positioned circumferentially of the pipe and spaced so as to provide a uniform coating of epoxy resin upon the pipe, or preferably are positioned on one side of the pipe with the coating composition being sprayed onto the pipe surface as the pipe, which is being rotated, passes axially through the coating station. A second, similar coating station is provided adjacent the first coating station, the second station being adapted to spray onto the epoxy-coated pipe the powdered, modified polyolefin employed in the instant invention. The coating of the powdered epoxy resin or the powdered, modified polyolefin or both may be aided through the use of electrostatic coating procedures in which the powdered material is provided with an electric charge. The coating stations as thus described may be partially enclosed within appropriate housings to avoid the escape of resin particles and/or contamination of the epoxy resin powder or modified polyolefin powder by the other.

While the method of the present invention is described herein with particular reference to the modified polyolefin being applied to the epoxy resin coating in the form of a powder, the modified polyolefin may also be applied in the form of molten polymer e.g. using extrusion coating techniques. For example, the modified polyolefin may be applied by use of an annular die through which the pipe is passed or by side extrusion in which molten polymer in the form of a tape or film is extruded onto rotating pipe.

A third coating station is used for the coating of the outer layer of polyolefin onto the pipe using extrusion techniques. For example, the polyolefin may be applied by use of an annular die through which the pipe is passed or by side extrusion in which molten polymer in the form of a tape or film is extruded onto rotating pipe. In the application of the coatings, a length of pipe to be coated passes axially and with optional rotation along a predetermined path sequentially through the coating stations. Before entering the first (epoxy resin composition) coating station, the pipe passes through a heating station where it is heated to an appropriate coating temperature in the range of from about 200 to about 250°C; this may be appropriately accomplished by means of induction heating, infrared heating or gas fired ovens. As the hot pipe passes through the coating stations, which are aligned with the heating station and with each other, it receives sequential coatings of the epoxy resin composition, the modified polyolefin and the outer coating of polyolefin.

In preferred embodiments of the invention, the epoxy resin powder has a particle size of up to about 250 microns, especially in the range of about 10 to about 150 microns. Similarly, the modified polyolefin powder has a particle size of up to about 350 microns, especially in the range of about 75 to about 175 microns. In further preferred embodiments of the invention, the epoxy resin powder has a melting point in the range of about 90 to 130"C, especially in the range of about 95 to 125"C. Similarly, the modified polyolefin powder has a melting point in the range of about 105 to 175"C, especially in the range of about 120 to 165°C.

The coating of epoxy resin has a thickness of at least about 200 microns, preferably a thickness in the range of about 300 to 800 microns, especially in the range of about 350 to 600 microns. Similarly, the modified polyolefin has a coating thickness of up to about 500 microns, for example 50 to 500 microns, especially 100 to 400 microns, preferably a thickness in the range of about 100 to 250 microns. The polyolefin has a thickness of at least about 300 microns e.g. 300 to 7000 microns, preferably in the range of about 400 to 1500 microns. It is critical to the present invention that the modified polyolefin be applied to the epoxy resin-coated surface of the pipe before the epoxy resin composition has substantially cured; preferably, the polyolefin powder is applied immediately, i.e. within about 5 to about 60 seconds, after application of the epoxy resin composition, and desirably within about 15 seconds following application to the pipe of the epoxy resin composition. This careful timing feature may be controlled, among other things, by controlling the temperature and thermal mass of the pipe, the speed at which the pipe moves through the coating stations and the spacing between the coating stations.

In the drawing. Figure 1 schematically shows a portion of a pipe coating apparatus of the type employed in the present invention. It will be understood that the entire pipe coating process employs various treatment stations both upstream and downstream from the apparatus shown in Figure 1. In a typical operation, stock lengths of pipe may be subjected to preheating, abrasive cleaning as by sandblasting and the like, sanding and grinding, and other surface conditioning operations before passing to the apparatus shown in Figure 1.

A section of pipe is shown at 12 in Figure 1 and is conveyed along an axial path of travel shown by the arrow 14 by means of conveyor rollers 16. The latter may be positioned at an angle to the axis of the pipe so as to impart an axial rotation to the pipe, as shown by the arrow 10. The section of pipe enters a heating station 18 in which the pipe is heated by various means such as passage through a gas fired oven and is thus heated to a temperature in the range of about 200-250^βC. The pipe then passes directly to an epoxy powder coating station 20. This station comprising a chamber having pipe entrance and exit ports and within which an epoxy powder of the type described above is sprayed upon the hot pipe surface. A schematic view of the chamber is shown in Figure 2, the chamber including a supply ring 22 about the pipe 12, the supply ring having a series of radially inwardly oriented spray nozzles 24 adjacent one side of the pipe to direct epoxy powder against the surface of the rotating pipe. Supply and exhaust tubes 26, 28, are provided to supply air-entrained epoxy resin composition powder to and to exhaust air from the chamber. In practice, the interior of the chamber becomes filled with a cloud of epoxy resin particles.

Exiting from the epoxy powder coating station 20, the pipe passes to a polyolefin coating station 30 which is exemplified as a powder coating station and may be substantially identical to the epoxy powder coating station 20. In this chamber, air-entrained modified polyolefin powder is brought into contact with the hot surface of the epoxy-coated pipe and coalesces upon that surface to the desired thickness. The pipe exits the polyolefin powder coating station 30, and curing of the epoxy resin coating layer continues as the pipe passes downstream in the direction of the arrow 14. If desired, an additional heating station 32 may be provided downstream from the modified polyolefin powder coating station, the station 32 typically employing infrared heating means to further heat and thus flow out the modified polyolefin layer. The pipe section then enters the third coating station 40 in which the outer coating of polyolefin is applied, normally using extrusion techniques. The pipe section 12 is then quenched and is subjected to rigorous inspection to insure integrity of the coating.

In a pipe coating operation, a typical speed of travel of the pipe section 12 along the path 14 may average about 24 feet (about 7.3 metres) per minute. If the polyolefin powder is to be applied to the epoxy-coated pipe within about 15 seconds following application of the epoxy resin composition, then the distance "d" between the epoxy powder and polyolefin powder stations must be approximately 6 feet (about 1.8 metres) . As noted above, it is preferable that the polyolefin powder be applied before substantial curing of the epoxy resin coating, but desirably after gelation. In practice, this can be accomplished by varying the temperature to which the pipe is heated, the linear speed with which the pipe passes through the coating stations, and the distance "d" between the epoxy and the polyolefin coating stations. Commonly, adjustments are made to the axial speed of the pipe or to the distance "d" between the coating stations 20, 30, which may be made movable along the axial path of travel 14 of the pipe so that the distance between them may be varied.

The thicknesses of the epoxy resin and the polyolefin coating will depend upon the flow rates of the respective powders to the pipe surface and the speed of the pipe through the coating stations. The flow rates of powdered epoxy and powdered modified polyolefin and the extrusion rate of the polyolefin for the outer coating may be adjusted as desired, as may the linear speed of pipe passing through the coating stations, the pipe speed, however, remaining strictly subject to the requirement that the polyolefin powder be applied to the epoxy resin composition layer before it has substantially cured but desirably after it has gelled. An example of a typical pipe coating operation under the present invention may employ steel pipe having an outer diameter of about 12 inches (about 30.5 cm) and a wall thickness of 0.312 inches (0.78 cm). Using apparatus generally of the type described in connection with Figure 1, the pipe may be rotated and provided with an axial speed of about 24 feet (about 7.3 metres) per minute. Upon passing through the heating station 18, which may be a gas fired oven, the pipe may be heated to a temperature of 240°C. The pipe then immediately enters the epoxy powder coating station 20, epoxy powder being uniformly applied by means of a spray to the pipe surface and coalescing to form a wet film. The epoxy resin composition applied may be that sold commercially by Valspar, Inc. under the designation D-1003LD, this powdered resin composition having gel time at 240°C of about 4-8 seconds. The distance "d" separating the epoxy and polyolefin coating stations may be approximately 4 feet (1.2 metres), this distance providing approximately 10 seconds between the epoxy and the polyolefin coating steps. In this manner, the polyolefin is applied to the epoxy resin coating approximately 2-6 seconds after the latter has gelled but well before curing has been completed. The modified polyolefin powder is applied to the epoxy resin-coated pipe at a rate sufficient to coat the epoxy to the required thickness. The modified polyolefin so applied may be Fusabond PMD 139 GBK or EMB 158, both manufactured by Du Pont Canada Inc. The length of pipe 12 is supported a sufficient distance downstream from the modified polyolefin coating station so that the modified polyolefin powder coating has coalesced into a wet film and has cooled to a solid before contacting downstream supporting rollers. The polyolefin applied as the outer coating may be an ethylene/butene-1 copolymer e.g. Sclair^® 35B polyolefin, manufactured by Du Pont Canada Inc. It is preferred that the polyolefin from which the modified polyolefin has been manufactured and the polyolefin of the outer coating be similar types of polymers e.g. both polypropylene or both polyethylene, which tends to result in improved compatibility and hence improved bond strength between the layer of modified polyolefin and the outer layer of polyolefin.

The resulting composite coating on the pipe is tightly adherent to the metal pipe surface and, because of the modified polyolefin intermediate coating and the polyolefin outer coating, is highly resistant to impact damage or damage from rough handling. It has been found to be substantially impossible to physically separate the modified polyolefin coating layer from the epoxy layer at ambient temperature, the two layers being intimately bonded together. Similarly, the layers of modified polyolefin and polyolefin cannot be readily separated. The thick epoxy resin coating layer provides the pipe with substantial resistance to cathodic disbondment and, together with the overlying polyolefin layers, serves to physically protect the pipe from damage of the type encountered in a pipeline laying operation.

Because of the inseparable nature of the layers that form the composite coating of the invention, and because of the manner in which the inseparable layers cooperate to contribute their particular desirable features to the composite coating, the coating is considerd to be an integral structure rather than a mere assemblage of layers.

By being placed in contact with the coalesced epoxy resin layer before the latter has completely cured, .the modified polyolefin (through its carboxyl or anhydride groups) reacts with and chemically bonds to the epoxy resin to provide an extremely adherent bond between these layers. The interface between the epoxy resin and the modified polyolefin is slightly diffuse, and attempts to separate these layers one from another along their interface has resulted in disruption of one layer or the other without evidence of interface separation.

The confronting surfaces of the modified polyolefin layer and the polyolefin outer layer are at least to some extent melt blended together during the application of the outer layer, and the bond between these layers is thus somewhat diffuse and may be extremely strong. Attempts to separate these layers along their interface have similarly resulted in disruption of the layers themselves, without interface separation.

Thus, the composite coating of the invention may be thought of as a sandwich structure in which the parts of the sandwich are intimately and inseparably joined in a cooperative manner. The epoxy resin layer bonds tenaciously to the metal surface to which it is applied. The intermediate modified polyolefin layer chemically bonds to the epoxy layer, and the polyolefin outer layer is melt blended at its interface into the outer surface of the modified polyolefin layer. The epoxy resin layer serves functionally to physically cover and protect the underlying metal surface, to adherently bond the composite coating to the metal layer, and to effectively and efficiently resist cathodic disbondment. The intermediate modified polyolefin layer serves to tenaciously adhere the epoxy resin layer to the outer polyolefin layer, and the intermediate layer should be of sufficient thickness e.g. at least 50 microns, to achieve this purpose. The intermediate layer may also in itself provide sufficient physical protection to the underlying epoxy resin layer. The outer polyolefin layer commonly will be the thickest layer, and serves to encapsulate and protect the layers beneath it and, accordingly, to protect the underlying metal surface. The composite coating therefore does not have three separate and distinct layers, even though the layers are applied separately. Rather, the composite coating that is formed functions as an integral coating having an inner portion (the epoxy resin) that is tenaciously bonded to the underlying metal surface and provides protection against cathodic disbondment, and an outer portion (formed from the intermediate modified polyolefin and the outer polyolefin layers) which is generally hydrophobic and waterproof and which exhibits sufficient flexibility as to enable it to absorb the physical abuse commonly encountered in pipe laying procedures while continuing to protect the underlying epoxy portion and the metal surface itself.

Metal pipe commonly used in petroleum pipeline applications may have a wall thickness in the range of about 2-25 mm and an outer diameter in the range of about 2.5-150 cm; for example, when heated to coating temperatures in the range of about 240°C, such pipe has sufficient thermal mass i.e. it has absorbed sufficient heat energy, so that its surface temperature falls quite slowly. Unless quenched, the temperature of such a typical section of pipe heated to 240°C and then coated in accordance with the invention falls to 220°C only after about 1-3 minutes. Thus, the more critical parameters involve the speed of the pipe through the coating stations and the space between the stations. The present invention is particularly intended for the coating of pipe intended for use in petroleum applications. However, the coated pipe may have uses in other applications in which protection against cathodic disbondment is important.

While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method of coating metallic pipe for use in buried pipelines to provide the pipe with resistance to impact damage and to cathodic disbondment, comprising: (a) heating the pipe to a temperature of at least 200^βC;

(b) applying to the outer surface of the heated pipe a powdered epoxy resin composition comprising an epoxy resin and a curing agent therefor, said powdered epoxy resin composition melting and coalescing upon the pipe to form a molten coating having a thickness of at least 200 microns;

(c) before complete curing of the epoxy resin, applying thereto a modified polyolefin, said modified polyolefin being a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms and which has been grafted with an ethylenically unsaturated organic carboxylic acid or anhydride, the modified polyolefin forming an adherent and protective coating on the epoxy coating and having a thickness in the range of up to 500 microns; and

(d) applying a molten layer of a polyolefin selected from the group consisting of homopolymers or copolymers of hydrocarbon alpha-olefins having 2-10 carbon atoms, said layer of polyolefin having a thickness of at least 300 microns.

2. The method of Claim 1 in which the epoxy resin composition has a softening point of at least 90"C.

3. The method of Claim 1 or Claim 2 in which the polyolefin of the outer layer is a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C₃-C₁₀ hydrocarbon alpha-olefins.

4. The method of any one of Claims 1-3 in which the modified polyolefin is applied after the epoxy resin composition has gelled upon the pipe surface.

5. The method of any one of Claims 1-4 in which the gel time of the epoxy resin composition is between 2 and 20 seconds at pipe temperatures of 250°C.

6. The method of any one of Claims 1-5 in which the time period between gelation of the epoxy resin composition and application step (c) is not greater than 90 seconds.

7. The method of Claim 1 in which the modified polyolefin is applied in step (c) to a thickness in the range of 100 to 250 microns.

8. The method of Claim 7 in which the epoxy coating has a thickness in the range of 200 to 400 microns.

9. The method of any one of Claims 1-8 in which the epoxy resin composition powder and the modified polyolefin powder have particle sizes of up to 250 microns.

10. The method of Claim 9 in which the modified polyolefin has a melting point in the range of 105 to

175°C.

11. The method of Claim 9 in which the epoxy resin composition has a melting point in the range of 90 to 130°C.

12. The method of Claim 11 in which the epoxy resin composition has a gel time in the range of 2-20 seconds at 250^βC.

13. The method of any one of Claims 1-12 in which the modified polyolefin is a modified homopolymer or copolymer of at least one of ethylene and propylene.

14. The method of Claim 13 in which the grafted ethylenically unsaturated carboxylic acid or anhydride is maleic acid or maleic anhydride.

15. The method of any one of Claims 1-14 in which the thickness of the outer coating of polyolefin is in the range of about 300 to 1500 microns.

16. The method of any one of Claims 1-15 in which the pipe has a thickness in the range of 2-25 mm.

17. Metal pipe suitable for buried pipeline use and bearing an outer composite coating resistant to impact damage and to cathodic disbondment, said coating comprising an inner layer of a cured epoxy resin composition having a thickness of at least 200 microns, an intermediate coating of a modified polyolefin bonded to the inner layer and having a thickness of up to 500 microns, and an outer layer in a thickness of at least 300 microns of a polyolefin that is a homopolymer or copolymer of hydrocarbon alpha-olefin having 2-10 carbon atoms.

18. The metal pipe of Claim 17 in which the polyolefin of the outer coating is a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C₃-C₁₀ hydrocarbon alpha-olefins.

19. A method of coating metal pipe to be used in the construction of a buried pipeline, comprising

(a) providing a heating station through which said pipe may be axially moved and heated;

(b) providing a first coating station in line with the heating station and capable of applying a powder coating to the exterior of the heated pipe;

(c) providing a second coating station in line with the first coating station for applying a subsequent coating; (d) providing a third coating station in line with the second coating station for applying a further coating;

(e) passing metal pipe through the heating station coating station, the pipe being heated to a temperature of at least 200°C; (f) applying a powdered epoxy resin composition to the pipe as it passes through the first coating station, the resulting coating melting, coalescing and subsequently gelling upon the surface of the pipe to a thickness of at least 200 microns;

(g) applying a modified polyolefin, to the pipe section as it passes through the second coating station, the modified polyolefin forming a polyolefin layer having a thickness of up to 500 microns, said modified polyolefin being a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms and which has been grafted with an ethylenically unsaturated organic carboxylic acid or anhydride; the axial speed of the pipe through said stations and the distance between the first and second coating stations being adjusted to cause the polyolefin to be applied in the second coating station before complete curing of the epoxy resin composition applied in the first coating station; and

(h) at the third coating station, applying a molten layer in a thickness of at least 300 microns of a homopolymer or copolymer of hydrocarbon alpha-olefins having 2-10 carbon atoms, to the pipe.

20. The method of Claim 19 in which the polyolefin of the outer layer is a polyolefin selected from the group consisting of homopolymers of ethylene, homopolymers of propylene and copolymers of ethylene and C₃-C₁₀ hydrocarbon alpha-olefins.

21. The method of Claim 19 or Claim 20 in which the modified polyolefin coating is applied after gelation of the coating of the epoxy resin composition applied in the first coating station.

22. The method of any one of Claims 19-21 in which the modified polyolefin is a powdered modified polyolefin.

23. The method of Claim 19 in which the modified polyolefin coating is applied before gelation of the coating of the epoxy resin composition applied in the first coating station.

24. The method of Claim 19 in which, in the coated metal pipe obtained, the epoxy and polyolefin layers are initmately bonded together.

25. The method of Claim 24 in which it is substantially impossible to physically separate the polyolefin layer from the epoxy layer at ambient temperatures.

26. The method of Claim 19 in which the temperature in step (f) , the axial speed of the pipe and the distance between the first and second coating stations are coordinated to ensure sufficient bonding between the polyolefin and epoxy layers so that physical separation of these layers is substantially impossible at ambient temperatures.