WO1994005495A1

WO1994005495A1 - Composition and method for coating metal substrates

Info

Publication number: WO1994005495A1
Application number: PCT/US1993/008064
Authority: WO
Inventors: James H. Reimer
Original assignee: Plastic Flamecoat Systems, Inc.
Priority date: 1992-09-03
Filing date: 1993-09-02
Publication date: 1994-03-17
Also published as: CA2094204A1; AU5094293A

Abstract

A field-applied, flame sprayed coating for metal substrates is disclosed that comprises a blend of approximately equal parts by weight of a fusion bonded epoxy and an ethylene methacrylic acid (EMAA) copolymer. The coating exhibits excellent resistance to disbondment when applied to cathodically protected substrates. The coating is desirably flame sprayed onto the clean metal substrate at a temperature of about 220 °F at a thickness of about 15 mils. Optionally, the subject coatings can be oversprayed with a flame sprayed-coating of up to about 10 mils of a powdered EMAA copolymer to achieve enhanced flexibility and resistance to impact or abrasion.

Description

COMPOSITION AND METHOD FOR COATING METAL SUBSTRATES

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to flame-sprayed coating systems, and more particularly, to a portable, field-applied coating and method useful for coating metal substrates such as pipelines. Pipeline coatings are required to meet specifications more stringent than required for some other applications. Since most pipelines utilize cathodic protection to reduce the rate of corrosion, cathodic disbondment testing is a critical test used to evaluate these coating. The coating and method disclosed herein meet or exceed typical cathodic disbondment specifications for pipeline coating service.

2. Description of Related Art

Flame sprayed coating systems for thermoplastic materials have previously been disclosed in United States Patent Nos. 4,632,309 and 4,934,595, and in pending application Ser. No. 760,866, filed September 16, 1991 , which are incorporated by reference herein. While the systems, compositions and coating methods disclosed in these foregoing patents and patent application have proved to be very useful for applying protective coatings to a wide variety of different articles, such as bridges, storage tanks, boat hulls, snow plows, conveying equipment, water treatment equipment, etc. , a specialized coating system, composition and method are needed for use on cathodically protected metal substrates such as girth welds on pipelines employing cathodic protection. While cathodic protection of underground pipelines does substantially eliminate or significantly reduce the corrosion due to electrochemical phenomena, this method does not prevent direct chemical attack.

The use of fusion bonded epoxy (FBE) coatings as pipe coatings is well known. Typically, these coatings are hard and brittle, with very low elongation, although their hardness provides very good abrasion and impact resistance. A major drawback to such coatings is that they are not readily applied in the field.

They are powder coatings typically applied in factory environments, due to the need for preheating ovens, and in some cases post-curing ovens, for proper application. Even with such coatings, it is necessary to leave the weld areas uncoated until the pipe is installed. Once the welds are made between adjacent pipe sections, expensive, elaborate equipment is needed to provide a comparable coating for the girth welds. In most cases, an inferior coating is applied around the girth welds in an attempt to complete the coating without such equipment.

Shrink sleeves, mastics, tapes, etc. , are examples of currently available girth weld coatings. Thus, the relatively low cost of applying high quality, fusion bonded epoxy coatings to piping in a factory do not extend to the field application of such coatings to the pipeline girth welds.

Flame-sprayed thermoplastic coatings as previously disclosed generally possess high elongation, good abrasion resistance, and excellent adhesion and flexibility when applied to clean metal substrates. However, the conventional flame-sprayed coatings do not provide the desired degree of resistance to cathodic disbondment when applied to cathodically protected substrates such as pipeline girth welds.

Coatings comprising epoxy resins that are said to be useful for protecting metal substrates from cathodic disbondment have previously been disclosed, for example, in United States Patent Nos. 4,782, 124 and 3,578,615, and in defensive publication T973015.

U.S. 3,578,615 discloses a fluidizable, heat-curable polyepoxide coating composition possessing improved cathodic disbonding resistance when applied to underground metallic piping. The use of synthetic resins and elastomers as fillers is also disclosed. While the use of fillers in amounts up to about 355 parts per one hundred parts by weight of the polyexpoxide (phr) are disclosed, the use of from about 10 to about 60 phr are recommended in order to optimize raw material costs without minimizing coating properties. Application techniques including fluidized beds, spraying as by a compressed air spray gun, and elctrostatic application are also disclosed.

U.S. 4,782, 124 discloses polycarbonate modified epoxy resins containing carbonate linkages between the epoxy resin and the transesterification induced chain scission products of the polycarbonate or polycarbonate oligomer. Application to a substrate using known methods such as powder dusting, fluidized bed processes, electrostatic powder spraying, electrostatic fluidized bed processes, and others is also disclosed.

U.S. Defensive Publication T973,015 discloses improved resistance to stress cracking or cathodic disbonding of an external polyolefin coating on a cathodically protected pipeline by including 1-18 percent, preferably 4-8 percent, by weight, based on the polyolefin, of cured epoxy resin in at least the region of the polyolefin coating adjacent the pipe. The preferred polyolefin is low-density polyethylene, but may also be an ethylene copolymer, high-density polyethylene, polypropylene or a propylene copolymer with up to 20 percent by weight of ethylene. The powdered composition may be coated by electrostatic spraying onto the heated pipe, e.g. at 190°C for low-density polyethylene, with possible subsequent application by extrusion or otherwise of polyolefin containing no epoxy resin. Strew-coating onto pipe heated to a higher temperature, e.g. 320°C, is also disclosed.

SUMMARY OF THE INVENTION The composition and method disclosed herein provide a non-toxic, high- build coating of up to 15 to 25 mils thickness without any cure time being required. Application is quick and easy, resulting in lower application costs, and the method of the invention does not produce any hazardous waste. The subject method and fusion bonded composition are useful for either new application to pipeline girth welds, or for repairing damage to existing fusion bonded coatings.

According to one embodiment of the invention, a single coating, preferably about 15 mils thick, of the preferred inventive composition (which by itself meets or exceeds standard pipeline specifications) is applied to a pipeline girth weld in the field using flame spray equipment as disclosed in U.S. Patent Nos. 4,632,309 or 4,934,595, and application Ser. No. 760,866. According to one preferred embodiment, the composition of the invention comprises a blend of cryogenically ground powder containing approximately equal parts by weight of fusion bonded epoxy and ethylene methacrylic acid (EMMA) copolymer.

Preferably, the epoxy and EMMA copolymer are each ground to a particle size of about 70 mesh prior to blending. Minor amounts of other ingredients such as UV stabilizer and pigment are optionally added to retard degradation in sunlight, improve appearance of the coating, and the like. According to another preferred embodiment of the invention, the coating as recited above is further overcoated with a flame-sprayed layer of EMAA copolymer to enhance the overall flexibility, impact resistance and abrasion resistance of the coating. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to a preferred embodiment of the invention, 1/8 inch pellets of natural-colored ethylene methacrylic acid copolymer, most preferably Nucrel^® 410 marketed by DuPont Chemical Corporation, are cryogenically ground to a particle size of about 70 mesh.

Fusion bonded epoxy, preferably Scotchkote^® 206N marketed by 3M Corporation, is likewise cryogenically ground to a particle size of about 70 mesh. Scotchkote^® 206N is a one-part, heat curable, thermosetting powdered epoxy coating designed to provide maximum corrosion protection of line pipe. The cryogenically ground powders thus produced are preferably blended in about equal parts by weight. Blending is preferably done using conventional, commercially available dry blending equipment such as a tumble blender, ribbon blender or vertical cone blender, and blending is continued until substantially homogeneous distribution is achieved. Optionally, about 0.5 percent by weight of the total blend of an ultraviolet light stabilizer, preferably Tinuvin^® 770 marketed by Ciba-Geigy Corp. , and about 4 percent by weight of the total blend of a commercially available pigment, preferably blue pigment C102, are blended together with the epoxy and EMAA copolymer.

The substrate to be coated is preferably clean and dry. Desirably, the substrate will be prepared by blast cleaning to a minimum of an SSPC SP-6

(Commercial Blast), with an anchor profile of between two and three mils. After blasting, the substrate should be cleaned with compressed air (provided from a source that is filtered for oil and water contamination) to remove any remaining powder or other paniculate matter and dried to remove moisture. Drying can usually be done by passing the lighted flame spray gun over the substrate prior to activating powder flow through the gun.

Prior to activating powder flow, the portion of the substrate that is to be coated first is desirably preheated to a temperature ranging from about 200 to about 220°F. Thereafter, except for heat-sink areas such as flanges, substrate areas adjacent to the work area being coated at a particular time will normally be preheated by thermal conductivity from the work area to the adjacent area.

The powder blend as described above is preferably applied to the clean, preheated substrate by flame spraying by the same general methods and with apparatus as disclosed in U.S. Patent Nos. 4,632,309 or 4,934,595, or in application Ser. No. 760,866, all of which are incorporated by reference herein. During application, the powder blend is preferably heated by the flame spray gun to a temperature ranging from about 200 to about 220°F (compared to a temperature ranging from about 170 to about 190°F for the EMAA copolymer alone).

A beneficial coating in accordance with the present invention is obtained by flame spraying the blended powder onto the substrate at thicknesses of about 15 mils. Further improvements in flexibility, impact resistance and abrasion resistance are desirably achieved by subsequently flame spraying a topcoating of the cryogenically ground EMAA copolymer powder over the epoxy/EMAA copolymer layer at an additional thickness of up to about 10 mils.

Repairability of these coatings is a major advantage over pure fusion bond epoxy coatings previously disclosed in the prior art. (Typical application of fusion bond epoxy requires that the substrate be heated to about 450°F, the powder applied, and the residual heat cures the coating.) These coatings will remelt to some degree due to the high levels of thermoplastic material, which gives a bonding agent for the patch material. No adhesion values have been determined to the patch area, but the "pocket knife test" indicates excellent adhesion. The overcoat adhesion test results indicate excellent adhesion at any repair sight. TESTING Samples were prepared using flat, carbon steel plates, which were cleaned by blasting according to Steel Structures Painting Council (SSPC) SP-6 (commercial blast) specifications. An anchor profile of about two mils was measured. The samples were preheated to 200-220 °F using a flame spraygun as manufactured by Plastic Flamecoat Systems, Inc. Once preheated, a flame sprayed coating of blended epoxy and EMAA copolymer as disclosed above was applied at an average thickness of about 15 mils. The coating material was preblended with a pigment to give it a dark blue appearance. A clear, or natural, overcoat of an EMMA copolymer-based composition

(marketed by Plastic Flamecoat Systems, Inc. under the tradename PF111) was flame sprayed onto half of the samples to evaluate the effect on physical properties. In so doing, the previously coated surface was preheated to a molten state (about 225 °F) before the overcoat was applied. Natural material was used so that any effects of overcoating on the undercoat could be viewed through the topcoat. No discoloration or off-gassing was observed. (In commercial applications, the topcoat may be colored to aid UV stability and for general appearance considerations.)

Cathodic disbondment testing (CDT) was a major consideration in evaluating the coated plates. All CDT's were conducted by Bell Evaluation Labs.

CDT results are reported in terms of zero and reduced adhesion radius, which are combined to report the total disbondment radius. All CDT's were conducted in accordance with ASTM G-95 and are reported in millimeters. These tests may be conducted at room temperature, or the test may be modified (accelerated) by evaluating performance at elevated temperatures.

Short term (24 hour and 14 day) testing was conducted at 150°F. These tests indicated excellent performance. Testing results are outlined in Table I. The 24 hour test at 150°F resulted in a total disbondment of one millimeter, while the 14 day test yielded a four to five millimeter disbondment radius. Twenty-four hour testing at 150°F was conducted on overcoated plates, yielding a one to two millimeter disbondment radius.

Long term (28 day) testing was conducted at room temperature on samples not having the EMAA copolymer overcoat. The total disbondment radius on these samples was 13 to 15 millimeters, within typical specification limits. Some color loss was seen in the testing area; however, coating integrity was not affected.

Flexibility was evaluated using the ASTM G10 test method. This test consists of bending coated plates (with and without overcoating) at room temperature and at freezing. The results are reported in degrees bend per pipe diameter. The temperature difference (75 °F vs. 32 °F) did not have a significant effect on the flexibility of the blended epoxy and EMAA copolymer coating, indicating excellent property retention. Overcoated samples exhibited a slight improvement in bending performance. The test results are presented in Table II. The results ranged between 14.19 and 15.07 bends per pipe diameter.

Other physical properties were also evaluated on the plates coated as described above. The test results are reported in Tables III, IV and V. In general, physical properties were enhanced on the plates where the blended epoxy and EMAA copolymer coating was overcoated with another layer of EMAA copolymer material. Impact properties were measured using the ASTM G14 test method. The impact rating of about 30 inch-pounds for the blended epoxy and EMAA copolymer-coated plate was exceeded by about 10 inch-pounds with the overcoated sample. Abrasion testing conducted in accordance with ASTM D1044 indicated a 30 percent improvement for the overcoated plate. Adhesion was determined by ASTM 4541 , testing the pull of strength of coatings using a portable adhesion tester. Plates flame-sprayed only with the blended epoxy and EMAA copolymer material exhibited cohesive failure at 550 and 600 psi. Plates in which the blended epoxy and EMAA copolymer material was overcoated with the additional layer of EMAA copolymer exhibited pull-off strengths of 750 and 900 psi, with adhesive failure see at the interface between the overcoat and the aluminum dolly. The interface between the steel substrate and the undercoating of flame-sprayed, blended epoxy and EMAA copolymer was unaffected.

TABLE I

CATHODIC DISBONDMENT TESTING (CDT) ASTM G-95

Electrolyte = 3% NaCl

Disbondment Radius

Sample Time Tem Volt Zero Reduced Total

A. csc ι ι:

B. CSC 215 24 hr 150°F 3V 1 1

1 0 1

TABLE II

FLEXIBILITY ASTM G-10

Sample Temp DFT Deg.Bend/Pipe Dia.

A. CSC 115 32°F 14.5-16.8 14.41

75°F 14.4-16.8 14.19

B. CSC 215 32°F 35.4-39.5 14.79

75°F 36.8-40.0 15.07

TABLE III IMPACT ASTM G-14

Sample Temp DFT Failure

A. CSC 115 72°F 26.3-34.1 > 30 in-lbs

B. CSC 215 72°F 45.0-51.7 > 40 in-lbs

TABLE IV

ABRASION ASTM D1044

Conditions: CS17 Wheel with lOOOg Load

TABLE V

ADHESION ASTM D4541 Samples Scored Prior to Testing

Sam le Adhe i n Failure

In summary, substrates having a flame-sprayed layer of blended epoxy and EMAA copolymer demonstrated excellent performance when subjected to standard pipeline coating tests. The flame-sprayed, blended epoxy and EMAA copolymer coating disclosed herein meets or exceeds typical performance specification limits for short and long term cathodic disbondment testing. Impact and abrasion properties are not significantly high, but the gain in elongation and flexibility are believed to be a good trade-off. This coating may be used as an effective, high build, one-step coating that is simple and cost effective for use in coating pipeline girth welds. Further improvement in impact resistance, abrasion resistance and adhesion can be realized by overcoating the base coat with a flame- sprayed layer of EMAA copolymer.

Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading the present disclosure, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled.

Claims

CLAIMS:

1. A coating for cathodically protected metal substrates, the coating consisting essentially of a field-applied, flame-sprayed blend comprising approximately equal parts by weight of cryogenically ground fusion-bonded epoxy and cryogenically ground ethylene methacrylic acid copolymer.

2. The coating of claim 1 wherein the blend comprises powder having a maximum particle size of about 70 mesh.

3. The coating of claim 1 , having a thickness of about 15 mils.

4. The coating of claim 1 , further comprising a field-applied, overcoat consisting essentially of flame-sprayed, cryogenically ground ethylene methacrylic acid copolymer.

5. The coating of claim 4 wherein the overcoat has a thickness of about 10 mils.

6. A composition comprising a blend of approximately equal parts by weight of cryogenically ground fusion-bonded epoxy and cryogenically ground ethylene methacrylic acid copolymer.

7. The composition of claim 6 wherein the epoxy and copolymer are cryogenically ground to a maximum particle size of about 70 mesh.

8. A method for coating a cathodically protected metal substrate comprising the steps of: providing a first powder consisting essentially of fusion-bonded epoxy that has been cryogenically ground to a particle size of about 70 mesh; providing a second powder consisting essentially of ethylene methacrylic acid copolymer that has been cryogenically ground to a particle size of about 70 mesh; blending the first powder and second powder in about equal parts by weight; and flame-spraying the resultant blend onto the substrate.

9. The method of claim 8 wherein the substrate is blast-cleaned prior to flame-spraying.

10. The method of claim 8 wherein the substrate is preheated to a temperature ranging from about 200 to about 220°F prior to flame-spraying.

11. The method of claim 8 wherein the blend is flame-sprayed onto the substrate at a thickness of about 15 mils.

12. The method of claim 8 wherein the flame-sprayed blend is subsequently overcoated with a flame-sprayed layer of ethylene methacrylic acid copolymer.

13. The method of claim 12 wherein the flame-sprayed blend is overcoated by an additional thickness of about 10 mils.