WO1997013589A1

WO1997013589A1 - Corrosion protection coating system

Info

Publication number: WO1997013589A1
Application number: PCT/US1996/015248
Authority: WO
Inventors: Richard M. Bymark; Kam C. Wu; Anne F. Miniely
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1995-10-10
Filing date: 1996-09-24
Publication date: 1997-04-17

Abstract

The invention provides epoxy resin compositions comprising dispersed adhesion promoters to enhance adhesion to polyolefin resins, and multilayer composite coating systems useful for corrosion prevention to metal substrates which comprise an underlayer formed from such epoxy resin compositions, and thereover, a fuser polyolefin resin.

Description

CORROSION PROTECTION COATING SYSTEM

Background ofthe Invention Field ofthe Invention The invention relates to corrosion protection of metal surfaces, especially steel pipes and reinforcing bars, by forming a multilayer polymeric coating over the metal wherein a strong bond develops between polymer layers rendering the use of tie layers unnecessary.

Composite coating systems comprise multiple layers of material coated atop each other over a metal substrate to form a moisture barrier that protects the metal from corrosion. These coating systems are well known in industries providing pipes, pipelines, rebar, and the like. Corrosion barriers for modern pipes and other metal products typically involve such composite construction. This is an increasing trend. Current composite coating systems typically comprise three layers, each contributing a particular benefit to the coating system as a whole. The underlayer or primer layer, i.e., the layer contacting the metal, is selected from polymers that show good adhesion to metals such as steel, e.g., epoxy resins. A moderately thick layer of polyethylene provides a good exterior layer for the composite coating, providing mechanical and moisture protection. Unfortunately, polyethylene does not bond well to epoxy material. This necessitates the use of a tie-layer inteφosed between the epoxy layer and the polyethylene layer.

WO 9,422,598 discloses a three layer coating comprising an epoxy primer layer, applied as a powder, a hard thermoplastic adhesive layer and an outer polyethylene sheath. In similar fashion, U.S. Patent No. 4,213,486 discloses the application of fused, powdered epoxy as a first layer over heated steel. A cover-sheet of polyethylene is coated with a pressure sensitive or hot-melt adhesive on the under surface in order to bond the polyethylene layer to the previously formed epoxy layer. Again, the final structure is a three layer protective coating where the intermediate layer is an adhesive. Japanese Patent Nos. JP 57, 113,871 and JP 56,168,862 also teach three layer protective coatings for steel. In these references, a bonding adhesive layer between polyethylene and epoxy layers comprises copolymers of ethylene with vinyl acetate, acrylic acid or maleic anhydride. The method of forming composite coatings over metal employs, e.g., electrostatic spray application of a fusion bonded epoxy powder, and melt-extrusion of both the tie-layer and the layer of polyethylene. alternative method of application, as disclosed in U.S. Patent Nos. 5,178,902 and 5,300,336, relies on spray-application of all three layers. Again, the under layer is epoxy, and the exterior layer polyethylene. The tie-layer comprises a fusion bonded epoxy powder mixed with either polyethylene powder or polyethylene plus a copolymer of polyethylene and maleic anhydride. The individual application requires that each previously applied layer remain in a heated molten condition during spray application ofthe next layer. Steel protective coatings described by Japanese Applications JP 3,058,834 and JP 2,008,043 use the same method of combining molten layers. This application method improves the bond between layers.

Elimination ofthe intermediate adhesive tie-layer offers cost savings in both materials and application time. Attempts to improve affinity between the polyolefins and epoxy coatings have typically included a chemically modified polyolefin top-coat. Japanese Patent JP 50,126,727 discloses a system wherein a maleic-anhydride- containing polyethylene sheet is bonded directly to an epoxy primer coat previously applied to the metal substrate by means ofthe application of pressure for five minutes at 270°C. There is, however, no evidence of commercial use for this or similar systems.

However, no disclosure exists of a two layer composite coating system where improvement of interlayer bonding is accomplished by means of chemical modification ofthe underlayer, e.g., the epoxy primer layer.

The current invention provides a commercially viable, economical two-layer composite corrosion-protective coating using modified epoxy compositions, which exhibit good adhesion between the layers, thus eliminating the tie layer. Modified epoxy coating powders ofthe invention have surface characteristics to which polyolefins, such as polyethylene and polypropylene bond strongly. Summary ofthe Invention The invention provides epoxy resin compositions to which polyolefin resins adhere well, and composite coating systems which provide a two layer composite coating system comprising a fused coating formed from such epoxy resin compositions, and coated thereover, a polyolefin resin.

More specifically, powdered epoxy resin compositions ofthe invention which bond to metal and adhere to a polyolefin comprise a thermoset epoxy resin and a dispersed adhesion promoter for said polyolefin. The promoter may be dispersed in a dispersion, or may be dispersed in particulate form, e.g., by mixing. Preferred powdered epoxy resin compositions ofthe invention comprise a thermoset epoxy resin and a dispersed adhesion promoter selected from the group consisting of oxidized polyethylene, maleated polyethylene, and maleated polypropylene.

Composite coatings systems ofthe invention comprise: a) a powdered epoxy composition comprising a thermoset epoxy resin and a dispersed adhesion promoter, and b) a polyolefin resin coated thereover.

Coated layers ofthe invention comprise a fused epoxy underlayer having been formed from a powdered epoxy composition and a dispersed adhesion promoter, overcoated by an exterior fused polyolefin layer, said epoxy layer having an adhesion to said metal of at least about 6.9 N/mm² adhesion to said polyolefin layer of at least about 3.5 N/mm.

The following terms have these meanings as used herein.

1. The term "composite coating system" means a multilayer coating system comprising several layers of material coated atop each other over a metal substrate to form a moisture barrier that protects the metal from corrosion.

2. The terms "epoxy" and "epoxy resin" mean a material containing at least one epoxide or glycidyl functionality.

3. The term "maleated polyethylene" and "maleated polypropylene" mean respectively a polyethylene or polypropylene resin having a maleic anhydride functional group. 4. The term "oxidized polyethylene" means polyethylene treated with ozone to create polar functional groups such as carboxylic groups.

5. The term "dispersed adhesion promoter" means 1) maleated or oxidized polyolefin phase dispersed in an epoxy disperson phase, or 2) maleated or oxidized polyolefin particles intermixed with an excess of epoxy powder.

All percents, parts and ratios herein are by weight unless specifically stated otherwise.

Detailed Description ofthe Invention Epoxy resin compositions useful for this invention are those which comprise certain dispersed adhesion promoters to enhance bonding between epoxy and polyolefin resins.

The improved bonding exhibited by these epoxy resins to polyolefin topcoats occurs because ofthe presence of these dispersed adhesion promoters in the molten epoxy layer. The epoxy composition forms protective surface coatings when applied to metal substrates and heated in the range of 180-250°C. A remarkably small amount of dispersed adhesion promoter, e.g. 0.5% to 5.0% is effective in increasing interlayer bond strength. Adhesion promoters such as oxidized or maleated polyolefins enhance bonding between epoxy and polyolefin layers. The resulting composite coating resists strongly any attempt to separate the layers. This is in sharp contrast to a similar combination of layers with the dispersed adhesion promoter omitted. Composite coatings easily peel apart in the absence ofthe promoter.

Adhesion promoters associate with the epoxy resin powder in several ways. When included during compounding ofthe epoxy resin powder, the adhesion promoter may be found inside a dispersed resin powder particle or at its surface. However, by compounding the epoxy resin first, then mixing-in the promoter, it is likely that each epoxy powder particle acquires a surface coating of adhesion promoter particles.

Compounded epoxy resins comprise a material containing an epoxide or glycidyl functionality, a catalyst, a cure accelerator, fillers and various additives. Materials with suitable epoxide or glycidyl group functionality include aliphatic epoxides such as aliphatic diepoxides and triepoxides, polyglycidyl ethers of aliphatic polymers, i.e., polyoxyalkylene extended polyols, such as Heloxy® 84, available from Shell or DER 732, available from Dow Chemical; bisphenol A, and substituted bisphenol A derivatives such as bisphenol A endcapped diglycidyl ether of bisphenol A, dihydric bisphenols, and blends thereof, and the like. Aromatic epoxides useful in compositions include diglycidyl ethers of bisphenol

A, diglycidyl ethers of bisphenol F and mixtures thereof, epoxy novolak, phenolic novolak and the like. Useful resins include monohydroxy and polyhydroxy compounds, and may be aliphatic or contain one or more aromatic rings, such as those disclosed in U.S. Patent 4,560,732, and EPO 0342 035, both of which are incoφorated by reference.

Useful epoxy resins for powder coating are solid resins formed from these epoxy materials, either singly or blends thereof. Typically, an epoxy resin has a softening point between about 50°C and about 130°C; preferred epoxy resins have an epoxy equivalent weight between about 550 and about 2000. Useful catalysts include, e.g., amine type curing agents such as dicyandiamide, methylene dianiline and dihydrazides, as well as anhydride type curing agents such as benzophenone tetracarboxylic dianhydride (BTDA), carboxyl terminated polyesters, and novolak resins and the like. The curing agents can be present in the coating composition in amounts which depend on the curing stoichiometry. Generally, the amount of curing agent in the composition will range between about 1 and 20% by weight.

Useful curing accelerators include metal salts such as stannous octoate, tertiary amines such as imidazoles and imidazole adducts with glycidyl ethers of bisphenol A. Commercially available products include e.g., Epon® P-101, available from Shell Chemical Company, and HULS #31. As desired, known accelerators for the production of epoxy resins may be incoφorated during compounding. Further adjuvants for the compounded epoxy resin include fillers and function-specific additives.

Useful fillers include calcium metasilicate, ground silica, various micas, calcium carbonate, magnesium oxide, barium sulfate and the like. The fillers can be present in amounts up to about 60% by weight. Function-specific additives include flow-modifiers such as polyacrylates and fluorocarbons, such as FC430 from 3M Company, and Modaflow III available from Monsanto Coφ. The flow control agents can be present in the coating composition in amounts up to about 2% by weight. Epoxy compositions ofthe invention may also contain minor amounts of conventional adjuvants such as dyes, colorants, pigments, leveling agents and the like, so long as the surface characteristics are not detrimentally affected by the type or amount ofthe promoter.

Pigments are frequently included both for identification, e.g., coating pipes used to carry various materials with different colors, and for aesthetic reasons. Useful pigments include e.g., titanium dioxide and various iron oxide pigments. When present, pigments can comprise up to about 60% by weight ofthe composition.

The epoxy composition may be prepared by conventional means such as melt- mixing using a heated two-roll mill or extruder.

such mixing, the composition is cooled, then pulverized by known means to a required range of particle size. The powdered epoxy resin is then treated with the dispersed adhesion promoters ofthe invention.

Alternatively, the use of an extruder or two roll mill provides compositions by combining the epoxy resin with dicyandiamine, a phenolic resin, a catalyst, pigments and fillers. Maleated polypropylene is added at the feed end ofthe extruder, thereafter extruding the compounded resins.

A remarkably small amount of adhesion promoter, e.g. 0.5% to 5.0% is effective in increasing interlayer bond strength. At this level, the adhesion ofthe applied fused epoxy layer to the exterior polyolefin layer is strong enough such that the polyolefin layer may fail internally rather than at the interface. At less than about 0.5%, the level of dispersed adhesion promoter is not sufficient to build strong adhesion; at levels much higher than 7-10%, the adhesion level ofthe fused epoxy layer to the steel begins to be adversely effected.

The epoxy composition may be applied to the metal substrate by spraying the powder composition onto a metal substrate heated to a temperature of between 180-

250°C, or the heated substrate may be dipped into a powder bed to form the coating. The coated substrate is then coated immediately with the polyolefin layer while the epoxy layer is still molten.

The top layer ofthe two layer composite coating system is a moisture impervious polymer such as a polyolefin, preferably polyethylene or polypropylene. This layer may be sprayed as a powder, extruded, or applied by other conventional means. This layer may also contain a variety of adjuvants, fillers, colorants, flow agents, and the like, as known in the art.

The following non-limiting examples provide evidence of composition and results of performance for epoxy resins ofthe invention that exhibit excellent adhesion to steel and very effective bonding to polyolefins.

Test Methods 180° Peel Adhesion Investigation of 180° peel adhesion provided quantitative information of bonding of fusion bonded epoxy (FBE) to a steel substrate and polyethylene (PE) to fusion bonded epoxy. A steel plate 7.62 cm x 15.24 cm x 1 mm, also referred to as a Q-panel, provided the metal surface that received the FBE coating. Q-panels, unwrapped after receipt from the supplier represented the test surfaces, without further treatment. Preparation of each panel involved lengthwise application of a of 2.54 cm wide Teflon® adhesive tape, approximately 7.50 cm long, along the approximate center-line ofthe Q-plate. Tape application left about 5.0 cm ofthe surface bare at one end ofthe plate. Coating of test plates with FBE and PE layers required an elevated temperature of about 228°C, obtained by placing a taped Q-plate on a hot¬ plate with the Teflon® tape facing outward from the hot-plate surface. Attainment of a suitable coating temperature at the surface ofthe taped Q-plate required at least 10 minutes in contact with the hot-plate. Use of a notched draw-down tool, required by the CSA Z245.23-M92. test method, produced a thin spread of molten FBE, lengthwise along the plate so that it covered taped and untaped portions ofthe Q-plate. PE powder, applied before the epoxy layer cured, simulated spraying of a powdered PE topcoat. Under the conditions described, heat associated with the FBE caused melting ofthe PE topcoat. Continued heating, for about 3 minutes or more, cured the FBE. Thereafter, heat removal allowed samples to cool to room temperature. Room temperature testing employed a 12 mm wide PE FBE composite strip cut lengthwise through the PE, FBE and Teflon® layers using a utility knife. The composite strip extended from the taped portion into the untaped end ofthe Q-plate. The length ofthe composite strip covering the Teflon® tape separated readily due to the release properties ofthe Teflon® surface. Thus separated, the composite strip provided a tab that facilitated testing of adhesion ofthe composite strip to the untaped surface ofthe Q-plate. Grasping the tab extending from the taped end ofthe Q-plate and pulling it back against itself caused the composite strip to form a loop with its free end positioned above the untaped end of the Q-plate. This cleared the end ofthe Q- plate, opposite the untaped end, for positioning vertically in the lower jaw of a 180° peel-tester. With the free end ofthe composite strip clamped in the upper jaw ofthe peel-tester, the sample was ready for testing. Gradual separation between upper and lower jaws caused a force to act on the sample in the section ofthe Q-plate where the FBE layer bonded directly to the steel surface. Force measurement with continuing jaw separation indicated, in Newtons/mm, the bond strength between layers.

Reference to standard test methods indicates that, for FBE/PE composite coatings, CSA-Z245.21 specifies a 180° peel adhesion of greater than 6 N/mm and DIN 30670 requires peel resistance of at least 3.5 N/mm.

90° Peel Adhesion Sample preparation of polypropylene protected steel bars

Application of an epoxy layer to a steel bar required pre-heating the bar to 205 °C (400°F) followed by its immediate immersion in a fiuidized bed of maleated polypropylene-containing epoxy powder. A cover layer of polypropylene, applied as a continuous sheet, in contact with the still molten, curing epoxy, formed a bond at the interface with the epoxy layer.

Bar samples, prepared as described above, have a 1.0 inch tab of polypropylene sheet extending beyond the end ofthe steel bar. The polypropylene sheet was large enough to allow preparation of a 1.0 inch- wide longitudinal strip cut from the tab end ofthe polypropylene sheet and extending 2.0 inches into a section where it bonded to the epoxy layer. The cuts, required in forming the strip, extended through the polypropylene layer and into the epoxy layer. The bar is of rectangular cross section with the polypropylene film bonded to one of its two major faces. Before measuring adhesion, the end ofthe bar farthest from the 1.0 inch tab was secured in a clamp, the length ofthe bar being positioned horizontally and the polypropylene film disposed on the underside ofthe bar. With the sample in this position a 11.35 Kg weight, attached to the end ofthe tab, exerted a force at the interface between the polypropylene sheet and the epoxy layer. The force acted at approximately 90° to the longitudinal axis ofthe steel bar. Adhesions greater than 4 Newtons/mm at 110°C represent satisfactory performance based upon European standard for external 3 -layer extruded polypropylene based coatings. Table 8 shows compositions and results for multilayer coatings using a polypropylene cover-layer. Samples 38 and 39 failed immediately at 23 °C when attaching the test weight to the polypropylene tab. .All subsequent samples, i.e. samples 40-51, passed the minimum adhesion requirement at 110°C.

Examples Example 1

A mixture of 99% epoxy resin Scotchkote®-6208 (an epoxy commercially available from 3M) and 1% Aqua Poly® 250, (a maleated polyethylene manufactured by Micro Powder Inc.) pre-blended in a food-blender then mixed (5-10 mins.) to a uniform powder in a paint shaker, provides the first layer of a metal protective composite coating. The uniform powder melts to form a continuous film when applied to the surface of a metal test plate heated at 230-235°C. A polyethylene topcoat, applied before the epoxy layer is gelled provides the water impervious, protective outer layer. The topcoat in this example was HPCC powder available from Shaw Industries Ltd. of Toronto, Canada. After cooling, a qualitative test ofthe bond between polyethylene and epoxy, requires a utility knife to probe between layers for evidence of separation associated with bond failure. Test results are shown in Table 1.

Cabosil® M5, available from Cabot Coφoration was mixed into the pulverized epoxy powder at a level of 0.2% in this and all further examples, as a flow agent. Example 2. and Comparative Examples C3-C4 Preparation of these examples and comparative examples was identical to Example 1, except for replacement of Aqua Poly® 250, available from Micropowder, with an oxidized polyethylene in Example 2, a copolymer of polyethylene and maleic anhydride in Example 3, and polyethylene wax in Example 4.

TABLE 1

Influence of Adhesion Promoter on Bonding

Between Epoxy and Polyethylene

Example 1 Example 2 Example C3 Example C4

99% SK-6208 99% SK-6208 99% SK-6208 99% SK-6208 99% SK-6208

1% Adhesion Aqua Poly 250 Oxidized PE* PE-Maleic Polyethylene Wax Promoting Maleated PE Anhydride Adhesion Promoter

Good Bonding Yes Yes

Bond Failure Yes Yes

^♦Oxidized polyethylene available from Aldrich, acid number = 28

Examples 5-15 Concentration Effect of Aqua-Polv® 250 (API The coated samples are subjected to the 180° peel adhesion test until a failure is achieved. The effects ofthe adhesion promoter can be seen on the interlayer adhesion. At less than about 0.5%, the adhesion is below the desired range of at least 6 N/mm, and the failure mode is consistently interfacial between the epoxy layer and the polyethylene layer as this is the weakest bond; at from 0.5% to about 5%, the failure mode is within the polyethylene layer itself, as the strength ofthe adhesion between the two layers is now stronger than the internal cohesion ofthe PE layer. As the amount of adhesion promoter exceeds 5%, the epoxy layer's adhesion to steel is reduced, and the failure mode is between the epoxy layer and the steel. TABLE 2

No. Scotchkote® -Aqua- Peel Adhesion Comments SK 6208 Poly® 250 (N/mm)

5C 100.00% 0.00% 1.00 FBEPE adhesive failure

6C 99.90% 0.10% 1.77 FBE PE adhesive failure

7C 99.80% 0.20% 2.50 FBE/PE adhesive failure

8 99.50% 0.50% 5.47 Evidence of failure in PE layer

9 99.25% 0.75% 12.40 Some cohesive failure in PE layer

10 99.00% 1.00% 14.80 Cohesive failure in PE layer

11 98.00% 2.00% 15.10 Cohesive failure in PE layer

12 96.50% 3.50% 16.00 Cohesive failure in PE layer

13 95.00% 5.00% 14.60 Some failure between FBE and steel

14 90.00% 10.00% small Failure between FBE and steel

15 98.8% 1.2% 6-10 FBE/PE adhesive and cohesive PE failure

Examples 16-24 Effect of Varying Epoxy in the Composition The data below show the effect of varying the epoxy resin in the epoxy composition. Again, every sample is subjected to 180° peel stress until a failure is achieved. For each epoxy resin, dramatic improvement in adhesion is shown when the adhesion promoter is added. In some cases, the adhesion becomes so strong that the polyethylene layer fails internally, in others the final failure is between the two layers or between the steel and the epoxy layer. In all cases, adhesion is below 1 N/mm without adhesion promoter and above 6 N/mm with addition ofthe adhesion promoter.

TABLE 3

Examples 25-29 Effect of Compounding Adhesion Promoters with FBE Formulations Examples 25-29 resulted from combining adhesion promoters with the epoxy composition in a single step using a two roll mill compounder having one roll cooled by water to between 4°C and 20°C while the other roll is heated to between 60°C and 90°C. This method allows adhesion promoters having larger particle size to be incoφorated into the epoxy coating composition. However, with this method only a small amount of adhesion promoter remains on the surface; the majority is distributed throughout the epoxy powder. As a result, most compositions made by this method do not show increased adhesion. Example 26 uses a large particle adhesion promoter; the mean particle size is greater than 100 microns; the compounder has less of an effect on such large particles and more remains on the surface to improve the adhesion. TABLE 4 Adhesion Promoters as Part of Epoxy Resin Composition

Composition 25 26 27 28 29

Epon® 2004 67.07% 67.07% 66.86% 66.86% 66.86%

Vansil® W-30 29.13% 29.13% 29.04% 29.04% 29.04%

Titanium Dioxide 0.96% 0.96% 0.96% 0.96% 0.96%

Green Toner 0.06% 0.06% 0.06% 0.06% 0.06%

Modaflow® Powder III 0.29% 0.29% 0.29% 0.29% 0.29%

2Phenyl-imidazole 0.38% 0.38% 0.38% 0.38% 0.38%

Dicyandiamide 0.96% 0.96% 0.96% 0.96% 0.96%

Aqua Poly® 250 1.15% 1.47%

ALPOL 1.15%

ALPLAC 1.47%

ALPJMA 1.47%

Peel Adhesion (N/mm) 3.94 14.90 3.62 1.68 2.71

Comment FBE/PE PE FBE/PE FBE/PE FBE/PE interface cohesive interfoce interfoce interface foilure foilure foilure foilure failure

ALPOL = Aldrich polyethylene, oxidized acid number = 28 ALPLAC = Aldrich poly(ethylene-co-acrylic acid) ALPJMA = Aldrich poly(ethylene-co maleic anhydride)

Examples 30-32 The topcoat used in all previous examples was applied as powdered polyethylene. Performance testing involved samples prepared on Q-plates as described previously.

In contrast, examples 30-32 either address performance of polyethylene films adhered to epoxy layers on Q-plates or either polyethylene film or polyethylene powder adhered to epoxy coated steel bar. The epoxy composition comprised 98.0% Scotchkote®-6208 3M epoxy and 2.0% Aqua-Poly 250. Preparation of samples 30 and 31 involved application of polyethylene film, heated to 230°C, to epoxy coated substrates while the epoxy material existed in a heated, molten condition. Steel bar, used for sample 32, received a coating of epoxy by placing the bar heated to 180- 250°C in a fiuidized bed of epoxy composition. Thereafter a similar procedure, using a polyethylene powder fiuidized bed, added a molten layer of polyethylene to the surface ofthe underlying epoxy. Sample testing occurred after the samples cooled. Samples prepared like example 31, but without Aqua-Poly 250, showed failure at the interface between polyethylene film and epoxy surface at less than 1.0 N/mm applied force.

TABLE 5 Metal Substrate and Polyethylene Topcoat Variation

No. Form of Steel PE Peel Adhesion Comment Topcoat

30 Q-Panel Film 4.0 N/mm Laminate after > 10 sees.

31 Steel Bar Film 7.8 N/mm Bars resting on hot PE film

32 Steel Bar Powder 15.7 N/mm PE cohesive foilure

Examples 33-36 Water Immersion and Cathodic Disbondment Testing Examples 33-36 provided information ofthe ability of coatings of epoxy compositions ofthe invention to survive immersion in water at 75°C and cathodic disbondment at 65°C as prescribed by standard test CSA-Z245.20-M92. Testing required sample preparation by heating steel bars to 232°C then dipping them into a fiuidized bed of epoxy composition according to Table 6. Epoxy coating thickness was between 12 and 15 mils (300 μm - 375 μm), each coating being cured for 3 minutes, at 232°C after dipping. Immersion in cold water quenched the samples after curing.

Results of hot water immersion reflect testing of duplicate samples with lower numbers preferred. For example, the CSA specification requires an adhesion rating of 3 or less after 2 days immersion at 75 °C, a cathodic disbondment radius less than 8.0 mm radius or less with conditions as indicated and no induced cracking when testing for flexibility at -30°C using 3 pipe diameter bend. TABLE 6 Functional Testing of Epoxy Coated Steel Bars

No. Scotchkote® Aqua- Hot Water Adhesion Cath. Disbondment Flexibility 6208 Poly 250 Days at 75°C Radius (mm) at 65°C

2 4 6 2 Days 7 Days

33 100% 0.0% 1,1 1,2 2,3 4.2 8.1 No crack

34 99.0% 1.0% 1,1 2,1 2,3 4.0 6.5 No crack

35 98.0% 2.0% 1,2 2,3 3,3 3.9 6.0 No crack

36 96.5% 3.5% 2,2 3,3 3,3 3.9 8.5 No crack

Example 37 A mixture of 98.8% epoxy resin Scotchkote-6208 and 1.2% Aqua Poly 250, prepared as shown in Example 1, provided the first layer, 250 μm thick (10 mils), of a metal protective composite coating. This layer formed on the bar during dipping of a grit-blasted steel bar, pre-heated to 232°C, in a fiuidized bed ofthe epoxy mixture. Immediately thereafter the heated, epoxy coated, bar received a coating of polyethylene, 250 μm - 380 μm (10 to 15 mils) thick, by immersion in a fiuidized bed of polyethylene powder (HPPC from Shaw). A post-cure process for the epoxy mixture required 3 minutes at 232°C. Thereafter the coated bar was quenched by immersion in cold water. The coated sample survived testing according to National Standards of Canada standard test method CSA- Z245.20-M92, cold bend testing at -30°C and cathodic disbondment at 65°C in 3% sodium chloride solution under a potential of 1.5 volts. Results of this testing appear in Table 7.

TABLE 7 Functional Testing of Composite Coated Samples

TEST RESULT AND COMMENT

2-day water soak at 75°C 1

7-day water soak at 75°C 1 FBE is much harder than Scotchkote®-6208 control

Cold bend at -30°C No crack or stress marks at 40X

7-day cathodic disbondment in 4.8 mm radius for blend 3% NaCl at 65°C and 1.5V (Comparison Scotchkote 6208 8.1 mm 206N-Slow 14 mm radius) Testing shows that the two-layer composite coating, comprising a first layer of a mixture of epoxy resin and adhesion promoter with a second layer of polyethylene, performed very well in test situations typical of those used to test pipeline coatings. It is considered remarkable that the testing, viewed as severe, did nothing to disrupt the bond at the interface between polyethylene and epoxy layers. This discovery effectively demonstrates achievement of multilayer protective coatings for metal with adequate interlayer bonding without a tie-layer.

Comparative Examples C38 and C39 Examples 40-51 Preparation of polypropylene containing epoxy powders

Examples 38-51 were formed using an extruder or two roll mill to mix epoxy compositions ofthe invention, combining epoxy resin with dicyandiamide, a phenolic resin, a catalyst, pigments, fillers and maleated polypropylene. The resulting material was ground to a powder having a particle size of about 44 μm.

As can be seen from the above results, compositions containing adhesion promoters according to the invention had good adhesion to the polyolefin substrates whereas the comparative examples did not adhere well to the polyolefin.

Claims

What is Claimed is:

1. A powdered epoxy resin composition that bonds both to metal and to a polyolefin comprising, a thermoset epoxy resin, and a dispersed adhesion promoter for said polyolefin.

2. A powdered epoxy resin composition according to claim 1 wherein said dispersed adhesion promoter is selected from the group consisting of oxidized polyethylenes and maleated polyethylenes, and maleated polypropylenes.

3. A two-layer composite coating system for a metal substrate comprising: a) a first layer comprising a powdered epoxy resin composition comprising a dispersed adhesion promoter capable of adhering to said metal substrate, and b) a second layer comprising a polyolefin resin.

4. A two-layer composite coating system according to claim 3 wherein said dispersed adhesion promoter is selected from the group consisting of oxidized polyethylenes, maleated polyethylenes, and maleated polypropylenes.

5. A two-layer composite coating system according to claim 4 wherein said dispersed adhesion promoter is selected from the group consisting of oxidized polyethylenes, and maleated polyethylenes.

6. A two-layer composite coating system according to claim 4 wherein said dispersed adhesion promoter comprises maleated polypropylenes.

7. A two-layer composite coating system according to claim 1 wherein said thermoset epoxy resin is formed from epoxide containing moieties selected from the group consisting of aliphatic epoxides, polyglycidyl ethers of aliphatic polymers, bisphenol A, substituted bisphenol A derivatives, bisphenol F, substituted bisphenol F derivatives and mixtures thereof.

8. A two-layer composite coating system according to claim 7 wherein said epoxy resin is formed from an epoxide containing moiety selected from the group consisting of diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, mixtures thereof, epoxy novolak, phenolic novolak and mixtures thereof.

9. A coated metal article having a two-layer composite coating system applied thereto, said layers comprising a) a fused epoxy resin underlayer, formed from a powdered epoxy resin composition comprising a dispersed adhesion promoter, and b) a polyolefin exterior layer, wherein said layers have an interfacial adhesion to said polyolefin layer of at least about 3.5 N/mm.

10. A coated metal article having a two-layer composite coating system applied thereto according to claim 9, wherein said epoxy underlayer was formed from a powdered epoxy resin comprising an adhesion promoter selected from the group consisting of oxidized polyethylenes, maleated polyethylenes, and maleated polypropylenes.