EPOXY COATING POWDERS

Abstract: Epoxy coating powders for application to substrates by fusion coating process. The resultant coating protects the substrates from corrosion when used in hostile environments. The epoxy coating powders comprise (a) unmodified epoxy resins having an average functionality greater than 3; (b) an aromatic polyfunctional amine curing agent; (c) at least 50 parts by weight of resin per hundred parts by weight of filler; and (d) a boron trichloride catalyst. The epoxy coating powders are particularly suitable for protecting oil field gear used down hole such as drill pipe, filter screens, production or gas recovery tubing and the like.

Description: Epoxy Coating Powders Technical Field This invention relates to high performance epoxy coating powders adapted for use in fusion coating processes. Coatings of these epoxy powders are highly resistant to chemical attack and are particularly well suited to extend the service life of oil field gear such as drill pipe, gas pipe, production tubing, transmission lines, sucker rods, filter screens and the like. Background Art The coating compositions of this invention are dry, free-flowing powders that may be used in fusion coating processes. "Fusion coating processes" are here defined as those coating processes in which coating powders are distributed over a substrate (which may be hot or cold) and heat, supplied from the substrate or an external source, fuses the powders into a .continuous protective film. Examples of fusion coating processes include fluidized bed, electrostatic spraying,, hot flocking (with or without electro¬ static spray), cloud .chambers, fluid transport of powder through pipe, and the like. When coating powders are based upon thermosetting resins, as is the case of the epoxy resins of this invention, sufficient heat in excess of that reσuired to fuse the powders must be available to cure the coatings and fully develop their physical and chemical properties. The coating compositions of this invention are distinguished by their unusual chemical and OMF physical properties and are especially adapted to protect substrates in demanding applications such as pipe coating for "down hole" use in the oil fields; e.g., drill pipe, filter screens, and production or gas recovery tubing. It is not uncommon to encounter temperatures as high as 200°C. and pressures exceeding 700 kg/cm 2 in deep oil wells and an effective coating must give corrosion protection in these environments from such things as acid forming gases, particularly carbon dioxide and hydrogen sulfide, corrosive drilling muds, and chemicals that may be used to free a "frozen" drill string or open pores in oil bearing strata. A good coating should also have sufficient flexibility to bend with the string during drilling and be tough enough to withstand mechanical abuse (e.g., impact and abrasion) encountered in drilling operations. Because the requirements for coatings for oil field pipe, particularly drill pipe, are so demanding, it is convenient, for purposes of exposition, to discuss the invention in this context, although it should be understood that it is not intended that the invention be so limited, as the coating compositions here disclosed will be of utility in other applications in which a combination of good chemical, physical or electrical properties is required. It has become common practice to coat drill pipe and related gear with protective polymeric coatings. When properly formulated and applied, these coatings can increase the service life of mild steel pipe many times over. The prior art has recognized that maximum corrosion resistance is obtained when coating compositions based upon phenolics or epoxy/ phenolics are used. The straight phenolic resins are superior in chemical and temperature resistance but lack in physical properties such as toughness and flexibility. Epoxy/phenolic resins, on the other hand, provide greater flexibility and toughness but they are not as chemically resistant as are straight phenolics. Phenolic and epox /phenolic coatings are usually applied as liquid coatings. Since only comparatively thin films can be applied from solution (e.g., 0.02 to 0.05 mm) at one time, a useful film thickness of from about 0.2 to 1 mm can only be achieved by performing a number of repetative steps including dipping or spraying the substrate with the liquid coating; evaporating volatile solvents or carriers; and at least partially curing the coating. Care must be exercised not to fully cure the resin until after the final coat has been applied, for other¬ wise, successive coating layers will not fuse together. More recently it has been recognized that fusion coating processes using dry powders offer a number of significant advantages over liquid systems. For example, powder coatings can be applied in film thickness of 1 mm or more in a single coating step. The energy requirements are reduced since only one bake cycle of 30 to 60 minutes is required as contrasted to multiple layer liquid coating systems which may require as many as six or more coats with each requiring an hour or c partial baking cycle. A further OMPI advantage of powder coatings ' is that they do not contain volatile solvents or carrier liquids that require costly collection and recovery systems to protect the environment. Unfortunately, it is not practical to apply phenolic based resin powders in fusion coating processes. Phenolics cure by condensation and the gaseous reaction products cause internal voids in the coating unless the coating is a relatively thin film. Epoxy resins, on the other hand, do not cure by condensation reactions, and are free from this problem. From the foregoing, it can be understood that epoxies, as compared with phenolics, have many advantageous features: they are tougher and more flexible, they do not cure by condensation reactions, they can be applied as thick films and, therefore, are suitable for use in fusion coating processes. It is. not surprising that a great deal of effort has been given to improving the chemical properties of the epoxies so that they can serve as effective replacements for phenolics in demanding applications such as drill pipe. It is known that the chemical resistance of an epoxy can be improved by increasing its functionality which results in a higher cross link density and a higher glass transition temperature of the cured resin. For example, U.S. patent 4,122,060 discloses an epichlorohydrin- bisphenol-A type resin that is modified by the inclusion of a minor amount of an epoxidized novolac resin having a functionality greater than 2. This coating is adapted for rapid cures _OMPI (e.g., less than two minutes ' ) by the use of bif nctional hydroxyl terminated curing agents. Coating powders such as disclosed in this patent are much improved in chemical resistance when compared with straight epichlorohydrin-bisphenol-A resins but fall considerably short of meeting the demands imposed, for example, upon drill pipe coatings. It is also known that the chemical resistance of epoxy resins can be further improved by increasing the amount of resins used having functionalities greater than 2. This gives rise to other problems since the use of the higher functionality resins in substantial quantities adversely effects the flexibility of the cured coating and its ability to adhere to a substrate. Loss of flexibility (embrittlement) results from the higher cross-link density. Poor adhesion is believed caused by the higher melt viscosity and faster curing times of high functionality resins which combine to reduce flow and prevent the melted resins from effectively wetting out the substrate. Disclosure of Invention Accordingly, it is an object of this invention to formulate epoxy coating powders which yield coatings having improved chemical resistance at elevated temperatures and pressures. Another object of this invention is to provide epoxy coating powders that will form adherent chemically resistant protective films over substrates Another object of this invention is to provide adherent, chemically resistant epoxy coatings that O PI have high cross-link densities, high glass transition temperatures and moderately good toughness and flexibility. A specific object of this invention is to provide coating powders that, when applied in fusion coating processes, will materially extend the service life of metal gear used in the oil fields, including such things as flood pipe, recovery pipe, drill pipe, production tubing, sucker rods and filter screens. These and other objects of this invention are achieved by formulating coating powders comprised of: 1. An unmodified epoxy resin or a mixture of epoxy resins having an average functionality of greater than 3 and more preferably greater than 4; 2. An aromatic polyfunctional amine curing agent; 3. At least 50 parts by weight per hundred parts of resin of a mineral filler; and 4. A boron trichloride catalyst. In addition to the foregoing ingredients, other materials such as pigments, flow promoters, and dry blend additives to ' improve powder fluidity may be added as is conventional and known to the prior art. In a preferred embodiment of this invention, minor amounts (e.g., less than 5 weight percent) of hydrophobic silica aerogels are included in the coating powders as described more fully in copending application Serial No. 107,019 filed on December 26, 1979, in the United States of America. OMFI BEST MODE FOR CARRYING OUT THE INVENTION A. The Composite System - Gel Time and Inclined Plate Flow If one was concerned only with the chemical properties of an epoxy coating powder, one would merely select a resin of the highest functionality available and so maximize the cross-link density. But to be useful in fusion coating processes, coating powders must also be formulated so that they will fuse into smooth continuous coatings and have useful physical properties. To achieve good overall performance, a number of compromises are often made in selecting the individual components of a. formulation. For example, balances must be established between such ihings as the f nctionality, molecular weight and melt viscosity " of the epoxy resin; the reactivity, functionality and chemical stability of the curing agent; the amount, particle size, surface area, surface treatment and chemical compatibility of the fillers; and the shelf life, reactivity and overall effect of a catalyst on the system. An essential property that must be considered *** when formulating a coating powder is the ability of the powder to fuse into a uniform, continuous and void free film. As a guide to formulation chemists, two relatively simple test procedures have been established to measure the ability of a coating powder to fuse over a substrate. One of these is gel time which provides a measure of the reactivity of a given system; and the other is inclined plate flow which is a combined measure of both the reactivity and melt viscosity of the coating powder. ASTM Specification D-3451 (14) defines a procedure for measuring gel time in which a piece of aluminum foil is placed on a hot plate and heated to a given temperature, e.g. 190°C. A small quantity of powder is dropped onto the heated foil and stroked with a tongue depressor until continuous and readily breakable filaments are formed when the depressor is lifted from the foil. The elapsed time for this to occur is measured in seconds and is the gel time. The Inclined Plate Flow Test is defined in ASTM D-3451 (17) . In this test a small pellet of powder is placed on a glass or tin plate and inserted into a heated oven, e.g. 150°C. After the pellets and the plate have reached temperature equilibrium, the oven rack on which the plate rests is tilted to a 65° angle without opening the oven. After 30 minutes, the plate is removed from the oven, allowed to cool to room temperature and the length of the flow measured in millimeters . -The distance the coating flows is dependent on the initial melt viscosity, rate of reaction, and the temperature at which the test is conducted. If the flow is too great, the coating may be expected to run and sag on a substrate; on the other hand, if the flow is too small, a rough, discontinuous, nonadherent film will probably result. By and large, as a general guide to formulating the coating powders of this invention, -g- acceptable gel times are between 10 to 300 seconds when measured at 205°C, and useful inclined plate flows are bewteen 15mm and 100mm when measured at 150°C. It is generally true that the gel time and the inclined plate flow decrease as the functionality of an epoxy resin increases. It is also true that the gel time and the inclined plate flow increase with a decrease in molecular weight and functionality of the epoxy resin. By judiciously balancing the amount of a high functionality resin used with the amount of a low functionality, low molecular weight resin used, a formulating chemist can arrive at coating powder that has a useful gel time and .. inclined plate flow. Further control over the gel time and the inclined plate flow can be achieved in the selection of hardening agents and catalysts of varying reactivity. Fillers can also play an important role in determining the melt viscosity and inclined plate flow of the coating material. Fillers generally increase the viscosity to a variable extent depending on the amount used, the particle size, the surface area and the surface chemistry of the filler. B. The Epoxy Resin To obtain the desired degree of chemical resistance, the average functionality of the epoxy resins useful in the practice of this invention should be at least 3, and, preferably, at least 4. A preferred class epoxy resins is the epoxy novolacs, which include both epoxy OMPI phenol novolacs and the epoxy cresol novolacs. These may be represented by the following general formula: in which n may be from 1 to abqut 5 or more, the epoxy functionality is n + 2 and y is 0 for the epoxy phenol novolac and 1 for the epoxy cresol no olac. Examples of other epoxies having functionalities of three or more include triglycidal p-aminophenol, triglycidal isocyanurate, tetra-p-hydroxyphenol- polyglycidal ether and the tetra functional epoxy based upon the reaction of ethylene dianaline with epichlorohydrin. The functionality of these resins makes them quite reactive and sufficient gel time and inclined plate flow may not be provided to yield satisfactory coatings. In this case it is useful to blend them with lower molecular weight, lower functionality (less than 2) epoxies such as the reaction products of epichlorohydrin and bisphenol-A having epoxide equivalent weights of less than 1,000 and Gardner- Holdt values in a range of G through V, as measured in a 40% solution of butyl carbitol. ' OM In the preferred practice of this invention, at least 50 parts per 100 parts of resin (herein¬ after phr) of an epoxy resin having a functionality of greater than 4 are mixed with not to exceed 50 phr of an epoxy resin having a functionality of 2 or less to provide a composite resin having an epoxy functionality of at least 3 and, more preferably, at least 4. For instance, in Example I, 75 phr of a resin with an epoxy functionality of 5.4 is mixed with 25 pph of a resin having an epoxy functionality of about 2 to yield a resin having an average functionality of 4.55. C. The Curing Agents The preferred curing agents for use in this invention are polyfunctional aromatic amines and particularly diamino diphenyl sulfone and bis (diaminophenyl) sulfone. The reasons for their selection are several fold and the most important one is that they cure at comparatively slow rates. The rapid rate at which high functionality epoxy novolac resins cure dictate the use of a slow curing agent in order to obtain long enough gel ' times and inclined plate flows. A second advantage to the use of these curing agents is that they are polyfunctional and will increase the cross link density of the coating. Lastly, it is believed that the chemical resistance of the coating is improved by the introduction of additional aromatic groups. The amount of curing agent used is not extremely critical but generally it is useful to approximate a stoichometric quantity based on the functionality of the epoxy resin to a tolerance of about + 20%. D. The Fillers Fillers are commonly included in coating powders to decrease raw material costs and to improve certain physical properties such as hardness and abrasion resistance. It has been believed by those skilled in the art that the use of fillers detracts from the chemical resistance of a pure epoxy coating, since fillers are generally more readily attacked by chemicals, particularly-strong acides, than are cured epoxies. It has now been discovered, -contrary to this conventional wisdom, that the performance of epoxy coatings, when exposed to corrosive environments at elevated temperatures and temperatures, can materially be increased through the use of selected mineral fillers in substantial amounts, i.e., at least 50 phr. As will be seen from the examples, filler loadings as high as 200 phr have proved quite useful and, it is believed, even higher levels can be used. The theoretical upper limit of the filler loading in a coating powder is the critical pigment volume concentration at which point there is no longer enough resin to wet out the pigment completely. Many suitable fillers are well known to the art, of which some of the more common are silica, barium sulphate, calcium carbonate, aluminum OMP silicate, calcium silicate, mica, and the like. The fillers should be in finely divided form, and, preferably their particle size should not exceed 10 microns. E. The Catalysts. It is not always necessary to include a catalyst in an epoxy coating composition if the reactivity of the epoxy resin and the curing agent is otherwise satisfactory. It is somewhat - of anomaly that the coating powders of the herein Examples which contain over 50 phr of a high functionality epoxy novalac resin, require a curing agent such as the dia ino diphenyl sulfone to slow down the reaction down, but, when this is done, the reaction rate becomes too slow, and a correction in the other direction must be made by adding a catalyst. One catalyst that has been found to be particularly effective in this system is a boron trichloride amine complex sold under the trade designation XU-213 by Ciba-Geigy. Only small amounts of less than 1% by weight are required. F. Other Ingredients. When coating powders are used to coat articles having sharp edges, a better edge coverage is obtained when a small amount, e.g. less than 2% by weight, of a material having a high surface area is dry blended into the coating powder. For example, fumed silica having an average diameter of 7 to 14 millimicrons and a surface area of between 200 and 400 square meters per gram is suitable, such as Cab-O-Sil® sold by the Cabot ,Corporation. The coating powders of this invention can be pigmented as desired, and any conventional inorganic, organic, or filler pigments can be used. The amounts are not critical other than they should be present in a sufficient amount to provide the desired color density. Flow control agents can be usefully included in the coating powders in amounts up to 2% by weight. Typical flow control agents are polyalkylacrylates in which the alkyl group contains between 2 and 8 carbon atoms, such as ModaFlow® sold by Monsanto. As previously mentioned, it is believed that hydrophobic silicas may contribute useful chemical properties and are included in preferred formulations. Example A coating powder was prepared in accordance with this invention by initially blending the following ingredients: Material phr Epoxy cresol novolac - 75 f nctionality 5.4 (Ciba-Geigy ECN 1299) Epoxy - epichlorohydrin-bisphenol- 25 A epoxide equivalent weight 875-1025; Gardner-Holdt viscosity R-U; functionality 2 (Ciba-Giegy Araldite 6084) Diaminodiphenyl sulfone - curing 20 agent (Ciba-Giegy Eporal) • Rl OMPI Material phr 4. BCI3 - amine complex - 1 catalyst (Ciba-Giegy XU-213) 5. CaSi0 3 - filler (Nyco Nycore 300) 125 6. iθ2 - pigment 10 7. Green Cr 2 θ3 - pigment 20 8. Hydrophobic silica aerogel 1 (Tulco, Inc. - Tullanox 500) The above materials were melt-mixed in an extruder and the extrudate was chopped and ground to a fine powder that all passed through a 60 mesh screen. 0.67 phr of an untreated hydrophilic silica aerogel was dry blended to promote powder flow. Steel test panels about 0.5 x 5 x 10 cm. were sand blasted, deburred, cleaned, degreased and primed. The panels were preheated in an oven to 190°C and then immersed in a fluidized bed of the above prepared coating powder. After a 5 second dip in the fluidized powder, the panels were postheated in an oven for 60 minutes at 220°C. Smooth, uniform coatings of approximately 10 mils were obtained. The coated panels were evaluated in autoclave tests in which the panels were immersed to 2/3 of their length in a given test solution and the solution was then purged with hydrogen sulfide. After the autoclave was sealed, it was charged with 140 psi of CO2 and the pressure was increased to 2,800 psi with air. The temperature was raised to 300°F. and these conditions were maintained for OMPI 24 hours. The test solutions used were four drilling muds including weighted lignσsulfate, Baroid K Plus (TM N. L. Baroid) , salt water polymer and fresh water polymer. In these tests, the coatings were found to be superior to the industry standards for coatings for oil field drill pipe applied from previously known epoxy powder, liquid phenolics and liquid epox /phenolics. The coated panels were bent and impacted and were found to have sufficient strength and toughness to withstand the abuse given drill pipe. The gel time of the coating powder was about 60 seconds when measured at 205°C and the inclined plate flow was about 85 mm when measured at 150°C. Two additional coating powders were prepared based upon the above formulation except in the first case 50 phr of both epoxy resins 1 and 2 were used and in the second case 90 phr of resin 1, 10 phr of resin 2 and 200 phr filler 5 were used. In both instances, the coatings applied to steel panels as above described were satisfactory, however, when tested in the autoclave the formulation having only 50 phr of the epoxy novolac did not perform as well as the one with 75 phr eopxy novolac and the formulation having 90 phr epoxy novolac out-performed the one with only 75 phr epoxy novolac.

Claims: Clai s 1. Epoxy coating powders adapted for application in fusion coating processes comprising a. unmodified epoxy resins having an average functionality greater than 3; b. an aromatic polyfunctional amine curing agent; c. at least 50 parts by wexght per hundred parts of resin of a finely divided filler; and d. a boron trichloride catalyst. 2. A coating powder according to Claim 1 wherein the epoxy resins include an epoxy novolac. 3. A coating powder according to Claim 3 wherein the epoxy novolac is present in an amount of at least 50 parts by weight per hundred parts epoxy resin. 4. A coating powder according to Claim 3 wherein the the epoxy novolac is present in amounts of at least 75 parts by weight per hundred parts epoxy resin. 5. A coating powder accorting to Claims 3 or 4 wherein the epoxy resins include, in addition to an epoxy-novolac, a resin which is a reaction product of epichlorohydrin and bisphenol-A. 6. A coating powder according to Claim 5 wherein the reaction products of epichlorohydrin and bisphenol-A have average epoxide equivalent weights of less than about 1,000 and Gardner Holdt bubble viscosities in a range of G through V as measured in a 40% solution of butyl carbitol. 7. A coating powder according to Claim 1 wherein the curing agent is diaminodiphenyl sulfone or bis (diaminophenyl) sulfone. 8. A coating powder according to Claim 1 wherein the filler is a mineral filler having a particle size less than 10 microns. 9. A coating powder according to Claim 1 in which the filler is present in an amount exceeding 100 parts by weight per hundred parts resin. 10. A coating powder according to Claim 1 in which the filler is present in an amount of at least 200 parts by weight per hundred parts resin. 11. A coating powder according to Claim 1 wherein the catalyst is a boron trichloride amine complex. 12. A coating powder according to Claim 1 wherein the gel time of the powders is between 10 seconds and 300 seconds when measured at 205°C. 13. A coating powder according to Claim 1 wherein the inclined plate flow of the powders is between 15 mm and 100 mm when measured at 150°C. 14. A coating powder according to Claim 1 additionally including up to 5 weight percent of a hydrophobic silicon aerogel. 15. A metal pipe which is protected from attack in corrosive environments characterized in that the pipe is coated with the coating powders of Claim 1. 16. A method of protecting metal pipe from corrosion which comprises coating the pipe in a fusion coating process using the coating powders of Claim 1.

Inventors: HOUSENICK J, RICHART D
Applicants: HOUSENICK J, POLYMER CORP, RICHART D
Application year: 1980
Main classification: C08L63
IPC codes: C08G59, C08L63, C09D5, C09D163