MXPA00000221A - Method for application of protective polymer coating - Google Patents

Abstract

A process for application of a protective coating to a steel, concrete, or wooden structure so as to provide protection against corrosion, weathering, or other environmental damage in which the surface to be protected is heated to a temperature in a range of about 75 DEG F. to about 150 DEG F. after which a liquid thermoset primer is applied to the heated material in two stages. The first portion is solidified by heating and then coated with a second portion, forming an uncured liquid thermoset outer primer layer. A melted polymer powder layer is then applied by flamespraying over the uncured liquid thermoset primer layer, forming an intermediate polymer powder layer embedded in the uncured liquid thermoset primer layer. The intermediate melted polymer powder layer is then heated to a flow temperature of the polymer powder and a second layer of melted polymer powder is applied over the intermediate polymer powder layer, forming an outer melted polymer powder layer which then cools to form the final protective coating

Description

METHOD FOR APPLICATION OF COATING POLYMER PROTECTOR Field of the Invention This invention relates to a method for applying polymer coatings to large substrate materials including steel, concrete, or wood structures for protection against corrosion, weathering, or other environmental damage. Included within the structures to which the method of this invention may be applied are buried steel pipe lines, for example, in the transmission and distribution of natural gas and oil. The method of this invention is particularly suitable for "in the field" applications and applications where the temperature maintenance of the substrate material to which the coatings are applied under. a level at which the integrity of the substrate material is essential is affected or a potentially hazardous condition is created.

Description of the Prior Art Protective coatings are used extensively to protect metal substrates, such as steel pipes and pipe lines, from corrosion and mechanical damage. Commercially available coatings broadly used for such substrates include epoxy-bonded epoxy (FBE) coatings. In the United States, FBE coatings are especially popular for pipe line protection due to their excellent anticorrosion properties, good adhesion to metal surfaces, and resistance to cathodic separation of the metal substrate. However, when used alone, Xos FBE coatings are prone to handling damage during pipe installation, and also exhibit relatively high moisture permeation. In this way, most coatings' of. FBE currently applied, especially in Europe, are an integral part of three-layer systems consisting of an epoxy primer (mainly FBE), a plastic copolymer adhesive, and an outer, plastic (polyolefin) cover for protection of the epoxy primer. The basic principle in three-layer systems is the use of a medium layer of adhesive to provide the bonding agent between the epoxy primer and the outer layer of plastic (polyolefin). Polyolefins are preferred for use as a protective layer because they have many of the missing qualities in isolated melt bonded epoxy coatings, such as superior impact strength, as well as improved waterproofing to moisture and many chemicals. Polyolefins are also easy to manufacture for coatings applied in the plant. However, due to their non-polarity, the polyol-olefins bind loosely to metal substrates. Still the use of adhesives, such as copolymers, in linking the polyolefin to the metal substrate has not been found to provide a coating with properties equal to the epoxy / metal bond in terms of resistance to hot water immersion and cathodic outcome. Another disadvantage of these systems, particularly when used in "field" applications in steel pipe lines, is the time that the preheating consumes up to 232aC and the number of different materials and application media required to apply the coating layer. In "field" applications, it is highly desirable to minimize the amount of equipment and the number of different materials to be applied. Other coating systems known to provide protection against both corrosion and chemical attack include fluoroplastic coatings that provide excellent protection against chemicals and are not attacked by either acids. strong or solvent. In addition to their well-known mechanical properties, such as high abrasion resistance and good elasticity, the thermal properties of fluoroplastics also allows them to be used just as they are, even when prolonged exposure is involved at temperatures up to 127SC. However, like other plastics, fluoroplastics exhibit both low adhesion to steel surfaces and permeability to gases, liquids and solutions, thus necessitating the application of relatively thick layers. A process for powder coating high temperature resistant surfaces with multilayer coatings of fluofaplastics is taught in the U.S. Patent. 4,999,221. The Patent of E.U.A. 4,510,007 teaches a method for coating steel pipes in which the pipe is heated to a temperature sufficient to cause a mixture of epoxy resin-curing agent powder subsequently applied to melt after which a double metal sheet of tubular configuration similar to The wood is extruded onto the previously coated object under the condition that the ethylene copolymer portion of the double or twin layer hose has been previously dried, and under the additional assumption that the temperature of. extrusion particularly of the external thermoplastic hose is on the scale of approximately 165aC to 1909C. - The implementation of this method requires preheating the steel pipes to a temperature between approximately 175 ° C and 275 ° C in order to ensure the fusion of the pulverized epoxy resin-curing agent powder mixture. A problem with this method is that temperatures on the required scale are difficult, if not impossible, to achieve, for example, in underground pipe lines in situ. Additionally, these temperatures are sufficiently high so that the integrity of any internal surface treatment, for example, internal piping coatings, could be compromised. The Patent of E.U.A. 4,345,004 teaches a process for forming an olefin resin film on a metal substrate comprising forming a multilayer coated film consisting of "an olefinic resin film as a surface layer portion and an epoxy resin film cured as a portion of the underlying layer in a metallic substrate by means of a single coating operation using a multilayer film-forming coating composition comprising as main resinous components a solid powder containing one. olefinic resin having a melt index, from 0.3 to 80 grams per 10 minutes, a solid powder containing a modified olefinic resin containing polar group having a melt index of 0.3 to 80 grams per 10 minutes, and a resinous material film forming comprising an epoxy resin having an average molecular weight number of about 350 to 4,000 and an equivalent equipment of 150 to 3,800 and a curing agent therefor, and then functionally bonding a coating material of olefinic resin to the surface layer of olefinic resin of the multilayer coated film. See also the U.S. Patent. No. 5,178,902 which teaches a method for applying and forming a protective composite coating on a metal substrate on which the substrate is heated. a temperature between about 175aC and 275eC and a spray coating of epoxy resin between 100 and 400 microns thick is applied to the outer surface of the heated substrate. A preblended powder coating of epoxy resin and polyolefin is applied directly to the epoxy resin coating, which forms an intermediate layer of interdispersed epoxy and polyolefin domains between about 100 and 400 microns thick. On .

In this case, the pulverized polyolefin is sprayed to produce a polyolefin sheath coating for the metallic substrate between 200 and 1,000 microns. esp & sor. According to one embodiment, the intermediate layer is formed by spraying pure epoxy resin powder and polyolefin powder from separate sources simultaneously onto the substrate. The application of a coating to a metal substrate previously heated to a temperature between about 71SC and about 116SC is often carried out by flame spray where a stream of finely divided, pneumatically transported thermoplastic material is driven through of a-. Flame towards the substrate surface to be coated. The thermoplastic material is melted by the heat of the flame and is deposited on the substrate surface where it cools and hardens to form a protective coating. Flame spray guns and processes employing flame spray are well known in the art. See, for example, the U.S. Patent. 5,211,990; Patent of E.U.A. 4, '962, 137; Patent of E.U.A. 5,041,713; Patent 'of E.U.A. 3,988,288; Patent of E.U.A. 4,985,278; and Patent of E.U.A. 4,276,390.

Russian Patent 407753 teaches a method for producing polymer coatings from thermoplastic powder materials wherein a heat-resistant, thermosetting resin based liquid primer is applied to a substrate and a thin layer of heated thermoplastic polymer powder it is sprayed on the unhardened sticky primer. After the primer has hardened at room temperature, the surface layer of the coating is heat treated, and additional layers of molten thermoplastic material are applied.A similar method for pipe line repair is taught in US Patent 5,792,518 A problem associated with the methods taught by both of these patents is raised by the requirement that the thin layer of thermoplastic polymer powder be applied to the primer layer before it has had an opportunity to harden. As a thermoplastic polymer, it is not uncommon for the uncured primer surface to be interrupted by pressure spraying resulting in the formation of air cavities within the primer that significantly reduce the overall strength and integrity of the primer layer. with the methods taught by these patents is the requirement that the.

Resin-based, thermally resistant, thermosetting resin primer hardens at room temperature before the surface layer of the polymer powder coating can be applied, making it in this manner unattractive for use in the field where it is not desirable to have workers waiting unproductively for hardening to occur, which, at lower ambient temperatures, could be for extended periods of time. The methods taught by these patents relate to the requirement that the resin-based, heat-resistant, thermosetting primer be applied at ambient temperatures as opposed to elevated temperatures.The flowability of the primer is substantially retarded at ambient temperatures making it difficult to Even another problem associated with conventional methods for coating steel, concrete or wood structures is related to the moie disposition between the structure and the coating, for example, the underground pipe lines, due to the temperature of the fluid that it flows through the same s, usually "sweat" when exposed to ambient temperatures. The application of protective coatings by conventional means results in some water that. is trapped between the protective coating and. the structure, thus negatively affecting the integrity of the interface between the protective coating and the structure to be protected.

SUMMARY OF THE INVENTION Accordingly, an object of this invention is to provide a process for applying protective polymer coatings to steel, concrete or wood structures using as a starting layer a liquid thermosetting primer, whose method overcomes the fluidity and time problems of Prolonged curing associated with the application of the primer to ambient temperatures Another object of this invention is to provide a method for applying polymer coatings to steel, concrete, or wood structures, which avoids the use of solvent based paints, the solvents which can negatively impact the integrity of the desired coating as a result of its volatilization during drying, and drastically shorten the time required to reach a finished state Still another object of this invention is to provide a process for applying a coating . protector to structures of steel, concrete or wood that can be carried out in situ. Still another object of this invention is to provide a method for applying a protective coating to buried steel pipe lines which are subsequently subjected to cathode protection voltages. These and other objects of this invention are achieved by a process for application of a protective coating to a substrate material in which a first portion of a liquid thermosetting primer is applied to the substrate and forced material, to be almost completely cured. In accordance with a particularly preferred embodiment of this invention, the substrate material is heated to a temperature on a scale of about 24aC to about 66SC. The heating of the substrate material in this manner not only improves the fluidity of the liquid thermosetting layer, but also evaporates any condensation present on the substrate surface. In fact, if the surface temperature of the substrate material is at or near the nebulization point of the surrounding air, the heating of the substrate material is required to eliminate the condensation. After the substantial curing of the first portion of thermosetting primer. liquid, a second portion of the liquid thermosetting primer is applied to the substrate material on. the partially cured liquid thermosetting primer, forming an uncured thermosetting primer layer. A molten polymer powder is then applied onto the uncured liquid thermosetting primer layer, forming a layer of intermediate molten polymer powder partially embedded in the uncured liquid thermosetting layer. The intermediate polymer powder layer is then heated to a flow temperature of the polymer powder and a second layer of molten polymer powder is applied thereon. cap . of molten polymer powder intermediate, forming an outer molten polymer powder layer. In accordance with a particularly preferred embodiment of this invention, the intermediate molten polymer powder layer and the outer molten polymer powder layer are applied by flame spray.

DESCRIPTION OF PREFERRED MODALITIES The method of this invention - produces coatings d? polymer on steel, concrete, or wood structures using powder thermoplastics, for example, polyolefin and liquid thermosetting primers, for example, epoxies. According to a particularly preferred embodiment of the method of this invention, the substrate material to which the coating d? The polymer is to be applied clean and then heated to a temperature on the scale of about 24aC to about 66SC after which a liquid thermosetting primer is applied., using a brush, roller or other appropriate device. The liquid thermosetting primers, such as those contemplated for use in the process of this invention, are typically cured at room temperature, even when curing can be accelerated by heating the thermosetting material. In accordance with a preferred embodiment of this invention, the first portion of liquid thermosetting primer is maintained at a temperature between about 66 ° C. and about 121 ° C. until it starts to cure. The curing of the thermosetting material results in thickening and, finally, hardening of the thermosetting material. As the material thickens, its fluidity becomes virtually non-existent. This condition is easily recognized because an object brought into contact with the thermosetting material will not adhere thereto. After the substantial curing of the first ..

In the liquid thermosetting primer portion, a second portion of liquid primer is applied to the substrate material on the cured liquid thermosetting primer, resulting in a total thermosetting primer layer thickness in the range of about 0.05 to about 0.51 millimeters. This second portion of liquid thermosetting primer layer, while still in a relatively liquid uncured state, is immediately coated with a thin layer of a partially melted thermoplastic powder by flame spray, producing a first layer of molten polymer powder in The scale is approximately 0.025 to 0.13 millimeters thick and embedded in the liquid primer. Before the thermosetting liquid primer has had time to cure, the first layer of thermoplastic powder is heated to a powder flow temperature, typically in the range of about 121aC to about 232aC, and an additional layer of molten thermoplastic powder it is applied on the first layer of thermoplastic powder by means of flame spray, producing an outer layer having a thickness in the scale of about 0.25 to about 2.03 millimeters. As indicated above, the good link. between the thermosetting primer and the partially melted thermoplastic powder requires that - the top portion of the thermo? ndur? cibl primer? be in a liquid form, substantially not hardened. However, the application of the partially melted thermoplastic powder to a thin layer of liquid, unhardened thermosetting primer causes the particles, as well as part of the driving gas stream that traps the molten powder, for example in the case of Flame spray, combustion products, flow into the thin layer of thermosetting primer, resulting in bubble formation therein and generally decreasing the structural integrity of the thermosetting primer layer. Consequently, in order to ensure the structural integrity of the thermosetting primer layer near the surface of the substrate and at the same time ensure the good bond d? the thermoplastic powder layer to the thermosetting primer layer, it is essential that the liquid thermosetting primer layer is applied to the substrate material in two stages as discussed above. That is, the first portion of the liquid thermosetting primer must be so well cured that the drive gas stream that traps the particles does not penetrate the first primer layer. thermosetting, but it will embed the molten powder in the second layer of thermosetting primer. Consequently, the distinctive features of the preferred modality of method d? This invention is the preheating of the substrate material to which the coating is to be applied which produces a more uniform and continuous thermosetting primer layer adjacent to the surface of the substrate material, the application of the liquid thermosetting primer layer to the substrate. two stages, and the immediate application of a thin layer of a partially melted thermoplastic powder to the uncured surface of the second portion of the thermosetting primer layer which serves as a transition zone between the thermosetting primer layer and a layer externally applied of molten powder. . The immediate application of the intermediate layer of thermoplastic powder improves the bonding of the powder layer to the thermosetting primer layer, the liquid thermosettable primers suitable for use in the process d? This invention comprises liquid resins selected from the group consisting of epoxy resins, urethane resins, and mixtures thereof, and curing agents, the resin and the curing agent being mixed in a ratio of about 1 to 1 to a ratio of approximately 5 to 1 in weight, respectively. In order to avoid the formation of bubbles of the primer layer during curing and the applion of plastic flame coating, the liquid thermosettable primers suitable for use in the process of this invention do not contain solvents. By the term "solvent" is meant any material that would evaporate during the curing and spraying by flame, resulting in the formation of bubbles within the primer layer. The top coating materials suitable for overcoating the uncured portion of the thermosetting primer layer include thermoplastic powders based on polyethylene or polypropylene. The key to making a strong adhesive bond between the liquid thermosetting primer layer and the thermoplastic layers is the applion of the partially molten transition powder cap to the portion of the uncured liquid thermosetting epoxy resin that is applied over the first portion. This can only be achieved using a flame spray technique. The applion of cold thermoplastic powder to a layer of thermosetting primer? uncured liquid, without the benefit of partial melting in a flame spray gun and the subsequent incrustation of the molten powder in the uncured liquid thermosetting primer layer, does not result in a good bond between the layers.

EXAMPLE This example is directed to the application of the method of this invention in a line of natural gas pipeline in service. To ensure good adhesion of the coating layers to the pipe, the surface of the pipe is cleaned, preferably by sandblasting, to a profile of 0.08 millimeters. Five (5) parts of Hempel 436US epoxy and one (1) part of Hempel 981US curing agent (both available from Hempel Coatings, Inc., of Houston, Texas) are mixed for three minutes, forming an appropriate liquid thermosetting primer. Approximately, 31.10 grams of the primer per 645.16 square centimeters of surface to be coated, to achieve a thickness of approximately 0.20 millimeters, are required. The portion of tubing to be coated is heated to a temperature between about 38aC and 43SC and held at this temperature for about 2 minutes. After the two minute heat holding period, the first portion of the liquid thermosetting primer layer is applied to the surface of the heated pipe. While the ST liquid thermosetting primer is applying-, it is important that the temperature of the uncoated portions d? The pipe surface that is still to be coated is maintained at the elevated temperature. After the application of the first portion of thermosetting primer? Full ST liquid, the surface temperature is gradually raised to a temperature of about 93 ° C and maintained at that temperature for about four minutes, the temperature then rises to about 1212 ° C and is maintained at that temperature for about three minutes . At this point, the first "" portion of liquid thermosetting primer must be solidifying. The surface temperature is allowed to cool to 43 ° C and a second portion of liquid thermosetting primer is applied. Immediately after d? the application of the second portion d? thermosetting primer '. liquid, while the upper portion is still in a liquid, uncured state, ST pulverizes by flame a thin layer of partially melted thermoplastic powder on the uncured portion of the thermosetting primer. The temperature of the coating system is raised, as necessary, to approximately 66SC and ST maintained at that temperature for approximately three minutes. then, the temperature of the ST system rises to the melting point of the polymer powder (about 1492C to about 232BC, depending on the powder being applied) at which temperature the additional thickness of the molten thermoplastic powder is sprayed by flame about the first layer d? thermoplastic powder Powders d? The polymer that has been found to be suitable for use in the process of this invention are GUARDIAN XLS, 35-15 and ET-20, available from PFS, Inc., and Eutectic Company. Although in the above specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional modalities. and "that certain of the details described herein may be varied considerably without departing from the basic principles of the invention.

Claims (6)

1. - A process for applying a protective coating to a substrate material, comprising: applying a first portion of a liquid thermosetting primer to a substrate material; solidifying the first portion of the liquid thermosetting primer; applying a second portion of the liquid thermosetting primer to the substrate material on the solidified liquid thermosetting primer, forming an uncured liquid thermosetting primer layer; Apply a powder d? molten polymer on the uncured liquid thermosetting primer layer, forming an intermediate polymer powder layer embedded in the uncured liquid thermosetting primer layer; heating the intermediate polymer powder layer to a flow temperature of the polymer powder; applying a second layer of the molten polymer powder on the intermediate polymer powder layer, forming an outer molten polymer powder layer; and cooling the outer molten polymer powder layer, forming a final coating layer.

2. A process according to claim 1, wherein the substrate is heated to a temperature on a scale of about 24SC to about 66eC before? of the application of the liquid thermosetting primer.

3. A process according to claim 1, wherein the substrate material is selected from the group consisting of metal, concrete and wood.

4. A process according to claim 1, wherein the powder layer, intermediate molten polymer and the outer molten polymer powder layers are applied by spraying. by flame.

5. A process according to claim 1, wherein the thermosetting primer comprises a diluent. 6.- A process of conformity with. Claim 1, wherein the liquid thermosetting primer is applied to the substrate by one of pouring, dispersing, brushing and spraying. 7, - A process according to claim 1, wherein the liquid thermosetting primer layer has a thickness on the scale of about 0.05 to about 1.02 millimeters. 8.- A process of compliance with. claim 1, wherein the intermediate polymer powder layer has a thickness in the range of about 0.02 to about 0.13 millimeters, 9. A process according to claim 1, wherein the outer molten polymer powder layer It has a thickness on the scale of about 0.25 to about 2.03 millimeters.