RU2723174C1 - Method of applying polytetrafluoroethylene coating on heat exchange elements - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention relates to methods of producing protective coatings on metal surfaces. Described is a method of applying a coating of polytetrafluoroethylene on heat exchange elements, which consists in degreasing the surface of a heat exchange element, drying it, then forming a protective layer by mechanical rubbing of the surface of the heat exchange element, at least in two directions, powdered polytetrafluoroethylene with particle size of up to 2 mcm, followed by annealing at temperature of 240–260 °C before fusion of polytetrafluoroethylene nanoparticles.EFFECT: technical result is improved anti-adhesion properties of heat exchange elements.4 cl, 2 tbl

Description

The invention relates to methods for producing protective coatings on metal surfaces and can be used to protect products and structures in contact with a medium containing corrosive ions, for example, in the chemical industry, in the food industry, in a marine climate, etc.

Polytetrafluoroethylene (Teflon, Fluoroplast, Fluorlon) is widely known - synthetic polymer, tetrafluoroethylene polymerization product, white solid. In polytetrafluoroethylene, only fluorine atoms are attached to the carbon skeleton, and the fluorine-carbon bond is one of the strongest, so the polymer has a combination of unique chemical and physical properties that cannot be found in any other material.

Due to its high heat, frost and chemical resistance, antifriction, anti-adhesive and exceptional dielectric properties, fluoroplast-4 is widely used as an anti-corrosion material in the chemical industry for the manufacture of apparatus, elements of distillation columns, heat exchangers, pumps, pipes, valves, cladding tiles, stuffing box packing and etc.

A known method of modifying the surface of metals and imparting functional properties to it by applying a superhydrophobic composite coating using electrochemical deposition, including the preparation of an aqueous suspension containing granules of polytetrafluoroethylene (PTFE), cetyl trimethylammonium bromide and fluorocarbon non-ionic surfactant FC-4430, obtain mixed electrolyte by introducing the prepared suspension into a nickel electrolyte with thorough stirring and electrophoresis-electrodeposition in a mixed electrolyte without stirring for 10-15 minutes, using a metal conductive surface as a cathode and a nickel plate as an anode. Further, in accordance with the method, the electrophoresis-electrodeposition process is stopped, the electrolyte is stirred for 1-2 minutes, the process is resumed and continued without interruption until the required coating thickness is achieved. The formed coating is washed and dried, heated at a temperature of 260-300 ° C for 30-60 minutes and cooled at room temperature (Chinese application No. 103526268, IPC C25D15 / 02; C25D5 / 50 (2006.01), 2014).

The disadvantage of this method is the complexity of its implementation. At the same time, coatings obtained in a known manner exhibit insufficiently high adhesive strength, which noticeably worsens over time.

There is a method of producing protective anti-corrosion wear-resistant coatings on aluminum and its alloys, including anodic oxidation using pulsating direct or alternating current with a density of up to 0.05 A / cm ² at a maximum voltage value of 450-500 V (effective voltage value of 75-130 V) aqueous electrolytes containing solutions and dispersions of fluorides and oxyfluorides, mainly titanium and / or zirconium, to obtain a protective layer of ceramics with a thickness of 3-6 μm containing Ti and Zr oxides, thermal spraying of the intermediate oxide layer and subsequent deposition of polytetrafluoroethylene or silicone to obtain a second protective layer a thickness of 10-15 microns (see US patent No. 7820300, IPC B32B3 / 10; C25D11 / 02; C25D11 / 04; C25D11 / 06; C25D11 / 30 (2006.01), 2010).

The disadvantage of this method is the complexity of the implementation, due to its multi-stage, as well as the need for independent selection in each case of the method of applying PTFE.

A known method of forming a coating having a sufficiently high adhesion and a long service life with lubricating properties, including the formation of an anode film on the surface of aluminum, magnesium or their alloys, its subsequent electrochemical or chemical treatment with an aqueous dispersion containing PTFE and a reactive surfactant, and drying the resulting PTFE film ( see Japanese Patent No. 4783124, IPC C25D11 / 18; C25D11 / 20; C25D11 / 24 (2010.01), 2011).

The disadvantage of this method is the insufficiently high adhesion of the obtained coatings to the surface of the metal being treated, due to the low porosity of the anode film, while using only drying in the absence of heat treatment leads to the formation of very loose coatings.

A known method of obtaining a protective polymer-containing coating on the surface of metal and alloy products, including plasma electrolytic oxidation (PEO) of a metal surface in an electrolyte containing soluble salts of organic and inorganic acids, to obtain a layer of oxide ceramic and subsequent deposition of polytetrafluoroethylene (PTFE) with heat treatment the resulting coating. PTFE is applied by electrophoresis from its aqueous dispersion, optionally containing sodium dodecyl sulfate and OP-10. (see RF patent No. 2569259, IPC C25D 11/00, C25D 11/18, C25D 13/06 (2006.01), 2015

The disadvantage of this method of coating is that, like all of the above, the method is based on the electrochemical processing of products.

A known method for producing composite coatings on aluminum and its alloys, including electrolytic oxidation with subsequent application of a polymer film. Electrolytic oxidation is carried out in the mode of microplasma discharges on the surface of the anode in the galvanostatic mode, at a constant current density of 0.1 1.0 A / dm2 and a voltage of 190 220 V in the electrolyte. The polymer film is formed by mechanical rubbing of powdered Teflon, followed by annealing to 280 - 340 ° C (see RF patent No. 2068037, IPC 6 C25D 11/18, 1996).

The disadvantage of this solution is the complexity of the preliminary preparation for the implementation of the coating. In addition, rubbing the powder with one layer does not make it possible to obtain a coating that provides maximum protection and high performance of the product on which it is applied, because during annealing, 2 to 5% of the mass weight of the applied powder is lost. This method is intended for coating aluminum utensils and creating a non-stick coating.

A known method of manufacturing a blade of the last stage of a turbine for use in a steam turbine engine, comprising: forming a blade of a steam turbine comprising a titanium-based alloy, applying high voltage to the leading edge of the blade in the electrolyte to obtain a transition layer of titanium oxide and the upper porous layer and sealing the upper the porous layer with a material selected from the group consisting of chromium, cobalt, nickel, polyimide, polytetrafluoroethylene and polyester. Moreover, the seal includes coating by spraying, dipping or coating with polymer powder and curing (see RF patent No. 2601674, IPC F01D 5/00, C25D 11/02, C25D 11/18, C25D 11/20, (2006.01), 2016 )

The disadvantage of this solution is the complexity of the preliminary preparation for the implementation of the coating. Moreover, the description and the formula do not provide information about how exactly the PTFE powder is applied and how it is cured.

None of the above solutions can be taken as a prototype.

The technical problem solved by the invention is to increase the release properties of the heat exchange elements due to the coating created on them, increase the service life of the heat exchange elements coated with them, and reduce the complexity of cleaning the heat exchange elements.

The technical problem posed is solved due to the fact that a method has been developed for coating polytetrafluoroethylene on heat exchange elements, which consists in degreasing the surface of the heat exchange element, drying it, then forming a protective layer by mechanically rubbing the surface of the heat exchange element in at least two directions, with powdered polytetrafluoroethylene with a particle size of up to 2 microns, after which annealing is carried out at a temperature of 240 - 260 ° C until the polytetrafluoroethylene nanoparticles are melted.

Degreasing is carried out with any soapy solution.

Drying is carried out at room temperature.

The rubbing direction during repeated application is chosen mutually perpendicular.

The technical result from the use of all the essential features of the invention is to increase the release properties of the heat exchange elements, as well as to increase the service life of the heat exchange elements, facilitate washing and descaling during technological maintenance.

By coating and forming a protective layer of polytetrafluoroethylene on the defatted and dried surface of the heat exchange element by mechanical rubbing of powdered polytetrafluoroethylene with a particle size of up to 2 μm, in at least two directions, followed by annealing at a temperature of 240 - 260 ° С until melting polytetrafluoroethylene nanoparticles, a heat-exchange element with enhanced wear-resistant and anti-adhesive properties is provided. The resource of his work as a whole is increasing, the interval between maintenance procedures is increasing. The heat exchange element coated with this method is easier to maintain, since there is no need for acid treatment or mechanical treatment to remove scale, as is usually done.

The scum is washed off with a stream of water from a heat-exchange element coated with this method according to this method, while the surface remains intact, the coating is firmly held on the surface of the product, and it can be used repeatedly.

In the process of mechanical rubbing on a defatted and dried surface of a polytetrafluoroethylene heat exchange element with a particle size of up to 2 μm, pores and roughness are closed by particles of polytetrafluoroethylene, which penetrates into them and adheres to the surface. During heat treatment under the influence of temperature from 240 to 260 ° С, the polytetrafluoroethylene particles are melted and a uniform protective layer is formed on the surface in the form of a film, characterized by high release properties.

Lowering the temperature below the specified limit of 240 ° C does not allow the particles to melt and create a uniform protective coating. In this case, an increase in temperature by more than 260 ° C leads to the fact that from the polytetrafluoroethylene with a particle size of up to 2 μm, a binding element, fluorine, begins to stand out. The selection of fluorine from polytetrafluoroethylene leads to the following negative consequences:

- firstly, to reduce the quality of the protective layer,

- secondly, it negatively affects the environment, requires the introduction of equipment to remove fluorine vapor and ensure the safe operation of the operator.

Exposure to temperature within the indicated limits makes it possible to obtain a film reliably held on the surface, since when heated, the adhesion of polytetrafluoroethylene with a particle size of up to 2 microns to the material of the heat exchange element is enhanced.

The relevance of the problem is due to the fact that lamellar collapsible heat exchangers are common in recent times. They are distinguished by intensive heat transfer, ease of manufacture, compactness, low hydraulic resistance, ease of installation and cleaning from contamination. They have a relief surface, which, like pipes, needs to be protected from scale buildup.

The implementation of the method.

Tests have been conducted, based on the processing of used heat exchanger plates (heat exchange elements) with polytetrafluoroethylene powder with a particle size of up to 2 microns “Dry lubricantFORTOL"By the described method. Why the following actions were carried out:

1. Disassembled two existing plate heat exchangers.

2. Since the tests were carried out on used heat-exchange elements, the heat exchanger plates were washed with a cleaning agent (using acid) from scale deposits - calcium and magnesium salts.

3. Next, the heat exchange elements were degreased with a soap solution and dried at room temperature.

4. After drying, polytetrafluoroethylene powder with a particle size of up to 2 μm was applied to the surface of the heat exchange elements of one heat exchanger. Rubbed the surface with a dry cloth in one direction. Then a polytetrafluoroethylene powder with a particle size of up to 2 μm was applied a second time and rubbed in the direction perpendicular to the previous treatment.

The heat exchange elements of the second heat exchanger were not processed.

5. Annealed the heat exchange elements treated with polytetrafluoroethylene powder with a particle size of up to 2 microns. Annealing was carried out using the following equipment: Professional building hair dryer BOSCH GHG 660 LCD or NK 6.6.6 / 3,5 I5 circulation furnace. Annealing was carried out at different temperature conditions. The time during which the heat-exchange elements were subjected to heat treatment ranged from 3 to 10 minutes, depending on the equipment used.

5.1. A part of the heat exchange elements (approximately one fourth part) was annealed at a temperature of 240 ° С until the polytetrafluoroethylene nanoparticles were melted for 10 min. Annealing was carried out using a NK 6.6.6 / 3.5 I5 Circulation Oven in which a temperature of 240 ° C was set.

5.2. Another part of the heat exchange elements (about one fourth part) was annealed at a temperature of 245 ° С until the polytetrafluoroethylene nanoparticles were melted for 7 min. Annealing was carried out using a NK 6.6.6 / 3.5 I5 Circulation Oven in which a temperature of 245 ° C was set.

5.3. The third part of the heat exchange elements was annealed at a temperature of 250 ° С until the polytetrafluoroethylene nanoparticles were melted for 5 min. Annealing was carried out using a Hair dryer professional building BOSCH GHG 660 LCD, in which the temperature was set at 250 ° C.

5.4. The remaining fourth part of the heat exchange elements was annealed at a temperature of 260 ° С until the polytetrafluoroethylene nanoparticles were melted for 3 min. Annealing was carried out using a Hair dryer professional building BOSCH GHG 660 LCD, in which the temperature was set at 260 ° C.

6. At the end of the processing of the heat exchange elements, the plate heat exchange units were assembled and launched in parallel to work.

7. After 5 months of operation, the efficiency of coating the surface of the heat-exchange elements with polytetrafluoroethylene with a particle size of up to 2 μm on the treated plates compared to untreated plates was evaluated. The results are shown in Table 1.

Conclusion: All heat transfer elements treated with polytetrafluoroethylene powder with a particle size of up to 2 μm, at different heating temperatures, in comparison with untreated heat transfer elements, showed the following results:

1. The release properties of the surface of the heat-exchange elements increased by about 70% (this indicator is confirmed by a layer of scale formed on the surface of the heat-exchange elements).

2. In this regard, the resource has increased, and the time between maintenance can be increased by 2 times.

3. Rinsing (descaling) from the heat exchange elements processed by the present method, produced by processing the heat exchange elements under running water. The need for chemical flushing has disappeared, which provides additional protection against corrosion.

4. Reduced equipment downtime heat exchange elements can be cleaned within 1 day compared to 4-10 days for untreated PTFE.

Tests were also conducted, based on the processing of new heat exchange elements with polytetrafluoroethylene powder with a particle size of up to 2 microns "Dry lubricantFORTOL"By the described method. Comparison was made with untreated heat exchange elements from the same plate heat exchange unit.

1. The number of new heat-exchange elements of the plate heat-exchange unit subjected to processing was 100 pcs. These heat exchange elements were degreased with a soap solution and dried at room temperature.

2. After drying, polytetrafluoroethylene powder with a particle size of up to 2 microns was applied to the surface of these heat exchange elements. Rubbed the surface with a dry cloth in one direction. Then a polytetrafluoroethylene powder with a particle size of up to 2 μm was applied a second time and rubbed in the direction perpendicular to the previous treatment.

The second part (123 pcs.) Of the heat exchange elements of this heat exchanger was not processed.

3. Annealed the heat exchange elements treated with polytetrafluoroethylene powder with a particle size of up to 2 microns. Annealing was carried out using the following equipment: Professional building hair dryer BOSCH GHG 660 LCD or NK 6.6.6 / 3,5 I5 circulation furnace. Annealing was carried out at different temperature conditions. The time during which the heat-exchange elements were subjected to heat treatment ranged from 3 to 10 minutes, depending on the equipment used.

3.1. A part of the heat exchange elements (approximately one fourth part) was annealed at a temperature of 240 ° С until the polytetrafluoroethylene nanoparticles were melted for 10 min. Annealing was carried out using a NK 6.6.6 / 3.5 I5 Circulation Oven in which a temperature of 240 ° C was set.

3.2. The other part of the heat exchange elements (approximately one fourth part) was annealed at a temperature of 250 ° С until the polytetrafluoroethylene nanoparticles were melted for 7 min. Annealing was carried out using a NK 6.6.6 / 3.5 I5 Circulation Oven, in which a temperature of 250 ° C was set.

3.3. The third part of the heat exchange elements was annealed at a temperature of 255 ° С until the polytetrafluoroethylene nanoparticles were melted for 5 min. Annealing was carried out using a Hair dryer professional building BOSCH GHG 660 LCD, in which the temperature was set at 255 ° C.

3.4. The remaining fourth part of the heat exchange elements was annealed at a temperature of 260 ° С until the polytetrafluoroethylene nanoparticles were melted for 3 min. Annealing was carried out using a Hair dryer professional building BOSCH GHG 660 LCD, in which the temperature was set at 260 ° C.

4. At the end of the processing, a plate heat-exchange unit was assembled, in which both the heat-exchange elements processed by the present method and not processed were installed. The heat exchanger unit was put into operation.

5. After half a month of operation, the heat-exchange unit was disassembled, and the efficiency of using the method for coating the surface of heat-exchange elements with polytetrafluoroethylene with a particle size of up to 2 μm on treated plates compared to untreated plates was evaluated.

All heat exchange elements treated with polytetrafluoroethylene powder with a particle size of up to 2 μm, at different heating temperatures, in comparison with untreated heat exchange elements, showed the following results:

1. The release properties of the surface of the treated heat transfer elements increased by about 70% (this indicator is confirmed by a layer of scale formed on the surface of the heat transfer elements).

2. In this regard, the resource has increased, and the time between maintenance can be increased by 2 times.

3. Rinsing (descaling) of the heat exchange elements processed by the present method, was carried out by processing the heat exchange elements under running water. There is no longer a need for chemical flushing and mechanical descaling, which provides additional protection against corrosion. From heat-exchange elements not subjected to processing, the scale had to be removed by machining.

The results are shown in Table 2.

The claimed method of coating polytetrafluoroethylene on heat exchange elements can be carried out using existing equipment.

The method can be used to obtain protective coatings on metal surfaces and can be used to protect products and structures in contact with a liquid medium, for example, in the food industry, in a marine climate, etc.

Table No. 1. Test sample t / o apparatus-ET100. Plates previously used for more than 1 year

At the time of the experiment, all the plates were mechanically cleaned and installed in regular places.

No.
p / p Brand, model of heat exchanger Total number of plates, pcs Number of PTFE machined plates, pcs Year
made Operating time
Month Degree of processing
PTFE, layer thickness, microns The thickness of the formed scale, mm Weight
scum
gr Hardware
for descaling The cleaning time of 1 plastic plug, min 1. Etra
ET-100 223 223 2017 6 10 3 25 Karcher 0.16 2. Etra
ET-100 223 - 2017 6 Not processing
huddled 6.5 85 Fur.
cleaning, grain sand. 300 fifteen

Table No. 2. Test sample t / o apparatus-ET100. New plates, not previously used

No.
p / p Brand, model of heat exchanger Total number of plates, pcs Number of plates treated with PTFE, pcs Year
made Operating time
month Degree of processing
PTFE, layer thickness, microns The thickness of the formed scale, mm Weight
scum
gr Hardware
for descaling The cleaning time of 1 plate, min 1. Etra
ET-100 223 100 2017 0.5 10 0.3 "0" within the margin of error Karcher 0.06 2. Not processed 1,1 fifteen Fur cleaning, grain sand. 300 3,5

Claims (4)

1. The method of applying a coating of polytetrafluoroethylene to heat exchange elements, which consists in degreasing the surface of the heat exchange element, drying it, then forming a protective layer by mechanically rubbing the surface of the heat exchange element in at least two directions with powdered polytetrafluoroethylene with a particle size of up to 2 μm, after which annealing is carried out at a temperature of 240 - 260 ° C until the polytetrafluoroethylene nanoparticles are melted. 2. The method according to claim 1, characterized in that the degreasing is carried out with any soapy solution. 3. The method according to claim 1, characterized in that the drying is carried out at room temperature. 4. The method according to claim 1, characterized in that the direction of rubbing during repeated application is chosen mutually perpendicular.