A method for preparing a steel bar coating loaded with an intelligent rust inhibitor

Abstract

The present invention discloses a preparation method of a steel bar coating loaded with an intelligent rust inhibitor, comprising the following steps: (1) etching halloysite for a period of time to obtain etched halloysite; soaking the above-etched halloysite in a rust inhibitor solution to obtain a halloysite intelligent rust inhibitor; (2) adding the halloysite intelligent rust inhibitor to a pretreatment solution, stirring evenly to obtain a pretreatment suspension; the steel bar is immersed in the pretreatment suspension, and after drying, a pretreatment layer is obtained on the surface of the steel bar. The present invention prepares a phytic acid composite coating system with active protective performance, in which the halloysite rust inhibitor in the pretreatment layer is firmly combined with the substrate, the thickness of the pretreatment layer is improved, and the density of the pretreatment layer is increased, and the synergistic anti-corrosion effect of the pretreatment layer and the intelligent rust inhibitor gives the coating system self-repairing performance. Therefore, the present invention is suitable for industrial production applications due to the advantages of simple process, low cost, and green environmental protection.

Description

Technical Field

[0001] The present invention relates to a method for preparing a surface pretreatment layer, and in particular to a method for preparing a steel bar coating loaded with an intelligent rust inhibitor, belonging to the fields of material surface treatment technology and steel bar corrosion resistance. Background Art

[0002] Improving the corrosion resistance of steel bars is one of the most important means of extending the durability of reinforced concrete structures. Among the many corrosion prevention and control measures, the construction of steel bar surface coating systems has become the most widely used and effective method. Although traditional coatings offer the advantages of ease of preparation and stability, they may not provide adequate corrosion protection once damaged. Therefore, inspired by the wound healing properties of organisms, rust inhibitors have been incorporated into smart coating systems to protect metal substrates from corrosion. However, direct addition of rust inhibitors can compromise the integrity and uniformity of the coating. Therefore, researchers have proposed using micro- / nanocontainers as carriers to load or encapsulate rust inhibitors before incorporating them into organic coatings to ensure the long-term reactivity and stability of smart coating systems. Furthermore, the surface pretreatment layer between the steel substrate and the coating in coating systems has garnered considerable attention. In recent years, organic acid pretreatment layers have become a research hotspot in surface pretreatment technologies. Among these, membranes are considered a novel "environmentally friendly" pretreatment layer, potentially replacing highly toxic chromate passivation layers and highly polluting phosphate conversion layers. However, phytic acid coatings also suffer from two major drawbacks: limited film thickness and inherent micro- and nano-scale cracks. Therefore, to overcome these limitations, combining smart rust inhibitors with pretreatment layer technology to create smart coating systems is crucial for mitigating steel corrosion and improving concrete durability. Summary of the Invention

[0003] Purpose of the invention: The present invention aims to provide a method for preparing a steel bar coating loaded with an intelligent rust inhibitor with high stability.

[0004] Technical solution: A method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to the present invention is characterized in that it comprises the following steps:

[0005] (1) etching halloysite for a period of time to obtain etched halloysite; soaking the etched halloysite in a rust inhibitor solution to obtain a halloysite intelligent rust inhibitor;

[0006] (2) adding the halloysite intelligent rust inhibitor to the pretreatment solution and stirring evenly to obtain a pretreatment suspension; immersing the steel bar in the pretreatment suspension and drying the steel bar to obtain a pretreatment layer on the surface;

[0007] The preparation method of the pretreatment solution is as follows: add 1 to 3 ml of phytic acid and 0.2 to 2 g of polyvinyl alcohol to every 100 mL of water, stir in a water bath at 75 to 85° C. for 3 to 7 hours, cool and let stand to room temperature, adjust the solution pH to 8 to 10 and let stand for 12 to 24 hours to obtain a pretreatment solution.

[0008] (3) The steel bar obtained in step (2) is immersed in an epoxy coating solution, taken out and allowed to stand, and a layer of epoxy coating is obtained on the pre-treated layer on the surface of the steel bar.

[0009] In the above technical solution, a technical method of combining an intelligent rust inhibitor with a pretreatment layer is adopted in the coating system, so that the steel bars have more significant corrosion resistance.

[0010] In order to further enhance the anti-corrosion and corrosion resistance of the coating system containing the pretreatment layer, in this technical solution, a pretreatment layer containing an intelligent rust inhibitor is constructed on the surface of the steel bar. Taking advantage of the fact that the pretreatment layer is close to the steel bar base, the intelligent rust inhibitor can act more efficiently on the steel bar surface after release to inhibit pitting corrosion.

[0011] Furthermore, the solute in the epoxy coating solution is epoxy resin, and the epoxy resin is one of E-44 and E-51.

[0012] Preferably, in step (1), the halloysite is nano-scale tubular halloysite.

[0013] Preferably, in step (1), the etchant used to etch the halloysite is a corrosive acid solution or a corrosive alkaline solution.

[0014] More preferably, the corrosive acid solution is dilute sulfuric acid, dilute hydrochloric acid, or acetic acid.

[0015] More preferably, the corrosive alkaline acid solution is one or a combination of sodium hydroxide solution, calcium hydroxide solution and red mud extract, wherein the red mud extract is prepared by stirring an aqueous solution of ordinary Bayer process red mud for 24 hours and filtering to obtain a transparent liquid with a pH of 13.1 to 13.6.

[0016] Furthermore, in step (1), the rust inhibitor in the rust inhibitor solution is one or more of molybdate, calcium lignin sulfonate, sodium citrate, and sodium dodecylbenzene sulfonate; and the concentration of the rust inhibitor in the rust inhibitor solution is 0.1 to 1M.

[0017] Furthermore, in step (1), the specific method of soaking the etched halloysite in the rust inhibitor solution is as follows: soaking the etched halloysite in the rust inhibitor solution, ultrasonically dispersing it for 5 to 30 minutes, and standing it under negative pressure at 20 to 40° C. for 1 to 3 days.

[0018] Furthermore, in step (1), the specific method of soaking the etched halloysite in the rust inhibitor solution is: soaking the etched halloysite in the rust inhibitor solution, ultrasonically dispersing it for 10 to 30 minutes, and standing it under negative pressure conditions at 20 to 40° C. for 2 to 3 days.

[0019] Preferably, the negative pressure condition is 0.5 to 0.8 MPa.

[0020] Preferably, during the preparation of the pretreatment solution, the stirring rate is 800 to 1500 r / min.

[0021] Furthermore, in step (2), the steel bars are pretreated and then immersed in a pretreatment suspension; the above-mentioned pretreatment method is: the steel bars are successively ground, polished, rinsed and degreased, and dried to obtain pretreated steel bars.

[0022] Preferably, in step (2), the reagent used to adjust the pH of the solution is hydroxide.

[0023] Furthermore, in step (2), the method for uniform stirring is: ultrasonication for 1 to 5 minutes and stirring and dispersing, the stirring temperature is 25±3° C., and the stirring rate is 100 to 200 r / min.

[0024] In a preferred embodiment, the preparation method of the steel bar coating loaded with intelligent rust inhibitor of the present invention includes three steps: the synthesis of intelligent rust inhibitor, the preparation of two steel bar pretreatment layers and the preparation of the coating system. The first step is the synthesis of the halloysite intelligent rust inhibitor, that is, the halloysite hollow tube is first etched with a corrosive acid and alkali solution to expand the volume inside the tube, and then the etched halloysite is immersed in the rust inhibitor aqueous solution by a negative pressure method to synthesize the halloysite rust inhibitor, which is then taken out and dried. The second step is to prepare a pretreatment layer on the surface of the steel bar, that is, phytic acid and polyvinyl alcohol are grafted to form a pretreatment liquid, and the halloysite rust inhibitor is added, and then the carbon steel surface is immersed in the pretreatment liquid to prepare a pretreatment layer containing the halloysite rust inhibitor, which is then taken out and dried. The third step is the preparation of anti-corrosion coated steel bars, that is, an epoxy coating is prepared on the surface of the steel bar with a pretreatment layer by a dip coating method, and the epoxy coating is left to dry.

[0025] In a preferred embodiment, the method for preparing a steel bar coating loaded with an intelligent rust inhibitor of the present invention specifically comprises three steps:

[0026] The first step is the synthesis of smart rust inhibitor:

[0027] 1.1) Immerse the untreated halloysite in a corrosive acid or alkali solution for etching, allow to stand at a negative pressure of 20-40°C for 1-3 days, remove, wash, and dry at room temperature for later use;

[0028] 1.2) Adding a rust inhibitor to 100 mL of deionized water to prepare an aqueous solution of the rust inhibitor, soaking the halloysite etched in 1.1) in the aqueous solution of the rust inhibitor, ultrasonically dispersing the solution for 5 to 30 minutes, allowing the solution to stand under negative pressure at 20 to 40° C. for 1 to 3 days, removing the solution, washing it, and drying it at room temperature to obtain a halloysite intelligent rust inhibitor;

[0029] The second step is the preparation of the steel bar pretreatment layer:

[0030] 2.1) Grind, polish and degrease the carbon steel surface;

[0031] 2.2) Add 1-3 ml of phytic acid and 0.2-2 g of polyvinyl alcohol to 100 mL of deionized water. Stir the mixed solution in a water bath at 75-85°C for 3-7 hours, cool it to room temperature, and then adjust the pH to a weakly alkaline environment of 8-10 with hydroxide and let it stand for 12-24 hours to obtain a pretreatment solution.

[0032] 2.3) adding the halloysite intelligent rust inhibitor prepared in 1.2) to the pretreatment solution prepared in 2.2), sonicating for 1 to 5 minutes and stirring to disperse the solution to obtain a pretreatment suspension;

[0033] 2.4) Fully immersing the steel bar surface in the pretreatment suspension in 2.1) and drying the steel bar surface to obtain a surface pretreatment layer;

[0034] The third step is the preparation of the coating system:

[0035] 3.1) Immerse the pre-treated layer obtained in the second step in the prepared epoxy coating solution, take it out and let it stand.

[0036] The present invention prepares a phytic acid composite coating system with active protective performance. The halloysite rust inhibitor in the pretreatment layer of the coating system is firmly bonded to the substrate, thereby increasing the thickness of the pretreatment layer and the density of the pretreatment layer. The synergistic anti-corrosion effect of the pretreatment layer and the intelligent rust inhibitor gives the coating system self-repairing performance.

[0037] Beneficial effects: Compared with the existing technology, the present invention has the following significant advantages: Compared with the traditional coating system with the addition of rust inhibitors, the intelligent rust inhibitor prepared by the present invention better maintains the stability and finish of the coating, while also ensuring the long-term effectiveness of the rust inhibitor performance. In addition, the combination of the intelligent rust inhibitor and the pretreatment layer technology increases the thickness of the pretreatment layer and brings the intelligent rust inhibitor closer to the steel substrate, giving the pretreatment layer active corrosion resistance. Finally, the preparation process of the pretreatment layer is simple, low-cost, green and environmentally friendly, and is suitable for industrial production applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 TEM images of halloysite at different stages in Example 1;

[0039] Figure 2 1 is the XRD pattern of halloysite at different stages in Example 1;

[0040] Figure 3 The EIS graphs of the smart coating steel bar samples in different embodiments were immersed in a saturated calcium hydroxide solution containing 3.5 wt% sodium chloride for 0.5 days;

[0041] Figure 4 Surface corrosion morphologies of the smart coated steel bar specimen prepared in Example 1 and the commercial coated steel bar specimen without a pretreatment layer of the comparative example, after being immersed in a saturated calcium hydroxide solution containing 3.5 wt % sodium chloride for 14 days. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0043] Example 1

[0044] The present invention provides a method for preparing a steel bar coating loaded with an intelligent rust inhibitor, comprising:

[0045] In the first step, 5g of halloysite was immersed in 200ml of 9.8% sulfuric acid aqueous solution, allowed to stand at 30°C and 0.8MPa negative pressure for 2 days, removed, washed, and dried at room temperature. The etched halloysite was then immersed in 100ml of 0.5M molybdate solution, ultrasonically dispersed for 10 minutes, allowed to stand at 30°C and 0.8MPa negative pressure for 2 days, removed, washed, and dried at room temperature. This product was named halloysite rust inhibitor and was ready for use.

[0046] like Figure 1 The following are TEM comparison images of halloysite at different stages. (a) is unetched halloysite, (b) is etched halloysite, and (c) is etched halloysite loaded with rust inhibitor. It can be seen that the inner cavity of halloysite is significantly enlarged and the rust inhibitor is successfully loaded. Figure 2 This is a comparison of XRD patterns of halloysite at different stages, where the peak of molybdate can be clearly seen.

[0047] In the second step, the steel bar surface was polished in stages using 180-2000# sandpaper and polished with a polishing solution. After rinsing with deionized water, the steel bar surface was ultrasonically treated with alcohol for 5 minutes, then air-dried and set aside. A phytic acid aqueous solution was prepared by adding 2ml of phytic acid to 100ml of deionized water. The solution was adjusted to a pH of 10 with sodium hydroxide and allowed to stand for 24 hours. 0.5g of polyvinyl alcohol was then added to the pH-adjusted phytic acid aqueous solution. The mixed solution was stirred in an 85°C water bath at a stirring rate of 1000 rpm for 4 hours, removed and allowed to cool to room temperature to obtain a phytic acid-polyvinyl alcohol pretreatment solution. The prepared halloysite rust inhibitor was then added to the phytic acid-polyvinyl alcohol pretreatment solution and ultrasonically dispersed for 1 minute to obtain a pretreatment suspension. Finally, the polished steel bar surface was immersed in the modified phytic acid-polyvinyl alcohol pretreatment solution for 30 seconds at 25°C and a stirring rate of 100 rpm. The steel bar with the phytic acid pretreatment layer was removed and air-dried.

[0048] In the third step, the prepared pre-treated layer steel bar sample is immersed in commercial E-44 epoxy coating liquid to obtain a smart coating steel bar sample containing a pre-treated layer.

[0049] Figure 3 This is the EIS of Example 1 and the comparative example. It can be seen that during the long-term corrosion process, the etched halloysite is loaded with more rust inhibitors and synergistically acts with the pretreatment layer to significantly improve the corrosion resistance of the steel bar. Figure 4 The surface corrosion morphology of the samples during 14 days of immersion was observed using an optical microscope (OM). The smart coated steel bar samples showed excellent corrosion resistance.

[0050] Example 2

[0051] The present invention provides a method for preparing a steel bar coating loaded with an intelligent rust inhibitor, comprising:

[0052] In the first step, 5g of halloysite was immersed in 200ml of red mud extract, allowed to stand at 40°C and 0.5MPa under negative pressure for three days, then washed and dried at room temperature. The etched halloysite was then immersed in 100ml of a 0.1M nitrite solution, ultrasonically dispersed for 20 minutes, allowed to stand at 40°C and 0.5MPa under negative pressure for three days, then washed and dried at room temperature. This product, designated halloysite rust inhibitor, was then prepared for future use.

[0053] In the second step, the steel bar surface was polished in stages using 180-2000# sandpaper and polished with a polishing solution. After rinsing with deionized water, the steel bar surface was ultrasonically treated with alcohol for 5 minutes, then air-dried and set aside. A phytic acid aqueous solution was prepared by adding 3ml of phytic acid to 100ml of deionized water. The solution was adjusted to a pH of 9 with calcium hydroxide and allowed to stand for 12 hours. Then, 1g of polyvinyl alcohol was added to the pH-adjusted phytic acid aqueous solution. The mixed solution was stirred in a 75°C water bath at a stirring rate of 800 rpm for 5 hours, removed and allowed to cool to room temperature to obtain a phytic acid-polyvinyl alcohol pretreatment solution. The prepared halloysite rust inhibitor was then added to the phytic acid-polyvinyl alcohol pretreatment solution and ultrasonically dispersed for 1 minute to obtain a pretreatment suspension. Finally, the polished steel bar surface was immersed in the modified phytic acid-polyvinyl alcohol pretreatment solution for 60 seconds at a stirring temperature of 25°C and a stirring rate of 100 rpm. The steel bar with the phytic acid pretreatment layer was removed and air-dried.

[0054] In the third step, the prepared pre-treated layer steel bar sample is immersed in commercial E-44 epoxy coating liquid to obtain a smart coating steel bar sample containing a pre-treated layer. Figure 3 This is the EIS of Example 2 and the comparative example. It can be seen that during the long-term corrosion process, the etched halloysite is loaded with more rust inhibitors and synergistically acts with the pretreatment layer to significantly improve the corrosion resistance of the steel bar.

[0055] Example 3

[0056] The present invention provides a method for preparing a steel bar coating loaded with an intelligent rust inhibitor, comprising:

[0057] In the first step, 5g of halloysite was immersed in 200ml of a 3.7% hydrochloric acid solution, allowed to stand at 20°C and 0.8 MPa under a negative pressure for three days, then washed and dried at room temperature. The etched halloysite was then immersed in 100ml of a 0.1M calcium lignin sulfonate solution, ultrasonically dispersed for 30 minutes, allowed to stand at 20°C and 0.8 MPa under a negative pressure for three days, then washed and dried at room temperature. This product, designated halloysite rust inhibitor, was then prepared for future use.

[0058] In the second step, the steel bar surface was polished in stages using 180-2000# sandpaper and polished with a polishing solution. After rinsing with deionized water, the steel bar surface was ultrasonically treated with alcohol for 5 minutes, then air-dried and set aside. A phytic acid aqueous solution was prepared by adding 3ml of phytic acid to 100ml of deionized water. The solution was adjusted to a pH of 9 with sodium hydroxide and allowed to stand for 24 hours. 2g of polyvinyl alcohol was then added to the pH-adjusted phytic acid aqueous solution. The mixed solution was stirred in a 75°C water bath at a stirring rate of 800 rpm for 5 hours, removed and allowed to cool to room temperature to obtain a phytic acid-polyvinyl alcohol pretreatment solution. The prepared halloysite rust inhibitor was then added to the phytic acid-polyvinyl alcohol pretreatment solution and ultrasonically dispersed for 1 minute to obtain a pretreatment suspension. Finally, the polished steel bar surface was immersed in the modified phytic acid-polyvinyl alcohol pretreatment solution for 120 seconds at a stirring temperature of 25°C and a stirring rate of 50 rpm. The steel bar with the phytic acid pretreatment layer was removed and air-dried.

[0059] In the third step, the prepared pre-treated layer steel bar sample is immersed in commercial E-44 epoxy coating liquid to obtain a smart coating steel bar sample containing a pre-treated layer. Figure 3 This is the EIS of Example 2 and the comparative example. It can be seen that during the long-term corrosion process, the etched halloysite is loaded with more rust inhibitors and synergistically acts with the pretreatment layer to significantly improve the corrosion resistance of the steel bar.

[0060] Example 4

[0061] The present invention provides a method for preparing a steel bar coating loaded with an intelligent rust inhibitor, comprising:

[0062] In the first step, 5g of halloysite was immersed in 200ml of a 4% sodium hydroxide aqueous solution, allowed to stand at 30°C and 0.8 MPa under a negative pressure for 3 days, then washed and dried at room temperature. The etched halloysite was then immersed in 100ml of a 0.2M phosphate solution, ultrasonically dispersed for 20 minutes, allowed to stand at 30°C and 0.8 MPa under a negative pressure for 3 days, then washed and dried at room temperature. This product was named halloysite rust inhibitor and was ready for use.

[0063] In the second step, the steel bar surface was polished in stages using 180-2000# sandpaper and polished with a polishing solution. After rinsing with deionized water, the steel bar surface was ultrasonically treated with alcohol for 5 minutes, then air-dried and set aside. A phytic acid aqueous solution was prepared by adding 1ml of phytic acid to 100ml of deionized water. The solution was adjusted to a pH of 9 with calcium hydroxide and allowed to stand for 18 hours. 0.2g of polyvinyl alcohol was then added to the pH-adjusted phytic acid aqueous solution. The mixed solution was stirred in an 85°C water bath at a stirring rate of 800 rpm for 5 hours, removed and allowed to cool to room temperature to obtain a phytic acid-polyvinyl alcohol pretreatment solution. The prepared halloysite rust inhibitor was then added to the phytic acid-polyvinyl alcohol pretreatment solution and ultrasonically dispersed for 2 minutes to obtain a pretreatment suspension. Finally, the polished steel bar surface was immersed in the modified phytic acid-polyvinyl alcohol pretreatment solution for 120 seconds at 25°C and a stirring rate of 100 rpm. The steel bar with the phytic acid pretreatment layer was removed and air-dried.

[0064] In the third step, the prepared pre-treated layer steel bar sample is immersed in commercial E-44 epoxy coating liquid to obtain a smart coating steel bar sample containing a pre-treated layer. Figure 3 This is the EIS of Example 4 and the comparative example. It can be seen that during the long-term corrosion process, the etched halloysite is loaded with more rust inhibitors and synergistically acts with the pretreatment layer to significantly improve the corrosion resistance of the steel bar.

Claims

1. A method for preparing a steel bar coating loaded with an intelligent rust inhibitor, characterized in that: The following steps are involved: (1) etching halloysite for a period of time to obtain etched halloysite; immersing the etched halloysite in a rust inhibitor solution to obtain a halloysite intelligent rust inhibitor; (2) Adding the halloysite intelligent rust inhibitor to the pretreatment solution and stirring evenly to obtain a pretreatment suspension; immersing the steel bar in the pretreatment suspension and drying the steel bar to obtain a pretreatment layer on the surface; The pretreatment solution is prepared by adding 1 to 3 ml of phytic acid and 0.2 to 2 g of polyvinyl alcohol to every 100 ml of water, stirring in a water bath at 75 to 85°C for 3 to 7 hours, cooling and allowing to stand to room temperature, adjusting the pH of the solution to 8 to 10, and allowing to stand for 12 to 24 hours to obtain a pretreatment solution. (3) The steel bar obtained in step (2) is immersed in an epoxy coating solution, taken out and allowed to stand, and a layer of epoxy coating is obtained on the pre-treated layer on the surface of the steel bar.

2. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: The solute in the epoxy coating solution is epoxy resin, and the epoxy resin is one of E-44 and E-51.

3. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (1), the halloysite is nano-scale tubular halloysite.

4. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (1), the etchant used to etch the halloysite is a corrosive acid solution or a corrosive alkaline solution.

5. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 4, wherein: The corrosive acid solution is dilute sulfuric acid, dilute hydrochloric acid or acetic acid.

6. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 4, wherein: The corrosive alkaline acid solution is one or a combination of sodium hydroxide solution, calcium hydroxide solution and red mud extract, wherein the red mud extract is prepared by stirring an aqueous solution of ordinary Bayer process red mud for a period of time and filtering to obtain a transparent liquid with a pH of 13.1-13.

6.

7. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (1), the rust inhibitor in the rust inhibitor solution is one or more of molybdate, calcium lignin sulfonate, sodium citrate, and sodium dodecylbenzene sulfonate; and the concentration of the rust inhibitor in the rust inhibitor solution is 0.1 to 1 M.

8. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (1), the specific method of soaking the etched halloysite in the rust inhibitor solution is as follows: soaking the etched halloysite in the rust inhibitor solution, ultrasonically dispersing it for 5 to 30 minutes, and standing it under negative pressure conditions at 20 to 40°C for 1 to 3 days.

9. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (2), the steel bars are pretreated and then immersed in the pretreatment suspension; the above-mentioned pretreatment method is: the steel bars are successively ground, polished, rinsed and degreased, and dried to obtain the pretreated steel bars.

10. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (2), the reagent used to adjust the pH of the solution is hydroxide.

11. The method for preparing a steel bar coating loaded with an intelligent rust inhibitor according to claim 1, wherein: In step (2), the method for uniform stirring is: ultrasonication for 1 to 5 minutes and stirring dispersion, the stirring temperature is 25±3°C, and the stirring rate is 100 to 200 r / min.

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