A detection method for yellowing of the coating film caused by nitrogen oxides in a powder coating curing furnace

Abstract

The present invention relates to the technical field of powder coatings and coating quality control, and specifically to a detection method for film yellowing caused by nitrogen oxides in a powder coating curing furnace, which includes the following steps: S1: Prepare test panels. Use a metal substrate, treat it with an aqueous solution of heavy metal compounds and dry it, then coat it with a light-colored powder coating with a customized formula, and bake it into a film in an electric heating oven to make the film on the test panel uneven; S2: Detect the samples. This method can easily judge whether the nitrogen oxides in the curing furnace exceed the acceptable level for powder coating; for the problem of film yellowing that occurs during the coating process of ordinary powder coatings, this method helps to find and judge the cause to exclude whether it is the yellowing caused by nitrogen oxides in the furnace, so as to take corresponding corrective and preventive measures.

Description

Technical Field

[0001] The present invention relates to the technical field of powder coatings and coating quality control, and particularly to a detection method for film yellowing caused by nitrogen oxides in a powder coating curing furnace. Background Art

[0002] Powder coatings have the advantages of high performance, high coating efficiency, low cost and environmental protection, and are widely used in many fields such as household appliances, furniture, building materials, IT and auto parts. In the coating industry, in order to respond to the environmental protection policy of "low-carbon emission reduction" and reduce energy consumption costs, enterprises have gradually phased out curing furnaces using liquid fuels (such as diesel) and solid fuels (such as firewood, coal, and biomass pellets) as heat sources, and replaced them with curing furnaces using gas fuels (such as natural gas, liquefied petroleum gas). Since gas is relatively clean, and in order to reduce heat loss, most gas curing furnaces are designed without installing heat exchangers like solid or liquid fuel curing furnaces, but use direct blast heating, which the coating industry calls a "direct-fired furnace". Relevant research papers and industry experience show that petrochemical fuels such as natural gas will produce a certain amount of nitrogen oxides (NOx) during combustion. If the concentration of nitrogen oxides in the curing furnace is too high, it is easy to cause the film of the baked product to turn significantly yellow, resulting in coating rework or even product return and scrapping.

[0003] For coating users, when batch yellowing of the product film occurs, it means that relatively large cost losses have been incurred. Therefore, it is necessary to detect in advance whether the concentration of nitrogen oxides in the curing furnace is too high and affects the coating quality, so as to take preventive measures in advance. On the other hand, there are also various factors for film baking yellowing. In addition to the oxidation yellowing of the film by nitrogen oxides, the heat resistance aging and oxidation resistance of the coating formulation raw materials, over-baking during coating, too thick substrate, bleeding and migration of the base color, and smoke from combustion exhaust gas may all cause the film to turn yellow. To sum up, whether it is to detect the nitrogen oxide content in the curing furnace in advance to prevent yellowing when spraying ordinary coating products, or to quickly identify the cause when film baking yellowing occurs during the coating process, it is necessary to design a rapid and intuitive method to detect the nitrogen oxides in the curing furnace. Summary of the Invention

[0004] The purpose of the present invention is to provide a detection method for film yellowing caused by nitrogen oxides in a powder coating curing furnace, so as to solve the problems raised in the above background art.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A detection method for film yellowing caused by nitrogen oxides in a powder coating curing furnace, characterized in that it includes the following steps:

[0006] S1: Prepare a test panel. Use a metal substrate, treat it with an aqueous solution of heavy metal compounds and dry it, then apply a light-colored powder coating with a customized formula, and bake it into a film in an electric heating oven to make the coating film of the test panel show an uneven state.

[0007] S2: Detect the sample. Put the test panel into the curing furnace to be tested, bake it according to the baking temperature and time of actual production. If the coating film shows obvious local yellowing, it proves that the content of nitrogen oxides in the curing furnace exceeds the tolerance range of ordinary powder coating, which is likely to cause yellowing of ordinary powder coating during baking.

[0008] Preferably, the test panel has a color-developing effect on nitrogen oxides in the curing furnace.

[0009] Preferably, the aqueous solution of heavy metal compounds in S1 is a soluble acid or soluble salt solution composed of one or more of heavy metal elements zirconium, zinc, molybdenum, copper, chromium, bismuth, tin, nickel, manganese, cobalt, cerium, and lanthanum.

[0010] The heavy metal concentration in the aqueous solution of heavy metal compounds is 0.005%-2%, and the pH value is 3.0-5.0 at 25°C.

[0011] Preferably, the CIE L value of the coating color of the light-colored powder coating in S1 is ≥62.

[0012] Preferably, the mass fraction ratio of the raw materials of the light-colored powder coating in S1 includes: 40-85 parts of resin, 0-20 parts of curing agent, 10-30 parts of titanium dioxide, 0-20 parts of filler, 0.1-5 parts of antioxidant, and 0-10 parts of other additives.

[0013] Preferably, the state that the coating film of the test panel shows uneven means that the surface of the test panel shows states of no coating, serious exposure of the coating film, semi-covering of the coating, basic covering of the coating, and complete covering at the same time.

[0014] Preferably, the antioxidant is a phenolic antioxidant.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] A detection method for yellowing of coating film caused by nitrogen oxides in a powder coating curing furnace proposed by the present invention can be used to simply judge whether the nitrogen oxides in the curing furnace exceed the acceptable level of powder coating; for the problem of yellowing of coating film that occurs during the coating process of ordinary powder coating, this method helps to find and judge the cause to exclude whether it is caused by nitrogen oxides in the furnace, so as to take corresponding corrective and preventive measures. Description of the Drawings

[0017] Figure 1 It is a comparison diagram of test panel A and test panel B in Embodiment 1 of the present invention.

[0018] Figure 2 This is a comparison diagram of Test Plate C and Test Plate D in Embodiment 2 of the present invention.

[0019] Figure 3 This is a comparison diagram of Test Plate E and Test Plate F in Embodiment 3 of the present invention. Specific Embodiments

[0020] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0021] Please refer to Figures 1 to 3 , the present invention provides a technical solution: a method for detecting the yellowing of the coating film caused by nitrogen oxides in a powder coating curing furnace, including the following steps:

[0022] S1: Prepare a test plate. The test plate has a color-developing effect on nitrogen oxides in the curing furnace. A metal substrate is used, which is treated with an aqueous solution of a heavy metal compound and dried, and then coated with a light-colored powder coating with a customized formula, and baked into a film in an electric heating oven to make the coating film of the test plate present an uneven state;

[0023] S2: Detect the sample. Put the test plate into the curing furnace to be tested, and bake it according to the baking temperature and baking time of actual production. If the coating film shows obvious local yellowing, it proves that the content of nitrogen oxides in the curing furnace exceeds the tolerance range of ordinary powder coating, and it is easy to cause yellowing of ordinary powder coating during baking.

[0024] The aqueous solution of the heavy metal compound in S1 is a soluble acid or soluble salt solution composed of one or more of the heavy metal elements zirconium, zinc, molybdenum, copper, chromium, bismuth, tin, nickel, manganese, cobalt, cerium, and lanthanum. The compound of zirconium element is preferably zirconium fluoride acid, the compound of zinc element is preferably zinc dihydrogen phosphate, and the compound of molybdenum element is preferably ammonium molybdate; the heavy metal concentration in the aqueous solution of the heavy metal compound is 0.005%-2%, and the pH value is 3.0-5.0 at 25°C. When treating the substrate with the aqueous solution, degrease, polish the metal substrate, clean the polishing dust, soak it and then take it out. The soaking time is 1 second - 60 minutes, preferably 2 - 5 minutes, and then directly drain without washing with water. The drying temperature condition is 30°C - 200°C, and the time is 0.5 - 120 minutes. The preferred temperature is 80°C - 120°C, and the time is 5 - 10 minutes.

[0025] The CIE L value of the coating color of the light-colored powder coating in S1 is ≥62, and its color is detected according to GB 11186.2-1989 "Methods for Measuring the Color of Paint Films - Part 2: Color Measurement", and the CIE L value of the paint film is ≥78; further, the CIE L value of the paint film is ≥90.

[0026] The mass fraction ratio of the raw materials of the light-colored powder coating in S1 includes: 40-85 parts of resin, 0-20 parts of curing agent, 10-30 parts of titanium dioxide, 0-20 parts of filler, 0.1-5 parts of antioxidant, the antioxidant is a phenolic antioxidant, and 0-10 parts of other additives. The resin is one or more of carboxyl polyester resin, hydroxyl polyester resin, epoxy resin, carboxyl acrylic resin, acrylate glycidyl ester copolymer, and hydroxyl acrylic resin; the curing agent is one or more of TGIC (triglycidyl isocyanurate), HAA (β-hydroxyalkylamide), pyromellitic acid and its anhydride, trimellitic acid and its anhydride, trimellitic acid, sebacic acid, dodecanedioic acid, blocked isocyanate; the titanium dioxide is one or more of anatase titanium dioxide and rutile titanium dioxide, preferably rutile titanium dioxide; other pigments and fillers are one or more of barium sulfate, calcium carbonate, kaolin, silica powder, mica powder, organic bentonite, zinc oxide, pearlescent pigment and aluminum powder pigment; the antioxidant is a phenolic antioxidant, including one or more of monophenols, polyphenols, phosphine phenols, and triazine phenols, such as antioxidant 1076 [octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], antioxidant 1010 [pentaerythritol tetrakis [β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate]], antioxidant 245 [diethylene glycol bis [3-(3-tert-butyl-5-methyl-4-hydroxyphenyl) propionate]], antioxidant 1098 [N,N'-hexamethylenebis(3,5-di-tert-butyl-4-hydroxybenzamide)], and antioxidant BHT (dibutylhydroxytoluene), and the dosage of the antioxidant is 0.2%-3%, preferably 0.5%-2%; other additives, including leveling agent, wetting promoter, benzoin, sanding agent, etc., are mainly used to adjust the appearance state and leveling property of the paint film. In the present invention, since the paint film for preparing the test sample does not have to be continuous and uniform, these additives can be added less or not at all.

[0027] For the preparation of the powder coating, all the raw materials are put into a high-speed mixer and mixed for 5-10 minutes to make the materials evenly mixed, then melt-extruded and pressed into sheet materials at 100-120 °C by an extruder, cooled and subjected to primary crushing, ground into powder, and sieved through a 120-200 mesh sieve to obtain the powder coating.

[0028] When the test plate is coated, it is coated by spraying with a high-voltage electrostatic spray gun or a friction electrostatic spray gun, and then baked in an electric oven to form a film. The baking conditions are 120°C-250°C, 1-60 minutes, preferably 150°C-180°C, 2-5 minutes. The prepared test plate coating presents an uneven state, which means that the surface of the test plate is simultaneously in the state of no coating, severe bottom exposure of the coating, semi-covering of the coating, basic covering of the coating and complete covering of the coating, wherein no coating refers to a coating thickness of 0 μm, severe bottom exposure of the coating refers to a coating thickness of 1-30 μm, semi-covering of the coating refers to a coating thickness of 31-50 μm, basic covering of the coating refers to a coating thickness of 51-60 μm, and complete covering refers to a coating thickness greater than 61 μm.

[0029] This method uses a metal substrate, after degreasing, grinding and chemical treatment and drying, coated with a customized light-colored powder coating, and baked in an electric oven to form a film. The prepared test plate coating shows an uneven thickness, transitioning from 0-100μm. When in use, the test plate is placed in the curing oven to be tested, and after baking according to the actual production baking temperature and baking time, if there is obvious local yellowing, it can be judged that there are more nitrogen oxides in the curing oven, which exceeds the general acceptance range of ordinary powder coatings, and is likely to cause yellowing quality problems of the coating.

[0030] The actual production baking temperature and baking time refer to the actual set temperature and time of the baking oven in the coating workshop. It is preferred that the surface temperature of the sprayed substrate reaches 180-200°C and the insulation time reaches 10-40 minutes.

[0031] The phenomenon of obvious local yellowing refers to the yellowing of the coating film when visually observing the test sample according to GB / T 9761-2008 "Visual colorimetry of paints and varnishes".

[0032] Embodiment 1:

[0033] A method for detecting yellowing of a coating film caused by nitrogen oxides in a powder coating curing furnace in this embodiment comprises the following steps:

[0034] 1. Composition and preparation of chemical treatment fluid:

[0035]

[0036] Add the above raw materials into the reaction kettle according to the formula, stir well and then check the pH value. If necessary, add sodium hydroxide or hydrochloric acid and stir well to adjust the pH value to 3.0-4.5 at 25°C.

[0037] 2. Chemical treatment of test panels

[0038] Use a rectangular aluminum alloy square tube with a thickness of 1.5 mm, wipe off the oil with alcohol, polish it with 400-mesh sandpaper and remove the polishing dust, soak it in the above-prepared chemical treatment for 5 minutes, take it out and drain it naturally, and then place it in an electric oven set at 120 °C to dry for 5 minutes.

[0039] 3. Preparation of powder coating

[0040] Powder coating formula:

[0041]

[0042]

[0043] Among them, the acid value of the carboxyl polyester resin is 33 ± 2 mgKOH / g.

[0044] Weigh various raw materials according to the formula, pour them into a high-speed mixer and mix for 6 - 8 minutes, then pour them into an extruder for melting and extrusion. The temperature of the first zone of the extruder is set at 95 - 120 °C, and the temperature of the second zone is set at 115 - 130 °C. The extruded material is pressed into thin slices by a tablet press, cooled and coarsely crushed to obtain coarse sheet materials.

[0045] Use an ACM vertical grinding mill to grind the coarse sheet materials into powder. The main grinding speed is set at 50 HZ, the secondary grinding speed is set at 40 HZ, and it is sieved through a 180-mesh rotary sieve to obtain the finished powder coating.

[0046] Take 2 of the above-mentioned rectangular aluminum alloy square tubes that have undergone chemical treatment, spray them with the finished powder coating. Note that spray 2 guns on the four edges to ensure good film coverage; spray 1 gun quickly in the middle to make the film coverage poor, and do not spray the inner wall. Put them into an electric oven and bake at 180 °C for 3 minutes, then take them out.

[0047] So far, a qualitative test plate for nitrogen oxides in a powder coating curing furnace has been prepared.

[0048] Put the 2 test plates prepared above into two gas curing furnaces A and B at the same time. The actual set furnace temperature is 205 °C, and the total time for entering and leaving the furnace is 33 - 36 minutes. The test results show that, refer to Figure 1 , there is no obvious yellowing on test plate A, and there is no obvious yellowing on the edges of test plate B, but there is obvious yellowing in the middle and on the inner wall. It is proved that the nitrogen oxide content in curing furnace A is very low and can meet the requirements for the coating of ordinary light-colored powder coatings; however, the nitrogen oxide content in curing furnace B is relatively high, and there is a risk of yellowing for the coating of ordinary light-colored powder coatings. It is necessary to check the fuel, burner and hot air circulation system, or switch to a customized powder coating resistant to gas yellowing.

[0049] Example 2:

[0050] A method for detecting film yellowing caused by nitrogen oxides in a powder coating curing furnace in this example is as follows:

[0051] 1. Composition and preparation of the chemical treatment solution:

[0052]

[0053] Add the above raw materials into the reaction kettle according to the formula amount, and the operation process and pH adjustment are the same as those in Example 1.

[0054] 2. Chemical treatment of the test plates

[0055] Soak and dry two aluminum plates with a thickness of 1.0 mm, and the operation process is the same as that in Example 1.

[0056] 3. Preparation of the powder coating

[0057] Powder coating formula:

[0058]

[0059]

[0060] Among them, the acid value of the carboxyl polyester resin is 70 ± 5 mgKOH / g, and the bisphenol A epoxy resin is E-12 epoxy resin.

[0061] Weigh, mix, extrude, coarsely crush, grind and sieve the powder according to the formula to obtain the finished powder coating, and the process parameters are the same as those in Example 1.

[0062] Take the two aluminum plates treated above, and spray the powder coating of Example 2 by high-voltage electrostatic spraying. Spray 2 guns at the upper end of the sample plate to make the coating cover well; spray 1 gun quickly at the lower end of the sample plate to make the coating cover slightly worse (locally slightly showing the bottom). In the same way as in Example 1, put it into the electric oven and bake at 180 °C for 3 minutes, then take it out to complete the preparation of the test plate.

[0063] Put the two test plates prepared above into the C electric baking oven and the D gas furnace for baking and curing respectively. The C electric oven is set at 200 °C and baked for 30 minutes; the D gas furnace is set at 200 °C, and the time for entering and leaving the furnace is 32 minutes. Refer to Figure 2 , the yellowing of the C test plate is not obvious, and the coating film color is uniform; there is no obvious yellowing in the part of the D test plate with a coating film higher than 80 microns, but obvious yellowing occurs in the part of the D test plate with a coating film lower than 60 microns. It is proved that the nitrogen oxide content in the D gas curing furnace is relatively high.

[0064] Example 3:

[0065] A method for detecting the yellowing of the coating film caused by nitrogen oxides in a powder coating curing furnace in this example is as follows:

[0066] 1. Composition and preparation of the chemical treatment solution:

[0067]

[0068] Add the above raw materials into the reactor according to the formula amount, and the operating process and pH adjustment are the same as in Example 1.

[0069] 2. Chemical treatment of test panels

[0070] Soak and dry 4 die-cast aluminum workpieces, and the operating process is the same as in Example 1.

[0071] 3. Preparation of powder coatings

[0072] Powder coating formula:

[0073]

[0074] Weigh, mix, extrude, coarsely crush, grind and sieve the powder according to the formula to obtain the finished powder coating, and the process parameters are the same as in Example 1.

[0075] Take the 4 die-cast aluminum workpieces treated above, and spray the powder coating of Example 3 by high-voltage electrostatic spraying. Spray 2 guns on the edges of the test panels to ensure good film coverage; when spraying at the corners of the test panels, rely on electrostatic shielding to make the film coverage poor (showing the base material). In the same way as in Example 1, put them into an electric oven and bake at 180 °C for 3 minutes, then take them out to complete the preparation of the test panels.

[0076] Put the 4 test panels prepared above into the E gas furnace and the F gas furnace, with 2 test panels in each furnace for baking and curing. Set the temperature to 205 °C, and the in-and-out furnace time is 31 - 32 minutes. Refer to Figure 3 , the test panels in the E furnace show no obvious yellowing; the film surface of the test panels in the F furnace turns slightly yellow, and the grooves turn significantly yellow. It is proved that the content of nitrogen oxides in the F gas curing furnace is relatively high.

[0077] After analyzing and testing a large number of coating yellowing cases, it is found that simple substances, inorganic acids, inorganic salts, oxides and complexes composed of certain heavy metal elements can cause uneven yellowing of uneven coatings under the condition of nitrogen oxides. Based on this, a detection method for yellowing of the coating film caused by nitrogen oxides in the powder coating curing furnace described in the present invention is proposed. In the field of powder coating, the present invention is similar to the function of a "qualitative test paper". By simply visually observing the color of the test panel, it is possible to quickly evaluate whether the content of nitrogen oxides in the curing furnace meets the coating quality requirements, which has the practical value of accuracy and convenience and fills the technical gap in the on-site visual detection and evaluation of nitrogen oxides in the powder coating curing furnace in the current powder coating field.

[0078] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting yellowing of coating film caused by nitrogen oxides in a powder coating curing furnace, characterized in that: The following steps are included: S1: Prepare a test plate, use a metal substrate, treat it with an aqueous solution of a heavy metal compound and dry it, then coat it with a customized light-colored powder coating, the coating color of the light-colored powder coating has a CIE L value of ≥62, and the mass fraction of the raw materials of the light-colored powder coating includes: 40-85 parts of resin, 0-20 parts of curing agent, 10-30 parts of titanium dioxide, 0-20 parts of filler, 0.1-5 parts of antioxidant, and 0-10 parts of other additives, and bake it in an electric oven to form a film, so that the coating of the test plate is uneven. The uneven coating of the test plate means that the surface of the test plate is simultaneously without coating, severely exposed coating, semi-covered coating, basically covered coating, and completely covered coating. The antioxidant is a phenolic antioxidant; S2: Test the sample. Put the test plate into the curing oven to be tested. After baking according to the actual production baking temperature and baking time, if the coating film shows obvious yellowing in some parts, it proves that the nitrogen oxide content in the curing oven exceeds the tolerance range of ordinary powder coating, which can easily cause ordinary powder coating to turn yellow during baking.

2. The method for detecting yellowing of coating film caused by nitrogen oxides in a powder coating curing furnace according to claim 1, characterized in that: The heavy metal compound aqueous solution in S1 is a soluble acid or soluble salt solution composed of one or more of the heavy metal elements zirconium, zinc, molybdenum, copper, chromium, bismuth, tin, nickel, manganese, cobalt, cerium, and lanthanum; The heavy metal concentration in the heavy metal compound aqueous solution is 0.005%-2%, and the pH value is 3.0-5.0 at 25°C.

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