CN112694775A

CN112694775A - Method for improving corrosion resistance of powder coating

Info

Publication number: CN112694775A
Application number: CN202011515222.9A
Authority: CN
Inventors: 祝京旭; 张辉
Original assignee: Tianjin Xidun Powder Paint Technology Co ltd
Current assignee: Tianjin Xidun Powder Paint Technology Co ltd
Priority date: 2020-12-19
Filing date: 2020-12-19
Publication date: 2021-04-23
Also published as: WO2022127115A1

Abstract

The invention relates to the field of preparation methods of high polymer materials, and particularly discloses a method for improving the corrosion resistance of a powder coating; comprises adding an anti-corrosion auxiliary agent into the powder coating; the anti-corrosion auxiliary agent comprises a first auxiliary agent, a second auxiliary agent, a third auxiliary agent, or a mixture of any two of the first auxiliary agent, the second auxiliary agent and the third auxiliary agent, or a mixture of the three auxiliary agents; the first aid comprises a nanoclay; the second auxiliary agent comprises a composition of barrier type pigment and inorganic salt type antirust pigment; the third auxiliary agent comprises a composition of carbon nano tubes and derivatives thereof and metal powder with a sacrificial anode effect. The method for adding the special compound anticorrosive additive into the common coating is adopted, the obtained powder coating has higher anticorrosive performance and wider application fields including common anticorrosion and heavy anticorrosion, and the method for directly adding the additive is more economic and effective than changing a resin curing agent and has stronger universality.

Description

Method for improving corrosion resistance of powder coating

Technical Field

The invention relates to the field of preparation of high polymer materials, in particular to a method for improving the corrosion resistance of a powder coating.

Background

The solvent-based paint is a mainstream product in the industries of anticorrosion and heavy-duty anticorrosion paint. In the manufacturing, construction and baking curing processes of the traditional solvent-based paint, a large amount of Volatile Organic Compounds (VOCs) volatilize into the air. Many of the widely used solvents are toxic and hazardous substances that are detrimental to the safety, health and environmental (HSE) of the manufacturing process, operators and surrounding environment. In recent years, water-based coatings have been greatly replaced by solvent-based coatings, but due to the limitations of slow drying, severe construction temperature and humidity conditions, high cost and the like, solvent-based coatings cannot be widely replaced.

Powder coatings have gained widespread use and rapid development over liquid coatings in their unique solvent-free manner of manufacture and construction. Its advantages are mainly expressed in "4E", ecological environmental protection (Ecology), excellent coating appearance (Excellence of finish), high Economy (Economy) and low Energy consumption (Energy).

Commonly used powder coating chemistries are 5 types, such as polyester (polyester/TGIC and polyester/HAA), epoxy (epoxy), polyester/epoxy hybrid (polyester/epoxy hybrid), polyurethane (polyurethane), and acrylic (i.e., acrylate). These 5 powder coating systems all have a certain corrosion protection, in particular epoxy powder coatings. However, ether bonds in the main chain of the epoxy resin are easily broken by Ultraviolet (UV) irradiation in sunlight, and thus, cannot be used as an outdoor top-coat paint. Although the corrosion resistance of the polyester powder coating is slightly inferior to that of the epoxy powder coating, the problem of sunlight pulverization does not exist; thus, the above systems all have certain drawbacks or limitations.

Disclosure of Invention

The invention aims to provide a method for improving the corrosion resistance of a powder coating, which maintains the foundation of unchanging a resin system in the formula of the existing powder coating by adding a specific corrosion-resistant auxiliary agent and greatly improves the corrosion resistance of the powder coating.

In order to achieve the technical purpose, the invention adopts the following technical scheme: a method for improving the anticorrosion performance of a powder coating comprises the steps of adding an anticorrosion auxiliary agent into the powder coating; the anti-corrosion auxiliary agent comprises a first auxiliary agent, a second auxiliary agent, a third auxiliary agent, or a mixture of any two of the first auxiliary agent, the second auxiliary agent and the third auxiliary agent, or a mixture of the three auxiliary agents; the first aid comprises a nanoclay; the second auxiliary agent comprises a composition of barrier type pigment and inorganic salt type antirust pigment; the third auxiliary agent comprises a composition of carbon nano tubes and derivatives thereof and metal powder with a sacrificial anode effect.

According to the invention, a method for adding a special compound anticorrosive additive into a common coating is adopted, so that the obtained powder coating has higher anticorrosive performance and wider application fields including common anticorrosion and heavy anticorrosion, and the method for directly adding the additive is more economic and effective than changing a resin curing agent and has stronger universality.

The invention can also be applied to sintering epoxy powder coating to further improve the anti-corrosion performance of the epoxy powder coating, and particularly, the invention solves the problem that the anti-corrosion performance of the powder coating cannot be improved by singly adopting a certain or a certain auxiliary agent under certain conditions through detailed electrochemical representation and mechanism research, and the ultrafine powder coating can greatly reduce the thickness of a coating film under the condition of not reducing the indexes of the surface smoothness, the gloss, the distinctness of image (DOI) and the like of the coating film in the application occasion of the powder coating with high appearance requirement, thereby reducing the use cost.

The formula of the anti-corrosion auxiliary agent is also suitable for a superfine ultrathin powder coating system, the anti-corrosion auxiliary agent is premixed with other formula components such as resin, a curing agent, pigment, filler and other auxiliary agents in the manufacturing process of the powder coating with the anti-corrosion performance improved by adding the anti-corrosion auxiliary agent, and the mixture is subjected to hot processing (co-extrusion in a molten state) by an extruder to achieve homogenization, and the subsequent powdering processing and construction process are the same as those of the common powder coating, so that the anti-corrosion auxiliary agent is high in practicability.

Drawings

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

FIG. 1 is a graph showing the results of electrochemical measurements (OCP) of comparative example 1 and example 7 in the present invention;

FIG. 2 is a graph showing the results of electrochemical measurements in comparative example 1 and example 7 (R) of the present invention_p)；

FIG. 3 is a graph showing the results of UV aging tests of comparative examples 1 to 10 in the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention are clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described and will be readily apparent to those of ordinary skill in the art without departing from the spirit of the present invention, and therefore the present invention is not limited to the specific embodiments disclosed below.

The invention relates to a method for improving the corrosion resistance of a powder coating, which comprises the steps of adding a corrosion-resistant auxiliary agent into the powder coating; the anti-corrosion auxiliary agent comprises a first auxiliary agent, a second auxiliary agent, a third auxiliary agent, or a mixture of any two of the first auxiliary agent, the second auxiliary agent and the third auxiliary agent, or a mixture of the three auxiliary agents;

the first aid comprises a nanoclay; the second auxiliary agent comprises a composition of barrier type pigment and inorganic salt type antirust pigment; the third auxiliary agent comprises a composition of carbon nano tubes and derivatives thereof and metal powder with a sacrificial anode effect.

Therefore, the method of the present invention is to add an anticorrosive auxiliary agent to a powder coating, and the anticorrosive auxiliary agent is applied to a polyester powder coating, an epoxy powder coating, a sintered epoxy powder coating, a polyester/epoxy mixed powder coating, a polyurethane powder coating, an acrylic powder coating, and the like, and the application range is not limited thereto, and is very wide.

In the invention, the first auxiliary agent comprises a composition of nano clay and inorganic salt type antirust pigment; the barrier type pigment is a lamellar structure pigment and comprises graphene and derivatives thereof, or micaceous iron oxide or mica powder; in the invention, the barrier pigment can also be a non-lamellar auxiliary agent, and comprises barium sulfate, kaolin and the like. The barrier pigment is a lamellar structure pigment and may also include a nanoclay.

The inorganic salt type antirust pigment comprises one or more metal salts of zinc phosphate, strontium phosphate, barium phosphate, aluminum phosphate, calcium phosphosilicate, strontium phosphosilicate, barium phosphosilicate, calcium borosilicate, strontium zinc phosphosilicate and lithium zinc phosphate.

In the invention, the graphene and the derivatives thereof comprise graphene, graphene oxide, reduced graphene oxide and graphene derivatives subjected to surface treatment; the nano clay is nano particles of silicate minerals, and comprises montmorillonite, bentonite andor organoclay.

The carbon nano tube and the derivatives thereof comprise one or more of a single-walled carbon nano tube, a multi-walled carbon nano tube, an oxidized carbon nano tube and a reduced oxidized carbon nano tube; the metal powder with the function of the sacrificial anode is metal powder containing one or more of zinc powder, magnesium powder and iron powder.

In the invention, the particle size range of the lamellar structure pigment and the inorganic salt antirust pigment is 5 nanometers to 300 micrometers; the dosage range of the lamellar structure pigment and the inorganic salt antirust pigment is 0.1 to 60 percent by weight. The particle size range of the metal powder with the function of the sacrificial anode is 5 nanometers to 300 micrometers, and the shape of the metal powder comprises a spherical shape and a lamellar shape; the amount of the metal powder having a sacrificial anode function is in the range of 0.1 to 90% by weight, and the amount of the carbon nanotube and its derivative is in the range of 0.1 to 60% by weight.

Therefore, in the present invention, the corrosion prevention auxiliary can enhance the corrosion prevention performance of the coating:

1. the lamellar blocking pigment/filler/auxiliary agent enhances the protective effect by increasing the tortuosity of the electrolyte penetrating into the metal substrate in a corrosive environment. For example, patent CN105907265 discloses an epoxy resin-based powder coating added with modified montmorillonite, so that the boiling resistance and the cathodic disbonding resistance of the coating are improved. The auxiliary agent adopted by the method does not need acidification and intercalation polymerization modification, so that the preparation method is simplified, and the universality of the montmorillonite is improved. Patent CN106752742 discloses an anticorrosive powder coating added with nano calcium carbonate, which improves the salt spray resistance and heat resistance, but does not indicate the applicable conditions of the method, such as whether the method is applicable to the case of having no mechanical damage, and whether the salt spray test is checked. Other components such as wollastonite and attapulgite exist in the formula, and the interaction between the components is not mentioned in the patent. Patent CN109971319 uses glass fiber and clay mineral to improve the weather resistance and corrosion resistance of polyester powder coating, and does not mention the performance of the additive combination in other powder coating systems such as epoxy. It is theorized that such coatings are most suitable for use in submerged environments, such as deep burial or immersion in water, and are not effective in resisting the development of post-scuffing corrosion.

2. The active passivation protective auxiliary agent, zinc phosphate and other salt compounds can be combined with metal ions generated by corrosion of a metal substrate to generate insoluble substances, and the local insoluble substances can be used as a passivation layer to protect the substrate and prevent further corrosion. For example, patent CN101864238 adopts zinc phosphate added into the powder paint formulation to improve the corrosion resistance and chipping resistance of the paint, but does not mention whether the paint can protect the coating without mechanical damage possibility. CN104893493 adopts zinc phosphate, calcium phosphate and other assistants and their combination to develop a high-performance heavy-duty anticorrosive powder coating. It is speculated from the principle that the coating is mostly used in an atmospheric corrosion environment and can resist the expansion of the corrosion of the coating after mechanical damage, but the improvement of the corrosion resistance of the coating which is only used in a water immersion or deep burying environment and has no possibility of continuous damage is limited.

3. The assistant with anode sacrificing function, such as zinc powder, etc. can react with iron to form electrochemical primary cell, zinc is used as anode for self-sacrifice, and iron is used as cathode for protecting iron from corrosion. In patent CN110066569, zinc powder with sacrificial anode effect is compounded with graphene, so that the content of the zinc powder in the coating is reduced, but no indication is given whether the reduced zinc powder has the protection effect of the sacrificial anode. Moreover, the evaluation was only carried out in epoxy powder coating systems, and the performance of the auxiliary combination in other powder coating systems, such as polyester, polyurethane, is not mentioned. Patent CN101560355 uses composite ferrotitanium powder and flaky zinc powder to prepare a heavy-duty anticorrosive antirust powder coating. The zinc flake powder content is very low, which may affect the sacrificial anode function. Patent CN103031024 adopts the cooperation of glass flake and zinc powder, has promoted the performance of ocean anticorrosive powder coating. The high-barrier glass flakes may reduce the conductivity between zinc powders, which affects their electrochemical activity. Such zinc-rich coatings have a high corrosion protection, but too high a zinc powder content (usually greater than 80% by mass) and other large additions of solid particulate aids can affect the processability of the powder coating and the mechanical properties of the film, such as substrate adhesion, impact resistance, etc.

In an exemplary embodiment, the specific processing steps of the corrosion protection assistant and the powder coating are as follows:

step 1: fully mixing and stirring the anti-corrosion auxiliary agent and the powder coating to form a mixture;

step 2: processing the mixture into tablets by an extruder or a tablet press;

and step 3: the flakes are ground with an air classifying mill and sieved to a predetermined particle size.

Experiments were carried out several times on different components of the corrosion protection aid, and several examples were carried out as follows:

[ example 1]

Adding the graphene and the zinc phosphate into raw materials of the polyester/TGIC powder coating according to the addition amounts of 0.5 percent and 1.0 percent respectively by mass proportion, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 2]

Adding the graphene and the zinc phosphate into raw materials of the polyester/epoxy mixed powder coating according to the addition amounts of 0.5 percent and 8.0 percent respectively by mass proportion, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) of less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 3]

Adding the nano clay and the zinc phosphate into the raw materials of the polyurethane type powder coating according to the addition amounts of 6.0 percent and 2.0 percent respectively by mass ratio, uniformly mixing according to the processing step 1, and preparing the anti-corrosion ultrafine powder coating with the median particle size (D50, V) less than or equal to 22 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 4]

Adding 3 mass percent of nano clay powder and 2 mass percent of zinc phosphate into raw materials of the polyester TGIC type powder coating, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 5]

Adding the nanoclay powder into the raw materials of the polyester TGIC type powder coating in an adding amount of 6% by mass, uniformly mixing according to the processing step 1, and preparing the anti-corrosion ultrafine powder coating with the median particle size (D50, V) of less than or equal to 22 mu m from the obtained mixture according to the processing steps 2 and 3.

[ example 6]

Adding the carbon nano tube and the zinc powder into the raw materials of the epoxy powder coating according to the addition amounts of 2.0 percent and 80.0 percent respectively by mass proportion, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) of less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 7]

Adding the carbon nano tube and the zinc powder into the raw materials of the polyester/TGIC powder coating according to the addition amounts of 2.0 percent and 40.0 percent respectively by mass ratio, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) of less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 8]

Adding the carbon nano tube and the zinc powder into the raw materials of the polyester/HAA powder coating according to the addition amounts of 2.0 percent and 40.0 percent respectively by mass ratio, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) of less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 9]

Adding the carbon nano tube and the zinc powder into the raw materials of the acrylic powder coating according to the addition amounts of 2.0 percent and 40.0 percent respectively by mass proportion, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) of less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

[ example 10]

Adding the nano clay, the zinc phosphate, the carbon nano tube and the zinc powder into the raw materials of the epoxy powder coating according to the mass ratio of 2.0 percent, 2 percent and 40 percent respectively, uniformly mixing according to the processing step 1, and preparing the anti-corrosion powder coating with the median particle size (D50, V) less than or equal to 40 mu m from the prepared mixture according to the processing steps 2 and 3.

Table 1 below, is a table of ingredients according to various examples of examples 1 to 10 above:

TABLE 1

For experimental comparison, a number of control experimental examples were also performed, as follows:

[ comparative example 1]

The raw materials of polyester TGIC powder coating are processed according to the same procedure to obtain powder coating with median particle size (D50, V)35 μm.

[ comparative example 2]

The same procedure was followed for the preparation of epoxy powder coating raw materials with 15% barium sulfate filler to produce a powder coating with a median particle size (D50, V) of 35 μm.

[ comparative example 3]

The raw materials of the mixed powder coating and 15 percent of barium sulfate filler are processed according to the same processing steps to prepare the superfine powder coating with the median particle size (D50, V) of 22 mu m.

[ comparative example 4]

Nanoclay was added to the other raw materials of the polyester/TGIC powder coating in an amount of 2.0% to 16.0% and the same processing steps were followed to produce a 22 μm median particle size (D50, V).

[ comparative example 5]

Graphene was added to the other raw materials of the polyester/TGIC powder coating in an amount of 2.0% to 16.0% and the same processing steps were followed to produce a superfine powder coating with a median particle size (D50, V) of 22 μm.

[ comparative example 6]

Zinc phosphate was added to the other raw materials of the polyester/TGIC powder coating in an amount of 2.0% to 16.0%, and an ultrafine powder coating having a median particle diameter (D50, V) of 22 μm was obtained according to the same processing procedure.

[ comparative example 7]

Other raw materials of the epoxy powder coating are mixed with 15% of barium sulfate filler, the obtained mixture is mixed with 20% of zinc powder by mass fraction and 80% of zinc powder by mass fraction, and the powder coating with the median particle size (D50, V) of 35 mu m is prepared according to the same processing steps.

Table 2 below, a table of the ingredients for a number of comparative examples:

TABLE 2

For the above examples to which the corrosion prevention aid of the present invention was added, the evaluation method and the test results were compared with those of the examples to which the corrosion prevention aid of the present invention was not added:

in the powder coating industry, powder coatings are tested for corrosion resistance primarily using the salt spray resistance (ASTM B117 and ASTM D1654) method. Its advantages are simple operation and interpretation. In the field of scientific research, Electrochemical testing tools, in particular Electrochemical Impedance Spectroscopy (EIS), can provide detailed information such as coating barrier properties and their changes over time that salt spray tests cannot provide. The research on the protective coating mechanism is greatly facilitated.

The experimental development work of the invention adopts an Electrochemical method to study the corrosion prevention effect and mechanism of various auxiliary agent combinations in detail, and the numerical values obtained by Electrochemical measurement include the absolute value (| Z |) of the impedance in the low frequency region on the Open Circuit Potential (OCP), the polarization resistance (Rp) and the Electrochemical Impedance Spectrum (EIS) spectral line. In the research, a numerical value corresponding to the frequency of 0.1Hz is used as an index of the shielding effect, and a higher numerical value indicates a better blocking effect. Rp refers to the direct current resistance of the whole system of the coating and the substrate in an OCP +/-10 mV area, and is close to the modular (| Z |) meaning of EIS low-frequency area impedance, and the higher the resistance, the denser the coating is, and the fewer defects are. The open circuit potential OCP qualitatively indicates the thermodynamic stability of the coating system. For barrier coatings, a higher OCP indicates more stable coating properties, i.e., less susceptibility to corrosion. For sacrificial anode type coatings, a lower OCP indicates a higher degree of activation of the auxiliary with sacrificial anode functionality, which is more beneficial for protecting the ferrous substrate.

1. Barrier type common powder coating and barrier type superfine powder coating

Comparative examples 1-7 (original powder coating formulation and formulation with certain auxiliary agents alone) and the powder coatings produced in examples 1-8 were constructed and tested using the same substrates (phosphated cold rolled steel panels, 76X 152X 0.81mm3), the same spray, curing conditions and equipment and to exactly the same dry film thickness of 60. + -. 3 μm for parallel comparison.

The hours of salt spray testing of the corrosion resistant powder coatings of control formulation examples 1-5 and formulation examples 1-8 are shown in Table 2, after salt spray testing (ASTM B117 and D1654, initial cross hatch width 0.5mm, average corrosion width 2mm required hours). Comparing the results, it is known that the use of a single lamellar barrier type auxiliary agent such as nanoclay or graphene has a limited effect of improving the corrosion resistance of the powder coating. The zinc phosphate is singly used, so that the hours of a grid-scribing salt spray test can be greatly increased, namely, a damaged area is repaired through passivation, and the coating stripping and the expansion process of substrate corrosion of the area near the scratch are greatly inhibited.

Table 3 below, the results of the salt spray test for the partial barrier powder coating:

TABLE 3

Through electrochemical tests, the electrochemical test results of the anticorrosive powder coatings in the comparative formula examples 1-6 and the formula examples 1-5 are as follows, and table 4 shows the electrochemical test results of the partial barrier powder coating, which are measured every 24 hours in 0-5 days:

TABLE 4

Comparing the results of comparative examples 4-6, it can be seen that the optimum formulation containing nanoclay has greatly improved lamellar barrier properties of the coating, i.e. the values of Rp and | Z |, and f ═ 0.1Hz are improved by 3 orders of magnitude, compared to comparative examples 1-3. The improvement of barrier properties is not obvious when zinc phosphate is added in different amounts because the particle size is small and the increase of tortuous paths in the coating is less.

The above salt spray test is completely inconsistent with the electrochemical results, because the salt spray test can only reflect the expansion of coating peeling and substrate corrosion under mechanical damage, while the electrochemical means characterizes the detailed process of gradual penetration of the electrolyte solution into the substrate in the coating with the vast majority of the coating area intact. The two are not to be confused. In addition, in the case of nanoclay alone, too high an amount of addition (from 4%) can result in more severe coating delamination and corrosion at the crosshatch due to the water-swelling of the nanoclay.

By adding the auxiliary agent with passivation performance such as zinc phosphate, the defect that the graphene and the nano clay can only play a role without mechanical damage is effectively compensated, and the defect is reflected in higher hours of a salt spray test, various electrochemical test indexes and a salt spray test result. In addition, other properties of the coating, such as specular gloss (ASTM D523), pencil hardness (ASTM D3363), distinctness of image (DOI, ASTM D5767), impact resistance (ASTM D2794), Taber abrasion resistance (ASTM D4060) and the like, can meet the requirements of industrial application, particularly, the pencil hardness can be improved by at least one unit after various additives are added, for example, the pencil hardness is improved from 2B to HB or F, the abrasion resistance is correspondingly improved, and the weight loss value after 500 times of Taber abrasion resistance can be reduced by 20%. 2. Sacrificial anode type heavy-duty anticorrosive powder coating

The powder coatings produced in comparative example 7 and examples 6 to 10 were constructed and tested with the same substrates, surface treatment, spray application, curing conditions and equipment and brought to exactly the same dry film thickness of 60. + -. 3 μm for parallel comparison.

The zinc-rich powder coatings have a stronger corrosion resistance than conventional powder coatings, as measured by salt spray tests (ASTM B117 and D1654). Unlike powder coating systems that use nanoclay alone, zinc powder has the same protective effect on mechanically damaged coatings, both due to the sacrificial anode effect and the fact that the corrosion products of the zinc powder can partially fill scratches and defects, preventing further damage to the coating and further corrosion of the substrate. The zinc powder must be interconnected in the coating to transfer electrons to the ferrous substrate. The carbon nano tube has certain conductivity, can communicate the zinc powder in the coating, enables the zinc powder to fully play a role, and can make up the defect of insufficient communication of the zinc powder particularly under the condition of reducing the zinc powder.

As shown in table 5 below, the results of the salt spray test for a portion of the sacrificial anode-type powder coating:

TABLE 5

The results of the electrochemical tests on the corrosion-inhibiting powder coatings of comparative example 7 and examples 6 to 10 are given in Table 6. The open circuit potential of the zinc-rich powder coating greatly reduced shows that the zinc-rich powder coating has a strong sacrificial anode effect, and the higher coating impedance shows that metal particles such as zinc powder and the like can also improve the barrier property of the coating. The carbon nanotubes can provide similar sacrificial anode protection function under the condition of reducing the content of zinc powder. The additional addition of solid particles also further increases the shielding effect of the coating, delaying the penetration of corrosive media. The zinc powder is relatively high in cost and high in hardness, and the high-addition zinc powder is unfavorable for the service life of a double-screw extruder for processing the powder coating.

As shown in table 6 below, the electrochemical test results of the partial sacrificial anode type powder coating are measured every 24 hours for 0-20 days:

TABLE 6

In the present invention, comparative example 1, i.e. polyester/TGIC varnish, example 7, i.e. a combination of 2% carbon nanotubes and 40% zinc powder; as shown in fig. 1, the results of electrochemical measurements (OCP) of comparative example 1 and example 7 are shown; as shown in FIG. 2, the results of electrochemical measurements (R) of comparative example 1 and example 7 are shown_p)。

Comparative example 1 only shows typical properties of a barrier-type coating, i.e., OCP and Rp decrease with time, indicating penetration of the electrolyte solution into the coating. And the coating is locally corroded at the later stage of the soaking test in the electrochemical impedance spectrum.

The open circuit potential of example 7 fluctuates widely with time, indicating uneven corrosion of the zinc powder in the coating from the surface to the substrate, with the conduction of the zinc powder (as reflected in the open circuit potential value) depending on the spatial distribution of the electrolyte penetrating into the coating. The lowest open circuit potential is below-900 mV and far below the critical value recommended by the literature, namely-800 mV, which proves that the coating has strong sacrificial anode anticorrosion effect. The low open circuit potential value also indicates that the carbon nano tube in the coating plays a role in conducting zinc powder. The polarization resistance of the formula sample plate steadily decreases along with time, and the gradual infiltration of the electrolyte and the corrosion of the zinc powder are reflected. The electrochemical detection result provides important basis for explaining the detailed change of the coating in the corrosive medium along with time.

3. Mechanical properties, UV and solar ageing resistance

The addition of zinc powder has been tested to enhance the pencil hardness (astm d3363) and impact resistance (astm d2794) of the powder coating. For example, comparative example 1 (polyester/TGIC varnish), 2 (epoxy, containing 15% barium sulfate), and 3 (mixed type, containing 15% barium sulfate) all had a pencil hardness of 2B (60 μm film thickness) and impact resistance values of 25, 20, and 5 kg-cm, respectively. After addition of 40% zinc powder, the pencil hardness of example 7 was B and the impact resistance value was 105kg cm. After addition of 80% zinc powder, the pencil hardness of example 6 was HB, and the impact resistance value was 40 kg. cm.

The coatings were tested for UV and solar accelerated weathering (Q-UV and Q-Sun) and examples 1, 3, 4, 5, 6, 7, 8 and 9 all had better UV and solar weathering than examples 2 (polyester/epoxy hybrid) and 10 (epoxy). The resulting gloss retention was the ratio of measured gloss to initial gloss, measured every 7 days. The higher the value, i.e., the lower the gloss reduction and the higher the gloss retention, the higher the UV aging resistance of the coating, as shown in FIG. 3, which is a graph of the UV aging test results of comparative examples 1 to 10 in the present invention.

The higher gloss retention of polyester and polyurethane powder coatings relative to epoxy powder coatings allows these two anticorrosion powder coating systems to be used in demanding outdoor sun exposure applications.

The powder coating prepared according to the application example of the invention can be normally used on carbon steel, cold rolled steel and aluminum substrates except phosphated steel plates, and shows good appearance, mechanical property and corrosion resistance.

Therefore, the method of the invention can be seen to enable the common powder coating to be used in occasions with high corrosion prevention requirements, and simultaneously further improve the performance of the heavy-corrosion-prevention zinc-rich powder coating. In the common powder coating, the anticorrosion powder coating based on polyester powder coating (polyester/TGIC, polyester/HAA and silicon modified polyester coating), polyurethane powder coating (polyurethane), acrylic powder coating (acrylic), fluorine-containing powder coating (such as powder coating using PVDF and FEVE as base materials) and the like can be used for outdoor application occasions of direct sunlight, and the application field of the powder coating is greatly expanded.

Although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that modifications may be made to the embodiments or portions thereof without departing from the spirit and scope of the invention.

Claims

1. A method for improving the corrosion resistance of a powder coating, comprising adding a corrosion protection aid to the powder coating; the anti-corrosion auxiliary agent comprises a first auxiliary agent, a second auxiliary agent, a third auxiliary agent, or a mixture of any two of the first auxiliary agent, the second auxiliary agent and the third auxiliary agent, or a mixture of the three auxiliary agents;

the first aid comprises a nanoclay;

the second auxiliary agent comprises a composition of barrier type pigment and inorganic salt type antirust pigment;

the third auxiliary agent comprises a composition of carbon nano tubes and derivatives thereof and metal powder with a sacrificial anode effect.

2. The method of claim 1, wherein: the anti-corrosion auxiliary agent is applied to polyester powder coatings, epoxy powder coatings, sintered epoxy powder coatings, polyester/epoxy mixed powder coatings, polyurethane powder coatings, acrylic powder coatings and fluorine-containing powder coatings.

3. The method of claim 1, wherein: the first auxiliary agent comprises a composition of nano clay and inorganic salt antirust pigment;

the barrier type pigment is a lamellar structure pigment and comprises graphene and derivatives thereof, or micaceous iron oxide or mica powder;

4. The method of claim 3, wherein: the graphene and the derivatives thereof comprise graphene, graphene oxide, reduced graphene oxide and graphene derivatives subjected to surface treatment;

the nano clay is nano particles of silicate minerals, and comprises montmorillonite, bentonite or organoclay.

5. The method of claim 1, wherein: the carbon nano tube and the derivatives thereof comprise one or more of a single-walled carbon nano tube, a multi-walled carbon nano tube, an oxidized carbon nano tube and a reduced oxidized carbon nano tube;

the metal powder with the function of the sacrificial anode is metal powder containing one or more of zinc powder, magnesium powder and iron powder.

6. The method of claim 3, wherein: the particle size range of the lamellar structure pigment and the inorganic salt antirust pigment is 5 nanometers to 300 micrometers.

7. The method of claim 3, wherein: the dosage range of the lamellar structure pigment and the inorganic salt antirust pigment is 0.1 to 60 percent by weight.

8. The method as claimed in claim 1, wherein the metal powder with sacrificial anode function has a particle size ranging from 5 nm to 300 μm and morphology including spherical and lamellar.

9. The method as claimed in claim 1, wherein the metal powder having a sacrificial anode effect is used in an amount ranging from 0.1% to 90% by weight, and the carbon nanotubes and their derivatives are used in an amount ranging from 0.1% to 60% by weight.

10. The method according to claim 1, characterized in that it comprises:

fully mixing and stirring the anti-corrosion auxiliary agent and the powder coating to form a mixture;

processing the mixture into tablets by an extruder or a tablet press;

the flakes are ground with an air classifying mill and sieved to a predetermined particle size.