Trivalent chromium surface treating agent for aluminum or aluminum alloy as well as preparation method and application of trivalent chromium surface treating agent

Abstract

The invention relates to a trivalent chromium surface treating agent for aluminum or aluminum alloy as well as preparation and application of the trivalent chromium surface treating agent. The invention provides a trivalent chromium surface treating agent for aluminum or aluminum alloy. The trivalent chromium surface treating agent comprises the following raw materials: a) a hexavalent chromium compound; b) an organic reducing agent containing a primary hydroxyl group, a secondary hydroxyl group and / or a phenolic hydroxyl group; c) hydrofluoric acid; d) inorganic acid, wherein the acidity coefficient pKa of the inorganic acid is less than the pKa of hydrofluoric acid; wherein the ratio of Cr in the hexavalent chromium compound to the maximum electron mole number theoretically provided by the organic reducing agent is 1.0: (3.0-5.0); the trivalent chromium is obtained through in-situ reduction of the hexavalent chromium compound. The hexavalent chromium can be completely and efficiently reduced into trivalent chromium in situ under the low-temperature reaction condition, the formula components are simple, the outstanding advantages of low energy consumption and low raw material cost are achieved, the hexavalent chromium is used for conducting surface treatment on an aluminum or aluminum alloy base material, a formed compact passivation film has good coating adhesive force, and the service life of the passivation film is prolonged. And the feather film of the tank cover can be effectively controlled within 0.1 mm.

Description

Technical Field

[0001] This invention relates to the field of metal surface treatment, specifically to trivalent chromium surface treatment agents for aluminum or aluminum alloys, their preparation and uses. Background Technology

[0002] Aluminum and aluminum alloys have seen increasing use in packaging materials in recent years due to their lightweight, ease of processing, good thermal conductivity and barrier properties, attractive appearance, and ease of recycling, especially in the food (beverage), pharmaceutical, and cosmetic industries. Taking the beverage industry as an example, the demand for aluminum or aluminum alloys in various can packaging materials has surged. Aluminum alloys are alloys made by adding small amounts of magnesium, manganese, and copper to aluminum as the base material. For example, 5052 and 5182 aluminum alloys are widely used in the production of can lids due to their high strength and good formability.

[0003] To improve the corrosion resistance of aluminum or aluminum alloy packaging materials, surface treatment, especially passivation, is usually required for the aluminum or aluminum alloy substrate. Passivation is a method of transforming the metal surface into a state that is not easily oxidized, thereby slowing down the corrosion rate. Through passivation, a dense film forms on the metal surface, which can be called a surface treatment film or passivation film. This film changes the surface state of the metal, causing a change in the metal's electrode potential, resulting in a corrosion-resistant passive state. Industrially, passivating agents are commonly used to passivate aluminum or aluminum alloy substrates. Currently, passivating agents are classified into three categories: hexavalent chromium passivating agents, trivalent chromium passivating agents, and chromium-free passivating agents. Among them, hexavalent chromium passivating agents, as traditional passivating agents, have outstanding advantages in corrosion resistance and other properties, but the toxicity of hexavalent chromium is harmful to human health and the environment; chromium-free passivating agents are the most environmentally friendly, but generally suffer from insufficient corrosion resistance; trivalent chromium passivating agents, relatively speaking, combine environmental friendliness and good corrosion resistance, and are currently the most widely researched and applied in the surface treatment of aluminum or aluminum alloy packaging materials.

[0004] However, trivalent chromium passivation still presents some challenges for aluminum alloy can lids, primarily insufficient coating adhesion and feathering defects at the lid's pull-open edge. Can lid production typically involves cleaning, passivation, coating application, curing, and stamping. To ensure good appearance and food safety, the coating applied to the aluminum or aluminum alloy substrate must have strong adhesion to the substrate to prevent paint peeling. Feathering refers to the phenomenon where the coating separates from the substrate at the edge of the lid after pasteurization. The requirements for feathering are particularly stringent for pull-open lids (RPT), typically requiring a feathering standard controlled within 0.1 mm. The passivation effect is the main factor affecting feathering. Currently available trivalent chromium passivating agents cannot control the feathering to within 0.1 mm.

[0005] Patent document CN108823559A discloses a trivalent chromium passivating agent for pretreatment of can lid packaging materials and its preparation method. It employs a chromic acid reduction method, improving the passivating agent's corrosion resistance and coating adhesion by adding components such as organophosphorus compounds, fluorotitanic acid, silane coupling agents, water-soluble polyurethane dispersants, and water-soluble modified alkyd resins to the formulation, effectively controlling the aluminum alloy can lid efflorescence to within 0.1 mm. However, it uses a conventional chromic acid reduction process, requiring heating the system to 80°C and holding it at that temperature for 6 hours. This significantly increases energy consumption and manpower during the heating-holding-cooling process. Furthermore, the complex formulation in this document increases production difficulty and cost, and the document does not provide any experimental data to prove that the disclosed passivating agent possesses the claimed technical effects. Summary of the Invention

[0006] The purpose of this invention is to provide a new trivalent chromium surface treatment agent for aluminum or aluminum alloys, which can effectively solve the problems of high production cost and high energy consumption of trivalent chromium passivating agents in the prior art, poor adhesion of the passivation film obtained by using it, and feathering defects in the can lid pull opening.

[0007] To achieve this objective, the present invention provides the following technical solution:

[0008] A trivalent chromium surface treatment agent for aluminum or aluminum alloys, the raw materials of which include:

[0009] a) Hexavalent chromium compounds;

[0010] b) An organic reducing agent, wherein the organic reducing agent contains primary hydroxyl, secondary hydroxyl and / or phenolic hydroxyl;

[0011] c) Hydrofluoric acid;

[0012] d) Inorganic acid, wherein the acidity coefficient pKa of the inorganic acid is less than that of hydrofluoric acid;

[0013] The ratio of the maximum theoretically provided molar number of electrons for Cr in the hexavalent chromium compound to that for the organic reducing agent is 1.0:3.0 to 5.0.

[0014] The trivalent chromium is obtained by in-situ reduction of the hexavalent chromium compound.

[0015] Furthermore, the present invention also provides a method for preparing the trivalent chromium surface treatment agent for aluminum or aluminum alloys of the present invention, wherein the preparation method employs a low-temperature reduction process.

[0016] Furthermore, the present invention also provides a surface treatment method for aluminum or aluminum alloy, which includes: applying the trivalent chromium surface treatment agent for aluminum or aluminum alloy of the present invention to the surface of a substrate made of aluminum or aluminum alloy, and heating and drying to form a surface treatment film of aluminum or aluminum alloy.

[0017] Furthermore, the present invention also provides a surface treatment film for aluminum or aluminum alloy, which is formed using the surface treatment method for aluminum or aluminum alloy of the present invention.

[0018] Furthermore, the present invention also provides a packaging material having a surface-treated film of aluminum or aluminum alloy according to the present invention.

[0019] This invention, through formulation design and control of the content of organic reducing agents, allows the components of the formulation to work synergistically, enabling hexavalent chromium to be completely and efficiently reduced to trivalent chromium in situ under low-temperature reaction conditions. Furthermore, the formulation is simple in composition and has the outstanding advantages of low energy consumption and low raw material cost. When used to treat aluminum or aluminum alloy substrates, the resulting dense passivation film has good coating adhesion, and the feathering on the can lid can be effectively controlled within 0.1 mm.

[0020] It should be noted that the above description does not disclose all embodiments of the present invention or all advantages of the present invention. Attached Figure Description

[0021] Figure 1 To observe the feather film test results of aluminum alloy substrates treated with the trivalent chromium surface treatment agent of the present invention after paint coating and boiling water test using an optical microscope. Detailed Implementation

[0022] The following describes embodiments of the present invention, but the invention is not limited thereto. The present invention is not limited to the configurations described below; various modifications can be made within the scope of the claims. Embodiments and examples obtained by appropriately combining the disclosed technical means in different embodiments and examples are also included within the technical scope of the present invention. Furthermore, all documents described in this specification are incorporated herein by reference.

[0023] Unless otherwise defined, the technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] In the context of describing this specification (especially in the context of the appended claims), the terms “a,” “an,” and “the,” and similar language will be interpreted to cover both the singular and plural, unless otherwise indicated herein or clearly contradicted by the context.

[0025] In this specification, the range of values ​​referred to as “value A ~ value B” or “value A - value B” refers to the range that includes the endpoint values ​​A and B and all ranges between the endpoints.

[0026] In this specification, the word "may" has both the meaning of performing a certain process and the meaning of not performing a certain process. In this specification, "optional" or "optionally" means that the event or situation described below may or may not occur, and the description includes both the case where the event occurs and the case where the event does not occur.

[0027] In this specification, the terms "some specific / preferred embodiments," "other specific / preferred embodiments," "some specific / preferred technical solutions," and "other specific / preferred technical solutions" refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to the described embodiment that are included in at least one of the embodiments described herein, and may or may not exist in other embodiments. Furthermore, it should be understood that these elements can be combined in any suitable manner in various embodiments.

[0028] In this specification, the term "comprising" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that comprises a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0029] In this specification, expressions such as "containing A, B and / or C" or "including A, B and / or C" mean containing A or B or C, or containing any two of A and C, or containing all three of A, B and C.

[0030] In this specification, the term "organic" refers to a carbon-containing compound.

[0031] In this specification, the term "inorganic" refers to compounds that do not contain carbon atoms except for carbonates and oxides of carbon.

[0032] In this specification, the term "polymer" refers to a molecule comprising a large number of similar units bonded together by covalent bonds.

[0033] In this specification, phrases such as “does not contain” or “does not include” are used. These phrases are not intended to exclude the presence of trace amounts of related compounds or chemical structures that may be present but were not intentionally used, such as the presence of environmental pollutants.

[0034] In this specification, the term "(meth)acrylic resin" refers to a product obtained by polymerizing a monomer composition containing at least one monomer selected from acrylic acid and its esters, methacrylic acid and its esters.

[0035] In this specification, the term "low temperature" means that the redox reaction of the present invention does not require heating by an external heat source, but does not exclude the possibility of the reaction system temperature rising due to the exothermic reaction of the present invention. In some specific embodiments of the present invention, "low temperature" means that the redox reaction does not exceed 40°C.

[0036] In this specification, the number of moles or molar amount of each element refers to the number of moles or molar amount of the element calculated based on the number of moles or molar amount of the component and the composition ratio of the element in the component, and does not refer to the number of moles or molar amount of free ions or molecules containing the element in the reaction system.

[0037] In this specification, the term "substrate" refers to aluminum or aluminum alloy material that has not been treated by the surface treatment methods described in this invention.

[0038] Trivalent chromium surface treatment agent

[0039] This invention provides a trivalent chromium surface treatment agent for aluminum or aluminum alloys, the raw materials of which include:

[0040] a) Hexavalent chromium compounds;

[0041] b) An organic reducing agent containing primary hydroxyl, secondary hydroxyl, and / or phenolic hydroxyl groups.

[0042] c) Hydrofluoric acid;

[0043] d) Inorganic acid, wherein the acidity coefficient pKa of the inorganic acid is less than that of hydrofluoric acid;

[0044] Wherein, the ratio of the maximum number of moles of Cr in the hexavalent chromium compound to the organic reducing agent theoretically provided is 1.0:3.0 to 5.0; preferably, the mole ratio is 1.0:3.2 to 4.8; more preferably, the mole ratio is 1.0:3.5 to 4.5.

[0045] The active material trivalent chromium (Cr(III)) in the trivalent chromium surface treatment agent of this invention is obtained by in-situ reduction of the hexavalent chromium (Cr(VI)) compound of this invention. "In-situ reduction" means reduction in place within the same system; trivalent chromium is obtained by reducing the hexavalent chromium compound under the action of an organic reducing agent, rather than adding the trivalent chromium compound to the system of this invention. Compared with schemes that directly use trivalent chromium compounds as the active material for passivation, this invention not only possesses the performance advantages of trivalent chromium passivating agents but also achieves significant improvements in raw material costs. Furthermore, because the formulation of this invention does not contain other strong oxidizing agents in the system, it also reduces the possibility of trivalent chromium converting to hexavalent chromium under certain conditions.

[0046] In the present invention, except for component c), each component of the other raw materials can be used alone or two or more of them can be used in combination in a desired ratio.

[0047] Component a) of the present invention is a hexavalent chromium compound, which has oxidizing property and can be reduced to trivalent chromium under the conditions of the present invention. In some embodiments of the present invention, the hexavalent chromium compound is a water-soluble hexavalent chromium compound, and its existence form in an aqueous solution is closely related to pH and concentration. For example, under strong acidic conditions (pH < 1), hexavalent chromium mainly exists in the form of H2CrO4 molecules. After the pH increases (1 < pH < 6), the hexavalent chromium in the solution mainly exists in the form of - HCrO4 - anionic form, and when the concentration of hexavalent chromium is high, HCrO4 2- can undergo dehydration condensation to form dichromate anions Cr2O7 2- . In some other embodiments of the present invention, the hexavalent chromium compound is selected from one or more of chromium trioxide (CrO3, chromic anhydride), chromic acid (H2CrO4) and / or hexavalent chromate. Further, the hexavalent chromate is a water-soluble hexavalent chromate, such as water-soluble chromate or dichromate. Examples include one or more of alkali metal chromates such as sodium chromate and potassium chromate; alkali metal dichromates such as sodium dichromate and potassium dichromate; magnesium chromate; ammonium dichromate. Preferably, the hexavalent chromium compound of the present invention is selected from chromium trioxide and / or chromic acid. The hexavalent chromium compound of the present invention can be obtained commercially.

[0048] A reducing agent is a substance that loses electrons in an oxidation-reduction reaction. Component b) of the present invention is an organic reducing agent, which contains primary hydroxyl groups, secondary hydroxyl groups and / or phenolic hydroxyl groups.

[0049] In some embodiments of the present invention, the organic reducing agent contains primary and / or secondary hydroxyl groups. When they react with hexavalent chromium (Cr(VI)), the primary hydroxyl groups are oxidized to aldehydes, acids, etc., while the secondary hydroxyl groups are oxidized to ketones, etc. In some embodiments, the organic reducing agent containing primary and / or secondary hydroxyl groups may undergo carbon chain cleavage, releasing carbon dioxide. The term "primary hydroxyl group" in the present invention refers to a functional group in an alcohol molecule where the carbon atom bonded to the hydroxyl group has two α-H groups (referring to the hydrogen atoms on the carbon atom bonded to the hydroxyl group), for example, the hydroxyl group in CH3CH2OH is a primary hydroxyl group; the term "secondary hydroxyl group" refers to a functional group in an alcohol molecule where the carbon atom bonded to the hydroxyl group has only one α-H group, for example, the hydroxyl group in CH3CH(CH3)OH is a secondary hydroxyl group. In the presence of hexavalent chromium, the α-H groups of primary or secondary hydroxyl groups can donate electrons to hexavalent chromium, causing the primary or secondary hydroxyl groups to be oxidized. The inventors have found that organic reducing agents containing only tertiary hydroxyl groups, such as citric acid, are not suitable for the system of the present invention. In some specific embodiments of the present invention, the organic reducing agent includes one or more of glucose, sucrose, gluconic acid, tartaric acid, ascorbic acid, mannitol, malic acid, isocitrate, and / or water-soluble salts corresponding to the above compounds; more preferably, an organic reducing agent molecule contains two or more primary hydroxyl groups and / or secondary hydroxyl groups, such as sucrose and / or gluconic acid, which facilitates the completion of the redox reaction in a shorter time. Such organic reducing agents of the present invention are commercially available.

[0050] In other embodiments of the present invention, the organic reducing agent contains a phenolic hydroxyl group. Although the phenolic hydroxyl group does not have α-H, the conjugation of the phenolic hydroxyl group with the benzene ring increases the electron cloud density at the ortho and para positions of the hydroxyl group. Under the action of hexavalent chromium, such as potassium dichromate (K2Cr2O7), the phenolic reducing agent can undergo an oxidation reaction to generate benzoquinone. In some specific embodiments of the present invention, the organic reducing agent includes one or more of tannic acid, gallic acid, and / or the water-soluble salts corresponding to the above compounds. Such organic reducing agents are also commercially available.

[0051] To achieve a complete conversion of hexavalent chromium to trivalent chromium at low temperatures, the inventors discovered the need to control the molar number of Cr in the hexavalent chromium compound and the maximum number of molar electrons theoretically provided by the organic reducing agent. The ratio of Cr in the hexavalent chromium compound to the maximum number of molar electrons theoretically provided by the organic reducing agent is 1.0:3.0–5.0; preferably, the ratio is 1.0:3.2–4.8, and more preferably, it is 1.0:3.5–4.5. If the maximum number of molar electrons theoretically provided by the organic reducing agent is too low, the redox reaction will be incomplete at low temperatures, resulting in residual hexavalent chromium in the system over a long period. If the maximum number of molar electrons theoretically provided by the organic reducing agent is too high, a large amount of organic reducing agent will remain, increasing raw material costs. In this invention, the maximum number of molar electrons theoretically provided by the organic reducing agent is related to the number of molar α-H atoms provided by the organic reducing agent and / or the number of molar hydrogen atoms at the ortho and para positions of the phenolic hydroxyl group. For organic reducing agents containing primary and / or secondary hydroxyl groups, the theoretical maximum number of moles of electrons provided by the organic reducing agent is 2 × the number of moles of α-H that the organic reducing agent can provide. For example, the theoretical maximum number of moles of electrons provided by 1 mol of sucrose is 2 × 11 = 22 mol. For organic reducing agents containing phenolic hydroxyl groups, the theoretical maximum number of moles of electrons provided by the organic reducing agent is the number of moles of ortho- and para-hydrogens in the phenolic hydroxyl group that the organic reducing agent can provide. For example, the theoretical maximum number of moles of electrons provided by 1 mol of phenol is 3 mol. In some embodiments of the present invention, the molar amount of the organic reducing agent is 1.2 times or more of the theoretical maximum number of moles of electrons provided by the organic reducing agent. More preferably, the molar amount of the organic reducing agent is between 1.2 and 3 times the theoretical maximum number of moles of electrons provided by the organic reducing agent. Even more preferably, the molar amount of the organic reducing agent is between 1.3 and 2.5 times the theoretical maximum number of moles of electrons provided by the organic reducing agent.

[0052] Component c) of this invention is hydrofluoric acid, which is an aqueous solution of hydrogen fluoride gas. Component c) is essential for achieving the objectives of this invention. Replacing component c) with other fluorides would prevent the redox reaction from being completed efficiently and completely at low temperatures, potentially leading to the presence of hexavalent chromium residue. In the system of this invention, when the hexavalent chromium is reduced to trivalent chromium, the trivalent chromium tends to combine with fluoride ions to form chromium trifluoride, which is soluble in hydrofluoric acid. In other words, the use of hydrofluoric acid effectively increases the solubility of trivalent chromium and reduces the likelihood of precipitating trivalent chromium phosphate. In some possible embodiments of this invention, the raw materials of the trivalent chromium surface treatment agent may include, in addition to component c), appropriate amounts of other water-soluble fluorides, such as sodium fluoride, potassium fluoride, sodium hydrogen fluoride, potassium hydrogen fluoride, ammonium hydrogen fluoride, etc., or one or more of these. To ensure the sufficiency of the redox reaction at low temperatures and to obtain a denser passivation film, in a preferred embodiment of this invention, the fluoride in the raw materials of the trivalent chromium surface treatment agent is only component c).

[0053] Component c) is commercially available, typically at a concentration of 40%. Regarding the content or amount of component c), in some preferred embodiments of the invention, the molar ratio of Cr in the hexavalent chromium compound to F in the hydrofluoric acid is 1:1.8 to 5.0, more preferably 1:2.0 to 4.0, and even more preferably 1:2.2 to 3.5. Excessive free hydrogen fluoride may lead to over-etching on the surface of the aluminum or aluminum alloy substrate. Insufficient free hydrogen fluoride will not effectively promote the dissolution of trivalent chromium, preventing complete reaction.

[0054] Component d) of this invention is an inorganic acid other than hydrofluoric acid, and the acidity coefficient pKa of this inorganic acid is lower than that of hydrofluoric acid. The lower the pKa value, the stronger the acidity. That is to say, the inorganic acid of this invention is stronger than hydrofluoric acid. In some embodiments of this invention, component d) is a strong or moderately strong inorganic acid stronger than hydrofluoric acid. Hydrofluoric acid generally has a pKa value of 3.17 when measured underwater at 25°C. In some embodiments of this invention, component d) can be selected from inorganic acids with a pKa value of not more than 3.0 when measured underwater at 25°C. Preferably, component d) is selected from one or more of phosphoric acid, sulfuric acid, nitric acid, hydrochloric acid, perchloric acid, hydroiodic acid, and hydrobromic acid. More preferably, the inorganic acid is selected from phosphoric acid and / or nitric acid, which can provide the acidic environment required by the system and also help to form a denser passivation film on the surface of aluminum or aluminum alloy. Regarding the content of component d), in some preferred embodiments of the present invention, the amount of component d) is usually excessive, and its addition amount is sufficient to satisfy the pH < 3.0 of the trivalent chromium surface treatment agent of the present invention.

[0055] In some embodiments of the present invention, the trivalent chromium surface treatment agent of the present invention further includes component e) a film-forming polymer, which can further improve the adhesion between the passivation film and subsequent coatings. There is no particular limitation on the type of component e), and in some embodiments of the present invention, the film-forming polymer is selected from one or more of polyurethane resins, epoxy resins, polyolefin resins, phenolic resins, polyester resins, (meth)acrylic resins, and (meth)acrylic urethane resins. Suitable polyurethane resins are typically addition polymerization products of organic compounds and polyisocyanates having at least two active hydrogen functional groups, for example, in the form of an aqueous dispersion. Polyurethane resins can be modified to be hydrophilically stable or have increased dispersibility in aqueous media by introducing cationic or anionic modifying groups or potential ionic groups that can be converted into cationic or anionic groups. Such polyurethane resins are generally referred to in the art as ionicly hydrophilically stable polyurethane resins. Alternatively, polyurethane resins can be modified by introducing nonionic hydrophilic modifying groups. Examples of epoxy resins include: epoxy compounds having two or more glycidyl groups; epoxy compounds having bisphenol A or bisphenol F as a backbone unit; epoxy resins obtained by reacting epoxy compounds having two or more glycidyl groups with diamines such as ethylenediamine to undergo cationization; and nonionic epoxy resins obtained by adding polyethylene glycol to the side chains of epoxy compounds having bisphenol A or bisphenol F as backbone units or other epoxy compounds having two or more glycidyl groups. Examples of polyolefin resins include polypropylene; polyethylene; modified polyolefins obtained by modifying polyolefins such as copolymers of propylene, ethylene, and α-olefins with unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid); and copolymers of ethylene and acrylic acid (methacrylic acid). Furthermore, copolymers obtained by copolymerizing with small amounts of other vinyl unsaturated monomers are also possible. As a method of water-based conversion, ammonia or amines can be used to neutralize the carboxylic acid introduced onto the polyolefin resin. As a (meth)acrylic resin, it can be obtained by addition polymerization of unsaturated (meth)acrylic acid monomers. The (meth)acrylic resin can be any resin among homopolymers or copolymers of (meth)acrylic acid monomers. In some embodiments of the invention, suitable (meth)acrylic resins are typically hydroxyl-containing (meth)acrylic resins, which can be copolymers of hydroxyl-containing polymerizable unsaturated monomers and at least one unsaturated monomer copolymerized with said hydroxyl-containing polymerizable unsaturated monomer, for example, in the form of an aqueous dispersion. These film-forming polymers are commercially available. The invention does not impose any particular limitation on the content of component e), and those skilled in the art can determine its content according to actual needs.

[0056] In some specific embodiments of the present invention, the pH value of the trivalent chromium surface treatment agent of the present invention is less than 3, preferably not greater than 2.8, and more preferably in the range of 1.5 to 2.5. If the trivalent chromium surface treatment agent is diluted during use, the pH range is preferably less than 2.5. In some embodiments of the present invention, the trivalent chromium surface treatment agent of the present invention may further include a pH buffer, such as one or more of the following buffering acids or salts: sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, ammonium dihydrogen phosphate, acetic acid, sodium acetate, etc., to mitigate pH changes in the trivalent chromium surface treatment agent. In other preferred embodiments of the present invention, the raw material of the trivalent chromium surface treatment agent of the present invention, except for components a) to e), is water.

[0057] The hexavalent chromium compound in the trivalent chromium surface treatment agent of this invention can be fully reduced to trivalent chromium. The trivalent chromium surface treatment agent of this invention, when tested according to the qualitative detection method for hexavalent chromium of this invention, can be determined to have no hexavalent chromium residue, thus meeting environmental protection requirements. In some specific embodiments of this invention, "no hexavalent chromium residue" means that the composition contains no more than 50 mg / L of free hexavalent chromium.

[0058] Preparation method of trivalent chromium surface treatment agent

[0059] This invention also provides a method for preparing the trivalent chromium surface treatment agent of this invention. The method for preparing the trivalent chromium surface treatment agent for aluminum or aluminum alloys of this invention includes,

[0060] Step A: Using water as a solvent, mix and dissolve a) hexavalent chromium compound, c) hydrofluoric acid and d) inorganic acid to obtain mixture I;

[0061] Step B: Under conditions where no external heat source is used to heat the reaction system, add the organic reducing agent (b) to the mixture I and stir to carry out the redox reaction.

[0062] The hexavalent chromium compound of the present invention can be completely and efficiently reduced in situ to trivalent chromium under low-temperature reaction conditions, which can significantly reduce energy consumption and is suitable for industrial use and promotion.

[0063] In some embodiments of the present invention, step A has no special restrictions on the mixing method and the order of addition. It is only necessary to fully dissolve a) hexavalent chromium compound, c) hydrofluoric acid and d) inorganic acid in water. The mixing and stirring in step A are carried out at room temperature (e.g., 15-30°C) without the need for an external heat source to heat the reaction system.

[0064] Step B of this invention is a redox reaction step, which does not require an external heat source to heat the reaction system. The organic reducing agent, under thorough stirring, undergoes a redox reaction with the hexavalent chromium compound. Heat is released upon the start of the reaction, and in some embodiments of this invention, carbon dioxide is also released. In some preferred embodiments of this invention, by controlling the addition rate of the organic reducing agent, the temperature of the redox reaction in step B does not exceed 40°C. More preferably, step B proceeds stably at 20–35°C. In some embodiments of this invention, the addition rate of the organic reducing agent is controlled so that the reaction temperature does not exceed 40°C, for example, it can be 0.5–2.0 kg / min. After completing the step of adding the reducing agent, stirring continues for 2–4 hours. In some preferred embodiments of this invention, stirring continues for 3 hours to allow the redox reaction to proceed more completely.

[0065] In some embodiments of the present invention, the preparation method of the present invention further includes:

[0066] Step C: Detect the presence of hexavalent chromium in the solution. If the result is negative, continue with the following steps.

[0067] Step D: Mix the film-forming polymer (e) to obtain the trivalent chromium surface treatment agent.

[0068] Step C can utilize conventional hexavalent chromium detection methods in the field, such as the method specified in GB / T 7467-1987 or the following qualitative detection methods for hexavalent chromium:

[0069] In some specific embodiments of the present invention, the specific steps of the qualitative detection method for hexavalent chromium are as follows: Take 2g of the test solution, dilute it 50 times with water, add 2mL of concentrated sulfuric acid and mix well, then add 2g of potassium iodide reagent and 1mL of 1% starch solution, and observe the color of the system. If the color of the system remains unchanged (it is determined that there is no hexavalent chromium residue in the system) or turns black (potassium iodide is oxidized to I2 by hexavalent chromium and turns black when it comes into contact with starch). If the system turns black, continue to add 0.1mol / L sodium thiosulfate solution (to reduce I2, and the black color disappears). If the black color disappears after adding 1 drop or less, it is determined that there is no hexavalent chromium residue in the system; if the black color disappears after adding more than 1 drop, it is determined that there is obvious hexavalent chromium residue.

[0070] If the test results show no hexavalent chromium residue, the redox reaction of the present invention is confirmed to be complete. The resulting reaction mixture solution does not require cooling and can be used for step D (i.e., mixing the film-forming polymer e) under stirring. There are no particular restrictions on the mixing method, resulting in the trivalent chromium surface treatment agent of the present invention.

[0071] The preparation method of the present invention has low energy consumption, low cost, high reaction efficiency, short time, and is easy to apply in industry.

[0072] Surface treatment methods for aluminum or aluminum alloys

[0073] The present invention also provides a surface treatment method for aluminum or aluminum alloy, which includes applying the trivalent chromium surface treatment agent for aluminum or aluminum alloy of the present invention to the surface of a substrate made of aluminum or aluminum alloy, and heating and drying to form a surface treatment film for aluminum or aluminum alloy.

[0074] The trivalent chromium surface treatment agent of the present invention is used to treat substrates made of aluminum or aluminum alloys. Examples of substrates made of aluminum or aluminum alloys include: sheet materials made of aluminum or aluminum alloys, packaging foils, etc.

[0075] In some embodiments of the present invention, the surface treatment method for aluminum or aluminum alloys of the present invention includes applying the above-described trivalent chromium surface treatment agent of the present invention to the surface of a substrate made of aluminum or aluminum alloy. In some embodiments of the present invention, in order to obtain better coating adhesion, the substrate made of aluminum or aluminum alloy is preferably pretreated before applying the trivalent chromium surface treatment agent of the present invention. The pretreatment steps include degreasing, rinsing, pickling, rinsing, and drying, preferably with a drying temperature in the range of 80-100°C. There are no particular limitations on the application method of the trivalent chromium surface treatment agent, and it is preferred to use a stainless steel doctor blade. In some embodiments of the present invention, the trivalent chromium surface treatment agent of the present invention can be applied directly or diluted before application. The contact time between the trivalent chromium surface treatment agent and the substrate surface is typically about 0.5-180 seconds. After coating, the substrate is heated and dried at a temperature of 60-250°C to form a surface treatment film on the aluminum or aluminum alloy, preferably at a temperature of 80-200°C. There are no special restrictions on the heating and drying methods. Intermittent or continuous hot air circulation drying ovens, conveyor belt hot air drying ovens, or electromagnetic induction heating ovens can be used. The air volume and air speed can be set according to actual needs.

[0076] This invention also provides a surface treatment film for aluminum or aluminum alloys, which is formed using the aforementioned surface treatment method for aluminum or aluminum alloys. In some embodiments of this invention, the surface treatment film is a passivation film formed on the surface of aluminum or aluminum alloys by a trivalent chromium surface treatment agent. The formation of the surface treatment film of this invention mainly involves the etching of the aluminum or aluminum alloy surface by an acidic trivalent chromium surface treatment agent. H ions are consumed on the aluminum plate surface, leading to an increase in the pH of the microenvironment and the formation of deposits such as chromium oxides. The composition of the surface treatment film of this invention is relatively complex, mainly including chromium oxides, and may also include deposits of chromium phosphate, chromium fluoride, and organic matter. In some specific embodiments of this invention, the surface treatment film of this invention has a weight of approximately 10–100 mg / m³ after drying. 2 Preferred concentration: 20–60 mg / m³ 2(Calculated using solids content and roller coating film thickness). If the weight of the surface treatment film is too high, it may affect processing performance; if the weight of the surface treatment film is too low, it may result in insufficient coating adhesion.

[0077] Packaging materials

[0078] This invention also provides a packaging material having the aforementioned surface-treated film of aluminum or aluminum alloy. In some preferred embodiments of this invention, the packaging material is a can packaging material, such as the body or lid material of a food can, a food container, or the outer casing material of a secondary battery. More preferably, the packaging material is a can lid material. In some embodiments of this invention, the packaging material is mainly obtained by passivating an aluminum or aluminum alloy substrate with the trivalent chromium surface treatment agent of this invention, drying it, coating it with a coating composition, and then curing it. This invention does not particularly limit the coating composition and the coating and curing methods. For the can lid material, the coating composition is preferably a solvent-based transparent coating composition, and the dry film thickness is preferably 10-30 μm, more preferably 15-20 μm. The packaging material of this invention has good coating adhesion and excellent coating tightness. In some specific embodiments of this invention, the surface treatment method of this invention can obtain a can lid that meets the feathering test. Further, the feathering width of this invention is less than 100 μm, preferably less than 80 μm, more preferably less than 70 μm, or there are no feathering defects.

[0079] The following embodiments are provided to illustrate the invention in more detail.

[0080] Implementation Plan 1:

[0081] A trivalent chromium surface treatment agent for aluminum or aluminum alloys, the raw materials of which include:

[0082] a) Hexavalent chromium compounds;

[0083] b) An organic reducing agent containing primary hydroxyl, secondary hydroxyl, and / or phenolic hydroxyl groups.

[0084] c) Hydrofluoric acid;

[0085] d) Inorganic acid, wherein the acidity coefficient pKa of the inorganic acid is less than that of hydrofluoric acid;

[0086] Wherein, the ratio of the maximum number of moles of Cr in the hexavalent chromium compound to the organic reducing agent theoretically provided is 1.0:3.0 to 5.0; preferably, the mole ratio is 1.0:3.2 to 4.8; more preferably, the mole ratio is 1.0:3.5 to 4.5;

[0087] The trivalent chromium is obtained by in-situ reduction of the hexavalent chromium compound.

[0088] Implementation Plan 2:

[0089] According to the trivalent chromium surface treatment agent of the first embodiment, wherein,

[0090] The trivalent chromium surface treatment agent further includes: e) a film-forming polymer; more preferably, the remaining component of the trivalent chromium surface treatment agent is water.

[0091] Implementation Plan 3:

[0092] According to the trivalent chromium surface treatment agent of the first or second embodiment, wherein,

[0093] The hexavalent chromium compound is selected from one or more of chromium trioxide, chromic acid, and / or hexavalent chromates.

[0094] Implementation Plan 4:

[0095] According to any of the embodiments 1 to 3, the trivalent chromium surface treatment agent, wherein,

[0096] The organic reducing agent is selected from one or more of glucose, sucrose, gluconic acid, tartaric acid, malic acid, tannic acid, gallic acid, isocitric acid, ascorbic acid, mannitol and / or their water-soluble salts.

[0097] Implementation Plan 5:

[0098] According to any of the embodiments 1 to 4, the trivalent chromium surface treatment agent, wherein,

[0099] The molar ratio of Cr in the hexavalent chromium compound to F in the hydrofluoric acid is 1.0:1.8 to 5.0, preferably 1.0:2.0 to 4.0, and more preferably 1.0:2.2 to 3.5.

[0100] Implementation Plan 6:

[0101] According to any of the embodiments 1 to 5, the trivalent chromium surface treatment agent, wherein,

[0102] The inorganic acid has an acidity coefficient pKa of no more than 3.0. Preferably, the inorganic acid is selected from one or more of phosphoric acid, sulfuric acid, nitric acid, hydrochloric acid, perchloric acid, hydroiodic acid, and hydrobromic acid. More preferably, the inorganic acid is selected from phosphoric acid.

[0103] Implementation Plan 7:

[0104] According to any of the embodiments 1 to 6, the trivalent chromium surface treatment agent, wherein,

[0105] The pH of the trivalent chromium surface treatment agent is less than 3.0.

[0106] Implementation Plan No. 8:

[0107] According to any of the embodiments 1 to 7, the trivalent chromium surface treatment agent, wherein,

[0108] The film-forming polymer is selected from one or more of polyurethane resin, epoxy resin, polyolefin resin, phenolic resin, polyester resin, (meth)acrylic resin and (meth)acrylic urethane resin.

[0109] Implementation Plan 9:

[0110] According to any of the embodiments 1 to 8, the trivalent chromium surface treatment agent, wherein,

[0111] The trivalent chromium surface treatment agent contains no hexavalent chromium residue.

[0112] Implementation Plan 10:

[0113] A method for preparing a trivalent chromium surface treatment agent for aluminum or aluminum alloys as described in any of embodiments 1 to 9, comprising,

[0114] Step A: Using water as a solvent, mix and dissolve a) hexavalent chromium compound, c) hydrofluoric acid and d) inorganic acid to obtain mixture I;

[0115] Step B: Under conditions where no external heat source is used to heat the reaction system, add the organic reducing agent (b) to the mixture I and stir to carry out the redox reaction.

[0116] Implementation Plan 11:

[0117] According to the method for preparing the trivalent chromium surface treatment agent described in the 10th embodiment, wherein,

[0118] The preparation method further includes:

[0119] Step C: Detect the presence of hexavalent chromium in the solution. If the result is negative, continue with the following steps.

[0120] Step D: Mix the film-forming polymer (e) to obtain the trivalent chromium surface treatment agent.

[0121] Implementation Plan 12:

[0122] The method for preparing the trivalent chromium surface treatment agent according to embodiment 10 or 11, wherein,

[0123] The temperature of the redox reaction in step B) is not higher than 40°C, and preferably the temperature of the redox reaction is 20-35°C.

[0124] Implementation Plan 13:

[0125] A surface treatment method for aluminum or aluminum alloys, comprising:

[0126] The trivalent chromium surface treatment agent for aluminum or aluminum alloy described in any of the embodiments 1 to 9 is applied to the surface of a substrate made of aluminum or aluminum alloy, and then heated and dried to form a surface treatment film of aluminum or aluminum alloy.

[0127] Implementation Plan 14:

[0128] A surface treatment film for aluminum or aluminum alloy is formed using the surface treatment method for aluminum or aluminum alloy described in embodiment 13.

[0129] Implementation Plan 15:

[0130] A packaging material having a surface-treated film of aluminum or aluminum alloy as described in embodiment 14, preferably, the packaging material is a can packaging material, more preferably, the packaging material is a can lid material.

[0131] The present invention will be explained in more detail below with reference to embodiments and comparative examples, and it should be understood that the present invention is not limited to these embodiments.

[0132] [Preparation of surface treatment agents]

[0133] Examples 1-4

[0134] Hexavalent chromium compound (chromium trioxide CrO3), hydrofluoric acid (HF), and phosphoric acid (H3PO4) were mixed and dissolved in deionized water. The mixture was stirred thoroughly to dissolve the chromium. The molar amount of Cr provided by chromium trioxide was 1.0 mol, the molar amount of F provided by hydrofluoric acid is shown in Table 1, and the amount of phosphoric acid added was 0.6 mol. An organic reducing agent was slowly added at room temperature (15–25 °C). The types of organic reducing agents and their theoretical maximum number of electrons provided are shown in Table 1. A redox reaction occurred under thorough stirring; the reaction was exothermic. By controlling the rate of addition of the organic reducing agent, the redox reaction could be stably controlled below 35 °C. After adding the organic reducing agent, stirring was continued for 3 hours. The solution was tested to confirm the absence of Cr-VI (hexavalent chromium) residue. The system could be mixed with polyacrylate under stirring without cooling to prepare a trivalent chromium surface treatment agent for aluminum or aluminum alloys, with a pH of 2.8.

[0135] The test solution uses the qualitative detection method for hexavalent chromium: Take 2g of the test solution, dilute it 50 times with water, add 2mL of concentrated sulfuric acid and mix well, then add 2g of potassium iodide reagent and 1mL of 1.0% starch solution. Observe the color of the system. If the color of the system remains unchanged (indicating no hexavalent chromium residue) or turns black (indicating that potassium iodide has been oxidized to hexavalent chromium), then the system is safe to use. 2，(It turns black upon contact with starch). If the system turns black, continue adding 0.1 mol / L sodium thiosulfate solution (reducing I2, the black color disappears). If the black color disappears after adding 1 drop or less, it is determined that there is no hexavalent chromium residue in the system; if the black color disappears after adding more than 1 drop, it is determined that there is significant hexavalent chromium residue.

[0136] Comparative Example 1

[0137] Compared to Example 4, the difference is that hydrofluoric acid was not added, while other conditions remained the same. After completing the step of adding the reducing agent, stirring was continued for 3 hours, and the solution was tested. Significant Cr-VI (hexavalent chromium) residue was observed. Stirring was continued for another 3 hours, and the solution was tested again; significant Cr-VI (hexavalent chromium) residue was still observed.

[0138] Comparative Example 2

[0139] Compared to Example 4, the difference lies in the absence of hydrofluoric acid and the heating of the reaction system during the addition of the reducing agent, resulting in a redox reaction temperature of 80°C. During the redox reaction, numerous dark green crystalline solids precipitated, and the solution tested showed significant Cr-VI (hexavalent chromium) residue.

[0140] Comparative Example 3

[0141] Compared to Comparative Example 2, the difference lies in replacing gluconic acid with hypophosphorous acid (H3PO2). The amount of hypophosphorous acid added is sufficient to ensure that the number of moles of electrons donated by gluconic acid and hypophosphorous acid are the same; all other conditions remain the same. During the redox reaction, many dark green crystalline solids were observed to precipitate, and the solution tested showed obvious Cr-VI (hexavalent chromium) residue.

[0142] Comparative Example 4

[0143] Compared to Example 4, the difference lies in replacing gluconic acid with hypophosphite (H3PO2). The amount of hypophosphite added is sufficient to ensure that the number of moles of electrons provided by gluconic acid and hypophosphite are the same; all other conditions remain the same. After completing the step of adding the reducing agent, stirring was continued for 3 hours, and the solution was tested. Significant Cr-VI (hexavalent chromium) residue was observed. Stirring was continued for another 3 hours, and the solution was tested again; significant Cr-VI (hexavalent chromium) residue was still observed.

[0144] Comparative Example 5

[0145] Compared to Example 1, the difference lies in reducing the amount of gluconic acid added, while other conditions remain the same. After completing the step of adding the organic reducing agent, stirring was continued for 3 hours, and the solution was tested. Significant Cr-VI (hexavalent chromium) residue was observed. Stirring was continued for another 3 hours, and the solution was tested again; significant Cr-VI (hexavalent chromium) residue was still observed.

[0146] Comparative Example 6

[0147] Compared to Example 4, only hydrofluoric acid was replaced with ammonium bifluoride (NH4HF2). The amount of ammonium bifluoride added was just enough to ensure that the molar number of F provided by hydrofluoric acid and ammonium bifluoride was the same; all other conditions remained the same. After completing the step of adding the organic reducing agent, stirring was continued for 3 hours, and the solution was tested. Significant Cr-VI (hexavalent chromium) residue was found. Stirring was continued for another 3 hours, and the solution was tested again; significant Cr-VI (hexavalent chromium) residue was still found.

[0148] Table 1

[0149]

[0150] Application of surface treatment agents

[0151] Pretreatment of aluminum alloy substrate: The aluminum alloy substrate (5 series, 200×200×0.3mm) is subjected to alkaline degreasing (Chemiter's Gardoclean S 5148), rinsed with water, pickled to remove ash (Chemiter's Gardacid P 4325), rinsed with water, and dried at 100℃ for later use.

[0152] Application of trivalent chromium surface treatment agent: The trivalent chromium surface treatment agent prepared in Example 4 was applied to the pretreated aluminum alloy substrate using a stainless steel doctor blade and dried at 100°C; after drying, the weight of the passivation film covering the surface of the aluminum alloy substrate was calculated to be approximately 50 mg / m². 2 .

[0153] [Paint Adhesion Performance Test]

[0154] An aluminum alloy substrate treated with a trivalent chromium surface treatment agent was coated with paint (Valspar polyester varnish) using a roller coating method and dried at 250°C to obtain a sample with a dry film thickness of 19 μm. The dry film thickness was measured using a Helmut-Fischer FMP20 instrument according to DIN EN ISO 2808:2007-05 (dated: May 2007), Method 12A - Magnetometer.

[0155] The prepared samples were subjected to the following three tests:

[0156] (1) The cross-cut test of the paint film was carried out in accordance with the national standard GB / T 9286-1998;

[0157] (2) T-bending test: The sample is bent flat at 180° with the painted side facing down, and then the degree of paint peeling at the bend is observed by applying adhesive tape.

[0158] (3) Feathering test: The sample was immersed in an 80℃ water bath for 40 minutes, and then the aluminum alloy substrate was torn along the rolling direction. The paint peeling at the fracture was observed using an optical microscope. The results are shown in [reference needed]. Figure 1 The width of the feather membrane at three locations was measured and the average value was taken.

[0159] Test results:

[0160] (1) Cross-cut test: Level 0 (the cut edges are completely smooth, and no squares are missing)

[0161] (2) T-bending test: 0T (no paint peeling after bending 180°)

[0162] (3) Feather width: 53.5 μm.

Claims

1. A trivalent chromium surface treatment agent for aluminum or aluminum alloys, characterized in that, The raw materials for the trivalent chromium surface treatment agent include: a) Hexavalent chromium compounds; b) An organic reducing agent, wherein the organic reducing agent contains primary hydroxyl, secondary hydroxyl and / or phenolic hydroxyl; c) Hydrofluoric acid; d) Inorganic acid, wherein the acidity coefficient pKa of the inorganic acid is less than that of hydrofluoric acid; Wherein, the ratio of the maximum number of moles of Cr in the hexavalent chromium compound to the organic reducing agent theoretically provided is 1.0:3.0 to 5.0; preferably, the mole ratio is 1.0:3.2 to 4.8; more preferably, the mole ratio is 1.0:3.5 to 4.5; The trivalent chromium is obtained by in-situ reduction of the hexavalent chromium compound.

2. The trivalent chromium surface treatment agent according to claim 1, characterized in that, The trivalent chromium surface treatment agent further includes: e) a film-forming polymer; more preferably, the remaining component of the trivalent chromium surface treatment agent is water.

3. The trivalent chromium surface treatment agent according to claim 1 or 2, characterized in that, The hexavalent chromium compound is selected from one or more of chromium trioxide, chromic acid, and / or hexavalent chromates.

4. The trivalent chromium surface treatment agent according to any one of claims 1 to 3, characterized in that, The organic reducing agent is selected from one or more of glucose, sucrose, gluconic acid, tartaric acid, malic acid, tannic acid, gallic acid, isocitric acid, ascorbic acid, mannitol and / or their water-soluble salts.

5. The trivalent chromium surface treatment agent according to any one of claims 1 to 4, characterized in that, The molar ratio of Cr in the hexavalent chromium compound to F in the hydrofluoric acid is 1.0:1.8 to 5.0, preferably 1.0:2.0 to 4.0, and more preferably 1.0:2.2 to 3.

5.

6. The trivalent chromium surface treatment agent according to any one of claims 1 to 5, characterized in that, The inorganic acid has an acidity coefficient pKa of not more than 3.

0. Preferably, the inorganic acid is selected from one or more of phosphoric acid, sulfuric acid, nitric acid, hydrochloric acid, perchloric acid, hydroiodic acid, and hydrobromic acid. More preferably, the inorganic acid is selected from one or more of phosphoric acid and / or nitric acid.

7. The trivalent chromium surface treatment agent according to any one of claims 1 to 6, characterized in that, The pH of the trivalent chromium surface treatment agent is less than 3.

0.

8. The trivalent chromium surface treatment agent according to any one of claims 1 to 7, characterized in that, The film-forming polymer is selected from one or more of polyurethane resin, epoxy resin, polyolefin resin, phenolic resin, polyester resin, (meth)acrylic resin and (meth)acrylic urethane resin.

9. The trivalent chromium surface treatment agent according to any one of claims 1 to 8, characterized in that, The trivalent chromium surface treatment agent contains no hexavalent chromium residue.

10. A method for preparing a trivalent chromium surface treatment agent for aluminum or aluminum alloys as described in any one of claims 1 to 9, comprising, Step A: Using water as a solvent, mix and dissolve a) hexavalent chromium compound, c) hydrofluoric acid and d) inorganic acid to obtain mixture I; Step B: Under conditions where no external heat source is used to heat the reaction system, add the organic reducing agent (b) to the mixture I and stir to carry out the redox reaction.

11. The method for preparing the trivalent chromium surface treatment agent according to claim 10, characterized in that, The preparation method further includes: Step C: Detect the presence of hexavalent chromium in the solution. If the result is negative, continue with the following steps. Step D: Mix the film-forming polymer (e) to obtain the trivalent chromium surface treatment agent.

12. The method for preparing the trivalent chromium surface treatment agent according to claim 10 or 11, characterized in that, The temperature of the redox reaction in step B) is not higher than 40°C, and preferably the temperature of the redox reaction is 20-35°C.

13. A surface treatment method for aluminum or aluminum alloys, comprising: The trivalent chromium surface treatment agent for aluminum or aluminum alloy as described in any one of claims 1 to 9 is applied to the surface of a substrate made of aluminum or aluminum alloy, and then heated and dried to form a surface treatment film of aluminum or aluminum alloy.

14. A surface treatment film for aluminum or aluminum alloy, formed using the surface treatment method for aluminum or aluminum alloy as described in claim 13.

15. A packaging material having a surface-treated film of aluminum or aluminum alloy as described in claim 14, preferably, the packaging material is a can packaging material, more preferably, the packaging material is a can lid material.

Images