Low temperature curing pure polyester powder coating for containers and method for the production thereof

Abstract

The application provides a low-temperature curing pure polyester powder coating for containers and a preparation method thereof, and relates to the technical field of special coatings. The low-temperature curing pure polyester powder coating for containers adopts a high-reactivity carboxyl-terminated pure polyester resin covalently modified by a weather-resistant monomer. The weather-resistant monomer is a functional monomer containing at least one carboxyl or hydroxyl in the molecular structure and capable of participating in polyester polycondensation reaction, so that the obtained coating has good weather resistance and low-temperature curing adaptability while maintaining the inherent advantages of the pure polyester system. It has been verified that the coating can be completely cured at 160 DEG C x 8 min, solving the problems of high energy consumption, uneven heating of large workpieces and damage of heat-sensitive components caused by high-temperature curing of 180-200 DEG C of traditional container powder coatings; the obtained coating has the performances of high salt mist resistance, high weather resistance and high impact resistance, and can meet the long-acting protection requirements of containers in the harsh marine environment.

Description

Technical Field

[0001] This invention relates to the field of special coatings technology, and in particular to a low-temperature curing pure polyester powder coating for containers and its preparation method. Background Technology

[0002] Containers are used in harsh environments such as marine salt spray, humidity, ultraviolet radiation, wide temperature range (-40℃ to 80℃), and frequent mechanical impacts for a long time. This places extremely high demands on the protective performance of the coating, which must simultaneously possess high salt spray resistance, high weather resistance, high mechanical strength, and long-term corrosion resistance.

[0003] Currently, traditional powder coatings for shipping containers generally use pure polyester systems, with curing conditions of 180℃~200℃ / 15~20min. This high-temperature curing process has the following drawbacks: (1) High energy consumption, and large container workpieces have large heat capacity, slow heating and uneven heating, which easily leads to over-baking of the corners and incomplete curing of the main body area; (2) High-temperature baking can easily cause heat-sensitive components such as sealant and insulation materials used in containers to age and fail; (3) The pure polyester system itself has low reactivity and is difficult to achieve full cross-linking under low temperature conditions. It generally suffers from problems such as incomplete curing, significant decrease in coating impact resistance, and deterioration of weather resistance. It is difficult to meet the requirements of low temperature curing and the harsh service environment of containers for the comprehensive performance of the coating. (4) Existing low-temperature curing solutions mostly use epoxy / polyester mixed powder coatings. Although the curing temperature can be reduced, the epoxy component has poor weather resistance and cannot meet the performance requirements of UV resistance, anti-chalking, gloss and color retention under long-term outdoor exposure of container exterior walls.

[0004] Therefore, it is imperative and urgent to develop a pure polyester powder coating that can be rapidly and completely cured at a relatively low temperature (160℃) to significantly reduce energy consumption while ensuring core properties such as high salt spray resistance, high weather resistance, and high impact resistance, and to be compatible with existing container coating production lines.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The primary objective of this invention is to provide a low-temperature curing pure polyester powder coating for containers. This coating can be rapidly and completely cured at 160°C, while also possessing the technical advantages of high corrosion resistance, high weather resistance, high toughness, and low energy consumption. It effectively alleviates the shortcomings of existing container powder coatings that require high-temperature curing and is fully adaptable to the industrialized preparation of container production lines.

[0007] The second objective of this invention is to provide a method for preparing a low-temperature curing pure polyester powder coating for containers.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: The present invention provides a low-temperature curing pure polyester powder coating for containers, the pure polyester powder coating comprising: a highly reactive carboxyl-terminated pure polyester resin covalently modified with weather-resistant monomers; The weather-resistant monomer is a functional monomer whose molecular structure contains at least one carboxyl or hydroxyl group and can participate in polyester polycondensation reaction. The highly reactive end-carboxyl pure polyester resin refers to polyester that is cured with TGIC or HAA at a curing temperature of <170℃ and a curing time of <10min.

[0009] Furthermore, the weather-resistant monomer is selected from at least one of 1,4-cyclohexanedicarboxylic acid, hydrogenated bisphenol A, 1,4-cyclohexanediethanol, neopentyl glycol monoacrylate, and 12-hydroxystearic acid.

[0010] Furthermore, the preparation method of the highly reactive end-carboxyl pure polyester resin includes: using diacid, diol and the weather-resistant monomer as raw materials, performing a normal pressure esterification-reduced compression copolymerization reaction, and then using acid excess end-capping after polycondensation to obtain the resin; Preferably, the acid overload end-capping refers to the addition of trimellitic anhydride, accounting for 6-10% of the total molar amount of the dicarboxylic acid, for end-capping when the acid value of the polycondensation reaction drops to 70% of the theoretical value.

[0011] Furthermore, the highly reactive end-carboxyl pure polyester resin has an acid value of 35~45 mgKOH / g, a glass transition temperature (Tg) of 61~68℃, and a softening point of 90~100℃.

[0012] Furthermore, the pure polyester powder coating comprises: Highly reactive carboxyl-terminated pure polyester resin, low-temperature curing agent, low-temperature catalyst, anti-corrosion pigment, functional filler, low-temperature leveling agent, degassing agent, antioxidant stabilizer and toughening agent; Optionally, the pure polyester powder coating may further include other additives, which are one or more combinations of antistatic agents, brighteners, and dispersants.

[0013] Furthermore, based on parts by weight, the pure polyester powder coating comprises: The composition includes 60-72 parts of highly reactive carboxyl-terminated pure polyester resin, 4.7-6.5 parts of low-temperature curing agent, 0.15-0.6 parts of low-temperature catalyst, 8-25 parts of anti-corrosion pigment, 12-30 parts of functional filler, 0.9-1.8 parts of low-temperature leveling agent, 0.4-0.9 parts of degassing agent, 0.2-0.5 parts of antioxidant stabilizer, 0.5-2 parts of toughening agent, and 0-1.2 parts of other additives. Preferably, the zinc phosphate content in the anti-corrosion pigment is 2 to 10 parts.

[0014] Furthermore, the low-temperature curing agent is selected from one or more of TGIC, HAA, blocked isocyanates, and polycarboxylic acid adducts, preferably TGIC; And / or, the low-temperature catalyst is a complex system of organotin compounds and quaternary ammonium salts, wherein the mass ratio of the organotin compounds to the quaternary ammonium salts is 1:1; Preferably, the organotin compound is dibutyltin dilaurate; Preferably, the quaternary ammonium salt is benzyltriethylammonium chloride or trioctylmethylammonium chloride; And / or, the anti-corrosion pigment includes one or more of zinc phosphate, aluminum tripolyphosphate, modified barium metaborate, black pigment, and rutile titanium dioxide; And / or, the functional filler includes one or more of 1250-1500 mesh barium sulfate, mica powder, and talc powder; And / or, the low-temperature leveling agent is a polyacrylate with a number average molecular weight of 4000-9000; And / or, the degassing agent is benzoin or a modified degassing agent, preferably BYK961 or Clariant 3910; And / or, the antioxidant stabilizer is obtained by compounding an antioxidant and a light stabilizer; Preferably, the antioxidant is selected from at least one of hindered phenols and phosphites; Preferably, the light stabilizer is selected from UV-531 or UV-9; And / or, the toughening agent is selected from carboxylated nitrile rubber and / or acrylate elastomers, preferably acrylate elastomers.

[0015] Furthermore, based on parts by weight, the pure polyester powder coating comprises: The composition includes: 60-72 parts of highly reactive carboxyl-terminated pure polyester resin, 4.7-6.5 parts of TGIC curing agent, 5-15 parts of rutile titanium dioxide, 0.25 parts of black pigment, 8-25 parts of zinc phosphate, 5.75 parts of 1250-mesh barium sulfate, 3 parts of mica powder, 0.9-1.8 parts of polyacrylate leveling agent, 0.4-0.9 parts of benzoin, 0.15-0.6 parts of low-temperature catalyst, 0.2-0.5 parts of antioxidant stabilizer, 0.5-2 parts of acrylate elastomer, and 0.2 parts of dispersant. Preferably, the pure polyester powder coating comprises, by weight parts: The composition includes: 65 parts highly reactive carboxyl-terminated pure polyester resin, 5 parts TGIC curing agent, 12 parts rutile titanium dioxide, 0.25 parts black pigment, 5 parts zinc phosphate, 5.75 parts 1250 mesh barium sulfate, 3 parts mica powder, 1.5 parts polyacrylate leveling agent, 0.6 parts benzoin, 0.4 parts low-temperature catalyst, 0.3 parts antioxidant stabilizer, 1 part acrylate elastomer, and 0.2 parts dispersant. Wherein: the low-temperature catalyst is composed of dibutyltin dilaurate and a quaternary ammonium salt, and the mass ratio of dibutyltin dilaurate to the quaternary ammonium salt is 1:1; The antioxidant stabilizer is composed of hindered phenolic antioxidant and UV-531, and the mass ratio of hindered phenolic antioxidant to UV-531 is 1:1.

[0016] This invention provides a method for preparing the above-mentioned low-temperature curing pure polyester powder coating for containers, the method comprising: The raw materials are mixed according to the formula to obtain a premix, which is then melt-extruded and granulated, and then screened to obtain a pure polyester powder coating.

[0017] Furthermore, the preparation method includes: (A) Premixing: Mix each component at room temperature according to the formula until the material is uniform and free of agglomeration to obtain a premix; (B) Melt extrusion: The premix is ​​extruded using a twin-screw extruder, and then cooled after extrusion into tablets; (C) Coarse crushing and fine grinding: The tablets are crushed and then ground to obtain the grinding material; (D) Grading and sieving: The abrasive is sieved to remove impurities to obtain the finished coating; Preferably, in step (B), the temperature of the first zone of the twin-screw extruder is 85~95℃, the temperature of the second zone is 105–115℃, and the screw speed is 350~550 r / min; Preferably, the grinding particle size D50 in step (C) is 35~45μm, more preferably 40~42μm; Preferably, the finished coating in step (D) is a powder that has passed through a 180-200 mesh sieve.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention provides a low-temperature curing pure polyester powder coating for containers. This coating uses a highly reactive carboxyl-terminated pure polyester resin covalently modified with weather-resistant monomers. The weather-resistant monomers are functional monomers containing at least one carboxyl or hydroxyl group in their molecular structure and capable of participating in polyester polycondensation reactions. This allows the resulting coating to maintain the inherent advantages of the pure polyester system while possessing excellent weather resistance and low-temperature curing adaptability. It can be fully cured at 160℃ for 8 minutes, solving the problems of high energy consumption, uneven heating of large workpieces, and damage to heat-sensitive components caused by the high-temperature curing of traditional container powder coatings at 180~200℃, as well as the poor low-temperature curing performance of the pure polyester system. The resulting coating also has high salt spray resistance (1000h), high QUV weather resistance (1000h), and high impact resistance, resulting in significant energy savings. It is compatible with existing container coating production lines and meets the long-term protection requirements of containers in harsh marine environments.

[0019] The present invention provides a method for preparing a low-temperature curing pure polyester powder coating for containers. The method includes: mixing raw materials according to a specified ratio to obtain a premix, followed by melt extrusion granulation, and then sieving to obtain the pure polyester powder coating. This preparation method has the technical advantages of simple processing and suitability for industrialized coating production. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] According to one aspect of the present invention, a low-temperature curing pure polyester powder coating for containers, the pure polyester powder coating comprising: a highly reactive terminal carboxyl pure polyester resin covalently modified with weather-resistant monomers; The weather-resistant monomer is a functional monomer whose molecular structure contains at least one carboxyl or hydroxyl group and can participate in polyester polycondensation reaction. The highly reactive end-carboxyl pure polyester resin refers to a polyester resin in which the curing temperature is <170℃ and the curing time is <10min when the polyester is cured with TGIC or HAA.

[0022] The present invention provides a low-temperature curing pure polyester powder coating for containers. This coating uses a highly reactive carboxyl-terminated pure polyester resin covalently modified with weather-resistant monomers. The weather-resistant monomers are functional monomers containing at least one carboxyl or hydroxyl group in their molecular structure and capable of participating in polyester polycondensation reactions. This allows the resulting coating to maintain the inherent advantages of the pure polyester system while possessing excellent weather resistance and low-temperature curing adaptability. It can be fully cured at 160℃ for 8 minutes, solving the problems of high energy consumption, uneven heating of large workpieces, and damage to heat-sensitive components caused by the high-temperature curing of traditional container powder coatings at 180~200℃, as well as the poor low-temperature curing performance of the pure polyester system. The resulting coating also has high salt spray resistance (1000h), high QUV weather resistance (1000h), and high impact resistance, resulting in significant energy savings. It is compatible with existing container coating production lines and meets the long-term protection requirements of containers in harsh marine environments.

[0023] It should be noted that the low-temperature curing pure polyester powder coating for containers in this application has excellent low-temperature and high-efficiency curing performance, heavy-duty corrosion protection and high weather resistance, mechanical properties, environmental friendliness, and process compatibility, as specifically demonstrated below: (I) The low-temperature curing pure polyester powder coating for containers of the present invention has low-temperature and high-efficiency curing performance. It adopts a highly reactive terminal carboxyl pure polyester resin covalently modified with weather-resistant monomers. The molecular chain ends are enriched with highly active and high-density carboxyl functional groups, the functional reaction sites are fully exposed and the steric hindrance is reduced. The introduction of weather-resistant monomers also moderately regulates the flexibility of polyester molecular chains, reduces the resin melting activation energy and cross-linking reaction activation energy, so that the terminal carboxyl groups and curing agents can quickly undergo cross-linking reaction at a lower temperature to complete the construction of a three-dimensional network structure, thereby meeting the overall baking uniformity requirements of large container workpieces. (II) The coating obtained by the above-mentioned coating of the present invention has heavy corrosion protection and high weather resistance: the weather-resistant monomer is embedded in the polyester main chain in the form of covalent bonds, introducing alicyclic or long-chain aliphatic structural units, eliminating conjugated aromatic rings and unsaturated bonds that are easily photo-oxidized and thermally degraded, which can effectively absorb and block ultraviolet light and inhibit resin photo-oxidative aging and damp heat yellowing; at the same time, the cross-linked network formed by the highly reactive terminal carboxyl group and the curing agent has high density and few defects, which can effectively block the penetration of water vapor, chloride ions and corrosive media, and combined with the synergistic effect of corrosion-inhibiting pigments and fillers, it can achieve long-term protection of the substrate; (III) The coating obtained by the above-mentioned coating of the present invention also has excellent mechanical properties: the weather-resistant monomer modification can improve the weather resistance and synergistically regulate the balance of rigidity and toughness of polyester resin; the alicyclic structure provides hardness and structural stability; the long-chain aliphatic structure enhances the internal friction of molecular chains and resistance to deformation; the high crosslinking density makes the molecular chains tightly entangled and the interfacial bonding force strong; the coating and the metal substrate form a dual effect of chemical bonding and physical anchoring, thereby ensuring the integrity of the coating under high-frequency impact and collision conditions during transportation and loading and unloading. (iv) The coating system of the present invention is a 100% solids pure polyester powder coating, which does not contain organic solvents, free formaldehyde and volatile organic components. No small molecule by-products are released during the curing process, which meets the requirements of environmentally friendly coating. Its melt viscosity and rheological properties are optimized after modification with weather-resistant monomers. Its melt flowability and electrostatic spraying charge are highly matched with traditional container powder coatings. It can be directly adapted to existing coating production lines without equipment modification. Uncoated powder can be recycled, screened and reused, which has good environmental protection and production economy.

[0024] In a preferred embodiment of the present invention, the weather-resistant monomer is selected from at least one of 1,4-cyclohexanedicarboxylic acid, hydrogenated bisphenol A, 1,4-cyclohexanediethanol, neopentyl glycol monoacrylate, and 12-hydroxystearic acid. In a preferred embodiment of the present invention, the method for preparing the highly reactive carboxyl-terminated pure polyester resin includes: The product is prepared by using diacid, diol and the weather-resistant monomer as raw materials, through a normal pressure esterification-reduced compression copolymerization reaction, and then by acid over-capping after polycondensation. In the preferred embodiment described above, the acid overload end-capping refers to the addition of trimellitic anhydride, accounting for 6-10% of the total molar amount of the dicarboxylic acid, for end-capping when the acid value of the polycondensation reaction drops to 70% of the theoretical value.

[0025] In a preferred embodiment of the present invention, the highly reactive terminal carboxyl pure polyester resin has an acid value of 35~45 mgKOH / g, a glass transition temperature Tg of 61~68℃, and a softening point of 90~100℃.

[0026] In a preferred embodiment of the present invention, the pure polyester powder coating comprises: highly reactive carboxyl-terminated pure polyester resin, low-temperature curing agent, low-temperature catalyst, anti-corrosion pigment, functional filler, low-temperature leveling agent, degassing agent, antioxidant stabilizer, and toughening agent. Optionally, the pure polyester powder coating includes other additives, which are one or more combinations of antistatic agents, brighteners, and dispersants.

[0027] In a preferred embodiment of the present invention, the pure polyester powder coating comprises, by weight parts: 60-72 parts of highly reactive carboxyl-terminated pure polyester resin (e.g., 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72 parts, or any value between 60 and 72 parts); 4.7-6.5 parts of low-temperature curing agent (e.g., 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5 parts, or any value between 4.7 and 6.5 parts); 0.15-0.6 parts of low-temperature catalyst (e.g. For example, the amounts can be 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, or 0.6 parts, or any value between 0.15 and 0.6 parts; 8 to 25 parts of anti-corrosion pigment (for example, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 parts, or any value between 8 and 25 parts); 12 to 30 parts of functional filler (for example, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or 2... 6, 27, 28, 29, or 30 parts, or any value between 12 and 30 parts; 0.9 to 1.8 parts of low-temperature leveling agent (e.g., 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, or 1.8 parts, or any value between 0.9 and 1.8 parts); 0.4 to 0.9 parts of degassing agent (e.g., 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 parts, or any value between 0.4 and 0.9 parts); 0.2 to 0.5 parts of antioxidant stabilizer (e.g., 0.2, 0.25, 0.3, 0.35, or 0.4 parts). 0.45, 0.5 parts, or any value between 0.2 and 0.5 parts; toughening agent 0.5 to 2 parts (e.g., 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 parts, or any value between 0.5 and 2.0 parts); other additives 0 to 1.2 parts (e.g., 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2 parts, or any value between 0 and 1.2 parts).

[0028] Preferably, the zinc phosphate content in the anti-corrosion pigment is 2 to 10 parts.

[0029] In a preferred embodiment of the present invention, the curing conditions for the low-temperature curing pure polyester powder coating for containers are 160℃×8min. After curing, the coating has an adhesion grade of 0, a hardness of ≥2H, an impact resistance of ≥100kg•cm on the front side, an impact resistance of ≥60kg•cm on the back side, salt spray resistance of 1000h, and QUV weather resistance of 1000h.

[0030] In a preferred embodiment of the present invention, the low-temperature curing agent is selected from one or more of TGIC, HAA, blocked isocyanate, and polycarboxylic acid adduct, preferably TGIC; In a preferred embodiment of the present invention, the low-temperature catalyst is a complex system of organotin compounds and quaternary ammonium salts, wherein the mass ratio of the organotin compounds to the quaternary ammonium salts is 1:1. Preferably, the organotin compound is dibutyltin dilaurate; Preferably, the quaternary ammonium salt is benzyltriethylammonium chloride or trioctylmethylammonium chloride.

[0031] In a preferred embodiment of the present invention, the anti-corrosion pigment includes one or more of zinc phosphate, aluminum tripolyphosphate, modified barium metaborate, black pigment, and rutile titanium dioxide. In a preferred embodiment of the present invention, the functional filler includes one or more of 1250-1500 mesh barium sulfate, mica powder, and talc powder; In a preferred embodiment of the present invention, the low-temperature leveling agent is a polyacrylate with a number average molecular weight of 4000-9000; In a preferred embodiment of the present invention, the degassing agent is benzoin or a modified degassing agent, and the modified degassing agent is preferably BYK961 or Clariant 3910; In a preferred embodiment of the present invention, the antioxidant stabilizer is obtained by compounding an antioxidant and a light stabilizer; Preferably, the antioxidant is selected from at least one of hindered phenols and phosphites; Preferably, the light stabilizer is selected from UV-531 or UV-9; And / or, the toughening agent is selected from carboxylated nitrile rubber and / or acrylate elastomers, preferably acrylate elastomers.

[0032] In a preferred embodiment of the present invention, the pure polyester powder coating comprises, by weight parts: The composition includes: 60-72 parts of highly reactive carboxyl-terminated pure polyester resin, 4.7-6.5 parts of TGIC curing agent, 5-15 parts of rutile titanium dioxide, 0.25 parts of black pigment, 8-25 parts of zinc phosphate, 5.75 parts of 1250-mesh barium sulfate, 3 parts of mica powder, 0.9-1.8 parts of polyacrylate leveling agent, 0.4-0.9 parts of benzoin, 0.15-0.6 parts of low-temperature catalyst, 0.2-0.5 parts of antioxidant stabilizer, 0.5-2 parts of acrylate elastomer, and 0.2 parts of dispersant. In the preferred embodiment described above, the pure polyester powder coating comprises, by weight parts: The composition includes: 65 parts highly reactive carboxyl-terminated pure polyester resin, 5 parts TGIC curing agent, 12 parts rutile titanium dioxide, 0.25 parts black pigment, 5 parts zinc phosphate, 5.75 parts 1250 mesh barium sulfate, 3 parts mica powder, 1.5 parts polyacrylate leveling agent, 0.6 parts benzoin, 0.4 parts low-temperature catalyst, 0.3 parts antioxidant stabilizer, 1 part acrylate elastomer, and 0.2 parts dispersant. Wherein: the low-temperature catalyst is composed of dibutyltin dilaurate and a quaternary ammonium salt, and the mass ratio of dibutyltin dilaurate to the quaternary ammonium salt is 1:1; The antioxidant stabilizer is composed of hindered phenolic antioxidant and UV-531, and the mass ratio of hindered phenolic antioxidant to UV-531 is 1:1.

[0033] According to one aspect of the present invention, a method for preparing the above-mentioned low-temperature curing pure polyester powder coating for containers, the method comprising: The raw materials are mixed according to the formula to obtain a premix, which is then melt-extruded and granulated, and then screened to obtain a pure polyester powder coating.

[0034] The present invention provides a method for preparing a low-temperature curing pure polyester powder coating for containers. The method includes: mixing raw materials according to a specified ratio to obtain a premix, followed by melt extrusion granulation, and then sieving to obtain the pure polyester powder coating. This preparation method has the technical advantages of simple processing and suitability for industrialized coating production.

[0035] In a preferred embodiment of the present invention, the preparation method includes: (A) Premixing: Mix each component at room temperature according to the formula until the material is uniform and free of agglomeration to obtain a premix; (B) Melt extrusion: The premix is ​​extruded using a twin-screw extruder, and then cooled after extrusion into tablets; (C) Coarse crushing and fine grinding: The tablets are crushed and then ground to obtain the grinding material; (D) Grading and sieving: The abrasive is sieved to remove impurities to obtain the finished coating; As an optional implementation, in step (B), the temperature of the first zone of the twin-screw extruder can be 85–95°C (e.g., 85°C, 86°C, 87°C, 88°C, 89°C, 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, or any value between 85 and 95°C), the temperature of the second zone can be 105–115°C (e.g., 105°C, 106°C, 107°C, 108°C, 109°C, 110°C, 111°C, 112°C, 113°C, 114°C, 115°C, or any value between 105 and 115°C), and the screw speed can be 350–550 rpm. r / min (for example, it can be 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550 r / min, or any value between 350 and 550 r / min).

[0036] As an optional implementation, in step (C), the particle size D50 of the ground powder can be 35–45 μm (for example, it can be 35 μm, 36 μm, 37 μm, 38 μm, 39 μm, 40 μm, 41 μm, 42 μm, 43 μm, 44 μm, 45 μm, or any value between 35–45 μm); in a preferred embodiment, the particle size D50 is 40~42 μm (for example, it can be 40 μm, 40.5 μm, 41 μm, 41.5 μm, 42 μm, or any value between 40~42 μm).

[0037] As an optional implementation, in step (D), the finished coating powder may be a powder that passes through a sieve of 180–200 mesh (for example, it may be a sieve of 180 mesh, 185 mesh, 190 mesh, 195 mesh, or 200 mesh, or it may be a sieve of any mesh size between 180 and 200 mesh).

[0038] The technical solution of the present invention will be further described below with reference to the embodiments.

[0039] Note: The amounts of each raw material used in the following embodiments and comparative examples of this application are all by weight. The manufacturers and models of the raw materials used are shown in Table 1. Raw materials not specified in Table 1 were all purchased from commercially available sources.

[0040] Table 1:

[0041] Examples 1-3 A low-temperature curing pure polyester powder coating for containers, by weight, is composed of the following raw materials (see Table 2).

[0042] Table 2:

[0043] The preparation method of the low-temperature curing pure polyester powder coating for containers is as follows: (1) Premixing: Add all the above components to a high-speed mixer and mix at room temperature (25±2℃) at a speed of 1200 r / min for 8 min until the material is uniform and there are no visible agglomerates; (2) Melt extrusion: The premixed material is fed into a twin-screw extruder, the temperature of zone one is set to 90℃, the temperature of zone two is set to 110℃, the screw speed is set to 450 r / min, and after extrusion and tableting, it is cooled to room temperature by cooling rollers; (3) Coarse crushing and fine grinding: After the cooled and compressed tablets are crushed, they are ultra-fine ground using an ACM mill to control the output particle size D50 to be 40 μm; (4) Grading and sieving: The grinding material is graded by an air classifier and sieved through a 200-mesh (74 μm) stainless steel screen to remove impurities and oversized particles, thus obtaining the finished powder coating.

[0044] Example 4 (without zinc phosphate) This embodiment is the same as Example 1, except that zinc phosphate in the raw material composition is replaced with an equal amount of titanium dioxide.

[0045] Example 5 (The low-temperature catalyst is only dibutyltin dilaurate) This embodiment is the same as in Example 1, except that the quaternary ammonium salt in the raw material composition is replaced with an equal amount of dibutyltin dilaurate.

[0046] Comparative Example 1 This comparative example is the same as Example 1, except that the "highly reactive carboxyl-terminated pure polyester resin prepared in Example 1" in the raw materials for preparing low-temperature curing pure polyester powder coating in Example 2 is replaced with "conventional polyester with an acid value of 25~30 mgKOH / g".

[0047] Experimental Example 1 Impact resistance and weather resistance are crucial parameters for shipping containers. In actual use, shipping containers endure extremely complex service environments: on the one hand, they are frequently subjected to impacts from spreaders, stacking pressure, and forklift collisions during dock handling; on the other hand, during ocean transport, the continuous friction, scraping, and violent collisions between containers due to waves can easily cause localized dents, deformation, and even weld cracks. Therefore, the coating must possess excellent toughness and energy dissipation capabilities to ensure structural integrity. Furthermore, containers are stacked and transported outdoors without shelter for extended periods, constantly exposed to high-intensity ultraviolet radiation from different climate zones worldwide, humid and hot salt spray, and diurnal temperature variations (-40℃ to 80℃), placing extremely high demands on the coating's resistance to chalking, gloss and color retention, inhibition of yellowing, and ability to block corrosive media penetration. The low-temperature curing pure polyester powder coating developed in this invention is designed specifically for these real-world conditions.

[0048] Furthermore, to verify the technical effect of the low-temperature curing pure polyester powder coating for containers of the present invention, performance tests were conducted on the coating samples prepared in Examples 2-6 and Comparative Examples 1 and 2.

[0049] The coating is prepared by spraying the epoxy base powder of Examples 1 to 5 and Comparative Example 1 onto a test plate made of JISG3125 SPA-H material with dimensions of 100nX100 mmX1.6m. The film thickness is 60~80um, and the curing process is 160℃ for 8 minutes.

[0050] The testing method includes: 1. The impact resistance test is conducted in accordance with section 7.1.3 of the current standard GB / T 20624.2-2006, that is, a 15.9 mm steel ball is used as the impact point, dropped from a height of 100 cm, with 5 points tested in each group, and the average value is taken for the drop hammer test; 2. Salt spray resistance shall be tested according to ASTM B117, with the first appearance of red rust on the substrate (Ri = 1) as the failure endpoint, and shall be checked every 24 hours. 3. QUV weather resistance is tested according to ISO 4892-3, using UVA-340 lamps, with an irradiance of 0.89 W / m²@340 nm, and a light / condensation cycle of 4 h / 4 h. The failure endpoint is defined as a light retention rate of <50% or a color difference ΔE ≥ 5.0.

[0051] 4. The gloss retention rate is measured at a 60° angle according to ASTM D523; 5. The total color difference ΔE shall be calculated according to ASTM D2244.

[0052] The expressions "1000h salt spray resistance" and "1000h QUV weather resistance" in Table 3 below all indicate that the sample did not meet the above failure criteria after 1000h of continuous testing under the corresponding standard.

[0053] See Tables 3 and 4 for specific results.

[0054] Table 3:

[0055] As can be seen from the above, the coatings prepared by the low-temperature curing pure polyester powder coatings for containers in Examples 1 to 3 of this application have curing conditions of 160℃×8min. After curing, the coatings have an adhesion grade of 0, a hardness of ≥2H, an impact resistance of ≥100kg•cm on the front side, an impact resistance of ≥60kg•cm on the back side, a salt spray resistance of 1000h, and a QUV weather resistance of 1000h.

[0056] Example 4, an embodiment of the coating without zinc phosphate, has the following curing conditions: 160℃×8min. After curing, the coating has an adhesion grade of 0, a hardness of ≥2H, an impact resistance of ≥100kg•cm on the front side, an impact resistance of ≥60kg•cm on the back side, salt spray resistance for 800h, and QUV weather resistance for 1000h.

[0057] In Example 5, the low-temperature catalyst in the coating is only dibutyltin dilaurate. Its curing conditions are 160℃×8min. After curing, the coating has an adhesion grade of 0, a hardness of ≥2H, an impact resistance of ≥100kg•cm on the front side, an impact resistance of ≥60kg•cm on the back side, a salt spray resistance of 1000h, and a QUV weather resistance of 1000h.

[0058] Table 4:

[0059] As shown in Tables 3 and 4, Examples 1 to 3, under the low-temperature curing condition of 160℃ / 8min, achieved comprehensive performance including adhesion grade 0, hardness 2H, front impact resistance 100 kg·cm, back impact resistance 60 kg·cm, salt spray resistance 1000 h, QUV weather resistance 1000 h (gloss retention rate 84%, color difference ΔE=0.73). In contrast, under the same conditions of 160℃ / 8min, the impact strength on the reverse side of Comparative Example 1 (using conventional polyester resin) dropped to 0 kg·cm (due to complete peeling of the coating causing reading failure), the hardness decreased to HB, and significant chalking and color difference (ΔE>1.8) appeared after 500 h of QUV curing. This fully demonstrates that the high comprehensive performance of the coating of the present invention under low temperature curing is not something that can be achieved by conventional components or processes.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A low temperature curing pure polyester powder coating for containers, characterized in that, The pure polyester powder coating comprises: a highly reactive, carboxyl-terminated pure polyester resin covalently modified with weather-resistant monomers; The weather-resistant monomer is a functional monomer whose molecular structure contains at least one carboxyl or hydroxyl group and can participate in polyester polycondensation reaction. The highly reactive end-carboxyl pure polyester resin refers to a polyester resin in which the curing temperature is <170℃ and the curing time is <10min when the polyester is cured with TGIC or HAA.

2. The low temperature curing pure polyester powder coating for containers according to claim 1, characterized in that, The weather-resistant monomer is selected from at least one of 1,4-cyclohexanedicarboxylic acid, hydrogenated bisphenol A, 1,4-cyclohexanediethanol, neopentyl glycol monoacrylate, and 12-hydroxystearic acid.

3. The low temperature curing pure polyester powder coating for containers according to claim 1, characterized in that, The preparation method of the highly reactive end-carboxyl pure polyester resin includes: using diacid, diol and the weather-resistant monomer as raw materials, performing a normal pressure esterification-reduced compression copolymerization reaction, and then using acid excess end-capping after polycondensation to obtain the resin. Preferably, the acid overload end-capping refers to the addition of trimellitic anhydride, accounting for 6-10% of the total molar amount of the dicarboxylic acid, for end-capping when the acid value of the polycondensation reaction drops to 70% of the theoretical value.

4. The low temperature curing pure polyester powder coating for containers according to claim 1, characterized in that, The highly reactive end-carboxyl pure polyester resin has an acid value of 35~45 mgKOH / g, a glass transition temperature (Tg) of 61~68℃, and a softening point of 90~100℃.

5. The low-temperature curing pure polyester powder coating for containers according to claim 1, characterized in that, The pure polyester powder coating includes: Highly reactive carboxyl-terminated pure polyester resin, low-temperature curing agent, low-temperature catalyst, anti-corrosion pigment, functional filler, low-temperature leveling agent, degassing agent, antioxidant stabilizer and toughening agent; Optionally, the pure polyester powder coating further includes additives, which are one or more combinations of antistatic agents, brighteners, and dispersants.

6. The low-temperature curing pure polyester powder coating for containers according to claim 5, characterized in that, The pure polyester powder coating comprises, by weight parts: The composition includes 60-72 parts of highly reactive carboxyl-terminated pure polyester resin, 4.7-6.5 parts of low-temperature curing agent, 0.15-0.6 parts of low-temperature catalyst, 8-25 parts of anti-corrosion pigment, 12-30 parts of functional filler, 0.9-1.8 parts of low-temperature leveling agent, 0.4-0.9 parts of degassing agent, 0.2-0.5 parts of antioxidant stabilizer, 0.5-2 parts of toughening agent, and 0-1.2 parts of other additives. Preferably, the zinc phosphate content in the anti-corrosion pigment is 2 to 10 parts.

7. The low-temperature curing pure polyester powder coating for containers according to claim 5, characterized in that, The low-temperature curing agent is selected from one or more of TGIC, HAA, blocked isocyanates, and polycarboxylic acid adducts, preferably TGIC; And / or, the low-temperature catalyst is a complex system of organotin compounds and quaternary ammonium salts, wherein the mass ratio of the organotin compounds to the quaternary ammonium salts is 1:1; Preferably, the organotin compound is dibutyltin dilaurate; Preferably, the quaternary ammonium salt is benzyltriethylammonium chloride or trioctylmethylammonium chloride; And / or, the anti-corrosion pigment includes one or more of zinc phosphate, aluminum tripolyphosphate, modified barium metaborate, black pigment, and rutile titanium dioxide; And / or, the functional filler includes one or more of 1250-1500 mesh barium sulfate, mica powder, and talc powder; And / or, the low-temperature leveling agent is a polyacrylate with a number average molecular weight of 4000-9000; And / or, the degassing agent is benzoin or a modified degassing agent, preferably BYK961 or Clariant 3910; And / or, the antioxidant stabilizer is obtained by compounding an antioxidant and a light stabilizer; Preferably, the antioxidant is selected from at least one of hindered phenols and phosphites; Preferably, the light stabilizer is selected from UV-531 or UV-9; And / or, the toughening agent is selected from carboxylated nitrile rubber and / or acrylate elastomers, preferably acrylate elastomers.

8. The low-temperature curing pure polyester powder coating for containers according to claim 7, characterized in that, The pure polyester powder coating comprises, by weight parts: The composition includes: 60-72 parts of highly reactive carboxyl-terminated pure polyester resin, 4.7-6.5 parts of TGIC curing agent, 5-15 parts of rutile titanium dioxide, 0.25 parts of black pigment, 8-25 parts of zinc phosphate, 5.75 parts of 1250-mesh barium sulfate, 3 parts of mica powder, 0.9-1.8 parts of polyacrylate leveling agent, 0.4-0.9 parts of benzoin, 0.15-0.6 parts of low-temperature catalyst, 0.2-0.5 parts of antioxidant stabilizer, 0.5-2 parts of acrylate elastomer, and 0.2 parts of dispersant. Preferably, the pure polyester powder coating comprises, by weight parts: The composition includes: 65 parts highly reactive carboxyl-terminated pure polyester resin, 5 parts TGIC curing agent, 12 parts rutile titanium dioxide, 0.25 parts black pigment, 5 parts zinc phosphate, 5.75 parts 1250 mesh barium sulfate, 3 parts mica powder, 1.5 parts polyacrylate leveling agent, 0.6 parts benzoin, 0.4 parts low-temperature catalyst, 0.3 parts antioxidant stabilizer, 1 part acrylate elastomer, and 0.2 parts dispersant. Wherein: the low-temperature catalyst is composed of dibutyltin dilaurate and a quaternary ammonium salt, and the mass ratio of dibutyltin dilaurate to the quaternary ammonium salt is 1:1; The antioxidant stabilizer is composed of hindered phenolic antioxidant and UV-531, and the mass ratio of hindered phenolic antioxidant to UV-531 is 1:

1.

9. A method for preparing a low-temperature curing pure polyester powder coating for containers according to any one of claims 1 to 8, characterized in that, The preparation method includes: The raw materials are mixed according to the formula to obtain a premix, which is then melt-extruded and granulated, and then screened to obtain a pure polyester powder coating.

10. The method for preparing low-temperature curing pure polyester powder coating for containers according to claim 9, characterized in that, The preparation method includes: (A) Premixing: Mix each component at room temperature according to the formula until the material is uniform and free of agglomeration to obtain a premix; (B) Melt extrusion: The premix is ​​extruded using a twin-screw extruder, and then cooled after extrusion into tablets; (C) Coarse crushing and fine grinding: The tablets are crushed and then ground to obtain the grinding material; (D) Grading and sieving: The abrasive is sieved to remove impurities to obtain the finished coating; Preferably, in step (B), the temperature of the first zone of the twin-screw extruder is 85~95℃, the temperature of the second zone is 105–115℃, and the screw speed is 350~550 r / min; Preferably, the grinding particle size D50 in step (C) is 35~45μm, more preferably 40~42μm; Preferably, the finished coating in step (D) is a powder that has passed through a 180-200 mesh sieve.

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