CN112384548A - Polyester resin, preparation method thereof, powder coating and workpiece - Google Patents

Abstract

The invention discloses a polyester resin and a preparation method thereof, a powder coating and a workpiece, wherein the raw materials of the polyester resin comprise polyhydric alcohol, amino alcohol, polybasic acid, an acidolysis agent and a catalyst; the structural formula of the alkamine is NH₂-R-OH, wherein R is a saturated hydrocarbon group containing 1 to 10 carbon atoms. Through the mode, the polyester resin disclosed by the invention has higher reactivity and frictional electrification property and higher Tg, and can be used for preparing low-temperature curing powder coating, especially low-temperature curing powder coating for spraying of a friction gun. Compared with the traditional low-temperature curing powder coating, the powder coating prepared from the polyester resin does not need to be additionally added with a curing accelerator and a frictional electrification agent, and the prepared powder coatingThe obtained powder coating has better storage stability, can reduce the phenomenon of smoke generation in the coating process and the furnace ash deposition in the flow line coating, and can realize the industrial application with lower cost.

Description

Polyester resin, preparation method thereof, powder coating and workpiece

Technical Field

The invention relates to the technical field of polyester resin synthesis, in particular to polyester resin, a preparation method thereof, powder coating and a workpiece.

Background

In recent years, the annual output of Chinese coatings exceeds 1800 million tons, wherein the annual output of solvent-based coatings is about 1200 million tons, the annual output of Volatile Organic Compounds (VOC) exceeds 600 million tons, and the coating is toxic and harmful, has high potential safety hazard, low utilization rate and waste of a large amount of resources and is one of important sources for causing atmospheric haze pollution. The powder coating is 100% solid powder coating which is composed of film-forming resin, curing agent, pigment and filler and auxiliary agent and does not contain solvent. Powder coatings are more environmentally friendly than paints or other coatings because they are solvent free and are a major development direction in the coating industry. The powder coating is widely applied to the fields of household appliances, electronic products, metal furniture and toys, automobiles and automobile accessories, electromechanical equipment, building aluminum profiles, highway guardrails, railway infrastructure and the like.

Although the powder coating has outstanding advantages, compared with liquid coatings, thermosetting powder coatings generally have higher curing temperature (180-200 ℃) and longer curing time (10-30 min), so that the powder coating can only be used on heat-resistant metal substrates and is not suitable for heat-sensitive substrates such as wood, paper, plastics and the like, and the application of the powder coating is greatly limited. The curing temperature of the powder coating is high, so that the energy consumption is high, the energy can be saved by about 10 percent when the baking temperature is reduced by 10 ℃ generally, the baking time is long, and the labor productivity is directly reduced. In addition, too high curing temperature and too long curing time may also affect the performance of the coated substrate, for example, the baking temperature of an aluminum hub used for high-grade cars is too high during coating, which may cause the mechanical properties of the aluminum hub to be seriously reduced. Therefore, the development of low-temperature curing powder coating becomes a necessary research direction in the coating industry.

Generally, the curing temperature is reduced or the curing time is shortened, and the powder coating is easy to have defects on flatness, so that the problems of orange peel, shrinkage cavity, poor appearance, incomplete curing and the like are caused. The low temperature curing of powder coatings (below 160 ℃) requires good leveling and reactivity of the polyester resin at low temperatures. However, good leveling is often accompanied by low viscosity and low Tg (glass transition temperature), and the Tg cannot be too low in order to ensure the storage stability of the resin, so that how to well balance the relationship between the stability of the polyester resin and the reactivity and Tg is a problem in the powder coating technology for realizing low-temperature curing.

In order to increase the reactivity of low-temperature-curing powder coatings, the prior art processes generally add large amounts of curing accelerators. For example, CN104277208A discloses a low-temperature curing polyester resin, which comprises the following raw material components in parts by mass: 7-9 parts of alcohol, 14-15 parts of acid, 0.02 part of esterification catalyst and 0.2-0.3 part of curing accelerator. CN106398482A discloses a low-temperature curing hammer tone powder coating, which comprises the following components in parts by mass: 25-40 parts of polyester resin, 25-40 parts of epoxy resin, 10-25 parts of titanium dioxide, 10-20 parts of filler, 0.1-0.3 part of hammer paint, 0-5 parts of pigment and 0.5-2.0 parts of low-temperature curing accelerator. Most of the curing accelerators are low molecular compounds, and when the curing accelerators are added into the powder coating, the powder coating is easy to agglomerate, and the resin in the coating can be catalyzed to slowly react with a curing agent in the storage process, so that the performance of a coating film is influenced.

In addition, electrostatic spraying and friction gun spraying are common in powder coating spraying processes. Electrostatic spraying is to make powder paint contact with electrostatic mechanism of high-voltage gun to make it carry electrostatic charge, and deposit it on the surface of the substrate to be coated under the action of the electric field given by the outside. Because the powder coating particles are more electrostatically charged, the charged particles accumulate to form a corona field, which can hinder the subsequent further deposition of the powder coating particles, i.e., the faraday effect. The Faraday effect can cause uneven coating and insufficient thickness of coating particles on a coated substrate, and can cause the phenomena of dead angles and difficult powdering of corners of complex workpieces.

The friction gun spraying is that the powder coating collides with a high polymer material (PTEF) in the gun wall to generate positive charges, the charged powder is sprayed to each part of a workpiece under the pushing of air flow and well attached to the surface of the workpiece, and a compact coating is formed after curing. The friction gun does not need a high-voltage electrostatic generator for spraying, and the Faraday effect can not be generated, so that a uniform coating can be sprayed on a complex workpiece, and the surface of the coating is smoother. However, the powder coating is required to have certain frictional electrification property by spraying of a friction gun, the dielectric constant of a gun wall material PTFE commonly used by a powder spray gun is 2, the dielectric constant of polyester resin in the powder coating is only about 3.0, and the dielectric constant of other auxiliary materials is smaller, so that the difference between the dielectric constant of the common powder coating and the dielectric constant of a material used by the friction gun is too small, and no obvious frictional electrification property exists.

In order to improve the triboelectric chargeability of powder coatings, it is generally necessary to add triboelectric charging agents, which are currently on the market either in solid or liquid form. For example, CN108299914A discloses an electrostatic additive for powder coatings, which is added in an amount of 0.15-0.35% of the powder coating. CN104277683A discloses a powder coating for friction gun spraying, which comprises the following substances in parts by weight: 30-60 parts of polyester resin, 4.5-30 parts of curing agent, 0.8-1.2 parts of flatting agent, 18-25 parts of pigment, 0-17 parts of inorganic filler, 0-5 parts of auxiliary agent and 0.1-0.4 part of friction agent. These triboelectric charging agents added to powder coatings in order to increase the reactivity of the powder coatings and to improve the triboelectric charging properties of the powder coatings are mostly small-molecule compounds, which have an adverse effect on the Tg and storage stability of the coatings; the coating is easy to volatilize in the coating baking and curing process to cause the phenomenon of smoke generation, a great amount of furnace dust can be deposited in a baking channel during the assembly line coating, the furnace dust needs to be cleaned regularly, and the production efficiency is influenced.

Disclosure of Invention

In order to solve at least one of the above technical problems, the present invention provides a polyester resin, a method for preparing the same, a powder coating, and a workpiece.

The technical scheme adopted by the invention is as follows:

in a first aspect of the present invention, there is provided:

the polyester resin comprises the following raw materials in percentage by mass: 18 to 40 percent of polyol, 3 to 10 percent of amino alcohol, 45 to 61 percent of polybasic acid, 5 to 15 percent of acidolysis agent and 0.015 to 0.15 percent of catalyst; the structural formula of the amino alcohol is NH₂-R-OH, wherein R is a saturated hydrocarbon group containing 1 to 10 carbon atoms. The saturated hydrocarbon group may specifically be an alkane group or a cycloalkyl group.

The raw materials of the polyester resin are added with specific amino alcohol, in the synthesis process of the polyester resin, the amino and the hydroxyl of the amino alcohol can respectively react with carboxyl, and finally the prepared polyester resin contains molecular chain segments: -CO-NH-R-OOC-.

According to some embodiments of the invention, the amino alcohol is selected from at least one of 2-aminoethanol, 3-amino-1-propanol, 1-amino-2-propanol, 4-amino-1-butanol, 2-amino-2-methyl-1-propanol, 5-amino-1-pentanol, 6-amino-1-hexanol.

According to some embodiments of the invention, the polyol is selected from at least one of neopentyl glycol, ethylene glycol, diethylene glycol, methylpropanediol, 2-ethyl-2-butyl-1, 3-propanediol, 1, 4-cyclohexanedimethanol, hexanediol, hydroxypivalic acid neopentyl glycol monoester, trimethylolpropane, trimethylolethane, ditrimethylolpropane, pentaerythritol.

According to some embodiments of the invention, the polybasic acid is selected from at least one of terephthalic acid, isophthalic acid, adipic acid, sebacic acid, 1, 4-cyclohexanedicarboxylic acid, hydrogenated bisphenol a, hexahydrophthalic anhydride.

According to some embodiments of the invention, the acid hydrolysis agent is selected from at least one of isophthalic acid, adipic acid, sebacic acid, 1, 4-cyclohexanedicarboxylic acid, hexahydrophthalic anhydride, trimellitic anhydride, pyromellitic anhydride.

According to some embodiments of the invention, the catalyst is selected from at least one of a tin-based catalyst, a germanium-based catalyst, and a titanium-based catalyst.

According to some embodiments of the invention, the tin-based catalyst is selected from at least one of monobutyl tin oxide, tin oxalate, monobutyl dihydroxy tin chloride; the germanium catalyst is selected from germanium dioxide; the titanium catalyst is at least one selected from tetrabutyl titanate, tetraisopropyl titanate, titanium dioxide and organic titanium chelate.

According to some embodiments of the present invention, the polyester resin has an acid value of 22 to 75mgKOH/g, a hydroxyl value of less than 10mgKOH/g, a glass transition temperature of 60 to 75 ℃, a melt viscosity of 2500 to 5500mPa.s at 200 ℃, and a reactivity of 50 to 200s at 180 ℃.

In a second aspect of the present invention, there is provided:

the process for producing a polyester resin according to the first aspect of the present invention comprises the steps of:

s1, adding polyhydric alcohol and amino alcohol into a reaction kettle, and heating to melt; then adding polybasic acid and catalyst, and carrying out esterification reaction under the condition of isolating oxygen to obtain an esterification product;

and S2, adding an acidolysis agent into the esterification product under the condition of isolating oxygen, and carrying out acidolysis reaction.

In step S1, after the esterification reaction is performed for a period of time, the degree of esterification reaction can be increased by vacuum pumping; in addition, after the acid hydrolysis reaction of step S2 is completed, the acid-base product obtained in step S2 may be subjected to vacuum polycondensation reaction by vacuum pumping to increase the relative molecular mass of the polyester resin.

The polyester resin can be applied to preparing powder coating, such as low-temperature curing powder coating, in particular to low-temperature curing powder coating for friction gun spraying. Accordingly, the present invention also provides a powder coating comprising any of the above polyester resins.

The specific preparation method of the powder coating can comprise the following steps: the polyester resin is respectively weighed and uniformly mixed with a curing agent, a filler, a pigment and an auxiliary agent according to a certain proportion, and then the mixture is melted, extruded, cooled, crushed and sieved to prepare the powder coating. Wherein, the pigment can be at least one of titanium dioxide, barium sulfate, calcium carbonate and Ciba green; the auxiliary agent comprises benzoin, a leveling agent and the like.

In addition, the above powder coating can be used for coating on the surface of a workpiece to form a protective coating. Therefore, the invention also provides a workpiece, and the surface of the workpiece is adhered with the coating formed by the powder coating.

The beneficial technical effects of the invention are as follows: the polyester resin has high reactivity and frictional electrification property and high glass transition temperature (Tg), and can be used for preparing low-temperature curing powder coating, especially low-temperature curing powder coating for friction gun spraying. Compared with the traditional low-temperature curing powder coating, the powder coating prepared by adopting the polyester resin does not need to additionally add a curing accelerator and a frictional electrification agent, has better storage stability, can reduce the smoke phenomenon in the coating process and the furnace ash deposition in the flow line coating, and can realize the industrial application with lower cost.

Detailed Description

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications of the present invention may be made by those skilled in the art after reading the teaching of the present invention, and such equivalents may fall within the scope of the present invention as defined in the appended claims.

Table 1 shows the specific raw material configurations (wherein the unit of each raw material is g) of the polyester resins of examples 1 to 5 of the present invention and comparative examples 1 to 2

TABLE 1 raw material arrangement of polyester resins of examples and comparative examples

The polyester resins of the above examples and comparative examples were prepared according to the following preparation methods, respectively:

examples 1-2 preparation procedure of polyester resin:

1) adding polyhydric alcohol and amino alcohol into a reaction kettle according to the raw material proportion of examples 1-2 in Table 1, heating until the materials are melted, adding polybasic acid and a catalyst, gradually heating to 230 ℃ under the protection of nitrogen to perform esterification reaction, wherein the acid value reaches 13-19 mgKOH/g, vacuumizing for 2h, and the acid value reaches 5-11 mgKOH/g to obtain an esterification product;

2) adding an acidolysis agent into the esterification product prepared in the step 1) under the protection of nitrogen, and carrying out acidolysis reaction for 3 hours at 230 ℃ to obtain an acidolysis product, wherein the acid value reaches 42-46 mgKOH/g;

3) carrying out polycondensation reaction on the acidolysis product obtained in the step 2) for 2h under the vacuum degree of-0.095 MPa, wherein the acid value reaches 31-36 mgKOH/g, the hydroxyl value is less than 10mgKOH/g, the melt viscosity at 200 ℃ is 4500-5500 mPa.s, and cooling to obtain the polyester resin.

Example 3 preparation procedure of polyester resin:

1) adding polyalcohol and alkamine into a reaction kettle according to the raw material ratio of the example 3 in the table 1, heating until the materials are melted, adding polybasic acid and a catalyst, gradually heating to 230 ℃ under the protection of nitrogen to perform esterification reaction, wherein the acid value reaches 13-19 mgKOH/g, vacuumizing for 1h, and the acid value reaches 5-11 mgKOH/g to obtain an esterification product;

2) adding an acidolysis agent into the esterification product prepared in the step 1) under the protection of nitrogen, and carrying out acidolysis reaction for 3 hours at 230 ℃ to obtain an acidolysis product, wherein the acid value reaches 62-66 mgKOH/g;

3) carrying out polycondensation reaction on the acidolysis product obtained in the step 2) for 2h under the vacuum degree of-0.095 MPa, wherein the acid value reaches 51-56 mgKOH/g, the hydroxyl value is less than 10mgKOH/g, the melt viscosity at 200 ℃ is 3000-4000 mPa.s, and cooling to obtain the polyester resin.

Example 4 preparation procedure of polyester resin:

1) adding polyalcohol and alkamine into a reaction kettle according to the raw material ratio of example 4 in table 1, heating until the materials are melted, adding polyacid and a catalyst, gradually heating to 230 ℃ under the protection of nitrogen to perform esterification reaction, wherein the acid value reaches 13-19 mgKOH/g, vacuumizing for 2h, and the acid value reaches 5-11 mgKOH/g to obtain an esterification product;

2) adding an acidolysis agent into the esterification product prepared in the step 1) under the protection of nitrogen, and carrying out acidolysis reaction for 3 hours at 230 ℃ to obtain an acidolysis product, wherein the acid value reaches 32-36 mgKOH/g;

3) carrying out polycondensation reaction on the acidolysis product obtained in the step 2) for 2h under the vacuum degree of-0.095 MPa, wherein the acid value reaches 22-25 mgKOH/g, the hydroxyl value is less than 10mgKOH/g, the melt viscosity at 200 ℃ is 4500-5500 mPa.s, and cooling to obtain the polyester resin.

Example 5 preparation procedure of modified polyester resin:

1) adding polyalcohol and alkamine into a reaction kettle according to the raw material ratio of example 5 in Table 1, heating until the materials are melted, adding polyacid and a catalyst, gradually heating to 240 ℃ under the protection of nitrogen to perform esterification reaction, wherein the acid value reaches 12-16 mgKOH/g, vacuumizing for 2h, and the acid value reaches 5-8 mgKOH/g to obtain an esterification product;

2) adding an acidolysis agent into the esterification product prepared in the step 1) under the protection of nitrogen, carrying out acidolysis reaction for 1.5h at 220 ℃, wherein the acid value reaches 68-75 mgKOH/g, the hydroxyl value is less than 10mgKOH/g, the melt viscosity at 200 ℃ is 2500-3500 mPa.s, and cooling to obtain the modified polyester resin.

Comparative example 1 preparation procedure of polyester resin:

the polyester resin of comparative example 1 was prepared according to the preparation method of example 1 according to the raw material ratio of comparative example 1 in table 1.

Comparative example 2 preparation procedure of polyester resin:

the polyester resin of comparative example 2 was prepared according to the preparation method of example 5 according to the raw material ratio of comparative example 2 in table 1.

The properties (including acid ester, hydroxyl number, viscosity, glass transition temperature, reactivity) of the polyester resins of examples 1-5 and comparative examples 1-2 above were tested, respectively. Wherein, the acid value and the viscosity are tested according to T/GDTL 004-2018; hydroxyl number was tested according to GB/T12008.3-2009; the glass transition temperature was tested according to GB/T19466.2; the reactivities of the examples 1-4 and the comparative example 1 were tested according to T/GDTL 004-; the reactivity of example 5 and comparative example 2 was tested according to Q/QTCL2-2014, and the curing agent was an epoxy resin. The test results are shown in table 2 below:

table 2, results of Performance test of polyester resins of examples and comparative examples

The polyester resins of examples 1 to 5 and comparative examples 1 to 2 above were used to prepare powder coatings, respectively. Specifically, the polyester resins of the above examples 1 to 5 and comparative examples 1 to 2 are respectively mixed with curing agent TGIC or epoxy resin, leveling agent GLP588, titanium dioxide, barium sulfate, benzoin and brightener 701, curing accelerator and friction agent according to the raw material ratio of the following table 3, and then the mixture is mixed uniformly, melted and extruded by a screw extruder, tabletted and crushed, and then the tabletted materials are crushed and sieved to obtain the powder coating. In table 3, the polyester resin of example i (i ═ 1,2,3,4,5) in table 1 above was used for the powder coating, the polyester resin of comparative example 1 was used for the powder coating of comparative example 1-m (m ═ 1,2,3), and the polyester resin of comparative example 2 was used for the powder coating of comparative example 2-n (n ═ 1,2, 3). The units of the raw materials in Table 3 are g.

TABLE 3 raw material formulation for powder coatings of examples and comparative examples

The powder coatings prepared in the above examples and comparative examples are sprayed on the iron plate after surface treatment by a friction gun, baked and cured at 160 ℃ for 15min to obtain a powder coating of 70-90 μm, and then the following performance tests are carried out on the powder coating:

(1) gel time was tested according to GB/T1699-1997;

(2) the appearance of the coating film was evaluated by visual inspection;

(3) gloss was tested according to GB/T9754-2007;

(4) the bending test was tested according to GB/T6742-2007;

(5) impact was tested according to GB/T1732-1993;

(6) reading and recording the friction current value by an ammeter on the friction spray gun equipment in the spraying process;

(7) aging test A1000 h xenon lamp artificial accelerated aging test was carried out according to GB/T1865-2009.

The performance of each powder coating was tested using the above test method, and the test results are shown in table 4:

TABLE 4, examples and comparative examples Performance test results for powder coatings spray formed from powder coatings

As can be seen from tables 1-4 above:

the polyester resins of comparative examples 1 and 2 had low reactivity at 180 ℃ without adding aminoalcohol to the starting materials; the polyester resins of comparative example 1 and comparative example 2 are respectively adopted to prepare the powder coating, if the strengthening accelerant and the friction agent are not used, the coating has too long gelling time, the mechanical property of the coating is poor, the curing is insufficient, the friction current value is small, and the powdering is poor; when a curing accelerator and an abrasive are added in the preparation of a powder coating, the coating gelling time is shortened and the friction current value is increased.

The polyester resin of the embodiments 1 to 5 of the invention has high reactivity and frictional electrification, and the powder coating prepared by using the polyester resin of the embodiments 1 to 5 does not need to additionally add a curing accelerator and a frictional electrification agent, so that the prepared powder coating has short reactivity, short gelling time and large frictional current, can realize low-temperature curing and friction gun spraying, and has excellent film coating performance. In addition, the polyester resin and the further prepared powder coating in the embodiments 1 to 5 have less added micromolecular auxiliary agent, so that the smoke generation phenomenon is not easy to occur in the baking process.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (10)

1. The polyester resin is characterized by comprising the following raw materials in percentage by mass: 18 to 40 percent of polyol, 3 to 10 percent of amino alcohol, 45 to 61 percent of polybasic acid, 5 to 15 percent of acidolysis agent and 0.015 to 0.15 percent of catalyst; the structural formula of the amino alcohol is NH₂-R-OH, wherein R is a saturated hydrocarbon group containing 1 to 10 carbon atoms.

2. The polyester resin according to claim 1, wherein the amino alcohol is at least one selected from the group consisting of 2-aminoethanol, 3-amino-1-propanol, 1-amino-2-propanol, 4-amino-1-butanol, 2-amino-2-methyl-1-propanol, 5-amino-1-pentanol, and 6-amino-1-hexanol.

3. The polyester resin according to claim 1, wherein the polyol is at least one selected from the group consisting of neopentyl glycol, ethylene glycol, diethylene glycol, methylpropanediol, 2-ethyl-2-butyl-1, 3-propanediol, 1, 4-cyclohexanedimethanol, hexanediol, hydroxypivalic acid neopentyl glycol monoester, trimethylolpropane, trimethylolethane, ditrimethylolpropane, and pentaerythritol.

4. The polyester resin according to claim 1, wherein the polybasic acid is at least one selected from the group consisting of terephthalic acid, isophthalic acid, adipic acid, sebacic acid, 1, 4-cyclohexanedicarboxylic acid, hydrogenated bisphenol a, and hexahydrophthalic anhydride.

5. The polyester resin according to claim 1, wherein the acid hydrolysis agent is at least one selected from the group consisting of isophthalic acid, adipic acid, sebacic acid, 1, 4-cyclohexanedicarboxylic acid, hexahydrophthalic anhydride, trimellitic anhydride, and pyromellitic anhydride.

6. The polyester resin according to claim 1, wherein the catalyst is at least one selected from the group consisting of a tin-based catalyst, a germanium-based catalyst, and a titanium-based catalyst.

7. The polyester resin according to any one of claims 1 to 6, wherein the polyester resin has an acid value of 22 to 75mgKOH/g, a hydroxyl value of less than 10mgKOH/g, a glass transition temperature of 60 to 75 ℃, a melt viscosity of 2500 to 5500mPa.s at 200 ℃ and a reactivity of 50 to 200s at 180 ℃.

8. The method for preparing polyester resin according to any one of claims 1 to 7, comprising the steps of:

s1, adding polyhydric alcohol and amino alcohol into a reaction kettle, and heating to melt; then adding polybasic acid and catalyst, and carrying out esterification reaction under the condition of isolating oxygen to obtain an esterification product;

and S2, adding an acidolysis agent into the esterification product under the condition of isolating oxygen, and carrying out acidolysis reaction.

9. A powder coating comprising the polyester resin according to any one of claims 1 to 7.

10. A workpiece having a surface to which a coating layer formed of the powder coating material according to claim 9 is attached.