A method for preparing polyester for powder coating

Abstract

The present invention belongs to the field of polyester technology and relates to a method for preparing a polyester for powder coatings. The reaction raw materials are sequentially subjected to an esterification reaction, an acidolysis reaction, and a pre-polycondensation reaction under the action of a catalyst. The esterification reaction has an esterification rate of 97-99.5%, a pressure of 0.1 MPa, and a reaction time of 3.5-4.4 hours. The catalyst is a hybrid titanium catalyst; the mass of the titanium element in the hybrid titanium catalyst is 100-170 ppm of the theoretical mass of the polyester for powder coatings. The hybrid titanium catalyst used in the present invention can successfully synthesize the polyester for powder coatings at normal pressure with high catalytic efficiency, high-temperature hydrolysis resistance, strong stability, and excellent performance of the synthesized polyester for powder coatings.

Description

Technical Field

[0001] The invention belongs to the technical field of polyester and relates to a method for preparing polyester for powder coating. Background Art

[0002] Powder coating is a specialized coating that uses a resin binder and curing agent as film-forming materials, supplemented with fillers, pigments, and functional additives, resulting in a 100% solids powder form. Compared to traditional coatings, this type of coating offers advantages: zero VOC (volatile organic compound) emissions, recyclability, energy savings, and superior film performance.

[0003] Powder coatings are primarily categorized into thermoplastic and thermosetting powder coatings. However, thermosetting powder coatings, particularly polyester systems, dominate the market. Polyesters, whether containing -COOH (carboxyl) or -OH (hydroxyl) functionalities, provide coatings with excellent appearance and mechanical properties, such as toughness and flexibility.

[0004] Polyesters for general-purpose powder coatings have excellent properties such as weather resistance and moisture resistance. They exhibit good performance in a variety of common application scenarios, different construction conditions, and on various coated substrates. Therefore, they are widely used in home appliances, automobiles and auto parts, architectural aluminum profiles, toys and other fields.

[0005] Catalyst selection plays a crucial role in polyester synthesis. Common catalysts for synthesizing polyesters for powder coatings include organotin catalysts and titanates. However, organotin catalysts are restricted in their use due to their heavy metal content. Titanate catalysts, such as tetrabutyl titanate (TBOT) and tetraisopropyl titanate, are widely used but have low catalytic efficiency at ambient pressure. Furthermore, titanate catalysts are susceptible to hydrolysis and exhibit instability, resulting in poor transparency in the resulting polyesters for powder coatings.

[0006] Patent ZL201811008954.1 discloses a polyester resin resistant to high humidity, heat, and radiation, as well as its preparation method and application. Its raw materials include neopentyl glycol, trimethylolpropane, 2-butyl-2-ethyl-1,3-propanediol, terephthalic acid, adipic acid, and isophthalic acid, while TBOT is used as a catalyst. However, the esterification reaction takes as long as 14.5 hours. The literature (Synthesis and Properties of Carboxyl-Terminated Polyesters and Semi-Crystalline Polyesters for Polyester / Epoxy Powder Coatings [D]. Guangzhou: South China University of Technology, 2010) uses ethylene glycol, neopentyl glycol, trimethylolpropane, terephthalic acid, isophthalic acid, adipic acid, and anhydride to prepare a polyester for powder coatings. When TBOT is used as a catalyst, the esterification reaction takes as long as 12 hours. Both studies demonstrate the low catalytic efficiency of titanate catalysts at ambient pressure.

[0007] Therefore, it would be of great significance to find a catalyst with high catalytic efficiency and low hydrolysis resistance under normal pressure and apply it to the preparation of polyester for powder coatings. The application of such a catalyst would bring the following advantages:

[0008] (1) Simplified production operations: No high-voltage equipment is required, which reduces equipment investment and maintenance costs, simplifies operating procedures, and improves safety. The simplified equipment structure also reduces operational difficulty and safety risks, ensuring the safety of personnel and facilities.

[0009] (2) Excellent product performance: The reaction conditions under normal pressure are mild and easy to control, enabling the precise synthesis of polyesters with specific structures and properties, thus ensuring the stability of product quality. The synthesized polyester has a regular molecular structure, which gives the coating film good comprehensive properties and further expands the scope of application.

[0010] (3) Effective cost control: Normal pressure reaction reduces energy consumption and energy expenditure, which meets the requirements of sustainable development. At the same time, material volatilization loss is reduced, the utilization rate of raw materials is improved, the cost is reduced, and the quality fluctuation is reduced.

[0011] (4) Significant environmental benefits: No special high-pressure additives are required, which reduces pollution emissions. The equipment sealing requirements are lowered, reducing the risk of leakage, conforming to the concept of green chemistry and promoting the sustainable development of the industry. Summary of the Invention

[0012] The purpose of the present invention is to solve the problems existing in the prior art, to find a catalyst with high catalytic efficiency and not easy to hydrolyze under normal pressure, and to apply the catalyst to the preparation of polyester for powder coating.

[0013] The premise for the application of polyester for powder coatings is that its esterification rate is high, the pre-condensation product is clear and transparent, and it meets a series of physical properties such as intrinsic viscosity, number average molecular weight, acid value, etc.

[0014] During the esterification process, the reaction between the acid and alcohol is initially heterogeneous, meaning the reactants and products are unevenly distributed throughout the system. However, as the reaction proceeds, particularly when the esterification rate exceeds 90%, the acid dissolves in the resulting ester, transforming the reaction into a homogeneous state and the system becoming clear. From the early to late stages of the reaction, a transition state exists between the starting materials and the products, a critical stage in the reaction process.

[0015] The polycondensation process involves the removal of small alcohols from the resulting intermediate esters to form polyesters. Transition state theory plays a key role in this process. According to this theory, reactant molecules do not directly form products through simple collisions; instead, they must first pass through a high-energy activated complex (transition state). Reaching this transition state requires overcoming a certain activation energy before conversion to the product. Activation energy is the minimum energy a reactant molecule must attain during the reaction in order to form an activated complex and ultimately convert to the product. Activation energy is typically represented by the symbol Ea, with units measured in joules (J) or kilocalories per mole (kcal / mol). The magnitude of the activation energy directly affects the reaction rate; the lower the activation energy, the faster the reaction rate.

[0016] The role of a catalyst in a chemical reaction is to lower the activation energy, making the reaction more likely to proceed. It provides a new reaction pathway with a lower activation energy than the original pathway. As a result, more reactant molecules are able to overcome the energy barrier and participate in the reaction, accelerating the reaction rate.

[0017] Under the same reaction system, the lower the activation energy of a catalyst, the higher its catalytic efficiency. To compare the activation energies of different catalysts (hybrid titanium catalyst (HCEO), TBOT, and tetraisopropyl titanate), the present inventors conducted a series of exploratory experiments.

[0018] The steps for preparing polyester for powder coating in the following experiments are roughly the same, the only difference being the type and amount of catalyst. The steps for preparing polyester for powder coating are as follows:

[0019] (a) feeding;

[0020] First, 2-methyl-1,3-propanediol and a catalyst are uniformly mixed to obtain a premix, and then, after preheating a reactor to 100°C, ethylene glycol, terephthalic acid, neopentyl glycol, the premix, trimethylolpropane, and trimethyl phosphate are added thereto;

[0021] (b) esterification reaction;

[0022] After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 200°C, and the pressure is maintained at 0.1 MPa for 3.5-7.1 hours. During this process, the oil temperature is adjusted to maintain the temperature at the top of the esterification tower at 95-100°C, and the temperature in the kettle is gradually raised to 260°C at a rate of 4°C / 10min;

[0023] (c) acid hydrolysis reaction;

[0024] After the temperature in the kettle was lowered to 225°C, isophthalic acid was added and reacted for 60 minutes;

[0025] (d) pre-polycondensation reaction;

[0026] Within 20 minutes, the temperature in the kettle was raised to 250°C and the pressure in the kettle was reduced to -101KPa;

[0027] (e) introducing nitrogen or an inert gas to lower the temperature in the autoclave, and cooling the discharged material to obtain polyester for powder coating;

[0028] In steps (a) to (e), the alcohol-acid ratio of the reaction system is 1.1:1; the amount of neopentyl glycol added is 55% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 30% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of isophthalic acid added is 1.1:1 of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added. ,3-propylene glycol, ethylene glycol and trimethylolpropane added in an amount of 2% mol; the amount of trimethylolpropane added is 1% mol of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added; the amount of trimethyl phosphate added is 0.3wt% of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added.

[0029] The exploration process is as follows:

[0030] (1) Experiments 1 to 4 were conducted, with the catalyst and dosage being HCEO (100 ppm, i.e., the mass of titanium element in the catalyst as a percentage of the theoretical mass of polyester for powder coating), HCEO (130 ppm), HCEO (150 ppm), and HCEO (170 ppm), respectively;

[0031] The esterification rate x (= actual esterification water output / theoretical esterification water output × 100%) corresponding to experiments 1 to 4 was plotted against the reaction time t into a scatter plot, and the fitting result was obtained. Figure 1 The four curves of

[0032] The esterification reaction time (time when the esterification rate reaches 100%) and product state corresponding to Experiments 1 to 4 are shown in Table 1 ;

[0033] Table 1

[0034] Catalyst name Esterification reaction time / min Product status HCEO (150ppm) 206 Clarity and transparency HCEO (170ppm) 230 Clarity and transparency HCEO (130ppm) 256 Clarity and transparency HCEO (100ppm) 264 Clarity and transparency

[0035] From the results, it can be seen that the catalytic efficiency is the fastest when the HCEO dosage is 150ppm;

[0036] (2) Experiments 5 to 8 were conducted, with the catalysts and dosages being HCEO (150 ppm), TBOT (150 ppm), tetraisopropyl titanate (150 ppm), and TBOT (100 ppm), respectively;

[0037] The esterification rate x (= actual esterification water output / theoretical esterification water output × 100%) corresponding to experiments 5 to 8 and the reaction time t were plotted into a scatter plot, and the fitting results were obtained. Figure 2 The four curves of

[0038] Get Figure 2 The temperature T in the kettle corresponding to each scattered point in the four curves;

[0039] respectively Figure 2 The four curves are processed (first-order derivative is substituted into the reaction time t) to obtain the slope K (representing the reaction rate constant) of each curve at each scattered point;

[0040] The lnK and T corresponding to experiments 5 to 8 are -1 Draw a scatter plot and get Figure 3 According to the Arrhenius formula, the slopes of the four straight lines are the four -E a / R, R is the molar gas constant, according to the four-E a / R and R can be calculated to get four E a , as shown in Table 2:

[0041] Table 2

[0042] Catalyst name <![CDATA[Activation energy E a / (kJ·mol －1 )]]> Esterification reaction time / min state HCEO (150ppm) 131.1 206 Clarity and transparency TBOT (150ppm) 338.15 305 opaque Tetraisopropyl titanate (150ppm) 470.65 360 Not clear and not transparent TBOT (100ppm) 340.26 427 Clarity and transparency

[0043] Comparing the esterification kinetics of several titanium-based catalysts revealed that the trends in activation energy and esterification reaction time were consistent. HCEO exhibited the lowest activation energy and the shortest esterification reaction time, resulting in a 30%-50% improvement in catalytic efficiency over TBOT. Furthermore, HCEO successfully synthesized clear and transparent polyester for powder coatings at atmospheric pressure. However, TBOT participates in the esterification reaction, resulting in unstable top temperatures ranging from 80°C to 100°C. The transparency of the resulting polyester for powder coatings gradually decreased with increasing TBOT dosage. At 150 ppm, product transparency became significantly poor. Furthermore, while the top temperature remained stable at around 80°C when catalyzed by tetraisopropyl titanate, no water molecules were produced, indicating incomplete esterification. Therefore, it cannot be used in this formulation or related applications.

[0044] The above research experiments show that the catalytic efficiency of HCEO is better than that of other catalysts. Therefore, the present invention attempts to use HCEO to prepare polyester for powder coatings. The technical solution of the present invention is as follows:

[0045] A method for preparing polyester for powder coatings comprises sequentially subjecting the reaction raw materials to esterification reaction, acidolysis reaction, and pre-polycondensation reaction under the action of a catalyst, wherein the esterification rate of the esterification reaction is 97-99.5%, the pressure of the esterification reaction is 0.1 MPa, and the esterification reaction time is 3.5-4.4 hours. The catalyst is a hybrid titanium catalyst, and the structural formula is as follows:

[0046] ;

[0047] The mass of titanium element in the hybrid titanium catalyst is 100-170ppm of the theoretical mass of polyester for powder coating. If the amount of the hybrid titanium catalyst is too low, the effect is not obvious, while if it is too high, it will lead to a decrease in esterification rate and reaction rate.

[0048] Although the hybrid titanium catalyst used in the present invention is a substance already disclosed in the prior art, the prior art only uses it as a polycondensation catalyst for the preparation of ordinary PET and PETG, and the polycondensation process is carried out under negative pressure. The working principle of the hybrid titanium catalyst is as follows: in the polyester polycondensation reaction, the titanium atom first undergoes an exchange reaction with the terminal hydroxyl group of the oligomer to form a metal alkoxide, and the oxygen in the hydroxyethyl ester group in the other molecule attacks the carbonyl carbon atom. The nucleophilic attack of the ester carbonyl oxygen on the central titanium atom increases the positive charge of the carbonyl carbon, thereby increasing the reaction rate.

[0049] The present invention applies it as an esterification catalyst for the first time to prepare polyester for powder coatings. The esterification process is carried out under normal pressure. The working principle of the hybrid titanium catalyst is: titanium atoms can introduce acidic centers. In the esterification reaction of terephthalic acid and ethylene glycol, the catalyst provides protons to protonate the carboxyl groups of terephthalic acid and the hydroxyl groups of ethylene glycol, forming a better electrophilic reagent and accelerating the reaction between the two to form an ester bond.

[0050] The present invention unexpectedly discovered that, in the esterification process of preparing polyester for powder coating, the catalytic efficiency of the hybrid titanium catalyst is significantly better than that of the traditional titanate catalyst.

[0051] As the preferred technical solution:

[0052] In the above-mentioned method for preparing polyester for powder coating, the reaction raw materials include terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol, and trimethylolpropane.

[0053] The multifunctional structure of 2-methyl-1,3-propanediol can regulate the structure of polyester molecular chains, thereby improving the flexibility of the resulting polyester. Compared to using traditional diols such as 1,4-butanediol, it can reduce the regularity of the polyester molecular chain, thereby reducing crystallinity, thus preventing problems such as caking during storage of powder coatings caused by excessive crystallinity. Furthermore, using 2-methyl-1,3-propanediol to synthesize polyesters helps improve the weather resistance of powder coatings. In outdoor environments, powder coatings made with polyester modified with 2-methyl-1,3-propanediol are resistant to UV rays, moisture, and other factors, and are less likely to powder or fade, extending the coating's service life. It also improves the leveling of powder coatings, resulting in a smoother and flatter surface, reducing surface defects and enhancing the appearance quality.

[0054] In the above-mentioned method for preparing polyester for powder coating, the alcohol-acid ratio of the reaction system is 1.1-1.2:1; the amount of neopentyl glycol added is 50-60% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 20-30% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol added; the amount of isophthalic acid added is 2-3% mol of the total amount of reaction raw materials added; and the amount of trimethylolpropane added is 1-2% mol of the total amount of reaction raw materials added.

[0055] In the above-mentioned method for preparing polyester for powder coating, the reaction system further contains a stabilizer (trimethyl phosphate). Adding the stabilizer can inhibit the oxidation reaction and reduce the occurrence of oxidative yellowing to a certain extent. The amount of the stabilizer added is 0.3-0.5wt% of the total amount of the reaction raw materials added.

[0056] The preparation method of the polyester for powder coating as described above comprises the following specific steps:

[0057] (a) feeding;

[0058] After preheating the reactor, ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, hybrid titanium catalyst, and stabilizer were added thereto;

[0059] (b) esterification reaction;

[0060] After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 190-200°C and the reaction is started. During this process, the oil temperature is adjusted to maintain the top temperature of the esterification tower at 95-105°C. At the same time, the temperature in the kettle is gradually raised to 245-260°C at a rate of 4°C / 10min.

[0061] (c) acid hydrolysis reaction;

[0062] After the temperature in the kettle is lowered to 220-230°C, isophthalic acid (as an acidolysis agent) is added and the reaction is continued for 60-90 minutes;

[0063] During the acid hydrolysis process, a capping reaction occurs to form a polyester resin mainly composed of terminal carboxyl groups. The reaction equation is as follows:

[0064] ;

[0065] (d) pre-polycondensation reaction;

[0066] Within 20-30 minutes, the temperature in the kettle is raised to 220-250°C and the pressure in the kettle is reduced to -101KPa;

[0067] The purpose of the prior art pre-condensation is to initially polymerize the monomers to form a prepolymer with a higher molecular weight. This process generally lasts for a long time and optimizes the molecular weight distribution. Unlike the prior art, the purpose of the present invention's pre-condensation is to extract the small molecules remaining in the system to further increase the molecular weight of the product. Therefore, the pre-condensation reaction temperature of the present invention is relatively low and the time is relatively short. This can avoid implosion while reducing side reactions, thereby obtaining a polyester for powder coatings with the desired molecular weight and viscosity.

[0068] (e) After nitrogen or inert gas is introduced to lower the temperature in the reactor, the discharged material flows into a mold for cooling to obtain polyester for powder coating.

[0069] In the above-mentioned method for preparing polyester for powder coating, in step (a), the preheating temperature is 80-100°C, and before feeding, 2-methyl-1,3-propanediol and the hybrid titanium catalyst are first mixed evenly, so that feeding can be facilitated.

[0070] The polyester for powder coating has a number average molecular weight of 4000-4800 g / mol, an intrinsic viscosity of 0.13-0.14 dL / g, an acid value of 18-20 mKOH / g, a glass transition temperature (Tg) of 60-62°C, and is clear and transparent.

[0071] Principle of the invention:

[0072] The reason why traditional titanate catalysts have low catalytic efficiency and are easily hydrolyzed at normal pressure is that the titanium atoms in titanate catalysts are connected to alkoxy groups through covalent bonds, forming a Ti-O bond with a certain polarity. This polarity makes the Ti-O bond susceptible to attack by nucleophiles. Under normal pressure conditions, the water molecules generated in the reactor act as nucleophiles, and the lone pair electrons on their oxygen atoms will be attracted by the positively charged titanium atoms in the Ti-O bond, thereby triggering a hydrolysis reaction and generating solid impurity particles such as titanium dioxide. These impurity particles not only affect the physical and chemical properties of the polymer, but may also lead to a series of problems such as increased oligomer content and yellowing of the color. In addition, titanate compounds themselves have high chemical activity and are easy to react with substances containing active hydrogen. Since water is a typical substance containing active hydrogen, it can easily react chemically with titanate, resulting in the hydrolysis of titanate. Reasons for low catalytic efficiency: (1) Reduction of active sites: Hydrolysis destroys the chemical bonds and active site structure of titanate, resulting in a significant reduction in the number of active sites; hydrolysis products such as titanium dioxide are deposited on the catalyst surface, covering the active sites and hindering the contact of reactants. (2) Chemical structure changes: Hydrolysis destroys the coordination environment of titanium atoms, causing them to lose their original electronic structure and spatial configuration, changing catalytic activity and selectivity; hydrolysis converts titanate into inactive or low-activity substances, which cannot effectively reduce the activation energy of the reaction. (3) Specific surface area reduction: Hydrolysis causes particle agglomeration, resulting in a decrease in the specific surface area of ​​the catalyst and a reduction in the contact area between the reactants and the catalyst.

[0073] The present invention uses a green, hydrolysis-resistant hybrid titanium catalyst to synthesize polyester for powder coatings, with high catalytic efficiency and stable reaction. The reasons are: (1) the hybrid titanium catalyst has an original nano-sheet structure and hybrid composition. This structure makes the molecular interaction strong, giving the catalyst excellent hydrolysis resistance, allowing it to participate in the esterification reaction, and its structure and performance are stable; (2) the hybrid titanium catalyst has segments similar to terephthalic acid and segments similar to diols. These two segments can respectively produce strong interactions with the reactants terephthalic acid and diols, thereby attracting the two to gather and collide at the catalyst, thereby promoting the esterification reaction. In addition, due to the presence of chain segments similar to diols, the catalyst can be well dispersed in the diols, and further evenly dispersed in the esterification system, greatly increasing the collision probability of terephthalic acid and diols near the catalyst, so the hybrid titanium catalyst has good catalytic activity.

[0074] Beneficial effects:

[0075] The present invention uses an environmentally friendly, hydrolysis-resistant hybrid titanium catalyst to synthesize polyester for powder coatings. The hybrid titanium catalyst can successfully synthesize polyester for powder coatings at normal pressure with high catalytic efficiency, which can be 30-50% higher than the catalytic efficiency of traditional titanate catalysts. In addition, the catalyst is resistant to high-temperature hydrolysis and has strong stability, and the synthesized polyester for powder coatings is clear and transparent. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 The graph is a relationship between the esterification rate and the reaction time under different dosages of hybrid titanium catalyst;

[0077] Figure 2 The graph is a relationship between esterification rate and reaction time under different catalysts and dosages;

[0078] Figure 3 lnK and T under different catalysts and dosages -1 relationship diagram. DETAILED DESCRIPTION

[0079] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0080] The following are the test methods for the relevant performance indicators in each embodiment and comparative example:

[0081] Number average molecular weight: tested using room temperature gel chromatography (PL-GPC50, Agilent, USA). The solvent was hexafluoroisopropanol and the concentration of the prepared solution was 1 mg / mL.

[0082] Intrinsic viscosity and acid value: According to GB / T 14190-2017 "Test method for fiber-grade polyester (PET) chips", the intrinsic viscosity and acid value of the test sample are tested.

[0083] Glass transition temperature: The test was performed using a Q20 high-temperature TA scanning calorimeter in a nitrogen atmosphere. The initial temperature was set at 30°C, held for 1 min, then heated to 300°C at a rate of 15°C / min, held for 5 min, then cooled to 30°C at a rate of 15°C / min, held for 5 min, and finally heated to 300°C at a rate of 15°C / min.

[0084] Example 1

[0085] A method for preparing polyester for powder coating, comprising the following steps:

[0086] (a) feeding;

[0087] First, 2-methyl-1,3-propanediol and a hybrid titanium catalyst (manufacturer: Shanghai Huiyi New Material Technology Co., Ltd., model: HCEO) were mixed evenly to obtain a premix. Then, after preheating the reactor to 80°C, ethylene glycol, terephthalic acid, neopentyl glycol, the premix, trimethylolpropane, and trimethyl phosphate were added to the reactor.

[0088] (b) esterification reaction;

[0089] After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 200°C and the pressure is maintained at 0.1 MPa for 4.4 hours. During this process, the oil temperature is adjusted to maintain the temperature at the top of the esterification tower at 95°C. At the same time, the temperature in the kettle is gradually raised to 260°C at a rate of 4°C / 10min.

[0090] The esterification rate of the esterification reaction was 97%;

[0091] (c) acid hydrolysis reaction;

[0092] After the temperature in the kettle was lowered to 230°C, isophthalic acid was added and reacted for 90 minutes;

[0093] (d) pre-polycondensation reaction;

[0094] Within 20 minutes, the temperature in the kettle was raised to 250°C and the pressure in the kettle was reduced to -101KPa;

[0095] (e) introducing nitrogen or an inert gas to lower the temperature in the autoclave, and cooling the discharged material to obtain polyester for powder coating;

[0096] In steps (a) to (e), the mass of the titanium element in the hybrid titanium catalyst is 100 ppm of the theoretical mass of the polyester for powder coating; the alcohol-acid ratio of the reaction system is 1.1:1; the amount of neopentyl glycol added is 50% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 30% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of isophthalic acid added is 100 ppm of the total amount of terephthalic acid, The amount of trimethylolpropane added is 2% mol of the total amount of isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added; the amount of trimethylolpropane added is 2% mol of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added; the amount of trimethyl phosphate added is 0.3wt% of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added.

[0097] The final polyester for powder coating has a number average molecular weight of 4000 g / mol, an intrinsic viscosity of 0.13 dL / g, an acid value of 18 mKOH / g, a glass transition temperature of 60° C., and is clear and transparent.

[0098] Example 2

[0099] A method for preparing polyester for powder coating, comprising the following steps:

[0100] (a) feeding;

[0101] First, 2-methyl-1,3-propanediol and a hybrid titanium catalyst (manufacturer: Shanghai Huiyi New Material Technology Co., Ltd., model: HCEO) were mixed evenly to obtain a premix. Then, after preheating the reactor to 90°C, ethylene glycol, terephthalic acid, neopentyl glycol, the premix, trimethylolpropane, and trimethyl phosphate were added to the reactor.

[0102] (b) esterification reaction;

[0103] After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 195°C and the pressure is maintained at 0.1 MPa for 4.27 hours. During this process, the oil temperature is adjusted to maintain the temperature at the top of the esterification tower at 100°C. At the same time, the temperature in the kettle is gradually raised to 255°C at a rate of 4°C / 10min.

[0104] The esterification rate of the esterification reaction was 98%;

[0105] (c) acid hydrolysis reaction;

[0106] After the temperature in the kettle was lowered to 225°C, isophthalic acid was added and reacted for 70 minutes;

[0107] (d) pre-polycondensation reaction;

[0108] Within 30 minutes, the temperature in the kettle was raised to 240°C and the pressure in the kettle was reduced to -101KPa;

[0109] (e) introducing nitrogen or an inert gas to lower the temperature in the autoclave, and cooling the discharged material to obtain polyester for powder coating;

[0110] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 130 ppm of the theoretical mass of polyester for powder coating; the alcohol-acid ratio of the reaction system is 1.2:1; the amount of neopentyl glycol added is 55% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 25% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of isophthalic acid added is 10% mol of terephthalic acid, 1,3-propanediol and ethylene glycol added. The amount of trimethylolpropane added is 1% mol of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added; the amount of trimethylolpropane added is 0.4 wt% of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added.

[0111] The final polyester for powder coating has a number average molecular weight of 4758 g / mol, an intrinsic viscosity of 0.134 dL / g, an acid value of 18.2 mKOH / g, a glass transition temperature of 61.1° C., and is clear and transparent.

[0112] Example 3

[0113] A method for preparing polyester for powder coating, comprising the following steps:

[0114] (a) feeding;

[0115] First, 2-methyl-1,3-propanediol and a hybrid titanium catalyst (manufacturer: Shanghai Huiyi New Material Technology Co., Ltd., model: HCEO) were mixed evenly to obtain a premix. Then, after preheating the reactor to 100°C, ethylene glycol, terephthalic acid, neopentyl glycol, the premix, trimethylolpropane, and trimethyl phosphate were added to the reactor.

[0116] (b) esterification reaction;

[0117] After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 190°C and the pressure is maintained at 0.1 MPa for 3.5 hours. During this process, the oil temperature is adjusted to maintain the top temperature of the esterification tower at 100°C. At the same time, the temperature in the kettle is gradually raised to 250°C at a rate of 4°C / 10min.

[0118] The esterification rate of the esterification reaction was 99.5%;

[0119] (c) acid hydrolysis reaction;

[0120] After the temperature in the kettle was lowered to 225°C, isophthalic acid was added and reacted for 60 minutes;

[0121] (d) pre-polycondensation reaction;

[0122] Within 20 minutes, the temperature in the kettle was raised to 240°C and the pressure in the kettle was reduced to -101KPa;

[0123] (e) introducing nitrogen or an inert gas to lower the temperature in the autoclave, and cooling the discharged material to obtain polyester for powder coating;

[0124] In steps (a) to (e), the mass of the titanium element in the hybrid titanium catalyst is 150 ppm of the theoretical mass of the polyester for powder coating; the alcohol-acid ratio of the reaction system is 1.1:1; the amount of neopentyl glycol added is 60% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 20% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of isophthalic acid added is 10% mol of terephthalic acid, The amount of trimethylolpropane added is 2% mol of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added; the amount of trimethylolpropane added is 0.5 wt% of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added.

[0125] The final polyester for powder coating has a number average molecular weight of 4800 g / mol, an intrinsic viscosity of 0.14 dL / g, an acid value of 20 mKOH / g, a glass transition temperature of 62° C., and is clear and transparent.

[0126] Comparative Example 1

[0127] A method for preparing polyester is different from Example 3 only in that the hybrid titanium catalyst is completely replaced by TBOT, and the mass of the titanium element in TBOT is the same as that in Example 3.

[0128] The esterification rate of the esterification reaction was 92%; the polyester finally obtained was white and opaque.

[0129] Compared with Example 3, Comparative Example 1 shows a significantly lower esterification rate and a significantly worsened transparency of the polyester. This is because the catalytic efficiency of the TBOT used in Comparative Example 1 is low at normal pressure (0.1 MPa), resulting in a lower esterification rate. Furthermore, TBOT is easily hydrolyzed to form white titanium dioxide particles, which are dispersed in the polyester system, resulting in a worsened transparency of the polyester.

[0130] Comparative Example 2

[0131] A method for preparing polyester is different from Example 3 only in that the hybrid titanium catalyst is completely replaced by tetraisopropyl titanate, and the mass of the titanium element in the tetraisopropyl titanate is the same as that in Example 3.

[0132] The esterification rate of the esterification reaction was 50%; the polyester finally obtained was white and opaque.

[0133] Compared with Example 3, Comparative Example 2 shows a significantly lower esterification rate and a significantly worsened transparency of the polyester. This is because the catalytic efficiency of the tetraisopropyl titanate used in Comparative Example 2 is low at normal pressure (0.1 MPa), resulting in a lower esterification rate. Furthermore, tetraisopropyl titanate is easily hydrolyzed to form white titanium dioxide particles, which are dispersed in the polyester system, causing the polyester to have poor transparency.

[0134] Example 4

[0135] A method for preparing polyester for powder coating, comprising the following steps:

[0136] (a) feeding;

[0137] First, 2-methyl-1,3-propanediol and a hybrid titanium catalyst (manufacturer: Shanghai Huiyi New Material Technology Co., Ltd., model: HCEO) were mixed evenly to obtain a premix. Then, after preheating the reactor to 100°C, ethylene glycol, terephthalic acid, neopentyl glycol, the premix, trimethylolpropane, and trimethyl phosphate were added to the reactor.

[0138] (b) esterification reaction;

[0139] After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 190°C and the pressure is maintained at 0.1 MPa for 3.8 hours. During this process, the oil temperature is adjusted to maintain the top temperature of the esterification tower at 105°C. At the same time, the temperature in the kettle is gradually raised to 245°C at a rate of 4°C / 10min.

[0140] The esterification rate of the esterification reaction was 99%;

[0141] (c) acid hydrolysis reaction;

[0142] After the temperature in the kettle was lowered to 220°C, isophthalic acid was added and reacted for 60 minutes;

[0143] (d) pre-polycondensation reaction;

[0144] Within 25 minutes, the temperature in the kettle was raised to 220°C and the pressure in the kettle was reduced to -101KPa;

[0145] (e) introducing nitrogen or an inert gas to lower the temperature in the autoclave, and cooling the discharged material to obtain polyester for powder coating;

[0146] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 170 ppm of the theoretical mass of polyester for powder coating; the alcohol-acid ratio of the reaction system is 1.15:1; the amount of neopentyl glycol added is 60% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 20% mol of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol added; the amount of isophthalic acid added is 10% mol of terephthalic acid, The amount of trimethylolpropane added is 1.5 mol% of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added; the amount of trimethylolpropane added is 0.5 wt% of the total amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane added.

[0147] The final polyester for powder coating has a number average molecular weight of 4753 g / mol, an intrinsic viscosity of 0.137 dL / g, an acid value of 18.7 mKOH / g, a glass transition temperature of 61.8° C., and is clear and transparent.

Claims

1. A method for preparing polyester for powder coating, comprising subjecting the raw materials to esterification reaction, acidolysis reaction and pre-polycondensation reaction in sequence under the action of a catalyst, wherein the esterification rate of the esterification reaction is 97-99.5%, characterized in that: The esterification reaction pressure is 0.1 MPa, the esterification reaction time is 3.5-4.4 hours, and the catalyst is a hybrid titanium catalyst with the following structural formula: The mass of titanium element in the hybrid titanium catalyst is 100-170 ppm of the theoretical mass of polyester for powder coating; The specific steps are as follows: (a) feeding; After preheating the reactor, ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, hybrid titanium catalyst, and stabilizer were added thereto; (b) esterification reaction; After nitrogen or inert gas is introduced to remove impurities and air in the kettle, the temperature in the kettle is raised to 190-200°C and the reaction is started. During this process, the oil temperature is adjusted to maintain the top temperature of the esterification tower at 95-105°C. At the same time, the temperature in the kettle is gradually raised to 245-260°C at a rate of 4°C / 10min. (c) acid hydrolysis reaction; After the temperature in the kettle is lowered to 220-230°C, isophthalic acid is added and reacted for 60-90 minutes; (d) pre-polycondensation reaction; Within 20-30 minutes, the temperature in the kettle is raised to 220-250°C and the pressure in the kettle is reduced to -101KPa; (e) introducing nitrogen or an inert gas to lower the temperature in the reactor, and then cooling the discharged material to obtain polyester for powder coating; The alcohol-acid ratio of the reaction system is 1.1-1.2:1; the amount of neopentyl glycol added is 50-60 mol% of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol added; the amount of 2-methyl-1,3-propanediol added is 20-30 mol% of the total amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol added; the amount of isophthalic acid added is 2-3 mol% of the total amount of the reaction raw materials added; and the amount of trimethylolpropane added is 1-2 mol% of the total amount of the reaction raw materials added. The acid value of polyester for powder coating is 18-20mKOH / g.

2. The method for preparing a polyester for powder coating according to claim 1, characterized in that: The amount of the stabilizer added is 0.3-0.5 wt% of the total amount of the reaction raw materials added.

3. The method for preparing polyester for powder coating according to claim 1, characterized in that: In step (a), the preheating temperature is 80-100° C. Before adding the materials, 2-methyl-1,3-propanediol and the hybrid titanium catalyst are first mixed evenly.

4. The method for preparing a polyester for powder coating according to claim 1, characterized in that: The polyester for powder coating has a number average molecular weight of 4000-4800 g / mol, an intrinsic viscosity of 0.13-0.14 dL / g, a glass transition temperature of 60-62°C, and is clear and transparent.

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