Preparation method of polyester for powder coating with low organotin content

Abstract

The invention belongs to the technical field of polyester, and relates to a method for preparing polyester for powder coating with low organic tin content, wherein the reaction raw materials are sequentially subjected to esterification reaction, acidolysis reaction and pre-polycondensation reaction under the action of a catalyst, the esterification rate of the esterification reaction is 97-99.5%, the pressure of the esterification reaction is 0.1MPa, the time of the esterification reaction is 2-5h, the catalyst is a mixture of an organic tin catalyst and a hybrid titanium catalyst, the mass of the tin element in the organic tin catalyst is 252-420ppm of the theoretical mass of the polyester for powder coating, and the mass of the titanium element in the hybrid titanium catalyst is 75-105ppm of the theoretical mass of the polyester for powder coating. The present invention not only improves the synthesis efficiency and quality of the polyester for powder coating through the application of innovative catalysts and the construction of a composite catalytic system, but also realizes effective control of the use of heavy metals.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyesters and relates to a method for preparing a polyester for powder coatings with a low organotin content. Background Art

[0002] Powder coatings, as a special existence different from traditional liquid coatings, are based on resin binders and curing agents, and at the same time incorporate fillers, pigments, and a series of functional additives, finally forming a powdery product in a 100% solid form. This innovative form not only completely eliminates the emission of volatile organic compounds (VOCs), but also realizes the recyclability of materials, significantly improves energy utilization efficiency, and endows the coating film with excellent performance.

[0003] In the field of powder coatings, thermoplastic powder coatings and thermosetting powder coatings constitute two core branches. However, thermosetting powder coatings, especially those based on polyester systems, have occupied a pivotal position in the market. Polyester, whether carrying carboxyl (-COOH) or hydroxyl (-OH) functional groups, can bring remarkable appearance effects and excellent mechanical properties to the coatings, such as high toughness and flexibility.

[0004] The polyester resins used in general-purpose powder coatings, with their excellent weather resistance, moisture resistance, and wide adaptability, have shown good performance in various application scenarios, variable construction conditions, and different substrate materials. Therefore, they are widely used in many fields such as household appliances, automotive and parts, building aluminum profiles, toys, etc., and have become an important and indispensable material in these industries.

[0005] In the synthesis process of polyester, the choice of catalyst is crucial. For the synthesis of polyester for powder coatings, the commonly used catalysts in the market at present include organotin-based and titanate-based ones, etc. However, titanate-based catalysts have relatively low catalytic efficiency under normal pressure and problems such as easy hydrolysis and insufficient stability, which directly lead to poor transparency of the synthesized polyester for powder coatings.

[0006] Patent applications CN104231240A and CN118027368A adopted an organotin catalyst (monobutyltin oxide), which not only has high catalytic efficiency and short reaction cycle, but also can smoothly carry out the esterification reaction under normal pressure conditions, thus significantly improving the production efficiency.

[0007] However, the mass of tin in the polyester powder coatings prepared in the above patent applications exceeds 0.1wt%. Since tin is a heavy metal with certain toxicity, the EU REACH regulations have imposed strict restrictions on its content in polyester for synthetic powder coatings, requiring that it should not exceed 0.1wt%. This restriction undoubtedly brings challenges to manufacturers: if the amount of organic tin catalyst used is reduced, although the tin content can be reduced, it will also prolong the esterification time, thereby affecting production efficiency and cost control. In addition, the catalytic efficiency of the above patent applications still needs to be further improved. Summary of the invention

[0008] The purpose of the present invention is to solve the problems existing in the prior art, that is, to find a catalyst with high catalytic efficiency and not easy to hydrolyze under normal pressure, and apply it to the preparation of polyester for powder coating. At the same time, the prepared polyester for powder coating must also comply with the relevant requirements of the EU REACH regulations.

[0009] 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.

[0010] During the esterification process, the reaction between the acid and the alcohol is initially heterogeneous, that is, the reactants and products are not evenly distributed in the system. However, as the reaction proceeds, especially when the esterification rate exceeds 90%, the acid is able to dissolve in the generated ester, and the reaction turns into a homogeneous reaction, and the system becomes clear. In the early to late stages of the reaction, there is a transition state between the raw materials and the products, which is a key stage in the reaction process.

[0011] The polycondensation process is to remove small molecule alcohols between the generated esters to form polyesters. In this process, the transition state theory plays an important role. According to this theory, the reactant molecules do not directly form products through simple collisions, but must first pass through a high-energy activated complex (transition state). To reach this transition state, a certain activation energy must be overcome before it can be converted into a product. Activation energy is the minimum energy that the reactant molecules must obtain during the reaction in order to form an activated complex and ultimately convert into a product. Activation energy is usually represented by the symbol Ea, and the unit is joule (J) or kilocalorie / mole (kcal / mol). The size of the activation energy directly affects the reaction rate. The smaller the activation energy, the faster the reaction rate.

[0012] The role of a catalyst in a chemical reaction is to lower the activation energy and make the reaction easier to proceed. It provides a new reaction path with a lower activation energy than the original reaction path. Therefore, more reactant molecules are able to overcome the energy barrier and participate in the reaction, thereby accelerating the reaction rate.

[0013] Under the same reaction system, the lower the activation energy corresponding to the catalyst, the higher its catalytic efficiency. To compare the activation energies of different catalysts, a series of exploratory experiments were conducted in this invention.

[0014] In the following experiments, the steps for preparing the polyester for powder coatings are generally the same, with the only differences being the type and dosage of the catalyst. The steps for preparing the polyester for powder coatings are as follows:

[0015] (a) Feeding;

[0016] First, 2-methyl-1,3-propanediol, a hybrid titanium catalyst (manufactured by Shanghai Huiyi New Materials Technology Co., Ltd., model HCEO), and monobutyltin oxide are mixed evenly to obtain a premix. Then, after preheating the reaction kettle to 100 °C, ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and the premix are added thereto;

[0017] (b) Esterification reaction;

[0018] After purging the impurity air in the kettle by introducing nitrogen or an inert gas, the temperature in the kettle is raised to 195 °C, and the reaction is carried out at a pressure of 0.1 MPa for 2 - 4.7 h. During this process, the oil temperature is adjusted to maintain the top temperature of the esterification tower at 100 °C, and at the same time, the temperature in the kettle is gradually raised to 260 °C at a rate of 5 °C / 10 min;

[0019] (c) Acidolysis reaction;

[0020] After lowering the temperature in the kettle to 210 °C, isophthalic acid is added and the reaction is carried out for 60 min;

[0021] (d) Pre-polycondensation reaction;

[0022] The temperature in the kettle is raised to 230 °C within 20 min while the pressure in the kettle is reduced to -101 KPa;

[0023] (e) After introducing nitrogen or an inert gas to lower the temperature in the kettle, the material is discharged and cooled to obtain the polyester for powder coatings with a low organotin content;

[0024] In steps (a) to (e), the molar ratio of alcohol to acid in the reaction system is 1.1:1; the addition amount of neopentyl glycol is 55% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 30% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of isophthalic acid is 3% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane; the addition amount of trimethylolpropane is 1% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane.

[0025] The exploration process is as follows:

[0026] Five groups of experiments were designed, and the corresponding catalysts for the five groups of experiments are as follows:

[0027] Experimental group 1: A mixture of a hybrid titanium catalyst and an organotin catalyst with a mass ratio of 5:5, denoted as Ti-Sn(5:5);

[0028] Experimental group 2: A mixture of a hybrid titanium catalyst and an organotin catalyst with a mass ratio of 6:4, denoted as Ti-Sn(6:4);

[0029] Experimental group 3: An organotin catalyst, denoted as Sn;

[0030] Experimental group 4: A hybrid titanium catalyst, denoted as Ti;

[0031] Experimental group 5: The same organotin catalyst as in experimental group 3, but the dosage is 1 / 2 of that in experimental group 3, denoted as Sn(1 / 2);

[0032] The esterification reaction time for the five groups of experiments is defined as the time when the esterification rate reaches 100%. The other conditions for the five groups of experiments are the same. The relationship between the esterification rate x (=actual esterification water output / theoretical esterification water output × 100%) and the reaction time t corresponding to experimental groups 1 to 5 was plotted as a scatter plot, and then non-linearly curve-fitted to a curve (as Figure 1 shown);

[0033] Since the solubility of terephthalic acid (PTA) in alcohol is very small, in the early heterogeneous phase of the esterification reaction (when the esterification rate reaches about 90%), the solution always remains in a saturated state of PTA; in this stage, the reaction rate constant is no longer affected by the reactant concentration, but only related to the temperature; the focus of the present invention is mainly concentrated in the range of 0% to about 95% of the esterification rate. Therefore, the reaction kinetics in this process can be described by a zero-order reaction equation; specifically, this equation can be expressed as follows:

[0034] dx / dt = K;

[0035] Wherein, x represents the esterification rate (%), t represents the reaction time (min), and K is the reaction rate constant (g / (mol·min));

[0036] Obtain Figure 1 The temperature T inside the kettle corresponding to each scatter point among the five curves;

[0037] Respectively process Figure 1 The five curves (first-order derivative and substitute the reaction time t) to obtain the slope K (representing the reaction rate constant) of each curve at each scatter point;

[0038] Plot the lnK corresponding to Experiments 1 - 5 against T - 1 as a scatter plot, and obtain Figure 2 The five straight lines;

[0039] According to the Arrhenius formula, the slope of the five straight lines is five - E a / R, where R is the molar gas constant. Based on the five - E a / R and R, five E a can be calculated, as shown in Table 1:

[0040] The Arrhenius formula is as follows:

[0041] lnK = ln A - E a / RT;

[0042] Wherein, A is the Arrhenius constant, E a is the activation energy (kJ / mol), R is the molar gas constant (J / (mol·K)); T is the reaction temperature (K);

[0043] According to the Arrhenius formula, the slope of the five straight lines is five - Ea / R, where R is the molar gas constant. Based on the five - Ea / R and R, five Ea can be calculated, as shown in Table 1:

[0044] Table 1

[0045]

[0046] As can be seen from the results, when comparing the esterification kinetic performances of several catalysts, we found that the activation energies of these catalysts and the lengths of time required for the esterification reaction showed a consistent trend of change. Among them, the activation energies of Ti-Sn(5:5) and Ti-Sn(6:4) were smaller and the esterification reaction times were shorter, so their catalytic efficiencies were higher than those of Sn, Ti, and Sn(1 / 2). During the reaction, the organotin catalyst first enables the reactant molecules to overcome the energy barrier and participate in the reaction, and then the hybrid titanium catalyst follows closely, enabling the whole reaction to continue along a path with a lower activation energy, thus accelerating the reaction rate.

[0047] The above exploration experiments show that the catalytic efficiency of the mixture of the organotin catalyst and the hybrid titanium catalyst is better than that of other catalysts. Therefore, the present invention attempts to use the mixture of the organotin catalyst and the hybrid titanium catalyst to prepare polyester for powder coatings. The technical solution of the present invention is as follows:

[0048] A method for preparing polyester for powder coatings with a low organotin content, in which the reaction raw materials are successively subjected to an esterification reaction, an acidolysis reaction, and a pre-polycondensation reaction under the action of a catalyst. The esterification rate of the esterification reaction is 97-99.5%, the pressure of the esterification reaction is 0.1 MPa, the time of the esterification reaction is 2-5 h, the catalyst is a mixture of an organotin catalyst and a hybrid titanium catalyst, and the mass of tin element in the organotin catalyst is 252-420 ppm of the theoretical mass of the polyester for powder coatings. The structural formula of the hybrid titanium catalyst is as follows:

[0049]

[0050] The mass of titanium element in the hybrid titanium catalyst is 75-105 ppm of the theoretical mass of the polyester for powder coatings.

[0051] 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 in the preparation of ordinary PET and PETG, and the polycondensation process is carried out under negative pressure. The action 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 generate metal alkoxide, and the oxygen in the hydroxyethyl group of another molecule attacks the carbonyl carbon atom. The nucleophilic attack of the ester carbonyl oxygen on the central titanium atom will increase the positive charge of the carbonyl carbon, thereby increasing the reaction rate.

[0052] The present invention first uses it as an esterification catalyst in the preparation of polyester for powder coatings, and the esterification process is carried out under normal pressure. The action principle of the hybrid titanium catalyst is as follows: the titanium atom can introduce acidic centers. In the esterification reaction of terephthalic acid and ethylene glycol, the catalyst provides protons to protonate the carboxyl group of terephthalic acid and the hydroxyl group of ethylene glycol, forming better electrophilic reagents and accelerating the reaction between the two to form ester bonds.

[0053] The present invention unexpectedly discovers that during the esterification process for preparing polyester for powder coatings, the catalytic efficiency of the hybrid titanium catalyst is significantly superior to that of traditional titanate catalysts.

[0054] If it is feasible to completely replace the organotin catalyst with the hybrid titanium catalyst, but the catalytic efficiency is still relatively low compared to using only the organotin catalyst. Therefore, the present invention considers compounding the hybrid titanium catalyst and the organotin catalyst to form a catalytic system, giving full play to the synergistic effect of the organotin catalyst and the hybrid titanium catalyst. The specific principle is as follows:

[0055] During the esterification process, there is a transition state between the raw materials and the esterified products. The smaller the activation energy, the faster the reaction rate, because the low activation energy makes it easier for the reactant molecules to overcome the energy barrier and reach the transition state for reaction. The catalyst can lower the activation energy of the chemical reaction, making the reaction easier to proceed. This is because the catalyst provides a new reaction path, and the activation energy of this path is lower than that of the original reaction path. In this way, more reactant molecules can overcome the energy barrier and participate in the reaction, thus accelerating the reaction rate.

[0056] When the hybrid titanium and the organotin catalyst are used together to catalyze the esterification reaction, due to the lower activation energy of the organotin catalyst, in the early stage for a period of time, under the catalysis of the organotin catalyst, the reactant molecules first overcome the energy barrier and continue the reaction along a path with a lower energy; as the reaction progresses, the hybrid titanium catalyst can also enable some reaction molecules to overcome the barrier and reach the transition state; because both of them catalyze the reaction simultaneously, the two paths overlap and optimize, resulting in a new path with a lower required energy. Thereafter, the reaction molecules will reach the transition state along this new path, and the reaction rate will be faster.

[0057] Experimental research finds that not only the catalytic efficiency is further improved, and the reaction rate is increased by up to 30% at the fastest compared to using only the organotin catalyst, but also the usage amount of the organotin is effectively reduced, making it not exceed 0.1 wt% (that is, the mass of tin element in the organotin catalyst is 330 - 420 ppm of the theoretical mass of the polyester for powder coatings), successfully meeting the requirements of the REACH regulation.

[0058] As a preferred technical solution:

[0059] For the preparation method of polyester for powder coatings with a low organotin content as described above, the organotin catalyst is monobutyltin oxide, dibutyltin oxide or stannous octoate.

[0060] For the preparation method of polyester for powder coatings with a low organotin content as described above, the reaction raw materials include terephthalic acid, isophthalic acid, neopentyl glycol, 2 - methyl - 1,3 - propanediol, ethylene glycol, and trimethylolpropane.

[0061] The multi-functional group structure of 2-methyl-1,3-propanediol can regulate the structure of the polyester molecular chain. By changing the molecular structure of the polyester, the synthesized polyester has better flexibility. Compared with the use of traditional diols such as 1,4-butanediol, it can reduce the regularity of the polyester molecular chain, thereby reducing crystallinity and avoiding problems such as caking of powder coatings during storage due to too high crystallinity. At the same time, the use of 2-methyl-1,3-propanediol to synthesize polyester helps to improve the weather resistance of powder coatings. In an outdoor environment, the powder coating made of polyester modified with 2-methyl-1,3-propanediol can resist the erosion of factors such as ultraviolet rays and moisture. The coating is not easy to powder and fade, extending the service life of the coating. It can also improve the leveling property of the powder coating, making the coating surface more flat and smooth, reducing surface defects and improving the appearance quality.

[0062] For the preparation method of a polyester for powder coatings with low organotin content as described above, the molar ratio of alcohol to acid in the reaction system is 1.2 - 1.3:1; the addition amount of neopentyl glycol is 50 - 60% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 20 - 30% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol; the addition amount of isophthalic acid is 3 - 4% mol of the total addition amount of reaction raw materials; the addition amount of trimethylolpropane is 1 - 2% mol of the total addition amount of reaction raw materials.

[0063] Due to the synergistic effect, the composite catalytic system of organotin catalyst and hybrid titanium catalyst has higher catalytic efficiency and can make the reaction proceed more fully and rapidly. In order to obtain a polyester with an ideal molecular weight distribution and properties, and at the same time avoid the product viscosity being too large due to too high degree of polymerization, it is necessary to set a larger molar ratio of alcohol to acid in the reaction system so that the proportion of polyol is larger. As the molar ratio of alcohol to acid increases, the number of terminal hydroxyl groups on the molecular chain will also increase accordingly. In order to obtain a polyester resin for powder coatings mainly with terminal carboxyl groups reaching the target, the amount of isophthalic acid required for the acidolysis reaction also needs to increase.

[0064] For the preparation method of a polyester for powder coatings with low organotin content as described above, the specific steps are as follows:

[0065] (a) Feeding;

[0066] After preheating the reaction kettle, add ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and catalyst into it.

[0067] (b) Esterification reaction;

[0068] After introducing nitrogen or inert gas to remove the impurity air in the kettle, raise the temperature in the kettle to 190 - 200 °C and then start the reaction. During this process, adjust the oil temperature to maintain the top temperature of the esterification tower at 95 - 105 °C, and at the same time, gradually raise the temperature in the kettle to 245 - 260 °C at a rate of 5 °C / 10 min;

[0069] The heating rate here is relatively fast because the formation and breakage of chemical bonds during the esterification reaction are accompanied by energy changes. The breakage of old chemical bonds in alcohol and acid molecules requires energy absorption, while the formation of new chemical bonds in the ester bond and water molecules releases energy. Generally speaking, the energy released by the formation of new bonds is greater than the energy absorbed by the breakage of old bonds. According to the law of conservation of energy, the entire reaction process shows heat release to the environment. The catalytic efficiency of the composite catalyst system is faster, the rate of new bond formation is faster, and the released heat is also more. Therefore, the temperature in the whole system will rise relatively fast;

[0070] (c) Acidolysis reaction;

[0071] After reducing the temperature in the kettle to 210 - 220 °C, add isophthalic acid (as an acidolysis agent) and react for 55 - 70 min;

[0072] The activation energy of the composite catalyst system is lower, enabling the reaction to occur under a relatively low energy state. Theoretically, the faster the catalytic efficiency, the lower the temperature required for the acidolysis reaction, because an efficient catalyst can make the reactant molecules reach the activation state required for the reaction more easily. Even at a lower temperature, it can ensure that a sufficient number of reactant molecules have effective collisions, thus smoothly carrying out the acidolysis reaction. This helps to save energy, reduce the requirements for equipment at high temperatures, and the possible side reactions;

[0073] During the acidolysis process, under the action of an efficient catalyst, the reactant molecules can be converted into products faster, thus greatly shortening the time required to reach the expected reaction degree. Therefore, the acidolysis time can be reduced when using the composite catalyst system;

[0074] A capping reaction occurs during the acidolysis process to form a polyester resin mainly with terminal carboxyl groups. The reaction equation is as follows:

[0075]

[0076] (d) Pre - polycondensation reaction;

[0077] Raise the temperature in the kettle to 230 - 245 °C within 20 - 30 min and at the same time reduce the pressure in the kettle to - 101 KPa;

[0078] The purpose of pre-polycondensation in the prior art is to preliminarily polymerize monomers to form prepolymers with relatively high molecular weights, which generally takes a long time and optimizes the molecular weight distribution. Different from the prior art, the purpose of pre-polycondensation in the present invention is to extract the residual small molecules in the system to further increase the molecular weight of the product. Therefore, the temperature of the pre-polycondensation reaction in the present invention is lower and the time is shorter. In this way, while avoiding explosive polymerization, side reactions can be reduced, and a polyester for powder coatings with the required molecular weight and viscosity can be obtained. The relatively low pre-polycondensation temperature is also because the catalytic efficiency of the compound catalyst system is faster and the activation energy is low, which can ensure that a sufficient number of reactant molecules undergo effective collisions at a lower temperature to achieve the required physical properties.

[0079] (e) After introducing nitrogen or an inert gas to lower the temperature in the kettle, the material is discharged and flows into a mold for cooling, and thus a polyester for powder coatings with a low organotin content is obtained.

[0080] In the preparation method of a polyester for powder coatings with a low organotin content as described above, in step (a), the preheating temperature is 80 - 100 °C. Before feeding, 2-methyl-1,3-propanediol and the catalyst are first mixed evenly, which can facilitate feeding.

[0081] In the preparation method of a polyester for powder coatings with a low organotin content as described above, the number-average molecular weight of the polyester for powder coatings with a low organotin content is 4800 - 5100 g / mol, the intrinsic viscosity is 0.14 - 0.15 dL / g, the acid value is 15 - 20 mKOH / g, the glass transition temperature (Tg) is 60 - 62 °C, and it is in a clear and transparent state.

[0082] Beneficial effects:

[0083] (1) The present invention first applies a hybrid titanium catalyst as an esterification catalyst to the preparation of a polyester for powder coatings. This innovative application breaks the usage limitations of traditional catalysts and provides new possibilities for the synthesis of polyester for powder coatings. During the esterification process under normal pressure, the hybrid titanium catalyst exhibits a catalytic efficiency superior to that of traditional titanate catalysts. This discovery not only increases the reaction rate but also helps to optimize the production process and improve production efficiency.

[0084] (2) By compounding a hybrid titanium catalyst and an organotin catalyst, the present invention gives full play to the synergistic effect of the two. This compound catalyst system not only further improves the catalytic efficiency, with the reaction rate being increased by up to 30% at most compared to using only the organotin catalyst alone, but also effectively reduces the usage amount of organotin, thus realizing a more efficient and environmentally friendly production process.

[0085] (3) The present invention successfully reduces the usage amount of the organotin catalyst and successfully meets the strict restrictions of the EU REACH regulation on the tin content in the polyester for synthetic powder coatings. Description of the drawings

[0086] Figure 1 It is a curve graph showing the relationship between the esterification rate and the reaction time of different catalysts;

[0087] Figure 2 It is a graph showing the relationship between lnK and T of different catalysts -1 of. Specific implementation manners

[0088] The present invention will be further described below in conjunction with specific implementation manners. It should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it 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 also fall within the scope defined by the appended claims of this application.

[0089] The following are the test methods for relevant performance indicators in each example and comparative example:

[0090] Number-average molecular weight: It was tested using a normal-temperature gel permeation chromatograph (PL-GPC50 type from Agilent Technologies, USA), with hexafluoroisopropanol as the solvent and a solution concentration of 1 mg / mL prepared.

[0091] Intrinsic viscosity, acid value: According to GB / T 14190-2017 "Test Methods for Fiber Grade Polyester (PET) Chips", the intrinsic viscosity and acid value of the sample to be tested were measured.

[0092] Glass transition temperature: It was tested using a Q20 type high-temperature TA scanning calorimeter. The test was carried out in a nitrogen atmosphere. The initial temperature was set at 30 °C, held for 1 min, heated at a rate of 15 °C / min to 300 °C, held for 5 min, then cooled at a rate of 15 °C / min to 30 °C, held for 5 min, and finally heated at a rate of 15 °C / min to 300 °C.

[0093] Example 1

[0094] A preparation method of a polyester for powder coatings with low organotin content, the specific steps are as follows:

[0095] (a) Feeding;

[0096] First, 2-methyl-1,3-propanediol, a hybrid titanium catalyst (manufactured by Shanghai Huiyi New Materials Technology Co., Ltd., model HCEO), and monobutyltin oxide were mixed evenly to obtain a premix. Then, after preheating the reaction kettle to 80 °C, ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and the premix were added thereto;

[0097] (b) Esterification reaction;

[0098] After introducing nitrogen or inert gas to remove the impurity air in the kettle, the temperature in the kettle is raised to 190 °C, and then the reaction is carried out at a pressure of 0.1 MPa for 2 h. During this process, the oil temperature is adjusted to keep the top temperature of the esterification tower at 100 °C, and at the same time, the temperature in the kettle is gradually raised to 250 °C at a rate of 5 °C / 10 min;

[0099] The esterification rate of the esterification reaction is 99.5%;

[0100] (c) Acidolysis reaction;

[0101] After the temperature in the kettle is lowered to 210 °C, isophthalic acid is added and the reaction is carried out for 55 min;

[0102] (d) Pre-polycondensation reaction;

[0103] The temperature in the kettle is raised to 230 °C within 20 min while the pressure in the kettle is lowered to -101 KPa;

[0104] (e) After introducing nitrogen or inert gas to lower the temperature in the kettle, the product is discharged and cooled to obtain a polyester for powder coatings with low organotin content;

[0105] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 75 ppm of the theoretical mass of the polyester for powder coatings with low organotin content, and the mass of tin element in monobutyltin oxide is 420 ppm of the theoretical mass of the polyester for powder coatings with low organotin content; the molar ratio of alcohol to acid in the reaction system is 1.1:1; the addition amount of neopentyl glycol is 55% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 25% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of isophthalic acid is 3% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane; the addition amount of trimethylolpropane is 1% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane.

[0106] The number-average molecular weight of the finally prepared polyester for powder coatings with low organotin content is 5001 g / mol, the intrinsic viscosity is 0.143 dL / g, the acid value is 17.9 mKOH / g, the glass transition temperature is 62 °C, and it is in a clear and transparent state.

[0107] Comparative Example 1

[0108] A method for preparing a polyester, which is only different from Example 1 in that: monobutyltin oxide is completely replaced by a hybrid titanium catalyst (the same as in Example 1), and the mass of titanium element in Comparative Example 1 is equal to the total mass of titanium element and tin element in Example 1.

[0109] The esterification rate of the esterification reaction is 88%; the finally prepared polyester is white and opaque.

[0110] Compared with Example 1, the esterification rate of the esterification reaction in Comparative Example 1 decreased significantly, and the transparency of the polyester became significantly worse. This is because when using an excessive amount of hybrid titanium catalyst alone, the catalytic efficiency is insufficient, resulting in incomplete esterification reaction and a decrease in the esterification rate of the esterification reaction. At the same time, using an excessive amount of hybrid titanium catalyst alone will over-catalyze the oxidation reaction to generate impurities with chromophoric groups such as aldehydes and ketones, and the metal titanium ions contained are likely to remain, and will also accelerate the side reaction to consume the active end groups of the molecular chain, making the polyester molecular chain unable to grow normally, resulting in poor transparency of the polyester.

[0111] Comparative Example 2

[0112] A method for preparing a polyester, which is only different from Example 1 in that: the hybrid titanium catalyst is completely replaced by monobutyltin oxide, and the mass of tin element in Comparative Example 2 is equal to the total mass of titanium element and tin element in Example 1.

[0113] The esterification rate of the esterification reaction is 94.2%.

[0114] Compared with Example 1, the esterification rate of the esterification reaction in Comparative Example 2 decreased significantly. This is because the reaction activation energy of the monobutyltin oxide catalyst is higher than that of the composite catalyst, and the reaction rate is slow, which is manifested as a decrease in the esterification rate.

[0115] Example 2

[0116] A method for preparing a polyester for powder coating with low organotin content, the specific steps are as follows:

[0117] (a) Feeding;

[0118] First, mix 2-methyl-1,3-propanediol, hybrid titanium catalyst (manufactured by Shanghai Huiyi New Materials Technology Co., Ltd., model HCEO) and monobutyltin oxide evenly to obtain a premix. Then, after preheating the reaction kettle to 90 °C, add ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and the premix to it;

[0119] (b) Esterification reaction;

[0120] After introducing nitrogen or inert gas to remove the impurity air in the kettle, raise the temperature in the kettle to 195 °C, and maintain the pressure at 0.1 MPa for 2.3 h. During this process, adjust the oil temperature to keep the top temperature of the esterification tower at 100 °C, and at the same time raise the temperature in the kettle from 250 °C at a rate of 5 °C / 10 min;

[0121] The esterification rate of the esterification reaction is 99%;

[0122] (c) Acidolysis reaction;

[0123] After reducing the temperature in the kettle to 215 °C, isophthalic acid is added and reacted for 60 min;

[0124] (d) Prepolymerization reaction;

[0125] Raise the temperature in the kettle to 240 °C within 25 min and reduce the pressure in the kettle to -101 KPa simultaneously;

[0126] (e) After introducing nitrogen or inert gas to reduce the temperature in the kettle, discharge and cool to obtain polyester for powder coatings with low organotin content;

[0127] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 90 ppm of the theoretical mass of polyester for powder coatings with low organotin content, the mass of tin element in monobutyltin oxide is 330 ppm of the theoretical mass of polyester for powder coatings with low organotin content; the molar ratio of alcohol to acid in the reaction system is 1.1:1; the addition amount of neopentyl glycol is 55% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 25% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of isophthalic acid is 3% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane; the addition amount of trimethylolpropane is 1% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane.

[0128] The number-average molecular weight of the finally obtained polyester for powder coatings with low organotin content is 5100 g / mol, the intrinsic viscosity is 0.146 dL / g, the acid value is 18.3 mKOH / g, the glass transition temperature is 60 °C, and it is in a clear and transparent state.

[0129] Example 3

[0130] A preparation method of polyester for powder coatings with low organotin content, the specific steps are as follows:

[0131] (a) Feeding;

[0132] First, mix 2-methyl-1,3-propanediol, hybrid titanium catalyst (manufacturer: Shanghai Huiyi New Material Technology Co., Ltd., model: HCEO) and dibutyltin oxide evenly to obtain a premix, and then preheat the reaction kettle to 100 °C, and add ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and the premix thereto;

[0133] (b) Esterification reaction;

[0134] After introducing nitrogen or inert gas to remove the impurity air in the kettle, when the temperature in the kettle rises to 200 °C, keep the pressure at 0.1 MPa and react for 4 h. During this process, adjust the oil temperature to maintain the top temperature of the esterification tower at 105 °C, and at the same time, gradually increase the temperature in the kettle to 245 °C at a rate of 5 °C / 10 min;

[0135] The esterification rate of the esterification reaction is 97%;

[0136] (c) Acidolysis reaction;

[0137] After lowering the temperature in the kettle to 220 °C, add isophthalic acid and react for 70 min;

[0138] (d) Pre-polycondensation reaction;

[0139] Raise the temperature in the kettle to 245 °C within 30 min and at the same time lower the pressure in the kettle to -101 KPa;

[0140] (e) After introducing nitrogen or inert gas to lower the temperature in the kettle, discharge and cool to obtain the polyester for powder coatings with low organotin content;

[0141] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 75 ppm of the theoretical mass of the polyester for powder coatings with low organotin content, and the mass of tin element in dibutyltin oxide is 420 ppm of the theoretical mass of the polyester for powder coatings with low organotin content; the alcohol-acid ratio of the reaction system is 1.2:1; the addition amount of neopentyl glycol is 60% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 30% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of isophthalic acid is 3.5% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane; the addition amount of trimethylolpropane is 1.5% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane.

[0142] The number-average molecular weight of the finally prepared polyester for powder coatings with low organotin content is 4854 g / mol, the intrinsic viscosity is 0.15 dL / g, the acid value is 20 mKOH / g, the glass transition temperature is 61.7 °C, and it is in a clear and transparent state.

[0143] Example 4

[0144] A preparation method of a polyester for powder coatings with low organotin content, the specific steps are as follows:

[0145] (a) Feeding;

[0146] First, 2-methyl-1,3-propanediol, a hybrid titanium catalyst (manufactured by Shanghai Huiyi New Materials Technology Co., Ltd., model HCEO), and stannous octoate are mixed evenly to obtain a premix. Then, after preheating the reaction kettle to 100 °C, ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and the premix are added thereto;

[0147] (b) Esterification reaction;

[0148] After purging the impurity air in the kettle by introducing nitrogen or an inert gas, the temperature in the kettle is raised to 200 °C, and then the pressure is maintained at 0.1 MPa for 5 h. During this process, the oil temperature is adjusted to keep the top temperature of the esterification tower at 95 °C, and at the same time, the temperature in the kettle is gradually raised to 260 °C at a rate of 5 °C / 10 min;

[0149] The esterification rate of the esterification reaction is 97%;

[0150] (c) Acidolysis reaction;

[0151] After lowering the temperature in the kettle to 220 °C, isophthalic acid is added and reacted for 70 min;

[0152] (d) Pre-polycondensation reaction;

[0153] The temperature in the kettle is raised to 245 °C within 30 min while the pressure in the kettle is reduced to -101 KPa;

[0154] (e) After introducing nitrogen or an inert gas to lower the temperature in the kettle, the material is discharged and cooled to obtain a polyester for powder coatings with a low organotin content;

[0155] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 75 ppm of the theoretical mass of the polyester for powder coatings with a low organotin content, and the mass of tin element in stannous octoate is 420 ppm of the theoretical mass of the polyester for powder coatings with a low organotin content; the molar ratio of alcohol to acid in the reaction system is 1.2:1; the addition amount of neopentyl glycol is 50% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 20% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol, and ethylene glycol; the addition amount of isophthalic acid is 4% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol, and trimethylolpropane; the addition amount of trimethylolpropane is 2% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol, and trimethylolpropane.

[0156] The number-average molecular weight of the polyester for powder coatings with low organotin content finally obtained is 4800 g / mol, the intrinsic viscosity is 0.14 dL / g, the acid value is 15 mKOH / g, the glass transition temperature is 60 °C, and it is in a clear and transparent state.

[0157] Example 5

[0158] A preparation method of polyester for powder coatings with low organotin content is as follows:

[0159] (a) Feeding;

[0160] First, mix 2-methyl-1,3-propanediol, a hybrid titanium catalyst (manufactured by Shanghai Huiyi New Materials Technology Co., Ltd., model HCEO), and monobutyltin oxide evenly to obtain a premix. Then, after preheating the reaction kettle to 95 °C, add ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane, and the premix into it.

[0161] (b) Esterification reaction;

[0162] After introducing nitrogen or inert gas to remove the impurity air in the kettle, raise the temperature in the kettle to 195 °C, and maintain the pressure at 0.1 MPa for 2.7 h. During this process, adjust the oil temperature to keep the top temperature of the esterification tower at 100 °C, and at the same time, gradually raise the temperature in the kettle to 250 °C at a rate of 5 °C / 10 min.

[0163] The esterification rate of the esterification reaction is 98%;

[0164] (c) Acidolysis reaction;

[0165] After lowering the temperature in the kettle to 220 °C, add isophthalic acid and react for 70 min;

[0166] (d) Pre-polycondensation reaction;

[0167] Raise the temperature in the kettle to 245 °C within 30 min while reducing the pressure in the kettle to -101 KPa;

[0168] (e) After introducing nitrogen or inert gas to lower the temperature in the kettle, discharge and cool to obtain the polyester for powder coatings with low organotin content;

[0169] In steps (a) to (e), the mass of titanium element in the hybrid titanium catalyst is 105 ppm of the theoretical mass of the polyester for powder coatings with low organotin content, and the mass of tin element in monobutyltin oxide is 252 ppm of the theoretical mass of the polyester for powder coatings with low organotin content; the alcohol-acid ratio of the reaction system is 1.1:1; the addition amount of neopentyl glycol is 55% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of 2-methyl-1,3-propanediol is 25% mol of the total addition amount of neopentyl glycol, 2-methyl-1,3-propanediol and ethylene glycol; the addition amount of isophthalic acid is 3% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane; the addition amount of trimethylolpropane is 1% mol of the total addition amount of terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol and trimethylolpropane.

[0170] The number-average molecular weight of the finally prepared polyester for powder coatings with low organotin content is 4800 g / mol, the intrinsic viscosity is 0.146 dL / g, the acid value is 18.1 mKOH / g, the glass transition temperature is 61.7 °C, and it is in a clear and transparent state.

Claims

1. A method for preparing polyester for powder coating with low organic tin content, wherein the reaction raw materials are subjected 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 pressure of the esterification reaction is 0.1 MPa, the time of the esterification reaction is 2-5 hours, the catalyst is a mixture of an organic tin catalyst and a hybrid titanium catalyst, the mass of the tin element in the organic tin catalyst is 252-420 ppm of the theoretical mass of the polyester for powder coatings, the mass of the titanium element in the hybrid titanium catalyst is 75-105 ppm of the theoretical mass of the polyester for powder coatings, and the structural formula of the hybrid titanium catalyst is as follows: 。 2. The method for preparing a polyester for powder coating with low organic tin content according to claim 1, characterized in that: The organotin catalyst is monobutyltin oxide, dibutyltin oxide or stannous octoate.

3. The method for preparing a polyester for powder coating with low organic tin content according to claim 1, characterized in that: The reaction raw materials include terephthalic acid, isophthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, ethylene glycol, and trimethylolpropane.

4. The method for preparing a polyester for powder coating with low organic tin content according to claim 3, characterized in that: The alcohol-acid ratio of the reaction system is 1.2-1.3:1; 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 3-4% 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.

5. The method for preparing a polyester for powder coating with low organic tin content according to claim 4, characterized in that: The specific steps are as follows: (a) feeding; After preheating the reaction kettle, ethylene glycol, terephthalic acid, neopentyl glycol, 2-methyl-1,3-propanediol, trimethylolpropane and a catalyst were added thereto; (b) esterification reaction; After nitrogen or inert gas is introduced to remove the 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, and the temperature in the kettle is gradually raised to 245-260°C at a rate of 5°C / 10min; (c) acid hydrolysis reaction; After the temperature in the kettle is lowered to 210-220°C, isophthalic acid is added and reacted for 55-70 minutes; (d) pre-polycondensation reaction; Within 20-30 minutes, the temperature in the kettle is raised to 230-245°C and the pressure in the kettle is reduced to -101KPa; (e) After nitrogen or inert gas is introduced to lower the temperature in the autoclave, the discharged material is cooled to obtain a polyester for powder coating having a low organic tin content.

6. The method for preparing a polyester for powder coating with low organic tin content according to claim 5, characterized in that: In step (a), the preheating temperature is 80-100° C., and before adding the materials, 2-methyl-1,3-propanediol and the catalyst are first mixed evenly.

7. The method for preparing a polyester for powder coating with low organic tin content according to claim 1, characterized in that: The polyester for powder coating with low organic tin content has a number average molecular weight of 4800-5100 g / mol, an intrinsic viscosity of 0.14-0.15 dL / g, an acid value of 15-20 mKOH / g, a glass transition temperature of 60-62° C., and is in a clear and transparent state.

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