Effect of Organic Tin Catalyst on the Synthesis of Polyester Resin for Powder Coatings
Powder coatings usually use polyester resin as the main raw material, and the performance of polyester has a significant impact on the coating. Polyester resin is formed by heating and condensation of binary acids and alcohols, and the performance of polyester mainly depends on the molecular weight and distribution of polyester resin. The polyester resin synthesized with neopentyl glycol has the characteristics of good weather resistance and high film strength, mainly used for advanced baking paint, self drying paint, etc. Polyester resin synthesized from ethylene glycol has good wear resistance and dimensional stability, and is widely used in fields such as fibers and engineering plastics. Due to the neopentyl structure in the molecule of neopentyl glycol, it plays a protective role in the molecular chain of polyester resin, resulting in excellent stability and flowability of powder coatings prepared from such resins. The synthesis of polyester can be controlled by adding catalysts to control the reaction rate, thereby controlling the molecular weight and distribution of polyester. During the reaction process, intense exothermic reactions can lead to the loss of polyols and incomplete reactions. Adjusting the ratio of binary alcohols to binary acids, using step heating processes, and selecting specific catalysts are also important ways to control the reaction process. There are various types of catalysts used for esterification reactions, including tin series, antimony series, and titanium series. The traditional polyester resin synthesis process usually uses organic tin compounds as esterification catalysts. Organic tin compounds are formed by the direct combination of carbon and tin elements to form metal organic compounds. As catalysts, there are few side reactions and do not affect the purity and quality of the product. The catalytic effect is good, and the organic tin catalyst has thermal stability and non corrosiveness. After the reaction is completed, there is no need for separation and post-treatment, which is convenient for production and can greatly shorten the process cycle. Organotin catalysts mainly consist of organotin compounds such as monobutyltin and dibutyltin, among which monobutyltin is the most widely used traditional high-efficiency organotin catalyst. This article uses five organic tin catalysts to catalyze the synthesis of two types of polyester resins, studies the catalytic performance of the five catalysts and their impact on polyester resins, and compares the catalytic rates of the same catalyst in the synthesis of two types of polyester resins.
1 Experiment
1.1 Raw materials
Neopentyl glycol, industrial grade, Wanhua Chemical Group Co., Ltd; Ethylene glycol, industrial grade, Shandong Hengxin Chemical Co., Ltd; Diethylene glycol, industrial grade, Guangzhou Canlian Chemical Co., Ltd; Terephthalic acid, industrial grade, Jinan Aochen Chemical Co., Ltd; PC9800, PC4100, PC779, PC380, and PC918 catalysts, Jiangsu Shunda New Materials Co., Ltd.
PC9800, PC4100, and PC779 are white powders, while PC380 and PC918 are slightly yellow oily liquids. PC4100 is monobutyl tin oxide, PC9800 and PC918 are chelates of tin, and PC779 and PC380 are derivatives of monobutyl tin oxide. Due to the non environmentally friendly nature of butyl tin, PC4100, PC779, and PC380 containing butyl tin components are subject to export restrictions, while PC9800 and PC918 do not contain harmful components such as butyl tin, making them new environmentally friendly catalysts.
1.2 Instruments
ZHHW intelligent temperature controller, Shanghai Pudu Technology Co., Ltd; WAY2S Digital Abbe Refractometer, Shanghai Precision Science Instrument Co., Ltd; PLGPC50 gel permeation chromatograph, Beijing Polytec Instrument Co., Ltd; SNBAI High Temperature Viscometer, Shanghai Junzhun Instrument Equipment Co., Ltd; YT4507B softening point tester, Shanghai Yutong Instrument Factory.
1.3 Synthesis of Two Polyester Resins
EG-PTA polyester resin is a polymerization system using ethylene glycol (EG), diethylene glycol (DEG), and terephthalic acid (PTA) as raw materials, while NPG-PTA polyester resin is a polymerization system using neopentyl glycol (NPG) and terephthalic acid (PTA) as raw materials. The synthesis process of EG-PTA polyester resin: Weigh a total mass of 1.6kg of ethylene glycol, diethylene glycol, and terephthalic acid according to the mass fractions of 18.93%, 15.37%, and 65.75%, respectively. The dosage of the five organic tin catalysts is 0.08% of the total mass. Pour the weighed ethylene glycol and diethylene glycol into a 2L four necked flask, raise the temperature to 85 ℃, and adjust the stirring speed to 150r/min. Then add the organic tin catalyst and terephthalic acid separately to the flask, raise the temperature to 180 ℃, and adjust the stirring speed to 200r/min. After refluxing for 40 minutes, connect the condensed water, raise the temperature in a stepwise manner to 240 ℃, and record the reaction time, reaction temperature, condensation temperature, and water output. The decrease in condensation temperature indicates that there is no water generated during the reaction process, which is considered as the endpoint of the reaction. The refractive index of the esterified water is measured. The synthesis process of NPG-PTA polyester resin is the same as that of EG-PTA polyester resin, with a total mass of 1.6kG of neopentyl glycol and terephthalic acid weighed according to their mass fractions of 40.19% and 59.81%, respectively. The dosage of the five organic tin catalysts is 0.08% of the total mass.
1.4 Performance testing
The acid value of the polymerization system shall be tested in accordance with GB/T 2895-2008; The viscosity of polyester is tested in accordance with GB/T 2794-1995; The softening point of polyester shall be tested in accordance with GB/T 4507-2014; The refractive index of esterified water was tested using an Abbe refractometer; The molecular weight of polyester was measured by gel permeation chromatography, tetrahydrofuran was used as mobile phase, the flow rate was set at 1mL/min, and the column temperature was 25 ℃.
2 Results and Discussion
2.1 Comparison of catalytic performance of five catalysts
Acid value is a commonly used indicator to measure the esterification reaction process, and the length of time it takes to reach the specified acid value reflects the catalytic efficiency of a catalyst. Figure 1 compares the catalytic efficiency of five catalysts in the EG PTA polymerization system. From Figure 1, it can be seen that throughout the reaction stage, the catalytic efficiency of PC4100, PC9800, and PC779 is higher than that of PC380 and PC918. Among them, PC779 has the highest catalytic efficiency, while PC918 has the lowest catalytic efficiency. The environmentally friendly catalyst PC9800 is only slightly lower than the traditional high-efficiency catalyst PC4100, and its later catalytic efficiency exceeds PC4100. Based on experimental experience, the acid value of polyester after complete esterification and condensation reaction is around 33 mg/g. In Table 1, it takes 6.4 hours for PC4100, PC9800, and PC779 to reach the specified acid value, 7.2 hours for PC380, and 8.0 hours for PC918. Considering the relationship between the reaction degree and time of the five catalysts in synthesis, PC4100, PC9800, and PC779 make the reaction speed faster.
The refractive index, as a standard for the purity of liquid substances, is one of the commonly used physical constants. The magnitude of the refractive index of esterification water indicates the amount of polyol loss during the esterification reaction process. From Table 1, it can be seen that the refractive indices of esterification water corresponding to the five catalysts are all within a reasonable range. Although the catalytic efficiency of PC779 is high, it can be seen from the refractive indices that its catalytic reaction consumes more polyols than PC9800 and PC4100. The catalytic effect of PC380 is relatively calm, and its refractive index is similar to that of PC9800 with high catalytic efficiency, making it the two catalysts with the least loss of polyols. In the early stage of the polycondensation reaction, the main process is the condensation of monomers into oligomers such as dimers and trimers. In the later stage of the reaction, these oligomers gradually condense into large molecules. Esterification reaction is a typical condensation polymerization reaction. The high catalytic efficiency in the early stage of the reaction can quickly reduce the acid value, thereby shortening the reaction time. In the later stage of the reaction, the viscosity of the system increases, making it difficult to dehydrate. The level of catalytic efficiency will directly affect the molecular weight and distribution of the final polyester resin. The catalytic efficiency of PC9800, PC4100, and PC779 is higher than that of the other two catalysts. Table 2 shows that the molecular weight of polyester synthesized by these three catalysts is also greater than that of the other two, indicating that efficient catalysts can increase the molecular weight of polyester resin. PC380 has a weak catalytic effect and a relatively slow reaction rate. The molecular weight of polyester resin is slightly lower, and the reaction tends to be more peaceful. The loss of polyols is less, and polyester resin with medium to low molecular weight and narrow molecular weight distribution can be obtained.
From Table 2, it can be concluded that the higher the molecular weight of synthesized polyester, the higher the viscosity and softening point of the polyester resin. If the melt viscosity of polyester resin is too high, its flowability will deteriorate, and after being made into powder, the coating will have serious peeling phenomenon, which affects the aesthetics. If the molecular weight of polyester is too small, although the melt fluidity of polyester resin is improved and the surface leveling of the coating film after powder production is good, the sagging phenomenon is severe, and the curing time will be extended, leading to a decrease in the mechanical properties of the coating film, and its storage stability is also affected. The average molecular weight of polyester resin generally ranges from 2000 to 5000. When the average molecular weight of polyester exceeds this range, its impact strength, adhesion, and flexibility cannot meet the technical specifications.
2.2 Comparison of catalytic rates of PC9800 in two types of polyester synthesis
Esterification reaction is a gradual polymerization process. In order to accurately and intuitively compare the catalytic rate of catalysts in the synthesis of EG-PTA and NPG-PTA polyester resins, the esterification rate is defined as the percentage of esterification water generated at each time point in the total water output. From Figure 2, it can be seen that there is a significant difference in the catalytic rate of PC9800 on the synthesis of two groups of polyester. The catalytic rate in the EG-PTA polymerization system is generally higher than that in the NPG-PTA polymerization system, and high molecular weight polyester can be obtained; In the NPG-PTA polymerization system, the catalytic rate is slow before and then fast, and higher molecular weight polyester can also be obtained. However, a large change in catalytic rate can lead to a wider molecular weight distribution of polyester, uneven molecular arrangement in the molecular chain, and unstable performance of polyester.
2.3 Differences in catalytic rates between PC4100 and PC779 in the synthesis of two types of polyester
Comparing Figures 2 and 3, it can be seen that the difference in catalytic rate between PC4100 and PC9800 in the two combinations is not as significant. The catalytic rate of PC4100 in the EG-PTA polymerization system is slightly higher than that of NPG-PTA, and the catalytic performance of both groups of synthesis is continuously efficient. However, the catalytic rate of PC4100 in the EG-PTA polymerization system is significantly less stable than that in the NPG-PTA polymerization system, ultimately affecting the molecular weight distribution of polyester, resulting in the stability of polyester being less than that of polyester synthesized in the NPG-PTA polymerization system.
PC4100 and PC779 are both traditional high-efficiency catalysts. From Figure 4, it can be seen that PC779 not only has a fast catalytic rate in the two combinations, but also has basically the same catalytic effect, indicating that PC779 maintains the same catalytic performance in the EG-PTA and NPG-PTA polymerization systems.
2.4 Differences in catalytic rates between PC380 and PC918 in the synthesis of two types of polyester
Comparing Figures 5 and 6, it can be seen that the catalytic performance of PC380 and PC918 in the early stage of the reaction is roughly the same in both syntheses. The catalytic rates of both in the EG-PTA polymerization system are higher than those in NPG-PTA. In the middle and late stages of the reaction, the catalytic rates of PC380 in the two combinations are similar, while the catalytic rate of PC918 in the NPG-PTA polymerization system is slightly higher than that of EG-PTA.
3 Conclusion
PC9800 and PC4100 have high catalytic efficiency in polyester synthesis, greatly shortening the process cycle, and are suitable for preparing high molecular weight and narrow molecular weight distribution polyesters. PC9800 is an environmentally friendly catalyst with better performance to some extent than PC4100; PC779 also has efficient catalytic performance, but the reaction is too intense, resulting in a significant loss of polyols; The catalytic efficiency of PC380 is average, the reaction is calm, and it is suitable for preparing polyester products with medium to low molecular weight and narrow molecular weight distribution; The catalytic efficiency of PC918 is poor, the reaction time is too long, and the catalytic effect is not ideal. The catalytic rates of the same catalyst were compared between the combination of EG-PTA and NPG-PTA, and the catalytic rates of PC779, PC918, and PC380 showed little difference between the two combinations; The catalytic rate of PC9800 used in the EG-PTA polymerization system is significantly higher than that of NPG-PTA; The catalytic rate of PC4100 used in the NPG-PTA polymerization system is smoother than that of EG-PTA, and the synthesized polyester product has superior stability.
