Effect of organotin catalysts on the synthesis of polyester resins for powder coatings

Effect of organotin catalysts on the synthesis of polyester resins for powder coatings
Powder coatings usually use polyester resin as the main raw material, and the performance of polyester has a great influence on the coating. Polyester resin is formed by heating condensation of dibasic acid and diol, and the performance of polyester mainly depends on the molecular weight size and molecular weight distribution of polyester resin. Polyester resin synthesized with neopentyl glycol has good weather resistance and high film strength, and is mainly used as high-level baking paint and self-drying paint. Polyester resins synthesized with ethylene glycol have good abrasion resistance and dimensional stability, and are widely used in the fields of fibers and engineering plastics. Because of the neopentyl structure in the molecule of neopentyl glycol, it plays a protective role in the molecular chain of polyester resin, which makes the powder coatings prepared from this kind of resin have excellent stability and fluidity. The synthesis of polyester can be controlled by adding catalysts to control the reaction rate, thus controlling the molecular weight and molecular weight distribution of polyester. During the reaction process, the exothermic nature of the violent reaction will lead to the loss of polyol and incomplete reaction, and by adjusting the ratio of diols and dicarboxylic acids, the use of step-up heating process and the selection of specific catalysts are also important ways to control the reaction process. There is a wide variety of catalysts used for esterification reactions, including tin, antimony and titanium. In the traditional polyester resin synthesis process, organotin compounds are usually used as esterification catalysts. Organotin compounds are formed by the direct combination of carbon and tin elements to form metal-organic compounds, as catalysts with few side reactions, do not affect the purity and quality of the product and other characteristics, the catalytic effect is good, and organotin catalysts are thermally stable and non-corrosive, the end of the reaction does not need to be separated, no need for post-treatment, convenient production, but also can greatly shorten the process cycle. Organotin catalysts are mainly monobutyltin (MBT) and dibutyltin (DBT) organotin compounds, of which MBT is a traditional high-efficiency organotin catalyst widely used. In this paper, five kinds of organotin catalysts are used to catalyze the synthesis of two kinds of polyester resins, to study the catalytic performance of the five kinds of catalysts as well as the effect on the polyester resins, and to compare the catalytic rate of the same kind of catalysts in the synthesis of two kinds of polyester.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 Auchen Chemical Co.

PC9800, PC4100 and PC779 are white powder, PC380 and PC918 are slightly yellow oily liquid. pc4100 is monobutyltin oxide, pc9800 and pc918 are chelates of tin, pc779 and pc380 are derivatives of monobutyltin oxide. Since butyltin is not environmentally friendly, PC4100, PC779 and PC380, which contain butyltin components, are subject to foreign export restrictions, while PC9800 and PC918 are new environmentally friendly catalysts as they do not contain harmful components such as butyltin.

1.2 Instrument

ZNHW Intelligent Temperature Control Instrument, Shanghai Purdue Science and Technology Co., Ltd; WAY2S Digital Abbe Refractometer, Shanghai Precision Scientific Instrument Co., Ltd; PLGPC50 Gel Permeation Chromatograph, Beijing Pritik Instrument Co.

1.3 Synthesis of two polyester resins

EG-PTA polyester resin is a polymerization system with ethylene glycol (EG), diethylene glycol (DEG) and terephthalic acid (PTA) as raw materials, and NPG-PTA polyester resin is a polymerization system with neopentyl glycol (NPG) and terephthalic acid (PTA) as raw materials. The synthesis process of EG-PTA polyester resin was as follows: according to the mass fractions of 18.93%, 15.37% and 65.75%, respectively, weighed a total mass of 1.6 kg of ethylene glycol, diethylene glycol and terephthalic acid, and the dosage of five kinds of organotin catalysts was 0.08% of total mass. The weighed ethylene glycol and diethylene glycol were poured into a 2L four-necked flask, heated up to 85 ℃, and the stirring speed was adjusted to 150 r/min. The organotin catalysts and terephthalic acid were added into the flask, and the temperature was heated up to 180 ℃, and the stirring speed was adjusted to 200 r/min. After refluxing for 40 min and then connecting condensate, the temperature was heated up to 240 ℃ in stepwise manner, and the reaction time, reaction temperature, condensation temperature and water output were recorded. The reaction time, reaction temperature, condensation temperature and water output were recorded. The decrease of condensation temperature indicated that there was no water generated during the reaction, which was regarded as the end point of the reaction, and the refractive index of esterified water was determined.The synthesis process of NPG-PTA polyester resin was the same as that of EG-PTA polyester resin, in which the total mass of neopentyl glycol and terephthalic acid of 1.6 kG was weighed according to the mass fractions of 40.19% and 59.81%, respectively, and the dosage of five kinds of organotin catalysts was 0.08% of the total mass. The dosage of all five organotin catalysts was 0.08% of the total mass.

1.4 Performance test

The acid value of the polymerization system was tested according to GB/T 2895-2008; the viscosity of the polyester was tested according to GB/T 2794-1995; the softening point of the polyester was tested according to GB/T 4507-2014; the refractive index of the esterified water was tested with an Abbe refractometer; the molecular weight of the polyester was tested with a gel refractometer. test; the molecular weight of polyester was tested by gel permeation chromatography with tetrahydrofuran as the mobile phase, the set flow rate of 1 mL/min, and the column temperature of 25 ℃.

2.1 Comparison of catalytic performance of five catalysts

Acid value is a common indicator of the progress of esterification reaction, and the length of time taken 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. As can be seen from Fig. 1, the catalytic efficiencies of PC4100, PC9800 and PC779 were higher than those of PC380 and PC918 throughout the reaction stage, with PC779 having a high catalytic efficiency and PC918 a low catalytic efficiency, whereas the environmentally friendly catalyst PC9800 was only slightly lower than that of the traditional high-efficiency catalyst PC4100, and its catalytic efficiency at the later stage was even higher than that of PC4100. According to the experimental experience, the acid value of polyester is around 33 mg/g after the esterification polycondensation reaction is complete. In Table 1, PC4100, PC9800 and PC779 all need 6.4 h to reach the specified acid value, PC380 needs 7.2 h, and PC918 needs 8.0 h. Combined with the relationship between the degree of reaction and the time in the synthesis of the five catalysts, PC4100, PC9800 and PC779 make the reaction faster.

Refractive index, as a criterion for the purity of a liquid substance, is one of the commonly used physical constants, and the magnitude of the refractive index of esterified water marks the amount of polyol loss during the esterification reaction. From Table 1, it can be seen that the refractive index of esterified water corresponding to the five catalysts are all within a reasonable range, although the catalytic efficiency of PC779 is high, the refractive index shows that its catalytic reaction consumes more polyol than PC9800 and PC4100. PC380 has a more peaceful catalytic effect, and its refractive index is similar to that of the catalytically efficient PC9800, and it is the two catalysts that have less loss of polyol. Polycondensation reaction is a process in which monomers are condensed into oligomers such as dimers and trimers, and these oligomers are slowly condensed into macromolecules at the later stage of the reaction. Esterification reaction is a typical polycondensation reaction, high catalytic efficiency in the early stage of the reaction can quickly reduce the acid value, thus shortening the reaction time, the viscosity of the system increases in the late stage of the reaction, and it is not easy to be dehydrated, the catalytic efficiency will have a direct impact on 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, and the molecular weight of synthesized polyester is also larger than that of the other catalysts, as can be seen from the Table 2. The molecular weight of the synthesized polyester is also larger than the other 2 catalysts, which proves that the high efficiency catalyst can increase the molecular weight of polyester resin. PC380 has a weaker catalytic effect, the reaction rate is slower, the molecular weight of the polyester resin is slightly lower, the reaction tends to be more calm, the polyol loss is less, and it can get the low molecular weight and narrow molecular weight distribution of the polyester resin.

From Table 2: the higher the molecular weight of the synthetic polyester, the higher the viscosity and softening point of polyester resin. If the melt viscosity of polyester resin is too large, its fluidity will become poor, and after made into powder, the coating crust phenomenon is serious, affecting the aesthetic. If the polyester molecular weight is too small, although the melt fluidity of polyester resin is improved, and the surface leveling of the coating film is better after being made into powder, the phenomenon of hanging is serious, and the curing time will be prolonged, resulting in a decrease in the mechanical properties of the coating film, and at the same time, its storage stability is also affected. The number average molecular weight of polyester resin is generally in the range of 2000 to 5000, when the number average molecular weight of polyester exceeds this range, its impact strength, adhesion and flexibility can not meet the requirements of the technical indicators.

2.2 Comparison of catalytic rate of PC9800 in two kinds of polyester synthesis

The esterification reaction is a gradual polymerization process, and in order to accurately and intuitively compare the catalytic rate of the catalyst in the synthesis of EG-PTA and NPG-PTA polyester resins, the esterification rate is defined as the percentage of the esterified water generated at each time point to the total water output. From Figure 2, it can be seen that: the effect of PC9800 on the catalytic rate in the synthesis of the two groups of polyester has obvious differences, the catalytic rate in the EG-PTA polymerization system is higher than that in the NPG-PTA polymerization system as a whole, and high molecular weight polyesters can be obtained; while in the NPG-PTA polymerization system the catalytic rate is slower in the beginning and then faster in the end, and higher molecular weight polyesters can be obtained as well, but a large change of the catalytic rate leads to a The molecular weight distribution of polyester is wider, the molecular arrangement in the molecular chain is not uniform, and the performance of polyester is not stable.

2.3 Difference between the catalytic rates of PC4100 and PC779 in the synthesis of two polyesters

The difference in the catalytic rate of PC4100 in the two syntheses is not as large as that of PC9800. The catalytic rate of PC4100 in the EG-PTA polymerization system is slightly higher than that of NPG-PTA, and the catalytic performance of the two syntheses is consistently and efficiently, but the catalytic rate of PC4100 in the EG-PTA polymerization system is obviously not as smooth as that of the NPG-PTA polymerization system, which will ultimately affect the molecular weight distribution of polyesters, leading to the stability of polyesters being less stable than that of the NPG-PTA polymerization system. distribution, resulting in a less stable polyester than that synthesized by the NPG-PTA polymerization system.

The catalytic rate of PC779 is not only fast, but also the catalytic effect is basically the same, indicating that PC779 maintains the same catalytic performance in both EG-PTA and NPG-PTA polymerization systems.

2.4 Differences in the catalytic rates of PC380 and PC918 in the synthesis of two kinds of polyesters

In the early stage of the reaction the catalytic properties of PC380 and PC918 were approximately the same in the two syntheses, and the catalytic rates of both were higher than those of NPG-PTA in the EG-PTA polymerization system, and in the middle and late stages of the reaction the catalytic rates of PC380 were similar in the two syntheses, whereas the catalytic rate of PC918 was slightly higher than that of EG-PTA in the NPG-PTA polymerization system.

 

3 Conclusions

The catalytic efficiency of PC9800 and PC4100 in polyester synthesis is high, which greatly shortens the process cycle and is suitable for the preparation of polyester with high molecular weight and narrow molecular weight distribution; PC9800 is an environmentally friendly catalyst, which has better performance than PC4100 to some extent; PC779 has the same high efficiency catalytic performance, but the reaction is too violent, and the loss of polyol is more; PC380 has the catalytic efficiency of The catalytic efficiency of PC380 is average, the reaction is calm, and it is suitable for the preparation of polyester products with medium-low molecular weight and narrow molecular weight distribution; the catalytic efficiency of PC918 is poor, and the reaction time is too long, so the catalytic effect is not ideal. The catalytic rate of the same catalyst was compared between the two syntheses of EG-PTA and NPG-PTA, and the catalytic rate of PC779, PC918 and PC380 had a small difference between the two syntheses; the catalytic rate of PC9800 used in the polymerization system of EG-PTA was significantly higher than that of NPG-PTA; the catalytic rate of PC4100 used in the polymerization system of NPG-PTA was smoother than that of The catalytic rate of PC4100 for NPG-PTA polymerization system was smoother than that of EG-PTA, and the stability of the synthesized polyester products was superior.