Analysis of the most important factors affecting t

Analysis of important factors affecting the quality of PET bottle grade slices (III)

3.2.1.1 the overall size of raw material slices

transesterification reaction (1) and esterification reaction (2) are reversible reactions. In order to move the balance to the direction of positive reaction, volatile small molecular products must be eliminated in time. When the small molecule by-products produced by solid-phase polycondensation leave chesolvay, they will first show the leading solutions used in the fields of seals, bearings and oil pumps. There are two processes, namely, the diffusion of small molecule by-products from the inside of the slice to the surface and the outward diffusion of small molecule by-products from the surface of the slice. Among them, the outward diffusion rate from the chip surface is related to the nitrogen temperature and flow. Relatively speaking, in SSP production, under relatively high temperature and flow, the diffusion rate of small molecular products in the chip is much slower than that from the chip surface. Therefore, in order to remove small molecular products as much as possible, the technology requires that the chips stay in the reactor for a long time. Because the diffusion path of small molecular products in small particles is shorter than that in large particles, it is easy to exclude, and the sample particles are small, the total surface area of particles increases, the heat transfer rate increases, and the reaction rate also accelerates. Therefore, in a certain range, the reaction rate of pet solid-phase polycondensation is inversely proportional to the particle size of raw material slices. However, if the particles are too small, it is easy to bond, which affects the reaction rate. In addition, the shape of particles will also affect the reaction rate. The particle shape is irregular, and it is easy to bond. Therefore, the requirements for granulation of basic slices are very high, and no abnormal slices can enter the solid-phase polycondensation system

3.2.1.2 color value of raw material slice

it can be seen from table 1 that the color value of raw material slice directly determines the color value of finished product slice. There are many factors that affect the color value of basic slice. Color and lustre is the most intuitive indicator of the quality of reaction slices. Its measurement is based on the principles of chromatography and photometry and the measurement standards of the international Wobbe Commission. It is usually measured with the color difference meter of Hunter (L, a, b) method, and l represents whiteness and brightness; A is the green/red index; B is the yellow index. There are many factors that affect the color of basic slices, mainly caused by the quality of raw materials, the type and content of additives, production technology, production process control and product quality differences [3]. At present, the more direct control method from the process is to change the amount of red agent and blue agent when the process is stable and the quality of raw and auxiliary materials is good, which can appropriately change the b value of slices. The factors affecting the color value of finished chips are more complex, but bottle grade chips have high requirements for the color value of products, so the process should be adjusted in time to meet the needs according to the requirements of users

3.2.1.3 content of IPA and DEG

the content of IPA and DEG in finished slices is controlled in the production of basic slices, and the content of both is basically unchanged in the process of solid-phase tackifying. The amount of IPA is very important for the tackifying of chips. The addition of IPA can inspire the developers, creatives and designers of customers. In order to reduce the regularity of pet macromolecule arrangement, so as to reduce the crystallinity of chips, one is to improve the processing performance during injection molding and bottle blowing, reduce the processing temperature, and the other is to increase the transparency of bottle embryos and bottles. However, the addition of IPA reduces the softening point and melting point of pet, which makes the temperature resistance of bottles worse and the mechanical strength lower. Therefore, the content of IPA should be properly adjusted and strictly controlled according to market requirements. At present, the company produces two kinds of bottle grade slices according to the requirements of users, one is ordinary carbonated beverage bottle grade slice, and the other is hot canned fruit juice beverage bottle grade slice. The latter requires good high temperature resistance, so in addition to making appropriate adjustments in the bottle blowing process, such as adding the heat treatment process and adjusting the temperature of the mold, In addition, the content of IPA in raw materials was appropriately reduced (by 1.5%, which is the weight percentage) to improve the crystallinity of pet and meet the requirements of temperature resistance of beverage bottles. In addition, the content of IPA also has a certain impact on solid-phase polycondensation. If the content of IPA is inappropriate, for example, when the content is too high, it will cause the chips to crystallize imperfectly in the pre crystallization and crystallizer, resulting in the bonding of chips in the tackifying process

the content of diethylene glycol is generally determined by the production process, and minor adjustments can also be made by adjusting the formula proportion (such as adjusting the ratio of EG and PTA). At present, the content of diethylene glycol in bottle grade slices to be produced on the market is about 1.1% ± 0.2% (weight percentage). In this range, a higher diethylene glycol content is conducive to the improvement of the heat resistance of chips. This is because the ether bond in diethylene glycol has a certain softness, which can improve the crystallization speed of pet, but this content should not be too high, because the existence of ether bond reduces the rigidity of pet molecules, reduces the melting point of pet, and is easy to cause the bonding of chips in the tackifying process. If the content is too high, it will also reduce the mechanical properties of slicing and bottle blowing

3.2.1.4 terminal carboxyl group

it can be seen from Figure 1 that under certain other conditions, the content of terminal carboxyl group is high, which is conducive to improving the reaction speed. It can be seen from the equation of SSP reaction that one is transesterification and the other is esterification. The high content of terminal carboxyl group is conducive to the esterification reaction between pet chains and improve the reaction rate. Raw material slice a

Figure 1 Effect of end carboxyl content on solid phase

end carboxyl is smaller than that of raw material slice B. end carboxyl increases the end activity of pet chain and accelerates the esterification reaction between pet chains. At the same time, the increase of h+ concentration in PET chips is also conducive to the autocatalytic effect of the catalyst, but the increase of carboxyl end group content will affect the subsequent processing performance of the chips, so it is required that the carboxyl end group of the basic chips and the swing angle limiting function should be controlled within a certain range, generally 30~40mol/t, Terminal carboxyl group of bottle grade slice [30mol/t.

3.2.1.5 other factors

the type and amount of various additives in raw material chips will also have a certain impact on the internal quality of finished chips. The production of bottle grade chips requires the addition of heat stabilizer polyphosphate, which is used to seal the end of PET molecular chain with phosphoric acid group to increase the thermal stability of pet chain, but because the phosphoric acid group may also be converted into pet crystal nucleating agent, especially for the injection molding and bottle blowing production of bottle grade chips Have a certain impact. During bottle blowing, oligomers, metal oxides (such as antimony trioxide), phosphates, etc. are nucleating agents for pet crystallization. Other low molecular compounds such as polyethylene glycol itself, although not nucleating, are a crystallization catalyst [4]. If the content of these substances in pet exceeds a certain amount, it will accelerate the crystallization speed of pet (that is, reduce the cold crystallization temperature), which will affect the quality of bottle blowing, cause white fog at the bottom or mouth of the bottle, and even affect the transparency of the whole bottle. Therefore, under the condition of ensuring chip quality and reaction speed (unit capacity), additives, including catalysts, should be less than more