PET is the most widely used beverage packaging material today. Because PET can be conveniently obtained into amorphous, highly transparent and easily stretchable PET products through rapid cooling, when used as packaging material, PET can be made into biaxially oriented packaging film, and high-strength and highly transparent stretch blow-molded bottles can be obtained from amorphous preforms. It can also be directly extruded or blow-molded into non-stretchable PET bottle source hollow containers. PET hollow containers, especially stretch blow-molded bottles, fully leverage the performance of PET, providing a good display effect on the contents. Their performance and cost are on par with other hollow containers. Therefore, when PET is used as packaging material, it is basically formed by stretch blow molding. Among them, the most widely used are small bottles ranging from tens of milliliters to 2 liters, and there are also large bottles with a capacity of 30 liters. Since the early 1980s, due to its light weight, easy molding, low price and ease of large-scale production, it has developed at an unstoppable pace since its introduction. In just about 20 years, it has developed into the world's leading form of beverage packaging. It is not only widely used in the packaging of carbonated beverages, bottled water, condiments, cosmetics, white spirits, dried fruit candies and other products, but also the specially treated hot-filled bottles can be used for the packaging of fruit juice and tea beverages. PET beer bottles processed with the most advanced technology are also entering the market, and aseptic filled PET bottles are also developing at a rapid pace. It can be said that technological progress is constantly expanding the application fields of PET bottles. They not only continue to expand their traditional markets in drinking water and carbonated beverages, but are also making an assault on the last battlefield of glass and aluminum can packaging for beer and other products.
The production process of PET bottle-grade chips mainly consists of two major parts. The first part is the production of basic chips, that is, polyester production. The production process of bottle-grade basic chips is basically the same as that of conventional chips. Meanwhile, to meet some performance requirements of bottle-grade chips, a third monomer IPA and some additives are added. The second part is the solid-phase tackification of the basic slices.
1. The external dimensions of the raw material slices
Both transesterification and esterification reactions are reversible. To shift the equilibrium towards the forward reaction direction, it is necessary to promptly remove volatile small-molecule products. There are two processes for the small-molecule by-products generated by solid-phase polycondensation to leave the section, namely, the process of the small-molecule by-products diffusing from the interior to the surface of the section and the process of diffusion from the surface to the outside of the section. Among them, the diffusion rate from the surface of the slice to the outside is related to the temperature and flow rate of nitrogen. Relatively speaking, in SSP production, under relatively high temperatures and flow rates, the diffusion rate of small-molecule products inside the slice is much slower than that from the surface of the slice to the outside. Therefore, in order to remove small-molecule products as much as possible, the process requires that the residence time of the slice in the reactor be longer. Because the diffusion path of small molecule products within small particles is shorter than that within large particles, they are easier to be excluded. Moreover, with smaller sample particles, the total surface area of the particles increases, the heat transfer rate rises, and the reaction rate also speeds up. Therefore, within a certain range, the reaction rate of solid-phase polycondensation of PET is inversely proportional to the particle size of the raw material chips. However, if the particles are too fine, they are prone to adhesion, which instead affects the reaction rate. In addition, the shape of the particles also affects the reaction rate. Irregular particle shapes are also prone to adhesion. Therefore, the granulation requirements for the basic slices are very high, and no abnormal slices should enter the solid-phase polycondensation system.
2. The color value of the raw material slices
The color value of the raw material slices directly determines the color value of the finished product slices. There are many factors that affect the color value of the basic slice. Color is the most direct indicator reflecting the quality of the section. Its measurement is based on the principles of chromatography and photometry as well as the metrological standards of the International Commission on Illumination. Usually,a colorimeter using the Hunter (L,a,b) method is used for measurement, where L represents whiteness and brightness. a is the green/red index; b represents the yellow index. There are many factors that affect the color of basic slices, mainly caused by differences in raw material quality, types and contents of additives, production processes, production process control and product quality [3]. At present, a relatively direct control method from the process perspective is that, under the condition of stable process and good quality of raw and auxiliary materials, the addition amount of red and blue degree agents can appropriately change the b value of the slices. The factors influencing the color value of finished product slices are more complex. However, bottle-grade slices have very high requirements for the color value of the product. Therefore, the process should be adjusted in a timely manner according to the user's requirements to meet the needs.
3.IPA and DEG content
The content of IPA and DEG in the finished slices is controlled during the production of the basic slices, and their contents remain basically unchanged during the solid-phase tackification process.
The amount of IPA is crucial for the viscosity increase of the chips. The addition of IPA is to reduce the regularity of the arrangement of PET macromolecules to a certain extent, thereby lowering the crystallization performance of the chips. Firstly, it can improve the processing performance during injection molding and blow molding, and lower the processing temperature. Secondly, it can increase the transparency of the preform and bottle. However, the addition of IPA lowers the softening point and melting point of PET, resulting in a decline in the heat resistance and mechanical strength of the produced bottles. Therefore, the content of IPA should be appropriately adjusted and strictly controlled in accordance with market demands. At present, the company has produced two types of bottle-grade slices according to the requirements of users: one is the bottle-grade slices for ordinary carbonated beverages, and the other is the bottle-grade slices for hot-canned juice beverages. The latter requires good high-temperature resistance. Therefore, in addition to making appropriate adjustments in the bottle blowing process, such as adding a heat treatment process and adjusting the temperature of the mold, In addition, the content of IPA was appropriately reduced in the raw materials (by 1.5%, which is a weight percentage) to increase the crystallinity of PET and meet the temperature resistance requirements of beverage bottles. In addition, the IPA content also has a certain impact on solid-phase polycondensation. If the IPA content is inappropriate, for instance, when it is too high, it will cause incomplete crystallization of the slices in the pre-crystallization and crystallizer, thereby resulting in the adhesion of the slices during the viscosity increase process.
The amount of diethylene glycol is generally determined by the production process and can also be slightly adjusted by regulating the formula ratio (such as adjusting the ratio of EG to PTA). Currently, the content of bottle-grade sliced diethylene glycol to be produced on the market is generally around 1.1%±0.2% (by weight percentage). Within this range, a higher content of diethylene glycol is beneficial for enhancing the heat resistance of the slices. This is because the ether bonds in diethylene glycol have a certain degree of softness, which can increase the crystallization rate of PET. However, this content should not be too high, as the presence of ether bonds reduces the rigidity of PET molecules and lowers the melting point of PET, making it prone to adhesion during the slice thickening process. If the content is too high, it will also reduce the mechanical properties during the slicing and bottle blowing process.
4. Terminal carboxyl group
Under certain other conditions, a high content of terminal carboxyl groups is conducive to increasing the reaction rate. From the equation of the SSP reaction, it can be seen that there are two types: transesterification and esterification. The high content of terminal carboxyl groups is conducive to the esterification reaction between PET chains and increases the reaction rate. In PET slices, an increase in H+ concentration is also beneficial to the self-catalytic effect of the catalyst. However, an increase in the content of terminal carboxyl groups will affect the subsequent processing performance of the slices. Therefore, the terminal carboxyl groups of basic slices should be controlled within a certain range, generally ranging from 30 to 40mol/t, while those of bottle-grade slices should be 30mol/t.
5. Other factors
The types and addition amounts of various additives in raw material slices will also have a certain impact on the intrinsic quality of the finished slices. The production of bottle-grade chips requires the addition of a heat stabilizer, polyphosphoric acid. The function of polyphosphoric acid is to seal the ends of the PET molecular chain with phosphate groups, enhancing the thermal stability of the PET chain. However, since phosphate groups may also transform into nucleating agents for PET crystals, it will particularly have a certain impact on the injection molding blow molding of bottle-grade chips. During the bottle blowing process, oligomers, metal oxides (such as antimony trioxide), phosphates, etc. are all nucleating agents for PET crystallization. In addition, some low-molecular-weight compounds, such as polyethylene glycol, although they do not have nucleating effects themselves, are crystallization catalysts. If the content of these substances in PET exceeds a certain level, it will accelerate the crystallization speed of PET (i.e., lower the cold crystallization temperature), which will affect the quality of bottle blowing, causing white fog at the bottom or mouth of the bottle, and even affect the transparency of the entire bottle. Therefore, under the condition of ensuring the quality of the slices and the reaction rate (the production capacity of the device), the amount of additives, including catalysts, should be less rather than more.
6. The influence of the process parameters of the pre-crystallizer and crystallizer on the properties of the product
The general temperature setting of the pre-crystallizer is 145 to 150℃(parameters provided by the foreign party). If the temperature is too low, due to the difficulty in removing water molecules in the form of crystalline water in the slices, the crystallization speed of the slices will be too slow, and the crystallization will be insufficient in a short time, which cannot meet the needs of production. However, the crystallization temperature should not be too high either, as as the temperature rises, the slices are prone to oxidation and degradation with the air inside the pre-crystallizer and the crystallizer, thereby affecting the color value of the product. The temperature setting of the mold is 170 to 175℃(parameters provided by the foreign party). If the temperature exceeds 175℃, as the residence time of the slices in the pre-crystallizer and the crystallizer increases, the color value will rise more sharply, while the crystallinity will change almost nothing. Of course, in actual production, excessive cooling cannot be used to obtain a better b value. Because when the temperature is low, insufficient crystallization of the slices will cause the slices to stick in the subsequent preheater and reactor, and the water in the crystalline state is also difficult to remove completely. This will affect the viscosity increase effect of the slices and thus the intrinsic quality of the finished slices. Only by producing good crystalline slices can good thickened slices be obtained. The so-called good crystallized slices mainly refer to the crystallinity of the slices reaching a certain value, such as the crystallinity coming out of the pre-crystallizer being ≥30%, the crystallinity at the outlet of the crystallizer being ≥40%, and the crystallinity at the outlet of the preheater being ≥45%. Otherwise, it will cause the slices to adhere during the thickening process. Another point is that the surface crystallization of the slices should be uniform.
7. The influence of process parameters of the preheater and reactor on product performance
These two stages increase the viscosity of the slices to varying degrees. There are two thermodynamic and kinetic influencing factors of solid-phase polycondensation reactions: reaction temperature and the degree to which small-molecule by-products diffuse outward from the sections. The first factor depends on the temperature control of nitrogen heating.
The influence of temperature on reactions always has both positive and negative aspects. On the positive side, raising the temperature can increase the reaction rate. Under the condition of a certain increase in viscosity, it can enhance the production capacity of the device. Additionally, under the condition of a certain output, it can also increase the increase in viscosity. However, an increase in temperature will be accompanied by an increase in side reactions, which in turn will affect the quality indicators of the product. Therefore, in actual production, it is necessary to find an appropriate temperature, taking into account two aspects. In this device, what truly determines the temperature of the reactor is the outlet temperature of the preheater. The temperature of the reactor can be controlled by changing the outlet temperature of the preheater and the flow rate of the cooling nitrogen at the bottom of the preheater. The inlet temperature of the reactor is gradually transferred downward, and the reaction of the system is also slow. The time for re-stabilization after a change is at least twice the reactor residence time. At the same time, the corresponding change in the viscosity value of the final product also requires time. Otherwise, the reaction rate will be uneven, resulting in uneven viscosity increase of the slices and thus affecting the subsequent processing performance of the slices.
The second factor depends on the nitrogen flow rate during the reaction and the specific surface area of the slices. Here, nitrogen is on the one hand a heating medium (especially in the preheater), and on the other hand a medium that removes small-molecule by-products. As mentioned earlier, there are two processes in which the small-molecule by-products generated by solid-phase condensation leave the section. Among them, the process of small-molecule diffusion from the surface to the outside is related to the nitrogen flow rate and temperature. Here, the nitrogen and the slices flow in opposite directions, which can enhance the heating effect and remove small-molecule by-products. The preheater of the BUHLER device adopts a roing-shaped structure, using nitrogen heating at the bottom and nitrogen circulation heating in the middle, which makes the heating more uniform and eliminates dead corners. In the reactor, as the slices are under higher pressure at the bottom, the inlet gas temperature at the bottom is controlled at a relatively low level of around 190 degrees, which makes the slices less likely to stick together. The flow rate of nitrogen, which is used as a heating medium, mainly depends on the reaction temperature and production load (i.e., the requirement of the gas-solid ratio). Under the condition that the temperature and load are constant, there is an limit value for the nitrogen flow rate. That is, after reaching this value, the increase in flow rate no longer accelerates the reaction rate because the gas-solid interface has reached adsorption equilibrium at this time. However, when the temperature rises, this equilibrium is disrupted. The concentration of small molecules at the gas-solid interface continues to decrease as the flow rate of nitrogen increases until a new equilibrium is reached.
There is another reason that affects the reaction rate of SSP, and that is the external driving force - the catalyst driving force. That is to say, the size of the catalyst content in the basic section, the catalyst content in section A is approximately two-thirds of that in section B. Among the factors influencing the catalytic effect of a catalyst, apart from the catalyst content, the reaction temperature is relatively important.
8. The influence of nitrogen purification systems on product properties
(1)Oxygen content
A small amount of instrument air is introduced into the nitrogen purification system to eliminate the small-molecule gaseous organic substances produced in the nitrogen system. As can be seen from equations 1-3, the main hydrocarbon in the reaction is ethylene glycol, and there are also some acetaldehyde, oligomers, etc. generated due to side reactions, which are catalytically oxidized by oxygen into carbon dioxide and water in the Pt/Pd catalytic bed of the catalytic reactor. However, the oxygen content must be strictly controlled because the presence of oxygen molecules will cause thermal degradation during the viscosity-increasing process, resulting in a deterioration of the product's color value, a decrease in viscosity, and an increase in terminal carboxyl groups. The oxygen content in the nitrogen gas coming out of the nitrogen purification system in this device is controlled within 10ppm. At present, based on the characteristics of nitrogen purification systems, in addition to catalytic oxidation, cold EG spray can also be used to remove small-molecule compounds from nitrogen. This method can eliminate the oxygen content in nitrogen, but it is not very effective in removing low-boiling-point small-molecule compounds such as acetaldehyde
(2) Nitrogen purification degree
The purity of nitrogen has a certain influence on the viscosity increase of slices and the quality of slices. Firstly, small molecule hydrocarbons in nitrogen can promote the viscosity increase reaction to move in the reverse direction, which is not conducive to the viscosity increase of the slices. At the same time, it will also affect the removal of acetaldehyde in the slices, thereby affecting the aldehyde content of the slices. However, due to the complexity of high-molecular reactions, the analysis of the influence of small molecules in nitrogen on the acetaldehyde content still needs further research.
(3) Dew point of the nitrogen system
At high temperatures, water molecules can easily cause hydrolysis of polyester macromolecules, thereby affecting product quality. Therefore, in solid-phase polycondensation production, it is necessary to control the dew point of the nitrogen system, that is, to control the water molecule content of the nitrogen system. For the BUHLER unit, the nitrogen dew point is required to be below -30 degrees Celsius, and for the SINCO unit, it is required to be below -40 degrees Celsius.
Conclusion
When PET bottle-grade chips are used as packaging materials, the main quality indicators include the following aspects: appearance quality, mechanical properties, processing performance, odorless and non-toxic. There are many and complex factors affecting the quality of chips, and the main factors are the aspects analyzed above. According to the user's requirements, the formula, process route and process conditions of the basic slices can be adjusted to adjust the above indicators, so as to meet the market needs. And make preparations for the localization of SSP production.
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