Important Factors Affecting the Quality of PET Bottle Grade Chips
Nowdays,PET is the most widely used beverage packaging material. Because PET can be conveniently cooled to get PET products which are basically in amorphous state, high transparency and easy to stretch, PET can be used as packaging material to make biaxial stretching packaging film, and can be used to get high strength and high transparency stretching blowing bottle from amorphous bottle blank, and can also be extruded or blown into non-stretching PE directly. T bottles are hollow containers. PET hollow containers, especially stretch blow molding bottles, give full play to the properties of PET, have a good display effect on the content, performance and cost of competing with other hollow containers. Therefore, PET as packaging materials are basically used in stretch blow molding, the most widely used are dozens of milliliters to 2 liters of small bottles, also has a capacity of 30 liters of bottles. Since the early 1980s, due to its light weight, easy formation, low price and easy to mass production, it has been an irresistible momentum of rapid development. In about 20 years, it has become the most important beverage packaging in the world. It is not only widely used in the packaging of carbonated beverages, bottled water, condiments, cosmetics, liquor, dry fructose and other products, but also can be used in the packaging of fruit juice and tea beverages after special treatment of hot-filled bottles. PET beer bottles treated with the most advanced technology are also entering the market, and aseptic filling PET bottles are also developing rapidly. It can be said that technological progress is expanding the application of PET bottles, not only to continue to expand the traditional market in drinking water and carbonated beverages, but also to beer and other glass and aluminum can packaging of the final position of the impact.
The production process of PET bottle-grade chip is mainly divided into two parts. The first part is the production of basic chip, that is, polyester production. The process of producing bottle-grade basic slices is basically the same as that of conventional slices. In order to satisfy some properties of bottle-grade slices, the third monomer IPA and some additives are added. The second part is the solid phase viscosity increasing of foundation slices.
1. size of raw material slicing
Both transesterification and esterification are reversible reactions. In order to shift the equilibrium to the positive direction, volatile small molecular products must be eliminated in time. The small molecule by-products produced by solid-state polycondensation can be separated from the chip by two processes: the diffusion of small molecule by-products from the inside to the surface of the chip and the diffusion from the surface of the chip. Relatively speaking, in the production of SSP, the diffusion rate of small molecule products inside the chip is much slower than that outside the chip surface at relatively high temperature and flow rate. Therefore, in order to get rid of small molecule products as far as possible, the technology requires the chip. The residence time in the reactor is longer. Because the diffusion path of small molecule 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 solid-state polycondensation of PET is inversely proportional to the particle size of the raw material chip. However, if the particles are too small, it is easy to bond, which will affect the reaction rate. In addition, the shape of particles will also affect the reaction rate. Irregular shape and easy to bond. Therefore, there is a high requirement for cutting the basal section, and no special section can be entered into the solid phase polycondensation system.
2. color value of raw material slices
The color value of raw slicing directly determines the color value of the product slicing. There are many factors that affect the color value of basal slices. Color is the most intuitive indicator of the quality of reaction slices. The measurement is based on chromatographic and photometric principles and the International Lighting Commission (ILC) measurement standards, usually measured by Hunt (L, a, b) colorimeter, L means whiteness, brightness; A is green / red index; B is yellow index. There are many factors affecting the color of basic slices, which are mainly caused by the quality of raw materials, the kinds and contents of additives, production technology, production process control and product quality differences. At present, the direct control method from the process is to change the addition of redness agent and blue agent under the condition of stable process and good quality of raw and auxiliary materials. The factors affecting the color value of the finished product slices are more complex, but the bottle-grade slices require high color value of the product, so the process should be adjusted in time to meet the needs of users.
3.IPA and DEG contents
The content of IPA and DEG in the finished product slice is controlled in the production of basic slice, and the content of IPA and DEG is basically unchanged in the process of solid phase tackification.
The amount of IPA is very important to the viscosity of PET chip. The purpose of adding IPA is to reduce the regularity of PET macromolecule arrangement so as to reduce the crystallinity of PET chip. But the softening point and melting point of PET were decreased by the addition of IPA, which made the thermostability and mechanical strength of bottles worse. Therefore, the content of IPA should be adjusted according to market requirements and strictly controlled. At present, the company produces two kinds of bottle-grade slices according to the user's requirements, one is ordinary carbonated beverage bottle-grade slices, the other is hot-canned fruit juice bottle-grade slices, the latter requires high temperature resistance, so in addition to making appropriate adjustments in the bottle blowing process, such as increasing the heat treatment process and adjusting the mold temperature. In addition, the content of IPA (1.5% reduced by weight percentage) was appropriately reduced in raw materials to improve the crystallinity of PET to meet the requirements of temperature resistance of beverage bottles. In addition, the content of IPA also has a certain impact on solid-state polycondensation, if the content of IPA is not appropriate, for example, when the content is too high, it will cause slices in the pre-crystallization and crystallizer crystallization imperfection, resulting in the adhesion of slices in the tackifying process.
Diethylene glycol content is generally determined by the production process, but also can be adjusted by adjusting the proportion of micro-adjustment (such as adjusting the ratio of EG to PTA). At present, the content of diethylene glycol in bottle-grade slices is about 1.1%+0.2%(weight percentage). In this range, higher diethylene glycol content is conducive to the improvement of the heat resistance of PET chips, which is due to the flexibility of the ether bond in diethylene glycol, which can improve the crystallization rate of PET, but this content can not be too high, because the existence of ether health reduces the rigidity of PET molecules, reduces the melting point of PET, and easily leads to slice viscosity. Bonding process. If the content is too high, it will also reduce the mechanical properties of slice blowing.
4. Terminal carboxyl group
Under certain other conditions, the high carboxyl group content is beneficial to increase the reaction rate. It can be seen from the equation of the SSP reaction that one is transesterification, the other is esterification, and the terminal carboxyl group content is high, which is favorable for the esterification reaction between the PET chains and the reaction rate. In the PET section, the increase of H+ concentration is also beneficial to the autocatalytic effect of the catalyst, but the increase of the terminal carboxyl group content will affect the subsequent processing performance of the slice, so the terminal carboxyl group of the basic slice is required to be controlled within a certain range, generally required to be 30. ~40mol/t, the terminal carboxyl group of the bottle-level slice [30mol/t.
5. Other factors
The type and amount of various additives in the raw material slice will also have a certain influence on the intrinsic quality of the finished slice. The production of bottle-level slicing requires the addition of a heat stabilizer polyphosphoric acid. The role of polyphosphoric acid is to seal the end of the PET molecular chain with a phosphate group and increase the thermal stability of the PET chain. However, the phosphate group may also be converted into a nucleating agent for PET crystal. In particular, it will have a certain impact on the injection molding of bottle-level slices. During the blowing process, oligomers, metal oxides (such as antimony trioxide), phosphates, etc. are all nucleating agents for PET crystallization, while other low molecular compounds such as polyethylene glycol itself do not have nucleation. However, it is a crystallization catalyst. If the content of these substances exceeds a certain content in PET, the crystallization rate of PET will be accelerated (ie, the cold crystallization temperature will be lowered), which will affect the quality of the blowing bottle, causing white fog at the bottom or mouth of the bottle, and may even affect the entire bottle. Transparency. Therefore, in the case of ensuring the quality of the slice and the reaction speed (device capacity), the additives, including the catalyst, should be less and less.
Effect of process parameters of pre-crystallizer and crystallizer on product properties
Generally, the pre-crystallizer temperature setting is 145~150°C (the parameter is provided by the foreign party). If the temperature is too low, it is difficult to remove the water molecules in the form of crystal water in the slice, which will cause the crystallization speed of the slice to be too slow. The crystallization is insufficient and cannot be adapted to the needs of production. However, the crystallization temperature should not be too high, because as the temperature rises, the dicing and the air in the pre-crystallizer and the crystallizer are susceptible to oxidative degradation to affect the color value of the product. The temperature setting of the crystallizer is 170~175 °C (the parameters are provided by the foreign party). If the temperature is higher than 175 ° C, as the residence time of the chips in the precrystallizer and the crystallizer increases, the color value rises more sharply, while the crystallinity hardly changes. Of course, in actual production, it is not possible to obtain a good b value by excessive cooling, because the temperature is low, the lack of crystallization of the slice causes the chip to occur in the subsequent preheater and the reactor, and the water in the crystalline state is difficult. Divide, this will affect the slice thickening effect and affect the intrinsic quality of the finished slice. Only good crystallized sections can be used to obtain good thickened sections. The so-called good crystal section mainly means that the crystallinity of the slice reaches a certain value, for example, the crystallinity from the precrystallizer is ≥30%, the crystallinity of the exit of the crystallizer is ≥40%, and the crystallinity of the outlet of the preheater is ≥45%. Otherwise it will cause the bonding of the slices during the thickening process; another point is that the surface of the slices is crystallized evenly.
7. Effect of process parameters of preheater and reactor on product performance
These two stages have different degrees of thickening of the slices. There are two factors affecting the thermodynamics and kinetics of the solid phase polycondensation reaction: the reaction temperature and the extent to which small molecular byproduct products diffuse outward from the section. The first factor depends on the nitrogen heating control temperature.
The effect of temperature on the reaction is always positive and negative. The positive side is that increasing the temperature can increase the reaction rate, and the productivity of the device can be increased under the condition that the viscosity increase is constant. In addition, the production can be increased under certain conditions. Thickness increase. However, an increase in temperature is accompanied by an increase in side reactions, which in turn affects the quality of the product. Therefore, in the actual production to find the appropriate temperature, taking into account two aspects. In this device, the temperature of the reactor is determined by the outlet temperature of the preheater. The temperature of the reactor can be controlled by varying the outlet temperature of the preheater and the flow of nitrogen at the bottom of the preheater. The inlet temperature of the reactor is slowly transferred downwards, and the reaction of the system is also slow. Once changed, the time for re-stabilization is at least twice the residence time of the reactor, and the viscosity of the corresponding final product is also changed. It takes time, otherwise, the reaction speed will be different, resulting in uneven thickening of the slice, which will affect the subsequent processing performance of the slice.
The second factor depends on the nitrogen flow rate at the time of the reaction and the specific surface area of the slice. Here, nitrogen is on the one hand a heating medium (especially in a preheater) and on the other hand a medium carrying away small molecular by-products. As mentioned earlier, the small molecular byproducts produced by solid phase polycondensation leave the section in two processes, wherein the outward diffusion of small molecules from the surface is related to nitrogen flow and temperature. The nitrogen and slicing here are reverse flow, which increases the heating effect and carries away small molecular by-products. The preheater of the BUHLER device uses a roof-like structure, which is heated by bottom nitrogen and intermediate nitrogen circulation to make the heating more uniform and no dead angle. In the reactor, because the slice is under pressure at the bottom, the bottom inlet temperature is controlled at a lower temperature of about 190 degrees, making the slices less likely to stick. As the medium for heating, the flow rate of nitrogen is mainly the reaction temperature and the production load (that is, the gas-solid ratio requirement). When the temperature and the load are constant, the nitrogen flow rate has a limit value, that is, after the value is reached, the flow rate is The increase no longer accelerates the reaction rate because the gas-solid interface reaches the adsorption equilibrium, but when the temperature rises, the equilibrium is broken, and the small molecule concentration at the gas-solid interface continues to decrease as the flow rate of nitrogen increases until the new Balance.
Another reason that affects the rate of SSP reaction is the external power - catalyst power. That is, the amount of catalyst in the base section, the catalyst content in the section A is about 2/3 of the section B. Among the factors affecting the catalytic effect of the catalyst, in addition to the catalyst content, among them, the reaction temperature is more important.
8. Effect of nitrogen purification system on product properties
(1) Oxygen content
A small amount of instrument air is introduced into the nitrogen purification system to eliminate the small molecular gaseous organic matter produced in the nitrogen system. It can be seen from Equation 1-3 that the main hydrocarbon in the reaction is ethylene glycol, and there are some The acetaldehyde, oligomer, etc. produced by the side reaction are catalytically oxidized by oxygen to carbon dioxide and water in the Pt/Pd catalytic bed of the catalytic reactor. However, the oxygen content therein must be strictly controlled because the presence of oxygen molecules causes thermal degradation during the thickening process, resulting in a deterioration in the color value of the product, a decrease in viscosity, and an increase in terminal carboxyl groups. The oxygen content of the nitrogen gas from the nitrogen purification system in the apparatus is controlled to be within 10 ppm. At present, according to the characteristics of the nitrogen purifying system, in addition to catalytic oxidation, a method of removing small molecular compounds in nitrogen can also be sprayed with cold EG, which can eliminate the oxygen content in nitrogen, but for low-boiling small-molecule compounds such as acetaldehyde. The removal effect is not good
(2) Degree of nitrogen purification
The purity of nitrogen has a certain effect on the thickening of the slices and the quality of the slices. First, small molecular hydrocarbons in nitrogen can promote the viscosity-increasing reaction to the reverse reaction direction, which is not conducive to the thickening of the slices. At the same time, it also affects the removal of acetaldehyde in the slices, thus affecting the aldehyde content of the slices, but the polymer reaction is very Complexity, analysis of the effect of small molecules in nitrogen on acetaldehyde content remains to be further studied.
(3) Dew point of the nitrogen system
At high temperatures, water molecules tend to hydrolyze polyester macromolecules and affect 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 BUHLER devices, the nitrogen dew point is required to be below -30 degrees and the SINCO device is required to be at -40 degrees.
In conclusion
When PET bottle grade chips are used as packaging materials, the main quality indicators have the following aspects: appearance quality, mechanical properties, processing properties, odorless and non-toxic, and many factors affecting the quality of the slices are also very complicated. The main factors are several of the above analysis. aspect. According to the user's requirements, the basic slice recipe, process route and process conditions can be adjusted to adjust the above indicators to meet market needs. And prepare for the localization of SSP production.
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