Abstract This paper analyzes the factors affecting the performance of PET bottle-level sections from three aspects: different production lines, basic sections and process conditions. By optimizing the process parameters and adjusting the formulation of the base slice to meet the market requirements, a further discussion was made to improve the quality of bottle-level slices. Key words: SSP; PET; Polyester; Bottle-level section Foreword 1 Basic conditions and reaction principle of continuous solid phase polycondensation production line 1.1 10R line 1.2 S2 line 1.3 Reaction principle 1.3.2 Main reactions HOOC??COOH + 2HOCH2CH2OH HOCH2CH2OOC??COOCH2CH2OH + 2H2O HOCH2CH2OOC? ?COOCH2CH2OH H--OCH2CH2OOC? -CO n OCH2CH2OH +(n-1)HOCH2CH2OH Main response: 1. Transesterification: PET-COO-CH2-CH2-OH + HO-CH2-CH2-OOC-PET → (1) K1 PET-COO-CH2-CH2-OOC-PET + HO-CH2-CH2-OH 2. Esterification: PET-COOH + HO-CH2-CH2-OOC-PET → (2) K2 PET-COO-CH2-CH2-OOC-PET + H2O Possible side effects: 1 PET chain degradation PET-COO-CH2-CH2OOC-PET → (4) K4 PET-COOH + CH2=CH-OOC-PET 2 Polycondensation reaction with vinyl PET-COO-CH=CH2 + HO-CH2-CH2-OOC-PET → (3) K3 PET-COO-CH2-CH2-OOC-PET + CH3CHO Acetaldehyde formation reaction PET-COO-CH=CH2 + HOOC-PET → K6 K5 (5) PET-COO-CH-OOC-PET+ CH3 → (6) K7 PET-CO-O-OC-PET + CH3CHO 3 Hydroxyl degradation PET-CH2-CH2-OH → PET-COOH + CH3CHO (8) K8 2 Raw materials and product testing Table 1 Contract indicators for raw materials and finished products index unit Indicator range Basic slice Finished slice Intrinsic viscosity Dg/l 0.620±0.015 0.870±0.015 Diethylene glycol content Wt.% [1.5 [1.5 IPA % [2.5 1.80±0.2% Carboxyl Mol/t [40 20±5 Acetaldehyde Mg/kg [200 [1.0 Water % 0.4 [0.5 Color value b / -3~-4 / b Increase / / [2.5 Color value L / Ã80 Ã82 20 grains Mg 350±10% / 3 Results and Discussion 3.1 Differences in Product Performance Caused by Different Production Lines Table 2 Comparison of Performance Specifications of Two Production Lines raw material Finished product (10R line) Finished product (S2 line) Intrinsic viscosity (dg/l) 0.619 0.872 0.865 Viscosity increase (dl/g) / 0.253 0.246 B value -3.92 -0.83 -0.8 L value 83.63 91.75 92.17 Acetaldehyde content (mg/kg) 150 0.9 0.5 Diethylene glycol(%) 1.09 1.21 1.21 Carboxyl (mol/t) 30 20.1 20.5 IPA (%) 1.8 1.8 1.8 Production load (t/d) / 81 81 3.2 Differences in product performance caused by different non-production lines 3.2.1 Effect of Raw Material Slice Properties on Product Performance 3.2.1.1 Value increase of raw material slice viscosity Table 3 Comparison of Product Performance of Different Raw Material Slice Production Raw material B Finished product B (S2 line) Raw material A Finished product A (S2 line) Intrinsic viscosity (dg/l) 0.627 0.873 0.619 0.865 Catalyst content (ppm) 280 / 190 / Viscosity increase / 0.246 / 0.246 Carboxyl (mol/t) 31.3 20.6 28 20.5 B value -2.48 -0.13 -3.92 -0.8 Production load (t/d) / 90.9 / 83 Note: (1) b is the slice yellow index, and the color values ​​are measured according to the international standard systems CIE-L*, a*, b*. The three measured values ​​represent the lightness (L*), the green/red index (a*), and the blue/yellow index (b*), respectively, representing the three-dimensional coordinates of the color measurement system. (4) The production process conditions for product A and product B are the same. 3.2.1.1 Both the transesterification reaction (1) and the esterification reaction (2) are reversible reactions. In order to shift the equilibrium toward the positive reaction direction, volatile small molecule products must be eliminated in time. The small-molecule byproducts produced by solid-phase polycondensation leave the section in two processes, namely the diffusion of small molecule byproducts from the inside of the section to the surface and the outward diffusion from the section surface. Among them, the out-diffusion rate from the slice surface is related to the nitrogen temperature and flow rate. Relatively speaking, in the SSP production, at a relatively high temperature and flow rate, the diffusion rate of the small-molecule product inside the slice is faster than the diffusion rate from the slice surface. It is much slower, so in order to try to get out of the small molecule product, the process requires that the residence time of the slice in the reactor be longer. Since the diffusion distance of the small molecule product in the small particle is shorter than that in the large particle, it is easily eliminated, and the sample particle is small, the total surface area of ​​the particle is increased, the heat transfer rate is increased, and the reaction rate is also accelerated. Therefore, in a certain range, the reaction rate of PET solid phase polycondensation is inversely proportional to the particle size of the raw material slice. However, if the particles are too small, they tend to bond, which affects the reaction rate. In addition, the shape of the particles also affects the reaction rate. Irregular particle shapes also tend to stick. Therefore, the requirements for the cutting of the base slice are very high, and there can be no abnormal slices into the solid phase polycondensation system. 3.2.1.2 From Table 1, it can be seen that the color value of the raw material slice directly determines the color value of the finished product slice. There are many factors that affect the color value of the basic slice. Color is the most intuitive index of the quality of the reaction slice. Its measurement is based on the principles of chromatography and photometry and the International Commission on Illumination standards. It is usually measured with the Hunter (L, a, b) color difference meter, and L represents the whiteness. , Brightness; a is green/red index; b is yellow index. There are many factors that affect the color of the base slice, which is mainly caused by the difference in raw material quality, additive type and content, production process, production process control, and product quality [3]. At present, the relatively straightforward control method from the process is to change the b value of the slice appropriately by changing the addition amount of the redness agent and the blueness agent under the conditions of stable process and good quality of raw and auxiliary materials. The factors affecting the color value of the finished product slice are more complicated, but the bottle-level slice has a high requirement for the color value of the product. Therefore, the user needs to adjust the process in time to meet the needs. 3.2.1.3 IPA and DEG content The content of IPA and DEG in the finished product slice is controlled in the production of the basic slice. The content of both in the solid phase and the viscosity increase is basically unchanged. How much IPA is critical for the viscosifying of slices, IPA is added to reduce the regularity of the alignment of PET macromolecules, thereby reducing the crystallization performance of the slices. First, the processing performance of injection molding and blowing bottles can be improved, and the processing temperature can be reduced. It can increase the transparency of bottle preforms and bottles. However, since the addition of IPA reduces the softening point and melting point of PET, the temperature resistance of the produced bottle deteriorates and the mechanical strength decreases. Therefore, the content of IPA should be properly adjusted according to market requirements and strictly controlled. At present, the company produces two types of bottle-level slicing according to the user's requirements. One is ordinary carbonated beverage bottle-level slicing, one is hot-canned juice drink bottle-level slicing, and the latter requires better heat resistance, so in addition to blowing the bottle The process has made appropriate adjustments, such as increasing the heat treatment process and adjusting the temperature of the mold. In addition, the IPA content of the material is also reduced (by 1.5%, this percentage is the weight percentage) to improve the PET. Crystallinity to meet the temperature resistance requirements of beverage bottles. In addition, the IPA content also has a certain influence on the solid phase polycondensation. If the IPA content is not appropriate, for example, if the content is too high, the slicing may be incomplete in the pre-crystallization and the crystallizer, thereby causing the sticking of the slices in the thickening process. The amount of diethylene glycol content is generally determined by the production process. It is also possible to make minor adjustments (for example, adjusting the ratio of EG and PTA) by adjusting the formula ratio. The content of diethylene glycol of the bottle-grade slice to be produced on the market is generally about 1.1%±0.2% (by weight). In this range, the higher content of diethylene glycol is conducive to the improvement of the heat resistance of the chips. This is due to the fact that the ether bond in diethylene glycol has a certain degree of softness and can increase the crystallization rate of PET, but this content should not be too high. Because the presence of Ether reduces the rigidity of PET molecules, lowers the melting point of PET, and can easily cause sticking of the thickening process of the slice. If the content is too high, the mechanical properties of the slice blowing can also be reduced. 3.2.1.4 Carboxyl It can be seen from Figure 1 that, under certain other conditions, the content of carboxyl groups is high, which helps to increase the reaction rate. From the equation of SSP reaction, one can see that one is transesterification, the other is esterification, and the content of carboxyl-terminal is high, which is favorable for the esterification reaction between PET chains and improves the reaction rate. Raw material slice A Fig.1 Effect of carboxyl content on solid phase Fig1 carboxyl effect on solid state 3.2.1.5 3.2.2 Effect of Solid Phase Polycondensation Process Conditions on Product Performance 3.2.2.1 Effect of Process Parameters of Pre-Crystal Mold and Crystallizer on Product Properties (take S2 ​​line as an example) The device's pre-crystallizer temperature setting is 145 ~ 150 °C (foreign to provide parameters), if the temperature is too low, because the water molecules in the form of crystallized water in the slice is difficult to remove, will cause the crystallization rate of the slice is too slow, short Crystallization is insufficient in time and cannot meet production needs. However, the crystallization temperature cannot be too high, because as the temperature rises, the air in the slicing and pre-crystallizer and the crystallizer is prone to oxidative degradation, thus affecting 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 dwell time of the chips in the pre-crystallizer and the crystallizer increases, the color value rises more drastically and the crystallinity changes little. Of course, in practical production, it is not possible to obtain a good b value by excessive cooling. Because the temperature is low, insufficient crystallization of the chips can cause sticking of the chips in the subsequent preheater and reactor, and the water in the crystalline state is also difficult. Dividing, this will affect the thickening effect of the slice and thus affect the intrinsic quality of the finished slice. Only good crystallized chips can produce good thickened sections. The so-called good crystallography mainly refers to the crystallinity of the slice reaches a certain value, for example, the crystallinity from the pre-crystallizer is ≥30%, the crystallinity at the crystallizer exit is ≥40%, and the crystallinity at the exit of the preheater is ≥45%. Otherwise, it will cause the adhesion of the slicing process in the viscosifying process; another point is that the surface of the slicing should be crystallized uniformly. 3.2.2.2 Effect of Process Parameters of Preheaters and Reactors on Product Performance Figure 2 Effect of reaction temperature on solid phase Fig2 temperature effect on solid state Fig. 3 Effect of niobium content on solid phase Fig3 antimony effect on solid state Table 3 Effect of spray EG temperature on nitrogen purity Tab3 effect of the temperature of EG on the purity of the N2 Spray EG temperature Acetaldehyde content EG content water molecule 9 2555.4 57.2 21.4 10 2572.9 63.3 22.8 11 2590.9 70.0 24.5 12 2607.4 77.4 26.3 13 2624.0 85.5 28.1 14 2639.2 94.4 30.1 15 2653.8 104.2 32.2 16 2667.7 114.9 34.4 17 2680.9 126.6 36.8 18 2693.3 139.5 39.3 19 2705.1 153.5 42.0 20 2715.2 168.7 44.9 The purity of nitrogen has a certain influence on the viscosifying of slices and the quality of slices. First, small molecules of hydrocarbons in nitrogen can promote the viscosifying reaction to move in the direction of the reverse reaction, which is not conducive to the thickening of the slice. At the same time, it also affects the removal of acetaldehyde in the slice, thus affecting the aldehyde content of the slice, but due to the high polymer reaction The analysis of the influence of complex, small molecules in nitrogen on the acetaldehyde content remains to be further studied. 4 Conclusion references [1]. Pan Zuren, Polymer Chemistry, 44; Chemical Industry Press; [2]. Pan Zuren, Polymer Chemistry, 45; Chemical Industry Press; [3]. Polyester Industry, 2000,, VOL.13, No.2; 42-43; [4]. Improved performance of PET processing for engineering plastics. Synthetic Technology and Applications, 1998, 3;31. Flower Essential Oil,Rose Essential Oil,Lavender Oil,Rosemary Oil Ji'An ZhongXiang Natural Plants Co.,Ltd. , https://www.zxessentialoil.com
Important factors affecting the quality of PET bottles
PET is the most widely used beverage packaging material today. Because PET can be easily cooled to obtain a substantially amorphous, highly transparent, easily stretchable PET product, PET can be used as a packaging material for both biaxially oriented packaging films and amorphous preforms. A high-strength, highly transparent stretch blow-molded bottle can be obtained, which can also be directly extruded or blown into a non-stretched PET bottle-sourced hollow container. PET hollow containers, especially stretch blow-molded bottles, give full play to PET performance, have a good display effect on the contents, and the performance and cost are in competition with other hollow containers. Therefore, PET as a packaging material is basically formed by stretch blow molding, of which the most widely used are small bottles of tens of milliliters to 2 liters, and large bottles of 30 liters in capacity. Since the early 1980s, due to its light weight, easy forming, low price and easy mass production, it has developed rapidly with unstoppable momentum since its inception. In just 20 years or so, it has become the world's leading beverage packaging form. It is not only widely used in the packaging of carbonated beverages, bottled water, spices, cosmetics, liquor, dried fruit candy and other products, but also specially processed hot filling bottles can be used for the packaging of fruit juices and tea beverages. PET beer bottles processed with state-of-the-art processes are also entering the market, and aseptic PET bottles are also in full swing. It can be said that technological progress is constantly expanding the application fields for PET bottles. It not only continues to expand its traditional market in drinking water and carbonated beverages, but it is also launching an impact on the final positions for packaging of glass and aluminum cans such as beer.
Yihua has two sets of continuous solid-phase polycondensation production lines, one is SINCO (referred to as 10R line) and the other is BUHLER (referred to as S2 line). The equipment and process flow of these two devices are as follows.
The main equipment includes pre-crystallizer, crystallizer, reactor and cooler. There is also a nitrogen purification system (NPU, SINCO patented) to remove nitrogen system moisture, hydrocarbons and some small molecular byproducts, to ensure the purity of nitrogen, and add a small amount of pure nitrogen, nitrogen recycling to reduce its Consumption. See Figure 1 for the process. All heating and cooling media are nitrogen, which can reduce the occurrence of side reactions.
The main equipment includes pre-crystallizer, crystallizer, preheater, reactor and cooler. There is also a nitrogen purification system. Preheaters and crystallizers are used as the heating medium for hot air, and the rest of the heating medium is nitrogen. See Figure 2 for the process.
1.3.1 Brief description of production process
The production process of PET bottle-level slices is mainly divided into two parts. The first part is the production of basic slices, namely polyester production. Yihua Company adopts the direct esterification method, and the process of producing bottle-level base chips is basically the same as the production of conventional chips. At the same time, in order to meet the performance of bottle-level chips, the third monomer IPA and some additives are added. The second part is the solid phase tackifying of the basic slice. At present, there are mainly three different processes: SINCO, BUHLER, and BEPEX processes. Our company uses SINCO and BUHLER processes.
The reaction equation is as follows:
Polyester reaction:
Solid phase polycondensation reaction:
2.1 Quality index of basic slice and finished slice
2.2 Test Methods
Intrinsic viscosity: Ubbelohde viscometer
Color value: color difference meter
Acetaldehyde content: gas chromatography
Diethylene glycol: gas chromatography
IPA: Gas Chromatography
Melting point: DSC
Crystallinity: density gradient method
Terminal carboxyl: titration
From Table 2, it can be seen that in the case of fixed raw material slicing and production, the viscosity increase of the 10R line is greater than the S2 line, which is mainly caused by the two systems themselves. In the production of the maximum load that satisfies the conditions, the reaction speed of the 10R line is faster than that of the S2 line. In the case of the same raw material, the reason is mainly the system itself. There are many factors that affect the reaction speed. From the perspective of thermodynamics, kinetics, and extrinsic power, there are about three factors, namely the reaction temperature, the removal rate of small molecules, and the extrinsic power—catalyst catalysis. S2 line reactor temperature is low, and there is no insulation layer, although there is a preheater to increase the viscosity, but the overall impact on the reaction rate, relatively speaking, the SINCO process, the reactor temperature is higher, and the material is higher The residence time at temperature is relatively long, so the reaction speed is faster than the S2 line. For polymer polymerization, the relationship between the total polymerization rate constant k and temperature T(K) follows the Arrhenius equation [1]:
k=Ae-E/RT
Since the polycondensation activation energy in the formula is a positive value [2], it indicates that the temperature increases and the rate constant increases.
Note: (1) b is the slice yellow index, and the color values ​​are measured according to the international standard systems CIE-L*, a*, b*. The three measured values ​​represent the lightness (L*), the green/red index (a*), and the blue/yellow index (b*), respectively, representing the three-dimensional coordinates of the color measurement system.
(2) The above data is the average of 30 data.
(3) The raw materials are the basic slices provided by the Second Factory.
The quality of raw material slicing is the key to the quality of the finished product. Therefore, each index of the basic slicing is an important factor affecting the finished product.
Tab.3 contrasting for products feature of the different basic chips
(2) The above data is the average of 30 data.
(3) Raw material A is the basic slice provided by the second factory, and raw material B is the base slice of the fourth factory.
In the case of a certain amount of production, the increase in the viscosity of the base section determines the final viscosity of the product, because the market has a high viscosity requirement, so in order to increase the output and to meet the market requirements, the sticking speed of the base section must be increased as much as possible. There are many factors that cause this difference in viscosity increase, and the accumulation of multi-day production results in the difference in the quality indexes of the two basic sections is the b-value of the carboxyl content of the two and the content of St+ in the base section. The effect of carboxyl content on solid phase polycondensation and products is given in 3.2.1.4. Of course, the factors affecting the solid phase viscosity increase rate are very complicated, and we must continue to summarize our experiences in production.
The carboxyl-terminated carboxyl groups are smaller than those of the raw material slice B. The carboxyl-terminated carboxyl groups increase the PET chain end activity and accelerate the esterification reaction between the PET chains. At the same time, the increase of H+ concentration in PET slices is also beneficial to the catalyst's autocatalytic effect, but the increase of carboxyl-terminated content will affect the follow-up processing performance of the slice. Therefore, it is required that the carboxyl-terminus of the basic slice be controlled within a certain range. General requirements At 30 to 40 mol/t, carboxy-terminal carboxyl groups [30 mol/t.
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 product slice. Production bottle-level slices need to add thermal stabilizer polyphosphoric acid, the role of polyphosphoric acid is to use phosphate to seal the PET molecular chain end, increase the PET chain thermal stability, but because the phosphate group may also be converted to PET crystal nucleating agent In particular, it will have a certain impact on the bottle-blade injection blow molding. During the blowing process, oligomers, metal oxides (eg, antimony trioxide), phosphates, and the like are all nucleating agents for PET crystals, while other low-molecular compounds such as polyethylene glycol itself do not have a nucleating effect. However, it is a crystalline catalyst [4]. If the content of these substances in PET exceeds a certain level, the crystallization rate of PET will be accelerated (ie, the cold crystallization temperature will be lowered). This will affect the quality of the blown bottle, causing a 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 chips and the reaction speed (apparatus production capacity), additives, including catalysts, should not be less.
These two stages have different degrees of viscosifying to the slice. The thermodynamic and kinetic influences of the solid phase polycondensation reaction are two factors: the reaction temperature and the extent to which the small-molecule byproduct product diffuses outward from the slice. The first factor depends on the nitrogen heating control temperature.
The influence of temperature on the reaction is always positive and negative. On the positive side, increasing the temperature can increase the reaction speed. It can increase the production capacity of the device under a certain range of increasing viscosity. In addition, it can also increase the output under certain conditions. Viscosity increase, see Figure 2 below. However, the increase in temperature will be accompanied by
With the increase of side effects, it will affect the quality of the product. Therefore, in the actual production to find the appropriate temperature, taking into account the two aspects. In this device, it is the outlet temperature of the preheater that really determines the reactor temperature. The temperature of the reactor can be controlled by changing the outlet temperature of the preheater and the flow of cooling nitrogen at the bottom of the preheater. The inlet temperature of the reactor is slowly transmitted downwards, and the reaction of the system is also slow. Once it is changed, the time of re-stabilization is at least twice the residence time of the reactor, and the corresponding viscosity value of the final product is also changed. It takes time. Otherwise, it will cause different reaction speeds, resulting in non-uniform tackiness of the slice, which will affect the subsequent processing performance of the slice.
The second factor depends on the nitrogen flow rate during the reaction and the specific surface area of ​​the section. Nitrogen here is on the one hand a heating medium (especially in a preheater) and on the other hand it is a medium which carries away small molecule by-products. As mentioned earlier, the small molecule by-products produced by solid-phase polycondensation leave the section in two processes, where the outward diffusion of small molecules from the surface is related to nitrogen flow and temperature. The nitrogen and chips here are in countercurrent flow, which can increase heating effect and carry away small molecule byproducts. The preheater of BUHLER device adopts a roof-like structure, which uses nitrogen heating at the bottom and intermediate nitrogen circulation heating to make the heating more uniform and no dead angle is generated. In the reactor, the bottom inlet temperature is controlled at a lower temperature of about 190 degrees because the slice pressure is higher at the bottom thereof, making the slicing difficult to bond. The nitrogen flow rate used as heating medium is mainly the reaction temperature and production load (ie gas-solid ratio requirement). Under certain temperature and load conditions, the nitrogen flow has a limit value, that is, after reaching this value, the flow rate 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 concentration of small molecules at the gas-solid interface continues to decrease as the nitrogen flow increases, until the new Balance.
Another reason that affects the SSP reaction rate is the external power, catalyst power, as shown in Figure 2. That is, the size of the catalyst in the base slice, and the content of the catalyst in the slice A is about 2/3 of the slice B. Of the factors that affect the catalytic effect of the catalyst, the most important one is the reaction temperature.
3.2.2.3 Effect of Nitrogen Purification System on Product Properties
(1) Oxygen content
The introduction of a small amount of instrument air into the nitrogen purification system is used to eliminate the small-molecule gaseous organic compounds produced in the nitrogen system. It can be seen from Equation 1-3 that the main hydrocarbon of the reaction is ethylene glycol, and some Acetaldehydes, oligomers and the like generated by side reactions are catalytically oxidized by oxygen to carbon dioxide and water in a catalytic reactor Pt/Pd catalyst bed. However, the oxygen content must be strictly controlled because the presence of oxygen molecules will cause thermal degradation during the tackification process, which will cause the product's color value to deteriorate, the viscosity to decrease, and the terminal carboxyl groups to increase. Oxygen content in nitrogen from the nitrogen purification system in this unit is controlled within 10 ppm. According to the characteristics of the nitrogen purification system, in addition to the catalytic oxidation, the method of removing the small molecule compounds in the nitrogen gas can be sprayed with cold EG. This method can eliminate the oxygen content in the nitrogen gas, but for low-boiling small molecule compounds such as acetaldehyde The removal effect is not good (see Table 3).
(2) nitrogen purification
(3) Dew point of nitrogen system
At high temperatures, water molecules tend to hydrolyze polyester macromolecules and affect product quality. Therefore, in solid phase polycondensation production, the dew point of the nitrogen system must be controlled, ie, the water molecule content of the nitrogen system is controlled. For the BUHLER device, the nitrogen dew point is required to be below -30 degrees, and the SINCO device is required to be at -40 degrees.
When using PET bottle-slices as packaging materials, the main quality indicators include the following aspects: appearance quality, mechanical properties, processing performance, odorless and non-toxic, and many factors that affect the quality of the slices are also very complex. The main factors are the above three analysis. aspect. According to user requirements, adjust the formula, process route and process conditions of the base slice to adjust the above indicators to meet market needs. And prepare for the localization of SSP production.
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