The packaging material is polyester, and the position of polyester in beverage packaging is undoubtedly important. In these packaging products, the highest quality requirements are carbonated beverages (such as Coca-Cola and Pepsi) bottles, one of which is the bottle's resistance to stress cracking, which has great theoretical and practical value for in-depth research. The production of carbonated beverages PET bottles is usually a two-step process. The typical production equipment is the Canadian HUSKEY injection molding machine and the French SIDEL bottle blowing machine. The first is the blanks: PET chips are dried at about 160°C for 5-6 hours, then the preforms are made by screw melt injection molding. The screw temperature is 285-290°C. The die is cooled by cooling water, the water temperature is 8-10°C, and the cooling time is about 3.5s. . After the preforms were placed for 24 hours, the bottles could be blow molded. Blowing: The preform is heated to about 110°C. After being stretched and blown into bottles, the mold is cooled by cooling water. The temperature at the bottom is 11°C. Carbonated beverages PET bottles usually have 500ml, 600ml, 1.25ml, 2L and several different capacities. The shape of the bottom of the bottle is quite complex, usually petal type, and its complex structure becomes the stress concentration point of the product, cracking position All of this happens here and becomes a key point in quality control. I. Requirements and testing methods for stress crack resistance of carbonated soft drinks PET bottles The stress cracking resistance test of carbonated beverage bottles was called QSCRT (Quick Stress Crack Resistance Test) at Coca-Cola Company. The process was as follows: Amount of citric acid (C6H8O7·H2O) and sodium bicarbonate (NaHCO3), fill with water, cap well, shake, make citric acid and sodium bicarbonate fully react, the volume of carbon dioxide generated is equivalent to the volume of the bottle 4.3 times. Next, the above bottle was placed in a 0.2% (mass percentage) NaOH solution (the solution temperature was 24C), and the NaOH solution submerged the bottom of the bottle. After 45 minutes, none of the 10 samples tested could leak, allowing cracks. In other words, if any of the 10 samples leaked within 45 minutes, the test was failed. The broken position occurs near the injection point at the bottom of the bottle. The time requirement of the newly blown bottle stress test is greater than 45 minutes. The bottle is prolonged with the storage time, and the leak time is shortened. The stress test time is required to be greater than 20 minutes after one month. Second, the mechanism of stress cracking Polymer micro-breaking process can be attributed to the following three situations: (a) chemical bond failure; (b) intermolecular slip; (c) van der Waals force or hydrogen bond damage. Through the theoretical analysis of the fracture strength of these three failure conditions, it is shown that: since the length of the actual polymer chain is limited, at the same time, the molecular chain will always have more or less non-oriented portion, so when the polymer is normally destroyed, First, the hydrogen bond or van der Waals force in the unoriented portion is destroyed, and then the stress is concentrated on the oriented main chain. Although the strength of the covalent bond is 10 to 20 times larger than the intermolecular force, it is directly subjected to the external force. With a small number of main chains, there will eventually be a break in chemical bonds. One of the main reasons for the stress is that during the molding process, due to the different cooling rates in the workpiece surface, the surface material contacts the mold wall with lower temperature and rapidly cools and solidifies into a hard shell, while the internal material is still in the molten state. The cooling shrinkage causes internal stresses in the part. The stress distribution caused by environmental factors is usually irregularly arranged, and it often directly develops into environmental stress cracking. According to different environmental factors, environmental stress cracking includes: solvent cracks, non-solvent cracks (including surface active substances such as alcohols and wetting agents), thermal stress cracking, and oxidative stress cracking. Among them, the stress cracking test caused by solvents and non-solvents has become an important method to study the stress and crack resistance of polymers. For example, the QSCRT test of carbonated beverage bottles is based on this principle. III. Influencing factors and solutions for stress cracking of carbonated beverages in polyester bottles The factors influencing the stress cracking of carbonated beverages in polyester bottles are quite complex. The authors summarized the quality of polyester raw materials in years of production, development, and after-sales service practices. The influencing factors and corresponding control methods in the two aspects of the bottle making process provide ideas for resolving this technical problem, with extremely strong pertinence and practical value, according to the influencing factors listed below, combined with actual conditions (such as different raw materials , models, bottles, and even weather, etc.) to make different process optimization, we give guidance in the use of the user, and achieved satisfactory results. 1. Polyester Quality Control (1) Controlling Polyester Crystallization Speed ​​(a) Through Copolymerization Modification: There are many technical know-hows. The crystallization rate of the polyester can be slowed by choosing the appropriate copolymerization component and the amount of addition. We have achieved remarkable results with the use of acid and alcohol copolymerization components in practice. (b) Through physical modification: The use of inorganic additives improves the post-processing properties of the polyester. There are many technical flaws. (c) The degree of crystallinity at the bottom of the bottle is the control point and is related to the raw material formulation. (2) Cleaner Production and Increased Polyester Purity (a) Use finer filtration. (b) Introduce online leak detection. (c) Modify the equipment structure to reduce dead angles. (d) Controlling the content of small molecules in polyesters. (e) Control other impurities. (3) The viscosity control (a) is suitably 0.86 to 0.88 dg/l. (b) Viscosity fluctuations should be less than 0.005dg/l. (c) Use medium to wide molecular weight distribution values. (d) Control the extent of solid phase polycondensation. (4) Bottle shrinkage The bottle with high shrinkage has a poor resistance to stress cracking, which is related to the design of the raw material formulation. (5) Control of diethylene glycol (DEG) content Diethylene glycol directly affects the flow properties of the polyester melt. The central value is controlled at 1.0% to 1.2%. 2. Injection molding process adjustment (1) Screw temperature The screw temperature was adjusted according to the melting point, the DEG content, and the addition amount of the copolymerization component. (2) The grammage of the preform is controlled by adjusting the injection molding process to control the weight of the preform, reducing the weight of the preform by 0.2 g, and significantly improving the stress. Adjust preform temperature distribution according to gram weight. (3) Crystallinity of the preform (a) The crystallinity of the preform should be as low as possible. (b) If the crystallinity of the preform is high, the preform heating temperature should be increased accordingly when it is blown. (c) Ensure proper cooling of the preform and proper drying of the material. 3. Blow process adjustment (1) Stretching speed (a) Select the proper stretching rate to ensure that the end point of the drawing is correct. If the bottom of the bottom of the bottle is off-center or the body of the bottle is white, you need to set the stretching rate again. (b) Apply the stress-strain curve of polyester. (2) Tensile temperature (a) The preform heating temperature is selected to be within 25 to 30°C above the glass transition temperature (Tg) of the polyester and varies depending on the viscosity, moisture, copolymerization components, and grammage. (b) The higher the viscosity, the higher the stretching temperature. (c) The higher the moisture, the lower the stretching temperature. (d) The higher the copolymerization component, the lower the stretching temperature. (e) The preform temperature profile changes with grammage. 4. An example of optimization of process conditions for solving the stress cracking problem The following (Table 1-5) is an example of process optimization based on the above-mentioned ideas on a domestically-made machine (Chende CJ120 injection molding machine, with Da Yilong CP22 blow molding machine). Bottle types of bottles (including 500ml, 1.25l, 2l, only the first two listed here) were fully qualified (QSCRT) (new bottle QSCRT time exceeded 60min). In the case of nozzles with a heating opening of 60%, the barrel temperatures of the 1, 2, 3, and 4 sections of the 500 ml and 1250 ml carbonated beverage polyester bottles were 275, 280, 280, and 275°C, respectively; 6 , 7, 8, 9, 10 blowing temperature control parameters (voltage) are 0. The intrinsic properties of polyesters are fundamental factors that affect the stress crack resistance of products, such as crystallinity, intrinsic viscosity, and molecular weight distribution. Choosing the right injection molding and blow molding process conditions is very important to improve the stress cracking resistance of polyester bottles. The influencing factors and control methods of stress cracking resistance summarized in this paper have achieved satisfactory results in practice and have practical application value. And reference significance.
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