Solution to the stress cracking of the hottest PET

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The solution of PET bottle stress cracking

the packaging material is polyester, which undoubtedly plays an important role in beverage packaging. Among these packaging products, the packaging bottles of carbonated beverages (such as Coca Cola and Pepsi) have the highest quality requirements. One of the indicators is the stress cracking resistance of the bottles, which has great theoretical and application value for its in-depth research.

the production of polyester packaging bottles of carbonated beverages usually adopts two-step method. The typical production equipment is huskey injection molding machine in Canada and Sidel bottle blowing machine in France

first, preform making: PET chips are dried at about 160 ℃ for 5 ~ 6h, and then bottle preforms are made by screw melt injection molding. The screw temperature is 285 ~ 290 ℃, and the mold is cooled by cooling water. The water temperature is 8 ~ 10 ℃, and the cooling time is about 3.5s. After the bottle embryo is placed for 24h, it can be blown into bottles

bottle blowing: the preform is heated to about 110 ℃, and then formed into a bottle after stretching and blowing. The mold is also cooled by cooling water, and the bottom water temperature is 11 ℃

carbonated beverage polyester packaging bottles usually have several different capacities, such as 500ml, 600ml, 1.25ml, 2L, etc. the shape of the bottom of the bottle is quite complex, usually petal type. Its complex structure has become the stress concentration point of the product, and the location of cracking has occurred here, which has become the key point of quality control

I. requirements and test methods for stress cracking resistance of carbonated beverage polyester packaging bottles

the stress cracking resistance test of carbonated beverage bottles is called qscrt (quick stress cracking resistance test) in Coca Cola company. The process is as follows:

add a corresponding amount of citric acid (C6H8O7 H2O) and sodium bicarbonate (NaHCO3) into the bottle, fill it with water, cover the bottle, shake it well, and make citric acid and sodium bicarbonate react fully, The volume of carbon dioxide generated is equivalent to 4.3 times of the volume in the bottle. Next, put the above bottle into NaOH solution with a concentration of 0.2% (mass percentage) (the solution temperature is 24 ℃), and the NaOH solution immerses the bottom of the bottle. After 45min, none of the 10 samples tested can leak and cracks are allowed. In other words, if any of the 10 samples leaks within 45min, the test is unqualified. The fracture occurred near the injection point at the bottom of the bottle

the stress test time of newly blown bottles is required to be greater than 45min. With the extension of storage time, the air leakage time of bottles is shortened, and the stress test time is required to be greater than 20min after 1 month

Second, the mechanism of stress cracking

the microscopic process of polymer fracture can be attributed to the following three cases:

(a) chemical bond failure

(b) intermolecular slippage

(c) van der Waals force or hydrogen bond failure

the theoretical analysis of the breaking strength of these three destruction cases shows that: because the length of the actual polymer chain is limited, and there will always be more or less non oriented parts of the molecular chain, when the polymer is normally destroyed, the hydrogen bond or van der Waals force in the non oriented part will be destroyed first, and then the stress will be concentrated on the oriented main chain, although the strength of the covalent bond is 10 ~ 20 times greater than the intermolecular force, However, due to the small number of oriented main chains that directly bear external forces, there will eventually be the fracture of chemical bonds

one of the main reasons for stress is that during the forming process, due to the different cooling rates in the surface of the parts, the surface materials contact the mold wall with low temperature, and quickly cool and solidify into a hard shell, while the internal materials are still in the molten state. With its cooling and shrinkage, internal stress is generated in the parts. The stress distribution caused by environmental factors is usually irregular, and it often directly develops into environmental stress cracking

according to different environmental factors, environmental stress cracking includes: solvent crack, non solvent crack (including alcohol, wetting agent and other surfactant), thermal stress cracking, oxidation stress cracking, etc. Among them, the stress cracking test caused by solvent and non solvent has become an important method to study the internal stress and cracking resistance of polymers. For example, the stress cracking resistance test of carbonated beverage bottles (qscrt) is based on this principle

III. influencing factors and solutions of stress cracking of polyester packaging bottles for carbonated beverages

the influencing factors of stress cracking of polyester packaging bottles for carbonated beverages are quite complex. The author summarizes the influencing factors and corresponding control methods of polyester raw material quality and bottle making technology in the practice of production, development and after-sales service for many years, and puts forward ideas for solving this technical problem, which has strong pertinence and practical value, According to the influencing factors listed below, combined with the actual situation (such as different raw materials, models, bottle types and even weather, etc.), we made different process optimization. We gave guidance in the user's use process and achieved satisfactory results

I. polyester quality control

① control the crystallization rate of polyester

(a) modification by copolymerization: there are many technical know-how. The crystallization rate of polyester can be slowed down by selecting appropriate copolymerization components and addition amount. In practice, we have achieved obvious results by using meta acid and alcohol copolymerization components

(b) physical modification: use inorganic additives to improve the post-processing properties of polyester. There are many technical know-how. (c) The crystallinity at each position of the bottle bottom is the control point, which is related to the raw material formula

② clean production, improve the purity of polyester

(a) use finer filtration

(b) introduce air tightness detection

(c) modify the equipment structure to reduce dead corners

(d) control the content of small molecules in polyester

(E) control other impurities

③ viscosity control

(a) 0.86 ~ 0.88dg/l is appropriate

(b) the viscosity fluctuation should be less than 0.005dg/l

(c) choose medium to wide molecular weight distribution values

(d) control the range of solid-phase polycondensation

④ bottle shrinkage

the resistance to stress cracking of bottles with large shrinkage becomes worse, which is related to the design of raw material formula

⑤ control the content of diethylene glycol (DEG)

diethylene glycol directly affects the flow properties of polyester melt. The central value is controlled at 1.0% - 1.2%

II. Adjustment of injection molding process

① screw temperature: adjust the screw temperature according to the melting point, DEG content and the amount of copolymer components

② weight of bottle blank: control the weight of bottle blank by adjusting the injection molding process, reduce the weight of 0.2g, significantly improve the stress, and adjust the temperature distribution of bottle blank according to the weight

③ the crystallinity of bottle blank is bridging with peroxide

a) the crystallinity of bottle blank should be as low as possible

b) if the crystallinity of the preform is high, the heating temperature of the preform should be increased correspondingly during bottle blowing

c) ensure the correct cooling of bottle blank and the correct drying of raw materials

III. blow molding process adjustment

① stretching speed

a) select the appropriate stretching rate to ensure that the stretching end point is correct. If the injection point at the bottom of the bottle deviates from the center or the bottle body turns white, the stretching rate needs to be reset

b) use the stress-strain curve of polyester

② tensile temperature

a) the heating temperature of bottle blank is selected within 25 ~ 30 ℃ above the glass transition temperature (TG) of polyester, which varies according to viscosity, moisture, copolymer components and gram weight

b) the higher the viscosity, the higher the tensile temperature

c) the higher the moisture content, the lower the tensile temperature

d) the higher the copolymer component, the lower the tensile temperature

e) the temperature distribution of bottle blank changes with the gram weight

IV. examples of optimization of process conditions to solve the problem of stress cracking

the following (table) is an example of our process optimization on domestic machines (Zhende cj120 injection molding machine, equipped with daylong cp22 bottle blowing machine) according to the above ideas. Several bottle types (including 500ml, 1.25l, 2L, only the first two are listed here) have all passed the test (qscrt) (the qscrt time of new bottles exceeds 60min). Under the condition that the heating opening of the nozzle is 60%, the barrel temperatures of section 1, 2, 3 and 4 in the production process of 500ml and 1250ml carbonated beverage polyester packaging bottles are 2752800, 280275 ℃ respectively; The blowing temperature control parameters (voltage) of sections 6, 7, 8, 9 and 10 are all 0

the inherent characteristics of polyester are the basic factors that affect the stress cracking resistance of products, such as crystallinity, intrinsic viscosity and molecular weight distribution. The selection of appropriate injection 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, which has practical application value and reference significance in promoting the transformation of military technology to civilian use

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