A material speciality, the chemical name of PET is polyethylene terephthalate. Since the first PET bottle was produced by DuPont in the United States in 1970, it has been widely used due to its excellent physical properties. Especially in the packaging of carbonated beverages, juice drinks and tea beverages, it is growing at a double-digit rate every year. This article discusses the blowing principle and process of PET beverage bottles.
1 PET Features and Performance
PET is a saturated thermoplastic polymer that is formed by esterification of terephthalic acid and ethylene glycol. There are three key temperature points in the heating process of PET. These three temperature points divide PET into four states. Each state represents the different arrangement of PET molecular chains and also determines the different properties of PET.
1.1 Glass transition temperature
The transition from a highly elastic state to an amorphous glass state is called glass transition. The temperature at this time (PET is about 80°C) is called the glass transition temperature, which reflects the movement of long molecular segments.
1.2 Crystallization temperature
As the temperature continues to increase and reaches 160°C, multiple molecular chains with intermolecular interactions undergo local rearrangements, producing spherical crystals. Because the rearrangement of the benzene ring in PET molecules is slow, the maximum degree of crystallization of PET is about 55%, and it can produce an orderly crystalline area of ​​cattle.
2 Biaxial stretching of PET bottles
After heating, the preform in the initially amorphous state becomes highly elastic, just like rubber. After biaxial stretching, macromolecular chains are oriented in the stretching direction to produce crystals. After blowing the bottle, the material becomes hard.
Since the stretching will eventually exceed the strain-enhancing limit to obtain a solid-state response, induced crystallisation will occur and the wall thickness of the bottle will be uniform. Once the strain-enhancement limit is approached, the strain increases in exponential form. In fact, the onset of strain hardening depends on the maximum strain value.
Cylindrical preforms are more easily stretched in the radial direction than in the axial direction, ie the intrinsic strain rate in the radial direction is greater than the intrinsic strain rate in the axial direction. Lead to preferential orientation in the axial direction. The axial orientation depends on the intrinsic viscosity of the material.
3 PET preform self-regulation
The stress distribution on the preform causes an orthotropic extension of the parts of the preform. This expansion is determined by the stress-enhancement factor.
Under the joint action of the stretching rod and the high-pressure gas, when the preform begins to deform, the weakest link is the hottest or thinnest wall, where the deformation begins first. When the strain-enhancing limit is reached, the intensity increases locally because of the induced crystallization.
Once the strength of the deformed area exceeds the strength of the undeformed area, the undeformed area begins to deform along the moving "bubble boundary".
This expansion is called self-regulating. Although the self-regulation only works when the strain enhancement limit is reached, the thickness of the bottle wall is controlled.
4 Biaxial orientation of industrial PET bottles
Plastics with the following conditions can be biaxially stretched under certain conditions.
1) Material properties
The material of the preform must be disordered (low crystallinity) to ensure proper orientation during biaxial stretching. In addition, since the highly viscous polyester has higher anisotropy (orthotropy orientation) before the critical stretch limit (before degradation occurs) under the same conditions, the intrinsic viscosity of the material must exceed the orientation requirement. value. In fact, the choice of intrinsic viscosity is based on the end use of the bottle. Highly viscous polyester (0.80 to 0.85) with good mechanical properties (creep) for blowing carbonated beverage bottles. For airless beverages such as mineral water, low viscosity polyester (0.70 to 0.78) is sufficient.
2) Geometry: The preform has a specific biaxial orientation ratio
The geometry of the preform contains the dimensions of the preform controlled by the intrinsic viscosity of the PET, the shape of the bottle, and the end use. In fact, the preform is determined by the biaxial elongation and the final wall thickness.
The intrinsic strain rate increases the internal stress. Internal stress tends to offset the expansion of the interior of the bottle. For carbonated beverage bottles, the internal stress offsets the internal gas pressure of the bottle, which is beneficial and should be increased as much as possible. On the other hand, for hot bottle bottling, in order to reduce the deformation of the bottle during cooling (negative pressure), it must be reduced as much as possible.
3) Temperature: The biaxially oriented temperature conditions of the semicrystalline material are: 1 above the glass transition temperature to obtain ductility allowing orientation. 2 below the crystallization temperature to avoid the formation of spherulite nuclei that impede orientation. The biaxially oriented PET temperature ranged from 90 to 120°C. For a defined biaxial stretching rate, the biaxially oriented temperature is mainly determined by the intended use of the final product. For carbonated beverage bottles, the temperature range is 90-100°C to increase induced stress. For hot cans, the temperature range is 110-120°C. Although the intrinsic strain rate is reached, the induced stress is also limited.
4) Stretching speed: The stretching speed must be very fast (500-1500 mm/s). To prevent de-orientation during stretching.
5) Cooling: After stretching, when the material cools below the glass transition temperature, the induced stress caused by rearrangement of the PET molecules is "frozen" in the wall of the bottle. This is beneficial for carbonated beverage bottles.
For hot fill bottles, the temperature must be maintained above the glass transition temperature to maintain the molecular orientation and relax the induced stress (f4d until it disappears). During this time, additional static crystallization (25% to 35%) was generated, strengthening the structure.
5 PET bottles
The main consideration of the bottle design is that drink sellers try to regard product packaging as a part of marketing, in addition to being limited by process and materials:
5.1 bottle design
Stretch blow molding techniques include issues related to bottle design, such as preventing dishing and localized elongation problems that may reduce the final mechanical properties of the bottle.
Should avoid sharp corners and wall thickness changes. In the transition section, stress concentration can be avoided by using a large corner radius.
5.2 items
The contents of the bottle are beverages or airless liquids; there are gas barrier performance requirements.
The bottom design must be considered for carbonated beverages or airless liquids. The petaloid bottom is used to withstand internal stress and induced stress. A variety of styles can be selected for the bottom of the bottle containing the airless liquid.
For the design of the bottle with the gas barrier performance, in addition to considering the barrier performance, consideration should be given to maintaining the taste and hygiene of the food after long-term storage.
For example, beer bottles must block UV rays and block oxygen and carbon dioxide infiltration (leakage). In fact it is possible to make multi-layered (EVOH) amber or green bottles.
5.3 filling method
Filling different ways, there are special technical requirements for the bottle. When using hot filling, the bottle must withstand negative pressure without losing strength.