Application of PET in the Beverage Packaging Industry

The application of PET in the beverage packaging sector has significantly propelled the rapid development of the beverage packaging industry. Concurrently, the growth of the beverage packaging industry has also provided expansive opportunities for the application of PET. Strictly controlling the PET injection molding and blow molding processes is key to ensuring the appearance and cost-effectiveness of PET bottles.

Characteristics of PET

PET is the condensation product of ethylene glycol and terephthalic acid, classified as a saturated thermoplastic polymer. PET molecules can exist in linear and semi-crystalline states.

The simplest production process for PET involves the reaction of terephthalic acid with ethylene glycol to form monomers (esterification), followed by polycondensation into the long-chain polymer PET. The degree of polymerization varies with temperature and pressure.

Like many plastics, PET undergoes three state transitions during processing: the glassy state, the high-elastic state, and the viscous-flow state. This involves three key temperature transitions: the glass transition temperature (Tg), the crystallization temperature (Tc), and the melting point (Tf).

The transition from the amorphous glassy state to the rubbery state is called the glass transition, indicating the initiation of long-chain segment movement. External heating increases the freedom of molecular motion; molecules frozen in the glassy state can now move. The glass transition depends on the morphology of PET. When the intrinsic viscosity (IV) is high, crystallization is more pronounced, molecular chain freedom is restricted, and Tg is higher.

As temperature increases, local spherulites gradually form, leading to the rearrangement of local molecular chains due to intermolecular forces, i.e., crystallization. For PET, the maximum crystallinity is about 55%. This limit is caused by the slow rearrangement of aromatic rings, which hinders the formation of crystalline regions.

If T < Tc, the viscosity of PET hinders chain segment movement towards order (preventing crystallization); if T > Tc, thermal effects hinder the formation of amorphous regions (promoting crystallization).

The melting point (Tf) is the temperature at which all crystals disintegrate.

Hydrolysis of PET

Solid PET is highly hygroscopic and readily absorbs moisture from the air. During storage, PET will absorb moisture until it reaches saturation with the environmental conditions. The saturation value can be as high as 0.6% by weight.

Typically, PET shipped from suppliers has a moisture content below 0.1% by weight. To achieve optimal product performance, it is necessary to reduce the moisture content to 0.004%, preferably below 30 ppm before melting.

Even very low moisture content in the resin can trigger a series of reactions:
When the temperature exceeds the PET melting point (approximately 250°C), water can rapidly cause polymer degradation (chemical chain scission due to hydrolysis), thereby reducing molecular weight, apparent viscosity, and related physical properties. In fact, hydrolysis begins at lower temperatures (e.g., 150°C) but at a slower rate, which increases with rising temperature. Under drying and molding conditions, the decrease in IV should not exceed 0.02 dl/g. An excessive drop in viscosity leads to increased crystallization rate, adversely affecting the transparency of the preform and resulting in decreased mechanical properties of the bottle, such as load-bearing strength and impact strength.

Thermal Degradation of PET

The influence of temperature on drying PET is complex. It affects not only the diffusion rate of moisture but also the chemical processes during drying, ultimately impacting resin properties. Considering potential hydrolysis and thermal processes is essential. As mentioned, the rate of hydrolysis accelerates above 150°C accompanied by a drop in IV. Because thermal transition processes are faster than diffusion processes, prematurely increasing the temperature during drying is detrimental.

Similarly, even if most moisture can be removed, excessively high temperatures (e.g., above 180°C) will lead to thermal degradation and thermal oxidation (in air drying systems). This causes polymer chain scission and the release of by-product substances, leading to decreased physical properties.

By-products include AA (acetaldehyde) components. Changes in physical properties manifest in the preform as hazy crystallization, decreased IV, and yellowing of the product.

Molding of PET Preforms

During the preform molding process, the ideal conditions are: achieving complete, rapid, and uniform melting at the lowest possible temperature and in the shortest possible time, minimizing the decrease in IV, generating as little AA as possible, and ensuring maximum transparency. Related process conditions include:

1. Temperature
   Molding temperature refers to the temperatures of the barrel and hot runner. During molding, only 30% of the heat comes from external heating, while 70% comes from internal friction heat. Therefore, besides appropriate heating, shear heat must be utilized effectively.

2. Injection and Holding Pressure
   Injection overcomes resistance in the flow channel to fill the mold with melt. For preforms, it’s best to have three stages of speed and pressure, decreasing sequentially.
   Injection speed too slow: insufficient shear, cooling before complete fill, resulting in incomplete filling or short shots. Too fast: inadequate venting in the mold cavity, leading to incomplete filling, sink marks, high AA.
   Holding pressure has two important functions: preventing melt backflow and ensuring cooling under pressure (improving cooling effectiveness). Too high: over-packing and mold deformation, higher internal stress, potential crystallization. Too low: sink marks, preform deformation (insufficient cooling), gate issues like pinholes, bubbles due to decreased cooling rate at the gate. Holding time must also be appropriate; too short can cause pinholes, stringing, etc.

3. Pressure Release
   Pressure release reduces pressure within the hot runner, preventing gate blockage and needle valve malfunction. Excessive release can cause sink marks, stringing, pinholes, etc.

4. Back Pressure
   Back pressure is the kneading force applied to the melt by the screw via the hydraulic system during screw rotation driven by the oil motor. Functions: enhances PET plastification, eliminates bubbles. Can be set to 0 at startup, then gradually increased after preforms are being produced stably. The suitable back pressure is when preforms are free of bubbles or blemishes. Excessively high back pressure leads to strong shear effects, causing molding defects, gate blockage, thermal degradation, etc.

5. Cushion
   The cushion is the residual amount in front of the screw after each injection shot. Too little causes molding defects; too much causes PET degradation. Generally, adjust gradually from small to large; the suitable amount is when preforms are not hazy or crystallized.

6. Cooling
   PET is opaque. Preform transparency relies on cooling. Poor cooling reduces the preform cooling rate, leading to sink marks, preform deformation, and affecting cycle time. To avoid this: perform water treatment, regularly clean water channels, check water flow and pressure, clean cores and cavities, etc.

Blow Molding Process

The blow molding process is a biaxial stretching process. During this process, PET chains undergo biaxial orientation, extension, and alignment, thereby enhancing the mechanical properties of the bottle wall, increasing tensile, tensile, and impact strength, and providing excellent gas barrier properties. Although stretching improves strength, over-stretching should be avoided. Control the blow-up ratio: radial ratio should not exceed 3.5–4.2, axial ratio should not exceed 2.8–3.1. Preform wall thickness should not exceed 4.5mm.

Blow molding is conducted between the glass transition temperature and the crystallization temperature, generally controlled between 90–120°C. Within this range, PET exhibits high elasticity. Rapid blow molding and cooling set the shape into a transparent bottle. In one-step machines, this temperature is determined by the cooling time during the injection process (e.g., Aoki blow molding machines), so the coordination between injection and blow molding stations is crucial.

The blow molding process involves: stretching – primary blow – secondary blow. These three actions occur in a very short time but must be well-coordinated, especially the first two steps, which determine the overall material distribution and the quality of the blown bottle. Therefore, adjust: stretch start timing, stretch speed, pre-blow start and end timing, pre-blow air pressure, pre-blow air flow, etc. If possible, it’s best to control the overall temperature distribution of the preform and the temperature gradient between the inner and outer walls of the preform.

During rapid blow molding and cooling, induced stress is generated within the bottle wall. For carbonated beverage bottles, this stress helps resist internal pressure and is beneficial. However, for hot-fill bottles, it must be sufficiently released above the glass transition temperature.

Common Problems and Solutions

1. Thick top, thin bottom: Delay pre-blow timing, or reduce pre-blow pressure and air flow.

2. Thick bottom, thin top: Opposite of the above.

3. Wrinkles below the neck ring: Pre-blow too late, pre-blow pressure too low, or this part of the preform is insufficiently cooled.

4. Whitening at the base: Preform too cold; over-stretching; pre-blow too early or pre-blow pressure too high.

5. ‘Magnifying glass’ effect at the base: Too much material at the base; pre-blow too late, pre-blow pressure too low.

6. Wrinkles inside the base: Base temperature too high (poor cooling at the gate); pre-blow too late, pre-blow pressure too low, air flow too small.

7. Entire bottle hazy (opaque): Insufficient cooling.

8. Local whitening: Over-stretching, local temperature too low, pre-blow too early, or contact with the stretch rod.

9. Eccentric base: Could be related to preform temperature, stretching, pre-blow, high-pressure blow. Lower preform temperature; increase stretch speed; check gap between stretch rod tip and base mold; delay pre-blow, reduce pre-blow pressure; delay high-pressure blow; check if preform is eccentric.

Conclusion

1. PET bottles are widely used in daily consumer goods. Furthermore, due to their light weight, excellent preservation properties, and technological trends emphasizing features like heat resistance and pressure resistance, PET bottles have become the mainstream packaging for beverages today. Many beverages requiring high-temperature sterilization for filling, such as flavored water, juice, and sports drinks, are also increasingly using PET bottle packaging.

2. PET bottles, due to their low environmental impact and low energy consumption, are gradually replacing traditional packaging materials in today’s environmentally conscious world. Possessing properties like heat resistance and pressure resistance, they are recently replacing various PVC bottles/packaging, aluminum cans, tin cans, glass bottles, etc., becoming the packaging material with the greatest growth potential.