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Hot fill bottles: Opting for diversity

For PET bottles, the HR process accommodates variations in product, bottle size, weight, fill temperature, blowing rate, and more. Sidel has been developing an array of solutions to respond to these many different packaging criteria.

Mold air cooling: From 120 deg C to 130 deg C
Mold air cooling: From 120 deg C to 130 deg C

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Before they are consumed as fresh or frozen drinks fruit juices isotonic beverages and teas are very often packaged while hot. Hot filling actually eliminates the microorganisms that threaten both product quality and consumer safety. Long the exclusive domain of the glass bottle these drinks are more and more frequently found in PET bottles. The containers are lighter tougher and better suited to today’s on-to-go consumption.

Some powerful technology underlies this market growth. Starting at 60°C PET softens and loses its shape. Furthermore the package is subject to hydrostatic pressure exercised by the weight of the hot liquid on the walls. The container must then withstand conveying while it’s still hot. The neck must not become distorted during capping or when it comes in direct contact with the hot liquid. Finally the cooling phase that follows filling causes the liquid volume to shrink and generates vacuum inside the bottle. This is amplified by condensation of air in the head space. Given these obstacles a method must be found to considerably boost the package’s heat resistance to maintain its physical integrity.

Developing hot fill solutions is no easy matter and implies a keen knowledge of the properties of PET material.

PET – In all its Forms

PET is a semi-crystalline thermoplastic. Depending on its transformation state PET can be found in a totally “amorphous” or a “semi-crystalline” form. In the latter case its morphology is comprised of amorphous and crystallized phases. In its amorphous state PET’s molecular chains are not organized sort of like a big ball of yarn. Conversely in its crystallized state PET’s molecular structure is a dense and organized arrangement of molecular chains the origin of which is double.

Some crystals form due to spherulitic crystallization when heat is applied at around 150°C. When PET is not stretched crystals appear around impurities and catalytic residue. Spherulitic in shape these crystals develop a threedimensional structure composed of molecular chains folded over a nucleus. PET’s three transition temperatures They are relatively large (around 6 microns) compared to the wavelength of light. Thus they cause the material to lose its clarity. Some crystals develop due to induced crystallization when the molecular chains are subjected to mechanical stretching during bi-orientation blowing. When a certain orientation ratio is reached the molecular chains align according to directions of orientation that bring the nuclei close together. Links are created that lead to the formation of parallelepiped crystals. Their very small size (about 60 angstroms or 1 times smaller than the spherulitic crystals) doesn’t alter the material’s transparency. The structure obtained by stretching is 14 Blowing & Coating Blowing & Coating 15 similar to a weaving and it is much stronger than the amorphous structure. By modifying PET’s molecular structure spherulitic or induced crystallizations improve PET’s mechanical strength and thermostability.

In contrast the amorphous phase remains heat sensitive. Beyond a certain temperature it tends to resume its original shape due to the memory effect. Amorphous PET softens when it nears this glass transition temperature which is around 75°C. Bottles produced using traditional techniques tend to lose their shape starting at around 60°C which makes the containers unsuitable for hot fill applications.

Finally PET is hygroscopic meaning that it absorbs moisture from the air. At the molecular level water has a lubricating effect. Sidel has conducted studies that have shown that by taking a few simple precautions the effect of water absorption in preforms is negligible. In contrast the phenomenon can also occur during package aging and depending on the amount of water absorbed can reduce a container’s thermostability.

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