PET film and what orientation locks in

PET film is made from polyethylene terephthalate, a polyester. That word matters more than it looks. The chain carries rigid aromatic terephthalate units, joined through ester linkages and short segments derived from ethylene glycol. A polyolefin chain has none of that. It is plain carbon and hydrogen. The ring is the difference, and most of what PET does well in packaging follows from it.

The second thing that defines PET film is what is done to it after extrusion. Almost all PET packaging film is biaxially oriented PET, written BOPET. It is stretched in both directions and then heat set. Those two steps decide how the film behaves for the rest of its life. An unstretched PET film and a biaxially oriented one are the same polymer with very different properties.

PET film properties at a glance

These are published values for polyethylene terephthalate. Several of them move with orientation and heat setting, which is unusual. For most packaging polymers the processing changes the film. Here it changes the numbers you would call material properties.

Two of those rows explain the role PET plays in almost every laminate. It is the stiffest and most heat resistant of the usual candidates, and in its standard form it does not seal. So it goes on the outside and something else goes on the inside.

A polyester, not a polyolefin

Polyethylene and polypropylene are hydrocarbons. Their chains are carbon and hydrogen, they are non polar, and nothing on the chain attracts anything else. PET is different in one decisive way: its chain contains ester groups and benzene rings.

The ring is rigid, so the chain cannot coil and fold as easily as a polyolefin chain. The crystal structure work that established this attributed PET’s high melting point to that rigidity rather than to unusually strong forces between the chains. The ester group is polar, so the surface has a real affinity for polar things. Ink, adhesive and a deposited metal layer anchor more readily on PET than on untreated polyethylene, although commercial print, coating and metallising grades are often still treated or primed.

Two different properties get confused here, so it is worth separating them. Water absorption is what the resin does before it is melted, and it is a processing problem. Water vapour barrier is what the finished film does in service, and it is a packaging property. A polymer can be hygroscopic and still be a reasonable moisture barrier once it is a film.

PET glass transition and what orientation changes

This attribute separates PET from both polyolefins, and it works in the opposite direction to polypropylene. Polypropylene has a glass transition near the freezing point of water, so it turns brittle in the cold. PET has one far above ambient. At room temperature the film is already below its glass transition. That is part of why an unsupported PET film feels crisp rather than soft, though thickness, orientation and crystallinity all contribute.

The figure is not fixed, and what moves it is the interesting part. Measurements on unoriented PET put the transition around 82 °C. Oriented film measures higher, and one biaxial study reports a value near 100 °C. Work across seven differently drawn films assigns values spread between roughly 70 and 94 °C. There is no single glass transition for BOPET. The number depends on the grade and on the thermal history.

What the shift does not buy on its own

So stretching does not only make the film stronger. It stiffens the amorphous chains and raises the temperature at which the film starts to move. That shift is not on its own what buys heat resistance, and it is worth saying so plainly. Research on heat setting notes that the deformation temperature of biaxially oriented PET rules it out of many hot fill applications, because it shrinks when reheated towards its glass transition. What earns a place in a hot pack is the combination of orientation, crystallinity and heat-setting history, not the transition on its own.

BOPET: biaxial orientation, heat setting and thermal shrinkage

BOPET is film stretched in the machine direction (MD) and the transverse direction (TD). Stretching aligns the chains and induces crystallinity, which is where the tensile strength and the flatness come from. Left there, though, the film has a memory. Heated above its glass transition it tries to return towards where it started, and it shrinks by a few per cent, irreversibly.

Heat setting is the step that reduces that memory. The film is held under tension at an elevated temperature, so the oriented structure crystallises in place. Film treated this way is sold as heat-set or heat-stabilised PET. Published work on polyester films reports substantial increases in crystalline fraction, crystallite size and glass transition, with a significant gain in dimensional stability.

What heat setting does not do

Two limits are worth knowing, because both are routinely stated too simply. Heat setting reduces residual shrinkage. It does not abolish it. Work on drawn PET found shrinkage was fully suppressed only once crystallinity reached about 43 per cent, a level only annealing achieves.

The second limit corrects a common assumption. The onset of shrinkage does not simply move up to whatever temperature the film was set at. A study comparing polyesters found that PET began to shrink from around 80 °C regardless of the heat-set temperature, unlike PEN, where the onset did follow the treatment. Dimensional stability therefore belongs in a specification as a temperature, a dwell time and a tolerance. It is not implied by the name of the polymer.

Why standard BOPET needs a sealant layer

A sealing layer has to soften and flow within the temperature and the dwell time a packaging line can give it, then hold the product while it cools. Two terms describe that duty. The seal initiation temperature is the lowest jaw temperature at which a usable seal forms. Hot tack is the strength the seal has while it is still hot, before it has cooled.

Both depend on how much of the sealant is molten at the jaw. Classic work on polyolefin films found that seal initiation coincides with the point where the amorphous fraction reaches roughly 77 per cent. A heat-set BOPET web is the opposite condition. It is crystalline by design, it melts between 255 and 260 °C, and it has been stabilised precisely so that it does not move under heat. It is engineered not to do what a sealant does.

Sealable polyesters do exist

This is a statement about standard BOPET, not an absolute rule about the polymer. Polyester seal layers and seal coatings exist, and lidding films use them. But in a conventional flexible laminate the division of labour holds, and it is the whole logic of a duplex. The polyester carries the print, the stiffness and the heat resistance on the outside. A polyolefin carries the seal on the inside.

That pairing has a consequence the material itself cannot solve. Two polymer families in one structure typically complicates mechanical recycling, which is the trade-off set out on PET/PE laminates. The alternative routes, where one family does both jobs, are the all-PE and all-PP structures.

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