PET Bottles and Heat Sensitivity
PET bottles after blow molding, with the tool opened to remove the finished molding
Introduction
A question frequently asked of us as manufacturers of PET bottles is “Can you put boiling water in a PET bottle?”.
No, you cannot put boiling water in a typical PET bottle, as there will be both destructive and maybe even health consequences.
Srlon is very active in this market sector, and we see some quite confused answers to this question so we figured we would clarify it categorically. PET is a complex material and its behavior can be quite varied, for reasons that are easy to explain.
First the health risk, which is small but worth considering. PET bottles are not made to be used under hot conditions, so when exposed to boiling water, the material may leach chemicals into the contents.
Though increasingly unlikely in food-safe grades of the polymer, this would pose possible harm to people and cannot be ruled out. All normal grades are now considered safe, but caution dictates that packaging for use with cold foodstuff (dinks, for example) should not be exposed to hot use.
Many drink products are packaged in PET and sources in the developing world may use less regulated grades of material than similar suppliers in more developed markets.
Secondly, a typical PET bottle will shrink and distort very heavily, when exposed to boiling water, destroying the shape.
Srlon has an extremely experienced and well established team in the PET bottle manufacturing space, with 17 years spent developing, tooling and manufacturing bottles in this and other materials, so you can trust our knowledge.
To help you with a detailed understanding about some of the behaviors of PET and why hot water is a bad idea in these bottles, we are going to go through some details that will provide you with secure knowledge that allows you to make fully informed decisions;
- Why PET bottles cannot hold boiling water without damage – looking at the chemistry and atomic structure of bottles to explain this.
- What the physical changes in a typical bottle will be.
- Why heat might have serious chemical effects that make it a bad idea to expose PET bottles to excessive temperatures, particularly in food related applications.
All about PET and Bottles
PET is a polyester, polyethylene terephthalate. It’s a really useful plastic with many attractive properties;
- PET is strong, with high impact-resistance, so it’s suitable for products that need to withstand both impact and steady loads.
- It is lightweight, which reduces shipping costs.
- The material is highly chemical resistant.
- When processed in the right way, it can withstand quite high temperatures.
- It offers excellent barrier properties against gasses and liquids.
- PET is naturally highly transparent.
- It is very versatile, working well with most plastic processing techniques.
- It is highly recyclable into new, high quality products.
- PET is approved for use in contact with food by the FDA and EFSA.
It is made into a huge range of products in single use and packaging that you encounter every day. These are made by vacuum forming, injection molding, blow molding and rotational molding.
PET grades in developed markets are tightly controlled for food grade applications, with a considerable burden of testing carried by the manufacturers and processors. However, two issues can affect this certainty; one is lower security and lower cost supply chains, whose material may get into the packaging of products you buy. The other is recycled materials which can be of unknown origin. Typically, recycled materials are used in food contact after very rigorous controls, but that isn’t a guarantee, so a cautious mindset is a wise choice for you and yours to remain safe.
Why then are some PET food-packages sometimes considered as one-time microwave safe for heating food, yet not able to withstand boiling water?
Virtually all PET bottles are made by blow molding, whose processing conditions make the bottle behave a little differently in two regions – the neck/thread and the bottle itself. We will go into detail about the cause and effect of the material properties, below.
Processing variations
The most common of these manufacturing methods is the type you are familiar with, bottles with a complex neck that typically includes a precise and high quality screw thread. In this type the screw top and a straight tube of plastic are first injection molded and then a second stage is carried out, where the tube is reheated and the shape of the bottle is blow molded from the straight tube material, leaving the neck/screw thread mostly undisturbed.
This process is injection-blow-molding or IBM. When PET is injection molded, it tends to form highly crystalline structures that are much more temperature resistant than the finished product from the blow molding stage.
A typical PET preform, ready for blow molding
The injection part of IBM makes a relatively heat resilient and highly crystalline part, but when it is reheated for blow molding, the crystallinity is greatly reduced, lowering the maximum service temperature. To blow mold the bottle part, the material must be heated to begin to soften, before air pressure pushes it outwards to match the cavity tool. That re-heating resets the crystallinity and the stretching builds in huge internal stresses which are frozen in place to shape the bottle.
When you put boiling water into a PET bottle, it heats it enough to begin to soften, so all of the stress blown/frozen into it relaxes and the material reverts to a stress free shape.
This is why material of a PET bottle will soften and distort, when a single use injection molded PET food tray (when molded to be ‘Microwave Safe’ and labeled accordingly) can be specified/manufactured to withstand limited or single use microwave conditions without any appreciable softening and with no BPA leeching.
A typical injection molded food tray is much more heat resilient than a blow
molded bottle of the same material
Some Technical Details
Despite PET having a melting point above 250°C and maximum service temperature of 150°C for more crystalline PET parts used in air, injection-blow-molded PET bottles will undergo severe shape changes when exposed to boiling water.
This is a consequence of the blow molding using the stretching of partially softened material to make the shape, and the heat of blow molding being enough to remove virtually all crystallinity.
The glass transition is the start temperature for the onset of softening, typically 61-85°C for various grades of the polymer. PET has a very long range of not-yet-melted temperatures before it transforms into a liquid.
This wide temperature range is useful in processing terms, as it means the material is both tolerant of molding condition variations and also able to be tuned by processing, to deliver varied characteristics.
So when the injection molded blank for a bottle is made, it is quite crystalline and will stay reasonably firm in hot water. When it is reheated to blow mold the bottle, all that crystallinity vanishes and the bottle is blown to shape. That makes the bottle both highly internally stressed and fully amorphous (non crystalline), lowering its temperature tolerance.
So that’s why a limited number of PET single use food trays will survive one-time microwaving with no damage or toxic consequences, but a PET bottle will lose all structure in contact with boiling water.
Conclusion
On that basis, there are good reasons why PET bottles will not tolerate exposure to boiling water, and you now probably know enough to feel secure in having the answer as to why.
PET remains a great material, despite this restriction – and where it must tolerate higher temperatures, for example in appropriately labeled ‘microwave safe’ food containers, choosing a low stress and high crystallinity production process DOES allow PET to survive these temperatures.
If you want to know more about this, please contact us so we can help. We have experience and we value sharing this without customers, potential customers and interested parties.
