Plastic bottle cap liner question

Hello, recently my team are trying to recycle plastic bottle caps for the HDPE plastic

However, the bottle caps of carbonated drinks contain a liner which seals the bottle to prevent the high pressure contents from escaping

Does anybody know what material this liner is made of? It is rubbery and much softer than the plastic of the bottle cap itself, and it seems to be similar across the different brands of soft drinks and sparkling mineral water we find here, the only difference being that some are a lighter or darker shade of grey

Thus far I have been removing the liner by cutting the cap in half and then prying it off from the exposed edge with a sharp knife, it is a laborious process, and I wonder how do the other recycling groups who focus on bottle caps deal with this, as they are a big proportion of the plastic bottle caps we encounter


Hi @carmatic

Just a thought; do you have any proof that the caps are actually HPDE?  I was wondering if they might actually be made from polypropylene – with maybe a silicone liner to act as a gasket.

Looking at your photos, the moulding details on the cap look slightly too thin compared to HDPE caps that I see over here – which suggests that they might be moulded out of a stronger polymer. If that is so, then maybe the material used will not yield like HDPE, and it would need an additional gasket to form a seal.

Ha, throwing the good old Shake-spear-e into the mix.

I think E-prime is a rather good moot answer to the ‘to be or not to be’ conundrum.


A complete language without ‘is’ might be a bit too much, but reading every ‘is’ as ‘=’ would make anybody more alert of where proof might be required.


But we digress.


Answer to the topic:

The plastic bottle cap liner is most likely “a soft grade of PE”.

(and beware the ‘is’)


*the person doing all the questioning, in general, nothing personal

Thanks for the clarification 😉

Ever heard of E-prime ?

Ah yes.  Reminded me of Hamlet’s soliloquy rewritten in E-Prime.

“Existing or not existing. That constitutes the questioning. Whether we postulate the greater nobility of tolerating in our neuro-linguistic systems the slings and arrows of what we perceive as outrageous fortunes or to take arms, legs, thoraxes, et cetera, against a sea of troubles and by opposing end them…

As Shakespeare would say “that soundeth compleat bollocks”  😉



QED: Question Everything Done

People might find you* annoying, but you’ll outlive them all 🙂


Ever heard of E-prime ?


*the person doing all the questioning, in general, nothing personal 😉

@donald – silicone was a bit of a wild guess (just from the look of it) – but a soft grade of PE would make sense for the gasket.

I guess this shows that some “rules of thumb”, like “bottle caps are always HDPE” cannot be relied upon  🙁


I think @frogfall might be right.

They appear to be Russian CoCo-caps (looked up the number), which have become ‘thinner’ recently.


“Most caps are made of polypropylene (#5 plastic), with some (like sports drink bottles) composed of high-density polyethylene (#2 plastic)”  – earth911


The liner however may not be silicone:

“Commonly used with plastic screw caps. F-217 is a soft polyethylene foam core covered on top and bottom with solid, clear polyethylene supplied in 0.050 inch thickness. F217 has become the industry standard due to its all purpose compatibility, resilient, compressible seal, cleanliness (no pulp dust) and economy. F217 liners have excellent chemical resistance and a low moisture transmission rate. F217 has good taste and odor resistance.” – wikipedia


Have not found the ‘smoking gun’, absolutely confirming the materials, but these seem likely…