Category: Microplastics

The Secret Plastic Liner in Your Drinks (and How to Avoid It)

11/10/2025

The Hidden Plastic Liner in Many Cups, Cans, and Bottles

(10 minute read)

Which Cups, Cans, and Bottles have Plastic Liners, and Which Ones Don't?

Many supposedly "eco-friendly" cups, cans, and bottles actually use a plastic liner on the inside. In addition to eventually ending up in landfills and adding to the growing microplastic problem, there are legitimate concerns over whether these plastic liners are healthy with long-term use.

In this post, we'll take a closer look to learn more about the plastic liners hidden in many common cups, cans, and bottles so you can make a more informed decision.

What's Next?

Part 1: Finding the plastic liner (experiment setup and recap)
Part 2: Learn whether this is even a problem from a health and environmental perspective
Part 3: Look into plastic-free alternatives

Video Review: Which Bottles have Plastic Liners?

If videos are more your thing, check out my Youtube video that covers most of the same information.

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Which Cups, Cans, and Bottles have Plastic Liners?

There are three types of popular containers that most people consider eco-friendly or plastic-free that actually have a plastic liner hidden on the inside.

Each of these has a plastic liner on the inside

Paper cups, like those commonly found at fast-food restaurants, gas stations, coffee shops, and public events
Aluminum cans, like those used for soda and beer
Aluminum bottles like Sigg

Experiment Setup

We're going to use a bit of science to find the plastic liner hidden in each of these containers. Drain cleaner contains sodium hydroxide, which reacts with raw aluminum to dissolve it, leaving the plastic liner behind. It also can saturate the paper in the paper cup, which eventually lets you peel it away.

What I used:

Three Plastic Containers: Purchased three inexpensive polypropylene plastic storage bins from the local dollar store. If you use a different type of plastic, do a little extra research to make sure the plastic will hold up well against the drain cleaner.
Drain Cleaner: Picked up two jugs, also from the local dollar store.
Sandpaper: Just one sheet is fine, and the coarser the better.
The Test Subjects: Used a Subway cup, but any paper cup should do. Same with the can. I used a Coke can, but any soda or beer can should work. Sigg is well-known as one of the best aluminum bottle companies, so I bought one of those off Amazon.

Total cost: ~$50 (mostly the Sigg)

Safety Notice!
Drain cleaner is a very caustic (basic) mixture that includes sodium hydroxide and sodium hypochlorate (bleach). These chemicals are very dangerous if used improperly. Always follow all safety requirements and recommendations from the manufacturer, including but not limited to gloves, safety goggles, and close-toed shoes.

Also, be sure to do this in a well-ventilated space. I did the experiment outside and placed a large plastic tote over everything when I was away from the experiment.

Sanding the aluminum can

First, prep the containers. The most important thing to do is sand the paint off the outside of the can. This lets the drain cleaner react more quiclky with the raw aluminum. I only sanded half of it because I was curious how much the paint resisted corrosion, but I'd recommend sanding it all the way around.

I also sanded the outside of the paper cup and aluminum bottle, but I'm not sure how much that impacted the results.

The paper cup does not have an outer plastic layer as far as I could tell, and my hope was that sanding it would break up the paper fibers some and quicken how fast they saturate and lose strength.

The bottle is made of anodized aluminum, and my hope was that the sandpaper would compromise the outer corrosion-resistant layer of aluminum to speed up the reaction.

Adding the drain cleaner

Next, center the cup, can, and bottle in the plastic containers.

Fill each with water or soda to prevent them from floating.

Then add the drain cleaner around the cup, can, and bottle, and set a timer.

Open the can and bottle to prevent pressure from building.

Making Anodized Aluminum (Warped Lab, YouTube)

Anodizing Aluminum (Warped Lab, YouTube)

What is Anodized Aluminum?

Anodizing is a process involving a caustic bath and electricity that makes a very thin outer oxide layer of the aluminum thicker and more corrosion resistant. To see how it works, check out this video.

See How it Works

The Hidden Plastic Liner in an Aluminum Can (90 Minutes After Starting)

Hydrogen gas bubbles

Here you can see a lot of bubbles coming off the can. These are hydrogen gas bubbles from the reaction between the sodium hydroxide and aluminum.

The aluminum is mostly dissolved

The aluminum on the sanded part of the can is almost completely dissolved. Pressing on the outside, you can see the folds in the plastic where the aluminum is completely dissolved.

Thin plastic film

Here's a section of the dark brown plastic film. It was very thin and easy easy to tear off.

At this point the cup looked saturated, but the paper still had a lot of strength to it. The aluminum bottle looked unchanged.

The Hidden Plastic Liner in a Paper Cup (4 Days After Starting)

Peeling off the paper

After four days the paper in the cup had broken down enough that I was able to peel it by gently using a wooden dowel. It came off pretty easily.

The plastic liner

Unlike the dark plastic liner from the aluminum can, this one is colorless and almost clear.

It's clear and fairly strong

It also felt thicker and much stronger than the aluminum can liner, taking significantly more force to tear it.

The Sigg aluminum bottle had started dissolving, but I let it go a few more days.

The Hidden Plastic Liner in an Aluminum Sigg Bottle (7 Days After Starting)

Corroded Sigg bottle

After 7 days, the Sigg bottle had dissolved enough around the bottom rim that you could easily see through it.

Brownish-gold plastic liner

I was also able to easily pry it open like a can opener. The brownish-gold color is the plastic liner on the inside.

Anodized aluminum and plastic layers

Here you can see the thicker aluminum sandwiched between the brown plastic liner on the inside and the thin layer of anodized aluminum on the outside.

Are Plastic Liners in Paper Cups, Soda Cans, and Aluminum Bottles Safe?

So now you've seen the plastic liner on these three containers, and it's a practical and inexpensive way for manufacturers to offer drinks. But the question is, is it safe? This is where it gets a little murky.

It's worth noting that all of the plastic liners you've seen in this video are classified as food safe. However, plastic containers have a long, troubled history with our health, usually after research over decades indicates that certain chemicals commonly used in plastics can have negative effects on your health.

BPAs and Phthalates

BPA and Phthalate health risks (Wikipedia and a Scientific Article)

BPAs and phthalates were all in the news in the late 2000s because of their effects on hormone regulation in your body, and that's what fueled the public's transition away from reusable plastic bottles like Nalgenes to stainless steel ones like Klean Kanteens.

Sigg BPA Controversy (Spokesman-Review)

Interestingly, Sigg was a haven at first for many Nalgene refugees, but then it came out that the company had used misleading marketing that made many loyal customers believe it was BPA-free when in fact it wasn't. And they only changed their formulation around 2008 after a lot of public backlash.

PFAS and Microplastics

PFAS health risks (ConsumerNotice.org)

More recently the spotlight is on PFAS and microplastics.

PFAS usually deals more with non-stick coatings like Teflon and isn't as relevant for bottles as far as I've read. You'd actually be more likely to have PFAS in your water supply, and you can see my deep dive into that topic in this other video.

The plastic liners, which eventually would break down into microplastics

Microplastics on the other hand are almost definitely a problem with any of these because just think about what happens when you throw them away. The paper will probably dissolve relatively quickly while the aluminum will take much longer to corrode, but either way you still have a plastic liner.

I suspect it will eventually break down into smaller and smaller pieces, contributing further to the microplastic problem in the environment. Multiply that by every restaurant, gas station, and all the other places you find them, and that's a lot of plastic being thrown away in cups, cans, and bottles that most people think are plastic free.

Which Bottles Avoid Plastic Liners

So what can you do?

When it comes to disposable paper cups and aluminum cans, it's best to avoid them as much as possible. You could recycle them, but the inefficiencies and issues with recycling programs in general could be the topic of a much longer post. The sad fact is that many things we think are recycled end up just being thrown away.

Case in point, the Subway where I got the cup for this experiment has the typical trash can near the door that's inside a cabinet. There are two holes on top, one for trash and one for recycling. But if you look inside, both holes drop into the same trash bin.

Bottles without Plastic Liners

The best options that avoid plastic liners are stainless steel and glass bottles.

Stainless steel and glass bottles avoid plastic liners

No stainless steel bottles I'm aware of have a plastic liner, so going with popular brands like Owala, YETI, Stanley, or Hydro Flask is a great option. Unless you have a nickel allergy, it's a really safe bet.

Glass does not contain plastic at all, and there are a bunch of those to choose from including Lifefactory, Ello, and W&P Porter.

Steel Straws for Owalas

Stainless Steel Straw Alternatives
If you get a bottle with a plastic straw, consider swapping out the standard straw for a stainless steel alternative. You can usually find replacements for most popular water bottle brands on Amazon, including this one for Owalas. They're usually from 3rd party brands.

JustOneBotl (Glass and Steel)

Combination Glass and Stainless Steel
One interesting option is JustOneBotl, which has a glass bottle that and a stainless steel bottle. You can use them separately, but the cool thing is the glass bottle fits inside the steel one.

It's the only options I've seen like this, and it lets you drink from glass while also having vacuum insulation.

Which Bottles Avoid Plastic Under the Lid?

If you really want to go all out and limit plastic contact with your water as much as possible, you could also look for a bottle with stainless steel under the lid.

Healthy Human with steel under lid

Some of the better stainless steel bottle options I'm aware of include Healthy Human, Ecovessel, and some Klean Kanteen bottles.

Hydro Flask's steel lid

Hydro Flask also makes a stainless steel version of their Flex Cap.

Mayu glass bottle with steel under lid

For glass, one I've seen for awhile is Mayu, but it looks like there are a lot more options on Amazon now

Unfortunately, this seems to be a growing sub-product niche that hasn't gone mainstream yet, and I've seen quite a few brands and options with stainless steel under the lid that have come-and-gone. Some of the links above were out of stock when I wrote this article, but I'm hoping they'll be back in stock soon so I still left the links in so you can check them yourself.

Hope this helps, and happy hydrating!

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Doing This May Remove 80% of Microplastics from your Drinking Water

3/3/2024

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There is a growing level of concern and awarness about microplastics in water, but a new study suggests that boiling water may remove many of these tiny particles.

What are Microplastics and Nanoplastics?

If you have not heard of microplastics in the past, the National Oceanic and Atmospheric Administration gives a quick recap here. (or just google "what are microplastics" and it should come up near the top of the list).

Essentially, microplastics are small pieces of plastics that are less than 5 millimeters long. They are caused when small plastics are washed away or discarded, and they can also be created when larger plastics break down over time. The widespread use of plastics combined with the difficult of filtering these small pieces has caused to them to become increasingly prevalent and widespread in our drinking water, lakes, and oceans.

And as you might expect, nanoplastics are similare but are even smaller.

Some microplastics with common examples include:

polystyrene
- foam-forms like in insulating coolers and cups, as well as packing peanuts
- hard forms include items like some disposable razors and CD cases
polyethylene (the most commonly produced plastic)
- "HDPE" (high-density polyethylene) is used to make many common containers, like milk jugs and detergent bottles, as well as industrial-size products like municipal water lines
- microbeads that are used as exfoliants in some health and beauty products, which were thankfully banned as part of the Microbead-Free Waters Act of 2015)
polypropylene (2nd most commonly produced plastic)
- found in many food-grade products (including many water bottle lids)
- some clothing (though polyester has become a common replacement)

Does Boiling Water Remove Microplastics?

The experiment is titled "Drinking Boiled Tap Water Reduces Human Intake of Nanoplastics and Microplastics." According the the abstract, the authors claim to:

"present evidenence that polystyrene, polyethylene, and polypropylene NMPs can coprecipitate with calcium carbonate (CaCO3) incrustants in tap water upon boiling. Boiling hard water (>120 mg L–1 of CaCO3) can remove at least 80% of polystyrene, polyethylene, and polypropylene NMPs size between 0.1 and 150 μm. Elevated temperatures promote CaCO3 nucleation on NMPs, resulting in the encapsulation and aggregation of NMPs within CaCO3 incrustants."

Essentially, this means that boiling water causes the plastics to combine with calcium cabonate (aka the "hard" in "hard water" that is in most people's homes), then the combined particles then drop to the bottom, leaving a layer of cleaner water on top.

Visualization of the Precipitation of Microplastics ("NMP") and Calcium Carbonate (credit to the authors Yu, Wang, Liu, Li, and Zeng)

I do not have full access to the article, but I would expect two followup questions should be considered that weren't covered in the abstract.

Does the water need to be filtered after boiling (likely), and if so, what type of filter would be effective to remove the precipitate?
How does this compare to other filtration methods, such as reverse osmosis?

Hydration News with BottlePro

At BottlePro, we keep an eye out for hydration and water bottle news, then we summarize it and post it here. Check back with us to stay in the loop!

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The Secret Plastic Liner in Your Drinks (and How to Avoid It)

Which Cups, Cans, and Bottles have Plastic Liners, and Which Ones Don't?

What's Next?

Video Review: Which Bottles have Plastic Liners?

Which Cups, Cans, and Bottles have Plastic Liners?

The Hidden Plastic Liner in a Paper Cup (4 Days After Starting)

The Hidden Plastic Liner in an Aluminum Sigg Bottle (7 Days After Starting)

Are Plastic Liners in Paper Cups, Soda Cans, and Aluminum Bottles Safe?

Which Bottles Avoid Plastic Liners

Doing This May Remove 80% of Microplastics from your Drinking Water

What are Microplastics and Nanoplastics?

Does Boiling Water Remove Microplastics?

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