[verisimi]

You know, this is simply not true.

I have found old buried plastic bags, from supermarkets - I remember the bag style from just a few years ago. The bags had severely degraded. When I tried to pick one up, it fell apart into small pieces, what integrity it had was gone. I've had the same experience with bags left in lofts - they degrade.

From my personal experience, I therefore assume that plastics already disintegrate after about 10 years, not 100 or 500 years, as you state.

[hombre_fatal]

You're just talking about a bag degrading into smaller and smaller plastic particles (microplastics) while the people above are talking about biodegrading into natural elements.

[verisimi]

You think I'm unfair in comparing a plastic bag to plastic packaging? If you follow this particular thread, they were talking about packaging.

[littlestymaar]

No, you're just confusing macroscopic level of degradation (the whole structure degrades) and microscopic degradation (molecules are being degraded).

The problem with plastic is that while the macroscopic structure can be altered in just a few years (depending on the conditions), the resulting parts aren't being metabolized away by micro-organisms and they remain as small plastic chunks, and then micro-plastic, then nano plastic, until they eventually break down entirely after decades, which is very unlike what happens with what we call biodegradable materials.

[verisimi]

So, in your view, although the bag I see is breaking up into small brittle pieces, there are even smaller pieces that are not been degraded? Is that a theory to you, or do you have first hand knowledge?

If I have understood your position correctly, this is certainly counterintuitive... because if there is not much plastic bag left, you'd think that whatever broke down the bag, would also be able to break down these smaller bits (microplastics). But you are saying that small bits of the bag remain for decades, even though I can't see them.. I just don't see why small bits of plastic bag would remain, when it is evident that something in the ground already degraded the plastic bag as a whole.

[littlestymaar]

> So, in your view, although the bag I see is breaking up into small brittle pieces, there are even smaller pieces that are not been degraded?

They are being degraded, just slowly.

> Is that a theory to you, or do you have first hand knowledge?

It's not a theory of mine, you can find the explanation in practically any explanation of what "microplastics" are.

> If I have understood your position correctly, this is certainly counterintuitive... because if there is not much plastic bag left, you'd think that whatever broke down the bag, would also be able to break down these smaller bits (microplastics)

That's correct, and it eventually happens, the process is just very slow, let me try explaining it to you:

Plastics are made of very long polymer chains, composed of the same small molecule (monomer) chained together millions of times. Let's assume we have a plastic for which half[1] of the connections break down in 50 years. If I'm not messing the math up, it means that every year you lose roughly 1% of the connections between the molecule. So, after one year, your chain of 1 million molecules have been cut in 10,000 smaller piece of 100 monomers. Then the next year, there's again 10,000 connections that will break (~1% of the remaining 990,000 connections), but this time it will just double the number of pieces (if you cut a piece 10 times, you and up with 11 pieces, if you have again 10 scissor hits on the next 11 pieces, you'll end up with 21 pieces).

Of course my model is not entirely accurate because I looked only at the 1D molecular structure, when plastics are actually 3D meshes of these chains, so multiple chains are holding one another and it slows down the macroscopic effect, but you should get the idea of why it first degrades relatively quickly at macro scale and then slower at smaller scale.

[1] half times are a good way to model degradations of chemical compounds, at least the “spontaneous” ones, but keep in mind it's a model: it's not flawless and it's not able to describe every degradation phenomenon (for instance it fails to describe degradation from microorganisms that can grow on a substrate they are degrading hence the “spontaneous” qualifier above).

[verisimi]

Thank you for taking the time to explain.

I get how these connections are meant to break down, though not to the detail you provided. It could be like you say, though I don't why the degradation wouldn't be concurrent - ie I don't get why half times are a good way to model plastic degradation, when all of the plastic is exposed to 'scissor hits' all of the times. Ie, once the scissor hits start to impact the bag, I would think the bag would soon fully disintegrate.

Assuming it is like you say, if a plastic bag is brittle, going to dust after just 10 years after being in the ground, (I'm using supermarket bag design as a means to figure out the age of the bag,) that is far from 50 years to break down half the connections. This is too say the metric to measure degradation seems wrong to me... Or perhaps the metric is the case in artificial, sterile conditions, which the ground is not.