[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.

[littlestymaar]

> 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.

They are all exposed to scissor hits all the time, but it's a stochastic scissor. You can think of every connection rolling many dices every second, and whenever they all land on 1 the molecule breaks. This kind of behavior leads to exponential decay, which is subject to half life.

> 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.

Why? You don't need to break all connections to break to dust. They are trillions of connections to break if you want to degrade the compound to its primitive molecule, but only thousands if you want to shred it to pieces. Keep in mind that if the monomer measures 1nm, a piece of plastic of 10um still has 10,000 connections. And if at the beginning you have a piece of plastic that's 10cm long (all reasoning are 1D here for simplicity), you just need to cut it 1000 times to get to 10um (so at 10um you're still much closer to the beginning than to the end, even though the plastic is now invisible).

> Or perhaps the metric is the case in artificial, sterile conditions, which the ground is not.

As I said before, it's a model and you're right it's not entirely accurate. Yet plastic doesn't have much enemies living in the ground, so it's pretty close to being sterile from the perspective of the plastic. And that's actually the problem we're facing right now.

[verisimi]

I still don't see why microbes or UV light or salt work in this linear process, where half lives come into play.

> Yet plastic doesn't have much enemies living in the ground

But this is the very point I'm making in my first post in this thread - that I found a fairly recent plastic bag (~10 years old) that was badly degraded.

Perhaps there aren't adverse conditions in the lab. Or perhaps the measurements are wrong? Perhaps these micro plastics are too small to see... But the info we are given should correlate with one's personal verification, I hope you agree.

[littlestymaar]

> why microbes or UV light or salt work in this linear process

It's not linear, it's exponential! And, as I said before microbes are special and are not well represented by half-life models, but they aren't too relevant for plastics (and that's the reason we call plastics ”non-biodegradable”)

> that I found a fairly recent plastic bags (~10 years old) that was badly degraded

I've spent the past two comments explaining why this happens, and why the plastic bag can appear degraded even though there's little degradation overall in the material itself.

It's OK that you don't understand the phenomenon, I'm probably at fault here for not explaining it well enough although I did my best. But if you don't understand it, please at least admit that it's what happening even if it sounds counter-intuitive to you.

It's not the lab conditions that are at fault here, it's your understanding of the mechanism and my limited abilities to explain them to you.

If you are curious I'm pretty sure you can find much better explanation on YouTube or elsewhere, as the internet is crowded with extraordinary science teachers. And if you aren't that curious (that's OK, life is short we cannot learn everything) then at least please admit the result and move on. Do not assume that “everybody else is wrong including polymer scientists, and microplastics don't exist because once I've seen a bag that was in bad shape”. It's just your intuition is just wrong, and that's OK.