[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.

[verisimi]

> 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.

Not at all - I thank you. I am curious and want to understand. I simply don't accept counter-intuitive claims without good reason. I accept that such things as counter-intuitive claims exist. But, even so, I need to know how to verify things personally.

So, if I see plastics that are turned to dust after just 10 years, but another is saying that the dust continues to degrade for 500 years, I can consider the claim - but I want to know how it can be verified. Once you are talking about stuff you can't see or confirm, you are in thrall to experts. Unfortunately, when information or tools are not available to others, this provides cover of darkness for all sorts of poor ideas.

I can imagine all sorts of claims... but I'm after knowledge, not hearsay. And knowledge is not a communal endeavour - it is personal, based in personal verification. To me, even scientific claims are hearsay - you of course will know about the reproducibility crisis, and will no doubt have countless examples of how science presumed to know one thing, then changed position. Of course, this is the scientific method - nothing is set in stone. But science present monolithic conclusions that might be wrong. And its not like scientific endeavours cannot be steered by money.

The philosophical issue in play here relates to knowledge. What is it that knows - is it a group or an individual? Can it be that one part of science state this or that to be true, and everyone else must then uniformly accept that pronouncement? A pronouncement that cannot be verified personally? Even though we know that all science is the output of flawed humans? Etc.

Thanks for the exchange.

[littlestymaar]

It's good to be skeptical and keep an open mind on stuff even when there's a well accepted “truth”. But on the flip hand you should be cautious not to let it evolve into anti-intellectualism and complete mistrust in scientific output (especially because calls to common sense are a very common manipulation tactics).

Science is a social thing, and as such it's far from an ethereal ideal of knowledge production. But at the same time, it's the best tool we have by far…

Maybe re-reading my explanation tomorrow or another day can help you grasp the phenomenon better ;).

Have a nice day.