Sciencemadness Discussion Board

Recycled plastic - what happens to plastic that was degraded by UV?

RogueRose - 8-6-2019 at 06:56

I've been taking some plastic stuff apart that has been outside and it is probably more brittle than glass in some parts which are actually black - which is supposed to be best for UV protection & the item was meant to be used outdoors. As it broke into 1000's of pieces I didn't find out what kind of plastic it is but I'm guessing maybe something like ABS or ABS/PC or maybe ABS/PS.

What I'm wondering is if this was shredded and put into an extruder, what would be the quality of the plastic that comes out after the extrusion process.

I'm wondering if the UV breaks the plastic down on a molecular level to create some other compound, or if it just breaks physical bonds between the molecules. If the latter is true, then heating would re-establish the bonds and basically make it good as new - kind of like ice melting and then re-freezing it in some way.

So does anyone know how this works and what the end product is? If it is permanent degradation of the molecule, what are the new products and what are their physical properties - meaning how much would it effect the strength of the recycled plastic.

I'm wondering if this degradation is the reason there has to be virgin plastic mixed in and maybe why some plastic is only used for things that don't require consistent strength/qualities - like carpets or padding or plastic deck lumber.

Ubya - 8-6-2019 at 07:56

From wikipedia:
Quote:

Common synthetic polymers that can be attacked include polypropylene and LDPE, where tertiary carbon bonds in their chain structures are the centres of attack. Ultraviolet rays interact with these bonds to form free radicals, which then react further with oxygen in the atmosphere, producing carbonyl groups in the main chain. The exposed surfaces of products may then discolour and crack, and in extreme cases, complete product disintegration can occur.
Many organic chemicals are thermodynamically unstable in the presence of oxygen, however, their rate of spontaneous oxidation is slow at room temperature. In the language of physical chemistry, such reactions are kinetically limited. This kinetic stability allows the accumulation of complex environmental structures in the environment. Upon the absorption of light, triplet oxygen converts to singlet oxygen, a highly reactive form of the gas, which effects spin-allowed oxidations. In the atmosphere, the organic compounds are degraded by hydroxyl radicals, which are produced from water and ozone.[3]

Photochemical reactions are initiated by the absorption of a photon, typically in the wavelength range 290-700 nm (at the surface of the Earth). The energy of an absorbed photon is transferred to electrons in the molecule and briefly changes their configuration (i.e., promotes the molecule from a ground state to an excited state). The excited state represents what is essentially a new molecule. Often excited state molecules are not kinetically stable in the presence of O2 or H2O and can spontaneously decompose (oxidize or hydrolyze). Sometimes molecules decompose to produce high energy, unstable fragments that can react with other molecules around them. The two processes are collectively referred to as direct photolysis or indirect photolysis


30 seconds on Google are enough to find the answer to your question

RogueRose - 9-6-2019 at 06:51

Quote: Originally posted by Ubya  
From wikipedia:
Quote:

Common synthetic polymers that can be attacked include polypropylene and LDPE, where tertiary carbon bonds in their chain structures are the centres of attack. Ultraviolet rays interact with these bonds to form free radicals, which then react further with oxygen in the atmosphere, producing carbonyl groups in the main chain. The exposed surfaces of products may then discolour and crack, and in extreme cases, complete product disintegration can occur.
Many organic chemicals are thermodynamically unstable in the presence of oxygen, however, their rate of spontaneous oxidation is slow at room temperature. In the language of physical chemistry, such reactions are kinetically limited. This kinetic stability allows the accumulation of complex environmental structures in the environment. Upon the absorption of light, triplet oxygen converts to singlet oxygen, a highly reactive form of the gas, which effects spin-allowed oxidations. In the atmosphere, the organic compounds are degraded by hydroxyl radicals, which are produced from water and ozone.[3]

Photochemical reactions are initiated by the absorption of a photon, typically in the wavelength range 290-700 nm (at the surface of the Earth). The energy of an absorbed photon is transferred to electrons in the molecule and briefly changes their configuration (i.e., promotes the molecule from a ground state to an excited state). The excited state represents what is essentially a new molecule. Often excited state molecules are not kinetically stable in the presence of O2 or H2O and can spontaneously decompose (oxidize or hydrolyze). Sometimes molecules decompose to produce high energy, unstable fragments that can react with other molecules around them. The two processes are collectively referred to as direct photolysis or indirect photolysis


30 seconds on Google are enough to find the answer to your question


Thanks for the quote, but this is kind of what I stated to a degree. Just out of curiosity, what did you search, what was the term or page this comes from. I searched 5 of the sentences and got a different result for each one, none of which had the sentence in it's entirety.


I see that there is breaking of the polymer by O and O3 and what I didn't know is if that is the only breakdown that occurs, or is there other breakdown, maybe a "lower order" that doesn't break the molecular bonds of the polymer, but the "physical" bonds between them - which might be re-constituted upon heating. Do you see what I'm asking, the difference between the breakdowns, because the two would result in very different outcomes, especially when it comes to recycling.

Sulaiman - 9-6-2019 at 07:59

Collect said degraded plastic, melt, pour, see what pours and what is left behind.
Quicker than an internet search ?

Ubya - 9-6-2019 at 08:38

i searched for "plastic uv degradation", the first link is wikipedia's page "UV degradation".
Quote:

 what I didn't know is if that is the only breakdown that occurs, or is there other breakdown, maybe a "lower order" that doesn't break the molecular bonds of the polymer, but the "physical" bonds between them - which might be re-constituted upon heating. Do you see what I'm asking, the difference between the breakdowns, because the two would result in very different outcomes, especially when it comes to recycling.


i see what you are asking, but the answer is right there, UV photons have enough energy to break chemical bonds, so the molecules are not the same anymore. if the energy is enough to break covalent bonds surely is enough to break intermolecular bonds (hydrogen bonds, dipole forces, van der waals forces etc), so the polymer is degraded both ways, but just the fact that the covalent bonds are gone or rearranged should give you a quite definite answer, different molecule=different properties, melting it won't restore its original properties

SWIM - 9-6-2019 at 08:59

Quote: Originally posted by Sulaiman  
Collect said degraded plastic, melt, pour, see what pours and what is left behind.
Quicker than an internet search ?


But will that answer the right question, or will it just tell you that partially oxidized plastic is soluble in molten plastic?

Or that partially oxidized plastic won't separate from an emulsion in melted plastic because of the viscosity?

Sulaiman - 9-6-2019 at 09:39

That's what the OP needs to determine ...
is the plastic re-uable ?

RedDwarf - 9-6-2019 at 10:35

We have a tendency to think of a particular plastic (eg polyethylene) as a particular compound with a specific formula but most polymers are a mixture of compounds with varying levels of chain length, molecular weight and branching. Polymer properties are closely linked to these three properties and the degree of variation of the molecules in the melt.
When polymers degrade under uv radiation chain length, molecular weight and branching all change (I've read of decomposition products of PE including methane, carbon dioxide and ethylene from terminal breakage but some polymers molecules will also break into larger components). This means that what you end up with might well be able to be melted down and described as polyethylene but it would it no way have the same properties as the material it started off as. This doesn't mean it would be completely useless but it's use would probably be limited.

Ubya - 9-6-2019 at 12:51

Quote: Originally posted by RedDwarf  
We have a tendency to think of a particular plastic (eg polyethylene) as a particular compound with a specific formula but most polymers are a mixture of compounds with varying levels of chain length, molecular weight and branching. Polymer properties are closely linked to these three properties and the degree of variation of the molecules in the melt.
When polymers degrade under uv radiation chain length, molecular weight and branching all change (I've read of decomposition products of PE including methane, carbon dioxide and ethylene from terminal breakage but some polymers molecules will also break into larger components). This means that what you end up with might well be able to be melted down and described as polyethylene but it would it no way have the same properties as the material it started off as. This doesn't mean it would be completely useless but it's use would probably be limited.



perfect explanation

SWIM - 9-6-2019 at 14:42

Your explanation took me back to listening to Alec Guinness explaining how to make an unbreakable fiber that never wears out in that old movie, The Man In The White Suit.

The movie that gave me an interest in chemistry when I was very young.
And an interest in the economics of technological innovation.

I still sometimes think of the distinctive sound his equipment made when I see a complicated lab apparatus.



[Edited on 9-6-2019 by SWIM]