Answering questions on polyethylene (1 Viewer)

[nightweaver066]

Say for these questions,
Explain how the structure and properties of polyethylene and polystyrene relate to the way each is used
Outline the chemical and physical processes involved in the production of a polyethylene bottle from a natural raw material

how would i answer them in relation to polyethylene? Would i talk about HDPE or LDPE? If neither, i'm not too sure how to generally talk about it.

Need some guidance...

 

[Kimyia]

For the structure/properties/uses question on polyethylene, I (personally) would talk about both HDPE and LDPE, their differences in structure (branching) and flexibility/rigidity/hardness and relate them to their differences in uses (LDPE: shopping bags -vs- HDPE: rubbish bins)
I think the second question would relate more to LDPE than HDPE but I'm not sure. If it was LDPE, then you'd mention the peroxide initiator, the high temp and high pressure.
Hope that helps maybe a little

 

[Carrotsticks]

Make sure when you're relating property to use, you actually explain it in detail.

Bad Answer: LDPE can be made to be flimsy (since there are different 'strengths' of LDPE), hence it is used for plastic bags.

Good Answer: The process of making LDPE creates a large amount of branching, which disallows an optimal amount of intermolecular bonding to occur between carbon chains. This results in it being less rigid (to the point of being flimsy in this case) and lightweight, whilst maintaining enough strength to support objects up to about 5-10kg. Hence, it is used for plastic bags.

 

[angelarr]

Second question:
Outline the chemical and physical processes involved in the production of a polyethylene bottle from a natural raw material

Take care with this because it specifies production "from a natural raw material" so you can't just start with ethylene as this is not a raw material. Things I think need to be included, depending on the mark value:
- Harvesting sugar cane or from cellulose to glucose
- Fermentation of glucose into ethanol and carbon dioxide
- Dehydration of ethanol into ethene and water
- Production of LDPE (initiation, etc etc)

 

[Kimyia]

Second question:
Outline the chemical and physical processes involved in the production of a polyethylene bottle from a natural raw material

Take care with this because it specifies production "from a natural raw material" so you can't just start with ethylene as this is not a raw material. Things I think need to be included, depending on the mark value:
- Harvesting sugar cane or from cellulose to glucose
- Fermentation of glucose into ethanol and carbon dioxide
- Dehydration of ethanol into ethene and water
- Production of LDPE (initiation, etc etc)

I didn't even pick up on the "raw material" bit. Thanks for pointing that out.
I wanted to just see if this was right with LDPE and HDPE:
Production of LDPE: uses a peroxide initiator to make free radicals. The free radicals break the down bond in the ethlyene monomers forming ethylene radicals. These radicals join up with others ethylene monomers, forming a chain until its stopped by an inhibitor or otherwise. This uses high temp and high pressure.
Production of HDPE: a catalyst (Ziegler-Natta or metallocene catalyst) is used. Ethlyene binds to its active site by donating electrons from its double bond which causes the double bond to break and a new monomer attaches, forming a chain. This involves lower temp and lower pressure.
I'm not entirely sure with the HDPE one though

 

[angelarr]

I didn't even pick up on the "raw material" bit. Thanks for pointing that out.
I wanted to just see if this was right with LDPE and HDPE:
Production of LDPE: uses a peroxide initiator to make free radicals. The free radicals break the down bond in the ethlyene monomers forming ethylene radicals. These radicals join up with others ethylene monomers, forming a chain until its stopped by an inhibitor or otherwise. This uses high temp and high pressure.
Production of HDPE: a catalyst (Ziegler-Natta or metallocene catalyst) is used. Ethlyene binds to its active site by donating electrons from its double bond which causes the double bond to break and a new monomer attaches, forming a chain. This involves lower temp and lower pressure.
I'm not entirely sure with the HDPE one though

I think your answer for LDPE is pretty good, but if you're able to be a little bit more specific and careful, that would be even better. The radicals do break down the ethylene monomer into ethylene radicals or activated monomers. In your answer you've said "these radicals join up to other ethylene monomers" which isn't entirely correct, though I know what you mean. I think it's better if you say these radicals join up to other ethylene radicals or "activated monomers" (the term I used throughout the course). And you can include they are stopped by an inhibitor when the desired chain length is obtained, or otherwise.

And about HDPE, to be honest, we didn't learn anything about it except Ziegler-Natta, lower temp and pressure, as well as it's different properties: minimal branching, crystalline structure, approx 1000 monomers to make up polymer chain, rigid.

If they're asking about production of a plastic bottle or something, I think it's fairly safe to just talk about LDPE.

 

[Kimyia]

I think your answer for LDPE is pretty good, but if you're able to be a little bit more specific and careful, that would be even better. The radicals do break down the ethylene monomer into ethylene radicals or activated monomers. In your answer you've said "these radicals join up to other ethylene monomers" which isn't entirely correct, though I know what you mean. I think it's better if you say these radicals join up to other ethylene radicals or "activated monomers" (the term I used throughout the course). And you can include they are stopped by an inhibitor when the desired chain length is obtained, or otherwise.

And about HDPE, to be honest, we didn't learn anything about it except Ziegler-Natta, lower temp and pressure, as well as it's different properties: minimal branching, crystalline structure, approx 1000 monomers to make up polymer chain, rigid.

If they're asking about production of a plastic bottle or something, I think it's fairly safe to just talk about LDPE.

Thanks heaps for that Should we know a specific temp or pressure, or it is suffice enough to say high/low temp and pressure?

 

[angelarr]

High temp and pressure + lower temp/pressure is enough!