Polymerisation! (1 Viewer)

NiMm

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hey im just wondering the correct (what must be included) answer for this dotpoint

Outline the steps in the production of polyethylene as an example of a commericially and industrially based polymer

and which dot point must we include information about LDPs and HDPs?
 

tennille

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This is the dot point that you include information about LDPE and HDPE.

HDPE: requires low gas pressure and low temp (~ 60 degrees). Requires a catalyst consisting of transition metal and organometallic compounds. This results in the formation of long unbranched and aligned molecules. Consequently, the chains can pack closer together, resulting in high density. HDPE is used in the manufacturing of pipes, plastic buckets, and containers to hold petrol, oil, etc

LDPE: high temp (~ 300 degrees) and high pressure (1000-3000 atm) is used along with an initiator such as peroxide (O-O). These conditions result in molecules with short branches and significant chain branching. Some of the hydrogen groups on carbon atoms are replaced by an alkyl group, meaning that close packing cannot occur, hence the low density. LDPE is used in plastic bags and flexible containers.
 

priesty

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it is also the dot point I belive where you should look at Initiation, Propogation and Termination (my HSC notes followed below). You will need equations and diagrams at least to go with these, so I suggest you look in Contexts or Conquering because I believe that they both have structure diagrams on this process of production. Hope this helps :)

Initiation
• The chain is initiated by a free radical, or can be initiated by an acid, alkali or by high pressures or high temperatures.
• Free radicals are molecules that contain unpaired electrons, which are produced by the reaction between some of the ethene and the oxygen initiator (or organic peroxide).

Propogation

• Each time a radical hits an ethene molecule a new longer free radical is formed.

Termination

• Eventually two free radicals hit each other producing a final molecule. The process stops here because no new free radicals are formed.
• An inhibitor can also be added.
• Because chain termination is a random process, polyethylene will be made up of all sorts of different lengths.


• There are two types of polyethylene; High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE).
• Low Density Polyethylene with extensive branching is produced under high temperatures and pressures. The ethene gas is gradually polymerized at a temperature of around 250oC and a final pressure of up to 300atm. Due to the branches it cannot pack well, hence it is more flexible and transparent and therefore it is used as Glad-Wrap and shopping bags.
• High Density Polyethylene with reduced to no branching is produced at lower temperatures (100 – 150oC) and pressures (30atm). To speed things up the Ziegler – Natta Catalyst is used and so due to no branches being present the chains can pack well. It is used for wheely bins, crates and solid containers due to it being very strong and tough. It is more crystalline and has a higher density than the branched-chain product.
• Polyethylene is commercial and industrially important as there are a large number of uses for them
 

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