polymer_technology

Polyurethane پلی یورتان ها

 

Example of synthesis of a polyurethane. Note the urethane groups -NH-(C=O)-O- linking the units of the product.

A polyurethane (IUPAC abbreviation PUR, but commonly abbreviated PU) is any polymer consisting of a chain of organic units joined by urethane (carbamate) links. Polyurethane polymers are formed through step-growth polymerization by reacting a monomer containing at least two isocyanate functional groups with another monomer containing at least two hydroxyl (alcohol) groups in the presence of a catalyst.

Polyurethanes are widely used in high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives and sealants, Spandex fibers, seals, gaskets, carpet underlay, and hard plastic parts (such as for electronic instruments).

Polyurethane products are often called "urethanes". They should not be confused with the specific substance urethane, also known as ethyl carbamate. Polyurethanes are neither produced from ethyl carbamate, nor do they contain it.

 

table of chain extenders and cross linkers [17]
hydroxyl compounds – difunctional molecules
  MW s.g. m.p. °C b.p. °C
ethylene glycol 62.1 1.110 -13.4 197.4
diethylene glycol 106.1 1.111 -8.7 245.5
triethylene glycol 150.2 1.120 -7.2 287.8
tetraethylene glycol 194.2 1.123 -9.4 325.6
propylene glycol 76.1 1.032 supercools 187.4
dipropylene glycol 134.2 1.022 supercools 232.2
tripropylene glycol 192.3 1.110 supercools 265.1
1,3-propanediol 76.1 1.060 -28 210
1,3-butanediol 92.1 1.005 - 207.5
1,4-butanediol 92.1 1.017 20.1 235
neopentyl glycol 104.2 - 130 206
1,6-hexanediol 118.2 1.017 43 250
1,4-cyclohexanedimethanol - - - -
HQEEGHGHGHHHTH - - - -
ethanolamine 61.1 1.018 10.3 170
diethanolamine 105.1 1.097 28 271
methyldiethanolamine 119.1 1.043 -21 242
phenyldiethanolamine 181.2 - 58 228
hydroxyl compounds – trifunctional molecules
  MW s.g. f.p. °C b.p. °C
glycerol 92.1 1.261 18.0 290
trimethylolpropane - - - -
1,2,6-hexanetriol - - - -
triethanolamine 149.2 1.124 21 -
hydroxyl compounds – tetrafunctional molecules
  MW s.g. m.p. °C b.p. °C
pentaerythritol 136.2 - 260.5 -
N,N,N',N'-tetrakis
(2-hydroxypropyl)
ethylenediamine
- - - -
amine compounds – difunctional molecules
  MW s.g. m.p. °C b.p. °C
diethyltoluenediamine 178.3 1.022 - 308
dimethylthiotoluenediamine 214.0 1.208 - -


This is a most exciting, page, surely it is! This is the page where you the netsurfer will learn how to make polyurethanes! Now, now, it's not that hard. It's quite simple actually. We'll prove it to you...

To start off, we make polyurethanes from two monomers, a diol and a diisocyanate. Can we get a look at them? Yes, there they are, right down below:

We can see them in 3-D by clicking here, if you like.

With the help of a little molecule called diazobicyclo[2.2.2]octane, or DABCO for short, we can make these two polymerize. When we stir the two monomers together with DABCO, something nifty happens.

DABCO is a very good nucleophile, that is, it has a pair of unshared electrons that would just love to attach themselves to a vulnerable nucleus. Remember, electrons have negative charges, and the nuclei of atoms have positive charges. And we all know that negative charges and positive charges attract. So DABCO's electron look around, and they find a nucleus on the alcohol hydrogens of the diol. These hydrogens are vulnerable, because they are bonded to oxygen atoms. Oxygen is electronegative. This is to say it pulls electrons away from other atoms. So it pulls electrons away from its neighbor the hydrogen atom. This leaves the positive charge of its nucleus unbalanced. The electrons would have balanced the positive charge with their own negative charges, they've been sucked away by the oxygen. This leaves a slight positive charge on the hydrogen.

Click here to see a movie of this reaction.
 

So DABCO's electrons see this and they just can't help themselves. They rush over and form a hydrogen bond between the hydrogen and DABCO's nitrogen. This H-bonding leaves a partial positive charge on the nitrogen, but more importantly, a partial negative charge on the oxygen. This partial negative charge makes the oxygen really hot. Being hot as it is, it wants to react with something

Would you like to know just what it will react with?

The oxygen has an excess of electrons, so it will react with something that is poor in electrons. If we look at our isocyanate, we can see that the carbon in the isocyanate group is sandwiched between two electronegative elements, oxygen and nitrogen. This means that this carbon is going to be very poor in electrons indeed. So our hot oxygen wastes no time in reacting with it. It throws a pair of electrons to that carbon, and a bond forms.

Click here to see a movie of this reaction.

Of course this pushes a pair of electrons out of the carbon-nitrogen double bond. This pair situates itself on the nitrogen, giving it a negative charge. Our oxygen meanwhile, having donated an electron pair, is left with a positive charge.

Now there isn't much that a nitrogen atom likes less than to have a negative charge. So it's going to try to get rid of it as soon as it can. The easiest way to do this is to donate that pair to our old friend, the alcohol hydrogen atom. This forms a bond between that hydrogen and the nitrogen.

Click here to see a movie of this reaction.

The electrons that the hydrogen had shared with the oxygen now belong to the oxygen alone. This eliminates that old positive charge that the oxygen was carrying. When it's all over we're left with a brand new urethane dimer.

If you want to see a movie of the whole urethane formation process, click here!

This urethane dimer has an alcohol group on one end, and an isocyanate group on the other, so it can react with either a diol or a diisocyanate to form a trimer. Or it can react with another dimer, or a trimer, or even a higher oligomers. In this way, monomers and oligomers combine and combine until we get a high molecular weight polyurethane.

For those of you keeping score at home, you'll notice that not only monomers react, but also dimers, trimers, and so on. This makes it a step growth polymerization. Also, because no small molecule by-products are produced, it is an addition polymerization.

Polymers Within Polymers

Sometimes, instead of using a small diol like ethylene glycol, we use a polyglycol, one with a molecular weight of about 2000.

This gives us a polymer within a polymer so to speak, and we have a polyurethane that looks something like this:

If you like, you can learn how to make a polyurethane foams.


REFERENCES OF THIS TEXT ARE : http://en.wikipedia.org/   &    http://www.pslc.ws/

+ نوشته شده در  پنجشنبه بیست و ششم فروردین 1389ساعت 20:15  توسط Ammar Ghasemian Azizi  |