ChemWiki - User contributions [en]

It:Polyurethane

2006-12-05T20:04:12Z

Yodf05: /* References */

=='''Introduction'''==

Polyurethane is any polymer with urethane linkage. It has many applications due to a wide variety of properties it possess, for examples, furniture cushioning, mattresses, textiles, refrigerated appliances, building blocks with integrated insulation, bonding foam, casting and surfacing, etc.

==='''Chemistry of Polyurethane'''===

A simple urethane is formed when nucleophilic attack of alcohol on isocyanates. Urethanes are hybrids between carbonates and ureas – half-esters and half-amides of carbonic acid.

The nucleophilic reaction of isocyanate and alcohol gives a simple urethane as shown below:

[[Image:Rxn_of_isocyanate.bmp|left]]

A prepolymer formed when a diisocyanate with diol. A long chain of polyurethane is formed when diisocyanates and polyols are reacted.

Reaction of diisocyanate and diol showing the urethane linkage:

[[Image:Prepolymer.jpg|left]]

==='''Solid Polyurethane Elastomer'''===
Elastomer is one type of some important polymers from polyurethane. Polyurethane elastomers are rubber-like materials that can be created with a wide variety of properties and molded into almost any shape.

===='''Properties'''====
The properties of polyurethane varied with the types of polyols, diisocyanates used in the reaction. However, they all have certain characteristic properties in common. Polyurethanes have a high wear resistant to solvents and environment degradation; they also exhibit high elasticity within the different hardness ranges.

The other factors affecting the properties of the polyurethane are the processes of manufacturing polyurethane:
#Hot cure systems - This method results in linear sequences which exhibits a relatively rigid geometry.
#Cold cure systems - A three-dimensional network of low crosslink density polyurethane is formed from the reaction.
#Reactive spray coatings

General properties of polyurethane elastomers:
*Mechanical wear resistance
*Resistance to light, air, ozone and ultraviolet radiation (especially polyester-based materials)
*Absence of extractable ingredients such as plasticizers
*Low degree of swelling in mineral oils and fats
*Good notched impact resistance

== '''Production of Solid Polyurethane Materials''' ==

Different methods can be used to mold solid polyurethanes. Castable polyurethanes are produced by pouring the blended liquid raw materials into a mold. Mixing and pouring these raw materials can be done manually or by using casting equipment.

Thermoplastic polyurethanes can be produced by injection molding, extruding or calendaring. By molding or “open heating” of rubber-like PU mixtures, production processes typically used in the rubber industry can be employed.

==='''PUR Cast Systems'''===
The oldest method of producing molded solid polyurethane parts is casting it into open molds. The liquid or molten components, which contain reactive NCO and OH or NH2 groups, are thoroughly mixed together and poured into open molds. It is essential to control material ratios and production conditions. Because further reaction will occurs in the mold as the mass solidifies. Hot or cold cure systems can be chosen according to production methods. Different method will determine the various chemical and physical properties desired in the end products. The physical properties of parts produced by the hot cure method are higher than those produced by room temperature cure. The majority of polyol components of hot-cast systems is based on polyester diols or polytetreamethylene glycols. Most cold cure systems use di- or tri-functional polypropylene glycol polyethers.

===Hot cure systems: Production and Processing===
Isocyanate-terminated MDI (4, 4’ diphenylmethane diisocyanate) or NDI (naphthalene diisocyanate) prepolymers are frequently prepared by the PU molders, so they can meet the manifold requirements in the different applications by individual adjustment of the formulation.

For hand mixing of the prepolymer, the polyol is heated, dewatered and placed in a reaction vessel. Liquid isocyanate is then added in one shot, generally in a molar excess. Since NDI cannot be added as a liquid to the polyol because of its high melting point (127°C), overheating of the prepolymer is prevented by heterogeneous reaction. This method of prepolymer formation is frequently used for the production of high monomer containing quasi-prepolymers of limited storage stability.

The mix ratio for the two components is determined by the properties (hardness) desired in the finished product.

===Polyurethane Cold Cure System===
Cold cure systems are mainly comprised of poly(oxypropylene)glycol or poly(oxyethylene-oxypropylene)glycol mixed ethers, sometimes from liquid polyesters or hydroxyl containing natural materials (castor oil). They are either processed by the prepolymer technique or the one-shot process. There is a slight difference between these two processes, because the prepolymer contains different amounts of monomeric isocyanates. However, in both processes, a mixture of long and short chain difunctional polyols, or partially branched polyols usually containing fillers, is used as the second component. This wide range of production processes leads to different properties in the finished product.

Comparing to the hot cure system, cold cure systems use prepolymers prepared on a large scale in a batch process. This large scale production will guarantee the required specified properties by accurate process control.

Cold cure systems can be done by hand mixing as well as by continuous or intermittent machine production. There are no basic differences between using a liquid diisocyanate and a prepolymer in a one shot method.

== References ==
1. "Polyurethane Handbook 2nd Edition", by Gunter Oertel;New York: Hanswer, 1993

It:Ziegler-Natta

2006-12-05T20:00:50Z

Yodf05: /* Reference */

== Introduction ==

A Ziegler – Natta catalyst is a method of producing unbranched, stereoregular vinyl polymers, such as linerar unbranched polyethylene and isotactic polypropylene. Ziegler – Natta catalysts are typically based on titanium chlorides and organometallic trialkyl aluminium based co-catalyst.

== Preparation of Catalyst ==

Most of the time the catalyst and co-catalysts pair are TiCl3 and Al(C2H5)2Cl, or TiCl4 with Al(C2H5)3.

[[Image:no1.bmp]]

In this case, the TiCl3 and Al(C2H5)2Cl system will be discussed. TiCl3 can arrange itself into a number of crystal structures. The one that is interested in is called α- TiCl3.

[[Image:no2.bmp]]

As it can be seen, there are six chlorine atoms coordinated to each titanium atom, with an octahedral geometry. In the interior of the crystal, each titanium is surrounded by six chlorines, but on the surface, a titanium atom is surrounded on one side by five chlorine atoms, and the other side by empty space.

[[Image:no3.bmp]]

Titanium has six empty orbitals, one 4s and five 3-d orbitals, in their outermost electron shells. The titanium atom on the surface of the crystal has an empty orbital.

[[Image:no4.bmp]]

Al(C2H5)2Cl , the co-catalyst, donates one of its ethyl groups to the impoverished titanium, but in the process, one of the chlorines leaves. Therefore, there is still an empty orbital on the
titanium.

[[Image:No5.JPG]]

The aluminium stays coordinated, though not covalently bonded, to the CH2 carbon atom of the ethyl group it just donated to the titanium. It also coordinates itself to one of the chlorine atoms adjacent to the titanium.

So then a vinyl monomer like propylene comes along. The two electrons in the π-system of the carbon-carbon double bond can be used to fill the empty orbital of the titanium. The propylene and the titanium form a complex.

[[Image:No6.JPG]]

===Alkene-metal complexes ===
A carbon-carbon double bond, is made up of a σ bond and a π bond.

[[Image:No7.JPG]]

The π bond consists of two π- orbitals. One is the π-bonding orbital shown in blue and the other is the π-antibonding orbital, shown in red. The π-antibonding orbital is too high in energy, so majority of the time it stays empty.

[[Image:No8.JPG]]

The green lobes are empty orbital and the pink lobes are one of the filled orbitals. The empty orbital is going to share a of pair electrons with the alkene’s π-bonding orbital.

[[Image:No9.JPG]]

== The Polymerization ==
==='''Isotactic polymerization'''===

It is believed that the first electron pairs shift is that pair from the carbon-carbon π-bond that is complexed with the titanium. It’s going to shift to form simple titanium-carbon bond. Then the electrons from the bond between the carbon of the ethyl group and the titanium will shift to form a bond between the ethyl group and the methyl-substitued carbon of the propylene monomer.

Next a migration will occur. The atoms rearrange themselves to form a slightly different structure as below:

[[Image:No10.JPG]]

The aluminium is complexed with one of the carbon atoms from the propylene monomer. As it is shown, titanium is once again with an empty orbital.

So when another propylene molecule comes along, the whole process will starts all over and the polymerization continues and resulting:

[[Image:No11.JPG]]

This will react with more propylene molecules and the polymer chain extends. All the methyl groups on the growing polymer are on the same side of the chain. With this mechanism, isotactic polypropylene is produce.

[[Image:No12.JPG]]

=== '''Syndiotactic polymerization''' ===
The mentioned catalyst system gives isotactic polymers, there are other systems tha can give syndiotactic polymers. The one that is focused on now is based on vanadium rather than titanium. That system is VCl4/Al(C2H5)2Cl. This complex will act similar to the titanium system. First the propylene will attack the vanadium and then the electrons will shift just like with titanium before and propylene is inserted between the metal and the ethyl group.

[[Image:No13.JPG]]

However, noted that when the second propylene adds to the chain, the chain changes position again. It’s back in the position where it started. The methyl groups of the first monomer in blue and the second monomer in red, they are on the opposite sides of the polymer chain. When the polymer chain is in one position the propylene monomer can only add so that the methyl group is on one side of the chain. When the chain is in the other position, propylene only adds the methyl group on the opposite side. Due to this switches positions with each propylene monomer added, the methyl groups are on alternating sides of the chain, producing a syndiotactic polymer.

[[Image:No14.JPG]]

'''Example of Titanium catalyst during polyermisation'''
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<inlineContents>HEADER CSD ENTRY TEWGUX
COMPND UNNAMED
AUTHOR GENERATED BY CONQUEST
CRYST1 17.034 11.534 19.534 90.00 108.49 90.00 P 21/n 4
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ATOM 86 H47 UNK 0 1 -8.836 -7.900 4.413 1.00 0.00
CONECT 1 2 10 11 12 13 14
CONECT 2 1
CONECT 3 0
CONECT 4 31
CONECT 5 31
CONECT 6 35
CONECT 7 35
CONECT 8 36
CONECT 9 36
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CONECT 11 1 16 17 32
CONECT 12 1 18 19 37
CONECT 13 1 22 26
CONECT 14 1 27
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CONECT 16 11 15 42 43
CONECT 17 11 18 44 45
CONECT 18 12 17 46 47
CONECT 19 12 20 48 49
CONECT 20 10 19 50 51
CONECT 21 10 22 52 53
CONECT 22 13 21 23
CONECT 23 22 24 54
CONECT 24 23 25 55
CONECT 25 24 26 56
CONECT 26 13 25 57
CONECT 27 14 28 29 30
CONECT 28 27 58 59 60
CONECT 29 27 61 62 63
CONECT 30 27 64 65 66
CONECT 31 4 5 67 68
CONECT 32 11 33 34 69
CONECT 33 32 70 71 72
CONECT 34 32 73 74 75
CONECT 35 6 7 76 77
CONECT 36 8 9 78 79
CONECT 37 12 38 39 80
CONECT 38 37 81 82 83
CONECT 39 37 84 85 86
CONECT 40 15
CONECT 41 15
CONECT 42 16
CONECT 43 16
CONECT 44 17
CONECT 45 17
CONECT 46 18
CONECT 47 18
CONECT 48 19
CONECT 49 19
CONECT 50 20
CONECT 51 20
CONECT 52 21
CONECT 53 21
CONECT 54 23
CONECT 55 24
CONECT 56 25
CONECT 57 26
CONECT 58 28
CONECT 59 28
CONECT 60 28
CONECT 61 29
CONECT 62 29
CONECT 63 29
CONECT 64 30
CONECT 65 30
CONECT 66 30
CONECT 67 31
CONECT 68 31
CONECT 69 32
CONECT 70 33
CONECT 71 33
CONECT 72 33
CONECT 73 34
CONECT 74 34
CONECT 75 34
CONECT 76 35
CONECT 77 35
CONECT 78 36
CONECT 79 36
CONECT 80 37
CONECT 81 38
CONECT 82 38
CONECT 83 38
CONECT 84 39
CONECT 85 39
CONECT 86 39
MASTER 0 0 0 0 0 0 0 0 86 0 86 0
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== Limitation ==

Ziegler-Natta doesn’t work for some kinds of monomers. For example, poly(vinylchoride) cannot be produced by Ziegler-Natta polymerization. This is because when the catalyst and the co-catalyst come together to form the initiating complex, radicals are being produced during the intermediate steps of the reaction, which will initiate free radical polymerization.

== Reference ==
1. Potapov, A.G. / Bukatov, G.D. / Zakharov, V.A., Journal of Molecular Catalysis. A, Chemical, Mar 2006

2. http://www.hyle.org/journal/issues/5/cerruti.htm

3. http://www.pslc.ws/mactest/ziegler.htm

It:Ziegler-Natta

2006-11-30T20:12:03Z

Yodf05: /* Alkene-metal complexes */