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Learning Center  Isocyanate Reactions

No. 1

No chemistry required (it would not hurt to know some)

 

Some chemistry or physics required

The learning center can make you familiar with some of the basic concept in polyurethane chemistry. This course is useful if you like to understand some of the formulation practices and techniques used with polyurethanes. There are two different versions of the course in parallel. On the left  side is a more practical oriented presentation with no or less chemistry. The right side is heavier on chemistry. You can of course jump from on side to the other. 

  The right side contains similar information as the left side but it contains formulations, equations and other good stuff. 

Introduction  

Polyurethanes are the reaction product of an isocyanate and a hydroxyl group. Poly indicates that more than one urethane group is involved. Typically diols or polyols (di is "two" functional and poly is more than 2) and a diisocyanate or polyisocyanate is used to prepare a polyurethane. A hydroxyl group is a reactive group which consist of one hydrogen and oxygen. Water for example consist of two hydrogen and one oxygen, it is also a hydroxyl functional compound and will therefore react with an isocyanates. (sorry I could not avoid some chemistry) What makes polyurethanes of so great interest is the fast reaction of a hydroxyl group with an isocyanate. Under appropriate conditions the reaction is completed in several seconds. It is possible to prepare a polymer from small molecules. A second aspect which is of great importance is the high stability of the isocyanate linkage to water under acidic and basic pH conditions. The urethane group also contributes excellent properties to a polymer. Depending on the composition of the polyurethane the polymer can have excellent water resistance, toughness, abrasion resistance, light stability and chemical resistance. For most polyurethane preparations a close control of the ratio of isocyanate to hydroxyl is required. The exact ratio is less sensitive if the polyol or the isocyanate is highly functional and a crosslinked network is prepared. 

 

 R-OH + R'-NCO 

Catalyst

Hydroxyl + isocyanate  

Urethane

Hydrolysis of Isocyanate 

 

+ H2O

+  ROH

   Isocyanate  +  Water 

Carbamic acid

R-NH2 + CO2

Carbamic acid unstable   Amine and carbon dioxide
Resistance properties (hydrolysis) Urethane groups are very resistant to water (hydrolysis) over a wide range of acid and basic conditions. 
     
 

In the presence of water the isocyanate reacts with water. This leads to formation of carbon dioxide and gassing or foaming.  The amine formed reacts very fast with the isocyanate forming an urea linkage. 

The reaction of an isocyanate group with  water or moisture is used in one component moisture cure coatings, adhesives or sealants as a cure mechanism. Prepolymer with isocyanate end groups are prepared. These polymers have to be stored under dry conditions and will cure in the presence of moisture. If the films are too thick or the moisture is high these systems can gas and foam.

Polyurethanes can also be stable to UV and outdoor exposure if aliphatic isocyanates are used to make a polymer. Polyurethanes prepared from aromatic isocyanates are very sensitive to UV degradation and have to covered or coated. 

 

H-O-H  +    R'-NCO

Catalyst

 

Water   +  Isocyanate  

Carbamic acid

R-NH2 +CO2

Carbamic acid unstable   Amine and carbon dioxide

R'-NH2 + R'-NCO

Amine + Isocyanate

  Urea linkage


Last edited on:

November 22, 2006

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