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Applications for polyurethanes are in foams for insulation, cushioning, elastomers, coatings and adhesives. In many applications the high reaction rates and the ease of formation of a polymer is an important characteristics. There are many different chemical reactions available to prepare polymers. Polyurethane chemistry has the advantage that polymer preparation at room temperature is possible. 

  The strength of polyurethane chemistry is the versatility in raw materials and the many different properties which can be obtained. Polyurethanes can be prepared from hard-soft, from elastomeric to rigid. The urethane linkages formed in polyurethane preparation are exceptionally stable to the elements. 

Introduction - General Information Chemistry
Polyurethanes  are the reaction product of a polyol with a polyisocyanate. A large selection of polyols and polyisocyanates are available, this permits the design of polymers for many applications. The larger uses of polyurethanes are in the preparation of polyurethane foams, elastomers, adhesives and coatings. Most polyurethanes are prepared from two components which are mixed before use. Besides polyol and polyisocyanate polyurethanes use catalysts to accelerate the reaction of the isocyanate with the polyol. For the preparation of foams it is also necessary to use a blowing agents to create the cell structure and a surfactant to stabilize the foam during the foaming stage. In addition to these basic components a polyurethane can also contain flame retardants, pigments or colorants, antioxidants.
 
Polyols are reactants which contains as functional groups hydroxyl groups. In urethane reactions primary hydroxyl groups, such as derived from ethylene oxide are faster reacting with an isocyanate than secondary hydroxyl groups, for example propylene oxide derived groups. Polyether polyols contain in the polymer backbone ether groups. These polyether polyols are very stable to hydrolysis under basic conditions, but can be attacked in an acidic environment. Polyether polyols are the main stay of the polyurethane market, they are used in foams, elastomers and coatings. Polyether polyols can be di-functional (diol) or they can be of higher functionality. In applications where high flexibility and elongation is required diols are used to create highly flexible and soft polyurethanes. Highly functional polyols are used in the preparation of hard or rigid polyurethanes. The products made this way are highly crosslinked and not soluble in a solvent. Besides polyether also polyester polyols are used in the preparation of polyurethanes. The ester linkage in the polyester makes the polyurethanes more sensitive to hydrolysis, but it is possible to prepare polyurethanes with improved exterior durability.
 
Polyisocyanates contain the very reactive isocyanate group. Polyisocyanates not only react with hydroxyl groups, but with other active hydrogen containing compounds such as water, primary and secondary amines, carboxylic acids and other compounds. Because of their reactivity, polyisocyanates are very toxic by inhalation, but are rather low toxic by ingestion.
 
Catalysts are required for most of the isocyanate-hydroxyl reactions. The reaction of a primary or secondary amine with an isocyanate is exceptionally fast and no catalysis is necessary. As catalysts both t-amines and also metal compounds can be used. Organotin compounds from type dibutyltin carboxylate (DBTDL) are the most common catalysts for the isocyanate-hydroxyl reaction. In the preparation of foams these catalysts are know as gelling catalysts. Most amines catalyze more the reaction of isocyanate with water and they are therefore called blowing catalysts. Other catalysts used in the preparation of urethane polymers are the trimerization catalysts which produce isocyanurates. Isocyanurates give improved flame retardancy are used in flame retardant foams. 
 
Blowing agents are at room temperature gases or low boiling liquids. During the exothermic reaction of the isocyanate-hydroxyl reaction the blowing agents evaporates and creates bubbles leading to foaming. It is also possible to saturate the polyol-polyisocyanate blend with a gas under pressure. The release of the pressure cause foaming during the polyurethane formation. Another approach is the formation of carbon dioxide from the reaction of the isocyanate with water. 
 
Surfactants are used to stabilize the foam during the preparation of polyurethane foams. The most common stabilizers are siloxane-polyether block polymers. These surfactants give a very low surface tension, in addition they also increase the surface viscosity which leads to an increase in foam stability.


Last edited on:

November 22, 2006

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