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No. 7

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

 

Some chemistry or physics required


Applications  
Polyurethane foams are used in applications such as cushioning or insulation. For most cushioning applications polyurethane foams are soft and elastic. Most foams used in insulation are hard and rigid foams. Often foams are prepared with additional properties, such as flame retardancy. 

A typical polyurethane is prepared by blending of a polyol component with a diisocyanate. The polyol component contains also the a volatile blowing agent, a surfactant foam stabilizer and a catalysts. Most foam preparation is done on automatic mixing equipment which can mix precisely different streams of all the components. Foams for insulation can be cast in place or cast in large equipment and later sliced in sheets. 

 

  Flexible foams are prepared from linear predominately di-functional polyol and diisocyanates. The polyols are of higher molecular weight and the Tg of the polyurethane is low. On the other hand the rigid foams are prepared from highly functional polyols and from di or polyisocyanates. These polyols are highly crosslinked and high in urethane content. Rigid foams can also be prepared with large levels of polyisocyanate and with the use of special catalysts which trimerize the isocyanate to an isocyanurate group. This isocyanate group can improve the flame retardancy of the foam.
 
Elastomers are linear polyurethane polymers with a low degree or no crosslinking. To achieve the high MW required the exact ration of hydroxyl and isocyanate has to be used. The preparation of isocyanate terminated prepolymers and extending this prepolymer with a diol is one of the methods used to obtain a good control of the molecular weight. Phase separation is used to achieve elastomeric properties in an elastomer. The diol or diamine used in chain extension of an isocyanate terminated prepolymer has a significant effect on phase separation. Depending on the size and the refractive index of the second phase the elastomer can be clear or hazy in appearance.    Diamines;  3,5-Diethyl-2,4-diaminotoluene 3,5-Dimethylthio-2,4-toluenediamine 4,4'-Methylenebis(2-chloraniline) (MOCA)
Diols; 1,4-Butanediol, 1,3-Propanediol, 
Coatings utilize polyurethanes in solvent and waterborne coatings. Applications are in abrasion resistant coatings, primers and in topcoats. For most coating applications the high reactivity of the isocyanate and the potential for low temperature crosslinking is important. The information on coatings will be expanded in the future.   Most coatings use as polymers polyester or acrylic backbones which are either crosslinked or chain extended with di or polyisocyanates. Coatings represents a large market for aliphatic light stable isocyanates. 
 
Adhesives are very similar in composition to elastomers. The only difference with an adhesive might be the application methods and how the adhesive is formulated.  


Last edited on:

November 20, 2006

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