Polyurethane

Polyurethane:

- Produced by the reaction of a polyfunctional isocyanate with a polyol or other reactant containing two or more groups reactive with isocyanate, most often hydroxyls.

- Hydroxyl-containing component covers a wide range of molecular weights and types, including polyester and polyether polyols. 

- Polyfunctional isocyanates can be aromatic, aliphatic, cycloaliphatic, or polycyclic in structure.

- Flexibility in the selection of reactants leads to the wide range of physical properties that allows polyurethanes to play an important role in the world market.

HISTORICAL DEVELOPMENT

- Discovered by Otto Bayer and co-workers in 1937.

- Vigorous research programme in the laboratories of I. G. Farben- industrie and its successor, Bayer AG, which led to the production of rigid foams, adhesives, and coatings.  

- The recent growth in the commercial importance of polyurethane products formed by reaction injection moulding (RIM). 

- RIM technology permits the conversion of polyurethane elastomers into automotive components, non- automotive products, including business-machine housings and parts for heavy-duty industrial equipment.

Isocyanates 

- Most important isocyanate used in the manufacture of polyurethane is 2,4-tolylene di-isocyanate (TDI) with 2,6 tolylene di-isocyanate and their derivatives.

- If toluene is dinitrated without separation a mixture of about 80% 2,4­dinitrotoluerle and 20% 2,6-dinitrotoluene is obtained. Nitration of   separated 2-nitrotoluene will yield a mixture of approximately 65% of the 2,4- and 35% of the 2,6-isomer 

- Next stage is the reduction of the nitro compounds to amines. Resultant amines are then reacted with phosgene.

- Carbamoyl chloride formed may then be decomposed to produce di­isocyanate.

- Isocyanates are toxic materials, highly reactive liquids and  care should be exercised in their use. 

Typical Di-isocyanate used in the manufacture of Polyurethane

MANUFACTURE OF POLYURETHANES      

POLYESTER POLYOLS

- Polyesters have several esters groups in their molecule.

- The main methods of manufacture are 

- Esterification of polycarboxylic acids with polyhydroxyl compounds, 

- Transesterification of polycarboxylic acid esters with polyhydroxyl compounds, 

- Polycondensation of hydroxycarboxylic acids, 

- Polymerization of lactones 

- If the diol is partially replaced by a triol such as triethylene glycol, a branched polyester results, the higher the triol content, the greater the degree of branching.

- The polyesters used for manufacturing polyurethanes have mean molecular weights of 2000 and hydroxyl values of 50 to 70.

Polyester polyols for Polyurethane

POLYETHER POLYOLS

- Polyethers are polymerization product of epoxides. 

- They are synthesized from propylene oxide or mixture of ethylene and propylene oxides.

- If a product with three or more active H-atoms instead of two is used, branched polyethers analogous to polyesters result 

- Polyethers used in PU chemistry have molecular weights of 300 to 6000 and functionalities of 2 to 8. 

Polyether Polyols for Polyurethane