Page 160 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 123

NH 2 NH 2 NH 2 NH 2
HNO 3 H 2 H CO
2
CH
HCl 2 CH 2

COCl 2
O O O
N N N

O
CH 2 CH 2
N N

MDI O PMDI

FIGURE 4.6 Synthesis of PMDI and MDI.

Polyurethanes and Polyureas

O O
C H C
H 3 3
N N N

O
(4.73)

N

O TDI

MDI and PMDI are both formed from the same reaction as shown in Figure 4.6
The hydroxyl-containing reactants are polyesters, polyethers, polycaprolactones, and diols. The
manufacture of the polyether and polycaprolactone macroglycols is given in Figure 4.7.
The formation of polyurethanes from reaction with polyols can be catalyzed by addition of ter-
tiary amines as shown in Figure 4.8
Production of foam was discovered when water was accidentally introduced into the reaction
mixture producing what the scientists called “imitation Swiss cheese.” Flexible foams are generally
made from TDI and longer-chained polyether triols. The use of trifunctional reactants is needed
to produce a three-dimensional product. These foams are generally water-blown, meaning that the
water added reacts with isocyanate end groups producing carbon dioxide gas giving open-celled
foam products. Isocyanates react with water producing unstable carbamic acids that decompose
forming diamines and carbon dioxide. Low-density flexible foams are used in bedding applications

and furniture while higher-density foams are used in automotive seating and semiflexible foams are

employed in automotive interior padding. Some of the flexible foams are used as carpet underlay

material.
Most rigid foam is made from PMDI and difunctional polyether polyols. The PMDI provides the
needed additional functionality offering an average functionality of 2.7 resulting in the formation

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