Page 161 - Carrahers_Polymer_Chemistry,_Eighth_Edition

124 Carraher’s Polymer Chemistry

R

O O
HO R 1
R
R
O
O
O
R R
O

FIGURE 4.7 Synthesis of polyether and polycaprolactone macroglycols.

R 2
R 2
R OH + N R 2 R O – + NH + R 2
R 2
R 2

R 2
– O
R O + R 1 N NH +
O
NH
R 2 R 2
–
O N
R 1 O R
O R
R 1
FIGURE 4.8 Tertiary amine catalyzed reaction between an alcohol and isocyanate.
of three-dimensional products. At times, the overall functionality is increased by introduction of
polyols with a functionality greater than two. The resulting foams are closed celled with outstand-
ing insulation properties. These foams are used as commercial insulations in the construction and
transportation industries. They can be used as a spray-on product insulating pipes and tanks and are
used in construction being sprayed on where insulation is needed.

Notice that open-celled products are employed where ready flexibility, softness, is needed while
closed-cell products are used when insulation is needed. Closed-celled materials retain heat/cold
while open-celled products allowed rapid heat exchange to occur.
Cross-linked PU coatings, elastomers, and foams can be produced using an excess of the diiso-
cyanate, which reacts with the urethane hydrogen producing an allophanate, or by incorporating
polyols such as glycerol in the reaction mixture. Larger polyols called macroglycols are used to
form segmented products, including Spandex used in clothing and undergarments. These products
are one basis for segmented elastomers, where aromatic PUs that form hydrogen bonding act as
the “hard” segment, and the polyols such as poly(ethylene glycol) (PEG) form the “soft” segment.
These segmented PUs can be thermoplastics or thermosets depending on whether cross-linking is
introduced.
Reaction–injection molding is increasing in importance and emphasizes the production of ther-
moset PU. Here, liquid monomers are mixed together under high pressure before injection into the
mold. Polymerization occurs within the mold. Most automotive dash panels are RIM produced.

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