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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. |
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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. |
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| Introduction -
General Information Chemistry |
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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.
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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Last edited on:
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November 22, 2006
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