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Prepolymer Preparation
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For many applications of isocyanates it is an advantage to prepare a
prepolymer by reacting a polyol with a diisocyanate. The preparation
of prepolymers raises the cost of polyurethane manufacture, but
reduces the risk of handling volatile isocyanates. The resulting
prepolymers still contain free diisocyanates. The amount of free
diisocyanate will depend on the ratio of hydroxyl/isocyanate and the
relative reactivity of the first and second isocyanate on a
diisocyanate. There are also commercially prepared prepolymers
available which have the excess of free diisocyanate removed by thin
film evaporation. Although these prepolymers are higher in cost, they
are saver to handle. If polyols and isocyanates with an average
functionality of >2 are reacted the prepolymer can gel.
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Calculation of isocyanate charge
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Simple
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Complex (several polyols)
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Components
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Charge lookup table
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Polyol
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Charge the polyol and heat under vacuum to 90°C.
Continue to heat under vacuum for two hours. Heating under vacuum will
remove the moisture from the polyol. Cool to 40-70°C under nitrogen.
For faster reacting isocyanates the lower temperature is recommended.
For MDI the higher reaction temperature can be used. At the higher
reaction temperature more side reactions are possible
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Diisocyanate
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Charge the diisocyanate and hold at 70-80°C
for one hour. Measure the free
isocyanate content.
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Repeat drying of the polyol as shown in the procedure 1, but instead
of feeding the diisocyanate into the reactor, charge the diisocyanate
into a separate reactor and feed the polyol into the isocyanate
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Sparge a suitable reactor equipped cooling and heating with nitrogen.
Charge the isocyanate and heat to 50-60°C.
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Slowly feed the polyol into the reactor. Control the addition
rate so that the temperature does not exceed 80-85°C.
Cool the reactor if necessary. Hold for an additional 3-4 hours.
Measure the free free
isocyanate content.
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