Prepolymer Preparation

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.  
Calculation of isocyanate charge Simple Complex (several polyols)

Components

Procedure 1

Charge lookup table
Polyol Charge the polyol and heat under vacuum to 90C. Continue to heat under vacuum for two hours. Heating under vacuum will remove the moisture from the polyol. Cool to 40-70C 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
Diisocyanate Charge the diisocyanate and hold at 70-80C for one hour. Measure the free isocyanate content

Procedure 2 

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
Sparge a suitable reactor equipped cooling and heating with nitrogen. Charge the isocyanate and heat to 50-60C.
Slowly  feed the polyol into the reactor. Control the addition rate so that the temperature does not exceed 80-85C. Cool the reactor if necessary. Hold for an additional 3-4 hours. Measure the free free isocyanate content
Procedure 1. This procedure is only recommended in the laboratory or in small volume if an isocyanate terminated polymer is to be prepared. The potential danger with this method is that any interruption of addition of the isocyanate can lead to gelation. The reaction is exothermic and using this addition method it is not possible to stop addition to control the exothermic reaction.
Procedure 2. This the recommended procedure of producing an isocyanate terminated prepolymer. The disadvantage of this method is that it requires two reactors for the preparation of the prepolymer.

Last edited on:

November 14, 2006

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