Semi-prepolymers process

Processes of Urethane Foam Preparation. Urethane foams can be prepared by the one-shot process, semi-prepolymer (quasi-prepolymer) process or prepolymer process. The one-shot fnrocess is most commonly used. The semi-prepolymer process is sometimes preferred because of the advantages of easy processing, stabilized-foam rise and lower exotherm. 

In principle, both the one-shot process and semi-prepolymer processes have been used for rigid-urethane-foam manufacturing. However, the monomeric TDI-based one-shot process was used only in the initial stage of the rigid-urethane-foam industry because of the toxicity problems of TDI and difficulties in controlling reactivity due to the high NCO percent. For these reasons TDI-prepolymers, blends of TDI prepolymers and polymeric isocyanates, and 100% polymeric isocyanate are most widely used. 

Before the prepolymer preparation, it is important to confirm that the polyols used have a low water content, e.g., less than 0.02% and a low pH value in order to prevent gelation in the prepolymer preparation. Table 23 shows rigid urethane-foam formulation by the semi-prepolymer process for obtaining a 30-kg/m density foam by hand mixing. 

Table 23 Formulation for Rigid Foam Made by the Semi-Prepolymer Process...

Crude TDI—One-Shot Process. In place of the TDI-semi-prepolymer process, a one-shot process using crude TDI is widely used for refrigerator insulation because of its lower cost, although the flowability of this system is inferior to the TDI-prepolymer process. 

Preparation. Urethane-modified isocyanurate foams are mostly fH epared by the one-shot process based on the jH inciple discussed previously in the urethane modification section. The semi-prepolymer process is used only in limited cases because of viscosity problems. This section describes several examples of formulations for producing block foams, slabstock foams, laminate foams, and spray foams. 

Polyurethane foams may be rigid, semi-rigid or flexible. They may be made from polyesters, polyethers or natural polyols such as castor oil (which contains approximately three hydroxyl groups in each molecule). Three general processes are available known as one-shot, prepolymer or quasi-prepolymer processes. These variations lead to 27 basic types of product or process, all of which have been used commercially. This section deals only with flexible foams (which are made only from polyesters and polyethers). Since prepolymers and... 

Prepolymer Process. Semi-prepolymers are prepared mostly by the reaction of TDI and a polyol to obtain a free NCO content of 5 to 10%. The polyols employed for the prepolymers are generally branched amine-or glycol-initiated polypropylene oxide-based polyether polyols having a molecular weight of ca. 600 to 4,000. Polyesters having molecular weights of about 1,000 to 2,000 can also be used. 

The ketimine of isophorone diamine is formed by reacting it with methyl isobutylketone, splitting off water in the process. When said ketimine is added to an isocyanate-terminated prepolymer based on IPDI, a semi-stable system is established with a pot life of several hours. The ketimine is a Schiff base and thus can react even in the absence of water. The complexities and advantages of this system are reviewed by Bock and Halpaap [75] ... 

The esterification reactor is usually not emptied completely after a batch is finished and a small amount of prepolymer is retained in the reactor. The reason for this is the solubility of TPA in EG and BHET, as discussed earlier in Section 3.1. During operation, the batch-wise prepared slurry is fed continuously into the esterification reactor while the esterification is already proceeding. For a significant part of the process time, the batch esterification reactor is operating semi-continuously. 

The preliminary operations of preparation of the initial components can be carried out in special centralized chemical production plants or directly in the work bays where the articles are manufactured. When the initial components are obtained in the form of semi-finished products, the manufacturing process is simplified. Semi-finished products are single- or, more often, two-component mixtures, especially die prepolymers used in polyurethane production. Mixtures include chemical reactants and mixtures of polymers or oligomers with different additives required for final items. 

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