Semi-prepolymers

Some examples of modified polyisocyanates are isocyanate-terminated quasi-prepolymers (semi-prepolymers), urethane-modified MDI, carbodiimide-modified MDI, isocyanurate-modified TDI, and isocyanurate-modified isophorone diisocyanate. 

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. 

The semi-prepolymer systems have the same advantages as the one-shot, two-component premix systems. 

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. 

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. 

In the preparation of TDI-based semi-prepolymer, TDI-80/20 and a polyol, preferably sucrose- or sorbitol-based polyol, are reacted to obtain about 30% free NCO-containing semi-prepolymers. 

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... 

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. 

Distribution of molecular weights in the commercial resins. Gel permeation chromatography (GPC) indicated that the liquid and semi-solid prepolymers (Epon 825, 828, and 834) were composed of... 

All the samples made from solid or semi-solid prepolymers had the same fine structure, with the size of the discontinuous phase in the range from 15 nm to 25 nm, whereas the samples made from liquid prepolymers had slightly larger discontinuous phases in the range from 25 nm to 50 nm. 

The silicone semi-lPNs consist of mixing a hydride-containing silicone prepolymer and a vinyl functionalized silicone polymer into a thermoplastic matrix such as PA, PBT, thermoplastic polyurethane (TPU) or styrene-ethylene/butylene-styrene (S-EB-S) block copolymer elastomer. The two silicone prepolymers co-react in the thermoplastic matrix during melt extrusion and injection molding to form a partially crosslinked network within the thermoplastic matrix. 

The adhesive used was a model semi-structural PU, similar to one described in an earlier work [7]. The two component formnlation consisted of methylene bis-4-cyclohexyl isocyanate prepolymer and trifunctional polyols with molecnlar masses in the range 500-900. A small amount of tin catalyst (UL-28, Witco Chemicals) was also present. The PU was used as control without additives and was modified by the following silanes ... 

CE resins are available in liquid, semi-solid or low-melting solid form. CE resins contain cyanate monomers or a prepolymer, which is prepared by a controlled... 

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