Prepolymer process

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 secondary hydroxyl groups of these poly(oxypropylene) glycol diols are less reactive than the primary hydroxyl groups of the earlier polyesters. At the time of the introduction of these polyethers, the catalysts then available were insufficiently powerful for one-shot processes to be practical and so these polymers have been used primarily in prepolymer processes. 

The waterborne prepolymer process is similar to the prepolymer synthesis described earlier, except that most of the waterborne prepolymers are based on aliphatic isocyanates and contain an internal emulsifier. There are several types of internal emulsifiers, both anionic and cationic. A good summary of these stabilizers is found elsewhere [56], The majority of the waterborne urethanes are anionic dispersions. An internal surfactant, such as dimethylolpropionic acid, is often incorporated into the prepolymer ... 

There is another method, called the quasi-prepolymer technique, which is similar to the full-prepolymer process but utilizes prereacted isocyanates in the 10-20% NCO range. This eases processing compared to full-prepolymer systems, requiring lower temperatures and volume ratios typically from 4 1 to nearly 1 1 (polyol to isocyanate). 

Step growth polymerization can also yield highly crosslinked polymer systems via a prepolymer process. In this process, we create a prepolymer through a step growth reaction mechanism on two of the sites of a trifunctional monomer. The third site, which is chemically different, can then react with another monomer that is added to the liquid prepolymer to create the crosslinked species. We often use heat to initiate the second reaction. We can use this method to directly create finished items by injecting a mixture of the liquid prepolymer and additional monomer into a mold where they polymerize to create the desired, final shape. Cultured marble countertops and some automotive body panels are created in this manner. 

PPDI PPG Prepolymer Processability initiating agent. An abbreviation for paraphenylene diisocyanate. Polypropylene glycol. The product from reacting a polyol with an isocyanate. The relative ease with which a raw or compounded polymer can be handled. 

Much of the success of these newer systems has been due to a unique prepolymer technology. It is this concept of prepolymer processing which has allowed the development of many new applications and the design of structural plastics with well-defined and highly desirable properties. 

Thus, prepolymer processing contrasts sharply with thermoplastic processing. Thermoplastics are polymerized to completion during the plastics manufacture. The fabricator apphes heat and pressure by means of extrusion, injection molding, or calendering to produce the finished article. 

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. 

Manufacturing Process. Both prepolymer and one-shot processes are available, but the polymeric isocyanate-based one-shot process is used in preference because of the easy processing due to the low viscosity of the system, relatively low toxicity of polymeric isocyanates, and fewer environmental problems. However, a disadvantage of the one-shot process is a possible risk of shrunken-foam formation due to its higher closed-cell content. In contrast, the TDI-based prepolymer process has advantages including better in-mold flowability and higher open-cell content. 

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. 

TDI Prepolymer Process. This process has the major advantage of better flowability in pour-in-place processes, e.g., household-refrigerator insulation, than crude TDI-based or polymeric MDl-based processes. An example of TDI-based prepolymers and a formulation for making rigid urethane foam on a small scale is shown below. 

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. 

The reactions by which urethane foam are produced can be carried out in a single stage (one-shot process) or in a sequence of several stages (prepolymer process and quasi-prepolymer process.) These variations led to 27 basic types of products or processes, all of which have been used commercially. 

In the one-shot process, all of the ingredients—isocyanate, polyol, blowing agent, catalyst, additives, etc.—are mixed simultaneously, and the mixture is allowed to foam. In the prepolymer method (Figure 1.29), a portion of the polyol is reacted with an excess of isocyanate to yield a prepolymer having isocyanate end groups. The prepolymer is then mixed with additional polyol, catalyst, and other additives to cause foaming. The quasi-prepolymer process is intermediate between the prepolymer and one-shot processes. 

Although the two-step prepolymer process is less important than the one-shot process, it has the advantage of low exotherms, greater flexibility in design of compounds, and reduced handling hazards. 

For a comparison of PTMEG in MDI/BDO cured elastomers the 3200-MW ultra-low monol PPG diol was chosen which contains about 20 wt% EO (Acclaim Polyol 3205) since it gives the best overall properties in this type of system. A 2000-MW PTMEG was used since it gives the best performance at an 80 Shore A hardness. Elastomers of 80 Shore A hardness were prepared using the prepolymer process. Table 9.14 summarises their prepolymer viscosities, elastomer processing characteristics and physical properties. 

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