Quasi-prepolymers

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... 

In addition to one-shot processes, quasi-prepolymer systems are used commercially with rigid polyether foams. The quasi-prepolymer is commonly produced using excess TDI rather than diphenylmethane di-isocyanate. Sinee the former isocyanate is light in colour and the latter dark, quasi-prepolymer foams... 

Polyester polyurethanes are usually based on a blend of a quasi-prepolymer (polyester/MDl) and a diol/polyester suitable for spray-up operation. An alternative is to use a solvent-containing system using blocked curatives to give an extended pot-life of 2 to 3 hours enabling them to be brush, roller or spray applied. 

Polyether polyurethanes are often based on a quasi prepolymer/diol blend reacted with a carbodiimide modified M.D.I. suitable for automatic mix-dispensers. 

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). 

The full-prepolymer, quasi-prepolymer, and one-shot techniques may appear to be quite similar, but they each have important bearing on equipment requirements and physical properties. The full-prepolymer method, for example, is die process of choice for making high-performance cast elastomers with superb dynamic properties, such as industrial bumpers and bushings. Quasi-prepolymers are used in less demanding applications like shoe soles, adhesives, and spray elastomers. One-shots make up the rest, including most foams and elastomers. 

Quasi-prepolymers, 236, 237 Quinoid resonance forms, 402 Quinone methide reactions, with... 

R. Wiser-Halladay. Polyurethane quasi prepolymer for proppant consolidation. Patent US 4920192, 1990. 

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. 

A self-releasing system based on an amine modified polyol to which had been added a proprietary IMR agent when formulated with the aromatic amine chain extender diethyltoluendlamine (DETDA) and MDI quasi-prepolymers demonstrated exceptional process and molded product attributes. The formulated systems Is shown In "Table I". 

ISOCYANATE 1. Quasi-prepolymer of MDI prepared by reacting small quantities of polyols with MDI. 

The best combination of physical properties for basic high modulus systems has been accomplished through the use of an amine extender in combination with a quasi-prepolymer modification of MDI. These systems are designated Bayflex"... 

Very recently Papa and Critchfield El9 7 described the prepara tion of foams based on a resole resin containing a high ortho-methylol content and a blend of quasi prepolymer of tolylene diiso cyanate, a phosphorus-containing polyol and polymeric isocyanate containing a minor portion of tris-(2-chloro-ethyl) phosphate. 

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. 

Quasi-prepolymer systems based on polyester polyols and polyether polyols are becoming important in shoe soling, the former being most wear resistant and the latter the easiest to process. 

Most urethane adhesives are based on urethane prepolymers. A prepolymer is made by reacting an excess of diisocyanate with a polyol to yield an isocyanate-terminated urethane as shown in Fig. 11. Prepolymers may have exeess isocyanate present ( quasiprepolymers ) or they may be made in a 2 1 stoichiometric ratio to minimize the amount of free isocyanate monomer present. Most moisture-cured prepolymers are based on 2 1 stoichiometric ratios. Two-component adhesives generally are based on quasi-prepolymers, which use the excess isocyanate to react with either chain extenders present in the other component or with the substrate surface. 

Uses Polyol for urethane elastomers and coatings Features Not rec. to make prepolymers or quasi-prepolymers Properties APHA200 clear liq. m.w. 700 sp.gr. 1.03 vise. 250 cps hyd. 

Uses Polyol for urethane applies, ind. tow- and high-dens, foams, coatings, adhesives, sealants, encapsulants, elastomers Features Not for use to make prepolymers or quasi-prepolymers Properties Pt-Co 300 clear liq. sp.gr. 1.09 vise. 270-3 cps acid no. 

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