Prepolymer method

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

There are many situations where isocyanates are derivatized before use, such as when MDI or TDI are reacted with high-molecular-weight, difunctional polyols to form low-NCO prepolymers used to make cast elastomers by the prepolymer method (Section 4.3.2.1). Because of their molecular weight, prepolymers can be of much higher viscosity than the starting isocyanates, and some are solid at room... 

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. 

Scheme 4.23 Two-component water-borne polyurethane by the prepolymer method.

Example 2. Two-Shot Cast Elastomer (Prepolymer Method). Synthesis of polyurethane elastomers via the prepolymer method is challenging and requires especial attention to details and techniques. This example is a relatively easy one, using a low-viscosity, 9.6% NCO MDI-based poly(tetramethylene ether) glycol prepolymer. It is adapted from the Bayer Product Information Bulletin for Baytec ME-090 prepolymer. 

NMR spectroscopy, 93. See also Proton NMR integrations Hoechst continuous process, 548 Homo-coupling reactions, aryl halide, 486-487 Homopolymers, 7 Hot-cast prepolymer method, 211 Hot phosgenation, 222 Houvink-Sakurada equation, 286 HTMAB. See Hexadecyltrimethylammonium bromide (HTMAB)... 

Prepolymer method for entrapment of biocatalyst developed by Fukui and Tanaka is considered to be one of the best immobilization methods. Characteristic features and advantages of the prepolymer method can be summarized as follows ... 

Synthesis of Polyurethanes. In the "prepolymer method" employed in this study, MDI (2 equivalents) and PPG (1 equivalent) were reacted at 60°C in the presence of 1% dibutyltin dilaureate as catalyst, in the melt. The course of the polymerizations was followed spectroscopically by observing the intensity of the — NCO peak in the IR (Scheme III). 

The other foaming process is referred to as the prepolymer method. The monomers are reacted to form a low molecular-weight prepolymer. Later the prepolymer is mixed with small amounts of water and is heated. The water reacts with the free isocyanate groups to liberate carbon dioxide, which foams the polyurethane as cross-linking starts. 

The one-prepolymer method involves one of the above prepolymers with two small reactants. The macrodiol is reacted with a diol and diisocyanate... 

The block lengths and the final polymer molecular weight are again determined by the details of the prepolymer synthesis and its subsequent polymerization. An often-used variation of the one-prepolymer method is to react the macrodiol with excess diisocyanate to form an isocyanate-terminated prepolymer. The latter is then chain-extended (i.e., increased in molecular weight) by reaction with a diol. The one- and two-prepolymer methods can in principle yield exactly the same final block copolymer. However, the dispersity of the polyurethane block length (m is an average value as are n and p) is usually narrower when the two-prepolymer method is used. 

Prepolymer method This consists of two steps. The diisocyanate and polyol are reacted together to form an intermediate polymer called prepolymer which is normally a thick viscous liquid. This prepolymer is... 

If one were to design a new polyurethane, the prepolymer method is doubtless the method that would be used. Prepolymer is a term of art that designates an intermediate process in the production of polyurethanes as we know them. Prepolymers are quite easy to produce in a laboratory. The isocyanate is slowly added to... 

We have focused our attention on the prepolymer method for polyurethane development because we feel that it offers the researcher the greatest control of the molecule. We hope to encourage the scientific community to investigate other nonclassical polyurethane tools. If the pin posc of a device is purely physical, the large polyurethane manufacturers and chemists are the best resources for expertise. If, however, the intent is to experiment with a new polymerization technique for a particular medical or environmental application, the researcher must be able to assemble the component parts along lines with which the polyurethane industry may not be familiar. 

We have described a process by which small quantities of foam can be made by the prepolymer method. A number of methods are available to bring the variables involved in prepolymer making under control. We assume that the starting point for such processes is the acquisition of commercial isocyanates, polyols, and additives. Other than for audit purposes, we will assume they arrive with certification that they are of the so-called urethane grade. In the case of polyols, this typically means they contain less than 0.01% water and have good color. Free acids and low metal and chloride contents are important considerations for isocyanates. Manufacturers are well aware of the problems that will arise if these contaminants are not controlled and the materials cannot be used with confidence. 

All hydrophilics are currently processed by the prepolymer method. The emulsification of the prepolymer and water are the primary determinants of cell size. The water also serves as a heat sink to moderate the temperature of the reaction. By adjusting the temperatures of the prepolymer and the water, one can control the kinetics described above. The mass of the water limits the destructive exotherm. 

With hydrophobic polyurethanes, the only option is adhesion. The polyol and isocyanate environments producing hydrophobic polyurethanes by the prepolymer method are thought to be too severe for hving cells. As noted earher, the weaker adhesion of adsorption to a hydrophobic polyurethane can be an advantage. Sanroman compared die adsorption and entrapment techniques and determined that the adsorption technique was superior based on citric acid productivity and operational stability. 

The high exotherm and the number of feeds have led to the popularity of the quasiprepolymer variation of the one-shot technique. Quasiprepolymers are a version of the standard prepolymer method, where some of the polyol is reacted with an excess of diisocyanate. This is later reacted with the remainder of the polyols, including the chain extender and some catalysts. The advantage of this scheme is that the exotherm is spread between the preparation of the quasiprepolymer and the final cured part. In MD1 systems, BDO is often used as the curative. As the molecular weight of BDOs is low, the amount added is low. By dissolving it in the polyol, a more even mix ratio can be obtained. 

The monomers 26 and 27 have also been used to synthesize segmented poly(ether urethane) from poly (propylene glycol) (PPG) and 4,4 -methylene-bis(phenyliso-cyanate) (MDI) [37]. In the prepolymer method employed, MDI (2 equiv.) and PPG (1 equiv.) were allowed to react at 60 °C in the presence of 1 mol% dibutyltin... 

In the prepolymer method for one-shot polyether flexible urethane foams the primary role of the silicone surfactant is to lower surface tension and to provide film (cell-wall) resilience. Resilient films prevent the collapse of the foam during foam rise and continue to stabilize it until the foam is self-supporting. A secondary, but nevertheless important role of the silicone surfactant is cell-size control. The silicones can be added to the formulation in any of the 2 to 6 streams usually fed to the mixing head in the one-shot process. Usually, however, the silicone is metered separately, in combination with the polyol, or added as a wa-ter/amine/silicone mixture. It can also be added in the fluorocarbon blowing agent (52). 

Polyurethane fibers (elastanes) can be made via either of the following two ways. In the first technique, the PU is first synthesized using the prepolymer method, and then the PU is spun from solution (approximately 30% PU) to form a filament or yarn. The yarn can either be collected as one long strand or be cut as it is collected. The second method, called reactive spinning, involves synthesizing the polyurethane and forming the filament all at the same time using a reactive bath to spin the fibers. ... 

Preparation of Elastomers. Two methods for preparing TPU elastomers are commonly used. In the prepolymer method, the polyol is prereacted with all of the MDI and the resultant prepolymer, often containing free MDI, is subsequently reacted with the chain extender. In the masterbatch method, the polyol and chain extender are premixed and then combined with the MDI,... 

Tanaka A, lida T (2001) Entrapment of biocatalysts by prepolymer methods. In Vulf-... 

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. 

After cross-linking polymerization, the grounded power was packed in chromatography columns using an acetonitrile-acetic acid solution then, assessment of affinity was examined for cinchonidine target molecule. Preliminary results proved that the prepolymer method presents potential utilization as an artificial receptor. 

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