Elastomers prepolymer technique

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. 

MDI- or NDI-based PUs usually have better mechanical properties than PUs obtained from aliphatic polyisocyanates. The effect of various diisocyanates on the thermal stability of their PU elastomers is given below for a polycap-rolactone (mol. wt. 2000)/diisocyanate/BDO type elastomer of molar ratio 1 2.6 1 and synthesized using the prepolymer technique. 

Both series of polyurethanes were prepared using a prepolymer technique in which reactants were mixed at 70 °C/1 hour, cast into molds at 105 °C/2 hours, and cured at 80 °C/14 hours. The BD/MDI hard segment contents ranged from 0% (transparent, colorless homopolyurethanes) to 30% w/w (opaque, white copolyurethanes). All elastomers were characterized using DSC, dynamic mechanical, and tensile stress-strain measurements. 

This prepolymer technique is frequently used in the manufacture of polyurethane elastomers, coatings, sealants, flexible foams, monocomponent polyurethanes, etc. 

Rigid polyurethane foams were first used as the core of sandwich structures in aircraft. Polyester polyols were used in combination with TDI, employing the prepolymer technique described for elastomers in Sections IV. A and IV.B. Major developments in manufacturing rigid polyurethane foams came with the introduction of MDI as the isocyanate eomponent and the use of chlorofluorocarbon blowing agents, especially... 

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. 

In designing a prepolymer for a specific use, this technique provides for more control of the chemistry. For elastomers, for instance, including some three-dimensional character changes the physical properties of the polymer. More cross-linking lowers elongation and increases strength. Cross-linking is not necessary for elastomers, but it is required for foams. 

The terminally bifunctional prepolymer, carboxy-terminated polyisobutylene, is prepared by the pyridine-modified ozonization of a copolymer of piperylene (2-4% ) and isobutylene. Figure 1 shows a block diagram of the process. The high molecular weight elastomer can be prepared by the conventional techniques of isobutylene-diene copoly-... 

Room-temperature crosslinkable polysiloxane elastomers (commonly called RTVs) are prepared by two techniques. In the first, chlorosiloxanes with functionality larger than two are added to hydroxy-terminated prepolymers. The products are subsequently crosslinkedby a second addition of polyfunctional compounds like tetraalkoxysilane in the presence of tin catalysts, like stannous octoate. Crosslinking occurs at room temperature. This reaction may vary from 10 minutes to 24 hours ... 

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