Prepolymer synthesis

Fig, 17. (a) Isocyanate-terminated polyurethane prepolymer synthesis, (b) Moisture cure of isocyanate-terminated polyurethane prepolymers, (c) Side reactions of isocyanate-terminated polyurethane prepolymers. 

Catalysts serve a dual purpose in one-component moisture-curing urethanes. The first purpose is to accelerate the prepolymer synthesis. The second purpose is to catalyze the curing reaction of the adhesive with moisture. The most common catalysts used to promote both prepolymer formation (NCO/OH) and later the adhesive curing reaction (NCO/H2O) are dibutyltin dilaurate and DMDEE ((tertiary amine. A stabilizer such as 2,5-pentanedione is sometimes added when tin is used, but this specific stabilizer has fallen from favor in recent years, due to toxicity concerns. DMDEE is commonly used in many one-component moisture-curing urethanes. DMDEE is one of the few tertiary amines with a low alkalinity and a low vapor pressure. The latter... 

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 ... 

In Fig. 11 spectra of resorcinol resins 36) are given. In comparison with prepolymers, it shows that the completely hardned resin is more susceptible than that of uncured resin to the conditions of prepolymer synthesis. Because of only moderate resolution in the aromatic region the spectral pattern is fairly similar for the two resins. 

Sec. 9-2a). The crosslinking process is referred to as drying and is directly dependent on the content of unsaturated fatty acid. The crosslinking reaction involves chemical reactions different from those involved in prepolymer synthesis. 

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. 

The length of the individual segments in the block copolymer is determined by the polyethylene glycol used for the prepolymer synthesis, whereas the length of the polystyrene blocks is controlled by the ratio [M]/[I]0 5 (see Fig. 4.1) and the temperature at which the block copolymerization is carried out56). 

At the end of the prepolymer synthesis, the diisocyanate excess depends on the initial excess and also on the reactivity ratio between the two isocyanate groups. 

In any case, the prepolymer synthesis results in somewhat better defined domains, characterized by the relative translucency of the prepolymer route material relative to the one-shot route, and as illustrated in the transition behavior. Figure 3. Better defined domains mean a purer rubber phase, which in turn may yield greater toughness to this form of the material. Important questions as to the development of intermediate Tg s with extensive interfacial boundary material (48) or on complete thermodynamic miscibility (49) remain for the future. 

Some examples of prepolymer synthesis are shown in the following schemes ... 

Uses Flexibilizer in metal decorative, automotive primers and topcoats, oveq)rint varnishes, aerospace coatings, and prepolymer synthesis Features Highest reactivity exc. adhesion to substrates fast cure Prr rerties Gardner < 1 color It. straw liq. sol. in alcohols, esters, ketones, xylene partly sol. in min. spirits low m.w. sp.gr. 1.12 dens. 9.3 Ib/gal vise. 10,000 cps acid no. 0.1 hyd. no. 230 100% act. polyester polyol 0.02% moisture K-Flex UD-320 [King Ind.]... 

Reaction Mechanism of Boron Modified Prepolymer Synthesis. The Source for the Dimethylene Ether Bridge is a Dimer and higher Oligomers, as Shown in the Transition State. 

The pH value used for prepolymer synthesis is an important parameter in order to obtain novolacs or resoles. Novolacs are formed if the reaction between phenol and methanai occurs in an acidic medium. Typical catalysts used are oxalic acid, sulfuric acid, and 4-methylbenzene sulfonic acid. The use of slight acidic conditions and bivalent metal salts (e.g. Ca, Zn, Mg, Pb, Co, Cu, Ni) to catalyze the phenol methanai reaction results in ortho, ortho -novolacs. Furthermore, no catalyst is necessary for the reaction of a substituted phenol with paraformaldehyde at temperatures between 150-200 C for the manufacture of novolacs. Resoles are formed when phenol reacts with an excess of methanai atpH values larger than 5. This process is catalyzed by inorganic salts, ammonia, and 1,3,5,7-tetraazatricyclo-... 

The prepolymer can be synthesized by a simple one-pot synthesis . During a first step an NCO-terminated prepolymer is prepared by a reaction of the polyol with an excess of TDI. The second step comprises the reaction of all remaining NCO groups with the silane 1. Phenyltrimethoxysilane 2 serves to reduce the viscosity of the prepolymer mixture. It has already been added during the prepolymer synthesis. 

PUs microstructure and mechanical behaviour are strongly dependent on the synthesis method employed. The various methods for producing PUs can be differentiated according to the medium of preparation (bulk, solution, water) and the addition sequence of reactants one-step or prepolymer synthesis routes). 

For the preparation of the PU elastomer containing simultaneously two different diisocyanates the macrodiol was reacted in different ways with two moles of MDI and two moles of DBDI. Finally the so-obtained different prepolymers were reacted with 2.64 moles of chain extender which was in all the cases ethylene glycol (EG). There were adopted three different modes of prepolymer synthesis leading to three different active PU oligomers. 

PUci Single step prepolymer synthesis which used a melt mixture of the two... 

PUc2 A two-step prepolymer synthesis where two moles of DBDI was first... 

Two step prepolymer synthesis where DBDI is first reacted, followed by the addition of MDI. 

The synthesis of the PUc2 polymers involved a two step prepolymer synthesis where the first introduced diisocyanate was DBDI, followed by the distribution of MDI in the prepolymer. The synthesis of PUc3 polymers was achieved similarly, except that the first introduced diisocyanate was MDI, followed by the distribution of DBDI in the prepolymer. 

Fig. 4.2 Stress-strain curves during loading at nominal strain-rate 0.0042 s for PUs with PEA as a soft phase. Material C was achieved by using a single step prepolymer synthesis using a melt mixture of the two isocyanates (MDl+DBDl) [280]...

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