Polyurethane, blocked isocyanates

This simple reaction is the bedrock of the polyurethane iadustry (see Urethane polymers). Detailed descriptions of the chemistry and process have been published (65—67). Certain carbamates are known to reversibly yield the isocyanate and polyol upon heating. This fact has been commercially used to synthesize a number of blocked isocyanates for elastomer and coating appHcations. 

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. 

Several poly(urea urethane) oligomers 28 (Figure 12) were prepared by one-component polycondensation of iV-(hydroxyalkyl)-2 -oxo-1,3-diazepane-l-carboxamides, which act as intramolecular blocked isocyanates <2005PLM 1459>. These oligomers are semicrystalline materials and their melting points show the odd/even effect observed earlier for [ ]-polyamides, [ ]-polyurethanes, poly(ester amide)s, and poly (amide urethane)s. Further analysis showed that the polymers are stable up to ca. 205-230 °C, the polymers with the lower number of methylene groups in the amino alcohol decomposing at the lowest temperature. 

Spindler and Frechet1391 prepared hyperbranched polyurethanes by step-growth polymerization (Scheme 6.8) of protected, or blocked , isocyanate AB2 monomers. The method is dependent on the thermal dissociation of a carbamate unit into the corresponding isocyanate and alcohol moieties.140,411 Decomposition temperatures range from ca. 250°C for alkyl carbamates to ca. 120°C for aryl carbamates.1401... 

Spindler, R., and Frechet, J. M. J. 1993. Synthesis and characterization of hyper-branched polyurethanes prepared from blocked isocyanate monomers by step-growth polymerization. Macromolecules, 26, 4809 1813. 

Phenol blocked isocyanates also have been used to prepare hyperbranched polyurethanes by a step-growth polycondensation mechanism, using DBTDL as a catalyst. 4... 

Because of the toxicity of isocyanates, there is interest in preparing blocked isocyanates and polyurethanes by a non-isocyanate process. Reaction of aliphatic diamines with cyclic carbonates (l,3-dioxolan-2-ones) results in the formation of /3-hydroxyalkyl carbamates. 

STRUCTURE 4.1 Hydrogen bonding in a hydroxyurethane. (Reprinted from E. Delebecq, J-P. Pascault, B. Boutevin, and F. Ganachaud, On the Versatility of Urethane/Urea Bonds Reversibility, Blocked Isocyanate, and Non-Isocyanate Polyurethane, Chem. Rev. 113 (2013) 80-118. With permission.)... 

Polyurethane Enamels. Small volumes of this enamel type are used in applications for which the temperature does not rise above approximately 105 C. These enamels are made from hydroxyl-bearing polyesters and blocked isocyanates. During the heating cycle in the enameling process the isocyanates become unblocked and react with hydroxyl groups of the polyesters to form a cross-linked polyurethane film. Wires so insulated can be directly soldered, without prior removal of the polyurethane insulation. 

Polyurethanes (single-component system) Block polyisocyanates and hydroxyl polymers Phenol-blocked isocyanates and hydroxyl groups Unblocking or removal of phenol to form a free isocynate followed by addition of hydroxyl groups to form urethane linkages 8 ... 

Blocked or partially blocked isocyanates have been extensively employed in cationic electrodeposition. Polymeric compositions employed for cationic electrodepositable coatings have been reviewed by Kordomenos and Nordstrom (82A). Matsunaga et al. (83) were the first to synthesize cationic thermosettable urethane resins. The system described was prepared from an isocyanato-terminated polyurethane prepolymer by reacting the free isocyanato groups with a tertiary amine that had at least three hydroxyl groups. The... 

Polyurethane systems are also formulated for surface-coating applications. A wide range of such products has become available. These include simple simple solutions of finished polymer (linear polyurethanes), One component systems containing blocked isocyanates, two-component systems based on polyester and isocyanate or polyether and isocyanate, and a variety of prepolymer and adduct systems. 

The idea of this invention is to improve resistance to heat and solvents by application of polyurethane system which is not reactive at room temperature but reacts during the gelling process because blocked isocyanate is unblocked at elevated temperature. The plastisol has stable viscosity for 10 days. The plastisol and technology can be used for many different purposes, including dip coating. 

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