Urethane prepolymer

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

Note that, in each case, water is split off to form each derivative. These blocked amines and blocked amino-alcohols can be stirred into isocyanate-terminated urethane prepolymers. Each system has a certain pot life. The pot life can vary from... 

In this way one mole of water reacts with two moles of isocyanate. For a urethane prepolymer with an equivalent weight of about 300, it only takes 9 grams of water to effectively consume... 

The same dibutyltin compounds are used in the industrial manufacture of poly(urethane) foams, the first step in which involves the addition of a polyether glycol to 2,4-diisocyanotoluene, to produce the urethane prepolymer having isocyanate end-groups. 

Polyurethane hydrogels derived from UV curable urethane prepolymer and hydrophilic monomers were prepared and their properties were evaluated. The urethane prepolymer used in this study contained well-defined hard segments centered with a polyether-based soft segment and end-capped with methacrylate groups. The hydrophilic monomers studied were 2-hydroxyethyl methacrylate (HEMA), N-vinyl pyrrolidone, and glycerol methacrylate. Methacryloxypropyl tris(trimethysiloxy) silane (TRIS) was also used in some cases to modify properties. All compositions were UV... 

A large exothermal effect resulting from chemical reactions is typical of these processes. When using oligomeric initial components, for example, formulations based on urethane prepolymers, epoxy resins, and lactams, self-heating may cause thermal decomposition. For this reason the correct choice of the initial solidification temperature is very important. 

A latent curable, urethane modified epoxy resin was found to provide high shear and peel strengths on metals. The modified epoxy is first reacted with an isocyanate-terminated urethane prepolymer and an acidic phosphorus compound.23... 

In this way one mole of water reacts with two moles of isocyanate. For a urethane prepolymer with an equivalent weight of about 300, it only takes 9 grams of water to effectively consume the prepolymer. This can lead not only to degraded physical properties, but it also forms gas bubbles in the proplnt which lead to voids and higher than predicted burning rates. This sensitivity to water content also exists with hydroxy-terminated polybutadiene polymers cured with... 

Given the above information, it is usually possible to determine likely end-uses for the prepolymer. Commercial supphers are not anxious to divulge full characterisations. Generally, functionality and particularly molecular weight information are withheld. Conversely, for a known end-use the polyurethane synthetic chemist designs a suitable characterisation and formulation for the urethane prepolymer. 

Xiao et al. (1995) developed a one-pack cross-linkable water-borne coating from anionic-blocked isocyanate-containing urethane prepolymers. The anionic-blocked urethane dispersions were prepared from poly(oxypropylene) glycol, polyarylpolyisocyanate, dimethylolpropionic acid and methylethyl ketoxime. The coatings based on these dispersions were reported to show good mechanical properties and chemical resistance. 

Feng and Waldman used 4,4 -diphenylmethane diisocyanate. The polyol is either 2000 or 4000 molecular weight polypropylene glycol (PPG) and the catalyst is dibutylin dilaurate (DBTOL). The NOC-terminated urethane prepolymers are made in the usual way by reacting an excess of methane diisocyanate with PPG in the presence of DBTDL at an NCO OH ratio of 1.4 2.0. The reactions are run at 70°C for about 3 h to reach a constant percentage of NCO. 

Scheme 1.10. Formation of urethane prepolymer and subsequent reaction with a diol or diamine extender to give a polyurethane or a polyurethane-polyurea. Adapted from Hepburn (1982).

Ho et al (1996) examined polyol or polysiloxane thermoplastic polyurethanes (TPUs) as modifiers in cresol-formaldehye novolac epoxy resins cured with phenolic novolac resin for computer-chip encapsulation. A stable sea-island dispersion of TPU particles was achieved by the epoxy ring-opening with isocyanate groups of the urethane prepolymer to form an oxazolidone. The flexural modulus was reduced by addition of TPU and also the Tg was increased due to the rigid oxazolidone structure. Mayadunne et al (1999) extended this work to a series of phenol- and naphthol-based aralkyl epoxy resins. 

Simultaneous IPNs are formed by homogeneously mixing together monomers, prepolymers, linear polymers, initiators, and crosslinkers, The monomers and prepolymers are simultaneously polymerized by independent reactions that differ enough to avoid interfering with each other. For example, a polyure-thane/polymethacrylate and a polyurethane/polystyrene were made in a process in which both monomers were prepolymerized, dissolved together, and reacted to form an IPN. Another urethane system was made from castor oil reacted with toluene diisocyanate and sebacic acid polyesters. The resultant urethane prepolymer was then mixed with polystyrene to form an IPN. 

Two-Package Coatings with Aminoformaldehvde Cure. One component consists of a hydroxyl-terminated urethane prepolymer while the other component is an alkyl ether (usually methyl or butyl ether) of a methylolmelamine or methylolurea derivative (melamine- or urea-formaldehyde condensation products or derivatives thereof) (140, 141). Various catalysts may be employed to accelerate the heat cure (. 125 C) and to lower the curing temperature. Curing results by splitting off of the respective alcohol, as shown ... 

Vultafoam. [General Latex Chem.] Urethane prepolymer rigid foam systems. 

Avadyne. [Herce Stevens] Water urethane prepolymer emulsion adhesive for laniination of film-to-film and film-to-metallized film structures. 

Vorite. [CasChem] Ifolymerized castor ofl or urethane prepolymers plasticizer, lubricant, penetrant, wetting agent, dispersant, couplbg solvent, adhe promoter for c ul lacquers, inks, adhesives, polish, caulks, leather dressing, hydraulic fluids, rubber conq>dbg., gasket cement... 

The method of immobilization in a polyurethane gel is at the same time a physical method of immobilization by inclusion and a chemical one. Indeed, the isocyanate groups of the urethane prepolymer react with-OH functions of water and with particular-NH2 functions of proteins located in the photosynthetic membrane structures. This chemical reaction is still reinforced by the addition of BSA to the prepolymer. 

Burkus [22] was one of the first to report the preparation of isocyanurate-containing urethanes from polyester urethane prepolymers reacted with a triethy-lamine propylene oxide trimerization catalyst. Burkus [15,15a] also reported the preparation of rigid foams by a similar trimerization process. 

---