Polyfunctional isocyanate

Over 40 chemical reactions are used in urethane chemistry. The six most common urethane reactions that are relevant to adhesives are shown in Fig, 1. The monomeric forms of the reactions are shown for simplicity s sake however, most commercially useful products for polyurethanes are based on polyfunctional isocyanates and polyfunctional alcohols or polyols . 

Antioxidant (Sherex Chemical Co.). Polyfunctional liquid isocyanate (Upjohn Co.)... 

The class of 1,3-dipolar cycloadditions embraces a variety of reactions that can accomplish the synthesis of a diverse array of polyfunctional and stereochemically complex five-membered rings.3 The first report of a 1,3-dipolar cycloaddition of a nitrone (a 1,3-dipole) to phenyl isocyanate (a dipolarophile) came from Beckmann s laboratory in 1890,4 and a full 70 years elapsed before several investigators simultaneously reported examples of nitrone-olefin [3+2] cycloadditions.5 The pioneering and brilliant investigations of Huisgen and his coworkers6 have deepened our under-... 

Rigid polyurethane foams can be made from either polyester or polyether prepolymers, which are crosslinked with polyfunctional isocyanates. The resulting foams are largely closed cell, with only about 5 to 10% of cells being open. Rigid polyurethane foams are widely used as insulation in commercial, residential, and industrial settings. 

Preparation of coatings as free films. The oligomeric esterdlols were mixed with the crosslinkers HMMM or polyfunctional isocyanate. The molar ratio esterdlol/HMMM was 2 1 leading to an OH/OCH3 ratio of 4 6. The OH/NCO ratio was 1 1. Some 1 wt% diethanolamine salt of p-toluene sulphonlc acid, respectively 0.2 wt% Dabco were used as catalyst. The coatings were applied to Bonder 101 plates which had been sprayed with a thin layer (1-2 pm) of teflon. 

Polyurethanes are macromolecules in which the constitutional repeating units (CRUs) are coupled with one another through urethane (oxycarbonylamino) groups. They are prepared almost exclusively by stepwise addition polymerization reactions of di- or polyfunctional hydroxy compounds with di- or polyfunctional isocyanates ... 

Rigid foams are based primarily on polyfunctional, low molecular weight alcohols and amines. Most global applications conventionally use polymeric isocyanates, TDI, or an undistilled grade of mixed TDI isomers. TDI prepolymers which have hydroxyl and isocyanate groups have been marketed as a low vapor pressure alternative to undistilled TDI. Density reduction is effected via the addition of chlorofluorocarbons, low molecular weight alkanes, or via the in situ generation of carbon dioxide. The resultant closed cell foams find applications as insulators in construction, appliance, transportation, pipeline, and tank end uses. 

As an example, the temperature rise for the formation of a polyurethane by reaction of a polymethylenpolyphenyl isocyanate (average functionality = 2.7), with a polyfunctional polyol based on sorbitol, using dibutyltin dilaurate (DBTDL) as a catalyst, is shown in Fig. 5.19 for two different catalyst concentrations (Marciano et al., 1982). 

A typical product has the following characteristics number average molecular weight, — 1800 carboxyl assay, — 1.1 meq./gram color, amber bulk viscosity (Brookfield synchrolectric viscometer) 70°F., 8000 poises, 100°F., 2000 poises, 150°F., 250 poises, and 250°F., 15 poises. The product can be cured with polyfunctional epoxides, aziridines, and isocyanates for applications such as thermoset and pressure sensitive adhesives, electronic component encapsulation, rocket propellant binders, epoxy flexibilizers, etc. Detailed vulcanization data can be obtained for the 1800 molecular weight material, EMD-590, from the Enjay Chemical Co., Cranford, N. J. 

Rigid Foam. This is made by mixing short polyfunctional polyol with di- or higher polyisocyanate, and foaming either with volatile liquid or with isocyanate and water. The largest use is in building insulation, with... 

Reactions of alkoxylated lignin with diisocyanates produce thermoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonreformable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. 

The choice of polyol, especially its size (moleeular weight), flexibility of its molecular structure, and functionality, has significant effect on the properties of the resultant polyurethanes. Varying isoeyanates also have major influenee on the properties of polyurethanes, sinee the reaetion of di- or polyfunctional isocyanates with polyols forms the polyurethanes. When there is excessive isocyanate with respect to diol, many secondary reactions may occur to create chemical cross-links between chains and network strueture in the polyurethanes. Thus pad properties can be eontrolled and fine-tuned through the control of the stoichiometric ratio of isocyanate to diol. 

When a drug or a non-active incorporated ingredient has plasticizing property, a further increase of cohesion with crosslinking of polymers is also a necessary approach. Practically, ionic crosslinkers such as aluminum acetylacetonate and chemical crosslinkers such as polyfunctional isocyanate are commonly used. The cross-linking occurs at the site of carboxyl groups or hydroxyl groups on the polymer side chain that may recover the cohesion of the adhesive. 

The structural variations possible in R and R make it possible to vary the toughness and elasticity of the polyurethane adhesive. Polyesters or polyethers are prepared with terminal hydroxyl groups that can then be reacted with difunctional or polyfunctional isocyanates. Polyurethane prepolymers can be formed in Reaction 1, with the desired terminal group produced by using one or other of the diol and diisocyanate reactants in excess. 

The first step in the production of complex polymers is to prepare segmented prepolymers by coupling a hydroxyl-terminated polyester or polyether to a polyfunctional isocyanate. This extends the molecular size through the urethane linkages. 

Other popular scaffolds have been derivatives of biphenyls which are now readily prepared using solid phase methods [265,464-469]. A solution phase approach has been to react a polyfunctional ised core containing reactive groupings - acid chlorides or isocyanates are easily prepared examples -with a mixture of reagents. One of the earliest synthetic examples showed the use of cubane tetracarboxylic acid chloride [151]. Two recent methods have used functionalised xanthene (library 71) [470] or diphenylmethane cores (library 72) [471] to identify DNA and urokinase receptor antagonists respectively. 

The use of a relatively small amount of polyfunctional reactants in the case of PURs these can be either isocyanates or polyols. 

The polycyclotrimerization of polyfunctional isocyanates (or NCO-terminated prepolymers) produces polymer networks containing heterocyclic perhydro-1,3,5-triazine-2,4,6-trione (isocyanurate) rings as crosslinks ... 

STRUCTURE 4.2 Example of polyfunctional cyclic carbonate-tricyclic carbonate on the base of propoxylated glycerin. (Reprinted from O. Figovsky and L. Shapovalov, Cyclocarbonate-Based Polymers Including Non-Isocyanate Polyurethane Adhesives and Coatings, in Encyclopedia of Surface and Colloid Science, ed. P. Somasundaran, vol. 3, 1633-1653. New York Taylor Francis, 2006. With permission.)... 

There are a number of syntheses leading to the formation of urethane polymers. However, the most important commercial route is the isocyanate addition polymerization, the reaction between di- and polyfunctional hydroxyl compounds such as hydroxy-terminated polyethers or polyesters and di- or polyisocyanates. When difunctional reactants are being used, linear polyurethanes are produced and the reaction can be schematically represented as follows ... 

The majority of the currently used coating systems is based on copolymers prepared from epoxy resins, polyfunctional amines, and partially blocked isocyanates (84-87). The resins are generally solubilized by the formation of cationic derivatives of amines by aminolysls of the carbamate in the partially blocked isocyanate. The principal applications of these cationic electrocoating systems are as automotive primers, in appliances, as primers and one-coat system.s, and in various other industrial applications. The main... 

Chain Extenders and Cross-linkers. In addition to the two principal components of most urethane coatings, isocyanate and polyol components, a number of di- or polyfunctional, active hydrogen components may be used as chain extenders or cross-linkers. The most important classes of compounds for this use are diols or polyols (monomers or oligomers), diamines, and alkanolamines. Typical examples of diols are ethylene, dlethylene, dlpropylene glycol, 1,4-butanedio1, 1,5-hexanediol, neopentyl glycol,... 

Two-Package Polyol Urethane Coatings (ASTM Type 5). Two-package polyol urethane coatings consist of isocyanate-terminated adducts of polymers that are cured by reaction with di- or polyfunctional hydroxyl-containing materials. The latter may consist of low- to medium-weight polyols with a polyester, polyether, polyether urethane, or castor oil backbone. When the two components (OH- and NC0-) are mixed together, they have only a limited pot life. Therefore, the components are mixed prior to application. Catalysts may be used to speed up the cure either for room temperature or oven cure. 

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