Multifunctional isocyanates

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

How does the number of multifunctional isocyanate units in a foamed polyurethane influence the rigidity of a foam ... 

LCB polymers can be formed by chemically linking preformed polymers (arm first or polymer first method) or by growing polymer chains from a multifunctional initiatior (core first method). In both cases living polymerization techniques are preferred because they provide better control over MW, MW distribution and the final branching architecture. However, highly selective coupling reactions e.g. with multifunctional isocyanates, or dicyclohexylcar-bodiimide (DCC) coupling, have also been successful. 

In the preparation of a prepolymer, every effort is made to prevent the formation of any unplanned branching such as biuret groups. The prepolymer is essentially linear except when some cross-link sites have been introduced by using a multifunctional isocyanate or triol. 

The creation of difunctional or multifunctional isocyanate molecules entirely based on renewable resource chemistry. 

Higher permanence to washing, dry cleaning and rubbing Crosslinking with functional groups on the fibre surface and by incorporation in the network of the reaction products of multifunctional isocyanates... 

Most investigators agree that the strength and durability of isocyanate-bound wood panels are due to the chemical reaction of the isocyanate group with wood hydroxyls as illustrated by the above equation. Thus, the multifunctional isocyanate molecule forms a chemically bonded bridge between two or more adjacent wood particles. This reaction is only one of several involving isocyanates that can and probably do occur in a hot press during formation of particleboards when isocyanate binder is used. A very important reaction is that of isocyanate with water to produce a very unstable carbamic acid which immediately decomposes to form a primary amine and COg ... 

The choice exists between using a one-part or a two-part formulation. One-part formulations are based on polyurethanes of very high molecular weight that are still soluble in petroleum-ester mixtures. Two-part adhesives are normally used where high cohesive strength and improved heat resistance are required and these comprise with the prepolymer terminated in some active group (such as polyether or polyester polyols) and as the second part either a multifunctional isocyanate (e.g., MDl) or an isocyanate... 

Isocyanate Foaming. Alternatively, PAI foams can be prepared by the reaction of multifunctional isocyanates with multifimctional imido carboxylic acids. " The reaction is catalyzed by tertiary amines. Best results are obtained with polymeric diisocyanates from the MDI type. Surfactants... 

In interfacial polymerization, the two reactants in a polycondensation meet at an interface and react rapidly. The substances used are multifunctional monomers. Generally used monomers include multifunctional isocyanates and multifunctional acid chlorides. The basis of this method is the classical Schottenn-Baumann reaction between an acid chloride and a compound containing an active hydrogen atom, such as an amine or alcohol, polyesters, polyurea, and polyurethane. Under the right conditions, thin, flexible waUs/sheU will be formed at or on the surface of the droplet or particle by polymerization of the reactive monomers. 

Aerogels are quasi-stable, low-density, three-dimensional assemblies of nanoparticles, which have usually poor mechanical properties. A facile one-pot synthesis of Kevlar-like aerogels based on the reaction of multifunctional isocyanates and carboxylic acids has been reported [57]. The materials exhibit an ultimate compressive strength, a high specific energy absorption, and a thermal conductivity like foamed PS. By a pyrolysis process at 800 °C, the materials can be converted to a porous, electrically conducting carbon with a high surface area. 

What are known as poly(carbodiimides) are structurally related to polyimines. Poly(carbodiimides) are obtained by elimination of carbon dioxide from multifunctional isocyanates under the influence of catalysts such as phospholene oxides ... 

Polyurethane finish n. An exceptionally hard and wear-resistant paint or varnish made by the reaction of polyols with a multifunctional isocyanate. 

A specific example of the process represented by Figure 4b occurs when a multifunctional isocyanate is dissolved in a liquid, water-immiscible core mate-... 

Isocyanates also react with each other to form dimers (metdiones) and trimers (isocyanurates). Formation of metdiones is catalyzed by phosphines. Formation of isocyanurates is catalyzed by quaternary ammonium compounds trimerization of aromatic isocyanates is catalyzed by tertiary amines. Uretdiones decompose thermally to regenerate isocyanates and are used as blocked isocyanates. Isocyanurates are stable and isocyanurates of diisocyanates are extensively used as multifunctional isocyanates. 

For elastomers, tolylene diisocyanate (TDI) and 4,4 -diphenylmethane diisocyanate (MDI) have come to be the principal multifunctional isocyanates used in polyurethane and polyurea manufacture. Tolylene diisocyanate is usually obtained as a commercial 80/20 by weight percent mixture of the 2,4- and the 2,6-isomers. A 65/35 mixture of these isomers is commercially available, as is the pure 2,4- isomer. The 2,4-isomer is used in selected coating applications and in the making of certain prepolymers. The 65/35 mixture of isomers is a result of separation of pure 2,4- isomer from the 80/20 mixed product of normal manufacture. 

A large number of multifunctional isocyanate curing agents are commercially available for crosslinking of the hydroxyl functional resin. These isocyarmte adducts are mostly derived from a limited number of aliphatic and aromatic monomeric isocyanates, whose stmctures can be seen in Figure 5-2. 

Solventless, multifunctional isocyanates are available based on aromatic diphenyl methene-4-4 -di-isocyanate (MDI) or aliphatic hexamethylene di-isocyanate (I dDI), although due to toxicity reasons, HMDI itself would not be used, but lower volatility adducts may be an alternative. In dried coatings, the water dispersible isocyanates undergo the usual reaction with active hydrogen groups. See Figure 7-18. (Also see isocyanate Chapter for more details on isocyanate reactions ). 

Carboxylic acid groups in combination with divalent metal ions, such as Ca " " or Zn " " gives crosshnking by chelate formation. Hydroxyalkyl acrylates are used when water dispersible, multifunctional isocyanates are taken for crosslinking. Monomers with carbonyl groups in the side chain are utilised for a curing mechanism with water-soluble acid... 

Alkyd binders - Alkyd resins are also used as core binders. Isocyanate-modified alkyd resins have excellent core making and molding properties in no-bake systems. They are cross-linked by multifunctional isocyanates as well as by oxydation and polymerization of the unsaturated allg d resins. Carbohydrates The mother liquor of dextrose crystallization (->hydFoI) is also used as a core binder. 

---