Isocyanate-terminated prepolymer

The first urethane reaction in Fig. 1 is used in two major ways in adhesives. In one case, a two-component adhesive usually employs a polyol and polyisocyanate with catalyst. This can react at room temperature to form the polyurethane. The second use of this reaction is to make an isocyanate-terminated prepolymer. Reacting a stoichiometric excess of isocyanate with polyol can produce an isocyanate-terminated prepolymer. A prepolymer is often made with an NCO/OH ratio of 2.0, as shown below, but the isocyanate ratio can range from 1.4 to over 8.0, depending upon the application ... 

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

Initially, the water slowly reacts with the isocyanate. However, the reaction can be catalyzed with an appropriate catalyst, such as dibutyltin dilaurate or a morpholine tertiary amine catalyst. The isocyanate will react with water to form a carbamic acid, which is unstable and splits off carbon dioxide, to produce a terminal amine end group (see p. 76 in [6]). This amine then reacts with more isocyanate-terminated prepolymer, as shown above, to form a polyurea. This process repeats itself, building up molecular weight and curing to become a polyurea-polyurethane adhesive. 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

The basis of most one-component, moisture-curing urethanes is the synthesis of an isocyanate-terminated prepolymer, as shown ... 

A tertiary amine such as triethylamine is then added to the isocyanate-terminated prepolymer (containing carboxylic acid groups). The tertiary amine reacts with the pendant carboxylic acid groups, forming a carboxylic acid salt. The presence of this salt, together with adequate stirring, allows the dispersion of the prepolymer in water by the so-called melt dispersion process [57]. 

Fig. 7. Room-temperature curing concept for a phenol-blocked isocyanate-terminated prepolymer.

The ketimine of isophorone diamine is formed by reacting it with methyl isobutylketone, splitting off water in the process. When said ketimine is added to an isocyanate-terminated prepolymer based on IPDI, a semi-stable system is established with a pot life of several hours. The ketimine is a Schiff base and thus can react even in the absence of water. The complexities and advantages of this system are reviewed by Bock and Halpaap [75] ... 

Polyaddition reactions based on isocyanate-terminated poly(ethylene glycol)s and subsequent block copolymerization with styrene monomer were utilized for the impregnation of wood [54]. Hazer [55] prepared block copolymers containing poly(ethylene adipate) and po-ly(peroxy carbamate) by an addition of the respective isocyanate-terminated prepolymers to polyazoesters. By both bulk and solution polymerization and subsequent thermal polymerization in the presence of a vinyl monomer, multiblock copolymers could be formed. 

We can make polyurethanes via one- or two-step operations. In the single-stage process, diols and isocyanates react directly to form polymers. If we wish to make thermoplastic linear polymers, we use only diisocyanates. When thermosets are required, we use a mixture of diisocyanates and tri- or polyisocyanates residues of the latter becoming crosslinks between chains. In the first step of the two-stage process, we make oligomers known as prepolymers, which are terminated either by isocyanate or hydroxyl groups. Polymers are formed in the second step, when the isocyanate terminated prepolymers react with diol chain extenders, or the hydroxyl terminated prepolymers react with di- or polyisocyanates. 

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 polyaddition reaction is influenced by the structure and functionality of the monomers, including the location of substituents in proximity to the reactive isocyanate group (steric hindrance) and the nature of the hydroxyl group (primary or secondary). Impurities also influence the reactivity of the system for example, acid impurities in PMDI require partial neutralization or larger amounts of the basic catalysts. The acidity in PMDI can be reduced by heat or epoxy treatment, which is best conducted in the plant. Addition of small amounts of carboxylic acid chlorides lowers the reactivity of PMDI or stabilizes isocyanate terminated prepolymers. 

Polyurethane adhesives are known for excellent adhesion, flexibiUty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terminated prepolymers with moisture or on the reaction with the substrate, eg, wood and cellulosic 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 appUcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Recently, hot melt PSA systems have been introduced and radiation curable PSA systems are at the commercial development stage. High solids (50%-70% by wt.) nonaqueous dispersion acrylic PSA systems have also been reported(1). Unlike the hot melt and radiation cured systems which require new capital outlay in coating head and/or curing (drying) equipment, BFG has developed PSA systems, based on Hycar 2100R reactive acrylic liquid polymers and isocyanate terminated prepolymer, which can be processed at 80% solids (by wt.) with equipment presently used in the PSA industry, namely, the reverse roll and knife-over-roll coater. 

Sometimes the system is formulated so that multiple cure mechanisms are possible and can occur sequentially or simultaneously. Compositions that rely on both epoxy and urethane chemistry are examples.62,94,95 These are compositions containing the diglycidyl ether of bisphenol A, an isocyanate or isocyanate-terminated prepolymer, amines or other reactants for either epoxy or isocyanates, and catalysts. 

Smith [5] prepared polynrethanes ionomers, (IV), where the material remained elastic between —20°C and 76°C by reacting isocyanate-terminated prepolymers with acid salt diols. 

If the polyurethane molecule is allowed to form a three-dimensional structure, it becomes insoluble and neither wet nor dry spinning processes can be used. A chemical spinning process must be used in this case. The trick is to spin the isocyanate-terminated prepolymer at a stage where it forms a viscous melt, and with the jets produced in an environment that contains a chain extender. This chain extender diffuses into the fibers and reacts to couple the prepolymer molecules (Couper, 1985). 

First-generation solventless polyurethane adhesives are one-component isocyanate terminated prepolymers formed by the reaction of MDI (4,4 methylene bis (phenyl isocyanate)), or other isocyanates with polyether and/ or polyester polyols. One-component 100% solids adhesives rely on moisture from the air or substrates or from induced moisture misting during the converting process, to cure the adhesive via an isocyanate/water reaction and subsequent polyurea-polyurethane polymer formation. Typically the high viscosity of the adhesive is such as to require adhesive delivery equipment and application rollers heated from 65-80 °C for use. They have a high level... 

Second-generation solventless polyurethane adhesives Two-component second-generation adhesive systems are the most widely used of the 100% solids products. These two-part adhesives comprise an isocyanate terminated prepolymer based on the reaction product normally of MDI and polyols (polyether or polyester) in the presence of excess isocyanate, and a polyol (polyether or polyester based). 

In two-component sealants the components consist of a base, a hydroxyl-terminated compound and a liquid isocyanate-terminated prepolymer. The prepolymer reacts with the base in the presence of a catalyst to form the polyurethane ... 

Because of their easy handling, 1-component spray foams are mainly in use in Europe. These foams are cured by a reaction of a mixture containing both isocyanate-terminated prepolymers and monomeric isocyanates with moisture. 

Preparation of Cast Pads. One hundred parts Adlprene LlOO (an Isocyanate terminated prepolymer containing polyoxytetra-methylene chain segments and having an equivalent weight of about 1025, manufactured by duPont) Is heated to 87-88 C while 12.5 parts MOCA, 4,4 -methylenebls(2-chloroanlllne), Is brought to 124-126. Degassing of the Adlprene LlOO and MOCA at a pressure of 1-2 Torr for about 5 and 1 minutes, respectively, follows. 

An Isocyanate terminated prepolymer such as Adlprene LlOO has a chemical structure (R represents isomeric forms of the tolylene radical) of the type ... 

Reaction with difunctional amines (see the next paragraph) allows the formation of poly (urethane-co-urea) and further extends the versatility of the segment architecture. For further control of the structure, a prepolymer is formed. The reaction, such as the polyurethane reaction shown above, is carried out with excess di-isocyanate so that an isocyanate-terminated prepolymer is obtained. The isocyanates used are typically aromatic, such as toluene di-isocyanate. This prepolymer is then reacted with a short-chain diol or diamine (for a polyurea) to form the final polymer. 

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