Terminal isocyanate groups, curing

Compatibility. Owing to the high reactivity of the isocyanate group, polyurethane propellants require a more sophisticated processing technique than the rather foolproof, carboxy-terminated polybutadiene aziri-dine and/or epoxy-cured propellant systems. Processing is even more complicated if bonding agents (see below) are present, which are used to bolster mechanical properties in practically any modern propellant. 

Emulsification occurs when the partially extended urethane is added with high-speed mixing to 3% aqueous acetic acid. Curing of the latex takes place either by reaction of water (150. 151) with the terminal isocyanate groups or by reaction with water-soluble diamines ... 

Vapor Permeation Curing. This technique is based on the reaction of a reactive isocyanate group in NCO-terminated polymers or adducts with a hydroxyl group containing compound or resin cured with either ammonia or an amine present in the vapor phase. 

The principal of formation of this type of polyurethane elastomer medium, based on a cationic urethane latex, is where an isocyanate-terminated prepolymer derived from either a polyester or polyether diol and toluene diisocyanate is first chain-extended with an alkyl diethanolamine to yield a relatively low molecular weight urethane capable of further chain-extending reactions. Emulsification occurs when the partially extended urethane is added with high-speed mixing to 3% aqueous acetic acid. Curing of the latex takes place either by reaction of water with the terminal isocyanate groups or by reaction with water-soluble diamines. 

Heat-curing systems are based on a combination of hydroxy-terminated polyesters or polyethers and blocked polyfunctional isocyanates. When the temperature is raised, the blocking agent is removed and the free isocyanate groups react with the hydroxy groups to effect cure. The usual blocking agent is phenol and a typical blocked isocyanate is prepared by reaction of trimethylol-propane (1 mole), tolylene diisocyanate (3 mole) and phenol (3 mole) ... 

The final type of polyurethane available is known as the two-part polyol system. One component contains the isocyanate containing compound such as a prepolymer or adduct and the second component is a hydroxy group terminated resin which may or may not contain a catalyst. The most commonly used hydroxy-terminated components are polyols, castor oil, hydroxy-terminated polyesters and some epoxy resins. By varying the components of such a system a wide range of cured properties can be achieved ranging from high flexibility to very hard or brittle. The curing reactions and properties of the different polyurethanes are summarised in Table 10.9. 

Isocyanate-terminated urethane prepolymers can be formed and cured into an elastomer by casting the prepolymer and allowing ambient moisture to react with the cast system s end groups. Moisture-cured sealants can be formulated and allowed to cure in a similar manner. 

Alternatively, polymers containing terminal isocyanate groups can also be prepared by copolymerization of lEM with other vinyl or acrylic monomers. These can then be cured through the isocyanate groups according to the mechanisms discussed earlier. " Development of a new monomer, m-isopropenyl-a,a-dimethylbenzyl isocyanate (19 m-TMI) has also been used in a similar... 

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. 

The prepolymer as received from the manufacturer has a simple chain that has been terminated with an isocyanate. The isocyanate ends with this magical NCO group. The NCO is the reactive part of it. The higher the percentage of NCO in the prepolymer, the harder the material will be. An 80 Shore A will have an NCO of approximately 3.1 to 3.2%, whereas a 75 Shore D will have an NCO content of about 11.2%. To obtain the chain extension, one must add an appropriate amount of an amine or diol curative. For every curative, there is a different amount that must be added. The manufacturers of the prepolymers and curatives will give the appropriate factors for mixing the polyurethane. The prepolymer must be heated before use. This is to reduce the viscosity of the material as well as to obtain the correct cure rate and complete cure time. 

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