Aromatic polyisocyanates

Diisocyanates/polyisocyanates Aliphatic and aromatic polyisocyanates are used to obtain PU networks for composite propellants. 

Aromatic polyisocyanates Aromatic polyisocyanates are primarily used for a wide variety of PU foamed plastics, elastomers, and adhesives. More than 90% of PUs are produced from aromatic polyisocyanates. The isocyanate group bonded to an aromatic ring is more reactive toward water or hydroxyl groups than that bonded to an aliphatic compound. The most important aromatic diisocyanates are also given in Figure 4.7. 

Figure 4.7 Structures of some aliphatic and aromatic polyisocyanates.

In the case of using aromatic polyisocyanate in making polyimide foams, crosslinking is increased by adding a highly functional polyol to react with part of the isocyanate so that the foam contains some urethane structure. A typical foam prepared in this way would have a density of 3.97 Ib/ft (63.5 kg/m ) and a K-factor of 0.26 Btu/(h) (fP) ( F/in). Foams of this type have been prepared in a range of densities of 2.5 to 18.5 Ib/ft (40 to 296 kg/m ) with compressive strengths of 25 to 1340 psi (172 to 9239 kPa) (8). 

Polyurethane. Polyurethanes are formed by mixing liquid aliphatic polyols with liquid aromatic polyisocyanates, which react readily. These mixes gel in several minutes at room temperature, but often benefit from final heat cure. Most polyurethanes are foamed by introducing and expanding gas bubbles when the reacting liquids have reached the optimum viscosity, then the cross-linking reaction is completed to stabilize the foam structure. 

Aliphatic and cycloaliphatic polyisocyanates can be used to produce lightfast, weather-resistant coatings for extremely severe conditions. Aromatic polyisocyanates are of limited suitability for exterior applications and react more quickly than aliphatic polyisocyanates. The different reactivities of aromatic and aliphatic polyisocyanates influence their use because they affect drying and curing. 

Aromatic Pis with anhydride end groups can be chain extended by reacting them with aromatic polyisocyanates." ... 

G. F. D Alelio. Chain-extending polyimides with aromatic polyisocyanates. US Patent 4 026 833, assigned to University of Notre Dame (Notre Dame, IN), May 31, 1977. 

Aliphatic polyisocyanate Aromatic polyisocyanate in ethyl acetate Contains no solvents... 

Chem. Descrip. Aromatic polyisocyanate adduct based on toluene diisocyanate dissolved in ethyl acetate Uses Mfg. of paints/coatings... 

Chem. Descrip. Aromatic polyisocyanate in toluene Uses Adhesion promoter in adhesives primer or adhesive additive for adhering elastomeric coatings to syn. fiber fabrics coated with syn. rubbers crosslinking agent for elastomers and plastics Features No ozone-depleting substances Properties Dk. brn. liq. mod. sol. in alcohol, toluene, chloroform si, sol. in petrol, ether pract. insol. in water dens. 1,05 0.02 mg/m flash pt. (PM) 4 C min,... 

Recently, a similar reaction was reported from aromatic isocyanates and imidazoles. 1,4-tetramethylene-A, A -diimidazoles were reacted with aromatic polyisocyanates to form thermoset polyamides ... 

PU3 A Aromatic polyisocyanate (XP-744, obtained from Bayer Corporation) with NCO OH of 1.03. 

Kamal MR. Preparation of non-aromatic polyisocyanates. US 3,465,023. General Mills 1965. 

It is reasonable to attribute this type of adhesion to thermally induced adsorption of the aromatic polyisocyanate on the metal surface, with linkages from the same isocyanate condensate simultaneously attaching to the elastomer through the diurethane vulcanization network. 

The most common polyisocyanate used in urethane sealant production is toluene diisocyanate (TDI). The TDI is usually prereacted to give an isocyanate-terminated prepolymer. Other aromatic polyisocyanates used in sealants include diphenylmethane diisocyanate (MDI) and polymethylene polyphenylene isocyanate (PAPI). All of the aromatic poly isocyanates contribute to yellowing of urethanes on exposure to light. 

For instance, blocked polyisocyanates based on aromatic polyisocyanates dissociate at lower temperatures than those based on aliphatic ones. Generally, the dissociation temperatures of blocked polyisocyanates based on commercially utilised blocking agents decrease in the order alcohols > s-caprolactam > phenols > methyl ethyl ketoxime > active methylene compounds. 

Toluene diisocyanate (TDl) is the most commonly used poly isocyanate in methane sealant production. All aromatic polyisocyanates cause urethanes to yellow on e q)osure to Hght. The yellowing problem can be eliminated by the use of aliphatic or cycloaliphatic polyisocyanates, but these are more expensive. 

Figure 16 illustrates polyisocyanates that are commonly used to prepare polyurethanes. The aromatic polyisocyanates are significantly less expensive than the aliphatic types. Again, this is due to the larger production volnmes of... 

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