Prepolymer isocyanate vapor pressure

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Prepolymeric HDI-BT paint formnlations, which generally contain 1% monomeric HDI, are now widely used for coatings applications, particularly in the automobile and airplane industries (Alexandersson et al. 1987 Karol 1986 Key-Schwartz 1992 Nielson et al. 1985 Rosenberg and Savolainen 1986 Rosenberg and Tuomi 1984). Consequently, many reported occupational exposures to HDI are actually exposures predominantly to HDI-BT (Karol 1986 Karol and Hauth 1982). Although isocyanate prepolymers are safer to use than the free monomers, primarily because of their lower vapor pressures, they can still pose a health risk to workers when inhaled in the aerosol form (Karol and Hauth 1982 Rosenberg and Tuomi 1984). There... 

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. 

Monitoring must be carried out to check that the levels of isocyanate vapors do not exceed the local limits. These limits vary from area to area. In some areas of the world, only the isocyanate level is specified and is not controlled by the type of isocyanate. Care must be taken, as the vapor pressure of different isocyanates may vary by a factor of 100 or more. Plants that may comply with the atmospheric isocyanate levels when using MDI-based material may have too high a level when using TDI-based prepolymers. 

The saturated analogue of MDI, 4,4 -dicy-clohexyl methane diisocyanate, has found limited use as an aiphatic isocyanate in adhesives. This material is known by a variety of names including Desmodur W (Bayer), hydrogenated MDI (or HMDI or Hl2MDI), reduced MDI (RMDI), and saturated MDI (SMDI), It is a low-viscosity liquid with a fairly high vapor pressure, so it too must be handled with care. In adhesive compositions, the diisocyanate usually is used to make an isocyanate functional prepolymer by reacting excess diisocyanate with a hydroxyl or amine functional polymer such as a polyester diol. 

Prepolymers are isocyanates and react like isocyanates, with several important differences. Prepolymers typically are much higher in molecular weight, are higher in viscosity, are lower in isocyanate content by weight percent, and have lower vapor pressures. Prepolymers are important to adhesives for a number of reasons. The desired polymeric structure of the adhesive can be built into the prepolymer, giving a more consistent... 

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