Isocyanates with two or more functional groups are required for the formation of polyurethane polymers. Volume wise, aromatic isocyanates account for the vast majority of global
diisocyanate production. Aliphatic and cycloaliphatic isocyanates are also important building blocks for polyurethane materials, but in much smaller
volumes. There are a number of reasons for this. First, the aromatically linked isocyanate group is much more reactive than the aliphatic one. Second, aromatic isocyanates are more economical to use. Aliphatic isocyanates are used only if special properties are required for the final product. For example, light stable coatings and
elastomers can only be obtained with aliphatic isocyanates. Even within the same class of isocyanates, there is a significant difference in reactivity of the functional groups based on steric
hindrance. In the case of 2,4-toluene diisocyanate, the isocyanate group in the para position to the methyl group is much more reactive than the isocyanate group in the ortho position.
Phosgenation of corresponding amines is the main technical process for the manufacture of isocyanates. The amine raw materials are generally manufactured by the hydrogenation of
corresponding nitro compounds. For example, toluenediamine (TDA) is manufactured from dinitrotoluene, which then converted to toluene diisocyanate (TDI). Diamino diphenylmethane or
methylenedianiline (MDA) is manufactured from nitrobenzene via aniline, which is then converted to diphenylmethane diisocyanate (MDI).
two most important aromatic isocyanates are toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). TDI consists of a mixture of the 2,4- and 2,6-diisocyanatotoluene isomers. The most
important product is TDI-80 (TD-80), consisting of 80% of the 2,4-isomer and 20% of the 2,6-isomer. This blend is used extensively in the manufacture of polyurethane flexible slabstock and molded
foam.12 TDI, and especially crude TDI and TDI/MDI blends can be used in rigid foam applications, but have been supplanted by polymeric MDI. TDI-polyether and TDI-polyester prepolymers
are used in high performance coating and elastomer applications. Prepolymers are available that have been vacuum stripped of TDI monomer, which greatly reduces their toxicity. Diphenylmethane
diisocyanate (MDI) has three isomers, 4,4'-MDI, 2,4'-MDI, and 2,2'-MDI, and is also polymerized to provide oligomers of functionality three and higher.
the 4,4'-MDI monomer is sold commercially as a single isomer. It is provided either as a frozen solid or flake, or in molten form, and is used to manufacture high performance prepolymers. Monomer
blends, consisting of approximately 50% of the 4,4'-isomer and 50% of the 2,4'-isomer, are liquid at room temperature and are used to manufacture
prepolymers for polyurea spray elastomer applications. 4,4'-MDI blends containing MDI uretonimine, carbodiimide, and allophonate moieties are also liquid at room temperature, and are used in the
manufacture of integral skin and microcellular foams. 4,4'-MDI-glycol prepolymers offer increased mechanical properties in the same applications, but
are prone to freezing at temperatures below 20�C. Polymeric MDI (PMDI) is used
in rigid pour-in-place, spray foam, and molded foam applications. Polymeric MDI that contains a very high portion of high-functionality oligomers is used to manufacture polyurethane and
polyisocyanurate rigid insulation boardstock. Modified PMDI, which contains high levels of MDI monomer, is used in the production of polyurethane flexible molded and microcellular foam. The
relative percentage of the 4,4'- and 2,4'- isomers is adjusted to change the reactivity and storage stability of the isocyanate blend, as well as the firmness and other physical properties of the
finished goods. Other aromatic isocyanate include p-phenylene diisocyante (PPDI), naphthalene diisocyanate (NDI), and o-tolidine diisocyanate (TODI).
most important aliphatic and cycloaliphatic isocyanates are 1,6-hexamethylene diisocyanate (HDI), 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethyl-cyclohexane (isophorone diisocyanate, IPDI), and
4,4'-diisocyanato dicyclohexylmethane (H12MDI). They are used to produce light stable, non-yellowing polyurethane coatings and elastomers. Because of their toxicity, aliphatic isocyanate
monomers are converted into prepolymers, biurets, dimers, and trimers for commercial use. HDI adducts are used extensively for weather and abrasion resistant coatings and lacquers. IPDI is used in
the manufacture of coatings, elastomeric adhesives and sealants. H12MDI prepolymers are used to produce high performance coatings and elastomers with optical clarity and hydrolysis
resistance. Other aliphatic isocyanates include cyclohexane diisocyanate (CHDI), tetramethylxylene diisocyanate (TMXDI), and 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI).
Well no one responded to my other entry, but what ever, I don't really care. It is finally summer and it is so exciting. June and July is filled up but August is completely empty. My dog tore his
acl AGAIN, but this time it was the other leg. He has to get another expensive surgery, less money for college!
It's been a little but since the last that I've written. Life is a lot easier right now. I guess that Christy and I are back together for now. I know that it's the wrong thing for me to still be
with her, but I am so addicted to her. And yes a little in love too.
She wants to keep sleeping with other people, but be with me. That's going to be hard to get used too. I am always wondering who she is with when I am not around. It's not as bad as not being with
her, but it's still pretty hard.
It's only a matter of time before she leaves again, so I had better have fun while I can.