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).
The 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.
Only 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).
The 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).
� Today I did a shoot with a baby girl named Eliska. She was so adorable. She had several different halloween costumes. She was so easy to work with and her mom and grandmother kept singing this really cute song to get her attention. I really enjoyed the session and am sure the images will be beautiful. I think my favorites are the ones of her with the colorful toule. There was a cat that she really loved. We tried to put the cat in the pail with her but it immediately jumped out before we could even snap off one picture. I was kind of sad about that because it would have been very halloween like. We did get a few with the cut on the bench with her but then it decided to get down when she started petting it a little less than it liked. She wasn't even 2 so she was learning to be nice to the kitty. She didn't quite have it down yet but it was very cute to see her try. There were some great areas at the pumpkin patch to shoot. The only thing is that in the barn some fly kept landing on the baby but I got lots of shots without it so that will be okay. There were huge pumpkins and colorful gourds so I think these will be perfect for halloween.
Okay, so I love this song called "Defying Gravity" from the broadway musical, Wicked. Whether it be the broadway version or the version Idina Menzel(Elphie) turned into a single for her CD, whenever I listen to it, during the�highest point�portion of the song, I get goose bumps. This song is so inspirational.
This song talks about, just what the title states, defying gravity. True, unless you are a bird, or some one who hasn't been detected by the Government to be able to levitate yet, then this is ALL just metaphorical.
However, the metaphorical sense is probably about the best sense, because it lifts up�your mood and just how you are!
To defy gravity, this means to not let things get you down! Defy whatever is getting you down, whether it be: life, parents, boyfriends, girlfriends, or even friends, work, school, the economic downturn this country has seen.
I have taken on this attitude in the last year, and it has helped TREMENDOUSLY! Life truly is what you make it, so when life gets you down, defy it! If you're flying solo, at least you're flying free!
Don't play by the rules of someone elses game...trust your instincts, close your eyes, and leap..Try defying gravity.
Don't accept limits just cause someone says they're so...There are some things you cannot change, but you won't know until you try...All of this and more is in the song. True, some words are in the broadway version that aren't in Menzel's version...
I have turned this song, "Defying Gravity," into a personal philosophy of defy gravity...I don't let things get me down. True, this mindset may make me seem cocky, but in truth this has made me more sure of myself, more confident, and a stronger person.
We all have the ability to defy gravity...sometimes it takes someone, to either push you over the edge to where you can't take it anymore, or someone to tell you that someone is opressing you so much that you need to defy them!
"Tell them how I am defying gravity, I'm flying high defying gravity..." Tell that to those who wish to keep you, your sould, your mind, your spirit grounded.
Defy Gravity, Defy life, Defy them all!
�today has been such a brillent day. and tommrow will be even better. we are going to the food festival in manchester agin. W are also going to �my in-laws �house which is in clifton england i call it riley mansion. It is so gorgeous there.They have a big garden. And there is a path that i walk down and visit the horses. They also have sheep. And the church we go to is also a favorite of mine. It is called st.john fischers. I think �it will be an exciting day. We plan on coming back on sunday. �I'am hoping that the weather will be really good.We take the bus. This time i am packing alot lighter then before as it is easier to carry that way.
�Last week we had a brillent party. there is alot of people in our family and about 20 of them were there.There was alot of terrific foods and it was so nice to visit.My sister in law also has a mini trampoline that she lets me use. �I have been on a weight loss program. I started doing belly dance for fun and excersize and lost from 165 american pounds to 126 in three months. That is from 12 stone in brittish weight to 9 stone in three months. and i am manageing to keep the weight off but latly i've been eating alot more so ill see if i can keep it off. �wish me luck!! i hope you have a brillent day take care
"Sometimes I try to do things and it just doesn't work out the way I wanted to.I get real frustrated and I try hard to do it and I take my time and it doesn't work out the way I wanted to.It's like I concentrate real hard and it doesn't work out.Everything I do and everything I try never turns out".
Add comment
