An isocyanate has the functional group with the formula R–N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. A di-isocyanate is an isocyanate that has two isocyanate groups. Di-isocyanates are manufactured for reactions with polyols in the production of polyurethanes.
Isocyanates are produced by treating amines with phosgene: RNH2 + COCl2 → RNCO + 2 HCl. Phosgene is a highly hazardous material, and the production of isocyanates requires special precautions and care.
Isocyanates are reactive toward a variety of nucleophiles including alcohols, amines, and water. Upon treatment with an alcohol, an isocyanate forms a urethane linkage: ROH + R'NCO → ROC(O)N(H)R' . If a di-isocyanate is treated with a compound containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are formed, which are known as polyurethanes.
In addition, isocyanates react with water (the hydroxyl) to form carbon dioxide: RNCO + H2O → RNH2 + CO2, which is used for the production of polyurethane to give polyurethane foams. The carbon dioxide functions as a blowing agent.
Chemistry – Isocyanates/Polyisocyanates
Of the roughly 85% of non-CASE applications, the polyisocyanates (PI) utilized are either Methylene Diphenyl Diisocyanate (MDI, CAS 101-68-8 and 9016-87-9) and Toluene Diisocyanate (TDI); 2,4-TDI (CAS: 584-84-9) and 2,6-TDI (CAS: 91-08-7).
The mixture of diisocyanates known as TDI consists of two isomers:
The starting material is methylbenzene (toluene). When it reacts with mixed acid (nitric and sulfuric), two isomers of nitromethylbenzene (NMB) are the main products.
Both TDI and MDI are aromatic compounds, and as such, provide relatively poor resistance to ultraviolet (sun)light, and therefore are not used in coatings applications where sunlight is particularly harsh, but rather in uses where UV is not a concern. TDI is mainly used to make flexible polyurethane foam that can be found in a wide range of products, including furniture, bedding, carpet underlay and packaging. TDI is also used in the manufacture of some coatings, sealants, adhesives and elastomers. TDI helps produce lighter automobile seating and headliners, saving weight and making vehicles more energy efficient. MDI is used primarily to make rigid polyurethane foams such as insulation for appliances such as refrigerators, and many other uses. Insulation made with MDI can help save heating and cooling costs, Vehicle parts like dashboards, steering wheels and bumpers are also made of MDI.
The starting materials are phenylamine (aniline) and methanal (formaldehyde) which react together to form a mixture of amines, known as MDA (methylenedianiline). This mixture reacts with carbonyl chloride (phosgene) to produce MDI in a similar way to the manufacture of TDI. MDI contains the diisocyanates isomers: 2, 4’-MDI, 2,2’-MDI and 4,4’-MDI. The term MDI refers to the mixture of the three isomers in Figure 3. They can be separated by distillation.
Less widely used, but still important, are the aliphatic diisocyanates, including hexamethylene diisocyanate (HDI), hydrogenated MDI (H12MDI), and isophorone diisocyanate (IPDI). However, even in coatings, only 3-5% utilize aliphatic PI, while the remaining 95-97% employ aromatics. HDI, H12MDI and IPDI are most often further reacted to form polyisocyanates, or pre-polymers, which are used to form color-stable polyurethane coatings and elastomers, that can significantly enhance a product’s appearance, lengthen its lifespan and offer high abrasion resistance.
Covestro (f.k.a. Bayer Material Sciences) and Evonik, are the main suppliers of H12MDI, while BASF and Covestro supply IPDI. There are several Asian companies that provide these as well.
There are specialty isocyanates as well. One type contains silane-termination, and are chiefly used in the manufacture and production of sealants and elastomers. In addition, there are hydrophilically-modified isocyanates, used primarly to produce TPU for medical applications such as catheters.
Coatings prepared with aliphatic diisocyanates can have excellent resistance to abrasion, as well as superior weathering characteristics, including gloss retention and resistance to yellowing and chalking, as well as lengthening the time between painting cycles. Chemical-resistant coatings made with aliphatic diisocyanates help commercial airliners maintain the durability and resistance needed to withstand harsh atmospheric conditions.
Dependent upon the application and environmental considerations, polyurethanes can be solventborne or waterborne. Although great strides have been made in waterborne polyurethane chemistry, they typically do not perform as well as their solventborne analogs. A market need and emergent trend is to produce either a 1-part (1K) polyurethane or/and a 2-part (2K) that cross-links without the use of polyisocyanates.
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