Aliphatic diisocyanates

For environmental reasons there has been interest in methods for manufacturing isocyanates without the use of phosgene. One approach has been to produee diurethanes from diamines and then to thermal eleave the diurethanes into diisocyanates and alcohols. Although this method has been used for the production of aliphatic diisocyanates such as hexamethylene diisocyanate and isophorone diisocyanate, for economic reasons it has not been adopted for the major aromatic isocyanates MDI and TDI. 

Polyurethane dispersions (PUD s) are usually high-performance adhesives based on crystalline, hydrophobic polyester polyols, such as hexamethylene adipate, and aliphatic diisocyanates, such as methylene bis(cyclohexyl isocyanate) (H12MDI) or isophorone diisocyanate (IPDI). These PUD s are at the more expensive end of the waterborne adhesive market but provide excellent performance. 

Aromatic diisocyanates such as toluene 2,4-diisocy-anate (TDI) and methylene di-p-phenylene isocyanate (MDl) are usually used. Aliphatic diisocyanate such as hexanediisocyanate (HDI), although it has the advantage that the TPU synthesized from it is softer and not prone to turning yellow, is seldom used due to its high cost. 

A different kind of oligoperoxide, polymeric per-oxycarbamate [19], was synthesized by reacting equimolar amounts of an aliphatic dihydroperoxide with an aliphatic diisocyanate. 

Synthesis of siloxane-urethane copolymers from various hydroxyalkyl-terminated PDMS oligomers and aliphatic diisocyanates, such as tetramethylene- and hexame-thylene diisocyanate and HMDI was reported 333,334). Reactions were conducted either in chloroform or 1,4-dioxane and usually low molecular weight, oily products were obtained. No data were available on the molecular weights or the thermal and mechanical properties of the copolymers obtained. These products were later cross-linked by a peroxide. Resulting materials were characterized by IR spectroscopy and water contact angle measurements for possible use as contact lenses. 

The isosorbide polyurethane based on the aliphatic diisocyanate P(I-HMDI) is flexible. It is a thermoplastic with a glass transition temperature of 110°C and softening temperature of 190°C. Both transitions are well below its degradation onset which occurs at approximately 260°C. It forms good films by evaporation from its solutions and colorless transparent compression moldings. 

TGA analysis shows that polymer degradation starts at about 235°C which corresponds to the temperature of decomposition of the cellobiose monomer (m.p. 239°C with decom.). Torsion Braid analysis and differential scanning calorimetry measurements show that this polymer is very rigid and does not exhibit any transition in the range of -100 to +250 C, e.g. the polymer decomposition occurs below any transition temperature. This result is expected since both of the monomers, cellobiose and MDI, have rigid molecules and because cellobiose units of the polymer form intermolecular hydrogen bondings. Cellobiose polyurethanes based on aliphatic diisocyanates, e.g. HMDI, are expected to be more flexible. 

A key factor in the preparation of polyurethanes is the reactivity of the isocyanates. Aromatic diisocyanates are more reactive than aliphatic diisocyanates, and primary isocyanates react faster than secondary or tertiary isocyanates. The most important and commercially most readily accessible diisocyanates are aliphatic and colorless hexamethylene-1,6-diisocyanate (HDI), isophorone diisocyanate (IPDI),and aromatic, brownish colored diphenylmethane-4,4 -diiso-cyanate (MDI), 1,5-naphthalenediisocyanate, and a 4 1 mixture of 2,4- and 2,6-toluenediisocyanates (TDI). 

Very little information exists on the toxicokinetics of HDI in animals. More information is available on the toxicokinetic and pharmacokinetics of aromatic diisocyanates. Since HDI is an aliphatic diisocyanate, no useful toxicokinetic extrapolations can be applied to derive information about the absorption, distribution, metabolism, and excretion behavior of HDI in animals. 

Isocyanates are produeed almost exclusively by the reaction of amines with phosgene (COCy, with the speeifie reaetion eonditions varying particularly for aromatic and aliphatic isocyanates (Chadwick and Cleveland 1981 Codd et al. 1972 Ulrich 1989). Aliphatic diisocyanates are produced by reaction of phosgene with either a slurry of the carbamate salts obtained in the reaction of the aliphatic diamines with earbon dioxide, or with a slimy of the amine hydrochloride (Ulrich 1989). Hexamethylene diisocyanate (HDI) is produeed by the reaction of phosgene with the amine salt (Chadwick and Cleveland 1981). The trimerie HDI biuret (HDI-BT), which has a low monomer content and is widely used in the formulation of exeeptionally high quality polymer coatings, is produced by controlled reaction of HDI with water, a water generator, or an amine (Chadwick and Cleveland 1981). 

Daicel Chemical Industries in Japan patented a promising phosgene-free process involving the reaction of an aliphatic diamine with dimethyl carbonate (DMC) to produce carbamate esters, which are then thermally converted to the corresponding aliphatic diisocyanates [38] (Scheme 5.4). It is noteworthy that this process could be a total phosgene-free process since the reactant, DMC, can be made directly from methanol and carbon dioxide (or urea) and eliminates the use of phosgene [39]. 

Urethanes obtained from aromatic diisocyanates undergo slow oxidation in the presence of air and ligh t, causing discoloration, which is unacceptable in some applications. Polyurethanes obtained from aliphatic diisocyanates are color-stable, although it is necessary to add antioxidants and nv-stabilizers to the formulation to maintain the physical properties with... 

Aliphatic Isocyanates. Aliphatic diisocyanates have traditionally commanded a premium price because the aliphatic amine precursors are more expensive than aromatic diamines. They are most commonly used in applications which support the added cost or where the long-term performance of aromatic isocyanates is unacceptable. Monofunctional aliphatic isocyanates, such as methyl and -butyl isocyanate, are used as intermediates in the production of carbamate-based and urea-based insecticides and fungicides (see Fungicides, agricultural Insect control technology). 

The isocyanates form the major part of the hard or rigid phase of the polyurethane. The three main isocyanates used in industry for castable materials are toluene diisocyanate (TDI), 4,4 diphenylmethane diisocyanate (MDI), and 1,5-naphthalene diisocyanate(NDI). Aliphatic diisocyanates form a small segment of the diisocyanate market. 

The aromatic diisocyanates are generally more reactive than the aliphatic diisocyanates. The position of the isocyanate group relative to surrounding groups controls the reactivity. 

Because of their much higher costs, aliphatic diisocyanates find use mainly in specialized areas where their special properties such as nonyellowing in light are of great importance. The nonyellowing is a result of the aliphatic structure of the isocyanate. There are no series of double bonds that cause the yellowing. 

The reactivity of aliphatic diisocyanates is low in comparison to aromatic isocyanates. It is a problem in the manufacturing stage when using the prepolymer route. Quasiprepolymers and one-shot reactions require the correct choice of curative and catalyst for the system to work. 

HMDI was originally produced by Du Pont as a coproduct in the manufacture of Qiana fiber. Du Pont subsequently sold the product to Bayer. In the 1990s MDA is hydrogenated by Air Products for Bayer (see Amines, AROMATIC-METHYLENEDIANILINE). Commercial HMDI is a mixture of three stereoisomers. Semicommercial aliphatic diisocyanates include / mf-cyclohexane-l,4-diisocyanate (CHDI) and w-tetramethylxylylene diisocyanate (TMXDI). A coproduct in the production of TMXDI is / -isopropenyl-a,a-dimethylbenzyl isocyanate (TMI), which can be copolymerized with other olefins to give aliphatic polyisocyanates. 

The toxicity of aliphatic diisocyanates also warrants monitoring exposure to its vapors. HDI has a moderate potential for acute systemic dermal toxicity rabbit dermal LD50 is 570 ml/Kg (57). However, HDI is severely irritating to the skin and eyes. Irritation, lacrimation, rhinitis, burning sensation to throat and chest, and coughing have all been reported in humans following acute inhalation exposure to HDI. HMDI has a low eye and dermal irritation potential, as well as a low potential for acute toxicity. Exposure to HMDI aerosol can cause dermal sensitization of laboratory animals. IPDI can cause skin sensitization reactions as well as eye irritation. The acute toxicity of diisocyanates in rats is shown in Table 12. 

An offering by Cytec Specialty Chemicals, the meta isomer of tetramethyl xylene diisocyanate (TMXDI) is interesting because it contains an aromatic ring, but the NCO groups themselves are aliphatic isocyanates and have reaction characteristics typical of aliphatic diisocyanates. It reacts even more sluggishly than the more standard aliphatic isocyanates because of steric interactions, making the reactions easier to control. Compounds such as dimethyl tin dilaurate, lead octoate, or tetrabutyl diacetyl distannox-ane have been shown to be effective catalysts for the isocyanate-hydroxyl reaction. The manufacturer claims that it is less toxic than many other isocyanates. 

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