Aliphatic isocyanate

Aliphatic Isocyanates. Aliphatic diisocvaiitcs have traditionally commanded a premium price hccause the aliphatic amine precursors arc more expensive than aromatic diamines. They are most commonly used m applications which support the added cost or where Ihe long-term performance of aromatic isocyanates is unacceptable. Moiiolunctional aliphatic isocyanates, such as methyl and n-butyl isueyanate. arc used as intermediates in the production of carbamate-based and urea-hased insecticides and fungicides. 

The rate of reaction depends on the structure of both the polyol and the isocyanate. Aliphatic polyols are more reactive than aromatic and more stable. Polyurethanes are described as any polymer chain that has been extended by reaction with di- or polyisocyanate. The isocyanate group (-NCO-) can react with most compounds containing an active hydrogen atom. 

Thermal stability of urethane group is relatively low. It was already stated that it depends on the groups to which it is attached, being the highest for aliphatic isocyanate aliphatic alcohol (approx, dissociation temperature 250 °C), then aliphatic alcohol aromatic... 

Benzoylbenzaldehyde reacts similarly with aryl isocyanates. Aliphatic aldehydes and ketones with a CH2 group adjacent to the carbonyl group react with aryl isocyanates at 200 °C to give heterocyclic compounds... 

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

Aliphatic isocyanates have a small but growing market application in thermoplastic polyurethanes (TPU). Medical appflcafions include wound dressings, catheters, implant devices, and blood bags. A security glass system using light-stable TPU as an inner layer is under evaluation for shatterproof automotive windshield appflcafions. 

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

V-Acylation of oxaziridine (54) is of more importance, yielding 2-acyloxaziridines which were unaccessible otherwise until recently. Oxaziridines (54) derived from cyclohexanone, butanone or benzaldehyde are acylated readily by acetic anhydride, acid chlorides or isocyanates. Oxaziridines from aliphatic aldehydes, too unstable to be isolated, may be trapped in situ by benzoylation (67CB2593). 

Other functional groups that are easily differentiated are cyanide (5c =110-120) from isocyanide (5c = 135- 150), thiocyanate (5c =110-120) from isothiocyanate (5c = 125 - 140), cyanate (5c = 105- 120) from isocyanate (5c = 120- 135) and aliphatic C atoms which are bonded to different heteroatoms or substituents (Table 2.2). Thus ether-methoxy generally appears between 5c = 55 and 62, ester-methoxy at 5c = 52 N-methyl generally lies between 5c = 30 and 45 and. S-methyl at about 5c = 25. However, methyl signals at 5c = 20 may also arise from methyl groups attached to C=X or C=C double bonds, e.g. as in acetyl, C//j-CO-. 

This tri-isocyanate is reported to impart good light stability and weather resistanee in polyurethane eoatings and is probably the most widely used aliphatic isocyanate. A number of other aliphatic polyisocyanates have been introduced recently in attempts to produce polyurethanes with improved light stability. Amine derivatives of diphenylmethane are made by reacting aniline of toluidines with formaldehyde. These can lead to a mixture of di-isoeyanates, the diphenylmethane di-isocyanates (MDIs) of commerce. Triphenylmethane-pp p"-tny tri-isocyanate is produced from leucorosaniline. 

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. 

As previously mentioned the initial research on polyurethanes was directed towards the preparation of fibre-forming polymers. Many poly hydroxy compounds and many di-isocyanates were used and the melting points of some of the more linear aliphatic polyurethanes produced cU"e given in Table 27.1. 

Isocyanate is added to provide an equivalent ratio of 1.5-2 isocyanatc/alcohol. The higher the ratio, the more free isocyanate, and diisocyanate monomer, and the lower the viscosity. Most common is 4,4 -methylene bisphenyl diisocyanate (MDl). Its saturated analog or other aliphatic isocyanates are used where light stability is critical. Most common is isophorone diisocyanate. 

R is an organic moiety, which can be aliphatic or aromatic. Most commonly used in adhesives are the aromatic isocyanates, e.g., methylene diphenylisocyanate (MDI) and toluene diisocyanate (TDI). These polyisocyanates and others will be discussed in Section 3. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

In certain niche applications, aliphatic isocyanates, such as isophorone diisocyanate (IPDI), hexamethylene diisoeyanate (HDI), methylene 4,4 -biscyclo-hexylisocyanate (H12MDI), and polymeric versions of these diisocyanates, are used, e.g., in instances where light stability or reduced reactivity is needed. These isocyanates usually cost more than the aromatic diisocyanates. Thus, they are used in adhesive areas that can Justify the higher costs. 

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. 

The waterborne prepolymer process is similar to the prepolymer synthesis described earlier, except that most of the waterborne prepolymers are based on aliphatic isocyanates and contain an internal emulsifier. There are several types of internal emulsifiers, both anionic and cationic. A good summary of these stabilizers is found elsewhere [56], The majority of the waterborne urethanes are anionic dispersions. An internal surfactant, such as dimethylolpropionic acid, is often incorporated into the prepolymer ... 

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. 

Molecular ion The molecular ions of aliphatic isocyanates are observed up to C8. 

Fragmentation Characteristic fragments of aliphatic isocyanates include m/z 56 (CH2NCO), 70 (CH2CH2NCO), 84 (CH2CH2CH2NCO), and so forth. 

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