Isocyanates commercially used

This simple reaction is the bedrock of the polyurethane iadustry (see Urethane polymers). Detailed descriptions of the chemistry and process have been published (65—67). Certain carbamates are known to reversibly yield the isocyanate and polyol upon heating. This fact has been commercially used to synthesize a number of blocked isocyanates for elastomer and coating appHcations. 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

Specialty Isocyanates. Acyl isocyanates, extensively used in synthetic appHcations, caimot be direcdy synthesized from amides and phosgene. Reactions of acid haUdes with cyanates have been suggested. However, the dominant commercial process utilizes the reaction of carboxamides with oxalyl chloride [79-37-8]. CycHc intermediates have been observed in these reactions which generally give a high yield of the desired products (86). 

Most commercial methods are based on the phosgenation of amines and their salts, a route first described by HentscheP in 1884. Only a few isocyanates are used commercially and the most important are ... 

Over 40 chemical reactions are used in urethane chemistry. The six most common urethane reactions that are relevant to adhesives are shown in Fig, 1. The monomeric forms of the reactions are shown for simplicity s sake however, most commercially useful products for polyurethanes are based on polyfunctional isocyanates and polyfunctional alcohols or polyols . 

British Drug Houses Ltd. or Matheson Laboratory reagent-grade phenyl isocyanate was used without further purification. When using other isocyanates, care should be taken to ensure their purity as the yield is greatly dependent upon this, commercially available p-nitrophenyl isocyanate being a case in point. 

Organic isocyanates are the major building blocks of urethane structural adhesives. They can be synthesized by a variety of routes, but most of the commercially available isocyanate compounds used in adhesives are made by the reaction of a precursor primary amine or amine salt with phosgene, followed by dehy-drohalogenation. The reaction with phosgene usually is carried out at a relatively low temperature, less than 60°C, and then the temperature is raised to 100-200°C to remove the HC1. 

MIC is a member of the isocyanate family of chemicals. The high chemical reactivity of isocyanates is central to their toxicity as well as commercial uses. No clinical use of isocyanates has so far been demonstrated. In view of these considerations, this section will elaborate in some detail the relationship between the structure of MIC and other isocyanates, and between their physicochemical properties and toxicities. 

Most of the commercially used isocyanates are diisocyanates and R is an aromatic ring. MIC is an exception its structure is H3C-N=C=0. The physicochemical properties of MIC differ from those of other isocyanates (Lowe, 1970 Tse and Pesce, 1978 Westcott, 1985 Worthy, 1985). Because of high chemical reactivity of MIC with alcohols, it serves as an intermediate in the production of the pesticide carbaryl. Diisocyanates are primarily used for the manufacture of polyurethanes. 

Polyurethanes are formed by polymerization of isocyanates with polyether polyols or polyester polyols. The products are quite versatile and can be manufactured as either foamed or elastomerics. In addition, a small amount of isocyanates are used for adhesives and coatings and a minor quantity of monoisocyanates are used as precursors for carbamate pesticides. Phosgene is a key reactant in the commercial processes for all of the isocyanates. 

Polyurethanes are thermoset polymers formed from di-isocyanates and polyfunctional compounds containing numerous hydroxy-groups. Typically the starting materials are themselves polymeric, but comprise relatively few monomer units in the molecule. Low relative molar mass species of this kind are known generally as oligomers. Typical oligomers for the preparation of polyurethanes are polyesters and polyethers. These are usually prepared to include a small proportion of monomeric trifunctional hydroxy compounds, such as trimethylolpropane, in the backbone, so that they contain pendant hydroxyls which act as the sites of crosslinking. A number of different di-isocyanates are used commercially typical examples are shown in Table 1.2. 

Blocked isocyanates offer a number of advantages to unblocked isocyanates. The traditional concern for moisture sensitivity can be addressed by blocking the isocyanate. Heat activation is then required, but most commercial adhesive applications can meet this requirement. Water-based dispersions and dispersions of the isocyanate in the polyol or other reactive media become possible using blocked isocyanates. There are a number of blocked isocyanates commercially available that could be used in adhesive... 

This innovative research has resulted in a commercial DuPont process that makes MIC and converts it in-situ to an agrichemical product. Consequently, the potential for exposure is greatly reduced. This trend in in-situ manufacture and derivatization is clearly the way of the future for hazardous chemicals. Non-phosgene routes to isocyanates, and use of solid acids to avoid HP and H2SO4 as alkylation catalysts are other examples of research in progress to minimize fur er the use of hazardous materials. 

Polyurethanes are one of the more commercially useful polymers and are a major commodity item. A series of papers have reported attempts to incorporate phosphazene oligomers into polyurethanes. Cyclic trimer (54) has free hydroxyl groups that was reacted with prepared isocyanate pre-polymer. The loading process increased thermal stability, lowered temperature resistance and hydrophobity however mechanical stability was slightly less. 

The common aliphatic diisocyanates are hexamethylene diisocyanate, hydrogenated (H12) 4,4 -dipenylmethane diisocyanate, isophorone diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate. Other aliphatic diisocyanates that are commercially used are lysine diisocyanate, methylcyclohexyl diisocyanate, isopropyhdine bis-(4-cyclohexyl isocyanate), and tetramethylene diisocyanate. Many additional polyfunctional isocyanates are described in the literature. 

Other example isocyanates commonly used in PUs applications are shown in Fig. 1.8. A series of model conventional aromatic diisocyanates is depicted 4,4-diphenylmethane diisocyanate (MDI), 1,5-naphthalene diisocyanate (NDI) and 2,4-tolylene diisocyanate (TDI), most common commercially as an 80 20 mixture of the... 

In spite of these successful experiments, there was little if any commercial use for polyurethanes. According to Dr. Bayer, the "great breakthrough" in polyurethane chemistry occurred in 1941 when Drs. Hoechtlen, Droste, and Bunge made a porous casting which those in the testing laboratory named an "imitation swiss cheese." The bubbles were the result of the production of CO2 from the reaction of isocyanate groups with unreacted carbo l... 

Polyurethanes were first discovered by Otto Bayer and coworkers at I. G. Farbenindustrie, Germany, in the late 1930s. The first products were obtained by reacting an aliphatic diisocyanate with an aliphatic diamine or diol. These materials soon found commercial uses and were marketed under the trade names of Irgamid U, for plastics, and Perlon U for synthetic fibers and bristles. Very soon after this, it was discovered that isocyanates could be used to bond rubber to metal, which in turn led to the development of urethane adhesives based on polyester diols these adhesives were commercialized under the trade name Polystal. For a more complete account of the history of polyurethanes, see refs. 2, 4, and 5. 

Monoisocyanates prepared by aminolysis of phosgene are predominantly used as intermediates in the manufacture of agricultural products such as herbicides and insecticides [207, 208]. The commercially used aliphatic isocyanates include methyl, propyl, isopropyl, butyl, isobutyl, octadecyl, and cydohexyl isocyanate. The aromatic isocyanates used as building blocks for agricultural chemicals include phenyl isocyanate and halogen-substituted phenyl isocyanates. 

When the AS 1160 resin is mixed with isocyanate, and cured for up to 24 h in dry (typically less than 15 % RH), a thixotropic polyurethane adhesive is formed. The adhesive is commercially used to bond metals, organic and inorganic materials, and is particularly useful for reducing the relative movement of components in multimaterial assemblies. The performance of the polyurethane adhesive requires the ultimate tensile strength (UTS) to be a minimum of 15 MPa when measured using butt tensile specimens by ASTM Test Method D2095. 

Applications. Isocyanates are used commercially to a minor extent in manufactured board as a specialty adhesive. They are used in particleboard, OSB, and waferboard for special products where color as well as exterior durability are important. 

---