Isocyanate vapour

Direct contact of the isocyanate vapours to the pump must be avoided. The fumes must be frozen out with a liquid nitrogen trap and/or a trap containing a plasticizer such as Messamol or a phthalate plasticizer. 

Gandevia, G. Studies ov ventilatory capacity and histamine response during exposure to isocyanate vapour in polyurethane foam manufacture. Br J Ind Med 20 204-09, 1963. 

The general hazards to health associated with isocyanates may be summarized as follows all isocyanates (both aliphatic and aromatic) if present above certain critical values adversely affect the respiratory system. Surveys by the International Isocyanate Institute (which is primarily a voluntary association of isocyanate chemical manufacturers) suggest that normal individuals can work a standard 8-h day in an atmosphere in which the TLV of the diisocyanate is observed in recent years these values have been revised downwards (see Table 14.1).t However, mild to severe asthmatic attacks will result in the majority of people exposed to excessive inhalation of isocyanate vapour and this must be recognized as an acute hazard also loss of lung capacity and reduction in forced respiratory response results. Once an individual has suffered such an attack, some sensitization must, in the majority of cases, be assumed to have occurred ... 

In the course of their work, operators need not be exposed to isocyanate vapours by inhalation if adequate precautions are taken. The vapour pressure of the isocyanate, the temperature resulting from the exothermic reaction of the isocyanate with hydroxyl groups and the elevated processing temperature of the components which is frequently necessary, all affect the concentration of the vapour. 

Protection from isocyanate vapours liberated during polyurethane manufacture is usually achieved by installing permanent exhaust ventilation units which either exhaust directly to the atmosphere or pass their exhaust fumes through scrubbers which extract the isocyanate vapour through a sodium carbonate spray tower before atmospheric exhaustion occurs. Continuous vertically positioned exhaust hoods are common where continuous conveyor lines are involved for localized extract situations, vertical down-draught or horizontal extract modes are much safer for operatives, being designed to remove all isocyanate vapour away from an operative s face and body. These latter situations apply particularly in the manufacture of cast-moulded and reaction-injection moulded products. 

Sampling from the mixing head for quality control or other purposes should be carried out in such a way as to prevent persons employed from being exposed to isocyanate vapour. In the absence of other precautions, suitable respiratory protective equipment should be used. 

Note Empty isocyanate drums should be decontaminated before disposal by filling them with sodium carbonate solution and leaving them to stand for 24 h with the bungs removed to allow carbon dioxide to escape. Care should be taken to avoid the inhalation of the isocyanate vapour displaced from the drum during filling. 

The isocyanate vapours are absorbed in an acid solution which converts the vapour into an amine. The amine is diazotized and coupled with V-naphthylethylene diamine and the colour measured in a spectrophotometer. Details of this common method are now given. 

Phenyl isocyanate is a colourless liquid, b.p. 164° or 55°/13 mm. its vapour is lacluymatory. The liquid reacts readily with water, yielding diphenyl urea, m.p. 241°, and hence must be protected from atmospheric moisture ... 

The next study of wood modification was that reported by Baird (1969), who performed vapour-phase reactions of spruce with ethyl, n-butyl, /-butyl, allyl and phenyl isocyanate (PhNCO). Unfortunately, DMF was used as a catalyst for the reactions, which resulted in polymerization of PhNCO in the cell wall of the wood, leading to unpredictable results. No evidence was presented in support of the contention that polymerization had occurred, and since this requires an anionic catalyst initiator, this is considered unlikely. However, the presence of side reactions when DMF is used in conjunction with isocyanates has already been mentioned. Greater success was reported when butyl isocyanate was reacted with wood (presumably a consequence of the lower reactivity of this isocyanate... 

Martins and Banks (1991) modified pine and wood samples with phenyl or butyl isocyanate and investigated the water vapour sorption properties of the modified wood. Modification resulted in a reduction in EMC at a given RH up to about 35 % WPG with butyl isocyanate, although reaction with phenyl isocyanate above 25 % WPG resulted in an increase in sorption due to cell wall damage caused by the reaction. The butyl derivative was more effective at reducing hygroscopicity over all the WPGs studied. 

Water vapour permeability. The most notable phenomenon overlooking the data presented In Table I Is that the water vapour permeabilities of the HMMM-based coatings are not widely different. The Isocyanate coatings show somewhat larger differences. GTG/lsocyanate and the coatings made from butanedlol and neopentyl glycol are more permeable. 

The heat of formation of [MolCOlg] has been determined as -960 + 12 kJ mol by measuring its heat of decomposition. The Mossbauer parameters for the 100 keV transition of in [W(CO)g] and some tungsten(vi) complexes have been measured and discussed in terms of known bonding and structure. Secondary ions [M (CO) ] (M = Mo, m = I or 2 M = W, m = 1—4 n = 0—14) formed by ion-molecule reactions have been observed in the mass spectra of the hexacarbonyls. A mixt u re of [Cr(CO) ] and [MolCO) ] vapours affords [CrMo(CO) ] ( = 5—7). [MofCOl ] and [WICO) ] catalyse the condensation of isocyanates with aldehydes to give imines in high yields. ... 

Figure 16.7—Ion mobility spectrometer. Ions enter the analyser tube by control of the polarity of the acceleration grid. An example of a recording in the repetitive mode (RIP Reactant Ion Peak) for tolyl isocyanate (TDI), the compound analysed, is shown. A commercial model of an Environmental Vapour Monitor is also shown (reproduced by permission of Grasby Electronics, UK). This instrument uses a GC column to improve compound identification.

The cyanogen halides polymerize on standing to the trimers, the cyanuric halides. Cyanamide is converted into the corresponding trimer, melamine, on heating to about 150°C. Isocyanic acid, however, polymerizes far more readily. If urea is distilled, isocyanic acid is formed but polymerizes to cyanuric acid, (NCOH)3, a crystalline solid, the vapour of which, on rapid cooling, yields isocyanic acid as a liquid which, above 0 C, polymerizes explosively to cyamelide, a white porcelain-like solid. This latter material is converted into salts of cyanuric acid by boiling with alkalis. These reactions are summarized in Chart 21.1. Cyanuric derivatives... 

The structure of the isocyanic acid molecule has been studied in the vapour and crystalline states. The H-N-C angle is close to 128° and the bond lengths... 

Experiment Heat c.c. of phenyl mustard oil in a test-tube with the same volume of yellow mercuric oxide for some time, until the oil boils. The yellow oxide is changed to the black sulphide, at the same time the extremely disagreeable odour of the phenyl isocyanate arises the vapour of the compound attacks the eyes, causing tears. 

Hydrogen selenide is probably a more poisonous gas and methyl isocyanate is certainly a more poisonous vapour (b.pt. 39 C). Bis(chloromethyl)ether is more poisonous in the chronic sense (OEL = 1 p.p.b.). Phosgene might, therefore, be more correctly described as the most poisonous gas used in any significant volume in industry. 

Liquid-vapour equilibria in the phosgene-hydrogen chloride system have also been studied in connection with the problems associated with the separation of these two materials during the production of isocyanates. The system has been examined under atmospheric pressure within the range of the boiling points of the materials, ca. 8 to -85 C. The temperature-composition curve is illustrated in Fig. 6.14 [789] it shows large deviations from ideality. 

The hydrogen chloride generated in the first step ensures that the carbamoyl chloride is normally the primary reaction product. However, if the hydrogen chloride is removed before the reaction products are cooled, then isocyanates are prepared directly [1897,1898]. The vapour phase preparations are especially suitable for the lower aliphatic carbamoyl chlorides, RNHCOCl R = Me, Et, Pr, CH2=CHCHj, Bu or CH3(CHj)4 [1897]. 

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