Monomeric diisocyanates

Isocyanate-Terminated Adducts and Polymers. Monomeric diisocyanates such as TDI, MDI, or Hi2MDI, because of their irritant characteristics, are seldom used in an unreacted form for the compounding of urethane coatings. They are normally converted into isocyanate-terminated polymers or adducts of polyols such as hydroxyl-terminated polyesters and polyethers, castor oil, etc. 

Metabolites of isocyanates in biological fluids and their protein adducts have been used as biomarkers of exposure. However, biomarkers are only available for a limited number of isocyanates at present. Several studies have suggested that metabolites of isocyanates in hydrolyzed urine may be used as an indicator of exposure over a work-shift, and some useful relationships between levels in air and urine have been presented for TDI, HDI, and MDI (as monomeric diisocyanates). > > > > Furthermore, metabolites of diisocyanates in hydrolyzed plasma and or as hemoglobin (Hb) adducts can be used as an indicator of accumulated dose over about... 

The analytical techniques employed for biomonitoring of isocyanates have primarily been GC and LC, often in combination with MS detection. These are techniques which are widely adapted in the field of bioanalysis. The corresponding diamine metabolites of the respective monomeric diisocyanates have in most studies been determined in urine or plasma, or as blood protein metabolites, after sample preparation often based on hydrolysis, extraction, and subsequent derivatization of polar functional groups, in order to increase the volatility in cases where GC was employed. -... 

PMDIs are less reactive than monomeric diisocyanate but are also less volatile. The polyisocyanurate produced from this material will be of type shown in Figure 4.33b. Some of the catalysts used for the polytrimerization reactions are alkali-metal phenolates, alcoholates and carboxylates, and compounds containing o-(dimethylaminomethyl)-phenol groups. 

Bonart R and Demmer P (1982) GPC determination of monomeric diisocyanate in prepolymer for segmented polyurethanes (in German). Colloid and Polymer Science 260 518-523. 

The chemistry of 2C-Polyurethane adhesives is outlined in Fig. 61. Cross-linking occurs after mixing a polyol-based, filled resin with an isocyanate-based hardener which can either be a monomeric diisocyanate or an isocyanate terminated prepolymer synthesised from a polyol with the isocyanate-containing compound. The active hydrogen groups, such as OH, NH, SH etc. react in an electrophilic manner with the isocyanate groups. 

The first reactant acts as the curing agent. Five major monomeric diisocyanates are commercially available [10] ... 

Figure 7.1 Synthesis of isocyanate terminated prepolymer from polyol and monomeric diisocyanate...

The monomeric diisocyanates are reacted with the hydroxyl groups at each end of the polymeric chains of the polyols (just discussed) to form the isocyanate-terminated prepolymers as shown in Figure 7.10. 

Polyurethanes are thermoset polymers formed from di-isocyanates and poly functional 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 poly ethers. 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 diisocyanates are used commercially typical examples are shown in Table 1.2. 

Embryotoxicity study of monomeric 4,4 -methylenediphenyl diisocyanate (MDI) aerosol after inhalation exposure in Wistar rats. Fundam Applied Toxicol 32 96-101, 1996... 

The two key isocyanates that are used in the greatest volumes for polyurethane polymers are toluene diisocyanate (TDl) and methylene diphenyl diisocyanate (MDl). Both isocyanates are produced first by nitration of aromatics (toluene and benzene, respectively), followed by hydrogenation of the nitro aromatics to provide aromatic amines. In the case of MDl, the aniline intermediate is then condensed with formaldehyde to produce methylene dianiline (MDA), which is a mixture of monomeric MDA and an oligomeric form that is typical of aniline/formaldehyde condensation products [2]. The subsequent reaction of phosgene with the aromatic amines provides the isocyanate products. Isocyanates can also be prepared by the reaction of aromatic amines with dimethylcarbonate [3, 4]. This technology has been tested at the industrial pilot scale, but is not believed to be practiced commercially at this time. 

In addition to their local irritant effects on the respiratory tract, the diisocyanates also have a propensity to induce an immimological response in some individuals, which is characterized by an asthma-like respiratory reaction, and will induce the formation of antibodies to both the monomeric and polymer forms of HDl. A few studies have examined the immimological effects of HDl toxicity in humans, with some data available from laboratory animal studies as well. 

Hexamethylene diisocyanate is a highly reactive synthetic chemical that is widely used in the production of polyurethane materials. There is no natural somee of HDI. All of the potential exposures to this compound are associated with the production, handling, use, and disposal of HDI and HDI-containing products or materials. Exposures to HDI are often associated with exposures to its prepolymers, especially to a trimeric biinetic prepolymer of HDI (HDI-BT) (see Figure 5-Ia), whieh is widely used as a hardener in automobile and airplane paints, and whieh typieally contains 0.5-1% unreacted HDI (Alexandersson et al. 1987 Hulse 1984 Karol and Hauth 1982). There is evidence that diisocyanate prepolymers may induce asthma at the same or greater frequency as the monomers (Seguin et al. 1987) therefore, there is a need to assess the potential for human exposme to prepolymeric HDI as well as monomeric HDI. Except for limited data on occupational exposures, no information was foimd in the available literature related to the potential for human exposure to prepolymers of HDI. 

No data were available to the Working Group with regard to preparations of monomeric 4,4 -methylenediphenyl diisocyanate alone. The following is a study of a preparation containing both monomeric and polymeric 4,4 -methylenediphenyl diisocyanate. 

Polymeric 4,4 -methylenediphenyl diisocyanate containing 44.8-50.2% monomeric 4,4 -mcthylcncdiphcnyl diisocyanate was tested for carcinogenicity by inhalation in rats. An increased incidence of lung tumours was observed. 

Soft blocks are composed of linear, dihydroxy poly ethers or polyesters with molecular weights between 600 and 3000. In a typical polymerization of a thermoplastic polyurethane elastomer, the macroglycol is end capped with the full amount of aromatic diisocyanate required in the final composition. Subsequently, the end-capped prepolymer and excess diisocyanate mixture reacts further with the required stoichiometric amount of monomeric diol to complete the reaction. The diol links the prepolymer segments together while excess diol and diisocyanate form short hard-block sements, leading to the (AB)n structure illustrated in Figure 1. Block lengths in (AB)n polymers are frequently much shorter than those in anionically synthesized ABA block copolymers. 

If the elapsed time between lamination and food contact is too short, imreacted monomeric isocyanate can be available to react with water to form low molecular weight amines. The presence of aromatic amines can be an issue with the use of MDl or toluene diisocyanate (TDl) containing isocyanate systems. Often aromatic isocyanates are selected to obtain good adhesion under moist and low temperature conditions. Aromatic isocyanates are also cheaper than aliphatic systems. The crystallinity of the film can reduce permeation rates of monomeric isocyanates into adjacent film layers. Other factors that can influence the rate/degree of cure (isocyanate reaction) include the following ... 

Poly(iminoborane)s [(-BR=N-)n] are a potential 7t-conjugated polymers, since boron-nitrogen bonds possess the double bond like nature owing to pTt-pjt interaction However, these materials are not fully characterized so far 10>. In order to incorporate the monomeric B-N bond into the conjugation path, we examined an alternating boration copolymerization between diisocyanates and diynes making use of different reactivity between haloboration and phenylboration reactions. 

All spray foams contain monomeric isocyanates, which can present toxicological and environmental hazards [2], Isocyanates like TDI (toluenediisocyanate) or MDI (methylendiphenyl-diisocyanate) are even suspected to cause cancer [2, 3]. 

The modification of a filler surface with isocyanates is a simple process which involves the reaction of hydroxyl groups on the filler surface with monomeric isocyanate. 2,4-toluene diisocyanate or hexamethylene diisocyanate are commonly used. Since isocyanates are bifunctional they can be further reacted with polyols to form a coating on the surface or they can be used for the reinforcement of polyurethane. A strong covalent bonding can be verified by controlled extraction with the solvent. Bound material will not be removed from the fillefs surface. 

Isocyanate Components. Aromatic Di- and Polyisocyanates. The most important monomeric aromatic diisocyanates used for coatings are tolylene diisocyanate (TDI) and 4,4 -methylene bis(phenyl isocyanates) (MDI). Tolylene diisocyanate, a colorless liquid (bp 120 °C at 10 mmHg), is generally used in 80/20 blends of the 2,4-and 2,6-lsomers. Pure 2,4-TDI isomer has also been employed for coatings. 

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