Monomeric isocyanates

In the reaction of trifluoromethylmercaptosulfenyl chloride with AgNCO the formation of only the monomeric isocyanate is observed (in 87% yield) (16) ... 

Difluorochloro- and fluorodichloromethanesulfenyl chlorides react directly with AgOCN to give the monomeric isocyanates, CF2CISNCO and CFCI2SNCO, whereas in benzene solution the corresponding iso-cyanurates are formed (61). Freshly prepared CFgClSNCO either tri-merizes or dimerizes to a uretidine dione within a month when kept at 20°C. 

PREPOLYMER. An adduct or reaction intermediate of a polyol and a monomeric isocyanate, in which either component is in considerable excess of the other, A polymer of medium molecular weight having reactive hydroxyl and —NCO groups. 

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 ... 

Summary As all conventional spray foams contain monomeric isocyanates, they can present toxicological hazards. Therefore, an isocyanate-ftee substitute for these foams has been developed. This isocyanate-free foam is cured by a crosslinking reaction of silane-terminated prepolymers (STPs). This silane crosslinking system exhibits some new properties, and several problems occur that are completely unknown with ordinary isocyanate spray foams. In particular, crack formation during foam curing turns out to be a crucial problem. Therefore it was necessary to develop modified STPs and to use special STP-propellant mixtures. 

Because of their easy handling, 1-component spray foams are mainly in use in Europe. These foams are cured by a reaction of a mixture containing both isocyanate-terminated prepolymers and monomeric isocyanates with moisture. 

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]. 

In another investigation, Tsuzuki et al. ( 8) found that trimerization of phenyl isocyanate occurs in the presence of ethylene carbonate and catalytic amounts of a tertiary amine. In this case, the trimer forms a complex with the carbonate which can react further at elevated temperatures by reverting to monomeric isocyanate. 

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. 

Weisfeld s order of reactivity is directly opposed to the order studied in this work, although the same test method was used in both experiments. Weisfeld used monomeric isocyanate and polyester while this study included carboxyl terminated polyurethane and prepolymerized isocyanate.(5) This distinction between monomeric and polymeric materials could explain the distinct contrast in catalytic activity. 

Expt. Monomeric isocyanate (ml) Solvent (ml) Initial temp (°C) Catalyst Yield (%) 7inh (0.5%) benzene... 

They contain monomeric isocyanate, which is toxic, and thus working safety must always be taken into account. 

Nevertheless all spray foams have the crucial disadvantage that they contain isocyanates, which are critical with respect to toxicological and enviromental impUcations [8]. Monomeric isocyanates like TDI (toluene diisocyanate) or MDI (methylenediphenyl diisocyanate) are even suspected of causing cancer [8, 9]. Although these foams are used even by nonprofessionals, to date there exist no suitable, less endangering substitutes. 

Conventional (single-component) polyurethane spray foams are cured by a reaction of prepolymers containing isocyanate moieties and monomeric isocyanates with moisture to generate a urea unit and CO2 [7]. The latter serves as propellant or — more often — as co-propellant besides being a physical blowing agent. 

Aryl isocyanates readily polymerize in the presence of catalysts into dimers containing 2 moles of the monomeric isocyanate. The reaction and structure is generally considered to be ... 

Isocyanate compounds are supplied in both monomeric as well as polymeric forms. The monomeric isocyanates are generally low MW liquid difunctional materials and are used as primary building blocks. Due to their greater toxicity, they must be handled very carefully, and generally, free monomer content is restricted in the... 

Since some catalysts initiate dimerization and trimerization under specific conditions, equilibria between monomeric isocyanates/catalyst, dimer/catalyst and trimer/catalyst exist. For example, interrupting the trimerization of phenyl isocyanate with a penta-substituted guanidine catalyst after short periods of time leads to the formation of the phenyl isocyanate dimer. ... 

A large number of multifunctional isocyanate curing agents are commercially available for crosslinking of the hydroxyl functional resin. These isocyarmte adducts are mostly derived from a limited number of aliphatic and aromatic monomeric isocyanates, whose stmctures can be seen in Figure 5-2. 

The final distillation stage is extremely important in order to ensure removal of any low molecular weight volatile monomeric isocyanates. 

However, polyurethane acrylic coatings have continued to be the subject of toxicological concern over the use of isocyanate compounds, although the commercialisation of higher molecular weight polyisocyanates has minimised the risk of exposure to monomeric isocyanates. These systems are only recommended for use in well ventilated areas with efficient air extraction. In the case of spraying, operators must wear masks with a separate air supply. 

Significant developments were reported in 1990 by Kokta, Maldas and co-workers at the University of Quebec" using dried hardwood (aspen) sawdust and cotton cellulose fiber in plasticized vinyl compounds. As might be expected, the best overall balance of properties resulted from use of the polymeric isocyanate polymethylenephenyl isocyanate (PMPI), followed by diisocyanates TDI and HMDI, with the least effect from a monomeric isocyanate. Use of isocyanates as adhesion promoters is discussed in Section 12.7. 

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