Isocyanate Levels

Free isocyanate vapors in the workplace are strictly controlled, because they are a health hazard. In the commercial production of prepolymers, the diisocyanates can be transferred in a closed system and the final prepolymer stripped of 

---

Amendments to the EC Directives regulating the reclassification of free isocyanate levels came into effect on January 1. Underthe new regulations, TDI prepolymers must be labelled according to the content of free TDI. In preparation for the new regulations, suppliers have been moving to offer finished products with lower levels of free TDI, if possible, to below the minimum required level for hazard labelling. 

Used to introduce chromophores into isocyanates suggested for use in determining isocyanate levels in air down to 0.2 ppm in a 20-1 air sample Reference 35... 

The air around the production area must be vented out, flowing in the direction away from the workers. The isocyanate level should be monitored by an isocyanate detector such as the Remote Intelligent Sensor (RIS) by AFC International, Inc. Detector badges also can be used. 

When prepolymers are manufactured, they do not always come out exactly the same. The viscosity and the isocyanate level can vary within the specified limits. The manufacturer determines the exact level of the NCO that is in that batch and supplies it on the container. 

Monitoring must be carried out to check that the levels of isocyanate vapors do not exceed the local limits. These limits vary from area to area. In some areas of the world, only the isocyanate level is specified and is not controlled by the type of isocyanate. Care must be taken, as the vapor pressure of different isocyanates may vary by a factor of 100 or more. Plants that may comply with the atmospheric isocyanate levels when using MDI-based material may have too high a level when using TDI-based prepolymers. 

The method for the determination of the isocyanate level is based on that given by Wright and Cummins (1969). 

Isocyanate Content Isocyanate levels in polyurethane preparations were measured on 2g samples taken from the polym-erizate and immediately cut into small pieces which were then promptly immersed in excess 0.05 N di(n-butyl)amine solution in dry tetrahyrofuran which was contained in an iodine flask. The gross weight of reagent plus flask was very accurately determined ahead of time to enable the exact weight of the added polyurethane sample to be subsequently determined by difference. Excess di(n-butyl)amine was titrated with 0.05N hydrochloric acid to the bromcresol green endpoint to enable calculation of the sample isocyanate concentration. 

Isocyanate Level During (2.0) Ratio Polymer Melt Polymerizations... 

Isocyanate Levels During 1-Decanol Shortstopped Polymerization of Polymers 17 18... 

Polymer formation may be incomplete, in particular if the labile isocyanate is prematurely decomposed at the time of polymerizaton. Also, a slight excess of isocyanate may be used in the manufacture of the polyurethane, and this is detected as a weak IR absorption band at 2280 cm due to the isocyanate group. The isocyanate group is a strong absorber, and even low levels of isocyanate can be detected in polyurethane. Monitoring of the isocyanate level is often used to assess the degree of polymerization and to measure the quality of the final product [74],... 

The manufacture of MDF, with a few exceptions, dupHcates the manufacture methods for dry-process hardboard, described at length hereia. One exception to it is that most MDF is made ia the medium-density range, 640—800 kg/m although small amounts are made at lower or higher densities. Second, the vast majority of MDF is made with UF resia adhesives with resia requhemeats ia the 7—11% range, and wax is usually added at the 0.50—0.75% level. A small amount of exterior-grade MDF is made with isocyanate resia. 

The isocyanates used with rigid foam systems are either polymeric MDI or specialty types of TDI. Both contain various levels of polymerized isocyanate groups which contribute to molecular weight per cross-link and also may affect reactivity due to steric hindrance of some isocyanate positions. 

Catalysis is usually accompHshed through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-/m(/V,/V-dimethylaminomethyl)phenol are used in the presence of high levels of cmde MDI to promote trimerization of the isocyanate and thus form isocyanurate ring stmctures. These groups are more thermally stable than the urethane stmcture and hence are desirable for improved flammabiUty resistance (236). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Similarly, thioalcohols and thiophenols react with isocyanates to form thiocarbamates. Although these reactions are generally found to be much slower than that of the corresponding alcohol, alkoxide catalysts have successfully been used to provide moderate levels of rate enhancement (68). 

Titrations with dibutylamine [111-92-2] can also be used to determine the NCO content of isocyanates and prepolymers. Generally, an excess of amine in a suitable solvent such as chlorobenzene [108-90-7] is added to the sample. The resulting solution is allowed to react and the unreacted amine is back- titrated with dilute hydrochloric acid. For low NCO content levels, a colorimetric method is often used. The isocyanate-containing species is titrated with amine and the unreacted amine is deterrnined using malachite green [569-64-2]. 

Since 1971, the overall demand for isocyanates has increased at a compounded rate of 12%. Although this level will not likely be sustained in the future due to the maturation of key appHcation markets, it is probable that additional growth will occur through the year 2000. This trend will likely iaclude a shift in emphasis from TDl to MDl and polymeric MDl-based materials. New growth opportunities in the constmction industry, stmctural appHcations, and growth in the automotive industry exist. Third-world markets are also anticipated to provide growth opportunities. 

For other recreational surfaces, such as mnning tracks, the installation techniques are quite different. Most are poured-in-place. An interlocking tile technique may be employed for tennis courts. In all cases, adequate provision for weathering and water drainage is essential. In general, the resiHent surfaces are installed over a hard base (see Fig. 4) that contains the necessary curbs to provide the finished level. Outdoors, asphalt is the most common base, and indoors, concrete. A poured-in-place polyurethane surface (14) is mixed on-site and cast from at least two components, an isocyanate and a filled... 

Whilst rigid closed-cell polyurethanes are excellent thermal insulators they do suffer from a limited and often unsatisfactory level of fire resistance, even in the presence of phosphorus-containing and halogen-containing fire retardants. Considerable promise is now being shown by the polyisocyanurates, which are also based on isocyanate chemistry. 

For all its benefits, the water/isocyanate reaction can be troublesome as well. All raw materials that go into a one-component moisture-cured adhesive must have very low moisture content, usually less than 0.05% water. If higher water levels are present, the adhesive can start curing in the reactor, causing an increase in viscosity, or, in extreme cases, gelation. 

Open times of two-component urethanes can vary widely, depending on the level of catalyst. Reaction times can vary from 90 s to over 8 h. Dibutyltin dilaurate is the most common catalyst employed to catalyze the urethane reaction. This is normally added to the polyol side. A tertiary amine may also be added in small amounts. Tin catalysts do not catalyze the amine/isocyanate reaction very well. Acids, such as 2-ethyl hexanoic acid, may be employed to catalyze the amine/isocyanate reaction where needed. 

In addition to the proteins discussed above, a large number of reactive chemicals used in industry can cause asthma and rhinitis. Hypersensitivity pneumonias have also been described. Isocyanates and acid anhydrides are industrial chemicals that cause occupational asthma. Acid anhydrides, such as phthalic anhydride, seem to cause mainly type I reactions, whereas the IgE-mediated mechanism explains only a part of the sensitizations to isocyanates. Several mechanisms have been suggested, but despite intensive research no models have been generally accepted. The situation is even more obscure for other sensitizing chemicals therefore, the term specific chemical hypersensitivity is often used for chemical allergies. This term should not be confused with multiple chemical sensitivity (MCS) syndrome, which is a controversial term referring to hypersusceptibility to very low levels of environmental chemicals. ... 

A more complete discussion of the kinetics of isocyanate crosslinking in the presence of humidity along with experimental verification of the rate equations used above has been given by van der Ven, et al. (18). Equations 38 and 39 can be integrated numerically for different values of isocyanate to hydroxy ratio and H. Plots of the consumption of hydroxy and isocyanate functionality are shown in Figure 2 for H-0 and H-0.66 assuming equal Initial Isocyanate and hydroxy levels. High humidity increases the rate of consumption of isocyanate and reduces the consumption of hydroxy. 

In general, coating systems are designed to achieve optimum properties at crosslinking levels short of 100% conversion in order to minimize cure time. The effect of Isocyanate to hydroxy ratio and humidity on crosslink density after a fixed cure time is shown in Figure 4. The fixed cure time has been arbitrarily defined as the time at which a coating with equal Isocyanate and hydroxy functionality reaches 85% conversion in the absence of humidity (H-0). If the initial ratio of isocyanate to hydroxy is less than or equal to 1, the crosslink density drops with increasing humidity. 

HPGPC also was used for quality control of incoming raw materials. Figure 8 shows the chromatograms of two different batches of blocked isocyanate crosslinkers. One was acceptable and the other was too reactive. As can be seen from the HPGPC traces, the level of the component eluted at retention volume 40 is much higher for CX-46 than for CX-48. This component was associated with free isocyanate functionality which in excess would make CX-46 too reactive. With this information, either the necessary adjustment for the presence of excessive free isocyanate functionality could be made or this particular batch from the supplier could be rejected. 

Another example involved a batch of isocyanate crosslinker which was too tacky. Upon comparing the HPGPC trace of this sample with that of a control as shown in Figure 9, it is seen that the major difference between these two samples was the level of free caprolactam. The high content of free caprolactam in sample CX-006 depressed the glass transition temperature (Tg) of the sample to such an extent that CX-006 became too tacky. This method of analysis has proved to be a reliable and useful technique for detecting low levels of free caprolactam in this type of oligomeric crosslinker. 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

---