Prepolymer isocyanate levels

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. 

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. 

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]. 

Important commercial isocyanates include the diisocyanate monomers toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and MDI-, TDI-, and HDI-based isocyanates (e.g., prepolymers and polyisocyanates). World-wide production volume is estimated at over 12 billion lb. Isocyanates (diisocyanates, polyisocyanates, and prepolymers) all cause similar health effects, most commonly asthma [32]. Isocyanates are reported to be the leading attributable cause of work-related asthma [16]. Isocyanates are potent sensitizers that can trigger a severe and potentially fatal asthma attack in sensitized persons at very low isocyanate exposure levels [16]. Toluene diisocyanate is reasonably anticipated to be a human carcinogen by National Toxicology Program. 

The last chemical characteristic is the attachment of ligands. Unlike the other properties (reservoir capacity and biocompatibility) that could be incorporated into a one-shot process, this aspect is most conveniently practiced at the prepolymer level. The philosophy is to use some of the isocyanate functionality of the prepolymer to attach active side chains. This is illustrated in two examples that we will discuss again. 

If the acidity of the prepolymer is not correct or if the material is stored at too high a temperature, the material will start to thicken. The level of isocyanate also falls under these conditions. 

Polyurethane prepolymers are supplied in a variety of sizes, from small 1-kg cans up to 1-ton IBC containers. The available isocyanate (%NCO) level should be marked on every container of conventional prepolymers. Quasiprepolymers do not have individual batch levels supplied, as the overall reaction levels are factory adjusted. 

Off-ratio mixes can be a result of the NCO level in the prepolymer being too low. If the prepolymer is stored at too high a temperature for an extended period, the isocyanate at the end of the chains reacts with itself to form multifunctional chains. Thus when the curative is added, it will be in great excess as a result of the isocyanates having used themselves up. 

Free isocyanate specimen. A measure of the free, unreacted diisocyanate monomer present in an isocyanate prepolymer. Low free isocyanate prepolymers are desirable, as they contain very low levels of volatile isocyanate, making them inherently safer for the end user. 

To prevent gelation and side reactions, it is normally recommended that the reaction be kept very slightly acid. The level is 0.33 microequivalents per gram of prepolymer. The acidity of the diisocyanates is normally specified in terms of parts per million of isocyanate acid in the diisocyanate. To convert to microequivalents ... 

First-generation solventless polyurethane adhesives are one-component isocyanate terminated prepolymers formed by the reaction of MDI (4,4 methylene bis (phenyl isocyanate)), or other isocyanates with polyether and/ or polyester polyols. One-component 100% solids adhesives rely on moisture from the air or substrates or from induced moisture misting during the converting process, to cure the adhesive via an isocyanate/water reaction and subsequent polyurea-polyurethane polymer formation. Typically the high viscosity of the adhesive is such as to require adhesive delivery equipment and application rollers heated from 65-80 °C for use. They have a high level... 

Third generation. Highest levels of shrinkage control. Accuracy of mold reproduction and surface smoothness Dual thickening systems containing isocyanate prepolymers Saturated polyesters Saturated polyester blends with pol5rvdnyl chloride Polyurethanes... 

Polyurethane reactive hot melts are 100% solid, hot-melt thermoplastic prepolymers that moisture cure slowly after application. Conventional hot melts are known for their quick setting, excellent green strength, ease of application, and low toxicity. Their primary limitation is low heat resistance (at elevated temperatures, the adhesive will soften and flow) and poor adhesion to some substrates, due to insufficient wetting. The use of a polyurethane prepolymer with low levels of free isocyanates as a hot melt offers distinct... 

The 8000-MW, ultra-low monol PPG is used particularly in the development of soft, plasticiser-free elastomers [16, 17]. The nse of this high MW, low polydispersity polyol allows for the preparation of low viscosity prepolymers with low isocyanate contents. Initial evalnations of these very soft elastomers showed lower than expected physical properties. The lower properties can be attributed to the very low hard-segment content of these polymers. The lack of hard segment (physical crosslinks) can be compensated for by the incorporation of low levels of chemical crosslinks (triol) [18]. This is accomplished by the addition of a 6000-MW, ultra-low monol triol (Acclaim Polyol 6300) into the polymer matrix. Table 9.7 shows the triol effect on prepolymer viscosities, elastomer processability and physical properties. It shonld be noted that very low levels of crosslinking are needed to improve the elastomer properties. 

A follow-up quantitative GC-MS analysis did not disclose excessive levels of TCPP in the unstained polyurethane foam, as compared with samples taken from other, problem-free enclosures. However, significant variability in the flame retardant amounts at different foam locations was detected. Therefore, based upon a follow-up review of the manufacturing process, it was hypothesized that the flame retardant had separated from the polyol prepolymer (since specific gravity of TCPP is close to 1.3 g/cc). It was not clear whether this separation was due to inadequate stirring, pauses in production of malfunction in the polyol / isocyanate mixhead. However, in the final analysis it was clear that the flame retardant on the ABS surface acted as a stress crack agent, and it was occasionally being delivered with the foam to the ABS surface in excess amounts. Polyurethane delivery equipment troubleshooting was subsequently undertaken by the enclosiu e manufacturer. 

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