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Analysis of Isocyanates

Isocyanate functionality and the quality of the isocyanate used for polyurethane polymerization are as critical to the final material result as the polyols. However, the characterization of isocyanates is in some ways easier since, with the exception of polymeric MDI (pMDI), they are monomeric and can be distilled to a pure state or possess an isomeric distribution that is relatively easy to characterize using modern analytical techniques. Another distinction with polyols, isocyanates are somewhat easier to titrate than alcohol groups since their reaction with amines is diffusion limited. Just as with polyols, ASTM provides several explicit standard methods for characterizing an isocyanate sample, and given the advantages of characterizing the isocyanate monomers and functionality, the tests exhibit superior precision and reliability to those for polyols. However, the reactivity of isocyanates does not permit the practitioner to trust the functionality per unit mass. Given the number of isocyanate-isocyanate (see Chapter 3) reactions and the facile reaction of isocyanate with opportunistic water, the quality of an isocyanate should always be regarded with suspicion if a reaction is not proceeding as anticipated.

There are a number of ASTM techniques applicable to isocyanates. Many of them relate to safety and air monitoring for isocyanates. Table 5.3 is a partial list of ASTM methods related to the use of polyisocyanates, the values of which might be included on a certificate of analysis from a manufacturer and relate to the reliability in use.

There are several other standards available associated with, for instance, measurement of viscosity, color, and density, but these are related more to manufacturing control than reliability in use. As alluded to, the reliability of these tests can be expected to be good as enumerated in Table 5.4.

Since the tests for isocyanate quality are expected to be relatively reliable, there is a reasonable expectation that the values attending a particular product were trustworthy at the time they were made. Problems in polyurethane properties that

TABLE 5.3 Conventional isocyanate specifications and AsTM procedures for measurement

Property

ASTM method

Percentage hydrolyzable chloride

D5523 potentiometric method following reaction of

sample with isopropanol to form HCl

Percent MDI monomer in pMDI

D7252 method of HPLC separation resulting in

and monomer

baseline resolution of species and well-determined

isomer distribution

elution times

Determination of isocyanate

D5155 titration of isocyanate with a secondary amine

content per unit mass

(dibutylamine) followed by titration of excess

amine

Standard specification for TDI

D1786 methods and expectations for TDI based on

purity, color, total chlorine, hydrolyzable chlorine,

isomer distribution

Isomer content of TDI

D4660 employs IR spectroscopy using standard

materials and internal standard for quantification

TABLE 5.4 Critical evaluation of the AsTM method derived property value in Table 5.1. Manufacturer numbers on well-established products are usually reliable. New materials made by new processes may be far less so

Property

Reliability

Percentage hydrolyzable chloride

Unknown but expected to be average

Percent MDI monomer in pMDI and monomer

Very good

isomer distribution

Determination of isocyanate content per unit mass

Very good

Standard specification for TDI

NA

Isomer content of TDI

Very good

may result from poor isocyanate quality are generally a reflection of poor storage conditions or unoptimized handling during the polymerization. It is often the case that a slight excess of isocyanate is added to the polymerization to account for loss of unit mass content of isocyanate. This addition can be from about a 1-5% excess isocyanate equivalence. Experience is the best guide here.

In many applications using pMDI, it is found that the performance of a particular manufacturer's product will consistently vary from that of another. While this could be a reflection of systematic handling differences, it is often the case that the composition of the pMDI is sufficiently different that the resulting polyurethane performance is significantly affected. Compositional differences can reflect numerous manufacturing process variables as well the amount of 4,4'-MDI monomer that is distilled from the crude product. Thus, the practitioner should understand that when using pMDI, it may be necessary to analyze according to ASTM 7252 for composition to understand the results and perhaps develop a more useful patent strategy when involved in innovation and patent application filing.

Analysis of pMDI

Composition ASTM 7252 is an explicit method to determine the composition of pMDI using high-performance liquid chromatography (HPLC). The method depends on the use of acetanilide as an internal standard and employs calibration curves based on pure standard samples obtained from well-known chemical suppliers. The precision of this method based on round-robin testing is about 1%. Example chromatograms are found in the standard. The percentage of a given isomer based on the chromatographic data using an internal standard based on integrated peak areas is given by Equation 5.5:

where Afmple is the area of the I isomer peak in the chromatogram, A*S is the area of the internal standard in the standard chromatogram, A*1 is the area of the I isomer in the standard chromatogram, Ajsā„¢ple is the area of the internal standard peak in the sample chromatogram, Wi"i is the weight of the I isomer in the standard solution, and W .is the weight of the unknown sample.

Many laboratories use gas chromatography (GC) for this analysis when HPLC is not convenient, employing a diphenylmethane internal standard, but this is not suggested in the ASTM method. GC analysis is considered to be reliable to a precision less than 1% standard deviation for these measurements [11]. Size-exclusion chromatography has also been demonstrated and found to be reasonably reliable with a precision of about 1% becoming slightly less reliable for quantitative determination of the higher-molecular-weight oligomers (pentamers and higher) [12].

 
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