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ANALYSIS OF REAGENTS FOR MAKING POLYURETHANES

Analysis of Polyols

There are several analyses that accompany virtually every batch of industrially produced polyol. These are indicated in Table 5.1 along with the relevant ASTM method developed to measure the property.

TABLE 5.1 Conventional polyol specifications and AsTM procedures for measurement

Property

ASTM method

Hydroxyl number

D4274 measurement by derivatization and titration

Hydroxyl number

D6342 measurement by near-infrared spectroscopy

Viscosity

D4878 measurement by Brookfield viscometry

Controlled polymerization

D6437 measures alkalinity remaining from glycol

rate (CPR)

polymerization that can affect subsequent isocyanate

polymerization

Unsaturation

D4671 titration method uses mercuric acetate in a

methanol solution

Cloud point

D2024 standard test method for nonionic surfactants

Water content

D4672—Karl Fischer colometric titration technique for

alcohols or E203 general Karl Fischer titration method

The application of these standard techniques will invariably result in a number whose precision can be well characterized by replicate samples. However, the ability to get optimal polymerization results will depend on the accuracy of the result. There is no a priori means of knowing if the analytical result a practitioner is handed is useful. The result of a poor analysis is a polyurethane with poor properties, and it will be ambiguous if the problem is a result of analysis or polymer design [1]. The practitioner should never simply accept the result of a single experiment and should always be suspicious of the polyol analysis, particularly the hydroxyl value. It may be necessary to bracket the polymerization, assuming the measurement of the property is wrong by as much as 10%. This may require a new polymerization attempt at the analyzed value and additional attempts at a ratio 10% greater and 10% less than the measured value. Improved polymer properties in either direction may require additional empirical iteration. Table 5.2 lists the polyol quality tests of Table 5.1 and a relative evaluation of the reliability of the test based on statistical analysis within labs and between labs (this information is usually provided within the standard). The discrepancy is often a result of systematic operator errors, differences in execution of the ASTM tests (i.e., catalysts, solvents, etc.), and the use of autotitration [1] versus manual titration techniques. The author has observed that unless an autotitrator is employed by a very experienced and methodical technician, the manual titration is more likely to provide a reproducible result.

Each of the tests in Table 5.1 can be potentially important; however, the hydroxyl number and the CPR value are the variables that can greatly affect the final result of the polyol use.

 
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