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Hydroxyl Number

Measurement of hydroxyl number is usually obtained by the derivatization and titration technique. The general procedure is to carry out an esterification reaction of the alcohol with an anhydride, usually phthalic or acetic (Fig. 5.1) [2]. The need to react all of the alcohol groups necessitates the use of an excess of anhydride, and a strong base catalyst (i.e., standard 0.5 N NaOHaq or imidazole). The reaction is often run in pyridine solvent and carried out at elevated temperatures (ca. 100 °C) for up to 2 h. A blank (i.e., no polyol) is run side by side, and the amount of acid is subsequently titrated using an indicator such as

TABLE 5.2 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



Hydroxyl number derivatization method

Below average

Hydroxyl number near-IR spectroscopy method

Above average


Above average

Controlled polymerization rate) (CPR)




Cloud point


Water content

Below average

Anhydride derivatization of polyols prior to titration of acid end groups for determination of OH number.

Figure 5.1 Anhydride derivatization of polyols prior to titration of acid end groups for determination of OH number.

phenolphthalein for colorimetric endpoint determination. The procedure is sufficiently exacting that operator experience and exactitude are a must to obtain usable results.

The hydroxyl number in units of milligrams KOH/gram of sample is calculated from Equation 5.1:

where A is the milliliters of NaOH required for titration of the blank, B is the milliliters of NaOH required for titration of the sample, N is the normality of the NaOH solution, 56.1 is the equivalent weight (grams/equivalent) of KOH, and W is the weight of the sample used. The weight percentage of OH in the sample is given by Equation 5.2 where 17 is the equivalent weight of the OH functionality [3]:

The application of an inaccurate hydroxyl number will result in materials with low polymer molecular weight and poor network formation resulting at the minimum in degraded break elongations and, at worst, a viscous mess.

When performed correctly, the precision of the measurement at the 95% confidence level is about 1% and is of similar reliability. It can be used for almost all polyols (polyether, polyester, polycarbonate, and amine initiated). Many common additives such as most fire retardants, antioxidants, and catalysts do not interfere with the precision or accuracy of the measurement. Excessive water (capable of hydrolyzing the anhydride and biasing the OH number to lower values) and primary or secondary amines, also able to react with anhydrides, do have the potential to interfere with the titrimetric evaluation of polyols. Lastly, if the sample contains intrinsic acidity or alkalinity prior to derivatization, the results must be corrected. The ASTM standard provides an explicit method for the correction of a sample having intrinsic acidity (biasing the result toward lower hydroxyl numbers) or alkalinity (biasing the result toward higher hydroxyl numbers) that can interfere with the titration of derivatized acid end groups.

An alternative means of measuring the percentage hydroxyl in a sample is by infrared (IR) spectroscopy. ASTM specifies the method and use of commercial near-IR instruments for the measurement. This technique is dependent on calibration curves of characteristic OH absorbance in the 5000-9000 cm-1 region. Absorption in this region is relatively weak, and there are relatively fewer resonances [4, 5]. While mid-IR spectra can also be used [6, 7], the weak absorption of the near IR is an advantage in this case since it allows thicker samples for transmission. As a secondary method, its reliability depends on the calibration curve. However, if a reliable calibration curve can be obtained and the instrumental application can be standardized, near-IR techniques can result in more reliable determination of hydroxyl values than titration techniques since human systematic error is minimized [8]. The greater potential reliability of the near-IR technique has been documented in industrial round-robin testing [9].

Modern IR techniques, especially attenuated total reflectance (ATR) spectros-copy, open up the potential for very rapid and convenient measurement of hydroxyl number and change of hydroxyl number during a reaction. The ATR technique eliminates problems associated with high absorbance causing instrumental response to no longer function in the detector's linear range. The experiment requires that the sample be dry and not have competing resonances due to N-H bonds. When these conditions are fulfilled, the O-H stretch resonance (between 3100 and 3600) can be isolated with baseline resolution (Fig. 5.2) from other absorption peaks, integrated (usually with instrument supplied software) and compared to an appropriate calibration curve constructed from structurally related polyols (Fig. 5.3).

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