Another standard polyol analysis is the CPR measurement providing an indication of the trace basicity left in the polyol following production. This is a particularly important variable for individuals involved in preparation of prepolymers (isocyanate-terminated polyols; see Section 2.1.6). The importance of CPR reflects the fact that the vast majority of polyols are produced using base catalysis. Although modern manufacturing practice has numerous means of nearly eliminating the free base left after production, there is normally a trace of basic activity left in the polyol sample being used for polyurethane or prepolymer preparation. This leftover base activity can have negative influences on prepolymer shelf stability due to the ability of base components to catalyze isocyanate-isocyanate reactions. The actual basic components present are not identified but are simply the total weak acid salts left in the sample. Since the amount of base active
Figure 5.2 Attenuated total reflectance FTIR spectroscopy of polyethylene glycol of molecular weight 200. The inset is a magnification of the hydrogen-bonded OH absorbance, which can be area integrated as part of a calibration curve.
Figure 5.3 Example calibration curve generated for FTIR determination of OH content of polyols.
elements left after manufacture and processing is usually small, a large sample of polyol must be employed for the test. The reported number is in units of milliequivalents of KOH/30 kg of sample or often 10 times that number. The typical CPR values are from about 0.1 to 1 [or 1-10]. The net CPR reflects that when mixed with an isocyanate, net acidity from the manufacture of isocyanate (in the form of HCl; see Chapter 2) can in part counterbalance the basicity of the polyol. Thus, the net CPR is given by Equation 5.3:
where is the active hydrolyzable chloride in the isocyanate. A high net isocyanate j j jo CPR will result in poor prepolymer shelf life and poor polyurethane formation due to uncontrolled catalysis. The CPR test itself is a titration to an indicated endpoint or readout of a potentiometric electrode. The calculation is based on Equation 5.4:
where V , is the volume of titrant (0.01 N HQ) to the last endpoint or potentio-sample v ' 1 1 metric brea^, Vbl , is the volume of titrant to the last break in blank titration, N is the normality of HCl, 30,000 is the conversion factor for 30 kg, and W is the weight of the sample. As indicated in Table 5.2, the precision of this test (±20%) is average at best based on round-robin industry testing and depending strongly on the skill of the practitioner. Thus, CPR measurement reliability should always be suspect if prepolymer shelf life stability is an issue.