Infrared Determination of Free Phenol in Phenol-Formaldehyde Resins

1.

JOSEPH J. SMITH, FRANK M. RUGG, AND HARRY M. BOWMAN Research Department, Bakelite Co., Division of Union Carbide and Carbon Corp., Bloomjield, N . J .

The most commonly employed methods for determining the free phenol content of phenol-formaldehyde resins involve separation of the phenol from the resin by distillation or extraction, operations that are time-consuming and often not quantitative. A rapid infrared method is based on the 14.4-micron phenol absorption in the spectrum of an acetone solution of the resin sample. The unreacted phenol in both heat-stable (novolac) and heat-reactive (resol) resins can be determined to within *0.3% of the total sample. This technique can be used to advantage in studying polycondensations of phenol and formaldehyde and the effects of free phenol on properties of phenolic resins.

EPENDIKG on the ratio of formaldehyde to phenol D charged and the reaction conditions employed, phenol- formaldehyde resins may be either heat-stable (novolac) or heat- reactive (resol). Both types of resins generally contain significant concentrations-2 to 15%-of unreacted phenol. As the un- reacted phenol content has a significant effect on some of the important properties of the resin, an accurate and rapid method of analysis is desirable.

The unreacted phenol content is customarily determined by separating the phenol from the resin and measuring the amount of phenol separated by a chemical method (3, 6, 7). Separation of the phenol is commonly effected by distillation or solvent extraction. When distillation is used, low phenol values are often obtained on heat-reactive resins as a result of further condensa- tion a t the elevated temperature employed. When solvent ex- traction is used, some of the loner molecular weight components of the resin are also separated with the phenol. Since these com- ponents generally react with the reagent-e.g., bromine-em- ployed to measure the amount of phenol separated, high values

/n SOLVENT - ACETONE

‘PHENOL

PHENOL-FREE NOVOLAC

NOMXAC CONTAINING 5.8% PHENOL

I 14.38

SOLVENT - ACETONE

PHENOL

PHENOL-FREE NOVOLAC

NOMXAC CONTAINING 8 5.8% PHENOL

I 14.38

u 14.5 .&.O 13.5

WMLENGTH - MICRONS

Figure 1. Acetone Solutions us. Air Cell path 0.24 mm.

W 0 z 2 H c z 4 c a

A

are obtained. Even when acceptable accuracy is achieved by these methods, results cannot usually be made available in less than 2.5 hours after a sample is submitted for analysis.

Because of these deficiencies, a rapid epectrophotometric method was sought, whicli could be applied directly to a solution of the resin. It is generally accepted that the formaldehyde reacts a t open ring positions on the phenol nuclei in the formation of heat-stable and heat-reactive resins (1). On this basis the unreacted phenol would be the only monosubstituted benzene structure in the resin. As a result, attention was focused on the infrared spectra of resins in the 14.0- to 14.5-micron region, where monosubstituted benzenes give a characteristic absorption.

EXPERIMENTAL

The spectra were obtained with a large-prism, double-beam, infrared spectrophotometer ( 6 ) , and the wave lengths assigned are considered to be accurate to ztO.01 micron. In order to avoid overlapping, the spectra in Figures 1, 2, and 3 were shifted along the transmittance axis.

The solvent was acetone (c.P. Baker’s analyzed grade); the

\\PHENOL-FREE RESOL

‘ RESOL CONTAINING 4.7% PHENOL I

u 0 14.5 14.0 13.5

WAVELENGTH - MICRONS

Figure 2. Acetone Solutions us. Air Cell path 0.24 mm.

497

resins were measured a t a concen- tration of 16.0 grams per 100 ml. of solution; and the sample cell thick- ness was 0.24 mm. Because of the large amount of energy absorbed or scattered by the resin solutions in this region of the spectrum, the slit widths employed were approxi- mately twice that normally used.

The purity of all reference com- pounds used was better than 99%. Heabreactive samples can be made phenol-free by various solvent ex- traction procedures, and heat-stable samples can be made phenol-free by either steam distillation or vac- uum distillation.

BASIS FOR THE METHOD

The monosubstituted benzene ring absorption maximum for phenol in acetone occurs at 14.38 microns. The reliability of using this ab- sorption for determining the unre- acted phenol in resins is shown in Figures 1 and 2, Absorption maxima are observed a t 14.38 microns in the spectra of representative heat-stable and heat-reactive samples. Of par- ticular significance is the absence of this absorption in the spectra ob- tained when the unreacted phenol * is removed from these same resins.

498 A N A L Y T I C A L C H E M I S T R Y

Because the use of solvent extraction to obtain a phenol-free resol may also remove such low molecular weight components as ealigenin m d the 2,2’-, 2,4’-, and 4,4’-dihydroxydiphenylmethanes the spectra of these compounds were also obtained. The spectra showed that no interference results from the presence of these compounds in the resins.

QUANTITATIVE ASPECTS

Analyses can be performed satisfactorily by use of either single- beam or double-beam pectrophotometers. Figure 3 illustrates typical single-beam and double-beam performance. A constant slit width was employed in all cases. The single-beam spectra, A , display a very sharp decrease in energy with increasing wave length. This results from decreasing energy from the source, increasing absorption by atmospheric carbon dioxide, absorption by both the resin and solvent, and absorption by the sodium chloride prism. This last effect is pronounced in the instrument employed, as the prism is large, having a 16-cm. base.

When the spectrum of a resin solution is obtained by a double- beam spectrophotometer, B , the contributions to the sloping background made by the change in energy from the Eource, absorption by atmospheric carbon dioxide, and absorption by the prism have been automatically compensated. On such a doublebeam spectrum reproducible positioning of the incident radiation line is much simpler. The shape of the spectrum can be further simplified by employing a compensating cell con- taining an acetone solution of a phenol-free resin in the reference beam of the instrument, C . In this way the contributions of the solvent and the resin are eliminated, thereby giving a spectrum of only the unreacted phenol present in the sample resin. little as 0.2% unreacted phenol in a resin can be determined in this manner.

pJ H.3Bw

A. SINGLE BEAN 8. DOUBLE BEAM . C. DOUBLE BEAM SAMPLE M. AIR SAMPLE YS PHENOL-

FREE RESIN

-WAVELENGTH

Figure 3. Comparison of Single-Beam and Double-Beam Spectra

By use of this latter double-beam technique, spectra were obtained for a series of known samples prepared by adding phenol to phenol-free heat-stable and heat-reactive resins. The results were employed to construct the calibration curve shown in Figure 4. The same calibration can be used in analyzing either heat-stable or heat-reactive samples. The “fit” of the points on this curve shows that the analytical results should be accurate to f0.3% of the total sample. Furthermore, by use of the infrared method, results can be made available 0.5 hour after B

sample is submitted for analysis.

APPLICATIONS OF METHOD

This method is being used to measure the unreacted phenol content of resins prepared from pure phenol and from phenols containing as much as 20% cresols. In these latter cases, ap-

0 RESOL RESIN

13 NOVOLAC RESIN

PPTH LENGTH OF SAMPLE CELL - 0.24 M U

0 2 4 6 B lo 1 2 1 4 1 6 L B PERCENT FREE PHENOL m PHENOLIC RESIN

Figure 4. Calibration Curie 16.0 grams of resin per 100 ml. of acetone solution i n sample cell e n d

acetone solution o f phenol-free resin i n reference cell

proximately 90% of the eresol piebent is composed of the ortho and para isomers which do not absorb a t the analytical wave length Although m-cresol displays an absorption maximum a t 1441 microns, it is less than half as active an absorber as phenol a t 14.38 microns. This observation, plus the fact that no more than 0 5% free m-cresol is normally found in auch resins, means that the error introduced by m-cresol will be much less than the exTerimenta1 error of 0.3% previously specified

hlthough this method was developed, and is primarily used, for measuring the unreacted phenol content of the resin before it is compounded for commercial use, analysis of other types of phenolic resin samples is indicated. Free phenol contents of molding powders and of molded pieces are customarily deter- mined by applying a chemical analysis to the aqueous extract of the h e l y divided sample ( 2 , 4). The infrared spectrum of that portion of the sample extracted by an appropriate organic solvent may give a more reliable value, provided pigments and other special additives do not present interfering absorptions.

This infrared method should also be helpful in studying the phenol-formaldehyde condensation reactions. As an illustration, the change in phenol content during the condensation of phenol with phenol alcohols can be followed. In some of these cases, the aater content of the samples may have to be reduced to prevent etching of rock salt cell plates and to minimize the infrared absorption by liquid a ater

ACKNOWLEDGMENT

The assistance of H. L. Bender and his coworkers, who pre- pared pure reference compounds, J. E. Potts, Jr., who prepared phenol-free resins, and Miss lf E Martin is gratefully ac- knoi? ledged.

LITERATURE CITED

(1) Caisnell, T. S., ”Phenoplasts, Their Structure, PropeI ties and Chemical Technology,” Chap 2, New York, Interscience Pub- lishers 1947

~ , ~

(2) Herman, A., ASTM Bull., K O . 111,33--1 (1941); ModernPlastics,

(3) Petrov, G., and Shmidt, Ya., Org. Chem. Ind. (U.S.S.R.), 2,102-4 18, 65, 82, 84 (August 1941).

(1936). (4) Redfarn, C. A., Brit. Plastics, 13, 139 (1949). (5) Robitschek, P., and Lewin, A, “Phenolic Resins,” p. 209, London,

Iliffe &- Sons, Ltd., 1950. (6) Rugg, F. M., Calvert, W. L., and Smith, J. J., J . Optical Soc.

Am., 41, 32-8 (1951). (7) Yimonds, H. R., Weith, A. ,J., and Bigelow, M. H., “Handbook

of Plastics,” 2nd ed., p. 1097, Ne-- York, D. Van Xostrand Co., 1949.

RECEIVED for review May 14, 1951. dccepted October 8, 1951. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Speo- troscopy. hZarch 7, 1951