Determination of Methylol Groups in Phenolic Resins GEORSE A. STENMARK
and
F. T. WElSS
Shell Development Co., Emeryville, Calif,
An accurate and simple method for the determination of methylol groups in phenol formaldehyde resins is described. Methylol groups and phenol condense in the presence of an acid catalyst to form water as one of the products. The water in the reaction mixture is titrated directly w i t h Karl Fischer reagent and water produced in the reaction is calculated after correction for w-ater in the sample and reagents. In contrast with previously available techniques, this method provides an accurate analysis using small quantities of sample, and thus facilitates examination of small scale laboratory preparations.
The direct titration procedure was applied using several reaction temperatures and varying reaction times. Acidic catalysts tested were either toluenesulfonic acid or boron trifluoride. The latter catalyst was introduced as a 26% boron trifluoride-phenol complex, a form superior from the standpoint of safety and convenience t o that of the gas. Xylene was also substituted for benzene to reduce the vapor pressure of the reagent and thus diminish its toxicity. The procedure tested was as follows. Place 25 ml. of reagent in a dry 100-ml. volumetric flask. .4dd a quantity of sample containing a total of 5 to 10 millimoles of methylol plus water. Stopper the flask and heat the mixture in an oven a t a fixed temperature for a measured length of time. Cool the mixture to room temperature, add 10 ml. of anhydrous 4 to 1 glycol-pyridine mixture and titrate with Fischer reagent t o the visual end point. Run a blank in an identical manner and determine the xater content of the sample by titration xith Fischer reagent.
S E E D has existed for an accurate and simple method for determination of methylol groups in phenolic resins. Severa1 techniques have been described in the recent literature but have not been found generally satisfactory. Halogenation procedures, involving the use of iodine (11)or bromine(f9, 88), suffer from variable substitution reactions involving the aromatic rings. Lilley and Osmond (18) describe a method utilizing a reaction with hydrogen bromide and titration of the released water with Fixher reagent. The reaction is sloa-, requiring as long as 24 hours for completion. hIartin (13) developed a method for methylol determination hased on a condensation reaction between methylol groups and phenol to form water as one of the products. The method involved reaction of the sample nith 100 to 500 grams of phenol in benzene solution with p-toluenesulfonic acid as a catalyst. The water produced was continuously distilled from the mixture as an azeotrope with benzene, collected as a lower layer, and measured in a calibrated receiver. Good recoveries were obtained in the analysis of benzyl alcohol, methylol phenols, methglol ureas, and trimethylolnitromethane. The disadvantages inherent in the Martin method are the large quantities of reagent and sample required for accurate volume measurement of the water produced. I n the analysis of small scale laboratory preparations, it is particularly imperative that samples be small. It appeared that the use of Karl Fischer reagent for measurpnient of the evolved water, as suggested by Martin, would result in a fiftyfold reduction in sample size and a considerable reduction in quantities of reagents required. Although this investigation is concerned primarily with the quantitative determination of methylol as a functional group, it is of interest to note the work of Freeman, who reported the separation and identification of individual methylol phenols by paper chromatography (Y),and used the latter technique in a kinetic study of the phenol-formaldehyde reactions (8).
Data obtained under varying reaction conditions in the analysis of benzyl alcohol and several substituted benzyl alcohols are shown in Table I. Excellent accuracy was observed with the horon trifluoride reagent a t 60' C., and variation of the reaction time between 1 and 3 hours had no effect upon the extent of reaction. Toluenesulfonic acid was the less active catalyst, as shown by a comparison of the values obtained with the two catalysts after 1 hour at 60' C. The extent of reaction, using toluenesulfonic acid catalyst, reached a maximum of approximately 9595, and increasing the temperature to 100' C. did not result i n increased reaction.
Table I.
Effect of Time, Temperature, and Catalyst on Reaction of 3Iethylol Groups with Phenol
(Reagents. A . 70 wt. % phenol. 28 wt. % benzene, 2 w t . % p-tolueneaulionic acid. E . 70 w t . % phenol, 29 wt. % xylene, 1 n t . "0 boron trifluoride) Reaction Conditions Tzmp., Time, React ion Coinpound Reagent C. hours % 25 1 Benzyl alcohol 25 60 60
2,6-bis(Hrdroxymeth,.I)4-niethyl phenol
o-Hydroxybenzyl alcohol p-Hydroxybenzyl alcohol Benshpdrol
67 67 100 60 60 60 60 60 60 60
GO
1 3
2 3
,
1 2 3 3
3 3 7
68 97.5, 94 7, 96 1, 96 9, 95 8, 95 2 9A 9
94.8 95.9, 94 9 99.6 99.6 99.0 96.2, 94.5, 93.7, 94.5, 94.8 99.6. 100.5 99.2, 99.4 100.2, 99.G 100 0, 99 fi
The method using toluenesulfonic acid catalyst was applied to known amounts of water t o test the possibility that low recoveries were due to physical loss of water. Low recoveries of the added nater, 94 to 97%, were obtained under all conditions, including a test in which the heating period was omitted. '4s the values shoa ed no consistent trend with severity of reaction conditions, the possibility of physical loss of water was excluded. The reason for the low recovery of water, corresponding to a loss of l t o 5 mg., is not known. It did not appear profitable t o pursue this question further, inasmuch as excellent recoveries were obtained with boron trifluoride. The effect of phenol concentration in the boron trifluoride reagent was studied with a view toward the possibility that a less corrosive reagent might be used. The results of these tests,
PRELIMINARY STUD1 ES
Initial tests were made using the reagent specified by Martin, consisting of 100 grams of phenol and 3 grams of p-toluenesulfonic acid dissolved in 50 ml. of benzene. Water produced in the reaction was measured bv azeotropic distillation with xylene and titration of the distillate with Fischer reagent or by direct titration of the reaction niixhre with Fischer reagent. The results obtained for benzyl alcohol by the distillation technique showed considerable variation in the recovery of water. The method was cumbersome and required special distillation equipment and considerable operator's time. As subsequent tests showed the more convenient direct titration t o be preferable, the distillation modification was abandoned.
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V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6 shown in Table 11, indicate that no reduction in accuracy was caused by reduction of the phenol concentration in the reagent from 74 to 17%. However, subsequent application of the method to phenolic resins showed that the higher phenol concentrations are necessary for solution of some samples.
Buret assembly. An all-glass buret system designed to exclude atmospheric moisture ( 1 7 ) . Safety oven, explosion-proof, capable of maintaining a temperature of 60" f 1" C. Ethylene glycol-pyridine mixture. Mix 1 volume of C.P. anhydrous p! ridine with 4 volumes of C.P. anhydrous ethylene glycol. Fischer reagent, prepared as described by Peters and Jungnickel ( 1 : ) .
Effect of Phenol Concentration on Extent of Reaction of Benzyl .41cohol
(CatalJ-st. Boron trifluoride, l , O q b . Keaction. Phenol Concentration Solvent in Reagent \%-t "; Benzene 74" Xylene 70 54 29 17 10
manner but omit the sample. Determine the water content of the original sample by titration with Fischer reagent ( 4 ) and the carbonyl content by the Fischer carbonyl method ( 1 5 ) . CA LCU L A T I 0 3
Calculate the methylol content of the sample by means of the folloning equation:
APPARATUS AND CHEMICALS
Table 11.
261
3 hours at 60" C.) Reaction of Benzyl Alcohol. OC 100.5 99 0 100.8
101.3 99.7 99.2, 96.9 ' Gaseous boron trifluoride used in preparation of reagent. Reagent in all other determinations prepared n.itti boron trifluoride-phenol romplex.
Phenol-boron trifluoride reagent. Add 100 ml. of C.P. xylene and 10 nil. of 26% boron trifluoride-phenol caomples to 200 grams of C.P. phenol in a 500-ml. glass-stoppered flask. The flask may be warmed on a hot plate to effect solution. Benzyl alcohol, Eastman Kodak Co. Assay by acetyl chloride method (eo), 99.4%. 2,6-Bis(hydroxymethyl)-4-methylphenol,laboratory preparation. Assay by acetic anhydride method ( 8 3 ) ) 97.4%. Melting point, 128" C.; melting point reported, 130" C. (24). Benzyl ether, Eastman Kodak Co. Hydroxyl content., by lithium aluminum hydride, using method similar to that of Hochstein (Y), less than 0.001 equivalent per 100 grams. Benzyl ethyl ether. Hydroxyl content, by acet'yl chloride method, 0.055 equivalent per 100 grams. Benzhydrol, Eastman Kodak Co. A S S : Lby ~ lithium aluminum hydride ( g ) , 100.5%. o-Hydroxybenzyl alcohol, Eastman Kodak Co., recrystallized from water and benzene. Assay~" hv lithium aluminum hvdride
Alethylo1 content, equivalent per 100 grams
=
where A = volume of Fischer reagent required for niethylol determination on sample, milliliters B = volume of Fischer reagent required for reagent blunli i u niethylol determinat.ion, milliliters F = standardization factor for Fischer reagent i n niilligrarn~ of water equivalent, to 1 ml. of reagent W = xeight of sample taken for methylol deterniinstiou. grams C = carbonyl content, equivalent per 100 grams D = m-ater content, weight yo. RESULTS AND DISCUSSION
Side Reactions. In view of the presence of considerable uiireacted fornialdehyde in the phenolic resins under consideration. the stoichionictry of the reaction of aldehydes in the method wa3 tested. The reactions of ketones, benzyl ethers, and isopropyl alcohol were also studied. The results of these tests are shown in Table 111. Formaldehyde, acetaldehyde, and benzaldehyde reacted to form 1 mole of water per mole of aldehyde, showing excellent agreement with theorj-. I n view of the observed stoichiometry, the methylol content of phenolic resins can be obtained by application of a correction for carbonyl content. The ketones tested, acetone and methyl isobutyl ketone, reacted to the estent of 7 . 3 and 0.2'%, respectively. The low reactivity of ketones is not a serious handicap, as ketones are not constituent3 of the phenolic resins under consideration. Isopropyl alcohol reacted to a negligible extent. Dibenzyl ethers reacted to produce almost 1 mole of water per mole, possiblv via a two-step reaction as follom: Oft
(91, 98.470.
Bis(2-hydroxybenzyl)etherj prepared by heating o-hydrosybenzyl al(8ohol a t 100" C. for 8 hours (13). Melting point, 119120" C . ; reported, 120-121" C. benzyl alcohol, prepared by hydrogenation of EastCo. p-hydroxyhenzaldehyde. Recrystallized from water. Assay by lithium aluminum hydride (,9), 99.5%. Melting point, 108" C.; reported, 110" C. Bis(2-hydroxypheny1)methanejobtained as one product from rtxaction of phenol and o-hydroxybenzyl alcohol. Infrared spectrum is consistent with that shown by Richards and Thompson ( 1 8 ) for the compound. hIelting point, 113-114" C.; reported, 1 1 9 O c. ( 2 ) . Bis(2-hydroxy-3,5-dimethylphenyl)mcthane,prepared by condensation of formaldehyde with 2,4-sylenol. Assay by acetic anhpdride (23), 100.370. Melting point', 148' C. ; report,ed, 14
