Table V. Determination of Solvent Composition in Dowex AG 50W-H+ Form Resin Phase after Equilibrationwith 1.94M HCI in Different Methanol-Water Mixtures Per cent solvent content fwiw) inside the resin Total solvent by drying HzOby K.F. MeOH by External difference titration solvent composition at 110 “C 36.68 11.19 25% MeOH 47.87 31.20 17.64 50% MeOH 48,84 29.69 47.04 17.35 80% MeOH
the Karl Fischer reagent is capable of reacting quantitatively with the water in these types of resins. The PMR method cannot be used for these resins as the spectra consist of one (unsymmetrical) peak only. An interesting application of the Karl Fischer titration method is in estimating the amounts of water and a n organic solvent sorbed by the resin when equilibrated with mixed solvents. Previous methods employed for such determinations (14-16) were time-consuming and involved either distillation or elution of the sorbed liquid and analysis of the composition of the mixture by measuring some physical property like refractive index, dielectric constant, etc. The solvent composition inside the resin can be evaluated by determining the total solvent content by the drying method and (14) 0. D. Bonner and Jane C. Moorefield, J. Phys. Chem., 58, 555 (1954). (15) H. P. Gregor, D. Nobel, and M. H. Gottlieb, ibid., 59, 10 (1955). (16) D. Reichenberg and W. F. Wall, J. Chem. SOC.,1956, 3364.
water alone by Karl Fischer titration. The per cent (w/w) of CHIOH and water sorbed by the Hfform of Dowex A G 50W resin when equilibrated with methanol-water mixtures were determined by us in connection with ion-exchange kinetic studies ( 2 ) and reproduced in Table V. Alternatively, uncertainties in weight loss during drying can be eliminated, especially for anion exchange resin systems by estimating the water fraction by Karl Fischer titration and the total composition of the mixture by one of the physical methods mentioned above. The titration method can also be expected to yield more accurate results when large amounts of volatile solute, e.g.,HCI, are contained within the resin bead along with water.
CONCLUSION It appears that the Karl Fischer titration method could be satisfactorily used Z;I estimate the moisture content in a majority of cation and anion exchange resins. It is capable of yielding reproducible and accurate values, provided the water content in the sample is -lox (w/w) or more. Since the titrations could be carried out to completion in a matter of minutes, it is particularly suitable while dealing with a large number of resin samples. The method does not suffer from the limitations encountered in the drying and PMR techniques.
ACKNOWLEDGMENT We are grateful to Dr. R. C. Vasishth, Research Director, Reichhold Chemicals (Canada) Ltd., Weston, Ont., for making available to us the use of Varian A60 NMR spectrometer. RECEIVED for review April 27, 1970. Accepted July 9, 1970. Work supported by the National Research Council of Canada and by a Postgraduate Scholarship awarded to N. S.
Quantitative Analysis of a Moisture-Cure Polyurethane Coating by Gel Permeation Chromatography N. D. Kornbau and D. C. Ziegler Research and DeveIopment Center, Armstrong Cork Company, Lancaster, Pa. I7604
UNTILTHE ADVENT of gel permeation chromatography (GPC), there was no single analytical tool that could separate both the low and the high molecular weight components of a complex mixture such as a wet (or solvent) coating. However, as illustrated by Bartosiewicz ( I ) , separations of this type can now be accomplished by the use of GPC, and the fractions provided by GPC can be analyzed by other means, such as infrared spectrophotometry. The identification of GPC fractions with infrared has been described in det-‘l by Quinn et al. (2). In this communication, the use of GPC as the primary means of investigating the constituents of a commercial moisture-cure polyurethane coating is reported. GPC is particularly applicable t o this work because of its
ability to separate each of the main constituents. Sample treatment during analysis is relatively mild since there is no heating or undesirable chemical treatment of the coating. Commercial polyurethane coatings are usually composed of three major ingredients : a solvent system, free diisocyanate, and prepolymer produced by the reaction of polyols with excess diisocyanate to form the urethane. After the coating has been applied, the solvent evaporates leaving a thin film of polyurethane. The free diisocyanate cross-links the prepolymer as the isocyanate groups react with moisture from the air. Typical reactions for the preparation and cure of a polyurethane coating (3) are: The reaction occurring between diisocyanate and the hy-
(1) R. L. Bartosiewicz,J. Paint Technol., 39,504-528 (1967). (2) E. J. Quinn, H. W. Osterhoudt, J. s. Heckles, and D. C. Ziegler, ANAL.CHEM., 40,547 (1968).
(3) J. H. Saunders and K. C. Frisch, “Polyurethanes: Chemistry and Technology, I. Chemistry (High Polymers XVI),” Interscience, New York, 1962.
1290
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
n
droxyl groups of the polyols which form the urethane prepolymer. 0
/I
(1)
+ (Hydroxyl) HO-R’ + R-NH-C-0-R’ (Isocyanate) (Urethane) R-NCO
The moisture-cure of the prepolymer forms an amine
+ HO-H (Isocyanate) (Water)
R”-NCO
(Amine)
1
30
+ (Gas) C02
R”NH2
(2)
which in turn reacts with an isocyanate group to yield a urea linkage, OlYMER(
0
+ H2N-R” (Isocyanate) (Amine) R-NCO
I1
+ R-NH-C-NH-R” (Urea)
joining molecules of the prepolymer. type reactions
(Isocyanate)
(3)
-
/I + R-”4-NH-R” (Urea)
A
(Catalyst)
0
Ii
R-N-C-N-R
”
I I (Biuret) c=o c=o
I I
(4)
I
NH
NH
R
R
I
H O R-NCO
(Isocyanate)
I /I + R-N-C--O-R’ (Urethane)
-
Figure 1. Polyurethane coating fractionated on low permeability limit columns
Biuret and allophanate
0 2 R-NCO
E l U l l O N VOLUME
A
(Catalyst)
0
11
R-N-C-0-R’
I I NH I
(A1lophanate)
C==O (5)
R
provide mechanisms for further cross-linking, upon the addition of heat or catalyst during the curing process. EXPERIMENTAL Apparatus and Conditions. Because of the reactive nature of isocyanate groups with moisture, the coating was handled in a dry box under a positive pressure of preparative grade dry nitrogen. The nitrogen in the dry box was recirculated through columns of anhydrous calcium sulfate to ensure a moisture-free atmosphere. This minimized the possibility of a change in the material due to reaction with moisture during the handling of the wet coating prior to gel permeation chromatography. The GPC elutions were made with a Waters Associates, Framingham, Mass., Model 100 gel permeation chromatograph. Two combinations of columns were employed for the sample fractionations. In the combination used to fractionate the prepolymer or higher molecular weight portion of the coating, four columns were used. The perqeability limit of each of three of these columns was 150,000 A, and the permeability limit of the remaining column
was 100,000 A. This combination of columns had 17,600 theoretical plates. In the combination used to separate the lower molecular weight components of the coating, five columns were used. The perFeability limits of these columns were one, 3,000 A; one, 800 A ; one, 400 A; and two, 100 A. This combination of columns had 19,300 theoretical plates. The permeability limits of all columns were supplied by Waters Associates, Inc. For each set of columns the number of theoretical plates was calculated from the injection of a benzene sample at a concentration of 1Z, for 15 seconds, as described by Determann ( 4 ) . The eluent was tetrahydrofuran. Elutions of 2 polyurethane solutions injected for 90 seconds were performed at ambient temperature and a flow rate of 1 ml per minute. A differential refractometer (Waters Associates) was used as the detector. Identification of fractions obtained from this type analysis was made on a Perkin-Elmer, Model 137B, Infracord Spectrophotometer and with a Cary Model 14 Spectrophotometer. Materials. A commercially available polyurethane coating was used as the basis for the development of the techniques described in this article. Materials used for identification of the various peaks investigated by GPC were methylene bis(4-cyclohexylene isocyanate) [Hylene W, commonly referred to as HMDI], supplied by E. I. du Pont de Nemours, reagent grade xylene, obtained from Fisher and 2,6-di-tertbutyl-4-methylphenol [Ionol] obtained from Aldrich Chemical Co. Procedure. Figure 1 shows a GPC chromatogram of the polyurethane coating which was fractionated on the low permeability limit columns. The peaks were identified as follows: SOLVENT.Identification of the solvent present in the coating was made by vacuum distillation of the wet coating and infrared examination of the distillate with a PerkinElmer, Model 1378, Infracord Spectrophotometer. After the identification of the solvent, quantitative determination of the solvent was made by eluting a known amount of this solvent and measuring the area under the peak so produced. From this value, a relationship was obtained between peak area and amount of solvent which in turn was used to calculate the amount of solvent in the coating. This value was then confirmed by gravimetric analysis. METHYLENE BIS (4-CYCLOHEXYLENE ISOCYANATE) AND 2,6D1-tert-BUTYL-4-METHYLPHENOL. The material causing Peak B in Figure 1 was identified after collecting the fraction of the material isolated via GPC and analyzing it by infrared spectrophotometry. The fraction, as collected, was mixed with spectrographic-grade KBr and the solvent (THF) was then removed by evaporation. A pellet was then formed from the remaining mixture and an infrared spectrogram was obtained. ~~
(4) H. Determann, “Gel Chromatography,” Springer-Verlag, New York, 1968, pp 69-70.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
1291
I .
.........:
:
3s
llUll0N VOlUYt
COHPOSIII P E A K (PRtPOLVHEl]
-
Figure 2. Polyurethane coating after reaction of isocyanate with ethanol
IlUllOW V U L U Y I
c
-
-
Figure 4. Polyurethane coating fractionated on high permeability limit columns PREPOLYMER. After the unreacted isocyanate in the coating was capped with ethanol, the prepolymer was eluted by GPC on both the high and the low permeability limit sets of columns. Elutions were performed as before and changes before and after capping were noted. Although no suitable calibration of elution volume in terms of molecular weight was obtained for the polyurethane fractionated on the high permeability limit columns, a polystyrene calibration was employed to give an estimation of the breadth of the polymer distribution.
Figure 3. Chromatograms comparing the unreacted and reacted polyurethane coating A known sample of Hylene W was also fractionated on the GPC to further substantiate the identity of the unknown material. In order to quantitatively analyze the HMDI peak, a means of preventing reaction of the isocyanate with moisture prior to measurement was necessary. It was observed that the isocyanate in the coating was extremely reactive with moisture. This became apparent when the tetrahydrofuran solution of the coating was re-eluted after approximately 24 hours and a chromatogram different from the original one was obtained. Preventing such changes was accomplished by reacting the isocyanate groups with excess ethanol, a procedure hereafter referred to as “capping.” Figure 2 shows a GPC chromatogram of the capped polyurethane coating fractionated on the low permeability limit columns. When reacted with ethanol, HMDI formed a new, higher molecular weight compound, methylene bis(4-cyclohexylene ethyl carbamate), which appeared at a lower elution volume than HMDI because of its higher molecular weight. Peak BQin Figure 2 resulted from this new compound. After the capping was completed, the methylene bis(4-cyclohexylene ethyl carbamate) was quantitatively analyzed through the use of GPC in a manner similar to that used for the solvent. After the isocyanate groups were reacted with ethanol, it was noted that a small peak of unreactive material still remained at the same elution volume at which the free HMDI peak had occurred. This material is represented by Peak B1 on Figure 2. A fraction of this material was collected and analyzed by infrared spectrophotometry. Once Peak B, on Figure 2 had been identified as Ionol, it was quantitatively analyzed through the use of GPC in a manner similar to that used for the solvent. 1292
RESULTS AND DISCUSSION Solvent. Infrared spectrograms of the distillate from the vacuum distillation of the wet coating and of reagent grade xylene were identical. Injection of the distillate into the GPC produced a peak elution volume identical to Peak A on Figure 1. The amount of solvent found by the GPC method was 59.0% and by the gravimetric analysis to be 58.7x. Methylene Bis (CCyclohexylene Isocyanate) and 2,6Ditert-Butyl-4Methylphenol. Infrared analysis of the material causing Peak B in Figure l yielded a strong absorbance at 4.38 microns, indicating the presence of isocyanate. A known sample of Hylene W, methylene bis (4-cyclohexylene isocyanate), was fractionated via GPC. This fraction appearing at the same elution volume and yielding an infrared spectrum identical to the fraction of Peak B in Figure 1, confirmed the presence of unreacted HMDI in the material causing Peak B. Via the area analysis obtained from the GPC chromatogram, it was determined that 4 , l X of the coating was HMDI. The infrared spectrogram of Peak B1in Figure 2 indicated the presence of 2,6-di-tert-butyl-4-methylphenol (Ionol). A known sample of Ionol was eluted on the GPC and found to produce a peak at the same elution volume as the material which caused Peak B1 in Figure 2. The sample of Ionol collected from the GPC fractionations was found to yield an infrared spectrum identical to that which caused Peak B1 in Figure 2. Quantitative analysis uia GPC found that 0 . 4 x of the coating was Ionol. This value was confirmed by ultraviolet measurements obtained on the Cary Spectrophotometer. Prepolymer. The solid line in Figure 3 represents a chromatogram of the uncapped coating on the low permeability limit columns. The dotted lines in the figure show the changes which result when the reactive isocyanate in the coating was capped with ethanol. The material causing composite Peak C in Figure 3 was confirmed to be isocyanate
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
terminated prepolymer when this material was found to react with ethanol during the capping operation. This reaction caused a shift in elution volume of the material, confirming the presence of reactive end groups as opposed to unreacted polyol. The chromatogram of the prepolymer, shown as composite Peak C in Figure 4, was obtained by fractionation on the high permeability limit columns. The long tail in the high molecular weight region indicates that the prepolymer had an extremely broad molecular weight distribution. Although no suitable calibration of elution volume in terms of absolute molecular weight was obtained for this polyurethane, ratios of i V w / J T n and iVz/iRwcould be estimated from a polystyrene calibration. By this means it was estimated nw/nn to be 15
ivn
and Rz/Rw to be 4.6 where is the number average molecular weight, ,Vw is the weight average molecular weight and Rzis the “z” average molecular weight.
ACKNOWLEDGEMENT The authors acknowledge Hans W. Osterhoudt for his valuable guidance and the work of the Analytical Department of Armstrong Cork Company for its supplementary analyses of materials. RECEIVED for review April 23,1970. Accepted June 26,1970. Presented at the 156th National Meeting, American Chemical Society, Atlantic City, N. J., September 1968.
Dielectrometric Titrations : Nonquantitative Reactions Robert Megargle,l George L. Jones, Jr., and Donald RosenthaP Department of Chemistry, Clarkson College of Technology, Potsdam, N. Y . 13676 THEMAJOR PRODUCT of the reaction of an acid HA with a base B in a low dielectric constant solvent is usually an ion pair BH+A-: B 4-HA BH+A(1) The reactions between picric acid (PiOH) and triethylamine or N,N-dimethylbenzylamine have been shown ( I ) to proceed quantitatively from left to right. The dielectrometric and spectrophotometric titration curves showed sharp breaks at the equivalence point. In this study, the interactions of PiOH and pyridine (Py) or 3-iodopyridine (3-IPy) were selected as examples of titration reactions which do not proceed quantitatively to the salt and which can be followed dielectrometrically and spectrophotometrically.
THEORETICAL TITRATION EQUATIONS If a measurable property P of a solution depends linearly upon the equilibrium concentration of each solute, Ci,then
When Equation 4 is solved for CSanL substituted into Equation 3,
{CAT
+ CBT+ Kf-’ - [(CAT+ CBT+ Kl-l)* 4CATCBT]li2]
(5)
The other root is extraneous because it requires CS to exceed C A T or CBT. If uA,U B , us, Kl, and Po are known, Equation 5 can be used to calculate a theoretical titration curve. For example, if acid is titrated with base, CATis the initial concentration of acid corrected for dilution and CBTis the number of moles of added titrant per liter of mixture. It was demonstrated ( I ) that the dielectric constant (e) of a dilute solution can be related to linear contributions from the various species by either of two equally applicable equations, both of which have the form of Equation 2 :
n
-€ =- -I
where Po is the constant contribution of the solvent, n is the number of solutes, and uf is the proportionality constant for the ith solute. In the usual case where acid, base, and salt coexist in solution, Equation 2 becomes
+
+
+
P = Po U A C A UBCB ~ s C s . (3) CA,CB,and Csrepresent the equilibrium concentrations of acid, base, and salt, respectively. If CATand CBTare the analytical concentrations of acid and base, the formation constant Kl of the salt is given by K f = - =c s cs CACB (CAT- CS)(CBT- csi
(4)
Present address, Department of Chemistry, University of Missouri, Columbia, Mo. 65201. To whom reprint requests should be addressed at Clarkson College of Technology. (1) R. Megargle, G. L. Jones, Jr., and D. Rosenthal, ANAL.CHEM., 41, 1214 (1969).
e -
e,-1
+ 7 pici 1
or (7) where e, is the dielectric constant of the solvent and pi and Y~ are the proportionality constants for the ith species. Equation 7 was used to derive dielectrometric titration equations for this study, since it is the simpler of the two. Hereafter, when dielectric constant data are being discussed, P = e, Po = e,, and ut = Y ~ .Spectrophotometric titration equations can be derived in the same manner, since Beer’s law also has the form of Equation 2 when P = A/b (absorbance/cell path length), P, = 0, and uf = ai (molar absorptivity). Measurement of P for solutions containing various concentrations of added acid or base can be used to determine uA, and uBfrom Equation 2. When reaction 1 is not quantitative, us cannot be determined directly because the salt is not the only species present in solution at equilibrium. Since the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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