200
ANALYTICAL CHEMISTRY
purposes. Since the concentration limits are given for the iron that is in the 20 ml. of U&mwn iron sample used in the procedure, the lower limit could be extended by increasing the concentration of 5-sulfoanthranilic acid in the reagent solution and using a volume of unknown iron solution that is larger than 20 nil. By this means it is reasonable to expect that a concentration range limit of 2 to 60 p.p.m. could be approached. I n addition, the lower limit probably could be extended significantly by the use of longer cells. This is especially promising, as the absorption of the reagent itself is so small.
LITERATURE CITED
(1) (2)
Funk, H., and D e m m e l , Lf.,z. anal. Chem. 9 6 , 3 8 5 (1934). Funk, H . , and Ditt, M., I b i d . , 91, 332 (1933).
3) Ibd.9
93, 241 (1933). (4) Funk, H., and R o m e r , F., Ibid., 101,85 (1935). (5) Harris, W. F., and S w e e t , T. R., J. Am. Chem. SOC.77, 2893 (1955). .
I
RECEIVED for review October 5 , 1955. Accepted November 14, 1956. Taken in part from a thesis presented to the Graduate School of The Ohio State University hy James M. Zehner in partial fulfillment of the requirements for the degree of master of science, August 26, 1955.
Infrared Analysis of Paint Vehicles Based on Alkyd-Nitrogen Resin Blends C.
D. MILLER
and
0.D. SHREVE
Research Laboratory, Fabrics and Finishes Department,
No satisfactory chemical method is available for estimating the urea formaldehydemelamine resin ratio in a paint vehicle comprising both of these in admixture with an alkyd resin. A method based on infrared absorbance measurements at 5.8, 6.1, and 12.25 microns on thin vehicle films has provided a solution of this problem and affords a rapid means for estimating each individual resin component in typical two- and three-component alkyd-nitrogen resin blends. Results obtained on synthetic mixtures of known composition indicate a degree of accuracy and precision sufficient for many practical applications.
C
OATING resin combinations comprising oil-modified alkyd resins blended with “nitrogen” resins are widely used in the formulation of industrial baking enamels. Classes of nitrogen resins available for such use include butylated urea-formaldehyde (UF) and butylated melamine-formaldehyde (MF). A solution of one or both of these resins in butyl alcohol or butyl alcoholhydrocarbon solvent is blended with a hydrocarbon solution of the alkyd and appropriate pigments are incorporated. The resulting enamel, when applied as a thin film and baked, undergoes further condensation and cross linking to produce a cured insoluble finish. The complete analytical characterization of a “wet” sample of such an enamel involves estimation of total pigment, total nonvolatile resin content, total solvent (by difference), and component analysis of each of these three fractions. The present discussion, however, is confined to the problem of quantitative estimation of the individual resinous film-forming components in the vehicle fraction. The first step in such an analysis involves separation of a sample of the resinous vehicle solution from the dispersed pigment phase by high speed centrifuging. I n the case of a two-component resin system (urea-formaldehyde-alkyd or melamine-formaldehyde-alkyd), the nitrogen resin content can be estimated from total nitrogen as determined by ASTM designation D 1013 ( 1 ) if the nitrogen content of the nitrogen resin present is known or can be assumed. The total alkyd content can then be estimated by difference. Alternatively, if the phthalate content of the alkyd present is known or can be assumed, the total phthalate content of the blend can usually be estimated by an ultraviolet spectrophotometric modification ( 5 ) of the well-known Kappelmeier procedure (8-4) and total alkyd calculated from this value. In the case of the three-component system (urea-formaldehyde-melamine-formaldehyde-alkyd), total alkyd may again be
E. 1.
du f o n t de Nemourr
& Co.,
Philadelphia, Pa.
estimated from total phthalate as determined by the spectrophotometric modification of the Kappelmeier method. Total nitrogen resin content may then be estimated by difference or approximated from nitrogen content. Estimation of the relative amounts of the two nitrogen resins in such a system, however, poses a special problem. Total nitrogen does not afford a satisfactory basis for such an estimate, as the individual resins do not differ sufficiently in nitrogen content. The authors are aware of no previously published method suitable for this purpose. Infrared spectrophotometry provides a solution to this problem and makes possible the rapid quantitative estimation of each resin component in typical t KO- and three-component blends. The method is based on infrared absorbance measurements a t selected wave lengths where the various resins exhibit unique absorption bands. Because butyl alcohol and hydrocarbon solvents interfere and removal of solvent followed by re-solution in
-
40-
20
-
8100
I
2 Ez
,017
f
60-
E c 5
P
-
ALKYD RESIN
’
I
I
I
I
I
I
I
40-
UREA-FORMALDEHYDE
20-
” 01
’
4I
‘
Figure 1.
’
6
I
MELAMINE-FORMALDEHYDE RESIN
6I
I
Microns
’
IO
I
’
12
Infrared spectra of resins
14 I
-
201
V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6 Table 1.
Absorptivity Ratios for Binary Systems Ratio of Absorptivity Values 0.88 0.32 0.27 0 . 2 8 (calcd.)
System VF/alkyd MF/UF hIF/alkyd
Table 11.
Relative .4bsorptivity Values for Individual Components Absorptivity Value 3.65 3.13 1.00
Component Alkyd (I) U F (11) M F (111)
Table 111.
Analysis of Known Blends % Composition
Present
Alkyd 52.0
MF
UF
37.4
10.6
59.4
0
40.6
58.5
41.5
59.1
91.6
19.3
35.0
41.3
23.7
47.7
27.0
25.3
39.2
60 8
Table IV. Sample 1 2
0
0
Alkyd 50.4 51.2 59 8 60.4 57.2 57.2 59.0 58.9 33.9 33.4 46.3 47.2 40.5 39.9
Found MF 39.5 39.2 0 0 42.8 42.8 21.1 22.3 46.4 45.3 27.2 27.0 59.5 60.1
UF 10.1 9.6 40.2 39.6 0 0 19.9 18.8 19.7 20.9 26.3 25.7 0 0
Analysis of Unknown Enamel Vehicles Infrared Analysis 71% alkyd 20% UF 9% M F 83% alkyd 17% U F
Chemical Analysis 68% alkyd 32% mixed nitrogen resins 85% a!kyd 15% nitrogen resin
infrared transparent solvents is not feasible, the spectral measurements are made on thin films prepared by spreading the resin solution on rock salt and drying in vacuum a t 60” C. for l hour. As calculations are based on absorbance ratios, the problem of exact control of film thickness is obviated. Figure 1 shows spectra of thin films of each of the three resins involved. The ester carbonyl absorption a t 5.8 microns rather than a characteristic aromatic band was chosen as the analytical wave length for the alkyd because of its intensity and sensitivity to the ester linkages in the oil modifier as well as those in the phthalate ester. The 6.1-micron amide carbonyl band in theureaformaldehyde spectrum and the 12.25-micron triazine ring band in the melamine-formaldehyde spectrum were selected for estimation of the two nitrogen resin components. The factors required to convert absorbance measurements to percentage composition were obtained from spectra run on films cast from solutions of resin mixtures of known composition. If absorbance ratio is plotted against concentration ratio for any two components and if the absorption laws are applicable, it follows directly that the elope of the line so obtained is the ratio of the appropriate absorptivity values. Figure 2 shows plots of these values which indicate that Beer’s law holds reasonably well in these condensed resinous systems. AEeach resin occurs twi’ce in this set of curves, the slope of any one line may be verified by calculation from the other two slopes, as shown in Table I. If the absorptivity value for any one of these bands is arbitrarily set at some convenient value, corresponding values for the remaining bands are readily obtained from the ratios of Table I.
Figure 2.
Absorbance ratios us. concentration ratios for binary mixtures
In this case, the absorptivity value for melamine-formaldehyde was chosen as 1.00 and the others evaluated as shown in Table 11. ANALYTICAL PROCEDURE
After calibration data of the type shown in Table I1 are obtained for the particular instrument and set of conditions to be employed, a sample of unknown quantitative composition is analyzed as follows.
A polished rock salt plate is uniformly coated with a thin continuous film of the resin solution or centrifuged paint vehicle. The solvent is removed by vacuum oven drying a t 60” C. and 1 mm. of mercury for 1 hour. The spectrum from 2 to 15 microns is first scanned qualitatively to determine the resin types present. If no extraneous material is observed, the spectrum is scanned from 13.5 to 11.5 microns and from 7 to 5 microns a t a film thickness such that the measured absorbance a t each analytical wave length falls in an accurately measurable range. Absorbance values A I , AII, and A I I I for alkyd (5.8 microns), urea-formaldehyde (6.1 microns), and melamine-formaldehyde (12.25 microns), respectively, are measured by the baseline method and composition is calculated as follows: A 100 -! a1 % alkyd = $ , & I
a1
’
a11
&I
a111
where U I ,U I I , and U I I I are the previously determined absorptivity values. The percentage of urea-formaldehyde and melamineformaldehyde are calculated from analogous formulas. The method was evaluated by analyzing a number of synthetic blends of known composition. Results are given in Table 111. Table IV compares results obtained on two commercial enamel vehicles of unknown composition with the corresponding values obtained by chemical methods. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of R. M. McNamara in obtaining the spectral data discussed herein. LITERATURE CITED (1) Am. SOC. T e s t i n g M a t e r i a l s , P h i l a d e l p h i a , “ S t a n d a r d s , ” Part 4, D . 330. 1952. -( 2 ) Kappelmeier, C. P. A . F d r b e r - Z t g . 40, 1141-2 (1935). (3) Ibid., 41, 161 (1936). (4) Kappelmeier. C. P. il., Paint, Oil Chern. Rev. 99, 12. 20. 22. 24 (1937). (5) Shrew, 0. D., and H e e t h e r , M. R., AXAL.CHEM.23, 441 (1951). RECEIVED for review August 27, 1955. Accepted October 28. 1955. Division of Paint, Plastics, and Printing Ink Chemistry, 128th Meeting ACS. Minneapolis. September 1955.
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