well as spectra of water solutions of As4010, the 960- cm -t band was assigned to the A s = O stretch of H3AsOt. The hydrate As4Q0.XHfO, if it exists, would be expected to have bands near 900 em -~ since the highest frequency AsO stretch of As4010 appears at a slightly higher frequency. No band was observed in this region and it is concluded that the hydrate is not an important species in water solutions of As4010.

Analysis of the spectra gives no indication of oxy- halides of arsenic. For example, for the potassium halides (C1, Br, and I) there was observed the same series of bands in the solutions of AsC13 and As40~. This suggests the bands observed are not due to AsX vibra- tions but to AsO. Only one oxyhalide of arsenic (AsOF) has been reported. This observation supports the suggestion that the oxyhalides are not stable and prob- ably have only transi tory existence.

Bands for AsC14- were not observed although some workers feel this ion does exist24 In all our solutions, no bands appear tha t cannot be assigned to species other than this ion. This ion should be stabilized by excess HC1 in AsC18 solutions, but it was observed that the IIC1 will not dissolve unless water is present. In addition, AsC13 is not soluble in liquid HC1.

I t was noted also that alkali halides are not soluble in AsC13 unless water is present. This further supports the conclusion that the AsC14- ion is very difficult to form.

1. H. Remy, Treatise on Inorganic Chemistry 1, 652 (1956). 2. F. A. Miller, G. L. Carlson, F. F. Bentley, and W. H. Jones,

Spectrochim. Acta 16, 135 (1960). 3. P. Paillette, A. Lankvanda, A. Hadni, V. J. Masson, and

Guerin H. Duc-Mauge, Bull. Soc. Chim. France 432 (1960). 4. K. A. Becker, K. Pleith, and I. N. Stranski, Progr. Inorg.

Chem. 4, i (1962). 5. R. S. Halford, J. Chem. Phys. 14, 8 (1946). 6. H. Gerding, H. Brederode, and H. de Decker, Ree. trav.

Chim. 61, 549 (1942). 7. G. E. Walrafen, J. Chem. Phys. 36, 90 (1962). 8. H. Gerding and H. C. J. de Decker, Rec. tray. Chim. 64, 191

(1945). 9. J. T. Braunholtz, G. E. Hall, G. Mann, and N. Sheppard,

J. Chem. Soe. 868 (1959). 10. M. FMk and P. A. Giguere, Can. J. Chem. 36, 1680 (1958). 11. G. E. Walrafen, J. Chem. Phys. 39, 1479 (1963). 12. P. Tarte, Spectroehim. Acta 23A, 2127 (1967). 13. R. A. Serway and S. A. Marshall, J. Chem. Phys. 45, 2309

(1966). 14. V. Gutmann, J. Chem. Phys. 63, 378 (1959).

Quantitative Infrared Analysis of Styrene-Acrylic-Melamine Resins

John T. Vandebergt DeSoto Inc., Research Center, Des Plaines, Illinois

(Received 16 June 1966)

This paper presents a method for rapid and accurate determination of individual components in modified thermosetting acrylic-coating resins. This technique is based on infrared-absorb- a~ce measurements at 1733 cm -1, 816 cm -1, and 700 cm -~ for the acrylate, melamine-formalde- hyde, and styrene components, respectively. The accuracy and precision obtained on typical system of knm~m composition have been found to be better than _+2% absolute for any one component. Furthermore, the analysis can be performed in minutes after the initial calibration work has been completed.

I N T R O D U C T I O N

The utili ty of modified thermosetting-acrylic resins has shown remarkable growth in recent years. Appli- sations of these coatings are especially abundant in the areas of home appliances, metal decorating, and most recently automobile finishes. This is largely because of superior properties regarding improved toughness, better resistance to softening at elevated temperatures, excellent weather resistance, and im- proved resistance to oil, grease, solvents, alkali, acids, and other chemicals.

A satisfactory infrared procedure for determination of the composition of a styrene-acrylic-melamine polymer mixture does not seem to have been reported in the literature. Therefore, the author proposes a

rapid infrared absorbance ratio procedure for estimat- ing each individual resin component in conventional two- and three- component styrene-acrylic-melamine resin systems. Samples of known compositions are required as standards for quanti tat ive calibration, but the method is specific, accurate, rapid, and useful. Miller and Shreve I along with Sterling and co-workers 2 have previously utilized a similar method to determine alkyd-nitrogen resin blends and methyl isopro- penyl ketone-butadiene-acrylonitrile terpolymers, respectively.

The most widely used spectroscopy methods for the determination of bound styrene are infrared absorp- tion, 3-9 nuclear magnetic resonanc@° and ultraviolet absorption21-~3 A pyrolysis technique in correlation

304 Volume 22, Number 4, 1968 APPLIED SPECTROSCOPY

with mass spectrometry ~4 has been employed to determine the styrene-acrylate ratio in styrene- acrylate copolymers; however, the method is not applicable in most cases. Index of refraction ~5,~6 was an early method proposed for determination of bound styrene; however, all interfering substances must be removed and accurate temperature equilib- rium must be attained. A wet-chemical method ~7 and a nitration procedure ~s have been developed to deter- mine bound styrene in various copolymer systems, but in some cases, interference products cannot be dis- tinguished from styrene. Previously, melamine-formal- dehyde resins have been determined in polymer systems using wet-chemical methods ~9-2~ and infrared absorptionY 2 Determination of the nitrogen content of a multicomponent resin system using a modified micro-Dumas nitrogen determination 23 permits the melamine-formaldehyde resin portion to be esti- mated, providing other nitrogen resins are not present in such a system. Good correlation between the micro- Dumas method and the new infrared procedure have been found. Acrylate polymers and copolymers have been determined by the classical Ziesel reaction and gas-chromatographic analysis, 24 by pyrolytic gas- chromatography techniques, 25-27 and with the aid of infrared spectroscopyY s-~° However, these methods are time consuming and interferences can exist which may limit their utility to specific applications.

The present discussion is restricted to the quantita 7 t ire determination of the individual resinous film- forming components in the vehicle portion of the coating. A prerequisite to the infrared analysis dictates that the resinous-vehicle solution be separated from the dispersed pigment portion. High speed centri- fuging usually causes the desired physical separation of these two phases.

I. EXPERIMENTAL

A. Apparatus

A Beckman IR-12 spectrophotometer was used to obtain the quantitative data. The instrument was purged with dry air to minimize water vapor.

Conditions were set as follows: SB/DB ratio, 1.2:1; slit, 2X standard program, i.e., 2.45 mm at 700 cm -~, 1.38 mm at 816 cm -~, and 0.375 mm at 1733 cm-1; period, 2; scan speed 150 cm -~ per min with speed suppression; slit control in select; percent transmission, 0-100.

Peak absorbances were measured from appropriate baselines with a Beckman absorbance scale ruler.

Samples were cast as films on cesium iodide crystals from approximately 10% (wt./vol.) solutions using three parts benzene and one part acetone as the solvent. Films were allowed to dry at room temperature for 15 min and then dried in a vacuum oven at 100°C for 15 min. Fihn thickness was regulated until the absorbance of the carbonyl band of the acrylic-resin portion of the system was between 0.7 and 1.0.

II. MATERIALS

All resin standards were commercial grade and were used without further purification. Resimene R-876 melamine-formaldehyde resin was obtained in xylene-butanol from the Monsanto Chemical Com- pany. Lucite 2041 ethyl methacrylate polymer resin and polystyrene PS-2 resin were obtained in powder form from E. I. DuPont De Nemours and Company and The Dow Chemical Company, respectively.

Typical styrene-acrylic-melamine resin systems were obtained from DeSoto Inc.

III. MATERIAL PREPARATION

Individual 10%-20% (wt./vol.) solutions of poly- styrene and poly (ethyl methacrylate) resin in xylene were prepared. The solutions were shaken on a mechanical shaker for approximately one h to accel- erate solubility; then filtered through glass sintered filters to remove any insoluble material. Percent solid (N.V.C.) values were determined on the melamine- formaldehyde resin solution and on the poly (ethyl methacrylate) resin and polystyrene resin solutions according to a slightly modified ASTM designation D 1259. ~ The same sample-weighing technique was used to make the required binary calibration blends. Binary solutions were diluted further to approximately 10% (wt./vol.) solutions using three parts benzene and one part acetone to facilitate ease of film casting.

IV. RESULTS

Films cast from solutions made by mixing p01y- styrene and poly (ethyl methacrylate) resin solutions, poly (ethyl methacrylate) and butylated melamine- formaldehyde resin solutions, and polystyrene and butylated melamine-formaldehyde resin solutions were satisfactory for use in obtaining the ratio of the appropriate absorptivity values. Ten standards were run for each of the three binary blends. Absorbance values Aa, Am, and A8 for acrylic (1733 cm-1), melamine-formaldehyde (816 cm-0, and styrene (700 em-0 resins, respectively, were measured by the base-

0.10

0.00 0.0(

• 5T YR EN FE.~EM A • U F /5TYREN E • M FJEMA

0,40 0.80 1.20 1.60 2 .00 CONC~ONCi~

FIG. 1. Plots of resin ratios to test adherence to Beer's law.

0,80

0.7O

0,60

0.50

'~H 0.40 <

0 .30

0.20

APPLIED SPECTROSCOPY 305

FIG. 2. Base-line application for determining absorption values. Styrene-acrylic-melamine thermosetting system.

line method and their ratio plotted vs the weight- concentration ratio. Figure 1 is the calibration curve obtained from known binary blends of the three forms of resins. The experimental slopes represented in Fig. 1 were calculated utilizing the method of least squares. Also, a correlation coefficient of +1 .0 was determined for each individual set of experimental data. This information indicates direct correlation of absorbance vs the weight-concentrat ion ratio. Figure 2 is an example of a typical modified thermosett ing- acrylic resin.

V. REVIEW OF INFRARED THEORY

Since the selected analytical wavelengths used in this analysis are free from overlap, the accuracy in this two- and three-component infrared analysis depends chiefly on the consistency of the absorptivi ty values among various acrylic components. Beer's law states tha t A~= a~bc~, where A ~ is the absorbance of component X, a, is its absorptivity, b is the sample thickness, and c~ is its concentration. For the modified acrylic-resin system under consideration, the following equations hold :

A~ = a,bca

for acrylic resin portion, (1)

A~ = asbc~

for styrene portion, (2)

Am = a,,bcm

for melamine-formMdehyde resin portion. (3)

The above conditions assume that different acrylate and methacrylate monomers would have silimar absorpt ivi ty values. Therefore, it can be seen tha t the absorpt ivi ty (a~) of the acrylic-resin portion is assumed constant regardless of the types or amounts of acrylate or methacrylate monomers used to synthesize the acrylic resin.

If Eqs. (2) and (3), respectively, are divided by Eq. (1) and Eq. (3) divided by Eq. (2), the following

relations are obtained:

A.~/A~, = asc~/a,c~,, (4)

A , , / A , = amC,,/a,c,, (5)

Am/As = a,,c,~/a.~c~. (6)

Since path length is not a factor in the calculations, the necessity for knowing the film thickness is elimi- nated by the ratio method. If absorbance ratiois plotted against concentration ratio for any two components, the slope of the line of the ratio of the specific absorp- t ivi ty coefficients is determined provided Beer's law is followed. Plots of these values are shown in Fig. 1 which indicate tha t Beer's law holds in this system. Each resin occurs twice in this set of curves. Since As = ~ A ~ i = ~a~jbci, from the previous equations, the following equations can be obtained:

am/a~ = (a,,/a.,)/ (a~/a~) (7)

where a~,/a~= 1/(a~/a,); a~/a~, = (am/aa) / (am/a.~) (8)

a,,/a, = (am/aa)/ (a~/aa). (9)

Therefore, the slope of any one line may be calcu- lated from the other two slopes. Table I shows the ratio of absorpt ivi ty values for binary systems. Assuming a m = l and substi tuting into Eq. (7), the relative absorpt ivi ty values for individual components may be calculated. Relative absorpt ivi ty values for individual components are listed in Table II. I t follows that C a + C , + C m = 1.0; this yields expressions for the concentrations in terms of specific absorbance values and specific absorptivi ty values. Component concentrations may be calculated using the following equations :

Ca ( % ) = (Aa/aaXlO0) ~ a~ + ~ , , ) ' (10)

C~ ( °~o)=(A~/a~M100) / (A"+A2+A" '~ , (11) / \ a , a~ am /

(12) Cm ( ~ o ) = ( A ~ / a m X l O 0 ) / ~ - + ~ a,,,/"

306 Volume 22, Number 4, 1968

Table I. Absorptivity ratios for binary systems.

Binary systems Rat io of absorp- % Relat ive error t i v i ty values

Found Calc.

Styrene/acrylic 0.609 0.610 0.16 Melamine-formaldehyde/acrylic 0.191 0.190 0.52 Melamine-formMdehyde/styrene 0.313 0.313 0.00

Table II. Re!ative absorptivity values fcr individual components.

Component Absorptivity value

Ethyl methaerylate polynler resin 5.24 Melamine-formaldehyde resin 1.00 Styrene resin 3.19

VI . D I S C U S S I O N

The validity of the infrared procedure depends on whether the 1733-cm -1 band of the acrylate (carbonyl stretch), 816-cm -~ band of the melamine-formaldehyde (aromatic bending), and 700-cm -t band of the styrene (aromatic bending) have reliable absorptivity values for various styrene-acrylic-melamine resin systems. Absorptivity values of some pure poly (acrylate) resins and poly (methacrylate) resins have been de- termined in this laboratory. The absorptivity values derived for different acrylate and methaerylate homo- polymer resins are not constant. Poly (ethyl meth- acrylate) resin was selected as a calibrating material because it possesses an absorptivity value which is

representative of the average absorptivity values found in the majori ty of the acrylic resins studied in this laboratory. Therefore, the determined meth- acrylate absorptivity employed in this work is most representative of common thermosett ing acrylic resins. In such cases, where a wider variety of acrylate and methaerylate monomers are employed, the method is still applicable if the data are plotted on a molar basis. This would then necessitate a earbonyl determination by some independent method. Methods such as the Zeisel-reaction and gas-chromatographic analysis, saponification followed by gas-chromatographic analy- sis, or both might be used for monomer identification. Other workers have suggested quanti tat ive infrared procedures for different systems 32 but have failed to indicate this possible discrepancy.

Some acrylate and methaerylate monomers that can be utilized to formulate typical thermosetting- acrylic resins are: methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate, hydroxy ethyl acrylate, acrylic acid, methyl methacrylate, ethyl methacrylate, butyl methaerylate, hydroxy ethyl methacrylate, and methaerylie acid. Combina- tions involving as many as six different monomers may be utilized while their concentrations may vary from approximately 1% to as much as 60% in the acrylic resins studied.

Since styrene-acrylic-melamine resins contain hydroxy and/or acid-functional groups for cross- linking purposes, their association with the earbonyl groups through hydrogen bonding can be followed by a characteristic shift in the infrared-absorption spec- trum. However, intramoleeular or intermoleeular

Table III. Infrared analysis of blended standard systems by absorbance ratio method.

Poiy (ethyl methacrylate) Polystyrene

Sample resin % % Error resin % % Error

Theor. Cale. Abs. Rel . Theo r . Cale. Abs. Rel.

Melamine- formald.ehyde

r e s i n

Theor. Calc.

~y~ Error

Abs. Rel.

1 66.4 65.7 0.7 1.05 3.1 2.9 0.2 6.45 30.5 31.4 0.9 2.95

2 60.4 60.5 0.1 0.17 7.7 7.6 0.1 1.30 31.1 31.8 0.7 2.25

3 74.9 74.7 0.2 0.27 9.0 9.1 0.1 1.[1 16.1 16.2 0.1 0.62 74.3 0.6 0.80 9.7 0.7 7.77 ]6.0 0.1 0.62

4 12.7 12.9 0.2 1.57 21.8 22.0 0.2 0.92 65.4 65.1 0.3 0.46 13.3 0.6 4.72 21.5 0.3 1.37 "65.2 0.2 0.31 12.9 0.2 1.57 22.0 0.2 0.92 65.1 0.3 0.46

5 50.3 50.2 0.1 0.20 2.4 2.9 0.5 20.85 47.0 46.9 0.1_ 0.21

Table IV. Infrared analysis of typical styrene-acrylate copolymers by absorbance-ratio method.

Sample Acrylate, % % Abs. error Styrene, %

Theor. Calc. Theor. Calc.

%Abs. error

l 95.0 93.5 1.5 5.0 6.5 2 53.0 51.2 1.8 47.0 48.8 3 70.0 70.8 0.8 30.0 29.2

1.5 1.8 0.8

APPLIED SPECTROSCOPY 307

Table V. Infrared analysis of typical melamine-formaldehyde and acrylic-resin blends by absorbance-ratio method.

Melamine-Form- Sample Acrylic resin, % % Abs. error aldehyde, % % Abs. error

Theor. Calc. Theor. Calc.

1 65.0 63.4 1.6 35.0 36.6 1.6

Micro-Dumas 35.8

2 64.0 63.7 0.3 36.0 36.3 0.3 Micro-Dumas

36.4

Table VI. Infrared analysis of typical resin plant styrene-acrylic-melamine-formaldehyde resin systems by absorbance-ratio method.

Melamine- Sample Acrylic formaldehyde

resin, % % Error Styrene % % Error resin, % % Error

Theor. Calc. Abs. Rel . Theor. Calc. Abs. Rel. Theor. Calc. Abs. Rel.

1 37.1 37.5 0.4 1.08 32.9 32.6 0.3 0.91

2 59.3 59.9 0.6 1.01 3.2 3.2 0.0 0.00

3 67.4 66.8 0.6 0.89 3.6 3.8 0.2 5.56

4 29.2 28.6 0.6 2.06 25.8 27.2 1.4 5.43

5 35.8 36.7 0.9 2.51 28.2 28.7 0.5 1.77

6 39.1 39.5 0.4 1.02 34.7 35.6 0.9 2.60

7 37.1 37.2 0.1 0.27 32.9 32.8 0.1 0.34

30.0 29.9 0.1 0.33 Micro-Dumas 30.6%

37.5 36.8 0.7 1.87 Micro-Dumas 39.4%

29.0 29.4 0.4 1.38 Micro-Dumas 29.2%

45.0 44.2 0.8 1.78 Micro-Dumas 45.5%

36.0 34.6 1.4 3.89 Micro-Dumas 36.8%

26.1 24.9 1.2 4.60 Micro-Dumas 28.0%

30.0 30.0 0.0 0.00 Micro-Dumas 31.4%

hydrogen bonding does not seem to have any great effect on the quanti tat ive accuracy of this method.

The essential base-lines tangent to the infrared curve were drawn from suitable points as indicated in Fig. 2. Since melamine-formaldehyde resin has a broad absorption band in the 600-cm -1 to 500-cm -1 region, its presence changes the relative position of the styrene base-line at higher melamine-formaldehyde resin concentrations. Therefore, the base-line involving the styrene absorption band was extended to approximately 425 cm -1 to facilitate a relatively constant and repro- ducible base-line.

As previously indicated, the proposed method is rapid, specific, and accurate. Table II1 shows the infrared analysis of blended s tandard systems by the absorbance-ratio method. Table IV and Table V con- tain examples of two-component systems to which this procedure has been applied. The accuracy of this

procedure on typical styrene-acrylic-melamine thermo- setting-resin systems is shown in Table VI. All listed samples are accurate to better than 2.0% absolute, based on determination of known systems. The micro- Dumas nitrogen method was used to confirm the melamine-formaldehyde resin concentration, and the results were in good agreement with the infrared procedure (Tables V and VI). Styrene was not determined by an independent method, but the acrylate portion was analyzed using the Zeisel reac- tion and gas chromatography. A forthcoming publica- tion will include data derived from these techniques.

I t is emphasized that more accurate data utilizing this procedure might be attained by altering the instrumental parameters. Some of these parameters would be to utilize: (1) a slower scanning speed; (2) a constant spectral slitwidth; (3) no speed suppres- sion, and (4) standardizing with actual acrylic and

308 Volume 22, Number 4, 1968

Table VII. Reproducibility of analysis of a typical thermosetting styrene-acrylic-melamine resin.

Film Styrene, v/v Acrylic Melamine- no. resin, % formaldehyde

d ~ resin, % d ~

d a

1 32.6 --1.24 37.5 +0.13 29.9 +1.12 2 33.0 --0.84 37.6 +0.23 29.4 +0.62 3 33.6 --0.24 37.4 +0.03 29.0 +0.22 4 34.0 +0.16 37.8 +0.43 28.2 --0.58 5 34.7 +0.86 37.1 --0.27 28.2 --0.58 6 34.2 +0.36 37.3 --0.07 28.5 --0.28 7 34.8 +0.96 36.9 --0.47 28.3 --0.48

Standard Deviation b : a=0.83 o-=0.34 0-=0.67

d =dev ia t i on b Equa t ion used to calculate s t anda rd devia t ion ~ = [~ ( X - X ) 2 / n - 1 ]] where

n = 7 .

m e l a m i n e - f o r m a l d e h y d e resin c o m p o n e n t s f o u n d in t he sample .

Since n o r m a l q u a l i t a t i v e in f r a red s p e c t r a a re u sua l ly o b t a i n e d in th is l a b o r a t o r y u n d e r t he p re - v ious ly s t a t e d i n s t r u m e n t a l p a r a m e t e r s , th is p ro - p o s e d p r o c e d u r e increases t he n u m b e r of s p e c t r a t h a t can be o b t a i n e d a n d al lows q u a n t i t a t i v e ana lys i s to be pe r fo rmed on s p e c t r a o b t a i n e d for q u a l i t a t i v e purposes .

Seven ana lyses of a single s a m p l e shown in T a b l e V I I i nd i ca t e t h a t the r e p r o d u c i b i l i t y of the m e t h o d is w i th in t he e x p e r i m e n t a l er ror . I n d i v i d u a l c o m p o n e n t dev i a t i ons and c a l c u l a t e d s t a n d a r d d e v i a t i o n s are l i s ted in th i s t ab le . B a s e d on a 9 5 % conf idence in t e rva l , the conf idence l imi t s for t he i n d i v i d u a l acry l ic - , s ty rene- , and m e l a m i n e - r e s i n po r t i ons are ~ 0 . 3 0 , ± 0 . 7 4 , a n d ± 0 . 6 0 , r e spec t ive ly .

This t e c h n i q u e has p r o v e d v e r y useful in th is l a b o r a t o r y . I t a ids the chemis t b y r a p i d l y d e t e r m i n i n g the r e l a t i ve a m o u n t s of c o m p o n e n t s in two or t h r ee c o m p o n e n t s t y r e n e - a c r y l i c - m e l a m i n e t h e r m o s e t t i n g - res in sys tems . Th is i n f o r m a t i o n can lend as s i s t ance to work in progress a n d can be espec ia l ly usefu l in q u a l i t y - c o n t r o l a p p l i c a t i o n s a n d c o m p e t i t o r - p r o d u c t eva lua t i on .

A C K N O W L E D G M E N T S

The ass i s t ance a n d c o o p e r a t i o n of m a n y m e m b e r s of th is l a b o r a t o r y are g r e a t l y a p p r e c i a t e d . Those dese rv -

ing spec ia l cons ide ra t i on are N. A. V i t t o r e for o b t a i n - ing s t a t i s t i c a l d a t a , J. E. Seraf in for he lp ing w i th t he l i t e r a t u r e search , a n d K. D. U d s t u e n for r u n n i n g the m i c r o - D u m a s n i t r o g e n d e t e r m i n a t i o n s . T h e sugges- t ions of L. C. A f r e m o w are a c k n o w l e d g e d and a p p r e c i a t e d .

*Paper presented at the Fifth National Meeting of the Society for Applied Spectroscopy.

tPresent address: Loyola University, Department of Chemistry, Chicago, Illinois 60626.

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20. 21. 22. 23. 24.

25.

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31.

32.

APPLIED SPECTROSCOPY 309