Evaluation of Alkane Sulfonic Acid Catalysts and Derivatives for Curing Activity in a Melamine/Acrylic Polyol Coating Formulation Using Fourier Transform Infrared Spectroscopy Dana Garcia, Stanley R, Sandier, Joseph Brennan, and Olivier Bousquet t ATOFINA Chemicals, Inc.*
INTRODUCTION A
lkane sulfonic acids and derivatives are a rela tively n e w class of acid curing agents and, de pending on their composition, can act as active or latent catalysts for curing melamine resin (reaction with the acrylic polyol added) based coatings for a variety of applications. They have the distinct feature of producing coatings for metal surfaces which provide good corrosion resistance, solvent resistance, hardness, and gloss at low catalyst loadings. These catalysts find applications in automotive clearcoats, basecoats, primers, and coatings for coils and cans as well as p o w d e r systems. U n d e r standing the mechanism of catalysis and qualifying per formance are critical parameters in the design of effective catalysts and making appropriate application selections. Fourier transform infrared spectroscopy (FTIR) tech niques provide a unique opportunity to study chemical changes 14 based on the complexity and specificity of the infrared spectrum. Major functional groups, w h o s e changes are associated with reaction progress, have well defined w a v e n u m b e r positions and shapes in the mid IR spectral range. These characteristics can be exploited to provide detailed information on reaction kinetics, extent of completion, and in m a n y cases mechanistic informa tion. As the reaction proceeds, the system's properties change and the IR measurements can serve as a probe for these changes. Furthermore, IR measurements m a y be able to replace more laborious physical property determi nations and thus serve as an efficient predictor. Literature examples include the w o r k of C. P. Yang and L. T. Lee 5on the cure of glycerin terminated urethane prepolymers, Y.S. Yang and L.J. Lee 6 on the cure of unsaturated polyes ters, C.A. PrydeTon imidization reactions, and D. Louden 8 on the acrylate cure in optical fiber coatings. In all cases, the studies were based on the selection of specific absor bances originating from functional groups involved in the chemical reaction. Disappearance and formation of species can thus be directly monitored. The cure reaction of melamine based systems has been extensively studied in the literature 915 as a result of its commercial relevance in industrial coatings applications. G'l ,i First A v e k ir,g ,:,f Prussia PA 1 9 4 0 6 tlndustrial intern fr,:,nl Ec,:,lo H a t i , : , n a l o
FfIRmeasurements ~u~y be able to replace more laborious physical property determinations and thus serve as an efficient scree~zing methodology in catalysis drive~z processes. This possibility was considered in evahu~ting alkane su!fonic acids catalyst performance to promote the cure qf a melamine resin with acr)/lic polyols. The example chosen here is an evahu~tion qf alkane sulfonic acid derivatives to measure catalytic efficiency qf the cure reaction between hexamethoxymethyl melamine (H M M M ) and an ac r)/l ic polyol. The IR measurements provided information on the extent q f reaction, via monitoring the disappearance q f the OH stretching mode at 3472 cm q associated with the acr)/lic polyol. This method may also be used to provide detailed information on reaction kinetics and insight into the cure meclumism. As the reaction proceeds, the coating's properties clumge and IR measureme~zts can serve as a probe for these changes. The s t u d i e s h a v e f o c u s e d on u n d e r s t a n d i n g the crosslinking mechanism, kinetics, and the influence of reactants' structure. M. Lazzara 9 discussed techniques to measure degree of conversion. A m o n g the techniques outlined were solid state NMR, FTIR, and continuous monitoring of reaction volatiles. The effect of catalyst level, reaction temperature, and polyol structure w a s investigated. The FTIR technique had significant limita tions due to the need to compare spectra acquired at variable temperature and the lack of experimental data to deconvolute the reaction effects from k n o w n tempera ture effects on the IR spectrum. 9 and co worker 1~
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COOR I
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studied the kinetics of acid catalysis e m p l o y i n g w e a k acid self catalysis to slow reaction rates. Both changes in the relative viscosity and IR absorbances associated w i t h O H and OCH3 g r o u p s w e r e employed. The reaction path w a y s involved transetherification and selGcondensation. P r i m a r y O H g r o u p s w e r e found to react faster than secondary O H groups. At cure temperatures below 140~ and w i t h p r i m a r y O H groups, transetherification w a s the d o m i n a n t pathway. Self condensation occurred at higher temperatures or w i t h secondary O H groups. The effect of the melamine crosslinker structure on the cure m e c h a nism and coating properties is discussed by Blank 11 and Lee. n Yaseen 13 examined similar effects associated w i t h the use of oligourethane diol rather than acrylic polyols. A detailed mechanistic and kinetic study of the melamine polyol reaction is found in the w o r k of Z i m m t and co workers. 14The rate of methanol evolution and gel forma tion (insoluble fraction) w a s utilized to provide kinetic data for two acrylic polyols reacting w i t h melamine. Two sulfonic acids and their dimethyloxazolidine salts were used as catalysts. The rates of methanol evalution were found to be d e p e n d e n t on the concentration and nature of the catalysts and proportional to [H § concentration. Ge lation d e p e n d e d on the chemical structure of the polyol. Methanol evolution w a s not found to be d e p e n d e n t on the gel content or the glass transition temperature at cure temperatures of 120~ The crosslinking reaction w a s
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also studied by Bauer and Budde.15 The extent of cure w a s determined u s i n g the O H absorbance of the polyol. A n expected rate increase w a s found w i t h increasing the reaction temperature and the catalyst concentration, and the a m o u n t of methanol evolved decreased w i t h increas ing conversion. A m o n g the catalysts utilized w e r e acids of v a r y i n g strength including para toluenesulfonic acid (PTSA) and the isopropylamine and triethylamine salts of PTSA. These w e r e found to reduce the cure rate as c o m p a r e d w i t h the free acid. This paper investigates the effect of the catalyst struc ture on the cure of h e x a m e t h o x y m e t h y l m e l a m i n e (HMMM) w i t h an acrylic polyol. The catalysts used are alkane sulfonic acids and amine blocked derivatives. Some of these classes of catalysts have been previously mentioned in the literature, 14,15but no systematic study of their effect w a s investigated. The structure of the reac tants is invariant to enable the evaluation of only catalytic efficiency. FTIR is utilized as a fast and reliable m e t h o d e l e g y for catalysts comparison.
EXPERIMENTAL
Materialsand Reagents The following catalysts were e m p l o y e d in this study: 9 methanesulfonic acid (MSA) from A T O F I N A C h e m i cals, Inc., 9 para toluenesulfonic acid (PTSA) from Aldrich, 9 dodecyl benzenesulfonic acid (DDBSA) from Acres Organics, 9 amine blocked alkanesulfonic acid (MCAT~M1219S andMCAT~M1300) from A T O F I N A Chemicals, Inc., 9 amine blocked para toluene sulfonic acid (BYKTM 460) from BYK Chemie, 9 amine blocked dodecylbenzenesulfonic acid (Nacure | 5543) from King Industries, 9 a terpolymer of styrene w i t h 2 hydroxyethyl methacrylate and methylmethacrylate (JoncryF ~ 500) from SC Johnson Polymer, and 9 h e x a m e t h o x y m e t h y l m e l a m i n e (HMMM) (Cymel | 303) from Cytec or Resimene | 547 from Solutia can be used.
SamplePreparation The samples were prepared by m i x i n g 70 parts acrylic polyol, w i t h 30 parts H M M M and the catalyst Call at 0.02 m o l e / k g resin mixture). Samples for each catalyst system w e r e deposited on a l u m i n u m foil covered glass slides. The films w e r e thin e n o u g h to afford absorbances below IRsaturationinthe spectral regionofinterest. The samples w e r e cured at 120~ for 0, 5, 10, 15, 20, 25, and 30 rain in a forced air oven.
FTIRMeasurements A n u m b e r of techniques are available for spectral acquisition in IR spectroscopy. For the purpose of this
Evaluation of Alkane Sulfonic Acid Catalysts study, w e have chosen g r a z i n g angle FTIR. G r a z i n g angle FTIR m17 is a reflection/absorption technique character ized by h i g h angles of incidence (85~ In a g r a z i n g angle experiment, the material is deposited as a thin film on a highly reflective metal surface. The incident I R b e a m that is reflected from the surface interacts w i t h the film yield ing an absorption spectrum. Spectra obtained u s i n g graz ing angle FTIR spectroscopy resemble transmittance spec tra (the technique is s o m e t i m e s referred to as double pass transmittance). The spectra were obtained on a Biorad 60A FTIR e q u i p p e d w i t h a g r a z i n g angle accessory. A DTGS detec tar w a s used, and 100 scans were co a d d e d at 4 cm 1 resolution. Each spectrum w a s baselined, offset to a zero baseline value, normalized u s i n g the absorbance value at 3027 cm 1 (aromatic C H stretch), and further normalized to 1 for the sample prior to curing (time 0 minutes). N o r m a l i z i n g the spectra eliminates the effect of v a r y i n g sample thickness and permits effective comparison be tween samples. The normalized peak absorbance of the O H stretch (MOO 3500 cm 1) w a s then obtained and plot ted against cure time to obtain a reaction profile for each catalyst. The choice of IR transition is similar to those in the w o r k of Bauer and Budde. 15 Film thicknessess are estimated at 1 3 ~Lm. Literature data has indicated that there is no dependence on thickness 15for films of 25 ~Lm. All IR m e a s u r e m e n t s w e r e p e r f o r m e d at RT.
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for m e l a m i n e based coatings, have been available for some time. They include catalysts such as PTSA, DDBSA, as well as other alkylated aromatic sulfonic acids. More recently, alkane sulfonic acids have become available, which are also active cure catalysts. They also directly catalyze the etherification reaction. Latent catalysts in clude v a r i o u s amine blocked sulfonic acids. In order for the reaction to occur, the deblocking reaction generating the active acid is required to occur first. The general structures for the two types of catalysts are s h o w n in
Figure 2. MEK Double Rub Test The methyl ethyl ketone (MEK) double rub test is a solvent rub technique adapted from a standardized test (ASTM D 5402 93). It is used for assessing the resistance of an organic coating to MEK, and this reflects the chemi cal changes (crosslinking) occurring d u r i n g the curing process. In our version of the test, the 7/:~ in. head of a double sided 3 / 4 and 7/:~ in. w r e n c h is w r a p p e d w i t h a piece of cheese cloth. The cheese cloth is d i p p e d into MEK just before the start and rubbed forward and b a c k w a r d over the coated surface. One forward and b a c k w a r d motion constitutes one double rub. The cheesecloth is d i p p e d into MEK every 25 double rubs until the coating layer gets d a m a g e d . The uncertainly of the results is + / 25 double rubs, and w e consider a resistance over 200 double rubs satisfactory.
Qualitative FTIR Results
The reference spectra for the two components of the coating, H M M M and the acrylic polyol are illustrated in Figure 3. The O H stretching vibration for the acrylic polyol is centered at 3500 cm 1 and is the d o m i n a n t spectral feature in this spectral region. The figure also s h o w s the aromatic C H stretching vibrations associated w i t h the styrene units. A s noted in the Experimental Section, these vibrations are utilized to normalize the spectra and compensate for any thickness effects. H M M M also exhibits a transition in this spectral region. It is
RESULTSAND DISCUSSION Chemistry and Catalysts The c h e m i s t r y i n v o l v e d in the c o a t i n g utilizes the crosslinking reaction between H M M M and the acrylic polyol. The reaction produces an etherification of the polyol h y d r o x y g r o u p s w i t h the m e t h o x y m e t h y l g r o u p s on the m e l a m i n e and liberates methanol. The schematics are illustrated in Figure 1. The use of a strong acid catalyst is required to produce a cure at a reasonably low tempera ture. Such a low b a k i n g temperature is desirable to save e n e r g y and to enable the coating of heat sensitive sub strates. Aromatic sulfonic acid cationic curing catalysts,
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Figure 3--Reference FTIRspectra for polyol (curve a) and HMMM (curve b).
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significantly weaker than the OH stretching vibration, m o s t likely an overtone, and its contribution at 3472 cm-~is reflected in a residual absorbance of approximately 0.2 normalized units. Thus, its detection is not an indication of incomplete cure. Furthermore, in control experiments, it was found that its intensity is invariant with the degro~ of cure and thermal treatment for a constant composition. Figures 4 and 5 illustrate composite plots of absorbance profiles as a function of cure ~rne at 120~ in the absence of catalyst and in the presence of MSA catalyst. In the absence of a catalyst, no change in the absorbance profile for the OH stretching vibration is observed, even after 30min cure. In contrast, when MSA is added, an immediate change is observed. Figure 5 also shows the high catalytic activity associated with an active catalyst such as MSA which does not require deblocking.
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Figure 6 illustrates the efficiency of a number of active and latent catalysts. All alkane sulfonic acid catalysts promote fast cure. The catalytic activity of amine blocked alkane sulfonic acids depends on the kinetics and thermodynamics of the deblocking reaction. Nacure 5543 undergoes fast deblocking, under the experimental conditions, and its activity is similar to that of free alkane sulfonic acids. For blocked catalysts exhibiting slower deblocking, the catalytic rate is decreased as illustrated for BYK 460 and MCAT 1219S. In his work, Bauers also noted slower rates for the amine salts for PTSA. An activity/latency diagram is shown in Figure 7 and schematically dassifies the catalysts. Based on such a diagram, an appropriate choice of catalysts can be made to tailor a specific application.
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The major interest in the grazing angle FHR methodology was to explore its applicability as a predicator for coating properties. Figure 8 illustrates a direct comparison between the IR normalized OH profile as a function of cure time and the results of the MEK double rub test. Shown in the figure, as the concentration of OH functionality in the film decreases, the number of double rubs necessary to damage the coating film on the steel panel increases. A good correlation between the two is established, especially at early cure times. At the latter stages of the reaction, IR is a less sensitive predicator. Small decreases in the concentration of OH groups may not translate into detectable changes in the intensity of the IR absorbance associated with the OH stretching vibration, but the resulting small increase in the degree of cure m a y affectproperties in a much more dramatic fashion. This is noted in Figure 8 where the MEK double rub test results continue to increase for the latter stages of the reaction. The IR measurement correspondence with the physical test (MEK double rub) opens the possibility of substituting IRmeasurementsfor catalyst performance screening. The FTIR technique results in significant ~me reductions for coating evaluation.
Evaluation of Alkane Sulfonic Acid Catalysts
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CONCLUSION The objective o f these e x p e r i m e n t s w a s to s t u d y the c u r i n g c h e m i s t r y o f m e l a m i n e resin c o a t i n g s u s i n g v a r i o u s catalysts b y m e a n s of FTIR techniques. The results illustrate the capabilities o f g r a z i n g angle FTIR spectros c o p y to m o n i t o r the extent of cure for the c o a t i n g s y s t e m s s t u d i e d . The d a t a s h o w s a decrease of the O H s t r e t c h i n g i n t e n s i t y at 3400 3500 cm l, w h i c h indicates a reaction b e t w e e n H M M M ( m e l a m i n e resin) a n d the acrylic p o l y o l f o r m i n g ether type l i n k a g e s w i t h m e t h a n o l g i v e n off. I n t e r m s of catalytic activity, M S A , P T S A , N a c u r e | 5543, a n d D D B S A cure the fastest. BYK TM 460 a n d iV[ACTTM 1219S are slower in p r o m o t i n g the cure, a f f o r d i n g a larger p r o c e s s i n g w i n d o w for these a p p l i c a t i o n s r e q u i r i n g flex ibility. The IR m e a s u r e m e n t s h a v e s h o w n g o o d corre s p o n d e n c e w i t h physical tests (MEK d o u b l e rub), o p e n i n g the p o s s i b i l i t y of s u b s t i t u t i n g IR m e a s u r e m e n t s for screening performance. The FTIR t e c h n i q u e s result in a s i g n i f i c a n t time r e d u c t i o n for e v a l u a t i o n a n d p r o v i d e o p p o r t u n i t i e s for classification o f the c h e m i s t r y i n v o l v e d in the curing.
References (1) Putzig, C.L, Leugers, M.A., McKelw,M.L.,Mitchell,G.E., Nyquist, R.A., Papenfuss, R.R., and Yurga, L., AtlaI. Chem., 64,270 (1992). (2) Putzig, C.L, Leugers, M.A., McKelw,M.L.,Mitchell,G.E., Nyquist, R.A., Papenfuss, R.R., and Yurga, L., AtlaI. Chem., 66, 26 (1994). (3) Claybottrn, M. and Turner, P., "Structure Property Rolations in Polymers," Advat~'es its Chemistry Series 236, p. 408, American Cheinical Sodety, 1993.
Figure 8--Comparison of IR and MEK double rub test performance for MCAT 1219S as a function of cure time at 12GC.
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(4) Carlson, G.M., N e a g , C.M., Kuo, C., a n d Provder, T., Fourier Tratasform hzfrar~d Swctroscopy Characteri=atiotz of Polymers, p. 197, P l e n u m Press, N e w York, 1987. (5) Yang, C. P. and Lee L. T., J. AppL Polym. Sci.,35, 449 (1988). (6) Yang, Y.S. and Lee, L.J., J. AppI. Polym. Sci.,37,2313(1989).
(7) Pryde, CA., J. Polym. Sci., Part A: Polym. Chem., 27, 711(1989). (8) Louden, D., J. AppI. Polym. Sci., 57,1575 (1985). (9) Lazzara, M.G., "Teclmiques to Measure Molamine/Polyol Reac tions in a Film," J,Z,HPNAL':'F C':'ATINGSTD2HN'Z'L'Z'G l, 56, No. 710, 19 (1984). (10) Yamamoto ,T., Nakamichi, T., and Ohe, O., "Kinetics of Carb oxylic Acid Catalyzed Melamine/Polvol Reactionin a Film," J,Z,HPNAL,:,F C,:,ATINGST'ECHN,E,L,E,Gl, 60, No. %2 (1988). (11) Blailk, W.J., "Polyuretlmne Oligomers for W a t e r b o r n e a n d H i g h Solids Coatings," J,:,upNAL,2,FC ,2,ATINGST E'=HN'E'L'E'Gl, 60, NO. 764, 43
(1988). (12) Hill, L.W. a n d Lee, S. B.," Effect of M e l m n i n t ~ F o r i n a l d e h y d e Struc ture on Cure Response of Thernloset Coatings," J,Z,HPNAL,Z,FC,:AT INGSTE,=HN,Z,L,Z,,31,71,NO. 897, 127 (1999).
(13) Haseobuddin, S., Raju, K.V.S.N., and Yaseen, M., "Cr osslink Den sity and Cure Window of Oligourethane Did/Melamine High Solids Coatings," J,:,vpNAL,Z,FC,Z,ATING3TE'=HN'Z'L'Z'Gl, 70, NO*879, 35 (1998). (14) Collett e, J.W., Cor cor an, P., Tmmenbatml, H.P., and Zimmt, W.S., J. AppL Polym. Sci., 32, 4209 (1986). (15) Bauer, D.R. and Budde, G.F.,J. AppI. Polym. Sci., 28,253 (1983). (16) Song,Y.P.,Petty,M.C.,andYarwood,J.,VibratiotmlSwctroscopy,1, 305 (1991). (17) Rabolt, J.F., Jurich, M., and Swalon, J.D., AppI. Swctros'., 39, 269 (1985). (18) Scl~otter, N.E. a n d Rabolt, J.F., AppI. Spectrosc., 39,994 (1985).
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