Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 440-444
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GENERAL ARTICLES Latent Acid Catalysts for Hydroxy/Melamine Coatings Mohlnder S. Chattha' and Davld R. Bauer Polymer Science Department, Engineering and Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Three alkyl p -toluenesulfonates, n -butyl, 2-hydroxypropy1, and 2-hydroxycyclohexyl, have been evaluated as latent catalysts for cure of hydroxy-melamlne high solids paint compositions. The rate of decomposition of these catalysts in paint solutions and in palnt films was found to be a function of the temperature, the structure of the alkyl group, and that of the composition of the paint. 2-Hydroxycyclohexyl p-toluenesulfonate is a more effective catalyst than the other two. This sulfonate decomposes to produce p -toluenesulfonlc acid In nonreactive solvents at elevated temperature; however, the decomposition rate is significantly enhanced when hydroxy solvents are employed. A number of volatile organic products resulting from its thermal decomposition have been identified. Mechanisms based on analysis of the decomposition products have been suggested. The hydroxy-melamine palnt composltions, which cure at 170 OC without a catalyst, can be cured at 140 OC with the use of this blocked catalyst. Shelf stabili of the paint compositions containing this catalyst is superior to the corresponding compositions formulated with conventional catalysts, e.g., p-toluenesulfonic acid, or with commercial blocked catalysts.
Introduction In hydroxy-melamine coating compositions, monomeric highly alkylated melamines provide higher solids than do polymeric alkylated melamines. Acid catalysts are invariably employed to cure high solids hydroxy-melamine compositions formulated with monomeric alkylated melamines (Blank, 1979; Chattha, 1980,1981a,b;Chattha and van Oene, 1982). Conventional catalysts (e.g., p-toluenesulfonic acid) cause the cross-linking reaction to occur even at room temperature, resulting in increased paint viscosity. The viscosity increase becomes a more serious problem at slightly elevated temperatures, as may occur during shipment or circulation. In addition to viscosity increase, acid catalysts tend to attack pigments and may cause gassing, color change, and flocculation. To overcome these problems, amine salts of strong organic acids are often used to catalyze hydroxy-melamine cross-linking reactions. Since amine salts may also protonate the amino groups of the melamines, room temperature hydroxy-melamine condensation may still proceed, again resulting in increased viscosity. In this paper a new class of blocked catalysts for hydroxy-melamine coatings is described which afford both excellent stability and efficient paint cure. Mechanisms and rates of decomposition of these latent catalysts are also discussed. Experimental Section Materials. Technical grade acetone, butyl acetate, methyl amyl ketone, toluene, and xylene were used as solvents and were dried over molecular sieves before use when employed in isocyanate-hydroxy reaction. Cyclohexene oxide, propylene oxide, and p-toluenesulfonic acid were obtained from Aldrich Chemical Co. Hexamethoxymethyl melamine (Cyme1 301) was obtained from American Cyanamid and was used as received. Isophoronediisocyanate (IPDI) was obtained from Veba-Chemie and
2-ethyl-l,3-hexanediolwas obtained from Eastman Kodak co. The hydroxy acrylic polymer N 2200) employed in this investigation was obtained from Ford Paint Plant, Mt. Clemens; this polymer was prepared from hydroxyethyl acrylate (30%), isobutyl methacrylate (45%) and styrene (25%). Butyl p-toluenesulfonate was obtained from ICN Pharmaceuticals; dibutyltin dilaurate, from M & T Chemicals; and catalyst Nacure X49-110, a blocked dinonylnaphthalene disulfonic acid, from King Industries. Synthesis of Dihydroxy Oligourethane. 2-Ethyl1,3-hexanediol(3504g) was dissolved in butyl acetate (1542 g) and was placed under nitrogen in a round-bottom flask equipped with a mechanical stirrer. Dibutyltin dilaurate (1.54 g) was mixed with isophoronediisocyanate (2664 g) and this mixture was added dropwise to the diol solution with continuous stirring. The temperature was allowed to rise to 50 "C and then was maintained at this temperature. After the addition was complete, the reaction mixture was stirred for 2 h to obtain the desired product. Synthesis of 2-Hydroxypropylp -Toluenesulfonate. Catalyst A. p-Toluenesulfonic acid (172 g) was taken up in 200 mL of diethyl ether and was cooled with a dry ice-acetone mixture. Propylene oxide (174 g) was added dropwise with continuous stirring and cooling. After the addition was complete, the reaction mixture was allowed to warm to room temperature (2 h) and was stirred overnight. The reaction mixture was dried over molecular sieves for 2 h and filtered. Ether and propylene oxide were evaporated under reduced pressure to obtain the desired product in essentially quantitative yield. Synthesis of 2-Hydroxycyclohexyl p -Toluenesulfonate. Catalyst B. A solution of p-toluenesulfonic acid (190 g) in 150 mL of acetone was placed under nitrogen in a round-bottom flask fitted with a dropping funnel. The flask was cooled with an ice-water mixture. Cyclohexene oxide (125 g) was dissolved in 50 mL of
0196-4321 I8311 222-0440$0l.50/0 0 1983 American Chemical Society
(an
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 441 Table I. Cure of Oligourethane-Cyme1301 Compositions at 140 "C/20 min and Viscosity Increase at 60 "C/16 h cure catalyst type
%
methyl amyl xylene ketone rubs
viscosity, s, no. 4 Ford cup init
final
2-hydroxycyclohexyl p-toluenesulfonate
1.22 Pass 25 15.0 15.2 1.32 P W 30 14.7 15.2 1.42 PW 50 14.5 15.2 50 14.4 15.1 1.42, H,O Pass -n-butyl p-toluenesulfonate 2.5 fail 10 14.3 14.7 3.5 Pass _2-hydroxypropyl p-toluenesulfonate 2.5 Pass 15 pass 40 14.4 15.0 3.5 p-toluenesulfonic acida 0.46 Pass 30 15.0 27.1 14.9 31.0 0.50 Pass 50 isopropylammonium p-toluenesulfonatea 1.98 Pass 30 15.0 25.5 2.15 Pass 50 14.9 29.7 Nacure X49-110a 1.35 Pass 25 15.0 18.1 1.46 Pass 50 14.9 20.5 a Amounts of these commercial catalysts which give poor methyl ethyl ketone resistance ( < 2 5 double rubs) are not included.
__
__
__
Table 11. Cure of Hydroxypolymer-Cyme1301 Compositions at 140 "C/20 min and Viscosity Increase at 60 "C/16 h cure catalyst type
%
xylene
methyl amyl ketone rubs
2-hydroxycyclohexyl p-toluenesulfonate
0.20 0.30 0.40 0.71 0.97 0.75 0.98 0.09 0.50 0.62 0.39 0.42
fail pass Pass fail pa= pas pas pas pas PW Pa= PW
3 20 50 5 15 10 45 50 40 50 35 50
n-butyl p-toluenesulfonate 2-hydroxypropyl p-toluenesulfonate p-toluenesulfonic acida isopropylammonium p-toluenesulfonate" Nacure X49-110a
viscosity, s, no. 4 Ford cup init
final
25.9 25.6 25.2
26.2 26.8 26.7
25.5
25.9
25.4 25.5 26.0 25.8 25.4 25.5
25.9 41.3 38.5 40.7 36.5 38.7
__ __
__ __
a Since the effectiveness of these catalysts in this system was already known, the amounts could be estimated to obtain a complete cure.
acetone; this solution was added dropwise to the acid solution with continuous cooling and stirring. After the addition was complete, the reaction mixture was stirred at ice-water temperature for 30 min and was then placed at room temperature. The product crystallized out as a white solid. The crystals were filtered and the filtrate was further reduced to obtain a second crop of the product; the total yield was 205 g (76%). The melting point, after recrystallization from an petroleum ether-acetone mixture, was found to be 94-96 "C (Roberts, 1968). Preparation of Isopropylammonium p -Toluenesulfonate. A solution of 190 g p-toluenesulfonic acid in 250 mL of acetone was placed in a three-necked 2-L flask. The flask was cooled with an ice-water mixture, and 59 g of isopropylamine was added dropwise with continuous stirring. After the addition was complete, the reaction mixture was stirred at room temperature for 1h and then part of the acetone was evaporated to obtain a white crystalline amino salt. It was recrystallized from acetone; melting point was 127.5-128 "C (lit. 128 "C)(Foxly and Short, 1947). P a i n t Cure a n d Stability Determination. Compositions of dihydroxy oligourethane, hydroxy polymer, and Cyme1 301 were prepared in butyl acetate. Increasing amounts of the catalysts were added; the resulting formulations were drawn on primed steel panels and were baked at 140 "C for 20 min. The paint cure was determined by placing a drop of xylene on the film for 1min. The film was considered cured when it did not show any spot after wiping off xylene. The degree of cure was
further determined by rubbing the film at a different spot with a cotton swab soaked in methyl ethyl ketone. The film was considered fully cured when it did not show any loss of gloss and integrity with 50 double rubs; this was an arbitrary standard set for comparison purposes. In order to evaluate paint stability, the paint viscosity was determined before and after aging at 60 "C for 16 h. These results are shown in Tables I and 11. Decomposition Studies of 2-Hydroxycyclohexyl p -Toluenesulfonate. Catalyst B. A weighed amount (2.5 g) of 2-hydroxycyclohexyl p-toluenesulfonate (1) was dissolved in methyl amyl ketone to obtain 100 mL of solution. A second solution (100 mL) of the same catalyst concentration was prepared in 15 g of 2-ethyl-1, 3-hexanediol, and methyl amyl ketone. A third solution of the same catalyst concentration was prepared in butyl diethyleneglycol acetate. Ten-mililiter samples of each solution were placed in conical flasks and were heated in an oven at 140 "C for various intervals of time. Upon removal from the oven, each sample was quickly cooled to 0 "C and dissolved in 20 mL of ethanol. The samples were then potentiometrically titrated with potassium hydroxide solution with a Brinkman Potentiograph E536. The amount of free acid was determined from the amount of potassium hydroxide consumed. The percentage decomposition was calculated from the theoretical amount of the acid produced upon complete decomposition of the blocked catalyst; the results are plotted in Figure 1. Thermal Decomposition of the Neat Blocked Cat-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 90
80
z
'0
9 60
a0
50
U
0" 8
40
20e I0
5
0
10
20
30
40
TIMEIMIN
50
60
1
Figure 1. Decomposition of 2-hydroxycyclohexylp-toluenesulfonate in: (0) methyl amyl ketone; (A)butyl diethyleneglycol acetate; (0) methyl amyl ketone and P-ethyl-l,3-hexanediol.
--
Table IV. Average Degree of Deblocking for Different D-Toluenesulfonates after 20 minu fraction deblocked 50 70 90 110 120 130 140
TIME
Table 111. Volatile Organic Products from the Decomposition of 2-Hydroxycyclohexyl p-Toluenesulfonat Determined by Gas Chromatography and Mass Spectrometry peak no.
peak area
compound
fraction
1
70271 348 273 26 689 3 1 579 748 121 1 6 808 124 931 34 67 2
benzene cyclohexene cyclohexadiene toluene cyclopentane carboxaldehyde cyclopentyl methanol cyclohexanone cyclopentyl toluene
0.050 0.248 0.019 0.022 0.534 0.012 0.089 0.025
3 4 5 6 7 8
Cat-A
Cat-B
0.005 0.016 0.02
0.002 0.012 0.055 0.19 0.25 0.32 0.35 0.38 22
0.05
150 170
0.06 0.06
EA
9
In hydroxy acrylic polymer. salt. In kcalimol.
Figure 2. Gas chromatograph of the volatile organic products resulting from thermal decomposition of pure 2-hydroxycyclohexyl p-toluenesulfonate.
alyst B. The decomposition was determined by combined gas chromatography and mass spectrometry. A weighed amount of the material (183 mg) was placed in a glass boat in a glass tube furnace held at 140 f 3 "C. Helium was passed through the furnace at 400 cm3/min and collected in a Tedlar bag. This resulted in a 12.9% weight loss from the sample. The sample darkened upon heating and a red-brown liquid residue remained. One milliliter of sample was removed from the bag and injected into a 50-m glass capillary column coated with SP-2250 liquid phase. The column was programmed from 0 to 230 "C at 5O/min. The peaks were then injected in mass spectrometer and the resulting spectra were identified. The compounds which were identified are only those which are volatile enough under these conditions to go through the chromatographic column; p-toluenesulfonic
30
Figure 3. Decomposition of 2-hydroxycyclohexylp-toluenesulfonate in paint film cured at: (A) 120 "c; (0)130 O C ; (0) 140 "c.
temp, "C
2
IO 15 20 25 B A K E TIME (min.)
salt b 0.06 0.09 0.15 0.30
0.40 0.41 8
PTSA-isopropylamine
acid could not be detected by this procedure. Fraction calculations assume unit relative molar response factors (Figure 2); the products are shown in Table 111. Decomposition of Blocked Catalysts in Paint Films. Since it is not feasible to titrate the acid generated during baking of a coating film, measurements of the actual rates of cross-linking were used to infer the degree of deblocking of the sulfonate catalysts to yield p-toluenesulfonic acid. The extent of cross-linking was followed by using infrared spectroscopy to measure the disappearance of the acrylic hydroxy band after baking for a given time (20 min unless otherwise noted) at a given temperature (Bauer and Dickie, 1980a,b). The extent of cross-linking can be related to a reaction rate which is proportional to catalyst concentration (Bauer and Budde, 1983),assuming that the reaction is catalyzed only by p-toluenesulfonic acid, average degree of unblocking after baking at temperature T for time t , A ( t , T ) ,can be determined by comparing the measured reaction rate for the sulfonate catalyst (Cat-X) with that previously determined for p-toluenesulfonic acid (PTSA).
The results are shown in Table IV and Figure 3.
Results and Discussion In the synthesis of blocked catalyst B, p-toluenesulfonic acid reacts with the epoxy group of cyclohexane oxide to product 2-hydroxycyclohexyl p-toluenesulfonate 1 (eq 1). For the cross-linking of hydroxy-melamine compositions, the hydroxy groups condense with the alkoxymethylmelamine groups to produce O-CH2-N linkages (eq 2). The
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, NO. 3, 1983
443
sulfonate do show a better cure (Tables I and 11), and hence a higher rate of decomposition, than n-butyl p toluenesulfonate; however, its rate of decomposition is still slow as seen from the cure response comparison with p toluenesulfonic acid (Tables I and 11). 2-Hydroxycyclohexyl p-toluenesulfonate 1 (Cat-B) was then selected for study as a model compound. This sulfonate has the hydroxy and tosylate group trans to each other as shown below (Roberts, 1968).
0
1
Ts =CH3-[---
0
OH
1 \N'
I
N A N FOC".rAN@(N/
\
(2)
acid catalyst needed for this condensation may, in principle, be obtained by the decomposition of alkyl p toluenesulfonates (tosylates) included in the paint composition. Alkyl tosylatss may thermally decompose through @-eliminationto produce p-toluenesulfonic acid according to eq 3. The elimination reaction may be asn
0
CHz=CHR
(3)
sisted by nucleophyllic species, such as melamines, present in the paint composition. Since there is a high concentration of hydroxy groups in the paint composition, part of the acid formation may take place through substitution reactions. 0 C H 3 0 " , . R
0
t R'OH
-
Tosylates structurally related and configurationally similar to 1 have been described to undergo solvolysis significantly faster than the corresponding cis-tosylates and substituted and unsubstituted alkyl tosylates (Gould, 1959). In addition to solvolysis with hydroxy moieties, tosylate 1 (Cat-B) undergoes thermal decomposition in nonreactive solvents (Figure 1) to generate p-toluenesulfonic acid. However, in half an hour at 140 OC,the amount of acid produced is only 8 to 9% of the theoretical amount. When 2-ethyl-1,3-hexanediol is included, the decomposition to generate p-toluenesulfonicacid is more than 80% complete in half an hour (Figure 1). In the first 10 min, the decQmposition is more than 50% as seen from the amount of the acid produced (Figure 1). This probably is due to displacement of the tosylate by hydroxy moieties (eq 4). The extent of decomposition of blocked catalysts in paint films can be estimated by comparing the amounts of blocked catalyst with the amount of PTSA necessary to achieve good solvent resistance. As shown in Tables I and 11, the average degree of decomposition after 20 min at 140 "C appears to range from 10 to 20% for Cat-A and 30 to 60% for Cat-B. The higher degree of decomposition of Cat-B is in keeping with the more rapid solvolysis of tosylates trans to hydroxy group relative to other tosylates. Both catalysts unblock more efficiently in the oligourethane coating than in the hydroxy polymer coating. This probably is due to the lower molecular weight and higher hydroxy concentration of oligourethane than that of the acrylic polymer. A more precise determination of the decomposition kinetics can be obtained from the infrared measurements of *the . ..extent . . of cross-linking. m1. .
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983
Cat-A and Cat-B (Tables I and 11). No difference in paint stability is observed in short term tests between Cat-A and Cat-B despite the higher activation energy of Cat-B. Paints formulated with Cat-A or Cat-B display superior paint stability to paints formulated at comparable cure response with PTSA, PTSA-amino salt, and Nacure X49-110. The poor stability of paints formulated with PTSA-amino salt is surprising in view of the fact that the activation energy for decomposition of the salt in paint films is similar to that for Cat-A. The acidity of the amine salt may be higher in the paint solution than in the dried paint film leading to a more rapid reaction in the paint solution than in the paint film. The kinetics of the deblocking of Cat-B in the coating was studied in more detail. Plots of A(t,!?') are shown in Figure 3 for three different temperatures. It can be seen that the degree of decomposition increases rapidly to a plateau value. This behavior can be compared to that for the decomposition of Cat-B in different solvents shown in Figure 1. The kinetics of deblocking in the coating seem to fall between that which occurs in a hydroxy containing solvent and that which occurs in a nonhydroxy solvent. It should be noted that during the cure of this coating, nearly all of the hydroxy functionality on the acrylic polymer is consumed. This may account for the observed deblocking kinetics in the coating. Infrared spectroscopy, potentiometric titrations (Figure l),and paint cure and stability evaluation (Tables 1and 11) determine the amount of p-toluenesulfonic acid produced in various solvents and paint compositions. A number of volatile organic products were also identified resulting from the thermal decomposition of pure 2hydroxycyclohexyl p-toluenesulfonate (Cat-B) Table 111). Separation of the products with gas chromatography is shown in Figure 2. The major constituent, cyclopentane carboxaldehyde,may result from the cation rearrangement as shown below. CHO
Cat-B
The aldehyde may also be produced by complete reversal of Cat-B to the epoxy and the acid and the protonation of the epoxy to generate the desired intermediate shown above. Cyclohexanone probably results from the thermal @elimination of the starting compound
6
H-%-royj]i s 0
The other products listed in Table I11 may result from a number of thermal decompositions, oxidations, reductions, and molecular rearrangements of blocked catalyst B and its products. Conclusions Alkyl p-toluenesulfonates decompose at elevated temperature to generate p-toluenesulfonic acid and thus catalyze the cure of hydroxy/melamine coatings. It is found that these catalysts do not decompose significantly at low temperatures (60 "C) and that paints formulated with these catalysts exhibit paint viscosity stability superior to paints formulated with commercially available blocked acid catalysts such as amine salts of sulfonic acids. The latent catalyst which had the best combination of stability at low temperature and rapid decomposition at high temperature was 2-hydroxycyclohexylp-toluenesulfonate. This catalyst decomposes slowly in nonreactive solvents at 140 "C. The rate of decomposition is accelerated in hydroxy solvents and in hydroxy-melamine coatings, indicating that solvolysis is an important mechanism in the decomposition of these adducts. Registry No. 1,15051-90-8;hydroxyethyl acrylate-isobutyl methacrylate-styrene terpolymer, 68541-33-3;2-ethyl-1,3-hexanediol-isophoronediisocyanate copolymer, 86129-56-8; 2hydroxypropyl p-toluenesulfonate, 54619-31-7; butyl p-toluenesulfonate, 778-28-9;benzene, 71-43-2;cyclohexene, 110-83-8;cyclohexadiene, 29797-09-9; toluene, 108-88-3; cyclopentanecarboxaldehyde, 872-53-7; cyclopentylmethanol, 3637-61-4; cyclohexanone, 108-94-1;cyclopentyltoluene, 86129-55-7.
Literature Cited Blank, W. J. J. Coat. Technd. 1979, 57(656), 61. Bauer, D. R.; Budde, G. F. J. App. Polym. Sci. 1983, 2 0 , 253. Bauer, D. R.: Dickie, R. A. J. fo/ym. Sci. folym. fhys. Ed. 1980a, 78. 1997. Bauer, D. R.: Dickie, R. A. J. folym. Sci., folym. fhys. Ed. I980b, 18, 2015. Chattha, M. S. (io Ford Motor Co.). U.S. Patent 4237241, 1980. Chattha. M. S.(to Ford Motor Co.), U S . Patent 4281 075, 1981a. Chattha, M. S. (to Ford Motor Co.), U.S. Patent 4307208, 1981b. Chattha, M. S.; van &ne, H. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 437. Foxly, P.; Short, W. F. J. Chem. SOC.1947, 382. Gould, E. S."Mechanism and Structure in Organic Chemistry"; HOC, Rinehart, and Winston: New York, 1959; p 575. Roberts, D. R. J. Org. Chem. 1988, 6 8 , 118.
Received for review November 22, 1982 Revised manuscript received March 7 , 1983 Accepted March 28, 1983
