monomers such as vinyl pyridine (7) were all evaluated. For curing in the range 280' to 300' F., however, tertiary amines or quaternary ammonium bases were preferred. To obtain properties of the free films of systems described, unpigmented polymer solutions were applied to tin foil stretched on glass plates. These panels were baked after which the coated tin foil was detached and cut into strips. The tin foil was then dissolved away from the polymer films by floating on a bath of mercury. Stress-strain curves were obtained for the films which were about 1-mil thick. Figure 2 is a typical curve for a copolymer containing vinyltoluene, ethyl acrylate, and acrylic acid
cross-linked with one equivalent of a low molecular weight epoxy resin.
Unpigmented Free-Film Properties Color Clarity Tensile strength, p.s.i. Elongation, yo Modulus, p.s.i.
Water white Excellent 11,000 5 2.7 x 105
5
% ELONGATION Figure 2. The tensile strength and per cent elongation were obtained from a stress-strain curve of a free film o f cross-linked copolymer
T o complete the discussion, lists of properties of an enamel and o i a crosslinked polymer film are given. References (1) Allenby, 0. C. W. (to Canadian Industries Ltd., Montreal, Quebec), U. S. Patent 2,662,870 (December 1953). (2) Murdock, J. D., Segall, G. H. (to Canadian Industries Ltd., Montreal, Quebec), Can. Patent 569,430 (January 1959). ( 3 ) Schecter, L., Wynstra, J., IND.ENG. CHEM.48, 86 (1956). (4) Segall, G. H., Bilton, J. A. (to Canadian Industries Ltd., Montreal, Quebec), U. S. Patent 2,604,453 (July 1952). (5) Segall, G. H., Cameron, J. L. (to Canadian Industries Ltd.? Montreal, Quebec), Zbid.,2,798,861 (July 1957). (6) Segall, G. H., Dixon, J. F. C. (to Canadian Industries Ltd., Montreal, Quebec), Zbid.: 2,604,457 (July 1952). (7) Segall, G. H., Dixon, J. F. C. ( t o Canadian Industries Ltd., Montreal, Quebec), Zbid., 2,604,464 (July 1952).
J. D. MURDOCK and G. H. SEGALL Canadian Industries Limited Central Research Laboratory McMasterville, Quebec, Canada
Acrylic Coatings Cross-linked with Amino Resins
DURING
the past decade, considerable progress has been made in the development of acrylic based resins as coating vehicles. The first of these to gain prominence were the acrylic emulsions which were introduced in the early fifties as vehicles for interior wall paint applications. In the mid fifties, organicsoluble thermoplastic acrylics were offered as vehicles for automotive finishes. With the late fifties came the introduction of several types of organic-soluble and water-soluble thermosetting acrylic resins for industrial finishes (7-4, 7, 9). This article describes the preparation of linear acrylic copolymers containing various amounts of methylolacrylamide, 6eta-hydroxyethyl methacrylate, or methacrylic acid. These monomers introduce into the linear copolymers pendant groups which are capable of reacting with aminoformaldehyde resins to yield thermosetting systems. There is also presented a comparison of the film properties of these thermosetting resins with conventional industrial finishes. An attempt is made to correlate the properties of these materials on the basis of their chemical composition and structure.
Experimental The types of monomers used in this study for copolymerization with the monomers containing the reactive pendant groups were restricted to the acrylate and methacrylate esters and the
466
styrenes since these materials readily copolymerize and in most cases are commercially available. M o n o m e r Abbreviations
HEMA = beta-hydroxyethyl methacrylate
MAM BA
= methylolacrylamide = butyl acrylate
St EtSt MeSt BMA
= styrene = ethylstyrene = methylstyrene = butyl methacrylate
Polymerization. This method was used to convert the monomers into polymers. Five hundred grams of a suitable solvent, usually a 50-50 xylene-butanol mixture or amyl acetate were introduced into a reaction flask equipped with agitator, thermometer, and condenser. The contents were equipped with agitator, thermometer, and condenser. The contents were heated to the reflux temperature whereupon 500 grams of a mixture of monomers containing 10 grams of peroxide catalyst [such as cumene hydroperoxide or 2,2-bis(tertbuty1peroxy)butanel were introduced dropwise over a period of about 1 to 2 hours while maintaining the reflux temperature. The reactants were held a t the reflux temperature for 5 to 6 hours. The resultant polymer solutions were usually clear and colorless and had viscosities ran ing from W - Z (Gardner-Hold3 a t about 507, solids depending on the solvent and catalyst employed. I n most cases, conversion of monomer to polymer was greater than 957&.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Preparation of Enamels and Films. The polymer solutions were converted into industrial white baking enamels via conventional methods with a TiOz pigment loading of 0.9 to 1 (pigmentbinder), Commercially available amino resins (butylated melamine-formaldehyde or benzoguanamine-formaldehyde condensates) were used at 10 to 40y0 levels to cross link the acrylic copolymers. The amounts for optimum film properties usually ranged between 20 to 30%. Films (about 1-mil dry) were made on bright and on bonderized steel from a spray gun and on glass with a draw blade applicator. Cure was effected by baking a t elevated temperatures. In most cases 30 minutes at 300' F. was sufficient for optimum properties. The baked films were examined for initial color and gloss, hardness, adhesion, flexibility, impact resistance, chemical resistance, and retention of color and gloss after exposure to elevated temperatures.
Results and Discussion Preliminary experiments indicated that combinations of styrene with butyl acrylate and with butyl methacrylate were the most suitable monomers to coreact with beta-hydroxyethyl methacrylate (HEMA) or with methacrylic acid. These combinations of monomers gave clear polymers which were compatible with amino resins and which possessed good film properties. A comonomer ratio of 3 moles of styrene to 1 mole of butyl methacrylate
THERMOSETTING ACRYLIC RESINS in combination with HEMA or with methacrylic acid at 25 mole % level was found to give the best over-all film properties. With methylolacrylamide, styrene as a modifying comonomer had to be avoided since it made the films too brittle to be of practical interest. A copolymer, whose composition was 30% methylolacrylamide and 7001, butyl acrylate, gave clear films whereas a similar product made from butyl methacrylate afforded hazy films. The best combination of film properties was obtained from a tripolymer containing equal portions by weight of methylolacrylamide, butyl acrylate, and butyl methacrylate. With the methylolacrylamide copolymers, amino resins were not required to effect cross linking. In fact, the addition of amino resins made the cured films too brittle to be considered as good industrial coatings. The best of the above acrylic polymers were compared as white industrial baking enamels. I n addition, a methacrylic acid analog of the Hema copolymer was also tested. A premium grade nonoxidizing alkyd-amino blend and an epoxy-urea resin blend were included as controls; the former is noted for its ability to maintain color and gloss and the latter for its ability to resist chemicals and detergents. Both the HEMA and the methacrylic acid copolymers were cured with a butylated benzoguanamine-formaldehyde resin. The methylolacrylamide copolymer required 2% phosphoric acid to catalyze cure a t 300" F. No amino resin was included since amino resins had no beneficial effect on the film properties of methylolacrylamide copolymers. The comparison which is listed in Table I revealed that: 0 The HEMA copolymers were far superior to the other acrylics, to the epoxy-urea blend, and to the alkydamino systems in retention of color and gloss when overbaked. This system also had the best initial gloss. The acrylics based on methacrylic acid and methylolacrylamide were similar to the alkyd-amino in color retention and slightly better in gloss retention after overbake. The epoxy-urea blend was by far the poorest in color retention. 0 The acrylics were similar to the alkyd-amino in knife scratch (adhesion) and flexibility, and like the alkyd-amino were much poorer than epoxy-urea in these properties. e I n chemical resistance, the HEMA system was considerably better than the other acrylics and the alkyd-amino and approached the performance of the epoxy-urea. The methacrylic acid copolymer showed excellent resistance to aqueous acetic acid but like the alkyd-amino and the methylolacrylamide
Table 1.
Comparison of Film Properties of Acrylic Resins with Alkyd-Amino and Epoxy-Urea Blends" Methylol Acrylamideb
Vehicle composition Copolymer or resin, yo Amino resin, % Film properties Color or yellowness factor Initial After 4 hrs. at 425' F. Gloss (60') Initial After 4 hrs. at 425O F. Knife scratch Front impact, in./lb. to fail Flexibility, '/*-in. mandrel Sward hardness Resistance to 5% NaOH, days to fail 50% HOAc, hrs. to fail Butyl acetate, hrs. to fail Rinso at 165' F., days to fail
Methacrylic HEMAC Acidd
EpoxyUrea'
AlkydAmino
100 0
70 30f
70 30
70 30f
-0.3 15.2
-2.5 2.8
-2.5 14.3g
+3.1 55
-1.3 16.7
85 74 7
90 708
86 57
12 0
100 95 7 2 0
50
60
5 0 0 56
>28 10 50
96 60 6 2 0 40
2-7
>21 48-173 >8 ll-15h
1-2 >I73
7
1
7 1-2
70 30f
10
>21
...
2-7 1-4
8
25
4-7
a All films baked a t 300' F. for 30 min. unless noted otherwise. Ratings: 10 = best or no failure; 0 = complete failure. Composition = 33 MAM/33.5 BA/33.5 BMA, wt. basis 01 42.2 MAM/30.4 BA/27.4 BMA mole basis. E Composition = 25 HEMA/56.3 St/18.7 BMA, mole basis. Composition = 25 methacrylic acid/56.3 St/18.7 BMA, mole basis. a Baked at 400° F. for 20 minutes. f Butylated benroguanamine-formaldehyde resin. 0 Overbake = 15 min. a t 500' F.; other experiments have shown this overbake to be approximately equivalent to 4 hours at 425' F. * Original bake of 20 min. at 400' F. gave a rating of 21 days.
copolymer it was readily attacked by aqueous alkali. ranged from 40 to Go in Sward hardness. Having shown that HEMA containing copolymers made better industrial baking enamels than the methylolacrylamide or the methacrylic acid containing copolymers, it was of interest to determine to what extent other constituents (styrene and butyl methacrylate) used in the HEMA formulation contributed to the performance of this system. Several variations wherein the styrene was replaced with methylstyrene or ethylstyrene and the butyl methacrylate was replaced with butyl acrylate were compared as white industrial baking enamels. Two other variations, an all butyl acrylateHEMA copolymer and an all butyl methacrylate-HEMA copolymer were included in this comparison. A premium grade saturated oil modified alkyd was also included as a reference point. Compositions and the results for color and gloss retentive properties after exposure to elevated temperatures are listed in Table 11. The data clearly dhow that copolymers based on butyl methacrylate and styrene changed the least with respect to color and gloss after exposure to elevated temperatures. The modifier which gave the greatest change was ethylstyrene followed by butyl acrylate. I n resistance to acetic acid, caustic, and soap solutions, styrene and substituted styrene modification was mark-
edly more resistant than the butyl acrylate modification. The butyl methacrylate was intermediate in this respect. I n flexibilitv and imDact resistance the reverse order was observed butyl acrylate > butyl methacrylate > styrene, methyl- and ethylstyrene. Our observed results are qualitatively consistent with what would be predicted on the basis of the reported behavior of model compounds representative of the structure of the polymers and of the behavior of homopolymers of the individual monomers. For example, resistance of the copolymers to acid, alkali,
Table 11. Changes in Color and Gloss of Acrylic Copolymers after Four Hours Exposure at 425" F." Lossin %of Color, Original Composition* A Yc Gloss
1
St-BA-HEMA 8.6 St-BMA-HEMA 3.7 MeSt-BMA-HEMA 5.8 EtSt-BMA-HEMA I 13.1 [50/22.5/27.5] J BA-HEMA 8.6 BMA-HEMA )[70/301d 1.9 Premium alkyd 18 " Initial bake, 30 min. at 300' F.
86 92 89 75 77 87 62 All com-
positions are on a weight basis. All compositions also contained a butylated benzoguanamine resin : weight ratio of polymer to amino resin i b 70/30. AY = YfInni(amber-blue) X 100. Ylnltlairwhere Y = green Same as (b) except weight ratio is 65/35
VOL. 53,
NO. 6
JUNE 1961
467
created as a result of cross linking. It deals only with the linkages between the cross links. This is reasonable since the amino resin and the amount of HEMA in the copolymer were held constant in those examples where comparative data were obtained. From Table 11, the amount of discoloration of the styrene copolymers increased in the order styrene < methylstyrene < ethylstyrene. If we ignore the backbone linkages of the polymer, the observed order is consistent with the reported relative rates of 1 and 7.9 for peroxy radical atrack on toluene and ethylbenzene, respectively ( 6 ) . From the similarity in structures between the styrene polymer backbone and cumene, which is reported to be attacked by a peroxy radical 13.3 times faster than toluene ( S ) , one would expect styrene polymer to have relatively poor oxidative stability. However, Hahn has reported polystyrene to be fairly stable to oxygen at 100' C. in contrast to poly(fi-'isopropylstyrene) which is readily oxidized at the isopropyl groups under these conditions ( 5 ) . One concludes, therefore, CH3
and soap or detergent solutions are in the expected order: styrene > butyl methacrylate > butyl acrylate. Obviously, styrene copolymers, having the least number of hydrolyzable groups, should have the better resistance. The greater resistance of the methacrylate copolymers in comparison with the acrylates is most likely attributable to the lower degree of accessibility around the hydrolyzable ester groups due to steric hindrance. Retention of color after exposure to overbake is also in the expected order: butyl methacrylate> styrene > methylstyrene > butyl acrylate > ethylstyrene. This order is based on the assumption that development of color is the result of thermal oxidative degradation. Degradation initiated by ultraviolet light naturally can be excluded since overbake is done in the dark. The rate of oxidation and hence color, and possibly gloss degradation is probably related to the ease of removing a hydrogen atom by a peroxy radical from a carbon atom activated by inductive and resonance effects. Once the polymer radical forms, it would rapidly react with oxygen to form a peroxy radical. The latter would rapidly abstract a hydrogen from polymer to form a hydroperoxide and a new radical. This chain reaction process is repeated. The chain reaction is maintained by thermal decomposition of the hydroperoxides. The initiating radical might be the result of homolytic scission of a weak segment in the polymer chain resulting from cross termination during the polymerization. A schematic presentation of this proposed degradation process is as follows (below). The preceding treatment ignores any participation in the degradation processes of the amino resin and the bonds
that unlike the 6.-C
I I
e
CH, cal derived from the styrene backbone is probably not stabilized by resonance due to steric hinderance by the adjacent bulky benzylidene groups which prevent planarity. The excellent color retentive properties of the butyl methacrylate copolymer is expected since the copolymer does not contain easily oxidizable hydrogens on carbon atoms adjacent to an activating group. Butyl acrylate polymers would be expected to be poorer than styrene polymers since the former can form
A
I
0 2
c---
L-+
+
-wCHOOH
I
polymer
-.-C-..-
COOBu
I
OOH
I
WM.JCWM-
I
X
X J.
polymer
+
--..C*-
0 2
-+
I
etc.
X
A l Colored decomposition products via radical intermediates
X = activating group (CsH6 or COOBu)
468
(
M.J.-C-CH~
equivalent to
radical
COOMe I formed by hydrogen abstraction from the backbone of the acrylate polymer has been found to
I
COOBu be ten times more reactive to ethyl benzene than a styrene polymer radical
-..-CHP. In
other words, PCHCH3
being more stable than wwC-CH3 COOMe I should be the easiest to form. Acknowledgment
Technical assistance in the preparation of some of the copolymers was given by R. Saxon and W. F. Hart. Enamels were prepared and evaluated by A. P. Baruch, W. L. Hensley, S. D. Kaminowski, and F. R. Spencer. Literature Cited
m-wCHOO* COOBu
radical, the radi-
radicals which are stabilized by resonance. This is supported by a publication which describes the ease with which polyethyl acrylate oxidizes and degrades when exposed to air or oxygen at 100" C. ( 8 ) . (It will be recalled that polystyrene is reported to be quite stable under similar conditions.) The better resistance to thermal discoloration sholz-n by the butyl acrylate copolymer in comparison with the ethylstyrene copolymer suggests the backbone of the former to be more resistant to peroxy radical attack than the side chain ethyl groups of the latter. The relative ease of hydrogen abstraction from these two positions can be estimated from chain transfer studies where the methacrylate polymer radical
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
X
(1) Berlin, A. A., others, U.S.S.R. Patent 123,311 (Oct. 23, 1959). (2) Chapin, E. C., Smith, R. F. ( t o Monsanto Chemical C o . ) , E. S. /Patent 2,899,404 (Aug. 11, 1959). (3) Daniel, J. H., Jr. (to American CyanamldCo.),Ibid.,2,906,724(Sept.29,1959). (4) Frazier, C., Cadwell. L. E., Ibtd., 2,681,897 (June 22, 1954). (5) Hahn, W., Lechtenbohmer, H , Makromol. Chem. 16, 50 (1955). (6) Russell, G. A., J . Am. Chem. SOC.78, 1047 (1956). (7) Sanders, P. F. (to E. I. du Pont de Nemours & Co.), U. S. Patents 2,787,603 (April 2, 1957) and 2,866,763 (Dec. 30, 1958). (8) Steele, R., Jacobs, H., J . Afifil. Polymer Sci. I, No. 4,86 (1959). (9) Vogel, H.A., Bittle, H. G. (to Pittsburgh Plate Glass Co.), Ibid. 2,870,116 (Jan. 20, 1959).
JOHN C. PETROPOULOS, CHARLES FRAZIER, and LEONARD E. CADWELL Stamford Research Laboratories American Cyanamid Co. Stamford, Conn.
