Copolymerization of Alkyd Silicones for Coatings C. ROBERT HILESl, BRAGE GOLDING, AND R. NORRIS SHREVE Purdue University, Lafayette, Ind.
A
LKYD-SILICONE coatings are comparatively new, being first mentioned in the literature in 1947 (1, 18, $0). Patterson (18, 19) reviewed the properties of alkyd-silicones and reported that they are intermediate between alkyd-melamine and pure silicone enamels in heat and alkali resistance, adhesion, hardness, and toughness. He also reported that varnishes made by chemical cocondensation of alkyds and silicones are usually superior to those made from cold-blend mixtures of the two. Practically the only details of the synthesis of alkyd-silicone varnishes which appeared in the literature until early in 1952 were those disclosed by Bowman and Evans ( 1 )in their British patent. They heated oil-modified alkyd resins with organosilanols in a solvent reflux process. The several patents issued since the early part of 1952 prepare alkyd-silicone varnishes either by the reaction of an alkyd resin having excess hydroxyl groups with an organoalkoxysilane ( 2 , 3, 16) or by reacting the silane with glycerol and then reacting this intermediate with an acidic compound or an acidic ester (6-9, I$, I S ) . The one exception found was in the patent of Millar ( 1 7 ) who used a process similar to that of Bowman and Evans. Among the recent reviews of the properties of alkyd-silicones are those of Hedlund ( I O , 11), Kress and Hoppens (IC), McGregor ( 1 6 ) ,and Pattison (2I). EXPERIMENTAL PROCEDURE
The varnishes were prepared in ordinary round-bottomed, three-necked flasks heated with an electric mantle. The reactions were run a t 200" C. under a carbon dioxide atmosphere, and with agitation. The resin apparatus is shown in Figure 1. The standard method for making the varnishes was to weigh the desired quantities of dibasic acid, fatty acid, and glycerol into the flask and heat a t the maximum rate to 200" C. Samples of the alkyd reactants were withdrawn a t 1/2-hour intervals t o determine the acid number. When the acid number dropped from approximately 300 a t the beginning of the reaction to less than 10, the organoalkoxysilane was added. The two-phase mixture was then checked for clarity a t 5-minute intervals. A clear homogeneous cold pill was usually obtained after 15 minutes. The reaction was continued a t 200" C. until gelation was imminent. This was determined by the cessation of cavitation around the stirrer. At this point the reaction was stopped by reducing the resin with high flash naphtha to approximately 50% solids. After cooling, the solids content of the varnish was adjusted t o 50%. A simple enamel formulation of varnish and rutile titanium dioxide in the ratio of 1:1 on a solids basis was used. Enamels were satisfactorily prepared both on ball mills and roller mills. The majority of enamels was made on a laboratory three-roll mill because of the versatility and speed of the mill. High flash naphtha was added to the enamel t o obtain a viscosity of 30 seconds as measured with a No. 4 Ford cup a t 80" F. The Gnished enamel was then centrifuged in a cup centrifuge a t 2500 r.p.m. to remove any oversize pigment particles. The finished enamels were sprayed on S.A.E. 1010, 20-gage 1
Present address, Lilly Varnish Co., Indianapolis, Ind.
cold-rolled steel panels and plate glass panels. The steel panels were degreased and treated with metal Prep, a commercial phosphate solution for preparing steel surfaces for enameling. The glass plates were washed with acetone. The enamels were sprayed onto the panels to obtain a dry film thickness of 1.9 =k 0.1 mils as measured with a magnetic filmthickness gage. Commercial film thicknesses range from 1 to more than 2 mils according to the desired amount of hiding. This thickness was chosen for this work in order to get optimum gloss retention and excellent hiding (see Figure 3). The films were cured for 1/2 hour a t 400 " F., and this bake was considered the initial point in the testing. The enamels were tested for effect of film thickness on enamel properties and for gloss and color retention, craze life, toluene resistance, alkali resistance, impact resistance, flexibility surface hardness, adhesion, and general appearance. FORMULATION NOMENCLATURE
I n discussing experimental results, the formulation nomenclature used should be kept in mind. It was assumed that the resins were composed of glyceryl siloxane and alkyd resin. The composition of the resins was then defined in terms of the silicone content (the percentage by weight of glyceryl organosiloxane in the totally reacted resin) and the oil length of the alkyd (the percentage by weight of fatty acid triglyceride in the alkyd portion). This method proved much more useful for correlating the results than did an equivalency basis. An additional benefit was that it is an adaptation of alkyd terminology and is therefore familiar to the coatings industry. A 350-gram batch of 50% silicone-50 oil length reacted resin would have the formulation: Grams Equivalentsa 0.41 Phthalic anhydride 68 0.I8 Lauric acid 82 0.41 Phenylethoxypolysiloxane I89 1.00 Glycerol 69 Formula given on arbitrary basis of one equivalent of glycerol.
This formulation gives 87.5 grams of glycerol phthalate, 87.5 grams of glycerol trilaurate, and 175 grams of glycerol phenylpolysiloxane. Theoretically, 15.5 grams of water and 42.5 grams of ethanol should be split out by condensation. These calculations are based on phenylethoxypolysiloxane having an equivalent weight of 204. This equivalent weight was based on the ethoxy content of the silicone-in this case an ethoxy to silicon ratio of 0.80. It was assumed that all the ethoxy groups were available for reaction with glycerol. Other resins were formulated by determining the desired amounts of the reacted glycerol phthalate, glycerol trilaurate, and glycerol phenylpolysiloxane, and calculating the required amounts of reactants from the chemical equations of the reactions. DEVELOPMENT O F VARNISH PROCEDURE
Several attempts were made to prepare varnishes according to the method of Bowman and Evans ( I ) . This consists of heating, by a solvent process, an oil-modified alkyd resin having an acid 1418
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
number of approximately 40 with organosilanols. Clear varnishes were obtained using as the silicone intermediates phenyl-, amyl-, nonyl-, and ethyltrichlorosilanes hydrolyzed to the silanols. However, enamels made from these varnishes had poor gloss and only fair color retention. It is believed that little copolymerization was obtained because of the great tendency of t h e silanols to condense t o silicones and their small tendency to react with the alkyd resin. Organotriethoxysilanes were also used as the silicone intermediate. These compounds had more of a tendency to react with excess alcohol in the alkyd portion but were not very satisfactory because they were relatively volatile, and i t was difficult to remove the water and ethanol of condensation without losing some of the silicone. The resulting products were not too satisfactory. For these reasons organoethoxypolysiloxanes were used in most of the work done in this investigation. These were formed by partially hydrolyzing and condensing organotriethoxysilanes to form low molecular weight silicone polymers containing residual ethoxy groups capable of reacting with the hydroxyl groups of alkyd resins. These compounds are nonvolatile, require less excess glycerol based on organic acid content, and when reacted with alkyd resins connect them to stable silicone nuclei. Most of the silicone intermediates used had ethoxy-to-silicon ratios of 0.8.
1419
total cooking time of No. 62 was 30 minutes, and the alkyd reaction of No. 6 was 135 minutes with the reaction continuing for 15 minutes after addition of phenylethoxypolysiloxane. Enamels were prepared from these varnishes and tested. T h e results are given in Table I.
EFFECT O F ORDER O F ADDITION
Homogeneous varnishes were obtained either by cooking all the alkyd and silicone ingredients together throughout the reaction or b y forming the alkyd resin first and then reacting this with the silicone. Distinct differences in properties resulted from the two methods of cooking. For example, varnishes having the following formulation were prepared b y both methods: Grams Phthalic anhydride Lauric acid Phenylethoxypolysiloxane Glycerol
91.5
49.5 193.0 76.0
Equivalents 0.50 0.10 0.38 1.00
In one case, the phthalic anhydride, lauric acid, phenylethoxypolysiloxane, and glycerol were loaded into the reaction flask at room temperature, and heated to 200" C. under agitation and an inert atmosphere. This temperature was maintained until gelation was imminent. The reaction was then stopped by adding high flash naphtha to the resin. The varnish had a color of 2 (Gardner color standards of 1933), a viscosity of A (GardnerH d t ) , and a n acid number of 75. I n the second case, the phthalic anhydride, lauric acid, and glycerol were placed in the reaction flask a t room temperature, heated to 200 C. under agitation and a n inert atmosphere, and held at this temperature until the acid number had decreased to 11. A t this time the phenylethoxypolysiloxane was added, 200" C. was regained, and the batch was held at this temperature until gelation appeared t o be imminent. The resin was then thinned with high flash na htha. This resin had a color of 6, a viscosity of C, and a n acianumber of 7. These varnishes were numbered 62 and 6, respectively. The Table I. Effect of Order of Addition on Enamel Properties Baked 60 Hr. Hardnessf at 400" F. Impact Alkali at 400' F. Craze YellowResistResist1/c 16 Enamel Life5 Glossa nessc anced ancee hr. hr. V-6 150 92 0.145 13 120 37 53 V-62 60 37 0.125 7 10 48 42 Heat life expressed as number of hours that an enamel film can be baked a t 40O0 F. before film integrity is destroyed and crazing or cracking occurs. b Expressed in reflection units based on mirror having a reflection of 1000; readings were taken with a gloss meter having a 60' reflectance angle. Expressed as a decimal on the basis of magnesium carbonate having a yellowness of 0 and yellow 1 . readings were taken with a reflectometer using tristimulus colorimetry: yeliowness calculated from: Yellowness = A B/ n -. d Expressed in inch-pounds and represents distance a 1-pound weight can fall on film without ruoturinz it. Expressed as number of Linutes film can withstand 5% aqueous sodium hydroxide without film failure due to cracking of film or chemical attack. f Expressed as per cent surface hardness of film with respect to plate glass as a standard, readings taken with a Sward hardness rocker.
-
Figure 1. Resin apparatus
The difference in alkali resistance is to be expected from the acid numbers of the varnishes. The large difference in gloss is typical of enamels prepared from varnishes cooked b y the two procedures. Enamels prepared from varnishes cooked according to the technique used for varnish No. 62 always chalked very badly. The tendency was much less pronounced when the alkyd resin was formed first and then reacted with the organoakoxypolysiloxane. Apparently the varnish coating the surface layer of pigment particles decomposes, leaving a chalklike layer of dust on the surface of the enamel. One explanation for the increased decomposition of varnishes having high acid numbers is that the phthalic half ester can easily revert to the alcohol and phthalic anhydride under the influence of heat, while the fully esterified phthalate does not depolymerize as easily. Another factor contributing to the poor gloss retention is that varnish No. 62 is probably not copolymerized to the extent that varnish No. 6 is copolymerized. The water of esterification can hydrolyze the ethoxy groups of the siloxane to silanols which tend to condense to the silicone structure. As will be shown later, mixtures of alkyds and silicones tend to be inferior in gloss to copolymers. The effect of temperature was investigated by cooking varnishes a t 190°, 200°, and 230" C. Resins cooked a t 190" C. tended to be darker than those cooked a t the other temperatures because of the long reaction time-200 minutes at 190' C. compared to 118 and 36 minutes a t 200" C. and 230" C., respectively. The silicone reaction is so fast at 230' C. that i t is difficult t o control. Therefore, the reactions were run a t 200' C. with the temperature manually controlled to f 3 " C. As a result of recent work, i t is believed that the best technique is to cook the alkyd a t approximately 200' C. and reduce the temperature to approximately 165" C. for the silicone reaction. It is possible to control the r e action's end point by viscosity measurements with this technique. EVIDENCE O F COPOLYMERIZATION
T h e statement that copolymerization occurred in the varnish reactions is based on the following evidence:
When the organoalkoxysilane is first added to the alkyd resin in the reaction vessel, the mixture is incompatible and samplee of the mixture are definitely two phase. As the reaction proceeds, the reaction mass becomes progressive1 clearer until finally cold-pill samples are completely clear a n J homogeneous. Either solubility is increasing as reaction proceeds in a highly functional reaction mass or the resin is becoming homogeneous because of copolymerization. The latter is much more robable. Another line of evidence is based on Flory's theory o f gelation ( 4 ) . This theory states that gelation occurs when a rigid lattice work formed by primary valence bonds extends throughout the reaction mass, immobilizing the mass and causing a large increase in viscosity. If an alkyd resin contains sufficient monobasic acid and excess glycerol to form only linear polymers, gelation will not occur. If organoalkoxysiloxane is added to this mass and gelation occurs, either the silicone itself or a eo olymer of the alkyd and the silicone is responsible for gelation. I r a copolymer is formed, it is reasonable to expect that the more highly the alkyd resin is reacted before addition of the silicone, the faster gelation will occur if copolymerization is taking place. Table I1 gives the reaction time for three different varnishes. The more highly reacted the alkyd resin was before addition of the silicone, the shorter the time for gelation. This strongly suggests that copolymerization was occurring. Table 11.
resin was 10, and the rate of removal of water was zero. It is probable that the alkyd resin mass consisted essentially of alkyd esters and unreacted glycerol hydroxyl groups. Phenylethoxysiloxane was added to this system which decreased the temperature to 160' C. As the temperature began to rise, ethanol split out of the reaction mass a t an increasing rate. The only explanation for this evolution of ethanol is that i t was split out by the reaction of phenylethoxysiloxane and the unreacted hydroxyl group? in the alkyd resin. Therefore, copolymerization was occurring.
Gelation Time as Function of Time of Alkyd Reaction before Silicone
Varnish Formulations I1 I EquivaEquivaGrams lents Grams lents Phthalic anhydride 95.0 0.51 ... 8O:O 0155 Adipic acid Lauric acid 49.5 o:io ... 8Q:O 0126 Capric acid Phenylethoxypolysiloxane 69.0 0.19 193.0 0.39 76.5 1.00 Glycerol 62.0 1.00
..
Varnish
Vol. 47, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
1420
Cookin Time before &cone Addition, Min.
Gelation Time after Silicone Addition, Min. 292 87 6 30 15 49 27
I11 Grams 70.0
Equivalents 0.44
86.0
o:io ..
134.0 67.0
0.36
... ...
1.00
Total Cooking Time, Min. 292 114 126 30 150 49 87
20
40
60
80
COOKING TIME
Figure 2.
100
120
- MINUTES
140
160
Condensate us. cooking time
DECOMPOSITION OF VARNISHES
One method for determining the heat stability of a resin is to measure its per cent weight loss when heated a t a certain temperaA third line of evidence is a study of the possible reactions. ture. The weight losses of the resins prepared in this work are Phenylethoxysiloxane was heated by itself and with dioctyl shown in Table 111. The values reported are the average of three phthalate without evolution of ethanol or evidence of further determinations made b y spraying the resin solutions on standard polymerization of the siloxane. Therefore i t is itself stable and stable in the presence of esters. However, when heated with panels a t 1-mil dry film thickness and measuring the weight % glycerol, ethanol rapidly split out, and gelation occurred. Figure loss versus time baked a t 450' F. 2 shows the condensate collected versus cooking time for an The weight loss depends primarily on the silicone content of the alkyd-silicone resin. The alkyd portion of the resin was cooked varnishes. This is best illustrated b y comparing the weight loss for 108 minutes at 200' C. At this point, the acid number of the of varnishes 810, 14, 15, 1, and the Dow Corninn silicone mixture of 40% De802 and 60% Table 111. Weight Loss and Properties of Varnishes DC804. These varnishes con(Varnishes prepared from phthalic anhydride, lauric acid, glycerol, and phenylethoxypolysilicone) tain 0, 25, 35, 50, and 1 0 0 ~ o Weight Loss (450' FJ, 70 color Viscosity silicone, respectively. Other Varnish Silicone, Oil After After (Gardner (GardnerAcid factors that determine the No. % Length '/Phr. 20 hr. 1933) Holt) NO. weight loss are the polyol s10a 0 30 12.0 64.7 .. .. 14 25 50 12.2 46.9 1 A' ' 15 (ethylene glycol varnishes lost 16 35 50 11.3 40.1 3 B9 far more weight than equiva1 50 50 9.9 22.5 2 A- 1 7 26 60 50 A-1 lent glycerol ones) and whether 100 .. i:e 5: 0 .1. .. .9. the alkyd-silicone is a mixture 6 50 30 9.2 19.9 6 C 7 1 50 50 9.9 22.5 2 A-1 7 or c o p o l y m e r . Varnishes 5 50 70 11.2 20.9 9 A-1 5 13 and 20 compared t o varnish 16C 50 50 10.9 30.7 6 A-3 11 1 exemplify this statement. 20d 50 50 12.8 34.2 .. 13" 50 50 16.8 40.2 2 A- i '7 Apparently varnishes contain12f 50 1 A-1 7 Plaskon ST856 ..50 .. 5:s 20: 0 3 q-X 2 ing phenyl- (60) and dimethyla Commercial alkyd. (40) ethoxypolysiloxane are b Dow-Corning silicone DC802-804. not as stable as those containC Phenyl-dimethyl (60:40) ethoxypolysilicone. d Cold-blend silicone-alkyd mixture. ing phenylethoxypolysiloxane, e Ethylene glycol used as polyhydric alcohol. f Excess glycerol, 15%. as shown in varnishes 16 and
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
1421
Decomposition products of resins heated t o 475' F. were collected. The principal product collected was phthalic anhydride. The other product was a yellow oily residue. This was unsaturated and did not contain carbonyl groups. Further analysis was unsuccessful. -4small amount of water and ethanol was also collected. VARYISH PROPERTIES
The physical properties of the varnishes are given in Table 111. T h e procedure used in cooking the varnishes gives low viscosities and low acid numbers. Plaskon ST856 must b e decidedly different because of the viscosity. The colors of the varnishes were, in general, light but were darker when the oil length was longer. Varnishes with low oil lengths were slightly more viscous than the others. ENAMEL PROPERTIES
It was found that the film thickness of the enamels exerted a significant influence on the properties of the enamels. The variation in yellowing, craze life, and gloss of the enamels with film thickness is shown in Figure 3. The large variations in properties make i t necessary to control the thickness of the films of the test enamels as accurately as possible. Three panels of each enamel were coated with films 1.9 =I= 0.1 mils thick, evaluated, and the average values obtained were reported. Gloss and Gloss Retention. Gloss values were measured with a photo-volt gloss meter that measured 60" specular gloss. Practically all declines in gloss occurred during the first few hours of baking at 400" F. Enamels attained their ultimate gloss after 100 hours. Therefore, gloss values of the enamels after baking for ' / z hour and for 100 hours represent initial and final gloss. These t n o values and the change in gloss are given'in Table IV. The two alkyd-silicone cold-blend mixtures reported (V-20 and 7'-25) had very poor gloss retention. This was probably due to increased decomposition of the mixtures in comparison with the (lopolymers and to incomplete homogeneity of the resins. The gloss rentention of the enamels became worse as the silicone content was decreased and the fatty acid content increased (Figure 4). Craze Life. I n this investigation, craze life is defined as the length of time in hours that an enamel can be baked at the designated temperature without film failure by cracking, checking, crazing, or loss of film integrity in any way. Craze life is very greatly influenced b y the baking temperature. All the enamels tested had craze lives in excess of 400 hours a t 350' F. and less than 3 hours at 500" F. The craze lives of the enamels reported in Table IV were ob-
F I L M THICKNESS- MILS
Figure 3.
Influence of film thickness
tained at 400' F. because a t this temperature t h e largest spread in craze lives was obtained. The effects of the oil length and the per cent silicone in enamels on craze life are shown in Figure 5, The decrease in craze life with increase in silicone content is rather surprising at first glance. It must be emphasized that this behavior may not be usual. Phenylethoxypolysiloxane was the primary silicone intermediate used in this work. This material is brittle and thus contributes toward the brittleness of copolymers containing this silicone. -4s the silicone content increases, the resins become more brittle and tend to craze. The effect of using a more flexible silicone can be ~~
Table IV.
Properties of Enamels
(Varnishes prepared from phthalic anhydride, lauric acid, glycerol, and phenylethoxysilicone) Enamel No. 810a 14 15 1
26 6 1 5 16: 13 12:
Silicone,
% 0
25 35
50 60 50 50 50
50 50
50 50 50
011 Length 30 50 50 50 50
30 50 70 50
50 50
Gloss and Gloss Retention Baked 400" F. Change 1/z hr. 100 hr. in gloss 91 2 89 81 53 28 81 75 6 79 75 4 84 82 2 95 89 6 79 75 4 77 67 10 86 82 4 83 67 16 82 82 0 76 35 41 100 41 59 89 73 16
20 50 25f 50 Plaskon ST-856 a Commercial soybean alkyd. b Phenyl-dimethyl (60: 40) ethoxypolysilicone used as silicone intermediate. c Ethylene glycol used as alcohol. d Excess glycenol, 15%. e Cold-blend mixture. f Cold-blend mixture; G E SR-82 silicone.
Craze Life at 400' F., Hours 400 210 110 60 50 150 60 40 110 90 80 180 400 350
Color Retention (Yellowness) a t 400' F. '/z hr. 100 hr. 0 085 0 330 0 050 0 270 0.050 0 220 0 070 0 185 0 070 0 160 0 055 0 I55 0 070 0 185 0 070 0 233 0 090 0 185 0 060 0 200 0 065 0 175 0 085 0 260 0 070 0 300 0 085 0 190
hr. 14 10 9 19 34 37 19 4 Taoky Tacky 9 5 20 46 1/2
Sward Hardness 1 hr. 16 hr. 18 41 37 50 24 43 44 52 46 48 56 53 44 52 19 45 4 45 0 44 22 35 23 49 26 38 54 42
INDUSTRIAL AND ENGINEERING CHEMISTRY
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seen by comparing enamel 16 with enamel 1. Other silicone intermediates would possibly give even better results. Enamels low in silicone decompose to weak, porous films that can expand and contract without crazing on heating and cooling.
20
30
PER
50
40
60
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in
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Figure 4.
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30
40
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OIL
LENGTH
l
I
Changes in gloss after baking for 100 hours at 400" F.
Color Retention. Color retention of the enamels was measured with a reflectometer equipped with tristimulus filters. The color of the ename!s was then calculated as the degree of yellowness according to the equation Yellowness =
A - B
-x--
CT
The degree of yellowness of the enamels is given in Table IV. The influence of oil length and silicone content on the yellowness of the enamels is shown in Figure 6. Extreme yellowing was evidenced with enamels that were alkyd-silicone mixtures, had high hydrocarbon chain (either fatty acid or adipic acid) content, or contained little or no silicone. Each of these conditions would be expected to contribute to decreased thermal stability. Enamels V-25 and V-20 are cold-blend mixtures having the same composition and silicone content as V-1. The better color of the latter illustrates again the protective influence of the silicone when copolymerized with alkyd resins. Toluene Resistance. A small pool of toluene covered with a watch-glass was placed on the surface of enamel films that had been baked for 16 hours a t 400' F. The time necessary for a detectable change in the film was observed. All the alkyd-silicone enamels were in contact with toluene for 48 hours and wiped dry with a rough cloth without marring the film. The pure silicone enamel, on the other hand, dissolved after 2 hours of contact with toluene. The straight alkyd resin softened slightly after 40 hours. Alkali Resistance. The alkali test was made in the same way as the toluene test except that 5% aqueous sodium hydroxide w&9 used. Alkyd-silicone enamels that had been cured for 16 hours a t 400" F. withstood contact with 5% alkali without failure by softening, bleaching, or loss of gloss. Failure occurred b y cracking of the enamel film, and the length of time for this to OCCLU depended primarily on freedom from dust or pigment particles imbedded in the film. Failure was usually evidenced in 4 to 9 hours. There were larger differences in alkali resistance when the films were not fully cured (baked 1hour a t 350' F.), After this curing schedule, enamels failed by bleaching and loss of gloss after '/2
Vol. 47, No. 2
hour. The bleaching effect was caused by extraction of color bodies, perhaps oxidized fatty acid molecules, from the film. Impact Resistance. The impact test consisted of dropping a 1-pound weight from different heights onto the enameled panels. The result of the test, reported as inch-pounds, represent the distance in inches required to rupture the enamel film. The test was made on the back and film sides of the panels and with or without deformation of the panel. None of the alkyd-silicone films ruptured under the impact of a 1-pound weight falling 20 inches onto the film when the impact did not involve deformation of the panel. When the impact involved deformation, produced by the impact occurring over a hole in the base plate of the instrument, the test measured a combination of brittleness and adhesion. I n general, the more flexible an enamel, the better it withstood this test. Only enamels containing the 60:40 phenyl-dimethyl silicone or composed of a cold-blend mixture of alkyd and General Electric's SR-82 silicone resin solution compared favorably with Plaskon ST-856. The impact resistance of the alkyd-silicone enamels was fair, indicating brittleness and only fair adhesion. When the impact was on the back side of the enameled panel, impacts of 3 inch-pounds caused star-shaped cracks in the film. Flexibility. Flexibility was measured by bending enameled panels 180' around a 3/4-inch mandrel. Two types of failure were observed: The film would break in one or two places and lift from the panel or many small cracks appeared without attendant lifting. l i
6 0 d
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fK
$
150
I W
LL
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0
50
20
30
PER
40
50
CENT
SILICONE
OIL
Figure 5 .
60
LENGTH
Craze life at 400' F.
As the oil length of the varnishes decreased, lifting of the film decreased, and the film tended to crack in many places without lifting. The use of adipic acid and ethylene glycol produced more flexible films, and lifting did not occur. The different kinds of silicone compounds did not appreciably influence the results of this test. Surface Hardness. The hardness of the enamels was measured with a Sward hardness rocker. Alkyd-silicone enamels tended to produce very hard surfaces with ultimate hardness between 40 and 50. Changing the con-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
stituents or proportions of ingredients of the varnish did not change the ultimate hardness significantly. The very hard films were probably due to the action of baking because even alkyd enamels attained hardnesses over 40 after baking a t 400' F. One of the more important applications of hardness readings is the information they give about the curing time of the enamels. Table IV gives the hardness of the enamels after '/z hour, 1 hour, and 16 hours of baking a t 400' F. The curing time of the enamels increased as the oil length increased. (See '/*hour hardnesses of V-6, V-18, V-1, and V-5-these enamels have oil lengths of 30, 40, 50, and 70, respectively.) Fifty oil length enamels containing phenylsilicone cured in ' 1 2 hour a t 400' F., but if phenyl-dimethyl silicone was used, the curing time was increased t o 4 hours.
PER CENT
I
s*
" w J0
w
*
SILICONE
2I O20 ,101
30
40
50
60
70
20
30
40
50
60
70
I
I
I
OIL
I
I
I
LENGTH
Figure 6. Effect of per cent silicone and oil length on yellowness after baking for 100 hours at 400" F.
Table V.
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Influence of Additives on Enamel Properties
(Basic varnish formulation: V-18 50% phenylsilicone-40 oil length; baking tedp., 400' F.) Gloss YellowCraze Hard(100 ness Life ness Plasticizer Hr.) (100 Hr.) Hou& (16 Hr.) 78 0.150 110 55 None 72 0.140 250 49 107 dioctyl phthalate 235 40 75 0.170 10.3 tricresyl phosphate 74 0.155 155 54 10% thioester Dolvmer I
79 78 84
0.175 0.150 0.140
266 110 85
.. ..
..
silicone and by increasing the oil length of the varnishes. The use of copolymer silicone showed promise. It was found that General Electric silicone resin solution 81129, a silicone plasticizer, was compatible with many alkyd-silicone varnishes, and enamels containing it did have superior properties, but the storage stability of these enamels was so low that the enamels lost their superior properties in a few weeks. Several linear ethylsilicone polymers were tried, but all were incompatible with the varnishes. A series of conventional plasticizers was used with promising results. As shown in Table V, dioctyl phthalate and tricresyl phosphate almost doubled the craze life without adversely affecting the gloss and color retention. Dioctyl phthalate was probably the best of the organic plasticizers used. Influence of Pigment Content. Enamels were prepared with pigment to resin ratios of 4:6, 1:1, and 6:4 on a weight basis. The results of evaluating these enamels are given in Table V. The craze life of the enamels increased with decreasing pigmentto-resin ratio. However, these enamels had inferior color retention. Pigment ratios of the order of 4:6 are probably the best. Influence of Driers. Cobalt naphthenate, lead naphthenate, zinc hexogen (the zinc salt of 2-ethylhexanoic acid), diethylenetriamine, and n-butyl acid phosphate were used as driers or curing accelerators. The results of using these are given in Table VI. Cobalt naphthenate and n-butyl acid phosphate were of value as curing catalysts in 0.1 and 1.0% amounts, respectively.
Adhesion and General Appearance. The adhesion of alkydsilicone enamels as measured by the impact and scratch test was only fair. All the enamels tended t o be too brittle. I n general, smooth, hard, glossy enamel films were produced by alkyd-silicone enamels. Adverse surface phenomena were encountered with some of the short oil enamels. The 30 oil length50% silicone enamel (V-6) had a tendency to flood. This was probably due to the varnish formulation, not dispersion technique, because duplicate runs also flooded. Forty oil length enamels containing 60 :40 phenyl-dimethyl silicone exhibited iridescence that did not disappear on prolonged baking. Poor storage stability was encountered with pure silicone enamels and with enamels having high acid numbers. Alkydsilicone enamels prepared from varnishes having low acid numbers were stored in glass jars for 6 months at room temperature without adverse effect on enamel properties.
1. Reacting the alkyd portion of the resin to a low acid number before addition of the silicone gave products superior to those formed when all the reactants were added a t the start of the reaction. 2. Gloss and color retention varied in proportion to the silicone content of the varnish, and craze life varied inversely with the silicone content. 3. Gloss, color retention, and craze life improve with decreasing oil length, and the optimum is in the range 30 to 40. Lower oil lengths present cooking and solubility roblems. 4. The alkyd silicone enamels had good aPkali resistance, toluene resistance, and were in general hard and rather brittle.
ENAMEL ADDITIVES
CONCLUSIONS
Summary. Certain generalizations may be made about the formulation of alkyd-silicone enamels made from the ingredients used in this investigation:
The alkyd-silicone enamels prepared in this investigation were T h e evaluation of any alkyd-silicone varnish in a heat resistant deficient in craze life, too brittle, and cured too slowly to be of enamel must be based on a composite of the enamels properties. practical use. Various enamel additives were tried in attempts to eliminate one or more of these deficiencies. Table VI. Influence of Curing Catalysts Plasticizers. The relatively (Varnish formulation: V-16, 50% phenyldimethylsilicone-50 oil length: baking temp., 400' F.) short craze life of the enamels Sward Hardness -'/a hr. >/I hr. 1 hr. 2 hr. 4 hr. 16 hr Catalyst was probably due, a t least in 35 45 part,- t o the-brittleness of the None Tacky 4 Tacky 9 23 36 44 0 0 . 1 7 Cobalt naphthenate Set to touch enamels. Internal plasticiza' 0.59 Lead naphthenate Tacky Tacky 3 6 15 54 3 8 22 Craeed Tacky 0 . 5 8 Zlnc hexogen Tacky tion was attempted by using 26 46 Tacky 0 6 Tacky 1.07 Diethylene triamine 60 :40 phenyl-dimethyl silicone 1 .08 n-butyl acid phosphate Tacky Tacky 10 26 42 54 in place of the straight phenyl-
1424
INDUSTRIAL AND ENGINEERING CHEMISTRY
Craze resistance and gloss and color retention are probably the three basic properties that any enamel must possess a t the required temperature level. For any particular application, a system must pass many specialized tests. No single term or expression was found that would give a clear picture of the changes occurring in an enamel film on prolonged heating. Considerable time was spent attempting t o show that copolymerization was occurring b y a transesterification reaction between the excess hydroxyl groups of the alkyd resin and the ethoxy groups of the siloxane. Although no direct chemical proof of the reaction was devised, it is believed that the evidence pointing to copolymerization is convincing. The three results cited to show copolymerization are all consistent with the hypothesis. An added factor is the difference in properties between known alkyd and silicone mixtures and those thought to be copolymers. The poor gloss retention of the mixtures is outstanding. The mixtures also discolor excessively considering their composition. The increased craze life of mixtures of an alkyd and phenylethoxypolysiloxane over a copolymer of the same composition is quite possibly peculiar to this system where the silicone is brittle. However, in this instance the difference between mixtures and copolymers shows up in craze life. The copolymers, being chemically united, tend to become hard and brittle, and tend to crack under the heating and cooling stresses. The mixtures, on the other hand, have stable silicone polymers imbedded in an alkyd matrix which decomposes and ruins the film integrity. The resulting film is not coherent enough to craze as easily as the copolymer film. One of the conclusions reached during this project was that the alkyd portion of the copolymer contributed most to degradation of properties and presented the best opportunity for improvement. A study of heat-resistant alkyd resins has subsequently been made, and the alkyds made with 2-ethylhexanoic acid, trimethylolethane, and phthalic anhydride have much better color and gloss retention than those made with lauric acid, glycerol, and phthalic anhydride. An alkyd-silicone resin containing 50% silicone was recently made along these lines, and it has better color retention than a commercial alkyd-silicone resin having 75% silicone. However, the gloss retention was not as good as the commercial resin. Results indicate that there is still room for substantial improvement in the alkyd portion of the alkyd-dicone copolymers.
Vol. 47, No. 7
ACKNOWLEDGMENT
The authors wish to express their appreciation to the Lilly Varnish Co. which sponsored the research and furnished many of the materials used; to Linde Air Products Co. for furnishing many of the organosilicon compounds and many helpful suggestions; and to Dow-Corning Co., General Electric Co., the Plaskon Division of Libbey-Owens-Ford, and Armour and Co. which obligingly furnished materials for this research. LITERATURE CITED
(1) Bowman, A., and Evans, F. hI., Brit. Patent 583,754 (Nov. 28, 1945). (2)
British Thompson-Houston Co., Ltd., Ibid., 650,241 (Feb.
21,
1951).
(3) Doyle, C. D., and Nelson, A . C., U. S. Patent 2,587,295 (Feb. 26, 1952). (4) Flory, P. S., J . Am. Chem. Soc., 63,3083 (1941). (5) Goodwin, J. T., and Hunter, M. J., U. S. Patent 2,584,341 (Feb. 5 , 1952).
(6) Ibid., 2,584,342. (7) Ibid., 2,584,343.
( 8 ) Ibid., 2,584,344. (9) Ibid., 2,589,243 (March 18, 1952). (10) Hedlund, R. C., Ofic. Digest Federation P a i n t & V a r n i s h Production Clubs, 26, 352, 356-7 (1954). (11) Hedlund, R. C., Org. F i n i s h i n g , 15, 16-18 (1954). (12) Hunter, M. J., and Rauner, L. A., U. S. Patent 2,584,351 (Feb. 5, 1952). (13) Ibid., 2,628,215(Feb. 10, 1953). (14) Kress, B. H., and Hoppens, H. A., Ofic.Digest Federation Paint & V a r n i s h Production Clubs, No. 333,689-99 (1952). (15) (16) (17) (18) (19) (20) (21)
Lawson, W. F., U. S. Patent 2,048,799(July 28, 1936). MeGregor, R. R., “Silicones and Their Uses,” pp. 112-16, 284, hIcGraw-Hill, New York, 1954. Millar, R. L., U. S. Patent 2,663,694(Dee. 22, 1953). Patterson, J. R . , IND.ENG.CHEM.,39, 1376 (1947). Patterson, J. R., Ofic. Digmt Federation P a i n t & V a r n i s h Production Clubs, No. 266, 144 (1947). Patterson, J. R., Org. F i n i s h i n g , 8, No.4 , 32-7 (1947). Pattison, E. S., P a i n t V a r n i s h Production, 42, No. 10,23-5, 81 (1952).
RECEIVED for review October 1 1 , 1954. ACCEPTED February 16, 1955. Division of Industrial and Engineering Chemistry, 126th Meeting, AC8, New York, September 1954.
Vanadium Oxides as Oxidation Catalysts G. L. SIMARD1, J. F. STEGER2, R. J. ARNOTT?, AND L. A. SIEGEL Research Division, .4merican Cyanamid C o . , Stamford, Conn.
T
HE selective catalytic oxidation of hydrocarbons-for example, of naphthalene or o-xylene to phthalic anhydrideis a process of considerable commercial importance. Approsiinately 300,000,000 pounds of phthalic anhydride were produced in the United States in 1 9 5 3 and 3 1 5 , 0 0 0 , 0 0 0 pounds have been projected for 1 9 5 4 . As normally operated, the process for the catalytic oxidation of naphthalene consists of passing a stream of vaporized hydrocarbon a t about 1 mole % concentration in air through a catalyst bed a t temperatures between 400’ and 500’ C. for contact times of a few tenths of a second. Typical yields are 70 to 90 pounds of phthalic anhydride per hundred pounds of naphthalene. Other products of reaction are carbon oxides, Present address, Schlumberger Well Surveying Corp., Ridgefield, Conn. Present address, Morton Salt Co., Chicago, 111. a Present address, The Texas Co., Houston, Tex. 1 2
water, and minor quantities of quinones and maleic anhydride. I n fixed bed operation, vanadium oxide-type catalysts supported on silicon carbide are frequently used. As an approach to the development of oxidation catalysts, a fundamental study of unpromoted vanadium oxides was undertaken. o-Xylene was chosen as the hydrocarbon for oxidation because i t permitted the use of a simple feed system. For the most part, the measurements were of bulk properties-i.e., properties of more regular structures than would be expected on the catalyst surface. A qualitative picture of the surface may nevertheless b e obtained since the arrangement of the surface atoms, although subject to considerable disorder, will tend toward the stable structures of the bulk oxides. The studies have been of a survey nature, but the results have helped clarify the catalytic behavior of vanadium oxides. The vanadium oxides formed in the oxidation of o-xylene are
