Chemistrv of Wash Primers J
HAROLD ROSENBLOOM Thompson & Co., Oakmont, Pa.
ASH primers are coating compositions that form both in-
d
organic and organic films on metal surfaces. The inorganic film is the reaction product of soluble constituents with the metal and begins t o form first. The organic part of the ccating is deposited over the inorganic film as solvent evaporates. In all probability, conventional corrosion inhibitive primers form inorganic films on metal surfaces in the course of exerting their protective action. Zinc chromate primers, i t is believed, function in this way, but the inhibitive chromate film is formed from the solution that results when water from the environment penetrates the coating. Wash primers, on the other hand, are a deliberate attempt to form a corrosion inhibitive inorganic coating-analogous to the well known aqueous metal treatments-from a solution of organic film-former.
one can discuss the reactions that occur between the wash primer constituents and the metal interface separately from those reactions that occur between the constituents; but practically, they cannot be separated, and both sorts of reactions will be considered as a part of the chemistry of wash primers. A typical WP-1 composition is given in (9): Base Grind Vinylite resin XYHL Basic zinc chromate Talc Isopropyl alcohol 99% or ethanol Butyl alcohol
Parts by Weight 7.2 6.9 1.1 48.7 16.1 80.00
Acid Diluent Phosphoric acid, 85%
500 xu. uu 100.00~0
>
0
I
Y
WP-I WP-I i n oxidationr csi stant s o l v e n t s
200 0
IO
20 30 TIME ( M I N S )
40
50
This is the most commonly used wash primer and is a reactive or unstable type. However, there are two other distinct types, both of which are stable. One of these is nonreactive and the other reacted. The chemistry of the nonreactive stable wash primer is radically different from that of the other types. It will not be discussed here, but this laboratory hopes to publish its study of this class in a later paper,
Figure 1. Effect of Solvent on Oxidation of WP-I Type Wash Primer
I
)*
Compositions of this type were described first by Whiting and Wangner (8). They found that when dilute, alcoholic phosphoric acid is added to a dispersion of zinc tetroxychromate (ZTO) in polyvinyl butyral solution, a primer having an unusually high order of adhesion to a variety of metals is formed. Furthermore, most of the commonly used film formers adhere to this primer, and the protective value of such systems is outstanding. Wash primers whose essential components are phosphoric acid, zinc tetroxychromate, and polyvinyl butyral are known widely and are used under the designation, WP-1. Bacon, Smith, and Rugg ( 1 ) have shown that electrolytic resistance may be used to evaluate the protective merits of coatings on steel immersed in sea water. Good protection is obtained a t resistances greater than 200 megohms and poor protection a t resistances lower than 1 megohm. One of the systems studied was the WP-1 wash primer and a red lead pigmented vinyl primer: 1.5 mils of WP-1 alone had a very low electrolytic resistr ance, and 3.2 mils of red lead primer alone had initially high resistance; continued immersion resulted in comparatively rapid loss of resistance and protection. However, 0.5 mil of WP-1 and 2.9 mils of primer maintained both a high order of resistance and protection throughout the same test period. The various authors mentioned have alluded t o the formation of an inorganic phosphate film on the metal surface to explain the unique adhesive and protective value of WP-1. However, no experimental evidence has been offered in support of this idea, and no attempt appears t o have been made to study the complex reactions that occur in the composition itself. For clarity,
-> 3 00 I
Y
LL
2 250
W
200
I50
2
3
4
5
6
PH Figure 2.
Relationship between E.M.F. and pH in Reacted Type Wash Primer EXPERIMENTAL
OXIDATIONREACTIONS, The primer composition given above shows a chromate, an acid, and an alcohol indicating that oxidation reactions must be important. Figure 1shows changes in the e.m.f. of two wash primers measured against time with the platinum-saturated calomel electrode system. The curve showing the rapid change in potential is for a typical wash primer con-
2561
’
Vol. 45, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
2562
taining easily oxidized primary and secondary alcohol. The curve showing practically no change is for the same composition except for solvent, which consisted of 97.5’% of oxidation resistant tsrt-butyl and tert-amyl alcohols and 2.5% methyl isobutyl ketone. The oxidation resistant wash primer showed none of the properties of the easily oxidized wash primer: it did not develop adhesion t o plain or galvanized steel or to aluminum; it lacked toughness, did not darken in color, and did not become insoluble in the original solvents. When, however, 10% of the tert-butyl alcohol was replaced by easily oxidized formaldehyde or n-butanol, the properties of this primer were indistinguishable from those of WP-1.
500
0 0
WP-I WASH PRIMER 100 grns 5%ALCOHOLIC 3.35 mi AQUEOUS K2Cr20, (1.21 %)
L
W
0
5
IO 15 20 T I M E (MINS)
25
30
Fieure 3. ComDarison of Changes in Oxidation PGential of WPll and Solution 07 &Cr207 i n Alcoholic H~POI The high oxidation potential of the wash primer in the oxidation resistant solvent indicates that the pigment was soluble, to some extent a t least, and supplied some hexavalent chromium to the alcoholic phosphoric acid solution. If this was the real situation, it means that phosphoric acid and hexavalent chrw mium will not produce a typical wash primer and that in order t o develop all the properties associated with the wash primer, some reduced form of chromium or some oxidation product of an alcohol or both are required. The oxidizing action of chromic acid has been represented by HCr04-
+ 7H+ + 3e
=
Cr+++
+ 4HzO
decrease in potential of only 49 mv. The remaining decrease must have occurred as a result of the decrease in the relative concentrations of Cr6+ t o Cra+. T h a t a relationship between pH and e.m.f. also exists and corresponds t o the theoretical equation in a qualitative way is shown by Figure 2. These data are for the reacted type of wash primer t h a t involves oxidation of an alcoholic polyvinyl butyral solution by a mixture of chromic and phosphoric acids; this type will be discussed in detail later. The reduction of chromic acid in this wash primer is in all likelihood near completion, and the ratio of Cr6+t o Cr3+ is exceedingly small but constant. Under these conditions, the effect of pH, as indicated by the curve, is in the direction predicted by Equation 2. Figure 3 compares the changes in e.m.f. with time of the WP-1 composition with those produced by adding a small amount of aqueous potassium dichromate t o a 5% solution of phosphoric acid in alcohol. The great similarity between the curves leaves little doubt t h a t the oxidation reaction of the wash primer is essentially the chromic acid oxidation of the solvent. It is important t o point out some additional observations in connection with the data of Figure 3. After 16 minutes’ reaction time when the e.m.f. was about 350 mv., the solution turned green, and 2 minutes later a green precipitate, presumably chrw mic phosphate, formed. The p H of the solution varied from pH 2.2 a t the start to p H 2.3 after 50 minutes’ reaction. Britton (3) reports t h a t in aqueous solutions, chromic phosphate begins to form an opalescence a t p H 4.64 and a flocculant precipitate at p H 5.65. The fact that salts precipitate from alcoholic solutions of acids a t a p H much lower than in the corresponding water solutions is important in the chemistry of wash primers.
50
1 WASH PRIMER MINUS PIGMENT
ri 20
I
I
I
I
I
(1)
and t h e potential by
According t o Luther (6) and Kolthoff (6),the actual behavior of the bichromate electrode differs from that which should be expected from the equations given No regular relationship exists between p H and e.m.f. Kolthoff agrees with the remark of Luther that “the measurement is attended with many difficulties.” The author has found no reason for injecting discord into this harmony. However, in the wash primer systems studied, there seems t o be qualitative agreement with the equation. The electrode potcntial decreases with increasing amounts of chromic ion at constant pH, and when the ratio of oxidized t o reduced chromium is constant, the e.m.f. of the electrode increases with increasing concentration of hydrogen ion. T h a t the decrease in oxidation potential of the wash primer with time is primarily the result of a decrease in the ratio of oxidized t o reduced chromium is a conclusion that can be reached by comparing the actual decrease in potential with t h a t which would have been produced by the change which occurs in p H alone. In Figure 1, the decrease in potential is about 195 mv. I n the same time interval the pH increased from 2.83 t o 3.18. If one assumes the ratio of Cr6f to Cr3+ t o have remained constant and Equation 2 t o hold, this change in (13’) corresponds t o a
Other facts of significance in connection with the oxidation reaction in the WP-1 type wash primer are the effects of water and of phosphoric acid concentration and the apparent effect of the polyvinyl butyral resin on the solubility of the zinc tetroxychromate pigment. EFFECT OF WATER. Zinc tetroxychromate, which has a solubility in water of about 0.2%, was dispersed in a water solution of phosphoric acid to give the following composition:
ZTO Water
(85%)
Per Cent 40.4 14.0 45.6
I n this example, the ratio of zinc tetroxychromate t o phosphoric acid is the same as that in the mash primer. The solution very quickly developed the orange color of chromate and had a p H of 5.8. When 100 grams of 95% ethanol were added to 100 grams of this mixture, the pH dropped to 5.5 presumably because of the effect of alcohol on the solubility of the chromate; but this pH was not low enough t o produce any signs of oxidation over a 4-hour period. When, however, the experiment was repeated using 95% ethanol as the only solvent, the p H of the mixture was 2.4 and
November 1953
3.0
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion found in the wash primer. This is shown by the data of Figure 4. And yet certain properties of the dry wash primer film suggest such a reaction as one possibility. For example, the water extract of the dry film has a pH of about 6, whereas the wash primer solution has a p H of about 2.7. A second possibility is that the higher pH of the dry film extract is simply the result of neutralization of phosphoric acid by the pigment. However, as indicated in the preliminary observations, reaction between pigment and phosphoric acid is negligible if the water content of solvent is low, These experiments suggest a method for determining the nature and extent of the reaction:
p H OF 1.0 % H 3 P 0 4 IN ETHANOL-WATER SOLUTlO NS
60 ao IO0 ETHANOL Figure 5. Effect of Composition of Solvent on pH of HaPo, in Ethanol-Water Solutions
0
40
20
WEIGHT
2563
%
oxidation occurred almost immediately after addition of the pigment t o the alcoholic phosphoric acid solution. The fact t h a t the pH of the water solution was 5.8 and that of the alcohol solution 2.4 can be explained only on the basis of the greater solubility of zinc tetroxychromate in aqueous phosphoric acid. EFFECTOF PHOSPHORIC ACID. When the concentration of phosphoric acid in the alcohol-pigment mixture was reduced from 14 t o about 3%, the solubility of zinc tetroxychromate was SO low that oxidation of solvent did not occur even after several months’ standing. Since the pH of the solution phase was about 2.2, oxidation would have been rapid if any chromate had been present. The odd thing APPARENTEFFECT OF POLYVINYL BUTYRAL. about the experiment just described is that if polyvinyl butyral is present in the same solution, the zinc tetroxychromate pigment is soluble, since oxidation occurs. This was shown by the data of Figure 1.
With simple titration methods as the analytical technique, practically all the phosphoric acid is recoverable from the wash primer constituents minus resin, providing the water content is low. However, when resin is present in the same composition, practicall none of the phosphoric acid is recoverable. These facts, togetger with the earlier observation that no reaction takes place between hosphoric acid and resin in the absence of pigment, form the chief experimental basis for establishing the nature of the reaction. When i t is recalled t h a t reduction of Cr6+ must occur in order to develop the properties of the wash primer, the picture of a familiar mechanism emerges, in which an unstable intermediate com ound is the necessary link between reactant and product. In %is case the intermediate link would appear t o be a compound of Cr6f)or Cr4+ with phosphoric acid.
EFFECT OF WATERON PIQMENT SOLUBILITY.Table I shows some changes that occur in the wash primer composition when everything is present except the resin. The variable studied is water content, which varies from that accounted for by the water present in the alcohol and phosphoric acid (1.63%),up to a maximum of 8.560/&
8 REACTIONS INYOLVING PIGMENT, POLYVINYL BUTYRAL, AND PHOSPHORIC ACID
1 Q
In the absence of zinc tetroxychromate pigment, polyvinyl butyral does not react with phosphoric acid in the concentra-
H3P04in io0 mi water 3
h
sr
TABLE I. WASHPRIMER COMPOSITION MINUSRESIN
I
Pigment component Zinc tetroxychromate Silica Methyl ethyl ketone Ethanol, 95% Total pigment component Acid component
HIPOI, 85% Water Butanol Total acid cornponent Total water. % fwt.)
(Effect of water content) Parts by Weight 13.75 2.30 41.60 42.45 100.00
A 4.84 8.56 11.60
0
of solution after 17 hours
Parts by Weight C D 4.84 4.84 2.14 1.07 18.02 19.09
25.06
25.00
25.00
25.00
8.56
4.92
3.28
2.46
Dark brown
E 4.84 0.00 20.16
25.00 1.63
3.25
Faint bluegreen
Faint bluegreen
10
15
20
25
30
Figure 6. Electrometric Titration of Phosphoric Acid in Water and in 95% Ethanol
5.98
coih
5
ml 0.100 N KOH
B 4.84 4.28 15.88
gm HjPOqin io0 ml
Faint bluegreen
Faint bluegreen
The effect of water on p H is apparent immediately after mixing the acid component with the pigment component and becomes progressively more apparent with time. Of course, the effect of water on an alcoholic solution of phosphoric acid is t o lower the pH, as shown by Figure 5. The fact that the p H of the mixture containing 8.56% water has risen from 1.9 t o 5.98, while that of the mixture containing 1.63% water has risen from 1.60 to 3.25, is the result of the greater solubility of the pigment in the mixture containing more water. REACTIONOF PHOSPHORIC ACID WITH POLYVINYL BUTYRAL IN A WASHPRIMER COMPOSITION.Phosphoric acid was determined by electrometric titration fist in the wash primer composition minus the resin and then in the complete wash primer. T h e latter determination was divided into two parts:
INDUSTRIAL AND ENGINEERING CHEMISTRY
2564
1. The extract obtained by precipitating the wash primer in water. 2. The alcoholic dispersion of the precipitate obtained in step 1.
Figure 6 is a curve showing p H versus ml. of 0.1 N potassium hydroxide for 0,100 gram of phosphoric acid in water and in alcohol. The formation of the mono- and dipotassium phosphates takes place at higher pH’s in alcohol than in the corresponding water solutions, and the titration produces high results. However, the end points are sufficiently distinct to be useful.
Vol. 45, No. 11
phosphoric acid appearing in the alcohol dispersion is about 1.5y0 of the total. While all the phosphoric acid is recoverable from the nlcoliol phase of the mash primer components minus resin, only about 2.2% is recoverable when resin is present. Phosphoric acid does not react with polyvinyl butyral in the absence of pigment nor in the presence of pigment when oxidation does not, occur. Furthermore, trivalent chromium does not produce the reaction. It must be concluded, then, that chromium in valence states intermediate between 6 and 3 are necessary for the reaction of phosphoric acid with poljvinyl butyral in the wash primer coinposition. THE CHROMIUM PHOSPHATE WASH PRIXER
typical example of the chromium phosphate wash primer (2) is as follows: Parts by Weigh1 1.40 10.95 10.95 62 I20 14.50
+.3% Aqueous CrOs Vinyl butyral 10.0% HaPO4(85y0)in acctone Ethanol (95%) Butanol
100.00
The reaction is carried out by first adding the chromic acid solution t o the phosphoric acid solution. This mixture is then added slowly with agitation to the ethanol solution of butyral maintained a t 45’ C. After reaction for 20 minutes, the mixture is cooled and thinned with butanol. The product is a clear, green solution. This reaction Yas studied-much along the same lines just discussed in connection with WP-1-both in the presence and absence of the repin.
rnl. O.IOON KOH
Figure 7. Eleotrometric Titration of Aged Wash Primer Components lMinus Resin Pigment does not neutralize HaPO4 when water aontent is low Titration of 2.075 grams of filtrate in 100 ml. of water Pigment Component A ZTO
Silica MEK 95 % Ethanol Total
13.75 2.30 41.50 42.45 100.00
L_
E 13.75 2.30 41.50 42.45
iOO.OO‘
Acid Component Added water 85 % IIsPOi Butanol
Total
A .8.56 4.84 11.60 25.00
-
E 0.00 4.84 20.16 25.00
Figure 10 shows the various phases that occur in the solveiits alone and in the solvents plus resin as the ratio of phosphoric acid to chromic acid is varied. Throughout the region where the solvents alone lead t o the formation of two phases, polgvinyl butyral plus the solvents lead to the formation of a bingle phase. R ~ a c ~ 1 o xWITH - s SOLVEXTS.The data of Figure 10 show that a solution of a chromic phosphate as well as solid chromic phosphate forms in the ethanol-butanol solvent in the region where the weight ratio of phosphoric acid to chromic anhydride is between 3.0 and 2.2. I t might be argued that the presence of resin in the solvent merely serves to inhibit the growth of large crystals and thus creates a very fine dispersion which appears to be a solution. REACTIONS IN PRESENCE OF POLYVINYL BUTYRAL.COMPLEX FORMATION. There is a distinction between the solutions that form in the presence and in the absence of resin. If the green
-
Figure 7 represents the titration of the filtrates from the aged wash primer components minus resin, the compositions of which are shown on the figure. From each composition 2.076 grams of filtrate were titrated in 100 ml. of water. Virtually all the phosphoric acid was recoverable from the composition containing no added water and practically none from that containing added water. Ten grams of aged wash primer of the same composition as E except for added vinyl butyral resin-ratio of resin to zinc tetroxychromate =0.92-were precipitated in 200 ml. of water. Figure 8 shows the titration of the extract. The first inflection point on the curve labeled “no added water” is missing altogether showing that what phosphate there was existed in the dihydrogen form. But only 0.68% of the added phosphoric acid is accounted for in the water extract. Titration of the alcohol dispersion of the precipitated resin and pigment is shown in Figure 9. From these data, the amount of
10
t
0
I
I
0.5
1.0
I
I
I
1.5 2.0 2.5 rnl. 0.100 N KOH
I
I
3.0
3.5
I
Figure 8. Electrometric Titration of Water Extract of Aged Wash Primer
November 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
2565
The weight ratio of phosphoric to chromic acid found in the resin phase was about 1.4. If this ratio is added IO initially to the polyvinyl butyral solution, gelation results. Phosphoric acid Water added in excess of this ratio is needed to mainm ISPERDISPER9 I 'SiON GEL SiON tain a solution. If this is so, then the SOLID., same viscosity effects, and ultimately GEL SOLN+ SOLID+ gelation, should be produced b y adding LVEm 8 GEL LVENT alcoholic potassium hydroxide to a wash -m J - I I I primer in which the ratio was initially 1 higher than 1.4, since free phosphoric a 7 acid would be removed as i n s o l u b l e potassium phosphate. ALCOHOL DISPERSION 6 A series of solutions having a weight RESIN-PIGMENT PPT. ratio of phosphoric to chromic acids of 2.0 was modified by adding alcoholic 5 potassium hydroxide t o produce p H values ranging from 2.7 to 6.0. As EtOH BuOH EtOHt %VENT shown by Figure 13, the viscosities of L ELOH 8, the solutions increased abruptly as the lI 2 SOLVENT RESIN p H increased, u p to about p H 4.0. At ml. 0.lOON K O H Figure 10. Phases that Form when this point there was a sudden and Figure 9. Electrometric Titration of Mixtures of Various Ratios of CrOa equally abrupt fall in viscosity followed Alcohol Dispersion of Wash Primer and HsPOa Are Reacted with PolyResin-Pigment Precipitate by a slight apparent increase. Howvinyl Butyral i n Ethanol-Butanol ever, no immediate gelation occurred. Solvent Blend and with Solvents Only The solutions were kept at room temDerature and observed from time to solution containing no resin is filtered to remove the solid phase time. After about 1 month's aging, gelation occurred in all and if sufficient butanol is present so that two phases are formed samples having a p H of 3.5 or greater. when the solution is mixed with water, the chromic phosphate is A possible explanation for this behavior may be found among extracted very rapidly in the water phase. the habits of complex-forming ions. The hydrolysis of cationic If the same experiment is repeated with the same composition chrome complexes always leads to the production of acid. Werexcept for the presence of resin, chromic phosphate cannot be ner, as quoted by McLaughlin and Theis (7), conceives the followextracted in the water phase, even when the water phase contains ing mechanism, in which complexly bound water groups are rea strong complexing agent-such as ethylenediamine tetraacetic placed by the more strongly held OHacid. Furthermore, chromic phosphate cannot be precipitated [(HzO)eCr]+++CisH+ OHfrom the resin solution by raising the p H t o 10.5 with alcoholic potassium hydroxide. [cr(%%)~] ++a2H + c i - (3) The evidence just discussed supports the idea that a chromephosphate is present in the green wash primer solution in the form Hydrolysis is therefore retarded by acid and promoted by alkali. of a stable complex that is joined in some manner t o polyvinyl I n the present case, some phosphate must be held in the chrobutyral. mium coordination sphere, since only a fraction of the phosphoric I n the following experiment, a series of chromic phosphate wash acid can be titrated. A complex similar t o the following may primers was prepared by the method already described. The exist in the wash primer solution ratio of phosphoric t o chromic acids was varied, keeping the ratio of total acid to polyvinyl butyral constant: IJ
.
1
+
P
+
*
+
+
After aging the solutions for 2 days, 20 grams of each ratio were stirred into 200 ml. of water. A clear solution containing excess phosphoric acid and a green resin precipitate formed. Phosphoric acid was estimated volumetrically by titration to the phenolphthalein end point. Since the chromic ion was readily detected by its color a t concentrations as low as 0.03%, the absence of color in the water extracts was taken as evidence that practically all the chromium remained in the resin precipitate. By subtracting the amount of phosphoric acid found in the water extract from that present in the wash primer before precipitation, the uantity of phosphoric acid in the resin could be estimated. T%ese results are found in Figure 11. Except for those ratios that lead to gelation, a constant ratio of chromic to phosphoric acids, independent of their concentrations in the solution, is found in the resin phase. The molar ratio of chromium t o phosphate in the precipitated resin phase is 5 t o 7. In the experiment just described, the total concentration of chromic and phosphoric acids added t o the polyvinyl butyral solution was kept constant as the ratio was varied. But as the proportion of chromic acid was increased, the amount of chromephosphate retained by the resin increased. This was reflected in a corresponding decrease in free or water-soluble phosphoric acid. The effect of this change on viscosity of the resulting wash primer is shown in Figure 12.
0
I
2
3
WUGHT RATIO H ~ P O ~ / C ~ OSOLUTION ~'I"
Figure 11. Relationship between Ratio of HBP04/CrQ3 Added to Solutions of Polyvinyl Butyral and Ratio Associated with Resin after Precipitation i n Water
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
2566
Excess phosphoric acid would favor equilibrium to the left and maintain the chrome phosphate in solution. Accordiiig t o this interpretation, hydrolysis results in the penetration of OHinto the coordination sphere of chromium. Chrome complexes are presumed to polymerize by means of a process known as olation, thus
This process is favored by heat, increasing basicity, increasing concentration, and time. It also is quite conceivable, then, that the increase in viscosity with pH reflects merely the polymerization of the chromium compound. However, polymerization of the chromium compound does not explain its insolubilizing effect on the butyral. One explanation of the insolubilizing effect is to assume that coordination between alcoholic 013 of butyral and chromium in the complex is possible. One such configuration is shown schematically in Figure 14. While this scheme explains the insolubilizing effect of the chromic phosphate complex, it does not offer a satisfying explanation for the effect of phosphoric acid on viscosity. A combination of the resin complex of Figure 14 with olation as shown in Equation 5 is more in harmony with the facts. With increasing pH, the chromic phosphate complex increases in basicity as shonrn by Equation 4. Polymerization of these units would then lead to increased viscosity. The following equation illustrates this possibility:
b '0
' 0
o/
Vol. 45, No. 11
Although the inorganic film that forms on steel cannot be seen, evidence for its existence may be obtained by following the change in potential of a steel electrode immersed in the WP-1 composition. Results of one such observation are shown in Figure 16. The initial noble potential can be explained by the initial presence of hexavalent chromium. As the concentration of hexavalent chromium is decreased by participation in the oxidation reaction, attack of the steel becomes greater, and the electrode becomes increasingly anodic. But then attack falls off, and the electrode becomes progressively more noble. This ran be interpreted as an increase -Cr(H20),]++++Clr- f HzO in the area of a cathodic inhibi(5) tive film (4). If adhesion were a function of inorganic film formation alone, one would expect an increase in adhesion paralleling film formation. Once the film was complete, additional acid should detract from adhesion as the result of its solubilizing effect on the film. If, however, acid also reacts with polyvinyl butyral, improvement should continue up to the point of complete reaction and then again fall off. The present set of curves may be interpreted in this light. For each metal studied, adhesion increased rapidly with acid content and reached a plateau, which can be attributed t o complete inorganic film formation. Increased adhesion beyond this point can be interpreted as the result of increased reaction with polyvinyl butyral and the maximum as the point of complete reaction. The variation in abruptness of the maxima might be assigned to the different solubilities of the inorganic coatings in phosphoric acid, which would depend upon the particular metal substrate. WP-1 STABILITY. A property of WP-1 of considerable practical importance is its lack of stability. After phoephoiic acid has been added to the composition, changes occur that ultimately prevent the primer from developing a high order of adhesion and that interfere with its protective value. The time requird for loss of these properties varies, but usually after a day or more they become fully apparent. In WP-1 compositions made without adding any water except that normally present in the alcohol and phosphoric acid, the loss of adhesion has been observed to coincide with increased pII and with the formation of a gelatinous material, presumably the result of olation of chromic chromate in the manner depicted in Equation 5 . This is a rather coarse material and when removed from the composition by filtration or centrifuging very pronounced improvement in adhefiion results. The gelatinous product appears to form preferentially around pigment particles where the concentration of chromate is highest. If this situation is
' 0 500
I
1
I
Q
MECHANISM OF WASH PRIMER ACTION
Some of the reactions that occur in wash primer compositions have been traced. A more speculative topic is how these reactions may account for some of the properties of the wash primer systems. ADHESIONOF WP-1. The adhesion of the WP-1 composition to metals is a function of acid concentration. This relationship is shown in Figure 15. If the adhesion of the WP-1 wash primer is attributed to the formation of an inorganic film on the metal surface to which the organic coating is bound through a chrome phosphate complex of the resin, a plausible explanation for the peculiarities of the data is found.
300
2
200
e m
SOLUTION
Ln -
>
I
I
400
" >:
I
1
IO0
E~0 50
20 10 "FREE" H3PO4(%o f TOTAL ADDED) Figure 12. Effect of H3P04 on Viscosity of Chromic Phosphate-Polyvinyl Butyral Complex 60
40
3
November 1953
*
INDUSTRIAL AND ENGINEERING CHEMISTRY
avoided by putting the composition in a pebble mill and grinding for several days, adhesion remains a t a high level providing the pH is adjusted t o the initial range of 2.7 to 3.0. ADHESIONOF CHROMIUM PHOSPHATE WASH PRIMER.The adhesive action of this wash primer appears to be of the same type as that of WP-1. Evidence for film formation is again obtained by following the potential of a platinum-steel electrode system immersed in the wash primer solution, as shown in Figure 17. Here, however, there is no initial noble potential. Presumably this is due to the absence of the passivating effect of hexavalent chromium, which is present in the WP-1 composition. The amount of free phosphoric acid-which previously has been shown t o be present in the chromium phosphate wash primer-can be conveniently adjusted by adding alcoholic potassium hydroxide t o the solution. The potassium phosphate formed is insoluble in the alcoholic medium and precipitates out of solution. A composition having an initial ratio of phosphoric acid to chromic anhydride of about 2 : l was adjusted in this manner over a range of pH varying from that of the unadjusted solution (pH 2.7) t o pH 6.0. Steel panels were coated with these compositions and immersed in distilled water at room temperature. Adhesion was estimated by noting the time required before the coating could be stripped from the surface by pressure-sensitive tape. This, of course, measure8 the effect of water on adhesion; but the information is useful in establishing the mechanism. If the excess phosphoric acid over that required to form the chrome-phosphate complex simply remains in the film in a water soluble form, adhesion should improve with increasing pH, since the excess acid is removed from the solution. However, as the data in Table I1 show, adhesion decreases with increasing pH. The excess phosphoric acid is necessary for adhesion, but presumably i t is insolubilized through reaction with the metal surface.
2567
R '0
R
I
C
' 0
0 '
I
' 0
I
R
R High Viscosity
Figure 14. Possible Architecture of Chromic PhosphatePolyvinyl Butyral Complexes
DISCUSSION
The properties of the WP-1 type wash primer appear to hinge more on the reduction of chromium than on any of the oxidation reactions that appear to occur., At first thought, one might suspect that the hydroxyl groups of polyvinyl butyral might be subject t o oxidation or that the acetal group would suffer hydrolysis and subsequent oxidation. These possibilities were checked qualitatively by adding chromic acid t o a solution of polyvinyl
12
8
4
a GALV STEEL 0
ALUMINUM
P
0
10 20 30 40 H3P04, WGT.% ON RESIN
50
Figure 15. Effect of HsPOl Concentration on Adhesion of WP-I to Several Metals
T A B L1~ 1. EFFECT OF p H
O N ADHESION RETENTION OF CrOr HIPOI WASHPRIMER TO STEEL
PH (Ad'usted with Alookolm KOH)
Immersion in Distilled Water at 22' C., Hoursa 2601 260
29
116C
47 4
2
3
4
5
4
6
PH Figure 13. Effect of pH on Viscosity of Chromic Phosphate Wash Primer
Failure measured by Bootoh Brand tape pull teat. b Partial failure. 0 Partial failure.21 houra. d pH adjusted with ZnO.
a
INDUSTRIAL AND ENGINEERING CHEMISTRY
2568
Vol. 45, No. 11
1
I2O-
POTENTlAL OF STEEL ELECTRODE IN Zn-CHROMATE TYPE WASH PRIMER
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STEEL ELECTRODE IN Cr03-H3P04
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0 L O G TIME (MINUTES) Figure 16. Change in Potential of Steel Electrode as Evidence for Inorganic Film Formation by WP-1 Wash Primer
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TIME (MINUTES) butyral in glacial acetic acid. Keither the acid alone nor the butyral solution was oxidized after several days’ contact with a concentration of chromic anhydride comparable to that which could be derived from the pigment. Presumably, oxidation of the solvent is the principal, if not only, oxidation reaction. Since ketones have been used as part of the solvent, the only new group to be introduced would be carboxyl. This aspect of the problem has not been evaluated specifically, except to note that small additions of acetic acid do not alter the properties of the wash primer. There are two important differences between the WP-1 type wash primer that contains zinc tetroxychromate and the chromium phosphate wash primer-greater insolubility and greater corrosion inhibiting value of the former. The first difference may be related to the presence of zinc in the WP-1 type. This assumption could not be confirmed by adding zinc oxide to the second type, which merely results in the formation of zinc phosphate and increased pH; nor could the greater insolubility be achieved by adding the dihydrogen phosphate salt, which does not affect the pH. The role of zinc is not understood. It is reasonable to attribute the greater corrosion inhibitive value of the WP-1 type to the presence of unreacted hexavalent chromium derived from the pigment. In both types of wash primer, phosphoric acid appears to have the same functions: it lowers the p H to a value where oxidation and attack of the metal substrate can occur; and it provides anions capable of forming insoluble compounds with the cation products of oxidation and metal dissolution. The most significant and generally interesting reaction is that involving chrome phosphate complex formation and the linkage of this with polyvinyl butyral. In the WP-1 type wash primer, the trivalent chromium originates from the reduction of chromate derived from the pigment, while in the chromic phosphate type it originates from reduction of chromic acid. The fact that the insolubilizing reaction is not duplicated merely by adding trivalent chromium-e.g., chromic sulfate-to polyvinyl butyral in alcoholic phosphoric acid, suggests the interesting possibility that intermediate oxidation states (&a+, Cr4f) promote the reaction between chromium and polyvinyl butyral. SUMMARY
Data have been presented that help construct a picture of the reactions that occur between wash primer constituents. Oxidation plays a dominant part. Chromate derived from the pigment is soluble in the alcoholic phosphoric acid solvent and is the
Figure 17. Change in Potential of Steel Electrode as Evidence for Inorganic Film Formation by Chromic Phosphate Wash Primer
sourcc of the oxidizing power of the composition. Phosphoric acid reacts with polyvinyl butyral but only under the influence of the reduction of chromium. The mechanism of this reaction has been further clarified by studying the oxidation of alcohol solutions of polyvinyl butyral by mixtures of chromic and phosphoric acids. In these systems, a chromic phosphate complex is formed. Data indicate that it is attached to the polyvinyl butyral chain and may function as a cross-linker between chains or may form polymers with similar groups along the chain. Phosphoric acid in excess of that required to form the complex inhibits cross linking or polymerizac tion of the complex or both. Changes in the potential of electrodes immersed in wash primer compositions have been used to follow the course of inorganic film formation. The adhesive properties of the wash primers have been attributed partly to the existence of this film. Since adhesion is a function of phosphoric acid concentration, it is also probably a function of the degree of reaction between phosphoric acid (or the chromic phosphate complex) and polyvinyl butyral. ACKNOWLEDGMENT
The assistance of E. P. Siciliano of this laboratory with much of the experimental work described is gratefully acknowledged. LITERATURE CITED
(1) Bacon, Charles R., Smith, Joseph J., and Rugg, Frank M., IND. ENG.CHEM., 40,161-7 (1948). (2) Bakelite Co., “Vinyl Butyral Resins for Coatings,” May 1952. (3) Britton, H. T. S., “Hydrogen Ions,” 2nd ed., p. 382, New York, D. Van Nostrand, Inc., 1932. (4) Gatty, O., and Spooner, E. C. R., “Electrode Potential Behavior of Corroding Metals in Aqueous Solutions,” p, 260, Oxford,
Clarendon Press, 1938.
( 5 ) Kolthoff, I. M., Chem. Weekblad, 16,450 (1919). (6) Luther, R., 2. physib. Chem., 30, 653 (1899). (7) McLaughlin, G. D., and Theis, E. R., “Chemistry of Leather Manufacture,” p. 411, ACS Monograph 101, New York,
Reinhold Pub. Co., 1945. (8) Whiting, L. R., and Wangner, P. F., U.S. Patent 2,525,107 (October 10,1950).
RECEIVED for review March 13, 1953.
ACCEPTED July 22, 1953.
