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INDUSTRIAL AND ENGINEERING CHEMISTRY
low rate of fdl to produce heavv-sectioned Dieces having minimum impressed strain, 3. High temperature and pressure are necessary t o obtain clwest mold reproduction. Mold shrinkage varied from 0.002 to 0.009 inch per inch and weights varied as much as 2% in 5 x I/* X */2 inch bars by changing molding technique. 4. Low rate of filling mold aids in forming bubble-free pieces. 5. PM-ty e cellulose acetate plastics (53% combined acetic acid) genera& require higher injection pressures than plastics of higher acetyl content t o avoid bubble formation in thick pieces. 6. Side gating is more effective than corner gating in producing ieces relatively free of strain. 7. #here are minimum pressures below which bubble formation consistently occurs in heavy moldings. 8. Increased mold temperature and longer periods in warm molds im rove the strain structure of plastics. 9. Sogent content in the test block prior to bakin (about 17%), baking temperature (320’F. approximately), and cooling pressure (500-1000 pounds per square inch) are critical factors in producing cellulose acetate sheet stock free of strain.
Vol. 36, No. S
ACKNOWLEDGMENT
Y
The assistance in preparing this paper by W. E. Gloor, C. A. Borton, and associates, and by J. S. Biddle in making the photographs, is gratefully acknowledged. LITERATURE CITED
(1) Clewell, J. (1937).
H.,and Paine, H. W., IND. ENG. CHEM.,29,
750
(2) Delmonte, J., and Dewar, W., Modern Plastics, 21, 121 (Nov.,
1943). (3) Ellis, C., and Simonds, H., Handbook of Plastics, p. 508. New York. D.Van Nostrand Co.. 1943. (4) Maoht,’M., Rahm, W. E., and Paine, H. W., IND.ENG.CHEM.. 33,663 (1941). PRESENTED before the Division of Paint, Varnish, and Plastics Chemistry at the 106th Meeting of the AMERICAN CHEMICAL SOCIETY, Pittsburgh. Pa.
Ammonium Ferrous PROTECTIVE PAINT FINISHES
T
HE growing recognition In a search for new pigment materials to improve the performance of metal protective in recent years of the paints, the behavior of ammonium ferrous phosphate (NH,FePOd.H20) has been investigated. This is an insoluble, greenish compound possessing a platelike structure desirability of i m p r o v i n g and a characteristic x-ray diffraction pattern. Evaluation of the product to date by metal .protective systems has stimulated research in this field various methods, including conventional panel tests, indicates that a definite improvement in performance can be obtained by its use in certain types of primer systems. by the many industrial groups Tests designed to demonstrate behavior under severe conditions of moisture condensafacing corrosion problems. tion likewise indicate its value especially in ferrous metal primers of the zinc yellow Considering only one line of attack-namely, protection type. The mechanism of the action of ammonium ferrous phosphate has not been positively established, but it probably functions both as an acid acceptor and oxygen by organic coatings-intensive e f f o r t s h a v e been m a d e acceptor and at the same time improves the impermeability of the film to water vapor. to develop and investigate improved products not only by manufacturers of paints, pigments, and vehicles, but centage of potassium. The composition is represented approxlikewise by organizations having a direct interest in the protecimately by the empirical formula K 2 0 . 4 Z n 0 .4Cr0a.3H20 (1). tion of metals. The use of this pigment has become well established’in primers Since such work has been concerned with both ferrous and nonfor nonferrous metals; during recent years it has been finding ferrous metals, each exposed under a wide variety of conditions, increased application in primers for steel, particularly in the it is obvious that the problem is one of unusual complexity. newer and more durable rapid-drying vehicles. Its use in Because of the magnitude of the field and the difficulties in primers for steel has been favored not only by its rust inhibitive evaluating research results, progress is relatively much slower than properties but also by its ease of grinding, good settling resisb in other comparable branches of technology. ance, and stability in the package. I t has a tendency toFrom the practical point of view ward water sensitivity due to its the problem of developing primer appreciable solubility; this property paints for a given use requires conA . M . Erskine, Godfrey Grimm, is manifested by a tendency t o blister under certain exposure conditions sideration of three equally important and S . C.Homing factors-namely, the pigment system, and requires c o n s i d e r a t i o n w h e n the vehicle, and the finished primer. PICMEtYTS DEPARTMENT, formulating the pigment in metal E. I. DU PONT D E NEMOURS & COMPANY, INC., Considering only the pigmentation protective paints. NEWARK, N. 3. A value of 26.1% “water soluble factor, certain points are worthy of comment. material” was reported (a) on the While many primers are formulated satisfactorily for certain basis of the A.S.T.M. test (D-126-36) which involves leachpurposes with inert, noninhibiting pigments such as iron oxide, ing a sample with a definite quantity of water. This figure there has been a definite trend toward the use of products with is equivalent to a concentration of 0.261 gram per 100 ml. positive rust-inhibitive properties-furnished, for example, by of solution and is in reasonable agreement with the results of zinc yellow. This pigment is a complex basic salt containing, experiments in this laboratory, in which the equilibrium soluin addition to ainc and hexavalent chromium, a substantial prebility (Le., the concentration of solution in equilibrium with
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1944
a n excess of solid phase) was determined by shaking a large excess of pigment with distilled water. No known quantitative relation exists between either leaching values or actual solubility, and the behavior of t h e pigment in a paint film under exposure conditions or when immersed in water.
\ 9
1 8
I
LD
27 36 45 54 EXPOSURE TIME. DAYS
63
Figure 1. Effect of Ammonium Ferrous Phosphate in Primer Paints Exposed on Structural Steel Sheet (Mill Scale Intact) A. Ammonium farroum phol.phate/dnc yellow, 30/70 (40% pigment volume) E . Red lea4 100% (80% pigment volume) C. Ammonium ferraum phomphate/rinc yeilow, zO/sO (40% pigment volume) D. Ammonium farroue phomphata/sinc yellow. 10/90 (40% pigment volume) E. Zinc yellow 100% (40% pigment volume) Yehielm,, raw limead oil( paint system, one primer coat (about 0.2 mil thick), no top coat) espoaure, miniature panel mdmturs ~~ndep.ptlon box
EVALUATION OF METAL PROTECTIVE SYSTEMS
The evaluation of metal protective pigments and primer systems in general represents one Df the most difEcult field8 of technical research, and progress is impeded t o an unusual degree by pitfalls of many kinds. Indeed, it is unsafe to draw final conclusions pending long-time observations of performance in the field. The difficulty in devising testing techniques t h a t will properly evaluate the different factors, such as effects of pigment and vehicle, condition of substratum, exposure conditions, pigmentbinder ratio and other paint variables, film thickness, etc., under t h e array of conditions encountered in t h e study of this field is evident. One typeof test comprises the application of primer paints with suitable controls t o portions of actual structures; such as bridges, tanks, or the like, under the conditions usually applicable in maintenance work. The structures are observed from time to time and the relative protective values noted and recorded, if desired, photographically. Exact control of all t h e factors i d u e n c i n g the results is difEcult, and the time required to show up significant differences! between the test systems may run into a matter of several years. Nevertheless, this procedure may yield valuable results and is widely used notwithstanding its limitations. A second type of test comprises painting panels of convenient size and deaign and exposing them to various environments, usually including conditions tending to promote early failure. Great care is neceaeary, not only in making certain t h a t the surfaces being painted are really comparable b u t also in maintaining equal film thickneas and controlling conditions during application. It is this control of application conditions which makes
457
tests of this type most useful. While somewhat artificial, panel tests are recognized as having definite value and are regularly used. Results are sometimes obtained more rapidly than in practical structure tests. A more severe test (4), which at t h e same time duplicates certain extreme conditions encountered in service, involves applying the test paints as relatively thin films to small panelse.g., 1 X 3 inch (2.5 X 7.6 cm.); they areexposed, usually without a topcoat, alternately to the ordinary atmosphere during t h e day and to conditions of positive water condensation at night. Failure by corrosion may occur in aa short a time aa 4-6 months, even with systems which behave satisfactorily in practice under ordinary conditions. Either of t h e above types of panels can be graded on a scale in which 10 represents perfect condition, and 0 represents complete failure. T h e intervening figures give a n approximate rep resentation of the percentage of area corroded. These gradings are similar to those obtained with the A.S.T.M.Photographic Reference Standards (D-610-41T) for evaluating the degree of resistance t o rusting with paints on iron or steel surfaces. The results obtained by any of the above methods can be supplemented by specific tests for pigment or paint film properties which may be of interest-for example, solubility, pH values, film permeability, adhesion, electrode potential measurements with corrosion test cells, etc. Unfortunately, the correlation of these specific testa with behavior in the field is frequently puzzling and interpretation of the d a t a is difficult. AMMONIUM FERROUS PHOSPHATE AS A PIGMENT MATERIAL &R
A search of the literature for novel materials of possible interest metal protective pigments, or aa adjuvants to metal protective
primer systems bmed on the pigments in current use, suggested the use of ammonium ferrous phosphate. This compound, first prepared over a century ago (S), is described as an insoluble greenish material possessing a platelike structure. While stable a t room temperature, i t waa found to be decomposed b y heat, giving ammonia and a ferrous phosphate which, in turn, oxidwed to the ferric compound. The composition, expressed in modern atomic units, waa established as NH,FePO,.H%O. The basis for considering ammonium ferrous phosphate as a metal protective pigment can be indicated briefly as follows: Like other nominally insoluble salts, it does have some slight
i
7
A5
L
I
EXPOSURE TIME, DAYS Figure 2. Effect of Ammonium Ferrous Phos hate in Primer Paints Exposed on Bright Auto Body 8t-l 1. Red lead in oil (30% pigment volume) 2. Ammonium faphaphate/sinc yeUow/iron oxlde/extender, 22/SS/lO/lS (40% pipment volume) 5. Ammonium fenvum phaphata/dnc yellow/Iron oxide/extender, 10/66/10/16 (40% pigment volume) 4. Zinc yellow/iron oxide/extender, 75/10/15 (40% plgment volume) 5. Iron oxide/extende.r. 85/15 (40 % pigment volume) VeMcler (1) Raw l i d oil; (2, 3, 4. 6) nuiyd/raw llrueed oil, 7S/W Paint t?yetmMa Single thin (about 0.2-mil) primer mat. no to.wt
INDUSTRIAL AND ENGINEERING CHEMISTRY
458
Vol. 36. No. 5
tlceepting properties. The compound itself is sufficiently low in solubility to warmnt evaluation as B component of the wtal pigmentation, particddy since its platelike character might conceivably impart impermeability and improved durability in a primer
am. In preliminary testa linseed oil paints were mnde up at 40% pigment volume and pigmented with blends of ainc yellow and ammonium ferrous p h a s
Figure 3. Topeostod Primer Films on Rusted Steel (above) and on Rusted Mill Scale Steel (below), Exposed 28 Monthr (45' South) in Florida Print*, Pigmar.lori"" 1. 5. Red lead. 1w% (30% pismcni rolumc) 2 , 6. Zinr ).sllow/imn orids/sitxndrr, 75/10/15 (40% pismrnf mluma) 3, 7. Like 2 a d 6, but I pa- of zinc yellow rsp1n-d hr ammonium frrmus ph0wh-k 4.8. Like 2 and 6, but 20 parts d zinc ynllar rsnlaoed by ammonium f*rruua phosphate Primer Vehtcla
1. S. Raw 1 i . u - d uil. 2 , 3, 4 uod 6, 7, 8.
Alk?d.'rnw linrcrcl oil. SU/5U
l'ep Coof. Blsck iron oxide and carbon blaoh in a 50-sallun L O Y ~r a c d a h
d i i b i l i l y sufficient IO give B m ni o n i iim , f e r r o u s , sncl phospliat,e ions, w d am-
monia could obviously be formed by hydnApsis UI. by reaction with alkaline m ~ t e riais whieli may be present in the film. It id k w r m 11?at corrosion oi Y;et irim is retarded by the pie*e*iee of a m m o n i a . F e r r o u s ion rould be enpeetrd not only I O repress tile solution of metallic iron by a mass uctiun eliect but ahoiild iioietion us an "oxygen ilec o p t o r " in t h e system. Phospha~e ion under cwtiLin conditions probably has B specific corrosion-inhibitive effect and, in m y ease, should have positive acid-
t >
IO
I
phate. These experimental compositions were exposed on structural steel (mill srale intact) by tho thin film toehniqne mentioned above, in the moisture condensation box, with ront.rol singlepigment primem cooulini r q zinc yellow and red lead. The rstilts can be seen moat clearly by reference to Figure 1, ahere tire eonditbo of tho panel is plotted sgainst the duyx of exposure. Under t h e e conditions the performance of nino yellow w ~ l sdefinitely improved hy the presence of ammonium ferroua p h a s phate, and the degree of improvement increased 8s tha proportion of ammonium fcrrom pharphate W&J incrensed. The proportion tor optimum performance in praetiesl systems has not been definitely established. However, in the light of present knowledge, 100% ammonium ferrous phosphate is generally unsatisfactory. It. should be undenrtood that the p&ts wed in these testa sre experimental compositions and do not neeesvsrily represent the optimum formulations for the type of expasue or pigment uswl. Figure 2 shows the results of similar tests wing systems containing, in addit,ion to the zinc yellow, iron
May. 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
459
oxide (foi hiding) and extender. The vehicle was a
blend of long-oil alkyd and linseed oil and the paints were applied to bright s l d An iron oxide primer was included 8s an additional control. This t-eries of tests lixewire showed marked improvement after the addition of SmmoniuM ferrous phoaphste, the improvement being more marked as the p m pation sas increased. These teats were supplemented by the usual fence expos~iia in Florida and Xew Jeney. Primer paints of the type discussed in the previous paragraph were s p plied in the laboratory to both clean and ruaty steel Figure 5. Toputateri Primer Films on Clean Steel, Exposed 9 Months at Moan Tide snd (45- south) Lwei in Florida b o t h looations with and Prime, Pigms,.torion rithaut a topcoat. Figure IS. U d b*d, llm% (so% pismcnt ro1ums1 3 shows the condition of the 14. Zinc y ~ l l o x / ~ i = n " * / * i t , n i v m dioxid+xandsc, s s / s l l s l u W% pismsnt volume1 IS. Uke 16. but. 15 pan- rior p1a)bxmplacsd by smmoniu.n ferrous phaphsts prerusted F l o r i d s panels after 2Emonth exposure. Prime, vehicle IS. Rnr lioeaad oil Figure 4 presents panels from 11.15. Phenolir/alkyd, 61/31 t,he aame series of paints exTop C h f . Umk B'P)~. 100% alkyd nosed without * tomost for 3 years on c l a m steel i n h'ew Jeney. The ainc yellow oontrol shows an improved performance CHEMICAL P ~ ~ s n ~ r eUnless s. uiiiitiud precautions are over this substratum although a little rusting is evident in the followed, B certain amount of the iron is oxidized to the ferric brush marks. With mmonium ferrous phoiiphate present, a condition and appews in tile product as hydrous ferric oxide, definite improvement can be seen. ferric phosphate, or possibly a double salt of tho latter with Figure 5 represen- B still different type of tesr-ntunoly, 9ammonium phosphate. The purity of tho product-i.e., the month immenion a t tide level in Florida. fled lead in oil is ammoniiim ferrous phosphate coiltent, which ordinarily run8 shown &s a control. The second paint -vasof the S a v y 52-1'-18 sLoiiL can bo determined most easily from the propor(INT) type (phenolio/alkyd vehicle) with zinc yellow mpresenttion of the tala1 iron in the ierrous dtste. Ilnterial of even 70% ing 559" of the total pigmentstion. Io the third pbnel p u t of purity (around 9% ferric iron) appears normal by hinetional tests. the zinc yellow in the latter forrnulu has been replaced by amTho following is a typical anslysis: monium ferrous phosphate. All of the pen& had a pigmsnted alkyd topcoat. Here again tho introduction of ammonium ferrous phosphate ha? resulted in improved performance. These results were comidered sufficiently outstanding to warrant proceeding with more praetioal character tests to establish the merit of ammonium ferrous phosphate under field conditions. Numerous evaluation &"dies of this sort, including the behavior of ammonium ferrous phosphate with pigments other than zinc When h a t e d above 100" C., ammonium ferrous phosphate i s yellow, are underway; the results continue to show promise. decomposed, the decomposition becoming more rapid a8 the PUEPARAI'ION Of AMMONIUM FERROUS YHOSYHATE
.Ammonium ierrous plioephate is prepared by the interaction of a ferrous salt with ammonium and phosphate ion-for example, by adding diammonium hydrogen phosphate. Formation of the compound spperently OCC~LTSin two distinct, step: the preeipitation of ferrous hydrogen phosphate, and the omversion of the latter to the ammonium salt, SH.FePOI.M,O. Tho former is 8 grayish flocculent precipitate; the latter is crystalline, and i t 8 flakelike psrticles give an opalescent appearance to the slurry. The desired reaot,ions~ e mover r s fsirly wide range of experimental conditions and are controlled largely by the laorors of slxslinity, ammonium ion conoentration, and conditions influencing oxidstion of the iron. The safe maximum pH is about 3.5; sbove this, nomsl ferrous phosphate forms snd ia converted with difficulry,if at all, to the ammonium salt.
temperntiire increases. A t 160' C. it loaes sll of its ammonia in about 16 hours; a t the same time, d l of che iron iB oxidized and appears as ferric phosphate. There is some evidence of decomposirion below 100" but it is V e r y slow. Ammonium ferrous phosphate i s slowly hydrolyzed in water wid is rapidly decomposed in alksline solutions with the formetion of ammonia. I n &is latter c-e, B black residue ia left, probably B hydrated ferrous oxide ( S ) . Tho pH of rtn tlqiieous suspension is &bout8.
e.,
P w ~ s r cPROPERTLES. ~~ Ammonium ferrous phosphate i s B somewhat bulky, graykh-green solid with s specific gravity of 2.5. ftisquiteinsoIoble,IOOml,of nsatnratedsolutionat25°C. containing 0.02 gram. Microscopic and electron micrograph studies show it to eomsiat of thin irregular plbtes ranging in diameter from 0.02 to 37 microns, and in tbiokness up to 0.3 micron (Figure 6). Its complete psrticle size distribution has not been determined.
4w
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Tbe refraciive index 01 ~nmioiiiumferroun phwphate is approximat,ely 1.6. Between crossed Nicols only part of the m a t e rid appears anisotropic, and it shows oblique entinct,ion. I t is probable th&t the whole of the meteriitl is anisotropio, but be-
Vol. 36. No. 5
permeability measurements on dctmhed filma have shorn ~n improvement in this property to result when ammonium ferrous phosphate is sdded to certain systems. Again, w a t e ~sensitivity as evidenced by blistering. loss of adhesion, eto., represents B complex set of phenomena snd the final result is due to no one cause. Several tests have indicated ammonium ferrous phosphate to decrease water sensitivity to B significant degree, and further resesrch along this line is warranted Mention should be made of the role of aoid-aocepting msterials in relation to corrosion occurring under a paint film due to soids liberated during drying of the psints or carried through the film during exposure l a industrial atmospheres. While not BO active BS the more soluble zinc yellow, ammonium ferrous phosphste has been shown to act as B solid-phase buffer and its beneficial notion in nome o ~ e is s probably due to this fact. Its psrticulsr advantage may he tbst the equilibrium pH value obtained with it is at.8 relatively high level, oompred with other substances used in this wsy 89 mid acceptors.
pp&m
Md"m Weak Very ws.k Wsak Wask Vary weak Medium Medium
Var weak
Figure 6. (+hour) Amrnoaium Ferrous Phosphate Dispersed m a Semialkyd Vehicle ( X 1000); (Below) X-Ray Diffriotion Pattern of Amnionium
Ver weak
Aluminum Filter)
Madivm Wsak Weak strong Went Very weak Weak
MK~== Very weak
Very weak vwu atrmg
Ferrous Phoiphate ((lapper Radiation, Nickel and
c~)useof the platelike habit, detection of chis is not possible. X-ray diffraction data point to the existence of s crystalline compound having a eharsoteristic structure independent of the mode of preparation. The diffmction pattern is shown in Figure 6 for a sample prepared under nitrogen and dried in B vacuum, and dais on interplanhr sp8eings are presented in Table I. They have been oompared with spacings for the other compounds which might be present and appear to be unique ACTION IN PAINT FILMS
No complete explsnation for the improved behavior of primers containing mmoniun ierrou phosphate c&n be given at this time, and it would be imprudent to postulat,e &hemechanism of its behavior. Nevertheless, it is of interest to record preliminary observations and to suggest possible factorsof speculative interest. Ammonium ferrouspho8phat.e. unlike chromate pigments aueh LW zinc yeUow, does not appear to have 8 speoi6o paeaivating aotion, according to evidence available at present. The Istter factor, however, may have definite importance under certain oonditiuns; for example, it may modify the effect of more active psssivating pigments. The @e 8tNCture of ammonium ferrous phosphste was mentioned sbove, and is not only evidedt during preCipitatiQn but can also be seen readily in a paint under the microscope (Figure 6). Flake pigments have 8 well established place in the formulation of metal protective paints; while leafing is less -ked than with mica OT aluminum,for example, this propercy may st least partly contribute to the enhanced performance which has been given by the sddition of 6mmonium ferrous phasphsts. Film impermeability to water and water vapor plays a large part in the protection obtsioed under snme experimental conditions. I t is probably related to the flake structure and lementioned sbove, but will obviously also be affected in great degree by the vehicle component. It i8 likewise B function of interdependent fwtors contributed by both pigment a n d v e h i c l e for euunple, reactivity or character of the 6Lm as determined by the pigment/binder ratio. matever the explanation, im-
Very weah Very weak Weak Very wenk Wssk. Very wt*k Very weak
There is little direct experimental evidence as yet bearing on the influence of ammonium ferrous phosphate coosidered simply 88 a ferrous salt. The p s i b l e function of ferrous ion ~9 &n iohibitor of anodic oorrasion of iron w&8 mentioned above and may serve at least. to explain the observed fsets partially. Furthermore, the relative e m with which ferrous salts we oxidized suggests that they may have value simply w oxygen acceptors. Moisture, seidity, sad oxygen have been recognized as the t h m mast important faotors contributing to cornsion. Ammonium ferrous phosphste may be unique in supplying, in o single compound, the properties required to minimize their iofluenre-namely, impermeability, scid soceptaoee, and oxygen acceptance. The validity of this hypothesis m w t obvio-uely be tasted by further research. It is hoped that a clear-cut a w e r will be afforded, not only by practical evalustions now in prop rean, but &o by more bssic studies an the influence of ammoninin ferrous phosphate on paint film properties. ACKNOWLEDGMEM'
Appre~ia~ion is expressed to members of the researoh staR of this lsbomkxy for sssiswce in various phases of the work. Aoknowledgment is also made to the Experimental Station of the Chemical Department, E. I. du Pont de Nemaura & Company. lnc., for the development of special testing methods. LlTERATURE CITED
(1) Brinsolara, A. A., Denslow. R. R.. and Rumbel. S. W.. IND.Ena. C-.. 29,666 (19371. (2) Kittelberper. W. W., I6id.. 34.363 (1942). (3) Otto. F. J., J. pro!& C h . , 2, 408 (1834). (4)
Patterson. 0.D.. a d Sloao. C . K.,I". 16, 234 (IW).
EN*.Cam m.. &I&.
Eo..
Pasarrrm bsfore the Division of Plint. Vllmuh, and Pllatie. Chsrnistr? st tbs 106th Meetiog of the Arrsm*s C B ~ ~ FSoczrrr, AL Piftaburph. R.
