INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1952 (4) (5)
Beard, E. E., U. S. Patents 2,476,950-1 (July 26, 1949). Brophy, J. E., Larson, J., and Militz, R. O., Trans. Am.
(15) Sac.
(16) (17)
Mech. Engrs., 70, 929 (1948).
Brophy, J. E., Larson, J., Singleterry, C. R., and Zisman, W. A., “Lubrication of Electric Motor Ball Bearings a t High Temperatures,” NRL Rept., to be published. (7) Brophy, J. E., Militz, R. O., and Zisman, W. A., Trans. Am.
(6)
Kaufman, G., Finn, W. J., and Harrington, R. J., I N D . ENG. CHEM.,ANAL.E D . , 11, 108 (1939). Kelly, C . I., Petroleum Times, 50, 903 (1946). Klemgard, E. N., “Lubricating Greases,” p. 686, New Y o r k , Reinhold, 1937.
(18) Lubrication, 35, 133 (1949).
(19) McGregor, R. R., and Warrick, E. L., U. S. Patents 2,389,802-7 (Nov. 27, 1945). (20) Merker, R. L., and Zisman, W. A., I N D . E N G . CHEM.,41, 2546 (1949). (21) Murphy, C. M.,Ravner, H., and Smith, N. L., Ibid., 42, 2479
SOC.Mech. Engrs., 68, 355 (1946). (8) Currie, C . C., and Hommel, M. C., I N D . E N G . CHEM.,42, 2452 (1950). (9) (10) (11)
563
Dahlen, M., Ibid., 31, 839 (1939). Dennison, G. H., U. S. Patents 2,398,315-6 (April 16, 1946). Farrington, B. B., and Humphries, R. L., I N D .E N G .CHEM.,31,
(1950). (22)
Murphy, C. M., Saunders, C. E., and Smith, D. C., Ibid., 4 2 , 2 4 6 2 (1950).
Ninos, N. J., Inst. Spokesman, 15, No. 3, 8 (June, 1951); IVavaZ Ordnance Rept. No. T-647-18, P50 #224 (June 4, 1947). (24) Singleterry, C . R., and Stone, E. E., J. CoZEoid Sci., 6, 171 (1951).
230 (1939).
(23)
(12)
Findlayson, C . M., and McCarthy, P. R., Inst. Spokesman, 14,
(13) (14)
Hain, G. M., A S T M Bull., No. 147, 86 (1947). Hamm, F. A., and Van Noxman, E. J., J . Applied Phys., 19, 1097
NO.2, 13-23 (1950).
,
(1948).
RECEIVED for review SepternLer 14,1951. ACCEPTED December 11, 1981 Presented in part before the Meeting of the National Lubricating Grease Institute. Chicago, Ill., October 30,1951.
Water Resistance of Coatings Containing Nitrogenous Resins -
HENRY GRINSFELDER Research Laboratories, Rohm & Haas Co., Philadelphia; P a .
FINISH may fail in any one of several ways, such as by tions, and formulation modifications. Finally, an attempt scratching, abrasion, under-film corrosion, fatigue, or chemiwas made t o obtain a more quantitative and fundamental cal deterioration. For some time paint chemists have concerned understanding of the effect of water on baked finishes by free-film themselves with the property of water resistance. It is difficult techniques. to point out the most important milestone along the road to imRESULTS AND DISCUSSION proved coatings, but the work of ’CVirsching(9) a t General Motors research laboratory is one. In a classic study about thirteen At the outset of this study an effort was made to establish a years ago, he pointed out the correlation between the amount of good and rapid test method for the studies undertaken. The moisture in the air as measured by dew formation and deterioracriteria used of a good test method were that i t is reproducible, tion of paint films. quantitatively accurate, inexpensive, simple to operate, and reSince then, the use of nitrogenous resins has assumed an imporlated to actual service. Actually, of course, to achieve all five tant place in the coatings field. According to recent figures repoints is often quite difficult, and the most difficult is to simulate leased by the United States Tariff Commission, the sale of ureain the laboratory the conditions of actual service. Even if this formaldehyde and melamine-formaldehyde resins for coatings is were possible, the time involved would be so great as to be impracpresently well over two million pounds per month on a solids tical. Generally, the industry has conducted accelerated tests basis. By using the proper conversion factor for specific gravity, by placing coated panels in water, exposing them t o humid atformulation, and film thickness, it has been calculated that close mospheres or combinations of water or water vapor and ultravioto one billion square feet of metal surface is being finished each let light. In most instances the films have been judged for detemonth in the United States with an enamel containing nitrogerioration by noting the blistering characteristics. For the most nous resins. This rather sizable area of surface is eventuallyfound part, that practice has been followed in much of the work reported in or near the homes and on such common items as refrigerators, kitchen cabinets, stoves, washing machines, autoTABLEI. RESULTS OF WATERIMMERSION TESTAT 165’ F. mobiles, metal Venetian blinds, bathPanels, one spray coat over solvent-sanded, solvent-rinsed furniture stock steel, unprimed. Back and room scales, and the like. All of these edges sealed with 100% phenolic 121/n gallon China-wood oil varnish are exposed to water a t one time or anNitrogenous resin type Urea Melamine Triazine other and in varying degrees, depending NonNonAlkyd resin type Oxidizing oxidizing oxidizing oxidizing Oxidizing Nonoxidizing on the vagaries and variability of the Ratiq, alkyd-nitrogen cleaning woman, maid, housekeeper, and resin 85-35 60-40 75-25 70-30 65-35 75-25 60-40 70-30 the family as a whole. 60” gloss (40-hour rePROCEDURE
The first phases of the investigation were concerned with laboratory methods of studying the effect of water on coatings. Later studies were made to determine the effect on water resistance of compositional changes in the nitrogen resins, variations in application condi-
sults) % loss AdhesionQ (16-hour results , % loss Pencil hardness (16-hour results), number of pencils softer than initially Blister appearance (16hour results) Over-all ratingb
70
70
56
11
52
0
0
100
100
82
23
6
23
8
10
8
7
4
3
0
5
1
2
9 9 5 1 8 7 6 4 (poorest) a Bell Laboratory Mar Adhesion Tester, pocket model, used. b See Table I1 for rating system.
3 2
4 5
3 1 (best)
2
2.4
2
564
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE 11. BLISTERRATINGSYSTELI
Rating 1 2 3 4
10
Blister Appearance None Scattered, minute blisters Uniform minute blisters. l/ls-inch diameter or less Scattered tinv blisters
Very, very large blisters, approximately 1-inch diameter
Vol. 44, No. 3
mers in water immersion results as well as the speed of the test, led the authors away from humidity exposure testing. Early in the development of the test method, aerated water was used in the 96" F. bath, but it was found that aerating the bath was a less severe test and offered no advantages, so that practice was discontinued. Establishment of an acceptable test method has been the subject of much study, particularly among the members of A.S.T.M. Committee D-1 ( 1 ) . From the work done in this committee and the above preliminary work in the author's laboratory, it was apparent that an appreciable uncontrollable variation in blistering is obtained in water or humidity exposure tests. The method finally adopted was t o immerse three panels prepared under identical conditions. At the end of the immersion period, the panels were rated according to degree of blistering by two or more independent inspectors. The ratings were then analyzed for statistical significance using the Kendall-Smith test, If the Kendall-Smith test showed that the results as a group were statistically significant, resort was then made to comparing two sets of panels, one against the other, using the findings of Dixon ( b ) . By Dixon's method, for samples of three, it is necessary for all three panels of one set to be rated superior to each panel of the second set for significance. Inasmuch as the urea resin gave the poorest results of the nitrogen resin types in the hot water test, it was believed that a study of its composition and variations of the composition would provide the most beneficial results. In all coating work of this nature there are many application and film properties of impoitance other than water resistance. Such properties as baking speed, enamel viscosity, film color, film gloss, adhesion, flexibility, and hardness are critical. In the experimental variations made on standard resins it was, there-
herein. For early orientation, a series of coatings containing nitrogenous resins was exposed to each of three types of laboratory exposures. VARIATION AXONG LABORATORY TESTMETHODS. In Table I are presented the results obtained when panels are immersed in water a t 165' F. The enamels were prepared to contain 50% pigment and 5070 binder on a solids content basis. The pigment in all cases was titanium dioxide, Type RA. Enamels were reduced to spray viscosity with xylene. Spray viscosity was selected as 22 seconds viscosity on a No. 4 Ford viscosity cup. All panels were solvent-sanded, solvent-rinsed furniture stock steel. KO primer was used. Back and edges of the panels were sealed with a 100% phenolic 121/2-gallonoil length China-wood oil varnish. Several trends may be observed from these results. First, the adhesion, with one exception, is more markedly reduced than is gloss, Generally, too, the triazine resin is superior t o the melamine resin, which in turn is superior to the urea resin in resisting water a t 165" F. The nonoxidizing alkyd is superior in its performance to the oxidizing- alkvd and in the two instances where only the alkyd content was varied, t h a t is, with the triazine resin, increasing the alkyd resin content detracts from the TABLE 111. RESULTS OF HOTTO COLDHUMIDITY CYCLE"TESTS performance of the coating. The sysh-itrogenous resin type Urea Melamine Triazine tem for rating blister appearance is given NonSonin Table 11. Alkyd resin type Oxidizing oxidizing Oxidizing oxidizing oxidizing Nonoxidizing In Table I11 are presented the results Ratiq, alkyd-nitrogen resin 65-35 60-40 75-25 70-30 65-35 75-25 60-40 70-30 obtained upon exposure of coated panels, 22 11 1 5 4 4 10 5 60' gloss, % loss Adhesion?, 73 loss 23 77 38 60 25 30 35 70 similar to those of Table I, to 6 cycles Pencil hardness, number from 120' to 14" F. of air of 100% relof pencils softer t h a n 5 3 2 2 2 3 2 initially 2 ative humidity. The results generally 8 4 5 1 2 7 6 Over-all rating 3 (best) correlate well with those of Table I. Six cyoles of exposure t o air a t 120' F. and 100% relative humidity a n d air a t 140' F. a n d 100% In Table IV are presented the appearrelative humidity. ance results obtained Then panels similar b Bell Laboratory Mar Adhesion Tester, pocket model, used. to those of Table I were exposed steadily to atmosphere a t 100' F. and 100% relative humidity. This test is OF 8 MONTHS OF STEADY EXPOSURE TO 100% RELATIVE HUMIDITY TABLE Iv. RESULTS commonly referred to as the "cook box" AT 100" F. test. Nitrogenous resin type Urea Melamine Triazine In Table V are the combined sumrllkyd resin type OxiNonOxiNonmary ratings from Tables I, 111, and IV. dizing oxidizing dizing oxidizing Oxidizing Nonoxidizing There does appear to be a slight degree Ratio, alkydnitrogen resin 65-35 60-40 75-25 70-30 65-35 75-25 60-40 70-30 of correlation between the three different Crow's feet 0.k. A few small 0.k. 0 . k . Intense Intense Appearance Dull checks clusters of minute tiny types of water exposure. However, it blisters bliaters blisters Rating 6 5 1 4 1 1 2 3 is also apparent t h a t "water resistance" is a term of many possible meanings. It would appear that a coating might be T A B L E v. REL.4TIVE RESISTANCE RATINGS more water resistant to the liquid form Triazine Nitrogenous resin type Urea LIelamine of water than to the vapor form. SonNonWhich form of water is the one encounOxidizing oxidizing Oxidizing oxidizing Oxidizing Nonoxidizing Alkyd resin type tered in actual service is indeterminate Ratiq, alkyd-nitrogen resin 65-35 60-40 75-25 70-30 65-35 75-25 60-40 70-30 a t this moment and probably varies from 8 7 6 4 2 5 1 2 165" F. water 3 8 4 5 1 2 7 120' t o 14' F. cycles 6 application to application. Cook box 6 5 1 4 1 1 2 3 It was decided to select the 16Over-all rating 7 8 4 6 1 2 3 4 hour immersion a t 165" F. as the (poorest) (best) screening test. Interest of many custo$
Q
March 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
fore, necessary not only to observe these properties, but to maintain them in their commercially acceptable form. In the data which follow, the general application, handling, and film properties are not reported. SOLVENT STUDIES. One of the easiest composition variables to study, and hence the first, was that of the solvent in which the resin is supplied. The results obtained are given in Table VI.
TABLE VIII.
565
EFFECTOF VARIOUSTYPESOF FORMALDEHYDE
Formulation and panel preparation, same as i n Table VI Low Salt
.Water resistancea, 16 hours at
TABLE VI.
SOLVENT BALANCE STUDIES
Water resistance, 16 hours a t 165' F. Solvent Balance Panel 1 Panel 2
Panel 3
165' F. 5 4 Pencil hardnessb B B 60° gloss, Photovolt 67 62 a See Table I1 for rating system. b See Table VI1 for rating system.
B
3
1 3B
61
75
Series A Xylene-butanol
5 5
20-30 25-25 32-18
5 4
5
6
Series B
Xylene-butanol-2-ethylhexanol 25-20-5 25-15-10 26-5-20
5 5 5
*
7
7
7
6 6
8
7 7 7
Formulation d r y basis 5 2 . 5 % piiment, 95 0 titanium dioxide dioxide, Tvpe Type RA zinc oxide, F Florence l o r e h e White Seal 47.5% binder, 70 0 oxidizing alkyd 30% urea formaldehyde resina Reduce to spray viscosity with xylol Panels prepared: Same as in Table I a A different urea resin was used i n series B than in series A.
5'$
TABLEVII. COMPARISON
O F n-BuTYL ALCOHOLTO CAPRYL ALCOHOL
Formulation, solids basis: 50% igment 95% titanium dioxide, Type RA 5% zinq oxide, Kadox 15; 50& bind&, 60% exidizing alkyd and 40% urea resin. Reduce with xylene t o spray viscosity. Panels: Triplicate panels, I spray coat over solvent-sanded, solventwashed furniture stock steel. Baked 30 minutes a t 300' F. Capryln-Butyl Ca ryl AlcoholBlend n-Butyl AlcoholZO/SO 10/90 Urea Resin 8 r e a Resin Water resistance", 16 5 5 hours a t 165' F. 1 3 B HB HB F Pencil hardnessa 59 61 60' gloss, Photovolt 69 60 a Table I1 rating system. b Pencil hardness scale : Softest +Harded 6B 5B 4 B 3B 2B B H B F H 2H 3 H
__
In this study, although slight differences in degree of blistering were obtained, the differences are not more than normal experimental variation. As the amount of butanol in series A increases, the viscosity decreases. I n series B, the use of 2-ethylhexanol reduced viscosity. While viscosity is not a significant property in regard t o the subject a t hand, it nevertheless is an important economic factor and must be considered in developing a resin. In Table VI1 is a comparison of two alcohols used in the alcoholation stage in making the resin. Based on these results it appears t h a t the choice of alcohol influences the water resistance. Other studies with alcohols more water soluble than butanol gave resins much poorer in resisting the water immersion test, so that efforts to improve the water resistance by moving in that direction are not advisable. FORMALDEHYDE VARIATIONS. One of the next ingredients considered worthy of investigation was the formaldehyde. Actually, there are several types of formaldehyde available. The four variations that were included in this study were a high salt content grade; a low salt content grade; a low salt content grade obtained by use of ion exchange resin treatment of the high salt content grade; and flake paraform. In all cases, the same type of urea formaldehyde resin was prepared and maintained as close chemically as was possible. The results of the study are presented in summation in Table VIII.
From these data it appears that treating the high salt content formaldehyde with ion exchange resins improves the water resistance. The ion exchange resin-treated formaldehyde is also superior to a normal low salt content grade. The resin based on flake paraformaldehyde is the best of all resins tested for water resistance and is the slowest curing as measured by pencil hardn e k It has the best Photovolt gloss. The low salt content grade of formaldehyde is superior in water resistance to the high salt content grade. CATALYSTS.The use of acid catalysts in the polycondensation of urea and formaldehyde is, of course, well known. The final study in this, the first phase of the program, was to investigate the effect of acid catalysts on the behavior, particularly the water resistance, of the paint films. The results are presented in Table IX. The use of phthalic and citric acid catalysts did not improve the curing speed of the resins, nor was the water resistance markedly altered. A slight indication of a n unfavorable nature exists for the use of citric acid. Doubling the concentration of regular catalyst caused instability in the enamel, so t h a t possible method of improving water resistance was discarded. Actually, trials were made t o stabilize the enamel system by the incorporation of small quantities of triethylamine. However, other difficulties were encountered, such as obnoxious odor and incompatibility with enamels containing zinc oxide. Comparing the results in series B and C, which were the same except for pigment content, it is noted that the formulations containing zinc oxide in the pigmentation were superior (but not statistically) in water resistance t o those based on straight titanium
TABLE IX. EFFECT OF VARIATION IN CONDENSATION CATALYST Water Resistance
(16 Hours a t 165' F,) ___-
Catalyst
Catalyst Content
Blister Rating" Pencil Panel 1 Panel 2 Panel 3 Hardnessb
Series A Regular Reg u 1a r Series B Regular Regular Regular and phthalic acida Regular and citric acid0
7
5 5
7 6
B
6
5
7
B
7
5
7
B
10 9
F F
10
F
Regular 1/2 regular amount Regular
6
Regular
Series C Regular Regular 5 8 Regular I / P regular 8 8 Regular and phthalic Regular 9 9 acid Regular and citric acid Regular 5 8 Formulation, solids basis igment. 100% titanium dioxide, Type RA 50% 60% Einder, 70% oxidizing alkyd 30 % urea resin Panels, prepared as in Table VI1 See Table I1 for rating system. b See Table VI1 for rating system. 0 Added to the resin after condensation.
8
B
F
INDUSTRIAL AND ENGINEERING CHEMISTRY
566
AND FILMHARDNESS TABLEX. WATERRESISTAXCE
Baking schedule effect Formula l a Water resistancec Hardnessd 7 B 6 6B 8 3B
Formula 2 b Water Baking Schedule resistancec Hardnessd 6 3B 30 minutes a t 250' F. 1 6B 7 minutes a t 300' F. 5 3B 9 minutes at 300' F. 8 B 2 3B 11 minutes a t 300' F. 4 B 1 5 minutes a t 300' F. 7 IIB 8 F 6 B 30 minutesat 300' F. a Pigment, 1 part, 100% titanium dioxide, Type RA. Binder, 0.9 p a r t , 70% oxidizing alkyd 30% urea resin. b Pigment, 1 part, b5% titanium dioxide, Type RA, 5 % zinc oxide, Florence White Seal. Binder 0.9 Dart. 70% oxidisine alkyd. 30% urea resin. c Table I1 rating s y s t e k . d Table VI1 rating system. I
_
TABLE XI. ALKYDRESINEFFECT Formulation: pigment, 1 part, 95% titanium dioxide, Type RA, 5 % zinc oxide, Florence White Seal. Binder, 0.9 part, alkyd resin, urea resin, balance t o loo%, a s indicated
60 70 Konoxidizing 60 70 a Table I1 rating system. b Table VI1 rating system.
Oxidizing
6 4 4
3
B
B F B
8 8 4 4
H F
F
B
dioxide. Also, the panels in series B were somewhat softer on a pencil test than those in series C. A similar relationship can be seen in Tables VI, VII, and VIII. This immediately raised the question as to the influence of such factors as curing time and temperature and formulation variables on the property of wat,er resistance. Consequent,ly, the next or second phase of the study was undertaken. APPLICATION COXDITIOXEFFECT.Examination of Table X shows that the use of formula 2 containing 5% zinc oxide greatly improves the water resistance of the urea resin tested, a t the same time considerably softening the film. LilieTvise, shortening the the baking schedule t,ends t,o improve the xater resistance in most, cases, but not to t,he extent of t'he improvement to be gained by the use of zinc oxide, Another interesting point noted is that while softer films usually are superior for water resistance, the pencil hardness value is not an accurate measure of the water resistance of the coating. Considerable differences in blistering tendency are not,ed in several of the films having equal pencil hardness values. From t'his it must be agreed that, the effect of zinc, oxide in the film goes beyond a mere sloring doxn of curing speed because of its basicit'y and ability to neutralize the acid catalyst. This is apparent when it is not,ed that many of the harder films containing zinc oxide are superior to softer films without zinc oxide. The general superiority of the zinc oxide films over the straight titanium dioxide films also bears this out. A comparison was next made to st'udg the ALKYDRESINS. influence of the alkyd resin by type and by amount on the water resistance of coatings. Based on the results shown in Table X I , it appears that a nonoxidizing alkyd is superior in 165' F. water resistance to the oxidizing-type alkyd under comparable conditions. Although not presented as data, it must be remembered that the nonoxidizing alkyds are poor in adhesion, but superior in gloss, color, color retention on heating, and alkali resist.ance as compared to the oxidizing alkyds. TJsually too, gloss and adhesion improve with increased alkyd concentration, while color does not. There is a slight tendency for the alkyds to give improved resistance as their concentration is increased. This is in contrast to the earlier findings when a triazine resin was used (see Table I). The effect of zinc oxide in the pigmentation is shown in more detail in Table XII. The results indicate that the use of zinc oxide is beneficial and confirm t h a t a concentration of 5% is as good as one of 10%. The narrow spread in results with the nonoxidis-
Vol. 44, No. 3
ing alkyd is in contrast to the wide spread with the oxi 3izing typp, indicating again that the latter is more senjitive to formulation and application variations. I t is necessary to mention that use of zinc oxide decreases the alkali resistance and color of the film and detracts from the already poor adhesion of the film containing nonoxidizing alkyd. PIGMESTATION.The final formulation variable investigated for its influence on water resistance was that of total pigment' content. The results of this study are presented in Table XIII. It appears that best water resistance is obtained a t low pigment cont,ent, when zinc oxide is present. But a t a zinc oxide content of 5'%) increased total pigment content appears to be beneficial a t the shorter baking schedule. Color was improved a.nd gloss deteriorated as the pigment content was increased. The work to this point indicates that the variables available to the paint formulator are a t least, as significant and influential, if not more so, than the variables a t the disposal of the resin supplier. FREEFILME x ~ ~ ~ ~ I ~ AInT order I o N .to obtain a better understanding of the mechanism causing loss of coating properties on exposure to water, a more fundamental study was initiated. It was hoped that the information so obtained would be useful in the development of baked finishes for longer time use. In general, tests of a more quantitative nature were also selected, in the hope of detecting incipient film deterioration before visible failure appeared. There are undoubtedly many factors that determine the behavior of a coat,ing when it is exposed to water. Much in the way of fundamental work on this subject has been contributed by Kit,telberger and Elm ( 3 , 6 - 7 ) . More recently, Mayne ( 8 ) has raised some questions regarding the contents of blisters formed during wat,er immersion. In a further effort to obtain a better understanding of the factors which may be of imporbance during t,he blistering process, balred enamels were prepared as free films and studied. It was hoped that a correlation might be found to exist between blist'ering and such properties as moisture vapor transmission, water absorption and extraction, volume change upon immersion, and strength change upon i.mmersion. Two series of panels were prepared as described in Table S I V . (Enamels numbered l;i, lB, and 1C are for successively increasing baking schedules using the same formulation. Enamels 2A and 2B contain different nitrogenous resins; the ratios of constituents and the baking schedules have been ma.intained constant. The variable between enamels 2R and 2C is the ratio of alkyd to tri-
T-kBLE
a
b
OF XII. EFFECT
ZINC
OXIDECOXTENT
Water resistances and hardnessb 15-minute bake a t 300' F. Formulations, same as Table X Zinc Oxide Content, % of Total Pigment 0% 5% Konoxidizing alkyd 60% alkyd 5(W 3(F) 70% alkyd 7 (F) 4(B) Oxidizing alkyd 60% alkyd O(F) 6(B) 70% alkyd 9(H) 3(B) Table I1 rating system. Table V I 1 rating system.
10 % 307)
...
6(B)
. .
TABLEXIII. EFFECTO F T O T ~PIGVEXT L CowmiPigment-Binder Ratio
45-55 52 6-47 45-55 52.5-47 45-55 52 5-47 46-55 52 5-47 a
5
Baking Schedule, Minutes a t 300° F. 15 15 30 30
5
15 15
5
30 30
5
Table I1 rating system.
ZnO. 0 0 0 0 6 0 5 0
Water Resistancea 7 9 9 9 6 4
7 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1952
567
proved to be unexpectedly quite difficult, particularly a t the
TABLE XIV. Series 1.
ENAMEL FORMULATIONS AND BAKINGSCHEDULES temperature of 165' F. At 165' F., immersed in water, enamel 1A had an ultimate 1 part pigment 100% titanium dioxide Type RA 0 . 9 part binder, bo% oxidizing-type alkyd,'30% urea resin tensile strength of less than 20 pounds per square inch. At this
Raking Schedule 1A 250' F. for 20 minutes 1B 250' F. for 90 minutes 1C 300' F. for 30 minutes
Baking schedule, 300' F. for 30 minutes
azine resin.) The coating films prepared for free-film studies wer applied to tin-surfaced steel panels and removed by mercury amalgamation ( 4 ) . Blistering studies made use of furniture stock steel and copper substrates. Blistering tests were conducted at 165' F. in tap water for an immersion time of 16 hours. The baking schedule and the nature of the nitrogenous resin are once again seen t o be important variables. Blistering over a copper substrate was not markedly different from that over steel. All films were cast to approximately a 2-mil dry film thickness. Tensile data were obtained using 0.5 X 0.5 inch specimens on a conventional IP-4 Scott Tensile Tester. Ultimate tensile strengths and Young's modulus results are shown in Table XV and the tendency of the films to blister upon water immersion is shown in Table XVI.
same loading, sample 1B broke after less than 5 hours. These results may be compared with ultimate tensile strengths of 1700 and 3100 pound per square inch for 1A and 1B a t 77" F. and 50% relative humidity. The films were observed to be much more flexible in the hot water than in the room air and also much more elastic. Creep of the films under hot water was found to be difficult to determine by the method that it had been hoped to use, and no results were obtained. It was rather interesting to observe that some films tended to shrink a t the start, as though a stress relaxation were occurring.
/ 'A
TABLEXV. TENSILEPROPERTIES OF BAKED ENAMELS AT 77" F. AND 50% RELATIVE HUMIDITY
Ename 1A 1B 1c 2A 2B 2c
Ultimate Tensile Strength, Lb./Sq. In. Standard Average deviation 1700 300 3100 1250 1040 3400 1400 3700 3100 1070 3200 950
Young's Modulus, Lb./Sq. In. Standard Average deviation ,17,200 2,000 74,700 25,000 102,700 19.000 33,500 8,000 27,000 7,000 30,400 15,900
During the course of this phase of the investigation, it was ascertained that the films behave very closely to the manner of an elastic material. They have a very small yield point at just about the ultimate and appear to obey Hook's law very nicely. From the ultimate tensile strength and Young's modulus data it is possible to calculate the area under the stress-strain curve, and hence obtain an indication of the film toughness. The toughness factor for the coatings was calculated to be 84, 64.5, 56.2, 204, 178, and 169, respectively. A correlation between blistering and film strength does not appear to exist. However, the films with the lowest elongation blistered the most. TABLEXVI.
Enamel 1A 1B
1C
a
2A 2B 2c See Table 11.
BLISTERING AND UNDER-FILMCORROSIONOF BAKEDENAMELS ON STEEL 16-hour immeraion in 1 6 5 O F. water Under-Film Blistering4 Corrosion, Rating 4 Third 6 Second 6 Second 4 Third 2 Best, least corrosion 2 Best
Film strength up to now has been determined on dry films only. It is highly possible that the strength of wet films is considerably different from that of dry films. Efforts were made to determine the tensile strength, elongation, and creep of wet films. This
Figure 1. Moisture-Vapor Transmission of Baked Enamels at 77' F.
Moisture-vapor transmission data are shown in Figure 1. These data were obtained a t 77" F. in a 50% relative humidity atmosphere using anhydrous calcium chloride in the moisturevapor transmission cup. I t may be noted that an increased bake causes a reduction in the moisture-vapor transmission in series I. Series 2 shows that this property may also be influenced by the nature of the nitrogenous resin. Water pickup and extraction data were obtained and these are shown in Table XVII. These data indicate that the undercured films are more water sensitive. There is no indication that equilibrium has been reached at 107 hours and it is likely that it is not the case. There does not appear to be any relationship between water pickup or extraction and blistering. There does, however, appear to be a close and direct relationship between moisture vapor transmissivity and water extractibles a t 107 hours. (A close relationship also may exist between water ab-
TABLEXVII. WATER PICKUP AND EXTRACTION OF BAKED ENAMELS AT 165" F. IMMERSED IN WATER Enamel 1A 1B 1C 2A
2B
18-Hour Immersion" Wb EO 12.82 ~ 7 % 4.3% 2.4%
107-Hour Immersiona
W 1.5% 3.3% 3.4% 4.1%
2C All values are &2% except as indicated. b W % net water pickup based on initial weight. 0 E = Yo extraction by water based on initial weight.
5
-
E
22.4% 6 . 9 %1 7 %
1.8% 2.0%
568
INDUSTRIAL AND ENGINEERING CHEMISTRY
sorption and moisture-vapor transmission, but more accurate data are needed to confirm this possibility.) $n attempt was made, without success, to determine the equilibrium moisture of the films, Although it was not successful because of the inherent error of the method selected, it mas interesting to observe that the free films are much easier to prepare into test specimens a t high than a t low relative humidities. Subsequent to the blistering test, film to substrate adhesion was examined qualitatively. One hour after removal of the steel panels from the hot water, it was found t h a t lA, l B , l C , and 2A had negligible, if any, adhesion. Enamels 2B and 2C had some residual adhesion. This decrease in adhesion permitted the films to be removed from their steel substrates. It was found t h a t under-film corrosion had stained the back enamel surface. Based upon the fraction of the area which had been stained, under-film corrosion ratings were attempted. These are shown in Table XVI. CONCLUSIONS
There is some correlation, although not precise, of the results between the three types of water test as t o their deleterious effects on nitrogen resin-containing alkyd coatings. Composition of the nitrogen resin influences the results markedly. The urea resin is much poorer for resisting water exposure than is the melamine or triazine resin. Some improvement can be obtained by altering the composition of the urea resin, but formulation changes appear to be equally as effective. No clear-cut understanding of the mechanism of blister formation is as yet available. -4hypothesis based on these and the
Vol. 44, No. 3
observations of others is that adhesion when wet exerts a dominant influence. The possibility that a film swells upon water absorption and expands because of this absorption, thereby exerting a delaminating influence, is not a remote one. Much remains to be done to study this subject further, both in the way of fundamental studies and from the practical application point of view, also. It may be noted, for example, that such an important variable as film thickness has not been mentioned and it is possible t h a t factor also plays a major role in film performance ACKKOWLEDGMEYT
Acknowledgment for much help and assistance in conducting the experiments, discussing the results, and preparing this paper for presentation are extended to Emory Slaght and to Samuel Gusman. LITERATURE CITED
Boylnn and Wray, A S T M BUZZ.,141,53-5 (March 1949). Dixon, W. J., Ann. Math. Statistics, 11, 119 (1940). Elm, A. C., Oflcial Digest Federation Paint & Varnish Production Clubs, NO.267, 197-288 (1947). Grinsfelder, H., Ibid., 312, 42-52 (January 1951). Kittelberger, W.W., IKD.ENG.CHEM.,34, 943-8 (1942). Kittelberger and Elm, Ibid.,38, 695-9 (1946). Ibid., 39,876-81 (1947). hlayne, J. E. O.,Oil d? Colour Chemists’ Assoc., 33,312-16,538-47 (JUIY 1950). Wirsching, R., Ibid.,33, 234-7 (1941). RECEIVED for review June 21, 1951. ACCEPTED November 14, 1951. Presented as part of the Symposium on ‘Crea, Melamine, and Related Resins before the Division of Paint, Varnish, and Plastics Chemistry, 119th hfeeting of the ERICAN AN CHEMICALSocIErY, Bost,on, &Lass.
BY LOW TEMPERATURE HYDRATIQN OF ALPHA-PHOSPHORUS(V) OXIDE R. N. BELL
L. F. AUDRIETH AND 0. F. HILL
Victor Chemical Works, Chicago Heights, I l l .
Unicersity of Illinois, Urbana, I l l .
YDRATION of the commercially available a- form of phosphorus(V) oxide may give a variety of products, depending upon the mole ratios of phosphorus pentoxide t o mater which are allo-ived t o react. The strong phosphoric acids, containing moIe than 72.401, phosphorus pentoxide and prepared by the high temperature hydration of phosphorus(V) oxide using P20s-Hz0 molar ratios greater than 1 t o 3, have been shonn t o consist of mixtures of poly- and metaphosphoric acids ( 3 ) . [The a- form of phosphorus(V) oxide is correctly represented as PlOlo. There are, however, three other recognized polymorphic forms which are more complex in nature. The empirical formula, PZO,, is used throughout this article, except where structural considerations demand otherwise.] It was therefore surprising t o find t h a t the low temperature hydration of a-phosphorus(V) oxide produces chiefly tetrametaphosphoric acid. Neutralization of this solution with sodium hydroxide has led t o t h e isolation of the tetrasodium salt, now commercially available for t h e first time (Cyclophos, Victor Chemical Works). Sodium tetrametaphosphate was apparently first prepared by Warschauer ( 1 1 ) , who obtained the compound by the treatment
of a suspension of the copper salt n-ith squeous sodium sulfide. The copper salb had been prepared by the interaction of powdered copper oxide with slightly more (5%) than t h e required quantity of phosphoric acid, followed b y slow and gradual heat,ing up to, but not exceeding, 450°C. Bonneman (4) prepared the compound by the 6ame procedure and determined its molecular weight by a cryoscopic procedure using fused sodium sulfate 10-hydrate. Bonneman also listed the characteristic x-ray diffraction spacings for both the 4-hydrate and the anhydrous salts. Three recent publications have dealt with the tetrametaphoEphate prepared by the Warschauer method. Conductance measurements of the acid derived from the salt by an ion exchange method have been presented as evidence for the existence of a discrete P a 0 1 2 ion (6). The cyclic structure of the tetrametaphosphate has been verified by x-ray analysis (1). Careful hydrolysis in alkaline solution has been stated t o yield tetraphosphate (10). Further evidence for the existence of a tetrametaphosphate has been presented by Raistrick and his coworkers ( 8 ) of the Slbright-Wilson Laboratories, who have prepared the sodium salt by a procedure similar to the method used by the authors.
