April 1949
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
is present, is dependent not only on its identity, but on the percentage in which it is present and the particular portion of the polymerization reaction under consideration. ACKNOWLEDGMENT
‘
The authors have had considerable help from many sources a n this problem, and this assistance is gratefully acknowledged. They are especially indebted t o the following contributors: D. H. Wheeler and H. Boyd of General Mills for the spectral analysis, crystallization saturates, and the sample of pure methyl linoleate; S. W. Gloyer of Pittsburgh Plate Glass for fractionated oils; 12. T. Milner for information on thiocyanogen values; and J. D. Parrish of the University of Minnesota Mines Experimcnt Station for the thermocouple calibration. LITERATIJRE C I T E D
(1) 12)
Adams, H. E., and Poweis, P. O., IND.ENG.CHEM.,36, 1124 (1944). American Oil Chemists Society, “Official and Tentative Methods,” 2nd ed., 1946.
749
(3) Bailey, A. E., “Industrial Oil and Fat Products,” p. 685, New
York, Interscience Publishers, 1945. ENG.CHEM., 30, 689 (1938). (4) Bradley, T. F., IND. (5) Cannegieter, D. D., Paint Oil Chem. Rev., 110, No. 4, 16 (1947). (6) Carrick, L. L., Brands, E. R., Cumminge, L. O., Kenney, J. E., and Pemberton. R. W.. Am. Paint J.,28, No. 30, 63: No. 39, 42 (1944). (7) Earle, F. R., and Milner, R. T., Oil & Soap, 17, 106 (1940). (8) Lambou, M. G., and Dollear, F. G., Ibid.,22, 226 (1945). (9) Ibid., 23, 97 (1946). 110) Knauss, C. A., and Smull, J. F., IND. ENC.CHEM., . . Lon=, J. S., 19 62 (1927). (11) Mitchell, J. H., Kraybill, H. R., and Zscheile, F. P., IND.ENG. CHEM.,ANAL.ED.,15, l(1943). (12) Painter, E. P., and Nesbitt, L. L., Ibid.,15, 123 (1943). (13) Paschke, R. F., and Wheeler, D. H., J. Am. Oil Chemists’ Soc., to be published. (14) Von Mikusoh, J. D., IND, ENG.CHEM.,32,106 (1940). RECEIVED February 26, 1948. Presented as a part of t h e Drying Oils Symposium under joint sponsorship of t h e University of Minnesota and the Minnesota Section, AMERICANCHEMICALSOCIETY,Minneavolis. Minn., March 27 t o 29, 1947.
MOISTURE=RESISTANT COATINGS FOR METAL WILLIAM F. SINGLETON AND WILLIAM C. JOHNSON E. I . du Pont de Nemours & Company, Inc., Philadelphia, P a . Data are presented for a range of polymeric coatings on permeability, adhesion to certain metals, and moisture resistance under alkaline conditions. Selected coatings were pigmented to determine the effect of the type and concentration of pigment. The polymers show a practical advantage over a phenolic-tung oil coating cause of superior resistance to alkali, although the permeability is in the same range as the phenolic varnish. Inert pigments offer some improvement over unpigmented films both in adhesion and permeability. Effect of pigmentation is less than would be obtained on oleoresinous films, and optimum pigment concentration is lower.
I
N THE formulation of moisture-resistant organic coatings, certain properties such as permeability and adhesion t o particular substrates furnish guides to the optimum composition for the purpose in mind. The composition will vary according t o the weight given the different properties for each purpose. This paper presents data from permeability, adhesion, and humidity cabinet tests obtained with a group of high polymer film-formers covering a wide range of chemical compositions. The specific problem was t o develop an improved sealing coat for metal objects which would be exposed continuously to moisture-saturated atmospheres in the presence of small concentrations of ammonia. A varnish (6) was already in use and became the control for other coatings developed in the investigation. This varnish consists of 50% p-phenylphenol formaldehyde resin and 50% tung oil in aromatic solvents. COMPOSITION OF COATINGS
The composition of the polymeric coatings is listed in column 2 of Table I. The polymers were put into solution in appropriate solvents a t a viscosity of 1.5 poises, The coating compositions
were intended for application by dip or spray and were force-dried 1 hour a t 150”F. (65” (3.). The viscosity of 1.5 poises is suitable for dipping or flowing. Where the films were t o be applied by spray, the viscosity was reduced further; the degree of dilution varied according t o the nature of the polymer. Several polymers atomized so poorly that coatings could not be prepared successfully by spray. Most of the polymers require plasticizing. Only two plasticizers were considered. Tricresyl phosphate was used t o improve the flexibility and adhesion. Where it was desired t o maintain maximum moisture resistance, liquid chlorinated biphenyl was used. I n all cases, these plasticizers were found t o be compatible with the resins in this series of tests. One of the important properties of a coating composition is the solids content at application viscosity, as this largely controls the thickness of the film. The solids content is listed in Table I, column 3. A number of the compositions gave films which were too weak or tacky. The general quality of each film is described in column 4 of Table I. PERMEABILITY OF UNPIGMENTED FILMS
Transmission rates for all films were determined in the saturated atmosphere over 1% ammonia solution. For this investigation, it was thought advisable t o make them a t 60” C. (140” F.), in order to include service conditions which would be met in the tropics. Since commercial permeability cups made of aluminum are rapidly attacked by ammonia, cups having dimensions of the Payne permeability cup (4) were made from stainless steel. Phosphorus pentoxide was used as the desiccant to absorb ammonia as well as water. The amount of ammonia absorbed was determined by Nessler’s method. The coatings were applied .. at 1 mil ( 2 5 ~ )dry thickness on nonmoisturepriof cellophane. The cellophane sheet is 0.9 mil (23p) thick. Specimens for test were cut with the film in place on cellophane.
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
750
TABLE I.
1
Coating 1 2
3 4 3
6
7 8 9
10
11
12
13 14 15 16 17 18
19 20 21 22 23 24 25 26
27 28 29
30 31 32 33 34 85 G
-
PROPERTIES O F UNPIGlfENTED FILMS
2 Binder, P a r t s 05-1433 phenol formaldehyde-tung oil varnish Vinyl chloride--vinyl acetate-maleic anhydride copolymer Polymer of h-0. 2 , 8 5 Liyiiid chlorinated biphenyl, 13 Polymer of S o . 2, 75 Liquid chlorinated biphenyl, 25 Polymer of N o . 2 , 65 Liquid chlorinated biphenyl, 35 Polvmer of 40.2. 85 Tricresyl phosphate. 1.5 Pnlvmer of No. 2. 7 5 Tricrewl phosphate, 25 Vinyl chloride-vinyl acetate coiiolymer, low viscosit5Polymer of 2 , 4 2 . 6 Polymer of No. 8, 42. 5 Tricresyl phosphate, 16 Polymer of ? i o . 2 , 3 7 . 5 Polymer of No. 8, 3 7 . 5 Tricresvl phosphate, 25 Vinvl chloride-1-invl acetate r o n o l v m ~ rnipdinin visTricresyl phosphate, 15 Vinyl chloride-vinyl acetate c o p o l y m ~ r niediuin , viscosity, 3 7 . 5 Polymer of No. 2 , 3 7 . 5 Tricresyl phosphate, 25 Polvvinvl butvral. low viscosity Polyvinyl butyral; medium viscosity vinylidene chloride-acrylonitrile copolymer Polymer of KO.15, 75 Tricresyl phosphate 25 Modified vinylidene'chloride polymer .4,7.5 Tricresyl phosghat,e, 25 Modified vinylidene chloride polymer B Polymer of No. 18, 75 Tricrcsvl Dhosohate. 25 n-Butyi nlate p o l y ~ n e rhigh , viscosity n-Butyl methacrylate polymer, low viscosity Ether resin, PRI-264 Ether resin, PRI-869 Polyamide resin (dimerized oil acids reacted with a diamine) Polyamide resin, 50 Chlorinated biphenyi resin, 50 20-cp. chlorinated synthetic rubber, 70 Liauid chiorinated biohenvl. 30 2 0 - c ~chlorinated . syniheti; rubber, 60 Liquid chlorinated biphenyl, 40 20-cp. chlorinated synthetic rubber, 65 Tricresyl phosphate, 35 20-cp. chlorinated synthetic rubber, 50 Chlorinated biphenyl resin, 50 125-cp. chlorinated aynthetic rubber, 70 Liquid chlorinated biphenyl, 30 Cyclized synthetic rubber Cyclized synthetic rubber, 75 Triciesyl hosphate, 25 But,yl r u b t e r Pol yisobutslene Gook resin good, E'
-
Vol. 41, No. 4
3 Solids at 1 5 Poises 52 20
4 Film Quality BIittle Good
5 6 Permeability Permeability t o Water, t o Ammonia, hlg./Sq. Mg,/Sq. Cm./Hr. Cm./Hr. 1.6 0.015 1.7 ...
7 Adhesion b y Knife Test" Cadmium Dural Copper Tin P P G P G P P G
8 tAdhesion o Aluminu m rJb./deal In. 0.5 1.5
20
Good
1.4
...
P
P
G
G
1.1
20
Good
1.3
...
P
F
G
G
1.3
19
Good
1.1
0.013
F
VP
G
G
1.1
20
Good
2.9
... ...
F
P
P
G
1.5
G
G
G
G
1.8
T'P
1-P
v 1' P
0 1 0.7
22
Good
3.5
18 22
Good Good
0.9 2.1
...
G
VP P
24
Good
3.0
0.047
G
Q
G
0
2.5
20
Good
1.6
...
G
G
c:
P
1, B
20
Good
3.i
G
G
c:
f
0.4
8 6 13 16
Good Good Good Good
5,2 2.9 0.6 1.5
E' F
8
o.'di3
1' P F VP
G
0.2 0.6
29
Soft
2.1
.**
19 24
Good Soft
0.6 1.9
0:013
21 31 20 20 30
Good Good Good Good Soft
6.7 5.6 2.6
. t I
I..
... ,..
.
.
I
... ...
2.1
... ...
1.6
P
P
P VP
G
F G VP P
P
1'
F
P
1.3
P
P P
1' F
P
x-P
0.3 1.6
1-' F
E
G G
G G 1-
0.3 0.2
P G
G VP P
G
rr
r P
F
G P
P
0.3 1.6
0.5
0.a 0.9
41
Soft
1.6
0,008
F
F
G
G
2.Q
37
Brittle
0.4
...
P
P
P
P
0.2
40
Britile
0.6
0.002
40
Fair
3.0
43
Brittle
0.4
23
Brittle
0.5
Rlitlle
0.4 1.5
18 24 8
7 23
Tacky Tacks Brittle
.
0.3 0.3 3.7
P
F
F
F
0.05
P
P
F
P
0.3
...
P
P
P
I'
0.2
...
P
P
P
P
0.2
I . .
..
.
I
j
0:024
+P
P
P
P
0.002
P
P
P
P
P
P
P
....~ .
..
..
I
.
.
..
P
0.1
0.1 0.8 0.1 0.2
fair, P = poor, and V P = very poor
Determinations of permeability usually are niadc ncar room temperature. The presence of cellophane has been shown to have a negligible effect on the permeability of films in the range of paint coatings under such conditions. Hon-ever, the question arises as to the effect of cellophane a t the higher temperature. Table I1 gives the results of permeability determinations for cellophane alone and for OS-1433 varniih at 25' and 60' C. The results are shon-n both as the transmission rate for water and ammonia through the film and as the permeability constant per millimeter of mercury vapor pressure. I n calculating the permeability constant, the partial pressure of water and ammonia over the solution has been estimated ( 5 ) as follows: K a t e r a t 60" C. Water at 25 ' C. Ammonia a t 60" C. Ammonia a t 25 ' C.
148 mm. of Hg 24
48 11
These represent the vapor pressure difference across the film, as the vapor pressure on the side of the desiccant is negligible. The permeability constant of the varnish doubles both for water and ammonia in the range from 25 O t o 60 O C. The permeability constant of uncoated cellophane does not change greatly over this temperature range; therefore, thp contribution of the cellophane
in hindering the passage of vapor through the composite film becomes greater a t the higher temperature, and it becomes necessary t o correct the observed transmission rate for the effect of the cellophane. 811 permeabilities in this paper have becn corrccted according to the formula:
where P is the corrected permeability of the coating, T is the total amount of vapor transmitted, and C i s the permeability of the cellophane, all in milligrams per square centimeter per hour. It is possible t o speculate on the continuity of these films from the fact that the permeability doubles for a 35" C. temperature change. DoCg, Ailren, and Mark (3) consider that the increase should be twofold foi a 10" rise in temperature, if the film is absolutely pore-free. Therefore, it appears t h a t transmission through thew air-dry films takes place through pores as wt.11 as by diffusion through the molecular network. The ratio of the permeability constant at the two ternpcratures is the same for ammonia as for water. Throughout the study there is a general correlation between +he permeability t o water and t o ammonia which makes it probable that absorbed water in
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1949
24-ST Duralumin sheet, vapor-degreased with trichloroethylene and wiped with muslin. 18-ounce cold rolled copper sheet cross-sanded with No. 400 emery paper and wiped with muslin wet with petroleum naphtha Hot-dip tin plate wiped with muslin wet with petroleum naphtha,
TABLE 11. EFFECT OF TEMPERATURE ON PERMEABILITY Transmission Transmission of Water, of Ammonia, Permeability Constant a t 1 0 Mil, Mg./Sq. Mg./Sq. Mg./Sq. Cm./Hr./Mm. Hg Film Cm.jHr. Cm.jHr. YWater.---Ammonia 25' C . 60' C . 25' C . 60' C. 25' C. 60' C. 25'C. 60°C. 05-1433 vainish 0 11 1 5 0 0017 0 013 0.006 0.010 0 00023 0 00043 0.025a 0 12& 0.13 Uncoatedoellophane 3 . 3 a 20" 0.12 0.0034 0.0040 a Transmission rate of cellophane a t 0.9 mil, t h e thickness of one sheet.
---.
the film contributes t o the passage of ammonia. The values for the permeability t o ammonia vapor may be quite different in a dry atmosphere. The permeability representative of unplasticized polymers is shown i n Figure 1. The data are taken from column 5 of Table I. The unplasticized polymers cover a tenfold range of permeability. One group of polymers including butyl methacrylate, polyvinyl butyraldehyde, and "Gook" resins are much more permeable than the phenolic varnish. For the purpose under investigation, they may be eliminated from further consideration because of their high permeability. On the other end of the scale the materials with the lowest permeability are the hydrocarbons-Butyl rubber and polyisobutylene. I n spite of their desirable permcability characteristics, they do not represent practical coating resins because they have a very low solubility and give soft, tacky films. I n the center of the scale are a group of polymers with permeability in the range of the phenolic varnish or slightly lower, and with sufficiently good film properties t o permit the formulation of practical coatings. This group in order of ascending permeability includes chlorinated synthetic rubber, cyclized rubber, vinylidene chloride-acrylonitrile polymer and other vinylidene chloride polymers, vinyl chloride-acetate interpolymer, t h e vinyl interpolymer modified with maleic anhydride, and a polyamide resin made from ethylenediamine and dimerized soybean acids. The permeability of all the coatings t o water vapor is shown in Table I, column 5. The permeability for ammonia was determined for selected coatings as shown in column 6. The introduction of plasticizers changes the permeability of the coating as shown i n Figure 2. I n general, the permeability is increased oy the use of tricresyl phosphate and is lowered or unchanged when chlorinated biphenyl is the plasticizer.
751
The panels were allowed to age at least a week and were tested for adhesion by scratching the film with a knife. The qualitative ratings for adhesion are reproduced in column 7 of Table I. A more quantitative measure of adhesion was obtained by a modification of the Courtney-Wakefield method ( 2 ) . I n this method the coating is applied t o aluminum foil and allowed t o dry. Two strips of the coated foil are then ceme itcd together in a press with a thermoplastic adhesive at 120" C. for 3 minutes. This forms a laminate in which thc two outer layers are aluminum foil, the two adjacent layers are the coating under teqt, arid the thermoplastic adhesive is a central layer. The test specimen is a section which has been trimmed t o a width of 1 or 2 inc,ie. (2.5 or 5 om.), depending on the degree of adhesion expectrd. The laminated section is then stripped apart in a Scott tensile strength tester, The force is expressed as pounds per lineal inch. The determination is made in duplicate and good agreement is generally obtained. The values obtained by this method are shown In Table I, column 8. The method has the disadvantage t h a t the film is subjected to high pressure and temperature for a shorr period, with the result t h a t the adhesion of thermoplastic films is improved.
.BUTYL
RUBBER
.POLYISOBUTYLENE 3. C H L O R I N A T E D
CYCLIZED ,V I
SYNTHETIC
RUBBER
RUBBER
C H LOR I D E
N Y LI D E N E
.MODIFIED INYL
A C R Y LON I T R I LE
VINYLIDENE
CHLORIDE
C H L O R I D E 1 ACETATE
8. P H E N O L I C
MALEIC
VARNISH
MODIFIED
C H L O R I D E / ACETATE
VINYL
ADHESION O F UNPIGMENTED FILMS For determination of adhesion, the coatings were applied at approximately 1-mil (25p) dry thickness on the following surfaces: Steel plated with 0.5 mil (13p) of cadmium, vapor-degreased with trichloroethylene and wiped with muslin.
TABLE111. Coating 1 36 37
Titanium oxide
..7 2
6
..
38 25
7 ..
39
7 3 2
40
41 27
..
42 43 44
7 3
a
2
Pigment Volume Zinc Zinc Flake oxide yellow talc
.. .. .. .
I
.. .. 10 .. .. * *
.. lo *.
..
Total
..
.. is ..
28
15
55 35
.. *.
28
..
35
28 22 15
35
18
..
28
18
15
35 35 35
..
..
.. ..
..
.. 22
Measure of cohesion rather t h a n adhesion.
..
..
35 35
..
PROPERTIES OF
NYL
BUTYRAL
3. N - B U T Y L
METHACRYLATE
0
,&i.
AT
600.
b
I
MIL
Figure 1. Permeability of Film-Forming Polymers
PIGMENTED FILXS
Permeability t o Water Vapor Binder Mg./Sq. Cm./H;. OS-1433 varnish Coating No. 1 Coating No. 1 Vinyl plasticized with with chlorinated biphenyl Coating No. 5 0.9 Polyamide-chlori1.6 nated biphenyl resin Coating No. 25 1.0 Coating No. 25 0.8 Coating No. 25 4.2 Plasticized ohlor!0.6 nated svnthetic rubber Coating No. 27 0.3 Coating No. 27 0.3 Coating No. 27 1.6
Adhesion by Knife Test C?dmium Dural Copper Tin
Adhesion t o Lb./Lineal Aluminumi n
0.5 0.5 0.4 3.0 2.1 2.0
G G G
P
G G F
F
G G G F
G G G F
2.4"
2.65 2.2 0.05
0.05
0.2 0.05
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
752 5
s Y
0
3.
i.1 a
00
eo
10
% M
Figure 2.
30
PLASTICIZER
o D I FI E 0
vi NY L
BY
40
WEIBHT
c H L O R I DE/ A c E T A T E
Effect of Plasticizer on Permeability
Vol. 41, No. 4
911 t,he polymeric coatings are superior t o the phenolic varnish in the humidity cahinet, although most show some loss of adhesion. The improvement that can be obtained is seen in Figurc 3. Figure 3 (above) is a photograph of the phenolic varnish on the four metal surfaces after exposure. Figure 3 (below) io a similar photograph for coating 10, vinyl plasticized with tricresyl phosp1iai.e. I t can he seen that the varnish film has curlod and torn on t,in plate arid Dural becausc it is no longer attached t,o tlie surface. On cadmium plate the film, which no longer adheres to the surface?has been fastened t o t)he panel with a. tab of Scotch t,ape for the photograph. The scratch marlis on thc other p:rl were made after exposure to determine adhesion. The failure of the phenolic varnish may be attributed to t l i c . drying oil whioll it contains. It is known that films which contuin drying oils hare considerahle quantities of free acid available t'o~ reaction under the alkaline conditions of teat,. The chief advantage of the polymers is the fact, that. they do not require drying oil to obrain satisfactory film properties. PI 641EUTED FI L3I S
The adheiion measuied by this method does not always follow the same order ab jn the knife test. The knife test depends on a more complicated set of properties, but is sometimes of greater practical significance. h further limitation of the measurement of adhesion by stripping from aluminum foil is that the adhesion to other metals may not correlate well viith the adhesion to aluminum in special cases. H U R I I D I ~ YCABlYUET EXPOSURE
T o have practical value, t,he coating should combine low permeability and satisfactory adhesion. I n addition to the control, eight coatings were select'ed as being representative of the most promising types of composition. One-mil films on metal panels were exposed for 2 weeks in a humidity cabinet a t GO" C. The atmosphere was saturated with the vapors of 1% ammonia solution.
COATIKG 1, 05-1433 VARNISH. The film failed badly on all metals in the humidity cabinet. On cadmium it blistered first and then became completely detached. On Dural and tin plate, it peeled from most of the surface, Ba,d loss of adhesion occurred on copper. COATING 5 , VINYLPLASTICIZED WITH CHLORINATED BIPHENYL. Some loss of adhesion occurred in the humidity cabinet on all metals. The degree of failure was much less than for the control varnish. COATING 10, VINYL PL.4sTICIZED T I T H TRICRESYL PHOSPHATE. This coating has good original adhesion t o all the metals but is The ca$I;nium and more permeable than the varnish control. Dural panels were unchanged a t the end of exposure. l h e copper was corroded at, scat,tered spots and the adhesion to tin plate \vas poor. C0.4TING 16, PLASTICIZED VINYLIDENE CI3LORIDE--.kCRYLOKIThis coating has poor original adhesion to all TRILE COPOLYMER. metals except copper. It shows blisters on copper and tin plate after exposure in the humidity cabinet with visible corrosion of the copper. There is no delectable change 011 Dural and cadmium. COhTING 19, PL.4sTICIZED lTIKYLIDENE CHLORIDE POLYMER. The film darkened in the humidity cabinet. It blistered on cadmium and Dural, allowed corrosion of copper, and peeled from tin plate. COATING 25, POLY.4NIDE-CHLORINATED BIPHENYLRESIN. The film lost adhesion on cadmium, Dural, and tin plate and blistered on copper. COATIXG 27, PLASTICIZED CHLORINATED SYNTHETIC RUBBER. The film whitened but was otherwise unharmed. The metal underneath was protected. COaTING 32, PLASTICIZED CYCLIZED SYNTHETIC RUBBER. This coating had poor initial adhesion. After exposure in the humidity cabinet i t flaked from cadmium and copper with visible corrosion of the copper. On Dural the film showed plastic checking. The film appeared t o be unchanged over tin plate. COATING 33, BUTYL RUBBER.This coating was included in the humidity cabinet exposure beca.use of it,s low permeability. It softened during exposure but, protected the metal except where the film was damaged mechanically.
In oleoresinous films thP introduction of pigment decreases t Iicl permeability of the films. The optimum concentration usually lies between 30 and 40% pigment volume. On analogy ivitli oleoresinous films, a number of selected coatings were pigmented with blends of four pigmcnts a t a pigment volume of 36%. The composition and properties of pigmented films are listed in Table 111. Flalcr talc was used as extender because it has the property of reducing permeability more than pigments with a spherica,l particle shape. Rutile titanium oxide was used as an inert opaque pigment. Zinc oxide was included in one blend becauscl it has some inhibitive effect on the corrosion of certain metals. Zinc yellow n-as introduced in another blend as a corrosion inhibitor, although it was recognized that its usefulness might be limited by the fact that it. is slightly wat,er soluble. The low chlorideelow sulfate grade of zinc yellow was used. Lead pigments were excluded by conditions of the problem not discuss4 here. The chief effect of pigmentation is t,o improve the apparent adhesion t o all metals in the test as judged by scratching with a knife. This is probably due to other factors than actual adhesion, as there is little increase in the measured adhesion to aluminum foil. A decrease in permeability ranging up to 50% of t'he original permeability is shown by pigment blends which do not contain zinc yellox-. Zinc yellow in the pigment. blend niakes all thc polymeric films more permeable. The increase in pcrmeability would be expected from the introduction of a waters-oluble material into a film which is itsclf substantially insoluble and resist,anf t'n water.
CADMIUM PLATE
Figure 3.
DURAL
COPPER
TIN PLATE
Coatings in Humidity Cabinet
(Above) Phenolic varnish; (below) vinyl
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
April 1949
753
0
AAYKE TALo
ZINC
YELLOW
.TITINIUM 1
FLAKE -
d
TITANIUM
OXIDE
M0’
OXIDE
4
YELLOW
ZINC
TALC V ”- O --- . :
I 0
IO
20
PIGMENT
Figure 4.
30
40
01
P e r m e a b i l i t y of P i g m e n t e d Vinyl C o a t i n g s
The effect of zinc yellow on the permeability of the polymeric films is contrary t o experience with oleoresinous paint films. I n common paint vehicles, zinc yellow lessens the permeability (1 ). An explanation for this action of zinc yellow can be suggested from the properties of oleoresinous films and the pigment. Oleoresinous films contain acid oxidation products which make the film water-sensitive. Zinc yellow acts as a base in the paint film t o convert the acids t o products which are less sensitive to water. At the samo time, the solubility of zinc yellow is relatively less important because the film already contains watersoluble material. The net effect of zinc yellow is to lessen permeability in such films. The pigment blend with zinc yellow in this study neither raises nor lowers the permeability of films from OS-1433 varnish. Films of this varnish are less permeable than films of commoii paint vehicles; presumably the two opposing actions of zinc yellow balance each other instead of showing a favorable net effect. P I G M E N T CON CENTKATION
A more detailed study of pigment concentration was made in coating 10, the vinyl vehicle which had given good results unpigmented in the humidity cabinet. Figure 4 illustrates the effect of flake talc, titanium oxide, and zinc yellow on the permeability of this coating. Flake talc lowers the permeability one third at the optimum concentration. The optimum is about 10% pigment volume. I n oleoresinous vehicles both the degree of improvement and the optimum concentration would be considerably higher. Titanium oxide has little effect up t o 40% pigment volume. I n oleoresinous vehicles i t would cause a definite lowering of permeability. Zinc yellow increases the permeability in polymeric vehicles which have low permeability unpigmented; in oleoresinous vehicles i t would decrease the permeabilicy. The adhesion of the pigmented coating is presented in Figure 5. Both talc and titanium oxide double the measured value of adhesion at the optimum concentration. The optimuin for talc is 10% pigment volume, the same as the optimum for permeability. The optimum for titanium oxide is about 3070. Zinc yellow has little effect at all concentrations.
20 PIGMENT
10
VOLUME
F i g u r e 5.
40
Adhesion of Pigmented Vinyl C o a t i n g s
Inert pigment at the optimum concentration presents some advantage over unpigmented films. The optimum appears to be different and probably to be lower than for oleoresinous vehicles of a more conventional type. Because the permeability of the vehicle is originally low, the lowering of permeability to be obtained by pigmentation is not so important as with oleoresinous vehicles, c ACKNOWLEDGMENT
This article is based upon work performed for the Office of Scientific Research and Development under Contract No. OEMsr-796. Permission t o publish the results of this investigation is gratefully acknowledged. Thanks are extended t o S. C. Horning for his modification of the Courtney-Wakefield test. LITERATURE C I T E D
(1) Burns, R. M., and Schuh, A. E., “Protective Coatings for Metals,” p. 323, New York, Reinhold Publishing Carp., 1939. (2) Courtney, R. P., and Wakefield, H. F., IND.ENG.CHEW,ANAL.
ED.,6,470 (1934). (3) Doty, P. M., Aiken, W. H., and Mark, H., I h i d . , 16, 686-9 (1 944) (4) New York Paint & Varnish Production Club, Natl. Paint, Varnish, Lacquer Assoc., Sci. Sect. Circ. 546, 75-8 (1937). (6) Perry, J. H., ed., Chemical Engineers’ Handbook, 1st ed., pp. 996,2116, New York, McGraw-Hill Book Co., 1934. (6) U. S. Navy, Bureau of Ordnance, Specification 09-1433(Jan. 10, 1944).
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RECEIVEDFebruary 27, 1948. Presented before the Division of P a i n t , Varnish, and Plastics Chemistry at t h e 110th Meeting of t h e AMEHICAN CHEMICAL SOCIETY,Chicago, Ill.
CORRECTIONS In the article entitled “Cellulose Ester Solutions” [ IND.ENQ. CHEM.,41, 105 (1949)] in Table I, in the fint group under “Methyl Alcohol-Ethylene Chloride, Atmospheric Pressure,” the subheading should be “yoEthylene Chloride” instead of “yoAcetone.”
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FORMULATING PRINCIPLES
The advantages t o be expected from polymeric resins in moisture-resistant coatings as compared with the best oleoresinous film lie chiefly in the fact that the polymers are more alkali-resistant. The permeability of most polymeric films dried a t or near room temperature is not markedly better than that of a good phenolic varnish. However, a better balance of film properties may be obtained at the same permeability by the proper choice of plasticizer.
30 VOLUME
I n the article on “Calculatiori of Liquid Densities a t Elevated Temperature” [IND.ENG.CHEM.,41,96 (1949)l in the tabulation a t the top of page 98, first column, the third line under Group I should read “Mononuclear aromatic hydrocarbons and halides, 2.” I n the Lauti6 equation in the first column Qn page 99, the -8.03 T , last term should be - ~ - - c
10
E. S. HANSON
Scale Formation on Laborator Evaporator F. 31. HILDEBRANDT AND IC. H. TC'_.IRREN 1'. S . Industrial Chemicals, Inc., Baltimore, Md.
It should be taken apart easily for ohsrivation or nieaiurement of the scale on t h e tube surface. Xeasurenient of scale formation by change in evaporation rate should be made easily. DESCRIPTIO\
O F APPARATUS
;1photograph of the appaiatub is shown in Fiyurc I
Figure 1.
Laboratory Evaporator Arranged for Natural Circulation
K E of the most serious problems to be solved in the evaporation of many materials is the drop in the coefficient of heat transfer because of the fouling of heat,ing surfaces by scale and occluded matter. Webre and Robinson ( I ? ) give data which show that in a n extreme case the over-all coefficient dropped, owing t o scale formaeion, from 1550 to 900 in less t h m 2 hours and then less rapidly to about 700. Other dat,a showing t h e heat transfer t o boiling liquids under varied conditions may be found in the literature ( 7 , 9, 10, 12, 16). The majority of these studies have been confined to plant scale equipment,. Laboratory data are difficult t o find arid are, in general, limited t o t'he investigation of a specific factor such as the film coefficient. This is due in part t,o the fact t h a t small scale equipment for evaporation studies is specialized for t,he study of these specific factors and in part to t,he feeling that laboratory apparatus is too far removed in size and operating conditions t,o give data valid for t,he larger evaporators. To experiment with full scale evaporators, however, is costly. illso such eyuiprrienl lacks flexibility, the machine is difficult to examine after use, and experimentation under production conditions is hard to interpret, a,s the uncontrollable factors of t h e larger operation affect the results erratically. JVith those things in mind attention vas turned t o the setting up of a small evaporator for laboratory studies of scale formatmion. Certain requirements for such an apparalus have been suggested by other workers ( 2 , 3,11, 18, 1 4 ) among which are the following: The apparatus should be designed t o evaporate small amounts of material under controlled conditions with t'he heat transfer tube constructed so that t,he scale forms on the outside where it can be observed readily.
il is t h e body of the evaporator constructed froin a 500-ml. Kjeldahl flask by adding a section to the neck and providing an outlet in the bulb and another outlet in thc neck about 1 inch from the bottom. B is another 500-ml. Kjeldahl with a tangential inlet, two outlets, one foi a thermonietei and the other near the top for vapor. The nccli of this flaqk is shortened and drawn down t o a reduced diameter for attachment of rubber tubing. C is a glass manifold with four outlets, interchangeably usrd for incoming feed, E , down-leg connection from B , or draining the apparatus. A , B, and C are connected by rubber tubing. G is a graduated receiver for water from R. condensed by bulb condenser, D. F is a graduated separatory funriel lor holding the feed and maintaining a supply at constant level in pump reservoir iV. Steam a t constant pressure enters the apparatus through regulator H . T h e gage, I , gives the pressure on the heater tube, and the condensate is removed by steam trap, K . The temperature of the steam is measured by a Weston thermometer, L. J1 is a braided metallic hose which admits steam t o the 0.5-inch brass evaporator tube, T . X is a rubber stopper fitted over tube T and into the neck of A . 0 is a steam trap connected t o a 0.125-inrh copper capillary tube within tube T, and drained by condensate discharge line P. Details of the oonstruction of the heater are shown in Figure 2. The construction and operation of feed pump R arc described in a previous publication (6). It consists of a rubber tube provided with stainless steel ball check valves. Pumping action is obtained by compressing the tube with a bar moved by means of ek motor-driven cam.
OPERATION O F EVAPORATOR
In actual operation, the evaporator is filled to the desired levd and the steam inlet valve opened. I n about 1 minute the liquid boils and circulation starts through the apparatus. From B, the catchall, the vapor goes to the condenser, D,and the oondensate is caught in a 100-ml. graduated cylinder. I n these experiments readings of the amount of condensate were made a t 5-minute intervals and the data obtained were used for plotting curves shown later in this paper. The liquid level is maintained relatively constant in the catchall (within about I inch)
simple and easily constructed laboratory apparatus designed for the study of evaporator scale formation is described. Results are presented graphically to show the decrease in the rate and amount of water cvaporated under controlled conditions as a coating of scale is formed on the heater tube, Experiments with calcium sulfate solutions and molasses stillage are described, and some of the more important factors affecting scale formation are emphasized. The apparatus is used to produce small amounts of evaporated material experimentally in the laboratory, in addition to its principal use as a tool for the investigation of the effect of various factors on scale fortnation.
