FLOW PROPERTIES OF PAINTS Leveling 0 R. W. ICEWISH AND D. F. WILCOCK The Lowe Brothers Company, Dayton, Ohio
M
ANY paints have a rather high consistency after stand-
ing undisturbed in the paint container for some time. On vigorous stirring they becomemuch thinner. After stirring is stopped, the paints gradually thicken. This property of thinning under agitation and then thickening when agitation ceases is called “thixotropy” (90). Under vigorous agitation of a paint, a minimum consistency may be attained which does not become less even when agitation increases. This minimum consistency is called the ‘imaximum fluidity” of the paint (IS). The leveling qualities of a paint apparently depend more on maximum fluidity and thixotropy than on any other physical property.
Flat wall paints were made according to a series of variations in a basic formula. Leveling tests were carried out on these paints, and the flow properties of the paints were measured on a plastometer of the Bingham and Green type and on a new kind of thixotrometer. The leveling test simulates the conditions of leveling in the case of a brush-applied paint film. A new method of judging extent of leveling is described. Evidence is given for concluding that measurements made with the plastometer are indicative of maximum fluidity.
Leveling Process Waring (88) derived an equation relating width and depth
of brush marks to surface tension and yield value of a paint. If a paint had no yield value, then complete leveling would be expected on the basis of the equation. McMillen found that on long standing some paints develop yield values (I@, but if any paint possesses a yield value immediately after it is subjected to a strong shearing action (stirring, brushing), such a fact has not been demonstrated. McMillen (18) concluded that the effects of yield value on leveling were relatively insignificant as compared with the effects of thixotropic changes.
must possess a relatively high fluidity under small shearing forces immediately after brushing, and thixotropic changes must not be so rapid as to retard or prevent leveling before gelation of the film begins. Pryce-Jones (89) expresses similar views. When a paint is brushed out, it is subjected to very high rates of shear (17). Under these conditions it is probable that much of the thixotropic structure is broken down, and that the consistency approaches the maximum fluidity of the paint. The steps, then, in the brushing and subsequent leveling of a paint film are: 1. The passage of the brush produces irregularities in the film. 2. The force of surface tension acting against the consistency of the paint tends to level out these irregularities. The available force decreases (98)as the extent of leveling increases. The initial rate of leveling depends on the maximum fluidity of the paint. 3. Thixotropic changes increase consistency, the initial rate of increase being very rapid (14). Hence, rate of leveling decreases rapidly a t first and then more slowly. 4. In some cases, before leveling has proceeded sufficiently to eliminate visible surface irregularities, the paint has reached a stage in which no appreciable further leveling can take place. In this experimental study an attempt has been made (a) to check the essential correctness of this outline of the leveling process, and (b) to investigate in more detail the relative effects of maximum fluidity and thixotropy on leveling. In order to do this it is necessary to have: a method of determining maximum fluidity, a thixotrometer, and a reliable
u UJ
THINNER CI
VEHICLE 20‘
IO
20 30 PIGMENT VOLUME PERCENT
I
40
FIGURBI 1. SURFACE TENSION-CONCENTRATION CURYES FOR PAINTS OF SERIES 1 (Above) AND OF SERIES2 (Below)
Gamble (6) says that since the surface tension values of various paint vehicles are of the same order of magnitude and can be modified only slightly, the consistency characteristics of a paint are the prime factors in determining leveling properties, In order to exhibit good leveling, a paint 76
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
leveling test. If maximum fluidity and thixotropy can be determined, then a reliable leveling test will show the part that each plays in the leveling of a paint film.
77
or may not be said to be due t o a yield value, depending on the definition of yield value (11) that one wishes to accept.
Plastometer Surface Tension Anomalies Haslam and Grady (9) found that the surface tension of a paint as measured by the du Nouy tensiometer increased with increasing pigment concentration, Figure 1 shows that similar results were obtained by the writers with two series of flat wall paints.
40
w
.._... 0.2 0.4 0.6 0.8 0.2 1.0 VERTICAL DISPLACEMENT-MM. FIQURE 2. FORCE-DISPLACEMENT CURVES FOR PAINTH-772 (DU NoiSm TENSIOMETER)
0
The plastometer used was similar to the original Bingham and Green apparatus (2, 7), but was simplified in design to facilitate the making of measurements and yet retain much of the accuracy of the original instrument. Figure 4 shows the general construction of the plastometer. Figure 5 shows the details of construction of the water bath, the paint container, and the capillary mounting. To determine maximum fluidity it is necessary to destroy thixotropic structure. This can be done by high rates of shear or by high shearing stresses since these imply high rates of shear. Freundlich and Abramson (4) showed that the viscosity of thixotropic materials is temporarily reduced by passing them through a capillary viscometer. There are two reasons for believing that thixotropic structure is temporarily destroyed during the passage of a paint through a plastometer of the Bingham and Green type: (a) A paint in which some degree of thixotropic structure has been built up is subjected a t the capillary entrance to very high shearing stresses. (b) The results obtained with the plastometer are reproducible and nearly independent of the time that the paint remains undisturbed in the paint container before the measurement is made.
A. Constant rate of force increase B . Final displacement under constant force C. Force-displacement for unpigmented vehicle
Obviously the explanation of these results should lie in the existence of a yield value. A yield value would oppose distortion of the surface with the result that the tension a t which the ring pulled away would be substantially higher for a pigmented material than for the unpigmented vehicle. The yield value would also oppose any downward movement, so that if the vertical motion of the ring were observed with increasing and decreasing tension, a type of hysteresis curve should be obtained. Figure 2 shows that such a curve is obtained, but that its shape depends on the manner and rate of application of the load. These curves were obtained by observing the motion of the end of the tensiometer arm, reading the displacement by means of the eyepiece micrometer scale of a microscope. The broken line (curve A ) represents the movement when the pull on the ring was increased in equal steps and one minute was allowed between steps, simulating a uniform rate of increase of force. The solid line (curve B) represents the movement when the pull was increased in equal steps, but time was allowed after each step until no perceptible movement took place in 30 seconds. Such a hysteresis curve is not obtained when surface tension measurements are made on the unpigmented vehicle (curve
C).
Because of the effect of rate of application of tension on the hysteresis curve, a study was made of the vertical displacement that occurred with time under constant tension. For a tension (30.1 dynes per cm., Figure 3, curve B ) above the surface tension of the unpigmented vehicle (28.2 dynes per cm.) but below that of the paint (35.9 dynes per cm.), the ring broke away when given sufficient time; but for a tension (22.1 dynes per cm., curve A) below the surface tension of the vehicle, the ring approached some maximum displacement but did not break free. From this we conclude that the hysteresis curves and the surface tension anomalies, such as Haslam and Grady reported and as were pointed out above, are due t o viscosity effects and the short lengths of time during which the surface tension measurements are made, The hysteresis curves and the surface tension anomalies may
I
I
I
I -
A
p:
w
I
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I
I
I
I
MINUTES FIGURE3. TIME-DISPLACEMENT CURVES UNDER CONSTANT LOADFOR PAINTH-772 (DU no if^ TENSIOMETER) A . Load, 22.1 dynes per om. B. Load, 30.1 dynes per am.
McMillen (12) found, 2 minutes after stirring, an apparent viscosity of 565 dyne seconds per sq. cm. a t a shearing stress of 0.287 dyne per sq. cm. for a paint that leveled well. With one of the paints studied (H-741, Figure 13A), the rate of flow in the plastometer was 0.00681 cc. per second a t a pressure of 695.5 grams per sq. cm. Assuming a viscosity of 565 dyne seconds per sq. cm. as the viscosity of this paint in the undisturbed state, then for a rate of flow of 0.00681 cc. per second the shearing stress a t the entrance to the capillary would be 211,000 dynes per sq. cm.
'2 (Poiseuille equation)
11 =
8 VL p = - -87V L ?r
where p
=
r4t
pressure, dynes/sq. cm., and 1 = time required for volume V to flow through a capillary of length L and radius r
If S is the shearing stress a t the walls of the capillary,
s = pr/2L
INDUSTRIAL AND ENGINEERING CHEMISTRY
78
FIGURE 5.
F I G U R E 4. PLASTOMETER
Substituting the Poiseuille equation in this equation,
s = rr4- -v
r3t
‘
R’ATER
VOL. 31, NO. 1
BATH, P A I X T CONTAINER, AND ClPILLARY hfOUNTINC,
tropic structure. Pryce-Jones (23) says that reproducibility is an indication of an absence of thixotropy, which implies that if paints are thixotropic, results with the plastometer will not be independent of time.] Bingham and Green (5) and Green (8) found that the apparent viscosity of a paint increases as shearing stress decreases. This may mean that the property which we are calling “maximum fluidity” decreases with decreasing shearing stress, and that because of the small forces of surface tension the maximum fluidities involved in the initial leveling of a paint film are substantially higher than those measured in the ranges of pressure ordinarily used in measurements made with the plastometer. Bingham’s statement (1) that the same flow curves were obtained with the plastometer for a given
This gives the shearing stress as a function of the viscosity. Then at the entrance to the capillary, S =
.
o’oo681
565 = 211,000 dynes/sq. cm.
3.1416 X (0.0285)3
Results obtained with paint H-838 show that for this paint, for the pressure used, measurements made with the plastometer are essentially independent of thixotropy. (Figure 1 4 0 shows that the paint is definitely thixotropic.) Since there were decreases in effective pressure due to decreasing hydrostatic head in the paint container, the rates of flow were corrected to the rate corresponding t o the initial effective pressure (711.3 grams per sq. cm.) by use of the slope of the flow curve obtained from previous measurements (Figure 130): d- = cc./sec.
dP
NO.
1 2
3
(cc./sec.) 0.0000347 (grams/sq. cm.)
R a t e of Flow Measured
Pressure CC./BBC. U./sq. cm.
0.01606 0.01590 0.01574
711.3 709.4 707.6
,A.
i
’,
\-.a’
FIGURE 6. PRINCIPLE OF OPERATION OF TWIXOTROMETER
THE
Angle A , a n d hence t h e torsion of the wire i kept prao. tioally constant by moving t h e c o & i n e r .
Flow Correction Cc./aec.
R a t e of Filling Time after Paint Flow Cor. Container Cc./sec. Min.
0.00000 0.00007 0.00013
0.01606 0.01597 0.01587
4 I9 60
The slight trend in rates of flow with time are attributed to an increase in resistance to flow at the capillary entrance caused by increase in thixotropic structure. However, the effect is so small that it may be said that the rates of flow are essentially independent of thixotropy, and hence that the maximum fluidity of the paint is being measured. [In contrast to these views, McMillen (16) does not believe that a material is subjected, in the capillary plastometer, t o a high enough rate of shear for a long enough time to destroy thixo-
materid either when using a series of increasing pressures or a series of decreasing pressures seems to imply that rate of flow in a capillary plastometer is independent of thixotropy over a wide range of shearing stresses. If this can be shown to be so, then maximum fluidity is a function of shearing stress.
Thixotrometer This thixotrometer is a departure from previous thixotrometer designs (5, 13, 18, 21, 25). The principle of operation of the thixotrometer is shown in Figure 6. The thixotrometer differs from the falling-ball viscometer in three ways: The driving force is supplied by the action of a horizontal torsion wire rather than by density difference; the position of the sphere is kept practically constant, the container being
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
WIRE MOUNTING ASD SPHERE FIGURE 7. TORSION
constant. (b) A considerable range of driving forces can be conveniently used. ( c ) The sphere moves toward a region not previously subjected to shear. Pryce-Jones (24) says that thixotropic measurements should be made a t constant rate of shear. It might be possible t o construct stress-time curves at constant rate of shear by making measurements with a series of driving forces and selecting points a t constant rate of travel from the curves obtained. [This assumes that constant rate of travel corresponds to constant rate of shear, which may not be true. Pryce-Jones (25) discusses this point in considerable detail. ] However, during the leveling of a paint film the force producing leveling decreases and the rate of shear also decreases. The use of a constant stress and a decreasing rate of shear (the method ordinarily used with the thixotrometer described) is a better approach to these conditions than the use of a constant rate of shear and a necessarily increasing shearing stress. Mention should be made that “elastic-recoil” effects such as those described by Pryce-Jones (27) have been observed. In these cases when the force that had been acting on the sphere was reduced to zero, the sphere moved back considerable Dortion of the ~~. distance that it had traveled through the paint.
a
moved instead; and the distances traveled b y the container are quite smallof the order of 0.2 inch (0.5 cm.). The use of torsion wire increases the accuracy of the measurement when small driving forces are used and facilitates the use of different driving forces. By keeping the position of the sphere constant, the torsion of the mire and hence the force acting on the sphere is kept constant.
Leveling Tests
In assembling the thixotrometer, the arm leading from the sphere (Figures 6 and 7) is disconnected, and AXD WATERBATH FIGURE 8. PAIXTCOXTAINER the paint container (Figure 8) is placed in the water bath (Figure 8) which is already in position on the metal guides (Figures 7 and 9). The sphere is inserted in the paint, and the arm is engaged in a clamp on the torsion wire (Figure 7). In using the thixotrometer the paint is stirred for a fixed length of time with a mechanical stirrer. The paint is poured into the paint container, and a stop watch is started the moment pouring is begun. The thixotrometer is assembled, and the micrometer (Figures 7 and 9) adjusted so that the arm is brought to a previously determined zero position as observed on the eyepiece scale of a microscope (Figure 9). At a fixed time after pouring, the torsion setting is made. Under the force due to torsion the sphere moves through the paint. By means of the micrometer the container is moved from time t o time so that the arm is brought back to the zero position. Readings of the micrometer indicate the distances the container has traveled. These distances and the corresponding times are recorded. Dimensions of various parts of the thixotrometer are: Container Glass sphere Torsion wire
79
The usual leveling test (6, 19) consists in (a) producing irregularities in the surface of the paint film, usually by means of a metal blade with a serrate edge and ( b ) judging the extent of leveling from the appearance of the surface after drying. I n these tests such a blade was used (Figures 10 and ll), but in judging the leveling, a cross section of the dried paint film was examined under the microscope.
Diameter 2 6 / 8 in. (6.7 om.), length 3 in. (7.6 cm.) Diameter 0.86 in. (2.18 om.) l o torsion = 12.6 dynes force on sphere
Most thixotrometers as well as this one measure change in viscosity after the material has been subjected to shear. In this thixotrometer thixotropic effects show up as a decrease in the rate of travel of the container with time. The thixotrometer has the following characteristics which appear to be advantageous: (a) ?he driving force is
FIGURE 9. ASSEMBLED THIXOTROYETER
INDUSTRIAL AND ENGINEERING CHEh'IISTRY
80
The blade used was made from a fine-tooth hack-saw blade with thirty-two teeth to the inch (2.5 cm.). The blade was ground down on emery paper and has the follon.ing dimensions: The teeth are spaced 0.79 mm. itpart, the width of the teeth where they touch the paper is 0.315 mm., and the triangular openings through which the paint goes are 0.263 mm. deep. The paint is applied on paper which has been given a sprsyed coat of black enamel. The enamel uresents a smoother surface than the paper and gives in cross s6ction a sharp black line by which the paint may be easily distinguished from the paper. The hlade is drawn throuvh the mint twice. the excess naint heins removed after each u&saae. 'The blade is then thoro&hly ~~~~~
~
~~
~
arsoersed. Thin sections cut from the ~araffinmounting may be
Number €1-745 H-746 H-747 H-748
VOL. 31, NO. 1 Nnmhar E-749 H-750 H-751
Per Cent 39.1 36.9 34.6 32.3
Per Cent 30.1 27.9 25.6
3. VARYING AMOUNTS OF ALUMINUM STEARATE. The pigment volume percentages based on total volume and the percentages of aluminum stearate based on the total weight of pigment were: Number H-767 H-768 €1-769
% A1 Stearate 0.0 0.0360 0 0732
Pigment Vol. % 32.13 32.14 32.16
4. VARYING VISCOSITY OF
Number
H-770 H-771 H-772 BODIED
DRYINU01-
VEEICLE. The pigment volume percentage in all ?he Dlroto&;oma6hs (Fieure r2) show the fdm as a series of white ridges agarnsi a bla& background. In evaluating the extent of leveling, the ratio of width to height of these ridges is det,emined. This value was called the "snread ratio." The &ad ratio increases with increased leveling
Paints Studied
The paints studied had the following composition: pigment Lithopone Die.tom.scsous earth Titanox C Aluminum stearate
% PY
Weight 79.4 9.1 11.4 0.146
Vehicle Raw lineeed oil Bodied drying oiia Thinner Drier-nonvolatile
8&
Pigment Vol. % 32.23 32.38 32.60
%AI
Stesrate 0.1464 0.292 0.584
USED IN
wa 32.16. The paints and the bodies and acid values of the oils used are given in Table I. caes
TAD^ I. BODIESAND ACIDV n ~ m OF s OILSIN PAINTS OF SERIES 4 Number H-835 H-na6 H-837 H-838 A439
Body of Oil i n GsrdnerHoldt Tnbe Untissted 22
+ 23 -
+
23 24 -
Seconds for Bubble to Reach First Mark OD Tube (25' C.)
Aoid Value
4i:4 54.0 60.5 70.7
1.04 11.15 10.00 9.55 9.39
of Oil
8.4 25.5 65.2
o m
Four variations in formulation were studied: 1. VARYING PIGMENT-VEHICL.E RATIO. The paints and the pigment volume percentages were: Number A-738 H-739 H-740 H-741
Per Cent 40.0 37.5 35.0 32.5
Number H-742 8-743 8-744
2. VARYING PIGMENT-VESICLE RATIO
c&age.
based on total volume were.
WITH
Per Cent 30.0 27.5 25.0
CONSTANT
.-
FIGURE 10 (Above). Bunm MOUNTINQ IWD PAPER HnmER FOR LEVELINQ TEST FIGVEE11 ( B e h ) . PEOTOMICR~QRAPE OF BLADE( X 25)
FI~WEE 12. PEOTOMICROQRAPH OF LEVELINGTESTEFOR SERIES 2 PAINTS ( X 25)
81
INDUSTRIAL AND ENGINEERING CHEMISTRY
JANUARY, 1939
FIGURE14 (Left).
RESULTS
OBTAINED WITH THE THIXOTROMETER FOR A FORCE OF 630 DYNESACTING ON THE SPHERE
A
A.
0
Series 1
C. Series 3
z 0 Y v)
B.
Series 2
D. Series 4
0.030 0
u'
The oil used in H-835 was a blend of untreated drying oils. The oil used in H-836 was a blend of the same composition, heat-bodied at 585' F. The oils of the higher viscosities were su c c e s si ve s a m p 1 es obtained from the heat-bodied oil by blowing at a temperature between 200' and 220' F.
V
V
2
-I LL
0.20
~~
0.090
0.10
Two pints (0.9 liter) of each of the members of the four series of paints were prepared. In each case one of the pints was used in the various tests, and the other was reserved for settling tests and for a final check.
Results of Flow Measurements
0
( c
500
1000
0.10
0.010 n
z
0
0 V
10
20
30 ,
IO
20
30
0.40
u w rc w
0.
.0.005
u
U
5
-I
L
0
ID
500
I
iooo
0.10
U
0.20 v)
yo.10
5 0
fl I NUT ES
0
5-00 PRESSURE,GRAMS PER 59. Cfi.
1000
FIGURE 13 (Left). RESULTS OBTAINED WITH THE PLASTOMETER A.
Series 1
C. Series 3
B . Series 2
D. Series 4
Figure 13 shows the results obtained with the plastometer for series 1, 2, 3, and 4, respectively. I n c o n n e c t i o n with leveling, three points in these sets of curves are significant: 1. In series 1 and 2, the paints show a nearly regular increase in viscosity (decrease in rate of flow) with increasing pigment concentration. 2. The differences between the various members of series 3 are slight. 3. The paint made with the untreated oils (H-835) in series 4 shows the lowest viscosity ( g r e a t e s t rate of flow). The dimensions of the capillary used in these tests were: effective radius, 0.0285 em.; length, 7.427 cm. (series 1) or 6.952 cm. (series 2, 3, 4). Figure 14 shows for the four series of paints the results obtained with the thixotrometer for a force of 630 dynes acting on the sphere. The setting of the torsion was made 10 minutes after the container was filled. From these results the following conclusions are indicated: 1. T h i x o t r o p y increases with increasing pigment concentration.
INDTJSTRIAI, AND ENGINEERING CHEMISTRY
82
VOL. 31, NO. 1
increasing viscosity will probably slow down the rate of flocculation by decreasing the rate of thermal agitation of the particles. With the oil used in paint €1-836, viscosity, acid value, and oxidation were increased over the values of these quantities for the raw oil; as a result, paint 11-836 is less thixotropic than II-835. With TI-837 the decrease in acid value of the oil (Table I) causes an increase in thixotropy, hut with H-838 and H-839 increasing viscosity and oxidation are more effective than decreasing acid value, and these paints are less thixotropic than 11.837. The spread ratios for the various paints as determined in the leveling test are: I'aint No. H-738 11-730 11-740 11-741
H-742 13.743 H-744
~ ~ ' I G V R 15. E Pii~,.rnniii.nor;~*~,,,,i OF L SEnlzs -I P.\INI-S ( X
E ~ L WTwrs G txm 25)
2. Paints helorv a concentration of 30 per cent pigment by volume are much less thixotropic than paints with higher pigment concent.rations. These results differ from those obtained with the plastorneter where the change from paint to paint varied in a fairly regular fashion. 3. Thixotropy increases with increasing aluminum stearate concentration. The differencesbetween successive inembers of the series are relatively greater than the differences found with the plastometer. 4. Thixotropy is not a simple function of oil viscosity. 5 . The thixotropy of paints inade with untreated oils is areatcr than that of paints made with heat-treated or blown oils. Here again there is a marked difference in these reLL sults as compared with the results = . W 2 obtained with tlie t:
3
Av. Spread Hstio
6.1 8.3
9.n 8.6 Complete sprend/ng Complete spresding Complete spreeding
Paint X o . ii-746 11-746 11-747
AT. Sniead Katio
H-749
Complete apread/op
0 .8 x 5 14.1
11-748 11.780
H-751
lG.6 Complete xpread!er Coniplete ~ c m a d i n g
The results shoiv irregularities which may he eliminated after further improvement of the test method. Nevertheless, the following generalities are indicated: 1. Ieveling Lmoriies progr&vely better as pigment concentration decreases. 2. At pigment concentrations of 30 per cent hy vohime and belou, complete leveling of the paints occurs. There is probably a connection between this result and tlie fact that the paints below this percentage are much less thixotropic than those above. 3. Increasing aluminum stearate concentration causes poorer leveling. On the basis of the results obtained with the plastometer, the difference in leveling over the entire series would be expected to be no greater than the difference between consecutive nienrhers of series 1 and 2 . The results oht,ained with the thixotrometer indicate that a considerable difference should be expected. 4. The paint made with the blend of untreated oils levels better than tlie paint made witli the bodied oils (Figure 15). 5. N o definite conclusion can be drawn with reference t,o the effects of blowing the bodied oil. Figures 16 to 19 show results obtained with the Gardner The conclusions to flowmeter for the four series of paint be drawn from these results are essen Ily the same as with tlie leveling tests. In particular, a different type of curve and a greater rate of flow is obtained with paint H-835 than with other meinhers of the same series.
Interpretation of Results
the oil. Further blowing of the oil gradiially decreases the thixotropy of the paints. Conditions of better dispersion tend to decrease thixotropy ($66). I n heat bodying an oil, polar-nonpolar type inolecules (good dispersing agents) may arise through the forination of free acids or by oxidation. I n hloxing a heat-bodied oil, acid value decreases, but polar-nonpola~type molecules may be formed by oxidation. Flocculation is cine of the accepted causes of thixotropic striictiire (10). Since a ccrtain amount of nrotion is necessary to bring pigment particles into position for flocculation,
Although paint If-835 is inore thixotropic than the other members of the same series, it shows better leveling in the leveliiig tests. This demonstrates that hoth maximum fluiclity and thixotropy iiifluence the leveling of a paint filni. It is a conrmon experience that, contrary t,o the results of the leveling test, paints made from untreated oils do not level as well as those from bodied oils. Drusli-outs of the series 4 paints show that, within the width of the hand eovcred by the brnsli, there is little observable difference iii levding. This is in agreement with the results obtained in the leveling test.. Jlowcver, at the edges of the hrosh a ridge is laid down along the path traveled by the brush. With p i n t IM35 tlie size of this ridge is greater than it is with the other paints (as a result of the greater maxiiniini
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
83
2. The initial rate of leveling of a paint corre-, sponds to the maximum fluidity of the paint. Subseq u e n t leveling d e p e n d s 3 upon the change of fluidity v) Ill with time. I V 3. I n the leveling out zz of surface irregularities in a paint film, the effects of maximum fluidity decrease I and the effects of thixotropy MINUTES increase with increasing size FIGURE 18. RESULTSOBTAIXEDWITH THE Of the within GARDNER FLOWMETER FOR SERIES 3 the size limits usually encountered. 4. Thixotropy decreases and maximum fluidity and leveling increase with decreasing pigment concentration. Of the flat wall paints studied, those which had concentrations below 30 per cent pigment by volume were much less thixotropic than the paints with higher pigment concentrations. Complete leveling in the leveling test occurred a t pigment concentrations of 30 per cent and lower.
5 10 15 TIME OF FLOW (MINUTES) FIGURE17. RESULTSOBTAINED WITH THE GARDNER FLOWMETER FOR SERIES2 I
4
3 2 I u 2 - 2
Acknowledgment The authors are indebted to E. W. Fasig for the suggestion loof this research. They wish to thank J. M. Purdy, W. L. MINUTES Evans, and others for their criticisms and suggestions, the FIGURE 19. RESULTSOBTAINED WITH THE GARDNER FLOWMETER Maintenance Department of the Lowe Brothers Company FOR SERIES 4 for construction of part of the apparatus used, and the Lowe Brothers Company for permission to publish. fluidity of this paint), and the ridge shows little tendency to level out. Further, in the case of a stippled film the irreguLiterature Cited larities remaining in the surface are greater with this paint than with the other members of the series. Obviously, , ” d ~ & ~ & ~ ~ Am. , ’ ~sot.~ Testing : Mathen, in the case of a small surface irregularity paint H-835 terials, 19, 11, 640 (1919). (3) Ibid., 19, 11, 657 (1919). levels better than the other members of the series, but in the (4) Freundlich, H., and Abramson, H. A. 2. physilc. Chem., 131, case of a large irregularity it does not level as well. 283 (1927). rate Of flow is dependent On maximum (5) Gamble, D.L,, I,yn, ENG,CHEM.,28, 1207 (1936). fluidity, the total flow will depend on how thixotropic Prop(6) Gardner, H. A,, “Physical and Chemical Examination of erties alter the consistency of the material. Since flow Paints, Varnishes, Lacquers and Colors,” 7th ed., p. 611, hinders the development of thixotropic structure, the slower Washington, D. C., Inst. of Paint and Varnish Research, 1935. (7) Green, Henry, PrOC. Am. SOC. Testing Materials, 209 11, 451 the rate of flow the rapid will be the development of (1920). thixotropic structure. (8) Ibid., 20, 11, 465 (1920). In the case of a paint applied to a horizontal surface, the (9) Haslam, G. S., and Grady, L. D., Jr., IND. ENQ.CHEV.,Anal. Ed., 2, 67 (1930). smaller a surface irregularity, the greater is the pressure caused by surface tension, and the largerthe irregularity, the (10) Hauser, E. A,, and Reed, c.E., Phys. Chem.9 41,930 (1937). (11) Houwink, R., “Elasticity, Plasticity, and Structure of Matter.” greater the hydrostatic pressure. Hence in passing from a p, 14, New York, Macmillan Co., 1937. (12) McMillen, E. L., IXD.ENQ.CHEX.,23, 678 (1931) very small to a very large surface irregularity, the pressure(13) hfcMillen, E. L., J . Rheol.9 3976 (1932). producing flow ranges from a higher to a lower and then to a higher pressure. I n the results obtained paint H-835 showed better flow than the other members of the same series under (16) Ibzd., 3, 190 (1932). conditions of higher pressure and poorer flow under condi(17) I b i d . , 3, 190-1 (1932). (18) Melsheimer, L. A., Oficial Digest Federatzon P a a n t & Varnish tions of lower pressure. For higher pressures-for example, P r o d u c t i o n CZubs, No. 166, 169 (Oct., 1937). with (brush marks) Or very large (Gardner flowmeter) (19) Pfund, A. H., Proc. Am. Soc. Testing Matermls, 25, 11, 401 surface irregularities-since flow took place rapidly, thixo(1925). tropic changes had less effect than maximum fluidity, and (20) Pryce-Jones, John, J . 0 2 1 Cdour Chem. Ansoc.. 17. 313 (1934). (21) Ibid.9 17, 324 (1934). H-835 showed better flow. For intermediate pressures-for example, with a not too large surface irregularity-because of ;:):;; ;:; the smaller rate of flow, thixotropic changes increased the (24) Ibid., 19, 299 (1936). consistency of H-835 to such an extent that the total flow (25) Ibid., 19, 301 (1936). (26) Ibid., 19, 311 (1936). was less than with the other members of the same series.
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Conclusions 1, Maximum fluidity and thixotropy are the fundamental factors involved in leveling.
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(27) Ibid., 19, 315 (1936). (28) Waring, R. K., J . Rheol., 2, 307 (1931).
RECEIVED April 23, 1938. Presented before t h e Division of Paint and Varnish Chemistry a t the 95th Meeting of the American Chemical Society, Dallas, Texas, Zpril 18 t o 22, 1938.
