INDUSTRIAL Ah'D ENCINEBRING CNEMISTRY
July, 1928
729
PIGMENT SETTLING SYMPOSIUM Paperr presented befme the Division of Peinf and Varnish Chemistry nt the 75th Meeting of the American Chemical Society. St Louis, Mo., April 10 to 19, 1928
Accelerated Tests for the Settling of Pigments in Paints' Sidney Werthan and R. H. Wien TIIS E-BWJ s ~ s a vZ i ~ cC O X P I N Y , PALUERTON. PA
Factors AffectingSettling Rate
extra thnner. The addition of t h i n n e r has a threefold d e e t : it increases the vehicle N THE development of an accelerated s e t t l i n g content, decreases the specific test, one must firstcong r a v i t y of the vehicle, and the factors involved and decreases its viscosity. De%tie bariations in these that creasing the pigment-vehicle ratio of a series of flat wall proinote settlmg tendrncies. These factors are tho properpaintsfrom 71:ZQ to W:4#by ties of the pigment and of the the addition of mineral spirits 7 chicle, pigment+ahioleratio, made it possible to obtain settype of container, and storage tling indications within 10 conditions. goods." days that were fairly representative of the settling 1 ~ . Auirie of the properties of the pigment that influence settling tendencies are the size of the regular products after 6 months' storage. (Figure 1) A very simple application of this type of test w to mix by the particles or the relative particle sizes of the various camponenk of the pigment mass, the density or relative den+ shaking without any milling, the pibment with the reduced ties, and the activity, which includes snrh properties a3 ba- vehir1e.l Sometimes the rrsults 80 obtained are mwleading, sicity. wettabihty, soap-forming, flocculating, dnpersing. and so it is preferable to add extra thinner to the regular paints. gel-forming tendencies. Those properties of the vrhicle, ~ a r i - Still, with certain formulas the mme relative results were ations in which influence settling tendenries, are specific obtained whethcr extra thinner was added to the paint or the gravity, 7iscosity. volatile-nonvolatile ratio, and activity, pigincnt merrly mixed with the reduced vehicle. The results which includes acidity, stability, wetting, dnpersmg, and mere also comparable nith the settling in the regular paints flocculating properties. after prolonged storage. Changing the pigment-vehicle ratio - that is, inerraiing the vehicle content-i? one of the simpleqt means of promoting settling Although the type of container does not influence setthiig, the i i v of tall cylin~lricallars inagnifiez the rrlative dlffwence.i between samples. Both rate of Fettling and suhaequent d i n g incrvase with heat As vttling IY priniarily due to the differences in the gra7itational forca on the varionb constltueiits of the paint, any incredbe in thi.; force ivill hasten settling.
I
Any test by which the settling tendencies of a paint can he determined before it is packed and distributed should he of value to the manufacturer. In this Paper the various factors affecting the settling rate or tendency are briefly considered, accelerated tests based on these factors are described and methods for examining the degree and nature of the settled cake are &en. Although the settling of pigment in barrels, drums, dipping tanks and the containers used with spraying and other Paint application equipment often causes difficulty, this paper is limited to the discussion of settling in small size packages or so-called "shelf
Testa and Applicatiana
.
(1 on these cliffereiit factors, various tests have bet:n ~levelopcd. The r:ite of settling in a paint CRIL ofteii he accolerated l q eliminating one of the pigment cmstitucnts~as, for example, a coarse, fibrous pigment, or one of low specific p a r i t y (such as fibrous talc), or one that has suspending properties dne to soap-forming tendencies (such as zinc oxide), or one that reacts with the vehicle to form a gel (Fucli as alunrinum stetirate). Tests of this kind arc usually quite limited in their application and should he adupt,cd only after careful preliminary experirnentalion has provcd that the change in the pigment formiila has only resiillcd in accelerating the normal settling of the halaiice of t,lie pigrnont constit.iients. I'erhaps the simplest iiieans of testing a specific forn~ula is to alter the vchiclc or the pigment-vehicle ratio by adding 2
I're,cetcd
1,y
s.Wrrthr".
Iidure I-series
of Plat Wall Painra Conlilioine Additions1 Mineral Splnfs
1t:ite of settling may be increased by eliminating a flocculating constituent, sue11 as an emnlsifyiiig agent, from the vrhicle. Increasing the dispersing properties of the vehicle by the addition of a small percentage of polymerized oil or a solvent powers has similar effect. discussed, any test based on inproperties of the vehicle necessitates preliminary invest,igation to establish the value of the results. A test snitable for general application should not necessi1:ite altering the product under examination. Objectionable settling is the combined result of migration of the pigment toward the bottom of the container and reactions within the paint. The centrifuge offers the simplest means of increasing the rate of migration of the pigment and heat is I. 1 test u i this kind her been sv~gertedand used ihy Doctor Schumaiin, 01 the Grand llapids Wood Finishiiig C~mpiiiiy.
730
I N D U ~ W I t I A LA N D ENGl NEEEING C H E M I S T R Y
the most logiesl agent fur :Lccclcrating the activities that produce caking. The rate of migration can be cont,rolled by regulating the speed of tho centrifuge, while, by a careful selection of temperature, the maximum acceleration of normal aging can be produced. The most promising accelerated test consists of a combination of centrifugal force and heat and the perfect test d e p e n d s on obtaining the correct balance. The ideal condition would he to apply the correct centrifugal speed, under the proper temperature, which would necessitate a con t r o 11e d temperature room or cabinet for the centrifuge. Satisfactory r e s u l t s can be obtained by interm i t t e n t l y centrifuging the varm sample. It is necessary to determine a suitable cycle, taking care that a temperat , u e is not used that is sufficiently high to cause reactions in the paint which normally would not occur. The cycle chosen will generally depend on the equipment available and the degree of acceleration Figure 2-penetrometer desired. At, present the miters are iising a %-hour cycle, which is quitc severe as only 4 to 5 days are required to reproduce conditions typical of 2 years' normal storage. Tests are niadc in snia,ll g h s jars with metal covers. These jars are approximately 6 cm. in diameter and 11 em. tali, r5-it.h a capacity of 180 cc. This jar was arbitrarily chosen because of the size of the cups on the centrifuge. In the present cycle the jar of paint is kept in a warm oven (65" to io" C.) overnight. (approximately 18 hours). The following morning the warm sample is centrifuged for 20 minutes at 1000r. p. m., returned to the oven Sor approximately 5 hours, cerit.riluged a g h for 20 uiiiiutea and, unless it is to be emmined, the cycle is repeated. A sample is brought to room temperature before being emminod. Although most of the work has been with flat wall paints, the results indicate that this test is applicable to other paints, enamels, or lacquers. Sufficient tests of miscellaneous products have not been made to determine whether the cycle and factor used in testing flat wall paints can be applied promiscuously in the examination of miscellaneous products. Methods for Examining and Measuring Degree of Settling The usual means of examining and t,he terms used for describing the degree of settling in a paint :irc often indefinite
Vol. 20, so.i
and inaccurate. The use of g l w jars makes it possible to measure accurately the amount of settled pigment or supernatant clear liquid and easy to detect any objectionable condition, such as a layer of gel or skin. The amount is usually not of such great importance as the condition of the settled material. The penetration tests used for asphalt, greases, and paste paints are not suitable for examining the settled material in a paint, which often varies from a soft paste in the upper portion to 8 hard, almost dry hyer at the bottom. A special penetrometer is used which consists of a vertically supported rod with & pan at the top for weights and a small plunger attached a t the bottom. (Figure 2) Three diBerent plungers, which vary in shape and size hut are of equal weight, are used. The tip of the plunger is brought in contact with the top surface of the settled material, released, and the time necessary to travel to the bottom of the container recorded. If complete penetration does not result, due to striking a harder layer of pigment, the test is repeated at another spot with the weight incresed. As an expediency, an individual test is considered completed when no m e a s u r a b l e movement of the plunger occurs irk 30 seconds. Tests are contiirued until complete pcnctratiim is o b t a i n e d , when the weight and time required are recorded. Bydetermining the load and time Fiaure3-seftled Painr Mixing Machine necessary to penetrate the settled cake with a t least two of the plungess, a mcamre is obtained OS the relative degree of packing. In a,ddition to the amount of supexnatant liquid and the hardness of the settled cake, the work required to rc-incorporate the settled pigment and t,he possibility of obtaining a smootb product are OS importance. In the writers' laboratory this information is obtained with a special stirring apparatus. (Figure 3) Three different mixing actisns arc produecd with it&a rot,ation of the stirrer on its own axis, a rotation of the stirrer around the longitudinal axis of the glass jar, and a vertical upand-down motion of the stirrer. A very rapid snd thorough mil;ing is obtained in a few mimltes and eTen the htrdest type of i:aked pigment can be re-iiicnrpossted.
Figure 3
Fbguie 1
Comparative R e a d r s between N ~ f u r a lSeftIlnB and Accelerated Seffling Photoxiaphed 3/20/28)
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1928
73 1
first pouring off the supernatant liquid and adding it gradually during mixing, while the other is mixed direct. A specky product in both cases is definite indication of granulation in the paint. Another property of interest is the condition of the settled pigment-that is, whether it is dispersed, flocculated, aggregated, or gelled. This information can be determined by microscopical examination. Indications are that microscopical investigation of the whole subject of settling offers rich possibilities. Simple examination of the pigment in T a b l e I-Flat W a l l Paints different portions of a settled paint often furnishes valuable (Shelf-aged-Examined March 20, 1928) leads as to the cause of settling and possible means of imDEPTHO F SUPER- PENETRATION proving the product. Although a large number of settled NATANT TEST0 MICROSCGPIC EXAMINApaints have been microscopically examined in this laboraPAINTDATEFILED LIQUID Plunger W t . Time TION OF SETTLED PIGCm. Grams Seconds MENT tory, the subject has not been sufficiently investigated to A 7/20/27 2.5 1 92 25 Top, partially floccuAged 8 months 2 . . . . . lated; bottom, partially Tarrant definite conclusions.
The time required to produce a perfectly mixed, smooth product can be used as a measure of the work necessary. One of the greatest objections to settling in a paint is that often when mixed by the consumer a specky product is obtained. Usually this is due t o improper or insufficient mixing. When duplicate samples are available, this machine affords an excellent means of determining whether a specky product would be the result of improper mixing or actually due to granulation of the paint. One of the samples is tested by
B
C
542 3 1 1042 2 ... 3 ...
15 flocculated 20 Top, flocculated;
2.6
1 2 3
242 542
30 21
3/2/26 Aged 2 years
2.7
1
1042
30
1/31/28
2.3
3/2/26 Aged 2 years
2.95
7/20/27 Aged 8 months
2 3 1 2 3 1 2 3
.. *.
...
.. ..
... ... ...
..
bottom, partially flocculated: considerable dispersed material apparently dry T o p , flocculated; hottom, dispersed, apparently quite d r y Top, partially flocculated; bottom, dispersed
..
Top, partially flocculated: bottom, partially flocculated 11/1/27 2.6 Top, dispersed; bottom, Aged 4 . 5 months 15 dispersed large pieces of . . inert, pigment fairly. dry. a T h e weight of the empty pan, rod, and plunger is 42 grams, so this represents t h e minimum weight t h a t can be applied; otherwise t h e weight given is the minimum tested t h a t produced complete penetration. IVhere results are not given, no tests were made. Aged 2 months
42 242 92 342
50
49 28
...
~~
T a b l e 11-Accelerated DGPTHOF STANDARD SUPERNATANT PENETRATION TEST Plunger Weight CYCLEa LIQUID
Hours 24
Cm. 1.0
Grams 1 2
3 4s
2.6
72
2.6
120
1
2 3
1
2 3
42 92 142 3-12 342 542 342 342 542
2.8
Results of Tests
In Tables 11, 111, and IV and also Figures 4 and 5 are results of accelerated settling of three typical flat wall paints, using a 24-hour cycle of heating and centrifuging. Paints A and B have fair non-settling properties and normally would not settle to an objectionable cake within a year. Paint C is not so satisfactory in this respect, settling more rapidly and to a harder cake. Table I covers tests on samples of the same paints which had settled during normal storage. They do not represent a sufficient number of different periods of aging to justify drawing definite relations between shelf storage and the
T e s t Using Centrifugal Force a n d H e a t (Flat Wall Paint A) RE-INCORMICROSCOPIC EXAMIN.&TIOS
Time
O F SETTLED P I G M E N T
PORATION
TEST
Seconds 2
Very little hard sediment a t bottom; entire mass flocculated
10
7 20 29 20 30 35 2s 32 9 17 20
Minutes 1
Top, flocculated; bottom, hard, d r y cake t h a t appears aggregated
1'/Y
Top, flocculated; bottom, some dispersed pigment a n d hard aggregates
l'/z
Top, flocculated: bottom, some dispersed pigment a n d hard aggregates
2
1 1542 Top, flocculated; bottom, thick, d r y layer partialiy floccu2'/r 2 ... .. lated a n d large amount of hard aggregdtion 3 144 3.1 1 15.12 Incomplete Top, small amount of dispersed pigment: bottom, very d r y 3 2 ... penetration and considerable amount of hard aggregates (granular) a Standard 24-hour cycle: Approximately 18 hours' heat 65-70° C.; 20 minutes' centrifuge a t 1000 r. p. m.: approximately 5 hours' heat 65-70° C.; 20 minutes' centrifuge a t 1000 r. p. m.
...
..
T a b l e 111-Accelerated DEPTHO F STANDARD SUPERNATANT CYCLE^ LIQUID Plunger Hours Cm.
24
1.0
48
1.5
1
1.7
2 3 1
72
1 2 3
2 3
96 120
1.9 1.9
1 2 3 +1
a
2.5
PENETRATION TEST Weight Time
Grams 42 92 242 142 242 342 542 542 542 742 942 ,..
1542
Seconds 2 20 15 22 9 9
TEST Minuies
1'/2
lo
Top, flocculated: bottom, thick layer of a r y . lumpy material, partly dispersed
2
20 12
Top, flocculated; bottom, d r y and lumpy material
2'12
Incomplete penetration
Top, flocculated; bottom, thick layer of dry material
3
TQP,flocculated; bottom, very dry and lumpy
25 33
..
...
1542
2
... ...
Incomplete penetration
..
3b (granular)
..
b On duplicate sample clear liquid was poured off, material mixed
minutes.
PIGMEKT
Top, flocculated; bottom, hard, d r y cake and hard aggregates
1
3 For standard cycle, see footnote t o Table 11.
PORATION
SETTLED
1
3
...
RE-INCORb 1 I C R O S C O P I C EXAhlINATION OF
Very little hard sediment a t bottom; entire mass flocculated
.. ..
7
144
Test U s i n g C e n t r i f u g a l F o r c e a n d H e a t (Flat Wall Paint B)
2 minutes, and vehicle added in small portions.
A smooth product was obtained in 6
INDUSTRIAL A N D ENGINEERING CHEMISTRY
732
Table IV-Accelerated
DEPTHO F SUPERNATANT PENETRATION TEST LIQUID Plunger Weight Time Cm. Grams Seconds 20 hours, 20 min.: 1.9 1 142 31 Oven, 18 hours 2 242 11 Centrifuge, 20 minutes 3 542 10 Room, 2 hours
RE-INCOR-
STANDARD CYcLEa
24 hours, 40 min.: Oven, 22 hours Centrifuge, 40 min. Room, 2 hours
3.6
43 hours: Oven, 40 hours Centrifuge, 60 min. Room, 2 hours
3.7
67 hours, 40 min.: Oven 64 hours Centhuge, 100 min. Room, 2 hours
4.0
1
342 542 1042
4 31
2 3
1
542 542 1042
1 2 3
1042 1042
68
2 3
Vol. 20, No. 7
T e s t Using Centrifugal Force a n d H e a t (Flat Wall Paint C) PORATION
hfICROSCOPIC EXAMINATION OF SETTLED PIGMENT
TEST
Minufes 2
Top, flocculated; bottom, flocculated
Top, flocculated; bottom, small amount of dispersed material
2'/1
11 16 Incomplete penetration
Top, flocculated; bottom, flocculated with some apparently dry pigment
2'/a
6
Top, flocculated and lumpy; bottom, hard aggregates and dispersed pigment that appears dry
9
...
*.
a See footnote t o Table 11.
accelerated cycle. Still, on comparison of the results of the penetration tests (ignoring the results on the sample of paint B which was tested after 8 months' storage and apparently falls somewhat out of line), one finds that each 24 hours of the cycle represents approximately 5 months' shelf storage. Several outside paints and enamels, which from previous observations were known not to develop settling even after several years of storage, were tested. No settling was ob-
5'12
(Formed lump, which dispersed very slowly)
tained in any case, after 7 days, when the tests were discontinued. Conclusion
The writers do not feel that either a perfected test or an ideal cycle has been submitted, but they do believe that the results obtained definitely demonstrate that a test applying heat and centrifugal force is an excellent tool with which the settling tendencies of a paint can be determined.
Settling of Pigments in House Paints
.
H. L. Beakes PEASLEE-GAULBERT Co., LOUISVILLE, KY.
0 STUDY the settling of pigments in house paints, (20 inrhes). The lowest opening for sampling was 3.81 single pigment paints were made up, using as the liquid cm. (1.5 inches) from the bottom. Each top opening is 90 per cent taw linseed oil, 3 per cent drier, and 7 per accordingly 44.45 cm. (17.5 inches) from the bottom of the cent mineral spirits. The pigments used were: zinc oxide can (inside measurement). (5 per cent leaded), sulfate white lead, Titanox, red lead All paints were made up one week before the final adjust(96 to 97 per cent), asbestine, silica, barytes, and china clay. ment to standard consistency. The paints were then filled Each of these pigments was ground separately. Two two- into the containers, a sample being retained of each paint. pigment paints were also made up from the above pigments: The following day samples were drawn from each opening (1) 50 per cent lead with 50 per cent zinc, and (2) 60 per cent and as often thereafter as seemed necessary. Determinations red lead with 40 per cent asbestine. These paints were of the pigment content are given in Table 11. adjusted to the same consistency using the Gardner-Parks It is noticeable that Titanox, which had been considered as mobilometer,' the consistency being that of the customary a pigment that settled out rapidly, does not show this tendency white house paint. (Table I) when used alone. Zinc alone and lead alone settled very All paints were filled into 10-gallon filler drums 31.11 by slowly, whereas the half-and-half mixture of these was very 53.34 cm. (12l/4 by 21 inches) and the container was filled rapid by comparison. To determine whether the lead was full. Each drum was equipped with five openings from dropping out, leaving the zinc in suspension, analyses were which samples could be drawn through 7-mm. (outside made of the pigment for zinc oxide content. (Table 111) No diameter) glass tubing. The tubing projected 89 cm. (3.5 change in zinc oxide content was found from that of the inches) into the container. The five openings for sampling original paint, indicating that the pigment is settling as a were arranged vertically, 10.16 cm. (4inches) from center to whole, instead of one constituent more than another. The center. The working height of the drum inside was 50.80 cm. specific gravity of the different constituents apparently is not a factor. 1 Paint Mfrs. Assocn. U. S., Tech. Circ. 866.
T
Table I-Mobilometer WEIGKT ZINC SULFATE OXIDE WHITE RED ADDEDTO TITANOX LEAD PLUNQER(5% LEADED) LEAD
D a t a for Paints C o n t a i n i n g Various P i g m e n t s
ASBESTINE SILICA BARYTES
CHINA CLAY
Crams (Temperature, 23.35' C. (74'
F.); weight of plunger, 79 grams)
50% LEAD 50% ZINC
60% REDLEAD 40% ASBESTINB
OUTSIDE WHITE HOUSE PAINT
