February, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2-Adjustment of cure is the most important problem. 3-Variation in quality excIusive of cure is of real importance. 4 - 4 .I. a t best cure gives a better idea of quality changes than slope, tensile, ultimate elongation, tensile product, energy of resilience, or coefficient of vulcanization. &-Tear resistance is an important property, particularly as a criterion for cure. &-In order to specify rubber quality it mill tie necessary t o specify the mixing used, until such time as the causes of variation are undefitood and means provided for eliminating or counteracting them.
Table XV-Stiffness Index Smoked Sheeis Rubber lots 3R 2s 3x 3H 3K 3 hl 3 I. 3G 1A 22 2Y 3P 3s 3u 3 iv 3N
-Slope--200 215 40 38 36 36 36 36 42 38 42 40 36 34 40 42 34 36 35 36 38 38 40 34 38 38 34 36 38 36 40 36 36 36
157 UI.
s. I.
230 38 34 36 36 34 34 38 38 34 36 34 38 36 36 38 3s
(best cure) 42 56 53 46 38 56 44 43 40 46 42 49 53 65 52 50
--
Slope in Pure Gum at 40 P o u n d s Pale Crcp-
Rubber lots
T
AA
U X Y
cc
W BB
VSIOR200 215 44 38 38 36 39 36 38 34 38 38 38 38 38 36 40 40
s. I.
230 36 34 34 32 34 36 36 36
(best cure)
I9 ' 21 25 24
21 21 18 21
The Settling and Packing of Mixed Paints' By William C. Arsem 226 STATESr., SCHENECTADY, N. Y .
In a mixed p a i n t t h e p i g m e n t grains a r e to s o m e exIt may be well to define the T HAS long been known t e n t deflocculated a n d dispersed by t h e free acids i n the terms used in this paper, bethat paints in storage vehicle, a n d t h e metallic soaps are m a i n t a i n e d i n t h e cause they are used by variundergo changes which sol condition by the s a m e agency. If no chemical ous workers in this field in a cause the pigment to settle t o changes t a k e place i n t h i s system, the p i g m e n t g r a i n s s e n s e s o m e w h a t different the bottom of the can, formm a y s e t t l e t h r o u g h t h e action of gravity, b u t will not f r o m t h e i r significance in ing a dense layer which is adhere, a n d soft s e t t l i n g will result. other fields. not easily incorporated with In a stored p a i n t slow chemical reaction between the supernatant liquid by Adsorption is used t o mean basic p i g m e n t s a n d free acids f o r m s basic soaps w i t h working and stirring. I n the an unequal distribution. of a be prelittle dispersing power. S o m e of t h e s o a p will past it appears that a purely substance between two phases cipitated as a r e s u l t of t h i s loss of free acid f r o m the c h e m i c a 1 explanation has of a heterogeneous mixture. As applied t o paint, we speak vehicle, a n d the p i g m e n t grains which a r e thus freed been sought for the phenomof the adsorption of a substance f r o m adsorbed dispersing a g e n t t e n d t o a d h e r e to each enon of packing in storage. from the vehicle by the pigment, o t h e r by virtue of t h e free surface valences. W h e n the Gardner2has indicated that having in mind a condition in removal of dispersing. a g e n t occurs a f t e r gravity has oils alone and in contact with which the free surface of the pigment is more or Iess comd o n e its work in bringing the particles close together, pigments may undergo hypletely covered over with molet h e y will a d h e r e m o r e or less strongly to e a c h o t h e r a n d drolysis in presence of moiscules of the adsorbed substance, h a r d settling will be the result. ture. Ware and Christman3 which are thus closer together have discussed the action of or in preater concentration than rosin and fatty acids OIL zinc oxide. They think that water and they are in the vehicle. The mechanism of adsorption, according to the latest views,6 involves an attraction of the molecules of adother substances added to paint as emulsifying agents hydro- sorbed substance by residual valences at the surface of a space lyze the oil, and that the fatty acids form insoluble zinc and lattice, so that we may conceive the molecules of adsorbed sublead soaps, which, if free to precipitate, form a skin. They stance as being attached to the surface of the pigment grains in a n show that a naphtha solution of rosin dissolves zinc oxide to oriented relation to the surface. Dispersion is the normal result of adsorption and peptization a clear solution and also dissolves the skin which forms on and should mean no more than a separation of the particles of paint, and they attribute the latter action to the formation of the dispersed phase from each other. The mechanism by which a soluble zinc resinate. Gardner4 has also again pointed this occurs is by no means clear, but the following explanation out that basic pigments can react with fatty acids and resins is offered without proof a t this time: Owing to the mutual solubility relations of the dispe'rsing in the paint vehicle to form soaps that are insoluble in naphtha agent and the solvent, the dispersing agent is probably not combut soluble to some extent in the presence of free fatty or pletely withdrawn from the solvent as a result of the adsorption resin acids. process, leaving a sharp surface of demarcation; but we may con-
I
The N a t u r e of Paint
Paint is a heterogeneous mixture of solids and liquids in which the phenomena of adsorption and peptization are very important. It offers a fertile field for the study of surface chemistry. The pigments have individual characteristics, and the liquids are often extremely complex. In order to direct attention to these factors, the ingredients of paints will be discussed in some detail. 1 Presented before the Section of Paint and Varnish Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925. 4 J. Franklin Inst., I??, 533 (1914). a Trans. A m . Insl. Chem. Eng., 9, 115 (1917). 4 Pain1 Mfrs'. Assoc. U.S.,Ie'h. Circ. 143 (1922).
ceive that each particle of pigment with the adsorbed molecules of dispersing agent always has an atmosphere of solvent which is bound or attached to the hydrocarbon chains of the dispersing agent by weak valence forces of the same kind as those concerned in ordinary solution processes. This leads to a conception of a definite structure for the interface between a dispersed solid phase and the liquid continuous phase. From a thermodynamic standpoint this view is admissible, as indicated by the following excerpt from a standard textbook:6
The boundary between two contiguous phases, which is known as a surface or an interface, IS not a mathematical boundary of only two-dimensional extension. There is a minute though finite region in which the prop6 8
Langmuir, J . A m . Chem. SOL, 38, 2221 (1916). Lewis and Randall, "Thermodynamics," 1st ed., p. 247.
15.8
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOl. 18, No. 2
erties differ appreciably from the properties in the interior of either phase, and~varyfrom point to point.
According to this view, we may recognize theoretically different degrees of dispersion of the pigment, depending on what proportion of the atomic spaces on the total surface of all the pigment grains is occupied by adsorbed molecules of dispersing agent, and what proportion of the solvent is bound by the solubility relations between it and the dispersing agent. In practice, the smoothness, flow, and stability of a paint would depend on these factors. Peptization is a special and extreme case of dispersion through which the dispersed phase is in very small particles or aggregates of a few molecules, so t h a t each system, consisting of a small solid (or liquid) aggregate surrounded by molecules of adsorbed substance, is small enough t o remain permanently suspended in the surrounding liquid phase. Flocculation. I n a system containing a peptized substance, the removal of some of the peptizing agent in any way tends to uncover some of the adsorbing surface of the peptized particles, and the active valences on these exposed surfaces tend to attract each other so that certain of the particles coalesce and the free surface is eliminated. This is what is meant by flocculation in the case of a fully peptized solid phase in a sol, and in this case some of the solid phase will be precipitated. As applied t o systems in which the dispersed particles are all of relatively large dimensions, as in a paint, flocculation causes adherence of adjacent pigment grains and loss of fluidity, although there is no coalescence to form large, continuous aggregates. The particles, although having some attraction for each other, do not adhere strongly together, but behave as though there were no lubrication at the points of contact.
When a pigment is placed in a vehicle containing nothing that can be adsorbed by it, the particles of pigment tend to adhere to each other or to flocculate. If the aggregates are broken up mechanically they will tend to reform. A mixture like this would not make a good paint because it would lack easy flow, although on standing the pigment probably would not settle into a hard, compact.mass. If a free acid is present, however, particularly if the pigment is strongly basic, the dispersing agent is adsorbed on the surface of the pigment grains,, and they thereupon become dispersed throughout the liquid and the mixture assumes more or less the character of paint. As long as nothing happens to disturb this arrangement, coalescence should not take place and the paint should remain soft and free-flowing, although the pigment may settle more or less, particularly if the amount of dispersing agent is small.
COOH
j ~
ZnO
j
............
~
Methyl Red, Adsorbed by Zinc Oxide
Composite pigments, such as lithopone and leaded zinc oxide, appear to be self-compensated by mutual adsorption of their constituents, and do not react clearly toward methyl red. The test with methyl red in naphtha is an extremely sensitive one for detecting acid impurities in basic pigments. Commercial white lead and zinc oxide show small pink specks, and commercial whiting becomes entirely pink owing to the presence of silica, while precipitated chalk gives a pure yellow color. Some common substances, including pigments, are classified thus by the methyl red test: ACID Kieselguhr Tripoli Talc Serpentine asbestos Amphibole asbestos Harytes Gypsum
BASIC Zinc oxide White lead
NEUTRAL O R DOUBTFUL Leaded zinc oxide
The classification of a solid as “acid” does not mean that it will not adsorb acid substances at all, because i t is conceivable that the surface of a solid lattice will exhibit both positive and negative valences. I n fact, i t can be shown that even acid pigments will adsorb fatty acids to some extent. Vehicles
A paint vehicle contains:
’
Drying oils-linseed, Free fatty acids Acid anhydrides Free resin acids Resins Metallic soaps
China wood, soy bean, etc.
Treated oils may be partly polymerized, or partly oxidized, or both. In any case there will be present molecular aggregates of two or more units, and also some free acid resulting from chemical changes taking place in the process. Pigments A sol condition often results in which the glyceride aggregates Any sokid substance has a t the surface unbalanced valences are peptized by the free acid. Free fatty acids may come from the original oil or may be which are manifested as power to adsorb certain classes of produced by treatment as just stated. substances. Free resin acids may come from any of the resinous inPigments may be classifled as acid or basic by their behavior toward solutions of dyes, and more particularly by gredients. The resins may include ordinary rosin, rosin ester gum, or means of amphoteric indicators such as methyl red. Asbestine, for instance, adsorbs the color from a water solution any of the commercial varnish gums. The metallic soaps include the resinates, linoleates, of a basic dye such as fuchsine or methyl violet. It will also become pink when placed in a pale yellow solution of neutral tungates, and other fatty acid and unsaturated acid salts of methyl red in naphtha. This behavior must mean that as- calcium, lead, manganese, cobalt, and zinc. These metallic bestine adsorbs methyl red in such a way that it is attached soaps are as a rule not soluble in naphtha but will form sols at the basic end of the molecule, which then changes its con- under particular conditions. Zinc oleate, made by dissolving figuration and becomes red just as it does in the presence of zinc oxide in the equivalent weight of hot oleic acid, is easily strong mineral acids. We may then class asbestine as an dispersed in naphtha, when freshly prepared, but the sol goes over into a gel within a few hours. The same zinc acid pigment. Pure zinc oxide in a neutral methyl red solution becomes oleate when 24 hours old will no longer disperse of itself into yellow, so that the indicator molecule must be attached to naphtha. Evidently the fresh soap is an unstable sol, which slowly changes into a gel, thus behaving like a gelatin sol the zinc oxide surface by its carboxyl group. which passes into a gel on standing. A soap can form a sol in the presence of free fatty acid CH~ 1 Sioz.x.+foi which appears to be adsorbed on the surfaces of the small j OH molecular aggregates so as to prevent their coalescence into \CHa I___..__.___.____________ I a gel. The stability of a soap sol depends on the concentraCOOH tion of the fatty acid in the solution, so that a stable sol may, Methyl Red, Adsorbed by Asbestine
I,VD USTRIAL A-ITDENGINEERING CHEMISTRY
February, 1926
on dilution, become cloudy by precipitation of flocks of gel, although in some cases considerable time is required for the change. It appears that there is a distribution of the peptizing agent between the solvent and the dispersed soap. To investigate this relation a certain obsolete type of factory liquid containing lead resinate and free acid was diluted with different amounts of naphtha, oleic acid being added in each case to just prevent precipitation, as shown by a faint opalescence. The relations between the concentration of acid and of lead resinate are shown in Figure 1. The smaller the amount of soap present the more acid is required to keep it peptized. The relation is: A
=
1.75 L - 0 . 4 2 8
This is a modification of the well-known Freundlich adsorption equation:
159
miscible liquid phases are produced, one high in the oxidized constituent and the other low. Dispersion of Pigments in Vehicles
The particles of pigment tend to adhere to each other by virtue of the external unbalanced valences which tend to unite the lattices of solid particles a t their points of contact. When a pigment is placed in a liquid, the particles will come together or flocculate unless the surface attraction is neutralized in some way. If a peptizing or dispersing agent is present the surface of the pigment particles adsorbs this substance so that every free valence a t the surface of the pigment tends to satisfy itself by attaching an equivalent of dispersing agent. Since we now think of adsorption as a polar phenomenon (Langmuir, Harkins), we may consider, in the case of a basic pigment which has adsorbed a fatty
1
r =acn-
m
m = mass of adsorbing substance (in this case the dispersed phase) y = mass of adsorbed substances (in this case the dispersing agent) c = concentration of adsorbed substance in this solution a t equilibrium
I n the case of a sol the dispersed substance is uniformly distributed in very small aggregates, to which the adsorbed substance, or dispersing agent, is attached by active groups. However, the greater part of the molecule of the dispersing agent remains virtually in solution, hence c may be taken as the total concentration of dispersing agent a t the critical point. Hence by making y = c, the equation becomes c
n -
Qmn-1
%‘LEADRESINATE -0.426 A= 1.75 L
or using A and L, as in Figure 1, we have Figure 1-Relatione
The explanation is that the additional solvent withdraws some of the peptizing agent from the surfaces of the units of the dispersed phase, and these free surfaces then coalesce to form larger aggregates, and these, if large enough, form a precipitate. I n the case of gloss oil a similar instability on dilution is observed, but the separation is very slow, requiring several days. A typical gloss oil may contain amounts of the several ingredients, which may be calculated as hTormal calcium resinate (or abietate) Free rosin Naphtha
Per cent 37 25
38
When some of this was diluted to 25 per cent by adding more naphtha, and allowed to stand several days, the washed and fused precipitate contained 3.37 per cent ash. Sormal calcium abietate Ca(C1gH2i02)2should yield 9.13 per cent ash. If one molecule of the normal soap were associated with two molecules of abietic anhydride, the chief constituent of rosin, as an acid soap or as an adsorption complex, the ash would be 3.24 per cent, a value very close to that found. An alternative explanation for the precipitation of gloss oil on dilution is that rosin itself in the fused or freshly dissolved condition is a sol which is stabilized by calcium resinate in concentrated solutions, but slowly separates from dilute solutions. It is known from other experiments that rosin separates in crystalline form from solutions in kerosene or transformer oil, and it is also known that calcium resinate is a good stabilizer for heavy metal soap sols. Certain blown oils contain constituents that are soluble in the balance of the oil, but when a thinner is added two im-
(L)
b e t w e e n Concentration of Acid a n d of Lead Resinate
acid, that the molecules of acid are all oriented so that the carboxyl groups are attached to the surface of the solid particle while the hydrocarbon chains extend outward perpendicular to the surface. Basic Soaps
By treating fatty acids dissolved in oils, and also oils with known acid content, with more than one equivalent of base such as white lead or basic zinc carbonate, basic soaps are obtained which as a rule are more soluble or more easily dispersed than the neutral or acid soaps. Basic zinc oleate has 2. Zn (OH)2. the formula Zn (CUH3302) When a n oil containing free acid, such as bodied linseed oil, is treated a t 200’ to 280” C. with basic zinc carbonate, 2ZnC03.3Zn(OH)2,as much as eight equivalents of zinc to one of the acid may be introduced, but when more than two equivalents have been added the neutral glyceride begins to be broken down, as is shown by the increase in apparent acid value. (It should be noted that most soaps except those of calcium, when tested for acid value by the usual method, react as free acids, because the bases set free by the standard alkali do not react alkaline to phenolphthalein.’) These basic soaps in most cases do not disperse pigments, and are only weakly adsorbed, although basic lead soaps do disperse zinc oxide somewhat. Slow Changes in Paints
“Soft-settling” and “hard-settling” are terms used to describe conditions in stored paints. When a paint is allowed to stand, the particles of pigment immediately begin to settle 7
Ware and Christman,
THIS J O U R N A L , 8, 996 (1916).
I@
INDUSTRIAL A N D ENGINEERING CHEMISTRY
at, a rate depending on their average size; and if the liquid contains no dispersing agent and the pigment particles consequently have no adsorbed layers, and, moreover, if they have no attraction for each other, they should settle freely and should eventually form a layer on the bottom in which the solid particles are closely packed together. If the particles are spherical and all of the same size, they should form a hexagonal, close-packed arrangement in which the solid particles occupy 74.04 per cent of the volume of the settled layer and the liquid between the solid particles occupies 25.96 per cent. I n an actual paint, however, these hypothetical conditions do not exist. I n the first place, each pigment particle is covered with a n adsorbed layer of dispersing agent, which prevents adhesion of adjacent particles. On the other hand, each pigment particle with its adsorbed layer of dispersing agent is to a large extent rendered incapable of attracting a neighboring particle of pigment similarly covered, since the surface valences are almost wholly neutralized. When gravity acts on such a paint, the particles slowly settle and approach each other as nearly as is possible without actual contact. A settled paint in this condition should stir up easily and be just as smooth as it was originally and would be classed as a soft-settled paint. On the other hand, if chemical changes take place by which the dispersing agent is destroyed or removed or ceases to function while the pigment is settling or after it has settled, the grains of pigment may then come into actual contact and be held by their normal attraction for each other, due to residual valences. When removal of dispersing agent occurs before the paint has settled, the phenomenon is known as flocculation and the pigment particles simply form large aggregates and the paint becomes less free-flowing. If the adsorbed dispersing agent is removed after the paint has settled, it becomes “packed” and is very difficult to stir up and would be classed as a hard-settled paint. Hard packing should be most pronounced and the packed pigment should be most dense when just enough dispersing agent is present to separate all the individual particles so that settling shall be as free as possible and the pigment layer shall occupy a minimum apparent volume. Thus, a paint which is well ground and contains a plentiful amount of acid or resinous dispersing agents and to all appearance behaves like a good paint when freshly prepared may be much more likely to get into a condition favoring hard packing if the complex systems formed are unstable and the pigment can eventually lose its adsorbed layer. We have now to show how the removal of dispersing agent and the change in the degree of deflocculation of the pigment can occur. I n a paint the partly deflocculated pigment and the metallic soap sol in the vehicle, both stabilized by the same free acid, form a highly sensitive system. As soon as a pigment is ground with a vehicle chemical and physical changes begin to take place. First, the pigment adsorbs fatty acid and possibly other substances from the vehicle. At this stage the paint consists of two unstable complex phases, one liquid and the other solid. The liquid phase has lost some of the dispersing agents which have previously stabilized the heavy metal soaps in the sol condition in the vehicle, so that the sols tend to revert more or less rapidly to the gel condition. As this change takes place the soaps may precipitate in a flocculent condition between the pigment grains as a blanket above the pigment or as a jelly or “pudding” throughout the entire mass of the paint. The solid phase, consisting of pigment grains with adsorbed fatty acid on their surfaces, is chemically unstable if there is any possibility of a reaction between the pigment and the dispersing agent. If the pigment is basic and the dispersing agent is acid, a slow reaction will take place, forming a
VOl. 18, No. 2
metallic soap. If the action between the basic pigment and the acid were to stop a t the point at which a neutral soap is formed this neutral soap might also act as a dispersing agent, but it has been shown that the basic pigment, if it is white lead or zinc oxide, will continue to act on the neutral soap until a basic soap is formed and this basic soap has very little dispersing action on a pigment derived from the same metal. We see, therefore, that as soon as the paint is once mixed unstable conditions immediately exist, and the inevitable changes toward chemical equilibrium tend to cause a loss of the characteristic properties of paint. Chemical Changes in Stored Paints Paints stored in cans were examined periodically for 22 months and determinations were made of acid value and ash content of the vehicle. The acid value is a measure, not only of the free acid, but of the metallic soaps. I n all cases the acid value decreased slightly with time, which was interpreted to mean a transition of a portion of the “drier” soaps from the sol to the flocculated or gel condition. The ratio of ash to total acid calculated as oleic acid was found to approach a constant value not far from 0.287, which is the ratio of zinc oxide to oleic acid in basic zinc oleate, Zn(ClsH330z)~.Zn(OH)~, and zinc oxide was found in the ash of the vehicle. It is considered highly probable that the zinc oxide in the paint slowly dissolves in the free acid, the normal end condition being that a soluble basic zinc soap is formed and all the acid neutralized. If there is no interference with the progress of this action, the ratio of the final ash content to the total combined and free acid should be 0.287 and the liquid should contain negligible amounts of the soaps of other metals which are thrown out when the liquid passes from the acid to the neutral condition. The vehicle in a stored paint thus finally loses most of its power to disperse pigments because of the loss of free acid and neutral soaps and the formation of basic soaps. Acknowledgment Acknowledgment is due to the Lowe Brothers Company, of Dayton, Ohio, for permission to publish this paper, and to D. A. Kohr, vice president, and to members of the staff for their cooperation and interest.
U. S. Gains Patent and Trade-Mark Relief Very material relief from former onerous conditions affecting American patent rights in foreign countries resulted from the International Convention for the Protection of Industrial Property held a t The Hague from October 8 t o November 6, a t which thirty-two countries were represented, according to Commissioner of Patents Thomas E. Robertson, chairman of the American delegation. Practical elimination of the risk of forfeiture of a patent for nonworking or for nonpayment of taxes through accidental causes is described by the patent commissioner as one of t h e changes of outstanding importance. An agreement was reached t h a t the period of three years, which must be allowed before any penalty can be imposed for nonworking, is to be reckoned from the date of the grant of the patent instead of the filing of the application. Another important development is the provision as to the cancellation of fraudulently registered trade-marks, furnishing “an effective remedy against piracy of well-known trade-marks.” The amendments with relation t o unfair competition will be of benefit, especially in those countries which have not developed, as have t h e United States and Great Britain, a well-defined system of jurisprudence with respect to unfair competition. While in Commissioner Robertson’s opinion the conference did not go so far on working patents and taxes thereon as t h e United States delegates proposed, the adoption of the present convention represents a long step forward in the direction of doing away entirely with requirements of working in order to maintain a patent in force and very materially diminishes the risk of a patentee Iosing his rights as in former years.
