Fundamental Studies of Paints Paint Durability as Affected by the Colloidal Properties of the Liquid Paint A. C. ELM,The New Jersey Zinc Company, Palmerton, Pa. the conception of the “critical HE results of paint and A large number of tests extending oz‘er a period point” of paints and its connecdrying oil research durof 2 years hat\e shown that the critical point qf a tion with p a i n t film p r o p e r ing the past, ten years inspecific paint system as determined according to ties. dicate that the drying of a n oil Wolff’s method indicates the pigment-binder ratio By empirical methods Wolff film consists of three overlapping which giws the best durability on exterior exposure. a n d h i s c o l l a b o r a t o r s (IO) stages: (1) oxidation, (2) polywere able to derive a m a t h e merization or association of the There is at present no evidence that the critical matical expression for the conoxidized oil molecules to colloidal points of dissimilar paint systems are indicative sistency of a paint as a funcdimensions, and (3) coagulation of their rdatiz‘e durabilities. tion of the pigment concentraof the colloidal system. As this W o l f s critical point method promises to betion. This equation, however, transition from the sol to the come a research tool of considerable value to the applies only w i t h i n a f a i r l y gel state progresses, the first, the narrow range of pigment-vehicle second, or the third reaction prepaint chemist. ratios in the neighborhood of dominates in turn. After the brushing consistency: film has dried, these reactions do not stop but continue a t an ever-decreasing rate. -4s TV - TI’, = k.pfar as the solidified film is concerned, it is now generally where T V = turboviscosity of the paint = the load in grams agreed that the third is the predominating reaction, and the necessary t o make the pan of the turboviscomfirst apparently is of minor importance. eter (Figure 1) fall through 1 meter in 10 seconds TVo = turboviscosity of the vehicle Long ( 2 ) and his collaborators (3) were able to show “that p = pigment concn., per cent by vol. ultimate analyses and heats of combustion of solid films or of k , n = parameters (referred to below as constants) the solid phase in the films do not change sufficiently with age to explain the definite changes in the film which lead to loss The constants k and n can easily be calculated from the of elasticity and finally to cracking and failure. Rather it turboviscosities of the vehicle and of two paints having differwould seem that the changes are more closely associated with ent but known pigment content. I n turn, the turboviscosity the transition of liquid phase to solid-in other words, with for any given pigment concentration can be calculated from colloidal reactions. k and n. All of these points fall on a parabolic curve-the *There is little doubt that the reactions which finally lead consistency curve-which is obtained when the turboviscosto paint failure on exterior exposure are primarily of a col- ity induced by the pigment (TV-TVO) is plotted against loidal nature. It is logical then to conclude that the colloidal the pigment concentration in per cent by volume ( p ) or make-up or structure of a paint film is a n influential factor in against the vehicle concentration in per cent by weight. By means of vector analysis, Wolff then derived an eauadetermining its behavior and life uDon exterior exposure. It may be assumed that the colloidal tion xvhich permits of the calculation anatomy of a pairit film is, to a large of that point on the curve which has degree, dependent upon the colloidal the s h o r t e s t vector. This point was p r o p e r t i e s of the liquid paint. The found to coincide quite well with the oil consistency p r o p e r t i e s of a paint, in concentration a t brushing consistency. turn, depend in part on these colloidal The equation is: properties and might be expected to f u r n i s h i n d i c a t i o n s of the colloidal X[lOOs X ( S - s)]’ = lOOSnli2 properties and hence the life of the film. [100s(100 - x)]*~~--1 It seems that such a connection be100s + X ( S - s) tween paint consistency and paint duraiyhere X = point of brushing consistency bility was suspected as early as a dozen (critical point), per cent by vears ago. Walker and Thompson (8) weight of oil S = sp. gr. of pigment hetermined the yield values and mos = sp. gr. of vehicle b i l i t i e s of a considerable number of p a i n t s b y m e a n s of the plastometer but were unsuccessful in “the interpreContinuing their work, these investitation of these results in c o n n e c t i o n gators found that the critical point is with the life and v a l u e of t h e p a i n t more than a mere measure of brushing films.” consistency and indicates the pigmentAt about the same time Wolff (9) and fixed binder ratio which yields optimum his collaborators in G e r m a n y , while paint properties from the point of view searching for a measure and definition of drying, water resistance, etc This of brushing consistency, uncovered some point, they claimed, is critical in so far interesting relationships between conas a deviation from it to either side will sistency and other paint p r o p e r t i e s result in markedly different and poorer which, in the course of a decade, led to FIGURE 1. TURBOVISCOMETER paint properties.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I n 1932 Wachholtz and Walther (7) published the results of some tests which lent considerable support to WOWShypotheses. On the basis of stress-strain measurements and swelling tests with white lead-linseed oil films, these authors concluded that basic carbonate white lead paints containing 29 to 30 per cent pigment by volume yield better films than higher or lower pigmentations. This pigment-oil ratio coincides with the critical point as determined according to Wolff’s method.
Vol. 26, No. 12
described above, however, is preferable since it takes into account all determinations, thus minimizing appreciably any experimental errors. The turboviscosity of the vehicle (TVO)is determined in the same manner. All data necessary for the calculation of the critical point of this particular paint system are then available, but before proceeding with the actual calculation of the critical point it is advisable to plot the experimental turboviscosity values (TP - TVo)against the corresponding pigment concentrations in per cent by volume ( p ) on logarithmic coordinate paper. The points thus obtained should fall on a straight line. Usually, however (Figure 3), they deviate from a straight line, the deviation being appreciable at the higher pigment concentrations. For the calculation of the critical point only those values which approach a straight-line relationship most closely should be selected. This requirement corresponds to Wolff’s suggestion that the paints used for the determination of the critical point should I have turboviscosities below 600. For a detailed description LOAD I N G R A M S of the procedure used in calculating- the critical point of a OF THE TURBOVISCOSITY FIGURE 2. DETERMINATION OF A PAINTSYSTEM paint system from the turboviscosity measurements the reader is referred to the Recently te Boekhorst ( I ) published the results of an in- literature (11). vestigation of the critical point of linseed oil paints which are As previously stated, in good agreement with Wolff’s and the present results.’ t h e c r i t i c a l p o i n t of All of these investigators have based their conclusions on paints was discovered in specific laboratory tests which may or may not be indicative an attempt t o find some of outdoor durability, It is obvious that the paint industry measure of brushing conwould derive real benefit from these investigations if it could s i s t e n c y a n d was first be reliably established that a similar relationship exists be- thought to be not more tween the critical point of a paint system as determined ac- t h a n t h a t . I n o t h e r cording to Wolff’s method and the durability of the same w o r d s , the consistency system on exterior exposure. The work described in this of linseed oil paints conpaper deals with this problem. taining the amounts of p i g m e n t indicated by EXPERIMENTAL PROCEDURE their critical points In early experiments the writer used a modified Kampf s h o u l d b e nearly conviscometer which had been obtained by replacing the cup stant. The critical conand stirrer of the instrument described by McMillen (6) with s i s t e n c i e s of the paint and S-shaped stirrer and a cylindrical cup of the same di- systems tested under this mensions as in Wolff’s instrument. Later experiments were investigation were calcumade using a n apparatus fashioned exactly after WOW’S lated by e n t e r i n g t h e turboviscometer with the exception that the rotating parts values for IC, n, and C p FIGURE3. RELATIONSHIP BEwere mounted in ball bearings to reduce frictional resistance (the pigment concentraTWEEN PIGMENT VOLUMEAND to a minimum. This apparatus is shown in Figure 1. TURBOVISCOSITY OF PAINTS tion a t the critical point) into the basic equation To determine the turboviscosity of a paint, the cup is Bled 30 mm. deep and the platform holding the cup is raised until T V - TTr, = k.p”. The values obtained, as Table I shows, are the stirrer is 5 mm. below the surface of the paint. Sufficient in the neighborhood of 200. This offers a possibility for deterweight is applied to make the pan fall through 1 meter in not mining the critical point of a raw linseed oil or similar paint by less than 5 and not more than 30 seconds. The pan should be allowed to traverse a distance of about 50 cm. or more before a graphical method. All that is needed is to read from the taking a reading in order to eliminate the effect of gravitational T V - TVo/p curve (Figure 3) the pigment concentration which acceleration. The load is increased or decreased, the string is would give a turboviscosity of about 200. The wide range rewound on the spool, and a second reading is then taken. of critical consistencies (160 to 260), on account of the steepThus a sufficient number of readings are taken between the time ness of the curve, corresponds to a comparatively narrow limits set above. range of pigment concentration, and, therefore, the values The readings are plotted on logarithmic graph paper (Figure thus obtained are satisfactory for all practical purposes. 2), using load as the abscissa and time as the ordinate. The Furthermore, the value obtained graphically is of considerable points thus obtained are connected by a straight line, and the help in the calculation of the critical point from Wolff’s equapoint at which this straight line intercepts the 10-second line tion which is best solved by trial and error. is the load necessary to make the pan fall through a distance From its name it might be assumed that the critical point of 1 meter in 10 seconds, or the turboviscosity of the paint of a paint system is a sharply defined point and that even the under test. The turboviscosities of several additional sam- slightest deviation from the pigment-vehicle ratio indicated ples of the same paint but differing from the first in their by i t would result in markedly poorer paint properties. This, pigment-vehicle ratios are determined in the same manner. however, is not the case, for the range of pigment concenZeidler and Rosen (11) suggest the use of a mathematical trations giving satisfactory durability is fairly broad. It equation for the calculation of the turboviscosity of a paint cannot be decided a t present whether this is due to the fact from two consistency determinations, The graphical method that the critical point is really not a point but a fairly wide range or whether the existing methods of testing paint dura1 Te Boekhorst’s paper appeared after the work reported i n this paper bility are not sufficiently accurate to permit of an exact dehad been completed.
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Shr tho purpose of deteriniiiing whether the criliea1 point as obtained I,?. vl'olfi's inetkod coincides with t h a t pigirient cone e n t r a t i o n wliich yields optimum (liirability, scve r d sets OS paints wcr(1 investigated. Their crit.ic a l p o i n t s u'cre deteriiiiiied and tile Imt pigirieiit-vehicle ratio f o v watlicr r e s i s t a n c e w a s found exposing paint.; prepared a t several pigment,-fixed binder ratios to outdoor aiid accelerated rreatheriiig (6). I'AiNT
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The paint,? m r c prepared b y g r i n d i n g stiff 1 on a blirec-roll i n i l l a n d t h i n n i n g m a l l portims of the pastes with additiiinal vehicle t u the desired pigment-vehicle ratio8 . Tho paints aere aged in the can for seve r a l meks to eliiriinate such iiifliiences a s p r o gressive wetting and flocculation and t o bring t.1iem t o a inure c < i i n parable basis, Leiore making critical point determiriatioils and preparing imnels for exposure. PANFAI'im,airmioh-s. In preparing the panels for exposure, three coats of jiaiiit were applied by brushing. Since the paints varied apjireciably in pigment concentration and consequently in consistency, special care had to be taken to apply coats OS equal thickness. Tho idea behind the tests did not perinit tliiniriirg either with vehicle or volatile thinner. A Sew check nieasurcnicnts with paints representing the cxtreme.- i n pigment-binder ratios indicated comparatively
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little difference in tile tliickiiess of the filins applied. Two sets {ifpanels, one Sor outdoor and one Sor accelerated exposure, were prepared in this manner. The accelerated exposures iicre carried to ultimate failure or to a point where distinctive differences between the various inembers of a set were apparent. Most of the outdoor exposures have not yet failed conipletely, the oldest set having been under exposure for 21 months. However, they are reported wherever failure 115s readied such a stage as to permit of drawing conclusions. ExBm-rLmmwr T m s . Rased on the assumption that paint films lose their distensibility much more rapidly than is indicated by the develop~nentof visible checks or orachs, W. W. Kittelherger of this laboratory has devised an ernbrittlement test which Iias been helpful in the early diagnosis of paint failure:
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immlier of tests is made. To illitstrate the type of e x p o s n r e results obtained, a few characteristic photomicrographs (XJO) arc rcprodueed in I4gnres 4 to 7 . Altliough, i t i niakiiig i~ pliotographic riwird oi' a panel, an attcinpt is made to select an area. representativc of tlje w h u l e p a n e l . i t is difficult t o c o n v e y exactly the same irnpression RS monld Be received
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consistency seems j n s t i ficd. The nature of the ACCELX:II.kTEU \~lirhTIiEK (REGULARCYCLE) relationship between the (Figures indicate peri.eiitage pigment by volume.) colloidal properties OS the liauid mint and itu film A film of paint, is ilpplied t o a zinc pmd by nleans of a spreader. ti86 l ~ c e rdiacnssed i at the beginning
