Document 5bjXNXx7XbDaBEZ4yRmVqxjve

PAINT RESEARCH BULLETIN RC-84 SUBJECT SOME ASPECTS OF. CRITICAL PIGMENT VOLUME RELATIONSHIPS by:'Wi K. Asbeck April 6, 1948 PAINT RESEARCH DEPARTMENT CHICAGO, ILLINOIS 0007-SWP-044722 N22984 Some Aspects of Critical Pigment Volume Relationships Paint Research Department W. X. Asbeck It is only in the last score of years that the significance of "ent/binder relationship has found widespread recognition in the paint itry.^ More and more the forraulator is making use of the concepts pigment volume rather than relying on the older methods of formulation ty weight relationships. It has been determined that substitutions paint formulation purely on the weight basis do not fill the expected uirements and it is only within very recent times that the recognition the value of a more accurate knowledge of pigment/binder relationships V; ^thrown considerable light on the mechanics of paint system substitutions. PIGMENT-VOLUME-CONCENTRATION P.V.C. = (VOLUME OF OPAQUE PIGMENTS + EXTENDERS) x 100 (VOLUME OF OPAQUE PIGMENTS + EXTENDERS) + (VOLUME OF NON-VOLATILE VEHICLE SOLIDS) EXAMPLE: P,V,C. = ... 0.9 + 2,5)------x 100 . (10+25) + (65) = _ X 100 100 = $5% r | VOLATILES I ~i | I RESINS AND OILS 35 EXTENDERS 10 0 OPAQUE PIGMENTS Figure 1 0007-SWP-044723 0007-SWP-000114093 The concept of "pigment volume concentration" has found almost versal acceptance in the paint industry. As illustrated in Figure 1, the. pigment volume concentration, or in short PVG of a paint is the volutric percentage of pigment present in the total solids of a paint system, itod excludes all volatiles from the calculation.; The PVC then, represents the volumetric percentage of pigmen.ts present in the dry paint film after thinner has volatilized. The amount of thinner present is immaterial from the standpoint of PVC and is mainly present to obtain the proper ainting consistency for the paint. If a series of paints containing the same constituents are made .with increasing values of PVC, ground to the same dispersion, and such factors as moisture permeability, rusting, blistering and gloss determined on: the dried samples, the typical results obtained .us a be represented by % the curves of Figure 2; Figure 2 CPVC Bad -h<(pQB Considerable Slight r. -p fa Absent 0007-SWP-044724 -3- As the figure illustrates, a more or less sharp break occurs in of the curves at a PVC of about 45$ for the specific combination of * ^ments and binders shown. The permeability of the samples of PVC above Arises very sharply, indicating a porous film/ The rusting character 's rise commensufately> the region of PVC's below 43$ showing good at preventive qualities while above this pointy the panels failed very pidly, as might be expected from the permeability data/ The blistering, = the other hand, is very bad at low PVC, improving with higher concen- tiorls of pigment/ The gloss curve too, runs somewhat parallel to the istering curve, showing high gloss characteristics for paint films of ^VC and a decrease of gloss with increasing pigment volume concen- '.m ration. It is obvious from Figure 2 that the breaks in the permeability'/ Sting, blistering and gloss curves all occur at a more or less definite at, about 43$ PVC for the pigment/binder combination shown. This point be designated as the critical pigment volume concentration, or in short, 3, for the pigment/binder'system involved. ^ gpvc is the trans itional point above or below which substantial differences in the appearance behavior of a paint film will be encountered/ From Figure 2 it is Cbvi- bs that different characteristics can be imparted to a paint containing the ?*je constituents by formulating above or below the CPVC. Factors Influencing CPVC If the CPVC of several series of samples is determined using the pigment but Various binders, it would appear that identical values of should result* This is not the case! As Figure 3 shows, the perme ability curve -- which is the more direct means of mensuring the pigment 0007-SWP-044725 0007-SWP-000114095 i'ume concentration above which a dried paint film becomes porous - breaks 'f. y sharply at a PVC of about 26% in the case of the formulation involving "base titanium dioxide and raw linseed oil, while the use of the same -ient and bodied linseed oil causes the break in the curve to advance :to cut 50% PVC. A change in the critical pigment volume concentration of ^r 10% has taken place by replacing raw linseed oil by bodied linseed oil. *now, a mixture of equal parts of the same raw and bodied linseed oils Is S' Sd in the formulation involving the same pigment, a value of CPVC practi- lally identical with that of the bodied linseed oil alone results instead .-Jr- .. value lying somewhere between the two as might have been expected', '3 is brought out graphically in curve "C,r of Figure 3. 40 35 30 4 < 25 -p>r->f 20 H abQaoD 15 10 P4 Moisture Permeability- Titanium Dioxide in A = Raw Linseed Oil B = Bodied Linseed Oil C = Raw + Bodied Linseed Oil 1:1 20 25 30 35 40 45 50 55 60 65 Pigment Volume Concentration Figure 3' 0007-SWP-044726 0007-SWP-000114096 -5That' this change in CPVC through the use of differing binders is .not limited to the use of TiOz pigment alone is shown in Figure 4v Here a `ibrous magnesium silicate pigment has been used with the same raw and bodied linseed oils and a combination of equal parts of the two as above a & very similar pattern results. The general values of CPVG have, however*, advanced to a higher region, namely 41% and 46$, and the differences between the two types of oil have been reduced to about 5%. .tat here again) the raw hseed oil gives the lower value of CPVG, while the esdied linseed oil and formulation using the combination of the two lies at substantially the value as that for the bodied linseed oil alone - Moisture Permeability Pigment Volume Concentration Figure 4 0007-SWP-044727 0007-SWP-000114097 -6- Froro the above it is apparent that not only the type and general pocking characteristics of the pigment, but their physical relationships to various vehicles play a major part in the determination of the critical pigment volume concentration of a pigment/binder system. That other factors than the above have an influence on CPVC is illustrated in Figure 5, Here, the same anatase Ti02 pigment as used above is dispersed in a serieB of binders consisting of increasing amounts of linseed fatty acids in alkali refined linseed oil. Curve "A" represents the moisture permeability of the Ti02 pigment in alkali refined linseed oil alone, rtB" the same with 1% of the fatty acid, "Cu 2%, and "D" of the fatty acid present in the dispersion on the basis of pigment volume. 20 25 30 35 40 45 50 55 60 65 Pigment Volume' Concentration Figure 5 0007-SWP-044728 0007-SWP-000114098 tHff 'eii >7- It is obvious that the CPVC has shifted a total of 7% from 6% for the combination involving anatase Ii03 and alkali refined linseed ill alone, to 45% for the samples to which 2% and 5% of the linseed fatty have been added. The l Gdded linseed fatty acid curve breaks at an mediate point, namely 39%. Changing the surface characteristics has ed a change in the packing of the pigment particles. It is particuitrly interesting, to note that the breaks in the 2% and 5;t curves are {practically coincidental, indicating that no additional advantage has been jfe achieved by the addition of more than 2% linseed fatty acid for the pigment/ [binder system involved, The explanation for this can probably be most satisfactorily found the fact that a selective adsorption from solution determination('5') rejg*. that the titanium dioxide involved in the formulation adsorbed 2.1% the linseed fatty acid upon it3 surface. Below this quantity, all the ce requirements of the pigment with regard to the acid are not filled, iving intermediate values of CPVC, while above this, the material is iyjierely in excess; 2.1% being sufficient to completely cover the surface of pigment. ' Another factor found to influence the critical pigment volume concentration was the degree to which the pigment in the ppint was dispersed, poorly ground paint results in a lower CPVC than a well ground one. Figfe 6 shows the results obtained by grinding an anatase Ti02 in bodied lin- IBeed oil to various Hegman Grind Gage values. It can be seen that although |^he differences in CPVC with various grinds are relatively small, a definite progression occurs with increased fineness of grind. 0007-SWP-044729 0007-SWP-000114099 Moisture Permeability Figure 6 The factors then, influencing the CPVC of a paint system are, ng others: 1. The fundamental packing characteristics of the pigment. 2. The type of binder employed. S. The types and amounts of agents present. 4, The fineness of grind of the system. 0007-SWP-044730 0007-SWP-000114100 -9- $-' CPVC p.nd Dispersion A survey of the above factors seems to yield only one common basis jjpon which these changes in CPVC can be placed; the degree of dispersion or gglomeration. Another factor supporting this is that an examination of the paints resulting from each of the above combinations of pigments and binders ' >. - i- eveals in the case of similar systems, that that system shoving the lower 'CPVC also shows more thixotropic effect while conversely that system with _'gher CPVC is decidedly "smoother" in its texture.' This undoubtedly results from differing degrees of agglomeration or dispersion of the pigments in the rious binders involved. * To ascertain the validity of this conclusion, a number of "agglom- 'ferate size" determinations were made by one of the well known settling methods.('?) A number of preliminary runs clearly indicated that the pigment articles participating in an individual agglomerate show a decided rigidity |nd resistance to being dispersed into the liquid medium used as the dilution uid, particularly if non-polar solvents are employed for this purpose. In..the course of the sedimentation investigations, no instance was noted .ere agglomerated systems showed a tendency to disperse in the solvent iquid, and conversely no dispersed system showed the tendency to ngglomern|e. It is of course necessary to use solvents compatible with the vehicle iployed in the paint. p- The sedimentation determination clearly indicated the CPVC of a given system to be a function of the degree of agglomeration or dispersion A which the system exists, A highly disperse system shows high CPVC, a Poorly disperse system shows low CPVC in relation to the fundamental packing .Pharacteriatips of the pigment. ' 0007-SWP-044731 0007-SWP-0001 -10- sS'-- ' Figure 7 t Dispersion 4 . .< L. . C V. >" jf ft 'J ' fc... ,v r. B = Raw G = Ks t / C Raw + Eodied 1:1 H = Raw + Eodied 1;1 D = Alkali Refined = Alkali Refined + 2$ Linseed Acids Figure 7 shows the values obtained for the sedimentation curves of the same anatase TiQ2 in raw linseed oil, bodied linseed oil, alkali refined inseed oil and alkali refined linseed oil with 2$ by volume of pigment linseed fatty acids added as well ns magnesium silicate in raw and bodied linSeed oils. The curves for both of the above pigments dispersed in a mixture Sf equal parts of raw and bodied linseed oil is also included. The degree P'ff dispersion in both cases, as determined by the sedimentation curves, is i|lost identical with that of the bodied linseed oils alone. This clearly explains the reason for finding the same values of CPVC for the pigments i dispersed in bodied linseed oil alone and in the 1:1 combination: both Systems show the same degree of dispersion of the pigment incorporated in ^hem. 0007-SWP-044732 0007-SWP-000114102 -11- K; The ordinate of Figure 7 represents the weight per cent of pig- pent sedimented based on a probability scale,'' 1 while the abscissa carries pTlogarithmic scale of a function <?f the reciprocal square root of the Sedimentation time.' This is proportional to the diameter of the particles.* It was found preferable to use this function rather than the calculated diameter of the agglomerates because of the difficulty of determining the ^absolute specific gravity of the agglomerates which is included in the gfactor (d^-dg) of Stokes'Law.' Because of the oil incoiporated in the pores gof the agglomerates, the specific gravity will be less than that of the Sk ** polid pigment particles, so that the factor (dx-d2) becomes too large where pi: ^ Agglomerated particles are concerned if the density of the solid pigment particles is taken for dx. A comparison of `Figure 7 with the Figures 3, 1 and 5 clearly ows the direct relationship between increased agglomerate 3ize and creased CPVC. ootnote: The basis for this calculation is Stokes'Law, which states:- : - D = Y-i 9 h 2t K x 1i/yr : D = diameter of particles; h = distance of fall; t - time of fall; - viscosity of liquid; dx and d2 = specific gravity of solid and liquid; g = gravity constant. 0007-SWP-044733 0007-SWP-000114103 -12- The Critical Pigment Volume Concentration Cell i' A clear understanding of the intrinsic dependency of CPVC upon relative degree of dispersion or agglomeration leads to the conclusion t the critical pigment volume concentration is determined by the packing racteristics of the pigment particles in relation to the binder employed the system,. The CPVC is that point in a pigment/vehicle system at which t sufficient hinder is present to completely fill the voids left between pigment particles incorporated in the film after volatilisation of all tier. It represents the densest packing of the pigment particles cornurate with the degree of dispersion of the system, , If the above conclusions are correct, and the agglomerates ess the rigidity indicated in the sedimentation determinations, a packsystem of pigments commensurate with that obtained in a dried paint film Id be produceable by other methods. One of the most obvious and simple would be to measure the volume of a pigment paste cake after all excess Ld has been removed by filtration. By determining the ratio of the volof thi3 cake to the true volume of the pigment present as determined the formulation of the original dispersion or other appropriate method, 'ue commensurate with the CPVC should result. 0007-SWP-044734 0007-SWP-000114104 The CPVC cell, r e developed in this laboratory, consists, ns 0 illustrates, of a standard ground glass joint to the top portion of the male section "A" of which.a glass or metal, fritted porous plate "B" has been firmly attached. The female section is constricted shortly above the ground glass portion to a narrow opening around which a calibration mark "D" .has been placed. The method of making a determina+ tion consists of diluting the paint to be tested in equal part with a high boiling naphtha or other suitable sol vent and transferring a measured volume of this into the top of the CPVC cell by means of a calibrated syringe. A vacuum 13 applied to the bottom of the CPVC - Cell cell and all excess liquid filtered out. Figure 8 The filtration can easily be observed through the glass walls of the cell. After all excess liquid has been re- ' ffloved, the vacuum is broken and the cell filled to the calibration mark *tth a measured volume of water introduced from a burette or measuring ^Pipette. Knowing the total volume of the cell from a previous calibration, the difference between the total volume and the measured volume represents the volume of the filter cake, including all voids. 0007-SWP-044735 0007-SWP-000114105 -14- ;i ; The results obtained by:this method check excellently with the .ues obtained by the permeability-draw down method, indicating that the inclusions concerning the packing characteristics nt the GPVC are correct. Ibe mean difference between the two methods runs about + 2$, well within l'e limits of experimental error. Some of the results cs applicable to this discussion are shown below in Table I. Table I Titanium Dioxide in: Raw Linseed Oil Bodied Linseed Oil Raw + Bodied Linseed Oil Alkali Refined Linseed Oil Alkali Refined Linseed Oil + 2% Linseed Fatty Acids CPVC Draw-down Cell 26% 27% 38$ 38$ 56% 38$ 56% 35$ 45% 43$ Magnesium Silicate in: Raw Linseed Oil 4156 Bodied Linseed Oil 46$ ' Raw + Bodied Linseed Oil 46$ Pigment/Binder Relationships % 39$ 47$ 46$ The recognition of the physical meaning of CPVC cs being that point at which just sufficient binder is present to completely fill the voids left between the particles of a pigment/binder system, leads to a ^necessary change in the concepts of pigment/binder relationships. Until j;the present the binder in a paint system has been classified into two .general types, namely the "bound" oil and "free" oil. The bound oil has r generally been considered to consist of an envelope of molecules firmly ,,adsorbed to the surface of the pigment particles from the vehicle and 0007-SWP-044736 0007-SWP-000114106 -15- jy, physically, be identified with the pigment. These molecules of binder Kre removed from the surface of the pigment only through considerable effort ^nd can- consequently be regarded as being truly "bound" to the pigment by "*j&v * Song physico-chemical forces. The quantity of. such binder firmly adsorbed )the solid particles can vary between low values of less than 1% to well >ve 10JS of pigment volume, depending upon the type of vehicle employed in tee system and the surface characteristics and area of the pigments. Some typical values are: anatase Ti02 in raw linseed oil - 2.5$; in bodied linseed WJL - 4,8%} in alkyd varnish - 10,355; magnesium silicate in raw linseed oil- g?0$; in bodied linseed oil - 3.1*; in alkyd varnish - 5.0$ of oil, by dgment volume adsorbed. The free binder has been considered to be all the re3t of the oil jgesent in the paint system at the formulated PVC. It is that oil which is aily removable from the system in the wet state by such simple methods as titration and can consequently be regarded as being truly "free." With the concept of CPVC this "free" portion of the binder must be ibdivided into two major parts, namely I, that portion which is required to 111 the interstices between the pigment particles when these are at their eldest degree of packing commensurate with their dispersion and II, that in ; - . . i.ftion of the binder which is in excess of this amount to the formula e'll ? ijj-i PVC; The first of this oil can be regarded as being "interstitial" " Mi- jglnder, and the second as "excess" binder. . P. These relationships can probably be most adequately represented /' figure 9. 0007-SWP-044737 0007-SWP-000114107 -16- Pigment/Binder Relationships o> I? e O> o coPrl &o(0 s & -PVC -Excess Oil <S-- Interstitial Oil 'Bound ^ /U6 Oil P* .Total Free Binder sfc' V 0$ Pigment 100$ Binder Volumetric $ Pigment -*f-Volumetric $ Binder 100^ Pigment G% Binder Figure 9 f**V . y , If a permeability vs. PVC curve is determined for a given pigment/ binder system, -the results can be represented by a typical graph as in figure 9 and as previously described in Figures 3, 4 and 5. The sharp break the permeability represents the CPVC, or that ratio of pigment to total solids where just sufficient binder is present to gi,ve a continuous solid/ liquid phase after all thinner has volatilized. The PVC of the paint is .determined by the formulation, while the bound oil can be evaluated by an ^adsorption determination. Mathematically, the system can be defined by the simple relation: Total Volume = PVC + Excess Oil + Interstitial- . Oil + Eound Oil apd: Excess Oil + Interstitial oil = Total Free Binder 0007-SWP-044738 0007-SWP-000114108 -17- Where agglomerated systems are encountered as for instance is pronounced case for Ti02 in raw linseed oil, it is necessary to sub divide the interstitial binder into two further groups, namely: , intra-agglomerate oil, and 2. extra-agglomerate oil, The first repre sents that binder which is present inside the pores of the individual ag glomerates, while the second is that which fills the spaces between them, or: Interstitial Oil = Intra-Agglomerate Oil + Extra-Agglomerate Oil. An evaluation of these two terms can be made with the assumption (hat the particles participating in an agglomerate are at their densest degree of packing, Consequently, in an isolated agglomerate the same Sir.'. pigment/vehicle ratio would exist as is obtained by a CPVC determination Ion a mono-disperse system of the same pigment. The arithmetical difference jp, gbetween the CBVC of the mono-disperse system and that of the agglomerated gsystem then represents the extra-agglomerate oil, and the difference between his and the interstitial oil is the intra-agglomerate oil. These rela tions are represented in graphical form in Figure 10, where the remainder [of the values conform to those of Figure 9, It . From Figure 10, it can easily be perceived that the extra-agglom- jpirate oil represents the difference in efficiency of packing between on Agglomerated and a completely dispersed pigment 3ystejn. For the latter, gfc* ||he intra-agglomerate oil simultaneously represents the interstitial pa; Joinder, Jhr. & A consideration of these factors immediately raises the question, L*Which portion of the vehicle is responsible for the wide divergence of IjFlgment packing behavior?" By the use of various vehicles and agents the &Same pigment system cen be induced to give high or low values of CPVC, 0007-SWP-044739 0007-SWP-000114109 10# Pigment Q% Border The answer can readily be found through the use of the CPVC cell, t, is possible to c ompletely substitute the total free binder of a paint system by a solvent and still retain the same packing characteristics as aracterize the original paint. The interstitial . binder and excess jinder consequently must be considered to play only a minor role in the establishment of the CPVC> while the bound oil is the main factor in etennining this value. The vehicle substitutions were carried out by diluting the origi5^ paint in equal part by volume with naphtha or other suitable solvent ?bd centrifuging out the pigment. The supernatant liquid is decanted and Replaced with the same volume of solvent. The pigment is then redispersed 0007-SWP-044740 0007-SWP-000114110 the solvent by rigorous shaking, r.nd repetitions of this -whole proce dure made until only traces of binder can be detected in the solutions. CPVC determination, by means of the CPVC cell, is then carried out on Jthis dispersion, while another is made on the original paint. The results M' '.obtained by these two methods check well within experimental error for ^systems exhibiting both highly agglomerated as well as highly dispersed Estates. That the adsorbed or bound oil is not removed from the surface pigment particles by simple dilution of this type has been pointed out previous authors,('5)'and has been further corroborated in the course of Jthis investigation. Conclusions The CPVC (Critical Pigment Volume Concentration) of a paint system is the transitional point above or below which substantial differences in the appearance and behavior of a paint, film will be encountered. It is that point in a pigment/vehicle system at which just sufficient binder 13 present to completely fill the voids left between the pigment particles incorporated in the film after volatilization of thinner. It represents the densest degree of packing of the pigr- ment particles commensurate with the degree of the dispersion of the system. The CPVC is influenced, among other factors, by: a. The fundamental packing characteristics of the pigment or combination of pigments involved, b. . The type of binder employed. c* The types and amounts of special agents present. d. The fineness of grind of the system. 0007-SWP-044741 0007-SWP-000114111 -20- Tbe CPVC is determined by the degree of dispersion of the pigment in the binder in combination with the fundamental packing characteristics of the pigment particles. An agglomerated system of pigments shows low CPVC while a highly dispersed system of the same pigments shows high CPVC. . In the concepts of pigment/binder relationships a number of new terms have been introduced to characterize the physical relations between pigments and vehicles. The Bound Oil is that portion of the vehicle firmly adsorbed to the surface of the pigments. .The Interstitial Oil is that portion of the binder required to fill the interstices between the pigment particles when these are at their densest degree of packing commensurate with their dispersion. The Excess Oil is that portion of the binder in excess of the Interstitial Oil. plus the Bound Oil.' In agglomerated systems, the Interstitial Oil is subdivided into two further parts. The Intra-Agglomerate Oil is that portion of the binder present inside the individual agglomerates* The Extra-Agglomerate Oil is that portion of the Binder filling the spaces between the agglomerates when these are close packed. The Bound Oil of a paint system characterizes the degree of ag glomeration or dispersion, and consequently the CPVC of the system. A CPVC Cell was developed to determine the CPVC of a paint system on a single sample in the wet state. 0007-SWP-044742 0007-SWP-000114112