Critical Pigment Volume Relationships W. K . ASBECK AKD MAURICE VAN LOO The S h e r w i n - T i l l i a m s Company, Chicago, I l l . ?’he critical pigment volume concentration (CPVC) is a fundamental physical transition point in a pigmentbinder system at which the appearance and behavior o f paint films change considerably. CPVC is influenced by such factors as fundamental packing characteristics of the pigments, type of binder employed, types and amounts of special agents present, and fineness of grind of the system. Degree of dispersion or agglomeration determines the CPVC of specific paints. New terms are introduced to characterize the physical relationships betw-een pigments and binders, and a new cell is described in wrhich the CPVC of a paint can be determined on a single sampIe in the wet state.
rust.ing, blistering, and gloss are determined on the dried samples, the results obbained can be represented by the typical curves of Figure 1. A more or less sharp break occurs in all t>hecurves at, a PVC of about 43% for the specific combination of pigments and binders shown. After an initial slight drop, t,he permeability of the samples of PVC above 43% rises sharply, which indkates a porous film. The rusting characteristics rise conimensurately ; the region of PVC’s below 43% show good rust preventive qunlit,ies, whereas above this point the panels fail rapidly, as might, be espect>ed from the permeability data. Blistering, on the ot,her hand, is very bad at low PVC but improves with higher concentrations of pigment. The gloss curve runs somewhat parallel to the blistering curve, showing high gloss characteristics for paint films of low PVC and a decrease of gloss with increasing pigment volume concentration. Figure 1 indicates t.hat the breaks in the permeability, rusting, blistering, and gloss curves occur at a more or less definite point, ahout 43% PVC for the particular pigment-binder combination shown. This point, can be designated the “critical pigment volume concent,ration” (CPVC) for the pigment-binder system involved (2, 6, 7). CPVC is the transition point above or below which substantial differences in the appearance and behavior of a paint film will be encountered. Figure 1 indicates that different characteristics can be imparted to a paint containing the same constituents by formulating above or below the CPT‘C;.
I
II’ CURRENT paint technology there i b a definite trend to approach formulating problems in terms of volume rather than weight relationships. Replacement of components purely on a weight basis often do not achieve the effects intended. Recognition in recent years of the value of more accurate knowledge of pigment-binder relationships has led to a Letter understanding of the mechanics of paint svstem substitutions ( 8 ) . The concept of pigment volumc concentration (PVC) has found almost universal acceptance in the paint industry. The PVC of a paint is the volumetric percentage of pigment present in the total solids of a paint system and excludes all volatiles from the calculation: PVC
=
+
(volume of hiding pigments extendcrs) (volume of hiding pigments extenders) (volume of nonvolatile vehicle solids)
[
+
+
1
X 100
If a series of paints containing the same constituents is prepared with increasing values of PVC and ground to the same degree of dispersion, and such factors as moisture permeability,
mixture of equal parts of the same raw and bodied linseed oils is used in the formulation involving the same pigment, a value of CPVC practically identical with that of the bodied
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PIGMENT VOLUME C 0N C EN TRATION Figure 1.
Effect of CYVC on Paint Characteristics
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July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
a combination of equal parts of the two, a similar pattern results. The general values of CPVC have, however, advanced to a higher region-41 and 46%-and the differences between the two types of oil have been reduced to about 50J0. Here again the raw linseed oil represents the lower value of CPVC, whereas the bodied linseed oil and the formulation using the combination of the two give substantially thesamevalueas that for the bodiedlinseedoilalone. The foregoing shows that, not only the type and
1471
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35 30
>-
7iironiJrn Dioxlae
25
b
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20
A = R o w L m s e e d 011
cg T IIJ 1 5
-
13 = Bodied L m s e e a 011
general packing characteristics of the pigments, but also their physical relationships to various t C = Raw 4 Bodied vehicles play a major part in the determination of the critical pigment volume concentration of a Q pigment-binder system. 5 Figure 4 illustrates the fact that other factors have an influence on CPVC. The same anatase 0 titanium dioxide pigment used earlier is dispersed in a series of binders consisting of increasing amounts 20 25 30 35 40 45 50 55 60 61 of linseed fatty acids in alkali-refined linseed oil. PIGMENT VOLUME CONCENTRATION CPVC shifts a total of 7% from 36% CPVC for the Figure 2. Effect of Binder on CPVC of Titanium Dioxide Films combination involving the pigment and alkalirefined linseed oil alone to 43% for the samples to This undoubtedly results from differing degrees of agglomeration which 2 and 5% of the linseed fatty acids have been added. Curve R for the addition of 1% linseed fatty acids breaks a t an interor dispersion of the pigments in the various binders involved. A number of agglomerate-size determinations were made by mediate point, 39%. The change in the surface characteristicshas caused a change in the packing of the pigment particles. The one of the well known settling methods (5) to ascertain the breaks in the 2 and 5% curves practically coincide, which indicates validity of this conclusion. Preliminary runs indicated that the that no further advantage has been achieved by adding more than pigment particles participating in an individual agglomerate show a decided rigidity and resistance to being dispersed into 2% linseed fatty acids for the pigment-binder system involved. The most satisfactory explanation can probably be found in the the liquid medium used as the dilution fluid, particularly if nonfact that a determination of selective adsorption from solution polar solvents are employed. In the course of the sedimentation investigations, no agglomerated system showed a tendency to (3)revealed that the titanium dioxide involved in the formulation adsorbed 2.1% of linseed fatty acids on its surface. Below this disperse in the solvent liquid; conversely, no dispersed system amount all the surface requirements of the pigment with regard showed the tendency to agglomerate. It is, of course, necessary to the acids are not filled, so that intermediate values of CPVC to use solvents compatible with the vehicle employed in the result; above 2.10J0 the material is merely in excess since 2.1% paint. is sufficient to cover the surface of this pigment completely. The sedimentation determinations indicate that the CPVC Another factor is the degree to which the pigment is dispersed of a given system is a function of the degree of agglomeration or in the paint. A poorly ground paint results in a lower CPVC dispersion in which the system exists. A highly dispersed system than a well ground paint. Figure 5 shows the results obtained by shows high CPVC; a poorly dispersed system shows low CPVC grinding anatase titanium dioxide in bodied linseed oil t o various in relation to the fundamental packing characteristics of the pigment, which are determined by such properties as size disHegman grind gage values ( 4 ) . Although the differences in tribution, shape, surface characteristics, etc., of the particles. CPVC with various grinds are relatively small, a definite progression occurs with increased fineness of grind. Figure 6 shows the sedimentation curves of the same anatase The factors which influence the CPVC of a paint system, are, then, among others: (1) fundamental packing characteristics of pigment, (2) type of I 40 binder employed, (3) types and amounts of special agents present, and (4) fineness of grind of the 3 5 -svstem.
5,
-
EFFECT O F DISPERSION
A survey of these factors seems to yield only one common element to which these changes in CPVC can be attributed-the degree of dispersion or agglomeration of the system. (The term “agglomerate” is used in this paper in the sense defined in the National Paint Dictionary: “An agglomerate is a group of two or more individual pigment particles that are held together so firmly by the force of adhesion that they tend to remain as an intact unit.”) Another factor supporting this conclusion is that, for the paints resulting from each of the above combinations of pigments and binders, the system showing the lower CPVC also shows more thixotropic effect. Conversely, the system with higher CPVC is decidedly smoother in texture.
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PIGMENT VOLUME CONCENTRATION Figure 3.
Effect of Binder on CPVC of Magnesium Silicate Films
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 41, No. 7
CRITICAL PIGMENT VOLUME CONCENTRATION CELL
40
An understanding of the intrinsic dependency of CPVC upon the relative degree of dispersion or agglomeration leads to the conclusion that 30 critical pigment volume concentration is determined by the packing characteristics of the pig25 ment particles and by the binder employed in 2k the system. CPVC is that point in a pigment2 20 vehicle system at which just sufficient binder is ID present to fill completely the voids left between 15 the pigment particles incorporated in the film after volatilization of all thinner. It represents the 10 densest packing of the pigment particles commensurate with the degree of dispersion of the system. by Pigment Volume 5 If these conclusions are correct, and the agglomerates possess the rigidity indicated in the sedi0 mentation determinations, a packing systein of 20 25 30 35 40 45 50 55 60 65 pigments analogous to that obtained in a dried PIGMENT VOLUME CONCENTRATION paint film should be producible by other methods. A simple way would be to measure the volume of a Figure 4. Effect of Varying Amounts of Linseed Fatty Acids in pigment paste cake after all excesq liquid had been Binder on CPVC of Titanium Dioxide Films removed by filtration. By determining the ratio of the volume of this cake to the true volume of the pigment present as determined from the formulation of the titanium dioxide in raw linseed oil, bodied linseed oil, alkalioriginal dispersion or ot,her appropriate method, a value equivrefined linseed oil, and alkali-refined linseed oil t o which has been alent to CPVC should result. added 2% of linseed fatty acids on the basis of pigment volume. Figure 7 illustrat,esthe CPVC cell, developed in this laboratory. Magnesium silicate in raw and bodied linseed oils is also included It consists of a standard ground-glass joint, t.o the top portion of as well as curves for both pigments dispersed in a mixture of equal parts of raw and bodied linseed oil. The degree of disthe male section, A , of which a glass or metal fritted porous plate, B , has been firmly attached. The female section is conpersion of both pigments in the blends of the oils, as determined stricted shortly above the ground-glass portion t o a narrow openby the sedimentation curves, is almost identical with that of the ing around which a calibration mark D has been placed. bodied linseed oils alone. This explains why the same values of The method of making a determination consists of diluting the CPVC are found for the pigments dispersed in bodied linseed oil paint with an equal volume of a high-boiling naphtha or other alone and in the 1: 1 combination: Both systems show the same suitable solvent and transferring a measured volume of the redegree of dispersion of the pigment incorporated in them. dumd paint into the top of the CPVC cell by a calibrated syringe. The ordinate of Figure G represents the weight per cent of pigA vacuum is applied to the bottom of the cell and all excess liquid ment sedimented, based on a probability scale ( 1 ) ; the abscissa filtered out. Filtration can be observed through the glass walls of carries a logarithmic scale of a function of the reciprocal square the cell. The solvent added to expedite filtrat'ion does not affect root of the sedimentation time. This is proportional to the dithe dispersion (CPVC of titanium dioxide in raw linseed oil, origiameter of the particles. The basis for this calculation is the nal paint 27%, diluted paint 27%). After all excess liquid has Stokes law, which states: been removed, the vacuum is broken and t,he cell filled to the calibration mark wit'h a measured volume of water introduced from a buret or measuring pipet. With the total volume of the cell 35
??
2
where D = diameter of particles h = distance of fall 1 = time of fall q = viscosity of liquid d l , d2 = specific gravity of solid and liquid, respectively g = gravity constant
It was found preferable to use the reciprocal square root of sedimentation time 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 thc factor ( d l - &) of the Stokes lan.. Because of the oil incorporated in the pores of the agglomerates, the specific gravity will be lesq than that of a solid pigment particle of the same size. The factor ( d l dz) consequently becomes too large where agglomerated particles are concerned if the density of the solid pigment particles is taken for d,. Comparison of Figure 6 with Figures 2, 3, and 4 shows the direct relationship between increased a g g l o m e r a t e size and d e c r e a s e d
-
CPVC.
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PIGMENT VOLUME CONCENTRATION Figure 5.
Effect of Fineness of Grind on CPVC of Titanium Dioxide Films
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1949
PIGM ENT-BINDER RE LATION SHIPS
99
a
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k! 2 cu
2 50 a
30 10
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A
.6 .8 I
2
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kx = f(D) Figure 6. Sedimentation Curves of Titanium Dioxide and Magnesium Silicate Films with Various Binders Titanium Dioxide in Linseed Oils A = bodied B = raw C = raw bodied, Ir 1 D E alkali-refined E alkali-refined 4- 2% linseed aoide
+-
-
Magnesium Silicate i n Linseed Oils F = bodied G = raw € =I raw f bodied, 1: 1
known from a previous calibration, the difference between the total volume and the measured volume represents the volume of the filter cake, including all voids. I n selecting the measuring liquid, the precaution must be observed to employ a poorly wetting fluid in relation to the filter cake. A poor wetter will not penetrate the pores of the filter cake and, consequently, will measure the total volume of the cake, including the voids left between the solid particles through the removal of the vehicle by the filtration. The results obtained by this method check well with the values obtained by the permeability-drawdown method; this indicates that the conclusions concerning the packing characteristics a t the CPVC are correct. The term “permeability-drawdown method” is used to designate moisture permeability determinations carried out on a cup similar to the Gardner-Van Heuckeroth type. The paint film is laid down uniformly on a permeable paper substrate by a drawdown gage. This contrasts with an electrical resistance-drawdown method developed in this laboratory. I n the latter the resistance across a similarly prepared paint film is measured by an alternating current resistance bridge after contacts are established on both sides and through the film by a weak electrolyte. The measurement is made in a glass cell similar to the permeability type but open at the point opposite from the end to which the film is clamped. The latter method is much faster than the former and is sufficiently accurate to detect with ease the break in the porosity of the increasing PVC paint ladder at the CPVC. The mean difference between the CPVC-cell and the permeability-drawdown methods runs about *2%, well within the limits of experimental error. Some of the results applicable to this discussion follow : Method Titanium dioxide in Raw linseed oil Bodied linseed oil Raw bodied linseed oil Alkali-refined linaeed oil Alkali-refined linseed oil
+
Drawdown
g% + 2% linseed fatty
CPVC Cell
The recognition of the physical meaning of CPVC as that point at which just sufficient binder is present to fill completely the voids left between the particles of a pigment-binder system leads to a necessary change in the concepts of pigment-binder relationships. Up to the present, the binder in a paint system has been classified in two general types-namely, “bound” and “free.” The bound portion of the vehicle has generally been considered to consist of a n envelope of molecules firmly adsorbed to the surface of the pigment particles and may, therefore, be identified physically with the pigment. These molecules of binder are removed from the surface of the pigment only through considerable effort and, consequently, can be regarded as being truly bound to the pigment by strong physicochemical forces. The quantity of such binder firmly adsorbed to the solid particles may vary between low values of less than 1% to well above 10% of pigment volume, depending upon th type of vehicle employed in the system and the surface characteristics and area of the pigments. Some typical values of vehicle components adsorbed, expressed in percentages on the basis of pigment volume, follow: 3
Titanium Dioxide 2.5 4.8 10.3
Raw linseed oil Bodied linseed oil Alkyd varnish
Magnesium Silicate 2.0 3.1 5.0
The “free” binder has been considered to be the remaining portion of the binder present in the paint system at the formulated PVC. It is that part of the binder which is easily removable from the system in the wet state by such simple methods as filtration and can, consequently, be regarded as being truly free. With the concept of CPVC this free portion of the binder must be subdivided into two major parts: Part I is that required to fill the interstices between the pigment particles when these are a t their densest degree of packing commensurate with their dispersion; part I1 is that in excess of this amount up to the formulated PVC. The first can be regarded as being interstitial binder, and the second as excess binder. These relationships are represented in Figure 8. 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 8 and as specifically described in F‘g ‘ 1 ures 2, 3, and 4. The sharp break in permeability represents CPVC, or that ratio of pigment to total solids at which just sufficient binder is present to give a continuous solid-liquid phase after all thinner has volatilized. The PVC of the paint is determined by the formulation; the bound vehicle can be evaluated by an adsorption determination. Mathematically the system can be defined by the simple relation:
27% 38
++
39 36 43
38 35
Total volume = PVC excess binder interstitial binder adsorbed binder
41 46 48
39 47
Excess binder interstitial binder = total free binder
+
43
+
46
A method of CPVC determination similar to that described but using a cell closed at the bottom and depending on centrifugal force to separate the pigments from the vehicle was also investigated but found to give results which did not check consistently with the values obtained by the permeability-drawdown and filter methods.
Figure 7. Critical Pigment Volume Concentration Cell
When the PVC is formulated above the CPVC, a binder deficiency will result. When agglomerated systems are encountered-for instance, titanium dioxide in raw linseed oil-it is necessary to subdivide the interstitial binder into
INDUSTRIAL AND ENGINEERING CHEMISTRY
1474
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0 X Pigment 100 % Binder
VOLUMETRIC % PIGMENT-
1 0 0 % Pigment
VOLUMETRIC % BINDER
0 % Binder
Figure 8. Representation of Interstitial a n d Excess Binder Locations for a Given Pigment-Binder S y s t e m two further groups-namely ,intra-agglomerate and extra-agglomerat,e. The first represents that binder which is present inside the pores of the individual agglomerates; the second is that which fills the spaces between them, or
Interstitial binder
=
+
intra-agglomerak binder extra-agglomerate binder
An evaluation of these two terms can be made with the assumption that the particles participating in an agglomerate approach their densest degree of packing. Consequently, in an isolated agglomerate the same pigment-vehicle ratio would exist as is obt,ained by a CPVC determination on a monodispersed syst,em of the same pigment. The arithmetical difference between the CPVC of the monodispersed system and that of teheagglomerated system then represents the extra-agglomerate binder, and the difference between this and the interstitial binder is the intra-agglomerate binder. These relations are represented in graphical form in Figure 9, where the remainder of the values conform t,o those of Figure 8. Figure 9 shows that the extra-agglomerate binder represents the difference in efficiency of packing between an agglomerated and a completely dispersed pigment system. For the latter, the intra-agglomerate binder simultaneously represents the interstitial binder. A consideration of these factors raises the question: Which portion of the vehicle is responsible for the wide divergence of pigment packing behavior? By the use of various vehicles and special agents the same pigment system can be induced to give high or low values of CPVC. The answer can be found through the use of the CPVC cell. It is possible to substitut’ecompletely the total free binder of a paint system by a solvent, and still retain the same packing relationships which characterize the original paint. The interstitial binder and excess binder, consequently, must be considered to play only a minor role in the establishment of the CPVC, while the bound vehicle is t,he main factor in determining this value. Vehicle substitutions are carried out by diluting the original paint with a n equal volume of naphtha or other suitable solvent and centrifuging out the pigment. The supernatant liquid is decanted and replaced with the same volume of solvent. The pigment is then redispersed into the solvent, by rigorous shaking. The procedure is repeated until only traces of binder can be detected in the solutions. A CPVC determination in the CPVC cell is then carried out on this dispersion, and another is made on the original paint. The results obtained by these two methods check well within experimental error for systems exhibiting both highly agglomerated and highly dispersed states. Some of the typical values obtained by the method of direct filtration of the paint in
0% Pigment 100 S. Binder
VOLUMETRIC % PfGMENT
Vol. 41, No. 7
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VOLUMETRIC % BINDER
1 0 0 %Pigmeni
0 % Binder
Figure 9. Representation of Intra-agglomerate and Extra-agglomerate Binder Locations for a Given P i g m e n t Binder S y s t e m contrast to the vehicle substitution met,hod are, respectively: titanium dioxide in raw linseed oil, 27 and 27%; titanium dioxide in bodied linseed oil, 37 and 38%; magnesium silicate extender in raw linseed oil, 43 and 42%; magnesium silicate extender in bodied linseed oil, 48 and 48%. It is obvious that, for paint.s which filter slowly, the vehicle substitution method can be employed advantageously to speed the CPVC determination by means of the CPVC cell. Ewing (3) pointed out that t,he adsorbed or bound vehicle is not removed from the surface of pigment particles by simple dilution, and this was corroborated further in the course of this investigation. CONCLUSIONS
1. The critical pigment volume concentration of a pairit. sya-
t.em is the transition point above or below which substant)ial differences in the appearance and b e h a ~ o of r a paint film will be encountered. It is that point in a pigment-vehicle system at which just sufficient binder is present to fill completely the voids left, between the pigment particles incorporated in the film after volatilization of thinner. It represents t8he densest degree of packing of the pigment particles commensurate m4t.h the degree of dispersion of the system. 2. CPVC is influenced, among other factors, by the fundamental packing characteristics of the pigment or combination of pigments involved, the. type of binder employed, the types and amounts of special agents present, and the fineness of grind of the system. 3. CPVC is determined by t,he degree of dispersion of the pigment in the binder and by the fundamental packing characteristics of t,he pigment part’icles. An agglomerated system of pigments shows low CPVC,, whereas a highly dispersed system of the same pigments shows high CPVC. 4. I n t,he concepts of pigment-binder relationships a number of new terms have been introduced to charact,erize the physical relations between pigments and vehicles. The “bound vehicle” solids is t’hat portion of the binder firmly adsorbed to the surface of the pigments. The “interstitial binder” is t,hat portion required to fill the interst,ices between the pigment particles when they are at their densest degree of packing commensurate with their dispersion. The “excess binder” is that portion of the binder in excess of the interstitial binder plus the bound vehicle solids. If the PVC of a paint system lies above the CPVC, a binder deficiency results. In agglomerated systems, the interstitial binder is subdivided into two further parts. The “intra-agglomerate binder” is that portion present inside the individual agglomcrates. The “extra-agglomerate binder” is that portion
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1949
filling the spaces between the agglomerates when these are closely packed. 5. The “adsorbed binder” of a paint system characterizes the degree of agglomeration or dispersion and, consequently, the CPVC of the system. 6. A CPVC cell (U. S. Patent applied for) has been developed to determine the CPVC of a paint system on a single sample in the wet. state. LITERATURE CITED
(1) Austin. J. B.. IND. ENQ.CHEM..ANAL.ED..11. 334 (19391 (2) Elm, A. C., O f i c h l Digest Federation Paint &‘Varn’ish Production Clubs, 267, 197 (1947). (3) Ewing, W.W., in “Symposium on New Methods for Particle Size
1475
Determination in Subsieve Range,” pp. 107-9,Am. SOC.Testing Materials, 1941. (4) Gardner, H. A,, and Sward, G. G., “Physical and Chemical Examination of Paints, Varnishes, Lacquers and Colors,” 10th ed., p. 277. 1946. (5) Martin, S. W., in “Symposium on New Methods for Particle Size Determination in Subsieve Ranee,” -DD._ 66-87. Am. Soc. Testing Materials, 1941. (6) Thynne, i. W. F., Paint Technol., 9,423 (1946). (7) Van Loo, Maurice, War Production Conference, Chicago, March 1945. (8) Wolff, H., Farben-Ztg., 34,2940 (1929). RECEIVED May 24, 1948. Presented a t the joint Technical Conference of the CHEMICAL SOCIETY, and the American AssociChicago Section, AMERICAN ation for the Advancement of Science, Chicago, December 26, 1947.
Polymerization of Monomers in Buna S-3 System J
J D. N. MARQUARDT, R. H. POIRIER’, AND L. B. WAKEFIELD Firestone Tire and Rubber Company, Akron, Ohio
A
butadiene-styrene polymer produced in the Buna S-3 system with materials available in this country was a stiff polymer which could be heat softened to any desired plasticity. The most distinctive differences between the Buna S-3 and GR-S polymerization system were the use of an alkylnaphthalene sulfonate emulsifier, limited modification with diisopropylxanthogen disulfide, and the recovery of the polymer at lower conversion. A tread wear about 50% better than GR-S has been obtained with a polymer prepared in the S-3 system. In view of these results, the Buna S-3 polymerization system was applied to the preparation of polybutadiene, polyisoprene, and copolymers of these monomers with styrene, monochlorostyrene, and dichlorostyrene. The resultant polymers, in most cases, were found to be superior in their cut growth resistance to similar polymers prepared in the GR-S recipe. The monomers maintained the same relative rates of polymerization although polymerization was somewhat slower’in the Buna S-3 than the GR-S recipe.
S
OON after the continental invasion in 1945, a Technical Intelligence Committee inspected synthetic rubber installations operated by the Germans. This mission reported that the German technique involved the production of an extremely stiff polymer which required heat softening before processing, in contrast to the American practice of making a polymer soft enough to be used directly without further plasticization. This practice was favored since it permitted the preparation of a superior polymer which could be softened to any desired plasticity. Specific differences from GR-S included the use of alkylnaphthalene sulfonates as emulsifiers in place of fatty acid soaps, the much more limited use of modifiers, and the isolation of the polymer at a lower conversion. American preparations made in the government program according to the German Buna 5-3process were found to be slightly slower in the heat softening rate. However, the cured stocks compared faxorably with those from the German polymer and were superior to a G R S control. The most conspicuous advantages resulting from the use of the S-3-type system were found to be greater resistance to cut growth and a tread wear approximately 50% better than shown by a GR-S charge. 1
Present address, Battellc Memorial Institute, Columbus, Ohio.
In view of the improvement in properties brought about by the use of the Buna S-3 recipe, the work was extended to polyisoprene and polybutadiene as well as to copolymers of these monomers with styrene, monochlorostyrene, and dichlorostyrene. This paper describes the preparation and evaluation of these polymers. For brevity the physical properties of the polymer or copolymer vulcanizates will be referred to as properties of the polymer or copolymer. EXPERIMENTAL METHODS
All bottle polymerizations were conducted in quart bottles rotated end-over-end in a water bath. The use of self-sealing cap liners made it possible to withdraw latex samples with a hypodermic needle throughout the course of the polymerization POLYMERIZATION FORMULAS Buna 8-3
GR-S ( 1 )
Butadiene Styrene Water Nekal B X
RRC
S O a D flakes Steario a i i d
...
0.4
0.4 o..o9a
... 0.3
...
0.5
45150 50 Conversion, % 60 75 a Diisopropylxanthogen disulfide was added incrementkse at 15, 30, and 45% conversion.
A sample of SA-178 from General Aniline and Film Corporation was substituted for the German Nekal BX which was unavailable in this country. At the desired conversion, the reaction was stopped with 2.0 parts of phenyl-p-naphthylamine. The latex was coagulated with a salt-acid solution (water 100, salt 10, acetic acid 5) and the rubber was dried at 70” C. Heat softening was conducted in a 130’ or 150’ C. forced draft oven for the time necessary to reduce the Williams plasticity value to about 3.5. The raw olymers were characterized by determination of Mooney a n f Williams plasticity values, solubility, molecular weight distribution, and osmotic molecular weight. The polymers were compounded in the following formula, cured a t 280” F and evaluated in standard laboratory tests: Sulfur Easy processing carbon black Stearic acid Bardol Pine tar oil
1.7
45.0
2.5 4.0 2.6
Phcnyl-p-naphthylamine Zinc oxide Santocure Polymer
0.0 2.4 1.2
100.0
The low temperature index values (2) given in the tables represent the temperature a t which the Young’s modulus reachec
