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assistance and criticism. They also wish t o thank the Eastern Experiment Station of t h e U. S. Bureau of Mines for t h e titanium metal and Owens-Corning Fiberglas Corp. for the glass sewing thread. LITERATURE CITED
(1) Am. SOC. Testing Materials, “A.S.T.M. Standards,’’ Part I1 (B 19244T), pp. 804-16
(1949). (2) Evans, U. R., “Metallic Corrosion, Passivity and Protection,” p. 599, London, Longmans, Green and Co., 1948. (3) Fontana, M. G., IND.ENG.CHEM.,40, 99-100A (October 1948). (4) Ibid., 41, 103-4A (February 1949). Figure 6. Corrosion Rates of Anodized and Unanodized Titanium (5) Gee, E. A., J. Trans. Electrochem. SOC.,96, in Mixed Solutions of 20% Hydrochloric Acid and Varied 19-21C (1949). Concentrations of Nitric Acid (6) Gee, E. A., Golden, L. B., and Lusby, W. E., Jr., IND.ENO.,CHEM.,41, 1668-73 (1949). (7) Gee, E. A., Long, J. R., and Waggamak, W. H., Materials & [Ti(SOa)z+z]-2xand [TiOz(S04),]-z”. The superiority of the Methods, 27, 75-8 (1948). anodized over the unanodized samples in sulfuric acid solutions of (8) Hutchinson, G. E., and Permar, P. H., Corrosion, 5 , 10 (1949). moderate strength is due t o the fact t h a t the neutral character of (9) Kroll, W. J., Metallwirtschaft, 18, 77 (1939). these solutions does not afford a n oxide film on the unanodized (10) Moore, R. L., and Anderson, R. C., J . Am. Chem. SOC., 67, samples and yet does not interfere with the protective film on the anodized ones. 167-71 (1945). (11) Pamfilov, A. V., and Khudyakova, T. A., Z h u r . Obshchei Khim., By adding a small percentage of nitric acid or other oxidizing 19, 1443-52 J1949). agent to a neutral or slightly reducing medium, the corrosion re(12) Uhlig, H. H., Corrosion Handbook,” p. 329, New York, John sistance of titanium is vastly increased. Wiley & Sons, 1948. ACKNOWLEDGMENT
The authors wish to express appreciation t o Francis M. Taylor and Hans B. Jonassen of Tulane University for invaluable
RECEIVED July 12, 1950. Based o n a thesis submitted b y E. M. Peres, Jr., t o the Graduate School of Tulane University in partla1 fulfillment of the requirements for the M.S. degree in chemical engineering.
Effect of Colloidal Noncarbon Pigments on Elastomer Properties ERNST SCHMIDT The Firestone Tire & Rubber Co., Akron, Ohio
CB
Commercial noacarbon pigments do not reinforce elastomers as effectively as certain carbon blacks. Most of the noncarbon pigments are of larger particle size than reinforcing carbon blacks. I t appeared, therefore, of interest to obtain information as to whether the inferiority of noncarbon pigments is due primarily to their insufficient degree of subdivision, or to their chemical nature. Pigment materials were obtained in a state of subdivision approaching or surpassing t h a t of reinforcing carbon blacks, by incorporating these materials in the form of colloidal solutions in elastomer latex. Using this technique, marked tensile and modulus reinforcement of GR-S type rubber is obtained with solid substances of widely differing chemical nature, such as stannic oxide, silica, Prussian blue, polystyrene, and casein. The most pronounced effects were obtained with the pigments of smallest particle size. Colloidal stannic oxide produces tensile and modulus reinforcement, which exceeds t h a t obtained with EPC black. The results indicate t h a t small particle size of the pigment is of prime importance in elastomer reinforcement, whereas the chemical nature of the pigment appears to be of secondary importance.
A
NUMBER of pigments other than carbon black are known, which reinforce certain physical properties of rubber t o various degrees but do not, in general, equal or surpass the reinforcing ability of carbon black. Numerous investigations of carbon blacks have revealed t h a t particle size is an important factor in reinforcement, and that the most effective carbon blacks have particle size diameters of not more than 250 t o 350 A, However, most commercial noncarbon pigments are of larger particle size and also differ from carbon blacks with regard t o particle shape and some of them with regard t o readiness to disperse in rubber. The question arises therefore, whether the inferiority of many noncarbon pigments is primarily due to their insufficient degree of subdivision and distribution, or t o their chemical nature. It was hoped t h a t an investigation of the reinforcing properties of a number of solid substances of widely differing chemical nature, in a state of subdivision and distribution approaching or surpassing that of reinforcing carbon blacks, would throw some light on this problem. The desired high degree of pigment dispersion and distribution was attempted by incorporating the pigments in the form of colloidal solution into rubber latices. Stannic oxide, silica, Prussian blue, polystyrene, and casein were used as potential reinforcing materials. GR-S was used in this study, because it shows more strikingly than natural rubber the effect of reinforcing pigments.
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test). The physical properties a t optimum cure, discussed in the following paragraph, were obtained by interpolation for curing times a t which the permanent set had reached a minimum value. PHYSICAL PROPERTIES OF VULCANIZATES
The tensile strength data in Figure 4 show that all pigments that were incorporated in the form of colloidal solutions increase the tensile strength. The highest tensile reinforcement is obtained with colloidal stannic oxide, which exceeds that produced by equal volume loadings of E P C carbon black a t optimum and at all volume Figure 2. Electron 3Iicrograph of loadings up to 20 to 25 volumes per 100 Prussian Blue volumes of polymer. Colloidal silica is about equivalent to E P C black at loadings PREPARATION OF up to 20 volumes, but gives lower tensile strength values COMPOUNDS than EPC a t optimum and higher loadings. However, if based on the cross section a t break, the tensile strength for optimum C o l l o i d a l solutions of s t a n n i c oxide, silica, PrusTABLEI. PROPERTIIX or COLLOID IL PIGMEKT So~c~ro~s sian blue, amConcen- Particlemonium caseinate, tration Size and a polystyrene of Sol, Diameter, Appearance % A. l a t e x , Ti-hich are Stannic oside l-ery Ion. electrolyte con- TTater 5.4 80-130 c h a r a c t e r i z e d iii tent. prepared by pepclear tization of purified Table I and by Sn(OH)4 with a m electron micromonia. concentrated by eva'poration graphs Figures 1 Silica Ludox ( D u P o n t Co.), Slightly 18.6 100-200 to 3, were mixed in low electrolyte conopalestent cent various proporSoluble Prus- Eimer & .%mend ... 10 200-400 tions with a GR-S sian blue T y p e I l a t e x of Polystyrene Latex prepared by eniulMilky 35 150-600 30.7Yc solids consion polymerization following procedure tent. employed in GR-S
Figure 1. Electron Micrograph of Silica Hydrosol
Figure 3. Electron Micrograph of Polystyrene Latex
The mixtures were poured on glass plates and dried a t room temperature. The residues were compounded on the mill according t o the recipe given in Table 11. Control compounds containing varying amounts of EPC black (easy processing carbon black Spheron 9) were prepared from GR-S which was obtained by drying portions of the same latex, as used for the other compounds. All compounds were cured a t 298" F. over a nide range of curing times and tested for the conventional stressstrain properties and permanent set a t break with the Scott tensile machine. Dynamic properties were tested with the Fircstone forced vibrator ( 3 ) and impact resilience by the ball rebound method. GR-S vulcanizates containing various quantities of colloidal silica were also tested for heat build-up with the Firestone flexometer ( 2 ) and for tear resistance (Winkclmann
production Solution of casein i n SHa solution
Casein
X
0
0 I
1) SILICA E.PC. CARBON BLACK PRUSSIAN BLUE POLYSTYRENE CASEIN
I
I
...
10
TABLE11. FORMULA OF GR-S C o ~ ~ o r r s nI,o.~uEI) s WITH DIFFERENT COLLOIDS Parts bv- Weinlit .100 Varying 2.4 1.8 2 5
GR-S type I
Pigment Sulfur Nercaptobensot hiazole Stearic acid Zinc oxide Phenyl-2-naphthylamine
A 0 0
,..
X
100-200 290 200-400 150 -600
0 0
0 I
Figure 5.
1
0.8
PIGMENT: PARTICLE DIAMETER A. COLLOIDAL STANNIC OXIDE 8 0 - 150 SILICA 100-200 PRUSSIAN BLUE POLYSTYRENE CASEIN I
I
200-400 150- 6 0 0 I
IO 20 30 40 VOLUMES PIGMENT PER IOO'VOLUMES GR-S
Tensile Strength Based on Cross Section at Break us. Volume Loading
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A
COLL. STANNIC OXIDE ’ SILICA CARBON BLACK x PRUSSIAN B L U E OPOLYSTYRENE
0
0 E.P.C.
0
COLLOIDAL S I L I C A
0
E.P.C. CARBON BLACK
P
4/
$300 d
I
0
I
10
I
20
I
30
40
VOLUMES PIGMENT PER 100 VOLUMES GR-S I
I
I
IO 20 3c VOLUMES PIGMENT PER 100 VOLUMES OR-!
30Oqo Modulus vs. Volume Loading
Figure 7.
1
Figure 6. Tear Resistance us. Volume Loading
i 0
F
loading is about equal t o that obtained with optimum E P C loading (see Figure 5 ) . Prussian blue ranks next with regard t o its ability t o increase the tensile strength of GR-S. Polystyrene and casein are only about half as effective as EPC. It can be seen from Figure 5 t h a t the volume loading which gives maximum tensile strength varies with the particle size of the pigment used, Colloidal stannic oxide and silica, which are of smaller particle size than E P C black, produce maximum tensile strength at lower volume loadings than EPC, while Prussian blue and polystyrene, which are of larger particle size than EPC, require higher volume loadings for maximum tensile reinforcement. The particle size of casein in the rubber matrix is believed to be considerably larger than in the original aqueous solution, and t o vary with volume loading. This is suspected t o be the cause of the relatively low degree of tensile strength reinforcement obtained with casein and of the large volume loading required for optimum tensile strength. The tear resistance of GR-S vulcanizates increases with increasing loadings of colloidal silica, in a manner and t o a n extent similar to that obtained with E P C black (see Figure 6). The modulus of GR-S vulcanizates, as measured with the Scott tensile machine, is increased by all pigments investigated (see Figure 7). The order of effectiveness of the pigments is somewhat different than with regard t o tensile reinforcement. Colloidal stannic oxide increases the modulus more than equal volumes of E P C black, except at very low volume loadings. Colloidal silica is much less effective than E P C black, Prussian blue, or casein at loadings of less than 15 volumes. At higher silica loadings, however, the modulus increases more rapidly than that of the other pigments shown. The dynamic modulus of GR-S vulcanizates containing relatively small quantities of colloidal silica gr casein is about equal or slightly below that of E P C loaded control compounds, but surpasses them at higher volume loadings, as shown in Figure 8. The order of these pigments with respect to their effect on internal friction is the same as t h a t with respect t o dynamic modulus, and is not affected by a change of the testing temperature. The impact resilience of GR-S vulcanizates containing colloidal stannic oxide or Prussian blue was found to be lower than t h a t of GR-S loaded with equal volumes of E P C black, and t o decrease with increasing loading in a manner similar to t h a t obtained with E P C (see Figure 9). The impact resilience at room temperature of GR-S vulcanizates is not affected by their silica or casein con-
goo0
COLLOIDAL SILICA 0 E.RC.CARBON B L A C K 0 CASEIN
I 0
I
I
I
I
IO 20 30 VOLUMES PIGMENT PER 100 VOLUMES G R - S
Figure 8. Dynamic Modulus at 100’ C. us. Volume Loading
A 3 0
x
40
0
m
0
W
COLLOIDAL STANNIC OXIDE SILICA PRUSSIAN B L U E POLYSTYRENE CASEIN
E
VOLUMES PIGMENT PER 100 VOLUMES GR-S Rebound at 72’ and 212” F. US.
Figure 9.
Volume Loading
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9 Lu 0
0
COLLOIDAL SILI.CA
0
E . P . C . CARBON BLACK
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600
-
3 !4
a
A
COLLOIDAL STANNIC OXIDE
W
0
I
X P R U S S I A N BLUE POLY S T Y R E N E p CASEIN
n
.
SILICA
-0
W
c 25
z z
3 K
I
I
L
10
20
3(
VOLUMES PIGMENT PER 100 VOLUMES OR-S
Figure 10. Running Temperature vs. Volume Loading
IO
/x
bc
0
10 20 30 40 50 VOLUMES PIGMENT PER 100 VOLUMES GR-S
Figure 11. Ultimate Elongation and Permanent Set z's. Volume Loading
tents and even increases with increasing loadings of polystyrene. These high rebound values are not necessarily indicative of low hysteresis, but are believed to be due to appreciable changes in hardness with increasing loading. The fact that the running temperature of the silica-loaded GR-S is higher than that of EPC-loaded control compounds (Figure 10) seems to be in accordance Jyith the view that the high impact resilience imparted by the colloidal silica is not due t o low hysteresis.
DISTRIBUTION OF COLLOIDAL SILICA IY RUBBER MATRIX
The remarkable increase of tensile strength and modulus of GR-S vulcanizates, which is obtained by incorporation of pigments of widely differing chemical nature in the form of colloidal solutions into rubber latex, suggests that these effects are due to the small particle size of the colloidal pigments. However, it had t o be recognized that the state of dispersion of these pigments in the rubber matrix would not necessarily equal that of the pigments in the original aqueous dispersion. I t appears likely that the colloidal pigment particles would undergo agglomeration during the drying process and subsequently some redispersion during milling of the rubber-pigment master batch, depending on the nature of the pigment. The possibility of such
Figure 12. Electron Micrograph of GR-S-Silica Sol Master Batch 23.7 volumes of silica
Unmilled, ashed
The permanent set values for GR-S vulcanizates containing various colloidal pigments, shown in Figure 11, are not directly comparable, because they were obtained with different stresses and extensions. Taking these factors into consideration, it can be seen that the recovery of the stannic oxide, silica, and Prussian blue-loaded vulcanizates is fairly high. Polystyrene and casein produce very high permanent set. These pigments are believed to form irregularly shaped secondary particles during drying of the rubber latex-pigment dispersion mixture and possibly also during milling of the dried residue. This might explain the high permanent set obtained with these pigments, as it is known (6) that vulcanizates loaded with pigments of nonisometric shape have a very high permanent set.
Figure 13. Electron Micrograph of Hevea-Silica Master Batch 23.7 volumes of silica Unmilled, ashed
changes of particle size was therefore investigated on rubbersilica master batches by means of the electron microscope. Mixtures of GR-S or Hevea latex with silica sol, containing 23.7 volumes of silica per 100 volumes of rubber, were diluted with water (100-fold). One droplet was placed on a collodioncovered specimen screen, dried, and ashed a t 600' C. in order to burn off the rubber and the supporting collodion film. Electron
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INDUSTRIAL AND ENGINEERING CHEMISTRY
micrographs of these samples (Figures 12 and 13) show a continuous silica structure, interrupted by circular holes. Determination of the size and size distribution of these holes gave approximate agreement with the histograms of the latices used, indicating t h a t the holes represent replicas of closely packed discrete latex particles of the original particle size. The silica structures in Figures 12 and 13 appear to be composed of small spherical particles, which are of about the same size as the silica particles in the original hydrosol (see Figure 1). (These silica particles cannot be distinguished quite so clearly in Figure 13 as Figure 12, because of the greater thickness of the specimen shown in the former.) It can be seen from Figures 12 and 13 that the primary colloidal silica particles are grouped in secondary structural elements, which correspond in size roughly t o the voids between the latex particles. The size of these secondary aggregates should therefore be expected t o depend on the size of the latex particles and on the volume of the pigment loading. I n the case of the GR-S latex used, which had a particle size of about 800 A., and with very small volume loadings, the secondary aggregates should approach the size of the individual colloidal silica payticles. T h a t the silica particles in the interstices between the latex particles (Figures 12 and 13) are not merely loosely grouped but actually in rigid connection is concluded from the fact that the electron microscopic specimens consist only of silica and are not held in place by a supporting film. Further observations indicating the existence of silica structures are mentioned below.
i
Figure 14. Electron Micrograph of Hevea-Silica Master Batch 23.7 volumes of silica Milled, ashed
c
The effect of milling on the distribution of the silica in the rubber matrix is illustrated in Figures 14 and 15. A sufficient quantity of the Hevea-silica master batch shown in Figure 13, in unmilled condition, was subjected t o milling. One portion of the milled master batch was treated for 1 week with a large excess of boiling toluene until it appeared t o be sufficiently well dispersed. The other portion was ashed a t 600°C. Electron micrographs of the ashed (Figure 14) and particularly of the toluene-dispersed sample (Figure 15) show that the silica structure had been broken up during milling into a great number of spherical silica particles of about the same size as present in the original hydrosol, as well as into clusters resembling the secondary silica structures or fragments thereof. It was also observed that the unmilled H e v e a d i c s master batch, which was very hard and turbid, became transparent and softer during milling. The views have been expressed that the formation of carbon networks (structures) produces anomalously high modulus reinforcement, while tensile reinforcement is more sensitive t o particle size of carbon b l a c h (1, 4, 6). Cohan ( 1 ) has pointed out that the rapid rise of modulus with increasing loadings of a structural carbon black ( H M F ) can be quantitatively correlated to a shape factor, estimated from electron micrographs of this carbon
683
black. The fact t h a t increasing loading of GR-S with colloidal silica and stannic oxide had little effect on t h e modulus within a n initial range of low volume loadings, while tensile strength showed a sharp increase within this range of loadings (see Figures 5 and
Figure 15. Electron Micrograph of Hevea-Silica Master Batch 23.1 volumes of silica Milled, dispersed i n toluene
7), seems t o indicate, according t o these views, t h a t colloidal silica and stannic oxide acted at low volume loadings like very small individual particles (of spherical shape). The rapid increase of modulus at higher loadings (Figure 7 ) might indicate an increased tendency of these pigments to form secondary structures. This possibility seems to be in accordance with the results of the electron microscopic investigation, described above. The tendency of colloidal pigment solutions to form aggregates during drying of their mixtures with GR-S latex, and their readiness t o redisperse during milling, should depend on the nature of the colloid. Casein should be expected t o form irreversible agglomerates, with little tendency to redisperse during milling into particles of the original colloidal size. It is obvious t h a t coagulation during mixing of the rubber latex with the colloidal pigment solution would affect the degree of pigment dispersion in the rubber matrix. The best results should therefore be obtained with stable colloidal solutions of pigments carrying electric charges of the the same sign as the latex and containing small quantities of electrolyte. The described method of pigment incorporation in the form of colloidal solution is believed t o be particularly useful for reinforcement studies with substances which cannot readily be obtained in form of powders of sufficiently small particle size. ACKNOWLEDGMENT
The electron microscopic work in this investigation was carried out by J. H. Daniel and R. B. Keller, Jr., Physics Division, Firestone Chemical & Physical Research Laboratories. The author wishes to express his thanks t o the Firestone Tire 8: Rubber Co. for permission to publish this w o r k ~ a n dto F. W. Stavely, J. W. Liska, and E. M. Glymph for their interest and helpful suggestions. LITERATURE CITED
(1) Cohan, L. H., I n d i a Rubber W o r l d , 117,343 (1947). (2) Cooper, L. V., IND.ENQ.CHEM.,ANAL.ED., 5, 350 (1933). (3) Dillon, J. H., Prettyman, I. B., and Hall, G. L., J . Applied P h l ~ s . , 15, 309 (1944). (4) Guth, E.,Rubber Technology Conference, London 1948, Paper 20. ( 5 ) Mullins, L., Ibid., Paper 4. (6) Parkinson, D., T r a n s . Inst. Rubber Ind., 19, 131 (1943). RECEIVED April 24, 1950. Presented before the Division of Rubber Chemistry at the 117th Meeting of the AMERICAN CHEMICAL SOCIETY, Detroit, Mioh.
