November. 1925
INDUSTRIAL A S D ESGIAVEERILVGCHEMISTRY
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The Physical Properties of Rubber‘ Microscopic Examination of Rubber By L. B. Sebrell, C. R. Park, and S. M. Martin, Jr. GOODYEAR TIRE& RUBBERCo., AKRON,OHIO
Particle Shape and Size HIS investigation has been undertaken in the belief The individual particle of Hevea latex has been reported that a clearer conception of the physical structure of rubber would result in its more intelligent treatment by various observer^^-^ as varying in size from 4 p in diameter in factory processing. The extraordinary physical properties down to particles a t or even below the lower limit of visiof rubber have been variously explained by a globular struc- bility. The particles have been described as varying in - ’ ~ particles ture, by a filamentary or brush heap structure, or by the shape from slightly ovoid to p e a r - ~ h a p e d , ~some configuration of the rubber molecule itself. I n view of the having quite long tails. globular condition of rubber in the latex, it has been sugThe present work verifies these findings. Particles have gested that this form may persist more or less unchanged been observed of all the shapes described by Hauser. I n Figure 1 are shown particles correduring the subsequent treat, ment to which the rubber , sponding in shape with the type said to be-most comis subjected. “Whether the A micrographic study has been made of latex and mon in new branches, leaf r u b b e r g l o b u l e s during the mechanism of its coagulation. New evidence has s t e m s , a n d leaves.* I n coagulation wholly coalesce been obtained relative to the shape of the latex parFigure 2 a t the upper edge to form one homogeneous ticles and their persistence after coagulation. The is a particle like that destructureless mass or results have been recorded in motion pictures and in scribed as found mostly in whether they only stick tophotomicrographs. The work of Hauser has been old trees. gether, keeping their origverified. The relation of these results to the probable inal globular form, is still Character of Particles structure of rubber as outlined in a previous coma subject of debate.”2 The munication has been pointed out. According to de Vries13 literature a t the present _ Iit is of little importance time continues contradicI whether latex is an emultory and u n c o n v i n c i n g . It is the writers’ intention, therefore, to review such evi- sion or a suspension since this criterion for classification dence as is a t hand and add to it where possible with the pur- has been replaced by that of solvation. From certain viewpose of clarifying the situation in general and determining points this’is perhaps true. Whether the latex particle is solid or liquid is, however, a matter of great interest as more definitely the following points: throwing light upon the physical behavior of massed rubber. (1) The form and consistency of the globule in Hevea rubber The viscosity-concentration relation of latex has been used latex. (2) Whether the particles persist during coagulation and dry- as evidence that latex is an emulsion-i. e., that the latex ing. particles are liquid. B ~ r r o w m a n ’states ~ that the typical (3) Whether the particles persist during milling or t o what viscosity-concentration curve of a suspension is a straight extent they are destroyed. (4) Whether they are destroyed by swelling and solution in line and that the deviation of latex from this behavior places it in the class of emulsions. The first statement has been solvents. ( 5 ) The effect of vulcanization. proved wrong by more recent data. The viscosity concenSatisfactory evidence has been obtained upon certain of these tration curve for a suspension is a hyperb01a.l~ The fluidity-concentration curve is a straight line. Borrowman’s points; others are less definitely settled. An attempt has been made to follow the microscopic data have been recalculated to fluidities and the data are changes in state which correspond to such macroscopic proc- plotted in Figure 3. The data fall upon a straight line, showesses as coagulation, drying of latex, and solution of rubber ing that they are a t least in accord with the laws governing in solvents. Photomicrographic studies have been made of suspensions of solid particles. The shape of the latex particle varies with the individual, materials before and after a given change, and in some but is persistent in any one particle over a long period of time. cases motion pictures have been used to record the transition. I n the course of the examination of a number of samples of
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Source of hlaterials and Apparatus
All the latex samples were ammonia-preserved latex from Hevea braziliensis. The photographic work was done with Bausch and Lomb standard equipment. A fluorite objective for oil immersion N. A. 1.3 was used throughout in conjunction with a 6x projection lens and Abbey substage condenser. Additional magnification was obtained by bellows extension. The source of illumination was an ordinary actinic carbon arc. Some of the observations were made with a B. & L. binocular microscope equipped with a Zeiss parabolic condenser. 1Presented before t h e Division of Rubber Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1025.
D e VrieS. “Estate Rubber,” 1920, p. 227
8 Henri, Comfit. rend., 144, 431 (1907); Caoutchouc & gutta-percha, 3, 510 (1906); Spence, “Lectures on India Rubber,” Liverpool, 1909, p. 203. 4 Bobilioff, Arch. Rubbercultuur, 3, 374, 405 (1919). 6 Spence, India Rubber J., 36, 233 (1908). e Gardner, in Torrey and Manders, “The Rubber Industry,” Vol. I, 1911, p. 225. 7 Loomis and Stump, Chem. Met. Eng., 29, IS4 (1923). 8 Hauser, India Rubber J., 68, 19 (1924). 0 Freundlich and Hauser, Kolloid-Z., 36, 15 (1925). 10 Hauser, India Rubber J., 68, 725 (1924). 11 Schidrowitz-Hauser, Ibid., 68, 455 (1924). 1’ Lloyd, private communication t o G. S. Whitby; Bogue, “Colloidal Behavior.” Vol. 2, 1924, p. 662. 18 Arch. Rubbercultuur, 7, 409 (1923). 1‘ Torrey and Manders, “The Rubber Industry,” Vol. I, 1911, p. 243. Borrowman does not state whether weight or volume concentration was used. It is assumed t h a t it was the volume concentration. 16 Bingham, “Fluidity and Plasticity,” 1922, 65, 199.
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htex, the writers have never observed a freely moving particle in the process of changing it.s shape or dividing into two smaller particles. Particles that have been under observation for as long as 15 or 20 minutes have given no indication of these phenomena which were described by Wight,rnan and 'I'rivelli.*6 Each individual particle is in constatit motion and sometimes rntates end for end in soiiic plaiie parallel to the axis of tlie optical system. This sometimes gives tlie impression that tlie particle is changing in shape from owid to sp1ierii:al and tlie rcmrse. This hchavior is very effectively sliown in the rnobion pictures. Arcording to Beadle and Ster ~ n s fnniim '~ of particles takes place in tlie latices of both ~ i in. gx 1900 ~ ~ ~ Castilloa and Ramhone funon ._ loagulation),8,vand it is described as similar to the fusion of oil droplets. They stato further t.hat the same sort of fusion occurs to a limited extent in Nevea latex. Loomis arid Stump' also report this phenomenon and describe it as the usual result, of coagulation of Hevea latex. Wightman and Trivclli'6 assert that particles sometirnes unite to form larger particles. The present miters ttave frequently nbserved two or more particles in the absence of coaylant cling together quite tenaciously but never actually to fuse to form a single larger particle. Tlic above-ment,ioned authors's state: "A particle moving rapidly for any reason will tend to take the pear shape." This is certainly in error, because invariably particles may be observed in the samc field traveliiig at the same rate, some going head first and some tail first in the same direction. The general behavior of the particles relative to persistence of form, together with the more detailed information given by Freuiidlich and Ilauser,ssg indicates that, as far as effective behavior is concerned, the particles may be regarded as plastic, semisolid masses. The ovoid shape is probably traceable to the process of formation from the lactiferous cells of the tree. This form may perhaps be retained owing to t.he formaI
,* Trm JOURNAL, 11, 164 (1025). 37
8th I n l r m . Cons. APPI. Chcm., 9, 17 (10121
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tion or a plustic d i d film, as showti by Wilaon'a until polymerkati(rn brings ahorit. the formatioti of the permanent skin desi:rihed by Frcundlich and Ilauser,y~"d t e r which the ihapc is quit,e definite and persistent. Drying
Wlieii Intes is evaporated in an open pan a film of c o r d erable tensile strength is formed on the surface. The mechanism of formation of this film has been observed as follows: h drop of latex diluted 1 : 10 was dropped on a clean slide and covered with a clean cover glass well pressed down. If ttic slidc was allowed to stand for 15 or 20 minutes, evaporation caused the interface between liquid and vapor to creep back under the cover glass. Tlie suspension became more cancentrated with referelice to latex particles, which were t.lien seen piling up at tlic interface. The particles do tint lose their identity during this process, and if it is carried far enough a film is formed which ltas tho characteristic properties of crude rubber. Such a film in the process of formation is sliown in Figures 4 and 5 . The actual process has been recorded by means of motion pictures. As the interface creeps back owing to evaporation, groups of particles are sometimes drawn out of the film and remain stuck to the slide or cover glass. Such groups, even when fairly dry, may be seen to consist nf the origiiial particles. An example is shown in Figure 6. Of course, this is a layer probably not over two particles deep and the structure is fairly evident. At times whole sections of the interfacial film are torn out and form trianylar patches or strips such as those shown in Figures 7 and 8. The structnre of these threads and patches may be clearly seen wlien they are first torn from the main piece. As drying proceeds, howover, the structure in tlie lighter central portion becomes less and less distinct and finally becomes quite clear. This portion is in direct contact with both the slide and the (!over glass and the surfaces are consequently smooth. Tlie edges of these patches still display their structure after the center has become clear, but the units are less distinct. The clearing takes place without any accompanying phenomena which would indicate the bursting and fusion of the particles to form a homogeneous niass. If any such process were taking place, it would undoubtedly be observable as IS
Mnlhewr, Colloid Symposium Monosrapb, 1913, p. 145.
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I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
a series of minute explosions like the bursting and fusion of oil droplets or bubbles in a foam. It is well known that many white and blue bodies owe their color or opaqueness to the scattering of light due to the difference in refractive index of the phases of the structural material.l0 Obviously, this is the explanation of the opaque whiteness of latex. During the drying process the water is removed from the protein layer between the particles and the rubber film becomes transparent. If in a dry transparent film the water is replaced to some extent in the protein layer between the particles, it is to be expected that the film will become white and opaque and that the structure will again be developed. The structure has been repeatedly developed in transparent, dried latex films in which it was otherwise almost invisible, and parallels the appearance of the milky whiteness characteristic of wet rubber. This method was largely used in preference to dyeing, because it was believed that fewer complications were introduced thereby. A soaked latex film is shown in Figure 9. Upoii drying the structure fades out again and the process may be repeated ad infinitum. When a thin film of latex is stretched the particles may be seen to elongate. The external portion of the particle, as reported by Freundlich and Hauser, has elastic properties similar to those of rubber. When an individual particle that has been stretched to about 600 to 700 per cent of its original length breaks from its point of attachment, it tends to return to its original globular condition. The elastic and retractive properties of the particle evidently reside in the external sheath of the particle. Although it is not necessary to assume that the skin has the same elastic properties as massed rubber, the material of the elastic film is unique in its properties of extensibility. , Note-Three hundred per cent increase in area of the surlace accounts for an increase of approximately 1000 per cent in the length as the particle changes from a sphere to an ellipsoid of revolution.
Structure of Rubber Obtained by Evaporation of Solutions
A benzene solution of slightly milled pale crepe was used which contained 7 grams of rubber to 100 grams of benzene. The solution had none of the characteristics of a gel, but was very viscous. A thin film of this solution was dried upon a slide. When the film was placed under the microscope it was practically clear. A drop of water was then run under the cover glass and the film was allowed to soak for about 30 minutes. A granular structure gradually developed during this time until the individual units of the structure could be seen at the edges of the cement. The order of magnitude of the granules was the same as that for latex particles. The development of the structure exactly paralleled the appearance of opaqueness in the film. The structure fades again upon drying as in the case of latex films and the process may be repeated. Such a film after soaking may be seen in Figures 10 and 11. This is in agreement with the observations of Schidrowitz.20 Coagulation
A drop of sulfuric acid diluted 1:1 with water was allowed to run under a cover glass into latex diluted 1: 25 with water. The Brownian motion of the latex particles was slowed up and the particles collected in clusters containing from 50 to many hundred particles (Figures 12 and 13). The agglutination or flocculation of particles in these figures represents no practical difference from the mechanism of formation of Bancroft, “Applied Colloid Chemistry,” 1921, pp. 182 and 188. J . SOC.Chem. Ind., 2S, 6 (1909); Fickendey;Kolloid-Z., 8, 43 (1911), could not confirm work of Schidrowitz; believed particles burst; used Castilloa. 10
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1’01. 17, No. 11
a true clot.21 Under practically identical conditions a clot is slowly formed in a beaker. It has been the opinion of Weber,22 in common with others, that the latex particles consist of a liquid which is not caoutchouc itself but a lower polymer of isoprene. During coagulation the particles were thought to coalesce to form a large droplet. It was further believed that with the fusion of the droplets some sort of polymerization took place and the material changed to a solid. This process was described for Castilloa. The experimental evidence in the case of Hevea latex is not in entire accord with this view. It indicates rather that the particles do not lose their identity upon coagulation. The appearance of the groups in Figures 12 and 13 indicates that the particles are still intact. An indirect but rather striking confirmation of this opinion is offered by the work of Hauser,lo Loomis and Stump,23and Green, who followed the vulcanization of latex particles during the S c h i d r o ~ i t z hot * ~ ~vulcaniza~~ tion process. The particles are reported to have become less easily deformable. It is improbable that such units would lose their identity upon coagulation. The coagulum so obtained is reported to have physical properties, tensile, resilience, etc., similar, although not equal, to those of rubber vulcanized in the usual way.20 These facts seem to be in agreement with the views of Gardner,G “that caoutchouc globules undergo no chemical change upon de-emulsification by chemical reagents, is sufficiently shown by the observation that if a little of the rubber coagulum obtained be diffused through benzine and this is afterwards evaporated off, microscopical examination of the rubber film obtained shows it to be composed of masses of globules identical in every respect with the globules in the latex.” Conclusions
The foregoing facts led to the belief that (1) ordinary rubber is a very closely packed mass of discrete latex globules; (2) the increase in plasticity upon milling is due to their partial destruction; (3) the decreased consistency of cements made from milled stocks is traceable to the same cause; (4) the peculiar stress-strain properties and grain effects are due at least in part to this structure; ( 5 ) the structure probably persists during vulcanization; (6) the destruction of the structure is accountable for the low tensile properties of overmilled rubber which has been cured t.0 a proper coefficient of vulcanization. Acknowledgment
The writers wish to thank Wm. B. Harsel for the use of a motion picture camera which made a part of this work possible. Thanks are also due A. M. Hamblet, of this laboratory, for helpful advice, and to R. P. Loveland of Eastman Kodak Company. 21 Audubert, Reo. gbn. colloides, 2, 321, 353 (1924). The relation between coagulation and agglutination is discussed at length. “One may consider coagulation as the ultimate state of agglutination since it is possible to pass from the second to the first of these phenomena in a progressive manner by addition of increasing quantities of the same reagent.” See also de Vries, India Rubber J . , 67, 677 (1924); the terms “coagulation,” “flocculation,” etc., are carefully defined. 22 Be?., 86, 3108 (1503); Gummt-Zfg., 17, 296 (1903); 19, 101 (1904). See de Jong and Tromp de Haas, Ber., ST, 3298 (1904); also Hinrichsen and Rindscher, Ibid., 42, 4329 (1909~. These writers disagree with Weber, de Haas and de Jong used Castilloa latex. Hinrichsen and Kindscher used Kickxia. Belgrave has studied the volume and heat changes occurring upon coagulation of Hevea and has come to the conclusion that no deepseated chemical change-e. g. polymerization, takes place. Malayan A gl. J . , 11, 348 (1923). 28 Chem. Met. Eng., 29, 540 (1923). 2‘ British Patent 193,451 (1923). 28 Davey, J . Soc. Chem Ind , 42, 473 (1523).
