INDUSTRIAL A N D ENGINEERING CHEMISTRY
1148
0.7 mm. and that of the kid leather 0.9 mm. The results are shown in Fig. 1. With increasing oil content, both the leathers show a decreasing tendency to hold water. I n a similar series kept a t 50 per cent relative humidity for 16 days, the same differences in water content were found. The most outstanding difference is the much greater tendency for the kid leather to stretch, which has long been widely appreciated in a qualitative way from actual wear, which explains why kid shoes lose their shape so much more readily than those of calf. The reason for this greater tendency to stretch is found in the fact that goat skins, like sheep skins, have a structure very much looser naturaIly than
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Vol. 16, No. 11
calf skin, which can be seen by examining cross sections of the raw skins. This loose structure also means that goat skins have less protein matter per unit cross section, which explains why their leathers are so much weaker than calf leathers, where the latter possess enough oil to overcome the internal friction of the fibers. The calf leather increases steadily in strength with increasing oil content, which has but little effect upon the strength of the kid leather. This may possibly be attributed to the difference in fundamental structure of the two skins, but it is not so clear why the tendency to stretch with increasing oil content shows a point of maximum with both leathers beyond which there is an apparent decrease.
Factors Determining the Reinforcing Value of Fillers in Compounded Rubber' By Herbert A. Endres THEGOODYEAR TIRE& RUBBERCo.,
AKRON,
OHIO
The reinforcing effects produced in vulcanized rubber by the incorporafion of fillers are determined by three basic factors-namely, particle size, shape, and the degree of wetting of thefiller by the rubber. Particle size is of paramount importance, and for fillers that are completely dispersed in rubber is a direct measure of reinforcing value. A filler cannot be completely dispersed in rubber on the mill unless the rubber wets it. The degree of wetting therefore determines the effectiw.particle size of the filler in rubber and the degree of attachment (adhesion) between the particles and the rubber. The strength of the bond between different fillers and oukanized rubber depends upon the degree of wetting, and there is no evidence that any differences exist in the magnitude of this adhesioe force between vulcanized rubber and the oarious fillers, which are chemically inactive during uulcanization, so long as the particles are in contact (wet) with the
rubber. By reducing the particle size of a filler as dispersed in rubber the reinforcing efect is correspondingly increased. I n presenting data on the reinforcing value of fillers it is therefore of greatest importance to sfate definitely the particle size of the material as present in the compound. Owing to a n alignment of the particles during the milling and calendering operations, anisotropicfillers show abnormalreinforcingefects. These effects are manifest. eoen though the filler is completely dispersed, and are not obliterated when the particle size is reduced to a state comparable with the finer fillers. The best reinforcing fillers are therefore those of smallest particle size, completely dispersed in and wet by the rubber and isotropic in particle shape. A n y material. whatever its chemical nature. that fulfils these requirements will be a good reinforcingfiller.
REVIEW of the literature will lead to the conclusion
I n studying the volume increase of compounded rubber under strain, Shippel6 found that this property ran roughly parallel with the mean diameter of the filler particles, zinc oxide being an exception. This volume increase he ascribed to the rubber pulling away from the filler particles and thus causing the formation of vacua running parallel to the line of force causing the strain. Green' confirmed this microscopically and also pointed out that fillers if not properly incorporated may exist in rubber in large agglomerated masses. Hardmans has shown this to be the case with antimony pigments. Several commonly used materials, such as iron oxide, antimony sulfides, whiting, lithopone, and light magnesium carbonate, are prepared by precipitation from solution. They are then filtered and dried, and during the drying process, as the water evaporates, the particles become agglomerated and cemented together. All the fillers in use a t the present time which are prepared by precipitation from solution are badly agglomerated and the smaller the particle size the worse the agglomeration. These fillers do not completely disperse in rubber on the mill, and compounds containing them all show large agglomerated masses (Figs. 1, 2, and 3). These agglomerates may be hard and behave like individual particles9 when compounds containing them are stretched (Fig. 4),or they may be soft and break up very easily when a
A
that the following factors determine the reinforcing effect which mineral compounding ingredients or fillers impart to vulcanized rubber: (1) particle size (specific surface); (2) degree of wetting (adhesive force); (3) particle shape. A relation exists between permanent set, abrasion resistance12hardness,S and the stress-strain relations of compounded rubber such as slope and energy of resilience. Since the stress-strain curve is the usual criterion for judging value of a filler, it alone will be considered here.
PARTICLE SIZE T h e relation between the particle size of a filler and its reinforcing effect in vulcanized rubber has been conclusively shown by Wiegand14who considers that the most direct and accurate way of determining the average fineness of a filler is to measure the area under the stress-strain curve in a standard mixing and compare the energy content with that of a known filler. Twiss5 prepared a precipitated barium sulfate which was considerably finer than the ordinary ground product and found that its reinforcing effect was also increased. 1 Presented before the Division of Rubber Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D. C . , April 21 to 26, 1924. 9 Greider, THISJOURNAL, 16, 504 (1923). 8 Ibid., 14, 385 (1922). . Chem. J., 4,160 (1920). a Rubber J., 66,651 (1923).
.
THISJOURNAG, 1!.4,33(1920). Ibid., 18, 1029 (1921). * I n d i a Rubber W o r l d , 68,711 (1923). B Green, Chem. Met. En&, 28, 53 (1923). 6
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INDUSTRIAL AND ENGINEERING C N E I I S T R Y
Sovember, 1924
1149.
strain is applied to the rubber (Fig. 5). The latter are very common and are shown by iron oxide, antimony sulfides, light magnesium carbonate, poor grades of carbon black, and others. Aside from causing the loss of the true reinforcing effect of t,he filler, these agglomerates have a very deleterious
in this way could he given here, hut they would he of little value because of their great dependence upon the method of. preparation of the fillers. Particle size and degree of disper-sion in rubber vary considerably in products from different. manufacturers, and each must he considered as a case by it-
effect on the phgsim.1 properties of the vulcanizat,e due to t.Be comparative ease with which t.hey are broken up. Buy means of completely dispersing these fillen will greatly increase their reinforcing value. This has been demonstrated in the case of light magnesium carbonate by Greider,'O who increased the dispersion of this filler by introducing a small amount of glue into the compound. The writer has dispersed this filler completely in rubber by other means, as was evidenced by a microscopic examination of thin sections. In Fig. F stress-strain curves are given for a compound containing 30 volumes of magnesium carbonate per 100 volumes of rubher. comsletrlr dissersed by a special proccss and incorporated bn the mil1 in the regular way. W h e n incorporated on the mill magnesium carbonate shows a maximum reinforcing effect at about 8 volumes per 100 volumes of rubber. A 30-volume s t o c k would therefore he highly over-loaded and badly agglomcrated. The stressstrain curve8 show . the pronounced effect oi an increased disRrc. 4-Rt:aeen CONTAINING WX~ING persion, and hence r;r.orrcnrao 300 PER CBM. A O O L O X ~ ~ A T Eincreased S specific surR B m v R AS SZNCLE PARTICLIS.x 630 face. Measurements of particle size are always made on completely dispersed preparations, using as a dispersing agent a liquid media, such as turpentine in the microscopic method of Green," or solutions of tannic acid, gum arabic, or other protective colloids, as in the ult.ramicroscopic and sedimentation methods. The fact that particle size as dispersed in rubber is not, necessarily the same as individual particle size, owing to agglomeration, has received hut little attention. The former can be obtained by microscopic measurements in thin sections containing the desired amount of filler incorporated in the usual way, and considering agglomerates as individual particles for this purpose. Measurements obtained
self. In general, it can be said that many of the badly agglomerated fillers, such as whiting and light magnesium carbonate, have a particle size in rubber many times greater than the individual particle size (Figs. 3 and 7). Data on individual particle size and particle size distribution are of value, however, as they show what can be expected in rubber if complete dispersion is obtained. When reinforcing effects do not correspond to ultimate particle size, agglomeration is the cause. When zinc oxide is heated, some of the particles grow at the expense of others, or if a smalf amount of lead is present the particles aggregate to forur denser masses. I n either case there is an increase in effective narticle size and a decrease in reinforcine effect. Luff" hea&d zinc oxide at 700" C . and found a substantial difference in the stress-strain curves before an8 after heating. Similarly, finelydivided c a r b o n becomes gritty when strongly heated, owing to agglomeration. Gas blacks formed at high temperatures, 1ooO" C. and above, although of small ultimate particle size are F S ~S .- F I L L ~ A C O L O ~ ~ E R A Tm Z ~R V S ~ Q RROPEN UP U N ~ SRm n m . X 530 badly agglomerated and graphitic in nature and do not disperse well when milled into rubber. Such a black is shown in Fig. 8. With this particular material the reinforcing effect, as judged by the stress at 300 per cent elongation, was only about 55 per cent of that of the best channel gas black. These points will be eonsidered in further detail, but for the present it can be definikly stated that the reinforcing value of fillers that are not chemically reactive during the cure is determined by their particle size ay dispersed in rubber, and as Wiegand'a has concluded, variation in the specific sudace of fillers is the predominating factor in theis behavior in vulcanized compounded rubber. Since substances that are in true solution in mbher have no
a@--. -
8. 81
Tars JOURNAL, 16, 151 (1924). J . ~ ~ ~ tnst.. ~ f i192, i 637 i ~ (1921).
3%
"The Chemistry of Rubber," 1924, p. 160. Txis Jooa~nr. IS, 118 (1921).
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INDUSTRIAL AND ENC ZNEERING' CIIEMISTR Y
1150
reinforcing effect in the ordinary sense, it follows that the reinforcing effect-part.icle size curve must go through a maxi~num.'~There is, therefore, a limit to which decrease in particle size can he effectively carried for maximum reinforeing rffect. In arriving at the particle size giving this maximum the chief difficulty is in ohtaininn a eom-
DECREE OP WETTIKC When a liquid is ad-
sorbed at the surface of a solid it forms a liquid film and wets the solid. The solid then adheres to thr liquid. Mirth has bt:en said eoneeriiirig tlie part played hy selective adsorntion OT I I prefercntial wetting i n % €hqeh,on J(iD the compounding of FIG 6 riihher. 1t i s gonernlly admitted that, there is some sort of a bond bet,wcen the filler particles and tlic rubber, but whet,lier the magnitude of this adhesive forre is the same in all cases or differs with the different fillers has been an open quesiioii. In an effort.to gain sorue insight into bliis question, the work of Green on the microscopic study of \ d o m e increase of compoundcd riibber under strain was extended to include all the mineral eo~npoundingingredients in coriiinon use. By noting t,he degree of strnin at the instxnt vacua fornia.t,ionbegan and also t,he length oE t,lie vacua for different elongations, it was hoped that somc definite iltformation relative to the strcngth of the union bet,weeii t.he particles and the rillher might. be obtained. In general, the larger the particle Llie less the strain necessary to piill the rubber from it. The marser materials, such as aluminium flake and asliestine, readily Yoparate from the riibher when it is siibjected t,o strain, while ilic smaller particles, such 5s zinc oxide and iroii oxide, show very few vacua eireii alien t.he ruhher is stretched early to the breaking point. Therefore the m a g d u d e of the rwlunie increase under strain seerns to he determined primarily by the part,icle size of the compounding i n g r e d i e n t s . Quant,itative measurements of volume increase have sliown this to he true, except in the case of ingredients that are chemically reactive during vulcanization. Since adsorption i s proporF,
