Sovember, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
1027
Influence of Pigments on Some Physical Properties of Unvulcanized Rubber‘ Harlan A . Depew THE XEW JERSEY Z I N C COMPANY, PALMERTOX, P A
T 18 generally known that unvulcanized rubber compounds containing fine pigments dissolve or, as some technologists prefer to say, disperse, slowly in solvents such 8s benzene.
pigment structure-and thereby destroys the semi-permeable membrane and increases the solubility. The very considerable increase in softness due to the addition of the stearic acid is in itself impressive evidence that a flocculated structure has been destroyed. Note-For t h e sake of clarity, t h e t e r m “solubility” has bemen used in this When a pigment is spoken of as dispersed or flocculated, paper t o denote t h e dispersion of t h e rubber compound in t h e solvent and the term “dispersion” has been used in discussing t h e distribution of pigment it does not mean that all the particles are dispersed or all in t h e rubber. flocculated. It is quite likely that in every mix containing The compounds are not really insoluble, as shown by the pigments there are aggregates, unmixed pigment, dispersed fact that fine pigment added to a rubber cement does not .pigment, and flocculates. The expression “flocculated” means that a large enough precipitate the cement; they percentage of the particles are just difficult to disperse. II is in this state to give the Beakers A and C in Figure The solubility of pigmented rubber in benzene can rubber properties, such as 1, containing carbon black be changed by altering the interfacial energy between s t i f f n e s s and insolubility, and a fine-particle-size zinc the rubber and the pigment, and this can be accomt h a t a r e characteristic of o x i d e , respectively. illusplished either by changing the surface of the pigment flocculated pigment. trate the insolubility. The or by changing the nature of the rubber-as, for exWhen pigments are milled stock compounded with an ample, by treating zinc oxide with sulfur trioxide gas into rubber, they are usuordinary zinc oxide shown and by adding organic acid to rubber. ally incorporated in small in beaker B is dissolving A plausible explanation has been offered as to why groups; then the mixing acmuch more rapidly. pigments cause rubber to become insoluble. In tion breaks up the groups Not#-In this report t h e finespecific cases it has been possible to predict the soluparticle-size zinc oxide has a parof p i g m e n t particles and bility of unvulcanized rubber containing pigments. ticle size of 0.15 micron a n d is coats the individual partiknown b y t h e brand name, Kadox. Data are presented showing the change in solubility cles w i t h rubber. If the T h e ordinary zinc oxide has a parof unvulcanized rubber during storage in contrast to wetting of the pigment by ticle size of about 0.30 micron and the constancy in consistency, which gives evidence of a is known a s X X Red. the rubber is poor, the inweak rubber structure that develops in storage. dividual particles will stick If this were all of the The influence of time and temperature of milling t o g e t h e r when they come story, the rate of solution on consistency and solubility is shown. would give a q u a l i t a t i v e into contact, and they will -__ then be spoken of as floccumethod of grading pigments 11 l a t e d . -4 c o n s i d e r a b l e w i t h i n a certain range of particle size, but factors other than particle size influence amount of dispersion and flocculation takes place when the the solubility. When stearic acid is added to @e com- rubber is hot and fluid during the early stages (3, 4’1of pounds, the solubility of the stock containing the fine-par- vulcanization. It is generally recognized that a quantitative relationship ticle-size zinc oxide is greatly increased, as shown in beaker CI, mhereaq the carbon black stock, shown in AI, remains cannot be developed between the size of the opening in a semi-permeable membrane and the size of particle that will insoluble. diffuse through it, but it seems reasonable that there should Theory of Insolubility of Pigmented Rubber be a t least an order-of-magnitude relationship in the case One explanation of the insolubility is that the pigments of pigmented unvulcanized rubber. Three flocculated spheriare in a flocculated state in the unvulcanized rubber and the cal pigment particles, 0.3 micron in diaemeter, will form an flocculent structure acts as a semi-permeable membrane opening that will allow a sphere 470 A. to pass through, which prevents the rubber aggregates from passing through. according to F. A. Steele of this company. This is the approxiThe common rubber solvents are known to be strong floccu- mate opening in an ordinary zinc oxide membrane, and publating agents for pigments and they would tend to keep the lished figureso(5) for the size of the rubber micelle (300-600 pigment in its flocculated state. The flocculated pigment X 100-200 A.) are of the same approximate size. Many of the openings are larger and allow rubber micelles particles, each surrounded by rubber except a t the point of contact, make threads and sheets which run irregularly to diffuse (2) through without difficulty. As they pass into throughout the compound and inter-mesh in every direction the solvent the network probably d r a m together, owing to making a network of pigment, so that it would be conceivable flocculating forces, and holds the remainder of the rubber for an electron, as an ewmple, to move from any part of thc even more tightly. This slow diffusion of unpigmented compound to any other part along the chain of pigment rubber through the larger openings in a semi-permeable membrane formed by a strongly flocculated pigment is to particles without leaving pigment. The addition of stearic acid (beakers AI, BI, and CI) dis- be expected. I n the case of a weakly flocculated pigment perses the ziiic. oxide-i. e., breaks down the flocculated such as ordinary zinc oxide, the flocculated structure is sufficient only to delay solution, and the experiments described Presented before t h e Dirision of Rubber Chemistry a t t h e 77th in this paper refer to the relative solubilities over very short Meeting of t h e American Chemical qocietv Columbus, Ohio April 29 t o May 3 1929 period. of timP
I
T i i e picces of rubber sliowii i n Figores 3 a1irl 4, after 10 : ~ i r i l 20 minutes in tlie solvent, respectively, demonstrate
that n rt!lat,ively Iiigh teinpcratiire and a short milling time, which does the least work on the eorrrpound, produced t l r r most iiisoliibie stock and that the reverse conditions resulted i n tlic most soluble. From Table I it is evident that time of ~iiilliiiga1Feets the soluhility, :is coiripnred with consistency. t,o t i relatively grentcr ilcgrix?tlitui teinpcratiire of milling. Table I -Consistency and Solubility Dafa Showin* EiTecr of Milling
" DI.iuiriiiaed at /oom l e i n j i e i a t i l i r i s "
Flaure ?--Influence ~f Pifimenc Concentration an Solublliry of U n v ~ l ~ ~ n l zRubber ed Pliot~graphtaken 2'12 hours niter immersion of rubber i n beiizcirr
ft, llright be tliought that t.iie incrensed rate (If sohitioir arid the decreased hardness are due to action of the stearic acid on the rubber rattier tlian to change in the interfacial tension. However, there is ariot.lrer way to change the interfacial tension between pigment aud rubber-by clranging tl-x composition of tlie surface of tlie pigment particle. This method, which slrould iufluence oirly the interfacial tension, u-as carried out by exposing the zinc oxide t.o the action of dilute sulfur trioxide gas and tliereby forming il surface layer of zinc sulfate. Beaker (XI in Figure 1 shows that rubber containing this acid-treated fiiie oxide is niore soloble tlian that containing the untreakd tine oxide. The unvulcaiiized stock is also much softer. If the picture of the semi-permeable ineinbraiic is correct, there are two methods of testing it: first, by reducing t.lic size of the opening in it tlirtiugh iiicreasiirg tire number of pigment particles-i. e., increasing the concentration pigment-and second, hy reducing tire size of the rubber aggregate?, as by severo nrilliiig. It is well known among rubber techologists that these two criteria will agme with tlie theory and t,lie data are prescrited for conipleteiiess i n tlie case of pigment loatling a i d I:ccau~;ctlie time a i d tcinperatrrre during thc inilling have t m n accurately deterinined, iiiakiiig the d;itil in the
i2"
1'. C2.%0
3" C.)
It must be admitted that milling stiould not only break &imi the rubber aggregates, but slrould also improve tlie clispersioii of the pignrent, and lience that increased pigment dispersion is responsible for soinc of the increasc in so1ubilit.y and tl,e decrease in coiisistency. Tliere is also some oxidation of the rubber, hut that does not need to be considered i n this paper. It would not seem probable that the pigment dispersion could cliange after the stock has cooled, and this is borne o u t by the fact that when tlie milled stock is allowed to stand for months the amount of flow under the relatively heavy
Fiaurr 3-Influence of Conditions of .Milling of B Stock on Its Soiubilify I'hotogrrph taken 10 mimitcs miter immersioia oi rubber in benzene
Influence of Pigment Concentration
Ikiin Figure 2 it will be seen tlrat E i m d i i c e s relatively i ~ i ~ ) I ~ uiri.uk,:$ ~lile i r i r i i t concentratiurr is high enoupli.
cil stocks if tlie pig-
Effect of Milling
order to study tire elkct of Imaking iiowii the riiiiber :rggregates, four condit,ioiis of inilliiir w i r e investigated using il stock containing 100 voluni~~s of ruiher aiid 30 m h n e s of ordinary zinc oxide: (1) 15 iriiiiiites wit,h tlie st0i.k a t 100° C.; (2) 60 minutes a t 100" C.; (3) I5 minirtr C.; (4) 60 minutos a t 60" C. The tempernbiirc control during milling was made possible by means of tlie contact resistance tliermometer described by Ashman and Homewood (0. In
Fi*urc 4-InRuonce
of Gondlrions uf Milling of a Stock on Ice Sulubiilfy
P i i o i o ~ a i i htakrri 20 sx~inutesaitri immersion oi rubber i n benzene
wci~litused iri the Williams press (slmsn by the t,Iiickllesn dt,w 3 niinutcs' coinprcssion) remains constairt (Figiire 5 ) .
Tiit! soliibility, on the otticr lmid, decreases sharply with time (Figure (i), wiiicli wo11l11 ii1dicat.e eit.her an increase in t.he size of tlie rubber aggregates or a devcloping structure. This st,ructure might he a rubber structure or it might be due t,o n flocculation, or precipitation, of protei11particles. That
INDUSTRIAL ALVD EA'GINEERING CHEMISTRY
Sovember, 1929
this newly developed structure is weaker than the original rubber structure is shown by the fact that the flow in the press is not changed by its presence. However, the forces that determine the percentage recovery on removal from the press are relatively small and the weak structure does affect these results soinenhat, as shown hy the erratic data given i n Figure 7 .
ELAPSED T/ME A F 7 E R M I L L I N G -
1029
necessity of considering special shapes and arrangements of the particles. Effect of Alkalinity
A water suspension containing fine-particle-size zinc oxide is slightly alkaline to phenolphthalein, and in order to learn whether the insolubility of the stock containing it might be due to this alkalinity, a series of tests was made using whiting, ordinary zinc oxide, and blanc fixe, together with 5 per cent of lime, 5 per cent of aniline, and 0.2 per cent of sodium hydroxide, each calculated on the rubber content of the stock. These tests showed that lime and sodium hydroxide decreased the rate of solution to only a slight degree, and that aniline had no effect. Tests with Pigments Other than Zinc Oxide
DAVS
Figure 5--Effect of Storage o n Consistency of Unvulcanized R u b b e r T h a t H a s B e e n Milled u n d e r DiRerent C o n d i t i o n s
The data in Tahle I1 show that the flon- in the press and the solubility of unpigmented rubber as determined from time t o time do not change appreciably during a week's qtorage. The early development of a structure on storage is manifested by the changing percentage recovery on removal from the press; the relatively large recovery was expected since thew n'as no pigment structure to prevent the recovery.
The conclusions in this paper have been based largely on the behavior of two zinc oxides and carbon black because there is considerable information regarding their particle
Insolubility of Vulcanized Rubber
g
Without entering into the merits of the various theorieb of vulcanization, or even endorsing the ideas brought out iii the brilliant paper by Stevens (6) t o the effect that vulcanization consists in the formation of particles of hard rubber,
$ c
s
IO
Figure 7-Effect of T i m e o n t h e Percentage Recovery a f t e r Removal f r o m t h e Consistency Press Determinations made ateroom temperature, 7.5' * 5' F (24 * 3 O C ) Figure 6-Effect of Storage o n Soluhilit? of Unvulcanized Rubber Determinations made at room temperature 75' t 5' F. (24' t 3 " C.)
it is pertinent to point out that the concept of a flocculated structure of CsHsS particles in vulcanized rubber would explain the insoluliility of vulcanized rubber and the increase in hardness of ruhlier during vulcanization, without any
' r H l C K N E S S A F T E R ,? LfXN. I N P R E E S A T 70'
RECOVERY 1 hfIN. A F T E R R E M O V A L F R O M PRESS.4T 7 5 ' * 5' F. (24' * 3' C.)
TIME AFTER MILLIKG
T I M E A F T E R MILLINO
c.
CONDITIONS
size, because of the difference in the surfaces, and because their rubber stocks give a good solubility end point on account of the opacity of the pigment, but a number of tests were also made on other pigments; precipitated whiting, blanc fixe, and clay giving decreasing solubility in the above order, as would be expected with decreasing average partic!e size. As a test case a magnesium carbonate stock was investigated. This pigment is very badly flocculated as shown
OF MILLING
-_ '\fin. '\fin. 15 15 15 60 a
0 0
C. C.
100 60
I1
1 Hour
22 Hours
.Mm .Mm.
Mm. Mm.
** ,, II O O 3 .. 1 16 6 3 2.82 1.67
4 4 .. 0 05 5 3.22 2.89 1.63
3 Days
7 Days
Mm.
Mm.
4.00 3 . 1199 2 . 86 86 1.57
4.14 3.33 2.8 888 1.60 1.60
-
1 Hour
R 13.66 9.34 8.87 1.24
22 Hours
3 Days
70
56
18.29 11.96 9.69 3.07
19.13 11.13 10.14 3.51
In the case of pigmented stock the solubility end point was the appearance of of a milky color.
a n d the end point chosen was complete dispersion,
7 Days
21.50 12.01 9.55 2.81
I
11111 1
SOLUBILITY-APPRO~. TIMEREQ U I R E D FOR C O M P L E T E DISPERS I O N O F I - C M . CUBP: A T 75'
*
50
rd.
(240 + 30
c.)=
T I M E A F T E R blILLTNO
1 Hour 22 Hours
3 Days 7 D a y s
Hours 44 44 44 20
Hours
Hours
45 45 24 16
43 43 24 16
10
Hours
38 38 20 12I/2
This could not be used in the case of milled pale crepe
Vol. 21. x o . 11
INDUSTRIAL A X D ENGINEERING CHEMISTRY
1030
by the dryness and hardness of the stock. I n spite of a particle size coarser than ordinary zinc oxide, the stock containing this sample of magnesium carbonate was decidedly insoluble as had been predicted on a basis of its poor wetting. In another test two lithopones were compared, the average particle size of one being approximately 0.3 to 0.35 micron and of the other approximately 0.4 to 0.5 micron, with aggregates considered as individual particles. These were compared for solubility in 20-volume stocks (20 volumes of pigment to 100 volumes of rubber). As was expected, the compound containing the finer lithopone required much longer (about three times) to go into solution than the one containing the coarser material. These two lithopones are
approximately representative of extremes in the particlesize range of commercial lithopones. A further consideration of the dispersion of pigments in unvulcanized rubber, for which the data are still very incomplete, would be helpful in bringing about a better understanding of the working qualities of factory stocks. Literature Cited (1) (2) (3) (4) (5) (6)
Ashman and Homewood, Indie Rubber U'orrd, 77, 59 (1928) Goodwin and Park, I N DEND.CHEJI.,20, 621 (1928). Depew, Rubber Age, 24, 378(1929). Grenquist, I N D . END.CHEM.,20, 1073 (1928). Meyer and Mark. Ber., 61B,1939 (1928). Stevens, J . SOC.Ckem. I n d . , 47, 37T (1928).
Explosive Limits of Industrial Gases' Jesse Yeaw ROCHESTER GAS & ELECTRIC CORPORATIOS, ROCHESTER, N.1'.
The relation between the composition and the exover, the results o b t a i n e d HROUGHOUT various plosive limits of widely varying complex inflammable must be directly applicable s t a g e s i n the manugas mixtures, generally encountered in the manuto large volumes. facture of city gas, sevfacture of city gas, has been shown. Coward and Jones (2) have era1 complex mixtures of inThe range of observation covers gas mixtures conflammable gases are generally found that, almost without taining from less than 12 per cent to more than 96 exception, a change of condie n c o u n t e r e d . It was beper cent combustible. The inflammable constituents tions which affects the exlieved that more information include varying amounts of hydrogen, carbon monplosive limits of one gas-air concerning the relation beoxide, methane, and other hydrocarbons. mixture will change the extween the composition and The effect on the explosive limits of changes in the plosive limits of any gas-air the explosive limits of these percentages of illuminants, methane, inert gases, and mixture in the same direcgases with air would prove the ratio of hydrogen to carbon monoxide has been tion, but not necessarily to useful in detecting explosive the same degree. They have pointed out and discussed. mixtures and eliminating unThe explosive limits of all the gases have been calshown that small changes in necessary hazards. culated from the analyses by means of a simple algetemperature and p r e s s u r e T h e chief constituents of braic formula first presented by LeChatelier and later have slight effects, that water these gases from the point of vapor affects the limits to a amplified and developed by Coward, Jones, and others. view of abundance are hydrovery small degree, that glass These calculated values agree with those found experigen, carbon monoxide, and bulbs 2 inches (5 cm.) or mentally remarkably well, and not only show that m e t h a n e . Ethylene, benm o r e in diameter give rethis work checks that of others in the field, but also zene, toluene, and a few other s u l t s a p p l i c a b l e t o large give further proof of the value of such a means of deh i g h e r hydrocarbons genervolumes, and that a strong termining dangerous conditions, particularly those ally c l a s s e d t o g e t h e r a s electric spark a t the bottom which abound in the production and distribution of i l l u m i n a n t s are found in center is a suitable method of smaller amounts in certain of city gas. ignition. the gases. Most of the work The temperatures of the gases under consideration are on explosive-limit mixtures of pure gases and air has been confined to the first three gases, and the limits of these gases are generally less than 100' C. except during the short period in which they are being produced in their respective ovens or fixed within a comparatively small range of accuracy. generators. Pressures are always as small as possible and T a b l e I-Widest Explosive L i m i t s Reported i n the Literature ( 3 ) rarely exceed a few centimeters of water. These gases all contain water vapor to very near the saturation point. PracGAS OR VAPOR n w r%s CENT GASOR VAPOR tically all explosive-limit work, however, has been done with Ethylene 3.2 to34.0 4.15 to 75.0 Hydrogen , water-saturated gases. Propylene 2.2 t o 9.7 1 2 . 5 t o 75 .O Carbon monoxide Acetylene 1.5 t o 80.5 4 . 9 to 15.4 Methane Apparatus Benzene 1 . 4 t o 8.0 2 . 5 t o 15.0 Ethane Toluene 1.3to 6.8 2 . 2 t o 7.3 ProDane A glass bulb of about 350 cc. capacity (8.9 cm. in diameter) was used as the explosion chamber. Mixtures of as near 200 Experimental Conditions cc. as practicable were introduced from a measuring buret, The object of the experimental work was to obtain the and atmospheric pressure was obtained by means of a leveling widest explosive limits or, in other words, the least amount of bulb using mercury as the fluid. Ignition was accomplished air or oxygen (upper limit) or the least amount of gas (lower by means of an electric discharge between platinum points limit) which could make a gas-air mixture dangerous. More- placed about 1 cm. above the surface of the mercury. A specially constructed buret of 200 cc. capacity, entirely en1 Received August 10, 1929. Presented before the Division of Gas closed in a large water jacket to maintain conatan&temperaand Fuel Chemistry at the 78th Meeting of the American Chemical Society, ture, was used to measure the gases. Minneapolis. Minn., September 9 t o 13, 1929.
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