December 1948
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
TABLE111. HIGH TEMPERATURE PLASTICIZER EFFECT Plasticizer (30%) Temp., ' C. b Viscosity, Poises Dimethyl phthalate Dimethyl phthalate Santicizer M-17 Santicizer M-17 Dioctyl phthalate Dioctyl phthalate
Cellulose Acetate 170 190
170 190 Vinylite VYNS 160 170
0.35 0.30 0.22 0.23
6 2 X 105 9 . 6 x 104 5 . 6 x 10s
0.23 0.20
3 . 7 x 104 7 . 1 X 108
6 . 7 X 10'
2325
many other effects (toughness, brittleness, etc.) on the mechanical and flow properties which are not taken into account in this relatively simple analysis. ACKNOWLEDGMENT
The writers wish to express their appreciation for the cooperation of J. H. Teeple, of Celanese Corporation of America, and H. W. Mohrman, of Monsanto Chemical Company, in supplying the samples of plasticized cellulose acetate. LITERATURE CITED
high temperature plasticizer. The absolute values of the viscosities are widely different. The viscosity values for the acetates are lo2to lo5 poises higher, depending on the plasticizer. The comparative curve for Vinylite VYNS in Figure 10 shows this difference graphically. Thus, a t processing temperatures the more significant difference lies in the relative values of the viscosities. The essential feature of the evaluation presented here is t h a t quantitative comparison of various plasticizer-resin systems can be made in the two important temperature regions-the general temperature range of application and the region of processing temperatures. It is recognized that a plasticizer may have
Clash, R. F., Jr., and Berg, R. M., M o d e r n Plastics, 21, 119 (1944). ( 2 ) Clash, k. IT., Jr., and Rynkiewicz, L. M., IND.ENG.CHEM., 36,279 (1944). (3) Conant, F. S., and Liska, J. W., J . A p p l i e d Phys., 15, 767 (1944). (4) Dienes, G. J., J. CoZZoid Sci., 2 , 131 (1947). (5) Dienes, G. J . , and Klemm, H. F., J. Applied Phw., 17, 458 (1946). ( 6 ) Tuckett, R. F., Tians. Faraday Soc., 39, 158 (1943); 40, 448 (1944). (1)
RECEIVED October 24, 1947. Presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 112th Meeting of the AMERICAN CHCIIICAL SOCIETY,New York, N. Y .
Carbon Black Dispersion in Rubber Effect of Fatty Acids ROSS E. MORRIS AND JOSEPH W. HOLLISTER Rubber Laboratory, Mare Island Naval Shipyard, Vallejo, Calif. Adsorption isotherms for stearic acid and other longchain aliphatic acids on various carbon blacks were determined. The solvent employed was heptane in most cases, although some work was done with benzene and cyclohexane. Considerably more acid is adsorbed by channel black from cyclohexane and heptane solutions than from benbene solutions. Adsorption isotherms on a molal basis for the acids on channel black coincide fairly closely. Stearic acid adsorption is generally higher for the finer particle size blacks, but the degree of adsorption is influenced also by the surface condition of the blacks. Cal-
culations indicate t h a t adsorbed stearic acid covers less than 20Yo of the surface of the hlaclc particles a t saturation. The effect of stearic acid on the dispersion of conducting furnace black and channel black in Butyl rubber was determined by a series of objective experiments. These experiments involved measuring the rate of incorporatioll of the blacks on a mill and determining the viscosity, plasticity, electrical resistivity, bound rubber, and tensile strength of Banbury-mixed batches. Paraffin was used as a control softener in all cases. Stearic acid apparently has little effect on dispersion of these blacks in Butyl rubber.
H E T H E R or not stearic acid assists in dispersing carbon black in rubber has not been settled. Blake (3)was the first to claim that fatty acids are dispersing agents for carbon black. He postulated a mechanism based on Langmuir's work with monomolecular films of fat acids on water (15) and calculated that the fatty acids naturally present in Hevea rubber, which he stated to be present to the extent of 2%, are just sufficient to form a monomolecular film on each particle of 30 volumes of channel black in 100 volumes of rubber. H e assumed that this monomolecular film serves to facilitate wetting the individual particles with rubber and prevents agglomeration or flocculation of the particles. Later developments, however, have invalidated Blake's calculations. Blake assumed that the diameter of the average channel-black particle is 2 0 0 , ~whereas ~ electron photomicrographs ( 7 ) have shown t h a t the actual average particle diameter is 2 8 , ~ . Therefore, 30 volumes of channel black have about seven times the
surface area calculated by Blake. The fatty acids in Hevea rubber, even if present to the extent of 27, and entirely concentrated on the surface of the particles, are insufficient to cover completely the surface of this quantity of channel black. Blake neglected to take into account the solubility of fatty acids in rubber. If the acids are soluble in rubber, not all of the acid molecules will concentrate at the rubber-black interface; some will remain in solution in the rubber. Morris (18) showed t h a t the solubility of stearic acid in Hevea rubber is aboui 1.5 parts in 100 parts of rubber at room temperature and increases rapidly as the temperature rises. The fatty acids naturally present i n Hevea rubber are known to consist of linoleic, oleic, and stearic acids (26), and the total amount is about 1 part per 100 parts rubber hydrocarbon (5'2). Linoleic and oleic acids probably are more soluble in rubber than stearic acid, so that these acids are present in quantities well below their respective solubility limits. It does not appear, therefore, that the fatty acids will
INDUSTRIAL AND ENGINEERING CHEMISTRY
2326
Vol. 40, No. 12
clearly in Figure 1. These curves show also that cyclohexane would have been a good choice for the solvent. The method of obtaining the plotted values is described in the following section. MATERIALS AND PROCEDURE
The heptane used in these experiments was Phillips’ n-heptane, stated to be 95 mole % minimum purity. The bensenc was Baker’s C.P. quality. The stearic, palmitic, myristic, lauric, pelargonic, snd oleic acids and the cyclohexane were Eastman’s best grades.
TABLEI. DESCRIPTION OF CARBOX BLACKS Same Thermax Rubber Makers Velvet Furnex Statex 93 Statex A Standard Micronex Super Spectra GRAMS
STEARIC ACID
PER
loo ML.
soLurioN
AT
Type Medium thermal Lampblack Semireinforcing furnace High modulus furnace Conduoting furnace Medium processing channel Color
Symbol hlT LB SRF HhlF CF MPC
C
EQUILIBRIUM
Figure 1. Adsorption of Stearic Acid from Various Solvents by -MPC Black
leave the rubber phase entirely in favor of the rubber-black interface, particularly a t the high temperatures attained during mixing. It is not customary in practice to rely on the naturally present fat acids when mixing Hevea tread stocks. Usually from 3 to 6 parts of stearic acid are added t o help disperse the carbon black and to activate the accelerator. Even when mixing tread stocks from GR-S, which initially contains 3.75 to 6.0% stearic acid (19),many manufacturers add an additional part of stearic acid. The question is: Do these amounts of stearic acid aid in dispersing carbon blacks? To answer this question the adsorption of fatty acids by channel black and other carbon blacks will first be examined, and then a more direct approach t o the problem will be considered,
ADSORPTIOIV OF F PTTY ACIDS BY CARBON BLACKS Fatty acid would have to be adsorbed on carbon black in order to aid the wetting and dispersing of the black particles by rubber. Analytical information of a qualitative nature has been obtained which proves that this adsorption does occur. Gehman and Field (11) found that when 48 parts of channel black were present in 100 parts of Hevea rubber, the x-ray diffraction ring due to crystalline stearic acid could not be observed for less than 2 or 3 parts of stearic acid, presumably because of the adsorption of stearic acid by the black. For the same loading of semireinforcing furnace black, 1or 2 parts of stearic acid were sufficient for detection; this indicated less adsorption than for channel black. The authors decided to measure the adsorption of stearic, palmitic, myristic, lauric, pelargonic, and oleic acids by channel black, and in addition to measure the adsorption of stearic acid by other carbon blacks used in rubber compound. Consideration was first given to measuring the adsorption of these acids using Hevea rubber as the continuous phase. It was soon found, however, that experimental difficulties made the results obtained of questionable significance. It ‘uTas then decided to use heptane as the continuous phase for several reasons: first, it is a hydrocarbon as are Hevea and GR-S rubbers; secondly, it is readily volatile; this facilitates analysis for the unadsorbed acid; and thirdly, heptane being an aliphatic hydrocarbon should be less adsorbed itself by the carbon black than an aromatic hydrocarbon or a polar solvent, therefore it should interfere less with adsorption of the acids. The advantage of using heptane in lieu of benzene is illustrated
The carbon blacks used are given in Table I with accepted symbols which will be used in this report. The blacks mere not dried or otherwise purified before use. Preliminary experiments were performed using P-33 and acetylene black; but further work with these blacks was canceled when it, was found that the extract from the P-33 colored the solvent, so much that the color of the indicator during titration could not be seen and that the acetylene black was so bulky that no free solvent remained for titration when a reasonable amount of black was used. The experimental procedure was as follows: 10 grams of the black were shaken for 2 hours with 60 ml. of solvent containing a known amount of acid in s cork-stoppered 4-ounce oil sample bottle. The bottle then was centrifuged for 15 minutes a t 1750 r.p.m. and 25 ml. of clear supernatant solution pipetted off. The solution mas evaporated in a beaker over a hot plate to remove the solvent, The acid residue was taken up in 25 ml. of warm ethanol and titrated with 0.1 N sodium hydroxide solution using phenolphthalein as the indicator. An experiment without acid was run with each black and the other titrations were corrected accordingly. In the cases of the LB and C blacks it was not convenient to use 10 grams for the absorption experiment because of their large bulk. Five grams of LB black and 2.6 grams of C black were used.
‘0
1
025
GPAUS
Figure 2.
AC13
350
075
PER
100 ML.
100 HEPTANE
125 SOLJTIOY
$50
A7
I75
2
t
EQUILIBRIUM
Adsorption of Acids from Heptane by MPC Black on Weight Basis
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1948
WETTING EFFIClENCY
0,0100
According to the Gibbs equation,
X
0.0090 X
Y
v
0
0.0060
0
f
where
0
X
-
O
A,.
0
o
m
00050 w Q
0
00040
.Hb
00030
i
0
Q 4
$
--kr C
1
STEARIC ACID P O L E I C ACID APALMlTlC ACID OMYRlSTlC ACID O L A U R I C ACID XP'PELARGONIC ACID
2 O0020b
is
00010
01 0
0010
0.020
0030
MOLES
Figure 3.
0040 PER
0050
0060
OF
LITER
0070
SOLUTION
0080 AT
0090
0100
0110
-'
d = change of surface tensiou a t interdc face with change of concentration of solute in solution, k = constant for isothermal adsorption, r = molal surface concentration of solute per unit area of interface (degree of adsorption), and c = molal concentration of solute in solution, the acids which are adsorbed by channel black to the same extent from heptane solutions of equal molal concentration cause the same decrease in surface tension at the interface between the channel black and the heptane. Such must be the case practically with the acids studied here in view of the near coincidence of the adsorption curves in Figure 3. Presumably, the same relation holds when rubber is the solvent instead of heptane, so that these acids should have almost an equal wetting action for channel black when they are dissolved in rubber in even molal quantities.
h 0
00070
w
-
dc
X
a
3
(Ir
0
00080
5
B
2327
0
EPUlLlERlUM
Adsorption of Acids from Heptane by MPC Black on Molal Basis
A11 experiments were conducted a t room temperature, which ranged between 77"and 87' F. I
RESULTS
1
f
10.01
The adsorption isotherms for the fatty acids on channel black using heptane as the solvent are plotted in Figure 2. It is evident that on a weight basis the adsorption tends t o be higher for the acids having the higher molecular weights. However, on a molal basis, as shown in Figure 3, the adsorption isotherms almost coincide. The adsorption isotherms for stearic acid from heptane on the different blacks are plotted in Figure 4. The various blacks differed widely in their absorption of stearic acid. The C black adsorbed far more stearic acid than the rubber blacks; in the case of the rubber blacks, adsorption decreased in the following order: CF, MPC, H M F , SRF, LB, MT.
-.
C
I
3l$p 01
o
a25
050
075
IM)
125
ORAMS BTEARIC ACID PER IQO ML. HEPTANE SOLUTION AT
175
IM
l
EauiLieRiuM
CORRELATION WITH CLASSICAL EQUATIONS
The classical equations for adsorption isotherms are the Freundlich equation and the Langmuir equation, expressed as follows: Freundlich equation: Log
m
= A
Langmuir equation: 2 '= -
c
+ B log c + DC
m
where x / m = grams acid absorbed per 100 grams black a t equilibrium; c = grams acid in 100 ml. solution; and A , B, C, and D are constants. The experimental values for adsorption of stearic acid from heptane on HMF black, pelargonic acid from heptane on MPC black, and stearic acid from cyclohexane on MPC black were substituted in these equations and the resulting data plotted as shown in Figures 5 and 6. If the data fitted the equations exactly, the points would fall on a straight line. Actually, straight lines were obtained with the Freundlich equation, but curves were obtained with the Langmuir equation. The curves from the Langmuir equation approached straight lines a t the higher concentrations.
5 GRAMS
STEARIC ACID
Figure 4.
PER
IOOML.
HEPTANE
SOLUTION
AT
EauiLiauiuM
Adsor tion of Stearic: Acid from Heptane Various Blacks
2328
INDUSTRIAL AND ENGINEERING CHEMISTRY
rb STEARIC ACID X
ia
7/
Vol. 40, No. 12
PELARGONIC
FROM H C P T L N C
O Y blVi
BLACK
AC D F R C M H E P T A Y E O N M P C B L A C K
0 STEARIC A C I D F R O M
CYCLOWEXANE ON MPC BLACK
A STEARIC ACID FROM HEPTANE ON H M F B L A c n Z PELARCONIC ACID FROM H E P T A N E ON MPC B L A C K 0
0 STEARIC A C I D FROM CYCLOHEXANE ON MPC B L A W
00
IO
12
14
LOC(6
Figure 5.
B
16
10
i
' 2 u
Ij;
10
12
I+
1.6
II
18
Plot of Data in Langmuir's Equation
microscope ( 7 , 26). The tabulated data indicate t h a t the dif-ferences in stearic acid adsorption may be accounted for either on the basis of surface condition as exemplified by pR, or on t'he basis of surface area. The higher the pH and/or the greater the surface area, the more stearic acid Kas adsorbed. From Figure 7, which is a plot of stearic acid adsorption from solutions of the same equilibrium concentrat,ion against surface area, i t appears that the logarithm of the adsorption varied directly as the surface area for blacks in the same pH range-that is, for blacks having pH 3 to 5 on the one hand a,nd blacks having p H 8 to 10 on t h e other hand.
TABLE 11. RELATIOXOF STEARIC ACID ADSORPTIONTO PROPERTIES OF BLACK
34
Stearic Acid Adsorption GramsI100 Grams BlackQ 0 09
J3
Black 32 Y AREA
n
BY GAS
5
6
08
IO 0%
-g -j0 8 9
0.6
D l
Figure 6.
100)
Plot of Data in Freundlich's Equation
02
01
0
40
e?
120
SO
ZOG
S q u d r e N i e t e r s s u r f a c e A r e a p e r G r d m of
ZSO
Black
Figure 7. Effect of Surface Area on Stearic Acid Adsorption from Heptane Heptane solution containing 0.50 gram stearic acid per 100 ml. at equilibrium
FACTORS INFLUENCING ADSORPTION
The wide differences in adsorption of stearic acid by the different blacks shown in Figure 4 can be explained a t least' partially by the differences lrnon-n to exist between the blacks in surface area per unit w i g h t and in surface condit,ion. One expreseion of surface condit.ion is the change in pH induced in distilled water by the black. The p H of the individual blacks Tvas determined by the proccdurc used by the Columbian Carbon Company ( 6 ) . This consisted of boiling a small sample of the black for 30 minutes with sufficient water to give a thin slurry. After cooling to room temperature the clear supernatant liquid was poured off and the remaining sludge m-as stirred unt,il uniform. The p H of the sludge was determined by means of a glass electrode. Table I1 shovis the relations between stearic acid adsorption, p R , and surface area. The surface areas TTere computed from the published dat,a on part,icle sizes obtained with the electron
pH
Surfacp Area, Sq. Meters/Gran?, 12
hlediuin thermal Lampblack 0.22 34 Gemireinforcing furnace 0.36 40 High modulus f u r n a c e 0.62 fi4 Conducting furnace 2.16 03 Medium processing channel 120 1.39 Color 10.1 260 a I n equilibrium with stearic acid solution in heptane containing 0.50 gram of stearic acid per 100 ml. solution.
Included in Figure 7 are points representing surface areas obtained from gas adsorption measurements ( 1 , 9 , WS). I n every case these surface areas are less than the corresponding areas calculated from measurements on electron photomicrographs. Hoviever, these ,points do n o t change the relation mentioned above; this relation was unexpected and no esplanation can be given for it a t the present time. I t had been aaticipated that the amount adsorbed rather than its logmit,hni would be proportional to the surface area for blacks in the same p1-1 range. SURFACE AVAILABLE FOR ADSORPTION
The amount, of stearic acid which could be adsorbed per 100 grams of black if a monoinolecular layer were formed over the entire surface of each black particle, can be calculated as follows: 5
-
W t max.
=
M.S. ____ 1 0 2 2 S.a
\There 4f = molecular weight of stearic acid, 284.6; S = surfacc? area of black in square meters per gram; AT = -4vogadro's num-
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
2329
dispersion consists of examining the black dispersion in batches which are identical except for the presence of fat acid. Goodwin and Park ( I S ) and Barron (2) attempted to do this by investigating the properties of vulcanized Hevea stocks which differed only in fatty acid content; but it does not seem t h a t reliable conclusions can be obtained with this approach because fatty acid is involved in the accelerator reaction and therefore affects the state of cure. The difference in properties resulting from the presence of fatty acid in stocks loaded with carbon black may be caused by the state of cure as well as the state of dispersion of the black. A better approach t o the problem was made by Parkinson ( 2 1 ) and Park and Morris (20). They determined the effect MAXIMUMSTEARIC ACIDADSORPTION of fatty acid on dispersion by comparison of mixing times and both TABLE 111. CALCULATED Maximum Stearic Acid Adsorption Surface macroscopic and microscopic examination of the raw stocks. Grams/100 Grams Black Covered, However, their conclusions were at variance: Parkinson found Black Calculated Found %" that channel black mixed easily into acetone-extracted Hevea 0.23 9 Medium thermal 2.7 5 0.37 Lampblack 7.6 rubber and, as far as could be determined, dispersed as well as 6 0.50b Semireinforcing furnace 8.9 in the untreated rubber; Park and Morris, on the other hand, 14 7 High modulus furnace 1.Ob 21 4.0b 19 Conducting furnace concluded that acetone-extracted Hevea rubber is a poor medium 7 Medium procejaing channel 27 2.0b 19 58 10.8b for the dispersion of channel black, and that stearic acid and Color certain other'materials improve the dispersion. I n view of the a 4ssuming monomolecular layer. b Estimated from projection of curves in Figure 4. contradictory conclusions of these investigators, it was felt t h a t further work in this field would not be amiss. The present authors decided t o use a series of objective tests OF STEARICACID ADSORPTIONS FROM TABLE IV. COMPARISON to evaluate the state of dispersion of M P C and C F blacks in HEPTANE SOLUTION AND BENZENE SOLUTION Butyl rubber (GR-I) in the absence of and in the presence of Stearic Acid Adsorbed per 100 Grams stearic acid. Butyl rubber was selected as the test medium Black From heptanea From benzenea instead of Hevea rubber because i t is more stable and contains Conducting furnace black 2.24 0.79 less nonrubber constituents. 1.44 0.41 iMedium processing channel black The Butyl rubber contained 1.2570 fatty acid computed as a Contained initially 1.01 grams stearic acid per 100 ml. solution. stearic acid. Apparently this fatty acid was largely in the form of a zinc salt inasmuch as 0.10% zinc also was present. According to Park and Morris, 1 part zinc stearate is sufficient to obtain improved dispersion of channel black in Hevea rubber initially The data reveal t h a t the stearic acid occupied less than 10% containing about O.6Y0 extractable material. They found the of the surface area of all blacks except C F and C blacks for which same for stearic acid, but also noted t h a t 4 parts stearic acid a value of 19% was found. produced still better dispersion. For t h a t reason and for convenience, the present authors elected to use unextracted Butyl SIGNIFICANCE O F DATA rubber and observe the effect of additional stearic acid on the dispersion of carbon blacks. It should be kept in mind that carbon blacks do not adsorb the Six properties or characteristics of the compounded rubber same amount of fatty acid from different solvents, as is shown in were used t o evaluate the state of dispersion. They were rate of Figure 1 for adsorption of stearic acid from solutions of stearic incorporation, viscosity, plasticity, electrical resistance, bound acid in cyclohexane, heptane, and benzene. It is shown also in rubber, and tensile strength. Table I V that C F black and M P C black adsorb approximately three times as much stearic acid from heptane as from benzene RATE OF INCORPORATION at equal initial concentrations. Consequently, the adsorption values given in this paper do not apply in a quantitative sense THEORY.Because stearic acid is adsorbed to some extent on t o the adsorption of fatty acids by carbon blacks in rubbers,natural channel black and conducting furnace black, the presence of or synthetic. There is no reason, however, for doubting that stearic acid in the rubber during incorporation of these blacks they do apply in a qualitative sense. brings about a reduction in the interfacial tension between the black particles and the rubber. If this reduction is a considerable of the original tension, then the stearic acid is a n active percentage E F F E C T O F STEARIC ACID ON DISPERSION O F wetting agent and should clearly hasten the incorporation of the CARBON BLACKS IN BUTYL RUBBER black. Having established t h a t carbon blacks do adsorb fatty acids, On the other hand, a faster rate of incorporation does not there remains the basic problem of whether or not fatty acids necessarily prove that stearic acid aids dispersion. Since the improve the dispersion of carbon blacks in rubber. Before black probably enters the rubber always in the form of agmixing into rubber, carbon black particles are in contact with glomerated particles, a good softener will increase the rate of each other forming graphlike clusters or chains (7), and much of incorporation by reducing the viscosity of the rubber. A more the remaining surface of the particles is undoubtedly covered fluid rubber is better able t o surround agglomerates even though with multilayers of adsorbed oxygen and other gases. I n order the outer surfaces of the agglomerates are not wetted by the to obtain complete dispersion in rubber, the contact between rubber. It follows that a substance which is both a softener and adjacent particles must be broken and all layers of adsorbed gas, a dispersing agent should facilitate incorporation of the black except perhaps the first layer, must be removed. Maximum more than a substance which is merely a softener. reinforcement of the rubber from the standpoints of tensile PROCEDURE. I n evaluating the dispersing power of stearic and tear strengths is obtained with complete dispersion of the acid for black by this means, allowance must be made for the black-that is, each black particle entirely surrounded by a softening effect of the acid. This can be done best by confilm of rubber. ducting control experiments wherein paraffin is used in lieu of Thedecisive test to determine whether or not fatty acids improve stearic acid. It will be shown later that paraffin is a somewhat ber, 6.06 X, and a = molecular area of stearic acid, about 21 square Angstroms. This calculation was performed for each of the blacks and the results are given in Table I11 with the optimum adsorption values found or estimated by projecting the curves in Figure 4, and the percentages of the surface area of the blacks which are occupied by stearic acid a t optimum adsorption. Figure 4 shows that only M T and L B blacks reached optimum adsorption in the range of stearic acid concentrations used.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2330 TABLEV.
Vol. 40, No. 12
stearic acid over paraffin; both materials incmased the ratc of FORXCLATIONS FOR RATEOF INCORPORATION incorporat'ion of the MPC black to the same extent, I n the case MEASUREMESTS of CF black, no experiments were performed with paraffin after
-A
Butyl rubber X P C black C F black Steario acid Paraffin
B 100 50
100 50
.. , . ..
Stock Designation C D 100 100 50 50
..
.. ..
, .
3
..
..
3
4
E
-__
100
F 100
50
50
..
'i
... I .
, .
it, was found that stearic acid was of so little benefit. These data indicate that stearic acid is not a dispersing agent for MPC black or CF black in Butyl rubber in the true scrire of t h e t,erin. VISCOSITY
RESCLTS O F RATEOF IKCOEPORATION EXPERIMENTS
TABLEVI. Black 3IPC MPC MPC NPC CF CF-
Stearic Acid, Parts 0
3 0 0 0
3
Paraffin, Parts 0 0
KO.
Average Time t o Incorporate Black, Seconds 250 230 230 240 260 230
of
Experiments 8 8 5
3
4 0 0
4 4 3
better softener for Butyl rubber than stearic acid; but paraffin would not be expected t o reduce t h e interfacial tension appreciably as it contains no polar groups? and therefore it should not act as a wetting agent for the black particles. The rates of incorporation of the black in the formulations given in Table V were determined. The experimental procedure x a s as follows: Cold water was turned full into both rolls of B 4 x 9 inch mill. The opening betneen the rolls was adjusted to 0.035 inch and 80 grams of Butyl rubber mere banded around the back roll. A small rolling bank was obtained. The Butyl rubber was allowed to roll for exactly 5 minutes by stopwatch; during this time the stearic acid or paraffin, if any, was addcd. Just before the end of the 5-minute period the band was given several cuts, alternating from each side. At the end of this period 40 grams of MPC black or CF black were added t o the bank. The opening between the rolls was increased t o 0.047 inch after 7 minutes. The band was not cut after adding the black to the bank; any black which passed between the rolls and fell in the pan was m e p t up hurriedly and replaced on the bank. The elapsed time when all of the black was incorporated and the band became glossy was noted. Several repeat experiments were run with each formulation. The temperature of the rolls was determined n-ith a band thermocouple immediately after incorporation of the black. The temperature of the front roll ranged from 68" to 79 O F. and of the back roll ranged from 74' to 84 ' F. Two measurements were made of the actual temperature of the Statex A stock by wrapping the sheet around a thermocouple; the values found were 104 and 106 F.
RESULTS. The incorporation times and other pertinent data are presented in Table VI. These data show no advantage of
THEORY.A poorly dispersed black is generally assumed t o form a structure or netiTork through the rubber matrix. Support for this assumptioil is given by the work of Bulgin (6) who has shown that the electrical conductivity of natural rubber containing acetylene black, ~t black noted for its structure, is decreased by remilling. The remilling improves the dispersion-that is, the contact between the black particles i s broken and the particles are surrounded with a n insulating layer of rubber. It is logical that the presence of structure in a colloidal dispersion will bring about a marked increase in the viscosity of the matrix. Here, then, is a method for evaluating the dispersing effect of st'earic acid; if the stearic acid reduces the viscosity of a rubber-black mixture, it would seem that the dispersion of the black is improved. Actually, the relation is not this simple ai, stearic acid is an effective softener and therefore reduces the viscosity of the rubber whether or not the b!ack is better dispersed. Control experiments utilizing paraffin are again needed. PROCEDURE. I n these experiments the effects of 5 parts paraffin or 5 parts stearic acid on the Mooney viscosity (17) of stocks containing no black and containing 50 parts MPC black or 50 parts C F black were determined. The formulations are given in Table VII. Two bat'ches each of Stocks A, B, and C and three batches each of Stocks D, E, F, G, H, and I were prepared. TABLEVII.
FORIWI~ATIONS FOR EVALUATIOKS A 100 ..
Hutylrubber MPC black C F black Paraffin Stearic acid
, ,
, .
..
B 100
.
, , ,
5
..
Stock Designation C D E-B 100 100 100 100 .. 50 50 50 , . ., .. ..
..
5
..
0
. . . .
, .
5
OF
DISPERSION
G
H T 100 100
100 ,
..
.
50 ,.
.
.
.
50 6 ,
50
..
5
The stocks mere mixed in a Model-B Banbury, using rotor Speeds of 68 r.p.m. and 77 r.p.m., respectively. The batch weight of each stock was adjusted to obtain a calculated finished batch volume of 1250 ml. The mixing cycle for all stocks v a s 10.5 minutes, with a 2-minute cooling period between batches. The ~~
TABLEVIII.
Stock A
Softener Kone
Batch 30.
value
None
1 2
43 \
B
Paraffin
Sone
Stearic acid
h-ont.
11
None
MPC
E
Paraffin
H I
Stearic acid
MPC
None
cr
Paraffin Stearic acid
Mooney Viscometer Viscoaity Rating -___-
Carbon Black
c
F
DATAFOR EVALUATISG DISPERSION OF CARBON BLACKS IN BUTYL RUBBER
CF CF
Batch
1
2
"I
1
32I
65
2 3
63 64 69'1
2
1
2 3 1 2
3
44
188\ 1071
2
7
Batrh
value
Bound Rubber, % Rntch
Averagr
Value
Average
17.4
17.8
Tensile Strength, Lb./Sq. Batch value Average
193
71 70 t
.4...1
37 81 84
68 71
326'
329,
322 331' 328, 330q 281
71
531 57 54 551
i
326
3.6 3.2 9.5 6.9 6.2
330
54 53 '"I,
, .
3.6 7.5
19.4 20 3 17.1 1.4 0 8
18.9
31.4 28.0 14 0 110 0 8.2 26 9
24.4
0 8
1
2 3 I
Suecific Rehivitv, Ohms ( X loG)
35 36 371 79
1
Average
Goodrich Plastoineter Stiffness Rating Ratch value Average
1.0
48.3
17.4 18.7 18.7 17.7 16.7 15.7 l5,l 14.8 13.5 15.4 16 1 20.7 18.6 21 6 13 5 16 4 11.0
17.7 15.2 1.5 0 20.3 13 6
49.1 32.1
53.4 40.7 45.5 42.1 43.1 45.8 42.2 29.7 30.2 30.5 25.2 27.4 26.3 25.4 26.2 28.1
51.5 42 8 43.7 30.1 26.3 26.6
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
Banbury was kept as cold as possible during mixing; the maximum stock temperatures according t o the thermocouple in the Banbury ranged from 137' t o 144" F. for the stock without added black and ranged from 150 O to 167 O F. for the stocks with added black. The black stocks without paraffin or stearic acid reached the higher temperatures. The stocks were sheeted off on a laboratory mill, then conditioned a t 82 F. for a t least 9 days befor testing. The Mooney viscometer was maintained at 212 O F. by blowing live steam through the platens. The rubber samples were warmed for 1 minute in the chamber before starting therotor. After the rotor had been turning for 5 minutes, readings were taken. The average reading in the next 5 minutes was recorded. RESULTS. The viscosity data for the 24 batches are given in Table VIII. These data show the following facts: MPC black increases the viscosity more than C F black. This is reasonable because of the finer particle size of the former black. Paraffin and stearic acid both decrease the viscosity of Butyl rubber. Paraffin is the better softener. Paraffin and stearic acid both decrease the viscosity of Butyl rubber containing either MPC or C F black. Paraffin is again somewhat more efficient in this respect than stearic acid. There is nothing in these data which indicates that stearic acid is a dispersing agent (as here defined) for carbon blacks. If stearic acid were a dispersing agent, the black stocks containing stearic acid would have been less viscous than the black stocks containing paraffin; this was not the case. PLASTICITY
THEORY.The theory for the effect of dispersion on viscosity applies here. Plasticity is probably a better property for showing the effect of poor dispersion than is viscosity because in the act of determining viscosity the rubber is churned, and this churning may improve the dispersion somewhat. The presence of structure, an evidence of incomplete dispersion, is signified by low plasticity ('7). If stearic acid is a dispersing agent for carbon blacks, it should yield softer stocks than the control material, paraffin. PROCEDURE. The plasticities of the nine stocks formulated in Table VI1 were determined with the Goodrich Plastometer (14). The samples, from which the cylindrical test specimens were cut, were molded from the original rough mill sheets. A positive mold was used and the molding time was 90 minutes a t 230" F. This procedure was necessary because uniform cylindrical specimens could not be cut from the rough mill sheets. The molding was done not less than 3 days after mixing, and the plasticities were determined 7 days after molding. The stocks were conditioned at 82' F. during the interim. The plasticities were determined a t a temperature of 100" F. The specimens were conditioned a t this temperature for 30 minutes and then subjected in turn to a load of 4 pounds (twice the usual plastometer load). Four minutes after applying the load, the thickness of the specimen was noted on the dial micrometer. Three specimens from each batch were tested.
RESULTS.The stiffnesses of the stocks (thickness of specimens in inches) are shown in Table VIII. These values are related inversely to the respective plasticities. These data lead to the same conclusions regarding plasticity as were given for viscosity. There is no indication that stearic arid is a dispersing agent for carbon black. ELECTRICAL R E S I S T A N C E
THEORY.It seems that the measurement of electrical resistance should be a direct method for evaluating the dispersion of carbon blacks. According to the work of Bulgin (6) electrical resistance is enhanced if the black particles are well dispersedthat is, surrounded by a film of rubber. Contrariwise, electrical resistance is diminished if black particles are poorly dispersedthat is, contact each other forming a chainlike structure through the rubber. Thus the directional effect of stearic acid on the
2331
electrical resistance of the carbon black compounds should show whether or not stearic acid improves dispersion. Stocks D, E, F, G, H, and I were used in this PROCEDURE, experiment. Four slabs, 6 inches s uare by 0.08 to 0.09 inch thick, were molded from each of the 8 r e e batches of each stock. This gave a total of 12 slabs from each stock. The slabs were prepared in a 4-cavity mold using a ram pressure of 70 tons, a temperature of 230" F., and a hot pressing time of 90 minutes. The mold was cooled before relieving the pressure. The slabs were talced and conditioned on galvanized trays for 7 days at 82" F. to permit internal stresses t o relieve themselves. After conditioning, one face of each slab was scrubbed with a rag wet with heptane and a sheet of lead foil, 0.0015'inch thick, was rolled down on the tacky surface, covering the entire face. This formed the bottom electrode. The top electrode was a circular sheet of lead foil, 4 inches in diameter, attached to the center of the opposite face in the same manner. A guard electrode was found t o be unnecessary. The rubber slabs with the attached electrodes were connected individually to a simple circuit consisting essentially of two 6-volt storage batteries in series, a voltmeter, and a microammeter. The current flowing through the rubber and the voltage drop across the rubber were noted. It was found necessary in some cases to insert a known resistance in series with the rubber to keep the ammeter readings in the same range. After the current and voltage measurements were made, the thickness of the rubber slab under the top electrode was measured at a number of points and the average value calculated. The specific resistivity of the rubber then was computed from the voltage, the current, and the average slab thickness. Allowance was made in this computation for other resistances in the circuit.
RESULTS. The data, presented in Table VIII, show considerable variability between the specific resistivities of replicate batches. The specific resistivity of the individual slabs from each batch, not recorded here, showed a similar variation. Nevertheless, the averaged data show a definite trend; both paraffin and stearic acid increased the resistances of the MPC and C F stocks, the latter to the greater extent. Whereas in the absence of these materials the resistance of the MPC stock was slightly higher than that of the C F stock, in their presence the CF stock tended to have the higher resistance. These results seem to indicate that both paraffin and stearic acid improved the dispersion somewhat, with stearic acid being the better dispersing agent. However, another and more probable explanation is that paraffin and stearic acid did not affect the initial dispersion but hindered to some extent the flocculation of black particles which occurred during the molding and conditioning periods. That this flocculation was taking place was evidenced by the gradual increase noted in the conductivity of the slabs after molding. The higher resistances in the presence of paraffin and stearic acid could have been due to the adsorption of these materials on the surface of the black particles. The adsorbed layers insulated each black particle from its neighbor. This suggests that paraffin was adsorbed by the blacks in spite of its nonpolar nature. ROUND RUBBER
THEORY.It has been reported that a mixture of rubber and carbon black is not completely dispersed by a solvent, but that the stock is swollen by the solvent and part of the rubber is extracted, leaving all of the carbon black in the swollen gel (4, 10, 94).
The rubber remaining with the carbon black is usually termed bound rubber. The proportion of bound rubber in volumes t o volume of carbon black increases as the particle size of the carbon black diminishes (7). In view of the relation between bound rubber and particle size, better dispersion of the black particles should increase t h e bound rubber. A poorly dispersed black contains agglomerates which are in effect equivalent to unit particles of larger size. For example, an agglomerate of channel black particles might
2332
INDUSTRIAL AND ENGINEERING CHEMISTRY
have the same effective size as one particle of medium thermal black. Stocks D to I, inclusive, were used in this exPROCEDURE. periment. Two &gram samples of each bat'ch of each stoclr mere tested, giving a total of 6 samples from each stock. Each sample vias cut into strips with about 0.0625-inch square cross section, accurately weighed, and immersed in 100 ml. of heptane in a 125ml. conical flask. Once every day for 7 days the flask was swirled gently; t,hen the liquid was decanted off t,hrough a 150-mesh moncl screen, and the residue was washed with small portions of heptane which also were poured t,hrough the screen. The original liquid and washings were evaporated almost to dryness in a beaker on i hot plate and dried t o constant weight in an oven a t 230" F. This gave the total extractables. I t mas necessary to correct the total extractables for its content of stearic acid and paraffin. I t vias assumed that all of the paraffin in stocks E and H was extracted by the heptane; but this assumption could not be applied to t,he stearic acid in these stocks and t,he other stocks bemuse part of the stearic acid remained adsorbed on the carbon black. Stearic acid in the dried residue mas found by extracting the residue with 50 nil. of boiling ethanol €or 30 minutes, cooling, and titrat,ing with 0.1 S sodium hydroxide solution. The extracted rubber equaled the total extract,ables less the contents of stearic acid and paraffin (for stocks E and H). The bound rubber equaled the rubber initially present in the sample (corrected for zinc stearate) minus the extracted rubber.
RESULTS.The bound rubber data given in Table VI11 shorn that CF black held lees rubber than t,he MPC black; t,hat stearic acid reduced the rubber held by both blacks; and that paraffin had no effect on the amount of rubber held by AIPC black but actually increased the amount' of rubber held by C F black. I t appears that stearic acid hindered the dispersion of both blacks, and that paraffin improved the dispersion of C F black. Holyever, before accepting these inferences relative to the effect of stearic acid on dispersion, consideration should be given to a possible effect of stearic acid on bound rubber itself. A tentative description of this effect)is as follows: When stearic acid is adsorbed a t t'he interface between a black particle and rubber, any adhesion which previously existed in that area b e t m e n t,he rubber and the particle is disrupted and the rubber now adheres to t'he hydrocarbon tails of the stearic acid molecules which extend from the particle like pins from a pin cushion. When the sample is immersed in heptane, the molecules of this solvent intrude between the stearic acid and rubber molecules and dislodge some of the latter, thereby diminishing the percent'age of bound rubber. Thus, a decrease in bound rubber in the presence of stearic acid is not' necessarily an indication of poorer dispersion of the black in the rubber. The authors noticed that Stock I, containing CF black and stearic acid, acted differently in heptane than the other stocks. The strips of the other stocks were swollen severely although they retained their shape and the heptane was clear after filtering through t,he 150-mesh screen. The samples of Stoc,k I also were swollen severely but were so disintegrated that, the individual pieces no longer could be seen. The heptane from these samples was found to contain an appreciable amount of free carbon black after filtering. This black was removed by centrifuging and decanting before determining the total extractables. The authors are of the opinion that t,he behavior of Stock I in heptane confirms thc mechanism postulated for the effect of stearic acid on bound rubber. It appears, therefore, that bound rubber is no measure of dispersion when stearic acid is present. TENSILE STREhGTH
THEORY.The tensile strength of vulcanized Butyl rubber is not improved by admixture with carbon blacks, but is lowered in measure with the coarseness of the black ( 8 ) . The authors planned to use the latter relation for evaluating the dispersion of CF and LIPC blacks in Butyl rubber. The assumption was
Vol. 40, No. 12
that a poorly dispersed fine black would affect the tensile strength of the unvulcanized rubber like a. coarser black-that is the lower the tensile strength, the poorer the dispersion. It was esscntial to take into account the weakening effect of stearic acid on the tensile strength of the Butyl rubber. The stearic acid which was not adsorbed on the carbon black would remain in the rubber phase and a c t as a soft,ener. Consequently, even though the acid improved the black dispersion, the over-all effect would be to reduce the tensile strength. Again it was necessary to use paraffin as a control additive. Paraffin was expected t,o have no effect on thix disperaian, b,it approxirnatc~1,y the same wealreniny effect on the rubber as stearic acid. This was the most practical test for the dispersing power of stearic acid because it evaluated an ultimate strength property. The usual reason for desiring a n improved dispersion of carbon black is to obtain better tensile strength, tear resist'ance, or abrasion resistance. PROCEDURE.Stocks D, E, F, G, H, and I were used in this experiment. The mixing of these stocks in the Banbury and t,he molding of the 6-inch square slabs have been described. Four slabs molded from each batch were tested. Five A.S.T.M. type ii dumbbell specimens were dried from each slab and pulled to break on a Scott L-6 tensile tester. This made a total of 20 specimens tested from each batch and 60 specimens tested from each stoclr. The temperature of tensile testing ranged from 79"
to82"F. RESULTS.The tensile specimens did not stretch uniformly along their constricted portion up to break as do vulcanized specimens; instead they exhibited a yielding or necking at, one point in the const,ricted portion as do metals when stretched to break. Iievertheless, the tensile stre~igt~hs of replicate specimens were uniform. The data for the batches (average of 20 breaks) and for the stocks (average of 60 breaks) are given in Tablc VIII. The data indicate that paraffin lox-ered the tensile strenglh of the MPC st'ock about 1547,and lowered the tensile strength of the C F stock about 1370. The reduction in tensile st,rength mas slightly less pronounced in both stocks when stcaric acid was present instead of paraffin. This difference between the effects of paraffin and stearic acid is not believed to be significant, particularly in view of the facts that part of the stearic acid is adsorbed by the black, leaving less t o soften the rubbcr, and stearic acid is not as efficient a softener as paraffin according to the Goodrich Plastometer results. Consequently, it does not appear from the tensile data that stearic acid is a more cfficient dispersing agent, than paraffin. DI SCCSSION
Only electrical resistivity of the six properties or characteristics examined gave any significant indication of improved dispersion caused by stearic acid. However, as previously pointed out,, the effect' of stearic acid on resistance can be explained also by assuming that, the absorbed acid breaks the contact between adjacent black particles \\-ithout' bringing about more equal distribution of the particles. The clusters and chains of particles are still present in the mass of the rubber but stearic acid molecules have intruded between adjacent particles in these formations so that good elcct'rical contact is no longer obtained. I n view of this likely explanation and the negative results given by the other five experiments, the a1.ithoi-s conclude that stcaric acid does not improve the dispersion of hlPC black or C F black in Butyl rubber. Indirect, support for this conclusion is given by certain obscrvations made during the adsorpt'ion experiments using heptane, benzene, and cyclohexane. These observations, which must apply at least in some degree to viscous hydrocarbons like Butyl rubber, are the following: First, all of t,he blacks tested appearod t,o be easily Fvetted by the fluid hydrocarbons without the intcrvention of fatty acids. Secondly, only a small part of the surface
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
area of the blacks was covered by fatty acid at saturation. Thirdly, a considerable proport,ion of the fatty acid remained in the solvent phase a t room temperature, and this proportion would he expected t o increase at higher temperatures in accordance with the usual behavior of adsorption equilibria. The second and third points are particularly significant. If there is a similarity between adsorption from Butyl rubber and adsorption from fluid hydrocarbons, it would seem that only a small part of the surface area of each black particle is covered with fatty acid a t the high temperatures (approximately 300’ F.) attained in practice when tlhe finer blacks are mixed into rubber. This small amount of adsorbed acid on the surface could hardly he effective in dispersing the particle. The authors believe that the results of this investigation indicate that fatty acid is not effective for dispersing blacks in other rubbers, particularIy those rubbers which are hydrocarbons such as GR-S, Hycar OS-IO, and natural rubber. BIBLIOGRAPHY (1) Amon, F. ET., S m i t h , W. I?., and Thornhill. F. S., IND.ENG. CHEM.,ASAI.. E D . , 15, 256 (1943). (2) B a r r o n , H., Zndia Rubber J . , 90, 638 (1935). (3) Blake, J. T., IND. E N G .CHEM., 20, 1054 (1928). (4) Boiry, F., Rev. gen. Caoutchoz~c,8 , 108 (1931). (5) Bulgin, D.. Trans. Inst. Rubber Ind., 21, 188 (1945). (6) Colurnbiaii Carbon Co., “Columbian Colloidal C a r b o n s , ” p. 180 ( 1 9 3 8 .
2333
(7) Columbian Carbon Co., “The Surface Area of Colloidal Carbons,” (1942). (8) Drogin, I., India Rubber World, 107, 42, (1942). E N G . CHEM.,ANAL. ED., (9) E m m e t t , P. H., a n d D e W i t t , T., IND. 1 3 , 2 8 (1941). (10) Fielding, J. H., IND.ENG.CHEM.,29, 880 (1937). (11) G e h m a n , S. D., a n d Field, J. E., I h i d . , 32, 1401 (1940). (12) Glasstone. S., “Textbook of Physical C h e m i s t r y , ” p. 1200, New York, D. Van N o s t r a n d Go., 1946. (13) Goodwin, N., and Park, C . R., IND. CHEM., 20, 621, 706 (1928). (14) K a r r e r , E., Dairies, J. M.,a n d Dioterich, E. O., Rub6er Chem. Technol.,3, 295 (1930). (15) Langmuir, I., J . A m . Chem. SOC.,39, 1848 (1917). (16) Zbid.,38, 2221 (1916); 40, 1361 (1918). (17) Mooney, M . , Rubber Chem. Techno!., 7, 564 (1934). E N G .CHEM.,24, 584 (1932). (18) Morris, T . C., IND. (19) Office of Rubber Reserve, Specifications for Government Synthetic Rubbers, effective J a n . 1, 1947. (20) P n r k , C. R., a n d Morris, V. N., INTI. E N G .CHEM.,27. 582 (1935). (21) Parkinson: D., Trans. Inst. Rubber IntJus., C , 263 (1930). (22) R o h e i t s , K. C., J . Rzhber Research J n p t . MaZaya, 7 , 46 (1936). (23) Smith, W. R., Thornhill, F. S.,and B r a y , R. I., IND.ENG. CHEM.,33, 1303 (1941). (24) Stamberger, P., Kazctschu/:, 7, 182 (1931). (25) Sweitzer, C. W., a n d Goodrich, W. C., Rubber Age (Ar. Y . ) , 55, 469 (1944). (26) W h i t b y , Dolid, and Yorston, J . Chem. S oc.. 129, 1448 (1926).
RECEIVED February 18, 1947. The opinions or assortations in this article are those of the authors and are not to be construed as official or reflecting the views of the Navy Department or naval service a t large.
DIPHASE METAL CLEANERS Relation of Emulsion Stability to Cleaning Eficiency IRVING REICH . ~ N DFOSTER DEE SNELL Eoster D. SmW, he., 29 W e s t 15th St., New York 11, N. Y.
A
materials preferentially we A compariaon is made between t w o classes of metal SOLUTION of 11% of by water. Steel parts cleaned triethanolamine oleate c l e a n e r e t h e unstable emulsion type cleaner and the in this way tend to rust stable type. Metal cleaning tests and umber dispersion in 89% mineral spirits constirapidly and surfaces are not tests were performed. In both cases diphase cleaners were chemically clean. tutes a diphase cleaner when 2. Vapor degreasing inmore effective than stable emulsion cleaners. This greater mixed with water and propvolves condensation of solvent effectiveness is a result of the heavy film of solvent with erly applied. A stable emulvapors upon the surface of the M-hich this cleaner coats the metal surfaces and the sion can be produced by very metal. The condensed solability of the unstable emulsion cleaner not only to wet vent drips back into a bath. vigorous agitation, by the adThe objections mentioned the soil and &Each it from the surface to be cleaned but to dition of more soap, or by disabove applyto this method. In disperse and suspend the soil and prevent redeposition. solving the oleic acid in the addition solid soil is often left mineral spirits and the triasaresidue on themetal which ethanolamine in water, then must then he hand wioed. 3. Alkali solution cleaning. The metal parts are generallv mixing the two phases. In any of these cases the efficiency of the allowed to soak in an alkali bath with or without water-solubiet cleaner is lost. Thus, contrary to widespread industrial pracdetergent’s. Spray machines may also be used. For rapid and tice, unstable rather than stable emulsions should be sought. effective cleaning high pH values are used. This leads to hazards for operating personnel and to danger of metal surfaces being Diphase cleaners are capable of great flexibility in solving attacked. Contamination builds up rapidly in the bath, resulting problems raised by special types of soil, special metals, and in redeposition of soil. It is difficult t o remove the last traces of specifications for finished surfaces. They can be used to leave alkali from the metal surface, and these can increase susceptithe metal surface chemically clean and ready for plating or other bility t o corrosion. 4. Emulsion cleaning should be divided into two categories. finishing operations, and also to place a rust-protective film A . The metal is treated with a solution of soap or other emuIon the metal surface. Their use in the latter way had an imsifying agents in an organic solvent. The emulsifying agents may portant bearing on production of metal parts during the war. aid loosening and solution or dispersion of the soil. The metal parts are then rinsed with water and generally a pressure stream is used. The solvent adhering to the metal surface largely emulsiMETHODS OF METAL CLEANING fies and is washed away. The flammability or toxicity hazards Important methods of metal cleaning include the following: of solvent-cleaning processes apply. The metal is subjected to the .simultaneous action of two phases only for an instant during rinsing. 1. Treatment with liquid soIvent can be done by spraying, B. The metal is soaked in or sprayed with a more or less stable dipping, or hand scrubbing. I t removes oil and grease rather emulsion, generally one of kerosene or a similar hydrocarbon fraceffectively hut is subjkct to the drawback of fire hazard or, if chlorinated solvents are used, high cost and toxicity. Liquidtion in water, stabilized by soap. Excess alkali may be present. Subsequently the metal is generally rinsed by water. solvent treatment does not remove water-soluble materials or
