May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY ACKNOWLEDGMENT
The authors are deeply indebted to a number of workers whose generous cooperation has made much of this work possible. They thank H. S. Halbedel and the Harshaw Chemical Co. for the hydrogen fluoride treatment of various sugars; Emma J. McDonald, H. H. Schlubach, and M. L. Wolfrom for reference samples of difructose anhydrides and their derivatives; M. L. Wolfrom and W. W. Binkley for column chromatography; and J, K. N. Jones of the University of Bristol, England, for his paper chromatography of methylated glucose anhydride mixtures. They take pleasure in thanking Sylvan Beer for assistance and Francine Schwartzkopf for microanalyses. LITERATURE CITED
Binkley and Wolfrom, Sci. Report Series, No. 10, New York, Sugar Research Foundation, Inc., 1948. Consden, Gordon, and Martin, Biochem. J.,38, 224 (1944). Erb and Zerban, IND.ENG.CHEM.,39, 1597 (1947).
1135
Hann and Hudson, J . Am. Chem. Soc., 66, 735 (1944). Haworth, Hirst, and Learner, J . Chem. SOC.,1927, 1044. Helferich and Peters, Ann., 494, 101 (1932). Jones, private communication. Kraisy, 2. Ver. deut. Zucker-Id., 71, 123 (1921). McDonald, private communication. Partridge and Westall, Biochem. J., 42, 238 (1948). Pictet and Chavan, H e b . Chim. Acta, 9,809 (1926). Sattler and Zerban, IND. ENG.CHEM.,34, 1180 (1942). Ibid., 37, 1133 (1945). Ibid., 41, 1401 (1949). Sattler and Zerban, J. Am. Chem. SOC.,72,3814 (1950). Sattler and Zerban, Sugar, 42, No. 12,26 (1947). Schlubach and Behre, Ann., 508, 16 (1933). Trevelyan, private communication. de Whalley, Albon, and Gross, Analyst, 76, 287 (1961). Wolfrom and Blair, J . Am. Chem. SOC.,70, 2406 (1948). Wolfrom, Binkley, Shilling, and Hilton, Ibid., 73, 3553 (1951). RECEIVED for review February 23, 1951. ACCEPTEDDecember 21, 1981. Presented in part a t the 114th Meeting of the AMERICANCHEMICAL SoCIETY, Portland, Ore.
Soiling and Soil Retention in Textile Fibers PRIMARY DEPOSITION OF GREASE-FREE CARBON BLACK ON CHOPPED COTTON FIBERS W. J. HART AND JACK COMPTON Institute of Textile Technology, Charlottesville, Vu.
I
N .4previous paper ( 1 ) it was shown that the formation and stability of carbon black-cotton fiber complexes was depende n t upon geometric and dimensional relationships between the fiber surface rugosities and carbon black particles in the aqueous soiling dispersions. It was also shown t h a t the formation of such complexes is a two-stage process involving the primary deposition of individual soil particles on the fiber surface and then agglomeration of soil particles on the primary soil deposit-fiber surface during the subsequent drying of the soil-fiber mixture. The preeent investigation is concerned with the effect of the following variables on the rate of primary deposition of carbon black soils on cotton fibers and the stability of the soil-fiber complex: time of exposure to the soiling mixtures; concentration of the soiling mixtures; ratio of soil-fiber weight at constant concentrations of soiling dispersions; effect of varying turbulence of mixing during soiling; primary particle size of the carbon black; qualitative effect of particle size distribution; presteeping of cotton fiber in water over the p H range of 3.5 t o 11 5 ; action of surfactants in the soiling mixtures; and temperature effects. EXPERIMENTAL
General Method. The cotton fiber used in this work was derived from 80 X 80 standard print cloth, desized, kiered, and bleached. Strips of this fabric were chopped in a Wiley mill using a 20-mesh screen to give a fiber length range of 0.1 t o 1 mm. as previously described (1). Carbon black dispersions were also repared in a manner similar t o that earlier described ( I ) , by cofioid milling the dry carbon blacks with water and Daxad 23 or other dispersants. Several of the carbon black dispersions used in this work were prepared from commercial carbon black pastes by simply diluting with solutions of the various dispersants included in this study. T h e chopped fiber was slurried with the carbon black dispwsions and stirred continuously during the designated exposure
periods. At the specified times, aliquots were withdrawn from the slurry and immediately diluted tenfold with water. Classification ( 1 ) b y decantation with hand stirring was carried out until a clear supernatant li uor was obtained. The fiber slurry was then reclassified in a $aring Blendor, b y subjecting it t o a procedure consisting of 10 seconds of stirring, followed by 10 seconds of standing and a final 20 seconds of stirring. Decantation was then continued until a clear supernatant liquor was again obtained. A fiber pad was made on an open Buchner funnel from the reclassified fiber slurry, dried, and reflectances were determined with a Photovolt reflectometer, Model 610, calibrated with National Bureau of Standards standard reflectance plates. An unsoiled fiber pad reflectance was in t h e range of 82 t o 86. T h e reflectance values reported are the observed average values which were obtained by reading both sides of a pad. Results were reproducible within A l . 0 reflectance units. The fiber soiling technique as described apparently minimizes secondary soil deposition and/or agglomerative build-up of soil in the soil fiber complex, and thus permits the study of the kinetics of primary deposition of soil particles. Variables Affecting Rate of Primary Soil Deposition. TIME of exposure of the chopped cotton fiber to the soiling dispersion was used as the independent variable in this work, and this variable will be given consideration while studying the other factors previously enumerated. CONCENTRATION OF CARBON BLACK. The effect of concentration of carbon black in the soiling dispersion on the rate of primary soil deposition is shown in Figures 1 and 2. Neo Spectra black beads were dispersed in water with Daxad ax the dispersing agent (15% of the weight of the carbon black) using a colloid mill. (For carbon black concentrations of 0.5% or lws a solution of Daxad containing 1.5 grams per liter was used a,s the dispersant medium.) In Figure 1 the reflectance valuee obtained after 2, 5, 10, 20, 40, OF EXPOSURE, The time
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1136
t 0
Minutes)
Figure 1.
Effect of Soil Concentration on Rate of Change of Reflectance
0
Vol. 44, No. 5
I
\
2.0
1.0
3.0
CONCENTRATION ( 9 . p . I.)
Figure 2.
Effect of Soil Concentration on Time Required to Obtain a Reflectance of 20
80, 180, 360, and 1440 minutes a t soil dispersion concentrations of
0.01, 0.02, 0.05, 0.10, 0.20, 0.5, 1.0, and 3% are given. The times required for the primary deposition of the carbon black on the chopped cotton fiber to give a reflectance value of 20 for Keo Spectra black dispersion concentrations of 0.20, 0.5, 1.0, and 3.0 are given in Figure 2. It is apparent that the rate of primary soil deposition increases as the carbon black soil concentration in the soiling dispersions increases (Figure 1)and that the time required to lower the reflectance of the cotton fiber from 82 to 20 decreases as the concentration of the soil dispersion increases (Figure 2). The relationships in neither case are linear. This may be regarded as further evidence t h a t the process of primary soil deposition is complex. A mechanism which qualitatively accounts for these phenomena v ill be given consideration.
-._ 50
40
=
-
@= I 5 0 0 ml. 8. 2 0 0 0 m i .
Neo Spectra black beads were colloid milled in an aqueous solution of 0.15% Daxad to give a concentration of 1% carbon black by weight. In the first study, 50 grams of dry chopped cotton fiber were niixed with 500, 1000, 1500, and 2000 ml, of the standard soit dispersion. Roil deposition rate curves were obtained by plotting time of fiber exposure to the soil dispersioii against classified fiber pad reflectance. It was found that a large decrease in deposition rate occurred when the volume of the soiling dispersion was increased from 500 to 1000 nil., whereas a t volumes above 1000 ml. a sloadecrease in deposition rate occurred as the volume increased (Figure 3). Since it is well known that cotton imbibes water readily, it wm first thought that. t,he posjible resulting increase of soil conceiitration in the dispersion wa8 responsible for the change in the rate of soil deposition on the fiber. The data obtained indicate that this factor m.ay be operative. However, as will subsequently be shown, it cannot be considered the entire explanation because of the complex nature of the system. To test the hypothesis that fiber imbibition of water causes a n increase in soiling dispersion concentration, thereby affecting the rate of soil deposition, a second series of tests were run in a-hich
20
0' 5 0 0 m i . 0 : 1000 ml. @- l500ml.
10
e8
i
TIMLIE ( M i n u t e s )
Figure 3. Effect of Soil Dispersion Volume on Kate of Change of Reflectance of Predried Fiber
The increasing rate of soil deposition, with increasing concentration of soil appears t o be characteristic of soil-fiber systems, since three other blacks of varying particle size, namely, Aquablaks B, H, and Molacco Black, were shown t o behave in a similar manner. CONCENTRATION OF CHOPPED COTTONFIBER. The effect of varying the soil-fiber weight ratios a t constant soil concentration was next investigated.
2 0 0 0 ml.
TIME
(Minutes)
Figure 4. Effect of Soil Dispersion Volume on Rate of Change of Reflectance of Water-Presteeped Cotton Fiber
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1952
200 grams of chopped cotton fiber were steeped in distilled water overnight; the excess moisture was removed on the Buchner funnel by pad formation. The fiber pad was then divided into four parts and soil deposition rate curves were determined a t four fiber slurry-soil dispersion volumes as described for the first series. These data are summarized in Figure 4. Soil deposition rates for soil dispersion volumes over the range of 1000 t o 2000 ml. agree with one another within experimental error, but once again there wm a large positive deposition rate differential when the soil dispersion volume was decreased to 500 ml. The concentration of the dispersing agent in the soil dispersions is a n important factor affecting fiber soiling rates and is probably related to the known stabilizing effect of the dispersing agent on the carbon black dispersions. It was suggested that at high fibervolume ratios, the fiber was adsorbing the dispersant, thereby changing the dispersion stability and leading t o increased deposition rates. With this possibility in mind a third series of tests were run in which 100 grams of cotton were steeped in 0.15% Daxad solution overnight, the excess liquor being removed on the Buchner funnel. Two rate curves were run on slurries prepared by mixing half the pad with 500 and 1000 ml. of the dispersion, respectively. The data are summarized in Figure 5. Once more a significantly greater soiling rate may be seen for the smaller volume.
-
I I11111l~
I I IIIIII]
I I I IIIIII
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VARYINGAGITATION TURBULENCE DURING SOILING. I n view of the foregoing, it was thought of interest t o further determine the magnitude of the effect produced by varying the violence of agitation during the period of soil deposition. I n this test, two identical fiber slurries were prepared using 50 grams of chopped cotton fiber by mixing with a liter of a 1% dispersion of Ne0 Spectra black in each case. I n one case, a variable speed mechanical stirrer was operated continuously at about 1000 r.p.m. in the soiling mixure; in the other case the slurry was stirred by hand occasionally with a stirring rod. The rate curves obtained are shown in Figure 6. After the initial mixing the soiling rate of the continuously stirred fiber slurry was much greater than for the one lightly and intermittently stirred.
I I IIIIIll
I I I I I1111
I
I
I I I1111
NSB-1%
30 W
-1 y.
w
a
’
-
Volume I%NSB. Dispersion
b = l O O O ml.
TIME
Figure 6.
0
-
I
I I I11111
-
TIME
I I I I I IIII
s
11111111l
8
I
2
(Mlnules 1
Figure 5. Effect of Soil Dispersion Volume on Rate of Change of Reflectance of 0.15% Daxad-Presteeped Cotton Fiber
Since the factors of soil concentration and stability were thus eliminated, the most likely factors remaining which might cause this effect are poMible changes of frictional forces and hydraulic stresses at the fiber surface as the slurry is diluted. It was noted that the slurries containing 50 grams of chopped cotton fiber in 500 ml. of soil dispersion were much thicker and lumpier in appearance than those of 1000 ml. or more in volume. The latter slurries had properties comparable t o those of free flowing liquids. During the stirring, friction between soiled fibers could break up weakly agglomerated soil flocculates deposited on the fiber surface and thereby increase the probability of soil deposition owing to an increase in the number of small particle aggregates. Soil exchange on the fiber surface would also be accelerated by the same action by displacement of weakly held particles from acceptor sites on t h e fiber surface.
(Minuter)
Effect of Agitation of Soil-Fiber Slurry on Rate of Changes of Reflectance
PRIMARY PARTICLE SIZEOF THE CARBON BLACK. It was found in previous work (1) that the primary particle size of the soiling black was a major factor in soil deposition and retention. (The average particle sizes of the carbon blacks used in this previous study were erroneously stated t o be in microns instead of the correct units, millimicrons.) To confirm this conclusion the rates of deposition of 13 carbon blacks whose primary particle size ranged from 13 t o 120 mp were investigated. Figure 7 summarizes t h e data. I n these tests, 1% carbon black soil dispersions were employed in preparing slurries containing 50 grams of dry chopped fiber per 1000 ml. of soil dispersion. The four curves are for different soiling times. A sharp break in each deposition curve occurs with the carbon black dispersion of primary particle size of 50 mp. When the primary particle size of the carbon blacks is greater than 50 mw the rate of primary soil deposition becomes small and much less affected by the particle size. Below 50 m p , soil deposition increases rapidly as primary particle size diminishes. This result would be expected from the known fine structure of the cotton fiber surface as shown in electron micrographs by various authors (a). Rugosities of 50 mp diameter and more are not common, but a very large number of crevices of lesser diameters exist. These data also support the theory of micro-occlusion as a major factor in permanent soil deposition and retention previously advanced ( I ), since onlyparticles whose diameters were less than t h a t of holes and rugosities in the fiber surface could be mechanically entrapped. PARTICLE SIZE DISTRIBUTION OF SOIL DISPERSIONS. It has already been shown t h a t the rate of primary soil deposition increases with the concentration of the soiling mixture. It has likewise been shown t h a t the rate of soiling increases as the particle size diminishes for primary particle sizes less than 50 mp.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the centrifuged carbon black dispersion-cotton fiber system has a lower soiling rate than the original dispersion but is considerably higher than 75 either of the two dispersions of equivalent carbon black contents, which may reasonably be assumed to have flatter distribution curves. The decrease in soiling rate as compared with that obtained with the original uncentrifuged carbon black dispersion is thought to be due t a a "sweep50 0 out" effect occurring during centrifuging, which z 4 would reduce somewhat the concentration of r w 0 h e particles. 1 PRESTEEPING C O T T O N FIBERS WITH WATER O F U W C : 5 min DIFFEREXT pH VALUES. In the work dealing E 25 w t h soil-fiber weight ratio, it was noted that 9 : 20min @ = leomin Iyater-presteeped fiber gave a lesser soiling rate than dry fiber. This effect was considered to be 8 360min largely due either to imbibition of water by the dry fiber which increased the concentration of the 0 carbon black in the dispersions or to adsorption 0 25 50 75 100 of the dispersant. It was thought possible that P R I M A R Y P A R T I C L E SIZE (m)l) specific adsorption of hydroxyl or hydrogen ions on the cellulosic fiber might also play an imEffect of Soil Prirnarr Particle Size on Rate of Change of Figure 7 . portant part in soil deposition. Reflectance The curves shown in Figure 9 were obtained from data resulting from the use of variously pretreated cotton fibers: dry cotton fiber; cott,on fiber presteeped I t would therefore follow that for fine particle blacks, 50 m p or 24 hours in distilled water, pH = 7.5; cotton fiber presteeped less, dispersions containing a larger percentage of primary par24 hours in 1% ammonium hydroxide, pH = 11.5; and cotton ticles or smaJl aggregates should show a higher soiling rate than fiber presteeped 24 hours in 1yoacetic acid, pH = 3.5. dispersions with flat distribution curves. The fiber slurries aft,er presteeping were formed into a pad on a This prediction was tested by centrifuging a 1y0dispersion of Buchner funnel and rinsed twice with distilled water to remove S e o Spectra black for 30 minutes a t 1850 r.p.m. in an Intermost of the adhering et.eep liquor. Rate c u r v a were then run by national centrifuge, SB-2, with a head radius of 6 inches. A the usual technique using 1000 ml. of a 1% dispersion of S e o large part of t,he dispersion (56%) was thrown down and the Spectra black mixed with 50 grams of chopped fiber. centrifiugate removed by decantation. Soil deposition rate It may be seen from.Figui-e 9 that the curve9 obtained when curves were then run and compared with the original dispersion, a using the variously prest.eeped cotton fibers are practically idendispersion of 0.44% Neo Spectra carbon black directly milled, tical. The soiling rate for the dry cotton fiber is significantly and a dispersion of 0.44% Neo Spectra carbon black prepared by greater than for t,he pretreated fibers. I n a similar manner, diluting the original dispersion (Figure 8). It may be seen that Figure 3 shows a small but progressively lessening decrease in soiling rate when the soiling dispersion volume is lo00 ml. and above for dry cotton, which does not appear (Figure 4) wit,li p r e steeped cotton fibers.
I
I
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I I I I Ill]
1
-
OsDirlilld H,O pH 7.5 O H - pHI1.S
Q). I% NH,
0
0 TIME
Figure 8.
Effect of Soil Particle Size Distribution on Rate of Change of Reflectance
Minutes)
Figure 9. Effect of pH of Cotton Fiber Presteeping Liquor on Rate of Change of Reaectance
May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1139
rate of formation of the soil-fiber complex and on the stability of the soil-fiber complex during classification. To observe the effect of temperature differences on the rate of soil deposition, 0.2 and 0.5% dispersions of Neo Spectra black, containing 5% of Daxad on the weight of the black as dispersant, were used. Fifty grams of chopped cotton fiber were slurried with 1 liter of each carbon black dispersion and deposition rate curves were obtained in the usual manner a t 20’ and 70” C. Classification was carried out a t 20”C. It may be readily seen that no significant difference in soil deposition rate was found (Figure 11). Soiling rate curves at 20” and 70“ C. m-ere similarly obtained using 1% Neo Spectra black dispersed in aqueous solutions of 0.02 and 0.04% CMC, respectively, as the soiling medium. Figure 12 shows that the dispersions containing 0.02% sodium carboxymethyl cellulose gave a negative thermal coefficient while the dispersions containing 0.04% sodium carboxymethyl cellulose gave a positive thermal coeffiFigure 10. Effect of Surfactant in Soiling Mixture of Soil Dispersion cient. on Rate of Change of Reflectance It thus becomes evident t h a t negative, positive, or zero thermal coefficients of soil deposition rate may be obtained in these systems, depending upon the nature 3’0effect on the rate of soil deposition appears when presteep and concentration of the dispersing agent. I t is suggested t h a t liquors of different pH’s are used. rhanges in temperature might give rise to either a decrease in SURFACTANTS IN THE SOILING MIXTURES. I n considering prisolvation or other stabilizing factors at the soil particle and fiber mary deposition of soil on cotton fiber it is obvious that a knowlsurfaces with an attending increase in the percentage of high edge of the effect of surfactants on such systems is of major imporenergy particles which could make close approach to or capture tance. (The term “surfactant” is used here as a condensation of of particles by the fiber surface more probable, or a n increase in “surface active agent” as recommended by General Aniline and average particle size because of destabilization effects which Film Corp.) would decrease the soiling rate as shown in Figure 8. The following technique was used in a series of tests designed to The balance between these opposing tendencies apparently condetermine the magnitude and nature of the effect. Five grams of trols the sign and magnitude of the thermal coefficient. the surfactant to be tested were added t o 1liter of a 1%dispersion As a preliminary to the determination of the thermal stability of Neo Spectm black and the mixture was stirred 15 minutes. of the soil-fiber complex, the thermal coefficient of soiling rate of Fifty grams of chopped cotton fiber were then added and a soil chopped cotton fiber was determined using a dispersion of deposition rate curve was obtained in the usual manner. Aquablak-S as the soiling medium as previously described. This The following surfactants were tested: Igepon T [CI.rHaaCON (CHa) C2H4S0&a]; Igepal C (alkyl aryl polyethylene glycol ether); Nacconol NRSF (an alkyl aryl sulfonate); Ivory Snow I I I J I I I I ~ I I I I IIll1 (fatty acid soap); Tide (heavy duty commercial synthetic detergent); and CMC, Hercules medium viscosity, Type 70 (sodium carboxymethyl cellulose). The results obtained are summarized in Figure 10. A rate curve for a cotton fiber control sample without added surfactant is given. It may be seen that the fatty acid soap, the commercial synthetic detergent, and the sulfonated amide were found t o be relatively poor additives for preventing soil deposition. The alkyl aryl sulfonate and the nonionic polyethylene oxide condensate are much better, but the sodium carboxymethyl cellulose is outstanding in this respect. Sodium carboxymethyl cellulose on checking further was shown to be three times as effective as a d i e persing agent for Neo Spectra carbon black as wm a formaldehyde naphthalenesulfonic acid condensate (Daxad 23) of the type widely used in preparing pigment and other solid particle dispersions. This technique gives valuable information concerning the efficiency of various surfactants in preventing soil deposition on textile fibers. TEMPERATURE. Previous evidence ( 1 ) indicated that the bonding of grease-free soil particles to a fiber surface is geometric in nature-Le., by micro-occlusion rather than by an energy funcT I M E I Minutes) tion such as sorption It was suggested that a study of thermal coefficients in these Figure 11. Effect of Temperature at Various Soil Concentrations on Rate of Change of Reflectance systems should afford a critical test of this hypothesis. Investigations were made of the effect of temperature changes on the Daxad used a0 dispersant
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
1140
dispersion was prepared by diluting the concentrated paste of Aquablak-S (45% solids) with a solution containing 1.5 grams of Daxad 23 per liter t o give a 1% dispersion. This system gives a large positive thermal coefficient of soil deposition as shown in
w
0
z U
iJ
W IA.
w ir
TIME
Yol. 44. No. 5
It is emphasized that the factors governing the formation of the soil-fiber complex need not be identical with those influencing its stability. Attention has, therefore, been given to both aspects of the problem in this study. The hypothesis has been advanced (1)that the mode of binding of grease-free soil particlee to the cotton fiber surface or the stability of the soil-fiber complex is primarily a geometric function, i.e., micro-occlusion, and not an energy function, Le., sorption. This does not exclude energy factors from a role in the formation of the complex, but these are of minor importance once the complex has been formed. This hypothesis finds additional confirmation in the various phenomena described in this report. The question concerning the process of formation of the soilfiber complex will be considered first. It may be concluded from the data presented that the primary deposition of soil on cotton fibers is initially a fast-rate process which decreases rapidly so that considerable time is required for completion (Figures 1 and 2 ) . The rate of primary soil deposition is only slightly affected by the concentration of chopped cotton fiber in the aqueous soiling slurry except in cases of very high concentrations where interfiber friction is apparently high during agitation (Figures 3, 4, and 5 ) . The rate of primary soil deposition increases rapidly with increasing violence of agitation of the aqueous soiling slurry as a reeult apparently of both increasing interfiber friction and hydraulic stresses (Figures 3, 4,5. and 6 ) .
(Minutes)
Figure 12. Effect of Temperature and Soil Dispersant (CMC) Concentration in Soil-Fiber Slurry on Rate of Change of Reflectance
Figure 13. Classification following soil deposition on the fiber was carried out a t 20” C. The thermal stability of the soil-fiber complex was determined by classifying the soiled fiber with water a t 20’ and 70“ C. and observing the changes in reflectance of the fiber pads. Chopped cotton fiber (100 grams) was mixed with 2 liters of a 1% dispersion of Aquablak-S at room temperature, and the rate of soil deposition was determined by withdrawing two aliquots a t each of the designated intervals (Figure 13) and by classifying with water a t the indicated temperatures. The rate curves are unaffected by the temperature of the water used in classification, hence, the soil-fiber complex has a zero temperature coefficient of stability. This is considered strong evidence for the hypothesis t h a t primary soil deposition a i d retention is geometric in nature. DISCUSSION
The term “primary soil deposition” is used to describe the process of deposition of soil particles on fiber surfaces from aqueous soiling dispersions under conditions such that the agglomerative accretion of soil is minimal. The study of primary soil deposition in this investigation has been limited to the simplest systems which could be devised-namely, chopped cotton fiber, carbon blacks, a minimum of dispersing agent, and water. Xevertheleas, the mechanism is complex, as is apparent from a consideration of the experimental results obtained. I n attempting to elucidate the mechanisms of primary soil deposition and retention on fiber surfaces, two fundamental questions arise concerning the formation and stability of the soilfiber complex, namely: 1. What is the nature of the process by which the soil-fiber complex is formed, or what are the main factors governing rate and probability of formation? 2. What is the nature of the bond between soil particles and t h e fiber surface, or what are the factors controlling the stability of the soil-fiber complex?
2P IO
0
~-
AOUABLAK S-I % O = S o i i e d at 2 0 ° C)=Soiled o t 7 0 °
@=CbssiCmdat20D 0 ~ ~ 0 9 i r i at e d7 0 °
11
0
80
E!
a -
TIME (Minutes)
-
Figure 13. Effect of Temperature on Rate of Soil Deposition and Retention of Cotton Fibers
The rate of primary soil deposition increases as the primary particle size of the carbon black dispersions decreases. The change of rate is slow above and rapid below a primary soil particle diameter of 50 my (Figure 7 ) . The rate of primary soil deposition increases for aqueous dispersions prepared from a given carbon black as the particle size distribution is changed to give a higher percentage of small particles (Figure 8). Presteeping of cotton fiber in water prior to introduction into the aqueous soiling dispersions decreases the rate of primary soil deposition. Changing the p H of the presteep liquor causes no change in the subsequent soiling rate of the fiber, but the addition
May 1952
INDUSTRIAL A N D ENGINEERING CHEMISTRY
of a surfactant t o the presteep liquor decreases the soiling rate to a greater extent than water alone (Figures 3, 4,5, apd 9). The rate of primary soil deposition is decreased by the presence of surfactants in the aqueous soiling slurry. The rate of soil deposition varies widely in otherwise comparable systems when various surfactants are added (Figure 10). Positive, negative, and zero thermal coefficients for the rate of formation of the soil cotton fiber complex may be obtained, depending upon the nature and concentration of stabilizing agent in the aqueous soil dispersions (Figures 11 and 12). T h e thermal coefficient of stability of the soil-cotton-fiber complex is zero, although the sensitivity of the complex t o mechanical agitation is very marked. Any process suggested for the mechanism of formation of the soil-fiber complex must be consistent with these phenomena. In view of the multiplicity of factors shown t o be operative in these systems no quantitative theory has been derived but the following qualitative conclusions are presented. The formation of the soil-fiber complex in aqueous soil dispersions is a rate process dependent upon the probability of c l o ~ approach of a soil particle to a site on the fiber surface in which it can be permanently occluded. A soil exchange process occurs as a result of the detachment of soil particles b y agitation during soiling. Particles are weakly occluded in acceptor sites whose geometric relationships to the size and shape of the particles renders them easily removable. This temporary blockage of acceptor sites has a marked effect upon the rate of soil deposition which is dependent on the severity of agitation. Factors which affect the probability of close approach of soil particle to fiber surface acceptor sites, such as changing the concentration of the soil of a given particle size distribution, changing the percentage of fine particles in dispersions of the same soil concentration, and changing the stability of the soil dispersion, will affect the rate of primary soil deposition. Similarly, factors which affect the availability of soil acceptor sites, such as changes i n severity of agitation, changing the degree of solvation of the fiber by the adsorption of surfactants, etc., will change the rate of soiling. Concerning the stability of the soil-fiber complex after formation, the very sharp break in the curve relating primary particle size of the carbon black soils to the variations in reflectance of the chopped cotton fiber after primary soil deposition (Figure 7) is difficult to explain if sorption is postulated as the mode of bonding, whereas if micro-occlusion is responsible, it follows w a natural consequence from the submicroscopic surface geometry of the cotton fiber (2). By use of the electron microscope, numerous crevices of 50 m r diameter or less may be seen on the surface of the cotton fiber, whereas larger ones are rare. It would be expected t h a t the number of soil particles attached to the fiber surface would increase very rapidly &s the soil primary particle size
1141
decreases below the value of 50 mp. This was found to be the case (Figure 7). The effect of temperature changes and variations in severity of agitation on the stability of the soil-fiber complex= is of considerable interest. As shown in Figure 13, the temperature coefficient of stability of the soil-fiber complex is zero. This is true for either the primary stage of classification involving gentle stirring or the final stage involving violent agitation with the Waring Blendor. I n contrast t o this observation is the loss of carbon black soil with attending increase of fiber pad reflectance when the second stage of classification is carried o u t with the Waring Blendor, regardless of the temperature of the classifying medium. An increase in temperature of about 2' C. for 500 ml. of fiber slurry is observed as a result of this energy input. The application of 1000 calories of mechanical energy thus markedly affects the stability of the soil-fiber complex with a decrease in the number of attached soil particles, whereas the maintenance of a 25,000-calorie heat content differential has no effect. The hypothesis of geometric bonding predicts t h a t such a n effect would be observed, but there is no type of energy bond that would explain the occurrence of such a phenomenon. CONCLUSIONS
The chief factors controlling the formation and stability of the soil-fiber complex are the geometric relationship betwern the sizes and shapes of the soil particles and the sizes and shapes of the functional rugosities upon the fiber surface, and the probability of close approach. The geometric bonds in the total soil-fiber complex formed before classifying vary in strength from practically zero to a magnitude such that disruption of the fiber is required for release of the soil particle. Soil particles remaining attached to the fiber after classification are held by geometric bonds stronger than the frictional or hydraulic stresses which can be imposed upon them. ACKNOWLEDGMENT
The authors wish to acknowledge t h e m i s t a n c e of William R. Musick in obtaining the experimental data and to W. F. Busse
for his helpful interest during the course of the work LITERATURE CITED
(1) C o m p t o n , Jack, and Hart, W. J., IND. ENG.CHEM.,43, 1664
(1951). (2) Kern,
S. F.,J . Polwner Sci., 1, No.
4, 259 (1946).
RECEIVED for review Maroh 20, 1951. ACCEPTED December 31, 1951. Presented before the Division of Colloid Chemistry at the 118th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, Ill.. Beptember 1950. Report of work done under contract with the U. S. Department of Agriculture and authorized by the Research and Marketing Act. Contract supervised by Southern Regional Research Laboratory of the Bureau of Agricultural and Industrial Chemistry. Mention of trade products in this paper does not imply their endorsement by the Department of Agriculture over similar products not mentioned.
