INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE 111. EVALCATION O F 30-PLATE OLDERSHAW COLGYN WITH MULTICOMPOXENT SYSTEYAND WITH BINARYSYSTEM No.
Components in Calibration Mixture 6 6
Overhead Rate. hfl./Hr. 750 1050
6Q
6a 6
1650 2300
No.
Theoretical Plates 15.8
2680 3000 6a 930 2 1070 2 1260 2 1910 2 2240 2 a In addition, small traces of oyclopentane
detected in these samples.
15.6 14.4 14.6 15.4
Plate Efficiency, % 52.7 52.0 48.0 48.7
51.3
14.7 49.0 15.0 50.0 15.2 50.7 15.2 50.7 15.5 51.7 15.6 52.0 and 2,2-dimethylbutane were
The number of theoretical plates for this column was constant over a loading range of 750 to 3000 ml. per hour. This is in agreement with previous work reported for this type of column (1 ). However, the general level of efficiencies reported herein is slightly lower than those previously reported. CONCLUSIONS
These experiments were made under the same conditions. The test mixtures contained saturated hydrocarbons of like mo-
Vol. 42, No. 7
lecular weight. The calculations were comparable and were based on vapor pressures obtained from the same source. Therefore, the variables inherent in different test mixtures, methods of treating data, and sources of volatility data were minimized. Since the number of theoretical plates remains constant (Table 111) for tests with systems containing 2 or 6 components, it was concluded that the efficiency of the Oldershaw laboratory column was not affected by the number of components present in the test mixture. ACKKOWLEDGMEKT
The authors gratefully acknowledge the assistance of several of their colleagues in the experimental work and calculations. including particularly J. W. Askins, D. M. Bartay, L. C. Carpenter, A. E. Krc, and T. J. McLean. Acknowledgment is especially made to Edward Gelus and Stanley Marple, Jr.., for their valuable suggestions and guidance. LITERATURE CITED
(1) Collins, Franc C., a n d L a n t s , Vernon, ISD. ENG.CHEM.,ANAL. ED., 18, 673 (1946). (2) Fenske, M. R.,IND. ENG.CHEJ?.,24, 482 (1932). (3) Lewis, W. K., a n d Matheson, G. L., Ibid., 24, 494 (1932). RECEIVED September 15, 1949.
Some Physical-Chemical Aspects of Cotton etergency LIMITATIONS OF PRESENT LABORATORY TESTING METHODS JOSEPH M. LAMBERT AND HERBERT L. SANDERS' Central Research Laboratory, General Aniline & F i l m Corporation, Easton, Pa.
A
review of the conventional testing methods has been made which show-ed that the present tests fail in many respects to simulate adequately actual use conditions. I t is pointed out that in practice cotton is soiled by complex mixtures rather than by large amounts of finely divided carbon black. Moreover, the nonlinear relationship between the reflectance and the amount of soil on the fabric indicates that even trace quantities of ingrained soil can reduce appreciably the whiteness of textiles. Also, cotton goods are normally soiled and laundered repeatedly throughout their lifetime in contrast to conventional laboratory wash tests which employ only a single cycle with unused cotton. Preliminary results are described which were obtained with several cotton detergents in multicycle wash tests in which roll towels were soiled in actual use, then washed in a home washing machine and measured in the laboratory. Available field tests made with these detergents essentially substantiated the results of this practical series of tests. Conventional carbon black-type swatches were included in the above washes but in this case the results failed to correlate with the actual performance data. The important distinction between precision and accuracy of laboratory detergency .data is illustrated by a typical example.
T
ECHNOLOGICAL advances in many fields have been dependent mainly on significant laboratory testing procedures. The laboratory tests not only guide in the development of improved products, but also furnish the basis for quality control in production. Standardization of dyestuffs and of most textile auxiliaries is being accomplished on a commercial scale by various empirical laboratory tests. Therefore, one can conclude that fairly indicative results can be obtained in numerous fields related to textile technology. VARIOUS TESTS ON COTTON
The procedures for testing the finishing treatments usually applied to cotton have been described in great detail in the manual of the American Association of Textile Chemists and Colorists ( 2 ) . Tests for dye fastness, waterproofing, shrinkproofing, etc., have been worked out empirically using a strictly practical approach. Also, the testing work related to dye application, where the scientific approach furnished most interesting results on the dyeing mechanism (48), has remained mainly on a technological level. Although there might be room for improvements in experimental procedures and for advances in instrumental techniques, the present test,ing methods have proved of definite value to both manufacturers and users of the related products. 1
Present address, Ninol Laboratories, Chicago, Ill.
July 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n the field of surface-active agents applied t o cotton, t,he empirical test for determining wetting power illustrates the *usefulness of a laboratory procedure which yields results of reasonable precision and accuracy. From a sufficient number of pertinent data, i t is then usually possible to make predictions as t o the performance of these agents in practice. Draves and Clarkson (IS), developed a relatively simple test which consists of a wetting rate measurement under controlled conditions. This test later critically evaluated by Draves (12), proved invaluable in the systematic synthetic programs of Caryl ( 7 ) and others (29),leading t o greatly improved wetting agents. It is understandable that successes like these have helped to establish a certain confidence in present testing methods. COTTON DETERGENCY TESTS
It might seem surprising that in the field of cotton detergency the results obtained have not generally been considered too satisfactory. This is the case even though there exists a wealth of scattered information on the theory of detergency summarized in several review articles (8, 26, 37, 43, 60). Many attempts have been made to enumerate in detail all the physical-chemical factors which might influence the detergency process. I n describing these factors, the authors discuss interesting experimental work which, a t least in part, might be of real significance. However, the relative importance of factors reportedly constituting the detergency process has not yet been assessed. Furthermore, very few experimental techniques have become available for quantitative determinations of the significant factors. The difficulty of assigning the proper degree of importance to a measurable factor is illustrated by the varying emphasis given to solubilization as a factor in detergent action. This factor has been considered of great importance by McBain (M), but Preston (33) f h d s t h a t solubilization plays only a secondary role in the usual washing process. Confirming the latter findings, dye solubilization data which had been obtained for a large number of commercial surface-active agents in this laboratory (24)could not be correlated with the detersive properties of the products. Some authors (10,16,37) have expressed the opinion t h a t the best method for evaluating detergency consists of a field trial under conditions of actual use. This procedure is recommended by Crowe (10)since results have often been found in the laboratory which are completely different from those obtained in textile mills and laundry plants under normal operating conditions. Flett (16) cautions the workers in this field not to render unqualified reports when using only laboratory tests for the evaluation of detergents. However, Schwartz and Perry (87) point out that practical tests have certain shortcomings since they usually have t o be run on a large scale with more or less fixed operating conditions. Therefore, a number of artificial detergency tests had to be devised. It has been stated frequently (9, 15,37,58) that the physicochemical theory of detergency has not advanced far enough for conducting anything but strictly empirical tests. Actually, most investigators aim at a duplication of the conditions encountered in practice. The commercial-type laboratory washing equipment furnishes satisfactory control of temperature and also some control of mechanical action. The most widely used instrument in the laboratory has been the Launder-Ometer (%, p. 90), but recently the Terg-0-Tometer (which is essentially a down-scaled home washing machine) was recommended as being faster and giving more reproducible results (27). I n fact, a prototype model of the latter instrument which has been in use in this laboratory for several years was found excellent in duplicating the mechanical action t o which a fabric is subjected in actual laundering. However, while the instrumental problems were relatively simple t o solve, great difficulties were encountered in formulating artificial soiling compositions. The main attention was directed toward the production of standardized soiled cotton samples which would give reproducible results. Sisley (58) has
1389
reviewed the soiling procedures used by the early investigators, and Clark and Holland (9) have presented a fairly complete account of more recent work. For the evaluation of soaps and synthetic detergents in cotton detergency, several types of soiling formulas have been used. Within the last few years Van Zile (4.4), Harris ( 1 9 ) , and Clark and Holland (9) have proposed detailed procedures for preparing soiled cotton using carbon black or colloidal graphite with mineral and vegetable oils in organic solvents. Some investigators (4, 9, 42) have studied extensively the uniformity and storage quality of the soiled cotton prepared in this manner, Results obtained b y using artificially soiled cotton in the Launder-Ometer have been presented in a large number of publications (3,6, 9, 10, 17, 21, 36). Many authors (9, 10, 16, $439)emphasize the limitations of the method and express hope that necessary improvements would be made in future testing procedures. Considering the very discouraging results reported by Crowe (IO),obtained by several cooperating laboratories in a b y e a r A.S.T.M. sponsored study, the over-all pessimism might be justified. Although great care had been exercised in duplicating the specified experimental conditions, it was impossible to decide which of the two synthetic detergents tested performed better. Some laboratories obtained one result, others the reverse, and a number of laboratories could not distinguish between the products. The most disturbing fact was t h a t some laboratories found that distilled water washed better than either of the two detergents a t 0.4y0concentration. Some of the difficulties experienced in the past have been obviously due t o a lack of reproducibility in the testing methods. Serious attempts have been made t o improve the reproducibility -i.e., increase the precision of the detergency tests. For example, Woodhead, Vitale, and Frantz (61)succeeded in increasing the precision of their tests by using a very large number of conventional swatches in a home washing machine. More recently, Powney and Feuell ( 3 1 )have shown t h a t improved reproducibility can be obtained by a new technique in which soiled chopped fibers replaced the soiled cloth swatches. However, no critical evaluation has been made of the accuracy obtainable by laboratory detergency tests. Accuracy should definitely be distinguished from precision. The inaccuracy of the present tests -Le., the inability of predicting the performance of a detergent in actual use on the basis of laboratory test results-has frequently been confused with a lack of precision. While precision can be improved by more rigid control of experimental conditions and a larger number of tests, inaccuracy cannot be eliminated because it appears to be an intrinsic fault of present testing methods. LIMITATION OF SINGLE-CYCLE T E S T S
In practically all published testing procedures, an attempt is made to determine the detersive effectiveness by a 1-cycle method consisting of a single soil application followed by washing under a controlled set of conditions. I n practice, cotton goods are soiled and washed repeatedly throughout the lifetime of the fiber. During the repeated soiling and washing cycles, considerable aging of the cotton occurs which causes profound physical and chemical changes in the cellulose. Tendering, for instance, is readily apparent from manual tear tests on old cotton which has gone through very many washing cycles. It follows that there takes place in normal wear and laundering a slow but constantly progressing deterioration of the fiber. Even after a few cycles, the characteristics of the fabric might be changed sufficiently to influence the detergency process. Although some investigators (84,42,46,47) have reported on multiple tests, the studies were restricted t o artificially soiled cotton and no resoiling took place between successive washings. The question arises whether a single-cycle test or the present multiple tests can ever measure significantly the complex interactions which occur in the many cycles encountered under practical use conditions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1390
LIJMITATION OF ARTIFICIAL SOILS
The artificial soiling mixtures recommended for cotton detergency tests contain mineral and vegetable oils, combined with relatively large quantities of carbon black, colloidal graphite, or other finely dispersed carbon. These heavily soiled mixtures are usually applied as dispersions in organic solvents or aqueous mediums. The conventional artificial soiling mixtures have been specifirally designed for single-cycle detergency tests. As Clark and Holland (9) pointed out, it was one of their aims in formulating soiling mixtures to have appreciable amounts of soil left after a single wash test. The relatively large quantities of colloidal carbon used lead to cotton which has a very low reflectance (about 20%). This value would correspond to a value of 5 on the visual value srale, such as given by the hlunsell color system, which place3 the cloth In the middle betneen white and black (Figure I )
1
2
3
4
5
6
7
8
9
Figure 1. Visual Value Scale as Given by Munsell Color System
It is a peculiar characteristic of such heavily soiled cotton that it contains, besides some loosely bound carbon, large ainount,s of tenaciously held carbon which is very difficult to remove. On the other hand, cotton soiled in actual use with natural soils which nornially have very lo~vcarbon black content contains only traces of ingrained soil. A possible explanation of t,he extreme fixation of the artificial soil can be found by considering the physical-chemical properties of carbon black. Almost all commercial grades of carbon black are characterized by exceedingly small particle size (around 0.1 p or below) and very large effective surface areas (up to hundreds of square meters per gram of carbon). The high reactivity of the carbon black surface was shown in studies on heats of adsorption ( 6 )which are particularl3high (in the order of 5 kg.-cal. per mole) for the first layer of adsorbate. Thus, superfine carbon particles will not, only penetrate the fiber more readily because of their small size, but can also be held there by strong forces due to their large surface area. It might be expected that aqueous dispersions of carbon black would give less tenacious soiling than solvent suspensions due to electrical charge effects. For instance, Tiegand (4.9) found a strong adsorption of hydroxyl ions onto the carbon particles. This indicates that in aqueous systems the carbon part,icles will carry a negative charge thus assuming the same polarity as the textile fiber. I n a nonaqueous medium the anionic adsorption might be absent and wit,h no repelling forces active, the carbon particles can enter the cellulose fiber more readily leading to abnormally permaneat soiling. The assuniptions do not seem to be borne out in practice, however, as shown by a comparative study on art,ificial soiling methods using oil as well as Tvater dispersions of carbon black which was recently published by Uterinohleri and co-workers (40). This discrepancy perhaps may be explained by the lesser swelling tendency of cotton in organic solvents preventing as deep a penetration of the carbon black into the fiber as is found in aqueous mediums. Ackley ( 1 ) has questioned the significance of detergency tests carried out with artificially soiled cloth of very low reflectance values. He pointed out that such a fabric after washing even in a good detergent will still be too gray to be acceptable in practice. On the other hand, the presence of some soil on swatches washed in a single-cycle test has been considered essential since the grading of t,he more efficient detergents would otherwise be impossible. However, the-ease of removal of the first, part of the soil
VOl. 42. No. 7
prohably does not correlate with the removal of the last, trace o f soil or the prevention of redeposition on a nearly clean fabric. I n the deposition tests proposed by many investigators (3, 9 , 18, 45-47), attempts have been made to obtain a measure for t h e suspending power of detergent solutions for heavy loads of artificial soil. Again, this suspending power which prevents the deposition of amounts of carbon black measurable in a single w s t might not be correlated with the suspending effect in a prac*tical washing operation. There, relatively small traces of soiling materials should be prevented from redepositing onto the cleaned fabric since after a number of cycles, their accumulation woul~i become objectionable. LIMITATION OF REFI.ECTANCE MEASUREMENTS
hlthough it, has been proposed to determine soil removal ~ ( J V calibrating or actual testing purposes by gravimetric ( 3 , 9, 2 1 ) 01 by turbidimetric (&-5-47) methods, reflectance measurements have been most commonly used. Some authors ( 1 9 , 8 6 ) assumed that the reflectance of the soiled or Tvashed cloth is a linear function 01’ the amount of soil on the fabric and have erroneously identifir,l the term “per cent detergency” (see Equation 2) with “per cent soil removal.” However, it was already recognized bb- I t h o d e sild Brainard ( 3 4 ) and confirmed by Utermohlen and \\’allaci~ ($5)that the reflectance cannot be taken as a direct measure of‘ the quantity of soil on the clot,h. In other empirical work, . b m SI rong and collaborators (3) determined gravimetrically thr amount of soil removed and related the data to reflectance measurcments. Theoretical treatments of this problem as related t o pigment and dyestuff systems have been presented in detail ( 2 2 . 3 2 ) , and some of the results were applied by Bacon and Smith ( 5 ) to detergency studies. Gtermohlen and Ryan ( 4 1 ) recentlJstudied t,he relations between reflectance and the amounts o” black iron oxide on cotton which were determined by cheniic,:il analyses. Kubelka and l l u n k ( 2 3 ) derived, from theoretical coiisidtwtions on the light distribution in “infinitelk- thick” layers of paiiir, the relation
or
where R is the reflectance; K!the absorption coefficient; and S , the scat,tering coefficient. Since K increases proportionally wit11 the amount of light-absorbing material-e.g., pigment or carbon black-added to the paint, while S remains essentially constant, K / S will be a linear function of the aniourit of coloring matter. Figure 2 was obtained from calculated values using the l i u M k a and Munk formula; reflertances of 20 and 85% were :issunied for the soiled and the clean fabric, respectively. Tlio validity of the curve shown in Figure 2 for fabrics was also ~ 1 1 firmed c:upc:rimentally in this laboratory, within one or two reflectance units, by a simple experiment ( 2 6 ) in which solutions (-ontaining known amounts of a nonsuhstantive dyestuff (Acid Orange S X j mere used for staining cotton swatches. By suhsequerit reHectance measurements in a General Electric specarrophotometer, the reflecetance values a t the absorption maximum ( X = 490 nip) could be correlated with the relative concentrations of the staining dye solutions. Referring to Figure 2 , it is interesting to note that the sensitivity of the reflectance value as a measure of the quantity of soil on the cloth greatly depends on the level a t which the test, is performed. Thus, plus or minus one reflect,ance unit a t the low reflecting end of the scale corresponds to about 5% more or less soil on the cloth; however, plus or minus one reflectance unit near the reflecting level of the clean fabric corresponds to only 0.3y0more or less soil on the fabric. I t follows that other experimental conditions being equal, the reproducibility of reflectance
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1950
readings on soiled fabric will be inversely related t o the reflectance level-Le., dark swatches give a deceptive appearance of reproducibility. This effect was unintentionally demonstrated in a recent paper by Powney and Feuell ( S I ) who presented duplicate determinations of reflectances in which much better agreement was obtained in the low reflectance range. 100
r
I
I
I
I
80
z W 0
z 60 4
c 0 W
i
1391
vestigators contained an aqueous carbon dispersion. This dispersion is also employed in the preparation of calibration curves for the photometer in order t o correlate light absorption values with absolute concentrations of carbon in the solution. How&er, it was noticed during extensive studies on the photometry of wash liquors in this laboratory (28)that serious errors can occur because of the great variability in the initial turbidities commonly found when soaps or sparingly soluble detergents are used. Up to 50’% more Aquadag (an aqueous dispersion of colloidal graphite) had to be added t o clear solutions than to turbid solutions for obtaining the same light absorption values. This effect is caused probably by the multiple scattering in the turbid medium leading to a higher effective absorbency per unit quantity of graphite. A still more serious error might occur in the photometry of wash liquors due to the chanGng state of dispersion of the carbon particles in various detergent solutions. Rhodes and Brainard ( 3 4 ) have reported that such changes in the size of the carbon particles took place in laboratory tests with artificial soils.
40
W
PRESENTATION OF DETERGENCY DATA
a
The most generally accepted method of presenting reflectance data has been to define per cent detergency, D , by
20
I I I I I I 0 0.2 0.4 0.8 OB 1.0 RELATIVE AMOUNT O f SOL OBITRARY UNITS)
Figure 2. Relation of Reflectance to Amount of Soil on Fabric
where R, is the reflectance of the washed fabric; R,, the reflectance of the soiled fabric; and Ro,the reflectance of the fabric before soiling. If all the soiled swatches have the same initial reflectance, the above expression can be reduced t o
Computed from Kubelka and Munk formula
Considering the prime requirement that a successful detergent has to fulfill-Le., t o wash the fabric as clean as possible-the photometry a t the high reflectance levels is of great importance The conventional photometric instruments have excellent sensitivity in this range, but in view of the experimental error inherent in the washing procedure, a large number of individual determinations would have to be made in order to obtaiii significant averages or detergency values of adequate precision. This fact may explain the universal tendency t o use very dark swatches in the conventional testing methods. The psychophysical response of the eye is opposite to thephotometer response. This is demonstrated in Figure 3 by the relation of reflectance to equal visual value steps of grayness aa given by the Munsell system. The curve was obtained by plotting data published by Newhall, Nickerson, and Judd (SO). In order to facilitate the visualization of these data, the actual Munsell specimens are reproduced in Figure 1. It can be seen that the washing ot heavily soiled swatches, although giving only small changes in reflectance readings (Figure 2 ) , still produces the visual effect of extensive soil removal to the eye. Advantage of this fact ha5 often been taken by presenting photographs of test swatches (15, 18, $0,50). Results obtained in actual laundering can be adequately expressed in terms of reflectance readings since cotton goods are washed to very high reflectances. For instance, in the extended niulticycle laundering tests with natural soiling (described below) it was found that the reflectances after washing never dropped below a value of 60%. As shown in Figure 3 throughout the range from 85 to SO%, the reflectance is correlated to the Munsell value units by a linear function. Therefore, degree of whiteness which is an important practical criterion of successful cotton detergency can be directly expressed in terms of differential reflectance readings. While the foregoing was restricted to reflectance measurements, the following will deal briefly with the limitation of turbidity measurements of wash liquors as suggested by Vaughn and collaborators (45-47). The artificial soiling mixture used by these in-
D = klRw
- k,
(3)
where kl and k~ are constants. It follows t h a t under these conditions per cent detergency is directly proportional t o the final reflectance of the washed fabric. This simple method of presentation was chosen for giving the experimental data t o be discussed in the following sections.
z W
o 2 4
t-
o W -I LL W
a
Figure 3. Relation of Reflectance to Equal Steps on Visual Black-GrayWhite Scale Plotted from Munsell value scale data (30)
The failure of D as defined in Equation 2 to give a true value of per cent soil removal was pointed out in the preceding section. D could be used with some justification as a measure of the power of the detergent itself, if it were not for the fact that swatches of different initial reflectance might also have their soil fixed t o varying degrees. The soil fixation t h a t usually determines the reflectance of artificially soiled cotton after washing is a property which can be determined only by systematic wash tests.
1392
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. LISTO F DETERGENTS AND % Concn. of Active Principle Detergent Type (Without Builders) A Built anionic 13 B Built nonionic 15 C Built soap 65 D Built anionic 24 E Nonionic 15 F Anionic 13 a Each value represents 70 determinations. li Each value represents 32 determinations.
SUMMARY O F
placed. Because of these fluctuations, the results from any one individual cycle were not considered significant and the reflectance readings obtained after washing in the third, fourth, and fifth; seventh, eighth and ninth; and eleventh, twelfth, and thirteenth cycles were averaged, respectively, and plotted in Figure 4. Various uncontrollable factors, such as the contaminants in the tap water or the quality of the commercial soap used, might have influenced the absolute values reported. However, a number of consumer surveys showed the same relative order of performance for the detergents as shown in these tests.
REFLECTANCE DaTA
Multieycle RollTowel Tests After 13 Cyclesn Std: Average deviation 1.0 73.6 72.8 1.8 70.7 1.2 69.7 1.1 66.9 1.6 65.9 1.1
Vol. 42, No. 7
Single-Cycle Swatch Tests Using Artificial Boilingb Std. Average deviation 38.1 3.3 39.6 3.4 40.5 2.8 34.6 2.7 42.5 2.9 37.6 4.2
MULTICYCLE ROLL-TOWEL TESTS
In view of the limitations of the conventional detergency tests, a practical laundering program was conducted under controlled laboratory conditions using a number of different detergents. This was carried out by using commercial roll towels, (75 feet long and 15 inches wide), which uiere placed in conventional dispensers in four different locations throughout this laboratory. [The specifications of the fabric were as follows: 12-inch Huck toweling; thread count, 75 warp (W) and 37 filling ( F ) ; weight, 5.7 ounces per square yard. ] The towels were soiled by actual use, and EVALUATION OF SIX DETERGENTS IN MULTCYCLE WASH TESTS USING ROLL TOWELS
MULTICYCLE ROU. (AFTER 13 CYCLES)
B
[ARTIFICIAL SOIL)
74 s
143
-
'
-55
Y f 72
41
Y $70
39
d
2
9
68
m
[
66
35
Yp: B
DETmGENT'X'
1 y
,tdI
,tE"
"FY
Figure 5. Cornparisan of Reflectance Readings Obtained with Six Detergents SINGLE-CYCLE T E S T S WITH CONVENTIONAL SWATCHES
a high degree of soiling was ensured by removing all soap or handwashing compounds and replacing them with borax. Each cycle of these multicycle wash tests consisted of soiling the towels (with rotation of their location) and taking about 70 reflectance readings before and after washing in a particular detergent. The details of the photometric measurements will be given elsewhere ( 3 5 ) . Briefly, it consisted of an integrating photometer composed of a photographic camera with the search unit of a conventional photometer placed against the ground glass upon which a 1-foot square area of the roll towel was imaged. The instrument was calibrated by means of surfaces having known reflectance values. A special rack facilitated the consecutive readings on each running foot of the roll towel which was marked off and numbered with hTissendye resist color. Six of the detergents used in this study are described as to type and approximate activity in Table I. The washing was accomplished in a home washing machine (Maytag) using 2.5 grams per liter of detergent in Easton tap water (hardness, 70 p.p.m.) a t 60' C. The results obtained for the six detergents A to F are summarized in Table I and Figure 4,showing average reflectance values of the washed towels after repeated soiling and washing cycles. The average reflectances after soiling varied considerably and were usually 5 to 8 reflectance units, but a t times only 2 reflectance units below the value for the clean towel. Moreover, possible variations in the nature of the soil could have occurred a t the different locations where the towels had been
For comparison, swatches soiled with carbon and oil, prepared according to the method described by Harris (19), were sewed onto the roll towels and washed for only one cycle. I n each run four replicate swatches were used. Since this was done for eight runs, a total of 32 swatches was washed with each detergent. I n accordance with the outline of the method, the soiled cotton was not kept for longer than 1 week before testing. This made it necessary to employ several different batches, since the multicycle tests extended over a period of several months. The extent of soiling obtained with the carbon black-type soil varied from batch to batch giving reflectance values for the soiled cloth from 11 to 19%. Therefore, each batch of soiled cloth was used with all six detergents so that these variations would not influence the relative test results. After a single washing in the home washing machine, the reflectance of the swatches always increased appreciably, but the data showed considerable scattering. By plotting the data it was found that the final reflectance values obtained were not correlated to the initial reflectance level of any particular batch, but varied a t random. COMPARISON O F RESULTS
The mean reflectance values for each detergent obtained by averaging the readings of the 32 swatches were plotted in the histogram shown in Figure 5. Also shown for comparison are the final reflectance values obtained for the same detergents in the roll-towel tests after 13 cycles of soiling and washing. The listing of the six detergents was made in order of decreasing efficiency according to the roll-towel test results. The average reflectance values are plotted in Figure 5 and also listed in Table I which includes the standard deviations for each set of determinations. The lack of correlation between the results obtained by testing the six detergents in multicycle roll-towel and single-cycle swatch tests is shown clearly in Figure 5 .
INDUSTRIAL AND E N G INEERING CHEMISTRY
July 1950
1393
CONCLUSIONS
The limitations of the present laboratory procedures for studying cotton detergency suggest that much greater stress than hitherto should be laid on multicycle tests. Furthermore, the removal of last traces of ingrained soil should be studied by washing to higher reflectance levels. An empirical approach t o a more realistic cotton detergency test based on these considerations will be given elsewhere (36). However, still another approach was taken in order t o obtain a better understanding of t h e underlying mechanism of cotton detergency which might greatly contribute to the solution of the testing problem. This involved an investigation of the interaction between the cellulose and the bath components during the washing process. This phase of the work is described in the following paper.
43
41
3s 31
35 33
ACKNOWLEDGMENT
SWATCH SET NO.
Figure 6.
SWATCH SET NO.
Precision and Accuracy Detergency Test Results
Comparison of single cycle swatch with multicycle roll-towel data
The authors would like t o thank E. J. Black for his valuable assistance in conducting the detergency test program. The help of Mrs. J. Gowen and of G. Wright in obtaining the photometric data and in applying statistical analyses is gratefully acknowledged. The comments of W. P. Utermohlen, Jr., who reviewed this paper, are also greatly appreciated. LITERATURE CITED
PRECISION AND ACCURACY OF DETERGENCY TEST RESULTS
Several authors (3, 14, 31) have recently applied statistical treatments to the data obtained in detergency testing for evaluating the precision of the results. Such an approach, and also the application of factorial designs to the study of multivariable detergency systems as proposed by Feuell and Wagg (14) are frequently indicated. Yet certain fundamental facts regarding the difference between the precision and accuracy are often overlooked in these treatments. I n his article on the control of accuracy and precision of industrial tests, Mitchell (28) has indicated the necessity of a “true value” for a definition of accuracy. Applying Mitchell’s terminology t o the data presented in this paper, the difference between precision and accuracy can best be demonstrated by using a more detailed graphic presentation of the results obtained for detergents D and E. Figure 6 shows the average reflectance values obtained with the two detergents as they were plotted in Figure 5. Included also are the individual reflectance readings obtained for each of the eight sets of artificially soiled swatches. These values show considerable scattering around the mean value, resulting in standard deviations of 2.7 and 2.9 reflectance units for D and E, respectively. Nevertheless, a statistical analysis indicated that the difference between the two mean values can be considered highly significant. By applying the student t t e s t t o the data and by determining the confidence level a.~ outlined by Davies (11), a P-value of 0.1% was obtained. [ P is defined as the probability that the given difference in the means is due to chance (sampling variations). I n this case the P-value of 0.1% indicates that there is only one chance in a thousand that the difference in the means is not real.] It follows that the precision of the data obtained with these carbonsoiled swatches is entirely adequate, if a sufficient number of replicate tests are made. It is of great practical importance, however, t o consider not only the precision, but also the accuracy of the results obtained with the artificially soiled swatches. Contrary t o the findings in the roll-towel test or in actual field trials, the swatch test showed E as being a much better detergent than D. Therefore, one must conclude that the swatch test although sufficiently precise, can be verj inaccurate in predicting the true performance of detergents. Improvements in precision and statistical treatments of data as proposed by several investigators (3, 81, 61) will not solve the fundamental problem of detergency testing if the accuracy cannot be improved simultaneously.
(1)Ackley, R.R., Ann. N . Y . A d . Sci., 46,519 (1946). (2) Am. Assoc. Textile Chemists Colorists, “Technical Manual and Year Book of the Am. Assoc. Textile Chemists Colorists,” Vol. XXV, New York, Howes Publishing Co., Inc., 1949. (3) Armstrong, L. J., et al., Am. Dyestuff Reptr., 37,596 (1948). (4)Bacon, 0.C.,Ibid., 34,556 (1945). (5) Bacon, 0.C., and Smith, J. E., IND.ENG.CHEM.,40, 2361 (1948). (6) Beebe, R. A., Briscoe, J., Smith, W. R., and Wendell, C. B., J. Am. Chem. Soc., 69, 95 (1947). (7) Caryl, C. R.,IND. ENG.CHEM.,33,731 (1941). (8) Chwala, A., “Textilhilfsmittel,” pp. 101-99, 438-43, Vienna, Julius Springer, 1939. (9) Clark, J. R., and Holland, V. B., Am. Dyestuff Reptr., 36, 734 (1947). (10) Crowe, J.B., Ibid., 32,237(1943). (11) Davies, 0.L., “Statistical Methods in Research and Production,’’ London, Oliver and Boyd, 1947. (12) Draves, C. Z.,Am. Dyestuff Reptr., 28,421,425 (1939). (13) Draves, C.Z., and Clarkson, R. G., Ibid., 20,201(1931). (14)Feuell, A. J., and Wagg, R. E., Research, 2,334 (1949). (15) Flett, L. H., Chem. Eng. News, 26, 1368 (1948). (16) Flett, L. H., Soap Sanit. Chemicals, 22,No.12,46 (1946). (17) Furry, M. S., McLendon, V. I., Mer, M. E., Am. Dyestuff Reptr., 37,751 (1948). (18) Gruntfest, I. J., and Young, E. M., J. Am. Oil Chemists’ SOC.,26, 236 (1949). (19) Harris, J. C., A.S.T.M. Bull. 140,76; 141,49 (1946). (20) Hersberger, A. B., and Neidig, C. P., Chem. Eng. News, 27,1646 (1949). (21) Holland, V. B., and Petrea, A., Am. Dyestuf Reptr., 32, 534 (1943). (22) Kubelka, P., and Munk, F., Z. tech. Physik, 12,593 (1931). (23) Lambert, J. M., and Ackerman, B. J., unpublished work, 1947. (24) Lambert, J. M., and Busse, W. F., J . Am. Oil Chemists’ SOC.,26, 289 (1949). and Martin, E. G., unpublished work, 1946. (25)Lambert, J. M., (26) McBain, J. W.,“Advances in Colloid Science, I,” edited by Kraemer, E. O., pp. 99-142, New York, Interscience Publishers, Inc., 1942. (27) McCutcheon, J. W., Soap Sanit. Chemicals, 25,No. 5,83 (1949). (28) Mitchell, J. A.. Anal. Chem.. 19, 961 (1947). (29) Nawiasky, P., and Sprenger, G. E. (to General Aniline & Film Corp.), U. S. Patent 2,315,375(March 30,1943). (30) Newhall, S. M., Nickerson, D., and Judd, D. B., J. Optical SOC. Am., 33,385 (1943). (31)Powney, J., and Feuell, A. J., Research, 2,331 (1949). (32) Preston, J. M., and Tsien, P. C., J. SOC.Dyers Colourists, 62,242 (1946). (33) Preston, W.C.,J . Phys. & Colloid Chem., 52,84 (1948). (34) Rhodes, F. H., and Brainard, S. W., IND.ENG.CHEM.,21, 60 (1929). l _ _ _ _ , _
(35) Sanders, H. L., and Lambert, J. M., J . Am. Oil Chemists’ SOC., 27,153 (1950). (38)Schwartz, A. M., Ibid., 26,212 (1949).
INDUSTRIAL AND ENGINEERING CHEMISTRY
1394
(37) Schwarta, A. M.,and Perry, J. W., “Surface Active Agents,” pp. 31C-84, New York, Interscience Publishers, Inc., 1949. 1\38)Sialey, J. P. (translated by Wood, P. J.), Am. Duestuff Reptr.. 36, 457 (1947). (39) Snell, F. D., Chenr. Eng. S e w s , 27, 2256 (1949). 140) Utermohlen, W. P., J r . , Fischer. E. K., Ryan. 11. E., and Campbell, G. H.. Teztile Research J . , 19, 489 (1949). (41) Gtermohlen, W. P., Jr., and Ryan, M. E., IND. ENG.CHEW,41,
2881 (1949).
(42) Gtermohlen, W.P., JT., and Wallace, E. L., Textile Reseccrch J . , 17, 670 (1947). (43) Valko, E., “Kolloidchemische Grundlagen der Textilvered-
Vol. 42, No. 7
Vaughn, T. H., and Smith, C. E., J . Am. Oil Chemists‘ SOC.,25, 44 (1948). (47) Vaughn, T. H., Vittone, A , , Jr., and Bacon, L. R., IND. ENG. CHEX,33, 1011 (1941). (48) Vickerstaff, T., Am. Dyestuf Reptr., 38, 305 (1949). (49) Wiegand, W. B., IND.ENG.CHEM.,29, 953 (1937). (50) Williams, E. T., Brown, C. B., and OaMey, H. B., “Wetting and Detergency,” 2nd ed., pp. 163-74, New York, Chemical Publishing Co. of T.Y.. Inc., 1939. (51) Woodhead, J. A., Vitale, P. T., and Frantz, A. J., Oal & Soap, 21, 333 (1944). (40)
lung,” pp. 581-646, Berlin, Julius Springer, 1937.
(44) Van Zile, B. S., Oil 6: Soap, 20, 55 (1943). (45) Vaughn, T. H., Hill, E. F., Smith, C. E., McCoy. 1,. K., and Simpson, J. E.. Ixn. ENG.CHEX, 41, 112 (1949).
RECEIVED S o v e m b e r 21, 1940. Presented beforc t h e Division of Colloid hleetinf of the A\rE:Rrcas CtrshrIcAL SOCIETY, Atlantio City, N . J.
Chemistry at the 116th
(Some Physical-Chemical Aspects of Cotton Detergency)
CATIONIC ADSORPTION AND EXCHANGE AS SHOWN BY RADIOCALCIUM TRACER STUDIES JOSEPH AI. LAMBERT Central Research Laboratory, General Aniline & F i l m Corporation, Easton, P a .
s o m e of the physical-chemical properties of cotton responsible for the complex interactions in practical detergency have been reviewed. The accessibility and acidic characteristics of cotton appear to be of particular importance. The role of adsorption as an important factor in detergency is illustrated by experimental adsorption data of surface-active agents on cotton. Radioisotope tracer methods are described for measuring the adsorption and exchange of calcium on cotton as it occurs in laboratory wash tests simulating hard water laundering. Results are presented which were obtained with several cotton detergents in multicycle wash tests. Varying amounts of calcium are adsorbed depending on the detergent and on the condition of the cloth (new or used cotton). -i tentative interpretation of these effects is offered as well as a discussion of possible extensions of the method.
I
S T H E preceding paper (bo) it was demonstrated that vcry
small quantities of ingrained soil, which are not removed from the cellulose fiber, play an important role in practical cotton detergency. It was also shown that the inaccuracy of present testing methods has led to many discrepancies between the evaluation of detergents in the laboratory and their performance in actual use. For further technological advances in this field which might lead t o greatly improved products, a more rational basis of cotton detergency t,esting has been considered essential. I t is also generally agreed that a better understanding of the washing mechanism would help in the formulation and possible solution of the problem. One of the important materials in cotton detergency is no doubt t,he cellulose fiber. Therefore, it was thought worthwhile to review first the physical and chemical properties of cotton and to search for a laboratory test which would measure some of the complex interactions between bath components and the fabric in the washing process. PHYSICAL CHEMISTRY OF COTTON
Cellulose chemistry, as one of the major branches of chemical scirnce and technology, has been a field of most intensive investigation; therefore, only a cursory review of some of the aspects pertinent to detergency can be given here. The chemical
and physical properties of native cellulose have been describetl extensively in monographs ( 1 2 , 14,28) as well as in the recent literature (4,16, 17, dZ, 24, SO, 43). Of particular importance for the understanding of the spatial aspects of soil retention arc: the fibrillar structure of the cotton fiber and the intermicellar hole and tube system as described by Mark ($4)and shown in electron micrographs by Kinsinger and Hock (17). Although the interaction of the colloidal soiling materials with the fiber might be expected to take place predominantly in the amorphous domains, accessibility in the physical and chemical sense will also influence that interaction. The chemical accessibility has been shown to be an exact experimental quantity which can be det,ermined by D20 exchange ( 2 , 1 ) or water sorption (16) experiments. The acidic properties of cotton cellulose have been studied by Sookne and Harris ( 3 6 ) who used a number of different techniques. By testing carefully prepared dewaxed and depectinized cott,on, these authors proved that many of the acidic characteristics of cotton are due to the pectic substance. However, the residual base-combining capacity of their highly purified and electrodialyzed cotton samples lead them to the conclusion that the acidic groups may be an integral part of the cellulose itself. In view of the slow, but steadily progressing deterioration of the cotton fibers during normal use (3: 3 2 ) and laundering (26, $6, 29), the chemical characteristics of oxycelluloses appear to be of even greater importance. In recent studies on the acidic properties of cotton cellulose anti derived oxycelluloses, Davidson and Neve11 (6,6)have shown that cations are readily absorbed from aqueous solutions. They conclude that the characteristic properties of acidic oxycelluloscs are most easily explained by the assumption that an exchange of ions can occur on carboxyl groups. The ion exchange process is represented by the following reactions which lead to a state of equilibrium:
RCOOH
+ l,l+S RCOOLI + H +
where RCOOH represents an acidic oxycellulose and M +, the cation. Besides silver and calcium ions, methylene blue was found t o furnish a suitable cation for quantitative adsorption measurements on various celluloses. Also unmodified cotton having a low carboxyl content showed cationic adsorption partly attributable to hydroxyl groups. The foregoing experimental
