IND UXTRIAL AND ENGINEERING CHEMISTRY
January 15, 1929
Table 11-Comparison
BEATING
TIME Hours
I
MULLENSTRENGTH FACTOR (100 X kg. per sq. cm. per kg. ream weinht) Pulp A
Pulp
Ratio
I
of Other Properties of Pulps A and B
FOLDING ENDURANCB
1 I
Pulp A
Pulp B
Ratio
No. double folds
sideration of these facts and Figures 1 to 4, in which the data are plotted, it is evident that the freeness-time data alone are suited to any considerable mathematical treatment, largely because such data are obtained by working with wet, almost fluid material under conditions where sampling is easy and accurate. The bursting strength data are next best, but cannot compare in accuracy with the freeness values and, except for only part of the range, the bursting strength bears no very simple relation to the beating time. It has been suggested3 that for the same pulp the rate of change of freeness with time affords a satisfactory means of comparing the performance of two beaters. Conversely, using the same beater, the rates of change of freeness with time should afford a valuable comparison of the ease with which two pulps may be beaten. Figure 1is a plot of freeness against time for the two pulps, A and B, considered here, and the slopes of these curves are the rates of change of freeness with respect to time. These slopes cannot be determined graphically with accuracy, but it is evident that they are given by the first derivatives of freeness with respect to time and are readily found once the equations connecting freeness and time are known. Fairly simple exponential equations were found to fit these curves very closely, as indicated by a comparison of actual and calculated freeness values listed in Table I. For pulp A the equation is F = 676 118 @Jo and the first derivative is d_F = -59 e-o.60 de For pulp B the equation is
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8
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Davis, Pulp PaDer Mag. Can., 24, 987 (1926).
II I
TBNSILE STRENGTH Pulp A K g . Der
Pulp B
w. cm.
Ratio
II
RATE
Pulp A
1
OF CHANGE OF FREBNESS
Pulp B
Ratio
Cc. aer hour
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F = 706 108.3 and the first' derivative is given by
Table I1 gives a comparison of values for the two pulps together with the ratio of the A to the B value a t various times of beating.
Conclusions From a consideration of the plots and tabular data, it is evident that pulp A is by far the superior pulp, that for pulp A the maximum fold is developed after beating 6 hours, that both less and more beating than that accomplished in 6 hours result in considerably decreased folding endurance, that shrinkage for both pulps becomes linear with time after 4 hours' beating, and that the bursting strength increases very little after 10 hours' beating. There is no reason why such comparative data could not be duplicated by the use of any beater. However, instead of beating to the time intervals of this test, i t would be necessary to beat to the freeness values reported a t these time intervals. The ratio of Mullen strength factors as shown in Table I1 varies from 2.42 to 1.85, or 24 per cent, the ratios of folding endurances from 250 t o 30, or 88 per cent; the ratios of tensile strengths from 2.5 to 1.5, or 40 per cent; the ratios of rates of change of freeness from 1.50 t o 1.77,or 18 per cent, which is the most constant. This, together with the ease, accuracy, and rapidity of the freeness tests and their susceptibility to mathematical treatment, leads to the suggestion that rate of change of freeness be used as a preliminary means of comparing pulps.
A Standard Method for Determining the Viscosity of Cellulose in Cuprammonium Hydroxide' BY THE COMMITTEE ON TEE VISCOSITY O F CELLULOSE, DIVISIONOF CELLULOSE CHEMISTRY. AMERICAN CHEMICAL SOCIETY
T
HE determination of the viscosity of the cellulose dissolved in cuprammonium hydroxide solutions has become a routine procedure among most users of chemical cellulose, and many of them include this viscosity in their specifications for purchasing the cellulose. Since, however, the viscosity may vary many hundred per cent according to the method of making the measurement, and since no two laboratories have adopted the same procedure, a committee was appointed by the Cellulose to determine Division of the AMERICANCHEMICALSOCIETY one procedure that could be satisfactorily used by all laboratories. A large amount of work was done by the various members 1 Received
September 10, 1928. Presented at the 74th Meeting of the American Chemieal Society, Detroit, Mich., September 5 to 10, 1927. At this meeting the Cellulose Division voted to adopt this method as a tentative standard, to be adopted later as an official method if it proved to be satisfactory.
of the committee, and descriptions of the methods used by most of the laboratories making these tests were obtained. The method described below is a composite of the various methods used, but i t is also practically the same as the methods used by some of the largest users of chemical cellulose. Since the method is merely a tentative one, and will surely not be the most satisfactory method for all users, some of the points are discussed at some length.
Solvent There are two common methods of making up cuprammonium solutions. One is to prepare dry cupric hydroxide powder and to add this and ammonia to the cellulose. This has generally been found unsatisfactory, however, and the following method is preferred: Clean copper turnings are placed in a glass tube about 26 inches (66 cm.) in length and 4 to 6 inches (10 to 15
ANALYTICAL EDITION
50
cm.) in diameter. Strong ammonia water (26 to 28 per cent "3) containing 10 grams of sucrose per liter is then poured in until the tube is nearly full, and air is bubbled up through it for several hours. It is well to have the tube surrounded with ice during this time. When the copper concentration is more than 3 per cent as shown by rough analysis, the solution is analyzed for copper and ammonia. To determine when the concentration is high enough for the analysis, add about 0.5 cc. of the cuprammonia solution to 50 cc. of ammonia water and compare the color with that of another tube prepared from a standard solution in the same manner. The cuprammonium solution is then diluted to the standard concentration. Dilution is made by addition of water containing 10 grams of sucrose per liter and the calculated amount of ammonia. The standard concentration is 30 * 2 grams of copper, 165 * 2 grams of ammonia, and 10 grams of sucrose per liter. This solution is stored in a dark, cold place when not needed. It may be kept for a month in this way. The analysis for copper may be done electrolytically or by weighing as copper oxide after evaporation and ignition, according to standard practice. The analysis for ammonia is sometimes done by direct titration, adding it to standard acid and titrating back with standard alkali, but this method gives somewhat too high results due to the formation of copper hydroxide. It is best to add strong alkali to a portion of the cuprammonium solution, distil into a standard acid, and titrate back with standard alkali. The chief reason for believing the solution prepared by the above method is superior to that made with the powder is that it will peptize a wider variety of celluloses in a greater range of concentrations. It often gives a clear solution when the powder gives a cloudy one. Also, since the amount of copper present in the solution affects the viscosity, it is better to have one fixed amount as in the solution type of solvent rather than variable amounts as is sometimes necessary when using the powder method. Figure I-DissolVA slight variation in copper concentration ing Bulb has no appreciable effect on the viscosity when that concentration is 3 per cent, but the effect is much greater with lower concentrations such as are used with the powdered cupric hydroxide. Sampling The cellulose used in making up a solution should be selected by careful sampling, as different portions of a bale may differ very much. There are teasing machines on the market which might be used for mixing cellulose, but many would not find it convenient to have one. A good way is to take samples from many parts of the bale, or bales, to be tested, tear them into fine shreds, and mix them carefully. This may be done by throwing the pieces on a table as they are shredded, then quartering and repeating until a sample of suitable size has been obtained. This gives a more representative sample and more uniform results than if the sample were selected from only one place. Careful sampling is essential if consistent results are to be obtained. Concentration of Cellulose The weight of cotton to be used in making up solutions should be carefully considered. The falling-sphere viscometer later described can be used with substances of from 30 to 300,000 centipoises, although its best range is from 100 to 10,000 centipoises. Celluloses vary so widely that no concentration can be chosen which will be sure to give viscosities within these limits. Therefore, some laboratories use 2.5 grams of cotton per 100 cc. of solution for ordinary cotton, 5.0 grams for very low-viscosity cottons, and 1.0 gram for high-viscosity cottons. The last two viscosities are then changed to the viscosities of solutions containing 2.5 grams of cotton by the application of Joyner's law.2 As this law is somewhat uncertain, it is better to use a single concentration. We recommend 2.5 grams of cotton per 100 cc. solvent, except in the case of very high viscosity cotton where 1.0 gram per 100 cc. is best. The viscosities that are too low to be measured with the falling-sphere viscometer are obtained with a special pipet described later. 2
Joyner, J. Chem. Soc., 121, 1523 (1922).
Vol. 1, No. 1 Allowance for Moisture in Cotton
I n weighing out the cottons it was found best to allow 5 per cent for moisture. Drying by heat decreases the viscosity in an irregular manner and it requires too much time to dry the cotton in a desiccator, or to determine the moisture in a separate sample. If the relative humidity varies between 35 and 60 per cent, the moisture content of the cotton will vary between 4 and 6 per cent, which would introduce an error in viscosity readings of about 3 per cent. If the room humidity is higher than 60 per cent or lower than 35 per cent, the cotton samples should be stored in a cabinet that can be kept at a humidity of about 50 per cent. Dissolving Bulb A convenient form of bulb for peptizing the cellulose is shown in Figure 1. It is about 80 cm. long and 4 cm. in diameter, with ends drawn out into tubes, one of them 2 cm. long and 1.3 cm. inside diameter and the other 2.0 cm. long and 0.6 cm. inside diameter.
Filling the Bulb in Absence of Air The cotton is placed in the bulb B, Figure 2. Pieces of heavywalled rubber tubing are slipped over the ends and securely wired. The end a is clamped and the other is fastened to the evacuating and filling apparatus by slipping the rubber tube onto opening b. By manipulating stopcocks c and d, the filling bulb is evacuated and flushed three or more times with hydrogen which has been purified by passing through the heated tube G containing platinized asbestos. After evacuating again, 100 cc. of cuprammonium hydroxide are allowed to enter the bulb B and a slight pressure is applied by turning stopcocks c and d to the hydrogen. The rubber tubing a t b is then clamped shut and the bulb slipped off the apparatus. It is then tumbled on a wheel rotating at about 10 r, p. m. until the cellulose is dissolved. Stopcock c should be especially ground, as the solution tends to eat through the grease. It is necessary to purify the hydrogen because there is usually a slight amount of oxygen left in the hydrogen whether it is from a tank or from another generator. Experiments conducted with purified and unpurified hydrogen, using cotton linters from the same batch, indicated that the viscosity may be lowered as much as 15 per cent by using hydrogen direct from a tank rather than the purified hydrogen. If hydrogen from a tank is used, a mercury-sealed safety valve will safeguard the apparatus
I
ll
c
Figure 2-Apparatus for Determining the Viscosity of Cotton in Cuprammonium Solution
January 15, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
against excessive pressures. Nitrogen, if carefully freed from oxygen, may be used in place of hydrogen, if it is preferred.
Falling-Sphere Viscometer The falling-sphere viscometer consists of a tube 30 cm. long and 1.4 t 0.05 cm. inside diameter, with the lower 4 cm.drawn down to 1 cm. outside diameter. It is etched every 5 cm., and placed inside a larger tube which serves as a water jacket. Water is pumped through this jacket from a constant-temperature bath maintained a t 25' * 0.1' C. If two 200-watt lamps are placed in the back of this tube, and if sheets of pasteboard with slits cut so as to allow the light to shine only through the blue solution itself are placed on each side of the tube, the readings are much easier to take. A piece of blue color filter has also been found to help, for it removes the mottled appearance of the solution, giving a sharper outline to the sphere. Care must be taken in using the lights, for strong light causes a decrease in the viscosity due to both photochemical action of the blue and ultra-violet light and to the heating effect of the infra-red rays. Therefore, the light should be turned on for only a few seconds a t a time while the sphere is being observed. An electric button placed on the floor serves as an excellent means of controlling the lights.
Spheres The spheres used are glass beads 3.175 * 0.05 mm. ( I / * inch) in diameter and as nearly spherical as possible. Their specific gravity should lie between 2.4 and 2.6. The spheres are calibrated in the viscometer with oil of known viscosity, such as may be obtained from the Bureau of Standards, and the constants calculated. Glass spheres are preferred to steel spheres because they are more easily seen in a dark solution. CALIBRATIONOF SPHERES-TOcalibrate the spheres, the viscometer tube is filled with the standard oil, the temperature adjusted, and the spheres dropped through. They are timed through the middle 15 cm. of the tube. The factor K is determined from the equation r] = K t (D d) where q is the viscosity, t the time, D the density of the sphere, and d that of the liquid. When one sphere has been selected by careful measurement it should be weighed and the other spheres chosen to nearly equal it in weight. They may then all be calibrated and only those selected which have the same constant within the required limits of error, depending on the precision demanded. This avoids the necessity of having a separate constant for each sphere.
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Measurement of Viscosity The falling-sphere viscometer is filled by connecting the dissolving bulb and the bottom of the viscometer tube by means of a U-tube, and then applying air pressure to the bulb. It has been shown that air does no harm a t this stage. The glass spheres are dropped through a small centering tube L,which extends a short distance below the surface of the liquid. This insures the sphere entering slowly without air bubbles. Care must be taken to have the axis of the viscosity tube perpendicular. The time required for the spheres to fall 15 cm. is taken and these times are averaged and multiplied by the constant obtained by calibrating the spheres and by the difference in density between the sphere and the solution. This appears to be better than stating the viscosities in terms of seconds or similar arbitrary units. The variations in the walls of the viscosity tube and in the spheres are taken care of by this method because of the calibration, except in so far as the plasticity of the solution may come into play, whereas they are neglected when the viscosities are stated in terms of seconds required for the spheres to fall a definite distance. Viscosity Pipet When the viscosity is very low the special pipet shown in Figure 3 should be used as the time of fall of the glass spheres becomes too low for accurate measurements. It is necessary to follow the dimensions given on the drawing, for they have been determined empirically so that the shearing force F applied by the pipet and the falling-sphere glass are nearly the same, thus making it possible to obtain the same apparent viscosity on a single solution by both methods of measuring viscosity. The pipet is inserted through the rubber tube at the upper end of the dissolving bulb until the lower end of the outer tube T is in exact contact with the liquid level. By pressing the rubber bulb R while the thumb is held over the outlet 0, the solution can be raised in the bulb S. The time taken for the liquid to fall from one etched mark, m, to the other, m', is the
51
measure of viscosity. It is converted into centipoises by means of the same formula as used in calibrating the pipet. The end of the tube T i s crimped in to prevent side play of this tube and to avoid breakage at the junction with the bulb. CALIBRATION OF PIPET-The pipet should be calibrated by timing the outflow of an oil of known viscosity. The constant K is obtained from the formula q =Kdt where q = viscosity, d = density of liquid, and t = time. In using the pipet the bulbs containing the cellulose solution are placed in a thermostat a t 25' C. for an hour and then held in clamps in the thermostat with their necks sticking out. Slight temperature changes caused by the fact that the pipet is not thermostated do not appear to affect the results. Error Due to Plasticity The procedure outlined above has been found to give very satisfactory r e s u 1t s with a wide variety of celluloses. It does not, however, give any indication of the plasticity of the solutions. But for routine measurem e n t s , w h i c h are made solely for determining the degree of breakdown of the cellulose, no evidence has yet been produced to show that plasticity m e a s u r e ments are significant.
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Precision Required
It is quite impossible for a committee to state the degree of precision which should be attained in these measurements. If all the precautions are followed, the actual determinations Figure 3-Cuprammonium Pipet should agree within 1 per Bore of capillary = 1.0 to 1.6 mm. Capacity of bulb S = 6 * 1 cc. cent. However, it is someOutside diameter of tube T,9 mm. times imaossible to obtain samples which are hiform to this degree, and, moreover, many users of cellulose do not require their viscosities to be maintained this closely. It seems best, therefore, to give the method with all the refinements required for the greatest accuracy, and to allow those laboratories which do not need extreme accuracy to omit those precautions which seem to them to be superfluous . Nole-Since the Committee feels that this method will need to be revised within a short time, it requests that users of the method or of any other method submit detailed criticisms of this report in order that the revised method may conform to the wishes of as many laboratories as possible. Criticisms should be sent to the chairman, E. K. Carver, Kodak Park, Rochester, N. Y.
E. K. CARVER,Chairman
E. C. BINGHAM
H. BRADSHAW C. S.VENABLE
Superiority in Corrosion Resistance Not Found in Leviathan's Plates That service conditions are in general more important than composition in influencing the corrosion of steel ship plate is the conclusion reached after a series of tests on representative materials by the Bureau of Standards. Prompted by the oft-repeated statement that the steel ship plate of the hull of the Leviathan has shown in service marked superiority in its resistance to corrosion by sea water, the bureau recently conducted a series of laboratory corrosion tests on this material, together with a number of comparison steels among which was included some other ship plate. The results of the tests have not confirmed or substantiated the claims which have been advanced for the outstanding superior corrosion resistance of the Leviathan ship plate. Differences in service conditions, the importance of the influence of which has not been fully appreciated, are probably the basis of the claims which have been made concerning the alleged superior quality of this material.
