Adsorption of Water by Papers at Elevated Temperatures

ADSORPTION OF WATER BY PAPERS AT ELEVATED TEMPERATURES C. C. HOUTZ AND D. A. McLEAN Bell Telephone Labwatwies, New York, New York Received June 4, 1958 INTRODUCTION

While a large quantity of data is available on the adsorption characteristics of water on various forms of cellulose, for the most part the important region of low equilibrium water vapor pressure and high temperature haa been neglected. Urquhart and Williams (13) have compiled much data on the adsorption of water on cotton over the complete humidity range and a t temperatures up to 110OC. Others (6, 9, 10) have covered similar conditions for other cellulosic materials. In the region first mentioned information regarding the quantity of water adsorbed on cellulosic materials is essential, as papers are frequently impregnated with waxes or oils a t from 100" to 15OoC.,under reduced pressures, for use as electrical insulation. Slight variation in the small quantities of water remaining influences greatly the insulation resistance and other properties of cellulosic materials (3). Accordingly, the equilibria have been extensively investigated between water and two types of paper, one processed from linen rag and the other from kraft wood pulp. Both papers are super-calendered condenser tissue less than 0.5 mil in thickness. The temperature of test vaned from 100°C. to 150°C., and equilibrium relative vapor pressures from 0 to 3 per cent were obtained. Data in this temperature and pressure range should also aid in advancing the knowledge of the mechanism of water adsorption on cellulose and of the structure of cellulose itself. EXPERIMENTAL

The sorption was measured by the addition of accurately measured quantities of water to the sample of known dry weight, subtracting the quantity of water existing as vapor in equilibrium with the sample from the quantity admitted to determine the amount adsorbed. For the present purpose this procedure is more accurate than the method of direct weighing, which cannot be used here because of the small quantities of water sorbed. The apparatus employed is shown schematically in figure 1, and is constructed entirely of glass. Bulb A, the volume of which between the stop 309

310

C. C. HOUTZ M D D.

A. MCLEAN

cocks was calibrated with water, is used to determine the volume of the whole and of separate sections of the completed apparatus. B is a mercury manometer, read with a cathetometer accurate to f 0.02 mm. of mercury. Corrections are applied to the manometer readings to remove the error caused by unequal meniscus heights. This is a small correction, seldom being greater than the error incurred in reading the manometer. C is a bulb of about 500-cc. volume, independently calibrated. It is employed in desorption measurements and has a short capillary tube attached to facilitate freeaing-out operations. D is a small bulb set on a capillary tube, in which the water from the system may be collected by free5ing out with dry ice and acetone. GI, Gz, and Gaare capillary tubes 85 cm. long

t -

TO PUMP

FIG.1. Schematic diagram of adsorption apparatus

and averaging 1.34mm. in diameter, accurately calibrated with a mercury thread. Water for the sorption experiments is distilled into the capillaries from the small bulb H, where it has been thoroughly degassed by alternate freezing and melting under vaCuum. Distillation from the bulb to the tubes is effected by a slight temperature difference, as too rapid distillation causes gaps to form in the water columns. Although in reading the water level in the capillary a small mirror is the only aid, the error of reading is only f 0.0002 g. of water, a n error which will be the same regardless of the total quantity of water evaporated from the tube, for the original water level in the capillary is held as reference throughout a series of measurements. This method, then, for measuring admitted water incurs a rather

ADSORPTION OF WATER BY PAPERS

311

large error for a single admission, possibly 2 per cent, but with additional admissions the percentage error decreases to a negligible value a t completion of a series of measurements. The method outlined above for measurement of water was used in obtaining a part of the data reported. However, certain difficulties of manipulation made it advisable to employ a second method, which for most purposes offers greater ease of operation. This is particularly true when the quantities of water measured are comparatively small, as in the work a t the higher temperatures. This second method makes use of the calibrated bulb and mercury well F. Water is admitted as vapor to the large bulb from the small bulb J, containing liquid water, purified as in the case of bulb H. The pressure of the water vapor in the bulb is read with the aid of a small mirror on the attached manometer. The water vapor is then displaced into the system by flushing out the bulb with mercury. This procedure incurs probably about the same error for a single measurement as the capillary method, but has the disadvantage that the error may be to some extent cumulative. Increased ease of operation, however, is a not inconsiderable factor in choosing between the two procedures. E is the tube containing the sample, and is immersed in a bath which is maintained a t constant temperature. Thermocouple measurements indicate that a regulation of f0.05"C. is obtained. The samples are taken from rolls of tissue and wound loosely into cylinders. They are weighed in the dry condition after being heated at 100°C. under vacuum for a t least 8 hr. This treatment is considered to give an essentially dry paper, although the treatment is necessarily arbitrary (l), for it appears in the light of the results obtained that in a theoretical sense the paper cannot be obtained in the dry state without disintegration occurring under the extreme conditione necessary to complete drying. The same treatment is given the sample a t the beginning of each series of measurements. Correction was made in all measurements for the thermal decomposition of the paper. This is a small correction a t 100°C., but becomes of increasing importance with increasing temperature. In one system employed, for example, the permanent gas pressure (gases other than water vapor) increased 0.07 mm. per hour, while 0.04 mg. of water was formed per hour a t 150°C. RESULTS

The two types of paper were examined over a similar range of conditions. Initial measurements were made a t lOO"C., the relative vapor premure varying from zero to the maximum obtainable, which was limited by the external temperature ahd was approximately 3.5 per cent. Additional series of measurements were made a t llO°C., 12OoC., 130°C., and 150°C.

312

C. C. HOUTZ AND D. A. M C L W

The relative vapor pressure range decreased with increasing temperature, with a maximum of 0.7 per cent available at 150°C. Graphical representation of the adsorption isotherms is shown in figure 2 for the linen rag paper, and in figure 3 for the wood pulp paper. The two sets of curves are alike in type, but the paper derived from wood pulp exhibits a considerably higher adsorption under all conditions.

RELATIVE VAPOR PRESSURE IN PER CENT

ho. 2. Adsorption of water by linen rag paper

It will be noticed that the 120OC. isotherm for the linen rag paper is missing, while the 110OC. isotherm for wood pulp paper does not appear to fit into the other curves of that family. The reason for the omission of the one and the error of the other is due to the fact that, in preparation of the sample for those particular series of measurements, the samples were subjected to temperatures above those a t which the measurements were made. The isotherms obtained in this low relative vapor pressure range may be expressed accurately by means of the Freundlich adsorption equation,


313

which has been extensively employed by Freundlich and others to express mathematically the adsorption of vapors by porous solids, and which in general form is (y

=

Ap""

where a is the quantity of adsorbed material per unit quantity of adsorbent, p is the pressure of the vapor in equilibrium with the adsorbent, and A

RELATIVE VAPOR PRESSURE I N PER CENT

FIQ.3. Adsorption of water by kraft process wood pulp paper and l/n are constants. The accuracy with which the isotherms may be expressed in this form is shown by the graphs of figures 4 and 5. Each isotherm, then, may be expressed by an equation of the Freundlich type, the equations differing only in the values of the constants. Table 1 lists the two constants log A and l/n with respect to the temperature for the isotherms measured on the two papers. For neither paper does the value of the constant A appear to be approaching a limiting value with

314

C. C. HOUTZ AND D. A. MCLEAN

increasing temperature. The constant l/n, however, approaches unity rapidly, reaching that value within 1 per cent below 15OoC. in the case of the linen paper. For the wood pulp paper, the curve of l/n against T is linear, in the region studied, as the curve of figure 6 indicates. Since l/n is not expected to become larger than unity, it may be assumed to remain constant a t all higher temperatures. Under this condition the isotherm itself is linear, and this has been observed to be the case in numerous vapor-adsorbent systems, notably for carbon dioxide adsorption on wood

LOG,()

P

FIG.4. Plot of curves of figure 2 on logarithmic scale

pulp, as determined by Salley (7). In the same paper Salley finds the isotherms for water adsorption on wood pulps to be linear at concentrations of adsorbed water of 0.00016 g. per gram and less. All of the isotherms obtained are adsorption isotherms, and no desorption measurements were made other than several of a qualitative nature. The few measurements made, however, indicate a definite hysteresis existing between the quantity of water held on adsorption and that held on desorption. The hysteresis was on the order of 5 per cent at 100OC. and 1.6 per cent R.H. This value is probably somewhat, smaller than the real

315


hysteresis, for, according to Pidgeon ( 5 ) , attainment of equilibrium accompanied by changing vapor pressure, which occurs as a consequence of the

4 LOGlO P

FIQ.5. Plot of curves of figure 3 on logarithmic scale TABLE 1 Values of log A and l / n PAPPEU

Linen. . . . . . . . . . . , . . . , , . ,

I

T

log A

l/n

100

-3.292 -3.552 -4.021 -4.375

0.802 0.872 0.989 0.991

-3.173 -3.383 -3.540 -3.757 -4.150

0.767 0.783 0.796 0.837 0.879

;;: 150

Kraft.. . . . . . . . . . . . . . , . . ,

100 110 120 130 150

procedure employed in this instance, tends to decrease the area of the hysteresis loop. However, the rapid attainment of equilibrium at 100°C.

316


and the large number of admissions of water required to obtain the isotherm, which is but 3 per cent of the entire length of the isotherm, minimize the error incurred, and the difference from the true hysteresis is probably small. In any event, the data indicate that hysteresis continues

TEMPERATURE IN DEGREES CENTIGRADE

FIG.6. Plot of constants from table for kraft process wood pulp paper

well into the region of slight adsorption and may be present at all moisture contents. The greater adsorption observed on the kraft wood pulp paper may be due in whole or in part to any one of a number of differences in the structure and composition of the two papers. Sufficient data are lacking, however, to attempt at present to assign the difference in adsorption to any

317


particular difference or differences in the papers. It is known, for instance, that the linen rag paper contains a much higher percentage of a-cellulose than does the wood pulp paper, but the differences among a-,8-, and y-cellulose are not well defined. However, the observed reproducibility of the isotherms eliminates the possibility of chemical reaction in the ordinary sense with either the cellulose of the paper or the impurities contained in the paper. Aleo, adsorption by lignin, which is present in greater quan-

2

RELATIVE VAPOR PRESSURE IN PER CENT

FIG.7. Decrease in adsorption upon preheating kraft process wood pulp paper

tity in the wood pulp paper, cannot be the cause of the greater adsorption by the wood pulp paper, unless the process of lignin removal alters greatly its capacity for adsorption, for Sebgrg (8) has shown that cuprammonium lignin has essentially the same adsorption as the wood from which it is obtained. Other variables in the paper composition cannot, however, as yet be eliminated or confirmed as to their quantitative effect on moisture adsorption

318

C. C. HOUTZ

AND D. A. MCLEAN

Mention was made previously of one isotherm being in error because the paper was dried at a temperature above that a t which the isotherm was obtained. That the temperature of drying, or “preheating,” has a large effect on the moisture adsorption is shown graphically in figure 7, where the isotherms for fresh paper at 100°C. and 120°C. are contrasted with the isotherm obtained a t 100°C. on paper dried a t about 170°C. While the curves shown are for the kraft wood pulp paper, the linen rag paper shows an almost identical decrease in adsorption. The per cent decrease is greater for the linen rag paper, however, since its adsorption in the fresh condition is smaller. This effect appears to be due to the temperature at which the adsorbent is dried, rather than the length of drying time. This conclusion is based on a series of qualitative measurements which showed the adsorption to be little affected by a reasonable increase in drying time. Data of a very similar nature have been obtained by Speakman and Stott (11) for the adsorption of water on wool and by Urquhart (12) for the adsorption of water on cotton. Heats of adsorption have been calculated from the data obtained by employing an equation derived by Freundlich (2). It is of the form

This equation can be altered to allow expression of AH in terms of a and T, making possible direct use of the isotherms and the log-log plots of the isotherms. This is done by substituting from the Freundlich adsorption equation log p =

log a - log A (1/n>

into the heat of adsorption equation to give

Values for l / n and log A , 8s tabulated, were obtained from the log-log plots of the isotherms. Values for the derivatives came from plots of these constants against temperature. Assumptions must necessarily be made in the derivation of this equation, which to some extent invalidate the results obtained. The principal unwarranted assumption is that the structure of the adsorbent does not undergo alteration as the equilibrium temperature is increased. As shown by the curves illustrating the effect of preheating, the adsorption decreases with increased drying temperatures, which indicates a change in the structure of the paper. It is not known as yet whether or not a stable structure

319


can be obtained by preheating to a temperature sufficiently high to include the entire range of measurements, and until this is ascertained the values given are of a tentative nature, although no more so than calculations made by other investigators from sorption data. Figure 8 illustrates the heat of adsorption for one gram-molecule of water on an infinite quantity of kraft process wood pulp paper containing the quantity a of adsorbed water. The solid parts of the curves are those values which fall within

0.001 0,002

0.003

0.004

a

0.005

0.006

0.007

0.008

GM./GM.

FIG.8. Heats of adsorption of water on kraft process wood pulp paper

the range of humidities covered, while the dotted portions are extrapolated by means of the equation. The change in heats of adsorption with ternperature is small and may not be real, although the increase in the heats with increasing temperatures does not appear unreasonable. The “heat of adsorption” values are arbitrarily chosen as the differences between the total heats evolved in adsorption and the heats of condensation of water vapor to liquid water. These are the quantities called by

320


Newsome and Sheppard (4) “heats of wetting.” It may be argued that the heat of crystallization of water should be subtracted also, but it is not known in what form comparable to free water the adsorbed water exists, SO that any subtraction of heats of crystallization calculated from existing data would be highly questionable, and the variation with temperature is likely small in comparison with the variation in heats of condensation of water with temperature. Curves for the heats of adsorption of water on the linen rag paper are not shown, as the constants obtained from the isotherms do not appear to be as reliable as those obtained from the kraft paper. It is fairly certain, however, that the slope of the AH versus CY ctlrve is greater, values of AH being substantially higher at moisture contents of 0.001 g. per gram and less, while the values fall off more sharply toward zero at the higher adsorptions. SUMMARY

1. The apparatus employed to obtain sorption isotherms is described, with particular attention to two methods of measuring the quantity of water admitted to the adsorbent. 2. Adsorption isotherms for water on two types of cellulosic tissue, one of linen derivation and the other of wood pulp, have been obtained for temperatures from 100°C. to 150°C. and for equilibrium water vapor pressures from 0 to 25 mm. of mercury. 3. The Freundlich adsorption equation is found to apply to the isotherms in the range studied. A table of the calculated constants is presented. 4. Hysteresis between adsorption and desorption equilibria is found under the conditions of the experiment, but no quantitative data are presented. 5. Drying paper at temperatures higher than the temperature a t which adsorption is subsequently measured is shown to cause a large diminution in the adsorption a t the temperature of measurement. 6. Tentative values for the heats of adsorption are calculated by means of a modification of Freundlich’s heat of adsorption equation, and curves are presented for the heats of adsorption on wood pulp paper with respect to the’quantity adsorbed. REFERENCES (1) DAVIDSON AND SHORTER: J. Textile Inst. 21, T165 (1930). (2) FREUNDLICH: Colloid and Capillary Chemistry, pp. 117-19, 135. E. P. Dutton and Co., Kew York (1922). (3) MURPHY AND LOWRY: J. Phys. Chem. 54,598 (1930). (4) NEWSOME AND SHEPPARD: J. Phys. Chem. 36, 3, 930 (1932).


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(5) PIDGEON: Can. J. Research 10, 713 (1934). (6) PIDQEON AND MAASS:J. Am. Chem. SOC.62, 1053 (1930). (7) SALLEY: Textile Research 6, 493 (1935). (8)SEBORG: Paper presented at meeting of TAPPI,New York, February 21, 1938. (9) SEBORG AND STAYM: Ind. Eng. Chem. 23, 1271 (1931). (10) SHEPPARD AND NEWSOME: Ind. Eng. Chem. 26,285 (1934). (11) SPEAKMAN AND STOTT: J. Textile Inst. 27, T186 (1936). (12) URQUHART: J. Textile Inst. 20, T125 (1929). (13) URQUHART AND WILLIAMS: J. Textile Inst. 16, T559 (1924).

Adsorption of Water by Papers at Elevated Temperatures

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