Measurement of Bound (Nonfreezing) Water by Differential Scanning

16

Measurement of

Bound

(Nonfreezing) Water by

Differential Scanning Calorimetry

Downloaded by UNIV OF LEEDS on October 4, 2015 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch016

SUBHASH DEODHAR Department of Paper Science and Engineering, University of Wisconsin, Stevens Point, WI 54481 PHILIP LUNER College of Environmental Science and Forestry, State University of New York, Syracuse, NY 13210 Various terms have been used to characterize the water associated with cellulose fibers. Bound water, imbibed water, water of constitution, adsorbed water, fiber saturation point are some of the terms that have been used to describe the water in pulps and papers. The origin of each term can be traced to either theoretical considerations or to the experimental method of measurement. Bound water has been the most popular term used to describe the associated water. Bulk water or free water is that portion of water not associated (or not bound) with the fibers. Two measurement techniques may not yield identical values of bound water. The non-existence of a sharp boundary between bound water and free water is one reason for such discrepancies. In addition, two methods may be measuring different physical phenomena. Measurement of Bound Water NMR (Nuclear Magnetic Resonance) (_1,_2,J3,4). This technique detects the mobility of protons in various energy states. The hydrogen atoms in bound water are at different energy levels than the hydrogen atoms in free water. These energy levels are measured and recorded in the form of NMR spectra. The bound water can be calculated from the NMR spectrum. NMR measurements may be done at any temperature. While NMR may be the most basic method for the measurement of bound water, i t requires expensive equipment, trained personnel, and considerable preparation for each experiment. These requirements are not frequently available to the researcher in the paper industry. DSC (Differential Scanning Calorimeter (4,_5,6) . When a wet pulp sample is cooled well below 0°C, the free water freezes but the bound water remains in the non-frozen state. When the frozen sample is heated in a calorimeter, the heat required to melt the frozen water can be measured. Non-frozen water, which is defined as the bound water, is the difference between the total water and the frozen water. The freezing of free water and non-freezing of 0-8412-0559-0/ 80/47-127-273$05.00/ 0 © 1980 American Chemical Society In Water in Polymers; Rowland, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

274

WATER IN

POLYMERS


bound water are thermodynamic phenomena, so the measurement i s abs o l u t e , but as the bound water i s c a l c u l a t e d by d i f f e r e n c e , the measurement i s i n d i r e c t . The bound water i s measured only at the f r e e z i n g p o i n t . The experimental procedure i s simpler than NMR. Solute E x c l u s i o n Method 07,8). The bound water i s defined as that water i n a swollen f i b e r s t r u c t u r e which does not act as a s o l v e n t f o r a c r i t i c a l s i z e s o l u t e molecule. E s s e n t i a l l y i t measures the volume of a l l the pores s m a l l e r than the c r i t i c a l pore s i z e . The c r i t i c a l s i z e f o r the pores or the s o l u t e molecules i s a r b i t r a r i l y s e t . As the thermodynamics of bound water does not enter, t h i s method i s not absolute. The s o l u t e e x c l u s i o n method i s a r e l a t i v e l y simple technique, which does not r e q u i r e expensive equipment but does r e q u i r e experimental p r e c i s i o n . Thermogravimetric or the Drying Rate Method (9,10). A wet f i b e r sample i s d r i e d under c o n t r o l l e d c o n d i t i o n s to o b t a i n the d r y i n g r a t e curve. The moisture content at the boundary of cons t a n t - r a t e d r y i n g p e r i o d and f a l l i n g r a t e d r y i n g p e r i o d i s defined as bound water. The d r y i n g r a t e s are h i g h l y dependent upon the d i f f u s i o n r a t e s and the geometry of the sample. These measurements may be made at any temperature. The bound water data on papermaking f i b e r s can be used i n e s t i m a t i n g the r a t e of d r y i n g i n the production of paper. The d r y i n g r a t e method i s not a high powered a n a l y t i c a l technique, but i t does provide d i r e c t measurement of bound water as i t i n f l u e n c e s d r y i n g of paper. WRV (Water Retention V a l u e ) . The water r e t a i n e d when f i b e r s are subjected to e x t e r n a l f o r c e i s known as the Water Retention Value. Water r e t a i n e d by surface t e n s i o n f o r c e s i n a d d i t i o n to adsorbed water may be i n v o l v e d i n t h i s determination. The methods used i n determining t h i s value are c e n t r i f u g i n g (water r e t a i n e d under standard c e n t r i f u g i n g ) (11,12), h y d r o s t a t i c t e n s i o n (water r e t a i n e d under standard tension) (13), and s u c t i o n (water r e t a i n e d under s u c t i o n or vacuum) (14). A l l these methods can be employed at any temperature and the experimental techniques are q u i t e simple. Experimental DSC Measurement. The non-freezing water of wet pulp and paper samples was determined by D i f f e r e n t i a l Scanning C a l o r i m e t r y . A wet sample was h e r m e t i c a l l y sealed i n a sample pan. The empty sample pan and the sealed pan were weighed. The sealed pan was q u i c k l y f r o z e n i n s i d e the DSC chamber to -40°C and s e v e r a l minutes were allowed f o r the system to come to e q u i l i b r i u m . The samp l e holder assembly was then heated at a r a t e of 5°C/min. A scanning speed of 5°C/min was found to give the optimum values of peak height and peak spread. This minimized the e r r o r s i n experimental measurements. A f t e r the DSC measurement, the pan was punctured

In Water in Polymers; Rowland, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


16.

DEODAR AND LUNER

Bound

Water

275

and heated i n a vacuum oven a t 1Q5°C t o o b t a i n the dry weight. A s t r i p chart recorder connected t o the Perkin-Elmer DSC ~ IB recorded the m e l t i n g behavior. A s t r a i g h t base l i n e was obtained when no p h y s i c a l change took p l a c e i n the sample. As soon as the f r o z e n water s t a r t e d m e l t i n g , i t absorbed heat and lowered the temperature. The instrument s u p p l i e d heat t o the sample so t h a t the sample temperature was maintained equal t o the r e f e r e n c e temperature. The amount o f heat s u p p l i e d t o e q u a l i z e the temperatures was recorded on the s t r i p chart r e c o r d e r . The t o t a l area under the DSC curve given by the r e c o r d e r i s p r o p o r t i o n a l t o the t o t a l heat s u p p l i e d t o e q u a l i z e the temperatures, i . e . , the heat r e q u i r e d to melt the f r o z e n sample. An i n t e g r a t o r performed the i n t e g r a t i o n a t the same time the curve was being t r a c e d . The i n t e g r a t o r was c a l i b r a t e d u s i n g d i s t i l l e d water as a sample. Thus, from the i n t e g r a t o r r e a d i n g , the heat r e q u i r e d to melt the f r o z e n sample was c a l c u l a t e d knowing the c a l i b r a t i o n f a c t o r . Each wet pulp o r paper specimen was c e n t r i f u g e d a t 3000 rpm f o r 30 minutes. The c e n t r i f u g a l f o r c e was e q u i v a l e n t t o 900g s. The c e n t r i f u g a l method o f water r e t e n t i o n v a l u e (WRV) under cond i t i o n s o f 900g and 30 minutes has been suggested by S c a l l a n and C a r l e s (12). Thus the water content o f the pulps was reduced t o the l e v e l o f WRV before determining the n o n - f r e e z i n g water. F o r each specimen, minimum o f 5 runs were made and the average o f f i v e samples i s reported as n o n - f r e e z i n g water. (The complete s e t o f r e s u l t s i n c l u d i n g the s t a t i s t i c a l a n a l y s i s i s a v a i l a b l e from the authors). f

Pulp P r e p a r a t i o n and C h a r a c t e r i s t i c s . S e v e r a l papermaking v a r i a b l e s were s e l e c t e d f o r s t u d y i n g the e f f e c t s on the non-freezi n g water. These were b e a t i n g , d r y i n g , p r e s s i n g , removal o f f i n e s , and a d d i t i o n o f s a l t s . A hardwood bleached pulp was used f o r the once-dried pulp w h i l e f o r the n e v e r - d r i e d p u l p , a 50% y i e l d spruce bleached k r a f t was used. The pulps were beaten i n a V a l l e y beater according t o TAPPI standard T-200 and handsheets made. The n o n - f r e e z i n g water v a l u e s were determined f o r both the pulp and the rewetted handsheets. The d e n s i t y , breaking l e n g t h , and e l a s t i c modulus o f the handsheets were a l s o measured by s t a n dard TAPPI procedures. The e f f e c t o f f i n e s was s t u d i e d f o r unbleached, n e v e r - d r i e d spruce s u l f i t e pulp ( y i e l d 57.6%). The f i n e s were removed u s i n g a 200-mesh screen. The n o n - f r e e z i n g water was measured f o r the pulp and the rewetted handsheets. To prepare a sheet from f i n e s , the f i n e s suspension was poured i n t o a f l a t d i s h and the water allowed to evaporate under ambient c o n d i t i o n s . The sheet d i d not d i s i n t e g r a t e on r e w e t t i n g . To study the e f f e c t o f a p p l i e d pressure on n o n - f r e e z i n g wat e r , handsheets were made i n a TAPPI standard sheet mold and manu a l l y pressed a t v a r i o u s pressures up t o 1000 p s i . The handsheets were pressed between a b l o t t e r and a s t e e l p l a t e . When pressed a t high p r e s s u r e s , the wet handsheets adhered s t r o n g l y t o the b l o t t e r



276

WATER IN

POLYMERS

and to the s t e e l p l a t e . To prevent the adhesion and f o r easy sepa r a t i o n of the sheet a f t e r p r e s s i n g , a f i l t e r paper was p l a c e d between the sheet and the s t e e l p l a t e . For the sake of u n i f o r m i t y , t h i s procedure was f o l l o w e d f o r a l l p r e s s u r e s , even though i t was not necessary a t low p r e s s u r e s . One group of handsheets was r e placed i n the water immediately a f t e r p r e s s i n g so the f i b e r s i n t h i s group were never allowed to dry. The remaining handsheets were d r i e d i n the humidity room under standard c o n d i t i o n s . The bound water was determined f o r these sheets a f t e r they were soaked i n water o v e r n i g h t . The d e n s i t y , breaking l e n g t h , and e l a s t i c modulus of the d r i e d sheets were determined by TAPPI standard methods. Non-freezing water measurements were made on s u l f i t e , k r a f t and mechanical pulps a t s e v e r a l l e v e l s of pressure. The s u l f i t e pulp was pulped from spruce to a 57.6% y i e l d . I t was unbleached, unbeaten and never d r i e d . The freeness of the d i s p e r s e d pulp was 635 ml CSF. The k r a f t pulp (50% y i e l d ) was pulped from the same spruce as the s u l f i t e pulp. I t was unbleached, unbeaten and never d r i e d . The freeness of the d i s p e r s e d pulp was 675 ml CSF. The groundwood was i n the dry l a p form and when d i s p e r s e d , i t had a freeness value of 465 ml CSF. E f f e c t of S a l t s on Non-freezing Water. The n o n - f r e e z i n g wat e r of pulps i n the presence of s a l t s was a l s o determined. The s a l t s added to the pulp were KNO3, C s C l , K l , MgSO^ L i C l , L12SO4, and A ^ i S O ^ ) ^ . The f i r s t three are considered s t r u c t u r e breakers w h i l e the l a s t f o u r are s t r u c t u r e makers. The pulp used was a 57.6% y i e l d spruce s u l f i t e pulp. I t was unbleached, unbeaten and never d r i e d . The pulp was d i s p e r s e d i n 1 M s a l t s o l u t i o n . To study the e f f e c t of c o n c e n t r a t i o n , 0.5 M and 0.1 M s a l t s o l u t i o n s were a l s o used f o r d i s p e r s i o n i n the case of KNO3 and L12SO4. The pH of the d i s p e r s i o n before and a f t e r s a l t a d d i t i o n was measured. The pH was not a d j u s t e d . The d i s p e r s i o n was s t i r r e d g e n t l y f o r a few hours by a magnetic s t i r r e r bar. The d i s p e r s i o n was c e n t r i fuged a t 3000 rpm f o r 30 minutes (900g s) as had done f o r other pulp samples before the n o n - f r e e z i n g water d e t e r m i n a t i o n . No a t tempt was made to wash the s a l t s from the p u l p s . f

C a p i l l a r y Condensation - F r e e z i n g P o i n t Depression As a consequence of s u r f a c e t e n s i o n , there i s a b a l a n c i n g pressure d i f f e r e n c e across any curved i n t e r f a c e . Thus, the vapor pressure over a concave l i q u i d s u r f a c e w i l l be s m a l l e r than t h a t over a corresponding f l a t s u r f a c e . This vapor pressure d i f f e r e n c e can be c a l c u l a t e d from the K e l v i n ' s equation: 2«V cos6 RT I n P / P = m

r

Q

Where P i s the vapor pressure i n the c a p i l l a r y of r a d i u s r , P i s the vapor pressure of f r e e water; G, V and S are s u r f a c e t e n s i o n , molar volume and contact angle of the water, r e s p e c t i v e l y . r

Q

m


16.

DEODAR AND LUNER

Bound

Water

277


I f i t i s assumed that an absorbed l a y e r o f water e x i s t s bef o r e c a p i l l a r y condensation takes p l a c e and that t h i s l a y e r cons i s t s o f ordered o r o r i e n t e d water molecules, then the contact angle i n K e l v i n ' s equation should be very c l o s e t o zero. With zero contact angle, vapor pressures i n the c a p i l l a r i e s are c a l c u l a t e d from K e l v i n ' s equation f o r c a p i l l a r i e s from 10 Â t o 200 Â, Table I . The vapor pressure o f water below 0 C ( 1 5 ) i s compared w i t h the vapor pressures i n the c a p i l l a r i e s t o o b t a i n the f r e e z i n g p o i n t s . F i g u r e 1 shows the r e l a t i o n between the f r e e z i n g p o i n t depression and the c a p i l l a r y r a d i u s . Table I : C a p i l l a r y r a d i i , vapor pressures and f r e e z i n g p o i n t of water from K e l v i n ' s equation.

Capillary Radius, A

Vapor Pressure, mm o f Hg

Freezing P o i n t , °C

σο

4.579

1000

4.526

-0.15

200

4.322

-0.8

150

4.258

-1.0

100

4.08

-1.6

75

3.925

-2.11

50

3.634

-3.15

40

3.41

-4.0

30

3.07

-5.4

20

2.368

-8.8

10

1.443

-15.0

0

The DSC curves o f the n o n - f r e e z i n g water f o r a l l pulp and paper samples gave a minimum m e l t i n g p o i n t o f -4°C. A f r e e z i n g p o i n t depression o f 4°C corresponds t o a 40 A r a d i u s c a p i l l a r y . Since f r e e z i n g o r m e l t i n g was not observed below -4°C, i t may be concluded that the water i n c a p i l l a r i e s s m a l l e r than 40 Â d i d not f r e e z e . A c r i t i c a l pore s i z e can be d e f i n e d as the l a r g e s t pore that can c a r r y 100% n o n - f r e e z i n g water. This value i s chosen as 40 A. The choice i s s u b j e c t i v e t o some extent.



278

WATER

IN

POLYMERS

C A P I L L A R Y RADIUS, A

Figure 1.

Freezing point of water in capillaries (from Kelvin's equation)

ε

700

600

500

400

300

200

100

F R E E N E S S , csf

Figure 2.

Effect of beating on once-dried Kraft pulp: f | j handsheets

pulp; (φ)

rewetted


16.

DEODAR AND LUNER

Bound

Water

279


R e s u l t s and D i s c u s s i o n E f f e c t o f Beating on Non-freezing Water. I t i s w e l l known t h a t b e a t i n g pulp r e s u l t s i n an i n c r e a s e d water r e t e n t i o n v a l u e . I t was t h e r e f o r e o f i n t e r e s t t o study the e f f e c t o f b e a t i n g on the n o n - f r e e z i n g water on s e v e r a l types o f p u l p s . F i g u r e 2 shows the changes i n the n o n - f r e e z i n g water and WRV o f a d r i e d hardwood k r a f t (commercial) pulp and handsheets made from t h i s pulp which was d r i e d and rewetted. Both the n o n - f r e e z i n g water and water r e t e n t i o n value f o r the pulp i n c r e a s e d on b e a t i n g . Two zones were observed, one from 700 t o 600 CSF and one from 300 CSF t o lower freeness v a l u e s . However, both n o n - f r e e z i n g water and WRV f o r the rewetted samples remained constant. F i g u r e 3 shows the same measurements f o r a bleached, neverd r i e d k r a f t (Spruce) pulp. I n c o n t r a s t t o F i g u r e 2, b e a t i n g d i d not a l t e r s i g n i f i c a n t l y the bound water values f o r the pulp o r the rewetted handsheet. However, the WRV f o r the pulp i n c r e a s e d w i t h the i n i t i a l b e a t i n g and was higher than the hardwood k r a f t pulp (Figure 2 ) . I t i s i n t e r e s t i n g t o note t h a t the n o n - f r e e z i n g water of the rewetted papers of the two pulps was constant and s i m i l a r , and a l s o that the beaten, once-dried hardwood pulp had a h i g h e r n o n - f r e e z i n g water value than the beaten n e v e r - d r i e d softwood pulp. The higher n o n - f r e e z i n g water values o f the f i n e s i n the hardwood pulp may be r e s p o n s i b l e f o r these r e s u l t s . I n f a c t , as shown i n Table I I , the non-freezing water o f f i n e s may be f o u r times the value o f f i b e r s . Thus, pulp f i n e s may c o n t r i b u t e s i g n i f i c a n t l y t o the n o n - f r e e z i n g water v a l u e s , but once d r i e d i n t o sheets, they become p a r t o f the f i b e r and do not c o n t r i b u t e t o the non-freezing water on r e w e t t i n g . Table I I : Non-freezing water f o r f i n e s , f i n e s - f r e e pulp and handsheets, the n e v e r - d r i e d s u l f i t e pulp beaten i n v a l l e y beater. W.R.V., g/g

Non-freezing water, g/g Sample

av.

st.dev.

av.

st.dev.

whole pulp

0.568

0.03

2.92

0.55

f i n e s - f r e e pulp

0.538

0.13

2.17

0.30

Handsheets from whole pulp

0.443

0.03

1.26

0.11

f i n e s - f r e e pulp

0.465

0.12

1.26

0.17

fines

1.85

-

0.611

0.07

once d r i e d

fines

13.4 2.68


0.4


280

WATER IN P O L Y M E R S

Figure 3.

Effect of beating on never-dried Kraft pulp: (Jl) pulp; (φ) rewetted handsheets

Figure 4.

Effect of pressing on sulfite sheet: (φ) sheets pressed and rewetted; (φ) pressed, dried, and rewetted



16.

DEODAR AND LUNER

Bound

Water

281

E f f e c t o f P r e s s i n g on Non-freezing Water. F i g u r e s 4 , 5 , and 6 show t h a t the n o n - f r e e z i n g water i n i t i a l l y i n c r e a s e s on pressed rewetted sheets and on pressed, d r i e d , and rewetted handsheets f o r s u l f i t e , k r a f t , and groundwood p u l p s . However, a t h i g h e r p r e s sures, the n o n - f r e e z i n g water v a l u e s decrease. This e f f e c t v a r i e s w i t h the type o f pulp. At t h i s p o i n t , the f o l l o w i n g p i c t u r e can be developed t o q u a l i t a t i v e l y account f o r these r e s u l t s . The n o n - f r e e z i n g water i n both pulps and paper sheets i s the r e s u l t o f the s t r o n g waterc e l l u l o s e s u r f a c e i n t e r a c t i o n . The extent o f t h i s i n t e r a c t i o n depends on the s u r f a c e c h a r a c t e r i s t i c s o f the f i b e r . Thus, hemic e l l u l o s e s i n f i b e r s and f i n e s would be expected t o g i v e h i g h e r n o n - f r e e z i n g water v a l u e s . Coupled w i t h these chemical e f f e c t s i s the p h y s i c a l presence o f pores i n pulps and handsheets. The c e l l w a l l i s composed o f a porous g e l - l i k e system ( 8 ) . I n a paper sheet, pores may a l s o e x i s t between the f i b e r elements as w e l l . The presence o f these s m a l l pores can a l s o r e s u l t i n n o n - f r e e z i n g water. Hence, the n o n - f r e e z i n g water i n pulp and paper appears as the r e s u l t o f both the p h y s i c a l presence o f pores and strong water-surface interactions. The i n c r e a s e i n the n o n - f r e e z i n g water v a l u e s i n F i g u r e 1 could o r i g i n a t e from the c r e a t i o n o f s m a l l pores d u r i n g the beating o f the d r i e d pulp as w e l l as from the f i n e s . L i t t l e e f f e c t on the n o n - f r e e z i n g water v a l u e i s observed a f t e r the paper i s d r i e d and rewetted. This i s the r e s u l t o f pore c l o s i n g during d r y i n g . In c o n t r a s t , no new s m a l l pores are c r e a t e d on b e a t i n g a neverd r i e d softwood pulp and thus the n o n - f r e e z i n g water shows no change. The i n c r e a s e i n n o n - f r e e z i n g water on p r e s s i n g ( F i g s . 4 , 5 , and 6 ) may be a t t r i b u t e d t o the c o n s o l i d a t i o n o f the pore s t r u c t u r e . F i r s t , a t the lower p r e s s u r e s , the l a r g e r pores ( > 4 0 1) where -water i s f r o z e n are c o n s o l i d a t e d t o pores below the c r i t i c a l pore s i z e ( 4 0 A ) . This leads t o an i n c r e a s e i n n o n - f r e e z i n g water. However, on f u r t h e r i n c r e a s e i n p l a t e p r e s s u r e , the nonf r e e z i n g water decreases. This decrease may be the d i r e c t r e s u l t of pore c l o s i n g as a r e s u l t o f d r y i n g between the b l o t t e r . I n deed, a f t e r p r e s s i n g a t 1 0 0 0 p s i , the moisture content f o r the k r a f t pulp decreased from 2 . 9 7 g/g t o 0 . 8 4 g/g o f dry p u l p . I t i s t h e r e f o r e very c o n c e i v a b l e t h a t some o f the s m a l l e r pores were c l o s e d d u r i n g moisture removal. Indeed, C a u l f i e l d and Weatherwax ( 1 6 ) have shown t h a t 20% o f water i s h e l d i n pores 1 2 Â o r l e s s . I t seems l i k e l y t h a t these pores are c l o s e d on p r e s s i n g and/or d r y i n g . Support f o r t h i s mechanism i s based on the f a c t t h a t the d r i e d and rewetted sheets show a s i m i l a r p a t t e r n w i t h p r e s s i n g as the non-dried sheets. The maxima i n the p r e s s i n g curves i n F i g u r e s 4 , 5 , and 6 i s thus b e l i e v e d t o be the r e s u l t o f two e f f e c t s , the c r e a t i o n o f s m a l l p o r e s < 4 0 1 and the c l o s i n g o f the < 1 2 A pores on p r e s s i n g . The d i f f e r e n c e i n the maxima between the f i g u r e s i s furthermore a f u n c t i o n o f the type o f pulp and i t s s t a t e of d i s p e r s i o n . Q


282

POLYMERS


WATER IN

Figure 6.

Effect of pressing on ground wood pulp: (J^) sheets pressed and rewetted; (φ) pressed, dried, and rewetted


16.

DEODAR AND LUNER

Bound

283

Water

|_2.0

|_i.o

> OC

0.85-1


0.75 J

0.65 J

Measurement of Bound (Nonfreezing) Water by Differential Scanning

Recommend Documents