Wetting and Penetration of Paper Surfaces - ACS Publications

---

22 Wetting and Penetration of Paper Surfaces

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

J. F. OLIVER Xerox Research Centre of Canada, Mississauga, Ontario, Canada L6J-5X6

High l e v e l image q u a l i t y s p e c i f i c a t i o n s demanded by non-impact technologies such as ink jet, e l e c t r o p h o tography and thermography, pose new requirements in the p r i n t i n g substrate which f o r economic reasons, will continue to be predominantly paper-based. Trad i t i o n a l s t a t i c methods f o r e v a l u a t i n g wetting and p e n e t r a t i o n behaviour are severely l i m i t e d however because of the r e l a t i v e s i z e o f the l i q u i d ink drops or dry toner p a r t i c l e s and the inherent physicochemic a l complexity of paper s t r u c t u r e s . To avoid many of the pitfalls a s s o c i a t e d with s t a t i c measurements a dynamic s o r p t i o n apparatus has been developed to study spreading and p e n e t r a t i o n d i r e c t l y a t f i b r e ­ - l e v e l r e s o l u t i o n . In t h i s article s o r p t i o n of an aqueous ink j e t ink on a wide v a r i e t y of commercial p r i n t i n g papers i s reported. The r a m i f i c a t i o n s o f these experiments are discussed i n the l i g h t of some e x i s t i n g t h e o r e t i c a l models f o r l i q u i d penet r a t i o n and in terms o f some s t r u c t u r a l aspects of paper s u b s t r a t e s . The porous nature of paper i s o f fundamental importance i n determining i t s p h y s i c a l i n t e r a c t i o n with l i q u i d s . An extensive review and d i s c u s s i o n of l i q u i d p e n e t r a t i o n i n paper has r e c e n t l y been covered i n an e x c e l l e n t s e r i e s of a r t i c l e s by Hoyland (1,2). Experimentally two major d i f f i c u l t i e s emerge which complicate i n t e r p r e t a t i o n of penetration data f o r aqueous l i q u i d s i n t o paper: ( i ) unambiguous d e s c r i p t i o n of the porous s t r u c t u r e ; and ( i i ) the e f f e c t of f i b r e s w e l l i n g on the pores. Moreover i n reprographic technologies, where the r e l a t i v e s i z e of paper f i b r e s with respect to ink j e t (IJ) drops o r toner p a r t i c l e s f o r example, are comparable, f i b r e i n t e r a c t i o n s obviously play a more s i g n i f i c a n t r o l e on the r e s u l t a n t image q u a l i t y , and thereby impose s p e c i a l demands upon s t r u c t u r e s with acceptable p r i n t i n g properties(3).Thus use of standard s t a t i c t e s t measurements o f wetting and p e n e t r a t i o n of 0097-6156/82/0200-0435$06.00/0 © 1982 American Chemical Society In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

436

REPROGRAPHIC

TECHNOLOGY

macroscopic drops such as contact angle and s i z e methods which are employed i n conventional p r i n t i n g technology, are of l i m i t e d value. To avoid these shortcomings we have developed a dynamic s o r p t i o n apparatus capable of studying spreading and penetration of I J drops d i r e c t l y at microscopic r e s o l u t i o n s . In t h i s a r t i c l e we report some experimental s t u d i e s on spreading and p e n e t r a t i o n of an aqueous I J ink on a v a r i e t y of commercial p r i n t i n g papers. The data are tested against e x i s t i n g simple t h e o r e t i c a l models f o r l i q u i d penetration i n porous media. No attempt has been made to develop or employ more rigorous models. C a p i l l a r y and D i f f u s i o n Models For Porous Media Penetration of a l i q u i d flowing under i t s own c a p i l l a r y pressure i n a h o r i z o n t a l c a p i l l a r y , or i n general, where g r a v i t y can be neglected, i s t h e o r e t i c a l l y described (4) by the LucasWashburn equation

*=

^

W

where % = penetration d i s t a n c e a f t e r time t, r = pore r a d i u s , Y = surface t e n s i o n , 6 = contact angle, and n. = l i q u i d v i s c o s i t y . A l t e r n a t i v e l y f o r the purpose of these experiments i t w i l l be more convenient to employ the volume V, of l i q u i d p e n e t r a t i n g i n t o the porous s t r u c t u r e a f t e r time t. Thus from [ l ] i t can be shown (4) that f o r c a p i l l a r i e s so small that the e x t e r n a l pressure i s n e g l i g i b l e i n comparison to the c a p i l l a r y pressure

1

where k i s a constant depending on the pore s t r u c t u r e of the p a r t i c u l a r system. In the f o l l o w i n g experiments i t i s assumed that throughout p e n e t r a t i o n the unabsorbed l i q u i d drop remaining on the s u r f a c e maintains a c i r c u l a r contact l i n e and s p h e r i c a l cap geometry. On t h i s b a s i s V can be c a l c u l a t e d from the contact angle G and base diameter of the drop a, according to Bikerman's (5) equation 7ra(2 - 3 cos9 + c o s 8 ) 24 s i n 6 3

=

3

[3]

The Lucas-Washburn equation i s the simplest equation to model the r a t e of c a p i l l a r y penetration i n t o a porous m a t e r i a l . I t i s derived from P o i s e u i l l e ' s i (4) f o r laminar flow of a Newtonian l i q u i d through c a p i l l a r i e s of c i r c u l a r c r o s s - s e c t i o n by assuming that the pressure drop (AP) across the l i q u i d - v a p o r i n t e r f a c e i s given by the Laplace-Young (6) equation. In p r a c t i c e , depending a

w

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

22.

Wetting, Penetration of Paper Surfaces

OLIVER

437

upon the complexity of the porous system the e f f e c t i v e pore radius r necessary to give the c o r r e c t flow r a t e according to [ l ] can vary considerably from the average c r o s s - s e c t i o n . To overcome t h i s shortcoming s e v e r a l supposedly more r e a l i s t i c models have been proposed. Cheever (7) f o r example developed a model to desc r i b e flow of polymer l i q u i d s i n porous substrates which i s given by the equation

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

2*Y_ __ d cos 6 t ( R - R

Q

)

2

M

= - ^

where R = drop radius a t zero time R = drop radius at time t d = h a l f s l i t width of pores. G i l l e s p i e (8) on the other hand developed an equation of the Lucas-Washburn type without s p e c i f i c reference to an e x p l i c i t pore model on the b a s i s of D'Arcy's law ( 6 ) . Assuming that AP was constant G i l l e s p i e derived the f o l l o w i n g equation f o r two dimens i o n a l r a d i a l spreading of a l i q u i d drop

R (R where

and

R , R = radius of s t a i n a t time zero and a f t e r time t , h = substrate t h i c k n e s s , 3 i s given by

spreading

bK Ycos0

where b = constant f o r the s u b s t r a t e , C = l i q u i d s a t u r a t i o n concentration i n the s u b s t r a t e , K = corresponding substrate p e r m e a b i l i t y a t C . G i l l e s p i e demonstrated the approximate v a l i d i t y o f [ 5 ] f o r spreading of drops of non-polar l i q u i d s on f i l t e r paper. More r e c e n t l y K i s s a ( 9 ) extended i t s a p p l i c a t i o n to the s o r p t i o n of various alkanes on n a t u r a l and s y n t h e t i c f a b r i c s . Rather than using drop r a d i i K i s s a measured drop areas which are more accurate p a r t i c u l a r l y f o r spreading on a n i s o t r o p i c s t r u c t u r e s such as t e x t i l e s o r paper. On t h i s b a s i s [p] can be w r i t t e n i n the general form

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

438

REPROGRAPHIC

TECHNOLOGY

where the exponents u, m and n are constant f o r a given s u b s t r a t e , and the c a p i l l a r y s o r p t i o n c o e f f i c i e n t a, given by 277TbK cos6

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

a =

8h2C

3

[8]

There have been a number of experimental s t u d i e s (7,8,10-13) reported f o r porous surfaces i n which l i q u i d p e n e t r a t i o n i s not c o n s i s t e n t with [ l ] [2] , [4] or [5] , and values of the exponent n, f o r example i n |_7] , are< 0.5. For paper the Lucas-Washburn c a p i l l a r y model g e n e r a l l y holds f o r p e n e t r a t i o n of non-swelling/ n o n - i n t e r a c t i v e l i q u i d s such as o i l - b a s e d p r i n t i n g inks and various organic s o l v e n t s (1). With water and other p o l a r l i q u i d s the s i t u a t i o n i s compounded by s w e l l i n g and hence the v a l i d i t y of [l] becomes more questionable. Since the i n t e r f i b r e voids are i d e a l i z e d as c y l i n d r i c a l c a p i l l a r i e s having e f f e c t i v e mean r a d i i equal to r , and 6 i s assumed constant with time, t h i s h a r d l y seems s u r p r i s i n g . However E v e r e t t et a l (14) based on t h e i r experimental s t u d i e s maintain that although [ l ] o r i g i n a t e s from a simple model i t i s not s t r i c t l y necessary f o r the r e a l pore system to resemble the model i n order that Washburn k i n e t i c s are followed. A l t e r n a t i v e l y an equation of the general form [ l ] can be obtained by t r e a t i n g p e n e t r a t i o n analogous to a molecular d i f f u s ion process (15,16). In t h i s case the p r o p e r t i e s of the pore system and p e n e t r a t i n g l i q u i d are incorporated i n t o a d i f f u s i o n c o e f f i c i e n t . However on the b a s i s of experimental data obtained f o r one-dimensional flow i n paper (17) there i s s t i l l a need to develop more r e a l i s t i c models f o r the pore geometry. Despite t h i s and p r i m a r i l y because s w e l l i n g i s a d i f f u s i o n process, Hoyland (2) contends that aqueous l i q u i d s penetrate paper more by t h i s process than by c a p i l l a r y a c t i o n . Commencing with F i c k ' s second law of d i f f u s i o n which r e l a t e s the d i f f u s i o n c o e f f i c i e n t D to a f u n c t i o n of the concentration change of d i f f u s i n g l i q u i d i n time t at any point x along the d i r e c t i o n of d i f f u s i o n :

Hoyland a r r i v e d at the f o l l o w i n g s o l u t i o n of [9] , to describe p e n e t r a t i o n of an aqueous l i q u i d by d i f f u s i o n

[10] where F = f r a c t i o n of the amount of penetrant taken up i n time t r e l a t i v e to the amount taken up a t i n f i n i t e time,

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

22.

Wetting, Penetration of Paper Surfaces

OLIVER

ra ,m ,m o t 0 0

439

= amount of penetrant present a t time zero, t and

°°,

h = the i n i t i a l thickness of paper. The approximate v a l i d i t y of these two models w i l l be examined where appropriate i n the f o l l o w i n g experiments by a n a l y z i n g the data with respect to [ 2 ] , [4], [7], and[lO] .

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

Experimental Part Apparatus. Spreading and p e n e t r a t i o n experiments of I J drops were performed i n a dynamic s o r p t i o n apparatus (shown schematica l l y i n Figure 1) c o n s i s t i n g of a drop-on-demand I J p r i n t e r (which t y p i c a l l y e j e c t e d drops i n the range 120-260ym diam. (18)), p r i n t i n g substrate sample holder and video system capable of simultaneously recording plan and p r o f i l e viewing of an I J drop from impact through to drying. Simultaneous r e c o r d i n g of the contact l i n e and drop p r o f i l e down to f i b r e l e v e l r e s o l u t i o n a t ^ 30 fps was achieved by means o f : two video cameras equipped with macro lenses; a screen s p l i t t e r ; s y n c h r o n i z i n g pulse generator; and video recorder. I n t e r f a c i n g the video recorder with an image analyzer enabled s t a t i s t i c a l a n a l y s i s of the sorption of many drops. The whole apparatus was mounted on a heavy metal block f r e e of extraneous v i b r a t i o n and surrounded by a removable Lexan enclosure which permitted constant temperature and humidity control. The apparatus was l o c a t e d i n a l a b o r a t o r y c o n t r o l l e d at 23± 1 C and r e l a t i v e humidity of 72 ± 2%. Temperature v a r i a tions i n the enclosure never exceeded ± 0.1°C and the r e l a t i v e humidity never v a r i e d more than ± 0.5% during a s i n g l e e x p e r i ment. Procedure. (a) C h a r a c t e r i z a t i o n of Paper Samples. A l l the paper samples i n v e s t i g a t e d were subjected to CPPA o r TAPPI standard t e s t measurements. These included c a l i p e r , S h e f f i e l d roughness, Gurley p o r o s i t y , and Cobb and Hercules s i z e . For the Hercules t e s t the a c t u a l ink j e t ink was used as the reference l i q u i d . In a d d i t i o n non-standard t e s t measurements which included p r o f i l o m e t r y and dynamic ink a b s o r p t i o n , were conducted on a Rank T a y l o r T a l y s u r f p r o f i l o m e t e r (Model 5M) and Bristow absorption apparatus (19), respectively. (b) Drop Formation; Narrow s t r i p s of the paper samples were cut from the center of an 8" x 11" sheet so that the machine d i r e c t i o n was lengthwise and perpendicular to the camera o p t i c axes when clamped ( f e l t s i d e toward j e t ) to the sample holder. I n d i v i d u a l I J drops were f i r e d by s i n g l e pulse a c t i v a t i o n a t a pre-focussed area a t l e a s t three drop diameters from the sample edge. Following drop s o r p t i o n , which v a r i e d from near i n s t a n taneous to s e v e r a l minutes depending upon the grade o f paper, the sample was moved approximately three drop diameters and f i r i n g of at l e a s t seven more drops repeated on adjacent unprinted areas.

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

REPROGRAPHIC

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

440

TECHNOLOGY

CAMERA MONITOR CAMERA 1

RECORDER MONITOR

Figure 1.

Schematic of dynamic sorption apparatus and recording system.

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

22.

OLIVER

Wetting, Penetration of Paper Surfaces

441

(c) Data A n a l y s i s : Frame by frame playback o f video r e c o r d ings of the drop contact l i n e and p r o f i l e h i s t o r y (3) were used to measure o r c a l c u l a t e the f o l l o w i n g parameters: d r y i n g time (trf), the time elapsed between the f i r s t video frame ( t ^ 0.03 sec) and complete disappearance of the drop meniscus beneath the paper s u r f a c e ; drop radius R and R f o r an approximately c i r c u l a r drop a f t e r time t and t r e s p e c t i v e l y ; drop area Ag and A; and contact angle 0 and 0 a f t e r time t anc* t . A l s o assuming that throughout p e n e t r a t i o n the unabsorbed l i q u i d drop remaining on the surface maintains a c i r c u l a r contact l i n e and s p h e r i c a l cap geometry, the volume V and Vp a f t e r time t and t j was c a l c u l a t e d from the contact angle 0 and base diameter of the drop p r o f i l e according to [ 3 ] . M a t e r i a l s . A v a r i e t y o f commercial uncoated and coated p r i n t i n g grade papers were s e l e c t e d f o r t h i s study (see Table I ) . These papers were evaluated with an aqueous dye-based ink of surface tension (y) approximately 60 mN.m". Q

Q

Q

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

O

0

t

1

Results and Discussion Dynamic Sorption of Ink J e t Ink Drops on Commercial Papers. Drying times ( t ^ ) and v a r i o u s parameters measured or derived from contact l i n e spreading, p r o f i l e development and p e n e t r a t i o n data for the v a r i o u s systems s t u d i e d are summarized i n Table I . A wide v a r i a t i o n occured i n the values of t ^ among the v a r i o u s papers s t u d i e d . The maximum d e v i a t i o n s i n t ^ based upon an a n a l y s i s of many drops, i n d i c a t e d an a p p r e c i a b l e v a r i a b i l i t y f o r a given paper. However the consistency o f the c a l c u l a t e d penetrat i o n volume, Vp, f o r drops included i n d e t a i l e d contact l i n e and p r o f i l e a n a l y s i s suggest that these d e v i a t i o n s r e s u l t from the s t r u c t u r a l v a r i a b i l i t y i n paper rather than drop-to-drop volume variations. This was s u b s t a n t i a t e d by the marked v a r i a t i o n s i n the f i n a l image shape observed among s e v e r a l drops p r i n t e d on the same paper sample. (a) Contact Line Development: Contact l i n e spreading contours revealed the complexity o f the surface and porous s t r u c t u r e of uncoated and coated papers. With the exception of samples K and P, where r a p i d spreading occurs p r i o r to the ' i n i t i a l period, the i n i t i a l contours f o r a l l other samples remained n e a r - c i r c u l a r and smooth. In part the l a t t e r r e s u l t s from the degree of s i z i n g and i s r e f l e c t e d by higher 0 values (see Table I ) . However even on these papers considerable d e t e r i o r a t i o n i n image q u a l i t y subsequently ensues p r i o r t o , o r a f t e r , t ^ . In c o n t r a s t the coated papers Y and C produced the highest q u a l i t y images even with a lower surface tension i n k . The d e t a i l e d h i s t o r y of contact l i n e development and i t s i m p l i c a t i o n s with respect to the paper s t r u c t u r e are reported elsewhere ( 3 ) ) . 1

O

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

442

REPROGRAPHIC

TECHNOLOGY

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

Table I . Drying times, i n i t i a l apparent contact angles and calculated penetration parameters for single drops of an aqueous I J ink on various commercial p r i n t i n g grade papers.

Ref.

PRINTING GRADE

t

d

n

( b )

0

V

O

( e )

3

sec.

deg. cm x 10~

a

( f )

D m/sec

p

a

5

x 10~

K

Inkjet (unsized)

0.02

0.22

40

a

H

newsprint

1.0

0.43

71.5

1.1

150

P

wet xerographic

4.0

0.15

32

1.3

2

J

rotogravure (coated)

6.4

0.14; 0.55

53

1.2

13

Y

offset (gloss coated)

20.0

0.20

49

2.6

a

V

wet xerographic

21.7

0.08

71

1.4

8

( i n t e r n a l l y sized) A

offset

43.7

0.08; 0.47

50

1.5

9

C

o f f s e t (coated)

81

0.10

57

1.7

0.4

dry xerographic

120

0.8

F

bond 150 (100% rag content)

(a) (b)

(d) (e) (f)

0.09; 0.50

73

1.8

0.005

77

1.5

1

Not resolvable. Exponent i n Equation 7 derived from F i g s . 2 and 3t which should equal 0.5 i f Lucas-Washburn c a p i l l a r y model i s v a l i d . Double values f o r some papers refer to a second or t h i r d stage of area development, respectively. Determined from high-speed cinematographic study (Ref. 18). Calculated from (3). D i f f u s i o n c o e f f i c i e n t derived from F i g s . 7 or 8 and calculated according to [10].

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

1 0

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

22.

OLIVER

Wetting, Penetration of Paper Surfaces

443

(b) Contact Area Development: Figures 2 and 3 show the cont a c t area data p l o t t e d according to Eq. [7] . Apart from systems H and F the corresponding slope values (n) are 0.5, and ( i i ) second region, t > 1 sec, i n which n< 0.5. Although there i s a discrepancy between the MD and CMD data caused by the a n i s o t r o p i c nature of the c o a t i n g , the d e r i v e d values of n f o r the same s o r p t i o n regions agree reasonably. The n o t i c e a b l y lower value of n (given i n Table 1) which i s derived from Figure 3 on the b a s i s of [7] i s probably due to a combination of f a c t o r s . F i r s t l y i n s u f f i c i e n t area a n a l y s i s was completed i n the i n i t i a l region thus values of n i n Table I correspond to the second or t h i r d spreading regions. Secondly i n the l i g h t of Figure 4 and drop a x i s r a t i o v a l u e s , [4] must be regarded as approximate even for system C. (c) P r o f i l e Development: Corresponding changes i n drop prof i l e s expressed i n terms of contact angle, 9, f o r systems with t 44 sec the r a t e of decrease i n 9 remained approximately l i n e a r and constant throughout. The average r a t e of change of 9 i s thus mainly dependent upon 9 and t ^ . With the exception of s i z e d papers ( i . e . F and Z), the r e l a t i o n s h i p between 9 and td i s q u i t e i n c o n s i s t e n t and o b v i o u s l y other s t r u c t u r a l f a c t o r s must be i m p l i c a t e d . I t should be emphasized that f o r many papers the l i q u i d drop contact l i n e became i n c r e a s i n g l y a n i s o t r o p i c beyond the i n i t i a l p e r i o d , i . e . t > 0.05. Thus 9 i s i n c r e a s i n g l y dependent upon the d i r e c t i o n of observation (20). (d) P e n e t r a t i o n : C a l c u l a t e d values of the f i n a l p e n e t r a t i o n volume, Vp, which i s assumed e q u i v a l e n t to the i n i t i a l drop volume c a l c u l a t e d from [3] are a l s o given i n Table I. The c u r v i l i n e a r behaviour of p l o t s of Vp v th i n d i c a t e d the i n a p p l i c a b i l i t y of [2] f o r both coated and uncoated papers. A l t e r n a t i v e l y Figures 7 and 8 show graphs o f the same data p l o t t e d i n terms of V^/Vp Q

Q

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

REPROGRAPHIC

)l 2.0

I

I

I

-1.5

-1.0

-0.5

I 0

TECHNOLOGY

I

I

I

0.5

1.0

1.5

log t (seconds) Figure 2. Drop area data plotted according to [7] for system with t < 22 s. d

-J -1.0

I -0.5

I 0

I 0.5

I

I

1.0

1.5

1— 2.0

log t (seconds) Figure 3. Drop area data plotted according to [7] for systems with t > 44 s. d

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

22.

OLIVER

Wetting, Penetration of Paper Surfaces

445

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

2.0 -

E

o

CMD

0.0 -2.0

-1.0

0.0

1.0

log t, sec

Figure 4. Radial spreading on coated paper (C) plotted according to [4].

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

446

REPROGRAPHIC

TECHNOLOGY

150

Figure 6.

Apparent contact angle (0) variations with time for systems with t 44 s.

d

1

t'

Figure 7.

2

172

(seconds )

Volume data for systems with t < 22 s plotted according to Hoyland's diffusion equation [10]. d

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

>

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

22.

OLIVER

447

Wetting, Penetration of Paper Surfaces

if

^

1.01

η

7

-

π

—

7

/ //

/// ///

0

4

8 1/

t *

Figure 8.

12 1

(seconds *)

Volume data for systems with /,, < 44 s plotted according to Hoyland's diffusion equation [10].

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

448

REPROGRAPHIC

TECHNOLOGY

according to the d i f f u s i o n equation [ l O ] . The i n i t i a l n o n - l i n e a r behaviour i s p a r t l y as a r e s u l t of the e r r o r i n v o l v e d i n c a l c u l a t i n g small values of V, and probably, as suggested by Hoyland (2), because the d i f f u s i o n c o e f f i c i e n t , D f o r p e n e t r a t i o n of aqueous l i q u i d s i s not constant at the commencement of s w e l l i n g . Values of D derived from the remaining l i n e a r p o r t i o n s of the graph and c a l c u l a t e d a c c o r d i n g to [lti] are a l s o given i n Table I. I t should be emphasised that the assumptions invoked by [3] may be reasonable during the ' i n i t i a l ' stages, but i n view of the a n i s o t r o p i c nature of both uncoated and to a l e s s e r degree coated papers ( 3 ) , i t s v a l i d i t y becomes more questionable as Vp i n creases. A l s o because s e v e r a l systems e x h i b i t e d show-through p r i o r to t ^ , values of D i n these experiments (which are based upon the c a l i p e r r a t h e r than the value of h at t = °°) are s u b j e c t to an a d d i t i o n a l e r r o r . In a d d i t i o n to the volume data, i t was a l s o p o s s i b l e to observe and estimate transverse spreading v e l o c i t y of the penet r a t i n g l i q u i d f r o n t during the i n c i p i e n t stages (3). P r e l i m i n a r y observations demonstrate some profound d i f f e r e n c e s i n the porous nature of uncoated versus coated s t r u c t u r e s and i n the r e l a t i v e c a p i l l a r y r e s i s t a n c e with l a t e r a l and transverse spreading of inks with d i f f e r e n t s u r f a c e tensions. One f u r t h e r important aspect i n v o l v e d with p e n e t r a t i o n , was that s h o r t l y before disappearance of the drop many paper samples became i r r e v e r s i b l y swollen. The amount of s w e l l i n g , expressed as the percentage i n c r e a s e i n the o r i g i n a l c a l i p e r was q u i t e a p p r e c i a b l e and ranged from ^5 to 25% for uncoated papers and V) to 10% f o r coated papers. (e) A p p l i c a t i o n of C a p i l l a r y and D i f f u s i o n Models f o r Drop S o r p t i o n : Despite wide v a r i a b i l i t y i n spreading, p e n e t r a t i o n and image development, a c o n s i s t e n t p a t t e r n of events emerge among the systems s t u d i e d . Consequently one may employ a g e n e r a l i z e d mechanism of s o r p t i o n to d e s c r i b e a l l these systems (3). I n i t i a l l y , s i g n i f i c a n t l a t e r a l l i q u i d drop spreading i s i n evidence for systems with low t ^ v a l u e s , but q u i c k l y diminishes as bulk c a p i l l a r y forces take e f f e c t . This i s s u b s t a n t i a t e d , at l e a s t for coated paper, by the i n i t i a l value of n > 0.5, derived from Figure 4 based on [4] . As was noted by Cheever (7) during t h i s stage the drop must be skimming over the s u r f a c e and s u b s t r a t e penetration i s n e g l i g i b l e . In the subsequent second and, i n a few systems, t h i r d stage, l i q u i d spreading terminates and p e n e t r a t i o n predominates. Papers with values of n ( d e r i v e d from F i g u r e s 2 or 3 and given i n Table 1) which are< 0.5 i n d i c a t e slower p e n e t r a t i o n than p r e d i c t e d by a simple c a p i l l a r y model. F r e d e r i c k and Bobalek's (13) explanat i o n f o r t h i s d e v i a t i o n from theory i s the i r r e g u l a r i t y of the spreading, f r o n t as opposed to a d i s c r e t e l i q u i d edge assumed i n Cheever's model. Rather, a s e l e c t i v e f i l l i n g process occurs i n the spreading zone, i . e . f i l l i n g of s m a l l e r , then l a r g e r , c a p i l l a r i e s which i s s i m i l a r to the spreading behaviour d e s c r i b e d by Gillespie. A good example i l l u s t r a t i n g t h i s type of behaviour i s

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

22.

OLIVER

449

Wetting, Penetration of Paper Surfaces

revealed i n s t u d i e s on microspreading of l i q u i d s on grooved surfaces (21). V a r i a t i o n s i n groove dimensions and hence c a p i l l a r y pressure produce i n t e r m i t t e n t c a p i l l a r y movement i n the grooves causing l o c a l p r o t r u s i o n s i n the contact l i n e . Apart from the aforementioned, other f a c t o r s c o n t r i b u t i n g to t h i s apparent increased porous r e s i s t a n c e (or decreased l i q u i d permeab i l i t y ) a r e the e f f e c t of : sharp f i b r e edges, pore t o r t u o s i t y , build-up o f a i r pressure p a r t i c u l a r l y i n 'dead-end pores, breakdown i n l i q u i d supply to spreading f r o n t , and f i b r e s w e l l i n g which tends to d i m i n i s h the average pore s i z e . F i n a l l y increase i n n shown by s e v e r a l systems during postd r y i n g (3) (see Table I) stems from subterranean a b s o r p t i o n processes e.g. i n t e r c o n n e c t i o n o f p a r t i a l l y f i l l e d pores and l o c a l i z e d surface f i b r e wicking. The values of n which i n s e v e r a l cases are c l o s e to the t h e o r e t i c a l value, 0.5, f o r a LucasWashburn type c a p i l l a r y model, suggest that the c o n d i t i o n of flow through completely f i l l e d and interconnected c a p i l l a r i e s to supply the spreading f r o n t , i s u l t i m a t e l y a t t a i n e d . T h i s f i n a l stage r e f l e c t s l a g i n the e q u i l i b r a t i o n of bulk and surface c a p i l l a r y forces. Deviations between the experimental r e s u l t s and the modified Lucas-Washburn c a p i l l a r y models demonstrate the l i m i t a t i o n s o f these t h e o r i e s f o r paper s t r u c t u r e s . Moreover, as discussed above, concentration gradients w i l l l i k e l y e x i s t w i t h i n the penetration zone during ink j e t p r i n t i n g so that AP, the c a p i l l a r y pressure, i s no longer constant and hence []l] l o s e s i t s v a l i d i t y . The i n a p p l i c a b i l i t y of £l] thus makes d e r i v a t i o n of an e f f e c t i v e pore r a d i u s , on the b a s i s of [4] and Figure 4, dubious. A l t e r n a t i v e l y on the b a s i s of a d i f f u s i o n type model, approximate values of the d i f f u s i o n c o e f f i c i e n t D, have been c a l c u l a t e d for these systems. Values o f D which f a l l i n the range o f 10~ to 10~ mm/sec are of the same order o f magnitude as those obtained by Hoyland (2) f o r the penetration of v a r i o u s aqueous l i q u i d s i n bleached softwood paper. Although H o y l a n d s d i f f u s i o n model takes i n t o c o n s i d e r a t i o n f i b r e s w e l l i n g during p e n e t r a t i o n i t s t i l l overlooks many o f the aforementioned s t r u c t u r a l complications unique to paper.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

1

2

5

1

R e l a t i o n s h i p Between Paper S t r u c t u r e and Ink S o r p t i o n . Values o f v a r i o u s p h y s i c a l t e s t measurements used to c h a r a c t e r i s e the d i f f e r e n t s t r u c t u r e s i n terms of ink r e c e p t i v i t y and image q u a l i t y are given i n Table I I . The standard as w e l l as nonstandard p h y s i c a l t e s t measurements, i n d i c a t e a poor c o r r e l a t i o n between s t r u c t u r a l and s o r p t i v e p r o p e r t i e s o f paper. At best methods such as Cobb s i z e provide only a q u a l i t a t i v e g u i d e - l i n e . The trend i n p o r o s i t y , roughness, and l i q u i d absorption measurements, i n terms of t j i s very i n c o n s i s t e n t f o r most papers. S i m i l a r l y Lyne (22) has found that Bendsten or S h e f f i e l d roughness t e s t s do not c o r r e l a t e with perception of l e t t e r p r e s s image u n i f o r m i t y , due to the r e l a t i v e l y l a r g e r areas monitored by these

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

116.3

Twin wire paper, wire side #1. Water completely soaked through sample. See Ref. (19). Trace 8mm long i n CMD.

Y

a b c d

18

234 15.1 1000+

160

26

1000+

30.1

89.8

J

26.0

14.4

32.3

b

b

b

b

79.9

53.1

62.9

25.3

56.6

27.8

61.4

55.3

262.9

K

83.3

H

53

662

48

102.5

Z

52

80

18.9

41

83

V

77

151

1000+

11.4

57.5

77.5

P

1*»5

35

164

146

97.0

F

a

3

2

Cobb Size g/m

174

71.3

C

151

Gurley Porosity sec/100cra

161

107.5

A

Sheffield Roughness F e l t Wire

157

Caliper

Ref.

441

0.7

1.2

1.8

443.5

8.8

2.3

153.0

29.4

29.6

Hercules Size sec.

5.5

64.2

-

38.9

4.9

10.1

-

16.7

3.3

6.8 10.7

6.7

1.7

5.4 6.3

7.6

5.4 1.8

2

8.5

2

0

b

b

b

b

b

Bristow A b s o r p t i o n Roughness Absorption Index Coeff. ml/m ml/m /S^

38.9

a

d

9.3

5.6

3.7

12.6

4.5

2.9

5.6

3.7

/Lim

Talysurf, R

Table I I . Standard and non-standard physical t e s t measurements o f various commercial grade p r i n t i n g papers (determined a t 23°C and 50% r e l a t i v e humidity).

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

8

o H m o x -z o r

-a X

7* >

73

o

22.

OLIVER

451

Wetting, Penetration of Paper Surfaces

methods. Parker P r i n t Surf roughness however was found to be s u p e r i o r , presumably because i t i s more s e n s i t i v e i n the s i z e range o f h a l f - t o n e dots, which can be comparable to I J drop images. For T a l y s u r f roughness i t i s doubtful whether other parameters i n a d d i t i o n to R (but not l i s t e d i n Table II) such as bearing r a t i o , waviness, peak-to-valley height, e t c . are o f more significance. On the other hand the means of generating 3- rather than 2-dimensional profilograms with the T a l y s u r f (23) would enable d e r i v a t i o n of more meaningful d i s t r i b u t i o n patterns of the a c t u a l drop contact area. Furthermore s i n c e a l l commercial papers are chemically heterogeneous, i . e . they c o n s i s t of c e l l u l o s e f i b r e s with v a r y i n g amounts of l i g n i n , h e m i c e l l u l o s e , organic and i n o r g a n i c papermaking a d d i t i v e s , and papermaker's a i d s , there i s a l s o an obvious need to map and q u a n t i f y t h e i r s u r f a c e chemical distribution. The s i g n i f i c a n c e of the l a t t e r parameters can be r e a l i s e d i f one considers the i n i t i a l drop impact. In a f i b r o u s s t r u c t u r e such as paper a l i q u i d i s unable to d i s c e r n d i f f e r e n c e s i n pore depth u n t i l e n t e r i n g the bulk s t r u c t u r e . P r i o r to p e n e t r a t i o n , f o l l o w i n g d i s s i p a t i o n of mechanical f o r c e s , drop wetting and spreading of an aqueous ink j e t drop w i l l be p r i m a r i l y determined by the combined e f f e c t s o f chemical heterogenity and p h y s i c a l roughness. The combination o f these e f f e c t s can be approximated (24) i n terms of Cassie and Baxter (25) and Shuttleworth and B a i l e y ' s (26) equations:

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

a

cos6 = f c o s 0 1

e

+ f

[ll]

2

where 0 r e f e r s to the apparent contact angle and 0 the e q u i l i brium contact angle and f and f to the s o l i d - l i q u i d area f r a c t i o n s ( p h y s i c a l and/or chemical) r e s p e c t i v e l y , and e

1

2

0 = 0 + 0 e

[12]

where 0 r e f e r s to the f i b r e edge angle. I t should be emphasized that on a porous surface such as paper, 0 represents the r e s u l tant of surface and bulk c a p i l l a r y processes (27). Also the contact l i n e and drop shape i n the contact zone w i l l be perturbed to an extent determined by the r a t i o of drop s i z e to surface roughness (28). Bearing these arguments i n mind i t i s not s u r p r i s i n g that values of 0 f o r most systems (see Table I) c o r r e l a t e poorly with data reported i n Table I I . U n t i l more meangingful surface p h y s i cochemical data are a v a i l a b l e the i n t e r p r e t a t i o n , o r a p p l i c a t i o n of contact angle measurements to develop s o r p t i v e models f o r paper, remains h i g h l y questionable. O

Conclusion A wide v a r i a t i o n i n the r a t e o f absorption d r y i n g and image q u a l i t y of an aqueous i n k j e t ink was apparent among the commer-

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

452

REPROGRAPHIC TECHNOLOGY

c i a l printing papers examined. Comparison of some of the sorption data with simple capillary and diffusion models reveal the present limitations of these theories when applied to paper structures. A more rigorous theoretical treatment of ink sorption in relation to the printing substrate requires further developments in characterizing the physicochemical properties of paper. The dynamic sorption apparatus clearly demonstrates the advantages of performing dynamic rather than static measurements of spreading and penetration at microscopic resolutions on substrates as complex as paper. The a b i l i t y to observe ink or toner/paper interactions in situ should be especially valuable for investigating other non-impact technologies. Aclmowledgments The author is grateful for the assistance of R. Forsyth with the dynamic sorption apparatus, A. Jones for physical test data and Xerox Corporation for granting permission to publish this a r t i c l e . Thanks are also due to Dr. M.B. Lyne (PAPRICAN) for providing Bristow absorption data. Literature Cited 1. Hoyland, R.W. and F i e l d , R. Paper Tech. Ind. 1976, 17, 213; 216; 292; 304; and 1977, 18, 7. 2. Hoyland, R.W. "Fibre-Water Interactions in Papermaking:, Tech. Div. BPBIF, London, 1978, 557. 3. Oliver, J.F., paper presented at the Seventh Fundamental Research Symposium: 'The Role of Research in Papermaking', Tech. Div. BPBIF, Cambridge, UK, Sept. 1981. 4. Washburn, E.W. Phys. Rev. 2nd Series, 1921, 17, 273. 5. Bikerman, J . J . Ind. Eng. Chem. Anal, ed 1941, 443. 6. Adamson, A.W. "Physical Chemistry of Surfaces", Interscience Pub., N . Y . , 1967. 7. Cheever, G.D. "Interface Convers. Polym. Coatings", Proc. Symp. 1967. Ed. by P. Weiss, 150-81. 8. Gillespie, T. J . Colloid Interface S c i . 1959, 14, 123. 9. Kissa, E. i b i d . 1981, 83, 265. 10. Greinacher, H. Z. Phys. Chem. 1959, 19, 101. 11. Mack, G.W. J . Oil Colour Chem. Ass. 1961, 44, 737. 12. Schickentanz, W. Powder Technol. 1974, 9, 49. 13. Frederick, W.J. and Bobalek, E . G . Ind. Eng. Chem. Fundam. 1975, 14, 40. 14. Everett, D . H . ; Haynes, J.M. ; R . J . M i l l e r , "Fibre-Water Interactions in Papermaking", Tech. Div. BPBIF, London, 1978, 519. 15. Ruoff, A.L.; Prince, D . C . ; Giddings, J.C.; Stewart, G.H. Roll. Zeit. 1959, 166, 144. 16. Rudd, D.F. J . Phys. Chem. 1060, 64, 1254.

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 20, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch022

22.

OLIVER

Wetting, Penetration of Paper Surfaces

453

17. Ruoff, A.L.; Stewart, G.H.; Giddings, J.C. Koll. Z e i t . 1960, 173, 14. 18. O l i v e r , J.F., Forthcoming P u b l i c a t i o n . 19. Bristow, J.A. Svensk, Papperstidn. 1963, 70, 623. 20. O l i v e r , J.F.; Huh, C.; Mason, S.G. J . Adhesion, 1977, 8, 223. 21. O l i v e r , J.F.; Mason, S.G. J . C o l l o i d I n t e r f a c e S c i . 1977, 60, 480. 22. Lyne, M.B., paper presented a t the Seventh Fund'1 Research Symposium: "The Role o f Research i n Papermaking", Tech. D i v . BPBIF, Cambridge, UK, Sept. 1981. 23. S a y l e s , R.S.; Thomas, T.R. J . Phys. E : Sci. Instrum. 1976, 9, 855. 24. O l i v e r , J.F.; Huh, C.; Mason, S.G. C o l l o i d s and Surfaces, 1980, 1, 79. 25. C a s s i e , A.B.D.; Baxter, S. Trans. Farad. Soc. 1944, 40, 546. 26. Shuttleworth, R.; B a i l e y , G.L.J. Disc. Farad. Soc. 1948, 3, 16. 27. O l i v e r , J.F.; Mason, S.G., "Fundamental P r o p e r t i e s o f Paper Related to i t s Uses", Tech. Div. BPBIF, London, 1976, 428. 28. Huh, C.; S c r i v e n , L.E. Symp: Contact Angle Phenom., ACS Mtg. LA, March 1971. RECEIVED

April 30, 1982

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.