Colloids and Surfaces in Reprographic Technology - American

8

Interfaces in Electrophotography

JOHN W. WEIGL

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch008

Xerox Corporation, Joseph C. Wilson Center for Technology, Webster, NY 14580

Practically all interesting effects in electrophotography occur at contact zones between d i f f e r e n t m a t e r i a l s . Examples will illustrate a v a r i e t y of i n t e r f a c e s and t h e i r functions. For instance, the photoconductive dielectric used i n xerography must be p r o v i d e d with s u r f a c e and base c o n t a c t s which b l o c k the injection of charge under h i g h electric field conditions. Contiguous l a y e r s o f photosensitive and charge t r a n s p o r t i n g m a t e r i a l s may be used to separate two essential functions required of xerographic photoreceptors--the p h o t o - g e n e r a t i o n o f mobile charge carriers, and the a m p l i f i c a t i o n o f c o n t r a s t p o t e n t i a l . The "toner" powder used to develop the l a t e n t charge p a t t e r n must itself be charged p r i o r to u s e , by a p r o c e s s which r e q u i r e s c l o s e c o n t r o l of its interaction with s e v e r a l o t h e r m a t e r i a l s i n the development system. Nonaqueous colloids, used as "liquid developers" for xerography or as imaging f l u i d for photoelectrophoresis depend on t h e i r s u r f a c e s f o r electrostatic and e n t r o p i c stabilization. The adhesion o f toner particles to p h o t o r e c e p t o r s , their removal i n the t r a n s f e r and c l e a n i n g s t e p , and the p r o c e s s e s by which they may be permanently a f f i x e d to the s u r f a c e o f the paper all involve the f o r m a t i o n and b r e a k i n g o f interfacial bonds. Finally, the e f f e c t o f electrostatic f i e l d s upon s u r f a c e energy has been utilized for a selective wetting 0097-6156/82/0200-0139$ 12.05/0 © 1982 American Chemical Society Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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development p r o c e s s , as w e l l as f o r d i s p l a y d e v i c e s and p h o t o p l a s t i c m i c r o f i l m s i n which the s u r f a c e o f the polymer l a y e r deforms i n response to local electrostatic field patterns.

E l e c t r o p h o t o g r a p h y i s l a r g e l y a s u r f a c e p r o c e s s , and most interesting steps involve interfaces between materials. In t h i s b r i e f survey we s h a l l attempt to p r o v i d e a rough o u t l i n e o f t h e s e , and to d e s c r i b e some o f the main i s s u e s . No attempt can be made to be d e t a i l e d or comprehensive. In the f o r t y - o d d years since Chester C a r l s o n ' s initial inventions, electrophotography has evolved p r e t t y much i n the d i r e c t i o n e n v i s i o n e d by him O ) • For c o n v e n t i o n a l x e r o g r a p h y , the image i s formed by the d e p o s i t i o n o f a b l a c k t h e r m o p l a s t i c powder on the s u r f a c e of a p h o t o c o n d u c t i v e d i e l e c t r i c s u r f a c e b e a r i n g an e l e c t r o s t a t i c charge p a t t e r n . The powder i s t r a n s f e r r e d to p l a i n paper and fused to i t to form the f i n a l image. T h i s i s the p r o c e s s used i n the u b i q u i t o u s x e r o g r a p h i c office copier (2). Significant variants exist: electrostatic charge p a t t e r n s may be t r a n s f e r r e d i n s t e a d o f powders, charged c o l l o i d s d i s p e r s e d i n d i e l e c t r i c l i q u i d s may be used as d e v e l o p e r s , images may be formed d i r e c t l y on p h o t o c o n d u c t i v e coated p a p e r ; or even by the d e p o s i t i o n o f e l e c t r i c a l l y photosensitive p a r t i c l e s (3) • We s h a l l r e t u r n to some o f these a l t e r n a t i v e s later—after d e s c r i b i n g the r o l e s o f i n t e r f a c e s i n c o n v e n t i o n a l dry t r a n s f e r xerography. Xerography

from the V i e w p o i n t o f the

Photoreceptor

The key p r o c e s s s t e p s may be i l l u s t r a t e d by a schematic c r o s s - s e c t i o n o f a t y p i c a l x e r o g r a p h i c c o p i e r (Figure 1). The p h o t o c o n d u c t o r drum i s r o t a t e d past a s e r i e s of process s t a t i o n s , where i t i s , in turn, c h a r g e d , exposed to a p r o j e c t e d image p a t t e r n , developed by an o p p o s i t e l y charged "toner" powder, then p r e s s e d l i g h t l y a g a i n s t an e l e c t r i c a l l y b i a s e d sheet o f paper to t r a n s f e r the image. Any r e m a i n i n g charge on the p h o t o r e c e p t o r may be n e u t r a l i z e d by an e r a s i n g lamp or an AC c h a r g i n g d e v i c e , a f t e r which the l a s t r e s i d u e o f the powder i s brushed o f f and the p h o t o r e c e p t o r i s ready f o r

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


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Figure 1.

Schematic of typical xerographic copier. Process elements are clustered about the photoconductor-coated drum.


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another c y c l e . In a modern c o p i e r , the whole s e r i e s o f s t e p s may take o n l y f i v e s e c o n d s . The powder p a t t e r n on the paper i s melted i n t o the f i b e r s to form a permanent image. Some o f the i n t e r f a c e s meeting the p h o t o r e c e p t o r are o b v i o u s : i t s s u r f a c e e n c o u n t e r s i o n i z e d gas which s e r v e s to d e p o s i t or remove corona c h a r g e , the d e v e l o p e r mass, the transfer sheet, and c l e a n i n g d e v i c e s . Other i n t e r f a c e s with e s s e n t i a l f u n c t i o n s are found i n the i n t e r n a l s t r u c t u r e o f the p h o t o r e c e p t o r i t s e l f . Let us now look at these i n some d e t a i l . Charging o f

the

Photoreceptor

The xerographic process places extraordinary demands upon the r e - u s a b l e p h o t o c o n d u c t i v e d i e l e c t r i c which i s the a c t i v e element o f the p h o t o r e c e p t o r (4, 5 ) . I t must be capable o f r e t a i n i n g the a p p l i e d s u r f a c e charge without s i g n i f i c a n t dark p o t e n t i a l l o s s between the c h a r g i n g and d e v e l o p i n g s t e p s . To produce adequate image c o n t r a s t , i n t e r n a l f i e l d s o f the o r d e r o f 20 to 50 V/yun are r e q u i r e d . I f i n e r t d i e l e c t r i c s c o u l d be u s e d , t h i s would p r e s e n t no p a r t i c u l a r problem: one c o u l d r e l y on a h i g h d e n s i t y o f deep t r a p s throughout the bulk o f the d i e l e c t r i c to r e t a i n charge i n the face o f the d r i v i n g field. Not so i n the x e r o g r a p h i c p h o t o r e c e p t o r . R e c a l l t h a t m a j o r i t y c a r r i e r s generated near the s u r f a c e i n the exposure s t e p must be f r e e to migrate from the top to the bottom of the d i e l e c t r i c l a y e r to enable imagewise p h o t o d i s c h a r g e i n the f r a c t i o n o f a s e c o n d , a v a i l a b l e between exposure and development. While the time s c a l e of the p r o c e s s does not r e q u i r e h i g h m o b i l i t y , deep t r a p s for the m a j o r i t y c a r r i e r must be excluded from the b u l k ; o t h e r w i s e , r e s i d u a l p o t e n t i a l and background w i l l b u i l d up as the p h o t o r e c e p t o r c y c l e s . T h e r e f o r e , the photoconduc t o r must not o n l y be f r e e of thermal c a r r i e r s but i t must be p r o t e c t e d a g a i n s t the dark i n j e c t i o n o f e x t e r n a l c h a r g e , as w e l l . Injection from the base e l e c t r o d e may be a v o i d e d by a t h i n b l o c k i n g d i e l e c t r i c — t y p i c a l l y 50 A . U . o f a dense oxide or p o l y m e r - - t h i n enough to a l l o w any accumulated r e s i d u a l charge to l e a k o f f between c y c l e s . The i n j e c t i o n b a r r i e r at the top s u r f a c e i s more d i f f i c u l t to u n d e r s t a n d . Although attempts have been made to d e p o s i t s u r f a c e charge by i n d u c t i o n or s u r f a c e



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c o n t a c t , these have been g e n e r a l l y much l e s s s u c c e s s f u l than the u n i p o l a r i o n stream from a corona d e v i c e . Indeed, C a r l s o n ' s i n v e n t i o n o f corona c h a r g i n g was one o f h i s most v a l u a b l e i n n o v a t i o n s ( 6 ) ( F i g u r e 2 ) . Evidently the corona i o n s can be made to d e p o s i t on the s u r f a c e i n such away as to f o r m a b l o c k i n g l a y e r . The mechanism f o r t h i s may w e l l d i f f e r from photoconduc t o r to p h o t o c o n d u c t o r . In the case o f z i n c o x i d e - b i n d e r l a y e r s t h e r e is extensive evidence that negative corona charge c o n s i s t s o f s t a b l y adsorbed oxygen a n i o n s ( F i g u r e 3 ) i which can be d i s c h a r g e d upon i l l u m i n a t i o n ( 5 , 7-11) . For s e l e n i u m i t has been found t h a t corona i o n s i n j e c t charge i n t o the p h o t o c o n d u c t o r , forming trapped charge s i t e s j u s t below the s u r f a c e (J.2, j_3) . Multi-Layer

Photoconductors

The xerographic photoreceptor converts the p r o j e c t e d image i n t o a charge p a t t e r n r e s i d i n g at the s u r f a c e o f the d i e l e c t r i c . To be d e v e l o p a b l e this e l e c t r o s t a t i c image must p r e s e n t c o n t r a s t p o t e n t i a l s o f the o r d e r o f 400-600V. S i n g l e layer photoconductors t h e r e f o r e serve two f u n c t i o n s : photogeneration of a mobile charge c a r r i e r c l o s e to the s u r f a c e o f i l l u m i n a t e d a r e a s , f o l l o w e d by t h e i r f i e l d - a s s i s t e d s e p a r a t i o n (JJO and t r a n s p o r t a c r o s s s u f f i c i e n t d i e l e c t r i c t h i c k n e s s to develop the d e s i r e d c o n t r a s t p o t e n t i a l ( 1 5 - j _ § ) . If a s i n g l e m a t e r i a l i s used i t must combine h i g h p h o t o sensitivity with e x c e l l e n t t r a n s p o r t p r o p e r t i e s and e x c e p t i o n a l l y h i g h d i e l e c t r i c s t r e n g t h (J_9) . Can one s e p a r a t e these f u n c t i o n s ? Yes! It is p o s s i b l e to use a l a y e r o f one m a t e r i a l as p h o t o - s e n s o r and another as v o l t a g e a m p l i f i e r . For example ( F i g u r e 4 ) , a s u b - m i c r o n f i l m o f amorphous s e l e n i u m s u f f i c e s to absorb v i s i b l e l i g h t and to generate mobile h o l e s . If such a l a y e r i s coated over a 15.* m l a y e r o f an a r o m a t i c polymer which has l i t t l e p h o t o s e n s i t i v i t y but e x c e l l e n t d i e l e c t r i c and t r a n s p o r t p r o p e r t i e s , one d e r i v e s an excellent photoreceptor (20-22). The composite has essentially the f u l l p h o t o s e n s i t i v i t y of selenium-s p e c t a c u l a r l y g r e a t e r than t h a t o f the polymer a l o n e . But i t a l s o has much b e t t e r d i e l e c t r i c p r o p e r t i e s and a high* degree of mechanical f l e x i b i l i t y . It can be f a b r i c a t e d i n t o b e l t s which run around s m a l l r o l l e r s and o f f e r c o n s i d e r a b l e advantage to the system d e s i g n e r . Similar results have been reported for inverted


144


CORONA WIRE

HIGH VOLTAGE DC CORONA POWER SUPPLY

L 1. PHOTOCONDUCTORBARRIER _ LAYER


METALLIC SUBSTRATE Figure 2.

Photoreceptor and corona charging process. 0 " LAYER 2

HOLES IN SURFACE TRAPS

Figure 3. Schematic representation of the charge deposited on two photoconductor materials: ZnO-binder dispersion (top) and amorphous selenium (bottom).

ip — -

Se

TWO-LAYER P/C

S lO" o o tr

3

x

\ \ \

o o aO/x SELENIUM o—oO-3txSe/7-6txPVK + — + 7 6ttPVK

\ (

+

+

3400

\

4200

5000

5800

WAVELENGTH(H)

Figure 4. Double layer photoreceptor-schematic structure, with selenium generator overlying a poly(N~vinyl carbazole) (PVK) transport layer (left). Performance of such a composite (O — O), compared to selenium (O O) and to PVK without generator layer (+ • - • -f j (right). (Reproduced, with permission, from Ref. 20. Copyright 1968, Pergamon Press, Ltd.)


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s t r u c t u r e s ( F i g u r e 5 ) , f o r a v a r i e t y o f o r g a n i c pigment g e n e r a t o r s , and f o r t r a n s p o r t l a y e r s i n which monomeric h o l e - or e l e c t r o n - t r a n s p o r t m o l e c u l e s are used in c r y s t a l l i n e form or i n polymer s o l u t i o n (23-26) . Materials f l e x i b i l i t y has thus been achieved-a l b e i t at the c o s t o f i n t r o d u c i n g an i n t e r n a l i n t e r f a c e w i t h i n the p h o t o r e c e p t o r s t r u c t u r e - - a n i n t e r f a c e which must be c a r e f u l l y c o n t r o l l e d i n the c o a t i n g p r o c e s s to prevent u n d e s i r a b l e i n t e r m i x i n g o f layers, delamin a t i o n , and the f o r m a t i o n o f b a r r i e r s impeding charge carrier injection. Pigment-Binder

Composites

A very d i f f e r e n t type o f i n t e r f a c e dominates the behavior of an important class of xerographic p h o t o c o n d u c t o r s c o n s i s t i n g o f p h o t o c o n d u c t i v e pigment dispersions in d i e l e c t r i c binders. F a m i l i a r examples i n c l u d e z i n c oxide ( 7 - 1 0 , 27) , and o t h e r i n o r g a n i c pigments (28-31 ) , as w e l l as o r g a n i c m i c r o c r y s t a l s dispersed in d i e l e c t r i c binders (32-36). In these systems, charge must t r a v e l through a l o o s e l y connected c h a i n o f pigment p a r t i c l e s ; the c o n c e n t r a t i o n r e q u i r e d f o r e f f e c t i v e c o n t a c t depends on p a r t i c l e shape, and v a r i e s from about 25 volume p e r c e n t f o r i s o t r o p i c chunks of z i n c oxide and cadmium s u l f i d e (7 , 9) to a mere 5 volume p e r c e n t f o r needle shaped c r y s t a l s o f p h t h a l o c y a n i n e (32) or filaments of a thiapyrylium-polycarbonate complex (36) ( F i g u r e 6 ) . The b i n d e r s e r v e s , not o n l y to p r o v i d e m e c h a n i c a l i n t e g r i t y and m o i s t u r e p r o o f i n g ; i t a l s o adds i n t e r f a c e s which s t r o n g l y a f f e c t e l e c t r i c a l performance. The m a t r i x m a t e r i a l i s c a r e f u l l y (and r a t h e r e m p i r i c a l l y ! ) selected to enable reliable charging, efficient p h o t o d i s c h a r g e (minimal t r a p p i n g ) , and s t a b l e r e p e t i tive c y c l i n g . A q u a s i - c r y s t a l l i n e complex of t h i a p y r y l i u m s a l t w i t h p o l y c a r b o n a t e , d i s p e r s e d i n a "doped" o u t e r r e s i n phase, appears to be a p a r t i c u l a r l y e l e g a n t s o l u t i o n (34-36) . Photoinduced d i s c h a r g e c u r v e s o f most p i g m e n t - b i n d e r composites tend to be S-shaped r a t h e r than c a p a c i t i v e , as are those f o r s e l e n i u m and o t h e r homogeneous p h o t o c o n d u c t o r s . T h i s has been t e n t a t i v e l y attributed to t r a p p i n g at inter-particle contacts, f o l l o w e d by a f i e l d - i n d u c e d t r a p e m p t y i n g , which r e s t o r e s the i n t e r f a c e to i t s i n i t i a l s t a t e (28, 32, 3 6 ) .


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TRANSPORT LAYER PHOTO-GENERATOR

Figure 5. Double layer photoreceptor—inverted structure. The photo-generator is protected against environmental damage by the active dielectric transport layer.

Figure 6. Cross-section (left) and schematic diagram illustrating charge transport through a chain of photoconductive pigment particles (A, B, C) dispersed in dielectric binder (right). Contact impedance and trapping at interfaces are critical to performance. (Reproduced, with permission, from Ref. 28. Copyright 1972, Walter de Gruyter & Co.)


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Z i n c o x i d e a l s o i l l u s t r a t e s one f u r t h e r i n t e r a c t i o n . S i n c e t h i s p h o t o c o n d u c t o r i n t r i n s i c a l l y o n l y responds to u l t r a v i o l e t r a d i a t i o n , i t i s commonly s e n s i t i z e d to v i s i b l e l i g h t by the a d s o r p t i o n o f a monolayer o f one or more dyes ( F i g u r e 7 ) . T h i s adds yet a n o t h e r e s s e n t i a l i n t e r f a c e to an a l r e a d y complex system ( 37-40) .


Xerographic Process

Steps

B l a c k " t o n e r " , which i s g e n e r a l l y used to d e v e l o p the electrostatic l a t e n t image, i s a d i s p e r s i o n o f 10$ carbon pigment i n a t h e r m o p l a s t i c . The powder i s u s u a l l y produced by shear m i l l i n g and h i g h - i m p a c t f r a c t u r e . Carbon, as w e l l as p l a s t i c , i s exposed at the s u r f a c e , and both m a t e r i a l s may t h e r e f o r e be expected to i n f l u e n c e the e l e c t r i c a l p r o p e r t i e s o f the composite (41-44 ) . Toner s e r v e s to t r a n s f o r m the e l e c t r o s t a t i c l a t e n t image on the p h o t o r e c e p t o r i n t o i n v i s i b l e image; t h e r e f o r e , p a r t i c l e s i z e and charge/mass r a t i o must be c l o s e l y c o n t r o l l e d : t y p i c a l v a l u e s are 12yum and 2 0 < * c / g , r e s p e c t i v e l y ; the latter corresponds to a density of some 300,000 e l e c t r o n i c u n i t charges per p a r t i c l e . F i g u r e 8 shows the x e r o g r a p h i c p r o c e s s from the v i e w p o i n t o f the t o n e r . The m a t e r i a l i s fed i n t o the development chamber, charged ( u s u a l l y by t r i b o e l e c t r i f i c a t i o n ) , and a p p l i e d to the p h o t o c o n d u c t o r s u r f a c e . Powder which adheres to charged a r e a s o f the l a t e n t image i s then t r a n s f e r r e d and fused to a sheet o f paper or f i l m ; the r e s i d u e i s c l e a n e d o f f . L e t us now c o n s i d e r these steps in greater d e t a i l . Development We shall illustrate the materials interfaces i n v o l v e d i n development f o r a s p e c i f i c system i n common use: the "two-component magnetic brush" ( 4 5 - 4 7 ) ( F i g u r e 9a). In such a d e v i c e , 1-2 weight p e r c e n t of t o n e r i s i n t i m a t e l y mixed w i t h r e l a t i v e l y massive " c a r r i e r " beads h a v i n g magnetic c o r e s and ( u s u a l l y ) t h i n d i e l e c t r i c coatings. A c y l i n d r i c a l s h e l l or s l e e v e r o t a t e s above the d e v e l o p e r r e s e r v o i r . Bar magnets mounted w i t h i n the s h e l l a t t r a c t the mixture o f t o n e r and magnetic beads and form them i n t o a l o o s e " b r u s h - l i k e " s t r u c t u r e , whose " b r i s t l e s " are r e a l l y c h a i n s of c a r r i e r particles, l o o s e l y l i n k e d by the magnetic f i e l d l i n e s ( F i g u r e 9 b ) .


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FLUORESCEIN DYE


\

5xiCT gDYE/g ZnO 4

NO DYE

400 440 480 520 560 600 WAVELENGTH (nryx.) 4

Figure 7. Photoresponse of zinc oxide-binder layer with (- • -X-) 5 X 10' g fluorescein dye and without ( ) sensitizing dye. (Reproduced, with permission, from Ref. 40. Copyright 1970, American Institute of Physics.)

FEED

FUSE •

i TRIB0-CHAR6E

TRANSFER TO PAPER

^DEVELOP-

TONER

'RECYCLE Figure 8.

J •

CLEAN

P/R

Toner interactions in xerography.


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8.

Figure 9. Magnetic brush (top) and magnetic brush "bristle" consisting of chain of large carrier beads covered with toner powder (bottom). Key: A, pickup magnet; and B, magnet. (Reproduced, with permission, from Ref. 50. Copyright 1974, Society of Photographic Scientists and Engineers.)



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As the r o l l r o t a t e s , i t a g i t a t e s the d e v e l o p e r mass by c o n t i n u o u s t u m b l i n g , c o l l a p s e , and r e - f o r m a t i o n o f these bead c h a i n s . Toner and c a r r i e r undergo r e p e a t e d bouncing c o n t a c t s and s e p a r a t i o n s , p r o v i d i n g ample o p p o r t u n i t i e s f o r charge exchange (55) . As the t i p of the brush sweeps a c r o s s the photoconductor s u r f a c e , a c o m b i n a t i o n o f m e c h a n i c a l impulses and the electric f i e l d from the l a t e n t image detaches toner from the c a r r i e r and a t t r a c t s i t to the charge p a t t e r n on the p h o t o c o n d u c t o r s u r f a c e (48, 4 9 ) . The r o l l i s u s u a l l y e l e c t r i c a l l y b i a s e d to augment the detachment t h r e s h o l d p r o v i d e d by the c a r r i e r c h a r g e , and to compensate f o r r e s i d u a l charge i n exposed a r e a s o f p h o t o c o n d u c t o r . Even a f t e r i n i t i a l d e p o s i t i o n , toner p a r t i c l e s a r e l i k e l y to be scavenged by the brush and r e d e p o s i t e d elsewhere on the l a t e n t image. I t i s beyond our scope to review the d e t a i l e d models f o r magnetic brush development which have been p u b l i s h e d (47 , 50-53) . C l e a r l y , the p r o c e s s i s dynamic, and must be o p t i m i z e d to the s p e c i f i c m a t e r i a l s , c o n f i g u r a t i o n , and development speed to be u s e d . S u f f i c i e n t c u r r e n t must flow from the development electrode through the d e v e l o p e r mass to charge the toner l a y e r c l o s e s t to the l a t e n t image and to compensate f o r a s i g n i f i c a n t f r a c t i o n of the charge on the photoconduc t o r . A c u r r e n t o f about 1 ma would be r e q u i r e d to n e u t r a l i z e a f u l l y charged l a t e n t image p a s s i n g through a 30 cm wide development n i p at 20 c m / s e c o n d . As Hays (.54) has shown, the e l e c t r i c a l i n t e r f a c e s i n v o l v e d i n the charge t r a n s p o r t through the d e v e l o p e r bed are q u i t e complex. C o n d u c t i o n through the magnetic " b r i s t l e s " i s l i m i t e d by the d i e l e c t r i c c o a t i n g on the beads as w e l l as by the t h i c k n e s s o f i n t e r v e n i n g t o n e r . Charge t r a n s p o r t by the forward motion o f charged toner p a r t i c l e s through the d e v e l o p e r mass i s impeded by p a r t i c l e c r o w d i n g , which i n t u r n depends upon f i e l d s t r e n g t h , and g e o m e t r i c a l f a c t o r s such as bead shape (50-52). S c h m i d l i n (56) has p o i n t e d out t h a t the commonly observed broad range o f van der Waals f o r c e s between toner and c a r r i e r p a r t i c l e s causes x e r o g r a p h i c d e v e l o p ment to r e q u i r e much h i g h e r e l e c t r i c f i e l d s f o r t o n e r r e l e a s e than those r e q u i r e d to e s t a b l i s h a sharp t h r e s h o l d . In f a c t , he argues t h a t i f the a d h e s i v e f o r c e s between these s u r f a c e s c o u l d be made s u f f i c i e n t l y s m a l l and u n i f o r m ( " n o i s e - f r e e " ) , i t should be p o s s i b l e to reduce e l e c t r o s t a t i c charge d e n s i t y r e q u i r e d f o r a g i v e n


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developed c o n t r a s t by a f a c t o r o f 5 0 - 1 0 0 . I f t h i s could be a c h i e v e d i n p r a c t i c e , i t would o f f e r an o p p o r t u n i t y to cut o p t i c a l exposure by n e a r l y two o r d e r s o f magnitude! A second a d v e r s e e f f e c t o f van der Waals a d h e s i o n i s a tendency o f c a r r i e r and p h o t o c o n d u c t o r s u r f a c e s to become coated w i t h a t h i n f i l m o f t o n e r r e s i d u e - - a f i l m which p r o g r e s s i v e l y i m p a i r s the e f f i c i e n c y o f toner c h a r g i n g and p h o t o r e c e p t o r p e r f o r m a n c e . S a l a n e c k et a l . ( 5 7 ) have r e p o r t e d t h a t m a t e r i a l t r a n s f e r amounting to as l i t t l e as 1 atom/10^ s u r f a c e s i t e s i s measurable as a s h i f t in " t r i b o e l e c t r i c " charging behavior! Charge Exchange Between

Carrier

and Toner

We now c o n s i d e r the i n t e r f a c e between c a r r i e r and toner, as these components interact i n a dynamic development s y s t e m . As we have n o t e d , t o n e r i s commonly fabricated by "jet impacting" a b r i t t l e pigmented t h e r m o p l a s t i c to form i r r e g u l a r l y shaped p a r t i c l e s o f about 12/u m c r o s s - s e c t i o n - - p a r t i c l e s whose surface comprises r e s i n and carbon b l a c k (2 , 43 , j>8) . The c a r r i e r c o r e s are magnetic ( f e r r i t e or s t e e l ) , t y p i c a l l y about 250y*m i n s i z e and t h e r e f o r e r e l a t i v e l y m a s s i v e . They are g e n e r a l l y c o a t e d w i t h a d i e l e c t r i c o x i d e or r e s i n , which p a r t i c i p a t e s i n the t r i b o - e l e c t r i f i c a t i o n p r o c e s s ( 2 , 47 ,

59) .

The a c t i o n of xerographic developer is highly leveraged: a mere 30 fc o f charge causes development by a 12 um s i z e d toner p a r t i c l e (59.) . While t h i s i s the b a s i s o f image a m p l i f i c a t i o n which g i v e s xerography " p r o j e c t i o n speed" ( e q u i v a l e n t to ASA 2 - 1 0 ) ( 2 , 3 , 60) i t i s a l s o i n e v i t a b l y a source o f image n o i s e ( 5 6 , 6 1 ) . S a t u r a t i o n charge on d i e l e c t r i c s i s reached at s u r f a c e d e n s i t i e s o f 1-10 n c / c m , c o r r e s p o n d i n g to o n l y one charge per 103 -10 * s u r f a c e atoms. Thus a r e v e r s a l o f the p o l a r i t y on l e s s than one t e n t h o f a p e r c e n t o f the s u r f a c e s i t e s on a t o n e r p a r t i c l e s u f f i c e s to double (or n u l l i f y ) the charge per p a r t i c l e ! 2

1

Developer m a t e r i a l s are chosen so as to a s s u r e t h a t f r i c t i o n a l e l e c t r i f i c a t i o n produces toner o f the d e s i r e d p o l a r i t y (which i s g e n e r a l l y o p p o s i t e t h a t o f the charged photoreceptor s u r f a c e ) . Because " r e a l " development systems are complex, and must o p e r a t e under h i g h l y v a r i a b l e c o n d i t i o n s , " p r a c t i c a l " m a t e r i a l s c h o i c e s must be made on an e m p i r i c a l b a s i s . Nonetheless, physical models have s e r v e d as u s e f u l g u i d e s . I n v e s t i g a t i o n s of



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"clean" model s u r f a c e s suggest t h a t charge exchange between metals and i n s u l a t o r s i s d r i v e n p r i m a r i l y by differences between accessible electronic energy levels. In metals these are r e a d i l y d e f i n e d , e . g . , i n terms o f contact potentials; and, indeed, charge t r a n s f e r to a common d i e l e c t r i c from a s e r i e s o f metals having d i f f e r e n t contact p o t e n t i a l s g i v e s f a i r l y p r e dictable results (62), provided that e f f e c t s like s u r f a c e o x i d a t i o n are taken i n t o account (6^3) . Even charge exchange between p a i r s o f d i e l e c t r i c s may be predicted--at least in a semiquantitative way--by r e f e r e n c e to t h e i r work f u n c t i o n s , r e l a t i v e to a common metal (62, 64) ( F i g u r e 10). S u c c e s s f u l attempts have been made to c o r r e l a t e "tribo" charging to chemical structure in model m a t e r i a l s ( F i g u r e 11). Cressman, Gibson and t h e i r c o workers (65, 66, 67, 154) have found p e r s u a s i v e c o r r e l a t i o n between the Hammet "sigma" f u n c t i o n (a measure o f s u b s t i t u e n t e l e c t r o n e g a t i v i t y ) of three s e r i e s of r i n g substituted aromatics with the ability of these m a t e r i a l s to a c c e p t e l e c t r i c charge from common metal contact electrodes. In polymers of xerographic i n t e r e s t , Duke and F a b i s h (68) have used s p e c t r o s c o p i c techniques to i d e n t i f y donor and a c c e p t o r states, c a p a b l e o f forming l o c a l i z e d m o l e c u l a r i o n s upon charge transfer. S u r p r i s i n g l y , such s t a t e s are s u f f i c i e n t l y s h a r p l y d e f i n e d to c o n t r o l charge exchange, as though they were communal energy l e v e l s - - e v e n though they are much too l o c a l i z e d to admit charge t r a n p o r t . While i t thus seems p l a u s i b l e t h a t charge exchange i n "dry" model systems is driven by d i f f e r e n c e s in e l e c t r o n i c c o n t a c t p o t e n t i a l , most " r e a l world" s u r f a c e s are covered by m o i s t u r e f i l m s which enable the exchange of any s o l u b l e i o n s which may be p r e s e n t . For example, a c c o r d i n g to the c l a s s i c experiments o f Knoblauch (J.44) and Rudge ( 145, 146 , 247) , a c i d i c s u r f a c e s g e n e r a l l y tend to charge inert materials positively, while basic s u r f a c e s are a source o f n e g a t i v e c h a r g e . Harper (149) suggests t h a t t h i s may w e l l be due to the s e l e c t i v e t r a n s f e r o f mobile p r o t o n s and OH" i o n s , r e s p e c t i v e l y . More work needs to be done on t h i s i n t e r e s t i n g s u b j e c t , which may have i m p o r t a n t i m p l i c a t i o n s f o r the use o f charge c o n t r o l agents i n x e r o g r a p h i c d e v e l o p e r m a t e r i a l s (150, 151). Model e x p e r i m e n t s , u s i n g i o n exchange r e s i n s and z e o l i t e s , might be most i n s t r u c t i v e !


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INSULATOR -TRAPS


(Q)

x

(b)

Figure 10. Energy levels at metal/dielectric interface: separated surfaces (a), blocking junction (b), and interface allowing efficient injection of electrons into insulator (c). (Reproduced, with permission, from Ref. 62. Copyright 1980, Taylor & Francis, Ltd.)




154

Figure 11. Correlation of triboelectric charging with electronegativity, as measured by Hammet sigma function. (Reproduced from Ref. 66. Copyright 1975, American Chemical Society.)


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O b s e r v a t i o n s t h a t c o n t a c t charge exchange can be q u i t e r a p i d r e l a t i v e to the d i e l e c t r i c r e l a x a t i o n times (63, 69 , 1 5 2 ) , and the d e m o n s t r a t i o n that "clean" polymers have w e l l d e f i n e d s e t s o f s u r f a c e s t a t e s (44, 58, 7 0 ) , suggest t h a t these l a t t e r may be more i m p o r t a n t than bulk s t a t e s f o r c h a r g i n g of d i e l e c t r i c s (62) . On the o t h e r hand, W i l l i a m s (TM, 72) has shown t h a t one can make bulk s t a t e s a c c e s s i b l e on a ten m i l l i s e c o n d time s c a l e by "doping" d i e l e c t r i c s to induce a b u l k c o n d u c t i v i t y of the o r d e r o f 10~ s i e / c m , and t h i s approach has found u s e f u l applications (73). P r a c t i c a l d e v e l o p e r systems a r e , o f c o u r s e , much more complex and o p e r a t e (at b e s t ! ) under steady s t a t e conditions controlled by mechanical agitation, geometry, ambient c o n d i t i o n s , and m a t e r i a l s aging. Thermodynamic equilibrium is neither reached nor recognizable. S u r f a c e s t a t e s are r e a d i l y a l t e r e d by a b r a s i o n , f i l m i n g , and o x i d a t i o n ( 6 3 ) . In t o n e r s , the network o f carbon pigment, which i s d i s t r i b u t e d on and through the polymer m a t r i x ( £ 3 , 44., 155) , must s u r e l y a l l o w i n t e r n a l s i t e s to p a r t i c i p a t e i n both the t r a n s f e r and s t o r a g e o f toner c h a r g e . F i n a l l y , as we have a l r e a d y n o t e d , the p r o c e s s r e q u i r e s t h a t t h e r e be n e g l i g i b l e charge t r a n s f e r between a l l components o f the d e v e l o p e r and the l a t e n t i m a g e - b e a r i n g p h o t o c o n d u c t o r . 1


in

1

Image T r a n s f e r A f t e r development, the toner p a t t e r n i s t r a n s f e r r e d to a sheet o f paper p l a c e d f a c e - t o - f a c e with the photoconductor. To overcome the e l e c t r o s t a t i c and van der Waals f o r c e s h o l d i n g the charged powder to the photoconductor s u r f a c e , an e l e c t r i c f i e l d i s a p p l i e d through the p a p e r , e . g . by means o f a corona spray or a b i a s e d , conformable s e m i c o n d u c t i v e r o l l e r ( F i g u r e 12). The paper must p r e s e n t an e l e c t r i c a l l y b l o c k i n g i n t e r face to the t o n e r . Paper p r o p e r t i e s are c r i t i c a l at t h i s stage (74) . W r i n k l e s and a s p e r i t i e s , i n excess of the toner p i l e h e i g h t (about 20-30*m) must be a v o i d e d . Paper conduct i v i t y must be kept w i t h i n bounds: e x c e s s i v e m o i s t u r e , absorbed i n the f i b e r s under h i g h h u m i d i t y c o n d i t i o n s , can not o n l y c o l l a p s e the t r a n s f e r f i e l d , but a l s o generate i o n s whose own t r a n s f e r a c r o s s the gap competes with t h a t o f the charged powder. On the o t h e r hand, the paper must not remain so h i g h l y charged t h a t i t c l i n g s




156



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to the p h o t o r e c e p t o r , or to t r a n s p o r t b e l t s and r o l l e r s which are used to remove the f i n a l copy from the processor. R e s i d u a l charge may be reduced by n e u t r a l i z i n g the back o f the paper by means o f an AC plasma (75) . Stripping may be assisted by mechanical means: i n t e r m i t t e n t a i r j e t s or g u i d i n g f i n g e r s may be a p p l i e d to the lead edge of the p a p e r , narrow t r a c k i n g b e l t s can be used to l i f t the edges of the s h e e t . Plastic-based photoconductor b e l t s can be made s u f f i c i e n t l y flexible to be t r a c k e d around a s m a l l diameter s t r i p p i n g r o l l ; here the s t i f f n e s s o f the sheet i t s e l f ensures r e l i a b l e s e p a r a t i o n (76) ( F i g u r e 13). As charged toner i s removed from the drum i n the e x i t n i p , the p o t e n t i a l between the photoconductor and powder s u r f a c e s r i s e s past the t h r e s h o l d f o r the i n i t i a t i o n o f Paschen d i s c h a r g e a c r o s s the a i r gap. Reverse charge t r a n s f e r , and l a t e r a l f i e l d s between toner and s u b s t r a t e at t h i s p o i n t can degrade both the d e n s i t y and the r e s o l u t i o n of the t r a n s f e r r e d image (62, 77) . Toner t r a n s f e r , as w e l l as c l e a n i n g , may be f a c i l i t a t e d by i n c l u d i n g s u b - m i c r o n flow a i d s and l u b r i c a n t s (78-81) . Although no d e t a i l e d model f o r t h e i r mode of a c t i o n appears to have been p u b l i s h e d , we s p e c u l a t e t h a t the powder l a y e r can be s p l i t , so as to l e a v e a monolayer o f c o l o r l e s s s i l i c a on the p h o t o r e c e p t o r w h i l e the toner i s l i f t e d o f f to the paper ( F i g u r e 14). The f o r c e s a d h e r i n g the innermost p a r t i c l e l a y e r to the photoconductor have been e x t e n s i v e l y d i s c u s s e d by Krupp (77 , 82) . I t would be i n t e r e s t i n g to extend S c h m i d l i n ' s model f o r a d h e s i o n c o n t r o l l e d xerographic development (4, 56) to the transfer process. Cleaning T r a n s f e r i s never q u i t e complete; toner r e s i d u e s , as w e l l as paper f i b e r s and s t r a y beads, must be removed from the photoconductor b e f o r e the l a t t e r can be r e c y c l e d . Krupp et a l . have c l a s s i f i e d the f o r c e s which adhere powders to s u r f a c e s i n g e n e r a l (77) and to x e r o g r a p h i c p h o t o r e c e p t o r s i n p a r t i c u l a r (8^) i n o r d e r of t h e i r d e c r e a s i n g range o f a c t i o n : 1) E l e c t r o s t a t i c f o r c e s ( a c t i n g on a s c a l e of many micrometers), 2) van der Waals f o r c e s (4-8 A . U . ) , and 3) S i n t e r i n g or i n t e r d i f f u s i o n ( l e s s than 4 A . U . ) .



158


T y p i c a l f o r c e s are o f the o r d e r o f 50 m i l l i d y n e s / p a r t i c l e (84). Amongst t h e s e , the e l e c t r o s t a t i c f o r c e s are the o n l y ones which can be d e l i b e r a t e l y reduced p r i o r to x e r o g r a p h i c c l e a n i n g : i t i s common p r a c t i c e to e l i m i n a t e residual charge on the photoreceptor by uniform illumination, and to neutralize toner charge by a t m o s p h e r i c i o n s s u p p l i e d by an AC c o r o t r o n . N e i t h e r o f these "erasure" s t e p s i s f u l l y e f f e c t i v e . For example, Nebenzahl et a l . (840 , who used K r u p p ' s centrifuge t e c h n i q u e to q u a n t i f y adhesion f o r c e s , found an e l e c t r i c f i e l d dependence f o r a d h e s i o n which was, s u r p r i s i n g l y , an o r d e r o f magnitude s m a l l e r than t h a t p r e d i c t e d from a simple d i p o l e model. The a u t h o r s noted t h a t , to be e f f e c t i v e , the b l a n k e t exposure and AC corona c u r r e n t used f o r n e u t r a l i z a t i o n must be kept w i t h i n w e l l - d e f i n e d o p e r a t i n g l i m i t s — i . e . , a range i n which the r e s i d u a l toner r e t a i n s as l i t t l e o f i t s o r i g i n a l charge as possible, without acquiring undesirable charge of r e v e r s e d p o l a r i t y ( F i g u r e 15). A complete model would t h e r e f o r e have to i n c l u d e l o c a l charge exchange between toner and p h o t o c o n d u c t o r under the c o r o t r o n - - a n e f f e c t whose q u a n t i f i c a t i o n w i l l be e x t r a o r d i n a r i l y d i f f i c u l t , because the very a c t o f removing toner i n the course o f the measurement would be expected to a l t e r the charge d i s t r i b u t i o n a c r o s s the i n t e r f a c e . M e c h a n i c a l f o r c e and s u r f a c e c h e m i c a l f i n e s s e must be used to overcome van der Waals f o r c e s , which may account f o r as much as 75$ o f the t o t a l a d h e s i o n (8^, 84) ( F i g u r e 16) . A r a p i d l y r o t a t i n g f i b e r brush i s o f t e n used to produce a l o c a l c l o u d o f t o n e r , which i s then drawn away through a vacuum f i l t e r . The b r i s t l e t i p s may be " f l i c k e d " a g a i n s t a bar or edge to f r e e them o f d u s t . T e f l o n f i b e r brushes (8j>) , s t e a r a t e wax d i s p e n s e r s (86) , and f l u o r o c a r b o n powder (87) have been e f f e c t i v e l y used to d e p o s i t t h i n low-energy f i l m s on the p h o t o c o n d u c t o r s u r f a c e to f a c i l i t a t e r e l e a s e . S c r a p e r b l a d e s a r e sometimes s u b s t i t u t e d as i n e x p e n sive alternatives, p a r t i c u l a r l y i n simple copiers ( F i g u r e 17). Here dry l u b r i c a n t s ( l i k e s i l i c a a e r o g e l , stearate, etc.) become i n d i s p e n s a b l e , to a v o i d blade s t i c k i n g , d e f o r m a t i o n , and s t r e a k i n g (78-81 , 8 8 - 9 3 ) . Toner may be r e c y c l e d i n t o the d e v e l o p e r s t a t i o n , but care must be taken to a l l o w i t s charge to be e q u i l i b r a t e d w i t h the f r e s h s u p p l y b e f o r e i t r e t u r n s to the d e v e l o p ment zone. Other c l e a n i n g systems, which have been


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PAPER — -

PHOTOCONDUCTOR

Figure 13.

Self-stripping of paper from sharply curved belt photoconductor.

^PAPER


ST—SILICA PHOTORECEPTOR

Figure 14. Illustration of hypothesis for role of silica in facilitating image transfer by a layer of fine silica aerogel which prevents intimate contact between toner and photoconductor surface.

B.G.

I I

IMAGE AREA

I

B.G. AFTER DEVELOPMENT

DURING TRANSFER

_ - ^ < S L j S ) ^ ^ L - _

TRANSFER ERASE

-.--. _] o CO CD

Colloids and Surfaces in Reprographic Technology - American

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