27
Magnetic Ink for Magnetic Ink Jet Printing
ZLATA KOVAC and CARLOS J. SAMBUCETTI
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch027
IBM, T. J. Watson Research Center, Yorktown Heights, NY 10598
This work w i l l describe a series of magnetic colloidal systems that were specifically developed for an application in ink jet printing technology. This application imposes a certain set of requirements such as: particle size 100 ± 50Å, magnetic moment of 25 emu/g or 35%w Fe O in colloidal dispersion, viscosity of 8-10 cps, non-toxic aqueous system, shelf l i f e of a few years, freeze-thaw s t a b i l i t y , fast drying (2 msec) and high optical density of magnetic ink on various papers. Experimental details of Fe O precipitation, choice of surfactants and additives used to give above properties of colloidal printing inks w i l l be given in d e t a i l . 3
3
4
4
Colloidal dispersions of ferromagnetic materials such as Fe, Co, Ni and their alloys, or ferrites in a liquid carrier are fluids with ferromagnetic properties; they are often referred to as ferrofluids. Due to their magnetic properties, such colloidal dispersions have^been used to reveal the structure of magnetic materials, to separate materials such as o i l s p i l l s from water, or to make ^ zero-leak high speed seals and self-lubricating bearings. Furthermore, precise magnetic controllability of such fluids makes them useful in ink jet printing. The basic concept in ink jet printing consists of an ink being supplied under pressure through a nozzle or an o r i f i c e . The ink jet is periodically interrupted to produce droplets, which impinge upon a sheet of moving paper. For printing purposes, i t is necessary that drops be uniform in size, equally spaced from each other, and be formed at a high rate (—10 /sec). The drops from the ink jet can be electrostatically charged gnd deflected to form a dot on a paper as described by Sweet and later reviewed in a paper by Kamphoefner. If a 0097-6156/82/0200-0543$06.00/0 © 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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REPROGRAPHIC T E C H N O L O G Y
magnetic ink i s used, then p r i n t i n g i s achieved by s e l e c t i o n and d e f l e g t i o n of the d r o p l e t s using a magnetic f i e l d gradient. The o b j e c t i v e of t h i s work was to develop a magnetic ink which could be used i n magnetic i n k j e t s f o r p r i n t i n g experiments. ^ Kaisey^and M i s k i s c z y , using Pappel's g r i n d i n g technique, prepared a sgable c o l l o i d a l suspension of magnetic p a r t i c l e s of 100A diameter i n kerosene, fluorcarbons and water, using o l e i c a c i d as a s t a b i l i z i n g agent. Using t h i s technique, the highest c o n c e n t r a t i o n they achieved was 13% by volume of F e , ^ i n kerosene and — 3 % by volume i n water. Reamers and K h a l a f a l l a precipitated Fe«0^ from an aqueous s o l u t i o n of f e r r i c and f e r r o u s ions with NH^OH and then coated Fe^O^ p a r t i c l e s i n s i t u with ammonium o l e a t e . A d d i t i o n of kerosene to t h i s mixture with heating and s t i r r i n g r e s u l t e d i n phase separation and formation of a s t a b l e c o l l o i d a l d i s p e r s i o n of an oleate-coated magnetic p a r t i c l e s - i n - k e r o s e n e phase and a s o l u b l e ammonium s a l t - i n - w a t e r phase. The concentration of Fe^O^, which they obtained i n kerosene was the same as the Kaiser obtained by g r i n d i n g . Kerosene-based f e r r o f l u i d i s u n s a t i s f a c t o r y when used i n an ink, due to spreading of ink drops on paper. In a d d i t i o n , a high concentration of magnetic p a r t i c l e s i n the d i s p e r s i n g medium i s d e s i r e d f o r magnetic ink j e t p r i n t i n g , since each drop of an i n k i s addressed according to i t s magnetic moment. Water-based inks are used i n e l e c t r o s t a t i c ink j e t p r i n t i n g due t h e i r low v i s c o s i t y , non-toxic and non-flammable nature. Due to the above f a c t s , i t becomes c l e a r that water-based inks having high magnetic moments are d e s i r a b l e f o r magnetic ink j e t p r i n t i n g . WATER-BASED INKS FOR INK JET Figure 1 i l l u s t r a t e s the p r i n c i p l e s of p r i n t i n g with a Magnetic Ink J e t , showing the sequences of i n k drop generation, d e f l e c t i o n , and placement on paper. In t h i s technology the f l u i d i t s e l f possesses an inherent magnetic moment so that d r o p l e t s of the i n k can be subjected to magnetic f i e l d gradients to d e f l e c t and p o s i t i o n these d r o p l e t s on paper according to prearranged p a t t e r n s . (Figure 1) As metals and metal oxides are not water s o l u b l e and s o l u b l e metal compounds which e x h i b i t ferromagnetism have very low magnetic s u s c e p t i b i l i t y , the i n k i s made of a d i s p e r s i o n of p a r t i c l e s . Therefore, the magnetic j e t uses an heterogeneous system: an aqueous magnetic c o l l o i d . For t h i s p a r t i c u l a r kind of c o l l o i d , the e s s e n t i a l parameters to consider on the p a r t i c l e s are: magnetic moment, p a r t i c l e s i z e , e l e c t r i c a l charge and chemical s t a b i l i t y . The use of c o l l o i d s i n which the n u c l e i are made of m a t e r i a l s of high magnetic moment, such as Fe, Co, N i have been considered.
Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Magnetic Ink
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Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch027
KOVAC A N D S A M B U C E T T I
Figure 1. Magnetic Ink Jet System. Drops of ink are steered by non-uniform magnetic fields. Deflection in the horizontal plane selects the drops for printing, vertical deflection by the deflector magnet controls placement of drops on paper.
Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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TECHNOLOGY
However, p a r t i c l e s of ferromagnetics, when s u f f i c i e n t l y small, are of s i n g l e domain character and possess an i n t r i n s i c permanent magnetic d i p o l e moment /x such that 2 3
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch027
2
M
/ a
=
( 1 )
18
where /x. magnetic d i p o l e moment, Ms s a t u r a t i o n magnetization, a p a r t i c l e r a d i u s . At a c e r t a i n p a r t i c l e s i z e the i n - l i n e d i p o l e - d i p o l e i n t e r a c t i o n energy becomes greater than the thermal energy and the p a r t i c l e s tend to agglomerate i n l i n e a r chains. For example, f o r Co, at p a r t i c l e s i z e greater than 50A the i n t e r a c t i o n energy overcomes KT. Furthermore, systems containing Fe or Ni as n u c l e i , present problems i n chemical s t a b i l i t y due to o x i d a t i o n and h y d r o l y s i s i n aqueous s o l u t i o n s . The magnetic c o l l o i d s considered here have been s y n t h e t i c a l l y prepared by p r e c i p i t a t i o n of magnetite (Fe^O^) and d i s p e r s i o n i n aqueous media. In t h i s type of aqueous c o l l o i d s s t a b i l i z a t i o n i s mainly due to formation of e l e c t r o s t a t i c double l a y e r . This s t a b i l i t y of these heterogeneous aqueous systems i s governed by the o v e r a l l energy of i n t e r a c t i o n E between p a r t i c l e s , which can g e n e r a l l y be expressed as E =
K.e"^
3
-
I electrostatic double l a y e r repulsion
r"K a?X" 0
I
3
4-
magnetic attraction
6
K X~ 1 0
l Van der Waals attraction
(2)
where K^, K«, K~ are constants, a i s p a r t i c l e r a d i u s , X i n t e r p a r t i c l e distance, and ^ the e l e c t r o k i n e t i c p o t e n t i a l which i s a f u n c t i o n of the charge of the double l a y e r around the p a r t i c l e s . The higher the value of K^e ^ , the more s t a b l e the system i s . The e l e c t r o s t a t i c mechanism was e s t a b l i s h e d by a t t a c h i n g to the p a r t i c l e s various charged s u r f a c t a n t s which give the p a r t i c l e s d i f f e r e n t values of T (Zeta p o t e n t i a l ) and determining suspension s t a b i l i t y by u l t r a c e n t r i c fugation. In a d d i t i o n , r e f e r r i n g to Eq 2, the s t a b i l i t y of the suspended p a r t i c l e s i s enhanced by forces of r e p u l s i o n due to (a) s t e r i c hindrance or e n t r o p i c r e p u l s i o n , and (b) hydration or i n general s o l v a t i o n of adsorbed s u r f a c t a n t molecules. The l a t t e r two e f f e c t s (although d i f f i c u l t to t r e a t mathematically) are known to combine i n some aqueous systems c r e a t i n g r e p u l s i v e forces and an energy b a r r i e r which s t a b i l i z e s the magnetic c o l l o i d s even i n the absence of l a r g e e l e c t r i c r e p u l s i v e f i e l d s . For example, s t a b l e inks have been made using only uncharged non-ionic s u r f a c t a n t s and h i g h l y hydrated polyoxyethylenated surfactant l a y e r s .
Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch027
27.
KOVAC
AND
SAMBUCETTI
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Magnetic Ink
F i g u r e s 2 and 3 show t y p i c a l r e s u l t s of c a l c u l a t i o n s of t o t a l energy of i n t e r a c t i o n E, f o r F e ^ p a r t i c l e s of 100A and 20θΧ diameter r e s p e c t i v e l y . The energy i s normalized over thermal energy KT. Values of Ε g r e a t e r than 10KT i n d i c a t e a s t a b l e system ( 1 ) . The a b c i s s a i s the r a t i o (S = R/r) between the i n t e r p a r t i c l e d i s t a n c e R and the r a d i u s of the p a r t i c l e s r_. The e l e c t r o s t a t i c r e p u l s i o n f o r c e was c a l c u l a t e d using the exgression of Verwey and Overbeek 15, using a constant Κ = 10 X as the constant Huckel parameter and v a r y i n g the zeta p o t e n t i a l ψ . The f o r c e of a t t r a c t i o n by Van der Waals energy was derived using the Hamaker expression f o r s ç ^ e r e s of a r b i t r a r y dimensions, with a ^ constant A = 10 . For the magnetic a t t r a c t i o n f o r c e s for o l e a t e covered Fe^O^ p a r t i c l e s with —12% o l e a t e an experimentally obtained value of M = 2430 gauss was used r a t h e r than the t h e o r e t i c a l value f o r bulk m a t e r i a l s . The surface l a y e r of Fe^O^ does ngt c o n t r i b u t e to the magnetic p r o p e r t i e s of the p a r t i c l e s , so the s a t u r a t i o n magnetization of the p a r t i c l e i s l e s s than that of the bulk Fe~0,. Furthermore, p r e c i p i t a t e d Fe^O, contains around 12% H^O, whicn f u r t h e r c o n t r i b u t e s to a lower value of M f o r p a r t i c l e s than f o r bulk Fe^O^, which i s 4500 gauss. S
From the above f i g u r e s , i t can be concluded that i t i s p o s s i b l e to o b t a i n a s t a b l e d i s p e r s i o n of Fe^O^ p a r t i c l e s i n water when ψ>60 mV and when r
