Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to an improved process for
removing unexposed diazo material from an aluminum lithographic
printing plate.
Lithographic printing techniques, using, for example,
silicated aluminum plates as described in Jewett et al patent
No. 2,714,066, or anodized and silicated aluminum plates as
described in Fromson patent No. 3,181,461, are widely used in
the printing industry.
A negative working plate is coated with an aqueous
solution of a diazo resin, dried and normally exposed through
a negative for 1 to 2 minutes. The exposed image areas become
water insoluble and the unexposed non-image areas remain water
soluble. The plate is conventionally developed with a litho-
graphic lacquer which consists of a two phase system, one phase
containing an hydrophilic gum in water and the other an oleo-
philic resin in a solvent. Upon application, the oleophilic
resin adheres to the exposed insoluble areas, while the water
in the aqueous phase dissolves away the unexposed soluble non-
image areas leaving a deposit of gum in the same areas. In
this way, the image areas are made oleophilic or ink receptive
and the gummed, non-image areas are made hydrophilic or ink
repellent.
Silicated aluminum plates and silicated anodized
aluminum plates (which can be pretreated to produce a grained
surface) have been found to have a negatively charged surface
which forms an ionic bond with a diazo resin which has an
opposite, positive charge. ~lile the unexposed, non-image
portions of the diazo in theory remain water soluble, in
practice it has been found that the dissolving power of water
is insufficient to overcome the ionic bond. Thus, residual
diazo remains in the non-image areas and the art has employed
gum arabic to mask the residual diazo, which remains oleophilic,
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to prevent scumming or ink pick-up in the non-image areas (also
referred to as the background~.
The presence of residual diazo in the background can
be demonstrated by exposing a diazo coated silicated aluminum
plate thru a negative, developing it with water and then re-
exposing the plate thru the same negative turned ninety degrees.
the twice exposed plate is again developed with water and then
rubbed with press ink. The first image appears as well as a
second, crossed image in the background of the first image.
Ionically bonded diazo that remains in the background is res-
ponsible and, unless masked with gum arabic, will result in
undesirable ink pick-up in the background long before the image
wears.
The use of stronger solvents such as benzyl alcohol
with water or by themselves has been proposed as well as
coupling diazo resins with various materials to make them
sparingly soluble in water and soluble in organic solvents
(cf. U.S. 3,300,309 and 3,790,556). However, solvents are
expensive and present serious disposal problems. The art has
long sought a truly water-developable plate.
It has also been proposed to alter the surface
characteristics of the aluminum plate to minimize or prevent
ionic bonding by the diazo resin (cf U.S. 3,220,832) but this
has drawbacks with respect to adhesion in the image areas.
While there are suggestions for using wetting agents
in lithographic developers and lacquers, the art has avoided
the use of anionic surfactants in diazo developers because it
is known that a chemical reaction takes place (cf. U.S.
3,790,556) between a diazo and an anionic surfactant. The
ink-loving reaction product is difficult to completely remove
and unless it is masked with a water-loving substance such as
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gum arabic, its presence in the background will cause undesirable
scumming or ink pick-up.
The present invention provides a developing system
using an aqueous developer that removes the soluble light
sensitive material completely. The invention also overcomes
ecological problems by providing an aqueous system which can
replace heretofore used organic solvent systems.
The invention provides a process for removing un-
exposed diazonium material from an aluminum lithographic
printing plate having a silicated aluminum substrate having a
hydrophilic, anionic, negatively charged surface. A layer of
a light sensitive, cationic, positively charged, water-soluble
diazonium material is bonded to the silicated substrate. The
plate is selectively exposed to actinic light in an image area
and unexposed diazonium material is substantially completely
removed leaving a clean, hydrophilic background on the plate
by contacting the plate after exposure with a solution consist-
ing essentially of an anionic material in water followed by
rinsing with water. The aqueous solution of anionic material
functions in two ways: first, anionic material couples (chemi-
cally reacts) with unexposed diazonium material and then, nearly
simultaneously, the solution dissolves the coupled product to
overcome the bond between the silicated substrate and the dia-
zonium material. The ability of the solution of anionic mate-
rial to dissolve the coupled product overcomes the affinity of
the diazonium material for the substrate and ensures removal of
unexposed material leaving a clean, hydrophilic background.
The aqueous solution has a concentration of anionic material
that is sufficient to couple with the unexposed diazonium
material and dissolve the coupled product. Stated differently,
the plate is contacted with an anionic material in water in a
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quantity and for a time sufficient to couple the anionic mate-
rial with the diazo and dissolve the coupled product from the
unexposed areas.
A plate developed according to the invention shows no
second image in the background upon reexposure indicating sub-
stantially complete removal of diazo from the unexposed areas.
Ionic development of dia~onium light sensitive com-
pounds depends on a chemical reaction between the positively
charged (cationic) diazo material and a negatively charged
(anionic) chemical entity in an aqueous ionizing medium. Upon
formation, the reaction product precipitates and if and when
the concentration of the anionic compound in the ionizing medium
is high enough, instant or simultaneous redissolution of the
reaction product occurs. In this way the reaction product is
prevented from repricipitating on the surface from which it
was removed.
The determining variables in formulating a suitable
developer are:
1. The anionic material
2. The cationic diazo material
3. Relative concentrations of both cationic diazo
and the anion.
4. The volume of the aqueous anionic solution and
its diffusion properties.
5. The ternperature, which affects the rate
reaction.
6. The pH
7. Physical agitation, which affects the micro
chemical diffusion by renewing surfaces and
homogenizing reaction gradients.
Suitable anionic materials are water soluble and in-
clude the alkali metal salts of alkylaryl sulfonates having 1
. 1
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to 20 carbon atoms in the alkyl portion and 6 to 14 carbon atoms
in the aryl portion, alkali metal salts of alkyl sulfonates
having 12 to 20 carbon atoms and ammonium and alkali metal salts
of sulfated higher fatty alcohols having 10 to 20 carbon atoms.
Anionic materials used in the invention are usually dissolved
in water and the concentration of the anionic material is suffi-
cient to form an ionically coupled product with the light sensi-
tive material and to dissolve the coupled product.
In more practical terms, it has been found that con-
centrations of anionic material of from 1 to 100% function
satisfactorily with diazo materials coated on a substrate from
solutions having a concentration of from 0.01 to 5%.
The rate of developing can also be accelerated by
heating the developer to a temperature in the range of from 18
to 90 C, preferably from 22 to 60 C.
Specific examples of anionic materials are given
herein together with a test to determine suitability. The
effectiveness of certain anionic surfactants can be improved
by adjusting the pH in the range of 2-10, preferably 2.5-9.
While a pH adjuster can be present, the presence of
other materials such as cationic materials that will interfere
with or prevent the coupling reaction between unexposed diazo
and the anionic material in the non-image areas and subsequent
dissolution of the coupled material in the non-image areas,
must be avoided.
As described in U.S. Patent No. 4,277,555 issued
July 7, 1981, cationic dyes can be applied to the anionically
charged substrate or incorporated in the light sensitive mate-
rial to provide a visible image on the plate. Suitable dyes
include basic cationic dyes such as Victoria Green, Rhodamine
B, Rhodamine 5GLD, Crystal Violet, extra pure APN, Paper slue R
and the like.
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Diazonium molecules are very strong cations and react
instantaneously with materials of the opposite charge, namely
anionic materials such as surfactants, dyes, polymers and the
like. Thus, when an unexposed diazonium material is treated
with an aqueous anionic solution such as sodium lauryl sulfate,
in water, a coupling reaction between diazonium material and
the anionic material takes place. It is thus believed that
there is a two-step sequence, first the coupling reaction bet-
ween the diazo and an anionic material followed by removal
(by dissolution) of the coupled material in the background by
the solution containing sufficient excess anionic material
(after the coupling reaction) for dissolution.
Cationic light sensitive materials used in the in-
vention are water-soluble diazonium materials having reactive
sites capable of coupling with an anionic material. For ex-
ample, 4-diazo diphenylamine condensed with a carbonyl compound
such as formaldehyde is capable of coupling with an anionic
material such as sodium lauryl sulfate. Such a condensed diazo
compound further coupled with an aromatic compound such as the
sulphonic acid of benzophenone to make it water insoluble cannot
undergo coupling according to the invention. Suitable diazonium
compounds are described, inter alia, in U.S. patents 2,063,631,
2,667,415, 2,677,498, 3,050,502 3,311,605, 3,163,633, 3,406,159
and 3,227,074.
The silicated aluminum substrate can be a single sheet
or a laminate and can be rigid or flexible. The preferred litho-
graphic substrate is anodized aluminum which may be pretreated
before anodizing to roughen or grain the surface, for example
using mechanical, chemical or electrochemical techniques as are
well known in the art and it may be post-treated after anodizing.
It is preferred to impart hydrophilicity and anionic charge by
silicating anodized aluminum as described in Fromson patent
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, ~
lls73ns
No. 3,181,461 or by silicating as described in Jewett et al.
patent No. 2,714,066. The term "silicated aluminum" is thus
used herein to describe silicated aluminum as per U.S.2,714,066
and silicated anodized aluminum as per U.S. 3,181,461.
After treatment with the anionic material, the image
can be reinforced with an oleophilic W curable material which
can be coated on and then cured. This is described in Canadian
application ~o. 338,014 filed October 19, 1979.
Suitable W curable materials are commercially avail-
able from a number of sources in the form of W curable inks,coatings, oligomers and monomers. Such commercially available
materials can be obtained from the following companies: Inmont
Corporation, Sinclair & Valentine, Celanese Chemical Company,
3-M Company, Desoto Chemical Company, Polymer Industries, Shell
Chemical, Mobile Chemical, W.R. Grace, Design Coat Company, and
Ware Chemical Corporation.
W curable materials including monomers and oligomers
are described in the following patents:
U.S. 3,297,745 1967
U.S. 3,380,381 1968
U.S. 3,673,140 1972
U.S. 3,770,643 1972
U.S. 3,712,871 1973
U.S. 3,804,736 1974
I'here are also materials that will cure upon exposure
to other sources of radiation, for example an electron beam.
These curable materials can be used in special applications in
place of the W material and are commercially available. Elec-
tron beam curable compositions are described in U.S. patents
3,586,526-30, 1971.
A teSt to determine whether a particular anionic
material is suitable is as follows:
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A 5% aqueous solution of the anionic material i8
prepared. An aluminum lithographic plate grained, anodized,
and silicated is coated with a 1% golution of a light sensitive
diazo condensation product (such as Fairmonfs Chemical Diazo
~o. 4). The coated plate is exposed through a negat~ve for a
relatively short period of time 5 to 10 seconds. The exposed
plate is immersed in the 5% solution of anionic material for
10 seconds. The plate is then rinsed thoroughly with water and
dried. The plate is then turned ninety degrees and exposed a
second time through the same negative. The twice exposed plate
is then lacquered with a standard lithographic lacquer (such as
Fairmont's Black Lacquer). If only a single image appears,the
anionic material is suitable. If a double, crossed image
appears, diazo is being left in the background of the first
image which means the anionic material is probably unsuitable.
However, higher concentrations should be tried before a final
conclusion on suitability is drawn.
Many different salts of anionic materials are suit-
able, these include sodiu~,lithium, ammonium, or triethanol
amine salts and the like. Examples of suitable anionic sur-
factants (and their commercial sources) are as follows:
1. Sodium Lauryl sulfate (Proctor & Gamble, Equex~MS,
Equex SP, Alcolac, Inc. Sipex SB).
2. Ammonium lauryl sulfate tAlcolac, Inc.,
SiponTML-22~.
3. Sodium lauryl ether sulfate (Alcolac, Inc.,
SiponTMES~,
4. Sodium dodecyl benzene sulfonate (Alcolac
Inc. Siponate ~ S-X0).
5. Ammonium lauryl ether sulfonate (Alcolac, Inc.
SiponTMEA).
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6. Triethanolamine lauryl sulfate (Alcolac, Inc.
sipon T~ T-6).
7. Sodium alkyl sulfate (Alcolac, Inc., Sipex TMOLS).
8. Sodium stearate (Emery Inds.).
9. Sodium palmitate (Emery Inds.).
10. Sodium oleate (Matlerson, Coleman & Bell).
11. Dioctyl sodium sulfosuccinate (Cyanamid,
~erosol OT).
12. Tetrasodium N-Cl, 2 dicarboxyethyl 1) -N-octadecyl
sulfosuccinate (Cyanamid, Aerosol 22).
13. Sodium Xylene sulfonate (~itco Chemical, Ultra SXS).
14. Sodium toluene sulfonate (Witco Chemical,
Ultra STS).
15. Sodium cumene sulfonate (Witco Chemical, Ultra
SCS hydrotrope).
16. Sodium dihexyl sulfosuccinate (Cyanamid
Aerosol AY-65).
17. Sodium diamyl sulfosuccinate (Cyanamid Aerosol
AY-65).
18. Anionic phosphate surfactant (Rohm & Haas Co.,
Triton QS-30).
19. Sodium alkylaryl polyether sulfate (Rohm & Haas
Co., Triton ~-30 conc.).
20. Phosphate surfactant, potassium salt (Rohm &
Haas Co., Triton MH-66).
21. Sodium alkylaryl polyether sulfonate (Rohm &
Haas Co., Triton ~ -200).
Sodium lauryl sulfate is preferred because of its
availability and cost.
It is preferred to overwhelm the exposed surface with
aqueous anionic material to couple the diazo layer and dissolve
the coupled product from the unexposed areas of the plate. This
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can be accomplished by immersion in the aqueous solution or by
flooding the exposed surface by cascading or spraying. Rubbing
by hand, normally employed with conventional developers is not
necessary and should be avoided to prevent the creation of
conditions under which the coupled product will come out of
solution in the developer and deposit in the background. Un-
even hand pressure can lead to this whereas uniform machine
pressure applied via sponges or brushes can be tolerated.
EXAMPLE 1 (CONTROL)
A problem common to most lithographic plates coated
with negative working condensation products of diazonium salts
is failure to completely remove the unexposed diazo in non-
image areas. To illustrate: a brush grained, anodized, sili-
cated plate is coated with a 3% solution of a water soluble
diazo resin (Fairmont's Diazo Resin No. 4). The plate is
exposed through a negative for 90 seconds with a Nu-arc flip-
top unit to a solid 6 or 7 on a Stouffer Step Guide. After
exposure the plate is immersed in running tap water, under
yellow lights, for eight hours. The plate is then wiped, dried,
and returned to the exposure unit. This time the negative is
turned at right angle to the previously exposed image. The
plate is reexposed for 90 seconds and developed using any avail-
able commercial developing lacquer. Upon development, it will
be noted that two distinct image patterns exist on the plate.
One from the first exposure and a second at a right angle to
the first.
EXAMPLE 2 (CONTROL)
A brush grained, anodized, silicated plate is coated
and exposed as in Example 1. The plate is developed with a
commercial developing lacquer such as Western's Jet Black or
Fairmont's Black lacquer. After developing, rinsing, and dry-
ing, the plate is reexposed with the negative turned at 90.
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The plate when inked all over results in a double image.
EXAMPLE 3
A plate is coated and exposed as in Example 1. The
plate is immersed in a 5% aqueous solution of sodiumlauryl sulfate
(SLS) for 10 seconds. The plate is then rinsed, dried, and
reexposed as in Example 1. After the second exposure the plate
is inked entirely. No second image results indicating that
the background has been thoroughiy cleaned, leaving no residual
diazo.
~XAMPLE 4
A brush grained, anodized, silicated plate is coated
with a 1% solution of diazo resin No. 4 (Fairmont Chemical Co.).
The plate is exposed for 10 seconds on a Nu arc flip top ex-
posure unit. The plate is developed by immersing it in a hot
(50C.) solution (5%) of SLS for 2 seconds. The plate is
rinsed, dried, and ready for press.
EXAMPLE 5
A plate is coated, exposed and processed as in Ex-
ample 3, except that the developer is acidified by adding 2.5
cc/liter of phosphoric acid. The plate is exposed a second
time with similar results.
EXAMPLE 6
Two plates are coated as in Example 1. The plates
are then exposed for 10 seconds. One plate is developed by
hand using a sponge, and rubbing a 5% solution of SLS vigorous'y
over the entire plate surface. After development the plate is
rinsed thoroughly and dried. The second plate is developed by
immersing the plate completely in a 5% solution of SLS for 10
seconds, rinsing and drying. Both plates are now tested for
background cleanliness by applying either lacquer or ink over
the entire surface. This test should be repeated many times
for statistical confirmation. It will be noted that invariably
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hand developed plate or plates developed via the application of
pressure to the surface be it scrubbing, rubbing, or buffing
will have undesirable background or scumming marks. Plates
processed via immersion will invariably have clean backgrounds.
EXAMPLE 7
Two plates are coated as in Example 1. These plates
are exposed for 10 seconds and one plate is spray developed
with a 5% SLS and the other is developed by cascading SLS over
the surface of the plate. In both instances clean, acceptable
plates resulted.
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