Process for graining an aluminum base lithographic plate and article thereof

Planar surfaces are roughened by graining with an aqueous slurry of unfused platy, crystalline alumina. Preferably, an aluminum base with is adapted to receive a light-sensitive coating thereon to make lithographic plate is grained with said aqueous slurry.

BACKGROUND 
This invention relates to the roughening of smooth planar surfaces such as 
metal or plastic and especially to the preparation of a grained aluminum 
base for lithographic printing plates, and more particularly to an 
anodized aluminum base having improved abrasion resistance and long 
press-life. 
The art of lithographic printing depends upon the immiscibility of grease 
and water, upon the preferential retention of a greasy image-forming 
substance by an image area, and upon the similar retention of an aqueous 
dampening fluid by a non-image area. When a greasy image is imprinted upon 
a suitable surface and the entire surface is then moistened with an 
aqueous solution, the image area will repel the water and the non-image 
area will retain the water. Upon subsequent application of greasy ink, the 
image portion retains ink whereas the moistened non-image area repels it. 
The ink on the image area is then transferred to the surface of a material 
on which the image is to be reproduced, such as paper, cloth and the like, 
via an intermediary, a so-called offset or blanket cylinder, which is 
necessary to prevent mirror-image printing. 
The type of lithographic plate to which the present invention is directed 
has a coating of a light-sensitive substance that is adherent to an 
aluminum base sheet. If the light-sensitive coating is applied to the base 
sheet by the manufacturer, the plate is referred to as a "presensitized 
plate". If the light-sensitive substance is applied to the base by the 
lithographer or trade plate-maker, the plate is referred to as a "wipe-on" 
plate. Depending upon the nature of the photosensitive coating employed, 
the treated plate may be utilized to reproduce directly the image to which 
it is exposed, in which case it is termed a positive-acting plate, or to 
produce an image complementary to the one to which it is exposed, in which 
case it is termed a negative-acting plate. In either case the image area 
of the developed plate is oleophilic and the non-image area is 
hydrophilic. 
In the case of a negative working plate the surface is coated with an 
aqueous solution of a conventional diazo salt. The plate is dried and 
exposed through a photographic negative. The exposed image areas decompose 
and become insoluble. The unexposed, nonimage areas remain soluble. The 
plate is developed with a lithographic lacquer which consists of two 
phases-- one phase containing an oleophilic resin and the other phase a 
hydrophilic gum. Upon application the oleophilic resin attaches to the 
exposed insoluble areas, while the hydrophilic phase cleans and protects 
the unexposed soluble nonimage areas. In this way the image areas are made 
oleophilic or ink-receptive and the nonimage areas are made hydrophilic or 
ink-repellent. 
A positive plate is generally one upon which the non-image area is the 
portion of the light-sensitive diazo compound exposed to light while the 
unexposed portion is either oleophilic or adapted to be converted by 
chemical reaction to a hardened oleophilic ink-receptive image area. 
In coating a metallic plate with a light-sensitive material it is desirable 
to provide an adherent, hydrophilic, abrasion resistance surface. This is 
best achieved by anodizing the surface of the aluminum base followed by a 
silicating treatment. In addition, the maximum lattitude between the 
oleophilic image areas and hydrophilic non-image areas is achieved. 
Anodized aluminum bases for lithographic printing plates are well known and 
commercially available. Such plates are described in U.S. Pat. No. 
3,181,461 issued May 4, 1965. 
Prior to anodizing, it is common practice to grain the surface of the 
aluminum to increase its surface area. Graining can be carried out by 
mechanically treating the aluminum, for example by brush graining or ball 
grained or it can be grained chemically or electrochemically. Slurry brush 
graining has grown in importance over the past 20 years and today 
approximately 75% of the lithographic plates produced in the U.S. are 
grained using this technique. 
Traditionally brush graining has been achieved by incorporating pumice or 
quartz into an aqueous slurry. These conventional abrasives are blocky 
and/or angular in shape thus presenting cutting edges for gouging or 
roughening surfaces in a random, nonuniform fashion. ("Brush Graining of 
Aluminum for Lithographic Printing Plates", J. H. Manhart, Alcoa). See 
also U.S. Pat. No. 3,891,516 issued June 24, 1975. 
The ultimate test of the efficiency of graining is the quality of printing 
and the useful life of the printing plate. A good grain holds the organic 
coating in the image area and it also holds more water in the non-image 
areas making the water balance on the press less critical. 
SUMMARY 
The present invention provides a process for roughening smooth planar 
surfaces such as metal or plastic surfaces by graining with an aqueous 
slurry of unfused, crystalline alumina having a flat plate-like particle 
configuration. In a preferred embodiment, the invention provides a 
grained aluminum base or an anodized aluminum base for making lithographic 
printing plates which is characterized by greatly improved abrasion 
resistance as compared to aluminum bases which are grained, prior to 
anodizing using conventional techniques. 
The process of the invention in one embodiment is thus an improvement in 
the process for making an aluminum base for use in making lithographic 
printing plates and involves graining the aluminum base with an aqueous 
slurry of unfused, platy, crystalline alumina. The graining is preferably 
carried out continuously on an aluminum web using a plurality of rotating 
brushes. The aluminum is preferably subsequently anodized and silicated 
before applying a light-sensitive coating. 
DESCRIPTION 
The particulate alumina used to grain in the invention is unfused, 
anhydrous, crystalline alumina having a plate-like or tablet-like particle 
configuration. The flat dimension is generally three to five times greater 
than the thickness. This form of alumina can be obtained from hydrated 
aluminas but generally it is made from alpha-alumina trihydrate. 
Alpha-alumina trihydrate is a crystalline material and in its natural state 
is known as gibbsite or hydrargillite. It forms the main constituent of 
certain bauxites, such as those found in America and Africa. Alpha-alumina 
trihydrate is obtained directly by the Bayer process, which consists in 
treating the bauxite with alkali, under pressure, followed by 
precipitation of the resulting sodium aluminate solution by dilution and 
seeding with already formed hydrate. The Bayer hydrate appears as grains 
of relatively spherical shape, measuring 50-100 microns, which are 
polycrystalline aggregates, the individual crystals of which may reach a 
size of 1-20 microns. 
Alpha-alumina trihydrate, when heated, begins to lose its water of 
constitution. Complete dehydration results in alpha-alumina as the final 
product. Unfused crystalline alumina obtained by dehydrating alpha-alumina 
trihydrate is a particulate material and the individual particles have a 
flat plate-like configuration the major dimension of which is generally 
five times greater than the minor dimension. They also tend to be 
hexagonal. 
The term "unfused" is used to describe that form of alpha-alumina obtained 
by dehydrating alpha-alumina trihydrate (or other hydrates) without 
exceeding the melting or fusion temperature of the anhydrous alumina. 
Stated differently, alpha-alumina trihydrate is calcined or dried to reach 
the anhydrous alpha-alumina product without destroying the crystallinity 
of the alpha-alumina. If the fusion or melting temperature of the 
alpha-alumina is exceeded, the product becomes amorphous and ball milling 
or grinding causes fractures and produces blocky or flinty particles which 
are quite different when used in brush graining as compared to unfused 
crystalline alumina as used in the present invention. 
Graining of aluminum according to the invention is preferably carried out 
continuously on a moving aluminum web using a plurality of rotating 
brushes. It is preferred to carry out the brush graining on a moving 
aluminum web using pairs of tandem brushes with an aqueous slurry of 
unfused crystalline alumina fed from recirculating sumps. Suitable 
graining equipment is commercially available from the Fuller Brush Company 
and was used in the examples described herein. 
The unfused, crystalline alumina obtained from alpha-alumina trihydrate is 
characterized by a hardness on the Mohs scale of 9 (Kirk-Othmer, 
Encyclopedia of Chemical Technology, 2nd Ed., Vol. 2, pp. 42-51). This 
material is used to form a slurry in water. From 3 to 6 lbs. of alumina 
from alpha-alumina trihydrate per gallon of water are generally employed, 
it being generally observed that graining efficiency is not increased when 
going above 6 lbs. per gallon. 
The geometry of unfused alumina is very unique. Unlike fused alumina, which 
is blocky or slivery in shape and quartz which is slivery or angular in 
shape; unfused alumina is hexagonal and platy. Unfused platy alumina is 
normally used for lapping, levelling or polishing of uneven or rough 
surfaces. This is achieved by a circular action whose force vector is 
normal to the surface. This action produces smooth surfaces. However, in 
accordance with this invention it has been found that unfused, platy 
alumina can be used to roughen a planar surface, e.g., metal sheets, if 
used with a rotary motion whose force vector is tangential to the surface 
of a moving web, specifically a brushing action. 
Anodizing following the graining operation of the invention may be carried 
out using known techniques to form a porous anodic oxide layer on the 
grained aluminum surface. Sulfuric acid is the preferred electrolyte. See 
Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 1, p. 978 
et seq. 
Cold rolled aluminum should be employed for forming printing plates 
according to the invention. Softer aluminum is not suitable because it 
will tear or rip when engaged by the lock-up device of a printing press. 
Preferred aluminum sheet generally has a temper of between H12 and H19 
where direct cold reduction is employed or between H22 and H27 where a 
combination of cold reduction and back annealing are employed, as 
specified by the American Aluminum Association in Aluminum Standards and 
Data, published by the Association. 
Aluminum printing plates can be made in any fashion known in the art, for 
example as taught by the following 
U.S. Pat. No. 2,714,066, Jewitt et al, July 26, 1955; 
U.S. Pat. No. 2,741,981, Frost, Apr. 17, 1956; 
U.S. Pat. No. 2,791,504, Plambeck, May 7, 1957; 
U.S. Pat. No. 3,062,648; Grawford, Nov. 6, 1962; 
U.S. Pat. No. 3,181,461, Fromson, May 4, 1965; 
U.S. Pat. No. 3,220,346, Strickler, Nov. 30, 1965; 
U.S. Pat. No. 3,280,734, Fromson, Oct. 25, 1966; and 
U.S. Pat. No. 3,338,164, Webers, Aug. 29, 1967. 
Especially preferred is an anodically oxidized aluminum base having an 
aluminum oxide surface which is initially porous after anodic oxidation 
and subsequently treated with an alkali metal silicate and sealed prior to 
application of a light-sensitive coating. This is the subject of U.S. Pat. 
No. 3,181,461 referred to above. 
It is preferred to continuously anodize aluminum after graining utilizing 
the anodizing techniques described in patents U.S. Pat. No. 3,865,700 
issued Feb. 11, 1975, and U.S. Pat. No. 3,920,525 issued Nov. 18, 1975. If 
desired, the aluminum base can be provided with a composite anodized and 
discontinuously electroplated surface prior to application of the 
light-sensitive coating as taught in patent U.S. Pat. No. 3,929,594 issued 
Dec. 30, 1975. 
The light-sensitive coating for making lithographic plates has one 
solubility in relation to a solvent in a state before exposure to actinic 
radiation and another solubility in relation to said solvent in another 
state after exposure to actinic radiation, said light-sensitive coating 
being soluble in said solvent in one of said states and being insoluble in 
said solvent in its other state. 
The light-sensitive layer or coating may be formed from a host of 
photochemical materials known in the art. Such light-sensitive materials 
include dichromated colloids, such as those based on organic colloids, 
gelatin, process glue, albumens, caseins, natural gums, starch and its 
derivatives, synthetic resins, such as polyvinyl alcohol and the like; 
unsaturated compounds such as those based on cinnamic acid and its 
derivatives, chalcone type compounds, stilbene compounds and the like; and 
photopolymerizable compositions, a wide variety of polymers including 
vinyl polymers and copolymers such as polyvinyl alcohol, polyvinyl 
acetals, polyvinyl acetate vinyl sorbate, polyvinyl ester acetal, 
polyvinyl pyrrolidone, polyvinyl butyrol, halogenated polyvinyl alcohol; 
cellulose based polymers such as cellulose-acetate hydrogenphthalate, 
cellulose alkyl ethers; ureaformalehyde resins; polyamide condensation 
polymers; polyethylene oxides; polyalkylene ethers, polyhexamethylene 
adipamide; polychlorophene, polyethylene glycols, and the like. Such 
compositions utilize as initiators carbonyl compounds, organic sulphur 
compounds, peroxides, redox systems, azo and diazo compounds, halogen 
compounds and the like. These and other photochemical materials including 
their chemistry and uses are discussed in detail in a text entitled 
[Light-Sensitive Systems, Jaromir Kosar, John Wiley and Sons, Inc., New 
York 1965. Diazo resins are particularly preferred. 
The light-sensitive coating is referred to for ease in understanding as 
being soluble in relation to a solvent before exposure to actinic 
radiation and insoluble with respect to said solvent after exposure to 
actinic radiation, it being understood that light-sensitive materials 
which behave in the opposite manner, that is first insoluble and then 
soluble after exposure, are within the purview of the present invention. 
The terms "soluble" and "insoluble" are intended to convey the meaning 
generally accepted and understood in the art of exposing and developing 
images utilizing light-sensitive systems. For example, a light-sensitive 
material is considered to be soluble when it can be readily removed by 
washing with a particular solvent at normal operating temperatures such as 
room temperature and insoluble when it is not removed upon exposure to a 
particular solvent under the same or similar temperature conditions. 
If desired, the light-sensitive printing plate can be provided with a 
tough, wear-resistant protective layer as taught in patent U.S. Pat. No. 
3,773,514 issued Nov. 20, 1973. If desired, graining with platy alumina 
can be carried out in combination with other abrasives such as quartz or 
conventional graining can precede or follow graining according to the 
invention.

The present invention wll be more fully understood from the following 
examples which are intended to illustrate the invention without limiting 
same. 
EXAMPLE 1 
Multiple graining units are installed in a continuous web anodizing line. 
The placement of these units relative to the entire line is after the 
degreasing section and prior to the anodizing section. Graining is 
achieved by supplying an aqueous, abrasive slurry at the point of contact 
between rotary brush and moving aluminum web. This is accomplished by 
sumps in which the slurry is stored, mixed, and circulated to the web 
where the work is done. For comparison, a slurry for one graining unit is 
prepared by adding FFF Pumice (5 lbs.) and 6/0 quartz (100 lbs.) to 30 
gallons of water under high speed agitation. A second sump is charged with 
5 lbs. FFF Pumice, 100 lbs. 7/10 quartz and 30 gallons of water. 
A coil of aluminum 0.012 inches thick, 24 inches wide is mounted on the 
production line. The speed of the web through the line is set at 50 
ft./minute. The sequence of processing is as follows: degreasing, rinsing, 
graining, cleaning, rinsing, anodizing rinsing, silicating, rinsing, and 
drying. 
Samples 17 in..times.6 in. are then taken for testing. The anodic oxide 
coat weight is determined by stripping and weighing-- this is typically 
1.2-1.5 mg./in..sup.2. A 17 in..times.6 in. sample is coated with a 
light-sensitive diazo compound (3% solution-Fairmont Chem. Co.) and dried. 
The sample is given a blanket exposure, 11/2 min., on a Nu-Arc exposure 
unit. The entire sample is then lacquered (solid) with black Lacquer 
(Fairmont Chem. Co.) and dried. The lacquered sample is then placed on a 
Gardner Straight Line Washability and Abrasion tester. Abrasion is 
accomplished with a nylon scouring pad manufactured by Metal Textiles 
Div., General Cable Corp. The pad (51/4 in..times.1.75 in.) is weighted 
with 13/4 lbs. The test is run for 150 cycles. The results of this 
abrasion test are compared with four standard samples empirically derived 
over many graining trials. They are designated poor-good-very 
good-excellent. This test is easy to judge because samples of lesser 
quality begin to show white scratch marks parallel to the direction of the 
abrasion. These marks are very obvious and easy to quantify because of the 
black background. In this example, using quartz, the results are rated 
poor (many white scratch marks) indicating a product of inferior quality. 
EXAMPLE 2 
A brush graining-anodizing trial similar to example 1 is run except the 
second graining unit is charged with 30 gals. of water and 100 lbs. white, 
unfused crystalline alumina made from alumina trihydrate having an average 
particle size of 18 microns. Samples are taken and tested in example 1. 
The abrasion test shows very good results (little or no scratch marks) 
indicating a product of good quality. 
EXAMPLE 3 
A brush graining-anodizing trial similar to example 1 is run except that 
the graining units are charged with identical slurries consisting of 30 
gals. water and 100 lbs. of Alumina MCA 820 (white, unfused alumina sold 
by the Norton Co.). 
Samples and tests are made similar to example 2. The abrasion tests give 
excellent results (no scratch marks) indicating a superior product. 
EXAMPLE 4 
A brush graining-anodizing run is made similar to example 3 except that the 
line speed is set at 100 ft./min. 
Samples and tests are run as in example 1. The oxide coat weight is 
approximately 50% lower, namely 0.8mg./in..sup.2 in example 1, and the 
abrasion tests show very good results indicating a product as good as that 
achieved in example 2. 
EXAMPLE 5 
A brush graining-anodizing run is made similar to example 1 except that the 
slurry charge in both sumps contain 100 lbs. of Alundum Abrasive, size 180 
(white friable fused Aluminum Oxide). This material is characterized by 
its blocky shape. (Purchased from the Norton Company). 
Samples taken and tests were run for abrasion resistance. The test showed 
poor abrasion resistance. 
EXAMPLE 6 
A brush graining-anodizing run is made similar to example 5 using a slurry 
charge of Micrograded, fused, Alundum, size 17.5 in one sump and fused 
Alundum 180 in the other. Abrasion tests showed similar results to that of 
example 5. 
EXAMPLE 7 
A brush graining-anodizing run was made using fused Alundum 180 in one sump 
and unfused MCA 820 in the other sump. The line speed was 55 ft./min. 
Samples were taken and abrasion tests were made. The test results were 
very good showing the unusual upgrading effect, on abrasion resistance, 
that the unfused alumina has in this process. 
EXAMPLE 8 
A brush graining trial similar to example 5 was run except that both sumps 
contained 30 gals of water and 100 lbs of Tabular Alumina, T-16,- 325 
mesh, purchased from Alcoa. Abrasion tests showed very good results. 
EXAMPLE 9 
A brush graining trial similar to example 5 was run using 100 lbs. of 
Calcined Alumina A-2 (Alcoa) in each sump. This material produced a grain 
that showed very good abrasion resistance as measured in example 1. 
EXAMPLE 10 
Copper, brass and stainless steel; Mylar, polystyrene, triacetate, 
polyacetate and vinyl sheets are surface roughened using the brush 
roughening techniques described in example 3. In each case the treated 
surfaces have a uniform grained appearance with a greatly increased 
surface area. The metallic surfaces act as substrates in electrolytic and 
chemical etching techniques where photo-sensitive resists or plain resin 
resists are used. On plastics, the graining technique is used to roughen 
the planar surfaces for anchoring subsequent coatings.