[0001] This invention relates to planographic printing and provides a substrate for a planographic
printing member and a planographic printing member
per se. The invention particularly, although not exclusively, relates to lithographic printing.
[0002] Lithographic processes involve establishing image (printing) and non-image (non-printing)
areas on a substrate, substantially on a common plane. When such processes are used
in printing industries, non-image areas and image areas are arranged to have different
affinities for printing ink. For example, non-image areas may be generally hydrophilic
or oleophobic and image areas may be oleophilic. In "wet" lithographic printing, a
dampening or fountain (water-based) liquid is, in general, applied initially to a
plate prior to application of ink so that it adheres to the non-image areas and repels
oil based inks therefrom. In "dry" printing, ink is repelled from non-image areas
due to their release property.
[0003] Image and non-image areas can be created by processes which include a step of exposing
a layer of image material on the surface of the substrate to radiation. The exposure
to radiation creates solubility differences in the image material corresponding to
image and non-image areas. During development, the more soluble areas are removed,
leaving a pattern on the substrate corresponding to the image.
[0004] The properties of lithographic plates are highly dependent on the substrate itself
and particularly its uppermost surface, since it is this surface which must bond with
image material prior to imaging of the plate but allow release of soluble image material
during development and, furthermore, it must be non-ink accepting and thereby define
non-image areas of the plate.
[0005] Other important properties affected by the substrate may include the following:
a) the shape of dots (or other printing areas) on the plate;
b) the resolution obtainable using the plate;
c) the range of dots obtainable using the plate;
d) the exposure latitude of the plate;
e) the ink-water balance of the plate;
f) the number of prints obtainable using (and, therefore, the durability of) the plate;
g) the speed of the plate;
h) the tendency of the plate to pick-up ink in non-image areas;
i) the aesthetics of the plate pre- and post-development.
[0006] The accurate reproduction of dots (or other printing areas) in terms of their size
and/or shape (e.g. properties a) to c)) is becoming more and more important for use
in applications such as stochastic printing and/or high quality colour printing. Accordingly,
plates that can accurately reproduce dots and have other advantageous properties are
highly desirable. In addition, there is some evidence of a movement in the printing
field towards plates with a wider exposure latitude (property d)). Properties such
as e), f) and h) are properties that always need to be optimised whilst property i)
is a desirable property to optimise, since good plate aesthetics may affect a printer's
perception of the quality of a plate and make it easier for the printer to inspect
the quality of an image produced, for example pre- or post-development.
[0007] One of the most common substrates used in lithographic printing comprises an aluminium
base layer which is treated to make it ready for use. For example, the aluminium may
be roughened, for example by electrograining, anodized and then conditioned by chemical
means, for example by treatment with water, a solution of phosphate or silicate salt,
or a polycarboxylic acid.
[0008] However, one problem associated with the use of an electrograined and anodized aluminium
substrate is its poor ability to accurately reproduce dots. Another problem is the
expense (both economically and environmentally) of preparing the substrate.
[0009] It is also well-known to prepare a substrate by applying a hydrophilic layer on a
support of, for example, aluminium or plastics. Numerous different hydrophilic layers
have been proposed which possess a whole range of chemistries and morphologies. However,
very few printing plates of the type described have been commercialised. Those that
have tend to have poor properties and are generally used for low quality, short run
length applications.
[0010] EP-A-514 312 discloses an offset printing plate comprising an aluminium substrate
and a ceramic layer of sodium and/or potassium silicate thereupon, the ceramic coating
having a roughness value Ra of between 0.3 and 0.9.
[0011] It is an object of the present invention to address problems associated with known
printing members.
[0012] According to a first aspect of the invention, there is provided a printing member
comprising:
a substrate which comprises a support and a hydrophilic layer having a surface roughness
(Ra) in the range 0.1 um to 2 um, said hydrophilic layer comprising a particulate
material and a binder material for the particulate material;
wherein said printing member carries information in a stochastic form.
[0013] It has been found that if the Ra is too low then the adhesion of the image layer
is poor and run length is accordingly low. On the other hand, if Ra is too high, then
properties a) to d) and g) are detrimentally affected.
[0014] various types of instruments are known for the measurement of Ra. For example Ra
may be measured using a Talkysurf Plus unit fitted with a 112/2564-430 head, supplied
by Rank Taylor Hobson Inc of Leicester, U.K.
[0015] The Ra may be at least 0.2 µm, suitably at least 0.25 µm, preferably at least 0.3
µm, more preferably at least 0.35 µm, especially at least 0.4 µm. The Ra may be less
than 1.5 µm, suitably less than 1 µm, preferably less than 0.8 µm, more preferably
less than 0.7 µm, especially less than 0.6 µm, most preferably less than 0.5 µm.
[0016] The Ra in a first direction, across the plate is preferably substantially the same
as the Ra perpendicular to said first direction.
[0017] Confirmation that the hydrophilic layer comprises a particulate material and a binder
can be made by the use of Scanning Electron Microscopy (SEM) as described in Assessment
5 hereinafter.
[0018] The hydrophilic layer may have a surface skewness (Ssk) of greater than -0.5, preferably
greater than -0.2, more preferably greater than 0, especially greater than 0.5, most
preferably greater than 1.0. The Ssk may be less than 2.0, preferably less than 1.5,
more preferably less than 1.4, especially less than 1.2.
[0019] Ssk may be measured using any suitable instrument. A stylus measuring instrument
is preferably used, such as a Rank Taylor Hobson Form Talysurf 3D unit fitted with
a stylus of radius 2 µm as described in Assessment 2 hereinafter.
[0020] Preferably, an elemental analysis of the hydrophilic layer, especially the binder
thereof, shows that it includes the elements silicon and oxygen. Preferably, elemental
analysis shows that the hydrophilic layer includes the element aluminium. Preferably,
the analysis shows the layer includes the element titanium. The elemental analysis
may be carried out using Energy Dispersive X-ray analysis (EDX), for example as described
in Assessment 6 hereinafter. The % of silicon measured as described in Assessment
6 is preferably in the range 10%-20%, more preferably 11% - 17%. The % of oxygen measured
the same way is preferably in the range 5% - 12%, more preferably 7% - 11%. The %
of aluminium measured in the same way is preferably in the range 40% - 60%, more preferably
45% - 55%. The % titanium is preferably in the range 15% - 30%, more preferably 19%
- 25% (when assessing the K alpha electron energy level).
[0021] The ratio of Ti : Al in the hydrophilic layer may be in the range 0.5 to 2, preferably
0.75 to 1.25, more preferably 0.9 to 1.1. The ratio of (Al+Ti) : si in the layer may
be in the range 1 to 10, preferably 2 to 6, more preferably 3 to 5.
[0022] Preferably, a reflectance FT-IR spectrum of the hydrophilic layer has at least one
peak in one (but preferably has at least one peak in each) of the following ranges:
1200 to 1300 cm
-1 (especially in the range 1220 to 1280 cm
-1), 1100 to 1200 cm
-1 (especially in the range 1130 to 1190 cm
-1) and 900 to 1000 cm
-1 (especially in the range 920 to 980 cm
-1).
[0023] Preferably, a UV-VIS absorbence spectrum shows that, on lowering the wavelength,
absorbence starts to increase rapidly at a wavelength in the range 380 to 430 nm,
preferably 390 to 420 nm and reaches a value of greater than 1.
[0024] Said hydrophilic layer preferably includes a material having Si-O bonds. Preferably,
the binder includes said material having Si-O bonds. Said binder material may be a
component of a polymeric material which includes Si-O bonds. Said polymeric material
may include -Si-O-Si-, especially -Si-O-Si-O- moieties.
[0025] At least 50 wt%, suitably at least 60 wt%, preferably at least 70 wt%, more preferably
at least 80 wt%, especially at least 90 wt% of said binder material is made up of
a polymeric material having Si-O bonds as described. Preferably, said binder material
consists essentially of a polymeric material having Si-O bonds as described.
[0026] Said binder material may make up at least 5 wt%, preferably at least 10 wt%, more
preferably at least 15 wt%, especially at least 20 wt% of said hydrophilic layer.
Said binder material may make up less than 50 wt%, preferably less than 40 wt%, more
preferably less than 30 wt%, especially less than 25 wt%, of said hydrophilic layer.
[0027] Said binder material may be derived or derivable from a silicate material for example
water glasses, metasilicates, orthosilicates, sesquisilicates and modified silicates
such as borosilicate and phosphosilicate. Said binder material is preferably derived
or derivable from a silicate solution.
[0028] Said binder material preferably includes less than 10 wt%, preferably less than 5
wt%, more preferably less than 1 wt%, especially substantially no, organic material,
for example polymeric organic material.
[0029] Said particulate material is preferably dispersed in said binder material. Suitably,
30 to 85 wt%, preferably 40 to 80 wt%, more preferably 50 to 80 wt%, especially 60
to 80 wt% of said hydrophilic layer is composed of said particulate material.
[0030] Said particulate material may be organic or inorganic. Organic particulate materials
may be provided by latexes or organosols or polymeric balls, such as of nylon. Inorganic
particulate materials may be selected from alumina, silica, silicon carbide, zinc
sulphide, zirconia, barium sulphate, talcs, clays (e.g. kaolin), lithopone and titanium
oxide.
[0031] Said particulate material may comprise a first particulate material. Said first material
may have a hardness of greater than 8 Modified Mohs (on a scale of 0 to 15), preferably
greater than 9 and, more preferably, greater than 10 Modified Mohs. Said first material
may comprise generally spherical particles. Alternatively, said material may comprise
flattened particles or platelets. Said first material may have a mean particle size
of at least 0.1 µm, preferably at least 0.5 µm and, more preferably at least 1 µm.
Said first material may have a mean particle size of less than 200 µm, suitably less
than 100 µm, preferably less than 45 µm, more preferably less than 20 µm, especially
less than 10 µm and, most preferably, less than 5 µm. The particle size distribution
for 95% of particles of the first material may be in the range 0.01 to 150 µm, preferably
in the range 0.05 to 75 µm, more preferably in the range 0.05 to 30 µm. Said first
material preferably comprises an inorganic material. Said first material preferably
comprises alumina which term includes Al
2O
3 and hydrates thereof, for example Al
2O
3.3H
2O. Preferably, said material is Al
2O
3.
[0032] Said hydrophilic layer may include at least 10 wt%, suitably at least 20 wt%, preferably
at least 25 wt%, more preferably at least 30 wt%, especially at least 35 wt% of said
first particulate material. Said hydrophilic layer may include less than 80 wt%, suitably
less than 70 wt%, preferably less than 60 wt%, more preferably less than 50 wt%, especially
less than 40 wt% of said first particulate material.
[0033] The ratio of the wt% of said first particulate material to binder material may be
in the range 0.5 to 2, preferably in the range 1 to 2, more preferably in the range
1.4.to 1.8.
[0034] Said particulate material may comprise a second particulate material. Said second
particulate material may have a mean particle size of at least 0.001 µm, suitably
at least 0.005 µm, preferably at least 0.01 µm, more preferably at least 0.05 µm,
especially at least 0.1 µm. Said mean particle size may be less than 200 µm, suitably
less than 100 µm, preferably less than 50 µm, more preferably less than 10 µm, especially
less than µm, most preferably less than 0.5 µm.
[0035] Said hydrophilic layer may include at least 10 wt%, suitably at least 20 wt%, preferably
at least 25 wt%, more preferably at least 30 wt%, especially at least 35 wt% of said
second particulate material. Said hydrophilic layer may include less than 80 wt%,
suitably less than 70 wt%, preferably less than 60 wt%, more preferably less than
50 wt%, especially less than 40 wt% of said second particulate material.
[0036] Said second material is preferably a pigment. Said second material is preferably
inorganic. Said second material is preferably titanium dioxide.
[0037] The ratio of the wt% of said second particulate material to binder material may be
in the range 0.5 to 2, preferably in the range 1 to 2, more preferably in the range
1.4 to 1.8.
[0038] Said first and second materials preferably define a multimodal, for example a bimodal
particle size distribution.
[0039] The ratio of the wt% of said first particulate material to said second particulate
material may be in the range 0.3 to 3, preferably 0.5 to 2, more preferably 0.75 to
1.33, especially about 1 : 1.
[0040] Said hydrophilic layer may include one or more additional materials for improving
its adhesion to a support, especially a plastics support. A preferred additional material
is organic and is preferably polymeric. Resins are preferred.
[0041] Said support may comprise a metal layer. Preferred metals include aluminium, zinc
and titanium, with aluminium being especially preferred. Said support may comprise
an alloy of the aforesaid metals. Other alloys that may be used include brass and
steel, for example stainless steel.
[0042] Said support may comprise a non-metal layer. Preferred non-metal layers include layers
of plastics, paper or the like. Preferred plastics include polyester, especially polyethylene
terephthlate.
[0043] Said support may include one or a plurality of layers. Where the support comprises
a plurality of layers, it may comprise a plastics, paper or textile layer and another
layer. Said other layer may be a metal layer, suitably of a type described above.
In this case, said support may comprise a metal to plastics or paper laminate; or
metal may be applied by other means to plastics or paper, for example by sputtering
or the like.
[0044] Said support may be any type of support used in printing. For example, it may comprise
a cylinder or, preferably, a plate. Said support may have a width of at least 10 cm,
suitably at least 20 cm, preferably at least 30 cm, more preferably at least 40 cm,
especially at least 50 cm. Said support may have a width of less than 300 cm, suitably
less than 200 cm, preferably less than 160 cm, more preferably less than 100 cm, especially
less than 80 cm.
[0045] Said support may comprise a web of material which may have a width as described above.
Preferably the web has a width in the range 0.7 m to 1.5 m.
[0046] Said support may have a length of at least 20 cm, suitably at least 40 cm, preferably
at least 60 cm. Said support may have a length of less than 300 cm, suitably less
than 250 cm, preferably less than 200 cm, more preferably less than 150 cm, especially
less than 105 cm. The support suitably does not have a length of about 35 cm.
[0047] Said support may have a thickness of at least 0.1 mm. Said support may have a thickness
of less than 0.6 mm.
[0048] Said support may be pretreated prior to the application of said hydrophilic layer
by one or more conventional methods used in the surface treatment of aluminium or
other supports, for example caustic etch cleaning, solvent etching, acid cleaning,
brush graining, mechanical graining, slurry graining, sand blasting, abrasive cleaning,
electrocleaning, solvent degreasing, ultrasonic cleaning, alkali non-etch cleaning,
primer coating, flame treatment, grit/shot blasting and electrograining. Details of
such methods are provided in: "The surface treatment and finishing of aluminium and
its alloys" S. Wernick, R. Pinner and P. G. Sheasby published by Finishing Publication
Ltd., ASM International, 5th edition 1987.
[0049] Said support may be provided with a roughened surface over which the hydrophilic
layer may be provided. Alternatively, a subbing layer or layers may be provided over
the support.
[0050] The substrate may have a whiteness (L) value of at least 75, preferably at least
80, more preferably at least 82. The substrate may have a whiteness value of less
than 100, preferably less than 90, more preferably less than 87.
[0051] The substrate may have a gloss value of at least 5, preferably at least 7, more preferably
at least 9. The substrate may have a gloss value of less than 20, preferably less
than 18, more preferably less than 15, especially less than 13.
[0052] Said hydrophilic layer may have an average thickness of less than 100 µm, suitably
less than 50 µm, preferably less than 20 µm, more preferably less than 10 µm, especially
less than 5 µm. In some cases, the layer may have an average thickness of less than
3 µm. Said hydrophilic layer may have an average thickness of greater than 0.1 µm,
suitably greater than 0.3 µm, preferably greater than 0.5 µm, more preferably greater
than 1 µm.
[0053] Said hydrophilic layer may include 1 to 20 g of material per metre squared of substrate.
Preferably said layer includes 3 to 20 g, more preferably 5 to 18 g, of material per
metre squared of substrate. Most preferably, said layer includes 8 to 16 g of material
per metre squared.
[0054] Preferably, substantially the entire hydrophilic layer can be removed by immersing
a support including the layer in a stripping solution at a temperature of 96°C for
20 minutes and rubbing cotton wool over the layer. The stripping solution may comprise
potassium dichromate (160g), phosphoric acid (85%, 460g) and water (2700g). Removal
of the layer as described may enable the weight of the hydrophilic layer to be assessed.
[0055] According to a second aspect, there is provided a printing member comprising a substrate
as described according to said first aspect and an image layer.
[0056] Unless otherwise stated or unless the context otherwise requires, the assessment
as to whether a substrate of a printing member is as described according to said first
aspect may involve removing the image layer, for example by exposure and/or development.
[0057] The term "image layer" includes a layer that can subsequently be partially removed
in order to define areas to be printed and includes a layer which already defines
areas to be printed. Said image layer may include one or a plurality of layers.
[0058] Said image layer is preferably arranged to be removed during or after exposure to
radiation, in order to define areas to be printed.
[0059] It has been found that printing members which are imaged using, for example UV radiation,
visible light thermal IR radiation can all benefit from using a substrate of the type
described, as can printing members prepared by depositing an image layer information-wise
on the substrate.
[0060] The printing member may be processible to a resolution of 10 µm or less, suitably
9 µm or less, preferably 8 µm or less, more preferably 7 µm or less, especially 6.5
µm or less.
[0061] Resolution is preferably assessed as described under point 2 in Praxis Report 34
(June 1996) (except that the modification described in Assessment 11 hereinafter is
followed) published by FOGRA Forschungsgesellschaft Druck e.V of Munich, Germany and
the contents of the report are incorporated herein by reference.
[0062] The printing member may be processible to give dots having a roundness of less than
2, preferably less than 1.8, more preferably less than 1.6, especially less than 1.4,
when the image is digitised to a resolution of 1.32 pixels.µm
-1.
[0063] Roundness may be assessed by capturing an image of exposed 5% UGRA dot screen areas,
determining the dot area (a) and the dot perimeter (b) and calculating roundness according
to the formula : roundness = b
2/4πa.
[0064] The printing member may have a broad exposure latitude, suitably of greater than
1.2, preferably greater than 1.3, more preferably greater than 1.35, especially greater
than 1.4.
[0065] Exposure latitude may be assessed as described under point 7 in the Praxis Report
34 referred to above (except that the modification described in Assessment 12 hereinafter
is preferably followed).
[0066] The printing member may have a broad dot range, following exposure and development
conditions that give rise to a Stouffer Clear 3, suitably of 99% or greater, preferably
99.5% or greater at one extreme; and 2.0% or less, preferably 1.0% or less at the
other extreme.
[0067] The dot range may be assessed as described under point 6 in the Praxis Report 34
referred to above.
[0068] The printing member may be for use in stochastic printing, wherein image areas include
dots of less than 20 µm maximum diameter, suitably less than 18 µm maximum diameter,
preferably less than 16 µm maximum diameter, more preferably less than 15 µm maximum
diameter, especially less than 14 µm maximum diameter are intended to be produced.
[0069] Said printing member is preferably capable of printing an area, for example a dot,
having a maximum diameter of less than 30 µm, preferably less than 25 µm, more preferably
less than 20 µm.
[0070] In a preferred embodiment, a method of preparing a substrate for a planographic printing
member may include the step of forming a hydrophilic layer on a support by contacting
the support with a fluid comprising a silicate solution in which particulate material
as described in any statement herein is dispersed.
[0071] Said silicate solution may comprise a solution of any soluble silicate including
compounds often referred to as water glasses, metasilicates, orthosilicates and sesquisilicates.
Said silicate solution may comprise a solution of a modified silicate for example
a borosilicate or phosphosilicate.
[0072] Said silicate solution may comprise one or more, preferably only one, metal or non-metal
silicate. A metal silicate may be an alkali metal silicate. A non-metal silicate may
be quaternary ammonium silicate.
[0073] Said silicate solution may be formed from silicate wherein the ratio of the number
of moles of Si species, for example SiO
2, to the number of moles of cationic, for example metal species is in the range 0.25
to 10, preferably in the range 0.25 to about 6, more preferably in the range 0.5 to
4.
[0074] Said silicate is preferably alkali metal silicate. In this case, the ratio of the
number of moles of SiO
2 to the number of moles of M
2O in said silicate, where M represents an alkali metal may be at least 0.25, suitably
at least 0.5, preferably at least 1, more preferably at least 1.5. Especially preferred
is the case wherein said ratio is at least 2.5. Said ratio may be less than 6, preferably
less than 5 and more preferably less than 4.
[0075] Preferred alkali metal silicates include lithium, sodium and potassium silicates,
with lithium and/or sodium silicate being especially preferred. A silicate solution
comprising only sodium silicate is most preferred.
[0076] Said fluid may comprise 2 to 30 wt% of silicate (e.g. dissolved sodium silicate solid),
preferably 5 to 20 wt%, more preferably 8 to 16 wt%. The fluid may be prepared using
10 to 60 wt%, preferably 30 to 50 wt%, more preferably 35 to 45 wt% of a silicate
solution which comprises 30 to 40 wt% silicate.
[0077] Said fluid may include 5 to 60 wt% of particulate material. Preferably, the fluid
includes 10 to 50 wt%, more preferably 15 to 45 wt%, especially 20 to 40 wt% of particulate
material.
[0078] The ratio of the weight of silicate to the weight of particulate material in the
fluid is preferably in the range 0.1 to 2 and, more preferably, in the range 0.1 to
1. Especially preferred is the case wherein the ratio is in the range 0.2 to 0.6.
[0079] Said fluid may include more than 20 wt%, preferably more than 30 wt%, more preferably
more than 40 wt%, especially more than 45 wt% water (including water included in said
silicate solution). Said fluid may include less than 80 wt%, preferably less than
70 wt%, more preferably less than 65 wt%, especially less than about 60 wt% water.
[0080] Where the fluid comprises a silicate and said particulate material comprises a first
material and a second material as described, the ratio of the wt% of silicate (e.g.
dissolved sodium silicate solid) to the wt% of said first material may be in the range
0.25 to 4, preferably in the range 0.5 to 1.5 and more preferably about 1. Similarly,
the ratio of the wt% of silicate to the wt% of said second material may be in the
range 0.25 to 4, preferably in the range 0.5 to 1.5 and more preferably about 1. The
ratio of the wt% of first material to the wt% of second material may be in the range
0.5 to 2, preferably in the range 0.75 to 1.5, more preferably about 1 to 1.
[0081] Said particulate material may include a third material which is preferably adapted
to lower the pH of the fluid. Said third material may be a colloid, suitably colloidal
silica or an inorganic salt, suitably a phosphate, with aluminium phosphate being
preferred. Where a third material is provided, preferably less than 30wt% more preferably
less than 20wt%, especially less than 10wt% of said particulate material is comprised
by said third material.
[0082] The pH of said fluid may be greater than 9.0, is preferably greater than 9.5 and,
more preferably, greater than 10.0. Especially preferred is the case wherein the pH
is greater than 10.5. The pH is suitably controlled so that the silicate remains in
solution and does not form a gel. A gel is generally formed when the pH of a silicate
solution falls below pH9. The pH of said fluid is preferably less than 14, more preferably
less than 13.
[0083] The fluid may include other compounds for adjusting its properties. For example,
the fluid may include one or more surfactants. Said fluid may include 0 to 1 wt% of
surfactant(s). A suitable class of surfactants comprises anionic sulphates or sulphonates.
The fluid may include viscosity builders for adjusting the viscosity of the fluid.
Said fluid may include 0 to 10 wt%, preferably 0 to 5 wt% of viscosity builder(s).
Also, the fluid may include dispersants for dispersing the inorganic particulate material
throughout the fluid. Said fluid may include 0 to 2 wt% of dispersant(s). A suitable
dispersant may be sodium hexametaphosphate.
[0084] Said fluid may have a viscosity of less than 100 centipoise when measured at 20°C
and a shear rate of 200s
-1 using a Mettler Rheomat 180 Viscometer incorporating a double gap measuring geometry.
Preferably, said viscosity is less than 50 centipoise, more preferably less than 30
centipoise when measured as aforesaid. Especially preferred is the case wherein the
viscosity is less than 20 centipoise.
[0085] Said fluid may be applied to said support by any suitable means which is preferably
non-electrochemical.
[0086] Said fluid may be applied to both sides of said support in order to form a hydrophilic
layer on both sides. A support with such a layer on both sides may be used to prepare
a double-sided lithographic plate. Alternatively, if such a support is used for a
single-sided plate, the side of the plate which does not carry an image layer may
be protected by the hydrophilic layer. Said fluid is preferably applied to only one
surface of said support.
[0087] Said fluid may be applied to said support to form a hydrophilic layer having an average
thickness after drying, of less than 20 µm, preferably less than 10 µm and, more preferably,
less than 5 µm. Especially preferred is the case wherein the average thickness is
less than 3 µm.
[0088] The method preferably includes the steps of providing suitable conditions for the
removal of water from the fluid after it has been applied to the support. Suitable
conditions may involve passive or active removal of water and may comprise causing
an air flow over the support and/or adjusting the humidity of air surrounding the
support. Preferably, the method includes the step of arranging the support in a heated
environment. The support may be placed in an environment so that its temperature does
not exceed 230°C, preferably does not exceed 200°C and, more preferably, does not
exceed 175°C. Especially preferred is the case wherein the support temperature does
not exceed 150°C.
[0089] The support may be arranged in the heated environment for less than 180 seconds,
preferably less than 120 seconds and, more preferably, less than 100 seconds.
[0090] The invention extends to a method of stochastic or colour printing, the method using
a printing member which includes a substrate comprising a support and a hydrophilic
layer having a surface roughness (Ra) in the range 0.1 um to 2 um, said hydrophilic
layer comprising a particulate material and a binder material for the particulate
material.
[0091] The invention extends to the use of a printing member comprising a substrate which
comprises a support and a hydrophilic layer having a surface roughness (Ra) in the
range 0.1 um to 2 um, said hydrophilic layer comprising a particulate material and
a binder material for the particulate material, in stochastic printing or colour printing.
[0092] The invention will now be described by way of Example with reference to the accompanying
figures, wherein:
Figure 1 is a reflectance FT-IR spectrum of the substrate of Example 1;
Figure 2 is a UV-VIS absorbence spectrum of the substrate of Example 1;
Figures 3 and 4 are SEMs of dots produced using the plates of respective Examples
1 and C1;
Figures 5 and 6 are computer generated masks of dots produced using the plates of
respective Examples 1 and C1; and
Figures 7 and 8 are resolution graphs for the respective plates of examples 1 and
C1.
A. Preparation of lithographic printing plate.
Example 1
Step 1
Preparation of Aluminium
[0093] A 0.2 mm gauge aluminium alloy sheet of designation AA1050 was cut to a size of 459
mm by 525 mm. The sheet was then immersed face up in a solution of sodium hydroxide
dissolved in distilled water (100g/l) at ambient temperature for 60 seconds and thoroughly
rinsed with water.
Step 2
Preparation of coating formulation
[0094] The following reagents are used in the preparation:
- Sodium silicate solution having a ratio SiO2 : Na2O in the range 3.17 to 3.45 (average about 3.3); a composition of 27.1 - 28.1 wt%
SiO2, 8.4 - 8.8 wt% Na2O, with the balance being water; and a density of about 75 Twaddel (°Tw), equivalent
to 39.5 Baumé (°Bé) and a specific gravity of 1.375.
- Deionised water having a resistivity of 5 Mohm.cm
- Al2O3 powder comprising alumina (99.6%) in the shape of hexagonal platelets. The mean particle
size is 3 µm. The powder has a hardness of 9 Moh (on a 0 - 10 hardness scale).
- Anatase titanium dioxide having a mean primary particle size of 0.2 µm.
[0095] Deionised water (150g; 40 wt%) was added to a 250ml beaker and sheared using a Silverson
high shear mixer. Titanium dioxide powder (53.29g; 14.21 wt%) was then added in portions
over a period of four minutes with the shearing continuing. Then, alumina powder (53.29g;
14.21wt%) was added in portions over a period of four minutes with the shearing continuing.
On completion of the addition, sodium silicate solution (118.43g; 31.58 wt%) was added
with shearing for a further three minutes. The viscosity of the liquid was found to
be about 10 centipoise when measured at 20°C and a shear rate of 200s
-1 using a Mettler Rheomat 180 Viscometer incorporating a double gap measuring geometry.
Step 3
Application of coating formulation
[0096] The coating formulation prepared in Step 2 was coated onto the aluminium sheet prepared
in Step 1 using a rotating Meyer bar coater (designation K303) to give a 12 µm wet
film thickness.
Step 4
Drying the formulation
[0097] The coated sheet prepared in Step 3 was placed in an oven at 130°C for 80 seconds.
The plate was then removed from the oven and allowed to cool to ambient temperature.
Step 5
Post-drying treatment
[0098] The dried sheet prepared in Step 4 was immersed in aluminium sulphate (0.1M) for
thirty seconds. The sheet was then spray rinsed for about twenty seconds using tap
water and fan dried.
Step 6
Application of light sensitive coating
[0099] A printing plate was produced from the sheet prepared in Step 5 by coating, using
a Meyer bar, a light sensitive material of the quinone diazide/novolak resin type
at a dry coating weight of 2 g/m
2. The light sensitive material was dried at 130°C for 80 seconds.
[0100] The light-sensitive coating includes a blue colour change dye which is arranged to
change from dark blue to turquoise green on exposure.
Comparative Example 1
[0101] This was a commercial positive plate sold under the Trade Mark HORSELL CAPRICORN
by Horsell Anitec of Leeds, England. The plate comprises an electrograined and anodised
substrate provided with a quinone diazide/novolak resin and a dye as described in
Example 1, Step 6.
Assessment of substrates used in the preparation of the plates of Examples 1 and C1
Assessment 1 - Surface Roughness (Ra)
[0102] This was measured using a Talysurf Plus unit fitted with a 112/2564-430 head, supplied
by Rank Taylor Hobson Inc of Leicester, U.K. For each example, the roughness was assessed
along a trace length of 15mm with respective measurements being taken in the direction
of the grain of the aluminium and perpendicular to the direction of the grain.
[0103] Results are provided in Table 1 wherein the Ra stated is an average of 204 runs.
TABLE 1
Example No. |
Ra in µm in grain direction |
Ra in µm perpendicular to grain direction |
1 |
0.47 |
0.47 |
C1 |
0.62 |
0.63 |
Assessment 2 - Surface Skewness (Ssk)
[0104] The surface skewness (Ssk) can be used to describe the shape of the surface height
distribution. For a Gaussian surface which has a symmetrical surface height distribution,
the skewness is zero. For an asymmetric surface height distribution, the skewness
may be negative if the distribution has a longer tail at its lower side of the mean
plane, or positive if the distribution has a longer tail at the upper side of the
mean plane.
[0105] A Rank Taylor Hobson Form Talysurf 3-D unit fitted with a stylus of radius 2 µm was
used to map an area of 200 µm by 200 µm for each of the examples. Results are provided
in Table 2.
TABLE 2
Measurement |
Example 1 |
Example C1 |
Sa, µm (mean deviation from plane) |
0.45 |
0.79 |
Sq, µm (root mean square deviation from plane) |
0.58 |
1 |
Sp, µm (maximum distance above mean plane) |
4.06 |
2.33 |
Sv, µm (maximum distance below mean plane) |
1.34 |
4.91 |
Ssk (skewness) |
1.08 |
-0.86 |
[0106] The positive value for the Ssk of Example 1 shows that the surface is peak dominated;
the negative value for Example C1 shows that the surface is pit dominated.
Assessment 3 - Whiteness (L)
[0107] This was measured using a Minolta CR-300 processor unit fitted with a CR-331 head,
supplied by Minolta U.K. Limited of Milton Keynes, England. The unit was initially
calibrated using a white tile supplied (CR-A46). The instrument was configured in
its Lab mode (i.e. CIE 1976 colour system used); used a D65 light source; a 2° observer
angle; and it was arranged to produce a value which is an average of three measurements.
Measurements were taken firstly with an orientation mark on the measuring head aligned
with the direction of the grain of the aluminium and, secondly, with the mark perpendicular
to the grain direction so as to give whiteness values in two directions. An average
of about 34 readings was taken. Results are provided in Table 3.
TABLE 3
Example No. |
L value in metal grain direction (mean, SD) |
L value perpendicular to metal grain direction (mean, SD) |
1 |
84.5, 0.14 |
84.39, 0.47 |
C1 |
78.54, 0.09 |
78.24, 0.06 |
Assessment 4 - Gloss
[0108] This was measured using a Minolta Multigloss 268 reflectometer supplied by Minolta
U.K. Limited of Milton Keynes, England. The unit was calibrated using an internal
standard which is related to a black glass standard with a defined index of refraction
(usually 1.567) which equals 100 units.
[0109] With a measurement angle of 85° (i.e. the angle from the normal to the plane of interest)
measurements were taken in the direction of and perpendicular to the grain as described
in Assessment 3. An average value from 3 scans was noted. Results are provided in
Table 4.
TABLE 4
Example No. |
Gloss units in metal grain direction (mean, SD) |
Gloss units perpendicular to metal grain direction (mean, SD) |
1 |
10.15, 0.48 |
10.4, 0.96 |
C1 |
18.96, 0.15 |
22.01, 0.4 |
Assessment 5 - Scanning Electron Microscopy (SEM)
[0110] A sample of substrate prepared as described in Example 1, Step 4, was stuck onto
aluminium pin stubs using a conductive paint (Electrodag - a colloidal silver suspension
in iso-butyl methylketone). The sample was sputter-coated with platinum using a Fisons
Instruments SEM coating system model SC510. The sputter coating was carried out in
a low pressure argon plasma for 120 seconds using a plasma current of 20 mA at 900
volts. Microscopy was carried out using the secondary electron detector of a Hitachi
S-4100 field emission scanning electron microscope. A condenser lens 8 was used with
an accelerating voltage of 10 keV and a working distance of 5 mm. The emission current
was 10 µA.
[0111] A micrograph was obtained using a magnification of x1000. A visual assessment of
this micrograph shows that the surface morphology is defined by particulate material
of two types:
spherical particles of size ∼0.2 µm; and
angular particles of size ∼3-4 µm.
[0112] It is also clear that the particles are held in a binder material.
Assessment 6 - Energy Dispersive x-ray Analysis (EDX)
[0113] A sample of substrate was prepared as described in Assessment 5 except that it was
sputter coated with platinum for only 90 seconds.
[0114] A Hitachi Scientific Instruments S-4100 field emission scanning electron microscope
was used with the following conditions: accelerating voltage 20 kV, condenser lens
8, extraction voltage 4.7 kV, emission current 20 µA, working distance 20 mm, beam
monitor aperture 2, objective aperture 1 and magnification 1000x.
[0115] Analysis was carried out using an Oxford Instruments Link ISIS 200 EDX system, thermally
cyclable SiLi ATW detector (1024 channels, 20 keV range) and software revision 1.04a.
The detector was calibrated using a cobalt calibration standard.
[0116] The X-ray analysis was carried out using the following acquisition set-up: fast counting
mode, process time 3, upper energy 30 keV, preset livetime 200, acquisition rate approximately
7000 counts per second operating with a 30% deadtime. Window integral measurements
were made using the "quant" requirement function of the software.
[0117] EDX elemental measurements were undertaken for 6 different areas on the sample. The
absorption peaks obtained in the EDX spectra were assessed to indicate the elements
present and integrated to give an indication of the relative amounts of each of the
elements.
[0118] The percentage of each element was calculated from: element integral/sum of integrals
of all elements noted.
[0119] Results are provided in Table 5
TABLE 5
Energy band (KeV) |
Element detected |
Absorption Band |
Mean percentage of |
1.388 - 1.587 |
Aluminium |
K alpha |
51.5 |
1.648 - 1.847 |
Silicon |
K alpha |
14.2 |
4.387 - 4.648 |
Titanium |
K alpha |
22.7 |
4.787 - 5.068 |
Titanium |
K beta |
3.2 |
0.477 - 0.608 |
Oxygen |
K alpha |
8.43 |
Assessment 7 - Reflectance FT-IR Spectroscopy
[0120] A sample of the substrate of Example 1 of length 10 cm and width 3 cm was cut from
a sheet prepared as described in Example 1, step 5 and subjected to reflectance FT-IR
using a Perkin Elmer System 2000 FT-IR unit, obtained from Perkin Elmer Limited of
England. The sample was mounted on a variable angle reflectance attachment 13969 supplied
by Graseby Specac Limited of Kent, England. Scanning of the sample was along the metal
grain direction and at a scan angle of 84°. A spectrum obtained is shown in figure
1. Peaks are noted at the following wavenumbers (cm
-1). The figures in brackets after each number represent the level of absorbence : 1619.54
(0.25), 1268.92 (0.30), 1239.08 (0.31), 1160.75 (0.25), 1037.66 (0.24), 996.63 (0.26),
944.41 (0.16), 873.54 (0.15), 634.82 (0.17), 549.03 (0.20) and 522.92 (0.20) and 399.83
(0.28).
Assessment 8 - UV-VIS Absorbence Spectrum
[0121] A UV-VIS absorbence spectrum was run on a sample of substrate prepared as described
in Example 1, Step 5 using a Perkin Elmer Lambda 15 UV/VIS spectrometer, fitted with
an integrating sphere attachment and being obtained from Bodenseewek Perkin-Elmer
GmbH of Uberlingen, Germany. The scanning speed was 240nm/min, the slit width was
2nm and the reference sample was 1050 aluminium alloy.
[0122] The spectrum is shown in Figure 2 from which it can be seen that, as the radiation
wavelength is lowered, absorbence increases rapidly from a value close to 0 at 390
nm to over 1 at 340 nm. Absorption is consistently high below 340 nm.
Assessment of plates prepared in Examples 1 and C1
Assessment 9 - Visual assessment of dot shapes
[0123] Plates of examples 1 and C1 were exposed on a Montakop 65 lightframe. Each plate
was imaged using a Printstar 21 Step Stouffer Wedge available from Horsell Anitec
and an UGRA Plate Control Wedge 1982 obtained from FOGRA of Munich, Germany. Each
plate was given sufficient exposure to give a clear Step 3 on the image of the Stouffer
wedge after development. The plates were developed at 20°C for 60 seconds using GREENSTAR
(Trade Mark) developer, sold by Horsell Anitec.
[0124] SEMs of dots produced from 5% UGRA dot screen areas were taken and these are shown
in figures 3 and 4 for plates of Examples 1 and C1 respectively. The difference in
roundness of the dots is easily appreciated: the dot on the plate of Example 1 is
substantially circular whereas that of Example C1 is highly irregular.
Assessment 10 - Mathematical roundness of dots
[0125] 5% UGRA dot screen areas of Examples 1 and C1, prepared as described in Assessment
9, were evaluated by capturing an image of 10 dots using a JVC KY-F55BE 3CCD colour
video camera fitted to an Olympus BX60 optical microscope set up for dark field illumination
and having an Olympus dark field lens (UMP lan fl; 20x/0.46 bd; ∝/O). The microscope
and lens were obtained from Olympus Optical Company (UK) Limited of London, England.
Computer generated masks were produced for image analysis and these are shown in figures
5 and 6 for respective examples 1 and C1 Image analysis was carried out using Image-Pro
Plus Version 1.3 for Windows obtained from Media Cybernetics of Maryland, USA. Image
resolution was found to be 1.32 pixels µm
-1. Each image was binarized to determine dot area (a) and dot perimeter (b) and the
roundness determined by the formula.
Results are provided in Table 6.
TABLE 6
Example |
Roundness |
Population Standard Deviation |
1 |
1.35 |
0.08 |
C1 |
1.99 |
0.20 |
[0126] The values of "a" for the Examples was found to be approximately equal.
Assessment 11 - Resolution
[0127] Resolution is a quality datum. It was assessed for the plates of Examples 1 and C1
as described under point 2 in Praxis Report 34 (June 1996) published by FOGRA Forschungsgesellschaft
Druck e.V of Munich, Germany, except that, whereas the Report states that: "A microline
segment is considered to be featured on the plate if more than half of the total line
lengths can be seen", for the Assessment, a microline segment was only considered
to be featured if greater than 90% of the total line length could be seen. The plates
were processed as described in Assessment 9.
[0128] Figures 7 and 8 provide resolution graphs for plates of Examples 1 and C1 respectively
from which it will be noted that the resolution of Example 1 is about 6 µm and that
of Example C1 is about 8 µm.
Assessment 12 - Exposure latitude
[0129] The exposure latitude of plates of Examples 1 and C1, processed as described in Assessment
9, was assessed as described under point 7 in Praxis Report 34 referred to above,
except that a microline segment was only considered to be featured if greater than
90% of the total line length can be seen as described in Assessment 11.
[0130] The exposure latitude of Example 1 was found to be 1.44; and that of Example C1 found
to be 1.32.
Assessment 13 - Dot Range
[0131] This was assessed by exposing to Clear 3 on a Stouffer 21 step wedge and as described
under point 6 in Praxis Report 34 referred to above on plates processed as described
in Assessment 9.
[0132] The dot range of Example 1 was found to be 0.5 to 99.5%; and that of Example C1 found
to be 1 to 99%.
Assessment 14 - Photo-coating Release
[0133] The plates were exposed at the lowest exposure (Stouffer Clear 0) and assessed after
being processed as described in Assessment 9. The plate of Example 1 appeared clear
of photocoat, whereas some photocoat was retained in the grain of the plate of Example
C1.
Assessment 15 - Ink-water balance
[0134] Plates prepared as described in Example 1 and C1 were run, side-by-side, on a Heidelberg
Speedmaster SM52 press with Z-roller fitted using Federal Tait Duo Laser Brilliance
80 gsm paper obtained from Rothera & Brereton of Leeds, England; Gibbon JCR Geneva
Black Ink, obtained from Gibbon JCR Ltd of Leeds, England; and a fount comprising
2% Emerald fount and 10% isosol obtained from Horsell Graphic Industries Ltd of Leeds,
England. The point at which the printed image/non-image boundary became blurred was
measured. The point for the plate of Example 1 was within 3% damp of that of Example
C1.
Assessment 16 - Run Length
[0135] After 500,000 impressions on the Heidelberg press referred to in Assessment 15, it
was found that:
- solid areas continued to print as well as a 5% dot area imaged from a 60 lines cm-1 screen.
- the background continued to print clearly;
- there were no signs of areas of the plate (e.g. the hydrophilic layer) breaking up
or falling off.
Assessment 17 - Plate Aesthetics
[0136] Visual assessment of the plates of Examples 1 and C1 before development showed that
Example 1 had better colour contrast between the image and non-image areas than Example
C1.
[0137] Following development, it was also noted that Example 1 had better colour contrast
between the two areas compared to Example C1.
Assessment 18 - Developer Resistance
[0138] Samples exposed as described in Assessment 9 were developed in a Mercury 650 Processor
using GOLDSTAR developer sold by Horsell Anitec for 20 seconds dwell time at 14°C
at one extreme; and for 60 seconds dwell time at 35°C at the other extreme. A shift
of 4 steps on the Ugra micron lines was seen for both Example 1 and Example C1. Nonetheless,
the plate of Example 1 held onto the 15 µm lines better than that of Example C1. One
concludes, therefore, that the plate of Example 1 has no problem compared to commercial
plates as regards developer resistance.
Assessment 19 - Stochastic Screens
[0139] It was found that the plate of Example 1 reproduced a stochastic screen very satisfactorily.
In the assessment, the lowest spot size of 16 µm diameter was held well at clear 3
Stouffer for Example 1, whereas this spot was virtually eliminated on the plate of
Example C1.
EXAMPLE 2
[0140] The procedure of Example 1 was repeated, excepted that Melinex 539 (biaxial polyethylene
terephthlate (PET) film provided with an anti-static coating (supplied by ICI Melinex
of Wilton, England)) was used instead of aluminium. The majority of properties of
the plate produced were found to be similar to those of Example 1.
EXAMPLE 3
[0141] The procedure of Example 1 was repeated, except that Simcote 400 (a paper obtained
from Samuel Grant Ltd of England) was used instead of aluminium. The majority of properties
of the plate produced were found to be similar to those of Example 1.
EXAMPLE 4
[0142] A Statistical Experimental Design (SED) was set up to assess the effects of varying
certain parameters in the formulation of Example 1 Step 2. In particular, the following
properties were assessed:
(a) The susceptibility of the image to rub off - this was assessed by applying 5ml
of TONE UP (a proprietary plate cleaner sold by Horsell Anitec) to a 5 × 5 cm area
of the plate of Example 1, followed by using a wad of damp tightly rolled cotton wool
to wipe across the image layer using a heavy downward pressure for a total of 50 passes.
The plate was rinsed in tap water and the rubbed area was examined for wear. A scale
was set wherein complete removal of the image layer was given a value of 100 and no
removal was given a value of 0.
(b) The susceptibility of the non-image area to staining by the dye in the photocoat
- this was assessed using the Minolta CR-300 processor unit referred to in Assessment
3 to assess the extent of any blue colour in developed non-image areas.
(c) The susceptibility of the non-image area to ink reception - this was assessed
on a developed non-image area coated with a gum and rinsed in tap water, followed
by removal of excess water using a rubber blade. A 2 ml drop of ink was applied to
a wad of cotton wool and this was wiped across the non-image area to allow the ink
to adhere to the non-image area if it could. The plate was then rinsed in tap water
to remove any loosely bound ink and the plate assessed.
[0143] With regard to (a), it was found that the susceptibility of the area to rub off decreased
with increasing the powder (i.e. alumina and titania) to silicate ratio. Additionally,
increasing the alumina content of the powder was found to decrease rub off.
[0144] with regard to (b), it was found that the susceptibility of the non-image area to
staining increased as the ratio of powder to silicate was increased.
[0145] With regard to (c), it was found that a reduction in the powder to silicate ratio
reduced ink reception.