Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELECTROLYTIC GRAINING
This invention relates to the electrolytic graining of
aluminium, aluminium alloys and aluminium lamina~es and mo.re
par~icularly, but not exclusively, is concerned with the
electrolytic graining of aluminium, alumini.um alloys or aluminium
laminates in the production of substrates suitable for use in the
manufacture of radiation sensitive plates in lithographic printing
plate production.
Radiation sensitive plates of the type with which this
invention is concerned conventionally consist of a substrate onto
which is coated a radiation sensitive composition. Image-wise
exposure of the plate to radiation causes the coating to change its
characteristics in the areas struck by radiation so that the
coating may be selectivel~ removed from the substrate in the non-
image areas by application of a suitable developer to leave aprinting image (or etch resistant axea) on the substrate. In the
case of the so-called negative-working devices, it is the non-
radiation struck areas of the coating which are removed. Those
parts of the coating which are not removed and which thus form the
printing image are ordinarily water-repellent and ink-receptive and
those parts of the substrate revealed on development are ordinarily
water-receptive and ink-repellent.
It will be apparent that the surface of the substrate should
be such that the printing image can ~trongly adhere thereto and
~ 25 such that i~ iB readily wettable with water. It is known to
; improve the adhesion of the printing image and to improve the
wetting characteristics of the non-image areas by roughening
(conventionally referred to as graining) -the substrate before
applying the radiation sensitive coating.
The coarseness or surface roughness of the grained substrate
can be characterised, for example, by measurement of a centre line
average (CLA).
The type of grain required for the substrate of a radiation
sensitive printing plate for lithographic printing plate production
depends upon the requirements of the final printing plate. Thus a
fine grain - i.e. shallow depressions - results in better
reproduction of half-tones whereas a coarse grain - i.e. deep
depressions - results in the non-image areas having better wetting
characteristics. In either case however it is important that the
depressions are evenly spaced over the substrate surface and that
they are close enough together so that peaks, rather than plateaux,
are formed between the depressions.
It is known to grain substrates in lithographic printing plate
production by electrolytic techniques. Graining is normally
effected by immersing the substrates in a suitable electrolyte and
subjecting them to a sine waveform alternating current.
Conventionally, hydrochloric acid has been used as the
electrolyts for graining aluminium and aluminium alloy substrates.
However, when using hydrochloric acid it is difficult to obtain a
fine homogeneou~ grain and it is therefore nece~sary carefully to
control the acid concentration of the electrolyte in order to
ensure consiætent results. This is particularly the case when
aluminium alloys such as 3103 aluminium manganese alloy are used as
the substrate. The use of such alloys for -the substra-te can be
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particularly advantageous due to their increased resistance to
tearing and cracking and to tempera-~ures in excess of 200C which
are used to harden the image on the printing plate and thus to
increase the printing run leng~h.
It is known to grain aluminium substrates using a~ the
electrolyte a mixture of hydrochloric and phosphoric acids. Whilst
this method can result in an even grain, an excessive amount of
smut is produced on the substrate which can cause the radiation
sensitive coating of the plate to become insolubilised during
storage of the plate. Thus the smu~ has normally to he removed.
A further disadvantage of using a hydrochloric acid/phosphoric acid
mixture as electrolyte is that the process is inflexible in respect
of the type of grain which can be produced.
; The use of hydrochloric acid or hydrochloric acid/phosphoric
acid mixtures is further disadvantageous when using certain
aluminium alloys since both these elec~rolytes attack the
impurities in the alloy and thus cause pi~ting of khe surface.
It is also known to use as the electrolyte hydrochloric acid
in combination with monocarboxylic acids having between 1 and 4
carbon atoms. By this method aluminium and aluminium alloy
substrates having a fine homogeneous grain structure can be
producedO However, complicated analytical techniques are required
to monitor the relative amounts of hydrochloric acid and
monocarboxylic acid. Moreover, the use of additives to the
hydrochloric acid electrolyte such as monocarboxylic acids can be
environmentally undesirable.
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It is an object of the present invention to provide a method
of electrolytically graining aluminium and aluminium alloys and
aluminium laminates which results in a fine homogeneous grain
structure and which obviates the need for complex chemical analysis
of the electrolyte.
It has surprisingly been found that in the electrolytic
graining of aluminium, aluminium alloys or aluminium laminates, a
fine homogeneous grain structure can be achieved by the use of an
alternating current having a square waveform rather than the
conventional alternating current having a sine waveform.
Accordingly the present invention provides a method of
electrolytically graining a sheet of aluminium, aluminium alloy or
aluminium laminate which comprises immersing the sheat in an
aqueous electrolyte and passing an alternating current through the
electrolyte wherein the alternating current has a square waveform.
Generally hydrochloric acid is used, and the concentration of
hydrochloric acid in the electrolyte will be from 3 to 20~ and
the electrolytic graining may preferably be effected at a voltage
of, for example, SV to 45V, particularly preferably from lOV to 35V
for 15 seconds to 4 minutes to give a surface roughness
characterised by a centre line average (CLA), as measured, for
example, by a Rank Taylor Hobson Talysurf 10, of from 0.3 to 1~0
microns. The electrolyte may be at any suitable temperature but
preferably from 25 to 34C. An alternative to the above is to use
nitric acid in which case concentrations of between 5 and 30gl~l may
he used.
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The frequency of the alternating square wave current will
preferably be from 20 ko lOOHz and particularly preferably from 40
to 70Hz. The voltage in each half cycle can be chosen as desired
within the preferred range. The preferred ratios of the voltaye in
the positive and negative half cycles are within the range of from
1:2 to 1:1, positive : negative. It is also possible to vary the
time period of each half-cycle whilst maintaining the fr~quency
within the preferred range. The preferred range for the ratio of
the time periods in the positive and negative half cycles is from
1:2 to 1:1, positive : negative.
The graining may be effected by immersing the aluminium,
a].uminium alloy or aluminium laminate sheet in the electrolyte, the
square waveform alternating current being passed ~hrough the
electrolyte using the sheet as an electrode. A second similar
sheet may be used as the second electrode. Alternatively the
graining may be effected as a continuous process by passing a
continuous web of aluminium/ aluminium alloy or aluminium laminate
through the electrolyte. In this case the electrodes used to
introduce the square waveform alternating current may, for example,
be carbon electrodes located near to the web.
After graining, the aluminium, aluminium alloy or aluminium
laminate may be anodised in a suitable electrolyte, preferably
using direct current. Thereafter the grained surface (or the
grained and anodised surface, as the case may be) of the sheet may
be coated with a radiation sensitive composition to form a
radiation sensitive plate. The radiation sensitive composition may
be a positive working composi~ion such as a mix~ure of a quinone
diazode and a novolak resin or a negative working composition, such
as a photopolymerisable resin. The radiation sensitive pla~e may
then be imagewise exposed and suitably processed to produce a
lithographic printing plate.
For a better understanding of tha invention, and to show how
the same may be carried in~o effect, reference will be made, by way
of example only, to the following figures in which:-
Figures la and lb illustrate the waveform associated
respectively with a sine waveform and a square waveform alternating
current,
Figures 2 to 5 are electron micrographs of electrolytically
grained sheets of 3103 grade aluminium-manganPse alloy, of which
Fiqure 2 shows a sheet of the alloy grained in accordance with
the present invention,
Figure 3 shows a sheet of the alloy grained in hydrochloric
acid electrolyte, using a sine wavefoxm alternating current,
Figure 4 shows a sheat of the alloy grained in hydrochloric
acid electrolyte with added monocarboxylic acid using a sine
waveform alternating current, and
Figure S shows a sheet of the alloy graîned in hydrochloric
acid electrolyte with added monocarboxylic acid using a square
waveform alternating current.
Figures 6 to 9 are electron micrographs of electrolytically
grained sheet of 1050 grade aluminium, of which
Figure 6 shows a sheet of the aluminium grained in accordance
with the present invention,
Figure 7 æhows a sheet of the zluminium grained in
hydrochloric acid electrolyte using a sine waveform alternating
current,
Figure 8 shows a sheet of the aluminium grained in
hydrochloric acid electrolyte with added mono~arboxylic acid using
a sine wa~eform alternating current, and
Figure 9 shows a sheet of the aluminium grained in
hydrochloric acid electrolyte with added monocarboxylic acid using
a square waveform alternating current.
10It can be appreciated that Figures 2 to 9, inclusive, are best
represented by photographs of the surface of the sheets described.
The following examples illustrate the invention:-
EXAMPLE 1
Sheets of 3103 grade aluminium-manganese alloy were degreased
15in 10 to 20gl~l sodium hydroxide for 30s ak 35 to 40C and rinsed.
The sheets were then elec~rolytically grained using hydrochloric
acid at a concentration of 7gl~l and a tempexature of 26 to 28C and
using a s~uare waveform alternating current at an applied voltage
of 16 to 18V and at a frequency of 50Hz. The resulting grained
sheets had a CLA of 0.6 to 0.8 microns. Part of the surface of one
sheet is shown in Figure 2.
CO~PARATIVE~EXAMPLE 1
Sheets of 3103 grade aluminium-manganese alloy were degreased,
rinsed and grained as in Example 1, but using a sine waveform
alternating current. The resulting grained sheets had a CLA of 0.~
to 0.8 microns. Part of the su.rface of one sheet is shown in
Figure 3.
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COMD ~
Sheets of 3103 grade aluminium-manganese alloy were degreased
and rinsed as in Example 1. The sheets were then elec~rolytically
grained using an electrolyte comprising 8 to 10gl~1 hydrochloric
acid and 15 to 30gl~l of a monocarboxylic acid at a temperature of
26 to 28C and using a sine waveform alternating current at an
applied voltage of 16 to 18V and frequency of 50Hz. The resulting
grained sheets had a CLA of 0.6 to 0.8 microns. Part of the
surface of one of the sheets is shown in Figure 4.
COMPARATIVE EXAMPLE 3
She~ts of 3103 grade aluminium-manganese alloy were degreased,
rinsed and grained as in Comparative Example 2, but using a square
waveform a.lternating current. The resulting grained sheets had a
CLA of 0.6 to 0.8 microns. Part of the surface of one of the
sheets is shown in Figure 5.
EXAMPLE 2
Sheets of 1050 grade aluminium (99.5%Al) were degreased,
rinsed and grained using the same conditions as Example 1. Part of
the surface of one of the sheets is shown in Figure 6.
COMPARATIVE EXAMPLE 4
Sheets of 1050 grade aluminium were degreased, rinsed and
grained using the same conditions as Comparative Example 1. Part
of the surface of one of the sheets is shown in Figure 7.
COMPARATIVE EXAMPLE 5
Sheets of 1050 grade aluminium were degreased, rinsed and
grained using the same conditions as Compara~ive Example 2. Part
of the surface of one of the sheetæ is shown in Figure 8.
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COMPARATIVE EXAM,P E 6
Sheets of 1050 grade aluminium were degreased, rinsed and
grained using the same conditions as Comparative Example 3. Part
of the surface of one of the sheets is shown in Figure 9.
EXAMPLE 3
Sheets of 3103 grade aluminium-manganese alloy were degreased
in 10 to 20gl-~ sodium hydroxide for 30 seconds at 35 to 40C and
rinsed. The sheets were then electrolytically grained using nitric
acid at a concentration of 16gl~l and a temperature of 26-28C. A
square waveform at a frequency of 50 Hz and voltage of 18-~OV was
used. The resulting grained sheets had a CLA of O.6 to O.8
microns.
COMPARATIVF: EXA~lPLE 7
Sheets of 3103 grade aluminium-manganese alloy were degreased,
rinsed and grained as in Example 7, but using a sine waveform. The
resulting grained sheets had a CLA of O.6 to O.8 microns.
Comparison of Figure 2 with Figure 3 and Figure 6 with Figure
7 clearly shows that when using a standard hydrochloric acid
electrolyte in the grain,ing of aluminium or aluminium alloys the
use of a square waveform alternating current instead of the
conventional sine waveform results in a significantly finer and
more homogeneous substrate surface.
Comparison of Figures 2 and 6 with Figures 4 and 8
respectively demonstrates that the grained aluminium or alumini~
alloy substrate obtained by use of a standard hydrochloric acid
electrolyte with a square waveform alternating current has an
equally fine and homogeneous surface as that obtained by use of a
mixed hydrochloric acid/monocarboxylic acid electrolyte and a ~ine
waveform alternating current. Furthermore it can be seen from
Figures 5 and 9 that no further advantage is gained by using a
mixed hydrochloric acid/monocarboxylic acid electrolyte with a
square waveform alternating current. Moreover, such a method is
disadvantageous because of the technical complexity of monitoring
the relative hydrochloric acid and monocarboxylic acid
concentrations.
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