IE910722A1

IE910722A1 - Electrolytic graining

Info

Publication number: IE910722A1
Application number: IE072291A
Authority: IE
Original assignee: Du Pont Uk
Priority date: 1990-03-06
Filing date: 1991-03-05
Publication date: 1991-09-11
Also published as: NO910882D0; AU7197691A; NO910882L; HU910713D0; FI910745A0; CA2037594A1; HUT57129A; GB9005035D0; EP0445959A1; ZA911605B; BR9100958A; JPH0770800A; FI910745A; CS58091A3

Abstract

A method of electrolytically graining aluminium, aluminium alloy or aluminium laminate, desirably in a substantially uniform manner, in particular for use as a lithographic printing plate substrate, is disclosed wherein the graining is effected using a square wave alternating current.

Description

ELECTROLYTIC GRAINING This invention relates to the electrolytic graining of aluminium, aluminium alloys and aluminium laminates and more particularly, but not exclusively, is concerned with the electrolytic graining of aluminium, aluminium 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 selectively removed from the substrate in the non-image areas by application of a suitable developer to leave a printing image (or etch resistant area) on the substrate. In the case of the so-called negative-working devices, it is the nonradiation 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 strongly adhere thereto and such that it is 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.

IE 91722 -2The 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 5 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 electrolyte for graining aluminium and aluminium alloy substrates. However, when using hydrochloric acid it is difficult to obtain a fine homogeneous grain and it is therefore necessary carefully to control the acid concentration of the electrolyte in order to ensure consistent results. This is particularly the case when aluminium alloys such as 3103 aluminium maganese alloy are used as the substrate. The use of such alloys for the substrate can be particularly advantageous due to their increased resistance to tearing and cracking and to temperatures in excess of 200 °C which are used to harden the image on the printing plate and thus to increase the printing run length.

It is known to grain aluminium substrates using as the electrolyte a mixture of hydrochloric and phosphoric IE 91722 -3acids. 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 smut has normally to be 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 electrolytes attack the impurities in the alloy and thus cause pitting of the 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 produced.

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.

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 IE 91722 -4./ 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 sheet 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 10 concentration of hydrochloric acid in the electrolyte will be from 3 to 20 gl“l and the electrolytic graining may preferably be effected at a voltage of, for example, 5V to 45V, particularly preferably from 10V 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 34°C. An alternative to the above is to use nitric acid in which case concentrations of between 5 and 30 gl-l may be used.

The frequency of the alternating square wave current will preferably be from 20 to 100Hz 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 voltage 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 half30 cycle whilst maintaining the frequency 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, aluminium alloy or aluminium laminate sheet in the electrolyte, the square waveform alternating IE 91722 -5current being passed through 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 composition such as a mixture of a guinone diazide and a novolak resin or a negative working composition, such as a photopolymerisable resin. The radiation sensitive plate may then be imagewise exposed and suitably processed to produce a lithographic printing plate.

For a better understanding of the invention, and to show how the same may be carried into 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-manganese alloy, of which Figure 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 waveform IE 91722 ι -6/ alternating current, Figure 4 shows a sheet of the alloy grained in hydrochloric acid electrolyte with added monocarboxylic acid using a sine waveform alternating current, and Figure 5 shows a sheet of the alloy grained 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 shows a sheet of the aluminium 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 monocarboxylic acid using a sine waveform alternating current, and Figure 9 shows a sheet of the aluminium grained in 20 hydrochloric acid electrolyte with added monocarboxylic acid using a square waveform alternating current.

The following examples illustrate the invention:EXAMPLE 1 Sheets of 3103 grade aluminium-manganese alloy 25 were degreased in 10 to 20gl-1 sodium hydroxide for 30s at 35 to 40°C and rinsed. The sheets were then electrolytically grained using hydrochloric acid at a concentration of 7gl-l and a temperature of 26 to 28°C and using a square 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.

COMPARATIVE 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 0.6 to 0.8 microns. Part of the surface of one sheet is shown in Figure 3.

IE 91722 -ΊCOMPARATIVE EXAMPLE 2 Sheets of 3103 grade aluminium-manganese alloy were degreased and rinsed as in Example 1. The sheets were then electrolytically grained using an electrolyte comprising 8 to 10gl~l hydrochloric acid and 15 to g+*l of a monocarboxylic acid at a temperature of 26 to 28 °C 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 Sheets of 3103 grade aluminium-manganese alloy were degreased, rinsed and grained as in Comparative Example 2, but using a square waveform alternating 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%A1) 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 Comparative Example 2. Part of the surface of one of the sheets is shown in Figure 8.

COMPARATIVE EXAMPLE 6 Sheets of 1050 grade aluminium were degreased, rinsed and grained using the same conditions as IE 91722 -8Comparative 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 5 were degreased in 10 to 20gl_l sodium hydroxide for 30 seconds at 35 to 40°C and rinsed. The sheets were then electrolytically grained using nitric acid at a concentration of 16gl“l and a temperature of 26-28°C.

A square waveform at a frequency of 50 Hz and voltage of 18-20V was used. The resulting grained sheets had a CLA of 0.6 to 0.8 microns.

COMPARATIVE EXAMPLE 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 0.6 to 0.8 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 graining 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 aluminium 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 sine 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.

Claims

1. A method of electrolytically graining a sheet of aluminium, aluminium alloy or aluminium laminate which method comprises immersing the sheet in an

2. A method according to claim 1 wherein the electrolyte comprises hydrochloric acid, at a

3. A method according to claim 1 wherein the electrolyte contains nitric acid at a concentration in the range of from 5 to 30gl - l.

4. A method according to claim 1, 2 or 3 wherein 15 the graining is effected at a voltage in the range of from 5 to 45V. 5. Anodised.

5. A method according to any previous claim wherein the graining is effected for from 15 seconds to 4 minutes. 20 5 aqueous electrolyte and passing an alternating current through the electrolyte, wherein the alternating current has a square waveform.

6. A method according to any previous claim wherein the frequency of said alternating current lies within the range of from 20 to 100Hz.

7. A method according to any preceding claim wherein said square wave has both a positive and a 25 negative half cycle and the ratio of the voltages in said positive and negative half cycle ranges from 1:1 to 1:2.

8. A method according to any preceding claim wherein said square waveform positive and negative half 30 cycles have a temporal ratio in the range of from 1:2 to 1:1.

9. A method according to any preceding claim wherein at least one electrode comprises a sheet of aluminium, aluminium alloy or aluminium laminate. 35 10. A method according to any preceding claim wherein a continuous web of aluminium, aluminium alloy IE 91722 -10or aluminium laminate is passed through the electrolyte.

10. As described in examples 1, 2 or 3. 10 concentration in the range of from 3 to 20 g/1.

11. A method according to any preceding claim including a stage in which the grained surface is then

12. A method according to any preceding claim including a final stage comprising coating the substrate with a radiation sensitive composition.

13. A method according to claim 1 substantially

14. A sheet of aluminium, aluminium alloy or aluminium laminate when grained according to the method of any one of claims 1 to 12.