Method of printing using a reimageable printing plate with an aluminum oxide surface

Abstract

A printing form precursor comprises a printing surface which comprises an inorganic metal compound, the printing surface being hydrophobic and capable of being made hydrophilic by energy but capable of becoming hydrophobic again, for reuse, if desired. An associated method of printing includes steps of subjecting the printing surface imagewise to energy so as to locally increase its hydrophilicity sufficient to make the surface differentiated in its acceptance of an oleophilic a printing ink; applying the ink to the printing surface and printing from the printing surface; causing or allowing the printing surface to undergo a reduction in hydrophilicity sufficient again to make the printing surface uniform in its acceptance of a printing ink; and, if wished, repeating these steps on multiple occasions. Thus the invention achieves the goal of providing a printing form precursor which does not need a chemical developer, and which can be used multiple times, to print different images.

Claims

1. A method of printing comprising: a) providing a printing form precursor having an aluminium oxide printing surface, uncoated by a developable image layer and uniform in its acceptance of an oleophilic printing ink; b) subjecting the printing surface imagewise to energy in the form of pulses of electromagnetic radiation of duration not greater than 110.sup.10 second, so as to increase the hydrophilicity of the printing surface, where subjected to energy, sufficient to make the surface differentiated in its acceptance and non-acceptance of the ink; and c) applying the ink to the printing surface and printing from the printing surface.

2. A method as claimed in claim 1, wherein step c) is followed by steps of d) causing or allowing the printing surface to undergo a reduction in hydrophilicity sufficient again to make the printing surface uniform in its acceptance of a printing ink; and e) repeating at least steps b) and c).

3. A method as claimed in claim 1, wherein the printing surface is anodized.

4. A method as claimed in claim 1, wherein the printing form precursor comprises the aluminium oxide printing surface on a metal base layer.

5. A method as claimed in claim 1, wherein the printing form precursor comprises the aluminium oxide printing surface on a plastics base layer.

6. A method as claimed in claim 1, wherein the printing surface is caused to undergo a reduction in hydrophilicity by the passage of time, under ambient conditions.

7. A method as claimed in claim 2, wherein the reduction in hydrophilicity in step d) is controlled wholly or in part by an external agency preferably selected from a surface treatment carried out in step a) or by being subjected to energy and/or a gaseous environment in step d).

8. A method as claimed in claim 1, wherein the printing surface remains sufficiently hydrophilic, after step b), for a printer to be able to use it for printing for a period of at least 4 hours and not greater than 72 hours, measured from the image-forming step b).

9. A method as claimed in claim 2, wherein step d) is repeated, along with steps b) and c); wherein each of steps b), c) and d) are carried out at least 3 times.

10. A method as claimed in claim 1, wherein step b) involves, in a single stage of operation, delivery of sufficient energy to cause said increase in the hydrophilicity of the printing surface.

11. A method as claimed in claim 1, wherein in step b) energy is delivered to the printing surface in two or more discrete stages, with the final stage causing the hydrophilicity of the printing surface to reach a desired level, and the previous stage, or stages, preparing the printing surface for that to happen.

12. A method as claimed in claim 1, wherein the pulses of electromagnetic radiation have a duration of at least 110.sup.18 second, and are delivered by a laser.

13. A method as claimed in claim 12, wherein the average frequency of the pulses is at least 100 pulses per second.

14. A method as claimed in claim 12, wherein the fluence is at least 1 mJ/cm.sup.2 and does not exceed 20,000 mJ/cm.sup.2 and the incubation number N is 1 or a larger number up to 10.

15. A method as claimed in claim 12, including subjecting an area of the printing surface to energy from two successive pulses from a pulsed energy beam.

16. A method as claimed in claim 12 including traversing a pulsed energy beam across the printing surface at a speed (V) determined according to one or more of the pulse repetition frequency (f) and the diameter (D) of the beam.

17. A method as claimed in claim 12, wherein the energy per pulse delivered in the method is between 0.1 J and 50 J.

18. The method of claim 17, wherein the energy per pulse is at least one of the following: between 0.1 J and 20 J, between 0.1 J and 10 J, between 0.1 J and 5 J, between 0.5 J and 50 J, between 0.5 J and 20 J, between 0.5 J and 5 J, between 1 J and 50 J, between 1 J and 20 J, or between 1 J and 5 J.

Description

EXAMPLE SET 1

(1) In this set of experiments the exposure of anodised aluminium sheets to ultra-fast (u-f) laser radiation was examined.

(2) This set of experiments started with freshly prepared aluminium oxide/aluminium substrate, 0.3 mm gauge (degreased, grained roughened, desmutted and anodised, without being post-anodically treated) has a contact angle with water of around 15. Contact angle means the angle between the surface of a drop of water and the printing surface of the substrate, where the water comes into contact with the printing surface.

(3) When the substrate was allowed to age for four or five days the contact angle increased, until it reached a maximum of around 70, as shown in Table 1 below. In other words the surface went from hydrophilic to hydrophobic.

(4) TABLE-US-00001 TABLE 1 Effect of ageing after production on contact angle of water on an aluminium oxide/aluminium substrate: Time after manufacture 5 mins 6 hours 24 hours 48 hours 96 hours 120 hours Contact 15 20 30 50 65 70 angle

(5) On exposure of an aged (>48 hours), hydrophobic, aluminium oxide/aluminium substrate to an ultra-fast laser beam (Clark ultra-fast laser operating under the following general conditions: wavelength of 775 nm, 30 m spot size, pulse width 180 fs and with an energy density (fluence) of around 225 mJ/cm.sup.2), the contact angle was reduced to 20 i.e. the exposed area became more hydrophilic. The contact angle then stayed fairly constant for some 12 hours and then started to increase fairly rapidly so that some 16-18 hours after exposure, the contact angle was around 70 once more and the printing surface was once again hydrophobic. This is shown by the results in Table 2 below.

(6) TABLE-US-00002 TABLE 2 Effect of time after u-f (ultra-fast laser) exposure on contact angle of water on an aluminium oxide/aluminium grained and anodised substrate: Time after exposure 5 mins 1 hour 4 hours 12 hours 16 hours 18 hours Contact 20 20 20 30 55 70 angle

(7) In further experimental work re-exposure of the printing surface described above >24 hours after the initial exposure and under laser conditions corresponding to those described above, again brought about a reduction in contact angle (i.e. an increase in hydrophilicity). This effect was observed for at least 5 exposure/re-exposure cycles.

(8) It has been observed that reversion (i.e. to a hydrophobic state) occurs more rapidly the more time a printing surface has been exposed, and further suggests that measures to advance or retard the reversion of the printing form precursor may be feasible.

(9) The results indicate the potential of u-f lasers to provide a reversible or rewriteable printing plate system.

EXAMPLE SET 2

(10) In this set of experiments the contact angle of water with anodised titanium oxide/titanium sheet, and the effect of u-f radiation, was examined.

(11) Anodised (40 gl.sup.1 sodium carbonate +2.5 gl.sup.1 sodium chloride, 30-32 C.) titanium sheet (having an initial surface layer of titanium oxide), no graining; no post-anodic treatment, had a contact angle with water droplets of around 70, a day or more after preparation. When exposed to the ultra-fast laser beam under the conditions described in Example Set 1, the contact angle reduced to 15-20 and the surface was rendered hydrophilic. After some 5 hours the contact angle had reverted back to 70. The results are set out in Table 3 below.

(12) TABLE-US-00003 TABLE 3 Effect of time after exposure on contact angle of anodised titanium oxide/titanium sheet: Time after exposure 5 mins 1 hour 2 hours 3 hours 4 hours 5 hours Contact 10 10 20 30 55 70 angle

(13) Comment:

(14) Thus the findings set out in Example Set 1 and Example Set 2, of the change in contact angle on the aluminium and titanium oxide surfaces is of significance for printing plates. The low levels of power required to produce the changes in contact anglealbeit with ultra-fast lasers where the peak energy delivery is extremely highand the accuracy and simplicity of the method using a u-f laser, show the capability for industrial application, and commercial value. The reversibility offers a prospective environmental and commercial advantage.

EXAMPLE SET 3

(15) Freshly degreased, non-grained and non-anodised aluminium oxide/aluminium is essentially hydrophilic and it has not been possible with ultra-fast laser exposure to introduce a change in hydrophilicity on the scale of hydrophobic to hydrophilic change observed with (several days old) grained and anodised material as described above. Nor is there any evidence with non-grained and non-anodised aluminium of any reversal phenomenon as described above. To check (i) if both graining and anodising are beneficial in effecting the changes in hydrophilicity and (ii) if they play a part in the reversal process, as described above, a series of experiments was carried out, under the conditions described in Example Sets 1 and 2, and with a substrate further described in Table 4 below. The results are recorded in Table 4 below.

(16) TABLE-US-00004 TABLE 4 Reversion of alternatively prepared aluminium oxide/aluminium substrates: Contact angle Reversion Contact angle Exposure after time Substrate prior to exposure energy (J) exposure (hours)* Grained only 70 2 70 hydrochloric 70 3 50 acid (8-10 gl.sup.1 70 4 20 3 at 30-33 C.) 70 6 20 9 Anodised 70 2 70 only 70 3 50 sulphuric 70 4 40 8 acid (150 gl.sup.1 70 6 20 >10 at 30-32 C.) Both grained 70 2 70 and 70 3 50 5 anodised 70 4 20 >12 (above 70 6 20 >14 conditions) *reversion time is defined as the time taken to revert to the original condition of hydrophobicity as determined by contact angle

(17) From the above tabulated results, it is apparent that, whilst hydrophilisation is achieved on an anodised only substrate, and reversion occurs, interesting results are obtained using a grained-only substrate; and that a combination of both graining and anodising is additionally beneficial in extending the reversion time. Also it is of interest to note the energy requirement to achieve the hydrophobic to hydrophilic switch. There is a preliminary indication that if the amount of energy delivered is just sufficient to achieve the switch from hydrophobic (contact angle 70) to hydrophilic (contact angle 20), the reversion time (to hydrophobic) is less than if an excess of energy is delivered. This is an indication of one method of controlling the reversion time.

EXAMPLE SET 4

(18) Further work has been undertaken to show that reversion times can be adjusted by the choice of substrate. A series of experiments was carried out as described in Example Sets 1, 2 and 3 above, and as further set forth in Table 5 below. Results are also set out in Table 5 below.

(19) TABLE-US-00005 TABLE 5 Reversion times achieved by different aluminium oxide/aluminium substrates: Contact Expo- Contact Contact Contact Contact angle sure angle angle angle angle prior to energy after ex- after after after Substrate exposure (J) posure 10 hrs 24 hrs 72 hrs Grained 70 6 20 70 70 70 and H.sub.2SO.sub.4 anodised (as per general conditions) As above + 70 6 20 20 35 70 K.sub.2ZrF.sub.6PAT (4.5 gl.sup.1, 46 C.) Grained 70 6 20 30 30 30 and H.sub.3PO.sub.4 anodised (250 gl.sup.1, 30-32 C.) + K.sub.2ZrF.sub.6PAT

(20) It may be noted that with the PAT (post-anodic treatment), imaged areas remain hydrophilic sufficiently long (up to 24 hours) to enable printing. This is not necessarily the case for non-PAT material. Similarly note that phosphoric acid anodised material the reversion time is further extended (up to 72 hours). These results confirm that it should be possible to adjust reversion times by selection of the substrate in order to meet the printer's needs.

(21) Reversion times also appear to change depending on whether or not the exposure is the first exposure or a subsequent (or re-writing exposure). Reversion times following exposures subsequent to the first appear to get shorter. The opportunity to have the means to adjust the reversion time by substrate choice therefore becomes available and this is potentially very useful from a commercial perspective. For example, it will be seen below that sulphuric acid anodising reversion times differ from phosphoric acid anodising reversion times and non-post anodically treated substrate reversion times differ from post anodically treated substrate reversion times. This preliminary study indicates that it should be possible to adjust reversion time to suit the needs of the printer.

(22) The effect of a post anodic treatment is also evidenced by the results shown in Table 6 below.

(23) TABLE-US-00006 TABLE 6 Contact angle measurements for PAT/non-PAT aluminium oxide/ aluminium substrates: Post anodic treatment No post anodic K.sub.2ZrF.sub.6 PAT Time after treatment (4.5 gl.sup.1, 46 C.) ultra-fast Exposed Exposed exposure Unexposed at 6 J unexposed at 6 J Before exp 75 20 75 20 1 hour.sup. 30 20 2 hours 70 20 5 hours 75 20 7 hours 75 20 9 hours 75 40 18 hours 75 60

(24) The substrates were grained, desmutted and anodised as described above. The results are of the initial exposure of the substrates to imaging radiation.

(25) TABLE-US-00007 TABLE 7 Reversion times Contact Exposure angle prior energy Contact Contact to (J) angle after Time angle Time exposures Each 1.sup.st to after 3.sup.rd to Substrate 1 and 3 exposure exposure >70 exposure >70 Grained 70 6 20 ~10 20 ~1.5 and hours hours H.sub.2SO.sub.4 anodised (con- ditions as above)

(26) The effect on reversion times of subsequent (to the initial) exposures of the same substrates can be illustrated by the results in the following Table 7.

(27) It is evident from the data in the table above, that the reversion time for the first exposure which is of the order of 10 hours, is significantly longer than a third exposure where the reversion time drops to approximately 1 hours. These results suggest that the use of modifiers, such as post-anodic treatments and phosphonic acid anodising may be useful in controlling reversion time.

EXAMPLES SET 5

(28) Exposure energy requirements were investigated to study obtaining the desired change in hydrophilicity without damaging the ultra-fast-exposed surface.

(29) Surface characterisation and weight loss studies of aluminium oxide/aluminium samples as described have indicated that at an exposure energy of about 4 J and a tracking speed of 15-20 mmsec.sup.1, a change from hydrophobic to hydrophilic can be effected with minimal anodic layer damage as evidenced by the SEM micrographs of FIGS. 1 and 2, and separately, contact angle measurements.

(30) In the SEM image of FIG. 1, it is apparent that the 25 J pulses cause some disruption to the anodic layer which is not evident in SEM image FIG. 2 where the ultra-fast exposure was 24 J pulses. Surface areas imaged with energies equivalent to the SEM image of FIG. 2 are rendered hydrophilic as determined by contact angle measurements.

(31) Possible Importance of Flat Top Beam Profile

(32) To establish best utilisation of the exposure energy of the ultra-fast laser beam and yet minimise anodic layer (or any surface) damage, it is believed that the ideal profile of the exposure beam, considered as an energytime graph, would be a top-hat or flat-top profile. It is highly likely with a conventional Gaussian beam profile that that proportion of the beam energy which constitutes the peak energy could damage the anodic layer whereas the remainder of the beam does not. It is expected therefore that a beam which has a more even energy distribution, for example, a beam with a flat-top profile, could be controlled more effectively to deliver hydrophilicity on exposure without causing significant damage to the anodic surface. Preliminary results indicate that this is indeed the case for a Gaussian beam whose profile has been modified by appropriately designed software.

(33) Possible Importance of Ultra-Fast Laser Wavelength

(34) Similarly, consideration has been given to the possibility that the wavelength of the incident ultra-fast laser radiation may be important with respect to the best utilisation of its energy. In effect it may be possible to identify a particular wavelength of incident radiation which maximises the hydrophilisation effect with minimal damage to the anodic layer (or surface layer) at a minimum energy. With this in mind, studies have been conducted on a HiQ picosecond ultra-fast laser under the following general conditions:

(35) Test sample: grained and anodised aluminium (standard conditions), no post-anodic treatment

(36) Pulse width: 10 picoseconds

(37) Pulse energy: 2 to 8 J

(38) Frequency range: 5 kHz to 20 kHz

(39) Spot size: 22 m

(40) Tracking speed: 100 to 400 mm/sec

(41) Wavelength: 532 nm (green visible) and 1064 nm (infra-red)

(42) TABLE-US-00008 TABLE 8 Potential effect of ultra-fast radiation wavelength Pulse Pulse Frequency = Pulse Frequency = Frequency = 5 kHz 10 kHz 20 kHz Tracking speed = Tracking speed = Tracking speed = Wave- 100 mm/sec 200 mm/sec 400 mm/sec length Energy Contact Energy Contact Energy Contact (nm) (J) angle () (J) angle () (J) angle () 532 2, 3, 4 >70 2, 3, 4 >70 2, 3, 4 >70 1064 2, 3, 4 >70 2, 3, 4 2, 3, 4 >70 532 5 17 5 28 5 63 1064 5 >70 5 >70 532 6 <10 6 <10 6 14 1064 6 >70 6 50 532 7 <10 7 <10 7 <10 1064 7 532 8 <10 8 <10 8 <10 1064 8.8 <20 8 45 532 1064 9.6 <10 10 <10 10 <10

(43) It is evident from the results tabulated above, that with the picosecond laser operating at 532 nm (green), the minimum energy to effect hydrophilisation is around a pulse energy of 6 J for each of the frequencies studied. However, for a similar set of pulse repetition frequencies but with the laser operating at 1064 nm (infra-red), a minimum pulse energy of 8 J operating at a frequency of 5 Hz (and a tracking speed of 100 mm/sec), is required.

(44) These results strongly suggest that there is some wavelength dependency for ultra-fast laser exposure with respect to introducing hydrophilicity into the printing form precursor as herein described. This dependency may well offer a means to effect hydrophilicity whilst simultaneously minimising unwanted surface defects (such as destruction or partial removal of the anodic layer).

(45) Possibility to Prime the Surface

(46) Further experimental work has shown that single pulse exposures at energies (say <or=3 J) which are below those causing anodic layer damage, as evidenced by scanning electron microscopy, may subsequently be rendered hydrophilic by a second low energy exposure; again in this way anodic layer damage is minimised. This gives rise to the idea of using a low energy blanket exposure (for example, ultra-fast laser exposure or plasma exposure), prior to low energy, imagewise exposure.

EXAMPLE SET 6

(47) There has been some evidence during the course of the experimental work that different exposure energies (i.e. different from the initial exposure) may be required to render an exposed area hydrophilic for second and subsequent exposures. This may be important when considering this new technology as the source of a re-useable and re-writeable printing plate. It may be that minimised exposure energy may facilitate a greater number of re-exposures as it is possible that surface damage to the alumina may, in this way, be minimised.

(48) General Exposure Conditions

(49) The grained and anodised (standard conditions set out above, no PAT) aluminium oxide/aluminium sheet was exposed to ultra-fast radiation using a Clark femtosecond laser operating as follows: pulse repetition frequency 1 kHz, wavelength 775 nm, a tracking speed of 20 mmsec.sup.1 spot size 30 m, and an exposure energy of 4-6 J per pulse.

(50) 1.sup.st exposure conditions: pulse repetition frequency 1 kHz, a tracking speed of 20 mmsec.sup.1, exposure energy 6 J.

(51) After the exposure the contact angle for the exposed area was measured and was found to be <10 i.e. the imaged area was completely hydrophilic. The plate sample was left in ambient conditions for 3 days during which time the imaged area reverted to fully hydrophobic i.e. the contact angle >70.

(52) 2.sup.nd Exposure Conditions

(53) The second exposure conditions were varied to study their effects. Sample 1 For sample 1, the exposure energy was reduced to 4 J, all other parameters were kept the same as for the 1.sup.st exposure condition. On exposure the imaged area became hydrophilic with contact angle <10. Sample 2 For sample 2, the tracking speed was increased to 30 mmsec.sup.1, all other parameters were kept the same as for the 1.sup.st exposure condition. On exposure the imaged area became hydrophilic with contact angle <10. Sample 3 For sample 3, the tracking speed was increased to 30 mmsec.sup.1 and the exposure energy was reduced to 4 J, other parameters were kept the same as for the 1.sup.st exposure condition. On exposure the imaged area became hydrophilic with contact angle 12.

(54) Conclusions

(55) The above results suggest that imagewise exposures subsequent to the initial exposure may require less incident radiation to effect a change from hydrophobic to hydrophilic. This may be brought about by, for example, adjusting (reducing) the exposure energy whilst maintaining tracking speed of by maintaining the exposure energy whilst increasing the tracking speed. These are important observations and may facilitate a potential increase in the number of image-print-reimage-reprint cycles as printing plate surface damage should be minimised.

EXAMPLE SET 7

Press Testing of a Once, Twice and Three Times Exposed Printing Form Precursor

(56) An electrochemically grained (hydrochloric acid), electrochemically anodised (sulphuric acid) and post-anodically treated (potassium hexafluorozirconate) aluminium oxide/aluminium sheet was allowed to reach its equilibrium point in terms of its hydrophilicity (contact angle 70 and effectively hydrophobic).

(57) The thus prepared printing form precursor was then imagewise exposed with an ultra-fast laser using the following conditions: femtosecond laser of pulse repetition frequency 1 kHz and wavelength 775 nm, a spot size of 30 m, a pulse energy of 6 J and a tracking speed of 20 mmsec.sup.1. The imaged plate was then mounted on a printing press without further treatment and a number of good copies printed to demonstrate a printing capability.

(58) Once the print test was completed, the plate was removed from the press and the printing ink removed using a proprietary plate cleaner. The plate was then allowed to equilibrate (effectively image areas were allowed to return to their hydrophobic state) before the plate was exposed for a second time utilising conditions equivalent to the first exposure. The plate was again mounted on the press and a good image was printed. The initial (first) image had completely reverted to its hydrophobic state and behaved (in printing terms) equivalently to other non-image areas.

(59) The entire process of removing the plate from the press, removing the ink from the plate and allowing to equilibrate was repeated for a second time. The plate was then given a third exposure identical to the first, mounted on the press and printed. A similar result to the second imaging was obtainedgood prints were again obtained. Previously exposed areas again behaved (in printing terms) equivalently to other non-image areas.

(60) The print tests demonstrate the capability of the discovered technology to provide a re-writeable and re-useable process less printing plate.

EXAMPLE SET 8

(61) As a possible means to retard or delay the reversion of exposed images (hydrophilic) to hydrophobic, the effects of different gaseous environments have been studied.

(62) Effect of Oxygen, Nitrogen and Helium on Reversion Time

(63) Immediately after image exposure (as described in Example 1) two samples (as described in Example 1) were sealed in separate plastic bags filled with oxygen, two in separate bags filled with nitrogen, three in separate bags filled with helium and one in a bag with no added gas. The three helium bags were then sealed in a large plastic bag also filled with helium. This was done to eliminate concerns about the possible excessive diffusion of helium from its sealed environment.

(64) The sample not under special gas (i.e. in air) was tested 12 hours after image exposure (contact angle 20) and then again after 20 hours by which time the contact angle was 70. At this time one sample from each of the three test gas atmospheres was removed from its gaseous test environment and its contact angle measured. All test samples had maintained a contact angle of <10.

(65) After 34 hours, however, the contact angles of the three test gas samples were re-measured and the contact angles were now found to be 70 i.e. the test samples had reverted to a hydrophobic condition.

(66) The second test gas samples which had continuously been held in oxygen, nitrogen and helium (2 samples in the case of helium) were then testedthe contact angles of the two helium samples had increased to 65, but the fresh oxygen and nitrogen samples still had contact angles of <10. However, by the time of the next tests some 48 hours after initial exposure, contact angles of 70 were obtained for both oxygen and nitrogen i.e. these test samples had fully reverted.

(67) Conclusions

(68) All three gasesoxygen, nitrogen and heliumslowed reversion delaying the return to the hydrophobic condition by some 14 hours; oxygen and nitrogen appeared to be marginally more effective than helium. However, despite the possibly limited efficacy of this approach, the results do suggest that there are likely to be means available to retard the reversion to a hydrophobic condition for printing form precursors.

EXAMPLE SET 9

(69) As a possible means to establish a route to ensuring a consistent time to first oleophilicity for the printing form precursor, a number of substrate parameters were studied, as set out below.

(70) Effects of Substrate Variants on Time to First Oleophilicity as Determined by Contact Angle Measurements

(71) The contact angles of different substrates at different times after manufacture are set out in Table 9 below

(72) General Aluminium Treatment Conditions: degreasing sodium hydroxide 8-10 gl.sup.1 at 35-45 C. graining hydrochloric acid 8-10 gl.sup.1 at 30-33 C. desmutting phosphoric acid 250 gl.sup.1 at 35 C. anodising sulphuric acid 150 gl.sup.1 at 30-32 C.

(73) TABLE-US-00009 Description Printing form precursor - time after manufacture of aluminium 0 hrs 24 hrs 48 hrs 90 hrs 114 hrs 138 hrs 162 hrs 184 hrs 220 hrs 244 hrs 298 hrs Degreased, <20 35 70 70 grained, desmutted only Flash <20 20 70 70 anodised Corona treated <20 <20 <20 70* 70 Degreased, <20 <20 <20 20 20 20 20 .sup.25 45 45 50 grained, desmutted, anodised and sealed.sup. Degreased, <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 grained, desmutted, anodised, Na silicate PAT Degreased, <20 <20 <20 <20 <20 <20 <20 <20 45 45 50 grained, desmutted, anodised, Na phosphate PAT Degreased, <20 <20 <20 <20 <20 <20 <20 <20 45 45 50 grained, desmutted, anodised, Na phosphate + NaF PAT Degreased, <20 20 20 50 50 70 70 grained, desmutted, anodised, no PAT Degreased 20 20 20 30 40 40 45 50 50 50 50 only, no other treatment *This result was 72 hours after corona treatment .sup.Sealing was achieved by treating grained, anodised aluminium (no PAT) in boiling water for 10 minutes.
Conclusions

(74) As is evidenced by the above results, for the post anodic treatments (PAT's), sodium silicate has still not even started to become oxophilic some 2 weeks after manufacture whereas sodium phosphate (NaH.sub.2PO.sub.4) both with and without sodium fluoride took more than 200 hours to reach a contact angle of >45; the sealed (boiling water) sample was almost identical. A non-post anodically treated control took some 100 hours to reach a similar state.

(75) Both flash anodising (raising voltage during electrochemical treatment from zero to 12 volts over 5 seconds with good agitation) and corona discharge gave similar results, taking 48 and 72 hours respectively. These results were similar to the 48 hours taken for the non-anodised control but significantly less than the 130 hours taken for the electrochemically anodised material.

(76) These results again suggest that there should be means available from a commercial perspective to control the time taken to reach the initial hydrophobicity (oleophilicity) and hence provide a printing plate precursors which have consistent starting properties.

EXAMPLE SET 10

(77) To further investigate the potential for the multiple exposure and multiple printing of an ultra-fast exposed aluminium plate, the following experiment was conducted. A grained and anodised aluminium plate (standard treatments as identified above) was exposed (exposure 1) using an ultra-fast laser (Clark ultra-fast laser operating under the following general conditions: frequency of 1 kHz, 50 m spot size, pulse width 240 femtoseconds and fluence of 225 mJ/cm.sup.2). The exposure target image comprised two 50% tint chequers and a non-printing image moat around the chequer patterns (this, to prevent the oxophilic surrounding areas swamping the non-printing image areas and masking any print differential). A simple offset press test (print test 1) was conducted on this as-imaged plate on a Heidelberg GTO press. Print testing took place within two and a half hours of the ultra-fast laser exposure being completed. After adjustment of ink water balance, 250 good quality prints were obtained, before printing was terminated.

(78) The plate was then removed from the press, excess ink was removed from the plate and the plate was reverted artificially to its hydrophobic state by heating at 150 C. for one hour followed by a relaxation period of 30 minutes under ambient conditions. The plate was then subjected to the same exposure conditions (exposure 2) as in exposure 1 above and again placed on the printing press. After ink water balance adjustments, good quality prints (print test 2) were again obtained. FIG. 3 is a photograph showing the print quality after 250 prints (from print test 2). It is clear from the photograph that the printed image is of good quality and that the print does not show any evidence of the original (first) exposure; suggesting that the first exposure image completely reverted to its original hydrophobic state, and that re-use as a printing plateinvolving re-exposure of a further image and a consequential stage of printing that further imageis entirely possible by way of this invention.

EXAMPLE SET 11

(79) Experiments were conducted with a nanosecond pulse laser to see if the same phenomenon was also apparent at longer laser radiation pulses (nanoseconds).

(80) Tests on a pulsed 10 W Ytterbium fibre nanosecond laser (IPG Photonics) using a Pryor (Yb) Pulsed Fibre Laser YF20 system were conducted. General exposure conditions were as follows:

(81) Average power=10-20 W

(82) Frequency=20-100 kHz

(83) Wavelength=1064 nm

(84) Spot size=60

(85) Pulse width=100 nanoseconds

(86) Pulse energy=1 mJ

(87) The exposure tests were undertaken on grained and anodised aluminium (standard conditions, no post-anodic treatment). Contact angle and reversion times are detailed below.

(88) TABLE-US-00010 Contact angle () Pulse On After 2 After 4 After 9 frequency (kHz) exposure hours hours hours 46 <15 20 20 >70 60 <15 40 70 >70

(89) It was observed that on nanosecond exposure the substrate, in exposed areas, became hydrophilic (as determined by contact angle measurement) and then over a period of time and dependent upon the pulse frequency, the exposed areas of substrate reverted to their hydrophobic state. The observations made, suggest that nanosecond laser exposure of alumina substrate could form the basis for generating a lithographic printing surface.

EXAMPLE SET 12

(90) Simple experiments with stainless steel (grade 304-18% Cr, 8% Ni) have shown that its typically hydrophobic surface (contact angle 70) can be rendered hydrophilic (contact angle 15) by exposure with a nanosecond laser (Pryor (Yb) Pulsed Fibre Laser YF20) which operates at a wavelength of 1064 nm and an average power of 20 W. The specific exposure conditions employed were as follows: pulse width 100 nS, pulse energy 1 mJ, spot size 60 and a frequency of 20 kHz. The thus-exposed surface then, over a period of time (4 to 5 hours), reverted to a hydrophobic state (contact angle 70). Subsequent re-exposure to investigate if a potential re-writeable capability also exists for stainless steel was carried out. Re-exposure with for example, a Clark femtosecond laser operating under the following conditions: wavelength of 775 nm, 30 m spot size, pulse width 180 fs, resulted in a hydrophilic surface again being generated (contact angle <20). Re-writeability (and hence re-use as a printing plate) with stainless steel thus appears to be viable.

(91) In this case the image layer is believed to be chromium oxide which naturally forms a passive protective layer on the surface of the stainless steel.

EXAMPLE SET 13

(92) A number of other metals (metallic compounds) have been examined in preliminary tests. The following general ultra-fast laser conditions were employed: HiQ picosecond laser operating at a wavelength of 355 nm, a pulse width of 10 ps, a pulse energy of 7 J, a spot size of 15 and a frequency of 5 kHz. All metallic samples were hydrophobic prior to exposure.

(93) Initial exposures were undertaken, observations made and recorded in the table below. Following the initial exposures, the samples were artificially reverted to their hydrophobic state by heating for 1 hour at 150 C. followed by a relaxation period of 30 minutes under ambient conditions before a second exposure was undertaken. The observations are recorded below.

(94) TABLE-US-00011 Observation on re-exposure Observation on initial (after reversion) - Metal/metal oxide Exposure - hydrophilic? hydrophilic? Copper (has copper yes yes oxide surface) Brass (has zinc oxide yes yes surface) Silver (tarnished - yes yes thought to be silver sulphide surface)