Relating to printing

Abstract

A method of preparing a printing form precursor for printing, or a printed circuit board precursor or a semiconductor precursor, the method comprising the step of applying electromagnetic radiation having a pulse duration of not greater than 110.sup.6 seconds, in an imagewise manner, to an imagable surface of the precursor. The imaging process may cause ablation of the coating of the precursor or permit its development in a developer. In each case the imaging radiation needs not be tuned to imaging chemistry (if any) present in the coating. Alternatively the imaging process may induce a change of hydrophilicity or hydrophobicity, or other change of state, of an uncoated substrate.

Claims

1. A method of preparing a printing form precursor for printing, the method comprising the step of applying electromagnetic radiation having a pulse duration of not greater than 110.sup.10 seconds and a fluence of at least 100 mJ/cm.sup.2, in an imagewise manner, to an imageable surface of the printing form precursor, wherein the imageable surface is aluminum oxide.

2. A method as claimed in claim 1, wherein the pulses are of duration at least 110.sup.18 seconds.

3. A method as claimed in claim 2, wherein the pulses are of duration at least 110.sup.15 seconds and not greater than 110.sup.12 seconds.

4. A method as claimed in claim 1, wherein an average frequency of the pulses is at least 100 pulses per second.

5. A method as claimed in claim 1, wherein a fluence does not exceed 20,000 mJ/cm.sup.2.

6. A method as claimed in claim 1, wherein an incubation number N in the method is 1 or a larger number up to 10.

7. A method as claimed in claim 1, wherein a profile of a laser beam applying the electromagnetic radiation is Gaussian, square or rectangular.

8. A method as claimed in claim 1, wherein the method employs, as an imaging device, a nanosecond laser, a femtosecond laser, or a picosecond laser.

9. A printing form having an imageable surface subjected to pulses of electromagnetic radiation having a pulse duration of not greater than 110.sup.10 seconds and a fluence of at least 100 mJ/cm.sup.2, with the result that selected portions are ink-accepting and the reciprocal portions are not ink-accepting, wherein the imageable surface is aluminum oxide.

Description

EXAMPLE SET 1

(1) In this set of experiments a range of commercially available printing plates were exposed to ultra-fast (u-f) laser radiation, and the threshold energy density (fluence) requirements for a) development and b) ablation were recorded.

(2) The printing plates were both analogue (conventional) and CtP (Computer to Plate, digital) commercial lithographic printing plates. Both the analogue plates (Fuji FPSE, Kodak New Capricorn) and the CtP plates (Agfa Amigo, and Rekoda Thermax) were exposed using a Clark ultra-fast laser operating under the following conditions: frequency of 1 kHz, 50 m spot size and pulse width of 240 femtoseconds (fs), and either 388 nm or 775 nm wavelength. The Agfa Amigo and the Fuji FPSE plates were also exposed using a Fianium laser, frequency of 500 kHz, 30 m spot size, pulse width of 10 picoseconds (ps), and 1064 nm wavelength. Development (when required) employed the developer recommended for the particular plate, under the standard conditions. Plate assessment used standard techniques well known to persons skilled in the art.

(3) The results are set out in Tables 1 to 3 below.

(4) TABLE-US-00001 TABLE 1 1. Clark femtosecond laser, 388 nm, 240 fs, 50 m spot size, 1 KHz: Track Energy Density Threshold Energy J speed (fluence) Plate for (per pulse) mm/sec mJ/cm.sup.2 Agfa Amigo Development 2 20 102 Ablation 3.5 10 178 Rekoda Development 1 10 51 Thermax Ablation 2 15 102 Fuji FPSE Development 1.27 20 65 Ablation 4.45 20 227 New Development 1.27 15 65 Capricorn Ablation No ablation 2 227 up to 4.45

(5) TABLE-US-00002 TABLE 2 2. Clark femtosecond laser, 775 nm, 240 fs, 50 m spot size, 1 KHz: Track Energy Density Threshold Energy J speed (fluence) Plate for Per pulse mm/sec mJ/cm.sup.2 Agfa Amigo Development 3.1 10 158 Ablation No ablation 10 280 up to 5.5 Rekoda Development 1.5 20 76 Thermax Ablation 3.1 10 158 Fuji FPSE Development 3.5 20 178 Ablation 5 100 255 New Development 1.27 15 65 Capricorn Ablation 4.45 2 227

(6) TABLE-US-00003 TABLE 3 3. Fianium Laser 1064 nm, 10 picosec, 30 m spot size: Threshold for development: Track Energy Density Energy J Speed (fluence) Plate (per pulse) mm/sec Hz mJ/cm.sup.2 Agfa Thermal 1.9 200 500K 269 0.24 50 20M 34 Fuji FPSE 2.9 100 500K 410 Note: Fuji FPSE starts to ablate at 2.9 J, 500 KHz, track speed 50 mm/sec.

(7) It has thus been shown that an ultra-fast (u-f) laser can be used to expose both analogue and CtP printing plates, independently of the wavelength the plates are sensitised to. They may be exposed to the extent that development can be carried out with a u-f laser at an energy density (fluence) of about 50-200 mJ/cm.sup.2 and ablation may take place at an energy density (fluence) of about 100-300 mJ/cm.sup.2. These u-f laser exposure requirements compare with traditional UV exposure needs of around 100-300 mJ/cm.sup.2 for analogue plates and 100-120 mJ/cm.sup.2 for CtP plates. Additionally, for ablation of commercial CtP thermal products, typically energy needs for laser diode exposure would be around 500 mJ/cm.sup.2.

EXAMPLE SET 2

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

(9) Freshly prepared aluminium substrate, 0.3 gauge (degreased, 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 substrate, where the water comes into contact with the substrate.

(10) If the substrate is allowed to age for four or five days the contact angle increases, until it reaches a maximum of around 70, as shown in Table 4 below.

(11) TABLE-US-00004 TABLE 4 Effect of ageing after production on contact angle of water on an aluminium substrate: Time after manufacture 5 mins 6 hours 24 hours 48 hours 96 hours 120 hours Con- 15 20 30 50 65 70 tact angle

(12) On exposure of an aged (>48 hours) aluminium substrate to an ultra-fast laser beam (Clark ultra-fast laser operating under the following general conditions: frequency of 1 kHz, 50 spot size, pulse width 240 fs and with an energy density (fluence) of around 225 mJ/cm.sup.2), the contact angle is reduced to 20 i.e. the exposed area becomes more hydrophilic. The contact angle then stays fairly constant for some 12 hours and then starts to increase fairly rapidly so that some 16-18 hours after exposure, the contact angle is once more around 70 and the plate is (relatively) hydrophobic. This is shown by the results in Table 5 below.

(13) TABLE-US-00005 TABLE 5 Effect of time after u-f exposure on contact angle of water on an aluminium substrate: Time after exposure 5 mins 1 hour 4 hours 12 hours 16 hours 18 hours Contact 20 20 20 30 55 70 angle

(14) Re-exposure of the aluminium plate described above after >24 hours after the initial exposure and under laser conditions corresponding to those described above, again brings about a reduction in contact angle (i.e. an increase in hydrophilicity). This effect has been observed for at least 5 exposure/re-exposure cycles.

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

EXAMPLE SET 3

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

(17) Anodised titanium sheet (having a surface of titania) a day or more after preparation has a contact angle of around 70. When exposed to the ultra-fast laser beam under the conditions described in Example Set 2, the contact angle reduces to 15-20 and the surface is rendered hydrophilic. After some 4-18 hours the contact angle reverts back to 70. The results are set out in Table 6 below.

(18) TABLE-US-00006 TABLE 6 Effect of time after exposure on contact angle of anodised 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
Comment:

(19) It is suggested that a contact angle in excess of 46 is good for imaged areas (hydrophobic or oleophilic) and less than 35 (ideally less than 25) is good for background areas (non-printing, hydrophilic), in a printing plate. Thus the findings set out in Example Set 3 and Example Set 4, of the change in contact angle on aluminium and titanium sheets is of significance for printing plates. The low amounts of energy required to produce the changes in contact angle, and 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 4

(20) 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 oleophilic surrounding areas swamping the non-printing image areas and masking any print differential). A simple offset press test (print test1) 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.

(21) 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. 1 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 5

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

(23) 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: Average power=10-20 W Frequency=20-100 kHz Wavelength=1064 nm Spot size=60 Pulse width=100 nanoseconds Pulse energy=1 mJ

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

(25) TABLE-US-00007 Contact angle () On After 2 After 4 After 9 Pulse frequency (kHz) exposure hours hours hours 46 <15 20 20 >70 60 <15 40 70 >70

(26) 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 6

(27) 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.

(28) 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 7

(29) 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.

(30) 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.

(31) TABLE-US-00008 Observation on initial Observation on re-exposure Exposure - (after reversion) - Metal/metal oxide hydrophilic? hydrophilic? Copper (has copper yes yes oxide surface) Brass (has zinc yes yes oxide surface) Silver (tarnished - yes yes thought to be silver sulphide surface)