EP1880861B1 - Aluminium strip for lithographic printing plate support - Google Patents

Description

The invention relates to a belt for producing a support for lithographic printing plates consisting of aluminum or an aluminum alloy, wherein the strip at least partially has a microcrystalline surface layer due to hot and / or cold rolling passes. Moreover, the invention relates to a method for characterizing a surface of a belt for the production of lithographic printing plate supports.

Tapes for making lithographic printing plate supports are produced after casting a corresponding aluminum alloy by rolling. Usually, the strip is produced by hot rolling a roll bar followed by cold rolling. After the tape has been made, it is degreased and wound onto a coil. The coil is pretreated by the lithographic printing plate manufacturer and then electrochemically roughened. Heretofore, the microcrystalline surface layer of the aluminum strip introduced by the rolling has largely been removed by the pretreatment, so that the microcrystalline surface layer no longer plays any role in terms of the subsequent electrochemical roughening. With increasing production speeds and therefore decreasing pickling depth in the upstream cleaning steps and during the electrochemical roughening occurs due the now relevant microcrystalline surface layer of the aluminum strips to more frequent production errors due to poor Aufrauergebnissen.

The EP 1 136 280 A2 relates to a support for lithographic printing plates and its production, in which prior to anodizing the surface of an aluminum sheet is pickled alkaline.

The EP 0 795 048 B1 describes a method of cleaning the surface of aluminum workpieces by anodizing.

In the EP 0 978 573 A2 discloses a support for aluminum alloy lithographic printing plates using cold rolling without intermediate annealing.

The EP 1598 138 A1 describes a method of texturing an aluminum sheet with a stamping die.

On this basis, the object of the present invention is to provide a ribbon for the production of lithographic printing plate supports which has an improved microcrystalline surface layer, so that higher production speeds are possible in the production of lithographic printing plate supports. In addition, the invention has for its object to propose a method for characterizing the surface quality of the microcrystalline surface layer of strips of aluminum or an aluminum alloy.

The object is achieved with an aluminum strip and a method for characterizing the surface of an aluminum strip having the features of independent claims 1 and 5.

According to a first teaching of the present invention, the above-derived object is achieved in that in a surface microprobe analysis after the mapping process of a surface region of the microcrystalline surface of the strip, the surface _portion with an intensity _ratio I / I _{bulk (avg)} of greater than 3 in the spectral range the K _α1 line of the X-ray emission spectrum of oxygen of the measured microcrystalline surface layer is less than 10%, preferably less than 7%, wherein in the areal microprobe analysis an excitation voltage of 15 kV, a beam current of 50 nA and a beam cross section of 1 .mu.m, at a Increment of 16.75 microns is used for the electron beam.
With a certain frequency and size of oxide particles in the microcrystalline surface layer, very good roughening properties can be achieved in the downstream production process for lithographic printing plate supports and overall production speeds can be increased. The usually the electrochemical roughening disturbing oxide particles are present in such a small number and size in the microcrystalline surface layer of the tape according to the invention, so that the microcrystalline surface layer can be roughened very well and thus with low material removal during electrochemical roughening due to high production speeds very good Aufrauergebnisse the production of printing plate supports can be achieved. In the areal microprobe analysis, a surface portion of the band is examined via an electron beam with an excitation voltage of 15 kV, a beam current of 50 nA and a beam cross section of 1 μm with a pitch of 16.75 μm. The electrons incident on the surface of the band generate X-ray brake radiation and characteristic X-ray emission spectra whose wavelength identifies the element present in the sample and whose intensity gives information about the concentration or frequency of the corresponding element in the measuring region of the electron beam cross section impinging on the surface to be measured. The highest intensities are found in the K _α1 lines of the X-ray emission spectra. Due to the excitation voltage of 15 kV, the penetration depth of the electrons is limited to 1 to 2 microns, so that only near-surface layers of the belt to the emission of the characteristic X-ray emission spectra excited become. In particular, the penetration depth of the electrons coincides with the values known from the literature for the thickness of the microcrystalline surface layer which is produced during hot rolling of the rolling bar and after cold rolling at final strip thicknesses of 0.15 to 0.5 mm is typically 1 to 2 μm (see For this: Lindseth I., "Optical total reflectance, near surface microstructure, and topography of rolled aluminum materials", PhD thesis, NTNO, Trontheim, Norway, 1999 ). The K _α1 line of the X-ray emission spectrum of oxygen now indicates the content of oxygen of oxidic compounds in the microcrystalline surface layer at the corresponding measurement point. By forming the ratio of the measured microsound signal of a microcrystalline surface layer and the averaged surface signal of a bulk material pickled surface layer of a tape, the substantially equal contributions of the relatively thin alumina film to the measured aluminum surfaces, which also contribute to the intensity of the characteristic oxygen spectrum, _averaged , so that the ratio I / I _{bulk (avg)} is essentially a measure of the proportion of oxygen atoms by rolled oxide particles in the region of the incident electron beam of the microcrystalline surface layer of the tape. The intensity of the micro-sonic signal can thus be used as a measure of the size of the oxide particles. Due to the penetration depth of the electrons of about 1 to 2 .mu.m, rolled-in-oxide particles, which have been identified as problematic with respect to electrochemical roughening, are detected in particular by rolling. Due to the limitation of the surface portions with I / I _{bulk (avg)} > 3 to less than 10%, preferably less than 7%, the tape _{according to} the invention for the production of Therefore lithographic printing plate supports a distribution of relatively small oxide particles, so that the tape according to the invention has very good Aufraueigenschaften.

According to the invention, the thickness of the tape is 0.15 to 0.5 mm and the thickness of the microcrystalline surface layer of the tape is about 0.5 to 2.5 μm.

A further increase in the process speeds in the electrochemical roughening of the tape for lithographic printing plate support can be ensured by the tape according to the invention in that in area microprobe analysis by the mapping method of a surface _{portion of} the tape, the area _ratio with an intensity _ratio I / I _{bulk (avg)} of greater than 4 in the spectral range of the K _α1 line of the X-ray emission spectrum of oxygen of the measured microcrystalline surface layer is less than 3%, preferably less than 2%. In this case, the microcrystalline surface layer of the strip according to the invention has an even smaller number of larger oxide particles which can disturb the electrochemical roughening or the preceding pretreatments.

Preferably, the band is made of an aluminum alloy of the type AA1050, AA1100 or AA3103. These aluminum alloys have already found wide application in terms of their suitability for the production of lithographic printing plate supports.

A further improvement in terms of strength and Aufraubarkeit tape for the production of lithographic printing plate supports can thereby That the aluminum strip consists of an aluminum alloy with the following proportions by weight of alloy components: 0.05% ≤ Si ≤ 0.1%, 0.4% ≤ Fe ≤ 1 %, Cu ≤ 0.04%, Mn ≤ 0.3%, 0.05% ≤ Mg ≤ 0.3%, Ti ≤ 0.04%, Residual Al with unavoidable impurities individually a maximum of 0.005% in total maximum of 0.15%.

According to a second teaching of the present invention, the above-described object is achieved by a method for characterizing a surface of a belt, in particular a belt for the production of lithographic printing plate carriers by performing a surface microprobe analysis of the microcrystalline surface layer according to the mapping method and the quality the surface of the band is evaluated on the basis of the measured intensity distribution in the spectral range of the K _α1 line of the X-ray emission spectrum of oxygen.

As already stated, the areal microprobe analysis offers the possibility of examining the microcrystalline surface layer for its composition and, in particular, by the distribution of the intensity distribution of the K _α1 line of the X-ray emission spectrum of oxygen over the area of oxide particles in the microcrystalline surface layer. Although a surface microprobe analysis according to the mapping of surfaces is already known. However, a quality evaluation of the surface area of an aluminum or aluminum alloy strip based on the intensity distribution in the spectral region of the _Kα1 line of the X-ray emission spectrum of oxygen with respect to the suitability for producing lithographic printing plate supports has not been performed hitherto. As already stated above, the suitability of the strip, in particular in downstream electrochemical roughening methods, can be reliably checked via the characterization method according to the invention.

The influence of the aluminum oxide film on the microcrystalline surface layer can, according to a first embodiment of the method according to the invention, be reduced in the measurement result by ascertaining from the measured intensity distribution of the surface layer an area _fraction having a specific value for the intensity _ratio I / I _{bulk (avg)} . In addition, a measure of the size of the oxide particles in the microcrystalline surface layer is provided by the intensity _ratio I / I _{bulk (avg) and} a measure of the frequency of the oxide particles over the surface portions with a specific intensity _ratio I / I _{bulk (avg)} posed. From the intensity ratio mentioned, this results in a combined measure of the size and area occupancy of the microcrystalline surface layer with oxide particles of a certain size. It has been found that in particular the combination of size and number of oxide particles in the microcrystalline surface layer, the subsequent electrochemical Can negatively influence Aufrauprozess, provided upstream pickling steps do not completely eliminate the microcrystalline surface layer or a roughened surface of bulk material is roughened.

If an excitation voltage of 5 to 20 kV, preferably 15 kV, a beam current of 10 to 100 nA, preferably 50 nA and a beam cross section of 0.2 to 1.5 .mu.m, preferably 1 .mu.m used for the electron beam, not only the penetration depth are limited to the electrons, but on the beam current and the beam cross-section excitation densities and X-ray emission intensities are achieved, which reduce measurement errors in the determination of the surface portions.

A measurement duration per measurement point of 0.3 to 1 s, preferably of 0.6 s, also contributes to this. In addition, the measuring time per measuring point ensures that a sufficiently large strip surface section can be measured in an adequate time.

Finally, it is preferable to use a linear focusing spectrometer with a crystal having a lattice plane distance 2d of 6 nm, preferably an LDE1H crystal. Usually, the crystal is arranged in linear focusing spectrometers on a Rowlandkreis with a small diameter, for example 100 mm. The spectrometer on the one hand by the linear focusing that the X-ray emission spectrum, which is emitted from the sample spot, with sufficient intensity in the detector, preferably a designed as a counter tube detector for X-rays, is bundled. Of the Crystal with a lattice plane distance 2d of 6 nm ensures that the K _α1 line of the X-ray emission spectrum of high-intensity oxygen is diffracted in a wavelength-selective manner in the direction of the optical detector via a Bragg reflection. This arrangement makes it possible in particular that even very small amounts of oxide particles deliver measurable K _α1 lines of the X-ray emission spectrum of oxygen.

Fig. 1

in einer schematischen Darstellung das linear fokussierende Spektrometer des gemäß einem Ausführungsbeispiel des erfindungsgemäßen Charakterisierungsverfahrens und

Fig. 2

ein Messergebnis eines Oberflächenabschnitts eines Bandes.

There are now a variety of ways to develop the tape of the invention for the production of lithographic printing plate supports and the inventive method for characterizing a strip of aluminum or an aluminum alloy and to design. For this purpose, on the one hand reference is made to the patent claims 1 and 5 subordinate claims, on the other hand, to the following description of embodiments in conjunction with the drawings. The drawing shows in

Fig. 1

in a schematic representation of the linearly focusing spectrometer of accordance with an embodiment of the characterization method according to the invention and

Fig. 2

a measurement result of a surface portion of a tape.

Fig. 1 shows the typical structure of the spectrometer of a microprobe analysis, in the present case a JEOL JXA 8200 microprobe was used, in which an electron beam 1 is deflected onto a sample 2. The electrons are excited with an excitation voltage of 15 kV, a beam current of 50 nA and a beam cross section of 1 micron to the sample 2 directed. In sample 2, the characteristic X-ray emission spectrum 3 is then generated, which is generated by electron transitions on the inner shells of the excited atoms. The wavelength of the emitted spectrum is therefore characteristic of each atom. This in Fig. 1 The linearly focused spectrometer shown has a curved crystal 4 for wavelength analysis, which reflects the X-radiation emitted by the specimen 2 in a wavelength-selective manner into the slit of a detector 5. The acceptance angle of the characteristic X-radiation α is 40 °. The position of the crystal 4 on the Rowland circle 6, which here has a diameter of 100 mm, is adjusted so that only the K _α1 line of the characteristic X-ray spectrum of oxygen in the detector is diffracted by Bragg reflection. After the number of X-ray pulses has been counted in the detector over a measuring time of 0.6 s, the sample is transported further by the increment of 16.75 μm and a next measuring point is measured.

The spectrometer has a crystal specially adapted for measuring the K _α1 line of the X-ray emission spectrum of oxygen and oriented for maximum intensity of the oxygen spectrum, a type LDE1H crystal having a lattice plane distance 2d of 6 nm. The penetration depth of the electrons into the sample 2 is about 1 to 2 μm at an excitation voltage of 15 kV. On each sample, a square area with an edge length of 5.025 mm was measured, with a step size of 16.75 μm being selected, so that a total of 900 measuring points were measured in the square surface. Fig. 2 shows the measurement results of the surface _{microprobe analysis} after the mapping process on a sample, each measuring point on the one hand a square area with an edge length of 16.75 microns and on the other hand, the measured intensity _ratio I / I _{bulk (avg) is} assigned. It can be seen in Fig. 2 the measured measured surface area intensity values converted into color values, which show the microscopic streaking in the rolling direction typical of the rolled strip surfaces investigated. This streakiness is attributed to a distribution of rolled surface particles in the rolling direction during rolling. Corresponding mappings were then evaluated with regard to their area _occupation with certain intensity ratios I / I _{bulk (avg)} .

A total of eight tape samples were tested, each tape sample consisting of an AA1050 aluminum alloy. The experimental setup for determining the size and frequency of the oxide particles in the microcrystalline surface layer was chosen as described above. In order to determine the influence of the microcrystalline surface layer in the measured intensity signal, in addition to a ninth sample consisting of an identical alloy, the microcrystalline surface layer was removed by ablating greater than 2 μm in a pickling step, the sample forming a typical aluminum oxide layer being about 1 Week stored, also carried out a two-dimensional microprobe analysis and a mean intensity _signal for the bulk material I _{bulk (avg)} determined. As an average intensity signal, 125 pulses were measured in 0.6 s at the above-mentioned excitation and detection conditions. The intensity values of the K _α1 line of the X-ray emission spectrum of oxygen measured on the samples became divided by the mean intensity value of the bulk material and assigned in a corresponding mapping of a square measurement area with an edge length of 16.75 microns. Subsequently, the areas in the 5.025 mm × 5.025 mm total measurement area were summed up, which have an intensity _ratio of I / I _{bulk (avg)} greater than 3 or greater than 4. The area proportions measured in samples Nos. 1 to 9 with an intensity _ratio of I / I _{bulk (avg)} greater than 3 or 4 are shown in Table 1 together with the averaged intensity _values I _avg measured on the samples. Table 1 Samples no. sample I _avg Area percentage in% with I / I _{bulk (avg)} > 3 Area percentage in% with I / I _{bulk (avg)} > 4 rating 1 comparison 577 84 52 - 2 comparison 358 23.9 6.8 - 3 comparison 325 12.8 3.3 - 4 invention 306 9.1 3.1 O 5 invention 293 6.0 2.0 O 6 invention 296 6.3 1.8 + 7 invention 223 0.2 0 ++ 8th invention 161 0.4 0.1 ++ 9 Bulk 125 0 0 ++

Subsequently, samples 1-9 or the associated strips were subjected to an electrochemical roughening and their behavior during the electrochemical roughening was evaluated.

It was found that Sample Nos. 1, 2 and 3 caused electrochemical roughening errors and no increase in process speed during electrochemical Allow for confidence. While Sample Nos. 1 and 2 were considered to be very poor (-) in terms of electrochemical roughening, so that homogeneous roughening could be achieved only at very high charge carrier input, the roughening of Sample No. 3 improved. However, Sample No. 3 did not show satisfactory roughening. All samples were subjected to conventional degreasing prior to measurement.

Less oxidic impurities on the surface and thus a satisfactory Aufraueigenschaften showed sample 4, on which a surface portion with an intensity _ratio of I / I _{bulk (avg)} greater than 3 or greater 4 was determined by 9.1% and 3.1%, respectively. Also the further samples 5 to 8 showed satisfactory (o), good (+) or very good (++) roughening properties. Overall, there is a clear correlation between the area fractions of the areas measured in a microprobe mapping with a certain intensity _ratio I / I _{bulk (avg)} and the Aufraueigenschaften the microcrystalline surface of the tape.

The results of the investigations were evaluated in such a way that the intensity _ratio I / I _{bulk (avg) is} a measure of the size of the oxide particles in the microcrystalline surface layer and their surface area corresponds to the frequency of oxide particles above a certain size. With a very small surface coverage with larger oxide particles, the Aufraueigenschaften the microcrystalline surface layer of the aluminum strip improve significantly.

The fact that the oxide particles represent the essential contribution to the measured distribution of the intensity _ratio I / I _{bulk (avg)} could be demonstrated by the sample No. 5. The sample No. 5 corresponds to the previously examined sample No. 2, which has additionally undergone a surface pickling selectively applied to the rolled-in particles. The surface of Sample No. 5 was pickled with a 10% H ₃ PO ₄ solution at 80 ° C for about 10 seconds. Since the phosphoric acid almost does not attack the aluminum matrix and selectively removes only the oxide particles, the area fraction with an intensity _ratio I / I _{bulk (avg)} greater than 4 could be reduced from 23.9% to 6.0%. The area _{fraction in the microprobe measurement} with an intensity _ratio I / I _{bulk (avg)} greater than 4 could be reduced by using the Phosphorsäurebeize of 6.8% to 2.0%. At the same time, it was possible to improve the properties with respect to an electrochemical roughening poorly from satisfactory to satisfactory.

For comparison, the measured values of the bulk sample 9 are shown in Table 1. By removing the microcrystalline surface layer, no larger oxide inclusions can be detected on the surface of the bulk sample 9. The measured values for the area proportions of I / I _{bulk (avg)} are consistently zero and the roughening was very good. The still measured intensity of the characteristic X-ray emission spectrum of the oxygen is attributed to the formation of a natural aluminum oxide layer on the surface. In order to achieve as practical a correction of the influence of the aluminum oxide layer on the measurement result of the microprobe measurement, the sample 9 after removal of the microcrystalline Surface layer stored for about 1 week, so that could form a sufficiently thick aluminum oxide layer. In the measuring time of 0.6 s selected in the present exemplary embodiment, an average intensity _signal I _{bulk (avg)} of 125 pulses was measured across the sample surface.

The improved electrochemical Aufraueigenschaften the inventive samples Nos. 4 to 8 are noticeable in particular in a reduced charge carrier entry for complete roughening during the electrochemical roughening of the surface of the samples. In this respect, a ribbon for lithographic printing plate supports can be provided, which allows higher process speeds in the electrochemical roughening or in the manufacture of lithographic printing plate supports.

Claims (9)

1. Strip for the production of a substrate for lithographic printing plates consisting of aluminium or an aluminium alloy, the strip having at least to some extent a microcrystalline surface layer due to hot and/or cold roll passes, characterised in that in a two-dimensional microprobe analysis according to the mapping method of a surface region of the microcrystalline surface of the strip, the surface portion with an intensity ratio I / I_bulk(avg) of more than 3 in the spectral range of the K_α1 line of the X-ray emission spectrum of oxygen of the measured microcrystalline surface layer is less than 10 %, preferably less than 7 %, wherein for the two-dimensional microprobe analysis, an excitation voltage of 15 kV, a beam current of 50 nA and a beam cross section of 1 µm with a step size of 16.75 µm for the electron beam is used, that the thickness of the strip is 0.15 to 0.5 mm, and that the thickness of the microcrystalline surface layer is 0.5 to 2.5 µm.
2. Strip according to claim 1, characterised in that in the two-dimensional microprobe analysis according to the mapping method of a surface portion of the strip, the surface portion with an intensity ratio I / I_bulk(avg) of more than 4 in the spectral range of the K_α1 line of the X-ray emission spectrum of oxygen of the measured microcrystalline surface layer is less than 3 %, preferably less than 2 %.
3. Strip according to claim 1 or claim 2, characterised in that the strip consists of an aluminium alloy of type AA1050, AA1100 or AA3103.
4. Strip according to claim 1 or claim 2, characterised in that the aluminium strip consists of an aluminium alloy having the following proportions of alloy constituents in percent by weight: $$0.05\%\le \mathrm{Si}\le 0.1\%,$$

   

   $$0.4\%\le \mathrm{Fe}\le 1\%,$$

   

   $$\mathrm{Cu}\le 0.04\%$$

   

   $$\mathrm{Mn}\le 0.3\%,$$

   

   $$0.05\%\le \mathrm{Mg}\le 0.3\%$$

   

   $$\mathrm{Ti}\le 0.04\%,$$

   

   
   remainder Al with unavoidable impurities
   individually maximum 0.005 %, in total maximum 0.15 %.
5. Method for the characterisation of a surface of a strip of aluminium or an aluminium alloy, in particular a strip according to claims 1 to 4, characterised in that a two-dimensional microprobe analysis of the microcrystalline surface layer is carried out according to the mapping method and the quality of the surface of the strip is assessed using the measured intensity distribution in the spectral range of the K_α1 line of the X-ray emission spectrum of oxygen.
6. Method according to claim 5, characterised in that a surface portion having a specific value for the intensity ratio I / I_bulk(avg) is determined from the measured intensity distribution of the surface layer.
7. Method according to claim 5 or claim 6, characterised in that an excitation voltage of from 5 to 20 kV, preferably 15 kV, a beam current of from 10 to 100 nA, preferably 50 nA and a beam cross section of from 0.2 to 1.5 µm, preferably 1 µm of the electron beam are used.
8. Method according to any one of claims 5 to 7, characterised in that a measurement duration per measuring point of from 0.3 to 1 s, preferably 0.6 s is selected.
9. Method according to any one of claims 5 to 8, characterised in that a linear-focussing spectrometer with a crystal having an interplanar spacing 2d of 6 nm, preferably an LDE1H crystal is used.

Images