Anodizing of aluminium is widely applied when a controllable morphology and properties of the surface are required. Anodic oxide films may be developed by appropriate selection of electrolyte and film-forming conditions for various applications in the fields of architecture, aerospace, electronics, packaging and printing. In the present study, the printability of aluminium with respect to anodizing conditions is discussed. In particular, AA1050 alloy specimens were anodized in either sulfuric acid or phosphoric acid at temperatures ranging from 10?C to 40?C, thereby affecting the porosity and anodic layer thickness. Both the porosity and oxide thickness increase with the temperature, whereas anodization in phosphoric acid produces thinner and more porous layer than that in sulfuric acid. After the anodization step, two different printing techniques were used (i.e. digital printing and screen printing). Printed specimens were characterized by means of colour parameters, microscopy, adhesion and light fastness test. Colour parameters and ink adhesion measurements indicate that both digital and screen printing techniques give a better print quality when the anodization step is conducted in the range of 20?C - 30?C.
Among the various types of printing substrates for special or external applications (panels, signage, barcodes, awards, labels, etc.), aluminium has gain a great interest due to its superior decorative and fashionable design appearance; it is also a light weighting, long-lasting, recyclable material that ensures contemporary aesthetic, durability, light fastness and resistance against chemicals.
Several processes are applied to modify aluminium surface properties and render it printable. The most effective one is the electrochemical treatment of anodizing. Anodizing of aluminium surface has received significant attention due to its diverse applications in various fields such as architecture, aerospace, electronics and packaging, but only a few special references are known in the field of printing and graphic arts. Such applications include lithography and various other special printing works [
The above mentioned uses of aluminium have enabled significant focus on fundamental aspects of anodic oxide film formation under controllable conditions in suitable electrolytes [
Many studies have been published on the modification of the porous structure as the anodizing conditions (i.e. voltage, time, temperature, electrolyte etc.) are altered [
The present work concerns aluminium as special printing substrate for specific applications under demanding conditions. Chemically pretreated ΑΑ1050 aluminium specimens were anodized in sulphuric or phosphoric acid solutions at various anodizing temperatures. Printing on anodized specimens by screen printing (using epoxy- catalytic inks) or inkjet printing (using UV curing inks) was followed by sealing in boiling water. Subsequently the CIEL*a*b* and DCMYK colour parameters of the imprint were evaluated and the optical and SEM micrographs were inspected. The relation of the characteristics (thickness and morphology) of the oxide layers formed under specific anodizing conditions with the printing quality of the anodized specimens, as well as the light fastness and adhesion of the imprint was investigated. The expected outcomes of the process described in the present paper are imprints of high quality and outdoor resistance and durability, on aluminum surface. Thus, a light weighting and recyclable material such as aluminum may be used more extensively.
Specimens of ΑΑ1050 aluminium alloy were degreased in acetone, etched for 1 min in a water solution containing 40 g/LNaOH at 40˚C rinsed with deionized water and immersed in 1:1 v/v concentrated HNO3 at room temperature for 1 min. After rinsing with deionized water and drying in a cool air stream the specimens were stored in a desiccator. Anodizing was carried out via a Delta Electronika Power Supply SM3004-D, at a constant voltage of 15 V in 1.8 M H2SO4 (S.A.) for 40 min or at 30 V in 0.4 M H3PO4 (P.A.) for 5 min at various temperatures in the range 10˚C - 40˚C. Current transients during anodizing were recorded via a multimeter (Keithley 2000) connected with a PC.
Anodized specimens were printed by a laboratorial hand operated screen printing machine or by an Oce Arizona 350 XT digital printing machine. Printing was performed using epoxy-catalytic inks or UV curing inks accordingly and then sealed in boiling deionized water.
A Spectro Eye (Gretag Macbeth) spectrophotometer was used for colour and print density measurements. Reflectance of printed specimens was measured with an Ocean Optics spectrophotometer (HR-2000modelequippedwith an optical fiber, Integrating Sphere, 50 mm, with glass trap, ISP-50-8-R-GT Micropack). Measurements were carried out via calibration with a standard Spectralon reflectance probe).
Printed specimens were tested against common chemical solvents (water, acetone, ethanol) and evaluated according to ASTM D3359 (Measuring Adhesion by tape test) and ASTM D3424 (Light fastness evaluating). An optical stereo-microscope Olympus 5261 10 X - 80 X connected with a Sony Ex Wave HAD camera and PVR Plus software was used for surface images of the printed specimens.
A JEOL JSM-6510 LV Scanning Electron Microscope (Oxford Instruments, 10mm2 Silicon Drift Detector-x-act) was used for observation of the treated specimens.
Image analysis was performed by using the Image J 1.50 b software in order to obtain quantitative information on the results of adhesion test on the printed surface of the specimens. The micrographs were first transformed to a suitable format, whereupon particle analysis was applied for the estimation of the percentage (%) of the removed printed surface area of the specimens by the tape test. Print quality of the specimens was evaluated by the same method.
The experimental procedure described above was performed for at least three times using distinctive specimens for each set of the selected experimental conditions. The obtained mean values of the measurements were used in order to ensure the reproducibility of the experiments.
The current density vs. time curves obtained during anodizing of AA1050 specimens in P.A. or S.A. at various temperatures are shown in Figure1. They show that a significant increase of current density occurs with increasing anodizing temperature. The values of the current density are about ten times higher in the case of S.A. compared to those of P.A. indicating that the consumed electric charge is much higher in the former case. Nevertheless, this charge results in an increase of the porous oxide layer thickness during the anodization process in both cases, while chemical dissolution of the oxide takes place leading to pore formation with an increased pore diameter towards the surface of the anodic alumina film, and accordingly to an increase of the film porosity especially at elevated temperatures [
The anodic oxide films formed on AA1050 specimens in P.A. and S.A. at 20ºC are shown (cross-sectional view) as SEM micrographs in
Since differences in the structure and porosity or roughness of the surface of anodic oxide films influence their optical properties [
In this figure, the observed λmax values in all cases are observed in the region of 450 nm, which lies in the blue region and this is consistent with the experimentally used blue printing ink. It is also observed that while all these curves show similar shapes, they also show slight differences in the absolute values of R %, which indicate certain differences in the structure and porosity or roughness of the surface of the films formed at various anodizing temperatures.
The influence of the different acidic solutions (P.A. and S.A.) on the printability of anodized aluminium specimens was evaluated by the colour parameters CIEL*a*b* and print density (DCMYK). Anodizing was carried out in both acidic solutions at various temperatures ranging from 10ºC to 40ºC. The specimens were subsequently printed with UV curing ink by digital print and the results are shown in
As can be seen in
uniformities of the ink layer and some uncovered areas are evident. The optical observations are in accordance with the results regarding the colour parameters, which indicate that anodized specimens show very good printability. Thus, in the case of specimens anodized in P.A. at a temperature range of 10ºC - 30ºC, these parameters showed a rather unaltered printability with values in the range of 2.21-2.26 (DC), 2.42-2.46 (DM), 1.94 - 2.01 (DY), and 1.31 - 1.35 (DK). These results are attributed to the fact that the anodic oxides formed in both P.A. and S.A. are of a highly porous and absorptive character, which permits the absorption of the ink and the formation of a uniform ink layer, as is confirmed from the images in
The results of the adhesion tape tests of the digital printed specimens anodized at different temperatures in P.A. and S.A. are presented in
The above findings are in good agreement with the values of the colour parameters
Anodizing T (ºC) | P.A. anodization R.A. % | S.A. anodization R.A. % |
---|---|---|
Non-anodized | 89.0 (0 B) | 89.0 (0 B) |
10 | 60.0 (1 B) | 90.1 (0 B) |
20 | 14.9 (3 B) | 91.1 (0 B) |
30 | 2.7 (4 B) | 2.3 (4 B) |
40 | 0.2 (4 B) | 0.0 (5 B) |
DCMYK, and especially of the Dc parameter, which is representative of the colour of the blue ink used in the case of specimens anodized at various temperatures in S.A. and printed by the digital-inkjet method, which are presented in
Considering the specimens anodized in P.A. and printed by screen print (the results are presented in
area covered by such aggregates was 2.3% for the specimen anodised at 30˚C, while for the other temperatures was in the range of 2.6% - 3.6%. However, specimens anodised for 3 min rather than 5 min showed lower values of density with the optimum value of 0.4 spots/mm2 in the case of anodized specimen at 30˚C while the average diameter of spots in all cases remained unchanged. The above results are in agreement with theories predicting that the anodic oxide films formed in phosphoric acid are very thin (about 3 μm), have large diameter pores [
From the above mentioned findings it is concluded that better imprints can be obtained in the case of screen print at anodizing temperatures slightly above room temperature applied for short anodization times. On the other hand, they are in good agreement with results of the adhesion tape tests on specimens anodized in P.A., which are presented in
The above findings are in good agreement with the ones shown in
Finally, in order to investigate further the influence of the anodizing temperature on the printability of the anodized substrate, specimens anodized in S.A. solution at 20ºC and 25ºC were selected, since previous results had shown them to be the most promising. In this case epoxy-catalytic inks were applied on the anodized specimens by the
Anodizing T (ºC) | R.A. % |
---|---|
(Non-anodized) | 0.4 (4B) |
10 | 0.3 (4B) |
20 | 0.3 (4B) |
30 | 0.2 (4B) |
40 | 1.0 (4B) |
screen printing method. Colour parameters CIEL*a*b* determined by spectrophotometry, SEM cross-sectional microphotographs, as well as surface images (X30) of the printed specimens were recorded in order to evaluate the ink layer film on anodized aluminium specimens in the above mentioned conditions and the results are presented in
It should be mentioned that all specimens anodized and printed with the epoxy- based inks applied by screen print method gave excellent adhesion tape test results (R.A. % values of 0.0 - 0.5). They also showed excellent light fastness and fastness against common chemical solvents (water, ethanol, acetone) indicating that the suggested method may be used as an excellent pretreatment process for aluminium surface prior to printing by either digital printing or screen print methods.
Commercially pure aluminum (AA1050) specimens were anodized and printed with
UV curing ink by digital and with epoxy-based inks by screen printing methods. As expected, it was observed that surface properties of anodic oxide films depended on the type of anodizing electrolyte and on anodizing temperature. A suitable modification of the surface of aluminum substrate results in an improvement of the quality and the outdoor resistance of the imprint. Specifically, the weatherability and resistance against chemicals as well as the protection from sunlight were improved.
In the case of P.A. anodization (thin anodic oxide films for internal applications), an increase of the anodizing temperature results in an improvement of print quality (colour parameters and adhesion test) of printed specimens by both methods and materials; in the case of epoxy-based inks applied by screen print method, the best results were obtained in the range of 20ºC - 30ºC. Printed specimens anodized at 30ºC gave also excellent results on print density and adhesion resistance especially in the case of the UV curing inks.
In the case of S.A. anodization (thick anodic films and protective against external effects), a slight increase of the electrolyte concentration or the anodizing temperature results in an improvement of print quality of specimens (colour parameters, adhesion and light fastness test, resistance against chemicals), especially when epoxy-based inks are applied by the screen print method. In the case of the UV curing ink printed specimens anodized at 30ºC gave also excellent results concerning the print density and the adhesion resistance.
Pretreatment of aluminium substrates under the conditions described in the present paper is expected to advance the use of the studied procedure, in order to improve the quality of the imprints, especially for outdoor and other demanding applications. The scientific outcomes may thus be combined with practical applications in the field of printing technology. As anodizing turns out to be an excellent treatment of aluminium surface prior to printing, especially in the case of specific applications (barcodes, labelling, panels, etc.) or when excellent adhesion, resistance properties and quality of printed specimens are required, clarification of the anodizing conditions would be of interest for further investigation. The influence of anodizing parameters combined with the various types of inks and printing techniques on print quality would be of great academic and commercial interest.
This research is funded by the Special Account for Research Grants of the TEI of Athens, in the framework of the Internal Programme for the Support of the TEI of Athens Researchers, for 2015. Authors would like to acknowledge Mrs. P. Gioti and Mr. A. Tsigonias for anodizing and printing of aluminium specimens, as well as A. Kordas for image processing at laboratories of TEI of Athens, Mrs. A. Mihailidou and Mr. D. Mantis for light fastness and adhesion tests of printed specimens at laboratories of Druckfarben Hellas SA.
Theohari, S., Iakovidis, I., Karampotsos, A. and Sianoudis, I. (2016) Printing on Anodized Aluminium Surface. Open Journal of Applied Sciences, 6, 783-795. http://dx.doi.org/10.4236/ojapps.2016.611069