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Application of laser ablation technique for removal of chemically inert organically modified silicate coatings
Progress in Organic Coatings 46 (2003) 250–258
Application of laser ablation technique for removal of chemically inert organically modified silicate coatings Tammy L. Metroke∗ , Elvira Stesikova, Kai Dou, Edward T. Knobbe Department of Chemistry, Environmental Institute, University Center for Laser and Photonics Research, Oklahoma State University, Stillwater, OK 74078, USA Received 10 December 2001; received in revised form 10 September 2002; accepted 7 November 2002
Abstract Removal of organically modified silicate (Ormosil) coatings from 2024-T3 aluminum alloy substrates has been investigated using a laser ablation technique utilizing a 308 nm excimer laser. Incorporation of UV-absorbing dye molecules containing λmax in the vicinity of the laser wavelength (butyl-PBD, Furan 2, Morin, HQSA) into the Ormosil thin film was found to facilitate coating removal. Ormosil thin films containing 0.1–0.5 mol% UV-absorbing dye molecules were subjected to laser treatment at various fluences ranging from 0.2 to 0.6 J/cm2 . The presence of the UV-absorbing dye molecules in the Ormosil thin film was found to facilitate coating removal at a lower fluence as compared to dye-free coatings as determined by scanning electron microscopy. The effectiveness of coating removal was found to depend on several parameters including laser fluence, number of pulses per spot, and dye concentration. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation technique; Ormosil; 2024-T3 aluminum alloy
1. Introduction Organic coating (OC) removal has been a topic of interest in recent years, due to increasing restrictions associated with the use of organic solvents in these processes. Traditionally, removal of organic polymer paint systems requires the use of abrasives or solvents. The former often results in damage to the metal substrate; the latter often utilizes hazardous or controlled-emission chemical agents, such as methylene chloride. In recent years, various coating removal technologies have been developed in an effort to produce efficient OC stripping without the generation of hazardous waste materials. For example, various dry media blasting procedures (dry ice, sodium bicarbonate, plastic media, wheat starch, etc.) have been found to be highly effective for OC removal from aluminum alloys commonly used in aerospace applications [1]. These methods are capable of broad-based paint stripping applications; generally, high-performance coatings may be removed while generating limited amounts of dry residue. Alternatively, cleaning methods such as coating pyrolysis [2], atmospheric-pressure pulsed-corona plasma [3], TEA-CO2 laser ablation [4], and UV laser irradiation [5] have also been investigated for OC ∗ Corresponding author. Tel.: +1-405-744-7316; fax: +1-405-744-7673. E-mail address: [email protected] (T.L. Metroke).
removal. The latter has been applied for cleaning of optical surfaces [6] and the removal of thin chromium films from glass substrates [7]. While these coating removal methods do not involve the use of solvents or generate large quantities of hazardous waste, they are limited in terms of efficiency and large-area processing capabilities. Organically modified silicate (Ormosil) coatings [8] have been applied to various substrates and investigated for a diverse range of high-performance applications including enhancement of abrasion resistance, antifogging, and corrosion resistance properties of the underlying materials [9]. Ormosil coatings are hybrid organic–inorganic materials typically produced through the hydrolysis and condensation of organically modified silanes with traditional alkoxide precursors [10]. Recent studies have shown that Ormosil films provide good corrosion protection as they form a dense, mechanically stable, chemically inert barrier layer on the metal surface [11]. Silane-based coatings are fundamentally different than OCs, due to the potential for these coatings to form covalent Si–O–Al bonds through silanol condensation reactions with the oxide layer on the aluminum alloy (AA) surface. The presence of these bonds produces coatings with high adhesion to metal surfaces. Adhesion of the Ormosil coating to AA substrates can exceed 2000 psi as reported previously [11d]. While partially responsible for the excellent performance characteristics of Ormosil coatings, high
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 1 9 5 - 9
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Fig. 1. Representation of Ormosil structures produced by direct incorporation of UV-absorbing dye molecules. Representation is not drawn to scale.
adhesion properties make removal of the high-performance coatings from the underlying metal difficult using common paint or coating stripping techniques. In this work, excimer laser irradiation was used to remove Ormosil coatings from 2024-T3 AA substrates. In order to make the coating materials more sensitive to the irradiation, UV-absorbing dye molecules were incorporated into the Ormosil film. The results of irradiation experiments revealed that UV-absorbing dye incorporation facilitates laser-induced coating removal. Effectiveness of coating removal was found to depend on laser fluence, number of pulses per spot, and UV-absorbing dye concentration.
coupons were wiped with hexanes and methanol. Subsequently, the substrates were soaked in aerated Oakite-164 alkaline cleaner1 solution for 10–15 min at 65 ◦ C and then in an acid-based Turco Smut-Go deoxidizing solution2 for 7–10 min at 25 ◦ C under rigorous air agitation. Each of these two steps was followed by thorough rinsing using tap water for 2 min. Ormosil films were dip coated onto polished 2024-T3 AA substrates using a speed-controlled dip coating apparatus with a withdrawal speed of approximately 4 cm/min. The coated AA substrates were allowed to dry at ambient conditions for at least 24 h prior to their characterization. 2.4. Characterization methods
2. Experimental 2.1. Materials Silicate precursors tetraethyl orthosilicate, TEOS, and 3glycidoxypropyl trimethoxysilane, GLYMO, were purchased from Gelest, Inc. or Aldrich. Laser dyes 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazol (butyl-PBD) and 2-(4-biphenylyl)-6-phenylbenzoxazotetrasulfonic acid potassium salt (Furan 2) were products of Lambda Physik GmbH, Germany. 2 ,3 ,4 ,5,7-Pentahydroxyflavone (Morin hydrate) and 8-hydroxyquinoline-5-sulfonic acid hydrate (HQSA) were obtained from Aldrich. All chemicals were used as received without further purification. 2.2. Preparation of Ormosils Ormosils were prepared by mixing 27.9 ml TEOS, 9.2 ml GLYMO, and 11.5 ml 0.05 M HNO3 . The mixture was allowed to stir for 1 h prior to the addition of 0.1–0.5 mol% UV-absorbing compound. 2.3. Coating techniques 2024-T3 AA substrates were freshly degreased and deoxidized using the following cleaning process. First, test
UV absorption spectra were acquired using a Fluoromax 3 UV spectrophotometer. Spectra were obtained under the following conditions: Morin (molarity (M) = 1.6 × 10−4 in ethanol); HQSA (M = 1.6 × 10−4 in water); butyl-PBD (M = 1.1×10−4 in propylene carbonate); Furan 2 (M = 9× 10−5 in water). Silanes and sols were analyzed as 10 vol.% solutions in ethanol. In order to assess the molar absorptivity (extinction coefficient) of Morin, solutions were prepared with concentrations ranging from 6 × 10−5 to 0.5 × 10−6 mol/l. A spectrum of each solution was acquired using pure ethanol solvent as a blank. 2.5. Laser irradiation Laser irradiation experiments were performed using a Lambda Physik Lextra 200i XeCl laser emitting 308 nm radiation with a pulse width of 15 ns. The laser output energy was measured using a Molectron J50 joulemeter probe in combination with a Tecktronic TDS 640A oscilloscope. The laser beam was focused onto the Ormosil coated 2024-T3 AA sample using a fused silica lens (focal length = 30 cm). 1 Supplied by Oakite Products, Inc., P.O. Box 602, 50 Valley Road, Berkeley Heights, NJ 07922, USA. 2 Supplied by Henkel Surface Technologies, 32100 Stephenson Hwy, Madison Heights, MI 48071, USA.
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A XY translation stage was used in order to vary the laser irradiation spot on the sample. Each sample was irradiated normal to the substrate, with each test specimen receiving various numbers of pulses per site ranging from 3 to 500. Variable power outputs with a repetition rate of 10 Hz were used to produce irradiation fluences varying from 0.2 to 0.8 J/cm2 , below the reported ablation threshold of 1.2 J/cm2 for 2024-T3 AA [12]. SEM images of pre- and post-irradiated GLYMO-TEOS Ormosil films were obtained using a Leica-Cambridge Stereoscan 360FE electron microscope operating at 20 kV with a working distance of 10 mm.
3. Results and discussion In the present study, excimer laser-induced coating removal was investigated for Ormosil coatings that were either doped or not doped with UV-absorbing dye molecules. UV-absorbing Ormosil coatings were prepared by direct incorporation of UV-absorbing dyes into the Ormosil sol prior to thin film deposition; this incorporation method is expected to produce films containing UV-absorbing dye molecules randomly dispersed in the Ormosil matrix. In order to maximize absorption characteristics, the specific dyes investigated during this study were chosen based on their characteristic absorptions in the vicinity of 308 nm, the wavelength of the laser used. A schematic representation of this coating system is shown in Fig. 1. Laser fluence, UV-absorbing dye concentration, and number of pulses per spot were investigated as variables affecting Ormosil coating removal. 3.1. Laser irradiation of non-doped GLYMO-TEOS Ormosil films In order to determine the baseline absorption characteristics of the Ormosil film, UV absorption spectra of TEOS, GLYMO, and a GLYMO-TEOS sol were collected. These solutions exhibited no significant absorptions in the region of interest (300–600 nm), indicating that the hybrid Ormosil films derived from them are generally transparent to UV irradiation at these wavelengths. As a control, hybrid Ormosil coatings that do not contain any UV-absorbing dye molecules were irradiated with a 308 nm excimer laser. SEM images of laser ablated GLYMO-TEOS Ormosil films (10 pulses per spot) free of any UV-absorbing groups are shown in Fig. 2. The results reveal insignificant film damage at 0.2 J/cm2 , more extensive film cracking and evidence of limited delamination at 0.4 J/cm2 , and complete film removal at 0.6 J/cm2 and above. The underlying AA substrate appeared to be undamaged by the laser irradiation at these fluences. These results suggest that the observed laser-induced film removal cannot be explained by characteristic UV absorption of the Ormosil, but rather may be attributed to the secondary effects caused by the laser treatment.
Fig. 2. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.4 J/cm2 , (C) 0.6 J/cm2 and (D) 0.8 J/cm2 . All images are shown at 200× magnification.
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Fig. 3. Structures of UV-absorbing dye molecules investigated.
Table 1 Pertinent UV absorption characteristics of selected compounds Compound
Solvent
Reported λmax (nm)
Observed λmax (nm)
Extinction coefficient (l/mol cm)
Butyl-PBD Furan 2 Morin
Propylene carbonate H2 O Ethanol
HQSA
H2 O
302 [13] 330 [13] 265 390 – –
306 330 260 360 250 287
4.35 × 104 4.79 × 104 3.85 × 104 2.61 × 104 – –
Fig. 4. UV–Vis absorbance spectra of solutions of: (a) Morin; (b) Furan 2; (c) butyl-PBD; (d) HQSA.
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3.2. Laser irradiation of GLYMO-TEOS Ormosil films containing UV-absorbing dye molecules In order to increase the efficiency of the laser-induced film removal, UV-absorbing dye molecules were incorporated into the GLYMO-TEOS Ormosil during the sol preparation stage by direct addition of the UV-absorbing dye into the reaction mixture. Various water or ethanol soluble compounds, exhibiting maximum UV absorption around 308 nm, i.e. the wavelength of the laser beam used, were investigated during this study. Molecular structures of dopant molecules are shown in Fig. 3. Pertinent absorption characteristics of the selected dye molecules are summarized in Table 1. Absorbance spectra of dye molecules under investigation are shown in Fig. 4. The presence of the UV-absorbing dye in the Ormosil produced absorption bands that substantially enhanced absorptivity at selected wavelengths. As a qualitative example of coating removal, Fig. 5 shows images of HQSA and Morin-doped Ormosils under a UV light. Areas that are Ormosil coated, fluoresce due to the presence of dye molecules in the coating. Non-coated areas, in which the coating has been removed by excimer laser irradiation, appear dark. SEM images of laser ablated GLYMO-TEOS Ormosil films containing butyl-PBD, Furan 2, and Morin are shown in Figs. 6–8, respectively. Fig. 6 depicts SEM images of GLYMO-TEOS Ormosil films containing 0.1 mol% butyl-PBD subjected to laser treatment at various laser fluences ranging from 0.2 to 0.6 J/cm2 using 10 pulses per spot. The SEM images of pre-irradiated Ormosil films were featureless and crack-free. At the lowest applied laser fluence, 0.2 J/cm2 , film damage in the form of partial film removal was observed. Laser treatment at 0.3 J/cm2 results in complete film removal. These observations suggest that UV-absorbing dye incorporation substantially enhances coating removal, since the presence of the dye molecules in the Ormosil films facilitates coating removal at a much lower laser fluence, as compared to the dye-free coatings depicted in Fig. 2. Similar behavior was observed for the laser treated GLYMO-TEOS Ormosil films containing 0.1 mol% Furan 2, which appeared to be considerably more damaged at 0.2 J/cm2 and completely removed at 0.3 J/cm2 , as shown in Fig. 7A and B, respectively. Fig. 8 depicts SEM images of GLYMO-TEOS Ormosil films containing 0.1 mol% Morin subjected to laser treatment at various laser fluences ranging from 0.2 to 0.6 J/cm2 using 10 pulses per spot. The presence of Morin in the Ormosil film appears to have only a marginal effect on film cracking and removal, as compared to the dye-free GLYMO-TEOS Ormosil. Only minimal enhancement of the laser stripping can be observed in these systems, as there is slightly more extensive film damage at 0.4 J/cm2 as compared to the non-doped GLYMO-TEOS Ormosil. Note that both butyl-PBD and Furan 2 result in complete removal of the films under these conditions, suggesting their higher effectiveness for facilitating coating removal.
Fig. 5. Images of: (a) HQSA- and (b) Morin-doped Ormosil films under a UV light. Areas which are Ormosil coated fluoresce due to the presence of UV-absorbing molecules in the coating. Non-coated areas, in which the coating has been removed by excimer laser irradiation, appear dark.
These results suggest that, for the same dye concentration in the Ormosil, 0.1 mol%, butyl-PBD and Furan 2 enhance laser-induced stripping of the film more effectively than Morin. This can be explained by the absorption characteristics of these molecules as listed in Table 1. Thus, the relatively low efficiency of Morin can be attributed to the fact that both its absorption maxima (265 and 390 nm) occur comparatively far away from the 308 nm laser emission wavelength. The measured extinction coefficient of Morin
T.L. Metroke et al. / Progress in Organic Coatings 46 (2003) 250–258
Fig. 6. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing 0.1 mol% of butyl-PBD after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.3 J/cm2 , (C) 0.4 J/cm2 and (D) 0.6 J/cm2 . All images are shown at 200× magnification.
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Fig. 7. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing 0.1 mol% of Furan 2 after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.3 J/cm2 , (C) 0.4 J/cm2 and (D) 0.6 J/cm2 . All images are shown at 200× magnification.
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Fig. 8. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing (I) 0.1 and (II) 0.5 mol% of Morin after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.4 J/cm2 , and (C) 0.6 J/cm2 . All images are shown at 200× magnification.
in ethanol solution at 308 nm is also low compared to the other dye molecules. Another aspect of interest in this study was to investigate the effect of the UV-absorbing dye concentration and number of pulses per spot on the efficiency of laser-induced film stripping. Fig. 8 shows the SEM images of results of laser treatment for the GLYMO-TEOS Ormosil films containing 0.1 and 0.5 mol% Morin in the sol–gel matrix. Whereas complete film removal is observed for both concentrations at the laser fluence of 0.6 J/cm2 , noticeable changes are observed at lower laser fluences. When all other factors are kept constant, the higher concentration of Morin in the Ormosil film yields more extensive film damage. Thus, at 0.4 J/cm2 , partial film removal is observed on approximately 60% of the total area
for the 0.1% Morin, and on approximately 80% of the total area for the 0.5% Morin in the Ormosil, respectively. This indicates that efficiency of the laser-induced UV-absorbing dye-assisted film removal is concentration dependent. Coating removal effectiveness was also investigated as a function of the number of pulses per spot. Fig. 9 depicts SEM images of GLYMO-TEOS Ormosil films containing 0.1 mol% Morin subjected to the laser treatment at 0.3 J/cm2 using various numbers of pulses per site. The results indicate that film damage is increased with augmentation of the pulse number. Thus, laser treatment produced after 10 pulses yields film cracking and insignificant removal of the coating (Fig. 9A), whereas after 100 pulses, a majority of the film is removed (Fig. 9B). Further progress in the film stripping
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pulses per site. The efficiency of this process can be estimated as an Ormosil coating removal rate of approximately 2 ft2 /min using commercially available lasers. The excimer laser-induced coating removal technique is able to remove the Ormosil coating without damaging the underlying metal substrate. Additionally, the laser stripping process does not rely on the use of organic solvents, thereby eliminating the generation of hazardous waste.
Acknowledgements The authors are grateful to Dr. Robert Parkhill and Mr. Steve Coleman for assistance in acquiring SEM images and Mr. Robert Ascio for sample preparation. This research was supported in part by Defense Logistics Agency and Air Force Office of Scientific Research whose contributions are gratefully acknowledged. References
Fig. 9. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing 0.1 mol% of Morin after irradiation using a 308 nm excimer laser, 0.3 J/cm2 laser fluence with various number of pulses: (A) 10, (B) 100, (C) 500. All images are shown at 200× magnification.
is observed after 500 pulses (Fig. 9C), though at this point, the coating is removed.
4. Conclusions The results of this study indicate that ORMOSIL coatings containing UV-absorbing dye molecules (butyl-PBD, Morin, HQSA, Furan 2) can be removed effectively using a laser-induced stripping technique. The presence of the UV-absorbing dye molecules was found to facilitate coating removal. The extent of the film removal depends strongly on laser fluence, dye concentration and number of laser
[1] K.E. Abbot, Dry media blasting for the removal of paint coatings on aerospace surfaces, Met. Finish. 33 (1996). [2] D.L. Lloyd, US Patent 5 571 335 (1996). [3] T. Yamamoto, J.R. Newsome, D.S. Ensor, Modification of surface energy, dry etching, and organic film removal using atmospheric-pressure pulsed-corona plasma, IEEE Trans. Ind. Appl. 31 (1995) 494. [4] C.A. Cottam, D.C. Emmony, A. Cuesta, R.H. Bradley, XPS-monitoring of the removal of an aged polymer coating from a metal substrate by TEA-CO2 laser ablation, J. Mater. Sci. 33 (1998) 3245. [5] Y.F. Lu, Y. Aoyagi, Jpn. J. Appl. Phys. 33 (1994) 430. [6] K. Mann, B. Wolff-Rottke, F. Muller, Cleaning of optical surfaces by excimer laser radiation, Appl. Surf. Sci. 96–98 (1996) 463. [7] S.K. Lee, K.K. Yoon, K.H. Whang, S.J. Na, Excimer laser ablation removal of thin chromium films form glass substrates, Surf. Coat. Technol. 113 (1999) 63. [8] H. Schmidt, New type of non-crystalline solids between inorganic and organic materials, J. Non-Cryst. Solids 73 (1985) 681. [9] (a) K.H. Haas, S. Amberg-Schwab, K. Rose, Functionalized coating materials based on inorganic–organic polymers, Thin Solid Films 351 (1999) 198; (b) J.D. Mackenzie, E.P. Bescher, Structures, properties, and potential applications of Ormosils, J. Sol–Gel Sci. Technol. 13 (1998) 371; (c) R. Kasemann, H. Schmidt, Coatings for mechanical and chemical protection based on organic–inorganic sol–gel nanocomposites, New. J. Chem. 18 (1994) 1117; (d) H. Schmidt, H. Wolter, Organically modified ceramics and their applications, J. Non-Cryst. Solids 121 (1990) 428; (e) H. Schmidt, Multifunctional inorganic–organic composite sol–gel coatings for glass surfaces, J. Non-Cryst. Solids 178 (1994) 302. [10] (a) C.J. Brinker, J.G.W. Scherer (Eds.), Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, Boston, 1990, pp. 787–790; (b) S. Sakka, K. Kamiya, J. Non-Cryst. Solids 42 (1980) 403; (c) J.D. Mackenzie, Structures and properties of Ormosils, J. Sol–Gel Sci. Technol. 1 (1994) 81. [11] (a) M. Atik, P. Lima Neto, L.A. Avaca, M.A. Aegerter, Sol–gel thin films for corrosion protection, Ceram. Int. 21 (1995) 403; (b) T.L. Metroke, R.L. Parkhill, E.T. Knobbe, Synthesis of hybrid organic–inorganic sol–gel coatings for corrosion resistance, Mater. Res. Soc. Symp. Proc. 576 (1999) 293;
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T.L. Metroke et al. / Progress in Organic Coatings 46 (2003) 250–258 (c) M. Guglielmi, Sol–gel coatings on metals, J. Sol–Gel Sci. Technol. 8 (1997) 443; (d) T.L. Metroke, R.L. Parkhill, E.T. Knobbe, Passivation of metal alloys using sol–gel-derived materials—a review, Prog. Org. Coat. 41 (2001) 233; (e) L.B. Reynolds, R. Twite, M. Khobaib, M.S. Donley, G.P. Bierwagen, Preliminary evaluation of the anticorrosive properties of aircraft coatings by electrochemical methods, Prog. Org. Coat. 32 (1997) 31; (f) R.L. Parkhill, E.T. Knobbe, M.S. Donley, Application and evaluation of environmentally compliant spray-coated Ormosil films as corrosion resistant treatments for aluminum 2024-T3, Prog. Org. Coat. 41 (2001) 261; (g) N.N. Voevdin, N.T. Grebasch, W.S. Soto, L.S. Kasten, J.T. Grant, F.E. Arnold, M.S. Donley, An organically modified zirconate film
as a corrosion-resistant treatment for aluminum 2024-T3, Prog. Org. Coat. 41 (2001) 287. [12] (a) R.L. Parkhill, E.T. Knobbe, Surface texturing of aluminum alloy 2024 Via excimer laser irradiation, SPIE Proc. 2992 (1997) 81; (b) D. Follstaedt, S. Picraux, S. Peercy, W. Wampler, Slip deformation and melt threshold in laser pulsed irradiated aluminum, Appl. Phys. Lett. 39 (1981) 327; (c) P. Peercy, in: C. Draper, P. Mazzoldi (Eds.), Solidification Dynamics and Microstructure of Metals in Pulsed Laser Irradiation, Laser Surface Pretreatment of Metals, 1986, pp. 57–58; (d) D. Follstaedt, Metallurgy and microstructure of Polse melted alloys, Mater. Res. Soc. Symp. Proc. 4 (1982) 377. [13] U. Brackmann, Lambdachrome Laser Dyes Data Sheets, 2nd Revised Edition, Lambda Physik GmbH, Germany, 1994.
Application of laser ablation technique for removal of chemically inert organically modified silicate coatings Tammy L. Metroke∗ , Elvira Stesikova, Kai Dou, Edward T. Knobbe Department of Chemistry, Environmental Institute, University Center for Laser and Photonics Research, Oklahoma State University, Stillwater, OK 74078, USA Received 10 December 2001; received in revised form 10 September 2002; accepted 7 November 2002
Abstract Removal of organically modified silicate (Ormosil) coatings from 2024-T3 aluminum alloy substrates has been investigated using a laser ablation technique utilizing a 308 nm excimer laser. Incorporation of UV-absorbing dye molecules containing λmax in the vicinity of the laser wavelength (butyl-PBD, Furan 2, Morin, HQSA) into the Ormosil thin film was found to facilitate coating removal. Ormosil thin films containing 0.1–0.5 mol% UV-absorbing dye molecules were subjected to laser treatment at various fluences ranging from 0.2 to 0.6 J/cm2 . The presence of the UV-absorbing dye molecules in the Ormosil thin film was found to facilitate coating removal at a lower fluence as compared to dye-free coatings as determined by scanning electron microscopy. The effectiveness of coating removal was found to depend on several parameters including laser fluence, number of pulses per spot, and dye concentration. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation technique; Ormosil; 2024-T3 aluminum alloy
1. Introduction Organic coating (OC) removal has been a topic of interest in recent years, due to increasing restrictions associated with the use of organic solvents in these processes. Traditionally, removal of organic polymer paint systems requires the use of abrasives or solvents. The former often results in damage to the metal substrate; the latter often utilizes hazardous or controlled-emission chemical agents, such as methylene chloride. In recent years, various coating removal technologies have been developed in an effort to produce efficient OC stripping without the generation of hazardous waste materials. For example, various dry media blasting procedures (dry ice, sodium bicarbonate, plastic media, wheat starch, etc.) have been found to be highly effective for OC removal from aluminum alloys commonly used in aerospace applications [1]. These methods are capable of broad-based paint stripping applications; generally, high-performance coatings may be removed while generating limited amounts of dry residue. Alternatively, cleaning methods such as coating pyrolysis [2], atmospheric-pressure pulsed-corona plasma [3], TEA-CO2 laser ablation [4], and UV laser irradiation [5] have also been investigated for OC ∗ Corresponding author. Tel.: +1-405-744-7316; fax: +1-405-744-7673. E-mail address: [email protected] (T.L. Metroke).
removal. The latter has been applied for cleaning of optical surfaces [6] and the removal of thin chromium films from glass substrates [7]. While these coating removal methods do not involve the use of solvents or generate large quantities of hazardous waste, they are limited in terms of efficiency and large-area processing capabilities. Organically modified silicate (Ormosil) coatings [8] have been applied to various substrates and investigated for a diverse range of high-performance applications including enhancement of abrasion resistance, antifogging, and corrosion resistance properties of the underlying materials [9]. Ormosil coatings are hybrid organic–inorganic materials typically produced through the hydrolysis and condensation of organically modified silanes with traditional alkoxide precursors [10]. Recent studies have shown that Ormosil films provide good corrosion protection as they form a dense, mechanically stable, chemically inert barrier layer on the metal surface [11]. Silane-based coatings are fundamentally different than OCs, due to the potential for these coatings to form covalent Si–O–Al bonds through silanol condensation reactions with the oxide layer on the aluminum alloy (AA) surface. The presence of these bonds produces coatings with high adhesion to metal surfaces. Adhesion of the Ormosil coating to AA substrates can exceed 2000 psi as reported previously [11d]. While partially responsible for the excellent performance characteristics of Ormosil coatings, high
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 1 9 5 - 9
T.L. Metroke et al. / Progress in Organic Coatings 46 (2003) 250–258
251
Fig. 1. Representation of Ormosil structures produced by direct incorporation of UV-absorbing dye molecules. Representation is not drawn to scale.
adhesion properties make removal of the high-performance coatings from the underlying metal difficult using common paint or coating stripping techniques. In this work, excimer laser irradiation was used to remove Ormosil coatings from 2024-T3 AA substrates. In order to make the coating materials more sensitive to the irradiation, UV-absorbing dye molecules were incorporated into the Ormosil film. The results of irradiation experiments revealed that UV-absorbing dye incorporation facilitates laser-induced coating removal. Effectiveness of coating removal was found to depend on laser fluence, number of pulses per spot, and UV-absorbing dye concentration.
coupons were wiped with hexanes and methanol. Subsequently, the substrates were soaked in aerated Oakite-164 alkaline cleaner1 solution for 10–15 min at 65 ◦ C and then in an acid-based Turco Smut-Go deoxidizing solution2 for 7–10 min at 25 ◦ C under rigorous air agitation. Each of these two steps was followed by thorough rinsing using tap water for 2 min. Ormosil films were dip coated onto polished 2024-T3 AA substrates using a speed-controlled dip coating apparatus with a withdrawal speed of approximately 4 cm/min. The coated AA substrates were allowed to dry at ambient conditions for at least 24 h prior to their characterization. 2.4. Characterization methods
2. Experimental 2.1. Materials Silicate precursors tetraethyl orthosilicate, TEOS, and 3glycidoxypropyl trimethoxysilane, GLYMO, were purchased from Gelest, Inc. or Aldrich. Laser dyes 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazol (butyl-PBD) and 2-(4-biphenylyl)-6-phenylbenzoxazotetrasulfonic acid potassium salt (Furan 2) were products of Lambda Physik GmbH, Germany. 2 ,3 ,4 ,5,7-Pentahydroxyflavone (Morin hydrate) and 8-hydroxyquinoline-5-sulfonic acid hydrate (HQSA) were obtained from Aldrich. All chemicals were used as received without further purification. 2.2. Preparation of Ormosils Ormosils were prepared by mixing 27.9 ml TEOS, 9.2 ml GLYMO, and 11.5 ml 0.05 M HNO3 . The mixture was allowed to stir for 1 h prior to the addition of 0.1–0.5 mol% UV-absorbing compound. 2.3. Coating techniques 2024-T3 AA substrates were freshly degreased and deoxidized using the following cleaning process. First, test
UV absorption spectra were acquired using a Fluoromax 3 UV spectrophotometer. Spectra were obtained under the following conditions: Morin (molarity (M) = 1.6 × 10−4 in ethanol); HQSA (M = 1.6 × 10−4 in water); butyl-PBD (M = 1.1×10−4 in propylene carbonate); Furan 2 (M = 9× 10−5 in water). Silanes and sols were analyzed as 10 vol.% solutions in ethanol. In order to assess the molar absorptivity (extinction coefficient) of Morin, solutions were prepared with concentrations ranging from 6 × 10−5 to 0.5 × 10−6 mol/l. A spectrum of each solution was acquired using pure ethanol solvent as a blank. 2.5. Laser irradiation Laser irradiation experiments were performed using a Lambda Physik Lextra 200i XeCl laser emitting 308 nm radiation with a pulse width of 15 ns. The laser output energy was measured using a Molectron J50 joulemeter probe in combination with a Tecktronic TDS 640A oscilloscope. The laser beam was focused onto the Ormosil coated 2024-T3 AA sample using a fused silica lens (focal length = 30 cm). 1 Supplied by Oakite Products, Inc., P.O. Box 602, 50 Valley Road, Berkeley Heights, NJ 07922, USA. 2 Supplied by Henkel Surface Technologies, 32100 Stephenson Hwy, Madison Heights, MI 48071, USA.
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A XY translation stage was used in order to vary the laser irradiation spot on the sample. Each sample was irradiated normal to the substrate, with each test specimen receiving various numbers of pulses per site ranging from 3 to 500. Variable power outputs with a repetition rate of 10 Hz were used to produce irradiation fluences varying from 0.2 to 0.8 J/cm2 , below the reported ablation threshold of 1.2 J/cm2 for 2024-T3 AA [12]. SEM images of pre- and post-irradiated GLYMO-TEOS Ormosil films were obtained using a Leica-Cambridge Stereoscan 360FE electron microscope operating at 20 kV with a working distance of 10 mm.
3. Results and discussion In the present study, excimer laser-induced coating removal was investigated for Ormosil coatings that were either doped or not doped with UV-absorbing dye molecules. UV-absorbing Ormosil coatings were prepared by direct incorporation of UV-absorbing dyes into the Ormosil sol prior to thin film deposition; this incorporation method is expected to produce films containing UV-absorbing dye molecules randomly dispersed in the Ormosil matrix. In order to maximize absorption characteristics, the specific dyes investigated during this study were chosen based on their characteristic absorptions in the vicinity of 308 nm, the wavelength of the laser used. A schematic representation of this coating system is shown in Fig. 1. Laser fluence, UV-absorbing dye concentration, and number of pulses per spot were investigated as variables affecting Ormosil coating removal. 3.1. Laser irradiation of non-doped GLYMO-TEOS Ormosil films In order to determine the baseline absorption characteristics of the Ormosil film, UV absorption spectra of TEOS, GLYMO, and a GLYMO-TEOS sol were collected. These solutions exhibited no significant absorptions in the region of interest (300–600 nm), indicating that the hybrid Ormosil films derived from them are generally transparent to UV irradiation at these wavelengths. As a control, hybrid Ormosil coatings that do not contain any UV-absorbing dye molecules were irradiated with a 308 nm excimer laser. SEM images of laser ablated GLYMO-TEOS Ormosil films (10 pulses per spot) free of any UV-absorbing groups are shown in Fig. 2. The results reveal insignificant film damage at 0.2 J/cm2 , more extensive film cracking and evidence of limited delamination at 0.4 J/cm2 , and complete film removal at 0.6 J/cm2 and above. The underlying AA substrate appeared to be undamaged by the laser irradiation at these fluences. These results suggest that the observed laser-induced film removal cannot be explained by characteristic UV absorption of the Ormosil, but rather may be attributed to the secondary effects caused by the laser treatment.
Fig. 2. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.4 J/cm2 , (C) 0.6 J/cm2 and (D) 0.8 J/cm2 . All images are shown at 200× magnification.
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Fig. 3. Structures of UV-absorbing dye molecules investigated.
Table 1 Pertinent UV absorption characteristics of selected compounds Compound
Solvent
Reported λmax (nm)
Observed λmax (nm)
Extinction coefficient (l/mol cm)
Butyl-PBD Furan 2 Morin
Propylene carbonate H2 O Ethanol
HQSA
H2 O
302 [13] 330 [13] 265 390 – –
306 330 260 360 250 287
4.35 × 104 4.79 × 104 3.85 × 104 2.61 × 104 – –
Fig. 4. UV–Vis absorbance spectra of solutions of: (a) Morin; (b) Furan 2; (c) butyl-PBD; (d) HQSA.
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3.2. Laser irradiation of GLYMO-TEOS Ormosil films containing UV-absorbing dye molecules In order to increase the efficiency of the laser-induced film removal, UV-absorbing dye molecules were incorporated into the GLYMO-TEOS Ormosil during the sol preparation stage by direct addition of the UV-absorbing dye into the reaction mixture. Various water or ethanol soluble compounds, exhibiting maximum UV absorption around 308 nm, i.e. the wavelength of the laser beam used, were investigated during this study. Molecular structures of dopant molecules are shown in Fig. 3. Pertinent absorption characteristics of the selected dye molecules are summarized in Table 1. Absorbance spectra of dye molecules under investigation are shown in Fig. 4. The presence of the UV-absorbing dye in the Ormosil produced absorption bands that substantially enhanced absorptivity at selected wavelengths. As a qualitative example of coating removal, Fig. 5 shows images of HQSA and Morin-doped Ormosils under a UV light. Areas that are Ormosil coated, fluoresce due to the presence of dye molecules in the coating. Non-coated areas, in which the coating has been removed by excimer laser irradiation, appear dark. SEM images of laser ablated GLYMO-TEOS Ormosil films containing butyl-PBD, Furan 2, and Morin are shown in Figs. 6–8, respectively. Fig. 6 depicts SEM images of GLYMO-TEOS Ormosil films containing 0.1 mol% butyl-PBD subjected to laser treatment at various laser fluences ranging from 0.2 to 0.6 J/cm2 using 10 pulses per spot. The SEM images of pre-irradiated Ormosil films were featureless and crack-free. At the lowest applied laser fluence, 0.2 J/cm2 , film damage in the form of partial film removal was observed. Laser treatment at 0.3 J/cm2 results in complete film removal. These observations suggest that UV-absorbing dye incorporation substantially enhances coating removal, since the presence of the dye molecules in the Ormosil films facilitates coating removal at a much lower laser fluence, as compared to the dye-free coatings depicted in Fig. 2. Similar behavior was observed for the laser treated GLYMO-TEOS Ormosil films containing 0.1 mol% Furan 2, which appeared to be considerably more damaged at 0.2 J/cm2 and completely removed at 0.3 J/cm2 , as shown in Fig. 7A and B, respectively. Fig. 8 depicts SEM images of GLYMO-TEOS Ormosil films containing 0.1 mol% Morin subjected to laser treatment at various laser fluences ranging from 0.2 to 0.6 J/cm2 using 10 pulses per spot. The presence of Morin in the Ormosil film appears to have only a marginal effect on film cracking and removal, as compared to the dye-free GLYMO-TEOS Ormosil. Only minimal enhancement of the laser stripping can be observed in these systems, as there is slightly more extensive film damage at 0.4 J/cm2 as compared to the non-doped GLYMO-TEOS Ormosil. Note that both butyl-PBD and Furan 2 result in complete removal of the films under these conditions, suggesting their higher effectiveness for facilitating coating removal.
Fig. 5. Images of: (a) HQSA- and (b) Morin-doped Ormosil films under a UV light. Areas which are Ormosil coated fluoresce due to the presence of UV-absorbing molecules in the coating. Non-coated areas, in which the coating has been removed by excimer laser irradiation, appear dark.
These results suggest that, for the same dye concentration in the Ormosil, 0.1 mol%, butyl-PBD and Furan 2 enhance laser-induced stripping of the film more effectively than Morin. This can be explained by the absorption characteristics of these molecules as listed in Table 1. Thus, the relatively low efficiency of Morin can be attributed to the fact that both its absorption maxima (265 and 390 nm) occur comparatively far away from the 308 nm laser emission wavelength. The measured extinction coefficient of Morin
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Fig. 6. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing 0.1 mol% of butyl-PBD after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.3 J/cm2 , (C) 0.4 J/cm2 and (D) 0.6 J/cm2 . All images are shown at 200× magnification.
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Fig. 7. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing 0.1 mol% of Furan 2 after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.3 J/cm2 , (C) 0.4 J/cm2 and (D) 0.6 J/cm2 . All images are shown at 200× magnification.
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Fig. 8. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing (I) 0.1 and (II) 0.5 mol% of Morin after irradiation using a 308 nm excimer laser, 10 pulses and laser fluence: (A) 0.2 J/cm2 , (B) 0.4 J/cm2 , and (C) 0.6 J/cm2 . All images are shown at 200× magnification.
in ethanol solution at 308 nm is also low compared to the other dye molecules. Another aspect of interest in this study was to investigate the effect of the UV-absorbing dye concentration and number of pulses per spot on the efficiency of laser-induced film stripping. Fig. 8 shows the SEM images of results of laser treatment for the GLYMO-TEOS Ormosil films containing 0.1 and 0.5 mol% Morin in the sol–gel matrix. Whereas complete film removal is observed for both concentrations at the laser fluence of 0.6 J/cm2 , noticeable changes are observed at lower laser fluences. When all other factors are kept constant, the higher concentration of Morin in the Ormosil film yields more extensive film damage. Thus, at 0.4 J/cm2 , partial film removal is observed on approximately 60% of the total area
for the 0.1% Morin, and on approximately 80% of the total area for the 0.5% Morin in the Ormosil, respectively. This indicates that efficiency of the laser-induced UV-absorbing dye-assisted film removal is concentration dependent. Coating removal effectiveness was also investigated as a function of the number of pulses per spot. Fig. 9 depicts SEM images of GLYMO-TEOS Ormosil films containing 0.1 mol% Morin subjected to the laser treatment at 0.3 J/cm2 using various numbers of pulses per site. The results indicate that film damage is increased with augmentation of the pulse number. Thus, laser treatment produced after 10 pulses yields film cracking and insignificant removal of the coating (Fig. 9A), whereas after 100 pulses, a majority of the film is removed (Fig. 9B). Further progress in the film stripping
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pulses per site. The efficiency of this process can be estimated as an Ormosil coating removal rate of approximately 2 ft2 /min using commercially available lasers. The excimer laser-induced coating removal technique is able to remove the Ormosil coating without damaging the underlying metal substrate. Additionally, the laser stripping process does not rely on the use of organic solvents, thereby eliminating the generation of hazardous waste.
Acknowledgements The authors are grateful to Dr. Robert Parkhill and Mr. Steve Coleman for assistance in acquiring SEM images and Mr. Robert Ascio for sample preparation. This research was supported in part by Defense Logistics Agency and Air Force Office of Scientific Research whose contributions are gratefully acknowledged. References
Fig. 9. SEM images of 2024-T3 AA substrates coated with GLYMO-TEOS Ormosil film containing 0.1 mol% of Morin after irradiation using a 308 nm excimer laser, 0.3 J/cm2 laser fluence with various number of pulses: (A) 10, (B) 100, (C) 500. All images are shown at 200× magnification.
is observed after 500 pulses (Fig. 9C), though at this point, the coating is removed.
4. Conclusions The results of this study indicate that ORMOSIL coatings containing UV-absorbing dye molecules (butyl-PBD, Morin, HQSA, Furan 2) can be removed effectively using a laser-induced stripping technique. The presence of the UV-absorbing dye molecules was found to facilitate coating removal. The extent of the film removal depends strongly on laser fluence, dye concentration and number of laser
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