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Treatment of polymers for subsequent metallization using intense UV radiation or plasma at atmospheric pressure
Surface and Coatings Technology 97 (1997) 372–377
Treatment of polymers for subsequent metallization using intense UV radiation or plasma at atmospheric pressure ¨ K. Pochner *, S. Beil, H. Horn, M. Blomer ¨ Fraunhofer-lnstitut fur Lasertechnik, Aachen, Germany
Abstract The activation of polymer surfaces in glow discharges and the deposition of metals from organo-metallic vapours, both at low pressure, are standard laboratory processes. The upscaling to industrial mass product applications is, however, hampered by cost and the time consumption needed for establishing a sufficient vacuum. Atmospheric pressure processes based on the same physical surface interactions show great promise as replacements of some steps in galvanic plating. Successful metallizations are reported after treatment of polymers in barrier discharges at atmospheric pressure. These may be applied directly to the surface of the workpiece or indirectly from within large-area monochromatic excimer UV lamps. Comparisons with excimer UV laser treatment are made. © 1997 Elsevier Science S.A. Keywords: Atmospheric pressure plasma; Dielectric barrier discharge; Metal deposition on plastics; Polymer surface treatment; Ultraviolet irradiation
1. Introduction Gas discharges at atmospheric pressure offer various advantages for surface treatment processes, combining selective photochemical and plasma-chemical reactions with simple large-area devices [1]. The kinetic energy of electrons in the plasma can be utilized for direct surface treatment in open configurations [2], or for the formation of excimers in lamp tubes to obtain monochromatic, high-intensity UV radiation [3,4]. Investigations of the mechanisms involved in the preparation and galvanic metal coating of polymer surfaces have been carried out using dielectric barrier discharges. The basic concepts of the devices used are illustrated in Fig. 1. Processes for galvanic coating are well established on an industrial scale for several types of polymer [5]. However, there are still materials like polyethylene (PE ) or polybutyleneterephtalate (PBT ) for which a satisfying method of metallization has not yet been found. It is the objective of this work to replace some of the most chemically aggressive steps in galvanic coating processes of polymers. Promising candidates to be substituted are: * Corresponding author. Tel: 0049 241 8906 137; Fax: 0049 241 8906 121; e-mail: [email protected] 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 20 5 - 3
the acid etch at the beginning used to prepare the polymer surface; $ the deposition of first metal atoms creating a thin electrically conductive film. The final electroplating step for reinforcement of the metallic layer will remain unchallenged, as it is both fast $
Fig. 1. Schematic arrangements using barrier discharges. Direct surface treatment, shown at the top, uses the surface being treated in place of a dielectric barrier. At the bottom, the principle of UV lamps is illustrated, where the discharge gap is filled with an excimer gas mixture and a wire mesh serves as a transparent electrode.
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and cheap. The common process and possible alternatives for single steps or a completely new sequence are illustrated in Table 1. The purpose of the etching step — besides cleaning the polymer parts — is to ensure sufficient bonding of metal layers on bulk material. Current processes use a wet chemical etch which results in topographical changes of the surface that allow for mechanical anchoring of the metal in the polymer. In addition to causing problems with handling large amounts of chemically aggressive substances, this method is only applicable to polymers which show some inhomogeneities that result in fissures or caverns after the etching procedure. A typical example is acrylbutadienstyrene (ABS ), which is a copolymer whose compounds can be selectively etched [6 ].
2. Experimental Dielectric barrier discharges have been used both for direct plasma treatment of surfaces as well as for indirect treatment via the production of UV radiation. Working in air or reactive gas mixtures at gas pressures in the order of 1000 hPa, barrier discharges are driven by a high voltage of 10 kV at frequencies of 15–150 kHz. The approximately sinusoidal voltage can be gated for control of the mean power. One of the electrodes is electrically insulated by a dielectric barrier which stops the current after a few tens of nanoseconds. The plasma forms short-lived filaments with 100 mm diameter and larger footprints of approximately 1.5 mm diameter on the sample surface. Due to the short duration of the breakdown, no significant heating of the gas volume takes place [7]. Depending on the parameters of the excitation, the filaments tend to appear at the same places, which leads to inhomogeneous treatment. This effect can be avoided by applying a gas flow to the discharge gap. In the first approach, barrier discharges have been realized directly on the surface of flat polymer samples. A high-voltage electrode, including the insulating dielectric barrier, is mounted 2–5 mm above the sample which has to be backed by the ground electrode. The highvoltage electrode is made up from a metal electrode
inside a water-cooled quartz tube (see Fig. 2). The gas discharge is produced in the gap between the tube electrode and the polymer, in direct contact to its surface. For the treatment of larger areas, the sample is moved underneath the electrode. Gating the excitation voltage allows for the treatment even of heat sensitive polymers and thin foils. Both the repetition frequency and the duration of the pulse trains can be adjusted. An electrical power of up to 50 W/cm of electrode length is used for generating the plasma. In the second approach, both electrodes are arranged concentrically, with the dielectrics forming an annular vessel, filled by an excimer gas mixture. Nearly monochromatic UV radiation of 172, 222 or 308 nm wavelength, depending on the kind of filling, passes through the grid-shaped outer electrode of this large-area set-up [8]. A laboratory unit with two discharge tubes in parallel homogeneously illuminates approximately 80×300 mm. For some years, UV excimer sources of this type have been commercially available [9,10]. The samples have been irradiated with these lamps in air at a distance of 100 mm and show warming to only 80 °C after 10 min. Due to the high absorption of the 172 nm radiation in oxygen, the experiments with this lamp have to be performed under a non-absorbing atmosphere, e.g. N or Ar. The treatment conditions for 2 each sample are given in Section 3. Fig. 3 shows a photograph illustrating the UV treatment. Excimer lasers, being an established tool for small-
Fig. 2. Cross-section of the experimental set-up used for direct plasma treatment of polymers at atmospheric pressure. The sample is moved perpendicular to the plasma curtain that forms below the electrode tube.
Table 1 Scheme of the metallization processes for polymers used today and possible replacements using atmospheric pressure plasma or UV radiation. Single steps may be replaced or the whole new sequence may be introduced
Cleaning of the polymer parts Functionalizing of the surface Seeding with palladium Thin electroless plating Electrolytical reinforcement
Conventional process
Alternative approach
Acid etching Acid etching Wet chemical deposition Nickel bath For example, copper
Washing with ultrasonic agitation UV irradiation or plasma treatment Wet chemical or photo deposition Nickel bath or photo deposition For example, copper
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3.1. Topography
Fig. 3. Treatment of plastic parts with excimer lamps. At the top the two discharge tubes of the lamp can be seen. The blue light is due to fluorescence effects in the quartz tubes and represents only a small fraction of the emitted radiation.
area surface treatment [11], have been investigated for comparison. In particular, a KrF excimer laser with a wavelength of 248 nm and an output energy per pulse of 250 mJ at a repetition rate of up to 400 Hz was used. The laser radiation passes a beam shaping integrator, which provides for homogeneous irradiation of 25×25 mm. For the treatment of larger areas, the laser spot can be scanned across the surface either by moving the sample or by deflecting the laser beam (see Fig. 4). The power density in the illuminated spot can be varied from intensities comparable with those of excimer lamps up to beyond the threshold where thermal effects occur.
Using a direct barrier discharge, micron-size cavities have been etched in ABS. These cavities show a size and structure quite similar to those obtained in conventional chemical etching (see Fig. 5). Additionally, an improvement of wettability has been detected, an effect that is not found after chemical etching (cf. Section 3.2). Experiments with mineral-filled polyamide (PA) have shown that by selective plasma etching, filler materials can be exposed, achieving a deeply clefted surface ( Fig. 6). All samples have been exposed several times to a plasma curtain in air for an equivalent of 12 s in total at an electrical input power of about 50 W/cm of electrode length. Using UV radiation from excimer lamps, no topographical changes are obtained for exposure times between 0.1 s and 20 min. With an excimer laser at integrated energy densities above 50 J/cm2, which melt or ablate material, an increase in surface area may result after re-crystallization of PBT. In the molten material generated by each pulse, internal stress that was created during the moulding process relaxes within the surface. After re-solidification the polymer shows a bulge structure, shown in Fig. 7. 3.2. Surface energy The wettability of polymer surfaces is strongly affected by plasma or UV treatment. Before treatment, the samples are cleaned in 0.1 M NaOH–ethanol mixtures at room temperature and subsequently dried under
3. Results and discussion Results have so far been obtained on topographical roughening, chemical modifications and on some spatially selective metal coatings of the polymer surfaces.
Fig. 4. Schematic set-up for laser treatment of polymers. The laser beam illuminates a square of up to 25×25 mm. Widespread treatment is achieved by scanning the laser spot across the surface.
Fig. 5. Scanning electron micrograph of plasma treated ABS. The cavern-like structure resembles the morphology obtained in conventional wet chemical etching.
K. Pochner et al. / Surface and Coatings Technology 97 (1997) 372–377
Fig. 6. PA after plasma treatment. The mineral filler was exposed by selective etching of the polymer matrix in a dielectric barrier discharge at atmospheric pressure.
argon. Many untreated polymer surfaces show hydrophobic behaviour with large contact angles (i.e. the angle between the surface and the edge of a droplet of de-ionized water) due to non-polar aromatic and aliphatic groups. After direct plasma treatment in air, contact angles usually decrease, as for PBT (saturation value 45°, untreated 75°), ABS (53°, untreated 75°), or PA (25°, untreated 61°). The surface energy after illumination of PBT with excimer lamps varies with the applied wavelength. Treatment with 308 nm leads to contact angles
Fig. 7. Scanning electron micrograph of laser treated PBT. Oriented bulges result from relaxation of internal stress after repeated melting and re-solidification.
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of not less than 42°, while sufficient irradiation with 222 and 172 nm results in angles below 20°. For excimer laser treatment, similar results are obtained. The sample handling before measurement of the contact angles may cause soaking and swelling effects due to solvents. This was verified by immersion or extended rinsing with water, which leads to a contact angle reduction of untreated PBT from 75 to 55°. The rise of the surface energy after UV treatment can be interpreted as the generation of active chemical groups on the surface, which interact with a polar solvent like water. Another hint in this direction might be the behaviour of UV irradiated surfaces with respect to organic dyes dissolved in ethanol–water mixtures. These dyes consist of large aromatic systems with small polar groups attached, but are insoluble in pure water. While the untreated polymer shows no interaction with the dye, after 222 nm exposure, a significant dyeing effect is visible (see Fig. 8, bottom). The intensity of the dyeing scales with the dose of the UV illumination. Small concentrations of dye molecules on the surface can be detected by yellow fluorescence under 222 nm illumination or by a change in the reflectivity of the polymer surface. Similar substances can be used for quantitative analysis in which the density of functional groups on the surface is determined by derivatization [12]. 3.3. Fluorescence A fluorescence signal of the treated PBT surface can also be detected without dyeing. For the observer’s eye, the plain polymer appears dark violet under weak UV illumination. The more the surface has been exposed to intense 308 or 222 nm light, the brighter blue the irradiated areas shine (cf. Fig. 8, top). The fluorescence signal can be measured with a monochromator if direct
Fig. 8. Dyeing and fluorescence of UV-treated PBT samples. The radiation dose in both photographs increases from left to right. A growing amount of dye is adsorbed (bottom) and the visible blue fluorescence under low power 222 nm illumination is enhanced (top).
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reflection of the UV light from the lamp into the optics of the monochromator is avoided. However, there are still some krypton lines in the visible region, which are emitted by the excimer gas filling of the lamp and being diffusely reflected by the polymer sample. Apart from that, two fluorescence bands have been found (Fig. 9). The untreated PBT shows fluorescence at a wavelength of about 340 nm which cannot be detected by visual inspection with the naked eye. After treatment with the intense UV radiation, an additional band at 450 nm appears, while the 340 nm signal decreases. Interpretation of these results may lead to a deeper understanding of the photo-induced chemical modifications of the surface. 3.4. Metallization Besides efforts to investigate the surface effects induced by UV light or plasma treatment, experiments on the metallization of PBT have been carried out. At present, commercial metallization baths with palladium seeding and nickel deposition are employed. The activation bath contains a wetting agent which leads to a uniform metallization, even for untreated polymers. However, the layers on the plain surfaces show bad adhesion and can be removed with adhesive tape. After a short treatment of about 90 s with 222 nm at 100 mm distance to the discharge tubes, a clear improvement of the adhesion can be achieved. Preliminary results from measurements of the adhesive forces show an improvement of sticking from 0.13 N/mm2 on untreated surfaces to 6–8 N/mm2 on the UV-treated areas. Apart from that, the metal layers on irradiated sites show a better durability to chemicals like acetone or 0.5 M sulphuric acid. Electrolytical reinforcement of the layers is possible. Similar results can be obtained by using the excimer
laser at low irradiation doses or by using the 308 nm lamp with exposure times extended to about 15 min. The main advantage of the 308 nm wavelength is being transmitted by deep-drawable mask foils, which allow for a selective UV treatment of polymer parts. From an optimization of the excimer wavelength to the desired photochemical process, significantly shorter treatment times are expected. These times can be further cut down by reducing the distance to the lamps, which was 100 mm. Utilizing the difference in adhesion between illuminated and masked sites, a subtractive structured metallization could already be achieved. Purely additive selective metallization results from passivation of polymer surfaces against palladium seeding after an exposure time of 10 min with 222 nm UV light. While the illuminated areas remain unplated, masked areas are metallized. Short additional UV irradiation of the entire surface is required for good adhesion. Preliminary results from the metallization of plasmatreated samples also show improved adhesion compared with untreated material. Metal layers on the rough surfaces are, however, more readily attacked by chemicals than UV treated parts.
4. Summary and conclusions The presented experiments show that an improvement of adhesion of metal coatings on PBT using UV or plasma treatment is possible. The treatment can be applied to large areas and requires no vacuum equipment or chemically aggressive substances. First attempts for structured metallization have been carried out successfully. While plasma and laser treatment both result in topographical changes of the surfaces, with incoherent excimer UV illumination only chemical modifications have been observed so far. These modifications can be detected by the dyeing and fluorescence properties of the polymers. More detailed investigations on the character of the surface modifications may allow further optimizations or a direct metal deposition on the polymer.
Acknowledgement
Fig. 9. Fluorescence spectra of PBT before and after UV treatment. The sample is illuminated with 222 nm at reduced power to excite the fluorescence radiation. The small peak at 222 nm emanates from diffuse reflection of the lamp into the monochromator.
This work was supported by the Bundesministerium ¨ fur Bildung, Wissenschaft, Forschung und Technologie (BMBF ) under grant number 13N6675. We also wish to acknowledge the financial contribution of Heraeus Noblelight GmbH and some helpful discussions with Dr Angelika Roth.
K. Pochner et al. / Surface and Coatings Technology 97 (1997) 372–377
References [1] K. Pochner, W. Neff, R. Lebert, Atmospheric pressure gas discharges for surface treatment, Surf. Coat. Technol. 7475 (1995) 394. [2] K. Pochner, Plasmabehandlung — Funkenregen reinigt Folien¨ oberflachen, Neue Verpackung, Huthig, Heidleberg, 1993, p. 14. [3] B. Eliasson, U. Kogelschatz, UV excimer radiation from dielectric barrier discharges, Appl. Phys. B 46 (1988) 299. [4] J.-Y. Zhang, I.W. Boyd, Efficient excimer ultraviolet sources from a dielectric barrier discharge in rare-gas/halogen mixtures, J. Appl. Phys. 80 (1996) 633. [5] R. Suchentrunk ( Ed.), Kunststoff-Metallisierung — Handbuch ¨ fur Theorie und Praxis, Leuze, Saulgau, 1991. [6 ] H. Ebneth ( Ed.), Metallisieren von Kunststoffen, Expert, Esslingen, 1995.
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[7] B. Eliasson, M. Hirth, U. Kogelschatz, Ozone synthesis from oxygen in dielectric barrier discharges, J. Phys. D 20 (1987) 1421. [8] B. Gellert, U. Kogelschatz, Generation of excimer emission in dielectrlic barrier discharges, Appl. Phys. B 52 (1991) 14. [9] Heraeus Noblelight GmbH, Excimer–UV–Strahler, Produktinformation 5C 6.93/N Ko, 1993. [10] A. Roth, Kunststoffe mit Excimer–UV–Strahlung vorbehandeln, ¨ Metalloberflache 50 (1996) 4. [11] H. Frerichs, J. Stricker, D.A. Wesner, E.-W. Kreutz, Laserinduced surface modification and metallization of polymers, Appl. Surf. Sci. 86 (1995) 405. ¨ [12] V.B. Ivanov, J. Behnisch, A. Hollander, F. Mehdorn, H. Zimmermann, Determination of functional groups on polymer surfaces using fluorescence labelling, Surf. Interface Anal. 24 (1996) 257.
Treatment of polymers for subsequent metallization using intense UV radiation or plasma at atmospheric pressure ¨ K. Pochner *, S. Beil, H. Horn, M. Blomer ¨ Fraunhofer-lnstitut fur Lasertechnik, Aachen, Germany
Abstract The activation of polymer surfaces in glow discharges and the deposition of metals from organo-metallic vapours, both at low pressure, are standard laboratory processes. The upscaling to industrial mass product applications is, however, hampered by cost and the time consumption needed for establishing a sufficient vacuum. Atmospheric pressure processes based on the same physical surface interactions show great promise as replacements of some steps in galvanic plating. Successful metallizations are reported after treatment of polymers in barrier discharges at atmospheric pressure. These may be applied directly to the surface of the workpiece or indirectly from within large-area monochromatic excimer UV lamps. Comparisons with excimer UV laser treatment are made. © 1997 Elsevier Science S.A. Keywords: Atmospheric pressure plasma; Dielectric barrier discharge; Metal deposition on plastics; Polymer surface treatment; Ultraviolet irradiation
1. Introduction Gas discharges at atmospheric pressure offer various advantages for surface treatment processes, combining selective photochemical and plasma-chemical reactions with simple large-area devices [1]. The kinetic energy of electrons in the plasma can be utilized for direct surface treatment in open configurations [2], or for the formation of excimers in lamp tubes to obtain monochromatic, high-intensity UV radiation [3,4]. Investigations of the mechanisms involved in the preparation and galvanic metal coating of polymer surfaces have been carried out using dielectric barrier discharges. The basic concepts of the devices used are illustrated in Fig. 1. Processes for galvanic coating are well established on an industrial scale for several types of polymer [5]. However, there are still materials like polyethylene (PE ) or polybutyleneterephtalate (PBT ) for which a satisfying method of metallization has not yet been found. It is the objective of this work to replace some of the most chemically aggressive steps in galvanic coating processes of polymers. Promising candidates to be substituted are: * Corresponding author. Tel: 0049 241 8906 137; Fax: 0049 241 8906 121; e-mail: [email protected] 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 20 5 - 3
the acid etch at the beginning used to prepare the polymer surface; $ the deposition of first metal atoms creating a thin electrically conductive film. The final electroplating step for reinforcement of the metallic layer will remain unchallenged, as it is both fast $
Fig. 1. Schematic arrangements using barrier discharges. Direct surface treatment, shown at the top, uses the surface being treated in place of a dielectric barrier. At the bottom, the principle of UV lamps is illustrated, where the discharge gap is filled with an excimer gas mixture and a wire mesh serves as a transparent electrode.
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and cheap. The common process and possible alternatives for single steps or a completely new sequence are illustrated in Table 1. The purpose of the etching step — besides cleaning the polymer parts — is to ensure sufficient bonding of metal layers on bulk material. Current processes use a wet chemical etch which results in topographical changes of the surface that allow for mechanical anchoring of the metal in the polymer. In addition to causing problems with handling large amounts of chemically aggressive substances, this method is only applicable to polymers which show some inhomogeneities that result in fissures or caverns after the etching procedure. A typical example is acrylbutadienstyrene (ABS ), which is a copolymer whose compounds can be selectively etched [6 ].
2. Experimental Dielectric barrier discharges have been used both for direct plasma treatment of surfaces as well as for indirect treatment via the production of UV radiation. Working in air or reactive gas mixtures at gas pressures in the order of 1000 hPa, barrier discharges are driven by a high voltage of 10 kV at frequencies of 15–150 kHz. The approximately sinusoidal voltage can be gated for control of the mean power. One of the electrodes is electrically insulated by a dielectric barrier which stops the current after a few tens of nanoseconds. The plasma forms short-lived filaments with 100 mm diameter and larger footprints of approximately 1.5 mm diameter on the sample surface. Due to the short duration of the breakdown, no significant heating of the gas volume takes place [7]. Depending on the parameters of the excitation, the filaments tend to appear at the same places, which leads to inhomogeneous treatment. This effect can be avoided by applying a gas flow to the discharge gap. In the first approach, barrier discharges have been realized directly on the surface of flat polymer samples. A high-voltage electrode, including the insulating dielectric barrier, is mounted 2–5 mm above the sample which has to be backed by the ground electrode. The highvoltage electrode is made up from a metal electrode
inside a water-cooled quartz tube (see Fig. 2). The gas discharge is produced in the gap between the tube electrode and the polymer, in direct contact to its surface. For the treatment of larger areas, the sample is moved underneath the electrode. Gating the excitation voltage allows for the treatment even of heat sensitive polymers and thin foils. Both the repetition frequency and the duration of the pulse trains can be adjusted. An electrical power of up to 50 W/cm of electrode length is used for generating the plasma. In the second approach, both electrodes are arranged concentrically, with the dielectrics forming an annular vessel, filled by an excimer gas mixture. Nearly monochromatic UV radiation of 172, 222 or 308 nm wavelength, depending on the kind of filling, passes through the grid-shaped outer electrode of this large-area set-up [8]. A laboratory unit with two discharge tubes in parallel homogeneously illuminates approximately 80×300 mm. For some years, UV excimer sources of this type have been commercially available [9,10]. The samples have been irradiated with these lamps in air at a distance of 100 mm and show warming to only 80 °C after 10 min. Due to the high absorption of the 172 nm radiation in oxygen, the experiments with this lamp have to be performed under a non-absorbing atmosphere, e.g. N or Ar. The treatment conditions for 2 each sample are given in Section 3. Fig. 3 shows a photograph illustrating the UV treatment. Excimer lasers, being an established tool for small-
Fig. 2. Cross-section of the experimental set-up used for direct plasma treatment of polymers at atmospheric pressure. The sample is moved perpendicular to the plasma curtain that forms below the electrode tube.
Table 1 Scheme of the metallization processes for polymers used today and possible replacements using atmospheric pressure plasma or UV radiation. Single steps may be replaced or the whole new sequence may be introduced
Cleaning of the polymer parts Functionalizing of the surface Seeding with palladium Thin electroless plating Electrolytical reinforcement
Conventional process
Alternative approach
Acid etching Acid etching Wet chemical deposition Nickel bath For example, copper
Washing with ultrasonic agitation UV irradiation or plasma treatment Wet chemical or photo deposition Nickel bath or photo deposition For example, copper
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K. Pochner et al. / Surface and Coatings Technology 97 (1997) 372–377
3.1. Topography
Fig. 3. Treatment of plastic parts with excimer lamps. At the top the two discharge tubes of the lamp can be seen. The blue light is due to fluorescence effects in the quartz tubes and represents only a small fraction of the emitted radiation.
area surface treatment [11], have been investigated for comparison. In particular, a KrF excimer laser with a wavelength of 248 nm and an output energy per pulse of 250 mJ at a repetition rate of up to 400 Hz was used. The laser radiation passes a beam shaping integrator, which provides for homogeneous irradiation of 25×25 mm. For the treatment of larger areas, the laser spot can be scanned across the surface either by moving the sample or by deflecting the laser beam (see Fig. 4). The power density in the illuminated spot can be varied from intensities comparable with those of excimer lamps up to beyond the threshold where thermal effects occur.
Using a direct barrier discharge, micron-size cavities have been etched in ABS. These cavities show a size and structure quite similar to those obtained in conventional chemical etching (see Fig. 5). Additionally, an improvement of wettability has been detected, an effect that is not found after chemical etching (cf. Section 3.2). Experiments with mineral-filled polyamide (PA) have shown that by selective plasma etching, filler materials can be exposed, achieving a deeply clefted surface ( Fig. 6). All samples have been exposed several times to a plasma curtain in air for an equivalent of 12 s in total at an electrical input power of about 50 W/cm of electrode length. Using UV radiation from excimer lamps, no topographical changes are obtained for exposure times between 0.1 s and 20 min. With an excimer laser at integrated energy densities above 50 J/cm2, which melt or ablate material, an increase in surface area may result after re-crystallization of PBT. In the molten material generated by each pulse, internal stress that was created during the moulding process relaxes within the surface. After re-solidification the polymer shows a bulge structure, shown in Fig. 7. 3.2. Surface energy The wettability of polymer surfaces is strongly affected by plasma or UV treatment. Before treatment, the samples are cleaned in 0.1 M NaOH–ethanol mixtures at room temperature and subsequently dried under
3. Results and discussion Results have so far been obtained on topographical roughening, chemical modifications and on some spatially selective metal coatings of the polymer surfaces.
Fig. 4. Schematic set-up for laser treatment of polymers. The laser beam illuminates a square of up to 25×25 mm. Widespread treatment is achieved by scanning the laser spot across the surface.
Fig. 5. Scanning electron micrograph of plasma treated ABS. The cavern-like structure resembles the morphology obtained in conventional wet chemical etching.
K. Pochner et al. / Surface and Coatings Technology 97 (1997) 372–377
Fig. 6. PA after plasma treatment. The mineral filler was exposed by selective etching of the polymer matrix in a dielectric barrier discharge at atmospheric pressure.
argon. Many untreated polymer surfaces show hydrophobic behaviour with large contact angles (i.e. the angle between the surface and the edge of a droplet of de-ionized water) due to non-polar aromatic and aliphatic groups. After direct plasma treatment in air, contact angles usually decrease, as for PBT (saturation value 45°, untreated 75°), ABS (53°, untreated 75°), or PA (25°, untreated 61°). The surface energy after illumination of PBT with excimer lamps varies with the applied wavelength. Treatment with 308 nm leads to contact angles
Fig. 7. Scanning electron micrograph of laser treated PBT. Oriented bulges result from relaxation of internal stress after repeated melting and re-solidification.
375
of not less than 42°, while sufficient irradiation with 222 and 172 nm results in angles below 20°. For excimer laser treatment, similar results are obtained. The sample handling before measurement of the contact angles may cause soaking and swelling effects due to solvents. This was verified by immersion or extended rinsing with water, which leads to a contact angle reduction of untreated PBT from 75 to 55°. The rise of the surface energy after UV treatment can be interpreted as the generation of active chemical groups on the surface, which interact with a polar solvent like water. Another hint in this direction might be the behaviour of UV irradiated surfaces with respect to organic dyes dissolved in ethanol–water mixtures. These dyes consist of large aromatic systems with small polar groups attached, but are insoluble in pure water. While the untreated polymer shows no interaction with the dye, after 222 nm exposure, a significant dyeing effect is visible (see Fig. 8, bottom). The intensity of the dyeing scales with the dose of the UV illumination. Small concentrations of dye molecules on the surface can be detected by yellow fluorescence under 222 nm illumination or by a change in the reflectivity of the polymer surface. Similar substances can be used for quantitative analysis in which the density of functional groups on the surface is determined by derivatization [12]. 3.3. Fluorescence A fluorescence signal of the treated PBT surface can also be detected without dyeing. For the observer’s eye, the plain polymer appears dark violet under weak UV illumination. The more the surface has been exposed to intense 308 or 222 nm light, the brighter blue the irradiated areas shine (cf. Fig. 8, top). The fluorescence signal can be measured with a monochromator if direct
Fig. 8. Dyeing and fluorescence of UV-treated PBT samples. The radiation dose in both photographs increases from left to right. A growing amount of dye is adsorbed (bottom) and the visible blue fluorescence under low power 222 nm illumination is enhanced (top).
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reflection of the UV light from the lamp into the optics of the monochromator is avoided. However, there are still some krypton lines in the visible region, which are emitted by the excimer gas filling of the lamp and being diffusely reflected by the polymer sample. Apart from that, two fluorescence bands have been found (Fig. 9). The untreated PBT shows fluorescence at a wavelength of about 340 nm which cannot be detected by visual inspection with the naked eye. After treatment with the intense UV radiation, an additional band at 450 nm appears, while the 340 nm signal decreases. Interpretation of these results may lead to a deeper understanding of the photo-induced chemical modifications of the surface. 3.4. Metallization Besides efforts to investigate the surface effects induced by UV light or plasma treatment, experiments on the metallization of PBT have been carried out. At present, commercial metallization baths with palladium seeding and nickel deposition are employed. The activation bath contains a wetting agent which leads to a uniform metallization, even for untreated polymers. However, the layers on the plain surfaces show bad adhesion and can be removed with adhesive tape. After a short treatment of about 90 s with 222 nm at 100 mm distance to the discharge tubes, a clear improvement of the adhesion can be achieved. Preliminary results from measurements of the adhesive forces show an improvement of sticking from 0.13 N/mm2 on untreated surfaces to 6–8 N/mm2 on the UV-treated areas. Apart from that, the metal layers on irradiated sites show a better durability to chemicals like acetone or 0.5 M sulphuric acid. Electrolytical reinforcement of the layers is possible. Similar results can be obtained by using the excimer
laser at low irradiation doses or by using the 308 nm lamp with exposure times extended to about 15 min. The main advantage of the 308 nm wavelength is being transmitted by deep-drawable mask foils, which allow for a selective UV treatment of polymer parts. From an optimization of the excimer wavelength to the desired photochemical process, significantly shorter treatment times are expected. These times can be further cut down by reducing the distance to the lamps, which was 100 mm. Utilizing the difference in adhesion between illuminated and masked sites, a subtractive structured metallization could already be achieved. Purely additive selective metallization results from passivation of polymer surfaces against palladium seeding after an exposure time of 10 min with 222 nm UV light. While the illuminated areas remain unplated, masked areas are metallized. Short additional UV irradiation of the entire surface is required for good adhesion. Preliminary results from the metallization of plasmatreated samples also show improved adhesion compared with untreated material. Metal layers on the rough surfaces are, however, more readily attacked by chemicals than UV treated parts.
4. Summary and conclusions The presented experiments show that an improvement of adhesion of metal coatings on PBT using UV or plasma treatment is possible. The treatment can be applied to large areas and requires no vacuum equipment or chemically aggressive substances. First attempts for structured metallization have been carried out successfully. While plasma and laser treatment both result in topographical changes of the surfaces, with incoherent excimer UV illumination only chemical modifications have been observed so far. These modifications can be detected by the dyeing and fluorescence properties of the polymers. More detailed investigations on the character of the surface modifications may allow further optimizations or a direct metal deposition on the polymer.
Acknowledgement
Fig. 9. Fluorescence spectra of PBT before and after UV treatment. The sample is illuminated with 222 nm at reduced power to excite the fluorescence radiation. The small peak at 222 nm emanates from diffuse reflection of the lamp into the monochromator.
This work was supported by the Bundesministerium ¨ fur Bildung, Wissenschaft, Forschung und Technologie (BMBF ) under grant number 13N6675. We also wish to acknowledge the financial contribution of Heraeus Noblelight GmbH and some helpful discussions with Dr Angelika Roth.
K. Pochner et al. / Surface and Coatings Technology 97 (1997) 372–377
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