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Short-time plasma pre-treatment of polytetrafluoroethylene for improved adhesion
Surface and Coatings Technology 142᎐144 Ž2001. 501᎐506
Short-time plasma pre-treatment of polytetrafluoroethylene for improved adhesion U K.-M. Baumgartner , J. Schneider, A. Schulz, J. Feichtinger, M. Walker ¨ Institut fur ¨ Plasmaforschung, Uni¨ ersitat ¨ Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
Abstract Fluoropolymers with their unique properties Žchemical inertness, thermal stability, low surface tension, mechanical stability . are used in many industrial applications. One disadvantage of these fluoropolymers is their poor adhesion to other materials. In this work a short-time plasma pre-treatment of polytetrafluoroethylene ŽPTFE. with a low pressure microwave plasma was investigated. The Plasmodul 䊛 source allows the pre-treatment of PTFE substrates with an ammonia plasma ŽNH 3 .. The newly developed plasma source Planartron 䊛 , which is derived from the Duo-Plasmaline 䊛 , was used for improving the adhesion properties of PTFE by generating oxygen and nitrogen plasmas. PTFE foils were modified on both sides by plasma treatment for typically 30 s. After bonding the foils to aluminum dollies using a 2-K-epoxy resin the bonding strength of the adhesive joint was measured directly by pull off tests. Some modified foils were additionally investigated by Fourier-transform infrared spectroscopy. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Industrial microwave plasma source; Polytetrafluoroethylene; Surface modification; Plasmodul 䊛 ; Planartron 䊛
1. Introduction Fluoropolymers with their unique properties are desired in many industrial applications. They are known for their chemical inertness, high thermal stability, low surface tension and high mechanical stability. These properties are important concerning engineering aspects, but pre-treatment of the polymers is normally required to achieve satisfactory adhesion. The widely accepted mechanism of polymer surface modification for improved adhesion properties is the formation of functional groups on the surface, according to the request of the used adhesive. Changes of the adhesion properties of PTFE substrates can be investigated by observing the wettability or the surface tension. In the experiment reported here a direct measurement of bond strength between polyU
Corresponding author. MUEGGE ELECTRONIC GmbH, Hochstrasse 4, 64385 Reichelsheim, Germany. Tel: q49-6164-930727. .. E-mail address: [email protected] ŽK.-M. Baumgartner ¨
mer and adhesive is employed by pull-off tests with aluminumrepoxyrPTFErepoxyraluminum adhesive joints w1,2x. Different types of pre-treatment of polyfluoroethylene ŽPTFE. are reported, like wet chemical treatment of the PTFE in a sodiumrammonia solution, UV-light, flame treatment, corona discharges and low pressure plasma modification w3,4x. Especially low pressure plasma treatment of PTFE is a suitable method for introducing functional groups on the PTFE surface for a subsequent bonding procedure w5᎐7x. Concerning economically and environmental aspects the surface treatment with a low pressure plasma is very advantageous. Especially the highly developed 2.45 GHz microwave plasma technology has reached a low cost standard that makes the plasma excitation by microwaves profitable. Homogeneous linearly extended plasmas can be produced by a specific arrangement of the emitting antenna where the microwave power is coupled from a linearly extended waveguide into the plasma w8x.
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 0 9 - 9
502
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
Recent investigations on a simple and inexpensive plasma source started with the Gigatron 䊛 w9x, where a linearly extended glass tube with a concentric metallic inner conductor was mounted in a low pressure chamber. One end was closed, the other one was connected to the microwave generator. The plasma is produced in the low pressure regime outside the tube. The microwaves propagate in the plasma as outer surface waves w10x. The axial homogeneity of the plasma was considerably improved by feeding the microwaves symmetrically from both ends of the tube. This DuoPlasmaline 䊛 forms a linearly extended and homogeneous plasma up to a length of several meters w11,12x. A combination of such plasmalines can be used to obtain a two-dimensional plasma array. In this work, a flexible laboratory plasma device, called Plasmodul 䊛 w13x, is presented. Here, the Plasmaline 䊛 principle is used for medium and small devices as test devices in plasma technology. A further simplification step applied to the Plasmodul 䊛 source leads to a completely new concept of plasma source, called Planartron 䊛 . An antenna structure is placed under a quartz plate at atmospheric pressure. The antenna is fed by 2.45 GHz microwaves. The microwaves mainly propagate along the antenna structure and form a homogeneous, planar plasma area in the low pressure regime on top of a quartz plate. The Planartron 䊛 source is designed for use in the modular plasma device Plasmodul 䊛 .
2. Experimental details 2.1. Plasma sources In this study two types of plasma sources were considered: the modular concept of Plasmodul 䊛 and a new extension called Planartron 䊛 . A view of the modular device Plasmodul 䊛 is shown in Fig. 1. The Plasmodul 䊛 downstream package includes the plasma source, two gas inlet systems, a reaction chamber and diagnostic ports. The device has a diameter of 35 cm and a height of 40 cm. Details of the mechanism of plasma generation and results about the measured electron densities and deposition rates were described previously w10,14,16x. The newly developed Planartron 䊛 source is derived from the Plasmaline 䊛 principle and allows the production of a planar and homogeneous plasma area. Fig. 2 shows a schematic view of the Planartron 䊛 arrangement, in this case especially designed for use as bottom module in the Plasmodul 䊛 device. The Planartron 䊛 device has a diameter of 35 cm with an active area of 20 cm in diameter. An antenna structure is placed under a quartz plate at atmospheric pressure. The antenna is fed by 2.45 GHz microwaves
Fig. 1. View of the Plasmodul 䊛 device with gas inlet system, plasma source, reaction chamber and diagnostic ports.
by a coaxial arrangement. Comparable with the Plasmaline 䊛 , the microwaves mainly propagate along the antenna structure. If the electric field strength exceeds the breakdown field strength the plasma ignites on the top of the quartz plate in the low pressure regime. The extension of the plasma area is mainly dependent on the applied microwave power, and a homogeneous plasma is produced. Electrically nonconducting substrates like PTFE can be placed directly onto the quartz plate and therefore are located in an
Fig. 2. Schematic view of the Planartron 䊛 source. The antenna structure is placed under a quartz plate. The shown arrangement is especially designed for use in Plasmodul 䊛.
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
503
area with high rate processes resulting in short time treatments. The Planartron 䊛 is fed with a maximum total power of 2 = 600 W. 2.2. PTFE treatment Commercially available sheets of PTFE were used as substrate material. Additionally first investigations on a PTFE compound with 25% glass fibers were done. The substrates were cut into pieces of approximately 2 = 2 cm2 . All PTFE substrates were modified at both sides by plasma exposition. The maximum process time was 10 min for each side. Pure PTFE was treated in the Plasmodul 䊛 device at a pressure of 0.2 mbar in an ammonia plasma. The substrates were placed 8 cm below the source and modified at a NH 3 flow rate of 100 sccm Žstandard cm3 miny1 .. The microwave power was 2 = 600 W. In the Planartron 䊛 device the pure PTFE substrates were placed directly onto the quartz plate and alternatively in a distance of 2 cm to the quartz plate. The PTFE substrates were processed at 0.2 mbar in a nitrogen plasma with a nitrogen flux of 100 sccm. The applied microwave power was 600 W. Tests on PTFE-glass fiber compound were performed in Planartron 䊛 device at 0.2 mbar in a nitrogen or alternatively in an oxygen plasma at a gas flow of 100 sccm. 2.3. Pull off test The PTFE foils modified at both sides by plasma treatment were bonded to aluminum dollies using a 2-K epoxy resin ŽUhu Plus sf.. The bond strength of the aluminumrepoxyrPTFErepoxyraluminum adhesive joints was measured by pull off tests with a special adhesion tester ŽElcometer. w1x. 2.4. FTIR spectroscopy IR spectra of the pre-treated PTFE substrates, the 2-K resin and the aluminum dollies after the adhesion tests were recorded on a Bruker Vector 22 with adapted reflectance unit. The measurements were performed at an angle of incidence of 75⬚ using parallel polarized IR light. For each spectrum, 32 scans with 8 cmy1 resolution were recorded.
3. Results and discussion 3.1. PTFE treated with Plasmodul 䊛 in a NH3 plasma In Fig. 3 the bond strength is plotted vs. the exposure time in an ammonia plasma. The figure shows, that a short-time plasma treatment in the range of 15 s results
Fig. 3. Bond strength in dependence of the plasma exposure time. The PTFE was treated with Plasmodul 䊛 in a NH 3 plasma.
in a maximum bond strength. The bond strength is more than one order of magnitude higher compared to the untreated material. In detail the bond strength increases strongly from 0.25 N mmy2 for the untreated fluoropolymer by a factor of 16 up to 3.5 N mmy2 after a plasma treatment of only 15 s. The value of 3.5 N mmy2 is the maximum bond strength. For treating times longer than 30 s the bonding strength strongly decreases to a value of 0.65 N mmy2 after 10 min, which is only an improvement by a factor of 2.6 compared with the untreated polymer. 3.2. PTFE treated with Planartron䊛 in a N2 plasma 3.2.1. Distance substrate᎐source 2 cm PTFE foils were treated with the Planartron 䊛 source by placing the substrates in a distance of 2 cm to the source. In Fig. 4 the bond strength is plotted vs. the exposure time in a nitrogen plasma. The figure shows, that the bond strength of PTFE treated with a nitrogen plasma in the Planartron 䊛 source increases strongly from 0.25 N mmy2 for the untreated material up to 5.9 N mmy2 Žfactor 23. after 50 s plasma treatment. For longer treating times the bond strength decreases to 2.6 N mmy2 after 200 s. 3.2.2. Distance substrate᎐source 0 cm PTFE foils were treated with the Planartron 䊛 source by placing the substrates directly onto the quartz plate of the source. The time dependent bond strength is shown in Fig. 5. After a short time treatment of 5 s the maximum bond strength of 5 N mmy2 is achieved. This represents a factor of 20 compared to the untreated material. Increasing the plasma exposition time again leads to a strong decrease in bond strength. It is remarkable, that already after a treatment of 5 s the same bond strength is achieved as treating the PTFE for 20 s in a
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K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
Fig. 4. Bond strength in dependence of the plasma exposure time. The PTFE was treated with the Planartron 䊛 source in a distance of 2 cm with a nitrogen plasma.
distance of 2 cm to the source. This means, that shortest plasma exposition times are achieved under the condition, that the substrates are directly placed on the Planartron 䊛 source. 3.3. Acti¨ ation of PTFE-glass fiber compound Some investigations were performed on a PTFE compound containing additionally to the fluoropolymer 25% of glass fibers ŽPTFE-GF.. For the tests with the PTFE-GF the Planartron 䊛 source was used. The bond strength of the untreated material is 0.8 N mmy2 and is slightly higher compared to pure PTFE Ž0.25 N mmy2 . due to mechanical forces among the adhesive and the glass fibers, which protrude some microns out of the surface of the PTFE-GF. Fig. 6 shows a microscopical picture of such an untreated PTFE-GF compound. The left half in the picture of Fig. 6 shows the formerly bonded area after
Fig. 5. Bond strength in dependence of the plasma exposure time. The PTFE was treated with the Planartron 䊛 source in direct contact Ž0 cm distance. with a nitrogen plasma.
Fig. 6. Microscopical picture of an untreated PTFE-GF compound. The area on the left half of substrate was formerly bonded to an aluminum dolly. The adhesive was removed completely without leaving damages.
the pull off tests, while the right half is completely unstressed. The adhesive is removed completely with the aluminum dolly from the untreated surface without leaving any damages. Only a small amount of glass fibers was torn away by the adhesive and explains the slightly increased bond strength. Fig. 7 shows a microscopic examination of a PTFEGF sample, which was treated for 5 min in an oxygen plasma and afterwards bonded to an aluminum dolly. After the pull off test the sample was wetted with a test ink. Again the left half of Fig. 7 represents the area after removing the adhesive joint, while the right half is unstressed. The test ink demonstrates, that the glass fibers are activated by the plasma process while the PTFE is completely unactivated and shows no wettability Žright side of Fig. 7.. The area with removed aluminum dolly contains almost no glass fibers and cannot be wetted at all Žleft side of Fig. 7.. This means,
Fig. 7. Microscopical picture of a plasma treated PTFE-GF substrate ŽO 2 plasma.. The glass fibers were activated and torn away from the surface as a result of the pull of test Žleft side of the substrate ..
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
that the glass fibers show a good adhesion property, which lead to an increased bond strength of 2.8 N mmy2 . The remaining surface after the pull off test is almost pure PTFE and does not contribute significantly to the total bond strength. For maximum bond strength it is necessary to activate both components, PTFE as well as the glass fibers. Fig. 8 shows a microscopic examination of PTFE-GF material treated in a nitrogen plasma for 5 min. Again the substrate was bonded to an aluminum dolly and tested with the adhesion tester. The test ink demonstrates, that the glass fibers as well as the PTFE were activated in the nitrogen plasma Žright side of Fig. 8, unstressed .. The left side of the picture in Fig. 8 shows the formerly bonded area. The glass fibers are torn away with the adhesive and also the activated PTFE was removed by the pull off test. The test area can not be wetted at all. The completely activated surface is reflected in the high bond strength of 6 N mmy2 .
505
Fig. 9. IR spectra of PTFE, 2-K epoxy resin ŽUhu plus sf. and additionally of the bond side of an aluminum dolly after pull off test.
terial. The maximum achievable bond strength is limited by the PTFE structure on the surface and subsurface after the plasma exposition.
3.4. FTIR in¨ estigations on plasma acti¨ ated PTFE 4. Conclusions Fig. 9 shows IR-spectra of plasma activated PTFE, of hardened 2-K-epoxy resin and additionally of a bonded aluminum dolly after removal from the PTFE substrate Žbonded side.. The spectra were recorded in the range from 400 cmy1 to 700 cmy1 . The most intense absorption peaks reported for CF2 modes in PTFE Ž553 cmy1 , 625 cmy1 and 638 cmy1 . are clearly identified w15x. Considering the IR-spectrum of the residue on a formerly bonded aluminum dolly, a superposition of the spectra recorded for PTFE and 2-K-epoxy resin, is found. This means, that the residue on the dolly not only consists of 2-K-epoxy resin, but also contains PTFE, torn away from the substrate ma-
Plasma pre-treatment of PTFE surfaces strongly influences the adhesion properties. New available plasma sources like Plasmodul 䊛 and Planartron 䊛 allow short time pre-treatment of PTFE for significant improved adhesion. Especially the Planartron 䊛 source represents a tool for high rate processes. The exposure times can be kept very short. The achieved bond strength is limited by the PTFE structure on the surface and subsurface after the plasma exposition.
Acknowledgements This work was performed using microwave equipment from MUEGGE ELECTRONIC GmbH, Hochstrasse 4, 64385 Reichelsheim, Germany. The authors are very grateful to K. Muegge and H. Muegge. The authors thank Professor Dr U. Schumacher from the Institut fur ¨ Plasmaforschung der Universitat ¨ Stuttgart for useful discussions. The authors would like to thank H. Petto for his technical assistance. References
Fig. 8. Microscopical picture of a plasma treated PTFE-GF substrate ŽN2 plasma.. The glass fibers as well as the PTFE were activated and both torn away from the surface as a result of the pull of test Žleft side of the substrate ..
w1x W. Petasch, K. Baumgartner, E. Rauchle, M. Walker, Surf. ¨ ¨ Coat. Technol. 59 Ž1993. 301᎐305. w2x W. Petasch, E. Rauchle, M. Walker, P. Elsner, Surf. Coat. ¨ Technol. 74r75 Ž1995. 682᎐688. w3x L.M. Siperko, R.R. Thomas, J. Adhesion Sci. Technol. 3 Ž1989. 157᎐173. w4x I. Mathieson, D.M. Brewis, I. Sutherland, R.A. Cayless, J. Adhesion 46 Ž1994. 49᎐56. w5x M. Kusabiraki, Jpn. J. Appl. Phys. 29 Ž1990. 2809᎐2814.
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w6x T. Hirotsu, S. Ohnishi, J. Adhesion 11 Ž1980. 57᎐67. w7x X. Xie, T.R. Gengenbach, H.J. Griessner, J. Adhesion Sci. Technol. 6 Ž1992. 1411᎐1431. w8x A. Ohl, Large area planar microwave discharges, in: C.M. Ferreira ŽEd.., Microwave Discharges: Fundamentals and Applications, NATO ASI Series, B, Plenum Press, New York, 1993, pp. 205᎐214. w9x W. Petasch, E. Rauchle, J. Weichart, H. Bickmann, Surf. Coat. ¨ Technol. 74r75 Ž1995. 200᎐205. w10x E. Rauchle, J. Phys. IV France 8 Ž1998. 99᎐108. ¨ w11x W. Petasch, E. Rauchle, H. Mugge, K. Mugge, Surf. Coat. ¨ ¨ ¨ Technol. 93 Ž1997. 112᎐118.
w12x German Patent, DE 19503205 C1. w13x German Patent, DE 19739894.4-33. w14x M. Walker, K.M. Baumgartner, A. Schulz, E. Rauchle, ¨ ¨ Proceedings of the 14th International Symposium on Plasma Chemistry, Prague, 3, Ž1999. 1427᎐1432. w15x P. Akavoor, W. Menezes, L.L. Kesmodel, G. Apai, W.P. McKenna, J. Vac. Sci. Technol. A 14 Ž1996. 95᎐103. w16x M. Walker, K.-M. Baumgartner, J. Feichtinger, M. Kaiser, A. ¨ Schulz, E. Rauchle, Vacuum 57 Ž2000. 387᎐397. ¨
Short-time plasma pre-treatment of polytetrafluoroethylene for improved adhesion U K.-M. Baumgartner , J. Schneider, A. Schulz, J. Feichtinger, M. Walker ¨ Institut fur ¨ Plasmaforschung, Uni¨ ersitat ¨ Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
Abstract Fluoropolymers with their unique properties Žchemical inertness, thermal stability, low surface tension, mechanical stability . are used in many industrial applications. One disadvantage of these fluoropolymers is their poor adhesion to other materials. In this work a short-time plasma pre-treatment of polytetrafluoroethylene ŽPTFE. with a low pressure microwave plasma was investigated. The Plasmodul 䊛 source allows the pre-treatment of PTFE substrates with an ammonia plasma ŽNH 3 .. The newly developed plasma source Planartron 䊛 , which is derived from the Duo-Plasmaline 䊛 , was used for improving the adhesion properties of PTFE by generating oxygen and nitrogen plasmas. PTFE foils were modified on both sides by plasma treatment for typically 30 s. After bonding the foils to aluminum dollies using a 2-K-epoxy resin the bonding strength of the adhesive joint was measured directly by pull off tests. Some modified foils were additionally investigated by Fourier-transform infrared spectroscopy. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Industrial microwave plasma source; Polytetrafluoroethylene; Surface modification; Plasmodul 䊛 ; Planartron 䊛
1. Introduction Fluoropolymers with their unique properties are desired in many industrial applications. They are known for their chemical inertness, high thermal stability, low surface tension and high mechanical stability. These properties are important concerning engineering aspects, but pre-treatment of the polymers is normally required to achieve satisfactory adhesion. The widely accepted mechanism of polymer surface modification for improved adhesion properties is the formation of functional groups on the surface, according to the request of the used adhesive. Changes of the adhesion properties of PTFE substrates can be investigated by observing the wettability or the surface tension. In the experiment reported here a direct measurement of bond strength between polyU
Corresponding author. MUEGGE ELECTRONIC GmbH, Hochstrasse 4, 64385 Reichelsheim, Germany. Tel: q49-6164-930727. .. E-mail address: [email protected] ŽK.-M. Baumgartner ¨
mer and adhesive is employed by pull-off tests with aluminumrepoxyrPTFErepoxyraluminum adhesive joints w1,2x. Different types of pre-treatment of polyfluoroethylene ŽPTFE. are reported, like wet chemical treatment of the PTFE in a sodiumrammonia solution, UV-light, flame treatment, corona discharges and low pressure plasma modification w3,4x. Especially low pressure plasma treatment of PTFE is a suitable method for introducing functional groups on the PTFE surface for a subsequent bonding procedure w5᎐7x. Concerning economically and environmental aspects the surface treatment with a low pressure plasma is very advantageous. Especially the highly developed 2.45 GHz microwave plasma technology has reached a low cost standard that makes the plasma excitation by microwaves profitable. Homogeneous linearly extended plasmas can be produced by a specific arrangement of the emitting antenna where the microwave power is coupled from a linearly extended waveguide into the plasma w8x.
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 0 9 - 9
502
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
Recent investigations on a simple and inexpensive plasma source started with the Gigatron 䊛 w9x, where a linearly extended glass tube with a concentric metallic inner conductor was mounted in a low pressure chamber. One end was closed, the other one was connected to the microwave generator. The plasma is produced in the low pressure regime outside the tube. The microwaves propagate in the plasma as outer surface waves w10x. The axial homogeneity of the plasma was considerably improved by feeding the microwaves symmetrically from both ends of the tube. This DuoPlasmaline 䊛 forms a linearly extended and homogeneous plasma up to a length of several meters w11,12x. A combination of such plasmalines can be used to obtain a two-dimensional plasma array. In this work, a flexible laboratory plasma device, called Plasmodul 䊛 w13x, is presented. Here, the Plasmaline 䊛 principle is used for medium and small devices as test devices in plasma technology. A further simplification step applied to the Plasmodul 䊛 source leads to a completely new concept of plasma source, called Planartron 䊛 . An antenna structure is placed under a quartz plate at atmospheric pressure. The antenna is fed by 2.45 GHz microwaves. The microwaves mainly propagate along the antenna structure and form a homogeneous, planar plasma area in the low pressure regime on top of a quartz plate. The Planartron 䊛 source is designed for use in the modular plasma device Plasmodul 䊛 .
2. Experimental details 2.1. Plasma sources In this study two types of plasma sources were considered: the modular concept of Plasmodul 䊛 and a new extension called Planartron 䊛 . A view of the modular device Plasmodul 䊛 is shown in Fig. 1. The Plasmodul 䊛 downstream package includes the plasma source, two gas inlet systems, a reaction chamber and diagnostic ports. The device has a diameter of 35 cm and a height of 40 cm. Details of the mechanism of plasma generation and results about the measured electron densities and deposition rates were described previously w10,14,16x. The newly developed Planartron 䊛 source is derived from the Plasmaline 䊛 principle and allows the production of a planar and homogeneous plasma area. Fig. 2 shows a schematic view of the Planartron 䊛 arrangement, in this case especially designed for use as bottom module in the Plasmodul 䊛 device. The Planartron 䊛 device has a diameter of 35 cm with an active area of 20 cm in diameter. An antenna structure is placed under a quartz plate at atmospheric pressure. The antenna is fed by 2.45 GHz microwaves
Fig. 1. View of the Plasmodul 䊛 device with gas inlet system, plasma source, reaction chamber and diagnostic ports.
by a coaxial arrangement. Comparable with the Plasmaline 䊛 , the microwaves mainly propagate along the antenna structure. If the electric field strength exceeds the breakdown field strength the plasma ignites on the top of the quartz plate in the low pressure regime. The extension of the plasma area is mainly dependent on the applied microwave power, and a homogeneous plasma is produced. Electrically nonconducting substrates like PTFE can be placed directly onto the quartz plate and therefore are located in an
Fig. 2. Schematic view of the Planartron 䊛 source. The antenna structure is placed under a quartz plate. The shown arrangement is especially designed for use in Plasmodul 䊛.
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
503
area with high rate processes resulting in short time treatments. The Planartron 䊛 is fed with a maximum total power of 2 = 600 W. 2.2. PTFE treatment Commercially available sheets of PTFE were used as substrate material. Additionally first investigations on a PTFE compound with 25% glass fibers were done. The substrates were cut into pieces of approximately 2 = 2 cm2 . All PTFE substrates were modified at both sides by plasma exposition. The maximum process time was 10 min for each side. Pure PTFE was treated in the Plasmodul 䊛 device at a pressure of 0.2 mbar in an ammonia plasma. The substrates were placed 8 cm below the source and modified at a NH 3 flow rate of 100 sccm Žstandard cm3 miny1 .. The microwave power was 2 = 600 W. In the Planartron 䊛 device the pure PTFE substrates were placed directly onto the quartz plate and alternatively in a distance of 2 cm to the quartz plate. The PTFE substrates were processed at 0.2 mbar in a nitrogen plasma with a nitrogen flux of 100 sccm. The applied microwave power was 600 W. Tests on PTFE-glass fiber compound were performed in Planartron 䊛 device at 0.2 mbar in a nitrogen or alternatively in an oxygen plasma at a gas flow of 100 sccm. 2.3. Pull off test The PTFE foils modified at both sides by plasma treatment were bonded to aluminum dollies using a 2-K epoxy resin ŽUhu Plus sf.. The bond strength of the aluminumrepoxyrPTFErepoxyraluminum adhesive joints was measured by pull off tests with a special adhesion tester ŽElcometer. w1x. 2.4. FTIR spectroscopy IR spectra of the pre-treated PTFE substrates, the 2-K resin and the aluminum dollies after the adhesion tests were recorded on a Bruker Vector 22 with adapted reflectance unit. The measurements were performed at an angle of incidence of 75⬚ using parallel polarized IR light. For each spectrum, 32 scans with 8 cmy1 resolution were recorded.
3. Results and discussion 3.1. PTFE treated with Plasmodul 䊛 in a NH3 plasma In Fig. 3 the bond strength is plotted vs. the exposure time in an ammonia plasma. The figure shows, that a short-time plasma treatment in the range of 15 s results
Fig. 3. Bond strength in dependence of the plasma exposure time. The PTFE was treated with Plasmodul 䊛 in a NH 3 plasma.
in a maximum bond strength. The bond strength is more than one order of magnitude higher compared to the untreated material. In detail the bond strength increases strongly from 0.25 N mmy2 for the untreated fluoropolymer by a factor of 16 up to 3.5 N mmy2 after a plasma treatment of only 15 s. The value of 3.5 N mmy2 is the maximum bond strength. For treating times longer than 30 s the bonding strength strongly decreases to a value of 0.65 N mmy2 after 10 min, which is only an improvement by a factor of 2.6 compared with the untreated polymer. 3.2. PTFE treated with Planartron䊛 in a N2 plasma 3.2.1. Distance substrate᎐source 2 cm PTFE foils were treated with the Planartron 䊛 source by placing the substrates in a distance of 2 cm to the source. In Fig. 4 the bond strength is plotted vs. the exposure time in a nitrogen plasma. The figure shows, that the bond strength of PTFE treated with a nitrogen plasma in the Planartron 䊛 source increases strongly from 0.25 N mmy2 for the untreated material up to 5.9 N mmy2 Žfactor 23. after 50 s plasma treatment. For longer treating times the bond strength decreases to 2.6 N mmy2 after 200 s. 3.2.2. Distance substrate᎐source 0 cm PTFE foils were treated with the Planartron 䊛 source by placing the substrates directly onto the quartz plate of the source. The time dependent bond strength is shown in Fig. 5. After a short time treatment of 5 s the maximum bond strength of 5 N mmy2 is achieved. This represents a factor of 20 compared to the untreated material. Increasing the plasma exposition time again leads to a strong decrease in bond strength. It is remarkable, that already after a treatment of 5 s the same bond strength is achieved as treating the PTFE for 20 s in a
504
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
Fig. 4. Bond strength in dependence of the plasma exposure time. The PTFE was treated with the Planartron 䊛 source in a distance of 2 cm with a nitrogen plasma.
distance of 2 cm to the source. This means, that shortest plasma exposition times are achieved under the condition, that the substrates are directly placed on the Planartron 䊛 source. 3.3. Acti¨ ation of PTFE-glass fiber compound Some investigations were performed on a PTFE compound containing additionally to the fluoropolymer 25% of glass fibers ŽPTFE-GF.. For the tests with the PTFE-GF the Planartron 䊛 source was used. The bond strength of the untreated material is 0.8 N mmy2 and is slightly higher compared to pure PTFE Ž0.25 N mmy2 . due to mechanical forces among the adhesive and the glass fibers, which protrude some microns out of the surface of the PTFE-GF. Fig. 6 shows a microscopical picture of such an untreated PTFE-GF compound. The left half in the picture of Fig. 6 shows the formerly bonded area after
Fig. 5. Bond strength in dependence of the plasma exposure time. The PTFE was treated with the Planartron 䊛 source in direct contact Ž0 cm distance. with a nitrogen plasma.
Fig. 6. Microscopical picture of an untreated PTFE-GF compound. The area on the left half of substrate was formerly bonded to an aluminum dolly. The adhesive was removed completely without leaving damages.
the pull off tests, while the right half is completely unstressed. The adhesive is removed completely with the aluminum dolly from the untreated surface without leaving any damages. Only a small amount of glass fibers was torn away by the adhesive and explains the slightly increased bond strength. Fig. 7 shows a microscopic examination of a PTFEGF sample, which was treated for 5 min in an oxygen plasma and afterwards bonded to an aluminum dolly. After the pull off test the sample was wetted with a test ink. Again the left half of Fig. 7 represents the area after removing the adhesive joint, while the right half is unstressed. The test ink demonstrates, that the glass fibers are activated by the plasma process while the PTFE is completely unactivated and shows no wettability Žright side of Fig. 7.. The area with removed aluminum dolly contains almost no glass fibers and cannot be wetted at all Žleft side of Fig. 7.. This means,
Fig. 7. Microscopical picture of a plasma treated PTFE-GF substrate ŽO 2 plasma.. The glass fibers were activated and torn away from the surface as a result of the pull of test Žleft side of the substrate ..
K.-M. Baumgartner et al. r Surface and Coatings Technology 142᎐144 (2001) 501᎐506 ¨
that the glass fibers show a good adhesion property, which lead to an increased bond strength of 2.8 N mmy2 . The remaining surface after the pull off test is almost pure PTFE and does not contribute significantly to the total bond strength. For maximum bond strength it is necessary to activate both components, PTFE as well as the glass fibers. Fig. 8 shows a microscopic examination of PTFE-GF material treated in a nitrogen plasma for 5 min. Again the substrate was bonded to an aluminum dolly and tested with the adhesion tester. The test ink demonstrates, that the glass fibers as well as the PTFE were activated in the nitrogen plasma Žright side of Fig. 8, unstressed .. The left side of the picture in Fig. 8 shows the formerly bonded area. The glass fibers are torn away with the adhesive and also the activated PTFE was removed by the pull off test. The test area can not be wetted at all. The completely activated surface is reflected in the high bond strength of 6 N mmy2 .
505
Fig. 9. IR spectra of PTFE, 2-K epoxy resin ŽUhu plus sf. and additionally of the bond side of an aluminum dolly after pull off test.
terial. The maximum achievable bond strength is limited by the PTFE structure on the surface and subsurface after the plasma exposition.
3.4. FTIR in¨ estigations on plasma acti¨ ated PTFE 4. Conclusions Fig. 9 shows IR-spectra of plasma activated PTFE, of hardened 2-K-epoxy resin and additionally of a bonded aluminum dolly after removal from the PTFE substrate Žbonded side.. The spectra were recorded in the range from 400 cmy1 to 700 cmy1 . The most intense absorption peaks reported for CF2 modes in PTFE Ž553 cmy1 , 625 cmy1 and 638 cmy1 . are clearly identified w15x. Considering the IR-spectrum of the residue on a formerly bonded aluminum dolly, a superposition of the spectra recorded for PTFE and 2-K-epoxy resin, is found. This means, that the residue on the dolly not only consists of 2-K-epoxy resin, but also contains PTFE, torn away from the substrate ma-
Plasma pre-treatment of PTFE surfaces strongly influences the adhesion properties. New available plasma sources like Plasmodul 䊛 and Planartron 䊛 allow short time pre-treatment of PTFE for significant improved adhesion. Especially the Planartron 䊛 source represents a tool for high rate processes. The exposure times can be kept very short. The achieved bond strength is limited by the PTFE structure on the surface and subsurface after the plasma exposition.
Acknowledgements This work was performed using microwave equipment from MUEGGE ELECTRONIC GmbH, Hochstrasse 4, 64385 Reichelsheim, Germany. The authors are very grateful to K. Muegge and H. Muegge. The authors thank Professor Dr U. Schumacher from the Institut fur ¨ Plasmaforschung der Universitat ¨ Stuttgart for useful discussions. The authors would like to thank H. Petto for his technical assistance. References
Fig. 8. Microscopical picture of a plasma treated PTFE-GF substrate ŽN2 plasma.. The glass fibers as well as the PTFE were activated and both torn away from the surface as a result of the pull of test Žleft side of the substrate ..
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