- Email: [email protected]
How acoustic cavitation can improve adhesion
Ultrasonics 52 (2012) 905–911
Contents lists available at SciVerse ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
How acoustic cavitation can improve adhesion J. Holtmannspötter a,⇑, M. Wetzel a, J.v. Czarnecki a, H.-J. Gudladt b a b
Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB), Institutsweg 1, 85435 Erding, Germany Bundeswehr University Munich, Department of Aerospace Engineering/IWK, W. Heisenberg-Weg 117, 85579 Neubiberg, Germany
a r t i c l e
i n f o
Article history: Received 30 October 2011 Received in revised form 18 February 2012 Accepted 19 February 2012 Available online 15 March 2012 Keywords: Power ultrasound Adhesive bonding Contamination tolerance Viscosity Bjerknes forces
a b s t r a c t In general, ultrasound is commonly used at low power level for non-destructive testing (NDT) and detection of delaminations in adhesive bonded structures. The present paper instead presents an approach where power ultrasound is used to improve interface formation prior to the bonding process and to ensure the quality of adhesive bonds by using acoustic cavitation in the liquid adhesive. Results from high-speed videos, rheological and thermal measurements and destructive testing of adhesive bonds with contaminated surfaces are presented and discussed. Power ultrasound can be used in general to improve adhesion and significantly to improve contamination tolerance and robustness of adhesive bonding processes. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Structural adhesive bonding is a joining technique that is successfully used in various fields. It becomes more and more popular due to the need for light-weight multi-material structures where thin materials have to be joined in a weight-efficient way [3]. In addition, for upcoming structures made of carbon fiber reinforced plastics and especially for repair issues, adhesive bonding is a joining technology the engineer would principally like to use. However, invisible surface contaminations on the substrate often lead to an adhesive interface failure that is accompanied by low fracture energy. Therefore, the adhesive bonding is often avoided for safety relevant applications. Unfortunately, a non-destructive test method that could detect weak bonds does not yet exist. It is a challenge to transform adhesive bonding into a contaminationtolerant and inherently robust process. Normally, a sufficient contamination tolerance of adhesive bonding can be improved by increasing the curing temperature. Up to now, contamination tolerance could not be obtained for room temperature curing adhesives because at that low temperature the necessary molecular transport process, e.g. the diffusion, is not effective enough and the adhesive cures too fast in comparison with hot curing adhesives. In this paper a new idea is presented for solving the competing interface transport problem for contamination in adhesive. By using power ultrasound, energy is provided to support a robust
⇑ Corresponding author. Tel.: +49 8122 9590 3610; fax: +49 8122 9590 3602. E-mail address: [email protected] (J. Holtmannspötter). 0041-624X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2012.02.013
interface formation even at low temperatures. Furthermore, the use of power ultrasound to improve adhesion in general is presented and discussed.
2. Experimental methods For the application, a commercial set of ultrasonic converter and ultrasonic generator from Herrmann Ultrasonic was used. The system consists of a converter, a booster and a sonotrode with integrated adhesive supply (Fig. 1). The digital generator controls the amplitude of the converter at a constant level of about 7 lm independent from the load on the contact (top) surface of the sonotrode. The amplitude is amplified with a mechanical booster. The sonotrode increases further the mass of the oscillating system and is used to couple the ultrasound into the liquid adhesive. To establish an uniform amplitude all over the contact surface, the geometry of the sonotrode was optimized using the finite element method. With a modal analysis, the dynamic behavior can be fitted in an iterative manner. The result is shown in Fig. 2. The final sonotrode was made in the form of a bar, where an elliptic tapering avoids lateral contraction. First experiments with block-form sonotrodes showed insufficient amplitudes especially in the corners. The developed new sonotrode has an uniform amplitude all over the contact surface at 35 kHz. To obtain information about the ultrasonic cleaning process at an interface, a transparent epoxy adhesive (UHU Endfest 300) was applied onto a glass substrate and the interface was observed with a high-speed color camera (Optronis CamRecord 1000) at a
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J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
Fig. 1. Ultrasonic adhesive application system consisting of sonotrode, booster and converter (from left to right).
Fig. 2. Sonotrode geometry and results of the modal analysis (total displacement).
frame rate up to 100,000/s from below. For illumination a cluster of 10 high-power LED (total of 35 W) were used. The camera recorded an adhesive filled gap of 0.1–1 mm between the glass substrate (bottom) and the sonotrode (top). To simulate a contaminated surface, lines and circles with a permanent marker (Edding 3000) were painted on the glass plate. For thermal imaging, the glass plate was replaced by a silizium wafer and an IR-Camera (FLIR SC 3000, 8–9 lm, 50 Hz) was taken to study the rise of temperature in the adhesive. For rheological measurements of the adhesive under influence of power ultrasound, a thermo fisher Rheostress 6000 rheometer was modified for plate–plate measurements. The temperature plate was substituted to an ultrasonic sonotrode to couple ultrasound in the gap of the plate–plate geometry. To show the application of ultrasound for adhesive bonding lap shear samples according to DIN EN 1465, made of carbon forced
reinforced plastics (Hexcel 8552/IM7), were tested. Surface preparation for CFRP was done by peel ply (Fiberflon 408.07P) removal and subsequent atmospheric plasma treatment (Plasmatreat OPENAIR). After surface preparation one side of the samples was dipped in diluted hydraulic fluid oil (NATO Code H515) to simulate contaminated surfaces. The samples were stored in an oven for 1 h at 130 °C before bonding to achieve a uniform contamination on the surfaces [7,6].
3. Results 3.1. Power ultrasound applied to liquid adhesives Complex cavitation structures of various forms can be observed inside the adhesive at 35 kHz (Fig. 3). Experiments with a high
J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
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Fig. 3. Depending on the viscosity, temperature, ultrasonic amplitude and adhesive various cavitation structures can be seen in high-speed videos (exposure time 1/1000 s).
magnification and short exposure time (<10 5 s) showed similar cavitation mechanism to those known from water [9,1], but in a smaller scale. The cavitation starts with single oscillation bubbles. The cavitation bubbles grow, collapse and are divided in smaller bubbles. But due to the high viscosity of the adhesive (50 Pa s at RT) the bubbles stay close together. The bubbles grow with each ultrasonic cycle but do not reach the same size as the initial bubble because of collisions with other bubbles. The cavitation bubbles appear directly in the gap with the presence of the ultrasonic oscillation and disappear after the oscillator is turned off. Contrary to liquids like water or ethanol very small bubbles (<10 lm) with a strong interaction due to primary and secondary Bjerknes forces appear in the liquid adhesive. Various dynamic structures with high stream velocities can be observed within an amplitude range of 1–40 lm. At the same amplitude in ethanol or water, only a massive cavitation without any real structure is visible. The bubble size in ethanol and water is, compared with adhesives, irregular and bigger. To obtain an area-wide cavitation in an epoxy adhesive ultrasound amplitudes of approximately 5 lm are needed. Locally there is a significant rise in temperature in the adhesive during sonification. In cavitation spots in the gap the adhesive temperature reaches up to 200° in parts of a second (Fig. 4). The
temperature depends mainly on the amplitude of the ultrasonic oscillation and the thermal conductivity of the contact surfaces. It can be explained by molecular friction and energy transformation from collapsing cavitation bubbles. Fig. 4 shows a sequence of the first 1.8 s after coupling in the power ultrasound. From the thermal images it is clearly visible, that the cavitation structures, that were already documented with the high speed camera, are the hottest areas. So cavitation is the main reason for the increase in temperature. For adhesive bonding, a good wetting of the surface by the adhesive is a must to establish adhesion. This requires a low viscosity of the liquid phase. But for most adhesives low viscosity is in direct conflict with the industrial process, where the adhesive has to stay in place and shape for a while after its application. To keep them in place until the final curing, structural adhesives therefore normally have a thixotropic rheological behavior and a higher viscosity (>1 Pa s). Due to the intense shear and the rise of temperature of the polymer during the sonification, the viscosity of the adhesive can be reduced for the moment of application. In rheological measurements, this effect can be clearly demonstrated. With a modified rotatory rheometer the adhesive was sheared for 100 ms at a high shear rate. Short pulses of ultrasound at low amplitude (to avoid cavitation) were then coupled into the adhesive. The result is a sudden and significant reduction in viscosity
Fig. 4. Thermal images of the cavitation process (35 kHz, amplitude was 4 lm). The images were taken through an IR-transparent silizium wafer.
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J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
Fig. 5. Results from rheological measurements. Ultrasonic pulses (100 ms, 300 ms, 500 ms; amplitude was 6 lm) cause a sudden reduction in viscosity of the sheared adhesive (100 s at c = 100 s 1).
of the adhesive (Fig. 5). The rise in temperature in the adhesive was low. 3.2. Power ultrasound applied to adhesive bonding The pictures from a high-speed-film (Fig. 6) show a glass plate with a permanent marker line (simulated contamination) covered with an epoxy adhesive. The video was recorded again through the glass substrate while ultrasonic energy was coupled into the adhesive from the top. To simulate an adhesion limiting surface contamination, a black line of permanent marker was applied in the center. Without ultrasound the adhesive would cure without adhesion to the substrate and removing the marking. The gap between the sonotrode and the glass surface was 0.5 mm. It takes about a second to remove the contamination completely by cavitation erosion. The contamination was eroded from the interface and diffused into the adhesive while the adhesive itself gets in direct contact to the substrate. The interface cleaning effect is clearly associated with the cavitation bubbles. Fig. 7 shows pictures from a high speed video
sequence of 300 ls with the local dynamic cavitation process starting to remove the contamination from the surface. In contrary to Fig. 6 the cavitation cleaning of the black permanent marker line on the glass substrate is done at lower ultrasound amplitude of 2 lm. The power level is here not sufficient to build up an areawide cavitation. Although the complete sonotrode oscillates with constant amplitude, it can be seen that only the arms of the cavitation clouds remove the marking from the surface. The oscillating adhesive itself does not remove the marking. Cleaning speed and efficiency depend mainly on the amount of amplitude, the adhesive and the gap width between sonotrode and the substrate. To determine optimized parameters, small circles were applied with the permanent marker on the glass plates. The time for the total cleaning of the circles was measured for different parameters. Fig. 8 shows the cleaning of the circular markings for two different parameter settings and the cleaning time dependence for the two main process parameters amplitude and gap height for water and the epoxy adhesive UHU Endfest 300. Using these optimized parameters, contamination tolerant adhesive bonding becomes possible [7] by the ultrasound
Fig. 6. Pictures taken from a high-speed video sequence to demonstrate the cleaning effectness of cavitation bubbles (15 lm amplitude) in adhesives. The black line of a permanent marker is removed in a second from the surface and the ink spreads into the adhesive by acoustic-driven cavitation.
J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
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Fig. 7. Pictures taken from a high-speed video sequence to demonstrate the cleaning mechanism. The black line of a permanent marker is removed by the acoustic-driven cavitation bubbles only.
Fig. 8. To measure the influence of the main process parameters gap height and ultrasonic amplitude, the time for cleaning simulated contamination (circles of a permanent marker) was measured. In the diagram below, the cleaning time for an epoxy adhesive and water is shown for varying gap height and ultrasonic amplitude.
supported adhesive application. By using this technique, specimens with contaminated surfaces (hydraulic oil) were bonded and showed high loads in destructive testing, similar to the clean reference samples. Furthermore, the performance of the bonds on clean specimens was improved by more than 10 percent (Fig. 9). In all cases US treated specimens show cohesive fracture behavior and shear strength close to the reference samples. The intensive cavitation in adhesives and liquids causes also visible modification on several substrate surfaces. Fig. 10 shows the cavitation erosion effect after 2, 10 and 30 s for two different liquids on pure aluminium. For water, considerable erosion of the surface is found. For ethanol, only light erosion is visible. With a scanning electron microscope (SEM) a deformation of the surface can be seen (Fig. 11) after ultrasound exposure. Due to the cavitation implosions the surface shows a microstructured deformation. Traces of the manufacturing of the aluminium (scratches, lines) are still visible. Even the small pits in figure are
not eroded away. Locally the material seems to be compressed heavily (Fig. 12). For polymer substrates no significant change of the surface topography was found. 3.3. Discussion The presented results show the potential for ultrasonic assisted processes to establish adhesion on difficult and contaminated surfaces. With massive acoustic cavitation induced in the liquid adhesive itself surfaces can be cleaned in a short time. A recontamination is securely excluded because the adhesive is used as coupling medium, cleaning agent and finally as the curing adhesive forming the bond. This new approach differs from existing processes (see for example [10]), where substrate cleaning is done in a conventional ultrasonic bath at a relatively low amplitude. Interface cleaning and contamination removal are the consequence of fluid streams and bubble implosions. Bubble implosions
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J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
Fig. 9. Lap shear strength of clean and contaminated adhesive bonded specimens with and without ultrasonic treatment. Specimens were contaminated with an adhesion limiting solution of petrol and hydraulic oil H- 515 (further results can be seen in [HCG]).
Fig. 10. Cavitation erosion on pure aluminium (35 kHz, 10 lm, gap height 0.5 mm) for different liquids after 2, 10 and 30 s.
Fig. 11. Scanning electron microscopy image (SEM) of an ultrasonic treated (10 s, water, 10 lm) surface of aluminium.
near surfaces remove adhesion preventing adsorbents by local deposition of energy during their implosion. By bubble interaction, mainly caused by Bjerknes forces, adhesion-limiting ingredients in the liquid are transported away from the boundary layer. The use of power ultrasound is interesting for adhesive bonding because of the possibility for ‘‘inline-interface-cleaning’’. The principle of such an ultrasound assisted adhesive application is shown in Fig. 13.
Fig. 12. High resolution SEM image of a cavitation erosion pit.
Thus, contamination tolerance can be significantly improved [2,5–7]. With this technology, adhesive bonding becomes much more robust and seems applicable for structural purposes with high safety requirements. The power ultrasound also disturbs inter-molecular forces of the fluid and increases the temperature. This results in a significant lower viscosity and better wetting of the substrates. Out gassing of adsorbents trapped in surface capillaries and surface modification within the liquid are further advantages. This effect can be used for
J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
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Fig. 13. Application of an adhesive assisted by ultrasonic energy. Illustration of the process (left) and real process demonstrator (right).
Fig. 14. Improvement of wetting of a degreased aluminium surface with power ultrasound.
an inline-surface preparation or to support wetting of materials with low surface energy. Fig. 14 shows the wetting of a water based adhesion primer on an aluminium substrate. The aluminium surface was degreased. With a short ultrasonic treatment (35 kHz, 10 lm) the surface is cleaned and eroded. The consequence is a perfect wetting, which is necessary for adhesion. 4. Summary In conclusion, the use of acoustic cavitation and the process presented to clean surfaces and to assist adhesion is a simple and powerful method for adhesive bonding and other adhesion-based processes. A very interesting topic to study is the initiation or acceleration of interfacial chemical reactions by ‘‘sonochemistry’’ [8,4] and to use its possibilities to allow new or to improve existing coating processes. This will also be topic of further investigation. Additionally the effects of power ultrasound with higher frequency (up to 2 MHz) in liquid polymers are being investigating.
References [1] W. Lauterborn, in: M.J. Crocker (Ed.), Handbook of Acoustics, 1998, pp. 235– 242. [2] J.v. Czanecki, GE Patent 103 12 815.8 and GE Patent 102 004013845.1. [3] W. Brockmann, P.L. Geiss, J. Klingen, K.B. Schroder, B. Mikhail, Adhesive Bonding: Materials, Applications and Technology, Wiley-VCH, Weinheim, 2009. [4] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. McNamara, M.M. Mdleleni, M. Wong, Roy Soc of London Phil Tr A 357 (1751) (1999) p. 335. [5] M.v. Hayek-Boelingen, Wege zum kontaminationstoleranten Kleben, PhD Thesis, University of the Bundeswehr, Faculty for Air and Aerospace Technology, Munich, 2004. [6] J. Holtmannspötter, Einsatz von Leistungsultraschall zu Verbesserung klebtechnischer Prozesse, PhD Thesis, University of the Bundeswehr, Faculty for Air and Aerospace Technology, Munich, 2010. [7] Holtmannspötter, J.v. Czarnecki, H.-J. Gudladt, The Use of Power Ultrasound Energy to Support Interface Formation for Structural Adhesive Bonding, International Journal of Adhesion and Adhesives, April 2010. [8] T.J. Mason, Sonochemistry, Oxford University Press, New York, 1999. [9] C.E. Brennen, Cavitation and Bubble Dynamics, Oxford University Press, 1995. [10] H. Keutner, Verfahren zur Herstellung von Farb- und Lackanstrichen. Deutsches Patent- und Markenamt, 1957.
Contents lists available at SciVerse ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
How acoustic cavitation can improve adhesion J. Holtmannspötter a,⇑, M. Wetzel a, J.v. Czarnecki a, H.-J. Gudladt b a b
Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB), Institutsweg 1, 85435 Erding, Germany Bundeswehr University Munich, Department of Aerospace Engineering/IWK, W. Heisenberg-Weg 117, 85579 Neubiberg, Germany
a r t i c l e
i n f o
Article history: Received 30 October 2011 Received in revised form 18 February 2012 Accepted 19 February 2012 Available online 15 March 2012 Keywords: Power ultrasound Adhesive bonding Contamination tolerance Viscosity Bjerknes forces
a b s t r a c t In general, ultrasound is commonly used at low power level for non-destructive testing (NDT) and detection of delaminations in adhesive bonded structures. The present paper instead presents an approach where power ultrasound is used to improve interface formation prior to the bonding process and to ensure the quality of adhesive bonds by using acoustic cavitation in the liquid adhesive. Results from high-speed videos, rheological and thermal measurements and destructive testing of adhesive bonds with contaminated surfaces are presented and discussed. Power ultrasound can be used in general to improve adhesion and significantly to improve contamination tolerance and robustness of adhesive bonding processes. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Structural adhesive bonding is a joining technique that is successfully used in various fields. It becomes more and more popular due to the need for light-weight multi-material structures where thin materials have to be joined in a weight-efficient way [3]. In addition, for upcoming structures made of carbon fiber reinforced plastics and especially for repair issues, adhesive bonding is a joining technology the engineer would principally like to use. However, invisible surface contaminations on the substrate often lead to an adhesive interface failure that is accompanied by low fracture energy. Therefore, the adhesive bonding is often avoided for safety relevant applications. Unfortunately, a non-destructive test method that could detect weak bonds does not yet exist. It is a challenge to transform adhesive bonding into a contaminationtolerant and inherently robust process. Normally, a sufficient contamination tolerance of adhesive bonding can be improved by increasing the curing temperature. Up to now, contamination tolerance could not be obtained for room temperature curing adhesives because at that low temperature the necessary molecular transport process, e.g. the diffusion, is not effective enough and the adhesive cures too fast in comparison with hot curing adhesives. In this paper a new idea is presented for solving the competing interface transport problem for contamination in adhesive. By using power ultrasound, energy is provided to support a robust
⇑ Corresponding author. Tel.: +49 8122 9590 3610; fax: +49 8122 9590 3602. E-mail address: [email protected] (J. Holtmannspötter). 0041-624X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2012.02.013
interface formation even at low temperatures. Furthermore, the use of power ultrasound to improve adhesion in general is presented and discussed.
2. Experimental methods For the application, a commercial set of ultrasonic converter and ultrasonic generator from Herrmann Ultrasonic was used. The system consists of a converter, a booster and a sonotrode with integrated adhesive supply (Fig. 1). The digital generator controls the amplitude of the converter at a constant level of about 7 lm independent from the load on the contact (top) surface of the sonotrode. The amplitude is amplified with a mechanical booster. The sonotrode increases further the mass of the oscillating system and is used to couple the ultrasound into the liquid adhesive. To establish an uniform amplitude all over the contact surface, the geometry of the sonotrode was optimized using the finite element method. With a modal analysis, the dynamic behavior can be fitted in an iterative manner. The result is shown in Fig. 2. The final sonotrode was made in the form of a bar, where an elliptic tapering avoids lateral contraction. First experiments with block-form sonotrodes showed insufficient amplitudes especially in the corners. The developed new sonotrode has an uniform amplitude all over the contact surface at 35 kHz. To obtain information about the ultrasonic cleaning process at an interface, a transparent epoxy adhesive (UHU Endfest 300) was applied onto a glass substrate and the interface was observed with a high-speed color camera (Optronis CamRecord 1000) at a
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J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
Fig. 1. Ultrasonic adhesive application system consisting of sonotrode, booster and converter (from left to right).
Fig. 2. Sonotrode geometry and results of the modal analysis (total displacement).
frame rate up to 100,000/s from below. For illumination a cluster of 10 high-power LED (total of 35 W) were used. The camera recorded an adhesive filled gap of 0.1–1 mm between the glass substrate (bottom) and the sonotrode (top). To simulate a contaminated surface, lines and circles with a permanent marker (Edding 3000) were painted on the glass plate. For thermal imaging, the glass plate was replaced by a silizium wafer and an IR-Camera (FLIR SC 3000, 8–9 lm, 50 Hz) was taken to study the rise of temperature in the adhesive. For rheological measurements of the adhesive under influence of power ultrasound, a thermo fisher Rheostress 6000 rheometer was modified for plate–plate measurements. The temperature plate was substituted to an ultrasonic sonotrode to couple ultrasound in the gap of the plate–plate geometry. To show the application of ultrasound for adhesive bonding lap shear samples according to DIN EN 1465, made of carbon forced
reinforced plastics (Hexcel 8552/IM7), were tested. Surface preparation for CFRP was done by peel ply (Fiberflon 408.07P) removal and subsequent atmospheric plasma treatment (Plasmatreat OPENAIR). After surface preparation one side of the samples was dipped in diluted hydraulic fluid oil (NATO Code H515) to simulate contaminated surfaces. The samples were stored in an oven for 1 h at 130 °C before bonding to achieve a uniform contamination on the surfaces [7,6].
3. Results 3.1. Power ultrasound applied to liquid adhesives Complex cavitation structures of various forms can be observed inside the adhesive at 35 kHz (Fig. 3). Experiments with a high
J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
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Fig. 3. Depending on the viscosity, temperature, ultrasonic amplitude and adhesive various cavitation structures can be seen in high-speed videos (exposure time 1/1000 s).
magnification and short exposure time (<10 5 s) showed similar cavitation mechanism to those known from water [9,1], but in a smaller scale. The cavitation starts with single oscillation bubbles. The cavitation bubbles grow, collapse and are divided in smaller bubbles. But due to the high viscosity of the adhesive (50 Pa s at RT) the bubbles stay close together. The bubbles grow with each ultrasonic cycle but do not reach the same size as the initial bubble because of collisions with other bubbles. The cavitation bubbles appear directly in the gap with the presence of the ultrasonic oscillation and disappear after the oscillator is turned off. Contrary to liquids like water or ethanol very small bubbles (<10 lm) with a strong interaction due to primary and secondary Bjerknes forces appear in the liquid adhesive. Various dynamic structures with high stream velocities can be observed within an amplitude range of 1–40 lm. At the same amplitude in ethanol or water, only a massive cavitation without any real structure is visible. The bubble size in ethanol and water is, compared with adhesives, irregular and bigger. To obtain an area-wide cavitation in an epoxy adhesive ultrasound amplitudes of approximately 5 lm are needed. Locally there is a significant rise in temperature in the adhesive during sonification. In cavitation spots in the gap the adhesive temperature reaches up to 200° in parts of a second (Fig. 4). The
temperature depends mainly on the amplitude of the ultrasonic oscillation and the thermal conductivity of the contact surfaces. It can be explained by molecular friction and energy transformation from collapsing cavitation bubbles. Fig. 4 shows a sequence of the first 1.8 s after coupling in the power ultrasound. From the thermal images it is clearly visible, that the cavitation structures, that were already documented with the high speed camera, are the hottest areas. So cavitation is the main reason for the increase in temperature. For adhesive bonding, a good wetting of the surface by the adhesive is a must to establish adhesion. This requires a low viscosity of the liquid phase. But for most adhesives low viscosity is in direct conflict with the industrial process, where the adhesive has to stay in place and shape for a while after its application. To keep them in place until the final curing, structural adhesives therefore normally have a thixotropic rheological behavior and a higher viscosity (>1 Pa s). Due to the intense shear and the rise of temperature of the polymer during the sonification, the viscosity of the adhesive can be reduced for the moment of application. In rheological measurements, this effect can be clearly demonstrated. With a modified rotatory rheometer the adhesive was sheared for 100 ms at a high shear rate. Short pulses of ultrasound at low amplitude (to avoid cavitation) were then coupled into the adhesive. The result is a sudden and significant reduction in viscosity
Fig. 4. Thermal images of the cavitation process (35 kHz, amplitude was 4 lm). The images were taken through an IR-transparent silizium wafer.
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J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
Fig. 5. Results from rheological measurements. Ultrasonic pulses (100 ms, 300 ms, 500 ms; amplitude was 6 lm) cause a sudden reduction in viscosity of the sheared adhesive (100 s at c = 100 s 1).
of the adhesive (Fig. 5). The rise in temperature in the adhesive was low. 3.2. Power ultrasound applied to adhesive bonding The pictures from a high-speed-film (Fig. 6) show a glass plate with a permanent marker line (simulated contamination) covered with an epoxy adhesive. The video was recorded again through the glass substrate while ultrasonic energy was coupled into the adhesive from the top. To simulate an adhesion limiting surface contamination, a black line of permanent marker was applied in the center. Without ultrasound the adhesive would cure without adhesion to the substrate and removing the marking. The gap between the sonotrode and the glass surface was 0.5 mm. It takes about a second to remove the contamination completely by cavitation erosion. The contamination was eroded from the interface and diffused into the adhesive while the adhesive itself gets in direct contact to the substrate. The interface cleaning effect is clearly associated with the cavitation bubbles. Fig. 7 shows pictures from a high speed video
sequence of 300 ls with the local dynamic cavitation process starting to remove the contamination from the surface. In contrary to Fig. 6 the cavitation cleaning of the black permanent marker line on the glass substrate is done at lower ultrasound amplitude of 2 lm. The power level is here not sufficient to build up an areawide cavitation. Although the complete sonotrode oscillates with constant amplitude, it can be seen that only the arms of the cavitation clouds remove the marking from the surface. The oscillating adhesive itself does not remove the marking. Cleaning speed and efficiency depend mainly on the amount of amplitude, the adhesive and the gap width between sonotrode and the substrate. To determine optimized parameters, small circles were applied with the permanent marker on the glass plates. The time for the total cleaning of the circles was measured for different parameters. Fig. 8 shows the cleaning of the circular markings for two different parameter settings and the cleaning time dependence for the two main process parameters amplitude and gap height for water and the epoxy adhesive UHU Endfest 300. Using these optimized parameters, contamination tolerant adhesive bonding becomes possible [7] by the ultrasound
Fig. 6. Pictures taken from a high-speed video sequence to demonstrate the cleaning effectness of cavitation bubbles (15 lm amplitude) in adhesives. The black line of a permanent marker is removed in a second from the surface and the ink spreads into the adhesive by acoustic-driven cavitation.
J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
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Fig. 7. Pictures taken from a high-speed video sequence to demonstrate the cleaning mechanism. The black line of a permanent marker is removed by the acoustic-driven cavitation bubbles only.
Fig. 8. To measure the influence of the main process parameters gap height and ultrasonic amplitude, the time for cleaning simulated contamination (circles of a permanent marker) was measured. In the diagram below, the cleaning time for an epoxy adhesive and water is shown for varying gap height and ultrasonic amplitude.
supported adhesive application. By using this technique, specimens with contaminated surfaces (hydraulic oil) were bonded and showed high loads in destructive testing, similar to the clean reference samples. Furthermore, the performance of the bonds on clean specimens was improved by more than 10 percent (Fig. 9). In all cases US treated specimens show cohesive fracture behavior and shear strength close to the reference samples. The intensive cavitation in adhesives and liquids causes also visible modification on several substrate surfaces. Fig. 10 shows the cavitation erosion effect after 2, 10 and 30 s for two different liquids on pure aluminium. For water, considerable erosion of the surface is found. For ethanol, only light erosion is visible. With a scanning electron microscope (SEM) a deformation of the surface can be seen (Fig. 11) after ultrasound exposure. Due to the cavitation implosions the surface shows a microstructured deformation. Traces of the manufacturing of the aluminium (scratches, lines) are still visible. Even the small pits in figure are
not eroded away. Locally the material seems to be compressed heavily (Fig. 12). For polymer substrates no significant change of the surface topography was found. 3.3. Discussion The presented results show the potential for ultrasonic assisted processes to establish adhesion on difficult and contaminated surfaces. With massive acoustic cavitation induced in the liquid adhesive itself surfaces can be cleaned in a short time. A recontamination is securely excluded because the adhesive is used as coupling medium, cleaning agent and finally as the curing adhesive forming the bond. This new approach differs from existing processes (see for example [10]), where substrate cleaning is done in a conventional ultrasonic bath at a relatively low amplitude. Interface cleaning and contamination removal are the consequence of fluid streams and bubble implosions. Bubble implosions
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J. Holtmannspötter et al. / Ultrasonics 52 (2012) 905–911
Fig. 9. Lap shear strength of clean and contaminated adhesive bonded specimens with and without ultrasonic treatment. Specimens were contaminated with an adhesion limiting solution of petrol and hydraulic oil H- 515 (further results can be seen in [HCG]).
Fig. 10. Cavitation erosion on pure aluminium (35 kHz, 10 lm, gap height 0.5 mm) for different liquids after 2, 10 and 30 s.
Fig. 11. Scanning electron microscopy image (SEM) of an ultrasonic treated (10 s, water, 10 lm) surface of aluminium.
near surfaces remove adhesion preventing adsorbents by local deposition of energy during their implosion. By bubble interaction, mainly caused by Bjerknes forces, adhesion-limiting ingredients in the liquid are transported away from the boundary layer. The use of power ultrasound is interesting for adhesive bonding because of the possibility for ‘‘inline-interface-cleaning’’. The principle of such an ultrasound assisted adhesive application is shown in Fig. 13.
Fig. 12. High resolution SEM image of a cavitation erosion pit.
Thus, contamination tolerance can be significantly improved [2,5–7]. With this technology, adhesive bonding becomes much more robust and seems applicable for structural purposes with high safety requirements. The power ultrasound also disturbs inter-molecular forces of the fluid and increases the temperature. This results in a significant lower viscosity and better wetting of the substrates. Out gassing of adsorbents trapped in surface capillaries and surface modification within the liquid are further advantages. This effect can be used for
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Fig. 13. Application of an adhesive assisted by ultrasonic energy. Illustration of the process (left) and real process demonstrator (right).
Fig. 14. Improvement of wetting of a degreased aluminium surface with power ultrasound.
an inline-surface preparation or to support wetting of materials with low surface energy. Fig. 14 shows the wetting of a water based adhesion primer on an aluminium substrate. The aluminium surface was degreased. With a short ultrasonic treatment (35 kHz, 10 lm) the surface is cleaned and eroded. The consequence is a perfect wetting, which is necessary for adhesion. 4. Summary In conclusion, the use of acoustic cavitation and the process presented to clean surfaces and to assist adhesion is a simple and powerful method for adhesive bonding and other adhesion-based processes. A very interesting topic to study is the initiation or acceleration of interfacial chemical reactions by ‘‘sonochemistry’’ [8,4] and to use its possibilities to allow new or to improve existing coating processes. This will also be topic of further investigation. Additionally the effects of power ultrasound with higher frequency (up to 2 MHz) in liquid polymers are being investigating.
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