Ultrasonic atomisation: A novel technique for surface coatings
S G Gaikwad, C Reddy V, and A B Pandit C h e m i c a l Ensineerin 8 D e p a r t m e n t , U n i v e r s i t y Institute of C h e m i c a l Technology, M a t u n g a , M u m b a i 4 0 0 0 1 9 , India
Keywords Atomisat[on, droplet, film thickness, surface coating, ultrasonic atom[sation
Summaries Ultrasonic atomisalion: A novel lechnique for surlace coatings In this work, ultrasonic atom[sation as a possible spray coating on a moving surface has been stud red. The experimental method employed here involved the spraying of ink vertically downwards onto a paper, which was moving laterally. It was found that the percentage area of the covered surface increased with an increase in the height of the ultrasound atom[set from the surface and the v[bra tional amplitude of the atomiser, while the liquid film thickness on the coated surface decreased with an increase in the height of the ultrasound atomiser, vibrational amplitude of the atomiser and the linear velocity of the moving surface. Also, it was observed that the percentage area as well as the liquid fil m thickness increasedwith an increase in the liquid flow rate irrespective of changes in other parameters.The increase in the viscosity of the spraying liquid reduced the area covered by the spray for surface coating, while it increased the coating film thickness. L'atomisalion ultrasonique : une nouvelle m~thode de rev~tir les surlaces Au cours de cette oeuvreor] a 6tudi6 I'atom[sation ultrasonique en rant que moyen possible de rev6t[r par pulv6risat[on les surfaces mobiles. La methode exp6rimentale employee ic[ a impliq@ rappl[ca tion vert[cale et vers le bas d'une encre pulveris@ sur un papier mouvant lat6ralement. On a trouv6 que la part[e rev6tuede la surface a augment~ en pourcentage au fur et ~ mesure que I'on a augment6 la distance de ratom[seur ultrasorl[que de la surface et [amplitude v[brationnelle de I'atom[seur, tandis que I'@a[sseur du feu[I liquide sur la surface rev6tue a d@ru au fur et ~ mesure que I'on a augment6 la hauteur de I'atom[seur, I'ampl[tude vibrat[onnelle de I'atom[seur et la veloc[t6 lin6aire de la surface mobile. Aussi il a ete observe que le pourcentage de la surface qu[ etaFtrevetu auss[ bbn que r@a[sseur du feuil liquide ont augment6 au fur et ~ mesure que ron a augment6 le taux d'@oulement liquide, malgr6 les variations dans le doma[ne des autres param~tres. ['augmentation de la v[scosit6 du I[quide pulv6ris6 a r6du[t la part[e de la surface qui ava[t 6t6 rev6tue, tandis qu'elle a augment6 I'@aisseur du feuil de rev6tement. UltraschalI-Alomisierung: eine neue Technik fur Anstriche Diese Arbe[t beschgft[gt sigh nit der n6gl[chen Anwendung yon UltraschalI-Ator][s[erung, un SprClhlacke auf eine sich bewegende Oberfl~che aufzubringen. Unter der experimentellen Methode, die in diesen Versuchen verwendet wurde, wurde e[ne Tinte senkrecht nach unten auf e[n Papier gesprE~ht,das yon Se[te zu Seite bewegt wurde. Wir stellten lest, dal3 der Anteil der besprE~htenOberfl~che m[t dem Abstand des UltraschalI-Atem[sators yen der Pap[ereberflgche,der V[bratiensamplirude des Atem[sators und der I[nearen Geschw[nd[gke[t des Pap[ereszunahm. W[r fanden auch, da6 sowohl der prozentuale Anteil der
[email protected] also auch die Dicke des FlClss[gfilmeszusammen mit der Fluls der T[nte unabh~ng[g yon Variat[onen in anderen Parametern zunahm. Die erh6hte V[skosit~t der gesprLihten Fli~ssigke[t reduzierte die besprLihte Oberfl~che, aber erh6hte die Filmdicke.
or correspondence contar A B Pandit Chemical Engineering Department, University Institute o[ Chemical Technology, Matunga, Mumbai 400019, India T~I: +91 22 24145616 Fax: +91 22 24145614
Ema[l:
[email protected]
Copyright OCCA2005
Surface Coatings International Part B: Coatings Transactions
Vol.88, B3, 157-230, September 2005
189
Ultrasonic atomisation: A novel technique for surface coatings S G Gaikwad, C Reddy V, and A B Pandit
Introduction Surface coatings lie both within the traditions of an ancient craft and on the leading edge of modern technology. The importance of coatings in modern technology is considerable and is on the increase as coatings become even more diverse and demands on them rise, both in terms of their range of applications and quality of performance. Looking back, surface coatings were almost synonymous with oleo-resinous paints. Now the applications of these have become wider and deeper, and no longer does one think of them solely for the decoration of houses or for the protection of engineering structures. Nowadays there are more sophisticated and demanding applications in such diverse fields as electronics, gas turbines, solar energy and other modern industries. Coatings of various kinds are applied to surfaces to modify important mechanical properties like wear, friction, corrosion resistance, adhesion, tensile strength, and optical properties such as absorptivity/reflectivity etc, with the aim of improving the suitability of materials for a specific application. Coatings are used both to remedy the deficiencies of substrates and to confer additional properties to them. Traditionally, they were used to protect wood, ceramics and metals. Coatings are no longer passive barriers but increasingly play an active role: chemically, biologically and electrically. There are various methods of application of a coating on a surface, such as brushing, spraying, dipping, flowing-coating, fluidised bed coating, knife coating, roller coating, calender coating, silk screen coating, tumbling barrel, and centrifugal coating, etc. Some of the above methods have been modified to permit the automatic application of the coating. 1,2Spraying is one of these methods. The basic principle of spraying is to atomise the coating liquid into a fine spray, with the spray being directed onto the object to be coated. The original spray apparatus used compressed air as the atomising medium, and although this method is still the most widely used, other methods and modifications have recently been developed. One of these methods is atomisation with an ultrasound atomiser. Atomisation is the disintegration of a liquid sheet or ligament into fine droplets in a gaseous phase. It is the process of the formation of small droplets. In the majority of these techniques, kinetic energy is imparted to the liquid to form a large surface area (fine droplets). The conventional way to atomise is to force the liquid at high velocity through a small aperture. When the liquid is allowed to flow atthe 190
tip of the vibrating surface (frequency > 20kHz) in the form of a thin film, the film breaks up into fine droplets. This process is known as ultrasonic atomisation. Very fine droplets are generated without the use of high liquid/gas pressure or atomising air. The most important advantage of the ultrasound atomisation over the conventional one is the need for less energy. For ultrasonic atomisation, the energy requirements are less than those of the conventional method producing uniform droplet sizes with very low kinetic energy. In ultrasonic atomisation, by changing the frequency and the energy input, the droplet size can be controlled and even with fluctuating liquid flow rates, the droplet size can be maintained. The low kinetic energy associated with droplets prevents rebounce and therefore the coating transfer efficiencies (percentage of liquid utilisation as a coated surface) increase substantially. There are two major hypotheses that explain the mechanism of liquid disintegration during the ultrasonic atomisation: namely the capillary wave hypothesis and the cavitation hypothesis. The capillary wave hypothesis is based on Taylor instability. 3 The cavitation hypothesis is generally applied to a high frequency and a high-energy (intensity) vibrational system.4~ In the present work, a study was carried out to explore the use of the ultrasound atorniser as a novel surface coating technique. The effect of various parameters on the coated liquid film thickness and the percentage area covered by the coating liquid on the surface were studied. The effects of the parameters such as liquid flow rate through the ultrasound atomiser, the amplitude of the ultrasound atomiser (energy intensity), the linear velocity of the coating surface (with which it was moved across the atomiser) and the height of the ultrasound atomiser from
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Experimentation Experi mental set-up The experimental set-up used for this study is schematically shown in Figure 1. The ultrasound atomiser of Vibra-cell was used for the purpose of these experiments. The maximum power output was 130 watts. Provision is made to adjust the power input to the transducer in terms of the percentage of the total power. Also, from the supplied energy output display, the power delivered at the atomising tip, irrespective of the input power, was indicated. In the controller, the normal frequency (50Hz) of the electrical current (AC) was first converted into an ultrasonic frequency (20000Hz) with the help of an ultrasonic generator. This high frequency current was applied to the piezo-electric transducer, which converted the frequency of the electrical signals into mechanical vibration of the same frequency based on the reverse piezo-electric principle. According to the piezoelectric principle, when an electric field is applied to the piezo-electric material, it changes its physical dimension. Thus, on applying a constantly changing electric field (AC), the dimensions of such a material constantly keep on changing, producing mechanical vibrations of the same frequency as the frequency of the electric field. These mechanical vibrations (ie ultrasound waves) are carried through an extender (which acts as a wave guide) to the tip of the probe where atomisation takes place. The nozzle body is fabricated from titanium because of its good acoustical properties, high tensile strength, and excellent corrosion resistance.
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the coating surface. Also, the effect of the physico-chemical properties of the liquid, namely its viscosity, has been studied.
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Surface Coatings International Part B: Coatings Transadions Vol.88, B3, 157-230, September 2005
Ultrasonic atomisatiom A novel technique for surface coatings S G Gall"
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Figure 14: Sample paper using glycerine solution
properties of the liquid were changed, the area covered by the spray and the liquid film thickness for surface coating also changed. With the increase in the viscosity of the liquid, the area covered by the spray decreased. For water, the area covered by the spray was more than for the glycerine solution, the latter having a higher viscosity and requiring greater energy for atomisation.
Acknowledgements q
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S G Gaikwad would like to acknowledge the fellowship from the University Grant Commission, and C Reddy V acknowledges R A Rajadhyaksha's innovative project award of MULCT for the work described in this article.
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References
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1. Oil and Colour Chemists Association, Australia, 'Paints and their applications', Surface Coatings, 2, 718 144, Chapman and Hall, 1984, ISBN 0 412 26710 1 2. Nylen P, Modern surface coatings: A
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Figure 15: Sample paper using water width of the band of the spray has increased with an increase in the amplitude. The spray became wider as the amplitude was increased, but the droplet size was reduced. 1~ Figure 15 shows the area coverage pattern of the spray of aqueous Rhodamine B solution. It was also observed that when the amplitude was increased, the bandwidth of the spray increased. The strip of the spray as observed at lower amplitude was scattered (more dispersed) throughout. As the amplitude and the liquid flow rate were increased, the band of the spray became dense at the centre and scattered at the boundary, indicating possibly the presence of a variable liquid film thickness on the atomising surface (higher at the liquid issuance point and lower near the edges).
Conclusions The present work describes a different technique for surface coating using an ultrasound atomiser. Observations were made to study the effect of different
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atomising parameters such as the height of the atomiser from the sample paper surface, liquid flow rate, vibrational amplitude of the atomiser, velocity of the sample paper across the spray and the physico-chemical properties of the liquid on the percentage covered area, and the liquid film thickness on the coated surface. With an increase in the height of the atomiser from the sample paper surface, the percentage area covered by the spray increased, while the liquid film thickness decreased. It was observed that the percentage area as well as the liquid film thickness of the surface coating increased with an increase in the liquid flow rate. As the vibrational amplitude of the atomiser was increased, it was observed that the percentage area covered also increased. However, the liquid film thickness decreased with an increase in the vibrational amplitude. It was seen that as the sample paper velocity was increased, the percentage area covered was not affected, but the liquid film thickness decreased. As the physico-chemical
textbook of chemistry and technology of paints, varnishes and lacquers, John Wiley and Sons, New York, 616-32, 1964 3. Taylor G I, 'The instability of liquid surfaces when accelerated in a direction perpendicular to their planes', Proceedings of the Royal SocA, CCI, 192 6, 1950 4. Sollner K, 'The mechanism of the formation of fogs by ultrasonic waves', Trans Faraday Soc, 32, 1532~, 1936 5. Eknadiosyants O K and B I Lin, 'Nature of the atomisation of liquids in an ultrasonic fountain', Soviet Physics-Acoustics, 12, 269, 1967 6. Lang R, 'Ultrasonic atomisation of liquids', The Journal of the Acoustic Society of America, 34, (1), 6-8, 1962 7. Mahulkar A V, 'Ultrasonic atomisation and cavitation', M Chem Eng Thesis, University of Mumbai, 2004 8. Balasubrahmanyam A, 'Studies in cavitation phenomenon multiple transducer system', M Chem Eng Thesis, University of Mumbai, 2003 9. Bellman R and R Pennington, 'Effects of surface tension and viscosity on Taylor instability', Quarterly of Applied Mathematics, 32, 244 61,1954 10. Pandit A B and R Rajan, 'Correlation to predict droplet size in ultrasonic atomisation', Ultrasonics, 39, 235-55, 2001
Surface Coatings International Part B: Coatings Transadions Vol.88, B3, 157-230, September 2005
