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Misting droplet size analysis in a metered film coater
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cherd
Misting droplet size analysis in a metered film coater G. Ascanio a,∗ , B. Taboada a , P.A. Tanguy b a b
´ CCADET, Universidad Nacional Autonoma de M´exico, 04510 DF, Mexico Department of Chemical Engineering, Ecole Polytechnique, Montreal H3C 3A7, Canada
a r t i c l e
i n f o
a b s t r a c t
Article history:
The high process speed as well as the low stress at which the web is submitted are two good
Received 17 July 2007
reasons for considering roll coating over other technologies for paper coating such as slot
Accepted 12 October 2007
coating or blade coating. However, due to the film splitting some filaments of coating fluid are formed and broken up at the exit of the transfer nip resulting in small droplets ejected to the surroundings. Such phenomenon, known as misting, appears usually at high speed
Keywords:
when using low concentrated coating fluids. The runnability of two coating colors has been
Misting droplet
evaluated by coating bond paper in a pilot roll coater at process speeds similar to those
Roll coating
encountered in the paper industry. Misting droplets size and distribution determined by
Extensional viscosity
image analysis have been related with the coating transfer efficiency and the extensional viscosity. It was demonstrated that high solids content suspensions having large extensional viscosity produce less misting especially at high speed resulting in higher coating effectiveness. © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Roll coating is nowadays a very profitable technology for paper coating at high speed. The advantages offered by a roll coater have been well recognized by the paper industry. The paper is not submitted to high stress as in blade coating resulting in a process with fewer interruptions (better runnability). On the other hand, the possibility of using high solids content formulations allows producing higher density coated papers requiring less coating steps. However, because the fluid splits at the exit of the transfer nip, the process is prone to two major runnability issues: misting and orange peel. Misting is defined as the ejection of droplets as a consequence of the break up of the filaments formed downstream the nip. Although misting affect neither the runnability nor the quality of the coated surface, it reduces the coverage. Because misting droplets are ejected to the surroundings roll coaters requires frequent cleansing operations. Misting phenomenon is a consequence of several process conditions, namely: coating speed, low solids content formulations, low viscosity (shear and extensional), high coat weights, among others (Triantafillopoulos and Smith, 1998).
∗
Some studies have been conducted with the aim of understanding the mechanisms leading to misting on metered size presses. Roper et al. (1997) proposed a model based on the lubrication approximation for the better understanding of misting in roll coaters operating a high speed. They found that droplets are larger as the centrifugal forces increase as a result of the coating speed. Although misting has been usually related with the rheology of the coating fluid, in particular with the high-shear viscosity, extensional viscosity plays a fundamental role on such runnability issue. Ascanio et al. (2005) found that suspensions having high solids content exhibit pronounced strain hardening at high strain rates resulting in low misting amount coating processes. The purpose of the present work is to highlight the relationship between misting droplet size and distribution and the extensional rheology of coating fluids.
2.
Methods and materials
Fig. 1 shows the pilot roll coater used to investigate the runnability of two coating colors. The machine is composed by two main sections: metering and transfer. The metering
Corresponding author. Tel.: +52 55 5622 8635; fax: +52 55 5550 0654. E-mail address: [email protected] (G. Ascanio). 0263-8762/$ – see front matter © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2007.10.016
216
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
Fig. 1 – Pilot roll coater.
section consists of a co-rotating rod of 36 mm diameter used as metering device and a feeding chamber for supplying the coating fluid onto the transfer roll. The transfer section is composed by the transfer roll and the backing roll. The two rolls are 800 mm in diameter and 500 mm long covered with a styrene butadiene elastomer layer. The gap is deformed by applying a load between the rolls by means of a pneumatic system. The backing roll entrains a paper loop 20 m long at the same speed of the transfer roll. The paper is coated on one side in one revolution and dried in several revolutions. The runnability of two coating colors was investigated using bond paper with a basis weight of 85 g/m2 . These coating formulations were prepared with delaminated kaolin clay as pigment, Dispex N40V (Ciba) as dispersant, carboxymethyl cellulose Finnfix 30 (Noviant) as thickener, and latex FC620NA (Dow Chemical) as binder. Table 1 shows the compositions of the coating colors used for the present work. An orifice rheometer was used for measuring the extensional properties of the fluids investigated. The operating principle of the orifice rheometer is full described elsewhere (see Ascanio et al., 2002). Misting droplets were collected on a sheet of black paper placed under the nip and the weight was measured before and after completion one paper loop revolution. The amount of misting is expressed in weight per surface unit per time unit taking into account that the collection time of ¨ droplets depends on the roll speed. As proposed by Gron et al. (1998), the transfer ratio can be used for analyzing the effectiveness of a roll coating process. Such a parameter is defined as the ratio of the coat weight and the premetered film amount (coat weight plus the amount of misting).
Table 1 – Composition of coating colors Compound
Results
From the extensional rheology tests, both coating colors exhibited strain hardening in the effective strain range corresponding to the roll speed investigated. In the effective strain rate range, corresponding to the coating speed, the apparent extensional viscosity can be calculated from the following power-law expressions for coating colors low solids content (LS) and high solids content (HS), respectively: E = 0.7774 0.8275 5 × 10−5 ¯˙ and E = 3 × 10−4 ¯˙ . Both solutions exhibited strain-hardening in the effective strain rate investigated, being more pronounced for the high concentrated coating color. The reader is referred to Ascanio et al. (2005, 2006) for a full description of the rheology of both suspensions. Concerning to the black sheet paper samples, Fig. 2 shows the photomicrographs obtained with the low solids content and high solids content coating colors at a coating speed of 500 and 1100 m/min. From a qualitative standpoint, the low concentrated coating color (LS) generates more misting for a coating speed of 1100 m/min. On the other hand, no a big difference is observed for a coating speed of 500 m/min. The photomicrographs were analyzed in terms of the droplets size by means of image analyzer software (Image Pro Plus 5.0, Media Cybernetics). Then, a macro program written in Visual Basic transferred the size data to an Excel in order to calculate their distribution. Fig. 3 shows the frequency as a function of droplet size of the four photomicrographs analyzed. Concerning to the coating speed, smaller droplets are ejected at 500 m/min. Droplets at least one order of magnitude bigger are produced as the speed is increased to 1100 m/min. However, more relevant findings can be obtained if the coating process is related with the color rheology. Table 2 summarizes the results from the image analysis in relation with the coating effectiveness in terms of the trans¨ et fer ratio and the extensional viscosity. As proposed by Gron al. (1998), the transfer ratio can be obtained from the following expression:
Coating fluid LS a
Pigment (pph) Dispersant (pph)a Thickener (pph)a Binder (pph)a Solids content (wt. %) a
3.
100 0.15 0.30 12 52
HS 100 0.15 0.30 12 62
Amounts based on 100 parts per hundred (pph) of delaminated kaolin clay.
transfer ratio =
CW M
where CW is the coat weight and M is the premetered film amount. On the other hand, the apparent extensional viscosity (E ) ¯˙ which depends on is a function of the effective strain rate (), the roll speed and the elastomer covered roll deflection as a result of the applied load between the rolls.
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
217
Fig. 2 – Photomicrographs of misting droplets: (a) LS at 500 m/min; (b) HS at 500 m/min; (c) LS at 1100 m/min; (d) HS at 1100 m/min.
Fig. 3 – Droplet size distribution: (a) LS at 500 m/min; (b) HS at 500 m/min; (c) LS at 1100 m/min; (d) HS at 1100 m/min.
218
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
Table 2 – Image analysis results in relation with the transfer ratio and apparent extensional viscosity Coating color
Roll speed (m/min)
Surface covered by droplets (%)
Transfer ratio (%)
E (Pa s)
LS
500 1100
0.375 14.414
86.6 46.2
0.221 0.409
HS
500 1100
0.671 1.098
92.5 66.8
2.269 4.362
As Table 2 shows, the transfer ratio increases when using coating colors having larger extensional viscosity, which is a result mainly of the solids concentration. It is important to note that the surface covered by the misting droplets is considerably reduced as the extensional viscosity is increased, resulting in a higher coating effectiveness. These results are in good agreement with the findings reported by Roper et al. (1997, 1998).
4.
Conclusions
The runnability of two coating suspensions with low and high solid content was assessed in a pilot coater in terms of the misting droplet size, the transfer ratio and the apparent extensional viscosity of coating colors. One of the main findings is that high solids content coating suspensions exhibiting a larger extensional viscosity generate less misting especially at high speed resulting in higher coating effectiveness in terms of the transfer ratio. Because the fluid flow through forward rolls is more sensitive to extension than shear; therefore the extensional viscosity is a more realistic criterion to analyze misting in a metered film coater, especially when both rolls rotate at the same speed and direction.
Acknowledgement The financial support of the National Science and Engineering Research Council of Canada is highly appreciated.
references
Ascanio, G., Carreau, P.J., Brito-De La Fuente, E. and Tanguy, P.A., 2002, Orifice flowmeter for measuring extensional rheological properties. Can J Chem Eng, 80(6): 1189–1196. Ascanio, G., Carreau, P.J. and Tanguy, P.A., 2006, High-speed roll coating of complex rheology fluids. Exp Fluids, 40(1): 1–14. ´ Ascanio, G., Carreau, P.J., Reglat, O. and Tanguy, P.A., 2005, Extensional rheology of coating suspensions in relation with misting in film coaters. Nordic Pulp Paper Res J, 20(1): 48–53. ¨ J., Sunde, H. and Nikula, E., 1998, Runnability aspects in Gron, high-speed film transfer coating. TAPPI J, 81: 157– 165. Roper, J.A., III, Bousfield, D.W., Urscheler, R. and Salminen, P., 1997, Observations and proposed mechanisms of misting on high-speed metered size press coaters, In TAPPI Coating Conference (TAPPI Press, Washington, D.C.), pp. 1–14. Roper, J.A., III, Salminen, P., Urscheler, R. and Moore, E., 1998, Optimization of formulation parameters to reduce misting and orange peel formation on metered film coaters, In TAPPI Coating Conference (TAPPI Press, Washington, D.C.), pp. 37–55. Triantafillopoulos, N.G. and Smith, M.K., 1998, Troubleshooting rheology problems in metered size press, In TAPPI Metered Size Press Forum (TAPPI Press, Washington, D.C.), pp. 13–35.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cherd
Misting droplet size analysis in a metered film coater G. Ascanio a,∗ , B. Taboada a , P.A. Tanguy b a b
´ CCADET, Universidad Nacional Autonoma de M´exico, 04510 DF, Mexico Department of Chemical Engineering, Ecole Polytechnique, Montreal H3C 3A7, Canada
a r t i c l e
i n f o
a b s t r a c t
Article history:
The high process speed as well as the low stress at which the web is submitted are two good
Received 17 July 2007
reasons for considering roll coating over other technologies for paper coating such as slot
Accepted 12 October 2007
coating or blade coating. However, due to the film splitting some filaments of coating fluid are formed and broken up at the exit of the transfer nip resulting in small droplets ejected to the surroundings. Such phenomenon, known as misting, appears usually at high speed
Keywords:
when using low concentrated coating fluids. The runnability of two coating colors has been
Misting droplet
evaluated by coating bond paper in a pilot roll coater at process speeds similar to those
Roll coating
encountered in the paper industry. Misting droplets size and distribution determined by
Extensional viscosity
image analysis have been related with the coating transfer efficiency and the extensional viscosity. It was demonstrated that high solids content suspensions having large extensional viscosity produce less misting especially at high speed resulting in higher coating effectiveness. © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Roll coating is nowadays a very profitable technology for paper coating at high speed. The advantages offered by a roll coater have been well recognized by the paper industry. The paper is not submitted to high stress as in blade coating resulting in a process with fewer interruptions (better runnability). On the other hand, the possibility of using high solids content formulations allows producing higher density coated papers requiring less coating steps. However, because the fluid splits at the exit of the transfer nip, the process is prone to two major runnability issues: misting and orange peel. Misting is defined as the ejection of droplets as a consequence of the break up of the filaments formed downstream the nip. Although misting affect neither the runnability nor the quality of the coated surface, it reduces the coverage. Because misting droplets are ejected to the surroundings roll coaters requires frequent cleansing operations. Misting phenomenon is a consequence of several process conditions, namely: coating speed, low solids content formulations, low viscosity (shear and extensional), high coat weights, among others (Triantafillopoulos and Smith, 1998).
∗
Some studies have been conducted with the aim of understanding the mechanisms leading to misting on metered size presses. Roper et al. (1997) proposed a model based on the lubrication approximation for the better understanding of misting in roll coaters operating a high speed. They found that droplets are larger as the centrifugal forces increase as a result of the coating speed. Although misting has been usually related with the rheology of the coating fluid, in particular with the high-shear viscosity, extensional viscosity plays a fundamental role on such runnability issue. Ascanio et al. (2005) found that suspensions having high solids content exhibit pronounced strain hardening at high strain rates resulting in low misting amount coating processes. The purpose of the present work is to highlight the relationship between misting droplet size and distribution and the extensional rheology of coating fluids.
2.
Methods and materials
Fig. 1 shows the pilot roll coater used to investigate the runnability of two coating colors. The machine is composed by two main sections: metering and transfer. The metering
Corresponding author. Tel.: +52 55 5622 8635; fax: +52 55 5550 0654. E-mail address: [email protected] (G. Ascanio). 0263-8762/$ – see front matter © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2007.10.016
216
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
Fig. 1 – Pilot roll coater.
section consists of a co-rotating rod of 36 mm diameter used as metering device and a feeding chamber for supplying the coating fluid onto the transfer roll. The transfer section is composed by the transfer roll and the backing roll. The two rolls are 800 mm in diameter and 500 mm long covered with a styrene butadiene elastomer layer. The gap is deformed by applying a load between the rolls by means of a pneumatic system. The backing roll entrains a paper loop 20 m long at the same speed of the transfer roll. The paper is coated on one side in one revolution and dried in several revolutions. The runnability of two coating colors was investigated using bond paper with a basis weight of 85 g/m2 . These coating formulations were prepared with delaminated kaolin clay as pigment, Dispex N40V (Ciba) as dispersant, carboxymethyl cellulose Finnfix 30 (Noviant) as thickener, and latex FC620NA (Dow Chemical) as binder. Table 1 shows the compositions of the coating colors used for the present work. An orifice rheometer was used for measuring the extensional properties of the fluids investigated. The operating principle of the orifice rheometer is full described elsewhere (see Ascanio et al., 2002). Misting droplets were collected on a sheet of black paper placed under the nip and the weight was measured before and after completion one paper loop revolution. The amount of misting is expressed in weight per surface unit per time unit taking into account that the collection time of ¨ droplets depends on the roll speed. As proposed by Gron et al. (1998), the transfer ratio can be used for analyzing the effectiveness of a roll coating process. Such a parameter is defined as the ratio of the coat weight and the premetered film amount (coat weight plus the amount of misting).
Table 1 – Composition of coating colors Compound
Results
From the extensional rheology tests, both coating colors exhibited strain hardening in the effective strain range corresponding to the roll speed investigated. In the effective strain rate range, corresponding to the coating speed, the apparent extensional viscosity can be calculated from the following power-law expressions for coating colors low solids content (LS) and high solids content (HS), respectively: E = 0.7774 0.8275 5 × 10−5 ¯˙ and E = 3 × 10−4 ¯˙ . Both solutions exhibited strain-hardening in the effective strain rate investigated, being more pronounced for the high concentrated coating color. The reader is referred to Ascanio et al. (2005, 2006) for a full description of the rheology of both suspensions. Concerning to the black sheet paper samples, Fig. 2 shows the photomicrographs obtained with the low solids content and high solids content coating colors at a coating speed of 500 and 1100 m/min. From a qualitative standpoint, the low concentrated coating color (LS) generates more misting for a coating speed of 1100 m/min. On the other hand, no a big difference is observed for a coating speed of 500 m/min. The photomicrographs were analyzed in terms of the droplets size by means of image analyzer software (Image Pro Plus 5.0, Media Cybernetics). Then, a macro program written in Visual Basic transferred the size data to an Excel in order to calculate their distribution. Fig. 3 shows the frequency as a function of droplet size of the four photomicrographs analyzed. Concerning to the coating speed, smaller droplets are ejected at 500 m/min. Droplets at least one order of magnitude bigger are produced as the speed is increased to 1100 m/min. However, more relevant findings can be obtained if the coating process is related with the color rheology. Table 2 summarizes the results from the image analysis in relation with the coating effectiveness in terms of the trans¨ et fer ratio and the extensional viscosity. As proposed by Gron al. (1998), the transfer ratio can be obtained from the following expression:
Coating fluid LS a
Pigment (pph) Dispersant (pph)a Thickener (pph)a Binder (pph)a Solids content (wt. %) a
3.
100 0.15 0.30 12 52
HS 100 0.15 0.30 12 62
Amounts based on 100 parts per hundred (pph) of delaminated kaolin clay.
transfer ratio =
CW M
where CW is the coat weight and M is the premetered film amount. On the other hand, the apparent extensional viscosity (E ) ¯˙ which depends on is a function of the effective strain rate (), the roll speed and the elastomer covered roll deflection as a result of the applied load between the rolls.
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
217
Fig. 2 – Photomicrographs of misting droplets: (a) LS at 500 m/min; (b) HS at 500 m/min; (c) LS at 1100 m/min; (d) HS at 1100 m/min.
Fig. 3 – Droplet size distribution: (a) LS at 500 m/min; (b) HS at 500 m/min; (c) LS at 1100 m/min; (d) HS at 1100 m/min.
218
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 215–218
Table 2 – Image analysis results in relation with the transfer ratio and apparent extensional viscosity Coating color
Roll speed (m/min)
Surface covered by droplets (%)
Transfer ratio (%)
E (Pa s)
LS
500 1100
0.375 14.414
86.6 46.2
0.221 0.409
HS
500 1100
0.671 1.098
92.5 66.8
2.269 4.362
As Table 2 shows, the transfer ratio increases when using coating colors having larger extensional viscosity, which is a result mainly of the solids concentration. It is important to note that the surface covered by the misting droplets is considerably reduced as the extensional viscosity is increased, resulting in a higher coating effectiveness. These results are in good agreement with the findings reported by Roper et al. (1997, 1998).
4.
Conclusions
The runnability of two coating suspensions with low and high solid content was assessed in a pilot coater in terms of the misting droplet size, the transfer ratio and the apparent extensional viscosity of coating colors. One of the main findings is that high solids content coating suspensions exhibiting a larger extensional viscosity generate less misting especially at high speed resulting in higher coating effectiveness in terms of the transfer ratio. Because the fluid flow through forward rolls is more sensitive to extension than shear; therefore the extensional viscosity is a more realistic criterion to analyze misting in a metered film coater, especially when both rolls rotate at the same speed and direction.
Acknowledgement The financial support of the National Science and Engineering Research Council of Canada is highly appreciated.
references
Ascanio, G., Carreau, P.J., Brito-De La Fuente, E. and Tanguy, P.A., 2002, Orifice flowmeter for measuring extensional rheological properties. Can J Chem Eng, 80(6): 1189–1196. Ascanio, G., Carreau, P.J. and Tanguy, P.A., 2006, High-speed roll coating of complex rheology fluids. Exp Fluids, 40(1): 1–14. ´ Ascanio, G., Carreau, P.J., Reglat, O. and Tanguy, P.A., 2005, Extensional rheology of coating suspensions in relation with misting in film coaters. Nordic Pulp Paper Res J, 20(1): 48–53. ¨ J., Sunde, H. and Nikula, E., 1998, Runnability aspects in Gron, high-speed film transfer coating. TAPPI J, 81: 157– 165. Roper, J.A., III, Bousfield, D.W., Urscheler, R. and Salminen, P., 1997, Observations and proposed mechanisms of misting on high-speed metered size press coaters, In TAPPI Coating Conference (TAPPI Press, Washington, D.C.), pp. 1–14. Roper, J.A., III, Salminen, P., Urscheler, R. and Moore, E., 1998, Optimization of formulation parameters to reduce misting and orange peel formation on metered film coaters, In TAPPI Coating Conference (TAPPI Press, Washington, D.C.), pp. 37–55. Triantafillopoulos, N.G. and Smith, M.K., 1998, Troubleshooting rheology problems in metered size press, In TAPPI Metered Size Press Forum (TAPPI Press, Washington, D.C.), pp. 13–35.