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In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy
Vibrational Spectroscopy 51 (2009) 152–155
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy Katja Heymann, Gabriele Mirschel, Tom Scherzer *, Michael R. Buchmeiser Leibniz Institute of Surface Modification, Permoserstr. 15, D-04318 Leipzig, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 February 2009 Received in revised form 7 April 2009 Accepted 8 April 2009 Available online 17 April 2009
We report on the development of a measuring method based on near-infrared (NIR) spectroscopy, which is able to determine the thickness of UV-cured coatings and which can be used for in-line monitoring in technical coating processes. In particular, acrylate coatings, which were applied to transparent polymer films with a typical thickness of 5–35 mm, were investigated. NIR spectra were recorded in transflection mode. Quantitative analysis of the spectral data was carried out with partial least square (PLS) regression. In-line measurements were performed on a pilot-scale roll coating machine at web speeds up to 50 m/min. It was shown that quantitative data with excellent precision (i.e. with a standard deviation lower than 1 mm) and high time resolution (2.5 spectra/s) can be obtained. ß 2009 Elsevier B.V. All rights reserved.
Keywords: NIR reflection spectroscopy Coating thickness In-line measurements Partial least square (PLS) regression Acrylic clear coatings
1. Introduction Process control has been used in chemical industry for a long time in order to control the reaction kinetics and thus the properties of the resulting products [1,2]. For many years, such systems have also been employed for the monitoring of batch, solution, or emulsion polymerization reactions [3–7] as well as for the control of thermal curing processes [8,9]. In contrast, only a few attempts have been made so far to monitor photochemically induced curing reactions [10–13], although UV polymerization of thin polymer coatings has become a rapidly growing technology with a wide range of applications such as scratch and abrasion resistant coatings, barrier layers, protective coatings, etc. [14]. In UV curing technology, functional coatings are usually made of mixtures of acrylates. The properties of the coatings do not only depend on the composition of the formulation, but also on the thickness of the applied layer. The thickness, in turn, is affected by several parameters such as viscosity, temperature, and web speed. This way, unintended fluctuations of the thickness can occur in technical coating processes. For this reason, the determination of the coating thickness is a very important issue in coating technology. Moreover, if the coating thickness is monitored in the process, it can be made sure that the thickness does not exceed a given maximum value which leads to a more economic use of the raw materials. On the other hand, the thickness must not fall below
* Corresponding author. Fax: +49 341 235 2584. E-mail address: [email protected] (T. Scherzer). 0924-2031/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2009.04.001
a specific minimum, because the properties of the coating and consequently the quality of the product depend on the layer thickness. However, up to now there is no analytical method which allows for a contact-free monitoring of the thickness of the applied acrylate layer in a continuous coating process. Most of the existing analytical methods, e.g. gravimetry, white-light interferometry, ellipsometry, optical or scanning electron microscopy (SEM) etc., are only suited for the off-line determination of the layer thickness in the laboratory. For example, white-light interferometry is a powerful analytical method which has been used for at least one decade to determine the thicknesses of thin transparent dielectric coatings [15,16] or dispersive media [17]. The thickness of the sample is calculated from the specific interference pattern caused by interfering light beams, which were reflected from the surface of the coating and the substrate, respectively. However, the method requires two significantly different refractive indices of coating and substrate, which limits the range of applications of this method. The refractive index of most polymers is around 1.5, i.e. the indices of typical UV-curable acrylate coatings and the polymer films used as substrates often differ only marginally. Moreover, the interference pattern of a moving web with a non-uniform coating can change very rapidly, which may impede or even prevent quantitative analysis. Ultimately, this may lead to destructive interference. Consequently, white-light interferometry is not a suitable method for the in-line determination of the layer thickness of UV-curable acrylic clear coats on transparent polymer foils.
K. Heymann et al. / Vibrational Spectroscopy 51 (2009) 152–155
Spectroscopic ellipsometry is another powerful method for the characterization of thin mono- or multilayers [18]. This method has already been used for the in-situ determination of the thickness of organic [19] and polymer layers [20], but mainly it is used for the in-situ monitoring of inorganic layers [18,21]. However, the typical thickness of these layers is in the range of some nanometers and does definitely not exceed some hundred nanometers. In contrast, the typical thickness of UV-cured coatings is in the dimension of some micrometers (5–20 mm) and, hence, much higher than it is usual for ellipsometry measurements. Currently, the control of the applied thickness in technical coating processes is mostly carried out off-line by gravimetric determination of the coating weight. However, it is apparent that this approach is not able to respond to sudden changes of the thickness, and thus it is poorly suited for process control. Therefore, there is a strong need for an analytical method, which is able to monitor the thickness of acrylic coatings in-line and which is capable for the use in process and quality control. As reported in a previous paper [12], near-infrared spectroscopy is a powerful contactless method, which can be used for in-line monitoring of the conversion in acrylate coatings. However, there are two facts which might be unfavourable for the use of NIR spectroscopy for the determination of the layer thickness: (i) the thickness of the coatings is in the range of only some micrometers, and hence, much lower than in most other NIR applications, and (ii) the absorption coefficients in the near-infrared are quite low. In this paper, we report on the development of a measuring method based on NIR reflection spectroscopy, which is able to determine the thickness of UV-cured coatings in-line.
153
2.4. NIR reflection spectroscopy NIR spectra were recorded with a Kusta 4004 P process spectrometer (LLA, Berlin, Germany). This instrument is based on a concave holographic grating and an InGaAs photodiode array detector with 256 pixels, which was set to cover a spectral range from 1470 to 1950 nm. The reflection probe head is linked to the spectrometer unit by a fiber optic cable. It contains a tungsten halogen lamp with a UV filter to cut-off the short-wavelength emission of the lamp. Spectra were taken in transflection mode using a ceramic plate as reflector, which was arranged behind the sample. A diffuser plate in front of the probe head suppresses interferences, which occur in thin transparent films such as 20 mm OPP foil. Moreover, the probe head is tilted against the web for the same reason. A more detailed description of the spectrometer system is given in ref. [22]. For in-line monitoring, the NIR probe head was mounted above the web immediately behind the UV lamp. Spectra were recorded continuously with a rate of about 150 spectra/min. 2.5. Determination of the coating thickness The nominal thickness of the coatings before irradiation was given by the gap of the Baker applicator used for application. The actual values of the coating thickness after irradiation were determined with a digital thickness gauge with a resolution of 0.2 mm (Heidenhain ND 221B, Dr. Johannes Heidenhain GmbH, Traunreut, Germany). 3. Results and discussion
2. Experimental 3.1. Quantitative analysis and calibration 2.1. Materials Acrylate coatings with different thicknesses were made from the following clear lacquer formulation: 60 wt% aliphatic urethane diacrylate (EB 270), 30 wt% amine modified polyether acrylate (EB 81), and 10 wt% tripropylene glycol diacrylate (TPGDA). All acrylates were obtained from Cytec Surface Specialities (Drogenbos, Belgium). Ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L, BASF, Ludwigshafen, Germany) was used as photoinitiator. It was added at a concentration of 3 wt%. 2.2. Preparation of coatings and UV irradiation Samples were prepared by application of the formulation to 20-
mm thick polypropylene foil (OPP), which had been pre-treated by corona discharge. Coating was carried out by using an automatic film applicator (SIMEX, Haan, Germany) and a set of Baker applicators (TQC GmbH, Haan, Germany), which allowed for a variation of the thickness from 5 to 50 mm. The coatings were irradiated under nitrogen in a UV curing unit equipped with a medium-pressure mercury lamp (120 W cm 1, IST Metz Strahlentechnik GmbH, Nu¨rtingen, Germany). The irradiation dose was 700 mJ cm 2 at a conveyer speed of 8 m/min and an intensity of 620 mW cm 2. 2.3. Roll coating and curing In-line monitoring studies were carried out on a pilot-scale roll coating machine at the IOM. It is equipped with a mediumpressure mercury lamp (PrintConcept UV Systeme, Ko¨ngen, Germany; 160 W cm 1, maximum power 8 kW, adjustable between 30 and 100%). Coating trials were carried out at different widths of the nip between the applicator rolls and at different web speeds.
In NIR spectroscopy, quantitative analysis of spectral data is generally carried out with multivariate calibration methods such as partial least square (PLS) regression [23], which are able to detect even minor differences in concentration. The use of such chemometric techniques first requires the setup of a specific calibration model. This model has to relate the spectral variation in the data to the parameter of interest, e.g. the thickness of the applied coating. Therefore, samples with coating thicknesses in the range from 5 to 50 mm, in steps of 5 mm, were prepared for calibration. For each specific thickness, ten samples were prepared in order to get sufficient samples for creating a stable and powerful calibration model. NIR spectra were recorded before and immediately after UV irradiation. Typically, about 1000 NIR spectra were accumulated during one scan, while moving the sample slowly through the focus of the probe beam. These single spectra were averaged before further processing. In this way, each sample was measured 10 times. Consequently, 100 mean value spectra per thickness were recorded. A typical spectrum is shown in Fig. 1. In coating technology, shrinkage of acrylates during the UVcuring process is a well-known effect. This effect leads to a decrease of the thickness of the cured coating in comparison to the uncured one. The shrinkage, which may amount up to 30%, is caused by the UV-induced cross-linking of acrylates during irradiation. In order to obtain correct reference values, the actual thickness of the coatings was determined with a digital thickness gauge. For the creation of a calibration model based on PLS regression, data were split in equal parts into a calibration and a validation set. The spectral range was limited to the region from 1470 to 1890 nm in order to exclude the influence of humidity, which absorbs beyond 1900 nm. Then, the PLS algorithm was applied to the spectra. After regression, the root mean square error of prediction
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K. Heymann et al. / Vibrational Spectroscopy 51 (2009) 152–155
Fig. 1. NIR reflectance spectrum of a 10 mm acrylic clear coat applied to a 20 mm OPP film.
Fig. 3. Prediction of the layer thickness of independent test samples using the PLS calibration shown in Fig. 2.
(RMSEP) and the coefficient of determination R2 were calculated. Moreover, several pre-processing methods such as normalization, first or second derivative, and baseline corrections were tested. These pre-treatments were applied individually or in combination to the spectra. Subsequently, the model with the lowest number of factors, the smallest RMSEP, and the highest R2 was selected. The resulting calibration curve is shown in Fig. 2. Normalization and the first derivative were applied as pre-treatments to the spectra. The underlying chemometric model was based on five factors. The performance of the developed model was tested with an additional set of independent test samples. The results are shown in Fig. 3. The coating thickness predicted with NIR spectroscopy is plotted against the thickness determined with the digital thickness gauge. The results clearly prove that the prediction can be carried out with high precision.
For in-line monitoring of the thickness of acrylic clear coats on OPP film, the probe head was mounted on a pilot-scale roll coating machine. It was installed behind the mercury lamp and close to a guide roll in order to minimize the influence of vibrations of the web. Furthermore, particular attention was paid to the geometric arrangement of probe head and film in order to make sure that it was identical to the arrangement in the lab. This is of utmost importance, since even very minor differences between both
arrangements would substantially affect the precision of the prediction. Important parameters, which influence the spectral response and consequently the predictive power, are the distances between light source and film or reflector, respectively, as well as the tilt angle of the probe head against the web. The predicting power of the calibration model was tested under in-line conditions at a line speed of 40 m/min. The coating was applied to OPP foil, and NIR spectra were recorded continuously right after UV curing. In order to obtain different coating thicknesses, the nip between the applicator rolls was varied. After the end of the roller application trial, the actual coating thickness was additionally determined off-line using the digital thickness gauge. The thickness values determined from the NIR spectra of each layer characterized by a specific thickness were averaged and plotted against the actual coating thickness (average of 40 data points). Results are shown in Fig. 4. The correlation coefficient (r2 = 0.97) and a standard deviation lower than 1 mm indicate that the calibration model is able to predict the thickness with high precision also under in-line conditions. In order to simulate changes of the thickness during a real coating process, the nip was stepwise reduced and increased. The web speed was again 40 m/min. Results are shown in Fig. 5. In addition to the results obtained by NIR spectroscopy, data determined with the digital thickness gauge are given. It can be seen that both the predicted and the actual thicknesses decrease or increase according to the changes of the nip. Thickness changes as
Fig. 2. PLS calibration curve for the thickness of UV-cured acrylic clear coatings on OPP film.
Fig. 4. Prediction of the layer thickness from in-line measurements on a roll coating machine using the PLS calibration shown in Fig. 2.
3.2. In-line monitoring of the thickness
K. Heymann et al. / Vibrational Spectroscopy 51 (2009) 152–155
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even small changes of the thickness of acrylic coatings under inline conditions. The time resolution of the data recording (2.5 spectra/s) is sufficient for in-line monitoring. If measurements at higher speeds are necessary, the recording rate of the NIR spectrometer can be further increased without a significant loss of quality of the spectra. Thus, process control at line speeds well above 100 m/min can be realized. 4. Conclusion
Fig. 5. In-line monitoring of the thickness upon stepwise decrease and increase of the nip between the applicator rolls of the coating machine at a web speed of 40 m/ min. For comparison, actual coating thicknesses, which were determined off-line, are given.
In this paper, it was demonstrated for the first time that the thickness of cured acrylic clear coatings can be monitored by NIR spectroscopy. Samples with a thickness in the range of 5–35 mm were studied. A multivariate chemometric method was used for calibration. Analysis was carried out with the aid of a PLS-model, which was based on five factors. The calibration model was found to show an excellent predicting power with an error of less than 1.0 mm. Furthermore, the calibration model was used successfully for in-line monitoring on a pilot-scale roll coating machine. Quantitative data were recorded with high precision at web speeds up to 50 m/min. A very close correlation between NIR-based and off-line thickness data was obtained. In summary, NIR spectroscopy has been proven to be able to monitor basic process parameters such as the coating thickness under various technological conditions. Consequently, this method could be effectively used for process and quality control in UV-curing technology. Acknowledgements The authors would like to thank U. Trimper and S. Pyczak for technical support. Financial support was provided by the AiF association under grant number KF 0189603FK6. References
Fig. 6. In-line monitoring of the thickness upon stepwise increase of the web speed and with constant nip between the applicator rolls. For comparison, actual coating thicknesses, which were determined off-line, are given.
low as 1.4 mm can be clearly detected by NIR spectroscopy. Thus, the close correlation between actual and predicted thickness as well as the immediate spectral response after any change in thickness suggest that the predicting power of the chemometric model is sufficient for in-line monitoring of the thickness of acrylic clear coatings. In coating technology, it is well-known that the thickness of the coating increases with increasing web speed. Therefore, the calibration model was also tested at different web speeds from 6 to 50 m/min (Fig. 6). Similar to the results shown in Fig. 5, the spectral response can be observed immediately after each change of the web speed. It can clearly be seen that the coating thickness significantly increases with increasing web speed, i.e. from about 4 mm at 6 m/min up to 11 mm at 50 m/min. Again, excellent correlation of the NIR spectroscopic data with the thickness values determined off-line was observed. The results demonstrate that NIR spectroscopy, in spite of the low absorption coefficients in the near-infrared, is able to detect
[1] J.B. Callis, D.L. Illmann, B.R. Kowalski, Anal. Chem. 59 (1987) 624A. [2] D.C. Hassel, E.M. Bowman, Appl. Spectrosc. 52 (1998) 18A. [3] S.Y. Chang, N.S. Wang, in: M.W. Urban, T. Provder (Eds.), Multidimensional Spectroscopy of Polymers, American Chemical Society, Washington, 1995, p. 147. [4] C. Kradjel, L. McDermott, NIR Analysis of Polymers, in: D.A. Burns, E.W. Ciurczak (Eds.), Handbook of Near Infrared Analysis, 2nd ed., Marcel Dekker, New York, 2001, p. 565. [5] H.W. Siesler, Y. Ozaki, S. Kawata, H.M. Heise (Eds.), Near-Infrared Spectroscopy: Principles, Instruments, Applications, Wiley-VCH, Weinheim, 2002. [6] A.F. Santos, F.M. Silva, M.K. Lenzi, J.C. Pinto, Polym. Plastics Technol. Eng. 44 (2005) 1. [7] D. Fischer, K. Sahre, M. Abdelrhim, B. Voit, V.B. Sadhu, J. Pionteck, H. Komber, J. Hutschenreuther, C. R. Chimie 9 (2006) 1419. [8] J. Mijovic´, S. Andelic´, J.M. Kenny, Polym. Adv. Technol. 7 (1995) 1. [9] L. Rey, J. Galy, H. Sauterau, G. Lachenal, D. Henry, J. Vial, Appl. Spectrosc. 54 (2000) 39. [10] H.W. Siesler, Macromol. Chem., Macromol. Symp. 52 (1992) 113. [11] A. Botella, J. Dupuy, A.A. Roche, H. Sautereau, V. Verney, Macromol. Rapid Commun. 25 (2004) 1155. [12] T. Scherzer, S. Mu¨ller, R. Mehnert, A. Volland, H. Lucht, Polymer 46 (2005) 7072. [13] T. Scherzer, M.R. Buchmeiser, Macromol. Chem. Phys. 208 (2007) 946. [14] J.P. Fouassier, J.F. Rabek (Eds.), Radiation Curing in Polymer Science and Technology, vol. 1–4, Elsevier, Applied Science, London, 1993. [15] U. Schnell, R. Da¨ndliker, S. Gray, Opt. Lett. 21 (1996) 528. [16] S.W. Kim, G.H. Kim, Appl. Opt. 38 (1999) 5968. [17] V.N. Kumar, D.N. Rao, J. Opt. Soc. Am. B 12 (1995) 1559. [18] H. Fujiwara (Ed.), Spectroscopic Ellipsometry: Principles and Applications, John Wiley & Sons, Chichester, 2007. [19] J. Sindu, S. Rudra, D. Lehmann, M. Friedrich, D.R.T. Zahn, Appl. Surf. Sci. 255 (2008) 694. [20] V. Svoboda, B.Y. Liaw, Pure Appl. Chem. 80 (2008) 2439. [21] U. Richter, W. Hentsch, H. Liepack, R. Kruse, R. Dittrich, Thin Solid Films 516 (2008) 4541. [22] T. Scherzer, R. Mehnert, H. Lucht, Macromol. Symp. 205 (2004) 151. [23] H. Martens, T. Næs (Eds.), Multivariate Calibration, Wiley, Chichester, 1989.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy Katja Heymann, Gabriele Mirschel, Tom Scherzer *, Michael R. Buchmeiser Leibniz Institute of Surface Modification, Permoserstr. 15, D-04318 Leipzig, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 February 2009 Received in revised form 7 April 2009 Accepted 8 April 2009 Available online 17 April 2009
We report on the development of a measuring method based on near-infrared (NIR) spectroscopy, which is able to determine the thickness of UV-cured coatings and which can be used for in-line monitoring in technical coating processes. In particular, acrylate coatings, which were applied to transparent polymer films with a typical thickness of 5–35 mm, were investigated. NIR spectra were recorded in transflection mode. Quantitative analysis of the spectral data was carried out with partial least square (PLS) regression. In-line measurements were performed on a pilot-scale roll coating machine at web speeds up to 50 m/min. It was shown that quantitative data with excellent precision (i.e. with a standard deviation lower than 1 mm) and high time resolution (2.5 spectra/s) can be obtained. ß 2009 Elsevier B.V. All rights reserved.
Keywords: NIR reflection spectroscopy Coating thickness In-line measurements Partial least square (PLS) regression Acrylic clear coatings
1. Introduction Process control has been used in chemical industry for a long time in order to control the reaction kinetics and thus the properties of the resulting products [1,2]. For many years, such systems have also been employed for the monitoring of batch, solution, or emulsion polymerization reactions [3–7] as well as for the control of thermal curing processes [8,9]. In contrast, only a few attempts have been made so far to monitor photochemically induced curing reactions [10–13], although UV polymerization of thin polymer coatings has become a rapidly growing technology with a wide range of applications such as scratch and abrasion resistant coatings, barrier layers, protective coatings, etc. [14]. In UV curing technology, functional coatings are usually made of mixtures of acrylates. The properties of the coatings do not only depend on the composition of the formulation, but also on the thickness of the applied layer. The thickness, in turn, is affected by several parameters such as viscosity, temperature, and web speed. This way, unintended fluctuations of the thickness can occur in technical coating processes. For this reason, the determination of the coating thickness is a very important issue in coating technology. Moreover, if the coating thickness is monitored in the process, it can be made sure that the thickness does not exceed a given maximum value which leads to a more economic use of the raw materials. On the other hand, the thickness must not fall below
* Corresponding author. Fax: +49 341 235 2584. E-mail address: [email protected] (T. Scherzer). 0924-2031/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2009.04.001
a specific minimum, because the properties of the coating and consequently the quality of the product depend on the layer thickness. However, up to now there is no analytical method which allows for a contact-free monitoring of the thickness of the applied acrylate layer in a continuous coating process. Most of the existing analytical methods, e.g. gravimetry, white-light interferometry, ellipsometry, optical or scanning electron microscopy (SEM) etc., are only suited for the off-line determination of the layer thickness in the laboratory. For example, white-light interferometry is a powerful analytical method which has been used for at least one decade to determine the thicknesses of thin transparent dielectric coatings [15,16] or dispersive media [17]. The thickness of the sample is calculated from the specific interference pattern caused by interfering light beams, which were reflected from the surface of the coating and the substrate, respectively. However, the method requires two significantly different refractive indices of coating and substrate, which limits the range of applications of this method. The refractive index of most polymers is around 1.5, i.e. the indices of typical UV-curable acrylate coatings and the polymer films used as substrates often differ only marginally. Moreover, the interference pattern of a moving web with a non-uniform coating can change very rapidly, which may impede or even prevent quantitative analysis. Ultimately, this may lead to destructive interference. Consequently, white-light interferometry is not a suitable method for the in-line determination of the layer thickness of UV-curable acrylic clear coats on transparent polymer foils.
K. Heymann et al. / Vibrational Spectroscopy 51 (2009) 152–155
Spectroscopic ellipsometry is another powerful method for the characterization of thin mono- or multilayers [18]. This method has already been used for the in-situ determination of the thickness of organic [19] and polymer layers [20], but mainly it is used for the in-situ monitoring of inorganic layers [18,21]. However, the typical thickness of these layers is in the range of some nanometers and does definitely not exceed some hundred nanometers. In contrast, the typical thickness of UV-cured coatings is in the dimension of some micrometers (5–20 mm) and, hence, much higher than it is usual for ellipsometry measurements. Currently, the control of the applied thickness in technical coating processes is mostly carried out off-line by gravimetric determination of the coating weight. However, it is apparent that this approach is not able to respond to sudden changes of the thickness, and thus it is poorly suited for process control. Therefore, there is a strong need for an analytical method, which is able to monitor the thickness of acrylic coatings in-line and which is capable for the use in process and quality control. As reported in a previous paper [12], near-infrared spectroscopy is a powerful contactless method, which can be used for in-line monitoring of the conversion in acrylate coatings. However, there are two facts which might be unfavourable for the use of NIR spectroscopy for the determination of the layer thickness: (i) the thickness of the coatings is in the range of only some micrometers, and hence, much lower than in most other NIR applications, and (ii) the absorption coefficients in the near-infrared are quite low. In this paper, we report on the development of a measuring method based on NIR reflection spectroscopy, which is able to determine the thickness of UV-cured coatings in-line.
153
2.4. NIR reflection spectroscopy NIR spectra were recorded with a Kusta 4004 P process spectrometer (LLA, Berlin, Germany). This instrument is based on a concave holographic grating and an InGaAs photodiode array detector with 256 pixels, which was set to cover a spectral range from 1470 to 1950 nm. The reflection probe head is linked to the spectrometer unit by a fiber optic cable. It contains a tungsten halogen lamp with a UV filter to cut-off the short-wavelength emission of the lamp. Spectra were taken in transflection mode using a ceramic plate as reflector, which was arranged behind the sample. A diffuser plate in front of the probe head suppresses interferences, which occur in thin transparent films such as 20 mm OPP foil. Moreover, the probe head is tilted against the web for the same reason. A more detailed description of the spectrometer system is given in ref. [22]. For in-line monitoring, the NIR probe head was mounted above the web immediately behind the UV lamp. Spectra were recorded continuously with a rate of about 150 spectra/min. 2.5. Determination of the coating thickness The nominal thickness of the coatings before irradiation was given by the gap of the Baker applicator used for application. The actual values of the coating thickness after irradiation were determined with a digital thickness gauge with a resolution of 0.2 mm (Heidenhain ND 221B, Dr. Johannes Heidenhain GmbH, Traunreut, Germany). 3. Results and discussion
2. Experimental 3.1. Quantitative analysis and calibration 2.1. Materials Acrylate coatings with different thicknesses were made from the following clear lacquer formulation: 60 wt% aliphatic urethane diacrylate (EB 270), 30 wt% amine modified polyether acrylate (EB 81), and 10 wt% tripropylene glycol diacrylate (TPGDA). All acrylates were obtained from Cytec Surface Specialities (Drogenbos, Belgium). Ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L, BASF, Ludwigshafen, Germany) was used as photoinitiator. It was added at a concentration of 3 wt%. 2.2. Preparation of coatings and UV irradiation Samples were prepared by application of the formulation to 20-
mm thick polypropylene foil (OPP), which had been pre-treated by corona discharge. Coating was carried out by using an automatic film applicator (SIMEX, Haan, Germany) and a set of Baker applicators (TQC GmbH, Haan, Germany), which allowed for a variation of the thickness from 5 to 50 mm. The coatings were irradiated under nitrogen in a UV curing unit equipped with a medium-pressure mercury lamp (120 W cm 1, IST Metz Strahlentechnik GmbH, Nu¨rtingen, Germany). The irradiation dose was 700 mJ cm 2 at a conveyer speed of 8 m/min and an intensity of 620 mW cm 2. 2.3. Roll coating and curing In-line monitoring studies were carried out on a pilot-scale roll coating machine at the IOM. It is equipped with a mediumpressure mercury lamp (PrintConcept UV Systeme, Ko¨ngen, Germany; 160 W cm 1, maximum power 8 kW, adjustable between 30 and 100%). Coating trials were carried out at different widths of the nip between the applicator rolls and at different web speeds.
In NIR spectroscopy, quantitative analysis of spectral data is generally carried out with multivariate calibration methods such as partial least square (PLS) regression [23], which are able to detect even minor differences in concentration. The use of such chemometric techniques first requires the setup of a specific calibration model. This model has to relate the spectral variation in the data to the parameter of interest, e.g. the thickness of the applied coating. Therefore, samples with coating thicknesses in the range from 5 to 50 mm, in steps of 5 mm, were prepared for calibration. For each specific thickness, ten samples were prepared in order to get sufficient samples for creating a stable and powerful calibration model. NIR spectra were recorded before and immediately after UV irradiation. Typically, about 1000 NIR spectra were accumulated during one scan, while moving the sample slowly through the focus of the probe beam. These single spectra were averaged before further processing. In this way, each sample was measured 10 times. Consequently, 100 mean value spectra per thickness were recorded. A typical spectrum is shown in Fig. 1. In coating technology, shrinkage of acrylates during the UVcuring process is a well-known effect. This effect leads to a decrease of the thickness of the cured coating in comparison to the uncured one. The shrinkage, which may amount up to 30%, is caused by the UV-induced cross-linking of acrylates during irradiation. In order to obtain correct reference values, the actual thickness of the coatings was determined with a digital thickness gauge. For the creation of a calibration model based on PLS regression, data were split in equal parts into a calibration and a validation set. The spectral range was limited to the region from 1470 to 1890 nm in order to exclude the influence of humidity, which absorbs beyond 1900 nm. Then, the PLS algorithm was applied to the spectra. After regression, the root mean square error of prediction
154
K. Heymann et al. / Vibrational Spectroscopy 51 (2009) 152–155
Fig. 1. NIR reflectance spectrum of a 10 mm acrylic clear coat applied to a 20 mm OPP film.
Fig. 3. Prediction of the layer thickness of independent test samples using the PLS calibration shown in Fig. 2.
(RMSEP) and the coefficient of determination R2 were calculated. Moreover, several pre-processing methods such as normalization, first or second derivative, and baseline corrections were tested. These pre-treatments were applied individually or in combination to the spectra. Subsequently, the model with the lowest number of factors, the smallest RMSEP, and the highest R2 was selected. The resulting calibration curve is shown in Fig. 2. Normalization and the first derivative were applied as pre-treatments to the spectra. The underlying chemometric model was based on five factors. The performance of the developed model was tested with an additional set of independent test samples. The results are shown in Fig. 3. The coating thickness predicted with NIR spectroscopy is plotted against the thickness determined with the digital thickness gauge. The results clearly prove that the prediction can be carried out with high precision.
For in-line monitoring of the thickness of acrylic clear coats on OPP film, the probe head was mounted on a pilot-scale roll coating machine. It was installed behind the mercury lamp and close to a guide roll in order to minimize the influence of vibrations of the web. Furthermore, particular attention was paid to the geometric arrangement of probe head and film in order to make sure that it was identical to the arrangement in the lab. This is of utmost importance, since even very minor differences between both
arrangements would substantially affect the precision of the prediction. Important parameters, which influence the spectral response and consequently the predictive power, are the distances between light source and film or reflector, respectively, as well as the tilt angle of the probe head against the web. The predicting power of the calibration model was tested under in-line conditions at a line speed of 40 m/min. The coating was applied to OPP foil, and NIR spectra were recorded continuously right after UV curing. In order to obtain different coating thicknesses, the nip between the applicator rolls was varied. After the end of the roller application trial, the actual coating thickness was additionally determined off-line using the digital thickness gauge. The thickness values determined from the NIR spectra of each layer characterized by a specific thickness were averaged and plotted against the actual coating thickness (average of 40 data points). Results are shown in Fig. 4. The correlation coefficient (r2 = 0.97) and a standard deviation lower than 1 mm indicate that the calibration model is able to predict the thickness with high precision also under in-line conditions. In order to simulate changes of the thickness during a real coating process, the nip was stepwise reduced and increased. The web speed was again 40 m/min. Results are shown in Fig. 5. In addition to the results obtained by NIR spectroscopy, data determined with the digital thickness gauge are given. It can be seen that both the predicted and the actual thicknesses decrease or increase according to the changes of the nip. Thickness changes as
Fig. 2. PLS calibration curve for the thickness of UV-cured acrylic clear coatings on OPP film.
Fig. 4. Prediction of the layer thickness from in-line measurements on a roll coating machine using the PLS calibration shown in Fig. 2.
3.2. In-line monitoring of the thickness
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even small changes of the thickness of acrylic coatings under inline conditions. The time resolution of the data recording (2.5 spectra/s) is sufficient for in-line monitoring. If measurements at higher speeds are necessary, the recording rate of the NIR spectrometer can be further increased without a significant loss of quality of the spectra. Thus, process control at line speeds well above 100 m/min can be realized. 4. Conclusion
Fig. 5. In-line monitoring of the thickness upon stepwise decrease and increase of the nip between the applicator rolls of the coating machine at a web speed of 40 m/ min. For comparison, actual coating thicknesses, which were determined off-line, are given.
In this paper, it was demonstrated for the first time that the thickness of cured acrylic clear coatings can be monitored by NIR spectroscopy. Samples with a thickness in the range of 5–35 mm were studied. A multivariate chemometric method was used for calibration. Analysis was carried out with the aid of a PLS-model, which was based on five factors. The calibration model was found to show an excellent predicting power with an error of less than 1.0 mm. Furthermore, the calibration model was used successfully for in-line monitoring on a pilot-scale roll coating machine. Quantitative data were recorded with high precision at web speeds up to 50 m/min. A very close correlation between NIR-based and off-line thickness data was obtained. In summary, NIR spectroscopy has been proven to be able to monitor basic process parameters such as the coating thickness under various technological conditions. Consequently, this method could be effectively used for process and quality control in UV-curing technology. Acknowledgements The authors would like to thank U. Trimper and S. Pyczak for technical support. Financial support was provided by the AiF association under grant number KF 0189603FK6. References
Fig. 6. In-line monitoring of the thickness upon stepwise increase of the web speed and with constant nip between the applicator rolls. For comparison, actual coating thicknesses, which were determined off-line, are given.
low as 1.4 mm can be clearly detected by NIR spectroscopy. Thus, the close correlation between actual and predicted thickness as well as the immediate spectral response after any change in thickness suggest that the predicting power of the chemometric model is sufficient for in-line monitoring of the thickness of acrylic clear coatings. In coating technology, it is well-known that the thickness of the coating increases with increasing web speed. Therefore, the calibration model was also tested at different web speeds from 6 to 50 m/min (Fig. 6). Similar to the results shown in Fig. 5, the spectral response can be observed immediately after each change of the web speed. It can clearly be seen that the coating thickness significantly increases with increasing web speed, i.e. from about 4 mm at 6 m/min up to 11 mm at 50 m/min. Again, excellent correlation of the NIR spectroscopic data with the thickness values determined off-line was observed. The results demonstrate that NIR spectroscopy, in spite of the low absorption coefficients in the near-infrared, is able to detect
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