Mass Production of Clear Barriers. Requirements on Vacuum Web Coaters for Quality Assurance. Rainer Ludwig, Applied Films, Alzenau, Germany.

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1 Mass Production of Clear Barriers Requirements on Vacuum Web Coaters for Quality Assurance Rainer Ludwig, Applied Films, Alzenau, Germany Abstract An increasing number of packages using Transparent Barrier Coatings, can be found in American stores. The success and acceptance of these coatings by the market will be based on the delivery of stable, high quality coatings. Compared to standard Al metallizing, monitoring quality is even more critical. For Al Metallization, the coating is visible and larger defects can even be seen by eye but with Clear Barriers, the coating is not visible. Defects cannot be easily detected and sometimes the question is arises: Is there any coating at all? Based on these concerns, production machines require a very robust and reliable process. More of the quality assurance has to be handled in the coating machine itself. This presentation will be focused on coating-quality requirements for Clear Barrier Production with the Electron Beam coating process, which is currently being used by the majority of the Clear Barrier producing companies. Introduction A basic requirement for all Web Coating Processes is the final appearance of the coated rolls. The products should be without scratches, wrinkles and other physical damages. State of the art web coaters should be able to guarantee these characteristics for standard substrates and processes easily. Another question is how to ensure the quality of the required functions of the coated layers Before discussing this, the required functions should be defined. In table 1, the basic requirements for Clear Barrier Coatings are summarized and compared to the basic requirements of Al Metallizing. Other requirements, like printability, behavior with regard to laminating, retort ability and environmental issues will be discussed later. Al Metallizing Clear Barrier Coating Barrier x x Light Protection x Content visible x Microwaveable x Metal free x Table 1 How to ensure the Layer Functions? As shown in table 1, the layers should provide a certain barrier; in addition there can be defined optical and electrical properties. It would be ideal to control all relevant functions, both cross- and lengthwise the substrate, inline while coating. Optical properties (transparency) can be measured and controlled inline, this is currently done in all state of the art coaters and is a standard tool. It is also possible to measure the electrical properties, for example to be insulating, but this is not really needed as soon the layers are

2 transparent. Fig. 1 shows the light transmission of 12 µm PET, uncoated and with different SiOx layers. Fig. 1: Spectral light transmittance For almost all transparent barrier coatings, there is an absorption in the uv-range. This effect can be used for inline layer measurement. It is not possible to measure barrier properties directly inline and since barrier is a critical functional requirement to most applications, alternatives need to be considered. Even offline, only limited spot measurements, which are very time consuming, can be done. Therefore even an indirect method for inline indication of barrier properties is helpful. For PET substrates, a correlation of barrier properties with the coating thickness is known. Fig.2 shows the dependence of OTR with coating thickness for different coatings. Fig. 2: OTR as function of coating thickness For all coatings ( Al, Al2O3, SiOx ) a stable barrier function starts at a thickness of about 20 nm. For Al, the OTR decreases further with increasing coating thickness. For Al2O3 and SiOx the OTR values stays constant with increasing thickness. That means, provided there is a stable process, and parameters like vacuum, web tension, substrate temperature, layer composition are controlled, the barrier properties should stay stable, as long as a certain minimum coating thickness can be guaranteed. This is a good basis for quality assurance of the barrier properties. Of course, the layer thickness cannot be measured directly inline, but there is a relationship again to the optical properties. This relationship has a direct correlation to the OD in case of Al. In case of oxides a relationship to the transparency is given for constant stoichiometry.

3 Transparent Barrier Coatings by Electron Beam Evaporation Electron Beam Evaporation is a very powerful and versatile process, but it also requires much higher investment cost, compared to resistance -, or radiation heated evaporation. EB process is a must, in case a special evaporation material requires such high power for evaporation. EB can be of advantage in other cases, due to the very simple and robust process which is based on the use of one single big crucible and the ease of closed loop inline control. Fig. 3 shows the principle of inline layer control in an EB web coater, using feed back control from inline transparency measurement. Fig. 3: closed loop control in EB web coater A scanning unit, directly installed in the electron beam gun moves the beam on the crucible surface. The power distribution on the crucible surface is controlled by the actual measured transparency value on the corresponding substrate surface. What is important for a stable Coating Quality? In this section, just a few quality related key issues will be discussed. To have a stable and reliable machine with proper winding and vacuum system is a mandatory precondition. Layer Uniformity To guarantee the layer thickness in an actual roll, across its widths and length, it is a must to have an inline measurement system for the relevant layer property. It was already mentioned, that for transparent barriers the measurement of the optical transparency at low wavelength can be used. An example for such a measurement unit is shown in Fig. 4

4 Fig. 4: Measurement system LMS XL Light from a light source outside of the vacuum is wave length selected (wavelength range 380 nm to 1000 nm), it is split up and transferred to multiple sensor heads across the substrate width. The substrate moves in between the end of the glass fibers and the sensors. The system can measure optical density up to OD 4. The measured values of the different heads can be used for a closed loop control with the beam scanning unit. This control is performed, using a special, patented software /1/. Uninterrupted Coating With high-voltage sources arcing always is an issue. Even with optimized eb guns, using separate pumped beam generator chambers and additional intermediate pumping between beam generator and process chamber, particles from the process may reach critical areas and cause arcing. Such arcs lead to a shutdown of the high voltage power supply and can result in uncoated areas on the substrate in the machine direction. Arcing cannot be avoided 100 % and shutdown of power supply will occur. But uncoated areas on the substrate can be avoided, if the shut down of the power supply is short enough. Such fast switching was ensured in older type power supplies by using high power tubes. Nowadays modular, switching type power supplies are used, which also can minimize the voltage interruption time to be in the range of several hundred microseconds /2 /. Such a power supply is shown in fig. 5. Fig. 5: High-Voltage Power Supply Pinholes With transparent barrier coating, there is a big advantage against Al metallizing. Even if pinholes are present, they are not visible! Usually pinholes are also not critical for the barrier function if the coated film is laminated afterwards. However, in case of printing directly on the coating, an invisible pinhole can become visible. For example a pinhole in a SiOx coating below a directly printed-on barcode can create problems. In addition to the standard counteractive measures like polished guiding rollers, with eb machines, pinholes can be minimized with an optimized beam scanning program; however, the most influencing factor is the evaporation material itself. Electrostatic Charging of the Substrate Coating of plastic film with insulating layers at high speed is more critical than coating with a conductive metal layer. In electron beam web coaters there is an additional charging up by reflected electrons. For discharging of the substrate prior leaving the coating drum, a special plasma tool has to be used.

5 Adhesion Adhesion of the coating to the substrate is a key issue for later processing like laminating or retorting. Adhesion can be influenced by inline pretreatment. A detailed description is given elsewhere /3/. Depending on the requirements, electron beam web coaters use either DC driven Magnetron glow discharge tools as shown in fig. 6 or RF driven Hollow Anodes as shown in fig. 7 Fig. 6: DC driven TreatMag The TreatMag /4/ is used in all kind of web coaters, such as, eb machines, sputter machines, capacitor web coaters and Al metallizers. By running Ar-plasma it can be used for cleaning of the substrate surface, with reactive gases like oxygen or nitrogen, a chemical modification of the substrate surface is possible. Fig. 8: Two Hollow Anodes in electron beam web coater Compared to the TreatMag, a Hollow Anode /5/ provides higher energetic particles to the substrate. Substrate Overheating Like in Al metallizers, the coating has to take place on a cooled drum. In eb coating, due to the reflected electrons, the substrate is charged up during coating with the effect of

6 better thermal contact between substrate and drum therefore thermal problems are lower compared to metallizers. Barrier Last but not least, the coating quality needs to be considered. If all above marked items are handled with care (the coating is uniform within the right thickness range, the coating is not interrupted and the substrate is overly charged, the substrate was not overheated and the adhesion is good), the barrier value range should be stable. However, stable at what value is a question which is mainly influenced by the substrate itself. By applying the same coating to different substrates, the OTR value can vary in the case of 12 µm PET between more than 3 to well below 1 cm 3 /m 2 day. For OPP the range is between 20 and several 100 cm 3 /m 2 day. In case of reactive evaporation of Al 2 O 3, there is an additional tool which can be influenced by the machine supplier. It can be shown; that with plasma assisted evaporation, the range of barrier values is stabilized at the lower end. Fig. 9 shows the patented arrangement of microwave tools in the evaporation chamber of an electron beam web coater with initiation of plasma in the evaporation cloud. Fig. 9: Microwave driven Plasma Source near Crucible Summary For critical quality assurance, particularly for non-visible coatings, it is necessary to have a detailed data logging for all relevant process parameters for each coated roll. This must include inline measured transparency data lengthwise and widthwise in order to have a direct control to determine the optical data are within specified range or not and also an indirect indication if the thickness is in the right range with respect to barrier. Other effects, which also may influence the quality, like pretreatment or discharging can not be measured directly. The process parameters of such equipment must be included in the data logging to document the stable function. Finally, with the ability to use fast switching power supplies or closed loop evaporation control, the amount of coated web without the specified range can be minimized. References /1/ M. Bähr et. al., New Scan and control System (ESCOSYS TM ) for High Power Electron Beam Techniques, Proc. Of PSE, 1996 /2/ A. Thiede et. al., New Type of High Voltage Power Supplies for Electron Beam Web Coaters, 61 th Ann. Techn. Conf. of the SVC, 149, 2003

7 /3/ R. Ludwig et. al., In Chamber Pretreatment for Vacuum Web Coaters, 48 th Ann. Techn. Conf. of the SVC, 2005 /4/ G. Loebig et. al., TreatMag, a New Tool for Inline Plasma Pre-treatment in Web Coaters for Packaging Applications, 41 th Ann. Techn. Conf. of the SVC, 502, 1998 /5/ M. Geisler et.al., rf Plasma Tool for Ion-Assisted Large-Scale and Sheet Processing, 44 th Ann. Techn. Conf. of the SVC, 482, 2001

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