Moisture control – a vital aspect of paper metallization

Moisture Control – A Vital Aspect of Paper Metallization

By Dr. Charles A. Bishop, C.A. Bishop Consulting, Ltd.

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Introduction

Producing metallized paper can be achieved through various methods. One approach involves laminating a 6-micron or thicker aluminum foil to a paper web. Another method, which uses significantly less aluminum, is vacuum-depositing metal onto paper similar to how it’s done on polymer substrates. Additionally, metallization can be transferred from a polymer web onto paper using hot or cold processes. This technique is more cost-effective for smaller areas, whereas the initial methods are preferred for full-surface coverage. Although the lamination of aluminum foil to paper is becoming less common, direct metallization continues to gain ground due to its superior energy efficiency.

It’s rather fascinating that paper can be metallized at all, especially considering that large paper rolls on a metallizer can weigh several tons. For instance, a roll 2.35 meters wide and 1.8 meters in diameter can weigh around 7 tons. In environments with 100% humidity and low temperatures, paper can contain up to 30% water by weight. Realistically, this water content is usually below 20%, but even so, in a vacuum, this represents a significant pumping load. Managing the moisture in the paper before it enters the vacuum system is crucial, as it involves removing approximately 150 kg of water per metric ton of paper, which translates to 150 billion cubic meters of water vapor. Reducing the moisture content to as low as 2% can be achieved by heating the paper in a dry atmosphere. This process not only calenders and coats the paper but also ensures it retains enough strength to endure lower moisture levels. Paper’s rough and porous structure leads to rapid initial moisture loss, typically about 1% within the first minute. However, as the surface moisture depletes, the outgassing rate decays exponentially, becoming limited by the diffusion rate of moisture from within the fibers.

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To help pump the moisture more effectively, it is typical to include cryopanels in the vacuum system. Most of the surface area of these pumps is located in the winding zone, but it is always good policy to have some portion of the cryopanel in the deposition zone [4]. The cryopanels are a surface that is kept at a low temperature so that the water-vapor molecules that hit the surface will stick and freeze into ice. However, this pumping is dependent upon the surface remaining at a low enough temperature to continue to encourage the water to stick and freeze. The ice that builds up can be as dense as an ice cube or porous as a snowflake or frost. deg C and, if there is a 10-mm layer of ice on the cryopanel, the front surface of the ice will be more than -80 deg C. The more porous the coating the higher the temperature. The pumping performance will drop to around 10 percent of the original for a frost layer of 5 mm. For an ice layer of 10 mm, the pumping speed may still be around 50 percent, and it may still be more than 10 percent of the original speed – at more than 25 mm.

Thus, the amount of cryopumping that is used for a paper metallizer may look to be excessive, but it is necessary when one considers the drop-off in performance. Typically, it is usually at least double that used on polymer-web metallizers. Often, part of the cryopanel surface will be used to help protect the diffusion pumps from having to pump a large amount of water vapor. To maintain a high performance of the pumps, it is necessary to gas-ballast any rotary backing pumps to prevent a build-up of water in the oil. With the amount of water available, the gasballasting may need to be more frequent than for polymer-web metallizers.

To minimize pumping time, it is important that the ice that is built up on the cryopanel is melted, drained and collected so that it can be disposed of before the next pumping cycle. This is done by heating the cryopanels at the time the system is prepared for venting. The pumps are shut off, the cryopanels are heated to melt the ice and the panels are arranged so that, as the ice melts, the water can drain into a collection vessel that can be removed once the system is at atmospheric pressure. It is vital that this water does not drip into the vacuum system where it would have to be pumped away during the next pumping cycle.

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