By Dr. Charles A. Bishop, C.A. Bishop Consulting, Ltd.
There are various methods of producing metallized paper. One consists of laminating an aluminum foil of 6-micron thickness or greater to a paper web. An alternative that uses on the order of 300x less aluminum is to directly metallize the paper in a roll-to-roll (R2R) vacuum-deposition system similar to those used to metallize polymer substrates. A final method of metallizing paper is to hot- or cold-transfer the metallizing from a polymer web onto paper. This process is economical more for small areas, whereas the first two methods are used when it is desired to cover the whole surface. I will ignore the lamination of aluminum foil to paper as it continues to be replaced by directly-metallized paper because it is a more energyefficient product.
Ialways find it amazing that paper can be metallized at all when you consider that, on a large metallizer, a roll of paper may easily weigh a few metric tons, i.e. a 2.35-meter-wide roll of 1.8-meter diameter will weigh 7 tons [1]. Paper may contain up to 30 percent of water by weight when the humidity is 100 percent and the temperature is low [2,3]. This is an extreme condition; more typically the water is well below 20 percent which, in a vacuum, would be a huge pumping load. This amount of water – if lost from the paper – would be approximately 150 kg per metric ton, which would convert to on the order of 150 billion cubic meters of water vapor to be pumped by the vacuum system. Thus, managing the moisture content of the paper prior to being loaded into the vacuum system is essential. This can be done by simply heating the paper in a dry atmosphere to reduce the moisture content to take calendered and coated paper substrates down to as low as 2-percent moisture content. The calendering and coating gives the paper some additional strength, allowing the moisture content to be lower without losing too much strength. Paper is rough and the structure is porous, giving it a very high surface area. This means that the moisture loss initially will be very quick, and typically 1-percent moisture will be lost within 1 min. But, as the surface is denuded of moisture, the continued loss of moisture will be ratelimited by the diffusion rate of the moisture from within the fibers to the surfaces, and so the outgassing rate of moisture decays exponentially with time.
The graph and tables in Figure 1 show how the relative humidity can change the moisture content of a roll of paper. This will not be a uniform change as the edges and outer surface will see any change of humidity first; whereas, the center of any roll will take much longer to equilibrate. On rolls of a couple of metric-tons of weight, this can take many hours – if not days. The graph of the weight change with relative humidity is a generic graph that will be different at different temperatures and for different paper
types. less than 5 percent. Some will FIGURE 1. The relationship between relative humidity and moisture content of paper 36 www.convertingquarterly.com • 2013 Quarter 1
FIGURE 2. A schematic showing the effect of calendaring on the paper roughness
FIGURE 3. The use of smoothing coatings to improve the specular reflectance of paper and also on the back surface to reduce any problem of curl
The management of moisture in paper is very important. As paper becomes increasingly fragile with continued loss of moisture, and if it loses too much, it becomes susceptible to web breaks. Following metallization, it is common to find the roll is immediately rewound to where the paper is subjected to a water spray so that it can be rehydrated to recover the strength to make it more robust for converters.
As paper is quite rough, the deposited metal will be conformal so that the resulting surface will have low specular reflectance and will appear metallic but matte. To improve the specular reflectance of the metal requires the paper surface to be smoothed. There are different methods of achieving this, each with different levels of smoothness.
Paper can be regarded as having different scales of roughness – having both large-scale and small-scale roughness. The paper can be calendered where the paper passes between nip rolls and then compressed, which has more effect on the regions where the paper is thicker with the peaks being smoothed, but the low points being left untouched (see Figure 2). This tends to make the uniformity of the paper thickness better, but it will not produce the best specular-reflectance paper. This can be a single-calendering or double-calendering process where the double calendering further improves the surface quality.
To further improve surface quality, it is necessary to add one or more coatings. Coatings containing fillers such as calcium carbonate or china clay are commonly used on papers to increase the opacity and smoothness of the paper surface, and this also can benefit the paper for metalllization. However, this too does not give the best specular reflectance because the filler can protrude from the surface and still give the surface an optical-scale roughness that results in diffuse reflectance. A second coating of a varnish or lacquer can be used to provide an optically smooth surface that will maximize the specular reflectance and more closely match the bright reflectance that is available from metallized polymer webs (see Figure 3).
...managing the moisture content of the paper prior to being loaded into the vacuum system is essential.
The coating (shown in Figure 4) also does affect the outgassing from the surface, reducing the rate of outgassing. This can lead to problems with the metallized paper as the two surfaces will have different moisture content. As the paper shrinks, moisture loss will result in the paper wanting to curl (shown in Figure 5). To minimize this problem, the backside of the paper also may be coated to restrict the outgassing from the back surface, making it similar to the front surface and, thus, minimizing the propensity to curl.
Following metalllization, there are other things that can be done to modify the metallized coating. Applying a clear lacquer will protect the aluminum coating. Alternatively, the lacquer can be transparent
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FIGURE 4. Optical microscopy images of a paper suitable for metallizing – both at the same magnification – with the front surface showing how smooth and featureless it can be made following coating.
FIGURE 5. Schematic shows how the loss or gain of moisture – particularly where the change is faster on one side – can lead to curl of the paper. This curl will be temporary. Once the moisture content is equalized the paper will return to being flat.
and colored to provide a colored, reflective metallic effect, or the lacquer can be translucent and colored to provide other surface or optical effects. Postmetallization embossing also can be done to give larger surface effects such as making the surface appear to have a linen weave. This gross distortion does tend to crack the various coatings.
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.
Having highlighted all of these problems relating to moisture, it is interesting to note that by using high-quality paper and preparing it well, it is possible to use paper as the substrate for electronics. Paper-based batteries and electronics can be manufactured where patterning down to 20-micron resolution can be deposited using organic chemical vapor deposition of conducting polymers in R2R vacuum-deposition processing [5]. Examples of photovoltaic devices have been demonstrated using this processing. n
1. Casey F. et al, “Design considerations of a high efficiency cryogenic pumping system for paper metalllization,” SVC 36th Ann Tech Conf Proc. N (1993) pp 213-219.
2. Prahl J.M., “Thermodynamics of paper fibre and moisture mixtures,” Ph.D. thesis, Harvard University, Cambridge, MA, USA 1968. 3. Andersson J.G., TAPPI Journal, 74 (6), 131 (1991). 4. Stenhouse J. & Spencer A.G., “Coating improvements through better control of water vapor in vacuum deposition,” Proceedings of 17th Vacuum Web Coating Conf. 2003, Session 2B, Section 1, Paper 3. 5. Barr C. M. et al, “ Direct monolithic integration of organic photovoltaic circuits on unmodified paper,” Adv. Mater. 2011, vol. XX, pp 1–6.
Dr. Charles A. Bishop holds a Bachelor’s degree in Materials Engineering with a Diploma in Industrial Studies. His research led to developing a process for manufacturing titanium-based bone implants for tendon location. He went on to obtain a Master’s degree and Ph.D. following further research into vacuum-deposition processes. Charles has more than 30 years of experience in vacuum deposition, mainly onto flexible webs. He has published two books and writes the “Vacuum Verbiage”Q&A technical column and Blog for this publication. Charles can be reached at +44-1509-502076, email: cabuk8@btinternet.com
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