By Charles A. Bishop, Ph.D., CEng., C.A.Bishop Consulting, Ltd.
Aluminum has been vacuum-deposited onto polymer films for more than 70 years. But the process has not radically changed since its introduction, and current market trends point out the difficulty metallized films have in energy consumption and end-product packaging recycling. This article covers the beneficial ideas behind metallized film and its positives for flexible packaging versus aluminum foil, how metal recovery depends on material thickness, how metallization traits impact recycling, and some potential solutions for economically recovering both the aluminum and recycling the polymer or paper substrate.
Introduction
Metallizing could be described as a “piggy in the middle” industry. Brand owners, product managers or packaging consultants are on one side and decide on the look of the packaging and performance of the packaging they wish to achieve. At the other end are other parts of the converting industry that take the metallized film and add further layers in the form of coatings, adhesives, films and printing to make the end package. Elsewhere, there can be government regulation that may restrict materials that may be used or may attempt to encourage materials that would increase the amount of material that could be recycled or reused and preferably not sent to landfill.
Some of these different goals may result in confusion about what to do. To minimize food waste, it seems logical to use packaging to extend the food lifetime by protecting the food from rodents, light, moisture and gases that may degrade the food. However, it is common to find food that has an excess of packaging, much of which ends up in landfill.
Among all this is a plethora of misinformation (or too much information) that adds to the confusion. Included in this can be errors from journalists who do not understand the technology they are describing, which can lead to mistakes. I have seen the thin, metallized-aluminum coating spuriously described as a foil or powder coating.
Why is aluminum of such interest
in flexible packaging?
Aluminum has been vacuum-deposited onto polymer films for over 70 years. The deposited aluminum not only has a bright metallic appearance but, if the polymer-substrate surface is smooth, can be highly reflective. This is popular in marketing and, with the right lighting, it can help attract more customers with the light reflection, catching the eye better than duller packages. Elsewhere in packaging, metal cans or tins have very good barrier properties to water, oxygen (or other gases) and light.
For many foods, their lifetime can be extended if the amount of water, oxygen or light that can reach the food can be minimized. The metal does not have to be as thick as a tin can to limit the ingress of water, oxygen or light. Aluminum can be made thin by squeezing a cast aluminum ingot between nip rolls to make the ingot thinner but longer. If the aluminum is passed through a succession of nip rolls, the gauge can be thinned down to as little as 6 microns. The process of crushing the aluminum between the nip rolls changes the crystal structure and makes the metal harder; so periodically, the metal must be heated for a time to allow recrystallization to make the metal more malleable to permit further thinning.
The result of this process can mean that any impurities in the metal casting can end up at grain boundaries, and when the aluminum gets to be very thin these impurities may drop out, leaving behind a pinhole in the foil. As the foil is made thinner, the number of pinholes increases. The pinholes may be elongated if there is any further rolling following the defect appearing [1]. The table published in Reference 1 shows that a foil of 7-micron thickness may have more than 400 pinholes/m2, and a maximum number of more than 1,500 pinholes/m2, whereas if the foil thickness is 25 microns, the expectation is there will be no pinholes in the foil.
Foils can tear during winding and also may be prone to wrinkling. It is common to find foils used a lamination layer in a multilayer-packaging product. The adhesive can fill the pinholes, reducing the loss of barrier performance that the pinholes otherwise would cause. The polymer film also can help minimize wrinkling and thus reduces problems such as flex-cracking of the foil and stress-cracking at bends or creases that might be necessary during wrapping of the material around the product to be packaged. The foil is duller and much less reflective than metallized aluminum.
Foils and foil laminates can be recycled in some aluminum-recovery plants where the other layers are incinerated and provide heat that translates into potential power generation. The aluminum can be recovered to be melted into a billet to be reused. This is one route where mixed-polymer, multilayer packaging can avoid landfill.
The beneficial ideas behind metallized film
Metallized polymer films use the idea that a thin, continuous layer of metal provides a barrier to water vapor, oxygen and light. This has the attraction that less aluminum will be cheaper and lighter than foil, and so there will be a cost-savings available. The metallized layer of aluminum metal is almost 1000x thinner than foil. Instead of a foil of 6 to 20 microns, the metallized aluminum usually is in the range of 20 to 40 nm.
The aluminum layer is coated directly onto a polymer or paper substrate in a roll-to-roll (R2R) vacuum-coating system where a web of substrate is wound around a water-cooled deposition drum. As the substrate moves around the drum, it passes through a zone where aluminum is evaporated from a molten pool and condenses (deposits) onto the substrate surface. The quality of the coated layer can be affected by the quality of the melted aluminum wire as well as the surface quality of the substrate. If the substrate surface is rough, the coating will be rough and less reflective and, if the substrate surface has particulates on the surface, they may result in pinholes, which will reduce the barrier performance of the coating.
The thickness of the coating also will affect the amount of light that will pass through. At ~18 nm, there will be 10% light transmittance; if the aluminum thickness is increased to ~32 nm, the light transmittance will be reduced to 1%; and at ~40 nm of aluminum the light transmittance will be down at 0.1%. These thicknesses are approximate because there are a number of factors that can affect the nucleation and growth of the coating which, in turn, affect the final structure and properties of the coating.
The advantage of such a thin metal coating is that flexibility is much better than for a foil. If light degradation is the most critical property required, the thickness of the aluminum coating becomes more critical. Thicknesses I have used are the final coating thickness, and this includes the oxide that forms on the aluminum surface. It has been shown [2,3] that an oxide will form at the air surface of the metallized aluminum but also at the aluminum substrate interface. This oxide will be on the order of 3.5 nm thick, irrespective of coating thickness. As aluminum oxide is transparent, it does not block light. Consequently, there needs to be sufficient metal deposited to provide enough to be converted to oxide and still leave enough left to provide sufficient light blocking to give the desired product lifetime.
Recovery success depends on thickness
The fact that aluminum has a large affinity for oxygen affects what happens to the packaging waste. Aluminum foil is too thick to be easily oxidized, and if ground up with the polymer, it would color the polymer if re-extruded, thus limiting how it could be reused. High-clarity polyester bottles can be recycled and reprocessed into a coextrusion where the center layer of the all-polyester coextrusion can be recycled polyester, and the clarity will be as good as if it was all-virgin polyester. But at 6 microns thick, foil can be accepted for the incineration process that recovers the aluminum as a new aluminum billet with the polymer being lost to reuse but still having some value as a source of heat to generate electricity.
Metallized aluminum has too little aluminum in the coating to be used in the incineration process to recover the metal, and this changes the economic balance of the process. Simply incinerating polymer to produce heat to generate electricity is not economical, and so metallized film is rejected as a suitable incoming feedstock. What has been proven acceptable is to grind up the metallized film and process it the same as unmetallized polymer. The very thin aluminum as it is processed becomes increasingly converted from the metal to the oxide (which is transparent) and becomes invisible within the polymer, so it does not affect the clarity of the extruded film.
Metallization traits impact recycling
While it has been proven that metallized layers are so thin that they are inconsequential to recycling, they still may be rejected as feedstock. Barrier aluminum-oxide-coated films already are transparent and, consequently, can pass as plain polymer film for recycling purposes. They may well be accepted in the feedstock because recyclers do not realize the film is coated. Metallized-polymer films often are laminated and look the same as unlaminated material.
Metallized film also may be detected as a metal object that could damage the processing equipment. To save the cost of sorting or risk letting a metal object through, it is easier and cheaper to reject everything that looks metallic. This may change in the future as recycling plants use improved sorting techniques that have better accuracy and speed in material identification. Although this would help more metallized film to be recycled, it does mean that the aluminum is lost to become a hidden filler in the polymer.
Six of one; half-dozen of the other
Currently, that appears to be the choice: Recover the aluminum at the expense of destroying the polymer or paper; or alternatively, if the aluminum content is small, recycle the polymer with the loss of the aluminum. There does not yet appear to be the option of economically both recovering the aluminum and recycling the polymer or paper.
Paper may appear to offer an advantage over polymers as a substrate in that, to recycle the paper, it has to be treated by liquids to form a slurry to produce a suitable feedstock for making new paper. As the aluminum can be removed by using sodium hydroxide, it would leave just the paper behind for recycling, and the aluminum could be recovered from the sodium-hydroxide solution. Unfortunately, metallized paper covers many different versions from directly metallized, metallized polymer laminated to the paper, and hot- or cold-stamped foiled paper. This also may include polymers, adhesives and coatings such as lacquers, adding to the difficulty of recycling and leading to some paper products being outright rejected for recycling.
Types of substrates and aluminum waste
produced by vacuum metallizing
There are different grades of scrap produced by the vacuum-metallizing process, some of which are relatively easy to recycle compared to others that present challenges.
The aluminum used in most metallizers is loaded into the system as a coil of wire on a spool. It is this wire that is fed to each of the individual thermally heated boats that evaporate the aluminum. There will be some wire scrap from loading the wire and feeding it through the guide tubes to direct the wire toward the evaporation boats. If the wire becomes kinked during setting up the process, it may need to be cut off to feed smoothly into the boats so that it does not wander offline or miss the boat altogether. It is impossible to ensure that all the aluminum wire is removed, and if the wire on the spool is not sufficient to complete a deposition cycle, it will be scrapped. Some of these wire oddments may be placed in the boat to help prime the boat before the wire-feed rate and the molten pool is fully established. Any wire not used can be recycled and is essentially the same quality as supplied.
The deposition process can vary in material efficiency from <50% to >80%, depending on system design and age. The aluminum that does not reach the substrate mainly is deposited onto deposition shields that periodically have the coating cleaned off. This stray deposition is not pure aluminum but is a mixture of aluminum, aluminum oxide and hydroxide, and it may have small quantities of any release coating used on the shield surfaces. This material takes the form of broken sheets several millimeters thick down to a fine powder produced as the sheets/flakes are broken. This debris has a high aluminum content and can be reprocessed back to pure aluminum.
The rest of the aluminum is deposited onto the substrate. There are various sources of scrap from coated substrates. Most films have an edge trim taken off to remove the masked edges used to prevent coating reaching the deposition drum. There also is the possibility of the substrate snapping during metallizing or for winding problems such as wrinkling, which may lead to lengths of film or part rolls being scrapped.
Problems with vacuum-metallized aluminum
According to the Aluminum Association, it takes just 5% of the energy to recycle aluminum as it does to make virgin aluminum from ore [4]. To this end, aluminum beverage cans widely are recycled, with Brazil topping the list with >90% of cans being recycled, followed by Japan with ~82% recycled, and across Europe >50% recycled. All of which is a good thing because the process is very simple. Just acid etch the cans to remove the print and lacquer and then remelt the clean cans. The incoming feedstock is >90% aluminum, and the processing is simple, making the rewards – rewarding.
Compare this to flexible packaging where the feedstock is Combined Packaging Waste (CPW) or Municipal Solid Waste (MSW). Here, recovery is complex, needing sorting and multiple steps to separate what may be multiple layers, including food waste, print, multiple mixed polymer layers, adhesives, metal as a thin film coating or thicker as a laminate foil, paper or paperboard. The aluminum content, if there’s foil in the package, may be up to 50%, but if it is a thin metallized layer, may be <1%. This can mean the process is costly and less rewarding.
What the numbers reveal
This all depends on how you look at the numbers (see Table 1). There are about 2,500 metallizers in the world today, which means >75,000 tonnes of aluminum on metallized film is used each year. A single, modern aluminum metallizer can produce ~375 million m2 of metallized film/yr. At a coating weight of 0.08 gsm, this represents 80 kg of aluminum per 1 million m2 or film or 30 tonnes of aluminum coating/yr. As mentioned above, the deposition efficiency may be as low as 50%, which would mean a requirement of 60 tonnes of aluminum/yr of which 30 tonnes would be immediate scrap from the debris taken off the deposition shields. If the deposition efficiency was ~75%, this would change to 40 tonnes of aluminum wire used each year and 10 tonnes becoming scrap. This still would mean that 30 tonnes/yr of aluminum in the coated material could end up in landfill because it is perceived as being too difficult or uneconomical to recycle. Where it is recycled, the aluminum is lost to reuse because it is degraded into transparent oxide and becomes, in effect, a filler in the recycled substrate.
Concerns for the future
By using solvents, multilayer films can be separated for easier recovery of the various components. An example would be separating a PE/Al/PET multilayer construction, where acetone can be used to separate the layers. The PET then can be depolymerized using a supercritical reactor containing ethanol [5]. This enables all materials to be recovered for recycling or reuse.
As governments around the world consider their goals for recycling and energy efficiency and savings, I am sure that packaging will again come under threat as so much continues to be sent to landfill. Incineration might convert some of the material into energy, but packaging likely still will be seen as wasteful.
Sorting waste still is one of the biggest challenges, not only at the sorting plants but also for consumers who find it hard to understand all the different printed information. I have packages that tell me the product is recyclable but to check local recycling preferences. When I check, I get pages of instructions including, “Yes, it can be recycled if I drive 10 miles to a specific collection site.” This is senseless as the cost of fuel to dispose of the package is disproportionate to the benefits of recycling, and so the temptation is to either put it in the recycling bin and let them sort it out or put it in the landfill bin.
What really is needed is an unambiguous way of coding the package so that sorting becomes fast, easy and accurate. This then gives no excuse for the packages not to be recycled. To make recycling easier, the use of mono-materials will help make processing more economical. The volume and security of material supply is improved, making it easier for recycling plants to be profitable. If the volumes are there, the economics of recovering thinner layers of aluminum become more attractive.
Resource-saving or energy-wasteful?
For many years, the metallizing industry has sold the idea that metallized film is almost as good as foil but is 1,000X thinner, which is environmentally better. Some would put the case that now metallized film is worse for the planet as it goes to landfill while foil can have the aluminum recovered and reused. In addition, metallizing can be regarded as having poor efficiency, with 25% to 50% of the aluminum being deposited on shields. The power to heat the boats also might be considered wasteful, with about one-third of the power being lost to heating the water to maintain good electrical contacts, another third being lost to radiant heating (ending up in heating the cooling water for the shields), and then leaving only about one-third of the power to evaporate the aluminum.
There have been other deposition sources used on R2R-coating systems that have >95% material efficiency [6,7,8]. These deposition sources are not suitable as a direct replacement for resistance-heated boats, and this always has been used as an excuse not to spend time and money to develop these sources. I think that now would be a good time to revisit these alternative technologies to see how they can be improved and made suitable for metallizing systems. At 95% material efficiency, aluminum use would be massively reduced from the 30 tonnes onto deposition shields down to 1.05 tonnes, as well as the energy required per m2 of coated film.
Using the potential material and energy-saving opportunity should be a good argument to get investment for developing improved source technology. This also would provide a very positive message that the metallizing industry is doing its part to minimize energy use and save material resources. Considering there already is investment in place trying to develop alternatives to metallization for high-barrier packaging with the goal of the packaging being 100% recyclable from sustainable source materials, it would appear metallizing is falling behind and needs to be more proactive in defending its current market position.
References
1. “The Impact of Foil Pinholes and Flex Cracks on the Moisture and Oxygen Barrier of Flexible Packaging,” Lee Murray, Alcan Packaging, Proceedings of TAPPI PLACE Conference, Nevada 2005.
2. “Evaporated Aluminum on Polyester: Optical and Electrical Properties as a Function of Thickness, Part 1,” N. Copeland and R. Astbury, Converting Quarterly 2011 Q1, pp. 48-52.
3. “Evaporated Aluminum on Polyester: Optical and Electrical Properties as a Function of Thickness, Part 2,” D.J. McClure and N. Copeland, Converting Quarterly 2011 Q2, pp. 66-68.
5. “PET and aluminum recycling from multilayer food packaging using supercritical ethanol,” S.L. Fávaro, A.R. Freitas, T.A. Ganzerli, A.G.B. Pereira, A.L. Cardozo, O. Baron, E.C. Muniz, E.M. Girotto, E. Radovanovic, Journal of Supercritical Fluids, 75, March 2013, pp. 138–143.
6. “Linear deposition sources for deposition processes,” Tai Joon Um, Young Cheol Joo, Sang Wook Lee, Kug Weon Kim, US Patent 2009/0142289 A1, June 4, 2004.
7. “Jet-vapor deposition, a novel coating technique with superior properties,” B Schmitz, R.Colin, M.Economopoulos, ATS, December 9, 1999, 99BSO 12 DOC, pp. 1-3.
8. “A novel roll coater for double-sided heavy deposition of magnesium onto PTFE films,” J.J. Fonseca, W. Gorton, Proceedings of 38th Annual Technical Conference, Society of Vacuum Coaters, 1995, pp. 64-70.
Charles A. Bishop, principal at UK-based C.A.Bishop Consulting, Ltd., 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 40+ years of experience in vacuum deposition, mainly onto flexible webs. He writes the “Vacuum Verbiage” Q&A technical column for this publication and moderates the online “Vacuum Web Coating” Technical Topics Channel. Charles can be reached at +44-1509-502076, email: cabuk8@btinternet.com.
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