Basics Multilayer
Basics Multilayer Packaging By: Michael Thielen The demands on film packaging have increased enormously in recent years. This applies both to the functional requirements regarding actual product protection as well as to its aesthetic specifications. Packaging today must be designed to appeal to all the senses. Visual appearance alone is no longer enough; the way packaging sounds and feels also matters to customers. Moreover, film packaging plays a vital role in terms of extending shelf life and providing aroma protection, especially in the food industry. For many packaging applications, only multilayer film products can provide the range of properties required. Such properties include flexibility, rigidity (tear and puncture resistance), UV and light protection, temperature resistance, optical appearance such as printing and gloss, sealability or specific barrier properties (moisture, oxygen, carbon dioxide, aromas etc). The properties of such multilayer film solutions can be modified as desired through the use of combinations of different films, with a broad range of possible combinations available to choose from. The flexibility in the structure of a multilayer film means that it can be used in a wide variety of industrial sectors, from the food industry or medical and chemical packaging to the cosmetics industry or in packaging for animal feed. [1, 2] Conventional multilayer film products The materials used for the different layers include polyethylene, PET, polyamides, blends of these resins and others, which are then combined with special barrier materials, such as for example EVOH. Multilayer films can be produced through coextrusion processes, in which two or more materials are extruded through a single die; or via a laminating process, in which different films are combined in a roller mill system. In many cases, special adhesives (e.g. based on polyurethanes or acrylics) are required to bond the different layers together. Despite all the above-mentioned benefits, multilayer plastic packaging waste represents a serious environmental concern, as separating the different layers of which the packaging is comprised at the end of life is impossible, virtually ruling out the possibility of recycling packaging of this kind. As a result, these multilayer packaging films usually end up in landfill or in being incinerated. Biobased and biodegradable multilayer film packaging Bioplastic multilayer films can offer solutions for this end-of-life problem. On the one hand, a multilayer structure made entirely from biobased plastics, even if these are not biodegradable, can be disposed of together with the residual household waste, and incinerated. The amount of (renewable) carbon dioxide emitted by these plastics during incineration is equal to the amount of carbon dioxide absorbed from the atmosphere during the growth phase of the plants from which these bioplastics are made. Hence, this end-of-life scenario is carbon- or climate neutral. Biobased plastics in waste-to-energy incineration represent a kind of renewable energy. If all the plastic films in a multilayer structure are certified compostable, the entire multilayer film, depending on its final thickness can also be compostable. “In recent years, together with raw material suppliers, we have developed more than 30 different laminates from biobased and/or compostable plastics,” says Patrick Gerritsen, co-founder and CEO of bio4Pack in Nordhorn, Germany. These are mainly 2 and 3-layer laminates. Examples are combinations of paper with cellulose-based Natureflex films by Futamura, paper with PLA based films, or combinations of different bioplastics. Combinations of cellulose films, PLA films, films of PBAT or PBS, and/or TPS (starch) can also be applied. “For enhanced barrier properties in most cases for example Natureflex NK or metallised Natureflex films are used,” Patrick explains. Metallisation is achieved through the chemical vapour deposition of aluminium; and if transparency is needed, aluminium oxide [3] coating can be used. In both cases, the coating is very thin and in the composting process, it behaves inert. There are, of course, many more biobased and /or biodegradable barrier solutions available on the market, such as solutions based on PVOH [4] or BVOH [5]. “In every case of application, the first questions that have to be clarified, concern the functionality of the final product that is needed,” says Patrick. Does customer want to have a fully biobased product? What barrier (against water vapour, oxygen, carbon dioxide or flavours) is needed? Is a printed paper layer desired on the outside of the packaging? Or is a glossy appearance desired for the outside? In that case, cellulose based films, printed on the inner side are often used. For the stability of the packaging, layers of PLA or cellulose-based films are often applied. The sealing layer depends, for example, on the weight of the filling good. Packages for lightweight products such tea, peppermint or chocolate can use a sealing layer of 34 bioplastics MAGAZINE [06/19] Vol. 14
Basics outer layer (e.g. paper) adhesive layer structural layer (e.g. PLA) adhesive layer inner layer (e.g. metallized cellulose) cellulose films. Heavier filling goods may require sealing layers of TPS or PLA/PBAT blends. On the other hand, it is important to know whether the sealing needs to be permanent, peelable or even reclosable. Adhesive layers As the different layers in a multilayer structure do not adhere well to one other, a special adhesive needs to be applied between the layers. Different solutions are available on the market, most of which are waterbased and include a high content of renewable materials [6, 7, 8]. Applications Application examples include potato chip bags made from metallised PLA and cellulose-based films, which are both fully biobased and compostable. An example from Germany is the packaging of dried food products, such as dried fruit or vegetables or nuts, where a combination of biobased PE and cellulose based films is used. This product is 4-star biobased (> 85 %) but not biodegradable/compostable. Coffee bags require a good barrier against oxygen and water vapour and a good permanent sealing layer. A typical structure for coffee bags is a combination of a PLA/PBAT blend for the sealing layer, with the barrier properties provided by a cellulose-based layer plus a metallised cellulose layer. “In many cases, coffee bags also have an injection moulded valve made of compostable PLA/PBAT,” Patrick adds. Conclusions Even if the topic of multilayer film packaging is complex, it shows that the different materials available on the market can be flexibly combined to fulfil almost every need in terms of mechanical and thermal stability, barrier, aesthetic appearance and sustainable end-of-life options. In other words, there is no need to use conventional plastic multilayer structures or, as is increasingly being put forward as a solution, to turn to the development of mono material solutions that in all likelihood would not offer all the required properties anyway. References: [1] http://www.karl-verpackungen.de/en/know-how-eng/laminate [2] https://www.toma-gmbh.de/verbundfolie/ [3] http://www.celplast.com/product-type/high-barrier-clear-packagingfilms/ [4] Thielen, M.: Multilayer transparent Barriere films, bioplastics MAGAZINE, vol 13, issue 04/2018 [5] Thielen, M.: PVOH for barrier applications, bioplastics MAGAZINE, vol 14, issue 05/2019 [4] http://biotak.com/products/ [5] https://packaging360.in/insights/an-insight-on----speciality-adhesives- in-plastic-packaging- [6] https://www.dispersions-pigments.basf.com/portal/basf/ien/dt.jsp?setC ursor=1_556487_615966 (Photos: Bio4Pack) bioplastics MAGAZINE [06/19] Vol. 14 35
