Basics Thermoforming of bioplastics Before the first bioplastics entered the market in significant quantities and qualities a few years ago, the industry no longer relied on the development of new plastic types over decades, but only on the production of polymer blends with the objective of maintaining the respective advantages and of eliminating the disadvantages. The ‘sustainability’ by reducing the thickness of mouldings was at the expense of the reduced recyclability of such multi-layer composite materials. Now, with the availability of new bioplastics, it is the task of the producers of extruded films and sheets, as well as of thermoformers, to ensure the processing methods, and this is where some, partly significant, differences arise compared to conventional plastics. Up until now bioplastics have been used as a replacement for conventional packaging, which means that they have to meet the requirements of the packaging industry. This is achieved by multi-layer structures, the use of special additives, but also by admixing non-bio materials. The knowledge acquired over the years when blending plastics helps in obtaining good and fast results. Today’s modern thermoforming machines offer a maximum possibility to meet the widely varying requirements of bioplastics. During the processing of bioplastics it is essential to deal with issues which did not arise during the processing of conventional thermoforming materials. In terms of quantity, the most commonly used bioplastic is PLA. Already in 1995, ILLIG GmbH & Co. KG (Heilbronn, Germany) conducted the first thermoforming tests with PLA films. In 1996, the first household containers with lids formed on an Illig-RDKP 72d were presented at Interpack in Düsseldorf, Germany. In both cases, the films came from OFoTec Folien GmbH (Nehren, Germany), the raw material from the first plant was provided by Cargill, USA. Processing in a thermoforming machine does not cause any problems for current raw material qualities. The producers have done their homework and the initial difficulties have been eliminated. Large dairies like Danone rely on PLA for their bio-packaging plastics. Since these products are mostly sold via the cold chain, the low heat deflection temperature of standard PLA imposes no limitations. Through the use of a silicone layer on the film, the low shrinkage of PLA film can be compensated for by better demouldability, as well as stack- and de-stackability. In addition, a reduction of the brittleness is possible with appropriate additives (see other articles in this issue). Thus, impact modifiers have already been added to Ofotec’s first films. It is generally possible to use the same tools that are used for commodities, also without preheating. Depending on the type of the moulded part, the mould temperatures can be lower than in case of polystyrene (PS). The poor thermal properties of standard PLA limit the range of possible applications. However, at present, there are small test quantities being developed with the objective of producing stereocomplex versions created with defined parts of L- and D-lactic acid. The developments with regard to specialties do not yet allow a large-scale operation in the packaging arena. Illig will be there when such versions are considered for testing. There are different scenarios for the waste issue. In principle, production waste such as punch scrap and rejects can be ground up and fed back to the production process (film extrusion). However, Ofotec has also processed punch scrap into foam sheets, even though there is still a significant lack of applications. The highest rates of growth are attributed to bioplastic PHA (PHB). The properties are ideal for their use as packaging plastics. The high temperature stability, the FDA approval, detailed reproduction accuracy/forming sharpness all make the plastics an excellent choice for higher temperature-stable packaging. A special feature is the minimum demoulding temperature which has to be adhered to. While the well-known thermoplastics follow the rule that “the colder the conditions for demoulding of the moulded part, the more dimensionally 56 bioplastics MAGAZINE [02/12] Vol. 7
Basics by Martin Barth Illig GmbH & Co. KG Heilbronn, Germany Ekkehard Adam OFoTec-Folien GmbH Nehren, Germany stable it will be”, these plastics have to be removed from the tool with a minimum residual heat. The new plastics used require a high crystallinity which ensures sufficient stability when using it. If the crystallization stage is passed through too quickly during the cooling process in the thermoforming machine, too few crystals arise and the plastic becomes very soft. If the moulded part is demoulded at a temperature above 60° C, crystals can begin to form over a longer period which leads to the desired stability. During processing the film tends to stick. This does not cause a problem with regard to the processing on an automatic roll-fed thermoformer where the film, held at the side by pins, is fed through the plant in chains. However, the processing on form-fill-seal lines is prevented. This machinery uses contact heating and this would directly stick to the plastics. The final stability of the thermoformed article is only reached after a few days. This means that the products require “Do not process before …” information. Many types of bioplastic, mainly the starch-based ones, can only be used if they include a defined water content. Due to the moisture, the thermoformed articles become ready to use and do not break on being subjected to the smallest load. But precisely this moisture level makes the processing with the well-known parameters of conventional plastics impossible. The water immediately begins to evaporate, forming blisters on the surface. In such cases, the thermoforming machine must be equipped with a special heating control. A very interesting plastic material for technical applications is manufactured by Tecnaro. Their plastics, based on lignin, offer ideal properties for thermoforming with highest precision on sheet processing machines. Based on the company’s environmentally aware thinking, Illig is not limited solely to a possible reduction of film thicknesses for material and thus resource saving and to the applicability of PLA films. Together with Ofotec, Illig devotes itself to the use of green HDPE made from sugar cane based bioethanol. This material can be recycled according to the established and proven procedures. Illig also takes care of these new materials. It should be reviewed to what extent possible changes of the settings in the automatic thermoforming machines are necessary and/or to what extent cycle times or stacking methods have to differ. Here, too, Ofotec was once again the partner and presented appropriate films. All-purpose tools have been applied as well as tray tools which are normally used to process PP films. It became apparent that the direct change from one film type to another is possible. This applies to unreinforced films. However, in the meantime, Ofotec is also working on highly filled versions. Here, the content of the plastics used – even though already renewable – is to be stretched with mineral filling for a further increase of the sustainability level. In this case the film can be heated more easily because the mineral filler has the task of transporting the heat inside the film and thus to distribute it more evenly. However, during the punching process, caution is advised and an optimization may be necessary. Promising tests are under way. There will be no universal solution when it comes to processing, but rather a lot of possibilities and challenges. Further development of thermoforming machines towards designing all-purpose machines for all thermoplastically processable plastics will ensure that it is possible to respond to any special aspects that may arise. Sophisticated machine technology, used here in some cases, is not only limited to the thermoforming machine and its tools regarding longitudinal and transverse stretching as well as temperature control of the transport chains, but it also covers additional components like regular and continuous unwinding from the roll, gentle stacking, a skeletal granulator adapted for smooth materials and a precisely adjustable punching technique. The central role is assigned to the heating. Unlike conventional thermoforming materials, an extremely accurate temperature control is necessary. In many cases, there are only a few degrees between ‘still too cold for processing’ and ‘decomposition is beginning’. www.illig.de www.ofotec.de bioplastics MAGAZINE [02/12] Vol. 7 57
