Foam Foamed blocks from
Foam Foamed blocks from NF-reinforced starch Biodegradable foamed blocks based on starch reinforced with natural fillers and produced using microwave radiation. Making materials lighter is one of the main challenges of society today because it involves cutting down on the use of raw materials, reducing fuel consumption in vehicles and increasing the thermal insulation performance of buildings, to name but a few examples. This is why foamed plastics, which can weigh even 40 – 45 times less than their solid counterparts, are slowly gaining more importance in our daily lives. They are produced from fossil-based polymers such as polystyrene, polyurethane, polyvinylchloride and polyolefins. However, another important challenge of our society and at the same time one strategic policy of the European Union [1] is to make our economy less dependent on fossilbased materials. The use of bioderived and biodegradable polymers for foaming applications could contribute to fulfilling this objective. Their global production, which was around 1.6 million tonnes in 2013, is expected to increase up to 6.7 million tonnes in 2018 [2]. Nevertheless, their use in the foam industry has been restricted so far due to their poor foaming performances and to their high cost. Although there are products in the market based on PLA foams such as food-packaging trays they have not reached a significant market share. This fact provides a unique opportunity for a natural polymer such as starch. As this polymer is easily extracted from plants such as cereals and tubers, its cost is lower than that of PLA and PHAs. Starch can act as a filler when used in its original form (semicrystalline granules) or can be transformed into a thermoplastic material, which is known as thermoplastic starch (TPS). TPS features properties similar to those of conventional fossil-based polymers used for foaming applications such as polystyrene and it is foamed by similar processes such as extrusion foaming [3]. In fact, the first starch-based foams produced by extrusion were snack foods (fig. 1) because foaming varies their texture and makes them more appetizing and crisp [4]. For quite some years starch foamed products have also been used as packages. Starch-based loose fill chips and boards, which are produced by extrusion foaming are widely employed for protective packaging applications (fig. 1). Moreover, food plates based on starch foams have been produced by baking a process similar to that employed for the production of waffles. More recently, a lab-scale foaming process based on microwave radiation has been developed by Cellmat Technologies (Valladolid, Spain). This company is a spinoff founded by the initiative of researchers from CellMat Laboratory (University of Valladolid). The process allows starch-based foamed blocks reinforced with natural fillers to be produced with lower energy consumption and lower cycle times than in the aforementioned processes. This process basically consists of three steps (fig. 2): firstly, starch is plasticized by water in a twin-screw extruder. Secondly, the pellets obtained are thermoformed in a hot-plates press so as to produce a solid sheet. This sheet is placed into a PTFE mould where the foaming process occurs. PTFE was chosen as the material for the mould because it avoids the starch sticking, it is heat-resistant and last but not least, it is transparent to microwave radiation. Finally, the mould with the solid TPS sheet inside is placed into a microwave oven chamber and microwave radiation at constant power is applied for a few seconds. During this short period of time, the interaction of microwaves with water molecules produces the heat necessary to soften the polymer matrix and to vaporize water, which in turn brings about the sudden expansion of the polymer. It is obvious that water is the most important element in this process because it acts not only as the plasticizer of starch but also as the blowing agent. Figure 1: Starch-based foam applications: cereals, snacks, loose-fill chips and boards. 32 bioplastics MAGAZINE [04/15] Vol. 10
Foam Starch undergoes several physical transitions in this process as shown in the SEM micrographs of figure 2. At the beginning, starch is a semi-cristalline granule (1), which cannot be processed by conventional plastic equipment because its degradation occurs before melting. For this reason, starch requires a transformation process (plasticization) in which the granule is disrupted by the action of a plasticizer (water in this case) and the high mechanical energy applied along the extruder barrel. An amorphous thermoplastic (2) with a glass transition temperature even lower than ambient temperature (depending on the amount of plasticizer used) is obtained. This flexible material turns into a dry foamed product due to water loss caused during the time in which microwaves are applied. Water goes from the polymer within the cell walls and struts to the interior of the cells expanding the material and in the end, stabilizing the cellular structure (3). By: Alberto Lopez Gil CellMat Technologies Centro de Tecnologías y Transferencia Aplicadas (CTTA) Valladolid, Spain These same physical transitions are undergone by starch when other foaming processes such as baking and extrusion foaming are used. However, the way in which heat transfer evolves is different. When microwaves are used, heat comes from multiple spots distributed along the solid precursor, which corresponds to the water molecules, and it is later transferred to the whole volume of the material by conduction (fig. 3). Therefore, heat is originated in the interior of the material and not in the exterior, which is the case in extrusion foaming and baking. Figure 2: Microwave foaming process developed by CellMat Technologies. 1 2 200 µm 40 µm Natural fillers Water Starch granules TPS pellets 40 µm 3 Plasticization of starch (TPS) and dispersion of natural fillers in a twin-screw extruder Thermoforming in a hot-plate press 2000 µm Expansion by microwaves inside a PTFE mold Foamed starch block reinforced with natural fillers TPS solid precursor bioplastics MAGAZINE [04/15] Vol. 10 33
