Rigid Packaging Flexible
Rigid Packaging Flexible Bio-foams Article contributed by Anneliese Kesselring, Fraunhofer UMSICHT, Oberhausen Christian Bonten, FKuR Kunststoff GmbH, Willich Foamed plastics have the advantage of a low density with comparably good material properties. There are different kinds of foaming procedures, (a) foaming of multi-component systems where the polymer (e. g. PUR) is generated only by chemical reaction and (b) physical foaming of thermoplastics in the melt (e. g. PE, PP, PS). Regardless of the different raw materials along with the different processing procedures involved, foamed materials are characterised by common features. Because of the entrapped gas mixture foamed plastics have a low density and a low thermal conductivity (high heat insulation). However, they still feature good mechanical properties per unit of weight and offer a substantial saving of material. The design variety is nearly as wide as that of injection moulded parts, but often with less tooling costs. The basic requirement for foaming of plastics is a free flow melt condition of the plastic before initiating the foaming process in order to enable the forming of bubbles. When the desired dimension of the bubbles has been reached this condition has to be fixed. For the foaming of thermoplastics this means that the thermoplastic itself has to possess a low melt viscosity at foaming start, which has to be increased rapidly by cooling for fixing the bubbles. For the formation of bubbles it is possible to make use of different blowing mechanisms. Either by using a chemical blowing agent which is introduced as an expanding by-product, by way of a decomposition product of a chemical reaction within the melt, or by physical blowing agents, e. g. through expansive vaporization of a low-boiling fluid or by means of mechanical mixing of air. The formation of bubbles in the melt implies that during the formation of expanding gases there are areas where a minimum quantity of molecules of these expanding gases is formed. By means of so-called ‘nuclei’ it is possible to create additional boundary layers where such minimum quantities of molecules can accumulate. Foaming biopolymers In order to foam biopolymers different requirements have to be met: on the one hand melt viscosities have to be distributed homogeneously and adjusted to be sufficiently low for the formation of bubbles; on the other hand, the viscosity has to be rapidly increased in order to fix the bubbles. Moreover, the chosen Figure 2: Small textured finishes Figure 1: Thermoformed foamed film with hinge 1 bioplastics MAGAZINE [03/09] Vol. 4
Rigid Packaging Figure 3: EPS foam structure (x100, reflected light); Photo: Fraunhofer UMSICHT Figure 4: Foam structure Bio-Flex® A 4100 CL (x100, reflected light); Photo: Fraunhofer UMSICHT blowing agent must not influence the bio character of this class of material. Not only chemical blowing agents, but also low-boiling fluids have to be chosen accordingly, since residues may remain in the polymer. To achieve a homogeneous distribution of the bubbles, besides a homogenous distribution of the blowing agent in the polymer, a homogeneous material performance is necessary. With varying molecular weights this is nearly impossible! Bio-Flex ® A 4100 CL is a bioplastic based on PLA that has been developed by FKuR together with Fraunhofer UMSICHT. It is composed of nearly 90 % renewable resources and is certified as biodegradable. Whereas with pure PLA only brittle foamed structures can be produced, Bio-Flex A 4100 CL allows the production of flexible foamed structures - even with moulded-in hinges (Fig. 1). The easy flow in its soft condition creates a small, even textured finish to avoid food sticking on the surface (Fig. 2). It is important to observe - by means of suitable temperature control - that the blowing agent does not discharge too early from the screw through the feed section. The wide process temperature range of Bio-Flex A 4100 CL allows the temperature to be controlled exactly according to the requirements of the foam. Bio-Flex with a very uniform distribution of the molecular mass enables a uniform foaming. With Bio- Flex A 4100 CL it is easy to produce foamed structures in series which are comparable in many properties and applications to, for instance, EPS (figures 3 and 4). Plastics, made by nature, for rigid packaging FKuR´s trade name Bio-Flex ® stands for copolyester blends based on PLA, which – depending on the respective grade – are composed of up to 100 % natural resources. Bio-Flex does not contain any starch or starch derivatives. These Bioplastics mostly replace conventional LDPE and HDPE as well as polystyrene (PS) and polypropylene (PP). www.fkur.com www.umsicht.fraunhofer.de advert bioplastics MAGAZINE [03/09] Vol. 4 1
