title: Structure-property relationships of polystyrene-based bead foams
Creators: name: Raps, Daniel
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Foam
subject: bead foam
subject: expanded polystyrene
subject: foam morphology
subject: mechanical properties
subject: fracture mechanics
subject: structure-property relationships
subject: thermal conductivity
subject: Schaum
subject: Partikelschaum
subject: expandiertes Polystyrol
subject: Schaummorphologie
subject: mechanische Eigenschaften
subject: Bruchmechanik
subject: Struktur-Eigenschaftsbeziehungen
subject: Wärmeleitfähigkeit
subject: 620

description: Expandable polystyrene (EPS, coll. "Styropor", "Unicell", “Styrofoam”) is one of the most used foams. It accounts for one quarter of the current foam production in terms of volume. EPS has a multi-scale morphology since EPS parts consist of many fused foamed beads. The foamed beads themselves contain cells, which are formed of walls and struts as well as the cell gas. In this frame, a variation in bead size causes a significant change of surface area, which needs to be moulded, whereas the variation in cell size leads to an altered micro-morphology (cell walls and struts). However, the effect of the multi-scale morphology on the properties has never been studied in a comprehensive manner to the knowledge of the author. Hence, the aim of the work is the establishment of structure-property relationships as well as the clarification of their fundamental mechanisms for the purpose to be able to tailor the structure for a specific application. Thus further material and cost savings could be possible. EPS and a bead foam made from a blend of polystyrene and polyethylene were studied regarding the effects of cell size, bead size and density on the mechanical and thermal behaviour. Furthermore, the distribution of polymer within a single bead was controlled by the introduction of a solid skin around the beads. As expected, density is the most important parameter for the foam’s properties. However, a smaller cell size at constant density in EPS leads to more orientation and annealing, which improves the modulus, strength and toughness. Furthermore, the size of polystyrene domains determines the fracture toughness as smaller domains (from smaller cells or a blend) enable more pronounced plastic deformation. Also, the thermal conductivity is improved by smaller cells as the radiative energy transfer is decreased (if the cell walls and struts are not too thin). The bead size at constant density changes the mechanical properties indirectly by affecting the moulding quality and macro-porosity. In terms of beads with a compact thick skin, a method to obtain beads with a 25 times thicker skin compared to “standard EPS” was successfully established to facilitate an internal sandwich effect. Unfortunately, a slight deterioration of the mechanical properties was observed with thicker skins. These insights provide valuable guidelines to enhance the properties of foams. For example, brittle polymers can be inherently toughened, if the domain size is decreased sufficiently, e.g. by smaller cells or by blending with other polymers. The bead size should be in a suitable range (EPS: 3 – 4 mm) as both too large and too small beads result in deteriorated mechanics. Although, the approach to facilitate an internal sandwich effect was not successful, the study shows that a local plasticisation of the beads´ surfaces with a solvent can enhance the moulding quality and thus the mechanics of the product.