Continuous direct melt foaming of a preceramic polymer using carbon dioxide: extrusion device and first results

Excerpt

In comparison to bulk materials, foams offer advantageous properties like low density, large surface area, high permeability, and low thermal conductivity. Therefore, foams are widely used in, e.g. thermal insulation, lightweight constructions, filtering, and catalysis. Prominent features of ceramic foams are high strength and stiffness, high temperature resistance, and high thermal shock resistance [1]. However, the foam properties depend strongly on the foam morphology and porosity which are influenced by the processing technique. The most important production routes incorporating preceramic polymers are the replica method, the template method, and direct foaming. The replica method uses an open cell polymer foam which is coated by a preceramic polymer slurry and subsequently pyrolysed leading to a positive foam structure [2‐6]. The template method is based on organic phases such as polymer microbeads or expendable microspheres mixed into a preceramic polymer and decomposed during pyrolysis leading to a negative foam structure [7‐18]. In the literature, there are also reports on combinations of the replica method and direct foaming [6] and the template method and direct foaming [19‐21]. The direct foaming approach can be classified in (a) physical foaming [22‐25], (b) chemical foaming [26‐31], and (c) a combination of physical and chemical foaming by co-blowing the preceramic polymer together with polyurethane [30, 32‐37]. In chemical foaming, either a suitable chemical foaming agent has to be found or the blowing is caused by volatile condensation products during crosslinking. In the latter case, however, the amount of “blowing agent” being available is given by the molecular structure of the preceramic polymer and the gas production is directly coupled with the crosslinking process. …