Early bubble coalescence in thermoplastic foaming

In the underlying literature, bubble coalescence has been so far observed at the latest foaming stages, where bubble growing causes large extensional loads on the polymer film between two neighboring bubbles and eventually leads to the film rupture and to the merging of the two bubbles in a single one. This phenomenon is responsible for the coarsening of the foam morphology and, in turn, the worsening of the final foam properties. Here, the unanticipated, indirect, observation of a bubble coalescence phenomenon occurring at the very early stage of foaming, when most of the blowing agent is still solubilized in the polymer, is reported. Likewise, this phenomenon is responsible for a significant coarsening of the foam morphology. With the aim of investigating this phenomenon, a novel pressure vessel for foaming was utilized, capable of imposing two different cooling histories on two polymer samples, foamed at the same time. Results are presented on polystyrene foamed with carbon dioxide.