Understanding the foamability of complex polymeric systems by using extensional

Abstract

Obtaining high density polyethylene (HDPE) based microcellular foams is a topic of interest due to the synergistic properties that can be obtained by the fact of achieving a microcellular structure using a polymer with a high number of interesting properties. However, due to the high crystallinity of this polymer the production of low density microcellular foams, by a physical foaming process, is not a simple task. In this work, the solution proposed to manufacture these materials consists on using crosslinked HDPEs, with different crosslinking degrees. In addition, the foaming conditions, i.e. the foaming time and the foaming temperature, have also been modified with the aim of analyzing and understanding the mechanisms taking place during the foaming process to control this process and therefore, to obtain cellular materials with low density and improved cellular structures. Results indicate that there is a time and a foaming temperature optima for which the nucleation rate and the cell growth rate prevails over other phenomena like cell coalescence and gas diffusion out the material. Therefore, with these foaming conditions the lowest densities and the best cellular structures are obtained. Moreover, when the gel content increases the amount of gas available for foaming increases and the rheological properties improve, as the polymers show strain hardening. As a consequence, the relative density decreases and the cell nucleation density increases. After this detailed study, it was concluded that cellular materials with relative densities of 0.37 and with cell sizes of approximately 2 microns can be produced using a saturation pressure of 5 MPa, a foaming time of 30 s, a foaming temperature of 160 ºC and a crosslinked HDPE with a gel content of 50 %.

Analysis of the composition-structure-properties relationship of open-cell polyolefin-based foams with tailored levels of gas-phase tortuosity

“…HDPE based cellular polymers with low cell sizes (around 50 microns) were obtained. All these results were supported by measuring the strain hardening (extensional rheological measurements) of the polymer irradiated with different doses [51][52][53].…”

“…It is important to mention here, that a lethal gas such as hydrogen cyanide is formed during the decomposition of polyurethane at high temperatures. The dangers and hazardous effects of hydrogen cyanide are well-known, being this fact a critical issue for some applications of PU foams [51].…”

Development of new lightweight green composites reinforced with nonwoven structures of flax fibres

“…On the one hand, in the classical homogeneous nucleation theory, nuclei formation in the liquid/viscous bulk is produced due to the aforementioned thermodynamic instability, and depends on the concentration of gas molecules, the activation energy to form the nuclei, the surface free energy in the polymer/bubble interface and, of course, on the temperature and the pressure drop. On the other hand, the heterogeneous nucleation requires another solid phase-such as impurities, fillers or other particles-since the lower surface energy of the interface between the polymer and these solid particles promotes the creation of the nuclei [95]. Therefore, the particles act as nucleation sites and can help to control the final cellular structure.…”

“…Due to that, the cell growth rate depends on the bubble radius, both the internal and external pressures affecting the bubble, the surface tension and the matrix viscosity, being the latter a key parameter in the cell growth process-the lower the viscosity, the faster the cell growth-. However, a variation of the viscosity is desirable, lower at the beginning for an easy promotion of the cell nucleation, and increasing meanwhile the foaming is produced in order to withstand the deformation, thus avoiding the excessive degeneration of the cell structure and cell ruptures [95].…”

“…This evolution requires to be stabilized by means of cooling of cross-linking; otherwise, the cellular structure can evolve up to its degeneration. The cell degeneration is mainly explained by three mechanisms, which are briefly described [95]:…”