Overcoming the Challenge of Producing Large and Flat Nanocellular Polymers: A Study with PMMA

Abstract

Although nanocellular polymers are interesting materials with improved properties in comparison with conventional or microcellular polymers, the production of large and flat parts of those materials is still challenging. Herein, gas dissolution foaming process is used to produce large and flat nanocellular polymethylmethacrylate samples. In order to do that, the foaming step is performed in a hot press. The methodology is optimized to produce flat samples with dimensions of 100 Â 100 Â 6 mm 3 , relative densities in the range 0.25-0.55 and cell sizes around 250 nm. Additionally, foaming parameters are modified to study their influence on the final cellular structure, and the materials produced in this paper are compared with samples produced by using a most conventional approach in which foaming step is conducted in a thermal bath. Results obtained show that an increment in the foaming temperature leads to a reduction in relative density and an increase of cell nucleation density. Moreover, differences in the final cellular structure for materials produced by both foaming routes are studied, proving that although there exist some differences, the mechanisms governing the nucleation and growing are the same in both processes, leading to the production of homogeneous materials with very similar cellular structures.J. Martín-de Le on, V. Bernardo, P. Cimavilla-Román, Dr. S. P erez-Tamarit, Prof. M. A. Rodríguez-P erez Cellular Materials Laboratory (CellMat) Universidad de Valladolid

Confined batch foaming of semi‐crystalline rubbery elastomers with carbon dioxide using a mold

“…Cylindrical molds were also used by Bao et al 16 to promote oriented pore F I G U R E 2 Schematic diagram of foaming system and components. The figure is adapted from Martin-de-Leon et al 28 [Color figure can be viewed at wileyonlinelibrary.com] formation in foams of polystyrene to improve mechanical properties.…”

“…In another study with PMMA films, solid‐state foaming with carbon dioxide was conducted to generate nanocellular samples using impermeable stainless‐steel plates inside a hot press 28 using a mold assembly as shown in Figure 2. Here, the PMMA samples were first saturated with carbon dioxide at 25°C and 310 bar for 24 hr.…”

“…Schematic diagram of foaming system and components. The figure is adapted from Martin‐de‐Leon et al 28 [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Numerous studies have already been presented in the literature where unique molds in batch foaming operations were used for a variety of applications such as controlling the pore isotropy, 16 restricting the diffusion of carbon dioxide in thin films, 17,18 controlling final foam geometries, [18][19][20][21][22][23][24][25] promoting the sintering of bead foams, 23 facilitating the removal of the sample, or permitting optical assessments of the foaming processes. 7,26,27 These molds can broadly be categorized as those with impermeable 7,[16][17][18]21,28 and permeable boundaries. 20,23,29,30 Despite their widespread use, the basic descriptions of mold geometries and their impact on foaming outcomes are often not provided or fully described.…”

The influence of cell size on the mechanical properties of nanocellular PMMA

“…After saturation, the pressure was rapidly released to atmospheric pressure with an instantaneous pressure drop rate close to 100 MPa s −1 for the samples saturated at T sat = 25°C and p sat = 31 MPa, whereas a pressure drop rate of 10 MPa s −1 to 70 MPa s −1 for all other saturation conditions. The samples were then foamed in a hot press (Remtex, Barcelona) at a foaming temperature equal to 60°C for a foaming time close to 60 s, resulting in flat foamed samples suitable for mechanical testing [17].…”