Developments on auxetic closed cell foam pressure vessel fabrications

Abstract

Auxetic foam can have higher indentation resistance, better protection under impact and higher vibration damping than conventional foam. Unlike auxetic open cell foam, with established, commercially viable options for manufacturing, methods for making auxetic closed cell foam are not established. We revisited pressure-vessel methods, proposed in 1996, for making auxetic closed cell foam. We processed low-density polyethylene foam for six hours at 400 to 700 kPa and 100 °C, causing foams to shrink by a factor of two to five. The volumetric compression kinked cell walls, producing negative Poisson’s ratios as low as -0.2 and Young’s moduli from 0.2 to 1.2 MPa. Trends between applied volumetric compression and Poisson’s ratio agree with those for open cell foam – initially decreasing to negative values as volume reduced by a factor of two after processing, then plateauing or slightly increasing as volume decreased by a factor of two to five. Foams of different sizes and shapes (15 to 75 mm sides) processed in the same conditions (700 kPa, 6 hours, 100 °C) shrank evenly in all three axes and had similar final volume ratios. We noticed a long settling period, of up to three months, where foams slowly shrank. Placing foam in a vacuum after processing reduced the settling period to within 24 hours.

Conversion of polystyrene foams into auxetic metamaterials

“…[ 32 ] But this trend has been reported several times in the literature for NPR foams. [ 7,8,21,22,30–32 ] Figure 6 also shows that NPR can be observed up to 65% of deformation. Unfortunately, no data about PS auxetic foams have been reported in the literature to compare.…”

“…Duncan et al reported on the preparation of auxetic foams via steam (water vapor) treatment, [ 21 ] and a pressure‐vessel method (vacuum). [ 22 ] The original foam was sheets of closed‐cell LDPE foam with a density of 60 kg/m 3 . Poisson's ratios as low as −0.3 in tension and −1.1 in compression were obtained, while only −0.2 was obtained for the vacuum method.…”

“…The main idea to produce auxetic foams from conventional ones is to convert the standard foam structure (honeycomb/polyhedral-shaped cells) into a re-entrant cellular structure. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] To this end, different treatments were proposed including mechanical, thermal, physical, and chemical methods, which can be performed alone or in combination. These processes involved volumetric compression, heating beyond the polymer's softening temperature and cooling while remaining under compression.…”

Variable Dual Auxeticity of the Hierarchical Mechanical Metamaterial Composed of Re‐Entrant Structural Motifs

“…Mechanical metamaterials [1,2,3,4,5,6,7,8,9,10,11,12,13] are rationally-designed structures capable of exhibiting a plethora of atypical mechanical properties that are rarely observed in the case of naturally-occurring materials. Some of the most commonly studied of such properties are negative Poisson's ratio [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] (auxetic behaviour), negative stiffness [30,31,32,33,34] and negative compressibility [35,36]. Over the last thirty years, it has been demonstrated that devices utilising materials exhibiting such characteristics can be used in the case of a variety of applications including protective devices [37,38], sports equipment [39] as well as biomedical [40,41] and vibration damping devices [42,43,44].…”

Investigating the Effect of Relative Humidity on the Mechanics and Dynamics of Open‐Cell Polyurethane Auxetic Foams

“…[28] Auxetic foams are mainly found in form of opencell configurations, although techniques to transform closed-cell foams by steam and pressure do also exist. [29][30][31] The most established manufacturing process to produce auxetic open-cell foams consists of a volumetric compression, annealing via heating, cooling, and relaxation. [32][33][34][35] Volumetric compression creates the typical re-entrant cell structures of auxetic materials, while heating and cooling provide the thermoforming of the compressed foam, so that the shape of the re-entrant cell structures stabilizes after the foam is released from compression.…”