Effects of Heat Exposure and Volumetric Compression on Poisson's Ratios, Young's Moduli, and Polymeric Composition During Thermo‐Mechanical Conversion of Auxetic Open Cell Polyurethane Foam

Abstract

The effects of thermo-mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open-cell polyurethane foam are related to changes in chemical bonding and cell structure. Applied volumetric compression, conversion temperature, and duration are reported on foam Young's modulus, Poisson's ratio, and structural stability. Fourier transform infrared spectral analysis on samples converted at and above 160 C strongly indicates a hydrogen bond interaction increase in urea groups (C¼O---H-N) and an increase in hydrogen bonding population. Spectral changes inferred soft segment degradation following extensive heat exposure (200 C, 180 min), specifically a shift and intensity change in CH 2 and C-O-C polyol bands and a broad baseline increase between 3600 and 2400 cm À1 . These changes are linked to (i) resistance to dimensional recovery over time and during re-heating; (ii) Poisson's ratio becoming negative, then zero in tension or marginally positive in compression; (iii) Young's Modulus reducing then increasing; (iv) mass loss, evidenced by thermogravimetric analysis increasing from 150 C. The minimum mean values of Poisson's ratio of %À0.2 (to 10% compression/tension) are comparable to other studies. All samples that retain their imposed compression over time are isotropic, with near constant Young's moduli and Poisson's ratio (to 10% compression/tension).

Experimental and computation assessment of thermomechanical effects during auxetic foam fabrication

“…While a distinct breakdown of the final NPR for each VCR in was not provided, the authors come to the same statistical conclusion that VCR is the most important factor between all the manufacturing parameters, even though the fabrication parameters between their study and this one wildly vary. The same behaviour can be further inferred from 13 , 59 , 61 where further variations in fabrication parameters (including specimen geometry, PPI, heating temperature and time) find that as VCR reduces so does the NPR.…”

Auxetics and Other Systems of Anomalous Characteristics

“…crystal of hard spheres with an array of [001]-nanochannels filled by hard spheres of another diameter' by J. Narojczyk et al [20] The simulations show that symmetry of such crystals changes from the cubic to tetragonal one. When spheres inside the inclusions are smaller than spheres forming the crystalline matrix, the changes of Poisson's ratio are qualitatively similar to the changes observed earlier in the Yukawa sphere crystal, that is, the introduction of nanochannels causes simultaneous decrease of the Poisson's ratio for [110] [1][2][3][4][5][6][7][8][9][10]-directions, and its increase for [110][001]-directions. Filling the nanochannels with spheres having diameters greater than that of the spheres in the crystalline matrix, causes a strong decrease of the minimum Poisson's ratio value.…”

“…Duncan and co-workers [3] in 'Effects of heat exposure and volumetric compression on Poisson's ratios, Young's moduli and polymeric composition during thermo-mechanical conversion of auxetic open cell polyurethane foam' report a detailed study into the effect of processing parameters on the mechanical properties, dimensional stability and bonding in polyurethane foams converted using the thermo-mechanical process originally reported by Lakes. Compressive and tensile Poisson's ratios and Young's moduli are presented as a function of conversion time and temperature and imposed compression level.…”

Validation of a Finite Element Modeling Process for Auxetic Structures under Impact

“…Full-field strain measurements were obtained using DIC, [83] as in previous research with sports equipment and auxetic foam, among other applications. [33,35,36,84,85] Speckle patterns were applied to the central region (37.5 Â 6.0 mm) of the face of the tensile samples prior to testing using matt black acrylic spray paint. A camera (Phantom Miro R110, Vision Research UK Ltd., Bedford, UK; resolution, 1280 Â 800 pixels; sample rate, 24 fps; lens, Nikon AF Nikkor 24-85 mm, f/2.8-4D IF) was used to film the pattern as the samples were stretched to a strain of 0.5 at a strain rate of 0.006 s À1 (Hounsfield HK10S Tensometer; load cell, 100 N; sample rate, 22.5 kHz).…”

“…Using compressed CO 2 in the conversion process has been identified as a potential method to increase sample sizes. [34] Duncan et al recently explored the effect of heat exposure and volumetric compression during fabrication on the properties of open-cell auxetic foam, [35,36] which could help facilitate investigations into the effect of Poisson's ratio on indentation resistance and impact performance. Closed-cell auxetic foams have been fabricated, [22] with a simpler fabrication process using steam penetration recently developed by Fan et al, [37] although its potential for sporting applications has not been investigated.…”