Approach to quantify the resistance of polymeric foams against thermal load under compression

Abstract

Nowadays, numerous techniques are used to quantify the resistance of cellular polymers against a thermal load. These techniques differ in significance and reproducibility and are all dependent on foam density, structure (i.e., cell size and -distribution) and sample geometry. Very different behaviors are expected for extrusion- and bead foams, as well as for amorphous and semi-crystalline polymers. Moreover, established tests use temperature ramps which would lead to temperature gradients within the sample and thus to faulty results. In this study, we developed a new approach from an engineering perspective to minimize these influences. In this approach, the resistance against the thermal load is derived from a steady creep test with defined temperature steps under a mechanical load, which is specifically set for each foam sample depending on its static compression behavior at room temperature. The two-stage test therefore combines (i) a standard quasi-static compression test at room temperature and (ii) a creep test with stepwise increased thermal loading. For each foam type, a rather low mechanical load (stress) is determined from the quasi-static compression test at room temperature; low enough to remain below the collapse strength and avoid irreversible deformation (i.e., buckling and/or breaking of the cell walls). This load is then applied in a creep test where the temperature is increased in defined steps from room temperature to a temperature close to T g or T m . The stepwise increase and holding of the temperature for a defined time enables a homogeneous temperature in the test specimen. The approach was applied to (i) polystyrene extrusion and bead foams (i.e., XPS and EPS), which have different foam structure, (ii) amorphous and semi-crystalline bead foams of polystyrene (EPS) and polypropylene (EPP), (iii) bead foams with different densities (30, 60, 120, and 210 kg/m3) and (iv) to a new type of bead foam made of the engineering polymer polybutylene terephthalate (E-PBT). The termination criterion for the test is defined as the temperature at which a relative compression of 10% is reached in the creep test with temperature steps. We suggest calling it the heat stability temperature T HS. For the studied foams, the procedure delivers characteristic T HS values that allow a good comparison between different polymer matrices and densities. The heat stability temperature T HS of amorphous PS foams (i.e., XPS and EPS) was determined to be 98 °C, which is close to the glass transition temperature T g . Using the same approach, values of 99–107 °C were determined for EPP and 186 °C for the semi-crystalline bead foam E-PBT.

Investigation of the Reaction Kinetics of Poly(butylene terephthalate) and Epoxide Chain Extender

“…[1][2][3] In particular, their enhanced thermal stability and mechanical resistance qualify them for use as construction materials in areas such as the automotive and electrical industries. [1,4,5] For lightweight structures, polyesters are used as foams to save weight while maintaining reliable mechanical properties. However, the foaming of these materials is challenging.…”

A unified approach for evaluating mechanical compression tests for polymer bead foams

“…The compression modulus is widely used in the literature to compare the linear elastic behavior of bead foams. 2,3,[6][7][8][9][10][11][12] However, an accurate automatic determination of E C has not been established, and therefore values for the same foam and conditions may vary depending on the laboratory where it was measured or even the person doing the manual determination of E C . For instance, for expanded polypropylene (EPP) foam with a density of 60 kg m À3 at room temperature, some reported values of E C (in MPa) include 3.34, 11 7.35, 6 and 8.34.…”

“…2,3,[6][7][8][9][10][11][12] However, an accurate automatic determination of E C has not been established, and therefore values for the same foam and conditions may vary depending on the laboratory where it was measured or even the person doing the manual determination of E C . For instance, for expanded polypropylene (EPP) foam with a density of 60 kg m À3 at room temperature, some reported values of E C (in MPa) include 3.34, 11 7.35, 6 and 8.34. 12 Among other problems, the manual determination of the maximum slope in the linear-elastic regime is prone to errors.…”

“…The compression modulus or elastic modulus () is the main parameter characterizing the linear‐elastic regime (region I in Figure 1) and is defined as the slope of the linear region in the stress–strain curve. The compression modulus is widely used in the literature to compare the linear elastic behavior of bead foams 2,3,6–12 . However, an accurate automatic determination of has not been established, and therefore values for the same foam and conditions may vary depending on the laboratory where it was measured or even the person doing the manual determination of .…”

Structure–Property–Processing Relations of Short‐Chain Branched Poly(butylene terephthalate) (PBT) with Biobased Comonomers

“…[ 3 ] However, PBT bead foams are not commercially established yet, and common established bead foams based on polystyrene (PS), or polypropylene (PP), withstand only intermediate temperatures (80 °C for PS and 110 °C for PP). [ 8 ] The successful preparation of PBT bead foams (so‐called expanded PBT, E‐PBT) would enable both, low density and strong mechanical integrity at elevated temperatures for highly advanced materials.…”