Analysis of density-dependent bead and cell structure of expanded polypropylene bead foams from X-ray computed tomography of different resolution

Abstract

Closed-cell bead foams with their hierarchical geometrical structure are a challenge for statistical reconstruction and finite element modelling. For the purpose of providing the fundamental micro- and meso-structural descriptors - wall thickness, cell as well as bead volume and sphericity - of expanded polypropylene bead foams of different density, 3D-images from X-ray computed tomography are analyzed. A detailed description of development and application of an image analysis methodology for the determination of feature distributions from CT-scans of different level of detail is provided. The methods are based on off-the-shelf algorithms provided by the open-source package distribution FIJI. It should be highlighted, that beside essential methods such as thresholding, euclidean distance and watershed transformation here the Trainable WEKA segmentation is applied for separating material phases in the images. Although the methods elaborated are generally very case sensitive, the reader benefits from the validation strategies applied, so that development of individual methods into the direction of reliability, repeatability and automation is supported.

On the determination of viscoelastic properties of EPP foam in dependence on pre-strain and loading direction

“…Cell wall bending and collapsing are considered to be the main drivers for the onset of nonlinear behavior. As these appear much earlier under compression than under tension and shear [8][9][10][11] different LVR limts are observed.…”

“…As the morphological properties like cell wall thickness and cell size as well as the described deformation mechanisms are randomly distributed, 4,9 the transitions between the three macroscopically observed stress-strain regions occur gradually. In addition, numerical simulations on closed-cell bead foams showed that the equivalent plastic strain on microscopic scale exceeds the macroscopically applied compressive strain by a factor of 10 easily.…”

Expanded beads of polyethylene moldable at low steam pressure

“…8 Notably, foam extrusion facilitates cost-effective production of smaller bead batches, amalgamating solubilization, compounding, and expansion within a single-step process. 9,10 This approach also offers a narrower distribution of foamed bead sizes through controlled extrudate swell, 11–14 coupled with facile density modification by adjusting blowing agent concentrations, 15 setting it apart from autoclave technology. 16 However, attaining precise process control and establishing a broad molding window remain formidable challenges, 17–20 resulting in the limited prevalence of extrusion-produced ePE in the market.…”