Polystyrene foams. II. Structure-impact properties relationships

Microfoamed Strands by 3D Foam Printing

We report the design, production, and characterization of microfoamed strands by means of a green and sustainable technology that makes use of CO₂ to create ad-hoc innovative bubble morphologies. 3D foam-printing technology has been recently developed; thus, the foaming mechanism in the printer nozzle is not yet fully understood and controlled. We study the effects of the operating parameters of the 3D foam-printing process to control and optimize CO₂ utilization through a maximization of the foaming efficiency. The strands’ mechanical properties were measured as a function of the foam density and explained by means of an innovative model that takes into consideration the polymer’s crystallinity content. The innovative microfoamed morphologies were produced using a bio-based and compostable polymer as well as polylactic acid and were then blown with CO₂. The results of the extensive experimental campaigns show insightful maps of the bubble size, density, and crystallinity as a function of the process parameters, i.e., the CO₂ concentration and temperature. A CO₂ content of 15 wt% enables the acquirement of an incredibly low foam density of 40 kg/m³ and porosities from the macro-scale (100–900 μm) to the micro-scale (1–10 μm), depending on the temperature. The foam crystallinity content varied from 5% (using a low concentration of CO₂) to 45% (using a high concentration of CO₂). Indeed, we determined that the crystallinity content changes linearly with the CO₂ concentration. In turn, the foamed strand’s elastic modulus is strongly affected by the crystallinity content. Hence, a corrected Egli’s equation was proposed to fit the strand mechanical properties as a function of foam density.

Thermal Energy Storage and Mechanical Performance of Crude Glycerol Polyurethane Composite Foams Containing Phase Change Materials and Expandable Graphite

The aim of this study was to enhance the thermal comfort properties of crude glycerol (CG) derived polyurethane foams (PUFs) using phase change materials (PCMs) (2.5–10.0% (wt/wt)) to contribute to the reduction of the use of non-renewable resources and increase energy savings. The main challenge when adding PCM to PUFs is to combine the low conductivity of PUFs whilst taking advantage of the heat released/absorbed by PCMs to achieve efficient thermal regulation. The solution considered to overcome this limitation was to use expandable graphite (EG) (0.50–1.50% (wt/wt)). The results obtained show that the use of PCMs increased the heterogeneity of the foams cellular structure and that the incorporation of PCMs and EG increased the stiffness of the ensuing composite PUFs acting as filler-reinforcing materials. However, these fillers also caused a substantial increase of the thermal conductivity and density of the ensuing foams which limited their thermal energy storage. Therefore, numerical simulations were carried using a single layer panel and the thermal and physical properties measured to evaluate the behavior of a composite PUF panel with different compositions, and guide future formulations to attain more effective results in respect to temperature buffering and temperature peak delay.

Preparation and mechanical properties of thermosetting epoxy foams based on epoxy/ 2-ethyl-4-methylimidazol system with different curing agent contents

Epoxy/2-ethyl-4-methylimidazol system with different curing agent content was completely cured for foaming, and the effect of a systematic variation in 2-ethyl-4-methylimidazol content on the crosslinking density of cured epoxy resins was investigated. It was found that the crosslinking density of completed cured epoxy reduced as the 2-ethyl-4-methylimidazol content increased in certain range of contents (10–50 mol%). Then the precursors were foamed by a batch foaming process with supercritical carbon dioxide. The cellular morphologies of foamed epoxy resins were analyzed by scanning electron microscopy. The results revealed that the reduced crosslinking density would improve the foamability of cured epoxy resin. The microcellular epoxy foams could be obtained by maintaining a moderate crosslinking density, which can be controlled by varying 2-ethyl-4-methylimidazol content. For the completely cured epoxy with different curing agent content, when the crosslinking density of epoxy resin was 232.40 mol m–3 (the 2-ethyl-4-methylimidazol content was 35 mol%) or lower, microcellular structure was obtained by adjusting the foaming conditions. The effects of foaming on the mechanical properties were also discussed. The results indicated that microcellular epoxy foams had higher impact strength but lower tensile strength and tensile modulus, validating that the introduction of microcellular structure in epoxy matrix was conducive to the improvement of the ductility of epoxy foams.

A novel lab-scale batch foaming equipment: The mini-batch

In this paper, we report the design of a new experimental apparatus for the study of the foaming process of thermoplastic polymers with physical blowing agents. The novel lab-scale batch foaming equipment is capable of achieving accurate control of the processing variables, namely, the temperature, the saturation pressure and the pressure drop rate and, furthermore, of allowing the achievement of very high pressure drop rates, the observation of the sample while foaming and the very fast extraction of the foamed sample. By recalling the considerations discussed by Muratani et al. ( J Cell Plast 2005; 24: 15), the design converged into a simple, cheap, and very small pressure vessel, thereby denoted as mini-batch. We herein describe the overall design path of the mini-batch, its characteristics, configurations, together with some examples of use with polystyrene and CO2 as the blowing agent.

Effect of stearic acid/organic montmorillonite on EVA/SA/OMMT nanocomposite foams by melting blending

The effects of stearic acid (SA)/organic montmorillonite (OMMT) on the morphology and mechanical properties of nanocomposite foams based on ethylene vinyl acetate (EVA) copolymer were studied. The dispersion of montmorillonite layers was characterized by both X-ray diffraction and transmission electron microscopy. The cellular microstructure of the foamed samples was observed by scanning electron microscope. The effects of SA/OMMT on the mechanical properties of the EVA-based foams were also investigated. It was found that the combined effects of the existence of SA/OMMT led to bimodal foam structure in EVA/SA/OMMT nanocomposite foams. Compared with pure EVA foams, the density of EVA/SA/OMMT foams became lower with the addition of SA/OMMT, while the peel strength and elongation-at-break of the samples were increased.

Tensile and impact behavior of polystyrene microcellular foams with bi-modal cell morphology

Bi-modal PS foams with various volume fractions of large cells ( fL), cell sizes and densities were prepared to investigate the effect of cell structures on the tensile and impact behaviors. The tensile results showed that for the similar density, the tensile strength and modulus decreased with the increase of fL, unless the cell size of large ones is smaller than 25 µm. Similarly, the impact experimental results showed that the impact strength decreased with increasing fL, unless the fL is in the range of 25–32%. It indicated that the bi-modal cell structure could lead to the better properties than that of uniform one, when the cell morphology was proper ( fL in the range of 25–32% and the cell size of large ones smaller than 25 µm). The SEM images of impact-fractured surface of bi-modal foams further confirmed that the cell morphology with fL of 32% was more favorable to the absorption of impact energy during the fracture process.

Mechanical Properties of Density Graded Foams: Tensile Properties

This work presents the production and characterization of functionally graded polymer foams produced by compression molding. For the conditions tested (temperature gradient, molding time, type and concentration of Expancel microbeads), it was possible to reach density reduction between 2 and 45% for symmetric and asymmetric polyethylene foams. Tensile properties (modulus and stress at 1% deformation) were then measured to relate with their respective density profiles. From the experimental data obtained, a simple finite element analysis was performed to determine the tensile properties of asymmetric foams. The results show that excellent predictions were obtained with a maximum deviation of 7% for all the conditions tested.

Effect of Prepolymerization Time on Morphology and Properties of Epoxy-modified Bismaleimide Foams

Epoxy-modified bismaleimide (BMI) foams were prepared through prepolymerization and foaming. Density-adjustable BMI foams with closed-cell structures were produced. The influence of prepolymerization time on bubble growth and morphology, as well as the compressive and heat resistant properties of the foams, were thoroughly investigated. BMI prepolymerization was an effective method for controlling the bubble growth rate, the bubble size and its distribution, and the foam density. As the prepolymerization time increased, the matrix viscosity, the number of cells per unit volume and the foam density increased, whereas, the bubble growth rate and cell size decreased, and the cell size distribution became narrower. The compressive strength and modulus increased with increasing prepolymerization time. The dimensional stability temperatures of the BMI foams were all above 200°C, as determined by their base materials, and were unaffected by prepolymerization time.

Foaming Behavior and Mechanical Properties of Microcellular PP/SiO2 Composites

Microcellular foamed polypropylene (PP)/SiO2 composites were prepared by using micro-SiO2 and nano-SiO2 particles. The effects of the particle size of SiO2 on the foaming behavior and mechanical properties of the composites were studied based on heterogeneous nucleation theory. The results showed that the silica facilitated the cell nucleation to some extent. The average cell size of 16.3 μm and the cell density of 4.46 × 107 cells/cm3 were achieved for the composite foam at the silica content of 4 wt.%. The plastic deformation of the PP was strongly constrained due to the presence of the nano-silica. The tensile and impact strength of the nano-SiO2 composite foam are larger than those of the micro-SiO2 composite foam due to the high crack propagation resistance in the microcellular PP/nano-SiO2 composite.

Polystyrene Foams as Core Materials Used in Vacuum Insulation Panel

This work examines the foam density, average cell sizes, and cell structure of four polystyrene (PS) blended with calcium carbonate (CaCO3) or low density polyethylene (LDPE) at various ratios. The PS/CaCO3 and PS/LDPE are foamed by carbon dioxide (CO2) under supercritical conditions. CaCO3 and LDPE can strongly influence the foam density, average cell sizes, and cell structure of PS foams. Based on the result of PS/CaCO3 foams and PS/LDPE foams, the thermal conductivity of large porous PS foams as core materials used in vacuum insulation panels (VIPs) is further studied. Large porous PS foams are produced by the mixture of CO2 and fluorocarbon or the mixture of CO2 and nitrogen (N2) as foaming agents. The content of open-cells in PS foams is affected by materials, foaming temperatures, and foaming agents. A higher content of open-cells in a porous PS foam can lead to a lower and more stable thermal conductivity of VIPs.