Foam | EPFL Graph Search

Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the regions of gas. Soap foams are also known as suds. Solid foams can be closed-cell or open-cell. In closed-cell foam, the gas forms discrete pockets, each completely surrounded by the solid material. In open-cell foam, gas pockets connect to each other. A bath sponge is an example of an open-cell foam: water easily flows through the entire structure, displacing the air. A sleeping mat is an example of a closed-cell foam: gas pockets are sealed from each other so the mat cannot soak up water. Foams are examples of dispersed media. In general, gas is present, so it divides into gas bubbles of different sizes (i.e., the material is polydisperse)—separated by liquid regions that may form films, thinner and thinner when the liquid phase drains out of the

Heat expandable biopolymers for one-step production of foam core sandwich composites

Yonghoon Yoon

The overall aim of this work has been to develop sustainable solid thermally activated foam precursors suitable for the in-line production of lightweight bio-based foam core sandwich structures with consolidated wood particle facings, using a non-inflammable, non-VOC (volatile organic compound) as a blowing agent. A preliminary study showed that expandable PLA (polylactide)/CO2 could be prepared by impregnation of PLA, a commercially available bio-based polyester, with supercritical CO2. However, the high processing temperatures, T, and pressures, P, associated with supercritical conditions resulted in premature foaming during the depressurization step, making it difficult to adapt this technique to the production of foam precursors in the particleboard process. In this thesis, PLA-based foam precursors were therefore prepared using impregnation in liquid CO2 at moderate T and P. It was subsequently shown that the resulting heat-expandable materials could be successfully implemented in a simple one-step batch process for the manufacture of foam core particleboard, an important first step towards full implementation in a continuous process at the industrial scale. The first part of the study involved determination of the optimum conditions for the impregnation of PLA with CO2, so as to prepare foam precursors that were not only stable with respect to foaming at ambient T and P but also expanded readily at elevated T. Absorption and desorption of CO2 from initially amorphous low and intermediate D-isomer content PLA was modeled using a concentration, c, dependent diffusion coefficient, D[c], such that D[c]=D[0]exp[Ac]. The empirical constants D[0] and A were obtained by modeling desorption results under various conditions. It was hence shown that CO2 uptake by PLA at about 5 MPa and 10 °C was associated with a step-like concentration gradient, consistent with the morphologies observed in PLA specimens treated with liquid CO2 for different times and then foamed. Further evidence in support of the model was obtained from observations of CO2-induced crystallization in low D-isomer content PLA. The high melting temperature, Tm, of around 170 °C of the resulting crystalline phase nevertheless hindered subsequent foam expansion at T at and around 100 °C, i.e. the range of T associated with the particleboard process, which is turn fixed by the evolution of water vapor from the wood particles used to form the facings. Crystallization could be avoided through use of fully amorphous PLA with an intermediate D-isomer content. Indeed, the significant reduction in the glass transition temperature, Tg, in the presence of high concentrations of CO2, meant that amorphous specimens saturated under the chosen impregnation conditions, i.e. containing close to 40 wt% CO2, tended to foam spontaneously at room temperature, which is inconsistent with the stability requirements of the particleboard process. However, impregnation for a limited time, followed by partial CO2 desorption at low T under conditions that could be inferred directly from the diffusion model, was shown to lead to solid foam precursors with a uniform CO2 content of around 0.1 g/g. These were not only stable at room temperature, but contained sufficient CO2 for the production of low density foams on heating. ...

EPFL2016

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.

EPFL2012

Synthesis of bone-like structured foams

Françoise Borcard, Sandrine Gerber

Here we present a processing route to produce multi-structured ceramic foams based on the combination of particle-stabilized foams with polymeric sponges to produce positive and negative templating structures. Polyester sponges are infiltrated with freshly produced calcium aluminate–alumina foams and upon sintering either positive templating structures are produced when wetting the sponges, or negative templating foams with a percolating pore network are obtained when completely filling the sponges. Additionally, by combining different layers of these particle-stabilized foam infiltrated sponges, various different structures can be produced, including sandwich structures, pore size gradients, and ceramic bone-like structures applying to different types of bone. The particle-stabilized foams used were in situ self-hardening calcium aluminate cement enriched alumina foams to obtain crack-free samples with pore interconnections and tailorable pore sizes.

Elsevier Sci Ltd2013