Making polyurethane foams from microemulsions

A remarkable correlation exists between the degree of expansion of polyurethane foams and the structure of the reacting premixes. Polyurethane foams obtained from reacting premixes containing microemulsions are highly expanded. The expansion rate is proportional to the volume fraction of microemulsion in the premix. The stability of premixes with and without microemulsion is completely different suggesting distinct creaming mechanisms. We apply this idea to synthesize polyurethane foams from microemulsions successfully. This approach can be used to rationalize the design of polyurethane formulations leading to highly expanded foams.     

Foaming of polymers with supercritical CO2: An experimental and theoretical study

Microcellular polystyrene (PS) foams and porous structures of the biodegradable poly(d,l-lactic acid) (P_d,lLA) were prepared with the batch foaming technique (pressure quench) using supercritical CO₂ as blowing agent. The effect of pressure, temperature and depressurization rate on the final porous structure was investigated. The results revealed that the size of the pores decreases and their population density increases with pressure increase, or decrease of temperature, and/or increase of the depressurization rate. The results were correlated by combining nucleation theory with NRHB model in order to account for and emphasize the physical mechanism related to nucleation of bubbles inside the supersaturated polymer matrix. A satisfactory agreement between correlations and experimental data was obtained indicating that the nucleation theory yields quantitative correlations when variables such as sorption, degree of plasticization, and surface tension of the system polymer-supercritical fluid are accurately described.     

Static indentation and unloading response of sandwich beams

This paper deals with analysis of foam core sandwich beams subject to static indentation and subsequent unloading (removal of load). Sandwich beams are assumed continuously supported by a rigid platen to eliminate global bending. An analytical model is presented assuming an elastic-perfectly plastic compressive behaviour of the foam core. An elastic part of indentation response is described using the Winkler foundation model. Upon removal of the load, an elastic unloading response of the foam core is assumed. Also, finite element (FE) analysis of static indentation and unloading of sandwich beams is performed using the FE code ABAQUS. The foam core is modelled using the crushable foam material model. To obtain input data for the analytical model and to calibrate the crushable foam model in FE analysis, the response of the foam core is experimentally characterized in uniaxial compression, up to densification, with subsequent unloading and tension until tensile fracture. Both models can predict load–displacement response of sandwich beams under static indentation and a residual dent magnitude in the face sheet after unloading along with residual strain levels in the foam core at the unloaded equilibrium state. The analytical and FE analyses are experimentally verified through static indentation tests of composite sandwich beams with two different foam cores. The load–displacement response, size of a crushed core zone and the depth of a residual dent are measured in the testing. A digital speckle photography technique is also used in the indentation tests in order to measure the strain levels in the crushed core zone. The experimental results are in good agreement with the analytical and FE analyses.     

Compressive mechanical properties of closed-cell aluminum foam–polymer composites

The compressive mechanical properties of two kinds of closed-cell aluminum foam–polymer composites (aluminum–epoxy, aluminum–polyurethane) were studied. The nonhomogeneous deformation features of the composites are presented based on the deformation distributions measured by the digital image correlation (DIC) method. The strain fluctuations rapidly grow with an increase in the compressive load. The uneven level of the deformation for the aluminum–polyurethane composite is lower than that for the aluminum–epoxy composite. The region of the preferentially fractured aluminum cell wall can be predicted by the strain distributions in two directions. The mechanical properties of the composites are investigated and compared to those of the aluminum foams. The enhancement effect of the epoxy resin on the Young’s modulus, the Poisson’s ratio and the compressive strength of the aluminum foams is greater than that of the polyurethane resin.     

Semi-rigid biopolyurethane foams based on palm-oil polyol and reinforced with cellulose nanocrystals

In this study, water-blown biopolyurethane (BPU) foams based on palm oil were developed and cellulose nanocrystals (CNC) were incorporated to improve the mechanical properties of the foams. In addition, the foams were compared with petroleum polyurethane (PPU) foam. The foam properties and cellular morphology were characterized. The obtained results revealed that a low-density, semi-rigid BPU foam was prepared using a new formulation. CNC as an additive significantly improved the compressive strength from 54 to 117 kPa. Additionally, cyclic compression tests indicated that the addition of CNC increased the rigidity, leading to decreased deformation resilience. The dimensional stability of BPU foams was increased with increasing CNC concentration for both heating and freezing conditions.Therefore, the developed BPU nanocomposite foams are expected to have great potential as core material in composite sandwich panels as well as in other construction materials.     

Modelling of polyurethane foam thermal degradation within annular region subject to fire conditions

Abstract

Nuclear materials are placed in shielded, stainless steel packaging for storage or transport. These drum type packages often employ a layer of foam, honeycomb, wood or cement that is sandwiched between thin metal shells to provide impact and thermal protection during hypothetical accidents, as those prescribed in the Code of Federal Regulations (10 CFR 71·73). The present work discusses the modelling of the thermal degradation of polyurethane (PU) foam within an annular region during an 800°C fire. Measurements and analysis by Hobbs and Lemmon [M. L. Hobbs and G. H. Lemmon: ‘Polyurethane foam response to fire in practical geometries’, Polym. Degrad. Stab., 2004, 84, 183–197.] indicate that at elevated temperatures, PU foam exhibits a two-stage, endothermic degradation. The first stage produces a degraded solid and a combustible gas; the second stage reaction consumes the degraded solid and produces another combustible gas. As a result, during a prolonged fire, a gas filled void develops beside the outer metal shell and grows inward toward the inner shell and the containment vessel. As a result of the radial symmetry in the drum geometry, a one-dimensional finite difference model is constructed for the annular foam region. Heat flux is applied to the inner surface to model the decay heat of the containment vessel contents. Thermal radiation and convection boundary conditions with a specified environmental temperature are applied to the outer surface. The material and reaction rate properties determined by Hobbs and Lemmon are applied to the foam. The annular region temperature and composition are determined as functions of radius and time after the environmental conditions are changed from room temperature to those of an 800°C fire. The effects of surface to surface radiation between the package’s outer skin and the undegraded foam and the reaction rate reduction due to material damage during the reaction are evaluated for fires lasting 20 h. The peak package liner temperature caused by a 30 min fire is predicted to be 129°C, well below the short term limit for containment vessel seal (377°C).     

Synthesis and properties of metal matrix composite foams based on austenitic stainless steels –titanium carbonitrides

Metal matrix composite foams based on 316L stainless steel and reinforced with TiC0.7N0.3 were produced by the replication method using polyurethane sponge as a template. The rheological properties of the slurry appeared to be the key issue in the preparation of the composite foams. A homogeneous distribution of TiC0.7N0.3 particles throughout the 316L matrix and a good interaction between the 316L matrix and TiC0.7N0.3 reinforcement particles were obtained. Compression strength results showed that TiC0.7N0.3 particles acted as the real reinforcement medium. The values of the compressive yield strength and the elastic modulus of the metal matrix composite foams increased significantly with increasing TiC0.7N0.3 content when compared to the open cell 316L stainless steel foams.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.