Silicon Oxycarbide Foams from a Silicone Preceramic Polymer and Polyurethane

Ceramic open-cell foams were obtained from a preceramic polymer (a silicone resin) and blown polyurethanes, by pyrolysis at 1200°C in nitrogen. Silicon carbide submicron powders were also added to the silicone resin to give SiOC + SiC composite foams. The morphology of the foams was dependent on the architecture of the blown polyurethanes. The crushing strength as well as the elastic modulus increased with increasing relative density, reaching values as high as 14 and 450 MPa, respectively. Some of the foams displayed an excellent thermal stability (resistance to oxidation in air and decomposition in inert atmosphere) up to elevated temperatures.     

Glass foam from window panes and bottle glass wastes

Glass foams are building materials that now compete with classic insulating polymeric and fiber materials for thermal enveloping. The low flammability, high chemical durability and thermal stability are distinct advantages over polymeric materials. The present paper proposes the possibility of producing glass foam using two types of recycled glass wastes (window panes and bottle glass) together with plaster wastes from used ceramic casting molds as foaming agent. Optical microscopy, measurements of apparent porosity and density, hydrolytic and chemical stability, as well as thermal conductivity were used in order to characterize the obtained glass foams as insulator materials for the building industry. The apparent porosity of glass foams ranges between 20.19–54.54% when using window glass wastes, and 18.77–51.75% with bottle glass wastes. Thermal conductivity was less than 0.25 W mK-1 for all the studied glasses. The obtained results confirm that there exists an alternative method for producing glass foams, for example, from glass wastes and used ceramic plaster molds, which are utilized as foaming agents with good chemical stability and insulating properties.     

Effect of Aging on the Stability of Ceramic Foams Prepared by Thermostimulated Sol-Gel Process

This work presents a new route of preparation of zirconium ceramic foams based on the thermostimulated sol-gel process. This method produces gelled bodies with up to 90% of porosity in the wet gel and can be used to make complex-shaped components. Unfortunately, the shrinkage during the drying step allows to a catastrophic reduction (>50%) of the foam porosity. To improve the foam stability we carried out a systematic study of the effect of gel foam aging on the drying process. Samples were aged in closed vessel at 25°C during different time period (from 6 to 240 h). The shrinkage and the mass loss during drying at 50°C were measured in situ, using a non-contact technique performed with a special apparatus. The results show that the total linear shrinkage decreases from 46% to 8% as the aging period increase from 6 to 240 h. This behavior is followed by a small change of total mass loss, from 42 to 54%. It indicates that by aging the structural stiffness of the foams increases due to secondary condensation reactions. Thus, by controlling the aging period, the porosity can be increased from 67 to 75% and the average size of mesopores of dried foams can be screened from 0.3 to 0.9 m. Finally, these results demonstrate that the thermostimulated sol-gel transition provides a potential route to ceramic foams manufacture.     

Polymer Foams Stabilized by Particles Adsorbed at the Air/Polymer Interface

In aqueous systems, partially hydrophobic particles are known to stabilize foams even in the absence of any added surfactant. This paper shows that the same principle can be applied to polymeric systems: particles that are partially wetted by a polymer melt can stabilize a foam of that polymer. The foam stability is attributable to the adsorption of the particles at the air/polymer interface. Remarkably, stable foams are realized even from polymers that are liquid at room temperature, and hence are otherwise unfoamable. The implications of this result to practical foaming operations are discussed.

     

Effect of electron beam radiation dose on the foam formation in pre-ceramic polymer

Methylsilicone resin as a polymer precursor for a SiOC ceramic material was cured and foamed by electron beam (EB) irradiation in air prior to the pyrolysis under an inert atmosphere. Methylsilicone foams were obtained without additional foaming agent when exposed to accelerated electrons with radiation doses up to 9 MGy and dose rate of 2.8 kGy/s. During irradiation the polymer was melted and simultaneously gaseous products were formed by the methyl group oxidation and by the poly-condensation crosslinking reactions. The formed gases could not escape from the molten polymer and began to aggregate into bubbles. The effect of the radiation dose on the polymer foam molecular structure, the gel fraction and the ceramic yield was analyzed. The results indicate that the maximum amount of crosslinking in methylsilicone, when EB radiation is used, occurred between 1.0 and 2.0 MGy radiation dose. Methylsilicone foams were pyrolysed in N₂ atmosphere at temperatures of 1200 and 1500 °C, resulting in amorphous SiOC and partially crystalline ceramic foams, respectively. A porosity of ~84% was achieved in the pyrolyzed foams, with cell size ranging from 30 to 300 μm and density of about 0.31 g cm^?3.     

Open-pore biodegradable foams prepared via gas foaming and microparticulate templating

Open-pore biodegradable foams with controlled porous architectures were prepared by combining gas foaming and microparticulate templating. Microparticulate composites of poly(epsilon-caprolactone) (PCL) and micrometric sodium chloride particles (NaCl), in concentrations ranging from 70/30 to 20/80 wt.-% of PCL/NaCl were melt-mixed and gas-foamed using carbon dioxide as physical blowing agent. The effects of microparticle concentration, foaming temperature, and pressure drop rate on foam microstructure were surveyed and related to the viscoelastic properties of the polymer/microparticle composite melt. Results showed that foams with open-pore networks can be obtained and that porosity, pore size, and interconnectivity may be finely modulated by optimizing the processing parameters. Furthermore, the ability to obtain a spatial gradient of porosity embossed within the three-dimensional polymer structure was exploited by using a heterogeneous microparticle filling. Results indicated that by foaming composites with microparticle concentration gradients, it was also possible to control the porosity and pore-size spatial distribution of the open-pore PCL foams.     

Compressive response of composite ceramic particle-reinforced polyurethane foam

Quasi-static and dynamic compressive tests are undertaken on the polyurethane (PU) foam and fumed silica reinforced polyurethane (PU/SiO₂) foam experimentally. The ceramic microspheres with varying mass fractions are adopted to mix with the PU/SiO₂ foam to fabricate the composite particle-reinforced foams. The effects of strain rate and particle mass fraction are discussed to identify and quantify the compressive response, energy-absorbing characteristic, and the associated mechanisms of the composite foams. The results show the initial collapse strength and plateau stress of the foams are improved significantly by reinforcing with the ceramic microsphere within 60 wt% at quasi-static compression. The rate sensitivity is observed on all the foams, but in different patterns due to the influence of ceramic microsphere. The compressive response affected by ceramic microsphere can be attributed to the particle cluster effect and stress wave propagation. Together with the deformation, the compressive characteristic experiences non-monotonic change from the low to high strain rates. The specific energy absorption (SEA) of the foam with 41 wt% ceramic microsphere show the largest magnitude at quasi-static compression. With the increasing strain rate, the ceramic reinforced foam exhibits superior energy absorption efficiency at high strain rates to that of the pure foams.     

Exploiting the Use of the Decarboxylative S-Alkylation Reaction to Produce Self-Blowing,Recyclable Polycarbonate Foams

Polymeric foams are widely used in many industrial applications due to their light weight and superior thermal, mechanical, and optical properties. Currently, increasing research efforts is being directed towards the development of greener foam formulations that circumvent the use of isocyanates/blowing agents that are commonly used in the production of foam materials. Here, a straightforward, one-pot method is presented to prepare self-blown polycarbonate (PC) foams by exploiting the (decarboxylative) S-alkylation reaction for in situ generation of the blowing agent (CO₂). The concomitant formation of a reactive alcohol intermediate promotes a cascade ring-opening polymerization of the cyclic carbonates to yield a cross-linked polymer network. It is shown that these hydroxyl-functionalized polycarbonate-based foams can be easily recycled into films through thermal compression molding. Furthermore, it is demonstrated that complete hydrolytic degradation of the foams is possible, thus offering the potential for zero-waste materials. This straightforward and versatile process broadens the scope of isocyanate-free, self-foaming materials, opening a new pathway for next-generation environmentally friendly foams.     

“Cooperative” Effects in the Destruction of Foam Column. Comparison of the Behavior of Polyhedral Two- and Three-Dimensional Foams

The kinetics of the destruction of polyhedral monolayer and bulk (three-dimensional) foams from solutions of nonionic surfactants and protein (lysozyme) was studied at various pressure drops Pand constant temperaturetand at various temperatures at P= const in order to elucidate the role of cooperative effects in a foam stability. It was established that lifetimes _p of monolayer and bulk foams prepared from surfactants at t= const decrease with an increase in pressure drop; however, the values of _p are always larger for the monolayer than for bulk foams. Avalanche-like destruction typical of foam prepared from surfactant solutions, which is observed at certain pressure drops in foam liquid phase and temperature, was not disclosed in the monolayer foam formed from the same solutions. For foams with structured adsorption layers (the foam from lysozyme solution), the lifetimes and character of destruction practically do not differ for monolayer and bulk foams. It was established that cooperative effect appears in very thin foam layers beginning with the bubble bilayer.     

Blocking Ability and Flow Characteristics of Nitrogen Foam Stabilized with Clay Particles in Porous Media

Particles-stabilized foams have received more attention in recent years due to their specific characteristics and advantages in contrast to conventional foams which were stabilized with surfactants. However, the rheology of particles-stabilized foam in consolidated cores was rarely studied. To investigate the feasibility of the particles-stabilized foam application in enhanced oil recovery, the blocking ability and flowing characteristics of foam stabilized with clay particles were investigated by using experimental cores. To do this, the foam resistance factor was studied as an index in this article. The effects of foam quality (gas velocity divided by total velocity), injection rate of foam, and the permeability of cores on the blocking ability of foams were investigated. Results showed that the blocking ability reached the peak value at the foam quality of 0.74. This indicated effective blocking ability as conventional foams performed in porous media. Moreover, the foams block the channels more effectively in high permeability cores, compared with low permeability ones. Finally, foams displayed shear-thinning property in porous media as injection rate increased.      

Synthesis of nanosized zirconium carbide from preceramic polymers by the facile one‐pot reaction

A novel preceramic polymer, polyzirconoxanesal (PZS), was synthesized efficiently by one‐pot protocol. The PZS polymer was characterized by the techniques of Fourier transform infrared spectra (FTIR), nuclear magnetic resonance (¹H‐NMR and ¹³C‐NMR) and gel permeation chromatography (GPC). The precursor exhibited excellent solubility and rheology for the processing of ceramic matrix fiber composites. Pyrolytic conversion of the preceramic polymer was studied by means of thermogravimetric analysis (TGA), X‐ray diffraction (XRD), scanning electron microscope (SEM) and element analysis. It is suggested that the polymer decomposed completely at 600°C and nanosized ZrC is formed at 1300°C for 2 hr in argon atmosphere with a yield of 68.5%. To the best of our knowledge, this is the lowest temperature reported in the literatures for preparing ZrC ceramics from preceramic polymers. The obtained ZrC particles exhibit spherical morphology with size ranged in 20–100 nm. Copyright © 2010 John Wiley & Sons, Ltd.     

Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent

Polyurethane rigid foams have been used for many applications such as pipelines insulation materials, automotive parts, solar water heater and construction materials[1,2], due to their desirable physical properties. Traditional rigid foam is made by the reaction of a polyol and 4,4′-diphenylmethane diisocyanate (MDI) with chlorofluorocarbons (CFCs), in particular tri- chlorofluoromethane (CFC-11) and/or HCFC-141b as blowing agents. However, the CFCs blowing agents contain halogens, whic…     

Separation and determination of trace amounts of zinc, lead, cadmium and mercury in tap and Qaroun lake water using polyurethane foam functionalized with 4-hydroxytoluene and 4-hydroxyacetophenone

A stable chelating sorbent was synthesized by covalently linking 4-hydroxytoluene or 4-hydroxyacetophenone with the polyurethane foam (PUF) through -NN- group. The synthesized chelating sorbents were characterized by IR and UV/vis measurements. The modified foams show excellent stability towards various solvents. Factors influencing the extraction process of Zn(II), Pb(II), Cd(II) and Hg(II) were studied and evaluated as a function of pH of metal ion solution and equilibration shaking time. The values of sorption capacity of metal ions (μg g⁻¹) were determined with the two types of bonded foams. The two phenolic bonded foams were studied comparatively. The potential applications of the two newly synthesized foams for the removal and separation of the examined metal ions from two natural water samples (drinking tap water and Qaroun lake water at Fayoum City, Egypt) were investigated. Precision (assessed as a relative standard deviation, R.S.D.) was also evaluated and found to be ≤7.3% (N = 5) with a detection limit under 0.46 μg L⁻¹.     

Surface forces in foam films from ABA block copolymer: a dynamic method study

The thinning of foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (average molecular weight 14,000 g/mol) is studied experimentally. The dependence of the surface forces on film thickness is obtained by the dynamic method of Scheludko and Exerowa.The total surface force measured in foam films (radius 60–70 m) from 10^–5 M (0.014 wt%) polymer solution with 0.1 M NaCl is positive at thicknesses from about 800 down to 460 . The electrostatic repulsion is negligible while the contribution of van der Waals attraction is small (within 15%). Therefore a positive surface force component predominates. Most probably it arises from steric interactions between the hydrophilic polyethylene oxide tails of the polymer. The dynamic method appears to be a suitable technique for exploring the stabilization of foam films from ABA copolymers.     

A steady‐state mass balance model of the polycarbonate–CO2 system reveals a self‐regulating cell growth mechanism in the solid‐state microcellular process

A simple mechanism regulating polymer mobility is demonstrated to determine initial and final growth states of solid‐state microcellular foams. This mechanism, governed by the extent of plasticization of the polymer by the dissolved gases, is examined with a mass balance model and results from foam growth experiments. Polycarbonate was exposed to CO₂, which acted as both a plasticizing gas and a physical blowing agent driving foam growth. The polycarbonate specimens were saturated to the equilibrium gas concentration at 25 °C for CO₂ pressures of 1–6 MPa in 1‐MPa increments. Equilibrated specimens were heated in a glycerin bath until thermal equilibrium was reached, and a steady foam structure was attained. Glycerin bath temperatures of 30–150 °C in 10 °C increments were examined. Using knowledge of gas solubility, the equation of state for CO₂, the effective glass‐transition temperature as a function of gas concentration, and a model for mass balance within a solid‐state foam, we demonstrate that foam growth terminates when sufficient gas is driven from the polycarbonate matrix into the foam cells. The foam cell walls freeze at the elevated bath temperature because of gas transport from the polycarbonate matrix and the associated rise in the polymer glass‐transition temperature to that of the heated bath. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 868–880, 2001     

Synthesis and pyrolysis of a novel preceramic polymer PZMS from PMS to fabricate high‐temperature‐resistant ZrC/SiC ceramic composite

As a valuable ultra‐high‐temperature ceramic (UHTC), ZrC was introduced to SiC ceramic for the preparation of high‐temperature‐resistant ZrC/SiC composite by a polymer‐derived method through the reaction between Cp₂Zr(CH=CH₂)₂ and polymethylsilane (PMS). The composition, structure, element distribution and pyrolysis process of the preceramic polymer polyzirconomethylsilane (PZMS) were investigated by nuclear magnetic resonance, infrared, gel permeation chromatography, X‐ray photoelectron spectroscopy, energy‐dispersive X‐ray spectroscopy, scanning electron microscopy and thermogravimetric analysis. The obtained ZrC/SiC ceramic composites had very good high‐temperature resistance with a weight loss of 7.1% after being subjected to temperatures ranging from 1200 to 2200°C, as the introduction of ZrC prevented the fast growth of crystalline β‐SiC. The ceramic composites prepared by this method were homogeneous with well‐distributed element components, and the ceramic yield reached as high as 78.4%. Copyright © 2013 John Wiley & Sons, Ltd.     

Foaming of trans‐polyisoprene using N2 as the blowing agent

Foaming of trans‐1,4‐polyisoprene (TPI) polymer was carried out through a batch process using nitrogen (N₂) as the blowing agent. TPI vulcanizates having varying crosslink densities were prepared by varying crosslinking agent content and curing time. The vulcanizates were then saturated with N₂ inside a pressure vessel at a pressure of 14 MPa and varying temperatures for 5 hours before effecting the foaming by rapidly quenching the pressure. The effects of varying the crosslinking agent content, silica filler content, and precuring time of the vulcanizates and the effects of varying the gas saturation temperature of foaming on the cell characteristics and physical properties of the foam prepared were investigated. The cells of the TPI foams had a spherical, closed structure. The density, expansion ratio, cell size, cell density, and tensile properties of the foams varied with varying crosslink density of the TPI vulcanizates as well as the saturation temperature of foaming. The important effects of crosslink density and saturation temperature on the N₂ solubility in the TPI matrix and thus on the foam expansion were discussed. The silica filler was found to be acting as a cell nucleating agent and reinforcing filler for the TPI foams.     

The study of the stability of aqueous three-phase fire-resistant foam in typical liquidus hydrocarbons

The utilization of solid particles in aqueous foam has a great potential in improving fire fighting efficiency. In this study, aqueous foam supported by micro fly-ash (FA) was prepared and its stability in a specific type of oil was characterized. Firstly, different amount of FA was added to study the influence of FA concentration on foamability. It showed that within a specific extent, foam expansion ratio increased with the increasing of FA concentration. And compared with conventional foams, oil resistance of FA stabilized foams, which was investigated by analyzing drainage rate and evolution process with a self-made apparatus, was remarkably improved when FA concentration exceed 4.8wt.%. Secondly, SiO₂ and Al₂O₃ particles with different median sizes were used to study the effect of particle size on stability. However, the smaller hydrophilic particles didn’t behave better as expected. Moreover, the foam stability in three hydrocarbons was evaluated in the same way. The results indicated that the short chain hydrocarbons had much stronger detrimental effect to both two-phase foam and three-phase foam. But overall, the three-phase foam stabilized by FA exhibited much better oil resistance, so it can be used as a promising material for pool fire extinguishing and prevention.GRAPHICAL ABSTRACT     

Macroporous SiC?MoSi2 ceramics from templated hybrid MoCl5–polymethylsilane

A molybdenum‐containing preceramic polymer, MoPMS, was synthesized for the first time by HCl elimination of polymethylsilane (PMS) and MoCl₅ at room temperature in tetrahydrofuran. The insoluble MoPMS prepared was embedded into the void spaces of a silica colloidal crystal template within the pot life of the polymer and successfully transformed to a three‐dimensionally long‐range‐ordered macroporous SiC? MoSi₂ ceramic after pyrolysis at 1400 °C in an argon atmosphere followed by template removal in HF. The bead‐inverse macroporous SiC? MoSi₂ ceramic, with a ceramic yield of about 88%, exhibits high temperature stability, high BET surface area, and semiconducting behavior. In addition, the macroporous SiC? MoSi₂ ceramic was used as a catalyst carrier for platinum–ruthenium coated on the surface of the pores. The preceramic polymer and the ceramic were characterized by IR, thermogravimetric analysis, X‐ray diffraction, scanning and transmission electron microscopy, and BET surface area. Copyright © 2005 John Wiley & Sons, Ltd.     

Crosslinked polyethylene foams, via EB radiation

Polyethylene foams, produced by radio-induced crosslinking, show a smooth and homogeneous surface, when compared to chemical crosslinking method using peroxide as crosslinking agent. This process fosters excellent adhesive and printability properties. Besides that, closed cells, intrinsic to theses foams, imparts opitmum mechanical, shocks and insulation resistance, indicating these foams to some markets segments as: automotive and transport; buoyancy, flotation and marine: building and insulation: packaging: domestic sports and leisure goods. We were in search of an ideal foam, by adding 5 to 15% of blowing agent in LDPE. A series of preliminary trials defined 203° C as the right blowing agent decomposition temperature. At a 22.7 kGy/dose ratio, the lowest dose for providing an efficient foam was 30 kGy, for a formulation comprising 10% of azodicarbonamide in LDPE, within a 10 minutes foaming time.