Prediction of the radiation term in the thermal conductivity of crosslinked closed cell polyolefin foams

The thermal conductivity and the cellular structure as well as the matrix polymer morphology of a collection of chemically crosslinked low‐density closed cell polyolefin foams, manufactured by a high‐pressure nitrogen gas solution process, have been studied. With the aid of a useful theoretical model, the relative contribution of each heat‐transfer mechanism (conduction through the gas and solid phases and thermal radiation) has been evaluated. The thermal radiation can be calculated by using a theoretical model, which takes into account the dependence of this heat‐transfer mechanism with cell size, foam thickness, chemical composition, and matrix polymer morphology. A simple equation, which can be used to predict the thermal conductivity of a given material, is presented. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 993–1004, 2000     

Thermal expansion of crosslinked closed-cell polyethylene foams

An experimental study on the thermal expansion of a collection of crosslinked low-density polyethylene (LDPE) foams with closed-cell structure is presented. The thermal characterization of these materials, the relationships between the linear thermal expansion coefficient and the structure of the foams, and the determination of the variables that can modify the thermal properties of these products are the goals of this work. The experimental results show that the linear thermal expansion coefficient decreases when the density of the foamed material increases. The gas expansion inside the cells is a mechanism that should be taken into account. Moreover, the thermal expansion also depends on the cellular structure. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2587–2596, 1998     

Applicability of the transient plane source method to measure the thermal conductivity of low‐density polyethylene foams

The thermal conductivity at constant pressure of a collection of crosslinked, closed‐cell polyethylene foams were measured at room temperature with the transient plane source (TPS) method. The experimental results were compared with those determined by a standard steady‐state technique. The results showed that the values measured by the TPS method follow the same trends as those measured by a heat‐flow meter. Therefore, with the TPS technique it is possible to observe the influence of structural characteristics such as cell size, black carbon content in foams, density, and so forth on thermal conductivity. However, the values obtained by the transient method were approximately 20% higher than those given by the standard method. Possible reasons for these variations are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1226–1234, 2004     

Extinction coefficient of polyolefin foams

The extinction coefficient of a collection of polyolefin foams was investigated experimentally and theoretically. Transmittance spectra were measured with Fourier transform infrared spectroscopy (FTIR) for samples of various thicknesses and different chemical compositions, densities, colors and structural characteristics. The extinction coefficients were then calculated by applying Beer's law. The results showed that the extinction coefficient decreased with the mean cell size and that this was the main structural parameter influencing the extinction coefficient of the foams under study. The experimental results agreed well with the Glicksman model. Moreover, the total thermal conductivity was calculated in terms of the Rosseland equation with an accuracy of 5%. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1608–1617, 2005     

Thermal expansion coefficient and bulk modulus of polyethylene closed‐cell foams

A regular Kelvin foam model was used to predict the linear thermal expansion coefficient and bulk modulus of crosslinked, closed‐cell, low‐density polyethylene (LDPE) foams from the polymer and gas properties. The materials used for the experimental measurements were crosslinked, had a uniform cell size, and were nearly isotropic. Young's modulus of biaxially oriented polyethylene was used for modeling the cell faces. The model underestimated the foam linear thermal expansion coefficient because it assumed that the cell faces were flat. However, scanning electron microscopy showed that some cell faces were crumpled as a result of foam processing. The measured bulk modulus, which was considerably smaller than the theoretical value, was used to estimate the linear thermal expansion coefficient of the LDPE foams. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3741–3749, 2004     

Radiation contribution to the thermal conductivity of plastic foams

The validity of two approaches widely used to determine the radiant thermal conductivity in plastic foams is discussed. While one approach is based on the solution of a geometric model, the other is derived from the experimental determination of the extinction coefficient. A comparison to recently reported experimental data shows that the geometric approach predicts values that are in good agreement. In contrast, values deduced from measurements of the mean extinction coefficient significantly underestimate the radiant thermal conductivity, an effect that can be traced to the way that the extinction coefficient is measured. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 190–192, 2005     

Preparation and dielectric properties of polyimide foams containing crosslinked structures

In order to obtain cellular materials with low dielectric properties, crosslinked polyimide foams were prepared using 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA), 4,4′‐oxydianiline (ODA) and 2,4,6‐triaminopyrimidine (TAP) as monomer via a poly(ester‐amine salt) precursor process. The structures of the precursors and the polyimide foams were characterized by thermogravimetric analysis (TGA) and FT‐IR, while the morphologies of the polyimide foams were viewed from scanning electron microscopy (SEM) measurements. The results revealed that the poly(ester‐amine salt) precursor containing TAP could successfully be converted to a crosslinked polyimide foam with relatively uniform cell structure. Also, the crosslinking of TAP improved the mechanical properties of foams in comparison with the non‐crosslinking systems. With increasing content of TAP, the dielectric constants of the polyimide foams decreased gradually. For the foam with TAP molar ratio at 15%, the dielectric constant was as low as 1.77 at the frequency of 10 kHz. Though the thermal resistance decreased slightly for crosslinked foams, the decomposition temperatures were still maintained above 520°C. Copyright © 2006 John Wiley & Sons, Ltd.     

Rigid polyurethane foams reinforced with disilanolisobutyl POSS: Synthesis and properties

This work reports the synthesis of rigid polyurethane (PU) foams modified by disilanolisobutyl polyhedral oligomeric silsesquioxane (DSIPOSS). This open‐cage nanostructure silsesquioxane has 2 hydroxyl groups and therefore can be chemically built directly in the PU backbone to form hybrid polyurethane‐POSS foam. Synthesis procedure using polymeric 4,4′‐diphenylmethane diisocyanate, polyetherol, and DSIPOSS has been elaborated, and the influence of POSS on the cell structure, closed cell content, apparent density, thermal conductivity, and compression strength of the rigid polyurethane composites has been evaluated. The hybrid composite foams containing 1.5 and 2.0 wt% DSIPOSS showed a reduced number of cells and an increased average area of foam cells in comparison with the unmodified PU, while the addition of 0.5wt% of DSIPOSS causes an increase in the number of cells of the foam as compared with the reference and thus a reduction in the average area of cells. X‐ray microtomography provided data on the porous structure of polyurethane hybrid materials, including reduction of the pore surface area. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy analysis revealed a good homogenization of DSIPOSS in polyurethane matrix. Thermogravimetric analysis results have shown that incorporation of POSS nanoparticles into PU foam does not significantly change the degradation process. The compressive strength of PUF‐POSS hybrids in the direction parallel and perpendicular to the direction of foam rise is greater than the strength of the reference foam already for the lowest DSIPOSS content.     

Constitutive modeling for characterizing the compressive behavior of PMMA open‐cell foams

A constitutive model for evaluating the compressive behavior of Poly(methyl‐methacrylate) (PMMA) open‐cell foams is herein proposed. Specifically, the study investigates the viscoelastic and viscoplastic behaviors of the PMMA open‐cell foams. The constitutive equation is expressed in terms of the following polymer and foam properties: elastic modulus, relative density, as well as the relaxation and densification constants. PMMA open‐cell foams are manufactured using a gas foaming/particulate leaching method and uniaxial compression tests are performed. The mechanical properties and compressive stress‐strain responses obtained from the experiments are compared with those predicted by the proposed constitutive model. The results suggest that the constitutive model is an apt one for assessing and evaluating the compressive behaviors of PMMA open‐cell foams. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 436–443, 2007     

Microcellular processing and relaxation of poly(ether ether ketone)

The semicrystalline microcellular closed‐cell foams are prepared by a two‐stage batch foaming process from poly(ether ether ketone) and characterized by scanning electronic microscopy. It can be observed that there are two kinds of cells with obviously different cellular sizes in the same transect and the distribution of larger cells (about 7 μm) looks like sandwich. The effects of foaming temperatures and transfer times on the cellular sizes and cell densities of porous materials were discussed. Particular emphasis was given to the effects of crystalline on the microcellular morphology. The relaxation mechanism of microcellular materials was systemically investigated by dynamic mechanics analysis. A plain on the storage modulus curve before T_g was observed due to the densification of cells. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2890–2898, 2007     

Foaming behavior,cellular structure and physical properties of polybenzoxazine foams

In this paper, polymer foams based on a benzoxazine resin have been successfully prepared using azodicarbonamide (ADC) as a chemical blowing agent and have been characterized regarding their foaming behavior, cellular structure, and physical properties. The effect of the ADC on the curing process of the resin was analyzed using differential scanning calorimetry and blowing agent decomposition was followed by thermogravitmetric analysis (TGA). The characterization of the cellular structure of the foamed samples was done using scanning electron microscopy. The mechanical properties of the foams were determined using compression tests and the thermal conductivity was assessed using the transient plane source method. The results indicated that the curing process and gas release took place in a similar time interval. The foams showed an isotropic cellular structure with relative densities in the range 0.35–0.60, and showed compressive strengths and compressive moduli in the range of 10–70 MPa and 400–1100 MPa, respectively. Thermal conductivities were in the range of 0.06–0.12 W m^?1K^?1. The findings in this paper demonstrate the possibility of producing polybenzoxazine foams using a simple process in which curing and foaming take place simultaneously. In addition, the mechanical characterization of these materials indicates that they are suitable for structural applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermophysical properties of polypropylene/aluminum composites

This article is dedicated to the study of the thermal parameters of composite materials. A nonlinear least‐squares criterion is used on experimental transfer functions to identify the thermal conductivity and the diffusivity of aluminum‐polymer composite materials. The density measurements were achieved to deduce the specific heat and thereafter they were compared to values given by differential scanning calorimetry measurement. The thermal parameters of the composite material polypropylene/aluminum were investigated for the two different types of aluminum filler sizes. The experimental data were compared with several theoretical thermal conductivity prediction models. It was found that both the Agari and Bruggeman models provide a good estimation for thermal conductivity. The experimental values of both thermal conductivity and diffusivity have shown a better heat transport for the composite filled with large particles. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 722–732, 2004     

Compressive stress relaxation of metallocene‐based polyolefin foams: Effect of gamma‐ray‐induced crosslinking

Metallocene‐based polyolefin (MPO) foams possess a closed‐cell structure which is in contrast to the open‐celled structure of polyurethane (PU) foams. In this study, we investigate the effects of gamma‐irradiation on the mechanical behavior of MPO foams using PU foam behavior as a basis. Compressive step‐strain experiments reveal a two‐step relaxation process in MPO foams, dominated by polymer chain relaxation at short times and gas diffusion from the closed cells at longer times. On the other hand, the relaxation in PU foams is similar to fully crosslinked polymers with the relaxation modulus reaching an equilibrium value after an initial decay. The closed‐celled structure of MPO foams lends to rapid stress relaxation and low structural recoverability upon application of compressive loads. Exposure to gamma radiation induces crosslinking in MPO foams and improves their resilience and recoverability. Stress relaxation tests reveal that nonradiated MPO foams show complete relaxation and structural loss at high temperatures. In contrast, radiated MPO foams show a significant retardation in relaxation kinetics and structural stability attributed to radiation‐induced crosslinking. Dynamic rheology and solvent‐extraction studies also support the results obtained from stress‐relaxation experiments. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1045–1056, 1999     

Kinetics study of oil sorption with open‐cell polypropylene/polyolefin elastomer blend foams prepared via continuous extrusion foaming

The objective of the present work was to study the sorption kinetics of open‐cell polypropylene/polyolefin elastomer (PP/POE) blend foams. First, open‐cell PP/POE foams of different cell structures were prepared by controlling the foaming temperature via a continuous extrusion foaming process. Second, the effect of the cell structures on the sorption process, rate, and capacity was studied. Pseudo‐first order and pseudo‐second order models were applied to study the sorption kinetics of the PP/POE foams for cyclohexane. Third, the sorption rate and sorption capacity by both volume and weight of the PP/POE foam for different oils and solvents were studied to show how the intrinsic properties of the testing oils and solvents affected the sorption performance. The results showed that the sorption with the PP/POE foams followed the pseudo‐second order kinetics model. Both the cell structures of the foams and the intrinsic properties of the testing oils and solvents affected the sorption performance. For the same testing oil, a higher open‐cell content in the foam was favorable for a higher sorption rate, and a higher void fraction was favorable for a higher sorption capacity. For the same foam, a lower viscosity of the testing oil was favorable for a higher sorption rate. The sorption capacity by volume was closely related to the viscosity of the testing oil, while both the viscosity and the density of the testing oil determined the sorption capacity by weight.     

An empirical model to predict temperature‐dependent thermal conductivity of amorphous polymers

Thermal conductivity in polymers has been theoretically and experimentally studied in good detail, but there is a need for more accurate models. Polymeric thermal conductivity exhibits a plateau‐like transition at temperatures around 10 K, which is not well accounted for by existing models. In this work, an empirical model that can predict temperature‐dependent thermal conductivity of amorphous polymers is developed. The model is based on kinetic theory and accounts for three sets of vibrational modes in polymers, and is developed using classical expressions, results of previous simulations, and experimental data. Fundamental material properties like density, monomer molecular weight, and speed of sound are the only input parameters. The model provides estimates for the locations of transitions between different sets of vibrational modes, an upper limit for the thermal conductivity, and temperature‐dependent thermal conductivity, which are in good agreement with experimental data. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1160–1170     

Study of slabstock flexible polyurethane foams based on varied toluene diisocyanate isomer ratios

The morphological features of three flexible slabstock polyurethane foams based on varied contents of 2,4 and 2,6 toluene diisocyanate (TDI) isomers are investigated. The three commercially available TDI mixtures, that is, 65:35 2,4/2,6 TDI, 80:20 2,4/2,6 TDI, and 100:0 2,4/2,6 TDI were used. The foams were characterized at different length scales with several techniques. Differences in the cellular structure of the foams were noted with scanning electron microscopy. Small‐angle X‐ray scattering was used to demonstrate that all three foams were microphase‐separated and possessed similar interdomain spacings. Transmission electron microscopy revealed that the aggregation of the urea phase into large urea‐rich regions decreased systematically on increasing the asymmetric TDI isomer content. Fourier transform infrared spectroscopy showed that the level of bidentate hydrogen bonding of the hard segments increased with the 2,6 TDI isomer content. Differential scanning calorimetry and dynamic mechanical analysis (DMA) were used to note changes in the soft‐segment glass‐transition temperature of the foams on varying the diisocyanate ratios and suggested that the perfection of microphase separation was enhanced on increasing the 2,6 TDI isomer content. The preceding observations were used to explain why the foam containing the highest content of the symmetric 2,6 TDI isomer exhibited the highest rubbery storage modulus, as measured by DMA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 258–268, 2003     

Polyurethane‐graphene nanocomposite foams with enhanced thermal insulating properties

The improvement of thermal insulating performance of polyurethane rigid foams is a crucial task for their use. In this work, the effect of graphene on these properties has been studied by preparing and testing unfilled, 0.3 and 0.5 wt% graphene‐filled polyurethane foams. It was found that graphene is able, at very low content (0.3 wt%), to reduce the radiative contribution of the initial thermal conductivity by both decreasing the cell size and increasing the extinction coefficient. Due to the low graphene contents considered, no concerns about the solid‐phase contribution of thermal conductivity arise. Polyurethane–graphene nanocomposite foams showed also slower aging rate with respect to unfilled foams. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrophilic polymer foams with well‐defined open‐cell structure prepared from pickering high internal phase emulsions

Open‐cell hydrophilic polymer foams are prepared through oil‐in‐water Pickering high internal phase emulsions (HIPEs). The Pickering HIPEs are stabilized by commercial titania (TiO₂) nanoparticles with adding small amounts of non‐ionic surfactant Tween85. The morphologies, such as average void diameter and interconnectivity, of the foams can be tailored easily by varying the TiO₂ nanoparticles and Tween85 concentrations. Further, investigation of the HIPE stability, emulsion structure and the location of TiO₂ nanoparticles in resulting foams shows that the surfactant tends to occupy the oil‐water interface at the contact point of adjacent droplets, where the interconnecting pores are hence likely to be formed after the consolidation of the continuous phase. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Use of blowing catalysts for integral skin polyurethane applications in a controlled molecular architectural environment: Synthesis and impact on ultimate physical properties

Polyurethane elastomers of a controlled molecular architecture were synthesized using a two‐step polymerization technique. The building blocks of the elastomeric materials included urea–urethane prepolymers end‐capped with diisocyanate groups and had an exact number of urea groups at both ends. Two‐dimensional bifurcated hydrogen‐bonding networks incorporating the urea groups were, with differential scanning calorimetric and dynamic mechanical thermal analyzer techniques, responsible for the increase in the glass‐transition temperature (T_g) of the hard block and sharp interface morphology between the pure “hard” domains and pure “soft” domains. The higher extent of the phase separation between the two phases contributed to higher elastic moduli for the hard blocks and higher tensile strength for the elastomeric samples. Higher elongation values were attributed to the liberation of the elastomeric chain ends that otherwise would have been constrained in the interface region. The higher T_g values of the hard blocks corresponded to an increase in the hardness values and a decrease in the tear‐strength values. The increase in the amount of urea groups within the hard segments, as a result of the increased amount of water and blowing catalyst, resulted in elastomeric foams with higher open‐cell content. This resulted in lower resilience values as measured using the pendulum rebound test and was attributed to the ability of the open cells to absorb and dissipate energy. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2526–2536, 2002     

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