Development of thermally conductive polymer matrix composites by foaming‐assisted networking of micron‐ and submicron‐scale hexagonal boron nitride

Thermally conductive polymer matrix composite (PMC) foams with effective thermal conductivities (k_eff) higher than their solid counterparts have been developed for the first time. Using a material system consists of low density polyethylene and micron‐scale or submicron‐scale hexagon boron nitride platelets as a case example, this article demonstrates that foaming‐assisted filler networking is a feasible processing strategy to enhance PMC's k_eff, especially at a low hBN loading. Parametric studies were conducted to identify the structure‐to‐property relationships between foam morphology (e.g., cell population density, cell size, and foam expansion) and the PMC foam's k_eff. In particular, there exists an optimal cell size to maximize the PMC foam's k_eff for foams with up to 50% volume expansion. However, an optimal cell size is absent for PMC foams with higher volume expansion. X‐ray diffraction (XRD) analyses reveal that both the presence of hBN platelets and foam expansion promoted the crystallization of LDPE phase. Moreover, the XRD spectra also provide evidence for the effect of foam expansion on the orientation of hBN platelets. Overall, the findings provide new directions to design and fabricate thermally conductive PMC foams with low filler contents for heat management applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42910.     

Improving polypropylene microcellular foaming through blending and the addition of nano‐calcium carbonate

This article reports an attempt to improve polypropylene (PP) microcellular foaming through the blending of PP with high‐density polyethylene (HDPE) as a minor component and the incorporation of nano‐calcium carbonate (nano‐CaCO₃) into PP and its blends with HDPE. Three HDPEs were selected to form three blends with a viscosity ratio less than, close to, or greater than unity. Two concentrations of nano‐CaCO₃, 5 and 20 wt %, were used. The blends and nanocomposites were prepared with a twin‐screw extruder. The foaming was carried out by a batch process with supercritical carbon dioxide as a blowing agent. The online shear viscosity during compounding and the dynamic rheological properties of some samples used for foaming were measured. The cell structure of the foams was examined with scanning electron microscopy (SEM), and the morphological parameters of some foams were calculated from SEM micrographs. The rheological properties of samples were used to explain the resulting cell structure. The results showed that the blend with a viscosity ratio close to unity produced a microcellular foam with the minimum mean cell diameter (0.7 μm) and maximum cell density (1.17 × 10¹¹ cells/cm³) among the three blends. A foamed PP/nano‐CaCO₃ composite with 5 wt % nano‐CaCO₃ exhibited the largest cell density (8.4 × 10¹¹ cells/cm³). © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Structures and properties of closed‐cell polyimide rigid foams

Closed‐cell polyimide rigid foams with different Calculated Molecular Weight (Calc'd M_n) and foam density (ρ) have been prepared by thermal foaming of nadimide‐endcapped imideoligomers (NAIO) powder. The NAIO powder was obtained by thermally treating of a PMR poly(amide ester) solution derived from the reaction of diethyl ester of 2,3,3′,4′‐biphenyltetracarboxylic dianhydride (α‐BPDE) and p‐phenylenediamine (p‐PDA) using monoethyl ester of cis‐5‐norbornene‐ endo‐2,3‐dicarboxylic acid (NE) as reactive endcapping agent in ethyl alcohol. Effect of Calc'd M_n and foam density (ρ) on mechanical and thermal properties of the polyimide rigid foams have been systematically investigated. It was found that the thermal foaming properties of NAIO powders were affected by the Calc'd M_n. The appreciate Calc'd M_n could yield polyimide foams with both high closed‐cell content (C_c) (>80%) and outstanding mechanical properties. Moreover, the thermal properties were reduced by increasing of Calc'd M_n and the mechanical properties improved gradually by increasing foam densities. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3282–3291, 2013     

Physical properties of water‐blown rigid polyurethane foams containing epoxidized soybean oil in different isocyanate indices

To explore the potential of isocyanate usage reduction, water‐blown rigid polyurethane foams were made by replacing 0, 20, and 50% of Voranoll® 490 in the B‐side of the foam formulation by epoxidized soybean oil (ESBO) with an isocyanate index ranging from 50 to 110. The compressive strength, density, and thermal conductivity of foams were measured. The foam surface temperature was monitored before and throughout the foaming reaction as an indirect indication of the foaming temperature. Increasing ESBO replacement and/or decreasing isocyanate index decreased the foam's compressive strength. The density of the foam decreased while decreasing the isocyanate index to 60. Further decrease in isocyanate index resulted in foam shrinkage causing a sharp increase in the foam density. The thermal conductivity of foams increased while decreasing the isocyanate index and increasing the ESBO replacement. Mathematical models for predicting rigid polyurethane foam density, compressive strength, and thermal conductivity were established and validated. Similar to compressive strength, the foaming temperature decreased while decreasing the isocyanate index and increasing the ESBO replacement. Because of the lower reactivity of ESBO with isocyanate, the rate of foaming temperature decrease with decreasing isocyanate index was in the order of 0% > 20% > 50% ESBO replacement. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Foaming of aliphatic polyester using chemical blowing agent

Poly(butylene adipate‐co‐succinate) (PBAS), a saturated aliphatic polyester cured by dicumyl peroxide (DCP), was prepared and the viscoelastic property was investigated. The viscosity of crosslinked PBAS increased, and it exhibited rubbery behavior as the content of curing agent was increased. The results suggested that the viscosity and elasticity of PBAS could be regulated by adding a small amount of DCP; hence, the processibility could be improved. Prior to foaming, a proper formulation of blowing agent (blowing agent/urea activator = 100:8 phr) was examined to prepare expanded PBAS foam. Low‐density PBAS expanded foams were prepared using a chemical blowing agent and DCP. The effect of the foaming temperature, additive content, and curing agent content on the blowing ratio and morphology of expanded PBAS foams was investigated. A closed‐cell structure PBAS foam of high blowing ratio (density about 0.05 g/cm³) could be obtained by adding 3 phr DCP. To manufacture expanded PBAS foam under 0.1 g/cm³ using a chemical blowing agent, the storage modulus of the matrix polymer should exceed the loss modulus by enough to stabilize growing bubbles. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2443–2454, 2001     

Foamability and viscosity behavior of extrusion foamed PLA–pulp fiber biocomposites

This study addresses the effect of fiber reinforcement, chain extension, and physical foaming agent type on foam morphology and viscosity behavior of pulp fiber reinforced poly(lactic acid) (PLA) biocomposites. PLA reinforced with 0, 10, and 20 wt % of bleached kraft pulp fibers with and without chain extender were foamed using two different physical foaming agents (carbon dioxide and isobutane) by extrusion foaming. Densities, foam morphologies, and viscosities were systematically analyzed and compared from the produced foams. As a conclusion, low-density foams are produced with both foaming agents and fiber levels, fiber addition limiting cell growth. Isobutane provides better dimensional stability with narrower cell size distribution, whereas carbon dioxide enables lower foaming temperature. Chain extension is essential to achieve foam with low density and good cell structure. Contrary to nonchain extended PLA, addition of fibers reduced the viscosity of chain extended PLA. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48202.     

Preparation of microcellular poly(ethylene‐co‐octene) rubber foam with supercritical carbon dioxide

In the past 3 decades, there has been great advancement in the preparation of microcellular thermoplastic polymer foams. However, little attention has been paid to thermoplastic elastomers. In this study, microcellular poly(ethylene‐co‐octene) (PEOc) rubber foams with a cell density of 2.9 × 10¹⁰ cells/cm³ and a cell size of 1.9 μm were successfully prepared with carbon dioxide as the physical blowing agent with a batch foaming process. The microcellular PEOc foams exhibited a well‐defined, closed‐cell structure, a uniform cell size distribution, and the formation of unfoamed skin at low foaming temperatures. Their difference from thermoplastic foam was from obvious volume recovery in the atmosphere because of the elasticity of the polymer matrix. We investigated the effect of the molecular weight on the cell growth process by changing the foaming conditions, and two important effect factors on the cell growth, that is, the polymer matrix modulus/melt viscoelastic properties and gas diffusion coefficient, were assessed. With increasing molecular weight, the matrix modulus and melt viscosity tended to increase, whereas the gas solubility and diffusion coefficient decreased. The increase in the matrix modulus and melt viscosity tended to decrease the cell size and stabilize the cell structure at high foaming temperatures, whereas the increase in the gas diffusion coefficient facilitated cell growth at the beginning and limited cell growth because most of the gas diffused out of the polymer matrix during the long foaming times or at high foaming temperatures. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of glass transition temperature and saturation temperature on the solid‐state microcellular foaming of cyclic olefin copolymer

Effect of glass transition temperature and saturation temperature on the solid‐state microcellular foaming of cyclic olefin copolymer (COC)—including CO₂ solubility, diffusivity, cell nucleation, and foam morphology—were investigated in this article. COCs of low T_g (78°C) and high T_g (158°C) were studied. Solubilities are 20–50% higher in high T_g COC than in the low T_g COC across the saturation temperature range. Diffusivities are about 15% higher on average in high T_g COC for temperatures up to 50°C. A much faster increase of diffusivity beyond 50°C is observed in low T_g COC due to it being in the rubbery state. Under similar gas concentration, high T_g COC starts foaming at a higher temperature. And the foam density decreases faster in low T_g COC with foaming temperature. Also, high T_g COC foams show about two orders of magnitude higher cell nucleation density than the low T_g COC foams. The effect of saturation temperature on microcellular foaming can be viewed as the effect of CO₂ concentration. Nucleation density increases and cell size decreases exponentially with increasing CO₂ concentration. Uniform ultramicrocellular structure with an average cell size of 380 nm was created in high‐T_g COC. A novel hierarchical structure composed of microcells (2.5 μm) and nanocells (cell size 80 nm) on the cell wall was discovered in the very low‐density high‐T_g COC foams. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42226.     

Effects of clay dispersion on the foam morphology of LDPE/clay nanocomposites

Intercalated and exfoliated low‐density polyethylene (LDPE)/clay nanocomposites were prepared by melt blending with and without a maleated polyethylene (PE‐g‐MAn) as the coupling agent. Their morphology was examined and confirmed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of clay content and dispersion on the cell morphology of nanocomposite foams during extrusion foaming process were also thoroughly investigated, especially with a small amount of clay of 0.05–1.0 wt%. This research shows the optimum clay content for achieving microcellular PE/clay nanocomposite foams blown with supercritical CO₂. It is found that < 0.1 wt% of clay addition can produce the microcellular foam structure with a cell density of > 10⁹ cells/cm³ and a cell size of ～ 5 μm. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2129–2134, 2007     

The effect of curing system on the properties of the thermoplastic elastomer foams of ethylene‐vinyl acetate copolymer and styrene‐butadiene and ethylene‐propylene‐diene monomer rubbers

Thermoplastic elastomer (TPE) foams have important application in electrical, toys, and other industries. Several foams were prepared by ethylene‐vinyl acetate copolymer (EVA) lonely, and in combination with styrene‐butadiene and ethylene‐propylene‐diene monomer rubbers (SBR and EPDM). The effects of crosslinking and foaming agents and EVA type on density and mechanical properties of the cured foams with two curing systems, peroxide and sulfur‐peroxide with potential use in automotive applications, were studied. The results showed that proposed compounds formulations were foamed properly. The viscosity of the EVA was a key factor for the density values of the formed foams. The densities of the cured foams with peroxide system with various SBR contents were higher when compared with cured foams with sulfur‐peroxide system. With increasing foaming agent, the densities of the foams were reduced for studied curing systems. The densities of the EVA–EPDM foams were lower than those of the EVA–SBR foams in the same studied conditions. Increasing rubber in foam formulation had adverse effect on tensile properties of the foams. The existence of the talc powder in foam formulation had important role on the shape and type of the formed cells and resulted in foams with mostly closed cells. The results of this study help the automotive article designer to produce suitable TPE foam. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45357.     

Graphene nanoribbons (GNRs) by unzipping MWCNTs for the improvement of PMMA microcellular foams

This work presents the cellular microstructures and properties of PMMA/graphene nanoribbons (GNRs) microcellular foams. GNRs were obtained by oxidative unzipping multiwalled carbon nanotubes and solvent thermal reduction in dimethylformamide (DMF), then they were mixed with PMMA to fabricate PMMA/GNRs nanocomposites by solution blending. Subsequently, supercritical carbon dioxide (scCO₂) as a friendly foaming agent was applied to fabricate PMMA/GNRs microcellular foam by a batch foaming in a special mold. The morphology of cell structure was analyzed by scanning electron microscopy and image software, showing that the addition of a smaller content of GNRs caused a fine cellular structure with a higher cell density (～3 × 10¹¹ cells/cm³) and smaller cell sizes (～1 μm) due to their remarkable heterogeneous nucleation effect. The mechanical testing of PMMA/GNRs microcellular foams demonstrated that the obtained GNRs also could be used as a reinforcing filler to increase the mechanical properties of PMMA foams. An improvement in the compressive strength of ～80% (about 39% increase standardized by specific compressive strength) was achieved by 1.5 wt % GNRs addition, and the thermal stability of PMMA/GNRs foams was enhanced too. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45182.     

Nanoplatelet reinforcement of cavity cell walls in polymer foams using carbon dioxide supercritical fluid

Reinforcing the cavity cell walls of polymer foams using nanoparticles can offer a new era for the property‐structure‐processing field in the development of functionalized ultra‐light components and devices manufactured from foam. When the nanoparticles are exfoliated in polymers, the viscosity substantially increases and thus mixing or foaming usually becomes almost impossible. We use CO₂ supercritical fluid (CO₂ SCF) for the mixing and foaming of poly(ethylene‐vinyl acetate) copolymer (EVA) with montmorillonite (MMT) nanoplatelets. The in situ evaporation of CO₂ induces robust cavity cells of the EVA/MMT nanocomposite foam in a stable form of spherical shapes, which are seldom achieved by other methods. As the bubble grows and becomes stabilized in CO₂ SCF, the exfoliated MMT nanoparticles are aligned at the cell walls by the Gibbs adsorption principle to minimize the surface energy at the gas–liquid interface and increase the rupture strength of the cavity walls. It is demonstrated that the developed methodology can be successfully used for foaming EVA containing high vinyl acetate (VA) content (>40%). Since EVA is too soft to construct cell walls of foam using conventional methods, the applicability of the developed methodology is extensively broadened for superior adhesion and compatibility with other materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46615.     

Porosity effect on microstructure,mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting

In this study, Ti₂AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k₁) and non‐Darcian (k₂) permeability coefficients displayed values in the range 0.30‐93.44 × 10^?11 m² and 0.39‐345.54 × 10^?7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti₂AlC foams.     

Microcellular foaming of low‐density polyethylene using nano‐CaCo3 as a nucleating agent

In this study, microcellular foaming of low‐density polyethylene (LDPE) using nano‐calcium carbonate (nano‐CaCO₃) were carried out. Nanocomposite samples were prepared in different content in range of 0.5–7 phr nano‐CaCO₃ using a twin screw extruder. X‐ray diffraction and scanning electron microscopy (SEM) were used to characterize of LDPE/nano‐CaCO3 nanocomposites. The foaming was carried out by a batch process in compression molding with azodicarbonamide (ADCA) as a chemical blowing agent. The cell structure of the foams was examined with SEM, density and gel content of different samples were measured to compare difference between nanocomposite microcellular foam and microcellular foam without nanomaterials. The results showed that the samples containing 5 phr nano‐CaCO₃ showed microcellular foam with the lowest mean cell diameter 27 μm and largest cell density 8 × 10⁸ cells/cm³ in compared other samples. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Extrusion of polystyrene nanocomposite foams with supercritical CO2

Intercalated and exfoliated polystyrene/nano‐clay composites were prepared by mechanical blending and in situ polymerization respectively. The composites were then foamed by using CO₂ as the foaming agent in an extrusion foaming process. The resulting foam structure is compared with that of pure polystyrene and polystyrene/talc composite. At a screw rotation speed of 10 rpm and a die temperature of 200°C, the addition of a small amount (i.e., 5 wt%) of intercalated nano‐clay greatly reduces cell size from 25.3 to 11.1 μm and increases cell density from 2.7 × 10⁷ to 2.8 × 10⁸ cells/cm³. Once exfoliated, the nanocomposite exhibits the highest cell density (1.5 × 10⁹ cells/cm³) and smallest cell size (4.9 μm) at the same particle concentration. Compared with polystyrene foams, the nanocomposite foams exhibit higher tensile modulus, improved fire retardance, and better barrier property. Combining nanocomposites and the extrusion foaming process provides a new technique for the design and control of cell structure in microcellular foams.     

A conductive foam: Based on novel poly(styrene‐b‐butadiene‐co‐styrene‐b‐styrene) tri‐block copolymer filled by carbon black

In this article, a conductive foam based on a novel styrene‐based thermoplastic elastomer called poly(styrene‐b‐butadiene‐co‐styrene‐b‐styrene) tri‐block copolymer S(BS)S was prepared and introduced. S(BS)S was particularly designed for chemical foaming with uniform fine cells, which overcame the shortcomings of traditional poly(styrene‐b‐butadiene‐b‐styrene) tri‐block copolymer (SBS). The preparation of conductive foams filled by the carbon black was studied. After the detail investigation of cross‐linking and foaming behaviors using moving die rheometer, the optimal foaming temperature was determined at 180°C with a complex accelerator for foaming agent. Scanning electron microscopy (SEM) images shown that the cell bubbles of conductive foam were around 30–50 µm. The conductivity of foams was tested using a megger and a semiconductor performance tester. SEM images also indicated that the conductivity of foams was mainly affected by the distribution of carbon black in the cell walls. The formation of the network of the carbon black aggregates had a contribution to perfect conductive paths. It also found that the conductivity of foams declined obviously with the foaming agent content increasing. The more foaming agent led to a sharp increasing of the number of cells (from 2.93 × 10⁶ to 6.20 × 10⁷ cells/cm³) and a rapid thinning of the cell walls (from 45.3 to 1.4 µm), resulting in an effective conductive path of the carbon black no forming. The conductive soft foams with the density of 0.48–0.09 g/cm³ and the volume resistivity of 3.1 × 10³?2.5 × 10⁵ Ω cm can be easily prepared in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41644.     

Polylactide foams prepared by a traditional chemical compression‐molding method

To reduce environmental pollution and oil shortages, biodegradable polylactide (PLA) from plants was used to replace synthetic plastic from petroleum. In this study, high‐melt‐viscosity PLA was achieved through the in situ reaction of carboxyl‐ended polyester (CP) and solid epoxy (SE) first; then, PLA foams were successfully prepared by a chemical compression‐molding method. The detailed foaming factors were also studied, including the decomposition temperature of the blowing agent, the foam temperature, and the open‐mold temperature. The results reveal that the obtained PLA foams had good water absorption and degradable properties, and the foam density was low as 0.16 g/cm³. Moreover, the effects of the CP/SE concentration and the AC content on the properties of the foams were also investigated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Batch foaming of SAN/clay nanocomposites with scCO2: A very tunable way of controlling the cellular morphology

This paper aims at elucidating some important parameters affecting the cellular morphology of poly(styrene-co-acrylonitrile) (SAN)/clay nanocomposite foams prepared with the supercritical CO₂ technology. Prior to foaming experiments, the SAN/CO₂ system has first been studied. The effect of nanoclay on CO₂ sorption/desorption rate into/from SAN is assessed with a gravimetric method. Ideal saturation conditions are then deduced in view of the foaming process. Nanocomposites foaming has first been performed with the one-step foaming process, also called depressurization foaming. Foams with different cellular morphology have been obtained depending on nanoclay dispersion level and foaming conditions. While foaming at low temperature (40 °C) leads to foams with the highest cell density (∼10¹²-10¹⁴ cells/cm³), the foam expansion is restricted (d∼0.7-0.8 g/cm³). This drawback has been overcome with the use of the two-step foaming process, also called solid-state foaming, where foam expansion occurs during sample dipping in a hot oil bath (d∼0.1-0.5 g/cm³). Different foaming parameters have been varied, and some schemes have been drawn to summarize the characteristics of the foams prepared - cell size, cell density, foam density - depending on both the foaming conditions and nanoclay addition. This result thus illustrates the huge flexibility of the supercritical CO₂ batch foaming process for tuning the foam cellular morphology.     

Solid‐state microcellular polycarbonate foams. I. The steady‐state process space using subcritical carbon dioxide

The process parameters for production of solid‐state microcellular polycarbonate using subcritical CO₂ were explored. Sufficiently long foaming times were used to produce foams, where cell growth had completed, resulting in steady‐state structures. A wide range of foaming temperatures and saturation pressures below the critical pressure of CO₂ were investigated, establishing the steady state process space for this polymer–gas system. Processing conditions are presented that produce polycarbonate foams where both the foam density and the average cell size can be controlled. The process space showed that we could produce foams at a constant density, while varying the cell size by and order of magnitude. At a relative density of 0.5, the average cell size could be varied from 4 to 40 μm. The ability to produce such a family of foams opens the possibility to explore the effect of microstructure, like cell size on the properties of cellular materials. It was found that the minimum foaming temperature for a given concentration of CO₂, determined from the process space, agrees well with the predicted glass transition temperature of the gas–polymer solution. A characterization of the average cell size, cell size distribution, and cell nucleation density for this system is also reported. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Polypropylene–nano‐silica nanocomposite foams: mechanisms underlying foamability,and foam microstructure,crystallinity and mechanical properties

We investigate the production and characterization of foams prepared from polypropylene (PP) as well as PP–silica nanocomposites containing different loadings of nano‐silica. This study was carried out to investigate the mechanisms underlying the production of foams with a regular cell structure through the use of nano‐scale fillers. Foaming was carried out in batch mode using an autoclave with CO₂ as the physical blowing agent; high pressures of the order of 14 MPa were achieved through a combination of active pressurization and the use of high foaming temperatures. The resulting PP nanocomposite foams were characterized in detail to quantify the effect of the nano‐silica loading on the foam density and mechanical, morphological and thermal properties. The addition of nano‐silica in PP resulted in the improvement of foam quality – as assessed from the well‐defined and regular cell structures with absence of cell coalescence – as well as an increase in expansion ratio and decrease in foam density. Careful analyses of trends in cell size, cell density and expansion ratio of the foams were correlated with measurements of melt rheology and nano‐filler morphology of the unfoamed specimens in order to identify subtle details regarding the role of silica nanoparticles in improving foam quality. © 2019 Society of Chemical Industry