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Highlights Syntactic foams are lightweight porous composites obtained by the addition of hollow glass microspheres (HGMs) to a polymer matrix. This work experimentally and numerically investigates the thermal behavior of epoxy‐based syntactic foams with enhanced multifunctionality produced by incorporating microencapsulated phase change materials (PCMs) as functional fillers, providing thermal energy storage and temperature stabilization due to its large latent heat of melting. Foams are fabricated using varying ratios of HGMs (0–40 vol%) and PCM (0–40 vol%) to achieve tuned densities and thermal properties with a total filler content of up to 40 vol%. A one‐dimensional numerical heat transfer model based on the enthalpy method is presented to simulate the thermal behavior of these materials. The model accurately incorporates the specific heat and thermal conductivity of the foam components, as well as the latent heat absorption during PCM melting (up to 68 J/g). The model is experimentally validated by demonstrating good agreement with the measurements of transient temperature profiles during heating tests on the foam samples. The validated tool is then leveraged to model the thermal response of the foams under a sinusoidally varying heat source, similar to the possible real applicative scenarios. These results can help optimize foam compositions balancing energy storage density, heat transfer properties, and structural support for lightweight energy storage systems. Potential applications include high‐efficiency thermal management in battery packs, electronic cooling, solar energy storage, and sustainable temperature‐controlled buildings. Foams with 0–40 vol% PCM and 0–40 vol% HGM show up to 68 J/g latent heat. 1D numerical model accurately captures thermal conductivity and PCM phase change. Validated model simulates thermal response under sinusoidal heat, optimizing foam composition. Foams act as thermal reservoirs, maintaining up to 5°C lower surface temperatures than epoxy. [ABSTRACT FROM AUTHOR]

Aluminum smelter waste (ASW) is a big contributor to landfill and its recycling has been of great interest. This study characterizes the ASW powder, manufactures aluminum matrix syntactic foams using ASW, and investigates their mechanical properties. Different particle size groups of the ASW powder contain different impurities and porosities, leading to different strengths. The mechanical behaviors of the ASW syntactic foams are similar to existing metal matrix syntactic foams. The compressive yield strength, plateau strength, and specific energy absorbed vary in the ranges of 111–398, 33–145 MPa, and 18–71 MJ m−3. The flexural strength and flexural modulus vary in the ranges of 8–138 MPa and 13–72 GPa. The peak impact strength and impact energy in drop‐weight impact vary in the ranges of 192–547 MPa and 16–29 J. Particle size and heat treatment of the ASW powder have significant effects on the properties and brittleness of the resultant syntactic foam. [ABSTRACT FROM AUTHOR]

Over the last ten years, magnesium (Mg)-based syntactic foams (SFs) have gained significant attention and their popularity continues to grow. This is because they possess unique properties such as high mechanical strength and are lightweight, making them potential candidates for applications in various industries, including aerospace, automotive, and biomedical (especially in orthopedics). This article reviews and discusses different fabrication techniques used in producing magnesium-matrix syntactic foams (Mg-MSFs). These techniques include stir casting, disintegrated melt deposition, powder metallurgy, and melt infiltration. The review comprehensively analyzes microstructure specifications, mechanical properties, and corrosion behavior exhibited by Mg-MSFs fabricated to date. The findings suggest that the properties of these foams, including microstructural characteristics, mechanical properties, and corrosion behavior, are significantly influenced by factors such as filler particle amounts and properties, Mg alloy-matrix specifications, fabrication techniques, process parameters, and post-processing treatments (such as annealing and sintering). These factors play a crucial role in determining the final characteristics of the syntactic foams. While Mg-MSFs hold substantial importance and potential, a limited body of research exists in this area. Therefore, more research is necessary to comprehensively understand these structures, which will facilitate their effective utilization in both industrial and biomedical applications.

This paper proposes an innovative multi-material approach for introducing auxetic behaviour to syntactic foams (SFs). By carefully designing the size, shape, and orientation of the SFs, auxetic deformation behaviour was induced. Re-entrant hexagon-shaped SF elements were fabricated using expanded perlite (EP) particles and a plaster of Paris slurry first. Then, an auxetic pattern of these SF elements was arranged within a stainless-steel casting box. The empty spaces between the SF elements were filled with molten aluminium alloy (A356) using the counter-gravity infiltration casting technique. The cast auxetic composite had a bulk density of 1.52 g/cm3. The cast composite was then compressed under quasi-static loading to characterise its deformation behaviour and to determine the mechanical properties, especially the Poisson's ratio. The cast composite deformation was auxetic with a Poisson's ratio of −1.04. Finite Element (FE) simulations were conducted to understand the deformation mechanism better and provide means for further optimisation of the geometry. [ABSTRACT FROM AUTHOR]

Herein, a new lightweight syntactic foam is reported with strong mechanical properties, unique multifunctionalities, and recyclability. Multifunctionality of materials and structures has gained ever‐increasing interest as an excellent approach to designing minimalistic systems. Inspired by nature, these materials can perform multiple functions besides bearing a load. Due to their shape‐changing and damage‐healing property, shape memory vitrimers (SMVs) are a great example of multifunctional materials readily exploited for many applications. Using nickel and silver‐plated hollow glass microbubbles (HGMs), an SMV‐based syntactic foam is introduced here that supplements the multifunctionality of SMVs with electrical conductivity and ferromagnetism, which enables a series of additional potentials such as strain sensing, damage monitoring, Joule heating, and electromagnetic interference shielding. Despite its low density and outstanding mechanical properties, this foam exhibits shape memory behavior, which can be triggered by an electrical current, and damage healing capability due to its reversible dynamic covalent bonds. Especially its recyclability makes recycling the expensive silver‐coated and nickel‐coated HGMs feasible, making this foam cost‐effective and environmentally sustainable. With its many features and economical manufacturability, this syntactic foam has a potential to be utilized in many applications, ranging from aerospace structures to biomedical devices to household items. [ABSTRACT FROM AUTHOR]

The current scenario focuses on lightweight and better energy-absorbing structural materials for critical engineering applications in aerospace, automobiles, marine, electronic, and implant materials for biomedical industries. The magnesium-based syntactic foams are the preferred choice of material in the same category due to their lightweight, higher plateau strength, better damping characteristics, and excellent energy absorption behavior. Many works were reported based on the polymer and aluminum-based syntactic foams and their developments. However, the rapidly increasing interest in magnesium-based syntactic foams and the adoption of the applications are still at a nascent stage. This proposed review work comprehensively insights the magnesium-based syntactic foam; various hollow filler material usages and summarizes the influence of physical and mechanical behavior on processing routes and future research opportunities for applications. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative multi-material approach for introducing auxetic behaviour to syntactic foams (SFs). By carefully designing the size, shape, and orientation of the SFs, auxetic deformation behaviour was induced. Re-entrant hexagon-shaped SF elements were fabricated using expanded perlite (EP) particles and a plaster of Paris slurry first. Then, an auxetic pattern of these SF elements was arranged within a stainless-steel casting box. The empty spaces between the SF elements were filled with molten aluminium alloy (A356) using the counter-gravity infiltration casting technique. The cast auxetic composite had a bulk density of 1.52 g/cm3. The cast composite was then compressed under quasi-static loading to characterise its deformation behaviour and to determine the mechanical properties, especially the Poisson’s ratio. The cast composite deformation was auxetic with a Poisson’s ratio of −1.04. Finite Element (FE) simulations were conducted to understand the deformation mechanism better and provide means for further optimisation of the geometry.

The present review introduces the analysis and gathering modern ideas about the novel polymer composites, namely polymer syntactic foams, known as extremely widely used multifunctional materials with unique properties that meet the requirements of advanced high-tech industries. The main classifications, methods of synthesis and characteristization of various types of polymer foams were summarized. Special attention has been paid to structural features and properties of polymer syntactic foams. The main types of polymer matrices and fillers used for synthesis of thermosetting syntactic foams were characterized in details. The influence of initial composition and fabrication techniques applied on morphology, physical-chemical properties and application of composite materials comprising polymer syntactic foams in high-tech industries such as marine and underwater, aerospace, defense and transport has been also established. The review also highlights the analysis of world markets, trends and forecasting the further development of syntactic foams, especially high-performanced ones, disclosing their advantages and disadvantages. Presumable ways of modification of the existing types of polymer syntactic foams as well as perspectiveness of further progress in this research area to obtain the improved high-performance materials are also outlined. [ABSTRACT FROM AUTHOR]

polypropylene (PP) syntactic foams (SFs) containing hollow glass microspheres (HGMs) possess low density and elevated mechanical properties, which can be tuned according to the specific application. A possible way to improve their multifunctionality could be the incorporation of organic Phase Change Materials (PCMs), widely used for thermal energy storage (TES) applications. In the present work, a PCM constituted by encapsulated paraffin, having a melting temperature of 57 °C, was embedded in a compatibilized polypropylene SF by melt compounding and hot pressing at different relative amounts. The rheological, morphological, thermal, and mechanical properties of the prepared materials were systematically investigated. Rheological properties in the molten state were strongly affected by the introduction of both PCMs and HGMs. As expected, the introduction of HGMs reduced both the foam density and thermal conductivity, while the enthalpy of fusion (representing the TES capability) was proportional to the PCM concentration. The mechanical properties of these foams were improved by the incorporation of HGMs, while they were reduced by addition of PCMs. Therefore, the combination of PCMs and HGMs in a PP matrix generated multifunctional materials with tunable thermo-mechanical properties, with a wide range of applications in the automotive, oil, textile, electronics, and aerospace fields. [ABSTRACT FROM AUTHOR]

The dielectric characteristics of syntactic foams based on Viksint K-18 sealant filled with hollow glass, ceramic, and phenol–formaldehyde microspheres are considered. The properties of syntactic foams in the microwave range of radio frequencies from 8 to 12 GHz have been studied. The influence of the concentration of hollow microspheres on the tangent of the dielectric-loss angle and dielectric permittivity has been studied. It is shown that the introduction of hollow microspheres leads to a decrease in the permittivity and dielectric-loss tangent of syntactic foams. [ABSTRACT FROM AUTHOR]

This work analyzes the microstructure, porosity, and mechanical properties of syntactic foams obtained infiltrating 3 mm in diameter Fe hollow spheres with an Al–12Si eutectic alloy. The hollow spheres were first sintered at 800, 900, and 1000 °C for 1h to obtain composite porous preforms, focusing on the effect of this sintering on the manufactured syntactic foams. Results showed that the increase in the sintering temperature led to an increase in the porosity from 51 to 56 %, also increasing the percentage of Fe in the solid part of the foams and decreasing the pore cell thickness. This led to significantly higher strength and a brittle behavior for the foams obtained using spheres sintered at 1000 °C, while the foams manufactured using lower temperatures presented a ductile behavior and lower mechanical properties. α (Al 8 Fe 2 Si) and β (Al 5 FeSi) were obtained during infiltration, decreasing the wall thickness of the Fe spheres. [Display omitted] • Syntactic foams successfully manufactured using preforms of sintered Fe hollow spheres. • Sintering temperature influenced the microstructure and the mechanical properties. • α (Al 8 Fe 2 Si) and β (Al 5 FeSi) were obtained during infiltration as reaction products. • Combined, sintering and infiltration controlled the mechanical properties. [ABSTRACT FROM AUTHOR]

Syntactic foams and their reinforcement with cellulose nanocrystals (CNC) in a thermosetting polyester resin (PR) matrix were studied in order to provide a pathway for lightweight composites for automotive applications with enhanced performance including thermomechanical properties and water uptake. A novel method for coating hollow glass spheres (HGS) with CNC was developed. Various combinations of functionalized CNC and surface modified HGS were studied to determine interactions between the two materials. Surface analysis techniques indicated that CNC were successfully deposited onto the HGS surface and remained at the surface after compounding with PR. Dynamic mechanical analysis of composites samples indicated a 30% increase in high temperature storage modulus between CNC coated HGS samples and uncoated HGS samples. Shifts in tan(δ) peak intensity and beta-relaxation temperature also indicated an increase in interphase volume around CNC coated HGS as compared to uncoated HGS samples. Coating HGS with CNC reduced the maximum water adsorption by as much as 40% indicating the potential for this material system in high moisture environments. [ABSTRACT FROM AUTHOR]

Syntactic foams, low density composites consisting of hollow microballoons or microspheres dispersed in a matrix, find application in various fields. The properties of these light weight composites can be easily tuned by suitably selecting the matrix and the hollow microsphere filler and their composition. Among the various matrices employed in syntactic foams, phenolic resins have enticed the researchers owing to their salient features viz. high thermal stability, high char yield, structural integrity etc. This review gives an overview of phenolic syntactic foams with a focus on various phenolic resin based syntactic foams and modified syntactic foams. Finally, applications of phenolic syntactic foams are also covered. [ABSTRACT FROM AUTHOR]

This paper presents the usage of spark plasma sintering (SPS) as a method to obtain aluminum-expanded perlite syntactic foams with high porosity. In the test samples, fine aluminum powder with flaky shape particles was used as matrix material and natural, inorganic, granular, expanded perlite was used as a space holder to ensure high porosity (35–57%) and uniform structure. SPS was used to consolidate the specimens. The structures were characterized by scanning electron microscopy and compression tests. Energy absorption (W~7.49 MJ/m3) and energy absorption efficiency (EW < 90%) were also determined. [ABSTRACT FROM AUTHOR]

Syntactic foams based on hollow ceramic microspheres and ceramic-forming binding polycarbosilane, capable of transitioning into silicon carbide at heightened temperatures are considered. The thermal processing of composites and the transition of polycarbosilane to silicon carbide are considered. The physicomechanical and thermophysical characteristics of syntactic foams were investigated. [ABSTRACT FROM AUTHOR]

The hollow glass microsphere (HGM) containing polymer materials, which are named as syntactic foams, have been applied as lightweight materials in various fields. In this study, carboxyl group-containing hyperbranched polymer (HBP) was added to a glass fiber (GF)-reinforced syntactic foam (RSF) composite for the simultaneous enhancement of mechanical and rheological properties. HBP was mixed in various concentrations (0.5–2.0 phr) with RSF, which contains 23 wt% of HGM and 5 wt% of GF, and the rheological, thermal, and mechanical properties were characterized systematically. As a result of the lubricating effect of the HBP molecule, which comes from its dendritic architecture, the viscosity, storage modulus, loss modulus, and the shear stress of the composite decreased as the HBP content increased. At the same time, because of the hydrogen bonding among the polymer, filler, and HBP, the compatibility between filler and the polymer matrix was enhanced. As a result, by adding a small amount (0.5–2.0 phr) of HBP to the RSF composite, the tensile strength and flexural modulus were increased by 24.3 and 9.7%, respectively, and the specific gravity of the composite was decreased from 0.948 to 0.917. With these simultaneous effects on the polymer composite, HBP could be potentially utilized further in the field of lightweight materials. [ABSTRACT FROM AUTHOR]

Fabrication of lightweight composites using thermally responsive expandable microspheres (EM) embedded in an elastomeric matrix to form syntactic foams offers significant opportunities to create functional and structural composites for diverse applications. The morphology of the EM-elastomer composite on the micro- and macro-scales is significantly influenced by the fabrication sequence and parameters (composition, expansion time/temperature, etc.). Herein, we report the morphological evolution of an EM-elastomer composite through the fabrication processes and elucidate the interaction of relevant fabrication parameters. Unlike the majority of published reports on hard EM composites, the in-situ expansion of the closed-cell foam structure within a soft matrix leads to larger void sizes, and the concomitant expansive force of the EM impacts the polymer chain mobility within the soft composite. At the same time, higher EM loading improves the mechanical properties; the evolved microstructure leads to a lightweight but stiff structure. For the first time the report details the thermal expansion of EM under applied pretension, leading to irreversible prestrain-dependent morphological changes toward anisotropic mechanical properties. Additionally, the composites are shaped under constrained expansion. The study provides a comprehensive guide and expands the pathways for the practical application of EM-embedded soft syntactic foams in aerospace, electronics, wearables, robotics, etc. [Display omitted] • Thermally expandable microspheres (EM) can undergo massive volume expansion when subjected to high temperatures. • In-vitro and in-situ expansion of EM and EM- elastomer composites are studied to elucidate their morphology and properties. • Tunable morphology, anisotropy, and shapability offer a versatile route to create ultralightweight functional materials. [ABSTRACT FROM AUTHOR]

Metal matrix syntactic foams (MMSFs) are advance hybrid materials which reflect synergetic combination of particle reinforced composites and close cell metal foams. Recently, MMSFs have become considerably popular for several industrial areas because of their low density, high compression strength, good ductility and perfect energy absorption ability. In this paper, Al 7075/bubble alumina syntactic foams were fabricated by recyclable pressure infiltration casting method. Macroscopic and microscopic investigations showed that almost flawless infiltration was obtained between 2 - 4 mm hollow bubble alumina spheres and total porosity values of fabricated foams varied between % 43.2 and % 45.6. As for mechanical analyses, all foam samples were subjected to quasi-static compression test (1 mm/min deformation rate) and their crucial properties such as compression strength, plateau strength, densification strain and energy absorption capacity were determined. Besides, aging heat treatment (T6) was applied to certain samples in order to explore probable effects of heat treatment on the mechanical responses. The outcomes indicated that there was a positive relationship between the aging treatment and compressive features. Under the compressive loading, even though T6 treated foams showed a tendency to brittle fracture, their as-cast variants exhibited ductile behavior. [ABSTRACT FROM AUTHOR]

The growing need for high thermal and mechanical resistant lightweight syntactic foams demands the designing of new materials. Herein, we report the successful fabrication of lightweight and high thermal resistance syntactic foams with excellent thermal and good mechanical properties. For this end, 10–50 wt% hollow glass microsphere (HGMS) containing polybenzoxazine based syntactic foams were prepared by using bisphenol‐A‐aniline (BA‐a) and phenol‐diaminodiphenylmethane (P‐ddm) benzoxazine via curing at 10 MPa compression molding. The curing behavior, tensile, flexural, compressive, and impact strength, thermal properties, and fracture morphology of the syntactic foams were studied by DSC, universal material testing machine, and drop hammer impact testing machine, TGA, and SEM, respectively. The curing temperatures of BA‐a and P‐ddm benzoxazine resins were slightly increased after blending the HGMS. The highest decline (59%) in the density and highest void contains (13.17%) was recorded for the poly(P‐ddm/HGMS50) sample. The excellent specific mechanical properties of the syntactic foams were observed on the 30 wt% HGMS loading. The thermal conductivity of the foams was gradually decreased and the lowest values of 0.106 and 0.0985 W/m.K were observed for poly(BA‐a/HMGS50) and poly(P‐ddm/HGMS50) sample, respectively. Furthermore, a gradual improvement in the thermal properties was observed as the loading of HGMS increased. [ABSTRACT FROM AUTHOR]

The paper presents the method of analysis and a comparison of the effectiveness of modified and conventional syntactic foams. The method employs the two-phase superposition approach recently developed for composite materials consisting of several concentric phases. Conventional syntactic foams consisting of spherical voids surrounded by thin glass shells embedded in the matrix are compared to modified foams with cylindrical or spheroidal voids. Modified syntactic foams analyzed in the paper include foams with cylindrical voids aligned along the applied stress or perpendicular to the stress and foams with randomly oriented cylindrical voids. It is demonstrated that while conventional syntactic foams with spherical voids absorb more energy than their modified counterparts, the stiffness of such foams is compromised due to the presence of voids to a larger degree than in modified foams. Accordingly, modified syntactic foams may appear a better compromise if a high energy absorption has to be combined with a prescribed large stiffness of the material. [ABSTRACT FROM AUTHOR]

A syntactic foam was prepared from an epoxy resin matrix and modified hollow glass microsphere fillers. Modification by silane coupling agents with different molecular structures was analyzed, and the optimal content of the silane coupling agent was determined. The results demonstrated that all silane coupling agents enhanced the adhesion between the hollow glass microspheres and epoxy resin matrix, resulting in enhanced water absorption, compressive performance, tensile performance, and bending performance compared to those prepared using unmodified hollow glass microspheres. Among silane coupling agents with different end groups, the one with a sulfhydryl end group exhibited optimal modification for hollow glass microspheres. Among the silane coupling agents with different backbone structures, the one with silanol groups exhibited the optimal modification of hollow glass microspheres. Additionally, the performance of the syntactic foams was optimal when 6% of the silanol-containing coupling agent was used. The results demonstrated that syntactic foams prepared with hollow glass microspheres modified by silane coupling agents exhibited improvements in water absorption, compressive performance, tensile performance, and bending performance, compared with those prepared using unmodified hollow glass microspheres. Among silane coupling agents with different end structures, the one with a sulfhydryl group as end group showed the best modification effect on hollow glass microspheres. The water absorption was 0.35%, the compressive strength was 62.15 MPa, the tensile strength was 40.15 MPa, and the bending strength was 53.17 MPa. Among silane coupling agents with different backbone structures, the one with silanol groupsbonds showed the best results. Its compressive strength was up to 64.15 MPa, the tensile strength was 35.47 MPa, and the bending strength was 53.99 MPa. [ABSTRACT FROM AUTHOR]

In recent years, there has been an increasing interest in the study of lightweight syntactic foams (SFs) because of their tailorable properties, quality characteristics and suitable applications in diverse technology industry. A considerable amount of literature has been published on SFs with most of these studies focusing on particular aspect like the matrices types (polymer, metal, ceramics, bio and hybrids); filler types (microparticles/microballoons/microspheres); nanoadditives types (nanotubes, nanofibers, nanoparticles); types of production techniques (powder metallurgy, infiltration and casting, mechanical molding and chemical deposition); and industry applications (aerospace, marine, building and construction, automotive and electrical and electronics). However, much of the literatures up to now has been research articles and few review articles. And there has been no attempt to comprehensively discuss SFs touching on the main research interest in a robust manner. Thus far, this study, brings a comprehensive insight toward the isolated research areas in SFs and then, compares and summarizes outcomes and inference from previous and current studies. And recommends on future research.

polypropylene (PP) syntactic foams (SFs) containing hollow glass microspheres (HGMs) possess low density and elevated mechanical properties, which can be tuned according to the specific application. A possible way to improve their multifunctionality could be the incorporation of organic Phase Change Materials (PCMs), widely used for thermal energy storage (TES) applications. In the present work, a PCM constituted by encapsulated paraffin, having a melting temperature of 57 °C, was embedded in a compatibilized polypropylene SF by melt compounding and hot pressing at different relative amounts. The rheological, morphological, thermal, and mechanical properties of the prepared materials were systematically investigated. Rheological properties in the molten state were strongly affected by the introduction of both PCMs and HGMs. As expected, the introduction of HGMs reduced both the foam density and thermal conductivity, while the enthalpy of fusion (representing the TES capability) was proportional to the PCM concentration. The mechanical properties of these foams were improved by the incorporation of HGMs, while they were reduced by addition of PCMs. Therefore, the combination of PCMs and HGMs in a PP matrix generated multifunctional materials with tunable thermo-mechanical properties, with a wide range of applications in the automotive, oil, textile, electronics, and aerospace fields.

The current focus on low-cost and high strength materials for advanced engineering applications attracted the research on the development of metal matrix syntactic foams. It is due to its highly tailorable properties. They are lightweight, higher ductility, quasi-static and dynamic compressive strength, better damping and energy absorption behavior and plateau strength. Different hollow particles were tried in the development of metal-based closed-cell syntactic foams such as cenosphere, ceramic, hollow glass particles, and expanded clay particles. Many researchers studied the physical and mechanical properties of polymer matrix syntactic foam. Very few reports are available on metal matrix syntactic foams, which remain a challenge to explore the properties. This present study provides a comprehensive insight into metal matrix syntactic foams, fabrication techniques and its influence on the mechanical properties, future scope, and its applications. [ABSTRACT FROM AUTHOR]

The syntactic foams are widely used in aeronautics, underwater vehicle structures, and oil drilling applications. These foams are being extensively utilized in naval applications wherein they are subjected to the compressive forces that are depth-dependent. Developing graded foams with better compressive behavior using three-dimensional printing (3DP) permits realizing complex geometrical structures with numerous advantages compared to conventional processing routes. The present work deals with 3DP of syntactic foams and their graded configuration by embedding (20, 40, and 60 vol%) glass microballoons (GMBs) in high density polyethylene (HDPE). It is noted that the modulus increases with the filler content. Specific properties of the graded foams exhibited superior response as compared to neat HDPE. Among functionally graded foams (FGFs), FGF-2 (20–40–60) showed the highest modulus and yield strength. FGFs exhibited better energy absorption among all the tested samples. GMBs are observed to be intact, and a seamless interface is seen in micrographs of 3D printed graded foams, making them candidate materials for lightweight structural applications.

This paper presents the usage of spark plasma sintering (SPS) as a method to obtain aluminum-expanded perlite syntactic foams with high porosity. In the test samples, fine aluminum powder with flaky shape particles was used as matrix material and natural, inorganic, granular, expanded perlite was used as a space holder to ensure high porosity (35–57%) and uniform structure. SPS was used to consolidate the specimens. The structures were characterized by scanning electron microscopy and compression tests. Energy absorption (W~7.49 MJ/m3) and energy absorption efficiency (EW < 90%) were also determined.

Bimodal poly(methyl methacrylate) (PMMA) foams are produced with a new method combining syntactic foam production and supercritical CO2 (scCO2) foaming. Bimodal pore structure offers several potential advantages to foam properties, such as reduced mass density and excellent thermal and sound insulation properties. To produce the foams, first PMMA composites are produced with hollow glass microspheres (HGMs) and octatrimethylsiloxy polyhedral oligomeric silsesquioxane (POSS) nanoparticles using a twin screw extruder. The composite syntactic foams are then processed with scCO2 to form bimodal pore structured PMMA-HGM-POSS composite foams. In the foam matrix, large pores form due to the CO2 diffusion in the matrix towards the voids between the matrix and HGM surfaces, and small pores are formed at the nucleation cites. Highly CO2-philic POSS nanoparticles contribute to scCO2 foaming as cell nucleators enhancing the foaming performance of the syntactic composites. The effects of process pressure, time, and concentrations of HGMs and POSS nanoparticles on the foam morphology are studied. The highest decrease achieved in the foam density compared to that of the bulk polymer is 69% with only 12% decrease in the hardness. [ABSTRACT FROM AUTHOR]

Polymer syntactic foams are transforming materials that will shape the future of next-generation aerospace and marine structures. When manufactured using traditional processes, like compression molding, syntactic foams consist of a solid continuous polymer matrix reinforced with stiff hollow particles. However, polymer matrix segregation can be achieved during the selective laser sintering process with thermoplastic polyurethane (TPU). It is uncertain what role hollow particles play in forming this matrix segregation and its impact on the corresponding mechanical properties of syntactic foams. We show that the size of the hollow particles controls the internal microscale morphology of matrix segregation, leading to counter-intuitive macroscale mechanical responses. Particles with diameters greater than the gaps between the cell walls of the segregated matrix get lodged between and in the walls, bridging the gaps in the segregated matrix and increasing the stiffness of syntactic foams. In contrast, particles with smaller diameters with higher particle crushing strength get lodged only inside the cell walls of the segregated matrix, resulting in higher densification stresses (energy absorption). We show that stiffness and densification can be tuned while enabling lightweight syntactic foams. These novel discoveries will aid in facilitating functional and lightweight syntactic foams for cores in sandwich structures. [ABSTRACT FROM AUTHOR]

The influence of types and parameters of hollow microspheres in the composition of syntactic foams on their structure and coefficient of thermal conductivity has been studied. By using structural and thermal analysis it has been found that the volume concentration and the size of the ceramic and glass hollow microspheres have a strong impact on the density and thermophysical properties of the thin syntactic foams coatings. It has been shown that the best heat insulating properties belong to syntactic foam with composition of 60 vol. % ceramic microspheres with particle size of 1 - 40 μm (k = 0.029 W/m·K, R = 0.008 (m2·K)/W) and with composition of 80 vol. % glass hollow microspheres with particle size of 9 - 25 μm (k = 0.087 W/m·K, R = 0.008 (m2·K)/W). The results demonstrate that application of syntactic foams as thin insulating coatings is appropriate and they are an energy efficient material with number of benefits compare with the common thermal insulators.

The hollow glass microsphere (HGM) containing polymer materials, which are named as syntactic foams, have been applied as lightweight materials in various fields. In this study, carboxyl group-containing hyperbranched polymer (HBP) was added to a glass fiber (GF)-reinforced syntactic foam (RSF) composite for the simultaneous enhancement of mechanical and rheological properties. HBP was mixed in various concentrations (0.5–2.0 phr) with RSF, which contains 23 wt% of HGM and 5 wt% of GF, and the rheological, thermal, and mechanical properties were characterized systematically. As a result of the lubricating effect of the HBP molecule, which comes from its dendritic architecture, the viscosity, storage modulus, loss modulus, and the shear stress of the composite decreased as the HBP content increased. At the same time, because of the hydrogen bonding among the polymer, filler, and HBP, the compatibility between filler and the polymer matrix was enhanced. As a result, by adding a small amount (0.5–2.0 phr) of HBP to the RSF composite, the tensile strength and flexural modulus were increased by 24.3 and 9.7%, respectively, and the specific gravity of the composite was decreased from 0.948 to 0.917. With these simultaneous effects on the polymer composite, HBP could be potentially utilized further in the field of lightweight materials.

Carbon nanofibers (CNFs) and halloysite nanotubes (HNTs) were incorporated in syntactic foams containing a 90% by volume homogeneous mixture of (20/80 wt%) glass/thermoplastic microballoons to enhance the mechanical and impact response properties. Tensile, compressive, and impact tests were employed to comparatively characterize the effect of nano-additive reinforcement on mechanical response properties. Compressive strength and modulus enhancements as large as 39% and 18%, respectively, were achieved with a 0.125 wt% addition of CNF and increases of 61% and 7%, respectively, were achieved with a 0.125 wt% addition of HNT. Tensile strength and modulus enhancements as large as 107% and 68%, respectively, were achieved with a 0.125 wt% addition of CNF and increases of 104% and 70%, respectively, were achieved with a 0.125 wt% addition of HNT. Impact analysis data were used to show that measured peak force increased and build-up time to peak force decreased with increasing CNF or HNT weight percentage due to stiffening of the matrix. The smallest increase observed in peak force was 20% for a 0.125 wt% addition of CNF and 17% for a 0.125 wt% addition of HNT. [ABSTRACT FROM AUTHOR]

Biological materials often employ hybrid architectures, such as the core–shell (C–S) motif present in porcupine quills and plant stems, to achieve unique specific properties and performance. Drawing inspiration from these natural materials, a new method to fabricate lightweight and stiff C–S architected filaments is reported. Specifically, a C–S printhead conducive to printing highly loaded fiber‐filled inks, as well as a new low‐density syntactic foam ink, are utilized to 3D‐print C–S architectures consisting of a syntactic epoxy foam core surrounded by a stiff carbon fiber‐reinforced epoxy composite shell. Effective printing of test specimens and structures with controlled geometry, composition, and architecture is demonstrated. The new foam ink exhibits density as low as 0.68 g cm−3 and C–S structures exhibit up to 25% higher specific flexural stiffness (E1/3/ρ) than either constituent alone. Finally, a new mechanical model is presented to predict this performance improvement while accounting for potential eccentricity of the core. [ABSTRACT FROM AUTHOR]

Polymeric syntactic foams are composites made from the mixture of Hollow Glass Microspheres (HGM) and polymer matrices. One of their main characteristics is their low density and the production of these composites using a matrix derived from renewable sources potentiates their development without neglecting sustainability. In this paper , the properties of High Density Polyethylene (HDPE)/HGM syntactic foams containing 1% and 5% w/w HGM and 5% w/w of a compatibilizer are assessed. The composites were prepared by two processing routes: single screw extruder and twin screw extruder. The morphology and mechanical properties (tensile and impact) of the syntactic foams thus manufactured were ascertained. Morphological analysis indicated that matrix/filler adhesion was poor for all samples and that the best HGM dispersions were obtained in twin screw extruded samples. Mechanical properties were affected by the processing route adopted and by the content of hollow glass microspheres added. Elastic modulus, tensile strength and strain were reduced by 20, 10 and 23%, respectively, in systems processed in a twin screw extruder. Impact strength was the exception, with an increase of more than 300%. Higher contents of hollow glass microspheres led to reductions in mechanical strength of the syntactic foams, varying from 5% for the elastic modulus to 50% for strain.

Physicomechanical, thermophysical, and electrical properties of syntactic foams based on ED-20 epoxy diane resin and hollow phenol-formaldehyde microspheres have been considered. The optimal amount of microspheres in the composition has been determined. It has been shown that hollow microspheres affect the adhesion and strength characteristics of the sealant and reduce the dielectric loss tangent in the centimeter UHF range, which allows one to recommend such syntactic foams as sealing compositions in radioelectronic equipment and antenna-feeder devices operating in the ultra-high-frequency radio range. [ABSTRACT FROM AUTHOR]

In this article, the co-continuous hollow glass microspheres (HGMs)/epoxy resin syntactic foams were prepared by filling the HGM preforms with epoxy resin. The effects of HGM wall thickness and solid loading on the density, open porosity, and morphology of HGM preform and HGM/epoxy resin syntactic foams were studied in detail. The results show that the HGM wall thickness had no effect on the open porosity of HGM preforms. However, the density of preforms increased with the true density of HGMs. The density of HGM/epoxy resin syntactic foams decreased with the contents of HGMs, and the measured density was essentially equal to the theoretical density. The compressive strength decreased with the contents of HGM, regardless of the HGMs wall thickness. However, the variation in compressive modulus was related to the HGMs' wall thickness. As the HGM solid loading increased, the compressive modulus of composites, prepared using thin-walled HGMs, decreased, while the composites prepared using thick-walled HGMs showed the opposite trend. The co-continuous HGM/epoxy resin syntactic foams possessed low density (0.567–1.002 g/cm3), and excellent compressive strengths (30.32–131.60 MPa). These properties were due to the elimination of matrix pores, and are a type of promising material in deep sea exploration area. [ABSTRACT FROM AUTHOR]

• Moisture aging-driven degradation in additively manufactured TPU and TPU-based syntactic foams, along with associated chemical and microphase morphology changes, is established. • When exposed to moisture for 90 days and then desorbed, the TPU foam dissociates and reorganizes hard segments to form more hard domain morphologies. • Prolonged moisture exposure of 160 days causes hydrolysis-induced chain scission that produces a homogeneous distribution of rigid hard segment fillers in the soft matrix, reducing peak damping. • The addition of GMB in TPU foams and moisture exposure alters the polymer's viscoelastic properties differently in sub-Tg and beyond due to various mechanisms, including the number of polymer chains that undergo configurational changes, hindrance of the free movement of polymer chains and debonding of GMBs Syntactic foams have found widespread usage in various applications including, marine, aerospace, automotive, pipe insulation, electrical cable sheathing, and shoe insoles. However, syntactic foams are often exposed to moisture when used in these applications that potentially alter their viscoelastic properties, which influences their long-term durability. Despite their significance, previous research has mainly focused on experimental studies concerning mechanical property changes resulting from filler loading and different matrix materials, overlooking the fundamental mechanisms resulting from moisture exposure. The current paper aims to bridge this gap in knowledge by elucidating the impact of long-term moisture exposure on TPU and TPU-based syntactic foam through multi-scale materials characterization approaches. Here, we choose a flexible syntactic foam manufactured using thermoplastic polyurethane elastomer (TPU) reinforced with glass microballoons (GMB) through selective laser sintering. Specifically, the research investigates the influence of moisture exposure time and the volume fraction of GMB on chemical and microphase morphological changes, along with their associated mechanisms. The study further examines how these microphase morphological changes manifest in viscoelastic properties. [ABSTRACT FROM AUTHOR]

Epoxy/hollow glass microsphere (HGM) syntactic foams (SFs) are peculiar materials developed to combine low density, low thermal conductivity, and elevated mechanical properties. In this work, multifunctional SFs endowed with both structural and thermal management properties were produced for the first time, by combining an epoxy matrix with HGM and a microencapsulated phase change material (PCM) having a melting temperature of 43 °C. Systems with a total filler content (HGM + PCM) up to 40 vol% were prepared and characterized from the mechanical point of view with a broad experimental campaign comprising quasi-static, impact, and fracture toughness tests. The experimental results were statistically treated and fitted with a linear model, to produce ternary phase diagrams to provide a comprehensive interpretation of the mechanical behaviour of the prepared foams. In quasi-static tests, HGM introduction helps to retain the specific tensile elastic modulus and to increase the specific compressive modulus. The brittle nature of HGMs decreases the Charpy impact properties of the SFs, while the PCM insertion improve their toughness. This result is confirmed in KIC and GIC tests, where the composition with 20 vol% of PCM shows an increase of 80% and 370% in KIC and GIC in to neat epoxy, respectively. The most promising compositions are those combining PCM and HGMs with a total particle volume fraction up to 40 vol%, thanks to their optimal combination of thermal management capability, lightness, thermal insulation, and mechanical properties. The ability to fine-tune the properties of the SFs, together with the acquired thermal energy storage (TES) capability, confirm the great potential of these multifunctional materials in automotive, electronics, and aerospace industries.

Syntactic foams (SFs) combining an epoxy resin and hollow glass microspheres (HGM) feature a unique combination of low density, high mechanical properties, and low thermal conductivity which can be tuned according to specific applications. In this work, the versatility of epoxy/HGM SFs was further expanded by adding a microencapsulated phase change material (PCM) providing thermal energy storage (TES) ability at a phase change temperature of 43 °C. At this aim, fifteen epoxy (HGM/PCM) compositions with a total filler content (HGM + PCM) of up to 40 vol% were prepared and characterized. The experimental results were fitted with statistical models, which resulted in ternary diagrams that visually represented the properties of the ternary systems and simplified trend identification. Dynamic rheological tests showed that the PCM increased the viscosity of the epoxy resin more than HGM due to the smaller average size (20 µm vs. 60 µm) and that the systems containing both HGM and PCM showed lower viscosity than those containing only one filler type, due to the higher packing efficiency of bimodal filler distributions. HGM strongly reduced the gravimetric density and the thermal insulation properties. In fact, the sample with 40 vol% of HGM showed a density of 0.735 g/cm3 (−35% than neat epoxy) and a thermal conductivity of 0.12 W/(m∙K) (−40% than neat epoxy). Moreover, the increase in the PCM content increased the specific phase change enthalpy, which was up to 68 J/g for the sample with 40 vol% of PCM, with a consequent improvement in the thermal management ability that was also evidenced by temperature profiling tests in transient heating and cooling regimes. Finally, dynamical mechanical thermal analysis (DMTA) showed that both fillers decreased the storage modulus but generally increased the storage modulus normalized by density (E′/ρ) up to 2440 MPa/(g/cm3) at 25 °C with 40 vol% of HGM (+48% than neat epoxy). These results confirmed that the main asset of these ternary multifunctional syntactic foams is their versatility, as the composition can be tuned to reach the property set that best matches the application requirements in terms of TES ability, thermal insulation, and low density.

Due to challenges in generating high-quality 3D speckle patterns for Digital Volume Correlation (DVC) strain measurements, DVC experiments often utilize the intrinsic texture and contrast of composite microstructures. One common deficiency of these natural speckle patterns is their poor durability under large deformations, which can lead to decorrelation and inaccurate strain measurements. Using syntactic foams as a model material, the effects of speckle pattern degradation on the accuracy of DVC displacement and strain measurements are assessed with both experimentally-acquired and numerically-generated images. It is shown that measurement error can be classified into two regimes as a function of the percentage of markers that have disappeared from the speckle pattern. For minor levels of damage beneath a critical level of damage, displacement and strain error remained near the noise floor of less than 0.05 voxels and 100 με, respectively; above this level, error rapidly increased to unacceptable levels above 0.2 voxels and 10,000 με. This transition occurred after 30%–40% of the speckles disappeared, depending on characteristics of the speckle pattern and its degradation mechanisms. These results suggest that accurate DVC measurements can be obtained in many types of fragile materials despite severe damage to the speckle pattern. [ABSTRACT FROM AUTHOR]

Syntactic foams comprising glass or thermoplastic microballoons have gained considerable attention in recent years due to mechanical and thermal properties that are advantageous for naval and aerospace applications. This work reports a method for producing syntactic foams with unusually high-volume fraction microballoon loadings (>0.74) and its utilization for the creation of "hybrid" epoxy resin-based syntactic foams comprising various mixtures of glass and thermoplastic microballoons. Microstructural analyses using X-ray micro-computed tomography provided non-destructive quantitative characterization of microballoon packing, confirming the high loading levels suggested by density measurements. By systematically varying the glass/thermoplastic microballoon ratio, it was shown that a range of mechanical properties can be engineered into these lightweight materials. The peak impact force of these syntactic foams can be significantly reduced (∼30% reduction) through combining glass and thermoplastic microballoons in a ratio where the thermoplastic microballoons are the dominant fraction but not the sole microballoon component. [ABSTRACT FROM AUTHOR]

Abstract Quasi static compression (10−1, 10−2 and 10−3 s−1 strain rates) and dynamic mechanical analysis (temperature sweep of 30–175 °C) of cenosphere/epoxy syntactic foams are investigated. Effect of cenosphere content (20, 40 and 60 vol %) and surface modification are presented. Quasi-static tests reveal lower modulus for neat epoxy samples as compared to all the syntactic foams. With increasing cenosphere content and strain rate, elastic modulus increases for all the tested conditions. Foams reinforced with surface modified cenosphere exhibit higher modulus as compared with untreated ones and neat epoxy. Energy absorption of samples increases with increasing cenosphere content and surface modification. Storage modulus of untreated and treated syntactic foams register higher values with increase in cenosphere content and are higher than the neat epoxy samples. Loss modulus of syntactic foams at room temperature are lower as compared with pure epoxy while damping of untreated and treated foams registered higher values as compared with neat resin. Scanning electron microscopy of the samples are performed for structure property correlations. Finally, property map for quasi-static compression is presented by comparing results of present work with the extracted values from literature. [ABSTRACT FROM AUTHOR]

Epoxy syntactic foams with different compositions of cenosphere were fabricated and characterised. The effect of loading cenosphere in epoxy syntactic foams was analysed. Good thermal stability of cenosphere–epoxy syntactic foams was established from the thermogravimetric analysis results. The completion of cure reaction at ambient temperature conditions was ascertained from differential scanning calorimetry results. Dynamic mechanical analysis revealed 114°C as the glass transition temperatures (Tg) for neat epoxy sample, which increased to 132°C with 50% loading of cenosphere. Cenosphere-filled epoxy syntactic foams had low density and low water absorption values when compared to the neat epoxy sample. Homogeneous distribution of the cenosphere particles was confirmed using scanning electron microscopy. The compression studies confirmed brittle failure of the syntactic foams. This was also supported by the scanning electron microscopic images. The incorporation of hollow cenosphere particles led to a decrease in the flexural strength. Syntactic foams with 30% loading of cenosphere exhibited best specific modulus and specific strength. The specific strength increased by 24% for T30 sample and specific modulus increased by 36% for T30 samples when compared to the neat epoxy sample. As the need for strong but lightweight thermally stable products is continually increasing, there is a great possibility for the utilisation of these cenosphere–epoxy syntactic foams as lightweight core for sandwich composites. [ABSTRACT FROM AUTHOR]