Epoxy Foam Research Articles

Research on failure resistance of vehicle’s skeletons

The skeleton of the means of transport must comply with the strictest criteria in terms of safety and failure resistance. The failure occurs for various reasons. However, the failure caused by external forces acting on the skeleton predominates. Such a type of failure results in the deformation of the whole skeleton structure or its parts. To reduce the failure, the skeleton must meet the requirements on ​​material, construction, strength, flexibility, statics, and other engineering areas. For this reason, among other things, an adequate strength of the skeleton construction in the context of its total weight must be aligned with number of criteria. There are different approaches and solutions to meet this criterion. One of the progressive approaches is based on a use of composite materials based on thin-walled profiles filled with polyurethane, epoxy, or other foam. The paper deals with the research of such thin-walled profiles properties in their use for skeleton construction of a specific type of means of transport. The aim of the paper is the failure analysis of closed steel profiles forming the frame of the means of transport, based on the analysis of different foam densities impact on the strength characteristics and failure size. The research on failure analysis was carried out using the 3-point bend test. From the point of view of failure size, unfilled profiles were compared and analyzed with each other, and then foam-filled profiles were selected, depending on the material used and its thickness, provided that the required mechanical properties are met. The results from research of a new vehicle skeleton model resulted in an overall weight reduction of 285.2 kg, which is 9.74%, while the required failure resistance of the skeleton had been achieved. This is a significant result, the conclusions of which can be applied in the design of various vehicles so that to increase their failure resistance in the context of vehicle’s weight reduction.

Fracture performance of fibre-reinforced epoxy foam

Low density aramid and carbon fibre-reinforced epoxy foam has been synthesised with the aim of improving mechanical properties, principally fracture performance. The foam properties measured were fracture energy, compressive strength, and density. The influence of fibre type, loading, and length was investigated. In addition, composite face-sheet bond tests were performed to ascertain how effective toughness transferred from individual component to composite structure. In general, the addition of fibres improved the mechanical performance of reinforced samples compared to the control foam. Increases in compressive strength were moderate whilst fracture energy was increased by up to 107% from 124 J/m2 to 256 J/m2 by the addition of 0.75 mm aramid fibres. Increased fracture energy of the foam and the presence of fibres on the foam surface, caused an increase in face-sheet bond propagation fracture toughness of 50% from 277 J/m2 to 416 J/m2.

Fracture performance of epoxy foam: Low density to bulk polymer

Epoxy foams with densities ranging from 180 to 500 kg/m3 were prepared and mechanically tested in compression, tension, and single-edge notched bending (SENB) configurations. Fracture results revealed a marked transition in behaviour at a critical density, between 227 kg/m3 and 249 kg/m3. Lower density foams failed at low SENB displacement, producing low toughness and fracture energy results, whereas higher density foams failed at higher SENB displacements, with correspondingly higher values of toughness and fracture energy. The stress-intensity factor increased monotonically with density, from 0.1 to 0.79 MPa m1/2. The fracture energy, GIc, of the foams reached values of up to 3.5 times that of the bulk polymer, 268 J/m2. Lower density foams below the transition in fracture behaviour exhibited a small number of large cells, caused by cell coalescence, and a wider cell size distribution than the denser foams. This distribution appears linked to the transition in fracture behaviour. The behaviour revealed in this paper raises the point whether in future design criteria, where foams are now often used in composite sandwich structures, allowance should be made for denser foams to be used as appreciable increases in fracture energy of the foam core are achievable.

Effect of vertical aligned carbon nanofiber/carbon nanotube on the mechanical, electrical and electromagnetic interference shielding properties of epoxy foams

In this study, one-dimensional (1D) carbonaceous blends containing multi-walled carbon nanotube (MWCNT) and carbon nanofiber (CNF) were added into epoxy resin (EP) to obtain hybrid powder though ball milling. Limited foam with vertically aligned nanofillers was fabricated through limited-foaming epoxy hybrid in a mold and Two-step foam was prepared by further freely foaming a preformed Limited foam in an oven. The cellular structure, compressive, electrically conductive and EMI shielding properties of these two kinds of epoxy composite foams were carefully investigated. Limited foam showed an isotropic electrical resistivity while the electrical anisotropy started to emerge after the second-step foaming. This result was ascribed to the twisty MWCNT bridged the aligned CNF to build conductive pathways in horizontal direction, thus eliminating the electrical anisotropy of Limited foam. However, these loose interconnections between MWCNT and CNF in horizontal direction were more seriously destroyed by the further growth of bubbles, consequently endowing the Two-step foam with electrical anisotropy. Adding CNF into EP/MWCNT foams not only improved the electromagnetic interference (EMI) shielding effectiveness (SE), which was due to the synergistic effect between hybrid nanofillers in enhancing the electrical conductivity, but also enlarged the anisotropy in EMI SE of both Limited foam and Two-step foam. As compared to anisotropic bubbles, the vertically aligned MWCNT/CNF in composite foams exhibited a weak effect on anisotropy in compressive strength. Moreover, a remarkably anisotropic expansion behavior of preformed foam during the second-step foaming was observed and this behavior can be tuned by the ball milling time and the content of vertically aligned nanofillers.

Anisotropic microcellular epoxy/rGO-SCF aerogel foam with excellent compressibility and superior electromagnetic interference shielding performance

Carbon aerogel shows great potential in electromagnetic interference shielding (EMI) application and is generally strengthened by epoxy impregnation for harsh working environment. However, it is still challenging to simultaneously achieve high mechanical robustness and EMI performance, especially taking low-cost and light-weight feature into account. In this work, a “gelation-press drying-impregnation & micro-foaming” method was systematically reported to target microcellular epoxy foam containing hybrid reduced graphene oxide (rGO)/short-carbon-fiber (SCF) aerogel. Herein, aerogel morphology was transformed from isotropic honey-comb structure into anisotropic corrugated structure by controllable press air-drying. A further supercritical carbon dioxide (scCO2) foaming process was implemented for density reduction. Moreover, the anisotropic microcellular epoxy(m-EP)/rGO-SCF foam (D = ∼0.48 μm, Nf = 5.02 × 1012 cell/cm3) shows low density (1.14 g/cm3), high electrical conductivity (2365.0 S/m), enhanced compressibility (σb = 243.0 MPa, εb = ∼45%) and outstanding EMI shielding effectiveness (95.5 dB) with a low thickness (2.0 mm) within X band (8.2–12.4 GHz). Therefore, our work provides a universal step-by-step and industrial-friendly approach to fabricate light-weight carbon-aerogel-based foams with anisotropic carbon structure and outstanding EMI performance.

Influence of ultrasonic-assisted supercritical carbon dioxide foaming process on microcellular thermosetting epoxy foams: Morphology and thermal-mechanical properties

The preparation of microcellular thermosetting epoxy foam with good cell morphology and uniform cell distribution is still a big challenge because of its highly cross-linking chemical networks, Herein, introducing ultrasound into the traditional supercritical carbon dioxide foaming process was reported to improve the foamability and thermal-mechanical properties of thermosetting epoxy resin. The influence of ultrasonic conditions on the cell morphology was investigated firstly, which indicated that higher ultrasonic power and controllable duty cycle resulted in outstanding foamability, with the average cell size decreasing to about 200 nm and cell density increasing to the level of 1014. The foaming mechanism with outstanding foamability of thermosetting epoxy is strongly related to the ultrasonic cavitation effect, which is positive to more uniform cell nucleating sites and then more small cells. Furthermore, the thermos-mechanical properties of foamed epoxy were investigated. The glass transition temperature and the storage modulus of the fabricated epoxy foams determined presents increasing tendency with the introduced higher ultrasonic power and duty cycle, which partially related to the high absorption of carbon dioxide by the mechanical effect of ultrasound and the enhanced thermal effect of ultrasonic field. Smaller cell size and higher cell density are also positive to the higher dynamic mechanical properties. This work provides a new strategy with the introduced ultrasound into the supercritical microcellular foaming for optimum the related cell morphology and also thermo-mechanical properties of polymeric foams.

Preparation of pros-foam sheets and their epoxy foams using the solid-state carbamate-foaming technique

The foaming of epoxy resin is a challenge process in optimizing the flow foaming and curing to obtain a homogeneous cellular structure. This study presents a new method for preparing of epoxy foams, which is called solid-state carbamate foaming technique. The epoxy resin was formulated based on epoxy, hardener (amine or polyamide) and carbamate. The first step is to precure an epoxy resin and hardener to form a solid-state pros-foam sheet. The second steps are foaming and complete curing. Carbamate decomposes to form CO 2 for foaming and revived amine for final curing. By changing the amine or polyamide hardener, and their ratio with carbamate, epoxy foam with density of 110–150 kg/m 3 after free expansion was achieved. Epoxy foams with 3 mm (density of 407–434 kg/m 3 ) and 4 mm thickness (density of 332–352 kg/m 3 ) were expanded in a mold. The final foams have a glass transition temperature of 119–157 °C and storage modulus at 30 °C of 90–210 MPa. • Solid-state pros-foam sheets are developed from epoxy, free diamine or polyamide and a carbamate. • The solid-state behavior is designed based on the rheological storage modulus. • Solid-state pros-foam sheets are able to freely expand without mold, and expand with mold according to the target density. • This is a new method to produce epoxy foams, which is called solid-state carbamate foaming technique.

Microscopic testing of carbon fiber laminates with shape memory epoxy interlayer

For the first time, microscopic testing has been performed on shape memory polymer composites (SMPCs) which were manufactured by commercial materials already used in aerospace. Results from micro-tests have been compared with those from conventional memory-recovery cycling on macro-scale. Two shape memory polymer composite (SMPC) laminates were fabricated with different shape memory (SM) interlayer: one in the form of an uncured epoxy powder and the other in the form of a thin epoxy foam. The latter, in particular has been studied to evaluate lightweight and stiff sandwich structures with SM properties. The assessment of the manufacturing process by a hot press moulding technique was assessed through micro scale analysis using SEM and MicroCT analysis. DMA analyses were carried out to understand the interaction mechanisms between raw constituents. A Vickers micro-indentation examination before and after heating was able to assess the shape recovery behaviour at the micro-scale level. A nano-instrumental indentation was used to characterise the shape memory response under different loads at elevated temperatures. Whilst an instrumented thermo-mechanical test allowed to investigate the shape memory behaviour at macro-scale level. Results allow identifying the recovery mechanisms at the micro-scale which are responsible for the shape memory performances at the macro-scale. The higher recovery ability of the SM foam is confirmed in comparison with bulk interlayers.

Lightweight epoxy foams prepared with arranged hollow-glass-microspheres/epoxy hollow spheres

The research on lightweight epoxy foams is of great significance to weight reduction for aerospace, aviation, shipbuilding, high-speed railway, and other fields. In this study, a novel lightweight epoxy foam with a macro-sized hollow structure was designed and prepared using arranged hollow glass microspheres/epoxy resin (HGMs/EP) hollow spheres as filler and EP as a matrix by template and molding method. The density and uniaxial compression properties of the prepared hollow spheres could be tuned between 0.14 and 0.27 g/cm3 and 0.30 ± 0.01 MPa to 2.39 ± 0.17 MPa through adjusting the shell thickness, respectively. The bending and compression performance of arranged hollow spheres reinforced epoxy foams were investigated, and the failure mechanism was analyzed as well. The highest specific flexural strength, the specific flexural modulus, the specific compression strength, and the specific compression modulus of obtained epoxy foams was 30.77 ± 3.04 MPa cm3/g, 2.66 ± 0.19 GPa cm3/g, 45.78 ± 1.58 MPa cm3/g, and 1.60 ± 0.26 GPa cm3/g, respectively. This work presents a novel syntactic epoxy foam with low density, good specific flexural strength, and specific compression strength with fewer hollow glass microspheres, which can provide a preparation method for lightweight structural materials.

Microchannel insulating foams comprising a multifunctional epoxy/graphene‐nanoplatelet nanocomposite

AbstractGraphene nanomaterials have demonstrated simultaneous enhancement of both the thermal and mechanical properties of many base polymer systems, but for application as a reinforcement in multifunctional thermally insulating polymer foam the graphene additive must not significantly increase the macroscopic thermal conductivity. In an effort to study and decouple these mechanical and thermal properties, low loadings of reduced graphene oxide nanoplatelets have been added into epoxy formulations to comprise the matrix structure of hollow microchannel nanocomposite foams. Substantial improvements of 226% were achieved in the microchannel foam specific flexural modulus at only 0.15% graphene nanoplatelet loading without compromising the foam flexural strength and while also maintaining the low thermal conductivity of the baseline epoxy foam material. These results support the use of these nanocomposite foams as mechanically reinforced thermal insulation, with the addition of the graphene nanoplatelets potentially providing additional multifunctional improvements to the foam such as reduced UV‐radiation transmittance, improved electrical surface conductivity for diminished static charge buildup, and/or lowering of the coefficient of thermal expansion for enhanced structural stability in extreme environments. This particular combination of multifunctional properties makes these materials well suited as high‐performance structural thermal insulation materials in support of next generation space system applications.

Fabrication and Mechanical Performance of Glass Fiber Reinforced, Three‐phase, Epoxy Syntactic Foam

AbstractIn the selection of materials for deep‐sea equipment, light weight and high strength are the most important criteria or requirements. In this paper, hollow glass microspheres (HGMS), expanded polystyrene (EPS), epoxy resin (EP) and curing agent were mixed to prepare hollow glass microspheres reinforced epoxy foam balls (HGMS‐EHS) by the rolling ball method. Glass fiber (GF) reinforced epoxy syntactic foam (GF‐ESF) was prepared by mixing HGMS‐EHS, HGMS, EP, GF, and curing agent using compression moulding method. By analyzing the mechanical properties and microscopic morphology of GF‐ESF, the results show that the addition of GF does enhance the compressive strength of epoxy syntactic foam. GF‐ESF has a density of 496 kg/m3, and its compressive strength is 30.48 MPa. By increasing the number of layers of HGMS‐EHS, the diameter of EPS, the density of HGMS and reducing the stacking volume of HGMS‐EHS, the density of ESF increased significantly thus subsequently increase the compressive strength. Therefore, the compressive strength and density should be considered comprehensively and find the best balance point in the process of preparing ESF. This paper could provide some suggestions for the preparation of deep‐sea epoxy composites.

Investigations on Epoxy-Carbamate Foams Modified with Different Flame Retardants for High-Performance Applications.

In transport sectors such as aviation, automotive and railway, materials combining a high lightweight potential with high flame retardant properties are in demand. Polymeric foams are suitable materials as they are lightweight, but often have high flammability. This study focuses on the influence of different flame retardants on the burning behavior of Novolac based epoxy foams using Isophorone Diamine carbamate (B-IPDA) as dual functional curing and blowing agent. The flame retardant properties and possible modifications of these foams are systematically investigated. Multiple flame retardants, representing different flame retardant mechanisms, are used and the effects on the burning behavior as well as mechanical and thermal properties are evaluated. Ammonium polyphosphate (APP), used with a filler degree of 20 wt.% or higher, functions as the best performing flame retardant in this study.

Enhanced ethanol production from lignocellulosic hydrolysate using Meyerozyma caribbica biofilm immobilized on modified epoxy foam

In comparison to planktonic cells, microbial biofilm immobilization is known to enhance ethanol production. To maintain this capability during long-term fermentation, the detachment of the biofilm must be prevented. In this study, the enhancement of the biofilm immobilization of the yeast Meyerozyma caribbica YLP01GX on bio-based epoxy foam (EF) was performed through plasma surface modification to improve the production of ethanol from oil palm empty fruit bunch hydrolysate. This newly synthesized EF serves as a suitable carrier due to its superior properties and cost-effectiveness. The modification could induce yeast cell attachment more rapidly with more extracellular polymeric substance (EPS) being produced in the presence of the yeast extract. Therefore, the immobilization period can be shortened. In comparison to untreated EF-immobilized cells (NEF-IC), the modified EF-immobilized cells (MEF-IC) showed an improved binding affinity for proteins in the EPS, which resulted in a higher adhesion force between the carrier and the yeast cell surface. The stable biofilm attachment on the modified EF can prevent biofilm detachment and improve the tolerance of the yeast to inhibitors in the hydrolysate, leading to enhanced ethanol production compared to NEF-IC. Additionally, the MEF-IC was reused several times with similar fermentation activity being observed. This approach offers a more efficient and economical fermentation process.

Synergetic effect between curing reaction and CO2 diffusion for microcellular epoxy foam preparation in supercritical CO2

Directly rapid depressurization foaming process has been successfully applied to prepare microcellular epoxy foams using supercritical CO2 as blowing agent. It is found that both curing reaction and CO2 diffusion are crucial for controlling cell morphology. For epoxy of tetraglycidyl-4,4′-diaminodiphenyl-methane (TGDDM) and 4,4-diaminodiphenylsulfone (4,4-DDS) system, the experimental and molecular simulation results displayed that higher CO2 pressure always promotes curing reaction, and CO2 diffusion is more sensitive to pressure at low curing degree, while limited by high curing degree. The foaming experimental results showed that there existed a suitable curing degree range, in which both unsaturated CO2 concentration and curing degree contribute to good cell structure. Higher CO2 pressure can broaden this foamable window. The microcellular epoxy foam with an average cell diameter of 14.6 µm and cell density over 109 cells/cm3 as well as volume expansion ratio of 14.5 has been obtained at 85% curing degree under 22 MPa CO2 pressure.

Dependence of the properties of epoxy foams on the composition

The features of the technological process of obtaining foams based on epoxy resin are considered. The influence of the concentration of the components of the composition on the physical and mechanical properties and morphology of the resulting materials is studied. The results of the study of the physical and mechanical properties of the developed epoxy foams are presented. The possibility of obtaining foams with a wide range of adjustable performance properties is shown.

Design and optimization of ultra-wideband planar multilayer absorber based on long-carbon fiber-loaded composites

This article presents a strategy for designing optimal microwave planar multilayer absorbers based on epoxy foam composites loaded with carbon fibers of 12 mm length. Firstly, the impedance gradient principle (gradual loaded composites) was adopted to realize two multilayer absorbers, of 125 mm thickness, using slightly loaded composites (0.0125 wt% < CFs < 0.075 wt%) and relative highly loaded composites (0 wt% < CFs < 0.4 wt%), respectively. The simulation of these absorbers shows that composites with very low CF rates are sufficient to achieve a very close absorption performance and bandwidth to that of the commercial absorber, in the entire studied frequency range (0.75–18 GHz). Secondly, the genetic algorithm optimizer is used to achieve a multilayer absorber that presents the best compromise between absorption performance and thickness. Different CF-loaded composites and layer thicknesses are therefore tested; a multilayer absorber with a total thickness of 98 mm is then proposed. This absorber shows a better reflection coefficient and a better compromise (absorption/total thickness) than that of the commercial absorber, while presenting a reduction of 22% in thickness. The presented simulation and measurement results confirm that a judicious choice of the composition and the thickness of each layer is necessary to optimize the absorption performance of a planar multilayer absorber. This paper also shows the advantage of using an optimizer to improve the absorption performance while reducing the total thickness of the absorber. Graphical abstract

New Insights on Expandability of Pre-Cured Epoxy Using a Solid-State CO2-Foaming Technique.

Foaming an epoxy is challenging because the process involves the curing reaction of epoxy and hardener (from monomer to oligomer, to a gel and a final three-dimensional crosslinked network) and the loading of gas phase into the epoxy phase to develop the cellular structure. The latter process needs to be carried out at the optimum curing stage of epoxy to avoid cell coalescence and to allow expansion. The environmental concern regarding the usage of chemical blowing agent also limits the development of epoxy foams. To surmount these challenges, this study proposes a solid-state CO2 foaming of epoxy. Firstly, the resin mixture of diglycidylether of bisphenol-A (DGEBA) epoxy and polyamide hardener is pre-cured to achieve various solid-state sheets (preEs) of specific storage moduli. Secondly, these preEs undergo CO2 absorption using an autoclave. Thirdly, CO2 absorbed preEs are allowed to free-foam/expand in a conventional oven at various temperatures; lastly, the epoxy foams are post-cured. PreE has a distinctive behavior once being heated; the storage modulus is reduced and then increases due to further curing. Epoxy foams in a broad range of densities could be fabricated. PreE with a storage modulus of 4 × 104–1.5 × 105 Pa at 30 °C could be foamed to densities of 0.32–0.45 g/cm3. The cell morphologies were revealed to be star polygon shaped, spherical and irregularly shaped. The research proved that the solid-state CO2-foaming technique can be used to fabricate epoxy foams with controlled density.

Uniaxial Compression Mechanical Properties of Foam Nickel/Iron-Epoxy Interpenetrating Phase Composites.

The damage process and failure mechanisms were analyzed by a series of quasi-static compressive experiments of seven materials including pure epoxy (EP), three different PPI (pores per linear inch) foam nickel-iron, and three different PPI foam nickel/iron-epoxy interpenetrating phase composites (IPC). Plotting the stress–strain curves of different materials, their change rules are discussed, then the effective elastic modulus and yield limit of the materials are provided, and the energy absorption properties of different materials are analyzed by the stress–strain curves. It was found that the effective elastic modulus and specific stiffness of the three IPC materials were higher than pure foam nickel-iron. The brittleness of epoxy can be obviously changed by selecting a suitable PPI foam nickel-iron composited with it. The unit volume energy absorption rate of foam nickel/iron-epoxy was significantly higher than pure epoxy and pure foam nickel-iron. It was also found that the energy absorption rate decreased with the increase in PPI. The stress relaxation rate decreased first and then increased with the increase in PPI. The creep behavior of the three composites was obvious in the creep elastic stage, and the creep rate increased with the increase in PPI. The creep rate decreased with the increase in PPI in the creep transition stage.

A novel temperature-sensitive expandable lost circulation material based on shape memory epoxy foams to prevent losses in geothermal drilling

Lost circulation will inevitably increase non-productive time (NPT) and well cost. The application of high-efficiency lost circulation materials (LCMs) plays an indispensable role in preventing and mitigating loss. The development of new LCMs based on stimuli-responsive materials is an important research direction and it is expected to realize the revolutionary development of LCMs. In recent years, the temperature-responsive plugging agent based on shape memory polymers has caused a lot of attention. In this paper, based on the shape memory epoxy polymer (SMEP) and hollow glass beads (HGBs), a shape memory epoxy foam (SMEF) was successfully synthesized. The results show that the shape memory temperature (51.6 °C - 97.5 °C) of the SMEPs has a good linear relationship with the content of curing agent 4, 4′-diaminodiphenyl methane (DDM) (12% - 20%), and the expansion ratio (0% - 120.2%) of the SMEFs has a good linear relationship with the content of HGBs (0% - 50%). The mechanism of the SMEFs was proposed. A novel temperature-sensitive and expandable plugging agent (SMP-LCM) has been developed. The plugging experiment shows that SMP-LCM has a good plugging effect on high permeability loss zone simulated by sand beds. Compared with conventional LCMs (e.g., nutshells or fibers), SMP-LCMs can eliminate the risk of plugging of downhole drilling tools and enhance the plugging effect.

Synthesis and Characterization of Dual-Functional Carbamates as Blowing and Curing Agents for Epoxy Foam

With regard to upcoming regulations of common chemical blowing agents for epoxy foams, carbamates provide suitable alternatives. They act as blowing agents releasing CO2 and cure epoxy resins after...