Thermally Expandable Microspheres Research Articles

Lantern-inspired capacitive pressure sensor with wide linear measuring range

In the field of pressure sensing, high sensitivity and linearity are two critical performance indicators, but striking a balance between them remains a prominent challenge for sensors. Existing solutions typically involve the structural design of electrodes or dielectric layers, often requiring complex and expensive techniques for designing microstructures. This study presents a novel flexible capacitive pressure sensor characterized by its low-cost nature, ease of manufacture, and scalability in design. The lantern accordion-like structure inspires the design of the dielectric layer and the porous structure is designed using thermally expandable microspheres (TEM). The synergy of surface microstructures and internal microstructures results in a high sensitivity of 3.07 × 10−3 kPa−1 within a wide linear response range of 0–100 kPa. The developed sensor is also evaluated for its weight detection capabilities, highlighting its promising weight and pressure data acquisition.

3D printed multifunctional hierarchical structured cellular silicones with ultraelasticity, extreme load-bearing capacity, shape morphing and sensing properties

Multifunctional lightweight cellular silicone with adjustable properties has aroused great interests in many fields. However, it remains a challenge to facilely prepare multifunctional lightweight porous silicones with high load-bearing capacity. Herein, this work developed a 3D printing technique to prepare lightweight hierarchical structured cellular silicones with macroscale lattice structure and microscale intra-strand close-cell porosities, which was achieved by the expansion of thermally expandable microspheres (TEM) with plastic shells dispersed in formulated silicones. The obtained silicone foam with hierarchical porosity distributions shows excellent mechanical properties, including extreme load-bearing capacity (load is more than 165000 times its weight), high elasticity (negligible stress and strain loss under 80% compression), and high cycle durability (less than 4% strain loss under 1000 compression cycles). Besides, the incorporation of conductive fillers of MWCNTs endowed the foam with multifunctional piezoresistive and temperature-sensing properties. Furthermore, by printing multiple mixture inks of varying expansion ratios, shape morphing ability was endowed to the printed foam, to achieve complex curvature geometry facilely, demonstrating excellent versatility and potential applications in manufacturing flexible and conformal electronics of this method.

Thermally expandable microspheres based on fully or partially bio‐based polymers

AbstractBio‐based or partially bio‐based thermally expandable microspheres (TEMs) were synthesized by suspension (co)polymerization of the bio‐based monomer α‐methylene‐γ‐valerolactone (MeMBL) together with acrylonitrile and/or methyl methacrylate to form expandable core/shell particles by encapsulating a hydrocarbon‐based blowing agent. The core/shell polymers were characterized with respect to their chemical structure, thermal expansion, and morphology. The obtained particles, TEMs, showed an increasing onset expansion temperature with increasing content of MeMBL owing to the high glass transition temperature of PMeMBL. As a result, bio‐based/partially bio‐based TEMs are achieved with high thermal stability and expansion properties which can be tailored for various applications.

Self‐Destructible Electromagnetic Interference Shielding Films

AbstractIn response to the increasing utilization of communication technology, there is a growing need for electromagnetic interference (EMI) shielding materials to adapt to diverse application scenarios. In this study, the self‐destructible EMI shielding films (SEFs) that possess the unique capability to reduce conductivity and shielding efficiency through temperature elevation are introduced. The SEFs effectively fulfill the requirement of triggering EMI shielding failure under specific conditions, which can be achieved by simply spraying a mixture of thermally expandable microspheres (TEMs), conductive fillers, and resin substrates. The segregated structures resulting from the incorporation of TEMs confer exceptional electrical conductivity and superior EMI shielding performance at low conductive filler content. When the ambient temperature surpasses the glass transition temperature of the TEM shell, the thermally expandable characteristics cause a disruption in the conductive path of the SEFs, leading to a significant decline in shielding effectiveness. Additionally, the SEFs exhibit outstanding Joule heating effects (90 °C) at low voltage (1.5 V) within a brief time frame (10 s). Importantly, when employed as Joule heaters, the SEFs possess an intrinsic safety mechanism that automatically ceases operation in the event of circuit overload, thus minimizing any potential risks.

Additive manufacturing of functionally graded foams: Material extrusion process design, part design, and mechanical testing

Functionally graded foams (FGFs) were in-situ fabricated via material extrusion (MEX) additive manufacturing (AM) process. Foamable filaments loaded with thermally expandable microspheres (TEMs) at 8.0 wt% was first fabricated using a single screw extruder. The correlations between the resultant foam density and process factors, namely nozzle temperature (NT) and flow rate (FR) were established using a statistical analysis and the density predictability of the model was verified by experiment. With concurrent control of NT and FR, FGFs with density ranges as high as 0.86 g cm−3 were achieved within a single print. Various FGFs were designed using linear, concave, and convex density gradient functions. The density-process correlation model was then used to obtain the NT and FR parameters needed to produce the density values as demanded by the part design. FGFs along with their single density foam (SDF) counterparts were successfully printed with good dimensional stability. Under quasi-static compression testing, all FGFs showed higher energy absorption capacity at low stress levels, as compared to their SDF counterparts. Moreover, under impact conditions, a significant loading direction dependency was found for the FGFs. Overall, this work demonstrates the AM feasibility of FGFs as a single print with tailored density profiles which can be used in generative design optimization of AM parts for enhanced mechanical performance and other functionalities.

Preparation of amphiphilic Janus nanosheets based on thermally expandable microspheres and evaluation of their physical and chemical properties

Janus nanosheets can reduce the interfacial tension of oil and water and are of great significance for improving the recovery efficiency of remaining oil. A novel strategy for preparing amphiphilic Janus nanosheets is presented in this paper. Trialkoxysilane is adsorbed on the surface of thermally expandable microspheres (TEMS), and Janus spheres (TEMS@SiO2) are obtained by sol-gel treatment. Dodecyltrimethoxysilane (DTES) is used to modify TEMS@SiO2, which is combined with the thermal expansion of TEMS and the breaking of the outer shell of Janus spheres to prepare amino/alkyl composite silicon-based amphiphilic Janus nanosheets (ASAJN). Characterization of ASAJN is carried out via FTIR, TG, SEM, TEM, and contact angle tester, with the results showing that ASAJN boasts amphiphilicity and two-sidedness, and a sheet structure with a thickness of 210 nm. The stability, wettability evaluation, emulsification properties, and interface properties of ASAJN nanofluids have been evaluated, with the results showing that ASAJN nanofluids boast good stability without delamination after 30 days. ASAJN nanofluid can convert oil-wet core slices into water-wettable ones. ASAJN nanofluid manifests excellent emulsification, with the 240-hour emulsification index of ASAJN nanofluid and silica fluid respectively at0.40 and 0.021 under ultra-low concentration (1000 mg/L). ASAJN is obviously able to reduce oil–water interfacial tension. At 1000 mg/L (60 ℃) concentration, the oil-water interfacial tension is reduced to 2.6 mN/m, while that of nano-silica at the same concentration is 15.1 mN/m. ASAJN flooding can significantly improve the recovery efficiency of low-permeability reservoirs, increasing the recovery efficiency by 32.8 % at a permeability of 7.5 × 10-3 μm2.

Investigation of expandable fillers for reversible adhesive bonding in photovoltaic modules

Photovoltaic (PV) is a sustainable energy source and an efficient tool towards CO2 neutrality. However, recycling of PV modules remains challenging due to the strong connection of the multi-material assembly. The present work aims at the development of disbonding concepts for adhesive connections used in PV modules to enable improved recyclability and repairability of such modules. To achieve stimuli-responsive adhesive connections, thermally expandable fillers were implemented in a condensation-curing alkoxy-based silicone adhesive. 10–50 wt% of expandable graphite (EG) as well as 10–50 wt% of thermally expandable microspheres (TEM) were incorporated to obtain a controlled expansion of the adhesive at elevated temperature. The expansion ratio in dependence on time and temperature was examined. Furthermore, nanoscale magnetite (Fe3O4) passivated with a layer of SiO2 was added to the formulation to trigger the expansion of TEMs, and therefore separation of adhesive bonds by inductive heating. In this approach an efficient heating and expansion of the adhesive layers was feasible by applying an external alternating magnetic field with a ring coil. Aging stability, influence of filler content and expansion of the adhesive layers were further evaluated in lap shear tests. The addition of functional fillers decreased the bond strength by approximately 50% for adhesive layers containing 50 wt% EG. In contrast, adhesives with 50 wt% TEM exhibited a higher bond strength (by 10%) compared to the unfilled silicone system. Thermally triggered expansion significantly lowered the bond strength (20–30%) for TEM-filled adhesives and led to separation for EG-filled adhesives. Temperature cycling tests had no influence on the filled and unfilled adhesive's strength, whereas after damp heat tests all samples exhibited a lower (less than 40% of the unaged, pristine reference samples) but similar bond strength.

Modifying explosive behavior with additives

AbstractThe influence of additives on the detonation velocity of a polyethylene wax/RDX formulation was examined. Additives included species of various shock impedance: glass microballoons; glass microspheres; polymethyl methacrylate (PMMA) microspheres; thermally expandable microspheres (TEMs); and PMMA microencapsulated pentaerythritol tetranitrate (PETN). Performance of the insensitive explosive 2,4‐dinitroanisole (DNAN) was enhanced by addition of PETN‐either neat or encapsulated, but encapsulation did not increase the sensitivity of the formulation. The energy contribution of the encapsulated PETN to the detonation front of the insensitive explosive 2,4‐dinitroanisole (DNAN) was also demonstrated. Present in 5 wt%, the encapsulated PETN allowed DNAN to sustain a reaction (5.36 km/s) at 13 mm, well below its critical diameter.

Modification of Hollow Expanded Microspheres with Superior Thermal Insulation Properties and Their Applications.

Thermally expandable microspheres (TEMs) are hollow polymeric particles in which a blowing gas has been encapsulated. This property makes them excellent for thermal insulation applications, such as lightweight fillers. This study has developed a viable technology for further improving thermal insulation properties in the field that needs excellent thermal insulation of textile fabrics. The ATO/TEMs composites were designed and prepared to reduce sunlight radiation by the charge gravity method. The test results showed that the ATO-coated TEMs effectively block thermal radiation from sunlight. The temperature difference between ATO/TEMs treated cotton and the uncoated cotton fabric was 9 °C, and the thermal conductivity coatings were 0.0432 W/m⋅K. The UPF value of ATO/TEMs (ILs) coated cotton fabric is 440, significantly higher than pure cotton. This approach can provide insight into the design of high-performance solar insulation composite structures.

Opposite Roles of Bacterial Cellulose Nanofibers and Foaming Agent in Polyhydroxyalkanoate-Based Materials.

In this work, an economically feasible procedure was employed to produce poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)-based foams. Thermally expandable microspheres (TESs) were used as a blowing agent, while bacterial cellulose (BC) nanofibers served both as a reinforcing agent and as a means of improving biocompatibility. PHBV was plasticized with acetyltributylcitrate to reduce the processing temperature and ensure the maximum efficiency of the TES agent. The morphological investigation results for plasticized PHBV foams showed well-organized porous structures characterized by a porosity of 65% and the presence of both large pores (>100 µm) and finer ones, with a higher proportion of pores larger than 100 µm being observed in the PHBV nanocomposite containing TESs and BC. The foamed structure allowed an increase in the water absorption capacity of up to 650% as compared to the unfoamed samples. TESs and BC had opposite effects on the thermal stability of the plasticized PHBV, with TESs decreasing the degradation temperature by about 17 °C and BC raising it by 3-4 °C. A similar effect was observed for the melting temperature. Regarding the mechanical properties, the TESs had a flexibilizing effect on plasticized PHBV, while BC nanofibers showed a stiffening effect. An in vitro cytotoxicity test showed that all PHBV compounds exhibited high cell viability. The addition of TESs and BC nanofibers to PHBV biocomposites enabled balanced properties, along with lower costs, making PHBV a more attractive biomaterial for engineering, packaging, or medical device applications.

Large size, sensitive response thermally expandable microspheres for the manufacture of suede‐like leather

AbstractA large size, sensitive response thermally expandable microspheres (TEMs) for suede‐like leather manufacture was prepared by suspension polymerization using acrylonitrile (AN), methyl methacrylate (MMA) and vinyl acetate (VAC) as the shell monomers and iso‐pentane as the foaming component. The effect of the interfacial tension and viscosity of polymerization system on the particle size of TEMs, and in turn, the effect of the particle size of TEMs on the expansion temperature, expansion ratio and thermal response performance were investigated in detail. The results showed that increasing the interface tension and viscosity of the polymerization system was beneficial for preparing large size TEMs. Under the optimized conditions of 300 rpm, 25 wt% iso‐pentane dosage, and 9:3:1 mass ratio of AN/MAA/VAC, the TEMs shown above 100 μm particle size and 4 times expansion ratio at 150°C. Moreover, TEMs with 18% encapsulation ratio (Er) of the foaming component can complete foaming in the narrow temperature range (ΔT ≤ 10°C), showing a sensitive thermal response. The mixture of waterborne polyurethane and quantitative TEMs was coated on the surface of synthetic leather base, and a new type of suede‐like leather was obtained after foaming, which can substitute genuine suede leather.

In Situ Foam 3D Printing of Microcellular Structures Using Material Extrusion Additive Manufacturing.

A facile manufacturing method to enable the in situ foam 3D printing of thermoplastic materials is reported. An expandable feedstock filament was first made by incorporation of thermally expandable microspheres (TEMs) in the filament during the extrusion process. The material formulation and extrusion process were designed such that TEM expansion was suppressed during filament fabrication. Polylactic acid (PLA) was used as a model material, and filaments containing 2.0 wt % triethyl citrate and 0.0-5.0 wt % TEM were fabricated. Expandable filaments were then fed into a material extrusion additive manufacturing process to enable the in situ foaming of microcellular structures during layer deposition. The mesostructure, cellular morphology, thermal behavior, and mechanical properties of the printed foams were investigated. Repeatable foam prints with highly uniform cellular structures were successfully achieved. The part density was reduced with an increase in the TEM content, with a maximum reduction of 50% at 5.0 wt % TEM content. It is also remarkable that the interbead gaps of mesostructure vanished due to the local polymer expansion during in situ foaming. The incorporation of TEM and plasticizer only slightly lowered the critical temperatures of PLA, that is, glass-transition, melting, and decomposition temperatures. Moreover, with the introduction of foaming, the specific tensile strength and modulus decreased, whereas the ductility and toughness increased severalfold. The results unveil the feasibility of a novel additive manufacturing technology that offers numerous opportunities toward the manufacturing of specially designed structures including functionally graded foams for a variety of applications.

Novel method to control explosive shock sensitivity: A mesoscale study to understand the effect of thermally expandable microsphere (TEM) inclusions in high explosives (HE) microstructure

When the void content and/or void structure of a high explosive (HE) is altered by some means (i.e., bulk heating or mechanical damage), the shock initiation behavior of the material changes. The ability to precisely predict the change in shock sensitivity after an HE has undergone microstructural changes is a crucial capability in multi-scale reactive flow models. Here, we utilize thermally expandable microspheres (TEMs) as a dopant in a polymer bonded explosive (PBX) matrix to alter the shock initiation properties in a controlled fashion. Using a mesoscale modeling approach, we evaluated how a single TEM (before and after thermal expansion) behaves under shock compression, as well as how the matrix PBX in the direct vicinity of the TEM is affected. We first examined the effect of an unexpanded TEM in the explosive matrix and found that its presence does not significantly perturb the bulk flow and by extension will not affect bulk sensitivity. Next, we examined the effect of an expanded TEM and found that its presence significantly perturbs the flow via hydrodynamic jetting, which causes a secondary shock wave with a strength that exceeds that of the incident wave. Finally, we showed that this secondary shock interacts with the downstream porosity to ignite a larger fraction of the overall pore volume, commensurate with the secondary shock strength and the affected volume, increasing the global (bulk) shock sensitivity.

Preparation and Properties of Thermally Expandable Microspheres for Foam Coating

A type of thermally expandable microspheres (TEMs) for foam coating was prepared by suspension polymerization with acrylonitrile (AN), methyl methacrylate (MMA), vinyl acetate (VAC) as shell polymer monomers and i-pentane as core foaming agent. The effects of an aqueous additive (Sodium Chloride, NaCl) on the size and distribution of TEMs, and the effects of crosslinking degree, i-pentane dosage and monomer mass ratio on the expansion property, expansion temperature and solvent-resistance of TEMs were investigated. The results showed that when the dosage of NaCl was close to the saturation solubility (30%), the dosage of crosslinking agent and alkane were about 0.09% and 7.4%, and the mass ratio of AN/MMA/VAC w rm distribution and good solvent resistance, the expansion diameter ratio was 5 times under 110~120 C, which meets the application requirements for foam coating of leather or synthetic leather.

Biocomposite foams based on polyhydroxyalkanoate and nanocellulose: Morphological and thermo-mechanical characterization

The application of bio-based and biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is restricted by its high cost and brittleness. In the present work, these deficiencies were overcome by the manufacture of PHBV foams using thermally expandable microspheres (TES). Nanocellulose (Nc) and a crosslinking agent were added to PHBV-TES to control the foam structure and to improve the mechanical properties. Foams with almost perfect pores, well embedded in the polymer matrix, were obtained by a simple melt molding process. The closed-cell foams have a density 2.5–2.7 times lower than that of PHBV. The addition of Nc increased the expansion ratio, cell density and porosity and also led to a more uniform cell size distribution. The incorporation of the crosslinking agent, together with Nc and TES, increased the glass transition temperature with about 7 °C and strengthened the PHBV–Nc interactions. PHBV foams showed a 1.7–3 times higher deformation compared to PHBV and absorbed up to 15 times more energy. The fully biodegradable PHBV–Nc foams obtained in this work exhibit an advantageous porosity, good specific mechanical properties and high energy absorption, being promising alternatives for insulation, packaging or biomedical application.

Thermal Expansion Behavior of Thermally Expandable Microspheres Prepared by Suspension Polymerization Using P(AN‐MMA‐MAA) Core/Shell

Thermally expandable microspheres (TEMs) have widespread industrial applications. In this study, high heat resistant and largely expandable TEMs have been synthesized by suspension polymerization. Microspheres were prepared with acrylonitrile (AN) as the main monomer. Initial thermomechanical analysis of TEMs containing AN and methyl acrylate (MA) showed good thermal expansion, leading to the investigation of the polymerization using various monomers at variable weight ratios. TEMs with co‐polymer AN, methyl methacrylate (MMA), and methacrylic acid (MAA) were investigated for the first time. They displayed wider heating range and larger expansion rate than the microspheres with MA monomer. The effect of varying amounts of monomer ratios on the thermal expansion behavior is discussed.

Preparation and Application of Conductive Polyaniline-Coated Thermally Expandable Microspheres.

The thermally expandable microspheres (TEMs) were prepared through suspension polymerization with acrylonitrile (AN), methyl methacrylate (MMA) and methyl acrylate (MA) as the main monomers. Simultaneously, iso-pentane, n-hexane, iso-octane and other low-boiling hydrocarbons were prepared as blowing agents under two conditions, including high-pressure nitrogen and atmospheric conditions. The above physical foaming microspheres have a core-shell structure and excellent foaming effects. A layer of polyaniline (PANI) was deposited on the surface of the prepared TEMs by emulsion polymerization to obtain conductive and heat-expandable microspheres. Afterwards, the foaming ink was prepared by mixing the conductive TEMs and water-based ink. Finally, a conductive three-dimensional picture was obtained by screen-printing technology. This paper specifically focuses on the effects of particle size, morphology and the thermal expansion properties of the microspheres. The present research methods expect to obtain microspheres with a high foaming ratio, uniform particle size and antistatic properties, which may be applied to physical foaming ink.

Suspension Polymerization of Thermally Expandable Microspheres Using Cinnamonitrile and Diethyl Fumarate as Crosslinking Agents

Thermally expandable microspheres (TEMs) were synthesized via suspension polymerization using acrylonitrile as the major monomer. Methyl methacrylate and methacrylonitrile were used as supporting co‐monomers, and 1,4‐butanediol dimethacrylate was used as a crosslinking agent. In addition, cinnamonitrile (CN) and diethyl fumarate (DEF) were investigated as monomers to improve the expansion performance of the TEMs. However, the expandability of the TEMs using CN was diminished. When TEMs were prepared with DEF as the crosslinking agent, the expandability was highly affected. From the thermomechanical analysis of TEMs with various crosslinking agents, DEF showed better performance as crosslinking agent, yielding increased expansion properties. This is the first report using DEF as a TEM crosslinking agent.

Synthesis and Compressive Response of Microcellular Foams Fabricated from Thermally Expandable Microspheres

Cellular foams are widely applied as protective and energy absorption materials in both civil and military fields. A facile and simple one-step heating method to fabricate polymeric foams is measured by adopting thermally expandable microspheres (TEMs). The ideal foaming parameters for various density foams were determined. Moreover, a mechanical testing machine and split Hopkinson bar (SHPB) were utilized to explore the quasi-static and dynamic compressive properties. Results showed that the cell sizes of the as-prepared TEMs foams were in the micrometer range of 11 μm to 20 μm with a uniform cell size distribution. All the foams exhibited good compressive behavior under both quasi-static and high strain rate conditions, and were related to both foam densities and strain rates. The compressive strength of the TEMs foams at 8400 s−1 was up to 4 times higher than that at 10−4 s−1. The effects exerted by the strain rate and sample density were evaluated by a power law equation. With increasing density, the strain rate effect was more prominent. At quasistatic strain rates below 3000 s−1 regime, initial cell wall buckling and subsequent cellular structure flattening were the main failure mechanisms. However, in the high strain rate (HSR) regime (above 5000 s−1), the foams were split into pieces by the following transverse inertia force.

Water as Blowing Agent: Preparation of Environmental Thermally Expandable Microspheres via Inverse Suspension Polymerization

ABSTRACTWater-encapsulated environment-friendly core–shell thermally expandable microspheres (TEMs) were prepared via inverse suspension polymerization under atmospheric pressure. Acrylonitrile (AN), methyl methacrylic acid (MMA), and methacrylate (MA) were used as monomers, and water was used as the core material. The influences of dispersants, monomer feed ratio, and cross-linking agent on the TEMs were systematically investigated. The water could disperse well in the continuous oil phase employing both 6 g spand 80 and 0.2 g calcium stearate as dispersants. When the feed ratio of AN/MMA/MA was set at 1:2:2 and 1,4-butanediol dimethacrylate was used as cross-linking agent, TEMs possessed excellent properties. The properties of melamine resin foamed by TEMs-encapsulated water were better than that of n-octane and TEMs-encapsulated n-octane.