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

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.

Lightweight and Highly Compressible Expandable Polymer Microsphere/Silver Nanowire Composites for Wideband Electromagnetic Interference Shielding

The exploration of lightweight and compressible electromagnetic interference (EMI) shielding materials with outstanding shielding effectiveness (SE) is still a tremendous challenge in the elimination of electromagnetic pollution. Lightweight and highly compressible expandable polymer microsphere/silver nanowire (EPM/AgNW) composites with micron-sized closed pores and an interfacial AgNW conductive network are fabricated via a facile thermal expansion process in an enclosed space. The EPM/AgNW composites with AgNW loading of 0.127 vol % show low density (0.061 g/cm³), high compressibility and compression strength (4.25 MPa at 92.6% of compressive strain), and high EMI SE (over 40 dB, 1 mm) at a wideband of 8-40 GHz. Their EMI SE can be improved to a record 111.5 dB by increasing the AgNW content to 0.340 vol %, which corresponds to the surface-specific SE (SSE/d; SE divided by density and thickness) up to 13433 dB·cm²/g. The EMI shielding mechanism is further discussed using the finite-element analysis software COMSOL Multiphysics, and the application of the EPM/AgNW composites is visually demonstrated via near-field shielding in a practical antenna radiation. The overall properties of light weight, high elasticity, excellent mechanical strength, and outstanding EMI shielding performance suggest that the as-prepared EPM/AgNW composites have a great potential for applications in modern electronics.

Expansion Properties and Diffusion of Blowing Agent for Vinylidene Chloride Copolymer Thermally Expandable Microspheres

Vinylidene chloride copolymer microspheres were synthesized by in situ suspension copolymerization of vinylidene chloride (VDC), methyl methacrylate (MMA), and/or acrylonitrile (AN) in the presence of a paraffin blowing agent. The effects of shell polymer properties including compositions, glass transition temperature (T_g), crosslinking degree, blowing agent type, and encapsulation ratio (E_r) on the expansion properties of copolymer microspheres were investigated. Moreover, the diffusion properties of blowing agent in copolymer microspheres were studied. The results show that VDC-MMA-AN copolymer microspheres exhibited excellent expansion properties, and the volume expansion ratio (E_v) and the apparent density were decreased over 40 times, but it was difficult to expand for the VDC-MMA copolymer microspheres. In addition, the moderately crosslinked inside of the polymer shell enhanced the E_v more than 30 and the stable expansion temperature range (T_r) was about 30 °C by adding 0.2–0.4 wt% of divinyl benzene. The T_g of the shell polymer must be higher than the boiling point of the blowing agent as a prerequisite; the lower the boiling point of the blowing agent, the higher the internal gas pressure driven microsphere expansion, and the wider the T_r. By increasing the E_r of blowing agent improved the E_v of the microspheres. The diffusion of pentane blowing agent in VDC-MMA-AN copolymer microspheres were divided into Fick diffusion and non-Fick diffusion.

Foaming of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with Supercritical Carbon Dioxide: Foaming Performance and Crystallization Behavior

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) samples were successfully foamed using supercritical carbon dioxide as a physical foaming agent. PHBV sheets were first saturated at 175 °C followed by a foaming process at different temperatures (145 to 165 °C) and different CO2 pressures (10 to 29 MPa). It was found that microcellular structures with average cell sizes ranging from 6 to 22 μm and cell densities ranging from 108 to 1.2 × 109 cells/cm3 could be controllably prepared by selecting suitable foaming conditions. To investigate crystallization behavior during the foaming process and explore the corresponding foaming mechanism, differential scanning calorimetry, wide angle X-ray diffraction, and small-angle X-ray scattering characterizations were carried out. Stretching behavior during the cell growth stage may increase the crystal nucleation rate, and the generated crystal nucleus accelerates the crystallization rate as well as thickens PHBV crystal lamellae.