Fabrication of high-expansion microcellular PLA foams based on pre-isothermal cold crystallization and supercritical CO2 foaming

Synthesis of High-Value Bio-Based Polyamide 12,36 Microcellular Foams with Excellent Dimensional Stability and Shape Recovery Properties

The search for alternatives to petroleum-based thermoplastic polyamide elastomers (TPAEs) has recently drawn great interest. In this study, a bio-massed TPAE, PA12,36, was synthesized using 1,12-dodecanediamine (DDA) and fatty dimer acid (FDA, PripolTM1009) precursors via catalyst and solvent-free melt polycondensation. The molecular structure and molecular weight of the PA12,36 were characterized by 1H NMR, FTIR, and GPC. PA12,36 displayed a low melting temperature of 85.8 °C, an initial degradation temperature of 425 °C, and a glass-transition temperature of 30.4 °C, whereas it sustained satisfactory tensile strength (10.0 MPa) and superior strain at break (1378%). Furthermore, PA12,36 was foamed by supercritical CO2, and the cell size, cell density, and porosity were determined. The entangled long-chained FDA component generated a physically crosslinked network, which promoted the melt viscosity of PA12,36 against elongations of foam cell growth and increased foamability significantly. As a result, uniform structured cellular foams with a cell diameter of 15-24 µm and high cell density (1011 cells/cm3-1012 cells/cm3) were successfully achieved. The foaming window was widened from 76 to 81 °C, and the expansion ratio was increased from 4.8 to 9.6. Additionally, PA12,36 foam with a physically crosslinked structure presented a better creep shape recovery percentage (92-97.9%) and sturdier dimensional stability. This bio-based PA12,36 foam is a promising candidate to replace petroleum-based thermoplastic elastomer foams for engineering applications, particularly shoe soles.

Fabrication of poly(lactic acid) foam with high expansion ratio and oriented cellular structure by restricting cold crystallization

The foaming behavior of semi-crystalline polymers is significantly affected by their crystallization. To achieve high expansion ratio of poly(lactic acid) (PLA) foams, we thought its cold crystallization should be restricted. Therefore, we used a short soaking time of CO2 to fabricate high-expansion PLA foams. Dynamic mechanical analysis of unfoamed PLA showed that only one rubbery plateau was observed owing to complete cold crystallization under a soaking time of 40 min at 10 MPa. The crystal morphology demonstrated that a short soaking time of 3 min could restrict the cold crystallization of PLA. Owing to plasticization of CO2, PLA crystallization of was accelerated at low temperatures (40-80 °C) but hindered at high temperatures (80-130 °C) at 10 MPa. Foaming results showed that under a soaking time of 3 min, a high expansion ratio exceeding 10 was achievable over a wide foaming temperature range of 90 to 115 °C because more amorphous regions were preserved at 10 MPa. In addition, the prepared foams presented an oriented cellular structure. Compared with the isotropic foam, the anisotropic foam exhibited higher compressive strength and heat resistance. The prepared PLA foams have good application prospects in the fields such as cushioning, packaging, and construction.

In Situ Nanofibrillar Polypropylene-Based Composite Microcellular Foams with Enhanced Mechanical and Flame-Retardant Performances

With the increasing demand for plastic components, the development of lightweight, high strength and functionalized polypropylene (PP) from a cost-effective and environmentally friendly process is critical for resource conservation. In situ fibrillation (INF) and supercritical CO2 (scCO2) foaming technology were combined in this work to fabricate PP foams. Polyethylene terephthalate (PET) and poly(diaryloxyphosphazene)(PDPP) particles were applied to fabricate in situ fibrillated PP/PET/PDPP composite foams with enhanced mechanical properties and favorable flame-retardant performance. The existence of PET nanofibrils with a diameter of 270 nm were uniformly dispersed in PP matrix and served multiple roles by tuning melt viscoelasticity for improving microcellular foaming behavior, enhancing crystallization of PP matrix and contributing to improving the uniformity of PDPP’s dispersion in INF composite. Compared to pure PP foam, PP/PET(F)/PDPP foam exhibited refined cellular structures, thus the cell size of PP/PET(F)/PDPP foam was decreased from 69 to 23 μm, and the cell density increased from 5.4 × 106 to 1.8 × 108 cells/cm3. Furthermore, PP/PET(F)/PDPP foam showed remarkable mechanical properties, including a 975% increase in compressive stress, which was attributed to the physical entangled PET nanofibrils and refined cellular structure. Moreover, the presence of PET nanofibrils also improved the intrinsic flame-retardant nature of PDPP. The synergistical effect of the PET nanofibrillar network and low loading of PDPP additives inhibited the combustion process. These gathered advantages of PP/PET(F)/PDPP foam make it promising for lightweight, strong, and fire-retardant polymeric foams.

Enhanced heat resistance and expansion ratio of biodegradable poly (lactic acid)/poly (butylene adipate-co-terephthalate) composite foams via synergistic effect of nucleating agent and chain extension

Environmentally friendly foams have been studied extensively to handle the plastic pollution caused by disposable tableware. Herein, biodegradable poly (lactic acid) (PLA) – poly (butylene adipate-co-terephthalate) (PBAT) composites were first fabricated by melt blending, in which a reactive chain extender (ADR) was incorporated to enhance the interfacial compatibility and melt strength, a nucleating agent (TMC) was applied to improve the crystallinity. Subsequently, the foaming behavior of the composites was investigated via supercritical CO2 foaming. Specifically, a spherulite and shish-kebab-like nanoporous structure was observed from PLA and PLA/TMC foams, respectively, when the soaking temperature was far below melting temperature. With an increase of foaming temperature, the unmelted completely crystals served as physical cross-linking points, also contributing to a high melt strength. Combined with chain extension reaction of ADR, the PLA–PBAT/ADR/TMC foams showed the largest cell diameter and expansion ratio, which were 19 μm and 8.9, respectively. Furthermore, the heat resistance of PLA–PBAT/ADR/TMC foams was also significantly improved, owing to the high crystallinity induced by TMC. Consequently, the composite foams with superior heat resistance and toughness were obtained to broaden its application as biodegradable disposable tableware for hot food.

Nanocomposite Foams with Balanced Mechanical Properties and Energy Return from EVA and CNT for the Midsole of Sports Footwear Application

Polymer foam that provides good support with high energy return (low energy loss) is desirable for sport footwear to improve running performance. Ethylene-vinyl acetate copolymer (EVA) foam is commonly used in the midsole of running shoes. However, EVA foam exhibits low mechanical properties. Conventional mineral fillers are usually employed to improve EVA's mechanical performance, but the energy return is sacrificed. Here, we produced nanocomposite foams from EVA and multi-walled carbon nanotubes (CNT) using a chemical foaming process. Two kinds of CNT derived from the upcycling of commodity plastics were prepared through a catalytic chemical vapor deposition process and used as reinforcing and nucleating agents. Our results show that EVA foam incorporated with oxygenated CNT (O-CNT) demonstrated a more pronounced improvement of physical, mechanical, and dynamic impact response properties than acid-purified CNT (A-CNT). When CNT with weight percentage as low as 0.5 wt% was added to the nanocomposites, the physical properties, abrasion resistance, compressive strength, dynamic stiffness, and rebound performance of the EVA foams were improved significantly. Unlike the conventional EVA formulation filled with talc mineral fillers, the incorporation of CNT does not compromise the energy return of the EVA foam. From the long-cycle dynamic fatigue test, the CNT/EVA foam displays greater properties retention as compared to the talc/EVA foam. This work demonstrates a good balanced of mechanical-energy return properties of EVA nanocomposite foam with very low CNT content, which presents promising opportunities for lightweight-high rebound midsoles for running shoes.

Green Processing of Neat Poly(lactic acid) Using Carbon Dioxide under Elevated Pressure for Preparation of Advanced Materials: A Review (2012-2022)

This review provides a concise overview of up-to-date developments in the processing of neat poly(lactic acid) (PLA), improvement in its properties, and preparation of advanced materials using a green medium (CO₂ under elevated pressure). Pressurized CO₂ in the dense and supercritical state is a superior alternative medium to organic solvents, as it is easily available, fully recyclable, has easily tunable properties, and can be completely removed from the final material without post-processing steps. This review summarizes the state of the art on PLA drying, impregnation, foaming, and particle generation by the employment of dense and supercritical CO₂ for the development of new materials. An analysis of the effect of processing methods on the final material properties was focused on neat PLA and PLA with an addition of natural bioactive components. It was demonstrated that CO₂-assisted processes enable the control of PLA properties, reduce operating times, and require less energy compared to conventional ones. The described environmentally friendly processing techniques and the versatility of PLA were employed for the preparation of foams, aerogels, scaffolds, microparticles, and nanoparticles, as well as bioactive materials. These PLA-based materials can find application in tissue engineering, drug delivery, active food packaging, compostable packaging, wastewater treatment, or thermal insulation, among others.

Applications and Challenges of Supercritical Foaming Technology

With economic development, environmental problems are becoming more and more prominent, and achieving green chemistry is an urgent task nowadays, which creates an opportunity for the development of supercritical foaming technology. The foaming agents used in supercritical foaming technology are usually supercritical carbon dioxide (ScCO₂) and supercritical nitrogen (ScN₂), both of which are used without environmental burden. This technology can reduce the environmental impact of polymer foam production. Although supercritical foaming technology is already in production in some fields, it has not been applied on a large scale. Here, we present a detailed analysis of the types of foaming agents currently used in supercritical foaming technology and their applications in various fields, summarizing the technological improvements that have been made to the technology. However, we have found that today's supercritical technologies still need to address some additional challenges to achieve large-scale production.

Self-assembled polylactic acid (PLA): Synthesis, properties and biomedical applications

The surface morphology and topography of cell culture substrates play an important role in cell proliferation and growth. Regulation of the surface microstructure allows the development of tissue culture media suitable for different cells. Polylactic acid (PLA) is a biobased and biodegradable (under defined conditions) polymer with low immunogenicity, non-toxicity, and good mechanical properties, which have facilitated their pharmaceutical and biomedical applications. This review summarizes recent advances in the synthesis and self-assembly of surface microstructure based on PLA materials and discusses their biomedical applications such as cell culturing and tissue engineering.

Production and Application of Polymer Foams Employing Supercritical Carbon Dioxide

Polymeric foams have characteristics that make them attractive for different applications. However, some foaming methods rely on chemicals that are not environmentally friendly. One of the possibilities to tackle the environmental issue is to utilize supercritical carbon dioxide ScCO2 since it is a “green” solvent, thus facilitating a sustainable method of producing foams. ScCO2 is nontoxic, chemically inert, and soluble in molten plastic. It can act as a plasticizer, decreasing the viscosity of polymers according to temperature and pressure. Most foam processes can benefit from ScCO2 since the methods rely on nucleation, growth, and expansion mechanisms. Process considerations such as pretreatment, temperature, pressure, pressure drop, and diffusion time are relevant parameters for foaming. Other variables such as additives, fillers, and chain extenders also play a role in the foaming process. This review highlights the morphology, performance, and features of the foam produced with ScCO2, considering relevant aspects of replacing or introducing a novel foam. Recent findings related to foaming assisted by ScCO2 and how processing parameters influence the foam product are addressed. In addition, we discuss possible applications where foams have significant benefits. This review shows the recent progress and possibilities of ScCO2 in processing polymer foams.

Progress in the Preparation, Properties, and Applications of PLA and Its Composite Microporous Materials by Supercritical CO2: A Review from 2020 to 2022

The development of degradable plastic foams is in line with the current development concept of being pollution free and sustainable. Poly(lactic acid) (PLA) microporous foam with biodegradability, good heat resistance, biocompatibility, and mechanical properties can be successfully applied in cushioning packaging, heat insulation, noise reduction, filtration and adsorption, tissue engineering, and other fields. This paper summarizes and critically evaluates the latest research on preparing PLA microporous materials by supercritical carbon dioxide (scCO₂) physical foaming since 2020. This paper first introduces the scCO₂ foaming technologies for PLA and its composite foams, discusses the CO₂-assisted foaming processes, and analyzes the effects of process parameters on PLA foaming. After that, the paper reviews the effects of modification methods such as chemical modification, filler filling, and mixing on the rheological and crystallization behaviors of PLA and provides an in-depth analysis of the mechanism of PLA foaming behavior to provide theoretical guidance for future research on PLA foaming. Lastly, the development and applications of PLA microporous materials based on scCO₂ foaming technologies are prospected.

Supercritical Fluid Technologies for the Incorporation of Synthetic and Natural Active Compounds into Materials for Drug Formulation and Delivery

Various active compounds isolated from natural sources exhibit remarkable benefits, making them attractive for pharmaceutical and biomedical applications, such as antioxidant, antimicrobial, and anti-inflammatory activities, which contribute to the treatment of cardiovascular diseases, neurodegenerative disorders, various types of cancer, diabetes, and obesity. However, their major drawbacks are their reactivity, instability, relatively poor water solubility, and consequently low bioavailability. Synthetic drugs often face similar challenges associated with inadequate solubility or burst release in gastrointestinal media, despite being otherwise a safe and effective option for the treatment of numerous diseases. Therefore, drug-eluting pharmaceutical formulations have been of great importance over the years in efforts to improve the bioavailability of active compounds by increasing their solubility and achieving their controlled release in body media. This review highlights the success of the fabrication of micro- and nanoformulations using environmentally friendly supercritical fluid technologies for the processing and incorporation of active compounds. Several novel approaches, namely micronization to produce micro- and nano-sized particles, supercritical drying to produce aerogels, supercritical foaming, and supercritical solvent impregnation, are described in detail, along with the currently available drug delivery data for these formulations.

Preparation and mechanism of lightweight wood fiber/poly(lactic acid) composites

The high density and poor thermal insulation of traditional wood-plastic composites limited the application in the field of building materials. In this paper, wood fiber (WF) and PLA were used as raw materials and azodicarbonamide was used as the foaming agent. Lightweight WF/PLA composites were prepared by the hot-pressing foaming method, aiming to obtain renewable, low-density material with high strength-to-weight ratio and thermal insulation performance. The results showed that after adding 20 % WF into PLA, the cell morphology was excellent and the cell size was uniform. The magnification reached the minimum value of 0.36 g/cm³ and the foaming magnification was 3.42 times. The impact strength and compressive strength were 3.16 kJ/m³ and 4.12 MPa, its comprehensive mechanical properties were outstanding. The thermal conductivity of foamed materials was 0.110-0.148 (W/m·K), which was significantly lower than that of unfoamed materials and common wood. Its excellent mechanical properties and thermal insulation can be suitable for application in the construction field to replace traditional wood.

Microcellular injection molded lightweight and tough poly (L-lactic acid)/in-situ polytetrafluoroethylene nanocomposite foams with enhanced surface quality and thermally-insulating performance

High-performance microcellular polymer foams have been widely used all over the world, while the excessive usage of petroleum-based polymers caused serious environmental problems. As the eco-friendly awareness is increasing significantly, poly (L-lactic acid) (PLLA), as a typical biomass polymer, has gradually attracted widespread attention. However, the slow crystallization and poor melt strength of PLLA lead to low foaming ability and thus limiting its industrial applications. Herein, a novel and scalable strategy by coupling in-situ fibrillation and mold-opening microcellular injection molding (MOMIM) was developed to fabricate lightweight and tough PLLA/polytetrafluoroethylene (PTFE) foams. Thanks to the reticulated in-situ PTFE nanofibrils with a diameter of 100-200 nm, the crystallization and viscoelasticity of PLLA were dramatically promoted, and further contributing to its foaming ability. The expansion ratio of the MOMIM PLLA/PTFE foam was increased by 86 % compared with the regular microcellular injection molded (RMIM) PLLA foam. Moreover, the lower foam density and the toughening effect of PTFE nanofibrils resulted in the outstanding ductility of the PLLA/PTFE foams, whose tensile elongation, flexural strength, and impact strength were maximally increased by 52 %, 28 %, and 48 %, compared with PLLA foams. More importantly, the thermally-insulating performance and surface quality of PLLA/PTFE foams were also greatly improved.

Fabrication of super-hydrophilic and highly open-porous poly (lactic acid) scaffolds using supercritical carbon dioxide foaming

Porous poly (lactic acid) (PLA)-based scaffolds have been widely used as a promising product in tissue engineering. However, it is still a challenge to prepare the PLA-based scaffolds with high expansion ratio, good hydrophilicity, and excellent cytocompatibility by a green and cost-effective fabrication approach. Herein, we prepared porous PLA-based scaffolds using carbon dioxide (CO₂) as the physical foaming agent. To improve the hydrophilicity and foaming behavior of PLA, poly (ethylene glycol) (PEG) was selected as a good additive to blend with PLA. It revealed that the introduction of PEG could improve the foaming behavior of PLA and promote the formation of opening cells via reducing the matrix strength of PLA. The obtained 3D PLA/PEG scaffolds exhibited high expansion ratio (9.1), high open-cell content (95.2%), and super-hydrophilicity (water contact angle 0°). Additionally, the mouse fibroblast NIH/3T3 cells with live/dead cell fluorescence staining assay was utilized to examine the biocompatibility of PLA/PEG scaffolds. The result demonstrated that the proliferation ratio of NIH/3 T3 cells on the surface of PLA/PEG scaffolds was higher than that of PLA scaffolds, indicating that the highly interconnected cell structure was conducive to cell adhesion and attachment. Consequently, such hydrophilic open-cell structure obtained by adding PEG into PLA possesses great potential for use in tissue engineering.

Flexibly Controlling the Polycrystallinity and Improving the Foaming Behavior of Polylactic Acid via Three Strategies

Controlling foamability plays the central role in preparing PLA foams with high performances. To achieve this, chain extension was often used to improve the rheological property of PLA resins; however, despite the availability of this approach, it often deteriorates the biodegradability of PLA and greatly increases the processing cost and complexity. Hence, we reported a special crystallization induction method to design PLA foams with a tunable cellular structure and a high expansion ratio. A novel crystallization-promoting agent combination (D-sorbitol, CO₂, and phenylphosphonic acid zinc salt) was used to induce PLA to enhance the chain interaction force and chain mobility and to provide crystallization templets. A series of PLAs with tunable stereocomplex (Sc)/α crystallinity and rapid non-isothermal crystallization ability were obtained. The effect of various crystallization properties on the foaming behavior of PLA was studied. The results demonstrated that proper crystallization conditions (a small spherulite size, a crystallinity of 6%, and rapid crystallization ability) could virtually contribute to the optimized cellular structure with the highest cell density of 4.36 × 10⁶ cell/cm³. When the Sc crystallinity was above 10%, PLA had a superior foamability, which thereby resulted in a high foaming expansion ratio of 16.2. A variety of cellular morphologies of PLA foams could be obtained by changing the foaming temperature and the crystallization property. The proposed crystallization-induced approach provided a useful method for controlling the cellular structure and the performances of the PLA foams.

Ultrahigh and Tunable Electromagnetic Interference Shielding Performance of PVDF Composite Induced by Nano-Micro Cellular Structure

Lightweight and efficient electromagnetic interference (EMI) shielding materials play a vital role in protecting high-precision electronic devices and human health. Porous PVDF/CNTs/urchin-like Ni composites with different cell sizes from nanoscale to microscale were fabricated through one-step supercritical carbon dioxide (CO₂) foaming. The electrical conductivity and electromagnetic interference (EMI) shielding performance of the composites with different cell sizes were examined in detail. The results indicated that the nanoscale cell structure diminishes the EMI shielding performance of the composite, whereas the microscale cell structure with an appropriate size is beneficial for improving the EMI shielding performance. A maximum EMI shielding effectiveness (SE) of 43.4 dB was achieved by the composite foams which is about twice that of the solid composite. Furthermore, as the supercritical CO₂ foaming process reduces the density of the composite by 25–50%, the EMI SSE (specific shielding effectiveness)/t(thickness) of the composite reaches 402 dB/(g/cm²), which is the highest value of polymer foam obtained to the best of the authors’ knowledge. Finally, compression tests were performed to show that the composites still maintained excellent mechanical properties after the supercritical CO₂ foaming process.

Preparation of high-expansion open-cell polylactic acid foam with superior oil-water separation performance

Environmentally friendly and non-toxic polylactic acid (PLA) foam has shown great application prospects in heat preservation, adsorption and other fields. However, it is still challenging to prepare high-expansion PLA foam. Herein, a cooling batch foaming process under supercritical CO₂ (sc-CO₂), based on pre-melted non-crystalline state, was proposed to prepare PLA foams with high expansion ratio. The CO₂ dissolved in the polymer melt will lower the crystallization temperature of PLA. Due to the lack of crystallization, the foaming temperature of PLA can be reduced, which increases the CO₂ saturation and helps foam. When foaming is triggered before crystallization, ultrahigh expansion foam can be produced. Based on pre-melting treatment, the maximum expansion ratio of PLA has reached 59.7-fold. At the same time, an open-pore structure is produced by this method, which can selectively absorb oil from water. In addition, the adsorption capacity of CCl₄ reaches 15 g/g, and there is no significant attenuation in 20 adsorption-desorption cycles. This work provides a green, solvent-free method to prepare biodegradable oil-adsorbing foam.