Past and present developments in polymer bead foams and bead foaming technology

Investigation of Recycled Expanded Polyamide Beads through Artificial Ageing and Mechanical Recycling as a Proof of Concept for Circular Economy

Car manufacturers are currently challenged with increasing the sustainability of their products and production to comply with sustainability requirements and legislation. One way to enhance product sustainability is by reducing the carbon footprint of fossil-based plastic parts. Particle foams are a promising solution to achieve the goal of using lightweight parts with minimal material input. Ongoing developments involve the use of expanded particle foam beads made from engineering plastics such as polyamide (EPA). To achieve this, a simulated life cycle was carried out on virgin EPA, including mechanical recycling. The virgin material was processed into specimens using a steam-free method. One series was artificially aged to replicate automotive life cycle stresses, while the other series was not. The mechanical recycling and re-foaming of the minipellets were then carried out, resulting in an EPA particle foam with 100% recycled content. Finally, the thermal and chemical material properties were comparatively analysed. The study shows that the recycled EPA beads underwent polymer degradation during the simulated life cycle, as evidenced by their material properties. For instance, the recycled beads showed a more heterogeneous molecular weight distribution (an increase in PDI from two to three), contained carbonyl groups, and exhibited an increase in the degree of crystallization from approximately 24% to 36%.

Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives

The last few decades have witnessed significant advances in the development of polymeric-based foam materials. These materials find several practical applications in our daily lives due to their characteristic properties such as low density, thermal insulation, and porosity, which are important in packaging, in building construction, and in biomedical applications, respectively. The first foams with practical applications used polymeric materials of petrochemical origin. However, due to growing environmental concerns, considerable efforts have been made to replace some of these materials with biodegradable polymers. Foam processing has evolved greatly in recent years due to improvements in existing techniques, such as the use of supercritical fluids in extrusion foaming and foam injection moulding, as well as the advent or adaptation of existing techniques to produce foams, as in the case of the combination between additive manufacturing and foam technology. The use of supercritical CO2 is especially advantageous in the production of porous structures for biomedical applications, as CO2 is chemically inert and non-toxic; in addition, it allows for an easy tailoring of the pore structure through processing conditions. Biodegradable polymeric materials, despite their enormous advantages over petroleum-based materials, present some difficulties regarding their potential use in foaming, such as poor melt strength, slow crystallization rate, poor processability, low service temperature, low toughness, and high brittleness, which limits their field of application. Several strategies were developed to improve the melt strength, including the change in monomer composition and the use of chemical modifiers and chain extenders to extend the chain length or create a branched molecular structure, to increase the molecular weight and the viscosity of the polymer. The use of additives or fillers is also commonly used, as fillers can improve crystallization kinetics by acting as crystal-nucleating agents. Alternatively, biodegradable polymers can be blended with other biodegradable polymers to combine certain properties and to counteract certain limitations. This work therefore aims to provide the latest advances regarding the foaming of biodegradable polymers. It covers the main foaming techniques and their advances and reviews the uses of biodegradable polymers in foaming, focusing on the chemical changes of polymers that improve their foaming ability. Finally, the challenges as well as the main opportunities presented reinforce the market potential of the biodegradable polymer foam materials.

Comparison of Mechanical Properties of Steam-Free and Steam-Based Specimens Made of Expanded Polypropylene Beads

Reducing the CO2 emissions of plastic parts is crucial in terms of sustainable product and process designs. Approaches include the use of recycled materials and reducing the energy demands of processes through more efficient technologies. In this context, this study shows the potential of the steam-free processing of particle foam beads into thin-walled moulded parts. Expanded polypropylene (EPP) particle foam beads have been processed in both a steam-free and steam-based process. For this purpose, specimens with different part densities and thicknesses were produced, the mechanical properties were investigated, and the surface quality was discussed. Specimens made of EPP with a part thickness of 5 to 20 mm and part densities of 60 to 185 g/L were produced steam-free. Lower part thicknesses and higher densities increase the mechanical properties. As the density increased, the homogeneity of the surfaces of the steam-free specimens also increased. In comparison, specimens with a thickness of 10 mm and part densities of 35 to 90 g/L were produced on a steam-based process. The results of the mechanical test were compared with those of the steam-free specimens. The steam-based specimens showed higher mechanical properties for the same density.

RF Welding of Dielectric Lossless Foam Particles by the Application of a Dielectric Heatable Coating with High Recycling Potential

Due to its chemical structure and the resulting dielectric properties, the processing of the commonly used particle foam material, expanded polypropylene (ePP), is limited. Processing within the radio-frequency welding process is therefore only possible with the use of processing aids. In this paper, a new approach for the use of a solid and dielectric heatable coating for the production of three-dimensional welded components out of ePP is presented. For this purpose, three different types of water-soluble polymer polyvinyl alcohol (PVA) were analyzed as potential coating materials. The thermal and dielectric properties of the coating were further adjusted by a modification with glycerol. The maximum amount of glycerol tested was 25% by volume. It influences both the temperature development in the radio-frequency (RF) welding process as well as the adhesive bond between the ePP foam particles. It is shown that the 120 °C approach in the RF welding process resulted in a cohesive bond between the coating layers. In this way, bonded plates can be produced. In mechanical tests with compression of 20%, the manufactured plates show sufficient load capacity. Furthermore, it can be shown that a separation of PVA and ePP by type, and thereby a separation of the foam particles, is possible with the use of hot water. This might open a new way for recycling of particle foams.

Effect of Janus Nanosheets in Polypropylene on Rheological Properties and Autoclave Foam Performance

Our experiment revealed that the addition of Janus nanosheets to polypropylene (PP) has a significant impact on the viscoelasticity of the composite system. Specifically, when 0.10 wt% of Janus nanosheets were added, the complex viscosity of the composite system increased. However, when we added less than 0.05 wt% of Janus nanosheets, there was a reduction in complex viscosity, which is known as the non-Einstein phenomenon. The Cole-Cole plot showed that the nanosheet network structure did not have a significant effect on the viscosity of the composite system. Additionally, we used carbon dioxide as a foaming agent to autoclave foaming using modified PP from Janus nanosheets, and the results demonstrated that increasing the number of Janus nanosheets decreased the apparent density and strengthened the cell structure of foaming beads, resulting in improved closed porosity.

Autoclave foaming and steam-chest molding of polypropylene/polybutene-1 blend bead foams and their crystallization and mechanical properties

Expanded polypropylene (EPP) foams have showed wide applications in our daily life, such as automotive and packaging. Usually, autoclave foaming combined with steam-chest molding is the main artwork to prepare the high-precision EPP foam products. However, the foaming behavior of EPP and the excessive pressure required for molding still need to be further improved, which is great significance for energy saving and cost saving, etc. Herein, this study finds that adding a certain amount of polybutene-1 (PB-1) into the PP can help to reduce the temperature and pressure required for foaming/molding, and to broaden the foaming temperature. For example, in order to make the foam beads bonding well and with the expansion ratio of 20, the molding pressure should be higher than 2.7 bar for Neat PP foams, but just 1.5 bar for PP/PB-1 mixtures. Moreover, the effects of PB-1 content on the crystallization properties and foaming/molding behaviors of the PP/PB-1 bead foams are illustrated, and then the mechanical properties are also studied. Furthermore, the low-pressure foaming strategy presented here is beneficial for reducing the barriers of energy consumption and promoting the development of new bead foam materials.

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.

Supercritical Fluid Microcellular Foaming of High-Hardness TPU via a Pressure-Quenching Process: Restricted Foam Expansion Controlled by Matrix Modulus and Thermal Degradation

High-hardness thermoplastic polyurethane (HD-TPU) presents a high matrix modulus, low-temperature durability, and remarkable abrasion resistance, and has been used in many advanced applications. However, the fabrication of microcellular HD-TPU foam is rarely reported in the literature. In this study, the foaming behavior of HD-TPU with a hardness of 75D was investigated via a pressure-quenching foaming process using CO₂ as a blowing agent. Microcellular HD-TPU foam with a maximum expansion ratio of 3.9-fold, a cell size of 25.9 μm, and cell density of 7.8 × 10⁸ cells/cm³ was prepared, where a high optimum foaming temperature of about 170 °C had to be applied with the aim of softening the polymer's matrix modulus. However, the foaming behavior of HD-TPU deteriorated when the foaming temperature further increased to 180 °C, characterized by the presence of coalesced cells, microcracks, and a high foam density of 1.0 g/cm³ even though the crystal domains still existed within the matrix. The cell morphology evolution of HD-TPU foam was investigated by adjusting the saturation time, and an obvious degradation occurred during the high-temperature saturation process. A cell growth mechanism of HD-TPU foams in degradation environments was proposed to explain this phenomenon based on the gas escape through the defective matrix.

Understanding the Influence of Gypsum upon a Hybrid Flame Retardant Coating on Expanded Polystyrene Beads

A low-cost and effective flame retarding expanded polystyrene (EPS) foam was prepared herein by using a hybrid flame retardant (HFR) system, and the influence of gypsum was studied. The surface morphology and flame retardant properties of the synthesized flame retardant EPS were characterized using scanning electron microscopy (SEM) and cone calorimetry testing (CCT). The SEM micrographs revealed the uniform coating of the gypsum-based HFR on the EPS microspheres. The CCT and thermal conductivity study demonstrated that the incorporation of gypsum greatly decreases the peak heat release rate (PHRR) and total heat release (THR) of the flame retarding EPS samples with acceptable thermal insulation performance. The EPS/HFR with a uniform coating and the optimum amount of gypsum provides excellent flame retardant performance, with a THR of 8 MJ/m2, a PHRR of 53.1 kW/m2, and a fire growth rate (FIGRA) of 1682.95 W/m2s. However, an excessive amount of gypsum weakens the flame retardant performance. The CCT results demonstrate that a moderate gypsum content in the EPS/HFR sample provides appropriate flame retarding properties to meet the fire safety standards.

Progress in the development of bead foams – A review

For a long time, the number of available bead foam variants limited to standard polymers which restricted their functionality mainly to packaging, thermal insulation (e.g. in construction) and shock absorption (e.g. in transportation). In particular, standard polymers such as expanded polystyrene, expanded polyethylene and expanded polypropylene were used for components requiring good insulating properties and high energy absorption at low cost. Mainly since the last two decades, new polymer variants have found their way into the world of bead foams and are currently adding further functionalities, such as sustainability, flame retardancy, increased thermal stability and enhanced mechanical performance (e.g. improvements in energy absorption and impact resistance). Versatile fields of application open up, revolutionizing both industry and design sectors. This review article emphasizes the special development progress of new bead foam variants and their processing technologies. Upcoming opportunities of digital methods for modelling and simulation are highlighted.

Foam 3D Printing of Thermoplastics: A Symbiosis of Additive Manufacturing and Foaming Technology

Due to their light-weight and cost-effectiveness, cellular thermoplastic foams are considered as important engineering materials. On the other hand, additive manufacturing or 3D printing is one of the emerging and fastest growing manufacturing technologies due to its advantages such as design freedom and tool-less production. Nowadays, 3D printing of polymer compounds is mostly limited to manufacturing of solid parts. In this context, a merged foaming and printing technology can introduce a great alternative for the currently used foam manufacturing technologies such as foam injection molding. This perspective review article tackles the attempts taken toward initiating this novel technology to simultaneously foam and print thermoplastics. After explaining the basics of polymer foaming and additive manufacturing, this article classifies different attempts that have been made toward generating foamed printed structures while highlighting their challenges. These attempts are clustered into 1) architected porous structures, 2) syntactic foaming, 3) post-foaming of printed parts, and eventually 4) printing of blowing agents saturated filaments. Among these, the latest approach is the most practical route although it has not been thoroughly studied yet. A filament free approach that can be introduced as a potential strategy to unlock the difficulties to produce printed foam structures is also proposed.

Expanded Beads of High Melt Strength Polypropylene Moldable at Low Steam Pressure by Foam Extrusion

We explore the foam extrusion of expanded polypropylene with a long chain branched random co-polypropylene to make its production process simpler and cheaper. The results show that the presence of long chain branches infer high melt strength and, hence, a wide foamability window. We explored the entire window of foaming conditions (namely, temperature and pressure) by means of an ad-hoc extrusion pilot line design. It is shown that the density of the beads can be varied from 20 to 100 kg/m³ using CO₂ and isobutane as a blowing agent. The foamed beads were molded by steam-chest molding using moderate steam pressures of 0.3 to 0.35 MPa independently of the closed cell content. A characterization of the mechanical properties was performed on the molded parts. The steam molding pressure for sintering expanded polypropylene beads with a long chain branched random co-polypropylene is lower than the one usually needed for standard polypropylene beads by extrusion. The energy saving for the sintering makes the entire manufacturing processes cost efficient and can trigger new applications.

"Toolbox" for the Processing of Functional Polymer Composites

The processing methods of functional polymer composites (FPCs) are systematically summarized in “Toolbox”. The relationship of processing method-structure-property is discussed and the selection and combination of tools in processing among different FPCs are analyzed. A promising prospect is provided regarding the design principle for high performance FPCs for further investigation.

ABSTRACT

Functional polymer composites (FPCs) have attracted increasing attention in recent decades due to their great potential in delivering a wide range of functionalities. These functionalities are largely determined by functional fillers and their network morphology in polymer matrix. In recent years, a large number of studies on morphology control and interfacial modification have been reported, where numerous preparation methods and exciting performance of FPCs have been reported. Despite the fact that these FPCs have many similarities because they are all consisting of functional inorganic fillers and polymer matrices, review on the overall progress of FPCs is still missing, and especially the overall processing strategy for these composites is urgently needed. Herein, a “Toolbox” for the processing of FPCs is proposed to summarize and analyze the overall processing strategies and corresponding morphology evolution for FPCs. From this perspective, the morphological control methods already utilized for various FPCs are systematically reviewed, so that guidelines or even predictions on the processing strategies of various FPCs as well as multi-functional polymer composites could be given. This review should be able to provide interesting insights for the field of FPCs and boost future intelligent design of various FPCs. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00774-5.

Two-Dimensional Correlation Analysis of iPP Bead Foaming Thermal Features Modeled by Fast Scanning Calorimetry

The bead foaming of semicrystalline polymers is a complex thermal process involving the formation of multiple crystalline populations, which serve the dual purposes of ensuring the structural integrity of beads while also allowing bead sintering at the interface. The quality of this "double melting peak" structure is determined by the temperature and duration of the isothermal treatment of the beads as well as the quenching rate following the isotherm. Currently, the intricacies of the quenching process are not very well-known due to the challenge of replicating these rapid cooling rates in a laboratory setting. Fast-scanning calorimetry was used to reproduce these conditions for isotactic polypropylene (iPP), revealing optimal quenching rates for the bead foaming of iPP. We further probed these thermal features using two-dimensional correlation analysis as a tool to understand the dynamics, interdependence, and relative contributions of multiple thermal events such as glass transition, mesophase formation, cold crystallization, and melting in response to the perturbation of the quenching rate.

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

In recent decades, quantum chemical calculations (QCC) have increased in accuracy, not only providing the ranking of chemical reactivities and energy barriers (e.g., for optimal selectivities) but also delivering more reliable equilibrium and (intrinsic/chemical) rate coefficients. This increased reliability of kinetic parameters is relevant to support the predictive character of kinetic modeling studies that are addressing actual concentration changes during chemical processes, taking into account competitive reactions and mixing heterogeneities. In the present contribution, guidelines are formulated on how to bridge the fields of computational chemistry and chemical kinetics. It is explained how condensed phase systems can be described based on conventional gas phase computational chemistry calculations. Case studies are included on polymerization kinetics, considering free and controlled radical polymerization, ionic polymerization, and polymer degradation. It is also illustrated how QCC can be directly linked to material properties.

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.

In-Situ Visualization of the Cell Formation Process of Foamed Polypropylene under Different Foaming Environments

In this paper, the dynamic foaming process of micro-foaming polypropylene (PP) in different foaming environments in real time was obtained via a visualization device. The relationship curve between cell number (n) and foaming time (t) was plotted, and then the nucleation kinetics of foam cells was analyzed. Results showed that the formation rate of cells changed obviously with the variation of melt temperature and the content of the foaming agent. The n-t curves presented a typical "S" shape, which indicated that the appearance of the cell number increased slowly in the initial foaming period, then increased rapidly in a short time, and finally maintained a certain value. When a certain pressure was applied to the PP melt, the external force had a great influence on the n-t curve. With the increasing external force, the rate of cell formation increased rapidly, and the shape of the n-t curve changed from "S" to "semi-S" without an obvious slow increase. The investigation of the n-t relationship in the PP dynamic foaming process under different foaming environments could provide effective bases for improving the foaming quality of injection molding foaming materials.

Compression Molding of Thermoplastic Polyurethane Foam Sheets with Beads Expanded by Supercritical CO2 Foaming

Expanded thermoplastic polyurethane (ETPU) beads were prepared by a supercritical CO₂ foaming process and compression molded to manufacture foam sheets. The effect of the cell structure of the foamed beads on the properties of the foam sheets was studied. Higher foaming pressure resulted in a greater number of cells and thus, smaller cell size, while increasing the foaming temperature at a fixed pressure lowered the viscosity to result in fewer cells and a larger cell size, increasing the expansion ratio of the ETPU. Although the processing window in which the cell structure of the ETPU beads can be maintained was very limited compared to that of steam chest molding, compression molding of ETPU beads to produce foam sheets was possible by controlling the compression pressure and temperature to obtain sintering of the bead surfaces. Properties of the foam sheets are influenced by the expansion ratio of the beads and the increase in the expansion ratio increased the foam resilience, decreased the hardness, and increased the tensile strength and elongation at break.

Insights into the Bead Fusion Mechanism of Expanded Polybutylene Terephthalate (E-PBT)

Expandable polystyrene (EPS) and expanded polypropylene (EPP) dominate the bead foam market. As the low thermal performance of EPS and EPP limits application at elevated temperatures novel solutions such as expanded polybutylene terephthalate (E-PBT) are gaining importance. To produce parts, individual beads are typically molded by hot steam. While molding of EPP is well-understood and related to two distinct melting temperatures, the mechanisms of E-PBT are different. E-PBT shows only one melting peak and can surprisingly only be molded when adding chain extender (CE). This publication therefore aims to understand the impact of thermal properties of E-PBT on its molding behavior. Detailed differential scanning calorimetry was performed on neat and chain extended E-PBT. The crystallinity of the outer layer and center of the bead was similar. Thus, a former hypothesis that a completely amorphous bead layer enables molding, was discarded. However, the incorporation of CE remarkably reduces the crystallization and re-crystallization rate. As a consequence, the time available for interdiffusion of chains across neighboring beads increases and facilitates crystallization across the bead interface. For E-PBT bead foams, it is concluded that sufficient time for polymer interdiffusion during molding is crucial and requires adjusted crystallization kinetics.

Effects of Waste Expanded Polypropylene as Recycled Matrix on the Flexural, Impact, and Heat Deflection Temperature Properties of Kenaf Fiber/Polypropylene Composites

Waste Expanded polypropylene (EPP) was utilized as recycled matrix for kenaf fiber-reinforced polypropylene (PP) composites produced using chopped kenaf fibers and crushed EPP waste. The flexural properties, impact strength, and heat deflection temperature (HDT) of kenaf fiber/PP composites were highly enhanced by using waste EPP, compared to those by using virgin PP. The flexural modulus and strength of the composites with waste EPP were 98% and 55% higher than those with virgin PP at the same kenaf contents, respectively. The Izod impact strength and HDT were 31% and 12% higher with waste EPP than with virgin PP, respectively. The present study indicates that waste EPP would be feasible as recycled matrix for replacing conventional PP matrix in natural fiber composites.

Microwave Foaming of Materials: An Emerging Field

In the last two decades, the application of microwave heating to the processing of materials has to become increasingly widespread. Microwave-assisted foaming processes show promise for industrial commercialization due to the potential advantages that microwaves have shown compared to conventional methods. These include reducing process time, improved energy efficiency, solvent-free foaming, reduced processing steps, and improved product quality. However, the interaction of microwave energy with foaming materials, the effects of critical processing factors on microwave foaming behavior, and the foamed product's final properties are still not well-explored. This article reviews the mechanism and principles of microwave foaming of different materials. The article critically evaluates the impact of influential foaming parameters such as blowing agent, viscosity, precursor properties, microwave conditions, additives, and filler on the interaction of microwave, foaming material, physical (expansion, cellular structure, and density), mechanical, and thermal properties of the resultant foamed product. Finally, the key challenges and opportunities for developing industrial microwave foaming processes are identified, and areas for potential future research works are highlighted.

Poly(ether imide)/Epoxy Foam Composites with a Microcellular Structure and Ultralow Density: Bead Foam Fabrication, Compression Molding, Mechanical Properties, Thermal Stability, and Flame-Retardant Properties

It is challenging to prepare ultralow-density microcellular foams based on high-performance polymers due to their low gas solubility and rigid polymer matrix. In this study, by applying microcellular foaming technology using CO₂/acetone as the blowing agent, ultralow-density poly(ether imide) (PEI) bead foams with an expansion ratio of 30-56 times and cell density of 10⁷-10⁹ cells/cm³ were fabricated, resulting from the enhanced plasticization effect of the mixed fluid. The slow diffusivity of acetone at room temperature ensured the saturated PEI beads to foam after desorption for more than 6 days, which potentially reduces the transportation cost of PEI bead foams significantly. A novel compression molding process was developed to prepare the molded PEI bead foams (MPEIs) using epoxy as a coating agent. The good infiltration character of epoxy on bead foams endowed the MPEIs with excellent mechanical properties, together with an ultralow density of 80-200 kg/m³, long-term dimensional stability at 160 °C, and excellent flame-retardant properties of V0 rating. These features made the MPEIs very promising for many advanced applications.

Expanded Polycarbonate (EPC)-A New Generation of High-Temperature Engineering Bead Foams

Bead foams serve in a wide variety of applications, from insulation and packaging to midsoles in shoes. However, the currently used materials are limited to somewhat low temperature or exhibit significant changes in modulus in the temperature range of many applications due to their glass transition. By comparison, polycarbonate (PC) exhibits almost constant mechanics for temperatures up to 130 °C. Therefore, it appears as an advantageous base material for bead foams. The aim of the publication is to provide comprehensive data on the properties of expanded PC (EPC) in comparison to already commercially available expanded polypropylene, EPP, and expanded polyethylene-terephthalate, EPET. A special focus is set on the thermo-mechanical properties as these are the most lacking features in current materials. In this frame, dynamic mechanical analysis, and tensile, bending, compression and impact tests at room temperature (RT), 80 °C, and 110 °C are conducted for the three materials of the same density. Already at RT, EPC exhibits superior mechanics compared to its peers, which becomes more pronounced toward higher temperature. This comes from the low sensitivity of properties to temperature as EPC is used below its glass transition. In summary, EPC proves to be an outstanding foam material over a broad range of temperatures for structural applications.

Microcellular foaming behavior of ether- and ester-based TPUs blown with supercritical CO2

The bead foaming behavior of ether- and an ester-based Tensor Processing Unit (TPU) resins were investigated in a lab-scale reactor using supercritical CO2 as the blowing agent. The samples were saturated at various saturation temperatures and the effects of hard segment crystallization during the saturation on the foaming behavior of the TPU samples were explored. The results revealed that the different HS crystallization tendencies and possible CO2 solubility differences in two TPU grades led to their different foaming behaviors. The ester-based TPU could be foamed within a wider saturation temperature range and revealed an easier cell growth and foam expansion while the ether-based TPU showed a more limited cell growth behavior and hence processing window. The effect of pre-annealing and hence the isothermally induced HS crystallization on the foaming behavior of the ether-based TPU and the influence of depressurization rate on the foaming behavior of ester-based TPU was also explored.