Foaming Temperature Research Articles

Microwave foaming of carbon dioxide saturated poly lactic acid

AbstractMicro cellular polymer foams find numerous applications such as in filtration, tissue scaffolds, insulation, and catalyst carriers. This study reports on a solvent free approach to fabricate microcellular poly lactic acid foams through a combination of additive manufacturing and microwave foaming. This approach provides the capability to foam polymers in a repeatable and controllable manner with minimal human intervention. Initially, samples are 3D printed, followed by gas saturation, and microwave foaming. The effect of microwave foaming parameters, namely power, temperature and time on the pore morphology was analyzed using a complete parametric study. The results show that microwave foaming can successfully generate porous structure with power and temperature being the main factors affecting the pore morphology. A combination of midrange power and high temperature results in foams with desired properties of small pore size and high porosity. The resulting foams have pore size as small as 120 μm with porosity of 78%. These foams fabricated using a solvent free approach find applications in biomedical field as three dimensional tissue scaffolds for drug testing and bio‐artificial organ development.

Research on the Influence of Environmental Pressure on the Combustion Characteristics and Fire Spread of Building External Wall Insulation Materials

Building external wall insulation materials are widely used in the construction industry, which is one of the main materials for building energy saving. In order to study the combustion characteristics and fire spread behaviour of typical exterior wall materials at different pressures. Four environmental pressures of 40KPa, 60KPa, 80KPa, and 101KPa were set in an experimental cabin, measuring the quality, temperature, heat radiation, and heat flow of rigid polyurethane foam (RPU) through experiments. The experimental results are as follows: the masses loss curve of RPU combustion approximately obeyed the quadratic curve, which was similar to the general solid fuel combustion law; the mass change was the most dramatic at 101KPa, the lower the pressure, the slower the mass change; as the pressure decreased, the flame temperature increased slightly, and the spread of the fire slowed down; as the pressure decreased, the peak value of the thermal radiation curve decreased, and the curve tends became flat, but the horizontal axis time increased, indicating that the total heat release remained the same.

Controlling foamability of polypropylene/γ-irradiated ethylene acrylic elastomer blends by extent of crosslinking and domain microstructure of elastomer

Foams of polypropylene/elastomer blends can often lead to softer foams which may not be desirable every time. Incorporating rigidity to the foams can often be made possible by preferentially crosslinking the elastomer phase prior to blending. Although foamability of polypropylene/elastomer blends has been understood in the scientific community, the influence of the extent of crosslinking in the elastomer phase is not yet understood well. The purpose of this investigation is to identify the influence of the extent of elastomer crosslinking and the blend morphological attributes (achieved by varying screw speed during melt mixing) on foamability of polypropylene/partially crosslinked elastomer blends. Crosslinking of ethylene-acrylic elastomer is carried out using gamma radiation with several doses (0, 12.5, 25, 50 kGy) before melt blending and, subsequently, 10 wt.% of the irradiated elastomers (prior optimized) are mixed with polypropylene in a micro-compounder at three different screw speeds. The microstructure development in blends is characterized by scanning electron microscopy. Frequency sweep rheological analysis is done for selected blends to identify the ease of foamability among the series of blends. Foaming of blends is done with supercritical carbon dioxide in batch mode at three different temperatures. The density reduction along with the microcellular morphology development of blends with foaming is analyzed with the screw speed, the extent of crosslinking, and foaming temperature; furthermore, the individual input parameters (the elastomer domain size, controlled by the screw speed and the extent of crosslinking, controlled by gamma radiation dose) are optimized based on the foam morphology. A uniform and good foamability were achieved at 155 and 160°C for blends with elastomers, irradiated at 12.5 and 25 kGy radiation doses. The lowest density foam (0.37 g/cc) was obtained for polypropylene with 12.5 kGy irradiated crosslinked elastomer mixed at 200 rpm at 160°C foaming temperature. The final elastomer domain dispositions within the foam morphologies are characterized and the plausible foaming mechanism is proposed.

Tuning High and Low Temperature Foaming Behavior of Linear and Long-Chain Branched Polypropylene via Partial and Complete Melting.

While existing foam studies have identified processing parameters, such as high-pressure drop rate, and engineering measures, such as high melt strength, as key factors for improving foamability, there is a conspicuous absence of studies that directly relate foamability to material properties obtained from fundamental characterization. To bridge this gap, this work presents batch foaming studies on one linear and two long-chain branched polypropylene (PP) resins to investigate how foamability is affected by partial melting (Method 1) and complete melting followed by undercooling (Method 2). At temperatures above the melting point, similar expansion was obtained using both foaming procedures within each resin, while the PP with the highest strain hardening ratio (13) exhibited the highest expansion ratio (45 ± 3). At low temperatures, the foamability of all resins was dramatically improved using Method 2 compared to Method 1, due to access to lower foaming temperatures (<150 °C) near the crystallization onset. Furthermore, Method 2 resulted in a more uniform cellular structure over a wider temperature range (120–170 °C compared to 155–175 °C). Overall, strong extensional hardening and low onset of crystallization were shown to give rise to foamability at high and low temperatures, respectively, suggesting that both characteristics can be appropriately used to tune the foamability of PP in industrial foaming applications.

Foamed warm mix asphalt mixture containing crumb rubber: Foaming optimization and performance evaluation

To address global concerns of global warming, environmental pollution, and rapid urban growth associated with the development of transportation infrastructure and the increasing accumulation of waste tires, warm mix asphalt and crumb rubber modified asphalt have been proposed as green technologies for environmentally-friendly pavement. However, limited research has devoted to the behaviors of the foamed warm mix asphalt with crumb rubber (FWMA-CR). This study aims to investigating the foaming condition of the FWMA-CR binder and evaluating performance of both FWMA-CR binder and mixture. It is recommended to fabricate the FWMA-CR binder at foaming temperature of 160 °C, water content of 2.0%, and air pressure of 0.70 MPa, achieving desirable foaming characteristics of expansion ratio, half-life, and foaming index. The manufacturing temperature of the FWMA-CR mixture is remarkably dropped by 17 °C in comparison with that of the hot mix asphalt mixture with crumb rubber (HMA-CR). It has also explored that the performance of the FWMA-CR binder is consistent with that of the mixture. The foamed rubberized asphalt binder and mixture exhibit better low-temperature performance and fatigue resistance, while present inferior high-performance performance in comparison with conventional rubberized asphalt binder and mixture. The statistical analysis verifies that the foaming imposes no significant impact on performance of rubberized asphalt. The environmental and economic analysis reveals that compared to hot mix asphalt mixture with crumb rubber, the production of the FWMA-CR mixture contributes to approximate 7.4% of energy saving, 0.78 $ of cost saving, and reduction of CO2 emission by 7.2% per 1 ton of mixture. The outcomes facilitate the production and usage of the foamed warm mix asphalt with crumb rubber.

Fabrication of a novel Three-Dimensional porous PCL/PLA tissue engineering scaffold with high connectivity for endothelial cell migration

As an artificial extracellular matrix, tissue engineering scaffolds promote tissue regeneration and repair. Therefore, the scaffold’s internal structure is crucial for cell adhesion, growth, and migration. In this paper, based on the supercritical CO2 foaming technology, a novel three-dimensional (3D) porous polycaprolactone/polylactic acid tissue engineering scaffold induced by heterogeneous nanofibres was prepared by introducing the second phase of polylactic acid electrospinning nanofibres. The scanning electron microscopy results showed that the prepared scaffolds exhibited obvious nanofibre structures and 3D reticular pore structures. The influence of foaming temperature on pore morphology was also systematically explored. Tensile tests and water contact angle tests showed that the novel scaffolds exhibited good mechanical properties and hydrophilicity. The results of cross-sectional human umbilical endothelial cell (HUVEC) culture showed that the scaffolds were biocompatible and significantly promote the adhesion, migration and permeability of HUVECs. Single-cell migration assays showed that the 3D porous scaffolds effectively promote HUVEC migration. Therefore, 3D porous scaffolds have enormous application potential in the field of vascular patches.

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.

Effect of rare earth nucleating agent on supercritical CO2 foaming behavior of block copolymerized polypropylene

The rare earth nucleating agent was used to modify block copolymerized polypropylene (PPB) in foaming process. The results show that the crystallization of PPB and the melting temperature of β-crystal increased gradually with increased β-crystal nucleating agent content. The total crystallinity decreased with amount of addition increasing, and the relative content of β-crystal increased first and then decreased. When β-crystal nucleating agent content was 0.4 wt%, the relative β-crystal content reached the maximum value of 95.27%, and the final crystal grain refinement significantly. The addition of rare earth β-crystal nucleating agent has a good effect on improving the uniformity of foam cells. Under the same content of β-crystal nucleating agent and pressure, the average cell diameter and expansion ratio increased with the saturation temperature increasing. After the foaming temperature reaches 155°C, the expansion ratio began to decrease, which was also consistent with the changed trend of relative β-crystal content. At the same content of temperature and relative β-crystal, as the foaming pressure increased, the cell diameter decreased gradually, and the expansion ratio increased first, and then decreased.

An experimental investigation of dynamic viscosity of foam at different temperatures

Foam viscosity is a key property to predict the foam flow in narrow channels and porous media. In this study, we investigated the effects of foam quality and temperature on the dynamic foam viscosity and how modifying the composition of colloidal system can alter the foam rheology. It was shown that the addition of nanoparticles and polymer can improve the foam stability at high-temperature conditions and increase the foam apparent viscosity. The results confirmed the foam apparent viscosity can be predicted using a power law model and the flow consistency index of the foam is a strong function of temperature and foam quality, while changes in the power law index with the variation in temperature and foam quality is insignificant.Finally, to predict the foam apparent viscosity, 3-D plots for the flow consistency and power law indices can be generated as a function of foam quality and temperature.

Ultralow Dielectric Constant Polyarylene Ether Nitrile/Polyhedral Oligomeric Silsesquioxanes Foams with High Thermal Stabilities and Excellent Mechanical Properties Prepared by Supercritical CO2

Dielectric materials with ultralow dielectric constant, excellent mechanical properties, and good thermal stability have a broader application prospect in microelectronic devices with the rapid growth of 5G communication systems. On this basis, a series of lightweight polyarylene ether nitrile (PEN)/PSS‐octamethyl substituted (POSS) foams have been prepared by supercritical carbon dioxide foaming technique combining the strategy of low dielectric constant POSS doping and the introduction of bubbles. The effects of the introduction of POSS and foaming temperature on the foaming behavior of PEN/POSS composites are studied. The density of PEN/POSS foam with 5 wt% POSS is as low as 0.208 g cm−3 and the homologous porosity is up to 82.1%, which realizes the lightweight preparation of engineering plastics. Due to the introduction of POSS and pores, the PEN/POSS foam exhibits an ultralow dielectric constant of 1.83 at 1 kHz, while the dielectric loss is as low as 0.0036 and high thermal stability (Td5 &gt; 453 °C). In addition, the specific modulus of PEN/POSS foam is up to 1.07 GPa cm−3 g−1 due to the high performance of the PEN. These excellent properties make PEN/POSS foams have great potential in the application of microelectronic devices.

Structure and morphology of thermoplastic polyamide 6 elastomers with different soft segment content and their foaming behavior using supercriticalCO2

AbstractThermoplastic polyamide elastomers (TPAEs) were synthesized using polyamide 6 (PA6) as hard segments and polyethylene glycol (PEG) as soft segments. The prepared TPAEs showed a superior mechanical property and thermal property, which the strength and elongation‐at‐break reached 35.50 ± 3.54 MPa and (316.05 ± 20.58)%, respectively, when the molecular weight of soft segment was 600. Furthermore, crystallization property and microphase separation of TPAEs were also investigated in detail, having a significant influence on the foaming behavior of the obtain elastomers. Owing to the restriction of crystalline hard segments, only a few cells formed in the amorphous soft segment regions at foaming temperature of 140°C, while it transformed to larger cells with a thin cell wall at foaming temperature of 160°C and above. When the molecular weight of soft segment increased from 300 to 1000, the average cell size of the foamed TPAEs at 160°C enlarged from 28.73 to 42.17 μm, while the cell density decreased from 5.26 × 108#/cm3to 4.25 × 108#/cm3, both expansion ratio and shrinkage ratio increased gradually.

Performance evaluation of thermally insulated building integrated photovoltaic roof

In this paper, the thermal performance of insulated BIPV roofs is studied compared to non-insulated BIPV roofs. The BIPV roof prototype is designed, developed, and experimentally evaluated for composite climatic conditions in the summer season. A surface temperature of PV module and different insulation Polyurethane foam, Rockwool, and Expanded Polystyrene were observed.It was seen that the PUF insulated BIPV roof has a decrement factor of 0.24 and a time lag of 1 h. The average bottom surface temperature of the insulated BIPV roofs is found between 37.4°C to 39°C.while, non-insulated BIPV has the average temperature of 46 °C. Further, heat gain and percentage heat reduction with respect to non-insulated BIPV roof are calculated. Also, the average Discomfort index ranged between 26 and 29 for all the cases of insulated BIPV roof. The study reveals that the thermal feasibility of the insulated BIPV roof is acceptable.

Preparing Polypropylene Auxetic Foam by a One‐Pot CO2Foaming Process

At present, the direct preparation of polymer foam with negative Poisson ratio (auxetic foam) from nonpolar polymer resins is still a challenge. Herein, a one‐pot CO2foaming method is established to directly prepare closed‐cell auxetic foams with different cellular structures (bimodal cell structure and homogeneous cell structure) from polypropylene (PP) resin by introducing hydrophilic additives (NaCl) into PP and regulating foaming conditions. The effects of foaming temperature, saturation time, and size of added NaCl on Poisson's ratio are studied. The comparison with PP/talc composites shows that the presence of NaCl plays a crucial role in the formation of the negative Poisson ratio of foam. Furthermore, the regulation of the size of negative Poisson ratio is achieved by controlling the amount of NaCl in PP/NaCl composites. This study has important reference significance for the preparation of nonpolar polymer auxetic foams.

Fabrication of Thermoplastic Polyurethane Foams with Wrinkled Pores and Superior Energy Absorption Properties by CO2 Foaming and Fast Chilling

AbstractPrecise control and delicate manipulation of pore morphology are challenges for supercritical CO2 (scCO2) foaming of polymer foams to boost their performances and extend their applications. Herein, a simple two‐step scCO2 foaming and fast chilling method is proposed to fabricate thermoplastic polyurethane (TPU) foams with unique wrinkly structures inside the pores by introducing a fast cooling procedure in an ice water bath right after the foaming process. The combined effects of the increase of TPU surface tension and the decrease of CO2 pressure inside the pores induce intense intermolecular stress on the pore walls, which trigger the formation of micro‐sized wrinkles on the pore walls. The wavelength of the wrinkly structure can be reduced from 2.8 to 1.1 µm by raising the foaming temperature, which is associated with the wrinkled structure changing from hexagon to labyrinth. Compared with conventional foams, wrinkled foams possess superior compressive modulus, strength, and energy absorption performance, and wrinkled foams with wrinkles display better energy absorption. This study provides a facile approach to produce wrinkled foams with controllable morphology and gives insights into the remarkable compressive performance of the wrinkled foams, which make them promising materials for energy absorption applications.

Novel Pb alloys based composite foams containing hybrid pores produced by liquid metallurgy for lightweight batteries

High-quality novel Pb alloys based composite foams reinforced with fly ash cenospheres of different diameters in a variety of volume fractions were successfully synthesized by stir casting. Several crucial parameters such as wetting between the cenospheres and molten Pb alloy, volume fraction and size of the cenospheres, melt temperature, rate of cenosphere addition, stirring time and speed, foaming agent amount, foaming temperature and foaming time were optimized to produce high-quality composite foams. The microstructure of the composite foams revealed uniform cell distribution and size with mostly closed cell structure. The cells were measured to be of the size range of 290–430 μm. Microscopy and spectroscopy of the composite foams indicated that limited or no interfacial reaction products are formed between fly ash cenospheres and Pb alloys. The specific discharge electrical capacity of the composite foams was higher than those of the Pb alloys used in conventional Pb-acid batteries. Moreover, the weights of the composite foam electrodes were much lower than those of the conventional lead alloys, providing benefits in transportation applications. The cyclic voltammetric and galvanostatic polarization results indicated that the rates of oxidation of Pb into PbSO4 in the composite foam were much lower than those in Pb-grids and the oxidation of the Pb in the positive and negative grids occurred at much lower voltage than that in composite foams.

Fabrication of wrinkled thermoplastic polyurethane foams by dynamic supercritical carbon dioxide foaming

Precise control and regulation of the cell structure in supercritical CO2 (scCO2) foaming is the key to boost the performance and extend the application of polymer foams. In this work, dynamic scCO2 foaming method is proposed to fabricate thermoplastic polyurethane (TPU) foams with unique wrinkly structures by introducing a continuous scCO2 flow field during scCO2 saturation. The scCO2 flow field applied on the TPU matrix generated intramolecular stress and resulted in the formation of wrinkles on the surface of cells. The structure of wrinkles was controlled by pivotal processing factors including foaming temperature, pressure, scCO2 flow rate, and scCO2 flow time. Attributing to the wrinkly structure, the wrinkled foams possessed superior compressive modulus, strength, recoverability, and energy absorption performance than conventional TPU foams, which make them a promising material for buffering and energy absorption applications. The dynamic scCO2 foaming method opens avenue for scCO2 foaming process involves dynamic field.

Study of internal porous structure formation of the powder metallurgically prepared aluminium foam

The internal pore wall structure formation and density play an important role in improving the mechanical and thermal properties of the closed-cell aluminium foams. The present research work aims to investigate the internal structure formation of the aluminium foam prepared by powder metallurgy and the uniformity of the distribution of the pores when the minimum amount of TiH2 is added. The foamable precursor of two different aluminium alloys (Al-1050 and A5083) is produced with a TiH2 gaseous agent of 0.05 wt.%. The parameters analysed include the density, pore wall formations, pore, and metal density distribution inside the structure with the help of X-ray tomography. Furthermore, the image-processing technique has been adopted to produce the 3D surrogate model of the foam for visual inspection and analysis. The obtained results show the importance of the amount of TiH2 addition and of the foaming furnace temperature in deciding the internal porous structure formation. Further, the pore morphology of lower porosity foams (in the range of 30-40 % porosity) of the two alloys produced at 690 °C furnace temperature is investigated with the help of developed surrogate models. The presence of micropores and uniformity of the distribution of pores found brings the idea of choosing the optimized structure of foam for thermal energy storage systems associated with PCM.

Effect of Vulcanization and CO2 Plasticization on Cell Morphology of Silicone Rubber in Temperature Rise Foaming Process.

Both vulcanization reaction and CO2 plasticization play key roles in the temperature rise foaming process of silicone rubber. The chosen methyl-vinyl silicone rubber system with a pre-vulcanization degree of 36% had proper crosslinked networks, which not only could ensure enough polymer matrix strength to avoid bubble rupture but also had enough dissolved CO2 content in silicone rubber for induced bubble nucleation. The CO2 diffusion and further vulcanization reaction occur simultaneously in the CO2 plasticized polymer during bubble nucleation and growth. The dissolved CO2 in the pre-vulcanized silicone rubber caused a temperature delay to start while accelerating further vulcanization reactions, but the lower viscoelasticity caused by either CO2 plasticization or fewer crosslinking networks was still the dominating factor for larger cell formation. There was a sudden increase in elastic modulus and complex viscosity for pre-vulcanized silicone rubbers at higher temperature because of the occurrence of further vulcanization, but CO2 plasticization reduced the scope of change of rheological properties, and the loss factor was close to 1 around 170 °C, which is corresponding to the optimum foaming temperature. The foamed silicone rubber had a higher cell density and smaller cell size at a higher temperature rising rate, which is due to higher CO2 supersaturation and faster vulcanization reaction. These results provide some insight into the coupling mode and effect of CO2 plasticization and vulcanization for regulating cell structure in foaming silicone rubber process.

Analysis of the Foaming Window for Thermoplastic Polyurethane with Different Hard Segment Contents

A series of thermoplastic polyurethanes (TPUs) with different amounts of hard segments (HS) (40, 50 and 60 wt.%) are synthesized by a pre-polymer method. These synthesized TPUs are characterized by Shore hardness, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and rheology. Then, these materials are foamed by a one-step gas dissolution foaming process and the processing window that allows producing homogeneous foams is analyzed. The effect of foaming temperature from 140 to 180 °C on the cellular structure and on density is evaluated, fixing a saturation pressure of 20 MPa and a saturation time of 1 h. Among the TPUs studied, only that with 50 wt.% HS allows obtaining a stable foam, whose better features are reached after foaming at 170 °C. Finally, the foaming of TPU with 50 wt.% HS is optimized by varying the saturation pressure from 10 to 25 MPa at 170 °C. The optimum saturation and foaming conditions are 25 MPa and 170 °C for 1 h, which gives foams with the lowest relative density of 0.74, the smallest average cell size of 4 μm, and the higher cell nucleation density of 8.0 × 109 nuclei/cm3. As a final conclusion of this investigation, the TPU with 50 wt.% HS is the only one that can be foamed under the saturation and foaming conditions used in this study. TPU foams containing 50 wt.% HS with a cell size below 15 microns and porosity of 1.4–18.6% can be obtained using foaming temperatures from 140 to 180 °C, saturation pressure of 20 MPa, and saturation time of 1 h. Varying the saturation pressure from 10 to 25 MPa and fixing the foaming temperature of 170 °C and saturation pressure of 1 h results in TPU foams with a cell size of below 37 microns and porosity of 1.7–21.2%.

Waste Wood Particles from Primary Wood Processing as a Filler of Insulation PUR Foams.

A significant part of the work carried out so far in the field of production of biocomposite polyurethane foams (PUR) with the use of various types of lignocellulosic fillers mainly concerns rigid PUR foams with a closed-cell structure. In this work, the possibility of using waste wood particles (WP) from primary wood processing as a filler for PUR foams with open-cell structure was investigated. For this purpose, a wood particle fraction of 0.315–1.25 mm was added to the foam in concentrations of 0, 5, 10, 15 and 20%. The foaming course of the modified PUR foams (PUR-WP) was characterized on the basis of the duration of the process’ successive stages at the maximum foaming temperature. In order to explain the observed phenomena, a cellular structure was characterized using microscopic analysis such as SEM and light microscope. Computed tomography was also applied to determine the distribution of wood particles in PUR-WP materials. It was observed that the addition of WP to the open-cell PUR foam influences the kinetics of the foaming process of the PUR-WP composition and their morphology, density, compressive strength and thermal properties. The performed tests showed that the addition of WP at an the amount of 10% leads to the increase in the PUR foam’s compressive strength by 30% (parallel to foam’s growth direction) and reduce the thermal conductivity coefficient by 10%.