Mechanical Properties Of Foams Research Articles

Microstructure and performances of rigid polyurethane foam prepared using double‐effect baking powder as a foaming agent

AbstractThe preparation of rigid polyurethane foam with uniform pore size is extremely challenging. In this work, double‐effect baking powder was used as foaming agent, and rigid polyurethane foam with regular morphology was successfully prepared through semi‐prepolymerization. The rigid polyurethane foam apertures with regular morphology were controlled by the secondary release of CO2 when the content of baking powder is 0.30 part. Combined with theoretical calculation and experimental test, it is found that the particle size distribution, wall thickness, and hole shape are uniform, regular, thin, polygonal, and few broken holes. The compression strength, contact angle, and surface energy of the rigid polyurethane foam are 33.36 kPa, 109.6°, and 21.8 mJ/m2 after adding 0.20 part of double‐effect baking powder, respectively. By adjusting the amount of double‐acting baking powder, the porosity, mechanical properties and foam rate of the polyurethane foams could be significantly improved. Due to the molecular solvation effect, rigid polyurethane foams will deform in the water and oil due phase, but it can remain stable for a long time at room temperature without solvent. It provides a new method for the preparation of rigid polyurethane foam, which is expected to be widely used in transportation and other fields.Highlights Baking powder was firstly used as a foaming modifier for rigid polyurethane. Baking powder releases CO2 twice to ensure uniform structure of polyurethane cell. Rigid polyurethane foam has long‐term stability and regular, uniform cell holes. The porosity and mechanical properties of foams could be significantly improved. Foaming mechanism of rigid polyurethane using baking powder was firstly revealed.

Sustainable foams from hemp, lignin, xylan, pectin, and glycerol: tunable via reversible citric acid crosslinking for absorption and insulation applications

This study investigates the development of sustainable multifunctional foams utilizing hemp stalk waste, lignin, xylan, pectin, glycerol, and citric acid. Using the freeze-drying method for foam formation in combination with industrial waste products and renewable resources, we emphasize a green, scalable material development approach. In total, 25 distinct formulations were prepared and methodically examined, mainly focusing on the roles of citric acid, pectin, and glycerol. Thermal crosslinking, conducted at 140°C, was analyzed using FTIR, confirming the formation of ester bonds. The microstructural characterization of the foams revealed distinct variations from nanofibrillar to microfibrillar structures based on composition. The bulk density of the foams ranged from 13 to 152 mg/cm3, and porosity values varied from 97 % to 99 % for most of the compositions. Foams showed up to 50 g/g water, 51 g/g rapeseed oil, and 46 g/g kerosine absorption. Foam absorption capacity changes were examined through 10 iterative cycles in water, demonstrating that most compositions retained near-original absorption capacities. Adding glycerol conferred exceptional hydrophobic properties to the foam surfaces, as evidenced by water contact angles ranging between 140° and 150°. The thermal conductivity of foams ranged from 0.040 to 0.046 W/mK. The mechanical properties of foams were assessed using compression testing, which showed highly tunable structures ranging from soft to rigid. This study illustrates the broad applicability of these foams, emphasizing their utility in thermal insulation, filtration systems, and environmental cleanup, among other potential uses.

Rigid polyurethane foams from commercial castor oil resins

Rigid polyurethane foams (RPUFs) are among the most used polymeric materials due to their tailorable properties. Greener and biobased polyol sources for synthesising RPUFs have been recently prototyped because of the ever-growing demand for more sustainable materials solutions. Castor Oil has also rapidly become one of the most successful alternatives to replace polyurethane fossil-based components. This study extensively investigates three types of RPUFs developed from commercially available Castor Oil-based resins, correlating their properties across different length scales. In particular, X-ray diffraction identified a characteristic peak whose intensity correlated with the apparent foam density, morphology (obtained from computed tomography scans) and mechanical properties. Furthermore, quasi-static compression and vibration transmissibility tests revealed distinctive anisotropic mechanical responses among the foams, with one type of RPUF consistently outperforming the others due to its lower porosity, reaching 13.3 MPa of compressive modulus. These findings demonstrate the possibility of tuning the mechanical properties of foams via compositional control.

Synthesis and characterization of foams generated via gel state frontal polymerization

AbstractIn conventional foams, anisotropy within the microcellular structure can only be achieved through confinement in one or more directions during the foaming process. Herein, we present a novel, versatile, and energy efficient method to create unique high‐performance foams where the anisotropy of the microcellular and the microcellular structure itself is not dictated by external confinement during the foaming process. To generate such foams, an initial crosslinked gel is first created where the crosslink density of the gel can be tuned by UV intensity and cure time. Next, the gel can be foamed while simultaneously creating a second network via radically induced cationic frontal polymerization (RICFP). In this case, the anisotropy in the foam is dictated by propagation front during the RICFP and the microcellular morphology and cell surface area are dictated by the crosslink density of the gel. These two unique qualities lead to rigid foams that can be generated from a gel without the need for confinement where the microcellular size and shape can be tuned by the crosslink density of the gel precursor. A one‐pot liquid system composed of miscible epoxies, acrylates, and chemical blowing agents (CBAs), allows for control over the foam physical, mechanical, and thermal properties with glass transition temperatures ranging from 74.7 to 125.8°C. Additionally, by patterning the crosslink density of the initial gel through controlled exposure of UV, complex microcellular structures can be formed that are not possible in conventional foaming processes. This positions gel frontal polymerization and foaming as an advanced technique to produce high performing anisotropic foams with a wide array of tunable physical, mechanical, and thermal properties.

The relationship between cellular microstructure and the mechanical properties of styrene/methyl methacrylate copolymer‐silica nanocomposite foams prepared by a supercritical CO2 blowing agent

AbstractStyrene/methyl methacrylate (St/MMA) copolymer‐silica nanocomposite foams were prepared using supercritical CO2 as blowing agent, and the relationships between their cellular microstructure and mechanical properties were investigated. The effects of nanosilica content, size, and surface chemistry on the foam's microstructure and properties were examined. Dispersion and distribution of silica nanoparticles with different surface chemistry in St/MMA copolymer nanocomposites were discussed. To determine the most suitable copolymer composition for incorporating silica nanoparticles, some operating conditions such as temperature and pressure were primarily examined to study their effects on the microstructure of resulting foams. In compressive mechanical properties of final foams, a considerably high value was obtained for the compressive strength of the foams of up to 9.5 MPa. The prepared low‐density (below 0.2 g/cm3) St/MMA copolymer microcellular foams can be used in green lost foam iron casting processes.Highlights Styrene/methyl methacrylate copolymer‐silica nanocomposite foams were prepared via supercritical CO2. Foams' cell size was changed by changing silica nanoparticle surface chemistry and size. The uniform cellular microstructure was achieved by nanosilica size increment. Increasing nanoparticle size improved the foam's mechanical properties. Low‐density foams were achieved for potential application in lost foam casting.

Biobased, reprocessable, self-blown non-isocyanate polyurethane foams: Influence of blowing agent structure and functionality

The ever-increasing scrutiny of polymer environmental issues calls for advancements in polymers derived from sustainable sources, which can include biobased precursors. However, biobased polymers often exhibit properties inferior to petrochemical-based materials. Here, we address this issue relevant in the development of polyhydroxyurethane (PHU) foams made from the tandem aminolysis-decarboxylation of dimer-acid-derived cyclic carbonates by probing the effects of thiol structure and functionality on foam properties. Incorporating difunctional, trifunctional, and tetrafunctional thiols, we observed that the foam morphology is independent of thiol structure. Using dynamic mechanical analysis, compressive testing, and hysteresis testing, we demonstrated enhancements of mechanical properties due to increase in cross-link density. Additionally, with the glass transition temperature well below room temperature, the foams are categorized as high-resiliency flexible foams. Then, we reprocessed the foams into bulk materials via hydroxyurethane dynamic chemistry, showing full recovery of cross-link density and demonstrating added sustainability and recyclability benefits of these materials. Moreover, the utility of the reprocessed films as dimensionally stable elastomers is demonstrated by the elevated-temperature creep arrest. Notably, cyclic carbonates derived from dimer acid also contain ester groups that can exchange during reprocessing. We performed stress relaxation experiments and demonstrated that transesterification plays a key role in tuning the relaxation phenomena. This study elucidates the modular control of PHU foam mechanical properties and demonstrates the tunable design and relaxation behaviors of biobased PHU foams.

Pyrolytic lignin fractionation for rigid polyurethane foams: Relationship between the Pyrolysate's chemical structure and foam properties

Pyrolytic lignin (PL) has emerged as a promising renewable material for the production of polyurethanes (PU) due to the high concentration of hydroxyl groups in its structure, being a potential substitute for petroleum-based polyols. On one hand, the high functionality and liquid form of PL favor its application as a polyol in PU formulations. On the other hand, the inherent heterogeneity of PL still imposes a challenge for the development of new materials with reproducible properties. In this work, aiming to overcome these limitations, an acetone-water fractionation process was successfully used to fractionate PL. The resulting fractions were analytically characterized by HSQC NMR, 31P NMR and rheological analysis, revealing significant structural and compositional differences. Afterwards, these fractions partially replaced an industrial polyether polyol in the formulation of rigid PU foams. Finally, microstructure and mechanical properties of these foams were evaluated by SEM and uniaxial compressive tests. The lignin-based foams exhibited reduced cell size and improved mechanical properties compared to the reference foam (based on a polyether polyol). Thus, the mechanical properties of PU foams can be enhanced through the selection of specific PL fractions. These results highlight the potential of PL fractions as a sustainable and viable alternative to industrial polyols for PU foams with improved properties.

Mechanical property—processing relations for SiC foams synthesized via polymer particle templating of polycarbosilane

AbstractSilicon carbide foams with an average pore diameter of 650 nm and an inter‐pore ligament thickness of 150 nm have been synthesized using spherical polymethylmethacrylate (PMMA) particle templating of a β‐SiC nanoparticle‐loaded polycarbosilane (PCS) preceramic polymer and the effect of crystallization temperature upon their microstructure and mechanical properties investigated. Differential scanning calorimetry and thermogravimetric analysis were used to investigate both the kinetics of PMMA decomposition and the influence of β‐SiC nanoparticles upon the mechanisms of PCS cure, pyrolysis, and partial crystallization. As the crystallization temperature was systematically increased, the inter‐pore ligament structure coarsened and nanopores developed within the ligaments between the β‐SiC nanoparticles. The foam's Young's modulus and compressive strength at first increased with crystallization temperature, reaching a maximum after processing at 1300˚C. However, further increases in temperature resulted in a rapid fall in both foam modulus and compressive strength. To gain insight into the fundamental processes responsible for the overall (macroscale) mechanical properties, models for open/closed cell foams were inverted and used in conjunction with the measured foam density, Young's modulus, and compressive strength to estimate the mechanical properties of the inter‐pore ligaments. This procedure indicated that changes to the ligament properties were responsible for the observed dependence of the foam mechanical properties upon crystallization temperature.

A unified approach for evaluating mechanical compression tests for polymer bead foams

AbstractIn the realm of material characterization, the mechanical properties of polymer foams play a pivotal role in shaping their applicability across diverse industries. In this pursuit, we present a novel approach to standardize and automate the assessment of key mechanical parameters for Expanded Polypropylene (EPP) foams using a self‐developed Python script, made freely available for the scientific community. We precisely determine the compression modulus, plateau onset, and onset of densification strain for EPP foams across various densities. The script's effectiveness is demonstrated through comparisons with manual evaluations and established standards, highlighting its superiority, consistency, and suitability for a wide range of materials and conditions. Moreover, the script enables the analysis of energy absorption, shedding light on the intricate relationship between density and energy dissipation. Finally, our approach was extended to other foams to provide insight on their mechanical properties. The automated methodology ensures accuracy, reproducibility, and efficiency, thereby advancing the understanding of foam materials and facilitating informed design decisions. This research contributes to laying a foundation for the standardized assessment of foam mechanical properties, which could potentially facilitate their effective use in other applications.

EFFECTS OF FOAMING PARAMETERS ON PHYSICAL PROPERTIES OF MICROWAVE-VULCANIZED NATURAL RUBBER LATEX FOAMS (IVCST2021)

This work reported the use of microwave vulcanization coupled with Dunlop process to prepared natural rubber latex foam (NRLF). Optimization of microwave heating for curing step was firstly carried out using a commercially available domestic microwave oven. The processing window and optimum condition for making NRLF were obtained. The optimum power and time of the irradiation were found at 600 W and 6 min, respectively. Using the optimum microwave vulcanization, effects of stirring time (2, 4, 6, 8, and 10 min) and speed (650, 950, 1250, 1550, and 1850 rpm) of foaming step on cell structure, bulk density, compressive properties, and hardness of NRLFs were investigated. Generally, the increases in the stirring time and speed in foaming step both gave rise to the volume of the air incorporated into the latex compound. This resulted in the increases in the bulk density and the number of cell but, the decrease in foam cell size. The stirring speed in a range of 950 to 1550 rpm was successfully used for preparing the NRLF. Too low and too high foaming speeds caused unstable latex compounds and failed to produce NRLF. The foam cell structure changed dramatically as the foaming speed increased. The foam mechanical properties, both compressive strength and hardness, were found to be linearly related to the foam bulk density.

Analysis of the effects of region and direction on the mechanical strength of polyurethane foams in white goods refrigerators

This research focuses on investigating the mechanical properties of polyurethane foam, specifically in the context of the refrigeration industry. Tensile, compression, and flexural tests were conducted to gather comprehensive data on the foam's mechanical behavior under various loading conditions. Samples were collected from different regions within a specific cabinet model to analyze the effects of region and direction of the foam growth on the foam's mechanical properties. The results revealed that samples collected from the top of the cabinet exhibited superior performance compared to those obtained from the bottom. The foam demonstrated isotropic behavior under compression, while exhibiting anisotropic behavior under tensile and flexure. When aligned parallel to the foam growth, the foam displayed slightly higher maximum stress in the tensile and flexural tests. However, the compression test showed comparable strength in both directions. These findings contribute to a better understanding and utilization of PU foam in various industrial sectors, particularly in refrigeration. The data obtained from this research can support the optimization of project designs and facilitate the development of more efficient and economically viable solutions.

Fire-Resistant Bio-based Polyurethane Foams Designed with Two By-Products Derived from Sugarcane Fermentation Process

There is a growing interest in replacing conventional fossil-based polymers and composites with waste-based materials and fillers for environmental sustainability. This study designed water-blown polyurethane rigid foams using two by-products from the Amyris fermentation process of producing β-farnesene. The distillation residue (FDR) served as the main polyol component in the foam’s formulation (PF), supplemented with 4.5% sugarcane bagasse ash (SCBA) as a fire-retardant filler (PFA). The study assessed the impact on foam properties. Based on the analysis of all compiled data (foam structure, mechanical, and thermal properties), it can be inferred that ash particles acted as nucleating points in the reaction media, leading to a reduction in foam density (from 134 to 105 kg/m3), cell size (from 496 to 480 nm), and thermal conductivity. The absence of chemical interaction between the ash filler and the polyurethane matrix indicates that the ash acts as a filler with a plasticizing effect, enhancing the polymer chain mobility. As a result, the glass transition temperature of the foam decreases (from 74 to 71.8 ºC), and the decomposition onset temperature is delayed. Although, the incorporation of 4.5% SCBA (grain size below 250 μm) was ineffective in the increment of the compressive strength, that small amount was enough to increase the foam’s specific strength from 1009 to 1149 m2/s2 suggesting that other factors (e.g. polyol feedstock, grain size, ash packing, etc.) are yet to be accounted. The flammability test results indicate that sugarcane bagasse ash improved the foam performance, reducing burning time from 251 to 90 s, time of extinguishment from 255 to 116 s, and burning length from 132 to 56.7 mm, meeting the fire protection standard UL 94, class HB. Despite the need for further improvement and detailed flammability evaluation, the results support the notion that polyurethane foams from renewable waste by-products offer a sustainable alternative to both edible and fossil-based sources. Additionally, sugarcane bagasse ash can be a suitable silica source for reinforcing composites with reduced flammability, potentially replacing harmful halogenated chemicals used for the same purpose.Graphical

Random Foam Meso-Model and Mechanical Properties of Foam Concrete

Random Foam Meso-Model and Mechanical Properties of Foam Concrete

Prediction of compressive mechanical properties of three-dimensional mesoscopic aluminium foam based on deep learning method

To achieve efficient and accurate prediction of the mechanical properties of aluminium foam, this study proposes a deep learning-based mechanical property prediction framework. The aluminium foam modelling plug-in based on the Voronoi model is selected to establish a three-dimensional solid model of aluminium foam, and the voxelization method is utilized to transform the geometric model of aluminium foam into a voxel model that can be recognized by the neural network. The mechanical properties of aluminium foam are calculated by the finite element method, including the stress‒strain response, densification strain and plateau stress. A deep learning sample dataset containing the aluminium foam voxel model and mechanical properties is established. The deep learning model based on a three-dimensional convolutional neural network (3D-CNN) is trained to identify the mesostructure features of aluminium foam, and a high-precision mapping relationship between the mesostructure and the macroscale mechanical properties is established. The research results show that the deep learning model established in this research can efficiently and accurately predict the mechanical properties of aluminium foam. Compared with conventional prediction methods, the prediction method based on deep learning has more advantages in efficiency and accuracy, and is an effective alternative to numerical simulation. In addition, the mechanical property prediction framework is highly scalable, extending the application scope of deep learning in the field of materials.

Physical and Mechanical Properties of Starch Foam Reinforced with Pineapple (Ananas comosus) Leaves Powder

Investigation of capability of high potential pineapple (Ananas comosus) leaves powder (PLP) as reinforcing material on the starch-based foam production has not been reported. This study was aimed to characterize the physical and mechanical properties of starch foam formed from the mixture of cassava starch and PLP. The foams were made using a baking process on a thermo-pressing machine at temperature of 170°C and 180°C of the lower and upper mold temperature during 120 seconds. The starch foams were formulated with four different concentrations (0, 5, 10, and 15%) of PLP as reinforcement and tested for their characteristics of mechanical (compressibility) strength, water adsorption, morphological structure, density, color appearance, and biodegradability. Addition of PLP up to 15% improved the physical and mechanical properties of the foams.

High Porosity Poly(ether ketone ketone): Influence of Solvents on Foam Properties

AbstractPoly(ether ketone ketone) (PEKK) is a semicrystalline high‐performance polymer with exceptional mechanical properties, high continuous operation temperature, and is insoluble in most common solvents. Porous PEKK, desired for biomedical applications, is produced by a high‐temperature thermally induced phase separation process using PEKK solutions in two high boiling aprotic solvents, 4‐phenylphenol and 9‐fluorenone, with concentrations up to 20 wt.%. It is demonstrated that the solvent choice has a pronounced influence on the phase separation behavior, which determines the foam morphology, physical and mechanical properties of PEKK foams. Porous PEKK with porosities ranging from 70% to 90%, specific surface areas up to 194 m2 g−1 and elastic moduli ranging from 35 to 100 MPa are produced.

Green Preparation of Lightweight, High-Strength Cellulose-Based Foam and Evaluation of Its Adsorption Properties.

In recent years, the application scope of most cellulose-based foams is limited due to their low adsorbability and poor recyclability. In this study, a green solvent is used to extract and dissolve cellulose, and the structural stability of the solid foam is enhanced by adding a secondary liquid via the capillary foam technology, and the strength of the solid foam is improved. In addition, the effects of the addition of different gelatin concentrations on the micro-morphology, crystal structure, mechanical properties, adsorption, and recyclability of the cellulose-based foam are investigated. The results show that the cellulose-based foam structure becomes compact, the crystallinity is decreased, the disorder is increased, and the mechanical properties are improved, but its circulation capacity is decreased. When the volume fraction of gelatin is 2.4%, the mechanical properties of foam are the best. The stress of the foam is 55.746 kPa at 60% deformation, and the adsorption capacity reaches 57.061 g/g. The results can serve as a reference for the preparation of highly stable cellulose-based solid foams with excellent adsorption properties.

Biochar as internal curing material to prepare foamed concrete

Biochar is one of the by-products of biomass pyrolysis, which has the characteristics of water absorption and low thermal conductivity, etc. This study produced foamed concrete incorporated rice husk biochar (RHBC) produced at 300℃, 500℃ & 700℃ pyrolysis (B300, B500, B700) with replacement rates of 1%, 3%, 5% and 7 %. The experimental results show that B700 has the highest mechanical properties of 11.3 MPa compressive strength and 2.82 MPa bending strength under its optimal addition rate (5% B700), which are much higher than the control group (5.6 MPa compressive strength and 1.23 MPa bending strength). Meanwhile, the resistance to humid environment and shrinkage resistance were shrinkage resistance improved the most. Instrumental analysis (TGA, XRD, heat flow, CT and SEM) revealed that the porous RHBC enhanced and extended the hydration processes, resulting in the formation of additional cementitious materials, and filled in the cracks, forming more homogeneous pore structure. In addition, the impact of RHBC on the foam’s and cement paste’s stability were studied. It can be concluded that the addition of RHBC enhances the bubble restraint force (Fc) by increasing the yield cement paste’s stress, thus reducing the bubble diameter size and bubble buoyancy (Fb), ultimately leading to higher stability of foam and better mechanical properties of foamed concrete.

Viscoelastic coarsening of quasi-2D foam

Foams are unstable jammed materials. They evolve over timescales comparable to their “time of use", which makes the study of their destabilisation mechanisms crucial for applications. In practice, many foams are made from viscoelastic fluids, which are observed to prolong their lifetimes. Despite their importance, we lack understanding of the coarsening mechanism in such systems. We probe the effect of continuous phase viscoelasticity on foam coarsening with foamed emulsions. We show that bubble size evolution is strongly slowed down and foam structure hugely impacted. The main mechanisms responsible are the absence of continuous phase redistribution and a non-trivial link between foam structure and mechanical properties. These combine to give spatially heterogeneous coarsening. Beyond their importance in the design of foamy materials, the results give a macroscopic vision of phase separation in a viscoelastic medium.

Flexural behavior and microstructural material properties of sandwich foam core under arctic temperature conditions

This study investigates three types of foam core materials used in composite sandwich structures at various densities: H60, H100, F50, F90, PN115, PN200 and PN250. Three-point bending test is conducted to determine relationships between material and flexural properties at both room and low temperature Arctic conditions. X-ray micro-computed tomography is utilized to observe the microstructural relationships between foam density and mechanical properties of the core. This study evaluates Arctic temperature effects on mechanical properties for various types of foam core at varying densities with the intention for future Arctic applications. Although foam core materials become more brittle at a lower temperature, their flexural stiffness and flexural strength are further increased. However, due to the enhanced brittleness, the energy required for fracture is significantly reduced at low temperature conditions. This study utilizes statistical analysis to create contour plots and linear regression equations to predict flexural properties as a function of temperature and foam density. Molecular dynamics simulation is employed to verify experimental results to elucidate the effect of temperature on material behavior. This work provides a deeper understanding of how flexural strength relates to foam density, adding to existing data on foam strength properties under compressive, shear and tensile loads.