The role of microstructure on the mechanical properties of polyurethane foams containing thermoregulating microcapsules

Rigid polyurethane foams with up to 50 wt% of microcapsules from LDPE-EVA containing Rubitherm^®RT27 were synthesized. The influence of microcapsules on the foams density, microstructure and mechanical resistance was studied. Cell size and strut and wall thicknesses were analyzed by SEM. The relationships between densities and foam microstructures with their Young's moduli and collapse stress were found by the Gibson and Ashby formulations and the Kerner equation for mechanical properties of composites. It was found a cell structure change from polyhedral closed-cells to spherical or amorphous open-cells. A good agreement between the experimental and theoretical data was observed but requiring a cell form factor. Thus, Fitting parameters confirmed the high trend of these microcapsules to be incorporated into the foam cell walls and the form factors depicted the abrupt change of cell morphology. Thus, these equations are suitable for predicting the mechanical properties of foams containing fillers of low mechanical resistance.     

Towards novel super‐elastic foams based on isoperene rubber: Preparation and characterization

A novel and conventional closed cell polyisoprene rubber (IR) foams were produced by a single step limited‐expansion and two step unlimited‐expansion foaming process, respectively. The effect of 3 to 12 part per hundred rubber (phr) of azodicarbonamide (ADC) foaming agent on their structure and properties of developed novel foams were studied. In developed novel foams, the density was strangely independent of ADC content; however, the cell sizes conversely related to ADC content and it decreased by 60% (555‐330 μm) and the internal cell pressure build up from 1 to 3.7 atm, which was related to pressure‐free foaming method. The both reasons of compressed gas trapped inside cells and constant density not only caused unique enhancement in novel foams mechanical properties as hardness and modulus but also improved their dynamic properties as hysteresis and elasticity. Results of conventional IR foams showed that, their foam density as well as dynamic and mechanical properties sharply decreased with increasing ADC content from 3 to 12 phr. For clear expression, in samples with 12 phr of ADC, novel developed foams have more foam density (180%), more hardness (240%), more modulus (290%), and smaller cell size (75%) than conventional foams. Finally, novel developed foams were super‐elastic material with no hysteresis and no plastic deformation while conventional foams had 40% hysteresis and 10% plastic deformation under the same compression conditions.     

X-ray CT microtomography and mechanical response of foamed polysiloxane elastomers

3D X-ray computer microtomography (CT) experiments have been performed to assess the microstructure of scaled cellular polysiloxane elastomers and to predict how key morphological features alter as a function of compressive loading. In the work reported here, full scale (nominally 600 μm pore size) and half scale (nominally 300 μm pore size) polydimethylsiloxane foams (M97) were prepared using extractable urea particles, and tested. CT test methodology was developed to image foam microstructure at different levels of compression. 1D magnetic resonance imaging (MRI) experiments have also been performed on full scale foams for baseline characterisation. Material porosity, bulk density and dynamic mechanical analysis (DMA) stress/strain responses as a function of compression were recorded. Our results show that undesirable engineering stress responses are evident when the material microstructure (cell size and shape) is non-uniform and complex. This is particularly evident when non-spherical urea particles are used, leading to undesirable scaled foam microstructures with mechanical responses that do not match that shown by ‘full scale’ versions. Through the use of X-ray CT and MRI, our studies have provided insights into the link between manufacturing, polymer architecture (cell size/shape) and mechanical response of scaled M97 cellular materials. The data collected will support materials FEA (finite element model) code development activities, as well as help identify how the material architecture can be modified to achieve more controlled and uniform mechanical responses.     

Phosphorus flame retardant polybenzoxazine foams based on renewable diphenolic acid

Flame retardant polybenzoxazine foams were prepared in a two step process, by heating mixtures of the benzoxazine derived from renewable diphenolic acid (DPA-Bz) with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or 9,10-dihydro-9-oxa-10-(1-hydroxy-1-methylethyl) phosphaphenanthrene-10-oxide (DOPO-2Me) as additives. In the first step partial curing was achieved at different times and temperatures. In the second step, these materials underwent self foaming when heated at 220 °C. By means of a factorial design 2³ the effect of curing conditions and type of additive on the foam density were evaluated. DOPO-2Me additive was found to partially react with the DPA-Bz leading to a decrease in the glass transition temperature of the materials. The cellular structure of the foams was characterized by scanning electron microscope in terms of cell size, cell size distribution, closed-cell content and anisotropy ratio. The presence of DOPO-2Me into the solid precursors and foams greatly influenced the thermal degradation and the flame retardancy properties as evaluated by TGA, LOI and UL-94 respectively.     

热塑性聚氨酯微孔发泡材料的表观密度与其力学性能的关系

用高压CO2流体通过升温发泡法制备了一系列不同表观密度的热塑性聚氨酯(TPU)微孔发泡材料,探究了TPU发泡材料的表观密度与其力学性能的关系.微孔发泡材料的泡孔结构和表皮结构由扫描电子显微镜表征;不同表观密度材料的力学性能利用万能材料试验机和旋转流变仪表征.研究发现:TPU微孔发泡材料的表观密度主要是由材料皮层厚度占比和泡孔层密度决定的,皮层厚度占比越小和泡孔面积占有率越高,泡沫的表观密度越小;微孔发泡材料在线性应变区的压缩模量E与材料表观密度ρ的关系为:E∝ρ1.7,符合泡沫材料压缩模量与表观密度呈指数关系的基本结论;循环压缩实验中,随微孔发泡材料表观密度减小,损耗百分比增大,残余应变减小;流变实验中,微孔发泡材料的模量随表观密度变化没有明显的变化,阻尼因子tanδ随泡沫表观密度变化不呈单一的规律性.同时,阐明了微孔发泡材料的压缩模量E和损耗百分比随表观密度变化的机理.     

Nutmeg filler as a natural compound for the production of polyurethane composite foams with antibacterial and anti-aging properties

Polyurethane (PU) composite foams were successfully reinforced with different concentrations (1 wt%, 2 wt%, 5 wt%) of nutmeg filler. The effect of nutmeg filler concentration on mechanical, thermal, antimicrobial and anti-aging properties of PU composite foams was investigated. PU foams were examined by rheological behavior, processing parameters, cellular structure (Scanning Electron Microscopy analysis), mechanical properties (compression test, impact test, three-point bending test, impact strength), thermal properties (Thermogravimetric Analysis), viscoelastic behavior (Dynamic Mechanical Analysis) as well as selected application properties (thermal conductivity, flammability, apparent density, dimensional stability, surface hydrophobicity, water absorption, color characteristic). In order to Disc Diffusion Method, all PU composites were tested against selected bacteria (Escherichia coli and Staphylococcus aureus). Based on the results, it can be concluded that the addition of 1 wt% of nutmeg filler leads to PU composite foams with improved compression strength (e.g. improvement by ~19%), higher flexural strength (e.g. increase of ~11%), improved impact strength (e.g. increase of ~32%) and comparable thermal conductivity (0.023–0.034 W m⁻¹ K⁻¹). Moreover, the incorporation of nutmeg filler has a positive effect on the fire resistance of PU materials. For example, the results from the cone calorimeter test showed that the incorporation of 5 wt% of nutmeg filler significantly reduced the peak of heat release rate (pHRR) by ca. 60% compared with that of unmodified PU foam. It has been also proved that nutmeg filler may act as a natural anti-aging compound of PU foams. The incorporation of nutmeg filler in each amount successfully improved the stabilization of PU composite foams. Based on the antibacterial results, it has been shown that the addition of nutmeg filler significantly improved the antibacterial properties of PU composite foams against both Gram-positive and Gram-negative bacteria.     

Conversion of low density polyethylene foams into auxetic metamaterials

Cellular polymers, such as polyethylene foams, are commonly used in the packaging industry. These materials have short service life and generate a high volume of waste after use. In order to valorize this waste and produce added-value applications, it is proposed to convert these materials into highly efficient energy absorption structures. This was done by modifying the original cellular morphology of the foams (spheroidal or polygonal) into a re-entrant structure to produce auxetic materials. This work presents an optimized process combining mechanical compression and solvent vapor evaporation-condensation leading to low density foams (77–200 kg/m³) having negative Poisson's ratios (NPR). Three series of recycled low density polyethylene (LDPE) foams with an initial density of 16, 21, and 36 kg/m³ were used to optimize the processing conditions in terms of treatment temperature, time, and pressure. From all the samples prepared, a minimum Poisson's ratio of −3.5 was obtained. To further characterize the samples, the final foam structure was analyzed to relate with mechanical properties and compare with conventional foams having positive Poisson's ratios. The results are discussed using tensile properties and energy dissipation which were shown to be highly improved for auxetic foams. Overall, the resulting foams can be used in several applications such as sport and military protection equipment.     

Effects of preparation conditions on properties of rigid polyurethane foam composites based on liquefied bagasse and jute fibre

Rigid polyurethane foams based on liquefied bagasse and reinforced with jute fibre were prepared. The effects of preparation conditions were investigated using a paper cup with a small horizontal section area as a mould. They were reflected in the foam height, which acted as a sensitive indicator. Density gradient existed in the foam rise direction and decreased from the bottom to top. Although the amount of blowing agent water was fixed, the foam height increased with stirring time after the addition of diphenyl methane diisocyanate, the isocyanate index and the catalyst content. This was partly due to the released heat that also contributed to the foam expansion. The relative intensity of the C─N stretching band at 1510 cm⁻¹ and the N─H out-of-plane bending band at 1527 cm⁻¹ in the FTIR spectrum reflected isocyanate reactions, which had a close relationship with the crosslink density. The normalized compressive strength was essentially attributed to the combined effects of the crosslink density and the thickness of cell walls and struts. Jute fibre enhanced the compressive strength only slightly due to poor interfacial adhesion between some fibres and the matrix.     

Effect of walnut shells and silanized walnut shells on the mechanical and thermal properties of rigid polyurethane foams

In the study walnut shells (WS) and silanized walnut shells (S_WS) were used as cellulosic fillers for novel polyurethane (PU) composite foams. The impact of 1, 2 and 5 wt% of WS and S_WS on the foaming parameters, mechanical and thermo-mechanical properties of obtained materials were evaluated. The results have shown that compared to untreated WS filler, the application of S_WS leads to PU foams with more regular structure and improved physico-mechanical behavior of PU materials. For example, compared to controlled WS_0 foam, PU foams enhanced with 1 wt% of the S_WS exhibited better mechanical properties, such as higher compressive strength (~15% of improvement), better impact strength (~6% of improvement), and improved tensile strength (~9% of improvement). The addition of S_WS improved the thermomechanical stability of PU foams. This work provides a better understanding of a relationship between the surface modification of the walnut shell filler and the mechanical, insulating and thermal properties of the PU composites. Due to these positive and beneficial effects, it can be stated that the use of WS and S_WS as natural fillers in PU composite foams can promote a new application path in converting agricultural waste into useful resources for creating a new class of green materials.     

Cellulose nanofibril (CNF) reinforced starch insulating foams

In this study, biodegradable foams were produced using cellulose nanofibrils (CNFs) and starch (S). The availability of high volumes of CNFs at lower costs is rapidly progressing with advances in pilot-scale and commercial facilities. The foams were produced using a freeze-drying process with CNF/S water suspensions ranging from 1 to 7.5 wt% solids content. Microscopic evaluation showed that the foams have a microcellular structure and that the foam walls are covered with CNF’s. The CNF’s had diameters ranging from 30 to 100 nm. Pore sizes within the foam walls ranged from 20 to 100 nm. The materials’ densities ranging from 0.012 to 0.082 g/cm³ with corresponding porosities between 93.46 and 99.10 %. Thermal conductivity ranged from 0.041 to 0.054 W/m-K. The mechanical performance of the foams produced from the starch control was extremely low and the material was very friable. The addition of CNF’s to starch was required to produce foams, which exhibited structural integrity. The mechanical properties of materials were positively correlated with solids content and CNF/S ratios. The mechanical and thermal properties for the foams produced in this study appear promising for applications such as insulation and packaging.     

The study of mechanical behavior and flame retardancy of castor oil phosphate-based rigid polyurethane foam composites containing expanded graphite and triethyl phosphate

The goal of this work was the synthesis of novel flame-retarded polyurethane rigid foam with a high percentage of castor oil phosphate flame-retarded polyol (COFPL) derived from renewable castor oil. Rigid flame-retarded polyurethane foams (PUFs) filled with expandable graphite (EG) and diethyl phosphate (TEP) were fabricated by cast molding. Castor oil phosphate flame-retarded polyol was derived by glycerolysis castor oil (GCO), H₂O₂, diethyl phosphate and catalyst via a three-step synthesis. Mechanical property, morphological characterization, limiting oxygen index (LOI) and thermostability analysis of PUFs were assessed by universal tester, scanning electron microscopy (SEM), oxygen index testing apparatus, cone calorimeter and thermogravimetric analysis (TGA). It has been shown that although the content of P element is only about 3%, the fire retardant incorporated in the castor oil molecule chain increased thermal stability and LOI value of polyurethane foam can reach to 24.3% without any other flame retardant. An increase in flame retardant was accompanied by an increase in EG, TEP and the cooperation of the two. Polyurethane foams synthesized from castor oil phosphate flame-retarded polyol showed higher flame retardancy than that synthesized from GCO. The EG, in addition to the castor oil phosphate, provided excellent flame retardancy. This castor oil phosphate flame-retarded polyol with diethyl phosphate as plasticizer avoided foam destroy by EG, thus improving the mechanical properties. The flame retardancy determined with two different flame-retarded systems COFPL/EG and EG/COFPL/TEP flame-retarded systems revealed increased flame retardancy in polyurethane foams, indicating EG/COFPL or EG/COFPL/TEP systems have a synergistic effect as a common flame retardant in castor oil-based PUFs. This EG/COFPL PUF exhibited a large reduction of peak of heat release rate (PHRR) compared to EG/GCO PUF. The SEM results showed that the incorporation of COFPL and EG allowed the formation of a cohesive and dense char layer, which inhibited the transfer of heat and combustible gas and thus increased the thermal stability of PUF. The enhancement in flame retardancy will expand the application range of COFPL-based polyurethane foam materials.     

The effect of post-consumer PET particles on the performance of flexible polyurethane foams

In this work, the use of post-consumer PET (polyethylene terephthalate), PET_pc, as reinforcement filler in flexible polyurethane foams was studied, with the aim of finding alternatives for the recycling of polymer packaging. Density, number of cells per linear centimeter, tensile resistance, strain at break and tear resistance of standard foams were compared to those of foams with PET_pc in the formulation, using 1.5 parts per hundred of polyol of PET_pc (granulometric range 0–297 μm). The produced foams were sectioned into top, mid-top, mid-bottom and bottom layers. Tensile resistance, strain at break and tear resistance of the reinforced foam surpassed those of the standard foam for all layers. The number of cells was constant but density increased towards the base of the block. In addition, the filled foams yielded better wear, compression set and compression resistance than the standard foam, whereas no significant variation in morphology (cell shape) was found.     

Ultra-lightweight cellulose foam material: preparation and properties

Ultra-lightweight cellulose foams were prepared by regeneration of sodium dodecyl sulfate (SDS)/cellulose/NaOH/urea blend solution via mechanical agitation and then freeze-drying. The morphology and properties of the blend solutions and foams were investigated via optical microscope, rheometer, BET and SEM. As a result, it was found that the inclusion complex structure between cellulose macromolecules and the solvent molecules was not destroyed. Moreover, the bubbles were about 20–50 μm in the solutions and larger (>100 μm) in the foams. Not only the micropores (bubbles) but also the nanopores could be observed in the wet and dried foams. The cellulose foams possessed ultra-low density of about 30 mg/cm³ and high specific surface area. The result of X-ray diffraction and Fourier transform infrared spectroscopy indicated that the cellulose foams were transited from cellulose I to cellulose II after dissolution and gelation. Bubbles inside the wet foams weakened the mechanical properties, but inversely increased the mechanical properties in the dried foams. Typical “J”-shaped curves were observed during the mechanical test, which revealed good compressive strength of dried foams. In this work, cellulose foams with ultra-lightweight and good mechanical properties were obtained, which exhibited great potentials for further development and comprehensive utilization of cellulose.     

Synthesis of activated carbon foams with high specific surface area using polyurethane elastomer templates for effective removal of methylene blue

Carbon foams have gained significant attention due to their tuneable properties that enable a wide range of applications including catalysis, energy storage and wastewater treatment. Novel synthesis pathways enable novel applications via yielding complex, hierarchical material structure. In this work, activated carbon foams (ACFs) were produced from waste polyurethane elastomer templates using different synthesis pathways, including a novel one-step method. Uniquely, the produced foams exhibited complex structure and contained carbon microspheres. The ACFs were synthesized by impregnating the elastomers in an acidified sucrose solution followed by direct activation using CO₂ at 1000 ℃. Different pyrolysis and activation conditions were investigated. The ACFs were characterized by a high specific surface area (S_BET) of 2172 m²/g and an enhanced pore volume of 1.08 cm³/g. Computer tomography and morphological studies revealed an inhomogeneous porous structure and the presence of numerous carbon spheres of varying sizes embedded in the porous network of the three-dimensional carbon foam. X-ray diffraction (XRD) and Raman spectroscopy indicated that the obtained carbon foam was amorphous and of turbostratic structure. Moreover, the activation process enhanced the surface of the carbon foam, making it more hydrophilic via altering pore size distribution and introducing oxygen functional groups. In equilibrium, the adsorption of methylene blue on ACF followed the Langmuir isotherm model with a maximum adsorption capacity of 592 mg/g. Based on these results, the produced ACFs have potential applications as adsorbents, catalyst support and electrode material in energy storage systems.     

Tungsten‐loaded SMP foam nanocomposites with inherent radiopacity and tunable thermo‐mechanical properties

Shape memory polymer (SMP) foams have been developed for use in neurovascular occlusion applications. These materials are predominantly polyurethanes that are known for their biocompatibility and tunable properties. However, these polymers inherently lack X‐ray visibility, which is a significant challenge for their use as implantable materials. Herein, low density, highly porous shape memory polyurethane foams were developed with tungsten nanoparticles dispersed into the foam matrix, at increasing concentrations, to serve as a radiopaque agent. Utilizing X‐ray fluoroscopy sufficient visibility of the foams at small geometries was observed. Thermal characterization of the foams indicated altered thermal response and delayed foam actuation with increasing nanoparticle loading (because of restricted network mobility). Mechanical testing indicated decreased toughness and strength for higher loading because of disruption of the SMP matrix. Overall, filler addition imparted x‐ray visibility to the SMP foams and allowed for tuned control of the transition temperature and actuation kinetics for the material. Copyright © 2015 John Wiley & Sons, Ltd.     

Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent

Polyurethane rigid foams have been used for many applications such as pipelines insulation materials, automotive parts, solar water heater and construction materials[1,2], due to their desirable physical properties. Traditional rigid foam is made by the reaction of a polyol and 4,4′-diphenylmethane diisocyanate (MDI) with chlorofluorocarbons (CFCs), in particular tri- chlorofluoromethane (CFC-11) and/or HCFC-141b as blowing agents. However, the CFCs blowing agents contain halogens, whic…     

Carbon nanofiber reinforced syntactic foams: Degradation mechanism for long term moisture exposure and residual compressive properties

Hollow particle filled polymer matrix composites, called syntactic foams, are challenging for studies related to environmental exposure and degradation mechanisms due to the possible role of matrix, particle–matrix interface, and particle material and wall thickness. In the current study, syntactic foams reinforced with 1 wt.% vapor grown carbon nanofibers (CNFs) are subjected to water immersion and characterized for residual compressive properties under quasi-static and high strain rates. The testing is conducted on four different types of syntactic foams, fabricated with glass hollow particles of two different densities: 220 and 460 kg/m³ in 30 and 50 vol.% quantities. After a period of 6 months, a maximum of 7% weight gain is observed in the worst performing syntactic foam. The exposed specimens are tested for residual compressive properties and the results are compared with the properties of dry specimens. The quasi-static compressive strength of CNF reinforced syntactic foams is found to decrease and the modulus remained unaffected due to the moisture exposure. The high strain rate compressive strength was 1.3–2.2 times higher for wet and dry specimens compared to the quasi-static strength of the same type of syntactic foams.     

Effect of crystallization on tensile mechanical properties of PET foam: Experiment and model prediction

Currently Polyethylene terephthalate (PET) foam is the most promising structural core materials, and the tensile mechanical properties are one of its important application indicators. Herein, environmental-friendly supercritical CO₂ (ScCO₂) extrusion foaming was adopted to prepare PET foam. Aiming at investigating the influence of crystals on the mechanical properties, isothermal treatment in the post-process was used to improve the crystallization process of PET foams. Due to the crystal perfection proceeds via migration and rejection of the structural defects at the crystallites induced by slow crystallization, the crystallinity increased rapidly with the rise of isothermal temperature, especially above the glass transition temperature (T_g). Qualitatively, it can be concluded that the crystalline phase contents have an intimate positive correlation with the tensile modulus, meanwhile, the shape ratio of the crystal have no significant effects on the tensile modulus. In addition, a coupling scheme of aggregate two-layered composite inclusion model and Simone-Gibson equation was first proposed to quantify the mathematical relationship between crystallization and tensile modulus of PET foam, which realized basic agreement.     

Multiscale description of polymeric foam behavior: A new approach based on discrete element modeling

The mechanical behavior of polymeric foams depends on several parameters, such as temperature, material density, and strain rate. The studied foams are multiscale materials; agglomerated beads (bead scale is millimetric) are composed of microscopic closed cells (a few tens of microns). The response of the material to dynamic loading consists of three regions: an elastic phase, a plastic phase, and densification. The first part of this work has been the identification of the behavior of these multiscale foams in terms of density and strain rate. Some results are presented in this paper. From these first dynamic results, the second step has been the observation and the analysis of the physical phenomena initiated during the yield plateau. Buckling of the bead and cell wall and strong damage localization were studied with several devices and techniques such as high-speed camera, SEM, and microtomography. The final objective is the development of a model adapted to the multiscale structure of the foam. The first step of this numerical approach consists in the modeling of the microstructure. Due to the microscopic discrete aspect of the foam, a Discrete Element Model has been developed to study the relationship between microscopic properties and the macroscopic behavior of foam. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 6, pp. 1037–1050. This article was submitted by the authors in English.     

Sound absorption properties of polyurethane foams derived from crude glycerol and liquefied coffee grounds polyol

The main objective of this study was to evaluate the sound absorption properties of rigid polyurethane foams (PUFs) produced from crude glycerol (CG) and/or liquefied coffee grounds derived polyol (POL). The lignin content of POL proved to have a major influence on the structure and mechanical properties of the foams. Indeed, the POL content increased the cell size of the foams and their stiffness, which subsequently influenced the sound absorption coefficients. The POL derived foam has slightly higher sound absorption coefficient values at lower frequencies, while the CG foam has higher sound absorption coefficient values at higher frequencies. In turn, the foam prepared using a 50/50 mixture of polyols presents slightly higher sound absorption coefficient values in the medium frequencies range due to a balance between the cell structure and the mechanical properties. The results obtained seem to suggest that the mechanisms involved in sound wave absorption depend on the formulation used to prepare the foams. Additionally higher POL contents improved the thermal stability of PUFs as well as their mechanical properties. From this work the suitability of CG and/or POL derived PUFs as sound absorbing materials has been proven.