Structure and properties of rigid foams derived from quebracho tannin

The first highly porous organic rigid foams based on condensed tannin extracted from quebracho tree (Schinopsis sp.) are reported. Samples having different densities have been prepared and characterised. Their porous structure and physical properties have been compared with those of foams prepared in similar conditions but based on mimosa or pine tannins. We show that the properties of quebracho foams are comparable, and in some cases better, than those made from other tannins. Quebracho and mimosa present very similar behaviours, attributed to the fact that the corresponding flavonoid units have identical reactive A-rings. In contrast, pine foams behave differently, due to their different A-ring. The results indicate that quebracho is thus an excellent alternative to mimosa for producing thermally insulating tannin foams presenting good mechanical performances.     

The microstructure of rigid polyurethane foams

A study has been made of the microstructure of rigid closed-cell polyurethane foams in the density range 35 to 420 kg m^–3. Existing models for the structure of foams of this type have been evaluated using optical and scanning electron microscope techniques. Foams with a density of the order of 35 kg m^–3 are shown to be best represented as a pentagonal dodecahedron. Medium density foams from 70 to 300 kg m^–3 have a structure described as rounded polyhedra and high-density foams from 300 to 420 kg m^–3 have an isolated spherical structure. A relationship between average cell size and density is given.     

Glass fiber reinforced rigid polyurethane foams

Rigid polyurethane foam (RPUF)/glass fiber composites have been fabricated from glass fiber, polymeric 4,4′-di-phenylmethane diisocyanate (PMDI) and polypropylene glycols (PPG) using HFC 365mfc as blowing agent. Thermal conductivity, glass transition and decomposition temperatures as well as the mechanical strengths of the foam increased with the addition of glass fiber. This indicates that an optimum fiber content should depend on the balance between the mechanical reinforcement and thermal insulation.The results were interpreted in terms of cell size, closed cell content, density, fiber dispersion and a simple series model for heat transfer of the composite foam.     

Quasistatic and dynamic crushability of polymeric foams in rigid confinement

An approach was developed for investigating the crushability behavior of epoxy-based, low-density structural polymeric foam (initial bulk density 0.81 g/cm³ was used for test illustration) under quasistatic and high strain rate conditions in rigid confinement. Quasistatic crushability tests were conducted in a steel confinement cell using an MTS material testing system and the high strain rate (dynamic) crushability behavior was investigated by placing a foam specimen in a steel confinement tube and then loading the specimen using two different split Hopkinson pressure bar systems, namely, a magnesium bar and steel bar. The dynamic deformation characteristics were obtained using a multi-step incremental loading procedure. It was found that these foams exhibited large uniform inelastic deformation during the confined loading. It is verified that multi-step incremental loading can be used to construct the complete stress–strain response curve for the specimens under both quasistatic and dynamic loading conditions. A phenomenological constitutive model was then applied to parametrically describe the crushability response and to determine the rate sensitivity of the foams. The rate sensitivity of yield stress was found to be around three under rigid confinement.     

玻璃微珠增强硬质聚氨酯泡沫塑料的压缩性能及热稳定性

研究了玻璃微珠增强硬质聚氨酯泡沫塑料的制备、微观结构、压缩性能和热稳定性。结果表明: 当玻璃微珠含量为10 %时, 增强泡沫塑料的压缩强度和压缩模量达到最大; 经过硅烷偶联剂表面处理的玻璃微珠增强的泡沫塑料的压缩强度和压缩模量提高幅度较大, 起始分解温度和峰值分解温度也有一定程度的提高。SEM、XPS 和EDS 分析表明: 增强泡沫塑料的泡孔密度增加、泡孔直径变小, 玻璃微珠表面与树脂基体间界面粘结状况良好, 玻璃微珠在树脂基体中均匀分散。这些因素是造成玻璃微珠增强泡沫塑料压缩性能和热稳定性具有较大改善的原因      

无卤添加型阻燃剂对硬质聚氨酯泡沫阻燃性能研究

初步探讨了无毒、无卤的添加型阻燃剂氢氧化铝、氢氧化镁和尿素对硬质泡沫燃烧性能的影响,结果表明,阻燃材料的单独和复配添加都对硬质聚氨酯泡沫的燃烧性能有一定的提高。红外光谱研究表明,阻燃剂没有参与硬质泡沫的聚合反应,对硬质泡沫的微观结构没有影响。差热分析对燃烧机理的研究表明,阻燃剂的添加有助于吸收硬泡燃烧过程中释放的热量,控制硬泡的放热速度,提高硬泡的燃烧性能。     

Impact deformation of rigid polymeric foams: experiments and FEA modelling

The ABAQUS Finite Element package was used to model the impact response of low-density polystyrene foam. Uniaxial compression impact data was used for the material model, rather than the required hydrostatic compression data. The response of the *FOAM material model differed significantly from the polystyrene foam response, especially in simple shear and tension. Predictions were made of the impact response of a pyramidal foam block, and rectangular blocks indented by various shapes, under plane strain conditions. The predicted deformation fields were the same as those in high-speed videos, when crack growth was absent. The predicted indentation forces close to experimental values, except for the small diameter indenter. Major crack growth, in some indentation impacts, caused the force to be significantly lower than that predicted.     

Fabrication and characterization of flexible high performance thermoplastic foams derived from rigid polyetherketoneketone via a VOC-free foaming method

There is a high demand for a volatile-organic-compound (VOC)-free approach to converting high performance polymers (HPPs), such as polyetherketoneketone (PEKK), to porous structures. This is particularly true for achieving flexible foams from these very rigid HPPs. This current work introduces a facile and environmentally benign approach to fabricating flexible PEKK foams via a hydration-induced foaming and desulfonation treatment. This foaming technique possesses three major advantages: energy efficiency, environment friendliness, and super flexibility. The resultant flexible PEKK foam has a uniform porous structure with a cell diameter of ca. 5 μm, as well as low mass density of ca. 0.42 g/cm³. Thermal analysis revealed that the porous PEKK showed a high thermal stability with a thermal degradation temperature of 520 °C, showing superiority over other reported flexible polymer foams. Compared with solid PEKKs fabricated by compression molding, the porous PEKK has a higher crystallinity and more thermodynamically stable crystal structures. According to nanoindentation analysis, the flexibility of porous PEKK may be a result of the better chain mobility in porous PEKK. The reduced modulus and hardness of porous PEKK are 0.73 ± 0.16 and 0.09 ± 0.03 GPa, respectively. The porous structures also efficiently reduced the dielectric constant from ca. 4.0 to 2.6 in a wide frequency range.     

Characterization of commercial rigid polyurethane foams used as bone analogs for implant testing

Mechanical properties and microstructure characterization of a series of graded commercial rigid polyurethane foams commonly used to mimic trabecular bone in testing orthopaedic devices is reported. Compressive testing conducted according to ASTM standard F1839-08, which requires large specimens (50.8 mm × 50.8 mm × 25.4 mm blocks) gave elastic modulus and compressive strength values ranging from 115 to 794 MPa and 4.7 to 24.7 MPa, respectively, for foams having densities of 0.240–0.641 g/cm³. All these results were within the requirements of the specification for the corresponding grades. Compression testing using smaller specimens (7.5 mm diameter × 15 mm) typical of testing bone, gave results in good agreement with those obtained in the standard tests. Microstructural measurements showed the average pore size ranged from 125 to 234 μm for densities ranging from 0.641 to 0.159 g/cm³, respectively. The relative modulus as a function of relative density of the foams fit well to the model of Gibson and Ashby. Cyclic testing revealed hysteresis in the lower density foams with a loading modulus statistically equivalent to that measured in monotonic testing. Shore DO durometry (hardness) measurements show good correlations to elastic modulus and compressive strength. The results suggest additional parameters to consider for the evaluation of polyurethane foams for bone analog applications.     

Modeling the mode I fracture toughness of anisotropic low-density rigid PUR and PIR foams

Low-density foams have to possess a sufficient resistance to cracking in order to ensure the mechanical integrity of foam materials in service, even when not intended for load-bearing applications. In this study, mode I fracture toughness in the foam rise direction has been experimentally characterized for anisotropic rigid commercial polyurethane foams as well as for polyisocyanurate foams produced using polyols derived from rapeseed oil and filled with a montmorillonite nanoclay. Rectangular parallelepiped unit-cell based scaling relations expressing foam toughness via its relative density, cell dimensions, geometrical anisotropy, and the ultimate tensile stress of the base polymer have been employed for prediction of foam toughness. Assuming a brittle fracture of foam struts, a conservative estimate of toughness is obtained. It is demonstrated that considering the yielding of foam struts at the crack front as the criterion of crack extension provides a closer estimate of foam toughness.     

湿度对玻璃微珠增强硬质聚氨酯复合泡沫塑料黏弹力学性能影响

实验研究了温度与湿度因素对玻璃微珠增强硬质聚氨酯复合泡沫塑料(玻璃微珠/RPUF)黏弹力学性能的影响。结果表明, 湿度对黏弹力学性能有显著的影响, 随着湿度的增加, 玻璃微珠/RPUF的储能模量减小, 刚度下降, 力学损耗因子值增加, 蠕变柔量增大, 柔韧性增强。湿度对玻璃微珠/RPUF弯曲蠕变性能的影响具有类似于时间-温度等效原理的等效关系, 并给出了60 ℃/50%RH参考温湿度下玻璃微珠/RPUF的蠕变主曲线和平移因子。     

Phosphorus-containing silane modified steel slag waste to reduce fire hazards of rigid polyurethane foams

Steel slag (SS) waste was microencapsulated by a 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived silane via sol–gel reaction to improve its compatibility with polymer as well as to enhance flame retardant efficiency. The modified steel slag (mSS) was combined with expanded graphite (EG) as flame retardant system for rigid polyurethane foams (RPUFs). The RPUF-3 sample with 10 wt% mSS and 10 wt% EG showed an optimal comprehensive performance including a compressive strength of 206.9 kPa, a thermal conductivity of 0.0304 W/(m·K), a limiting oxygen index (LOI) value of 24.0% and UL-94 V-0 classification. The peak heat release rate (pHRR) and total heat release (THR) of the RPUF-3 dropped by 55% and 47%, respectively. By comparison, the RPUF-6 sample with 10 wt% SS and 10 wt% EG exhibited poorer flame-retardant behaviors. The superior flame retardancy of RPUF-3 over RPUF-6 could be ascribed to phosphorus-silicon complex in mSS promoted charring that provided better barrier effect during combustion.     

Ceramic foams from preceramic polymers

The direct foaming of preceramic polymer mixtures, followed by high temperature pyrolysis in an inert atmosphere, can be employed to fabricate ceramic foams with impressive strength, stiffness, thermomechanical and thermochemical durability, and electromagnetic properties. Flexural strength and stiffness were superior to those of foams produced using conventional replication process technologies. Excellent strength retention and thermal fatigue resistance was exhibited during long term static and cyclic thermal exposure to 1200 °C in air. Further improvements can be realized through judicious selection of the preceramic polymers and pre-pyrolysis process modifications. Fabrication of foams with graded porosity and multifunctionality, via the incorporation of electrically-, thermally-, and magnetically-active fillers was demonstrated. This direct foaming technology offers substantial opportunity for the near-net shape fabrication of lightweight, inorganic foam structures with tailored thermal, elastic, mechanical, electrical and magnetic characteristics for high temperature applications. Electronic Publication     

Steel slag waste combined with melamine pyrophosphate as a flame retardant for rigid polyurethane foams

To explore the potential application of industrial waste, steel slag powder in combination with melamine pyrophosphate (MPP) was adopted to improve the flame retardancy of rigid polyurethane foam (RPUF). The incorporation of steel slag slightly reduced the thermal conductivity of the resulting flame-retardant RPUF samples. The addition of MPP and/or steel slag did not significantly alter the thermal stability in terms of T_-10% and T_max but did obviously increase the T_-50% value, suggesting the improved thermal resistance of the residues. The coaddition of MPP and steel slag into RPUF resulted in higher LOI values and lower peak heat release rates than the samples incorporating either MPP or steel slag alone. The superior flame retardancy could be attributed to MPP promoting char formation, which then acted as a barrier at the beginning of RPUF thermal decomposition; simultaneously, the thermally stable inorganics in the steel slag powder strengthened the thermal resistance of this char layer.     

Soft-type polyurethane foams derived from molasses

Molasses (ML)-based soft-type polyurethane (PU) foams were successfully prepared by controlling evolved heat during chemical reaction. Two kinds of isocyanate, poly(phenylene methylene) polyisocyanate (MDI) and tolylene diisocyanate (TDI), and polypropylene glycol with a long molecular chain length were utilized to control the chemical reaction. The hydroxyl group in ML was used as the reaction site and soft-type PU foams were synthesized at isocyanate (NCO)/hydroxyl group (OH) ratios of 1.05. Mechanical properties of the above foams were controlled by changing the mixing ratio of MDI and TDI. Pore size and distribution were measured by scanning electron microscopy. With increasing thickness of cell wall, compression strength and modulus increased. Thermal properties of PU foams were investigated by differential scanning calorimetry, thermogravimetry, and thermal conductivity measurements. Two-step glass transition temperatures were observed at around ca. ?55 and 80 °C, regardless of kind of isocyanate. The low temperature side glass transition is attributed to the molecular motion of long oxyethylene chains and the high temperature side transition is caused by rigid components including saccharide components. Thermal decomposition of PU foams started from ca. 270 °C. Thermal conductivity of soft-type PU was observed in a range from 0.034 to 0.035 J s^?1 m^?1 K^?1.     

Thermal stability and fire behaviour of flame retardant high density rigid foams based on hydromagnesite-filled polypropylene composites

The present work deals with the development of new rigid polypropylene composite foams filled with high amounts of flame-retardant systems based on synthetic hydromagnesite, a basic magnesium carbonate obtained from an industrial by-product. A partially-interconnected cellular structure with a cell size around 100 μm was obtained for the hydromagnesite-filled PP foams. A 40% reduction of this cell size was observed when a small amount of a combination of montmorillonite and graphene layered nanoparticles was added to the hydromagnesite. The combination of hydromagnesite with an intumescent additive (ammonium polyphosphate) and layered nanoparticles led to improved thermal stability. In particular, the intumescent additive delayed the beginning of the thermal decomposition temperature and the layered nanoparticles split the second step of thermal decomposition in a third peak observed at higher temperatures. Improved flame retardancy, measured by means of cone calorimetry, was observed in the samples containing the intumescent additive. A novel normalized parameter, called foam efficiency ratio (FER), which takes into account the expansion ratio of the foam and the relation of its fire properties with that of the base solid, was also analyzed.     

New classes of tough composite materials—Lessons from natural rigid biological systems

The structures and properties of a new class of composite materials, containing a predominantly high volume fraction ceramic or glass phase, combined with minor organic (adhesive) phases, have been studied. These composites have unusual combinations of mechanical properties, such as stiffness, strength, and toughness. They are based on the architecture of a rigid natural material, the nacre structure, such as those found in the shells of the abalone Haliotis rufescens, and those of other mollusk shells. The mechanisms underlying these properties have also been studied. Analogs (utilizing high-performance engineering materials), that mimic many of the mechanisms underlying those superior combinations of properties, have been built. The results of the foregoing investigations are discussed. It was found that the toughness of segmented composite beams which have high volume fractions of ceramic (89 v / o) exceeded those of continuous layered beams, as well as the monolithic ceramic (alumina) on which they are based.