Thermal conductivity anisotropy in polypropylene foams prepared by supercritical CO2 dissolution

Different relative density polypropylene foams were prepared by means of two foaming processes: chemical foaming by compression moulding and physical foaming by high pressure CO₂ dissolution. By controlling the foaming parameters, such as blowing agent concentration, foaming temperature, pressure drop and pressure drop rate, it was possible to regulate the cellular structure, foams showing from markedly isotropic-like cellular structures to ones with highly-elongated cells in the vertical foam growth direction (honeycomb-like cell orientation). The thermal conductivity was measured using the transient plane source method. Using this technique, it was possible to measure the global conductivity and the thermal conductivity in both the axial and radial directions of a given sample. Results show that the global thermal conductivity of foams was mainly regulated by their relative density. In addition, the honeycomb-like cell orientation of the CO₂ dissolution foams resulted in considerably higher values in axial direction when compared to radial, demonstrating that there was a direct influence of cellular structure on the thermal conduction behaviour of these foams, enabling the development of new polypropylene foams with direction-dependent thermal properties.     

The compressive properties of expandable microspheres/epoxy foams

Expandable microspheres/epoxy foams with different densities and microstructures were prepared by changing the foaming temperature and the precuring extent. The microstructure of foams reveals a homogeneous distribution of cells at high precuring extent and high foaming temperature, while small cells size at high precuring extent and low foaming temperature. Furthermore, the compressive properties of epoxy foams were investigated. The compressive strength and modulus of the foam exhibited a power-law dependence with respect to density. By optimizing the foaming temperature and the precuring extent, epoxy foams with homogeneous cells and stable compressive property can be obtained. Fracture surface showed that deformed microspheres and less debris were observed at relatively high-density foams.     

Macro- and micro-cellular porous ceramics from preceramic polymers

Macrocellular and microcellular SiOC open cell ceramic foams were fabricated from a preceramic polymer. Macrocellular foams, with a cell size ranging from about 100–600 μm and a bulk density ranging from about 0.25–0.58 g/cm³, depending on the processing parameters, were fabricated using a direct foaming approach. Microcellular foams, with a cell size of about 8 μm, were fabricated using poly(methyl methacrylate)microbeads as sacrificial templates. The bulk density ranged from about 0.31–0.48 g/cm³, depending on the amount of microbeads in the starting material. The compression strength of the foams increased with increasing relative density, and microcellular foams possessed a 2–5 times higher crushing strength than macrocellular foams of similar density.     

加热条件对炭泡沫材料孔结构和性能的影响

以AR沥青为原料,利用高压釜在不同恒温条件下制备了炭泡沫,并测定了其孔结构、体积密度、显气孔率、压缩强度、常温热导率以及微晶参数.结果表明:相对于短恒温时间,长恒温时间制得的炭泡沫孔径大(412nm)、显气孔率高(83.82%)、体积密度小(0.34g/cm~3)、压缩强度高(4.92MPa),多孔连通结构更丰富.经过石墨化处理后,石墨泡沫呈现出较高的常温热导率(71.34W/(m·K))和较小的层片间距d_(002)(0.33556nm).石墨泡沫的常温比导热率能达到210(W·(m·K)~(-1)) /(g·cm~(-3)),是铜的5倍,铝的4倍.     

Preparation of Microcellular Epoxy Foams through a Limited‐Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

3D cross‐linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited‐foaming method is introduced for the preparation of microcellular epoxy foams (Lim‐foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim‐foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free‐foaming process (Fre‐foams). Surprisingly, however, the T_g of Lim‐foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross‐linking reaction during post‐curing.     

Processing of closed-cell silicon oxycarbide foams from a preceramic polymer

A novel processing route for fabricating closed-cell ceramic foams has been developed. The strategy for making the ceramic foams involves: (i) forming some shapes using a mixture of preceramic polymer and expandable microspheres by a conventional ceramic forming method, (ii) foaming the compact by heating, (iii) cross-linking the foamed body, and (iv) transforming the foamed body into ceramic foams by pyrolysis. By controlling the microsphere content and the pyrolysis temperature, it was possible to adjust the porosity ranging from 56 to 85%.     

应用于包装领域的明胶泡沫性能研究

目的 制作和表征基于明胶的生物基可堆肥降解泡沫材料,并应用于包装领域。方法 明胶泡沫通过机械发泡和在周围环境中干燥制成。研究明胶含量、表面活性剂含量以及发泡温度对泡沫最大发泡倍率(MER)、收缩、密度、结构以及压缩性能的影响。此外,研究不同明胶含量样品的导热率。结果 研究的3个因素对泡沫性能和结构有显著影响。MER值和收缩是黏度相关,并极大地影响泡沫密度、力学性能以及热导率。增加明胶含量制造出了密度和压缩强度更高的泡沫(由于MER值更低)。表面活性剂质量分数从0.75%增加到1.5%由于发泡性提升造成泡沫密度轻微下降。然而,进一步将表面活性剂质量分数提升至3%造成黏度显著增加、MER值下降,从而导致泡沫密度增加。更高的发泡温度可以得到更高的MER,但是由于液态泡沫稳定时间更长,收缩程度更大,泡沫密度更大。结论 明胶泡沫展现出作为低密度传统塑料泡沫(密度小于30 kg/m³)环保替代品极具潜力的性能。研究成功实现了明胶泡沫的低热导率〔0.038～0.039 W/(m.K)〕和相对较低的收缩程度。     

SiOC ceramic foams synthesized from electron beam irradiated methylsilicone resin

A new method to prepare silicon oxycarbide (SiOC) foams has been developed and it consists of electron beam irradiation of a methylsilicone preceramic polymer followed by pyrolysis in an inert atmosphere. Methylsilicone resin foams were prepared by simultaneous curing and foaming, without the addition of calalysts or blowing agents. The polymer precursor was irradiated with 1.5 MeV EB up to a dose of 7.0 MGy and at a dose rate of 2.8 kG/s, in air. During irradiation the polymer melted, due to rapid increase in temperature, and simultaneously crosslinked by interaction with the ionizing radiation. Crosslinking occurred mainly by poly-condensation reactions and gaseous condensation products were released. The latter acted as an intrinsic foaming agent in the molten polymer. Foams obtained with radiation doses higher than 3.5 MGy showed a high degree of crosslinking with a ceramic yield of over 89% at 1,000 °C. Pyrolysis at 1,200–1,500 °C resulted in SiOC ceramic foams with dense struts and walls, with bulk density around 0.3 g/cm³ and total porosity of 84%. Foams pyrolyzed at 1,200 °C revealed compression strength of 6.8 MPa.     

高密度石墨泡沫及其石蜡复合材料的热物理性能(英文)

以中间相沥青和添加中间相炭微球的沥青为原料,调整发泡压力和发泡温度制备沥青泡沫,经1273K炭化和2973K石墨化,制备了高密度石墨泡沫。为了进一步提高石墨泡沫的密度,采用573 K的沥青反复浸渍炭化未添加中间相炭微球的沥青在1273K下所制的泡沫炭,再经2973K石墨化获得增密度后的石墨泡沫。而后制备了相应石墨泡沫/石蜡复合材料。研究了石墨泡沫热物理性能的影响因素和石墨泡沫/石蜡复合材料的热行为。研究表明:沥青组分、发泡温度和发泡压力决定了石墨泡沫的结构和热物理性能,而石墨泡沫的热导率决定了复合材料的热行为。与石蜡相比,石墨泡沫/石蜡复合材料的热扩散系数提高了768至1588倍。石墨泡沫/石蜡复合材料的潜热与石蜡的质量分数成正比。该复合材料是快速响应电子散热材料的良好选择。     

Fabrication of pure SiC ceramic foams using SiO2 as a foaming agent via high-temperature recrystallization

A novel method was developed to produce the pure silicon carbide foams via the high-temperature recrystallization with the presence of a novel foaming agent-SiO₂. In this method, SiO₂ reacts with SiC to produce the gases in the silica liquid at high temperature, which leads to the formation of foams. The foams consist of the directional and interconnecting SiC crystals, and numerous intercommunicating pores that are located between them. The phase of foams was identified as 6H-SiC without the presence of SiO₂ since SiO₂ particles could react completely with SiC particles and vaporize from the sample at high temperature. The total porosity, weight loss and volume expansion rate can be increased with increasing SiO₂ contents while the three-point bending strength decreasing. The porosity of SiC foam with 25 wt.% SiO₂ as a foaming agent exhibits the maximum value while the three-point bending strength shows minimum value correspondingly. The sintered samples presented the porosities of 61-81%, the bending strength from 1.5 MPa to 4.8 MPa, and the volume expansion rate from 17.4% to 65%. This research can develop the theory for the preparation of SiC ceramics foams with controlled structure.     

煤基炭泡沫孔结构调控

以肥煤镜质组富集物为前驱体, 采用高压渗氮法制备煤基炭泡沫, 研究了发泡温度、发泡压力和发泡时间对炭泡沫孔结构的影响。利用SEM观察炭泡沫的孔胞形貌, 同时利用Nano Measurer分析软件统计SEM照片孔胞直径分布和孔喉直径分布以及平均孔径。结果表明: 微孔塑料成核理论可以定性解释炭泡沫的孔结构变化趋势。发泡温度的升高导致成核密度增加, 同时导致气体在胶质体的溶解度降低, 不利于孔胞长大。发泡压力的增大导致炭泡沫的孔胞密度增加, 临界成核半径降低, 同时加剧了热聚合反应, 导致胶质体的粘度增大, 不利于孔胞长大。发泡时间的延长会使热聚合更加充分, 影响胶质体粘度, 进而影响孔结构。     

Thermal evolution of a silicone resin/polyurethane blend from preceramic to ceramic foam

Ceramic foams, prepared by the pyrolysis of a foamed blend of a methylsilicone preceramic polymer and a polyurethane, exhibit excellent mechanical properties. The thermal evolution of process to produce from the foamed blend (weight ratio of 1 to 1) to ceramic foam was investigated from room temperature to 1400°C. Firstly, the methylsilicone preceramic polymer was characterized with various techniques. Secondly, the weight decrease and the degradation gas from the unpyrolyzed foamed blend, the phase morphology change, the compositional change, and the dimensional change were investigated. The main variation of characteristics of the foamed blend was observed in the temperature range 400 to 600°C, where the largest weight loss occurred in TGA, for most of the measurements. At these temperatures, the decomposition of the polyurethane phase is mostly completed, and the polymer-to-ceramic conversion of the silicone resin is under way. The phase-morphological analysis surprisingly showed that the polyurethane was dispersed as particles in a methylsilicone preceramic polymer matrix, although originally polyurethane was intended to be used as a sacrificial template matrix. The polyurethane domain particles gradually aggregated and tended to disappear as the temperature increased, and the ceramic foam walls and struts appeared to be dense (for pyrolysis temperature <1400°C). These features can be explained assuming that the preceramic polymer matrix deformed during the decomposition of the polyurethane and the polymer-to-ceramic conversion.     

Controlling bubble density in MWNT/polymer nanocomposite foams by MWNT surface modification

Polymer nanocomposite foams are promising substitutes for polymeric foams. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Creating polymer foams with controlled foam morphology is of great importance for controlling foam properties. The foam morphology is influenced by the foaming conditions and filler properties. For carbon nanotube/polymer composite foams, dispersion state and aspect ratio of the carbon nanotubes have been shown to influence the bubble density and bubble size. In the current study, the influence of carbon nanotube surface chemistry on the bubble density of multi-walled carbon nanotube/poly(methyl methacrylate), MWNT/PMMA, nanocomposite foams was investigated. The surface of the MWNTs with controlled aspect ratio was covalently modified with glycidyl phenyl ether (GPE). Surface modified MWNT/PMMA nanocomposite foams were produced using a supercritical carbon dioxide foaming process. At constant MWNT concentration, the bubble density of polymer nanocomposite foams filled with GPE surface modified MWNT was found to be several times higher than that of polymer nanocomposite foams filled with nitric acid treated MWNT. After the MWNTs were modified with GPE, the surface chemistry of the MWNT became the dominant factor in determining the bubble density while the MWNT aspect ratio became less influential.     

基于气体捕捉法的泡沫Ti-6Al-4V等温发泡规律研究

为了确定气体捕捉法制备泡沫Ti-6Al-4V等温发泡过程中孔隙率和微观孔洞的变化规律,在不同发泡温度及发泡时间下制备了泡沫Ti-6Al-4V.运用阿基米德原理对泡沫Ti-6Al-4V的孔隙率进行测量,通过OM和SEM对其微观特征进行观察.研究表明:泡沫Ti-6Al-4V的孔隙率及孔径均随等温发泡温度升高而增加;但当发泡温度大于950℃时,孔隙率和孔径均减小,且孔洞形态由球形变成多边形,这是由于基体内生成大尺寸β相造成的.增加发泡时间能以促进孔洞长大的方式提高泡沫Ti-6Al-4V的孔隙率,球形孔洞数量随着发泡时间的增加逐渐增多.经950℃/10 h发泡得到了孔隙率34.2%、孔径平均值156μm、孔洞为球形且分布弥散的泡沫Ti-6Al-4V.     

Preparation and properties of hollow glass bead filled silicone rubber foams with low thermal conductivity

Silicone rubber foams filled with various content and different particle size of hollow glass bead (HGB) were prepared by compression molding. It was revealed that compared with silica filled silicone rubber foams, HGB filled materials achieved higher foaming extent, lower thermal conductivity, and lower hardness, which can be significant for thermal insulation materials. For HGB filled materials, the morphology indicated the average cell size decreased with higher HGB content and larger particle size of HGB. The density, thermal conductivity, hardness and tensile strength increased with higher HGB content and larger particle size of HGB.     

Thermographic Monitoring of Aluminium Foaming Process

A novel method for measuring the temperature distribution and evolution of metal foams in the molten state is proposed. Foamable AlSi9 precursor material containing 0.6 wt% TiH₂ was foamed, kept at high temperatures and solidified while its temperature distribution was monitored by a thermographic camera. Free foaming and foaming inside a closed mould were carried out and direct and screened IR monitoring have been tested. Different heating conditions were applied giving rise to homogeneous and inhomogeneous temperature distributions. The effect of oxidation was studied on a piece of pure aluminium for reference purposes. The error sources of the measured temperature were analysed. Direct monitoring of foams was shown to be associated to serious problems with quantitative temperature measurement, while screened monitoring yielded promising and accurate quantitative results.     

Characterization of rigid polyurethane foams containing microencapsulated Rubitherm? RT27. Part I

Rigid polyurethane foams (RPU foams) and phase change materials (PCMs) are widely used in buildings for thermal insulation and thermal storage, respectively. The combination of both materials could increase energy savings, leading to more energy efficient housing. In this work, PU foams were produced incorporating different percentages of microcapsules containing Rubitherm^? RT27. Microcapsules added to the foam had a high influence on the foaming process and also on the foam properties. It was observed that the increase of foam microcapsules content decreases the final foam height but increases its density and thermal energy storage (TES) capacity. On the other hand, an increase of the foam microcapsules content up to 5 wt% led to decrease the reduced compressive strength (RS) and modulus (RE) in 7 and 25%, respectively. Higher contents had a sharply negative impact on mechanical properties.     

双马来酰亚胺泡沫的微观形貌与结构控制研究

以偶氮二甲酰胺(AC)为发泡剂制备了改性双马来酰亚胺(BMI)泡沫,用扫描电镜(SEM)对泡沫的微观形貌进行观察,研究泡沫的发泡过程及不同条件下泡沫的泡孔结构,包括密度、孔径、单位体积的泡孔数目、发泡倍率等。结果表明:改性的BMI泡沫是一种闭孔结构泡沫,其构型为排泄型十二面体。可通过发泡体系的黏度、温度和发泡剂含量控制BMI泡沫的结构,随发泡体系黏度的增加,泡沫密度,成核密度N0和单位体积的泡孔数目Nf增加,泡孔直径减小,均匀性变好。泡沫密度随发泡剂AC含量提高而降低,当AC含量超过7%(质量分数)时,泡沫密度反而上升。随发泡温度提高,泡沫密度降低,孔径增大,泡沫成型稳定性变差。     

先驱体转化法制备多孔陶瓷的发展现状

多孔陶瓷材料因其优异的性能在各种领域的应用越来越广泛,其制备方法也不断的发展.先驱体转化法制备多孔陶瓷是20世纪末才出现的一种新型工艺,它具有烧结温度低、成型工艺简单、所得制品强度高等优点,引起了科学技术界的广泛兴趣.根据所得多孔陶瓷的形态,先驱体转化法制备多孔陶瓷大致可分为两类:制备本征结构的多孔陶瓷,制备泡沫陶瓷.本文介绍了先驱体转化制备这两类多孔陶瓷的工艺、结构和性能的研究现状,以及其存在的急需解决的问题.     

The relationship between thermal decomposition properties of titanium hydride and the Al alloy melt foaming process

Using a new temperature programmed decomposition (TPD) theory and related experimental technique, a set of thermal decomposition kinetics equations of titanium hydride can be acquired by separating and simulating its TPD spectrum. According to these equations, the relation curve of decomposition quantity and time for titanium hydride at temperature of 940 K is obtained and the result coincides well with the Al alloy melt foaming process, which provides a scientific basis for controlling the Al alloy melt foam and then the Al alloy foams with different pore structure are successfully prepared.