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.     

聚氨酯基气凝胶隔热材料研究进展

聚氨酯基气凝胶隔热材料是一类新型隔热材料。聚氨酯分子结构的可设计性和气凝胶独特纳米多孔三维网络结构的有机结合能使聚氨酯基气凝胶材料具有更好的隔热性能。本文介绍了聚氨酯气凝胶、聚脲气凝胶和聚氨酯增强无机物气凝胶材料的研究现状,重点介绍其在隔热性能方面的研究进展。     

High strain recovery with improved mechanical properties of gelatin–silica aerogel composites post-binding treatment

Silica aerogels are very light and highly porous materials that are intriguingly and complexly networked with large internal surface area, high hydrophobicity with extremely low density and thermal conductivity. These features make them ideal choice for applications as thermal and acoustics insulators or as optical, electrical, and energy storing devices. However, their exploitation for structural applications is primarily inhibited by their brittleness. The brittleness of the silica aerogels makes their processing and handling difficult. Volumetric shrinkage occurs, which becomes more apparent at elevated temperatures. While there are hybrid silica aerogels doped with materials such as polymer, ceramics, metals in the market, the improvements in the mechanical properties are compromised with tremendous increase in density and reduction in the insulation performance. Post-synthesis binding treatment of silica aerogels composites are not extensively explored due to the chemically inert trimethylsilyl (TMS) terminal groups on the surface of the hydrophobic silica aerogels. This paper discusses a unique fabrication method of developing a ductile silica aerogel composite solid via post-synthesis binding treatment. Gelatin–silica aerogel (GSA) and GSA–sodium dodecyl sulfate (SDS) composite blocks were produced by mixing hydrophobic aerogel granulates in a gelatin–SDS foamed solution by frothing method. The entire fabrication process and grounds for using a controlled % of gelatin as the main binder and SDS as an additive are explained. The compression testing of the blocks is presented. The associated strain recovery—an unusual phenomenon with brittle silica aerogels, observed upon unloading is highlighted and studied. The microstructure and surface characterization of these composites was examined via FESEM/EDX and XPS/ESCA, respectively. The dependency of process variables involved were analyzed through analysis of variance (ANOVA) model. Empirical models that relate the composition of gelatin, aerogel, and SDS to achieve the optimal strain recovery with the associated compressive modulus and strength and density are established. The transition from brittleness to ductility is measured in terms of compressive stress versus strain behavior for various mass fractions of gelatin and SDS. The test data presented indicate analogous behavior of these to creep-like behavior of a material typically identified as the primary, secondary, and tertiary stages. The rationale and mechanisms behind such creep-like three stages are explained using schematic diagrams.     

Aramid fibers reinforced silica aerogel composites with low thermal conductivity and improved mechanical performance

Aramid fibers reinforced silica aerogel composites (AF/aerogels) for thermal insulation were prepared successfully under ambient pressure drying. The microstructure showed that the aramid fibers were inlaid in the aerogel matrix, acting as the supporting skeletons, to strengthen the aerogel matrix. FTIR revealed AF/aerogels was physical combination between aramid fibers and aerogel matrix without chemical bonds. The as prepared AF/aerogels possessed extremely low thermal conductivity of 0.0227 ± 0.0007 W m⁻¹ K⁻¹ with the fiber content ranging from 1.5% to 6.6%. Due to the softness, low density and remarkable mechanical strength of aramid fibers and the layered structure of the fiber distribution, the AF/aerogels presented nice elasticity and flexibility. TG–DSC indicated the thermal stability reaching approximately 290 °C, can meet the general usage conditions, which was mainly depended on the pure silica aerogels. From mentioned above, AF/aerogels present huge application prospects in heat preservation field, especially in piping insulation.     

耐高温氧化铝气凝胶隔热复合材料研究进展

氧化铝气凝胶是一种高孔隙率、低密度、高比表面积、耐高温和低热导的纳米多孔材料, 在高温隔热领域(如航天飞行器热防护系统、工业窑炉保温材料等)具有广阔的应用前景。但是, 纯氧化铝气凝胶因耐温性(1000 ℃以上)、力学性能和高温隔热性能相对较差难以直接应用, 需要引入增强相和遮光组分制备成气凝胶复合材料以进行改善。本文对耐高温氧化铝气凝胶的制备、氧化铝气凝胶隔热复合材料的制备及性能等方面的最新研究进展进行了综述。研究人员通过原位掺杂改性、沉积改性、有机链和炭涂层改性等方法提高了氧化铝气凝胶的热稳定性。在氧化铝气凝胶中引入晶须、颗粒、多孔骨架和纤维等增强相, 能够大幅提高其力学性能; 纤维和遮光剂的协同作用, 能够提高氧化铝气凝胶抑制红外辐射的能力, 显著降低高温热导率。本文还提出了后续的研究方向：对氧化铝气凝胶的密度、微观结构进行精细调控, 再引入合适的异质元素和遮光剂,以进一步提高气凝胶的热稳定性和复合材料的隔热性能;深入研究复合材料在高温下结构和性能的演化, 以及氧化铝气凝胶和增强相之间的相互作用。作为一种新型的隔热材料, 氧化铝气凝胶复合材料将在高温隔热领域发挥其优势并逐步实现广泛应用。     

玻璃纤维增强体形式对酚醛泡沫性能的影响

针对酚醛泡沫脆性大、强度低等缺点,采用3种不同增强形式的玻璃纤维增强体,即短切玻璃纤维(SGF)、酚醛树脂浸渍固化的玻璃纤维针(GFN)及三维间隔连体织物,对酚醛泡沫进行增强.研究了纤维含量和纤维长度对酚醛泡沫压缩性能的影响规律,对比了不同增强形式纤维增强酚醛泡沫复合材料的压缩性能与保温性能.结果表明:当SGF长度为3 mm,与基体质量比为5%时,SGF增强酚醛泡沫的比压缩强度最佳,较纯泡沫的提高了21%;GFN(长度5 mm,与基体质量比为25%)增强酚醛泡沫的比强度提高8%;三维间隔连体织物增强酚醛泡沫的比强度虽略有下降,但其压缩强度(0.239 MPa)达到了承重类酚醛泡沫的要求.SGF和GFN增强的酚醛泡沫的热导率与纯酚醛泡沫的相比略有上升,但仍符合高效保温材料的要求;三维间隔连体织物增强酚醛泡沫的热导率上升明显.     

Enhanced acoustic damping in flexible polyurethane foams filled with carbon nanotubes

Flexible polyurethane (PU) foams, with loading fractions of up to 0.2 wt% carbon nanotubes (CNTs), were made by free-rising foaming using water as blowing agent. Electron microscopy revealed an open cellular structure and a homogeneous dispersion of CNTs, although the incorporation of nanofiller affected the foaming process and thus the final foam density and cellular structure. The compressive response of the foams did not show an unambiguous improvement with CNT content due to the variable foam structure. However, dense films generated by hot pressing the foams indicated a significant intrinsic reinforcement of the polymer, even at low loadings of CNTs. Most significantly, CNTs were found to increase the acoustic activity monotonically at concentrations up to 0.1 wt%.     

Investigation of the effect of rice husk derived Si/SiC on the morphology and thermal stability of carbon composite aerogels

Highly porous carbon aerogels were prepared by pyrolyzing the novolac–silica aerogels. The silica phase was extracted from rice husk ash (RHA). The polymer aerogel was synthesized via the novel method of sol–gel polymerization in solvent vapor-saturated atmosphere. This method removes the need for supercritical drying and reduces the shrinkage of aerogels during drying stage and also has much lower process time compared to the conventional sol–gel method. In the next step, polymer composite aerogels become carbon/silica and carbon/silica/silicon carbide composites in pyrolysis (800 °C) and carbothermal reduction (1500 °C) stages, respectively. The characterization of the prepared composite aerogels was performed by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses, respectively. Thermal and mechanical properties of the samples were investigated by differential scanning calorimetry (DSC) and compressive strength analysis. The resultant composite aerogels show a nanostructure with high porosity (above 82%) and low density (below 0.3 g cm^− 3). Si mapping images showed the good distribution of silica phase throughout the carbon matrix. Also the rate of oxidation for carbon composites decreased by silica incorporation and oxidation temperature increased about 20% by adding RHA silica. Compressive strength of composite samples increased about 25% by increasing RHA silica phase content.     

Ultrasonic measurements as a probe of3He/4He phase separation in aerogels

We have used ultrasonic velocity and attenuation measurements to study the phase separation of³He/⁴He mixtures confined in a silica aerogel with a porosity of 87%. We used both shear and longitudinal sound and varied the frequency between 4 and 20 MHz. The superfluid transition is accompanied by a velocity increase due to decoupling and by a critical attenuation peak which increases with frequency. At the phase separation there are changes in the velocity and attenuation, and hysteresis on thermal cycling. We show some recent results and discuss how they relate to the phase diagram inferred from torsional oscillator and heat capacity measurements on helium mixtures in aerogels.     

碳纳米纤维增强聚酰亚胺复合气凝胶的合成与性能EI北大核心CSCD

以4,4′-二氨基二苯醚(ODA)、均苯四甲酸二酐(PMDA)为单体,酸化碳纳米纤维(a-CNF)为增强材料,采用溶胶-凝胶方式成型,运用冷冻干燥技术制备PI复合气凝胶。对复合气凝胶的形貌、隔热、吸波以及压缩性能等进行表征分析。研究结果表明:随着a-CNF含量的增加,PI复合气凝胶的收缩率从45.52%降至35.32%,密度也随之从0.084 g/cm^(3)降至0.069 g/cm^(3),气凝胶孔洞分布呈增大增宽趋势。a-CNF的引入有效抑制了PI复合气凝胶的收缩率,热导率降低;整个体系的导电损耗增加,同时由于气凝胶的多孔结构提供了较好的阻抗匹配,使得PI复合气凝胶的反射损耗(RL)在8.3 GHz达到-9.7 dB。加入质量分数为15%的CNF/PI复合气凝胶压缩强度和压缩模量分别是纯PI气凝胶的近1.5倍和2倍。     

Tailoring mechanical properties of aerogels for aerospace applications

Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.     

Development of rigid bio-based polyurethane foam reinforced with nanoclay

Integration of organic nanoclay into bio-based polyurethane (PU) foam is a promising alternative to enhance the foam’s properties via green technology. In this paper, modified diaminopropane montmorillonite (DAP-MMT) nanoclay was introduced into palm oil-based PU foam at different weight loadings, namely, 0, 2, 4, 6, 8, and 10 wt.%, in order to investigate the effects on the mechanical and thermal properties of the foam. Several tests and characterizations were carried out to study the surface morphology, density, compressive strength and thermal stability of the foam. It was found that foam exhibited an exfoliated or intercalated microstructure based on the DAP-MMT contents. The X-ray diffraction analysis showed that below 4 wt.%, the foams displayed exfoliated structures while beyond the value, the foams exhibited the intercalated morphologies. Closed cells with different cell sizes were observed when the DAP-MMT contents were varied. Meanwhile, thermal stability and compressive strength of foams increased with increasing DAP-MMT contents up to 4 wt.%, as shown by thermogravimetry analysis and compression test, respectively.     

氧化铝气凝胶的合成与性能

氧化铝气凝胶具有大比表面积、高孔隙率以及比氧化硅气凝胶更优异的热稳定性, 在高温隔热和催化领域有广阔的应用前景, 但其复杂的合成工艺和较低的强度限制了其应用。本文首先介绍氧化铝气凝胶合成工艺, 指出溶胶-凝胶过程中存在的问题, 不同干燥方式的特点以及最新发展的干燥工艺; 然后介绍了近年来氧化铝气凝胶在比表面积、热稳定性、强度、导热系数等性能改善方面所取得的代表性研究成果; 最后介绍了氧化铝气凝胶在高温催化、隔热领域的应用, 并展望了未来氧化铝气凝胶的发展方向以及应关注的问题。     

Silica-based aerogels as aggregates for cement-based thermal renders

Subcritically dried silica-based aerogels were synthesized by design to be used as aggregates for lightweight cement-based thermal renders. The molecular and pore structure of the aerogels and of the corresponding renders were correlated with their thermal conductivities. A subcritical hybrid aerogel proved to have advantages over a supercritical commercial one, since the particle size distribution may be controlled, it is more hydrophobic, and imparts higher specific surface areas and total pore volumes to the renders. Good stabilization of the hybrid aerogels within the aqueous cement paste, without affecting the final renders’ structure, was accomplished by using an anionic surfactant. The efficient range of aerogel contents for thermal insulation purposes (above 60 vol% of total aggregate) was optimized using an inorganic subcritical aerogel. Thermal conductivities as low as ∼0.085 W.m⁻¹.K⁻¹ and densities of 410 kg.m⁻³ were achieved by total replacement of silica sand with a designed hybrid aerogel.     

Effect of surface modifications of carbon black (CB) on the properties of CB/polyurethane foams

The effect of surface modifications of carbon black (CB) on its dispersion in polyether polyol and CB/polyurethane (PU) foams and the properties of the CB/PU composite foams was investigated. Pristine CB (p-CB) was oxidized in nitric acid to obtain oxidized CB (o-CB), and then by the esterification reaction between the carboxyl groups of o-CB and the hydroxyl groups of polyether polyol, polyol grafted CB (g-CB) was obtained. Optical microscopy, scanning and transmission electron microscopy observations showed that surface modifications effectively improved the dispersion of CB in polyether polyol and in the final composite foams. Compared with the p-CB/PU foams, the o-CB/PU and g-CB/PU composite foams exhibited improved conductivities, storage moduli, and increased glass transition temperatures. The compressive strengths of the p-CB/PU and o-CB/PU composite foams decreased with the increase of filler contents, but g-CB has no negative effect on the compressive strength even at a filler content as high as 8 phr. Furthermore, the cell sizes for the o-CB/PU and g-CB/PU composite foams were more uniform than those of p-CB/PU foams.     

轻质高效保温材料掺杂硅气凝胶

利用正硅酸甲脂（ＴＭＯＳ）为原料的溶胶－凝胶过程，摸索了不同反应条件下形成凝胶的规律，并选用ＴｉＯ２粉末及玻璃纤维作为掺杂剂，采用超临界干燥处理制备出掺ＴｉＯ２的硅气凝胶，通过对红外光谱以及不同温度和气压条件下热导率的测量，讨论了不同成份配比以及相应的热传输过程对材料热导率的影响。结果表明，密度为２６０ｋｇ／ｍ＾３的掺杂硅气凝胶在８００Ｋ时的热导率为０．０３８ｗ／ｍ．ｋ，是一种新型的轻质高效保温材     

微纤化程度对MFC气凝胶的性质和包装性能的影响

目的 深入研究制备过程中微纤化程度对微纤化纤维素（MFC）气凝胶的性质及导热、保温、缓释等性能的影响。根据相应性能数据将该气凝胶应用于包装领域,解决精油在包装内释放速率过快而造成精油浓度过高和短时间内消耗完毕的问题。方法 采用TEMPO(2,2,6,6-四甲基哌啶-1-氧自由基)/NaBr/NaClO氧化体系预处理针叶木漂白硫酸盐浆,通过改变高压均质次数和低温真空冷冻干工艺制备不同微纤化程度的MFC气凝胶,利用FT-IR、XRD、SEM和BET等技术分析气凝胶的结构性质,运用应力-应变测试、导热系数测试、TGA测试和GC-MS分析等方法分别对气凝胶的压缩、保温、耐热和缓释性能进行研究。结果 随着微纤化程度的增加,MFC气凝胶的结晶度逐渐增加,气凝胶的比表面积呈现先减小后增加的趋势,范围为17.643～35.171m2/g;气凝胶的压缩强度呈现增加趋势,增幅为15.35%;均质次数为10的气凝胶的耐热性最好,均质次数为8的气凝胶次之。结论 均质次数的增加引起MFC微纤化程度提高,不同微纤化程度的MFC气凝胶具有不同的内部结构性质,以及压缩、保温、耐热和精油负载缓释性能。     

平压平模切版中聚氨酯泡沫塑料的应用研究

研究了钢刀周长、钢刀连接点等因素对钢刀与纸板摩擦力的影响,测试了多种聚氨酯泡沫塑料的性能,得出了聚氨酯弹垫的压缩模量与硬度的关系,并通过分析得出了小面积封闭钢刀处聚氨酯弹垫的压缩模量的计算公式.研究结果将有助于包装企业正确选用模切弹垫.     

溶胶配比对碳纤维增强炭气凝胶隔热复合材料力学性能的影响

以异丙醇(I)为溶剂、 六次甲基四胺(H)为催化剂, 配制间苯二酚(R)-糠醛(F)的醇溶胶, 经浸渍纤维预制件、凝胶老化、超临界干燥和炭化制得碳纤维增强炭气凝胶隔热复合材料。研究了溶胶配比对碳纤维增强炭气凝胶隔热复合材料密度、微观结构和力学性能的影响规律。结果表明:随着异丙醇与间苯二酚物质的量之比增大, 碳纤维增强炭气凝胶隔热复合材料的密度逐渐降低, 基体炭气凝胶内及与碳纤维形成的界面内孔径增大, 大孔数量增多, 碳纤维增强炭气凝胶隔热复合材料的强度降低。当异丙醇与间苯二酚物质的量之比由18增加到28时, 压缩强度由2.498 MPa(应变10%)降至0.716 MPa(应变10%), 拉伸强度由2.019 MPa降至1.001 MPa, 弯曲强度由3.984 MPa降至1.818 MPa。随着六次甲基四胺与间苯二酚物质的量之比增大, 碳纤维增强炭气凝胶隔热复合材料的密度先增大后减小, 基体炭气凝胶内及与碳纤维形成的界面内孔径先减小后增大, 大孔数量先减少后增加, 碳纤维增强炭气凝胶隔热复合材料的强度先增大后减小。当六次甲基四胺与间苯二酚物质的量之比为0.008 5时, 碳纤维增强炭气凝胶隔热复合材料的密度最大, 强度最大, 其压缩强度为1.066 MPa(应变10%), 拉伸强度为1.256 MPa, 弯曲强度为3.556 MPa。      

Graphene coating makes carbon nanotube aerogels superelastic and resistant to fatigue

Lightweight materials that are both highly compressible and resilient under large cyclic strains can be used in a variety of applications. Carbon nanotubes offer a combination of elasticity, mechanical resilience and low density, and these properties have been exploited in nanotube-based foams and aerogels. However, all nanotube-based foams and aerogels developed so far undergo structural collapse or significant plastic deformation with a reduction in compressive strength when they are subjected to cyclic strain. Here, we show that an inelastic aerogel made of single-walled carbon nanotubes can be transformed into a superelastic material by coating it with between one and five layers of graphene nanoplates. The graphene-coated aerogel exhibits no change in mechanical properties after more than 1?×?10(6) compressive cycles, and its original shape can be recovered quickly after compression release. Moreover, the coating does not affect the structural integrity of the nanotubes or the compressibility and porosity of the nanotube network. The coating also increases Young's modulus and energy storage modulus by a factor of ～6, and the loss modulus by a factor of ～3. We attribute the superelasticity and complete fatigue resistance to the graphene coating strengthening the existing crosslinking points or 'nodes' in the aerogel.