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。     

Effect of nano-Calcium Carbonate on microcellular foaming of polypropylene

Using supercritical carbon dioxide as the physical foaming agent, a new batch process was carried out to prepare microcellular polypropylene (PP) and polypropylene/nano-Calcium Carbonate (PP/nano-CaCO₃) foams. Four concentrations of nano-CaCO₃, 3, 5, 7, and 10 wt% were used. The cell structure of foams and advantages of this new process were investigated and explained by thermal properties. Results showed that the foamed PP/5 wt% nano-CaCO₃ produced a microcellular foam with the minimum mean cell diameter (9.55 μm) and maximum cell density (1.50 × 10⁹ cells/cm³) among the four blends. Some unfoamed regions were observed in nanocomposite foams because nano-CaCO₃ could accelerate crystallization in cooling and cryostat stage. The new process took much less time (2.5 h) to foam and had much broader foaming temperature range (about 55 °C). But the foaming temperature range decreased after blending nano-CaCO₃ into PP matrix because nano-CaCO₃-induced isothermal and non-isothermal crystallization at higher temperature. In addition, the cell growth effect on variations of volume expansion ratio in PP/nano-CaCO₃ nanocomposites could be neglected comparing with the heterogeneous cell nucleation effect.     

THE NUCLEATION OF MICROCELLULAR THERMOPLASTIC FOAM: PROCESS MODEL AND EXPERIMENTAL RESULTS

Microcellular polymer foams exhibit greatly improved mechanical properties compared to standard foams due to the formers' small bubble size. Typical microcellular foams have bubbles with diameters on the order of 10 microns and volume reductions of 30 to 40%. The presence of these bubbles acts to increase the impact strength of a microfoamed structure to six or seven times that of solid parts of the same linear dimensions due to crack blunting and increased craze initiation in the cell walls.

The first step in designing techniques to manufacture parts of microcellular foam is a complete understanding of the bubble nucleation process. To this end, a theoretical model for the nucleation of microcellular foams in thermoplastic polymers has been developed and experimentally confirmed. This model explains the effect of various additives and processing conditions on the number of bubble nucleated. At levels of secondary constituents below their solubility limits, an increase in the concentration of the additive or the concentration of gas in solution with the polymer increases the number of bubbles nucleated. Nucleation in this region is homogeneous. Above the solubility limit of additives, nucleation is heterogeneous and takes place at the interface between second phase inclusions and the polymer. The number of bubbles nucleated is dependent on the concentration of heterogeneous nucleation sites and their relative effect on the activation energy barrier to nucleation.     

Polyethylene ionomer-based nano-composite foams prepared by a batch process and MuCell® injection molding

To understand the correlation between foamability and melt rheology of polyethylene-based ionomers having different degrees of the neutralization and corresponding nano-composites, we have conducted the foam processing via a batch process in an autoclave and microcellular foam injection molding (FIM) process using the MuCell^® technology. We have discussed the obtainable morphological properties in both foaming processes. All cellular structures were investigated by using field emission scanning electron microscopy. The competitive phenomenon between the cell nucleation and the cell growth including the coalescence of cell was discussed in light of the interfacial energy and the relaxation rate as revealed by the modified classical nucleation theory and rheological measurement, respectively. The FIM process led to the opposite behavior in the cell growth and coalescence of cell as compared with that of the batch process, where the ionic cross-linked structure has significant contribution to retard the cell growth and coalescence of cell. The mechanical properties of the structural foams obtained by FIM process were discussed.     

A novel technology to manufacture biodegradable polylactide bead foam products

Bead foaming technology with double crystal melting peak structure has been recognized as a promising method to produce high-performance low-density foams with complex geometries. Polylactide (PLA) bead foaming has been of the great interest of researchers due to its origin from renewal resources and biodegradability. However, due to the PLA’s low melt strength and slow crystallization kinetics, the attempts have been limited to the manufacturing methods used for expanded polystyrene (EPS). In this study, we developed microcellular PLA bead foams with double crystal melting peak structure in a large content using a lab-scale autoclave system followed by molding of the beads. PLA bead foams were produced with expansion ratios and average cell sizes ranging from 6 to 31-fold and 6 to 50 μm, respectively. The high-melting point crystals generated during gas-saturation significantly affected the expansion ratio and cell density of the PLA bead foams by enhancing the PLA’s melt strength and promoting cell nucleation around the crystals. The tensile properties of the molded EPLA bead foams showed that EPLA bead foams with double crystal melting peak can be a promising substitute not only for EPS but also for expanded polypropylene (EPP) bead foams.     

微孔塑料物理发泡新技术

微孔塑料因其独特的结构性能和工艺合于环保要求而成为研究热点忧核过程和泡孔长大的控制是微孔发泡中的研究重点。文章阐述了泡核形成的均相成核,固液界面诱导的非均相成核以及空穴成核的机理和模型。在成核过程中,泡核的临界半径和成核速率占用 位,提高体系的过饱和度和压力降有利于泡核临界半径的减小和成核速率的提高。     

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。     

微孔聚丙烯的分形特征

泡孔结构具有分形特征,可以用分形维数来描述其形态复杂性。文中应用MATLAB编写的数盒子算法,计算了应用超临界二氧化碳作为发泡剂间歇发泡制备的微孔聚丙烯的盒维数,并讨论了二值化阈值对分形维数的影响。结果表明,聚丙烯微孔结构具有典型的分形特征,分形维数与微孔聚丙烯的泡孔密度有关,在一定阈值下,分形维数随着泡孔密度的增加而增加。分形维数是发泡聚丙烯微孔结构的几何特性之一,体现了聚丙烯在特定条件下的发泡能力。     

微孔发泡聚合物泡孔形貌控制的研究进展

使用超临界流体技术制备微孔发泡聚合物,泡孔形貌是影响微孔聚合物性能的关键因素。针对微孔发泡聚合物泡孔形貌控制,即增加泡孔密度、减小泡孔直径、均化泡孔尺寸分布,从工艺条件、基体性质、共混改性、纳米添加剂、外加力场等影响因素出发,综述了近年来微孔发泡聚合物泡孔形貌控制的研究进展;最后展望了微孔发泡聚合物的前景。     

Controlling foam morphology of polystyrene via surface chemistry,size and concentration of nanosilica particles

Controlling cell morphologies of polymeric foams is an important part of controlling foam properties. In this study, the effects of particle size, particle content, and particle surface chemistry on cell nucleation in nanosilica/polystyrene (PS) composites are investigated. A theoretical hypothesis on the effect of nanoparticle size on cell nucleation in PS matrix foam was examined. The surface chemistry of nanosilica particles was studied by modifying them with Vinyltriethoxysilane (VTES) silane coupling agent. The microcellular porous materials of neat and composite PS were prepared by batch foaming technique (pressure quench) using supercritical carbon dioxide (ScCO₂) as a blowing agent. It was found that the size of the pores decreases and the cell density increases with the decrease in nanosilica size and the increase of silica loading. It was also observed that the surface treatment of the nanosilica particles have substantial effect on the decrease of the cell size and the increase of the cell density.     

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.     

模压法制备微孔发泡聚碳酸酯片材

为制备采用微孔挤出法、微孔注射法及常规发泡方法难以制备的薄型微孔发泡聚碳酸酯(PC)片材,首次采用具有制备周期短、工艺简单、操作容易、制备价格低廉等优点的模压法,通过快速降温降压制备了薄型微孔发泡PC片材,并探讨了加工参数对泡孔结构的影响,利用显微镜对泡孔结构进行了表征.实验结果表明:随着发泡时间的增加,泡孔尺寸先增加后恒定不变,泡孔密度先增加后降低;随发泡压力的增加,泡孔尺寸快速减小后变化不大,泡孔密度先快速增加后变化较小;随着发泡温度的增加,泡孔尺寸快速增加,泡孔密度快速降低;随活化比的增加,泡孔尺寸先减小后增加,泡孔密度则先增加后降低.通过控制发泡时间、发泡压力、发泡温度、活化比等加工参数可以控制微孔发泡PC的泡孔结构.     

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.     

聚丁二酸丁二醇酯离聚物微孔发泡性能

聚丁二酸丁二醇酯(PBS)综合性能优越,但因熔体黏度低而难以用于制备微孔塑料。论文采用离子化改性提高PBS熔体黏度,用于超临界二氧化碳(sc-CO_2)发泡。将丁二酸与丁二醇预缩聚后,添加二乙醇胺盐酸盐和二异腈酸六亚甲基酯(HDI)进行扩链反应,制备氨基离子浓度(UIC)为1%~5%的PBS离聚物(PBSUI),再以sc-CO_2发泡制备PBSUI微孔塑料。采用动态流变仪和X射线衍射测试PBSUI的流变行为和结晶性能,随着UIC的增加,由离子簇聚集产生的物理交联提高了PBSUI的熔体黏度和松弛时间,发泡前后PBSUI结晶度和球晶的直径均降低。扫描电镜结果表明,随着UIC增加,PBSUI泡孔由圆形转变为多边形,泡孔平均直径(D)、壁厚和开孔率下降,而泡孔密度(N_f)和发泡倍率则上升。当UIC为3%,PBSUI的D为2.05μm,N_f达1.73×10~(10)cm~(-3),发泡倍率超过10,由离子簇聚集引起的物理交联和异相成核作用,显著提高了PBSU的发泡性能,获得了泡孔形态良好的微孔泡沫塑料。     

环氧树脂微孔材料的制备与性能

采用以超临界CO₂为物理发泡剂的固态间歇发泡技术制备了环氧树脂微孔材料,利用SEM和DSC研究了环氧树脂微孔材料的制备工艺,分析了发泡前后环氧树脂的力学性能和介电性能。结果表明:环氧树脂片材的预固化度为75%~85%时,气体浓度达到5.11%~5.43%,气体饱和时间为48 h,泡孔排列紧密尺寸均匀。发泡温度的提高和发泡时间的延长会使环氧树脂微孔材料的平均泡孔直径逐渐增大,泡孔密度逐渐降低。当环氧树脂片材预固化度为75%、发泡温度为120 ℃、发泡时间为10 s时,环氧树脂微孔材料的平均泡孔直径为10.6 μm,泡孔密度为1.03×109 个/cm3,泡孔呈均匀致密的球形或多边形结构。与未发泡材料相比,环氧树脂微孔材料的断裂伸长率和冲击强度分别提高了43%和39%,介电常数降低了42%,介电损耗降低了50%。      

泡沫金属中池沸腾传热的研究进展

介绍了泡沫金属的结构特性,总结了泡沫金属中池沸腾的气泡生长速度、气泡直径和气泡生长现象等传热特点,以及泡沫金属的孔隙率、孔密度等参数对池沸腾传热的影响,并指出了泡沫金属中沸腾传热的研究方向。     

Microcellular foams of thermoplastic vulcanizates (TPVs) based on waste ground rubber tire powder

With the increased adoption of thermoplastic vulcanizates (TPVs) in automotive weather seal systems, the foams of TPVs present an important milestone in providing key applications such as trunk and door seals. In this study, microcellular foams of TPV based on waste ground rubber tire powder (WGRT) were investigated. In order to investigate the relationship between processing conditions and structure of TPV foams, we first prepared the thermoplastic vulcanizates of PP-g-MA/WGRT, then the samples were saturated with carbon dioxide and the saturated specimens were expanded during the pressure-quench process. The results indicated that the microcellular structure was dependent on the processing conditions. Cell size increased with saturation temperature, whereas cell density and relative density decreased. Different nucleation processes were produced with saturation pressure.     

Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part I: Cell structure

Tannin-based rigid foams and derived glasslike carbon foams are new, lightweight, cellular materials, prepared from 95% natural precursors. They are mainly based on bark extracts that are cross-linked with a little of formaldehyde, in the presence of furfuryl alcohol, blowing agent and acid catalyst. Their carbonaceous counterparts are obtained by pyrolysis in inert atmosphere. Various processing and composition parameters were varied, in order to observe the resultant effects on the pore structure, i.e., cell morphology, apparent density, homogeneity, and surface area. Especially, the amounts of foaming agent, strengthener and additives (nanoclay filler) were changed, and the influences of mould diameter and compression stress during foaming were investigated as well. The foams are found to be slightly orthotropic materials whose pore structure is mainly controlled by the amount of blowing agent, leading to an easily tuneable linear cell density that typically ranges from 50 to 250 pores per inch. All the other parameters have much lower influence.     

AN/MAA共聚物泡沫塑料泡体结构研究

通过烘箱自由发泡制备丙烯腈(AN)/甲基丙烯酸(MAA)共聚物泡沫塑料,讨论了该泡沫塑料泡孔平均孔径随泡沫塑料密度变化的规律,研究了成核剂和应力对泡沫塑料泡孔尺寸的影响,通过光学显微和电子扫描显微观察了该泡沫塑料的整体结构和微观结构.结果表明: AN/MAA共聚物泡沫塑料泡孔平均孔径随密度的增加而减小;碳酰胺能起到成核剂的作用,显著减小泡沫塑料泡孔尺寸;可发泡共聚物的应力发白现象能改善其发泡成核效果并使泡孔孔径大大减小;该共聚物泡沫塑料具有高闭孔率、各向同性的特征,其泡壁为三至七边形,泡棱由三个泡壁相交而成,泡壁体积分数随密度的增加而减小.     

Fabrication and investigation of influence of CaCO3 as foaming agent on Al-SiCp foam

Closed cell aluminum foams have been used in various disciplines of engineering. Aluminum foams provide high strength with the advantage of low weight. In the current research, CaCO₃ is used as a foaming agent for producing closed-cell aluminum foams. For the fabrication of homogenous foam, optimization of process parameters was done. The effect of SiC as a thickening agent on structural property of foams viz. density and porosity have been inspected. Foams with density 0.40–0.86 g/cm³ were produced. The produced foams were studied under axial compression tests for evaluating mechanical properties. It can be inferred from the results that by adding 3 wt.% CaCO₃, the uniform viscosity of melt was achieved and a homogeneous foam structure is achieved with optimum porosity. Also, 5 wt.% addition of CaCO₃ in melt and stirring speed at 1400 rpm tend to increase porosity and decrease cell wall thickness. The optimum values for thickening agent SiC, foaming agent CaCO₃ at stirring speed 1400 rpm were found out to be 15 wt.%, 3 wt.%. The effect of relative density, the addition of thickening and foaming agent is studied.