Crystallinity is a controlling parameter in the development of microcellular foam final structure. In this research, using a well-controlled continuous microcellular foamed sheet production system, the effect of crystallinity on the final structure of the microcellular foam is studied. To produce microcellular foamed sheets, different levels of the supercritical carbon dioxide (ScCO2) is dissolved in the polymer at high pressure and the foamed material is stabilized using a four roll apparatus at different roll temperatures. Crystallization occur by delay, however this lag time can be controlled by controlling temperature gradient. The higher decrease in the roll temperature results in higher temperature gradient which increases the crystallization rate during the cell growth, causing bimodal cell structures which indicates secondary nucleation. Moreover, as the amount of gas increases, the cell formation time increases, hence, at even higher temperatures we observed the effect of secondary nucleation, resulting in the occurrence of bimodal cell structures. 相似文献
Supercritical CO2 as a blowing agent has attracted increasing interest in the preparation of microcellular polyamide 6 (PA6) foams. In this work, we developed the supercritical CO2-assisted method to prepare a series of different microcellular PA6 foams by controlling its crystallization properties in two steps and carefully investigated the corresponding crystallization properties of modified PA6 and foams using various techniques. Initially, a multifunctional epoxy-based chain extender (CE) was used to produce high-melt strength-modified PA6 with improved foaming ability; then, the resulting PA6 was foamed to prepare the microcellular foams of PA6 using supercritical CO2 as a blowing agent in a batch foaming route. The CE effectively enhanced the melt strength of PA6, and CE usage was optimized to obtain a threshold of high branching without crosslinking. The number of crystals was also adjusted by the saturation temperature. Furthermore, these crystals that formed during the saturation process served as high-efficiency bubble nucleating agents and then limited the growth of bubbles at the same time. The microcellular foams of PA6 were successfully obtained with a cell size of 10.0 μm, and cell density of 2.0 × 109 cells/cm3 at the saturation temperature of 225°C. 相似文献
In the development of weapons, the current trend is to replace incombustible constituent elements with combustible ones. The traditional porous combustible objects are composed of nitrocellulose as energetic component, which is highly sensitive and inflammable. Formulations composed of high content RDX and inert polymer binder were employed to replace the tradional ones. This paper reports the fabrication process of microcellular combustible objects with skin‐core structure using supercritical CO2 (SC‐CO2) as foaming agent. The objects were foamed in designed foaming molds with expansion ratios of 1.1, 1.2 and 1.35. The influence of foaming temperature, saturation pressure, expansion ratio and RDX content on porous structure was investigated by scanning electron microscopy (SEM). Thermogravimetric analysis was conducted and the results revealed a two‐stage decomposition process of RDX and binder. Performance in terms of heat resistance and moisture resistance was evaluated and compared with the traditional ones. A comparative study indicated that microcellular combustible objects are superior to traditional ones in respect of their survivability. 相似文献
The effect of CO2‐induced crystallization on the mechanical properties, in particular the yield and the ultimate stresses, of polyolefins is studied. PP and SEBS copolymer blends are used as examples and foamed after sorption of CO2 at temperatures below Tm. CO2 sorption thickens the crystalline lamellae and consequently increases Tm from 160 to 178 °C for both pure PP and PP/SEBS blend systems. Foams with an average cell size smaller than 250 nm retain the ultimate stress at the level of the polymer before foaming, even without the effect of CO2‐induced crystallization. Including CO2‐induced crystallization, the yield and the ultimate stresses of the foam can be improved by 30 and 50% over solid PP and by 22 and 40%, for solid PP/SEBS blends, respectively.
Closed-cell polycarbonate foams were prepared using a two-step foaming process, which consisted of the initial dissolution of supercritical CO2 (scCO2) into PC foaming precursors and their later expansion by heating using a double contact restriction method. The effects of the parameters of both CO2 dissolution and heating stages on the cellular structure characteristics as well as on the physical aging of PC in the obtained foams were investigated. A higher amount of CO2 was dissolved in PC with increasing the dissolution temperature from 80 to 100 °C, with similar CO2 desorption trends and diffusion coefficients being found for both conditions. PC foams displayed an isotropic-like microcellular structure at a dissolution temperature of 80 °C. It was shown that it is possible to reduce their density while keeping their microcellular structure with increasing the heating time. On contrary, when dissolving CO2 at 100 °C and later expanding, PC foams presented a cellular morphology with bigger cells and with an increasingly higher cell elongation in the vertical growth direction with increasing the heating time. Comparatively, PC foams obtained by dissolving CO2 at 100 °C presented a more marked physical aging after CO2 dissolution and foaming, although this effect could be reduced and ultimately suppressed with increasing the heating time. 相似文献
In this article, PA6/poly(tetrafluoroethylene) (PTFE) composites were prepared by internal mixer with high rotor speed. The existence of PTFE nano-fibrillation network structure was observed by scanning electron microscopy (SEM) analysis. The effect of PTFE on crystallization and rheological behavior of PA6 was evaluated. The result showed that the PTFE fibrils improved the crystallization properties of PA6 and do not change the crystal structure. The PTFE effectively enhanced the melt strength of PA6 by fibrillation. The PA6/PTFE composites were then foamed assisted by supercritical CO2. The PTFE was used as cell nucleating agent, crystal nucleating agent and melt strength enhancement agent in the foaming process. Finally, the microcellular PA6 foams were successfully obtained with the cell density higher than 109 cells/cm3, the cell size of ca. 14 μm and the volume expansion ratio of 16. 相似文献
In this study we investigate the solid-state batch foaming of polyetherimide (PEI) using sub-critical CO2 as a blowing agent. We report on the gas diffusion for various saturation pressures in this system. Foaming process characterization is reported detailing conditions used to create microcellular and nanocellular PEI foams of 40% and higher relative density. Gas sorption, foaming, and resultant morphologies are analyzed and compared to previously reported results on PEI thin films. It was found that equilibrium gas concentrations for PEI sheet begin to significantly exceed that of films for CO2 pressures above 3 MPa. A large solid-state foaming process window has been identified that allows for the creation of either microcellular or nanocellular structures at comparable density reductions. A transition from micro-scale cells to nano-scale cells was observed at gas concentrations in the range of 94–110 mg CO2/g PEI. Additionally, a hierarchical structure was observed which consisted of nanocellular structures internal to microcells. The PEI–CO2 system offers the unique opportunity to compare and contrast the bulk properties of nanofoams and microfoams. 相似文献
Biomaterials are materials that can be biodegradable or obtained from renewable resources. Among them, poly (lactic acid) (PLA) and thermoplastic starch (TPS) represent an interesting alternative to replace petro-sourced thermoplastics. In this study, blends made by TPS addition to PLA were subjected to a foaming process using supercritical CO2-aided extrusion. Extruder die temperature and CO2 content were the most prominent parameters explaining the structure of the foams obtained. Both parameters were intimately linked since the CO2 flow depends on the melt temperature, the lower the temperature, the higher the CO2 solubility. Therefore, the die temperature was chosen to pilot the process. Whatever the experimental conditions, a 50/50 (in wt%) blend was poorly foamed due to the strong incompatibility between both biopolymers. However, the blend made of 80 wt% PLA and 20 wt% TPS gave evenly foamed samples. In terms of expansion and type of porosity this blend behaved like pure PLA with high porosity, up to 96%, and the presence of a threshold die temperature separating a close cell porosity at lowest temperatures and an open cell structure above the threshold. This temperature threshold was however significantly lower to that obtained with pure PLA. 相似文献
The effects of process variables on the microcellular structure and crystallization of foamed polypropylene (PP) with supercritical CO2 as the foaming agent were investigated in this article. The cell size increased and the cell density reduced with increased foaming temperature. Differently, both the cell diameter and cell density increased as saturation pressure increased. DSC curves showed that the melting peak was broadened when supercritical CO2 foaming PP. Furthermore, the width at half-height of the melting peak increased, the melting peak moved to higher temperature, and the melting point and crystallinity enhanced as the foaming temperature lowered and the saturation pressure enhanced. 相似文献