微孔聚乳酸支架材料的热稳定性与动态热机械特性

采用无溶剂二氧化碳固态发泡技术,在2.5、3.5、4.0和5.0 MPa饱和压力下制备了泡孔孔径为350-20μm的聚乳酸支架材料.利用热重分析技术、动态热机械分析技术和扫描电子显微镜技术,测定了材料的起始分解温度、分解速率、储存/损耗模量和损耗因子等参数,并利用Kissinger、Ozawa-Doyle和Vyazovkin方程进行了热分解动力学计算,推算了氮气环境下材料的降解时间和使用寿命.结果表明,随着发泡压力的减小,支架材料的泡孔孔径增大,材料的柔韧性增强,表观活化能降低,降解时间缩短.     

Preparation and properties of polyimide/chopped carbon fiber composite foams

In this study, a series of reinforced polyimide (PI)/carbon fiber (CF) composite foams were fabricated through thermal foaming of polyester ammonium salt (PEAS) precursor powders. The PEAS precursor powders containing different contents of chopped CF were synthesized from benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) and 4,4′‐diaminodiphenyl ether (ODA). The effects of different CF loadings on foaming behavior of PEAS/CF composite precursor powders, final cellular morphology, and physical properties of PI composite foams were investigated. The results revealed that the chopped CF acted as nucleation agent in the foaming process. The dispersion of CF can be evaluated using digital microscope. It is interesting to find that the chopped CF were highly oriented along the direction of cell arrises. As a result, the mechanical properties of PI foams were significantly enhanced owing to the incorporation of chopped CF. Furthermore, the thermal stability of PI composite foams were also slightly improved owing to fine dispersion of CF. In addition, the PI/CF composite foam shows uniform cell size distribution and the best comprehensive physical properties as chopped CF loading at around 6 wt%. Copyright © 2016 John Wiley & Sons, Ltd.     

Increased expansion ratio,cell density,and compression strength of microcellular poly(lactic acid) foams via lignin graft poly(lactic acid) as a biobased nucleating agent

Green and renewable foaming poly(lactic acid) (PLA) represents one of the promising developments in PLA materials. This study is the first to use the lignin graft PLA copolymer (LG‐g‐PLA) to improve the foamability of PLA as a biobased nucleating agent. This agent was synthesized via ring‐opening polymerization of lignin and lactide. The effects of LG‐g‐PLA on cell nucleation induced by the crystallization, rheological behavior, and foamability of PLA were evaluated. Results indicated that LG‐g‐PLA can improve the crystallization rate and crystallinity of PLA, and play a significant nucleation role in the microcellular foam processing of PLA. LG‐g‐PLA improved the foam morphology of PLA, obtaining a reduced and uniform cell size as well as increased expansion ratio and cell density. With the addition of 3 wt% LG‐g‐PLA content, the PLA/LG‐g‐PLA foams increased the compressive strength 1.6 times than that of neat PLA foams. The improved foaming properties of PLA via a biobased nucleating agent show potential for the production and application of green biodegradable foams.     

Fabrication of open‐porous PCL/PLA tissue engineering scaffolds and the relationship of foaming process,morphology, and mechanical behavior

Tissue engineering scaffolds should provide a suitable porous structure and proper mechanical strength, which is beneficial for the delivery of growth factor and regulation of cells. In this study, the open‐porous polycaprolactone (PCL)/poly (lactic acid) (PLA) tissue engineering scaffolds with suitable porous scale were fabricated using different ratios of PCL/PLA blends. At the same time, the relationship of foaming process, morphology, and mechanical behavior in the optimized batch microcellular foaming process were studied based on the single‐factor experiment method. The porous structures and mechanical strength of the scaffolds were optimized by adjusting foaming parameters, including the temperature, pressure, and CO₂ dissolution time. The results indicated that the foaming parameters influence the cell morphology, further determine the mechanical behavior of PCL/PLA blends. When the PCL content is high, with the increase of temperature and time, the cell diameter and the elastic modulus increased, and the tensile strength and elastic modulus increased with the increase of the average cell size, and decreased as the increase of the cell density. While when the PLA content was high, the cell diameter showed the same trend, and the tensile strength and elastic modulus were higher, and the elongation at break was lower, and tensile strength and elastic modulus decreased with the increase of the average cell size and increased with the increase of cell density. This work successfully fabricated optimized porous PCL/PLA scaffolds with excellent suitable mechanical properties, pore sizes, and high interconnectivity, indicating the effectiveness of modulating the batch foaming process parameters.     

Effect of Supercritical Carbon Dioxide Conditions on PVDF/PVP Microcellular Foams

Supercritical carbon dioxide (ScCO₂) was used as a physical foaming agent to prepare poly(vinylidene f luoride)/poly(N-vinyl pyrrolidone) (PVDF/PVP) microstructure material. The effects of foaming conditions including saturation pressure, foaming temperature and foaming time on PVDF/PVP foams morphology, thermal and electrical behavior were systematically investigated by scanning electron microscope, differential scanning calorimeter and broadband dielectric spectrometer. Small cell and low cell density were achieved at low pressure of 12 MPa, as increasing saturation pressure, the average cell size increased first, and then decreased. The competition between the cell growth and cell nucleation played an important role in average cell size, which was directly related to ScCO₂ processing conditions. With increasing foaming temperature, cell size was increased and cell density was decreased, in a nearly linear manner. The variation of foaming time was considered to be closely related to the time for cells to grow. Thus, the results revealed that the average cell size enhanced with extending foaming time. The thermal properties of PVDF/PVP composites are slightly inf luenced by foaming parameters, and the dielectric constant of PVDF/PVP composite foams decreased with increasing volume expansion ratio.     

Glass transition temperature of polyurethane foams derived from lignin by controlled reaction rate

Sodium ligninosulfonate (LS)-based polyurethane (PU) foams were prepared using three kinds of ethylene glycols, diethylene glycol, triethylene glycol or polyethylene glycol. Two kinds of industrial NaLS, acid-based and alkaline-based NaLS, were mixed with various ratios, and foaming reactions were controlled. Mixing, cream, and rise time were used as an index of foaming reaction. Mixing time was defined as the time interval from adding isocyanate to detection of evolved heat under stirring, cream time as the time interval from termination of stirring to starting of foaming, and rise time as the time interval from starting to completion of foaming. The above reaction time increased with increasing amount of acid base NaLS content in polyols. Apparent density, compression strength and compression modulus of PU foams linearly increased with reaction time. Thermal decomposition temperature was measured by thermogravimetry and glass transition temperature by differential scanning calorimetry. Glass transition temperature can be controlled in a temperature range from 310 to 390 K by changing the mixing rate of two kinds of LS and molecular mass of ethylene glycols. It was found that mechanical and thermal properties of PU foams are controllable through the foaming reaction rate using two kinds of industrial lignin.     

Foaming behavior,cellular structure and physical properties of polybenzoxazine foams

In this paper, polymer foams based on a benzoxazine resin have been successfully prepared using azodicarbonamide (ADC) as a chemical blowing agent and have been characterized regarding their foaming behavior, cellular structure, and physical properties. The effect of the ADC on the curing process of the resin was analyzed using differential scanning calorimetry and blowing agent decomposition was followed by thermogravitmetric analysis (TGA). The characterization of the cellular structure of the foamed samples was done using scanning electron microscopy. The mechanical properties of the foams were determined using compression tests and the thermal conductivity was assessed using the transient plane source method. The results indicated that the curing process and gas release took place in a similar time interval. The foams showed an isotropic cellular structure with relative densities in the range 0.35–0.60, and showed compressive strengths and compressive moduli in the range of 10–70 MPa and 400–1100 MPa, respectively. Thermal conductivities were in the range of 0.06–0.12 W m^?1K^?1. The findings in this paper demonstrate the possibility of producing polybenzoxazine foams using a simple process in which curing and foaming take place simultaneously. In addition, the mechanical characterization of these materials indicates that they are suitable for structural applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Extrusion foaming of semi-crystalline PLA and PLA/thermoplastic starch blends

Low density open-cell foams were obtained from polylactic acid (PLA) and from blends of PLA with thermoplastic starch (TPS) using CO(2) as a blowing agent. Two unexpected features were found. First, a 2D cavitation process in the fractured cell walls was unveiled. Elliptical cavities with dimensions in the 100-300 nm range were aligned perpendicular to large cell cracks clearly exhibiting 2D crazing prior to macroscopic cell rupture. Secondly, a significant crystallization rate increase associated with the CO(2) foaming of PLA was discovered. While the PLA used in this study crystallized very slowly in isothermal crystallization, the PLA foams exhibited up to 15% crystallinity, providing evidence that CO(2) plasticization and the biaxial stretching upon foam expansion provided conditions that could increase the crystallization rate by several orders of magnitude.     

偶氮复合发泡剂对交联聚氯乙烯泡沫结构和性能的影响

采用异氰酸酯和环氧树脂作为交联剂,偶氮二异丁腈（AIBN）和偶氮二甲酰胺（AC）作为发泡剂制备了交联聚氯乙烯泡沫材料。 通过扫描电子显微镜分析了两种发泡剂的粒径、比例以及用量对泡孔结构的影响。 结果表明,AIBN的粒径对泡沫的泡孔结构影响不大,但其用量增加导致泡沫密度减小;AC的粒径对泡孔结构产生一定的影响,最优的粒径范围为8～20 μm,并且随着用量增加,泡孔尺寸变小,但对密度影响不大。 采用DSC研究了AIBN和AC在预混料中的分解情况,提出了两种发泡剂影响泡沫的泡孔结构的机理。 对泡沫材料力学性能的分析表明,泡沫的强度随密度的增加而增加,剪切变形则在60～80 kg/m3密度范围内出现最大值。     

Poly(lactic acid)/organoclay nanocomposites: Thermal,rheological properties and foam processing

In this study, polymer nanocomposites based on poly(lactic acid) (PLA) and organically modified layered silicates (organoclay) were prepared by melt mixing in an internal mixer. The exfoliation of organoclay could be attributed to the interaction between the organoclay and PLA molecules and shearing force during mixing. The exfoliated organoclay layers acted as nucleating agents at low content and as the organoclay content increased they became physical hindrance to the chain mobility of PLA. The thermal dynamic mechanical moduli of nanocomposites were also improved by the exfoliation of organoclay; however, the improvement was reduced at high organoclay content. The dynamic rheological studies show that the nanocomposites have higher viscosity and more pronounced elastic properties than pure PLA. Both storage and loss moduli increased with silicate loading at all frequencies and showed nonterminal behavior at low frequencies. The nanocomposites and PLA were then foamed by using the mixture of CO₂ and N₂ as blowing agent in a batch foaming process. Compared with PLA foam, the nanocomposite foams exhibited reduced cell size and increased cell density at very low organoclay content. With the increase of organoclay content, the cell size was decreased and both cell density and foam density were increased. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 689–698, 2005     

Towards novel super‐elastic foams based on isoperene rubber: Preparation and characterization

A novel and conventional closed cell polyisoprene rubber (IR) foams were produced by a single step limited‐expansion and two step unlimited‐expansion foaming process, respectively. The effect of 3 to 12 part per hundred rubber (phr) of azodicarbonamide (ADC) foaming agent on their structure and properties of developed novel foams were studied. In developed novel foams, the density was strangely independent of ADC content; however, the cell sizes conversely related to ADC content and it decreased by 60% (555‐330 μm) and the internal cell pressure build up from 1 to 3.7 atm, which was related to pressure‐free foaming method. The both reasons of compressed gas trapped inside cells and constant density not only caused unique enhancement in novel foams mechanical properties as hardness and modulus but also improved their dynamic properties as hysteresis and elasticity. Results of conventional IR foams showed that, their foam density as well as dynamic and mechanical properties sharply decreased with increasing ADC content from 3 to 12 phr. For clear expression, in samples with 12 phr of ADC, novel developed foams have more foam density (180%), more hardness (240%), more modulus (290%), and smaller cell size (75%) than conventional foams. Finally, novel developed foams were super‐elastic material with no hysteresis and no plastic deformation while conventional foams had 40% hysteresis and 10% plastic deformation under the same compression conditions.     

Effects of monomer structures on the foaming processes and thermal properties of polyimide foams

Four polyimide (PI) foams were prepared from polyamide acid precursors. The effects of monomer structures on the foaming processes and thermal properties of PI foams were investigated. The foaming processes of PI foams were observed by a self‐made visualization device. The thermal properties of four PI foams were studied by the methods of dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetry/differential thermogravimetry (TG/DTG) analysis. The results indicated that the inflation onset temperatures and maximum inflation degrees of four precursors increased from 123 to 171°C and decreased from 28 to 15 times with the increasing rigidity of the precursor molecule, respectively. The glass transition temperatures, the 5% weight loss temperatures, the decomposed activation energies, and pre‐exponential factors of PI foams increased with increase in the rigidity of monomer. Copyright © 2009 John Wiley & Sons, Ltd.     

应用超临界CO2制备微孔聚丙烯的微孔形貌

研究了应用超临界CO2技术制备微孔聚丙烯时发泡条件和聚丙烯(PP)的熔体强度对微孔形貌的影响。结果表明:在一定的饱和压力下,随着温度的升高,PP的变形能力改善,有利于泡孔的长大。随着饱和压力的增加,PP的熔点降低,升高压力和升高温度具有一定的等同作用。由于CO2在PP内分散的不同,高压低温时得到的泡孔比高温低压时得到的泡孔要规整。降压速率对泡孔形貌的影响因饱和压力的大小而异,饱和压力较高时随着降压速率的提高,孔密度增加,泡孔形貌经历了一个从球体到多面体转变的过程。由于PP熔体强度较低,在发泡温度和PP熔点之间非常接近时,CO2气体容易冲破孔壁而使泡孔呈开孔结构。     

Effect of Blowing Agent Type in Rigid Polyurethane Foam

Rigid polyurethane foams have been fabricated from polymeric MDI and polypropylene glycols (PPG) synthesized with two different initiator compositions using two different types of blowing agents, viz., the conventional HCFC 141b and environmently friendly HFC 365 mfc. It was found that the two blowing agents gave identically the same cream time, gel time, and tack‐free time. The HFC 365 mfc gave foams with smaller cell size, greater core density and compression strength, whereas HCFC 141b gave better dimensional stability and thermal insulation. For the same type of blowing agent, the initiator containing more toluene diamine gave greater core density, compression strength and thermal insulation     

Foaming of trans‐polyisoprene using N2 as the blowing agent

Foaming of trans‐1,4‐polyisoprene (TPI) polymer was carried out through a batch process using nitrogen (N₂) as the blowing agent. TPI vulcanizates having varying crosslink densities were prepared by varying crosslinking agent content and curing time. The vulcanizates were then saturated with N₂ inside a pressure vessel at a pressure of 14 MPa and varying temperatures for 5 hours before effecting the foaming by rapidly quenching the pressure. The effects of varying the crosslinking agent content, silica filler content, and precuring time of the vulcanizates and the effects of varying the gas saturation temperature of foaming on the cell characteristics and physical properties of the foam prepared were investigated. The cells of the TPI foams had a spherical, closed structure. The density, expansion ratio, cell size, cell density, and tensile properties of the foams varied with varying crosslink density of the TPI vulcanizates as well as the saturation temperature of foaming. The important effects of crosslink density and saturation temperature on the N₂ solubility in the TPI matrix and thus on the foam expansion were discussed. The silica filler was found to be acting as a cell nucleating agent and reinforcing filler for the TPI foams.     

新型PES微孔材料的制备及性能研究

合成了新型双烯丙基聚醚砜(PES), 采用超临界CO2作为物理发泡试剂制备微孔材料, 研究了不同发泡温度、饱和压力、发泡时间和放气时间等因素对微孔形貌的影响. 结果表明, 发泡温度在110~170 ℃之间, 随着温度的升高, 泡孔直径增加, 泡孔密度在140 ℃达到一个最大值; 随着饱和压力的升高, 泡孔直径减小, 泡孔密度增大; 发泡时间和放气时间对微孔直径和密度影响不大; 研究了在不同辐照剂量下微孔材料的交联性能, 结果表明, 在600 kGy辐照剂量以下, 交联效果不明显, 在800 kGy以上, 随着辐照剂量的增大, 凝胶含量增加, 辐照后的样品在265 ℃热处理10 min, 仍能保持完好的微孔结构.     

Absorption and foaming of plastics using carbon dioxide

In this study, carbon dioxide was used as a foaming agent for common plastics, such as acrylonitrile–butadiene–styrene (ABS) polymer, polystyrene (PS), polypropylene (PP), high-density polyethylene (HDPE), and high impact polystyrene (HIPS). Carbon dioxide was first absorbed by the sample plastics placed within a pressure vessel at various pressure levels and absorption time intervals. The Henry’s constant of the absorbed carbon dioxide in the plastics was determined. The diffusion coefficient of carbon dioxide in polymer was also identified by curve-fitting with the relationship between the absorbed amount and time. The results showed that ABS, PS, and HIPS absorbed more gas than did PP and HDPE, because PP and HDPE exhibit higher crystallinity. Generally, a polymer can take up saturation absorption of gas under higher pressure. After absorption, the foaming process occurred at various temperatures and time intervals. The cell structure, density, and size of the plastic foams were then investigated using scanning electron microscopy. A longer foaming period and higher temperature increase the size of the cell and decrease the cell density (the number of bubbles per unit volume). A dense skin layer without bubbles appeared directly adjacent to the surface of the foamed plastics. Its thickness decreased if the foaming process took place at higher temperatures.     

智能水基泡沫研究进展

作为典型的软物质,水基泡沫因具有较小的粒径、较大的比表面积和良好的流动性而广泛应用于洗涤剂、化妆品、食品工程、油气开采等领域。在实际应用中,泡沫的稳定性起着制约性作用。近年来,在环境因素刺激下,能在稳定和非稳定状态之间转变的可控智能泡沫引起了极大关注。针对近年来智能水基泡沫的研究进展,本文综述了基于温度、磁场、光、pH和CO₂响应等智能水基泡沫体系,讨论了不同类型的智能水基泡沫的形成机理及相应性能,展望了智能水基泡沫的应用前景和发展方向。     

Effects of the hydroxyl value of polyol in rigid polyurethane foams

Rigid polyurethane foams (RPUFs) have been fabricated from crude MDI (CMDI) and polypropylene glycols (PPGs) of various hydroxyl values (OHVs) of 300–600 with an environmentally friendly blowing agent (HFC 365mfc). The closed cell content, compression strength, and dimensional stability of the foam increased with increase in the OHV due to the increased crosslink density, which was evidenced from the increased glass transition temperature (T_g). The cream time, gel time, tack‐free time, and density of the foam showed a minimum at 500OHV. The decrease and increase around the minimum were, respectively, interpreted in terms of increased mixture mobility and extensive allophanate crosslinks, which retard gelling as well as foaming reactions. The thermal conductivity also showed a small minimum at 500OHV. Copyright © 2008 John Wiley & Sons, Ltd.     

The two steps thermal decomposition of titanium hydride and two steps foaming of Al alloy

1 Introduction Al alloy foam with closed pores prepared by melt foaming, realizing the lightness, high specific strength and multifunction of structure material, is becoming one of the hotspots[1―11]. In order to meet the demand of high-tech, preparing s…