发泡工艺对超临界CO2／PP微孔发泡泡孔形态的影响

研究了超临界CO2／PP微孔发泡过程中发泡温度和饱和压力对结晶性聚合物PP泡孔形态的影响。结果表明，温度对泡孔形态影响很大，温度升高，熔体黏度和表面张力降低，泡孔变大，泡孔密度减小。与发泡温度相比，CO2饱和压力对泡孔结构的影响较小。压力太低，CO2的溶解度小，泡孔壁太厚，泡孔分布不均匀。随着压力升高，CO2的溶解度增加，熔体黏度减小，所以泡孔直径和泡孔密度都增加，泡孔壁变薄。     

高发泡聚丙烯材料的研究

以偶氮二甲酰胺(AC)为发泡剂,采用化学交联法,使聚丙烯(PP)在发泡之前交联以提高熔体强度,并利用模压法制备了高发泡PP材料。结果表明:当AC含量为2份时,PP泡沫材料的表观密度降到0.2×103kg/m3,泡孔微观形态良好,但泡沫材料力学性能较差。     

Cell coalescence suppressed by crosslinking structure in polypropylene microcellular foaming

A series of crosslinked polypropylene samples with increased melt strengths were prepared via a copolymerization reaction, followed by melt processing. These crosslinked PP samples (PP‐Cs) were foamed by a temperature rising process using supercritical CO₂ as the physical blowing agent. The introduction of crosslinking structure resulted in PP‐Cs foams with well‐defined closed cell structure, decreased cell size, and increased cell density in comparison with a linear PP, which were attributed to the suppressed cell coalescence due to the significant increase in melt strength of PP‐Cs. Further increasing the crosslinking degree tended to enhance the suppression effect on the cell coalescence, and hence increase the cell density of PP foams under the same foaming conditions, especially at the longer foaming times. The well‐defined closed cell structure was observed at the foaming temperature of 170–250°C and saturation pressure of 12–20 MPa. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

加工参数对微孔PC泡孔性能的影响

采用模压法成功制备了薄型微孔聚碳酸酯（PC）片材，研究了加工参数（发泡时间、发泡压力和发泡温度）对泡孔性能的影响，结果表明：泡孔尺寸随发泡时间的延长先减小后增大，随发泡压力的增加而减小，随发泡温度的增加而增大；泡孔密度的变化趋势随与泡孔尺寸的变化趋势相反；相对密度则随加工参数的增加而降低。通过比较各加工参数对泡孔性能的影响，可以发现发泡时间对微孔PC片材泡孔性能影响最大，且最佳的发泡时间约为8min。     

组合发泡剂对聚丙烯发泡行为的影响研究

采用物理发泡剂和化学发泡剂的组合发泡剂对聚丙烯(PP)和高熔体强度聚丙烯(HMSPP)在自制的单螺杆串联单螺杆挤出发泡机组上进行挤出发泡试验。通过真密度计/开闭孔率测定仪和扫描电子显微镜对发泡制品的密度、发泡倍率和泡孔形态进行测试。研究结果表明,采用组合发泡剂后,大部分PP和HMSPP发泡制品的泡孔密度提高,发泡倍率增加,泡孔尺寸分布更加均匀,泡体结构优于单独使用物理发泡剂或化学发泡剂的发泡制品。     

Strategy for the preparation of lightweight polypropylene/polyethylene-octene elastomer composite foams with different phase morphologies using supercritical carbon dioxide

Polypropylene (PP)/polyethylene-octene elastomer (POE) composites with a “sea-island” structure and a cocontinuous structure were prepared. With the selection of a suitable foaming temperature, the supercritical carbon dioxide foaming of PP/POE composites with different phase morphologies occurred only in the POE phase. The effects of the POE content, foaming temperature, pressure, and number of layers on the cell size, cell density, apparent density, foaming layer density, and foaming ratio under different phase morphologies were investigated by scanning electron microscopy, polarized optical microscopy, differential scanning calorimetry, and dynamic thermomechanical analysis. This article provides a novel approach for foaming PP at a low temperature. For PP/POE-blended composites with a “sea-island” structure, the foaming temperature is as low as 80 °C, and for PP/POE alternating multilayered composites with a cocontinuous structure, foaming can occur at 40 °C. Compared with the conventional methods for foaming PP, this method avoids the problems of a high foaming temperature, a narrow range of the foaming temperature, and a low melt strength of the PP. Thus, the PP foaming method was successfully improved, yielding a new technique for the preparation of lightweight PP. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48157.     

A strategy for the preparation of closed‐cell and crosslinked polypropylene foam by supercritical CO2 foaming

We report the preparation of a closed‐cell polypropylene (PP) foam material by supercritical carbon dioxide foaming with the assistance of γ‐ray radiation crosslinking. Styrene–ethylene–butadiene–styrene (SEBS) copolymer was added to PP to enhance radiation crosslinking and nucleation. Radiation effects on the foaming of the PP/SEBS blend with different ratios were investigated. A significant improvement in the foaming of the crosslinked PP/SEBS blend was achieved as compared to pristine PP. The cell density of the crosslinked PP/SEBS foam greatly increased at a dose of 10 kGy and a high closed‐cell ratio was obtained. The tensile strength of the crosslinked PP/SEBS foams (10 kGy) was improved from 14 to 20.7 MPa compared to pristine PP foam (0 kGy). In addition, the crosslinked PP/SEBS blend exhibited a wider foaming temperature window (10 °C) as compared to the non‐crosslinked ones (4 °C). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45809.     

Morphology and Properties of Polypropylene Foaming Sheet with Alternating Multilayered Structure

In this work, the foaming sheet was designed as alternating multilayered foam/film structure of foaming layers and film layers. The foaming layer contained polypropylene (PP)/high density polyethylene (HDPE)/Talc ternary composites. The film layer contained PP only. The rheological data showed that the melt elasticity of PP was obviously improved by the addition of HDPE and talc. The results exhibited that the alternating multilayered structure was well kept and hardly influenced by the foaming layers, and then the mechanical properties were obviously improved. The cell in the alternating multilayered sheet with 16 layers was smaller and more homogenous than that in pure PP foaming sheet.     

Characterization of LDPE and LDPE/EVA blends crosslinked by electron beam irradiation and foamed with chemical foaming agent

In this work, low density polyethylene and its blend with poly (ethylene vinyl acetate) (LDPE/EVA) (80/20) were mixed with different concentration of Azodicarbonamide (ACA) as a foaming agent. The specimens were crosslinked by electron beam irradiation in presence of 1,6-Hexandiol diacrylate (HDDA). The foam structure was obtained by heating the crosslinked sheets at 225 °C. The effect of ACA content and irradiation dose on the gel percent, tensile strength, cell density, and thermal stabilities was investigated. The results showed clearly that the increasing of ACA content reduces the stress and strain at break and increase the gel content. On the other hand, the foaming degree increases with increasing the ACA content and decreases with increasing irradiation dose.     

成核剂对PVC涂覆制品泡孔形态的影响

选择不同拓扑结构的无机成核剂纳米二氧化硅和有机蒙脱土,研究其对PVC增塑糊涂覆发泡制品泡孔形态、结构的影响。通过扫描电子显微镜(SEM)观察样品泡孔形态,采用图像分析手段计算泡孔直径、发泡倍率及泡孔密度,以探明成核剂对泡孔形态结构的影响规律及机制。结果表明:与大多数聚合物熔融体系物理发泡不同,球型纳米二氧化硅对偶氮二甲酰胺(AC发泡剂)主导下的PVC增塑糊涂覆工艺化学发泡过程影响不明显,而插层、团聚分散于PVC增塑糊发泡体系中的有机蒙脱土会增加涂覆制品的泡孔直径,并降低发泡倍率及泡孔密度。     

Study of the cellular structure heterogeneity and anisotropy of polypropylene and polypropylene nanocomposite foams

This article presents the analysis of the processing parameters influence on the foaming behavior and cellular structure of PP‐montmorillonite foams. Polypropylene nanocomposites containing 5.0 phr of an organically‐modified montmorillonite (MMT) were initially melt‐compounded in a twin‐screw extruder with azodicarbonamide (ADC) and later foamed using a one‐step compression‐molding process. The cellular structure and morphology of the foams was assessed using both scanning and transmission electron microscopies. A time‐dependant double‐effect was observed during foaming: (1) first of all, the melt strength of the polymer, too high for shorter times, not allowing full cell growth, and too low for high foaming times due to thermal oxidation; (2) and polymer degradation, clearly observed for very high foaming times, directly affecting polymer's melt resistance. Comparatively, PP‐MMT foams exhibited a broader foaming time processing window, a more isometric type of cellular structure and decreased open‐cell contents, indicating an effective nucleation and cell wall stabilization induced by the exfoliated MMT particles. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Effect of foaming temperature and rubber grades on properties of natural rubber foams

Foaming temperature and grade of dry natural rubber were varied to evaluate their effects on the morphology and mechanical properties of natural rubber (NR) foams. Three different grades of NR were used; namely ENR‐25, SMR‐L, and SMR‐10. NR foams from these grades were produced at three different foaming temperatures, i.e. 140, 150, and 160°C. The study was carried out using formulated compositions containing sodium bicarbonate as the chemical blowing agent and were expanded using conventional compression molding technique via a heat transfer foaming process. The NR foams were characterized with respect to their relative foam density, density of crosslinking, cell size, compression stress, and compression set. Increase in foaming temperature resulted in lower relative density and larger cell size. It was also discovered that the crosslink density slightly decrease with increasing foaming temperature. For mechanical properties, the highest foam density resulted in the highest compression stress. Compression stress at 50% strain increased with increasing foaming temperature and ENR‐25 foam has the highest compression stress among the produced foams. The results showed that the morphology, physical, and mechanical properties of the rubber foams can be controlled closely by the foaming temperature and rubber grades. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Solid‐state foaming of isotactic polypropylene and its composites with spherical or fibrous poly(butylenes terephthalate)

In this article, the foaming behavior of isotactic polypropylene (iPP) and its composites with spherical or fibrous poly(butylenes terephthalate) (PBT) using supercritical CO₂ as a blowing agent were investigated. Their foaming performances were also compared in relation to the crystal morphology and rheological behavior of PP. Results demonstrate that crystal structures significantly impacted the cell structures of foams. At relatively low temperature, microcells appeared at the centers of PP spherulites where the melting started. Particularly, bi‐modal cell structure formed in the foamed PP with increasing temperature. However, in the foamed PP composites with spherical or fibrous PBT, this structure almost disappeared due to the smaller PP spherulites. In foaming PP/PBT composites, the heterogeneous nucleation of spherical or fibrous PBT was effective at reducing cell size as well as improving cell density and cell uniformity. The fibrous PBT also acted as scaffolds for preserving cell shapes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41801.     

Effect of the high‐density polyethylene melt index on the microcellular foaming of high‐density polyethylene/polypropylene blends

The effect of high‐density polyethylene (HDPE)/polypropylene (PP) blending on the crystallinity as a function of the HDPE melt index was studied. The melting temperature and total amount of crystallinity in the HDPE/PP blends were lower than those of the pure polymers, regardless of the blend composition and melt index. The effects of the melt index, blending, and foaming conditions (foaming temperature and foaming time) on the void fractions of HDPEs of various melt indices and HDPE/PP blends were also investigated. The void fraction was strongly dependent on the foaming time, foaming temperature, and blend composition as well as the melt index of HDPE. The void fraction of the foamed 30:70 HDPE/PP blend was always higher than that of the foamed 50:50 HDPE/PP blend, regardless of the melt index. The microcellular structure could be greatly improved with a suitable ratio of HDPE to PP and with foaming above the melting temperature for long enough; however, using high‐melt‐index HDPE in the HDPE/PP blends had a deleterious effect on both the void fraction and cell morphology of the blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 364–371, 2004     

Foaming behavior of microcellular foam polypropylene/modified nano calcium carbonate composites

A new process was used to prepare microcellular foams with supercritical carbon dioxide as the physical foaming agent in a batch. The foaming temperature range of the new process was about five times broader than that of the conventional one. Characterization of the cellular structure of the original polypropylene (PP) and PP/nano‐CaCO₃ (nanocomposites) foams was conducted to reveal the effects of the blend composition and processing conditions. The results show that the cellular structure of the PP foams was more sensitive to the foaming temperature and saturation pressure variations than that of the nanocomposite foams. Uniform cells of PP foams are achieved only at a temperature of 154°C. Also, the low pressure of 20 MPa led to very small cells and a low cell density. The competition between the cell growth and cell nucleation played important role in the foam density and was directly related to the foaming temperature. Decreasing the infiltration temperature depressed the initial foaming temperature, and this resulted in significantly larger cells and a lower cell density. A short foaming time led to a skin–core structure; this indicated that a decrease in the cell size was found from skin to core, but the skin–core structure gradually disappeared with increasing foaming time. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effects of Process Variables on Microcellular Structure and Crystallization of Polypropylene Foams with Supercritical CO2 as the Foaming Agent—A Study of Microcellular Foaming of Polypropylene

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.     

A Taguchi analysis on structural properties of polypropylene microcellular nanocomposite foams containing Fe2O3 nanoparticles in batch process

Polypropylene (PP) as a thermoplastic polymer has been foamed using batch foaming process. CO₂ is used as the blowing agent of the foaming. Ferrous oxide nanoparticles (nano Fe₂O₃) are also added as reinforcement. Effect of different parameters including nanoparticle weight percentage, foaming temperature and time on the structural properties of PP/nano Fe₂O₃ nanocomposites is investigated using Taguchi approach. Scanning electron microscope results depict that an appropriate microcellular structure is obtained with the cell density of 10⁹ cells/cm³ and almost 1 μm of cell size. Analysis of variance results indicated that foaming temperature is the most significant parameter on the structural properties. Cell density and expansion ratio are decreased by increasing foaming temperature. This phenomenon could be due to the reducing melt strength of polymer/gas mixture. It was also inferred that adding 2 wt% of nanoparticles leads to 80% improvement in cell density while cell size and expansion ratio was decreased.     

Effects of formulation and processing parameters on the morphology of extruded polypropylene‐(waste ground rubber tire powder) foams

This paper presents an experimental study of the foaming behavior of polypropylene (PP)/(waste ground rubber tire powder) (WGRT) blends when using a chemical blowing agent in an extrusion foaming process. The effects of formulations (i.e., WGRT content, blowing agent content, compatibilizer) and the processing parameters (i.e., die temperature, screw speed) on the void fraction, average cell size, cell density, and cell morphology of the PP/WGRT foams were investigated. The blowing agent loading affected the cell structure of the foams and the average cell size, and the void fraction increased with increasing blowing agent loading. Both increasing the screw speed and decreasing the die temperature could establish a high pressure drop in the extruder die, and these were beneficial to the foaming extrusion. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers     

Preparation, characterization, and supercritical carbon dioxide foaming of polystyrene/graphene oxide composites

Graphene oxide (GO) was prepared by oxidation of graphite using the Hummers method, and was modified by isocyanate to obtain dispersed GO sheets in dimethylformamide. Polystyrene (PS)/GO composites were prepared by solution blending, and their morphologies and properties were characterized. The addition of GO increased the glass transition temperature of the PS/GO composites. The storage modulus and thermal stability of the composites were also improved compared with PS. Foams of PS and PS/GO composites were prepared by supercritical carbon dioxide foaming. The composite foams exhibited slightly higher cell density and smaller cell size compared with the PS foam, indicating the GO sheets can act as heterogeneous nucleation agents.     

LDPE共混改性PP的结晶行为及模压发泡效果

采用单螺杆挤出机制备了低密度聚乙烯(LDPE)共混改性聚丙烯(PP)可发性粒料,并通过模压发泡工艺得到改性PP发泡材料;考察了发泡剂的热分解特性以及LDPE的含量对共混体系的熔融/结晶行为、晶体结构和发泡性能的影响。结果表明:与纯偶氮二甲酰胺(AC)相比,复合发泡剂的分解温度下降了45℃;LDPE的引入没有改变PP的晶型结构,但降低了共混体系的结晶度;当LDPE的含量为15%～20%时,LDPE/PP共混体系的发泡效果最佳。