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Cell structure evolution and the crystallization behavior of polypropylene / clay nanocomposites foams blown in continuous extrusion / Wentao Zhai in Industrial & engineering chemistry research, Vol. 49 N° 20 (Octobre 2010) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 49 N° 20 (Octobre 2010) . - pp. 9834–9845 Titre : Cell structure evolution and the crystallization behavior of polypropylene / clay nanocomposites foams blown in continuous extrusion Type de document : texte imprimé Auteurs : Wentao Zhai, Auteur ; Takashi Kuboki, Auteur ; Lilac Wang, Auteur Année de publication : 2011 Article en page(s) : pp. 9834–9845 Note générale : Chimie industrielle Langues : Anglais (eng) Mots-clés : Crystallization  Nanocomposites Résumé : In this study, linear homopolypropylene/clay (HPPC) nanocomposite foams with a high expansion ratio of about 18 and a high cell density of about 1.7 × 108 cells/cm3 were produced using an extrusion foaming method with CO2 as the physical blowing agent. The result was much better than pure HPP foams with expansion ratios of 1.7−2.2 and cell densities of 103−105 cells/cm3 obtained even at the same foaming conditions. The nanoclays had a half-exfoliated structure in the HPP matrix, and their presence dramatically affected the viscoelastic properties of HPP melt and foaming behaviors. It was found that the introduction of a small amount of nanoclay significantly increased the cell morphology of HPP foams at low die temperatures, where the cell wall was very thin and cell distribution was uniform. With an increase in nanoclay content of up to 5 wt %, cell morphology was improved gradually at broader die temperatures. Based on the cell morphology results, a suitable foaming window for clay content and die temperature was established. The mechanisms behind these phenomena are discussed from the perspective of cell nucleation and coalescence. Microstructures were found in the cell walls of HPP and HPPC nanocomposite foams, and they tended to evolve with cell wall thickness, depending on the die temperatures. Scanning electron microscopy (SEM) observation of foams and solvent-etched foams revealed that the microstructures in the cell walls were formed by covering large-sized crystals and that the absence of microstructures was due to the presence of small-sized crystals in the cell walls. A distribution of crystal sizes was observed across the foamed samples, which was affected by the die temperature and the introduction of nanoclay. The possible reasons were elaborated by considerations of temperature gradient. DSC tests indicated that the foaming process induced a low-temperature peak (Tm1) and its heat of fusion (ΔHm1) tended to evolve with the die temperature and the introduction of nanoclay. DEWEY : 660 ISSN : 0888-5885 En ligne : http://pubs.acs.org/doi/abs/10.1021/ie101225f [article] Cell structure evolution and the crystallization behavior of polypropylene / clay nanocomposites foams blown in continuous extrusion [texte imprimé] / Wentao Zhai, Auteur ; Takashi Kuboki, Auteur ; Lilac Wang, Auteur . - 2011 . - pp. 9834–9845. Chimie industrielle Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 49 N° 20 (Octobre 2010) . - pp. 9834–9845 Mots-clés : Crystallization  Nanocomposites Résumé : In this study, linear homopolypropylene/clay (HPPC) nanocomposite foams with a high expansion ratio of about 18 and a high cell density of about 1.7 × 108 cells/cm3 were produced using an extrusion foaming method with CO2 as the physical blowing agent. The result was much better than pure HPP foams with expansion ratios of 1.7−2.2 and cell densities of 103−105 cells/cm3 obtained even at the same foaming conditions. The nanoclays had a half-exfoliated structure in the HPP matrix, and their presence dramatically affected the viscoelastic properties of HPP melt and foaming behaviors. It was found that the introduction of a small amount of nanoclay significantly increased the cell morphology of HPP foams at low die temperatures, where the cell wall was very thin and cell distribution was uniform. With an increase in nanoclay content of up to 5 wt %, cell morphology was improved gradually at broader die temperatures. Based on the cell morphology results, a suitable foaming window for clay content and die temperature was established. The mechanisms behind these phenomena are discussed from the perspective of cell nucleation and coalescence. Microstructures were found in the cell walls of HPP and HPPC nanocomposite foams, and they tended to evolve with cell wall thickness, depending on the die temperatures. Scanning electron microscopy (SEM) observation of foams and solvent-etched foams revealed that the microstructures in the cell walls were formed by covering large-sized crystals and that the absence of microstructures was due to the presence of small-sized crystals in the cell walls. A distribution of crystal sizes was observed across the foamed samples, which was affected by the die temperature and the introduction of nanoclay. The possible reasons were elaborated by considerations of temperature gradient. DSC tests indicated that the foaming process induced a low-temperature peak (Tm1) and its heat of fusion (ΔHm1) tended to evolve with the die temperature and the introduction of nanoclay. DEWEY : 660 ISSN : 0888-5885 En ligne : http://pubs.acs.org/doi/abs/10.1021/ie101225f

Fabrication of lightweight microcellular polyimide foams with three-dimensional shape by CO2 foaming and compression molding / Wentao Zhai in Industrial & engineering chemistry research, Vol. 51 N° 39 (Octobre 2012) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 51 N° 39 (Octobre 2012) . - pp. 12827-12834 Titre : Fabrication of lightweight microcellular polyimide foams with three-dimensional shape by CO2 foaming and compression molding Type de document : texte imprimé Auteurs : Wentao Zhai, Auteur ; Weiwei Feng, Auteur ; Jianqiang Ling, Auteur Année de publication : 2012 Article en page(s) : pp. 12827-12834 Note générale : Industrial chemistry Langues : Anglais (eng) Mots-clés : Molding  Compression  Foaming  Carbon  dioxide  Morphology  Foam Résumé : Microcellular structure endows polymeric foams with the improved mechanical properties, but the preparation of lightweight microcellular polyimide (PI) foams with a large size is challenging and inefficient, because of low gas solubility, high stiffness, and an extremely long saturation time. In this study, PI foam was prepared by solid-state microcellular foaming technology with the compressed CO2 as a physical blowing agent and tetrahydrofuran as coblowing agent The presence of coblowing agent was verified to increase the gas sorption of PI, causing a dramatic increase in the expansion ratio of miaocellular PI beads from 2.9 to 15.7. Using a novel compression molding process, the prepared PI foams were molded into the 3-D shaped products. Before the molding, the foamed PI beads were coated by poly(ether imide) (PEI)/chloroform solution. The contact angle tests indicated that PEI/chloroform could infiltrate well PI foams' surface, which facilitated the formation of strong interbead bonding between bead foams. The thickness of the coated PEI layer and the interbead bonding regions were the important parameters to adjust the bending and compression properties of the molded PI foam (MPI) product. The experimental results indicated that the bending strength and compression strength (at 10% strain) of MPI sample with density of 137.7 kg/m3 were 1.27 and 1.59 MPa, respectively. ISSN : 0888-5885 En ligne : http://cat.inist.fr/?aModele=afficheN&cpsidt=26419239 [article] Fabrication of lightweight microcellular polyimide foams with three-dimensional shape by CO2 foaming and compression molding [texte imprimé] / Wentao Zhai, Auteur ; Weiwei Feng, Auteur ; Jianqiang Ling, Auteur . - 2012 . - pp. 12827-12834. Industrial chemistry Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 51 N° 39 (Octobre 2012) . - pp. 12827-12834 Mots-clés : Molding  Compression  Foaming  Carbon  dioxide  Morphology  Foam Résumé : Microcellular structure endows polymeric foams with the improved mechanical properties, but the preparation of lightweight microcellular polyimide (PI) foams with a large size is challenging and inefficient, because of low gas solubility, high stiffness, and an extremely long saturation time. In this study, PI foam was prepared by solid-state microcellular foaming technology with the compressed CO2 as a physical blowing agent and tetrahydrofuran as coblowing agent The presence of coblowing agent was verified to increase the gas sorption of PI, causing a dramatic increase in the expansion ratio of miaocellular PI beads from 2.9 to 15.7. Using a novel compression molding process, the prepared PI foams were molded into the 3-D shaped products. Before the molding, the foamed PI beads were coated by poly(ether imide) (PEI)/chloroform solution. The contact angle tests indicated that PEI/chloroform could infiltrate well PI foams' surface, which facilitated the formation of strong interbead bonding between bead foams. The thickness of the coated PEI layer and the interbead bonding regions were the important parameters to adjust the bending and compression properties of the molded PI foam (MPI) product. The experimental results indicated that the bending strength and compression strength (at 10% strain) of MPI sample with density of 137.7 kg/m3 were 1.27 and 1.59 MPa, respectively. ISSN : 0888-5885 En ligne : http://cat.inist.fr/?aModele=afficheN&cpsidt=26419239

Nanosilica addition dramatically improves the cell morphology and expansion ratio of polypropylene heterophasic copolymer foams blown in continuous extrusion / Wentao Zhai in Industrial & engineering chemistry research, Vol. 50 N° 12 (Juin 2011) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 50 N° 12 (Juin 2011) . - pp. 7282-7289 Titre : Nanosilica addition dramatically improves the cell morphology and expansion ratio of polypropylene heterophasic copolymer foams blown in continuous extrusion Type de document : texte imprimé Auteurs : Wentao Zhai, Auteur ; Chul B. Park, Auteur ; Marianna Kontopoulou, Auteur Année de publication : 2011 Article en page(s) : pp. 7282-7289 Note générale : Chimie industrielle Langues : Anglais (eng) Mots-clés : Extrusion  Foam  Expansion  Morphology Résumé : Currently, the preparation of polypropylene (PP) foam with a well-defined cell structure and a high expansion ratio is receiving increased attention. However, the present technical problems such as poor cell nucleation ability and weak melt strength of polymer resin, hinder the broader use of linear PP in foam production. In this study, a PP heterophasic copolymer with a linear structure was selected together with nanosilica to challenge the fabrication of PP foam with uniform cell structure, high cell density, and a high expansion ratio using CO2 as a physical blowing agent. Scanning electron microscopy (SEM) observation indicated that silica particles tended to aggregate in the PP matrix, but the multisilica aggregates with sizes from 80 to 350 nm were well dispersed in PP because of the addition of a coupling agent (CA). PP foam exhibited poor cell morphology and low cell densities of ca. 104―5 cells/cm3at different die temperatures. An introduction of a small amount of nanosilica, 0.5 wt % and 1 wt %, dramatically improved the foaming behavior of PP, where the cell structure distribution of the resultant foams was uniform, and the cell density and foam expansion were high (i.e., 108―9 cells/cm3 and 16.9―19.5, respectively). Furthermore, the presence of nanosilica clearly broadened the foaming window of PP. By further increasing silica content, however, the foaming behavior of PP/silica nanocomposites became poor, especially at slightly higher die temperatures (i.e., above 140 °C), even though a high silica loading increased the number of heterogeneous nucleation sites. The effect of foaming on the dispersion of nanosilica in the PP matrix was also investigated. DEWEY : 660 ISSN : 0888-5885 En ligne : http://cat.inist.fr/?aModele=afficheN&cpsidt=24239042 [article] Nanosilica addition dramatically improves the cell morphology and expansion ratio of polypropylene heterophasic copolymer foams blown in continuous extrusion [texte imprimé] / Wentao Zhai, Auteur ; Chul B. Park, Auteur ; Marianna Kontopoulou, Auteur . - 2011 . - pp. 7282-7289. Chimie industrielle Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 50 N° 12 (Juin 2011) . - pp. 7282-7289 Mots-clés : Extrusion  Foam  Expansion  Morphology Résumé : Currently, the preparation of polypropylene (PP) foam with a well-defined cell structure and a high expansion ratio is receiving increased attention. However, the present technical problems such as poor cell nucleation ability and weak melt strength of polymer resin, hinder the broader use of linear PP in foam production. In this study, a PP heterophasic copolymer with a linear structure was selected together with nanosilica to challenge the fabrication of PP foam with uniform cell structure, high cell density, and a high expansion ratio using CO2 as a physical blowing agent. Scanning electron microscopy (SEM) observation indicated that silica particles tended to aggregate in the PP matrix, but the multisilica aggregates with sizes from 80 to 350 nm were well dispersed in PP because of the addition of a coupling agent (CA). PP foam exhibited poor cell morphology and low cell densities of ca. 104―5 cells/cm3at different die temperatures. An introduction of a small amount of nanosilica, 0.5 wt % and 1 wt %, dramatically improved the foaming behavior of PP, where the cell structure distribution of the resultant foams was uniform, and the cell density and foam expansion were high (i.e., 108―9 cells/cm3 and 16.9―19.5, respectively). Furthermore, the presence of nanosilica clearly broadened the foaming window of PP. By further increasing silica content, however, the foaming behavior of PP/silica nanocomposites became poor, especially at slightly higher die temperatures (i.e., above 140 °C), even though a high silica loading increased the number of heterogeneous nucleation sites. The effect of foaming on the dispersion of nanosilica in the PP matrix was also investigated. DEWEY : 660 ISSN : 0888-5885 En ligne : http://cat.inist.fr/?aModele=afficheN&cpsidt=24239042

Steam - chest molding of expanded polypropylene foams / Wentao Zhai in Industrial & engineering chemistry research, Vol. 49 N° 20 (Octobre 2010) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 49 N° 20 (Octobre 2010) . - pp. 9822-9829 Titre : Steam - chest molding of expanded polypropylene foams : 1. DSC simulation of bead foam processing Type de document : texte imprimé Auteurs : Wentao Zhai, Auteur ; Young-Wook Kim, Auteur ; Chul B. Park, Auteur Année de publication : 2011 Article en page(s) : pp. 9822-9829 Note générale : Chimie industrielle Langues : Anglais (eng) Mots-clés : Differential  scanning  calorimetry  Foam  Molding  Water  vapor Résumé : To investigate the evolution of melting behavior during the steam-chest molding process of expanded polypropylene (EPP) bead, a differential scanning calorimetry (DSC) test involving fast heating, isothermal treatment, and fast cooling was used to simulate the bead foam processing. The EPP bead foam had two original melting peaks: a low melting peak, Tmlow, and a high melting peak, Tmhigh. A new melting peak, Tmi, was induced in the DSC curves, resulting from the heating process, when the treatment temperature was 80—150 °C. The Tmi was about 7 °C higher than the treatment temperature and tended to increase linearly with the increased treatment temperature. A new melting area and further a melting peak, Tmc, were observed in DSC curves, resulting from the cooling process, when the treatment temperatures were higher than 135 °C. The high sensitivity of the Tmi and Tmc to treatment temperature provided a clue to the actual temperature inside the mold during the steam-chest molding process. Different steam pressures were applied during EPP bead processing, and the melting behavior of molded EPP samples was measured to check the processing thermal history. A comparison study was done between the DSC simulations and the actual results to understand the melting behavior evolution of EPP bead foam during processing. Some fundamental issues in the steam-chest molding processing, such as the actual steam temperature, the temperature gradient across the mold cavity, and the accurate pressure parameter used to describe the actual processing temperature were alsc studied based on the DSC simulation. DEWEY : 660 ISSN : 0888-5885 En ligne : http://cat.inist.fr/?aModele=afficheN&cpsidt=23325804 [article] Steam - chest molding of expanded polypropylene foams : 1. DSC simulation of bead foam processing [texte imprimé] / Wentao Zhai, Auteur ; Young-Wook Kim, Auteur ; Chul B. Park, Auteur . - 2011 . - pp. 9822-9829. Chimie industrielle Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 49 N° 20 (Octobre 2010) . - pp. 9822-9829 Mots-clés : Differential  scanning  calorimetry  Foam  Molding  Water  vapor Résumé : To investigate the evolution of melting behavior during the steam-chest molding process of expanded polypropylene (EPP) bead, a differential scanning calorimetry (DSC) test involving fast heating, isothermal treatment, and fast cooling was used to simulate the bead foam processing. The EPP bead foam had two original melting peaks: a low melting peak, Tmlow, and a high melting peak, Tmhigh. A new melting peak, Tmi, was induced in the DSC curves, resulting from the heating process, when the treatment temperature was 80—150 °C. The Tmi was about 7 °C higher than the treatment temperature and tended to increase linearly with the increased treatment temperature. A new melting area and further a melting peak, Tmc, were observed in DSC curves, resulting from the cooling process, when the treatment temperatures were higher than 135 °C. The high sensitivity of the Tmi and Tmc to treatment temperature provided a clue to the actual temperature inside the mold during the steam-chest molding process. Different steam pressures were applied during EPP bead processing, and the melting behavior of molded EPP samples was measured to check the processing thermal history. A comparison study was done between the DSC simulations and the actual results to understand the melting behavior evolution of EPP bead foam during processing. Some fundamental issues in the steam-chest molding processing, such as the actual steam temperature, the temperature gradient across the mold cavity, and the accurate pressure parameter used to describe the actual processing temperature were alsc studied based on the DSC simulation. DEWEY : 660 ISSN : 0888-5885 En ligne : http://cat.inist.fr/?aModele=afficheN&cpsidt=23325804

Steam-chest molding of expanded polypropylene foams. 2. mechanism of interbead bonding / Wentao Zhai in Industrial & engineering chemistry research, Vol. 50 N° 9 (Mai 2011) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 50 N° 9 (Mai 2011) . - pp. 5523–5531 Titre : Steam-chest molding of expanded polypropylene foams. 2. mechanism of interbead bonding Type de document : texte imprimé Auteurs : Wentao Zhai, Auteur ; Young-Wook Kim, Auteur ; Dong Won Jung, Auteur Année de publication : 2011 Article en page(s) : pp. 5523–5531 Note générale : Chimie industrielle Langues : Anglais (eng) Mots-clés : Mechanism  Interbead  bonding Résumé : Expanded polypropylene (EPP) bead foams were processed using steam-chest molding. Interbead bonds formed during processing and was characterized by observing both the product surface and the fracture surfaces using scanning electron microscopy (SEM) and by measuring the tensile properties of specimens. It was found that both the degree of interbead bonding and the tensile strength had a direct relationship to the steam pressure/temperature and that high steam temperature enhanced the crystallization behavior of EPP beads during the cooling process. A formation mechanism of interbead bonding during processing was proposed based on the crystal melting behaviors, as discussed in the first article of this series (Zhai et al. Ind. Eng. Chem. Res. 2010, 49, 9822). Possible parameters influencing the formation of interbead bonding, namely, steam temperature/pressure and bead foam expansion ratio, were investigated in this study. The effect of postannealing of the EPP products on the shrinkage and crystallinity of the molded EPP products was also addressed. DEWEY : 660 ISSN : 0888-5885 En ligne : http://pubs.acs.org/doi/abs/10.1021/ie101753w [article] Steam-chest molding of expanded polypropylene foams. 2. mechanism of interbead bonding [texte imprimé] / Wentao Zhai, Auteur ; Young-Wook Kim, Auteur ; Dong Won Jung, Auteur . - 2011 . - pp. 5523–5531. Chimie industrielle Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 50 N° 9 (Mai 2011) . - pp. 5523–5531 Mots-clés : Mechanism  Interbead  bonding Résumé : Expanded polypropylene (EPP) bead foams were processed using steam-chest molding. Interbead bonds formed during processing and was characterized by observing both the product surface and the fracture surfaces using scanning electron microscopy (SEM) and by measuring the tensile properties of specimens. It was found that both the degree of interbead bonding and the tensile strength had a direct relationship to the steam pressure/temperature and that high steam temperature enhanced the crystallization behavior of EPP beads during the cooling process. A formation mechanism of interbead bonding during processing was proposed based on the crystal melting behaviors, as discussed in the first article of this series (Zhai et al. Ind. Eng. Chem. Res. 2010, 49, 9822). Possible parameters influencing the formation of interbead bonding, namely, steam temperature/pressure and bead foam expansion ratio, were investigated in this study. The effect of postannealing of the EPP products on the shrinkage and crystallinity of the molded EPP products was also addressed. DEWEY : 660 ISSN : 0888-5885 En ligne : http://pubs.acs.org/doi/abs/10.1021/ie101753w

Ultrasonic irradiation enhanced cell nucleation in microcellular poly (lactic Acid) / Jing Wang in Industrial & engineering chemistry research, Vol. 50 N° 24 (Décembre 2011) 

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