Documents disponibles écrits par cet auteur

Crystallization kinetics of linear and long - chain - branched polylactide / Mohammadreza Nofar in Industrial & engineering chemistry research, Vol. 50 N° 24 (Décembre 2011) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 50 N° 24 (Décembre 2011) . - pp. 13789–13798 Titre : Crystallization kinetics of linear and long - chain - branched polylactide Type de document : texte imprimé Auteurs : Mohammadreza Nofar, Auteur ; Wenli Zhu, Auteur ; Chul B. Park, Auteur Année de publication : 2012 Article en page(s) : pp. 13789–13798 Note générale : Chimie industrielle Langues : Anglais (eng) Mots-clés : Kinetics Résumé : In this study, the non-isothermal cold crystallization and isothermal melt crystallization of both linear and long-chain-branched (LCB) polylactide (PLA) were investigated using a differential scanning calorimeter (DSC). Talc was used as a nucleating agent to promote crystallization. The effects of chain branching on PLA’s cold crystallization kinetics at different heating rates and on PLA’s melt crystallization kinetics under different temperatures were studied by using Avrami analysis. The results showed that LCB-PLAs have faster cold and melt crystallization rates than those of linear PLA, since branched chains can play a role of nucleating site. Talc is a powerful nucleating agent, especially for linear PLA, either in cold crystallization or melt crystallization process. It was seen that addition of talc to PLA improves the crystallinity of PLA samples with more linear structure, more effectively because of its role of crystal nucleation. In PLA samples with more branched structure, talc has the least effect on crystallinity suggesting that the branched structure dominated crystallization already regardless of the presence of talc. Isothermal melt crystallization experimental results also showed that branched PLAs crystallized much faster than linear PLA and talc could increase the melt crystallization rate of linear PLA, but not that of PLA with a more branched structure. DEWEY : 660 ISSN : 0888-5885 En ligne : http://pubs.acs.org/doi/abs/10.1021/ie2011966 [article] Crystallization kinetics of linear and long - chain - branched polylactide [texte imprimé] / Mohammadreza Nofar, Auteur ; Wenli Zhu, Auteur ; Chul B. Park, Auteur . - 2012 . - pp. 13789–13798. Chimie industrielle Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 50 N° 24 (Décembre 2011) . - pp. 13789–13798 Mots-clés : Kinetics Résumé : In this study, the non-isothermal cold crystallization and isothermal melt crystallization of both linear and long-chain-branched (LCB) polylactide (PLA) were investigated using a differential scanning calorimeter (DSC). Talc was used as a nucleating agent to promote crystallization. The effects of chain branching on PLA’s cold crystallization kinetics at different heating rates and on PLA’s melt crystallization kinetics under different temperatures were studied by using Avrami analysis. The results showed that LCB-PLAs have faster cold and melt crystallization rates than those of linear PLA, since branched chains can play a role of nucleating site. Talc is a powerful nucleating agent, especially for linear PLA, either in cold crystallization or melt crystallization process. It was seen that addition of talc to PLA improves the crystallinity of PLA samples with more linear structure, more effectively because of its role of crystal nucleation. In PLA samples with more branched structure, talc has the least effect on crystallinity suggesting that the branched structure dominated crystallization already regardless of the presence of talc. Isothermal melt crystallization experimental results also showed that branched PLAs crystallized much faster than linear PLA and talc could increase the melt crystallization rate of linear PLA, but not that of PLA with a more branched structure. DEWEY : 660 ISSN : 0888-5885 En ligne : http://pubs.acs.org/doi/abs/10.1021/ie2011966

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

Strategies to estimate the pressure drop threshold of nucleation for polystyrene foam with carbon dioxide / Siu N. Leung in Industrial & engineering chemistry research, Vol. 48 N°4 (Février 2009) 

Public ISBD [article]  in Industrial & engineering chemistry research > Vol. 48 N°4 (Février 2009) . - pp. 1921–1927 Titre : Strategies to estimate the pressure drop threshold of nucleation for polystyrene foam with carbon dioxide Type de document : texte imprimé Auteurs : Siu N. Leung, Auteur ; Anson Wong, Auteur ; Chul B. Park, Auteur Année de publication : 2009 Article en page(s) : pp. 1921–1927 Note générale : Chemical engineering Langues : Anglais (eng) Mots-clés : Pressure  drop  threshold  Polymer  foaming  processes  Bubble  nucleation Résumé : Two methods to predict the pressure drop threshold (ΔPthreshold) to initiate bubble nucleation in polymer foaming processes are developed. One method uses the modified nucleation theory developed in our previous work, while the other utilizes computer simulations to model the growth profiles of the first observable bubbles in batch foaming experiments. These two approaches have shown reasonably good agreement qualitatively with each other in their ΔPthreshold predictions. Moreover, the effects of the pressure drop rate, the gas content, and the processing temperature on ΔPthreshold are demonstrated. It was found that the pressure drop rate does not have a significant effect on ΔPthreshold, while increasing the gas content or the processing temperature leads to a decrease in ΔPthreshold. En ligne : http://pubs.acs.org/doi/abs/10.1021/ie800079x [article] Strategies to estimate the pressure drop threshold of nucleation for polystyrene foam with carbon dioxide [texte imprimé] / Siu N. Leung, Auteur ; Anson Wong, Auteur ; Chul B. Park, Auteur . - 2009 . - pp. 1921–1927. Chemical engineering Langues : Anglais (eng) in Industrial & engineering chemistry research > Vol. 48 N°4 (Février 2009) . - pp. 1921–1927 Mots-clés : Pressure  drop  threshold  Polymer  foaming  processes  Bubble  nucleation Résumé : Two methods to predict the pressure drop threshold (ΔPthreshold) to initiate bubble nucleation in polymer foaming processes are developed. One method uses the modified nucleation theory developed in our previous work, while the other utilizes computer simulations to model the growth profiles of the first observable bubbles in batch foaming experiments. These two approaches have shown reasonably good agreement qualitatively with each other in their ΔPthreshold predictions. Moreover, the effects of the pressure drop rate, the gas content, and the processing temperature on ΔPthreshold are demonstrated. It was found that the pressure drop rate does not have a significant effect on ΔPthreshold, while increasing the gas content or the processing temperature leads to a decrease in ΔPthreshold. En ligne : http://pubs.acs.org/doi/abs/10.1021/ie800079x