Vinyl chloride polymerization in microdroplet reactor - Les thÃ¨ses en ...

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Chapter I : Bibliographic review variety of hydrolysis degree (measured as a function of degree of hydrolysis of the acetates group), is generaly used in commercial production of PVC. The degree of hydrolysis and molecular weight of PVA determines the protective action and therefore the final size, shape and porosity (morphology) of the polymer particles. Also it determines the positioning of PVA at the surface of the VCM droplets, as shown in Figure I- 6: Hydrophilic segment H 2 O OH OH OH OH OH OH OH OH OH OH OH OAc OAc OH OH OH OAc OAc OH OAc OH OAc OAc OAc OAc OH OH OAc OAc Hydrophobic MVC OAc segment VCM OH OH OH Figure I- 6 : PVA suspending agent positioned at the interface monomer - water The so-called primary suspending agent is employed to regulate the size of the polymer particles. This PVA is a block copolymer with a hydrolysis degree over 70%. Due to its pronounced hydrophilic character, this kind of agent is situated mainly at the outside of the droplet and is able to stabilize it well enough to control coalescence. The resulting final PVC particles are of the same size as the initial VCM droplet, which makes this PVA very suitable for regulating the particle size. A secondary suspending agent, on the other hand, acts on the porosity of the polymer particles. PVA used as secondary suspending agent is a copolymer with a medium degree of hydrolysis, usually ranging from 20 to 60%. This PVA is more withdrawn into the droplet and therefore has a lower surface tension, which makes it less able to stabilize the initial droplet. This way the droplet will break down into several smaller droplets, which will aggregate again during polymerization, as they are not stabilized well enough against flocculation. The final particles thus formed are larger than the initial droplets and have an irregular shape and a high porosity caused by the voids within the aggregates (Ormondroyd, 1987). Figure I- 7 illustrates the effect of each suspending agent on the grain porosity and dimensions. 26
Chapter I : Bibliographic review b) 200 Grain porosity (%) 40 30 a) 150 Mean grain size (µm) 60 70 80 Degree of hydrolysis (%) 120 Figure I- 7: Effect of the degree of hydrolysis of PVA’s on grain porosity (a) and mean grain size (b) of PVC. When a combination of the primary and secondary suspending agents is used, it is possible to obtain intermediate conditions. The initial droplets will still break down, but to a smaller extent. These droplets will aggregate again during polymerization, resulting in a final particle of a multicellular structure, formed by several coalesced droplets. I.D.2.b Buffer A buffer salt such as sodium biphosphate is generally used to prevent any lowering of pH of the aqueous phase due to HCl loss from PVC during polymerization. The neutralization of HCl counteracts thermal degradation during polymerization and corrosion of the reactor wall or tubing. Furthermore, the neutralization of HCl prevents a secondary reaction taking place, such as the acid catalyzed hydrolysis of the ester groups of PVA. This undesired reaction could lead to substantial differences in the final particle size and size distribution of the PVC grains. There are no easy ways to measure the exact value of pH during the reaction. It is assumed that the amount of buffer introduced at the beginning in the reactor (0.08 % wt based on water) suffices so that the reaction medium does not become too acid (pH around 5-6). I.D.2.c Inhibitor of polymerization The VCM polymerization reaction is a highly exothermic reaction, therefore emergency situations such as power failures, temperature runaways or agitation system problems can occur. A temperature runaway would cause an increase in both reaction rate and pressure, leading even to an explosion. To avoid the risks caused by an acceleration of the process, it is necessary to completely stop the reaction. 27
Page 1: THÈSE En vue de l'obtention du DOC
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Page 7 and 8: TABLE OF CONTENTS LIST OF SYMBOLS..
Page 9: VI. TWO-PHASE LITERATURE MODEL FOR
Page 12 and 13: List of symbols M g Mass of monomer
Page 14 and 15: List of symbols BA BMA ClBu CRP DCH
Page 17 and 18: Introduction and outline INTRODUCTI
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Chapter II: Development of the micr
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CHAPTER III: ON-LINE KINETIC MONITO
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Chapter III: On-line kinetic monito
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CHAPTER IV: MORPHOLOGIC CHARACTERIS
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Chapter IV: Morphologic characteris
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GENERAL CONCLUSION AND PERSPECTIVES
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General conclusion and perspectives
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General conclusion and perspectives
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Appendix 1 APPENDIX 1 Vinyl chlorid
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Appendix 1 Kidney, stomach and skin
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Appendix 1 EPA. Integrated Risk Inf
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Appendix 2 APPENDIX 2 Modelling of
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Appendix 2 Conversion degree 1 0,9
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Appendix 2 K 11 /γ m = 0 V ) m (T
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Bibliographic references BIBLIOGRAP
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Bibliographic references C Cameron,
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Bibliographic references Ehrfeld W.
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Bibliographic references K Kamachi
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Bibliographic references McAdams W.
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