Microparticle Design in Polyolefin Reaction Engineering and Drug Delivery
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Date
2019
Publication Type
Doctoral Thesis
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yes
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Abstract
In Ziegler-Natta catalysis, the catalyst particle size has a strong influence not only on catalyst performance but also on the morphology and particle size distribution of the final polymer particles. Fundamental insight into the catalyst particle formation process is therefore of industrial importance when addressing specific requirements in the final products.
In the first part of this work, we fully characterize a single-step catalyst preparation process, which comprises the reactive precipitation of a MgCl2-supported Ziegler-Natta catalyst (ZNC) through decomposition of the hetero bi-metallic complex, MgCl2∙Ti(OR)4, by addition of ethyl aluminum dichloride (EADC). We track the evolutions of both the concentrations of the metals (Mg, Ti, Al) as well as Cl in the liquid phase and the size of the formed catalyst particles.
It is observed that the liquid phase composition is governed by the EADC feed rate under fully Cl-starved conditions. The process can be divided into two stages: The first stage is dominated by the precipitation of the MgCl2-based support, and the second stage involves complex adsorption-precipitation of the Ti-complexes. The growth of the catalyst particle size occurs only in the first stage and is controlled by the aggregation and breakage events during the MgCl2 precipitation. It follows that the hydrodynamic stress in the reactor plays the essential role in controlling the catalyst size. In the second stage, no further particle growth occurs, not only because of the depletion of Mg in the liquid phase but also because of the increased stability of the particles against aggregation, which is most likely caused by adsorbing Ti-complexes.
Finally, we have performed polymerization tests with the prepared catalysts and found that the size distribution (SD) of the polymer particles indeed closely replicates the one of the used catalyst particles.
Within the second part of this work, we apply sonofragmentation to break the ZNC particles dispersed in hexane. Our main goals are first to monitor the breakage induced by the ultrasonic power in order to understand the kinetics of breakage and the effect of sonofragmentation on the colloidal stability of the ZNC particles. Second, we explore the effect of sonofragmentation on the polymerization performance of the ZNC particles.
It is found that sonofragmentation can not only reduce the size but also narrow the SD of the ZNC particles. From the ethylene polymerization tests, it is observed that the catalyst yield of the sonofragmented ZNC is substantially higher than that of the untreated one and that the obtained polymer particles exhibit a smaller average size and a narrower SD.
The third part of this work discusses the development of a microparticulate carrier system for the delivery of an antiseptic molecule. Nosocomial infections remain a serious treat even for patients in highly industrialized countries. These infections are mostly occurring on surgical sites and wounds, on entry sites of catheters, tubes and other indwelling devices. The involvement of multi-resistant bacterial strains in these infections makes them particularly difficult to cure with the antibiotics presently available. In this sense, medical devices providing local antiseptic action over an extended period of time may represent an opportunity to prevent such infections. One way to achieve this goal is the encapsulation of active molecules into bioresorbable polymer matrices, which can locally release the compound of interest along a predefined time span and do not need to be removed since they naturally degrade in vivo.
Spray drying is used to encapsulate the broad band antiseptic octenidine hydrochloride (OHC) into a set of poly(D,L-lactide) (PDLLA) and poly(D,L-lactide-co-glycolide) (PLGA) carrier materials with different molecular weights and chain end-groups, with efficiencies on the order of 90 %. For each material, the efficacy of entrapment and the ability to provide extended release time is evaluated through release experiments over 24 h and up to three weeks respectively. In addition, the effect of enzymes on the release profile is assessed through the addition of the serine protease Proteinase-K.
It is demonstrated that the carrier materials bearing acid end-groups provide a significantly larger entrapment efficacy with respect to their ester counterparts independently of the carrier composition and molecular weight. Furthermore, it is found that only the PLGA carriers bearing acid end-groups provide extended release over the tested time frame. Conversely, the addition of Proteinase-K triggers steady release of the samples encapsulated with PDLLA bearing an acid-end, whereas it is shown that enzymatic degradation is negligible for the ones bearing ester-ends.
Finally, it is demonstrated on cultures of Staphylococcus epidermidis that the encapsulated and subsequently released OHC has conserved its antiseptic activity, which supports the potential applicability of the approach.
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Examiner : Morbidelli, Massimo
Examiner : Grass, Robert
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ETH Zurich
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Subject
microparticles; polyolefins; drug delivery; catalysis
Organisational unit
03451 - Morbidelli, Massimo (emeritus) / Morbidelli, Massimo (emeritus)